U.S. patent application number 15/566246 was filed with the patent office on 2018-04-12 for hot-rolled steel sheet and method for producing the same.
The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Hiroyuki KAWATA, Yasumitsu KONDO, Mutsumi SAKAKIBARA, Natsuko SUGIURA, Yasuaki TANAKA, Takafumi YOKOYAMA.
Application Number | 20180100213 15/566246 |
Document ID | / |
Family ID | 57127115 |
Filed Date | 2018-04-12 |
United States Patent
Application |
20180100213 |
Kind Code |
A1 |
TANAKA; Yasuaki ; et
al. |
April 12, 2018 |
HOT-ROLLED STEEL SHEET AND METHOD FOR PRODUCING THE SAME
Abstract
A hot-rolled steel sheet according to the present embodiment has
a chemical composition containing, in mass %: C: 0.07 to 0.30%, Si:
more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0.030% or less, S:
0.010% or less, Al: 0.01 to less than 1.0%, N: 0.01% or less, O:
0.01% or less, and one or more types selected from the group
consisting of Sb, Sn and Te in a total amount of 0.03 to 0.30%,
with a balance being Fe and impurities, and satisfying Formula (1).
The micro-structure of the hot-rolled steel sheet includes ferrite
and pearlite in a total amount of 50 area % or more. The tensile
strength of the hot-rolled steel sheet is 900 MPa or less.
Si+Mn.gtoreq.3.20 (1) Where, a content (mass %) of a corresponding
element is substituted for each symbol of an element in Formula
(1).
Inventors: |
TANAKA; Yasuaki;
(Nishinomlya-shi, Hyogo, JP) ; SAKAKIBARA; Mutsumi;
(Ota-shi, Oita, JP) ; YOKOYAMA; Takafumi;
(Futtsu-shi, Chiba, JP) ; KAWATA; Hiroyuki;
(Kisarazu-shi, Chiba, JP) ; SUGIURA; Natsuko;
(Kimitsu-shi, Chiba, JP) ; KONDO; Yasumitsu;
(Chiba-shi, Chiba, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
57127115 |
Appl. No.: |
15/566246 |
Filed: |
April 14, 2016 |
PCT Filed: |
April 14, 2016 |
PCT NO: |
PCT/JP2016/061991 |
371 Date: |
October 13, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 6/008 20130101;
C22C 38/32 20130101; C22C 38/002 20130101; C22C 38/16 20130101;
C22C 38/38 20130101; B21B 2001/225 20130101; C22C 38/34 20130101;
C21D 8/0226 20130101; C21D 9/46 20130101; C21D 2211/009 20130101;
B21B 1/22 20130101; C22C 38/06 20130101; C22C 38/12 20130101; C21D
8/0263 20130101; C22C 38/60 20130101; C21D 2211/002 20130101; C21D
6/002 20130101; C22C 38/02 20130101; C21D 6/001 20130101; B21B 1/26
20130101; C22C 38/005 20130101; C22C 38/08 20130101; C22C 38/28
20130101; C22C 38/008 20130101; C22C 38/001 20130101; C21D 2211/008
20130101; C22C 38/14 20130101; C21D 6/005 20130101; C21D 2211/005
20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 8/02 20060101 C21D008/02; C21D 6/00 20060101
C21D006/00; C22C 38/38 20060101 C22C038/38; C22C 38/34 20060101
C22C038/34; C22C 38/32 20060101 C22C038/32; C22C 38/60 20060101
C22C038/60; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12; C22C 38/16 20060101 C22C038/16; C22C 38/08 20060101
C22C038/08; C22C 38/02 20060101 C22C038/02; C22C 38/06 20060101
C22C038/06; C22C 38/00 20060101 C22C038/00; B21B 1/22 20060101
B21B001/22 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 15, 2015 |
JP |
2015-083603 |
Apr 15, 2015 |
JP |
2015-083604 |
Claims
1. A hot-rolled steel sheet having a chemical composition
consisting of, in mass %: C: 0.07 to 0.30%, Si: more than 1.0 to
2.8%, Mn: 2.0 to 3.5%, P: 0.030% or less, S: 0.010% or less, Al:
0.01 to less than 1.0%, N: 0.01% or less, O: 0.01% or less, Sb:
0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to 0.30%, Nb: 0 to 0.15%, Cr: 0
to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 1.0%, B: 0 to
0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0 to 0.30%, Se: 0 to
0.30%, Te: 0 to 0.30%, Ge: 0 to 0.30%, As: 0 to 0.30%, Ca: 0 to
0.50%, Mg: 0 to 0.50%, Zr: 0 to 0.50%, Hf: 0 to 0.50%, and rare
earth metal: 0 to 0.50%, with a balance being Fe and impurities,
and satisfying Formula (1): Si+Mn.gtoreq.3.20 (1) where, a content
(mass %) of a corresponding element is substituted for each symbol
of an element in Formula (1).
2. The hot-rolled steel sheet according to claim 1, containing, in
mass %, one or more types of element selected from a group
consisting of: Ti: 0.005 to 0.15%, V: 0.001 to 0.30%, and Nb: 0.005
to 0.15%.
3. The hot-rolled steel sheet according to claim 1, containing, in
mass %, one or more types of element selected from a group
consisting of: Cr: 0.10 to 1.0%, Ni: 0.10 to 1.0%, Mo: 0.01 to
1.0%, W: 0.01 to 1.0%, and B: 0.0001 to 0.010%.
4. The hot-rolled steel sheet according to claim 1, containing, in
mass %, Cu: 0.10 to 0.50%.
5. The hot-rolled steel sheet according to claim 1, containing, in
mass %, one or more types of element selected from a group
consisting of Sn, Bi, Se, Te, Ge and As in a total amount of 0.0001
to 0.30%.
6. The hot-rolled steel sheet according to claim 1, containing, in
mass %, one or more types of element selected from a group
consisting of Ca, Mg, Zr, Hf and rare earth metal in a total amount
of 0.0001 to 0.50%.
7. The hot-rolled steel sheet according to claim 1, comprising: an
Sb concentrated layer having a thickness of 0.5 .mu.m or more
between a surface and scale.
8. The hot-rolled steel sheet according to claim 1, wherein: in a
micro-structure of the hot-rolled steel sheet, a total area
fraction of ferrite and pearlite is 75% or more; and a tensile
strength of the hot-rolled steel sheet is 800 MPa or less.
9. The hot-rolled steel sheet according to claim 1, wherein: in a
micro-structure of the hot-rolled steel sheet, a total area
fraction of bainite and martensite is 75% or more; and a tensile
strength of the hot-rolled steel sheet is 900 MPa or more.
10. The hot-rolled steel sheet according to claim 1, wherein: in a
micro-structure of the hot-rolled steel sheet, a total area
fraction of bainite and martensite is 75% or more; and a tensile
strength of the hot-rolled steel sheet is 800 MPa or less.
11. The hot-rolled steel sheet according to claim 1, wherein: a
thickness of an internal oxidized layer of the hot-rolled steel
sheet is 5 .mu.m or less.
12. The hot-rolled steel sheet according to claim 8, wherein: a
scale thickness on a surface of the hot-rolled steel sheet is 10
.mu.m or less.
13. The hot-rolled steel sheet according to claim 8, wherein: a
decarburization layer thickness in an outer layer of the hot-rolled
steel sheet is 20 .mu.m or less.
14. The hot-rolled steel sheet according to claim 9, wherein: a
scale thickness on a surface of the hot-rolled steel sheet is 7
.mu.m or less.
15. A production method for producing a hot-rolled steel sheet
comprising: a process of preparing a steel material having a
chemical composition consisting of, in mass %: C: 0.07 to 0.30%,
Si: more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0.030% or less, S:
0.010% or less, Al: 0.01 to less than 1.0%, N: 0.01% or less, O:
0.01% or less, Sb: 0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to 0.30%,
Nb: 0 to 0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%, W: 0
to 1.0%, B: 0 to 0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0 to
0.30%, Se: 0 to 0.30%, Te: 0 to 0.30%, Ge: 0 to 0.30%, As: 0 to
0.30%, Ca: 0 to 0.50%, Mg: 0 to 0.50%, Zr: 0 to 0.50%, Hf: 0 to
0.50%, and rare earth metal: 0 to 0.50%, with a balance being Fe
and impurities, and satisfying Formula (1): Si+Mn.gtoreq.3.20 (1)
where, a content (mass %) of a corresponding element is substituted
for each symbol of an element in Formula (1), a process of heating
the steel material to 1100 to 1350.degree. C., and thereafter
performing hot rolling of the steel material so as to form a steel
sheet; and a process of coiling the steel sheet at a temperature of
600 to 750.degree. C., wherein in a micro-structure of the
hot-rolled steel sheet, a total area fraction of ferrite and
pearlite is 75% or more; and a tensile strength of the hot-rolled
steel sheet is 800 MPa or less.
16. A production method for producing a hot-rolled steel sheet
comprising: a preparation process of preparing a steel material
having a chemical composition consisting of, in mass %: C: 0.07 to
0.30%, Si: more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0.030% or
less, S: 0.010% or less, Al: 0.01 to less than 1.0%, N: 0.01% or
less, O: 0.01% or less, Sb: 0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to
0.30%, Nb: 0 to 0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%,
W: 0 to 1.0%, B: 0 to 0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0
to 0.30%, Se: 0 to 0.30%, Te: 0 to 0.30%, Ge: 0 to 0.30%, As: 0 to
0.30%, Ca: 0 to 0.50%, Mg: 0 to 0.50%, Zr: 0 to 0.50%, Hf: 0 to
0.50%, and rare earth metal: 0 to 0.50%, with a balance being Fe
and impurities, and satisfying Formula (1): Si+Mn.gtoreq.3.20 (1)
where, a content (mass %) of a corresponding element is substituted
for each symbol of an element in Formula (1), a hot rolling process
of heating the steel material to 1100 to 1350.degree. C., and
thereafter performing hot rolling of the steel material so as to
form a steel sheet, and cooling the steel sheet to a coiling
temperature; and a process of coiling the steel sheet after the
cooling, at a temperature of 150 to 600.degree. C., wherein in a
micro-structure of the hot-rolled steel sheet, a total area
fraction of bainite and martensite is 75% or more; and a tensile
strength of the hot-rolled steel sheet is 900 MPa or more.
17. A production method for producing a hot-rolled steel sheet
comprising: a process of preparing a steel material having a
chemical composition consisting of, in mass %: C: 0.07 to 0.30%,
Si: more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0.030% or less, S:
0.010% or less, Al: 0.01 to less than 1.0%, N: 0.01% or less, O:
0.01% or less, Sb: 0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to 0.30%,
Nb: 0 to 0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%, W: 0
to 1.0%, B: 0 to 0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0 to
0.30%, Se: 0 to 0.30%, Te: 0 to 0.30%, Ge: 0 to 0.30%, As: 0 to
0.30%, Ca: 0 to 0.50%, Mg: 0 to 0.50%, Zr: 0 to 0.50%, Hf: 0 to
0.50%, and rare earth metal: 0 to 0.50%, with a balance being Fe
and impurities, and satisfying Formula (1): Si+Mn.gtoreq.3.20 (1)
where, a content (mass %) of a corresponding element is substituted
for each symbol of an element in Formula (1), a process of heating
the steel material to 1100 to 1350.degree. C., and thereafter
performing hot rolling of the steel material so as to form a steel
sheet; a process of coiling the steel sheet at a temperature of 150
to 600.degree. C.; and a process of tempering the steel sheet after
coiling, at a temperature of 550.degree. C. or more, wherein in a
micro-structure of the hot-rolled steel sheet, a total area
fraction of bainite and martensite is 75% or more; and a tensile
strength of the hot-rolled steel sheet is 800 MPa or less.
18. The hot-rolled steel sheet according to claim 10, wherein a
scale thickness on a surface of the hot-rolled steel sheet is 7
.mu.m or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hot-rolled steel sheet
and a method for producing the hot-rolled steel sheet.
BACKGROUND ART
[0002] In order to compatibly achieve both collision safety and a
reduction in weight of automobiles, the application of high
strength steel sheets to automobiles is progressing. A high
strength steel sheet contains a large amount of alloying elements
to increase strength. In particular, a high strength steel sheet
having a tensile strength of 980 MPa or more contains a large
amount of Si and Mn.
[0003] A high strength steel sheet is usually produced by the
following method. First, a slab is hot rolled to produce a
hot-rolled steel sheet, which is then coiled in a coil shape. Next,
the hot-rolled steel sheet is pickled, cold-rolled, and
annealed.
[0004] The temperature of the hot-rolled steel sheet when being
coiled into a coil shape (hereunder, referred to as "coiling
temperature") may be raised to enhance the cold workability of the
hot-rolled steel sheet. If the coiling temperature is high, an
internal oxidized layer is formed in the vicinity of an outer layer
of the hot-rolled steel sheet. The internal oxidized layer is
formed with a thickness of several tens of .mu.m from the surface
of the base metal of the hot-rolled steel sheet toward the center
of the plate thickness. The internal oxidized layer reduces the
surface properties, formability and weldability of the steel sheet
after cold rolling (cold-rolled steel sheet). Therefore, the
internal oxidized layer is removed before cold rolling, by
subjecting the hot-rolled steel sheet to a pickling treatment.
[0005] Further, during production of the hot-rolled steel sheet, an
oxide film (scale) is formed on the surface of the hot-rolled steel
sheet. The scale reduces the surface properties, formability and
weldability of the steel sheet. Therefore, similarly to the
internal oxidized layer, the scale is also removed by subjecting
the hot-rolled steel sheet to a pickling treatment.
[0006] However, if the internal oxidized layer or the scale is
thick, an excessive workload is applied in the pickling treatment
for the hot-rolled steel sheet. In addition, if the internal
oxidized layer or the scale remains, as described above, the
surface properties, formability and weldability of the cold-rolled
steel sheet are reduced. Furthermore, the internal oxidized layer
or the scale peel off during forming of the cold-rolled steel
sheet, and cause surface defects such as indentation defects.
[0007] The internal oxidized layer is formed as a result of an
alloying element in the base metal being selectively oxidizing. Si
and Mn are easily oxidized. Therefore, an internal oxidized layer
is liable to arise in a hot-rolled steel sheet in which the content
of Si and Mn is high. Scale is similarly liable to become thick on
a hot-rolled steel sheet having a high Si and Mn content.
[0008] In addition, as a time period in which the steel sheet has a
high temperature continues, the thicknesses of the internal
oxidized layer and scale increase. If the coiling temperature is
raised in order to enhance the cold workability of the hot-rolled
steel sheet as described above, an internal oxidized layer is more
liable to arise, and is liable to be thick. This situation
similarly applies with respect to scale.
[0009] Technology for suppressing the formation of the above
described internal oxidized layer and scale is proposed in
JP62-13520A (Patent Literature 1), JP2010-535946A (Patent
Literature 2), JP2013-253301A (Patent Literature 3), JP2011-184741A
(Patent Literature 4), JP2011-231391A (Patent Literature 5),
JP2012-036483A (Patent Literature 6), JP2013-216961A (Patent
Literature 7), JP2013-103235A (Patent Literature 8), JP2010-503769A
(Patent Literature 9), JP2011-523441A (Patent Literature 10),
JP2015-113505A (Patent Literature 11), JP2004-332099A (Patent
Literature 12), JP2013-060657A (Patent Literature 13) and
JP2011-523443A (Patent Literature 14).
[0010] According to Patent Literature 1, an antioxidant agent is
coated onto a steel sheet surface. It is described in Patent
Literature 1 that by this means the formation of an internal
oxidized layer and scale is suppressed.
[0011] According to Patent Literature 2, a hot-rolled steel sheet
is coiled at a comparatively low temperature of 530 to 580.degree.
C. It is described in Patent Literature 2 that by this means the
formation of an oxidized layer is suppressed.
[0012] According to Patent Literature 3, a hot-rolled steel sheet
after rolling is coiled at a temperature between 750.degree. C. and
600.degree. C. to form a coil. After coiling, the coil is
maintained for 10 to 30 minutes, and thereafter the hot-rolled
steel sheet is cooled while dispensing the coil. Subsequently, when
the temperature of the hot-rolled steel sheet reaches 550.degree.
C. or less, the hot-rolled steel sheet is coiled again to form a
coil. It is described in Patent Literature 3 that in this case the
oxidized layer can be thin.
[0013] According to Patent Literatures 4 to 6, a heat treatment or
a cooling treatment is performed on a steel sheet after hot rolling
or after coiling in an atmosphere in which the oxygen concentration
is reduced. It is described in the aforementioned Patent
Literatures 4 to 6 that the heat treatment or cooling treatment in
the atmosphere in which the oxygen concentration is reduced is
effective to reduce scale and an internal oxidized layer.
[0014] According to Patent Literature 7, descaling is performed
prior to coiling on a hot-rolled steel sheet after hot rolling, to
thereby remove oxide scale from the surface thereof. By removing
the oxide scale, an oxygen supply source that is utilized for
formation of an internal oxidized layer during coil cooling
decreases. It is described in Patent Literature 7 that,
consequently, not only the scale but also the internal oxidized
layer decreases.
[0015] In Patent Literature 8, a cooling method is proposed for
uniformizing an internal oxidation amount of a hot-rolled steel
sheet within a preferable range across the width direction, in the
longitudinal direction thereof.
[0016] On the other hand, in Patent Literatures 9 to 14, technology
that is different from the technology disclosed in the above
described Patent Literatures is proposed. According to Patent
Literature 9, internal oxidation is suppressed by appropriately
controlling the alloy elements of a steel and the conditions for
heat treatment of a hot-rolled steel sheet. Specifically, according
to Patent Literature 9, Sb of a content of 0.001 to 0.1% is
contained in the steel, the steel sheet is reheated to 1100 to
1250.degree. C. and subjected to hot rolling, and coiled at a
temperature of 450 to 750.degree. C. Thereafter, the hot-rolled
steel sheet is subjected to pickling and cold rolling, and is
annealed at 700 to 850.degree. C. By this means, formation of an
internal oxidized layer is suppressed.
[0017] In Patent Literature 10, technology is proposed to
appropriately control alloy elements to suppress formation of an
oxide, and thereby improve a plating property. According to Patent
Literature 10, a steel slab is used that contains 0.005 to 0.1% of
Sb, and in which the relation between the contents of Ni, Mn, Al
and Ti is adjusted. The steel slab is subjected to hot working, and
hot rolled and coiled at 500 to 700.degree. C. In addition, the
steel slab is subjected to pickling, cold rolling and annealing. By
this means, internal oxidation is suppressed.
[0018] According to Patent Literature 11, a slab containing 0.02 to
0.10% of Sb is subjected to hot rolling, pickling, cold rolling,
annealing and cooling. In this case, the finish rolling temperature
for the hot rolling is 800 to 1000.degree. C., and the draft during
the cold rolling is set to 20% or more. In addition, annealing is
performed under conditions of being held for 60 seconds or more in
a temperature range of 750 to 900.degree. C. in an atmosphere in
which a dew-point temperature is -35.degree. C. or less. After
annealing, cooling is performed to 300.degree. C. or less at an
average cooling rate of 30.degree. C./sec or more, followed by
tempering. By this means internal oxidation is suppressed.
[0019] Patent Literatures 12 to 14 describe suppressing scale by
appropriately adjusting a content of Si, a heating temperature of a
slab, a temperature of finish rolling and a coiling temperature and
the like.
[0020] However, even when the respective technologies described in
Patent Literatures 1 to 14 are implemented, in some cases a deep
internal oxidized layer is formed or thick scale is formed.
CITATION LIST
Patent Literature
Patent Literature 1: Japanese Patent Application Publication No.
62-13520
Patent Literature 2: National Publication of International Patent
Application No. 2010-535946
Patent Literature 3: Japanese Patent Application Publication No.
2013-253301
Patent Literature 4: Japanese Patent Application Publication No.
2011-184741
Patent Literature 5: Japanese Patent Application Publication No.
2011-231391
Patent Literature 6: Japanese Patent Application Publication No.
2012-036483
Patent Literature 7: Japanese Patent Application Publication No.
2013-216961
Patent Literature 8: Japanese Patent Application Publication No.
2013-103235
Patent Literature 9: National Publication of International Patent
Application No. 2010-503769
Patent Literature 10: National Publication of International Patent
Application No. 2011-523441
Patent Literature 11: Japanese Patent Application Publication No.
2015-113505
Patent Literature 12: Japanese Patent Application Publication No.
2004-332099
Patent Literature 13: Japanese Patent Application Publication No.
2013-060657
Patent Literature 14: National Publication of International Patent
Application No. 2011-523443
SUMMARY OF INVENTION
Technical Problem
[0021] An objective of the present invention is to provide a
hot-rolled steel sheet in which formation of an internal oxidized
layer and scale is suppressed.
Solution to Problem
[0022] A hot-rolled steel sheet according to the present embodiment
has a chemical composition consisting of, in mass %, C: 0.07 to
0.30%, Si: more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0.030% or
less, S: 0.010% or less, Al: 0.01 to less than 1.0%, N: 0.01% or
less, O: 0.01% or less, Sb: 0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to
0.30%, Nb: 0 to 0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%,
W: 0 to 1.0%, B: 0 to 0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0
to 0.30%, Se: 0 to 0.30%, Te: 0 to 0.30%, Ge: 0 to 0.30%, As: 0 to
0.30%, Ca: 0 to 0.50%, Mg: 0 to 0.50%, Zr: 0 to 0.50%, Hf: 0 to
0.50% and rare earth metal: 0 to 0.50%, with a balance being Fe and
impurities, and satisfying Formula (1):
Si+Mn.gtoreq.3.20 (1)
[0023] where, a content (mass %) of a corresponding element is
substituted for each symbol of an element in Formula (1).
Advantageous Effects of Invention
[0024] In the hot-rolled steel sheet of the present embodiment,
formation of an internal oxidized layer and scale is
suppressed.
BRIEF DESCRIPTION OF DRAWINGS
[0025] FIG. 1 is a list view showing SEM images of cross-sections
subjected to vital etching, oxygen mapping images obtained using
EPMA at the SEM image regions, and Sb mapping images with respect
to a steel with a high Si and high Mn content that does not contain
Sb (steel not containing Sb), and a steel containing Sb that is a
steel with a high Si and high Mn content that contains 0.1% of
Sb.
[0026] FIG. 2 is a view illustrating the relation between an Sb
content (.times.10.sup.-3%) and the thickness (.mu.m) of an
internal oxidized layer for a case where the amount of Sb contained
in a steel with a high Si and high Mn content was varied and
hot-rolled steel sheets were produced.
[0027] FIG. 3 shows SEM images of the vicinity of an outer layer in
the above described steel not containing Sb and steel containing
Sb.
DESCRIPTION OF EMBODIMENTS
[0028] The present inventors conducted investigations and studies
regarding an internal oxidized layer and scale with respect to a
steel with a high Si and high Mn content, and obtained the
following findings.
[0029] In an outer layer of a hot-rolled steel sheet after coiling,
an internal oxidized layer is formed within the base metal (ground
metal), and scale is formed adjoining the surface.
[0030] It is considered that the internal oxidized layer and scale
are formed by the following mechanism. Oxygen ions penetrate into
the hot-rolled steel sheet through grain boundaries of a
near-surface portion of the hot-rolled steel sheet and the surface
of the hot-rolled steel sheet. An internal oxidized layer is formed
as a result of the oxygen ions that penetrated into the inside of
the hot-rolled steel sheet oxidizing iron of the base metal. On the
other hand, iron ions in the base metal move to the surface of the
hot-rolled steel sheet through the grain boundaries. Scale is
formed as a result of the Fe that moved to the surface being
oxidized.
[0031] It is effective to block the movement path (grain boundaries
and surface) of the oxygen ions and iron ions in order to suppress
formation of an internal oxidized layer and scale. If elements that
easily segregate at the grain boundaries and surface (hereunder,
referred to as "segregation elements") are contained in a
hot-rolled steel sheet, the segregation elements segregate at the
surface and grain boundaries of the hot-rolled steel sheet and
suppress movement of oxygen ions and iron ions. Therefore,
penetration of oxygen ions into the inside of the hot-rolled steel
sheet can be suppressed. In addition, movement of iron ions to the
hot-rolled steel sheet surface can be suppressed. As a result,
formation of an internal oxidized layer and scale can be
suppressed.
[0032] The segregation elements are, for example, P, B, and Sb.
However, although P and B segregate at grain boundaries and block
the movement path of oxygen ions and iron ions, they also reduce
the mechanical properties of the hot-rolled steel sheet.
[0033] On the other hand, Sb segregates at the surface of the
hot-rolled steel sheet. Therefore, the present inventors produced a
hot-rolled steel sheet from steel with a high Si and high Mn
content, which also contained Sb, and examined the thickness of
scale and an internal oxidized layer.
[0034] FIG. 1 shows, with respect to a conventional steel with a
high Si and high Mn content that does not contain Sb (hereunder,
referred to as "steel not containing Sb") and a steel containing Sb
that is a conventional with a high Si and high Mn content, which
also contains 0.10% of Sb, SEM images at a cross-section in the
vicinity of the surface, oxygen mapping images obtained using EPMA
at the SEM image regions, and Sb mapping images. The steel not
containing Sb contained, in mass %, C: 0.185%, Si: 1.8%, Mn: 2.6%,
P: 0.01%, S: 0.002%, Al: less than 0.03%, N: 0.003%, O: 0.0009%,
and Ti: 0.005%, with the balance being Fe and impurities. The steel
containing Sb was steel for which 0.10% of Sb was added to the
chemical composition of the steel not containing Sb. For each of
these steels, a hot-rolled steel sheet was formed by hot rolling in
a similar manner to the conventional method. The aforementioned
microstructure observation and EPMA mapping was performed with
respect to the prepared hot-rolled steel sheets.
[0035] Referring now to the SEM images in FIG. 1, in the steel not
containing Sb, scale 10 was formed on the steel sheet surface, and
an internal oxidized layer 20 was formed in the base metal. On the
other hand, in the steel containing Sb, although the scale 10 was
formed, the thickness thereof was thinner than the scale 10 of the
steel not containing Sb. Further, in the steel containing Sb, the
internal oxidized layer 20 was not observed. As a result of
performing oxygen mapping using EPMA, in the steel not containing
Sb, oxygen was observed in the scale 10 and the internal oxidized
layer 20 (white region and gray region in the drawing). In
contrast, in the steel containing Sb, oxygen was observed in only
the region in which the scale 10 was formed (white region in the
drawing).
[0036] In addition, Sb mapping was performed using EPMA. As a
result, in the steel containing Sb, a layer 30 containing Sb (white
region in the drawing; hereunder referred to as "Sb concentrated
layer") was observed at the interface between the scale 10 and the
base metal.
[0037] As described above, in a case where Sb is contained in steel
with a high Si and high Mn content, an Sb concentrated layer is
formed. It is considered that, because of the formation of the Sb
concentrated layer, the situation is as follows. In a case where
steel with a high Si and high Mn content contains a suitable amount
of Sb, in a hot rolling process, an Sb concentrated layer is formed
at the interface (surface of the hot-rolled steel sheet) between
scale and the base metal. The Sb concentrated layer blocks the
penetration of oxygen ions into the base metal. Consequently, iron
in the base metal is not oxidized, and it is difficult for an
internal oxidized layer to form. The Sb concentrated layer also
suppresses movement of iron ions contained in the base metal to the
scale. Consequently, growth of the scale is suppressed, and the
thickness of the scale is thin.
[0038] Thus, the Sb concentrated layer functions as a so-called
"barrier layer" that blocks the movement of oxygen ions and iron
ions. Therefore, by formation of the Sb concentrated layer, the
penetration of oxygen ions into the base metal from the scale after
coiling of the hot-rolled steel sheet can be suppressed. In
addition, movement of iron ions to the scale from the base metal
can be suppressed. Therefore, the formation of an internal oxidized
layer and scale is suppressed.
[0039] A barrier layer like the Sb concentrated layer is not formed
even if P and B that are elements segregable at grain boundaries
are contained in steel with a high Si and high Mn content.
Accordingly, Sb is suitable for suppressing scale and an internal
oxidized layer.
[0040] FIG. 2 is a view illustrating the relation between the Sb
content (.times.10.sup.-3%) and the thickness (.mu.m) of an
internal oxidized layer in a case where the Sb amount contained in
a steel with a high Si and high Mn content is varied and hot-rolled
steel sheets are produced (coiling temperature of 750.degree. C.).
Referring to FIG. 2, it is found that the thickness of the internal
oxidized layer decreases noticeably as the Sb content increases.
Further, in a case where the Sb content is 0.03% or more, although
the thickness of the internal oxidized layer decreases as the Sb
content increases, the margin of the decrease is not as great as
when the Sb content is less than 0.03%. That is, in the relation
between the thickness of the internal oxidized layer and the Sb
content, an inflection point exists in the vicinity of an Sb
content equal to 0.03%.
[0041] In the present embodiment, the Sb concentrated layer also
suppresses movement of carbon contained in the base metal, and not
just movement of oxygen ions and iron ions. Consequently, it is
easy to maintain a uniform micro-structure in the plate thickness
direction, and the obtainment of strength in a cold-rolled steel
sheet after cold rolling and annealing is facilitated.
[0042] FIG. 3 shows SEM images of the vicinity of an outer layer in
the above described steel not containing Sb and steel containing
Sb. Referring to FIG. 3, it is found that in the steel not
containing Sb, a decarburization layer 40 is formed in the outer
layer. On the other hand, in the steel containing Sb in which the
Sb concentrated layer is formed at the interface between the base
metal and the scale, a decarburization layer is not formed.
Accordingly, the Sb concentrated layer can also suppress the
movement of carbon in the base metal, and not just suppress the
movement of oxygen ions and iron ions.
[0043] A hot-rolled steel sheet according to the present embodiment
that was completed based on the above described findings has a
chemical composition consisting of, in mass %, C: 0.07 to 0.30%,
Si: more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0.030% or less, S:
0.010% or less, Al: 0.01 to less than 1.0%, N: 0.01% or less, O:
0.01% or less, Sb: 0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to 0.30%,
Nb: 0 to 0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%, W: 0
to 1.0%, B: 0 to 0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0 to
0.30%, Se: 0 to 0.30%, Te: 0 to 0.30%, Ge: 0 to 0.30%, As: 0 to
0.30%, Ca: 0 to 0.50%, Mg: 0 to 0.50%, Zr: 0 to 0.50%, Hf: 0 to
0.50% and rare earth metal: 0 to 0.50%, with a balance being Fe and
impurities, and satisfying Formula (1):
Si+Mn.gtoreq.3.20 (1)
[0044] where, a content (mass %) of a corresponding element is
substituted for each symbol of an element in Formula (1).
[0045] The above described chemical composition may also contain
one or more types of element selected from the group consisting of
Ti: 0.005 to 0.15%, V: 0.001 to 0.30% and Nb: 0.005 to 0.15%.
[0046] The above described chemical composition may also contain
one or more types of element selected from the group consisting of
Cr: 0.10 to 1.0%, Ni: 0.10 to 1.0%, Mo: 0.01 to 1.0%, W: 0.01 to
1.0% and B: 0.0001 to 0.010%.
[0047] The above described chemical composition may also contain
Cu: 0.10 to 0.50%.
[0048] The above described chemical composition may also contain
one or more types of element selected from the group consisting of
Sn, Bi, Se, Te, Ge and As in a total amount of 0.0001 to 0.30%.
[0049] The above described chemical composition may also contain
one or more types of element selected from the group consisting of
Ca, Mg, Zr, Hf and rare earth metal in a total amount of 0.0001 to
0.50%.
[0050] A hot-rolled steel sheet according to the present embodiment
includes an Sb concentrated layer having a thickness of 0.5 .mu.m
or more between the surface and scale.
[0051] In the micro-structure of the above described hot-rolled
steel sheet, a total area fraction of ferrite and pearlite may be
75% or more, and the tensile strength of the hot-rolled steel sheet
may be 800 MPa or less.
[0052] In the micro-structure of the above described hot-rolled
steel sheet, a total area fraction of bainite and martensite may be
75% or more, and the tensile strength of the hot-rolled steel sheet
may be 900 MPa or more.
[0053] In the micro-structure of the above described hot-rolled
steel sheet, a total area fraction of bainite and martensite may be
75% or more, and the tensile strength of the hot-rolled steel sheet
may be 800 MPa or less.
[0054] Preferably, the thickness of an internal oxidized layer of
the hot-rolled steel sheet is 5 .mu.m or less.
[0055] Preferably, in the above described hot-rolled steel sheet
having a micro-structure in which a total area fraction of ferrite
and pearlite is 75% or more and which has a tensile strength of 800
MPa or less, a scale thickness is 10 .mu.m or less.
[0056] Preferably, in the above described hot-rolled steel sheet
having a micro-structure in which a total area fraction of ferrite
and pearlite is 75% or more and which has a tensile strength of 800
MPa or less, a decarburization layer thickness in an outer layer of
the hot-rolled steel sheet is 20 .mu.m or less.
[0057] Preferably, in the above described hot-rolled steel sheet
having a micro-structure in which a total area fraction of bainite
and martensite is 75% or more and which has a tensile strength of
900 MPa or more, a scale thickness is 7 .mu.m or less.
[0058] Preferably, in the above described hot-rolled steel sheet
having a micro-structure in which a total area fraction of bainite
and martensite is 75% or more and which has a tensile strength of
800 MPa or less, a scale thickness is 7 .mu.m or less.
[0059] A production method for producing the above described
hot-rolled steel sheet having a micro-structure in which a total
area fraction of ferrite and pearlite is 75% or more and which has
a tensile strength of 800 MPa or less includes: a process of
preparing a steel material having the above described chemical
composition; a process of heating the steel material to 1100 to
1350.degree. C. and thereafter performing hot rolling so as to form
the steel material into a steel sheet; and a process of coiling the
steel sheet at a temperature of 600 to 750.degree. C., preferably
650 to 750.degree. C., and more preferably 700 to 750.degree.
C.
[0060] A production method for producing the above described
hot-rolled steel sheet having a micro-structure in which a total
area fraction of bainite and martensite is 75% or more and which
has a tensile strength of 900 MPa or more includes: a preparation
process of preparing a steel material having the above described
chemical composition; a hot rolling process of heating the steel
material to 1100 to 1350.degree. C., thereafter performing hot
rolling so as to form the steel material into a steel sheet, and
cooling the steel sheet to a coiling temperature; and a process of
coiling the steel sheet after cooling, at a temperature of 150 to
600.degree. C., preferably 350 to 500.degree. C., and more
preferably 400 to 500.degree. C.
[0061] A production method for producing the above described
hot-rolled steel sheet having a micro-structure in which a total
area fraction of bainite and martensite is 75% or more and which
has a tensile strength of 800 MPa or less includes: a preparation
process of preparing a steel material having the above described
chemical composition; a hot rolling process of heating the steel
material to 1100 to 1350.degree. C., thereafter performing hot
rolling so as to form the steel material into a steel sheet, and
cooling the steel sheet to a coiling temperature; a process of
coiling the steel sheet after cooling, at a temperature of 150 to
600.degree. C., preferably 350 to 500.degree. C., and more
preferably 400 to 500.degree. C.; and a process of tempering the
steel sheet after coiling at a temperature of 550.degree. C. or
more.
[0062] Hereunder, hot-rolled steel sheets according to the present
embodiments are described in detail.
First Embodiment
[Chemical Composition]
[0063] The chemical composition of the hot-rolled steel sheet
according to the present embodiment contains the following
elements. The symbol "%" with respect to the chemical composition
means "percent by mass" unless specified otherwise.
[0064] C: 0.07 to 0.30%
[0065] Carbon (C) forms retained austenite in the hot-rolled steel
sheet and enhances the strength and formability of the steel. If
the C content is too low, the aforementioned effect will not be
obtained. On the other hand, if the C content is too high, the
strength of the hot-rolled steel sheet will be too high and a cold
rolling property will be reduced. If the C content is too high, the
weldability of the steel will also decrease. Therefore, the C
content is from 0.07 to 0.30%. A preferable lower limit of the C
content is 0.10%, more preferably is 0.12%, and further preferably
is 0.15%. A preferable upper limit of the C content is 0.25%, and
more preferably is 0.22%.
[0066] Si: More than 1.0 to 2.8%
[0067] Silicon (Si) suppresses the formation of iron-based carbides
and facilitates formation of retained austenite. The strength and
formability of the steel is improved by formation of retained
austenite. If the Si content is too low, the aforementioned effect
will not be obtained. On the other hand, if the Si content is too
high, an internal oxidized layer will grow noticeably and the
surface properties of the hot-rolled steel sheet will decrease. If
the Si content is too high, the hot-rolled steel sheet will also
become brittle and the ductility will decrease. Therefore, the Si
content is from more than 1.0 to 2.8%. A preferable lower limit of
the Si content is 1.3%, and more preferably is 1.5%. A preferable
upper limit of the Si content is 2.5%, and more preferably is
2.0%.
[0068] Mn: 2.0 to 3.5%
[0069] Manganese (Mn) increases the strength of the steel sheet. If
the Mn content is too low, a large amount of soft micro-structure
is formed during cooling after annealing, and the strength is
lowered. On the other hand, if the Mn content is too high, coarse
Mn concentrated parts form at a central portion of the plate
thickness and the steel becomes brittle. Consequently, a slab that
is cast is liable to crack. If the Mn content is too high, the
weldability of the steel also decreases. If the Mn content is too
high, the hot-rolled steel sheet will also harden and a cold
rolling property will decrease. Therefore, the Mn content is from
2.0 to 3.5%. A preferable lower limit of the Mn content is 2.2%,
more preferably is 2.3%, and further preferably is 2.5%. A
preferable upper limit of the Mn content is 3.2%, and more
preferably is 3.0%.
[0070] P: 0.030% or Less
[0071] Phosphorus (P) segregates at a central portion of the plate
thickness of the steel sheet and embrittles a weld zone. Therefore,
the P content is 0.030% or less. A low P content is preferable.
However, making the P content low increases the production costs.
Therefore, when taking the production cost into consideration, the
lower limit of the P content is, for example, 0.0010%.
[0072] S: 0.010% or Less
[0073] Sulfur (S) reduces the weldability of the steel. S also
reduces the producibility during casting and heat rolling. S also
combines with Mn to form MnS, and reduces the ductility and stretch
flangeability of the steel. Therefore, the content of S is 0.010%
or less. A preferable upper limit of the S content is 0.005%, and
more preferably is 0.0025%. A lower limit of the S content is not
particularly limited. However, when taking the production cost into
consideration, the lower limit of the S content is, for example,
0.0001%.
[0074] Al: 0.01 to Less than 1.0%
[0075] Aluminum (Al) suppresses formation of iron-based carbides
and facilitates formation of retained austenite. The strength and
formability of the steel is enhanced by the formation of retained
austenite. Al also deoxidizes the steel. If the Al content is too
low, the aforementioned effects are not obtained. On the other
hand, if the Al content is too high, the weldability of the steel
decreases. Therefore, the Al content is from 0.01 to less than
1.0%. A preferable lower limit of the Al content is 0.02%. A
preferable upper limit of the Al content is 0.8%, and more
preferably is 0.5%. In the present description, the "Al" content
means the content of "sol. Al" (acid-soluble Al).
[0076] N: 0.01% or Less
[0077] Nitrogen (N) forms coarse nitrides and reduces the ductility
and stretch flangeability of the steel. N is also a cause of the
occurrence of blowholes during welding. Therefore, a low N content
is preferable. The N content is 0.01% or less. A preferable upper
limit of the N content is 0.005%. The lower limit of the N content
is not particularly limited. However, when taking the production
cost into consideration, the lower limit of the N content is, for
example, 0.0001%.
[0078] O: 0.01% or Less
[0079] Oxygen (O) forms oxides and reduces the toughness and
stretch flangeability of the steel. Therefore, a low O content is
preferable. The O content is 0.01% or less. A preferable upper
limit of the O content is 0.008%, and more preferably is 0.006%.
The lower limit of the O content is not particularly limited.
However, when taking the production cost into consideration, a
preferable lower limit of the O content is, for example,
0.0001%.
[0080] Sb: 0.03 to 0.30%
[0081] Antimony (Sb) is, as described above, an element that easily
segregates at the surface of the steel. Sb forms an Sb concentrated
layer in the surface (interface between scale and base metal) of
the hot-rolled steel sheet during hot rolling. The Sb concentrated
layer suppresses penetration of oxygen ions into the inside of the
hot-rolled steel sheet from grain boundaries that are exposed on
the surface of the hot-rolled steel sheet. The Sb concentrated
layer also suppresses movement of iron ions contained in the base
metal to scale. Therefore, formation of an internal oxidized layer
in the hot-rolled steel sheet and the growth of scale are
suppressed. Sb also restricts movement of C to suppress formation
of a decarburization layer.
[0082] If the Sb content is too low, it is difficult to form an Sb
concentrated layer and the above described effects are not
obtained. On the other hand, if the Sb content is too high, the
workability of the steel sheet decreases. Further, if the Sb
content is too high, the mechanical properties of the hot-rolled
steel sheet also decrease. Therefore, the Sb content is from 0.03
to 0.30%. A preferable lower limit of the Sb content is 0.05%, more
preferably is 0.07%, further preferably is 0.10%, and further
preferably is 0.11%. A preferable upper limit of the Sb content is
0.25%, and more preferably is 0.20%.
[0083] The chemical composition of the above described hot-rolled
steel sheet further satisfies Formula (1):
Si+Mn.gtoreq.3.20 (1)
[0084] where, a content (mass %) of a corresponding element is
substituted for each symbol of an element in Formula (1).
[0085] If the total content of Si and Mn is less than 3.20%,
retained austenite will not be stable during annealing performed
after cold rolling. In this case, there is a possibility that the
strength or ductility of the steel sheet after annealing will be
low. Therefore, the lower limit of the total content of Si and Mn
is 120%. In this case, the strength and ductility of the steel
sheet will be high even after cold rolling and annealing. The lower
limit of the total content of Si and Mn is preferably 3.50%. On the
other hand, if the total content of Si and Mn is 5.0% or less, a
phase transformation delay during annealing can be suppressed.
Therefore, carbon (C) sufficiently concentrates in untransformed
austenite, and the retained austenite is more stable. Accordingly,
an upper limit of the total content of Si and Mn is preferably
5.0%, and more preferably is 4.5%.
[0086] The balance of the chemical composition of the hot-rolled
steel sheet of the present embodiment is Fe and impurities. Here,
the term "impurities" refers to elements that, during industrial
production of the hot-rolled steel sheet, are mixed in from ore or
scrap used as a raw material, or from the production environment or
the like, and that are allowed within a range that does not
adversely affect the hot-rolled steel sheet according to the
present embodiment.
[Optional Elements]
[0087] The chemical composition of the hot-rolled steel sheet that
is described above may contain optional elements that are described
hereunder in addition to the above described essential elements.
The chemical composition need not contain optional elements.
[0088] The above described chemical composition may contain one or
more types of element selected from the group consisting of Ti, V
and Nb as a substitute for a part of Fe. Each of Ti, V and Nb is an
optional element, and each element increases the strength of the
steel.
[0089] Ti: 0 to 0.15%
[0090] Titanium (Ti) is an optional element, and need not be
contained in the steel. In a case where Ti is contained, Ti forms
carbo-nitrides and increases the strength of the steel. Ti also
contributes to fine-grain strengthening of the steel by suppressing
growth of ferrite grains. Ti also contributes to dislocation
strengthening of the steel through suppression of
recrystallization. However, if the Ti content is too high,
carbo-nitrides are excessively formed and the formability of the
steel decreases. Therefore, the Ti content is 0 to 0.15%. A
preferable upper limit of the Ti content is 0.10%, and more
preferably is 0.07%. A preferable lower limit of the Ti content is
0.005%, more preferably is 0.010%, and further preferably is
0.015%.
[0091] V: 0 to 0.30%
[0092] Vanadium (V) is an optional element, and need not be
contained in the steel. In a case where V is contained, similarly
to Ti, V increases the strength of the steel by precipitation
strengthening, fine-grain strengthening and dislocation
strengthening. However, if the V content is too high,
carbo-nitrides precipitate excessively and the formability of the
steel decreases. Therefore, the V content is from 0 to 0.30%. A
preferable upper limit of the V content is 0.20%, and more
preferably is 0.15%. A preferable lower limit of the V content is
0.001%, and more preferably is 0.005%.
[0093] Nb: 0 to 0.15%
[0094] Niobium (Nb) is an optional element, and need not be
contained in the steel. In a case where Nb is contained, similarly
to Ti and V, Nb increases the strength of the steel by
precipitation strengthening, fine-grain strengthening and
dislocation strengthening. However, if the Nb content is too high,
carbo-nitrides precipitate excessively and the formability of the
steel decreases. Therefore, the Nb content is from 0 to 0.15%. A
preferable upper limit of the Nb content is 0.10%, and more
preferably is 0.06%. A preferable lower limit of the Nb content is
0.005%, more preferably is 0.010%, and further preferably is
0.015%.
[0095] The above described chemical composition may contain one or
more types of element selected from the group consisting of Cr, Ni,
Mo, W and B as a substitute for a part of Fe. Each of Cr, Ni, Mo, W
and B is an optional element, and each element increases the
strength of the steel.
[0096] Cr: 0 to 1.0%
[0097] Chromium (Cr) is an optional element, and need not be
contained in the steel. In a case where Cr is contained, Cr
suppresses phase transformation at a high temperature and increases
the strength of the steel. However, if the Cr content is too high,
the workability of the steel decreases and productivity is reduced.
Therefore, the Cr content is from 0 to 1.0%. A preferable lower
limit of the Cr content is 0.10%.
[0098] Ni: 0 to 1.0%
[0099] Nickel (Ni) is an optional element, and need not be
contained in the steel. In a case where Ni is contained, Ni
suppresses phase transformation at a high temperature and increases
the strength of the steel. However, if the Ni content is too high,
the weldability of the steel decreases. Therefore, the Ni content
is from 0 to 1.0%. A preferable lower limit of the Ni content is
0.10%.
[0100] Mo: 0 to 1.0%
[0101] Molybdenum (Mo) is an optional element, and need not be
contained in the steel. In a case where Mo is contained, Mo
suppresses phase transformation at a high temperature and increases
the strength of the steel. However, if the Mo content is too high,
the hot workability of the steel decreases and productivity is
reduced. Therefore, the Mo content is from 0 to 1.0%. A preferable
lower limit of the Mo content is 0.01%.
[0102] W: 0 to 1.0%
[0103] Tungsten (W) is an optional element, and need not be
contained in the steel. In a case where W is contained, W
suppresses phase transformation at a high temperature and increases
the strength of the steel. However, if the W content is too high,
the hot workability of the steel decreases and productivity is
reduced. Therefore, the W content is from 0 to 1.0%. A preferable
lower limit of the W content is 0.01%.
[0104] B: 0 to 0.010%
[0105] Boron (B) is an optional element, and need not be contained
in the steel. In a case where B is contained, B suppresses phase
transformation at a high temperature and increases the strength of
the steel. However, if the B content is too high, the hot
workability of the steel decreases and productivity is reduced.
Therefore, the B content is from 0 to 0.010%. A preferable upper
limit of the B content is 0.005%, and more preferably is 0.003%. A
preferable lower limit of the B content is 0.0001%, more preferably
is 0.0003%, and further preferably is 0.0005%.
[0106] The above described chemical composition may contain Cu as a
substitute for a part of Fe.
[0107] Cu: 0 to 0.50%
[0108] Copper (Cu) is an optional element, and need not be
contained in the steel. In a case where Cu is contained, Cu
precipitates in the steel as fine particles and increases the
strength of the steel. However, the weldability of the steel will
decrease if the Cu content is too high. Therefore, the Cu content
is from 0 to 0.50%. A preferable lower limit of the Cu content is
0.10%.
[0109] The above described chemical composition may contain one or
more types of element selected from the group consisting of Sn, Bi,
Se, Te, Ge and As as a substitute for a part of Fe. Each of Sn, Bi,
Se, Te, Ge and As is an optional element, and each element
suppresses formation of an internal oxidized layer.
[0110] Sn: 0 to 0.30%
[0111] Bi: 0 to 0.30%
[0112] Se: 0 to 0.30%
[0113] Te: 0 to 0.30%
[0114] Ge: 0 to 0.30%
[0115] As: 0 to 0.30%
[0116] Tin (Sn), bismuth (Bi), selenium (Se), tellurium (Te),
germanium (Ge) and arsenic (As) are optional elements, and need not
be contained in the steel. If contained, these elements suppress
formation of an internal oxidized layer by suppressing segregation
of Mn and Si. However, if the content of these elements is too
high, the formability of the steel decreases. Therefore, the Sn
content is from 0 to 0.30%, the Bi content is from 0 to 0.30%, the
Se content is from 0 to 0.30%, the Te content is from 0 to 0.30%,
the Ge content is from 0 to 0.30% and the As content is from 0 to
0.30%. A preferable upper limit of the Sn content is 0.25%, and
more preferably is 0.20%. A preferable upper limit of the Bi
content is 0.25%, and more preferably is 0.20%. A preferable upper
limit of the Se content is 0.25%, and more preferably is 0.20%. A
preferable upper limit of the Te content is 0.25%, and more
preferably is 0.20%. A preferable upper limit of the Ge content is
0.25%, and more preferably is 0.20%. A preferable upper limit of
the As content is 0.25%, and more preferably is 0.20%. A preferable
lower limit of the Sn content is 0.0001%. A preferable lower limit
of the Bi content is 0.0001%. A preferable lower limit of the Se
content is 0.0001%. A preferable lower limit of the Te content is
0.0001%. A preferable lower limit of the Ge content is 0.0001%. A
preferable lower limit of the As content is 0.0001%. Note that,
when two or more types of element selected from the group
consisting of Sn, Bi, Se, Te, Ge and As are to be contained in the
steel, it is preferable to make the total content thereof from
0.0001 to 0.30%.
[0117] The above described chemical composition may contain one or
more types of element selected from the group consisting of Ca, Mg,
Zr, Hf and rare earth metal (REM) as a substitute for a part of Fe.
Each of these elements is an optional element, and each element
increases the formability of the steel.
[0118] Ca: 0 to 0.50%
[0119] Mg: 0 to 0.50%
[0120] Zr: 0 to 0.50%
[0121] Hf: 0 to 0.50%
[0122] Rare earth metal (REM): 0 to 0.50%
[0123] Calcium (Ca), magnesium (Mg), zirconium (Zr), hafnium (Hf)
and rare earth metal (REM) are each an optional element and need
not be contained in the steel. If contained, these elements enhance
the formability of the steel. However, if the content of these
elements is too high, the ductility of the steel decreases.
Therefore, the Ca content is from 0 to 0.50%, the Mg content is
from 0 to 0.50%, the Zr content is from 0 to 0.50%, the Hf content
is from 0 to 0.50%, and the rare earth metal (REM) content is from
0 to 0.50%. A preferable lower limit of the Ca content is 0.0001%,
more preferably is 0.0005%, and further preferably is 0.001%. A
preferable lower limit of the Mg content is 0.0001%, more
preferably is 0.0005%, and further preferably is 0.001%. A
preferable lower limit of the Zr content is 0.0001%, more
preferably is 0.0005%, and further preferably is 0.001%. A
preferable lower limit of the Hf content is 0.0001%, more
preferably is 0.0005%, and further preferably is 0.001%. A
preferable lower limit of the rare earth metal (REM) content is
0.0001%, more preferably is 0.0005%, and further preferably is
0.001%. Note that, when two or more types of element selected from
the group consisting of Ca, Mg, Zr, Hf and rare earth metal (REM)
are to be contained in the steel, it is preferable to make the
total content thereof from 0.0001 to 0.50%.
[0124] The term "REM" as used in the present description refers to
one or more types of element selected from Sc, Y, and lanthanoids
(elements with atomic numbers 57 through 71 from La to Lu). The
term "REM content" refers to the total content of these
elements.
[Micro-Structure]
[0125] The micro-structure of the hot-rolled steel sheet of the
present embodiment is not particularly limited. The micro-structure
of the hot-rolled steel sheet of the present embodiment, for
example, mainly consists of ferrite and pearlite. Specifically, in
the micro-structure, the combined area fraction of ferrite and
pearlite is 75% or more. In the micro-structure, a region (balance)
other than ferrite and pearlite is one or more types of
micro-structure selected from the group consisting of bainite
(including tempered bainite), martensite (including tempered
martensite) and retained austenite.
[0126] If the total area fraction of ferrite and pearlite in the
micro-structure is 75% or more, the strength of the hot-rolled
steel sheet can be suppressed. In this case, the cold workability
is enhanced.
[0127] The area fraction of the respective phases can be determined
by the following methods.
[Area Fraction of Ferrite and Pearlite]
[0128] The hot-rolled steel sheet is cut along a plane
perpendicular to the rolling direction. The cut surface is mirror
polished. Of the entire area of the cut surface that was mirror
polished, a width-wise central portion of the hot-rolled steel
sheet (range of .+-.10 mm in the width direction from the center in
the width direction) that is a range of .+-.5 mm from a position
that is equal to 1/4 of the plate thickness from the surface is
defined as an observation region. The observation region is
corroded with a nital etching reagent. After corrosion, an
arbitrary range of 200 .mu.m.times.150 .mu.m of the observation
region is photographed using a scanning electron microscope (SEM).
Ferrite and pearlite are identified using the image of the region
that was photographed (hereunder, referred to as "photographing
region"). The total of the areas of ferrite and pearlite that are
identified is determined, and the determined total area is divided
by the sum total of the areas of the entire photographing region to
obtain the total area fraction (%) of ferrite and pearlite. The
areas of ferrite and pearlite are measured using a mesh method or
image processing software (product name: ImagePro).
[Area Fraction of Bainite and Martensite]
[0129] A method for measuring the area fraction of bainite and
martensite is as follows. A photographing region (200
.mu.m.times.150 .mu.m) that is the same as in the above described
method for measuring an area fraction of ferrite and pearlite is
photographed using an electron backscattering diffraction method
(EBSD method) to generate a photographic image.
[0130] A portion excluding pearlite and retained austenite is
extracted from the photographic image by image processing. With
respect to a low temperature transformation phase of the remaining
region, 15 degrees is defined as a threshold value of an
orientation difference with adjacent grains, and grains are
identified. With respect to each identified grain, the average
visibility of the Kikuchi Diffraction pattern in the grains (Grain
Average Image Quality: GAIQ) is converted into numerical values. A
histogram of area fractions is created with respect to the GAIQ
values that were converted into numerical values. In a case where
the created histogram has two peaks, a distribution on a side on
which the GAIQ is high is taken as originating from bainite, and a
distribution on a side on which the GAIQ is low is taken as
originating from martensite. The total area fraction of grains
having a GAIQ identified as originating from bainite is defined as
a bainite area fraction. In a case where two peaks are overlapping
in the histogram, grains that are identified by the GAIQ up to a
boundary at which the distributions overlap are defined as bainite,
and the area fraction of the bainite is determined.
[0131] A value (%) obtained by deducting the sum total (%) of the
aforementioned ferrite area fraction, pearlite area fraction and
bainite area fraction as well as a retained austenite area fraction
that is described later from 100(%) is defined as the area fraction
of martensite.
[Area Fraction of Retained Austenite]
[0132] The area fraction of retained austenite is determined by
X-ray diffractometry. Specifically, in a photographing region (200
.mu.m.times.150 pin) that is the same as in the above described
method for measuring an area fraction of ferrite and pearlite, the
proportion of retained austenite is determined experimentally by
X-ray diffractometry using the property that the reflection surface
intensity differs between austenite and ferrite. A retained
austenite area fraction V.gamma. is determined using the following
formula based on an image obtained by X-ray diffractometry using
the K.alpha. ray of Mo:
V.gamma.=(2/3){100/(0.7.times..alpha.(211)/.gamma.(220)+1)}+(1/3){100/(0-
.78.times..alpha.(211)/.gamma.(311)+1)}
[0133] where, .alpha.(211) represents the reflection surface
intensity at a (211) surface of ferrite, .gamma.(220) represents
the reflection surface intensity at a (220) surface of austenite,
and .gamma.(311) represents the reflection surface intensity at a
(311) surface of austenite.
[Tensile Strength]
[0134] A preferable tensile strength of the hot-rolled steel sheet
of the present embodiment is 800 MPa or less, and more preferably
is 700 MPa or less. Because the tensile strength is low, the cold
workability is enhanced. Although a lower limit of the tensile
strength is not particularly limited, for example, the lower limit
is 400 MPa. The tensile strength can be determined by a tensile
testing method for metal materials in accordance with JIS Z 2241
(2011).
[Regarding Sb Concentrated Layer]
[0135] As described above, an Sb concentrated layer is formed at an
interface between a base metal surface and scale of a hot-rolled
steel sheet. The existence or non-existence of the Sb concentrated
layer can be observed by electron probe microanalysis (EPMA).
Specifically, the hot-rolled steel sheet is cut along a plane
perpendicular to the rolling direction, and of the entire cut
surface, an arbitrary region of 50 .mu.m in the width
direction.times.45 .mu.m in the depth direction of the hot-rolled
steel sheet among a width-wise central portion (range of .+-.10 mm
in the width direction from the center in the width direction) that
includes the surface is defined as an observation region. A sample
including the observation region is extracted. Mapping analysis
using EPMA is conducted with respect to the observation region. A
portion at which an Sb concentration is 1.5 times or more greater
than the region average is defined as an Sb concentrated layer. In
a case where an Sb concentrated layer is confirmed in 90% or more
of the width (50 .mu.m) of the observation region, it is determined
that an Sb concentrated layer is formed.
[0136] The thickness of the identified Sb concentrated layer is
measured at a pitch of 5 .mu.m in the width direction of the
observation region, and an average value thereof is defined as the
thickness of the Sb concentrated layer. A preferable thickness of
the Sb concentrated layer is 0.5 .mu.m or more, more preferably is
1.0 .mu.m or more, and further preferably is 1.5 .mu.m or more.
[0137] At a high temperature, Sb segregates at grain boundaries and
the surface of the steel, and in particular there is a strong
tendency for Sb to segregate and concentrate at the surface. Even
in a case where scale covers the base metal, Sb segregates strongly
toward the base metal surface. To sufficiently induce segregation
of Sb and promote the formation of an Sb concentrated layer, it is
preferable to retain the hot-rolled steel sheet in a high
temperature region for a long time period. The Sb concentrated
layer is also formed during hot rolling, and is extended by
rolling. Therefore, as described above, the finish rolling
temperature is preferably a high temperature.
[Thickness of Internal Oxidized Layer]
[0138] In the hot-rolled steel sheet of the present embodiment,
because an Sb concentrated layer is formed, the thickness of an
internal oxidized layer is suppressed. A preferable thickness of
the internal oxidized layer is 5 .mu.m or less.
[0139] The internal oxidized layer is measured by the following
method. A small piece that includes a part of the surface of the
hot-rolled steel sheet is cut out from an arbitrary position within
a width-wise central portion (range of .+-.10 mm in the width
direction from the center in the width direction) of the hot-rolled
steel sheet. Of the entire surface of the small piece, a
cross-section that is perpendicular to the rolling direction
(hereunder, referred to as "observation face") is mirror polished.
The observation face is subjected to C vapor deposition. After the
C vapor deposition, a portion in the vicinity of the surface of the
observation face is photographed for arbitrary visual fields at a
magnification of .times.1000 using a field-emission scanning
electron microscope (FE-SEM) to obtain images (each visual field is
200 .mu.m.times.180 .mu.m). The thickness (.mu.m) of the internal
oxidized layer is determined based on the obtained images. Oxides
of Si and Mn arise in the base metal in the internal oxidized
layer. Therefore, the scale, the internal oxidized layer and the
base metal can be easily distinguished by means of a backscattered
electron image obtained by a backscattered electron detector that
is normally mounted in a common SEM.
[0140] In the obtained image, a distance from the interface of the
scale and base metal to the lowest edge of the internal oxidized
layer is determined at each interval of 10 .mu.m in the rolling
direction. This measurement is performed for an arbitrary three
visual fields, and the average value of the obtained distances is
defined as the internal oxidized layer thickness (.mu.m).
[Scale Thickness]
[0141] In the hot-rolled steel sheet of the present embodiment,
because the Sb concentrated layer is formed, formation of scale is
also suppressed. A preferable thickness of the scale is 10 .mu.m or
less.
[0142] The scale thickness is measured by the following method.
Images are obtained using an FE-SEM, similarly to when measuring
the thickness of the internal oxidized layer. In the obtained
images (it is sufficient to use the same images as when measuring
the internal oxidized layer), the scale is identified, and a
distance between an uppermost edge of the scale and the interface
is determined at each interval of 10 .mu.m in the rolling
direction. This measurement is performed for an arbitrary three
visual fields, and the average value of the obtained distances is
defined as the scale thickness (.mu.m).
[Thickness of Decarburization Layer]
[0143] In the hot-rolled steel sheet of the present embodiment,
because the Sb concentrated layer is formed, the thickness of a
decarburization phase is also suppressed. A preferable thickness of
the decarburization layer is 20 .mu.m or less.
[0144] The decarburization layer is measured by the following
method. A small piece that includes a part of the surface of the
hot-rolled steel sheet is cut out from an arbitrary position within
a width-wise central portion (range of .+-.10 mm in the width
direction from the center in the width direction) of the hot-rolled
steel sheet. Line analysis of the C-K.alpha. line is performed by
means of EPMA with respect to the surface of the small piece, and C
strengths (line analysis results) in the depth direction from the
surface of the steel sheet are obtained. Among the obtained line
analysis results, a distance from a position at which the C
strength is smallest in the steel sheet to a depth position at
which a difference between the average C strength (C strength of
the base metal) of the steel sheet and the smallest C strength in
the steel sheet is 98% is defined as the thickness (.mu.m) of the
decarburization layer.
[0145] As described above, in the hot-rolled steel sheet of the
present embodiment, an Sb concentrated layer suppresses formation
of an internal oxidized layer. The Sb concentrated layer also
suppresses formation of scale. Furthermore, the Sb concentrated
layer suppresses formation of a decarburization layer. In addition,
in the hot-rolled steel sheet of the present embodiment, a total
area fraction of ferrite and pearlite in the micro-structure is 75%
or more. Therefore, the tensile strength is suppressed to 800 MPa
or less, preferably 700 MPa or less, and the hot-rolled steel sheet
is thus excellent in cold workability.
[0146] The hot-rolled steel sheet of the present embodiment may
also be subjected to descaling that is described later. In this
case, an area fraction of the surface of island-shaped scale that
is caused by fayalite and formed on the surface is lowered.
Consequently, a pickling property is further enhanced.
[Production Method]
[0147] An example of a method for producing the above described
hot-rolled steel sheet will now be described. The production method
includes a preparation process, a hot rolling process and a coiling
process.
[Preparation Process]
[0148] In the preparation process, a steel material having the
above described chemical composition is prepared. Specifically,
molten steel having the above described chemical composition is
produced. A slab as the steel material is produced using the molten
steel. The slab may be produced by a continuous casting process.
Alternatively, an ingot may be produced using the molten steel, and
the ingot may be subjected to blooming to produce the slab.
[Hot Rolling Process]
[0149] The prepared steel material (slab) is heated. The heating
temperature is from 1100 to 1350.degree. C. Preferably, the heating
time period is set to 30 minutes or more. The heated slab is
subjected to hot rolling using a roughing mill and a finish rolling
mill and made into a steel sheet. The roughing mill includes a
plurality of roll stands that are arranged in a single row, with
each roll stand having a pair of rolls. The roughing mill may be a
reverse-type roughing mill. The finish rolling mill includes a
plurality of roll stands that are arranged in a single row, with
each roll stand having a pair of rolls.
[Descaling]
[0150] During the hot rolling, descaling may also be performed on
the steel sheet that is being subjected to rolling, by means of one
or a plurality of high-pressure water descaling devices that are
installed between the plurality of roll stands (roughing mill or
finish rolling mill). The descaling is preferably performed on the
steel sheet having a temperature of 1050.degree. C. or more. In
this case, Fe.sub.2SiO.sub.4 (fayalite) arising on the surface of a
steel with a high Si and high Mn content, as in a steel sheet
having the chemical composition of the present embodiment, can be
effectively removed. If fayalite remains, island-shaped scale will
be formed on the surface of the hot-rolled steel sheet. If
island-shaped scale remains on the surface of the hot-rolled steel
sheet, it is difficult to remove the scale during pickling.
Further, if fayalite remains, indentation defects arise during cold
rolling, and such defects may mar the external appearance of the
cold-rolled steel sheet. Fayalite can be removed by performing
descaling.
[0151] In a case where one or a plurality of high-pressure water
descaling devices are installed between finishing roll stands
during finish rolling, it is preferable that a steel sheet (rough
bar) in a state after rough rolling and before finish rolling is
heated to 1050.degree. C. or more by a heating apparatus arranged
in the vicinity of the entrance side of the initial roll stand of
the finish rolling mill. The method of heating the rough bar is not
particularly limited. For example, the rough bar is heated by an
induction heating apparatus or a reflow furnace.
[Finish Rolling Temperature FT]
[0152] In hot rolling, the surface temperature of the steel sheet
of the exit side of the final stand of the finish rolling mill is
defined as a finish rolling temperature FT (.degree. C.). A
preferable finish rolling temperature FT (.degree. C.) is the
A.sub.r3 transformation temperature +50.degree. C. or more. If the
finish rolling temperature FT is less than the A.sub.r3
transformation temperature +50.degree. C., rolling resistance of
the steel sheet increases and productivity decreases. In addition,
the steel sheet is rolled in a two-phase region of ferrite and
austenite. In this case, the micro-structure of the steel sheet
forms a layered micro-structure and the mechanical properties
decrease. Therefore, the finish rolling temperature FT is the
A.sub.r3 transformation temperature +50.degree. C. or more. A
preferable finish rolling temperature FT is more than 920.degree.
C., and more preferably is 950.degree. C. or more.
[0153] After finish rolling is completed, the steel sheet is cooled
to the coiling temperature. The cooling method is not particularly
limited. The cooling methods are, for example, water cooling,
forced air-cooling and allowing cooling.
[Coiling Process]
[0154] The hot-rolled steel sheet produced in the hot rolling
process is coiled to form a coil. A surface temperature (hereunder,
referred to as "coiling temperature") CT of the hot-rolled steel
sheet when starting coiling of a coil is preferably from
600.degree. C. to 750.degree. C.
[0155] If the coiling temperature CT is too high, formation of an
internal oxidized layer in the hot-rolled steel sheet is promoted.
On the other hand, if the coiling temperature CT is too low, in
steel that contains a large amount of Si, as in the case of the
hot-rolled steel sheet of the present embodiment, the strength of
the hot-rolled steel sheet will be too high and the cold rolling
property will decrease.
[0156] If the coiling temperature CT is made a temperature in the
range of 600.degree. C. to 750.degree. C., an increase in the
strength of the hot-rolled steel sheet can be suppressed, and
formation of an internal oxidized layer in the steel composition
defined in the present embodiment is suppressed. The coiling
temperature CT is preferably from 650.degree. C. to 750.degree. C.,
and more preferably is from 700.degree. C. to 750.degree. C.
[0157] The hot-rolled steel sheet of the present embodiment can be
produced by the above described processes. Note that, the above
described production method is one example of a method for
producing a hot-rolled steel sheet in which a total area fraction
of ferrite and pearlite is 75% or more, and a method for producing
the hot-rolled steel sheet of the present embodiment is not limited
thereto.
Second Embodiment
[0158] The micro-structure of the hot-rolled steel sheet may be a
micro-structure that consists mainly of bainite and martensite.
Specifically, a combined area fraction of bainite and martensite
may be 75% or more.
[Micro-Structure]
[0159] The chemical composition of a hot-rolled steel sheet
according to the present embodiment (second embodiment) is the same
as the chemical composition of the hot-rolled steel sheet of the
first embodiment, and satisfies Formula (1). If the chemical
composition does not satisfy Formula (1), the ductility of the
cold-rolled steel sheet may decrease. If Formula (1) is satisfied,
excellent ductility is obtained in a cold-rolled steel sheet after
annealing also.
[0160] On the other hand, the micro-structure of the hot-rolled
steel sheet of the present embodiment is different from the first
embodiment. In the micro-structure of the hot-rolled steel sheet of
the present embodiment, a combined area fraction of bainite and
martensite is 75% or more.
[0161] A region (balance) other than bainite and martensite is one
or more types of micro-structure selected from the group consisting
of ferrite, pearlite and retained austenite. In the present
embodiment, preferably tempering is performed on the hot-rolled
steel sheet after coiling. By this means, the strength of the steel
sheet can be reduced to a certain extent, and the cold workability
can be enhanced while maintaining a certain degree of strength. If
tempering is performed, the bainite is mainly tempered bainite, and
the martensite is mainly tempered martensite. Methods for measuring
the area fractions of the respective phases in the micro-structure
are the same as in the first embodiment.
[Tensile Strength]
[0162] The hot-rolled steel sheet of the present embodiment has the
above described chemical composition and micro-structure. In a case
where tempering is not performed after coiling, the tensile
strength of the hot-rolled steel sheet of the present embodiment is
900 MPa or more.
[0163] On the other hand, if tempering is performed after coiling,
the tensile strength of the hot-rolled steel sheet is 800 MPa or
less. In this case the cold workability can be enhanced, and the
load placed on the equipment system during cold rolling can be
reduced. Although the lower limit of the tensile strength is not
particularly limited, for example, the lower limit is 400 MPa. The
tensile strength is determined by a method that is in accordance
with JIS Z 2241 (2011).
[Production Method]
[0164] An example of a method for producing the hot-rolled steel
sheet according to the present embodiment will now be described.
The production method includes a preparation process, a hot rolling
process and a coiling process. In comparison to the production
method of the first embodiment, in the present embodiment the
coiling temperature CT in the coiling process differs from the
first embodiment. Preferably, tempering is also performed after the
coiling process. The other processes are the same as in the first
embodiment.
[Coiling Process]
[0165] The steel sheet produced in the hot rolling process is
coiled to form a coil. If the surface temperature (coiling
temperature) of the steel sheet when starting coiling of a coil is
too low, the strength of the steel sheet increases and the load
placed on a coiling apparatus becomes large. Therefore, the surface
temperature (coiling temperature) CT of the steel sheet when
starting coiling is from 150 to 600.degree. C., preferably from 350
to 500.degree. C., and more preferably from 400.degree. C. to
500.degree. C.
[Tempering]
[0166] Because the coiling temperature CT of the hot-rolled steel
sheet of the present embodiment is 600.degree. C. or less,
preferably 500.degree. C. or less, the hardness of the hot-rolled
steel sheet is high. Therefore, tempering may be performed to lower
the strength and enhance the cold rolling property. In the
tempering, the steel sheet after coiling is tempered at a
temperature of 550.degree. C. or more (Ac1 transformation
temperature or less). If the tempering time period is too short, it
is difficult to obtain the above described effect. On the other
hand, if the tempering time period is too long, the effect
saturates. Therefore, a preferable tempering time period is 0.5 to
8 hours in a temperature range of 550.degree. C. or more.
[0167] The hot-rolled steel sheet of the second embodiment can be
produced by the above described processes.
[0168] Note that tempering need not be performed. In a case where
tempering is not performed also, in the micro-structure of the
hot-rolled steel sheet, a combined area fraction of bainite and
martensite is 75% or more, and the balance is one or more types of
micro-structure selected from the group consisting of ferrite,
pearlite and retained austenite. However, the bainite
micro-structure and martensite micro-structure in a case where
tempering is not performed are not micro-structures that are mainly
composed of tempered bainite and tempered martensite, but rather
are micro-structures mainly composed of bainite and martensite that
contain some tempered bainite and tempered martensite formed during
the coiling process.
[0169] In a case where tempering is not performed, the tensile
strength of the hot-rolled steel sheet is 900 MPa or more. A
hot-rolled steel sheet that has not been subjected to tempering is
useful in particular in a case where a high tensile strength is
required as a hot-rolled steel sheet and the like.
[0170] Note that the above described production method is one
example of a method for producing a hot-rolled steel sheet in which
the total area fraction of bainite and martensite is 75% or more,
and a method for producing the hot-rolled steel sheet of the
present embodiment is not limited to the above described
method.
Other Embodiments
[0171] In the above described first and second embodiments, the
micro-structure is defined. However, the micro-structure of a
hot-rolled steel sheet of the present embodiments is not
particularly limited. As long as the above described chemical
composition and Formula (1) are satisfied, an Sb concentrated layer
can be formed and formation of an internal oxidized layer and/or
scale can be suppressed while maintaining the necessary workability
and strength.
[0172] The above described production method is merely one example.
Accordingly, in some cases the hot-rolled steel sheets of the first
and second embodiments can also be produced by other production
methods.
[0173] Note that, in the finish rolling of the hot rolling process,
it is preferable to make the average cooling rate (hereunder,
referred to as "ADFT") from the temperature of the steel sheet at
the time when the final descaling is performed until the
temperature of the steel sheet reaches 800.degree. C. by cooling
after the finish rolling, a rate of 10.degree. C./sec or more. In
this case, formation of scale on the hot-rolled steel sheet surface
can be further suppressed.
EXAMPLES
[0174] Examples of the hot-rolled steel sheet of the present
invention will now be described. The conditions adopted in the
Examples are merely one example of conditions adopted to confirm
the operability and advantageous effects of the present invention.
Therefore, the present invention is not limited to this one example
of the conditions. The present invention can adopt various
conditions as long as the objective of the present invention is
achieved without departing from the gist of the present
invention.
Example 1
[0175] Molten steels having the chemical compositions shown in
Table 1 were produced.
TABLE-US-00001 TABLE 1 Steel Chemical Composition (Unit Is Mass
Percent: Balance Is Fe And Impurities) Type C Si Mn P S Al N O Sb
Ti V Nb Cr Ni Mo A 0.076 1.10 2.39 0.007 0.0011 0.0420 0.0034
0.0014 0.10 -- -- -- -- -- -- B 0.120 1.97 2.01 0.011 0.0016 0.0240
0.0022 0.0011 0.20 -- -- -- -- -- -- C 0.182 1.75 2.70 0.009 0.0010
0.0330 0.0034 0.0012 0.05 -- -- -- -- -- -- D 0.185 1.80 2.60 0.010
0.0020 0.0300 0.0030 0.0009 0.07 0.005 -- -- -- -- -- E 0.152 1.20
2.11 0.008 0.0021 0.0310 0.0026 0.0019 0.03 0.062 -- -- 0.12 -- --
F 0.152 1.10 2.24 0.007 0.0023 0.0400 0.0016 0.0029 0.03 -- -- --
-- 0.24 -- G 0.181 1.20 2.41 0.015 0.0034 0.0380 0.0019 0.0015 0.03
-- -- -- -- -- -- H 0.178 1.20 2.42 0.014 0.0036 0.0410 0.0027
0.0018 0.03 -- -- -- -- -- -- I 0.182 1.10 2.39 0.009 0.0024 0.0190
0.0020 0.0011 0.03 -- -- -- -- -- -- J 0.179 1.10 2.37 0.013 0.0023
0.0680 0.0028 0.0022 0.04 -- -- -- -- -- -- K 0.179 1.10 2.14 0.009
0.0019 0.0350 0.0025 0.0019 0.07 -- 0.083 -- -- -- -- L 0.180 1.10
2.45 0.001 0.0026 0.0980 0.0044 0.0019 0.07 -- -- -- -- -- -- M
0.200 1.10 2.30 0.010 0.0010 0.0330 0.0030 0.0015 0.07 -- -- -- --
-- -- N 0.175 1.75 2.70 0.009 0.0011 0.0330 0.0022 0.0021 0.05
0.005 -- -- -- -- -- O 0.190 1.75 2.80 0.010 0.0010 0.0330 0.0022
0.0014 0.11 0.005 -- -- -- -- -- P 0.180 2.20 3.10 0.010 0.0010
0.0330 0.0030 0.0015 0.01 -- -- -- -- -- -- Q 0.150 2.00 2.60 0.010
0.0020 0.0300 0.0030 0.0009 -- R 0.429 1.10 2.50 0.007 0.0011
0.0270 0.0022 0.0016 -- -- -- -- -- -- -- S 0.155 2.88 2.21 0.016
0.0033 0.0240 0.0029 0.0020 -- 0.034 -- -- -- -- -- T 0.142 1.10
0.70 0.012 0.0038 0.0080 0.0034 0.0026 0.40 -- -- -- -- -- -- U
0.250 1.10 5.61 0.014 0.0030 0.2900 0.0034 0.0019 0.02 -- -- -- --
-- -- Chemical Composition (Unit Is Mass Percent: Balance Is Fe And
Impurities) Steel Sn, Bi, Se, Type B Cu Te, Ge, As Ca Mg REM Si +
Mn Remarks A -- -- -- -- -- 3.49 Inventive Example B -- -- -- -- --
3.98 Inventive Example C -- -- -- -- -- 4.45 Inventive Example D --
-- -- -- -- 4.40 Inventive Example E 0.0014 -- -- -- -- 3.31
Inventive Example F -- 0.12 -- -- -- 3.34 Inventive Example G -- --
0.0006 -- -- 3.61 Inventive Example H -- -- -- -- Ce: 0.0013 3.62
Inventive Example I -- -- -- 0.0013 -- 3.49 Inventive Example J --
-- -- -- Nd: 0.0007 3.47 Inventive Example K -- -- -- -- -- 3.24
Inventive Example L -- -- -- -- -- 3.55 Inventive Example M -- --
Sn: 0.01 -- -- -- 3.40 Inventive Example N -- -- Se: 0.05 -- -- --
4.45 Inventive Example O -- -- Ge: 0.02 -- -- -- 4.55 Inventive
Example As: 0.03 P -- -- Bi: 0.02 -- -- -- 5.30 Comparative Example
Te: 0.04 Q 4.60 Comparative Example R -- -- -- -- -- 3.60
Comparative Example S 0.0009 -- -- -- -- 5.09 Comparative Example T
-- -- -- -- -- 1.80 Comparative Example U -- -- -- -- -- 6.71
Comparative Example (Note) Underlined Numerical Values Indicate
That The Values Are Outside The Range Of The Present Invention.
[0176] Referring to Table 1, steel types A to O are within the
range of the chemical composition of the steel material of the
present embodiments. On the other hand, the chemical compositions
of steel types P to U are outside the range of the chemical
composition of the steel material of the present embodiments.
[0177] Steel materials (ingots) were produced by an ingot-making
process using the above described molten steel. Hot-rolled steel
sheets were produced by hot rolling the steel materials under the
hot rolling conditions (heating temperature (.degree. C.) and
finish rolling temperature FT (.degree. C.)) shown in Table 2 using
a hot rolling mill for testing that was composed of a plurality of
hot rolling stands. In addition, a heat history equivalent to a
coil that was coiled at a coiling temperature CT (.degree. C.)
shown in Table 2 was imparted to the respective hot-rolled steel
sheets after hot rolling by an N.sub.2-purged annealing
furnace.
[0178] Note that, a reheating furnace simulating a rough bar heater
was installed on the entrance side of the finish rolling mill, and
reheating of the hot-rolled steel sheets was conducted under the
conditions shown in Table 2. Further, a high-pressure water
descaling device was disposed between roll stands of the finish
rolling mill, and descaling was performed with respect to steel
sheets undergoing finish rolling. The surface temperatures
(descaling temperatures) of the respective steel sheets immediately
before performing descaling were as shown in Table 2.
TABLE-US-00002 TABLE 2 Heating Reheating Descaling Test Steel
Temperature Temperature Temperature FT CT Number Type (.degree. C.)
(.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) Remarks 1 A
1280 1100 1050 970 655 Inventive Example 2 B 1250 1100 1080 930 655
Inventive Example 3 C 1280 1090 1050 970 655 Inventive Example 4 C
1250 1100 1060 970 700 Inventive Example 5 C 1220 1120 1085 970 720
Inventive Example 6 C 1280 -- 980 850 450 Inventive Example 7 D
1250 1170 1100 970 730 Inventive Example 8 D 1280 1010 1050 990 500
Inventive Example 9 E 1200 1110 1070 930 700 Inventive Example 10 F
1200 1140 1100 930 655 Inventive Example 11 G 1200 1080 1050 930
700 Inventive Example 12 H 1200 1090 1080 930 700 Inventive Example
13 I 1250 1060 1050 900 655 Inventive Example 14 J 1220 1080 1060
930 700 Inventive Example 15 K 1250 1100 1080 900 720 Inventive
Example 16 L 1220 1110 1100 950 730 Inventive Example 17 M 1280
1120 1080 950 700 Inventive Example 18 N 1230 1080 1050 950 700
Inventive Example 19 O 1280 1090 1060 980 700 Inventive Example 20
P 1200 1080 1060 980 700 Comparative Example 21 Q 1280 1060 1060
950 750 Comparative Example 22 R 1200 1070 1060 930 400 Comparative
Example 23 S 1200 -- 1000 950 650 Comparative Example 24 T 1250
1100 1050 930 680 Comparative Example 25 U 1200 1080 1050 900 655
Comparative Example
[Internal Oxidized Layer Thickness and Scale Thickness Measurement
Test]
[0179] A small piece was cut out from a width-wise central portion
(range of .+-.10 mm in the width direction from the center in the
width direction) of the hot-rolled steel sheet of each test number.
Of the entire surface of the small piece, a cross-section
(hereunder, referred to as "observation face") perpendicular to the
rolling direction was mirror polished. The observation face was
subjected to C (carbon) vapor deposition. After the C vapor
deposition, a portion in the vicinity of the surface of the
observation face was photographed using a field-emission scanning
electron microscope (FE-SEM) and images were obtained. The
thickness (.mu.m) of the internal oxidized layer and the scale
thickness (.mu.m) were determined by the above described method
using the obtained images.
[Decarburization Layer Measurement Test]
[0180] A small piece was cut out from a width-wise central portion
(range of .+-.10 mm in the width direction from the center in the
width direction) of the hot-rolled steel sheet of each test number.
Line analysis of C-K.alpha. line was performed by means of EPMA
with respect to the surface of the small piece. Among the obtained
line analysis results, a distance from a position at which the C
strength was smallest in the steel sheet to a depth position at
which a difference between the average C strength (C strength of
the base metal) of the steel sheet and the smallest C strength in
the steel sheet was 98% was defined as the thickness (.mu.m) of the
decarburization layer.
[Sb Concentrated Layer Measurement Test]
[0181] The presence or absence of an Sb concentrated layer and the
thickness (.mu.m) of an Sb concentrated layer were measured by the
measurement method described in the first embodiment.
[Cold Rolling Property Evaluation Test]
[0182] No. 5 tensile test coupons specified in the JIS Standards
were taken at a width-wise central portion of the respective steel
sheets that from a position that was 1/4 of the thickness from the
surface. Parallel portions were made parallel in the rolling
direction. A tension test in accordance with JIS Z 2241 (2011) was
performed using the respective tensile test specimens at normal
temperature (25.degree. C.) in the atmosphere, and a tensile
strength (MPa) was obtained.
[Pickling Property Evaluation Test]
[0183] A test specimen including part of the steel sheet surface
was taken from a width-wise central portion of the steel sheet of
each test number. In each test specimen, a region including part of
the steel sheet surface was 50 mm in width.times.70 mm in length. A
pickling test was performed on each test specimen. In the pickling
test, the test specimen was immersed in an 8% hydrochloric acid
aqueous solution heated to 85.degree. C., and scale on the surface
of the test specimen was removed. A time period (pickling
completion time period) taken to remove scale from the entire
surface of the test specimen was measured. The pickling property
was determined as being excellent if the pickling completion time
period was within 60 seconds.
[Test Results]
[0184] The test results are shown in Table 3.
TABLE-US-00003 TABLE 3 Sb Internal Micro- Tensile Decarburization
Concentrated Oxidized structure Strength Layer Scale Layer Layer
Test (Area %) TS Thickness Thickness Thickness Thickness Pickling
Number F P Other (MPa) (.mu.m) (.mu.m) (.mu.m) (.mu.m) Property
Remarks 1 89 11 -- 621 0.7 5.0 2.3 0.0 .largecircle. Inventive
Example 2 84 16 -- 623 0.7 5.0 2.9 0.0 .largecircle. Inventive
Example 3 46 54 -- 652 0.7 5.0 1.4 0.8 .largecircle. Inventive
Example 4 46 54 -- 617 1.1 5.0 1.4 0.8 .largecircle. Inventive
Example 5 46 54 -- 618 2.0 5.0 1.5 0.8 .largecircle. Inventive
Example 6 0 0 B: 75, 967 0.2 5.0 1.1 0.8 .largecircle. Inventive
Example M: 18, Rg: 7 7 53 47 -- 593 1.5 5.0 1.9 0.2 .largecircle.
Inventive Example 8 0 0 B: 100 823 0.1 5.0 1.5 0.2 .largecircle.
Inventive Example 9 78 22 -- 650 1.1 7.0 1.1 4.0 .largecircle.
Inventive Example 10 75 25 -- 577 0.7 9.0 0.8 5.0 .largecircle.
Inventive Example 11 71 29 -- 529 1.1 9.0 1.1 4.0 .largecircle.
Inventive Example 12 71 29 -- 523 2.1 6.0 1.1 4.0 .largecircle.
Inventive Example 13 69 31 -- 590 0.7 6.0 0.9 1.8 .largecircle.
Inventive Example 14 70 30 -- 562 1.9 6.0 1.2 0.2 .largecircle.
Inventive Example 15 70 30 -- 639 2.1 5.0 1.8 0.2 .largecircle.
Inventive Example 16 70 30 -- 641 1.9 5.0 1.7 0.2 .largecircle.
Inventive Example 17 66 34 -- 497 1.1 5.0 2.0 0.8 .largecircle.
Inventive Example 18 47 53 -- 570 2.1 5.0 1.3 0.0 .largecircle.
Inventive Example 19 43 57 -- 531 1.1 5.0 2.6 0.0 .largecircle.
Inventive Example 20 75 25 -- 643 1.1 5.0 0.4 20.0 X Comparative
Example 21 71 29 -- 596 186.0 75.0 0.0 40.0 X Comparative Example
22 0 0 B: 52, 904 5.0 100.0 0.0 10.0 X Comparative M: 31, Example
Rg: 17 23 85 15 -- 643 65.0 100.0 0.0 30.0 X Comparative Example 24
77 23 -- -- 0.9 3.0 5.3 0.0 .largecircle. Comparative Example 25 0
0 M: 92, -- 69.0 8.0 0.2 8.0 .largecircle. Comparative Rg: 8
Example
[0185] In Table 3, the item "micro-structure" shows the
micro-structure at a depth position at 1/4 of the plate thickness
at a width-wise central portion (range of .+-.10 mm in the width
direction from the center in the width direction) of the steel
sheet. In each of the micro-structures shown in Table 3, an area
fraction (%) of ferrite in the respective micro-structures is
described in an "F" column, and an area fraction of pearlite is
described in a "P" column. The area fractions of phases other than
ferrite and pearlite are described in an "Other" column. The
character "B" denotes the area fraction (%) of bainite (including
tempered bainite), and the character "M" denotes the area fraction
(%) of martensite (including tempered martensite). "Rg" denotes the
area fraction (%) of retained austenite. The area fraction of each
phase was measured by the above described measurement method.
[0186] The item "tensile strength TS" shows the tensile strength TS
(MPa) at a width-wise central portion of the steel sheet. The cold
rolling property was determined as excellent if the tensile
strength was 800 MPa or less.
[0187] An "O" mark in the "pickling property" column in Table 3
indicates that the pickling completion time period was within 60
seconds. An "x" mark indicates that the pickling completion time
period was more than 60 seconds.
[0188] Referring to Table 3, the chemical compositions of test
numbers 1 to 19 were appropriate, and also satisfied Formula (1).
Therefore, an Sb concentrated layer was confirmed. The thickness of
each of the Sb concentrated layers was 0.5 .mu.m or more. In
addition, the scale thickness was 10 .mu.m or less, and the
thickness of an internal oxidized layer was 5 .mu.m or less. Thus,
the internal oxidized layer and scale could be suppressed.
Consequently, the pickling property of the respective chemical
compositions of test numbers 1 to 19 was excellent. Further, the
thickness of the decarburization layer was 20 .mu.m or less.
[0189] In test numbers 1 to 5, test number 7 and test numbers 9 to
19, the production conditions were suitable for formation of
ferrite and pearlite. Therefore, in the micro-structure of the
hot-rolled steel sheet of each of these test numbers, the total
area fraction of ferrite and pearlite was 75% or more. Therefore,
the tensile strength was 800 MPa or less.
[0190] In test number 6 and test number 8, because the coiling
temperature CT was from 150 to 600.degree. C., the total area
fraction of bainite and martensite in the micro-structure was 75%
or more, and the tensile strength was 900 MPa or more.
[0191] On the other hand, in steel type P used in test number 20,
the Sb content was too low. Therefore, the thickness of the Sb
concentrated layer was less than 0.5 .mu.m, and the thickness of
the internal oxidized layer was more than 5 .mu.m. Consequently,
the pickling property was poor.
[0192] Steel type Q used in test number 21 did not contain Sb.
Therefore an Sb concentrated layer was not confirmed. Consequently,
the thickness of an internal oxidized layer was more than 5 .mu.m,
and the scale thickness was more than 10 .mu.m. Therefore, the
pickling property was poor. In addition, the decarburization layer
thickness was more than 20 .mu.m.
[0193] In steel type R used in test number 22, the C content was
too high. In addition, Sb was not contained therein. The coiling
temperature CT was also too low. Consequently, the micro-structure
mainly consisted of bainite and martensite, and did not contain
ferrite and pearlite. Therefore, the tensile strength was more than
800 MPa. In addition, because there was no Sb concentrated layer,
the thickness of the internal oxidized layer was more than 5 .mu.m,
and the scale thickness was more than 10 .mu.m. Therefore, the
pickling property was poor.
[0194] In steel type S used in test number 23, the Si content was
too high, and Sb was not contained therein. Consequently, an Sb
concentrated layer was not formed, and the thickness of the
internal oxidized layer was more than 5 .mu.m and the scale
thickness was more than 10 .mu.m. Therefore, the pickling property
was poor. In addition, the decarburization layer thickness was more
than 20 .mu.m.
[0195] Note that, in test number 23, heating with a rough bar
heater was not performed during hot rolling. Consequently, the
descaling temperature was less than 1050.degree. C. As a result,
the ratio of island-shaped scale was more than 6%.
[0196] In steel type T used in test number 24, the Mn content was
too low, the Al content was too low, and the Sb content was too
high. Consequently, embrittlement of the steel material was severe
and the evaluation test was abandoned. In test number 25, the Mn
content was high. Consequently, embrittlement of the steel material
was severe and the evaluation test was abandoned.
Example 2
[0197] Molten steels having the chemical compositions shown in
Table 4 were produced.
TABLE-US-00004 TABLE 4 Steel Chemical Composition (Unit Is Mass
Percent: Balance Is Fe And Impurities) Type C Si Mn P S Al N O Sb
Ti V Nb Cr Ni Mo A 0.155 1.25 2.23 0.009 0.0025 0.0586 0.0042
0.0016 0.05 -- -- -- -- -- -- B 0.176 1.71 2.49 0.014 0.0014 0.0588
0.0042 0.0023 0.11 -- -- -- -- -- -- C 0.178 1.41 2.13 0.013 0.0031
0.0540 0.0039 0.0028 0.15 -- -- -- -- -- -- D 0.151 1.19 2.24 0.008
0.0033 0.0608 0.0032 0.0028 0.12 0.062 -- -- 0.12 -- -- E 0.148
1.65 2.73 0.013 0.0022 0.0770 0.0022 0.0013 0.08 -- -- -- -- 0.24
-- F 0.169 1.90 2.08 0.013 0.0012 0.0722 0.0020 0.0019 0.22 -- --
-- -- -- -- G 0.186 1.21 2.83 0.009 0.0024 0.0837 0.0043 0.0029
0.08 -- -- -- -- -- -- H 0.118 1.67 2.42 0.008 0.0013 0.0790 0.0041
0.0014 0.13 -- -- -- -- -- -- I 0.138 1.32 2.68 0.013 0.0029 0.0742
0.0038 0.0019 0.18 -- -- -- -- -- 0.14 J 0.159 1.79 2.03 0.012
0.0025 0.0905 0.0038 0.0026 0.11 -- 0.083 -- -- -- -- K 0.182 1.74
2.84 0.008 0.0032 0.0859 0.0033 0.0021 -- -- 0.053 -- -- -- -- L
0.113 1.40 2.42 0.008 0.0022 0.0232 0.0030 0.0026 0.41 0.005 -- --
-- -- -- M 0.110 1.86 2.68 0.013 0.0011 0.0974 0.0028 0.0011 0.004
0.005 -- -- -- -- -- N 0.131 1.02 2.05 0.012 0.0034 0.0926 0.0026
0.0016 0.15 -- -- -- -- -- -- O 0.172 0.93 2.11 0.008 0.0013 0.0252
0.0021 0.0027 0.03 -- -- -- -- -- -- P 0.169 1.88 1.55 0.015 0.0028
0.0205 0.0018 0.0012 0.08 -- -- -- -- -- -- Q 0.121 2.96 2.21 0.011
0.0014 0.0320 0.0043 0.0023 0.15 -- -- -- -- -- -- R 0.123 1.70
3.99 0.008 0.0032 0.0321 0.0041 0.0028 0.09 -- -- -- -- -- -- S
0.185 1.75 2.50 0.010 0.0020 0.0350 0.0035 0.0016 0.02 -- -- -- --
-- -- Steel Chemical Composition (Unit Is Mass Percent: Balance Is
Fe And Impurities) Type B Cu Sn Bt Te Ca Mg REM Si + Mn Remarks A
-- -- -- -- -- -- -- -- 3.48 Inventive Example B -- -- -- -- -- --
-- -- 4.20 Inventive Example C -- -- -- -- -- -- -- -- 3.55
Inventive Example D 0.0014 -- -- -- -- -- -- -- 3.42 Inventive
Example E -- 0.12 -- -- -- -- -- -- 4.38 Inventive Example F -- --
-- 0.0008 -- 0.0006 -- -- 3.98 Inventive Example G -- -- -- -- 0.05
-- 0.0013 -- 4.04 Inventive Example H -- -- -- -- -- -- -- Nd:
0.0007 4.09 Inventive Example I -- -- -- -- -- -- -- -- 4.00
Inventive Example J -- -- -- -- -- -- -- -- 3.81 Inventive Example
K -- -- -- -- -- -- -- -- 4.58 Comparative Example L -- -- -- -- --
-- -- -- 3.82 Comparative Example M -- -- -- -- -- -- -- -- 4.54
Comparative Example N -- -- -- -- -- -- -- -- 3.07 Comparative
Example O -- -- -- -- -- -- -- -- 3.04 Comparative Example P 0.0046
-- -- -- -- -- -- -- 3.43 Comparative Example Q -- -- -- -- -- --
-- -- 5.17 Comparative Example R -- -- -- -- -- -- -- -- 5.69
Comparative Example S -- -- -- -- -- -- -- -- 4.25 Comparative
Example (Note) Underlined Numerical Values Indicate That The Values
Are Outside The Range Of The Present Invention.
[0198] Using the above described molten steel, steel materials
(ingots) were produced by an ingot-making process. Steel sheets
were produced by hot rolling the steel materials under the hot
rolling conditions (heating temperature (.degree. C.) and finish
rolling temperature FT (.degree. C.)) shown in Table 5 using a hot
rolling mill for testing. In addition, a heat treatment that
simulated coiling at a coiling temperature CT (.degree. C.) shown
in Table 5 was performed on the respective steel sheets after hot
rolling. Specifically, the steel sheets were stacked and charged
into a furnace that was set to the coiling temperature CT (.degree.
C.). The inside of the furnace was a nitrogen atmosphere, and the
steel sheet surface was in a state in which the surface was
blocked-off from the atmosphere. That is, the surface state of the
steel sheet was equal to the surface state of a coil obtained by
actual production. After holding the steel sheet inside the furnace
for 30 minutes at the coiling temperature CT (.degree. C.), the
steel sheet was gradually cooled to room temperature at 20.degree.
C./hour.
TABLE-US-00005 TABLE 5 Sb Internal Concentrated Oxidized Heating
Steel Layer Layer Test Steel Temperature FT CT Micro- Thickness
Thickness TS EL Number Type (.degree. C.) (.degree. C.) (.degree.
C.) structure (.mu.m) (.mu.m) (MPa) (%) Remarks 1 A 1250 950 750 F
+ P 1.5 1 540 11.9 Inventive Example Of Present Invention 2 B 1250
950 750 F + P 2.4 0 633 10.7 Inventive Example Of Present Invention
3 C 1250 950 750 F + P 2.8 0 596 11.1 Inventive Example Of Present
Invention 4 D 1250 950 750 F + P 2.5 0 682 10.0 Inventive Example
Of Present Invention 5 E 1250 950 750 F + P 2.2 0 620 11.1
Inventive Example Of Present Invention 6 F 1250 950 750 F + P 3.4 0
585 10.1 Inventive Example Of Present Invention 7 G 1250 950 750 F
+ P 2.2 0 599 11.6 Inventive Example Of Present Invention 8 H 1250
950 750 F + P 2.6 0 596 10.9 Inventive Example Of Present Invention
9 I 1250 950 750 F + P 3.0 0 585 11.7 Inventive Example Of Present
Invention 10 J 1250 950 750 F + P 2.4 0 566 10.0 Inventive Example
Of Present Invention 11 K 1250 950 750 F + P 0 47 648 10.5
Comparative Example 12 L 1250 950 750 F + P 5.3 0 -- -- Comparative
Example 13 M 1180 950 750 F + P 0 34 616 10.4 Comparative Example
14 N 1250 950 750 F + P 2.8 0 534 8.8 Comparative Example 15 O 1250
950 750 F + P 1.1 4 548 8.7 Comparative Example 16 P 1250 950 750 F
+ P 2.0 0 562 6.7 Comparative Example 17 Q 1250 950 750 F + P 2.7 0
682 7.8 Comparative Example 18 R 1250 950 750 B + M 2.3 0 1317 3.2
Comparative Example 19 S 1200 950 750 F + P 03 25 640 10.2
Comparative Example
[Ferrite and Pearlite Area Fraction Measurement Test]
[0199] By the same method as in Example 1, the total of the area
fractions of ferrite and pearlite in the steel sheet (hot-rolled
steel sheet) after hot rolling was measured. The results are shown
in Table 5. In the "steel micro-structure" column in Table 5, "F+P"
indicates that the total area fraction of ferrite and pearlite in
the micro-structure of the hot-rolled steel sheet was 75% or more.
In the "steel micro-structure" column in Table 5, "B+M" indicates
that the total area fraction of bainite and martensite in the
micro-structure of the hot-rolled steel sheet was 75% or more.
[Internal Oxidized Layer Thickness Measurement Test]
[0200] The thickness of an internal oxidized layer in the steel
sheet after hot rolling (hot-rolled steel sheet) of each test
number was measured by the same method as in Example 1.
Specifically, a small piece including a part of the surface of the
hot-rolled steel sheet was cut out from a width-wise central
portion of the hot-rolled steel sheet. Of the entire surface of the
small piece, a cross-section (hereunder, referred to as
"observation face") perpendicular to the rolling direction was
mirror polished. The observation face was subjected to C (carbon)
vapor deposition. After the C vapor deposition, a portion in the
vicinity of the surface of the observation face was photographed at
an observation magnification of .times.1000 using a field-emission
scanning electron microscope (FE-SEM) and images were obtained. The
thickness of the internal oxidized layer was measured by the above
described method based on the obtained images. The results are
shown in Table 5.
[0201] Note that scale is a layer that is formed when iron ions at
the exterior of the hot-rolled steel sheet are oxidized. On the
other hand, an internal oxidized layer is a layer that contains
oxides of Si and Mn and is formed inside the hot-rolled steel
sheet. Therefore, scale, an internal oxidized layer and the base
metal can be easily distinguished using a common SEM.
[Tension Test]
[0202] The tensile strength TS of the hot-rolled steel sheet of
each test number was measured by a method in accordance with JIS Z
2241 (2011). The results are shown in Table 5. In Table 5, the
symbol "-" in the "TS (MPa)" column indicates that cracking
occurred at an edge of the hot-rolled steel sheet and measurement
was not possible.
[Uniform Elongation Measurement Test]
[0203] The hot-rolled steel sheet of each test number was
cold-rolled at a draft of 50%. After cold rolling, each steel sheet
was subjected to annealing. The annealing was performed under the
following conditions. The steel sheet was heated to an HC
temperature (Ae.sub.3 temperature +10.degree. C.) at an average
heating rate of 5.degree. C./second, and the steel sheet was
subjected to annealing for 90 seconds at this HC temperature.
Thereafter, the steel sheet was gradually cooled at a cooling rate
of 2.degree. C./sec to an AC temperature (HC temperature
-120.degree. C.). In addition, the steel sheet was rapidly cooled
at 80.degree. C./sec from the AC temperature to 420.degree. C.
After being held at 420.degree. C. for 300 seconds, the steel sheet
was allowed to cool to room temperature. A tension test was
performed by a method in accordance with JIS Z 2241 (2011) on the
steel sheet after annealing. During the tension test, a length that
the test specimen stretched until necking occurred in the test
specimen (a section in which the test specimen exhibited uniform
elongation) was measured. The obtained length was divided by the
length of the test specimen and the result was adopted as a uniform
elongation EL. The results are shown in Table 5. In Table 5, the
symbol "-" in the "EL (%)" column indicates that cracking occurred
at an edge of the steel sheet and measurement was not possible.
[Test Results]
[0204] Referring to Table 4 and Table 5, the chemical compositions
of test numbers 1 to 10 were appropriate. In addition, the
production conditions of test numbers 1 to 10 were appropriate.
Therefore, in the micro-structure of the hot-rolled steel sheets of
test numbers 1 to 10, the total area fraction of ferrite and
pearlite was 75% or more. Further, in the hot-rolled steel sheets
of test numbers 1 to 10, an Sb concentrated layer with a thickness
of 0.5 .mu.m or more was formed. Furthermore, the thickness of an
internal oxidized layer was 5 .mu.m or less, and thus formation of
an internal oxidized layer was suppressed.
[0205] In addition, the tensile strength of the hot-rolled steel
sheets of test numbers 1 to 10 was 800 MPa or less, and the
workability during cold rolling was excellent. The uniform
elongation of the cold-rolled steel sheets of test numbers 1 to 10
was 10.0% or more, indicating excellent workability after cold
rolling also.
[0206] Steel type K used for test number 11 did not contain Sb.
Consequently, in the hot-rolled steel sheet of test number 11, an
Sb concentrated layer was not formed and an internal oxidized layer
had a thick thickness of 47 .mu.m.
[0207] In steel type L used for test number 12, the Sb content was
too high. Consequently, cracking occurred at an edge of the
hot-rolled steel sheet of test number 12, and a tension test could
not be performed. Therefore, the workability during cold rolling
was poor.
[0208] In steel type M used for test number 13, the Sb content was
too low. Consequently, an internal oxidized layer in the hot-rolled
steel sheet of test number 13 had a thick thickness of 34
.mu.m.
[0209] In steel type N used in test number 14, a total content of
Si and Mn was 3.07%, and thus Formula (1) was not satisfied.
Consequently, in the cold-rolled steel sheet of test number 14, in
comparison to test numbers 1 to 10 in which the total area fraction
of ferrite and pearlite was 75% or more similarly to test number
14, the uniform elongation EL was a low value of less than 10%.
[0210] In steel type O used in test number 15, the Si content was a
low value of 0.93%. In addition, in steel type O, the total content
of Si and Mn was 3.04%, and thus Formula (1) was not satisfied.
Therefore, the uniform elongation of the cold-rolled steel sheet of
test number 15 was 8.7%, which was low in comparison to test
numbers 1 to 10 in which, similarly to test number 15, the total
area fraction of ferrite and pearlite was 75% or more.
[0211] In steel type P used in test number 16, the Mn content was a
low value of 1.55%. Consequently, the uniform elongation of the
cold-rolled steel sheet of test number 16 was 6.7%, which was low
in comparison to test numbers 1 to 10 in which, similarly to test
number 16, the total area fraction of ferrite and pearlite was 75%
or more.
[0212] In steel type Q used in test number 17, the Si content was a
high value of 2.96%. Consequently, the uniform elongation of the
cold-rolled steel sheet of test number 17 was 7.8%, and the
workability was poor in comparison to test numbers 1 to 10 in
which, similarly to test number 17, the total area fraction of
ferrite and pearlite was 75% or more.
[0213] In steel type R used in test number 18, the Mn content was a
high value of 3.99%. Consequently, the uniform elongation of the
cold-rolled steel sheet of test number 18 was 3.2%, and the
workability was poor.
[0214] In steel type S used in test number 19, the Sb content was a
low value of 0.02%. Consequently, in the hot-rolled steel sheet of
test number 19, the thickness of the Sb concentrated layer was less
than 0.5 .mu.m, and the internal oxidized layer had a thick
thickness of 25 .mu.m.
Example 3
[0215] Molten steels having the chemical compositions shown in
Table 4 were produced.
[0216] Using the above described molten steel, steel materials
(ingots) were produced by an ingot-making process. Steel sheets
were produced by hot rolling the steel materials under the hot
rolling conditions (heating temperature (.degree. C.) and finish
rolling temperature FT (.degree. C.)) shown in Table 6 using a hot
rolling mill for testing. In addition, a heat treatment that
simulated coiling at a coiling temperature CT (.degree. C.) shown
in Table 6 was performed on the respective steel sheets after hot
rolling. Specifically, the steel sheets were stacked and charged
into a furnace that was set to the coiling temperature CT (.degree.
C.). The inside of the furnace was a nitrogen atmosphere, and the
steel sheet surface was in a state in which the surface was
blocked-off from the atmosphere. That is, the surface state of the
steel sheet was equal to the surface state of a coil obtained by
actual production. After holding the steel sheet inside the furnace
for 30 minutes at the coiling temperature CT (.degree. C.), the
steel sheet was gradually cooled to room temperature at 20.degree.
C./hour. In addition, with respect to the steel sheets of test
numbers other than test numbers 2, 5, 7, 13 and 15, tempering was
performed at a tempering temperature (.degree. C.) and for a
tempering time period (hr) as shown in Table 6. In Table 6, the
column "tempering time period (hr)" shows the time period for which
the relevant steel sheet was kept at the tempering temperature
shown in Table 6.
TABLE-US-00006 TABLE 6 Sb Concentrated Heating Tempering Tempering
Micro-structure Layer Test Steel Temperature FT CT Temperature Time
(Area Fraction (%)) Thickness Number Type (.degree. C.) (.degree.
C.) (.degree. C.) (.degree. C.) Period (hr) F P B M .gamma. (.mu.m)
1 A 1250 960 450 700 0.5 -- -- 88 12 -- 1.3 2 A 1250 980 400 -- --
-- -- 68 22 10 0.8 3 B 1250 970 450 650 1.0 -- -- 86 14 -- 1.9 4 C
1250 980 450 600 8.0 -- -- 85 15 -- 2.5 5 C 1250 950 450 -- -- --
-- 86 -- 14 2.2 6 D 1250 980 450 700 0.5 -- -- 88 12 -- 2.1 7 D
1250 960 450 -- -- 1 -- 88 -- 11 1.8 8 E 1250 930 440 700 0.5 -- --
85 15 -- 1.7 9 F 1250 970 150 700 0.5 3 -- -- 98 2 3.3 10 G 1250
955 460 700 0.5 -- -- 84 16 -- 1.7 11 H 1250 930 460 700 0.5 1 --
89 11 -- 2.2 12 I 1250 950 440 680 0.5 -- -- 89 11 -- 2.8 13 I 1250
950 450 -- -- -- -- 89 -- 11 2.6 14 J 1250 950 460 700 0.5 1 -- 87
11 -- 2.0 15 A 1250 950 450 -- -- -- -- 88 -- 12 0.8 16 K 1250 950
450 700 1.0 -- -- 80 20 -- 0.0 17 L 1250 950 450 650 0.5 -- -- 91 9
-- 5.7 18 M 1200 950 450 650 1.0 -- -- 89 11 -- 0.0 19 N 1250 950
480 700 0.5 17 -- 83 -- -- 2.5 20 O 1250 980 450 700 0.5 3 -- 86 11
-- 1.0 21 P 1250 950 480 700 0.5 30 -- 70 -- -- 1.7 22 Q 1250 950
450 700 0.5 -- -- 88 12 -- 2.5 23 R 1250 950 450 700 0.5 -- -- 40
60 -- 1.8 24 S 1200 950 450 650 1.0 -- -- 85 15 -- 0.2 Internal
Oxidized Layer Scale Test Thickness Thickness TS EL Number (.mu.m)
(.mu.m) (MPa) (%) Remarks 1 0 4.8 621 12.5 Inventive Example Of
Present Invention 2 0 4.8 1095 8.9 Inventive Example Of Present
Invention 3 0 2.7 748 11.3 Inventive Example Of Present Invention 4
0 4.8 781 11.6 Inventive Example Of Present Invention 5 0 4.8 955
6.9 Inventive Example Of Present Invention 6 0 4.1 618 11.1
Inventive Example Of Present Invention 7 0 4.1 948 12.3 Inventive
Example Of Present Invention 8 0 5.1 610 12.1 Inventive Example Of
Present Invention 9 0 5.7 628 11.5 Inventive Example Of Present
Invention 10 0 4.7 654 13.1 Inventive Example Of Present Invention
11 0 2.6 590 12.2 Inventive Example Of Present Invention 12 0 3.7
666 12.5 Inventive Example Of Present Invention 13 0 3.7 930 7.8
Inventive Example Of Present Invention 14 0 4.1 621 10.9 Inventive
Example Of Present Invention 15 0 4.8 1095 9.8 Inventive Example Of
Present Invention 16 30 9.8 608 11.2 Comparative Example 17 0 5.1
-- 12.7 Comparative Example 18 22 12.3 517 10.0 Comparative Example
19 0 6.9 597 9.2 Comparative Example 20 0 6.6 635 9.3 Comparative
Example 21 0 5.7 637 7.5 Comparative Example 22 0 4.7 587 7.9
Comparative Example 23 0 5.6 597 3.2 Comparative Example 24 14 8.3
660 11.0 Comparative Example
[Bainite and Martensite Area Fraction Measurement Test]
[0217] The area fraction of bainite and martensite in the
hot-rolled steel sheet was measured by the above described method.
The results are shown in Table 6. In the "micro-structure" columns
in Table 6, "F" shows the area fraction of ferrite, "P" shows the
area fraction of pearlite, "B" shows the area fraction of bainite,
"M" shows the area fraction of martensite, and ".gamma." shows the
area fraction of austenite.
[Internal Oxidized Layer Thickness and Scale Thickness Measurement
Test]
[0218] The internal oxidized layer thickness and scale thickness of
the hot-rolled steel sheet of each test number were measured by the
same method as in Example 1. The results are shown in Table 6.
[Sb Concentrated Layer Thickness Measurement Test]
[0219] The presence or absence of an Sb concentrated layer and the
thickness (.mu.m) of an Sb concentrated layer were measured for the
hot-rolled steel sheet of each test number by the same method as in
Example 1. The results are shown in Table 6.
[Tension Test]
[0220] The tensile strength TS (MPa) of each test number was
measured by the same method as in Example 1. The results are shown
in Table 6. In Table 6, the symbol "-" in the "tensile strength"
column indicates that cracking occurred at an edge of the
hot-rolled steel sheet and measurement was not possible.
[Uniform Elongation Measurement Test]
[0221] The uniform elongation EL of each test number was measured
by the same method as in Example 2. The results are shown in Table
6.
[Test Results]
[0222] Referring to Table 4 and Table 6, the chemical compositions
of test numbers 1 to 15 were appropriate. In addition, the
production conditions of test numbers 1 to 15 were appropriate.
Consequently, in the micro-structure of the hot-rolled steel sheets
of test numbers 1 to 15, the total area fraction of bainite and
martensite was 75% or more. In the hot-rolled steel sheets of test
numbers 1 to 15, an Sb concentrated layer having a thickness of 0.5
.mu.m or more was also confirmed. As a result, the thickness of the
internal oxidized layer was 5 .mu.m or less, and formation of an
internal oxidized layer was suppressed. In addition, the scale
thickness of the hot-rolled steel sheets of test numbers 1 to 15
was 7 .mu.m or less, and scale was suppressed.
[0223] In test numbers 1, 3, 4, 6, 8 to 12 and 14, tempering was
performed. Consequently, the tensile strength TS was 800 MPa or
less and the uniform elongation EL was 10% or more, and excellent
workability was obtained after cold rolling. On the other hand, in
test numbers 2, 5, 7, 13 and 15, tempering was not performed.
Consequently, the tensile strength was 900 MPa or more and
excellent strength was obtained.
[0224] In contrast, steel type K used in test number 16 did not
contain Sb. Consequently, an Sb concentrated layer was not formed.
As a result, the thickness of an internal oxidized layer was more
than 5 .mu.m, and the scale thickness was more than 7 .mu.m.
[0225] In steel type L used in test number 17, the Sb content was
0.41%, which was too high. Consequently, in test number 17,
cracking occurred at an edge of the hot-rolled steel sheet, and the
workability was poor. Therefore a tension test could not be
performed.
[0226] In steel type M used in test number 18, the Sb content was
0.004%, which was too low. Therefore, an Sb concentrated layer was
not formed in the hot-rolled steel sheet of test number 18.
Consequently, the thickness of the internal oxidized layer was more
than 5 .mu.m, and the scale thickness was more than 7 .mu.m.
[0227] In steel type N used in test number 19, the total content of
Si and Mn was 3.07%, and thus Formula (1) was not satisfied.
Therefore, even though tempering was performed, the uniform
elongation EL was less than 10%.
[0228] In steel type O used in test number 20, the Si content was a
low value of 0.93%. In addition, in steel type O, the total content
of Si and Mn was 3.04%, and thus Formula (1) was not satisfied.
Consequently, even though tempering was performed, the uniform
elongation EL was less than 10%.
[0229] In steel type P used in test number 21, the Mn content was a
low value of 1.55%. Consequently, in the micro-structure, the area
fraction of ferrite was 30%, and the combined area fraction of
martensite and bainite was less than 75%. As a result, even though
tempering was performed, the uniform elongation EL was less than
10%.
[0230] In steel type Q used in test number 22, the Si content was a
high value of 2.96%. Consequently, even though tempering was
performed, the uniform elongation EL was less than 10%.
[0231] In steel type R used in test number 23, the Mn content was a
high value of 3.99%. Consequently, even though tempering was
performed, the uniform elongation EL was less than 10%.
[0232] In steel type S used in test number 24, the Sb content was a
low value of 0.02%. Therefore, the thickness of an Sb concentrated
layer was less than 0.5 .mu.m. Consequently, the thickness of an
internal oxidized layer was more than 10 .mu.m, and the scale
thickness was more than 7 .mu.m.
[0233] Embodiments of the present invention have been described
above. However, the above described embodiments are merely examples
for implementing the present invention. Accordingly, the present
invention is not limited to the above described embodiments, and
the above described embodiments can be appropriately modified
within a range that does not deviate from the technical scope of
the present invention.
* * * * *