U.S. patent application number 14/764818 was filed with the patent office on 2015-12-24 for high-strength hot-rolled steel sheet and method for manufacturing the same (as amended).
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Chikara Kami, Katsumi Nakajima, Kazuhiko Yamazaki.
Application Number | 20150368741 14/764818 |
Document ID | / |
Family ID | 51261979 |
Filed Date | 2015-12-24 |
United States Patent
Application |
20150368741 |
Kind Code |
A1 |
Nakajima; Katsumi ; et
al. |
December 24, 2015 |
HIGH-STRENGTH HOT-ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING
THE SAME (AS AMENDED)
Abstract
There are provided a high-strength hot-rolled steel sheet having
high burring formability and a method for manufacturing the
high-strength hot-rolled steel sheet. A high-strength hot-rolled
steel sheet having high burring formability contains, on a mass
percent basis, C: 0.06% or more and 0.13% or less, Si: less than
0.5%, Mn: more than 0.5% and 1.4% or less, P: 0.05% or less, S:
0.005% or less, N: 0.01% or less, Al: 0.1% or less, Ti: 0.05% or
more and 0.25% or less, and V: more than 0.15% and 0.4% or less
such that S, N, Ti, and V satisfy Ti.+V.gtoreq.0.35 (wherein
Ti..dbd.Ti--N.times.(48/14)-S.times.(48/32), and S, N, Ti, and V
denote the amounts (% by mass) of the corresponding elements), the
remainder being Fe and incidental impurities, wherein the
high-strength hot-rolled steel sheet has a microstructure in which
a ferrite phase fraction is more than 90%, a carbide containing Ti
is precipitated, and 70% or more of the carbide has a grain size of
less than 9 nm.
Inventors: |
Nakajima; Katsumi; (Tokyo,
JP) ; Yamazaki; Kazuhiko; (Tokyo, JP) ; Kami;
Chikara; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
51261979 |
Appl. No.: |
14/764818 |
Filed: |
January 23, 2014 |
PCT Filed: |
January 23, 2014 |
PCT NO: |
PCT/JP2014/000337 |
371 Date: |
July 30, 2015 |
Current U.S.
Class: |
148/533 ;
148/332; 148/333; 148/337; 148/602 |
Current CPC
Class: |
C22C 38/12 20130101;
C22C 38/02 20130101; C21D 2211/004 20130101; C23C 2/06 20130101;
Y02P 10/20 20151101; Y02P 10/212 20151101; C22C 38/001 20130101;
C22C 38/00 20130101; C22C 38/14 20130101; C22C 38/28 20130101; C21D
2211/005 20130101; C22C 38/04 20130101; C21D 8/0226 20130101; C22C
38/06 20130101; C22C 38/16 20130101; C23C 2/28 20130101; C22C
38/002 20130101; C23C 2/02 20130101; C22C 38/08 20130101; C22C
38/24 20130101; C22C 38/54 20130101; C21D 9/46 20130101; C23C 2/40
20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/28 20060101 C22C038/28; C22C 38/24 20060101
C22C038/24; C22C 38/16 20060101 C22C038/16; C22C 38/14 20060101
C22C038/14; C22C 38/12 20060101 C22C038/12; C22C 38/08 20060101
C22C038/08; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C23C 2/06 20060101 C23C002/06; C23C 2/28 20060101
C23C002/28; C23C 2/40 20060101 C23C002/40; C21D 8/02 20060101
C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2013 |
JP |
2013-016457 |
Claims
1. A high-strength hot-rolled steel sheet comprising, on a mass
percent basis: C: 0.06% or more and 0.13% or less, Si: less than
0.5%, Mn: more than 0.5% and 1.4% or less, P: 0.05% or less, S:
0.005% or less, N: 0.01% or less, Al: 0.1% or less, Ti: 0.05% or
more and 0.25% or less, and V: more than 0.15% and 0.4% or less
such that S, N, Ti, and V satisfy the following formula (1), the
remainder being Fe and incidental impurities, wherein the
high-strength hot-rolled steel sheet has a microstructure in which
a ferrite phase fraction is more than 90%, a carbide containing Ti
is precipitated, and 70% or more of the carbide has a grain size of
less than 9 nm. Ti.+V.gtoreq.0.35 (1) wherein
Ti..dbd.Ti--N.times.(48/14)-S.times.(48/32), and S, N, Ti, and V
denote the amounts (% by mass) of the corresponding elements.
2. The high-strength hot-rolled steel sheet according to claim 1,
wherein 5Q% by mass or more of Ti is precipitated as precipitates
containing Ti having a grain size of less than 20 nm.
3. The high-strength hot-rolled steel sheet according to claim 1,
further comprising Nb: 0.002% or more and 0.1% or less on a mass
percent basis.
4. The high-strength hot-rolled steel sheet according to claim 1,
further comprising at least one of Cu: 0.005% or more and 0.2% or
less, Ni: 0.005% or more and 0.2% or less, Cr: 0.002% or more and
0.2% or less, Mo: 0.002% or more and 0.2% or less, and Sn: 0.005%
or more and 0.2% or less on a mass percent basis.
5. The high-strength hot-rolled steel sheet according to claim 1,
further comprising B: 0.0002% or more and 0.003% or less on a mass
percent basis.
6. The high-strength hot-rolled steel sheet according to claim 1,
further comprising at least one of Ca: 0.0002% or more and 0.005%
or less and REM: 0.0002% or more and 0.03% or less on a mass
percent basis.
7. A method for manufacturing a high-strength hot-rolled steel
sheet, comprising: heating steel having a composition according to
claim 1 to 1100.degree. C. or more, hot-rolling the steel at a
finish-rolling temperature of (Ar.sub.3+25.degree. C.) or more and
at a total reduction ratio of 60% or less at last two finish
rolling stands, cooling the hot-rolled steel sheet at an average
cooling rate of 40.degree. C./s or more, and coiling the hot-rolled
steel sheet at a coiling temperature in the range of 520.degree. C.
to 680.degree. C.
8. A method for manufacturing a high-strength hot-rolled steel
sheet, comprising: heating steel having a composition according to
claim 1 to 1100.degree. C. or more, hot-rolling the steel at a
finish-rolling temperature of (Ar.sub.3+25.degree. C.) or more and
at a total reduction ratio of 60% or less at last two finish
rolling stands, cooling the hot-rolled steel sheet at an average
cooling rate of 40.degree. C./s or more, coiling the hot-rolled
steel sheet at a coiling temperature in the range of 500.degree. C.
to 640.degree. C., annealing the hot-rolled steel sheet at a
soaking temperature of 760.degree. C. or less after pickling, and
plating the hot-rolled steel sheet by immersing the hot-rolled
steel sheet in a molten zinc bath.
9. The method for manufacturing a high-strength hot-rolled steel
sheet according to claim 8, wherein the plating treatment is
followed by alloying treatment.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is the U.S. National Phase application of PCT
International Application No. PCT/JP2014/000337, filed Jan. 23,
2014, and claims priority to Japanese Patent Application No.
2013-016457, filed Jan. 31, 2013, the disclosures of each of these
applications being incorporated herein by reference in their
entireties for all purposes.
FIELD OF THE INVENTION
[0002] Aspects of the present invention relate to a high-strength
hot-rolled steel sheet having high burring formability and a method
for manufacturing the high-strength hot-rolled steel sheet. A
high-strength hot-rolled steel sheet, according to one aspect of
the present invention, is mainly used in automotive body
components, for example, structural parts, such as members and
frames of automotive bodies, and chassis parts, such as
suspensions. However, the present invention is not limited to these
aspects and/or applications.
BACKGROUND OF THE INVENTION
[0003] In recent years, for weight saving of automotive bodies,
high-strength steel sheets have been actively used as materials for
automotive parts. High-strength steel sheets are widely used as
automotive structural parts. For further weight saving of
automotive bodies, there is a strong demand for application of
high-strength steel sheets not only to structural parts but also to
chassis parts in which hot-rolled steel sheets are generally
used.
[0004] Most of automotive parts made of steel sheets are formed
into predetermined shapes by press forming or burring forming.
However, in general, higher-strength steel sheets have lower
workability. Thus, high-strength steel sheets for use in automotive
parts must have high workability as well as desired strength. In
particular, because chassis parts are formed by severe processing,
both high strength and workability must be satisfied. In
particular, applicability of high-strength steel sheets to these
parts and mass productivity of these parts often depend on burring
formability.
[0005] Various microstructure control and reinforcement methods
have been used to improve the workability of high-strength
hot-rolled steel sheets. For example, these methods include use of
a complex structure of ductile ferrite and hard martensite, use of
a bainite microstructure, and precipitation strengthening of a
ferrite microstructure. However, high-strength hot-rolled steel
sheets having sufficient workability that can be applied to parts
to be subjected to severe burring forming, such as chassis parts,
cannot be manufactured in the related art. Thus, there is a demand
for high-strength hot-rolled steel sheet having high
workability.
[0006] To address such a demand, Patent Literature 1 describes a
hot-rolled steel sheet that has a composition including, on a mass
percent basis, one or two of C: 0.01% or more and 0.20% or less,
Si: 1.5% or less, Al: 1.5% or less, Mn: 0.5% or more and 3.5% or
less, P: 0.2% or less, S: 0.0005% or more and 0.009% or less, N:
0.009% or less, Mg: 0.0006% or more and 0.01% or less, 0: 0.005% or
less, and Ti: 0.01% or more and 0.20% or less, and Nb: 0.01% or
more and 0.10% or less and has a structure consisting essentially
of a bainite phase. In the technique proposed in Patent Literature
1, the steel sheet has a structure consisting essentially of a
bainite phase, and Mg sulfide is used to decrease the size of (Ti,
Nb)N. Thus, the high-strength hot-rolled steel sheet has a strength
of more than 980 N/mm.sup.2.
[0007] Patent Literature 2 describes a hot-rolled steel sheet that
has a composition including, on a mass percent basis, C: 0.01% or
more and 0.10% or less, Si: 2.0% or less, and Mn: 0.5% or more and
2.5% or less, as well as one or two or more of V: 0.01% or more and
0.30% or less, Nb: 0.01% or more and 0.30% or less, Ti: 0.01% or
more and 0.30% or less, Mo: 0.01% or more and 0.30% or less, Zr:
0.01% or more and 0.30% or less, and W: 0.01% or more and 0.30% or
less, V, Nb, Ti, Mo, Zr, and W being 0.5% or less in total, and has
a microstructure in which the bainite fraction is 80% or more. In
the technique proposed in Patent Literature 2, the steel sheet has
a structure consisting essentially of bainite, and V, Ti, and/or Nb
carbide causes precipitation strengthening of the bainite.
[0008] Patent Literature 3 describes a high-strength hot-dip
galvanized steel sheet that includes a hot-rolled steel sheet as a
substrate. The hot-rolled steel sheet has a composition including,
on a mass percent basis, C: 0.07% or more and 0.13% or less, Si:
0.3% or less, Mn: 0.5% or more and 2.0% or less, P: 0.025% or less,
S: 0.005% or less, N: 0.0060% or less, Al: 0.06% or less, Ti: 0.10%
or more and 0.14% or less, and V: 0.15% or more and 0.30% or less,
and has a structure consisting essentially of a ferrite phase in
which a desired volume percentage of fine carbide having an average
grain size of less than 10 nm is dispersedly precipitated. The
technique proposed in Patent Literature 3 can be used to
manufacture a high-strength hot-dip galvanized steel sheet having a
tensile strength of 980 MPa or more.
PATENT LITERATURES
[0009] PTL 1: Japanese Unexamined Patent Application Publication
No. 2005-120437
[0010] PTL 2: Japanese Unexamined Patent Application Publication
No. 2009-84637
[0011] PTL 3: Japanese Unexamined Patent Application Publication
No. 2011-225978
SUMMARY OF THE INVENTION
[0012] However, in the techniques proposed in Patent Literatures 1
and 2, the hot-rolled steel sheets, which have a structure
consisting essentially of a bainite phase, have low ductility.
Thus, high-strength hot-rolled steel sheets having sufficient
burring formability that can be applied to automotive parts cannot
be obtained. The technique proposed in Patent Literature 1 is not a
practical technique that can be applied to mass-produced parts,
such as automotive parts, because of the use of expensive Mg.
[0013] In the technique proposed in Patent Literature 3, the steel
sheet has a microstructure in which fine carbide is dispersed in a
matrix consisting essentially of a ferrite phase. Thus, the
technique proposed in Patent Literature 3 can be used to
manufacture a high-strength steel sheet. The burring formability of
this high-strength steel sheet was tested by a method according to
the Japan Iron and Steel Federation standard, as shown by an
example.
[0014] However, burring formability required for mass production of
automotive parts is not studied in the related art, including the
technique proposed in Patent Literature 3.
[0015] Burring formability of steel sheets has been principally
evaluated in a hole-expanding test by a method according to the
Japan Iron and Steel Federation standard. However, it is difficult
to say that the hole-expanding test accurately simulates a punching
process and a hole-expanding process in mass production of
automotive parts in actual production lines. Thus, there is a
problem that steel sheets that are experimentally shown to have
good burring formability according to the standard often suffer
from processing defects in mass production of automotive parts.
[0016] In particular, evaluation of workability in a laboratory
alone is insufficient for mass production of parts. It is necessary
to ensure workability of materials also in consideration of
variations in processing conditions in mass production. Such
problems are not investigated in the related art. Thus, the
resulting high-strength hot-rolled steel sheets do not necessarily
have desired strength and workability required for mass production
of automotive parts, particularly burring formability (hereinafter
also referred to as mass production burring formability). For
example, it is very difficult to manufacture high-strength
hot-rolled steel sheets that satisfy severe mass production burring
formability required by automobile manufacturers and parts
manufacturers using the techniques proposed in Patent Literatures 1
to 3.
[0017] As described above, many studies have been made on
hot-rolled steel sheets having high stretch-flangeability (burring
formability). However, techniques utilizing a known bainite phase
or techniques utilizing carbide in a structure consisting
essentially of ferrite cannot realize good mass production burring
formability.
[0018] It is an aim of an aspect of the the present invention to
advantageously solve the problems of the related art and provide a
high-strength hot-rolled steel sheet having a tensile strength (TS)
of 900 MPa or more and having high burring formability,
particularly high mass production burring formability, and a method
for manufacturing the high-strength hot-rolled steel sheet.
[0019] The "mass production burring formability" herein is
evaluated as a burring ratio measured in a hole-expanding test
using a 60-degree conical punch after punching with a 50-mm.phi.
punch (clearance of stamping: 30%) and is different from burring
formability evaluated as a .lamda. value determined by a known
hole-expanding test method, for example, a hole-expanding test
method according to the Japan Iron and Steel Federation
standard.
[0020] In order to solve the problems described above, the present
inventors studied mass production burring formability. Burring
formability has been evaluated as a .lamda. value, for example,
measured by a hole-expanding test method according to the Japan
Iron and Steel Federation standard. In this case, the punch
diameter is 10 mm.phi.. However, the present inventors found that
burring formability in actual mass production settings for parts is
not correlated with the .lamda. value measured in laboratories
according to the Japan Iron and Steel Federation standard. It was
found that burring formability evaluated in a new hole-expanding
test that includes hole-expanding using a 60-degree conical punch
after punching with a 50-mm.phi. punch (clearance of stamping: 30%)
is closely correlated with mass production punchability and mass
production burring formability.
[0021] The present inventors also extensively studied various
factors that contribute to high strength and workability,
particularly mass production burring formability, of hot-rolled
steel sheets by evaluating mass production burring formability in
the new hole-expanding test.
[0022] More specifically, extensive studies have been made on means
for improving mass production burring formability of a hot-rolled
steel sheet based on a structure consisting essentially of a
ductile ferrite phase while reinforcing the hot-rolled steel sheet,
with consideration given to all the precipitates that can be
precipitated in the hot-rolled steel sheet, such as nitrides,
sulfides, carbides, and complex precipitates thereof (for example,
carbonitride).
[0023] As a result, it was found that a hot-rolled steel sheet
having a tensile strength of 900 MPa or more that satisfies severe
mass production burring formability required in actual automotive
part production lines can be manufactured by optimizing the total
amount of V in the hot-rolled steel sheet and Ti (Ti.) that
contributes to the formation of carbide and increasing the
percentage of carbide having a grain size of less than 9 nm in
carbide precipitated in the hot-rolled steel sheet. It was also
found that mass production burring formability can be further
improved by controlling the size of not only carbide but also all
the precipitates that can be precipitated in a hot-rolled steel
sheet (nitrides, sulfides, carbides, and complex precipitates
thereof).
[0024] The present inventors also studied means for achieving a
desired size of precipitates that are precipitated in a hot-rolled
steel sheet (nitrides, sulfides, carbides, and complex precipitates
thereof), that is, a size required to impart desired strength
(tensile strength of 900 MPa or more) and good mass production
burring formability to the hot-rolled steel sheet. As a result, it
was found that it is beneficial to properly control the Mn content
and S, N, Ti, and V contents of a hot-rolled steel sheet and
optimize the hot rolling conditions and the cooling and coiling
conditions after hot rolling.
[0025] The present invention was completed, in part, in view of
these findings, and the following examples of the present invention
are provided below. [0026] [1] A high-strength hot-rolled steel
sheet, containing: on a mass percent basis, C: 0.06% or more and
0.13% or less, Si: less than 0.5%, Mn: more than 0.5% and 1.4% or
less, P: 0.05% or less, S: 0.005% or less, N: 0.01% or less, Al:
0.1% or less, Ti: 0.05% or more and 0.25% or less, and V: more than
0.15% and 0.4% or less such that S, N, Ti, and V satisfy the
following formula (1), the remainder being Fe and incidental
impurities, wherein the high-strength hot-rolled steel sheet has a
microstructure in which a ferrite phase fraction is more than 90%,
a carbide containing Ti is precipitated, and 70% or more of the
carbide has a grain size of less than 9 nm.
[0026] Ti.+V.gtoreq.0.35 (1)
[0027] In the formula (1),
Ti..dbd.Ti--N.times.(48/14)-S.times.(48/32), and S, N, Ti, and V
denote the amounts (% by mass) of the corresponding elements.
[0028] [2] The high-strength hot-rolled steel sheet according to
[1], wherein 50% by mass or more of Ti is precipitated as
precipitates containing Ti having a grain size of less than 20 nm.
[0029] [3] The high-strength hot-rolled steel sheet according to
[1] or [2], further containing Nb: 0.002% or more and 0.1% or less
on a mass percent basis. [0030] [4] The high-strength hot-rolled
steel sheet according to any one of [1] to [3], further containing
at least one of Cu: 0.005% or more and 0.2% or less, Ni: 0.005% or
more and 0.2% or less, Cr: 0.002% or more and 0.2% or less, Mo:
0.002% or more and 0.2% or less, and Sn: 0.005% or more and 0.2% or
less on a mass percent basis. [0031] [5] The high-strength
hot-rolled steel sheet according to any one of [1] to [4], further
containing B: 0.0002% or more and 0.003% or less on a mass percent
basis. [0032] [6] The high-strength hot-rolled steel sheet
according to any one of [1] to [5], further containing at least one
of Ca: 0.0002% or more and 0.005% or less and REM: 0.0002% or more
and 0.03% or less on a mass percent basis. [0033] [7] A method for
manufacturing a high-strength hot-rolled steel sheet, including:
heating steel having a composition according to any one of [1] and
[3] to [6] to 1100.degree. C. or more, hot-rolling the steel at a
finish-rolling temperature of (Ar.sub.3+25.degree. C.) or more and
at a total reduction ratio of 60% or less at last two finish
rolling stands, cooling the hot-rolled steel sheet at an average
cooling rate of 40.degree. C./s or more, and coiling the hot-rolled
steel sheet at a coiling temperature in the range of 520.degree. C.
to 680.degree. C. [0034] [8] A method for manufacturing a
high-strength hot-rolled steel sheet, comprising: heating steel
having a composition according to any one of [1] and [3] to [6] to
1100.degree. C. or more, hot-rolling the steel at a finish-rolling
temperature of (Ar.sub.3+25.degree. C.) or more and at a total
reduction ratio of 60% or less at last two finish rolling stands,
cooling the hot-rolled steel sheet at an average cooling rate of
40.degree. C./s or more, coiling the hot-rolled steel sheet at a
coiling temperature in the range of 500.degree. C. to 640.degree.
C., annealing the hot-rolled steel sheet at a soaking temperature
of 760.degree. C. or less after pickling, and plating the
hot-rolled steel sheet by immersing the hot-rolled steel sheet in a
molten zinc bath. [0035] [9] The method for manufacturing a
high-strength hot-rolled steel sheet according to [8], wherein the
plating treatment is followed by alloying treatment.
[0036] Aspects of the present invention provide a high-strength
hot-rolled steel sheet having a tensile strength of 900 MPa or more
and high burring formability such that the high-strength hot-rolled
steel sheet can be subjected to processing in mass production of
automotive parts. Thus, in one embodiment, a high-strength
hot-rolled steel sheet can be applied to structural parts, such as
members and frames of automotive bodies, and chassis parts, such as
suspensions. Thus, exemplary embodiments of the present invention
may contribute greatly to weight saving of these parts.
[0037] Aspects of the present invention can provide a hot-rolled
steel sheet having a tensile strength of 900 MPa or more and good
mass production burring formability. Thus, the high-strength
hot-rolled steel sheet can be applied not only to automotive parts
but also to other applications. Thus, embodiments of the present
invention have industrially advantageous effects.
DETAILED DESCRIPTION OF THE INVENTION
[0038] Embodiments of the present invention will be further
described below.
[0039] A high-strength hot-rolled steel sheet according to one
embodiment contains, on a mass percent basis, C: 0.06% or more and
0.13% or less, Si: less than 0.5%, Mn: more than 0.5% and 1.4% or
less, P: 0.05% or less, S: 0.005% or less, N: 0.01% or less, Al:
0.1% or less, Ti: 0.05% or more and 0.25% or less, and V: more than
0.15% and 0.4% or less such that S, N, Ti, and V satisfy the
following formula (1), the remainder being Fe and incidental
impurities, wherein the high-strength hot-rolled steel sheet has a
microstructure in which a ferrite phase fraction is more than 90%,
a carbide containing Ti is precipitated, and 70% or more of the
carbide has a grain size of less than 9 nm.
Ti.+V.gtoreq.0.35 (1)
[0040] In the formula (1),
Ti..dbd.Ti--N.times.(48/14)-S.times.(48/32), and S, N, Ti, and V
denote the amounts (% by mass) of the corresponding elements.
[0041] First, the composition of a hot-rolled steel sheet according
to embodiments of the present invention will be described below.
Unless otherwise specified, the percentages of the components are
on a mass percent basis.
C: 0.06% or More and 0.13% or Less
[0042] C is an important element that forms an appropriate carbide
in a hot-rolled steel sheet and secures the strength of the steel
sheet. In order to achieve the desired tensile strength (900 MPa or
more), the C content is 0.06% or more. However, a C content of more
than 0.13% results in poor workability and undesired burring
formability of a hot-rolled steel sheet. Thus, the C content is
0.06% or more and 0.13% or less, preferably 0.07% or more and 0.12%
or less.
Si: Less than 0.5%
[0043] A Si content of 0.5% or more results in very low surface
quality of a hot-rolled steel sheet, which adversely affects
fatigue characteristics, chemical conversion treatability, and
corrosion resistance. Si increases the ferrite transformation
temperature and thereby adversely affects the formation of fine
precipitates, which may be desirable in embodiments of the present
invention. Thus, the Si content is less than 0.5%, preferably
0.001% or more and less than 0.1%, more preferably 0.001% or more
and less than 0.05%.
Mn: More than 0.5% and 1.4% or less
[0044] Mn is an important element. Mn influences precipitation of a
carbide containing Ti, which is beneficial in embodiments of the
present invention, through control of austenite-to-ferrite
transformation temperatures.
[0045] In the case of a hot-rolled steel sheet containing Ti, a
carbide containing Ti is mainly precipitated by
austenite.fwdarw.ferrite transformation in cooling and coiling
steps after finish rolling in a hot-rolled steel sheet
manufacturing process. Among carbides precipitated in a hot-rolled
steel sheet, fine carbide contributes to high strength of the
hot-rolled steel sheet, but coarse carbide does not contribute to
high strength and adversely affects the workability of the
hot-rolled steel sheet.
[0046] A high austenite-ferrite transformation temperature results
in precipitation of a carbide containing Ti in a high-temperature
region and consequently coarsening of the carbide containing Ti.
Thus, in order to decrease the size of the carbide containing Ti,
it is preferable to decrease the austenite-ferrite transformation
temperature.
[0047] Mn is an element that has an effect of decreasing the
austenite-ferrite transformation temperature. A Mn content of 0.5%
or less results in an insufficient decrease in the
austenite-ferrite transformation temperature. As a result, a
carbide containing Ti is coarsened, and it is difficult to provide
a high-strength hot-rolled steel sheet having high mass production
burring formability. A Mn content of more than 1.4% results in
increased Mn segregation in the central portion in the thickness
direction. This center segregation impairs a punched surface before
burring forming and is therefore responsible for low mass
production burring formability. Thus, the Mn content is more than
0.5% and 1.4% or less, preferably more than 0.7% and 1.4% or less,
more preferably more than 1.0% and 1.4% or less.
P: 0.05% or Less
[0048] P is beneficial for low workability of a hot-rolled steel
sheet due to segregation. Thus, the P content is 0.05% or less,
preferably 0.001% or more and 0.03% or less. In the case of a
galvanized steel sheet formed by galvanizing treatment of a
hot-rolled steel sheet, the P content is preferably 0.005% or more,
more preferably 0.01% or more, in terms of platability.
S: 0.005% or Less
[0049] S forms a sulfide and decreases the workability of a
hot-rolled steel sheet. Thus, the S content is 0.005% or less,
preferably 0.0001% or more and 0.003% or less, more preferably
0.0001% or more and 0.0015% or less.
N: 0.01% or Less
[0050] An excessively high N content of more than 0.01% results in
the formation of a large amount of nitride in a hot-rolled steel
sheet manufacturing process, low hot ductility, and very low
burring formability of a hot-rolled steel sheet due to coarsening
of nitride. Thus, the N content is 0.01% or less, preferably
0.0001% or more and 0.006% or less, more preferably 0.0001% or more
and 0.004% or less.
Al: 0.1% or Less
[0051] Al is a useful element as a deoxidizing agent for steel.
However, an Al content of more than 0.1% makes casting of steel
difficult and results in a large amount of residual inclusion in
steel and low surface quality and workability of a hot-rolled steel
sheet. Thus, the Al content is 0.1% or less, preferably 0.001% or
more and 0.06% or less.
Ti: 0.05% or More and 0.25% or Less
[0052] Ti is animportant element in aspects the present invention.
Ti forms fine carbide and contributes to increased strength of a
hot-rolled steel sheet. In order to achieve the desired strength of
a hot-rolled steel sheet (tensile strength of 900 MPa or more), the
Ti content is 0.05% or more. However, a Ti content of more than
0.25% tends to result in the remaining coarse carbide in a
hot-rolled steel sheet. Coarse carbide has no strength increasing
effect and greatly impairs the workability, toughness, and
weldability of a hot-rolled steel sheet. Thus, the Ti content is
0.05% or more and 0.25% or less, preferably 0.08% or more and 0.20%
or less.
V: More than 0.15% and 0.4% or Less
[0053] V is also an important element. V forms fine carbide and
contributes to increased strength of a hot-rolled steel sheet. In
order to achieve the desired strength of a hot-rolled steel sheet
(tensile strength of 900 MPa or more), the V content is more than
0.15%. However, a V content of more than 0.4% is not worth the
cost. Thus, the V content is more than 0.15% and 0.4% or less,
preferably more than 0.15% and 0.35% or less.
[0054] A hot-rolled steel sheet according to the present invention
contains S, N, Ti, and V in the ranges described above so as to
satisfy the formula (1). The formula (1) is satisfied in order to
achieve high strength and good mass production burring formability
of a hot-rolled steel sheet and is a useful indicator for aspects
of the present invention. In the formula (1),
Ti..dbd.Ti--N.times.(48/14)-S.times.(48/32), and S, N, Ti, and V
denote the amounts (%) of the corresponding elements.
Ti.+V.gtoreq.0.35 (1)
[0055] As described below, in one embodiment, predetermined amounts
of Ti and V, which are carbide formation elements, are added to
steel, and carbide in steel is dissolved by heating before hot
rolling. These elements are mainly precipitated as carbides during
coiling after hot rolling. However, Ti and V added to steel do not
entirely contribute to the formation of carbide. In particular,
part of Ti added to steel is likely to be consumed by forming
nitride or sulfide. This is because Ti is likely to form nitride or
sulfide rather than carbide in a higher temperature region than the
coiling temperature. Thus, Ti forms nitride or sulfide before the
coiling step in the production of a hot-rolled steel sheet. Thus,
the minimum amount of Ti that can contribute to the formation of
carbide out of Ti added to steel can be represented by
Ti.(.dbd.Ti--N.times.(48/14)-S.times.(48/32)).
[0056] A hot-rolled steel sheet cannot have the desired strength
(tensile strength of 900 MPa or more) at Ti.+V of less than 0.35.
Ti.+V of less than 0.35 tends to result in precipitation of coarse
nitride or sulfide in a hot-rolled steel sheet, which results in
low mass production burring formability. Thus, in an embodiment,
Ti.+V is 0.35 or more, preferably 0.355 or more. However, Ti.+V of
more than 0.46 may result in excessively high strength of a
hot-rolled steel sheet and low workability. Thus, Ti.+V is
preferably 0.46 or less.
[0057] Provided are base components in an example of a hot-rolled
steel sheet according to aspects of the present invention. A
hot-rolled steel sheet may contain Nb: 0.002% or more and 0.1% or
less, if desired.
[0058] Nb is effective in decreasing the size of crystal grains and
improving the toughness of a hot-rolled steel sheet. Thus, Nb may
be added as desired. In order to produce such an effect, the Nb
content is preferably 0.002% or more. However, a Nb content of more
than 0.1% is not worth the cost. Thus, the Nb content is preferably
0.002% or more and 0.1% or less, more preferably 0.002% or more and
0.08% or less.
[0059] A hot-rolled steel sheet may contain at least one of Cu:
0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or
less, Cr: 0.002% or more and 0.2% or less, Mo: 0.002% or more and
0.2% or less, and Sn: 0.005% or more and 0.2% or less, if
desired.
[0060] Cu, Ni, and Sn are elements that contribute to high strength
of a hot-rolled steel sheet and may be added, if desired. In order
to produce such an effect, the Cu content is preferably 0.005% or
more, the Ni content is preferably 0.005% or more, and the Sn
content is preferably 0.005% or more. However, a Cu, Ni, or Sn
content of more than 0.2% may result in surface layer cracking
during hot rolling in the production of a hot-rolled steel sheet.
Thus, the Cu content is preferably 0.005% or more and 0.2% or less,
more preferably 0.005% or more and 0.1% or less. The Ni content is
preferably 0.005% or more and 0.2% or less, more preferably 0.005%
or more and 0.15% or less. The Sn content is preferably 0.005% or
more and 0.2% or less, more preferably 0.005% or more and 0.1% or
less.
[0061] Cr and Mo are carbide formation elements, contribute to high
strength of a hot-rolled steel sheet, and may be added, if desired.
In order to produce such an effect, the Cr content is preferably
0.002% or more, and the Mo content is preferably 0.002% or more.
However, a Cr or Mo content of more than 0.2% is not worth the
cost. Thus, the Cr content is preferably 0.002% or more and 0.2% or
less, more preferably 0.002% or more and 0.1% or less. The Mo
content is preferably 0.002% or more and 0.2% or less, more
preferably 0.002% or more and 0.1% or less.
[0062] A hot-rolled steel sheet may contain B: 0.0002% or more and
0.003% or less, if desirable.
[0063] B is an element that retards austenite-ferrite
transformation of steel. B decreases the precipitation temperature
of a carbide containing Ti by suppressing austenite-ferrite
transformation and contributes to a reduction in the size of the
carbide. In order to produce such an effect, the B content is
preferably 0.0002% or more. However, a B content of more than
0.003% results in a strong bainite transformation effect of B,
making it difficult for a hot-rolled steel sheet to have a
structure consisting essentially of a ferrite phase. Thus, the B
content preferably 0.0002% or more and 0.003% or less, more
preferably 0.0002% or more and 0.002% or less.
[0064] A hot-rolled steel sheet may contain at least one of Ca:
0.0002% or more and 0.005% or less and REM: 0.0002% or more and
0.03% or less, if desired.
[0065] Ca and REM are elements that are effective in morphology
control of an inclusion in steel and contribute to improved
workability of a hot-rolled steel sheet. In order to produce such
an effect, the Ca content is preferably 0.0002% or more, and the
REM content is preferably 0.0002% or more. However, a Ca content of
more than 0.005% or a REM content of more than 0.03% may result in
an increased inclusion in steel and low workability of a hot-rolled
steel sheet. Thus, the Ca content is preferably 0.0002% or more and
0.005% or less, more preferably 0.0002% or more and 0.003% or less.
The REM content is preferably 0.0002% or more and 0.03% or less,
more preferably 0.0002% or more and 0.003% or less.
[0066] The remainder is Fe and incidental impurities. Examples of
the incidental impurities include W, Co, Ta, Sb, Zr, and O. The
amount of each of the incidental impurities may be 0.1% or
less.
[0067] Next, examples of the microstructure of a hot-rolled steel
sheet according to aspects of the present invention will be
described below.
[0068] A hot-rolled steel sheet according to one embodiment has a
microstructure in which the ferrite phase fraction is more than
90%, a carbide containing Ti is precipitated, and 70% or more of
the carbide has a grain size of less than 9 nm. Preferably, 50% by
mass or more of Ti in a hot-rolled steel sheet is precipitated as
precipitates having a grain size of less than 20 nm.
Ferrite Phase Fraction: More than 90%
[0069] The burring formability of a hot-rolled steel sheet can be
effectively improved when the hot-rolled steel sheet has a
microstructure including a ductile ferrite phase. In order to
achieve high mass production burring formability, the ferrite
fraction in the microstructure of a hot-rolled steel sheet may be
more than 90%, preferably more than 92%, more preferably more than
94% by area. It is desirable that the ferrite grains have a
polygonal shape from the perspective of burring formability. In one
embodiment, it is also desirable that the ferrite grain size be as
small as possible. The hot-rolled steel sheet preferably has a
single phase structure of ferrite in terms of burring formability.
In order to improve punchability, the ferrite fraction is
preferably 98% by area or less, more preferably 97% by area or
less.
[0070] A hot-rolled steel sheet may have a microstructure other
than the ferrite phase, such as cementite, pearlite, bainite,
martensite, and/or retained austenite. Although excessive amounts
of these microstructures in the steel sheet impair burring
formability, these microstructures may constitute approximately
less than 10% by area in total. Appropriate amounts of these
microstructures in the hot-rolled steel sheet contribute to
improved punchability before burring forming and consequently
contribute to improved burring formability. Thus, the
microstructures other than the ferrite phase preferably constitute
2% or more and less than 8% by area, more preferably 3% or more and
less than 6% by area.
Carbide Containing Ti
[0071] In one embodiment, the desired strength (tensile strength of
900 MPa or more) of a hot-rolled steel sheet is achieved by
precipitation of a carbide containing Ti in the hot-rolled steel
sheet. The carbide containing Ti is mainly precipitated carbide
resulting from austenite.fwdarw.ferrite transformation in the
cooling and coiling steps after finish rolling in a hot-rolled
steel sheet manufacturing process.
[0072] In order to make the maximum use of the precipitation
strengthening effect and optimize the balance between strength and
workability (mass production burring formability), it is beneficial
to reduce the size of a carbide containing Ti precipitated in a
hot-rolled steel sheet. As a result of extensive studies, the
present inventors found that 70% by number or more, preferably 80%
or more, of a carbide containing Ti. has a grain size of less than
9 nm may achieve the desired characteristics. The "carbide
containing Ti" includes complex carbides containing Ti and at least
one of V, Nb, Cr, and Mo as well as Ti carbide.
Precipitates Containing Ti
[0073] The size of precipitates containing Ti can be controlled to
further improve the mass production burring formability of a
hot-rolled steel sheet.
[0074] As described above, in an example of a hot-rolled steel
sheet made of steel containing Ti, in addition to carbide (carbide
containing Ti) that contributes to high strength of a hot-rolled
steel sheet, nitride, carbonitride, and sulfide containing Ti are
precipitated. In the production of a hot-rolled steel sheet, these
nitride, carbonitride, and sulfide are precipitated faster than
carbide containing Ti. Thus, nitride, carbonitride, and sulfide
containing Ti are precipitated in a higher temperature range than
carbide and are therefore easily coarsened and tend to impair mass
production burring formability.
[0075] As a result of extensive studies, the present inventors
found that the control of the amount and size of these precipitates
is very effective in improving mass production burring formability.
In order to produce such an effect, 50% by mass or more, more
preferably 60% by mass or more and 85% by mass or less, still more
preferably 65% by mass or more and 80% by mass or less, of Ti in a
hot-rolled steel sheet is preferably precipitated as precipitates
containing Ti having a grain size of less than 20 nm. The
precipitates containing Ti having a grain size of less than 20 nm
are mostly carbide containing Ti and also include nitride,
carbonitride, and sulfide containing Ti.
[0076] The precipitates containing Ti may be precipitates of Ti
carbide, Ti nitride, Ti sulfide, and/or Ti carbonitride, and/or
complex precipitates, such as complex carbide, complex nitride,
complex sulfide, and/or complex carbonitride containing Ti and at
least one of V, Nb, Cr, and Mo.
[0077] Even when precipitates having a grain size of 20 nm or more
out of precipitates containing Ti are precipitated, it is surmised
that a proper amount of precipitates contribute to improved
punchability before burring forming and consequently contribute to
improved burring formability.
[0078] Formation of a coated layer on a surface of a hot-rolled
steel sheet in order to impart corrosion resistance does not reduce
the advantages of the present invention. The type of coated layer
formed on a surface of a hot-rolled steel sheet is not particularly
limited and may be galvanic electroplating or hot-dip plating. The
hot-dip plating may be hot-dip galvanization. The coated layer may
also be galvannealed steel, which was subjected to alloying
treatment after plating.
[0079] An exemplary method for manufacturing a hot-rolled steel
sheet according to aspects of the present invention will be
described below.
[0080] Aspects of the present invention include heating steel
having the composition described above to 1100.degree. C. or more,
hot-rolling the steel at a finish-rolling temperature of
(Ar.sub.3+25.degree. C.) or more and at a total reduction ratio of
60% or less at last two finish rolling stands, cooling the
hot-rolled steel sheet at an average cooling rate of 40.degree.
C./s or more, and coiling the hot-rolled steel sheet at a coiling
temperature in the range of 520.degree. C. to 680.degree. C.
[0081] The steel may be melted by any method, for example, in a
converter, electric furnace, or induction furnace. After that,
secondary smelting is preferably performed with vacuum degassing
equipment. Subsequent casting is preferably performed in a
continuous casting process in terms of productivity and quality. A
blooming method can also be used. A slab (steel) to be casted may
be a general slab having a thickness in the range of approximately
200 to 300 mm or a thin slab having a thickness of approximately 30
mm. In the case of a thin slab, rough rolling may be omitted. A
slab after casting may be subjected to hot direct rolling or may be
subjected to hot rolling after reheating in a furnace.
Steel Heating Temperature: 1100.degree. C. or More
[0082] Steel thus produced is subjected to hot rolling. It is
desirable to heat the steel (slab) before hot rolling and
redissolve carbide in the steel. At a steel heating temperature of
less than 1100.degree. C., carbide may not be redissolved in the
steel, and desired fine carbide may be difficult to form in the
cooling and coiling steps after hot rolling. Thus, the steel
heating temperature is 1100.degree. C. or more, preferably
1200.degree. C. or more, more preferably 1240.degree. C. or
more.
[0083] However, an excessively high steel heating temperature
results in excessively accelerated oxidation of the surface of a
steel sheet and very poor surface quality and adversely affects the
workability of a hot-rolled steel sheet. Thus, the steel heating
temperature is preferably 1350.degree. C. or less.
[0084] After heating of steel, the steel is subjected to hot
rolling, which is composed of rough rolling and finish rolling. The
rough rolling conditions are not particularly limited. As described
above, when steel is a thin slab, rough rolling may be omitted. In
the finish rolling, the finish-rolling temperature is
(Ar.sub.3+25.degree. C.) or more, and the total reduction ratio at
last two stands of a finish rolling mill is 60% or less.
Finish-Rolling Temperature: (Ar.sub.3+25.degree. C.) or More
[0085] At a finish-rolling temperature of less than
(Ar.sub.3+25.degree. C.), austenite.fwdarw.ferrite transformation
in the cooling and coiling steps after hot rolling is ferrite
transformation from unrecrystallized austenite grains. In such a
case, desired fine carbide cannot be formed, and a hot-rolled steel
sheet may not have a desirable strength (tensile strength of 900
MPa or more). Thus, the finish-rolling temperature is
(Ar.sub.3+25.degree. C.) or more, preferably (Ar.sub.3+40.degree.
C.) or more. However, an excessively high finish-rolling
temperature results in coarsening of crystal grains and adversely
affects the punchability of a hot-rolled steel sheet. Thus, the
finish-rolling temperature is (Ar.sub.3+140.degree. C.) or
less.
[0086] The Ar.sub.3 transformation point herein refers to a
transformation temperature at a change point of a thermal expansion
curve measured in a thermecmastor test (thermo-mechanical
simulation test) at a cooling rate of 5.degree. C./s.
Total Reduction Ratio at Last Two Finish Rolling Stands: 60% or
Less
[0087] When the total reduction ratio at last two finish rolling
stands exceeds 60%, this results in increased residual strain and
accelerates ferrite transformation from unrecrystallized austenite
grains. Thus, the total reduction ratio at last two stands of a
finish rolling mill is 60% or less, preferably 50% or less.
Average Cooling Rate: 40.degree. C./s or More
[0088] When the average cooling rate in cooling after hot rolling
is less than 40.degree. C./s, this results in a high ferrite
transformation temperature. As a result, carbide is precipitated in
a high temperature region, desired fine carbide cannot be formed,
and a hot-rolled steel sheet may not have a desirable
strength(tensile strength of 900 MPa or more). Thus, the average
cooling rate is 40.degree. C./s or more, preferably 50.degree. C./s
or more. However, at an excessively high average cooling rate, it
may be difficult to achieve the desired ferrite microstructure.
Thus, the average cooling rate is 150.degree. C./s or less.
[0089] The average cooling rate herein refers to the average
cooling rate between the finish-rolling temperature and the coiling
temperature.
[0090] In one embodiment, a carbide containing Ti is precipitated
in a period from immediately before coiling to the beginning of the
coiling step by decreasing the ferrite transformation temperature
so as to be close to the coiling temperature at the average cooling
rate. This can prevent precipitation and coarsening of the carbide
containing Ti in a high temperature region. Thus, the resulting
hot-rolled steel sheet can contain precipitated fine carbide.
Coiling Temperature: 520.degree. C. to 680.degree. C.
[0091] As described above, in one embodiment, fine carbide
containing Ti is mainly precipitated in a period from immediately
before coiling to the beginning of the coiling step. Thus, in order
to precipitate a large amount of fine carbide containing Ti, the
coiling temperature is controlled in a temperature range suitable
for precipitation of the carbide containing Ti. At a coiling
temperature of less than 520.degree. C. or more than 680.degree.
C., fine carbide that contributes to high strength of steel is not
sufficiently precipitated, and the hot-rolled steel sheet may not
have the desired strength. For these reasons, the coiling
temperature ranges from 520.degree. C. to 680.degree. C.,
preferably 550.degree. C. to 650.degree. C.
[0092] In one embodiment, a hot-rolled steel sheet after coiling
may be subjected to pickling and annealing treatment and then to
plating treatment by immersion in a molten zinc bath. After the
plating treatment, the hot-rolled steel sheet may be subjected to
alloying treatment. When the plating treatment is performed, the
coiling temperature ranges from 500.degree. C. to 640.degree. C.,
and the soaking temperature for the annealing treatment is
760.degree. C. or less.
Coiling Temperature: 500.degree. C. to 640.degree. C.
[0093] A higher coiling temperature facilitates the formation of an
internal oxidation layer in a hot-rolled steel sheet. The internal
oxidation layer is responsible for plating defects. In particular,
a coiling temperature of more than 640.degree. C. results in low
plating quality. On the other hand, a low coiling temperature is
preferred in order to prevent plating defects. However, a coiling
temperature of less than 500.degree. C. results in insufficient
precipitation of a carbide containing Ti, and a hot-rolled steel
sheet cannot have the desired strength. Thus, when plating
treatment is performed after coiling, the coiling temperature
ranges from 500.degree. C. to 640.degree. C., preferably
520.degree. C. to 600.degree. C.
Soaking Temperature: 760.degree. C. or Less
[0094] As described above, when the coiling temperature is lowered
for plating treatment, fine carbide that contributes to high
strength of a hot-rolled steel sheet (carbide containing Ti) may be
insufficiently precipitated during coiling. Thus, in one
embodiment, the desired strength (e.g., tensile strength of 900 MPa
or more) of a hot-rolled steel sheet after plating treatment is
achieved by precipitating fine carbide (carbide containing Ti)
during annealing treatment before the plating treatment. When the
soaking temperature for annealing treatment exceeds 760.degree. C.,
precipitated carbide (carbide containing Ti) is coarsened, and a
hot-rolled steel sheet has low strength. Thus, the soaking
temperature for the annealing treatment is 760.degree. C. or less,
preferably 740.degree. C. or less. In order to promote
precipitation of fine carbide (carbide containing Ti), the soaking
temperature for annealing treatment is preferably 600.degree. C. or
more. The holding time at the soaking temperature preferably ranges
from 10 to 1000 seconds.
[0095] After the annealing treatment, the steel sheet is immersed
in a hot-dip galvanizing bath to form a hot-dip galvanized layer on
the surface of the steel sheet. After immersion in the hot-dip
galvanizing bath, the steel sheet may be subjected to alloying
treatment. The annealing treatment and plating treatment are
preferably performed in a continuous hot-dip galvanizing line.
[0096] The type of plating is not limited to hot-dip galvanization
or galvannealing described above and may be electrogalvanizing.
[0097] The plating treatment conditions, the alloying treatment
conditions, and other manufacturing conditions are not particularly
limited and may be general conditions.
EXAMPLES
[0098] Steel slabs (Nos. A to P) containing the components listed
in Table 1 and having the Ar.sub.3 transformation point listed in
Table 1 were heated to a temperature in the range of 1200.degree.
C. to 1290.degree. C., and hot-rolled steel sheets (Nos. 1 to 22)
were formed under the hot-rolling conditions listed in Table 2. The
hot-rolled steel sheets had a thickness in the range of 1.2 to 3.2
mm. The Ar.sub.3 transformation points listed in Table 1 were
determined as described above. Part of the hot-rolled steel sheets
(Nos. 3, 4, 9, 10, 14, 16, and 18 to 20) were subjected to pickling
and were then subjected to annealing treatment at a soaking
temperature listed in Table 2 and hot-dip galvanizing treatment in
a hot-dip galvanization line. In the hot-dip galvanizing treatment,
each of the hot-rolled steel sheets subjected to the annealing
treatment was immersed in a galvanizing bath (0.1% by mass Al--Zn)
at 480.degree. C., and a hot-dip galvanized layer was formed on
both faces of the steel sheet at 45 g/m.sup.2. Part of the
hot-rolled steel sheets (Nos. 9, 10, 14, 16, and 18 to 20) were
subjected to the hot-dip galvanizing treatment and then alloying
treatment. The alloying treatment temperature was 520.degree.
C.
[0099] Test specimens were taken from the hot-rolled steel sheets
(Nos. 1 to 22) and were subjected to microstructure observation, a
tensile test, and a hole-expanding test. The microstructure
observation method and various test methods were as follows:
(i) Microstructure Observation
Ferrite Phase Fraction
[0100] Scanning electron microscope (SEM) test specimens were taken
from the hot-rolled steel sheets. A vertical cross section of each
of the test specimens parallel to the rolling direction was
polished and was subjected to nital etching. SEM photographs were
taken in 10 visual fields at a quarter thickness in the depth
direction and at a magnification ratio of 3000. A ferrite phase and
a non-ferrite phase were separated by image analysis. The fraction
of each of the phases (area fraction) was determined.
Carbide Containing Ti
[0101] Thin film samples were prepared from the hot-rolled steel
sheets (at a quarter thickness in the depth direction). Photographs
were taken in 10 visual fields with a transmission electron
microscope at a magnification ratio of 200,000.
[0102] The number of carbide grains containing Ti (N.sub.0) was
determined from the photographs. The grain size of each carbide
grain containing Ti was determined as an equivalent circle diameter
by image processing. The number of carbide grains having a grain
size of less than 9 nm (N.sub.1) out of the carbide grains
containing Ti was determined. The ratio of the number of carbide
grains having a grain size of less than 9 nm to the number of
carbide grains containing Ti (N.sub.1/N.sub.0.times.100(%)) was
calculated from these numbers (N.sub.0 and N.sub.1).
Precipitates Containing Ti
[0103] Precipitates were extracted from the hot-rolled steel sheets
by constant-current electrolysis using an AA electrolyte solution
(an ethanol solution of acetylacetone-tetramethylammonium
chloride). The extract was passed through a filter having a pore
size 20 nm. Precipitates having a size of less than 20 nm were
separated in this manner, and the amount of Ti in the precipitates
having a size of less than 20 nm was measured by
inductively-coupled plasma optical emission spectrometry (ICP). The
ratio (percentage) of Ti in the precipitates having a size of less
than 20 nm was determined by dividing the amount of Ti in the
precipitates having a size of less than 20 nm by the amount of Ti
in the hot-rolled steel sheet.
(ii) Tensile Test
[0104] Three JIS No. 5 test pieces for tensile test were taken from
each of the hot-rolled steel sheets such that the tensile direction
is a direction perpendicular to the rolling direction. The tensile
strength and total elongation were measured in a tensile test
(strain rate: 10 mm/min) according to JIS Z 2241 (2011). Each of
the hot-rolled steel sheets was subjected to measurements three
times in the tensile test. The tensile strength (TS) and total
elongation (El) were averages of the three measurements.
(iii) Hole-Expanding Test (Evaluation of Mass Production Burring
Formability)
[0105] Test specimens (size: 150 mm.times.150 mm) were taken from
the hot-rolled steel sheets. A hole having an initial diameter
d.sub.0 was formed in each of the test specimens by punching with a
50-mm.phi. punch (clearance of stamping: 30%). The hole was then
expanded by inserting a conical punch having a vertex angle of 60
degrees into the hole from the punched side. A hole diameter
d.sub.1 at which a crack penetrated the steel sheet (test specimen)
in the thickness direction was measured. The burring ratio (%) was
calculated using the following equation.
Burring ratio (%)={(d.sub.1-d.sub.0)/d.sub.0}.times.100
[0106] A burring ratio of 30% or more was considered to be high
mass production burring formability.
[0107] Table 3 shows the results.
TABLE-US-00001 TABLE 1 Chemical composition (mass %) Ar.sub.3 Steel
C Si Mn P S Al N Ti V Others Ti* + V (.degree. C.) Remarks A 0.061
0.11 1.28 0.045 0.0040 0.048 0.0018 0.210 0.162 -- 0.360 858
Inventive steel B 0.069 0.46 1.11 0.023 0.0004 0.019 0.0035 0.188
0.193 Nb: 0.008, Sn: 0.02 0.368 860 Inventive steel C 0.075 0.03
0.85 0.021 0.0016 0.038 0.0033 0.159 0.221 B: 0.0008 0.366 845
Inventive steel D 0.081 0.19 1.40 0.009 0.0005 0.033 0.0019 0.134
0.238 -- 0.360 845 Inventive steel E 0.085 0.01 1.01 0.014 0.0009
0.044 0.0027 0.131 0.251 -- 0.371 825 Inventive steel F 0.093 0.03
0.93 0.013 0.0014 0.024 0.0062 0.124 0.254 Ni: 0.18 0.355 831
Inventive steel G 0.099 0.03 0.72 0.005 0.0004 0.066 0.0011 0.095
0.366 Cr: 0.04 0.457 829 Inventive steel H 0.107 0.27 1.03 0.011
0.0009 0.042 0.0042 0.088 0.333 Mo: 0.09 0.405 832 Inventive steel
I 0.116 0.01 0.59 0.017 0.0011 0.046 0.0037 0.074 0.355 Nb: 0.010,
Ca: 0.0013 0.415 836 Inventive steel J 0.128 0.09 1.38 0.030 0.0014
0.039 0.0039 0.056 0.321 Cu: 0.04, Ni: 0.08 0.360 821 Inventive
steel K 0.074 0.08 0.49 0.013 0.0011 0.049 0.0048 0.177 0.225 --
0.384 853 Comparative steel L 0.104 0.03 1.46 0.011 0.0010 0.038
0.0031 0.121 0.289 -- 0.398 810 Comparative steel M 0.085 0.01 1.01
0.013 0.0009 0.047 0.0039 0.125 0.227 -- 0.337 830 Comparative
steel N 0.089 0.02 1.39 0.010 0.0007 0.041 0.0035 0.119 0.229 --
0.335 816 Comparative steel 0 0.086 0.22 1.40 0.025 0.0043 0.033
0.0065 0.144 0.251 -- 0.366 860 Inventive steel P 0.078 0.18 1.37
0.024 0.0035 0.033 0.0075 0.155 0.241 REM: 0.001 0.365 850
Inventive steel Ti* = Ti - N .times. (48/14) - S .times. (48/32)(S,
N and Ti in Formulae (1) and (2) denote the amounts (mass %) of the
corresponding elements)
TABLE-US-00002 TABLE 2 Manufacturing conditions for hot-rolled
steel sheet Total reduction Soaking Finish- ratio at last Average
temperature Hot-rolled Plating Sheet Heating rolling two finish
cooling Coiling for annealing steel sheet treatment thickness
temperature temperature rolling passes rate temperature treatment
No. Steel *2 (mm) (.degree. C.) (.degree. C.) (%) (.degree. C.)
(.degree. C.) (.degree. C.) Remarks 1 A -- 3.2 1200 900 55 65 680
-- Example 2 B -- 2.3 1230 950 45 80 650 -- Example 3 B GI 1.4 1220
915 50 90 580 655 Example 4 B GI 1.4 1220 920 45 80 595 770
Comparative example 5 C -- 2.6 1280 880 40 50 590 -- Example 6 D --
2.0 1250 930 50 100 615 -- Example 7 D -- 2.0 1250 825 60 125 635
-- Comparative example 8 D -- 2.0 1250 910 75 95 650 -- Comparative
example 9 E GA 2.3 1240 905 60 90 610 690 Example 10 E GA 2.3 1240
900 50 85 450 660 Comparative example 11 E -- 2.6 1240 930 50 35
670 -- Comparative example 12 F -- 1.8 1280 925 40 110 635 --
Example 13 G -- 1.6 1290 920 35 140 630 -- Example 14 H GA 2.9 1260
905 55 55 635 610 Example 15 I -- 1.2 1240 865 45 90 575 -- Example
16 J GA 2.0 1220 910 50 75 520 740 Example 17 K -- 2.0 1250 890 55
95 695 -- Comparative example 18 L GA 2.3 1250 900 40 90 515 615
Comparative example 19 M GA 2.9 1260 915 45 45 590 695 Comparative
example 20 N GA 2.6 1250 910 50 75 505 710 Comparative example 21 O
-- 2.0 1250 930 50 100 615 -- Example 22 P -- 2.0 1250 930 50 100
615 -- Example *2) "--" represents no plating treatment. "GI"
represents hot-dip galvanizing treatment. "GA" represents hot-dip
galvanizing treatment and alloying treatment.
TABLE-US-00003 TABLE 3 Microstructure of hot-rolled steel sheet
Number percentage of carbide having grain size of Mechanical
properties of less than hot-rolled steel sheet Hot-rolled 9 nm out
of carbide Percentage of Ti Tensile Total steel sheet Ferrite phase
containing Ti precipitate having strength elongation No. fraction
(area %) *3 (%) grain size of 20 nm (%) TS (MPa) El (%) Burring
ratio (%) Remarks 1 91 (P: 9) 70 50 915 25 56 Example 2 95 (P: 5)
85 80 936 22 54 Example 3 98 (B: 2) 80 75 929 24 58 Example 4 92
(P: 4, B: 4) 55 40 884 17 24 Comparative example 5 95 (P: 1, B: 4)
75 50 951 21 51 Example 6 98 (P: 2) 90 85 1007 20 59 Example 7 91
(P: 9) 40 40 885 13 14 Comparative example 8 92 (P: 8) 50 50 881 12
16 Comparative example 9 97 (P: 3) 75 65 1019 19 61 Example 10 91
(B: 2, M: 7) 65 35 893 9 13 Comparative example 11 81 (P: 19) 25 50
864 11 11 Comparative example 12 98 (P: 2) 80 80 998 18 44 Example
13 99 (P: 1) 85 85 1198 14 40 Example 14 91 (P: 9) 70 55 1181 16 39
Example 15 95 (P: 1, B: 4) 75 55 1195 17 33 Example 16 94 (B: 6) 75
50 997 19 53 Example 17 91 (P: 9) 30 30 854 16 21 Comparative
example 18 98 (B: 2) 70 30 1099 14 23 Comparative example 19 93 (P:
7) 65 40 932 16 26 Comparative example 20 97 (B: 2, M: 1) 70 40 945
17 28 Comparative example 21 98 (P: 2) 80 45 942 20 32 Example 22
98 (P: 2) 70 40 915 20 31 Example *3) The percentages of
microstructures other than ferrite phase are shown in parentheses.
P: pearlite (including cementite), B: bainite, M: martensite
(including retained austenite).
[0108] The hot-rolled steel sheets according to examples (Nos. 1 to
3, 5, 6, 9, 12 to 16, 21, and 22) had the desired tensile strength
(900 MPa or more) and good mass production burring formability. By
contrast, the hot-rolled steel sheets according to comparative
examples (Nos. 4, 7, 8, 10, 11, and 17 to 20), which were outside
the scope of the present invention, did not have the desired high
strength or sufficient burring ratios.
* * * * *