U.S. patent application number 15/541897 was filed with the patent office on 2018-01-11 for high-strength plated steel sheet having excellent plating properties, workability, and delayed fracture resistance, and method for producing same.
This patent application is currently assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.. The applicant listed for this patent is KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.. Invention is credited to Yuichi FUTAMURA, Muneaki IKEDA, Michiharu NAKAYA.
Application Number | 20180010226 15/541897 |
Document ID | / |
Family ID | 56414778 |
Filed Date | 2018-01-11 |
United States Patent
Application |
20180010226 |
Kind Code |
A1 |
IKEDA; Muneaki ; et
al. |
January 11, 2018 |
HIGH-STRENGTH PLATED STEEL SHEET HAVING EXCELLENT PLATING
PROPERTIES, WORKABILITY, AND DELAYED FRACTURE RESISTANCE, AND
METHOD FOR PRODUCING SAME
Abstract
The high-strength plated steel sheet of the present invention
has a plated layer on the surface of a base steel sheet and
contains predetermined steel components. The steel sheet includes,
in the order from the interface of the base steel sheet and the
plated layer towards the base steel sheet: a soft layer having a
Vickers hardness that is 90% or less of the Vickers hardness at a
portion t/4 of the base steel sheet, where t is a sheet thickness
of the base steel sheet: and a hard layer containing martensite,
bainite, and ferrite in predetermined ranges. The average depth D
of the soft layer is 20 .mu.m or greater, and the average depth d
of an internal oxide layer is 4 .mu.m or greater and smaller than
D.
Inventors: |
IKEDA; Muneaki;
(Kakogawa-shi, JP) ; FUTAMURA; Yuichi;
(Kakogawa-shi, JP) ; NAKAYA; Michiharu;
(Kakogawa-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD. |
Kobe-shi |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA KOBE SEIKO SHO
(KOBE STEEL, LTD.
Kobe-shi
JP
|
Family ID: |
56414778 |
Appl. No.: |
15/541897 |
Filed: |
January 5, 2016 |
PCT Filed: |
January 5, 2016 |
PCT NO: |
PCT/JP2016/050070 |
371 Date: |
July 6, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/0205 20130101;
C21D 8/0263 20130101; Y10T 428/12611 20150115; C21D 8/0278
20130101; C22C 38/04 20130101; Y10T 428/12799 20150115; C22C 38/32
20130101; C23C 2/40 20130101; Y10T 428/12792 20150115; Y10T
428/24967 20150115; C22C 38/12 20130101; C22C 38/54 20130101; C23C
30/00 20130101; C23G 1/08 20130101; Y10T 428/265 20150115; C23C
2/28 20130101; C21D 9/46 20130101; C22C 38/46 20130101; C22C 38/42
20130101; C22C 38/08 20130101; C22C 38/16 20130101; C22C 38/28
20130101; C22C 38/20 20130101; C22C 38/06 20130101; C22C 38/001
20130101; Y10T 428/12972 20150115; C21D 1/74 20130101; C21D 8/0247
20130101; C22C 38/02 20130101; C22C 38/002 20130101; B32B 15/04
20130101; C22C 38/22 20130101; C22C 38/24 20130101; Y10T 428/1259
20150115; C22C 38/26 20130101; C23F 17/00 20130101; Y10T 428/1266
20150115; Y10T 428/263 20150115; C21D 8/0226 20130101; Y10T
428/12667 20150115; B32B 15/043 20130101; C23C 2/06 20130101; B32B
15/013 20130101; C23C 2/02 20130101; C22C 38/44 20130101; Y10T
428/12583 20150115; C21D 8/0236 20130101; C22C 38/18 20130101; C22C
38/50 20130101; C23G 1/06 20130101; Y10T 428/2495 20150115; B32B
15/18 20130101; Y10T 428/264 20150115; C22C 38/60 20130101; C23C
30/005 20130101; C22C 38/14 20130101; C22C 38/40 20130101; C22C
38/48 20130101 |
International
Class: |
C23C 2/40 20060101
C23C002/40; C23C 2/06 20060101 C23C002/06; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 9/46 20060101
C21D009/46; C23C 2/28 20060101 C23C002/28; C21D 8/02 20060101
C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 9, 2015 |
JP |
2015-003671 |
Aug 11, 2015 |
JP |
2015-159212 |
Claims
1. A high-strength plated steel sheet, comprising: a hot-dip
galvanized or galvannealed layer on a surface of a base steel
sheet, wherein the base steel sheet comprises, in mass %, C: 0.05
to 0.25%; Si: 0.25 to 3%; Mn: 1.5 to 4%; P: more than 0% to 0.1% or
less; S: more than 0% to 0.05% or less; Al: 0.005 to 1%; and N:
more than 0% to 0.01% or less, the high-strength plated steel sheet
sequentiaih has, from an interface of the base steel sheet and the
hot-dip galvanized or galvannealed layer towards the base steel
sheet: an internal oxide layer containing at least one oxide
selected from the group consisting of Si and Mn; a soft layer
including the internal oxide and having a Vickers hardness of 90%
less of a Vickers hardness at a portion t/4 of the base steel
sheet, where t is a sheet thickness of the base steel sheet; and a
hard layer containing martensite and bainite in a proportion of
from 60 area % or more to less than 95 area %, and polygonal
ferrite in a proportion of from more than 5 area % to 40 area % or
less, an average depth D of the soft layer is 20 .mu.m or greater,
an average depth d of the internal oxide layer is 4 .mu.m or
greater and smaller than the D, and the high strength plated steel
sheet has a tensile strength of 980 MPa or higher.
2. The high-strength plated steel sheet according to claim 1,
wherein the base steel sheet further contains, in mass %, at least
one of (a) to (c): (a) at least one selected from the group
consisting of Cr: more than 0% to 1% or less, Mo: more than 0% to
1%, or less and B: more than 0% to 0.01% or less; (b) at least one
selected from the group consisting of Ti: more than 0% to 0.2% or
less, Nb: More than 0% to 0.2 or less and V: more than 0% to 0.2%
or less; and (c) at least one selected from the group consisting of
Cu: more than 0% to 1% or less and Ni: more than 0% to 1% or
less.
3. The high-strength plated steel sheet according to claim 1,
wherein the average depth d of the internal oxide layer and the
average depth D of the soft layer satisfy D>2d.
4. The high-strength plated steel sheet according to claim 2,
wherein the average depth d of the internal oxide layer and the
average depth D of the soft layer satisfy D>2d.
5. A method for producing the high strength plated steel sheet
according to claim 1, the method comprising: coiling, at a
temperature of 600.degree. C. or higher, a steel sheet containing,
in mass %, C: 0.05 to 0.25%, Si: 0.25 to 3%, Mn: 1.5 to 4%, P: more
than 0% to 0.1% or less, S: more than 0% to 0.05% or less, Al:
0.005 to 1%, and N: more than 0% to 0.01% or less; subsequently
pickling and cold rolling the steel sheet such that the internal
oxide layer with the average depth d of 4 .mu.m or more remains;
subsequently oxidizing the steel sheet at an air ratio of 0.9 to
1.4 in an oxidation zone; subsequently soaking the steel sheet at a
temperature of (Ac.sub.1 point +50.degree. C.) to (Ac.sub.3 point
20.degree. C.) in a reduction zone; and subsequently cooling the
steel sheet at an average cooling rate of 5.degree. C./sec or
higher down to a cooling stop temperature.
6. A method for producing the high-strength plated steel sheet
according to claim 1, the method comprising: coiling, at a
temperature of 500.degree. C. or higher, a steel sheet containing
in mass %. C: 0.05 to 0.25%, Si: 0.25 to 3%, Mn: 1.5 to 4%, P: more
than 0% to 0.1% or less, S: more than 0% to 0.05% or less, Al:
0.005 to 1%, and N: more than 0% to 0.01% or less: subsequently
keeping the steel sheet at a temperature of 500.degree. C. or
higher for 80 minutes or longer; subsequently pickling and cold
rolling the steel sheet such that the internal oxide layer with the
average depth d of 4 .mu.m or more remains; subsequently oxidizing
the steel sheet at an air ratio of 0.9 to 1.4 in an oxidation zone;
subsequently soaking the steel sheet at a temperature of (Ac.sub.1
point +50.degree. C.) to (Ac.sub.3 point +20.degree. C.) in a
reduction zone; and subsequently cooling the steel sheet at an
average cooling rate of 5.degree. C./sec or higher down to a
cooling stop temperature.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high-strength plated
steel sheet having a tensile strength of 980 MPa or higher and
having excellent plalability; excellent formability in terms or
bendability, hole expandability, and balance between strength and
ductility; and excellent delayed fracture resistance, and also
relates to a method for producing the steel sheet. The term plated
steel sheet of the present invention denotes both of a hot-dip
galvanized steel sheet and a hot-dip galvannealed steel sheet.
BACKGROUND ART
[0002] Hot-dip galvanized steel sheets and hot-dip galvannealed
steel sheets that are widely used in the fields of automobiles,
transport aircrafts and the like, are required to have, in addition
to high strength, also excellent formability in terms of
bendability, hole expandability (stretch flangeability), and
balance between strength and ductility, and to exhibit moreover
excellent delayed fracture resistance.
[0003] Adding a substantial amount of strengthening elements such
as Si or Mn into steel is effective in order to secure high
strength and formability. However, Si and Mn are readily oxidizable
elements, and hot-dip galvanizing wettability is significantly
impaired by Si oxides, Mn oxides, and complex oxides of Si and Mn,
that are formed on the surface. This poor wettability gives rise to
problems such as bare spots and the like.
[0004] Various technologies have therefore been proposed with a
view to enhancing the formability and so forth of plated steel
sheets that contain large amounts of Si and/or Mn.
[0005] For instance, Patent Literature 1 discloses a hot-dip
galvanized steel sheet having a tensile strength of 590 MPa or
higher and being excellent in bendability and corrosion resistance
in worked portions. In further detail, Patent Literature 1
discloses the feature of significantly speeding up the growth of a
decarburized layer with respect to the growth of an internal oxide
layer that is formed from the interface of the steel sheet and a
galvanized layer towards the steel sheet, so as to enable
suppression of bending cracks and damage to a galvanized coating
that are caused by the internal oxide layer. Further, Patent
Literature 1 discloses a surface-near structure in which the
thickness of the internal oxide layer in a ferrite region formed by
decarburization is controlled so as to be thin.
[0006] Further, Patent Literature 2 discloses a hot-dip galvanized
steel sheet excellent in fatigue durability, resistance to hydrogen
embrittlement (synonymous with delayed fracture resistance) and
bendability, the steel sheet having a tensile strength of 770 MPa
or higher. In further detail, a steel sheet portion in Patent
Literature 2 is configured to have a soft layer directly in contact
with the interlace with a galvanized layer, and a soft layer in
which ferrite is set to be the structure of highest area ratio.
Further, Patent Literature 2 discloses a hot-dip galvanized steel
sheet in which a thickness D of the soft layer, and a depth d, from
a galvanized layer/base iron interface, of an oxide that contains
one or more from among Si and Mn and that is present in a surface
layer of the steel sheet, satisfy d/4.ltoreq.D.ltoreq.2d.
[0007] Patent Literature 3 discloses a high-strength cold-rolled
steel sheet having a maximum tensile strength of 700 MPa or higher
and having excellent bendability. In further detail, Patent
Literature 3 discloses that the steel sheet surface layer can be
softened by performing a decarburization process and that, even
with a high-strength cold-rolled steel sheet having a maximum
(ensile strength of 700 MPa, an excellent bendability as if the
steel sheet were a low-strength steel sheet can be obtained.
[0008] Patent Literature 4 discloses a high-strength hot-dip
galvanized steel sheet having excellent delayed fracture resistance
and moreover, even with a thin sheet, having little anisotropy of
delayed fracture resistance, without deteriorating the ductility
and strength. In further detail, Patent Literature 4 discloses that
in order to prevent delayed fracture starting from a surface layer
portion of a matrix material steel sheet, the surface layer portion
of the matrix material steel sheet is made into a decarburized
layer having little amount of hard structure and that line oxides
functioning as hydrogen trap sites are dispersed at a high density
in the decarburized layer.
[0009] Patent Literature 5 discloses a high-strength steel sheet
having a maximum tensile strength of 900 MPa or higher by which
excellent moldability and excellent hydrogen embrittlement
resistance can be obtained. In further detail, Patent Literature 5
discloses that excellent hydrogen embrittlement resistance (delayed
fracture resistance) as if the steel sheet were a low-strength
steel sheet can be obtained because the steel sheet surface layer
has a decarburized layer (softened layer) which is softer than an
inside of the steel sheet.
CITATION LIST
Patent Literatures
[0010] Patent Literature 1: Japanese Unexamined Patent Publication
No. 2011-231367
[0011] Patent Literature 2: Japanese Patent No. 4943558
[0012] Patent Literature 3: Japanese Patent No. 5454746
[0013] Patent Literature 4: Japanese Patent No. 5352793
[0014] Patent Literature 5: Japanese Unexamined Patent Publication
No. 2011-111675
SUMMARY OF INVENTION
Problems to be Solved by the Invention
[0015] As described above, various technologies, have been proposed
for enhancing the formability and so forth of plated steel sheets
that contain large amounts of Si and Mn. However, it would be
desirable to provide a technology that combines all of various
characteristics demanded in a plated steel sheet, namely, high
strength of 980 MPa or higher; platability; formability in terms of
bendability, hole expandability, and balance between strength and
ductility; and delayed fracture resistance.
[0016] In the light of the above issues; it is an object of the
present invention to provide a hot-dip galvanized steel sheet and a
hot-dip galvannealed steel sheet of 980 MPa or higher, the steel
sheet being excellent in platability; formability in terms of
bendability, hole expandability, and balance between strength, and
ductility; and delayed fracture resistance, as well as a method for
producing the steel sheet.
[0017] A high-strength plated steel sheet according to the present
invention having a tensile strength of 980 MPa or higher and
attaining the above goal is a plated steel sheet having a hot-dip
galvanized layer or a hot-dip galvannealed layer on a surface of a
base steel sheet, wherein (1) the base steel sheet contains, in
mass %, C: 0.05 to 0.25%; Si: 0.25 to 3%; Mn: 1.5 to 4%; P: more
than 0% to 0.1% or less; S: more than 0% to 0.05% or less; Al:
0.005 to 1%; and N: more than 0% to 0.01% or less, the balance
being iron and inevitable impurities; (2) the plated steel sheet
sequentially has, from an interface of the base steel sheet and the
hot-dip plated layer, towards the base steel sheet; an internal
oxide layer containing at least one oxide selected from the group
consisting of Si and Mn; a soft layer including the internal oxide
layer and having a Vickers hardness of 90% or loss of a Vickers
hardness at a portion t/4 of the base steel sheet, where t is a
sheet thickness of the base steel sheet; and a hard layer
containing martensite and bainite: 60 area % or more to less than
95 area %, and polygonal ferrite: more than 5 area % to 40 area %
or less, wherein the high-strength plated steel sheet satisfies: an
average depth D of the soft layer being 20 .mu.m or greater, and an
average depth d of the internal oxide layer being 4 .mu.m or
greater and smaller than the D; and a tensile strength being 980
MPa or higher.
[0018] In a preferred embodiment of the present invention, the base
steel sheet further contains, in mass %, at least one of (a) to (c)
below: (a) at least one selected from the group consisting of Cr:
more than 0% to 1% or less, Mo: more than 0% to 1% or less and B:
more than 0% to 0.01% or less; (b) at least one selected from the
group consisting of Ti: more than 0% to 0.2% or less, Nb: mare than
0% to 0.2%, or less and V: more than 0% to 0.2% or less; and (c) at
least one selected from the group consisting of Cu: more than 0% to
1% or less and Ni: more than 0% to 1% or less.
[0019] In a preferred embodiment of the present invention, the
average depth d of the internal oxide layer and the average depth D
of the soft layer satisfy the relationship D>2d.
[0020] A production method of the present invention that allows
attaining the above goal is a method for producing any of the
high-strength plated steel sheets set forth above, the method
having, in order: a hot rolling step of coiling, at a temperature
of 600.degree. C. or higher, a hot-rolled steel sheet having the
aforementioned steel components; a step of pickling and cold
rolling the steel sheet such that there remain the internal oxide
layer with the average depth d of 4 .mu.m or more; a step of
oxidizing the steel sheet at an air ratio in a range of 0.9 to 1.4
in an oxidation zone; a step of soaking the steel sheet by keeping
within a range (Ac.sub.1 point +50.degree. C.) to (Ac.sub.3 point
+20.degree. C.), in a reduction zone; and a step of, after the
soaking cooling the steel sheet at an average cooling rate of
5.degree. C/sec or higher over a range down to a cooling stop
temperature.
[0021] Another production method of the present invention that
allows attaining the above goal is a method for producing any of
the high-strength plated steel sheets set forth above, the method
having, in order: a hot rolling step of coiling, at a temperature
of 500.degree. C. or higher, a hot-rolled steel sheet having the
aforementioned steel components; a step of keeping the steel sheet
in a temperature region of 500.degree. C. or higher for 80 minutes
or longer; a step of pickling and cold rolling the steel sheet such
that there remain the internal oxide layer with the average depth d
of 4 .mu.more: a step of oxidizing the steel sheet at an air ratio
in the range 9 to 1.4, in a oxidation zone; a step of soaking the
steel sheet by keeping within a range of (Ac.sub.1 point 50.degree.
C.) to (Ac.sub.3 point +20.degree. C.), in a reduction zone; and a
step of, after the soaking, cooling the steel sheet at an average
cooling rate of 5.degree. C./sec or higher over a range down to a
cooling stop temperature.
[0022] The plated steel sheet of the present invention is
configured to have, sequentially from an interface of a plated
layer and a base steel sheet towards the base steel sheet; an
internal oxide layer having at least one oxide selected from the
group consisting of Si and Mn; a soft layer including the region of
the internal oxide layer; and a hard layer which is other than the
soft layer (and which contains martensite and bainite: 60 area % or
more to less than 95 area % and polygonal ferrite; more than 5 area
% to 40 area % or less, as a matrix phase structure). In
partictilar, the internal oxide layer is utilized as a hydrogen
trap site by controlling the average depth d of the internal oxide
layer to be 4 .mu.m or greater, and accordingly, it becomes
possible to suppress hydrogen embrittlement effectively, and to
obtain a high-strength plated steel sheet having a tensile strength
of 980 Pa or higher and being excellent in all of formability in
terms of bendability, hole expandability, and balance between
strength and ductility; and delayed fracture resistance.
Preferably, the relationship between the average depth d of the
internal oxide layer and the average depth D of the soft layer at
includes the region of the internal oxide layer is controlled
properly, whereby bendability and delayed fracture resistance in
particular are further enhanced.
BRIEF DESCRIPTION OF DRAWINGS
[0023] FIG. 1 is a diagram for explaining schematically a layer
configuration of a plated steel sheet of the present invention,
from an interface of a plated layer and a base steel sheet towards
the base steel sheet.
[0024] FIG. 2 is an explanatory diagram for measuring an average
depth d of an internal oxide layer of the plated steel sheet of the
present invention.
[0025] FIG. 3 is a diagram for explaining measurement positions of
Vickers hardness, used to establish an average depth D of a soft
layer.
DESCRIPTION OF EMBODIMENTS
[0026] The inventors conducted repetitive researches focusing in
particular on a layer configuration from the interface of a plated
layer and a base steel sheet towards the base steel sheet, with a
view to providing a high-strength plated steel sheet from a base
steel sheet that contains a large amount of Si and Mn, the plated
steel sheet having a high strength of 980 MPa or higher and being
excellent in all of platability, formability, delayed fracture
resistance, and impact absorption properties. As a result, the
inventors found that (a) by prescribing the layer configuration to
be, from the interface of the plated layer and the base steel sheet
towards the base steel sheet: a soft layer including an internal
oxide layer having at least one oxide selected from the group
consisting of Si and Mn, and a hard layer which is other than the
soft layer and which contains martensite and bainite: 60 area % or
more to less than 95 area %, and polygonal ferrite: more than 5
area % to 40 area % or less, as illustrated in the schematic
diagram of FIG. 1 described below, and (b) by controlling the
average depth d of the internal oxide layer to be 4 .mu.m or
greater, the internal oxide layer can function as a hydrogen trap
site, and it becomes possible to suppress hydrogen embrittlement
effectively, thereby achieving the intended purpose, and that (c)
preferably by properly controlling the relationship between the
average depth d of the internal oxide layer and the average depth D
of the soft layer that includes a region of the internal oxide
layer, bendability and delayed fracture resistance in particular
are further enhanced, thereby completing the present invention.
[0027] In the present specification, the term plated steel sheet
denotes both of a hot-dip galvanized steel sheet and a hot-dip
galvannealed steel sheet. Further, in the present specification,
the term base steel sheet denotes a steel sheet before formation of
a hot-dip galvanized layer and a hot-dip galvannealed layer, and is
distinguished from the above plated steel sheet.
[0028] In the present specification, the term high strength denotes
a tensile strength of 980 MPa or higher.
[0029] In the present specification, the feature of having
excellent formability indicates being excellent in bendability,
hole expandability, and balance between strength and ductility. In
further detail, a steel sheet satisfying the acceptance criteria in
the Examples described below upon measurement of the foregoing
characteristics, in accordance with the methods described in the
Examples, will be referred to as a steel sheet having "excellent
formability".
[0030] The plated steel sheet of the present invention, as
described above, has a hot-dip galvanized layer or a hot-dip
galvannealed layer (which may hereafter be referred to simply as
plated layer) on the surface of a base steel sheet. The
characterizing feature of the present invention lies in having a
layer configuration (A) to (C) shown below, in this order, from the
interface of the base steel sheet and the plated layer towards the
base steel sheet: [0031] (A) an internal oxide layer containing at
least one oxide selected from the group consisting of Si and Mn,
where an average depth d of the internal oxide layer is 4 .mu.m or
greater and is smaller than an average depth D of the soft layer
set forth in (B) below. [0032] (B) a soft layer including the above
internal oxide layer and satisfying having a Vickers hardness of
90% or less of the Vickers hardness at a portion t/4 of the base
steel sheet, where t is the sheet thickness of the base steel
sheet, and an average depth D of the soft layer is 20 .mu.m or
greater. [0033] (C) a hard layer consists of a structure which
contains martensite and bainite: 60 area % or more to less than 95
area %, and polygonal ferrite: more than 5 area % to 40area % or
less.
[0034] Hereafter, the layer configuration (A) to (C), being the
characterizing feature of the present invention, will be
successively described in detail with reference to FIG. 1. As
illustrated in FIG. 1 the layer configuration of the base steel
sheet 2 in the plated steel sheet of the present invention has the
(B) soft layer 4 and the (C) hard layer 5 which is located further
inside the base steel sheet 2 than the soft layer 4, as viewed from
the interface of the plated layer 1 and the base steel sheet 2
towards the base steel sheet 2. The (B) soft layer 4 includes the
(A) internal oxide layer 3. The soft layer 4 and the hard layer 5
are present contiguously.
[0035] (A) Internal Oxide Layer
[0036] First, a portion directly in contact with the interface of
the plated layer 1 and the base steel sheet 2 has the internal
oxide layer 3 having an average depth d of 4 .mu.m or greater. The
term average depth herein denotes an average depth from the
interface, and a detailed measurement method of the average depth
will be explained further on with reference to FIG. 2 in the
Example section given below.
[0037] The internal oxide layer 3 consists of an oxide that
includes at least one from among Si and Mn, and of a depletion
layer of Si and Mn having little solid-solution Si and/or
solid-solution Mn in the periphery as a result of the formation of
the oxides of Si and Mn.
[0038] The most distinctive feature of present invention involves
controlling the average depth d of the internal oxide layer 3 to a
thickness of 4 .mu.m or greater. By doing so, it becomes possible
to exploit the internal oxide layer as a hydrogen trap site and to
suppress hydrogen embrittlement, while enhancing bendability, hole
expandability, and delayed fracture resistance.
[0039] During annealing (i.e. during an oxidation-reduction step in
a below-described continuous hot-dip galvanizing line), an oxide
film having Si oxides, Mn oxides, and complex oxides of Si and Mn
readily forms on the base steel sheet surface, and platability is
hampered, in a base steel sheet having a large amount of readily
oxidizable elements such as Si and Mn, as in the present invention.
Therefore, countermeasure methods are known that involve annealing
(reduction annealing) in an atmosphere containing hydrogen, after
generation of a Fe oxide film through oxidation of the base steel
sheet surface in an oxidizing atmosphere. A further method involves
controlling the atmosphere inside a furnace, to thereby fix readily
oxidizable elements in the interior of a base steel sheet surface
layer, in the form of oxides, and reduce the amount of readily
oxidizable elements being in solid solution in the base steel sheet
surface layer, to prevent as a result formation of an oxide film of
readily oxidizable elements on the base steel sheet surface.
[0040] However, results of studies by the inventors have revealed
that, in the oxidation-reduction methods generally resorted to in
order to plate a base steel sheet containing a large amount of Si
and Mn, hydrogen, in the hydrogen atmosphere during reduction
intrudes into the base steel sheet, giving rise to an impairment of
bendability and hole expandability due to hydrogen embrittlement;
and that the use of at least one oxide selected from the group
consisting of Si and Mn is effective in order to improve such
impairment. In further detail, it has been found out that the above
oxides prevent intrusion of hydrogen into the base steel sheet
during reduction and are thus useful as hydrogen trap sites that
allow improving bendability, hole expandability, and delayed
fracture resistance, and that, in order to effectively elicit the
above effect, it is essential to form the internal oxide layer
thicker such that the average depth d of the internal oxide layer
including the oxides is 4 .mu.m or greater.
[0041] In the present invention, the upper limit of the average
depth d of the internal oxide layer is at least smaller than the
average depth D of the (B) soft layer described below. Preferably,
the upper limit of the above d is 30 .mu.m or smaller. That is
because although prolonged keeping in a high-temperature region
after hot-rolled coiling is required in order to thicken the
internal oxide layer, the upper limit takes on roughly the above
preferred value, due to productivity and equipment constraints. The
above d is more preferably 18 .mu.m or smaller, and yet more
preferably 16 .mu.m or smaller. The above d is preferably 6 .mu.or
greater, more preferably 8 .mu.m or greater.
[0042] Further, in the present invention, preferably, the average
depth d of the internal oxide layer is controlled in such a way so
as to satisfy a relational expression D>2d in a relationship
with the average depth D of the (B) soft layer described below.
Bendability and delayed fracture resistance, particularly the
bendability, are further enhanced as a result. In contrast, Patent
Literature 2 described above discloses, a hot-dip galvanized steel
sheet that satisfies d/4.ltoreq.D.ltoreq.2d for a presence depth d
of an oxide and a thickness D of a soft layer that substantially
correspond to the average depth d of the internal oxide layer and
the average depth D of the soft layer described in the present
invention, so that the directionality of control is completely
different from that of the relational expression (D>2d)
prescribed in the present invention. Patent Literature 2 discloses
the feature of controlling the range of the presence depth d of an
oxide while basically satisfying the relationship
d/4.ltoreq.D.ltoreq.2d described above, but does not involve at all
the basic idea of controlling the average depth d of the internal
oxide layer to a thickness of 4 .mu.m or greater, as in the present
invention. Needless to say, Patent Literature 2 fails to disclose
the effect of the present invention that is elicited as a result,
i.e. effective functioning as hydrogen trap sites and affording
enhanced bendability, hole expandability, and delayed fracture
resistance.
[0043] In order to control the average depth d of the internal
oxide layer to be 4 .mu.m or more in the present invention, it is
necessary to control to 4 .mu.m or more the average depth of the
internal oxide layer in the cold-rolled steel sheet before passing
through a continuous hot-dip galvanizing line. The details are
described below in the section relating to a production method.
Specifically, the internal oxide layer after pickling and cold
rolling goes on to become an internal oxide layer in a plated steel
sheet that is obtained eventually after passage through a
galvanizing/galvannealing line, as in the Examples described
below.
[0044] (B) Soft Layer
[0045] As illustrated in FIG. 1, in the present invention, the soft
layer 4 is a layer including a region of the above (A) internal
oxide layer 3, and satisfying having a Vickers hardness of 90% or
less of the Vickers hardness at a portion t/4 of the base steel
sheet 2. The detailed measurement method of the Vickers hardness is
explained in the below-described Example section.
[0046] The soft layer has a soft structure of lower Vickers
hardness than that of the hard layer (C) described below, exhibits
excellent deformability, and accordingly, affords in particular
enhanced bendability. That is, although the surface layer portion
of the base steel sheet is an origin of cracks during bending work,
bendability is particularly improved through formation of a
predetermined soft layer on the base steel sheet surface layer, as
in the present invention. Moreover, forming the soft layer allows
preventing the oxide inside (A) from becoming an origin of cracks
during bending work, and to enjoy only the benefits of the oxide
acting as a hydrogen trap site, as described above. As a result not
only bendability but also delayed fracture resistance is further
enhanced.
[0047] The average depth D of the soil layer is set to be 20 .mu.m
or greater in order to effectively elicit the effect derived from
forming such a soft layer. The above D is preferably 22 .mu.m or
greater, and more preferably 24 .mu.m or greater. When the average
depth D of the soft layer is excessively large, on the other hand,
the strength of the plated steel sheet itself drops, and
accordingly, it is preferable to set the upper limit of the average
depth D to be 100 .mu.m or smaller, The above D is more preferably
60 .mu.m or smaller.
[0048] (C) Hard Layer
[0049] In the present invention, as illustrated in FIG. 1, the hard
layer is formed on the base steel sheet 2 side of the (B) soft
layer 4 and consists of a structure which contains martensite and
bainite: 60 area % or more to less than 95 area %, and polygonal
ferrite: more than 5 area % to 40 area % or less. The martensite of
the hard layer 5 may be tempered. The larger a sum area ratio of
bainite and martensite is (that is, the smaller the area ratio of
ferrite is), the more the strength tends to be enhanced. The
smaller a sum area ratio of bainite and martensite is (that is, the
larger the area ratio of ferrite is), the more the ductility tends
to be enhanced. Further, when the area ratio of ferrite is small,
the balance between strength and elongation is hampered.
Accordingly, it is recommended that the preferable area ratios of
these structures are properly set in consideration of the
relationship to the desired characteristics. For instance, in view
of enhancing strength, a sum area ratio of bainite and martensite
is preferably 80 area % or greater, and a sum area ratio of ferrite
is 20 area % or smaller. Further, in consideration of enhancing the
balance between strength and elongation, a sum area ratio of
bainite and martensite is preferably 70 area % or smaller, and a
sum area ratio of ferrite is 30 area % or greater.
[0050] Besides the above-described structure, the hard layer may
include structures, for instance residual austenite (.gamma.),
pearlite, and the like, that may be unavoidably mixed in during
production, in amounts that do not impair the effect of the present
invention. Such a structure occupies 15 area % at the most, and the
smaller the better. Such structures are notated in the Table 3
below as "Other".
[0051] It is sufficient that the hard layer in the present
invention contains bainite and martensite within a rare of 60 area
% or more to less than 95 area % in a sum area, as described above,
and the ratio of each of bainite and martensite is not limited at
all. This is because, in the present invention, the above effect
produced by formation of the hard layer is elicited as long as the
aforementioned requirements are satisfied. Accordingly, the hard
layer can satisfy any of the relationships of bainite
>martensite, bainite=martensite, and bainite<martensite, as
long as the above requirements are satisfied. Furthermore, a mode
in which the hard layer is composed of bainite alone and martensite
is not contained at all; and a mode in which, conversely, the hard
layer is composed of martensite alone and bainite is not contained
at all, are both comprised within the scope of the present
invention. From the above viewpoint, bainite and martensite were
observed without distinguishing the two from each other in the
Examples given below, and only a sum area was measured. The result
thereof is shown in Table 3.
[0052] The layer configuration from the interface of the plated
layer and the base steel sheet towards the base steel sheet, which
is the most significant characterizing feature of the present
invention, has been thus explained above.
[0053] The steel components that are used in the present invention
will be explained next.
[0054] The plated steel sheet of the present invention contains C:
0.05 to 0.25%, Si: 0.25 to 3%, Mn: 1.5 to 4%, P: more than 0% to
0.1% or less, S: more than 0% to 0.05% or less, Al: 0.005 to 1% and
N: more than 0% to 0.01% or less, the balance being iron and
inevitable impurities.
[0055] C: 0.05 to 0.25%
[0056] C has the effect of enhancing hardenability and of hardening
martensite, by virtue of which C is an important element in terms
of strengthening steel. In order to effectively bring out that
effect, the lower limit of the amount of C is set to be 0.05% or
more. A preferred lower limit of the amount of C is 0.08% or more,
more preferably 0.10% or more. When C is added in an excessive
amount, however, the hardness difference between the soft phase and
the hard phase increases to degrade formability and delayed
fracture resistance, so that the upper limit of the amount of C is
set to be 0.25%. A preferred upper limit of the amount of C is 0.2%
or less, more preferably 0.18% or less.
[0057] Si: 0.25 to 3%
[0058] Si is an effective element in terms of increasing the
strength of steel and enhancing formability, through solid-solution
strengthening. Further, Si has the effect of generating an internal
oxide layer and suppressing hydrogen embrittlement. In order to
effectively bring out that effect, the lower limit of the amount of
Si is set to be 0.25% or more. A preferred lower limit of the
amount of Si is 0.3% or more, more preferably 0.5% or more, and yet
more preferably 0.7% or more. However, Si is a ferrite-generating
element, and when Si is added in an excessive amount, generation of
ferrite cannot be suppressed, so that the hardness difference
between the soft phase and the hard phase increases, and
formability decreases. Further, platability as well is impaired,
and hence the upper limit of the amount of Si is set to be 3%. A
preferred upper limit of the amount of Si is 2.5% or less, more
preferably 2.0% or less.
[0059] Mn is a hardenability-enhancing element that suppresses
ferrite and bainite and contributes to increasing the strength
through generation of martensite. In order to effectively bring out
that effect, the lower limit of the amount of Mn is set to be 1.5%
or more. A preferred lower limit of the amount of Mn is 1,8% or
more, more preferably 2.0% or more. When Mn is added in an
excessive amount, however, platability decreases, and segregation
becomes conspicuous. A further concern is the resulting promotion
in P grain boundary segregation. Accordingly, the upper limit of
the amount of Mn is set to be 4%. A preferred upper limit of the
amount of Mn is 3.5% or less.
[0060] P: more than 0% to 0.1% or less
[0061] As a solid-solution strengthening element, P is a useful
element for strengthening steel. In order to effectively bring out
that effect, the lower limit of the amount of P is set to exceed
0%. If the addition amount is excessive, however, not only
formability but also weldability and toughness might become
impaired, and accordingly, the upper limit of the addition amount
is set to be 0.1% or less. The smaller the amount of P is, the
better it is. The amount of P is preferably 0.03% or less, more
preferably 0.015% or less.
[0062] S: more than 0% to 0.05% or less
[0063] S is an element of unavoidable presence and forms sulfides
such as MnS, giving rise to origins of cracks and the concern of
impaired formability. Accordingly, the upper limit of the amount of
S is set to be 0.05% or less. The smaller the amount of S is, the
better it is. The amount of S is preferably 0.01% or less, more
preferably 0.008% or less.
[0064] Al: 0.005 to 1%
[0065] Al acts as a deoxidizing agent. Also, by bonding with N to
form AlN thereby, Al has the effect of enhancing formability and
delayed fracture resistance by making the grain size of austenite
finer. In order to effectively bring out that effect, the lower
limit of the amount of Al is set to be 0.005% or more. A preferred
lower limit of the amount of Al is 0.01% or more, more preferably
0.02% or more. When Al is added in an excessive amount, however,
inclusions of alumina and the like increase, and both formability
and toughness are impaired as a result. Accordingly, the upper
limit of the amount of Al is set to be 1%. A preferred upper limit
of the amount of Al is 0.8% or less, more preferably 0.6% or
less.
[0066] N: more than 0% to 0.01% or less
[0067] N is an element of unavoidable presence which, if
excessively contained, impairs formability. Further, BN,
precipitates are formed when B (boron) is added to the steel, and
the hardenability-enhancing effect of B is thus hampered.
Accordingly, the content of N should be reduced as much as
possible. Therefore, the upper limit of the amount of N is set to
be 0.01% or less. A preferred upper limit of the amount of N is
0.008% or less, more preferably 0.005% or less.
[0068] The plated steel sheet of the present invention contains the
above components, the balance being iron and inevitable
impurities.
[0069] Further, the optional elements shown below can be
incorporated in the present invention.
[0070] At least one element selected from the group consisting of
Cr: more than 0% to 1% or less, Mo: more than 0% to 1% or less, and
B: more than 0% to 0.01% or less
[0071] These elements are effective elements in terms of enhancing
the strength of the steel sheet. The foregoing elements can be
incorporated singly or in combinations of two or more elements.
[0072] In further detail, Cr enhances hardenability and contributes
to increasing strength. Further, Cr suppresses generation and
growth of cementite, and contributes to improving bendahility. In
order to effectively bring out that effect, the lower limit of the
amount of Cr is set to be 0.01% or more. However, platability
decreases when Cr is added in an excessive amount. Further, Cr
carbides are generated excessively, and formability is impaired.
Accordingly, a preferred upper limit of the amount of Cr is set to
be 1% or less, more preferably 0.7% or less and yet more preferably
0.4% or less.
[0073] Mo is effective in increasing strength, and accordingly, a
preferred lower limit of the amount of Mo is set to be 0.01% or
more. However, even when M is added in excess, the effect of Mo
levels off while giving rise to an increase in costs. Accordingly,
a preferred upper limit of the amount of Mo is set to be 1% or
less, more preferably 0.5% or less and yet more preferably 0.3% or
less.
[0074] As in the case of Mn, B is a hardenability-enhancing element
that suppresses ferrite and bainite, and that generates martensite,
contributing thus to enhancing strength. In order to effectively
bring out that effect, the lower limit of the amount of B is set to
be 0.0002% or more, more preferably 0.0010% or more. However, an
excessive amount of B results in poorer hot formability, so that a
preferred upper limit of the amount of B is set to be 0.01% or
less, more preferably 0.0070% or less and yet more preferably
0.0050% or less.
[0075] At least one element selected from the group consisting of
Ti: more than 0% to 0.2% or less, Nb: more than 0% to 0.2% or less
and V: more than 0% to 0.2% or less
[0076] These elements are effective elements in enhancing
formability and delayed fracture resistance by making the structure
finer. The foregoing elements can be added singly or in
combinations of two or more elements.
[0077] In order to effectively bring out the effect of the
elements, the lower limits of Ti, Nb and V are each set to 0.01% or
more. However, as ferrite is generated to impair formability when
the content of any one of these elements is excessive, a preferred
upper limit of the amount of each element is set to be 0.2% or
less, more preferably 0.15% or less, and yet more preferably 0.10%
or less, for all of these elements.
[0078] At least one element selected from the group consisting of
Cu: more than 0% to 1% or less and Ni: more than 0% to 1% or
less
[0079] Cu and Ni are effective elements in terms of increasing
strength. The foregoing elements may be added singly or may be used
in combination.
[0080] In order to effectively bring out the effect of the
elements, the lower limits of Cu and Ni are each set to be 0.01% or
more. However, as hot formability decreases when the content of any
one of these elements is excessive, a preferred upper limit of the
amount of each element is set to be 1% or less, more preferably
0.8% or less and yet more preferably 0.5% or less, for all of these
elements.
[0081] The steel components of the present invention have been
explained above.
[0082] Methods for producing the plated steel sheet of the present
invention will be explained next. The production methods of the
present invention include a first method that involves pickling,
without temperature keeping, immediately after hot-rolled coiling,
and a second method that involves pickling after temperature
keeping following the hot-rolled coiling. The lower limit of the
hot-rolled coiling temperature varies between the first method
(without temperature keeping) and the second method (with
temperature keeping), depending on the presence or absence of
temperature keeping. However, other steps are identical in the
methods. The details are as described below.
[0083] [First Production Method (Without Temperature Keeping)]
[0084] The first production method according to the present
invention can be divided roughly into: a hot rolling step; a
pickling and cold rolling step; and an oxidation step, a reduction
step, and a galvanizing/galvannealing step in a continuous hot-dip
galvanizing line (CGL (Continuous Galvanizing Line)). A
characterizing feature of the present invention is to include the
following steps, in order: a hot rolling step of coiling a steel
sheet that satisfies the above steel components at a temperature of
600.degree. C. or higher, to obtain as a result a hot-rolled steel
sheet having an internal oxide layer formed therein; a step of
pickling and cold rolling the steel sheet such that there remain
the internal oxide layer with the average depth d of 4 .mu.m or
more; a step of oxidizing the steel sheet at an air ratio in the
range of 0.9 to 1.4, in an oxidation zone; a step of soaking the
steel sheet by keeping within a range of (Ac.sub.1 point
+50.degree. C.) to (Ac.sub.3 point +20.degree. C.), in a reduction
zone; and a step of, after the soaking, cooling the steel sheet at
an average cooling rate of 5.degree. C./sec or higher over a range
down to a cooling stop temperature.
[0085] The steps will be explained next in succession.
[0086] Firstly, there is prepared a hot-rolled steel sheet that
satisfies the above steel components. Hot rolling may be carried
out according to an ordinary method. Preferably, for instance, the
heating temperature is set to lie in the range of about 1150 to
1300.degree. C. in order to prevent coarsening of austenite grains.
Preferably, the finish rolling temperature is controlled to lie
roughly in the range of 850 to 950.degree. C.
[0087] In the present invention, it is important to control the
coiling temperature after hot rolling to be 600.degree. C. or
higher. As a result, an internal oxide layer is formed on the base
steel sheet surface and a soft layer as well is formed through
decarburization; hence, it becomes possible to obtain a desired
internal oxide layer and a desired soft layer in the steel sheet
after galvanizing/galvannealing. The internal oxide layer and the
soft layer are not sufficiently generated in a case where the
coiling temperature is lower than 600.degree. C. Further, strength
of the hot-rolled steel sheet increases, and cold ductility drops.
A preferred coiling temperature is herein 620.degree. C. or higher,
and more preferably 640.degree. C. or higher. When the coiling
temperature is excessively high, however, black skin scale grows
excessively and cannot be dissolved by pickling, so that the upper
limit is preferably set to be 750.degree. C. or lower.
[0088] Next, the hot-rolled steel sheet thus obtained is subjected
to pickling and cold rolling in such a manner that there remain the
internal oxide layer with the average depth d of 4 .mu.m or more.
As a result, there remains not only the internal oxide layer but
also the soft layer, and in consequence, also the desired soft
layer can be readily generated after galvanizing/galvannealing.
Controlling the thickness of an internal oxide layer through
control of pickling conditions is a known feature. Specifically,
the temperature and time of pickling may be properly controlled in
such a manner that the desired thickness of the internal oxide
layer can be ensured in accordance with, for instance, the type,
concentration and so forth of the pickling solution that is
used.
[0089] For instance, a mineral acid such as hydrochloric acid,
sulfuric acid, nitric acid, or the like can be used as the pickling
solution.
[0090] Generally, a higher concentration and/or a higher
temperature of the pickling solution, and longer pickling time,
tend to produce a thinner internal oxide layer through dissolution.
Conversely, removal of black skin scale layer through pickling
becomes insufficient when the concentration or temperature of the
pickling solution is low and the pickling time is short.
Accordingly, it is recommended to control the concentration so as
to lie in the range of about 3 to 20%, the temperature in the range
of 60 to 90.degree. C., and the time in the range of about 35 to
200 seconds, when using for instance hydrochloric acid.
[0091] The number of pickling baths is not particularly limited,
and a plurality of pickling baths may be used herein. For instance,
a pickling suppressant, i.e. inhibitor such as an amine, or a
pickling promoter or the like may be added to the pickling
solution.
[0092] After pickling, cold rolling is performed in such a manner
that there remain the internal oxide layer with the average depth d
of 4 .mu.m or more. Preferably, the cold rolling conditions are
controlled in such a manner that a cold rolling ratio lies in the
range of about 20 to 70%.
[0093] Oxidation and reduction are performed next.
[0094] In detailed terms, firstly oxidation is carried out at an
air ratio in the range of 0.9 to 1.4, in an oxidation zone. The
term air ratio denotes herein a ratio of the amount of air actually
supplied with respect to the amount of air stoichiometrically
necessary in order to completely burn off a combustion gas that is
supplied. An air ratio higher than 1 entails an excess of oxygen,
while an air ratio lower than 1 entails a shortage of oxygen. In
the examples described below, CO gas is used as the combustion
gas.
[0095] Decarburization is promoted through oxidation in an
atmosphere having an air ratio lying in the above range, whereby
the desired soft layer is formed, and bendability is improved.
Further, it becomes possible to generate a Fe oxide film on the
surface and to suppress generation of a complex oxide film or the
like that is detrimental to platability. When the air ratio is
lower than 0.9, decarburization is insufficient, and a sufficient
soft layer is not formed, so that bendability is impaired as a
result. Further, generation of the Fe oxide film becomes
insufficient, and generation of for instance the above complex
oxide film cannot be suppressed, so that platability is impaired as
a result. The air ratio has to be controlled to be 0.9 or higher,
and is preferably controlled to be 1.0 or higher. When, on the
other hand, the air ratio is so high as to exceed 1.4, the Fe oxide
film is generated excessively, and cannot be sufficiently reduced
in a subsequent reduction furnace, which hinders platability. The
air ratio has to be controlled to be 1.4 or lower, and is
preferably controlled to be 1.2 or lower.
[0096] In the oxidation zone, it is particularly important to
control the air ratio. Ordinarily used methods can be resorted to
herein as regards other conditions. For instance, a preferred lower
limit of the oxidation temperature is 500.degree. C. or higher,
more preferably 750.degree. C. or higher. The upper limit of the
oxidation temperature is 900.degree. C. or lower, more preferably
50.degree. C. or lower.
[0097] Next, the oxide film is reduced in a hydrogen atmosphere, in
a reduction zone. In order to obtain a hard layer containing
bainite and martensite, and ferrite in predetermined ranges, a
soaking treatment is performed by keeping in, the range of
(Ac.sub.1 point +50.degree. C.) to (Ac.sub.3 point +20.degree. C.).
Ferrite becomes excessive in amount when the soaking temperature is
lower than (Ac.sub.1 point +50.degree. C.), while ferrite becomes
insufficient in amount when the soaking temperature exceeds
(Ac.sub.3 point +20.degree. C.). A preferred soaking temperature
lies in the range of (Ac.sub.1 point +100.degree. C.) or higher to
Ac.sub.3 point'C. or lower.
[0098] In the present invention, the Ac.sub.1 point is calculated
on the basis of Expression (i) below. The brackets [ ] in the
Expression denote content (mass %) of the elements. The Expression
is disclosed in "The Physical Metallurgy of Steels" (William C.
Leslie, Published by Maruzen, page 273).
Ac.sub.1 (.degree.
C.)=723-10.7.times.[Mn]-16.9.times.[Ni]+29.1.times.[Si]+16.9.times.[Cr]+2-
90.times.[As]+6.38.times.[W] (i)
[0099] In the present invention, the Ac.sub.3 point is calculated
on the basis of Expression (ii) below. The brackets [ ] in the
expression denote content (mass %) of the elements. The expression
is disclosed in "The Physical Metallurgy of Steels" (William), C.
Leslie, Published by Maruzen, page 273).
Ac.sub.3 (.degree.
C.)=910-203.times.[C].sup.1/2-15.2.times.[Ni]+44.7.times.[Si]+104.times.[-
V]+31.5.times.[Mo]+13.1.times.[W]-{30.times.[Mn]+11.times.[Cr]+20.times.[C-
u]-700.times.[P]-400.times.[Al]-120.times.[As]-400.times.[Ti]}
(ii)
[0100] In the present invention, the keeping time at the above
soaking temperature is preferably set to be 10 seconds or longer.
When the keeping time is shorter than 10 seconds, reduction is
insufficient to hamper platability. The keeping time is more
preferably 30 seconds or longer, yet more preferably 50 seconds or
longer. The keeping time during the soaking treatment is not
particularly limited from the above viewpoint. However, in
consideration of productivity and so forth, the keeping time is
preferably controlled to be about 100 seconds or shorter, more
preferably about 80 seconds or shorter.
[0101] In the reduction furnace, it is particularly important to
control the soaking temperature and the keeping time at the above
soaking temperature. Ordinarily used methods can be resorted to
herein as regards other conditions. Preferably, for instance, the
atmosphere of the reduction zone includes hydrogen and nitrogen,
and the hydrogen concentration is controlled to lie in the range of
about 5 to 25 vol %. Further, the dew point is preferably
controlled to lie in the range of -30 to -60.degree. C.
[0102] Cooling is performed next. Specifically, after the soaking,
cooling is performed at an average cooling rate of 5.degree. C./sec
or higher over a range down to a cooling stop temperature. By doing
so, it becomes possible to control the area ratio of ferrite to be
within a predetermined range. The average cooling rate is
preferably 8.degree. C./sec or higher, more preferably 10.degree.
C./sec or higher. The upper limit of the average cooling rate is
not particularly limited, but is preferably controlled to be about
100.degree. C./sec or lower, for instance in consideration of ease
of control of the base steel sheet temperature and equipment cost.
A more preferred average cooling rate is 50.degree. C./sec or
lower, and yet more preferably 30.degree. C./sec or lower.
[0103] The cooling, stop temperature may be as low as within a
temperature region such that ferrite is not generated, and the
steel sheet is preferably cooled down, for instance, to 550.degree.
C. or lower. A preferable lower limit of the cooling stop
temperature may be, for example, 400.degree. C. or higher, more
preferably 430.degree. C. or higher, and yet more preferably
460.degree. C. or higher.
[0104] In the present invention, it is important to control the
average cooling rate at least until the cooling stop temperature,
and a cooling method thereafter is not limited to the
above-described one. For instance, when the steel sheet is to be
heated to a plating bath temperature in performing hot-dip
galvanization after cooling, the steel sheet may be cooled down to
a temperature lower than the aforementioned preferable cooling stop
temperature (for instance, see No. 26 of Table 2 given below). Or
else, the steel sheet may be water-quenched after being cooled to a
predetermined temperature.
[0105] Thereafter, hot-dip galvanizing is carried out in accordance
with an ordinary method. The hot-dip galvanizing method is not
particularly limited, and for instance a preferred lower limit of
the plating, bath temperature is 400.degree. C. or higher, more
preferably 440.degree. C. or higher. Moreover, a preferred upper
limit of the plating bath temperature is 500.degree. C. or lower,
more preferably 470.degree. C. or lower. The composition of the
plating bath is not particularly limited, and a known hot-dip
galvanizing bath may be used herein.; Further, cooling conditions
after the hot-dip galvanizing are not particularly limited, and for
instance the average cooling rate down to ordinary temperature is
preferably controlled to be about 1.degree. C./sec or higher, and
more preferably 5.degree. C./sec or higher. The upper limit of the
average cooling rate is not particularly limited, but is preferably
controlled to be about 50.degree. C./sec or lower, for instance in
consideration of ease of control of the base steel sheet
temperature and equipment cost. The average cooling rate is
preferably 40.degree. C./sec or lower, and more preferably
30.degree. C./sec or lower.
[0106] Furthermore, in accordance with the needs, an alloying
process may be carried out in accordance with an ordinary method,
whereby a hot-dip galvannealed steel sheet is obtained. The
conditions of the alloying process as well are not particularly
limited. Preferably, for instance, hot-dip galvanizing is carried
out under the above conditions, and thereafter the temperature is
kept in the range of about 500 to 600.degree. C., in particular in
the range of about 530 to 580.degree. C., for about 5 to 30
seconds, in particular for about 10 to 25 seconds. Below the above
ranges, alloying is insufficient. On the other hand, above the
above ranges, alloying progresses excessively, and plating peeling
may occur during press forming of the galvannealed steel sheet.
Moreover, ferrite is generated readily in such a case. The alloying
process may be carried out using for instance a heating furnace,
open fire, or an infrared heating furnace. The heating means is not
particularly limited, and may be for instance a conventional means
such as gas heating or induction-heater heating, i.e. heating using
a high-frequency induction heating device.
[0107] The alloying process is followed by cooling ine accordance
with an ordinary method, whereby a hot-dip galvannealed steel sheet
is obtained. The average cooling rate down to ordinary temperature
is preferably controlled to be about 1.degree. C./sec or
higher.
[0108] [Second Production Method (With Temperature Keeping)]
[0109] The second production method according to the present
invention includes, in this order: a hot rolling step of coiling a
hot-rolled steel sheet that satisfies the above steel components,
at a temperature of 500.degree. C. or higher; a step of keeping the
steel sheet in a temperature region of 500.degree. C. or higher for
80 minutes or longer; a step of pickling and cold rolling the steel
sheet such that there remain the internal oxide layer with the
average depth d of 4 .mu.m or more; a step of oxidizing the steel
sheet at an air ratio in the range of 0.9 to 1.4, in an oxidation
zone; a step of soaking the steel sheet by keeping within a range
(Ac.sub.1 point +50.degree. C.) to (Ac.sub.3 point +20.degree. C.),
in a reduction zone; and a step of after the soaking, cooling the
steel sheet at an average cooling rate of 5.degree. C./sec or
higher over a range down to a cooling stop temperature. The second
production method differs from the first production method
described above only as regards two features, namely in that, in
the second production method, the lower limit of the coiling
temperature after hot rolling is set to be 500.degree. C. or
higher, and in that a temperature keeping step is provided after
the hot-rolling step. Accordingly, only the above differences will
be explained below. Steps identical to those of the first
production method may be referenced to the first production method
above.
[0110] The reason for providing the temperature keeping step as
shown above is to enable prolonged keeping in a temperature region
that allows for oxidation through temperature keeping, so as to
expand the lower limit of the coiling temperature range within
which there are obtained the desired internal oxide layer and soft
layer. A further advantage herein is the increased homogeneity of
the base steel sheet, resulting from a smaller temperature
difference between the surface layer and the interior of the base
steel sheet.
[0111] In the second production method, firstly the coiling
temperature after hot rolling is controlled to be 500.degree. C. or
higher. As explained in detail further on, the lower limit of the
coiling temperature can be set herein to be lower than that of the
first production method described above, i.e. 600.degree. C., since
in the second production method there is provided the subsequent
temperature keeping step. A preferred coiling temperature is
540.degree. C. or higher, and more preferably 570.degree. C. or
higher. A preferred upper limit of the coiling temperature is
identical to that of the first production method described above,
and is preferably set to be 750.degree. C. or lower.
[0112] Next, the hot-rolled steel sheet thus obtained is kept in a
temperature region of 500.degree. C. or higher for 80 minutes or
longer. The desired internal oxide layer can be obtained as a
result. Preferably, the temperature of the hot-rolled steel sheet
is kept by placing the hot-rolled steel sheet for instance in a
thermally-insulated apparatus, in such a way so as to effectively
bring out the effect derived from temperature keeping. The above
apparatus used in the present invention is not particularly
limited, as long as the device is made up of a thermally insulating
material. Preferred materials that can be used as the thermally
insulating material include ceramic fibers and the like.
[0113] The temperature mast be kept in a region of 500.degree. C.
or higher for 80 minutes or longer in order to elicit effectively
the above effect. A preferred temperature is herein 540.degree. C.
or higher, and more preferably 560.degree. C. or higher. A
preferred time is 100 minutes or longer, more preferably 120
minutes or longer. Upper limits of the temperature and time are
preferably controlled to be roughly 700.degree. C. or lower and 500
minutes or shorter, for instance in consideration of pickling
properties and productivity.
[0114] The first and second production methods according to the
present invention have been explained above.
[0115] The plated steel sheet of the present invention obtained in
accordance with the above production methods may be further
subjected to various coating and coat-grounding treatments, for
instance chemical conversion treatments such as a phosphate
treatment, and organic coating treatments, for instance formation
of an organic coating film such as a film laminate.
[0116] Coating materials that can be used in the above various
coating schemes include known resins, for instance, epoxy resins,
fluororesins, silicone acrylic resins, polyurethane resins, acrylic
resins, polyester resins, phenol resins, alkyd resins, melamine
resins, and the like. Taking corrosion resistance into account,
preferred among the foregoing are epoxy resins, fluororesins, and
silicon acrylic resins. A curing agent may be used together with
the resins. The coating material may contain known additives, for
instance coloring pigments, coupling agents, leveling agents,
sensitizers, antioxidants, UV stabilizers, flame retardants, and
the like.
[0117] The form of the coating material in the present invention is
not particularly limited, and there can be used coating materials
in any form, for instance, solvent-based coating materials, aqueous
coating materials, aqueous dispersion type coating materials,
powder coating materials, electrodeposition coating materials, and
the like. Neither the coating method is particularly limited, and
may involve dipping, roll coating, spraying, curtain flow coating,
electrodeposition coating, or the like. The thickness of the
coating layer such as a plated layer, an organic coating film, a
chemical conversion coating, a coating film, or the like may be set
as appropriate in accordance with the intended application.
[0118] The high-strength plated steel sheet of the present
invention has ultra-high strength and boasts excellent formability
(bendability and hole expandability) as well as excellent delayed
fracture resistance, and can therefore be used in automotive strong
parts, for instance, in crash parts such as front and rear
side-members, crash boxes, and the lake; pillars such as center
pillar reinforcements and the like; as well as car body components
such as roof rail reinforcements, side sills, floor members, kick
sections, and the like.
[0119] The present application claims the right of priority based
on Japanese Patent Applications No. 2015-003671 filed on Jan. 9,
2015 and No. 2015-159213 file don Aug. 11, 2015. The entire
contents of the specification of Japanese Patent Applications No.
2015-003671 filed on Jan. 9, 2015 and No. 2015-159212 file don Aug.
11, 2015 are incorporated in the present application by
reference.
[0120] Hereafter, the present invention will be explained more
specifically by way of Examples; however, the invention is not
limited by the following Examples and can be carried out while
including additional modifications within a scope conforming to the
gist disclosed heretofore and hereinafter, all such modifications
being encompassed within the technical scope of the invention.
EXAMPLES
[0121] Slabs having the component composition given in Table 1, the
balance being iron and inevitable impurities, were heated to
1250.degree. C., and were hot-rolled down to 2.4 mm at a finish
rolling temperature of 900.degree. C., followed by coiling at the
temperature given in Table 2.
[0122] Some of the examples, i.e. Nos. 27 to 29, 38 to 40 and 42,
were placed thereafter in a ceramic fiber heat-insulating
apparatus, where the temperature was kept under the conditions
given in Table 2. The keeping time at a temperature of 500.degree.
C. or higher was measured using a thermocouple attached to the
outer periphery of the coil.
[0123] Each hot-rolled steel sheet thus obtained was then pickled
under the conditions below, and was thereafter cold-rolled at a
cold rolling ratio of 50%. The sheet thickness after cold rolling
was 1.2 mm.
[0124] The pickling solution was 10% hydrochloric acid, the
temperature was 82.degree. C. and the pickling time as given in
Table 2.
[0125] Next, annealing (oxidation, reduction) and cooling were
performed in a continuous hot-dip galvanizing line under the
conditions given in Table 2. The temperature of an oxidation
furnace disposed in the continuous hot-dip galvanizing line was
controlled to be 800.degree. C., the hydrogen concentration in a
reduction furnace was controlled to 20 vol %, with the balance
being nitrogen and inevitable impurities, and the dew point was
controlled to be -45.degree. C. The keeping time at the soaking
temperature given in Table 2 was set to be 50 seconds in all
instances.
[0126] Thereafter, except for the following No. 26, dipping into a
galvanizing bath at 460.degree. C. was carried out, followed by
cooling down to room temperature at an average cooling rate of
10.degree. C./sec, thereby to obtain a hot-dip galvanized steel
sheet (GI) (No. 25). With respect to hot-dip galvannealed steel
sheets (GA), dipping into the above galvanizing bath was carried
out to perform hot-dip galvanizing, followed by heating to
500.degree. C. Then, after keeping at this temperature for 20
seconds to perform an alloying process, cooling down to room
temperature at an average cooling rate of 10.degree. C./sec was
carried out (Nos. 1 to 24 and 27 to 42).
[0127] In No. 26, after cooling down to a cooling stop temperature
of 250.degree. C. given in Table 2, heating to 460.degree. C. was
carried out, followed by dipping into a galvanizing bath, thereby
to obtain a GA steel sheet in the same manner as shown above.
[0128] The plated steel sheets, i.e. GI or GA, thus obtained were
evaluated for the below-described characteristics. As described
below, the average depth of the internal oxide layer was measured
not only in each plated steel sheet, but was likewise measured, for
reference, in the base steel sheet after pickling and cold rolling.
The purpose of this is to confirm that the desired average depth of
the internal oxide layer is obtained already in the cold-rolled
steel sheet before annealing, through control of for instance the
coiling to temperature and pickling conditions after hot
rolling.
[0129] (1) Measurement of the Average Depth D of the Internal Oxide
Layer of Plated Steel Sheets
[0130] Taking as W the sheet width of each plated steel sheet, a
test piece having a size of 50 mm.times.50 mm was sampled from a
W/4 portion, and thereafter the O amount, Fe amount, and Zn amount
from the plated layer surface were analyzed and quantified by
GD-OES [(Glow Discharge-Optical Emission Spectroscopy)]. In further
detail, the surface of the test piece was high-frequency sputtered
within an Ar glow discharge region, using a GO-OES device of
GD-PROFILER 2 GDA 750, by HORIBA Ltd., and the respective emission
lines of the sputtered elements O, Fe and Zn in the Ar plasma were
resolved continuously, to measure as a result the respective
element content profiles in the depth direction of the base steel
sheet. The sputtering conditions were, as described below, and the
measurement region was set to extend to a depth of 50 .mu.m from
the plated layer surface.
[0131] (Sputtering Conditions) [0132] Pulsed sputtering frequency:
50 Hz [0133] Anode diameter (analysis surface area): 6 mm diameter
[0134] Discharge power: 30 W [0135] Ar gas pressure: 2.5 hPa
[0136] The results of the analysis are illustrated in FIG. 2. As
illustrated in FIG. 2, the interface of the plated layer and the
base steel sheet was set at the position from the plated layer
surface at which the Za amount and the Fe amount were equal. The
average value of the O amount at each measurement position in a
depth in the range of 40 to 50 .mu.m from the plated layer surface
was taken as the bulk O amount average value, and a range being
0.02% higher than that (i.e. O amount.gtoreq.(bulk O amount average
value +0.02%)) was defined as the internal oxide layer. The maximum
depth of that range was taken as the depth of the internal oxide
layer. The same test was performed using three test pieces, and the
average of the foregoing was taken as the average depth d of the
internal oxide layer.
[0137] (2) Measurement of the Internal Oxide Layer Depth After
Pickling-Cold Rolling (Reference)
[0138] The average depth of the internal oxide layer was calculated
in the same way as in (1), but using herein the base steel sheet
after pickling-cold rolling.
[0139] (3) Measurement of the Average Depth D of the Soft Layer
[0140] A test piece having a size of 20 mm.times.20 mm was sampled
from an exposed W/4 portion, being a cross-section perpendicular to
the direction of the sheet width W of each plated steel sheet.
Thereafter, the test piece was embedded in resin, and Vickers
hardness was measured from the interface of the plated layer and
the base steel sheet towards the interior of the base steel sheet
at a sheet thickness t. The hardness was measured at a load of 3 gf
using a Vickers hardness tester. In detail, measurements were
performed at pitches of 5 .mu.m inward in the sheet thickness,
starting at a measurement position at a depth of 10 .mu.m inward in
the sheet thickness, from the interface of the plated layer 1 and
the base steel sheet 2, as illustrated in FIG. 3. The Vickers
hardness was measured down to a depth of 100 .mu.m. The spacing
between measurement points; that is, the distance
between.times.and.times.in FIG. 3, was set to be 15 .mu.m or
greater at the lowest. The Vickers hardness was measured by n=1 for
each depth, to investigate the hardness distribution in the inward
direction of the sheet thickness. The Vickers hardness at a portion
t/4 of the base steel sheet was also measured (n=1) under a load of
1 kgf, using a Vickers hardness tester. A region having a Vickers
hardness of 90% or less with respect to that of the portion t/4 of
the base steel sheet was considered to be a soft layer, and the
depth of this soft layer was calculated. The same process was
carried out at 10 sites in one same test piece, and the average was
taken as the average depth D of the soft layer.
[0141] (4) Method for Measuring the Structure Fraction of the
Plated Steel Sheets
[0142] A W/4 portion, being a cross-section perpendicular to the
direction of the sheet width W of the plated steel sheet, was
exposed, and the cross-section was polished and then
electropolished; thereafter, the cross-section was corroded with
nital, and was observed by SEM (Scanning Electron Microscope). The
observation position was set to a t/4 position, where t is the
sheet thickness of the base steel sheet, the observation
magnifications were set to 2000 times, and the observation region
to 40 .mu.m.times.40 .mu.m. The metal structure micrographs
captured by SEM were subjected to image analysis, to measure the
respective area ratios of martensite and bainite (the two were not
distinguished from each other), and ferrite. In Table 3,
.alpha.=ferrite, and (B+M)=(bainite+martensite). In Table 3, the
area fraction of the "Other" structure was calculated by
subtracting the area ratios of martensite and bainite, and ferrite
from 100 area %. The observations were carried out arbitrarily in
three fields of view, and the average value of the foregoing was
calculated.
[0143] (5) Measurement Method in a Tensile Test
[0144] JIS 13B tensile test pieces were sampled in such a manner
that the direction perpendicular to the rolling direction of the
plated steel sheet and the longitudinal direction of the test
pieces were parallel, and the tensile strength (TS), yield stress
(YS), and elongation (EL) in the C direction were measured
according to JIS Z2241. The yield ratio YR (YS/TS) was calculated
from TS and YS.
[0145] In the Examples, those test pieces having a tensile strength
TS of 980 MPa or higher were rated as of high strength
(acceptable).
[0146] Further, TS.times.FL was calculated from the tensile
strength and the elongation obtained in the above-described manner.
In the present Examples, those test pieces having TS.times.EL of
17000 or more were rated as having excellent balance between
strength and ductility (acceptable).
[0147] (6) Bending Work Test
[0148] 20 mm.times.70 mm test pieces were cut out of the plated
steel sheets in such a manner that the direction perpendicular to
the rolling direction of the plated steel sheets and the
longitudinal direction of the test pieces were parallel, and a
90.degree. V.-bending test was carried so that a bending ridge line
coincided with the longitudinal direction. The test was performed
by modifying the bending radius R as appropriate, and there was
worked out the minimum bending radius Rmin that allowed for bending
work without cracks occurring in the test pieces.
[0149] Bendability was evaluated for each tensile strength TS, on
the basis of Rmin/t, which is the quotient of Rmin divided by the
sheet thickness t of the base steel sheet. The details are as
follows. Bendability was not evaluated (marked as "-" Table 3) for
test pieces in which TS did not satisfy the acceptance criteria
being 980 MPa or higher. [0150] Rmin/t<1.5 was deemed as
acceptable when TS was 980 MPa or higher and lower than 1080 MPa.
[0151] Rmin/t<2.5 was deemed as acceptable when TS was 1080 MPa
or higher and lower than 80 MPa. [0152] Rmin/t<3.2 was deemed as
acceptable when TS was 1180 MPa or higher.
[0153] (7) Delayed Fracture Resistance Test
[0154] A W/4 portion, being a cross-section perpendicular to the
direction of the sheet width W of each plated steel sheet, was
exposed, and a 150 mm (W).times.30 mm (L) test piece was cut out
and was U-bent at a minimum bending radius; thereafter, the test
piece was fastened with bolts, and the outer surface of the U-bent
test piece was loaded with a tensile stress of 1000 MPa. To measure
tensile stress, a strain gauge was affixed to the outside of the
U-bent test piece, and strain was convened to tensile stress.
Thereafter, the edges of the U-bent test piece were masked, and the
test piece was electrochemically charged with hydrogen. Hydrogen
charging was carried out herein through immersion in a mixed
solution of 0.1M-H.sub.2SO.sub.4 (pH=3) and 0.01M-KSCN, under
conditions of roam temperature and constant current of 100
.mu.A/mm.sup.2.
[0155] In the results of the hydrogen charge test, instances with
no cracking in 24 hours were rated as acceptable, i.e. of excellent
delayed fracture resistance.
[0156] (8) Hole Expansion Test
[0157] A hole expansion test was carried out according to the Japan
Iron and Steel Federation Standard JFS T 1001, to measure .lamda..
In further detail, holes having a diameter of 10 mm were punched in
the plated steel sheet; thereafter a 60' conical punch was pressed
into the hole, with the periphery in a restrained state, and the
diameter of the hole at the crack initiation limit was measured. A
limit hole expansion ratio .lamda. (%) was worked out on the basis
of the expression below. Instances where .lamda. was 25% or higher
were rated, as acceptable, i.e. of excellent hole
expandability.
Limit hole expansion ratio .lamda.(%)={(Df-D0}.times.100
[0158] In the expression, Df denotes the diameter (mm) of the hole
at the crack initiation limit, and D0 denotes the diameter (min) of
the initial hole.
[0159] (9) Appearance of the Plated Steel Sheets
[0160] The appearance of the plated steel sheets was observed
visually, and platability was evaluated on the basis of the
occurrence or absence of bare spots.
[0161] The results are summarized in Table 2 and Table 3.
TABLE-US-00001 TABLE 1A Steel Components (mass %) Ac.sub.3 Ac.sub.1
No. type C Si Mn P S Al N Cr Mo B Ti Nb V Cu Ni (.degree. C.)
(.degree. C.) 1 A 0.12 1.10 2.65 0.011 0.002 0.042 0.0037 834 727 2
B 0.13 1.21 2.32 0.012 0.001 0.040 0.0045 0.25 843 738 3 C 0.16
1.58 2.12 0.010 0.001 0.040 0.0048 0.14 863 746 4 D 0.15 1.87 2.22
0.008 0.002 0.040 0.0044 0.0032 870 754 5 E 0.13 0.87 2.45 0.009
0.002 0.040 0.0039 0.04 840 722 6 F 0.09 0.64 3.32 0.012 0.002
0.040 0.0038 0.08 803 706 7 G 0.14 1.55 2.34 0.012 0.001 0.040
0.0045 0.03 861 743 8 H 0.19 1.43 1.87 0.011 0.001 0.040 0.0042
0.17 850 745 9 I 0.15 1.12 2.08 0.012 0.002 0.040 0.0047 0.16 841
731 10 J 0.15 0.55 2.50 0.009 0.001 0.037 0.0036 0.15 0.05 0.0032
0.02 810 715 11 K 0.10 1.50 2.45 0.010 0.001 0.042 0.0039 0.32 0.07
0.0030 0.02 0.02 870 746 12 L 0.12 0.76 2.45 0.010 0.001 0.520
0.0039 0.08 0.0017 0.02 1022 720 13 M 0.26 1.32 1.79 0.012 0.002
0.040 0.0035 836 742 14 N 0.17 0.10 2.33 0.011 0.002 0.040 0.0044
785 701 15 A 0.12 1.10 2.65 0.011 0.002 0.042 0.0037 834 727 16 A
0.12 1.10 2.65 0.011 0.002 0.042 0.0037 834 727 17 A 0.12 1.10 2.65
0.011 0.002 0.042 0.0037 834 727 18 A 0.12 1.10 2.65 0.011 0.002
0.042 0.0037 834 727 19 A 0.12 1.10 2.65 0.011 0.002 0.042 0.0037
834 727 20 A 0.12 1.10 2.65 0.011 0.002 0.042 0.0037 834 727
TABLE-US-00002 TABLE 1B Steel Components (mass %) Ac.sub.3 Ac.sub.1
No. type C Si Mn P S Al N Cr Mo B Ti Nb V Cu Ni (.degree. C.)
(.degree. C.) 21 A 0.12 1.10 2.65 0.011 0.002 0.042 0.0037 Z 834
727 22 A 0.12 1.10 2.65 0.011 0.002 0.042 0.0037 834 727 23 A 0.12
1.10 2.65 0.011 0.002 0.042 0.0037 834 727 24 A 0.12 1.10 2.65
0.011 0.002 0.042 0.0037 834 727 25 A 0.12 1.10 2.65 0.011 0.002
0.042 0.0037 834 727 26 A 0.12 1.10 2.65 0.011 0.002 0.042 0.0037
834 727 27 A 0.12 1.10 2.65 0.011 0.002 0.042 0.0037 834 727 28 A
0.12 1.10 2.65 0.011 0.002 0.042 0.0037 834 727 29 A 0.12 1.10 2.65
0.011 0.002 0.042 0.0037 834 727 30 O 0.15 0.33 2.70 0.014 0.002
0.039 0.0035 791 704 31 P 0.15 0.26 2.45 0.009 0.002 0.041 0.0043
0.15 792 707 32 Q 0.15 0.34 2.43 0.010 0.002 0.038 0.0045 0.10 0.09
0.0028 0.02 807 709 33 R 0.13 0.45 2.47 0.010 0.002 0.038 0.0032
0.15 0.04 819 712 34 S 0.12 0.32 2.45 0.011 0.002 0.040 0.0028 0.15
0.05 803 709 35 T 0.11 0.36 2.41 0.012 0.002 0.044 0.0034 0.15 0.02
0.03 819 710 36 U 0.15 0.49 2.51 0.013 0.002 0.042 0.0027 0.15 0.06
808 713 37 V 0.15 0.34 2.49 0.015 0.002 0.034 0.0036 0.15 0.11
0.0031 0.02 0.03 807 709 38 O 0.15 0.33 2.70 0.014 0.002 0.039
0.0035 791 704 39 Q 0.15 0.34 2.43 0.010 0.002 0.038 0.0045 0.10
0.09 0.0028 0.02 807 709 40 V 0.15 0.34 2.49 0.015 0.002 0.034
0.0036 0.15 0.11 0.0031 0.02 0.03 834 709 41 W 0.16 0.27 2.55 0.015
0.002 0.038 0.0038 790 704 42 W 0.16 0.27 2.55 0.015 0.002 0.038
0.0038 790 704
TABLE-US-00003 TABLE 2A Coiling Keeping Average depth temperature
tune in a of internal after hot temperature Pickling oxide layer
after Oxidation Steel rolling Temperature region of 500.degree. C.
or time pickling-cold furnace Ac.sub.1 + 50 No. type (.degree. C.)
keeping higher (min) (sec) rolling (.mu.m) air ratio (.degree. C.)
1 A 670 No -- 45 12 1.1 777 2 B 670 No -- 45 13 1.1 788 3 C 670 No
-- 45 12 1.1 796 4 D 670 No -- 45 12 1.1 804 5 E 670 No -- 45 10
1.1 772 6 F 670 No -- 45 9 1.1 756 7 G 670 No -- 45 13 1.1 793 8 H
670 No -- 45 12 1.1 795 9 I 670 No -- 45 11 1.1 781 10 J 670 No --
45 8 1.1 765 11 K 670 No -- 45 12 1.1 796 12 L 670 No -- 45 12 1.1
770 13 M 670 No -- 45 13 1.1 792 14 N 670 No -- 45 2 1.1 751 15 A
580 No -- 45 2 1.1 777 16 A 720 No -- 45 15 1.1 777 17 A 670 No --
45 12 1.1 777 18 A 670 No -- 45 12 1.1 777 19 A 670 No -- 45 12 1.1
777 20 A 670 No -- 45 12 0.8 777 Average depth d of Depth D Soaking
Average Cooling stop internal oxide layer of soft layer Ac.sub.3 +
20 temperature cooling rate temperature after plating after plating
No. (.degree. C.) (.degree. C.) (.degree. C./sec) (.degree. C.)
(.mu.m) (.mu.m) D/2 d 1 854 820 10 450 12 29 1.21 2 863 820 10 450
14 30 1.07 3 883 820 10 450 12 30 1.25 4 890 820 10 450 13 31 1.19
5 860 820 10 450 11 33 1.50 6 823 820 10 450 10 29 1.45 7 881 820
10 450 13 29 1.12 8 870 820 10 450 13 27 1.04 9 861 820 10 450 12
29 1.21 10 830 820 10 450 9 28 1.56 11 890 820 10 450 12 28 1.17 12
1042 820 10 450 12 28 1.17 13 856 820 10 450 13 37 1.42 14 805 800
10 450 2 20 5.00 15 854 820 10 450 2 17 4.25 16 854 820 10 450 15
36 1.20 17 854 900 10 450 12 28 1.17 18 854 840 10 450 12 29 1.21
19 854 770 10 450 12 27 1.13 20 854 820 10 450 12 15 0.63
TABLE-US-00004 TABLE 2B Coiling Keeping Average depth temperature
time in a of internal after hot temperature Pickling oxide layer
after Oxidation Steel rolling Temperature region of 500.degree. C.
or time pickling-cold furnace Ac.sub.1 + 50 No. type (.degree. C.)
keeping higher (min) (sec) rolling (.mu.m) air ratio (.degree. C.)
21 A 670 No -- 45 12 0.9 777 22 A 580 No -- 45 1 0.7 777 23 A 580
No -- 45 1 1.3 777 24 A 670 No -- 45 12 1.1 777 25 A 670 No -- 45
12 1.1 777 26 A 670 No -- 45 12 1.1 777 27 A 570 Yes 180 45 14 1.1
777 28 A 470 Yes 180 45 2 1.1 777 29 A 570 Yes 60 45 2 1.1 777 30 O
690 No -- 45 6 1.1 754 31 P 690 No -- 45 4 1.1 757 32 Q 690 No --
45 6 1.1 759 33 R 690 No -- 45 6 1.1 762 34 S 690 No -- 45 5 1.1
759 35 T 690 No -- 45 5 1.1 760 36 U 690 No -- 45 7 1.1 763 37 V
690 No -- 45 6 1.1 759 38 O 690 Yes 180 45 10 1.1 754 39 Q 690 Yes
180 45 11 1.1 759 40 V 690 Yes 180 45 10 1.1 759 41 W 690 No -- 45
4 1.1 754 42 W 690 Yes 180 45 8 1.1 754 Average depth d of Depth D
Soaking Average Cooling stop internal oxide layer of soft layer
Ac.sub.3 + 20 temperature cooling rate temperature after plating
after plating No. (.degree. C.) (.degree. C.) (.degree. C./sec)
(.degree. C.) (.mu.m) (.mu.m) D/2 d 21 854 820 10 450 12 22 0.92 22
854 820 10 450 1 7 3.50 23 854 820 10 450 2 28 7.00 24 854 820 2
450 12 29 1.21 25 854 820 10 450 12 29 1.21 26 854 820 10 250 12 29
1.21 27 854 820 10 450 14 34 1.21 28 854 820 10 450 3 15 2.50 29
854 820 10 450 2 17 4.25 30 811 790 6 450 6 27 2.25 31 812 790 4
450 4 26 3.25 32 827 790 6 450 6 28 2.33 33 839 790 6 450 6 29 2.42
34 823 790 5 450 5 27 2.70 35 839 790 5 450 5 26 2.60 36 828 790 7
450 7 27 1.93 37 827 790 6 460 6 26 2.17 38 811 790 10 450 10 32
1.60 39 827 790 11 450 11 32 1.45 40 854 790 10 450 10 31 1.55 41
810 790 10 450 4 25 3.13 42 810 790 10 450 8 30 1.88
TABLE-US-00005 TABLE 3A Hard layer structure Delayed Steel (area %)
YS TS EL YR .lamda. Bendability fracture Bare Type of No. type
.alpha. B + M Other (MPa) (MPa) (%) (%) (%) Rmin/t Evaluation TS
.times. EL resistance spots plating 1 A 25 74 1 765 1212 13.0 63 24
2.08 Acceptable 15756 Acceptable No GA 2 B 31 69 0 776 1234 13.1 63
25 2.08 Acceptable 16165 Acceptable No GA 3 C 34 65 1 775 1256 12.9
62 22 2.08 Acceptable 16202 Acceptable No GA 4 D 27 71 2 805 1279
12.3 63 23 1.67 Acceptable 15732 Acceptable No GA 5 E 22 75 3 754
1197 13.3 63 26 2.08 Acceptable 15920 Acceptable No GA 6 F 16 83 1
812 1276 11.5 64 29 2.08 Acceptable 14674 Acceptable No GA 7 G 37
62 1 799 1267 12.9 63 26 1.67 Acceptable 16344 Acceptable No GA 8 H
26 74 0 800 1287 12.5 62 22 1.67 Acceptable 16088 Acceptable No GA
9 I 19 79 2 665 1078 14.4 62 32 1.25 Acceptable 15523 Acceptable No
GA 10 J 12 86 2 845 1301 11.4 65 26 2.08 Acceptable 14831
Acceptable No GA 11 K 35 63 2 795 1243 13.0 64 28 1.67 Acceptable
16159 Acceptable No GA 12 L 35 63 2 765 1223 13.6 63 22 2.08
Acceptable 16633 Acceptable No GA 13 M 16 82 2 912 1454 10.6 63 15
3.75 Unacceptable 15412 Unacceptable No GA 14 N 8 92 0 768 1185
11.6 65 18 3.75 Unacceptable 13746 Unacceptable No GA 15 A 23 76 1
767 1224 12.7 63 17 3.33 Unacceptable 15545 Unacceptable Yes GA 16
A 24 75 1 745 1209 12.9 62 27 1.67 Acceptable 15596 Acceptable No
GA 17 A 0 99 1 879 1276 10.7 69 35 1.67 Acceptable 13653 Acceptable
No GA 18 A 9 90 1 823 1254 11.7 66 30 1.67 Acceptable 14672
Acceptable No GA 19 A 72 27 1 545 968 19.4 56 17 0.83 -- 18779
Acceptable No GA 20 A 24 75 1 778 1218 12.8 64 18 3.33 Unacceptable
15590 Unacceptable Yes GA
TABLE-US-00006 TABLE 3B Hard layer structure Delayed Steel (area %)
YS TS EL YR .lamda. Bendability fracture Bare Type of No. type
.alpha. B + M Other (MPa) (MPa) (%) (%) (%) Rmin/t Evaluation TS
.times. EL resistance spots plating 21 A 24 75 1 769 1216 12.9 63
22 2.50 Acceptable 15686 Acceptable No GA 22 A 24 75 1 776 1226
12.8 63 17 3.75 Unacceptable 15693 Unacceptable Yes GA 23 A 24 75 1
761 1211 13.1 63 18 2.92 Acceptable 15864 Unacceptable Yes GA 24 A
45 52 3 675 1155 15.2 58 18 2.50 Unacceptable 17556 Acceptable No
GA 25 A 25 74 1 781 1199 13.0 65 27 2.08 Acceptable 15587
Acceptable No GI 26 A 25 74 1 802 1209 13.1 66 34 2.08 Acceptable
15838 Acceptable No GA 27 A 24 75 1 743 1193 13.2 62 30 1.67
Acceptable 15748 Acceptable No GA 28 A 23 76 1 751 1208 13.2 62 17
3.33 Unacceptable 15946 Unacceptable Yes GA 29 A 24 75 1 791 1221
13.1 65 18 3.33 Unacceptable 15995 Unacceptable Yes GA 30 O 11 89 0
812 1230 11.5 66 21 2.92 Acceptable 14145 Acceptable No GA 31 P 13
87 0 800 1213 11.7 66 22 2.92 Acceptable 14192 Acceptable No GA 32
Q 19 81 0 823 1243 12.1 66 23 2.92 Acceptable 15040 Acceptable No
GA 33 R 31 69 0 756 1181 12.8 64 25 2.92 Acceptable 15117
Acceptable No GA 34 S 19 81 0 712 1123 13.2 63 28 2.08 Acceptable
14824 Acceptable No GA 35 T 29 71 0 690 1087 14.5 63 29 2.08
Acceptable 15762 Acceptable No GA 36 U 23 77 0 799 1254 11.8 64 23
2.50 Acceptable 14797 Acceptable No GA 37 V 18 82 0 812 1247 11.4
65 24 2.92 Acceptable 14216 Acceptable No GA 38 O 14 86 0 786 1217
11.9 65 29 2.08 Acceptable 14482 Acceptable No GA 39 Q 22 78 0 802
1229 11.8 65 28 2.08 Acceptable 14502 Acceptable No GA 40 V 20 80 0
795 1221 12.1 65 30 2.08 Acceptable 14774 Acceptable No GA 41 W 10
90 0 799 1211 11.6 66 21 2.92 Acceptable 14048 Acceptable No GA 42
W 12 88 0 772 1201 11.7 64 24 2.08 Acceptable 14052 Acceptable No
GA
[0162] The following observations arise from the Tables.
[0163] Firstly, examples Nos. 1 to 12, 16, 18, 21, 25, 26, 27, and
30 to 42 satisfied the requirements of the present invention, and
all exhibited good strength, formability [bendability, hole,
expandability (.lamda.) and balance between strength and ductility
(TS.times.EL)], delayed fracture resistance, and platability. In
particular, No. 1 (D/2d=1.21) in which the average depth d of the
internal oxide layer and average depth D of the soft layer
satisfied the relationship D>2d (i.e. value of "D/2d" greater
than 1 in Table 2), exhibited better bendability than No. 21
(D/2d=0.92), which did not satisfy the above relationship.
[0164] In contrast, in No. 13, being an example in which a steel
type M of Table 1 with a large amount of C was used, bendability,
.lamda., and delayed fracture resistance were low.
[0165] In No. 14, being an example in which a steel type N of Table
1 with a small amount of Si was used, the internal oxide layer
failed to be generated sufficiently. As a result, bendability,
.lamda., balance between strength and ductility, and delayed
fracture resistance were low.
[0166] In No. 15, being an example in which the coiling temperature
during hot rolling was low though a steel type A of Table 1 having
steel components satisfying the requirements of the present
invention was used, the average depth of the internal oxide layer
after pickling-cold rolling was shallow, and accordingly the
average depth d of the internal oxide layer and the average depth D
of the soft layer after galvanizing/galvannealing were shallow. As
a result, bendability, .lamda., delayed fracture resistance, and
platability were low.
[0167] In No. 17, being an example in which the soaking temperature
was high though the steel type A of Table 1 having steel components
satisfying the requirements of the present invention was used,
ferrite was not generated at all, and accordingly, balance between
strength and ductility was low.
[0168] In No. 19, being an example in which the soaking temperature
was low though the steel type A of Table 1 having steel components
satisfying the requirements of the present invention was used,
ferrite was generated excessively, and also the sum amount of (B+M)
was low, so that the desired hard layer was not obtained. For this
reason, TS was low, and .lamda. as well was low.
[0169] In No. 20, being an example in which the air ratio in the
oxidation furnace was low though the steel type A of Table 1 having
steel components satisfying the requirements of the present
invention was used, the iron oxide film was not generated
sufficiently, and platability was low. Further, the soft layer was
not generated sufficiently. As a result, bendability, .lamda., and
delayed fracture resistance were low.
[0170] In No. 22, being an example in which the coiling temperature
during hot rolling was low and the air ratio in the oxidation
furnace was low though the steel type A of Table 1 having steel
components satisfying the requirements of the present invention was
used, the average depth of the internal oxide layer after
pickling-cold rolling was shallow, and accordingly the average
depth d of the internal oxide layer and the average depth D of the
soft layer after galvanizing/galvannealing were shallow. As a
result, .lamda., bendability, delayed fracture resistance, and
platability were low.
[0171] In No. 23, being an example in which the coiling temperature
during hot rolling was low though the steel type A of Table 1
having steel components satisfying the requirements of the present
invention was used, the average depth of the internal oxide layer
after pickling-cold rolling was shallow, and accordingly the
average depth d of the internal oxide layer after
galvanizing/galvannealing were shallow. As a result, .lamda.,
delayed fracture resistance, and platability were low.
[0172] In No. 24, being an example in which the average cooling
rate after soaking was low though the steel type A of Table 1
having steel components satisfying the requirements of the present
invention was used, ferrite was generated excessively during
cooling, and also the sum amount of (B+M) was low, so that the
desired hard layer was not obtained. As a result, .lamda. and
bendability were low.
[0173] In No. 28, being an example in which the coiling temperature
during hot rolling was low though the steel type A of Table 1
having steel components satisfying the requirements of the present
invention was used, the average depth of the internal oxide layer
after pickling-cold rolling was shallow, and accordingly the
average depth d of the internal oxide layer and the average depth D
of the soft layer after galvanizing/galvannealing were shallow. As
a result, bendability, delayed fracture resistance, and platability
were low.
[0174] In No. 29, being an example in which the temperature keeping
time was insufficient though the steel type A of Table 1 having
steel components satisfying the requirements of the present
invention was used, the average depth of the internal oxide layer
after pickling-cold rolling was shallow, and accordingly the
average depth d of the internal oxide layer and the average depth D
of the soft layer after galvanizing/galvannealing were shallow. As
a result, bendability, .lamda., delayed fracture resistance, and
platability were low.
REFERENCE SIGNS
[0175] 1 plated layer [0176] 2 base steel sheet [0177] 3 internal
oxide layer [0178] 4 soft layer [0179] 5 hard layer
* * * * *