U.S. patent application number 14/004809 was filed with the patent office on 2014-01-02 for steel sheet for hot stamped member and method of production of same.
This patent application is currently assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION. The applicant listed for this patent is Jun Maki, Hiroyuki Tanahashi. Invention is credited to Jun Maki, Hiroyuki Tanahashi.
Application Number | 20140004378 14/004809 |
Document ID | / |
Family ID | 46879372 |
Filed Date | 2014-01-02 |
United States Patent
Application |
20140004378 |
Kind Code |
A1 |
Tanahashi; Hiroyuki ; et
al. |
January 2, 2014 |
STEEL SHEET FOR HOT STAMPED MEMBER AND METHOD OF PRODUCTION OF
SAME
Abstract
A steel sheet for obtaining a member which is excellent in
fatigue characteristics equal to ordinary high strength steel sheet
of the same strength even if applying the hot stamping process and
a method of production of the same are provided. Steel sheet for a
hot stamped member which includes composition which contains, by
mass %, C: 0.15 to 0.35%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.3%, Al:
0.01 to 0.5%, and a balance of Fe and unavoidable impurities, and
limit the impurities to P: 0.03% or less, S: 0.02% or less, and N:
0.1% or less, wherein that a standard error of Vicker's hardness at
a position of 20 .mu.m from the steel sheet surface in the sheet
thickness direction is 20 or less. This steel sheet is produced by
a recrystallization-annealing step of a first stage of heating a
cold rolled steel sheet, which is obtained by hot rolling steel
containing the above composition and then cold rolling it, by an
average heating rate of 8 to 25.degree. C./sec from room
temperature to 600 to 700.degree. C., then a second stage of
heating by an average heating rate of 1 to 7.degree. C./sec to 720
to 820.degree. C.
Inventors: |
Tanahashi; Hiroyuki;
(Chiyoda-ku, JP) ; Maki; Jun; (Chiyoda-ku,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Tanahashi; Hiroyuki
Maki; Jun |
Chiyoda-ku
Chiyoda-ku |
|
JP
JP |
|
|
Assignee: |
NIPPON STEEL & SUMITOMO METAL
CORPORATION
Tokyo
JP
|
Family ID: |
46879372 |
Appl. No.: |
14/004809 |
Filed: |
March 16, 2012 |
PCT Filed: |
March 16, 2012 |
PCT NO: |
PCT/JP2012/056917 |
371 Date: |
September 12, 2013 |
Current U.S.
Class: |
428/653 ;
148/320; 148/330; 148/332; 148/333; 148/336; 148/337; 148/531;
148/533; 148/579; 148/621; 148/645; 428/659 |
Current CPC
Class: |
C21D 8/0273 20130101;
C21D 8/0247 20130101; C21D 8/0226 20130101; C23C 2/28 20130101;
C21D 9/48 20130101; C22C 1/02 20130101; Y10T 428/12757 20150115;
C21D 9/46 20130101; C22C 38/16 20130101; C22C 38/06 20130101; C23C
2/06 20130101; C22C 38/04 20130101; C22C 38/38 20130101; C22C 38/12
20130101; C23C 2/12 20130101; C21D 1/18 20130101; C21D 1/673
20130101; C22C 38/001 20130101; C22C 38/08 20130101; C23C 2/02
20130101; C21D 8/0205 20130101; C22C 38/14 20130101; C22C 38/28
20130101; C22C 38/32 20130101; Y10T 428/12799 20150115; C22C 38/02
20130101; C21D 6/00 20130101; C21D 8/0236 20130101 |
Class at
Publication: |
428/653 ;
148/320; 428/659; 148/579; 148/621; 148/645; 148/531; 148/533;
148/330; 148/332; 148/333; 148/336; 148/337 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/16 20060101 C22C038/16; C22C 38/14 20060101
C22C038/14; C22C 38/00 20060101 C22C038/00; C22C 38/08 20060101
C22C038/08; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/38 20060101
C22C038/38; C22C 38/12 20060101 C22C038/12 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 18, 2011 |
JP |
2011-060893 |
Claims
1. Steel sheet for a hot stamped member which includes composition
which contains, by mass %, C: 0.15 to 0.35%, Si: 0.01 to 1.0%, Mn:
0.3 to 2.3%, Al: 0.01 to 0.5%, and a balance of Fe and unavoidable
impurities, and limit the impurities to P: 0.03% or less, S: 0.02%
or less, and N: 0.1% or less, wherein a standard deviation of
Vicker's hardness at a position of 20 .mu.m from the steel sheet
surface in the sheet thickness direction is 20 or less.
2. The steel sheet for a hot stamped member as set forth in claim 1
which further contains, by mass %, one or more of elements selected
from Cr: 0.01 to 2.0%, Ti: 0.001 to 0.5%, Nb: 0.001 to 0.5% B:
0.0005 to 0.01%, Mo: 0.01 to 1.0% W: 0.01 to 0.5%, V: 0.01 to 0.5%,
Cu: 0.01 to 1.0%, and Ni: 0.01 to 5.0%.
3. The steel sheet for a hot stamped member as set forth in claim 1
which has on the surface of said steel sheet one of a 5 .mu.m to 50
.mu.m thick Al plating layer, a 5 .mu.m to 30 .mu.m thick
galvanized layer, or a 5 .mu.m to 45 .mu.m thick Zn--Fe alloy
layer.
4. A method of production of steel sheet for a hot stamped member
comprising recrystallization-annealing cold rolled steel sheet
which includes composition which contains, by mass %, C: 0.15 to
0.35%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.3%, Al: 0.01 to 0.5%, and a
balance of Fe and unavoidable impurities, and limit the impurities
to P: 0.03% or less, S: 0.02% or less, and N: 0.1% or less, in
which step, including a first stage of heating by an average
heating rate of 8 to 25.degree. C./sec from room temperature to a
temperature M (.degree. C.) and then a second stage of heating by
an average heating rate of 1 to 7.degree. C./sec to a temperature S
(.degree. C.), wherein the temperature M (.degree. C.) is 600 to
700 (.degree. C.) and the temperature S (.degree. C.) is 720 to 820
(.degree. C.).
5. The method of production of steel sheet for a hot stamped member
as set forth in claim 4 wherein said steel further contains, by
mass %, one or more of Cr: 0.01 to 2.0%, Ti: 0.001 to 0.5%, Nb:
0.001 to 0.5% B: 0.0005 to 0.01%, Mo: 0.01 to 1.0% W: 0.01 to 0.5%,
V: 0.01 to 0.5%, Cu: 0.01 to 1.0%, and Ni: 0.01 to 5.0%.
6. The method of production of steel sheet for a hot stamped member
as set forth in claim 5 wherein a hot rolling rate in said hot
rolling step is 60 to 90%, while a cold rolling rate of said cold
rolling step is 30 to 90%.
7. The method of production of steel sheet for a hot stamped member
as set forth in claim 4 which further includes, after said
recrystallization-annealing step, a step of dipping said steel
sheet in an Al bath to form an Al plating layer on the surface.
8. The method of production of steel sheet for a hot stamped member
as set forth in claim 4 which further includes, after said
recrystallization annealing step, a step of dipping said steel
sheet in a Zn bath to form a galvanized layer on the surface.
9. The method of production of steel sheet for a hot stamped member
as set forth in claim 4 which further includes, after said
recrystallization-annealing step, a step of dipping said steel
sheet in a Zn bath to form a galvanized layer on the surface, then
further heating to 600.degree. C. or less to form a Zn--Fe alloy
layer on said surface.
Description
TECHNICAL FIELD
[0001] The present invention relates to steel sheet for a hot
stamped member which is suitable for the hot stamping method, one
of the shaping methods giving a high strength member, and a method
of production of the same.
BACKGROUND ART
[0002] In the field of automobiles, construction machinery, etc.,
vigorous efforts are being made to reduce weight by use of high
strength materials. For example, in automobiles, the amount of use
of high strength steel sheet has been steadily increasing for the
purpose of cancelling out the increase in vehicle weight
accompanying the improvements in impact safety and performance and
furthermore improving fuel efficiency to reduce the amount of
emission of carbon dioxide.
[0003] In the trend toward expanded use of such high strength steel
sheet, the biggest problem, unavoidable when raising the strength
of steel sheet, is the rise of the phenomenon called "degradation
of the shape fixability". This phenomenon is the general term for
loss of ease of obtaining a target shape due to the increase in the
amount of springback after shaping accompanying higher strength. To
solve this problem, working steps which were unnecessary with low
strength materials (materials with shape fixabilities which are
excellent or not a problem) (for example, restriking) have been
performed or the product shapes have been changed.
[0004] As one method for dealing with this situation, the hot
shaping method called the "hot stamping method" has come under
attention. This heats a steel sheet (worked material) to a
predetermined temperature (generally, the temperature resulting in
an austenite phase) to lower the strength (that is, facilitate
shaping), then shapes it by a die of a lower temperature than the
worked material (for example room temperature) to thereby easily
impart a shape and simultaneously utilize the temperature between
the two for rapid cooling heat treatment (quenching) so as to
secure the strength of the shaped product.
[0005] Several arts relating to steel sheet suitable for such a hot
stamping method and method of shaping the same have been
reported.
[0006] PLT 1 shows steel sheet obtained by controlling the amounts
of elements which the steel sheet contains and the relationship
among the amounts of the elements to predetermined ranges so as to
give a member which is excellent in impart characteristics and
delayed fracture characteristic after hot shaping (synonymous with
hot stamping).
[0007] PLT 2, in the same way as the above, discloses a method
comprising making the amounts of elements which the steel sheet
contains and the relationship among the amounts of the elements to
predetermined ranges and heating before shaping the steel sheet in
a nitriding atmosphere or a carburizing atmosphere so as to obtain
a high strength part.
[0008] PLT 3 describes means for prescribing the composition and
microstructure of steel sheet and limiting the heating conditions
and shaping conditions so as to obtain hot pressed parts with a
high productivity.
[0009] Recently, the hot stamping method has become widely
recognized for its usefulness. Members for which its application
has been studied have become much more diverse. Among these, for
example, there are parts, such as underbody parts of automobiles,
where not only the strength of the parts, but also the fatigue
characteristic is an important, necessary characteristic.
[0010] The fatigue characteristic of steel sheet is improved
together with the static strength, so steel sheet (product) made
high in strength by the hot stamping method also can be expected to
exhibit a commensurate fatigue characteristic, if compared with
steel sheet of the same strength not using the hot stamping method
(high strength steel sheet produced by controlling the composition
or method of production of the strength steel sheet, below, called
"ordinary high strength steel sheet"), it became clear that
depending on the production conditions, the fatigue characteristics
of the former were inferior to the latter.
[0011] Studied in detail, it was discovered that compared with the
deviation in hardness of the surfacemost part of "ordinary high
strength steel sheet", the deviation in hardness of the surfacemost
part of steel sheet (product) raised in strength by using the hot
stamping method is larger. It was concluded that this deviation in
hardness might be related to the fatigue characteristic.
[0012] The relationship between the deviation in hardness and the
fatigue characteristic is not necessarily clear, but in a high
strength member which is produced by the hot stamping method (for
example, a tensile strength of 1500 MPa or more), the effect of the
notch sensitivity on the fatigue characteristic is extremely large,
so it is guessed that this deviation in hardness might be an
indicator comparable to the flatness of a surface layer.
[0013] Therefore, the inventors studied the art for reducing as
much as possible the deviation in hardness after hot stamping and
as a result discovered that the deviation in surface layer hardness
of the steel sheet before hot stamping has an impact. No literature
has been found which studies steel sheet for hot stamping use from
such a perspective.
[0014] PLT 1 discusses steel sheet for hot shaping use where all of
Ni, Cu, and Sn are essential, wherein the impact characteristics
and the delayed fracture characteristic are improved, but does not
allude to the fatigue characteristic or the deviation in surface
layer hardness before hot stamping.
[0015] PLT 2 relates to the art of heating in a carburizing
atmosphere so as to raise the strength of a shaped part, but does
not allude to the fatigue characteristic or the deviation in
surface layer hardness before hot stamping. Heating in a
carburizing atmosphere is essential. Compared with heating in the
air, the production costs rise. Further, when using carbon monoxide
as the source of carbon, there is a concern that tremendous costs
would be required for securing the safety of operations. It is
believed that this art is not easily workable.
[0016] PLT 3 also does not allude to the fatigue characteristic and
the deviation in surface layer hardness before hot stamping.
[0017] As opposed to this, as art for obtaining steel sheet for hot
stamping use which has a fatigue characteristic of the same extent
as "ordinary high strength steel sheet", there is PLT 4. Further,
while as art inherent to the case of use of steel sheet which has
been galvanized, PLT 5 is known as art for improving the fatigue
characteristic of a member which is produced by the hot stamping
method.
[0018] PLT 4 discloses to make fine particles which contain Ce
oxides disperse slight inward from the steel sheet surface so as to
improve the fatigue characteristic after hot stamping, but advanced
steelmaking art is required, so there is the problem that even a
person skilled in the art would not necessarily find it easy to
work it.
[0019] The art of PLT 5 relates to facilities for hot stamping
technology. There is the problem that without new capital
investment, even a person skilled in the art could not enjoy its
benefits.
In this way, steel sheet for hot stamping use for obtaining steel
sheet (product) made high in strength by hot stamping, which
enables fatigue characteristics of the same extent as "ordinary
high strength steel sheet" of the same strength to be secured
relatively easily, has been sought, but no art which solves this
problem has been found.
CITATIONS LIST
Patent Literature
[0020] PLT 1: Japanese Patent Publication No. 2005-139485A
[0021] PLT 2: Japanese Patent Publication No. 2005-200670A
[0022] PLT 3: Japanese Patent Publication No. 2005-205477A
[0023] PLT 4: Japanese Patent Publication No. 2007-247001A
[0024] PLT 5: Japanese Patent Publication No. 2007-182608A
SUMMARY OF INVENTION
Technical Problem
[0025] The present invention, in view of the above situation, has
as its object the provision of steel sheet for a hot stamped member
which enables the production of a product of high strength steel
sheet which has an excellent fatigue characteristic of the same
extent as high strength steel sheet which is produced by
controlling the composition of the steel sheet or method of
production ("ordinary high strength steel sheet") when producing a
product by applying the hot stamping method to steel sheet and of a
method of production of the same.
Solution to Problem
[0026] The inventors engaged in intensive research to solve this
problem. As a result, they discovered that making the deviation in
hardness near the surface layer of steel sheet before hot stamping
within a predetermined range is extremely effective for improving
the fatigue characteristic of the steel sheet after hot stamping
(product). They discovered that such steel sheet can be obtained by
controlling the conditions when recrystallization-annealing the
cold rolled steel sheet, conducted repeated tests, and thereby
completed the present invention.
[0027] The gist of the invention is as follows:
[0028] (1) Steel sheet for a hot stamped member which includes
composition which contains, by mass %,
C: 0.15 to 0.35%,
Si: 0.01 to 1.0%,
Mn: 0.3 to 2.3%,
Al: 0.01 to 0.5%, and
[0029] a balance of Fe and unavoidable impurities, and limit the
impurities to P: 0.03% or less, S: 0.02% or less, and N: 0.1% or
less, wherein a standard deviation of Vicker's hardness at a
position of 20 .mu.m from the steel sheet surface in the sheet
thickness direction is 20 or less.
[0030] (2) The steel sheet for a hot stamped member as set forth in
(1) which further contains, by mass %, one or more of elements
selected from
Cr: 0.01 to 2.0%,
Ti: 0.001 to 0.5%,
Nb: 0.001 to 0.5%
B: 0.0005 to 0.01%,
Mo: 0.01 to 1.0%
W: 0.01 to 0.5%,
V: 0.01 to 0.5%,
Cu: 0.01 to 1.0%, and
Ni: 0.01 to 5.0%.
[0031] (3) The steel sheet for a hot stamped member as set forth in
(1) or (2) which has on the surface of the steel sheet one of a 5
.mu.m to 50 .mu.m thick Al plating layer, a 5 .mu.m to 30 .mu.m
thick galvanized layer, or a 5 .mu.m to 45 .mu.m thick Zn--Fe alloy
layer.
[0032] (4) A method of production of steel sheet for a hot stamped
member comprising recrystallization-annealing cold rolled steel
sheet which includes composition which contains, by mass %,
C: 0.15 to 0.35%,
Si: 0.01 to 1.0%,
Mn: 0.3 to 2.3%,
Al: 0.01 to 0.5%, and
[0033] a balance of Fe and unavoidable impurities, and limit the
impurities to P: 0.03% or less, S: 0.02% or less, and N: 0.1% or
less, in which step, including a first stage of heating by an
average heating rate of 8 to 25.degree. C./sec from room
temperature to a temperature M (.degree. C.) and then a second
stage of heating by an average heating rate of 1 to 7.degree.
C./sec to a temperature S (.degree. C.), wherein the temperature M
(.degree. C.) is 600 to 700 (.degree. C.) and the temperature S
(.degree. C.) is 720 to 820 (.degree. C.).
[0034] (5) The method of production of steel sheet for a hot
stamped member as set forth in (4) wherein the steel further
contains, by mass %, one or more of
Cr: 0.01 to 2.0%,
Ti: 0.001 to 0.5%,
Nb: 0.001 to 0.5%
B: 0.0005 to 0.01%,
Mo: 0.01 to 1.0%
W: 0.01 to 0.5%,
V: 0.01 to 0.5%,
Cu: 0.01 to 1.0%, and
Ni: 0.01 to 5.0%.
[0035] (6) The method of production of steel sheet for a hot
stamped member as set forth in (4) or (5) wherein a hot rolling
rate in the hot rolling step is 60 to 90%, while a cold rolling
rate of the cold rolling step is 30 to 90%.
[0036] (7) The method of production of steel sheet for a hot
stamped member as set forth in any one of (4) to (6) which further
includes, after the recrystallization-annealing step, a step of
dipping the steel sheet in an Al bath to form an Al plating layer
on the surface.
[0037] (8) The method of production of steel sheet for a hot
stamped member as set forth in any one of (4) to (6) which further
includes, after the recrystallization-annealing step, a step of
dipping the steel sheet in a galvanization bath to form a
galvanized layer on the surface.
[0038] (9) The method of production of steel sheet for a hot
stamped member as set forth in any one of (4) to (6) which further
includes, after the recrystallization-annealing step, a step of
dipping the steel sheet in a Zn bath to form a galvanized layer on
the surface, then further heating to 600.degree. C. or less to form
a Zn--Fe alloy layer on the surface.
Advantageous Effects of Invention
[0039] The steel sheet for a hot stamped member of the present
invention can be produced by a known steelmaking facility. Further,
a shaped part which is obtained using the steel sheet for a hot
stamped member of the present invention for shaping by widespread
hot stamping facilities (hot stamped members) has a fatigue
characteristic equal to "ordinary high strength steel sheet" of the
same strength, so has the effect of expanding the scope of
application of hot stamped members (parts).
BRIEF DESCRIPTION OF INVENTION
[0040] FIG. 1 is perspective view which shows a sheet press die for
hot stamping which is used for the examples of the present
invention.
[0041] FIG. 2 is a view which shows fatigue test pieces.
[0042] FIG. 3 is a perspective view which shows locations of
measurement of hardness in a test piece for hardness measurement
use of the same dimensions as the crack growth region of the
fatigue test piece which is shown in FIG. 2.
[0043] FIG. 4 is a graph which shows the correlation between the
fatigue limit ratio and standard deviation of hardness before hot
stamping of steel sheet for a hot stamped member of Example 1.
[0044] FIG. 5 is a perspective view which schematically shows steel
sheet (member) which is formed into a hat shape by the hot stamping
method.
[0045] FIG. 6 is a graph which shows the correlation between the
fatigue limit ratio and standard deviation of hardness before hot
stamping of steel sheet for a hot stamped member of Example 2.
DESCRIPTION OF EMBODIMENTS
[0046] The inventors engaged in research using steel sheet which
contains, by mass %, C: 0.23%, Si: 0.5%, and Mn: 1.6% to prepare a
hot stamped member and evaluated its characteristics. They
discovered that the fatigue characteristic is one of the same but
that there are hot stamped members which are the same in
composition of the steel sheet and almost the same in tensile
strength, but differ in fatigue characteristic. Therefore, they
investigated the differences of these in detail, whereupon they
learned that there are differences in the deviation in hardness
near the surface layers of hot stamped members. Accordingly, they
further changed the composition and recrystallization conditions of
cold rolled steel sheet over a broad range to investigate the
fatigue characteristic of hot stamped members and discovered that
there is a strong correlation between the fatigue characteristic of
hot stamped members and the deviation in surface hardness of the
same and that to obtain a hot stamped member which is excellent in
fatigue characteristic, it is effective to make the various in
surface hardness of steel sheet before hot stamping within a
predetermined range and that further to obtain such steel sheet, it
is possible to control the conditions when
recrystallization-annealing cold rolled steel sheet to a
predetermined range.
[0047] Details will be explained in the examples, but the inventors
used these test findings as the basis to experimentally clarify the
suitable range of deviation in hardness and the annealing
conditions and thereby completed the present invention.
[0048] Composition of Steel Sheet
[0049] First, the composition of steel sheet will be explained.
Here, the "%" in the composition mean mass %.
[0050] C: 0.15 to 0.35%
C is the most important element in increasing the strength of steel
sheet by hot stamping. To obtain a 1200 MPa or so strength after
hot stamping, 0.15% or more has to be included. On the other hand,
if over 0.35% is included, deterioration of toughness is a concern,
so 0.35% is made the upper limit.
[0051] Si: 0.01 to 1.0%
Si is a solution strengthening element. Up to 1.0% can be
effectively utilized. However, if more than that is included,
trouble is liable to occur at the time of chemical treatment or
coating after shaping, so 1.0% is made the upper limit. The lower
limit is not particularly limited. The effect of the present
invention can be obtained. However, reduction more than necessary
just raises the steelmaking load, so the content is made the level
of inclusion due to deoxidation, that is, 0.01% or more.
[0052] Mn: 0.3 to 2.3%
Mn is an element which functions as a solution strengthening
element in the same way as Si and also is effective for raising the
hardenability of steel sheet. This effect is recognized at 0.3% or
more. However, even if over 2.3% is included, the effect becomes
saturated, so 2.0% is made the upper limit.
[0053] P: 0.03% or less, S: 0.02% or less
The two elements are both unavoidable impurities. They affect the
hot workability, so have to be limited to the above ranges.
[0054] Al: 0.01 to 0.5%
Al is suitable as a deoxidizing element, so 0.01% or more should be
included. However, if included in a large amount, coarse oxides are
formed and the mechanical properties of the steel sheet are
impaired, so the upper limit is made 0.5%.
[0055] N: 0.1% or less
N is an unavoidable impurity. It easily bonds with Ti or B, so has
to be controlled so as not to reduce the targeted effect of these
elements. 0.1% or less is allowable. The content is preferably
0.01% or less. On the other hand, reduction more than necessary
places a massive load on the production process, so 0.0010% should
be made the target for the lower limit.
[0056] Cr: 0.01 to 2.0%
Cr has the effect of raising the hardenability, so can be suitably
used. This effect becomes clear at 0.01% or more. On the other
hand, even if over 2.0% is added, this effect becomes saturated, so
2.0% is made the upper limit.
[0057] Ti: 0.001 to 0.5%
Ti is an element which acts to stably draw out the effect of B,
explained later, through the formation of its nitride, so can be
effectively used. For this reason, 0.001% or more has to be added,
but if excessively added, the nitrides become excessive and
deterioration in toughness or shear surface properties is invited,
so 0.5% is made the upper limit.
[0058] Nb: 0.001 to 0.5%
Nb is an element which forms carbonitrides and raises the strength,
so can be effectively used. This effect is recognized at 0.001% or
more, but if over 0.5% is included, the controllability of the hot
rolling is liable to be impaired, so 0.5% is made the upper
limit.
[0059] B: 0.0005 to 0.01%
B is an element which raises the hardenability. The effect becomes
clear at 0.0005% or more. On the other hand, excessive addition
leads to deterioration of hot workability and a drop in the
ductility, so 0.01% is made the upper limit.
[0060] Mo: 0.01 to 1.0%, W: 0.01 to 0.5%, V: 0.01 to 0.5%
These elements all have the effect of raising the hardenability, so
can be suitably used. The effect becomes clear in each case at
0.01% or more. On the other hand, it is an expensive element, so
the concentration where the effect becomes saturated is preferably
made the upper limit. For Mo, this is 1.0%, while for W and V, it
is 0.5%.
[0061] Cu: 0.01 to 1.0%
Cu has the effect of raising the strength of the steel sheet by
addition of Cu in 0.01% or more. However, excessive addition
detracts from the surface quality of the hot rolled steel sheet, so
1.0% is made the upper limit.
[0062] Ni: 0.01 to 5.0%
Ni is an element which has the effect of raising the hardenability,
so can be effectively used. The effect becomes clear at 0.01% or
more. On the other hand, it is an expensive element, so 5.0% where
the effect becomes saturated is made the upper limit. Further, it
also acts to suppress the drop in the surface quality of the hot
rolled steel sheet due to Cu, so inclusion simultaneously with Cu
is desirable.
[0063] Note that in the present invention, the composition other
than the above consist of Fe, but unavoidable impurities which
enter from the scrap and other melting materials or the
refractories etc. are allowed.
[0064] Deviations in Steel Sheet Surface Hardness
The deviations in steel sheet surface hardness will be
explained.
[0065] First, the method of determining (measuring) the hardness of
the steel sheet surface will be explained.
[0066] The hardness of the steel sheet surface ideally should be
measured by a hardness meter (for example Vicker's hardness meter)
with the steel sheet surface facing upward and with the sheet
thickness direction matched with the vertical direction, but to
clearly determine indentations (measure dimensions of indentations
precisely), the surface (measurement surface) has to be polished or
other certain work is necessary. In such work (for example,
mechanical polishing), at least several dozen .mu.m or so are
removed from the original surface. Further, even if removing part
of the surface using an acid etc. to chemically polish it, there is
no difference. Rather, the smoothness is often degraded. Therefore,
using such a technique to determine (measure) the hardness of the
steel sheet surface is not practical.
[0067] Therefore, the inventors decided to determine the hardness
at a cross-section parallel to the sheet thickness direction of the
steel sheet. By doing so, the steel sheet surface can be measured
without working it (without removing the steel sheet surface).
However, in this case as well, the position able to be measured by
a hardness meter in this way is inside from the surface a slight
amount in the sheet thickness direction. For this reason, as a next
best solution, the inventors attempted to obtain information on a
portion close to the surface by making an indentation by as low a
load as possible.
[0068] Specifically, refer to FIG. 3. First, the measurement
surface (steel sheet cross-section) was polished to a mirror
finish. A Vicker's hardness meter was used with a test load (load
pushing in indenter) of 10 gf, a pushing time of 15 seconds, and a
measurement position in the sheet thickness direction of 20 .mu.m
from the steel sheet surface. The "hardness of the steel sheet" as
used in the Description indicates the hardness determined based on
the above technique.
[0069] Further, the hardness of the steel sheet surface in steel
sheet which has as a surface layer of the steel sheet either an Al
plating layer, galvanized layer, and Zn--Fe alloy layer was
measured at a position 20 .mu.m from the boundary (interface)
between the plating layer and the steel sheet.
[0070] For example, the Al plating layer of the steel sheet which
is used in the examples is deemed to be comprised of an outside
layer which has Al as its main composition and an inside (steel
sheet side) layer which is believed to be a reaction layer of Al
and Fe, so the hardness was measured at a position 20 .mu.m from
the boundary of the inside layer and the steel sheet in the sheet
thickness direction and this was used as the surface hardness of
the steel sheet.
[0071] Next, the galvanized layer of the steel sheet which is used
in the examples is deemed to be comprised of two layers of an
outside layer which has Zn as its main composition and an inside
layer which is a reaction layer of Al which was added in a fine
amount in the Zn bath and Fe, so the hardness was measured at a
position 20 .mu.m from the boundary of the inside layer and the
steel sheet in the sheet thickness direction and this was used as
the surface hardness of the steel sheet.
[0072] Further, the Zn--Fe alloy layer of the steel sheet which is
used in the examples is deemed to be comprised of a plurality of
alloy layers which are comprised of Zn and Fe, so the hardness was
measured at a position 20 .mu.m from the boundary of the
inside-most layer and the steel sheet in the sheet thickness
direction and this was used as the surface hardness of the steel
sheet.
[0073] For the purpose of finding the deviation in hardness, the
above measurement was performed in the region corresponding to the
fatigue crack growth region (21) of the fatigue test piece which is
shown in FIG. 2. FIG. 3 is a perspective view which shows the
location of measurement of the hardness. The indenter of the
Vicker's hardness meter was pushed in at a position of 20 .mu.m
from the surface or the steel sheet or the interface of the steel
sheet and the plating layer in the sheet thickness direction. This
operation, as shown in FIG. 3, was performed at indentation
intervals of 0.1 mm in a direction parallel to the surface of the
steel sheet at 300 points per measurement sample (over 30 mm by
measurement length) (first measurement surface). Further, the same
operation was performed at another location 5 mm from the first
measurement surface taken in advance (second measurement
surface).
[0074] The hardnesses were found for the total 600 points in this
way. The standard deviation using this as the population was
calculated and used as an indicator of the deviation.
[0075] Note that the above measurement length of 30 mm and the two
locations 5 mm apart were determined so as to match with the crack
growth region of the fatigue test piece which is explained
later.
[0076] In the experiment which is explained in the examples,
samples with a fatigue limit ratio after hot stamping of 0.4 or
more and ones with a ratio below that were compared for deviation
in hardness of the steel sheet surface, whereupon in the former,
the standard deviation was 40 or less. Therefore, the inventors
proceeded with more detailed investigations, whereupon it became
clear that the deviation in hardness after hot stamping has a
standard deviation of 40 or less when the deviation in hardness of
the steel sheet before hot stamping, determined by a similar
technique, has a standard deviation of 20 or less.
[0077] In the present invention, the standard deviation of the
Vicker's hardness at a position 20 .mu.m from the steel sheet
surface in the sheet thickness direction was defined as 20 or less
based on such experimental findings.
[0078] Method of Production of Steel Sheet for Hot Stamped
Member
Finally, the method of production of steel sheet for a hot stamped
member of the present invention will be explained.
[0079] The steel sheet for a hot stamped member of the present
invention is processed in the accordance with the usual methods by
the steps of steelmaking, casting, hot rolling, pickling, and cold
rolling to obtain cold rolled steel sheet. The composition is
adjusted to the above-mentioned scope of the present invention in
the steelmaking step, the steel is cast to a slab in the continuous
casting step, then the slab is started to be hot rolled at for
example a 1300.degree. C. or less heating temperature. The rolling
is ended around 900.degree. C. The coiling temperature can be
selected as, for example 600.degree. C. etc. The hot rolling rate
may be made 60 to 90%. The cold rolling is performed after the
pickling step. The rolling rate can be selected from 30 to 90% in
range.
[0080] The annealing step for recrystallizing the cold rolled steel
sheet which was produced in this way is extremely important. The
annealing step is performed using a continuous annealing facility
and is comprised of two stages of a first step of heating by an
average heating rate of 8 to 25.degree. C./sec from room
temperature to the temperature M (.degree. C.) and a second stage
of then heating by an average heating rate of 1 to 7.degree. C./sec
down to a temperature S (.degree. C.). Here, the temperature M has
to be 600 to 700(.degree. C.), and the temperature S has to be 720
to 820(.degree. C.). These conditions are determined based on the
results of the experiment which is explained in the examples which
are described below.
[0081] The reason why, when recrystallization-annealing under these
conditions, the standard deviation of the Vicker's hardness which
was measured at a position of 20 .mu.m from the steel sheet surface
in the sheet thickness direction is 20 or less, that is, steel
sheet with a small deviation in hardness is obtained, is not
necessarily clear, but the distribution of crystal grain size is
preferably as uniform as possible and the dimensions and
distribution of carbides are also preferably similarly as uniform
as possible, so the following may be guessed from the viewpoint of
the distribution of recrystallized particle size and the dimensions
and distribution of carbides.
[0082] The recrystallization process of cold rolled steel sheet is
complicated, so it is not suitable to separate and independently
discuss the meanings of the heating rate for the phenomenon called
recrystallization and the highest heating temperature at that
heating rate.
[0083] Therefore, first, regarding the first stage, for example,
consider the case where the heating rate is small and where it is
large with respect to a certain single temperature M (.degree. C.).
It is believed that in the former case, that is, when the heating
rate is small, the density of recrystallization nuclei is
(relatively) low and the individual recrystallized grains freely
grow, but in the high temperature region near M (.degree. C.), fine
recrystallized grains are produced from the remaining
non-recrystallization region and, at the stage where the
temperature of the steel sheet reaches M (.degree. C.),
(relatively) large crystal grains and small crystal grains are
mixed.
[0084] On the other hand, it is believed that in the case of the
latter, that is, when the heating rate is large, the density of
recrystallized grain nuclei is high, a large number of
recrystallized grains grow at a fast rate, and the grain boundaries
become closer and further, in the high temperature region near M
(.degree. C.), the recrystallized grains compete in growth and as a
result crystal grains which have specific crystal orientations grow
while eating away at crystal grains which have other crystal
orientations, so at the stage when reaching M (.degree. C.), it is
believed there are large crystal grains and small crystal grains
mixed together. Therefore, a combination of the suitable heating
rate and M (.degree. C.) whereby the recrystallized grains become
close in grain boundaries at the stage where the temperature
reaches M (.degree. C.) becomes necessary for achieving a more
uniform distribution of recrystallized particle sizes. The 8 to
25.degree. C./sec of the average heating rate of the first stage
and the 600 to 700.degree. C. of the temperature M (.degree. C.)
are believed to correspond to these suitable conditions.
[0085] Next, to control competition of growth of recrystallized
grains after the temperature of the steel sheet reaches M (.degree.
C.), the heating rate of the second stage has to be made smaller
than the first stage. Further, in the temperature region from the
temperature M (.degree. C.) to the temperature S (.degree. C.),
reformation of carbides due to the diffusion of carbon becomes
active, so the combination of the setting of the highest
temperature S (.degree. C.) of the annealing step and the heating
rate up to that temperature has important meaning.
[0086] When the heating rate is small for one S (.degree. C.), the
carbides which were present at the temperature M (.degree. C.)
uniformly grow, so it may be that a steel sheet results in which
carbides of various dimensions which were present in the stage when
reaching the temperature M (.degree. C.) are present in various
ways. On the other hand, when the heating rate is large, small
carbides disappear and large carbides grow and therefore the
dimensions of the carbides become closer to uniform ones relatively
speaking, but the density becomes small. Therefore, unevenness of
hardness of the steel sheet is caused due to the carbides. As
opposed to these, when the combination of the heating rate and the
temperature S (.degree. C.) of the second stage is suitable, the
small carbides grow preferentially and it may be that a steel sheet
results in which relatively uniform dimension carbides are
dispersed at a suitable density, so the unevenness of hardness of
the steel sheet due to carbides becomes uneven. The 1 to 7.degree.
C./sec of the heating rate of the second stage and the 720 to
820.degree. C. of the temperature S correspond to such suitable
conditions.
[0087] After reaching the temperature S, the temperature S may be
held for a short time or the next cooling step may be immediately
shifted to. When holding the temperature S, from the viewpoint of
coarsening of the crystal grains, the holding time is preferably
180 seconds or less, more preferably 120 seconds or less.
[0088] The cooling rate from the temperature S in the cooling step
is not particularly limited, but 30.degree. C./sec or more rapid
cooling is preferably avoided. Therefore, the cooling rate from the
temperature S is less than 30.degree. C./sec, preferably 20.degree.
C. or less, more preferably 10.degree. C. or less. Steel sheet for
hot stamping use is often sheared to a predetermined shape and then
used for hot stamping. This is because it is feared that rapid
cooling raises the shear load and lowers the production
efficiency.
[0089] After annealing, the sheet may be cooled down to room
temperature. During cooling, it may be dipped in a hot dip Al bath
to form an Al plating layer.
[0090] The hot dip Al bath may contain 0.1 to 20% of Si.
[0091] The Si which is contained in the Al plating layer affects
the reaction of Al and Fe which occurs during heating before hot
stamping. Excessive reaction is liable to detract from the press
formability of the plating layer itself. On the other hand,
excessive control of the reaction is liable to invite adherence of
Al on the press forming die. To avoid such a problem, the content
of Si in the Al plating layer is preferably 1 to 15%, more
preferably 3 to 12%.
[0092] Further, during the cooling after annealing, the sheet was
dipped in a hot dip galvanization bath to form a galvanized
layer.
[0093] Furthermore, the sheet was dipped in a hot dip galvanization
bath to form a galvanized layer, then was heated to 600.degree. C.
or less to form a Zn--Fe alloy layer.
[0094] The hot dip galvanization bath could contain 0.01 to 3% of
Al.
[0095] The existence of Al has a strong affect on the reaction of
Zn and Fe. When forming a galvanized layer, the reaction layer of
the Fe and Al becomes an obstacle and suppresses mutual dispersion
of Zn and Fe. On the other hand, a Zn--Fe alloy layer is comprised
of a Zn-rich alloy layer (.zeta.-phase, .delta..sub.1-phase) and
Fe-rich alloy layer (.GAMMA..sub.1-phase, .GAMMA.-phase), but the
former is rich in adhesion with the base iron, but the workability
is degraded, while the latter is excellent in workability, but is
insufficient in adhesion. Therefore, it is necessary to suitably
control the ratio of composition of these four phases to satisfy
the targeted properties (giving preference to adhesion, giving
preference to workability, or balancing the two etc.) This can be
performed by including in the hot dip galvanization bath 0.01 to 3%
of Al so as to enable control of the diffusion of Fe. What sort of
concentration to use may be selected by the manufacturer in
accordance with the ability or objective of the production
facility.
[0096] The thicknesses of the Al plating layer, galvanized layer,
and Zn--Fe alloy layer do not influence the fatigue characteristic
of the steel sheet after hot stamping or the fatigue characteristic
of the parts, but if excessively thick, the press formability is
liable to be affected. As shown in the examples, when the thickness
of the Al plating layer is over 50 .mu.m, the phenomenon of galling
is recognized. When the thickness of the Zn plating layer exceeds
30 .mu.m, adhesion of the Zn to the die frequently occurs. When the
thickness of the Zn--Fe alloy layer is over 45 .mu.m, scattered
cracking of the alloy layer is seen, and the productivity is
otherwise impaired. Therefore, the thicknesses of the layers are
preferably made Al plating layer: 50 .mu.m or less, galvanized
layer: 30 .mu.m or less, and Zn--Fe alloy layer: 45.mu.m or
less.
[0097] When these plating layers are thin, there is no problem at
all in shapeability, but from the viewpoint of the corrosion
resistance, which is aimed at imparting these plating layers, the
lower limits of the plating layers are preferably made as follows:
That is, the limits are the Al plating layer: preferably 5 .mu.m or
more, more preferably 10 .mu.m or more, the galvanized layer:
preferably 5 .mu.m or more, more preferably 10 .mu.m or more, and
the Zn--Fe alloy layer: preferably 5 .mu.m or more, more preferably
10 .mu.m or more.
EXAMPLES
[0098] Below, examples will be used as the basis to explain the
present invention in detail.
Example 1
[0099] Steels "a" to "f" which have the composition which is shown
in Table 1 were produced and cast. The slabs were heated to
1250.degree. C. and supplied to a hot rolling step where they were
hot rolled at a final temperature of 900.degree. C. and a coiling
temperature of 600.degree. C. to obtain thickness 3.2 mm steel
sheets. These hot rolled steel sheets were pickled, then cold
rolled to obtain thickness 1.6 mm cold rolled steel sheets.
[0100] The cold rolled steel sheets were recrystallized and
annealed under the conditions of i to xviii described in Table 2 to
obtain the steel sheets for hot stamped members 1 to 32 which are
shown in Table 3. From part, two test pieces for measurement of the
hardness before hot stamping were obtained. The positions for
sampling the test pieces were made positions 5 mm separated in the
width direction of the obtained steel sheet for hot stamped
member.
[0101] The average heating rate 1 (first stage) and average heating
rate 2 (second stage) in Table 2 respectively show the average
heating rates from room temperature to temperature M (.degree. C.)
and the average heating rate from temperature M (.degree. C.) to
the temperature S (.degree. C.).
[0102] These steel sheets for hot stamped members were held at
900.degree. C. for 10 minutes, then were sandwiched by the test-use
sheet press die which was shown in FIG. 1 and hot stamped. Each
type of steel sheet for a hot stamped member was used hot stamping
10 pieces. From one among these, two tensile test pieces based on
the provisions of JIS No. 5 and two test pieces for measurement of
hardness (same procedure as with hot stamping) were obtained. From
the remaining nine, two fatigue test pieces which are shown in FIG.
2 each, for a total of 18, were obtained. The method of working for
obtain test pieces was electrodischarge machining.
[0103] A tensile test was performed to find the tensile strength
.sigma..sub.B (average value of two tensile test pieces). On the
other hand, 18 test pieces were used to run a plane bending fatigue
test and determine the 1.times.10.sup.7 cycle fatigue strength
.sigma..sub.W. The test conditions were a stress ratio of -1 and a
repetition rate of 5 Hz.
[0104] The test pieces for measurement of hardness were polished to
a mirror finish at cross-sections parallel to the rolling
directions of cold rolled steel sheets both before and after hot
stamping.
[0105] The hardness at 20 .mu.m inside from the surfaces of these
test pieces in the sheet thickness direction was measured using a
Vicker's hardness meter (HM-2000 made by Mitsutoyo). The pushing
load was made 10 gf, the pushing time was made 15 seconds, and the
measurement interval in the direction parallel to the surface made
0.1 mm for measurement of 300 points.
[0106] Two test pieces were measured in the same way. The standard
deviation of hardness was calculated from the data of the Vicker's
hardness of a total of 600 points.
[0107] Table 3 shows the steel number, processing conditions,
standard deviation of hardness before hot stamping, tensile
strength .sigma..sub.B (average of two), strength .sigma..sub.W,
fatigue limit ratio .sigma..sub.W/.sigma..sub.B, and standard of
hardness after hot stamping. The correlation between the fatigue
limit ratio .sigma..sub.W/.sigma..sub.B and the standard deviation
of hardness before hot stamping is shown in FIG. 4.
[0108] It was learned that the tensile strength .sigma..sub.B of
steel sheet after hot stamping is almost entirely unaffected by the
recrystallization-annealing conditions in steel sheet of the same
composition (code "b"). On the other hand, the fatigue
characteristics (.sigma..sub.W/.sigma..sub.B) were strongly
affected by the recrystallization-annealing conditions.
[0109] In steel sheets using the annealing conditions iii, iv, vii,
viii, xv, and xviii of the present invention, relatively high
fatigue characteristics, that is, a 0.4 or more fatigue limit ratio
(.sigma..sub.W/.sigma..sub.B), could be obtained in the range of
about 1200 to 1500 MPa in tensile strength. As opposed to this, in
steel sheets which were annealed under conditions outside the scope
of the present invention, the obtained fatigue limit ratio was a
low level of about 0.3.
[0110] This difference is due to the fact that the fatigue limit
ratio is correlated with the standard deviation of hardness after
hot stamping. Simultaneously, it clearly depends on the standard
deviation of the hardness before hot stamping. As shown in Nos. 1
to 6, 8, 9, 12, 13, 16, 17, 20, 21, and 23 to 28, it became clear
that when the standard deviation of the hardness is 2 or less, a
hot stamped member which has an excellent fatigue characteristic
(high fatigue limit ratio) is obtained.
[0111] Further, as the conditions of recrystallization-annealing
for obtaining steel sheet with a standard deviation of hardness
before hot stamping of 20 or less, there are a first stage of
heating by an average heating rate of 15 to 25.degree. C./sec from
room temperature to a temperature M (.degree. C.) and a second
stage of then heating by an average heating rate of 2 to 5.degree.
C./sec to the temperature S (.degree. C.). It became clear that M
is 620 to 680 (.degree. C.) and S is 780 to 820(.degree. C.).
TABLE-US-00001 TABLE 1 Steel no. C Si Mn P S Al N Others a 0.25 0.7
1.9 0.02 0.002 0.03 0.004 Ti: 0.03, B: 0.003 b 0.23 0.5 1.6 0.02
0.002 0.03 0.003 c 0.21 0.3 1.4 0.02 0.002 0.03 0.002 B: 0.004 d
0.20 0.2 1.2 0.02 0.002 0.03 0.004 Cr: 0.2, Ti: 0.02, B: 0.002 e
0.18 0.2 1.3 0.02 0.002 0.03 0.003 Cr: 1.4, Ti: 0.02, B: 0.002 f
0.15 0.3 1.1 0.02 0.002 0.03 0.003 Cr: 0.1, B: 0.004 Units are mass
%.
TABLE-US-00002 TABLE 2 Average Average Condition heating rate Temp.
M heating rate Temp. no. 1 (.degree. C./sec) (.degree. C.) 2
(.degree. C./sec) S (.degree. C.) Cooling conditions i 20 650 3 800
No holding. Cooling Inv. ex. by average cooling rate 6.degree.
C./sec to 670.degree. C., holding for 10 seconds, then air cooling
to room temperature. ii 25 590 3 800 No holding. Cooling Comp. ex.
by average cooling rate 6.degree. C./sec to 670.degree. C., holding
for 10 seconds, then air cooling to room temperature. iii 25 600 3
800 No holding. Cooling Inv. ex. by average cooling rate 6.degree.
C./sec to 670.degree. C., holding for 10 seconds, then air cooling
to room temperature. iv 8 700 3 800 No holding. Cooling Inv. ex. by
average cooling rate 6.degree. C./sec to 670.degree. C., holding
for 10 seconds, then air cooling to room temperature. v 8 710 3 800
No holding. Cooling Comp. ex. by average cooling rate 6.degree.
C./sec to 670.degree. C., holding for 10 seconds, then air cooling
to room temperature. vi 15 650 7 830 No holding. Cooling Comp. ex.
by average cooling rate 6.degree. C./sec to 670.degree. C., holding
for 10 seconds, then air cooling to room temperature. vii 15 650 7
820 No holding. Cooling Inv. ex. by average cooling rate 6.degree.
C./sec to 670.degree. C., holding for 10 seconds, then air cooling
to room temperature. viii 15 650 2 720 No holding. Cooling Inv. ex.
by average cooling rate 6.degree. C./sec to 670.degree. C., holding
for 10 seconds, then air cooling to room temperature. ix 15 650 2
710 No holding. Cooling Comp. ex. by average cooling rate 6.degree.
C./sec to 670.degree. C., holding for 10 seconds, then air cooling
to room temperature. x 7 600 4 800 No holding. Cooling Comp. ex. by
average cooling rate 6.degree. C./sec to 670.degree. C., holding
for 10 seconds, then air cooling to room temperature. xi 8 600 4
800 No holding. Cooling Inv. ex. by average cooling rate 6.degree.
C./sec to 670.degree. C., holding for 10 seconds, then air cooling
to room temperature. xii 25 700 3 800 No holding. Cooling Inv. ex.
by average cooling rate 6.degree. C./sec to 670.degree. C., holding
for 10 seconds, then air cooling to room temperature. xiii 26 700 3
800 No holding. Cooling Comp. ex. by average cooling rate 6.degree.
C./sec to 670.degree. C., holding for 10 seconds, then air cooling
to room temperature. xiv 20 650 0.5 720 No holding. Cooling Comp.
ex. by average cooling rate 6.degree. C./sec to 670.degree. C.,
holding for 10 seconds, then air cooling to room temperature. xv 20
650 1 720 No holding. Cooling Inv. ex. by average cooling rate
6.degree. C./sec to 670.degree. C., holding for 10 seconds, then
air cooling to room temperature. xvi 20 650 7 820 No holding.
Cooling Inv. ex. by average cooling rate 6.degree. C./sec to
670.degree. C., holding for 10 seconds, then air cooling to room
temperature. xvii 20 650 8 820 No holding. Cooling Comp. ex. by
average cooling rate 6.degree. C./sec to 670.degree. C., holding
for 10 seconds, then air cooling to room temperature. xviii 20 650
3 800 Holding for 10 Inv. ex. sec., then air cooling to room
temperature Underlined FIGURES indicate outside scope of present
invention.
TABLE-US-00003 TABLE 3 Standard Standard deviation of
.sigma..sub.W/.sigma..sub.B deviation of hardness (fatigue hardness
Steel Processing before hot .sigma..sub.B .sigma..sub.W limit after
hot No. no. conditions stamping (MPa) (MPa) ratio) stamping 1 a i
10 1510 619 0.41 27 Inv. ex. 2 b i 9 1508 603 0.40 22 Inv. ex. 3 c
i 6 1501 630 0.42 20 Inv. ex. 4 d i 8 1498 614 0.41 21 Inv. ex. 5 e
i 11 1503 646 0.43 27 Inv. ex. 6 f i 7 1422 597 0.42 24 Inv. ex. 7
b ii 30 1512 484 0.32 46 Comp. ex. 8 b iii 12 1506 602 0.40 20 Inv.
ex. 9 b iv 16 1489 610 0.41 23 Inv. ex. 10 b v 29 1502 451 0.30 42
Comp. ex. 11 b vi 24 1499 465 0.31 44 Comp. ex. 12 b vii 13 1505
647 0.43 19 Inv. ex. 13 b viii 11 1516 637 0.42 22 Inv. ex. 14 b ix
24 1511 453 0.30 43 Comp. ex. 15 b x 32 1522 502 0.33 51 Comp. ex.
16 b xi 16 1518 638 0.42 24 Inv. ex. 17 b xii 19 1512 650 0.43 26
Inv. ex. 18 b xiii 33 1507 452 0.30 49 Comp. ex. 19 b xiv 29 1500
480 0.32 46 Comp. ex. 20 b xv 12 1496 598 0.40 22 Inv. ex. 21 b xvi
11 1506 617 0.41 25 Inv. ex. 22 b xvii 27 1503 496 0.33 45 Comp.
ex. 23 a xviii 10 1510 634 0.42 19 Inv. ex. 24 b xviii 6 1512 605
0.40 12 Inv. ex. 25 c xviii 8 1503 601 0.40 14 Inv. ex. 26 d xviii
13 1509 649 0.43 24 Inv. ex. 27 e xviii 18 1499 600 0.40 27 Inv.
ex. 28 f xviii 11 1418 610 0.43 22 Inv. ex. Underlined FIGURES
indicate outside scope of present invention.
Example 2
[0112] Steels 2a to 2h which have the composition which is shown in
Table 4 were produced and cast. The slabs were hot rolled under the
same conditions as Example 1 to obtain thickness 3.0 mm steel
sheets. These hot rolled steel sheets were pickled, then cold
rolled to 1.2 mm.
[0113] These steel sheets were recrystallized and annealed under
conditions of i, ix, and xviii of Table 2 to obtain steel sheets
for hot stamped members.
[0114] From these steel sheets, test pieces for measurement of
hardness were obtained by the same procedure was in Example 1.
[0115] These steel sheets for a hot stamped member were held at
900.degree. C. for 5 minutes, then were formed to hat shapes which
are shown in FIG. 5 by the hot stamping method. As shown in this
figure, fatigue test pieces which are shown in FIG. 2 and JIS No. 5
tensile test pieces were obtained from the top parts of the
hats.
[0116] These test pieces were used by the same procedure as in
Example 1 to find the standard deviation of hardness before hot
stamping and the tensile strength .sigma..sub.B (average of two)
and 1.times.10.sup.7 cycle fatigue strength .sigma..sub.W of the
steel sheet after hot stamping (member).
[0117] Table 5 should these results. The correlation between the
fatigue limit ratio .sigma..sub.W/.sigma..sub.B and the standard
deviation of the hardness before hot stamping is shown in FIG.
6.
[0118] In steel sheets for a hot stamped member which were
recrystallized and annealed using conditions i and xviii in the
scope of the present invention, even if steel sheets which contain
Mo, W, V, Cu, and Ni, the deviation in hardness of the surface
layer before hot stamping had a standard deviation of 20 or less.
Further, if using these, it became clear that a hot stamped member
with a fatigue limit ratio of 0.4 or more, that is, excellent in
fatigue characteristic, was obtained.
[0119] On the other hand, in steel sheets which were recrystallized
and annealed using the condition ix which is outside the scope of
the present invention, the deviation in hardness of the surface
layer before hot stamping has a standard deviation of over 20. The
fatigue limit ratio of the hot stamped members obtained by using
these was 0.26 to 0.31. It became clear the fatigue characteristic
was inferior.
TABLE-US-00004 TABLE 4 Steel Composition (mass %) no. C Si Mn P S
Al N Others 2a 0.35 0.3 1.0 0.02 0.004 0.03 0.004 Cr: 0.2, Ti:
0.01, B: 0.002, Cu: 0.1, Ni: 0.1 2b 0.31 0.5 1.2 0.02 0.004 0.03
0.004 Cr: 0.5, Ti: 0.02, B: 0.004, Nb: 0.02, Mo: 0.2 2C 0.28 1.0
1.7 0.02 0.004 0.03 0.004 W: 0.2, Ni: 2.0 2d 0.25 0.8 1.9 0.02
0.004 0.03 0.004 Ti: 0.03, B: 0.003, Mo: 0.2, Ni: 1.0 2e 0.23 0.6
1.6 0.02 0.004 0.03 0.003 Mo: 0.1, W: 0.5, V: 0.5 2f 0.21 0.4 1.4
0.02 0.004 0.03 0.002 B: 0.004, Mo: 0.1, V: 0.5 2g 0.20 0.3 1.2
0.02 0.004 0.03 0.004 Cr: 0.2, Ti: 0.02, Mo: 0.2, W: 0.4 2h 0.18
0.3 1.3 0.02 0.004 0.03 0.003 Cr: 1.4, Ti: 0.02, B: 0.002, Mo: 0.1,
V: 0.2
TABLE-US-00005 TABLE 5 Standard deviation of hardness
.sigma..sub.W/.sigma..sub.B Steel Processing before hot
.sigma..sub.B (fatigue limit No. no. conditions stamping (MPa)
.sigma..sub.W (MPa) ratio) 29 2a i 18 1794 718 0.40 Inv. ex. 30 2a
ix 40 1790 465 0.26 Comp. ex. 31 2a xviii 19 1802 721 0.40 Inv. ex.
32 2b i 16 1706 682 0.40 Inv. ex. 33 2b ix 37 1696 441 0.26 Comp.
ex. 34 2b xviii 18 1711 702 0.41 Inv. ex. 35 2C i 15 1598 639 0.40
Inv. ex. 36 2C ix 30 1592 430 0.27 Comp. ex. 37 2C xviii 14 1590
636 0.40 Inv. ex. 38 2d i 15 1492 612 0.41 Inv. ex. 39 2d ix 26
1500 435 0.29 Comp. ex. 40 2d xviii 5 1498 614 0.41 Inv. ex. 41 2e
i 9 1492 597 0.4 Inv. ex. 42 2e ix 31 1502 421 0.28 Comp. ex. 43 2e
xviii 10 1516 622 0.41 Inv. ex. 44 2f i 12 1508 603 0.4 Inv. ex. 45
2f ix 36 1512 469 0.31 Comp. ex. 46 2f xviii 19 1522 609 0.4 Inv.
ex. 47 2g i 14 1496 613 0.41 Inv. ex. 48 2g ix 33 1504 406 0.27
Comp. ex. 49 2g xviii 13 1526 641 0.42 Inv. ex. 50 2h i 14 1506 602
0.4 Inv. ex. 51 2h ix 32 1512 454 0.3 Comp. ex. 52 2h xviii 15 1528
642 0.42 Inv. ex. Underlined FIGURES indicate outside scope of
present invention.
Example 3
[0120] Steels 3a to 3d which have the composition which is shown in
Table 6 were produced and cast. The slabs were hot rolled under the
same conditions as Example 1 to obtain thickness 2.5 mm steel
sheets. These hot rolled steel sheets were pickled, then cold
rolled to 1.2 mm.
[0121] These steel sheets were heated by an average heating rate of
19.degree. C./sec up to 655.degree. C., then were heated by an
average heating rate of 2.5.degree. C. to 800.degree. C., then were
immediately cooled by an average cooling rate of 6.5.degree.
C./sec. Further, they were dipped in a 670.degree. C. hot dip Al
bath (containing 10% of Si and unavoidable impurities), taken out
after 5 seconds, adjusted in amount of deposition by a gas wiper,
then air cooled down to room temperature.
[0122] From the obtained steel sheets, the same procedure as in
Example 1 was used to obtain test pieces for measurement of
hardness. To measure the hardness, the hardness at a position 20
.mu.m from the boundary of the inside layer of the Al plating layer
(reaction layer of Al and Fe) and the steel sheet was measured by
the same procedure as in Example 1. At the time of this
measurement, the thickness of the Al plating layer (total of two
layers) was also measured. The range of measurement of thickness
was made the same length 30 mm as the range of measurement of
hardness. Seven points were measured at measurement intervals of 5
mm at each of the first measurement surface and second measurement
surface for a total of 14 measurement positions. The average value
was found.
[0123] These steel sheets were hot stamped into hat shapes by the
same procedure as in Example 2. The heating conditions were holding
at 900.degree. C. for 1 minute.
[0124] From the top parts of the hats, fatigue test pieces which
are shown in FIG. 2 and JIS No. 5 tensile test pieces were
obtained.
[0125] These test pieces were used to find the tensile strength
.sigma..sub.B (average of two) and 1.times.10.sup.7 cycle fatigue
strength .sigma..sub.W. Table 7 shows the results.
[0126] In all examples, excellent steel sheet for a hot stamped
member with a fatigue limit ratio of 0.4 or more was obtained, but
in Nos. 57, 62, 67, and 72 where the thickness of the Al plating
layer exceeded 50 .mu.m, a galling phenomenon occurred at a high
frequency at the long wall parts of the hat shape. In examples of
50 .mu.m or less, no galling phenomenon occurred at all. Therefore,
it was judged that the upper limit of thickness when Al plating the
steel sheet surface is 50 .mu.m or less.
TABLE-US-00006 TABLE 6 Steel no. C Si Mn P S Al N Others 3a 0.33
0.09 1.8 0.01 0.004 0.04 0.003 Cr: 0.2, Mo: 0.2, Cu: 0.1, Ni: 0.05
3b 0.25 0.18 1.4 0.01 0.004 0.04 0.003 Cr: 0.002, Ti: 0.02. B:
0.003, Mo: 0.2, W: 0.1, V: 0.1 3C 0.22 0.12 1.3 0.02 0.008 0.03
0.004 Cr: 0.13, Ti: 0.03, Nb: 0.02, B: 0.002 3d 0.15 0.33 1.0 0.02
0.008 0.03 0.004 B: 0.0005 Units are mass %.
TABLE-US-00007 TABLE 7 Standard deviation
.sigma..sub.W/.sigma..sub.B of hardness (fatigue Thickness of Steel
before hot .sigma..sub.B .sigma..sub.W limit Al plating No. no.
stamping (MPa) (MPa) ratio) layer (.mu.m) 53 3a 17 1784 714 0.40
16.0 Inv. ex. 54 3a 18 1789 716 0.40 22.2 Inv. ex. 55 3a 16 1801
720 0.40 33.9 Inv. ex. 56 3a 14 1792 717 0.40 48.6 Inv. ex. 57 3a
14 1790 716 0.40 51.0 Comp. ex. 58 3b 12 1516 652 0.43 15.1 Inv.
ex. 59 3b 15 1520 638 0.42 19.6 Inv. ex. 60 3b 19 1524 671 0.44
34.2 Inv. ex. 61 3b 18 1522 685 0.45 49.6 Inv. ex. 62 3b 20 1534
614 0.40 54.7 Comp. ex. 63 3C 11 1502 631 0.42 14.5 Inv. ex. 64 3C
14 1509 649 0.43 20.1 Inv. ex. 65 3C 9 1513 635 0.42 34.6 Inv. ex.
66 3C 13 1519 668 0.44 49.2 Inv. ex. 67 3C 18 1524 610 0.40 55.3
Comp. ex. 68 3d 10 1318 554 0.42 17.2 Inv. ex. 69 3d 10 1326 557
0.42 20.4 Inv. ex. 70 3d 8 1320 554 0.42 30.2 Inv. ex. 71 3d 14
1314 539 0.41 42.0 Inv. ex. 72 3d 15 1310 537 0.41 53.6 Comp. ex.
Underlined FIGURES indicate outside scope of present invention.
Example 4
[0127] Steels 3a to 3d which have the composition which is shown in
Table 6 were produced and cast. The slabs were hot rolled under the
same conditions as Example 1 to obtain thickness 2.5 mm steel
sheets. These hot rolled steel sheets were pickled, then cold
rolled to 1.2 mm.
[0128] These steel sheets were heated by an average heating rate of
19.degree. C./sec up to 655.degree. C., then were heated by an
average heating rate of 2.5.degree. C. to 800.degree. C., then were
immediately cooled by an average cooling rate of 6.5.degree.
C./sec. Further, they were dipped in a 460.degree. C. hot dip
galvanization bath (containing 0.15% of Al and unavoidable
impurities), taken out after 3 seconds, adjusted in amount of
deposition by a gas wiper, then air cooled down to room
temperature.
[0129] From the obtained steel sheets, the same procedure as in
Example 1 was used to obtain test pieces for measurement of
hardness. To measure the hardness, the hardness at a position 20
.mu.m from the boundary of the inside layer of the Zn plating layer
(reaction layer of Al and Fe) and the steel sheet was measured by
the same procedure as in Example 1. At the time of this
measurement, the thickness of only the Zn plating layer may also be
measured. The range of measurement of thickness was made the same
length 30 mm as the range of measurement of hardness. Seven points
were measured at measurement intervals of 5 mm at each of the first
measurement surface and second measurement surface for a total of
14 measurement positions. The average value was found.
[0130] These steel sheets were hot stamped into hat shapes by the
same procedure as in Example 2. They were heated to 880.degree. C.
and held for 5 seconds, then air-cooled down to 700.degree. C. and
pressed.
[0131] From the top parts of the hats, fatigue test pieces which
are shown in FIG. 2 and JIS No. 5 tensile test pieces were
obtained.
[0132] These test pieces were used to find the tensile strength
.sigma..sub.B (average of two) and 1.times.10.sup.7 cycle fatigue
strength .sigma..sub.W. Table 8 shows the results.
[0133] In all examples, excellent steel sheet for a hot stamped
member with a fatigue limit ratio of 0.4 or more was obtained, but
in Nos. 77, 82, 87, and 92 where the thickness of the galvanized
layer exceeded 30 .mu.m, adhesion of Zn was observed at a high
frequency in the die. In examples of 30 .mu.m or less, no adhesion
of Zn occurred at all. Therefore, it was judged that the upper
limit of thickness when galvanizing the steel sheet surface is 30
.mu.m or less.
TABLE-US-00008 TABLE 8 Stan- dard devia- tion of hard- ness before
.sigma..sub.W/.sigma..sub.B Thickness hot (fatigue of Steel stamp-
.sigma..sub.B .sigma..sub.W limit galvanized No. no. ing (MPa)
(MPa) ratio) layer (.mu.m) 73 3a 17 1785 714 0.40 6.1 Inv. ex. 74
3a 17 1788 715 0.40 12.5 Inv. ex. 75 3a 16 1802 721 0.40 23.8 Inv.
ex. 76 3a 13 1794 718 0.40 28.6 Inv. ex. 77 3a 15 1793 717 0.40
31.0 Comp. ex. 78 3b 12 1516 652 0.43 11.1 Inv. ex. 79 3b 15 1522
639 0.42 19.6 Inv. ex. 80 3b 19 1534 675 0.44 24.8 Inv. ex. 81 3b
18 1532 689 0.45 29.0 Inv. ex. 82 3b 20 1545 618 0.40 33.7 Comp.
ex. 83 3c 10 1518 638 0.42 10.3 Inv. ex. 84 3c 14 1536 660 0.43
17.2 Inv. ex. 85 3c 9 1524 640 0.42 19.6 Inv. ex. 86 3c 14 1539 677
0.44 29.3 Inv. ex. 87 3c 18 1544 618 0.40 32.3 Comp. ex. 88 3d 10
1336 561 0.42 11.2 Inv. ex. 89 3d 12 1342 564 0.42 17.4 Inv. ex. 90
3d 8 1318 554 0.42 20.2 Inv. ex. 91 3d 13 1320 541 0.41 28.0 Inv.
ex. 92 3d 15 1330 545 0.41 33.4 Comp. ex. Underlined FIGURES
indicate outside scope of present invention.
Example 5
[0134] Steels 3a to 3d which have the composition which is shown in
Table 6 were produced and cast. The slabs were hot rolled under the
same conditions as Example 1 to obtain thickness 2.5 mm steel
sheets. These hot rolled steel sheets were pickled, then cold
rolled to 1.2 mm.
[0135] These steel sheets were heated by an average heating rate of
19.degree. C./sec up to 655.degree. C., then were heated by an
average heating rate of 2.5.degree. C. to 800.degree. C., then were
immediately cooled by an average cooling rate of 6.5.degree.
C./sec. Further, they were dipped in a 460.degree. C. hot dip
galvanization bath (containing 0.13% of Al, 0.03% of Fe, and
unavoidable impurities), taken out after 3 seconds, adjusted in
amount of deposition by a gas wiper, then heated to 480.degree. C.
to form an Zn--Fe alloy layer, then air cooled down to room
temperature.
[0136] From the obtained steel sheets, the same procedure as in
Example 1 was used to obtain test pieces for measurement of
hardness. To measure the hardness, the hardness at a position 20
.mu.m from the boundary of the inner-most layer of the Zn--Fe alloy
layer (reaction layer of Zn and Fe) and the steel sheet was
measured by the same procedure as in Example 1. At the time of this
measurement, the total thickness of the Zn--Fe alloy layer (which
was comprised of four layers) was also measured. At the time of
this measurement, the thickness of the Al plating layer (total of
two layers) was also measured. The range of measurement of
thickness was made the same length 30 mm as the range of
measurement of hardness. Seven points were measured at measurement
intervals of 5 mm at each of the first measurement surface and
second measurement surface for a total of 14 measurement positions.
The average value was found.
[0137] These steel sheets were hot stamped into hat shapes by the
same procedure as in Example 2. They were heated to 880.degree. C.
and held for 5 seconds, then air-cooled down to 700.degree. C. and
pressed.
[0138] From the top parts of the hats, fatigue test pieces which
are shown in FIG. 2 and JIS No. 5 tensile test pieces were
obtained.
[0139] These test pieces were used to find the tensile strength
.sigma..sub.B (average of two) and 1.times.10.sup.7 cycle fatigue
strength .sigma..sub.W. Table 9 shows the results.
[0140] In all examples, excellent steel sheet for a hot stamped
member with a fatigue limit ratio of 0.4 or more was obtained, but
in Nos. 97, 102, 107, and 112 where the thickness of the Zn--Fe
alloy layer exceeded 45 .mu.m, fine cracks occurred in the alloy
layer after pressing. In examples of 45 .mu.m or less, no fine
cracks formed at all. Therefore, it was judged that the upper limit
of thickness when forming a Zn--Fe alloy layer on the steel sheet
surface is 45 .mu.m or less.
TABLE-US-00009 TABLE 9 Standard deviation
.sigma..sub.W/.sigma..sub.B of hardness (fatigue Thickness of Steel
before hot .sigma..sub.B .sigma..sub.W limit Zn--Fe alloy No. no.
stamping (MPa) (MPa) ratio) layer (.mu.m) 93 3a 17 1773 727 0.41
15.0 Inv. ex. 94 3a 16 1777 711 0.40 22.2 Inv. ex. 95 3a 17 1802
739 0.41 31.5 Inv. ex. 96 3a 14 1786 714 0.40 39.9 Inv. ex. 97 3a
13 1772 709 0.40 46.0 Comp. ex. 98 3b 12 1505 632 0.42 15.7 Inv.
ex. 99 3b 18 1519 638 0.42 21.6 Inv. ex. 100 3b 19 1513 651 0.43
39.2 Inv. ex. 101 3b 18 1502 661 0.44 44.6 Inv. ex. 102 3b 14 1518
622 0.41 49.7 Comp. ex. 103 3C 11 1506 633 0.42 14.5 Inv. ex. 104
3C 14 1503 646 0.43 20.8 Inv. ex. 105 3C 9 1500 645 0.43 34.6 Inv.
ex. 106 3C 12 1506 633 0.42 42.2 Inv. ex. 107 3C 19 1510 619 0.41
45.3 Comp. ex. 108 3d 17 1307 523 0.40 15.2 Inv. ex. 109 3d 11 1313
551 0.42 18.4 Inv. ex. 110 3d 8 1320 554 0.42 30.6 Inv. ex. 111 3d
14 1314 539 0.41 42.9 Inv. ex. 112 3d 15 1310 537 0.41 48.6 Comp.
ex. Underlined FIGURES indicate outside scope of present
invention.
REFERENCE SIGNS LIST
[0141] 11a top die [0142] 11b bottom die [0143] 12 steel sheet
[0144] 21 fatigue crack growth region [0145] 51 test piece sampling
position
* * * * *