n -* AA516
- 1 -
DESCRIPTION
Title of Invention: Steel Sheet for Hot Stamped Member
and Method of Production of Same
5
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
10 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
15 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
20 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
25 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
30 (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"
35 has come under attention. This heats a steel sheet
(worked material) to a predetermined temperature
(generally, the temperature resulting in an austenite
- 2 -
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
5 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
10 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
15 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
20 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.
25 [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
30 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
35 characteristic is an important, necessary characteristic.
[0010] The fatigue characteristic of steel sheet is
improved together with the static strength, so steel
- 3 -
^^ 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
5 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
10 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
15 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
20 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
25 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
30 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.
35 [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
- 4 -
1^ 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
5 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
10 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.
15 [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
20 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
25 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
30 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
35 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
- 5 -
II 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
5 been sought, but no art which solves this problem has
been found.
Citations List
Patent Literature
[0020] PLT 1: Japanese Patent Publication No. 2005-
10 139485A
PLT 2: Japanese Patent Publication No. 2005-200670A
PLT 3: Japanese Patent Publication No. 2005-205477A
PLT 4: Japanese Patent Publication No. 2007-247001A
PLT 5: Japanese Patent Publication No. 2007-182608A
15 Summary of Invention
Technical Problem
[0021] 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
20 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
25 producing a product by applying the hot stamping method
to steel sheet and of a method of production of the same.
Solution to Problem
[0022] The inventors engaged in intensive research to
solve this problem. As a result, they discovered that
30 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
35 obtained by controlling the conditions when
recrystallization-annealing the cold rolled steel sheet,
conducted repeated tests, and thereby completed the
- 6
^p present invention.
[0023] The gist of the invention is as follows:
[0024] (1) Steel sheet for a hot stamped member which
includes composition which contains, by mass%,
5 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
10 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
15 position of 20 |^m from the steel sheet surface in the
sheet thickness direction is 20 or less.
[0025] (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
20 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%
25 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%.
[0026] (3) The steel sheet for a hot stamped member as
30 set forth in (1) or (2) which has on the surface of the
steel sheet one of a 5 jjin to 50 |jm thick Al plating
layer, a 5 j^m to 30 |Jin thick galvanized layer, or a 5 |j.m
to 45 i^m thick Zn-Fe alloy layer.
[0027] (4) A method of production of steel sheet for a
35 hot stamped member comprising recrystallization-annealing
cold rolled steel sheet which includes composition which
7 -
# contains, by mass%,
C: 0.15 to 0.35%,
Si: 0.01 to 1.0%,
Mn: 0.3 to 2.3%,
5 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
10 N: 0.1% or less,
in which step, including
a first stage of heating by an average heating rate of 8
to 25°C/sec from room temperature to a temperature M (°C)
and
15 then a second stage of heating by an average heating rate
of 1 to 7°C/sec to a temperature S (°C) ,
wherein the temperature M (°C) is 600 to 700 (°C) and
the temperature S (°C) is 720 to 820 (°C) .
[0028] (5) The method of production of steel sheet for
20 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%
25 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
30 Ni: 0.01 to 5.0%.
[0029] (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
35 to 90%.
[0030] (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 recrystallizationannealing
step, a step of dipping the steel sheet in an
Al bath to form an Al plating layer on the surface.
5 [0031] (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 recrystallizationannealing
step, a step of dipping the steel sheet in a
galvanization bath to form a galvanized layer on the
10 surface.
[0032] (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 recrystallizationannealing
step, a step of dipping the steel sheet in a Zn
15 bath to form a galvanized layer on the surface, then
further heating to 600°C or less to form a Zn-Fe alloy
layer on the surface.
Advantageous Effects of Invention
[0033] The steel sheet for a hot stamped member of the
20 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
25 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
30 [0034] FIG. 1 is perspective view which shows a sheet
press die for hot stamping which is used for the examples
of the present invention.
FIG. 2 is a view which shows fatigue test pieces.
FIG. 3 is a perspective view which shows locations of
35 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
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^ FIG. 2.
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
5 member of Example 1.
FIG. 5 is a perspective view which schematically shows
steel sheet (member) which is formed into a hat shape by
the hot stamping method.
FIG. 6 is a graph which shows the correlation between the
10 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
[0035] The inventors engaged in research using steel
15 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
20 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
25 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
30 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
35 within a predetermined range and that further to obtain
such steel sheet, it is possible to control the
conditions when recrystallization-annealing cold rolled
- 10 -
steel sheet to a predetermined range.
[0036] 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
5 hardness and the annealing conditions and thereby
completed the present invention.
[0037] Composition of Steel Sheet
First, the composition of steel sheet will be explained.
Here, the "%" in the composition mean mass%.
10 [0038] 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
15 included, deterioration of toughness is a concern, so
0.35% is made the upper limit.
[0039] 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
20 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
25 raises the steelmaking load, so the content is made the
level of inclusion due to deoxidation, that is, 0.01% or
more.
[0040] Mn: 0.3 to 2.3%
Mn is an element which functions as a solution
30 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.
35 [0041] 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
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jlf above r a n g e s.
[0042] Al: 0.01 t o 0.5%
Al is suitable as a deoxidizing element, so 0.01% or more
should be included. However, if included in a large
5 amount, coarse oxides are formed and the mechanical
properties of the steel sheet are impaired, so the upper
limit is made 0.5%.
[0043] N: 0.1% or less
N is an unavoidable impurity. It easily bonds with Ti or
10 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
15 be made the target for the lower limit.
[0044] 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
20 effect becomes saturated, so 2.0% is made the upper
limit.
[0045] 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
25 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.
30 [0046] 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
35 liable to be impaired, so 0.5% is made the upper limit.
[0047] B: 0.0005 to 0.01%
B is an element which raises the hardenability. The
- 12 -
« 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.
5 [0048] 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
10 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%.
[0049] Cu: 0.01 to 1.0%
15 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.
20 [0050] 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
25 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.
[0051] Note that in the present invention, the
30 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.
[0052] Deviations in Steel Sheet Surface Hardness
35 The deviations in steel sheet surface hardness will be
explained.
[0053] First, the method of determining (measuring)
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^ the hardness of the steel sheet surface will be
explained.
[0054] The hardness of the steel sheet surface ideally
should be measured by a hardness meter (for example
5 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
10 surface) has to be polished or other certain work is
necessary. In such work (for example, mechanical
polishing) , at least several dozen f^m or so are removed
from the original surface. Further, even if removing part
of the surface using an acid etc. to chemically polish
15 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.
[0055] Therefore, the inventors decided to determine
20 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
25 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.
30 [0056] 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
35 position in the sheet thickness direction of 20 |am from
the steel sheet surface. The "hardness of the steel
sheet" as used in the Description indicates the hardness
- 14 -
y determined based on the above technique.
[0057] 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
5 layer, and Zn-Fe alloy layer was measured at a position
20 |Jin from the boundary (interface) between the plating
layer and the steel sheet.
[0058] For example, the Al plating layer of the steel
sheet which is used in the examples is deemed to be
10 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 )xm from the
boundary of the inside layer and the steel sheet in the
15 sheet thickness direction and this was used as the
surface hardness of the steel sheet.
[0059] 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
20 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 |am from the boundary of the inside layer and the steel
sheet in the sheet thickness direction and this was used
25 as the surface hardness of the steel sheet.
[0060] 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
30 position 20 )Jin 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.
[0061] For the purpose of finding the deviation in
hardness, the above measurement was performed in the
35 region corresponding to the fatigue crack growth region
(21) of the fatigue test piece which is shown in FIG. 2.
- 15 -
3^ 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 |j,m from
the surface or the steel sheet or the interface of the
5 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
10 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).
[0062] The hardnesses were found for the total 600
15 points in this way. The standard deviation using this as
the population was calculated and used as an indicator of
the deviation.
[0063] Note that the above measurement length of 30 mm
and the two locations 5 mm apart were determined so as to
20 match with the crack growth region of the fatigue test
piece which is explained later.
[0064] 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
25 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
30 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 technigue, has a
standard deviation of 20 or less.
[0065] In the present invention, the standard
35 deviation of the Vicker's hardness at a position 20 |^m
from the steel sheet surface in the sheet thickness
direction was defined as 20 or less based on such
- 16 -
J^ experimental findings.
[0066] Method of Production of Steel Sheet for Hot
Stamped Member
Finally, the method of production of steel sheet for a
5 hot stamped member of the present invention will be
explained.
[0067] 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
10 rolling, pickling, and cold rolling to obtain cold rolled
steel sheet. The composition is adjusted to the abovementioned
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
15 hot rolled at for example a 1300°C or less heating
temperature. The rolling is ended around 900°C. The
coiling temperature can be selected as, for example 600°C
etc. The hot rolling rate may be made 60 to 90%. The cold
rolling is performed after the pickling step. The rolling
20 rate can be selected from 30 to 90% in range.
[0068] 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
25 two stages of a first step of heating by an average
heating rate of 8 to 25°C/sec from room temperature to the
temperature M (°C) and a second stage of then heating by
an average heating rate of 1 to 7°C/sec down to a
temperature S (°C) . Here, the temperature M has to be 600
30 to 700 (°C), and the temperature S has to be 720 to
820 (°C) . These conditions are determined based on the
results of the experiment which is explained in the
examples which are described below.
[0069] The reason why, when recrystallization-
35 annealing under these conditions, the standard deviation
of the Vicker's hardness which was measured at a position
- 17
W of 20 \xm 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
5 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
10 dimensions and distribution of carbides.
[0070] 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
15 and the highest heating temperature at that heating rate.
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 (°C) . It is believed that in the former
20 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 (°C), fine
recrystallized grains are produced from the remaining
25 non-recrystallization region and, at the stage where the
temperature of the steel sheet reaches M(°C), (relatively)
large crystal grains and small crystal grains are mixed.
[0071] On the other hand, it is believed that in the
case of the latter, that is, when the heating rate is
30 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 (°C), the
recrystallized grains compete in growth and as a result
35 crystal grains which have specific crystal orientations
grow while eating away at crystal grains which have other
- 18 -
yr crystal orientations, so at the stage when reaching M
(°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 (°C)
5 whereby the recrystallized grains become close in grain
boundaries at the stage where the temperature reaches M
(°C) becomes necessary for achieving a more uniform
distribution of recrystallized particle sizes. The 8 to
25°C/sec of the average heating rate of the first stage
10 and the 600 to 700°C of the temperature M (°C) are
believed to correspond to these suitable conditions.
[0072] Next, to control competition of growth of
recrystallized grains after the temperature of the steel
sheet reaches M (°C), the heating rate of the second stage
15 has to be made smaller than the first stage. Further, in
the temperature region from the temperature M (°C) to the
temperature S (°C), reformation of carbides due to the
diffusion of carbon becomes active, so the combination of
the setting of the highest temperature S (°C) of the
20 annealing step and the heating rate up to that
temperature has important meaning.
[0073] When the heating rate is small for one S (°C) ,
the carbides which were present at the temperature M (°C)
uniformly grow, so it may be that a steel sheet results
25 in which carbides of various dimensions which were
present in the stage when reaching the temperature M (°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
30 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 (°C)
35 of the second stage is suitable, the small carbides grow
- 19 -
M 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
5 uneven. The 1 to 7°C/sec of the heating rate of the second
stage and the 720 to 820°C of the temperature S (°C)
correspond to such suitable conditions.
[0074] After reaching the temperature S, the
temperature S may be held for a short time or the next
10 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,
[0075] The cooling rate from the temperature S in the
15 cooling step is not particularly limited, but 30°C/sec or
more rapid cooling is preferably avoided. Therefore, the
cooling rate from the temperature S is less than 30°C/sec,
preferably 20°C or less, more preferably 10°C or less.
Steel sheet for hot stamping use is often sheared to a
20 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.
[0076] After annealing, the sheet may be cooled down
to room temperature. During cooling, it may be dipped in
25 a hot dip Al bath to form an Al plating layer.
[0077] The hot dip Al bath may contain 0.1 to 20% of
Si.
[0078] The Si which is contained in the Al plating
layer affects the reaction of Al and Fe which occurs
30 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
35 content of Si in the Al plating layer is preferably 1 to
15%, more preferably 3 to 12%.
- 20 -
'^g [0079] Further, during the cooling after annealing,
the sheet was dipped in a hot dip galvanization bath to
form a galvanized layer.
[0080] Furthermore, the sheet was dipped in a hot dip
5 galvanization bath to form a galvanized layer, then was
heated to 600°C or less to form a Zn-Fe alloy layer.
[0081] The hot dip galvanization bath could contain
0.01 to 3% of Al.
[0082] The existence of Al has a strong affect on the
10 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 (^-phase, 6i-phase) and Fe-rich alloy layer
15 (Fi-phase, F-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
20 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
25 sort of concentration to use may be selected by the
manufacturer in accordance with the ability or objective
of the production facility.
[0083] The thicknesses of the Al plating layer,
galvanized layer, and Zn-Fe alloy layer do not influence
30 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 (im, the phenomenon of
35 galling is recognized. When the thickness of the Zn
plating layer exceeds 30 [im, adhesion of the Zn to the
- 21 -
^J die frequently occurs. When the thickness of the Zn-Fe
alloy layer is over 45 )Jin, scattered cracking of the
alloy layer is seen, and the productivity is otherwise
impaired. Therefore, the thicknesses of the layers are
5 preferably made Al plating layer: 50 )im or less,
galvanized layer: 30 fom or less, and Zn-Fe alloy layer:
45 |im or less.
[0084] When these plating layers are thin, there is no
problem at all in shapeability, but from the viewpoint of
10 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 )Lim or more,
more preferably 10 |Ltm or more, the galvanized layer:
15 preferably 5 |am or more, more preferably 10 ^m or more,
and the Zn-Fe alloy layer: preferably 5 |am or more, more
preferably 10 (xm or more.
Examples
[0085] Below, examples will be used as the basis to
20 explain the present invention in detail.
Example 1
Steels "a" to "f" which have the composition which is
shown in Table 1 were produced and cast. The slabs were
25 heated to 1250°C and supplied to a hot rolling step where
they were hot rolled at a final temperature of 900°C and a
coiling temperature of 600°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
30 steel sheets.
[0086] 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.
35 From part, two test pieces for measurement of the
- 22 -
y 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.
5 [0087] 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 (°C) and the average heating
rate from temperature M (°C) to the temperature S (°C).
10 [0088] These steel sheets for hot stamped members were
held at 900°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
15 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.
20 The method of working for obtain test pieces was
electrodischarge machining.
[0089] A tensile test was performed to find the
tensile strength OB (average value of two tensile test
pieces). On the other hand, 18 test pieces were used to
25 run a plane bending fatigue test and determine the 1x10^
cycle fatigue strength aw. The test conditions were a
stress ratio of -1 and a repetition rate of 5Hz.
[0090] The test pieces for measurement of hardness
were polished to a mirror finish at cross-sections
30 parallel to the rolling directions of cold rolled steel
sheets both before and after hot stamping.
[0091] The hardness at 20 |am 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
35 Mitsutoyo). The pushing load was made 10 gf, the pushing
time was made 15 seconds, and the measurement interval in
- 23 -
y the direction parallel to the surface made 0.1 mm for
measurement of 300 points.
[0092] Two test pieces were measured in the same way.
The standard deviation of hardness was calculated from
5 the data of the Vicker's hardness of a total of 600
points.
[0093] Table 3 shows the steel number, processing
conditions, standard deviation of hardness before hot
stamping, tensile strength a^ (average of two), strength
10 Gw, fatigue limit ratio (JW/CJBA and standard of hardness
after hot stamping. The correlation between the fatigue
limit ratio O^/GB and the standard deviation of hardness
before hot stamping is shown in FIG. 4.
[0094] It was learned that the tensile strength QB of
15 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 {OW/OB) were
strongly affected by the recrystallization-annealing
20 conditions.
[0095] In steel sheets using the annealing conditions
i, ill, iv, vii, viii, xv, and xviii of the present
invention, relatively high fatigue characteristics, that
is, a 0.4 or more fatigue limit ratio (o^^/cfB) r could be
25 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.
30 [0096] 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,
35 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
- 24 -
^C less, a hot stamped member which has an excellent fatigue
characteristic (high fatigue limit ratio) is obtained.
[0097] Further, as the conditions of
recrystallization-annealing for obtaining steel sheet
5 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°C/sec from room
temperature to a temperature M (°C) and a second stage of
then heating by an average heating rate of 2 to 5°C/sec to
10 the temperature S (°C) . It became clear that M is 620 to
680 (°C) and S is 780 to 820 (°C) .
[0098] Table 1
steel
no.
a
b
c
d
e
f
C
0.25
0.23
0.21
0.20
0.18
0.15
Si
0.7
0.5
0.3
0.2
0.2
0.3
Mn
1.9
1.6
1.4
1.2
1.3
1.1
P
0.02
0.02
0.02
0.02
0.02
0.02
S
0.002
0.002
0.002
0.002
0.002
0.002
Al
0.03
0.03
0.03
0.03
0.03
0.03
N
0.004
0.003
0.002
0.004
0.003
0.003
Others
Ti : 0.03, B: 0.003
B: 0.004
Cr: 0.2, Ti: 0.02, B: 0.002
Cr: 1.4, Ti: 0.02, B: 0.002
Cr: 0.1, B: 0.004
Units are mass%.
- 25
V [0099] Table 2
Condition
no.
i
ii
iii
iv
V
vi
vii
viii
ix
X
xi
xii
xiii
xiv
XV
xvi
xvii
xviii
Average
heating rate
1 (°C/sec)
20
25
25
8
8
15
15
15
15
7
8
25
26
20
20
20
20
20
Temp.
M
(°C)
650
590
600
700
710
650
650
650
650
600
600
700
700
650
650
650
650
650
Average
heating rate
2 (°C/sec)
3
3
3
3
3
7
7
2
2
4
4
3
3
0.5
1
7
8
3
Temp.
S (°C)
800
800
800
800
800
830
820
720
710
800
800
800
800
720
720
820
820
800
Cooling conditions
No holding. Cooling
by average cooling
rate 6°C/sec to
670°C, holding for
10 seconds, then
air cooling to room
temperature.
Ditto
Ditto
Ditto
Ditto
Ditto
Ditto
Ditto
Ditto
Ditto
Ditto
Ditto
Ditto
Ditto
Ditto
Ditto
Ditto
Holding for 10
sec., then air
cooling to room
temperature
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Underlined figures indicate outside scope of present invention.
- 26 -
[0100] Table 3
10
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
Steel
no.
a
b
c
d
e
f
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
a
b
c
d
e
f
Processing
conditions
i
i
i
i
i
i
ii
iii
iv
V
vi
vii
viii
ix
X
xi
xii
xiii
xiv
XV
xvi
xvii
xviii
xviii
xviii
xviii
xviii
xviii
Standard
deviation of
hardness
before hot
stamping
10
9
6
8
11
7
3^
12
16
29
2A
13
11
21
32
16
19
33
21
12
11
27
10
6
8
13
18
11
(MPa)
1510
1508
1501
1498
1503
1422
1512
1506
1489
1502
1499
1505
1516
1511
1522
1518
1512
1507
1500
1496
1506
1503
1510
1512
1503
1509
1499
1418
(MPa)
619
603
630
614
646
597
484
602
610
451
465
647
637
453
502
638
650
452
480
598
617
496
634
605
601
649
600
610
(fatigue
limit
ratio)
0.41
0.40
0.42
0.41
0.43
0.42
0.32
0.40
0.41
0.30
0.31
0.43
0.42
0.30
0.33
0.42
0.43
0.30
0.32
0.40
0.41
0.33
0.42
0.40
0.40
0.43
0.40
0.43
Standard
deviation of
hardness
after hot
stamping
27
22
20
21
27
24
46
20
23
42
44
19
22
43
51
24
26
49
46
22
25
45
19
12
14
24
27
22
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
15
Underlined figures indicate outside scope of present invention.
[0101]
Example 2
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.
[0102] 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.
[0103] From these steel sheets, test pieces for
measurement of hardness were obtained by the same
procedure was in Example 1.
[0104] These steel sheets for a hot stamped member
- 27 -
yfj were held at 900°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
5 pieces were obtained from the top parts of the hats.
[0105] 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
QB (average of two) and 1x10^ cycle fatigue strength aw of
10 the steel sheet after hot stamping (member).
[0106] Table 5 should these results. The correlation
between the fatigue limit ratio aw/as and the standard
deviation of the hardness before hot stamping is shown in
FIG. 6.
15 [0107] 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
20 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.
[0108] On the other hand, in steel sheets which were
25 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
30 these was 0.26 to 0.31. It became clear the fatigue
characteristic was inferior.
28 -
P [0109] Table 4
Steel
no.
2a
2b
2C
2d
2e
2f
2g
2h
Composition (mass%)
C
0.35
0.31
0.28
0.25
0.23
0.21
0.20
0.18
Si
0.3
0.5
1.0
0.8
0.6
0.4
0.3
0.3
Mn
1.0
1.2
1.7
1.9
1.6
1.4
1.2
1.3
P
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
S
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
Al
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
N
0.004
0.004
0.004
0.004
0.003
0.002
0.004
0.003
Others
Cr: 0.2, Ti: 0.01, B:
0.002, Cu: 0.1, Ni: 0.1
Cr: 0.5, Ti: 0.02, B:
0.004, Nb: 0.02, Mo: 0.2
W: 0.2, Ni: 2.0
Ti: 0.03, B: 0.003, Mo:
0.2, Ni: 1.0
Mo: 0.1, W: 0.5, V: 0.5
B: 0.004, Mo: 0.1, V: 0.5
Cr: 0.2, Ti: 0.02, Mo: 0.2,
W: 0.4
Cr: 1.4, Ti: 0.02, B:
0.002, Mo: 0.1, V: 0.2
[0110]
No.
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
Steel
no.
2a
2a
2a
2b
2b
2b
2C
2C
2C
2d
2d
2d
2e
2e
2e
2f
2f
2f
2g
2g
2g
2h
2h
2h
Table
Processing
conditions
i
ix
xviii
i
ix
xviii
i
ix
xviii
i
ix
XVlll
i
ix
xviii
i
ix
xviii
i
ix
xviii
i
ix
xviii
5
Standard
deviation of
hardness
before hot
stamping
18
40
19
16
37
18
15
30
14
15
26
5
9
31
10
12
36
19
14
33
13
14
32
15
CTB
(MPa)
1794
1790
1802
1706
1696
1711
1598
1592
1590
1492
1500
1498
1492
1502
1516
1508
1512
1522
1496
1504
1526
1506
1512
1528
CT„ (MPa)
718
465
721
682
441
702
639
430
636
612
435
614
597
421
622
603
469
609
613
406
641
602
454
642
CTW/CTB
(fatigue limit
ratio)
0.40
0.26
0.40
0.40
0.26
0.41
0.40
0.27
0.40
0.41
0.29
0.41
0.4
0.28
0.41
0.4
0.31
0.4
0.41
0.27
0.42
0.4
0.3
0.42
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Underlined figures
[0111]
Example 3
S t e e l s 3a t o 3d
in Table 6 were
i n d i c a t e outside scope of present invention.
which have the composition which is shown
produced and cast. The slabs were hot
- 29 -
^^' 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.
[0112] These steel sheets were heated by an average
5 heating rate of 19°C/sec up to 655°C, then were heated by
an average heating rate of 2.5°C to 800°C, then were
immediately cooled by an average cooling rate of
6.5°C/sec. Further, they were dipped in a 670°C hot dip Al
bath (containing 10% of Si and unavoidable impurities),
10 taken out after 5 seconds, adjusted in amount of
deposition by a gas wiper, then air cooled down to room
temperature.
[0113] From the obtained steel sheets, the same
procedure as in Example 1 was used to obtain test pieces
15 for measurement of hardness. To measure the hardness, the
hardness at a position 20 |^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
20 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
25 the first measurement surface and second measurement
surface for a total of 14 measurement positions. The
average value was found.
[0114] These steel sheets were hot stamped into hat
shapes by the same procedure as in Example 2. The heating
30 conditions were holding at 900°C for 1 minute.
[0115] 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.
[0116] These test pieces were used to find the tensile
35 strength G^ (average of two) and 1x10^ cycle fatigue
strength aw. Table 7 shows the results.
- 30 -
t-
10
[0117] 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 |j.m, a
galling phenomenon occurred at a high frequency at the
long wall parts of the hat shape. In examples of 50 jjin 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 pin or less,
[0118] Table 6
steel
no.
3a
3b
3C
3d
C
0.33
0.25
0.22
0.15
Si
0.09
0.18
0.12
0.33
Mn
1.8
1.4
1,3
1.0
P
0.01
0.01
0.02
0.02
S
0.004
0.004
0.008
0.008
Al
0.04
0.04
0.03
0.03
N
0.003
0.003
0.004
0.004
Others
Cr: 0.2, Mo: 0.2, Cu:
0.1, Ni: 0.05
Cr: 0.002, Ti: 0.02. B:
0.003, Mo: 0.2, W: 0.1, V: 0.1
Cr: 0.13, Ti: 0.03, Nb:
0.02, B: 0.002
B: 0.0005
Units are mass%.
[0119] Table 7
No.
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
Steel
no.
3a
3a
3a
3a
3a
3b
3b
3b
3b
3b
3C
3C
3C
3C
3C
3d
3d
3d
3d
3d
Standard deviation
of hardness
before hot
stamping
17
18
16
14
14
12
15
19
18
20
11
14
9
13
18
10
10
8
14
15
CTB
(MPa)
1784
1789
1801
1792
1790
1516
1520
1524
1522
1534
1502
1509
1513
1519
1524
1318
1326
1320
1314
1310
(MPa)
714
716
720
717
716
652
638
671
685
614
631
649
635
668
610
554
557
554
539
537
aw/cJe
(fatigue
limit
ratio)
0.40
0.40
0.40
0.40
0.40
0.43
0.42
0.44
0.45
0.40
0.42
0.43
0.42
0.44
0.40
0.42
0.42
0.42
0.41
0.41
Thickness of
Al plating
layer (fim)
16.0
22.2
33.9
48.6
51.0
15.1
19, 6
34.2
49.6
54.7
14.5
20.1
34.6
49.2
55.3
17.2
20.4
30.2
42.0
53.6
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
15
Underlined figures indicate outside scope of present invention.
[0120]
Example 4
- 31 -
^' 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
5 sheets were pickled, then cold rolled to 1.2 ram.
[0121] These steel sheets were heated by an average
heating rate of 19°C/sec up to 655°C, then were heated by
an average heating rate of 2.5°C to 800°C, then were
immediately cooled by an average cooling rate of
10 6.5°C/sec. Further, they were dipped in a 460°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.
15 [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 jjm from the boundary of the
inside layer of the Zn plating layer (reaction layer of
20 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
25 measurement of hardness. Seven points were measured at
measurement intervals of 5 ram at each of the first
measurement surface and second measurement surface for a
total of 14 measurement positions. The average value was
found.
30 [0123] These steel sheets were hot stamped into hat
shapes by the same procedure as in Example 2. They were
heated to 880°C and held for 5 seconds, then air-cooled
down to 700°C and pressed.
[0124] From the top parts of the hats, fatigue test
35 pieces which are shown in FIG. 2 and JIS No. 5 tensile
test pieces were obtained.
32 -
C
10
15
[0125] These test pieces were used to find the tensile
strength GQ (average of two) and 1x10^ cycle fatigue
strength aw. Table 8 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. 77, 82, 87, and 92 where
the thickness of the galvanized layer exceeded 30 fxm,
adhesion of Zn was observed at a high frequency in the
die. In examples of 30 |am 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 |Lim or less.
[0127] Table 8
No.
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
Steel
no.
3a
3a
3a
3a
3a
3b
3b
3b
3b
3b
3c
3c
3c
3c
3c
3d
3d
3d
3d
3d
Standard
deviation
of hardness
before hot
stamping
17
17
16
13
15
12
15
19
18
20
10
14
9
14
18
10
12
8
13
15
CTB
(MPa)
1785
1788
1802
1794
1793
1516
1522
1534
1532
1545
1518
1536
1524
1539
1544
1336
1342
1318
1320
1330
CTw
(MPa)
714
715
721
718
717
652
639
675
689
618
638
660
640
677
618
561
564
554
541
545
CT„/aB
(fatigue
limit
ratio)
0.40
0.40
0.40
0.40
0.40
0.43
0.42
0.44
0.45
0.40
0.42
0.43
0.42
0.44
0.40
0.42
0.42
0.42
0.41
0.41
Thickness
of
galvanized
layer (|am)
6.1
12.5
23.8
28.6
31.0
11.1
19.6
24.8
29.0
33.7
10.3
17.2
19.6
29.3
32.3
11.2
17.4
20.2
28.0
33.4
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp.
ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp.
ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp.
ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp.
ex.
Underlined figures indicate outside scope of present invention.
[0128]
Example 5
Steels 3a to 3d
in Table 6 were
which have the compositi
produced and cast. The s
on which is shown
labs were hot
- 33 -
^ 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.
[0129] These steel sheets were heated by an average
5 heating rate of 19°C/sec up to 655°C, then were heated by
an average heating rate of 2.5°C to 800°C, then were
immediately cooled by an average cooling rate of
6.5°C/sec. Further, they were dipped in a 460°C hot dip
galvanization bath (containing 0.13% of Al, 0.03% of Fe,
10 and unavoidable impurities), taken out after 3 seconds,
adjusted in amount of deposition by a gas wiper, then
heated to 480°C to form an Zn-Fe alloy layer, then air
cooled down to room temperature.
[0130] From the obtained steel sheets, the same
15 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 [im 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
20 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.
2 5 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
30 positions. The average value was found.
[0131] These steel sheets were hot stamped into hat
shapes by the same procedure as in Example 2. They were
heated to 880°C and held for 5 seconds, then air-cooled
down to 700°C and pressed.
35 [0132] From the top parts of the hats, fatigue test
pieces which are shown in FIG. 2 and JIS No. 5 tensile
- 34 -
10
test pieces were obtained.
[0133] These test pieces were used to find the tensile
strength QB (average of two) and 1x10^ cycle fatigue
strength aw. Table 9 shows the results.
[0134] 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
|a,m, fine cracks occurred in the alloy layer after
pressing. In examples of 45 |j,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 |am or less.
[0135] Table 9
15
No.
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
Steel
no.
3a
3a
3a
3a
3a
3b
3b
3b
3b
3b
3C
3C
3C
3C
3C
3d
3d
3d
3d
3d
Standard deviation
of hardness
before hot
stamping
17
16
17
14
13
12
18
19
18
14
11
14
9
12
19
17
11
8
14
15
CTB
(MPa)
1773
1777
1802
1786
1772
1505
1519
1513
1502
1518
1506
1503
1500
1506
1510
1307
1313
1320
1314
1310
(MPa)
727
711
739
714
709
632
638
651
661
622
633
646
645
633
619
523
551
554
539
537
(fatigue
limit
ratio)
0.41
0.40
0.41
0.40
0.40
0.42
0.42
0.43
0.44
0.41
0.42
0.43
0.43
0.42
0.41
0.40
0.42
0.42
0.41
0.41
Thickness of
Zn-Fe alloy
layer (nm)
15.0
22.2
31.5
39.9
46.0
15.7
21.6
39.2
44.6
49.7
14.5
20.8
34.6
42.2
45.3
15.2
18.4
30.6
42.9
48.6
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Underlined figures indicate outside scope of present invention.
20
Reference Signs List
[0136]
11a top die
lib bottom die
- 35 -
12 steel sheet
21 fatigue crack growth region
51 test piece sampling position

^EL^
- 36 -
CLAIMS
Claim 1. Steel sheet for a hot stamped member which
includes composition which contains, by mass%,
C: 0.15 to 0.35%,
5 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
10 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 |^m from the steel sheet surface in the
15 sheet thickness direction is 20 or less.
Claim 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
20 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%
25 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%.
30 Claim 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 |j,m to 50 |am thick Al plating layer, a 5
|j,m to 30 |am thick galvanized layer, or a 5 |xm to 45 ^m
thick Zn-Fe alloy layer.
35
Claim 4. A method of production of steel sheet for a hot
- 37 -
^ stamped member comprising recrystallization-annealing
cold rolled steel sheet which includes composition which
contains, by mass%,
C: 0.15 to 0.35%,
5 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
10 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
15 to 25°C/sec from room temperature to a temperature M (°C)
and
then a second stage of heating by an average heating rate
of 1 to 7°C/sec to a temperature S (°C) ,
wherein the temperature M (°C) is 600 to 700 (°C) and
20 the temperature S (°C) is 720 to 820 (°C) .
Claim 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
25 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%
30 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%.
35 Claim 6. The method of production of steel sheet for a
hot stamped member as set forth in claim 5 wherein a hot
10
- 3^
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%.
Claim 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.
Claim 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
15 galvanized layer on the surface.
Claim 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
20 step of dipping said steel sheet in a Zn bath to form a
galvanized layer on the surface, then further heating to
600°C or less to form a Zn-Fe alloy layer on said surface.