U.S. patent application number 16/110069 was filed with the patent office on 2018-12-20 for steel sheet for hot stamping, method for production thereof, and hot stamping steel material.
This patent application is currently assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION. The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Hiroyuki TANAHASHI, Toshimasa TOMOKIYO.
Application Number | 20180363109 16/110069 |
Document ID | / |
Family ID | 49116745 |
Filed Date | 2018-12-20 |
United States Patent
Application |
20180363109 |
Kind Code |
A1 |
TANAHASHI; Hiroyuki ; et
al. |
December 20, 2018 |
STEEL SHEET FOR HOT STAMPING, METHOD FOR PRODUCTION THEREOF, AND
HOT STAMPING STEEL MATERIAL
Abstract
A hot stamping steel material, which secures good hydrogen
embrittlement resistance even when the steel sheet after hot
stamping is subjected to processing leading to remaining of stress,
such as piercing and which is easily practicable, wherein the steel
sheet has the chemical composition of: C: 0.18 to 0.26%; Si: more
than 0.02% and not more than 0.05%; Mn: 1.0 to 1.5%; P: 0.03% or
less; S: 0.02% or less; Al: 0.001 to 0.5%; N: 0.1% or less;O: 0.001
to 0.02%; Cr: 0 to 2.0%; Mo: 0 to 1.0%; V: 0 to 0.5%; W: 0 to 0.5%;
Ni: 0 to 5.0%; B: 0 to 0.01%; Ti: 0 to 0.5%; Nb: 0 to 0.5%; Cu: 0
to 1.0%; and balance: Fe and impurities, in terms of % by mass, the
concentration of a Mn-containing inclusion is not less than 0.010%
by mass and less than 0.25% by mass, and the number ratio of a Mn
oxide to the inclusion having a maximum length of 1.0 to 4.0 pm is
10.0% or more.
Inventors: |
TANAHASHI; Hiroyuki; (Tokyo,
JP) ; TOMOKIYO; Toshimasa; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
NIPPON STEEL & SUMITOMO METAL
CORPORATION
Tokyo
JP
|
Family ID: |
49116745 |
Appl. No.: |
16/110069 |
Filed: |
August 23, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14382704 |
Sep 3, 2014 |
|
|
|
PCT/JP2013/055992 |
Mar 5, 2013 |
|
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16110069 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/14 20130101;
C22C 38/001 20130101; C22C 38/32 20130101; C22C 38/38 20130101;
C22C 38/08 20130101; C22C 38/54 20130101; Y10T 428/12757 20150115;
C22C 38/16 20130101; C21D 9/48 20130101; C23C 2/02 20130101; C22C
38/06 20130101; C21D 8/0436 20130101; C22C 38/12 20130101; C22C
38/02 20130101; C22C 38/18 20130101; C21D 8/0463 20130101; B21B
1/26 20130101; C21D 2211/004 20130101; C23C 2/28 20130101; C22C
38/002 20130101; C22C 38/28 20130101; C22C 38/22 20130101; C23C
2/40 20130101; C23C 2/06 20130101; C22C 38/24 20130101; C21D 9/46
20130101; Y10T 428/12799 20150115; C23C 2/12 20130101; C22C 38/04
20130101 |
International
Class: |
C22C 38/38 20060101
C22C038/38; C23C 2/06 20060101 C23C002/06; C23C 2/02 20060101
C23C002/02; C22C 38/16 20060101 C22C038/16; C22C 38/22 20060101
C22C038/22; C22C 38/24 20060101 C22C038/24; B21B 1/26 20060101
B21B001/26; C22C 38/08 20060101 C22C038/08; C22C 38/32 20060101
C22C038/32; C22C 38/28 20060101 C22C038/28; C22C 38/18 20060101
C22C038/18; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C22C 38/54 20060101
C22C038/54; C22C 38/06 20060101 C22C038/06; C21D 9/46 20060101
C21D009/46; C23C 2/40 20060101 C23C002/40; C23C 2/28 20060101
C23C002/28; C23C 2/12 20060101 C23C002/12 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 7, 2012 |
JP |
2012-050935 |
Claims
1. A method for production of a steel sheet for hot stamping, the
method comprising: hot-rolling a steel piece having a chemical
composition of: C: 0.18 to 0.26%; Si: more than 0.02% and not more
than 0.05%; Mn: 1.0 to 1.5%; P: 0.03% or less; S: 0.02% or less;
Al: 0.001 to 0.5%; N: 0.1% or less; O: 0.0010 to 0.020%; Cr: 0 to
2.0%; Mo: 0 to 1.0%; V: 0 to 0.5%; W: 0 to 0.5%; Ni: 0 to 5.0%; B:
0 to 0.01%; Ti: 0 to 0.5%; Nb: 0 to 0.5%; Cu: 0 to 1.0%; and
balance: Fe and impurities, in terms of % by mass; coiling the
steel piece at a temperature of 690.degree. C. or higher to form a
hot-rolled steel sheet; and cold-rolling the hot-rolled steel sheet
at a draft of 10 to 90% to form a cold-rolled steel sheet.
2. The method for production of a steel sheet for hot stamping
according to claim 1, wherein the chemical composition comprises
one or more selected from the group consisting of Cr: 0.01 to 2.0%;
Mo: 0.01 to 1.0%; V: 0.01 to 0.5%; W: 0.01 to 0.5%; Ni: 0.01 to
5.0%; and B: 0.0005 to 0.01%, in terms of % by mass.
3. The method for production of a steel sheet for hot stamping
according to claim 1, wherein the chemical composition comprises
one or more selected from the group consisting of Ti: 0.001 to
0.5%; Nb: 0.001 to 0.5%; and Cu: 0.01 to 1.0%, in terms of % by
mass.
4. The method for production of a steel sheet for hot stamping
according to claim 1, further comprising, immersing the cold-rolled
sheet in an aluminum hot-dipping bath to form an aluminum
hot-dipping layer on the surface of the cold-rolled steel
sheet.
5. The method for production of a steel sheet for hot stamping
according to claim 1, further comprising, immersing the cold-rolled
sheet in a hot-dip galvanizing bath to form a hot-dip galvanized
layer on the surface of the cold-rolled steel sheet.
6. The method for production of a steel sheet for hot stamping
according to claim 1, further comprising, immersing the cold-rolled
sheet in a hot-dip galvanizing bath, and then heating the
cold-rolled sheet at a temperature of 600.degree. C. or lower to
form an alloyed hot-dip galvanized layer on the surface of the
cold-rolled steel sheet.
Description
[0001] This application is a Divisional of U.S. application Ser.
No. 14/382,704, filed Sep. 3, 2014, which is the U.S. National
Phase of PCT/JP2013/055992, filed Mar. 5, 2013, which claims
priority under 35 U.S.C. 119(a) to Japanese Patent Application No.
2012-050935, filed Mar. 7, 2012, the contents of all of which are
incorporated by reference, in their entirety, into the present
application.
TECHNICAL FIELD
[0002] The present invention relates to a steel sheet for hot
stamping, a method for production thereof, and a hot stamping steel
material.
BACKGROUND ART
[0003] In the field of transportation equipment such as
automobiles, an attempt is extensively made to reduce the mass by
using high-strength materials. For example, in automobiles, use of
high-strength steel sheets has been steadily increased with an
intention to improve collision safety and enhance functionality
without increasing the car body mass, and also improve fuel
efficiency to reduce emissions of carbon dioxide.
[0004] In this movement for expansion of use of high-strength steel
sheets, the biggest problem is manifestation of a phenomenon called
"degradation of shape fixability", which is more likely to occur as
the strength of the steel sheet is increased. The phenomenon is
more likely to occur as the spring back amount after forming
increases with strength enhancement, and the phenomenon causes such
an additional problem specific to high-strength steel sheets that
it is not easy to obtain a desired shape.
[0005] For solving the problem, it is necessary in a usual method
for forming a high-strength steel sheet additionally to carry out
an unnecessary processing step (e.g. restriking) for a low-strength
material free from the problem of degradation of shape fixability,
or to change the product shape.
[0006] As one method for solving such situations, a hot-forming
method called a hot stamping method has received attention. The hot
stamping method is a method in which a steel sheet (processed
material) is heated to a predetermined temperature (generally the
temperature that serves as an austenite phase), and stamped by a
die having a temperature (e.g. room temperature) lower than the
temperature of the processed material with the strength of the
processed material decreased for facilitating forming, whereby a
desired shape can be easily provided, and also a rapid cooling heat
treatment (quenching) using a difference in temperature between the
processed material and the pressing is performed to increase the
strength of a product after forming.
[0007] In recent years, the hot stamping method has been recognized
for its usefulness, and a wide range of steel materials have been
considered to be applied. Examples thereof include steel materials
that are used under a severe corrosive environment, like automobile
undercarriage components, and steel materials provided with
perforated portions for the purpose of joining other components.
Thus, steel materials obtained by the hot stamping method have been
required to have not only strength but also hydrogen embrittlement
resistance.
[0008] This is because while it is generally known that hydrogen
embrittlement resistance is reduced with strength enhancement of
steel materials, a steel material obtained by the hot stamping
method generally has high strength, and therefore in application of
the hot stamping method to the steel material, the steel material
is exposed to a corrosive environment to accelerate ingress of
hydrogen into the steel, and massive residual stress occurs as
processing such as punching is performed, thus raising the
possibility that hydrogen embrittlement occurs.
[0009] From such a viewpoint, a technique intended to secure
hydrogen embrittlement resistance has also been proposed for steel
materials whose strength is enhanced by the hot stamping method.
For example, Patent Literature 1 discloses a technique concerning a
steel sheet having resistance to delayed rupture (the same meaning
as hydrogen embrittlement resistance) by including at a
predetermined density one or more of oxides, sulfides, composite
crystallized products and composite precipitated products of Mg
having an average particle size in a predetermine range. Patent
Literature 2 discloses a technique in which the punching
characteristic is improved by performing punching (perforation) in
a high-temperature state (hot) after heating for hot stamping and
before pressing, so that delayed rupture resistance is
improved.
PRIOR ART LITERATURE
Patent Literatures
[0010] [Patent Literature 1] JP2006-9116A [0011] [Patent Literature
2] JP2010-174291A [0012] [Patent Literature 3] JP2006-29977A
SUMMARY OF THE INVENTION
Problems to Be Solved by the Invention
[0013] Although the technique disclosed in Patent Literature 1 is
an excellent technique, but it is a technique in which Mg that is
not easily included in general is made to exist in the steel, and a
product containing Mg is highly controlled. Therefore, a more
easily practicable technique is desired.
[0014] The technique disclosed in Patent Literature 2 is a
technique based on hot perforation in which punching (perforation)
is performed in a high-temperature state (hot) after heating for
hot stamping and before pressing. Accordingly, high dimensional
accuracy cannot be secured in a steel material after hot stamping.
Further, the shape capable of being formed by the technique is
restricted. Therefore, it is difficult to expand the range of
applications (components) of the hot stamping method by the
technique disclosed in Patent Literature 2.
[0015] Thus, there has not been proposed a technique which secures
good hydrogen embrittlement resistance even when processing leading
to remaining of stress, such as perforation, is performed after hot
stamping and which is easily practicable.
[0016] Accordingly, an object of the present invention is to
provide a steel sheet for hot stamping, which secures good hydrogen
embrittlement resistance even when a steel material after hot
stamping is subjected to processing leading to remaining of stress,
such as perforation; a method for production thereof which can
easily be performed; and a hot stamping steel material.
Means for Solving the Problems
[0017] For achieving the object described above, the present
inventors have extensively conducted studies as described below.
The present inventors have given attention to a Mn-containing
inclusion and a Mn oxide which are relatively easily generated in
the steel, and come up with a new idea of securing good hydrogen
embrittlement resistance by making these substances serve as a trap
site for diffusible hydrogen and non-diffusible hydrogen.
[0018] Then, steel sheets for hot stamping have been prepared under
various conditions and subjected to a hot stamping method, and for
the obtained steel materials, strength and ductility as fundamental
characteristics as well as hydrogen embrittlement resistance and
toughness have been examined. As a result, it has been newly found
that good hydrogen embrittlement resistance can be secured in the
steel material after hot stamping by increasing the concentration
of the Mn-containing inclusion and the number ratio of the Mn oxide
to the Mn-containing inclusion having a predetermined size.
[0019] On the other hand, such a problem has been newly found that
when the concentration of the Mn-containing inclusion is
excessively increased, a reduction in toughness becomes apparent in
the steel material after hot stamping. That is, it has been newly
found when the concentration of the Mn-containing inclusion falls
within a predetermined range and the number density of the Mn oxide
to the Mn-containing inclusion having a predetermined size is equal
to or greater than a predetermined value, good hydrogen
embrittlement resistance can be secured and good toughness can be
secured even when the steel material after hot stamping is
subjected to processing leading to remaining of stress, such as
punching.
[0020] Then, it has been newly found that by increasing the coiling
temperature in a hot rolling step as compared to conventional
techniques and performing cold rolling in conditions for production
of the steel sheet for hot stamping, the concentration of the
Mn-containing inclusion can be made fall within a predetermined
range and the number ratio of the Mn oxide to the Mn-containing
inclusion having a predetermined size can be made equal to or
greater than a predetermined value. [0018]
[0021] The present invention has been devised based on the
above-described new findings, and the subject thereof is as
follows.
[0022] (1) A steel sheet for hot stamping, wherein the steel sheet
has the chemical composition of: C: 0.18 to 0.26%; Si: more than
0.02% and not more than 0.05% ; Mn: 1.0 to 1.5%; P: 0.03% or less;
S: 0.02% or less; Al: 0.001 to 0.5%; N: 0.1% or less; O: 0.0010 to
0.020%; Cr: 0 to 2.0%; Mo: 0 to 1.0%; V: 0 to 0.5%; W: 0 to 0.5%;
Ni: 0 to 5.0%; B: 0 to 0.01%; Ti: 0 to 0.5%; Nb: 0 to 0.5%; Cu: 0
to 1.0%; and balance: Fe and impurities, in terms of % by mass, the
concentration of a Mn-containing inclusion is not less than 0.010%
by mass and less than 0.25% by mass, and the number ratio of a Mn
oxide to the inclusion having a maximum length of 1.0 to 4.0 .mu.m
is 10.0% or more.
[0023] (2) The steel sheet for hot stamping according to (1),
wherein the chemical composition includes one or more selected from
the group consisting of Cr: 0.01 to 2.0%; Mo: 0.01 to 1.0%; V: 0.01
to 0.5%; W: 0.01 to 0.5%; Ni: 0.01 to 5.0%; and B: 0.0005 to 0.01%,
in terms of % by mass.
[0024] (3) The steel sheet for hot stamping according to (1) or
(2), wherein the chemical composition includes one or more selected
from the group consisting of Ti: 0.001 to 0.5%; Nb: 0.001 to 0.5%;
and Cu: 0.01 to 1.0%, in terms of % by mass.
[0025] (4) The steel sheet for hot stamping according to any one of
(1) to (3), wherein the steel sheet includes on a surface thereof
an aluminum hot-dipping layer having a thickness of 50 .mu.m or
less.
[0026] (5) The steel sheet for hot stamping according to any one of
(1) to (3), wherein the steel sheet includes on a surface thereof a
hot-dip galvanized layer having a thickness of 30 .mu.m or
less.
[0027] (6) The steel sheet for hot stamping according to any one of
(1) to (3), wherein the steel sheet includes on a surface thereof
an alloyed hot-dip galvanized layer having a thickness of 45 .mu.m
or less.
[0028] (7) A method for production of a steel sheet for hot
stamping, the method including: a hot rolling step of hot-rolling a
steel piece having the chemical composition of: C: 0.18 to 0.26%;
Si: more than 0.02% and not more than 0.05%; Mn: 1.0 to 1.5%; P:
0.03% or less; S: 0.02% or less; Al: 0.001 to 0.5%; N: 0.1% or
less; O: 0.0010 to 0.020%; Cr: 0 to 2.0%; Mo: 0 to 1.0%; V: 0 to
0.5%; W: 0 to 0.5%; Ni: 0 to 5.0%; B: 0 to 0.01%; Ti: 0 to 0.5%;
Nb: 0 to 0.5%; Cu: 0 to 1.0%; and balance: Fe and impurities, in
terms of % by mass, and then coiling the steel piece at a
temperature of 690.degree. C. or higher to form a hot-rolled steel
sheet; and a cold rolling step of cold-rolling the hot-rolled steel
sheet at a draft of 10 to 90% to form a cold-rolled steel
sheet.
[0029] (8) The method for production of a steel sheet for hot
stamping according to (7), wherein the chemical composition
includes one or more selected from the group consisting of Cr: 0.01
to 2.0%; Mo: 0.01 to 1.0%; V: 0.01 to 0.5%; W: 0.01 to 0.5%; Ni:
0.01 to 5.0%; and B: 0.0005 to 0.01%, in terms of % by mass.
[0030] (9) The method for production of a steel sheet for hot
stamping according to (7) or (8), wherein the chemical composition
includes one or more selected from the group consisting of Ti:
0.001 to 0.5%; Nb: 0.001 to 0.5%; and Cu: 0.01 to 1.0%, in terms of
% by mass.
[0031] (10) A method for production of a steel sheet for hot
stamping, wherein the steel sheet for hot stamping, which is
obtained by the production method according to any one of (7) to
(9), is immersed in an aluminum hot-dipping bath to form an
aluminum hot-dipping layer on the surface of the steel sheet.
[0032] (11) A method for production of a steel sheet for hot
stamping, wherein the steel sheet for hot stamping, which is
obtained by the production method according to any one of (7) to
(9), is immersed in a hot-dip galvanizing bath to form a hot-dip
galvanized layer on the surface of the steel sheet.
[0033] (12) A method for production of a steel sheet for hot
stamping, wherein the steel sheet for hot stamping, which is
obtained by the production method according to any one of (7) to
(9), is immersed in a hot-dip galvanizing bath, and then heated at
a temperature of 600.degree. C. or lower to form an alloyed hot-dip
galvanized layer on the surface of the steel sheet.
[0034] (13) A hot stamping steel material, wherein the hot stamping
steel material has the chemical composition of: C: 0.18 to 0.26%;
Si: more than 0.02% and not more than 0.05%; Mn: 1.0 to 1.5%; P:
0.03% or less; S: 0.02% or less; Al: 0.001 to 0.5%; N: 0.1% or
less; O: 0.0010 to 0.020%; Cr: 0 to 2.0%; Mo: 0 to 1.0%; V: 0 to
0.5%; W: 0 to 0.5%; Ni: 0 to 5.0%; B: 0 to 0.01%; Ti: 0 to 0.5%;
Nb: 0 to 0.5%; Cu: 0 to 1.0%; and balance: Fe and impurities, in
terms of % by mass, the concentration of a Mn-containing inclusion
is not less than 0.010% by mass and less than 0.25% by mass, and
the number ratio of a Mn oxide to the inclusion having a maximum
length of 1.0 to 4.0 gm is 10.0% or more.
[0035] (14) The hot stamping steel material according to the above
(13), wherein the chemical composition includes one or more
selected from the group consisting of Cr: 0.01 to 2.0%; Mo: 0.01 to
1.0%; V: 0.01 to 0.5%; W: 0.01 to 0.5%; Ni: 0.01 to 5.0%; and B:
0.0005 to 0.01%, in terms of % by mass.
[0036] (15) The hot stamping steel material according to (13) or
(14), wherein the chemical composition includes one or more
selected from the group consisting of Ti: 0.001 to 0.5%; Nb: 0.001
to 0.5%; and Cu: 0.01 to 1.0%, in terms of % by mass.
Effects of the Invention
[0037] According to the present invention, good hydrogen
embrittlement resistance can be secured even when processing
leading to remaining of stress, such as punching, is performed
after hot stamping, and practice is easy, so that the range of
applications (components) of the hot stamping method can be
expanded.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 is a view illustrating a relationship between the
amount of diffusible hydrogen and the time until rupture.
[0039] FIG. 2 is a view showing a hot stamping method and a die
used in examples.
[0040] FIG. 3 is a view showing an aspect of a constant load test
piece used in examples.
[0041] FIG. 4 is a view showing an aspect of a steel sheet (member)
pressed into a hat shape.
MODES FOR CARRYING OUT THE INVENTION
[0042] (1) Chemical Composition
[0043] The reason for specifying the chemical compositions of a
steel sheet for hot stamping (hereinafter, also referred to as the
"present invention steel sheet") and a hot stamping steel material
(hereinafter, also referred to as the "present invention steel
material") according to the present invention will be described.
The "%" in the following descriptions means "% by mass".
[0044] <C: 0.18 to 0.26%>
[0045] C is an element that is the most important in increasing the
strength of a steel sheet by a hot stamping method. When the C
content is less than 0.18%, it is difficult to secure a strength of
1500 MPa or more after hot stamping. Therefore, the C content is
0.18% or more.
[0046] On the other hand, when the C content is more than 0.26%,
ductility after hot stamping becomes poor and it is difficult to
secure a total elongation of 10% or more. Therefore, the C content
is 0.26% or less.
[0047] <Si: More Than 0.02% and Not More Than 0.05%>
[0048] Si is an element that is important in controlling the
concentration of a Mn-containing inclusion and the number ratio of
a Mn oxide to the inclusion having a maximum length of 1.0 to 4.0
.mu.m. When the Si content is 0.02% or less, generation of the Mn
oxide is excessively accelerated, and the concentration of the
Mn-containing inclusion reaches 0.25% or more, so that toughness
may be significantly reduced. Therefore, the Si content is more
than 0.02%. On the other hand, when the Si content is more than
0.05%, generation of the Mn oxide is excessively suppressed, and
the number ratio of the Mn oxide to the Mn-containing inclusion
having a maximum length of 1.0 to 4.0 .mu.m is less than 10.0%, so
that it is difficult to obtain good hydrogen embrittlement
resistance with stability. Therefore, the Si content is 0.05% or
less.
[0049] <Mn: 1.0 to 1.5%>
[0050] Mn is an element that is the most important in the present
invention. Mn acts to enhance hydrogen embrittlement resistance by
forming a Mn-containing inclusion in the steel. Remaining Mn that
has not formed the inclusion acts to enhance hardenability. When
the Mn content is less than 1.0%, it is difficult to ensure that
the concentration of the Mn-containing inclusion is 0.010% by mass
or more. Therefore, the Mn content is 1.0% or more. On the other
hand, when the Mn content is more than 1.5%, the effect from the
above-mentioned action is saturated, thus being economically
disadvantageous, and mechanical characteristics may be deteriorated
due to segregation of Mn. Therefore, the Mn content is 1.5% or
less.
[0051] <P: 0.03% or Less>
[0052] P is an element that is generally contained as an impurity.
When the P content is more than 0.03%, hot processability is
significantly deteriorated. Therefore, the P content is 0.03% or
less. The lower limit of the P content does not have to be
particularly specified, but is preferably 0.001% or more because
excessive reduction causes a considerable burden on the
steel-making process.
[0053] <S: 0.02% or Less>
[0054] S is an element that is generally contained as an impurity.
When the S content is more than 0.02%, hot processability is
significantly deteriorated. Therefore, the S content is 0.02% or
less. The lower limit of the S content does not have to be
particularly specified, but is preferably 0.0005% or more because
excessive reduction causes a considerable burden on the steel
production process.
[0055] <Al: 0.001 to 0.5%>
[0056] Al is an element that acts to consolidate the steel by
deoxidization. When the Al content is less than 0.001%, it is
difficult to perform sufficient deoxidization. Therefore, the Al
content is 0.001% or more. On the other hand, when the Al content
is more than 0.5%, generation of the Mn oxide is excessively
suppressed, and it is difficult to secure the later-described Mn
oxide ratio, so that it is difficult to secure good hydrogen
embrittlement resistance. Therefore, the Al content is 0.5% or
less.
[0057] <N: 0.1% or Less>
[0058] N is an element that is generally contained as an impurity.
When the N content is more than 0.1%, N is easily bound with Ti and
B which are the later-described optional elements to consume the
elements, so that the effects of these elements are reduced.
Therefore, the N content is 0.1% or less, preferably 0.01% or less.
The lower limit of the N content does not have to be particularly
specified, but is preferably 0.001% or more because excessive
reduction causes a considerable burden on the steel-making
step.
[0059] <O: 0.0010 to 0.020%>
[0060] O forms a Mn oxide in the steel, which acts to enhance
hydrogen embrittlement resistance by serving as a trap site for
diffusible hydrogen and non-diffusible hydrogen. When the 0 content
is less than 0.0010%, generation of the Mn oxide is not
sufficiently accelerated, and the number ratio of the Mn oxide to
the Mn-containing inclusion is less than 10.0%, so that good
hydrogen embrittlement resistance cannot be obtained with
stability. Therefore, the 0 content is 0.0010% or more. On the
other hand, when the 0 content is more than 0.020%, a coarse oxide
is formed in the steel to degrade mechanical characteristics of the
steel material. Therefore, the 0 content is 0.020% or less.
[0061] The present invention steel sheet and the present invention
steel material have the above-described components as an essential
component composition, and may further contain one or more of Cr,
Mo, V, W, Ni, B, Ti, Nb and Cu as necessary.
[0062] <Cr: 0 to 2.0%>, <B: 0 to 0.01%>, <Mo: 0 to
1.0%>, <W: 0 to 0.5%>, <V: 0 to 0.5%> and <Ni: 0
to 5.0%>
[0063] These elements all act to enhance hardenability. Therefore,
one or more of these elements may be contained. However, when B is
contained in an amount exceeding the above-mentioned upper limit,
hot processability is degraded and ductility is reduced. When Cr,
Mo, W, V and Ni are contained in an amount exceeding the
above-mentioned upper limit, the effect from the above-mentioned
action is saturated, thus being economically disadvantageous.
Therefore, the upper limits of the contents of B, Cr, Mo, W, V and
Ni are each as described above. For more reliably obtaining the
effect from the above-mentioned action, it is preferred that the B
content is 0.0005% or more, or the content of any of Cr, Mo, W, V
and Ni elements is 0.01% or more. Ni acts to suppress degradation
of the surface property of the hot-rolled steel sheet by Cu, and
therefore it is preferred that Ni is also contained when
later-described Cu is contained.
[0064] <Ti: 0 to 0.5%>, <Nb: 0 to 0.5%> and <Cu: 0
to 1.0%>
[0065] Ti, Nb and Cu all act to increase strength. Therefore, one
or more of these elements may be contained. However, when the Ti
content is more than 0.5%, generation of the Mn oxide is
excessively suppressed, and it is difficult to secure the
later-described Mn oxide ratio, so that it is difficult to secure
good hydrogen embrittlement resistance. Therefore, the Ti content
is 0.5%. When the Nb content is more than 0.5%, controllability of
hot rolling may be impaired. Therefore, the Nb content is 0.5% or
less. When the Cu content is more than 1.0%, the surface property
of the hot-rolled steel sheet may be impaired. Therefore, the Cu
content is 1.0% or less. For obtaining the effect from the
above-mentioned action more reliably, it is preferred that any of
Ti (0.001% or more), Nb (0.001% or more) and Cu (0.01% or more) is
contained. Since Ti is preferentially bound with N in the steel to
form a nitride, and thereby inhibits B from being wastefully
consumed by forming a nitride, so that the effect by B can be
further increased, it is preferred that Ti is also contained when
the above-mentioned B is contained.
[0066] The balance includes Fe and impurities.
[0067] (2) Inclusion
[0068] Next, the reason for specifying the concentration of the
Mn-containing inclusion and the number ratio of the Mn oxide to the
Mn-containing inclusion having a maximum length of 1.0 to 4.0 pm in
the present invention steel sheet and the present invention steel
material will be described.
[0069] <Concentration of Mn-Containing Inclusion: Not Less Than
0.010% by Mass and Less Than 0.25% by Mass>
[0070] The Mn-containing inclusion plays an important role in
suppression of hydrogen embrittlement together with the number
ratio of the Mn oxide to the later-described Mn-containing
inclusion having a maximum length of 1.0 to 4.0 gm. When the
concentration of the Mn-containing inclusion is less than 0.010%,
it is difficult to obtain good hydrogen embrittlement resistance.
Therefore, the concentration of the Mn-containing inclusion is
0.010% or more. On the other hand, when the concentration of the
Mn-containing inclusion is 0.25% or more, toughness may be reduced.
Therefore, the concentration of the Mn-containing inclusion is less
than 0.25%.
[0071] The concentration of the Mn-containing inclusion is
determined in accordance with the following procedure. That is, a
steel sheet is electrolyzed at a constant current in an
electrolytic solution with acetylacetone and tetramethylammonium
dissolved in methanol, a filter having a pore diameter of 0.2 .mu.m
is used to collect residues, the mass of the residues is divided by
an electrolysis amount (mass of the steel sheet lost by
electrolysis), and the obtained value is multiplied by 100 to be
described in terms of a percentage. It is confirmed that the
inclusion extracted by the electrolysis method contains Mn by EDS
(energy dispersive X-ray spectroscopy) with a SEM (scanning
electron microscope).
[0072] <Number Ratio of Mn Oxide to Number of Mn-Containing
Inclusions Having Maximum Length of 1.0 to 4.0 .mu.m: 10.0% or
More>
[0073] The number ratio of the Mn oxide to the Mn-containing
inclusion having a maximum length of 1.0 to 4.0 .mu.m plays an
important role in suppression of hydrogen embrittlement together
with the Mn-containing inclusion described above. When the number
ratio of the Mn oxide to the number of Mn-containing inclusions
having a maximum length of 1.0 to 4.0 .mu.m is less than 10.0%, it
is difficult to obtain good hydrogen embrittlement resistance.
Therefore, the number ratio of the Mn oxide to the number of
Mn-containing inclusions having a maximum length of 1.0 to 4.0
.mu.m is 10.0% or more.
[0074] The number ratio of the Mn oxide to the number of
Mn-containing inclusions having a maximum length of 1.0 to 4.0
.mu.m is determined in accordance with the following procedure. The
cross section of a steel sheet is observed with a SEM, and
inclusions having a maximum length (e.g. the length of the longer
side when the inclusion is rectangular, and the length of the major
axis when the inclusion is elliptical) of 1.0 to 4.0 .parallel.m
are selected, and defined as examination objects. These inclusions
are subjected to EDS analysis, and those for which a characteristic
X-ray from Mn and a characteristic X-ray from O (oxygen) are
detected at the same time are judged as the Mn oxide.
Observation/analysis is performed in a plurality of visual fields
until the total number of examined objects exceeds 500, and the
number ratio of the Mn oxide to the total number of examined
objects is defined as a number ratio of the Mn oxide.
[0075] Here, the reason why the maximum length of inclusions to be
examined is 1.0 .mu.m or more is that with a smaller inclusion,
accuracy of analysis of constituent elements by EDS becomes
insufficient. Here, the reason why the maximum length of inclusions
to be examined is 4.0 .mu.m or less is that a larger inclusion is a
union etc. of a plurality of different inclusions, so that
constituent elements (combinations thereof) are not uniquely
defined by EDS analysis sites.
[0076] (3) Plating Layer
[0077] The present invention steel sheet and the present invention
steel material may be a surface-treated steel sheet or a
surface-treated steel material with plating layer formed on a
surface thereof for the purpose of improvement of corrosion
resistance, etc. The plating layer may be hot-dipping layer or may
be an electroplating layer. Examples of the hot-dipping layer
include hot-dip galvanized layers, alloyed hot-dip galvanized
layers, aluminum hot-dipping layers, Zn--Al alloy hot-dipping
layers, Zn--Al--Mg alloy hot-dipping layers and Zn--Al--Mg--Si
alloy hot-dipping layers. Examples of the electroplating layer
include zinc-electroplating layers and Zn-Ni alloy-electroplating
layers.
[0078] The thickness of the plating layer is not particularly
limited from the viewpoint of hydrogen embrittlement resistance and
toughness. For the present invention steel sheet, however, it is
preferred to restrict the upper limit of the thickness of the
plating layer from the viewpoint of press formability. For example,
the thickness of the plating layer is preferably 50 .mu.m or less
from the viewpoint of galling resistance in the case of aluminum
hot-dipping, the thickness of the plating layer is preferably 30
.mu.m or less from the viewpoint of suppressing adhesion of Zn to a
die in the case of hot-dip galvanizing, and the thickness of the
plating layer is preferably 45 .mu.m or less from the viewpoint of
suppressing occurrence of cracking of an alloy layer in the case of
alloying hot-dip galvanizing. On the other hand, it is preferred to
restrict the lower limit of the thickness of the plating layer from
the viewpoint of corrosion resistance. For example, in the case of
aluminum hot-dipping and hot-dip galvanizing, the thickness of the
plating layer is preferably 5 .mu.m or more, more preferably 10
.mu.m or more. In the case of alloying hot-dip galvanizing, the
thickness of the plating layer is preferably 10 .mu.m or more, more
preferably 15 .mu.m or more.
[0079] (4) Method for Production of Present Invention Steel
Sheet
[0080] A method for production of the present invention steel sheet
will be described. The present invention steel sheet can be
produced by a production method including: a hot rolling step of
hot-rolling a steel piece having the above-mentioned chemical
composition, and then coiling the steel piece at a temperature of
690.degree. C. or higher to form a hot-rolled steel sheet; and a
cold rolling step of cold-rolling the hot-rolled steel sheet at a
draft of 10 to 90% to form a cold-rolled steel sheet. Here,
steel-making conditions and casting conditions in production of the
steel piece and conditions for cold rolling applied to the
hot-rolled steel sheet may conform to a usual method. Pickling
performed before cold-rolling the hot-rolled steel sheet may
conform to a usual method.
[0081] The form of the inclusion described above is obtained by
hot-rolling a steel piece having the above-mentioned chemical
composition, then coiling the steel piece at a temperature of
690.degree. C. or higher to form a hot-rolled steel sheet, and
cold-rolling the hot-rolled steel sheet at a draft of 10 to 90%.
Therefore, recrystallization annealing after cold rolling is not
necessary from the viewpoint of hydrogen embrittlement resistance
and toughness after hot stamping. However, it is preferred that
after cold rolling, recrystallization annealing is performed to
soften the steel sheet from the viewpoint of processability of
blanking and pre-forming etc. which are performed before the steel
sheet is subjected to hot stamping. A plating layer may be provided
after recrystallization annealing for the purpose of improvement of
corrosion resistance, etc. When the hot-dipping is performed, it is
preferred to perform hot-dipping treatment performed using
continuous hot-dipping equipment subsequent to recrystallization
annealing.
[0082] The reason why a steel sheet for hot stamping, which is
capable of providing a hot stamping steel material having good
hydrogen embrittlement resistance and toughness, is obtained by the
above-described production method is not necessarily evident, but
this is considered to be related to a generation state of cementite
and a microstructure in the hot-rolled steel sheet before being
subjected to cold rolling. That is, cementite is crushed together
with other inclusions in the cold rolling step as a post-step of
the hot rolling step, but depending on a size thereof, the size and
the dispersion state after crushing and a generation state of gaps
between the cementite and the steel vary. Depending on the strength
(hardness) of the microstructure, the hardness difference between
the microstructure and the inclusion varies, and this also affects
the state of the inclusion and gaps. Moreover, both the cementite
and microstructure affect the state of inclusions that are not
crushed but deformed.
[0083] The present inventors presume that by hot-rolling a steel
piece having the above-mentioned chemical composition and then
coiling the steel piece at a temperature of 690.degree. C. or
higher, and cold-rolling the thus obtained hot-rolled steel sheet
at a draft of 10 to 90%, a generation state of cementite and a
microstructure are exquisitely combined, and as a result, the form
of the inclusion described above can be secured, so that good
hydrogen embrittlement resistance and toughness can be
obtained.
[0084] The upper limit of the coiling temperature is not
particularly restricted from the viewpoint of securing both
hydrogen embrittlement resistance and toughness. However, the
coiling temperature is preferably 850.degree. C. or lower from the
viewpoint of suppressing an increase in crystal grain size of the
hot-rolled steel sheet to reduce anisotropy of mechanical
properties such as stretchability or suppressing an increase in
scale thickness to reduce a burden of pickling. The draft in the
cold rolling step may be appropriately selected according to a
capacity of equipment and a sheet thickness of the hot-rolled steel
sheet.
[0085] Production conditions other than those described above have
little influence on hydrogen embrittlement resistance and
toughness. For example, in the hot rolling step, a temperature of
1200 to 1250.degree. C. as a temperature of the steel piece
subjected to hot rolling, a draft of 30 to 90%, and a finishing
temperature of around 900.degree. C. may be selected.
[0086] When recrystallization annealing is performed, the annealing
temperature is desired to be 700 to 850.degree. C. from the
viewpoint of moderately softening the steel sheet, but for the
purpose of characterizing other mechanical properties, the
annealing temperature may be lower than 700.degree. C., or may be
higher than 850.degree. C. After recrystallization annealing, the
steel sheet may be directly cooled to room temperature, or may be
immersed in a hot-dipping bath in the process of cooling to room
temperature to form a hot-dipping layer on the surface of the steel
sheet.
[0087] When hot-dipping is aluminum hot-dipping, Si may be
contained in a concentration of 0.1 to 20% in an aluminum
hot-dipping bath. Si contained in the aluminum hot-dipping layer
affects the reaction between Al and Fe, which takes place during
heating before hot stamping. From the viewpoint of moderately
suppressing the above-mentioned reaction to secure press
formability of the plating layer itself, the content of Si in the
bath is preferably 1% or more, further preferably 3% or more. On
the other hand, from the viewpoint of moderately accelerating the
above-mentioned reaction to suppress deposition of Al on a press
die, the content of Si in the bath is preferably 15% or less,
further preferably 12% or less.
[0088] When hot-dipping is hot-dip galvanizing, the steel sheet is
immersed in a hot-dip galvanizing bath, and then cooled to room
temperature, and when hot-dipping is alloying hot-dip galvanizing,
the steel sheet is immersed in a hot-dip galvanizing bath, then
heated at a temperature of 600.degree. C. or lower and thereby
subjected to alloying treatment, and then cooled to room
temperature. Al may be contained in a concentration of 0.01 to 3%
in the hot-dip galvanizing bath. Al affects the reaction between Zn
and Fe. When hot-dipping is hot-dip galvanizing, mutual diffusion
of Zn and Fe can be suppressed by the reaction layer of Fe and Al.
When hot-dipping is hot-dip galvanizing, it can be utilized for
performing control to a suitable plating composition from the
viewpoint of processability and plating adhesion. These effects
from Al are exhibited by ensuring that the concentration of Al in
the hot-dip galvanizing bath is 0.01 to 3%. Therefore, the
concentration of Al in the hot-dip galvanizing bath may be selected
according to a capacity of equipment involved in production, and a
purpose.
[0089] (5) Method for Production of Present Invention Steel
Material
[0090] The present invention steel material can be obtained by
subjecting the present invention steel sheet using a usual
method.
[0091] Embodiments of the present invention described above are
merely illustrative, and various changes may be made in claims.
EXAMPLES
[0092] As tests common in examples below, details of a hydrogen
embrittlement accelerating test and measurement of a critical
diffusible hydrogen amount for evaluating hydrogen embrittlement
resistance and details of a Charpy impact test for evaluating
toughness will be first described.
[0093] Diffusible hydrogen was introduced into a test piece (steel
sheet) by a cathode charge method in an electrolytic solution. That
is, the test piece was used as a cathode and platinum electrode
arranged around the test piece was used as an anode, a
predetermined current density was passed between both the former
and the latter to generate hydrogen on a surface of the test piece,
and hydrogen was encouraged to diffuse to the inside of the test
piece. An aqueous solution formed by dissolving NH.sub.4SCN and
NaCl in pure water in concentrations of 0.3% and 3%, respectively,
was used as an electrolytic solution.
[0094] Tension corresponding to residual stress as another factor
to cause hydrogen embrittlement was applied by a "lever type"
constant load tester using a weight (hereinafter, referred to as a
"constant load test"; test piece is referred to as a "constant load
test piece"). The constant load test piece was notched. A time
until the test piece was ruptured was recorded, and the test piece
was quickly collected after being ruptured. The electrolytic
solution was removed, and a diffusible hydrogen amount was
immediately measured by a temperature rising hydrogen analysis
method using a gas chromatograph. A cumulative emission amount from
room temperature to 250.degree. C. was defined as a diffusible
hydrogen amount.
[0095] By changing the current density while fixing the applied
tension, a relationship between a diffusible hydrogen amount and a
time until rupture as shown in FIG. 1 is determined. Here,
".smallcircle." with an arrow indicates that the test piece had not
ruptured even after elapse of a preset time. A period of 96 hours
was employed as a set time. A median between a minimum value
H.sub.min of the diffusible hydrogen amount of a ruptured test
piece (".circle-solid." in FIG. 1) and a maximum value H.sub.max of
the diffusible hydrogen amount of an unruptured test piece was
defined as a critical diffusible hydrogen amount Hc. That is,
Hc=(Hmin+Hmax)/2. Patent Literature 3 (JP2006-29977A) discloses a
similar test method.
[0096] Hydrogen embrittlement resistance of a steel sheet with the
plating on the surface was evaluated based on presence/absence of
cracking by observing hole walls in a piercing test conducted with
the clearance being changed. That is, a steel sheet having a sheet
thickness of t (mm) was pierced with holes of 10 mm.phi.. At this
time, the diameter Dp of a punch was fixed to 10 mm, and the inner
diameter Di of a die was changed, so that the
clearance=(Di-Dp)/2t.times.100 ranged from 5% to 30%.
Presence/absence of cracking in hole walls was examined, and a
steel sheet free from cracking was judged as a steel sheet
excellent in hydrogen embrittlement resistance. The number of
piercing was 5 or more per clearance, and all the hole walls were
examined.
[0097] Toughness was evaluated by a Charpy impact test conforming
to JIS Z 2242 irrespective of presence/absence of plating. The test
piece was shaped in conformity with the No. 4 test piece in JIS Z
2202, and the thickness of the test piece was determined according
to a steel sheet to be evaluated. The test was conducted in a range
of -120.degree. C. to 20.degree. C. to determine a ductility
brittleness transition temperature. [0072]
Example 1
[0098] A steel piece having the chemical composition shown in Table
1 was casted. The steel piece was heated to 1250.degree. C. and
hot-rolled to form a 2.8 mm-thick hot-rolled steel sheet at a
finishing temperature of 870 to 920.degree. C. The coiling
temperature was set to 700.degree. C. The steel sheet was pickled,
and then cold-rolled at a draft of 50% to obtain a cold-rolled
steel sheet having a sheet thickness of 1.4 mm. The cold-rolled
steel sheet was subjected to recrystallization annealing such that
the steel sheet was held at a temperature ranging from 700.degree.
C. to 800.degree. C. for 1 minute and air-cooled to room
temperature, thereby obtaining a sample material (steel sheet for
hot stamping).
[0099] A test piece of 50.times.50 mm was taken from each sample
material, and electrolyzed at a constant current in an electrolytic
solution with acetylacetone and tetramethylammonium dissolved in
methanol. The current value was set to 500 mA, and the electrolysis
time was set to 4 hours. A filter having a pore diameter of 0.2
.mu.m was used to collect residues, and the mass of the residues
was divided by an electrolysis amount, and described in terms of a
percentage. In this way, the concentration of a Mn-containing
inclusion was determined.
[0100] The cross section of the sample material was observed with a
SEM, and analyses of the inclusion, i.e. counting, dimension
measurement and examination of constituent elements by EDS were
performed. In this way, a number ratio of a Mn oxide to the
inclusion having a maximum length of 1.0 to 4.0 .mu.m was
determined.
[0101] Each sample material was held in the air at 900.degree. C.
for 3 minutes, and then sandwiched between experimental flat press
dies shown in FIG. 2, so that hot stamping was performed. That is,
as shown in FIG. 2, a steel sheet 22 was processed by an upper die
21a and a lower die 21b. An average cooling rate to 200.degree. C.
as measured by providing a thermocouple was about 70.degree. C./s.
A JIS No. 5 tensile test piece, a constant load test piece shown in
FIG. 3 and a Charpy impact test piece were taken from the steel
material after hot stamping.
[0102] The constant load test was conducted by applying a tension
corresponding to 90% of a tensile strength determined in the
tensile test. The current density was set to 0.01 to 1
mA/cm.sup.2.
[0103] Diffusible hydrogen was measured at a heating rate of
100.degree. C./hour.
[0104] The Charpy impact test was conducted at a test temperature
of 20.degree. C., 0.degree. C., -20.degree. C., -40.degree. C.,
-60.degree. C., -80.degree. C., -100.degree. C. and -120.degree.
C., and a ductility brittleness transition temperature was
determined from a change in absorbed energy.
[0105] For the test piece taking direction, the tensile direction
was made perpendicular to the rolling direction of the steel sheet
in the case of the tensile test piece and the constant load test
piece, and the longitudinal direction was made parallel to the
rolling direction in the case of the Charpy test piece. The sheet
thickness of the tensile test piece was set to 1.4 mm, and the
sheet thickness of other test pieces was set to 1.2 mm by grinding
both surfaces. The results are shown in Table 2.
TABLE-US-00001 TABLE 1 CHEMICAL COMPOSITION (UNIT: % BY MASS,
BALANCE: Fe AND IMPURITIES) STEEL C Si Mn P S Al N O OTHERS REMARKS
a 0.18 0.015 1.5 0.02 0.004 0.001 0.004 0.007 -- COMPARATIVE STEEL
b 0.18 0.025 1.5 0.02 0.004 0.001 0.004 0.007 Cr: 0.2, Ti: 0.001,
B: 0.0035 RELEVANT STEEL c 0.18 0.045 1.5 0.02 0.004 0.003 0.004
0.007 Nb: 0.01, B: 0.0035 RELEVANT STEEL d 0.18 0.055 1.5 0.02
0.004 0.003 0.004 0.007 Cr: 0.2, Ti: 0.005, B: 0.0025 COMPARATIVE
STEEL e 0.22 0.015 1.2 0.02 0.004 0.001 0.003 0.0006 Cr: 0.01, B:
0.0025 COMPARATIVE STEEL f 0.22 0.025 1.2 0.02 0.002 0.005 0.003
0.005 -- RELEVANT STEEL g 0.22 0.025 1.2 0.02 0.002 0.003 0.003
0.009 B: 0.0025 RELEVANT STEEL h 0.22 0.025 1.2 0.02 0.002 0.003
0.003 0.012 Ti: 0.01, B: 0.005 RELEVANT STEEL i 0.24 0.025 1.0 0.01
0.002 0.005 0.003 0.007 Cr: 0.2 RELEVANT STEEL j 0.24 0.030 1.0
0.01 0.002 0.005 0.003 0.007 Ti: 0.01, B: 0.003 RELEVANT STEEL k
0.24 0.035 1.0 0.01 0.002 0.005 0.003 0.021 Ti: 0.01 COMPARATIVE
STEEL l 0.24 0.030 0.9 0.01 0.002 0.005 0.003 0.003 Nb: 0.1
COMPARATIVE STEEL m 0.26 0.010 1.5 0.02 0.004 0.6 0.003 0.010 Nb:
0.03 COMPARATIVE STEEL n 0.26 0.025 1.0 0.02 0.002 0.001 0.002
0.007 Cr: 0.2, B: 0.0030 RELEVANT STEEL o 0.26 0.035 1.0 0.02 0.002
0.003 0.003 0.015 -- RELEVANT STEEL p 0.26 0.030 1.0 0.02 0.004
0.003 0.004 0.010 Cr: 1.0, Ti: 0.03, B: 0.005 RELEVANT STEEL
UNDERLINES IN THE TABLE INDICATE THE VALUES FALL OUT OF THE RANGE
SPECIFIED IN THE PRESENT INVENTION
TABLE-US-00002 TABLE 2 Mn-CONTAINING INCLUSION HAVING MAXIMUM
LENGTH OF 1.0 TO 4.0 .mu.m CONCENTRATION NUMBER DUCTILITY OF NUMBER
OF NUMBER RATIO OF BRITTLENESS Mn-CONTAINING OBSERVED OF Mn NUMBER
TENSILE TRANSITION INCLUSION INCLUSIONS OXIDES OF Mn STRENGTH Hc
TEMPERATURE No. STEEL (% BY MASS) (NUMBER) (NUMBER) OXIDES (%)
(MPa) (ppm) (.degree. C.) REMARKS 1 a 0.26 501 261 52.1 1502 0.74
-35 COMPARATIVE EXAMPLE 2 b 0.15 500 69 13.8 1510 0.96 69 PRESENT
INVENTION EXAMPLE 3 c 0.12 512 52 10.2 1512 0.90 70 PRESENT
INVENTION EXAMPLE 4 d 0.10 508 49 9.6 1514 0.45 -55 COMPARATIVE
EXAMPLE 5 e 0.13 501 21 4.2 1542 0.30 70 COMPARATIVE EXAMPLE 6 f
0.16 504 136 27.6 1545 0.92 -68 PRESENT INVENTION EXAMPLE 7 g 0.14
502 172 34.3 1540 0.91 -66 PRESENT INVENTION EXAMPLE 8 h 0.18 500
181 36.2 1546 0.94 -67 PRESENT INVENTION EXAMPLE 9 i 0.15 500 124
24.8 1577 0.90 -71 PRESENT INVENTION EXAMPLE 10 j 0.13 503 139 27.6
1570 0.92 -68 PRESENT INVENTION EXAMPLE 11 k 0.32 502 208 41.5 1562
0.72 -29 COMPARATIVE EXAMPLE 12 l 0.11 500 45 9.0 1566 0.32 -65
COMPARATIVE EXAMPLE 13 m 0.02 500 7 1.4 1582 0.22 -31 COMPARATIVE
EXAMPLE 14 n 0.18 500 121 24.2 1590 0.89 -61 PRESENT INVENTION
EXAMPLE 15 o 0.22 500 154 30.8 1596 0.90 -60 PRESENT INVENTION
EXAMPLE 16 p 0.17 507 115 22.7 1598 0.84 -62 PRESENT INVENTION
EXAMPLE UNDERLINES IN THE TABLE INDICATE THE VALUES FALL OUT OF THE
RANGE SPECIFIED IN THE PRESENT INVENTION
[0106] In every example, the steel sheet after hot stamping showed
a tensile strength of 1500 MPa or more. Samples Nos. 2, 3, 6 to 10
and 14 to 16 in which both the concentration of the Mn-containing
inclusion and the number ratio of the Mn oxide to the inclusion
having a maximum length of 1.0 to 4.0 .mu.m fell within the range
specified in the present invention had good hydrogen embrittlement
resistance and toughness with the critical diffusible hydrogen
amount Hc of 0.84 ppm or more and the ductility brittleness
transition temperature of -60.degree. C. or lower.
[0107] On the other hand, samples Nos. 1 and 11 in which the
concentration of the Mn-containing inclusion fell out of the range
specified in the present invention were poor in toughness with the
ductility brittleness transition temperature being much higher as
compared to present invention examples having a comparable tensile
strength. Samples Nos. 4, 5, 12 and 13 in which the number ratio of
the Mn oxide to the inclusion having a maximum length of 1.0 to 4.0
.mu.m fell out of the range specified in the present invention were
poor in hydrogen embrittlement resistance with the Hc being
significantly smaller as compared to present invention examples.
The sample No. 13 has a much higher ductility brittleness
transition temperature as compared to present invention examples
having a comparable tensile strength although the concentration of
the Mn-containing inclusion falls within the range specified in the
present invention. It is thought that because of the fact that the
Al content is high (falls out of the range specified in the present
invention), an Al-based oxide is contained in a high
concentration.
Example 2
[0108] A steel piece having the chemical composition shown in Table
3 was casted. The steel piece was heated to 1250.degree. C. and
hot-rolled to form a 3.0 mm-thick hot-rolled steel sheet at a
finishing temperature of 880 to 920.degree. C. The coiling
temperature was set to 700.degree. C. The steel sheet was pickled,
and then cold-rolled at a draft of 50% to obtain a cold-rolled
steel sheet having a sheet thickness of 1.5 mm. The cold-rolled
steel sheet was subjected to recrystallization annealing such that
the steel sheet was held at a temperature ranging from 700.degree.
C. to 800.degree. C. for 1 minute and air-cooled to room
temperature, thereby obtaining a sample material (steel sheet for
hot stamping). A concentration of a Mn-containing inclusion and a
number ratio of a Mn oxide to the inclusion having a maximum length
of 1.0 to 4.0 .mu.m were determined in the same manner as in
Example 1. Further, a sample material was held in the air at
900.degree. C. for 5 minutes, and then pressed into a hat shape
shown in FIG. 4 using a hot stamping method. An average cooling
rate to 200.degree. C. as measured by providing a thermocouple was
about 35.degree. C./s. From a test piece taking position 41 (hat
head portion) shown in FIG. 4, a JIS No. 5 tensile test piece, a
constant load test piece and a Charpy impact test piece were taken.
The relationship between the test piece taking direction and the
steel sheet rolling direction was same as that in Example 1. The
sheet thickness of the tensile test piece was set to 1.5 mm, and
the sheet thickness of other test pieces was set to 1.3 mm by
grinding both surfaces. The constant load test was conducted by
applying a tension corresponding to 90% of a tensile strength
determined in the tensile test. The current density was set to 0.01
to 1 mA/cm.sup.2. Diffusible hydrogen was measured at a heating
rate of 100.degree. C./hour. The Charpy impact test was conducted
at a test temperature of 20.degree. C., 0.degree. C., -20.degree.
C., -40.degree. C., -60.degree. C., -80.degree. C., -100.degree. C.
and -120.degree. C., and a ductility brittleness transition
temperature was determined from a change in absorbed energy. The
results are shown in Table 4.
TABLE-US-00003 TABLE 3 CHEMICAL COMPOSITION (UNIT: % BY MASS,
BALANCE: Fe AND IMPURITIES) STEEL C Si Mn P S Al N O OTHERS REMARKS
2a 0.22 0.015 1.2 0.02 0.002 0.005 0.003 0.005 V: 0.05 COMPARATIVE
STEEL 2b 0.22 0.025 1.2 0.02 0.002 0.005 0.003 0.005 V: 0.5
RELEVANT STEEL 2c 0.22 0.025 1.2 0.02 0.002 0.005 0.003 0.005 Mo:
0.2 RELEVANT STEEL 2d 0.22 0.025 1.2 0.02 0.002 0.005 0.003 0.005
W: 0.2 RELEVANT STEEL 2e 0.22 0.025 1.2 0.02 0.002 0.005 0.003
0.005 W: 0.05 RELEVANT STEEL 2f 0.22 0.025 1.2 0.02 0.002 0.005
0.003 0.005 Cu: 0.5, Ni: 0.3 RELEVANT STEEL 2g 0.22 0.025 1.2 0.02
0.002 0.005 0.003 0.005 Mo: 0.1, W: 0.2, V: 0.2 RELEVANT STEEL 2h
0.22 0.025 1.2 0.02 0.002 0.005 0.003 0.005 B: 0.002, Mo: 0.1, V:
0.2 RELEVANT STEEL 2i 0.22 0.030 1.6 0.02 0.007 0.001 0.003 0.025
B: 0.002, Nb: 0.5 COMPARATIVE STEEL 2j 0.22 0.055 0.6 0.01 0.002
0.003 0.003 0.007 B: 0.002, Cu: 1.0, Ni: 0.5 COMPARATIVE STEEL 2k
0.22 0.025 1.2 0.02 0.002 0.005 0.003 0.005 B: 0.003, Mo: 1.0
RELEVANT STEEL 2l 0.22 0.025 1.2 0.02 0.002 0.005 0.003 0.005 Nb:
0.2, V: 0.5 RELEVANT STEEL 2m 0.22 0.060 1.2 0.02 0.002 0.003 0.003
0.005 B: 0.002, V: 0.5 COMPARATIVE STEEL 2n 0.22 0.025 1.2 0.02
0.002 0.002 0.003 0.0007 B: 0.004, Cu: 0.5, Ni: 0.5 COMPARATIVE
STEEL 2o 0.22 0.025 1.2 0.02 0.002 0.005 0.003 0.005 B: 0.002, Nb:
0.2, W: 0.2, V: 0.3 RELEVANT STEEL 2p 0.22 0.025 0.6 0.01 0.002
0.001 0.003 0.007 B: 0.003, Mo: 0.2, V: 0.3 COMPARATIVE STEEL
UNDERLINES IN THE TABLE INDICATE THE VALUES FALL OUT OF THE RANGE
SPECIFIED IN THE PRESENT INVENTION
TABLE-US-00004 TABLE 4 Mn-CONTAINING INCLUSION HAVING MAXIMUM
LENGTH OF 1.0 TO 4.0 .mu.m CONCENTRATION NUMBER DUCTILITY OF NUMBER
OF NUMBER RATIO OF BRITTLENESS Mn-CONTAINING OBSERVED OF Mn NUMBER
OF TENSILE TRANSITION INCLUSION INCLUSIONS OXIDES Mn OXIDES
STRENGTH Hc TEMPERATURE No. STEEL (% BY MASS) (NUMBER) (NUMBER) (%)
(MPa) (ppm) (.degree. C.) REMARKS 17 2a 0.27 501 113 22.6 1580 0.60
-48 COMPARATIVE EXAMPLE 18 2b 0.15 500 125 25.0 1585 0.98 68
PRESENT INVENTION EXAMPLE 19 2c 0.14 512 109 21.3 1588 0.96 -67
PRESENT INVENTION EXAMPLE 20 2d 0.19 508 126 24.8 1592 0.96 -68
PRESENT INVENTION EXAMPLE 21 2e 0.16 504 119 23.6 1590 0.96 -69
PRESENT INVENTION EXAMPLE 22 2f 0.12 500 110 22.0 1586 0.91 -65
PRESENT INVENTION EXAMPLE 23 2g 0.10 500 118 23.6 1587 1.92 -67
PRESENT INVENTION EXAMPLE 24 2h 0.13 502 109 21.7 1591 1.00 68
PRESENT INVENTION EXAMPLE 25 2i 0.39 511 302 59.1 1600 0.56 36
COMPARATIVE EXAMPLE 26 2j 0.005 500 40 7.9 1602 0.55 -65
COMPARATIVE EXAMPLE 27 2k 0.15 500 134 26.8 1588 0.95 -65 PRESENT
INVENTION EXAMPLE 28 2l 0.12 503 123 24.5 1589 1.04 -70 PRESENT
INVENTION EXAMPLE 29 2m 0.007 504 49 9.8 1594 0.60 -65 COMPARATIVE
EXAMPLE 30 2n 0.18 500 103 4.3 1590 0.35 -68 COMPARATIVE EXAMPLE 31
2o 0.16 512 151 29.4 1587 1.05 -71 PRESENT INVENTION EXAMPLE 32 2p
0.02 502 47 9.4 1584 0.61 -69 COMPARATIVE EXAMPLE UNDERLINES IN THE
TABLE INDICATE THE VALUES FALL OUT OF THE RANGE SPECIFIED IN THE
PRESENT INVENTION
[0109] In every example, the steel sheet after hot stamping showed
a tensile strength of 1580 MPa or more. Among them, samples Nos. 18
to 24, 27, 28 and 31 in which both the concentration of the
Mn-containing inclusion and the number ratio of the Mn oxide to the
inclusion having a maximum length of 1.0 to 4.0 .mu.m fell within
the range specified in the present invention had good hydrogen
embrittlement resistance and toughness with the Hc of 0.91 ppm or
more and the ductility brittleness transition temperature of
-65.degree. C. or lower.
[0110] On the other hand, samples Nos. 17 and 25 in which the
concentration of the Mn-containing inclusion exceeded the range
specified in the present invention were poor in toughness and had
ductility brittleness transition temperatures much higher as
compared to present invention examples. Samples Nos. 26, 29, 30 and
32 in which the number ratio of the Mn oxide to the inclusion
having a maximum length of 1.0 to 4.0 .mu.m fell out of the range
specified in the present invention is apparently poor in hydrogen
embrittlement resistance and had the Hc smaller as compared to
present invention examples. The sample No. 25 has a small He
although the number of Mn oxides falls within the range specified
in the present invention. This is thought that because of the fact
that the Mn content and the O content are high (fall out of the
range specified in the present invention), the distribution of the
size of the Mn oxide is biased to the side of the larger size as
compared present invention examples, and therefore the number of
gaps between the Mn oxide and the steel is small.
Example 3
[0111] A steel piece having the chemical composition shown in Table
5 was casted. The steel piece was heated to 1200.degree. C. and
hot-rolled to form a 2.0 to 4.0 mm-thick hot-rolled steel sheet at
a finishing temperature of 880 to 920.degree. C. The steel sheet
was coiled at a plurality of coiling temperatures while conditions
for cooling on a cooling bed (ROT) were controlled. The steel sheet
was pickled, and then cold-rolled at a draft of 50% to obtain a
cold-rolled steel sheet. The cold-rolled steel sheet was subjected
to recrystallization annealing such that the steel sheet was held
at 700.degree. C. to 800.degree. C. for 1 minute and air-cooled to
room temperature, thereby obtaining a sample material (steel sheet
for hot stamping). A concentration of a Mn-containing inclusion and
a number ratio of a Mn oxide to the Mn-containing inclusion having
a maximum length of 1.0 to 4.0 .mu.m were determined in the same
manner as in Example 1. Hot stamping was performed using a flat die
identical to that in Example 1. A tensile test piece, a constant
load test piece and a Charpy impact test piece were taken from the
steel sheet after hot stamping in the same manner as in Example 1.
For the sheet thickness of the test piece, the tensile test piece
had a sheet thickness identical to that of the cold-rolled steel
sheet, and other test pieces had a sheet thickness obtained by
grinding both surfaces of the cold-rolled steel sheet to a depth of
0.1 mm. A constant load test, measurement of diffusible hydrogen
and a Charpy impact test were also performed in the same manner as
in Example 1. The finishing sheet thickness of the hot-rolled
sheet, the coiling temperature, the results of examining the
inclusion, hydrogen embrittlement resistance (Hc) and toughness are
collectively shown in Table 6.
TABLE-US-00005 TABLE 5 CHEMICAL COMPOSITION (UNIT: % BY MASS,
BALANCE: Fe AND IMPURITIES) STEEL C Si Mn P S Al N O OTHERS 3a 0.20
0.025 1.0 0.02 0.004 0.003 0.003 0.005 B: 0.004 3b 0.26 0.025 1.5
0.02 0.004 0.003 0.003 0.007 Cr: 1.0, Mo: 0.2, W: 0.2, V: 0.5
TABLE-US-00006 TABLE 6 Mn-CONTAINING INCLUSION HAVING MAXIMUM
LENGTH CONCENTRATION OF 1.0 TO 4.0 .mu.m OF NUMBER HOT- Mn- NUMBER
OF NUMBER RATIO OF ROLLED COOLING CONTAINING OBSERVED OF Mn NUMBER
OF SHEET TEMPERATURE INCLUSION INCLUSIONS OXIDES Mn OXIDES No.
STEEL THICKNESS (T) (% BY MASS) (NUMBER) (NUMBER) (%) 33 3a 2.8 700
0.15 500 89 17.8 34 3a 2.8 690 0.16 500 73 14.6 35 3a 2.8 680 0.14
504 47 9.4 36 3a 3.2 710 0.14 500 78 15.6 37 3a 3.2 700 0.16 501 67
13.4 38 3a 3.2 680 0.13 500 45 9.0 39 3a 4.0 720 0.17 507 77 15.2
40 3a 4.0 690 0.15 500 57 11.4 41 3a 4.0 660 0.15 502 46 9.4 42 3b
2.0 710 0.19 500 85 17 43 3b 2.0 690 0.20 508 81 15.9 44 3b 2.0 670
0.18 500 45 8.9 45 3b 2.4 750 0.20 503 58 11.5 46 3b 2.4 700 0.21
500 52 10.3 47 3b 2.4 645 0.18 500 48 9.6 48 3b 3.2 740 0.19 500 82
16.3 49 3b 3.2 710 0.22 500 70 13.9 50 3b 3.2 680 0.21 500 49 9.8
DUCTILITY BRITTLENESS TENSILE TRANSITION STRENGTH Hc TEMPERATURE
No. (MPa) (ppm) (.degree. C.) REMARKS 33 1508 0.90 -66 PRESENT
INVENTION EXAMPLE 34 1516 0.89 -67 PRESENT INVENTION EXAMPLE 35
1520 0.48 -47 COMPARATIVE EXAMPLE 36 1503 0.92 -68 PRESENT
INVENTION EXAMPLE 37 1510 0.90 -65 PRESENT INVENTION EXAMPLE 38
1518 0.44 -45 COMPARATIVE EXAMPLE 39 1500 0.58 -69 PRESENT
INVENTION EXAMPLE 40 1506 0.91 -70 PRESENT INVENTION EXAMPLE 41
1514 0.46 -44 COMPARATIVE EXAMPLE 42 1596 1.06 -60 PRESENT
INVENTION EXAMPLE 43 1600 1.03 -59 PRESENT INVENTION EXAMPLE 44
1606 0.68 -40 COMPARATIVE EXAMPLE 45 1587 1.01 -61 PRESENT
INVENTION EXAMPLE 46 1613 0.98 -63 PRESENT INVENTION EXAMPLE 47
1622 0.70 -43 COMPARATIVE EXAMPLE 48 1594 1.07 -59 PRESENT
INVENTION EXAMPLE 49 1601 1.02 -58 PRESENT INVENTION EXAMPLE 50
1618 0.69 -41 COMPARATIVE EXAMPLE UNDERLINES IN THE TABLE INDICATE
THE VALUES FALL OUT OF THE RANGE SPECIFIED IN THE PRESENT
INVENTION
[0112] The tensile strength of the steel sheet after hot stamping
was independent of the finishing sheet thickness, and the steel 3a
showed a tensile strength of 1500 to 1520 MPa and the steel 3b
showed a tensile strength of 1587 to 1622 MPa. When comparing
samples having the same sheet thickness, it is shown that the
tensile strength tends to increase as the coiling temperature
decreases, and therefore it is thought that the strength of the
sample material is affected by the coiling temperature. The
concentration of the
[0113] Mn-containing inclusion fell within the range specified in
the present invention in every example, but in samples Nos. 35, 38,
41, 44, 47 and 50 of comparative examples in which the coiling
temperature fell out of the range specified in the present
invention, the number ratio of the Mn oxide to the Mn-containing
inclusion having a maximum length of 1.0 to 4.0 pm fell out of the
range specified in the present invention (less than 10%), and
accordingly the He was significantly smaller compared to two
present invention examples with the same finishing thickness of the
same steel, leading to poor hydrogen embrittlement resistance, and
also the ductility brittleness transition temperature was higher
compared to two present invention examples with the same finishing
thickness of the same steel, leading to poor toughness. In view of
the fact that in all of these comparative examples, the
concentration of the Mn-containing inclusion fell within the range
specified in the present invention, it is thought that in these
comparative examples, crushing of the Mn oxide was insufficient, so
that gaps capable of serving as a trap site for diffusible hydrogen
could not be sufficiently secured, and therefore the value of He
became small, and the ductility brittleness transition temperature
was increased because an inclusion stretched without being crushed
remained. Samples Nos. 33, 34, 36, 37, 39, 40, 42, 43, 45, 46, 48
and 49 of present invention examples in which the coiling
temperature fell within the range specified in the present
invention were excellent in both hydrogen embrittlement resistance
and toughness.
Example 4
[0114] A steel piece having the chemical composition shown in Table
7 was produced. The steel piece was formed into a 2.8 mm-thick
hot-rolled steel sheet under the conditions same as those in
Example 1, and the steel sheet was pickled, and then cold-rolled
(draft: 50%) into a steel sheet having a sheet thickness of 1.4 mm.
The cold-rolled steel sheet was heated to 655.degree. C. at an
average heating rate of 19.degree. C./s, subsequently heated to 730
to 780.degree. C. at an average heating rate of 2.5.degree. C./s,
immediately cooled at an average cooling rate of 6.5.degree. C./s,
immersed in an aluminum-plating bath (containing Si in a
concentration of 10% and impurities) at 670.degree. C., and taken
out after 5 seconds. The deposition amount was adjusted with a gas
wiper, followed by air-cooling the steel sheet to room temperature.
Analysis of the inclusion of the obtained steel sheet was performed
in the same manner as in Example 1. In the same manner as in
Example 2, the steel sheet was hot-stamped into a hat shape, and a
JIS No. 5 tensile test piece, a piercing testing test piece and a
Charpy impact test piece were taken from the hat portion. For
heating conditions for hot stamping, the steel sheet was held at
900.degree. C. for 1 minute, nitrogen containing hydrogen in a
concentration of 3% was set as an atmosphere, and the dew point was
set to 0.degree. C.
[0115] Analysis results related to the inclusion are shown in Table
8, and test results related to the hot stamp material are
collectively shown in Table 9.
TABLE-US-00007 TABLE 7 CHEMICAL COMPOSITION (UNIT: % BY MASS,
BALANCE: Fc AND IMPURITIES) STEEL C Si Mn P S Al N O OTHERS 4a 0.20
0.025 1.5 0.02 0.004 0.003 0.0025 0.007 Cr: 1.0, B: 0.004 4b 0.22
0.025 1.3 0.02 0.002 0.003 0.0025 0.006 B: 0.003, Mo: 0.2, W: 0.1,
V: 0.1 4c 0.24 0.040 1.1 0.02 0.002 0.003 0.0025 0.007 Nb: 0.02
TABLE-US-00008 TABLE 8 Mn-CONTAINING INCLUSION HAVING MAXIMUM
LENGTH OF CONCENTRATION 1.0 TO 4.0 .mu.m THICKNESS OF NUMBER OF Al
Mn- NUMBER OF RATIO OF PLATING CONTAINING OBSERVED NUMBER OF NUMBER
OF LAYER INCLUSION INCLUSIONS Mn OXIDES Mn OXIDES No. STEEL (.mu.m)
(% BY MASS) (NUMBER) (NUMBER) (%) 51 4a 16.1 0.15 500 60 12.0 52 4a
22.1 0.16 500 64 12.8 53 4a 33.8 0.15 500 63 12.6 54 4a 48.7 0.17
500 66 13.2 55 4a 51.1 0.15 502 63 12.5 56 4b 15.2 0.11 500 73 14.6
57 4b 19.7 0.13 500 70 14.0 58 4b 34.1 0.11 504 71 14.1 59 4b 49.5
0.13 500 86 17.2 60 4b 54.8 0.12 500 74 14.8 61 4c 14.3 0.15 500 56
11.2 62 4c 20.0 0.15 500 61 12.2 63 4c 34.7 0.17 500 55 11.0 64 4c
49.3 0.16 500 57 11.4 65 4c 55.4 0.15 500 66 13.2 UNDERLINES IN THE
TABLE INDICATE THE VALUES FALL OUT OF THE SUITABLE RANGE SPECIFIED
IN THE PRESENT INVENTION
TABLE-US-00009 TABLE 9 NUMBER OF DUCTILITY CRACKS IN BRITTLENESS
TENSILE HOLE WALL TRANSITION HOT STRENGTH PORTION TEMPERATURE
STAMPING No. STEEL (MPa) (NUMBER) (.degree. C.) STATE 51 4a 1510 0
-62 GOOD 52 4a 1512 0 -69 GOOD 53 4a 1519 0 -67 GOOD 54 4a 1508 0
-68 GOOD 55 4a 1511 0 -61 GALLING 56 4b 1540 0 -67 GOOD 57 4b 1543
0 -61 GOOD 58 4b 1546 0 -69 GOOD 59 4b 1539 0 -66 GOOD 60 4b 1544 0
-66 GALLING 61 4c 1563 0 -64 GOOD 62 4c 1560 0 -61 GOOD 63 4c 1559
0 -60 GOOD 61 4c 1561 0 -62 GOOD 65 4c 1558 0 -63 GALLING
[0116] In every example, the concentration of the Mn-containing
inclusion and the number ratio of the Mn oxide to the Mn-containing
inclusion having a maximum length of 1.0 to 4.0 .mu.m fell within
the range specified in the present invention, and therefore
cracking did not occur in hole walls in the piercing test and the
ductility brittleness transition temperature was -60.degree. C. or
lower, so that a steel sheet (member) having both hydrogen
embrittlement resistance and toughness was obtained, but in samples
Nos. 55, 60 and 65 in which the thickness of the Al-plating layer
was more than 50 .mu.m, galling occurred in the hat-shaped
longitudinal wall portion with high frequency. On the other hand,
in samples Nos. 51 to 54, 56 to 59 and 61 to 64 in which the
thickness of the Al-plating layer was 50 .mu.m or less, galling did
not occur at all in the hat-shaped longitudinal wall portion.
Example 5
[0117] A steel piece having the chemical composition shown in Table
7 was formed into a 2.8 mm-thick hot-rolled steel sheet under the
conditions same as those in Example 1, and the steel sheet was
pickled, and then cold-rolled into a steel sheet having a sheet
thickness of 1.2 mm. The cold-rolled steel sheet was heated to
655.degree. C. at an average heating rate of 19.degree. C./s,
subsequently heated to 730 to 780.degree. C. at an average heating
rate of 2.5.degree. C./s, immediately cooled at an average cooling
rate of 6.5.degree. C./s, immersed in a hot-dip galvanizing bath
(containing Al in a concentration of 0.15% and impurities) at
460.degree. C., and taken out after 3 seconds. The deposition
amount was adjusted with a gas wiper, followed by air-cooling the
steel sheet to room temperature. Analysis of the inclusion of the
obtained steel sheet was performed in the same manner as in Example
1. In the same manner as in Example 2, the steel sheet was
hot-stamped into a hat shape, and a JIS No. 5 tensile test piece, a
piercing test piece and a Charpy impact test piece were taken from
the hat portion. For heating conditions for hot stamping, the steel
sheet was held at 900.degree. C. for 1 minute, nitrogen containing
hydrogen in a concentration of 3% was set as an atmosphere, and the
dew point was set to 0.degree. C. Analysis results related to the
inclusion are shown in Table 10, and test results related to the
hot stamp material are collectively shown in Table 11.
TABLE-US-00010 TABLE 10 Mn-CONTAINING INCLUSION HAVING MAXIMUM
LENGTH OF CONCENTRATION 1.0 TO 4.0 .mu.m THICKNESS OF NUMBER OF Mn-
NUMBER OF RATIO OF GALVANIZED CONTAINING OBSERVED NUMBER OF NUMBER
OF LAYER INCLUSION INCLUSIONS Mn OXIDES Mn OXIDES No. STEEL (.mu.m)
(% BY MASS) (NUMBER) (NUMBER) (%) 66 4a 6.3 0.15 500 66 13.2 67 4a
12.7 0.16 500 63 12.6 68 4a 23.6 0.15 500 68 13.6 69 4a 28.8 0.17
500 65 13.0 70 4a 31.1 0.15 500 60 12.0 71 4b 11.3 0.11 500 71 14.2
72 4b 19.4 0.13 500 75 15.0 73 4b 24.6 0.11 505 78 15.4 74 4b 29.2
0.13 500 66 13.2 75 4b 33.5 0.12 500 70 14.0 76 4c 10.1 0.15 500 65
13.0 77 4c 17.5 0.15 500 61 12.2 78 4c 19.8 0.17 500 58 11.6 79 4c
29.1 0.16 500 54 10.8 80 4c 32.5 0.15 500 69 13.8 UNDERLINES IN THE
TABLE INDICATE THE VALUES FALL OUT OF THE SUITABLE RANGE SPECIFIED
IN THE PRESENT INVENTION
TABLE-US-00011 TABLE 11 NUMBER OF DUCTILITY CRACKS IN BRITTLENESS
TENSILE HOLE WALL TRANSITION HOT STRENGTH PORTION TEMPERATURE
STAMPING No. STEEL (MPa) (NUMBER) (.degree. C.) STATE 66 4a 1499 0
-65 GOOD 67 4a 1504 0 -69 GOOD 68 4a 1503 0 -61 GOOD 69 4a 1507 0
-68 GOOD 70 4a 1511 0 -64 Zn ADHERED 71 4b 1543 0 -66 GOOD 72 4b
1561 0 -61 GOOD 73 4b 1566 0 -69 GOOD 74 4b 1569 0 -66 GOOD 75 4b
1567 0 -62 Zn ADHERED 76 4c 1640 0 -64 GOOD 77 4c 1646 0 -68 GOOD
78 4c 1640 0 -62 GOOD 79 4c 1645 0 -62 GOOD 80 4c 1652 0 -62 Zn
ADHERED
[0118] In every example, the concentration of the Mn-containing
inclusion and the number ratio of the Mn oxide to the Mn-containing
inclusion having a maximum length of 1.0 to 4.0 .mu.m fell within
the range specified in the present invention, and therefore
cracking did not occur in hole walls in the perforation test and
the ductility brittleness transition temperature was -60.degree. C.
or lower, so that a steel sheet (member) having both hydrogen
embrittlement resistance and toughness was obtained, but in samples
Nos. 70, 75 and 80 in which the thickness of the galvanized layer
was more than 30 adhesion of Zn to the die occurred with high
frequency. On the other hand, in samples Nos. 66 to 69, 71 to 74
and 76 to 79 in which the thickness of the galvanized layer was 30
.mu.m or less, adhesion of Zn to the die did not occur at all.
Example 6
[0119] A steel piece having the chemical composition shown in Table
7 was formed into a 2.8 mm-thick hot-rolled steel sheet under the
conditions same as those in Example 1, and the steel sheet was
pickled, and then cold-rolled (draft: 50%) into a steel sheet
having a sheet thickness of 1.4 mm. The cold-rolled steel sheet was
heated to 655.degree. C. at an average heating rate of 19.degree.
C./s, subsequently heated to 730 to 780.degree. C. at an average
heating rate of 2.5.degree. C./s, immediately cooled at an average
cooling rate of 6.5.degree. C./s, immersed in a hot-dip galvanizing
bath (containing Al in a concentration of 0.13%, Fe in a
concentration of 0.03% and impurities) at 460.degree. C., and taken
out after 3 seconds. The deposition amount was adjusted with a gas
wiper, the steel sheet was then heated to 480.degree. C. form an
alloyed hot-dip galvanized layer, and then air-cooled to room
temperature. Analysis of the inclusion of the obtained steel sheet
was performed in the same manner as in Example 1. In the same
manner as in Example 2, the steel sheet was hot-stamped into a hat
shape, and a JIS No. 5 tensile test piece, a piercing test piece
and a Charpy impact test piece were taken from the hat portion. For
heating conditions for hot stamping, the steel sheet was held at
900.degree. C. for 1 minute, nitrogen containing hydrogen in a
concentration of 3% was set as an atmosphere, and the dew point was
set to 0.degree. C. Analysis results related to the inclusion are
shown in Table 12, and test results related to the hot stamp
material are collectively shown in Table 13.
TABLE-US-00012 TABLE 12 Mn-CONTAINING INCLUSION HAVING MAXIMUM
LENGTH OF 1.0 TO 4.0 .mu.m CONCENTRATION NUMBER THICKNESS OF RATIO
OF OF Zn--Fe Mn- NUMBER OF NUMBER ALLOY CONTAINING OBSERVED NUMBER
OF OF LAYER INCLUSION INCLUSIONS Mn OXIDES Mn OXIDES No. STEEL
(.mu.m) (% BY MASS) (NUMBER) (NUMBER) (%) 81 4a 15.1 0.15 501 66
13.2 82 4a 22.5 0.16 501 68 13.6 83 4a 31.4 0.15 500 63 12.6 84 4a
39.7 0.17 500 61 12.2 85 4a 46.2 0.15 502 63 12.5 86 4b 15.5 0.11
510 75 14.7 87 4b 21.1 0.13 502 79 15.7 88 4b 39.3 0.11 504 80 15.9
89 4b 44.4 0.13 500 86 17.2 90 4b 49.5 0.12 500 70 14.0 91 4c 14.1
0.15 500 59 11.8 92 4c 20.6 0.15 500 63 12.6 93 4c 34.7 0.17 500 54
10.8 94 4c 42.1 0.16 504 59 11.7 95 4c 45.4 0.15 500 60 12.0
UNDERLINES IN THE TABLE INDICATE THE VALUES FALL OUT OF THE
SUITABLE RANGE SPECIFIED IN THE PRESENT INVENTION
TABLE-US-00013 TABLE 13 NUMBER OF DUCTILITY CRACKS IN BRITTLENESS
TENSILE HOLE WALL TRANSITION HOT STRENGTH PORTION TEMPERATURE
STAMPING No. STEEL (MPa) (NUMBER) (.degree. C.) STATE 81 4a 1500 0
-62 GOOD 82 4a 1507 0 -62 GOOD 83 4a 1499 0 -60 GOOD 84 4a 1503 0
-68 GOOD 85 4a 1507 0 -60 VERY SMALL CRACKS GENERATED 86 4b 1569 0
-67 GOOD 87 4b 1614 0 -66 GOOD 88 4b 1619 0 -69 GOOD 89 4b 1612 0
-63 GOOD 90 4b 1608 0 -60 VERY SMALL CRACKS GENERATED 91 4c 1681 0
-64 GOOD 92 4c 1647 0 -61 GOOD 93 4c 1641 0 -68 GOOD 94 4c 1646 0
-62 GOOD 95 4c 1653 0 -60 VERY SMALL CRACKS GENERATED
[0120] In every example, the concentration of the Mn-containing
inclusion and the number ratio of the Mn oxide to the Mn-containing
inclusion having a maximum length of 1.0 to 4.0 .mu.m fell within
the range specified in the present invention, and therefore
cracking did not occur in hole walls in the piercing test and the
ductility brittleness transition temperature was -60.degree. C. or
lower, so that a steel sheet (member) having both hydrogen
embrittlement resistance and toughness was obtained, but in samples
Nos. 85, 90 and 95 in which the thickness of the alloyed hot-dip
galvanized layer was more than 45 .mu.m, very small cracks were
generated in the alloy layer after pressing. On the other hand, in
samples Nos. 81 to 84, 86 to 89 and 91 to 94 in which the thickness
of the alloyed hot-dip galvanized layer was 45 .mu.m or less, very
small cracks were not generated at all in the alloy layer after
pressing.
INDUSTRIAL APPLICABILITY
[0121] According to the present invention, good hydrogen
embrittlement resistance can be secured even when processing
leading to remaining of stress, such as piercing, is performed
after hot stamping, and practice is easy, so that the range of
applications (components) of the hot stamping method can be
expanded. Accordingly, the present invention is highly usable in
steel sheet processing industries.
REFERENCE SIGNS LIST
[0122] 21a upper die
[0123] 21b lower die
[0124] 22 steel sheet
[0125] 41 test piece taking position CLAIMS
* * * * *