U.S. patent application number 15/039339 was filed with the patent office on 2017-02-02 for hot formed steel sheet component and method for producing the same as well as steel sheet for hot forming.
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 Koutarou HAYASHI.
Application Number | 20170029914 15/039339 |
Document ID | / |
Family ID | 53199177 |
Filed Date | 2017-02-02 |
United States Patent
Application |
20170029914 |
Kind Code |
A1 |
HAYASHI; Koutarou |
February 2, 2017 |
HOT FORMED STEEL SHEET COMPONENT AND METHOD FOR PRODUCING THE SAME
AS WELL AS STEEL SHEET FOR HOT FORMING
Abstract
A hot formed steel sheet component including: a chemical
composition including in terms of mass % C at from 0.100% to
0.340%, Si at from 0.50% to 2.00%, Mn at from 1.00% to 3.00%, P at
0.050% or less, S at 0.0100% or less, sol. Al at from 0.001% to
1.000%, and N at 0.0100% or less, with a remainder consisting of Fe
and impurities; and a steel structure including ferrite, at least
one of tempered martensite or tempered bainite, and martensite,
wherein an area rate of ferrite is from 5% to 50%, a total area
rate of tempered martensite and tempered bainite is from 20% to
70%, an area rate of martensite is from 25% to 75%, a total area
rate of ferrite, tempered martensite, tempered bainite and
martensite is 90% or more, and an area rate of retained austenite
is from 0% to 5%.
Inventors: |
HAYASHI; Koutarou; (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: |
53199177 |
Appl. No.: |
15/039339 |
Filed: |
November 28, 2014 |
PCT Filed: |
November 28, 2014 |
PCT NO: |
PCT/JP14/81514 |
371 Date: |
May 25, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/06 20130101;
C22C 38/04 20130101; C22C 38/16 20130101; B21D 22/022 20130101;
C21D 8/0221 20130101; C22C 38/00 20130101; B21D 35/005 20130101;
C22C 38/18 20130101; C21D 8/0447 20130101; C22C 38/005 20130101;
C22C 38/12 20130101; C21D 2211/008 20130101; C22C 38/002 20130101;
C22C 38/14 20130101; C21D 8/0436 20130101; C21D 9/46 20130101; C22C
38/38 20130101; C22C 38/02 20130101; C21D 2211/002 20130101; C21D
1/18 20130101; C21D 2211/005 20130101; C22C 38/08 20130101; C22C
38/001 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/38 20060101 C22C038/38; C22C 38/16 20060101
C22C038/16; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12; C21D 1/18 20060101 C21D001/18; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 8/02 20060101
C21D008/02; B21D 22/02 20060101 B21D022/02; C22C 38/08 20060101
C22C038/08 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2013 |
JP |
2013-247814 |
Claims
1. A steel component after hot pressing, comprising: a chemical
composition comprising, in terms of mass %, C at from 0.100% to
0.340%, Si at from 0.50% to 2.00%, Mn at from 1.00% to 3.00%, P at
0.050% or less, S at 0.0100% or less, sol. Al at from 0.001% to
1.000%, and N at 0.0100% or less, with a remainder consisting of Fe
and impurities; and a steel structure comprising ferrite, at least
one of tempered martensite or tempered bainite, and martensite,
wherein an area rate of ferrite is from 5% to 50%, a total area
rate of tempered martensite and tempered bainite is from 20% to
70%, an area rate of martensite is from 25% to 75%, a total area
rate of ferrite, tempered martensite, tempered bainite and
martensite is 90% or more, and an area rate of retained austenite
is from 0% to 5%.
2. The steel component after hot pressing according to claim 1,
wherein the chemical composition comprises at least one selected
from the group consisting of Ti at 0.200% or less, Nb at 0.200% or
less, V at 0.200% or less, Cr at 1.000% or less, Mo at 1.000% or
less, Cu at 1.000% or less, Ni at 1.000% or less, B at 0.0025% or
less, Ca at 0.0100% or less, Mg at 0.0100% or less, REM at 0.0100%
or less, Zr at 0.0100% or less, and Bi at 0.0100% or less, in terms
of mass %.
3-5. (canceled)
6. A steel sheet for hot pressing, comprising: a chemical
composition comprising, in terms of mass %, C at from 0.100% to
0.340%, Si at from 0.50% to 2.00%, Mn at from 1.00% to 3.00%, P at
0.050% or less, S at 0.0100% or less, sol. Al at from 0.001% to
1.000%, and N at 0.0100% or less, with a remainder consisting of Fe
and impurities; and a steel structure comprising ferrite with an
aspect ratio of 2.0 or less, and at least one of martensite or
bainite, wherein an area rate of ferrite is from 5% to 50%, a total
area rate of martensite and bainite is from 45% to 90%, and a total
area rate of ferrite, martensite, and bainite is 90% or more.
7. The steel sheet for hot pressing according to claim 6, wherein
the chemical composition comprises at least one selected from the
group consisting of Ti at 0.200% or less, Nb at 0.200% or less, V
at 0.200% or less, Cr at 1.000% or less, Mo at 1.000% or less, Cu
at 1.000% or less, Ni at 1.000% or less, B at 0.0025% or less, Ca
at 0.0100% or less, Mg at 0.0100% or less, REM at 0.0100% or less,
Zr at 0.0100% or less, in terms of mass %, and Bi at 0.0100% or
less, in terms of mass %.
8-10. (canceled)
11. A method for producing a steel component after hot pressing,
the method comprising: heating the steel for hot pressing according
to claim 6 to a temperature range of 720.degree. C. or higher but
lower than an Ac.sub.3 point; performing hot pressing within a time
period of from 3 sec to 20 sec, during which the steel sheet is
exposed to air cooling from the end of the heating until the
initiation of the hot pressing; and cooling to a temperature range
not above an M.sub.S point at an average cooling rate of from
10.degree. C./sec to 500.degree. C./sec.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hot formed steel sheet
component to be used, for example, in a machine structural
component such as an automobile body structural component, and to a
method for producing the same as well as to a steel sheet for hot
forming. Specifically, the invention relates to a hot formed steel
sheet component having superior ductility and bendability together
with a high tensile strength, and to a method for producing the
same as well as to a steel sheet for hot forming for yielding the
same.
BACKGROUND ART
[0002] In recent years, for the sake of weight reduction of an
automobile, it has been strived to enhance the strength of a steel
to be used for a body so as to reduce an employed weight. In the
case of a thin steel sheet used broadly for an automobile, when the
strength of a steel sheet is increased, the press formability is
compromised so that production of a component with a complicated
shape becomes difficult. Specifically, there occurs a problem, for
example, that the ductility is lowered so as to cause a fracture at
a highly processed region, or that the springback or wall warpage
becomes significant so as to impair dimensional accuracy.
Therefore, it is not easy to produce such a component by press
forming using a steel sheet with a high strength, especially with a
tensile strength in a range of 980 MPa or more. Although a high
strength steel sheet can be processed not by press forming but by
roll forming, it can be applicable only to a component having a
uniform cross-section in the longitudinal direction.
[0003] On the other hand, by a method called as hot pressing, by
which a heated steel sheet is press-formed as disclosed in Patent
Document 1, a component having a complicated shape can be formed
with high dimensional accuracy, because a steel sheet is soft and
highly-ductile at a high temperature. Further, when a steel sheet
is heated up to an austenite single phase region and then quenched
(hardened) in a mold, enhancement of the strength of a component
can be achieved at the same time due to a martensitic
transformation. Therefore, such a hot pressing method is a superior
forming method, which can attain a high strength of a component and
superior formability of a steel sheet simultaneously.
[0004] Further, Patent Document 2 discloses a pre-press quenching
method, by which a steel sheet is formed in advance to a
predetermined shape at room temperature, heated up to an austenite
region, and then quenched in a mold to achieve a higher strength of
a component. Since such a pre-press quenching method, which is an
embodiment of hot pressing, can suppress deformation to be caused
by a thermal strain by restraining the component with a mold, it is
a superior forming method that secures a higher strength of a
component and high dimensional accuracy at the same time.
[0005] However, ductility has come to be also required recently for
a hot steel sheet component, and conventional art as represented by
Patent Document 1 or Patent Document 2, in which the steel
structure is substantially a martensite single phase, confronts a
problem that such a requirement cannot be satisfied.
[0006] With such a background, a hot pressed steel sheet component
that is allegedly superior in terms of high strength and ductility
owing to the two phase structure of ferrite and martensite, for
which a steel sheet is heated to a two phase temperature range of
ferrite and austenite, pressed while keeping the two phase
structure, and quenched in a mold, is disclosed in Patent Document
3. However, under such a two phase heating condition, a steel
structure is apt to become nonuniform, and the bendability and the
toughness of a hot pressed steel sheet component may be
deteriorated and the impact absorption characteristic may be
impaired extremely.
[0007] Meanwhile, Patent Document 4 discloses a hot pressed steel
sheet component, which is yielded by heating a steel sheet having a
steel structure with 80 volume-% or more of martensite or bainite
at an Ac.sub.1 transformation point or higher, and then quenching
it in a mold to have a structure containing from 3 to 20 volume-%
of retained austenite, from 30 to 97 volume-% of tempered
martensite or tempered bainite, and from 0 to 67 volume-% of
martensite, and is allegedly superior in terms of high strength and
ductility.
[0008] Furthermore, Patent Document 5 discloses a high strength
pressed component satisfying that the area rate of martensite with
respect to the entire steel sheet structure is from 10% to 85%, 25%
or more of the martensite is tempered martensite, the content of
retained austenite is from 5% to 40%, the area rate of bainitic
ferrite in bainite with respect to the entire steel sheet structure
is 5% or more, and a total of the area rate of martensite, the area
rate of retained austenite, and the area rate of bainitic ferrite
in the bainite with respect to the entire steel sheet structure is
65% or more.
[0009] Patent Document 6 discloses a steel sheet for hot pressing
in which a total fraction of bainite and martensite is 80% by area
or more.
[0010] Further, Patent Document 7 discloses a steel sheet for hot
pressing in which the fraction of ferrite is 30% by area or
more.
CITATION LIST
Patent Literature
[0011] Patent Document 1: GB Patent No. 1490535 [0012] Patent
Document 2: Japanese Patent Application Laid-Open (JP-A) No.
H10-96031 [0013] Patent Document 3: JP-A No. 2010-65292 [0014]
Patent Document 4: JP-A No. 2012-237066 [0015] Patent Document 5:
International Publication No. WO2011/111333 [0016] Patent Document
6: JP-A No. 2013-185243 [0017] Patent Document 7: JP-A No.
2013-185248
SUMMARY OF INVENTION
Technical Problem
[0018] It has become clear through investigation by the present
inventors that by making the steel structure of a steel sheet for
hot pressing contain mainly bainite or martensite, not only the
ductility of a hot pressed steel sheet component as described, for
example, in Patent Literature 4, but also the toughness is
improved. However, even by such regulation of the structure of a
component, deterioration of the bendability cannot be resolved, and
a bending crack in a component to appear at a buckling region
during impact deformation cannot be prevented. This drawback
becomes obvious when the tensile strength of a steel becomes high
(for example, 980 MPa or more). With respect to a hot pressed steel
sheet component with a high tensile strength (for example, tensile
strength of 980 MPa or more) and superior in bendability in
addition to ductility, a product itself has been heretofore not yet
proposed, let alone establishment of a production technology.
[0019] Similarly, even when the whole hot formed steel sheet
components including a roll-formed component besides a hot pressed
steel sheet component are surveyed, with respect to a hot formed
steel sheet component with a high tensile strength (for example,
tensile strength of 980 MPa or more) and superior in bendability in
addition to ductility, a product itself has been heretofore not yet
proposed, let alone establishment of a production technology.
[0020] A specific object of the invention is to provide a hot
pressed steel sheet component, which has a high tensile strength
and is superior in ductility and bendability after hot pressing,
not available according to conventional art as described above, and
a method for producing the same as well as a steel sheet for hot
pressing for yielding the same. By generalization, the invention is
also applicable to hot forming provided with a means for cooling a
steel sheet simultaneously during or immediately after forming as
in the case of hot pressing. Therefore, a specific object of the
invention is to provide a hot formed steel sheet component superior
in ductility and bendability while having a high tensile strength
after hot forming, and a method for producing the same as well as a
steel sheet for hot forming for yielding the same.
Solution to Problem
[0021] The inventors studied diligently for improving the ductility
and the bendability of a hot formed steel sheet component with a
high tensile strength. As the result, the following novel knowledge
has been acquired. Namely, a steel sheet for hot forming including
a chemical composition in which Si is actively added to specific
amounts of C and Mn, and also including a steel structure
containing ferrite and at least one of martensite or bainite is to
be utilized. Further, a heat treatment condition for hot forming
optimum to the steel sheet for hot forming is to be applied. By the
above means, differently from a conventional hot formed steel sheet
component, the steel structure can be made to include a dual phase,
which contains no, or not more than 5% of, retained austenite in
terms of area rate, and contains ferrite, at least one of tempered
martensite or tempered bainite, and martensite at predetermined
area rates. As a result, a novel knowledge has been acquired that a
hot formed steel sheet component superior in ductility and
bendability while having a high tensile strength can be produced,
when the chemical composition and the steel structure are
present.
[0022] The invention based on the knowledges is as follows:
[1] A hot formed steel sheet component comprising:
[0023] a chemical composition comprising in terms of mass % C at
from 0.100% to 0.340%, Si at from 0.50% to 2.00%, Mn at from 1.00%
to 3.00%, P at 0.050% or less, S at 0.0100% or less, sol. Al at
from 0.001% to 1.000%, and N at 0.0100% or less, with a remainder
consisting of Fe and impurities; and
[0024] a steel structure comprising ferrite, at least one of
tempered martensite or tempered bainite, and martensite, wherein an
area rate of ferrite is from 5% to 50%, a total area rate of
tempered martensite and tempered bainite is from 20% to 70%, an
area rate of martensite is from 25% to 75%, a total area rate of
ferrite, tempered martensite, tempered bainite and martensite is
90% or more, and an area rate of retained austenite is from 0% to
5%.
[2] The hot formed steel sheet component according to Paragraph
[1], wherein the chemical composition comprises one kind or two or
more kinds selected from the group consisting of Ti at 0.200% or
less, Nb at 0.200% or less, V at 0.200% or less, Cr at 1.000% or
less, Mo at 1.000% or less, Cu at 1.000% or less, and Ni at 1.000%
or less, in terms of mass % instead of a part of the Fe. [3] The
hot formed steel sheet component according to Paragraph [1] or
Paragraph [2], wherein the chemical composition comprises B at
0.0025% or less, in terms of mass % instead of a part of the Fe.
[4] The hot formed steel sheet component according to any one of
Paragraph [1] to Paragraph [3], wherein the chemical composition
comprises one kind or two or more kinds selected from the group
consisting of Ca at 0.0100% or less, Mg at 0.0100% or less, REM at
0.0100% or less, and Zr at 0.0100% or less, in terms of mass %
instead of a part of the Fe. [5] The hot formed steel sheet
component according to any one of Paragraph [1] to Paragraph [4],
wherein the chemical composition comprises Bi at 0.0100% or less,
in terms of mass % instead of a part of the Fe. [6] A steel sheet
for hot forming comprising:
[0025] a chemical composition comprising in terms of mass % C at
from 0.100% to 0.340%, Si at from 0.50% to 2.00%, Mn at from 1.00%
to 3.00%, P at 0.050% or less, S at 0.0100% or less, sol. Al at
from 0.001% to 1.000%, and N at 0.0100% or less, with a remainder
consisting of Fe and impurities; and
[0026] a steel structure comprising ferrite with an aspect ratio of
2.0 or less, and at least one of martensite or bainite, wherein an
area rate of ferrite is from 5% to 50%, a total area rate of
martensite and bainite is from 45% to 90%, and a total area rate of
ferrite, martensite, and bainite is 90% or more.
[7] The steel sheet for hot forming according to Paragraph [6],
wherein the chemical composition comprises one kind or two or more
kinds selected from the group consisting of Ti at 0.200% or less,
Nb at 0.200% or less, V at 0.200% or less, Cr at 1.000% or less, Mo
at 1.000% or less, Cu at 1.000% or less, and Ni at 1.000% or less,
in terms of mass % instead of a part of the Fe. [8] The steel sheet
for hot forming according to Paragraph [6] or Paragraph [7],
wherein the chemical composition comprises B at 0.0025% or less, in
terms of mass % instead of a part of the Fe. [9] The steel sheet
for hot forming according to any one of Paragraph [6] to Paragraph
[8], wherein the chemical composition comprises one kind or two or
more kinds selected from the group consisting of Ca at 0.0100% or
less, Mg at 0.0100% or less, REM at 0.0100% or less, and Zr at
0.0100% or less, in terms of mass % instead of a part of the Fe.
[10] The steel sheet for hot forming according to any one of
Paragraph [6] to Paragraph [9], wherein the chemical composition
comprises Bi at 0.0100% or less, in terms of mass % instead of a
part of the Fe. [11] A method for producing a hot formed steel
sheet component, the method comprising: heating the steel sheet for
hot forming according to any one of Paragraph [6] to Paragraph [10]
to a temperature range of 720.degree. C. or higher but lower than
an Ac.sub.3 point; performing hot forming within a time period of
from 3 sec to 20 sec, during which the steel sheet is exposed to
air cooling from the end of the heating until the initiation of the
hot forming; and cooling to a temperature range not above an
M.sub.S point at an average cooling rate of from 10.degree. C./sec
to 500.degree. C./sec.
Advantageous Effects of Invention
[0027] A technologically valuable effect that a hot formed steel
sheet component, which, as hot formed, has a high tensile strength,
and is superior in ductility a well as bendability, can be at last
put into practical use, has been attained by the invention. A hot
formed steel sheet component according to the invention exhibits
extremely superior collision characteristics, such that it can
absorb an impact through a bending deformation even at a collision
causing most severe plastic deformation. Therefore, a hot formed
steel sheet component according to the invention is especially
suitable for producing a structural component of an automobile
body, however it is naturally applicable to another use such as a
machine structural component.
BRIEF DESCRIPTION OF DRAWINGS
[0028] FIG. 1 is a photograph showing an example of a steel
structure according to the invention.
DESCRIPTION OF EMBODIMENTS
[0029] Next, reasons behind limitations of each range imposed
according to the invention will be described. In this regard, hot
forming will be described below taking hot pressing, which is a
specific embodiment thereof, as an example. A numerical range
expressed by "x to y" includes the values of x and y in the range
as the minimum and maximum values, respectively.
[0030] 1. Chemical Composition
[0031] Firstly, reasons behind the above definition of the chemical
composition of a hot formed steel sheet component according to the
invention (hereinafter also referred to simply as "steel sheet
component") and a steel sheet for hot forming according to the
invention (hereinafter also referred to simply as "steel sheet")
will be described. In the following descriptions, "%" representing
the content of each alloy element means "mass %" unless otherwise
specified.
[0032] (C at 0.100% to 0.340%)
[0033] C is a very important element, which enhances the
hardenability of a steel and predominantly decides the strength
after hot pressing (after quenching). When the C content is less
than 0.100%, it becomes difficult to secure a high tensile strength
(for example, tensile strength of 980 MPa or more) after hot
pressing (after quenching). Therefore the C content should be
0.100% or more, and is preferably 0.120% or more. Meanwhile, when
the C content is above 0.340%, martensite after hot pressing (after
quenching) becomes rigid, such that not only deterioration of the
bendability becomes remarkable, but also the ductility declines.
Therefore, the C content should be 0.340% or less. From the
viewpoint of weldability, the C content is preferably 0.300% or
less and still more preferably 0.280% or less.
[0034] (Si at 0.50% to 2.00%)
[0035] Si is a very effective element for enhancing the ductility
of a steel heated into a two phase temperature range of ferrite and
austenite, and securing a stable strength after hot pressing (after
quenching). When the Si content is less than 0.50%, the effects are
difficult to obtain. Therefore, the Si content should be 0.50% or
more. From the viewpoint of improvement of weldability, the Si
content is preferably 0.70% or more, and still more preferably
1.10% or more. Meanwhile, when the Si content is above 2.00%, the
effect of the above action becomes saturated, which is economically
disadvantageous, and defective plating occurs frequently due to a
remarkable decrease in plating wettability. Therefore, the Si
content should be 2.00% or less. From the viewpoint of suppression
of surface defects in a hot formed steel sheet component, the Si
content is preferably 1.80% or less, and still more preferably
1.50% or less.
[0036] (Mn at 1.00% to 3.00%)
[0037] Mn is a very effective element for improving the
hardenability of a steel and securing a strength after hot pressing
(after quenching). However, when the Mn content is less than 1.00%,
not only it becomes very difficult to secure a high tensile
strength (for example, tensile strength of 980 MPa or more) after
hot pressing (after quenching), but also the bendability may be
compromised. Therefore, the Mn content should be 1.00% or more. For
securing the action more stably, the Mn content is preferably 1.10%
or more, and still more preferably 1.20% or more. Meanwhile, when
the Mn content is above 3.00%, a steel structure after hot pressing
(after quenching) shows an obvious band caused by Mn segregation,
which compromises the toughness and remarkably deteriorates the
collision characteristics. Therefore, the Mn content should be
3.00% or less. From the viewpoint of the productivity of hot
rolling and cold rolling, the Mn content is preferably 2.50% or
less and still more preferably 2.40% or less.
[0038] By defining C, Si and Mn within the above ranges, the steel
structure of a steel sheet for hot forming can be made to a steel
structure with a dual phase containing ferrite and at least one of
martensite or bainite. Further, by defining the heating conditions
during hot pressing according to the invention, the steel structure
of a hot formed steel sheet component can be made to a desired
steel structure with a dual phase.
[0039] (P at 0.050% or Less)
[0040] Although P is generally an impurity contained in a steel,
since it has an action to enhance the strength of a steel sheet by
solid solution strengthening, it may be added actively. However,
when the P content is above 0.050%, deterioration of the
weldability becomes conspicuous. Therefore, the P content should be
0.050% or less. The P content is preferably 0.018% or less. For
securing an effect of the action, the P content is preferably
0.003% or more.
[0041] (S at 0.0100% or Less)
[0042] S is an impurity contained in a steel, and the content is
preferably as low as possible from a viewpoint of weldability. When
the S content is above 0.0100%, deterioration of the weldability
becomes conspicuous. Therefore, the S content should be 0.0100% or
less. The S content is preferably 0.0030% or less, and still more
preferably 0.0015% or less. From the viewpoint of desulfurization
costs, the S content is preferably 0.0006% or more.
[0043] (Sol. Al (Soluble Al) at 0.001% to 1.000%)
[0044] Al is an element having an action to make the quality of a
steel robust through deoxidation. When the sol. Al content is less
than 0.001%, it becomes difficult to obtain the action. Therefore,
the sol. Al content should be 0.001% or more, and is preferably
0.015% or more. Meanwhile, when the sol. Al content is above
1.000%, deterioration of the weldability becomes conspicuous, and
further an oxide-type inclusion increases so that deterioration of
a surface condition becomes also conspicuous. Therefore, the sol.
Al content should be 1.000% or less, and is preferably 0.080% or
less. In this regard, sol. Al means acid-soluble Al, which is not
in a form of an oxide such as Al.sub.2O.sub.3, and soluble in an
acid.
[0045] (N at 0.0100% or Less)
[0046] N is an impurity contained in a steel, and the content is
preferably as low as possible from a viewpoint of weldability. When
the N content is above 0.0100%, deterioration of the weldability
becomes conspicuous. Therefore, the N content should be 0.0100% or
less, and is preferably 0.0060% or less. From a viewpoint of
denitrification cost, the N content is preferably 0.0020% or
more.
[0047] [Impurities]
[0048] Impurities refer to ingredients contained in a source
material or ingredients entered during a process of production,
which are ingredients that are not intentionally added to a steel
sheet component or a steel sheet for hot forming.
[0049] The chemical compositions of a steel sheet component and a
steel sheet for hot forming according to the invention may contain
further at least one kind of the elements described below.
[0050] (One Kind or Two or More Kinds Selected from the Group
Consisting of Ti at 0.200% or Less, Nb at 0.200% or Less, V at
0.200% or Less, Cr at 1.000% or Less, Mo at 1.000% or Less, Cu at
1.000% or Less, and Ni at 1.000% or Less)
[0051] Each of the elements is an element having an effect of
securing a stably high strength after hot pressing (after
quenching). Therefore, one kind or two or more kinds of the
elements may be added. However, with respect to Ti, Nb and V, when
any of them is contained beyond 0.200%, not only hot rolling and
cold rolling may become difficult, but also securement of a stably
high strength may become difficult. Therefore, preferably the Ti
content, the Nb content, and the V content are respectively 0.200%
or less. With respect to Cr, when the content exceeds 1.000%,
securement of a stably high strength may become difficult.
Therefore, the Cr content is preferably 1.000% or less. With
respect to Mo, when the content exceeds 1.000%, hot rolling and
cold rolling may become difficult. Therefore, the Mo content is
preferably 1.000% or less. With respect to Cu and Ni, when the
content of any of them exceeds 1.000%, an effect of the action is
apt to be saturated so that the economy may become disadvantageous,
and further hot rolling or cold rolling may become difficult.
Therefore, preferably the Cu content, and the Ni content are
respectively 1.000% or less.
[0052] For securing an effect of the action, it is preferable to
satisfy at least one of Ti at 0.003% or more, Nb at 0.003% or more,
V at 0.003% or more, Cr at 0.005% or more, Mo at 0.005% or more, Cu
at 0.005% or more, or Ni at 0.005% or more.
[0053] In other words, the lower limit of the Ti content is
preferably 0.003%. The lower limit of the Nb content is preferably
0.003%. The lower limit of the V content is preferably 0.003%. The
lower limit of the Cr content is preferably 0.005%. The lower limit
of the Mo content is preferably 0.005%. The lower limit of the Cu
content is preferably 0.005%. The lower limit of the Ni content is
preferably 0.005%.
[0054] (B at 0.0025% or Less)
[0055] B is an element having an action to enhance the toughness of
a steel. Therefore, B may be added. However, when B is added in an
amount exceeding 0.0025%, it may become difficult for the steel
structure in a steel sheet for hot forming to contain ferrite, and
the ductility and the bendability of a hot formed steel sheet
component may be deteriorated. Therefore, the B content should be
preferably 0.0025% or less. Further, for securing an effect of the
action, the B content is preferably 0.0003% or more.
[0056] (One Kind or Two or More Kinds Selected from the Group
Consisting of Ca at 0.0100% or Less, Mg at 0.0100% or Less, REM at
0.0100% or Less, and Zr at 0.0100% or Less)
[0057] Any of these elements is an element having an action to
enhance the toughness through contribution to control of inclusion,
especially to microdispersion of inclusions. Therefore, one kind or
two or more kinds of the elements may be added. However, when the
content of any element exceeds 0.0100%, deterioration of a surface
condition may become conspicuous. Therefore, the content of each
element is preferably 0.0100% or less. For obtaining an effect of
the action more securely, the content of at least one of the
elements should be preferably 0.0003% or more. Namely, preferably
the lower limits of the Ca content, the Mg content, the REM
content, and the Zr content are respectively 0.0003%.
[0058] In this regard, REM represents at least one kind of a total
of 17 elements including Sc, Y, and lanthanoids. The above REM
content means a total content of at least one kind of the elements.
A lanthanoid is industrially added in a form of a misch metal.
[0059] (Bi at 0.0100% or Less)
[0060] Bi is an element having an action to make the structure
uniform and enhance the bendability. Therefore, Bi may be
contained. However, when Bi is added more than 0.0100%, the hot
working property may be deteriorated so that hot rolling may become
difficult. Therefore, the Bi content should be preferably 0.0100%
or less. For obtaining an effect of the action more securely, the
Bi content should be preferably 0.0003% or more.
[0061] 2. Steel Structure of Hot Formed Steel Sheet Component
[0062] Next, the steel structure of a hot formed steel sheet
component according to the invention will be described.
[0063] A hot formed steel sheet component according to the
invention includes a steel structure containing ferrite, at least
one of tempered martensite or tempered bainite, and martensite at
the following predetermined area rates. In other words, the steel
structure may contain one of tempered martensite or tempered
bainite, or contain both of the same. Further, the steel structure
contains no retained austenite, or contains the same only not more
than 5% by area rate.
[0064] FIG. 1 shows an example of a steel structure according to
the invention. The steel structure shown in FIG. 1 is a steel
structure containing ferrite, tempered martensite, and martensite,
but not containing retained austenite.
[0065] (Area Rate of Ferrite at 5% to 50%)
[0066] When the area rate of ferrite is less than 5%, the ductility
and the bendability decline. Therefore, the area rate of ferrite
should be 5% or more, and is preferably 15% or more. Meanwhile,
when the area rate of ferrite is above 50%, the bendability
declines. Therefore, the area rate of ferrite should be 50% or
less, and is preferably 40% or less,
[0067] The aspect ratio of ferrite is preferably 2.0 or less from a
viewpoint of suppression of decline in bendability. When the aspect
ratio of ferrite exceeds 2.0, the anisotropy of ferrite (crystal
grain of ferrite) increases, and such ferrite may constitute an
origin of stress concentration, and the bendability may decline.
Therefore, the aspect ratio of ferrite is preferably 2.0 or less,
and more preferably 1.8 or less. Meanwhile, when the aspect ratio
of ferrite approaches closer to 1.0, the anisotropy of ferrite
(crystal grain of ferrite) decreases further, therefore the lower
limit of the aspect ratio of ferrite is preferably 1.0. However,
from a viewpoint of enhancement of the yield strength of a steel
sheet component after hot pressing, the lower limit of the aspect
ratio of ferrite is preferably 1.2.
[0068] An aspect ratio of ferrite is a value measured by the method
described precisely in Example presented below.
[0069] (Total Area Rate of Tempered Martensite and Tempered Bainite
at 20% to 70%)
[0070] When the total area rate of tempered martensite and tempered
bainite is less than 20%, the bendability declines. Therefore, the
total area rate of tempered martensite and tempered bainite should
be 20% or more, and is preferably 30% or more. Meanwhile, when the
total area rate of tempered martensite and tempered martensite is
above 70%, the ductility declines. Therefore, the total area rate
of tempered martensite and tempered bainite should be 70% or less,
and is preferably 50% or less.
[0071] (Area Rate of Martensite at 25% to 75%)
[0072] By forming martensite in a steel, the strength after hot
pressing (after quenching) can be enhanced. When the area rate of
martensite is less than 25%, it becomes difficult to secure a high
tensile strength (for example, tensile strength of 980 MPa or more)
after hot pressing (after quenching). Therefore, the area rate of
martensite should be 25% or more. Meanwhile, when the area rate of
martensite is above 75%, the ductility declines. Therefore, the
area rate of martensite should be 75% or less, and is preferably
50% or less.
[0073] In this regard, "martensite" means both of as-quenched
martensite, and martensite after age hardening formed by
age-hardening as-quenched martensite. Namely, the "area rate of
martensite" means the total area rate of as-quenched martensite,
and martensite after age hardening formed by age-hardening
as-quenched martensite.
[0074] (Total Area Rate of Ferrite, Tempered Martensite, Tempered
Bainite, and Martensite at 90% or More)
[0075] Basically, a hot formed steel sheet component according to
the invention has a structure containing ferrite, tempered
martensite, tempered bainite, and martensite. However, depending on
a production condition, as a phase or a structure other than the
above, one kind or two or more kinds of bainite, retained
austenite, cementite, or pearlite may be mixed in. In this case,
when the percentage of such a phase or a structure other than
ferrite, tempered martensite, tempered bainite, and martensite
exceeds 10%, an intended characteristic may not be obtained due to
an influence of the phase or structure. Therefore, the mixture of a
phase or a structure other than ferrite, tempered martensite,
tempered bainite, and martensite should be 10% or less, and is
preferably 5% or less. Namely, the total area rate of ferrite,
tempered martensite, tempered bainite, and martensite should be 90%
or more, and preferably 95% or more. The upper limit of the total
area rate of ferrite, tempered martensite, tempered bainite, and
martensite is 100%.
[0076] (Area Rate of Retained Austenite at 0% to 5%)
[0077] With respect to a phase or a structure other than ferrite,
tempered martensite, tempered bainite, and martensite, when
especially retained austenite is mixed (retained) more than 5% in
terms of area rate, the bendability declines. Therefore, retained
austenite should not be contained, or even if it should be
contained, the area rate of retained austenite should be 5% or
less, and is preferably 3% or less. The area rate of retained
austenite is most preferably 0%.
[0078] The area rate of each phase and structure in the steel
structure of a hot formed steel sheet component is a value measured
by the method described precisely in Example presented below.
[0079] A steel sheet component according to the invention means a
component hot formed from a steel sheet, and includes, for example,
a steel sheet component formed by hot pressing. As a typical
example, there is a door guard bar to be used as an automobile body
structural component. Further, for an automobile use, there is, for
example, a bumper reinforcement. As a machine structural component,
there is a hot formed steel pipe for a building structure produced
from a steel sheet as a source material.
[0080] 3. Mechanical Properties
[0081] It is preferable that a hot formed steel sheet component
according to the invention has a tensile strength (TS) of 980 MPa
or more, which is adequate to contribute to weight reduction of an
automobile.
[0082] 4. Production Method
[0083] Next, a preferable method for producing a hot formed steel
sheet component according to the invention having the above
characteristics will be described.
[0084] In order to attain favorable ductility and bendability for a
hot formed steel sheet component according to the invention, while
securing a high tensile strength (for example, tensile strength of
980 MPa or more), the steel structure after hot pressing (after
quenching) should better, as described above, not be a martensite
single phase, but rather be a dual phase structure, in which the
area rate of ferrite is from 5% to 50%, the total area rate of
tempered martensite, and tempered bainite is from 20% to 70%, the
area rate of martensite is from 25% to 75%, the total area rate of
ferrite, tempered martensite, tempered bainite, and martensite is
90% or more, as well as the area rate of retained austenite is from
0% to 5%.
[0085] In order to obtain a steel structure for a hot formed steel
sheet component according to the invention, as a steel sheet (steel
sheet for hot forming), which is a source material for hot forming,
a steel sheet including the above chemical composition, and a steel
structure (dual phase structure) containing ferrite with an aspect
ratio of 2.0 or less, and at least one of martensite or bainite, in
which structure the area rate of ferrite is from 5% to 50%, the
total area rate of martensite and bainite is from 45% to 90%, the
total area rate of ferrite, martensite, and bainite is 90% or more,
is preferably used. Then, preferably the steel sheet (steel sheet
for hot forming) is heated into a temperature range of 720.degree.
C. or higher but lower than an Ac.sub.3 point, then performed to
hot pressing within a time period of from 3 sec to 20 sec, during
which the steel sheet is exposed to air cooling from the end of the
heating until the initiation of the hot pressing, and then cooled
to a temperature range not above an M.sub.S point at an average
cooling rate of from 10.degree. C./sec to 500.degree. C./sec.
[0086] By hot pressing a steel sheet for hot forming including the
chemical composition and the steel structure under the above
conditions, a hot formed steel sheet component having a desired
steel structure after hot pressing, with a high tensile strength
(for example, tensile strength of 980 MPa or more), and superior in
ductility and bendability can be obtained.
[0087] (Steel Structure of Steel Sheet for Hot Forming)
--Aspect Ratio of Ferrite at 2.0 or Less--
[0088] When the aspect ratio of ferrite is above 2.0, the aspect
ratio of ferrite in a steel structure of a steel sheet component
after hot pressing may also exceed 2.0, and further the ferrite
area rate in a steel sheet component after hot pressing may fall
below 5%, because ferrite is transformed excessively to austenite
during heating. When the aspect ratio of ferrite of steel sheet
component exceeds 2.0, the anisotropy of ferrite (crystal grain of
ferrite) increases and constitutes an origin of stress
concentration, so that the bendability may decline. Therefore, the
aspect ratio of ferrite should be 2.0 or less, and is preferably
1.8 or less. Meanwhile, when the aspect ratio of ferrite approaches
closer to 1.0, the anisotropy of ferrite (crystal grain of ferrite)
decreases further, therefore the lower limit of the aspect ratio of
ferrite is preferably 1.0. However, from a viewpoint of enhancement
of the yield strength of a steel sheet component after hot
pressing, the lower limit of the aspect ratio of ferrite is
preferably 1.2.
[0089] An aspect ratio of ferrite is a value measured by the method
described precisely in Example presented below.
--Area Rate of Ferrite at 5% to 50% --
[0090] When the area rate of ferrite is less than 5%, the area rate
of ferrite in a steel structure of a steel sheet component after
hot pressing may also become less than 5%. Therefore, the area rate
of ferrite should be 5% or more, and is preferably 15% or more.
Similarly, when the area rate of ferrite is above 50%, the area
rate of ferrite in a steel structure of a steel sheet component
after hot pressing may also exceed 50%. Therefore, the area rate of
ferrite should be 50% or less, and is preferably 45% or less.
--Total Area Rate of Martensite and Bainite at 45% to 90%--
[0091] When the total area rate of martensite and bainite is less
than 45%, the total area rate of tempered martensite and tempered
bainite in the steel structure of a steel sheet component after hot
pressing may become less than 20%. Further, the area rate of
martensite in the steel structure of a steel sheet component after
hot pressing may become less than 25%. Therefore, the total area
rate of martensite and bainite should be 45% or more, and is
preferably 50% or more. Similarly, when the total area rate of
martensite and bainite is above 90%, the total area rate of
tempered martensite and tempered bainite in the steel structure of
a steel sheet component after hot pressing may also exceed 70%.
Further, the area rate of martensite in the steel structure of a
steel sheet component after hot pressing may exceed 75%. Therefore,
the total area rate of martensite and bainite should be 90% or
less, and is preferably 80% or less.
--Total Area Rate of Ferrite, Martensite, and Bainite at 90% or
More--
[0092] When the total area rate of ferrite, martensite, and bainite
is less than 90%, mixture of a phase or a structure other than
ferrite, tempered martensite, tempered bainite, and martensite in
the steel structure of a steel sheet component after hot pressing
may exceed 10%. Especially, the area rate of retained austenite may
exceed 5%. Therefore, the total area rate of ferrite, martensite,
and bainite should be 90% or more, and is preferably 93% or more.
The upper limit of the total area rate of ferrite, martensite, and
bainite is 100%.
[0093] An area rate of each phase and structure in the steel
structure of a steel sheet for hot forming is a value measured by
the method described precisely in Example presented below.
[0094] (Production of Steel Sheet for Hot Forming)
[0095] A steel sheet for hot forming may be any of a hot-rolled
steel sheet, a cold-rolled steel sheet, and a coated steel sheet.
Examples of a coated steel sheet include aluminum coated steel
sheet, and zinc coated steel sheet.
[0096] A hot-rolled steel sheet having the above steel structure
can be produced in a hot rolling step, by defining C, Si and Mn
within the ranges in terms of the chemical composition, and
completing finish-rolling at from 850.degree. C. to 930.degree. C.,
retaining the product in process in a range from 740.degree. C. to
660.degree. C. for 3 sec or more, and winding the same in a
temperature range of 450.degree. C. or less. Further, a cold-rolled
steel sheet having the above steel structure can be produced, after
cold rolling, by heating the product in process at from 780.degree.
C. to 900.degree. C., and then cooling the same at an average
cooling rate of 10.degree. C./sec or more in an annealing step. A
coated steel sheet having the above steel structure can be
produced, after production of the hot-rolled steel sheet or the
cold-rolled steel sheet, by performing a well known plating
treatment on a surface of the hot-rolled steel sheet or the
cold-rolled steel sheet.
[0097] (Heating of Steel Sheet for Hot Forming: Heating to
Temperature Range of 720.degree. C. or Higher but Lower than an
Ac.sub.3 Point)
[0098] Heating of a steel sheet for hot forming is performed up to
a temperature of 720.degree. C. or higher but lower than an
Ac.sub.3 point. In this regard, the Ac.sub.3 point (.degree. C.) is
a temperature defined by the following empirical Formula (i), which
is lower than the Ac.sub.3 point (.degree. C.) of an austenite
single phase.
Ac.sub.3=910-203.times.(C.sup.0.5)-15.2.times.Ni+44.7.times.Si+104.times-
.V+31.5.times.Mo-30.times.Mn-11.times.Cr-20.times.Cu+700.times.P+400.times-
.sol. Al+50.times.Ti (i)
[0099] In this case the symbols of elements in Formula (i)
represent the contents of the respective elements (by mass %) in
the chemical composition of a steel sheet. In this regard, Formula
(i) is calculated by putting the content of an element not
contained in a steel sheet as 0 (0 mass %).
[0100] When a heating temperature is less than 720.degree. C.,
austenitization becomes insufficient and martensite is not
contained in a hot pressed steel sheet, so that securement of a
high tensile strength (for example, tensile strength of 980 MPa or
more) after hot pressing (after quenching) becomes difficult.
Therefore, a heating temperature should be 720.degree. C. or
higher, and is preferably 750.degree. C. or higher. Meanwhile, when
a heating temperature is not lower than an Ac.sub.3 point, even if
a steel sheet is exposed to air cooling thereafter, the area rate
of martensite in a steel structure after hot pressing (after
quenching) exceeds 75%, and deterioration of the ductility becomes
remarkable. Therefore, a heating temperature should be not higher
than an Ac.sub.3 point, and is preferably not higher than an
Ac.sub.3 point -30.degree. C.
[0101] Although there is no particular restriction on a heating
rate up to 720.degree. C. and a heating time for retention in the
temperature range, they are preferably in the following ranges
respectively.
[0102] The average heating rate in heating up to 720.degree. C. is
preferably from 0.2.degree. C./sec to 100.degree. C./sec. When the
average heating rate is 0.2.degree. C./sec or more, high
productivity can be secured. Further, when the average heating rate
is 100.degree. C./sec or less, the heating temperature can be
regulated easily, even in a case in which heating is conducted in
an ordinary furnace.
[0103] The heating time in a temperature range of 720.degree. C. or
higher and lower than an Ac.sub.3 point is preferably from 2 min to
10 min. In this regard, a heating time is a time period from a time
point, when the temperature of a steel sheet reaches 720.degree.
C., to a time point of completion of heating. Specifically, the
time point of completion of heating means, in the case of furnace
heating, a time point, when a steel sheet is taken out of a heating
furnace, and in the case of Joule heating or induction heating, a
time point, when the power supply is cut off. When the heating time
is 2 min or more, the strength after hot pressing (after quenching)
can be made more stable. When the retention time is 10 min or less,
the structure of a steel sheet component can be micronized further,
so that the toughness of a steel sheet component can be further
improved.
[0104] (Time Period from Completion of Heating to Initiation of Hot
Pressing, During which Steel Sheet being Exposed to Air Cooling: 3
Sec to 20 Sec)
[0105] In general, a steel sheet for hot forming is transported
after heating in a heating furnace to a hot press. In this case,
for example, during extraction from a heating furnace, or during
transportation to or loading on a hot press, a steel sheet may be
partly exposed to air cooling. Since ferrite is newly formed or
grown during such air cooling, the time duration of air cooling has
influence on tensile strength. Therefore, for securing stably a
high strength after hot pressing (after quenching), the time
duration of air cooling should be preferably short. Especially,
when the time period from completion of heating to initiation of
hot pressing, during which a steel sheet is exposed to air cooling,
is above 20 sec, the tensile strength of a steel sheet component
after hot pressing (after quenching) decreases, or even when a high
tensile strength (for example, tensile strength of 980 MPa or more)
is secured, carbon concentration in austenite becomes conspicuous
and martensite transformed region is apt to crack, so that the
bendability declines. Therefore, the time period from completion of
heating to initiation of hot pressing, during which a steel sheet
is exposed to air cooling, should be 20 sec or less, and is
preferably 16 sec or less. Meanwhile, austenite formed during
heating has precipitated in an acicular form. Since a part of the
precipitated austenite transforms during cooling to ferrite and the
form of the austenite changes gradually from an acicular form to a
spherical form, when hot pressing (quenching) is conducted within a
time period of less than 3 sec from completion of heating to
initiation of hot pressing, during which a steel sheet is exposed
to air cooling, to cause martensitic transformation, an acicular
martensitic transformed region constitutes an origin of stress
concentration, so that not only the bendability declines, but also
retained austenite is apt to be formed. Therefore, the time period
from completion of heating to initiation of hot pressing, during
which a steel sheet is exposed to air cooling, should be 3 sec or
more, and is preferably 7 sec or more, more preferably 10 sec or
more.
[0106] In this regard, the time period allowing exposure to air
cooling can be regulated by regulating a transportation time from
extraction out of a heating furnace to a press mold, which is
ordinarily exposed to air cooling.
[0107] (Average Cooling Rate to Temperature Range of M.sub.S Point
or Lower: From 10.degree. C./Sec to 500.degree. C./Sec)
[0108] When a steel sheet for hot forming is hot-pressed and cooled
to a temperature range of an M.sub.S point (M.sub.S point=starting
temperature of martensitic transformation) or lower at an average
cooling rate of from 10.degree. C./sec to 500.degree. C./sec,
diffusional transformation is suppressed. When the average cooling
rate is less than 10.degree. C./sec, bainitic transformation
advances excessively. Alternatively, pearlitic transformation
occurs, so that the area rate of martensite, which is a reinforcing
phase, cannot be secured, and a high tensile strength (for example,
tensile strength of 980 MPa or more) after hot pressing (after
quenching) is difficult to secure. Possibly, austenite is
stabilized, so that the bendability declines. Therefore, the
average cooling rate in the temperature range should be 10.degree.
C./sec or more, and is preferably 30.degree. C./sec or more.
Meanwhile, when the average cooling rate is over 500.degree.
C./sec, it becomes extremely difficult to maintain the soaking of a
steel sheet component, and the strength becomes unstable.
Therefore, the average cooling rate should be 500.degree. C./sec or
less, and is preferably 200.degree. C./sec or less.
[0109] In this regard, an average cooling rate is a value obtained
by dividing the deference between a temperature for performing hot
pressing (.degree. C.) and an Ms point (.degree. C.) by a time
period required from the temperature for performing hot pressing
(.degree. C.) to the Ms point (.degree. C.).
[0110] In this regard, during cooling extremely large heat
evolution due to phase transformation occurs after reaching
400.degree. C., and therefore adequate cooling rate may not be
secured by the same cooling method to be used in a temperature
range not lower than 400.degree. C. Therefore, cooling from
400.degree. C. to an M.sub.S point is required to be performed more
strongly than cooling down to 400.degree. C., and preferably
performed as specified below. By a hot pressing method, cooling is
achieved ordinarily by a steel-made mold at normal temperature or
several tens of degrees Celsius. Therefore, for changing a cooling
rate, a mold dimension may be changed so as to change the heat
capacity. Further, the cooling rate can be changed also by changing
a mold material to a different metal (for example, copper). In a
case in which the mold dimension cannot be changed, a water-cooled
mold may be used to change the cooling rate by changing the cooling
water amount. Further, using a mold, on which several grooves are
cut in advance, the cooling rate may be changed by flowing water in
the groves during pressing, or the cooling rate may be also changed
by lifting a press interrupting pressing and flowing water in
between. Furthermore, the cooling rate may be also changed by
changing a mold clearance so as to change the contact area with a
steel sheet. As a means for changing the cooling rate, for example,
above and below 400.degree. C., the following means are
conceivable.
(1) Immediately after arriving at 400.degree. C., a component is
transferred to a mold having a different heat capacity or a mold at
a room temperature condition, to change a cooling rate. (2) In the
case of a water-cooled mold, immediately after arriving at
400.degree. C., the water flow rate in the mold is changed to
change a cooling rate. (3) Immediately after arriving at
400.degree. C., water is flown between a mold and a component and
the water flow rate is changed to change a cooling rate.
[0111] There is no particular restriction on a form of forming by a
hot pressing method according to the invention. Examples thereof
include bending, draw forming, stretch-expand forming, bore
expansion forming, and flange forming. Suitable one may be selected
appropriately according to a kind of an intended hot formed steel
sheet component. Typical examples of a hot formed steel sheet
component include a door guard bar and a bumper reinforcement,
which are automobile reinforcing components, as described
above.
[0112] A hot formed steel sheet component according to the
invention is characterized in that it is superior in ductility and
bendability. As ductility to withstand practical use, total
elongation in a tensile test of 12% or more is preferable, and
total elongation of 14% or more is still more preferable. As
bendability, a limit bending radius of 5 t or less in a V-bend test
with a tip angle of 90.degree. is preferable.
[0113] A hot formed steel sheet component after hot pressing may be
subjected to a shotblasting treatment in order to remove a scale.
The shotblasting treatment has an effect of introducing a
compression stress in a surface, and therefore offers an advantage
of suppressing a delayed fracture and also enhancing the fatigue
strength.
[0114] Although in the above description, hot forming has been
described taking hot pressing, which is its specific embodiment, as
an example, the invention is applicable similarly to hot pressing
also to hot forming provided with a means for cooling a steel sheet
simultaneously with or immediately after forming, for example to
roll forming.
EXAMPLES
[0115] Examples according to the invention will be described,
provided that the invention be in no way restricted by
Examples.
[0116] Steel sheets having chemical compositions set forth in Table
1 were used as test materials. Each of the steel sheets was
prepared by heating a slab ingoted in a laboratory at 1,250.degree.
C. for 30 min, then, except test materials No. 6 and No. 22,
subjected to hot rolling, such that finish-rolling is completed in
a range of from 880.degree. C. to 910.degree. C., and the material
is retained in a range of from 720.degree. C. to 680.degree. C. for
5 sec, to yield a 2.6 mm-thick hot-rolled steel sheet. After hot
rolling, the sheet was cooled by water spraying down to 420.degree.
C. or lower, and then cooled slowly at 20.degree. C./hour to room
temperature, simulating a step for winding a hot rolled sheet in a
temperature range of 420.degree. C. or lower.
TABLE-US-00001 TABLE 1 Chemical composition (unit: mass %,
remainder: Fe and impurities) Steel C Si Mn P S sol. Al N Ti Nb V
Cr Mo A 0.167 1.20 1.51 0.014 0.0013 0.035 0.0042 0.150 B 0.200
0.25 1.42 0.014 0.0015 0.033 0.0046 0.021 C 0.139 1.46 2.03 0.012
0.0009 0.029 0.0043 0.023 D 0.171 1.17 1.88 0.014 0.0013 0.037
0.0046 0.017 E 0.202 1.23 1.64 0.013 0.0011 0.033 0.0040 0.033 F
0.195 1.28 1.45 0.011 0.0010 0.040 0.0039 G 0.152 1.18 1.59 0.011
0.0011 0.027 0.0042 0.015 H 0.362 1.89 1.24 0.014 0.0012 0.041
0.0039 I 0.163 1.52 1.78 0.009 0.0012 0.033 0.0041 J 0.155 1.21
1.63 0.011 0.0014 0.038 0.0042 0.025 K 0.158 1.22 1.66 0.013 0.0009
0.029 0.0046 L 0.134 1.02 0.81 0.011 0.0015 0.029 0.0043 M 0.183
1.35 1.72 0.009 0.0014 0.033 0.0042 N 0.202 1.23 1.64 0.013 0.0011
0.033 0.0040 O 0.154 1.24 1.51 0.010 0.0012 0.041 0.0044 0.032 P
0.084 1.12 2.44 0.013 0.0011 0.036 0.0048 Q 0.163 1.19 1.62 0.012
0.0011 0.036 0.0040 0.032 R 0.162 1.13 1.47 0.012 0.0008 0.039
0.0039 0.100 Chemical composition (unit: mass %, remainder: Fe and
impurities) A.sub.C3 Ms Steel Cu Ni B Ca Mg REM Zr Bi (.degree. C.)
(.degree. C.) A 858 429 B 0.0012 812 419 C 859 428 D 0.0009 847 418
E 0.0035 848 411 F 0.0010 858 421 G 0.0009 855 436 H 861 348 I
0.0020 862 424 J 858 434 K 855 431 L 876 471 M 0.100 0.100 0.0010
848 415 N 847 411 O 0.0010 865 438 P 852 441 Q 859 431 R 0.0010 862
435
[0117] A thus obtained hot-rolled steel sheet had a complex
structure of ferrite and martensite, or of ferrite and bainite.
[0118] Meanwhile, the hot rolling conditions for test materials No.
6 and No. 22 are different from the above conditions. Test material
No. 6 simulated a step for winding a hot-rolled sheet at room
temperature by retaining the sheet in a range of from 740.degree.
C. to 660.degree. C. for 2 sec, and cooling the same by water
spraying to room temperature. Test material No. 22 simulated a step
for winding a hot-rolled sheet at 670.degree. C., by cooling the
sheet by water spraying to 670.degree. C., and then cooling the
same slowly at 20.degree. C./hour to room temperature.
[0119] A part of a hot-rolled steel sheet obtained as above was
freed from a scale by pickling, then subjected to cold rolling to a
sheet thickness of 1.6 mm, heated at from 780.degree. C. to
900.degree. C., and annealed under condition of cooling at an
average cooling rate of 30.degree. C./sec. However, test material
No. 27 was heated at 920.degree. C., and annealed under condition
of cooling at an average cooling rate of 30.degree. C./sec.
[0120] Each of the area rates of ferrite, martensite, and bainite
in a steel sheet to be subjected to hot pressing was measured
applying an EBSP (Electron Back Scatter Pattern) method.
Specifically, cross-sections both in the rolling direction and in
the direction vertical to the rolling direction were sliced out
from a steel sheet to be subjected to hot pressing. The sliced out
cross-sections were subjected to polishing and nital etching. Next,
using a scanning electron microscope (SEM) equipped with an EBSP
detector (trade name QUANTA 200, produced by FEI Company), an IQ
image (image quality map: magnification 2000.times.) of EBSP was
obtained for each sliced out cross-section by an EBSP analysis.
Then, the respective area rates of ferrite, martensite, and bainite
were determined as an average value of area rates measured
respectively based on respective IQ images of EBSP for both
cross-sections in the rolling direction and in the direction
vertical to the rolling direction. In this regard, for an EBSP
analysis the following conditions were set: acceleration voltage=25
kV, working distance=15 mm, and scan step=0.2 .mu.m.
[0121] Further, an aspect ratio of ferrite in a steel sheet to be
subjected to hot pressing was measured as follows. Specifically,
cross-sections both in the rolling direction and in the direction
vertical to the rolling direction were sliced out from a steel
sheet to be subjected to hot pressing. The sliced out
cross-sections were subjected to polishing and nital etching. Next,
using a scanning electron microscope (SEM) equipped with an EBSP
detector (trade name QUANTA 200, produced by FEI Company), an IQ
image (image quality map: magnification 2000.times.) of EBSP was
obtained for each sliced out cross-section by an EBSP analysis.
Then, the aspect ratio of ferrite was determined as an average
value of aspect ratios of each 50 ferrite crystal grains measured
based on each IQ image of EBSP for cross-sections both in the
rolling direction and in the direction vertical to the rolling
direction. In this regard, for an EBSP analysis the following
conditions were set: acceleration voltage=25 kV, working
distance=15 mm, and scan step=0.2 .mu.m.
[0122] The steel structures of steel sheets to be subjected to hot
pressing are shown in Table 2.
TABLE-US-00002 TABLE 2 Test material Kind of Aspect ratio Area rate
of Area rate of Area rate of No. Steel steel sheet of ferrite
ferrite (%) martensite (%) bainite (%) *1 *2 1 A Hot-rolled 1.4 36
64 0 64 100 steel sheet 2 A Hot-rolled 1.3 36 64 0 64 100 steel
sheet 3 A Coated steel 1.3 28 5 65 70 98 sheet 4 B Hot-rolled 1.5
35 0 60 60 95 steel sheet 5 C Hot-rolled 1.4 29 0 66 66 95 steel
sheet 6 C Hot-rolled 1.6 32 0 64 64 96 steel sheet 7 D Hot-rolled
1.3 0 100 0 100 100 steel sheet 8 E Hot-rolled 1.4 0 100 0 100 100
steel sheet 9 F Hot-rolled 1.2 32 68 0 68 100 steel sheet 10 F
Hot-rolled 1.2 32 68 0 68 100 steel sheet 11 G Hot-rolled 1.4 22 78
0 78 100 steel sheet 12 H Hot-rolled 1.3 0 5 78 83 83 steel sheet
13 I Hot-rolled 1.6 37 0 56 56 93 steel sheet 14 I Hot-rolled 2.1
40 60 0 60 100 steel sheet 15 I Cold-rolled 1.3 44 56 0 56 100
steel sheet 16 I Cold-rolled 1.4 44 56 0 56 100 steel sheet 17 J
Hot-rolled 1.3 35 0 60 60 95 steel sheet 18 J Cold-rolled 1.6 43 57
0 57 100 steel sheet 19 K Hot-rolled 1.4 34 66 0 66 100 steel sheet
20 K Hot-rolled 1.3 34 66 0 66 100 steel sheet 21 K Cold-rolled 1.5
42 50 5 55 97 steel sheet 22 K Coated steel 1.7 36 6 52 58 94 sheet
23 L Hot-rolled 1.4 54 37 3 40 94 steel sheet 24 M Hot-rolled 1.3
33 67 0 67 100 steel sheet 25 M Hot-rolled 1.3 33 67 0 67 100 steel
sheet 26 N Hot-rolled 1.2 45 0 32 32 77 steel sheet 27 N Hot-rolled
1.4 41 59 0 59 100 steel sheet 28 N Hot-rolled 1.6 41 59 0 59 100
steel sheet 29 O Hot-rolled 1.4 39 0 55 55 94 steel sheet 30 P
Hot-rolled 1.5 57 43 0 43 100 steel sheet 31 Q Hot-rolled 1.3 37 63
0 63 100 steel sheet 32 Q Cold-rolled 1.9 21 79 0 79 100 steel
sheet 33 R Hot-rolled 1.4 28 0 68 68 96 steel sheet 34 R
Cold-rolled 1.4 9 91 0 91 100 steel sheet *1: The total area rate
of martensite and bainite (%) *2: The total area rate of ferrite,
martensite, and bainite (%)
[0123] The obtained steel sheets were heated in a gas furnace at an
air fuel ratio of 0.85 and under the conditions set forth in Table
3. Then the heated steel sheets were taken out of the heating
furnace, and after an air cooling time until hot pressing (time
period from extraction of a sheet out of the furnace to placement
of the same into a mold, namely time period in which a steel sheet
is exposed to air cooling between the completion of heating and the
initiation of hot forming) regulated to a time as set forth in
Table 3, subjected to hot pressing using a flat plate steel-made
mold. Then, after hot pressing, the steel sheets were cooled at an
average cooling rate set forth in Table 3 down to 150.degree. C.,
which was not higher than an M.sub.S point, while keeping the steel
sheets in contact with the mold, and thereafter taken out from the
mold and left standing to allow cooling, thereby completing various
test steel sheets (such a test steel sheet is hereinafter referred
to as "hot-pressed steel sheet").
[0124] Cooling was performed 1) after cooling the periphery of a
mold with cooling water, 2) after cooling in a mold, which had been
at normal temperature, or 3) after cooling in a heated mold, by
cooling the periphery of a mold with cooling water. An average
cooling rate down to 150.degree. C. was determined by attaching a
thermocouple to an edge of a steel sheet to be subjected to hot
pressing, and reading the temperature. In this regard, a heating
time means a time period from a time point when a steel sheet
reaches 720.degree. C. after placement of the same in a furnace,
until the same is taken out from the furnace. Meanwhile, in
Examples 6, 18 and 25, various test steel sheets were prepared by
conducting gas cooling at a predetermined cooling rate after air
cooling for a predetermined time period, for simulating a hot
pressing condition, under which a cooling rate is changed using a
mold with grooves,
[0125] The area rates of ferrite, tempered martensite, tempered
bainite, and martensite of a hot pressed steel sheet were measured
identically with the respective area rates of ferrite, martensite,
and bainite of a steel sheet to be subjected to hot pressing,
applying an EBSP (Electron Back Scatter Pattern) method. The
results are shown in Table 4.
[0126] The aspect ratio of ferrite of a hot pressed steel sheet was
measured identically with the aspect ratio of ferrite of a steel
sheet to be subjected to hot pressing.
[0127] The mechanical properties of a hot pressed steel sheet were
examined as follows. The results are also shown in Table 4.
[0128] JIS No. 5 test piece for tensile test was sampled from each
steel sheet in the direction normal to the rolling direction, and a
tensile test was carried out to measure TS (tensile strength) and
El (total elongation).
[0129] Further, a rectangular sample was cut from each steel sheet
allowing a bending ridge line to be directed normal to the rolling
direction, and a surface thereof was machined to prepare a bending
test piece with a thickness of 1 mm, a width of 30 mm, and a length
of 60 mm. The test piece was subjected to a V-bend test with a tip
angle of 90.degree., and tip radii of 5 mm, 4 mm, and 3 mm for
evaluating the bendability. In this regard, the machined surface
constituted an inner surface of a bend. The surface of a bend after
the test was examined visually, and rated according to the
following rating criteria.
--Rating Criteria of Bendability--
[0130] A: After a V-bend test with a tip radius of 4 mm, a crack is
not recognized. B: After a V-bend test with a tip radius of 4 mm, a
microcrack or necking is recognized. C: After a V-bend test with a
tip radius of 4 mm, a crack is recognized. D: After a V-bend test
with a tip radius of 5 mm, a crack is recognized.
[0131] Steel sheets produced in the present Example were not
hot-pressed with a mold, however experienced the same heat history
as a hot pressed steel sheet component. Therefore, the mechanical
properties of a steel sheet were substantially the same as those of
a hot pressed steel sheet component having the same heat
history.
[0132] An underlined value in Table 1 to Table 4 means that a
content, a condition, or a mechanical property expressed by the
value is outside the scope of the invention.
TABLE-US-00003 TABLE 3 Average cooling Heating rate rate to from
room Air temperature Test temperature Heating Heating cooling range
of Ms material to 720.degree. C. temperature time time point or
less No. (.degree. C./sec) (.degree. C.) (min) (sec) (.degree.
C./sec) 1 12 800 5 10 70 2 12 900 4 15 70 3 12 800 5 10 70 4 12 775
5 10 70 5 12 800 5 10 70 6 12 800 5 10 15 7 12 800 5 10 70 8 12 790
5 10 70 9 12 800 5 10 70 10 12 800 5 10 400 11 12 800 5 10 70 12 12
790 5 10 70 13 12 800 5 10 70 14 12 840 5 10 70 15 12 800 5 10 70
16 12 820 5 25 70 17 12 800 5 10 70 18 12 800 5 10 8 19 12 800 5 10
70 20 12 680 5 10 70 21 12 800 5 10 70 22 12 800 5 10 70 23 12 800
5 10 70 24 12 800 5 10 70 25 12 800 5 10 5 26 12 800 5 10 70 27 12
775 5 5 70 28 12 775 5 17 70 29 12 800 5 10 70 30 12 800 5 10 70 31
12 800 5 10 70 32 12 770 8 1 70 33 12 800 5 10 70 34 12 740 5 10
70
TABLE-US-00004 TABLE 4 Steel structure Test Steel structure and
Area rate (%) Ferrite Mechanical properties material Tempered
Tempered Retained aspect TS EI No. Ferrite martensite bainite
Martensite austenite *3 *4 ratio (MPa) (%) Bendability 1 25 39 0 36
0 39 100 1.4 1174 15 A Inventive Example 2 18 0 0 82 0 0 100 1.3
1275 10 D Comparative Example 3 15 2 50 33 0 52 100 1.3 1136 14 A
Inventive Example 4 15 0 47 37 0 47 99 1.4 1102 11 A Comparative
Example 5 22 0 46 32 0 46 100 1.4 1089 17 A Inventive Example 6 23
0 40 33 4 40 96 1.5 1027 16 B Inventive Example 7 0 58 0 42 0 58
100 1.3 1245 9 D Comparative Example 8 0 62 0 38 0 62 100 1.4 1305
11 D Comparative Example 9 23 44 0 33 0 44 100 1.2 1194 16 A
Inventive Example 10 22 45 0 33 0 45 100 1.2 1243 14 B Inventive
Example 11 15 49 0 36 0 49 100 1.3 1142 17 A Inventive Example 12 0
0 55 45 0 55 100 1.2 1486 7 D Comparative Example 13 26 0 38 36 0
38 100 1.5 1186 15 A Inventive Example 14 4 28 0 68 0 28 100 2.1
1198 8 D Comparative Example 15 32 33 0 35 0 33 100 1.3 1203 14 A
Inventive Example 16 43 21 0 23 2 21 87 1.3 968 14 A Comparative
Example 17 24 0 40 36 0 40 100 1.3 1256 14 A Inventive Example 18
31 31 0 31 7 37 93 1.5 1043 18 D Comparative Example 19 22 43 0 35
0 43 100 1.4 1178 16 A Inventive Example 20 34 66 0 0 0 66 100 1.3
848 16 A Comparative Example 21 33 28 3 36 0 31 100 1.4 1146 15 A
Inventive Example 22 28 0 39 33 0 39 100 1.5 1137 14 A Inventive
Example 23 52 18 0 23 0 18 93 1.4 925 15 D Comparative Example 24
26 39 0 35 0 39 100 1.2 1203 14 A Inventive Example 25 30 37 0 4 3
37 71 1.3 878 24 A Comparative Example 26 38 0 18 44 0 18 100 1.2
1278 13 D Comparative Example 27 36 30 0 34 0 30 100 1.3 1175 13 C
Inventive Example 28 38 31 0 31 0 31 100 1.3 1136 17 C Inventive
Example 29 25 0 43 32 0 43 100 1.3 1246 15 A Inventive Example 30
35 29 0 36 0 29 100 1.4 946 16 A Comparative Example 31 24 42 0 34
0 42 100 1.3 1143 15 A Inventive Example 32 17 51 0 26 6 51 100 1.8
1126 14 D Comparative Example 33 18 0 45 37 0 45 100 1.3 1189 14 A
Inventive Example 34 7 85 0 8 0 85 100 1.3 949 10 A Comparative
Example *3: The total area rate of tempered martensite and tempered
bainite (%) *4: The total area rate of ferrite, tempered
martensite, tempered bainite, and martensite (%)
[0133] The test materials No. 1, 3, 5, 6, 9, 10, 11, 13, 15, 17,
19, 21, 22, 24, 27, 28, 29, 31, and 33 as Inventive Examples in
Table 4 are steel sheet components of Inventive Examples, namely
hot pressed steel sheet components, satisfying all the requirements
according to the invention. Any of the hot pressed steel sheet
components of Inventive Examples as hot-formed has a tensile
strength as high as 980 MPa or more, and superior in ductility as
well as bendability.
[0134] Meanwhile, with respect to the test material No. 2, since
the heating temperature of the steel sheet exceeded the upper limit
of the range defined according to the invention, a desired
structure could not be obtained and the ductility and the
bendability were inferior.
[0135] With respect to the test material No. 4, since the Si
content was below the lower limit of the range defined according to
the invention, the ductility was inferior.
[0136] With respect to the test material No. 7, since the steel
sheet to be subjected to hot pressing and the hot pressed steel
sheet component did not have the structure defined according to the
invention, the ductility and the bendability were inferior.
[0137] With respect to the test material No. 8, a desired structure
was not obtained for the steel sheet to be subjected to hot
pressing and the hot pressed steel sheet component, and the
ductility and the bendability were inferior.
[0138] With respect to the test material No. 12, since the C
content exceeded the upper limit of the range defined according to
the invention, and the steel sheet to be subjected to hot pressing
and the hot pressed steel sheet component did not have the
structure defined according to the invention, the ductility and the
bendability were inferior.
[0139] With respect to the test material No. 14, a desired
structure was not obtained for the steel sheet to be subjected to
hot pressing and the hot pressed steel sheet component, and the
ductility and the bendability were inferior.
[0140] With respect to the test materials No. 16, 20, and 25, since
the air cooling time, the heating temperature, and the average
cooling rate fell outside the respective ranges defined according
to the invention, a desired structure was not obtained for the hot
pressed steel sheet component, and a targeted tensile strength was
not obtained.
[0141] With respect to the test material No. 18, since the average
cooling rate fell outside the range defined according to the
invention, a desired structure was not obtained for the hot pressed
steel sheet component, and the bendability was inferior.
[0142] With respect to the test material No. 23, since the Mn
content was below the lower limit of the range defined according to
the invention, and the steel sheet to be subjected to hot pressing
and the hot pressed steel sheet component did not have the
structure defined according to the invention, a targeted tensile
strength was not obtained and the bendability was inferior.
[0143] With respect to the test material No. 26, since the steel
sheet to be subjected to hot pressing and the hot pressed steel
sheet component did not have the structure defined according to the
invention, the bendability was inferior.
[0144] With respect to the test material No. 30, since the C
content was below the lower limit of the range defined according to
the invention, a targeted tensile strength was not obtained.
[0145] With respect to the test material No. 32, since the air
cooling time fell outside the range defined according to the
invention, a desired structure was not obtained for the hot pressed
steel sheet component, and the bendability was inferior.
[0146] Further with respect to the test material No. 34, since the
steel sheet to be subjected to hot pressing and the hot pressed
steel sheet component did not have the structure defined according
to the invention, the tensile strength was low, and the ductility
was inferior.
[0147] The entire disclosures of Japanese Patent Application No.
2013-247814 are hereby incorporated by reference.
[0148] All the literature, patent application, and technical
standards cited herein are also herein incorporated to the same
extent as provided for specifically and severally with respect to
an individual literature, patent application, and technical
standard to the effect that the same should be so incorporated by
reference.
* * * * *