DESCRIPTION
IIOT FORMED STEEL SHEET COMPONENT AND METIIOD FOR PRODUCING THE
SAME AS WELL AS STEEL SI-IEET FOR HOT FORMING
Technical Field
[0001] The present invention relates to a hot formed steel sheet component to be used, fol
example, in a machine structural component such as at1 aotomnobile body structural component,
and to a method for producing the same as well as to a steel sheet for hot fonning.
Specificallj: 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 wdl as to a steel sheet for hot forming for yielding the same.
Background hrt
[0002] In recent years, for the sake of weiglit 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 becornes 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 wvall 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] 011 the other hand, by a method called as hot pressing, by wvliiclich a heated steel sheet
is press-formed as disclosed in Patent Document 1, a con~poneiiht aving a complicated shape
can be formed with high dimensional accuracy, because a steel sheet is soft and highly-ductile
at a high temperature. Furthel; \vheri a steel sheet is heated up to an austenite siugle phase
region and then quenched (hardened) in a mold, enhancement of the strength of a co~nponent
can be achieved at the salile time due to a maltensitic transfonnation. Therefore, such a hot
pressing nlethod is a superior forming method, which can attain a high strength of a
compolle~lta nd superior fornlability of a steel sheet simn~iltaneously.
[0004] Further, Patent Document 2 discloses a pre-press quenching method, by which a steel
1
sheet is fornled in advance to a predetermined shape at room te~nperaturc, heated up to an
austenite region, and then quenched in a nlold to achieve a higher strength of a component.
Since such a pre-press quenching nletllod, which is an enlbodi~nent 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.
COO061 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 Act transformation point or highel; 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 fiom 0 to 67 w~olume-% of
tnartensite, 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 fiom 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.
Patent Document 6 discloses a steel sheet for bot pressing in wllich a total fiaction of
bainite and martensite is 80% by area or more.
Further, Patent Document 7 discloses a steel sheet for hot pressing in \vhicb the
fraction of ferrite is 30% by area or more,
2
Citatiori List
Patent Literature
[0009]
Patent Docume~lt 1 : GB Patent No. 1490535
Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. H10-9603 1
Patent Docun~ent 3: JP-ANo. 2010-65292
Patent Docunie~i4t : JP-ANo. 2012-237066
Patent Document 5: International Publication No.WO20111111333
Patent Docunient 6: JP-ANo. 2013-1 85243
Patent Docunient 7: JP-ANo. 2013-185248
SUMMARY OF INVENTION
Teclmical Proble~n
[0010] It has become clear through i~~vestigatiobuy the present inventors that by making the
steel structure of a steel sheet for hot pressing contain tnainly 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 touglmess 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.
[0011] Similarly, even when the \vhole hot for~ineds teel sheet components including a
roll-fornied component besides a hot pressed steel sheet component aar surveyed, with respect
to a hot for~neds teel sheet conlponent with a high tensile strength (for example, tensile
strength of 980 MPa or more) and superior in bendability ill addition to ductility, a product
itself has been heretofore not yet proposed, let alone establisli~uento f a productioil
technology.
[0012] 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 accordiilg to conventional art as described above, and a method fo-r producing
the sane as \veil as a steel sheet for hot pressing for yielding the same. By generalization,
the itlve~xtioliis also applicable to hot foxming provided with a means for cooling a steel sheet
3
sitnultaneously during or immediately after fortning as in the case of liot pressing. Therefore,
a specific object of the invention is to provide a liot formed steel sheet cotnponent superior in
ductility and bendability wl~ileh aving a high tensile stretigth after hot forming, and a method
for producing the same as \veil as a steel sheet for liot forniing for yielding the same.
Solution to Problem
[0013] The inventors studied diligently for improving the ductility and the bendability of a
hot formed steel sheet conlponerlt with a high tensile strength. As the result, the following
novel knowledge has been acquired. Namely, a steel sheet for hot fortning including a
chemical cotnposition in which Si is actively added to specific arnounts 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 fsotn a conver~tional 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 maltensite or tempered bainite, and mastensite 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 coniposition and the steel structure are present.
[0014] The invention based on the knowledges is as follows:
[I] A hot formed steel sheet component comprising:
a cliemical composition comprising in terms of mass% C at from 0.100% to 0.340%,
Si at from 0.50% to 2.00%, Mn at fsom 1.00% to 3.00%, P at 0.050% or less, S at 0.0100% or
less, sol. A1 at fsom 0.001% to l.OOO%,atld N at 0.0100% or less, with a remainder consisting
of Fe and impurities; and
a steel sttucture comprising feirite, at least one of te~npered martensite or tempered
bainite, and maltensite, wherein an area rate of ferrite is fsom 5% to 50%, a total area rate of
tempered niartensite and tempered bainite is from 20% to 70%, an area rate of martensite is
fioni 25% to 75%, a total area rate of fenite, tempered mal-tensite, tempered bainite and
nlartensite is 90% or more, and an area rate of retained austenite is from 0% to 5%.
[OOlS]
[21 The hot formed steel sheet component according to Paragraph [I], wherein the
chemical cot~iposition cotnprises one kind or two or more kinds selected fsom 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, atld Ni at 1.000% or less, in ter~nso f mass%
instead of a part of the Fe.
4
[00 161
131 The hot formed steel sheet conlponent according to Paragraph [l] or Paragraph [2],
wherein the chemical composition comprises B at 0.0025% or less, in terms of mass% instead
of a part of tlie Fe.
[00 171
[4] The hot formed steel sheet component according to any one of Paragraph [I] to
Paragraph [3], wherein the chemical conipositiori comprises one kind or two or xilore 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.
[0018]
[5] The hot formed steel sheet conlponent according to any one of Paragraph [I] to
Paragraph [4], wherein the chemical compositioil comprises Bi at 0.0100% or less, in terms of
mass% instead of a part of the Fe.
[00 191
[6] A steel sheet for hot forming comprising:
a chemical composition comprisi~igin 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%, Pat 0.050% or less, S at 0.0100% or
less, sol. A1 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 fe~ritew ith an aspect ratio of 2.0 or less, and at least one
of martensite or bainite, wherein an area rate of ferrite is fiom 5% to 50%, a total area rate of
niartensite and bainite is from 45% to 90%, and a total area rate of ferrite, martensite, and
bainite is 90% or more.
[0020]
[71 The steel sheet for hot for~ning according to Paragraph 161, wherein the chemical
composition comprises one kind or two or more kinds selected fkom 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 temis of mass% instead of
a part of the Fe.
[0021]
P I The steel sheet for hot fomiing according to Paragraph [6] or Paragraph 171, 12iercin
the clien~icacl omposition comprises B at 0.0025% or less, in teniis of mass% instead of a part
of the Fe.
[0022]
[9] The steel sleet for hot forming according to any one of Paragraph [6] to Paragraph
5
[8], wvherein the chemical co~~~positcionill prises 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 ternls of mass% instead of a part of the Fe.
[0023]
[lo] The steel sheet for hot forn~iuga ccording to any one of Paragraph [6] to Paragraph
[9], wherein the chemical cornposition comprises Bi at 0.0100% or less, in tern~s of mass%
instead of a part of the Fe.
[0024]
[l I] A method for producing a hot fornled steel sheet component, the nlethod comprising:
heating the steel sheet for hot forming according to any one of Paragraph [6] to Paragraph
[lo] to a temperature range of 720°C or higher but lower than an Ac3 point; performing hot
fornling 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 Ms point at an average cooliilg rate of from 10"CIsec to
5OO"CIsec.
Advantageous Effects of Invention
[0025] A technologically valuable effect that a hot formed steel sheet component, ~vhicha, s
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
cotnponent according to the invention exhibits extremely superior collisioll characteristics,
such that it can absorb an impact thro:ough 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 structnlral conlponent of an automobile
body, however it is naturally applicable to another use such as a machine structural
component.
BRIEF DESCRIPTION OF DRAWINGS
[0026]
[Figure 11 Figure 1 is a photograph showing an example of a steel structltre according
to the invention.
DESCRIPTION OF EMBODIMENTS
100271 Next, reasons behind limitations of each range imposed according to thc iillvention
will be described. In this regard, hot forming will be described below taking hot pressing,
which is a specific elnbodilnent thereof, as an exatnple. A numerical range expressed by "x
6
to y" includes the values of s and 11 in the range as the minimum and maximum values,
respectively.
100281 1. Chemical Colnposition
Firstly, reasons behind the above definition of the che~nicalc olnpositio~o~f 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 fornling according to the invention
(hereinafter also referred to simply as "steel sheet") will be described. Ln the following
descriptions, "%" representing the content of each alloy element means "mass%" unless
otherwise specified.
[0029] (C at 0.100% to 0.340%)
C is a very itnportant 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 Inore preferably 0.280% or
less.
[0030] (Si at 0.50% to 2.00%)
Si is a vely effective element for enhancing the ductility of a steel heated into a two
phase telnperature 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 econotnically 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.
[003 11 (Mn at 1.00% to 3.00%)
Mn is a very effective elemcnt for improving the hardenability of a steel and securing
a strength after hot pressing (after quenching). Iiowevel; when the A h content is less than
7
1.00%, not only it becomes very difficult to secure a high tensile strength (for example,
tensile strength of 980 MPa or more) afrcr hot pressing (after quenching), but also tlle
bendability map be compron~ised. Therefore, the Mn content should bc 1.00% or more,
For securing the action more stably, the Mu 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 att obvious band caused by Mn
segregation, which cotnpro~nises the toughness and re~narkably deteriorates the collision
characteristics. Therefore, the Mn content should be 3.00% or less. From the vie\iyoint 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.
[0032] 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 mattensite or bainite. Further, by defining the heating conditions during hot
pressing according to the invention, the steel structure of a hot formed steel sheet conlponent
can be made to a desired steel structure with a dual phase.
[0033] (Pat 0.050% or Less)
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.
Howvever, 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.
[0034] (S at 0.0100% or Less)
S is an in~purityc ontained in a steel, and the content is preferably as low as possible
fro111 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 inore preferably 0.0015% or less.
From the viewpoint of desulfi~rizationc osts, the S content is preferably 0.0006% or more.
[0035] (Sol. Al (Soluble Al) at 0.001% to 1.000%)
A1 is an element having an action to make the quality of a steel robust tlxough
deoxidation. When the sol. A1 content is less than 0.001%, it becotnes difficult to obtain the
action. Therefore, the sol. A1 content should be 0.001% or more, and is preferably 0.015%
or more. Meanwhile, when the sol. A1 content is above 1.000%, deterioration of the
v,~eldability becotnes conspicuous, and fi~rther an oxide-type inclusion increases so that
deterioration of a surface condition becotnes also conspicuous. Therefore, the sol. Al
8
contcnt should be 1.000% or less, and is preferably 0.080% or less. In this regard, sol. A1
~neansa cid-soluble Al, which is not in a fom~o f an oxide such as A1203,a nd soluble in an
acid.
[0036] (N at 0.0100% or Less)
N is an impurity contained in a steel, and the content is preferably as low as possible
frotii 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.
[0037] [Impurities]
Impurities refer to ingtedients contained in a source material or ingredients entered
during a process of productiot~, which are ingredients that are not intentionally added to a
steel sheet component or a steel sheet for hot forming.
[0038] The chernical conipositions 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.
[0039] (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, aud Ni at 1.000% or Less)
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 then1 is
contained beyond 0.200%, not only hot rolling and cold rolling may beconle 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%, securetnent of a stably high strength may become
diffictdt. Therefore, the Cr content is preferably 1.000% or less. With respect to Mo, wlien
the content exceeds 1.000%, hot rolling and cold rolling n~apb ecotne difficult. Therefore,
the Mo content is preferably 1.000% or less. With respect to Cu and Ni, when the content of
any of thern exceeds 1.000%, an effect of the action is apt to be saturated so that the economy
nlay becotne disadvantageous, and fi~rtherh ot rolling or cold rolling may beconle diflicult.
Therefore, preferably the Cu content, and the Ni corltent are respectively 1.000% or less.
[0040] 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.
9
111 othcr words, the lo\ver limit of the Ti content is preferably 0.003%. The lo\ver
linlit of the Nb content is preferably 0.003%. The lower li~niot f the V content is preferably
0.003%. The lower linlit 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%.
[0041] (B at 0.0025% or Less)
B is an element having an action to enhance the toughness of a steel. Therefore, B
may be added. I-Iowevel; when B is added in an anlount exceeding 0.0025%, it rnay beconle
difficult for the steel structure in a steel sheet for hot fornling to contain ferrite, and the
ductility and the bendability of a hot formed steel sheet conlponent 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.
[0042] (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)
Any of these elements is an element having an action to enhance the tolighness
through contribution to control of it~clusion, 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 elenlent 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%.
In this regard, REM represents at least one kind of a total of 17 elements including Sc,
Y, and lanthanoids. The above REM content nmeans a total content of at least one kind of the
elements. Alanthanoid is industrially added in a form of a misch metal.
[0043] (Bi at 0.0100% or Less)
Bi is an clernent having an action to make the structure uniform and enhance the
bendability. Therefore, Bi may be contained. However, wllen Bi is added more than
0.0100%, the hot working property may be deteriorated so that hot rolling may become
difficult. Tlterefore, the Bi content should be preferably 0.01 00% or less. For obtaining an
effect of the action more securely, the Bi content should be preferably 0.0003% or more.
100441 2. Steel Structure of 1-lot Formed Steel Sheet Conlponcnt
Nest, the steel structure of n hot forn~ed steel sheet conlponent according to the
invention \\rill be described.
10
A hot for~ned steel sheet co1i1po11ent according to tlic invention includes a steel
structure containing ferrite, at least one of tempered martensite or tenlpered bainite, and
ma~tensitea t the following predetermined area rates. I11 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.
[0045] Figure 1 shows an example of a steel structure according to the invention. The steel
structure sllown in Figure 1 is a steel structure containing ferrite, te~npered martensite, and
martensite, but not containing retained austenite.
[0046] (Area Ratc of Ferrite at 5% to 50%)
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,
[0047] The aspect ratio of fersite 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. Tlerefore, the aspect ratio of fesrite is
preferably 2.0 or less, and more preferably 1.8 or less. Meanwhile, when the aspect ratio of
ferrite approaches closer to 1 .O, 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.
An aspect ratio of ferrite is a value ~neasured by the method described precisely in
Example presented below.
[0048] (Total Area Rate of Tempered Martensite and Tempered Bainite at 20% to 70%)
Whell the total area rate of tenlpered martensite and tenlpered bainite is less than
2096, the bendability declines. Therefore, the total area rate of tenlpered martensite and
te~nperedb ainite sliould be 20% or more, and is preferably 30% or inore. Meanwhile, u~lien
the total area rate of tempered martensite and ten~peredm artensite is above 70%, the ductility
declines. Therefore, the total area rate of tenlpered martensite and tempered bainite sliould
be 70% or less, and is preferably 50% or less.
[0049] (Area Rate of Martensite at 25% to 75%)
By for~ningm ai-tcnsite in a steel, the strength after hot pressing (after quenching) can
be enhanced. When the area rate of n~artensite is less than 25%, it beconles difficult to
11
secure a high tensile strength (for example, tensile strength of 980 MPa or more) after hot
pressing (after quenching). Therefore, the area rate of tnartensite sl~ouldb e 25% or more.
Meanwhile, wl~enth e area rate of martensite is above 75%, the ductility declines. Therefore,
the area rate of rnartensite should be 75% or less, and is preferably 50% or less.
[0050] In this rcgard, "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-quenclled martensite, and martensite after
age hardening formed by age-hardening as-quenched martensite.
[0051] (Total Area Rate of Ferrite, Tempered Martensite, Tempered Bainite, and Martensite
at 90% or More)
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 bair~ite, and martensite exceeds lo%, an intended characteristic may not
be obtained due to an inhence 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
mattensite, 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 mattetisite, tempered bainite,
and martensite is 100%.
[0052] (Area Rate of Retained Anstenite at 0% to 5%)
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 austenitc is most
preferably 0%.
[0053] The area rate of each phase and structure in the steel structure of a hot formed steel
sheet conlponent is a value measured by the tnethod described precisely in Example presented
below.
[0054] A steel sheet component according to the invention means a component hot fonncd
fro111 a steel sheet, and includes, for exanlple, a steel sheet component formed by hot prcssing.
As a typical example, there is a door guard bar to be used as an autonlobile body structural
12
conlponent. Further, for an autotnobile use, there is, for example, a buml~er reinforcement.
As a nlachine stractoral component, there is a hot fornted steel pipe for a building structure
produced from a steel sheet as a source material.
[OOSS] 3. Mechanical Properties
It is preferable that a hot formed steel sheet conlponent according to the inventioti
has a tensile strength (TS) of 980 MPa or more, which is adequate to contribute to xveight
reduction of an automobile.
[0056] 4. Production Method
Next, a preferable method for producing a hot formed steel sheet component
accorditig to tlie inventio~havingth e above characteristics will be described.
[0057] In order to attain favorable ductility and bendability for a hot formed steel sheet
component according to the invention, w21ile 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
st~aciore,in which tlie area rate of ferrite is from 5% to 50%, the total area rate of tempered
martensite, and tenlpered bainite is from 20% to 70%, the area rate of martensite is frotn 25%
to 75%, the total area rate of ferrite, tenlpered martensite, tempered bainite, and martensite is
90% or more, as well as the area rate of retained austenite is from 0% to 5%.
[0058] In order to obtain a steel structure for a hot formed steel sheet cotnponent 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, atid 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 SO%, the total
area rate of ~~lal-tensiatned bainite is fro111 45% to 90%, the total area rate of ferrite, tnartensite,
and bainitc is 90% or more, is preferably used. Then, preferably the steel slteet (steel sheet
for hot forming) is heated into a temperature range of 720°C or higher but lower than an Ac3
point, then performed to hot pressing within a time period of from 3 sec to 20 sec, during
wl~ichth e 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 Ms point at an
average cooling rate of from 10"CIsec to 5OO"CIsec.
[0059] By hot pressing a steel sheet for hot forming including the ct~elnicacl omposition and
the steel structure under the above conditions, a hot formed steel sheet component having a
desired steel strnctnre after hot pressing, with a high tensile strength (for example, tensile
strength of 980 A4Pa or illore), and superior in ductility and bendability can be obtaiticd.
[OOGO] (Steel Structure of Steel Sheet for I-Iot Forn~ing)
13
-Aspect Ratio of Ferrite at 2.0 or Less -
When the aspect ratio of ferrite is above 2.0, the aspect ratio of ferrite in a steel
structure of a steel sheet co~~~polalfet~eri th ot pressiug may also cxceed 2.0, and further the
ferrite area rate in a steel sheet colnponellt afier hot pressing may fall below 596, because
ferrite is transfonncd excessively to aostenite during heating. When the aspect ratio of
ferrite of steel sheet colnponetlt 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 fenite should be 2.0 or less, and is preferably 1.8 or less.
Meat~wliilew, hen the aspect ratio of ferrite approaches closer to 1.0, the anisotropy of ferrite
(crystal grain of ferrite) decreases fiuthel; 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.
[006l] An aspect ratio of ferrite is a value measured by the method described precisely in
Exatnple presented belo~v.
[0062]
-Area Rate of Ferrite at 5% to 50% -
When the area rate of ferrite is less than 5%, the area rate of ferrite in a steel structure
of a steel sheet componellt 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 slieet
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.
[0063]
- Total Area Rate of Martensite and Bainite at 45% to 90% -
When the total area rate of martensite and baillite is less than 45%, the total area rate
of tempered martensite and tempered bainite in the steel structure of a steel sheet componet~t
after hot pressing may become less than 20%. Further, the area rate of niartel~sitein the steel
structure of a steel slieet comporlent after hot pressing may become less than 25%.
Therefore, the total area rate of maltensite and bairlite should be 45% or more, and is
preferably 50% or more. Similarlj: when the total area rate of martensite arid hainite is
above 90%, tic total area rate of tempered martensite and tempered bainite in the steel
structure of a steel slieet con~ponenat ftcr hot pressing may also exceed 70%. Further, the
area rate of inartensite in the-steel structure of a steel sheet component after hot pressing may
exceed 75%. Therefore, the total area rate of martcnsite and bainite should be 90% or less,
14
and is preferably 80% or less.
[0064]
-Total Area Rate of Ferrite, Martensite, and Rainite at 90% or More -
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
n~arteusitein 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, marteasite, and bainite should be 90% or more, and is preferably 93% or more.
The upper liinit of the total area rate of ferrite, inartensite, and bainite is 100%.
[0065] An area rate of each phase and stlxcture in the steel stiucture of a steel sheet for hot
forniing is a value measured by the method described precisely in Example presented below.
100661 (Production of Steel Sheet for Hot Forn~ing)
A steel sheet for hot forming may be any of a hot-rolled steel sheet, a cold-lolled
steel sheet, and a coated steel sheet. Examples of a coated steel sheet include aluminum
coated steel sheet, and zinc coated steel sheet.
[0067] A hot-rolled steel sheet having the above steel structure call be produced in a hot
rolling step, by defining C, Si and Mn within the ranges in terms of the chemical con~position,
and completing finish-rolling at from 850°C to 930°C, retaining the product in process in a
range from 740°C to 660'C for 3 sec or more, and winding the same it1 a temperature range of
450°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°C to 900aC, and
then coolitig the same at an average cooling rate of 1O0C/sec or more in an amlealing 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 perfornling a well known plating
treatment on a surface of the hot-rolled steel sheet or the cold-rolled steel sheet.
[0068] (Heating of Steel Sheet for Hot For~ning:I- Ieating to Terrtperature Range of 720°C or
Higher but Lower than an Acj Point)
Heating of a steel sheet for hot forming is performed up to a tcniperature of 720°C or
higher but lower than an Acj point. 111 this regard, the A c ~po int ('C) is a temperature
defined by the following empirical Fornmla (i), ~vhichis lower than the Ac3 point ('C) of ail
austenite single phase.
Acj =910 - 203x(co5)- 15.2xNi + 44.7xSi f 104xV -I- 31.5xMo - 30xMn- 1lxCr -
20xCu + 700xP + 400xsol. Al + 5OxTi (9
111 this case .the symbols of elements in Fonnula (i) represent thc contents of the
respective elenlents (by mass%) in the cliemical co~ilpositiono f a steel sheet. In this regard,
15
Forn~nla (i) is calculated by putting the content of an eleincnt uot contaiucd in a steel sheet as
0 (0 mass%).
[0069] When a heating temperature is less than 720eC, austenitization becomes insufficient
and martensite is not contained in a lot 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) becor~iesd ifficult. Therefore, a heating temperature should be 720'C or highel;
and is preferably 750°C or highee Meanwhile, when a heating temperature is not lower than
an Ac3 point, even i f a steel sheet is exposed to air cooling thereaftel; 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 anAc3 point, and is preferably not higher than an Ac3 point - 30'C.
[0070] Although there is no particular restriction on a heating rate up to 720°C and a heating
time for retention in the temperature range, they are preferably in the following ranges
respectively.
[0071] The average heating rate in heating up to 720'C is preferably from 0.2"C/sec to
100"Clsec. When the average heating rate is 0.2"Clsec or more, high productivity can be
secured. Further, when the average heating rate is IOO"C/sec or less, the heating temperature
can be regulated easily, even in a case in which heating is conducted in an ordinaly furnace.
[0072] The heating time in a temperature range of 720°C or higher and lower than an Acj
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°C, to a time point of
completion of heating. Specifically, the time point of completion of heating means, in the
case of filrnace heating, a tiine 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, \vlien the power supply is cut
off. When the heating time is 2 min or more, the strength after hot pressing (after
quenching) can be lnade more stable. When the retention time is 10 niin or less, the
stmcture of a steel sheet component can be micronized fi~rthers, o that the toughness of a steel
sheet component can be further improved.
100731 (Time Period fiom Con~pletion of Heating to Initiation of Hot Pressing, during
Which Steel Sheet Being Exposed to Air Cooling: 3 sec to 20 sec)
In general, a steel sheet for hot forming is transported after heating in a heating
filrnace to a hot press. In this case, for example, during extraction from a heating hrnace, or
during transportation to or loading on a hot press, a steel sheet inay be partly exposed to air
cooling. Since ferrite is newly formed or gro\vn during such air cooling, the titne duration of
air cooling has influence on tensile strength. 'I'herefore, for securing stably a high strength
16
after hot pressing (aftcr quenching), tlie time duration of air cooling should be preferably short.
Especially, when the time period from conlpletion 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 aftcr hot pressing (after quenching) decreases, or even \vhen 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 fiom 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 fornled
during heating has precipitated in an acicular fonn. Since a part of the precipitated austenite
transforms during cooling to ferrite and the forin 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 ~nartensitic 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 fiom completion of heating to initiation of hot pressing, during which a steel sheet is
exposed to air cooling, sl~ould be 3 sec or more, and is preferably 7 sec or more, more
preferably 10 sec or more.
In this regard, the time period allo~vinge xposure to air cooling can be regulated by
regulating a transportation time from extraction out of a heating fiirnace to a press mold,
which is ordinarily exposed to air cooling.
[0074] (Average Cooling Rate to Temperature Range of Ms Point or Lower: from 10"CIsec
to SOO"C/sec)
When a steel sheet for hot forming is hot-pressed and cooled to a temperature range
of an Ms point (Ms point = starting teniperature of niartensitic transformation) or lower at an
average cooling rate of from 10"CIsec to 5OO0C/sec, diffusional transfornlation is suppressed.
When tlie average cooling rate is less than 10aC/sec, bainitic transfornlation advances
excessively. Alternatively, pearlitic transformation occurs, so that the area rate of martensite,
\vhich 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, anstenite is stabilized, so that the bendability declines. Therefore, the avel.age
cooling rate in the temperature range should be 10aC/sec or more, and is preferably 30"CIscc
or tnore. Meanwhile, when the average cooling rate is over 500eC/sec, it becomes extre~iiely
difficult to niaintain the soaking of a steel sheet component, and the strength beconies
17 .
unstable. Thcreforc, thc average cooling rate should be 5OO"CIsec or less, and is preferably
200"CIsec or less.
In this regard, an average cooling rate is a value obtained by dividing the defcrence
between a temperature for performing hot prcssing ("C) and an Ms point ("C) by a time period
required from the temperature for performing hot pressing ("C) to the Ms point ("C).
[0075] In this regard, during cooling extren~ely large heat evolution due to phase
transformation occurs after reaching 400°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°C.
Therefore, cooling from 400°C to an Ms point is required to be performed more strongly than
cooling down to 400'C, and preferably perfornled,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 nlold 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 it1 the groves during
pressing, or the cooling rate tnay be also changed by lifting a press interrupting pressing and
flowing water in between. Futthermore, the cooling rate may be also changed by chauging a
mold clearance so as to change the contact area with a steel sheet. As a means for changing
the cooling rate, for exanlple, above and below 400°C, the following means are conceivable.
[0076]
(1) Irnnlediately after arriving at 400aC, a component is transferred to a tnold 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°C, the water
flow rate in the mold is changed to change a cooling rate.
(3) Immediately after arriving at 4OO0C, water is flown between a mold and a component
and the water flow rate is changed to change a cooling rate.
[0077] There is no particular restriction 011 a fort11 of forming by a hot pressing method
according to the invention. Examples thereof include betiding, draw forn~ings, tretch-expand
forming, bore expansion forn~ing, and flange forming. Suitable one may be selected
appropriately according to a kind of an intended hot fornled steel sheet co~nponent. Typical
examples of a hot formed steel sheet component include a door guard bar and a bumper
reinforcement, which are automobile reinforcing co~nponentsa, s described above.
(00781 A hot formed steel sheet con~ponent according to the invention is characterized in
18
that it is superior in ductility and bendability. As d~ictilityt o withstand practical use, total
elongation in a tensile test of 12% or more is preferable, arid total elongation of 14% or more
is still more preferable. As bendability, a limit bending radius of St or less in a V-bend test
with a tip angle of 90" is preferable.
[0079] A hot formed stecl sheet conlponent 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 fi.acture and also enhancing the fatigue strength.
[0080] Although in the above description, hot forniing has been described taking hot
pressing, which is its specific embodiment, as an example, the illvention is applicable
similarly to hot pressing also to hot fornling provided with a means for cooling a steel sheet
simultaneousl~w~it h or ilnmediately after forming, for example to roll fonning.
Examples
[0081] Examples according to the invention will be described, provided that the invention be
in no way restricted by Examples.
(00821 Steel sheets having chemical conlpositions 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"C for 30 miti, 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°C to 910"C, and the material is retained
in a range of from 720°C to 680°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°C or lower, and then
cooled slowly at 20"CIhour to room temperature, simulating a step for winding a hot rolled
sheet in a temperature range of 420°C or lower.

[0084] A thus obtained hot-rolled steel sheet had a complex structure of ferrite and
martensite, or of ferrite and bainite.
[0085] Mean\vhihile, 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 tenlperature by retaining the sheet in a range of from 740°C to 660°C
for 2 sec, and cooling the same by water spraying to room temperature. Test material No. 22
sirllulated a step for windilig a hot-rolled sheet at 670"C, by cooling the sheet by \vater
spraying to 670°C, and then cooling the same slo~vlya t 20"Chlour to room temperature.
[0086] Apart 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°C to 900"C,
and annealed under condition of cooling at an average cooling rate of 30"Clsec. I-Iowevel;
test material No. 27 was heated at 920"C, and annealed under condition of cooling at an
average cooling rate of 30"C/sec.
[0087] 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 2000x) of
EBSP was obtained for each sliced out cross-section by an EBSP analysis. Then, the
respective area rates of ferrite, maltensite, 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 veltical to the rolling direction. In
this regard, for an EBSP analysis the following conditions were set: acceleration voltage =
25kV, working distance = 15 mm, and scan step = 0.2 pm
[0088] Furfher, an aspect ratio of ferrite in a steel sheet to be subjected to hot pressing was
measured as follo\vs. 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. Tlle sliced out cross-sections were subjected to polishing and nital etching.
Next, using a scaiu~inge lectron microscope (SEM) equipped with an EBSP detector (trade
uame QUANTA 200, produced by FEI Conlpany), an 1Q image (image quality map:
magllification 2000x) of EBSP was obtained for each sliced out cross-section by an EBSP
analysis. Then, the aspect ratio of Eelrite was detern~ineda s an average value of aspect ratios
of each 50 ferrite crystal grains nleasured based on each IQ image of EBSP for cross-sections
21
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 = 25kV,
wvorking distance = 15 inm, and scan step = 0.2 p111.
[0089] The steel structures of steel sheets to be subjected to hot pressing arc shown in Table
[Table 21
CL
Hot-rolled
steel sheet 1.4 29 0 66 66 95 --
0 64 64 96
Test material
No.
I
-
2
3
4
nor-roue0
steel slleet 1.4 36 64 0 64
I I I I
lot-rolled
;tee1 sheet 1.3 5 78 83 -83
. .. .
100
, 1 1 1 I
lot-rolled
tee1 sheet -2. I 40 60 0 60 100 .. . . . I
..
5 C
6 C
J -. .. . 8
'2 .- . 3 1 . , ,
A
2 1
Cold-rolled
steel sheet 1.5 42 50 5 55 97 - -
22
Coated steel
c,.^^+ 1.7 36 6 52 58 94
A
A
- B
~ot-rolled
drpl -hrrt 1 1.3 ( 36 1 64 1 0 1 6 4 1 1 0 0 1
Coated sleel
rhnnt 1 1.3 1 28 1 5 1 65 1 70 1 98 1
>LCC, >,,I
-23 -I, steel -her1
24 -
25
26
22
h4
M
Hot-rolled
S ~ ~CP I~ ~ S + 1 1.3 1 33 1 67 1 0 1 6 7 1 1 0 0 1
Hut-ruueo
stad -t..... 1 1.3 1 33 1 67 1 0 6 7 1 100 1
I lot-rolled
steel sheet 32 -32 -77
* 1 : The total area rate of martensite and bainite (%)
*2: The total area rate of ferrite, martensite, and bainite (%)
[0091] The obtained steel sheets were heated in a gas furnace at an air fuel ratio of 0.85 aid
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 conlpletion 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 fol-th in Table 3 down to 150"C, which was not higher than an Ms
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 cotnpleting various test steel sheets (such
a test steel sheet is hereinafter referred to as "hot-pressed steel sheet").
[0092] 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 cooli~lg
rate down to 150°C was deter~ninedb y attaching a thermocouple to an edge of a steel sheet to
be subjected to hot pressing, and reading the temperature. I11 this regard, a heating time
means a time period from a time point when a steel sheet reaches 720°C after placement of the
samc in a furnace, until the same is taken out from the furnace. Meanwhile, in Examples 6,
IS and 25, various test steel sheets were prepared by conducting gas cooling at a
predetermined cooling rate after air cooling for a predetertnined time period, for simulating a
hot pressing condition, under ~vhicha cooling rate is changed using a mold with grooves,
[0093] The area rates of ferrite, tempered martensite, tempered bainite, and ma~fensite of a
hot pscssed 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
23
.The aspect ratio of ferrite of a hot pressed steel sheet was measured identically with
the aspect ratio of fenite of a steel sheet to be subjected to hot pressing.
[0094] The mechanical properties of a hot pressed steel sheet were examined as follows.
The results are also show11 in Table 4.
[0095] JIS No. 5 test piece for tensile.test was sampled fiom each steel sheet in the direction
nornial to the rolling direction, and a tensile test was carried out to measure TS (tensile
strength) and El (total elongation).
[0096] Further, a rectangular sample was cut fsom each steel sheet allowing a bending ridge
line to he 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°, and tip radii of 5
mnm, 4 tnm, 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 ratulg criteria.
- Rating Criteria of Bendability -
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 nlm, a tnicrocrack 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 nnn, a crack is recognized.
[0097] 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 conlponent.
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.
[0098] 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.
[0099]
[Table 31
Tesl Heating rate fro111 room I [eating tcmpenture Heating time Air cooling Average cooling rate to tcn~perature
mafcrial temperature to 720pC
PC) (min)
ti~tle range of Ms point or less
- No. (C/scc) (set) ('C/sec)
1 I?

[OlOO]
[Table 41
*3: The total area rate of te~npcredm altensite and tenlpered bainite (%)
*4: The total area rate of ferrite, tempered martensite, tempered bainite, and tnartensite (%)
[0101] ThetestmaterialsNo. 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 Exaniples,
namely hot pressed steel sheet components, satisfying all the requirements according to the
invention. Any of the hot pressed steel sheet colnponents of Inventive Examples as
hot-formed has a tensile stret~gtha s high as 980 MPa or more, and superior in ductility as wvell
as bendability.
[0102] 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.
[0103] 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.
[0104] With respect to the test material No. 7, since the steel sheet to be subjected to hot
pressing and the hot pressed steel sheet co~nponent did not have the structure defined
according to tlie invention, the ductility and the bendability were inferior,
[0105] With respect to the test illaterial 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.
[0106] With respect to the test ~nateriaNl o. 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 stecl sheet component did not have the structure defined
according to the invention, the ductility and the bendability were inferior.
[0107] 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 tlie
ductility and the bendability \yere inferior.
[0108] 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 thc hot pressed steel sheet
27
colilponent, and a targeted tensile strength was not obtained.
[O109] 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 tlie liot
pressed steel sheet component, and the bendability was inferior.
[0110] 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 bendabilitp
was inferior
[Olll] 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.
[0112] 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.
[0113] With respect to tlie 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.
[0114] 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 tlie invention, the tensile strength was lowv, and the ductility was inferior.
[0115] The entire disclosures of Japanese Patent Application No. 2013-247814 are hereby
incorporated by reference.
All tlle 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 effcct that the same
should be so incorporated by reference.
CLAIMS
[Claim 11
A hot formed steel sheet component, cotnprising:
a chemical composition comprising, in terns 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. A1 at from 0.001% to 1.000%, and N at 0.0100% or less, wvit11 a
remainder consisting of Fe and impurities; and
a steel structure comprising ferrite, at least one of tempered lnartensite or tempered
bainite, and martensite, wherein at1 area rate of ferrite is from 5% to 50%, a total area rate of
tempered martensite and tempered bainite is frotn 20% to 70%, an area rate of martensite is
from 25% to 75%, a total area rate of fel.rite, tenlpered martensite, tempered bainite and
martensitc is 90% or more, and an area rate of retained austenite is from 0% to 5%.
[Claim 21
The hot formed steel sheet component according to claim I, 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 telms of mass%, instead
of a part of the Fe.
[Claim 31
The hot forrned steel sheet con~ponenat ccording to claim 1 or claim 2, wherein the
chemical composition comprises B at 0.0025% or less, in terms of mass%, instead of a part
of the Fe.
[Claim 41
The hot formed steel sheet component according to any one of claint 1 to claim 3,
wherein the che~nicalc omposition 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, REh4 at 0.0100% or
less, and Zr at 0.01 00% or less, in terms of mass%, instead of a part of the Fe.
[Claim 51
The hot formed steel sheet colnponent according to any one of claim 1 to claim 4,
wherein the chetnical composition colnprises Bi at 0.01 00% or less, in terms of mass%,
instead of a part of the Fe.
[Claim 61
A steel sheet for hot forniing, cotnprising:
29
a chemical conlpositio~cl ornpsising, in terlns of ~nass%C, at from 0.100% to
0.340%, Si at fro111 0.50% to 2.00%, h4n at from 1.00% to 3.00%, Pat 0.050% or less, S at
0.0100% or less, sol. A1 at from 0.001% to 1.000%, and N at 0.0100% or less, with a
remainder comprising Fe and impurities; and
a steel structure comprising ferrite with an aspect ratio of 2.0 or less, and at least one
of maltensite or bainite, wherein an area rate of ferrite is from 5% to SO%, a total area rate of
martensite and bainite is from 45% to 90%, and a total area rate of ferrite, inartensite, and
bainite is 90% or luore.
[Claim 71
The steel sheet for hot fosnling according to claim 6, wherein the chemical
co~npositionc omprises one kind or hvo 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%, illstead of
a part of the Fe.
[Claim 81
The steel sleet for hot forming according to clai~n6 or claim 7, wherein the cllemical
composition comprises B at 0.0025% or less, in terms of mass%, instead of a part of the Fe.
[Claim 91
The steel sheet for hot fornling according to any one of claim 6 to claim 8, wherein
the clle~nicalc omposition comprises one kind or two or more kinds selected from tl~egr oup
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 tile Fe.
[Claim 101
The steel sheet for hot for~ninga ccording to any one of claim 6 to claim 9, wherein
the che~nicacl onlposition comprises Bi at 0.0100% or less, in terms of mass%, instead of a
part of the Fe.
[Claim 111
A lnethod for producing a liot fonncd steel shcct co~nponentt,h e n~ethodc onnprising:
heating the steel sheet for hot forming according to any one of clainl6 to claim 10 to a
temperature range of 720°C or higlrer but lower than an Ac3 point; performing hot for~ning
within a time period of from 3 sec to 20 sec, during whicll the stecl sheet is es~~ostcod a ir
cooling from the end of the heating until the initiation of the hot forming; and cooling to a
te~nperaturer ange not above an I\& point at an average cooling rate of from 1O"CIsect o
SOO"C/sec.