Docn~nentT ype] Specification
[Title of the Invention] HOT-FORMED MEMBER AND METHOD OF
MANUFACTURING SAME
[Teclmical Field of the Invention]
[OOOl]
The present invention relates to a hot-formed member for use in tnachine
structural components such as structural components for the body of an automobile,
and a method of manufacturing the hot-formed member. Specifically, the invention
relates to a hot-formed meniber which has excellent ductility and impact resistance
while having a tensile strength of 900 MPa or more, and a method of manufacturing
the hot-formed member.
[Related Put]
[0002]
In recent years, there has been a lot of effort to improve the strength of steel
used in the bodies of automobiles in order to reduce the use weight of the steel. As
for steel sheets widely used in technical fields related to automobiles, press formability
decreases with an increase in the strength of the steel sheet, arid it becomes difficult to
manufacture a member having a complicated shape. Specificallj: there is a problem
in that since the.ductility of the steel sheet decreases with an increase in the strength of
the steel sheet, breaking occurs in a region having a high working degree in the
member, and/or flle spring-back and the wall camber of the nle~nberin crease, and thus
the shape accuracy of the ~ileniberd eteriorates. Accordingly, it is not easy to
manufacture a member having a co~nplicateds hape by applying press forming to a
steel sheet having a high strength, particularly, a tensile strength of 900 MPa or more.
It is possible to work a steel sheet having a high strength by roll forming, not by press
forming. I-Iolvever, roll forming can be applied only to a ~ncthodo f manufacturing a
member having a unifonn cross-section in a longitudinal direction.
[0003]
As shown in Patent Docnment 1, a member having a conlplicated shape can
be formed with high shape accuracy from a high-strength steel sheet it] a method called
hot pressing for press-forming a heated steel sheet. This is because, in the hot
pressing process, the steel sheet is formed in a state of beit~gh eated at a high
temperature, and thus the steel sheet dnring working is soft and has high ductility.
Ful-thermore, in the hot pressing, by heating the steel sheet in an austenite single phase
region before the pressing, and rapidly cooling (hardened) the steel sheet in a die after
the pressing, it is possible to achieve an increase in the strength of the member due to
~nartensiticts ansformation. Accordingly, the hot pressing method is at1 excelletit
forming method capable of simultaneously securing an increase in the strength of a
member and the formability of a steel sheet.
[0004]
Patent Document 2 discloses a pre-press quenching method in which a steel
sheet is previously formed into a predetermined shape at room temperature, and a
member obtained is heated to an austenite region and rapidly cooled in a die to achieve
an increase in the strength of the member. In the pre-press quenching method mhich
is an aspect of hot pressing, the deformation of tlic member due to distortion by heating
can be suppressed by restraining the member with the die. The pre-press quenching ,
method is an excellent forming method capable of increasing the strength of the .
member and of obtainitig high shape accuracy.
[OOOS]
However, in recent years, there has also been a demand for hot-formed
~nenlbersto have ductility, and there is a problem in that in the related art represented
by Patent Documents 1 and 2 in which the metallographic nlicrostr~lctttrcis
substantially a martensite single phase, it is not possible to meet such a demand.
[0006]
Patent Docu~nent3 discloses a member whicli is regarded to be excellent in
ductility and have a dual phase microstructure including ferrite and martensite by
heating a steel sheet having the C content limited to 0.1% or less in an austenite single
phase region and by performing hot pressing. In this manner, when the steel sleet is
heated in an austenite single phase region, the member has a uniform metallographic
microstructure. However, as is obvious from the description of examples of Patent
Document 3, the C content is limited to 0.1% or less in the enember described in Patent
Document 3, and thus the tensile strength of the member is at most 700 MPa.
Therefore, the member does not have a sufficient strength for contributing to a
reduction it1 the weight of an automobile.
[0007]
Patent Document 4 discloses a member ~vhichh as a tensile strength of 980
MPa or more and excellent ductility and which has a dual phase microstructure,
specifically, a two phase microstructure of ferrite and martensite by heating a steel
sheet having a large amount of Cr added thereto to an austenite single phase region, to
transform a part of the austenite into ferrite before or after pressing. However, when
using steel having a large amount of Cr added thereto as disclosed in Patent Document
4, carbide such as cementite and M23C6 formed in the steel is poorly solid-soluble
during heating, and thus it is necessary to perform heating for a long period of time to
secure stable mechanical properties. Furthermore, since a long period of time is
required for the ferritic transformation, a new manufacturing process in which liolding
is performed for a long period of time is needed after heating to an austenite single
phase region in order to form the two phase n~icrostructure. Accordingl~t~h,e method
described therein is a method xvhich significantly impairs productivity in addition to an
increase in the cost for the manufacturing of the hot-formed melnbel; and is not
suitable as a rnass production techno log^^.
[0008]
Patent Docunlents 5 to 7 disclose members which bas a high strength and
excellent ductility with a two phase microstructure of ferrite and martensite and in
which each of the ferrite and the martensite has an average grain size of 7 pm or less
by heating a cold-rolled steel sheet having an average grain size (average grain size of
ferrite, or in the case in which a second phase is further included, average grain size of
ferrite and the second phase) of 15 p n o~r less to form a two phase microstructure of
ferrite and austenite, pressing the cold rolled steel sheet while maintaining the
microstructure, and rapidly cooling the cold rolled steel sheet in a die.
[Prior Art Documents]
[Patent Docutnents]
[0009]
[Patent Document 11 British Patent Publication No. 1490535
[Patent Document 21 Japanese Unexa~nined Patent Application, First
Publication No. H10-96031
[Patent Docun~ent3 1 Published Japanese Translation No. 201 0-521584 of
the PCT International Publication
[Patent Document 41 Japanese Unexa~ninedP atent Application, First
Publication No. 2010-13 1672
[Patent Document 51 Japanese Unexa~ninedP atent Application, First
Publication No. 201 0-65293
[Patent Document 61 Japanese Unexami~icd Patent Application, First
Publication No. 2010-65292
[Patent Document 71 Japanese Unexamined Patent Application, First
Publication No. 201 0-65295
[Disclosure of the In\fention]
[Problems to be Solved by the Invention]
[OOlO]
As described in Patent Documents 5 to 7, the n~etallographicm icrostmcture of
the steel sheet subjected to hot pressing has an influence on the metallographic
microstructure of the hot-formed member. Particularly, as sho\vn in Patent
Documents 5 to 7, making a fine metallographic microstructure is an important
microstructure co~itroml ethod contributing to an improvement in the ductility of the
member.
[OOll]
The inventors have tiewly found that the impact resistance of the member can
be improved by refining and uniformizing the microstructure of the steel sheet to be
subjected to hot forming. The inventors have further found that in order to refine and
uniformize the microstructure of the steel sheet to be subjected to hot fanning, it is
required that the hot-rolled steel sheet is cold-rolled aud is annealed at a
recrystallizatinn.temperature. , .
[OO 121
Regarding this, in the methods disclosed in Patent Docu~nents5 to 7, the
annealing temperature is controlled to be near an Acl temperature in order to refine the
microstmcture of the steel sheet to bc subjected to hot pressing. According to the
knowledge of the i~lventorsa, large alnount of unrecrystallized ferrite remains in the
steel sheet subjected to hot pressing under such manufactnring conditions. Since the
unrecrystallized ferrite is not recrystallized even when heated in a two phase
temperature range in which ferrite and austenite coexist, the microst~uch~arfet er hot
pressing is extremely non-uniform. In addition, the methods disclosed in Patent
Documents 5 to 7 also assume that the steel sheet contains a large amount of Ti.
Since Ti acts to prevent the ferrite from being recrystallized, there is a concern that a
large amount of unrecsystallized ferrite may remain when a large amount of Ti is
contained. However, the technologies disclosed in Patent Documents 5 to 7 do not
suggest any ideas for controlling the unrecrystallized ferrite. Accordingly, in the
methods disclosed in Patent Documents 5 to 7, although the microstructure of the hotformed
member can be made fme and the ductility thereof can be improved, the impact
resistance thereof is significantly insufficient.
[00 131
As described above, a mass production technology that provides members
which are manufactured by hot forming and have a tensile strength of 900 MPa or
more aud excellent ductility and impact resistance has not been established yet.
[0014]
A specific object-of the present im~entionis to provide a hot-formed lnetnber
having excellent ductility and impact ~.esistancea nd a tensile strength of 900 MPa or
more, that could not bemass-produced in the past as described above, and a method of
manufacturing the hot-fosnled member.
[Means for Solving the Problem]
[0015]
The inventors have conducted an intensive study to improve the ductility and
the impact resistance of a hot-formed member having a tensile strengtl~o f 900 MPa or
more, and as a result, obtained novel knowvledge that the ductility and the impact
resistance of a hot-fanned member are improved by (1) setting the Ti content in the
hot-formed member to a limited range and (2) making the metallographic
microst~~~ctoufr teh e hot-formed member as a fine and uniform metallographic
microstmcture consisting of ferrite and martensite. In addition, the inventors have
obtained novel knowledge that a hot-formed member having such a metallographic
microstructure is achieved by using a steel sheet having the above-described chemical
composition and having a fine and uniform metallographic microstructure as a steel
sheet to be subjected to hot forming, and by adjusting the heat treatment conditions in
the hot forming.
[0016]
The present invention is contrived based on such knowledge, and the gist
thereof is as follows.
(1) A hot-formed member according to an aspect of tlie present invention
has a chemical co~npositionc ontaining, by mass%: 0.10% to 0.40% of C; 0% to 2.0%
of Si; 1.0% to 3.0% of Mn; 0.05% or less of P; 0.01% or less of S; 0.001% to 1.0% of
sol. Al; 0.050% to 0.30% of Ti; 0.01% or less of N; 0% to 0.4% of Nb; 0% to 0.4% of
V, 0% to 1 .O% of Cr; 0% to 1 .O% of Mo; 0% to 1.0% of Cu; 0% to 1 .O% of Ni; 0% to
0.01% of Ca; 0% to 0.01% of Mg; 0% to 0.01% of REM; 0% to 0.01% of Zr; 0% to
O.Ql% of B; 0% lo 0.01% of Bi; and tlie balance of Fe andimpurities, wherein the hot-
, formed member has a metallographic microstructure which has, in terms of area%,
10% to 90% of a ferrite, 0% to 2.0% of an unrecrystallized ferrite, 10% to 90% of a
mattensite is, in ~vhiclai total area ratio of the ferrite and the martensite is 90% to
loo%, and in wvhicli an average grain size of the ferrite is 0.5 p n to 5.0 pm, and a
tensile strength is 900 MPa to 1800 MPa.
[0017]
(2) In thc hot-formed member according to (I), the chemical composition
may contain, by mass%, one or two or more selected ftom the group consisting of
0.003% to 0.4% of Nb, 0.003% to 0.4% of V, 0.005% to 1 .O% of Cr, 0.005% to 1 .O%
of Mo, 0.005% to 1.0% of Cu, and 0.005% to 1 .O% of Ni.
[0018]
(3) In the hot-formed member according to (1) or (2), the chemical
composition may contain, by mass%, one or two or more selected from the group
consisting of 0.0003% to 0.01% of Ca, 0.0003% to 0.01% of Mg, 0.0003% to 0.01% of
REM, and 0.0003% to 0.01% of Zr.
[0019]
(4) In the hot-formed member according to any one of (1) to (3), the
chemical con~positionm ay contain, by mass%, 0.0003% to 0.01% of B.
[0020]
(5) In the hot-formed member according to any one of (1) to (4), the
chemical composition may contain, by mass%, 0.0003% to 0.01% of Bi.
[0021]
(6) A method of nianufacturing a hot-fonned member according to atlother
aspect of the present invention includes: heating a base steel sheet having the same
,, chemical co~npositiona s that of the hot-fonned rneniber according to any one of (1) to
(5) and a metallographic microstructure in which the amount of an ulxecrystallized
ferrite is 0 area% to 2.0 areayo and an average grain size of a ferrite is 0.5 vln to 7.0
pm to a temperature range of 720°C to lower than an Ac) temperature; then
maintaining a temperature of the base steel sheet for 1 minute to 20 minutes in the
temperature range of 720°C to lower than the ACJ temperature; then hot-forming the
base steel sheet; and then cooling the base steel sheet under the conditions in ~vhicha n
average cooling rate is 20°C/sec to 500°C/sec in a temperature range of 600°C to 150°C.
[0022]
(7) A method of manufacturing a hot-formed member according to still
another aspect of the present invention includes: heating a base steel sheet having the
same chemical composition as that of the hot-formed metnber according to any one of
(1) to (5) and a n~etallographicm icrostructure in which an unrecrystallized ferrite is
more than 2.0 area% and an average grain size of a ferrite is 0.5 pm to 7.0 ym to a
temperature range of an ACJ temperature to the ACJ temperature + 100°C; then
maintaining a ten~peratureo f the base steel sheet for 30 seconds or longer and shorter
than 20 minutes in the ten~peraturer ange of the ACJt emperature to the Ac3 temperature
+ 100°C; then hot-forming the base steel sheet; and then cooling the base steel sheet
under the conditions in which an average cooling rate is 3OClsec to 20°Clsec in a
temperature lange of the Ac3 temperature to 600°C.
[0023]
(8) In the method of manufacturing a hot-formed member according to (7)
or (X), the base steel sheet may be one selected from the group consisting of a coldrolled
steel sheet, a hot-dip galvanized steel sheet, and a galvannealed steel sheet.
[Effects of the Invention]
[0024] ..
According to the present invention, technically valuable effects such that
practical application of a hot-formed member having excellent ductility and it~lpact
resistance and a tensile strength of 900 MPa or more is achieved, and the first mass
production of such a hot-formed member becomes possible.
[Brief Descriptio~ol f the Drav,lit~g]
[0025]
FIG. I is a flowchart illustrating a ~nanufacturingtn ethod according to the
present invention..
[E~nbodimentso f the Invention]
[0026]
Next, a hot-formed niember and a method of tna~n~facturinthge hot-formed
member according to an embodiment of the present invention, v,~l~ichha ve been
achieved based on the above-described knowledge, will be described. In the
follo~vingd escription, regarding hot forming, hot pressing that is a specific aspect
thereof will be used as an example for description. However, when the substantially
same manufacturing conditions as those disclosed in the following description are
achieved, forming methods other than hot pressing, for example, roll forming and the
like may be e~nplopeda s a hot forming method. The tensile strength of the hotformed
member according to this embodiment is 900 MPa to 1800 MPa. In order to
reduce the weights of machi~les tn~eturacl omponents of automobiles and the like, the
tensile strength of the material thereof is required to be set to 900 MPa or more. In
addition, in order to prevent brittle fracture of a steel sheet fsom occurring, it is
necessary to set tlie tensile strengt11 to 1800 MPa or less. In this embodiment, such a
tensile strength is achieved by appropriately controlling the atnoutlt of various alloy
elements such as C and the manufacturing metliod.
[0027]
1. Chemical Composition
First, tlie reasons \vl~yth e ehetnieal cotnposition of a hot-formed ~neniber
according to an ctnbodime~lot f the present invention is specified as described above
will be described. In the following description, the sy~nbol"%in dicating the
amount of each alloy element means "mass% unless specifically noted. Since the
chemical cornposition of steel does not change even when the steel is subjected to hot
fonning, the amount of the respective elements in a base steel sheet before hot forming
is the same as that in a hot-formed member after hot forming.
[OOZS]
(C: 0.10% to 0.40%)
C is a very important element which increases hardenability of steel and has
the strongest influence on the strength after hardening. When the C content is less
than 0.10%, it is dificult to secure a tensile strength of 900 MPa or more after
hardening. Accordingly, the C content is set to 0.10% or more. In order to more
securely obtain the above-described effects, the C content is preferably 0.11% or more.
When the C content is more than 0.40%, the impact resistance of a hot-formed member
may significantly deteriorate, and weldability of the hot-formed member may decrease.
Accordingly, the C content is set to 0.40% or less. The C content is preferably set to
0.28% or less from the viewpoint of weldability.
[0029]
(Si: 0% to 2.0%)
The hot-formed member according to this en~bodimendt oes not essentially
contain Si. Accordingly, the lower limit of the Si content is 0%. However, Si is an
elenlent which acts to increase the strength after hardening while not deteriorating or
improving ductility. When the Si content is less than 0.001%, it is difficult to obtain
the above-described actions. Accordingly, in order to obtain the above-described
effects, the Si content may be set to 0.001% or more. When the Si content is set to
0.05% or more, ductility is further improved. Accorditigly, the Si content is
- 11 -
preferably set to 0.05% or more. When the Si content is more than 2.0%, the effects
obtained by the above-described actions are saturated and economic disadvantages are
caused. Moreover, surface properties significantly deteriorate. Accordingly, the Si
content is set to 2.0% or less, and is preferably 1.5% or less.
[0030]
(Mn: 1.0% to 3.0%)
Mn is a very effective element for increasing hardenability of steel and stably
securing the strength after hardening. However, when the Mn content is less than
1.0% or less, the effects thereof cannot be sufficiently obtained, and it is very diflicult
to secure a tensile strength of 900 MPa or more after hardening. Accordingly, the Mn
content is set to 1.0% or more. When the Mn content is 1.6% or more, a tensile
strength of 980 MPa or more can be secured after hardening. Therefore, the Mn
content is preferably set to 1.6% or more. When the Mn content is more than 3.0%,
the hot-formed member has a non-uniform nietallogsaphic microstnlcture, and impact
resistance significantly deteriorates. Accordingly, the Mn content is set to 3.0% or
less. When a tensile strength of a base steel sheet before application of hot fortning is
reduced, productivity in a later hot forming process is improved. In order to obtain
this effect, the Mn content is preferably set to 2.4% or less.
[0031] . . .
(P: 0.05% or Less)
In g e n d , Pisan impurity inevitably contained in steel. However, in this.,
embodiment, since P acts to increase the strength of the hot-formed member by solidsolution
strengthening, P may be actively contained. However, when the P content is
more than 0.05%, weldability of the hot-formed member significantly deteriorates.
Accordingly, the P content is set to 0.05% or less. In order to more securely prevent a
- 12 -
deterioration in the weldability of the hot-formed member, the P content is preferably
set to 0.02% or less. In addition, in order to tnore securely obtain the above-described
actions, the P content is preferably set to 0.003% or more. However, even when the P
content is 0%, necessary characteristics for solving the problems can be obtained.
Therefore, it is not necessary to limit the lower limit of the P content. That is, the
lower limit of the P content is 0%.
[0032]
(S: 0.01% or Less)
S is an impurity contained in steel. The S content is preferably as small as
possible in order to iniprove weldability. When the S content is tnore than 0.01%, the
weldability is unacceptably significantly deteriorated. Accordingly, the S content is
set to 0.01% or less. In order to more securely prevent the deterioration in the
weldability, the S content is preferably set to 0.003% or less, and more preferably
0.0015% or less. Since the S content is preferably as small as possible, it is not
necessary to specify the lower limit of the S content. That is, the lower limit of the S
content is 0%.
[0033]
(sol. Al: 0.001% to 1.0%)
sol. Al-indicates solid solution A1 existing in a solid solution state in steel.
A1 is an element acting to deoxidize steel, and is an elenlent which prevents a
carbonitride forming element such as Ti from being oxidized to promote the formation
of carbonitride. Due to these actions, generation of surface defects on the steel call be
suppressed, and the yield of the steel can be improved. When the sol. A1 content is
less than 0.001%, it is difficult to obtain the above-described actions. Accordingly,
the sol. A1 content is set to 0.001% or more. In order to more securely obtain the
above-described actions, the sol. A1 contellt is preferably set to 0.01 5% or more.
When the sol. A1 col~teli~s tm ore than 1.0%, weldability of the hot-formed member
significantly decreases, and the amount of oxide-based inclusioris illcreases in the hotformed
member, whereby surface properties of the hot-formed member significatltly
deteriorate. Accordingly, the sol. A1 content is set to 1 .O% or less. In order to more
securely avoid the above-described phenomenon, the sol. Al content is preferably
0.080% or less.
[0034]
(Ti: 0.050% to 0.30%)
Ti is an importa~ite lement in this embodiment. Since fine precipitates that
are Ti carbide, Ti nitride, and/or Ti carbonitride are formed in the hot-formed member
when Ti is contained, the metallographic microstructure after hardening can be made
fine, atid whereby the ductility of the hot-formed member is sigtiificantly improved.
When the Ti content is less than 0.050%, the metallographic microstructure after
hardening is not made fine, and thus the ductility ca~itiobt e improved. Accordingly,
the Ti content is set to 0.050% or more. The Ti content is preferably 0.070% or more.
When tlie Ti content is tilore than 0.30%, coarse carbotiitride is formed during casting
and during hot rolling, and thus the impact resistance of the hot-formed member
significantly deteriorates. Accordingly, the Ti content is set to 0.30% or less. The Ti
content is preferably 0.25% or less, and more preferably 0.20% or less.
[OD351 '- ,.
(N: 0.01% or Less)
N is an impurity contait~edi n steel. In order to improve weldability, the N
content is preferably as small as possible. When the N content is more than 0.01%,
the weldability of tlie hot-formed member is unacceptably significantly deteriorated.
Accordingly, the N content is set to 0.01% or less. In order to more sccurcly avoid
the deterioration in the weldability, the N content is preferably 0.006% or less. Since
the N content is prefcrably as small as possible, it is not necessary to specify the lower
limit of the N content. That is, the lower limit of the N content is 0%.
[0036]
The cl~emicacl omposition of the hot-formed member according to this
embodiment includes the balance of Fe and impurities. The impurities mean
compone~itsth at are incorporated from raw materials such as ore or scrap, or due to
various factors in the manufacturing process in the industrial manufacturing of steel,
and are permitted within a range in which this embodiment is not affected. Howevel;
. the hot-fonned member according to this embodiment ]nay further contain, as arbitrary
components, elements described below. Since necessary characteristics for solving
the problems can be obtained even when the arbitrary elements to be described below
are not contained in the hot-formed membel; it is not necessary to limit the lower limit
of the amount of the arbitrary elements. That is, the lower limit of the amount of each
, arbitrary element is 0%.
[0037]
(One or Two or More Selected from Group Consisting of 0% to 0.4% of Nb,
0% to 0.4% of V, 0% to 1.0% of Cr, 0% to 1 .O% of Mo, 0% to 1.0% of Cu, and 0% to
1 .O% of Ni)
All these elemellts are effective elements for increasing hardenability of steel
and stably securing the strength of the hot-fonned member after hardening.
Accordingly, one or two or more of these elements may be contained in the hot-formed
member. However, when Nb and V are contained in an amount of more than 0.4%,
respectively, it is dificult to perform hot rolling and cold rolling in the manufacturing
process. Furthermore, when Nb and V are contained in an amoullt of Inore than 0.4%,
respectively, the hot-formed member after hardening is likely to have a ilon-unifor111
microstructure, and thus the impact resistance of the hot-fanned nle~nbers ignificantly
deteriorates. In addition, ~vllenC s, Mo, Cu, and Ni are contained in an amount of
lllore than 1.0%, respectively, the effects obtained by the above-described actions are
saturated and eco~lomicd isadvantages are caused. Moreover, it is difficult to perfon11
hot rolling and cold rolling in the manufacturing process. In order to tnore securely
obtain the effects caused by the above-described actions, at least one of 0.003% or
more of Nh, 0.003% or more of V, 0.005% or more of Cr, 0.005% or more of Mo,
0.005% or more of Cu, and 0.005% or more of Ni is preferably co~ltained.
[0038]
(One or Two or More Selected from Group Consisting of 0% to 0.01% of Ca,
0% to 0.01% of Mg, 0% to 0.01% of REM, and 0% to 0.01% of Zr)
All these elements are elements contributing to control of inclusions,
particularly, fine dispersion of inclusions, and acting to increase low-temperature
toughness of the hot-formed member. Accordingly, one or two or more of these
elements may he contained. However, when any of these elements is contained in an
amount of more that1 0.01%, surface properties of the hot-formed member may
deteriorate. According15 in a case it1 which the elements are contained, the anlount
of each element is set as described above. In order to Illore securcly obtain the effects
caused by the above-described actions, the amount of each element to be added is
preferably set to 0.0003% or more.
Here, the term "REM" indicates total 17 elements including Sc, Y, and
lantl~anoida, nd the "REM conte~lt"m eans the total amount of these 17 elements. In
the case in which lanthanoid is used as REM, the REM is industrially added in the
form of misch metal.
[0039]
(B: 0% to 0.01%)
B is an element acting to increase low-temperature toughness of the hotformed
member. Accordingly, B may be contained in the hot-formed member.
However, when B is contained in aji amount of more than 0.01%, the hot workability
of a base steel sheet deteriorates, and it becomes difficult to perform hot rolling.
Accordingly, in a case in which B is contained in the hot-formed membel; the B
content is set to 0.01% or less. In order to more securely obtain the effects caused by
the above-described actions, the B content is preferably set to 0.0003% or more.
[0040]
p i : 0% to 0.01%)
Bi is an element uniformizing the metallographic microstructure of the hotformed
member and acting to increase impact resistance of the hot-formed member.
Accordingly, Bi may be contained in the hot-formed member. However, when Bi is
contained in an amount of more than 0.01%, hot workability of a base steel sheet
deteriorates, and it becomes difficult to perfonn hot rolling. Accordingly, in the case
in which Bi is contained in the hot-fornled membel; the Bi content is set to 0.01% or
less. In order to more securely obtain the effects caused by the above-described '
actions, the Bi content is preferably set to 0.0003% or more.
,., [0041] . A.
2. Metallographic Microstructure oCHot-Formed Member
Next, the metallographic n~icrostructureo f the hot-formed member according
to this embodiment w i l l be described. In the following description, the symbol " %
indicating the alnount of each n~etallographicm icros~~~ctmureean s "area% unless
specifically noted.
Tlie configuration of the riletallographic microstructure to be described below
is the configuration at approximately 112t to 114t of a sheet thickness, and not in a
central segregation portion. The central segregation portion may have a different
metallographic tnicrostruch~refr om a representative ~netallographicm icrostructure of
steel. However, the central segregation portion is a very small region with respect to
tlie entire sheet thickness, and has little influence on the mechanical properties of the
steel. That is, the metallographic microstructure of the central segregation portion
does not represent the metallographic tnicrostructure of the steel. Accordingly, the
metallographic microstructure of the hot-formed member according to this
embodiment is specified at approximately 112t to 114 of the sheet thickness and not in
the central segregation portion. The "position at 112t" indicates a position at a depth
of 112 of a thickness t of the ~netnberfr om a surface of the hot-fo~medm ember, and the
"position at 114t" indicates a position at a depth of 114 of the thickness t of the member
from the surface of the hot-formed membes.
In this embodiment, ferrite plastically deformed by rolling, elongated in a
rolling direction, and then remaining without being recrystallized is called
"unrecrystallized ferrite". In this embodiment, ferrite other than the unrecrystallized
ferrite is called ''fem'te" or "tiormal ferrite". Tlie tern1 "unrecrystaliized ferrite" is a
term well known to those skilled in the art. The nornlal ferrite includes recrystallized
.. ferrite generated-byr. ecrystallization and trausfor~nedf errite generated by phase
transfor~nationa nd the like.
In grains of the utlrecrystallized ferrite, crystal orientation continuously
cliatiges due to tlie plastic deforniatioti by rolling. The crystal orientation it1 grains of
the normal ferrite is alinost unifonn, and crystal orientations between the tiornial ferrite
grains adjacent to each otlter are different fiom each other. Due to such a difference,
the unrecrystallized ferrite has higher hardness than the normal ferrite.
Since the umecrystallized ferrite has a shape elongated in the rolling direction, the
unrecrystallized ferrite and the normal ferrite can be discriminated from each otlier by
observing the metallographic microstructures with a microscope. In addition, since
the unrecrystallized ferrite and tlte nor~ttafle rrite are different in the state of the crystal
orientation, the unrecrystallized ferrite and the nonnal ferrite can be discriminated
from each other by analyzing crystal orientation measurement data of an electron back
scattering pattern (EBSP) of the ~netallographicm icrostructure through a kernel
average misorientation method (KAM method). In this embodiment, ferrite with an
aspect ratio of 4 or more is the unrecrystallized ferrite, and fenite with an aspect ratio
of less than 4 is normal ferrite.
[0042]
(Area Ratio of Ferrite: 10% to 90%)
When the area ratio of the ferrite is less than lo%, the grains of the ferrite are
not adjacent to each otlter. That is, most ferrite grains are isolated, and the ductility of
the hot-formed member cannot be improved. Accordingly, the area ratio of the fessite
is set to 10% or more. When the area ratio of the ferrite is more than 90%, the area
ratio of the martensite is less than lo%, and as will be described later, it is difficult to
secure a tensile strength of 900 MPa or more after hardening. Accordingly, the area
.. ratio of the ferrite is set to 90% or less. The ratio between the ferrite and the
mattensite is not particularly limited as long as {he area ratio of the ferrite is within the
above-described range. However, the ferrite is preferably 25% to 85%, and the
martensite is preferably 15% to 75%.
[0043]
(Area Ratio of Unrecrystallized Ferrite: 0% to 2.0%)
When the unrecrystallized ferrite remains in the ~~ietallograplimici crostmcture
of the hot-formed meniber, the strength of the hot-formed member after hardening
increases, however, since the metallographic microstructure is extremely non-uniform,
the ductility and the impact resistance of the hot-formed member extre~~~deeltyer iorate.
Specifically, in the case in which the area ratio of the unrecrystallized ferrite is more
than 2.0%, desired ductility and impact resistance cannot be obtained. Accordingly,
the area.ratio of the utirecrystallized ferrite of the hot-formed member is set to 2.0% or
less (including 0%).
[0044]
(Area Ratio of Maltensite: 10% to 90%)
The strength of the hot-formed member after hardening can be increased by forming
the martensite in the metallographic microst~uctureo f the hot-formed member. When
the area ratio of the martensite is less than lo%, it is difficult to secure a tensile
strength of 900 MPa or more after hardening. Accordingly, the area ratio of the
martensite is set to 10% or more. When the area ratio of the martensite is more than
90%, the area ratio of the ferrite (recrystallized ferrite) is less than lo%, and as
described above, the ductility cannot be improved. Accordingly, the area ratio of the
tnartensite is set-tu90% or less.
[0045]
(Total&= Ratio of Ferrite and Marteusite: 90% to 100%)
The hot-fonned rne~nbera ccording to this embodiment has a metallographic
tnicrostr~~ctumrea inly including ferrite and martensite. However, in accordance with
manufacturing conditions, one or two or more of bainite, residual austenite, cementite,
and pearlite tnay be incorporated in the metallographic !nicrostructure as a phase or
n~icrostructurco ther than the ferrite and the martensite. In this case, when tlie area
ratio of the phase or ~netallographicm icrostructure other than tlie ferrite and the
martensite is more than lo%, target n~echanicapl roperties may not be obtained due to
the influence of these phases or metallographic n~icrostructures. Accordingly, the
area ratio of tlie phase or microst~uctt~oreth er than the ferrite and the ~nartensiteis less
than 10%. That is, the total area ratio of the ferrite and the martensite is set to 90% or
more. Since it is not necessary to specify the upper limit of the total area ratio of the
ferrite and the martensite, the upper limit of the total area ratio of tlie ferrite and the
martensite is 100%.
[0046]
The method of measuring the area ratio of each phase in the metallographic
microstructure is well known to those skilled in the art, and can be measured using a
conventional method in this emboditnent. As will be shown later in examples, in this
embodiment, test pieces are prepared from the hot-formed member in a direction in
which a base steel sheet that is a raw material of the hot-formed member is rolled and
in a direction perpendicular to the rolling direction. Next, metallographic
microstructures of a cross-section in the rolling direction and a cross-section
perpendicular to the rolling direction in the test piece are photographed by an electron
microscope. Electron nlicrographs of regions of 800 pm x 800 pm (800-square-pm
regions) obtained as described above are analyzed to calculate area ratios of
onrecrystallized ferrite, ferrite, and martensite. By using the electron microscope, -
ferrite grains and ~nartensiteg rains can be easily discriminated from surrounding
microstructures. In addition, ferrite grains can be discriminated from unrecrystallized
ferrite grains by calculating aspect ratios of grains from shapes of the grains, by
recognizing ferrite grains having an aspect ratio of 4 or Inore as unrecrystallized ferrite
grains, and by recognizing ferrite grains having an aspect ratio of less than 4 as ferrite
grains.
[0047]
(Average Grain Size of Ferrite: 0.5 pm to 5.0 pm)
By refining the metallographic microstructure after hardening, the strength,
ductility, and impact resistance after hardelling can be increased. The average grain
size of the fer~iteis set to 5.0 pm or less in order to secure good ductility and impact
resistance while tnaintaining a tensile strength to 900 MPa or more. Since the
average grain size of the ferrite is preferably as small as possible, it is not necessary to
specify the lower limit of the average grain size of tlie ferrite. However, the
substantial lower limit of the average grain size of the ferrite is about 0.5 pm in
consideration of a manufactt~ringe quipment capacity.
[0048]
The hot-formed member according to this embodinlent represents a member
hot-formed from a base steel sheet, and includes, for example, a steel member formed
by hot pressing. Representative hot-formed tnembers include colnponents for the
body structure of an auutomobile such as a door guard bar and a bumpcr reinforcement,
and hot-formed steel pipes for building construction.
[0(149] -
3. Manufacturing Method
Next, a pxeferred.niethod of manufacturing the hot-fortned ~nelnbera ccording
to this embodiment having tlie above-described features will be describcd. I11 the
following description, the ssy~nbol"%" indicating the amount of each metallographic
microstrl~cturem eans "area%" unless specifically noted.
The configuration of the metallographic microstruct~ureto be described below
is the configuration at about 112t to 1/4t of a sheet thickness, and not in a central
segregation portion. The central segregation portion may have a different
~nctallographicm icrostructure fiom a representative metallograpluc microstructure of
steel. Howevel; the central segregation poition is a very small region with respect to
tlie entire sheet thickness, and has little influence on tlie mechanical properties of the
steel. That is, the metallographic n~icrostructureo f the central segregation portion
does not represent the metallographic microstructure of tlie steel. Accordingly, the
metallographic microstmctnre of the hot-formed member according to this
embodinlent is specified at about 112t to 114t of the sheet thickness and not in the
central segregation portion.
[0050]
In order to obtain a hot-formed member having a tensile strength of 900 MPa
or more and having excellent ductility and impact resistance, a hot-formed member
after hardening is required to have a metallographic microstructure (final
metallographic microstructure ) in which, in terms of areaY0, 10% to 90% of ferrite,
0% to 2.0% of tinrecrystallized ferrite, and 10% to 90% of martensite are included, the
total area ratio of the ferrite and the mastensite is 90% or more, and the average grain
size of the ferrite is 5.0 pm or less.
[0051]
In this embodiment, in order to obtain such a final n~etallograpluc
microstructure, fhen~etallographicm icrostructure of a base steel sheet (also called
"start steel sheet") before being subjected to hot press fanning is previously adjusted to
a predetermined state, and hot pressing is perfomled uuder predetermined hot press
fosming conditions.
[0052]
3-1. In Case in Which Atnoont of U~uecfystallizedF e~~iinte B ase steel
sheet is 0 Area% to 2.0 Area%
In order to obtain a hot-formed member having the above-described
metallographic microstructure, a steel sheet having the same chemical composition as
that of the above-described hot-formed member and having a metallographic
microstructure in which unrecrystallized ferrite is 0 area% to 2.0 area% and the
average grain size of the ferrite is 0.5 pm to 7.0 ptn is prepared as a base steel sheet.
A base steel sheet in which the amount of unrecrystallized ferrite is 2.0 area% or less is
obtained by, for example, performing a recrystallization annealing treatmet~to n a steel
sheet after cold rolling for a sufficient period of time. A cold-rolled steel sheet, a hotdip
galvanized cold-rolled steel sheet, and a galvannealed steel sheet having a
metallographic microstructure in which uurecrystallized ferrite is 2.0 area% and the
average grain size of ferrite is 0.5 pm to 7.0 pm can be manufactured by, for example,
annealing a cold-rolled steel sheet in a temperature range of (Ac3 temperature - 20°C)
or higher.
[0053]
The steel sheet for hot pressing prepared in this manner, which is a base steel
sheet having the same chemical composition as that of the above-described hot-formed
metnber and having~wmetallographicm icrostructure in which unrecrystallized ferrite is
2.0 area% or less and the average grain size of the ferrite is 0.5 pm to 7.0 pm, is
subjected to liot press forming in accordance with the following conditions. Since the
area ratio of the unrecrystallized ferrite of the base steel sheet is lin~itedto 2.0 area% or
less, the metallographic microstmch~reo f the hot-formed member does not become a
non-uniform microst~uctnre. In addition to such an advantage, since the
metallographic microstructure of the base steel sheet is a fine microstructure, the
ductility and itnpact resistance of the hot-formed nletnber can be significantly
improved by the manufacturing method according to this embodinlent. Although it is
not necessary to specify the lower limit of the unrecrystallized ferrite, the
unrecrystallized ferrite is preferably as small as possible, and thus the lower limit of
the unrecrystallized ferrite is substantially 0%.
The area ratio of each metallographic microstruct~ureo f the above-described base steel
sheet can be obtained through the same method as the method of obtaining an area
ratio of each metallographic microstmcture of the hot-formed tnetnbel:
[0054]
The base steel sheet prepared as described above is heated to a temperature
range of 720°C to lower than the ACJ temperature in a heating process. Next, the
temperature of this base steel sheet is maintained for 1 minute to 20 minutes in a
temperature range of 720°C to lower than the Ac3 temperature in a holding process, and
then the steel sheet is hot-pressed in a hot forming process. Then, in a cooling
process, the steel sheet is cooled under the conditions in \vhich the average cooling rate
is 20°C/sec to 500°C/sec in a temperature range of 600°C to 150°C. According to the
manufacturing method according to this embodiment, tlie base steel sheet can be
processed for a sho~pte riod of time without being heated in an austenite single phase
region. ."
[OOSS]
'4Metallographic Microstructure of Base steel sheet) ,.
As the base steel sheet to be subjected to hot pressing, a cold-rolled steel sheet
or a hot-dip galvanized cold-rolled steel sheet having the same chemical composition
as that of the lot-formed steel sheet and having a metallographic microstructure in
~vhichu nrecrystallized ferrite is 2.0 area% or less and the average grain size of the
ferrite is 0.5 kun to 7.0 [Em can be used.
[0056]
According to this etnboditnent, the chemical conlposition of the base steel
sheet is specified as described above, and particularly, C, Mn, and Ti are specified
within specific ranges, respectively. Accordingly, the above-described base steel
sleet can be easily obtained by sufficiently perfortning recrystallization annealing
under normal conditions.
[0057]
By hot-pressing the base steel sheet having the above-described
metallographic microstructure tunder heat treatment conditions to be described later, a
hot-formed member having a desired metallographic microstructure, a tensile strength
of 900 MPa or more, and excellent ductility and impact resistance can be obtained.
[OOSS]
As described already, the cold-rolled steel sheet and the hot-dip galvanized
cold-rolled steel sheet having the above-described metallographic microstructure can
be manufactured by, for example, annealing in a temperature range of (Ac3 temperature
- 20°C) or higher.
[0059]
(Heating Temperature of Base steel sheet: Temperature Range of 720°C to
Lower Than Ac3 Temperature)
(Holding Ten1peratul.e and Holding Time of Base steel sheet: Holding for 1
Minute to 20 Mintdes in Temperature Range of 720°C to Lower Than Ac3
Temperature)
In a process of heating the base steel sheet in the hot forming process, the base
steel sheet is heated to a temperature range of 720°C to lower than the Ac3 temperature
PC). In a process of holdi~lgth e base steel sheet, the tetnperature of the base steel
sheet is maintained for 1 minute to 20 minutes in the above-described tetnperature
range, that is, in a tcrnperatu~er ange of 720°C to lower than the ACJt emperature. The
ACJ temperature is a temperature specified by the following Formula (i) obtained by an
experiment, and in the case in \vliich steel is heated in a temperature range of the ACJ
tenlperature or higher, the nnletallographic microstructure of the steel is an austenite
single phase.
[0060]
Ac3 = 910 - 203 x (co5- )1 5.2 x Ni + 44.7 x Si + 104 x V + 31.5 x Mo -30 x
Mn- 11 xCr-20xCu+700xP+400xsol.Al+50xTi ....( i)
Here, the chemical sytnbol in the above forlnula indicates the amount (unit:
mass%) of each element in the chemical composition of the steel sheet. "sol. Al"
indicates a concentration (unit: mass%) of solid solution Al.
[0061]
When the heating temperature in the heating process and the holding
temperature in the holding process are lower than 720°C, the metallographic
microsttucture of the base steel sheet is a microst~~~ctculorese to a ferrite single phase,
and it is difficult to secure a tensile strength of 900 MPa or more after hardening.
Accordingly, the heating telnperature and the holding temperature are set to 720°C or
higher. When the heating te~nperaturein the heating process and the holding
temperature in the holding process are equal to or higher than the Ac3 temperature, the
metallographic microstructure of the hot-formed member after hardening is a
martensite single phase, and the ductility of the hot-fomed member significaatly
deteriorates. Accordingl~: the heating temperature and the holding temperature are
lower than the Ac3 temperature.
[0062]
In addition, when the holding time in the holding process is shorter than 1
minute, undissolved carbide such as cementite remains in the hot-fos~lled member, and
the impact resistance of the hot-formed member deteriorates. Accordingly, the
holding time is set to 1 mitiute or longer. On the other hand, when the holding time is
longer than 20 minutes, productivity is reduced, and surface properties of the hotformed
member deteriorate due to the generation of scale and zinc-based oxide.
Accordingly, the holding time is set to 20 minutes or shorter.
[0063]
At this time, it is not particularly necessary to linlit the average heatit~gra te up
to a temperature range of 720°C to the Ac3 temperature in the heating process, but the
rate is preferably set to 0.2°C/sec to 100°C/sec. By setting the average heating rate to
0.2°C/sec or higher, higher productivity can be secured. In addition, by setting the
average heating rate to 100°C/sec or lowel; the heating temperature is easily controlled
in the case in which the heating is performed using a normal furnace. However, when
using high-frequency heating or the like, it is possible to co~ltrotlh e heating
temperature with high accuracy even wvhen the heating is performed at a heating rate of
higher than 100°C/sec.
[0064]
(Average Cooling Rate in Temperature Range of 600°C to 150°C: 20°C/sec
'. to 500°C/sec) -
The cooli~lgin a te~llperaturer ange of 600°C to 150°C is performed such that
diffi~sivetr ansformation does not occur, Wlie~tlh e average cooling rate in the above
temperature range is lower than 2O0C/sec, bainitic transfor~llatione xcessively proceeds,
the al-ea ratio of maltensite that is a phase (strengtliei~i~pigh ase) strengthening the
strength of the hot-for~l~erdn en~berc annot be secured, and thus it is difficult to secure
a tensile strength of 900 MPa or more after hardening. Accordingly, the average
cooling rate in the above tempcrature range is set to 20°C/sec or highet It is difficult
to increase the average cooling rate in the above temperature range to be higher than
5OO0C/sec using normal equipment. Accordingly, the average cooli~~ragte in the
above temperature range is set to 500°C/sec or lower. The average cooling rate in the
above tetnperature range is preferably 20O0C/sec or lower.
[0065]
During the cooling, heat generation by phase transformation is very large in a
temperature range of 600°C or lower. Accordingly, in a temperature range of 600°C
or lowel; a sufficient cooling rate may not be secured in the same cooling method as
the cooling method in a temperature range of 600°C or higher. Therefore, it is
necessary to strongly perform cooling from 600°C to 150°C compared to cooling to
600°C, and specifically, the cooling is preferably performed as described below.
[0066]
In the hot pressing method, in general, cooling is achieved when a die having
a room temperature or a temperature of about several tens of OC immediately before
hot pressing takes heat from the hot-formed member. Accordingly, in order to change
the cooli~~ragte ;a heatcapacity of the die may be changed by changing a size of the
die. In addition, the cooling rate can also be changed by changing the material of the
die to dissinlilarrn& (for example, copper). In the case in wvhich the size of the die
cannot be changed, the cooling rate can also be changed by using a fluid cooliag-type
die and by changing the flow rate of a coolaut. In addition, the cooling rate can also
be changed by using a die in which several grooves are previously cut and by passing a
coolant (water or gas) in the groove during pressitlg. In addition, the cooling rate can
also be changed by separating a die fio~nth e hot-formed nletnber by operating a press
tnachine during pressing arid by passing a coolatit therebetween. Furthermore, the
cooling rate can also be changed by changing the contact area between a die and the
steel sheet (hot-fonned member) by changing a clearance of the die. In view of the
above facts, the following methods can be considered to be methods of cl~atigingth e
cooling rate at about 600°C.
[0067]
(1) Immediately after reaching 60OoC, the hot-formed member is moved to a
die having a different heat capacity or a die in a room temperature state to change the
cooling rate;
(2) In the case of a fluid cooling-type die, immediately after reaching 600°C,
the flow rate of a coolant in the die is changed to change the cooling rate; and
(3) Immediately after reaching 600°C, a coolant is allowed to pass between a
die and the member, and the flow rate thereof is changed to change the cooling rate.
[O068]
3-2. In Case in Whichhount of Unsecrystallized Ferrite in Base steel
sheet is More Than 2.0 Area%
In the case in which the amount of the unsecrystallized ferrite in the base steel
sheet is 2.0 area%-or less, a hot-fortned nlember having a predetermined
metallographic microstructure can be obtained through the above-described method.
However, even wheathe amount of.the unrecrystallized ferrite in the base steel sheet is .
more than 2.0 area%, a hot-fornled ~llen~bhearv ing a predetermined metallographic
microstructure can be obtained tlxough the following method.
In order to obtain a hot-formed member having the above-described
metallographic microstructure, a steel sheet having the same chemical composition as
that of the above-described hot-fomied member and having a metallographic
microstructure in which the average grain size of ferrite is 7.0 pm or less and the
amount of unrecrystallized ferrite is more than 2.0 area% is prepared as a base steel
sheet. A cold-rolled steel sheet, a hot-dip galvanized cold-rolled steel sheet, and a
galvannealed steel sheet having a ~netallograpl~mici crostructure in which the average
grain size of ferrite is 7.0 pm or less and the amount of u~lrecrystallizedf errite is more
than 2.0 area% can be manufactured by, for example, annealing a cold-rolled steel
sheet in a temperature range of lower than (Ac3 temperature - 20°C). The base steel
sheet prepared as described above is hot-pressed after being held for 30 seconds or
longer and shorter than 20 minutes in a temperature ralige of the Ac3 temperature to the
Acj temperature + 100°C, and is cooled at an average cooling rate of 3"CIsec to
2O0C/sec in a temperature range of the ACJ temperature to 60OoC.
[0069]
(Metallographic Microstructure of Base Steel Sheet)
As the base steel sheet to be subjected to hot pressing, a cold-rolled steel sheet
or a hot-dip galvanized cold-rolled steel sheet having the same chemical composition
as that of the hot-formed member and having a metallographic microstructure in which
the average grain size of ferrite is 7.0 pm or less and the amount of unsecrystallized
ferrite is more than2.0 area% can be used.
[0070]
By hot-pressing the base steel sheet having the above-described . .
nietallographic microstmcture under heat treatment conditions to be described later, a
hot-fornied member having a desired metallographic microstructure, a tensile strength
of 900 MPa or more, and excellent ductility and impact resistance can be obtained.
[0071]
(Heating Temperature of Base steel sheet: Temperature Range of Ac3
Temperature to Ac3 Telllpe~ture+ 10OoC)
(Holding Temperature and Holding Time of Base steel sheet: Holding for 30
Seconds or Longer and Shorter Than 20 Minutes in Temperature Range ofAc3
Tenlperature to Ac3 Temperature + 1 OO°C)
The steel sheet to be-subjected to hot pressing is heated by being held for 30
seconds or longer and shorter than 20 minutes in a temperature range of the Ac3
temperature ("C), which is specified by the above-described Experimental Formula (i),
to the Ac, temperature + 100°C.
[0072]
When the holding temperature is lower than the Ac3 temperature, more than
2% of u~ecrystallizedfe rrite remains in the hot-formed member, and the
metallographic microstructure becomes non-uniform. Accordingly, the holding
temperature is set to the ACJ temperature or higher. When the holding temperature is
equal to or higher than the Ac3 temperature + 100°C, intergranular oxide is generated in
the metallographic microstructure, and the impact resistance of the hot-formed member
significantly decreases. Accordingly, the holding temperature is set to the Ac3
temperatlure + I OO°C or lower.
[0073].
In addition, when the holding time is shorter than 30 seconds, the strength of
the base steel sheet largely scatters. Since conditio~lsu nder which such a .
phenomenon occurs are not suitable for mass production, the lloldiug time is set to 30
seconds or longer. On the other hand, when the holding time is 20 minutes or longer,
austenite grains excessively grow, and the tnetallographic tnicrostructure beco~nesn onuniform,
and thus, the impact resistance of the hot-fortned member significantly
decreases. Accordingly, the holding tinie is set to be shorter than 20 minutes.
[0074]
At this time, the heating rate up to a temperature range of the Ac3 temperature
to tl1eAc3 telnperature + 100°C is preferably 0.2"C/sec to 100°C/sec. By setting the
average heating rate to 0.2"C/sec or higher, higher productivity can be secured. In
addition, by setting the average heating rate to 100°C/sec or lo\ve~; the heating
temperature is easily controlled in the case in which the heating is performed using a
normal furnace. However, when using high-frequency heating or the like, it is
possible to control the heating temperature with high accuracy even when the heating
is perfonned at a heating rate of higher than 100°C/sec.
[0075]
(Average Cooling Rate in Temperature Range of Ac3 Temperature to 600°C:
3OCIsec to 20°C/sec)
The cooling in a temperature range of the Ac3 temperature to 600°C is
performed such that the average cooling rate is 3"CIsec to 20°C/sec. When the
average cooling rate in the above temperature range is lo~verth an 3OC/sec,
intergranular oxide is generated in the metallographic microstructure, and the impact
resistance of the hot-formed member significantly decreases. Accordingly, the
average cooling-rate in the above te~nperaturer ange is set to 3OClsec or higher. When
the average cooling rate in the above temperature range is higher than 20°C/sec, the
amount of ferrite .in the hot-fonned member is insufficient. Therefore, the average
cooling rate in the above temperature range is set to 20°C/sec or lower. The average
cooling rate in a temperature range of lower than 600°C is set to 2O0C/sec to 500°C/sec.
[0076]
In this embodiment, thc aspect of the forming ill the hot pressing method is
not particularly limited. Examples of the aspect ofthe fortning include bending,
drawing, stretch fornling, hole expansion for~ninga, nd flanging. A preferred form
tnay be appropriately selected among the above-described forms of the forming in
accordance with the kind and the shape of a target hot-forned metnbet: Examples of
the material of a base steel sheet to be subjected to the hot pressing method in this
embodiment include a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a
galvannealed steel sheet.
[0077]
Representative examples of the hot-formed member include a door guard bar
and a bumper reinforcement which are reinforcing components for an automobile.
For example, in the case in which the hot-formed member is a bumper reinforcement,
the above-described base steel sheet \vhich is a galvannealed steel sheet having a
predetermined length is prepared, and under the above-described conditions, working
such as bending may be sequentially performed in a die.
[0078]
The hot-formed member according to this embodiment is characterized by
excellent ductility and excellent impact resistance. The hot-formed member
according to this embodiment preferably has such ductility that a total elongation in a
tension test is 10% or more. In addition, the hot-formed member according to this
embodiment preferably has such impact resistance that an impact value in a Charpp
test at O°C is 20 ~ / c morm~o re. The hot-formed member having such mechanical
properties is realized by satisfying the above-described specificatiotis related to the
chemical composition and the metallographic microstructure.
[0079]
After the hot forming such as hot pressing, a shot blasting treatment is
generally perfor~uedo n the hot-formed member for the purpose of removing scale.
This shot blasting treatment is effective for introducing a con~pressives tress to a
suiface of a lneniber to be treated. Accordingly, performing the sllot blasting
treatment on the hot-formed member has advantages in that delayed fracture is lirnited
in the hot-fornied member and the fatigue strength of tlie hot-formed member is
improved.
[0080]
111 hot forming accompanied with pre-forming, for example, hot pressing, it is
preferable that the base steel sheet is as soft as possible, and the base steel sheet has
high ductility. For example, the tensile strength of the base steel sheet is desirably
800 MPa or less.
[0081j
In tile above description, the hot forming has been described using hot
pressing wl~ichis a specific aspect thereof, but the manufacturing method according to
this embodiment is not limited to the hot press forming. Similarly to hot pressing, the
manufacturing method according to this embodiment can be applied to all hot forming
processes having a means ~vl~icoho ls a steel sheet simultaneously or immediately
after forming. Examples of the hot forming include roll forming.
Examples
(00821
Exanlples of the present invention will be described. ..
Base steel sheets (sheet thickness t: 1.2 mm) having chemical compositions
shown in Table 1 and metallographic microstructures and tensile strengths shown in
Table 2, respectively, were subjected to hot pressing.
[0083j
These base steel sheets are steel sheets (cold-rolled steel sheets in Table 2)
manufactured by subjecting slabs tlianufactured in a laboratory to hot rolling, cold
rolling, and recrystallization annealing. Using a plating simulator, some steel sheets
were subjected to a hot dip galvanizing treatment (plating adhesion amount per side:
60 g/~n2a)n d an galvannealed treatnnent (plating adhesion amount per side: 60 g/m2, Fe
content in coating film: 15 mass%). These are hot-dip galvanized cold-rolled steel
sheets and galvannealed steel sheets in Table 2. Steel sheets as cold-rolled, and not
subjected to recrystallization annealing (full hard in Table 2), were also used as base
steel sheets.
[0084]
These steel sheets were cut into sizes of a thickness of 1.2 mm, a width of 100
mm, and a length of 200 mnl, and heated and cooled under conditions in Table 3. A
thermocouple was adhered to the steel sheet, and a cooling rate was also measured.
The "average heating rate" in Table 3 indicates an average of heating rates in a
temperature range of a room temperature to 720°C. The "holding time" in Table 3
indicates a period of time in which the steel is held in a temperature range of 720°C or
higher. The "cooling rate *1" in Table 3 indicates an average coolitlg rate from at1
Ac3 temperature to 600°C in the case in which the heating temperature is the Ac3
temperature or highel; and indicates an average cooling rate from the heating
temperature to 600C in the case in which the heating temperature is lower than the Ac3
'Senlperature. The "cooling rate *2" is an average cooling rate in a temperature range
of 600C to 150°C. The steel shccts obtained under the various ~nailufacturitlg
conditions were subjected to a tensile test, a Charpy test, and n~etallographic
microstructure observation. The steel sheet members prepared in this exarnple are not
subjected to hot pressing by a die, but undergo the same thermal history as a hotformed
member. Therefore, the steel sheets have substantially the same n~echanical
properties as the hot-formed member having the same tliermal history.
[0085]
(Tensile Test)
A JIS No. 5 tensile test piece of \vl~ichth e longitudinal direction was
pespetidicular to a rolling direction was prepared from each steel sheet, and a tensile
strength (TS) and a total elongation (EL) were measured. Samples having a TS of
900 MPa or more and an EL of 10% or more were determined to be accepted.
[0086]
(Impact Resistance)
Four steel sheets having a thickness of 1.2 mm were laminated and threadably mounted,
and then V-notch test pieces were prepared and subjected to a Charpy impact test.
The impact resistance was evaluated as "good" in the case in which the impact value at
0°C was 20 ~/cm' or highee In the case in which the impact value at O°C does not
reach 20 ~/ctn', it was evaluated as "poor".
[0087]
(Area Ratios of Ferrite, U~irecrystallizedF errite, and Martensite, and Average
Grain Size of Ferrite)
Test pieces were prepared fsom the base steel sl~eetsa nd the heat-treated steel
,, sheets in a rolling direction of the base steel s11eet.and the heat-treated steel sheet and
in a direction perpendicular to the rolling direction. Next, metallographic
microstructures of a cross-section in the rolling direction and a cross-section
perpendicular to the rolling direction in the test piece \\we pliotographed by an
electron microscope. Electron tnicrographs of regions of 800 pm x 800 ptn obtained
as described above were analyzed to calculate area ratios of unrecrystallized ferrite,
ferrite, and maltensite.
(Description of Test Results)
Results of these tests are sliown in Tables 4 and 5.
[0089]
In Tables 1 to 5, underlined numerical values indicate that the amount,
conditions, or mechanical properties shown by the numerical value are out of the range
of the present invention.
and 39, which are invention examples, have excellent ductility and impact resistance.
[0091]
Sample No. 3 had poor ductility and poor impact resistance since the average
pain size of ferrite of the base steel sheet was out of the range specified in the present
invention. Sample No. 13 had poor ductility and poor impact resistance since the
man~~facturinmge thod specified in the present invention was not applied. Sample No.
14 had poor ductility since the manufacturing method specified in the present invention
was not applied. Sanlple Nos. 9 and 26 had poor impact resistance since the chemical
composition was out of tlie range specified in the present invention. Sample Nos. 10
.A and 17 could not-obtain a target tensile strength since the manufacturing conditions
were out of the range specified in the present in\fention, and thus a desired
microstructure could not be obtained.
[0092]
Sample No. 18 had poor ductility since the manufacturing conditions werc out
of tlie range specified in the present invention, and thus a desired microstructure could
not be obtained.
[0093]
Sample Nos. 28 and 32 could not obtain a target tensile strength since the
chemical compositio~w~a s out of the range specified in the present invention.
[0094]
Sample No. 34 had poor ductility since the chemical composition was out of
the range specified in the present invention, and thus a desired microstructure could not
be obtained.
[0095]
Sample No. 8 had poor ductility since the heating temperature was higher than
the Ac3 temperah~re. Sample No. 12 could not obtain a target tensile strength since
the heating tetnperah~rew as lower than 720°C. In Sample No. 20, surface scale was
generated beyond the allowable extent since the holding time was out of the range
specified in the present invention. Sample No. 30 could not obtain target itnpact
resistance since the holding time at 750°C or higher was out of tlie range specified in
the present invention. In Sample No. 24, a total area ratio of ferrite and martensite of
the hot-formed member \vas out of the range specified in the present invention, and the
tensile strength was insufficient since the cooling rate at a tetnperature lower than
600°C was out of the range specified in the present invention. I11 Sample No. 35,
surface scale nras generated beyond the allowable extent since the Si content was out
of the range specified in the present invention. In Sa~npleN o. 38, surface scale \\cis
generated beyond the allowable extent since the A1 content \\'as out of the range
specified in tlie present invention. Sample No. 40 had poor impact resistance since
the Ti content was out of the range specified in the present invention.
100961
[Table 11
[Table 21
*: T ks teel sh~ctuuea tler co!d rolling.
Accordingly, it w a s not possible to masure the grain size of ferrite.
[009S]
[Table 31
* 1: The arerage cooling rate from the Ac3 point to 600'C; and when the heating tenwefahue is lo\ser than the Ac3
poult:
'2: The avmgz cooling