IIOT-FOIWED STEEL SIHEET MEMBER
Technical Field
[0001] The present specification relates to a hot-formed steel shcct member formed by
hot-forming a steel sheet.
Background Art
[0002] A high-strength steel sheet having high tensile strength has been widely applied to
the field of an automotive steel sheet in order to achieve both weight saving for an
improvement in fuel consumption and an improvement in collision resistance. Howevcr, the
high strength causes deterioration in the press formability of the steel sheet, which makes it
difficult to produce products having complicated shapes.
[0003] As a result, for example, the high strength of the steel sheet disadvantageously causes
deterioration in ductility, which causes breaking at a site having a high degree of processing,
and disadvantageously causes deterioration in dimension accuracy or the like because of
increased spring back and wall warpage. Therefore, a steel sheet having high strength,
particularly tensile strength of 780 MPa or more, is not easily press-formed to a product
having a complicated shape.
[0004] Then, recent years, for example, as disclosed in Japanese Patent Application
Laid-Open (JP-A) No. 2002-102980, a hot stamp technique is adopted as a technique for
press-forming a material which is hard to form such as a high-strength steel sheet. The hot
stamp technique is a hot forming technique for heating and forming a material provided for
forming. Since the steel sheet is formed and quenched at the same time in the technique, the
steel sheet is soft and has favorable formability during forming, and the formed member after
forming can have strength higher than that of a steel sheet for cold forming.
[0005] Japanese Patent Application Laid-Open (JP-A) No. 2006-213959 discloses a steel
member having tensile strength of 980 MPa.
Japanese Patent Application Laid-Open (JP-A) No. 2007-3 1481 7 discloses that a hot
pressed steel sheet member having excellent tensile strength and toughness is obtained by
decreasing a cleanliness level and segregation degrees of P and S.
SUMMARY OF DISCLOSURE
[0006] The metal material described in JP-ANo. 2002-102980 has insufficient hardcnability
during hot press, as a result of which the metal material has poor hardness stability. The
steel sheets having excellent tensile strength and touglmess arc disclosed in JP-A No.
2006-213959 and J1'-ANo. 2007-314817, but room for an improvement in local deformation
characteristics is lert.
[0007] An objective of embodiments of thc specification is to provide a hot-formed steel
sheet member having excellent hardness stability and local deformability. In many cases, a
steel sheet membcr which is hot-formed is not a flat sheet but a formed body, and is referred
to as "a hot-formed steel shect member" including a case in which the hot-formed steel sheet
member is the formed body in the specification.
[0008] According to one aspect of the present specification, there is provided a hot-formed
steel sheet membcr having a chemical conlposition consisting of, in terms of mass %, from
0.08 to 0.16% of C, 0.19% or less of Si, from 0.40 to 1.50% of Mn, 0.02% or less ofP, 0.01%
or less of S, from 0.01 to 1.0% of sol. Al, 0.01% or less of N, from 0.25 to 3.00% of Cr, from
0.01 to 0.05% of Ti, from 0.001 to 0.01% of B, from 0 to 0.50% of Nb, from 0 to 2.0% of Ni,
from 0 to 1 .O% of Cu, from 0 to 1.0% of Mo, from 0 to 1 .O% of V, from 0 to 0.005% of Ca,
and a remainder consisting of Fe and impurities,
wherein a total volume fraction of martensite, tempered martensite, and bainite is
50% or more, and a volume fraction of ferrite is 3% or less,
an average grain size of priory grains is 10 pm or less, and
a number density of residual carbides which are present is 4 x lo3 per mm2 or less
BRIEF DESCRIPTION OF DRAWINGS
[0009] Fig. 1 is a schematic view showing a shape of a mold in hat forming in Examples.
Fig. 2 is a schematic view showing a shape of a formed body obtained by
hot-forming in Examples.
Fig. 3 is a schematic view showing a shape of a notch tensile test piece in Examples.
DESCRIPTION OF EMBODIMENTS
[0010] The present inventors have conducted studies earnestly to provide a hot-formed steel
sheet member having excellent hardness stability and local deformability, and resultantly
obtained the findings described below.
[001 I] (1) Fine priory grains in the hot-formed steel sheet member delay the occurrence and
connection of voids, which provides an improvement in local deformability. Therefore, thc
fine priory grains are preferable.
(2) In a case in which a number of residual carbides a e present in the hot-formed
steel shect member, hardenability after hot-forming may be deteriorated to cause deterioration
2
in hardness stability, and the residual ca~hidess erve as thc occurrence sourcc of'voids to cause
deterioration in the local defornability Therefore, the number density of the residual
carbides is preserahly reduccd.
[0012] En~bodimentso f the specification is based on the findings. According to one aspect
of the embodiments,
(1) thcre is provided a hot-formed steel sheet member having a chemical
composition, consisting of, in terms of mass %, from 0.08 to 0.16% of C, 0.19% or less oESi,
from 0.40 to 1.50% of Mn, 0.02% or less of P, 0.01% or less of S, from 0.01 to 1.0% of sol.
Al, 0.01% or less of N, from 0.25 to 3.00% of Cr, from 0.01 to 0.05% of Ti, from 0.001 to
0.01% of B, from 0 to 0.50% of Nb, from 0 to 2.0% of Ni, from 0 to 1.0% of Cu, from 0 to
1.0% of Mo, from 0 to 1.0% of V, from 0 to 0.005% of Ca, and a remainder consisting of Fe
and impurities,
wherein a total volume fraction of martensite, tempered martensite, and bainite is
50% or more, and a volume fraction of ferrite is 3% or less,
an average grain size of prior y grains is 10 pm or less, and
a number density of residual carbides which are present is 4 x lo3 per mm2 or less.
[0013] (2) In the hot-formed steel sheet member of (I), the cheniical composition preferably
includes one or more selected from the group consisting of, in terms of mass %, from 0.003 to
0.50% of Nb, from 0.01 to 2.0% of Ni, from 0.01 to 1.0% of Cu, from 0.01 to 1.0% of Mo,
from 0.01 to 1.0% of V, and from 0.001 to 0.005% of Ca.
i00141 (3) In the hot-formed steel sheet member of (1) or (2), a value of a cleanliness level
of steel speciiied by JIS G 0555 (2003) is preferably 0.08% or less.
[0015] (4) In any one of the hot-formed steel sheet members of (1) to (3), a segregation
degree a of Mn represented by the following formula (i) is preferably 1.6 or less,
a = [maximum Mn concentration (mass %) at a central part of a sheet
thiclcness]l[average Mn concentration (mass %) at a 114 depth position of the sheet thickness
from a surface] ... (i).
[0016] (5) In any one of the hot-formed steel sheet members of (1) to (4), the steel sheet
member preferably has a surface on which a plating layer is formed.
(6) In any one of the hot-formed steel sheet members of (I) to (5), the steel sheet
member preferably has a tensile strength of 1.0 GPa or more.
1 [0017] Hereinafter, the embodiments will be described in detail.
I
I [OO 181 (A) Chemical Composition
The reason why the content of each element is limited will be described below. In
the description bclow, the symbol "%" of the content of each element means "mass % .
3
1001 91 C: from 0.08 to 0.16%
(: is an element important Tor improving thc hardenability oS stccl and for sccuring
the strength aaftr quenching. Since C is an austenite-forming element, it has a function to
suppress strain-induced ferrite transfor~nationd uring high strain formation. l'his lnalies it
easy to obtain a stable hardness distribution in a steel sheet member after hot-forming. The
C content of less than 0.08% makes it difficult to secure tensile strength of 1.0 GPa or more
after quenching and to obtain the above-mentioned effect. Therelore, the C content is set to
0.08% or more. The C content exceeding 0.16% causes an excessive increase in the strength
after quenching to cause deterioration in local deformability. Therefore, the C content is set
to 0.16% or less. The C content is preferably 0.085% or more, and more preferably 0.9% or
more. The C content is preferably 0.15% or less, and more preferably 0.14% or less.
[0020] Si: 0.19% or less
Si is an element having a function to suppress scale formation during high
temperature heating for hot-forming. However, the Si content exceeding 0.19% causes a
remarkable increase in a heating temperature required for austeuite transformation during
hot-forming. This causes an increase in cost required for a heat treatment, and insufficient
quenching due to insufficient heating. Si is a ferrite-forming element. Thereby, a too high
Si content is apt to produce strain-induced ferrite transformation during high strain formation.
This causes a local decrease in the hardness of the steel sheet meinher after hot-forming,
which makes it difficult to obtain a stable hardness distribution. Furthermore, a significant
amount of Si causes deterioration in wettability in a case in which a hot-dip plating treatment
is performed, which may cause non-plating. Therefore, the Si content is set to 0.19% or less.
The Si content is preferably 0.15% or less. In a case in which the above-mentioned effect is
desired to be obtained, the Si content is preferably 0.01% or more.
[0021] Mn: from 0.40 to 1.50%
Mn is an element useful for improving the hardenability of a steel sheet and for
stably securing the strength after hot-forming. The Mn content of less than 0.40% makes it
difficult to obtain the above-mentioned effect. Therefore, the Mn content is set to 0.40% or
more. The Mn content exceeding 1.50% produces coarse MnS, which becomes a factor for
deterioration in local deformability. Therefore, the Mn content is set to 1.50% or less. The
Mn content is preferably 0.80% or more, and preferably 1.40% or less.
I ~ [0022] P: 0.02% or less
! Since P is an element contained as impurities, and has functions to make it possible
to improve the hardenability of steel and to stably secure the strength of the steel after
quenching, P may be positively contained. Howevcr, thc P content exceeding 0.02% causes
4
remarkable deterioration in local defornlability. 'I'he~.eforet, he P content is set to 0.02% or
less. The P contcnt is preferably 0.01% or less. Although the lower limit ofthe P content
is not particularly li~iliteda, n excessive reductioll in the P content causes a remarlmble
increase in cost. For this reason, the P content is preferably set to 0.0002% or more.
100231 S: 0.01% or less
S is an elcmcnt contained as impurities, and causing deterioration in local
deformability. The S content exceeding 0.01 % causes remarkable deterioration in the local
deformability. Therefore, the S content is set to 0.01% or less. Although the lower limit or
the S content is not particularly limited, an excessive reduction in the S content causes a
remarlcable increasc in cost. Therefore, thc S content is preferably set to 0.0002% or morc.
[0024] sol. Al: from 0.01 to 1.0%
sol. A1 is an element having a function to enable soundness of steel by deoxidizing
molten steel. The sol. A1 content of less than 0.01% causes insufficient deoxidation.
Furthermore, since the sol. A1 is also an element having functions to improve the
hardenability of a steel sheet and to stably secure the strength after quenching, the sol. A1 may
be positively contained. Therefore, the sol. A1 content is set to 0.01% or more. However,
the sol. A1 content exceeding 1 .O% provides a small effect obtained by the function, and
unnecessarily causes an increase in cost. For this reason, the sol. A1 content is set to 1.0% or
less. The sol. A1 contcnt is preferably 0.02% or more and preferably 0.2% or less.
[0025] N: 0.01% or less
N is an element contained as impurities, and causing deterioration in toughness.
The N content exceeding 0.01% forms coarse nitride in steel, which causes remarkable
deteriorations in local deformability and toughness. Therefore, the N content is set to 0.01%
or less. The N contcnt is preferably 0.008% or less. Although the lower limit of the N
content need not be particularly limited, an excessive reduction in the N content causes a
remarkable increase in cost. For this reason, the N content is preferably set to 0.0002% or
more, and more preferably 0.0008% or more.
[0026] Cr: from 0.25 to 3.00%
Cr is an element having a function to improve the hardenability of steel. Therefore,
! Cr is a particularly important element in an embodiment in which the Mn content is limited to
I
1.50% or less. Cr is an austenite-forming element, and has a function to suppress
strain-induced ferrite transformation during high strain formation. Therefore, Cr is
contained, which makes it easy to obtaii~a stable hardness distribution in a steel sheet inember
!
after hot-forming. The Cr content of !ess than 0.25% cannot sufficiently provide the
above-mentioned effect. Therefore, the Cr content is set to 0.25% or more. The Cr content
5
excccdiilg 3.00% causes Cr to be incrassated in carbonates in carbonates in the steel, which
delays the solid solution of the carbides in a heating step in the case ol'being provided for
hot-forming to cause deterioration in the hardenability. 'I'hercforc, the Cr content is set to
3.00% or less. The Cr content is preferably 0.30% or more, and more prererably 0.40% or
more. The Cr content is preferably 2.50% or less, and more preferably 2.00% or less.
[0027] Ti: from 0.01 to 0.05%
Ti is an element having a function to suppress the recrystallization of austenite grains
in a case in which a steel sheet for hot-forming is heated to an ACJ point or more and provided
for hot-forming. Furthermore, Ti has a function to form fine carbides to suppress the grain
growth of the austenite grains, thereby providing fine grains. For this reason, Ti has a
function to largely improve the local deformability of a hot-formed steel sheet member.
Since Ti is preferentially bonded to N in steel, Ti suppresses the consumption of B due to the
precipitation of BN, as a result of which Ti has a function to improve hardenability due to B.
Therefore, the Ti content is set to 0.01% or more. However, the Ti content exceeding 0.05%
causes an increase in the amount of precipitation of Tic, which causes the consumption of C,
thereby causing a decrease in the strength after quenching. For this reason, the Ti content is
set to 0.05% or less. The Ti content is preferably 0.015% or more. The Ti content is
preferably 0.04% or less, and more preferably 0.03% or less.
[0028] B: from 0.001 to 0.01%
B is an element having functions to makes it possible to improve the hardenability of
steel and to stably secure the strength after quenching. Therefore, in an embodiment in
which the Mn content is limited to 1.50% or less, B is a particularly important element. l l e
B content of less than 0.001% cannot sufficiently provide the above-mentioned effect.
Therefore, the B content is set to 0.001% or more. The B content exceeding 0.01% causes
the saturation of the above-mentioned effect, and deterioration in the local deformability of a
quenched part. Therefore, the B content is set to 0.01% or less. The B content is
preferably 0.005% or less.
[0029] The hot-formed steel sheet members of the embodiments have a chemical
composition consisting of the elements or C to B and the remainder consisting of Fe and
impurities.
[0030] The "impurities" herein are elements which are mixed in by various factors in raw
materials such as ore or scrap and in a production process when a steel sheet is produced on
an industrial scale, and are allowed to bc contained within the range such that the elements do
not exert an adverse influence on the embodiments
1003 11 The hot-formed steel sheet member of the embodiments may further contain one or
6
more elements selccted from the group consistiilg oTNb, Ni, C:u, Mo, V. and Ca in amo~mtsto
be described below in addition to the abovc-mentioned clcments.
[0032) Nb: from 0 to 0.50%
Nb is an element having functions to suppress recrystallization in a case in which a
steel sheet for hot-forming is heated to an Ac) point or more and provided for hot-forming,
and to form fine carbides to suppress the grain growth, thereby providing line austenite
grains. For this reason, Nb has a function to largely improve the local defornlability of a
hot-formed steel sheet member. Therefore, Nb may be contained if necessary. However,
the Nb content exceeding 0.50% causes an increase in the amount of precipitation of NbC to
cause the consumption of C, thereby causing a decrease in the strength after quenching. For
this reason, the Nb content is set to 0.50% or less. The Nb content is preferably 0.45% or
less. In a case in which the above-mentioned effect is desired to be obtained, the Nb content
is preferably set to 0.003% or more, and more preferably 0.005% or more.
[0033] Ni: from 0 to 2.0%
Since Ni is an element effective in improving the hardenability of steel sheet and in
stably securing the strength after quenching, Ni may be contained if necessary. However, the
Ni content exceeding 2.0% provides a small effect, which unnecessarily causes an increase in
cost. For this reason, the Ni content is set to 2.0% or less. The Ni content is preferably
1.5% or less. In a case in which the above-mentioned effect is desired to be obtained, the Ni
content is preferably set to 0.01% or more, and more preferably 0.05% or more.
100341 Cu: from 0 to 1 .O%
Since Cu is an element effective in improving the hardenability of steel sheet and in
stably securing the strength after quenching, Cu may be contained if necessary. However,
the Cu content exceeding 1 .O% provides a small effect, which unnecessarily causes an
increase in cost. For this reason, the Cu content is set to 1 .O% or less. The Cu content is
preferably 0.5% or less. In a case in which the above-mentioned effect is desired to be
obtained, the Cu content is preferably set to 0.01% or more, and more preferably 0.03% or
more.
[0035] Mo: from 0 to 1.0%
Mo is an element having a function to form fine carbides in a case in which a steel
sheet for hot-forming is heated to an Ac3 point or more and provided for hot-forming to
suppress the grain growth, thereby providing fine austenite grains. Mo has also an effect of
largely improving the local deformability of a hot-formed steel sheet member. For thcse
reasons, Mo may be contained if necessary. However, the Mo content exceeding 1 .O%
causes the saturation of the effect, which unnecessarily causes an increase in cost
7
l'hcrcforc. the Mo content is sct lo 1 .O% or less. 'She Mo content is preferably 0.7% or less.
I11 a case in which the above-mentioned erfect is desired to be obtained. the Mo content is
preferably set to 0.01% or more, and marc preferably 0.04% or more.
100361 V: from 0 to 1 .O%
Since V is an element cffeclive in improving the hardenability of steel sheet and in
stably securing the strength after quenching, V may be contained if necessary. However, the
V content exceeding 1 .O% provides a small effect, which unnecessarily causes an increase in
cost. For this reason, the V content is set to 1 .O% or less. The V content is preferably
0.08% or less. In a case in which the effect is desired to be obtained, the V content is
preferably set to 0.01% or more, and more prcferably 0.02% or morc.
[0037] Ca: from 0 to 0.005%
Since Ca is an element having an effect of grain refining of inclusions in steel to
improve the local deformability after quenching, Ca may be contained if necessary.
However, the Ca content exceeding 0.005% causes the saturation of the effect, which
unnecessarily causes an increase in cost. Therefore, the Ca content is set to 0.005% or less.
The Ca content is preferably 0.004% or less. In a case in which the effect is desired to be
obtained, the Ca content is preferably set to 0.001% or more, and more preferably 0.002% or
more.
[0038] (B) Metal Structure
In the embodiments, in order to improve local deformability, variations in hardness in
the metal structure after hot-forming is preferably suppressed. Since an increased hardness
difference in the structure serves as the starting point of voids, the mixture of a
low-temperature transformation structure such as hard martensite or bainite and a soft ferrite
structure is preferably suppressed as much as possible. Therefore, it is preferable that the
hot-formed steel sheet members of the embodiments mainly have a low-temperature
transformation structure, and has a metal structure having a ferrite volume fraction of 3% or
less.
[0039] The metal structure mainly having a low-temperature transformation structure means
a metal structure in which the total volume fraction of martensite, tempered martensite, and
bainite is 50% or more. The tempered martensite herein means martensite transformed
during quenching and tempered by automatic tempering, and martensite subjected to low
temperature tempering such as a coating baking process after quenching. The volume
fraction of the low-temperature transfomied structure in the metal structure is preferably 80%
or more, and more preferably 90% or morc.
[0040] Since residual austenite improves ductility according to the TRIP effect, the residual
8
austenite is ul~eventhllyc oulaincd. ~lowcvcri,u arlensite t~.ansfol.rned& om auslenite is hard,
which serves as the starting point orvoids. Therefore, the volume haction ofthe rcsidual
austenitc contained in the metal structure is preferably 10% or less.
100411 Segregation Degree a of Mn: 1.6 or less
n = [maximum Mn concentration (mass %) at a central part of a sheet
thickness]/[average Mn concentration (mass %) at 114 depth position of sheet thickness from a
surface] ... (i)
At thc central part of the section of the sheet thickness ofthe hot-formed steel sheet
member, center segregation occurs, which incrassates Mn. Therefore, MnS concentrates on
the center as inclusions, which is apt to causc the formation of hard martensite. This causes
a difference in hardness between the hard martensite and its circumference, as a result of
which the local deformability is deteriorated. Particularly, in a case in which the value of the
segregation degree a of Mn represented by the formula (i) exceeds 1.6, thc local deformability
is remarkably deteriorated. Therefore, in order to improve the local deformability, the a
value of the hot-formed steel sheet member is preferably set to 1.6 or less. In order to
further improve the local deformability, the a value is more preferably set to 1.2 or less.
100421 The segregation of Mn in the steel sheet is mainly controlled by a steel sheet
composition (particularly an impurity contcnt) and a continuous casting condition, and is not
substantially changed after and before hot-rolling and hot-forming. Therefore, the inclusions
and segregation situation of the steel sheet for hot-forming are almost the same as those ofthe
hot-formed steel sheet member manufactured by hot-forming the steel sheet for hot-forming.
Since the a value is not largely changed by hot-forming, the a value of the hot-formed steel
sheet member can also be set to 1.6 or less by setting the a value of the steel shcet for
hot-forming to 1.6 or less. The a value of the hot-formed steel sheet member can also be set
to 1.2 or less by setting the a value to 1.2 or less.
100431 The maximum Mn concentration at a central part of the sheet thickness is obtained
by the following method. The central part of the sheet thickness of the steel sheet is
subjected to line analysis using an electron probe microanalyzer (EPMA). Three measured
values are selected in higher order from the analysis results, and the average value thereof is
calculated. The average Mn concentration at the 114 depth position of the sheet thickness
from the surface is obtained by the following method. Similarly, ten places are analyzed at
the 114 depth position of the steel sheet using EPMA, and the average value thereof is
calculated.
100441 Cleanliness Level: 0.08% or less
In a case in which A-based, B-based, and C-based inclusions described in JIS G 0555
9
(2003) exist in large amounts in the steel sheet member, the inclusions are apt to servc as the
starting point of breaking. In a case in which the inclusions are increased, crack propagation
easily occurs, which causes dcterioration in the local deformability. Particularly, in thc case
of the hot-formed steel sheet member having tensile strength of 1.0 GPa or more, the
cxistence fraction of the inclusions is prefcrahly suppressed low. In a case in which the
value of the cleanliness level ofthe steel specified by JIS G 0555 (2003) cxceeds 0.08%, the
amount of the inclusions is large, which makes it difficult to secure practically sufficient local
deformability. Therefore, the value of the cleanliness level of the steel sheet for hot-forming
is preferably set to 0.08% or less. The value of the cleanliness level is more preferably set to
0.04% or less in order to further improve the local deformability. The value ofthe
cleanliness level of thc steel is obtained by calculating the area percentages of the A-based,
B-based, and C -based inclusions.
100451 Since the valuc of the cleanliness level is not largely changed by hot-forming, the
value of the cleanliness level of the hot-formed steel sheet member can also be set to 0.08% or
less by setting the value of the cleanliness level of the steel sheet for hot-forming to 0.08% or
less. The value of the cleanliness level of the hot-formed steel sheet member can also be set
to 0.04% or less by setting the value of the cleanliness level of the steel sheet for hot-forming
to 0.04% or less.
[0046] In the embodiments, the value of the cleanliness level of the steel sheet for
hot-forming or the hot-formed steel sheet member is obtained by the following method. Test
materials are cut from five places of the steel sheet for hot-forming or the hot-formed steel
sheet member. In a case in which the sheet thickness of the steel sheet for hot-forming or the
hot-formed steel sheet member is defined as t, the cleanliness level is investigated at each of
positions of 1\81, 1/4t, 112, 3/4t, and 718t in the direction of the sheet thickness of each of the
test materials by a JIS-G-0555 method. The largest value (lowest cleanliness property) of
the cleanliness level in each of the sheet thicknesses is used as the value of the cleanliness
level of the test material.
[0047] Average Grain Size of Priory Grains: 10 pm or less
In a case in which a priory grain size in the hot-formed steel sheet member is
decreased, the local deformability is improved. In a steel sheet mainly containing
i martensite, voids occur at priory grain boundaries and boundaries of the lower structures in
I
i grains. Howevel; grain refining of priory grains can suppress the occurrence of the voids,
i
i
I and improve the local deformability for delaying connection. In a case in which the average
grain size of the priory exceeds 10 pm,'this effect cannot be exhibited. Therefore, the
average grain size of the priory grains in the hot-formed steel sheet member is set to 10 pm or
10
less. 111 ordcr to pcrSorm grain relining of thc priory grains, it is effective to dectease a
heating terupcrzture, aild to delay the dissolution of carbides during heating Lo suppress the
grain growth.
[0048] The average grain size of the priory grains can be measured using a method specified
by IS0643. That is, the number of crystal grains in a measured view is measured. The
average arca of the crystal grains is obtained by dividing the area of the measured view by thc
number orthe crystal grains, and the crystal grain size in an equivalent circular diameter is
calculated. At that time, it is preferable that the grain on the boundary of the view is
measured as 112, and a magnification ratio is adjusted so that the number of the crystal grains
is set to 200 or more. A plurality of views arc preferably measured in order to improve
accuracy.
[0049] Residual Carbides: 4 x lo3 per mm2 or less
In the case of hot-lhrming, sufficient hardenability can be secured by the resolution
of carbides generally existing in steel. However, a part of the carbides may remain without
being resolved. The residual carbides have an effect of suppressing the growth of y grains in
holding heating during hot-forming by pinning. Therefore, the residual carbides desirably
exist during holding heating. As the residual carbides are decreased after hot-forming, the
hardenability is improved, which can provide the securement of high strength. Therefore, it
is preferable that the number density of the residual carbides can be reduced in a case in
which the holding heating is completed.
[0050] In a case in which a number of residual carbides exist, the hardenability after
hot-forming may be deteriorated, and the residual carbides serve as the occurrence source of
voids to cause deterioration in local deformability. Particularly, in a case in which the
number density of the residual carbides exceeds 4x 1 o3 per mm2, the hardenability after
hot-forming may be deteriorated. Therefore, the number density of the residual carbides
existing in the hot-formed steel sheet member is preferably 4x10~pe r mm2 or less.
[005 11 (C) Plating Layer
The high-strength hot-formed steel sheet member according to the embodiments may
have a surface on which a plating layer is formed for the purpose of an improvement in
corrosion resistance, or the like. The plating layer may be an elcctroplating layer, and may
be a hot-dip plating layer. Examples of the electroplating layer include electrogalvanizing,
electric Zn-Ni alloy plating, and electric Zn-Fe alloy plating. Examples of the hot-dip
plating layer include hot dip galvanizing, alloyed hot dip galvanizing, molten aluminum
plating, molten Zn-A1 alloy plating, molten Zn-AI-Mg alloy plating, and molten Zn-Al-Mg-Si
alloy plating. A plating deposition amount is not particularly limitcd, and may be adjusted
11
within a general rangc.
LO0521 (D) Method fol. Manufacturing Steel Sheet Sot. Hot-Forming
The manufactuiing conditions or the steel sheet for hot-forming wed for
manufacturing the steel shcet member lor hot-forming according to the embodiments are not
particularly limited, but the steel sheet for hot-forming can be suitably manufactured by using
a manufacturing method to be shown bclow.
[0053] The stcel having the above-mentioned chemical composition is melted in a furnace,
and a slab is then produced by casting. In order to set the cleanliness level of the steel sheet
to 0.08% or less, it is desirable to set the heating temperature of molten steel to a temperature
higher by 5°C or more than the liquidus-line tcnlperature of the steel in a case in which the
molten steel is continuously cast, and to suppress the amount of the molten steel to be cast per
unit time to 6 tlmin or less.
[0054] In a case in which the amount to be cast per unit time of the molten steel exceeds 6
tlmin during continuous casting, the molten steel is fast stirred in a mold. Thereby,
inclusions are apt to be trapped by a solidifying shell, which causes an increase in the
inclusions in the slab. In a case in which the molten steel heating temperature is less than a
temperature higher by 5°C than the liquidus-line temperature, the viscosity of the molten steel
is increased, and thereby, the inclusions are less likely to float in a continuous-casting
machine. As a result, the inclusions in the slab are increased, which is apt to cause
deterioration in cleanliness property.
[0055] The molten steel is cast with the molten steel heating temperature set to 5'C or higher
from the liquidus-line temperature of the molten steel and the amount of the molten steel to be
cast per unit time set to 6 tlmin or less, which is less likely to cause the introduction of the
inclusions into the slab. As a result, the amount of the inclusions at the stage in which the
slab is produced can be effectively decreased, which can easily achieve the steel sheet
cleanliness level of 0.08% or less.
[0056] In a case in which the molten steel is continuously east, the molten steel heating
temperature is more desirably set to a temperature higher by 8°C or more than the
liquidus-line temperature, and the amount of the molten steel to be cast per unit time is more
desirably set to 5 tlmin or less. By setting the molten steel heating temperature to a
temperature higher by 8°C or more than the liquidus-line temperature, and setting the amount
of the molten steel to be cast per unit time to 5 tlmin or less, the cleanliness level is easily set
to 0.04% or less, wh~chis desirable.
[0057] In order to suppress the concentration of MnS causing deteiioration in local
deformability, a center segregation reducing treatment is desirably perlbrmed to reduce the
12
center segregatio~io f Mn. Examples of the ccntcr segregation reducing trealrnenl include a
mcthod ofdischat.ging moltcil steel in which Mn is incrassated in an unsolidiiled layer besore
a slab is completely solidilied.
[0058] Specifically, molten steel in which Mn before being completely solidified is
incrassated can be discharged by a treatment such as electromagnetic stirring or unsolidified
layer reduction. The electromagnetic stirring trcatment can bc performed by stirring
unsolidified molten steel at from 250 to 1000 gausses, for example. The unsolidified layer
reduction treatment can be performed by reducing a last solidified part at the slope of about 1
mdm, for example.
[0059] The slab obtained by the above-mentioned method may be subjected to a soaking
treatment if necessary. By performing the soaking treatment, segregated Mn is diffused,
which can provide a reduction in a segregation degree. A preferable soaking temperature in
a case in which the soaking treatment is performed is from 1200 to 1300°C, and a preferable
soaking time is from 20 to 50 hours.
[0060] Then, the slab is hot-rolled. As hot-rolling conditions, from the viewpoint of more
uniformly producing carbides, it is preferable that a hot-rolling initiation temperature is set to
a temperature region of from 1000 to 1300°C, and a hot-rolling completion temperature is set
to 850°C or higher. A winding temperature is prcferably higher from the viewpoint of
processability. However, in a case in which the winding temperature is too high, scale
formation causes a decrease in yield, and thereby the winding temperature is preferably from
500 to 650°C. A hot-rolled steel sheet obtained by hot-rolling is subjected to a descale
treatment by pickling or the like.
[0061] In the embodiments, in order to perform grain refining of priory grains after
hot-forming and to reduce the number density of residual carbides, the hot-rolled steel sheet
subjected to the descale treatment is preferably annealed to produce a hot-rolled annealed
steel sheet.
[0062] In order to provide the fine priory grain size after hot-forming, the growth of they
grains is preferably suppressed by the carbides in solution. However, in order to improve the
hardenability, to secure the high strength, and to suppress the occurrence of voids in the
hot-formed steel sheet member, the number density of the residual carbides is preferably
reduced.
[0063] In order to provide the fine priory grain size in the hot-formed steel sheet member
and to reduce the number density of the residual carbides, the form of the carbides existing in
the steel sheet beforc hot-forming and the incrassating degree of eleinenls in the carbides arc
important. It is desirable that the carbides are finely dispersed. However, since the
13
cat-bides are fast dissolved in the case, a grain growth suppressing el'l'ect cannoi be expected.
111 a case in which elements such as Mn and Cr arc incrassated in the carbides, the carbides are
less likcly to be solved. Therefore, it is desirable that the carbides in the steel sheet before
hot-forming are finely dispersed, and the incrassating degree of the elements in the carbides is
highcn
100641 The form of the carbides can be controllcd by adjusting the annealing condition after
hot-rolling. Specifically, it is preferable that the annealing temperature is sct to an Ac1 point
or less and the Ac1 point-100°C or higher, and an annealing time is 5 hours or less.
[0065] In a case in which a winding temperature after hot-rolling is set to 55OoC or lower,
the carbides are likely to be finely dispersed. However, since the incrassating degree of the
clements in the carbides is also decreased, the incrassating of the elements is advanced by
annealing.
[0066] In a case in which the winding temperature is 550°C or higher, perlite is generated,
and the incrassating of the elements to the carbides in the perlite is advanced. In this case, in
order to divide the perlite to disperse the carbides, annealing is performed.
[0067] As the steel sheet for hot-formed steel sheet member in the embodiments, the
above-mentioned hot-rolled annealed steel sheet, a cold-rolled steel sheet obtained by
cold-rolling the hot-rolled annealed steel sheet, or a cold-rolled annealed steel sheet obtained
by annealing the cold-rolled steel sheet can be used. A treating step may be selected if
appropriate according to the request level of the accuracy of the sheet thickness of a product,
or the like. Since the carbides are hard, the form of the carbides is not changed even in a
case in which cold-rolling is performed, and the existence form before cold-rolling is
maintained even after cold-rolling.
[0068] The cold-rolling may be performed using a usual method. From the viewpoint of
securing favorable flatness, a reduction ratio in the cold-rolling is preferably set to 30% or
more. In order to avoid an excessive load, the reduction ratio in the cold-rolling is
preferably set to 80% or less.
[0069] In a case in which the cold-rolled steel sheet is annealed, it is desirable that the
cold-rolled steel sheet is preliminarily subjected to a treatment such as degreasing. The
annealing is preferably performed at an Acl point or less, for hours or less, preferably for 3
hours or less for the purpose of cold-rolling strain lessening.
[0070] (E) Method for Forming Plating Layer
As described above, the hot-formed steel sheet member according to the
embodiments may have a surface on whlch a plating layer is formed for the purpose of an
improvement in corrosion resistance, or thc like. The plating layer is desirably formed on
14
thc stccl shect before being subjected to hot-forming. In a case in which zinc-bascd plating
is applicd to the sulfate of thc steel sheet, lnoltcn zinc-bascd plating is preferably applicd in a
continuous hot dip galvanidng line from thc viewpoint of productivity In the case,
annealing may be performed before a plating treatment in the continuous hot dip galvanizing
line. Only a plating treatment may be performed without being annealed with a heat holding
temperaturc sct to a low temperature. An alloyed molten zinc sheetd sheet steel may be
provided by performing an alloying heat treatment after hot dip galvanizing. The zinc-based
plating can also be applied by electroplating. The zinc-based plating can be applied to at
least a part of the surface of the steel material. However, generally, the zinc-based plating is
cntirely applied to one surface or both surfaces of the steel sheet.
[0071] (F) Method for Manufacturing Hot-Formed Steel Sheet Membcr
By hot-forming the steel sheet for hot-forming, a high-strength hot-formed steel sheet
member can be obtained. From the viewpoint of suppressing the grain growth, the heating
rate of the steel sheet during hot-forming is desirably 2OoC1sec or higher, and more preferably
50°Clsec or higher. The heating temperaturc of the steel sheet during hot-forming is
desirably set to a temperature of more than an Ac: point and 1050°C or lower. In a case in
which the heating temperature is the Ac: point or less, ferrite, perlite, or bainite remains in the
steel sheet without providing an austenite single phase state before hot-forming. As a result,
desired hardness may not be obtained without providing the metal structure mainly containing
martensite after hot-forming. This causes not only an increase in a variation in hardness of
the hot-formed steel sheet member but also deterioration in local deformability.
[0072] In a case in which the heating temperature exceeds 1050°C, the austenite is coarse,
which may cause deterioration in the local deformability of the steel sheet member.
Therefore, the heating temperature ofthe steel sheet during hot-forming is preferably set to
1050°C or lower. In a case in which a heating time is less than 1 min, the single-phasing of
the austenite may be insufficient even if heating is performed. Furthermore, since the
dissolution of the carbides is insufficient, the number density of the residual carbides is
increased even if the y grain size is refined. In a case in which the heating time exceeds 10
min, the austenite is coarse, which may cause deterioration in the local deformability of the
hot-formed steel sheet member. Thercfore, the heating time of the steel sheet during
hot-forming is desirably set to from 1 to 10 min.
[0073] In a case in which a hot-forming initiation temperature is less than the Ar: point,
ferrite transformatio~st~a rts. Therefore, even if forcible cooling is then performed, the
structure mainly containing martensite may not be provided. Thercforc, the hot-forming
initiation temperature is desirably the Ari point or more. Rapid cooling is desirably
15
peri'ormed at ihe cooling ratc of 1O0C/sec or higller aftcr hot-formillg, and rapid cooling is
more desirably performed at thc rate of2O0C/scc or higher. l'hc uppcr limit of thc cooli~lg
rate is not particularly specilied.
LO0741 In order to obtain a hot-formed steel sheet member having a metal structure mainly
containing martensite having a less variation in hardness, the steel sheet alier hot-lbrming is
desirably rapidly cooled until the surhce temperature of the steel sheet becomes 350°C or
lower. A cooling end temperature is preferably set to 100°C or lower, and more preferably
room temperature.
[0075] Hereinafter, the embodiments will be more specifically described with reference to
Examples, but the present invention is not limited to these Examples.
Examples
[0076] Steel having chemical components shown in Table 1 was melted in a test converter,
and subjected to continuous casting in a continuous casting testing machine, to produce slabs
each having a width of 1000 mm and a thickness of 250 mm. Symbol " used in Table 1
means departing from the composition range of the embodiments. Under conditions shown
in Table 2, the heating temperature of molten steel and the amount of molten steel lo be cast
per unit time were adjusted. The cooling rate of each of the slabs was controlled while the
water amount of a secondary cooling spray band was changed. A center segregation
reducing treatment is performed by carrying out soft reduction at the slope of 1 mmlm using
rolls in a solidified terminal part and discharging the incrassated molten steel of a last
solidified part. A part of the slabs were then subjected to a soaking treatment under
conditions of 1250°C and 24 hours.

[0078] [Table 21
degree of
annehot.forming aling1
molten steel
number
density
winding tensile of
amount
of
hot-rolling
siab center
test
number
1
2
3
notch
temperature
880
880
880
4 B hot-rolling 1550 3.5 YES 1250"Cx24h 510 620 1 NO NO 880 90 1345 419 399 20 94.8 2 0 3.2 8.6 0 02 1.2 16x1031 6.8 m: - - - - - ~ - p - - ~ ~ ~ ~ ~ - - 5 B coid-rolling 1520 5.5 YES NO 510 620 1 YES NO 880 90 1351 422 403 19 94.5 2.1 3.4 9.5 0.09 1.3 2.0~103 CEoxmapmeprleast~ v1 ~! -- ----p----ppp-p
6 B cold-roiling 1550 3.5 YES 125WCx24h 510 620 1 YES NO 820. 90 1230 420 404 16 87.0 9.6 3.0 5.5 0.03 Exampies
7 C cold-rolling 1550 5.1 YES 125O0Cx24h 510 620 1 YES YES 880 90 1169 376 359 16 95.9 1.0 3.1 5.4 0.02
8 D coid-rolling 1550 5.5 YES 125PCx24h 510 620 t YES YES 880 50 1384 429 405 20 94.6 2.0 3.4 8.7 0.02 1.2 i l x l 0 i 6.5 ixa?lples p--p-p-p--
620 1 YES YES 1100 90 1356 426 408 18 96.0 0.0 4.0 20.1' 0.02
Comparative
9 D cold-roiling 1550 5.5 YES 125O0Cx24h 510
-
10 E cold-rolling 1550 3.6 YES 125PCx24h 510 620 1 YES NO 880 90 1305 407 387 20 94.6 1.2 4.2 9.2 0.02 1.1 14xi03 7.2 ' Examples 1
strength
(MPa)
1236
1226
1230
variation in
hardness
anneaiing
heating
temperature
("C)
- 1550
1550
1550
soaking
treatment
125PCx24h
i250"Cx24h
NO
I
I
hoiding
time
(h)
90
90
90
-
temperature
rc'
510
510
510
steel
type
I A
A
molten
steel to
be cast
(iimin)
4.3
8
43
steel sheet
hot-rolliq
cold-roiiing
temperature
rC)
YES
prior
cold-rolling
NO
YES
YES
segregation
reducing
treatment
YES
YES
A I cold roli~ng NO - '
HSeo
391
386
388
metal structure
time
(h)
~omparatlve~
after
cold-roiiing
NO
YES
NO
volume fraction
11
12
13
NO
HSja
371
363
364
Of
low-temperature
transformation
structure (%)
pp 95.6
95.4
95.8
~ ~ - ~ p ~ 620 ; 1
AHv
_i
20
23
24
voiume
620
620
segregation residual elongation
grain ievei (%) a carbioes (%,
size
mm2)
!
-
8.8 0.02
8.3 0.09 Examples 1
E
F
G
880
voiume
fraction
of
ferrite
(%)
1.2
1.5
1.3
1
1 9.0
of
residual 1 Y (%I
3.2
3.1
2.9
coid-roiling
cold-rolling
mid-rolling
90
14
15
1.7 1 . 8 ~ 9
-
0.02
265
394
409
Cornparativi~
1550
1550
1550
1255
16 i* hot-rolling 1550 3.9 YES 1250mCx24h 510 ~~~~ 620 1 NO NO 880 90 1384 429
G
H
17 J* cold-rolling 1550 2.8 NO NO 510 620 1 YES NO 880 90 1236 391 0.4 4.8 8.4
18 K*coid-roiling 1550 2.7 YES 1250aCx24h 510 620 1 YES YES 11.0 2.5 6.5
3.6
2.1
5.2
389
5.6
. 6.6
95
18
20
cold-roiiing
cold-roiiing
Comparative
Examples
4q8
7.3
5.1
19 L* coid-roiling 1550 5.1 YES 1250"~~24h- 620 1 YES NO 2.0 3.8 10.2'
20 , M' cold-rolling 1550 4.5 YES 125PCx24h 510 620 1 YES NO 880 90 1242 394 222 172 97.7 0.5 1.8 8.9
0.04
0.02
YES
YES
YES
350
Comparative
Exelnples
Examples
81.0
94.7
94.8
1550
1550
0.02
002
I
1.2
125PCx24h
i25PCx24h
125PCx24h
39
18.1
1.3
0.6
1.1 4.6
1.2 07xlO? 7.8
2.9~103
5.2
3.9
680
510
510
94.2
0.9
4.0
4.6
YES
YES
620
620
3.5
5.1
9.3
8.8
125VCx24h
1250'Cx24h
1
1
2.3
0.02
0.03
0.09
510
510
YES
YES
6.5
1.1
1.1
1.1
650
620
8.5~10"
18x10?
23x10'
NO .
YES
0.02
20
1
880
880
1.1
YES
YES
90
90
4 5x103
YES
YES
5,,
1222
1276
880
880
384
-402 -
90
90
362
384
1055
1315
360
412
22
2
94.8
94.3
1.3
0.5
3.9
5.2
86
8.4
0.02
0.02
1.2
1.1
1 . 5 x 1 0 V . 2
2 9 m 7.8
Examples ' Examples 1
Examples I
100791 The obtained slabs wcre hot-rolled with a hot-rolling tcstiilg machine, to produce
3.0-n~m-thickh ot--rolleds tecl sheets. Each of the hot-rollcd stcel shccts was wound, thcn
subjected to piclsling, and further anllcaled. A part of the steel sheels wcre furthcr
cold-rolled with a cold-rolling testing machine, to produce 1.5-mm-thick cold-rollcd stcel
sheets. Furthermore, a pa12 of the cold-rolled steel sheets were annealed at 6OO0C lor 2 h to
obtain cold-rolled annealed steel sheets.
[0080] Then, as shown in Fig. 1 and Fig. 2, the steel sheets 1 for hot-fortning were subjected
to hot pressing (hat forming) with a mold (punch 11, dice 12) using a hot pressing test
apparatus, to obtain hot-formed steel sheet members 2. The steel sheets were heated at
various surface temperatures rainging from 820°C to 1 10O0C in a heating furnace, held at the
temperatures for 90 seconds, then talcen out from the heating furnace, immediately subjected
to hot pressing with the mold with a cooling device, and subjected to a quenching treatment
simultaneously with forming. The hot-formed steel sheet members were evaluated as
follows. The evaluation results are shown in Table 2. In Table 2, "hot-rolling" mcans a
3.0-mm-thick-hot-rolled steel sheets sub,jected to hot-rolling, and "cold-rolling" means a
1.5-mm-thick-cold-rolled steel sheet obtained by further cold-rolling the hot-rolled steel
sheets. Symbol * means departing from the range of the embodiments.
[0081]
A JlS No. 5 tensile test pieces were obtained from the rolling right-angle direction of
the hot-formed steel sheet members, and subjected to a tensile test according to JIS 22241
(201 1) to measure tensile strength (TS).
[0082]
The hot-formed steel sheet members were cut to samples so that the ccntral part of
the sheet thickness of sections parallel to the rolling direction, of the hot-formed steel sheet
members were viewing surfaces, and the samples were then subjected to mirror polishing.
Then, the samples were subjected to Nital corrosion, and the metal structures of five views of
each of the samples were observed using a scanning electron microscope (magnification ratio:
2000). By subjecting the obtained microphotograph to an image treatment, the area fraction
of ferrite was obtained. It was used as the volulnc fraction of ferrite. The volume fraction
of residual austenite in the metal structure was obtained using X diffraction (XRD). The
balance thereof was calculated as the volume fraction of a low-temperature transformation
structure. The residual y volume fraction was obtained from the intensity ratio of diffraction
intensity Ia(200) of (200) of ferrite, diffraction mtensity la(211) of (21 1 ) of ferrite, diffraction
intensity Iy (220) of (220) of austenite, and diffraction intensity ly (3 11) of (31 1) of austenite
according to X diffraction using a Mo bulb after chcinically polishing the 118 inner layer of
19
the sheet thickl~cssf rom the surface of cacli oftlie steel sheets.
Vy(volume %) - 0.25 x {ly(220)1(1.35x lu(200) + Iy(220)) i-ly-(2 20)1(0.69 x In(211) '
Iy(220)) -I- ly(3 11)1(1.5 x In(200) + ly(3 11)) + Iy(3 11)1(0.69 x In(211) + ly(3 11)))
[0083]
Test matcrials were cut from five places of the hot-formed steel sheet members.
The cleanliness level was investigated at cach of positions of 1/81, 114t, 1/2t, 3/4t, and 7/81
with respect to the sheet thickness t of each of the test materials by a point counting method
The largest value (lowest cleanliness property) of the cleanliness level in each of the sheet
thicknesses was used as the value of the cleanliness level of the test material.
[0084]
The central part of the sheet thickness of the hot-formed steel sheet member was
subjected to line analysis using EPMA. Three measured values were measured at high order
from the analysis results, and the average value thereof was then calculated to obtain the
maximum Mn concentration in the central part of the sheet thickness. Ten places were
analyzed using EPMA at the 114 depth position of the sheet thickness from the surface of the
hot-formed steel sheet member, to obtain the average value thereof. The average Mn
concentration at the 114 depth position of the sheet thickness from the surface was obtained.
The segregation degree a of Mn was obtained by dividing the maximum Mn concentration in
the central part of the sheet thickness by the average Mn concentration at the 114 depth
position of the sheet thickness from the surface.
[0085]
The average grain size of the priory grains in the hot-formed steel sheet member was
obtained by measuring the number of crystal grains in a measured view, dividing the area of
the measured view by the number of the crystal grains to obtain the average area of the crystal
grains, and calculating a crystal grain size in an equivalent circular diameter. At that time,
the grain on the boundary of the view was mcasured as 112, and an observation magnification
ratio was adjusted if appropriate so that the number of the crystal grains was set to 200 or
more.
[0086]
The surface of the hot-formed steel sheet member was corroded using a picral liquid,
and magnified in a size of 2000 times with a scanning electron microscope. A plurality of
views were observed. At this time, the number of views in which carbides existed was
counted to calculate the number per 1 mm2.
[0087]
The following test was performed in order to evaluate hardness stability. Steel
sheets for hot-forming were heated at 10°C/sec to 900°C by a heat treatment simulator, and
then held for 150 sec. Then, ihe steel sheets for hot-forming were cooled at the cooling rates
of about 8O0C/sec and 1O0C/sec to room temperature. Each of the samples was subjected to
a Vickers hardness test at the 114 position of the sheet thiclmess of the section. Hardness
measurement was performed based on JIS Z 2244 (2009) at five points with a test force set to
9.8 N, and the average thereof was obtained. The average value of the hardnesses at the
cooling rate of about 80°C/sec and the average value of the hardnesses at the cooling rate of
1O0C/sec were defined as I-ISso and HSlo, and the difference AHv thereof was used as the
index of the hardness stability.
[0089] In order to evaluate the hardness stability and local deformability of each of the
samples, the samples having AI-IV of 50 or 1ess.and notch elongation of 6% or more were
determined to be favorable.
[0090] As shown in Table 2, the test number 2 had a steel composition satisfying the range
of the embodiments, but the amount of molten steel to be cast per unit time was large.
Thereby, the value of the cleanliness level exceeded 0.08%, which resulted in poor local
deformability.
Since the test number 3 was not subjected to a center segregation reducing treatment
and a soaking treatment, the segregation degree of Mn exceeded 1.6, which resulted in poor
local deformability.
I Since the test number 5 had a low molten steel heating temperature, the value of the
i cleanliness level exceeded 0.08%, which resulted in poor local deformability.
I
! Since the test number 6 had a low hot-Iorming temperature, the voliune fraction of
fcil-ite exceeded 3% after hot-forming, which resulted in poor hardness stability.
Furthermore, the number density of residual carbidcs was also as high as 8 . 0l~o3 per mm2,
2 1
which resulted iri poor local deforinability.
Since the Lest number 9 had a high heating teniperaturc dixing hot-forming, the prior
y grain size was increased, which resulted in poor local deformability.
Since the test number 11 had a high wiuding temperature after hot-rolling, the dcnsity
of residual carbides was increased, which resulted in poor local deformability.
Since the test number 14 had a high annealing temperature after hot-rolling and a
long annealing time, the volume fraction of ferrite exceeded 3% after hot-forming, which
resulted in poor hardness stability. The insufficient dissolution of carbides caused an
increase in the density of residual carbides, which resulted in poor local deformability.
Since the test number 16 had an S content exceeding the upper limit value of the
range ofthe embodiments, the value of thc cleanliness level exceeded 0.08%, which resulted
in poor local deformability.
Since the test number 17 had a Mn content exceeding the upper limit value of the
range of the embodiments, the segregation degree of Mn exceeded 1.6, which resulted in poor
local deformability.
Since the test number 18 had an Si content exceeding the upper limit value of the
range of the emhodiments, an A3 point was increased, and the volume fraction of ferrite
exceeded 3% after hot-forming, which resulted in poor hardness stability.
The test numher 19 had a C content exceeding the upper limit value ofthe range of
the embodiments, which resulted in poor local deformability.
The test numher 20 had a Cr content lower than the range of the embodiment, which
resulted in poor hardness stability.
[0091] The test numbers 1,4,7, 8, 10, 12, 13, and 15 satisfying the range of the
emhodiments were excellent in both hardness stability and local deformability.
[0092] The entire disclosures ofJapan Patent Application No. 2014-101443 filed in May 15,
2014 and Japan Patent Application No. 2014-101444 filed in May 15,2014 are incorporated
herein by reference.
All publications, patent applications, and technical standards described herein are
herein incorporated by reference to the same extent as if each individual publication, patent
application, or technical standard was specifically and individually indicated to be
incorporated by reference.
[0093] As described above, the various typical embodiments have been described, but the
illvention is not limited to these embodiments. The range of the invention is limited by only
thc following claims.
1. A hot-formed steel sheet member having a chemical composition consisting of, 1
in terms of mass %, from 0.08 to 0.16% of C, 0.19% or less of Si, from 0.40 to 1.50% of Mn,
0.02% or less of P, 0.01% or less of S, from 0.01 to 1.0% of sol. Al, 0.01% or less of N, from i :
0.25 to 3.00% of Cr, from 0.01 to 0.05% of Ti, from 0.001 to 0.01% of B, from 0 to 0.50% of
Nb, from 0 to 2.0% of Ni, from 0 to 1.0% of Cu, from 0 to 1 .O% of Mo, from 0 to 1 .O% of V,
from 0 to 0.005% of Ca, and a remainder consisting of Fe and impurities, i I
wherein a total volume fraction of martensite, tempered martensite, and bainite is
50% or more, anda volume f~actiono f ferrite is 3% or less, !
an average grain size of priory grains is 10 pm or less, and
a number density of residual carbides which are present is 4 x lo3 per mm2 or less.
2. The hot-formed steel sheet membkr according to claim 1, wherein the chemical
composition comprises one or more selected from the group consisting of, in terms of mass
%, from 0.003 to 0.50% of Nb, from 0.01 to 2.0% of Ni, from 0.01 to 1.0% of Cu, from 0.01
to 1.0% of Mo, from 0.01 to 1.0% of V, and from 0.001 to 0.005% of Ca.
3. The hot-formed steel sheet member according to claim 1 or 2, wherein a value of
a cleanliness level of steel specified by JIS G 0555 (2003) is0.08% or less.
4. The hot-formed steel sheet member according to any one of claims 1 to 3,
wherein a segregation degree a of Mn represented by the following formula (i) is 1.6 or less,
a = [maximum Mn concentration (mass %) at a central part of a sheet
thickness]/[average Mn concentration (mass %) at a 114 depth position of the sheet thickness
from a surface] ... (i).
5. The hot-formed steel sheet member according to any one of claims 1 to 4,
wherein the steel sheet member has a surface on which a plating layer is formed.
6. The hot-formed steel sheet member according to any one of claims 1 to 5,
wherein the steel sheet member has tensile strength of 1.0 GPa or more.