[Title of the Invention] STEEL MATERIAL
[Technical Field]
[0001] The present invention relates to a steel material, and concretely
5 relates to a steel material suitable for a material of an impact absorbing
member in which an occurrence of crack when applying an impact load is
suppressed, and fu~ther,a n effective flow stress is high. This application is
based upon and claims the benefit of priority of the prior Japanese Patent
Application No. 2012-161 730, filed on July 20, 2012, the entire contents of
10 which are incorporated herein by reference.
[Background Art]
[0002] In recent years, fiom a point of view of global environmental
protection, a reduction in weight of a vehicle body of automobile has been
required as a part of reduction in C02 emissions fiom automobiles, and a
15 high-strengthening of a steel material for automobile has been aimed. This
is because, by improving the strength of steel material, it becomes possible to
reduce a thickness of the steel material for automobile. Meanwhile, a social
need with respect to an improvement of collision safety of autolnobile has
been further increased, and not only the high-strengthening of steel material
20 but also a development of steel material excellent in impact resistance when a
collision occurs during traveling, has been desired.
[0003] Here, respective postions of a steel material for automobile at a
time of collision are deformed at a high strain rate of several tens (s-') or more,
so that a high-strength steel material excellent in dynamic strength property is
25 required.
[0004] As such a high-strength steel material, a low-alloy TRIP steel
having a large static-dynamic difference (difference between static strength
and dynamic strength), and a high-strength multi-phase structure steel
material such as a multi-phase structure steel having a second phase mainly
formed of ma~tensitea, re known.
5 [OOOS] Regarding the low-alloy TRIP steel, for example, Patent
Document 1 discloses a strain-induced transformation type high-strength steel
sheet (TRIP steel sheet) for absorbing collision energy of automobile
excellent in dynamic deformation propesty.
[0006] Further, regarding the multi-phase structure steel sheet having the
10 second phase mainly formed of ma~tensite, inventions as will be described
below are disclosed.
[0007] Patent Document 2 discloses a high-strength steel sheet having an
excellent balance of strength and ductility and having a static-dynamic
difference of 170 MPa or more, the high-strength steel sheet being formed of
15 fine ferrite grains, in which an average grain diameter ds of nanocrystal grains
each having a crystal grain diameter of 1.2 pm or less and an average c~ystal
grain diameter dL of microclystal grains each having a c~ystagl rain diameter
of greater than 1.2 pm satisfy a relation of dL / ds 2 3.
[0008] Patent Document 3 discloses a steel sheet fo~medo f a dual-phase
20 structure of martensite whose average grain diameter is 3 pm or less and
ma~tensitew hose average grain diameter is 5 pm or less, and having a high
static-dynamic ratio.
[0009] Patent Document 4 discloses a cold-rolled steel sheet excellent in
impact absorption property containing 75% or more of ferrite phase in which
25 an average grain diameter is 3.5 pm or less, and a balance composed of
tempered martensite.
[0010] Patent Document 5 discloses a cold-rolled steel sheet in which a
prestrain is applied to produce a dual-phase structure formed of ferrite and
martensite, and a static-dynamic difference at a strain rate of 5 x lo2 to 5 x
10' / s satisfies 60 MPa or more.
5 [0011] Further, Patent Document 6 discloses a high-strength hot-rolled
steel sheet excellent in impact resistance propesty formed only of hard phase
such as bainite of 85% or more and martensite.
[Prior A1-t Document]
[Patent Document]
10 [0012] Patent Document 1: Japanese Laid-open Patent Publication No.
H11-80879
Patent Document 2: Japanese Laid-open Patent Publication No.
2006-161077
Patent Document 3: Japanese Laid-open Patent Publication No.
15 2004-84074
Patent Document 4: Japanese Laid-open Patent Publication No.
2004-277858
Patent Document 5: Japanese Laid-open Patent Publication No.
2000-17385
20 Patent Document 6: Japanese Laid-open Patent Publication No.
H11-269606
[Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[OO 131 However, the conventional steel materials being materials of
25 impact absorbing members have the following proble~ns. Specifically, in
order to improve an impact absorption energy of an impact absorbing member
(which is also simply referred to as "member", hereinafter), it is essential to
increase a strength of a steel material being a material of the impact absorbing
member (which is also simply referred to as "steel material", hereinafter).
[00 141 However, as disclosed in "Journal of the Japan Society for
5 Technology of Plasticity" vol. 46, No. 534, pages 641 to 645, that an average
load (F,J determining an impact absorption energy is given in a manner that
F,a (oY . t2) I 4, in which oY indicates an effective flow stress, and t
indicates a sheet thickness, the impact absorption energy greatly depends on
the sheet thickness of steel material. Therefore, there is a limitation in
10 realizing both of a reduction in thickness and a high impact absorbency of the
impact absorbing member only by increasing the strength of the steel
material.
[0015] Here, the flow stress corsesponds to a stress required for
successively causing a plastic deformation at a start or after the start of the
15 plastic deformation, and the effective flow stress means a plastic flow stress
which takes a sheet thickness and a shape of the steel material and a rate of
strain applied to a member when an impact is applied into consideration.
[00 161 Incidentally, for example, as disclosed in pamphlet of
International Publication No. WO 20051010396, pamphlet of International
20 Publication No. WO 20051010397, and pamphlet of International Publication
No. WO 2005/010398, an impact absorption energy of an impact absorbing
member also greatly depends on a shape of the member.
[0017] Specifically, by optimizing the shape of the impact absorbing
member so as to increase a plastic deformation workload, there is a possibility
25 that the impact absorption energy of the impact absorbing member can be
dramatically increased to a level which cannot be achieved only by increasing
the strength of the steel material.
[0018] However, even when the shape of the impact absorbing member is
optimized to increase the plastic deformation workload, if the steel material
has no deformability capable of enduring the plastic deformation workload, a
5 crack occurs on the impact absorbing member in an early stage before an
expected plastic deformation is completed, resulting in that the plastic
deformation workload cannot be increased, and it is not possible to
dramatically increase the impact absorption energy. Fusther, the occurrence
of crack on the impact absorbing member in the early stage may lead to an
10 unexpected situation such that another member disposed by being adjacent to
the impact absorbing member is damaged.
[0019] In the conventional techniques, it has been aimed to increase the
dynamic strength of the steel material based on a technical idea that the
impact absorption energy of the impact absorbing member depends on the
15 dynamic strength of the steel material, but, there is a case where the
deformability is significantly lowered only by aiming the increase in the
dynamic stsength of the steel material. Accordingly, even if the shape of the
impact absorbing member is optimized to increase the plastic deformation
workload, it was not always possible to dramatically increase the impact
20 absorption energy of the impact absorbing member.
[0020] Further, since the shape of the impact absorbing member has been
studied on the assumption that the steel material manufactured based on the
above-described technical idea is used, the optimization of the shape of the
impact absorbing member has been studied, &om the first, based on the
25 deformability of the existing steel material as a premise, and thus the study
itself such that the deformability of the steel material is increased and the
shape of the impact absorbing member is optimized to increase the plastic
deformation workload, has not been done sufficiently so far.
[0021] The present invention has a task to provide a steel material
suitable for a material of an impact absorbing member having a high effective
5 flow stress and thus having a high impact absolption energy and in which an
occurrence of crack when an impact load is applied is suppressed, and a
manufacturing method thereof.
[Means for Solving the Problems]
[0022] As described above, in order to increase the impact absolption
10 energy of the impact absorbing member, it is important to optimize not only
the steel material but also the shape of the impact absorbing member to
increase the plastic deformation workload.
[0023] Regarding the steel material, it is important to increase the
effective flow stress to increase the plastic deformation workload while
15 suppressing the occussence of crack when the impact load is applied, so that
the shape of the impact absorbing member capable of increasing the plastic
deformation workload can be optimized.
100241 The present inventors conducted ealnest studies regarding a
method of suppressing the occussence of crack when the impact load is
20 applied and increasing the effective flow stress regarding the steel material to
increase the impact absolption energy of the impact absorbing member, and
obtained new findings as will be cited hereinbelow.
[0025] [Improvement of impact absorption energy]
(1) In order to increase the impact absolption energy of the steel
25 material, it is effective to increase the effective flow stress when a true strain
of 5% is given (which will be described as "5% flow stress", hereinafter).
[0026] (2) In order to increase the 5% flow stress, it is effective to
increase a yield strength and a work hardening coefficient in a low-strain
region.
[0027] (3) In order to increase the yield strength, it is required to perform
5 refining of steel structure.
[0028] (4) In order to increase the work hardening coefficient in the
low-strain region, it is effective to efficiently increase a dislocation density in
the low-strain region.
[0029] (5) In order to efficiently increase the dislocation density in the
10 low-strain region, it is effective to increase a proportion of small-angle grain
boundaries (grain boundaries with misorientation angle of less than 15") in
ciystal grain boundaries. This is because, although a high-angle grain
boundaiy easily becomes a sink (place of annihilation) of piled-up
dislocations, the dislocation is easily accumulated in the small-angle grain
15 boundaiy, and for this reason, by increasing the proportion of the small-angle
grain boundaries, it becomes possible to efficiently increase the dislocation
density even in the low-strain region.
[0030] [Suppression of occurrence of crack when impact load is applied]
(6) When a crack occurs on the impact absorbing member at the time
20 of applying the impact load, the impact absorption energy is lowered.
Further, there is also a case where another member adjacent to the impact
absorbing member is damaged.
[0031] (7) When the strength, particularly the yield strength of the steel
material is increased, a sensitivity with respect to a crack at the time of
25 applying the impact load (which is also referred to as "impact crack",
hereinafter) (the sensitivity is also refei~edt o as "iinpact crack sensitivity",
hereinafter) becomes high.
[0032] (8) In order to suppress the occurrence of impact crack, it is
effective to increase a uniform ductility, a local ductility and a fsacture
toughness.
5 [0033] (9) In order to increase the uniform ductility, it is effective to
produce a multi-phase structure made of ferrite as a main phase and a balance
formed of a second phase containing one or two or more selected from a
group consisting of bainite, mastensite and austenite.
[0034] (10) In order to increase the local ductility, it is effective to make
10 the second phase to be a soft one, and to provide a plastic deformability equal
to a plastic deformability of ferrite being the main phase to the second phase.
[0035] (1 1) In order to increase the fiacture toughness, it is effective to
refine fe~riteb eing the main phase and the second phase.
[0036] The present invention is made based on the above-described new
15 findings, and a gist thereof is as follows.
LOO371 [ll
A steel material having a chemical composition of, by mass%, C:
greater than 0.05% to 0.2%, Mn: 1% to 3%, Si: greater than 0.5% to 1.8%, Al:
0.01% to 0.5%, N: 0.001% to 0.015%, Ti or a sum of V and Ti: greater than
20 0.1% to 0.25%, Ti: 0.001% or more, Cr: 0% to 0.25%, Mo: 0% to 0.35%, and
a balance: Fe and impurities, includes a steel structure being a multi-phase
structure having a main phase made of ferrite of 50 area% or more, and a
second phase containing one or two or more selected fsom a group consisting
of bainite, martensite and austenite, in which an average nanohardness of the
25 above-described second phase is less than 6.0 GPa, and when a boundary
where a misorientation of clystals becomes 2' or more is defined as a grain
boundary, and a region surrounded with the grain boundary is defined as a
crystal grain, an average grain diameter of all ciystal grains in the
above-described main phase and the above-described second phase is 3 pm or
less, and a proportion of a length of small-angle grain boundaries where the
5 misorientation is 2' to less than 15" in a length of all grain boundaries is 15%
or more.
[00381 [21
The steel material according to [l] contains, by mass%, one or two
selected from a group consisting of Cr: 0.05% to 0.25%, and Mo: 0.1% to
10 0.35%.
[Effect of the Invention]
[0039] According to the present invention, it becomes possible to obtain
an impact absorbing member capable of suppressing or eliminating an
occusrence of crack thereon when an impact load is applied, and having a
15 high effective flow stress, so that it becomes possible to dramatically increase
an impact absorption energy of the impact absorbing member. By applying
the impact absorbing member as above, it becomes possible to fiu-ther
improve a collision safety of a product of an automobile and the like, which is
industrially extremely useful.
20 [Brief Description of the Drawings]
[0040] [FIG. 11 FIG. 1 illustrates a temperature history in continuous
annealing heat treatment;
[FIG. 21 FIG. 2 is a graph illustrating a relationship of a hardness of a second
phase and a stable buckling ratio obtained by an axial crush test with respect
25 to an average grain diameter, in which 0 indicates that a stable buckling
occurs with no occurrence of crack, A indicates that a crack occurs with a
probability of 112, and X indicates that a crack occurs with a probability of
212, and an unstable buckling occurs; and
[FIG. 31 FIG. 3 is a graph illustrating a relationship between an average grain
diameter and an average crush load obtained by the axial crush test.
5 [Mode for Canying out the Invention]
[0041] Hereinafter, the present invention will be described in detail.
I. Chemical composition
Note that " % in the following description regarding the chemical
composition means "mass%, unless othe~wisen oted.
10 [0042] (1) C: greater than 0.05% to 0.2%
C has a function of facilitating a generation of bainite, martensite and
austenite contained in a second phase, a function of improving a yield
strength and a tensile strength by increasing a strength of the second phase,
and a function of improving the yield strength and the tensile strength by
15 strengthening a steel through solid-solution strengthening. If a C content is
0.05% or less, it is sometimes diff~culto achieve an effect provided by the
above-described functions. Therefore, the C content is set to be greater than
0.05%. On the other hand, if the C content exceeds 0.2%, there is a case
where martensite and austenite are excessively hardened, resulting in that a
20 local ductility is significantly lowered. Therefore, the C content is set to
0.2% or less. Note that the present invention includes a case where the C
content is 0.2%.
[0043] (2) Mn: 1% to 3%
Mn has a function of facilitating a generation of the second phase
25 typified by bainite and maitensite, a hnction of improving the yield strength
and the tensile strength by strengthening the steel through solid-solution
strengthening, and a function of improving the local ductility by increasing a
strength of ferrite though solid-solution strengthening and by increasing a
hardness of ferrite under a condition where a high strain is applied. If a Mn
content is less than 1%, it is sometimes difficult to achieve an effect provided
5 by the above-described functions. Therefore, the Mn content is set to 1% or
more. The Mn content is preferably 1.5% or more. On the other hand, if
the Mn content exceeds 3%, there is a case where martensite and austenite are
excessively generated, resulting in that the local ductility is significantly
lowered. Therefore, the Mn content is set to 3% or less. The Mn content is
10 preferably 2.5% or less. Note that the present invention includes a case
where the Mn content is 1% and a case where the Mn content is 3%.
[0044] (3) Si: greater than 0.5% to 1.8%
Si has a function of improving a uniform ductility and the local
ductility by suppressing a generation of carbide in bainite and mastensite, and
15 a function of improving the yield strength and the tensile strength by
strengthening the steel though solid-solution strengthening. If a Si content
is 0.5% or less, it is sometimes difficult to achieve an effect provided by the
above-described functions. Therefore, the Si amount is set to be greater than
0.5%. The Si amount is preferably 0.8% or more, and is more preferably 1%
20 or more. On the other hand, if the Si content exceeds 1.8%, there is a case
where austenite excessively remains, and the impact crack sensitivity
becomes significantly high. Therefore, the Si content is set to 1.8% or less.
The Si content is preferably 1.5% or less, and is more preferably 1.3% or less.
Note that the present invention includes a case where the Si content is 1.8%.
25 [0045] (4) Al: 0.01% to 0.5%
A1 has a function of suppressing a generation of inclusion in a steel
through deoxidation, and preventing the impact crack. However, if an A1
content is less than 0.01%, it is difficult to achieve an effect provided by the
above-described function. Therefore, the A1 content is set to 0.01% or more.
On the other hand, if the A1 content exceeds 0.5%, an oxide and a nitride
5 become coarse, which facilitates the impact crack, instead of preventing the
impact crack. Therefore, the A1 content is set to 0.5% or less. Note that the
present invention includes a case where the A1 content is 0.01% and a case
where the A1 content is 0.5%.
[0046] (5) N: 0.001% to 0.015%
10 N has a function of suppressing a grain growth of austenite and ferrite
by generating a nitride, and suppressing the impact crack by refining a
stsucture. However, if a N content is less than 0.001%, it is difficult to
achieve an effect provided by the above-described function. Therefore, the
N content is set to 0.001% or more. On the other hand, if the N content
15 exceeds 0.015%, a nitride becomes coarse, which facilitates the impact crack,
instead of suppressing the impact crack. Therefore, the N content is set to
0.015% or less. Note that the present invention includes a case where the N
content is 0.001% and a case where the N content is 0.015%.
[0047] (6) Ti or sum of V and Ti: greater than 0.1% to 0.25%
20 Ti and V have a function of generating carbides such as Tic and VC in
the steel, suppressing a growth of coarse clystal gains thsough a pinning
effect with respect to a grain growth of ferrite, and suppressing the impact
crack. Ful-ther, Ti and V also have a function of improving the yield strength
and the tensile strength by strengthening the steel thsough precipitation
25 strengthening realized by Tic and VC. If a content of Ti or a sum of V and
Ti is 0.1% or less, it is difficult to achieve these functions. Therefore, the
content of Ti or the sum of V and Ti is set to be greater than 0.1%. The
content is preferably 0.15% or more. On the other hand, if the content of Ti
or the sum of V and Ti exceeds 0.25%, Tic and VC are excessively generated,
which increases the impact crack sensitivity, instead of lowering the impact
5 crack sensitivity. Therefore, the content of Ti or the sum of V and Ti is set to
0.25% or less. The content is preferably 0.23% or less. Note that the
present invention includes a case where the content of Ti or the sum of V and
Ti is 0.25%.
[0048] (7)Ti:0.001%ormore
10 Further, these functions are exhibited more significantly when 0.001%
or more of Ti is contained. Therefore, it is prerequisite that Ti of 0.001% or
more is contained. Although the V content may be 0%, it is preferably set to
0.1 % or more, and is more preferably set to 0.1 5% or more. From a point of
view of a reduction in the impact crack sensitivity, the V content is preferably
15 set to 0.23% or less. Ful-ther, the Ti content is preferably set to 0.01% or less,
and is more preferably set to 0.007% or less.
[0049] Further, it is also possible that one or two of Cr and Mo is (are)
contained as an optionally contained element.
[0050] (8) Cr: 0% to 0.25%
20 Cr is an optionally contained element, and has a function of increasing
a hardenability and facilitating a generation of bainite and martensite, and a
function of improving the yield strength and the tensile strength by
strengthening the steel through solid-solution strengthening. In order to
more securely achieve these functions, a content of Cr is preferably 0.05% or
25 more. However, if the Cr content exceeds 0.25%, a martensite phase is
excessively generated, which increases the impact crack sensitivity.
Therefore, when Cr is contained, the content of Cr is set to 0.25% or less.
Note that the present invention includes a case where the content of Cr is
0.25%.
[0051] (9) Mo: 0% to 0.35%
5 Mo is, similar to Cr, an optionally contained element, and has a
function of increasing the hardenability and facilitating a generation of bainite
and martensite, and a function of improving the yield strength and the tensile
strength by strengthening the steel though solid-solution strengthening. In
order to more securely achieve these functions, a content of Mo is preferably
10 0.1% or more. However, if the Mo content exceeds 0.35%, the martensite
phase is excessively generated, which increases the impact crack sensitivity.
Therefore, when Mo is contained, the content of Mo is set to 0.35% or less.
Note that the present invention includes a case where the content of Mo is
0.35%.
15 [0052] The steel material of the present invention contains the
above-described essential contained elements, further contains the optionally
contained elements according to need, and contains a balance composed of Fe
and impurities. As the impurity, one contained in a raw material of ore,
scrap and the like, and one contained in a manufacturing step can be
20 exemplified. However, it is allowable that the other components are
contained within a range in which the properties of steel material intended to
be obtained in the present invention are not inhibited. For example,
although P and S are contained in the steel as impurities, P and S are desirably
limited in the following manner.
25 [0053] P: 0.02% or less
P makes a grain boundamy to be fiagile, and deteriorates a hot
workability. Therefore, an upper limit of P content is set to 0.02% or less.
It is desirable that the P content is as small as possible, but, based on the
assumption that a dephosphorization is performed within a range of actual
manufacturing steps and manufacturing cost, the upper limit of P content is
5 0.02%. The upper limit is desirably 0.015% or less.
[0054] S: 0.005% or less
S makes the grain boundaly to be fiagile, and deteriorates the hot
workability and ductility. Therefore, an upper limit of P content is set to
0.005% or less. It is desirable that the S content is as small as possible, but,
10 based on the assumption that a desulfurization is performed within a range of
actual manufacturing steps and manufacturing cost, the upper limit of S
content is 0.005%. The upper limit is desirably 0.002% or less.
[0055] 2. Steel structure
(1) Multi-phase structure
15 A steel structure related to the present invention is made to be a
multi-phase stsucture having ferrite with fine crystal grains as a main phase,
and a second phase containing one or two or more of bainite, martensite, and
austenite with fine ciystal grains, in order to realize both of an increase in
effective flow stress by obtaining a high yield strength and a high work
20 hardening coefficient in the low-strain region, and an impact crack resistance.
[0056] If an area ratio of ferrite being the main phase is less than 50%,
the impact crack sensitivity become high, and the impact absorption property
is lowered. Therefore, the area ratio of ferrite being the main phase is set to
50% or more. An upper limit of the area ratio of fel-site is not particularly
25 defined. If a propoltion of the second phase is lowered in accordance with
an increase in a proportion of ferrite being the main phase, a strength and a
work hardening ratio are lowered. Therefore, the upper limit of the area
ratio of ferrite (in other words, a lower limit of area ratio of the second phase)
is set in accordance with a strength level.
[0057] The second phase contains one or two or more selected from a
5 group consisting of bainite, martensite and austenite. There is a case where
cementite and perlite are inevitably contained in the second phase, and such
an inevitable stlucture is allowed to be contained if the stlucture is 5 area% or
less. In order to increase the strength, the area ratio of the second phase is
preferably 35% or more, and is more preferably 40% or more.
10 [0058] (2) Average grain diameter of fel-site (main phase) and second
phase: 3 pm or less
In the steel material being an object of the present invention, an
average grain diameter of all crystal grains of ferrite and the second phase is
set to 3 pm or less. Such a fine structure can be obtained through a device in
15 rolling and heat treatment, and in that case, both of the main phase and the
second phase are refined. Fulther, in such a fine structure, it is difficult to
determine an average grain diameter regarding each of ferrite being the main
phase and the second phase. Accordingly, in the present invention, the
average grain diameter of the entire ferrite being the main phase and second
20 phase, is defined.
[0059] If an average grain diameter of ferrite in a steel having ferrite as a
main phase is refined, the yield strength is improved, and accordingly, the
effective flow stress is increased. If a ferrite grain diameter is coarse, the
yield strength becomes insufficient, and the impact absorption energy is
25 lowered.
[0060] Further, the refining of the second phase such as bainite,
martensite and austenite improves the local ductility, and suppresses the
impact crack. If the grain diameter of the second phase is coarse, when an
impact load is applied, a brittle fracture easily occurs in the second phase,
resulting in that the impact crack sensitivity becomes high.
5 [0061] Therefore, the above-described average grain diameter is set to 3
pm or less. The average grain diameter is preferably 2 pm or less.
Although the above-described average grain diameter is preferably finer, there
is a limitation in the refining of fel~iteg rain diameter realized through normal
rolling and heat treatment. Further, when the second phase is excessively
10 refined, there is a case where the plastic deformability of the second phase is
lowered, which lowers the ductility, instead of increasing the ductility.
Therefore, the above-described average grain diameter is preferably set to 0.5
pm or more.
[0062] (3) Proportion of length of small-angle grain boundaries where
15 misorientation is 2" to less than 15" in length of all grain boundaries: 15% or
more
A grain boundary plays a role of any one of a dislocation generation
site, a dislocation annihilation site (sink) and a dislocation pile-up site, and
exerts an influence on a work hardening ability of the steel material. Out of
20 the grain boundaries, a high-angle grain boundary where a misorientation is
15" or more easily becomes the annihilation site of piled-up dislocations.
On the other hand, in a small-angle grain boundaiy where the misorientation
is 2" to less than 15", the annihilation of dislocation hardly occurs, which
contributes to an increase in dislocation density. Therefore, in order to
25 increase the work hardening coefficient in the low-strain region to increase
the effective flow stress, there is a need to increase a proportion of the
small-angle grain boundaries described above. If a proportion of a length of
the above-described small-angle grain boundaries is less than 15%, it is
difficult to increase the work hardening coefficient in the low-strain region to
increase the effective flow stress. Therefore, the proportion of the length of
5 the above-described small-angle grain boundaries is set to 15% or more.
The proportion is preferably 20% or more, and is more preferably 25% or
more. Although it is preferable that the propoltion of the small-angle grain
boundaries described above is as high as possible, there is a limitation in a
proportion of small-angle interface capable of being included in a normal
10 polycrystal. Specifically, it is realistic to set the proportion of the length of
the small-angle grain boundaries described above to 70% or less.
[0063] The propostion of the small-angle grain boundaries is determined
by conducting an EBSD (electron backscatter diffiaction) analysis at a
position of 114 depth in a sheet thickness of a cross section parallel to a rolling
15 direction of a steel sheet. In an EBSP analysis, several tens of thousands of
measurement regions on a surface of a sample are mapped at equal intervals
in a grid pattern, and a c~ystaol rientation is determined in each grid. Here, a
boundaly where a misorientation of clystals between adjacent grids becomes
2" or more is defined as a grain boundaiy, and a region sui-sounded with the
20 grain boundary is defined as a clystal grain. If the misorientation becomes
less than 2", a clear grain boundaly is not formed. Out of the all grain
boundaries, a grain boundaiy where the misorientation is 2" to less than 15" is
defined as a small-angle grain boundary, and a proportion of a length of the
small-angle grain boundaries where the misorientation is 2" to less than 15"
25 with respect to a length of total sum of grain boundaries is determined. Note
that regarding an average grain diameter of ferrite (main phase) and the
second phase, a number of clystal grains defined in a similar manner (regions
each sut~oundedw ith a grain boundaiy where the misorientation becomes 2"
or more) is counted in a unit area, and based on an average area of the crystal
grains, the average grain diameter can be determined as a circle-equivalent
5 diameter.
[0064] (4) Average nanohardness of second phase: less than 6.0 GPa
When the hardness of the second phase such as bainite, mastensite and
austenite is increased, the local ductility is lowered. Concretely, if an
average nanohardness of the second phase exceeds 6.0 GPa, the impact crack
10 sensitivity is increased due to the decrease in the local ductility. Therefore,
the average nanohardness of the second phase is set to 6.0 GPa or less.
[0065] Here, the nanohardness is a value obtained by measuring a
nanohardness in a grain of each phase or structure by using a nanoindentation.
In the present invention, a cube corner indenter is used, and a nanohardness
15 obtained under an indentation load of 1000 pN is adopted. The hardness of
the second phase is desirably low for improving the local ductility, but, if the
second phase is excessively softened, a material strength is lowered.
Therefore, the average nanohardness of the second phase is preferably greater
than 3.5 GPa, and is more preferably greater than 4.0 GPa.
20 [0066] 3. Manufacturing method
In order to obtain the steel material of the present invention, it is
preferable that VC and Tic are properly precipitated in a hot-rolling step and
a temperature-raising process in a heat treatment step, a growth of coarse
crystal grains is suppressed by the pinning effect provided by VC and Tic,
25 and an optimization of multi-phase structure is realized by subsequent heat
treatment. In order to achieve this, it is preferable to perform manufacture
through the following manufacturing method.
[0067] (1) Hot-rolling step and cooling step
A slab having the above-described chemical composition set to have a
temperature of 1200°C or more, is subjected to multi-pass rolling at a total
5 reduction ratio of 50% or more, and hot rolling is completed in a temperature
region of not less than 800°C nor more than 950°C. After the completion of
the hot rolling, the resultant is rolled at a cooling rate of 600°C/second or
more, and after the completion of the rolling, the resultant is cooled to a
temperature region of 700°C or less within 0.4 seconds (this cooling is also
10 referred to as primary cooling), and then retained for 0.4 seconds or more in a
temperature region of not less than 600°C nor more than 700°C. After that,
the resultant is cooled to a temperature region of 500°C or less at a cooling
rate of less than 100°C/second (this cooling is also referred to as secondary
cooling), and then further cooled to a room temperature at a cooling rate of
15 0.03"C/second or less, thereby obtaining a hot-rolled steel sheet. The last
cooling at the cooling rate of 0.03"C/second or less is cooling performed on
the steel sheet which is coiled in a coil state, so that in a case where the steel
sheet is a steel strip, by coiling the steel stsip after the secondaly cooling, the
last cooling at the cooling rate of 0.03"C/second or less is realized.
20 [0068] Here, in the above-described primary cooling, after the hot rolling
is practically completed, rapid cooling is conducted to a temperature region of
700°C or less within 0.4 seconds. The practical completion of hot rolling
means a pass in which the practical rolling is conducted at last, in the rolling
of plurality of passes conducted in finish rolling of the hot rolling. For
25 example, in a case where the practical final reduction is conducted in a pass
on an upstream side of a finishing mill, and the practical rolling is not
conducted in a pass on a downstream side of the finishing mill, the rapid
cooling (primaty cooling) is conducted to the temperature region of 700°C or
less within 0.4 seconds after the rolling in the pass on the upstream side is
completed. Ful-ther, for example, in a case where the practical rolling is
5 conducted up to when the pass reaches the pass on the downstream side of the
finishing mill, the rapid cooling (primaty cooling) is conducted to the
temperature region of 700°C or less within 0.4 seconds after the rolling in the
pass on the downstream side is completed. Note that the primaty cooling is
basically conducted by a cooling nozzle disposed on a run-out-table, but, it is
10 also possible to be conducted by an inter-stand cooling nozzle disposed
between the respective passes of the finishing mill.
[0069] Each of the cooling rate (600°C/second or more) in the
above-described primary cooling and the cooling rate (less than
lOO"C/second) in the above-described secondary cooling is set based on a
15 temperature of a surface of sample (surface temperature of steel sheet)
measured by a thetmotracec A cooling rate (average cooling rate) of the
entire steel sheet in the above-described primary cooling is estimated to be
about 20O0C/second or more, as a result of conversion from the cooling rate
(60O0C/second or more) based on the surface temperature.
20 [0070] By the above-described hot-rolling step and cooling step, the
hot-rolled steel sheet in which the carbide of V (VC) and the carbide of Ti
(Tic) are precipitated at high density in the ferrite grain boundaty, is obtained.
It is preferable that an average grain diameter of VC and Tic is 10 nm or
more, and an average intergranular distance of VC and Tic is 2 pm or less.
25 [0071] (2) Cold-rolling step
The hot-rolled steel sheet obtained by the above-described hot-rolling
step and cooling step may be directly subjected to a later-described heat
treatment step, but, it may also be subjected to the later-described heat
treatment step after being subjected to cold rolling.
100721 When the cold rolling is performed on the hot-rolled steel sheet
5 obtained by the above-described hot-rolling step and cooling step, the cold
rolling at a reduction ratio of not less than 30% nor more than 70% is
performed, to thereby obtain a cold-rolled steel sheet.
[0073] (3) Heat treatment step (steps (Cl) and (C2))
A temperature of the hot-rolled steel sheet obtained by the
10 above-described hot-rolling step and cooling step or the cold-rolled steel sheet
obtained by the above-described cold-rolling step is raised to a temperature
region of not less than 750°C nor more than 920°C at an average temperature
rising rate of not less than 2"C/second nor more than 20°C/second, and the
steel sheet is retained in the temperature region for a period of time of not less
15 than 20 seconds nor more than 100 seconds (annealing in FIG. I).
Subsequently, heat treatment in which the resultant is cooled to a temperature
region of not less than 440°C nor more than 550°C at an average cooling rate
of not less than 5"CIsecond nor more than 20°C/second, and retained in the
temperature region for a period of time of not less than 30 seconds nor more
20 than 150 seconds, is perfolmed (overaging 1 to overaging 3 in FIG. 1).
[0074] If the above-described average temperature rising rate is less than
2"C/second, the grain growth of fei-site occurs during the temperature rising,
resulting in that the crystal grains become coarse. On the other hand, if the
above-described average temperature rising rate is greater than 20°C/second,
25 the precipitation of VC and Tic during the temperature rising becomes
insufficient, resulting in that the crystal gain diameter becomes coarse,
instead of becoming fine.
[0075] ' If the temperature retained after the above-described temperature
rising is less than 750°C or greater than 920°C, it is difficult to obtain an
intended multi-phase structure.
5 [0076] If the above-described average cooling rate is less than
5°C/second, a ferrite amount becomes excessive, and it is difficult to obtain a
sufficient strength. On the other hand, if the above-described average
cooling rate is greater than 20°C/second, a hard second phase is excessively
generated, resulting in that the impact crack sensitivity is increased.
10 [0077] The retention after the above-described cooling is important to
facilitate softening of the second phase to secure the average nanohardness of
the second phase of less than 6.0 GPa. In a case where the condition such
that the retention is perfoimed in the temperature region of not less than
440°C nor more than 550°C for a period of time of not less than 30 seconds
15 nor more than 150 seconds, is not satisfied, it is difficult to obtain a desired
propel-ty of the second phase. There is no need to set the temperature to be a
fixed temperature during the retention, and the temperature can be changed
continuously or in stages as long as it is within the temperature region of not
less than 440°C nor more than 550°C (refer to overaging 1 to overaging 3
20 illustrated in FIG. 1, for example). From a point of view of controlling the
small-angle grain boundaly and the precipitates of V and Ti, the temperature
is preferably changed in stages. Specifically, the above-described treatment
is treatment corresponding to so-called overaging treatment in continuous
annealing, in which in an initial stage of the overaging treatment step, it is
25 preferable to increase the proportion of small-angle grain boundaries by
performing retention in an upper bainite temperature region. Concretely, it
is preferable to perform the retention in a temperature region of not less than
480°C nor more than 580°C. After that, in order to make Ti and V remained
in the fel-rite phase and the second phase in a supersaturated manner to be
precipitated, the retention is perfoimed in a temperature region of not less
5 than 440°C nor more than 480°C to generate a precipitation nucleus, and then
the retentionis performed in a temperature region of not less than 480°C nor
more than 550°C to increase a precipitation amount. A fine carbide such as
VC precipitated in the ferrite phase and the second phase improves the
effective flow stress, so that it is desirable to cause the precipitation at high
10 density through the above-described overaging treatment.
[0078] The hot-rolled steel sheet or the cold-rolled steel sheet
manufactured as above may be used as it is as the steel material of the present
invention, or a steel sheet, cut from the hot-rolled steel sheet or the cold-rolled
steel sheet, on which appropriate working such as bending and presswork is
15 performed according to need, may also be employed as the steel material of
the present invention. Ful-ther, the steel material of the present invention
may also be the steel sheet as it is, or the steel sheet on which plating is
performed after the working. The plating may be either electroplating or hot
dipping, and although there is no limitation in a type of plating, the type of
20 plating is normally zinc or zinc alloy plating.
[Examples]
[0079] An experiment was conducted by using slabs (each having a
thickness of 35 mm, a width of 160 to 250 rnm, and a length of 70 to 90 mm)
having chenlical compositions presented in Table 1. In Table 1, "-" means
25 that the element is not contained positively. An underline indicates that a
value is out of the range of the present invention. A steel type E is a
comparative example in which a total content of V and Ti is less than the
lower limit value. A steel type F is a comparative example in which a
content of Ti is less than the lower limit value. A steel type H is a
comparative example in which a content of Mn is less than the lower limit
5 value. In each of the steel types, a molten steel of 150 kg was produced in
vacuum to be cast, the resultant was then heated at a kmace temperature of
1250°C, and subjected to hot forging at a temperature of 950°C or more, to
thereby obtain a slab.
[OOSO]
[Table I]
[0081] Each of the above-described slabs was reheated at 1250°C within
1 hour, and after that, the resultant was subjected to rough hot rolling in 4
passes by using a hot-rolling testing machine, the resultant was hrther
subjected to finish hot rolling in 3 passes, and after the completion of rolling,
5 primary cooling and cooling in two stages were conducted, to thereby obtain a
hot-rolled steel sheet. Hot-rolling conditions are presented in Table 2. The
prima~y cooling and the secondaly cooling right after the completion of
rolling were conducted by water cooling. By completing the seconda~y
cooling at a coiling temperature presented in Table, and letting a coil cool, the
10 cooling to a room temperature at a cooling rate of 0.03°C/second or less was
realized. A sheet thickness of each of the hot-rolled steel sheets was set to 2
mm.
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[0083] A part of the hot-rolled steel sheets was subjected to cold rolling,
and then all of the steel sheets were subjected to heat treatment by using a
continuous annealing simulator with a heat pattern presented in FIG. 1 and
under conditions presented in Table 3. In the present examples, the reason
5 why the temperature retention (referred to as overaging in the examples) after
cooling was performed from the annealing temperature was conducted at
three stages of different temperatures as presented in FIG. 1 and Table 3, is
because the propostion of small-angle grain boundaries and the precipitation
density of VC carbide are made to be increased.
10
IS I 50% I 10 1 SSO 1 30 1 10 1 460 I M I 464 1 22 1 500 1 15
16 1 50% I 10 $70 30 1 10 1 460 M 464 ?? 500 15
[0085] Regarding the hot-rolled steel sheets and the cold-rolled steel
sheets obtained as above, the following examination was conducted.
First, a JIS No. 5 tensile test piece was collected from a test steel sheet
in a direction perpendicular to a rolling direction, and subjected to a tensile
5 test, thereby determining a 5% flow stress, a maximum tensile strength (TS),
and a uniform elongation (u-El). The 5% flow stress indicates a stress when
a plastic deformation occurs in which a strain becomes 5% in the tensile test,
the 5% flow stress has a propoltionality relation with the effective flow stress,
and becomes an index of the effective flow stress.
10 [0086] A hole expansion test was conducted to determine a hole
expansion ratio based on Japan Iron and Steel Federation standard JFST
1001-1996 except that reamer working was performed on a machined hole to
remove an influence of a damage of end face.
[0087] The EBSD analysis was conducted at a position of 114 depth in a
15 sheet thickness of a cross section parallel to a rolling direction of the steel
sheet. In the EBSD analysis, a boundary where a misorientation of c~ystals
became 2" or more was defined as a grain boundaly, an average grain
diameter was determined without distinguishing between a main phase and a
second phase, and a grain boundary surface misorientation map was created.
20 Out of all grain boundaries, a grain boundaly where the misorientation was 2"
to less than 15" was defined as a small-angle grain boundaly, and a proportion
of a length of small-angle grain boundaries where the misorientation was 2"
to less than 15" with respect to a length of total sum of grain boundaries was
dete~mined. Further, an area ratio of ferrite was determined from an image
25 quality map obtained by this analysis.
[0088] A nanohardness of the second phase was detesmined by a
nanoindentation method. A section test piece collected in a direction parallel
to the rolling direction at a position of 1/4 depth in a sheet thickness was
polished by an emery paper, the resultant was subjected to mechanochemical
polishing using colloidal silica, and then ful-ther subjected to electrolytic
5 polishing to remove a worked layer, and then the resultant was subjected to a
test. The nanoindentation was cal-sied out by using a cube corner indenter
under an indentation load of 1000 yN. An indentation size at this time is a
diameter of 0.5 ym or less. The hardness of the second phase of each
sample was measured at randomly-selected 20 points, and an average
10 nanohardness of each sample was determined.
[0089] Further, an square tube member was produced by using each of
the above-described steel sheets, and an axial crush test was conducted at a
collision speed in an axial direction of 64 kmlh, to thereby evaluate a collision
absorbency. A shape of a cross section perpendicular to the axial direction
15 of the square tube member was set to an equilateral octagon, and a length in
the axial direction of the square tube member was set to 200 mm. The
evaluation was conducted under a condition where each member was set to
have a sheet thickness of 1 mm, and a length of one side of the
above-described equilateral octagon (length of straight poltion except for
20 cul-ved poltion of corner poi-tion) (Wp) of 16 mm. Two of such square tube
members were produced from each of the steel sheets, and subjected to the
axial crush test. The evaluation was conducted based on an average load
when the axial crush occul~ed(a verage value of two times of test) and a stable
bucking ratio. The stable buckling ratio corresponds to a proportion of a
25 number of test bodies in which no crack occui-sed in the axial crush test, with
respect to a number of all test bodies. Generally, the possibility in which the
crack occurs in the middle of the crush is increased when an impact
absorption energy is increased, resulting in that a plastic deformation
workload cannot be increased, and there is a case where the impact absolption
energy cannot be increased. Specifically, no matter how high the average
5 crush load (impact absorbency) is, it is not possible to exhibit a high impact
absorbency unless the stable buckling ratio is good.
[0090] Results of the examination described above (steel structure,
mechanical properties, and axial cmsh properties) are collectively presented
in Table 4.
10 Further, a relationship of the hardness of the second phase and the
stable buckling ratio with respect to an average grain diameter of each of test
numbers 1 to 16, is illustrated by graph in FIG. 2. FIG. 3 is a graph
illustrating a relationship between the grain diameter and the average crush
load.
15
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(%) (W) ("/.I PHASE wno Mbm2)
(Ghl
smumm TEKSILE .I\> HOLE
DSAXSlOh'PROPERTIES
AXIAL tFCSX
PROPERIT
[0092] As can be understood from Table 4, FIG. 2 and FIG. 3, in the steel
material related to the present invention, the average load when the axial
crush occurs is high to be 0.29 k ~ l m mor~ m ore. Further, a good axial crush
propel-ty is exhibited such that the stable buckling ratio is 212. Therefore, the
5 steel material related to the present invention is suitably used as a material of
the above-described crush box, a side member, a center pillar, a rocker and the
like.
[Name of Document] Claims
[Claim 11 A steel material having a chemical composition of, by mass%,
C: greater than 0.05% to 0.2%,
Mn: 1% to 3%,
Si: greater than 0.5% to 1.8%,
Al: 0.01% to 0.5%,
N: 0.001% to 0.015%,
Ti or a sum of V and Ti: greater than 0.1% to 0.25%,
Ti: 0.001% or more,
10 Cr: 0% to 0.25%,
Mo: 0% to 0.35%, and
a balance: Fe and impurities, the steel material comprising
a steel structure being a multi-phase stsucture having a main phase
made of ferrite of 50 area% or more, and a second phase containing one or
15 two or more selected from a group consisting of bainite, martensite and
austenite, wherein:
an average nanohardness of the second phase is less than 6.0 GPa; and
when a boundary where a misorientation of crystals becomes 2" or
more is defined as a grain bounda~y, and a region surrounded with the grain
20 boundaly is defined as a crystal grain, an average grain diameter of all c~ystal
grains in the main phase and the second phase is 3 pm or less, and a
propostion of a length of small-angle grain boundaries where the
misorientation is 2" to less than 15" in a length of all grain boundaries is 15%
or more.
25 [Claim 21 The steel material according to claim 1, wherein
one or two selected from a group consisting of Cr: 0.05% to 0.25%,
and Mo: 0.1% to 0.35% idare contained, by mass%.