TECHNICAL FIELD
[0001]
The present invention relates to a steel sheet and a method for producing the steel
sheet.
BACKGROUND ART
[0002]
From the viewpoint of reducing a weight of an automobile body and ensuring
collision safety, the application of a high-strength steel sheet as a steel sheet for an
automobile has been sought. Members for an automobile include reinforcing members
such as a bumper or a door guard bar as well as skeleton members such as a pillar, a sill,
and a member. A high-strength steel sheet applied to these members is required to have
such a collision resistance that can ensure safety of passengers at the time of collision
(e.g., Patent Documents 1 to 3). Here, the collision resistance refers to properties having
high reaction force properties and enabling absorbing energy at the time of crash
deformation without causing a brittle fracture even when a member significantly deforms
at the time of the crash deformation.
[0003]
As a steel sheet excellent in energy absorption properties, a DP steel sheet having
a duplex micro-structure of ferrite and martensite (e.g., Patent Document 4) or a TRIP
steel sheet (transformation induced plasticity steel sheet) having a steel micro-structure
of ferrite and bainite as well as retained y (e.g., Patent Document 5) is used. Further,
steel sheets and members having a steel micro-structure made mainly of martensite and
having high yield stresses are disclosed (e.g., Patent Documents 6 to 8).
LIST OF PRIOR ART DOCUMENTS
3
PATENT DOCUMENT
[0004]
Patent Document 1: JP2009-185355A
Patent Document 2: JP2011-111672A
Patent Document 3: JP2012-251239A
Patent Document 4: JP11-080878A
Patent Document 5: JP11-080879A
Patent Document 6: JP2010-174280A
Patent Document 7: JP2013-117068A
Patent Document 8: JP2015-175050A
NON PATENT DOCUMENT
[0005]
Non-Patent Document 1: "Atlas for Bainitic Microstructures Vol. 1 ", 1992, The
Iron and Steel Institute of Japan, p. 4
Non-Patent Document 2: Tadashi Maki, "Tekko no soshiki seigyo (in Japanese)
(Microstructure control in steels)", 2015, Uchida Rokakuho
Non-Patent Document 3: Liu Xiao, et al., "Lattice-parameter variation with
carbon content of martensite. I. X-ray-diffraction experimental study", Physical Review
B, 52 (1995), pp. 9970-9978
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0006]
However, the DP steel sheet or the TRIP steel sheet described in Patent
Document 4 or 5 provides a low yield stress and insufficient reaction force properties,
and additionally, a crack occurs in some cases at the time of crash deformation from its
end face formed by shearing punching, failing to obtain a predetermined amount of
energy absorption.
[0007]
In addition, although the steel sheets described in Patent Documents 6 to 8
4
having a steel micro-structure made mainly of martensite provide a high yield stress,
when the steel sheet is formed into a member, brittle cracking occurs in some cases at the
time of crash deformation at a stress concentration such as a punching end face or a
portion at which the sheet is bent, failing to absorb collision energy sufficiently.
[0008]
The present invention has an objective to provide a steel sheet that exerts good
reaction force properties when an impact load is applied to a shaped component from the
steel sheet, is unlikely to cause a crack from an end face of the component or a region of
the component bent at the time of the impact, and has a yield stress of 1000 MPa or more,
and to provide a method for producing the steel sheet.
SOLUTION TO PROBLEM
[0009]
The present inventors conducted intensive studies about a technique to solve the
problems described above, and consequently came to obtain the following findings.
[0010]
(a) By optimizing a crystal structure of martensite and further decreasing its
block size to a certain value or less, it is possible to prevent or reduce the occurrence and
the propagation of a crack from a stress concentration at the time of a fast and large
deformation.
[0011]
(b) By optimizing components and optimizing a martensitic transformation
starting temperature Ms, it is possible to prevent or reduce the occurrence and the
propagation of a crack from a stress concentration at the time of a fast deformation.
[0012]
(c) By having a high yield stress in addition to preventing or reducing the
occurrence of a crack, high reaction force properties and impact energy absorption ability
can be obtained.
[0013]
The present invention is made based on such fmdings and has a gist of the
5
following steel sheet and the following method for producing the steel sheet.
[0014]
(1) A steel sheet having a chemical composition consisting of, in mass%:
C: 0.14 to 0.60%,
Si: more than 0% to less than 3.00%,
AI: more than 0% to less than 3.00%,
Mn: 5.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
N: 0.015% or less,
B: 0 to 0.0050%,
Ni: 0 to 5.00%,
Cu: 0 to 5.00%,
Cr: 0 to 5.00%,
Mo: 0 to 1.00%,
W: 0 to 1.00%,
Ti: 0 to 0.20%,
Zr: 0 to 0.20%,
Hf: 0 to 0.20%,
V: 0 to 0.20%,
Nb: 0 to 0.20%,
Ta: 0 to 0.20%,
Sc: 0 to 0.20%,
Y: 0 to 0.20%,
Sn: 0 to 0.020%,
As: 0 to 0.020%,
Sb: 0 to 0.020%,
Bi: 0 to 0.020%,
Mg: 0 to 0.005%,
Ca: 0 to 0.005%, and
6
REM: 0 to 0.005%,
with the balance: Fe and impurities, and
satisfYing following formulas (i) to (v), wherein
a value ofMs expressed by a following formula (vi) is 200 or more,
a steel micro-structure contains, in volume%:
martensite: 85% or more, and
retained austenite: IS% or less,
with the balance: bainite,
an average block size of martensite and bainite: 3. 0 IJlD. or less,
an average axial ratio of martensite and bainite: 1.0004 to 1.0100, and
a yield stress is 1000 MPa or more:
Si +AI:<;; 3.00 (i)
C X Mn :<;; 0.80 (ii)
Mn + Ni + Cu + 1.3Cr + 4(Mo + W) ~ 0.80 (iii)
0.003 :<;; Ti + Zr+ Hf+ V + Nb + Ta+ Sc + Y :<;; 0.20 (iv)
Sn +As+ Sb + Bi :<;; 0.020 (v)
Ms = 546 x exp(-1.362 x C)- 11 x Si- 30 x Mn- 18 x Ni- 20 x Cu- 12 x Cr-
8(Mo + W) (vi)
where symbols of elements represent contents (mass%) of the elements in the
steel sheet, and in a case where an element is not contained, zero is assigned to its symbol.
[0015]
(2) The steel sheet according to the above(!), wherein an average particle size
of iron carbides included in the steel micro-structure is 0.005 to 0.20 J.Lm.
[0016]
(3) The steel sheet according to the above (1) or (2), wherein the steel sheet
includes a plating layer on a surface of the steel sheet.
[0017]
(4)Amethod for producing the steel sheet according to any one of the above (1)
to (3), wherein
a cast piece having the chemical composition according to the above (I) is
7
subjected to a hot-rolling step, a cold-rolling step, an annealing step, and a heat treatment
step in this order,
in the hot-rolling step, the steel sheet is cooled to room temperature at an average
cooling rate for a range from a rolling finish temperature to 650°C set at 8°C/s or more,
in the annealing step, the steel sheet is held within a temperature range from an
Ac3 point to (Ac3 point + 1 00)°C for 3 to 90 s, and
an average cooling rate for a range from 700°C to (Ms point - 50)°C is set at
10°C/s or more, and
in the heat treatment step,
in a case where the Ms point is 250°C or more,
a holding time for a temperature range from (Ms point + 50) to 250°C is set at
1 00 to 1 0000 s, and
in a case where the Ms point is less than 250°C,
a holding time for a temperature range from (Ms point + 80) to 100°C is set at
100 to 50000 s:
where the Ms point (0C} and the Ac3 point (0 C} are expressed by following
formulas, where symbols of elements represent contents (mass%) of the elements in the
steel sheet, and in a case where an element is not contained, zero is assigned to its symbol.
Ms = 546 x exp(-1.362 x C)- 11 x Si- 30 x Mn- 18 x Ni- 20 x Cu- 12 x Cr-
8(Mo + W) (vi)
Ac3 = 910-203 x C05 + 44.7(Si + Al)- 30 x Mn + 700 x P- 15.2 x Ni- 26 x Cu
- 11 x Cr + 31.5 x Mo (vii)
[0018]
(S)Amethod for producing the steel sheet according to any one of the above (1)
to (3), wherein
a cast piece having the chemical composition according to the above ( 1) is
subjected to a hot-rolling step, an annealing step, and a heat treatment step in this order,
in the hot -rolling step, the steel sheet is cooled to room temperature at an average
cooling rate for a range from a rolling finish temperature to 650°C set at 8°C/s or more,
in the annealing step, the steel sheet is held within a temperature range from an
8
Ac3 to (Ac3 + 100)°C for 3 to 90s, and
an average cooling rate for a range from 700°C to (Ms - 50)°C is set at 1 0°C/s
or more, and
in the heat treatment step,
in a case where the Ms point is 250°C or more,
a holding time for a temperature range from (Ms + 50) to 250°C is set at 100 to
10000 s, and
in a case where the Ms point is less than 250°C,
a holding time for a temperature range from (Ms + 80) to 100°C is set at 100 to
50000 s:
where the Ms point (0C) and the Ac3 point (°C) are expressed by following
formulas, where symbols of elements represent contents (mass%) of the elements in the
steel sheet, and in a case where an element is not contained, zero is assigned to its symbol.
Ms = 546 x exp(-1.362 x C)- 11 x Si- 30 x Mn- 18 x Ni- 20 x Cu- 12 x Cr-
8(Mo + W) (vi)
Ac3 = 910-203 x C0·5 + 44.7(Si + Al)- 30 x Mn + 700 x P- 15.2 x Ni- 26 x Cu
- 11 x Cr + 31.5 x Mo (vii)
[0019]
(6)Amethod for producing the steel sheet according to any one of the above (1)
to (3), wherein
a cast piece having the chemical composition according to the above ( 1) is
subjected to a hot-rolling step and a heat treatment step in this order,
in the hot-rolling step, a rolling finish temperature is set at a Ar3 point or more,
and
an average cooling rate for a range from a rolling finish temperature to (Ms -
50)°C is set at 1 0°C/s or more, and
in the heat treatment step,
in a case where the Ms point is 250°C or more,
a holding time for a temperature range from (Ms +50) to 250°C is set at 100 to
10000 s, and
9
in a case where the Ms point is less than 250°C,
a holding time for a temperature range from (Ms + 80) to 1 00°C is set at 100 to
50000 s:
where the Ms point (°C) and the An point ("C) are expressed by following
formulas, where symbols of elements represent contents (mass%) of the elements in the
steel sheet, and in a case where an element is not contained, zero is assigned to its symbol.
Ms = 546 x exp(-1.362 x C)- 11 x Si- 30 x Mn- 18 x Ni- 20 x Cu- 12 x Cr-
8(Mo + W) (vi)
Ar3 = 910-310 x C + 33 x Si- 80 x Mn- 55 x Ni- 20 x Cu- 15 x Cr- 80 x Mo
(viii)
ADVANTAGEOUS EFFECT OF INVENTION
[0020]
According to the present invention, it is possible to obtain a high-strength steel
sheet that exerts good reaction force properties when an impact load is applied to a shaped
component from the steel sheet, is unlikely to cause a crack from an end face of the
component or a region of the component bent at the time of the impact, and has a yield
stress of 1000 MPa or more.
BRIEF DESCRIPTION OF DRAWING
[0021]
[Figure 1] Figure 1 is a diagram for describing a shape of a test piece used for a
collision test.
DESCRIPTION OF EMBODIMENT
[0022]
Requirements of the present invention will be described below in detail.
[0023]
(A) Chemical Composition
Reasons for limiting a content of each element are as follows. In the following
10
description, a symbol "%" for each content means "mass%".
[0024]
C: 0.14 to 0.60%
C (carbon) is an element that has effects of improving strength and refming a
block size. In order to maintain a yield stress oflOOO MPa, a contentofC is set at 0.14%
or more. On the other hand, if the content of C is more than 0.60%, an Ms point
decreases, and an average axial ratio to be described below tends to increase. As a result,
at the time of crash deformation, a brittle fracture occurs at a stress concentration,
decreasing impact energy absorption ability. The content ofC is therefore set at 0.14 to
0.60%. The content of C is preferably 0.15% or more, more preferably 0.18% or more,
and is preferably 0.50% or less.
[0025]
Si: more than 0% to less than 3.00% and AI: more than 0% to less than 3.00%,
and
Si +AI!> 3.00 (i)
Si (silicon) and AI (aluminum) are elements useful in deoxidizing steel and has,
in the present invention, an effect of increasing an average axial ratio of martensite, an
effect of preventing or reducing the formation of iron carbide, and an effect of decreasing
a block size of martensite, thereby preventing or reducing cracking in a member at the
time of crash deformation to improve energy absorption ability. In order to obtain an
effect of the deoxidation, Si and AI are to be contained at more than 0% each. Si and AI
are preferably contained at 0.01% or more each.
[0026]
However, if their total content is more than 3.00%, a tendency of a brittle fracture
to occur at the time of crash deformation increases, thereby decreasing impact energy
absorption ability. The total content of Si and AI is therefore set at 3.00% or less. The
total content is preferably 2.50% or less. A lower limit of the total content is not limited
to a particular value, but in order to obtain the effect of decreasing the block size reliably,
the total content is preferably 0.10% or more.
[0027]
11
Mn: 5.00% or less
Mn (manganese) is an element that has effects of preventing or reducing the
formation of ferrite and improving yield stress and is additionally useful in controlling
the average axial ratio. However, if a content ofMn is more than 5.00%, the Ms point
decreases, and the average axial ratio to be described below tends to increase. As a
result, at the time of crash deformation, a brittle fracture occurs at a stress concentration,
decreasing impact energy absorption ability. The content ofMn is therefore set at 5.00%
or less. The content ofMn is preferably 4.00% or less, 3.00% or less, or 2.00% or less.
In order to obtain the effect reliably, Mn is preferably contained at 0.01% or more.
[0028]
C X Mn :<;; 0.80 (ii)
The product of the contents of C and Mn is a parameter that correlates with a
brittle fracture at a stress concentration at the time of crash deformation. If the value of
C x Mn is more than 0.80, the brittle fracture tendency increases, and thus the value is set
at 0.80 or less. This value is preferably 0.60 or less, more preferably 0.40 or less.
[0029]
P: 0.030% or less
P (phosphorus) is an element that contributes to the improvement of strength.
However, if a content ofP is more than 0.030%, a grain boundary fracture tendency at the
time of crash deformation increases, thereby decreasing impact energy absorption ability.
The content of P is therefore set at 0.030% or less. From the viewpoint of resistance
weldability, the content ofP is preferably 0.020% or less. A lower limit of the content
is not limited to a particular value, but reducing the content to less than 0.001% leads to
an increase in a production cost, and thus 0.001% is practically the lower limit.
[0030]
S: 0.0050% or less
S (sulfur) is an impurity element, and if a content of S is more than 0.0050%, a
fracture occurs from a punched portion or a bent portion at the time of a crash. The
content of S is therefore set at 0.0050% or less. The content of S is preferably 0.0040%
or less or 0.0030% or less. A lower limit of the content is not limited to a particular
12
value, but reducing the content to less than 0. 0002% leads to an increase in a production
cost, and thus 0.0002% is practically the lower limit.
[0031]
N: 0.015% or less
N (nitrogen) is an element available for controlling the average axial ratio.
However, if a content ofN is more than 0.015%, a toughness of the steel sheet decreases,
resulting in a tendency of cracking to occur from a stress concentration at the time of a
crash. The content of N is therefore set at 0.015% or less. The content of N is
preferably 0.010% or less or 0.005% or less. A lower limit of the content is not limited
to a particular value, but reducing the content to less than 0.001% leads to an increase in
a production cost, and thus 0.001% is practically the lower limit.
[0032]
B: 0 to 0.0050%
B (boron) is an element that has an effect of increasing a hardenability of the
steel sheet and therefore may be contained when necessary. However, if a content of B
is more than 0.0050%, cracking may occur at the time of crash deformation. The content
ofB is therefore set at 0.0050% or less. The content ofB is preferably 0.0040% or less
or 0.0030% or less. A lower limit of the content ofB is not limited to a particular value
and may be 0%, but when obtaining the effect described above is intended, the content of
B is preferably 0.0003% or more.
[0033]
Ni: 0 to 5.00%, Cu: 0 to 5.00%, Cr: 0 to 5.00%, Mo: 0 to 1.00%, and W: 0 to
1.00%, and
Mn + Ni + Cu + 1.3Cr + 4(Mo + W) ~ 0.80 (iii)
As with Mn, Ni (nickel), Cu (copper), Cr (chromium), Mo (molybdenum), and
W (tungsten) are elements that have effects of preventing or reducing the formation of
ferrite and improving yield stress and are additionally useful in controlling the average
axial ratio. Thus, one or more elements selected from these elements may be contained.
In order to obtain this effect, contents of these elements need to satisfy Formula (iii).
[0034]
13
From the viewpoint of stably preventing or reducing the formation of ferrite and
bainite, the left side value of Formula (iii) described above is preferably 1.00 or more.
An upper limit of the left side value is not limited to a particular value, but if the left side
value is more than 4.00, the Ms point decreases, and the average axial ratio to be described
below tends to increase. As a result, at the time of crash deformation, a brittle fracture
may occur at a stress concentration, decreasing impact energy absorption ability. The
left side value of Formula (iii) described above is preferably 4.00 or less.
[0035]
In addition, the contents ofNi and Cu are each preferably 4.00% or less, more
preferably 3.00% or less, still more preferably 1.00% or less. The content of Cr is
preferably 3.00% or less, more preferably 1.00% or less. The contents ofMo and Ware
each preferably 0.80% or less, more preferably 0.60% or less.
[0036]
Ti: 0 to 0.20%, Zr: 0 to 0.20%, Hf: 0 to 0.20%, V: 0 to 0.20%, Nb: 0 to 0.20%,
Ta: 0 to 0.20%, Sc: 0 to 0.20%, andY: 0 to 0.20%, and
0.003 :S: Ti + Zr+ Hf+ V + Nb + Ta+ Sc + Y :S: 0.20 (iv)
These elements have an effect of decreasing a block size of martensite and an
effect of preventing or reducing the formation of iron carbide, thereby preventing or
reducing the occurrence and the propagation of a crack from a stress concentration at the
time of crash deformation. Thus, at least one or more of these elements are contained,
and their total content is set at 0.003% or more. On the other hand, if the total content
is more than 0.20%, alloy precipitate precipitates in a large quantity, and thus cracking
tends to occur at the time of crash deformation; therefore, the total content is set at 0.20%
or less. The total content is preferably 0.010% or more.
[0037]
Sn: 0 to 0.020%, As: 0 to 0.020%, Sb: 0 to 0.020%, and Bi: 0 to 0.020%, and
Sn +As+ Sb + Bi::;; 0.020 (v)
Sn (tin), As (arsenic), Sb (antimony), and Bi (bismuth) are elements each of
which is used for obtaining a predetermined steel micro-structure, and therefore one or
more elements selected from these elements may be contained when necessary.
14
However, if their total content is more than 0.020%, a grain boundary fracture tendency
at the time of crash deformation increases; therefore, an upper limit of the total content is
set at 0.020%. A lower limit of the total content is not limited to a particular value, but
reducing the total content to less than 0.00005% leads to an increase in a production cost,
and thus 0.00005% is practically the lower limit.
[0038]
Mg: 0 to 0.005%, Ca: 0 to 0.005%, and REM: 0 to 0.005%
Mg (magnesium), Ca (calcium), and REM (rare earth metal) are elements each
of which has an action that controls morphology of oxides and sulfides, and therefore one
or more elements selected from these elements may be contained when necessary.
However, if a content of any one of the elements is more than 0.005%, the effect provided
by the addition of the element levels off, and energy absorption ability at the time of crash
deformation decreases; therefore, the content of any one of the elements is set at 0.005%
or less. Any one of the contents of Mg, Ca, and REM is preferably 0.003% or less.
When obtaining the effect described above is intended, one or more elements selected
from Mg: 0.001% or more, Ca: 0.001% or more, and REM: 0.001% or more are
preferably contained.
[0039]
Here, in the present invention, REM refers lanthanoids, which are 15 elements,
and the content of REM means a total content of the lanthanoids. In industrial practice,
the lanthanoids are added in a form of misch metal.
[0040]
Value of Ms: 200 or more
Ms means a martensitic transformation starting temperature (°C). If a Ms point
of a steel sheet is less than 200°C, an axial ratio increases, and it becomes difficult for the
configuration according to the present invention to prevent or reduce brittle fracture at
the time of crash deformation. For that reason, a value ofMs is set at 200 or more. The
value of Ms is preferably 220 or more.
Ms = 546 x exp(-1.362 x C)- 11 x Si- 30 x Mn- 18 x Ni- 20 x Cu- 12 x Cr-
8(Mo + W) (vi)
15
[0041]
In the chemical composition of the steel sheet according to the present invention,
the balance is Fe and impurities. The term "impurities" as used herein means
components that are mixed in the steel sheet in producing the steel sheet industrially due
to raw materials such as ores and scraps, and various factors of a producing process and
that are allowed to be mixed in the steel sheet within ranges in which the impurities have
no adverse effect on the present invention.
[0042]
(B) Steel Micro-Structure
A steel micro-structure of a steel sheet according to an embodiment of the present
embodiment will be described. In the following description, the symbol "%" means
"volume%".
[0043]
Martensite: 85% or more
Making the steel micro-structure mainly of martensite is indispensable for
ensuring a yield stress of 1000 MPa or more. If a volume ratio of martensite is less than
85%, it becomes difficult to ensure the yield stress: 1000 MPa or more. For that reason,
the volume ratio of martensite is set at 85% or more. In order to ensure the yield stress
stably, the volume ratio of martensite is preferably 90% or more. Note that the
martensite should be construed as including tempered martensite, that is, a martensite
with carbides formed therein. In addition, the morphology of martensite may be any
one oflath, butterfly, twin, lamella, and the like.
[0044]
Retained austenite: 15% or less
Retained austenite is a steel micro-structure that is useful in improving
formability and improving impact energy absorption properties. However, if its volume
ratio is more than 15%, there are a tendency of yield stress to decrease and a tendency of
brittle cracking to occur at the time of crash deformation. For that reason, the volume
ratio of retained austenite is set at 15% or less. The volume ratio of retained austenite
is preferably 12% or less. A lower limit of the volume ratio is not limited to a particular
16
value, but the volume ratio is preferably 0.1% or more.
[0045]
The remainder of the steel micro-structure is bainite. Here, the bainite includes
lower bainite and upper bainite, and additionally, bainitic ferrite (a.0B) described in Non
Patent Document 1 is categorized as the bainite. Note that tempered martensite is
difficult to subject to structure separation from bainite in some cases even with Reference
Document 1. In such a case where the structure separation is difficult, the bainite is
considered as martensite to calculate a structure separation fraction. Although there is
no need to place an upper limit on an area fraction of the bainite being the balance, the
area fraction is practically 15% or less, preferably 10% or less.
[0046]
A volume ratio of a steel micro-structure is determined according to the
following procedure. First, a 1/4 thickness portion of a surface of each steel sheet
parallel to a rolling direction and a thickness direction of the steel sheet is mirror-polished
and subjected to Nital etching. The surface is then subjected to structure observation
under a scanning electron microscope (SEM) or further a transmission electron
microscope (TEM), using a photograph of a steel micro-structure obtained by capturing
the steel micro-structure, the point counting method or image analysis is performed to
determine area fractions of martensite and bainite, and the area fractions are used as
volume ratios. In addition, the volume ratio of the retained austenite is determined by
the X-ray diffraction method. An area of a region to be observed is set at 1000 l!m2 or
more when a SEM is used, or 100 l!ffi2 or more when a TEM is used.
[0047]
Further, in the present invention, an average block size and an average axial ratio
of martensite and bainite are also defined as follows.
[0048]
Average block size of martensite and bainite: 3.0 !LID or less
A block size of martensite influences the occurrence and the propagation of a
brittle fracture at the time of crash deformation; the smaller a value of the block size is,
the better impact properties are obtained. If the average block size is more than 3.0 !Lffi,
17
a fracture of the sheet may occur at a bent portion at the time of crash deformation;
therefore, the average block size is set at 3.0 J.Uil or less. The average block size is
preferably 2. 7 J.Lm or less, 2.5 J.Uil or less, or 2.4 J.Lm or less.
[0049]
Here, the block size will be described. As shown in a table in p. 223 of NonPatent
Document 2, martensite and bainite can be classified as being made up of 24
different crystal units (variants) as their substructures. One of methods for grouping
these 24 variants is a method using Bain groups, which are described in p. 223 of NonPatent
Document 2, by which martensite and bainite can be classified into three crystal
units. The block size in the present invention indicates an average size of group grains
when the classification is performed using these Bain groups.
[0050]
The average block size is measured according to the following procedure.
First, each steel sheet is cut such that its surface parallel to its rolling direction and its
thickness direction serves as an observation surface, and the cross section is measured
between a 1/4 sheet-thickness position and a 1/2 sheet-thickness position of the cross
section by the EBSD method within a region having an area of 5000 J.Lm2 or more. A
step size of the measurement is set at 0.2 J.Lm. Then, based on crystal orientation
information obtained by the EBSD measurement, orientations are classified on the basis
of the three Bain groups, their images are displayed, and a size of a crystal unit is
determined by the cutting method described in Appendix 2 of ns G 0552.
[0051]
Average axial ratio of martensite and bainite: 1.0004 to 1.0100
A crystal structure of a portion of the steel micro-structure other than the retained
austenite, that is, martensite and bainite, influences cracking behavior at a stress
concentration and a bent portion at the time of crash deformation. It is particularly
necessary to appropriately adjust an average axial ratio of martensite and bainite, which
have a tetragonal crystal structure. Here, the axial ratio is a value expressed by c/a,
where a and c denotes a-axis and c-axis lattice constants in a tetragonal crystal structure,
respectively. The reason that a magnitude of the axial ratio c/a is associated with
18
cracking behavior at the time of a fast and large deformation in a collision test is unclear,
but crystal lattice strain may exert some influence on the cracking behavior.
[0052]
If the average axial ratio is less than 1.0004, cracking may occur at the time of
crash deformation, or there arises a tendency to resist absorption of impact energy. On
the other hand, if the average axial ratio is more than 1.0100, there arises a tendency of a
brittle fracture to occur from an end face or a bent portion of a member at the time of
crash deformation. For that reason, the average axial ratio is set at 1.0004 to 1.0100.
From the viewpoint of ensuring the yield stress stably, the average axial ratio is preferably
1.0006 or more. Further, in order to prevent or reduce cracking at the time of crash
deformation more reliably, the average axial ratio is preferably 1.0007 or more. On the
other hand, from the viewpoint of increasing the absorption of impact energy, the average
axial ratio is preferably 1.0080 or less.
[0053]
Here, the average axial ratio of martensite and bainite is measured by the X-ray
diffraction method according to the following procedure. At this time, the average axial
ratio c/a is to be determined by any one of the following two methods depending on
whether diffraction lines of tetragonal iron or cubic iron are split. Here, an area of a
region on a sample irradiated with X-ray is set at 0.2 mm2 or more.
[0054]
(a) In a case where a 200 diffraction line and a 002 diffraction line are split
clearly into two
The pseudo-Voigt function is used to perform peak separation of diffraction lines
from a {200} plane, a lattice constant calculated from a 200 diffraction angle is denoted
by a, a lattice constant calculated from a 002 diffraction angle is denoted by c, and their
ratio is determined as the average axial ratio c/a.
[0055]
(b) In a case where the diffraction lines are not split clearly into two
A lattice constant calculated from a diffraction angle of a diffraction from a
{200} plane is denoted by a, a lattice constant calculated from a diffraction angle from a
19
{ 110} plane is denoted by c', and their ratio c' /a is approximated as the average axial ratio
c/a (see Non Patent Document 3).
[0056]
Average particle size of iron carbides: 0.005 to 0.20 IJlD.
Iron carbide may be contained in a steel micro-structure of a steel sheet
according to another embodiment of the present invention. If an average particle size of
iron carbides is more than 0.20 IJ.ID., a fracture from a bent portion tends to accelerate
during crash deformation; on the other hand, if the average particle size of iron carbides
is less than 0. 005 J.Lm, a brittle fracture from a bent portion during crash deformation tends
to accelerate. For that reason, the average particle size of iron carbides is preferably
0.005 to 0.20 IJ.ID.. Note that the iron carbide may contain, in addition to Fe, alloying
elements such as Mn and Cr.
[0057]
An average particle size of iron carbides in martensite and bainite is measured
by observing their structures under a SEM and a TEM in a region having an area of 10
IJlD. 2 or more. Fine iron carbides that cannot be identified with the TEM are measured
by the atom probe method. In this case, the measurement is to be performed on five or
more iron carbides.
[0058]
(C) Plating layer
A steel sheet according to another embodiment of the present invention may
include a plating layer on its surface. A composition of the plating is not limited to a
particular composition, and any one of hot-dip galvanizing, galvannealing, and
electroplating may be employed.
[0059]
(D) Mechanical properties
Yield stress: 1000 MPa or more
If the yield stress is less than 1000 MPa, an advantage of reducing a member
weight provided by making the member thin-wall, and the yield stress is therefore set at
1000 MPa or more. Here, the yield stress is determined to be a flow stress (0.2% proof
20
stress) at 0.002 strain when a tensile test is performed in conformance with ns z 2241
2011.
[0060]
Although there is no particular limitation imposed on a tensile strength, the
tensile strength is preferably 1400 MPa or more from the viewpoint of enhancing impact
energy absorption properties.
[0061]
(E) Producing method
Although there is no particular limitation on conditions for producing the steel
sheet according to the present invention, the steel sheet can be produced by subjecting a
cast piece having the chemical composition described above to processing including steps
described below as (a) to (c). Each of the methods will be described in detail.
[0062]
Note that the cast piece can be obtained by a conventional method from a molten
steel having the chemical composition described above. The cast piece to be subjected
to hot rolling is not limited to a particular cast piece. That is, the cast piece may be a
continuously cast slab or a cast piece produced by a thin slab caster. In addition, the
method is applicable to a process such as continuous-casting direct-rolling, in which hot
rolling is performed immediately after casting.
[0063]
In the following description, the Ms point (°C), the Ac3 point (°C), and the Ar3
point (°C) are expressed by the following formulas, where symbols of elements represent
contents (mass%) of the elements in the steel sheet, and in a case where an element is not
contained, zero is assigned to its symbol.
Ms = 546 x exp(-1.362 x C)- 11 x Si- 30 x Mn- 18 x Ni- 20 x Cu- 12 x Cr-
8(Mo + W) (vi)
Ac3 = 910-203 x C05 + 44.7(Si + Al)- 30 x Mn + 700 x P- 15.2 x Ni- 26 x Cu
- 11 x Cr + 31.5 x Mo (vii)
Ar3 = 910-310 x C + 33 x Si- 80 x Mn- 55 x Ni- 20 x Cu- 15 x Cr- 80 x Mo
(viii)
21
[0064]
(a) Method including hot-rolling step, cold-rolling step, annealing step, and heat
treatment step
The cast piece described above is subjected to a hot-rolling step, a cold-rolling
step, an annealing step, and a heat treatment step in this order. In this case, a resultant
steel sheet is a cold-rolled steel sheet. Each of the steps will be described in detail.
[0065]
In the hot-rolling step, the cast piece is first heated. The heating temperature is
not limited to a particular temperature but is preferably set at 1200°C or more so that alloy
carbo-nitride that has precipitated during casting or rough rolling is remelted.
[0066]
After the heating, hot rolling is performed. At this time, an average cooling
rate for the range from a rolling finish temperature to 650°C is set at 8°C/s or more. If
the average cooling rate is less than 8°C/s, a block size of martensite in a finished product
increases, resulting in a deterioration in impact properties. Thereafter, the steel sheet is
coiled. The coiling temperature is not limited to a particular temperature but is
preferably 630°C or less. After being coiled, the steel sheet is further cooled to room
temperature.
[0067]
Subsequently, the steel sheet is subjected to treatment such as pickling then cold
rolling. As conditions for the cold rolling, the number of rolling passes and a rolling
reduction need not be particularly specified, and the conditions are only required to
conform to the conventional method.
[0068]
In the annealing step, the steel sheet subjected to the cold rolling is retained
within the temperature range from the Ac3 point to (Ac3 point+ 1 00)°C for 3 to 90 s. If
the annealing temperature is less than the Ac3 point, a predetermined amount of martensite
cannot be obtained, and if the annealing temperature is more than (Ac3 point+ 100)°C,
block size increases. In addition, if the retention time for the temperature range is less
than 3 s, the predetermined amount of martensite is not obtained, and a yield stress of
22
1000 MPa or more cannot be obtained. On the other hand, if the retention time is more
than 90 s, the block size increases. From the viewpoint of decreasing the block size, the
annealing temperature is preferably as low as possible and is preferably (Ac3 point +
80)°C or less. In addition, the retention time is preferably 10 s or more and is preferably
60s or less.
[0069]
After being retained within the temperature range for a predetermined time
period, the steel sheet is cooled under a condition that an average cooling rate for the
range from 700°C to (Ms point - 50)°C is 1 0°C/s or more. If this average cooling rate
is less than 10°C/s, the predetermined amount of martensite cannot be obtained, resulting
in the yield stress to decrease, and further, the block size increases, resulting in a tendency
of cracking to occur at the time of impact deformation. In a case where the setting of
the average axial ratio at 1.0007 or more is intended for preventing or reducing cracking
at the time of crash deformation more reliably, the average cooling rate is preferably
20°C/s or more. Note that a temperature at which this cooling is stopped is only required
to be (Ms - 50)°C or less and is not limited to a particular temperature but is preferably
1 00°C or more from the viewpoint of resistance to fracture.
[0070]
In the heat treatment step, a heat treatment that results in the following thermal
history is performed, according to Ms calculated from the chemical composition of the
steel sheet. Note that the following heat treatment may be performed subsequently to
stopping the cooling, or heating may be performed to a degree that does not exceed an
upper limit of a temperature range in the heat treatment step described below subsequently
to stopping the cooling.
[0071]
In a case where the Ms point calculated from the chemical composition of the
steel sheet is 250°C or more, a holding time for the temperature range from (Ms point +
50) to 250°C is set at 100 to 10000 s. If the holding time is less than 100 s, the average
axial ratio may exceed a predetermined value, causing a brittle fracture in a collision test
or failing to obtain a predetermined yield stress. On the other hand, if the holding time
23
is more than 10000 s, the average axial ratio becomes less than a predetermined value,
and additionally iron carbides coarsen, resulting in a tendency of cracking to occur at the
time of a crash. The holding time is preferably 400 s or more and is preferably 5000 s
or less. In particular, in a case where the setting of the average axial ratio at 1.0007 or
more is intended for preventing or reducing cracking at the time of crash deformation
more reliably, the holding time is more preferably 1500 s or less.
[0072]
In a case where the Ms point calculated from the chemical composition of the
steel sheet is less than 250°C, a holding time for the temperature range from (Ms point+
80) to 1 00°C is set at 100 to 50000 s. If the holding time is less than 100 s, the average
axial ratio may exceed the predetermined value, causing a brittle fracture in a collision
test. On the other hand, if the holding time is more than 50000 s, the average axial ratio
becomes less than a predetermined value, and additionally iron carbides coarsen, resulting
in a tendency of cracking to occur at the time of a crash. The holding time is preferably
400 s or more and is preferably 30000 s or less, more preferably 10000 s or less.
[0073]
(b) Method including hot-rolling step, annealing step, and heat treatment step
The cast piece described above is subjected to a hot-rolling step, an annealing
step, and a heat treatment step in this order. In this case, a resultant steel sheet is a hotrolled
steel sheet. Each of the steps will be described in detail.
[0074]
In contrast to the steps described in (a), the cold-rolling step is not performed in
the present step. In the annealing step, ferrite being a parent phase is recrystallized while
the cold-rolled steel sheet is heated from room temperature to the annealing temperature,
and crystallographic texture develops. Under the influence of this preferential
orientation of the crystal orientations, crystallographic texture also develops in austenite
that exists in retention within the temperature range from the Ac3 point to (Ac3 point +
100)°C. By the development of the crystallographic texture, when austenite with a
biased orientation transforms to martensite, crystals of the martensite are formed and
grow in a particular direction.
24
[0075]
In addition, since the formation and the growth of crystals of the martensite
causes the steel to expand, the steel sheet expands biasedly in the particular direction
macroscopically. However, allowing a steel strip to expand or deform freely in the
annealing step leads to a decrease in strip running properties; therefore, a tension is
usually applied to straighten a shape of the steel sheet and to keep the stability of strip
running.
[0076]
Note that if martensitic transformation occurs in a state where such an excessive
tension is applied, a residual stress is applied in the steel sheet, and it becomes difficult
to obtain an effect of preventing or reducing cracking. In addition, if the residual stress
in the steel sheet increases, a crack that occurs when the steel sheet is deformed is likely
to form and propagate. For that reason, from the viewpoint of preventing or reducing
cracking at the time of crash deformation more reliably, the cold-rolling step is preferably
omitted; that is, with an aim of preventing the development of crystallographic texture by
the recrystallization of ferrite being a parent phase in the annealing step to align the crystal
orientations on a random basis, the steel sheet according to an embodiment of the present
invention is preferably a hot-rolled steel sheet.
[0077]
In the hot-rolling step, the cast piece is first heated. The heating temperature is
not limited to a particular temperature but is preferably set at 1200°C or more so that alloy
carbo-nitride that has precipitated during casting or rough rolling is remelted.
[0078]
After the heating, hot rolling is performed. At this time, an average cooling
rate for the range from a rolling finish temperature to 650°C is set at 8°C/s or more. If
the average cooling rate is less than 8°C/s, a block size of martensite in a finished product
increases, resulting in a deterioration in impact properties. Thereafter, the steel sheet
may be coiled or need not be coiled but may be cooled to room temperature. In addition,
after the cooling, the steel sheet may be subjected to treatment such as pickling or may
be subjected to flattening.
25
[0079]
In the annealing step, the steel sheet subjected to the hot rolling is retained within
the temperature range from the Ac3 point to (Ac3 point+ 100)°C for 3 to 90s. If the
annealing temperature is less than the Ac3 point, a predetermined amount of martensite
cannot be obtained, and if the annealing temperature is more than (Ac3 point+ 100)°C,
block size increases. In addition, if the retention time for the temperature range is less
than 3 s, the predetermined amount of martensite is not obtained, and as a result, a yield
stress of 1000 MPa or more cannot be obtained. On the other hand, if the retention time
is more than 90 s, the block size increases. From the viewpoint of decreasing the block
size, the annealing temperature is preferably as low as possible and is preferably (Ac3
point + 80)°C or less. In addition, the retention time is preferably 10 s or more and is
preferably 60 s or less.
[0080]
After being retained within the temperature range for a predetermined time
period, the steel sheet is cooled under a condition that an average cooling rate for the
range from 700°C to (Ms point - 50)°C is 1 0°C/s or more. If this average cooling rate
is less than 10°C/s, the predetermined amount of martensite cannot be obtained, resulting
in the yield stress to decrease, and further, the block size increases, resulting in a tendency
of cracking to occur at the time of impact deformation. In a case where the setting of
the average axial ratio at 1.0007 or more is intended for preventing or reducing cracking
at the time of crash deformation more reliably, the average cooling rate is preferably
20°C/s or more. Note that a temperature at which this cooling is stopped is only required
to be (Ms - 50)°C or less and is not limited to a particular temperature but is preferably
1 00°C or more from the viewpoint of resistance to fracture.
[0081]
In the heat treatment step, a treatment that results in the following thermal history
is performed, according to Ms calculated from the chemical composition of the steel
sheet. Note that the following heat treatment may be performed subsequently to
stopping the cooling in the annealing step, or heating may be performed to a degree that
does not exceed an upper limit of a temperature range in the heat treatment step described
26
below subsequently to stopping the cooling.
[0082]
In a case where the Ms point calculated from the chemical composition of the
steel sheet is 250°C or more, a holding time for the temperature range from (Ms point +
50) to 250°C is set at 100 to 10000 s. If the holding time is less than 100 s, the average
axial ratio may exceed a predetermined value, causing a brittle fracture in a collision test
or failing to obtain a predetermined yield stress. On the other hand, if the holding time
is more than 10000 s, the average axial ratio becomes less than a predetermined value,
and additionally iron carbides coarsen, resulting in a tendency of cracking to occur at the
time of a crash. The holding time is preferably 400 s or more and is preferably 5000 s
or less. In particular, in a case where setting of the average axial ratio at 1.0007 or more
is intended for preventing or reducing cracking at the time of crash deformation more
reliably, the holding time is more preferably 1500 s or less.
[0083]
In a case where the Ms point calculated from the chemical composition of the
steel sheet is less than 250°C, a holding time for the temperature range from (Ms point+
80) to 100°C is set at 100 to 50000 s. If the holding time is less than 100 s, the average
axial ratio may exceed the predetermined value, causing a brittle fracture in a collision
test. On the other hand, if the holding time is more than 50000 s, the average axial ratio
becomes less than a predetermined value, and additionally iron carbides coarsen, resulting
in a tendency of cracking to occur at the time of a crash. The holding time is preferably
400 s or more and is preferably 30000 s or less, more preferably 10000 s or less.
[0084]
(c) Method including hot-rolling step and heat treatment step
The cast piece described above is subjected to a hot-rolling step and a heat
treatment step in this order. In this case, a resultant steel sheet is a hot-rolled steel sheet.
Each of the steps will be described in detail

We claim:
1. A steel sheet having a chemical composition consisting of, in mass%:
C: 0.14 to 0.60%,
Si: more than 0% to less than 3.00%,
Al: more than 0% to less than 3.00%,
Mn: 5.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
N: 0.015% or less,
B: 0 to 0.0050%,
Ni: 0 to 5.00%,
Cu: 0 to 5.00%,
Cr: 0 to 5.00%,
Mo: 0 to 1.00%,
W: 0 to 1.00%,
Ti: 0 to 0.20%,
Zr: 0 to 0.20%,
Hf: 0 to 0.20%,
V: 0 to 0.20%,
Nb: 0 to 0.20%,
Ta: 0 to 0.20%,
Sc: 0 to 0.20%,
Y: 0 to 0.20%,
Sn: 0 to 0.020%,
As: 0 to 0.020%,
Sb: 0 to 0.020%,
Bi: 0 to 0.020%,
Mg: 0 to 0.005%,
Ca: 0 to 0.005%, and
42
REM: 0 to 0.005%,
with the balance: Fe and impurities, and
satisfYing following formulas (i) to (v), wherein
a value ofMs expressed by a following formula (vi) is 200 or more,
a steel micro-structure contains, in volume%:
martensite: 85% or more, and
retained austenite: IS% or less,
with the balance: bainite,
an average block size of martensite and bainite: 3.0 IJlD. or less,
an average axial ratio of martensite and bainite: 1.0004 to 1.0100, and
a yield stress is 1000 MPa or more:
Si +AI:<;; 3.00 (i)
C X Mn :<;; 0.80 (ii)
Mn + Ni + Cu + 1.3Cr + 4(Mo + W) ~ 0.80 (iii)
0.003 :<;; Ti + Zr+ Hf+ V + Nb + Ta+ Sc + Y :<;; 0.20 (iv)
Sn +As+ Sb + Bi :<;; 0.020 (v)
Ms = 546 x exp(-1.362 x C)- 11 x Si- 30 x Mn- 18 x Ni- 20 x Cu- 12 x Cr-
8(Mo + W) (vi)
where symbols of elements represent contents (mass%) of the elements in the
steel sheet, and in a case where an element is not contained, zero is assigned to its symbol.
2. The steel sheet according to claim 1, wherein an average particle size of iron
carbides included in the steel micro-structure is 0.005 to 0.20 J.Lm.
3. The steel sheet according to claim 1 or claim 2, wherein the steel sheet
includes a plating layer on a surface of the steel sheet.
4. A method for producing the steel sheet according to any one of claim I to
claim 3, wherein
a cast piece having the chemical composition according to claim 1 is subjected
43
to a hot-rolling step, a cold-rolling step, an annealing step, and a heat treatment step in
this order,
in the hot-rolling step, the steel sheet is cooled to room temperature at an average
cooling rate for a range from a rolling fmish temperature to 650°C set at 8°C/s or more,
in the annealing step, the steel sheet is held within a temperature range from an
Ac3 point to (Ac3 point + 1 00)°C for 3 to 90 s, and
an average cooling rate for a range from 700°C to (Ms point - 50)°C is set at
10°C/s or more, and
in the heat treatment step,
in a case where the Ms point is 250°C or more,
a holding time for a temperature range from (Ms point + 50) to 250°C is set at
1 00 to 1 0000 s, and
in a case where the Ms point is less than 250°C,
a holding time for a temperature range from (Ms point + 80) to 100°C is set at
100 to 50000 s,
where the Ms point (0C} and the Ac3 point (0 C} are expressed by following
formulas, where symbols of elements represent contents (mass%) of the elements in the
steel sheet, and in a case where an element is not contained, zero is assigned to its symbol:
Ms = 546 x exp(-1.362 x C)- 11 x Si- 30 x Mn- 18 x Ni- 20 x Cu- 12 x Cr-
8(Mo + W) (vi)
Ac3 = 910-203 x C05 + 44.7(Si +AI)- 30 x Mn + 700 x P- 15.2 x Ni- 26 x Cu
- 11 x Cr + 31.5 x Mo (vii).
5. A method for producing the steel sheet according to any one of claim 1 to
claim 3, wherein
a cast piece having the chemical composition according to claim 1 is subjected
to a hot-rolling step, an annealing step, and a heat treatment step in this order,
in the hot-rolling step, the steel sheet is cooled to room temperature at an average
cooling rate for a range from a rolling fmish temperature to 650°C set at 8°C/s or more,
in the annealing step, the steel sheet is held within a temperature range from an
44
Ac3 to (Ac3 + 100)°C for 3 to 90s, and
an average cooling rate for a range from 700°C to (Ms - 50)°C is set at 1 0°C/s
or more, and
in the heat treatment step,
in a case where the Ms point is 250°C or more,
a holding time for a temperature range from (Ms + 50) to 250°C is set at 100 to
10000 s, and
in a case where the Ms point is less than 250°C,
a holding time for a temperature range from (Ms + 80) to 100°C is set at 100 to
50000 s,
where the Ms point (0C) and the Ac3 point (°C) are expressed by following
formulas, where symbols of elements represent contents (mass%) of the elements in the
steel sheet, and in a case where an element is not contained, zero is assigned to its symbol:
Ms = 546 x exp(-1.362 x C)- 11 x Si- 30 x Mn- 18 x Ni- 20 x Cu- 12 x Cr-
8(Mo + W) (vi)
Ac3 = 910-203 x C0·5 + 44.7(Si + Al)- 30 x Mn + 700 x P- 15.2 x Ni- 26 x Cu
- 11 x Cr + 31.5 x Mo (vii).
6. A method for producing the steel sheet according to any one of claim 1 to
claim 3, wherein
a cast piece having the chemical composition according to claim 1 is subjected
to a hot-rolling step and a heat treatment step in this order,
in the hot-rolling step, a rolling finish temperature is set at a Ar3 point or more,
and
an average cooling rate for a range from a rolling finish temperature to (Ms -
50)°C is set at 1 0°C/s or more, and
in the heat treatment step,
in a case where the Ms point is 250°C or more,
a holding time for a temperature range from (Ms +50) to 250°C is set at 100 to
10000 s, and