Specification
1
[Name of Document] DESCRIPTION
[Title of the Invention] HIGH-STRENGTH COLD-ROLLED STEEL SHEET
HAVING EXCELLENT LOCAL DEFORMABILITY AND
MANUFACTURING METHOD THEREOF
[Technical Field]
[0001] The present invention relates to a high-strength cold-rolled steel
sheet having excellent local deformability for bending, stretch flanging,
burring, and the like, and is mainly used for automobile parts and the like.
This application is based upon and claims the benefit of priority of the
prior Japanese Patent Application No. 2011-089250, filed on April 13, 2011,
the entire contents of which are incorporated herein by reference.
[Background Art]
[0002] In order to abate emission of carbon dioxide gas from automobiles,
a reduction in weight of automobile vehicle bodies has been promoted by
using high-strength steel sheets. Further, in order also to secure the safety of
a passenger, a high-strength steel sheet has been increasingly used for an
automobile vehicle body in addition to a soft steel sheet. In order to further
promote the reduction in weight of automobile vehicle bodies from now on, a
usage strength level of the high-strength steel sheet has to be increased more
than conventionally, and in order to use the high-strength steel sheet for an
underbody part, for example, local deformability for burring has to be
improved.
[0003] However, when a steel sheet is increased in strength in general,
formability decreases, and as shown in Non-Patent Document 1, uniform
elongation important for drawing and bulging decreases. In contrast to this,
as shown in Non-Patent Document 2, there is disclosed a method of securing
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uniform elongation even with the same strength by making a metal structure
of a steel sheet complex.
[0004] Meanwhile, there is also disclosed a metal structure control
method of a steel sheet that improves local ductility typified by bending, hole
expanding, and burring. Non-Patent Document 3 discloses that controlling
inclusions, making a structure uniform, and further decreasing hardness
difference between structures are effective for bendability and hole
expanding.
[0005] This is to improve hole expandability by making a structure
uniform by structure control, but in order to make a structure uniform, as
shown in Non-Patent Document 4, a heat treatment from an austenite single
phase becomes a basis of manufacture. Further, in order to achieve strength
and ductility, Non-Patent Document 4 also discloses a technique in which
metal structure control is performed by cooling control after hot rolling,
precipitates are controlled, and a transformation structure is controlled,
thereby obtaining appropriate fractions of ferrite and bainite.
[0006] Meanwhile, Patent Document 1 discloses a method in which a
finishing temperature of hot rolling, a reduction ratio and a temperature range
of finish rolling are controlled, recrystallization of austenite is promoted,
development of a rolled texture is suppressed, and crystal orientations are
randomized, thereby improving strength, ductility, and hole expandability.
[Prior Art Document]
[Patent Document]
[0007] Patent Document 1: Japanese Laid-open Patent Publication No.
2009-263718
[Non-patent Document]
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[0008] Non-Patent Document 1: Kishida, Nippon Steel Technical Report
(1999) No. 371, p. 13
Non-Patent Document 2: O. Matsumura et al., Trans. ISIJ (1987) vol.
27, p. 570
Non-Patent Document 3: Kato et al., Steelmaking Research (1984) vol.
312, p. 41
Non-Patent Document 4: K. Sugimoto et al., ISIJ International (2000)
Vol. 40, p. 920
[Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[0009] As described above, the main cause of deterioration of local
deformability is various "non-uniformities" of hardness difference between
structures, non-metal inclusions, a developed rolled texture, and the like.
The most effective one among them is the hardness difference between
structures described in Non-Patent Document 3 above, and as another
effective controlling factor, the developed rolled texture described in Patent
Document 1 can be cited. These elements are mixed in a complex manner,
and the local deformability of a steel sheet is determined. Therefore, for
maximizing an increased margin of the local deformability by texture control,
structure control is performed in a combined manner, and it is necessary to
eliminate the non-uniformity ascribable to the hardness difference between
structures as mush as possible.
[0010] Thus, the present invention is to provide a high-strength
cold-rolled steel sheet having excellent local deformability capable of
improving local ductility of the high-strength steel sheet and also capable of
improving anisotropy in the steel sheet by making a metal structure in which
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an area ratio of bainite is 95% or more, together with controlling a texture,
and a manufacturing method thereof.
[Means for Solving the Problems]
[0011] According to the conventional knowledge, as described above, the
improvements of hole expandability, bendability, and the like have been
performed by inclusion control, making precipitates fine, structure
homogenization, turning structures into a single phase, a decrease in hardness
difference between structures, and the like. However, these are not sufficient,
so that an effect on anisotropy is concerned in a high-strength steel sheet to
which Nb, Ti, and the like are added. This causes problems that other
forming factors are sacrificed, the direction in which a blank before forming
is taken is limited, and the like, and use is also limited.
[0012] Thus, the present inventors, in order to improve hole
expandability and bending workability, newly focused attention on the effect
of a texture of a steel sheet and examined and studied its functional effect in
detail. As a result, they revealed that by controlling intensities of respective
orientations of a specific crystal orientation group, the local deformability
improves drastically without the elongation and strength decreasing greatly.
The point where emphasis should be placed is that they also revealed that an
improved margin of the local deformability by the texture control greatly
relays on a steel structure, a metal structure in which an area ratio of bainite is
95% or more is made, and thereby the improved margin of the local
deformability is maximized on the basis that the strength of the steel is
secured. Additionally, they found that in a structure in which intensities of
respective orientations of a specific crystal orientation group are controlled,
the size of a grain unit greatly affects the local ductility.
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[0013] Generally, in a structure in which low-temperature generating
phases (bainite, martensite, and the like) are mixed, the definition of crystal
grains is extremely vague and quantification of them is difficult. In contrast
to this, the present inventors found it possible to solve the problem of the
quantification of crystal grains if a "grain unit" of crystal grains is determined
in the following manner.
[0014] The "grain unit" of crystal grains determined in the present
invention is determined in the following manner in an analysis of orientations
of a steel sheet by an EBSP (Electron Back Scattering Pattern). That is, in
an analysis of orientations of a steel sheet by an EBSP, for example,
orientations are measured at 1500 magnifications with a measured step of 0.5
um or less, and a position at which a misorientation between adjacent
measured points exceeds 15° is set to a boundary between crystal grains.
Then, a region surrounded with this boundary is determined to be the "grain
unit" of crystal grains.
[0015] With respect to crystal grains of the grain unit determined in this
manner, a circle-equivalent diameter d is obtained and the volume of crystal
grains of each grain unit is obtained by 4/3Tid3. Then, a weighted mean of
the volume is calculated and a mean volume diameter (Mean Volume
Diameter) is obtained.
[0016] The present invention is constituted based on the previously
described knowledge and the gist thereof is as follows.
[1]
A high-strength cold-rolled steel sheet having excellent local deformability
contains:
in mass%,
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C: not less than 0.02% nor more than 0.20%;
Si: not less than 0.001% nor more than 2.5%;
Mn: not less than 0.01%o nor more than 4.0%;
P: not less than 0.001% nor more than 0.15%;
S: not less than 0.0005% nor more than 0.03%;
Al: not less than 0.001% nor more than 2.0%;
N: not less than 0.0005% nor more than 0.01%; and
O: not less than 0.0005% nor more than 0.01%; in which Si + Al is limited to
less than 1.0%, and
a balance being composed of iron and inevitable impurities, in which
an area ratio of bainite in a metal structure is 95% or more,
at a sheet thickness center portion being a range of 5/8 to 3/8 in sheet
thickness from the surface of the steel sheet, an average value of pole
densities of the {100}<011> to {223}<110> orientation group represented by
respective crystal orientations of {100}<011>, {116}<110>, {114}<110>,
{113}<110>, {112}<110>, {335}<110>, and {223}<110> is 4.0 or less, and a
pole density of the {332}<113> crystal orientation is 5.0 or less, and
a mean volume diameter of crystal grains in the metal structure is 7 urn or
less.
[2]
The high-strength cold-rolled steel sheet having excellent local deformability
according to [1], in which
to crystal grains of the bainite, a ratio of the crystal grains in which a ratio of
a length dL in a rolling direction to a length dt in a sheet thickness direction:
dL/dt is 3.0 or less is 50% or more.
[3]
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The high-strength cold-rolled steel sheet having excellent local deformability
according to [1], further contains:
one type or two or more types of
in mass%,
Ti: not less than 0.001% nor more than 0.20%,
Nb: not less than 0.001% nor more than 0.20%,
V: not less than 0.001%) nor more than 1.0%, and
W: not less than 0.001% nor more than 1.0%.
[4]
The high-strength cold-rolled steel sheet having excellent local deformability
according to [1], further contains:
one type or two or more types of
in mass%,
B: not less than 0.0001% nor more than 0.0050%,
Mo: not less than 0.001% nor more than 1.0%,
Cr: not less than 0.001% nor more than 2.0%,
Cu: not less than 0.001% nor more than 2.0%,
Ni: not less than 0.001% nor more than 2.0%,
Co: not less than 0.0001% nor more than 1.0%,
Sn: not less than 0.0001% nor more than 0.2%,
Zr: not less than 0.0001% nor more than 0.2%, and
As: not less than 0.0001% nor more than 0.50%.
[5]
The high-strength cold-rolled steel sheet having excellent local deformability
according to [1], further contains:
one type or two or more types of
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in mass%,
Mg: not less than 0.0001% nor more than 6.010%,
REM: not less than 0.0001 % nor more than 0.1 %, and
Ca: not less than 0.0001% nor more than 0.010%.
[6]
The high-strength cold-rolled steel sheet having excellent local deformability
according to [1], in which
on the surface, a hot-dip galvanized layer or an alloyed hot-dip galvanized
layer is provided.
[7]
A manufacturing method of a high-strength cold-rolled steel sheet having
excellent local deformability, includes:
on a steel billet containing:
in mass%,
C: not less than 0.02% nor more than 0.20%;
Si: not less than 0.001%) nor more than 2.5%;
Mn: not less than 0.01% nor more than 4.0%;
P: not less than 0.001% nor more than 0.15%;
S: not less than 0.0005% nor more than 0.03%;
Al: not less than 0.001% nor more than 2.0%;
N: not less than 0.0005% nor more than 0.01%; and
O: not less than 0.0005%) nor more than 0.01%; in which Si + Al is limited to
less than 1.0%, and
a balance being composed of iron and inevitable impurities,
performing first hot rolling in which rolling at a reduction ratio of 40% or
more is performed one time or more in a temperature range of not lower than
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1000°C nor higher than 1200°C;
setting an austenite grain diameter to 200 jam or less by the first hot rolling;
performing second hot rolling in which rolling at a reduction ratio of 30% or
more is performed in one pass at least one time in a temperature region of not
lower than a temperature Tl + 30°C nor higher than Tl + 200°C determined
by Expression (1) below;
setting the total reduction ratio in the second hot rolling to 50% or more;
performing final reduction at a reduction ratio of 30% or more in the second
hot rolling and then starting primary cooling in such a manner that a waiting
time t second satisfies Expression (2) below;
setting an average cooling rate in the primary cooling to 50°C/second or more
and performing the primary cooling in a manner that a temperature change is
in a range of not lower than 40°C nor higher than 140°C;
performing cold rolling at a reduction ratio of not less than 30% nor more
than 70%;
performing holding for 1 to 300 second/seconds in a temperature region of
Ae3 to 950°C;
performing secondary cooling at an average cooling rate of not less than
10°C/s nor more than 200°C/s in a temperature region of Ae3 to 500°C; and
performing an overaging heat treatment in which holding is performed for not
shorter than t2 seconds satisfying Expression (4) below nor longer than 400
seconds in a temperature region of not lower than 350°C nor higher than
500°C.
Tl (°C) = 850 + 10 x (C + N) x Mn + 350 x Nb + 250 x Ti + 40 x B + 10 x
Cr+100xMo + 1 0 0 x V - ( l )
t ^ 2.5 x tl - (2)
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Here, tl is obtained by Expression (3) below.
tl = 0.001 x ((Tf - Tl) x Pl/100)2 - 0.109 x ((Tf - Tl) x Pl/100) + 3.1 - (3)
Here, in Expression (3) above, Tf represents the temperature of the steel billet
obtained after the final reduction at a reduction ratio of 30% or more, and PI
represents the reduction ratio of the final reduction at 30% or more.
log(t2) = 0.0002(T2 - 425)2 + 1.18 ... (4)
Here, T2 represents an overaging treatment temperature, and the maximum
value of t2 is set to 400.
[8]
The manufacturing method of the high-strength cold-rolled steel sheet having
excellent local deformability according to [7], in which
the total reduction ratio in a temperature range of lower than Tl + 30°C is
30% or less.
[9]
The manufacturing method of the high-strength cold-rolled steel sheet having
excellent local deformability according to [7], in which
the waiting time t second further satisfies Expression (2a) below.
t < t l - ( 2 a )
[10]
The manufacturing method of the high-strength cold-rolled steel sheet having
excellent local deformability according to [7], in which
the waiting time t second further satisfies Expression (2b) below.
tl ^ t S tl x 2.5 - (2b)
[11]
The manufacturing method of the high-strength cold-rolled steel sheet having
excellent local deformability according to [7], in which
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the primary cooling is started between rolling stands.
[12]
The manufacturing method of the high-strength cold-rolled steel sheet having
excellent local deformability according to [7], in which
when heating is performed up to the temperature region of Ae3 to 950°C after
the cold rolling, an average heating rate of not lower than room temperature
nor higher than 650°C is set to HR1 (°C/second) expressed by Expression (5)
below, and
an average heating rate of higher than 650°C to Ae3 to 950°C is set to HR2
(°C/second) expressed by Expression (6) below.
HR1 ^ 0.3 ... (5)
HR2 ^ 0.5 x HR1 ... (6)
[13]
The manufacturing method of the high-strength cold-rolled steel sheet having
excellent local deformability according to [7], further includes:
forming a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on
the surface.
[Effect of the Invention]
[0017] According to the present invention, it is possible to obtain a
high-strength cold-rolled steel sheet having excellent local deformability for
bending, stretch flanging, burring, and the like by controlling a texture and a
steel structure of the steel sheet.
[Brief Description of the Drawings]
[0018]
[FIG. 1] FIG. 1 shows the relationship between an average value of
pole densities of the {100}<011> to {223}<110> orientation group and a
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sheet thickness/ a minimum bend radius;
[FIG. 2] FIG. 2 shows the relationship between a pole density of the
{332}<113> crystal orientation and the sheet thickness/the minimum bend
radius;
[FIG. 3] FIG. 3 shows the relationship between the number of times of
rolling at 40% or more in rough rolling and an austenite grain diameter in the
rough rolling;
[FIG. 4] FIG. 4 shows the relationship between a reduction ratio at Tl
+ 30 to Tl + 200°C and the average value of the pole densities of the
{100}<011> to {223}<110> orientation group;
[FIG. 5] FIG. 5 shows the relationship between the reduction ratio at
Tl + 30 to Tl + 200°C and the pole density of the {332}<113> crystal
orientation;
[FIG. 6] FIG. 6 is an explanatory view of a continuous hot rolling line;
[FIG. 7] FIG. 7 shows the relationship between strength and hole
expandability of present invention steels and comparative steels; and
[FIG. 8] FIG. 8 shows the relationship between the strength and
bendability of the present invention steels and the comparative steels.
[Mode for Carrying out the Invention]
[0019] Hereinafter, the contents of the present invention will be explained
in detail.
[0020] (Crystal orientation)
First, there will be explained an average value of pole densities of the
{100}<011> to {223}<110> orientation group and a pole density of the
{332}<113> crystal orientation at a sheet thickness center portion being a
range of 5/8 to 3/8 in sheet thickness from a surface of a steel sheet.
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[0021] In a cold-rolled steel sheet of the'present invention, an average
value of pole densities of the {100}<011> to {223}<110> orientation group
and a pole density of the {332}<113> crystal orientation at a sheet thickness
center portion being a range of 5/8 to 3/8 in sheet thickness from the surface
of the steel sheet are particularly important characteristic values.
[0022] As shown in FIG. 1, when X-ray diffraction is performed at the
sheet thickness center portion being the range of 5/8 to 3/8 in sheet thickness
from the surface of the steel sheet to obtain pole densities of respective
orientations, the average value of the pole densities of the {100}<011> to
{223}<110> orientation group is less than 4.0 and it is possible to satisfy a
sheet thickness/a bend radius ^ 1.5 that is required to work a framework
part to be required recently. Additionally, when in a steel structure, 95% or
more of a bainite fraction is satisfied, the sheet thickness/the bend radius ^
2.5 is satisfied. When hole expandability and small limited bendability are
required, the average value of the pole densities of the {100}<011> to
{223}<110> orientation group is desirably less than 3.0.
[0023] When this value is 4.0 or more, anisotropy of mechanical
properties of the steel sheet becomes strong extremely, and further local
deformability only in a certain direction is improved, but a material in a
direction different from it deteriorates significantly, resulting in that it
becomes impossible to satisfy the sheet thickness/the bend radius ^ 1.5.
On the other hand, when this value becomes less than 0.5, which is difficult to
be achieved in a current general continuous hot rolling process, the
deterioration of the local deformability is concerned.
[0024] The {100}<011>, {116}<110>, {114}<110>, {113}<110>,
{112}<110>, {335}<110>, and {223}<110> orientations are included in this
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orientation group.
[0025] The pole density is synonymous with an X-ray random intensity
ratio. The pole density (X-ray random intensity ratio) is a numerical value
obtained by measuring X-ray intensities of a standard sample not having
accumulation in a specific orientation and a test sample under the same
conditions by X-ray diffractometry or the like and dividing the obtained X-ray
intensity of the test sample by the X-ray intensity of the standard sample.
This pole density can be measured by any one of X-ray diffraction, an EBSP
(Electron Back Scattering Pattern) method, and an ECP (Electron Channeling
Pattern) method.
[0026] As for the pole density of the {100}<011> to {223}<110>
orientation group, for example, pole densities of respective orientations of
{100}<011>, {116}<110>, {114}<110>, {112}<110>, and {223}<110> are
obtained from a three-dimensional texture (ODF) calculated by a series
expansion method using a plurality (preferably three or more) of pole figures
out of pole figures of {110}, {100}, {211}, and {310} measured by the
method, and these pole densities are arithmetically averaged, and thereby the
pole density of the above-described orientation group is obtained.
Incidentally, when it is impossible to obtain the intensities of all the
above-described orientations, the arithmetic average of the pole densities of
the respective orientations of {100}<011>, {116}<110>, {114}<110>,
{112}<110>, and {223}<110> may also be used as a substitute.
[0027] For example, for the pole density of each of the above-described
crystal orientations, each of intensities of (001)[1-10], (116)[1-10],
(114)[1-10], (113)[1-10], (112)[1-10], (335)[1-10], and (223)[1-10] at a 2 =
45° cross-section in the three-dimensional texture may be used as it is.
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[0028] Further, due to the similar reason, the pole density of the
{332}<113> crystal orientation of the sheet plane at the sheet thickness center
portion being the range of 5/8 to 3/8 in sheet thickness from the surface of the
steel sheet has to be 5.0 or less as shown in FIG. 2. As long as it is 3.0 or
less desirably, the sheet thickness/the bend radius ^ 1 . 5 that is required to
work a framework part to be required recently is satisfied. Additionally,
when in the steel structure, 95% or more of the bainite fraction is satisfied, the
sheet thickness/the bend radius ^ 2.5 is satisfied. On the other hand, when
the pole density of the {332}<113> crystal orientation is greater than 5.0, the
anisotropy of the mechanical properties of the steel sheet becomes strong
extremely, and further the local deformability only in a certain direction is
improved, but the material in a direction different from it deteriorates
significantly, resulting in that it becomes impossible to securely satisfy the
sheet thickness/the bend radius ^ 1.5. Further, when the pole density
becomes less than 0.5, which is difficult to be achieved in a current general
continuous hot rolling process, the deterioration of the local deformability is
concerned.
[0029] The reason why the pole densities of the above-described crystal
orientations are important for shape freezing property at the time of bending
working is not necessarily obvious, but is inferentially related to slip behavior
of crystal at the time of bending deformation.
[0030] With regard to the sample to be subjected to the X-ray diffraction,
EBSP method, or ECP method, the steel sheet is reduced in thickness to a
predetermined sheet thickness from the surface by mechanical polishing or
the like. Next, strain is removed by chemical polishing, electrolytic
polishing, or the like, and the sample is fabricated in such a manner that in the
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range of 5/8 to 3/8 in sheet thickness, an appropriate plane becomes a
measuring plane. For example, on a steel piece in a size of 30 mm<|) cut out
from the position of 1/4 W or 3/4 W of the sheet width W, grinding with fine
finishing (centerline average roughness Ra: 0.4a to 1.6a) is performed. Next,
by chemical polishing or electrolytic polishing, strain is removed, and the
sample to be subjected to the X-ray diffraction is fabricated. With regard to
the sheet width direction, the steel piece is desirably taken from, of the steel
sheet, the position of 1/4 or 3/4 from an end portion.
[0031] As a matter of course, the pole density satisfies the
above-described pole density limited range not only at the sheet thickness
center portion being the range of 5/8 to 3/8 in sheet thickness from the surface
of the steel sheet, but also at as many thickness positions as possible, and
thereby local ductility performance (local elongation) is further improved.
However, the range of 5/8 to 3/8 from the surface of the steel sheet is
measured, to thereby make it possible to represent the material property of the
entire steel sheet generally. Thus, 5/8 to 3/8 of the sheet thickness is
prescribed as the measuring range.
[0032] Incidentally, the crystal orientation represented by {hkl}
means that the normal direction of the steel sheet plane is parallel to
and the rolling direction is parallel to . With regard to the crystal
orientation, normally, the orientation vertical to the sheet plane is represented
by [hkl] or {hkl} and the orientation parallel to the rolling direction is
represented by (uvw) or . {hkl} and are generic terms for
equivalent planes, and [hkl] and (uvw) each indicate an individual crystal
plane. That is, in the present invention, a body-centered cubic structure is
targeted, and thus, for example, the (111), (-111), (1-11), (11-1), (-1-11),
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(-11-1), (1-1-1), and (-1-1-1) planes are equivalent to make it impossible to
make them different. In such a case, these orientations are generically
referred to as {111}. In an ODF representation, [hkl](uvw) is also used for
representing orientations of other low symmetric crystal structures, and thus it
is general to represent each orientation as [hkl](uvw), but in the present
invention, [hkl](uvw) and {hkl} are synonymous with each other.
The measurement of crystal orientation by an X ray is performed according to
the method described in, for example, Cullity, Elements of X-ray Diffraction,
new edition (published in 1986, translated by MATSUMURA, Gentaro,
published by AGNE Inc.) on pages 274 to 296.
[0033] (Mean volume diameter of crystal grains)
The present inventors earnestly examined texture control of a
hot-rolled steel sheet. As a result, it was found that under the condition that
a texture is controlled as described above, the effect of crystal grains in a
grain unit on the local ductility is extremely large and the crystal grains are
made fine, thereby making it possible to obtain drastic improvement of the
local ductility. Incidentally, as described above, the "grain unit" of the
crystal grains is determined in a manner that the position at which a
misorientation exceeds 15° is set as a boundary of crystal grains in an analysis
of orientations of the steel sheet by the EBSP.
[0034] As above, the reason why the local ductility improves is not
obvious. However, it is conceivably because when the texture of the steel
sheet is randomized and the crystal grains are made fine, local strain
concentration to occur in micron order is suppressed, homogenization of
deformation is increased, and strain is dispersed uniformly in micron order.
[0035] As there are more large crystal grains even though the number of
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them is small, the deterioration of the local ductility becomes larger.
Therefore, the size of the crystal grains is not an ordinary size mean, and a
mean volume diameter defined as a weighted mean of volume is strongly
correlated with the local ductility. In order to obtain this effect, the mean
volume diameter of the crystal grains needs to be 7 urn or less. It is
desirably 5 urn or less in order to secure the hole expandability at a higher
level. Incidentally, the method of measuring crystal grains is set as
described previously.
[0036] (Equiaxial property of crystal grains)
As a result of further pursuit of the local ductility, the present
inventors also found that when equiaxial property of the crystal grains is
excellent on the condition that the above-described texture and the size of the
crystal grains are satisfied, the local ductility improves. As an index
indicating this equiaxial property, with respect to the crystal grains expressed
by the grain unit, a ratio of the grains excellent in equiaxial property in which
dL/dt, being a ratio of, of the crystal grains, a length dL in a cold rolling
direction to a length dt in a sheet thickness direction, is 3.0 or less needs to be
at least 50% or more to all the bainite grains.
[0037] (Chemical composition)
Subsequently, there will be described limiting conditions of
components. Incidentally, % of each content is mass%.
[0038] C: not less than 0.02% nor more than 0.20%
The lower limit of C is set to 0.02% in order to have 95% or more of
bainite in the steel structure. Further, C is an element increasing strength, to
thus be preferably set to 0.025%) or more in order to secure the strength. On
the other hand, when the C content exceeds 0.20%, weldability is sometimes
19
impaired, and workability sometimes deteriorates extremely due to an
increase in a hard structure, and thus the upper limit is set to 0.20%. Further,
when the C content exceeds 0.10%, formability deteriorates, so that the C
content is preferably set to 0.10% or less.
[0039] Si: not less than 0.001% nor more than 2.5%
Si is an element effective for increasing mechanical strength of the
steel sheet, but when Si becomes greater than 2.5%, the workability
deteriorates and a surface flaw occurs, so that this is set to the upper limit.
Further, when the Si content is large, a chemical conversion treatment
property decreases, so that Si is preferably set to 1.20% or less. On the other
hand, it is difficult to set Si to less than 0.001% in a practical steel, so that this
is set to the lower limit.
[0040] Mn: not less than 0.01 % nor more than 4.0%
Mn is also an element effective for increasing the mechanical strength
of the steel sheet, but when Mn becomes greater than 4.0%, the workability
deteriorates, so that this is set to the upper limit. On the other hand, it is
difficult to set Mn to less than 0.01% in a practical steel, so that this is set to
the lower limit. Further, when elements such as Ti that suppress occurrence
of hot cracking caused by S are not sufficiently added except Mn, the Mn
amount satisfying Mn/S ^ 20 in mass% is desirably added. Further, Mn is
an element that, with an increase in the content, expands an austenite region
temperature to a low temperature side, improves hardenability, and facilitates
formation of a continuous cooling transformation structure having excellent
burring workability. This effect is not easily exhibited when the Mn content
is less than 1%, so that 1% or more is desirably added.
[0041] P: not less than 0.001% nor more than 0.15%
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S: not less than 0.0005% nor more than 0.03%
With regard to the upper limits of P and S, P is set to 0.15% or less
and S is set to 0.03% or less respeetively. This is to prevent deterioration of
the workability and cracking at the time of hot rolling or cold rolling. With
regard to the lower limits of P and S, P is set to 0.001%) and S is set to
0.0005%), as values applicable in current general refining (including
secondary refining).
[0042] Al: not less than 0.001 % nor more than 2.0%
For deoxidation, 0.001% or more of Al is added. When deoxidation
is needed sufficiently, 0.01%) or more is preferably added. Further, Al is also
an element significantly increasing a y to a transformation point. However,
when it is too much, the weldability deteriorates, so that the upper limit is set
to 2.0%. It is preferably set to 1.0% or less.
[0043] N: not less than 0.0005% nor more than 0.01%
O: not less than 0.0005% nor more than 0.01 %
N and O are impurities, and are both set to 0.01%> or less in order to
prevent the workability from deteriorating. The lower limits of the both
elements are set to 0.0005%) that is applicable in current general refining
(including secondary refining). However, they are preferably set to 0.001%
or more in order to suppress an extreme increase in steelmaking cost.
[0044] Si + Al: less than 1.0%
When Si and Al are contained excessively, cementite precipitation
during an overaging treatment is suppressed and the fraction of retained
austenite becomes too large, so that the total added amount of Si and Al is set
to less than 1%).
[0045] Ti: not less than 0.001% nor more than 0.20%
21
Nb: not less than 0.001% nor more than 0.20%>
V: not less than 0.001% nor more than 1.0%
W: not less than 0.001 % nor more than 1.0%
Further, when the strength is obtained by precipitation strengthening,
it is preferred to generate fine carbonitride. For obtaining precipitation
strengthening, it is effective to add Ti, Nb, V, and W, and one type or two or
more types of them may be contained.
[0046] In order to obtain this effect by adding Ti, Nb, V, and W, it is
necessary to add 0.001% of Ti, 0.001% of Nb, 0.001% or more of V, and
0.001% or more of W. When the precipitation strengthening is particularly
needed, it is desired to add 0.01% or more of Ti, 0.005% or more of Nb,
0.01% or more of V, and 0.01% or more of W. However, even when they are
added excessively, the strength increase is saturated, and additionally
recrystallization after hot rolling is suppressed, to thereby make it difficult to
perform crystal orientation control after cold rolling annealing, so that Ti
needs to be set to 0.20% or less, Nb needs to be set to 0.20% or less, V needs
to be set to 1.0% or less, and W needs to be set to 1.0% or less.
[0047] B: not less than 0.0001% nor more than 0.0050%
Mo: not less than 0.001% nor more than 1.0%
Cr: not less than 0.001% nor more than 2.0%
Cu: not less than 0.001% nor more than 2.0%
Ni: not less than 0.001% nor more than 2.0%
Co: not less than 0.0001% nor more than 1.0%
Sn: not less than 0.0001% nor more than 0.2%
Zr: not less than 0.0001 % nor more than 0.2%
As: not less than 0.0001% nor more than 0.50%
22
When the strength is secured by increasing the hardenability of the
structure to perform second phase control, it is effective to add one type or
two or more types of B, Mo, Cr, Cu, Ni, Co, Sn, Zr, and As. In order to
obtain this effect, it is necessary to add 0.0001% or more of B, 0.001% or
more of each of Mo, Cr, Cu, and Ni, and 0.0001% or more of each of Co, Sn,
Zr, and As. However, when they are added excessively, the workability is
deteriorated by contraries, so that the upper limit of B is set to 0.0050%), the
upper limit of Mo is set to 1.00%), the upper limit of each of Cr, Cu, and Ni is
set to 2.0%), the upper limit of Co is set to 1.0%, the upper limit of each of Sn
and Zr is set to 0.2%, and the upper limit of As is set to 0.50%.
[0048] Mg: not less than 0.0001% nor more than 0.010%
REM: not less than 0.0001% nor more than 0.1%
Ca: not less than 0.0001%o nor more than 0.010%)
Mg, REM, and Ca are important elements to be added for improving
local formability and making inclusions harmless. In order to obtain this
effect, the lower limit of each of them is set to 0.0001%). On the other hand,
excessive additions lead to deterioration of cleanliness, so that 0.010%) is set
as the upper limit of Mg, 0.1 % is set as the upper limit of REM, and 0.010%)
is set as the upper limit of Ca.
[0049] (Metal structure)
Next, there will be explained a metal structure of the cold-rolled steel
sheet of the present invention.
[0050] The metal structure of the cold-rolled steel sheet of the present
invention has a bainite area ratio of 95%> or more and is preferably a bainite
single phase. This is because the metal structure is composed of bainite,
thereby making it possible to achieve the strength and the hole expandability.
23
Further, this structure is generated by transformation at relatively high
temperature, to thus have no necessity to be cooled down to low temperature
when being manufactured, and is a preferred structure also in terms of
material stability and productivity.
[0051] As the balance, 5% or less of pro-eutectoid ferrite, pearlite,
martensite, and retained austenite is allowed. Pro-eutectoid ferrite has no
problem as long as it is precipitation-strengthened sufficiently, but
pro-eutectoid ferrite sometimes becomes soft depending on the chemical
composition, and when the area ratio becomes greater than 5%, the hole
expandability slightly decreases due to hardness difference from bainite.
Further, when an area ratio of pearlite becomes greater than 5%, the strength
and the workability sometimes deteriorate. When area ratios of martensite
and retained austenite to be strain-induced transformed to martensite become
1% or more and greater than 5% respectively, an interface between bainite
and a structure harder than bainite becomes a starting point of cracking and
the hole expandability deteriorates.
[0052] Thus, as long as the area ratio of bainite is set to 95% or more, the
area ratio of pro-eutectoid ferrite, pearlite, martensite, and retained y being the
balance becomes 5% or less, so that the strength and the hole expandability
are well balanced. However, it is necessary to set martensite to less than 1%
as described above.
[0053] Here, bainite in the present invention is a microstructure defined
as a continuous cooling transformation structure (Zw) positioned at an
intermediate stage between a microstructure containing polygonal ferrite and
pearlite to be generated by a diffusive mechanism and martensite to be
generated by a non-diffusive shearing mechanism, as is described in The Iron
24
and Steel Institute of Japan, Society of basic research, Bainite Research
Committee/Edition; Recent Research on Bainitic Microstructures and
Transformation Behavior of Low Carbon Steels - Final Report of Bainite
Research Committee (in 1994, The Iron and Steel Institute of Japan).
[0054] That is, the continuous cooling transformation structure (Zw) is
defined as a microstructure mainly composed of Bainitic ferrite (a°B),
Granular bainitic ferrite (aB), and Quasi-polygonal ferrite (aq), and further
containing a small amount of retained austenite (yr) and Martensite-austenite
(MA) as is described in the above-described reference literature on pages 125
to 127 as an optical microscopic observation structure.
[0055] Incidentally, similarly to polygonal ferrite (PF), an internal
structure of aq does not appear by etching, but a shape of aq is acicular, and it
is definitely distinguished from PF. Here, on the condition that of a targeted
crystal grain, a peripheral length is set to lq and a circle-equivalent diameter is
set to dq, a grain having a ratio (lq/dq) of them satisfying lq/dq ^ 3.5 is aq.
[0056] The continuous cooling transformation structure (Zw) of the
present invention is defined as a microstructure containing one type or two or
more types of a°B, aB, aq, yr, and MA. Incidentally, the total content of yr
and MA being small in amount is set to 3% or less.
[0057] There is sometimes a case that this continuous cooling
transformation structure (Zw) is not easily discerned by observation by
optical microscope in etching using a nital reagent. In such a case, it is
discerned by using the EBSP-OIM™.
[0058] The EBSP-OIM (Electron Back Scatter Diffraction
Pattern-Orientation Image Microscopy, registered trademark) method is
constituted by a device and software in which a highly inclined sample in a
25
scanning electron microscope SEM (Scanning Electron Microscope) is
irradiated with electron beams, a Kikuchi pattern formed by backscattering is
photographed by a high-sensitive camera and is image processed by a
computer, and thereby a crystal orientation at an irradiation point is measured
for a short time period.
[0059] In the EBSP method, it is possible to quantitatively analyze a
microstructure and a crystal orientation of a bulk sample surface, and as long
as an area to be analyzed is within an area capable of being observed by the
SEM, it is possible to analyze the area with a minimum resolution of 20 nm,
depending on the resolution of the SEM. The analysis by the EBSP-OIM
method is performed by mapping an area to be analyzed to tens of thousands
of equally-spaced grid points for several hours. It is possible to see crystal
orientation distributions and sizes of crystal grains within the sample in a
polycrystalHne material. In the present invention, one discernible from a
mapped image with a misorientation between packets defined as 15° may also
be defined as the continuous cooling transformation structure (Zw) for
convenience.
[0060] Further, the structural fraction of pro-eutectoid ferrite was
obtained by a KAM (Kernel Average Misorientation) method being equipped
with the EBSP-OIM. The KAM method is that a calculation, in which
misorientations among pixels of adjacent six pixels (first approximations) of a
certain regular hexagon of measurement data, or 12 pixels (second
approximations) positioned outside the six pixels, or 18 pixels (third
approximations) positioned further outside the 12 pixels are averaged and an
obtained value is set to a value of the center pixel, is performed with respect
to each pixel.
26
[0061] This calculation is performed so as not to exceed a grain boundary,
thereby making it possible to create a map representing an orientation change
within a grain. That is, this map represents a distribution of strain based on a
local orientation change within a grain. Note that as the analysis condition
in the present invention, the condition of which in the EBSP-OIM, the
misorientation among adjacent pixels is calculated is set to the third
approximation and one having this misorientation being 5° or less is
displayed.
[0062] In the present invention, pro-eutectoid ferrite is defined as a
microstructure up to a planar fraction of pixels of which misorientation third
approximation is calculated to be 1° or less as described above. This is
because polygonal pro-eutectoid ferrite transformed at high temperature is
generated in a diffusion transformation, and thus a dislocation density is small
and strain within the grain is small, and thus, a difference within the grain in
the crystal orientation is small, and according to the results of various
examinations that have been performed so far by the present inventors, a
volume fraction of polygonal ferrite obtained by observation of optical
microscope and an area fraction of an area obtained by 1° of the
misorientation third approximation measured by the "KAM method
substantially agree with each other.
[0063] (Manufacturing method)
Next, there will be described a manufacturing method of the
cold-rolled steel sheet of the present invention. In order to achieve excellent
local deformability, it is important to form a texture having predetermined
pole densities and to manufacture a steel sheet satisfying conditions of
making crystal grains fine and equiaxial property and homogenization of
27
crystal grains. Details of manufacturing conditions for satisfying them at the
same time will be described below.
[0064] A manufacturing method prior to hot rolling is not limited in
particular. That is, subsequent to melting by a shaft furnace, an electric
furnace, or the like, secondary refining may be variously performed, and next
casting may be performed by normal continuous casting, or casting by an
ingot method, or further a method such as thin slab casting. In the case of
continuous casting, it is possible that a cast slab is once cooled down to low
temperature and thereafter is reheated to then be subjected to hot rolling, or it
is also possible that a cast slab is subjected to hot rolling continuously. A
scrap may also be used for a raw material.
[0065] Further, in hot rolling, it is also possible that sheet bars are bonded
after rough rolling to be subjected to finish rolling continuously. On this
occasion, it is also possible that rough bars are coiled into a coil shape once,
stored in a coyer having a heat insulating function according to need, and
uncoiled again to then be joined.
[0066] (First hot rolling)
A slab extracted from a heating furnace is subjected to a rough rolling
process being first hot rolling to be rough rolled, and thereby a rough bar is
obtained. A high-strength steel sheet having excellent local deformability of
the present invention is obtained when the following requirements are
satisfied. First, an austenite grain diameter in the rough bar after the rough
rolling, namely before the finish rolling is important and the austenite grain
diameter before the finish rolling is desirably small, and it became clear that
the austenite grain diameter of 200 urn or less greatly contributes to making
grains fine in the grain unit and homogenization of a main phase.
28
[0067] In order to obtain this austenite grain diameter of 200 um or less
before the finish rolling, as shown in FIG. 3, in rough rolling in a temperature
region of not lower than 1000°C nor higher than 1200°C, rolling is performed
one time or more at a reduction ratio of at least 40% or more.
[0068] As the reduction ratio and the number of times of reduction are
larger, fine grains can be obtained, and in order to efficiently obtain this effect,
the austenite grain diameter is desirably set to 100 urn or less, and in order to
achieve it, rolling at 40% or more is desirably performed two times or more.
However, when in the rough rolling, the reduction ratio is greater than 70%
and rolling is performed greater than 10 times, there is a concern that the
temperature decreases or a scale is generated excessively.
[0069] In this manner, the decrease in the austenite grain diameter before
the finish rolling is effective for the improvement of the local deformability
through control of recrystallization promotion of austenite in the finish rolling
later, making grains fine, and making grains equiaxial of the grain unit in a
final structure. It is supposed that this is because an austenite grain
boundary after the rough rolling (namely before the finish rolling) functions
as one of recrystallization nuclei during the finish rolling.
[0070] In order to confirm the austenite grain diameter after the rough
rolling, a sheet piece before being subjected to the finish rolling is desirably
quenched as much as possible, and the sheet piece is cooled at a cooling rate
of 10°C/s or more, and the structure of a cross section of the sheet piece is
etched to make austenite grain boundaries appear, and the austenite grain
boundaries are measured by an optical microscope. On this occasion, at 50
or more magnifications, 20 visual fields or more are measured by image
analysis or a point counting method.
29
[0071] (Second hot rolling)
After the rough rolling process (first hot rolling) is completed, a finish
rolling process being second hot rolling is started. The time between the
completion of the rough rolling process and the start of the finish rolling
process is desirably set to 150 seconds or shorter.
[0072] . In the finish rolling process (second hot rolling), a finish rolling
start temperature is desirably set to 1000°C or higher. When the finish
rolling start temperature is lower than 1000°C, at each finish rolling pass, the
temperature of the rolling to be applied to the rough bar to be rolled is
decreased, the reduction is performed in a non-recrystallization temperature
region, the texture develops, and thus the isotropy deteriorates.
[0073] Incidentally, the upper limit of the finish rolling start temperature
is not limited in particular. However, when it is 1150°C or higher, a blister
to be the starting point of a scaly spindle-shaped scale defect is likely to occur
between a steel sheet base iron and a surface scale before the finish rolling
and between passes, and thus the finish rolling start temperature is desirably
lower than 1150°C.
[0074] In the finish rolling, a temperature determined by the chemical
composition of the steel sheet is set to Tl, and in a temperature region of not
lower than Tl + 30°C nor higher than Tl + 200°C, the rolling at 30% or more
is performed in one pass at least one time. Further, in the finish rolling, the
total reduction ratio is set to 50% or more. By satisfying this condition, at
the sheet thickness center portion being the range of 5/8 to 3/8 in sheet
thickness from the surface of the steel sheet, the average value of the pole
densities of the {100}<011> to {223}<110> orientation group becomes less
than 4.0 and the pole density of the {332}<113> crystal orientation becomes
30
5.0 or less. This makes it possible to obtain the local deformability of a final
product.
[0075] Here, Tl is the temperature calculated by Expression (1) below.
Tl (°C) = 850 + 10 x (C + N) x Mn + 350 x Nb + 250 x Ti + 40 x B +
lOxCr+lOOxMo+lOOxV - (1)
C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the
element (mass%).
[0076] FIG. 4 and FIG. 5 each show the relationship between a reduction
ratio in each temperature region and a pole density of each orientation. As
shown in FIG. 4 and FIG. 5, heavy reduction in the temperature region of not
lower than Tl + 30°C nor higher than Tl + 200°C and light reduction at Tl or
higher and lower than Tl + 30°C thereafter control the average value of the
pole densities of the {100}<011> to {223}<110> orientation group and the
pole density of the {332}<113> crystal orientation at the sheet thickness
center portion being the range of 5/8 to 3/8 in sheet thickness from the surface
of the steel sheet, and thereby the local deformability of the final product is
improved drastically, as shown in Tables 2 and 3 of Examples to be described
later.
[0077] This Tl temperature itself is obtained empirically. The present
inventors learned empirically by experiments that the recrystallization in an
austenite region of each steel is promoted on the basis of the Tl temperature.
In order to obtain better local deformability, it is important to accumulate
strain by the heavy reduction, and the total reduction ratio of 50% or more is
essential. Further, it is desired to take reduction at 70% or more, and on the
other hand, if the reduction ratio greater than 90% is taken, securing
temperature and excessive rolling load are as a result added.
31
[0078] When the total reduction ratio in the temperature region of not
lower than Tl + 30°C nor higher than Tl + 200°C is less than 50%, rolling
strain to be accumulated during the hot rolling is not sufficient and the
recrystallization of austenite does not advance sufficiently. Therefore, the
texture develops and the isotropy deteriorates. When the total reduction
ratio is 70% or more, the sufficient isotropy can be obtained even though
variations ascribable to temperature fluctuation or the like are considered.
On the other hand, when the total reduction ratio exceeds 90%, it becomes
difficult to obtain the temperature region of Tl + 200°C or lower due to heat
generation by working, and further a rolling load increases to cause a risk that
the rolling becomes difficult to be performed.
[0079] In the finish rolling, in order to promote the uniform
recrystallization caused by releasing the accumulated strain, the rolling at
30%o or more is performed in one pass at least one time at not lower than Tl +
30°C nor higher than Tl + 200°C.
[0080] Incidentally, in order to promote the uniform recrystallization, it is
necessary to suppress a working amount in a temperature region of lower than
Tl + 30°C as small as possible. In order to achieve it, the reduction ratio at
lower than Tl + 30°C is desirably 30% or less. In terms of sheet thickness
accuracy and sheet shape, the reduction ratio of 10% or less is desirable.
When the isotropy is further obtained, the reduction ratio in the temperature
region of lower than Tl + 30°C is desirably 0%.
[0081] The finish rolling is desirably finished at Tl + 30°C or higher. In
the hot rolling at lower than Tl + 30°C, the granulated austenite grains that
are recrystallized once are elongated, thereby causing a risk that the isotropy
deteriorates.
t
32
[0082] That is, in the manufacturing method of the present invention, in
the finish rolling, by recrystallizing austenite uniformly and finely, the texture
of the product is controlled and the local deformability such as the hole
expandability or the bendability is improved.
[0083] A rolling ratio can be obtained by actual performances or
calculation from the rolling load, sheet thickness measurement, or/and the like.
The temperature can be actually measured by a thermometer between stands,
or can be obtained by calculation simulation considering the heat generation
by working from a line speed, the reduction ratio, or/and like. Thereby, it is
possible to easily confirm whether or not the rolling prescribed in the present
invention is performed.
[0084] When the hot rolling is finished at Ar3 or lower, the hot rolling
becomes two-phase region rolling of austenite and ferrite, and accumulation
to the {100}<011> to {223}<110> orientation group becomes strong. As a
result, the local deformability deteriorates significantly.
[0085] In order to make the crystal grains fine and suppress elongated
grains, a maximum working heat generation amount at the time of the
reduction at not lower than Tl + 30°C nor higher than Tl + 200°C, namely a
temperature increased margin by the reduction is desirably suppressed to
18°C or less. For achieving this, inter-stand cooling or the like is desirably
applied.
[0086] (Primary cooling)
After the final reduction at a reduction ratio of 30% or more is
performed in the finish rolling, primary cooling is started in such a manner
that a waiting time t second satisfies Expression (2) below,
t ^ 2.5 x t l - ( 2)
33
Here, tl is obtained by Expression (3) below.
tl = 0.001 x ((Tf - Tl) x Pl/100)2 - 0.109 x ((Tf - Tl) x Pl/100) + 3.1 - (3)
Here, in Expression (3) above, Tf represents the temperature of a steel billet
obtained after the final reduction at a reduction ratio of 30% or more, and PI
represents the reduction ratio of the final reduction at 30% or more.
[0087] Incidentally, the "final reduction at a reduction ratio of 30% or
more" indicates the rolling performed finally among the rollings whose
reduction ratio becomes 30% or more out of the rollings in a plurality of
passes performed in the finish rolling. For example, when among the
rollings in a plurality of passes performed in the finish rolling, the reduction
ratio of the rolling performed at the final stage is 30% or more, the rolling
performed at the final stage is the "final reduction at a reduction ratio of 30%
or more." Further, when among the rollings in a plurality of passes
performed in the finish rolling, the reduction ratio of the rolling performed
prior to the final stage is 30% or more and after the rolling performed prior to
the final stage (rolling at a reduction ratio of 30% or more) is performed, the
rolling whose reduction ratio becomes 30% or more is not performed, the
rolling performed prior to the final stage (rolling at a reduction ratio of 30%
or more) is the "final reduction at a reduction ratio of 30% or more."
[0088] In the finish rolling, the waiting time t second until the primary
cooling is started after the final reduction at a reduction ratio of 30% or more
is performed greatly affects the austenite grain diameter. That is, it greatly
affects an equiaxed grain fraction and a coarse grain area ratio of the steel
sheet.
[0089] When the waiting time t exceeds tl x 2.5, the recrystallization is
already almost completed, but the crystal grains grow significantly and grain
34
coarsening advances, and thereby an r value and the elongation are decreased.
[0090] The waiting time t second further satisfies Expression (2a) below,
thereby making it possible to preferentially suppress the growth of the crystal
grains. Consequently, even though the recrystallization does not advance
sufficiently, it is possible to sufficiently improve the elongation of the steel
sheet and to improve fatigue property simultaneously.
t < tl - (2a)
[0091] At the same time, the waiting time t second further satisfies
Expression (2b) below, and thereby the recrystallization advances sufficiently
and the crystal orientations are randomized. Therefore, it is possible to
sufficiently improve the elongation of the steel sheet and to greatly improve
the isotropy simultaneously.
tl ^ t ^ tl x 2.5 - (2b)
[0092] Here, as shown in FIG. 6, on a continuous hot rolling line 1, the
steel billet (slab) heated to a predetermined temperature in the heating furnace
is rolled in a roughing mill 2 and in a finishing mill 3 sequentially to be a
hot-rolled steel sheet 4 having a predetermined thickness, and the hot-rolled
steel sheet 4 is carried out onto a run-out-table 5. In the manufacturing
method of the present invention, in the rough rolling process (first hot rolling)
performed in the roughing mill 2, the rolling at a reduction ratio of 40% or
more is performed on the steel billet (slab) one time or more in the
temperature range of not lower than 1000°C nor higher than 1200°C.
[0093] The rough bar rolled to a predetermined thickness in the roughing
mill 2 in this manner is next finish rolled (is subjected to the second hot
rolling) through a plurality of rolling stands 6 of the finishing mill 3 to be the
hot-rolled steel sheet 4. Then, in the finishing mill 3, the rolling at 30% or
35
more is performed in one pass at least one time in the temperature region of
not lower than the temperature Tl + 30°C nor higher than Tl + 200°C.
Further, in the finishing mill 3, the total reduction ratio becomes 50% or
more.
[0094] Further, in the finish rolling process, after the final reduction at a
reduction ratio of 30% or more is performed, the primary cooling is started in
such a manner that the waiting time t second satisfies Expression (2) above or
either Expression (2a) or (2b) above. The start of this primary cooling is
performed by inter-stand cooling nozzles 10 disposed between the respective
two of the rolling stands 6 of the finishing mill 3, or cooling nozzles 11
disposed in the run-out-table 5.
[0095] For example, when the final reduction at a reduction ratio of 30%
or more is performed only at the rolling stand 6 disposed at the front stage of
the finishing mill 3 (on the left side in FIG. 6, on the upstream side of the
rolling) and the rolling whose reduction ratio becomes 30% or more is not
performed at the rolling stand 6 disposed at the rear stage of the finishing mill
3 (on the right side in FIG. 6, on the downstream side of the rolling), if the
start of the primary cooling is performed by the cooling nozzles 11 disposed
in the run-out-table 5, a case that the waiting time t second does not satisfy
Expression (2) above or Expressions (2a) and (2b) above is sometimes caused.
In such a case, the primary cooling is started by the inter-stand cooling
nozzles 10 disposed between the respective two of the rolling stands 6 of the
finishing mill 3.
[0096] Further, for example, when the final reduction at a reduction ratio
of 30%o or more is performed at the rolling stand 6 disposed at the rear stage
of the finishing mill 3 (on the right side in FIG. 6, on the downstream side of
36
the rolling), even though the start of the primary cooling is performed by the
cooling nozzles 11 disposed in the run-out-table 5, there is sometimes a case
that the waiting time t second can satisfy Expression (2) above or Expressions
(2a) and (2b) above. In such a case, the primary cooling may also be started
by the cooling nozzles 11 disposed in the run-out-table 5. Needless to say,
as long as the performance of the final reduction at a reduction ratio of 30%
or more is completed, the primary cooling may also be started by the
inter-stand cooling nozzles 10 disposed between the respective two of the
rolling stands 6 of the finishing mill 3.
[0097] Then, in this primary cooling, the cooling that at an average
cooling rate of 50°C/second or more, a temperature change (temperature
drop) becomes not less than 40°C nor more than 140°C is performed.
[0098] When the temperature change is less than 40°C, the recrystallized
austenite grains grow and low-temperature toughness deteriorates. The
temperature change is set to 40°C or more, thereby making it possible to
suppress coarsening of the austenite grains. When the temperature change is
less than 40°C, the effect cannot be obtained. On the other hand, when the
temperature change exceeds 140°C, the recrystallization becomes insufficient
to make it difficult to obtain a targeted random texture. Further, a ferrite
phase effective for the elongation is also not obtained easily and the hardness
of a ferrite phase becomes high, and thereby the elongation and the local
deformability also deteriorate. Further, when the temperature change is
greater than 140°C, an overshoot to/beyond an Ar3 transformation point
temperature is likely to be caused. In the case, even by the transformation
from recrystallized austenite, as a result of sharpening of variant selection, the
texture is formed and the isotropy decreases consequently.
37
[0099] When the average cooling rate in the primary cooling is less than
50°C/second, as expected, the recrystallized austenite grains grow and the
low-temperature toughness deteriorates. The upper limit of the average
cooling rate is not determined in particular, but in terms of the steel sheet
shape, 200°C/second or less is considered to be proper.
[0100] Further, in order to suppress the grain growth and obtain more
excellent low-temperature toughness, a cooling device between passes or the
like is desirably used to bring the heat generation by working between the
respective stands of the finish rolling to 18°C or lower.
[0101] The rolling ratio (reduction ratio) can be obtained by actual
performances or calculation from the rolling load, sheet thickness
measurement, or/and the like. The temperature of the steel billet during the
rolling can be actually measured by a thermometer being disposed between
the stands, or can be obtained by simulation by considering the heat
generation by working from a line speed, the reduction ratio, or/and like, or
can be obtained by the both methods.
[0102] Further, as has been explained previously, in order to promote the
uniform recrystallization, the working amount in the temperature region of
lower than Tl + 30°C is desirably as small as possible and the reduction ratio
in the temperature region of lower than Tl + 30°C is desirably 30% or less.
For example, in the event that in the finishing mill 3 on the continuous hot
rolling line 1 shown in FIG. 6, in passing through one or two or more of the
rolling stands 6 disposed on the front stage side (on the left side in FIG. 6, on
the upstream side of the rolling), the steel sheet is in the temperature region of
not lower than Tl + 30°C nor higher than Tl + 200°C, and in passing through
one or two or more of the rolling stands 6 disposed on the subsequent rear
38
stage side (on the right side in FIG. 6, on the downstream side of the rolling),
the steel sheet is in the temperature region of lower than Tl + 30°C, when the
steel sheet passes through one or two or more of the rolling stands 6 disposed
on the subsequent rear stage side (on the right side in FIG. 6, on the
downstream side of the rolling), even though the reduction is not performed
or is performed, the reduction ratio at lower than Tl + 30°C is desirably 30%
or less in total. In terms of the sheet thickness accuracy and the sheet shape,
the reduction ratio at lower than Tl + 30°C is desirably a reduction ratio of
10% or less in total. When the isotropy is further obtained, the reduction
ratio in the temperature region of lower than Tl + 30°C is desirably 0%.
[0103] In the manufacturing method of the present invention, a rolling
speed is not limited in particular. However, when the rolling speed on the
final stand side of the finish rolling is less than 400 mpm, y grains grow to be
coarse, regions in which ferrite can precipitate for obtaining the ductility are
decreased, and thus the ductility is likely to deteriorate. Even though the
upper limit of the rolling speed is not limited in particular, the effect of the
present invention can be obtained, but it is actual that the rolling speed is
1800 mpm or less due to facility restriction. Therefore, in the finish rolling
process, the rolling speed is desirably not less than 400 mpm nor more than
1800 mpm.
[0104] Incidentally, after this primary cooling, coiling is performed at an
appropriate temperature and a hot-rolled original sheet can be obtained. In
the present invention, the microstructure of the cold-rolled steel sheet is
mainly formed by cold rolling later, or a heat treatment after cold rolling.
Thus, a cooling pattern to the coiling does not have to be strictly controlled
very much.
39
[0105] (Cold rolling)
The hot-rolled original sheet manufactured as described above is
pickled according to need to be subjected to cold rolling at a reduction ratio of
not less than 30%> nor more than 70%>. When the reduction ratio is 30%> or
less, it becomes difficult to cause recrystallization in heating and holding later,
resulting in that the equiaxed grain fraction decreases and further the crystal
grains after heating become coarse. When rolling at over 70%> is performed,
a texture at the time of heating is developed, and thus the anisotropy becomes
strong. Therefore, the reduction ratio is set to 70% or less.
[0106] (Heating and holding)
The cold-rolled steel sheet is thereafter heated up to a temperature
region of Ae3 to 950°C and is held for 1 to 300 second/seconds in the
temperature region of Ae3 to 950°C in order to make an austenite single phase
steel or a substantially austenite single phase steel. By this heating and
holding, work hardening is removed. In order to heat the steel sheet after the
cold rolling up to the temperature region of Ae3 to 950°C in this manner, an
average heating rate of not lower than room temperature nor higher than
650°C is set to HR1 (°C/second) expressed by Expression (5) below, and an
average heating rate of higher than 650°C to Ae3 to 950°C is set to HR2
(°C/second) expressed by Expression (6) below.
HR1 ^ 0.3 ... (5)
HR2 ^ 0.5 x HR1 ... (6)
[0107] The hot rolling is performed under the above-described condition,
and further the primary cooling is performed, and thereby making the crystal
grains fine and randomization of the crystal orientations are achieved.
However, by the cold rolling performed thereafter, the strong texture develops
40
and the texture becomes likely to remain in the steel sheet. As a result, the r
value and the elongation of the steel sheet decrease and the isotropy decreases.
Thus, it is desired to make the texture that has developed by the cold rolling
disappear as much as possible by appropriately performing the heating to be
performed after the cold rolling. In order to achieve it, it is necessary to
divide the average heating rate of the heating into two stages expressed by
Expressions (5) and (6) above.
[0108] The detailed reason why the texture and properties of the steel
sheet are improved by this two-stage heating is unclear, but this effect is
thought to be related to recovery of dislocation introduced at the time of the
cold rolling and the recrystallization. That is, driving force of the
recrystallization to occur in the steel sheet by the heating is strain
accumulated in the steel sheet by the cold rolling. When the average heating
rate HR1 in the temperature range of not lower than room temperature nor
higher than 650°C is small, the dislocation introduced by the cold rolling
recovers and the recrystallization does not occur. As a result, the texture that
has developed at the time of the cold rolling remains as it is and the properties
such as the isotropy deteriorate. When the average heating rate HR1 in the
temperature range of not lower than room temperature nor higher than 650°C
is less than 0.3 °C/second, the dislocation introduced by the cold rolling
recovers, resulting in that the strong texture formed at the time of the cold
rolling remains. Therefore, it is necessary to set the average heating rate
HR1 in the temperature range of not lower than room temperature nor higher
than 650°C to 0.3 (°C/second) or more.
[0109] On the other hand, when the average heating rate HR2 of higher
than 650°C to Ae3 to 950°C is large, ferrite existing in the steel sheet after the
41
cold rolling does not recrystallize and non-recrystallized ferrite in a state of
being worked remains. When the steel containing C of 0.01% or more in
particular is heated to a two-phase region of ferrite and austenite, formed
austenite blocks growth of recrystallized ferrite, and thus non-recrystallized
ferrite becomes more likely to remain. This non-recrystallized ferrite has the
strong texture, and thus the properties such as the r value and the isotropy are
adversely affected, and this non-recrystallized ferrite contains a lot of
dislocations, and thus the ductility is deteriorated drastically. Therefore, in
the temperature range of higher than 650°C to Ae3 to 950°C, the average
heating rate HR2 needs to be 0.5 x HR1 (°C/second) or less.
[0110] Further, at the two-stage average heating rate as above, the steel
sheet is heated up to the temperature region of Ae3 to 950°C and is held for 1
to 300 second/seconds in the temperature region of Ae3 to 950°C. If the
temperature is lower than this range or the time is shorter than this range, the
fraction of the bainite structure does not become 95% or more in a secondary
cooling process thereafter, and the increased margin of the local ductility by
the texture control decreases. On the other hand, if the steel sheet is
continuously held at higher than 950°C or longer than 300 seconds, the crystal
grains become coarse, and thus an area ratio of the grains having 20 um or
less increases. Incidentally, Ae3 [°C] is calculated by Expression (7) below
by the contents of C, Mn, Si, Cu, Ni, Cr, and Mo [mass%]. Incidentally,
when the selected element is not contained, the calculation is performed with
the content of the selected element [mass%] set as zero.
Ae3 = 911 - 239C - 36Mn + 40Si - 28Cu - 20Ni - 12Cr + 63Mo ... (7)
[0111] Incidentally, in this heating and holding, the holding does not
mean only the isothermal holding, and it is sufficient if the steel sheet is
42
retained in the temperature range of Ae3 to 950°C. As long as the steel sheet
is in the temperature range of Ae3 to 950°C, the temperature of the steel sheet
may be changed.
[0112] (Secondary cooling)
Thereafter, secondary cooling is performed down to a temperature of
500°C or lower so that an average cooling rate in a temperature region of Ae4
to 500°C may become not less than 10°C/s nor more than 200°C/s. When a
secondary cooling rate is less than 10°C/s, ferrite is generated excessively,
thereby making it impossible to bring the fraction of the bainite structure to
95% or more, and resulting in that the increased margin of the local ductility
by the texture control decreases. On the other hand, even when the cooling
rate is set to greater than 200°C/s, controllability at a cooling finishing
temperature deteriorates significantly, and thus the cooling rate is set to
200°C/s or less. Preferably, an average cooling rate at HF (a heating and
holding temperature) to 0.5HF + 250°C is set not to exceed an average
cooling rate at 0.5HF + 250°C to 500°C in order to securely suppress ferrite
transformation and pearlite transformation.
[0113] (Overaging heat treatment)
In order to promote bainite transformation, an overaging heat
treatment is performed in a temperature range of not lower than 350°C nor
higher than 500°C subsequently to the secondary cooling. A holding time in
this temperature range is set to t2 seconds or longer that satisfies Expression
(4) below according to an overaging treatment temperature T2. However, in
consideration of an applicable temperature range of Expression (4), the
maximum value of t2 is set to 400 seconds.
log(t2) = 0.0002(T2 - 425)2 + 1.18 ... (4)
43
[0114] Incidentally, in this overaging heat treatment, holding does not
mean only the isothermal holding, and it is sufficient if the steel sheet is
retained in the temperature range of not lower than 350°C nor higher than
500°C. For example, the steel sheet may be once cooled to 350°C to then be
heated up to 500°C, or the steel sheet may also be cooled to 500°C to then be
cooled down to 350°C.
[0115] Incidentally, even when a surface treatment is performed on the
high-strength cold-rolled steel sheet of the present invention, the effect of
improving the local deformability does not disappear, and for example, a
hot-dip galvanized layer, or an alloyed hot-dip galvanized layer may be
formed on the surface of the steel sheet. In this case, the effect of the present
invention can be obtained even when any one of electroplating, hot dipping,
deposition plating, organic coating film forming, film laminating, organic
salts/inorganic salts treatment, non-chromium treatment,, and so on is
performed. Further, the steel sheet according to the present invention can be
applied not only to bulge forming but also to combined forming mainly
composed of bending working such as bending, bulging, and drawing.
Example
[0116] Next, examples of the present invention will be explained.
Incidentally, conditions of the examples are condition examples employed for
confirming the applicability and effects of the present invention, and the
present invention is not limited to these condition examples. The present
invention can employ various conditions as long as the object of the present
invention is achieved without departing from the spirit of the invention.
Chemical compositions of respective steels used in examples are shown in
Table 1. Respective manufacturing conditions are shown in Table 2 and
44
Table 3. Further, structural constitutions and mechanical properties of
respective steel types under the manufacturing conditions in Table 2 are
shown in Table 4. Structural constitutions and mechanical properties of
respective steel types under the manufacturing conditions in Table 3 are
shown in Table 5. Incidentally, each underline in Tables indicates that a
numeral value is outside the range of the present invention or is outside the
range of a preferred range of the present invention.
[0117] As examples, there will be explained results of examinations using
steels A to T satisfying the components of claims of the present invention and
using comparative steels a to i, which have the chemical compositions shown
in Table 1. Incidentally, in Table 1, each numerical value of the chemical
compositions means mass%.
[0118] These steels were cast and then as they were, or were reheated
after once being cooled down to room temperature and were heated to a
temperature range of 1000°C to 1300°C, and then were subjected to hot
rolling under the conditions of Table 2 and Table 3, and the hot rolling was
finished at an Ar3 transformation temperature or higher. Incidentally, in
Table 2 and Table 3, English letters A to T and English letters a to i that are
added to the steel types indicate to be the respective components of Steels A
to T and a to i in Table 1.
[0119] In the hot rolling, first, in rough rolling being first hot rolling,
rolling was performed one time or more at a reduction ratio of 40% or more in
a temperature region of not lower than 1000°C nor higher than 1200°C.
However, with respect to Steel types B2, H3, and J2 in Table 2, and Steel
types B2', H3', and J2' in Table 3, in the rough rolling, the rolling at a
reduction ratio of 40% or more in.one pass was not performed. The number
45
of times of reduction and each reduction ratio (%) in the rough rolling, and an
austenite grain diameter (^m) after the rough rolling (before finish rolling) are
shown in Table 2 and Table 3.
[0120] After the rough rolling was finished, the finish rolling being
second hot rolling was performed. In the finish rolling, rolling at a reduction
ratio of 30% or more was performed in one pass at least one time in a
temperature region of not lower than Tl + 30°C nor higher than Tl + 200°C,
and in a temperature range of lower than Tl + 30°C, the total reduction ratio
was set to 30% or less. Incidentally, in the finish rolling, rolling at a
reduction ratio of 30% or more was performed in a final pass in the
temperature region of not lower than Tl + 30°C nor higher than Tl + 200°C.
[0121] However, with respect to Steel types G2, H4, and M3 in Table 2
and Steel types G2', H4', and M3' in Table 3, the rolling at a reduction ratio
of 30% or more was not performed in the temperature region of not lower
than Tl + 30°C nor higher than Tl + 200°C. Further, with regard to Steel
types F3 and H6 in Table 2 and Steel types F3' and H6' in Table 3, the total
reduction ratio in the temperature range of lower than Tl + 30°C was greater
than 30%.
[0122] Further, in the finish rolling, the total reduction ratio was set to
50%) or more. However, with regard to Steel types G2, H4, and M3 in Table
2 and Steel types G2', H4', and M3' in Table 3, the total reduction ratio was
less than 50%.
[0123] Table 2 and Table 3 show, in the finish rolling, the reduction ratio
(%) in the final pass in the temperature region of not lower than Tl + 30°C
nor higher than Tl + 200°C and the reduction ratio in a pass at one stage
earlier than the final pass (reduction ratio in a pass before the final) (%).
46
Further, Table 2 and Table 3 show, in the finish rolling, the total reduction
ratio (%) in the temperature region of not lower than Tl + 30°C nor higher
than Tl + 200°C and a temperature Tf after the reduction in the final pass in
the temperature region of not lower than Tl + 30°C nor higher than Tl +
200°C. Incidentally, the reduction ratio (%) in the final pass in the
temperature region of not lower than Tl + 30°C nor higher than Tl + 200°C
in the finish rolling is particularly important, to thus be shown in Table 2 and
Table 3 as PL
[0124] After the final reduction at a reduction ratio of 30% or more was
performed in the finish rolling, primary cooling was started before a waiting
time t second exceeding 2.5 x tl. In the primary cooling, an average cooling
rate was set to 50°C/second or more. Further, a temperature change (a
cooled temperature amount) in the primary cooling was set to fall within a
range of not less than 40°C nor more than 140°C.
[0125] Under the manufacturing conditions shown in Table 2, after the
final reduction at a reduction ratio of 30% or more was performed in the
finish rolling, the primary cooling was started before the waiting time t
second exceeding tl (t < tl). On the other hand, under the manufacturing
conditions shown in Table 3, after the final reduction at a reduction ratio of
30%o or more was performed in the finish rolling, the primary cooling was
started before the waiting time t second exceeding a range of tl or longer to
2.5 x tl (tl ^ t ^ tl x 2.5). Incidentally, ['] (dash) was added to each
reference numeral of the steel types following the manufacturing conditions
shown in Table 3 in order to distinguish the ranges of the waiting time t
second.
[0126] However, with respect to Steel type H13' shown in Table 3, the
47
primary cooling was started after the waiting time t second exceeded 2.5 x tl
since the final reduction at a reduction ratio of 30% or more in the finish
rolling. With regard to Steel type M2 in Table 2 and Steel type M2' in Table
3, the temperature change (cooled temperature amount) in the primary cooling
was less than 40°C, and with regard to Steel type HI2 in Table 2 and Steel
type HI2' in Table 3, the temperature change (cooled temperature amount) in
the primary cooling was greater than 140°C. With regard to Steel type H8 in
Table 2 and Steel type H8' in Table 3, the average cooling rate in the primary
cooling was less than 50°C/second.
[0127] Table 2 and Table 3 show tl (second) and 2.5 x tl (second) of the
respective steel types. Further, Table 2 and Table 3 show the waiting time t
(second) from completion of the final reduction at a reduction ratio of 30% or
more to start of the primary cooling, t/tl, the average cooling rate (°C/second)
in the primary cooling, and the temperature change (cooled temperature
amount) (°C).
[0128] After the primary cooling, coiling was performed and hot-rolled
original sheets each having a thickness of 2 to 5 mm were obtained. Table 2
and Table 3 show the coiling temperature (°C) of the respective steel types.
[0129] " Next, the hot-rolled original sheets were each pickled to then be
subjected to cold rolling at a reduction ratio of not less than 30% nor more
than 70% to a thickness of 1.2 to 2.3 mm. However, with regard to Steel
types E2 and L2 in Table 2 and Steel types E2' and L2' in Table 3, the
reduction ratio of the cold rolling was less than 30%. Further, with regard to
Steel type HI 1 in Table 2 and Steel type HI 1' in Table 3, the reduction ratio of
the cold rolling was greater than 70%. Table 2 and Table 3 show the
reduction ratio (%) in the cold rolling of the respective steel types.
48
[0130] After the cold rolling, heating was performed up to a temperature
region of Ae3 to 950°C and holding was performed for 1 to 300
second/seconds in the temperature region of Ae3 to 950°C. Further, in order
to perform the heating up to the, temperature region of Ae3 to 950°C, an
average heating rate HRl(°C/second) of not lower than room temperature nor
higher than 650°C was set to 0.3 or more (HR1 ^ 0.3), and an average
heating rate HR2(°C/second) of higher than 650°C to Ae3 to 950°C was set to
0.5 x HR1 or less (HR2 ^ 0.5 x HR1).
[0131] However, with regard to Steel types C2 and G3 in Table 2 and
Steel types C2' and G3' in Table 3, a heating temperature was lower than Ae3.
Further, with regard to Steel type H10 in Table 2 and Steel type H10' in Table
3, the heating temperature was higher than 950°C. With regard to Steel type
N2 in Table 2 and Steel type N2' in Table 3, the holding time in the
temperature region of Ae3 to 950°C was longer than 300 seconds. Further,
with regard to Steel type E2 in Table 2 and Steel type E2' in Table 3, the
average heating rate HR1 was less than 0.3 (°C/second). With regard to
Steel types C2, H6, and H8 in Table 2 and Steel types C2\ H6\ and H8' in
Table 3, the average heating rate HR2 (°C/second) was greater than 0.5 x
HR1. Table 2'and Table 3 show As3 (°C), the heating temperature (°C), the
holding time (second), and the average heating rates HR1 and HR2
(°C/second) of the respective steel types.
[0132] After the heating and holding, secondary cooling was performed
at an average cooling rate of not less than 10°C/s nor more than 200°C/s in a
temperature region of Ae3 to 500°C. However, with regard to Steel type H2
in Table 2 and Steel type H2' in Table 3, the average cooling rate in the
secondary cooling was less than 10°C/s. Table 2 and Table 3 show the
49
average cooling rate (°C/second) in the secondary cooling of the respective
steel types.
[0133] After the secondary cooling, an overaging heat treatment was
performed for not shorter than t2 seconds nor longer than 400 seconds in a
temperature region of not lower than 350°C nor higher than 500°C.
However, with regard to Steel type H9 in Table 2 and Steel type H9' in Table
3, a heat treatment temperature of the overaging was lower than 350°C, and
with regard to Steel types A2 and 12 in Table 2 and Steel types A2' and 12' in
Table 3, the heat treatment temperature of the overaging was higher than
500°C. Further, with regard to Steel type D2 in Table 2 and Steel type D2' in
Table 3, a treatment time of the overaging was shorter than t2 seconds, and
with regard to Steel types A2, H9, and 12 in Table 2 and Steel types A2', H9and 12' in Table 3, the treatment time of the overaging was longer than 400
seconds. Table 2 and Table 3 show the heat treatment temperature of the
overaging, t2 (second), and the treatment time (second) of the respective steel
types.
[0134] In all the cases of Table 2 and Table 3, after the overaging heat
treatment, skin pass rolling at 0.5% was performed and material evaluation
was performed.
[0135] Table 4 and Table 5 show an area ratio (structural fraction) (%) of
bainite, pearlite, pro-eutectoid ferrite, martensite, and retained austenite in a
metal structure of the respective steel types. Incidentally, Table 4 shows the
structural constitutions and the mechanical properties of the steel types
following the manufacturing conditions in Table 2. Further, Table 5 shows
the structural constitutions and the mechanical properties of the steel types
following the manufacturing conditions in Table 3. Incidentally, with regard
50
to the structural fraction in Table 4 and Table 5, B means bainite, P means
pearlite, F means pro-eutectoid ferrite, M means martensite, and rA means
retained austenite. Table 4 and Table 5 show, of the respective steel types,
an average value of pole densities of the {100}<011> to {223}<110>
orientation group, a pole density of the {332}<113> crystal orientation, a
mean volume diameter of crystal grains (size of a grain unit) (um), and a ratio
of crystal grains having dL/dt of 3.0 or less (equiaxed grain ratio) (%).
Further, Table 4 and Table 5 show, of the respective steel types, tensile
strength TS (MPa), an elongation percentage El (%), a hole expansion ratio X
(%) as an index of the local deformability, and a limit bend radius by 60°
V-shape bending (a sheet thickness/a minimum bend radius). In a bending
test, C-direction bending (C-bending) was performed. Incidentally, a tensile
test and a bending test were based on JIS Z 2241 and Z 2248 (a V block 90°
bending test). A hole expansion test was based on the Japan Iron and Steel
Federation standard JFS T1001. The pole density of each of the crystal
orientations was measured using the previously described EBSP at a 0.5 urn
pitch on a 3/8 to 5/8 region at sheet thickness of a cross section parallel to the
rolling direction.
[0136] As indexes of the hole expandability and the bendability,
satisfying TS ^ 440 MPa, El ^ 15%, X ^ 90%, and the sheet
thickness/the bend radius ^ 2.5 were set as conditions. It is found that
only ones satisfying the prescriptions of the present invention can have both
the excellent hole expandability and bendability as shown in FIG. 7 and FIG.
8.
[0137]
[Table 1]
HH ETER
BEFORE
FKEH
ROLLHG
BO
83
80
245.
85
87
75
77
83
8B
85
130
80
85
86
83
85
90
225.
87
85
B6
140
85
89
89
89
89
78
80
85
2JL5.
150
150
120
123
75
76
79
82
BO
85
83
92
77
63
B8
85
82
80
83
85
86
80
85
88
REDUCTDN
RATI)
ATTl 0 0
TO TI * :oor
fHKN
ROLL N O
89
B9
BG
BG
BO
85
80
80
75
B9
85
75
85
85
AS.
85
89
89
89
35
65
89
B9
89
89
89
89
69
85
B5
80
87
75
75
B5
85
85
85
22
85
85
65
B5
80
85
85
85
85
65
B5
85
BG
85
85
85
85
REDUCTDN
RAID OF
PASS BEFORE
FKAL
ATTl+ 30
TO Tl • ;O0"C
FNEH
ROLLNGt /%
40
40
40
40
40
40
40
40
40
35
40
35
40
40
25
40
40
40
40
25
40
40
40
40
40
40
40
40
40
40
40
40
30
30
35
35
45
45
15
40
40
40
35
40
45
45
45
45
45
45
45
45
46
45
45
45
REDIICTDK
RATPOf
FMALFAS5
ATTl * 30
TO Tl . lOOt
fKBH
ROLLHG)
40
40
35
35
40
35
40
40
40
35
40
35
40
40
25.
40
40
40
40
25
40
40
40
40
40
40
40
10 "
40
40
40
40
30
30
35
35
40
40
25
40
40
40
35
40
45
45
45
45
45
45
45
45
45
45
45
45
REDIICTDN
It AT D ATT
TO
LOIERTKAX
Tl + S O t
fKEH
ROLLHG)
10
0
0
10
0
a
0
0
0
0
0
0
40.
0
0
0
0
0
0
0
0
ID
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
10
0
0
0
0
0
0
0
0
0
0
0
TEMPERATURE
AFTER FBAl
REPUCTDN AT
3010RKORE
FNSH
ROLLHG)
/•c
1022
960
956
948
971
980
1001
966
930
948
981
952
943
902
931
907
982
975
980
950
964
962
991
972
980
980
980
960
965
948
901
943
924
925
1005
1007
1050
1051
1055
1027
1016
977
1033
1001
995
1033
1009
986
980
981
1029
1048
977
1032
980
1013
PI :
REPUCTDN
RAID OFFMA
REPUC1DN AT
101 OR MORE
fKEH
ROLLNG)
40
40
35
35
40
35
40
40
40
35
40
35
40
40
25
40
40
40
40
25
40
40
40
40
40
40
40
40
40
40
40
40
30
30
35
35
40
40
25.
40
40
40
35
40
45
45
45
45
45
45
45
45
45
45
45
45
„,
0.18
1.13
0.67
Oil I
0.29
034
0.21
0.65
0.73
0.61
0.44
1.18
1.21
1.49
1.44
135
0.14
0.16
037
1.04
0.23
0.25
0.13
0.1 B
0.14
0.14
0.14
0.14
0.21
0 39
131
0.44
1.24
122
0.16
0.16
0.29
0 30
0.26
0.20
0.15
0.15
0.20
0.13
0.23
054
0.40
0.14
0.13
0.14
0.14
0.16
0.13
0.69
0.14
0.43
„=,
0.44
2.82
169
2.03
0.72
0.86
052
1.62
1*2
1.52
1.11
2.95
3.03
3.73
3.61
337
034
0.40
0.93
2£0
057
061
0 33
0.44
0.36
035
035
035
0.54
0.96
3.27
1.09
3.10
3.05
0.41
039
0.73
0.78
0.64
0.50
036
037
0.51
0 33
058
1.34
1.00
035
0 33
034
034
0.40
0.34
1.72
034
1.08
THE FROM
COItPLETENOF
FHALROLLHG AT
301.0 IK ORE TO
PRIIART
COOLUG
0.12
O.BB
0.00
0.60
0.20
0.30
0.12
0.20
0.60
0.52
030
0.96
1.05
1.20
0.70
1.30
0.12
0.14
0.35
0.B2
0.21
0.20
0.11
0.15
0.12
0.12
012
0.12
0.20
0.25
1.30
0.40
0.90
1.20
0.15
0.15
0.25
0.25
0.20
0.12
0.12
0.14
0.16
0.12
0.17
0.40
0.20
0.10
0.10
0.10
0.10
0.10
0.10
0.50
0.10
0.31
«
0.68
0.78
0.K9
0.74
0.70
0.87
0.58
031
0.82
0.85
0.68
0.81
0.87
0.80
0.4 B
0.96
0.87
0.B7
0.95
0.79
0.92
0.81
0.84
0.65
0.64
0.86
0.86
0.86
0.93
0.65
0.99
0.91
0.73
0.98
0.91
0.96
0.86
0.83
0.78
0.60
0.63
0.94
0.88
0.91
0.74
0.74
0.50
0.71
0.76
0.73
0.73
0.62
0.74
0.73
0.73
0.71
COOING
KATE H
PRNART
COOLHG
f.'.i
55
55
100
Km
100
100
60
60
80
80
120
120
120
110
110
110
70
70
70
70
70
70
70
•in
70
70
70
70
GO
60
60
60
65
65
65
65
75
75
75
85
65
70
70
70
70
70
70
80
80
80
80
80
80
80
60
60
TEMPERATURE
HECREASE
AMOUNTH
PRIIARf
COOLUG
60
50
40
50
70
50
90
60
50
40
90
too
60
50
40
50
100
BO
100
90
120
100
70
90
70
70
70
160
40
100
50
40
50
50
80
60
80
5
80
70
70
100
80
100
70
130
no
60
60
60
60
60
60
60
60
60
COLHG
TEMPERATURE
. • *U
550
550
450
450
400
450
700
700
500
520
480
550
500
450
500
500
500
500
500
500
500
500
500
500
500
500
500
500
390
450
400
500
500
500
500
500
500
550
550
500
300
700
700
700
700
700
700
500
500
500
500
500
500
500
500
500
COLD
ROLLHG
RATD
60
60
45
45
70
70
45
45
50
IS
60
60
60
60
60
60
50
50
50
50
50
50
50
GO
50
50
£5
50
70
70
50
50
45
45
50
IE
65
65
65
50
SO
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
,.,
2.10
1.70
140
1.40
1.60
1.40
1*0
2.20
1.60
Q2S.
1.70
1.20
130
1*0
2.10
2.00
0.40
2.20
2.40
230
3.00
2.60
150
2.70
130
130
130
130
1.10
2.50
3.00
2.10
2.10
150
1.60
230
I JO
2.00
2 00
150
1.70
1.50
130
1.90
150
2.00
230
2.10
150
1 10
150
1.20
1.20
120
130
150
HR:
0.90
0.70
055
050
0.60
1,00,
0.70
0.90
0.65
0.07
0.70
0.60
0.60
0*5
1.00
0.90
0.10
1.00
1.00
1.00
1.40
1.40
0.70
4.30
0.45
0.45
0.45
0.45
050
130
1.00
1.00
1.00
0*5
0.75
1X10
0.70
0*0
0*0
0.70
0*0
0.70
050
0.60
0*0
0*0
1.00
0.84
059
034
054
039
039
0 39
0.44
054
A«J
846
846
845
845
815
815
618
618
843
843
805
B05
605
816
816
816
829
629
829
829
829
829
829
B29
629
829
829
829
872
B72
845
845
816
816
910
910
840
840
840
812
B12
847
829
812
636
625
901
874
786
795
825
620
634
793
798
928
HEATHC
TEMPERATURE
/ 'C
890
890
900
900
B70
22£
900
900
B70
870
850
850
850
870
670
750
850
850
850
650
650
850
850
850
650
960
850
850
900
900
B50
850
860
880
920
920
870
870
870
850
650
880
880
880
880
880
910
880
880
8B0
880
880
B80
860
860
930
HO LP MO
THE
90
90
150
150
30
30
30
30
60
60
40
40
40
60
60
60
60'
60
60
60
60
60
60
60
60
60
60
60
90
90
60
60
90
90
60
60
90
90
90
60
500
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
AVERAGE
COOLHG
RATE FROM
A«3T0
wot
/"C/i
16
15
13
13
2B
100
17 '
17
16
16
13
13
13
17
17
22
16
2
16
16
16
16
16
16
19
19
19
19
14
11
19
19
13
13
13
13
18
18
18
15
15
18
18
18
16
18
IB
18
18
18
16
18
IB
18
18
18
OVERACKG
HEAT
TREATMENT
TEMPERATURE
,"C
480
550
400
400
400
400
350
350
400
400
380
380
380
450
450
450
450
450
450
450
450
450
450
450
3Jltt
450
450
450
500
fiPJt
450
450
400
400
450
450
400
400
400
450
450
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
c ,
61
400
20
20
20
20
202
202
20
20
38
38
38
20
20
20
20
20
20
20
20
20
20
20
400
20
20
20
202
400
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
HO LUNG
THE
250
450
400
400
400
400
350
150,
350
350
200
200
200
200
200
200
300
300
300
300
300
300
300
300
450
300
300
300
300
450
400
400
400
350
350
350
350
300
300
300
300
300
300
300
300
300
300
300
300
300
300
300
300
300
300
300
•
54
[0139]
[Table 3]
4Qr
STEEL
TYPE
Al'
A2'
B l '
B2'
CI'
C2'
or
D2*
E l '
E2'
F l '
F2"
F3'
G ] '
C2"
C3'
HI'
H2'
H3'
H4'
H5'
H6'
H7'
H8'
H9'
H10"
Hir
H12'
H13"
n*
D'
JI'
J 2 '
Kl"
K2'
LI'
L2'
H I '
112'
U3'
Nl'
N2'
or
PI'
Q l '
Rl'
SI"
T l '
a ] '
b l '
c l '
d l '
« ] '
f l '
t l '
h i '
fl'
Tl/T
903
903
867
867
866
866
887
887
855
855
889
889
889
858
858
858
853
853
853
853
853
853
853
853
853
853
853
853
853
852
852
851
851
853
853
866
866
882
882
882
870
870
852
853
861
852
867
851
*59
856
854
914
939
B51
963
853
. 853
NUMBER OFTUES
OFREtlUCTDNAT
4
TO C23K110>
ORENTATDN
CROUP
2.6
2.0
2.1
12
2.6
5,5
2.6
2.9
2.7
12
2.4
3.1
12
2.2
4.6
15
2.0
2.0
12
18
3.2
11
2.4
1.3
2.1
3.1
15
12
2.0
2.3
2.2
11
3.2
2.5
2.3
4.1
2.1
2.1
11
2.4
2.2
2.1
2.1
2.4
2.7
23
2.5
2.3
2.2
2.3
12
13.
2.3
51
2.3
2.3
POLE
DENSITY OP
G32KH3>
3.0
2.3
1.9
5£
3.5
67.
3.5
3.8
3.3
51
3.2
4.2
57
2.8
5^6
51
2.5
2.6'
si. 55
3.9
51
3.4
1.7
2.6
3.8
5£
51
2.2
2.6
2.8
55
4.1
3.3
3.2
5J.
2.3
2.4
55
3.0
0.6
2.6
2.7
3.1
3.4
2.8
3.2
2.9
2.8
2.8
66
5ji
2.9
M
2.9
2.9
SEEOF
GRAN UNIT
/ vn
2.3
2.7
5.4
4.2
1.9
5.2
3.9
1.7
5.1
85
2.2
4.0
3.3
2.8
4.8
5.5
4.2
4.1
4.7
§jS
6.8
3.6
4.0
LI
3.9
10.0
7.0
4.2
3.3
1.1
6.1
12
4.9
10.0
5.2
M
3.5
2J>
BA
2.9
£3
5.3
5.2
5.7
3.9
4.1
2.2
3.6
4.0
3.7
4.0
3.3
3.8
4.0
3.7
3.8
EQUHXED
GRAN
RATD
%
59
67
68
34
64
47
65
62
64
37
64
54
35
66
54
45
67
66
40
55
62
3d
55
74
66
70
52
65
69
66
67
47
50
58
60
25
68
68
57
64
67
67
67
65
59
66
62
65
66
66
66
64
65
66
65
65
TS/MPa
791
829
696
702
737
454
985
1119
539
532
961
973
952
858
875
634
782
656
786
778
777
793
787
777
917
796
784
792
626
649
480
500
895
865
606
591
956
968
971
811
817
589
624
920
708
767
629
427
855
952
985
951
568
1099
683
864
El/S
22
21
21
21
24
32
16
15
28
20
18
15
15
20
16
22
20
24
18
13
16
19
19
9
17
19
20
19
27
29
29
25
15
9
24
19
17
12
11
21
13
25
24
15
22
20
28
39
18
16
16
17
29
14
24
18
l /%
151
58
173
80
130
£2
91
51
169
58
90
95
64
140
51
81
152
90
87
64
109
94
132
54
51
103
55
80
148
98
183
101
102
11
159
Zi
112
39
22
143
63
175
167
92
146
144
183
105
53
38
44
54
66
22
11
56
SHEET
THTKNESS/
UNUUU
BEND
3.5
16
5.4
01
2.6
M
3.2
.U>
3.2
12
2.6
2.7
G\5
4.2
0.7
M
4.7
21
08
12
2.8
07
3.2
2.5
16
2.9
M
OS
3.8
m 4.1
M
2.6
2.6
3.5
Ql
4.3
2.6
QA
3.7
2.7
4.4
4.3
3.6
3.0
4.0
3.5
11
14
12
M
OS
11
Oj>
14
1.4
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT KVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT KVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATNG STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COM PARAT1VE STEEL
COMPARATNE STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
PRESENT NVENTDN STEEL
PRESENT NVENTDN STEEL
PRESENT NVENTDN STEEL
PRESENT NVENTDN STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARAT1VE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATNE STEEL
COMPARATNE STEEL
COMPARATNE STEEL 1
[0141]
[Table 5]
57
STEEL
TYPE
Al'
A2'
Bl'
B2'
CI'
C2'
Dl'
D2'
El'
E2'
Fl"
F2'
F3'
Gl'
C2'
G3'
HI"
112'
H3'
114'
115'
116'
H7"
H8"
119'
1110'
tin'
H12'
H13'
IT
E'
Jl'
J2'
Kl'
K2'
LI'
L2'
Ml'
M2'
M3'
NT
N2'
or
PI'
or Rl"
sr Tl'
al'
bl'
c l '
dl'
e l '
fl'
gl'
hi'
a'
STRUCTURAL
FRACTDN
B+1XP+3W
B+5%P+5*F
B+1KF
B
B + lW
F+30KB
B+4%lA
B+8M
B + 1XM
B + 1XM
B+3SF
B+1SF+DW
B+5KF
B+3KF
B
B+35SF
B+4%F
B +30XF+2SP
B+3KF
B+3KF
B+4KF
B+2HF
B+3XF
B+35SF
B+HF+IMP
B+2XF
B+4%F
B+4XF
B+2*F
B
F+33XB + 12W
B+5»F
B+2*F+2%P
B + 1SM
B+2SSF
B
B+2%F
B
B+3%F
B
B+3XF
B
B
B+2*F
B+3%F
B
B
B
B+26%F
B+5XP
B
B
B
B+2SF
B
B
B + 10M+5%rA
AVERAGE
VALUE OF POLE'
DENS1TIS OF
tooKon>
TO E23KU0>
ORENTATDN
GROUP
2.5
2.0
2.0
12
2.5
££
2.5
2.8
2.6
12
2.3
3.0
U
2.2
15
15
1.9
2.0
12
U
3.1
12
2.3
1.2
2.0
3.1
16
12
\2
2.0
2.3
2.2
11
32
2.4
2.3
11
2.1
2.1
11
2.4
2.2
2.0
2.1
2.4
2.7
2.2
2.5
2.3
2.2
2.2
12
12
2.2
SJ,
2.2
2.3
POLE
DENSITY OF
632K113)
3.0
2.2
1.9
5J>
3.5
61
3.4
3.8
3.3
51
3.1
4.2
SI
2.7
5,5
5J.
2.4
2.5
Li
55
3.9
M
3.4
1.7
2.5
3.8
JL6
5J.
1.4
2.2
2.5
2.7
SS
4.0
3.2
3.1
51
2.3
2.3
55
2.9
0.6
2.6
2.6
3.0
3.4
2.8
3.1
2.8
2.7
2.7
6,6
JL8
2.8
62
2.8
2.9
SEEOF
GRAN UNIT
2.5
2.9
5.6
4.4
2.1
5.4
4.1
1.9
S3
H
2.4
42
35
3.0
5.0
5.7
4.4
4.3
4.9
90
7.0
3.8
42
93
4.1
10.2
12
4.4
10.2
3.5
1.3
6.3
1A
5.1
10.2
5.4
9J)
3.7
9 !
M
3.1
U
5.5
5.4
5.9
4.1
43
2.4
3&
42
3.9
42
3.5
4.0
4.2
3.9
4.0
EQURXED
GRAN
RATD
%
59
67
68
24
64
il
65
62
64
27
64
54
as 66
54
45
67
66
40
55
62
34
55
74
66
70
52
65
70
69
66
67
il
50
58
60
25
68
68
57
64
67
67
67
65
59
66
62
65
66
66
66
64
65
66
65
65
TS/MPa
716
747
640
645
673
444
874
983
512
507
854
864
847
771
785
589
709
607
713
706
705
718
714
706
818
721
711
718
708
583
602
465
481
801
776
567
554
850
860
862
733
738
553
581
821
649
698
586
422
768
847
874
846
536
967
629
776
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23
22
22
21
25
33
17
16
28
21
19
16
16
21
17
22
21
25
19
U
17
19
20
9
18
20
21
20
21
28
30
30
25
16
9
25
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18
11
12
22
13
26
25
16
23
21
29
39
19
17
17
18
29
15
25
19
A/%
158
£1
181
£4
136
£2
97
55
173
52
93
100
fil
148
52
84
159
94
91
51
114
98
138
56
51
108
58
84
SS
153
101
186
103
107
44
164
73
119
11
as 150
66
181
173
98
152
151
189
88
Sfi
40
IS
51
£8
21
74
59
SHEET
THEKNESS/
M Nil UK
BEND
3.5
Lfi
5.4
22
2.6
21
3.2
IS
3.2
12
2.6
2.7
23
42
21
M
4.7
21
Ojj
12
2.8
21
3.2
2.5
L6
2.9
21
29
2.5
3.8
M
4.1
25
2.6
2.6
3.5
21
4.3
2.6
0j>
3.7
2.7
4.4
4.3
3.6
3.0
4.0
3.5
11
11
12
06
OS
11
2£
11
1.4
PRESENT KVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
PRESENT MVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
PRESENT NVENTDN STEEL
PRESENT NVENTDN STEEL
PRESENT NVENTDN STEEL
PRESENT NVENTDN STEEL
PRESENT NVENTDN STEEL
PRESENT NVENTDN STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
*
58
[Explanation of Codes]
[0142]
1
2
3
4
5
6
10
11
continuous hot rolling line
roughing mill
finishing mill
hot-rolled steel sheet
run-out-table
rolling stand
inter-stand cooling nozzle
cooling nozzle 11
59
[Name of Document] Claims
[Claim 1] A high-strength cold-rolled steel sheet having excellent local
deformability comprising:
in mass%,
C: not less than 0.02% nor more than 0.20%;
Si: not less than 0.001% nor more than 2.5%;
Mn: not less than 0.01%> nor more than 4.0%;
P: not less than 0.001% nor more than 0.15%;
S: not less than 0.0005% nor more than 0.03%;
Al: not less than 0.001% nor more than 2.0%;
N: not less than 0.0005% nor more than 0.01%; and
O: not less than 0.0005% nor more than 0.01%; in which Si + Al is limited to
less than 1.0%, and
a balance being composed of iron and inevitable impurities, wherein
an area ratio of bainite in a metal structure is 95% or more,
at a sheet thickness center portion being a range of 5/8 to 3/8 in sheet
thickness from the surface of the steel sheet, an average value of pole
densities of the {100}<011> to {223}<110> orientation group represented by
respective crystal orientations of {100}<011>, {116}<110>, {114}<110>,
{113}<110>, {112}<110>, {335}<110>, and {223}<110> is 4.0 or less, and a
pole density of the {332}<113> crystal orientation is 5.0 or less, and
a mean volume diameter of crystal grains in the metal structure is 7 um or
less.
[Claim 2] The high-strength cold-rolled steel sheet having excellent local
deformability according to claim 1, wherein
to crystal grains of the bainite, a ratio of the crystal grains in which a ratio of
60
a length dL in a rolling direction to a length dt in a sheet thickness direction:
dL/dt is 3.0 or less is 50% or more.
[Claim 3] The high-strength cold-rolled steel sheet having excellent local
deformability according to claim 1, further comprising:
one type or two or more types of
in mass%,
Ti: not less than 0.001% nor more than 0.20%,
Nb: not less than 0.001% nor more than 0.20%,
V: not less than 0.001% nor more than 1.0%, and
W: not less than 0.001% nor more than 1.0%.
[Claim 4] The high-strength cold-rolled steel sheet having excellent local
deformability according to claim 1, further comprising:
one type or two or more types of
in mass%,
B: not less than 0.0001% nor more than 0.0050%,
Mo: not less than 0.001% nor more than 1.0%,
Cr: not less than 0.001% nor more than 2.0%,
Cu: not less than 0.001% nor more than 2.0%,
Ni: not less than 0.001% nor more than 2.0%,
Co: not less than 0.0001% nor more than 1.0%,
Sn: not less than 0.0001% nor more than 0.2%,
Zr: not less than 0.0001% nor more than 0.2%, and
As: not less than 0.0001% nor more than 0.50%.
[Claim 5] The high-strength cold-rolled steel sheet having excellent local
deformability according to claim 1, further comprising:
one type or two or more types of
61
in mass%,
Mg: not less than 0.0001% nor more than 0.010%,
REM: not less than 0.0001% nor more than 0.1%, and
Ca: not less than 0.0001% nor more than 0.010%.
[Claim 6] The high-strength cold-rolled steel sheet having excellent local
deformability according to claim 1, wherein
on the surface, a hot-dip galvanized layer or an alloyed hot-dip galvanized
layer is provided.
[Claim 7] A manufacturing method of a high-strength cold-rolled steel
sheet having excellent local deformability, comprising:
on a steel billet containing:
in mass%,
C: not less than 0.02% nor more than 0.20%;
Si: not less than 0.001%) nor more than 2.5%;
Mn: not less than 0.01 % nor more than 4.0%;
P: not less than 0.001% nor more than 0.15%;
S: not less than 0.0005% nor more than 0.03%;
Al: not less than 0.001% nor more than 2.0%;
N: not less than 0.0005% nor more than 0.01 %; and
O: not less than 0.0005%) nor more than 0.01%; in which Si + Al is limited to
less than 1.0%>, and
a balance being composed of iron and inevitable impurities,
performing first hot rolling in which rolling at a reduction ratio of 40% or
more is performed one time or more in a temperature range of not lower than
1000°C nor higher than 1200°C;
setting an austenite grain diameter to 200 urn or less by the first hot rolling;
62
performing second hot rolling in which rolling at a reduction ratio of 30% or
more is performed in one pass at least one time in a temperature region of not
lower than a temperature Tl + 30°C nor higher than Tl + 200°C determined
by Expression (1) below;
setting the total reduction ratio in the second hot rolling to 50% or more;
performing final reduction at a reduction ratio of 30% or more in the second
hot rolling and then starting primary cooling in such a manner that a waiting
time t second satisfies Expression (2) below;
setting an average cooling rate in the primary cooling to 50°C/second or more
and performing the primary cooling in a manner that a temperature change is
in a range of not lower than 40°C nor higher than 140°C;
performing cold rolling at a reduction ratio of not less than 30% nor more
than 70%;
performing holding for 1 to 300 second/seconds in a temperature region of
Ae3 to 950°C;
performing secondary cooling at an average cooling rate of not less than
10°C/s nor more than 200°C/s in a temperature region of Ae3 to 500°C; and
performing an overaging heat treatment in which holding is performed for not
shorter than t2 seconds satisfying Expression (4) below nor longer than 400
seconds in a temperature region of not lower than 350°C nor higher than
500°C.
Tl (°C) = 850 + 10 x (C + N) x Mn + 350 x Nb + 250 x Ti + 40 x B + 10 x
Cr+lOOxMo+lOOx V - ( l )
t ^ 2.5 x tl - (2)
Here, tl is obtained by Expression (3) below.
tl = 0.001 x ((Tf - Tl) x Pl/100)2 - 0.109 x ((Tf - Tl) x Pl/100) + 3.1 - (3)
63
Here, in Expression (3) above, Tf represents the temperature of the steel billet
obtained after the final reduction at a reduction ratio of 30% or more, and PI
represents the reduction ratio of the final reduction at 30% or more.
log(t2) = 0.0002(T2 - 425)2 +1.18 ... (4)
Here, T2 represents an overaging treatment temperature, and the maximum
value of t2 is set to 400.
[Claim 8] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent local deformability according to claim 7, wherein
the total reduction ratio in a temperature range of lower than Tl + 30°C is
30% or less.
[Claim 9] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent local deformability according to claim 7, wherein
the waiting time t second further satisfies Expression (2a) below.
t < t l - ( 2 a )
[Claim 10] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent local deformability according to claim 7, wherein
the waiting time t second further satisfies Expression (2b) below.
tl ^ t ^ tl x 2.5 - (2b)
[Claim 11] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent local deformability according to claim 7, wherein
the primary cooling is started between rolling stands.
[Claim 12] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent local deformability according to claim 7, wherein
when heating is performed up to the temperature region of Ae3 to 950°C after
the cold rolling, an average heating rate of not lower than room temperature
nor higher than 650°C is set to HR1 (°C/second) expressed by Expression (5)
64
below, and
an average heating rate of higher than 650°C to Ae3 to 950°C is set to HR2
(°C/second) expressed by Expression (6) below.
HR1 ^ 0.3 ...(5)
HR2 S 0.5xHRl ...(6)
[Claim 13] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent local deformability according to claim 7, further
comprising:
forming a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on
the surface.