[Name of Document] DESCRIPTION
[Title of the Invention] HIGH-STRENGTH COLD-ROLLED STEEL SHEET
HAVING EXCELLENT UNIFORM ELONGATION AND HOLE
EXPANDABILITY AND MANUFACTURTNG METHOD THEREOF
[Technical Field]
[0001] The present invention relates to a high-strength cold-rolled steel
sheet having excellent uniform elongation and hole expandability that is
mainly used for automobile parts and the like, and a manufacturing method
thereof.
This application is based upon and claims the benefit of priority of the
prior Japanese Patent Application No. 201 1-095254, filed on April 21, 201 1,
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,
the strength of the high-strength steel sheet has to be increased more than
conventionally.
[0003] In order to use the high-strength steel sheet for an underbody part,
for example, burring workability has to be improved in particular. However,
when a steel sheet is increased in strength in general, formability decreases,
and uniform elongation important for drawing and bulging decreases.
[0004] In Non-Patent Document 1, there is disclosed a method in which
austenite is made to remain in a steel sheet structure to secure uniform
elongation. Further, in Non-Patent Document 2, there is disclosed a method
of securing uniform elongation with the same strength by making a metal
structure of a steel sheet complex.
[0005] Meanwhile, there is also disclosed control of a metal structure that
improves local ductility necessary for 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 improvement of bendability and hole
expandability.
[0006] This is a method to improve the hole expandability by making a
structure uniform by structure control, but in order to make a structure
uniform, a heat treatment from an austenite single phase becomes a basis as
disclosed in Non-Patent Document 4.
[0007] In order to attain achievement of strength and ductility,
Non-Patent Document 4 discloses that a transformation structure is controlled
by cooling control, thereby obtaining appropriate fractions of ferrite and
bainite. However, all the cases are to improve local deformability relying on
the structure control, and desired properties are greatly affected by how the
structure is formed.
[0008] Meanwhile, as a method of improving a material of a hot-rolled
steel sheet, there is disclosed a technique of increasing a reduction amount in
continuous hot rolling. This is what is called a technique of making crystal
grains fine, in which heavy reduction is performed at as low temperature as
possible in an austenite region and non-recrystallized austenite is transformed
to ferrite, to achieve making crystal grains of ferrite, which is the main phase
of a product, fine.
[0009] Non-Patent Document 5 discloses that by this grain refining,
increasing strength and increasing toughness are aimed. However,
Non-Patent Document 5 pays no attention to the improvement of hole
expandability, which is desired to be solved by the present invention, and does
not disclose also a means applied to a cold-rolled steel sheet.
[Prior Art Document]
mon-Patent Document]
[OOl 01 Non-Patent Document 1: Takahashi, Nippon Steel Technical
Report (2003) No. 378, p. 7
Non-Patent Document 2: 0. 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., (2000) Vol. 40, p. 920
Non-Patent Document 5: Nakayama Steel Works, Ltd. NFG Catalog
[Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[OOl 11 As described above, performing structure control including
inclusions is the main method for improving local ductility performance of a
high-strength steel sheet. However, since the structure control is performed,
form of precipitates and fractions of ferrite and bainite need to be controlled,
and it is essential to limit a metal structure to be a base.
[0012] Thus, the present invention has a task to improve uniform
elongation and burring workability of a high-strength steel sheet and improve
also anisotropy in the steel sheet by controlling the fractions and form of a
metal structure to be a base and controlling a texture. The present invention
has an object to provide a high-strength cold-rolled steel sheet having
excellent uniform elongation and hole expandability that solves this task, and
a manufacturing method thereof.
[Means for Solving the Problems]
[0013] The present inventors earnestly examined a method of solving the
above-described task. As a result, it was found that when rolling conditions
and cooling conditions are controlled to required ranges to form a
predetermined texture and steel sheet structure, a high-strength cold-rolled
steel sheet having excellent isotropic workability can be thereby
manufactured.
[0014] The present invention is made based on the above-described
knowledge and the gist thereof is as follows.
[00 1 51
111
A high-strength cold-rolled steel sheet having excellent uniform elongation
and hole expandability contains:
in mass%,
C: 0.01 to 0.4%;
Si: 0.001 to 2.5%;
Mn: 0.001 to 4.0%;
P: 0.001 to 0.15%;
S: 0.0005 to 0.03%;
Al: 0.001 to 2.0%;
N: 0.0005 to 0.01%; and
0: 0.0005 to 0.01%; in which Si +A1 is limited to less than 1.0%, and
a balance being composed of iron and inevitable impurities, in which
at a sheet thickness center portion being a range of 518 to 318 in sheet
thickness from the surface of the steel sheet, an average value of pole
densities of the {100)<011> to (2231-4 10> orientation group represented by
respective crystal orientations of { 100)<0 11>, (1 1614 lo>, (1 14)<1 lo>,
{113)<110>, {112)<110>, {335}<110>, and {223}<110> is 5.0 or less, and a
pole density of the {332)<113> crystal orientation is 4.0 or less,
a metal structure contains 5 to 80% of ferrite, 5 to 80% of bainite, and 1% or
less of martensite in terms of an area ratio and the total of martensite, pearlite,
and retained austenite is 5% or less, and
an r value (rC) in a direction perpendicular to a rolling direction is 0.70 or
more and an r value (r30) in a direction 30" from the rolling direction is 1.10
or less.
II21
The high-strength cold-rolled steel sheet having excellent uniform
elongation and hole expandability according to [I], in which
an r value (rL) in the rolling direction is 0.70 or more and an r value (r60) in a
direction 60' from the rolling direction is 1.10 or less.
[31
The high-strength cold-rolled steel sheet having excellent uniform
elongation and hole expandability according to [I], in which
in the metal structure, a mean volume diameter of crystal grains is 7 pm or
less, and an average value of a ratio of, of the crystal grains, a length dL in the
rolling direction to a length dt in a sheet thickness direction: dL/dt is 3.0 or
less.
[41
The high-strength cold-rolled steel sheet having excellent uniform
elongation and hole expandability according to [I], fbrther contains:
one type or two or more types of
in mass%,
Ti: 0.001 to 0.2%,
Nb: 0.001 to 0.2%,
B: 0.0001 to 0.005%,
Mg: 0.0001 to 0.01%,
Rem: 0.0001 to 0.1%,
Ca: 0.0001 to 0.01%,
Mo: 0.001 to 1.0%,
Cr: 0.001 to 2.0%,
V: 0.001 to 1.0%,
Ni: 0.001 to 2.0%,
Cu: 0.001 to 2.0%,
Zr: 0.0001 to 0.2'36,
W: 0.001 to 1.0%,
As: 0.0001 to 0.5%,
Co: 0.0001 to 1.0%,
Sn: 0.0001 to 0.2%,
Pb: 0.001 to 0.1%,
Y: 0.001 to 0.10%, and
Hf 0.001 to 0.10%.
[51
The high-strength cold-rolled steel sheet having excellent uniform
elongation and hole expandability according to [I], in which
on the surface, hot-dip galvanizing is performed.
[61
The high-strength cold-rolled steel sheet having excellent uniform
elongation and hole expandability according to [I], in which
after the hot-dip galvanizing, an alloying treatment is performed at 450 to
600°C.
[71
A manufacturing method of a high-strength cold-rolled steel sheet
having excellent uniform elongation and hole expandability, includes:
on a steel billet containing:
in mass%,
C: 0.01 to 0.4%;
Si: 0.001 to 2.5%;
Mn: 0.001 to 4.0%;
P: 0.001 to 0.15%;
S: 0.0005 to 0.03%;
Al: 0.001 to 2.0%;
N: 0.0005 to 0.01%; and
0: 0.0005 to 0.01%; in which Si + A1 is limited to less than 1.0%, and
a balance being composed of iron and inevitable impurities,
performing fxst 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 pm 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 T1 + 30°C nor higher than T1 + 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 pre-cold rolling 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 less than 40°C nor more than 140°C;
performing cold rolling at a reduction ratio of not less than 30% nor more
than 70%;
performing heating up to a temperature region of 700 to 900°C and
performing holding for not shorter than 1 second nor longer than 1000
seconds;
performing post-cold rolling primary cooling down to a temperature region of
580 to 750°C at an average cooling rate of 12"C/second or less;
performing post-cold rolling secondary cooling down to a temperature region
of 350 to 500°C at an average cooling rate of 4 to 300°C/second; 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.
T1 ("C) = 850 + 10 x (C + N) x Mn + 350 x Nb + 250 x Ti + 40 x B + 10 x
Cr + 100 x Mo + 100 x V --- (I)
Here, C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the
element (mass%).
t 5 2.5 x tl (2)
Here, tl is obtained by Expression (3) below.
tl = 0.001 x ((Tf - Tl) x ~11100-) 0~.1 09 x ((Tf - Tl) x P1/100) + 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 P1
represents the reduction ratio of the final reduction at 30% or more.
log(t2) = O.O002(T2 - 42512 + 1.18 .. . (4)
Here, T2 represents an overaging treatment temperature, and the maximum
value of t2 is set to 400.
[81
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent uniform elongation and hole expandability according to [7],
further includes:
after performing the pre-cold rolling primary cooling, performing
pre-cold rolling secondary cooling down to a cooling stop temperature of
600°C or lower at an average cooling rate of 10 to 300°C/second before
performing the cold rolling, and performing coiling at 600°C or lower to
obtain a hot-rolled steel sheet.
P I
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent uniform elongation and hole expandability according to [7],
in which
the total reduction ratio in a temperature range of lower than T1 + 30°C is
30% or less.
[lo1
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent uniform elongation and hole expandability according to [7],
in which
the waiting time t second further satisfies Expression (2a) below.
t < tl ..- (2a)
[Ill
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent uniform elongation and hole expandability according to [7],
in which
the waiting time t second further satisfies Expression (2b) below.
[I21
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent uniform elongation and hole expandability according to [7],
in which
post-hot rolling primary cooling is started between rolling stands.
[I31
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent uniform elongation and hole expandability according to [7],
in which
when the heating is performed up to the temperatyre region of 700 to 900°C
after the cold rolling, an average heating rate of not lower than room
temperature nor higher than 650°C is set to HR1 ("Clsecond) expressed by
Expression (5) below, and
an average heating rate of higher than 650°C to 700 to 900°C is set to HR2
("Clsecond) expressed by Expression (6) below.
HR1 2 0.3 ... (5)
HR2 2 0.5 x HR1 ... (6)
[I41
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent uniform elongation and hole expandability according to [7],
further includes:
performing hot-dip galvanizing on the surface.
[I51
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent uniform elongation and hole expandability according to [14],
further includes:
performing an alloying treatment at 450 to 600°C after performing the hot-dip
galvanizing.
[Effect of the Invention]
[0016] According to the present invention, it is possible to provide a
high-strength cold-rolled steel sheet that is not large in anisotropy even when
Nb, Ti, andlor the like arelis added and has excellent uniform elongation and
hole expandability.
[Brief Description of the Drawings]
[0017]
{FIG. 11 FIG. 1 is an explanatory view of a continuous hot rolling line.
[Mode for Carrying out the Invention]
[0018] Hereinafter, the present invention will be explained in detail.
[0019] First, there will be explained a high-strength cold-rolled steel
sheet having excellent uniform elongation and hole expandability of the
present invention, (which will be sometimes called a "present invention steel
sheet" hereinafter).
[0020] (Crystal orientation)
In the present invention steel sheet, an average value of pole densities
of the {100)<011> to {223)<110> orientation group at a sheet thickness
center portion being a range of 518 to 318 in sheet thickness from the surface
of the steel sheet is a particularly important characteristic value. As long as
the average value of the pole densities of the {100)<011> to {223)<110>
orientation group is 5.0 or less when X-ray diffi-action is performed at the
sheet thickness center portion being the range of 518 to 318 in sheet thickness
from the surface of the steel sheet to obtain pole densities of respective
orientations, it is possible to satisfy a sheet thicknessla bend radius 2 1.5
that is required to work a framework part to be required in recent years.
[002 11 When the above-described average value exceeds 5 .O, 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 satisfl the sheet thicknesslthe bend radius 2
1.5.
[0022] The average value of the pole densities of the {100)<011> to
{223)<110> orientation group is desirably 4.0 or less. When more excellent
hole expandability and small limited bendability are required, the
above-described average value is desirably 3.0 or less.
[0023] On the other hand, when the above-described average value
becomes less than 0.5, which is difficult to be achieved in a current general
continuous hot rolling process, deterioration of the local deformability is
concerned, so that the above-described average value is preferably 0.5 or
more.
[0024] The {100)<011>, {116)<110>, {114)<110>, {113)<110>,
(1 12)<1 lo>, (3351-4 lo>, and (2231-4 10> orientations are included in
the{100)<011> to {223)<110> 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 is measured by using a device of X-ray difiaction, EBSD
(Electron Back Scattering Diffi-action), or the like. Further, it can be
measured by an EBSP (Electron Back Scattering Pattern) method or an ECP
(Electron Channeling Pattern) method. It may be obtained fiom a
three-dimensional texture calculated by a vector method based on a pole
figure of {110), or may also be obtained from a three-dimensional texture
calculated by a series expansion method using a plurality (preferably three or
more) ofpole figures out ofpole figures of {110), {loo), {211), and (310).
COO261 For example, for the pole density of each of the above-described
crystal ' orientations, each of intensities of (00 1)[1-101, (1 16)[1-101,
(114)[1-101, (113)[1-101, (112)[1-101, (335)[1-101, and (223)[1-101 at a 42 =
45" cross-section in the three-dimensional texture (ODF) may be used as it is.
[0027] The average value of the pole densities of the {100)<011> to
(2231x1 10, orientation group is an arithmetic average of the pole densities of
these orientations. When it is impossible to obtain all the intensities of these
orientations, the arithmetic average of the pole densities of the respective
orientations of {100)<011>, {116)<110>, {114)<110>, {112}<110>, and
(2231x1 10> may also be used as a substitute.
[002 81 Further, due to the similar reason, a pole density of the
{332)<113> crystal orientation of a sheet plane at the sheet thickness center
portion being the range of 518 to 318 in sheet thickness from the surface of the
steel sheet has to be 4.0 or less. As long as it is 4.0 or less, it is possible to
satisfy the sheet thicknesslthe bend radius 2 1.5 that is required to work a
framework part to be required in recent years. It is desirably 3.0 or less.
[0029] When the pole density of the {332)<113> crystal orientation is
greater than 4.0, 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 thicknesslthe bend radius 2 1.5. On the other hand, 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, so that the pole density of the {332}<113> crystal
orientation is preferably 0.5 or more.
[0030] 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.
[003 11 The sample to be subjected to the X-ray diffraction is fabricated in
such a manner that the steel sheet is reduced in thickness to a predetermined
sheet thickness by mechanical polishing or the like, and next strain is
removed by chemical polishing, electrolytic polishing, or the like, and in the
range of 518 to 318 in sheet thickness from the surface of the steel sheet, an
appropriate plane becomes a measuring plane. 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 518 to 318 in sheet
thickness from the surface of the steel sheet, but also at as many thickness
positions as possible, and thereby the uniform elongation and the hole
expandability are further improved. However, the range of 518 to 318 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, 518
to 318 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 (Ill), (-Ill), (1-ll), (11-I), (-1-11),
(-1 1 -I), (1 -1 -I), 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 (1 11 ) . 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.
LO0331 (r value)
An r value (rC) in a direction perpendicular to the rolling direction is
important in the present invention steel sheet. As a result of earnest
examination, the present inventors found that good hole expandability and
bendability cannot always be obtained even when the pole densities of the
various crystal orientations are in the appropriate ranges. In order to obtain
good hole expandability and bendability, the ranges of the above-described
pole densities need to be satisfied, and at the same time, rC needs to be 0.70
or more. The upper limit of rC is not determined in particular, but if it is
1.10 or less, more excellent hole expandability can be obtained.
COO341 An r value (r30) in a direction 30" from the rolling direction is
important in the present invention steel sheet. As a result of earnest
examination, the present inventors found that good hole expandability and
bendability cannot always be obtained even when the pole densities of the
various crystal orientations are in the appropriate ranges. In order to obtain
good hole expandability and bendability, the ranges of the above-described
pole densities need to be satisfied,-and at the same time, r30 needs to be 1.10
or less. The lower limit of r30 is not determined in particular, but if it is 0.70
or more, more excellent hole expandability can be obtained.
[0035] As a result of earnest examination, the present inventors found
that if in addition to the pole densities of the various crystal orientations, rC,
and r30, an r value (rL) in the rolling direction and an r value (r60) in a
direction 60" from the rolling direction are rL 2 0.70 and r60 5 1.10
respectively, better hole expandability can be obtained.
[0036] The upper limits of rL and r60 are not determined in particular,
but if rL is 1.00 or less and r60 is 0.90 or more, more excellent hole
expandability can be obtained.
[0037] The above-described r values can be obtained by a tensile test
using a JIS No. 5 tensile test piece. Tensile strain to be applied is normally 5
to 15% in the case of a high-strength steel sheet, and the r values may be
evaluated in a range of the uniform elongation. Incidentally, the direction in
which bending working is performed varies depending on parts to be worked,
and thus it is not particularly limited, and in the case of the present invention
steel sheet, the similar bendability can be obtained even when the present
invention steel sheet is bent in any one of the directions.
COO381 Generally, a texture and the r values are correlated with each other,
but in the present invention steel sheet, limitation on the pole densities of the
crystal orientations and limitation on the r values are not synonymous with
each other, and unless both the limitations are satisfied at the same time, good
hole expandability cannot be obtained.
[0039] (Metal structure)
Next, there will be explained limiting reasons related to a metal
structure of the present invention steel sheet.
[0040] The structure of the present invention steel sheet contains 5 to
80% of ferrite in terms of an area ratio. Due to the existence of ferrite
having excellent deformability, the uniform elongation improves, but when
the area ratio is less than 5%, good uniform elongation cannot be obtained, so
that the lower limit is set to 5%. On the other hand, when ferrite being
greater than 80% in terms of an area ratio exists, the hole expandability
deteriorates drastically, so that the upper limit is set to 80%.
[0041] Further, the present invention steel sheet contains 5 to 80% of
bainite in terms of an area ratio. When the area ratio is less than 5%,
strength decreases significantly, so that the lower limit is set to 5%. On the
other hand, when bainite being greater than 80% exists, the hole expandability
deteriorates significantly, so that the upper limit is set to 80%.
[0042] In the present invention steel sheet, as the balance, the total area
ratio of 5% or less of martensite, pearlite, and retained austenite is allowed.
[0043] An interface between martensite and ferrite or bainite becomes a
starting point of cracking to thus deteriorate the hole expandability, so that
martensite is set to 1% or less.
[0044] Retained austenite is strain-induced transformed to be martensite.
An interface between martensite and ferrite or bainite becomes a starting
point of cracking, to thus deteriorate the hole expandability: Further, when a
lot of pearlite exists, the strength and workability are sometimes impaired.
Therefore, the total area ratio of martensite, pearlite, and retained austenite is
set to 5% or less.
[0045] (Mean volume diameter of crystal grains)
In the present invention steel sheet, it is necessary to set a mean
volume diameter of crystal grains in a grain unit to 7 pm or less. When
crystal grains having a mean volume diameter of greater than 7 pm exist, the
uniform elongation is low and further the hole expandability is also low, so
that the mean volume diameter of the crystal grains is set to 7 pm or less.
[0046] Here, conventionally, 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.
[0 0471 The "grain unit" of crystal grains determined in the present
invention is determined in the following manner in an analysis of the
orientations of the steel sheet by an EBSP (Electron Back Scattering Pattern).
That is, in an analysis of the orientations of the steel sheet by an EBSP, for
example, the orientations are measured at 1500 magnifications with a
measured step of 0.5 pm 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.
[0048] With respect to the crystal grains of the grain unit determined in
this manner, a circle-equivalent diameter d is obtained and the volume of the
crystal grains of each grain unit is obtained by 4/3rrd3. Then, a weighted
mean of the volume is calculated and the mean volume diameter (Mean
Volume Diameter) is obtained.
[0049] As there are more large crystal grains even though the number of
them is small, deterioration of local ductility becomes larger. Therefore, the
size of the crystal grains is not an ordinary size mean, and the 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 pm or less. It is desirably 5 pm 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.
[005 01 (Equiaxial property of crystal grains)
Further, as a result of earnest examination, the present inventors found
that when a ratio of, of the crystal grains in the grain unit, a length dL in the
rolling direction to a length dt in a sheet thickness direction: dL/dt is 3.0 or
less, the hole expandability improves greatly. This physical meaning is not
obvious, but it is conceivable that the shape of the crystal grains in the grain
unit is similar to a sphere rather than an ellipsoid, and thus stress
concentration in grain boundaries is alleviated and thus the hole expandability
improves.
[0051] Further, as a result of earnest examination, the present inventors
found that when an average value of the ratio of the length dL in the rolling
direction to the length dt in the sheet thickness direction: dL/dt is 3.0 or less,
good hole expandability can be obtained. When the average value of the
ratio of the length dL in the rolling direction to the length dt in the sheet
thickness direction: dL/dt is greater than 3.0, the hole expandability
deteriorates.
COO521 (Chemical composition)
Next, there will be explained reasons for limiting a chemical
composition of the present invention steel sheet. Incidentally, % according
to the chemical composition means mass%.
LO0531 C: 0.01 to 0.4%
C is an element effective for improving mechanical strength, so that
0.01% or more is added. It is preferably 0.03% or more, and is more
preferably 0.05% or more. On the other hand, when it exceeds 0.4%, the
workability and weldability deteriorate, so that the upper limit is set to 0.4%.
It is preferably 0.3% or less, and is more preferably 0.25% or less.
[0054] Si: 0.001 to 2.5%
Si is an element effective for improving the mechanical strength.
However, when Si becomes greater than 2.5%, the workability deteriorates
and further a surface flaw occurs, so that 2.5% is set to the upper limit. On
the other hand, it is difficult to decrease Si to less than 0.001% in a practical
steel, so that 0.001% is set to the lower limit.
[0055] Mn: 0.001 to 4.0%
Mn is also an element effective for improving the mechanical strength,
but when Mn becomes greater than 4.0%, the workability deteriorates, so that
4.0% is set to the upper limit. It is preferably 3.0% or less. On the other
hand, it is difficult to decrease Mn to less than 0.001% in a practical steel, so
that 0.001% is set to the lower limit. When elements such as Ti that
suppress occurrence of hot cracking caused by S are not sufficiently added
except Mn, Mn satisfying Mn/S 2 20 in mass% is desirably added.
[0056] P: 0.001 to 0.15%
The upper limit of P is set to 0.15% in order to prevent the
deterioration of the workability and cracking at the time of hot rolling or cold
rolling. It is preferably 0.04% or less. The lower limit is set to 0.001%
applicable in current general refining (including secondary refining).
[0057] S: 0.0005 to 0.03%
The upper limit of S is set to 0.03% in order to prevent deterioration
of the workability and cracking at the time of hot rolling or cold rolling. It is
preferably 0.01% or less. The lower limit is set to 0.0005% applicable in
current general refining (including secondary refining).
[0058] Al: 0.001 to 2.0%
For deoxidation, 0.001% or more of A1 is added. Further, A1
significantly increases a y to a transformation point, to thus be an effective
element when hot rolling at an Ar3 point or lower is directed in particular, but
when it is too much, the weldability deteriorates, so that the upper limit is set
to 2.0%.
[0059] N and 0: 0.0005 to 0.01%
N and 0 are impurities, and both elements are set to 0.01% or less in
order to prevent the workability from deteriorating. The lower limits are
each set to 0.0005% applicable in current general refining (including
secondary refining).
[0060] Si + Al: less than 1 .O%
When Si and A1 are contained excessively in the present invention
steel sheet, precipitation of cementite during an overaging treatment is
suppressed and the fraction of retained austenite becomes too large, so that
the total added amount of Si and A1 is set to less than 1%.
[0061] In the present invention steel sheet, one type or two or more types
of Ti, Nb, B, Mg, Rem, Ca, Mo, Cr, V, W, Zr, Cu, Ni, As, Co, Sn, Pb, Y, and
Hf, being elements that have been used up to now may be contained in order
to improve the hole expandability by controlling inclusions to make
precipitates fine.
[0062] Ti, Nb, and B are elements to improve the material through
mechanisms of fixation of carbon and nitrogen, precipitation strengthening,
structure control, fine grain strengthening, and the like, so that according to
needs, 0.001% or of Ti is added, 0.001% or more of Nb is added, and
0.0001% or more of B is added. Ti is preferably 0.01% or more, and Nb is
preferably 0.005% or more.
[0063] However, even when they are added excessively, no significant
effect is obtained, and the workability and manufacturability deteriorate
instead, so that the upper limit of Ti is set to 0.2%, the upper limit of Nb is set
to 0.2%, and the upper limit of B is set to 0.005%. B is preferably 0.003%
or less.
[0064] Mg, Rem, and Ca are elements to make inclusions harmless, so
that the lower limit of each of them is set to 0.0001%. Mg is preferably
0.0005% or more, Rem is preferably 0.001% or more, and Ca is preferably
0.0005% or more. On the other hand, when they are added excessively,
cleanliness of the steel deteriorates, so that the upper limit of Mg is set to
0.01%, the upper limit of Rem is set to 0.1%, and the upper limit of Ca is set
to 0.01%. Ca is preferably 0.01% or less.
[0065] Mo, Cr, Ni, W, Zr, and As are elements effective for increasing the
mechanical strength and improving the material, so that according to need,
0.001% or more of Mo is added, 0.00 1% or more of Cr is added, 0.001 % or
more of Ni is added, 0.001% or more W is added, 0.0001% or more of Zr is
added, and 0.0001% or more of As is added. Mo is preferably 0.01% or
more, Cr is preferably 0.01% or more, Ni is preferably 0.05% or more, and W
is preferably 0.0 1 % or more.
[0066] However, when they are added excessively, the workability is
deteriorated by contraries, so that the upper limit of Mo is set to 1.0%, the
upper limit of Cr is set to 2.0%, the upper limit of Ni is set to 2.0%, the upper
limit of W is set to 1.0%, the upper limit of Zr is set to 0.2%, and the upper
limit of As is set to 0.5%. Zr is preferably 0.05% or less.
[0067] V and Cu, similarly to Nb and Ti, are elements effective for
precipitation strengthening, and are elements causing less deterioration of the
local deformability ascribable to strengthening by addition than Nb and Ti, so
that V and Cu are elefnents more effective than Nb and Ti when high strength
and better hole expandability are required. Therefore, the lower limits of V
and Cu are both set to 0.001%. They are each preferably 0.01% or more.
[0068] However, when they are added excessively, the workability
deteriorates, so that the upper limit of V is set to 1.0% and the upper limit of
Cu is set to 2.0%. V is preferably 0.5% or less.
[0069] Co significantly increases the y to a transformation point, to thus
be an effective element when hot rolling at the point or lower is directed
in particular. In order to obtain an addition effect, 0.0001% or more is added.
It is preferably 0.001% or more. However, when it is added excessively, the
weldability deteriorates, so that the upper limit is set to 1.0%. It is
preferably 0.1% or less.
[0070] Sn and Pb are elements effective for improving wettability and
adhesiveness of galvanizing, so that 0.0001% or more of Sn is added and
0.001% or more of Pb is added. Sn is preferably 0.001% or more.
However, when they are added excessively, a flaw is likely to occur at the
time of manufacture, and fbrther toughness decreases, so that the upper limit
of Sn is set to 0.2% and the upper limit of Pb is set to 0.1%. Sn is preferably
0.1% or less.
[0071] Y and Hf are elements effective for improving corrosion
resistance. When the elements are each less than 0.001%, an addition effect
is not obtained, so that the lower limits of them are set to 0.001%. On the
other hand, when they each exceed 0.10%, the hole expandability deteriorates,
so that the upper limit of each of the elements is set to 0.10%.
[0072] (Manufacturing method)
Next, there will be explained a manufacturing method of the present
invention steel sheet, (which will be sometimes called a "present invention
manufacturing method" hereinafter). In order to achieve excellent uniform
elongation and hole expandability, it is important to form a texture that is
random in terms of pole densities and to control conditions of structural
fractions of ferrite and bainite and form dispersion. Hereinafter, details will
be explained.
[0073] A manufacturing method prior to hot rolling is not limited in
particular. That is, subsequently to melting by a shaft furnace, an electric
furnace, or the like, secondary refining may be variously performed, and then
casting may be performed by normal continuous casting, or by an ingot
method, or further by thin slab casting, or the like. In the case of a
continuous cast slab, it is possible that a continuous 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 continuous cast slab is subjected to hot
rolling continuously after casting. Incidentally, a scrap may also be used for
a raw material of the steel.
[0074] (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. The present invention steel sheet needs to satisfy the following
requirements. First, an austenite grain diameter after the rough rolling,
namely an austenite grain diameter before finish rolling is important. The
austenite grain diameter before the finish rolling is desirably small, and the
austenite grain diameter of 200 pm or less greatly contributes to making
crystal grains fine and homogenization of crystal grains, thereby making it
possible to finely and uniformly disperse martensite to be formed in a process
later.
[0075] In order to obtain the austenite grain diameter of 200 pm or less
before the finish rolling, it is necessary to perform rolling at a reduction ratio
of 40% or more one time or more in the rough rolling in a temperature region
of 1000 to 1200°C.
[0076] The austenite grain diameter before the finish rolling is desirably
100 pm or less, and in order to obtain this grain diameter, rolling at 40% or
more is performed two times or more. However, when in the rough rolling,
the reduction is greater than 70% or rolling is performed greater than 10 times,
there is a concern that the rolling temperature decreases or a scale is generated
excessively.
[0077] In this manner, when the austenite grain diameter before the finish
rolling is set to 200 pm or less, recrystallization of austenite is promoted in
the finish rolling, and through the formation of the texture and
uniformalization of the grain unit, uniform elongation and hole expandability
of a fmal product are improved.
[0078] 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. The austenite grain
diameter after the rough rolling is confinned in a manner that a steel sket
piece before .being subjected to the finish rolling is quenched as much as
possible, (which is cooled at 10°C/second or more, for example), and a cross
section of the steel sheet piece is etched to make austenite grain boundaries
appear, and the austenite grain boundaries are observed by an optical
microscope. On this occasion, at 50 or more magnifications, the austenite
grain diameter of 20 visual fields or more is measured by image analysis or a
point counting method.
[0079] (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.
[0080] 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.
[0081] 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 11 50°C.
[0082] In the finish rolling, a temperature determined by the chemical
composition of the steel sheet is set to T1, and in a temperature region of not
lower than T1 + 30°C nor higher than T1 + 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 518 to 318 in sheet
thickness from the surface of the steel sheet, the average value of the pole
densities of the (1 00}<0 1 1> to (22314 10> orientation group becomes 5.0 or
less and the pole density of the {332)<113> crystal orientation becomes 4.0
or less. This makes it possible to secure the uniform elongation and the hole
expandability of the final product.
[0083] Here, T1 is the temperature calculated by Expression (1) below.
T1 ("C)=850+10x(C+N)xMn+350xNb+250xTi+40xB+
10xCr+100xMo+100xV -..(I)
C, N, Mn, Nb, Ti, By Cr, Mo, and V each represent the content of the
element (mass%).
[0084] Heavy reduction in the temperature region of not lower than T1 +
30°C nor higher than T1 + 200°C and light reduction at lower than T1 + 30°C
thereafter control the average value of the pole densities of the (1 00)<0 1 1 > 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 518 to 318
in sheet thickness from the surface of the steel sheet, and thereby the uniform
elongation and the hole expandability of the final product are improved
drastically, as shown in Examples to be described later.
[0085] This T1 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 T1 temperature.
In order to obtain better uniform elongation and hole expandability, it is
important to accumulate strain by the heavy reduction, and the total reduction
ratio of 50% or more is essential in the finish rolling. 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.
[0086] When the total reduction ratio in the temperature region of not
lower than T1 + 30°C nor higher than T1 + 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 T1 + 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.
[0087] In the finish rolling, in order to promote the uniform
recrystallization caused by releasing the accumulated strain, the rolling at
30% or more is performed in one pass at least one time at not lower than T1 +
3 0°C nor higher than T1 + 200°C.
[0088] Incidentally, in order to promote the uniform recrystallization, it is
necessary to suppress a working amount in a temperature region of lower than
T1 + 30°C as small as possible. In order to achieve it, the reduction ratio at
lower than T1 + 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 TI + 30°C is desirably 0%.
[0089] The finish rolling is desirably finished at T1 + 30°C or higher. In
the hot rolling at lower than T1 + 30°C, the granulated austenite grains that
are recrystallized once are elongated, thereby causing a risk that the isotropy
deteriorates.
[0090] 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 uniform elongation and the hole
expandability are improved.
[009 11 A rolling ratio can be obtained by actual performances or
calculation fi-om the rolling load, sheet thickness measurement, orland 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 fi-om a line speed, the reduction ratio, orland like. Thereby, it is
possible to easily confm whether or not the rolling prescribed in the present
invention is performed.
[0092] 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 (1 00)<011> to {223)<110> orientation group becomes strong. As a
result, the uniform elongation and the hole expandability deteriorate
significantly.
[0093] 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 T1 + 30°C nor higher than T1 + 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.
[0094] (Pre-cold rolling primary cooling)
After final reduction at a reduction ratio of 30% or more is performed
in the finish rolling, pre-cold rolling primary cooling is started in such a
manner that a waiting time t second satisfies Expression (2) below.
t S 2.5 x tl (2)
Here, tl is obtained by Expression (3) below.
tl = 0.001 x ((Tf - Tl) x ~11100-) 0~.1 09 x ((Tf - TI) x P11100) + 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 P1
represents the reduction ratio of the final reduction at 30% or more.
[0095] 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."
[0096] In the finish rolling, the waiting time t second until the pre-cold
rolling 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.
[0097] When the waiting time t exceeds tl x 2.5, the recrystallization is
already almost completed, but the crystal grains grow significantly and grain
coarsening advances, and thereby the r values and the elongation are
decreased.
[0098] 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)
[0099] 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 5 t 5 t l x 2.5 -.- (2b)
[O 1001 Here, as shown in FIG. 1, 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 20% 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.
[O 1011 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
more is performed in one pass at least one time in the temperature region of
not lower than the temperature T1 + 30°C nor higher than T1 + 200°C.
33
Further, in the finishing mill 3, the total reduction ratio becomes 50% or
more.
[O102] Further, in the finish rolling process, after the final reduction at a
reduction ratio of 30% or more is performed, the pre-cold rolling 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 pre-cold rolling 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.
[0103] 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. 1, 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. 1, on the downstream side of the rolling), if the
start of the pre-cold rolling 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 pre-cold rolling 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.
[O104] Further, for example, when the final reduction at a reduction ratio
of 30% 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. 1, on the downstream side of
the rolling), even though the start of the ire-cold rolling 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 pre-cold
rolling 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
pre-cold rolling 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.
[0105] Then, in this pre-cold rolling 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.
[O106] 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 insuficient
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 uniform elongation and the
hole expandability also deteriorate. Further, when the temperature change is
greater than 140°C, an overshoot tobeyond an Ar3 transformation point
temperature is likely to be caused. In the case, even by the transformation
fi-om recrystallized austenite, as a result of sharpening of variant selection, the
texture is formed and the isotropy decreases consequently.
[0107] When the average cooling rate in the pre-cold rolling 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.
[0108] 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 fmish rolling to 18OC or lower.
[0109] The rolling ratio (reduction ratio) can be obtained by actual
performances or calculation from the rolling load, sheet thickness
measurement, orland 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, orland like, or
can be obtained by the both methods.
[O 11 01 Further, as has been explained previously, in order to promote the
uniform recrystallization, the working amount in the temperature region of
lower than TI + 30°C is desirably as small as possible and the reduction ratio
in the temperature region of lower than T1 + 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. 1, in passing through one or two or more of the
rolling stands 6 disposed on the fiont 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 T1 + 30°C nor higher than T1 + 200°C, and in passing 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),
the steel sheet is in the temperature region of lower than T1 + 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. 1, on the
downstream side of the rolling), even though the reduction is not performed
or is performed, the reduction ratio at lower than T1 + 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 TI + 30°C is desirably a reduction ratio of
I 10% or less in total. When the isotropy is further obtained, the reduction
ratio in the temperature region of lower than T1 + 30°C is desirably 0%.
[O 11 11 In the present invention manufacturing method, 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.
[O 1 121 (Pre-cold rolling secondary cooling)
In the present invention manufacturing method, it is preferred that
after the pre-cold rolling primary cooling, pre-cold rolling secondary cooling
should be performed to control the structure. The pattern of the pre-cold
rolling secondary cooling is also important.
[O 1 131 The pre-cold rolling secondary cooling is desirably performed
within three seconds after the pre-cold rolling primary cooling. When the
time to the start of the pre-cold rolling secondary cooling after the pre-cold
rolling primary cooling exceeds three seconds, the austenite grains become
coarse and the strength and the elongation decrease.
[0114] In the pre-cold rolling secondary cooling, the cooling is performed
down to a cooling stop temperature of 600°C or lower at an average cooling
rate of 10 to 300°C/second. When the stop temperature of this pre-cold
rolling secondary cooling is higher than 600°C and the average cooling rate of
the pre-cold rolling secondary cooling is less than 10°C/second, there is a
possibility that surface oxidation advances and the surface of the steel sheet
deteriorates. When the average cooling rate exceeds 300°C/second,
martensite transformation is promoted to drastically increase the strength,
resulting in that subsequent cold rolling becomes difficult to be performed.
[0 1 151 (Coiling)
After being obtained in this manner, the hot-rolled steel sheet can be
coiled at 600°C or lower. When a coiling temperature exceeds 600°C, the
area ratio of ferrite structure increases and the area ratio of bainite does not
become 5% or more. In order to bring the area ratio of bainite to 5% or
more, the coiling temperature is preferably set to 600°C or lower.
[O 1 161 (Cold rolling)
A 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.
[O 1 1 71 (Heating and holding)
The steel sheet that has been subjected to the cold rolling (a
cold-rolled steel sheet) is thereafter heated up to a temperature region of 700
to 900°C and is held for not shorter than 1 second nor longer than 1000
seconds in the temperature region of 700 to 900°C. By this heating and
holding, work hardening is removed. When the steel sheet after the cold
rolling is heated up to the temperature region of 700 to 900°C in this manner,
an average heating rate of not lower than room temperature nor higher than
650°C is set to HR1 ("Clsecond) expressed by Expression (5) below, and an
average heating rate of higher than 650°C to the temperature region of 700 to
900°C is set to HR2 ("Clsecond) expressed by Expression (6) below.
HR1 2 0.3 ... (5)
HR2 5 0.5 x HR1 ... (6)
[0 11 81 The hot rolling is performed under the above-described condition,
and further post-hot rolling 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 and the texture becomes likely to remain in the steel sheet.
As a result, the r values 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.
[O 1 191 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 (OCIsecond) or more.
[0120] On the other hand, when the average heating rate HR2 of higher
than 650°C to the temperature region of 700 to 900°C is large, ferrite existing
in the steel sheet after the 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 a strong texture, to thus adversely affect the
properties such as the r values and the isotropy, and this non-recrystallized
Therefore, in the temperature range of higher than 650°C to the temperature
region of 700 to 900°C, the average heating rate HR2 needs to be 0.5 x HR1
("Clsecond) or less.
[0121] Further, when a heating temperature is lower than 700°C or a
I holding time in the temperature region of 700 to 900°C is shorter than one
I second, reverse transformation fiom ferrite does not advance sufficiently and
in subsequent cooling, a bainite phase cannot be obtained, resulting in that
sufficient strength cannot be obtained. On the other hand, when the heating
temperature is higher than 900°C or the holding time in the temperature
region of 700 to 900°C is longer than 1000 seconds, the crystal grains become
coarse and the area ratio of the crystal grains each having a grain diameter of
200 pm or more increases.
[O 1221 (Post-cold-rolling primary cooling)
After the heating and holding, post-cold rolling primary cooling is
performed down to a temperature region of 580 to 750°C at an average
cooling rate of 12"C/second or less. When a finishing temperature of the
post-cold rolling primary cooling exceeds 750°C, ferrite transformation is
promoted to make it impossible to obtain 5% or niore of bainite in terms of an
area ratio. When the average cooling rate of this post-cold rolling primary
cooling exceeds 12"C/second and the finishing temperature of the post-cold
rolling primary cooling is lower than 580°C, the grain growth of ferrite does
not advance suficiently to make it impossible to obtain 5% or more of ferrite
in terms of an area ratio.
[O 1231 (Post-cold rolling secondary cooling)
After the post-cold rolling primary cooling, post-cold rolling
secondary cooling is performed down to a temperature region of 350 to 500°C
at an average cooling rate of 4 to 300°C/second. When the average cooling
rate of the post-cold rolling secondary cooling is less than 4"CIsecond or the
post-cold rolling secondary cooling is fmished at a temperature of high'& than
500°C, pearlite transformation advances excessively to create a possibility
that 5% or more of bainite cannot be obtained finally in terms of an area ratio.
Further, when the average cooling rate of the post-cold rolling secondary
cooling is greater than 300°C/second or the post-cold rolling secondary
cooling is finished at a temperature of lower than 350°C, martensite
transformation advances and there is a risk that the area ratio of martensite
becomes greater than 1%.
[0 1241 (Overaging heat treatment)
Subsequently to the post-cold rolling secondary cooling, an overaging
heat treatment is performed in a temperature range of not lower than 3 5 0 " ~
nor higher than 500°C. A holding time in this temperature range is set to t2
seconds satisfying Expression (4) below according to an overaging treatment
temperature T2 or longer. However, in consideration of an applicable
temperature range of Expression (4), the maximum value of t2 is set to 400
seconds.
log(t2) = O.O002(T2 - 425)' + 1.18 ... (4)
[0125] Incidentally, in this overaging heat treatment, the holding does not
mean only 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.
[0126] 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 hole expandability 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
saltslinorganic 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 bulging forming but also to combined forming mainly
composed of bending working such as bending, bulging, and drawing.
[0127] When hot-dip galvanizing is performed on the present invention
steel sheet, an alloying treatment may be performed after the galvanizing.
The alloying treatment is performed in a temperature region of 450 to 600°C.
When an alloying treatment temperature is lower than 450°C, the alloying
does not advance sufficiently, and when it exceeds 600°C, on the other hand,
the alloying advances too much and corrosion resistance deteriorates.
Therefore, the alloying treatment is performed in the temperature region of
450 to 600°C.
Example
[0 1281 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 Tables 2 and 3.
Further, structural constitutions and mechanical properties of respective steel
types under the manufacturing conditions in Tables 2 and 3 are shown in
Tables 4 and 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. Further, in Table 2 to
Table 5, English letters A to T and English letters a to i that are added to the
steel types indicate to be components of Steels A to T and a to i in Table 1
respectively.
[0129] There will be explained results of examinations using invention
steels "A to T" and using comparative steels "a to h," which have the
chemical compositions shown in Table 1. Incidentally, in Table 1, each
numerical value of the chemical compositions means mass%.
[0130] These steels were cast and then as they were, or were heated to a
temperature region of 1000 to 1300°C after once being cooled down to room
temperature, and thereafter were subjected to hot rolling, cold rolling, and
cooling under the conditions shown in Table 2 and Table 3.
[0131] 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 A3, E3, and M2, in the rough rolling, the
rolling at a reduction ratio of 40% or more in one pass was not performed.
The number of times of reduction at a reduction ratio of 40% or more and
each reduction ratio (%) in the rough rolling, and an austenite grain diameter
(pm) after the rough rolling (before finish rolling) are shown in Table 2.
Incidentally, a temperature T1 ("C) and a temperature Acl ("C) of the
respective steel types are shown in Table 2.
[01321 After the rough rolling was finished, the fmish 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 T1 + 30°C nor higher than T1 + 200°C,
and in a temperature range of lower than T1 + 30°C, the total reduction ratio
was set to 30% or less. Incidentally, in the fmish rolling, rolling at a
reduction ratio of 30% or more in one pass was performed in a final pass in
the temperature region of not lower than T1 + 30°C nor higher than T1 +
200°C.
[0133] However, with respect to Steel types A4, A5, A6, and B3, the
rolling at a reduction ratio of 30% or more was not performed in the
temperature region of not lower than T1 + 30°C nor higher than T1 + 200°C.
Further, with regard to Steel types P2 and P3, the total reduction ratio in the
temperature range of lower than T1 + 30°C was greater than 30%.
[0134] Further, in the finish rolling, the total reduction ratio was set to
50% or more. However, with regard to Steel types A4, A5, A6, B3, and C3,
the total reduction ratio in the temperature region of not lower than T1 + 30°C
nor higher than T1 + 200°C was less than 50%.
[0135] Table 2 shows, in the finish rolling, the reduction ratio (%) in the
final pass in the temperature region of not lower than T1 + 30°C nor higher
than T1 + 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) (%). Further, Table 2
shows, in the fmish rolling, the total reduction ratio (%) in the temperature
region of not lower than T1 + 30°C nor higher than T1 + 200°C, a
temperature (OC) after the reduction in the final pass in the temperature region
of not lower than T1 + 30°C nor higher than T1 + 200°C, and a maximum
working heat generation amount ("C) at the time of the reduction in the
temperature region of not lower than T1 + 30°C nor higher than T1 + 200°C.
[0136] After the final reduction in the temperature region of not lower
than T1 + 30°C nor higher than T1 + 200°C was performed in the finish
rolling, pre-cold rolling primary cooling was started before a waiting time t
second exceeding 2.5 x tl. In the pre-cold rolling primary cooling, an
average cooling rate was set to 50°C/second or more. Further, a temperature
change (a cooled temperature amount) in the pre-cold rolling primary cooling
was set to fall within a range of not less than 40°C nor more than 140°C.
[0137] However, with respect to Steel type 52, the pre-cold rolling
primary cooling was started after the waiting time t second exceeded 2.5 x tl
since the final reduction in the temperature region of not lower than T1 +
30°C nor higher than T1 + 200°C in the finish rolling. With regard to Steel
type T2, the temperature change (cooled temperature amount) in the pre-cold
rolling primary cooling was less than 40°C, and with regard to Steel type 53,
the temperature change (cooled temperature amount) in the pre-cold rolling
primary cooling was greater than 140°C. With regard to Steel type T3, the
average cooling rate in the pre-cold rolling primary cooling was less than
50°C/second.
[O 13 81 Table 2 shows tl (second) of the respective steel types, the waiting
time t (second) from the final reduction in the temperature region of not lower
than T1 + 30°C nor higher than T1 + 200°C to the start of the pre-cold rolling
primary cooling in the finish rolling, tltl, the temperature change (cooled
amount) ("C) in the pre-cold rolling primary cooling, and the average cooling
rate ("C/second) in the pre-cold rolling primary cooling.
[0139] After the pre-cold rolling primary cooling, pre-cold rolling
secondary cooling was perfonned. After the pre-cold rolling primary
cooling, the pre-cold rolling secondary cooling was started within three
seconds. Further, in the pre-cold rolling secondary cooling, the cooling was
performed down to a cooling stop temperature of 600°C or lower at an
average cooling rate of 10 to 300°C/second, coiling was performed at 600°C
or lower, and hot-rolled original sheets each having a thickness of 2 to 5 mm
were obtained.
[O 1401 However, with regard to Steel type D3, three seconds passed until
the pre-cold rolling secondary cooling was started after the pre-cold rolling
primary cooling. Further, with regard to Steel type D3, the average cooling
rate of the pre-cold rolling secondary cooling was greater than 300°C/second.
Further, with regard to Steel type E3, the cooling stop temperature of the
pre-cold rolling secondary cooling (coiling temperature) was higher than
600°C. Table 2 shows, of the respective steel types, the time(second) to the
start of the pre-cold rolling secondary cooling after the pre-cold rolling
primary cooling, the average cooling rate("C/second) of the pre-cold rolling
secondary cooling, and the cooling -stop temperature("C) of the pre-cold
rolling secondary cooling (coiling temperature).
[0141] 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%. However, with regard to Steel type T4, the reduction ratio of the
cold rolling was less than 30%. Further, with regard to Steel type T5, the
reduction ratio of the cold rolling was greater than 70%. Table 3 shows the
reduction ratio (%) of the cold rolling of the respective steel types.
[0142] After the cold rolling, heating was performed up to a temperature
region of 700 to 900°C and holding was performed for not shorter than 1
second nor longer than 1000 seconds. Further, when the heating was
performed up to the temperature region of 700 to 900°C, an average heating
*
rate HR1("C/second) of not lower than room temperature nor higher than
650°C was set to 0.3 or more (HR1 2 0.3), and an average heating rate
HR2("C/second) of higher than 650°C to 700 to 900°C was set to 0.5 x HR1
or less (HR2 5 0.5 x HR1).
[0143] However, with regard to Steel type Al, a heating temperature was
higher than 900°C. With regard to Steel type Q2, the heating temperature
was lower than 700°C. With regard to Steel type Q3, a heating and holding
time was shorter than one second. With regard to Steel type Q4, the heating
and holding time was longer than 1000 seconds. Further, with regard to
Steel type T6, the average heating rate HR1 was less than 0.3 ("Clsecond).
With regard to Steel type T7, the average heating rate HR2 ("Clsecond) was
greater than 0.5 x HR1. Table 3 shows the heating temperature ("C) and the
average heating rates HR1 and HR2 ("Clsecond) of the respective steel types.
[0144] After the heating and holding, post-cold rolling primary cooling
was performed down to a temperature region of 580 to 750°C at an average
cooling rate of 12"C/second or less. However, with regard to Steel type A2,
the average cooling rate in the post-cold rolling primary cooling was greater
than 12"C/second. Further, with regard to Steel type A2, a stop temperature
of the post-cold rolling primary cooling was lower than 580°C, and with
regard to Steel type K1, the stop temperature of the post-cold rolling primary
cooling was higher than 740°C. Table 3 shows, of the respective steel types,
the average cooling rate ("Clsecond) and the cooling stop temperature (OC) in
the post-cold rolling primary cooling.
[0145] Subsequently to the post-cold rolling primary cooling, post-cold
rolling secondary cooling was performed down to a temperature region of 350
to 500°C at an average cooling rate of 4 to 300°C/second. However, with
regard to Steel type A5, the average cooling rate of the post-cold rolling
secondary cooling was less than 4"C/second. With regard to Steel type P4,
the average cooling rate of the post-cold rolling secondary cooling was
greater than 300°C/second. Further, with regard to Steel type A2, a stop
temperature of the post-cold rolling secondary cooling was higher than 500°C,
and with regard to Steel type G1, the stop temperature of the post-cold rolling
secondary cooling was lower than 350°C. Table 3 shows the average
cooling rate("C1second) in the post-cold rolling secondary cooling of the
respective steel types.
[0 1461 Subsequently to the post-cold rolling secondary cooling, an
overaging heat treatment (OA) was performed at the stop temperature of the
post-cold rolling secondary cooling. The range of the temperature of this
overaging heat treatment (OA) (stop temperature of the post-cold rolling
secondary cooling) was set to not lower than 350°C nor higher than 500°C.
Further, the time of the overaging heat treatment (OA) was set to not shorter
than t2 seconds nor longer than 400 seconds. However, with regard to Steel
type A2, a heat treatment temperature of the overaging was higher than 500°C,
and with regard to Steel type G1, the heat treatment temperature of the
overaging was lower than 350°C. Further, with regard to Steel type Dl, a
treatment time of the overaging was shorter than t2 seconds, and with regard
to Steel types C2 and GI, the treatment time of the overaging was longer than
400 seconds. Table 3 shows the heat treatment temperature of the overaging
("C), t2 (second), and the treatment time (second) of the respective steel
types.
lo1471 After the overaging heat treatment, skin pass rolling at 0.5% was
performed and material evaluation was performed. Incidentally, on Steel
type Sl, a hot-dip galvanizing treatment was performed. On Steel type TI,
an alloying treatment was performed in a temperature region of 450 to 600°C
after galvanizing.
[O 1481 Table 4 shows area ratios (structural fractions) (%) of ferrite,
bainite, pearlite, martensite, and retained austenite in a metal structure of the
respective steel types, and, of the respective steel types, a mean volume
diameter dia(average value) of crystal grains (pm), and a ratio of, of the
crystal grains, a length dL in the rolling direction to a length dt in the sheet
thickness direction: dL/dt. Table 5 shows, of the respective steel types, an
average value of pole densities of the (1 00)<011> to {223)<1 lo> orientation
group and a pole density of the {332)<113> crystal orientation at a sheet
thickness center portion being a range of 518 to 318 in sheet thickness fiom the
surface of the steel sheet. Incidentally, the structural fraction was evaluated
by the structural fraction before the skin pass rolling. Further, Table 5 shows,
as mechanical properties of the respective steel types, tensile strength TS
(MPa), uniform elongation u-El (%), an elongation percentage El (%), and a
hole expansion ratio h (%) as an index of the local deformability. Table 5
shows rC, rL, r30, and r60 each being the r value.
[0149] Incidentally, a tensile test was based on JIS Z 2241. 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 pm pitch on a 318 to 518 region
at sheet thickness of a cross section parallel to the rolling direction. Further,
as indexes of the uniform elongation and the hole expandability, TS x EL was
set to 8000 (MPa.%) or more, and desirably set to 9000 (MPa.%) or more,
and TS x h was set to 30000 (MPa.%) or more, preferably set to 40000
(MPa.%) or more, and still more preferably set to 50000 (MPa.%) or more.
Ti Nb B Me. Rem Ca Mo Cr N W Zr As V Cu Co Sn Pb . Y Hf NOTE
- 0.00 - - - - - - - - - - - - - - - - - INVPlnON STEEL
- 0.00 0.005 - - - - - - - - - - - - - - - - INVPlTiON STEEL
- 0.04 - - - - - - - - - - - - - - - - - INVENTIONSTEEL
- 0.04 - - - 0.002 - - - - - - - - - - - - - INVEtiTlON STEEL
- 0.02 - - 0.002 - - - - - - - - - - - - - - INWTlON S'IEEL
- 0.02 - - 0.002 - - - - - - - - - - - - - - INVENTON STEEL
0.0210.00 0.002 - - - 0.03 0.35 - - - - - - - - - - - INWONSTEEL
0.0210.00 0.002 - - - 0.03 0.35 - - - - - - - - - - - INVPmON S7PL
- 0.02 - 0 - 0 0015 - - - - - - 0.03 - - - - - - I N W O N STEEL
0.1 0.02 - 0 - 0.00l5 - - - - - - 0.03 - . - - - - N W O N STEEL - - - 0 - - 0.1 - - - - - 0.1 - - - - - - INVPlTiONS7EFL - 0.05 - 0 - 0.002 0.1 - - - - - 0.1 - - - - - - INMNnONSTEEL
0.036 0.089 0.001 - - - - - - - - - - - - - - 0 - WVEWYONSTEa
0.089 0.036 0.001 - - - - - - - - - - - - - - - 0 INVENTONSTEEL
0.042 0.121 9E-04 - - - - - - - 0 - - - - 0 - - - INVENTON STEEL
0.042 0 121 9E44 - 0 004 - - - - - - - - - INVPmON STEEL - - - - - - - - - 0.1 - - - - - - - - - INVFNnONSTEEL
- - - - - - - - 0.1 - - - - - - - - - - INVENTONSTEEL
- - - - - - - - - - - - - - - - 0 - - INVPlTlONSTEa
0.12 - - - - - - - - - - 0..002 - 0.2 - - - - - INVPFnONSTEEL
- - - - - - - - - - - - - - - - -c o ~.p m n SwTE EL
- -1.5 - - - - - - - - - - - - - - - - - COMPARATNE STEEL - - - - - - - - - - - - - - - - - - COMPARATIVE STEa
- - - - - - - - - - - M - - - - - - COMPARATIVESTEEL
- - - - - - - - - - - - - - LZ - - - - COMPARATIVESTEEL
- - - - - - - - - - - - - - - - - - COMPARATNESTEEL - - - - - - - - - - - - - - - - - - COMPARATNESTEEL
- - - - - - - - - - - - - - - - - - - COMPARAnVESTEEL
TII'C C Si Mn P S AI N 0
A 851 0.0700.08 1.30 0.0150.0040.0400.00260.00320.12
B 851 0.070 0.08 1.30 0.015 0.004 0.040 0.0026 0.0032
C 865 0.080 0.31 1.35 0.012 0.005 0.016 0.00320.0023
D 865 0.080 0.31 1.35 0.012 0.005 0.016 0.00320.0023
E 858 0.060 0.87 1.20 0.009 0.004 0.038 0.00330.0026
F 858 0.060 0.30 1.20 0.009 0.004 0.500 0.00330.0026
G 865 0.210 0.15 1.62 0.012 0.003 0.026 0,00330,0021
H 865 0.210 0.90 1.62 0.012 0.003 0.026 0.00330.0021
I 861 0.035 0 67 1.88 0.015 0.003 0.045 0.0028 00029
J 886 0035 0.67 1.88 0.015 0.003 0.045 0.00280.0029
K 875 0.1800.482.72 0.0090.0030.0500.00360.00220.53
L 892 0.1800.482.72 0.0090.0030.0500.00360.00220.53
M 892 0.060 0.11 2.12 0.01 0.005 0.033 0.0028 0.0035
N 886 0.060 0.11 2.12 0.01 0.005 0.033 0.0028 0.0035
0 903 0.040 0.13 1.33 0.01 0.005 0.038 0.00320.0026
P 903 0.040 0.13 1.33 0 01 0.005 0.038 0.0036 0.0029
Q 852 0.1800.500.90 0.0080.W30.0450.00280.00290.55
R 852 0.1800.30 1.30 0.08 0.0020.0300.00320.00220.33
S 852 0.180 0.21 1.30 0.01 0.002 0.650 0.00320.0035
T 880 0.0350.02 1.30.0.01 0.0020.0350.00230.00330.06
a 856 9.450 0.52 1.33 0.003 0.045 0.0026 0.0019
b 1376 0.072 0.15 1.42 0.014 0.004 0.036 0.00220.0025
c 851 0.1100.231.120.0210.0030.0260.W250.00230.26
d 1154 0250 0.23 1.56 0.024 0.034 0.00220.0023
e 854 0.250 0.23 1.54 0.02 0.002 0.038 0.00260.00320.27
f 854 0.250 0.21 1.54 0.02 0.002 0.034 0.00260.0023
g 853 0.220 0.2 1.53 0.015 0.004 0.031 0.00280.0026
h 852 0.180 2.30 0.90 0.008 0003 0.045 0 . 0 0 2 8 0 . 0 0 2 2 E
Si+AI
0.12
0.33
0.33
0.91
0.80
0.18
0.93
0.72
0.72
0.14
0.14
0.17
0.17
0.86
0.57
0.19
0.26
0.24
0.23

FERRm B m P!3RLlTE MARTENSITE RETAINED7
S ! FRACTION FRACTION FRACTION FRACTIOW FRACTION dia b) dl(p) dT b) mEy3 NOTE
Ph Ph Ph Wh Ph
Al 57.2 39.5 3.1 0.1 0.1 230.0 235.9 213.8 1.1 COMPARAT(VESTEFl.
A Z M -97.4 0.2 0.3 0.1 5.8 5.4 3.2 1.7 TaMPA3
59.2 40.0 0.1 0.4 0.3 10.0 9.6 9.6 1 .O m-,
A4 62.9 36.0 0.2 0.6 0.3 Q 7.6 2.3 33 coMpARATlvEsTEEL
A5 56.2 33.4 0.1 -10.1 0.2 -8.0 7.6 1 9 u! - A6 61.6 38.0 0.1 0.1 0.2 E! 7.5 0.8 coMpARAnvEsTa
BI 60.6 39.0 0.1 0.1 0.2 5.3 4.9 2.7 1.8 PRESENTINVENTIONSTEEL
B2 55.0 44.0 0.1 0.7 0.2 5.8 5.4 2.5 2 1 PRESENTEWENIlONSTEEL
B3 60.7 37.0 0.1 0.9 1 3 -8.0 7.6 1.8 3J - CI 64.0 35.0 0.1 0.6 0.3 5.5 5.1 2.6 1.9 PRESENTINVENTIONSTEEL
C2 60.0 42 0.1 0.8 34.8 6.1 5.7 2.4 .-2.M3
C3 65.4 33.0 0.4 0.9 0.3 5.7 5.3 2.5 2.1 COMPARATlVESTEn
Dl 53 8 6.0 0.1 -39.8 0.3 5.4 5.0 2.0 2.-6O C
D2 58.0 38.0 3.1 0.8 0.1 6.1 6.9 4.6 1.5 PRESENI INVENTION STEEL
D3 42.3 57.0 0.1 0.5 0.1 11.0 11.8 7.8 1.5 coMpARATIvEsTFEI.
El 55.5 41.9 2.1 0.4 0.1 6.0 6.8 3.3 2.1 PRESENT INVENTION STEEL
E2 53.1 42.7 4.0 0.0 0.2 5.3 6.1 3 3 1.8 PRESENIINVENTIONSTEEL
E3 67.2 28.0 3.7 0.9 0.2 10.9 11.7 7.8 1.5 - F1 55.5 41.9 1.5 0.9 0.2 6.0 6.8 5.6 1.2 PRESENT EWENIlON STEEL
F2 53.1 43.0 3.1 0.5 0.3 5.3 6.1 3.2 1.9 PRESENIINVENTIONSTEEL
F3 53.3 44.7 1.5 0.3 0.2 6.0 6.8 3.9 1.7 PRESENTINVENTIONSTEEL
GI 57.4 23 0.2 -40.2 0.2 5.3 6.1 3.2 1.9 T a F i
G2 59.8 36.0 3.7 0 3 0.2 6.4 7.2 7.0 1.0 IXP-,
HI 56.2 40.0 3.2 0.5 0.1 6.0 6.8 3.0 2.2 PRESENT INVENTION STEEL
I1 50.9 46.0 2.7 0.2 0.2 6.1 6.9 3.3 2.1 PRESENTINVENTIONSTEEL
12 67.9 30.0 1.3 0.5 0 3 6.2 7.0 2.5 2.8 PRESENT EWENIlON STEEL
I3 56.7 40.0 2.4 0.6 0.3 9.1 4.5 2.0 lmy.lBR-
11 52.8 45.0 1.5 0.5 0.2 6.1 6.0 2.7 2.2 PRESENTINVENTIONSTEEL
12 58.0 40.0 1.7 0.1 0.2 U 8.9 4.1 2.2 COMPARATIVESTEEI.
13 53.1 43.0 3.5 0.2 0.2 6.2 6.1 5.0 1.2 r n ! L
K1 202 LC? ZJ 0.1 0.1 6.0 5.9 3.4 1.7 CQh6-L
LI 47.3 52.1 0.2 0.3 0.1 6.0 6.3 3.6 1.7 PRESENT INVENTION STEEL
M1 64.2 35.0 0.3 0.4 0.1 5.6 5.9 2.0 2.9 PRESENT INVENTION STEEL
M2 53.9 43.0 2.8 0 2 0.2 83 8.6 5.7 1.5 EM!-
N1 56.4 39.0 4 1 0.3 0.2 5.6 5.9 2.9 2.0 PRESENTINVENTIONSTEEL
01 58.1 38.0 3.3 0.4 0.2 5.1 5.4 3.2 1.7 PRESENT INVENTION STEEL
02 62.1 33.0 4.3 0.4 0.2 83 8.6 2.4 3A coMpARATlvEsTEEL
PI 56.9 40.0 2.7 0.3 0.1 5.1 5.4 2.1 2.5 PRESENTINVENTIONSTEEL
P2 64.0 35.0 0.1 0.6 0.3 2.5 2.8 0 6 fl - P3 58.0 38.0 3.1 0.8 0.1 2.8 2.9 0.7 COMPARATlVESTEEL
P4 43.2 I& 0.1 -55.4 0.3 5.3 6.1 3.3 1.8 T a F k
QI 59.7 38.0 2.1 0.1 0.1 5.2 5.5 2.4 2.3 PRESENT INVENTION STEEL
Q2 8.622 tt U 0.2 0.2 6.1 6.9 3.3 2.1 - Q3 78.9 -1.5 0.1 -193 0.2 5.3 4.9 2.7 1.8
Q4 67.9 30.0 1.3 0.5 0.3 220.5 221.0 220.0 2.8 - RI 63.3 34.5 2.0 0.1 0.1 5.1 5.4 2.1 2.6 PRESENT INVENTION STEEL
R2 63.1 35.2 1.3 0.2 0.2 4.1 4.4 1.8 2.5 PRESENTINVENTIONSTEEL
R3 61.8 35.7 2.1 0.2 0.2 4.2 4.5 1.9 2.4 PRESENT INVENTION STEEL
R4 58.9 38.9 1.9 0.1 0.2 4.0 4.3 1.9 2.3 PRESENT INVENTION STEEL
S1 57.4 40.0 2.4 0.1 0.1 5.2 5.5 3.3 1.7 PRESENT INVENTIONSTEEL
S2 59.4 39.2 1.1 0.2 0.1 4.0 4.3 1.7 2.5 PRESENT INVENTION STEEL
S3 58.8 39.0 1.9 0.1 0.2 4.0 4.3 2.0 2.2 PRESENTINVENTIONSTEEL
S4 52.9 45.2 1.6 0.1 0.2 4.1 4.4 1.6 2.8 PRESENT EWENIlON STEEL
TI 61.6 36.0 2.2 0.1 0.1 5.5 5.8 3.5 1.7 PRESENT INVENTION STEEL
T2 61.5 36.5 1.8 0.1 0.1 -8.6 8.9 2.4 32 COMPARATIVESTEn
n 61.0 38.0 0.8 0.1 0.1 &S 8.8 2.5 3.5 COMPARATIVESTFEI.
T4 56.9 40.3 2.1 0.4 0.3 U 9.3 0.9 10.0 !X)l-
T5 61.4 37.9 0.4 0.2 0.1 4.0 4.3 1.9 2.3 COMPARA'ITVESTER.
T6 60.6 38.6 0.5 0.2 0.1 3.8 4.1 1.8 2.3 TaF-,
n 59.0 39.8 0.5 0.4 0.3 4.4 4.7 2.8 1.7 COMPARATNESTE~
al
bl
cl
dl
el
f l
gl
hl
CRACKING OCCURRED DURING HOT ROLLING
- COMPARATIVESTFEI. -
[O 1 541 [Table 51
AVERAGE VALUE
OF POLE DENS~~~ESP OLEmSrrY ",! TS *EL(%) E(%) PA) OF (lW)dll> OF (332JC11" r CRYSTAL rL rm r60 mn
TO (W)
ORIENTATION GROW ORIENTATION
Al 645 LO 12 44.0 2.9 2.6 0.79 0.84 1.10 1.10 TOMPARA7TVFSTFFI.
A2 560 6 9 36.0 1.7 2 0 0.74 0.79 1.06 1.04 COMPARATTVFSTER.
M 830 11 15 86.6 2.9 2.4 0.74 0.79 0.97 0.98 - A4 751 12 18 44.0 1.8 2.4 W 0.63 1.21 U COMPARATTVFSTER.
A7 886 14 20 43.0 2.9 2.4 0.74 0.79 0.97 0.98 - A6 779 13 18 39.0 2.9 2.4 0.74 0.79 0.97 0.98 COMPARATTVFSTER.
Bl 804 13 I8 91.7 1.5 1.7 0.71 0.76 1.03 1.02 PRESENI INVENIIONSTEEL
82 914 I4 19 82.8 2.1 2.6 0.71 0.76 1.07 1.05 PRESENI INVENIIONSTEEL
83 797 I3 I8 45.0 3.7 1.6 U4Q3UL2ZC1
737 I2 18 95.4 1.7 2.5 0.71 0.76 1.03 1.02 PRESENI IMIP(IW3NSTEEL
C2 814 I3 22 65.2 2.4 2.8 0.71 0.76 1.05 1.04 coMpARATIvEsTFEI.
0 708 I2 17 96.6 1.9 2.7 &&! Q3 LY 1.01-
Dl I083 11 I5 48.0 2.7 3.0 0.71 0.76 1.03 1.02 COMPARATTYFSTFR.
D2 855 I3 19 85.4 1.7 1.6 0 71 0.76 1.05 1.04 PRESENC INVENIIONSTZEL
D3 I168 15 22 55.0 1.9 1.9 0.71 0.76 1.05 1.04
El 904 I4 19 82.6 2.1 2.5 0.72 0.77 1.07 1.06 PRESENTINVENIIONSTEEL
E2 956 . 14 20 78.5 1.8 2.2 0 72 0.77 1.09 1.07 PRESENI INVENIIONSTEEL
W 668 12 17 90.0 3.3 3.4 0.72 0.77 1.06 1.04 cmmmmam
FI 900 14 19 83.4 1.4 1.3 0.72 0.77 1.07 1.05 PRESENI INVENIIONSTEEL
F2 954 14 20 78.4 2.1 2.1 0.72 0.77 1.08 1.07 PRESENI INVENIIONSTEEL
F3 947 14 20 80.4 43 2.0 0.85 0.90 1.44 1.35 PRESM IMIP(Il0NSTEEL
GI 1073 9 13 62.6 1.6 2.6 0.71 0.76 1.03 1.02 - G2 817 13 I9 39.0 52 &@ 0.74 1.23 J.&3-
HI 891 14 19 82.4 2.2 2.7 0.70 0.75 1.02 1.02 PRESENI INVENIIONSTEEL
11 997 14 20 75.9 2.5 2.2 0.72 0.77 1.07 1.05 PRESM INVENTIONSTEEL
U 657 12 I7 99.5 3 1 3.1 0.74 0.79 1.11 1.09 PRESM INVENIIONSTEEL
!3 881 14 I9 46.0 2.3 1.6 0.74 0.79 1.09 1.09 CDMPARATTYFSTFFl.
I1 959 14 20 78.9 2.0 2.7 0.72 0.77 1.07 1.06 PRESENT INVENIIONSTEEL
12 854 I3 I9 44.0 2.5 2.4 0.74 0.79 1.09 1.09 - 13 953 I4 20 39.0 4.8 4.3 Q.55 &@ 1.09 1.09 - K1 365 I6 22 32.0 1.5 2.2 0 70 0.75 1.05 1.04 - L1 853 12 17 85.9 1.5 2 2 0.71 0.76 1.06 1.04 PRESENI INVENIIONSTEEL
MI 727 12 17 95.5 2.9 3.6 0.70 0.75 1.04 1.03 PRESENI INVENIIONSTEEL
hi2 936 14 20 38.0 4.1 0.9 0.88 0.93 1.04 1.03 COMPnRnTNEsrm
N1 883 14 19 82.9 19 2.6 0.70 0.75 1.05 1.04 PRESENT INVENTIONSTEEL
01 852 13 19 85.8 18 2.0 0.70 0.75 1.03 1.02 PRESENT INVENIIONSTEEL
02 764 13 18 41.0 26 -4.4 055 0.60 L?6 IdZ - PI 873 13 19 84.1 2.2 3.3 0.71 0.76 1.04 1.03 PRESENIINVENIlONSTEa
P2 1051 9 10 26.1 dl 5.a L @ s l . % t l b l ~ -
P3 I042 9 10 25.8 69 S 0.48 055 1.49 - P4 I113 6 7 23.1 1.8 22 0.72 0.77 1.09 1.07 - QI 818 13 19 88.9 2.3 2.7 0.71 0.76 1.04 1.03 PRFSENImIOWSTEEL
Q2 485 I2 13 55.0 2.5 2.2 0.72 0.77 1.07 1.05 - Q3 568 I0 I1 51.2 1.5 1.7 0.71 0.76 1.03 1.02 MMPARATIYFSTm.
Q4 657 11 12 34.0 3.1 3.1 0.74 0.79 1.11 1.09 - RI 752 13 18 93.3 2.6 3.1 0.71 0.76 1.04 1.03 PRESENI WvENTIONSTEEL
R2 1080 17 24 74.0 2.6 3.0 0.69 0.74 1.03 1.04 PRESM INVENIIONSTEEL
R3 1073 17 24 73.7 2.5 2.9 0.77 0.82 1.02 1.02 PRESENI WVENTIONSTEEL
R4 1060 16 23 74.3 2.3 2.6 0.72 0.77 1.04 1.03 PRESENIINVENIIONSTEEL
Sl 868 I3 I9 85.8 1.6 2.1 0.71 0.76 1.05 1.04 PRESM INVENIIONSTEEL
S2 1020 16 22 77.0 2.0 2.4 0.72 0.77 1.05 1.03 PRESENI INVENIIONSTEEL
S3 I050 16 23 74.9 1.9 2.3 0.71 0.76 1.02 1.03 PRESM INVENIIONSTEEL
S4 1020 15 21 75.2 1.5 1.8 0.78 0.83 1.08 1.04 PRESENIIMIP(I1ONSTEEL
TI 780 13 18 92.1 1.8 1.9 0.71 0.76 1.07 1.05 PRESENC INVENIIONSTEEL
R 720 12 17 39 5d 4.6 052 057 1.47 139 - T3 735 12 17 41 ii 42 ~ O S ~ ~ M M P A R A T I Y F S T F F 1 .
T4 986 I5 21 36 5.5 G pii9,PPmmCOMPARATTVFSTER.
T5 998 I6 22 35 LZ Ll e 5 e Q h 3 U l ~ M M P A R A T I Y F S T m .
T6 898 I4 20 32 6J 4.6 P 6 1 e 6 n ~ I A l ~
'8I7 8 0 6 9 3 3 U wmmur
al - bl - cl - CRACKJNG OCCURRED DURING HOT ROLLING - el - n - 31 cmmmmam
hl -
[Industrial Applicability]
[0155] As described previously, according to the present invention, it is
possible to provide a high-strength cold-rolled steel sheet that is not large in
anisotropy even when Nb, Ti, and/or the like arelis added and has excellent
uniform elongation and hole expandability. Thus, the present invention is
the invention having high industrial applicability.
[Explanation of Codes]
[0 1561
1 continuous hot rolling line
2 roughing mill
3 finishing mill
4 hot-rolled steel sheet
5 run-out-table
6 rolling stand
10 inter-stand cooling nozzle
11 cooling nozzle 11

Name of Document] Claims
[Claim 11 A high-strength cold-rolled steel sheet having excellent
uniform elongation and hole expandability comprising:
in mass%,
C: 0.01 to 0.4%;
Si: 0.001 to 2.5%;
Mn: 0.001 to 4.0%;
P: 0.001 to 0.15%;
S: 0.0005 to 0.03%;
Al: 0.001 to 2.0%;
N: 0.0005 to 0.01%; and
0: 0.0005 to 0.01%; in which Si + A1 is limited to less than 1.0%, and
a balance being composed of iron and inevitable impurities, wherein
at a sheet thickness center portion being a range of 518 to 318 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)<01 1>, (1 16)<1 lo>, (1 1414 lo>,
{113)<110>, {112)<110>, {335)<110>, and {223)<110> is 5.0 or less, and a
pole density of the {332)<113> crystal orientation is 4.0 or less,
a metal structure contains 5 to 80% of ferrite, 5 to 80% of bainite, and 1% or
less of martensite in terms of an area ratio and the total of martensite, pearlite,
and retained austenite is 5% or less, and
an r value (rC) in a direction perpendicular to a rolling direction is 0.70 or
more and an r value (r30) in a direction 30" fiom the rolling direction is 1.10
or less.
[Claim 21 The high-strength cold-rolled steel sheet having excellent
uniform elongation and hole expandability according to claim 1, wherein
an r value (rL) in the rolling direction is 0.70 or more and an r value (r60) in a
direction 60" fiom the rolling direction is 1.10 or less.
[Claim 31 The high-strength cold-rolled steel sheet having excellent
uniform elongation and hole expandability according to claim 1, wherein
in the metal structure, a mean volume diameter of crystal grains is 7 pm or
less, and an average value of a ratio of, of the crystal grains, a length dL in the
rolling direction to a length dt in a sheet thickness direction: dL/dt is 3.0 or
less.
[Claim 41 The high-strength cold-rolled steel sheet having excellent
uniform elongation and hole expandability according to claim 1, further
comprising:
one type or two or more types of
in mass%,
Ti: 0.001 to 0.2%,
Nb: 0.001 to 0.2%,
B: 0.0001 to 0.005%,
Mg: 0.0001 to 0.01%,
Rem: 0.0001 to-O.l%,
Ca: 0.0001 to 0.01%,
Mo: 0.001 to 1.0%,
Cr: 0.001 to 2.0%,
V: 0.001 to 1.0%,
Ni: 0.001 to 2.0%,
Cu: 0.001 to 2.0%,
Zr: 0.0001 to 0.296,
W: 0.001 to 1.0%,
As: 0.0001 to 0.5%,
Co: 0.0001 to 1.0%,
Sn: 0.0001 to 0.2%,
Pb: 0.001 to 0.1%,
Y: 0.001 to 0.10%, and
Hfi 0.001 to 0.10%.
[Claim 51 The high-strength cold-rolled steel sheet having excellent
uniform elongation and hole expandability according to claim 1, wherein
on the surface, hot-dip galvanizing is performed.
[Claim 61 The high-strength cold-rolled steel sheet having excellent
uniform elongation and hole expandability according to claim 5, wherein
after the hot-dip galvanizing, an alloying treatment is performed at 450 to
600°C.
[Claim 71 A manufacturing method of a high-strength cold-rolled steel
sheet having excellent uniform elongation and hole expandability,
comprising:
on a steel billet containing:
in mass%,
C: 0.01 to 0.4%;
Si: 0.001 to 2.5%;
Mn: 0.001 to 4.0%;
P: 0.001 to 0.15%;
S: 0.0005 to 0.03%;
Al: 0.001 to 2.0%;
N: 0.0005 to 0.01%; and
0: 0.0005 to 0.01%; in which Si + A1 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 pm 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 T1 + 30°C nor higher than T1 + 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 pre-cold rolling 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°Clsecond or more
and performing the primary cooling in a manner that a temperature change is
in a range of not less than 40°C nor more than 140°C;
performing cold rolling at a reduction ratio of not less than 30% nor more
than 70%;
performing heating up to a temperature region of 700 to 900°C and
performing holding for not shorter than 1 second nor longer than 1000
seconds;
performing post-cold rolling primary cooling down to a temperature region of
580 to 750°C at an average cooling rate of 12"C/second or less;
performing post-cold rolling secondary cooling down to a temperature region
of 350 to 500°C at an average cooling rate of 4 to 30O0C1second; 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.
T1 ("C)=850+10x(C+N)xMn+350xNb+250xTi+40xB+10x
Cr+ 100 x Mo + 100 x V-.a (1)
Here, C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the
element (mass%).
t 5 2.5 x tl - a - (2)
Here, tl is obtained by Expression (3) below.
tl = 0.001 x ((Tf - Tl) x ~11100)' - 0.109 x ((Tf - Tl) x P11100) + 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 P1
represents the reduction ratio of the final reduction at 30% or more.
log(t2) = O.O002(T2 - 425)' + 1.18 ... (4)
Here, T2 represents an overaging treatment temperature, and the maximum
value of t2 is set to 400.
[Claim 81 The manufacturing method of the high-strength cold-rolled
steel sheet having excellent uniform elongation and hole expandability
according to claim 7, further comprising:
after performing the pre-cold rolling primary cooling, performing
pre-cold rolling secondary cooling down to a cooling stop temperature of
600°C or lower at an average cooling rate of 10 to 300°C/second before
performing the cold rolling, and performing coiling at 600°C or lower to
obtain a hot-rolled steel sheet.
[Claim 91 The manufacturing method of the high-strength cold-rolled
steel sheet having excellent uniform elongation and hole expandability
according to claim 7, wherein -
the total reduction ratio in a temperature range of lower than T1 + 30°C is
30% or less.
[Claim 101 The manufacturing method of the high-strength cold-rolled
steel sheet having excellent uniform elongation and hole expandability
according to claim 7, wherein
the waiting time t second further satisfies Expression (2a) below.
t < tl (2a)
[Claim 111 The manufacturing method of the high-strength cold-rolled
steel sheet having excellent uniform elongation and hole expandability
according to claim 7, wherein
the waiting time t second further satisfies Expression (2b) below.
tl 5 t i tl x 2.5 (2b)
[Claim 121 The manufacturing method of the high-strength cold-rolled
steel sheet having excellent uniform elongation and hole expandability
according to claim 7, wherein
post-hot rolling primary cooling is started between rolling stands.
[Claim 131 The manufacturing method of the high-strength cold-rolled
steel sheet having excellent uniform elongation and hole expandability
according to claim 7, wherein
when the heating is performed up to the temperature region of 700 to 900°C
after the cold rolling, an average heating rate of not lower than room
temperature nor higher than 650°C is set to HRl ("Clsecond) expressed by
Expression (5) below, and
an average heating rate of higher than 650°C to 700 to 900°C is set to HR2
(OC/second) expressed by Expression (6) below.
HRl 2 0.3 ... (5)
HR2 _I 0.5 x HR1 ... (6)
[Claim 141 The manufacturing method of the high-strength cold-rolled
steel, sheet having excellent uniform elongation and hole expandability
according to claim 7, further comprising:
performing hot-dip galvanizing on the surface.
[Claim 151 The manufacturing method of the high-strength cold-rolled
steel sheet having excellent uniform elongation and hole expandability
according to claim 14, further comprising:
performing an alloying treatment at 450 to 600°C after performing the hot-dip
I galvanizing.