[Name of Document] DESCRIPTION
[Title of the Invention] HIGH-STRENGTH COLD-ROLLED STEEL SHEET
-- HAVING EXCELLENT STRETCH FLANGEABILITY AND PRECISION
PUNCHABILITY AND MANUFACTURING METHOD THEREOF
5 [Technical Field]
[0001] The present invention relates to a high-strength cold-rolled steel
sheet having excellent stretch flangeability and precision punchability, and a
manufacturing method thereof.
This application is based upon and claims the benefit of priority of the
10 prior Japanese Patent Application No. 2011-164383, filed on July 27, 2011,
the entire contents of which are incorporated herein by reference.
[Background Art]
[0002] In order to abate emission of carbon dioxide gas from automobiles,
a reduction in weight of automobile vehicle bodies has been promoted by
15 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 fiirther
promote the reduction in weight of automobile vehicle bodies from now on, it
is necessary to increase the level of usage strength of a high-strength steel
20 sheet more than conventionally. However, when a high-strength steel sheet
is used for an outer panel part, cutting, blanking, and the like are often applied,
and fiirther when a high-strength steel sheet is used for an underbody part,
working methods accompanied by shearing such as punching are often
applied, resulting in that a steel sheet having excellent precision punchability
25 has been required. Further, workings such as buning have also been
increasingly performed after shearing, so that stretch flangeability is also an
important property related to working. However, when a steel sheet is
increased in strength in general, punching accuracy decreases and stretch
flangeability also decreases.
[0003] With regard to the precision punchability, as is in Patent
5 Documents 1 and 2, there is disclosed that punching is performed in a soft
state and achievement of high strength is attained by heat treatment and
carburization, but a manufacturing process is prolonged to thus cause an
increase in cost. On the other hand, as is in Patent Document 3, there is also
disclosed a method of improving precision punchability by spheroidizing
10 cementite by annealing, but achievement of stretch flangeability important for
working of automobile vehicle bodies and the like and the precision
punchability is not considered at all.
[0004] With regard to the stretch flangeability to achievement of high
strength, a steel sheet metal structure control method to improve local
15 elongation is also disclosed, and Non-Patent Document 1 discloses that
controlling inclusions, making structures uniform, and ftirther decreasing
difference in hardness between structures are effective for bendability and
stretch flangeability. Further, Non-Patent Document 2 discloses a method in
which a finishing temperature of hot rolling, a reduction ratio and a
20 temperature range of finish rolling are controlled, recrystallization of austenite
is promoted, development of a rolled texture is suppressed, and crystal
orientations are randomized, to thereby improve strength, ductility, and stretch
flangeability. -
From Non-Patent Documents 1 and 2, it is conceivable that the metal
25 structure and rolled texture are made uniform, thereby making it possible to
improve the stretch flangeability, but the achievement of the. precision
punchability and the stretch flangeability is not considered at all.
[Prior Art Document]
[Patent Document]
[0005] Patent Document 1: Japanese Patent Publication No. H3-2942
5 Patent Document 2: Japanese Patent Publication No. H5-14764
Patent Document 3: Japanese Patent Publication No. H2-19173
[Non-Patent Document]
[0006] Non-Patent Document 1: K. Sugimoto et al., [ISIJ International]
(2000) Vol. 40, p. 920
10 Non-Patent Document 2: Kishida, [Nippon Steel Technical Report]
(1999) No. 371, p. 13
[Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[0007] Thus, the present invention is devised in consideration of the
15 above-described problems, and has an object to provide a cold-rolled steel
sheet having high strength and having excellent stretch flangeability and
precision punchability and a manufacturing method capable of manufacturing
the steel sheet inexpensively and stably.
[Means for Solving the Problems]
20 [0008] The present inventors optimized components and manufacturing
conditions of a high-strength cold-rolled steel sheet and controlled structures
of the steel sheet, to thereby succeed in manufacturing a steel sheet having
excellent strength, stretch flangeability, and precision punchability. The gist
is as follows.
25 [0009] [1]
A, high-strength cold-rolled steel sheet having excellent stretch
flangeability and precision punchability contains:
in mass%,
C: greater than 0.01% to 0.4% or less;
Si: not less than 0.001% nor more than 2.5%);
5 Mn: not less than 0.001 %> nor more than 4%;
P: 0.001 to 0.15% or less;
S: 0.0005 to 0.03% or less;
Al: not less than 0.001% nor more than 2%);
N: 0.0005 to 0.01% or less; and
10 a balance being composed of iron and inevitable impurities, in which
in a range of 5/8 to 3/8 in sheet thickness from the surface of the steel sheet,
an average value of pole densities of the {100]<011> to {223}<110>
orientation group represented by respective crystal orientations of
{100}<011>, {116}<110>, {114}<110>, {113}<110>, {112}<110>,
15 {335}<110>, and {223}<110> is 6.5 or less, and a pole density of the
{332}<113> crystal orientation is 5.0 or less, and
a metal structure contains, in terms of an area ratio, greater than 5% of
pearlite, the sum of bainite and martensite limited to less than 5%, and a
balance composed of ferrite.
20 [2]
The high-strength cold-rolled steel sheet having excellent stretch
flangeability and precision punchability according to [1], in which
further, Vickers hardness of a pearlite phase is not less than 150 HV nor more
than 300 HV.
25 [3]
The high-strength. cold-rolled steel sheet having excellent stretch
flangeability and precision punchability according to [1], in which
further, an r value in a direction perpendicular to a rolling direction (rC) is
0.70 or more, an r value in a direction 30° from the rolling direction (r30) is
1.10 or less, an r value in the rolling direction (rL) is 0.70 or more, and an r
5 value in a direction 60° from the rolling direction (r60) is 1.10 or less.
[4]
The high-strength cold-rolled steel sheet having excellent stretch
flangeability and precision punchability according to [1], further contains:
one type or two or more types of
10 inmass%,
Ti: not less than 0.001% nor more than 0.2%,
Nb: not less than 0.001% nor more than 0.2%,
B: not less than 0.0001% nor more than 0.005%,
Mg: not less than 0.000l%o nor more than 0.01%),
15 Rem: not less than 0.0001% nor more than 0.1%,
Ca: not less than 0.0001 % nor more than 0.01%,
Mo: not less than 0.001%) nor more than l%o,
Cr: not less than 0.001%) nor more than 2%,
V: not less than 0.001% nor more than 1%),
20 Ni: not less than 0.001 % nor more than 2%o,
Cu: not less than 0.001%) nor more than 2%,
Zr: not less than 0.0001% nor more than 0.2%,
W: not less than 0.00 l%o nor more than 1%),
As: not less than 0.0001% nor more than 0.5%,
25 Co: not less than 0.0001 % nor more than 1 %,
Sn: not less than 0.0001% nor more than 0.2%,
Pb: not less than 0.001% nor more than 0.1%,
Y: not less than 0.001% nor more than 0.1%), and
Hf: not less than 0.001% nor more than 0.1%.
[5]
5 The high-strength cold-rolled steel sheet having excellent stretch
flangeability and precision punchability according to [1], in which
further, when the steel sheet whose sheet thickness is reduced to 1.2 mm with
a sheet thickness center portion set as the center is punched out by a circular
punch with O 10 mm and a circular die with Wo of a clearance, a shear
10 surface percentage of a punched edge surface becomes 90% or more.
[6]
The high-strength cold-rolled steel sheet having excellent stretch
flangeability and precision punchability according to [1 ], in which
on the surface, a hot-dip galvanized layer or an alloyed hot-dip galvanized
15 layer is provided.
[7]
A manufacturing method of a high-strength cold-rolled steel sheet
having excellent stretch flangeability and precision punchability, includes:
on a steel billet containing:
20 in mass%,
C: greater than O.OP/o to 0.4% or less;
Si: not less than 0.001% nor more than 2.5%;
Mn: not less than 0.001% nor more than 4%;
P: 0.001 to 0.15% or less;
25 S: 0.0005 to 0.03% or less;
Al: not less than 0.001%) nor more than 2%;
N: 0.0005 to 0.01% or less; 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
5 1000°C nor higher than 1200°C;
setting an austenite grain diameter to 200 \xm or less by the first hot rolling;
performing second hot rolling in which rolling at a reduction ratio of 30%) or
more is performed in one pass at least one time in a temperature region of not
lower than a temperature Tl determined by Expression (1) below + 30°C nor
10 higher than T1+200°C;
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 cooling in such a manner that a
waiting time t second satisfies Expression (2) below;
15 setting an average cooling rate in the pre-cold rolling cooling to 50°C/second
or more and setting a temperature change to fall within a range of not less
than 40°C nor more than 140°C;
performing cold rolling at a reduction ratio of not less than 40%) nor more
than 80%;
20 performing heating up to a temperature region of 750 to 900°C and
performing holding for not shorter than 1 second nor longer than 300 seconds;
performing post-cold rolling primary cooling down to a temperature region of
not lower than 580°C nor higher than 750°C at an average cooling rate of not
less than l°C/s nor more than 10°C/s;
25 performing retention for not shorter than 1 second nor longer than 1000
,-. seconds under the condition that a temperature decrease rate becomes l°C/s
or less; and
performing post-cold rolling secondary cooling at an average cooling rate of
5°C/s or less.
Tl (°C) = 850 + 10 X (C + N) X Mn + 350 X Nb + 250 x Ti + 40 X B + 10 X
5 Cr + 100 X Mo + 100 X V - Expression (1)
Here, C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the
element (mass%).
t ^ 2.5 X tl ••• Expression (2)
Here, tl is obtained by Expression (3) below.
10 tl = 0.001 X ((Tf - Tl) X Pl/100)^ - 0.109 x ((Tf - Tl) x Pl/100) + 3.1 -
Expression (3)
Here, in Expression (3) above, Tf represents the temperature of the steel billet
obtained after the final reduction at a reduction ratio of 30% or more, and PI
represents the reduction ratio of the final reduction at 30% or more.
15 [8]
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent stretch flangeability and precision punchability according to
[7], in which
the total reduction ratio in a temperature range of lower than Tl + 30°C is
20 30% or less.
[9]
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent stretch flangeability and precision punchability according to
[7], in which
25 the waiting time t second further satisfies Expression (2a) below.
t < tl ••• Expression (2a)
[10]
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent stretch flangeability and precision punchability according to
[7], in which
5 the waiting time t second fiirther satisfies Expression (2b) below.
tl ^ t ^ tl X 2.5 - Expression (2b)
[11]
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent stretch flangeability and precision punchability according to
10 [7], in which
the pre-cold rolling cooling is started between rolling stands.
[12]
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent stretch flangeability and precision punchability according to
15 [7], further includes:
performing coiling at 650°C or lower to obtain a hot-rolled steel sheet after
performing the pre-cold rolling cooling and before performing the cold
rolling.
[13]
20 The manufacturing method of the high-strength cold-rolled steel sheet
having excellent stretch flangeability and precision punchability according to
[7], in which
when the heating is performed up to the temperature region of 750 to 900°C
after the cold rolling, an average heating rate of not lower than room
25 temperature nor higher than 650°C is set to HRl (°C/second) expressed by
Expression (5) below, and -
10
an average heating rate of higher than 650°C to 750 to 900°C is set to HR2
(°C/second) expressed by Expression (6) below.
HRl ^ 0.3 ... Expression (5)
HR2 ^ 0.5 X HRl ... Expression (6)
5 [14]
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent stretch flangeability and precision punchability according to
[7], further includes:
performing hot-dip galvanizing on the surface.
10 [15]
The manufacturing method of the high-strength cold-rolled steel sheet
having excellent stretch flangeability and precision punchability according to
[14], further includes:
perfonning an alloying treatment at 450 to 600°C after performing the hot-dip
15 galvanizing.
[Effect of the Invention]
[0010] According to the present invention, it is possible to provide a
high-strength steel sheet having excellent stretch flangeability and precision
punchability. When this steel sheet is used, particularly, a yield when the
20 high-strength steel sheet is worked and used improves, cost is decreased, and
so on, resulting in that industrial contribution is quite prominent.
[Brief Description of the Drawings]
[0011] [FIG. 1] FIG. 1 is a view showing the relationship between an
average value of pole densities of the {100}<011> to {223}<110> orientation
25 group and tensile strength x a hole expansion ratio;
[FIG. 2] FIG. 2 is a view showing the relationship between a pole density of
11
the {332}<113> orientation group and the tensile strength x the hole
expansion ratio;
[FIG. 3] FIG. 3 is a view showing the relationship between an r value in a
direction perpendicular to a rolling direction (rC) and the tensile strength x
5 the hole expansion ratio;
[FIG. 4] FIG. 4 is a view showing the relationship between an r value in a
direction 30° from the rolling direction (r30) and the tensile strength x the
hole expansion ratio;
[FIG. 5] FIG. 5 is a view showing the relationship between an r value in the
10 rolling direction (rL) and the tensile strength x the hole expansion ratio;
[FIG. 6] FIG. 6 is a view showing the relationship between an r value in a
direction 60° from the rolling direction (r60) and the tensile strength x the
hole expansion ratio;
[FIG. 7] FIG. 7 shows the relationship between a hard phase fraction and a
15 shear surface percentage of a punched edge surface;
[FIG. 8] FIG. 8 shows the relationship between an austenite grain diameter
after rough rolling and the r value in the direction peipendicular to the rolling
direction (rC);
[FIG. 9] FIG. 9 shows the relationship between the austenite grain diameter
20 after the rough rolling and the r value in the direction 30° from the rolling
direction (r30);
[FIG. 10] FIG. 10 shows the relationship between the number of times of
rolling at 40% or more in the rough rolling and the austenite grain diameter
after the rough rolling;
25 [FIG. 11] FIG. 11 shows the relationship between a reduction ratio at Tl + 30
to Tl + 150°C and the average value of the pole densities of the {100}<011>
12
to {223}<110> orientation group;
[FIG. 12] FIG. 12 is an explanatory view of a continuous hot rolling line;
[FIG. 13] FIG. 13 shows the relationship between the reduction ratio at Tl +
30 to Tl + 150°C and the pole density of the {332}<113> crystal orientation;
5 and
[FIG. 14] FIG. 14 shows the relationship between a shear surface percentage
and strength x a hole expansion ratio of present invention steels and
comparative steels.
[Mode for Carrying out the Invention]
10 [0012] Hereinafter, the contents of the present invention will be explained
in detail.
[0013] (Crystal orientation)
In the present invention, it is particularly important that in a range of
5/8 to 3/8 in sheet thickness from the surface of a steel sheet, an average value
15 of pole densities of the {100}<011> to {223}<110> orientation group is 6.5
or less and a pole density of the {332}<113> crystal orientation is 5.0 or less.
As shown in FIG. 1, as long as the average value of the {100}<011> to
{223}<110> orientation group when X-ray diffraction is performed in the
sheet thickness range of 5/8 to 3/8 in sheet thickness from the surface of the
20 steel sheet to obtain pole densities of respective orientations is 6.5 or less
(desirably 4.0 or less), tensile strength x a hole expansion ratio ^ 30000 that
is required to work an underbody part to be required immediately is satisfied.
When the average value is greater than. 6.5, anisotropy of mechanical
properties of the steel sheet becomes strong extremely, and further hole
25 expandability only in a certain direction is improved, but a material in a
direction different from, it significantly deteriorates, resulting in that it
# 13
becomes impossible to satisfy the tensile strength x the hole expansion ratio
^ 30000 that is required to work an underbody part. On the other hand,
when- the average value becomes less than 0.5, which is difficult to be
achieved in a current general continuous hot rolling process, deterioration of
5 the hole expandability is concerned.
[0014] The {100}<011>, {116}<110>, {114}<110>, {113}<110>
{112}<110>, {335}<110>, and {223}<110> orientations are included in
the {100}<011 > to {223}< 110> orientation group.
[0015] The pole density is synonymous with an X-ray random intensity
10 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.
15 This pole density is measured by using a device of X-ray diffraction, EBSD
(Electron Back Scattering Diffraction), or the like. Further, it can also be
measured by an EBSP (Electron Back Scattering Pattern) method or an ECP
(Electron Channeling Pattern) method. It may be obtained from a
three-dimensional texture calculated by a vector method based on a pole
20 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) of pole figures out of pole figures of{110}, {100}, {211}, and {310}.
[0016] For example, for the pole density of each of the above-described
crystal orientations, each of intensities of (001)[1-10], (116)[1-10],
25 (114)[1-10], (113)[1-10], (112)[1-10], (335)[1-10], and (223)[1-10] at a ({)2 =
. 45° cross-section in the thi-ee-dimensional texture (ODF) may be used as it is.
Ip
14
[0017] The average value of the pole densities of the {100}<011> to
{223}<110> orientation group is an arithmetic average of the pole densities of
the above-described respective orientations. When it is impossible to obtain
the intensities of all the above-described orientations, the arithmetic average
5 of the pole densities of the respective orientations of {100}<011>,
{116}<110>, {114}<110>, {112}<110>, and {223}<110> may also be used
as a substitute.
[0018] Further, due to the similar reason, as long as the pole density of
the {332}<113> crystal orientation of a sheet plane in the range of 5/8 to 3/8
10 in sheet thickness from the surface of the steel sheet is 5.0 or less (desirably
3.0 or less) as shown in FIG. 2, the tensile strength x the hole expansion ratio
^ 30000 that is required to work an underbody part to be required
immediately is satisfied. When this is greater than 5.0, the anisotropy of the
mechanical properties of the steel sheet becomes strong extremely, and further
15 the hole expandability 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 tensile strength x the hole
expansion ratio ^ 30000 that is required to work an underbody part. On
the other hand, when the pole density becomes less than 0.5, which is difficult
20 to be achieved in a current general continuous hot rolling process, the
deterioration of the hole expandability is concerned.
[0019] The reason why the pole densities of the above-described crystal
orientations are important for improving the hole expandability is not
necessarily obvious, but is inferentially related to slip behavior of crystal at
25 the time of hole expansion working.
[0020] With regard to the sample to be subjected to the X-ray diffraction.
• 15
the steel sheet is reduced in thickness to a predetermined sheet thickness from
the surface by mechanical polishing or the like, and next strain is removed by
chemical polishing, electrolytic polishing, or the like^ and at the same time,
the sample is adjusted in accordance with the above-described method in such
5 a manner that, in the range of 3/8 to 5/8 in sheet thickness, an appropriate
plane becomes a measuring plane, and is measured.
[0021] As a matter of course, limitation of the above-described pole
densities is satisfied not only in the vicinity of 1/2 of the sheet thickness, but
also in as many thickness ranges as possible, and thereby the hole
10 expandability is further improved. However, the range of 3/8 to 5/8 in sheet
thickness from the surface of the steel sheet is measured, to thereby make it
possible to represent the material property of the entire steel sheet generally.
Thus, 5/8 to 3/8 of the sheet thickness is prescribed as the measuring range.
[0022] Incidentally, the crystal orientation represented by {hkl}
15 means that the nornial 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
20 equivalent planes, and [hkl] and (uvw) each indicate an individual crystal
plane. That is, in the present invention, a body-centered cubic structure is
targeted, and thus, for example, the (111), (-111), (l-ll), (H-l), (-1-11),
(rll-1), (1-1-1), and (-1-1-1) planes are equivalent to make it impossible to
make them different. In such a case, these orientations are generically
25 referred to as {111}. In an ODF representation, [hkl](uvw) is also used for
, : representing orientations of other low symmetric crystal structures, and thus it
16
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 in
accordance with the method described in, for example, Cullity, Elements of
5 X-ray Diffraction, new edition (published in 1986, translated by
MATSUMURA, Gentaro, published by AGNE Inc.) on pages 274 to 296.
[0023] (rvalue)
An r value in a direction perpendicular to the rolling direction (rC) is
important in the present invention. That is, as a result of earnest
10 examination, the present inventors found that good hole expandability cannot
always be obtained even when only the pole densities of the above-described
various crystal orientations are appropriate. As shown in FIG. 3,
simultaneously with the above-described pole densities, rC needs to be 0.70
or more. The upper limit of rC is not deteraiined in particular, but if (rC) is
15 1.10 or less, more excellent hole expandability can be obtained.
[0024] An r value in a direction 30° from the rolling direction (r30) is
important in the present invention. That is, as a result of earnest
examination, the present inventors found that good hole expandability cannot
always be obtained even when X-ray intensities of the above-described
20 various crystal orientations are appropriate. As shown in FIG. 4,
simultaneously with the above-described X-ray intensities, r30 needs to be
1.10 or less. The lower limit of r30 is not determined in particular, but if r30
is 0.70 or more, more excellent hole expandability can be obtained.
[0025] As a result of earnest examination, the present inventors further
25 found that if in addition to the X-ray random intensity ratios of the
above-dcEcribed various ciystal orientations, rC, and r30, as shown in FIG. 5
17
and FIG. 6, an r value in the rolling direction (rL) and an r value in a direction
60° from the rolling direction (r60) are rL ^ 0.70 and r60 ^ 1.10
respectively, the tensile strength x the hole expansion ratio ^30000 is
better satisfied.
5 The upper limit of the above-described rL value and the lower limit of
the r60 value 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.
[0026] The above-described r values are each evaluated by a tensile test
using a JIS No. 5 tensile test piece. Tensile strain only has to be evaluated in
10 a range of 5 to 15% in the case of a high-strength steel sheet normally, and in
a range of uniform elongation. By the way, it has been Icnown that a texture
and the r values are coiTelated with each other generally, but in the present
invention, the already-described limitation on the pole densities of the crystal
orientations and the limitation on the r values are not synonymous with each
15 other, and unless both the limitations are satisfied simultaneously, good hole
expandability cannot be obtained.
[0027] (Metal structure)
Next, there will be explained a metal structure of the steel sheet of the
present invention. The metal structure of the steel sheet of the present
20 invention contains, in terms of an area ratio, greater than 5% of pearlite, the
sum of bainite and martensite limited to less than 5%, and a balance
composed of ferrite. In the high-strength steel sheet, in order to increase its
strength, a complex structure obtained by providing a high-strength second
phase in a ferrite phase is often used. The structure is normally composed of
25 ferrite-pearlite, ferrite-bainite, ferrite-martensite, or the like, and as long as a
second phase fi-action is fixed, as there are more low-temperature
18
transformation phases each having the hard second phase whose hardness is
hard, the strength of the steel sheet improves. However, the harder the
low-temperature- transformation phase is, the more prominent a difference in
ductility from ferrite is, and during punching, stress concentrations of ferrite
5 and the low-temperature transformation phase occur, so that a fracture surface
appears on a punched portion and thus punching precision deteriorates.
[0028] Particularly, when the sum of bainite and martensite fractions
becomes 5% or more in terms of an area ratio, as shown in FIG. 7, a shear
surface percentage being a rough standard of precision punching of the
10 high-strength steel sheet falls below 90%. Further, v^'hen the pearlite fraction
becomes 5% or less, the strength decreases to fall below 500 MPa being a
standard of the high-strength cold-rolled steel sheet. Thus, in the present
invention, the sum of the bainite and martensite fractions is set to less than
5%, the pearlite fraction is set to greater than 5%, and the balance is set to
15 ferrite. Bainite and martensite may also be 05. Thus, as the metal structure
of the steel sheet of the present invention, a form made of pearlite and ferrite,
a form containing pearlite and ferrite and further one of bainite and martensite,
and a form containing pearlite and ferrite and further both of bainite and
martensite are conceived.
20 [0029] Incidentally, when the pearlite fraction becomes higher, the
strength increases, but the shear surface percentage decreases. The pearlite
fraction is desirably less than 30%. Even though the pearlite fraction is 30%,
90% or more of the shear surface percentage can be achieved, but as long as
the pearlite fraction is less than 30%, 95% or more of the shear surface
25 percentage can be achieved and the precision punchability improves more.
[0030] (Vickers. hardness of the pearlite phase)
19
The hardness of the pearlite phase affects a tensile property and the
punching precision. As Vickers hardness of the pearlite phase increases, the
strength improves, but -when the Vickers hardness of the pearlite phase
exceeds 300 HV, the punching precision deteriorates. In order to obtain
5 good tensile strength-hole expandability balance and punching precision, the
Vickers hardness of the pearlite phase is set to not less than 150 HV nor more
than 300 HV. Incidentally, the Vickers hardness is measured by using a
micro-Vickers measuring apparatus.
[0031] Further, in the present invention, the precision punchability of the
10 steel sheet is evaluated by the shear surface percentage of a punched edge
surface [= length of a shear surface/(length of a shear surface + length of a
fracture surface)]. The steel sheet whose sheet thickness is reduced to 1.2
mm with a sheet thickness center portion set as the center is punched out by a
circular punch with O 10 mm and a circular die with 1% of a clearance, and
15 measurements of the length of the shear surface and the length of the fracture
surface with respect to the whole circumference of the punched edge surface
are performed. Then, the minimum value of the length of the shear surface
in the whole circumference of the punched edge surface is used to define the
shear surface percentage.
20 Incidentally, the sheet thickness center portion is most likely to be
affected by center segregation. It is conceivable that if the steel sheet has
predetermined precision punchability in the sheet thickness center portion,
predetermined precision punchability can be satisfied over the whole sheet
thickness.
25 [0032] (Chemical components of the steel sheet)
Next, there will, .be explained -reasons for limiting chemical
20
components of the high-strength cold-rolled steel sheet of the present
invention. Incidentally, % of a content is mass%.
[0033] C: greater than 0.01 to 0.4%
C is an element contributing to increasing the strength of a base
5 material, but is also an element generating iron-based carbide such as
cementite (FcsC) to be the starting point of craclcing at the time of hole
expansion. When the content of C is 0.01% or less, it is not possible to
obtain an effect of improving the strength by structure strengthening by a
low-temperature transformation generating phase. When greater than 0.4%
10 is contained, center segregation becomes prominent and iron-based carbide
such as cementite (FcaC) to be the starting point of cracking in a secondary
shear surface at the time of punching is increased, resulting in that the
punchability deteriorates. Therefore, the content of C is limited to the range
of greater than 0.01% to 0.4% or less. Further, when the balance with
15 ductility is considered together with the improvement of the strength, the
content of C is desirably 0.20% or less.
[0034] Si: 0.001 to 2.5%
Si is an element contributing to increasing the strength of the base
material and also has a part as a deoxidizing material of molten steel, and thus
20 is added according to need. As for the content of Si, when 0.001%) or more
is added, the above-described effect is exhibited, but even when greater than
2.5% is added, an effect of contributing to increasing the strength is saturated.
Therefore, the content of Si is limited to the-range of not less than 0.001% nor
more than 2.5%. Further, when greater than 0.1% of Si is added. Si, with an
25 increase in the content, suppresses precipitation of iron-based carbide such as
cementite in the material structure and contributes to improving the strength
21
and to improving the hole expandability. Further, when Si exceeds 1%, an
effect of suppressing the precipitation of iron-based carbide is saturated.
Thus, the desirable range of the content of Si is greater than 0.1 to 1%.
[0035] Mn: 0.01 to 4%
5 Mn is an element contributing to improving the strength by
solid-solution strengthening and quench strengthening and is added according
to need. When the content of Mn is less than 0.01%, this effect cannot be
obtained, and even when greater than 4% is added, this effect is saturated.
For this reason, the content of Mn is limited to the range of not less than
10 0.01%) nor more than 4%. Further, in order to suppress occurrence of hot
cracking by S, when elements other than Mn are not added sufficiently, the
amount of Mn allowing the content of Mn ([Mn]) and the content of S ([S]) to
satisfy [Mn]/[S] ^ 20 in mass% is desirably added. Further, Mn is an
element that, with an increase in the content, expands an austenite region
15 temperature to a low temperature side, improves hardenability, and facilitates
foraiation of a continuous cooling transformation structure having excellent
burring. When the content of Mn is less than 1%, this effect is not easily
exhibited, and thus 1% or more is desirably added.
[0036] P: 0.001 to 0.15% or less
20 P is an impurity contained in molten iron, and is an element that is
segregated at grain boundaries and decreases toughness with an increase in its
content. For this reason, the smaller the content of P is, the more desirable it
is, and when greater than 0.15% is contained, P adversely affects workability
and weldability, and thus P is set to 0.15%) or less. Particularly, when the
25 hole expandability and the weldability are considered, the content of P is
desirably 0.02% or less. The lower limit is set to 0.001% apphcable in
22
current general refining (including secondary refining).
[0037] S: 0.0005 to 0.03% or less
S is an impurity contained in molten iron, and is an element that not
only causes cracking at the time of hot rolling but also generates an A-based
5 inclusion deteriorating the hole expandability when its content is too large.
For this reason, the content of S should be decreased as much as possible, but
as long as S is 0.03% or less, it falls within an allowable range, so that S is set
to 0.03% or less. However, it is desirable that the content of S when the hole
expandability to such extent is needed is preferably 0.01% or less, and is more
10 preferably 0.005% or less. The lower limit is set to 0.0005%) applicable in
cun-ent general refining (including secondary refining).
[0038] Al: 0.001 to 2%
For molten steel deoxidation in a refining process of the steel, 0.001%
or more of Al needs to be added, but the upper limit is set to 2% because an
15 increase in cost is caused. Further, when Al is added in very large amounts,
non-metal inclusions are increased to make the ductility and toughness
deteriorate, so that Al is desirably 0.06% or less. It is further desirably
0.04% or less. Further, in order to obtain an effect of suppressing the
precipitation of iron-based carbide such as cementite in the material structure,
20 similarly to Si, 0.016% or more is desirably added. Thus, it is more
desirably not less than 0.016% nor more than 0.04%.
[0039] N: 0.0005 to 0.01% or less
The content of N should be decreased as much as possible^but as long
as it is 0.01% or less, it falls within an allowable range. In terms of aging
25 resistance, however, the content of N is further desirably set to 0.005% or less.
-...-The lower limit is set to 0.0005% applicable in cuiTent general refining
23
(including secondary refining).
[0040] Further, as elements that have been used up to now for controlling
inclusions and making precipitates fine so that the hole expandability should
be improved, one type or two or more types of Ti, Nb, B, Mg, Rem, Ca, Mo,
5 Cr, V, W, Zr, Cu, Ni, As, Co, Sn, Pb, Y, and Hf may be contained.
[0041] Ti, Nb, and B 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 need, 0.001%) of Ti,
0.001%) of Nb, and 0.0001%) or more of B are desirably added. Ti is
10 preferably 0.01%), and Nb is preferably 0.005%o or more. However, even
when they are added excessively, no significant effect is obtained to instead
make the workability and manufacturability deteriorate, 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.
15 [0042] Mg, Rem, and Ca are important additive elements for making
inclusions harmless. The lower limit of each of the elements is set to
0.000 l%o. As their preferable lower limits, Mg is preferably 0.0005%), Rem
is preferably 0.001%, and Ca is preferably 0.0005%. On the other hand,
their excessive additions lead to deterioration of cleanliness, so that the upper
20 limit of Mg is set to 0.01%, the upper limit of Rem is set to 0.1%o, and the
upper limit of Ca is set to 0.01 %>. Ca is preferably 0.01 % or less.
[0043] Mo, Cr, Ni, W, Zr, and As each have an effect of increasing the
mechanical strength and improving the material, so that according to need,
0.001% or more of each of Mo, Cr, Ni, and W is desirably added, and
25 0.000 l%o or more of each of Zr and As is desirably added. As their
. preferable lower limits. Mo is preferably 0.0 l%o, Cr is preferably 0.0L%), Ni is
24
preferably 0.05%, and W is preferably 0.01%. 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%o, the
upper limit of Ni is set tp 2.0%), the upper limit of W is set to 1.0%), the upper
5 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.
[0044] V and Cu, similarly to Nb and Ti, are additive elements that are
effective for precipitation strengthening, have a smaller deterioration margin
of the local ductility ascribable to strengthening by addition than these
10 elements, and are more effective than Nb and Ti when high strength and better
hole expandability are required. Therefore, the lower limits of V and Cu are
set to 0.001%). They are each preferably 0.01% or more. Their excessive
additions lead to deterioration of the workability, so that the upper limit of V
is set to 1.0%o and the upper limit of Cu is set to 2.0%. V is preferably 0.5%)
15 or less.
[0045] Co significantly increases a y to a transformation point, to thus be
an effective element when hot rolling at an Ars point or lower is directed in
particular. In order to obtain this effect, the lower limit is set to 0.0001%).
It is preferably 0.001%) or more. However, when it is too much, the
20 weldability deteriorates, so that the upper limit is set to 1.0%). It is
preferably 0.1 % or less.
[0046] Sn and Pb are elements effective for improving wettability and
adhesiveness of a plating property, and 0.0001% and 0.001%) or more can be
added respectively. Sn is preferably 0.001%) or more. However, when they
25 are too much, a flaw at the time of manufacture is likely to occur, and further
a decrease, in toughness is caused, so that the upper limits are set to 0.2%) and
25
0.1 % respectively. Sn is preferably 0.1 % or less.
[0047] Y and Hf are elements effective for improving corrosion
resistance, and 0.001% to 0.10% can be added. When they are each less
than 0.001%, no effect is confirmed, and when they are added in a manner to
5 exceed 0.10%, the hole expandability deteriorates, so that the upper limits are
set to 0.10%.
[0048] (Surface treatment)
Incidentally, the high-strength cold-rolled steel sheet of the present
invention may also include, on the surface of the cold-rolled steel sheet
10 explained above, a hot-dip galvanized layer made by a hot-dip galvanizing
treatment, and further an alloyed galvanized layer by performing an alloying
treatment after the galvanizing. Even though such galvanized layers are
included, the excellent stretch flangeability and precision punchability of the
present invention are not impaired. Further, even though any one of
15 surface-treated layers made by organic coating film forming, film laminating,
organic salts/inorganic salts treatment, non-chromium treatment, and so on is
included, the effect of the present invention can be obtained.
[0049] (Manufacturing method of the steel sheet)
Next, there will be explained a manufacturing method of the steel
20 sheet of the present invention.
In order to achieve excellent stretch flangeability and precision
punchability, it is important to forai a texture that is random in terms of pole
densities and to manufacture a steel sheet satisfying the conditions of the r
values in the respective directions. Details of manufacturing conditions for
25 satisfying these simultaneously will be described below.
[0050] A manufacturing method prior to hot rolling is not limited in
26
particular. That is, subsequently to melting by a shaft furnace, an electric
furnace, or the like, it is only necessary to variously perform secondary
refining, thereby performing adjustment so as to have the above-described
components and next to perform casting by normal continuous casting, or by
5 an ingot method, or further by thin slab casting, or the like. In the case of
continuous casting, it is possible that a cast slab is once cooled down to low
temperature and thereafter is reheated to then be subjected to hot rolling, or it
is also possible that a cast slab is subjected to hot rolling continuously. A
scrap may also be used for a raw material.
10 [0051] (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 steel sheet of the present invention needs to satisfy the
following requirements. First, an austenite grain diameter after the rough
15 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 )im 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
20 later.
[0052] In order to obtain the austenite grain diameter of 200 |.im or less
before the finish rolling, it is necessary to perfonn rolling at a reduction ratio
of 40% or more one time or more in.-the rough rolling in a temperature region
ofl000tol200°C.
25 [0053] The austenite grain diameter before the fmish rolling is desirably
100 f.im or less, and in order to obtain this grain diameter, rolling at 40% or
27
more is performed two times or more. However, when in the rough rolling,
the reduction is greater than 70% and rolling is performed greater than 10
times, there is a concern that the rolling temperature decreases or a scale is
generated excessively.
5 [0054] In this manner, when the austenite grain diameter before the finish
rolling is set to 200 |im or less, recrystallization of austenite is promoted in
the finish rolling, and particularly, the rL value and the r30 value are
controlled, resulting in that it is effective for improving the hole
expandability.
10 [0055] 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 confirmed in a manner that a steel sheet
piece before being subjected to the finish rolling is quenched as much as
15 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
20 point counting method.
[0056] In order that rC and r30 should satisfy the above-described
predetennined values, the austenite grain diameter after the rough rolling,
namely before the finish rolling is important. - As shown in FIG. 8 and FIG. 9,
the austenite grain diameter before the finish rolling is desirably small, and it
25 turned out that as long as it is 200 ]xm or less, rC and r30 satisfy the
above-described values.
28
[0057] (Second hot rolling)
After the rough rolling process (first hot rolling) is completed, a finish
rolling process being second hot rolHng is started. The time between the
completion of the rough rolling process and the start of the finish rolling
5 process is desirably set to 150 seconds or shorter.
[0058] 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
10 decreased, the reduction is performed in a non-recrystallization temperature
region, the texture develops, and thus isotropy deteriorates.
[0059] 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
15 between a steel sheet base iron and a surface scale before the finish rolling
and between passes, and thus the finish rolling start temperature is desirably
lower than 1150°C.
[0060] In the finish rolling, a temperature determined by the chemical
composition of the steel sheet is set to Tl, and in a temperature region of not
20 lower than Tl + 30°C nor higher than Tl + 200°C, rolling at 30% or more is
perfomied 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, in
the range of 5/8 to 3/8 in sheet thickness from the surface of the steel sheet,
the average value of the pole densities of the {100}<011> to {223}<110>
25 orientation group becomes 6.5 or less and the pole density of the {332}<113>
crystal orientation becomes 5.0 or less. This, makes it possible to secure the
29
excellent flangeability and precision punchability.
[0061] Here, Tl is the temperature calculated by Expression (1) below.
Tl (°C) = 850 + 10 X (C + N) X Mn +-350 x Nb + 250 x Ti + 40 x B +
10 X Cr + 100 X Mo + 100 X V - Expression (1)
5 C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the
element (mass%). Incidentally, when Ti, B, Cr, Mo, and V are not contained,
the calculation is performed in a manner to regard Ti, B, Cr, Mo, and V as
zero.
[0062] In FIG. 10 and FIG. 11, the relationship between a reduction ratio
10 in each temperature region and a pole density in each orientation is shown.
As shown in FIG. 10 and FIG. 11, heavy reduction in the temperature region
of not lower than Tl + 30°C nor higher than Tl + 200°C and light reduction
at Tl or higher and lower than Tl + 30°C thereafter control the average value
of the pole densities of the {100}<011> to {223}<110> orientation group and
15 the pole density of the {332}<113> crystal orientation in the range of 5/8 to
3/8 in sheet thickness from the surface of the steel sheet, and thereby hole
expandability of a final product is improved drastically, as shown in Tables 2
and 3 of Examples to be described later.
[0063] The Tl temperature itself is obtained empirically. The present
20 inventors learned empirically by experiments that the recrystallization in an
austenite region of each steel is promoted on the basis of the Tl temperature.
In order to obtain better 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%)
25 or more, and on the other hand, if the reduction ratio greater than 90%) is
taken, securing temperature and excessive rolling addition are as a result
30
added.
[0064] When the total reduction ratio in the temperature region of not
lower than Tl + 30°C nor higher than Tl + 200°C is less than 50%, rolling
strain to be accumulated during the hot rolling is not sufficient and the
5 recrystaliization 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
10 difficult to obtain the temperature region of Tl + 200°C or lower due to heat
generation by working, and further a rolling load increases to cause a risk that
the rolling becomes difficult to be performed.
[0065] In the finish rolling, in order to promote the uniform
recrystaliization caused by releasing the accumulated strain, the rolling at
15 30%) or more is performed in one pass at least one time at not lower than Tl +
30°C nor higher than Tl + 200°C.
[0066] Incidentally, in order to promote the uniform recrystaliization
caused by releasing the accumulated strain, it is necessary to suppress a
working amount in a temperature region of lower than Tl + 30°C as small as
20 possible. In order to achieve it, the reduction ratio at lower than Tl + 30°C
is desirably 30%) or less. In terms of sheet thiclaiess accuracy and sheet
shape, the reduction ratio of 10%) or less is desirable. When the hole
expandability is further emphasized, the reduction ratio in the temperature
region of lower than Tl + 30°C is desirably 0%o.
25 [0067] The finish rolling is desirably finished at Tl + 30°C or higher. If
the.reduction ratio in the temperature region of Tl or higher, and lower than
31
Tl + 30°C is large, the recrystallized austenite grains are elongated, and if a
retention time is short, the recrystallization does not advance sufficiently, to
thus make the hole expandability deteriorate. That is, with regard to the
manufacturing conditions of the invention of the present application, by
5 making austenite recrystallized uniformly and finely in the finish rolling, the
texture of the product is controlled and the hole expandability is improved.
[0068] A rolling ratio can be obtained by actual performances or
calculation from the rolling load, sheet thickness measurement, or/and the like.
The temperature can be actually measured by a thermometer between stands,
10 or can be obtained by calculation simulation considering the heat generation
by working from a line speed, the reduction ratio, or/and like. Thereby, it is
possible to easily confirm whether or not the rolling prescribed in the present
invention is performed.
[0069] The hot rollings perfonned as above (the first and second hot
15 rollings) are finished at an Arj transformation temperature or higher. When
the hot rolling is finished at Ar3 or lower, the hot rolling becomes two-phase
region rolling of austenite and ferrite, and accumulation to the {100}<011> to
{223}<110> orientation group becomes strong. As a result, the hole
expandability deteriorates significantly.
20 [0070] In order to obtain better strength and to satisfy the hole expansion
^ 30000 by setting rL in the rolling direction and r60 in a direction 60° from
the rolling direction to rL ^ 0.70 and r60 ^ 1.10 respectively, a maximum
working heat generation amount at the time of the reduction at not lower than
Tl + 30°C nor higher than Tl + 200°C, namely a temperature increased
25 margin (°C) by the reduction is desirably suppressed to 18°C or less. For
achieving this, inter-stand cooling or the like is desirably applied. ,.
32
[0071] (Pre-cold rolling cooling)
After final reduction at a reduction ratio of 30% or more is performed
in the finish rolling, pre-cold rolling cooling is started in such a manner that a
waiting time t second satisfies Expression (2) below.
5 t ^ 2.5 X tl ••• Expression (2)
Here, tl is obtained by Expression (3) below.
tl = 0.001 X ((Tf - Tl) X Pl/100)^ - 0.109 x ((Tf - Tl) x Pl/100) + 3.1 -
Expression (3)
Here, in Expression (3) above, Tf represents the temperature of a steel billet
10 obtained after the final reduction at a reduction ratio of 30%) or more, and PI
represents the reduction ratio of the final reduction at 30% or more.
[0072] 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
15 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
20 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%
25 or more) is the "final reduction at a reduction ratio of 30% or more."
[0073] In the finish rolling, the waiting time t second until the pre-cold
33
rolling 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.
5 [0074] 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.
[0075] The waiting time t second further satisfies Expression (2a) below,
10 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 ••• Expression (2a)
15 [0076] At the same time, the waiting time t second ftirther 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.
20 tl ^ t ^ tl X 2.5 - Expression (2b)
[0077] Here, as shown in FIG. 12, 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
25 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)
34
performed in the roughing mill 2, the rolling at a reduction ratio of 40% or
more is performed on the steel billet (slab) one time or more in the
temperature range of not lower than 1000°C nor higher than 1200°C.
[0078] The rough bar rolled to a predetermined thickness in the roughing
5 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 Tl + 30°C nor higher than Tl + 200°C.
10 Further, in the finishing mill 3, the total reduction ratio becomes 50% or
more.
[0079] Further, in the finish rolling process, after the final reduction at a
reduction ratio of 30% or more is perfonned, the pre-cold rolling primary
cooling is started in such a manner that the waiting time t second satisfies
15 Expression (2) above or either Expression (2a) or (2b) above. The start of
this pre-cold rolling 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.
[0080] For example, when the final reduction at a reduction ratio of 30%
20 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. 12, 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. 12, on the downstream side of the rolling), if the
25 start of the pre-cold rolling 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
35
satisfy Expression (2) above or Expressions (2a) and (2b) above is sometimes
caused. In such a case, the pre-cold rolling 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.
5 [0081] 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. 12, on the downstream side of
the rolling), even though the start of the pre-cold rolling cooling is performed
by the cooling nozzles 11 disposed in the run-out-table 5, there is sometimes a
10 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 cooling
may also be started by the cooling nozzles 11 disposed in the mn-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 cooling may
15 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.
[0082] Then, in this pre-cold rolling 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
20 performed.
[0083] 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
25 less than 40°C, the effect cannot be obtained. On the other hand, when the
temperature change exceeds 140°C, the recrystallization becomes insufficient
36
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 hole expandability also
deteriorates. Further, when the temperature change is greater than 140°C, an
5 overshoot to/beyond the Ar3 transformation point temperature is likely to be
caused. In the case, even by the transformation from recrystallized austenite,
as a result of sharpening of variant selection, the texture is formed and the
isotropy decreases consequently.
[0084] When the average cooling rate iri the pre-cold rolling cooling is
10 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.
[0085] Further, as has been explained previously, in order to promote the
15 uniform recrystallization, the working amount in the temperature region of
lower than Tl + 30°C is desirably as small as possible and the reduction ratio
in the temperature region of lower than Tl + 30°C is desirably 30% or less.
For example, in the event that in the finishing mill 3 on the continuous hot
rolling line 1 shown in FIG. 12, in passing through one or two or more of the
20 rolling stands 6 disposed on the front stage side (on the left side in FIG. 12, on
the upstream side of the rolling), the steel sheet is in the temperature region of
not lower than Tl + 30°C nor higher than Tl + 200°C, and in passing through
one or two or more of the rolling stands 6 disposed on the subsequent rear
stage side (on the right side in FIG. 12, on the downstream side of the rolling),
25 the steel sheet is in the temperature region of lower than Tl + 30°C, when the
steel sheet passes through one or two or npre of the rolling stands-6 disposed
37
on the subsequent rear stage side (on the right side in FIG. 12, on the
downstream side of the rolling), even though the reduction is not performed
or is performed, the reduction ratio at lower than Tl + 30°C is desirably 30%
or less in total. In terms of the sheet thickness accuracy and the sheet shape,
5 the reduction ratio at lower than Tl + 30°C is desirably a reduction ratio of
10% or less in total. When the isotropy is further obtained, the reduction
ratio in the temperature region of lower than Tl + 30°C is desirably 0%.
[0086] In the manufacturing method of the present invention, a rolling
speed is not limited in particular. However, when the rolling speed on the
10 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 elongation are
decreased, and thus the elongation 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
15 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. Further, in the hot rolling, sheet bars may also be bonded after
the rough rolling to be subjected to the finish rolling continuously. On this
occasion, the rough bars may also be coiled into a coil shape once, stored in a
20 cover having a heat insulating fLinction according to need, and uncoiled again
to be joined.
[0087] (Coiling)
After being obtained in this manner, the hot-rolled steel sheet can be
coiled at 650°C or lower. When a coiling temperature exceeds 650°C, the
25 area ratio of ferrite structure increases and the area ratio of pearlite does not
become greater than 5%.
38
[0088] (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 40% nor more than 80%. When the reduction ratio is 40% or less,
5 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 80% is performed,
the texture is developed at the time of heating, and thus the anisotropy
becomes strong. Therefore, the reduction ratio of the cold rolling is set to
10 not less than 40% nor more than 80%.
[0089] (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 750
to 900°C and is held for not shorter than 1 second nor longer than 300
15 seconds in the temperature region of 750 to 900°C. When the temperature is
lower than this or the time is shorter than this, reverse transformation from
ferrite to austenite does not advance sufficiently and in the subsequent cooling
process, the second phase cannot be obtained, resulting in that sufficient
strength cannot be obtained. On the other hand, when the temperature is
20 higher than this or the holding is continued for 300 seconds or longer, the
crystal grains become coarse.
[0090] When the steel sheet after the cold roiling is heated up to the
temperature region of 750 to 900°C in this manner, an average heating-rate of
not lower than room temperature nor higher than 650°C is set to HRl
25 (°C/second) expressed by Expression (5) below, and an average heating rate
of higher, than 650°G to the temperature region of 750 to 900°C is set,to-HR2
39
(°C/second) expressed by Expression (6) below.
HRl ^ 0.3 ... Expression (5)
HR2-^ 0.5 X HRl ... Expression (6)
[0091] The hot rolling is performed under the above-described condition,
5 and further the pre-cold rolling 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
10 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 perfonned 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.
15 [0092] 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
reciystallization to occur in the steel sheet by the heating is strain
20 accumulated in the steel sheet by the cold rolling. When the average heating
rate HRl 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
25 such as the isotropy deteriorate. When the average heating rate HRl in the
temperature range of not lower than room temperature nor higher than 650°C.
40
is less than 0.3°C/second, the dislocation introduced by the cold rolling
recovers, resulting in that the strong texture fonned at the time of the cold
rolling remains. Therefore, it is necessary to set the average heating rate
HRl in the temperature range of not lower than room temperature nor higher
5 than 650°C to 0.3 (°C/second) or more.
[0093] On the other hand, when the average heating rate HR2 of higher
than 650°C to the temperature region of 750 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
10 containing C of greater than 0.01% in particular is heated to a two-phase
region of ferrite and austenite, fonned 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
15 non-recrystallized ferrite contains a lot of dislocations, to thus deteriorate the
elongation drastically. Therefore, in the temperature range of higher than
650°C to the temperature region of 750 to 900°C, the average heating rate
HR2 needs to be 0.5 x HRl (°C/second) or less.
[0094] (Post-cold rolling primary cooling)
20 After the holding is performed for a predetermined time in the
above-described temperature range, post-cold rolling primary cooling is
performed down to a temperature region of not lower than 580°C nor higher
than 750°C at an average cooling rate of not less than l°C/s nor more than
10°C/s.
25 [0095] (Retention)
After the post-cold rolling primary cooling is completed, retention is
41
performed for not shorter than 1 second nor longer than 1000 seconds under
the condition that a temperature decrease rate becomes 1 °C/s or less.
[0096] (Post-cold rolling secondary cooling)
After the above-described retention, post-cold rolling secondary
5 cooling is performed at an average cooling rate of 5°C/s or less. When the
average cooling rate of the post-cold rolling secondary cooling is larger than
5°C/s, the sum of bainite and martensite becomes 5% or more and the
precision punchability decreases, resulting in that it is not favorable,
[0097] On the cold-rolled steel sheet manufactured as above, a hot-dip
10 galvanizing treatment, and fiirther subsequently to the galvanizing treatment,
an alloying treatment may also be performed according to need. The hot-dip
galvanizing treatment may be performed in the cooling after the holding in
the temperature region of not lower than 750°C nor higher than 900°C
described above, or may also be performed after the cooling. On this
15 occasion, the hot-dip galvanizing treatment and the alloying treatment may be
performed by ordinary methods. For example, 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
20 advances too much and the corrosion resistance deteriorates.
Example
[0098] 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
25 present invention is not limited to these condition examples. The present
invention can employ various conditions as long as the object of the present
9 42
invention is achieved without departing from the spirit of the invention.
Chemical components of respective steels used in examples are shown in
Table 1. Respective manufacturing conditions are shown in Table 2.
Further, structural constitutions and mechanical properties of respective steel
5 types under the manufacturing conditions in Table 2 are shown in Table 3.
Incidentally, each underline in each Table 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.
[0099] There will be explained results of examinations using Invention
10 steels "A to U" and Comparative steels "a to g," each having a chemical
composition shown in Table 1. Incidentally, in Table 1, each numerical
value of the chemical compositions means mass%. In Tables 2 and 3,
English letters A to U and English letters a to g that are added to the steel
types indicate to be components of Invention steels A to U and Comparative
15 steels a to g in Table 1 respectively.
[0100] These steels (Invention steels A to U and Comparative steels a to
g) were cast and then were heated as they were to a temperature region of
1000 to 1300°C, or were cast to then be heated to a temperature region of
1000 to 1300°C after once being cooled down to room temperature, and
20 thereafter were subjected to hot rolling, cold rolling, and cooling under the
conditions shown in Table 2.
[0101] 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.
25 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.
I& 43
Table 2 shows, in the rough rolling, the number of times of reduction at a
reduction ratio of 40% or more, each reduction ratio (%), and an austenite
grain diameter (|Lim) after the rough rolling (before finish rolling).
Incidentally, a temperature Tl (°C) and a temperature Acl (°C) of the
5 respective steel types are shown in Table 2.
[0102] After the rough rolling was finished, the finish rolling being
second hot rolling was performed. In the finish rolling, rolling at a reduction
ratio of 30% or more was performed in one pass at least one time in a
temperature region of not lower than Tl + 30°C nor higher than Tl + 200°C,
10 and in a temperature range of lower than Tl + 30°C, the total reduction ratio
was set to 30%) or less. Incidentally, in the finish rolling, rolling at a
reduction ratio of 30%o or more in one pass was performed in a final pass in
the temperature region of not lower than Tl + 30°C nor higher than Tl +
200°C.
15 [0103] However, with respect to Steel types A9 and C3, the rolling at a
- reduction ratio of 30%) or more was not perfomied in the temperature region
of not lower than Tl + 30°C nor higher than Tl + 200°C. Further, with
regard to Steel type A7, the total reduction ratio in the temperature range of
lower than Tl + 30°C was greater than 30%o.
20 [0104] Further, in the finish rolling, the total reduction ratio was set to
50%o or more. However, with regard to Steel type C3, the total reduction
ratio in the temperature region of not lower than Tl + 30°C nor higher than
Tl + 200°C was less than 50%.
[0105] Table 2 shows, in the finish rolling, the reduction ratio (%)) in the
25 final pass in the temperature region of not lower than Tl + 30°C nor higher
than Tl + 200°C and the reduction ratio in a pass atone stage earlier than the
44
final pass (reduction ratio in a pass before the final) (%). Further, Table 2
shows, in the finish rolling, the total reduction ratio (%) in the temperature
region of not lower than Tl + 30°C nor-higher than Tl + 200°C, a
temperature (°C) after the reduction in the final pass in the temperature region
5 of not lower than Tl + 30°C nor higher than Tl + 200°C, a maximum
working heat generation amount (°C) at the time of the reduction in the
temperature region of not lower than Tl + 30°C nor higher than Tl + 200°C,
and the reduction ratio (%) at the time of reduction in the temperature range
of lower than Tl + 30°C.
10 [0106] After the final reduction in the temperature region of not lower
than Tl + 30°C nor higher than Tl + 200°C was performed in the finish
rolling, pre-cold rolling cooling was started before a waiting time t second
exceeding 2.5 x tl. In the pre-cold rolling cooling, an average cooling rate
was set to 50°C/second or more. Further, a temperature change (a cooled
15 temperature amount) in the pre-cold rolling cooling was set to fall within a
- range of not less than 40°C nor more than 140°C.
[0107] However, with respect to Steel types A9 and J2, the pre-cold
rolling 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 Tl +
20 30°C nor higher than Tl + 200°C in the finish rolling. With regard to Steel
type A3, the temperature change (cooled temperature amount) in the pre-cold
rolling primary cooling was less than 40°C, and with regard to Steel type B3,
the temperature change (cooled temperature amount) in the pre-cold rolling
cooling was greater than 140°C. With regard to Steel type A8, the average
25 cooling rate in the pre-cold rolling cooling was less than 50°C/second.
. [0108] Table 2 shows tl (second) of the respective steel.types, the waiting
45
time t (second) to the start of the pre-cold rolling cooling since the final
reduction in the temperature region of not lower than Tl + 30°C nor higher
than Tl + 200°C in the finish rolling, t/tl, the temperature change (cooled
amount) (°C) in the pre-cold rolling cooling, and the average cooling rate in
5 the pre-cold rolling cooling (°C/second).
[0109] After the pre-cold rolling cooling, coiling was performed at 650°C
or lower, and hot-rolled original sheets each having a thickness of 2 to 5 mm
were obtained.
[0110] However, with regard to Steel types A6 and E3, the coiling
10 temperature was higher than 650°C. Table 2 shows a stop temperature of the
pre-cold rolling cooling (the coiling temperature) (°C) of the respective steel
types.
[0111] Next, the hot-rolled original sheets were each pickled to then be
subjected to cold rolling at a reduction ratio of not less than 40% nor more
15 than 80%). However, with regard to Steel types A2, E3, 13, and M2, the
reduction ratio of the cold rolling was less than 40%. Further, with regard to
Steel type C4, the reduction ratio of the cold rolling was greater than 80%).
Table 2 shows the reduction ratio (%o) of the cold rolling of the respective
steel types.
20 [0112] After the cold rolling, heating was perfomied up to a temperature
region of 750 to 900°C and holding was performed for not shorter than 1
second nor longer than 300 seconds. Further, when the heating was
perfonned up to the temperature region of 750 to 900°C, an average heating
rate HRl of not lower than room temperature nor higher than 650°C
25 (°C/second) was set to 0.3 or more (HRl ^ 0.3), and an average heating rate
. .Pm2.of higher than 650°C to 750 to 900°C (°C/second) was setto.0.5 x HRl
# 46
or less (HR2 ^ 0.5 x HRl). Table 2 shows, of the respective steel types, a
heating temperature (an annealing temperature), a heating and holding time
(time to start of post-cold rolling primary cooling) (second), and the average
heating rates HRl and HR2 (°C/second).
5 [0113] However, with regard to Steel type F3, the heating temperature
was higher than 900°C. With regard to Steel type N2, the heating
temperature was lower than 750°C. With regard to Steel type C5, the
heating and holding time was shorter than one second. With regard to Steel
type F2, the heating and holding time was longer than 300 seconds. Further,
10 with regard to Steel type B4, the average heating rate HRl was less than 0.3
(°C/second). With regard to Steel type B5, the average heating rate HR2
(°C/second) was greater than 0.5 x HRl.
[0114] After the heating and holding, the post-cold rolling primary
cooling was performed down to a temperature region of 580 to 750°C at an
15 average cooling rate of not less than l°C/s nor more than 10°C/s. However,
with regard to Steel type A2, the average cooling rate in the post-cold rolling
primary cooling was greater than 10°C/second. With regard to Steel type C6,
the average cooling rate in the post-cold rolling primary cooling was less than
l°C/second. Further, with regard to Steel types A2 and A5, a stop
20 temperature of the post-cold rolling primary cooling was lower than 580°C,
and with regard to Steel types A3, A4, and M2, the stop temperature of the
post-cold rolling primary cooling was higher than 750°C. Table 2 shows, of
the respective steel types, the average cooling rate (°C/second) and the
cooling stop temperature (°C) in the post-cold rolling primary cooling.
25 [0115] After the post-cold rolling primary cooling was performed,
retention was performed for not shorter than 1 second nor longer than 1000.
# 47
seconds under the condition that a temperature decrease rate becomes l°C/s
or less. Table 2 shows a retention time (time to start of the post-cold rolling
primary cooling) of the respective steels.
[0116] After the retention, post-cold rolling secondary cooling was
5 performed at an average cooling rate of 5°C/s or less. However, with regard
to Steel type A5, the average cooling rate of the post-cold rolling secondary
cooling was greater than 5°C/second. Table 2 shows the average cooling
rate (°C/second) in the post-cold rolling secondary cooling of the respective
steel types.
10 [0117] Thereafter, skin pass rolling at 0.5% was performed and material
evaluation was performed. Incidentally, on Steel type Tl, a hot-dip
galvanizing treatment was performed. On Steel type Ul, an alloying
treatment was performed in a temperature region of 450 to 600°C after
galvanizing.
15 [0118] Table 3 shows area ratios (structural fractions) (%) of femte,
pearlite, and bainite + martensite in a metal structure of the respective steel
types, and an average value of pole densities of the {100}<011> to
{223}<110> orientation group and a pole density of the {332}<113> crystal
orientation in a range of 5/8 to 3/8 in sheet thickness from the surface of the
20 steel sheet of the respective steel types. Incidentally, the structural fraction
was evaluated by the structural fraction before the skin pass rolling. Further,
Table 3 showed, as the mechanical properties of the respective steel types, rC,
rL, r30, and r60 being respective r vales, tensile strength TS (MPa), an
elongation percentage El (%), a hole expansion ratio X (%) as an index of
25 local ductility, TS x X, Vickers hardness of pearlite HVp, and a shear surface
percentage (%). Further, it showed presence or absence of the galvanizing
f^ 48
treatment,
[0119] 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
TlOOl. The pole density of each of the crystal orientations was measured
5 using the previously described EBSP at a 0.5 [xm pitch on a 3/8 to 5/8 region
at sheet thickness of a cross section parallel to the rolling direction. Further,
the r value in each of the directions was measured by the above-described
method. With regard to the shear surface percentage, each of the steel sheets
whose sheet thickness was set to 1.2 mm was punched out by a circular punch
10 with O 10 mm and a circular die with 1% of a clearance, and then each
punched edge surface was measured. vTrs (a Charpy fracture appearance
transition temperature) was measured by a Chaipy impact test method based
on JIS Z 2241. The stretch flangeability was determined to be excellent in
the case of TS X A- ^ 30000, and the precision punchability was determined
15 to be excellent in the case of the shear surface percentage being 90% or more.
The low-temperature toughness was detennined to become poor in the case of
vTrs = higher than -40.
[0120] As shown in FIG. 14, it is found that only ones satisfying the
conditions prescribed in the present invention have the excellent precision
20 punchability and stretch flangeability.
[0121] [Table 1]
[0122] [Table 2]
[0123] [Table 3]
TABLE1
TP
A
B
C
D
E
F
G
H
I
J
K
L
M
N
0
P
Q
R
S
T
U
a
b
c
d
e
f
g
T1/°C
851
851
865
865
858
858
865
865
861
886
875
892
892
886
903
903
852
852
852
852
880
856
1376
851
1154
854
854
853
C
0.070
0.070
0.080
0.080
0.060
0.060
0.210
0.210
0.035
0.035
0.180
0.180
0.060
0.060
0.040
0.040
0.180
0.180
0.180
0.180
0.035
0.450
0.072
0.110
0.250
0.250
0.250
0.220
Si
0.08
0.08
0.31
0.31
0.87
0.30
0.15
0.90
0.67
0.67
0.48
0.48
0.11
0.11
0.13
0.13
0.50
0.30
2.30
0.21
0.02
0.52
0.15
0.23
0.23
0.23
0.21
0.2
Mn
1.30
1.30
1.35
1.35
1.20
1.20
1.62
1.62
1.88
1.88
2.72
2.72
2.12
2.12
1.33
1.33
0.90
1.30
0.90
1.30
1.30
1.33
1.42
1.12
1.56
1.54
1.54
1.53
P
0.015
0.015
0.012
0.012
0.009
0.009
0.012
0.012
0.015
0.015
0.009
0.009
0.01
0.01
0.01
0.01
0.008
0.08
0.008
0.01
0.01
0.26
0.014
0.021
0.024
0.02
0.02
0.015
S
0.004
0.004
0.005
0.005
0.004
0.004
0.003
0.003
0.003
0.003
0.003
0.003
0.005
0.005
0.005
0.005
0.003
0.002
0.003
0.002
0.002
0.003
0.004
0.003
0.12
0.002
0.002
0.004
Al
0.040
0.040
0.016
0.016
0.038
0.500
0.026
0.026
0.045
0.045
0.050
0.050
0.033
0.033
0.038
0.038
0.045
0.030
0.045
0.650
0.035
0.045
0.036
0.026
0.034
0.038
0.034
0.031
N
0.0026
0.0026
0.0032
0.0032
0,0033
0.0033
0.0033
0.0033
0.0028
0.0028
0.0036
0.0036
0.0028
0.0028
0.0032
0.0036
0.0028
0.0032
0,0028
0.0032
0.0023
0.0026
0.0022
0.0025
0.0022
0.0026
0.0026
0.0028
0
0.0032
0.0032
0.0023
0.0023
0.0026
0.0026
0.0021
0.0021
0.0029
0.0029
0.0022
0.0022
0.0035
0.0035
0.0026
0.OO29
0.0029
0.0022
0.0022
0.0035
0.0033
0.0019
0.0025
0.0023
0.0023
0.0032
0.0023
0.0026
Ti
--
--
-
-
0.021
0.021
-
0.1
-
-
0.036
0.089
0.042
0.042
-
---
0.12
-
--
-
-
-
-
Nb
0.00
0.00
0.04
0.04
0.02
0.02
0.00
0.00
0.02
0.02
-
0.05
0.089
0.036
0.121
0.121
-
----
-
15
-
--
--
B
-
0.005
-
0.0000
-
-
0.0022
0.0022
-
--
-
0.0012
0.0012
0.0009
0.0009
--
-
-
-
--
---
--
Mg
--
---
---
0.002
0.002
0.002
0.002
--
---
------
0.15
-
-
--
Rem
----
0.0015
0.0015
--
----
---
0.004
-
------
-
-
-
--
Ca
--
-
0.002
-
---
0.0015
0.0015
-
0.002
---
-
-
--------
-
-
-
Mo
--
---
-
0.03
0.03
--
0.1
0.1
--
-----
• -
---
-
-
-
-
-
Cr Ni W
- - -
- - -
- - -
- - -
- - -
- - -
0.35 -
0.35 -
- - -
- - -
0 - -
0 - -
- - -
- - -
Zr As V
--
- -
- - -
- - -
- - -
- - -
- - -
_ - -
0.029
0.029
0.1
0.1
- - -
- - -
- - - 0.001 - 0.00
- - -
- 0.1
0.1
- - -
- - -
- - -
--
-
Si) - -
-
- - -
- - -
-
_
- - -
- ~ -
- - -
0..002
---
L5
-
- - -
- - -
Gu,Co.Sn,Pb,Y,Hf
--
------
---
-
Y: 0.004
Hf: 0.003
Sn: 0.002
Co: 0.003
-
--
Pb: 0.003
Cu:0.2
--
--
Co:12
PbiOS
YiOa
NOTE
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
INVENTION STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
Acl
711.42
711.42
717.58
717.58
735.48
718.89
715.95
737.77
722.38
722.38
707.86
707.86
703.52
703.52
712.55
712.55
727.92
717.82
780.3
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0.70
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1.03
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1.07
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517
573
517
521
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546
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735
750
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CRACKING OCCURRED DURING HOT ROLLING
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17
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16
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90.5
40.6
42.3
43.0
40.2
36.5
41.9
62.0
35.0
86.4
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43.0
41.0
55.0
57.3
34.3
31.4
30.2
42.0
60.2
63.1
63.4
73.2
71.0
36.0
50.7
42.5
40.1
61.1
31.0
62.2
50.4
46.0
395
55.1
35.2
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399
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-70
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-80
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-80
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-90
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-100
-40
-50
-40
-60
-80
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-60
-30
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TS-JL
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25334
22123
29541
20783
20915
21662
32302
18340
50366
51024
28220
27262
26937
50215
52258
29910
29328
27331
35994
54601
53923
58835
60317
60066
28296
36707
29793
27188
62444
27404
64875
42941
34500
29309
49259
29779
35178
64045
62504
37412
28500
45149
24430
35964
29765
37252
48441
51559
74455
71833
52385
IIV,
163
143
124
201
133
142
170
173
180
190
227
140
197
208
151
150
150
159
151
156
152
151
162
294
232
176
166
154
137
164
157
201
156
142
142
153
151
162
251
291
198
156
236
241
185
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175
353
378
184
196
165
SilEAR
SURFACE
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100
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86
88
46
76
90
91
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100
100
84
90
89
98
97
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100
94
100
100
75
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84
72
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64
91
100
100
91
100
80
100
90
90
100
74
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94
100
88
100
92
93
100
100
100
PRESENT INVENTION STEEL
COMPARATIVE STEEL
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COUPAHATIVF STFFL
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PRESENT INVENTION STEEL
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COMPARATIVE STEEL
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COMPARATIVE STEEL
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PRESENT INVENTION STEEL
PRESENT INVENTION STEEL
COMPARATIVE STEEL
PRESENT INVENTION STEEL
COMPABATIVF STEEL
PRESENT INVENTION STEEL
PRESENT INVENTION STEEL
PRESENT INVENTION STEEL
PRESENT INVENTION STEEL
COMPARATIVF STFFL
PRESENT INVENTION STEEL
COMPARATIVE STFFL
PRESENT INVENTION STEEL
COMPARATIVE STEEL
PRESENT INVENTION STEEL
PRESENT INVENTION STEEL
PRESENT INVENTION STEEL
PRESENT INVENTION STEEL
PRESENT INVENTION STEEL
PRESENT INVENTKJN STEEL
COMPARATIVF STFFL
COMPARATIVF STEEL
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^
yu!ii*i.«(»."ji<.>« Mi,l.H!tl!aB!millw

i,-> 49 ±, ^ .'U>.r > '-v'^lGI^^ A t i l l i(t
[Name of Document] What is claimed is ^i 1 p £B 1914
[Claim 1] A high-strength cold-rolled steel sheet having excellent stretch
flangeability and precision punchability comprising:
in mass%,
5 C: greater than 0.01 % to 0.4% or less;
Si: not less than 0.001% nor more than 2.5%;
Mn: not less than 0.00 P/o nor more than 4%;
P: 0.001 to 0.15% or less;
S: 0.0005 to 0.03% or less;
10 Al: not less than 0.001 % nor more than 2%;
N: 0.0005 to 0.01% or less; and
a balance being composed of iron and inevitable impurities, wherein
in a range of 5/8 to 3/8 in sheet thickness from the surface of the steel sheet,
an average value of pole densities of the {100}<011> to {223}<110>
15 orientation group represented by respective crystal orientations of
{100}<011>, {116}<110>, {114}, -{113}<110>, {112}<110>,
(335}<110>, and {223}<110> is 6.5 or less, and a pole density of the
{332}<113> crystal orientation is 5.0 or less, and
a metal structure contains, in terms of an area ratio, greater than 5% of
20 pearlite, the sum of bainite and martensite limited to less than 5%, and a
balance composed of ferrite.
[Claim 2] The high-strength cold-rolled steel sheet having excellent
stretch tlangeability and precision punchability according to claim 1, wherein
further, Vickers hardness of a pearlite phase is not less than 150 HV nor more
25 than 300 HV.
[Claim 3] The high-strength cold-rolled steel sheet having excellent
> n '^ *
^ ^*^' 21 FEB 2014
stretch flangeability and precision punchability according to claim 1, wherein
further, an r value in a direction perpendicular to a rolling direction (rC) is
0.70 or more, an r value in a direction 30° from the rolling direction (r30) is
1.10 or less, an r value in the rolling direction (rL) is 0.70 or more, and an r
5 value in a direction 60° from the rolling direction (r60) is 1.10 or less.
[Claim 4] The high-strength cold-rolled steel sheet having excellent
stretch flangeability and precision punchability according to claim 1, further
comprising:
one type or two or more types of
10 inmass%,
Ti: not less than 0.001% nor more than 0.2%,
Nb: not less than 0.001% nor more than 0.2%,
B: not less than 0.0001%) nor more than 0.005%,
Mg: not less than 0.0001% nor more than 0.01%,
15 Rem: not less than 0.0001% nor more than 0.1%,
Ca: not less than 0.0001 % nor more than 0.01 %,
Mo: not less than 0,001%) nor more than 1%,
Cr: not less than 0.001%) nor more than 2%),
V: not less than 0.001%) nor more than 1%),
20 Ni: not less than 0.001 % nor more than 2%,
Cu: not less than 0.001%) nor more than 2%o,
Zr: not less than 0.000 P/o nor more than 0.2%,
W: not less than 0.001%) nor more than l%o,
As: not less than 0.0001%) nor more than 0.5%),
25 Co: not less than 0.0001%) nor more than l%o,
Sn: not less than 0.000 l%o nor more than 0.2%,-
Pb: not less than 0.001 % nor more than 0.1 %,
Y: not less than 0.001% nor more than 0.1%, and
Hf: not less than 0.001 % nor more than 0.1 %.
[Claim 5] The high-strength cold-rolled steel sheet having excellent
5 stretch flangeability and precision punchability according to claim 1, wherein
further, when the steel sheet whose sheet thickness is reduced to 1.2 mm with
a sheet thickness center portion set as the center is punched out by a circular
punch with O 10 mm and a circular die with 1% of a clearance, a shear
surface percentage of a punched edge surface becomes 90% or more.
10 [Claim 6] The high-strength cold-rolled steel sheet having excellent
stretch flangeability and precision punchability according to claim 1, wherein
on the surface, a hot-dip galvanized layer or an alloyed hot-dip galvanized
layer is provided.
[Claim 7] A manufacturing method of a high-strength cold-rolled steel
15 sheet having excellent stretch flangeability and precision punchability,
comprising:
on a steel billet containing:
in mass%),
C: greater than 0.01% to 0.4% or less;
20 Si: not less than 0.001% nor more than 2.5%;
Mn: not less than 0.001% nor more than 4%;
P: 0.001 to 0.15% or less;
. S: 0.0005 to 0.03% or less;
Al: not less than 0.001% nor more than 2%;
25 N: 0.0005 to 0.01% or less; and
a balance being composed of iron and inevitable impurities,
2 , FEB Mtt
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 fim or less by the first hot rolling;
5 performing second hot rolling in which rolling at a reduction ratio of 30%) or
more is performed in one pass at least one time in a temperature region of not
lower than a temperature Tl determined by Expression (1) below + 30°C nor
higher than Tl+200°C;
setting the total reduction ratio in the second hot rolling to 50%) or more;
10 performing final reduction at a reduction ratio of 30% or more in the second
hot rolling and then starting pre-cold rolling cooling in such a manner that a
waiting time t second satisfies Expression (2) below;
setting an average cooling rate in the pre-cold rolling cooling to 50°C/second
or more and setting a temperature change to fall within a range of not less
15 than 40°C nor more than 140°C;
perfonning cold rolling at a reduction ratio of not less than 40%) nor more
than 80%;
perfomiing heating up to a temperature region of 750 to 900°C and
performing holding for not shorter than 1 second nor longer than 300 seconds;
20 performing post-cold rolling primary cooling down to a temperature region of
not lower than 580°C nor higher than 750°C at an average cooling rate of not
less than l°C/s nor more than 10°C/s;
performing retention for not shorter than 1 second nor long-er than 1000
seconds under the condition that a temperature decrease rate becomes l°C/s
25 or less; and
. . performing post-cold rolling secondary cooling at an average cooling rate of
^^^ r-^^pSHwJ^-rmmis
5°C/s or less.
Tl (°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 - Expression (1)
Here, C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the
5 element (mass%).
t ^ 2.5 X tl — Expression (2)
Here, tl is obtained by Expression (3) below.
tl = 0.001 X ((Tf - Tl) X Pl/100)^ - 0.109 x ((Tf - Tl) x Pl/100) + 3.1 -
Expression (3)
10 Here, in Expression (3) above, Tf represents the temperature of the steel billet
obtained after the final reduction at a reduction ratio of 30% or more, and PI
represents the reduction ratio of the final reduction at 30% or more.
[Claim 8] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent stretch flangeability and precision punchability
15 according to claim 7, wherein
the total reduction ratio in a temperature range of lower than Tl + 30°C is
30% or less.
[Claim 9] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent stretch flangeability and precision punchability
20 according to claim 7, wherein
the waiting time t second further satisfies Expression (2a) below.
t < tl ••• Expression (2a)
[Claim 10] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent stretch flangeability and precision punchability
25 according to claim 7, wherein
- the waiting time t second further satisfies Expression (2b) below. ,
IS
tl ^ t ^ tl X 2.5 - Expression (2b) I » ' ^
[Claim 11] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent stretch flangeability and precision punchability
according to claim 7, wherein
5 the pre-cold rolling cooling is started between rolling stands.
[Claim 12] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent stretch flangeability and precision punchability
according to claim 7, further comprising:
performing coiling at 650°C or lower to obtain a hot-rolled steel sheet after
10 performing the pre-cold rolling cooling and before performing the cold
rolling.
[Claim 13] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent stretch flangeability and precision punchability
according to claim 7, wherein
15 when the heating is performed up to the temperature region of 750 to 900°C
after the cold rolling, art average heating rate of not lower than room
temperature nor higher than 650°C is set to HRl (°C/second) expressed by
Expression (5) below, and
an average heating rate of higher than 650°C to 750 to 900°C is set to HR2
20 (°C/second) expressed by Expression (6) below.
HRl ^ 0.3 ... Expression (5)
HR2 ^ 0.5 X HRl ... Expression (6)
[Claim 14] The manufacturing method of the high-strength cold-rolled
steel sheet having excellent stretch flangeability and precision punchability
25 according to claim 7, further comprising:
performing hot-dip galvanizing on the surface. - -
-•Avfi'6«l«
fte M»
[Claim 15] The manufacturing method of the high-strength xold-rolled
steel sheet having excellent stretch flangeability and precision punchabihty
according to claim 14, further comprising:
performing an alloying treatment at 450 to 600°C after performing the hot-dip
galvanizing.