Technical Field [0001] The present invention relates to a hot-rolled steel sheet, a cold-rolled steel sheet,
10 and a galvanized steel sheet which are excellent in terms of local deformability, such as bending, stretch flange, or a burring working, have a small orientation dependency of formability, and are used mainly for automobile components and the like, and methods of manfiiacturing the same. The hot-rolled steel sheet includes a hot-rolled strip that serves as a starting sheet for the cold-rolled steel sheet, the galvanized steel sheet, or the
15 like.
Priority is claimed on Japanese Patent Application No. 2010-169670, filed July 28,2010, Japanese Patent Application No. 2010-169627, filed July 28,2010, Japanese Patent Application No. 2011-048236, filed March 4,2011, Japanese Patent Apphcation No. 2010-169230, filed July 28,2010, Japanese Patent Application No. 2011-048272,
20 filed March 4,2011, Japanese Patent Application No. 2010-204671, filed September 13, 2010, Japanese Patent Application No. 2011-048246, filed March 4,2011, and Japanese Patent Application No. 2011-048253, filed March 4,2011, the contents of which are incorporated herein by reference.
25 Background Art

2 [0002]
An attempt is being made to reduce the weight of an automobile frame through
use of a high-strength steel sheet in order to suppress the amount of carbon dioxide
exhausted from an automobile. In addition, a high-strength steel sheet as well as a soft
5 steel sheet has been frequently used for automobile frames from the viewpoint of
securing the safety of passengers. However, in order to fiirther reduce the weight of an
automobile frame in the fiiture, it is necessary to increase the level of operational strength
of a high-strength steel sheet compared to the related art.
[0003]
10 However, in general, an increase in the strength of a steel sheet results in a
decrease in the formability. For example, Non Patent Document 1 discloses that an
increase in strength degrades uniform elongation which is important for drawing or
stretch forming.
Therefore, in order to use a high-strength steel sheet for underbody components
15 of an automobile frame, components that contribute to absorption of impact energy, and
the like, it becomes important to improve local deformability, such as local ductility that
contributes to formability, such as burring workability or bending workability.
[0004]
In contrast to the above, Non Patent Document 2 discloses a method in which
20 uniform elongation is improved by complexing the metallic structure of a steel sheet even
when the strength is maintained at the same level.
[0005]
In addition, Non Patent Document 3 discloses a metallic structure control
method in which local deformability represented by bending properties, hole expanding
25 workability, or burring workability is improved through inclusion control, single

^ 3
structure formation, and, furthermore, a decrease in the hardness difference between
structures. The above method is to improve hole expanding properties by forming a
single structure through structure control, and, in order to form a single structure, a
thermal treatment from an austenite single phase serves as the basis of the manufacturing
5 method as described in Non Patent Document 4.
[0006]
In addition, Non Patent Document 4 discloses a technique in which metallic
structure is controlled through the control of cooling after hot rolling, and precipitates
and deformed structures are controlled so as to obtain ferrite and bainite at an appropriate
10 proportion, thereby satisfying both an increase in the strength and securement of
ductility.
However, all of the above techniques are a method of improving local
deformability through structure control, which is significantly influenced by base
structure formation.
15 [0007]
Meanwhile, even for improvement of material quality through an increase in the
rolling reduction in a continuous hot rolling process, related art exists, which is a
so-called grain refinement technique. For example, Non Patent Document 5 describes a
technique in which large reduction is carried out at an extremely low temperature range
20 in an austenite range, and non-recrystallized austenite is transformed into ferrite so that
the crystal grains of ferrite which is the main phase of the product are refined, and the
strength or toughness increases due to the grain refinement. However, Non Patent
Document 5 pays no attention to improvement of local deformability which is the object
of the present invention.
25

^ 4
Citation List
Non Patent Documents
[0008]
[Non Patent Document 1] "Nippon Steel Corporation Technical Report," by 5 Kishida(1999)No.371,p. 13
[Non Patent Document 2] "Trans. ISIJ," by O. Matsumura et al. (1987) Vol. 27, P570
[Non Patent Document 3] "Steel-manufacturing studies," by Kato et al. (1984)
Vol. 312, p. 41
10 [Non Patent Document 4] "ISIJ International," by K. Sugimoto et al. (2000)
Vol.40, p. 920
[Non Patent Document 5] NFG Catalog, Nakayama Steel Works, Ltd.
Summary of Invention 15 Technical Problem [0009]
As described above, structure control including inclusion control was a main solution for improving the local deformability of a high-strength steel sheet. However, since the solution relied on structure control, it was necessary to control the proportion or 20 form of structures, such as ferrite and bainite, and the base metallic structure was limited. [0010]
Therefore, in the present invention, control of a texture is employed instead of control of the base structure, and a hot-rolled steel sheet, a cold-rolled steel sheet, and a galvanized steel sheet which are excellent in terms of the local deformability of a 25 high-strength steel sheet, and have a small orientation dependency of formability, and a

5 method of manfuacturing the same are provided by controlling the size or form of crystal
grains and texture as well as the kinds of phases.
Solution to Problem
5 [0011]
According to the knowledge in the related art, hole expanding properties, bending properties, and the like were improved through inclusion control, precipitation refinement, structure homogenization, formation of a single structure, a decrease in the hardness difference between structures, and the like. However, with the above
10 techniques alone, the main structure composition will be limited. Furthermore, in a case in which Nb, Ti, and the like which are typical elements that significantly contribute to an increase in strength are added in order to increase the strength, since there is a concern that anisotropy may increase extremely, it is necessary to sacrifice other forming factors or limit the direction in which blanks are taken before forming, thereby limiting uses.
15 [0012]
Therefore, the present inventors newly paid attention to the influence of the texture in a steel sheet in order to improve hole expanding properties or bending workability, and investigated and studied the effects in detail. As a result, the inventors clarified that local deformability is drastically improved by controlling the X-ray random
20 intensity ratio of the respective orientations of a specific crystal orientation group from a hot rolling process, and, furthermore, controlling the r value in a rolling direction, the r value in the direction perpendicular to the rolling direction, and the r value in a direction that forms an angle of 30° or 60° with respect to the rolling direction. [0013]
25 The present invention was constituted based on the above finding, and the

6 present invention employed the following measures in order to solve the above problems
and achieve the relevant object.
(1) That is, a hot-rolled steel sheet according to an aspect of the present
invention contains, by mass%, C: 0.0001% to 0.40%, Si: 0.001% to 2.5%, Mn: 0.001%
5 to 4.0%, P: 0.001% to 0.15%, S: 0.0005% to 0.03%, Al: 0.001% to 2.0%, N: 0.0005% to 0.01%, and O: 0.0005% to 0.01%, and further contains one or two or more of Ti: 0.001% to 0.20%, Nb: 0.001% to 0.20%, V: 0.001% to 1.0%, W: 0.001% to 1.0%, B: 0.0001% to 0.0050%, Mo: 0.001% to 1.0%, Cr: 0.001% to 2.0%, Cu: 0.001% to 2.0%, Ni: 0.001% to 2.0%, Co: 0.0001% to 1.0%, Sn: 0.0001% to 0.2%, Zr: 0.0001% to 0.2%, As: 0.0001% to
10 0.50%, Mg: 0.0001% to 0.010%, Ca: 0.0001% to 0.010%, and REM: 0.0001% to 0.1% and balance composed of iron and inevitable impurities, in which an average value of an X-ray random intensity ratio of a {100} <011> to {223} <110> orientation group at least in a thickness central portion that is in a sheet thickness range of 5/8 to 3/8 from a steel sheet surface is 1.0 to 6.0, an X-ray random intensity ratio of a {332} <113> crystal
15 orientation is 1.0 to 5.0, rC which is an r value in a direction perpendicular to a rolling direction is 0.70 to 1.10, and r30 which is an r value in a direction that forms an angle of 30° with respect to the rolling direction is 0.70 to 1.10. [0014]
(2) In addition, in the aspect according to the above (1), furthermore, rL which is
20 an r value in the rolling direction may be 0.70 to 1.10, and r60 which is an r value in a
direction that forms an angle of 60° with respect to the rolling direction may be 0.70 to 1.10.
[0015]
(3) In addition, in the aspect according to the above (1) or (2), furthermore, one
25 or two or more of bainite, martensite, pearlite, and austenite are present in the hot-rolled

^ 7
steel sheet, and a proportion of grains having a dL/dt which is a ratio of a length in the
rolling direction dL to a length of a sheet thickness direction dt of 3.0 or less in crystal
grains in the structures may be 50% to 100%.
[0016]
5 (4) In the aspect according to the above (1) or (2), an area proportion of crystal
grains having a grain diameter of more than 20 [xm in a total area of a metallic structure in the hot-rolled steel sheet may be 0% to 10%.
[0017]
(5) A cold-rolled steel sheet according to an aspect of the present invention is a
10 cold-rolled steel sheet obtained through cold rolling of the hot-rolled steel sheet
according to the above (1), in which the average value of the X-ray random intensity ratio of a {100} <011> to {223} <110> orientation group at least in the thickness central portion is 1.0 to less than 4.0, the X-ray random intensity ratio of a {332} <113> crystal orientation is 1.0 to 5.0, rC which is the r value in a direction perpendicular to the rolling 15 direction is 0.70 to 1.10, and r30 which is the r value in a direction that forms an angle of 30° with respect to the rolling direction is 0.70 to 1.10. [0018]
(6) In the aspect according to the above (5), rL which is an r value in the rolling
direction may be 0.70 to 1.10, and r60 which is an r value in a direction that forms an
20 angle of 60° with respect to the rolling direction may be 0.70 to 1.10. [0019]
(7) In the aspect according to the above (5) or (6), furthermore, one or two or
more of bainite, martensite, pearlite, and austenite are present in the cold-rolled steel
sheet, and a proportion of grains having a dL/dt which is a ratio of a length in the rolling
25 direction dL to a length of a sheet thickness direction dt of 3.0 or less in crystal grains in

8 the structures may be 50% to 100%.
[0020]
(8) In the aspect according to the above (5) or (6), an area proportion of crystal
grains having a grain diameter of more than 20 |am in a total area of a metallic structure
5 in the cold-rolled steel sheet may be 0% to 10%. [0021]
(9) A galvanized steel sheet according to an aspect of the present invention is a
galvanized steel sheet further having a galvanized coating layer or a galvanealed coating
layer on a surface of the cold-rolled steel sheet according to the above (5), in which the
10 average value of the X-ray random intensity ratio of a {100} <011 > to {223} < 110>
orientation group at least in the thickness central portion is 1.0 to less than 4.0, the X-ray random intensity ratio of a {332} <113> crystal orientation is 1.0 to 5.0, rC which is the r value in a direction perpendicular to the rolling direction is 0.70 to 1.10, and r30 which is the r value in a direction that forms an angle of 30° with respect to the rolling direction is
15 0.70 to 1.10.
[0022]
(10) In the aspect according to the above (9), rL which is an r value in the rolling
direction may be 0.70 to 1.10, and r60 which is an r value in a direction that forms an
angle of 60° with respect to the rolling direction may be 0.70 to 1.10.
20 [0023]
(11) In a method of manufacturing the hot-rolled steel sheet according to an
aspect of the present invention, first hot rolling in which an ingot or slab which contains,
by mass%, C: 0.0001% to 0.40%, Si: 0.001% to 2.5%, Mn: 0.001% to 4.0%, P: 0.001%
to 0.15%, S: 0.0005% to 0.03%, Al: 0.001% to 2.0%, N: 0.0005% to 0.01%, and O:
25 0.0005% to 0.01%, and further contains one or two or more of Ti: 0.001% to 0.20%, Nb:

0.001% to 0.20%, V: 0.001% to 1.0%, W: 0.001%) to 1.0%, B: 0.0001% to 0.0050%, Mo: 0.001% to 1.0%, Cr: 0.001% to 2.0%, Cu: 0.001% to 2.0%, Ni: 0.001%) to 2.0%, Co: 0.0001% to 1.0%, Sn: 0.0001%) to 0.2%), Zr: 0.0001% to 0.2%, As: O.OOOP/o to 0.50%, Mg: 0.0001% to 0.010%, Ca: 0.0001% to 0.010%, and REM: 0.0001% to 0.1% and 5 balance composed of iron and inevitable impurities is rolled at least once at a rolling
reduction ratio of 20% or more is carried out in a temperature range of 1000°C to 1200°C, an austenite grain diameter is set to 200 ^mi or less, second hot rolling in which a total of rolling reduction ratios is 50% or more is carried out in a temperature range of T1+30°C to T1+200°C, third hot rolling in which a total of rolling reduction ratios is less than 30% 10 is carried out in a temperature range of T1°C to TH-30°C, and hot rolling ends at an Ar3 transformation temperature or higher.
Here, Tl is a temperature determined by steel sheet components, and expressed by the following formula 1.
Tl (°C) = 850+10x(C + N)xMn + 350xNb + 250xTi + 40xB + 10xCr +
15 lOOxMo+lOOxV - (Formula 1)
[0024]
(12) In the aspect according to the above (11), in the second hot rolling in the
temperature range of TH-30°C to T1+200°C, the ingot or slab may be rolled at least once
at a rolling reduction ratio of 30% or more in a pass.
20 [0025]
(13) In the aspect according to the above (11) or (12), in the first hot rolling in a
temperature range of 1000°C to 1200°C, the ingot or slab may be rolled at least twice at a
rolling reduction ratio of 20% or more, and the austenite grain diameter may be set to 100
[am or less.

^ 10
[0026]
(14) hi the aspect according to the above (11) or (12), in a case in which the pass
in which the rolling reduction ratio is 30% or more in the temperature range of T1+30°C
to T1+200°C is defined as a large reduction pass, a waiting time t from completion of a
5 final pass of the large reduction pass to initiation of cooling may employ a configuration that satisfies the following formula 2.
tl to {223} <110> orientation group and the sheet

14 thickness/ minimum bending radius of a hot-rolled steel sheet.
FIG. 2 is a view showing the relationship between the average value of an X-ray
random intensity ratio of a {332} <113> crystal orientation and the sheet thickness/
minimum bending radius of the hot-rolled steel sheet.
5 FIG. 3 is a view showing the relationship between rC which is an r value in a
direction perpendicular to a rolling direction and the sheet thickness/ minimum bending
radius of the hot-rolled steel sheet.
FIG. 4 is a view showing the relationship between r30 which is an r value in a
direction that forms an angle of 30° with respect to the rolling direction and the sheet
10 thickness/ minimum bending radius of the hot-rolled steel sheet.
FIG. 5 is a view showing the relationship between rL which is an r value in the
rolling direction and the sheet thickness/ minimum bending radius of the hot-rolled steel
sheet.
FIG. 6 is a view showing the relationship between r60 which is an r value in a
15 direction that forms an angle of 60° with respect to the rolling direction and the sheet
thickness/ minimum bending radius of the hot-rolled steel sheet.
FIG. 7 is a view showing the relationship between the average value of the X-ray
random intensity ratio of a {100} <011> to {223} <110> orientation group and the sheet
thickness/ minimum bending radius of a cold-rolled steel sheet.
20 FIG. 8 is a view showing the relationship between the average value of the X-ray
random intensity ratio of the {332} <113> crystal orientation and the sheet thickness/
minimum bending radius of the cold-rolled steel sheet.
FIG. 9 is a view showing the relationship between rC which is the r value in the
direction perpendicular to the rolling direction and the sheet thickness/ minimum bending
25 radius of the cold-rolled steel sheet.

^ 15
FIG. 10 is a view showing the relationship between r30 which is the r value in
the direction that forms an angle of 30° with respect to the rolling direction and the sheet
thickness/ minimum bending radius of the cold-rolled steel sheet.
FIG. 11 is a view showing the relationship between rL which is the r value in the
5 rolling direction and the sheet thickness/ minimum bending radius of the cold-rolled steel
sheet.
FIG. 12 is a view showing the relationship between r60 which is the r value in
the direction that forms an angle of 60° with respect to the rolling direction and the sheet
thickness/ minimum bending radius of the cold-rolled steel sheet.
10 FIG. 13 is a view showing the relationship between the average value of the
X-ray random intensity ratio of a {100} <011> to {223} <110> orientation group and the
sheet thickness/ minimum bending radius of a galvanized steel sheet.
FIG. 14 is a view showing the relationship between the average value of the
X-ray random intensity ratio of the {332} <113> crystal orientation and the sheet
15 thickness/ minimum bending radius of the galvanized steel sheet.
FIG. 15 is a view showing the relationship between rC which is the r value in the
direction perpendicular to the rolling direction and the sheet thickness/ minimum bending
radius of the galvanized steel sheet.
FIG. 16 is a view showing the relationship between r30 which is the r value in
20 the direction that forms an angle of 30° with respect to the rolling direction and the sheet
thickness/ minimum bending radius of the galvanized steel sheet.
FIG. 17 is a view showing the relationship between rL which is the r value in the
rolling direction and the sheet thickness/ minimum bending radius of the galvanized steel
sheet.
25 FIG 18 is a view showing the relationship between r60 which is the r value in

^ 16
the direction that forms an angle of 60° with respect to the rolling direction and the sheet
thickness/ minimum bending radius of the galvanized steel sheet.
FIG. 19 is a view showing the relationship between the austenite grain diameter after rough rolling and rC which is the r value in the direction perpendicular to the rolling 5 direction in the hot-rolled steel sheet.
FIG. 20 is a view showing the relationship between the austenite grain diameter after rough rolling and r30 which is the r value in the direction that forms an angle of 30° with respect to the rolling direction in the hot-rolled steel sheet.
FIG. 21 is a view showing the relationship between the number of times of 10 rolling at a rolling reduction ratio of 20% or more in rough rolling and the austenite grain diameter after the rough rolling.
FIG. 22 is a view showing the relationship between a total rolling reduction ratio in a temperature range of TH-30°C to T1+200°C and the average value of the X-ray random intensity ratio of a {100} <011> to {223} <110> orientation group in the 15 hot-rolled steel sheet.
FIG. 23 is a view showing the relationship between a total rolling reduction ratio
in a temperature range of T1°C to lower than TH-30°C and the average value of the
X-ray random intensity ratio of a {100} <011> to {223} <110> orientation group in the
hot-rolled steel sheet.
20 FIG. 24 is a view showing the relationship between a total rolling reduction ratio
in a temperature range of T1+30°C to T1+200°C and the X-ray random intensity ratio of the {332} <113> crystal orientation in the hot-rolled steel sheet.
FIG. 25 is a view showing the relationship between a total rolling reduction ratio in a temperature range of T1°C to lower than T1+30°C and the X-ray random intensity

17 ratio of the {332} <113> crystal orientation in the hot-rolled steel sheet.
FIG. 26 is a view showing the relationship among a maximum temperature
increase amount of the steel sheet between the respective passes during rolling in a
temperature range of T1+30°C to T1+200°C, a waiting time from completion of a final
5 pass of the large reduction pass to initiation of cooling in a case in which the pass in
which the rolling reduction ratio is 30% or more in the temperature range of T1+30°C to
T1+200°C is defined as a large reduction pass, and rL which is the r value in the rolling
direction in the hot-rolled steel sheet.
FIG. 27 is a view showing the relationship among a maximum temperature
10 increase amount of the steel sheet between the respective passes during rolling in a
temperature range of T1+30°C to TH-200°C, a waiting time from completion of a final
pass of the large reduction pass to initiation of cooling in a case in which the pass in
which the rolling reduction ratio is 30% or more in the temperature range of T1+30°C to
T1+200°C is defined as a large reduction pass, and r60 which is the r value in the
15 direction that forms an angle of 60° with respect to the rolling direction in the hot-rolled
steel sheet.
FIG. 28 is a view showing the relationship between the austenite grain diameter
after the rough rolling and rC which is the r value in the direction perpendicular to the
rolling direction in the cold-rolled steel sheet.
20 FIG. 29 is a view showing the relationship between the austenite grain diameter
after the rough rolling and r30 which is the r value in the direction that forms an angle of
30° with respect to the rolling direction in the cold-rolled steel sheet.
FIG. 30 is a view showing the relationship between the rolling reduction ratio of
T1+30°C to T1+200°C and the average value of the X-ray random intensity ratio of a

^ 18
{100} <011> to {223} <110> orientation group in the cold-rolled steel sheet.
FIG. 31 is a view showing the relationship between the total rolling reduction
ratio in a temperature range of TH-30°C to T1+200°C and the X-ray random intensity
ratio of the {332} <113> crystal orientation in the cold-rolled steel sheet.
5 FIG. 32 is a view showing the relationship between the austenite grain diameter
after the rough rolling and rC which is the r value in the perpendicular direction to the rolling direction in a galvanized steel sheet.
FIG. 33 is a view showing the relationship between the austenite grain diameter after the rough rolling and r30 which is the r value in the direction that forms an angle of 10 30° with respect to the rolling direction in the galvanized steel sheet.
FIG. 34 is a view showing the relationship between the total rolling reduction
ratio in a temperature range of T1+30°C to TH-200°C and the average value of the X-ray
random intensity ratio of the {100} <011> to {223} <110> orientation group in the
galvanized steel sheet.
15 FIG. 35 is a view showing the relationship between the total rolling reduction
ratio in a temperature range of T1°C to lower than TH-30°C and the average value of the X-ray random intensity ratio of the {100} <011 > to {223} < 110> orientation group in the galvanized steel sheet.
FIG. 36 is a view showing the relationship between the total rolling reduction 20 ratio in a temperature range of TH-30°C to T1+200°C and the X-ray random intensity ratio of the {332} <113> crystal orientation in the galvanized steel sheet.
FIG. 37 is a view showing the relationship between the total rolling reduction ratio in a temperature range of T1°C to lower than T1+30°C and the X-ray random intensity ratio of the {332} <113> crystal orientation in the galvanized steel sheet.

^ 19
FIG. 38 is a view showing the relationship among a maximum temperature
increase amount of the steel sheet between the respective passes during rolling in a
temperature range of T1+30°C to T1+200°C, the waiting time from completion of a final
pass of the large reduction pass to initiation of cooling in a case in which the pass in
5 which the rolling reduction ratio is 30% or more in the temperature range of T1+30°C to
T1+200°C is defined as a large reduction pass, and rL which is the r value in the rolling
direction in the galvanized steel sheet.
FIG. 39 is a view showing the relationship among a maximum temperature increase amount of the steel sheet between the respective passes during rolling in a 10 temperature range of T1+30°C to T1+200°C, a waiting time from completion of a final pass of the large reduction pass to initiation of cooling in a case in which the pass in which the rolling reduction ratio is 30% or more in the temperature range of T1+30°C to T1+200°C is defined as a large reduction pass, and r60 which is the r value in the direction that forms an angle of 60° with respect to the rolling direction in the galvanized 15 steel sheet.
FIG. 40 is a view showing the relationship between strength and hole expanding properties of the hot-rolled steel sheet of the embodiment and a comparative steel.
FIG. 41 is a view showing the relationship between strength and bending
properties of the hot-rolled steel sheet of the embodiment and the comparative steel.
20 FIG. 42 is a view showing the relationship between strength and the anisotropy
of formability of the hot-rolled steel sheet of the embodiment and the comparative steel.
FIG. 43 is a view showing the relationship between strength and hole expanding properties of the cold-rolled steel sheet of the embodiment and the comparative steel.
FIG. 44 is a view showing the relationship between strength and bending

^ 20
properties of the cold-rolled steel sheet of the embodiment and the comparative steel.
FIG. 45 is a view showing the relationship between sfrength and the anisbfropy
of formability of the cold-rolled steel sheet of the embodiment and the comparative steel.
5 Description of Embodiments [0040]
Hereinafter, an embodiment of the present invention will be described in detail. 1. Regarding a hot-rolled steel sheet
(1) An average value of the X-ray random intensity ratio of a {100} <011> to 10 {223} <110> orientation group in a sheet thickness cenfral portion that is in a sheet
thickness range of 5/8 to 3/8 from the surface of a steel sheet, an X-ray random intensity ratio of a {332} <113> crystal orientation:
The average value of the X-ray random intensity ratio of a {100} <011> to {223} <110> orientation group in a sheet thickness central portion that is in a sheet 15 thickness range of 5/8 to 3/8 from the surface of the steel sheet is a particularly important characteristic value of the embodiment. [0041]
As shown in FIG. 1, if the average value of the {100} <011> to {223} <110> orientation group is 6.0 or less when X-ray diffraction is carried out on a sheet surface in 20 the sheet thickness central portion that is in a sheet thickness range of 5/8 to 3/8 from the surface of the steel sheet so that the intensity ratios of the respective orientations with respect to a random specimen are obtained, d/Rm which is a sheet thickness/minimum bending radius necessary for working of underbody components or skeleton components is 1.5 or more. Furthermore, in a case in which hole expanding properties or small limit 25 bending characteristic is required, d/Rm is desirably 4.0 or less, and more desirably less

^ 21
than 3.0. When d/Rm is more than 6.0, the anisotropy of the mechanical characteristics
of the steel sheet becomes extremely strong, and, consequently, even when local
deformability in a certain direction improves, material qualities in directions different
from the above direction significantly degrade, and therefore it becomes impossible for
5 the sheet thickness/ minimum bending radius to be greater than or equal to 1.5. In a
case in which a cold-rolled steel sheet or hot-rolled strip which is a starting sheet for a
galvanized steel sheet is used, the X-ray random intensity ratio is preferably less than 4.0.
[0042]
Meanwhile, while it is difficult to realize in a current ordinary continuous hot
10 rolling process, when the X-ray random intensity ratio becomes less than 1.0, there is a
concern that local deformability may degrade.
[0043]
Furthermore, due to the same reason, if the X-ray random intensity ratio of the
{332} <113> crystal orientation in the sheet thickness central portion that is in a sheet
15 thickness range of 5/8 to 3/8 from the surface of the steel sheet is 5.0 or less as shown in
FIG. 2, the sheet thickness/minimum bending radius necessary for working of underbody
components is 1.5 or more. The sheet thickness/minimum bending radius is more
desirably 3.0 or less. When the sheet thickness/minimum bending radius is more than
5.0, the anisotropy of the mechanical characteristics of the steel sheet becomes extremely
20 strong, and, consequently, even when local deformability improves only in a certain
direction, material qualities in directions different from the above direction significantly
degrade, and therefore it becomes impossible for the sheet thickness/ minimum bending
radius to be greater than or equal to 1.5. Meanwhile, while it is difficult to realize in a
current ordinary continuous hot rolling process, when the X-ray random intensity ratio
25 becomes less than 1.0, there is a concern that the local deformability may degrade.

22 [0044]
The reason is not absolutely evident why the X-ray random intensity ratio of the
above crystal orientation is important for shape freezing properties during bending
working, but it is assumed that the X-ray random intensity ratio of the crystal orientation
5 has a relationship with the slip behavior of crystals during bending working.
(2) rC which is the r value in the direction perpendicular to the rolling direction:
rC is important in the embodiment. That is, as a result of thorough studies, the
inventors found that favorable hole expanding properties or bending properties cannot
always be obtained even when only the X-ray random intensity ratios of the above
10 variety of crystal orientations are appropriate. As shown in FIG 3, in addition to the
X-ray random intensity ratio, rC should be 0.70 or more.
[0045]
When the upper limit of rC is set to 1.10, more favorable local deformability can
be obtained.
15 (3) r30 which is the r value in the direction that forms an angle of 30° with
respect to the rolling direction:
r30 is important in the embodiment. That is, as a result of thorough studies, the
inventors found that favorable local deformability cannot be always obtained even when
only the X-ray random intensity ratios of the above variety of crystal orientations are
20 appropriate. As shown in FIG. 4, in addition to the X-ray random intensity ratio, r30
should be 1.10 or less.
[0046]
When the lower limit of r30 is set to 0.70, more favorable local deformability
can be obtained.
25 (4) rL which is the r value in the rolling direction and r60 which is the r value in

24 in the crystal grains. When the fraction is less than 50%, bending properties R in an L
direction which is the rolling direction or a C direction which is the direction
perpendicular to the rolling direction degrade.
The respective structures can be determined as follows.
5 Pearlite is specified through structure observation using an optical microscope.
Next, a crystal structure is determined using an electron back scattering diffraction
(EBSD), and a crystal having an fee structure is determined to be austenite. Ferrite,
bainite, and martensite having a bcc structure can be recognized through Kernel Average
Misorientation with which EBSP-OIM^^ is equipped, that is, through a KAM method.
10 In the KAM method, among measurement data, the orientation differences of 6 closest pixels of a regular hexagonal pixel, of 12 second closest pixels outside the closest pixels, or of 18 third closest pixels outside the second closest pixels are averaged, and a value is computed by carrying out calculation in which the averaged value is used as the value of the central pixel on the respective pixels. A map that represents an orientation change
15 in a grain can be prepared by carrying out the calculation within grain boundaries. The map represents a distribution of strain based on the local orientation change in the grain.
In the examples of the present invention, as a condition under which the orientation difference between adjacent pixels in EBSP-OIM^'^ is calculated, the orientation difference was set to 5° or less with respect to the third closest pixel, and a
20 pixel having an orientation difference with respect to the third closet pixel of more than 1° was defined as bainite or martensite which is a product of low-temperature transformation, and a pixel having an orientation difference with respect to the third closet pixel of 1° or less was defined as ferrite. This is because polygonal pro-eutectic ferrite transformed at a high temperature is generated through diffusion transformation,
25 and therefore the dislocation density is small, and strain in the grain is small so that the

^ 25
difference of crystal orientations in the grain is small, and the ferrite volume fraction
obtained from a variety of investigations that the inventors have carried out using optical
microscope observation and the area fraction obtained at an orientation difference with
respect to a third closest pixel of 1° measured through the KAM method, approximately
5 match.
[0050]
(6) Fraction of crystal grains having a grain diameter of more than 20 |im:
Furthermore, it was found that the bending properties are strongly influenced by
the equiaxed properties of crystal grains, and the effect is large. The reasons are not
10 evident, but it is considered that a mode of bending deformation is a mode in which strain
locally concentrates, and a state in which all crystal grains are uniformly and equivalently
strained is advantageous for bending properties. It is considered that, in a case in which
there are many crystal grains having a large grain diameter, even when crystal grains are
sufficiently made to be isotropic and equiaxed, crystal grains locally strain, and a large
15 variation appears in the bending properties due to the orientation of the locally strained
crystal grains such that degradation of the bending properties is caused. Therefore, in
order to suppress localization of strain and improve the bending properties by the efifect
of being made isotropic and equiaxed, the area fraction of crystal grains having a grain
diameter of more than 20 |.im is preferably smaller, and needs to be 0% to 10%. When
20 the area fraction is larger than 10%), the bending properties deteriorate. The crystal
grains mentioned herein refer to crystal grains of ferrite, pearlite, bainite, martensite, and
austenite.
[0051]
The present invention is generally applicable to hot-rolled steel sheets, and, as
25 long as the above limitations are satisfied, local deformability, such as the bending

^ 26
workability or hole expanding properties of a hot-rolled steel sheet, drastically improves
without the limitation on combination of structures.
[0052]
2. Regarding a cold-rolled steel sheet
5 (1) An average value of the X-ray random intensity ratio ofa{100}<011>to
{223} <110> orientation group in a sheet thickness central portion that is in a sheet
thickness range of 5/8 to 3/8 from the surface of a steel sheet, and an X-ray random
intensity ratio of a {332} <113> crystal orientation:
The average value of the X-ray random intensity ratio of a {100} <011> to
10 {223} <110> orientation group in a sheet thickness central portion that is in a sheet
thickness range of 5/8 to 3/8 from the surface of the steel sheet is particularly important
the embodiment.
[0053]
As shown in FIG. 7, if the average value of the {100} <011> to {223} <110>
15 orientation group is less than 4.0 when an X-ray diffraction is carried out on a sheet
surface in the sheet thickness central portion that is in a sheet thickness range of 5/8 to
3/8 from the surface of the steel sheet so that the intensity ratios of the respective
orientations with respect to a random specimen are obtained, a sheet thickness/minimum
bending radius necessary for working of skeleton components is 1.5 or more.
20 Furthermore, in a case in which hole expanding properties or a small limit bending
characteristic is required, the sheet thickness/minimum bending radius is desirably less
than 3.0. When the sheet thickness/minimum bending radius is 4.0 or more, the
anisotropy of the mechanical characteristics of the steel sheet becomes exfremely sfrong,
and, consequently, even when local deformability in a certain direction improves,
25 material qualities in directions different from the above direction significantly degrade,

[0057]
(2) rC which is the r value in the direction perpendicular to the rolling direction:
rC is important in the embodiment. That is, as a result of thorough studies, the
inventors found that favorable hole expanding properties or bending properties cannot be 5 always obtained even when only the X-ray random intensity ratios of the above variety of crystal orientations are appropriate. As shown in FIG 9, in addition to the X-ray random intensity ratio, rC should be 0.70 or more. [0058]
When the upper limit of rC is set to 1.10, more favorable local deformability can 10 be obtained.
[0059]
(3) r30 which is the r value in the direction that forms an angle of 30° with
respect to the rolling direction:
r30 is important in the embodiment. That is, as a result of thorough studies, the
15 inventors found that favorable local deformability cannot be always obtained even when
only the X-ray random intensity ratios of the above variety of crystal orientations are
appropriate. As shown in FIG. 10, in addition to the X^ray random intensity ratio, r30
should be 1.10 or less.
[0060]
20 When the lower limit of r30 is set to 0.70, more favorable local deformability
can be obtained. [0061]
(4) rL which is the r value in the rolling direction and r60 which is the r value in
the direction that forms an angle of 60° with respect to the rolling direction:
25 Furthermore, as a result of thorough studies, the inventors found that, in addition

29 to the X-ray random intensity ratios of the above variety of crystal orientations, rC, and
r30, when, furthermore, rL in the rolling direction is 0.70 or more, and r60 which is the r
value in the direction that forms an angle of 60° with respect to the rolling direction is
1.10 or less as shown in FIGS. 11 and 12, the sheet thickness/minimum bending radius is
5 equal to or greater than 2.0. -v
[0062]
When the rL and the r60 are set to 1.10 or less and 0.70 or more respectively,
more a favorable local deformability can be obtained.
[0063]
10 Meanwhile, generally, it is known that there is a correlation between a texture
and the r value, in the cold-rolled steel sheet according to the embodiment, the limitation
on the X-ray intensity ratio of the crystal orientation and the limitation on the r value are
not identical to each other, and favorable local deformability carmot be obtained as long
as both limitations are satisfied at the same time.
15 [0064]
(5) dL/dt ratios of bainite, martensite, pearlite, and austenite grains:
As a result of further investigating local deformability, the inventors found that,
when the texture and the r value are satisfied, and further the equiaxed properties of
crystal grains are excellent, the direction dependency of bending working almost
20 disappears. As an index that indicates the equiaxed properties, it is important that the
fraction of grains that have a dL/dt, which is a ratio of dL which is the length of crystal
grains in the structure in the cold-rolling direction to dt which is the length in the sheet
thickness direction, of 3.0 or less, and are excellent in terms of equiaxed properties is
50% to 100% in the crystal grains. When the fraction is less than 50%, bending
25 properties R in an L direction which is the rolling direction or in a C direction which is

30 the direction perpendicular to the rolling direction degrade.
The respective structures can be determined as follows.
Pearlite is specified through structure observation using an optical microscope.
Next, a crystal structure is determined using electron back scattering diffraction (EBSD),
5 and a crystal having an fee structure is determined to be austenite. Ferrite, bainite, and
martensite having a bcc structure can be recognized through Kernel Average
Misorientation with which EBSP-OIM^^ is equipped, that is, through a KAM method.
In the KAM method, among measurement data, the orientation differences of 6 closest
pixels of a regular hexagonal pixel, of 12 second closest pixels outside the closest pixels,
10 or of 18 third closest pixels outside the second closest pixels are averaged, and a value is computed by carrying out calculation in which the averaged value is used as the value of the central pixel on the respective pixels. A map that represents an orientation change in a grain can be prepared by carrying out the calculation within grain boundaries. The map represents a distribution of strain based on the local orientation change in the grain.
15 In the examples of the present invention, as a condition under which the
orientation difference between adjacent pixels in EBSP-OIM^'^, the orientation difference was set to 5° or less with respect to the third closest pixel, and a pixel having an orientation difference with respect to the third closet pixel of more than 1° was defined as bainite or martensite which is a product of low-temperature transformation,
20 and a pixel having an orientation difference with respect to the third closet pixel of 1 ° or less was defined as ferrite. This is because polygonal pro-eutectic ferrite transformed at a high temperature is generated through difiusion transformation, and therefore the dislocation density is small, and strain in the grain is small so that the difference of crystal orientations in the grain is small, and the ferrite volume fraction obtained from a
25 variety of investigations that the inventors have carried out using optical microscope

31 observation and the area fraction obtained at an orientation difference third closest pixel
of 1° measured through the KAM method approximately match.
[0065]
(6) Fraction of crystal grains having a grain diameter of more than 20 |im:
5 Furthermore, it was found that the bending properties are strongly influenced by
the equiaxed properties of crystal grains, and the effect is large. The reasons are not
evident, but it is considered that bending deformation is a mode in which strain locally
concentrates, and a state in which all crystal grains are uniformly and equivalently
strained is advantageous for bending properties. It is considered that, in a case in which
10 there are many crystal grains having a large grain diameter, even when crystal grains are sufficiently made to be isotropic and equiaxed, crystal grains locally strain, and a large variation appears in the bending properties due to the orientation of the locally strained crystal grains such that degradation in the bending properties is caused. Therefore, in order to suppress localization of strain and improve the bending properties through the
15 effect of making isotropic and equiaxed, the area fraction of crystal grains having a grain diameter of more than 20 ^im is preferably smaller, and needs to be 0% to 10%. When the area fraction is larger than 10%, the bending properties deteriorate. The crystal grains mentioned herein refer to crystal grains of ferrite, pearlite, bainite, martensite, and austenite.
20 [0066]
The present invention is generally applicable to cold-rolled steel sheets, and, as long as the above limitations are satisfied, local deformability, such as the bending workability or hole expanding properties of a cold-rolled steel sheet, drastically improves without limitation on combination of structures.
25 [0067]

»

33 rolling process, when the X-ray random intensity ratio becomes less than 1.0, there is a
concern that local deformability may degrade.
[0069]
Furthermore, due to the same reason, if the X-ray random intensity ratio of the
5 {332} <113> crystal orientation in the sheet thickness central portion that is in a sheet
thickness range of 5/8 to 3/8 from the surface of the steel sheet is 5.0 or less as shown in
FIG. 14, the sheet thickness/minimum bending radius necessary for working of
underbody components is 1.5 or more. The sheet thickness/minimum bending radius is
more desirably 3.0 or less. When the sheet thickness/minimum bending radius is more
10 than 5.0, the anisotropy of the mechanical characteristics of the steel sheet becomes
extremely strong, and, consequently, even when local deformability improves only in a
certain direction, material qualities in directions different from the above direction
significantly degrade, and therefore it becomes impossible to reliably satisfy the sheet
thickness/ minimum bending radius > 1.5. Meanwhile, while it is difficult to realize in
15 a current ordinary continuous hot rolling process, when the X-ray random intensity ratio
becomes less than 1.0, there is a concern that local deformability may degrade.
[0070]
The reason is not absolutely evident why the X-ray random intensity ratio of the
above crystal orientation is important for shape freezing properties during bending
20 working, but it is assumed that the X-ray random intensity ratio of the crystal orientation
has a relationship with the slip behavior of crystals during bending working.
[0071]
rC which is the r value in the direction perpendicular to the rolling direction:
rC is important in the embodiment. That is, as a result of thorough studies, the
25 inventors found that favorable hole expanding properties or bending properties cannot be

34 always obtained even when only the X-ray random intensity ratios of the above variety of
crystal orientations are appropriate. As shown in FIG. 15, in addition to the X-ray
random intensity ratio, rC should be 0.70 or more.
When the upper limit of rC is set to 1.10, more favorable local deformability can
5 be obtained.
[0072]
r30 which is the r value in the direction that forms an angle of 30° with respect
to the rolling direction:
r30 is important in the embodiment. That is, as a result of thorough studies, the
10 inventors found that favorable hole expanding properties or bending properties caimot be
always obtained even when only the X-ray random intensity ratios of the above variety of
crystal orientations are appropriate. As shown in FIG. 16, in addition to the X-ray
random intensity ratio, r30 should be 1.10 or less.
When the lower limit of r30 is set to 0.70, more favorable local deformability
15 can be obtained.
[0073]
rL which is the r value in the rolling direction, and r60 which is the r value in the
direction that forms an angle of 60° with respect to the rolling direction:
Furthermore, as a result of thorough studies, the inventors found that, in addition
20 to the X-ray random intensity ratios of the above variety of crystal orientations, rC, and
r30, when, furthermore, rL in the rolling direction is 0.70 or more, and r60 which is the r
value in the direction that forms an angle of 60° with respect to the rolling direction is
1.10 or less as shown in FIGS. 17 and 18, the sheet thickness/minimum bending radius
will be greater than or equal to 2.0.
25 When the rL value and the r60 value are set to 1.10 or less and 0.70 or more.

35 respectively, more favorable local deformability can be obtained.
[0074]
Meanwhile, generally, it is known that there is a correlation between a texture
and the r value, in the galvanized steel sheet according to the present invention, the
5 limitation on the X-ray intensity ratio of the crystal orientation and the limitation on the r
value are not identical to each other, and favorable local deformability cannot be obtained
as long as both limitations are not satisfied at the same time.
The present invention is generally applicable to galvanized steel sheets, and, as
long as the above limitations are satisfied, local deformability, such as the bending
10 workability or hole expanding properties of a galvanized steel sheet, drastically improves
without limitation on a combination of structures.
[0075]
Main orientations included in the {100} <011 > to {223} <110> orientation
group are {100} <011>, {116} <110>, {114} <110>, {113} <110>, {112} <110>, {335}
15 <110>,and{223}<110>.
[0076]
The X-ray random intensity ratios of the respective orientations can be measured
using a method, such as X-ray diffraction or electron back scattering diffraction (EBSD).
Specifically, the X-ray random intensity may be obtained ifrom a 3-dimensional texture
20 computed through a vector method based on the {110} pole figure or a 3-dimensional
texture computed through a series expansion method using a plurality of pole figures
(preferably three or more) among {110}, {100}, {211}, and {310} pole figures.
[0077]
For example, as the X-ray random intensity ratios of the respective crystal
25 orientations in the EBSD method, the intensities of (001) [1-10], (116) [1-10], (114)

36 [1-10], (113) [1-10], (112) [1-10], (335) [1-10], and (223) [1-10] in a (|)2 = 45° cross
section of a 3-dimensional texture may be used as they are. The 1 with bar above which
indicates negative 1 is expressed by -1.
In addition, the average value of the {100} <011> to {223} <110> orientation
5 group is the arithmetic average of the respective orientations. In a case in which the
intensities of all of the above orientations cannot be obtained, the intensities may be
replaced with the arithmetic average of the respective orientations of {100} <011>,
{116} <110>, {114} <110>, {112} <110>, and {223} <110>
[0078]
10 For measurement, a specimen provided for X-ray diffraction or EBSD is
subjected to mechanical polishing or the like so that the steel sheet is reduced from the
surface to be a predetermined sheet thickness, next, strain is removed through chemical
polishing or electrolytic polishing, and, at the same time, the specimen is adjusted
through the above method so that an appropriate surface in a sheet thickness range of 5/8
15 to 3/8 becomes a measurement surface. The specimen is desirably taken from a location
of a 1/4 or 3/4 width from the end portion in the sheet width direction.
[0079]
It is needless to say that, when the limitation on the X-ray intensity is satisfied
not only at the vicinity of 1/2 of the sheet thickness but also at as many thicknesses as
20 possible, local deformability becomes more favorable. However, since, generally, the
material characteristics of the entire steel sheet can be represented by measuring the sheet
thickness central portion that is in a sheet thickness range of 5/8 to 3/8 from the surface
of the steel sheet, the average value of the X-ray random intensity ratios of the {100}
<011> to {223} <110> orientation group in the sheet thickness central portion that is in a
25 sheet thickness range of 5/8 to 3/8 from the surface of the steel sheet and the X-ray

37 random intensity ratio of the {332} <113> crystal orientation are specified. The crystal
orientation that is represented by {hkl} indicates that the normal direction of the
sheet surface is parallel with {hkl}, and the rolling direction is parallel to .
[0080]
5 In addition, the respective r values are evaluated through tensile tests in which
JIS No. 5 tensile test specimens are used. In the case of a high-strength steel sheet,
tensile strain may be evaluated in a range of 5% to 15% using a range of uniform
elongation.
[0081]
10 Since a direction in which bending working is carried out varies by components
to be worked, the direction is not particularly limited; however, according to the present
invention, the same characteristics can be obtained in all bending directions.
[0082]
The dL/dt and grain diameter of pearlite can be obtained through a binarization
15 and a point counter method in structure observation using an optical microscope.
In addition, the grain diameters of ferrite, bainite, martensite, and austenite can
be obtained by measuring orientations, for example, at a magnification of 1500 times and
a measurement step (pitch) of 0.5 |am or less in an analysis of steel sheet orientations
through the EBSD method, specifying locations at which the orientation difference
20 between adjacent measurement points exceeds 15° as grain boundaries, and obtaining a
diameter of the equivalent circle. At this time, the lengths of a grain in the rolling
direction and the sheet thickness direction are obtained at the same time, thereby
obtaining dL/dt.
[0083]
25 Next, conditions for limiting the steel sheet components will be described. %

38 for contents is mass%.
Since the cold-rolled steel sheet and galvanized steel sheet of the present
invention use the hot-rolled steel sheet of the present invention as a raw sheet, the
components of a steel sheet will be as follows for all of the hot-rolled steel sheet, the
5 cold-rolled steel sheet, and the galvanized steel sheet.
[0084]
C is a basically included element, and the reason why the lower limit is set to
0.0001% is to use the lower limit value obtained from practical steel. When the upper
limit exceeds 0.40%, workability or weldability deteriorates, and therefore the upper
10 limit is set to 0.40%. Meanwhile, since excessive addition of C significantly
deteriorates spot weldability, the upper limit is more desirably set to 0.30% or lower.
[0085]
Si is an effective element for enhancing the mechanical strength of a steel sheet,
and, when the content exceeds 2.5%, workability deteriorates, or surface defects are
15 generated, and therefore the upper limit is set to 2.5%. On the other hand, since it is
difficult to include Si at less than 0.001% in practical steel, the lower limit is set to
0.001%.
[0086]
Mn is an effective element for enhancing the mechanical strength of a steel sheet,
20 and, when the content exceeds 4.0%,the workability deteriorates, and therefore the upper
limit is set to 4.0%. On the other hand, since it is diflFicult to include Mn at less than
0.001 % in practical steel, the lower limit is set to 0.001%. However, in order to avoid
an extreme increase in steel-manufacturing costs, the lower limit is desirably set to 0.01%
or more. Since Mn suppresses generation of ferrite, in a case in which it is intended to
25 include a ferrite phase in a structure so as to secure elongation, the lower limit is

39 desirably set to 3.0% or less. In addition, in a case in which, other than Mn, elements
which suppress generation of hot cracking caused by S, such as Ti, are not added, Mn is
desirably added at an amount so that Mn/S becomes equal to or larger than 20 in terms of
mass%.
5 [0087]
The upper limits of P and S are 0.15% or less and 0.03% or less respectively in
order to prevent deterioration of workability or cracking during hot rolling or cold rolling.
The respective lower limits are set to 0.001% for P and 0.0005% for S which are values
obtainable through current ordinary purification (including secondary purification).
10 Meanwhile, since extreme desulfiirization significantly increases the costs, the lower
limit of S is more desirably 0.001% or more.
[0088]
For deoxidizing, Al is added at 0.001% or more. However, in a case in which
sufficient deoxidizing is required, Al is more desirably added at 0.01% or more. In
15 addition, since Al significantly increases the y-^a transformation point from y to a, Al is
an effective element in a case in which hot rolling particularly at Ar3 point or lower is
oriented. However, when Al is excessive, weldability deteriorates, and therefore the
upper limit is set to 2.0%.
[0089]
20 N and O are impurities, and are both set to 0.01% or less so as to prevent
workability from degrading. The lower limits are set to 0.0005% which is a value
obtainable through current ordinary purification (including secondary purification) for
both elements. However, the contents of N and O are desirably set to 0.001% or more
in order to suppress an extreme increase in steel-manufacturing costs.
25 [0090]

40 Furthermore, in order to enhance the mechanical strength through precipitation
strengthening, or to control inclusions or refine precipitates for improving local
deformability, the steel sheet may contain one or two or more of any of Ti, Nb, B, Mg,
REM, Ca, Mo, Cr, V, W, Cu, Ni, Co, Sn, Zr, and As which have been thus far used. In
5 order to achieve precipitation strengthening, it is effective to generate fine carbonitrides,
and addition of Ti, Nb, V, or W is effective. In addition, Ti, Nb, V, and W also have an
effect of contributing to refinement of crystal grains as solid solution elements.
[0091]
In order to obtain the effect of precipitation strengthening through addition of Ti,
10 Nb, V, or W, it is necessary to add 0.001% or more of Ti, 0.001% or more of Nb, 0.001% or more of V, or 0.001% or more of W. In a case in which precipitation strengthening is particularly required, it is more desirable to add 0.01% or more of Ti, 0.005% or more of Nb, 0.01% or more of V, or 0.01% or more of W. Furthermore, Ti and Nb have an effect of improving material quality through mechanisms of fixation of carbon and
15 nitrogen, structure control, fine grain strengthening, and the like in addition to precipitate strengthening. In addition, V is effective for precipitation strengthening, causes less degradation of local deformability induced from strengthening due to addition than Mo or Cr, and an effective addition element in a case in which a high strength and better hole expanding properties or bending properties are required. However, even when the
20 above elements are excessively added, since the effect of an increase in strength is
saturated, and, furthermore, recrystallization after hot rolling is suppressed such that it is difficult to control crystal orientation, it is necessary to add Ti and Nb at 0.20% or less and V and W at 1.0% or less. However, in a case in which elongation is particularly required, it is more desirable to include V at 0.50% or less and W at 0.50% or less.
25 [0092]

41 In a case in which the hardenability of a structure is enhanced, and a second
phase is controlled so as to secure strength, it is effective to add one or two or more of B,
Mo, Cr, Cu, Ni, Co, Sn, Zr, and As. Furthermore, in addition to the above effect, B has
an effect of improving material quality through mechanisms of fixation of carbon and
5 nitrogen, structure control, fine grain strengthening, and the like. In addition, in
addition to the effect of enhancing the mechanical strength. Mo and Cr have an effect of
improving material quality.
In order to obtain the above effects, it is necessary to add B at 0.0001% or more.
Mo, Cr, Ni, and Cu at 0.001% or more, and Co, Sn, Zr, and As at 0.0001% or more.
10 However, in contrast, since excessive addition deteriorates workability, the upper limit of
B is set to 0.0050%, the upper limit of Mo is set to 1.0%, the upper limits of Cr, Ni, and
Cu are set to 2.0%, the upper limit of Co is set to 1.0%, the upper limits of Sn and Zr are
set to 0.2%, and the upper limit of As is set to 0.50%. In a case in which there is a
strong demand for workability, it is desirable to set the upper limit of B to 0.005%> and
15 the upper limit of Mo to 0.50%. In addition, it is more desirable to select B, Mo, Cr,
and As among the above addition elements fi^om the viewpoint of costs.
[0093]
Mg, REM, and Ca are important addition elements that detoxify inclusions and
further improve local deformability. The lower limits for obtaining the above effect are
20 0.0001% respectively; however, in a case in which it is necessary to control the shapes of
inclusions, Mg, REM, and Ca are desirably added at 0.0005% or more respectively.
Meanwhile, since excessive addition results in degradation of cleanness, the upper limits
of Mg, REM, and Ca are set to 0.010%, 0.1%, and 0.010% respectively.
[0094]
25 The effect of improving local deformability is not lost even when a surface

42 treatment is carried out on the hot-rolled steel sheet and cold-rolled steel sheet of the
present invention, and the effects of the present invention can be obtained even when any
of electroplating, hot dipping, deposition plating, organic membrane formation, film
laminating, an organic salts/ inorganic salts treatment, non-chromium treatment, and the
5 like is carried out.
In addition, the galvanized steel sheet of the present invention has a galvanized
layer by carrying out a galvanizing treatment on the surface of the cold-rolled steel sheet
of the present invention, and galvanizing can obtain the effects both in hot dip
galvanizing and electrogalvanizing. In addition, the galvanized steel sheet of the
10 present invention may be produced as a zinc alloy-plated steel sheet mainly used for
automobiles by carrying out an alloying treatment after galvanizing.
Additionally, the effects of the present invention are not lost even when a surface
treatment is further carried out on the high-strength galvanized steel sheet of the present
invention, and the effects of the present invention can be obtained even when any of
15 electroplating, hot dipping, deposition plating, organic membrane formation, film
laminating, an organic salts/ inorganic salts treatment, non-chromium treatment, and the
like is carried out.
[0095]
2. Regarding the manufacturing method
20 Next, the method of manufacturing a hot-rolled steel sheet according to the
embodiment will be described.
In order to realize excellent local deformability, it is important to form a texture
having a predetermined X-ray random intensity ratio, satisfy the conditions for the r
values in the respective directions, and control the grain shapes. Details of the
25 manufacturing conditions for satisfying the above will be described below.

43 [0096]
A manufacturing method preceding hot rolling is not particularly limited. That is, subsequent to ingoting using a blast furnace., an electric fumace, or the like, a variety of secondary purifications are carried out, then, the ingot may be cast through a method, 5 such as ordinary continuous casting, an ingot method, or thin slab casting. In the case of continuous casting, the ingot may be once cooled to a low temperature, reheated, and then hot-rolled, or a cast slab may also be hot-rolled as it is after casting without cooling the cast slab to a low temperature. Scraps may be used as a raw material.
[0097]
10 The hot-rolled steel sheet according to the embodiment is obtained in a case in
which the following conditions are satisfied.
[0098]
In order to satisfy the above predetermined values of rC of 0.70 or more and r30 of 1.10 or less, the austenite grain diameter after rough rolling, that is, before finishing 15 rolling is important. As shown in FIGS. 19 and 20, the austenite grain diameter before finishing rolling may be 200 |am or less.
[0099]
In order to obtain an austenite grain diameter before finishing rolling of 200 pm or less, in the rough rolling, it is necessary to carry out rolling in a temperature range of 20 1000°C to 1200°C and carry out rolling once or more at a rolling reduction ratio of at least 20% or more in the temperature range as shown in FIG. 21. However, in order to further enhance homogeneity and enhance elongation and local deformability, it is desirable to carry out rolling once or more at a rolling reduction ratio of at least 40% or more in a temperature range of 1000°C to 1200°C.

44 [0100]
The austenite grain diameter is more desirably set to 100 |im or less, and, in
order to achieve the austenite grain diameter of 100 )im or less, it is desirable to carry out
rolling twice or more at a rolling reduction ratio of 20% or more. Desirably, rolling is
5 carried out twice or more at a rolling reduction ratio of 40% or more. As the rolling
reduction ratio and the number of times of rolling increase, smaller grains can be
obtained, but there is a concern that the temperature may decrease or scales may be
excessively generated when the rolling exceeds 70% or the number of times of the rough
rolling exceeds 10 times. As such, a decrease in the austenite grain diameter before
10 finishing rolling is effective to improve local deformability through acceleration of
recrystallization of austenite during subsequent finishing rolling, particularly through
control of rL or r30.
[0101]
The reason why refinement of the austenite grain diameter has an influence on
15 local deformability is assumed to be that austenite grain boundaries after the rough
rolling, that is, austenite grain boundaries before the finishing rolling flinction as one of
recrystallization nuclei during the finishing rolling.
In order to confirm the austenite grain diameter after the rough rolling, it is
desirable to cool a sheet piece that is about to be fmishing-roUed as rapidly as possible.
20 The sheet piece is cooled at a cooling rate of 10°C/s or more, the structure on the cross
section of the sheet piece is etched, austenite grain boundaries are made to appear, and
the austenite grain diameter is measured using an optical microscope. At this time, the
austenite grain diameter is measured at a magnification of 50 times or more at 20 sites or
more through an image analysis or a point counter method.
25 [0102]

46 [0104]
In order to obtain more favorable local deformability, it is important to
accumulate strain through the large reduction or repeatedly recrystallize the structure
every rolling. In order to accumulate strain, the total rolling reduction ratio is 50% or
5 more, and desirably 70% or more, and, furthermore, an increase in the temperature of the
steel sheet between passes is desirably set to 18°C or lower. Meanwhile, the total
rolling reduction of more than 90% is not desirable from the viewpoint of temperature
securement or excessive rolling load. Furthermore, in order to enhance the
homogeneity of a hot-rolled sheet, and enhance the local deformability to the extreme,
10 among the rolling passes in a temperature range of T1+30°C to T1+200°C, at least one pass is carried out at a rolling reduction ratio of 30% or more, and desirably at 40% or more. Meanwhile, when the rolling reduction ratio exceeds 70% in a pass, there is a concern that the shape may be impaired. In a case in which there is a demand for more favorable workability, it is more desirable to set the rolling reduction ratio to 30% or
15 more in the final 2 passes. [0105]
Furthermore, in order to accelerate uniform recrystallization through releasing of accumulated strain, it is necessary to suppress as much as possible the working amount in a temperature range of T1°C to lower than T1+30°C after the large reduction at T1+30°C
20 to T1+200°C, and the total rolling rate at T1°C to lower than TH-30°C is set to less than 30%. A rolling reduction ratio of 10% or more is desirable from the viewpoint of the sheet shape, but a rolling reduction ratio of 0% is desirable in a case in which local deformability matters more. When the rolling reduction ratio at T1°C to lower than TH-30°C exceeds a predetermined range, recrystallized austenite grains are expanded.

47 and, when the retention time is short, recrystallization does not sufficiently proceed, and
the local deformability deteriorates. That is, in the manufacturing conditions according
to the embodiment, it is important to uniformly and fiinely recrystallize austenite during
finishing rolling so as to control the texture of a hot-rolled product in order to improve
5 local deformability, such as hole expanding properties or bending properties.
[0106]
When rolling is carried out at a lower temperature than the temperature range
specified above or at a larger rolling reduction ratio than the specified rolling reduction
ratio, the texture of austenite develops, and the X-ray random intensity ratios in the
10 respective crystal orientations, such as the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group at least in a thickness central portion that is in a sheet thickness range of 5/8 to 3/8 from a steel sheet surface of 6.0 or less and the X-ray random intensity ratio of the {332} <113> crystal orientation of 5.0 or less, cannot be obtained in the finally obtained hot-rolled steel sheet.
1-5 [0107]
Meanwhile, when rolling is carried out at a higher temperature than the specified temperature range or at a smaller rolling reduction ratio than the specified rolling reduction ratio, grain coarsening or duplex grains results, and the area fraction of crystal grains having a grain diameter of larger than 20 jim increases. Whether or not the
20 above-specified rolling is carried out can be determined firom rolling reduction ratio, rolling load, sheet thickness measurement, or the like through actual performance or calculation. In addition, since the temperature can be also measured if a thermometer is present between stands, and calculation simulation in which working heat generation and the like are considered from line speed, rolling reduction ratio, and the like is available,
25 whether or not the above-specified rolling is carried out can be determined using either or

45 In addition, in order to achieve an average value of the X-ray random intensity
ratio of the {100} <011> to {223} <110> orientation group in a thickness central portion
that is in a sheet thickness range of 5/8 to 3/8 from the steel sheet surface and an X-ray
random intensity ratio of the {332} <113> crystal orientation in the above value ranges,
5 based on the Tl temperature described in the formula 1 which is determined by the steel
sheet components in the finishing rolling after the rough rolling, working is carried out at
a large rolling reduction ratio in a temperature range of T1+30°C to T1+200°C, desirably
in a temperature range of T1+50°C to T1+100°C, and working is carried out at a small
rolling reduction ratio in a temperature range of T1°C to lower than T1+30°C.
10 According to the above, the local deformability and shape of a final hot-rolled product can be secured. FIGS. 22 to 25 show the relationships between the rolling reduction ratios in the respective temperature ranges and the X-ray random intensity ratios of the respective orientations. [0103]
15 That is, as shown in FIGS. 22 and 24, large reduction in a temperature range of
T1+30°C to TH-200°C and subsequent light rolling at T1°C to lower than T1+30°C as shown in FIGS. 23 and 25 control the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group in a thickness central portion that is in a sheet thickness range of 5/8 to 3/8 fi-om the steel sheet surface and the X-ray
20 random intensity ratio of the {332} <113> crystal orientation so as to drastically improve the local deformability of the final hot-rolled product.
The Tl temperature is experimentally obtained, and the inventors found from experiments that recrystallization in the austenite range of the respective steels is accelerated with the Tl temperature as a basis.

48 both of temperature and calculation simulation.
[0108]
The hot rolling carried out in the above manner ends at a temperature of Ar3 or
higher. When the end temperature of the hot rolling is lower than Ar3, since two-phase
5 region rolling in an austenite area and a ferrite area is included, accumulation into the
{100} <011> to {223} <110> orientation group becomes strong, and, consequently, local
deformability significantly degrades.
[0109]
As long as rL and r60 are 0.70 or more and 1.10 or less respectively, furthermore,
10 favorable sheet thickness/minimum bending radius > 2.0 is satisfied. In order to
achieve the sheet thickness/minimum bending radius > 2.0, in a case in which a pass in
which the rolling reduction ratio is 30% or more in the temperature range of T1+30°C to
TH-200°C is defined as a large reduction pass, a waiting time t (seconds) from
completion of the final pass of the large reduction pass to initiation of cooling satisfies
15 the formula 2, and the temperature increase of the steel sheet between the respective
passes is desirably 18°C or lower.
FIGS. 26 and 27 show the relationship among the temperature increase amount
of the steel sheet between the passes during rolling at T1+30°C to TH-200°C; the waiting
time t; and rL and r60. In a case in which the temperature increase of the steel sheet
20 between the respective passes at T1+30°C to T1+200°C is 18°C or lower, and t satisfies
the formula 2, it is possible to obtain uniform recrystallized austenite having an rL of
0.70 or more and an r60 of 1.10 or less.
When the waiting time t exceeds tlx2.5, grain coarsening proceeds, and
elongation significantly degrades. In addition, when the waiting time t is shorter than tl.

49 anisotropy increases, and the equiaxed grain proportion decreases.
[0110]
In a case in which the temperature increase of the steel sheet at T1+30°C to
T1+200°C is too low to obtain a predetermined rolling reduction ratio in a range of
5 T1+30°C to TH-200°C, recrystallization is suppressed. In addition, in a case in which
the waiting time t (seconds) does not satisfy the formula 2, grains are coarsened by the
time being too long, recrystallization does not proceed by the time being too short, and
sufficient local deformability cannot be obtained.
[0111]
10 A cooling pattern after rolling is not particularly limited. The effects of the
present invention can be obtained by employing a cooling pattern for controlling the
structure according to the respective objects.
[0112]
During hot rolling, a sheet bar may be joined after rough rolling, and finishing
15 rolling may be continuously carried out. At this time, a rough bar may be once rolled
into a coil shape, stored in a cover having a heat-retention function as necessary, and
again rolled back, whereby the rough bar is joined.
In addition, rolling may be carried out after hot rolling.
[0113]
20 Skin pass rolling may be carried out on the hot-rolled steel sheet according to
necessity. Skin pass rolling has an effect of preventing the stretcher strain which
occurrs during working forming or flatness correction.
[0114]
The structure of the hot-rolled steel sheet obtained in the embodiment mainly
25 includes ferrite, but may include pearlite, bainite, martensite, austenite, and compounds

50 such as carbonitrides, as metallic structures other than ferrite. Since the crystal structure
of martensite or bainite is the same as or similar to the crystal structure of ferrite, the
above structures may be a main component instead of ferrite.
[0115]
5 Further, the steel sheet according to the present invention can be applied not
only to bending working but also to combined forming composed mainly of bending,
overhanging, drawing, and bending working.
[0116]
Next, the method of manufacturing a cold-rolled steel sheet according to the
10 embodiment will be described. In order to realize excellent local deformability, in a
steel sheet that has undergone cold rolling, it is important to form a texture having a
predetermined X-ray random intensity ratio, satisfy the conditions of the r values in the
respective directions, and control grain shapes. Details of the manufacturing conditions
for satisfying the above will be described below.
15 [0117]
A manufacturing method preceding hot rolling is not particularly limited. That
is, subsequent to ingoting using a blast fumace, an electric furnace, or the like, a variety
of secondary purifications are carried out, then, the ingot may be cast through a method,
such as ordinary continuous casting, an ingot method, or thin slab casting. In the case
20 of continuous casting, the ingot may be once cooled to a low temperature, reheated, and
then hot-rolled, or a cast slab may also be hot-rolled as it is after casting without cooling
the cast slab to a low temperature. Scraps may be used as a raw material.
[0118]
The cold-rolled steel sheet having excellent local deformability according to the
25 embodiment is obtained in a case in which the following conditions are satisfied.

51 [0119]
In order for rC and r30 to satisfy the above predetermined values, the austenite grain diameter after rough rolling, that is, before finishing rolling is important. As shown in FIGS. 28 and 29, the austenite grain diameter before finishing rolling is 5 desirably small, and the above values are satisfied when the austenite grain diameter is 200 nm or less.
[0120]
In order to obtain an austenite grain diameter before finishing rolling of 200 \im or less, as shown in FIG 21, it is necessary to carry out the rough rolling in a temperature 10 range of 1000°C to 1200°C and carry out rolling once or more at a rolling reduction ratio of at least 20% or more. As the rolling reduction ratio and the number of times of rolling increase, smaller grains can be obtained.
[0121]
The austenite grain diameter is more desirably set to 100 )im or less, and, in 15 order to achieve the austenite grain diameter of 100 |xm or less, it is desirable to carry out rolling twice or more at a rolling reduction ratio of 20% or more. Desirably, rolling is carried out twice or more at a rolling reduction ratio of 40% or more. As the rolling reduction ratio and the number of times of rolling increase, smaller grains can be obtained, but there is a concern that the temperature may decrease or the scales may be 20 excessively generated when the rolling exceeds 70% or the number of times of the rough rolling exceeds 10 times. As such, a decrease in the austenite grain diameter before finishing rolling is effective to improve local deformability through acceleration of recrystallization of austenite during subsequent finishing rolling, particularly through control ofrLor r30.

52 [0122]
The reason why refinement of the austenite grain diameter has an influence on
local deformability is assumed to be that austenite grain boundaries after the rough
rolling, that is, austenite grain boundaries before the finishing rolling, fiinction as one of
5 recrystallization nuclei during the finishing rolling. In order to confirm the austenite
grain diameter after the rough rolling, it is desirable to cool a sheet piece that is about to
be finishing-rolled as rapidly as possible. The sheet piece is cooled at a cooling rate of
IO°C/s or more, the structure on the cross section of the sheet piece is etched, austenite
grain boundaries are made to appear, and the austenite grain diameter is measured using
10 an optical microscope. At this time, the austenite grain diameter is measured at a magnification of 50 times or more at 20 sites or more through an image analysis or a point counter method. [0123] In addition, in order to achieve an average value of the X-ray random intensity
15 ratio of the {100} <011 > to {223} < 110> orientation group in a thickness central portion that is in a sheet thickness range of 5/8 to 3/8 fi-om the steel sheet surface, and an X-ray random intensity ratio of the {332} <113> crystal orientation in the above value ranges, based on the Tl temperature determined by the steel sheet components in the finishing rolling after the rough rolling, working is carried out at a large rolling reduction ratio in a
20 temperature range of T1+30°C to TH-200°C, desirably in a temperature range of
T1+50°C to T1+100°C, and working is carried out at a small rolling reduction ratio in a temperature range of T1°C to lower than T1+30°C. According to the above, the local deformability and shape of a final hot-rolled product can be secured. FIGS. 30 to 31 show the relationships between the rolling reduction ratios in the temperature range of

53 T1+30°C to T1+200°C and the X-ray random intensity ratios of the respective
orientations.
[0124]
That is, large reduction in a temperature range of T1+30°C to T1+200°C and 5 subsequent light rolling at T1°C to lower than T1+30°C as shown in FIGS. 30 and 31 control the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group in a thickness central portion that is in a sheet thickness range of 5/8 to 3/8 from the steel sheet surface, and the X-ray random intensity ratio of the {332} <113> crystal orientation so as to drastically improve the local deformability 10 of the final hot-rolled product as shown in Tables 7 and 8 below. The Tl temperature is experimentally obtained, and the inventors found from experiments that recrystallization in the austenite range of the respective steels is accelerated with the Tl temperature as a basis.
[0125]
15 Furthermore, in order to obtain more favorable local deformability, it is
important to accumulate strain through the large reduction, and the total rolling reduction ratio is 50% or more, more desirably 60% or more, and still more desirably 70% or more. On the other hand, a total rolling reduction ratio exceeding 90% is not desirable from the viewpoint of temperature securement or excessive rolling loads. Furthermore, in order 20 to enhance the homogeneity of a hot-rolled sheet, and enhance the local deformability to the extreme, among the rolling passes in a temperature range of T1+30°C to T1+200°C, in at least one pass, rolling is carried out at a rolling reduction ratio of 30% or more, and desirably at 40% or more. Meanwhile, when the rolling reduction ratio exceeds 70% in a pass, there is a concern that the shape may be impaired, hi a case in which there is a

54 demand for more favorable workability, it is more desirable to set the rolling reduction
ratio to 30% or more in the final 2 passes.
[0126]
Furthermore, in order to accelerate uniform recrystallization through releasing of 5 accumulated strain, it is necessary to suppress as much as possible the working amount in a temperature range of T1°C to lower than T1+30°C after the large reduction at T1+30°C to T1+200°C, and the total rolling rate at TrC to lower than T1+30°C is set to less than 30%. A rolling reduction ratio of 10% or more is desirable fi^om the viewpoint of the sheet shape, but a rolling reduction ratio of 0% is desirable in a case in which local 10 deformability matters more. When the rolling reduction ratio at T1 °C to lower than T1+30°C exceeds a predetermined range, recrystallized austenite grains are expanded, and, when the retention time is short, recrystallization does not sufficiently proceed, and the local deformability deteriorates. That is, in the manufacturing conditions according to the embodiment, it is important to uniformly and finely recrystallize austenite during 15 finishing rolling so as to control the texture of a hot-rolled product in order to improve local deformability, such as hole expanding properties or bending properties.
[0127]
When rolling is carried out at a lower temperature than the temperature range specified above or at a larger rolling reduction ratio than the specified rolling reduction 20 ratio, the texture of austenite develops, and the X-ray random intensity ratios in the respective crystal orientations, such as the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group at least in a thickness central portion that is in a sheet thickness range of 5/8 to 3/8 from a steel sheet surface of less than 4.0 and the X-ray random intensity ratio of the {332} <113> crystal orientation of

55 5.0 or less, cannot be obtained in the finally obtained cold-rolled steel sheet.
[0128]
Meanwhile, when rolling is carried out at a higher temperature than the specified
temperature range or at a smaller rolling reduction ratio than the specified rolling
5 reduction ratio, grain coarsening or duplex grains results, and the area fraction of crystal
grains having a grain diameter of larger than 20 |j,m increases. Whether or not the
above-specified rolling is carried out can be determined from the rolling reduction ratio,
rolling load, sheet thickness measurement, or the like through actual performance or
calculation. In addition, since the temperature can also be measured if a thermometer is
10 present between stands, and calculation simulation in which working heat generation and
the like are considered from line speed, rolling reduction ratio, and the like is available,
whether or not the above-specified rolling is carried out can be determined using either or
both of temperature and calculation simulation.
[0129]
15 The hot rolling carried out in the above manner ends at a temperature of Ar3 or
higher. When the end temperature of the hot rolling is lower than Ar3, since two-phase
region rolling in an austenite area and a ferrite area is included, accumulation into the
{100} <011> to {223} <110> orientation group becomes sfrong, and, consequently, local
deformability significantly degrades.
20 [0130]
As long as rL and r60 are 0.70 or more and 1.10 or less respectively, furthermore,
favorable sheet thickness/minimum bending radius is greater than or equal to 2.0 is
satisfied. In order to achieve the sheet thickness/minimum bending radius of greater
than or equal to 2.0, the temperature increase of the steel sheet between the respective
25 passes during rolling at T1+30°C to T1+200°C is desirably suppressed to 18°C or lower.

56 and it is desirable to employ cooling between stands, or the like.
[0131]
Furthermore, cooling after rolling at the final rolling stand of rolling mill in a
temperature range of TH-30°C to T1+200°C has a strong influence on the grain diameter
5 of austenite, which has a strong influence on the equiaxed grain proportion and coarse
grain proportion of a cold-rolled and annealed structure. Therefore, in a case in which a
pass in which a rolling reduction ratio is 30% or more in a temperature range of T1+30°C
to T1+200°C is defined as a large reduction pass, it is necessary for the waiting time t
from completion of the final pass of the large reduction pass to initiation of cooling to
10 satisfy the formula 4. When the time being too long, grains are coarsened and
elongation significantly degrades. When the time being too short, recrystallization does
not proceed and sufficient local deformability cannot be obtained. Therefore, it is not
possible for the sheet thickness/minimum bending radius is greater than or equal to 2.0.
[0132]
15 In addition, a cooling pattern after hot rolling is not particularly specified, and
the effects of the present invention can be obtained by employing a cooling pattern for
controlling the structure according to the respective objects.
[0133]
During hot rolling, a sheet bar may be joined after rough rolling, and finishing
20 rolling may be continuously carried out. At this time, a rough bar may be once rolled
into a coil shape, stored in a cover having a heat-retention fimction as necessary, and
again rolled back, whereby the rough bar is joined.
[0134]
On the steel sheet for which the hot rolling has been completed, cold rolling is
25 carried out at a rolling reduction ratio of 20% to 90%. At a rolling reduction ratio of

57 less than 20%, it becomes difficult to cause recrystallization in a subsequent armealing
process, and aimealed crystal grains are coarsened and the equiaxed grain proportion
decreases. At a rolling reduction ratio of more than 90%, since a texture develops
during annealing, anisotropy becomes strong. Therefore, the rolling reduction ratio is
5 set to 20% to 90% of cold rolling.
[0135]
The cold-rolled steel sheet is, then, held in a temperature range of 720°C to
900°C for 1 second to 300 seconds. When the temperature is less than 720°C or the
holding time is less than 1 secnd, reverse transformation does not proceed sufficiently at
10 a low temperature or for a short time, and a second phase cannot be obtained in a
subsequent cooling process, and therefore a sufficient strength cannot be obtained. On
the other hand, when the temperature exceeds 900°C or the cold-rolled steel sheet is held
for 300 seconds or more, crystal grains coarsen, and therefore the area fraction of crystal
grains having a grain diameter of 20 |im or less increases. After that, the temperature is
15 decreased to 500°C or less at a cooling rate of 10°C/s to 200°C/s from 650°C to 500°C.
When the cooling rate is less than 10°C/s or the cooling ends at higher than 500°C,
pearlite is generated, and therefore local deformability degrades. On the other hand,
even when the cooling rate is set to more than 200°C/s, the effect of suppressing pearlite
is saturated, and, conversely, the controllability of the cooling end temperature
20 significantly deteriorates, and therefore the cooling rate is set to 200°C/s or less.
[0136]
The structure of the cold-rolled steel sheet obtained in the embodiment includes
ferrite, but may include pearlite, bainite, martensite, austenite, and compounds such as
carbonitrides, as metallic structures other than ferrite. However, since pearlite

58 deteriorates local deformability, the content of pearlite is desirably 5% or less. Since
the crystal structure of martensite or bainite is the same as or similar to the crystal
structure of ferrite, the structure may mainly include any of ferrite, bainite, and
martensite.
5 [0137]
Further, the cold-rolled steel sheet according to the present invention can be
applied not only to bending working but also to combined forming composed mainly of
bending, overhanging, drawing, and bending working.
[0138]
10 Next, the method of manufacturing a galvanized steel sheet according to the
embodiment will be described.
In order to realize excellent local deformability, in a steel sheet that has
undergone a galvanizing treatment, it is important to form a texture having a
predetermined X-ray random intensity ratio, satisfy the conditions of the r values in the
15 respective directions. Details of the manufacturing conditions for satisfying the above
will be described below.
A manufacturing method preceding hot rolling is not particularly limited. That
is, subsequent to ingoting using a blast furnace, an electric furnace, or the like, a variety
of secondary purifications are carried out, then, the ingot may be cast through a method,
20 such as ordinary continuous casting, an ingot method, or thin slab casting. In the case
of continuous casting, the ingot may be once cooled to a low temperature, reheated, and
then hot-rolled, or a cast slab may also be hot-rolled as it is after casting without cooling
the cast slab to a low temperature. Scraps may be used as a raw material.
[0139]
25 The galvanized steel sheet having excellent local deformability according to the

59 embodiment is obtained in a case in which the following conditions are satisfied.
[0140]
Firstly, in order for rC and r30 to satisfy the above predetermined values, the austenite grain diameter after rough rolling, that is, before finishing rolling is important. 5 As shown in FIGS. 32 and 33, the austenite grain diameter before finishing rolling is desirably small, and the above values are satisfied when the austenite grain diameter is 200 }xm or less.
[0141]
In order to obtain an austenite grain diameter before finishing rolling of 200 \im 10 or less, as shown in FIG 21, it is necessary to carry out the rough rolling in a temperature range of 1000°C to 1200°C and carry out rolling once or more at a rolling reduction ratio of at least 20% or more. However, in order to further enhance homogeneity and enhance elongation and local deformability, it is desirable to carry out rolling once or more at a rolling reduction ratio of at least 40% in terms of a rough rolling reduction ratio 15 in a temperature range of 1000°C to 1200°C.
[0142]
In order to obtain austenite grains of 100 |j,m or less which are more preferable, one or more times of rolling, a total of two or more times of rolling at a rolling reduction ratio of 20% or more is flirther carried out. Desirably, rolling is carried out twice or 20 more at 40% or more. As the rolling reduction ratio and the number of times of rolling increase, smaller grains can be obtained, but there is a concern that the temperature may decrease or scales may be excessively generated when the rolling exceeds 70% or the number of times of the rough rolling exceeds 10 times. As such, a decrease in the austenite grain diameter before finishing rolling is effective to improve local

60 deformability through acceleration of recrystallization of austenite during subsequent
finishing rolling, particularly through control of rL or r30.
[0143]
The reason why refinement of the austenite grain diameter has an influence on 5 local deformability is assumed to be that austenite grain boundaries after the rough
rolling, that is, austenite grain boundaries before the finishing rolling function as one of recrystallization nuclei during the finishing rolling.
In order to confirm the austenite grain diameter after the rough rolling, it is desirable to cool a sheet piece that is about to be finishing-roiled as rapidly as possible. 10 The sheet piece is cooled at a cooling rate of 10°C/s or more, the structure on the cross section of the sheet piece is etched, austenite grain boundaries are made to appear, and the austenite grain diameter is measured using an optical microscope. At this time, the austenite grain di^ameter is measured at a magnification of 50 times or more at 20 sites or more through an image analysis or a point counter method. Furthermore, the austenite 15 grain diameter is desirably 100 \im or less in order to enhance local deformability.
[0144]
In addition, in order to achieve an average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group in athickness central portion that is in a sheet thickness range of 5/8 to 3/8 from the steel sheet surface and an X-ray 20 random intensity ratio of the {332} <113> crystal orientation in the above value ranges, based on the Tl temperature determined by the steel sheet components specified in the formula 1 in the finishing rolling after the rough rolling, working is carried out at a large rolling reduction ratio in a temperature range of T1+30°C to T1+200°C, desirably in a temperature range of T1+50°C to T1+100°C, and working is carried out at a small rolling

61 reduction ratio in a temperature range of T1°C to lower than T1+30°C. According to
the above, the local deformability and shape of a final hot-rolled product can be secured.
FIGS. 34 to 37 show the relationships between the rolling reduction ratios in the respective temperature ranges and the X-ray random intensity ratios of the respective 5 orientations.
[0145]
That is, large reduction at a total rolling reduction ratio of 50% or more in a temperature range of T1+30°C to T1+200°C as shown in FIGS. 34 and 36 and subsequent light rolling at a total rolling reduction ratio of less than 30% or more at T1°C 10 to lower than T1+30°C as shown in FIGS. 35 and 37 control the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group in a thickness central portion that is in a sheet thickness range of 5/8 to 3/8 fi'om the steel sheet surface, and the X-ray random intensity ratio of the {332} <113> crystal orientation so as to drastically improve the local deformability of the final hot-rolled product. The 15 Tl temperature is experimentally obtained, and the inventors and the like found fi'om experiments that recrystallization in the austenite range of the respective steels is accelerated with the Tl temperature as a basis.
[0146]
Furthermore, in order to obtain more favorable local deformability, it is 20 important to accumulate strain through the large reduction or repeatedly recrystallize the structure every rolling. For strain accumulation, the total rolling reduction ratio needs to be 50% or more, more desirably 60% or more, and still more desirably 70% or more, and the temperature increase of the steel sheet between passes is desirably 18°C or lower. On the other hand, achieving a rolling reduction ratio of more than 90% is not desirable

62 from the viewpoint of temperature securement or excessive rolling load. Furthermore,
in order to enhance the homogeneity of a hot-rolled sheet, and enhance the local
deformability to the extreme, among the rolling passes in a temperature range of
TH-30°C to T1+200°C, in at least one pass, rolling is carried out at a rolling reduction
5 ratio of 30% or more, and desirably at 40% or more. Meanwhile, when the rolling
reduction ratio exceeds 70% in a pass, there is a concern that the shape may be impaired.
In a case in which there is a demand for more favorable workability, it is more desirable
to set the rolling reduction ratio to 30% or more in the final 2 passes.
[0147]
10 Furthermore, in order to accelerate uniform recrystallization through releasing of
accumulated strain, it is necessary to suppress as much as possible the working amount in a temperature range of T1°C to lower than TH-30°C after the large reduction at T1+30°C to TH-200°C, and the total rolling rate at T1°C to lower than TH-30°C is set to less than 30%. A rolling reduction ratio of 10% or more is desirable from the viewpoint of the
15 sheet shape, but a rolling reduction ratio of 0% is desirable in a case in which local deformability is focused. When the rolling reduction ratio at T1°C to lower than T1+30°C exceeds a predetermined range, recrystallized austenite grains are expanded, and, when the retention time is short, recrystallization does not sufficiently proceed, and the local deformability deteriorates. That is, in the manufacturing conditions according
20 to the embodiment, it is important to uniformly and finely recrystallize austenite during finishing rolling so as to control the texture of a hot-rolled product in order to improve local deformability, such as hole expanding properties or bending properties. [0148] When rolling is carried out at a lower temperature than the temperature range

63 specified above or at a larger rolling reduction ratio than the specified rolling reduction
ratio, the texture of austenite develops, and the X-ray random intensity ratios in the
respective crystal orientations, such as the average value of the X-ray random intensity
ratio of the {100} <011> to {223} <110> orientation group at least in a thickness central
5 portion that is in a sheet thickness range of 5/8 to 3/8 from a steel sheet surface of less
than 4.0, and the X-ray random intensity ratio of the {332} <113> crystal orientation of
5.0 or less, cannot be obtained in the finally obtained galvanized steel sheet. Meanwhile,
when rolling is carried out at a higher temperature than the specified temperature range
or at a smaller rolling reduction ratio than the specified rolling reduction ratio, grain
10 coarsening or duplex grains results, and, consequently, local deformability significantly degrades. Whether or not the above-specified rolling is carried out can be determined from rolling reduction ratio, rolling load, sheet thickness measurement, or the like through actual performance or calculation. In addition, since the temperature can be also measured if a thermometer is present between stands, and calculation simulation in
15 which working heat generation and the like are considered from line speed, rolling reduction ratio, and the like is available, whether or not the above-specified rolling is carried out can be determined using either or both of temperature and calculation simulation.
[0149]
20 The hot rolling carried out in the above manner ends at a temperature of Ar3 or
higher. When the end temperature of the hot rolling is lower than Ar3, since two-phase region rolling in an austenite area and a ferrite area is included, accumulation into the {100} <011>to {223} <110> orientation group becomes strong, and, consequently, local deformability significantly degrades.
25 [0150]

64 Furthermore, as long as rL and r60 are 0.70 or more and 1.10 or less respectively,
furthermore, the sheet thickness/minimum bending radius is greater than or equal to 2.0.
In order to achieve the sheet thickness/minimum bending radius of greater than or equal
to 2.0, in a case in which a pass in which a rolling reduction ratio is 30% or more in a
5 temperature range of T1+30°C to T1+200°C is defined as a large reduction pass, it is
important for the waiting time t (seconds) from completion of the final pass of the large
reduction pass to initiation of cooling to satisfy the formula 6.
[0151]
FIGS. 38 and 39 show the relationship among the temperature increase of the 10 steel sheet during rolling at T1+30°C to T1+200°C, the waiting time t, rL, and r60.
The waiting time t satisfying the formula 6 and, furthermore, suppression of the temperature increase of the steel sheet at T1+30°C to T1+200°C to 18°C or lower in the respective passes are effective to obtain uniformly recrystallized austenite.
[0152]
15 Further, in a case in which the temperature increase at T1+30°C to T1+200°C is
too low such that a predetermined rolling reduction ratio cannot be obtained in a range of T1+30°C to T1+200°C, recrystallization is suppressed, and, in a case in which the waiting time t does not satisfy the formula 6, by the time being too long, grains are coarsened and, by the time being too short, recrystallization does not proceed and 20 sufficient local deformability cannot be obtained.
[0153]
A cooling pattern after hot rolling is not particularly specified, and the effects of the present invention can be obtained by employing a cooling pattem for controlling the structure according to the respective objects. However, when the winding temperature

65 exceeds 680°C, since there is a concern that surface oxidation may proceed or bending
properties after cold rolling or annealing may be adversely influenced, the winding
temperature is set to a temperature from room temperature to 680°C.
[0154]
5 During hot rolling, a sheet bar may be joined after rough rolling, and finishing
rolling may be continuously carried out. At this time, a rough bar may be once rolled
into a coil shape, stored in a cover having a heat-retention function as necessary, and
again rolled back, whereby the rough bar is joined. Skin pass rolling may be carried out
on the hot-rolled steel sheet as necessary. Skin pass rolling has an effect of preventing
10 stretched strain occurring during working forming or flatness correction.
[0155]
In addition, the steel sheet for which the hot rolling has been completed is
subjected to pickling, and then cold rolling at a rolling reduction ratio of 20% to 90%.
When the rolling reduction ratio is less than 20%, there is a concern that sufficient
15 cold-rolled recrystalhzed structures may not be formed, and mixed grains may be formed.
In addition, when the rolling reduction ratio exceeds 90%, there is a concern of rupture
due to cracking. The effects of the present invention can be obtained even when a heat
treatment pattern for controlling the structure in accordance with purposes is employed as
the heat treatment pattern of annealing.
20 [0156]
However, in order to obtain a sufficient cold-rolled recrystallized equiaxed
structure and satisfy conditions in the ranges of the present application, it is necessary to
heat the steel sheet to a temperature range of at least 650°C to 900°C, anneal the steel
sheet for a holding time of 1 second to 300 seconds, and then carry out primary cooling
25 to a temperature range of 720°C to 580°C at a cooling rate of 0.1 °C/s to 100°C/s. When

66 the holding temperature is lower than 650°C, or the holding time is less than 1 second, a
sufficient recovered recrystallized structure cannot be obtained. In addition, when the
holding temperature exceeds 900°C, or the holding time exceeds 300 seconds, there is a
concern of oxidation or coarsening of grains. In addition, when the cooling rate is less
5 than 0. l°C/s, or the temperature range exceeds 720°C in the temporary cooling, there is a
concern that a sufiBcient amount of transformation may not be obtained. In addition, in
a case in which the cooling rate exceeds 100°C/s, or the temperature range is lower than
580°C, there is a concern of coarsening of grains and the like.
After that, according to an ordinary method, a galvanizing treatment is carried 10 out so as to obtain a galvanized steel sheet.
[0157]
The structure of the galvanized steel sheet obtained in the embodiment mainly includes ferrite, but may include pearlite, bainite, martensite, austenite, and compounds such as carbonitrides, as metallic structures other than ferrite. Since the crystal structure 15 of martensite or bainite is the same as or similar to the crystal structure of ferrite, the structure may mainly include any of ferrite, bainite, and martensite.
The galvanized steel sheet according to the present invention can be applied not
only to bending working but also to combined forming composed mainly of bending,
overhanging, drawing, and bending working.
20 [Example 1]
[0158]
The technical content of the hot-rolled steel sheet according to the embodiment will be described using examples of the present invention.
[0159]

67 The results of studies in which steels of AA to Bg having the component
compositions shown in Table 1 were used as examples will be described.
[0160]
[Table 1]
5 [0161]
The steels were cast, reheated as they were or after being cooled to room
temperature, heated to a temperature range of 900°C to 1300°C, and then hot-rolled
under the conditions of Table 2 or 3, thereby, finally, obtaining 2.3 mm or 3.2 mm-thick
hot-rolled steel sheets.
10 [0162]
[Table 2]
[0163]
[Table 3]
[0164]
15 Table 1 shows the chemical components of the respective steels. Tables 2 and 3
show the respective manufacturing conditions, and Tables 4 and 5 show structures and mechanical characteristics.
As an index of local deformability, the hole expanding rate and the limit bending radius through 90° V-shape bending were used. In bending tests, C-direction bending 20 and 45°-direction bending were carried out, and the rates were used as the index of the orientation dependency of formability. Tensile tests and the bending tests were based on JIS Z2241 and the V block 90° bending tests of JIS Z2248, and hole expanding tests were based on the Japan Iron and Steel Federation standard JFS TlOOl, respectively. The X-ray random intensity ratio was measured using the EBSD at a 0.5 [jm pitch with

68 respect to a 1/4 location from the end portion in the width direction in a sheet thickness
central portion in a 5/8 to 3/8 area of a cross section parallel to the rolling direction, hi
addition, the r values in the respective directions were measured through the above
methods.
5 [0165]
[Table 4]
[0166]
[Table 5]
[Example 2]
10 [0167]
The technical content of the cold-rolled steel sheet according to the embodiment will be described using examples of the present invention.
[0168]
The resuhs of studies in which steels of CA to CW having the component 15 compositions shown in Table 6 which satisfied the components specified in the claims of the present invention and comparative steels of Ca to Cg were used as examples will be described.
[0169]
20 [Table 6]
[0170]
The steels were cast, reheated as they were or after being cooled to room temperature, heated to a temperature range of 900°C to 1300°C, then, hot-rolled under the conditions of Table 7, thereby obtaining 2 mm to 5 mm-thick hot-rolled steel sheets. 25 The steel sheets were pickled, cold-rolled into a thickness of 1.2 mm to 2.3 mm, and

69 annealed under the annealing conditions shown in Table 7. After that, 0.5% scan pass
rolling was carried out, and the steel sheets were provided for material quality evaluation.
[0171]
5 [Table 7]
[0172]
Table 6 shows the chemical components of the respective steels, and Table 7
shows the respective manufacturing conditions. In addition. Table 8 shows the
structures and mechanical characteristics of the steel sheets. As an index of local
10 deformability, the hole expanding rate and the limit bending radius through V-shape
bending were used. In bending tests, C-direction bending and 45°-direction bending
were carried out, and the rates were used as the index of the orientation dependency of
formability. Tensile tests and the bending tests were based on JIS Z2241 and the V
block 90° bending tests of JIS Z2248, and hole expanding tests were based on the Japan
15 Iron and Steel Federation standard JFS T1001, respectively. The X-ray random
intensity ratio was measured using the EBSD at a 0.5 ^im pitch with respect to a 1/4
location from the end portion in the width direction in a sheet thickness central portion in
a 5/8 to 3/8 area of a cross section parallel to the rolling direction. In addition, the r
values in the respective directions were measured through the above methods.
20 [0173]
[Table 8] [Example 3] [0174]
The technical content of the galvanized steel sheet according to the embodiment 25 will be described using examples of the present invention.

70 [0175]
The results of studies in which steels of DA to DL having the component
compositions shown in Table 9 were used as examples will be described.
5 [0176]
[Table 9] [0177]
The steels were cast, reheated as they were or after being cooled to room temperature, heated to a temperature range of 900°C to 1300°C, then, cold-rolled under 10 the conditions of Table 10, thereby obtaining 2 mm to 5 mm-thick hot-rolled steel sheets. The steel sheets were pickled, cold-rolled into a thickness of 1.2 mm to 2.3 mm, annealed under the annealing conditions shown in Table 10, and continuously subjected to annealing and a galvanized coating or galvanealed coating treatment using a galvanized coating bath. After that, 0.5% scan pass rolling was carried out, and the steel sheets 15 were provided for material quality evaluation. [0178] [Table 10] [0179]
Table 9 shows the chemical components of the respective steels. Table 10 shows 20 the respective manufacturing conditions, and Table 11 shows the structures and mechanical characteristics of the steel sheets under the respective manufacturing conditions.
As an index of local deformability, the hole expanding rate and the limit bending
radius through 90° V-shape bending were used. Tensile tests and the bending tests were
25 based on JIS Z2241 and the V block 90° bending tests of JIS Z 2248, and hole expanding

71 tests were based on the Japan Iron and Steel Federation standard JFS TlOOl, respectively
The X-ray random intensity ratio was measured using the EBSD at a 0.5 urn pitch with
respect to a 1/4 location from the end portion in the width direction in a sheet thickness
central portion in a 3/8 to 5/8 area of a cross section parallel to the rolling direction. In
5 addition, the r values in the respective directions were measured through the above
methods.
[0180]
[Table 11]
[0181]
10 As shown in, for example, FIGS. 40,41,42,43,44, and 45, steel sheets
satisfying the specifications of the present invention had excellent hole expanding
properties, bending properties, and small forming anisotropy. Furthermore, steel sheets
manufactured in the desirable condition ranges exhibited superior hole expanding rate
and bending properties.
15
Industrial Applicability
[0182]
As described above, according to the present invention, without limiting the
main structure configuration, it is possible to obtain a hot-rolled steel sheet, a cold-rolled
20 steel sheet, and a galvanized steel sheet which are excellent in terms of local
deforambility and have a small orientation influence of foraiability even when Nb, Ti and
the like are added by controlling the texture in addition tocontroUing the sizes and shapes
of crystal grains.
[0183]
25 Therefore, the present invention is highly useful in the steel-manufacturing

•

72 industry.
In addition, in the present invention, the strength of the steel sheet is not
specified; however, since formability degrades as the strength increases as described
above, the effects are particularly large in the case of a high-strength steel sheet, for
example, a case in which the tensile strength is 440 MPa or more.

^

n3

Table 1 Chemical components (mass%) (1/4)
T1/°C C Si Mn P S AI N 0 Ti Nb
AA 851 0.070 0.08 1.30 0.015 0.004 0.040 0.0026 0.0032 - -
AB 865 0.080 0.31 1.35 0.012 0.005 0.016 0.0032 0.0023 - 0.041
AC 858 0.060 0.87 1.20 0.009 0.004 0.038 0.0033 0.0026 - 0.021
AD 865 0.210 0.15 1.62 0.012 0.003 0.026 0.0033 0-0021 0.021 -
AE 861 0.035 0.67 1.88 0.015 0.003 0.045 0.0028 0.0029 - 0.021
AF 875 0.180 0.48 2.72 0.009 0.003 0.050 0.0036 0.0022 - -
AG 892 0.060 0.11 2.12 0.010 0.005 0.033 0.0028 0.0035 0.036 0.089
AH 903 0.040 0.13 1.33 0.010 0.005 0.038 0.0032 0.0026 0.042 0.121
AI 855 0.350 0.52 1.33 0.260 0.003 0.045 0.0026 0.0019 - -
AJ 1376 0.072 0.15 1.42 0.014 0.004 0.036 0.0022 0.0025 - M
AK 851 0.110 0.23 1.12 0.021 0.003 0.026 0.0025 0.0023 - -
AL 1154 0.250 0.23 1.56 0.024 0.120 0.034 0.0022 0.0023 - -
BA 864 0.078 0.82 2.05 0.012 0.004 0.032 0.0026 0.0032 0.02 0.02
BE 852 0.085 0.75 2.25 0.012 0.003 0.035 0.0032 0.0023 - -
BC 866 0.110 0.10 1.55 0.02 0.004 0.038 0.0033 0.0026 - 0.04
BD 863 0.350 1.80 2.33 0.012 0.003 0.710 0.0033 0.0021 0.02
BE 859 0.120 0.22 1J5 0.015 0.003 0.025 0.0055 0.0029 - 0.02
BF 884 0.068 0.50 3.20 0.122 0.002 0.040 0.0032 0.0038 0.03 0.07
BG 858 0.130 0.24 1.54 0.010 0.001 0.038 0.0025 0.0029 - 0.02
BH 899 0.035 0.05 2.20 0.010 0.020 0.021 0.0019 0.0023 0.15 0.03
BI 852 - 0.090 1.25 1.88 0.014 0.002 0.030 0.0030 0.0030 - -
BJ 852 0.115 1.10 1.46 0.008 0.002 0.850 0.0034 0.0031 - -
BK 861 0.144 0.45 2.52 0.007 0.001 0.021 0.0024 0.0031 0.03 -

-^M

Table 1 Chemical components (mass%) (2/4)
B MR Rem Ca Mo Cr w As V Others | Note
AA - - - - - - - - - 1 Invention steel
AB - - - - - - - - - - Invention steel
AC - - 0.0015 - - - - - - - Invention steel
AD 0.0022 - - - 0.03 0.35 - - - - Invention steel
AE - 0.002 - 0.0015 - - - - 0.029 - Invention steel
AF - 0.002 - - 0.10 - - - 0.10 - Invention steel
AG 0.0012 - - - - - - - - Invention steel
AH 0.0009 - - - - - - - - - Invention steel
AI - - - - - - - - - - Comparative steel
AJ - - - - - - - - - - Comparative steel
AK - 0.150 - - - - - - - - Comparative steel
AL - - - - - M - - 2.50 - Comparative steel
BA - - - - - - - - - - Invention steel
BE - - - - - - - - - Co:0.5% Sn:0.02% Invention steel
BC - - - - - - - - - - Invention steel
BD 0.0020 - 0.0035 - - - - - - - Invention steel
BE - - - - - - - - - - Invention steel
BF - - 0.0044 - - 0.10 - - - - Invention steel
BG - - - - - - - - - Invention steel
BH - - 0.0005 0.0009 0.05 - - Invention steel
BI - - - - - - - - - - Invention steel
BJ - - - - - - - - - - Invention steel
BK - - - - - ■ " - - Cu:0.5%, Ni:0.25%, ZnO.02% Invention steel 1

>^

Table 1 Chemical components (mass %) (3/4)
T1/°C C Si Mn P S Al N O Ti Nb
BL 853 0.190 1.40 1.78 0.011 0.002 0.018 0.0032 0.0028 - -
BM 866 0.080 0.10 1.40 0.007 0.002 1.700 0.0033 0.0034 - -
BN 852 0.062 0.72 2.82 0.009 0.002 0.035 0.0033 0.0022 - -
BO 885 0.120 0.80 2.20 0.008 0.002 0.035 0.0022 0.0035 0.05 -
BP 873 0.190 0.55 2.77 0.009 0.002 0.032 0.0033 0.0036 0.04 -
BQ 852 0.082 0.77 1.82 0.008 0.003 0.025 0.0032 0.0031 - -
BR 875 0.030 1.00 2.40 0.005 0.001 0.033 0.0022 0.0011 0.05 0.01
BS 852 0.077 0.45 2.05 0.009 0.003 0.025 0.0029 0.0031 - -
BT 861 0.142 0.70 2.44 0.008 0.002 0.030 0.0032 0.0035 0.03 -
BU 876 0.009 0.10 1.40 0.006 0.001 0.003 0.0033 0.0024 0.10 -
BV 853 0.150 0.61 2.20 0.011 0.002 0.028 0.0021 0.0036 - -
BW 1043 0.120 0.17 2.26 0.028 0.009 0.033 0.0027 0.0019 - -
Ba 860 0.440 0.50 2.20 0.008 0.002 0.035 0.0021 0.0012 - -
Bb 854 0.080 0.45 4.50 0.200 0.002 0.034 0.0041 0.0015 - -
Be 914 0.080 0.35 2.00 0.008 0.002 0.033 0.0042 0.0034 0.25 -
Bd 939 0.070 0.35 2.40 0.008 0.002 0.035 0.0035 0.0026 - 0.25
Be 851 0.090 0.10 1.00 0.008 0.040 0.036 0.0035 0.0022 - -
Bf 952 0.070 0.21 2.20 0.008 0.002 0.033 0.0023 0.0036 - -
Bg 853 0.140 0.11 1.90 0.008 0.002 0.032 0.0044 0.0035 - -

-^6

Table 1 Chemical components (mass%^ (4/4)
B MR Rem Ca Mo Cr W As V Others Note
BL 0.0002 - - - - - - - - - Invention steel
BM - - - 0.0022 - - - - 0.15 - Invention steel
BN - - - - - - - - - - Invention steel
BO - - - - - - - 0.01 0.20 - Invention steel
BP - 0.006 - - 0.022 - - - 0.05 - Invention steel
BQ 0.0002 - - - - - - - - - Invention steel
BR - 0.004 0.004 ■ - - 0.80 - - - - Invention steel
BS - - - - - - - - - Invention steel
BT 0.0002 - - - - - - - - - Invention steel
BU - - - - 0.01 - - - - - Invention steel
BV - 0.004 0.005 - - - - - - - Invention steel
BW - - - - 0.90 - - - - - Invention steel
Ba - - - - - - - - - - Comparative steel
Bb - - - - - - - - - - Comparative steel
Be - - - - - - - - - - Comparative steel
Bd - - - - - - - - - - Comparative steel
Be - - - - - - - - - - Comparative steel
Bf - 0.020 - - - - - - 1.10 - Comparative steel
Bg - - 0.15 . - - .. - - - - - Comparative steel

"1

-f-

Table 2 Manufacturing conditions(l/2
Steel type Tiyc Number of times of
rolling of 20% or more at
1000°Cto 1200°C Rolling
reduction
rate of 20%
or more at
1000°C to
1200''C
/% Austenite grain
diameter /^im Total
rolling
reduction
rate at
TI+30°C to
T1+200°C
/% Temperature
increase
during
rolling at
T1+30°C to
T1+200''C
/°C
1 AA 851 1 20 150 85 15
2 AA 851 2 45/45 90 95 5
3 AB 865 2 45/45 80 75 15
4 AB 865 2 45/45 80 85 18
5 AC 858 2 45/45 95 85 13
6 AC 858 2 45/45 95 95 14
7 AD 862 3 40/40/40 75 80 16
8 AE 858 2 45/40 95 80 17
9 AE 858 1 50 120 80 18
10 AF 875 3 40/40/40 70 95 18
11 AG 892 3 40/40/40 65 95 10
12 AH 903 2 45/45 70 90 13
13 AH 903 2 45/45 95 85 15
14 AF 875 3 40/40/40 70 65 20
15 AG 892 1 50 120 75 20
16 AG 892 1 50 120 60 21
17 AH 903 ' 1 50 120 65 19
18 AH 903 1 50 120 35 12
19 AA 851 2 45/45 90 45 20
20 AB 865 2 45/45 80 45 15
21 AV 858 2 40/45 95 75 12
22 AG 892 0 - 350 45 30
23 AE 858 1 50 120 80 40 .
24 AA 851 0 - 250 65 18
25 AC 858 0 - 300 85 13
26 AI 855 Cracked during hot rolling
27 AJ 1376 Cracked during hot rolling
28 AK 851 Cracked during hot rolling
29 AL 1154 Cracked during hot rolling

^

i

Table 2 Manufacturing conditions(2/2)
Steel type Total
rolling
reduction
rate at
TI°Cto
lower than
Tl+SCC
/% Tf:
Temperature
after final
pass of
heavy rolling
pass
/"C PI: Rolling
reduction rate
of final pass
of heavy
rolling pass
/% tl 2.5xtl t: Waiting
time from
completio
n of heavy
rolling
pass to
initiation
of cooling
/s 1/tl Winding
temperature
/"C
I 10 935 40 0.57 1.41 0.8 1.41 600
2 0 892 35 1.74 4.35 2 1.15 50
3 25 945 37 0.76 1.90 1 1.32 600
4 5 920 31 1.54 3.86 2.3 1.49 50
5 15 955 31 0.73 1.82 1 1.38 600
6 0 934 40 0.71 1.78 1 1.41 500
7 25 970 30 0.62 1.56 0.9 1.45 600
S 5 960 30 0.70 1.75 1 1.42 300
9 15 921 30 1.40 3.50 2 1.43 200
10 0 990 30 0.53 1.32 0.7 1.32 500
11 0 943 35 1.46 ■ 3.65 2.1 1.44 600
12 0 1012 40 0.25 0.63 0.3 1.19 500
13 10 985 40 0.61 1.52 0.9 1.48 600
14 25 965 34 0.70 1.75 0.9 1.28 500
15 15 993 30 0.71 1.77 0.8 1.13 500
16 20 945 45 1.06 2.64 1.1 1.04 600
17 15 967 38 1.05 2.63 1.5 1.43 500
18 45 880 30 3.92 9.79 5 1.28 100
19 45 930 30 1.08 2.69 5 4.64 600
20 45 1075 30 0.20 0.50 oa 0.50 600
21 45 890 30 2.15 5.36 1.3 0.61 600
22 35 910 35 2.44 6.09 M 0.21 400
23 35 860 40 3.02 7.54 9 2.98 600
24 20 850 30 3.13 7.83 03 0.10 800
25 25 890 30 2.15 5.36 2.2 1.03 600
26 Cracked during hot rolling
27 Cracked during hot rolling
28 Cracked during hot rolling
29 Cracked during hot rolling

^^

Table 3 Manufacturing conditions(l/2)
Steel type JVC Number oi times of
rolling of 20% or more at
lOOCC to
noo-'c Rolling
reduction
rate of
20% or
more at
1000°Cto
1200«'C
/% Austenite grain
diameter /jim Total
rolling
reduction
rate at Tl+BO^C
to
T1+200''C
/% Temperature
increase
during
rolling at
T1+30''C to
T1+200''C
BAl BA 864 2 45/45 80 85 17
BBl BB 852 2 45/45 85 80 13
BB2 BB 852 2 45/45 80 85 16
BCl BC 866 2 45/45 80 85 16
BDl BD 863 1 50 120 85 14
BE2 BE 859 2 45/45 80 80 16
BFl BF 884 2 45/45 75 85 15
BF2 BF 884 1 50 110 80 13
BGl BG 858 3 40/40/40 80 80 15
BHl BH 899 2 45/45 80 80 12
BIl BI 852 2 45/45 75 90 12
BI2 BI 852 2 45/45 75 80 16
BJl BJ 852 3 40/40/40 85 85 15
BJ2 BJ 852 2 45/45 75 80 13
BKl BK 861 3 40/40/40 85 90 13
BK2 BK 853 3 40/40/40 85 90 12
BLl BL 853 2 45/45 80 85 14
BL2 BL 853 2 45/45 80 80 17
BMl BM 866 1 30 140 65 12
BNl BN 852 2 45/45 75 70 12
BOl BO 885 2 45/45 80 60 15
BPl BP 873 2 45/45 75 85 13
BQl BQ 852 2 45/45 80 80 16 . ,
BRl BR 875 2 45/45 75 85 12
BSl BS 852 2 45/45 80 85 12
BS2 BS 852 2 45/45 75 80 15
BTl BT 861 2 45/45 80 95 16
BT2 BT 861 2 45/45 85 80 12
BUI BU 876 2 45/45 75 85 12
BVl BV 853 2 45/45 85 80 11
BWI BW 1043 1 50 120 80 16
Bal Ba 860 2 45/45 75 90 16
Bbl Bb 854 1 50 120 85 12
Bel Be 914 2 45/45 75 90 13
Bdl Bd 939 2 45/45 75 85 12
Bel Be 851 2 45/45 80 65 11
Bfl Bf 952 2 45/45 80 70 12
Bgl B? 853 2 45/45 75 60 12

s-
Table 3 Manufacturing conditions(2/2)
Steel type Total
rolling
reduction
rate at
Trcto
lower than
Tl+SCC
/% Tf:
Temperatu
re after
final pass of heavy rolling
pass
/°C PI:
Rolling
reduction
rate of final
pass
of heavy
rolling
pass
/% tl 2.5xtl t: Waiting
timefixim conpletion
ofheavy rolling pass to initiation
of cooling Is t/tl Winding
temperature
/°C
BAl 0 984 45 0.13 0.33 0.28 2.15 500
BBl 0 982 40 0.14 0.34 0.29 2.10 500
BB2 0 922 45 0.66 1.65 1.15 1.75 500
BCl 0 966 45 0.22 0.55 0.37 1.68 600
BDl 0 963 40 0.34 0.85 0.49 1.44 600
BE2 0 929 45 0.66 1.65 1.15 1.75 600
BFl 15 944 45 0.89 2.22 1.04 1.17 500
BF2 0 954 40 0.83 2.08 6.00 7.21 500
BG2 0 958 45 0.22 0.55 0.37 1.68 600
BHl 20 959 40 1.06 2.65 1.21 1.14 500
BIl 0 952 40 0.34 0.85 0.49 1.44 600
BI2 0 922 45 0.66 1.65 1.15 1.75 600
BJl 0 962 45 0.15 0.39 0.30 1.97 600
BJ2 0 922 40 0.83 2.08 1.46 1.75 600
BKl 0 961 40 0.34 0.85 0.49 1.44 550
BK2 0 923 40 0.83 2.08 0.98 1.18 600
BLl 0 953 45 0.22 0.55 0.37 1.68 600
BL2 r 0 923 50 0.51 1.28 0.66 1.29 600
BMl 10 966 40 0.34 0.85 0.49 1.44 500
BNl 0 952 40 0.34 0.85 0.49 1.44 550
BOl 0 985 45- 0.22 0.55 0.37 1.68 600
BPl 0 973 40 0.34 0.85 0.49 1.44 600
BQl 0 952 45 0.22 0.55 0.37 1.68 600.
BRl 0 985 40 0.24 0.60 0.39 1.63 500
BSl 0 992 40 0.13 0.33 028 2.14 550
BS2 0 922 45 0.66 1.65 0.81 1.23 550
BTl 15 961 45 0.22 0.55 0.37 1.68 500
BT2 0 931 40 0.83 2.08 0.98 1.18 500
BUI 10 976 40 0.34 0.85 0.49 1.44 500
BVl 0 953 40 0.34 0.85 0.49 1.44 600
BWl 10 1083 45 1.46 3.66 1.61 1.10 550
Bal 0 960 45 0.22 0.55 0.37 1.68 600
Bbl 0 954 40 0.34 0.85 0.49 1.44 600
Bel 0 994 40 0.64 1.59 0.79 1.24 600
Bdl 0 999 40 1.06 2.65 1-21 1.14 600
Bel 0 951 40 0.34 0.85 0.49 1.44 600
Bfl 0 1012 1 40 1.06 2.65 1.21 1.14 600
Bgl 0 1 953 1 40 1 0.34 0.85 0.49 1.44 600

i)

Table 4 The structure and mechanical characteristics of the respective steels in the respective manufacturing conditions (1/2)
Steel type X-ray random
intensity ratio of
{100}<011>to
{223}<110>
orientation group X-ray random
intensity ratio
of{332}<113> rL rC r30 r60 coarsened
grain
area
ratio
/%
1 2.6 2.2 0.88 0.87 1.04 1.05 5
2 2.2 2.1 0.92 0.90 0.96 0.98 1
3 2.9 2.S 0.87 0.79 1.05 1.05 5
4 2.7 2.7 0.90 0.85 1.02 1.03 4
5 3.5 3.2 0.78 0.75 0.98 1.00 6
6 3.0 2.8 0.83 0.85 0.95 0.98 4
7 5.2 4.1 0.70 0.70 1.08 1.09 7
8 2.9 2.7 0.85 0.90 1.06 1.05 5
9 3.5 2.9 0.75 0.95 1.02 1.10 5
10 3.0 3.0 0.72 0.75 1.05 1.08 6
n 2.9 3.0 0.72 0.74 1.07 1.09 6
12 2.9 2.6 0.71 0.72 1.06 1.08 3
13 3.0 2.9 0.73 0.72 1.10 1.08 5
14 5.4 4.6 0.66 0.73 1.10 ,1.20 5
15 3.7 3.5 0.65 0.75 1.05 1.19 4
16 5.4 4.5 0.58 0.70 1.10 1.26 1
17 5.4 3.0 QM 0.75 1.02 1.15 5
18 1=2 6=4 0.54 0.67 L24 1.31 3
19 M 5J. 0.69 0.79 lAl 1.15 22
20 M 5=2 0.56 0.65 1.25 1.19
21 12 M 0.65 0.68 1.18 1.15
22 M 5=4 0.52 0.65 1.22 L3Q
23 H 6=4 ML 0.65 1.15 1.23 16
24 5.4 5=6 0.59 0.75 1.05 1.21
25 5.2 5=4 0.68 0.72 1.15 1.10 4
26 Cracked during hot rolling
27 Cracked during hot rolling
28 Cracked during hot rolling
2, Cracked during hot rolling

8^
Table 4 The structure and mechanical characteristics of the respective steels in the respective manufacturing conditions (2/2)
Steel type equiaxed
grain rate
/% TS /MPa EI. /% X /% TsX A. /MPa-% Sheet
thickness/
minimum
bending
radius 45''-direction
bending/
C-direction
bending
ratio Note
1 74 445 34 145 64525 3.2 1.1 Invention steel
2 80 450 38 180 81000 3.3 1.0 Invention steel
3 72 605 25 95 57475 3.2 1.2 Invention steel
4 73 595 24 115 68425 2.3 1.1 Invention steel
5 75 595 29 85 50575 2.7 1.2 Invention steel
6 78 600 28 90 54000 2.3 1.1 Invention steel
7 72 650 19 75 48750 2.1 1.5 Invention steel
8 72 625 21 135 84375 3.3 1.1 Invention steel
9 72 635 19 118 74930 3.2 1.2 Inveotioa steel
10 78 735 15 75 55125 2.5 1.4 Invention steel
11 77 810 19 85 68850 2.3 1.4 Invention steel
12 78 790 21 140 110600 2.7 1.4 Invention steel
13 74 795 20 140 111300 2.3 1.4 Invention steel
14 69 765 14 60 45900 M 1.6 Invention steel
15 74 825 18 70 57750 M 1.5 Invention steel
16 70 835 17 65 54275 1.5 1.8 Invention steel
17 67 830 17 125 103750 1.5 1.5 Invention steel
18 59 805 19 60 48300 1.1 2.0 Invention steel
19 29 465 34 85 39525 L2 1.5 Comparative steel
20 70 635 24 65 41275 12 1.9 Comparative steel
21 79 640 26 45 28800 12 1.7 Comparative steel
22 73 845 16 45 38025 M 2.0 Comparative steel
23 57 670 16 75 50250 L2 1.8 Comparative steel
24 81 405 30 70 28350 Li 1.6 Comparative steel
25 78 650 27 50 32500 ii 1.5 Comparative steel
26 Cracked during hot rolling Comparative steel
27 Cracked during hot rolling Comparative steel
28 Cracked during hot rolling Comparative steel 1
29 Cracked during hot rolling Comparative 1 steel 1

3

Table 5 The structure and mechanical characteristics of the respective in the respective manufacturing conditions (1/4) steels
Steel type X-ray random intensity ratio of
{100} <011>to
{223}
<110>
orientation
group X-ray random intensity
ratio of{332}
<113> rL TC r30 r60 coarsened
grain
area
ratio
/%
BAl 2.3 2.4 0-83 0.84 0.85 0.88 9
BBl 2.4 2.4 0.84 0.85 0.86 0.89 9
BB2 2.8 2.8 0.79 0.81 0.90 0.92 6
BCl 2.8 2.9 0.78 0.80 0.91 0.93 6
BDl 3.5 3.1 0.83 0.84 0.99 0.99 5
BE2 2.8 2.8 0.79 0.81 0.90 0.92 6
BFl 3.3 M 0.72 0.75 0.97 0.98 2
BF2 1.1 1.2 0.95 0.95 0.99 1.01 30
BGl 2.8 2.8 0.78 0.80 0.91 0.93 6
BHl 3.4 3.4 0.72 0.76 0.97 0.98 2
BIl 3.0 3.2 0.74 0.77 0.94 0.95 5
BI2 2.7 2.8 0.78 0.80 0.90 0.92 6
BJl 2.6 2.6 0.82 0.83 0.88 0.91 8
BJ2 2.7 2.8 0.78 0.80 0.90 0.92 7
BKl 3.1 3.2 0.76 0.79 0.95 0.96 5
BK2 3.4 3.4 0.73 0.76 0.99 0.99 3
BLl 2.8 2.9 0.78 0.80 0.91 0.93 6
BU 3.2 3.2 0.74 0.77 0.95 0.96 2
BMl 3.7 2.9 0.87 0.87 0.99 0.99 5
BNl 3.0 3.0 0.74 0.77 0.92 0.94 5
BOl 2.8 2.6 0.78 0.80 0.89 0.91 6
BPl 3.0 3.1 0.74 0.77 0.94 0.95 5

u

Table 5 The structure and mechanical characteristics of the respective steels in the respective manufacturing conditions (2/4)

CLAIMS
1. A hot-rolled steel sheet comprising, by mass%:
C: 0.0001% to 0.40%;
5 Si: 0.001% to 2.5%;
Mn: 0.001% to 4.0%;
P: 0.001% to 0.15%;
S: 0.0005% to 0.03%;
Al: 0.001% to 2.0%;
10 N: 0.0005% to 0.01%;
0:0.0005% to 0.01%;
and further comprising one or two or more of:
Ti: 0.001% to 0.20%;
Nb: 0.001% to 0.20%;
15 V: 0.001% to 1.0%;
W: 0.001% to 1.0%;
B: 0.0001% to 0.0050%;
Mo: 0.001% to 1.0%;
Cr: 0.001% to 2.0%;
20 Cu: 0.001% to 2.0%;
Ni: 0.001% to 2.0%;
Co: 0.0001% to 1.0%;
Sn: 0.0001% to 0.2%;
Zr: 0.0001% to 0.2%;
25 As: 0.0001% to 0.50%;
Mg: 0.0001% to 0.010%;
Ca: 0.0001% to 0.010%; and
REM: 0.0001% to 0.1%;
and balance composed of iron and inevitable impurities,
5 wherein an average value of an X-ray random intensity ratio ofa{100}<011>
to {223} <110> orientation group at least in a sheet thickness central portion that is in a
sheet thickness range of 5/8 to 3/8 from a steel sheet surface is 1.0 to 6.0, an X-ray
random intensity ratio of a {332} <113> crystal orientation is 1.0 to 5.0; and
rC which is an r value in a direction perpendicular to a rolling direction is 0.70
10 to 1.10, and r30 which is an r value in a direction that forms an angle of 30° with respect
to the rolling direction is 0.70 to 1.10.
2. The hot-rolled steel sheet according to Claim 1,
wherein rL which is an r value in the rolling direction is 0.70 to 1.10, and r60
15 which is an r value in a direction that forms an angle of 60° with respect to the rolling
direction is 0.70 to 1.10.
3. The hot-rolled steel sheet according to Claim 1 or 2,
wherein one or two or more of bainite, martensite, pearlite, and austenite are
20 present in the hot-rolled steel sheet, and a proportion of grains having a dL/dt, which is a
ratio of a length in the rolling direction dL to a length of a sheet thickness direction dt, of
3.0 or less in crystal grains in the structures is 50% to 100%.
4. The hot-rolled steel sheet according to Claim 1 or 2,
25 wherein an area proportion of crystal grains having a grain diameter of more
than 20 |j,m in a total area of a metallic structure in the hot-rolled steel sheet is 0% to
10%.
5. A cold-rolled steel sheet obtained through cold rolling of the hot-rolled steel
5 sheet according to Claim 1,
wherein the average value of the X-ray random intensity ratio of the {100}
<011> to {223} <110> orientation group at least in the sheet thickness central portion is
1.0 to less than 4.0, the X-ray random intensity ratio of the {332} <113> crystal
orientation is 1.0 to 5.0; and
10 rC which is the r value in a direction perpendicular to the rolling direction is
0.70 to 1.10, and r30 which is the r value in a direction that forms an angle of 30° with
respect to the rolling direction is 0.70 to 1.10.
6. The cold-rolled steel sheet according to Claim 5,
15 wherein rL which is an r value in the rolling direction is 0.70 to 1.10, and r60
which is an r value in a direction that forms an angle of 60° with respect to the rolling
direction is 0.70 to 1.10.
7. The cold-rolled steel sheet according to Claim 5 or 6,
20 wherein one or two or more of bainite, martensite, pearlite, and austenite are
present in the cold-rolled steel sheet, and a proportion of grains having a dL/dt, which is
a ratio of a length in the rolling direction dL to a length of a sheet thickness direction dt,
of 3.0 or less in crystal grains in the structures is 50% to 100%.
25 8. The cold-rolled steel sheet according to Claim 5 or 6,
wherein an area proportion of crystal grains having a grain diameter of more
than 20 \xm in a total area of a metallic structure in the cold-rolled steel sheet is 0% to
10%.
5 9. A galvanized steel sheet further comprising a galvanized coating layer or a
galvanealed coating layer on a surface of the cold-rolled steel sheet according to Claim 5,
wherein the average value of the X-ray random intensity ratio of the {100}
<011> to {223} <110> orientation group at least in the sheet thickness central portion is
1.0 to less than 4.0, the X-ray random intensity ratio of the {332} <113> crystal
10 orientation is 1.0 to 5.0; and
rC which is the r value in a direction perpendicular to the rolling direction is
0.70 to 1.10, and r30 which is the r value in a direction that forms an angle of 30° with
respect to the rolling direction is 0.70 to 1.10.
15 10. The galvanized steel sheet according to Claim 9,
wherein rL which is an r value in the rolling direction is 0.70 to 1.10, and r60
which is an r value in a direction that forms an angle of 60° with respect to the rolling
direction is 0.70 to 1.10.
20 11. A method of manufacturing the hot-rolled steel sheet, the method
comprising,
first hot rolling carried out at least once at a rolling reduction ratio of 20% or
more in a temperature range of 1000°C to 1200°C, and an austenite grain diameter is set
to 200 |im or less, wherein
an ingot or slab containing, by mass%:
C: 0.0001% to 0.40%,
Si: 0.001% to 2.5%,
Mn: 0.001% to 4.0%,
5 P: 0.001% to 0.15%,
8:0.0005% to 0.03%,
Al: 0.001% to 2.0%,
N: 0.0005% to 0.01%,
0:0.0005% to 0.01%,
10 and further comprising one or two or more of:
Ti: 0.001% to 0.20%,
Nb: 0.001% to 0.20%,
V: 0.001% to 1.0%,
W: 0.001% to 1.0%,
15 B: 0.0001% to 0.0050%,
Mo: 0.001% to 1.0%,
Cr: 0.001% to 2.0%,
Cu: 0.001% to 2.0%,
Ni: 0.001% to 2.0%,
20 Co: 0.0001% to 1.0%,
Sn: 0.0001% to 0.2%,
Zr: 0.0001% to 0.2%,
As: 0.0001% to 0.50%,
Mg: 0.0001% to 0.010%,
25 Ca: 0.0001% to 0.010%, and
REM: 0.0001% to 0.1%
and balance composed of iron and inevitable impurities;
second hot rolling in which a total of rolling reduction ratios is 50% or more is
carried out in a temperature range of T1+30°C to TH-200°C;
5 third hot rolling in which a total of rolling reduction ratios is less than 30% is
carried out in a temperature range of T1°C to lower than TH-30°C; and
hot rolling ends at an Ar3 transformation temperature or higher,
where, Tl is a temperature determined by steel sheet components, and expressed
by the following formula 1.
10 Tl(°C) = 850+10x(C+N)xMn + 350xNb + 250xTi + 40xB+10xCr +
lOOxMo+lOOxV - (Formula 1)
12. The method of manufacturing a hot-rolled steel sheet according to Claim
11,
15 wherein, in the second hot rolling in tiie temperature range of T1+30°C to
TH-200°C, the ingot or slab is rolled at least once at a rolling reduction ratio of 30% or
more in a pass.
13. The method of manufacturing a hot-rolled steel sheet according to Claim
20 11 or 12,
wherein, in the first hot rolling in a temperature range of 1000°C to 1200°C, the
ingot or slab is rolled at least twice at a rolling reduction ratio of 20% or more, and the
austenite grain diameter is set to 100 |j,m or less.
14. The method of manufacturing a hot-rolled steel sheet according to Claim
11 or 12,
wherein, in a case in which the pass in which the rolling reduction ratio is 30%
or more in the temperature range of T1+30°C to T1+200°C is defined as a large reduction
5 pass, a waiting time t from completion of a final pass of the large reduction pass to
initiation of cooling employs a configuration that satisfies the following formula 2,
t l < t < t l x 2 . 5 ••• (Formula2)
where tl is expressed by the following formula 3;
tl = 0.001 X ((Tf-Tl) X Pl)2 - 0.109 X ((Tf-Tl) x PI) + 3.1
10 (Formula 3)
where Tf represents a temperature after the final pass, and PI represents a
rolling reduction ratio in the final pass.
15. The method of manufacturing a hot-rolled steel sheet according to Claim
15 14,
wherein a temperature of the steel sheet increases by 18°C or less between the
respective passes of the second hot rolling in the temperature range of T1+30°C to
T1+200°C.
20 16. A method of manufacturing a cold-rolled steel sheet, the method
comprising,
pickling, after the end of the hot rolling the hot-rolled steel sheet obtained
through the method of manufacturing the hot-rolled steel sheet according to Claim 11 at
the Ar3 transformation temperature or higher;
cold-rolling at 20% to 90%;
annealing at a temperature range of 720°C to 900°C for a holding time of 1
second to 300 seconds;
acceleration-cooling at a cooling rate of 10°C/s to 200°C/s from 650°C to
5 500°C;and
helding at a temperature of 200°C to 500°C.
17. The method of manufacturing a cold-rolled steel sheet according to Claim
16,
10 wherein, in the second hot rolling in the temperature range of T1+30°C to
T1+200°C, rolling at a rolling reduction ratio of 30% or more in a pass is carried out at
least once.
18. The method of manufacturing a cold-rolled steel sheet according to Claim
15 16 or 17,
wherein, in the first hot rolling in the temperature range of 1000°C to 1200°C,
rolling at a rolling reduction ratio of 20% or more is carried out at least twice, and the
austenite grain diameter is set to 100 )am or less.
20 19. The method of manufacturing a cold-rolled steel sheet according to Claim
16 or 17,
wherein, in a case in which the pass in which the rolling reduction ratio is 30%
or more in the temperature range of TH-30°C to T1+200°C is defined as a large reduction
pass, a waiting time t from completion of a final pass of the large reduction pass to
initiation of cooling employs a configuration that satisfies the following formula 4,
t l < t < t l x 2 . 5 ••• (Formula4)
where tl is expressed by the following formula 5;
tl = 0.001 X ((Tf- Tl) X ?lf - 0.109 X ((Tf- Tl) x PI) + 3.1
5 (Formula 5)
where Tf represents a temperature after the final pass, and PI represents a
rolling reduction ratio in the final pass.
20. The method of manufacturing a cold-rolled steel sheet according to Claim
10 19,
wherein a temperature of the steel sheet increases by 18°C or less between the
respective passes of the second hot rolling in the temperature range of T1+30°C to
T1+200°C.
15 21. A method of manufacturing a galvanized steel sheet, the method
comprising,
a winding in a temperature range of 680°C to room temperature, after the end of
the hot rolling the hot-rolled steel sheet obtained through the method of manufacturing
the hot-rolled steel sheet according to Claim 11 at the Ar3 transformation temperature or
20 higher;
pickling;
cold-rolling at 20% to 90%;
heating to a temperature range of 650°C to 900°C;
annealing for a holding time of 1 second to 300 seconds;
cooling at a cooling rate of 0.1 °C/s to 100°C/s from 720°C to 580°C; and
galvanizing treating.
22. The method of manufacturing a galvanized steel sheet according to Claim
5 21,
wherein, in the second hot rolling in the temperature range of TH-30°C to
T1+200°C, rolling at a rolling reduction ratio of 30% or more in a pass is carried out at
least once.
10 23. The method of manufacturing a galvanized steel sheet according to Claim
21 or 22,
wherein, in the first hot rolling in the temperature range of 1000°C to 1200°C,
rolling at a rolling reduction ratio of 20% or more is carried out at least twice, and the
austenite grain diameter is set to 100 p,m or less.
15
24. The method of manufacturing a galvanized steel sheet according to Claim
21 or 22,
wherein, in a case in which the pass in which the rolling reduction ratio is 30%
or more in the temperature range of T1+30°C to TH-200°C is defined as a large reduction
20 pass, a waiting time t from completion of a fmal pass of the large reduction pass to
initiation of cooling employs a configuration that satisfies the following formula 6,
t l < t < t l x 2 . 5 ••• (Formula6)
where tl is expressed by the following formula 7;
tl -0.001 X ((Tf-Tl) X Pl)^-0.109 X ((Tf-Tl) x PI)+ 3.1
(Formula?)
where Tf represents a temperature after the final pass, and PI represents a
rolling reduction ratip in the final pass.
5 25. Themethodof manufacturing a galvanized steel sheet according to Claim
• • . wherein a temperature of the steel sheet increases by 18°C or less between the
• respective passes of the second hot rolling in the temperature range of TH-30°C to