[0001]The present invention relates to a high-carbon steel
sheet which is suitably used as, for example, a material for
10 deep drawing and to a method for manufacturing the high-
carbon steel sheet.
Background Art [0002]
15 A high-carbon steel sheet that is used as a material for
deep-drawn formed article is required (i) to be soft so that the forming load can be reduced during the deep drawing. Further, such a high-carbon steel sheet is generally required (ii) to have small in-plane anisotropy of the Lankford value
20 (r-value) since it is preferable that the deep-drawn formed
article can be manufactured so that the height of a vertical
wall part of the deep-drawn formed article is as uniform as
possible, without exhibiting variations, in the
circumferential direction of the deep-drawn formed article.
25 Thus, the techniques as described in, for example, Patent

NT19328/PCT
- 2 -Literatures 1 to 4 have been examined. [0003]
The technique of Patent Literature 1 provides a carbon
steel sheet that reduces variation in height of a vertical wall
5 part of a deep-drawn formed article. The carbon steel sheet
contains, in % by mass, C: 0.15% to 2.0%, Si: not more than 0.40%, Mn: not more than 0.5%, P: not more than 0.3%, S: not more than 0.03%, and Cr: not more than 2.0%, and the remainder consists of Fe and unavoidable impurities.
10 Further, in the carbon steel sheet, carbides are dispersed in
ferrite such that the carbide spheroidization ratio is not less than 90% and the mean carbide grain size is not less than 0.4 μm. This carbon steel sheet has an anisotropy Δr of -1.0 to 1.0.
15 [0004]
The technique of Patent Literature 2 provides a high-carbon steel sheet that is formed into, for example, automotive parts, particularly a high-carbon steel sheet presenting good dimensional accuracy after forming of
20 cylindrical parts and after heat treatment. This high-carbon
steel sheet contains, in % by mass, C: 0.25% to 0.60%, Mn: 0.20% to 1.50%, Cr: 0.60% or less, and further, as needed, Ti: 0.010% to 0.060%, and B: 0.0003% to 0.0050%. Further, in the relation between the X-ray integrated intensity ratio
25 of the (222) plane and the (200) plane and the C amount of

NT19328/PCT
- 3 -
the high-carbon steel sheet, this high-carbon steel sheet
satisfies (222)/(200) < 5.5-5×C(%) and thus presents good
circularity of the formed articles and good circularity after
quenching.
5 [0005]
The technique of Patent Literature 3 provides a high-carbon steel sheet with small in-plane anisotropy which can be applied to parts that require high dimensional accuracy in forming process and that are subjected to heat treatment
10 such as quenching, tempering, and the like, and a method
for manufacturing the high-carbon steel sheet. This high-carbon steel sheet has a component system where C: 0.2% to 1.5%, Si: 0.10% to 0.35%, Mn: 0.1% to 0.9%, P: not more than 0.03%, S: not more than 0.035%, Cu: not more than
15 0.03%, Ni: not more than 0.025%, and Cr: not more than
0.3%. The carbide mean grain size of this high-carbon steel sheet is less than 0.5 μm. This high-carbon steel sheet is such that the in-plane anisotropy index Δr of the r-value is more than -0.15 and less than 0.15.
20 [0006]
The technique of Patent Literature 4 provides a high-carbon cold-rolled steel band as below and a method for manufacturing the high-carbon cold-rolled steel band. A steel material is used which has a steel composition
25 containing C: 0.25% to 0.75%, sol. Al: 0.01% to 0.10%, and

NT19328/PCT - 4 -
N: 0.0020% to 0.0100% and satisfying 2<(sol. Al/N)<20. This
steel material is subjected to hot rolling at a winding
temperature of 550°C to 680°C. After pickled, the steel
material is subjected to cold rolling at a reduction rate of
5 20% to 80%, and then to box annealing and temper rolling
in the temperature range of 650°C to Ac1. The high-carbon
cold-rolled steel band obtained satisfies the condition that
the mean grain size of in-steel carbide is not less than 0.5
um and the spheroidization ratio>90%. In the texture of the
10 high-carbon cold-rolled steel band, the relation between the
X-ray integrated intensity ratio of the (222) plane and the (200) plane and the C amount of the high-carbon steel sheet satisfies (222)/(200) > 6-8.0×C(%). In the high-carbon steel band, the mean r-value>0.80 and the in-plane anisotropy
15 index Ar is within ±0.020.
Citation List
[Patent Literature]
[0007]
20 [Patent Literature 1]
Japanese Patent Application Publication Tokukai No. 2018-141184
[Patent Literature 2]
Japanese Patent Application Publication Tokukai No.
25 2005-097659

NT19328/PCT
- 5 -[Patent Literature 3]
Japanese Patent Application Publication Tokukai No. 2003-089846
[Patent Literature 4]
5 Japanese Patent Application Publication Tokukai No.
2000-328172
Summary of Invention
Technical Problem
10 [0008]
However, in recent years, it is required for deep-drawn formed articles that not only the height of the vertical wall part is as uniform as possible in the circumferential direction of the formed article (the variation in height is
15 small) but also the variation in sheet thickness of the
vertical wall part is small in the circumferential direction of the formed article. Thus, it is an object of an aspect of the present invention to provide, as a material having suitable material properties for obtaining such deep-drawn formed
20 articles, a high-carbon steel sheet that is soft and has small
in-plane anisotropy of the r-value, and a method for manufacturing the high-carbon steel sheet.
Solution to Problem [0009]
25 As a result of diligent studies, the inventors of the

NT19328/PCT
- 6 -
present invention arrived at the invention of the present
application by obtaining a novel finding about means of
allowing a high-carbon steel sheet containing C: not less
than 0.65% by mass and not more than 1.60% by mass both
5 to be softened (yield stress of not more than 400 MPa) and
to have small in-plane anisotropy of the r-value. More
specifically, the inventors of the present invention have
found that utilizing annealing through heating to a
temperature which is equal to or higher than the Ac1
10 transformation point enables a high-carbon steel sheet to be
softened appropriately and to have ferrite with random orientation effectively made in the metallographic structure. In addition, a method for manufacturing the high-carbon steel sheet in accordance with an aspect of the present
15 invention is applicable to a cold-rolled sheet obtained by
applying cold rolling (finishing cold rolling). It is thus possible to manufacture a high-carbon steel sheet that is soft and has small in-plane anisotropy of the r-value by using, as a material, semi-finished products of high-carbon
20 steel sheets that are made of various types of steels and that
have undergone various pretreatments. [0010]
That is, a high-carbon steel sheet in accordance with an aspect of the invention is a high-carbon steel sheet
25 including C: not less than 0.65% by mass and not more than

NT19328/PCT - 7 -
1.60% by mass, wherein the high-carbon steel sheet has a
yield stress of not more than 400 MPa, an in-plane
anisotropy index Δr of an r-value of not less than -0.05 and
not more than 0.05, and a difference between rmax and rmin
5 of not more than 0.1, wherein Δr=(r0-2r45 +r90)/2, and r0, r45,
and r90 are Lankford values in a 0° direction, a 45° direction, and a 90° direction, respectively, with respect to the rolling direction. The values rmax and rmin are the maximum value and the minimum value among r0, r45, and r90.
10 [0011]
Furthermore, a method for manufacturing a high-carbon steel sheet in accordance with an aspect of the invention includes: a cold rolling step of subjecting, to cold rolling at a rolling reduction rate of not less than 25%, a hot-
15 rolled steel sheet or an annealed steel sheet including C: not
less than 0.65% by mass and not more than 1.60% by mass to obtain a cold-rolled sheet; and an annealing step of subjecting the cold-rolled sheet to annealing such that the cold-rolled sheet is heated at a rate of temperature rise of
20 not less than 30°C/h in a temperature range from 400°C to
650°C, and the cold-rolled sheet is then held at an annealing temperature which is equal to or higher than an Ac1 transformation point, wherein the annealing temperature in the annealing step is not lower than the Ac1 transformation
25 point + 10°C and not more than the Ac1 transformation point

NT19328/PCT
- 8 -+ 60°C.
Advantageous Effects of Invention [0012]
An aspect of the present invention provides, as a
5 material having suitable material properties for obtaining
deep-drawn formed articles, a high-carbon steel sheet that
is soft and has small in-plane anisotropy of the r-value and
a method for manufacturing the high-carbon steel sheet.
10 Brief Description of Drawings
[0013]
(a) of Fig. 1 is a view for explaining a method for
manufacturing a high-carbon steel sheet in accordance with
an embodiment of the present invention, (b) of Fig. 1 is a
15 view for explaining a cold rolling step, and (c) of Fig. 1 is a
view for explaining a manner of annealing of a cold-rolled coil.
Fig. 2 is a photograph showing a formed article
obtained by press forming with use of a high-carbon steel
20 sheet in Present Invention Example.
Fig. 3 is a photograph showing a formed article obtained by press forming with use of a high-carbon steel sheet in Comparative Example.
25 Description of Embodiments

NT19328/PCT
- 9 -[0014]
The following description will discuss embodiments of
the present invention. Note that the following descriptions
are for better understanding the gist of the present invention,
5 and are not intended to limit the scope of the present
invention, unless otherwise specified. Furthermore, "A to B" in the present application indicates "not less than A and not more than B". [0015]
10 To begin with, the outline of the finding obtained by
the inventors of the present invention will be discussed as below. [0016]
First, a means of softening a high-carbon steel sheet
15 will be discussed. Common annealing of a high-carbon steel
sheet involves spheroidization of cementite (Fe3C) at a temperature which is lower than the Ac1 transformation point. In order to further soften the high-carbon steel sheet, spheroidization and coarsening of cementite are carried out
20 by annealing utilizing heating to a temperature which is
equal to or higher than the Ac1 transformation point. In this annealing, the heating to a temperature which is equal to or higher than the Ac1 transformation point partially dissolves the cementite to form a metallographic structure in which
25 undissolved cementite is dispersed in austenite, and then

NT19328/PCT
- 10 -
slow cooling is carried out. During the slow cooling, the
undissolved cementite grows and transformation progresses
in which the austenite dissolves into ferrite and cementite.
By carrying out slow cooling until the transformation is
5 completed, a metallographic structure is obtained in which
spheroidal and coarse cementite is dispersed in ferrite. As a result, the high-carbon steel sheet is softened. [0017]
Next, a basic means of achieving small in-plane
10 anisotropy of the Lankford value (hereinafter referred to as
the r-value) of a high-carbon steel sheet will be discussed.
In general, the r-value in a certain direction of a steel sheet
depends on the orientation state of crystallographic
orientation (the degree of orientation in a particular
15 direction) of each of many crystal grains of ferrite present in
the metallographic structure of the steel sheet. The
crystallographic orientation of the crystal grains in the
metallographic structure of the steel sheet is made by cold
rolling and recrystallization annealing. In this case, a high-
20 carbon steel sheet in which many recrystallized grains in a
particular crystallographic orientation are formed (having
texture) has large in-plane anisotropy of the r-value. On the
other hand, a high-carbon steel sheet in which many
recrystallized grains in random crystallographic orientation
25 are formed has small in-plane anisotropy of the r-value.

NT19328/PCT
- 11 -[0018]
A high-carbon steel sheet has a metallographic
structure in which a large amount of cementite is dispersed
in ferrite. When the high-carbon steel sheet undergoes cold
5 rolling, the strain caused by the cold rolling accumulates
mainly at the ferrite grain boundaries and the
ferrite/cementite interface. Then, when the high-carbon
steel sheet after cold rolling is subjected to recrystallization
annealing at or below the Ac1 transformation point, strain-
10 free recrystallized ferrite forms from the ferrite grain
boundaries and ferrite/cementite interface at which strain
has been accumulated. The recrystallized ferrite grows with
the passage of time. In this case, (i) ferrite having texture is
formed from the ferrite grain boundaries, and (ii) ferrite in
15 random orientation are formed from the ferrite/cementite
interface.
[0019]
Accordingly, the inventors of the present invention
conceived of, in applying recrystallization annealing to a
20 high-carbon steel sheet, increasing the ratio of recrystallized
ferrite forming from the ferrite/cementite interface can
improve the in-plane anisotropy of the r-value of the high-
carbon steel sheet after having been treated.
[0020]
25

NT19328/PCT
- 12 -
A high-carbon steel sheet in accordance with an
embodiment of the present invention that has been arrived
at based on the finding above will be discussed first, before
a method for manufacturing the high-carbon steel sheet in
5 accordance with an embodiment of the present invention will
be discussed in detail. [0021]
(Steel composition)
The following description will present steel
10 composition (chemical composition) of the high-carbon steel
sheet in accordance with an embodiment of the present invention will be described below. [0022]
(C)
15 The invention is directed to a high-carbon steel having
a C (carbon) content of not less than 0.65% by mass and not
more than 1.60% by mass in the steel. C is the most basic
alloying element for carbon steel, and the carbon content
greatly varies the amount of cementite and the
20 metallographic structure that is formed when it is heated to
a temperature which is equal to or higher than the Ac1
transformation point. Steel having a C content of less than
0.65% by mass has a small amount of cementite, and, when
heated to a temperature which is equal to or higher than the
25 Ac1 transformation point and held at or above the Ac1

NT19328/PCT
- 13 -
transformation point, will have a ferrite structure that is not
of a single-phase austenite and will have residual ferrite.
Accordingly, in a steel sheet having a C content of less than
0.65% by mass, ferrite grains having texture remain in a
5 crystal structure after annealing.
[0023]
On the other hand, if the C content exceeds 1.60% by mass, a hot-rolled steel sheet or an annealed steel sheet before being subjected to cold rolling will be hard and the
10 work hardening during the cold rolling will considerably
harden a cold-rolled sheet obtained after cold rolling. This makes the cold rolling difficult, lowers manufacturability and handleability, and makes it impossible to obtain sufficient ductility after final annealing. It is then difficult
15 to apply the steel sheet to parts that are worked to high
degrees. Accordingly, from the standpoint of providing a
material steel sheet presenting both appropriate
manufacturability and workability, the present invention is directed to high-carbon steel having a C content of not less
20 than 0.65% by mass and not more than 1.60% by mass.
[0024]
The C content is preferably not less than 0.7% by mass and not more than 1.2% by mass. The C content is preferably not less than 0.7% by mass for the purpose of forming a
25 single-phase austenite by heating to or above the Ac1

NT19328/PCT
- 14 -
transformation point, and allowing undissolved cementite to
readily remain in an amount suitable for spheroidization.
The C content is preferably not more than 1.2% by mass in
applications requiring higher workability.
5 [0025]
(Si)
Si (silicon) is an alloying element that acts as a deoxidizer in the process of manufacturing a steel sheet. This action cannot be obtained sufficiently if an Si content is less
10 than 0.02% by mass. On the other hand, Si is one of elements
that considerably affect the workability of an annealed steel
sheet. Excessively adding Si hardens ferrite due to solid-
solution strengthening, and may cause cracks during
forming of the annealed steel sheet. Further, increased Si
15 contents tend to cause scale defects on the surface of a steel
sheet during the manufacturing process, and thus lower surface quality of the steel sheet. Accordingly, Si is added in such a manner that the Si content is not more than 0.50% by mass. Thus, the Si content is preferably not less than
20 0.02% by mass and not more than 0.50% by mass, and more
preferably not less than 0.10% by mass and not more than 0.40% by mass. [0026]
(Mn)
25 Mn (manganese) is an alloying element that improves

NT19328/PCT
- 15 -
hardenability of steel and is added as needed. If an Mn
content exceeds 1.0% by mass, the steel sheet will be
hardened and its workability lowers. The Mn content is
preferably not more than 1.0% by mass, and more preferably
5 not less than 0.1% by mass and not more than 0.5% by mass.
[0027]
(Cr)
Cr (chromium) is an element that improves
hardenability of steel and increases tempering softening
10 resistance, and is added as needed. However, steel having a
high Cr content exceeding 1.8% by mass will not be easy to
soften even when annealed, and its workability before
hardening lowers. When Cr is added, it is therefore desirable
that the Cr content falls within a range of not more than
15 1.8% by mass. The Cr content is preferably from 0.1% by
mass to 1.6% by mass. [0028]
(P and S)
P (phosphorus) and S (sulfur) are alloying elements
20 that reduce toughness. Accordingly, to improve the
toughness, it is preferable that P and S contents are as low
as possible. When toughness of high-carbon steel parts used
as various mechanical parts is to be ensured, the P content
and the S content are each permitted up to 0.03% by mass.
25 The P content and the S content are each preferably not more

NT19328/PCT
- 16 -than 0.025% by mass, and more preferably not more than 0.020% by mass. [0029]
The present invention can be applied to steel to which
5 elements as shown below have been added in order to
improve properties such as hardenability and toughness. As
to the ranges not hindering formability, Mo can be added to
not more than 0.5% by mass, Cu can be added to not more
than 0.3% by mass, Ni can be added to not more than 2.0%
10 by mass, Al can be added to not more than 0.1% by mass, Ti
can be added to not more than 0.3% by mass, V can be added
to not more than 0.3% by mass, Nb can be added to not more
than 0.5% by mass, and B can be added to not more than
0.01% by mass.
15 [0030]
The remainder other than the above-mentioned
components contains Fe and unavoidable impurities. The
unavoidable impurities mean components that are difficult
to remove, such as O and N. These components are
20 unavoidably mixed in during stages to produce a steel piece
(slab). [0031]
(Properties)
The high-carbon steel sheet in accordance with an
25 embodiment of the present invention is such that a yield

NT19328/PCT
- 17 -
stress at room temperature is not more than 400 MPa, an in-
plane anisotropy index Δr of the Lankford value is not less
than -0.05 and not more than 0.05, and a difference between
rmax and rmin is not more than 0.1. Such mechanical
5 properties are realized by manufacturing a high-carbon steel
sheet in accordance with an embodiment of the present invention by a method (conditions) described later so that the high-carbon steel sheet has a metallographic structure and metallic texture formed of a particular annealed
10 structure.
[0032]
(i) Yield stress
The high-carbon steel sheet in accordance with an embodiment of the present invention is such that cementite
15 particles in the metallographic structure are relatively
spheroidal and coarse, and an interval between the cementite
particles is relatively large. A high-carbon steel sheet is such
that, as the interval between the cementite particles in the
metallographic structure is larger (as the number of
20 cementite particles per unit volume is smaller), soft ferrite
continuously exists in a larger part (region), and the steel
sheet can deform more easily when being worked. As a result,
the high-carbon steel sheet in accordance with an
embodiment of the present invention presents a yield stress
25 of not more than 400 MPa at room temperature (e.g. 20°C to

NT19328/PCT
- 18 -25°C). The yield stress may be measured by the testing method of JIS Z2241, for example. [0033]
(ii) Planar anisotropy index of Lankford value
5 The Lankford value (r-value) is an indicator that is
used to evaluate deformation anisotropy in a plate width direction and a plate thickness direction during working of a metal material. The Lankford value is also referred to as a plastic working strain ratio. Specifically, in a tensile test
10 using a plate-like test piece, the r-value of the plate-like test
piece is obtained based on the plate widths and plate thicknesses before and after the tensile test. However, since it is difficult to accurately grasp variation in plate thickness of a thin plate like a steel sheet (e.g. with a plate thickness
15 of about 1 mm), the r-value is obtained as below based on
the assumption that the volume is unchanged before and after the plastic working. [0034]
r=ln(W/W0)/ln(L0•W0/L•W)
20 where W0 and L0 are a plate width and a gauge length,
respectively, in the parallel portion of the plate-like test piece before the tensile test. Further, W and L are a plate width and a gauge length, respectively, in the parallel portion of the plate-like test piece after the tensile test.
25 [0035]

NT19328/PCT
- 19 -
The tensile test is usually carried out such that a
tensile strain of 10 to 20% is caused by the test, and the r¬
value obtained by this test is called the Lankford value. For
the high-carbon steel sheet in accordance with an
5 embodiment of the present invention, too, the Lankford value
is obtained based on the results of the tensile test that is carried out such that the tensile strain is 10% to 20%. In the description provided hereinafter in this specification, the r¬value means the Lankford value.
10 [0036]
Then, the in-plane anisotropy index Δr is obtained according to the following equation: [0037]
Δr=(r0-2r45+r90)/2.
15 The high-carbon steel sheet in accordance with an
embodiment of the present invention is manufactured through various rolling and annealing processes. Then, in the plane of the sheet, based on the rolling direction in the rolling work (the direction in which rotating rolls push out
20 the steel sheet), the r-value in a 0° direction with respect to
the rolling direction is referred to as r0. In the same way, r45 and r90 are the r-value in a 45° direction and the r-value in a 90° direction in the sheet plane, respectively, with respect to the rolling direction.
25 [0038]

NT19328/PCT
- 20 -
In the high-carbon steel sheet of the embodiment,
ferrite exists in random crystallographic orientation in the
metallographic structure, and the in-plane anisotropy index
Δr of the Lankford value is not less than -0.05 and not more
5 than 0.05. A value of Δr that is closer to 0 means smaller in-
plane anisotropy. [0039]
(iii) Maximum value and minimum value of Lankford
value
10 If the values of r0, r45, r90 become larger in this order
and if r0=0.8, r45=1, and r90=1.2, for example, then the value
of Δr is 0 (in the range of not less than -0.05 and not more
than 0.05). However, a difference between the maximum
value among the values of r0, r45, and r90 and the minimum
15 value among the values of r0, r45, and r90 is 0.4, in which
case it can be said that the in-plane anisotropy is large in
fact. Accordingly, the high-carbon steel sheet in accordance
with an embodiment of the present invention defines that the
absolute value of the difference between the maximum value
20 among the values of r0, r45, and r90 and the minimum value
among the values of r0, r45, and r90 is not more than 0.1.
[0040]

A method for manufacturing a high-carbon steel sheet
25 that is soft and has small in-plane anisotropy in accordance

NT19328/PCT
- 21 -
with an embodiment of the present invention will be
described with reference to Fig. 1. (a) of Fig. 1 is a view for
explaining a method for manufacturing a high-carbon steel
sheet in accordance with an embodiment of the present
5 invention, and shows an example of an annealing cycle. In
(a) of Fig. 1, the horizontal axis indicates time t and the vertical axis indicates temperature TE. The annealing cycle shown in (a) of Fig. 1 is merely an example, and specific annealing conditions (temperature control) can be changed
10 appropriately within a range where the conditions explained
below are satisfied. In Fig. 1, areas (1) to (5) surrounded by dotted lines are used as reference numerals for explaining the states at the respective time points. [0041]
15 As shown in (a) of Fig. 1, the method for manufacturing
a high-carbon steel sheet in accordance with an embodiment of the present invention includes: a cold rolling step (S1) of subjecting cold rolling to a hot-rolled steel sheet or an annealed steel sheet that is to be annealed; a first
20 temperature raising step (S2) of heating the cold-rolled sheet
in a heating furnace to raise the temperature up to around the Ac1 transformation point; and a second temperature raising step (S3) of, following the first temperature raising step, raising the temperature of the cold-rolled sheet to an
25 annealing temperature which is equal to or higher than the

NT19328/PCT
- 22 -
Ac1 transformation point. The method for manufacturing a
high-carbon steel sheet in accordance with an embodiment
of the present invention further includes: following the step
S3, a soaking step (S4) of heating the cold-rolled sheet to the
5 annealing temperature and holding the temperature; and a
slow cooling step (S5) of lowering the temperature of the annealed sheet that has been annealed through the steps S2 to S4. In this specification, the steps S2 to S5 above are collectively referred to as an annealing step. After the step
10 S5, the annealed sheet is cooled to room temperature to
obtain the high-carbon steel sheet in accordance with an embodiment of the present invention. These steps will be described below. [0042]
15 (Cold rolling step)
First, (i) a hot-rolled steel sheet that has undergone
hot rolling and then acid cleaning to remove scales, or (ii) an
annealed steel sheet obtained by subjecting primary
annealing to this hot-rolled steel sheet, is prepared. This
20 hot-rolled steel sheet or annealed steel sheet can be
produced by a common method. In general, a hot-rolled steel sheet or annealed steel sheet is manufactured as a coil. The primary annealing may be a process of holding the hot-rolled steel sheet at, for example, a temperature that is lower than
25 the Ac1 transformation point or a temperature that is equal

NT19328/PCT
- 23 -
to or higher than the Ac1 transformation point, so as to
spheroidize the cementite in the metallographic structure.
The hot-rolled steel sheet and the annealed steel sheet have
a steel composition of the above-described high-carbon steel
5 sheet in accordance with an embodiment of the present
invention. [0043]
The hot-rolled steel sheet has a metallographic
structure that is mainly formed of lamellar pearlite. The
10 annealed steel sheet has a metallographic structure mainly
formed of ferrite and spheroidized cementite. The hot-rolled
steel sheet and the annealed steel sheet have small strain
accumulated in the metallographic structure.
[0044]
15 In a method for manufacturing a high-carbon steel
sheet in accordance with an embodiment of the present
invention, the hot-rolled steel sheet or the annealed steel
sheet is subjected to cold rolling (finishing rolling). (b) of Fig.
1 is a view for explaining the cold rolling step S1.
20 [0045]
As shown in (b) of Fig. 1, a coil 1 of such a hot-rolled
steel sheet or annealed steel sheet is subjected to cold rolling
with a rolling reduction rate (reduction rate) of not less than
25% by using a cold rolling machine 2, to produce a cold-
25 rolled coil 3 of cold-rolled sheet. The cold rolling machine 2

NT19328/PCT
- 24 -
can be the one generally used for finishing rolling, and is,
for example, a Sendzimir cold-rolling machine or a tandem
rolling machine.
[0046]
5 When the hot-rolled steel sheet or annealed steel sheet
is subjected to cold rolling with a rolling reduction rate of
not less than 25%, recrystallized grains are formed not only
from the ferrite grain boundaries but also from the
ferrite/cementite interface during the occurrence of
10 recrystallization in the metallographic structure of the cold -
rolled sheet. It is not necessary to set a particular upper limit of the rolling reduction rate in the cold rolling step S1. However, if the rolling reduction rate exceeds 70%, then the steel sheet undergoes considerable work hardening, and an
15 increased number of passes of cold rolling leads to an
increase in cost. Further, in some cases, problems like cracks at the edges of the steel sheet may occur. [0047]
Further, in the cold rolling step S1, if cold rolling with
20 a rolling reduction rate exceeding 50% enables accumulation
of sufficient strain at the ferrite/cementite interface, then
the ratio of recrystallized ferrite forming from the
ferrite/cementite interface can be increased. However, the high-carbon steel sheet in accordance with an embodiment
25 of the present invention is likely to become very hard due to

NT19328/PCT
- 25 -work hardening, and it may therefore be difficult to apply rolling with a rolling reduction rate exceeding 50%. [0048]
Accordingly, in a method for manufacturing the high-
5 carbon steel sheet in accordance with an embodiment of the
present invention, the rolling reduction rate in the cold
rolling step S1 is preferably not more than 70% and more
preferably not more than 50%.
[0049]
10 (First temperature raising step)
(c) of Fig. 1 is a view for explaining a manner of annealing of the aforementioned cold-rolled coil 3 (i.e., the cold-rolled sheet). As shown in (c) of Fig. 1, the cold-rolled coil 3 is placed in a heating furnace 4, and the inside of the
15 furnace is heated so that the cold-rolled coil 3 is subjected
to box annealing (batch annealing). That is, the processes in the first temperature raising step S2 to the slow cooling step S5 are carried out within the heating furnace 4. The cold-rolled coil 3 (i.e., the cold-rolled sheet) to be annealed is
20 hereinafter referred to as an annealing target material.
[0050]
A relation between conditions defined in the first
temperature raising step S2 in accordance with an
embodiment of the present invention and the state ((2) in (a)
25 of Fig. 1) of the metallographic structure and

NT19328/PCT
- 26 -
crystallographic orientation of the annealing target material
will be described below. In the first temperature raising step
S2, heating is carried out in the temperature range from
400°C to 650°C at a rate of temperature rise of not less than
5 30°C/h. If the rate of temperature rise in the temperature
range from 400°C to 650°C is low, only the recovery of strain
progresses before the recrystallization temperature is
reached, and the formation of recrystallized grains having random orientation from the ferrite/cementite interface is
10 hindered. When the temperature is raised up to 650°C at a
rate of temperature rise of not less than 30°C/h, a lot of recrystallized grains are formed from the ferrite/cementite interface. While the recrystallization temperature of a high-carbon steel sheet is affected by the degree of work strain
15 and the alloying elements, heating to 650°C generally
completes recrystallization. Accordingly, the effect of
improving anisotropy is not affected even if slow heating or/and soaking is performed in the temperature range from 650°C to a temperature which is lower than the Ac1
20 transformation point, before heating the annealing target
material to the temperature which is equal to or higher than
the Ac1 transformation point. Accordingly, the first
temperature raising step S2 may include a process(es) of performing slow heating, soaking, or the like, after the
25 temperature has been raised to 650°C at a rate of

NT19328/PCT
- 27 -temperature rise of not less than 30°C/h. [0051]
(Second temperature raising step)
The state (3) of the metallographic structure and
5 crystallographic orientation of the annealing target material
in the second temperature raising step S3 in accordance with an embodiment of the present invention will be described below. In the second temperature raising step S3, the annealed target is heated to a temperature which is equal to
10 or higher than the Ac1 transformation point. In general,
when a high-carbon steel is heated to a temperature which is equal to or higher than the Ac1 transformation point, cementite is dissolved, and austenite is thus formed. In the second temperature raising step S3 in accordance with an
15 embodiment of the present invention, the ferrite having
random orientation formed from the ferrite/cementite
interface during the recrystallization in the temperature rise in the first temperature raising step S2 preferentially transforms into austenite.
20 [0052]
It is known that, in transformation from ferrite to
austenite, the formed austenite has a particular
crystallographic orientation relationship with the original
ferrite. Thus, the austenite formed in the second
25 temperature raising step S3 in accordance with an

NT19328/PCT
- 28 -
embodiment of the present invention has a similar
orientation relation to the ferrite in random orientation (the
austenite retains the crystallographic orientation of the
ferrite before transformation).
5 [0053]
(Soaking step)
A relation between conditions defined in the soaking step S4 of this embodiment and the state (4) of the metallographic structure and crystallographic orientation of
10 the annealing target material will be described below. In the
soaking step S4, the austenite having been formed in the second temperature raising step S3 grows as the cementite dissolved by soaking at or above the Ac1 transformation point, and a metallographic structure of austenite and
15 undissolved cementite is formed. Accordingly, even if, in the
recrystallization, recrystallized ferrite having texture is formed from the ferrite grain boundaries, austenite having an orientation relationship with this ferrite is not formed. This is because, due to the growth of the austenite formed
20 in the second temperature raising step S3, recrystallized
ferrite having texture is absorbed by this austenite
(transforms as if it is taken in). [0054]
The temperature for soaking in the soaking step S4,
25 which is equal to or higher than the Ac1 transformation point,

NT19328/PCT
- 29 -
will be referred to as an annealing temperature. The
annealing temperature in the soaking step S4 in accordance
with an embodiment of the present invention is not lower
than Ac1 transformation point + 10°C and not higher than
5 Ac1 transformation point + 60°C. The state of existence
(density) of undissolved cementite after the soaking step S4 determines the size of cementite particles of the high -carbon steel sheet in accordance with an embodiment of the present invention. This is because, in the slow cooling step S5, the
10 undissolved cementite grows spheroidally and coarsely and
austenite transforms into ferrite. [0055]
Since the amount of cementite after the annealing is determined by the C content, the interval between cementite
15 particles (the number of cementite particles per unit volume)
is determined when the size of cementite particles is determined. A continuous portion of soft ferrite becomes larger as the interval between cementite particles is larger, and then the steel can easily deform when worked. That is,
20 the annealed steel sheet becomes softer as the interval
between cementite particles becomes larger. [0056]
If the heating temperature which is equal to or higher
than the Ac1 transformation point is less than Ac1
25 transformation point + 10°C, then cementite dissolves

NT19328/PCT
- 30 -
insufficiently, in which case there will be a larger number of
undissolved cementite particles per unit volume and the
annealed steel sheet is not softened sufficiently.
[0057]
5 On the other hand, if the annealing temperature in the
soaking step S4 exceeds Ac1 transformation point + 60°C,
then cementite dissolves excessively and undissolved
cementite particles will become small in number or disappear. In the slow cooling step S5, undissolved cementite grows
10 during the slow cooling and phase transformation progresses,
forming a coarse, spheroidal cementite structure. This phase transformation is a phenomenon that accompanies diffusion of elements. Accordingly, if undissolved cementite particles are small in number, i.e. if the interval between cementite
15 particles is large, then elements cannot diffuse to
undissolved cementite in locations away from undissolved cementite. In this case, cementite core will be newly formed to form pearlite that is a lamellar structure of ferrite and cementite. Pearlite structure has inferior workability and
20 may experience breakage, for example in deep drawing. Thus,
in order to ensure workability of high-carbon steel sheet, it is necessary to set the annealing temperature in the soaking step S4 in the range of not lower than Ac1 transformation point + 10°C and not higher than Ac1 transformation point
25 + 60°C.

NT19328/PCT
- 31 -[0058]
(Slow cooling step)
A relation between conditions defined in the slow
cooling step S5 in accordance with an embodiment of the
5 present invention and the state (5) of the metallographic
structure and crystallographic orientation of the annealed target will be described below. In the slow cooling step S5, slow cooling from the heating temperature which is equal to or higher than the Ac1 transformation point is carried out.
10 With a decrease in temperature, undissolved cementite grows
spheroidally and coarsely, and austenite transforms into ferrite. In this process, the formed ferrite has a particular crystallographic orientation relationship with the original austenite. Accordingly, the ferrite in the final annealed
15 structure has the same crystallographic orientation (i.e.,
random orientation) as the ferrite formed from the
ferrite/cementite interface during the recrystallization. [0059]
In order to achieve sufficient spheroidization of
20 cementite, it is preferable to perform slow cooling at a
cooling rate of 5 to 30°C/h after the heating at or above the Ac1 transformation point, until the phase transformation is completed. If the cooling rate is less than 5°C/h, the annealing takes a very long time and hinders productivity. If
25 the cooling rate is higher than 30°C/h, then the diffusion of

NT19328/PCT
- 32 -
elements cannot occur enough and pearlite may be formed
even if sufficient undissolved cementite is left. From the
standpoint of productivity and workability of a steel sheet,
the cooling rate in the slow cooling step S5 is preferably not
5 less than 5°C/h and not more than 30°C/h.
[0060]
As described above, annealing utilizing heating to or above the Ac1 transformation point is performed with appropriate control of the temperature raising rate and the
10 annealing temperature, so that a high-carbon steel sheet
with improved in-plane anisotropy of the r-value is obtained. [0061]
(Advantages of invention)
In the present invention, in the annealing after cold
15 rolling with a rolling reduction rate of not less than 25%, the
cold-rolled sheet is heated in the temperature range from 400°C to 650°C at a rate of temperature rise of not less than 30°C/h, and then annealed at the annealing temperature which is not less than Ac1 transformation point + 10°C and
20 not more than Ac1 transformation point + 60°C. This has
achieved improvement of the in-plane anisotropy of the r¬value of the steel sheet of high-carbon steel. Specifically, a high-carbon steel sheet is obtained which has a yield stress of not more than 400 MPa and small in-plane anisotropy of
25 the r-value. The in-plane anisotropy index Δr of the r-value

NT19328/PCT
- 33 -
of the high-carbon steel sheet according to the present
invention is not less than -0.05 and not more than 0.05, and
a difference between rmax and rmin is not more than 0.1.
Formed articles with reduced variations in thickness and
5 diameter are obtained by using the high-carbon steel sheet
according to the invention in deep drawing. [0062]
(Remarks)
The present invention is not limited to the
10 embodiments described above, but can be altered by a person
skilled in the art within the scope of the claims. The present
invention also encompasses, in its technical scope, any
embodiment derived by combining technical means disclosed
in the above description.
15 [Example 1]
[0063]
Table 1 shows chemical compositions and Ac1
transformation points of prepared sample steels.

NT19328/PCT
- 34 -
[0064]
[Table 1]

Example Steel type Chemical components (% by mass) Ac1 transformation point (°C)

C Si Mn P S Cr Other elements

Comparative Example A 0.47 0.26 0.83 0.019 0.013 - - 722
Present Invention Example B 0.67 0.25 0.89 0.024 0.017 0.15 - 723
Present Invention Example C 0.76 0.05 0.54 0.011 0.012 0.52 - 727
Present Invention Example D 0.82 0.33 0.48 0.013 0.010 0.15 - 730
Present Invention Example E 0.83 0.09 0.36 0.006 0.005 - - 722
Present Invention Example F 0.88 0.25 0.42 0.011 0.003 0.48 Nb: 0.24 734
Present Invention Example G 1.02 0.21 0.39 0.009 0.005 - Ni: 0.98 708
Present Invention Example H 1.09 0.22 0.37 0.010 0.009 1.56 - 752
Present Invention Example I 1.20 0.10 0.54 0.019 0.011 - - 720
Present Invention Example J 1.56 0.12 0.40 0.003 0.006 0.06 - 723
Comparative Example K 1.90 0.20 0.33 0.009 0.014 - - 725

NT19328/PCT
- 35 -[0065]
Steels having the components in Table 1 were
subjected to hot rolling, and the steel sheets thus obtained
were pickled to remove scales. The hot-rolled steel sheets
5 obtained were subjected to primary annealing under the
condition (a) or (b) below. Some of the steel sheets were not subjected to the primary annealing but subjected to a subsequent step (primary annealing: none). Condition (a): Held at a temperature obtained by [Ac1
10 transformation point - 100°C to Ac1 transformation point]
for 10 h to 60 h
Condition (b): Held at a temperature obtained by [Ac1 transformation point to Ac1 transformation point + 50°C] for 4 h to 20 h, and then slowly cooled to or below the Ar1 point
15 at a cooling rate of not more than 30°C/h.
Note that the condition (b) above included cases in which the steels were held at a temperature which is equal to or lower than the Ac1 transformation point after and/or before being heated and held at or above the Ac1 transformation point.
20 [0066]
Further, the hot-rolled steel sheets and the annealed steel sheets that have been annealed under the condition (a) or (b) were subjected to finishing cold rolling at various rolling reduction rates and then finishing annealing under
25 various annealing conditions. Table 2 below shows the

NT19328/PCT
- 36 -
rolling reduction rates in the finishing cold rolling and the
annealing cycles of the finishing annealing. Then, the yield
stress and the r-value in-plane anisotropy of the annealed
sheets obtained were measured.
5 [0067]
The tensile test was carried out on JIS No. 5 test pieces for tensile test which were prepared so as to have three directions of L (rolling direction), D (45° with respect to the rolling direction), and T (90°with respect to the rolling
10 direction), a gauge length in the parallel portion of 50 mm,
and a plate thickness of 1.0 mm. In the tensile test, 10% tensile elongation was applied, the plate width in an area between gauge marks at the time of application of 10% tensile elongation was measured, and the r-value was
15 calculated by the following equation:
[0068]
r=ln(WX/W0)/ln(L0•W0/LX•WX) where W0 and L0 denote respectively the plate width and the gauge length before the test, respectively, and WX and LX
20 denote respectively the plate width and the gauge length
after the application of the 10% tensile elongation. [0069]
As an indicator of r-value in-plane anisotropy, the Δr value of each test specimen was calculated by the following
25 equation:

NT19328/PCT
- 37 -
Δr value=(r0-2r45+r90)/2
A Δr value closer to 0 indicates smaller anisotropy. Subscript
x of rx indicates the cutting direction of the test piece with
respect to the rolling direction. For example, r 45 indicates
5 the r-value measured with the test piece sampled in the 45°
direction with respect to the rolling direction. [0070]
Further, a difference rmax-rmin between the maximum
value rmax and minimum value rmin of the r-values in each
10 direction was calculated to evaluate the in-plane anisotropy
of the r-value. Furthermore, the yield stress was measured as an indicator of softening. [0071]
Table 2 shows rolling reduction rates of the finishing
15 cold rolling, finishing annealing conditions, r-value in-plane
anisotropy of an annealed material, and yield strength. [0072]

NT19328/PCT
- 38 -
[Table 2]

No. Steel type Primary annealing Rolling
reduction rate
of finishing
cold rolling
(%) Finishing annealing Ar value rmax-rmin Yield stress (MPa) Example

Rate of temperature rise in 400-650°C (°C/h) Annealing cycle

1 A Condition (a) 30 40 [740°C×5h]->15°C/h->630°C 0.11 0.36 304 Comparative Example
2 B Condition (b) 16 35 [690°C×5hM750°C×5h]->10°C/h->630°C 0.11 0.18 373 Comparative Example
3

40

-0.03 0.03 369 Present Invention Example
4



[710°C×20h] 0.17 0.31 425 Comparative Example
5 C None 28 10 [760°C×5h]->20°C/h->630°C 0.10 0.14 322 Comparative Example
6


50
-0.03 0.03 325 Present Invention Example
7 D Condition (a) 35 40 [755°C×7h]->10°C/h->650°C 0.05 0.06 317 Present Invention Example
8



[800°C×7h]->10°C/h->650°C -0.06 0.08 453 Comparative Example
9 E None 40 30 [750°C×7h]->5°C/h->650°C -0.02 0.02 299 Present Invention Example
10



[750°C×7h]->50°C/h->650°C -0.05 0.06 441 Comparative Example
11 F Condition (a) 30 20 [710°C×10hM755°C×7h] ->15°C/h->630°C 0.10 0.16 332 Comparative Example
12


40
0.01 0.02 338 Present Invention Example
13



[690°C×10h] 0.14 0.24 410 Comparative Example
Note: Underlined parts indicate falling outside the ranges specified by the present invention.

NT19328/PCT
- 39 -
[Table 2] (Cont.)

No. Steel type Primary annealing Rolling
reduction rate
of finishing
cold rolling
(%) Finishing annealing Ar value rmax-rmin Yield stress (MPa) Example

Rate of temperature rise in 400-650°C (°C/h) Annealing cycle

14 G Condition (b) 13 35 [700°C×15hM745°C×10h] ->10°C/h->630°C 0.10 0.13 356 Comparative Example
15

50

-0.03 0.03 348 Present Invention Example
16 H Condition (a) 30 15 [720°C×15hM790°C×8h]->5°C/h->680°C 0.13 0.20 336 Comparative Example
17


30
0.03 0.04 327 Present Invention Example
18


50
0.02 0.03 331 Present Invention Example
19
Condition (b) 40 30 [770°C×20h]->10°C/h->660°C 0.01 0.04 329 Present Invention Example
20



[730°C×35h] 0.19 0.36 377 Comparative Example
21 I Condition (a) 28 40 [745°C×15h]->15°C/h->650°C 0.02 0.03 339 Present Invention Example
22 J Condition (b) 30 40 [745°C×15h]->15°C/h->650°C 0.02 0.03 375 Present Invention Example
23 K Condition (b) 30 35 [750°C×20h]->5°C/h->660°C -0.04 0.04 468 Comparative Example
Note: Underlined parts indicate falling outside the ranges specified by the present invention.

NT19328/PCT
- 40 -[0073]
As shown in Table 2, Comparative Example No. 1 using
the steel type A with the C content lower than the C content
in the range falling within the scope of the present invention,
5 presented Δr being 0.11 and rmax-rmin being 0.36, which
indicates large in-plane anisotropy, even when cold rolling and annealing were carried out within the scope of the present invention. Comparative Example No. 23 using the steel type K with the C content higher than the C content in
10 the range falling within the scope of the present invention,
presented small in-plane anisotropy but yield stress as high as 468 MPa, when cold rolling and annealing were carried out within the scope of the present invention. It is found that sufficient softening through annealing is not achieved.
15 [0074]
In Comparative Examples (Nos. 2 and 14), the rolling reduction rate of cold rolling was lower than 25%, In Comparative Examples (Nos. 5, 11, and 16), the annealing after cold rolling was carried out such that the rate of
20 temperature rise in the range from 400°C to 650°C is less
than 30. In Comparative Examples (Nos. 4, 13, and 20) annealing not involving heating to or above the Ac1 transformation point was carried out. In all of these examples, the Δr value and rmax-rmin fall outside the scope of
25 the present invention. This indicates large in-plane

NT19328/PCT
- 41 -anisotropy. [0075]
Furthermore, in Comparative Example No. 8, the
heating temperature was high even when annealing utilizing
5 heating to or above the Ac1 transformation point was
performed in the Comparative Example No. 10, the cooling rate was high even when annealing utilizing heating to or above the Ac1 transformation point was performed. In these Examples, the yield stress exceeds 400 MPa, and formability
10 is thus poor. On the other hand, in Examples (Nos. 3, 6, 7,
9, 12, 15, 17, 18, 19, 21, and 22) in which steel components, cold rolling reduction rates, and annealing conditions are in the ranges falling within the scope of the present invention, both Δr value and rmax-rmin are in the ranges falling within
15 the scope of the present invention, and in-plane anisotropy
is small.
[Example 2] [0076]
A deep drawing test was carried out using the steel
20 sheets of Comparative Example No. 19 and Present Invention
Example No. 20 shown in Table 2. The plate thickness of the test pieces was 1 mm as in Example 1, and the blank diameter was 84 mm. The punch diameter was 40 mm, the shoulder R was 5 mm, and the die shoulder R was 5 mm. The
25 thickness of the vertical wall of a deep-drawn formed article

NT19328/PCT
- 42 -
at positions at a height of 25 mm from the bottom of the
deep-drawn formed article was measured using a micrometer.
The measurement positions were a total of eight positions at
a pitch of 45°, with the rolling direction of the material being
5 0°. In addition, the maximum and minimum values of the
diameter of the deep-drawn formed article at a height of 25
mm from the bottom of the deep-drawn formed article were
also measured. The measurement was carried out using a
laser width measuring instrument, while rotating the deep-
10 drawn formed article.
[0077]
Figs. 2 and 3 show the appearances of the deep-drawn
formed articles. (a) of Fig. 2 is a photograph showing the
deep-drawn formed article obtained by press forming with
15 use of the high-carbon steel sheet (No. 19) in Present
Invention Example, and (b) of Fig. 2 is a photograph of an
enlarged main part of the deep-drawn formed article. (a) of
Fig. 3 is a photograph showing the deep-drawn formed article
obtained by press forming with use of the high-carbon steel
20 sheet (No. 20) in Comparative Example, and (b) of Fig. 3 is a
photograph of an enlarged main part of the deep-drawn formed article. [0078]
As shown in Fig. 3, in a deep-drawn formed article 100
25 prepared with use of Comparative Example No. 20, relatively

NT19328/PCT
- 43 -
large height variations are observed in the vertical wall of
the deep-drawn formed article 100. On the other hand, in a
deep-drawn formed article 10 prepared with use of Present
Invention Example No. 19, it is found that height variations
5 in the vertical wall are very small.
[0079]
Table 3 shows the maximum and minimum values of
the thickness of the vertical wall at the 25 mm position from
the bottom of each deep-drawn formed article.
10 [0080]
[Table 3]
(mm)

Maximum value Minimum value Max – Min
No. 19 1.006 0.985 0.021
No. 20 1.032 0.968 0.064
[0081]
It is found that, as compared to Comparative Example
15 No. 20, Present Invention Example No. 19 presents a small
thickness variation in the circumferential direction of the deep-drawn formed article. [0082]
Table 4 shows the maximum and minimum values of
20 the diameter at the 25 mm position from the bottom of each
deep-drawn formed article. [0083]

- 44 -[Table 4]

NT19328/PCT

(mm)

Maximum value Minimum value Max – Min
No. 19 42.189 42.164 0.025
No. 20 42.206 42.088 0.118
[0084]
It is found that, as compared to Comparative Example
5 No. 20, Present Invention Example No. 19 presents a small
variation in the diameter of the deep-drawn formed article. As described above, it is found that a high-carbon steel sheet in accordance with the present invention achieve small in-plane anisotropy of the r-value and very small variations in
10 thickness and diameter in the circumferential direction of
the deep-drawn formed article obtained by deep drawing. Reference Signs List [0085] 1: coil
15 2: cold rolling machine
3: cold-rolled coil 4: heating furnace

WE CLAIMS

A high-carbon steel sheet comprising C: not less than
0.65% by mass and not more than 1.60% by mass,
5 wherein said high-carbon steel sheet has a yield stress
of not more than 400 MPa, an in-plane anisotropy index Δr
of an r-value of not less than -0.05 and not more than 0.05,
and a difference between rmax and rmin of not more than 0.1,
wherein Δr=(r0-2r45+r90)/2,
10 where r0 is a Lankford value in a 0° direction with
respect to a rolling direction,
r45 is a Lankford value in a 45° direction with respect to the rolling direction,
r90 is a Lankford value in a 90° direction with respect
15 to the rolling direction,
rmax is a maximum value among the Lankford values r0, r45, and r90, and
rmin is a minimum value among the Lankford values r0, r45, and r90. 20
Claim 2
The high-carbon steel sheet according to claim 1,
further comprising, in % by mass, Si: not less than 0.02%
and not more than 0.50%, Mn: not more than 1.0%, P: not
25 more than 0.03%, S: not more than 0.03%, and Cr: not more

NT19328/PCT
- 46 -than 1.8%.
Claim 3
A method for manufacturing a high-carbon steel sheet,
5 comprising:
a cold rolling step of subjecting, to cold rolling at a
rolling reduction rate of not less than 25%, a hot-rolled steel
sheet or an annealed steel sheet comprising C: not less than
0.65% by mass and not more than 1.60% by mass to obtain
10 a cold-rolled sheet; and
an annealing step of subjecting the cold-rolled sheet
to annealing such that the cold-rolled sheet is heated at a
rate of temperature rise of not less than 30°C/h in a
temperature range from 400°C to 650°C, and the cold-rolled
15 sheet is then held at an annealing temperature which is
equal to or higher than an Ac1 transformation point,
wherein the annealing temperature in the annealing step is not lower than the Ac1 transformation point + 10°C and not higher than the Ac1 transformation point + 60°C. 20
Claim 4
The method according to claim 3, wherein the hot-
rolled steel sheet or the annealed steel sheet further
comprises, in % by mass, Si: not less than 0.02% and not
25 more than 0.50%, Mn: not more than 1.0%, P: not more than

NT19328/PCT
- 47 -0.03%, S: not more than 0.03%, and Cr: not more than 1.8%.