METHOD FOR PRODUCING COLD-ROLLED STEEL SHEET
Technical Field
The present invention relates to a method for producing a cold-rolled steel
5 sheet. More particularly, it relates to a method for producing a cold-rolled steel
sheet that is used in various shapes formed by press forming or the like process,
especially, a high-tensile cold-rolled steel sheet that is excellent in ductility, work
hardening property, and stretch flanging property.
1 0 Background Art
In these days when the industrial technology field is highly fractionalized,
a material used in each technology field has been required to deliver special and
high performance. For example, for a cold-rolled steel sheet that is worked by
press forming and put in use, more excellent formability has been required with
15 the diversification of press shapes. In addition, as a high strength has been
required, the use of a high-tensile cold-rolled steel sheet has been studied. In
particular, concerning an automotive steel sheet, in order to reduce the vehicle
body weight and thereby to improve the fuel economy from the perspective of
global environments, a demand for a high-tensile cold-rolled steel sheet having
20 thin-wall high formability has been increasing remarkably. In press forming, as
the thickness of steel sheet used is smaller, cracks and wrinkles are liable to
occur. Therefore, a steel sheet further excellent in ductility and stretch flanging
property is required. However, the press formability and the high strengthening
of steel sheet are characteristics contrary to each other, and therefore it is difficult
25 to satisfy these characteristics at the same time.
As a method for improving the press formability of a high-tensile coldrolled
steel sheet, many techniques concerning grain refinement of microstructure
have been proposed. For example, Patent Document 1 discloses a
method for producing a very fine grain high-strength hot-rolled steel sheet that is
30 subjected to rolling at a total draft of 80% or higher in a temperature region in the
vicinity of Ar3 point in the hot-rolling process. Patent Document 2 discloses a
method for producing an ultrafine ferritic steel that is subjected to continuous
rolling at a draft of 40% or higher in the hot-rolling process.
By these techniques, the balance between strength and ductility of hotrolled
steel sheet is improved. However, the above-described Patent
5 Documents do not at all describe a method for making a fine-grain cold-rolled
steel sheet to improve the press formability. According to the study conducted
by the present inventors, if cold rolling and annealing are performed on the finegrain
hot-rolled steel sheet obtained by high reduction rolling being a base metal,
I
I I the crystal grains are liable to be coarsened, and it is difficult to obtain a cold-
I
I 10 rolled steel sheet excellent in press formability. In particular, in the
manufacturing of a composite-structure cold-rolled steel sheet containing a lowtemperature
transformation producing phase or retained austenite in the metallic
structure, which must be annealed in the high-temperature region of Acl point or
higher, the coarsening of crystal grains at the time of annealing is remarkable,
15 and the advantage of composite-structure cold-rolled steel sheet that the ductility
is excellent cannot be enjoyed.
Patent Document 3 discloses a method for producing a hot-rolled steel
sheet having ultrafine grains, in which method, rolling reduction in the dynamic
recrystallization region is performed with a rolling reduction pass of five or more
20 stands. However, the lowering of temperature at the hot-rolling time must be
decreased extremely, and it is difficult to carry out this method in a general hotrolling
equipment. Also, although Patent Document 3 describes an example in
which cold rolling and annealing are performed after hot rolling, the balance
between tensile strength and bore expandability is poor, and the press formability
25 is insufficient.
Concerning the cold-rolled steel sheet having a fine structure, Patent
Document 4 discloses an automotive high-strength cold-rolled steel sheet
excellent in collision safety and formability, in which retained austenite having
an average crystal grain size of 5 pm or smaller is dispersed in ferrite having an
30 average crystal grain size of 10 pm or smaller. The steel sheet containing
retained austenite in the metallic structure exhibits a large elongation due to
transformation induced plasticity (TRIP) produced by the martensitizing of
austenite during working; however, the bore expandability is impaired by the
formation of hard martensite. For the cold-rolled steel sheet disclosed in Patent
Document 4, it is supposed that the ductility and bore expandability are improved
by making ferrite and retained austenite fine. However, the bore expanding
5 ratio is at most 1.5, and it is difficult to say that sufficient press formability is
provided. Also, to enhance the work hardening index and to improve the
collision safety, it is necessary to make the main phase a soft ferrite phase, and it
is difficult to obtain a high tensile strength.
Patent Document 5 discloses a high-strength steel sheet excellent in
10 elongation and stretch flanging property, in which the secondary phase consisting
of retained austenite andlor martensite is dispersed finely within the crystal
grains. However, to make the secondary phase fine to a nano size and to
disperse it within the crystal grains, it is necessary to contain expensive elements
such as Cu and Ni in large amounts and to perform solution treatment at a high
15 temperature for a long period of time, so that the rise in production cost and the
decrease in productivity are remarkable.
Patent Document 6 discloses a high-tensile hot dip galvanized steel sheet
excellent in ductility, stretch flanging property, and fatigue resistance property, in
which retained austenite and low-temperature transformation producing phase
20 are dispersed in ferrite having an average crystal grain size of 10 pm or smaller
and in tempered martensite. The tempered martensite is a phase that is effective
in improving the stretch flanging property and fatigue resistance property, and it
is supposed that if grain refinement of tempered martensite is performed, these
properties are further improved. However, in order to obtain a metallic
25 structure containing tempered martensite and retained austenite, primary
annealing for forming martensite and secondary annealing for tempering
martensite and further for obtaining retained austenite are necessary, so that the
productivity is impaired significantly.
Patent Document 7 discloses a method for producing a cold-rolled steel
30 sheet in which retained austenite is dispersed in fine ferrite, in which method, the
steel sheet is cooled rapidly to a temperature of 720°C or lower immediately after
being hot-rolled, and is held in a temperature range of 600 to 720°C for 2
seconds or longer, and the obtained hot-rolled steel sheet is subjected to cold
rolling and annealing.
Patent Document
5 Patent Document 1 : JP 58- 123 823 A1
Patent Document 2: JP 59-2294 13 A1
Patent Document 3: JP 1 1-1 52544 A1
Patent Document 4: JP 1 1-6 1326 A1
Patent Document 5: JP 2005-179703 A1
10 Patent Document 6: JP 2001-192768 A1
Patent Document 7: ~ 0 2 0 0 71/5 5 4 1 A 1
Summary of Invention
The above-described technique disclosed in Patent Document 7 is
15 excellent in that a cold-rolled steel sheet in which a fine grain structure is formed
and the workability and thermal stability are improved can be obtained by a
process in which after hot rolling has been finished, the work strain accumulated
in austenite is not released, and ferrite transformation is accomplished with the
work strain being used as a driving force.
20 However, due to needs for higher performance in recent years, a coldrolled
steel sheet provided with a high strength, good ductility, excellent work
hardening property, and excellent stretch flanging property at the same time has
come to be demanded.
The present invention has been made to meet such a demand.
25 Specifically, an objective of the present invention is to provide a method for
producing a high-tensile cold-rolled steel sheet having excellent ductility, work
hardening property, and stretch flanging property, in which the tensile strength is
780 MPa or higher.
The present inventors performed detailed investigations of the influence of
30 chemical composition and manufacturing conditions exerted on the mechanical
properties of a high-tensile cold-rolled steel sheet. In this description, symbol
-2 -
e
"%" indicating the content of each element in the chemical composition of steel
means mass percent.
A series of sample steels had a chemical composition consisting, in mass
percent, of C: more than 0.020% and less than 0.30%, Si: more than 0.10% and
5 3.00% or less, Mn: more than 1.00% and 3.50% or less, P: 0.10% or less, S:
0.010% or less, sol.Al: 2.00% or less, and N: 0.010% or less.
A slab having the above-described chemical composition was heated to
1200°C, and thereafter was hot-rolled so as to have a thickness of 2.0 mm in
various rolling reduction patterns in the temperature range of AT3 point or higher.
10 After being hot-rolled, the steel sheets were cooled to the temperature region of
780°C or lower under various cooling conditions. After being air-cooled for 5
to 10 seconds, the steel sheets were cooled to various temperatures at a cooling
rate of 90°C/s or lower. This cooling temperature was used as the coiling
temperature. After the steel sheets had been charged into an electric heating
15 fbrnace held at the same temperature and had been held for 30 minutes, the steel
sheets were furnace-cooled at a cooling rate of 20°C/h, whereby the gradual
cooling after coiling was simulated. Some of the hot-rolled steel sheets thus
obtained were heated to various temperatures, and thereafter were cooled,
whereby hot-rolled and annealed steel sheets were obtained. The hot-rolled
20 steel sheets or the hot-rolled and annealed steel sheets were subjected to pickling
and cold-rolled at a draft of 50% so as to have a thickness of 1.0 mm. Using a
continuous annealing simulator, the obtained cold-rolled steel sheets were heated
to various temperatures and held for 95 seconds, and thereafter cooled to obtain
annealed steel sheets.
25 From each of hot-rolled steel sheets, hot-rolled and annealed steel sheets,
and annealed steel sheets, a test specimen for structure observation was sampled.
By using a scanning electron microscope (SEM) equipped with an optical
microscope and an electron backscatter diffraction pattern (EBSP) analyzer, the
metallic structure was observed at a position deep by one-fourth of thickness
30 from the surface of steel sheet, and by using an X-ray diffractometry (XRD)
apparatus, the volume ratio of retained austenite was measured at a position deep
by one-fourth of thickness from the surface of annealed steel sheet. Also, from
the annealed steel sheet, a tensile test specimen was sampled along the direction
perpendicular to the rolling direction. By using this tensile test specimen, a
tension test was conducted, whereby the ductility was evaluated by total
elongation, and the work hardening property was evaluated by the work
hardening index (n value) in the strain range of 5 to 10%. Further, from the
annealed steel sheet, a 100-mm square bore expanding test specimen was
sampled. By using this test specimen, a bore expanding test was conducted,
whereby the stretch flanging property was evaluated. In the bore expanding test,
a 10-mm diameter punched hole was formed with a clearance being 12.5%, the
punched hole was expanded by using a cone-shaped punch having a front edge
angle of 60°, and the expansion ratio (bore expanding ratio) of the hole at the
time when a crack penetrating the sheet thickness was generated was measured.
As the result of these preliminary tests, the findings described in the
following items (A) to (I) were obtained.
(A) If the hot-rolled steel sheet, which is produced through a so-called
immediate rapid cooling process where rapid cooling is performed by water
cooling immediately after hot rolling, specifically, the hot-rolled steel sheet is
produced in such a way that the steel is rapidly cooled to the temperature region
of 780°C or lower within 0.40 second after the completion of hot rolling, is coldrolled
and annealed, the ductility and stretch flanging property of annealed steel
sheet are improved with the rise in annealing temperature. However, if the
annealing temperature is too high, the austenite grains are coarsened, and the
ductility and stretch flanging property of annealed steel sheet may be deteriorated
abruptly.
(B) By controlling the hot-rolling conditions, the grains each having a bcc
structure and the grains each having a bct structure (hereinafter, these grains are
also generally called "bcc grains") in the hot-rolled steel sheet or the hot-rolled
and annealed steel sheet, which is obtained by annealing the said hot-rolled steel
sheet, (in the present invention, the hot-rolled steel sheet subjected to annealing
is referred to as a "hot-rolled and annealed steel sheet") are made fine, which
restrains the coarsening of austenite grains that may occur when annealing is
performed at high temperatures after cold rolling. The reason for this is
unclear; however, it is presumed to be attributable to the fact that, since the
crystal grain boundary of bcc grains functions as a nucleation site of austenite on
account of transformation at the annealing time after cold rolling, the nucleation
frequency is raised by the refinement of bcc grains, and even if the annealing
5 temperature is high, the coarsening of austenite grains is restrained.
(C) If iron carbides are precipitated finely in the hot-rolled steel sheet or
the hot-rolled and annealed steel sheet, the coarsening of austenite grains that
may occur when annealing is performed at high temperatures after cold rolling is
restrained. The reason for this is unclear; however, it is presumed to be
10 attributable to the fact that (a) since iron carbides function as a nucleation site in
the reverse transformation to austenite during annealing after cold rolling, as the
iron carbides precipitate more finely, the nucleation frequency is raised, and the
austenite grains are made fine, and (b) since the undissolved iron carbides
restrain the grain growth of austenite, the austenite grains are made fine.
15 (D) If the final roll draft of hot rolling is increased, the coarsening of
austenite grains that may occur when annealing is performed at high
temperatures after cold rolling is restrained. The reason for this is unclear;
however, it is presumed to be attributable to the fact that (a) with the increase in
final roll draft, the bcc grains in the hot-rolled steel sheet or the hot-rolled and
20 annealed steel sheet is made fine, and (b) with the increase in final roll draft, the
iron carbides are made h e , and the number density thereof increases.
(E) In the coiling process after immediate rapid cooling, if the coiling
temperature is raised to a temperature exceeding 400°C, the coarsening of
austenite grains that may occur when annealing is performed at high
25 temperatures after cold rolling is restrained. The reason for this is unclear;
however, it is presumed to be attributable to the fact that since the grains of hotrolled
steel sheet are made fine by immediate rapid cooling, with the rise in
coiling temperature, the precipitation amount of iron carbides in the hot-rolled
steel sheet increases remarkably.
(F) Even if the hot-rolled steel sheet produced with the coiling temperature
being made a low temperature of lower than 400°C in the coiling process after
immediate rapid cooling is subjected to hot-rolled sheet annealing in which the
hot-rolled steel sheet is heated to the temperature region of 300°C or higher, the
coarsening of austenite grains that may occur when annealing is performed at
high temperatures after cold rolling is restrained. The reason for this is unclear;
however, it is presumed to be attributable to the fact that since the lowtemperature
transformation producing phase in the metallic structure of hotrolled
steel sheet is made fine by immediate rapid cooling, if the hot-rolled steel
sheet is annealed, iron carbides precipitate finely within the low-temperature
transformation producing phase.
(G) As the Si content in the steel increases, the effect of preventing the
coarsening of austenite grains becomes stronger. The reason for this is unclear;
however, it is presumed to be attributable to the fact that with the increase in Si
content, the iron carbides are made fine, and the number density thereof increases.
(H) If the steel sheet is soaked at a high temperature while the coarsening
of austenite grains is restrained and is cooled, a metallic structure is obtained in
which the main phase is a fine low-temperature transformation producing phase,
the secondary phase contains fine retained austenite, and coarse austenite grains
are few.
Figure 1 is a graph showing the result of investigation of grain size
distribution of retained austenite in an annealed steel sheet obtained by hotrolling
under the conditions of the final roll draft of 42% in thickness decrease
percentage, the rolling finishing temperature of 900°C, the rapid cooling stop
temperature of 660°C, and the immediate rapid cooling process of 0.16 seconds
from rolling completion to rapid cooling stop, and cold rolling with the coiling
temperature of 520°C, followed by annealing at a soaking temperature of 850°C.
Figure 2 is a graph showing the result of investigation of grain size distribution
of retained austenite in an annealed steel sheet obtained by hot-rolling a slab
having the same chemical composition by using an ordinary method without the
immediate rapid cooling process, and by cold rolling and annealing the hot-rolled
steel sheet. From the comparison of Figure 1 and Figure 2, it can be seen that,
for the annealed steel sheet produced through a proper immediate rapid cooling
process (Figure l), the formation of coarse austenite grains is restrained, and
retained austenite is dispersed finely.
(I) The cold-rolled steel sheet having such a metallic structure exhibits not
only high strength but also excellent ductility, work hardening property, and
stretch flanging property.
From the above-described results, it was revealed that a hot-rolled steel
5 sheet or a hot-rolled and annealed steel sheet having a fine metallic structure,
which is obtained by hot-rolling a steel containing a certain amount or more of Si
with the final draft being increased, thereafter by subjecting the hot-rolled steel
sheet to immediate rapid cooling, by either coiling the steel sheet at a high
temperature or coiling the steel sheet at a low temperature and then by subjecting
I
10 the steel sheet to hot-rolled sheet annealing, is cold-rolled, and the obtained cold-
I
i rolled steel sheet is annealed at a high temperature, and thereafter is cooled,
I
I whereby a cold-rolled steel sheet excellent in ductility, work hardening property,
and stretch flanging property, which has a metallic structure such that the main
phase is a low-temperature transformation producing phase, the secondary phase
15 contains fine retained austenite, and coarse austenite grains are few, can be
I
i produced.
In one aspect, the present invention provides a method for producing a
cold-rolled steel sheet having a metallic structure such that the main phase is a
low-temperature transformation producing phase, and the secondary phase
20 contains retained austenite, characterized in that the method has the following
processes (A) and (B) (first invention):
(A) a cold-rolling step in which a hot-rolled steel sheet having a chemical
composition consisting, in mass percent, of C: more than 0.020% and less than
0.30%, Si: more than 0.10% and at most 3.00%, Mn: more than 1.00% and at
25 most 3 SO%, P: at least 0. lo%, S: at most 0.01 0%, sol.Al: at least 0% and at most
2.00%, N: at most 0.010%, Ti: at least 0% and less than 0.050%, Nb: at least 0%
and less than 0.050%, V: at least 0% and at most 0.50%, Cr: at least 0% and at
most 1.0%, Mo: at least 0% and at most 0.50%, B: at least 0% and at most
0.010%, Ca: at least 0% and at most 0.010%, Mg: at least 0% and at most
30 0.010%, REM: at least 0% and at most 0.050%, and Bi: at least 0% and at most
0.050%, the remainder of Fe and impurities, wherein the average grain size of the
grains having a bcc structure and the grains having a bct structure surrounded by
- Jd-
, e
a grain boundary having an orientation difference of 15" or larger is 6.0 pm or
smaller, is subjected to cold rolling to form a cold-rolled steel sheet; and
(B) an annealing process in which the cold-rolled steel sheet is subjected
to soaking treatment in the temperature region of (Ac3 point - 40°C) or higher,
5 thereafter cooled to the temperature region of 500°C or lower and 300°C or
higher, and is held in that temperature region for 30 seconds or longer.
The hot-rolled steel sheet is preferably a steel sheet in which the average
number density of iron carbides existing in the metallic structure is 1.0 x 10-
'lPm2 or higher.
10 In another aspect, the present invention provides a method for producing a
cold-rolled steel sheet having a metallic structure such that the main phase is a
low-temperature transformation producing phase, and the secondary phase
contains retained austenite, characterized in that the method has the following
processes (C) to (E) (second invention):
15 (C) a hot-rolling process in which a slab having the above-described
chemical composition is subjected to hot rolling such that the roll draft of the
final one pass is higher than 15%, and rolling is finished in the temperature
region of AT3 point or higher to form a hot-rolled steel sheet, and the hot-rolled
steel sheet is cooled to the temperature region of 780°C or lower within 0.4
20 seconds after the completion of the rolling, and is coiled in the temperature
region of higher than 400°C;
(D) a cold-rolling process in which the hot-rolled steel sheet obtained by
the above-described process (C) is subjected to cold rolling to form a cold-rolled
steel sheet; and
25 (E) an annealing process in which the cold-rolled steel sheet is subjected to
soaking treatment in the temperature region of (Ac3 point - 40°C) or higher,
thereafter cooled to the temperature region of 500°C or lower and 300°C or
higher, and is held in that temperature region for 30 seconds or longer.
In still another aspect, the present invention provides a method for
30 producing a cold-rolled steel sheet having a metallic structure such that the main
-He
phase contains retained austenite, characterized in that the method has the
following processes (F) to (I) (third invention):
(F) a hot-rolling process in which a slab having the above-described
chemical composition is subjected to hot rolling such that the rolling is finished
5 in the temperature region of AT3 point or higher to form a hot-rolled steel sheet,
and the hot-rolled steel sheet is cooled to the temperature region of 780°C or
lower within 0.4 seconds after the completion of the rolling, and is coiled in the
temperature region of lower than 400°C;
1
I
I (G) a hot-rolled sheet annealing process in which the hot-rolled steel sheet
I 10 obtained by the process (F) is subjected to annealing such that the hot-rolled steel
I
! I
sheet is heated to the temperature region of 300°C or higher to form a hot-rolled
I and annealed steel sheet;
(H) a cold-rolling process in which the hot-rolled and annealed steel sheet
1 is subjected to cold rolling to form a cold-rolled steel sheet; and
15 (I) an annealing process in which the cold-rolled steel sheet is subjected to
soaking treatment in the temperature region of (Ac3 point - 40°C) or higher,
thereafter cooled to the temperature region of 500°C or lower and 300°C or
higher, and is held in that temperature region for 30 seconds or longer.
In the metallic structure of the cold-rolled steel sheet, the secondary phase
20 preferably contains retained austenite and polygonal ferrite.
In the cold-rolling process (A), (D) or (H), the cold rolling is preferably
performed at a total draft exceeding 50%.
In the annealing process (B), (E) or (I), preferably, the soaking treatment is
performed in the temperature region of (Ac3 point - 40°C) or higher and lower
25 than (Ac3 point + 50°C), and/or the cooling is performed by 50°C or more at a
cooling rate of lower than 10.O°C/s after the soaking treatment.
In the preferred mode, the chemical composition further contains at least
one kind of the elements (% means mass percent) described below.
One kind or two or more kinds selected from a group consisting of Ti: at
30 least 0.005% and less than 0.050%, Nb: at least 0.005% and less than 0.050%,
and V: at least 0.0 10% and at most 0.50%; and/or
0
One kind or two or more kinds selected from a group consisting of Cr: at
most 0.20% and at most 1.0%, Mo: at least 0.05% and at most 0.50%, and B: at
least 0.00 10% and at most 0.0 10%; andlor
One kind or two or more kinds selected from a group consisting of Ca: at
5 least 0.0005% and at most 0.010%, Mg: at least 0.0005% and at most 0.010%,
REM: at least 0.0005% and at most 0.050%, and Bi: at least 0.0010% and at most
0.050%.
According to the present invention, a high-tensile cold-rolled steel sheet
having sufficient ductility, work hardening property, and stretch flanging
10 property, which can be used for working such as press forming, can be produced.
Therefore, the present invention can greatly contribute to the development of
industry. For example, the present invention can contribute to the solution to
global environment problems through the lightweight of automotive vehicle body.
1 5 Brief Description of Drawings
[Figure 11 Figure 1 is a graph showing grain size distribution of retained
I austenite in an annealed steel sheet produced through an immediate rapid cooling
process.
[Figure 21 Figure 2 is a graph showing grain size distribution of retained
20 austenite in an annealed steel sheet produced without an immediate rapid cooling
process.
Description of Embodiments
The metallic structure and chemical composition in a high-tensile cold-
25 rolled steel sheet produced by the method in accordance with the present
invention, and the rolling and annealing conditions and the like in the method in
accordance with the present invention capable of producing the steel sheet
efficiently, steadily, and economically are described in detail below.
30 1. Metallic structure
The cold-rolled steel sheet of the present invention has a metallic structure
such that the main phase is a low-temperature transformation producing phase,
1 -&- I @
and the secondary phase contains retained austenite. This is because such a
metallic structure is preferable for improving the ductility, work hardening
I
I property, and stretch flanging property while the tensile strength is kept. If the
I
1 main phase is polygonal ferrite that is not a low-temperature transformation 1 5 producing phase, it is difficult to assure the tensile strength and stretch flanging
property.
The main phase means a phase or structure in which the volume ratio is at
the maximum, and the secondary phase means a phase or structure other than the
main phase. The low-temperature transformation producing phase means a
10 phase and structure formed by low-temperature transformation, such as
martensite and bainite. As a low-temperature transformation producing phase
other than these, bainitic ferrite and tempered martensite are cited. The bainitic
ferrite is distinguished from polygonal ferrite in that a lath shape or a plate shape
is taken and that the dislocation density is high, and is distinguished from bainite
15 in that iron carbides do not exist in the interior and at the interface. This lowtemperature
transformation producing phase may contain two or more kinds of
phases and structures, for example, martensite and bainitic ferrite. In the case
where the low-temperature transformation producing phase contains two or more
kinds of phases and structures, the sum of volume ratios of these phases and
20 structures is defined as the volume ratio of the low-temperature transformation
producing phase.
To improve the ductility, the volume ratio of retained austenite to total
structure preferably exceeds 4.0%. This volume ratio fbrther preferably
exceeds 6.0%, still further preferably exceeds 9.0%, and most preferably exceeds
25 12.0%. On the other hand, if the volume ratio of retained austenite is excessive,
the stretch flanging property deteriorates. Therefore, the volume ratio of
retained austenite is preferably lower than 25.0%, hrther preferably lower than
18.0%, still hrther preferably lower than 16.0%, and most preferably lower than
14.0%.
3 0 In the cold-rolled steel sheet having a metallic structure such that the main
phase is a low-temperature transformation producing phase, and the secondary
phase contains retained austenite, if the grains of retained austenite are made fine,
the ductility, work hardening property, and stretch flanging property are
improved remarkably. Therefore, the average grain size of retained austenite is
preferably made smaller than 0.80 pm. This average grain size is further
preferably made smaller than 0.70 pm, still further preferably made smaller than
0.60 pm. The lower limit of the average grain size of retained austenite is not
subject to any special restriction; however, in order to make the average grain
size 0.15 pm or smaller, it is necessary to greatly increase the final roll draft of
hot rolling, which leads to a remarkably increased production load. Therefore,
the lower limit of the average grain size of retained austenite is preferably made
larger than 0.15 pm.
In the cold-rolled steel sheet having a metallic structure such that the main
phase is a low-temperature transformation producing phase, and the secondary
phase contains retained austenite, even if the average grain size of retained
austenite is small, if coarse retained austenite grains exist in large amounts, the
work hardening property and stretch flanging property are liable to be impaired.
Therefore, the number density of retained austenite grains each having a grain
size of 1.2 pm or larger is preferably made 3.0 x 1 0 - ~ / ponr ~lo wer. This
number density is further preferably 2.0 x 10-~lpmor~ l ower, still further
preferably 1.5 x 10 '~1prno~r lower, and most preferably 1.0 x 10 -~1prnor~ l ower.
To further improve the ductility and work hardening property, the
secondary phase preferably contains polygonal ferrite in addition to retained
austenite. The volume ratio of polygonal ferrite to total structure preferably
exceeds 2.0%. This volume ratio further preferably exceeds 8.0%, still further
preferably exceeds 13.0%. On the other hand, if the volume ratio of polygonal
ferrite is excessive, the stretch flanging property deteriorates. Therefore, the
volume ratio of polygonal ferrite is preferably lower than 27.096, further
preferably lower than 24.0%, and still further preferably lower than 18.0%.
As the grains of polygonal ferrite are finer, the effect of improving the
ductility and work hardening property increases. Therefore, the average crystal
grain size of polygonal ferrite is preferably made smaller than 5.0 pm. This
average crystal grain size is further preferably smaller than 4.0 pm, still further
preferably smaller than 3.0 pm.
To further improve the stretch flanging property, the volume ratio of
tempered martensite contained in the low-temperature transformation producing
phase to total structure is preferably made lower than 50.0%. This volume ratio
is further preferably lower than 3 5.0%, still hrther preferably lower than 10.0%.
To enhance the tensile strength, the low-temperature transformation
producing phase preferably contain martensite. In this case, the volume ratio of
martensite to total structure preferably exceeds 4.0%. This volume ratio further
preferably exceeds 6.0%, still hrther preferably exceeds 10.0%. On the other
hand, if the volume ratio of martensite is excessive, the stretch flanging property
deteriorates. Therefore, the volume ratio of martensite to total structure is
preferably made lower than 15.0%.
The metallic structure of the cold-rolled steel sheet in accordance with the
present invention is measured as described below. The volume ratios of lowtemperature
transformation producing phase and polygonal ferrite are determined.
Specifically, a test specimen is sampled from the steel sheet, and the longitudinal
cross sectional surface thereof parallel to the rolling direction is polished, and is
corroded with nital. Thereafter, the metallic structure is observed by using a
SEM at a position deep by one-fourth of thickness from the surface of steel sheet.
By image processing, the area fractions of low-temperature transformation
producing phase and polygonal ferrite are measured. Assuming that the area
fraction is equal to the volume ratio, the volume ratios of low-temperature
transformation producing phase and polygonal ferrite are determined. The
average grain size of polygonal ferrite is determined as described below. A
circle corresponding diameter is determined by dividing the area occupied by the
whole of polygonal ferrite in a visual field by the number of crystal grains of
polygonal ferrite, and the circle corresponding diameter is defined as the average
grain size.
The volume ratio of retained austenite is determined as described below.
A test specimen is sampled from the steel sheet, and the rolled surface thereof is
chemically polished to a position deep by one-fourth of thickness from the
surface of steel sheet, and the X-ray diffraction intensity is measured by using an
XRD apparatus.
The grain size of retained austenite and the average grain size of retained
austenite are measured as described below. A test specimen is sampled fiom
the steel sheet, and the longitudinal cross sectional surface thereof parallel to the
rolling direction is electropolished. The metallic structure is observed at a
position deep by one-fourth of thickness from the surface of steel sheet by using
a SEM equipped with an EBSP analyzer. A region that is observed as a phase
consisting of a face-centered cubic crystal structure (fcc phase) and is surrounded
by the parent phase is defined as one retained austenite grain. By image
processing, the number density (number of grains per unit area) of retained
austenite grains and the area fractions of individual retained austenite grains are
measured. From the areas occupied by individual retained austenite grains in a
visual field, the circle corresponding diameters of individual retained austenite
grains are determined, and the mean value thereof is defined as the average grain
size of retained austenite.
In the structure observation using the EBSP, in the region of 50 pm or
larger in the sheet thickness direction and 100 pm or larger in the rolling
direction, electron beams are applied at a pitch of 0.1 pm to make judgment of
phase. Also, among the obtained measured data, the data in which the
reliability index is 0.1 or more are used for grain size measurement as effective
data. Also, to prevent the grain size of retained austenite from being
undervalued by measurement noise, only the retained austenite grains each
having a circle corresponding diameter of 0.15 pm or larger is taken as effective
grains, whereby the average grain size of retained austenite is calculated.
In the present invention, the above-described metallic structure is defined
at a position deep by one-fourth of thickness from the surface of steel sheet in the
case of cold-rolled steel sheet, and at a position deep by one-fourth of thickness
of steel sheet, which is a base material, from the boundary between the base
material steel sheet and a plating layer in the case of plated steel sheet.
As the mechanical property that can be realized based on the feature of the
above-described metallic structure, to assure the shock absorbing property, the
steel sheet of the present invention preferably has a tensile strength (TS) of 780
MPa or higher, further preferably has that of 950 MPa or higher, in the direction
I -HI
@
perpendicular to the rolling direction . Also, to assure the ductility, the TS is
preferably lower than 1 180 MPa.
When the value obtained by converting the total elongation (Elo) in the
direction perpendicular to the rolling direction into a total elongation
5 corresponding to the sheet thickness of 1.2 mm based on formula (1) below is
taken as El, the work hardening index calculated by using the nominal strains of
two points of 5% and 10% with the strain range being made 5 to 10% in
conformity to Japanese Industrial Standards JIS 22253 and the test forces
corresponding to these strains is taken as n value, and the bore expanding ratio
10 measured in conformity to Japan Iron and Steel Federation Standards JFST100 1
is taken as h, from the viewpoint of press formability, it is preferable that the
value of TS x El be 15,000 MPa% or higher, the value of TS x n value be 150
MPa or higher, and the value of TS" x h be 4,500,000 MP~"% or higher.
El = Elo x (1 .2/b)OV.2.. (1)
15 in which Elo is the actually measured value of total elongation measured by using
JIS No. 5 tensile test specimen, to is the thickness of JIS No. 5 tensile test
specimen used for measurement, and El is the converted value of total elongation
corresponding to the case where the sheet thickness is 1.2 mm.
TS x El is an index for evaluating the ductility from the balance between
20 strength and total elongation, TS x n value is an index for evaluating the work
hardening property from the balance between strength and work hardening index,
and TS'.? x h is an index for evaluating the bore expandability from the balance
between strength and bore expanding ratio.
It is fbrther preferable that the value of TS x El be 19,000 MPa% or higher,
25 the value of TS x n value be 160 MPa or higher, and the value of TS'.~x A be
5,500,000 MP~'.~o%r h igher. It is still further preferable that the value of TS x
El be 20,000 MPa% or higher, the value of TS x n value be 165 MPa or higher,
and the value of TS'.? x h be 6,000,000 MP~'.?%or higher.
Since the strain occurring when an automotive part is press-formed is
30 about 5 to lo%, the work hardening index was expressed by n value for the strain
range of 5 to 10% in the tension test. Even if the total elongation of steel sheet
is large, the strain propagating property in the press forming of automotive part is
-a -
0
insufficient when the n value is low, and defective forming such as a local
thickness decrease occurs easily. Also, from the viewpoint of shape fixability,
the yield ratio is preferably lower than 80%, further preferably lower than 75%,
and still Wher preferably lower than 70%.
5
2. Chemical composition of steel
C: more than 0.020% and less than 0.30%
If the C content is 0.020% or less, it is difficult to obtain the abovedescribed
metallic structure. Therefore, the C content is made more than
10 0.020%. The C content is preferably more than 0.070%, further preferably
more than 0.10%, and still hrther preferably more than 0.14%. On the other
hand, if the C content is 0.30% or more, not only the stretch flanging property of
steel sheet is impaired, but also the weldability is deteriorated. Therefore, the C
content is made less than 0.30%. The C content is preferably less than 0.25%,
15 further preferably less than 0.20%, and still further preferably less than 0.17%.
Si: more than 0.10% and 3.00% or less
Silicon (Si) has a function of improving the ductility, work hardening
property, and stretch flanging property through the restraint of austenite grain
20 growth during annealing. Also, Si is an element that has a function of
enhancing the stability of austenite and is effective in obtaining the abovedescribed
metallic structure. If the Si content is 0.10% or less, it is difficult to
achieve the effect brought about by the above-described function. Therefore,
the Si content is made more than 0.10%. The Si content is preferably more than
25 0.60%, further preferably more than 0.90%, and still further preferably more than
1.20%. On the other hand, if the Si content is more than 3.00%, the surface
properties of steel sheet are deteriorated. Further, the chemical conversion
treatability and the platability are deteriorated remarkably. Therefore, the Si
content is made 3.00% or less. The Si content is preferably less than 2.00%,
30 further preferably less than 1 .80%, and still further preferably less than 1.60%.
In the case where the later-described A1 is contained, the Si content and
the sol.Al content preferably satisfy formula (2) below, further preferably satisfy
formula (3) below, and still hrther preferably satisfy formula (4) below.
in which, Si represents the Si content (mass%) in the steel, and sol.Al represents
the content (mass%) of acid-soluble Al.
10
Mn: more than 1.00% and 3.50% or less
Manganese (Mn) is an element that has a function of improving the
hardenability of steel and is effective in obtaining the above-described metallic
structure. If the Mn content is 1.00% or less, it is difficult to obtain the above-
15 described metallic structure. Therefore, the Mn content is made more than
1.00%. The Mn content is preferably more than 1 SO%, further preferably more
than 1.80%, and still further preferably more than 2.10%. If the Mn content
becomes too high, in the metallic structure of hot-rolled steel sheet, a coarse lowtemperature
transformation producing phase elongating and expanding in the
20 rolling direction is formed, coarse retained austenite grains increase in the
metallic structure after cold rolling and annealing, and the work hardening
property and stretch flanging property are deteriorated. Therefore, the Mn
content is made 3.50% or less. The Mn content is preferably less than 3.00%,
hrther preferably less than 2.80%, and still hrther preferably less than 2.60%.
25
P: 0.10% or less
Phosphorus (P) is an element contained in the steel as an impurity, and
segregates at the grain boundaries and embrittles the steel. For this reason, the
P content is preferably as low as possible. Therefore, the P content is made
30 0.10% or less. The P content is preferably less than 0.050%, further preferably
less than 0.020%, and still further preferably less than 0.0 15%.
-26 e
S: 0.010% or less .
Sulfur (S) is an element contained in the steel as an impurity, and forms
sulfide-base inclusions and deteriorates the stretch flanging property. For this
reason, the S content is preferably as low as possible. Therefore, the S content
5 is made 0.0 10% or less. The S content is preferably less than 0.005%, fiather
preferably less than 0.003%, and still further preferably less than 0.002%.
sol.Al: 2.00% or less
Aluminum (Al) has a function of deoxidizing molten steel. In the present
10 invention, since Si having a deoxidizing function like A1 is contained, A1 need
not necessarily be contained. That is, the sol.Al content may be close to 0%
unlimitedly. In the case where sol.Al is contained for the purpose of promotion
of deoxidation, 0.0050% or more of sol.Al is preferably contained. The sol.Al
content is further preferably more than 0.020%. Also, like Si, A1 is an element
15 that has a function of enhancing the stability of austenite and is effective in
obtaining the above-described metallic structure. Therefore, A1 can be
contained for this purpose. In this case, the sol.Al content is preferably more
I
1
I than 0.040%, further preferably more than 0.050%, and still hrther preferably
I more than 0.060%. On the other hand, if the sol.Al content is too high, not only
i
20 a surface flaw caused by alumina is liable to occur, but also , the transformation
I
I point rises greatly, so that it is difficult to obtain a metallic structure such that the
1
I main phase is a low-temperature transformation producing phase. Therefore,
I
I the sol.Al content is made 2.00% or less. The sol.Al content is preferably less
I
I
I than 0.60%, hrther preferably less than 0.20%, and still further preferably less
I
I 25 than 0.10%.
N: 0.010% or less
Nitrogen (N) is an element contained in the steel as an impurity, and
deteriorates the ductility. For this reason, the N content is preferably as low as
30 possible. Therefore, the N content is made 0.010% or less. The N content is
preferably 0.006% or less, further preferably 0.005% or less.
The steel sheet produced by the method in accordance with the present
invention may contain elements described below as optional elements.
One kind or two or more kinds selected from a group consisting of Ti: less than
0.050%, Nb: less than 0.050%, and V: 0.50% or less
Ti, Nb and V each have a function of increasing the work strain by means
of the restraint of recrystallization in the hot-rolling process, and have a function
of making the metallic structure of hot-rolled steel sheet fine. Also, these
elements precipitate as carbides or nitrides, and have a fbnction of restraining the
coarsening of austenite during annealing. Therefore, one kind or two or more
kinds of these elements may be contained. However, even if these elements are
contained excessively, the effect brought about by the above-described function
saturates, being uneconomical. Rather, the recrystallization temperature at the
time of annealing rises, the metallic structure after annealing becomes uneven,
and the stretch flanging property is also impaired. Furthermore, the
precipitation amount of carbides or nitrides increases, the yield ratio ascends, and
the shape fixability also deteriorates. Therefore, the Ti content is made less
than 0.050%, the Nb content is made less than 0.050%, and the V content is
made 0.50% or less. The Ti content is preferably less than 0.040%, hrther
preferably less than 0.030%. The Nb content is preferably less than 0.040%,
further preferably less than 0.030%. The V content is preferably 0.30% or less,
further preferably less than 0.050%. To surely achieve the effect brought about
by the above-described function, either of Ti: 0.005% or more, Nb: 0.005% or
more, and V: 0.010% or more is preferably satisfied. In the case where Ti is
contained, the Ti content is fbrther preferably made 0.010% or more, in the case
where Nb is contained, the Nb content is fbrther preferably made 0.0 10% or
more, and in the case where V is contained, the V content is further preferably
made 0.020% or more.
One kind or two or more kinds selected from a group consisting of Cr: 1 .O% or
less, Mo: 0.50% or less, and B: 0.010% or less
Cr, Mo and B are elements that have a function of improving the
hardenability of steel and are effective in obtaining the above-described metallic
structure. Therefore, one kind or two or more kinds of these elements may be
contained. However, even if these elements are contained excessively, the
5 effect brought about by the above-described function saturates, being
uneconomical. Therefore, the Cr content is made 1 .O% or less, the Mo content
is made 0.50% or less, and the B content is made 0.010% or less. The Cr
content is preferably 0.50% or less, the Mo content is preferably 0.20% or less,
and the B content is preferably 0.0030% or less. To more surely achieve the
10 effect brought about by the above-described function, either of Cr: 0.20% or
more, Mo: 0.05% or more, and B: 0.0010% or more is preferably satisfied.
One kind or two or more kinds selected from a group consisting of Ca: 0.010%
or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% or less
15 Ca, Mg and REM each have a function of improve the stretch flanging
property by means of the regulation of shapes of inclusions, and Bi also has a
function of improve the stretch flanging property by means of the refinement of
solidified structure. Therefore, one kind or two or more kinds of these elements
may be contained. However, even if these elements are contained excessively,
20 the effect brought about by the above-described function saturates, being
uneconomical. Therefore, the Ca content is made 0.010% or less, the Mg
content is made 0.010% or less, the REM content is made 0.050% or less, and the
Bi content is made 0.050% or less. Preferably, the Ca content is 0.0020% or
less, the Mg content is 0.0020% or less, the REM content is 0.0020% or less, and
25 the Bi content is 0.010% or less. To more surely obtain above-described
function, either of Ca: 0.0005% or more, Mg: 0.0005% or more, REM: 0.0005%
or more, and Bi: 0.00 10% or more is preferably satisfied. The REM means rare
earth metals, and is a general term of a total of 17 elements of Sc, Y, and
lanthanoids. The REM content is the total content of these elements.
30
3. Production conditions
(Cold-rolling process in first invention)
In the cold-rolling process, a hot-rolled steel sheet having the abovedescribed
chemical composition, in which the average grain size of grains having
a bcc structure and the grains having a bct structure (as described already, these
grains are generally called "bcc grains") surrounded by a grain boundary having
5 an orientation difference of 15" or larger is 6.0 pm or smaller, and preferably,
furthermore, the average number density of iron carbides existing in the metallic
structure is 1.0 x 10"/pm2 or higher, is cold-rolled to form a cold-rolled steel
sheet.
Herein, the average grain size of bcc grains is calculated by the method
10 described below. A test specimen is sampled from the steel sheet, the
longitudinal cross sectional surface thereof parallel to the rolling direction is
electropolished, and the metallic structure is observed by using a SEM equipped
with an EBSP analyzer at a position deep by one-fourth of thickness fiom the
surface of steel sheet. A region that is observed as the phase consisting of a
15 body-centered cubic crystal type crystal structure and is surrounded by a
boundary having an orientation difference of 15" or larger is taken as one crystal
grain, and the value calculated by formula (5) below is taken as the average grain
size of bcc grains. In this formula, N is the number of crystal grains contained
in the average grain size evaluation region, Ai is the area of the i-th (i = 1, 2, . .,
20 N) crystal grain, and di is the circle corresponding diameter of i-th crystal grain.
[Expression 11
25 The crystal structure of martensite is strictly a body-centered tetragonal
lattice (bct); however, in the grain size evaluation of the present invention,
martensite is also handled as the bcc phase because in the metallic structure
evaluation using the EBSP, the lattice constant is not considered,
In the structure evaluation by using the EBSP in this embodiment, the
phase of a region having a size of 50 ym in the sheet thickness direction and of
100 pm in the rolling direction (the direction perpendicular to the sheet thickness
direction) is judged by controlling the electron beams at a pitch of 0.1 pm.
5 Among the obtained measured data, the data in which the reliability index is 0.1
or more is used for grain size measurement as effective data. Further, to
prevent the grain size from being undervalued by measurement noise, in the
evaluation of bcc grains, unlike the before-described case of retained austenite,
the above-described grain size calculation is performed by taking only the bcc
10 grains each having a grain size of 0.47 pm or larger as effective grains.
The reason why the crystal grain size is defined by taking the grain
boundary having an orientation difference of 15" or larger as an effective grain
boundary is that the grain boundary having an orientation difference of 15" or
larger becomes an effective nucleation site of reverse transformation austenite
15 grains, whereby the coarsening of austenite grains at the time of annealing after
cold rolling is restrained, and the nucleation site contributes greatly to the
improvement in workability of cold-rolled steel sheet. Also, in the case where
the structure of hot-rolled steel sheet is a mixed grain size structure in which fine
grains and coarse grains are intermixing, the portion of coarse grains easily
20 coarsens at the time of annealing after cold rolling, so that the ductility, work
hardening property, and stretch flanging property are deteriorated. In the case
where the grain size of such a mixed grain size structure is evaluated by the
cutting method used generally as the evaluation of crystal grain size of metallic
structure, the influence of coarse grains may be undervalued. In the present
25 invention, as a calculation method of crystal grain size considering the influence
of coarse grains, the above-described formula (9, in which the individual areas
of crystal grains are multiplied as a weight, is used.
The amount of iron carbides existing in the steel sheet is defined by the
average number density (unit: numberiPm2), and the average number density of
30 the iron carbides is measured as described below. A test specimen is sampled
from the steel sheet, the longitudinal cross sectional surface thereof parallel to the
rolling direction is polished, and the metallic structure is observed by using an
optical microscope or a SEM at a position deep by one-fourth of thickness from
the surface of steel sheet. The composition analysis of precipitates is made by
using an Auger electron spectroscope (AES), the precipitates containing Fe and
C as constituent elements are taken as iron carbides, and the number density of
iron carbides in the metallic structure is measured. In the number density
evaluation of iron carbides of the present invention, observation was
accomplished in five visual fields of lo2 pm2 at a magnification of ~5000t,h e
number of iron carbides existing in the metallic structure in each visual field was
measured, and the average number density was calculated from the mean value
of the five visual fields. The iron carbides means compounds consisting mainly
of Fe and C, and Fe3C, Fe3(C, B), Fe23(C, B)6, Fe2C, Fe2.2C, Fe2,&, and the like
are cited as iron carbides. In order to efficiently restrain the coarsening of
austenite, the iron carbide is preferably Fe3C. Also, a steel component such as
Mn and Cr may be dissolved in these iron carbides.
For the hot-rolled steel sheet to be subjected to cold rolling, in the case
where the average grain size of bcc grains calculated by the above-described
method exceeds 6.0 pm, the metallic structure after cold rolling and annealing is
coarsened, and the ductility, work hardening property, and stretch flanging
property are impaired. Therefore, the average grain size of bcc grains is made
6.0 pm or smaller. This average grain size is preferably 4.0 pm or smaller, and
firther preferably 3.5 pm or smaller.
For the hot-rolled steel sheet to be subjected to cold rolling, the average
number density of iron carbides existing in the metallic structure is preferably 1.0
x 10"/pm2 or higher. Thereby, the coarsening of austenite in the annealing
process after cold rolling is restrained, and the ductility, work hardening property,
and stretch flanging property of cold-rolled steel sheet can be improved
remarkably. The average number density of iron carbides is further preferably
5.0 x 10-'/~rnor~ h igher, still firther preferably 8.0 x 10-'lPm2o r higher.
The kinds and volume ratios of the phase and structure forming the hotrolled
steel sheet are not defined especially, and one kind or two or more kinds
selected from a group consisting of polygonal ferrite, acicular ferrite, bainitic
ferrite, bainite, pearlite, retained austenite, martensite, tempered bainite, and
tempered martensite may be intermixed. However, a softer hot-rolled steel
sheet is preferable in that the load of cold rolling is alleviated and the cold rolling
ratio is further increased, whereby the metallic structure after being annealed can
be made fine.
5 The above-described method for producing a hot-rolled steel sheet is not
defined especially; however, it is preferable that the hot-rolling process in the
second invention, described later, or the hot-rolling process in the third invention,
described later, be adopted. The above-described hot-rolled steel sheet may be
a hot-rolled and annealed steel sheet subjected to annealing after being hot-rolled.
10 The cold rolling itself may be performed pursuant to an ordinary method.
Before cold rolling, the hot rolled steel sheet may be descaled by pickling or the
like means. In the cold rolling, in order to promote recrystallization and
homogenize the metallic structure after cold rolling and annealing, thereby
hrther improving the stretch flanging property, the cold rolling ratio (the total
15 draft in cold rolling) is preferably made 40% or higher, hrther preferably made
more than 50%. Thereby, the metallic structure after annealing is made further
fine, and the aggregate structure is improved, so that the ductility, work
hardening property, and stretch flanging property are hrther improved. From
this viewpoint, the cold rolling ratio is further preferably made more than 60%,
20 most preferably made more than 65%. On the other hand, if the cold rolling
ratio is too high, the rolling load is increased, and it is difficult to perform rolling.
Therefore, the upper limit of cold rolling ratio is preferably made lower than 80%
hrther preferably made lower than 70%.
25 (Annealing process in first invention)
The cold-rolled steel sheet obtained by the above-described cold-rolling
process is annealed after being subjected to treatment such as degreasing
pursuant to a publicly-known method as necessary. The lower limit of soaking
temperature in annealing is made (Ac3 point - 40°C) or higher. This is for the
30 purpose of obtaining a metallic structure such that the main phase is a lowtemperature
transformation producing phase, and the secondary phase contains
retained austenite. To increase the volume ratio of low-temperature
transformation producing phase and to improve the stretch flanging property, the
soaking temperature is preferably made higher than (Ac3 point - 20°C), and
further preferably made higher than Ac3 point. However, if the soaking
temperature is too high, austenite is coarsened excessively, and the formation of
5 polygonal ferrite is restrained, so that the ductility, work hardening property, and
stretch flanging property are liable to deteriorate. Therefore, the upper limit of
soaking temperature is preferably made lower than (Ac3 point + 100°C), fixther
preferably made lower than (Ac3 point + 50°C), and still hrther preferably made
lower than (Ac3 point + 20°C). Also, to promote the formation of fine
10 polygonal ferrite and to improve the ductility and work hardening property, the
upper limit of soaking temperature is preferably made lower than (Ac3 point +
50°C), hrther preferably made lower than (Ac3 point + 20°C).
The holding time at the soaking temperature (the soaking time) need not
be subject to any special restriction; however, to attain stable mechanical
1 5 properties, the holding time is preferably made longer than 15 seconds, hrther
preferably made longer than 60 seconds. On the other hand, if the holding time
is too long, austenite is coarsened excessively, so that the ductility, work
hardening property, and stretch flanging property are liable to deteriorate.
Therefore, the holding time is preferably made shorter than 150 seconds, further
20 preferably made shorter than 120 seconds.
In the heating process in annealing, to homogenize the metal structure
after annealing by means of the promotion of crystallization and to improve the
stretch flanging property, the heating rate from 700°C to the soaking temperature
is preferably made lower than 10.O°C/s. This heating rate is hrther preferably
25 made lower than 8.0°C/s, still further preferably made lower than 5.0°C/s.
In the cooling process after soaking in annealing, to promote the formation
of fine polygonal ferrite and to improve the ductility and work hardening
property, cooling is preferably performed by 50°C or more from the soaking
temperature at a cooling rate of lower than 10.O°C/s. This cooling rate after
30 soaking is preferably lower than 5.0°C/s, further preferably lower than 3.0°C/s,
and still firther preferably lower than 2.0°C/s. To fbrther increase the volume
ratio of polygonal ferrite, cooling is performed by 80°C or more from the soaking
temperature at a cooling rate of lower than 10.O°C/s. The cooling is performed
further preferably by 100°C or more, still further preferably by 120°C or more.
To obtain a metallic structure such that the main phase is a lowtemperature
transformation producing phase, the cooling in the temperature
range of 650 to 500°C is preferably performed at a cooling rate of 15"C/s or
higher. To perform cooling in the temperature range of 650 to 450°C at a
cooling rate of 15"CIs or higher is mher preferable. With the increase in the
cooling rate, the volume ratio of the low-temperature transformation producing
phase increases. Therefore, a cooling rate higher than 30°C/s is hrther
preferable, and a cooling rate higher than 50°C/s is still further preferable. On
the other hand, if the cooling rate is too high, the shape of steel sheet is
deteriorated. Therefore, the cooling rate in the temperature range of 650 to
500°C is preferably made 200°C/s or lower, further preferably made lower than
150°C/s, and still further preferably made lower than 130°C/s.
Further, to obtain retained austenite, the steel sheet is held in the
temperature region of 500 to 300°C for 30 seconds or longer. In order to
enhance the stability of retained austenite and to improve the ductility, work
hardening property, and stretch flanging property, the holding temperature region
is preferably made 475 to 320°C. The holding temperature region is further
preferably made 450 to 340°C, still fkther preferably made 430 to 360°C. Also,
as the holding time is made longer, the stability of retained austenite increases.
Therefore, the holding time is preferably made 60 seconds or longer, further
preferably made 120 seconds or longer, and still further preferably made 300
seconds or longer.
In the case where an electroplated steel sheet is produced, after the coldrolled
steel sheet produced by the above-described method has been subjected to
well-known preparations as necessary to purifL and condition the surface,
electroplating has only to be performed pursuant to an ordinary method. The
chemical composition and mass of deposit of plating film is not subject to any
special restriction. As the kind of electroplating, electro zinc plating, electro-
Zn-Ni alloy plating, and the like are cited.
In the case where a hot dip plated steel sheet is produced, the steel sheet is
treated in the above-described method up to the annealing process, and after
being hold in the temperature region of 500 to 300°C for 30 seconds or longer,
the steel sheet is heated as necessary, and is immersed in a plating bath for hot
5 dip plating. In order to enhance the stability of retained austenite and to
improve the ductility, work hardening property, and stretch flanging property, the
holding temperature region is preferably made 475 to 320°C. The holding
temperature region is further preferably made 450 to 340°C, still further
preferably made 430 to 360°C. Also, as the holding time is made longer, the
10 stability of retained austenite increases. Therefore, the holding time is
preferably made 60 seconds or longer, further preferably made 120 seconds or
longer, and still further preferably made 300 seconds or longer. The steel sheet
may be reheated after being hot dip plated for alloying treatment. The chemical
composition and mass of deposit of plating film is not subject to any special
15 restriction. As the kind of hot dip plating, hot dip zinc plating, alloying hot dip
zinc plating, hot dip aluminum plating, hot dip Zn-A1 alloy plating, hot dip Zn-
Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, and the like are cited.
The plated steel sheet may be subjected to suitable chemical conversion
treatment after being plated to hrther enhance the corrosion resistance. In place
20 of the conventional chromate treatment, the chemical conversion treatment is
preferably performed by using a non-chrome type chemical conversion liquid
(for example, silicate-based or phosphate-based).
The cold-rolled steel sheet and plated steel sheet thus obtained may be
subjected to temper rolling pursuant to an ordinary method. However, a large
25 elongation percentage of temper rolling leads to the deterioration in ductility.
Therefore, the elongation percentage of temper rolling is preferably made 1 .O%
or smaller, further preferably made 0.5% or smaller
(Hot-rolling process in second invention)
30 A steel having the above-described chemical composition is melted by
publicly-known means and thereafter is formed into an ingot by the continuous
casting process, or is formed into an ingot by an optional casting process and
thereafter is formed into a billet by a billeting process or the like. In the
continuous casting process, to suppress the occurrence of a surface defect caused
by inclusions, an external additional flow such as electromagnetic stirring is
preferably produced in the molten steel in the mold. Concerning the ingot or
billet, the ingot or billet that has been cooled once may be reheated and be
subjected to hot rolling. Alternatively, the ingot that is in a high-temperature
state after continuous casting or the billet that is in a high-temperature state after
billeting may be subjected to hot rolling as it is, or by retaining heat, or by
heating it auxiliarily. In this description, such an ingot and a billet are generally
called a "slab" as a raw material for hot rolling. To prevent austenite from
coarsening, the temperature of the slab that is to be subjected to hot rolling is
preferably made lower than 1250°C, hrther preferably made lower than 1200°C.
The lower limit of the temperature of slab to be subjected to hot rolling need not
be restricted specially, and may be any temperature at which hot rolling can be
finished at AT3 point or higher as described later.
The hot rolling is finished in the temperature region of AT3 point or higher
to make the metallic structure of hot-rolled steel sheet fine by means of
transformation of austenite after the completion of rolling. If the temperature of
rolling completion is too low, in the metallic structure of hot-rolled steel sheet, a
coarse low-temperature transformation producing phase elongating and
expanding in the rolling direction is formed, the metallic structure after cold
rolling and annealing is coarsened, and the ductility, work hardening property,
and stretch flanging property is liable to be deteriorated. Therefore, the
finishing temperature of hot rolling is preferably made AT3 point or higher and
higher than 820°C, hrther preferably made Ar3 point or higher and higher than
850°C, and still further preferably made AT3 point or higher and higher than
880°C. On the other hand, if the hot rolling finishing temperature is too high,
the accumulation of work strain is insufficient, and it is difficult to make the
metallic structure of hot-rolled steel sheet fine. Therefore, the hot rolling
finishing temperature is preferably lower than 950°C, further preferably lower
than 920°C. Also, to lighten the production load, it is preferable that the
finishing temperature of hot rolling be raised and thereby the rolling load be
reduced. From this viewpoint, the finishing temperature of hot rolling is
preferably made AT3 point or higher and higher than 780°C, further preferably
made AT3 point or higher and higher than 800°C.
In the case where the hot rolling consists of rough rolling and finish rolling,
to finish the finish rolling at the above-described temperature, the rough-rolled
material may be heated at the time between rough rolling and finish rolling. It
is desirable that by heating the rough-rolled material so that the temperature of
the rear end thereof is higher than that of the front end thereof, the fluctuations in
temperature throughout the overall length of the rough-rolled material at the start
time of finish rolling are restrained to 140°C or less. Thereby, the homogeneity
of product properties in a coil is improved.
The heating method of the rough-rolled material has only to be carried out
by using publicly-known means. For example, a solenoid type induction
heating apparatus is provided between a roughing mill and a finish roiling mill,
and the temperature rising amount in heating may be controlled based on, for
example, the temperature distribution in the lengthwise direction of the roughrolled
material on the upstream side of the induction heating apparatus.
Concerning the roll draft of hot rolling, the roll draft of the final one pass
is made higher than 15% in thickness decrease percentage. The reason for this
is that the work strain amount introduced to austenite is increased, the metallic
structure of hot-rolled steel sheet is made fine, the metallic structure after cold
rolling and annealing is made fine, and the ductility, work hardening property,
and stretch flanging property are improved. The roll draft of the final one pass
is preferably made higher than 25%, hrther preferably made more than 30%, and
still hrther preferably made more than 40%. If the roll draft is too high, the
rolling load increases, and it is difficult to perform rolling. Therefore, the roll
draft of the final one pass is preferably made lower than 55%, further preferably
made lower than 50%. To reduce the rolling load, so-called lubrication rolling
may be performed in which rolling is performed while a rolling oil is supplied
between a rolling roll and a steel sheet to decrease the friction coefficient.
After hot rolling, the steel sheet is cooled rapidly to the temperature region
of 780°C or lower within 0.40 seconds after the completion of rolling. The
-He
reason for this is that the release of work strain introduced to austenite by rolling
is restrained, austenite is transformed with the work strain being used as a driving
force, the metallic structure of hot-rolled steel sheet is made fine, the metallic
structure after cold rolling and annealing is made fine, and the ductility, work
5 hardening property, and stretch flanging property are improved. As the time up
to the stop of rapid cooling is shorter, the release of work strain is restrained.
Therefore, the time up to the stop of rapid cooling afier the completion of rolling
is preferably within 0.30 seconds, fbrther preferably within 0.20 seconds. As
the temperature at which rapid cooling stops is lower, the metallic structure of
10 hot-rolled steel sheet is made finer. Therefore, it is preferable that the steel
sheet be rapidly cooled to the temperature region of 760°C or lower after the
completion of rolling. It is hrther preferable that the steel sheet be rapidly
cooled to the temperature region of 740°C or lower after the completion of
rolling, and it is still Mher preferable that the steel sheet be rapidly cooled to the
15 temperature region of 720°C or lower afier the completion of rolling. Also, as
the average cooling rate during rapid cooling is higher, the release of work strain
is restrained. Therefore, the average cooling rate during rapid cooling is
preferably made 300°C/s or higher. Thereby, the metallic structure of hot-rolled
steel sheet can be made still finer. The average cooling rate during rapid
20 cooling is further preferably made 400°C/s or higher, and still further preferably
made 600°C/s or higher. The time from the completion of rolling to the start of
rapid cooling and the cooling rate during the time need not be defined specially.
The equipment for performing rapid cooling is not defined specially;
however, on the industrial basis, the use of a water spraying apparatus having a
25 high water amount density is suitable. A method is cited in which a water spray
header is arranged between rolled sheet conveying rollers, and high-pressure
water having a sufficient water amount density is sprayed from the upside and
downside of the rolled sheet.
After the stop of rapid cooling, the steel sheet is coiled in the temperature
30 region of higher than 400°C. Since the coiling temperature is higher than
400°C, iron carbides precipitate sufficiently in the hot-rolled steel sheet. The
iron carbides have an effect of restraining the coarsening of metallic structure
- a3/-
e
after annealing. The coiling temperature is preferably higher than 500°C,
further preferably higher than 550°C, and still fbrther preferably higher than
580°C. On the other hand, if the coiling temperature is too high, in the hotrolled
steel sheet, ferrite is coarse, and the metallic structure after cold rolling and
5 annealing is coarsened. Therefore, the coiling temperature is preferably made
lower than 650°C, further preferably made lower than 620°C. The conditions
from the stop of rapid cooling to the coiling are not defined specially; however,
after the stop of rapid cooling, the steel sheet is preferably held in the
temperature region of 720 to 600°C for one second or longer. Thereby, the
10 formation of fine ferrite is promoted. On the other hand, if the holding time is
too long, the productivity is impaired. Therefore, the upper limit of holding
time in the temperature region of 720 to 600°C is preferably made within 10
seconds. After being held in the temperature region of 720 to 600°C, the steel
sheet is preferably cooled to the coiling temperature at a cooling rate of 20°C/s or
15 higher to prevent the coarsening of formed ferrite.
For the hot-rolled steel sheet obtained by the above-described hot rolling,
the average grain size of bcc grains calculated by the above-described method is
preferably 6.0 pm or smaller, further preferably 4.0 pm or smaller, and still
further preferably 3.5 pm or smaller.
20 Also, the average number density of iron carbides existing in the metallic
structure is preferably 1.0 x 10-l/pm2 or higher, fbrther preferably 5.0 x 10"/pm2
or higher, and still further preferably 8.0 x 10"lpm2 or higher.
(Cold-rolling process in second invention)
25 The hot-rolled steel sheet obtained by the above-described hot rolling is
cold-rolled pursuant to an ordinary method. Before the cold rolling, the hotrolled
rolled steel s h e e ~ b d e s c a l ebdy pickling or the like means. In the cold
rolling, to homogenize the metallic structure after cold rolling and annealing by
means of promotion of recrystallization, and to hrther improve the stretch
30 flanging property, the cold rolling ratio is preferably made 40% or higher, further
preferably made higher than 50%. Thereby, the metallic structure after
annealing is made still finer, and the aggregate structure is improved, so that the
-id
a
ductility, work hardening property, and stretch flanging property are further
improved. From this viewpoint, the cold rolling ratio is further preferably made
more than 60%, most preferably made more than 65%. On the other hand, if
the cold rolling ratio is too high, the rolling load is increased, and it is difficult to
5 perform rolling. Therefore, the upper limit of cold rolling ratio is preferably
made lower than 80%, hrther preferably made lower than 70%.
(Annealing process in second invention)
The cold-rolled steel sheet obtained by the above-described cold rolling is
10 annealed in the same way as the annealing process in the first invention.
(Hot-rolling process in third invention)
Up to hot rolling and subsequent immediate rapid cooling, the hot-rolling
process in the third invention is the same as that in the second invention. After
15 the stop of rapid cooling, the steel sheet is coiled in the temperature region of
lower than 400°C, and the obtained hot-rolled steel sheet is subjected to hotrolled
sheet annealing.
By making the coiling temperature lower than 400°C, at the time of next
hot-rolled sheet annealing, iron carbides can be precipitated finely, and the
20 metallic structure after cold rolling and subsequent annealing is made fine. The
coiling temperature in this case is preferably lower than 300°C, further preferably
lower than 200°C, and still further preferably lower than 100°C. The coiling
temperature may be room temperature.
The hot-rolled steel sheet coiled at a temperature lower than 400°C as
25 described above is subjected to degreasing and the like treatment as necessary
pursuant to a publicly-known method, and thereafter is annealed. The annealing
performed on a hot-rolled steel sheet is called hot-rolled sheet annealing, and the
steel sheet having been subjected to the hot-rolled sheet annealing is called a hotrolled
and annealed steel sheet. Before the hot-rolled sheet annealing, the steel
30 sheet may be descaled by pickling or the like means. With the increase in
heating temperature in the hot-rolled sheet annealing, Mn or Cr is concentrated in
iron carbides, and the function of preventing the coarsening of austenite grains
-6
@
due to iron carbides is increased. Therefore, the lower limit of heating
temperature is made higher than 300°C. The lower limit of heating temperature
is preferably made higher than 400°C, further preferably made higher than 500°C,
and still further preferably made higher than 600°C. On the other hand, if the
5 heating temperature is too high, the coarsening and re-dissolving of iron carbides
occur, and the effect of preventing the coarsening of austenite grains is impaired.
Therefore, the upper limit of heating temperature is preferably made lower than
750°C, further preferably made lower than 700°C, and still further preferably
made lower than 650°C.
10 The holding time in the hot-rolled sheet annealing need not be subject to
any special restriction. For the hot-rolled steel sheet produced through a
suitable immediate rapid cooling process, the metallic structure is h e , the
precipitation sites of iron carbides are many, and iron carbides precipitate rapidly.
Therefore, the steel sheet need not be held for a long period of time. Long
15 holding time degrades the productivity. Therefore, the upper limit of holding
time is preferably shorter than 20 hours, firther preferably shorter than 10 hours,
and still further preferably shorter than 5 hours.
For the hot-rolled and annealed steel sheet obtained by the abovedescribed
method, the average grain size of bcc grains calculated by the above-
20 described method is preferably 6.0 ym or smaller, further preferably 4.0 pm or
smaller, and still further preferably 3.5 pm or smaller.
Also, the average number density of iron carbides existing in the metallic
structure is preferably 1.0 x 10-'/~mo*r higher, further preferably 5.0 x 10-'lPm2
or higher, and still further preferably 8.0 x 10-'/~rnor~ h igher.
25
(Cold-rolling process in third invention)
The hot-rolled steel sheet obtained by the above-described hot rolling is
cold-rolled in the same way as the cold-rolling process in the second invention.
30 (Annealing process in third invention)
The cold-rolled steel sheet obtained by the above-described cold rolling is
annealed in the same way as the annealing process in the first and second
inventions.
The following examples merely illustrate the present invention, and do not
5 intend to limit the present invention.
Example 1
Example 1 describes an example of the case where in the metallic structure
of hot-rolled steel sheet, the average grain size of bcc grains surrounded by a
10 grain boundary having an orientation difference of 15" or larger is 6.0 pm or
smaller.
By using an experimental vacuum melting furnace, steels each having the
chemical composition given in Table 1 were melted and cast. These ingots
were formed into 30-mm thick billets by hot forging. The billets were heated to
15 1200°C by using an electric heating furnace and held for 60 minutes, and
thereafter were hot-rolled under the conditions given in Table 2.
Specifically, by using an experimental hot-rolling mill, 6-pass rolling was
performed in the temperature region of Ar3 point or higher to finish each of the
billets into a steel sheet having a thickness of 2 to 3 mm. The draft of the final
20 one pass was set at 12 to 42% in thickness decrease percentage. After hot
rolling, the steel sheet was cooled to a temperature of 650 to 720°C under various
cooling conditions by using a water spray. Successively, after having been
allowed to cool for 5 to 10 seconds, the steel sheet was cooled to various
temperatures at a cooling rate of 60°C/s, and these temperatures were taken as
25 coiling temperatures. The steel sheet was charged into an electric heating
furnace that was held at that temperature, and was held for 30 minutes.
Thereafter, the gradual cooling after coiling was simulated by furnace-cooling
the steel sheet to room temperature at a cooling rate of 20°C/h, whereby a hotrolled
steel sheet was obtained.
30 A test specimen for EBSP measurement was sampled from the obtained
hot-rolled steel sheet, and the longitudinal cross sectional surface thereof parallel
to the rolling direction was electropolished. Thereafter, the metallic structure
was observed at a position deep by one-fourth of thickness from the surface of
steel sheet, and by image analysis, the average grain size of bcc grains was
measured. Specifically, as an EBSP measuring device, OIM(TM)S
manufactured by TSL Corporation was used, electron beams were applied at a
5 pitch of 0.1 ym in a region having a size of 50 ym in the sheet thickness direction
and 100 pm in the rolling direction, and among the obtained measured data, the
data in which the reliability index was 0.1 or more was used as effective data to
make judgment of bcc grains. With a region surrounded by a grain boundary
having an orientation difference of 15" or larger being made one bcc grain, the
10 circle corresponding diameter and area of individual bcc grain were determined,
and the average grain size of bcc grains was calculated pursuant to the
aforementioned formula (5). In calculating the average grain size, the bcc
grains each having a circle corresponding diameter of 0.47 pm or larger were
made effective bcc grains. As described before, in the metallic structure
15 evaluation using the EBSP, the lattice constant is not considered. Therefore,
grains each having a bct (body-centered tetragonal lattice) structure such as
martensite are also measured together. Therefore, the bcc grains include both of
the grains having a bcc structure and the grains having a bct structure.
The obtained hot-rolled steel sheet was pickled to form a base metal for
20 cold rolling. The base metal was cold-rolled at a cold rolling ratio of 50 to 60%,
whereby a cold-rolled steel sheet having a thickness of 1.0 to 1.2 mm was
obtained. By using a continuous annealing simulator, the obtained cold-rolled
steel sheet was heated to 550°C at a heating rate of 10°C/s, thereafter being
heated to various temperatures given in Table 2 at a heating rate of 2"C/s, and
25 was soaked for 95 seconds. Subsequently, the steel sheet was cooled to various
cooling stop temperatures given in Table 2 with the average cooling rate from
700°C being 60°C/s, being held at that temperature for 330 seconds, and
thereafter was cooled to room temperature, whereby an annealed steel sheet was
obtained.
[Table 11
Note) 1. Ac, point was determined from thermal expansion change at the time when cold-rolled steel sheet was heated at 2"CIs.
2. Ar3 point was determined from thermal expansion change at the time when cold-rolled steel sheet was heated to 900°C
and thereafter was cooled at 0.01 "CIS.
Af3 point
("C)
698
742
753
742
752
764
74 1
742
736
750
73 1
754
768
78 1
758
786
783
772
775
74 1
Ac3 point
Steel
A
B
C
D
E
F
G
H
I
J
K
L
M
N
0
P
Q
R
S
T
0.124
0.145
0.147
0.145
0.149
0.146
0.166
0.174
0.176
0.175
0.175
0.184
0.203
0.197
0.198
0.197
0.150
0.151
0.149
0.148
0.05*
0.99
0.98
1.25
1.49
1.25
1.51
1.26
1.26
1.25
1.30
1.28
1.28
1.26
1.26
1.28
1.51 - -
1.50
1.25
1.26
2.97
2.49
2.48
2.49
2.48
2.48
2.53
2.50
2.5 1
2.50
2.53
2.24
1.93
1.92
2.22
2.24
2.5 1
2.52
2.47
2.48
0.01 1
0.012
0.01 1
0.010
0.010
0.009
0.010
0.008
0.008
0.008
0.008
0.009
0.009
0.009
0.009
0.009
0.008
0.009
0.009
0.009
0.003
0.004
0.003
0.001
0.001
0.001
0.001
0,001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.031
0.029
0.030
0.049
0.050
0.150
0.048
0.050
0.05 1
0.050
0.045
0.050
0.051
0.140
0.143
0.15 1
0.052
0.047
0.152
0.141
0.0041
0.0048
0.0038
0.0030
0.0035
0.0032
0.0032
0.0032
0.003 1
0.0033
0.0030
0.0032
0.0027
0.0033
0.003 1
0.0029
0.0034
0.0031
0.0033
0.0030
Others
Nb:O.Ol 1
Nb:0.0 10
Nb:O.Ol 1
Nb:O.O 13
Nb:O.O 1 1
Tk0.021
Nb:0.0 10
Nb:O.Ol 1
Nb:O.Ol 1
Nb:0.010
Nb:O.Ol 1
Nb:O.Ol 1 C~0.30
V:0.11 REM:0.0006
Bi:0.008
Ca:0.0009 Mg:0.0007
Mo:O. 10 B:0.00 15
("C)
792
836
840
846
862
874
856
839
843
848
849
854
855
870
855
848
872
862
864
877
1) Sheet thickness of hot-rolled steel sheet. 2) Time from rolling completion to rapid cooling stop. 3) Average cooling rate during rapid cooling.
A test specimen for SEM observation was sampled from the annealed steel
sheet, and the longitudinal cross sectional surface thereof parallel to the rolling
direction was polished. Thereafter, the metallic structure was observed at a
position deep by one-fourth of thickness from the surface of steel sheet, and by
5 image processing, the volume fractions of low-temperature transformation
producing phase and polygonal ferrite were measured. Also, the average grain
size (circle corresponding diameter) of polygonal ferrite was determined by
dividing the area occupied by the whole of polygonal ferrite by the number of
crystal grains of polygonal ferrite.
10 Also, a test specimen for XRD measurement was sampled from the
annealed steel sheet, and the rolled surface down to a position deep by one-fourth
of thickness from the surface of steel sheet was chemically polished. Thereafter,
an X-ray diffraction test was conducted to measure the volume fraction of
retained austenite. Specifically, RINT2500 manufactured by Rigaku
15 Corporation was used as an X-ray diffiactometer, and Co-Ka beams were
applied to measure the integrated intensities of a phase (1 lo), (200), (21 1)
diffraction peaks and y phase (1 1 l), (200), (220) diffraction peaks, whereby the
volume fraction of retained austenite was determined.
Furthermore, a test specimen for EBSP measurement was sampled from
20 the annealed steel sheet, and the longitudinal cross sectional surface thereof
parallel to the rolling direction was electropolished. Thereafter, the metallic
structure was observed at a position deep by one-fourth of thickness from the
surface of steel sheet, and by image analysis, the grain size distribution of
retained austenite and the average grain size of retained austenite were measured.
25 Specifically, as an EBSP measuring device, OIM(TM)5 manufactured by TSL
Corporation was used, electron beams were applied at a pitch of 0.1 pm in a
region having a size of 50 pm in the sheet thickness direction and 100 pm in the
rolling direction, and among the obtained data, the data in which the reliability
index was 0.1 or more was used as effective data to make judgment of fcc phase.
30 With a region that was observed as the fcc phase and was surrounded by a parent
phase being made one retained austenite grain, the circle corresponding diameter
of individual retained austenite grain was determined. The average grain size of
retained austenite was calculated as the mean value of circle corresponding
diameters of individual effective retained austenite grains, the effective retained
austenite grains being retained austenite grains each having a circle
corresponding diameter of 0.15 pm or larger. Also, the number density (NR)
per unit area of retained austenite grains each having a grain size of 1.2 pm or
larger was determined.
The yield stress (YS) and tensile strength (TS) were determined by
sampling a JIS No. 5 tensile test specimen along the direction perpendicular to
the rolling direction from the annealed steel sheet, and by conducting a tension
test at a tension rate of 10 mrnlmin. The total elongation (El) was determined as
follows: a tension test was conducted by using a JIS No. 5 tensile test specimen
sampled along the direction perpendicular to the rolling direction, and by using
the obtained actually measured value (Elo), the converted value of total
elongation corresponding to the case where the sheet thickness is 1.2 mm was
determined based on formula (1) above. The work hardening index (n value)
was determined with the strain range being 5 to 10% by conducting a tension test
by using a JIS No. 5 tensile test specimen sampled along the direction
perpendicular to the rolling direction. Specifically, the n value was calculated
by the two point method by using test forces with respect to nominal strains of
5% and 10%.
The stretch flanging property was evaluated by measuring the bore
expanding ratio (A) by the method described below. From the annealed steel
sheet, a 100-mm square bore expanding test specimen was sampled. A 10-mm
diameter punched hole was formed with a clearance being 12.5%, the punched
hole was expanded from the shear drop side by using a cone-shaped punch
having a front edge angle of 60°, and the expansion ratio of the hole at the time
when a crack penetrating the sheet thickness was generated was measured. This
expansion ratio was used as the bore expanding ratio.
Table 3 gives the metallic structure observation results and the
performance evaluation results of the cold-rolled steel sheet after being annealed.
In Tables 1 to 3, mark "*" attached to a symbol or numeral indicates that the
symbol or numeral is out of the range of the present invention.
18
19
20
21
22
23
- 24
25
26
27
I) Cold rolling ratio: Total draft of cold rolling; 2) NR: Number density of retained austenite grain having grain size of 1.2 prn or larger; 3) El: Total elongation converted so as to correspond to 1.2-mm
thickness, A: Bore expanding ratio, n value: work hardening index
L
M
M
N
0
P
Q
R
S
T
1 .O
1 .O
1 .O
I .O
1 .O
1 .O
1 .O
1 .O
1 .O
1 .O
50
50
50
50
50
50
50
50
50
50
77
65
61
61
74
85
77
77
84
73
13
I0
13
14
12
11
8
9
9
10
10
25
26
25
14
4
15
14
7
17
0.51
0.54
0.62
0.65
0.55
0.43
0.42
0.41
0.43
0.47
2.0
4.7
4.8
4.5
2.3
0.7
2.9
2.8
1.4
2.5
0.014
0.018
0.025
0.028
0.021
0.008
0.006
0.007
0.007
0.010
457
569
575
527
693
571
587
535
699
534
937
985
901
879
993
1071
1011
986
1061
999
22.3
22.6
26.4
27.1
22.2
19.3
21.5
21.6
20.3
22.8
0.243
0.172
0.184
0.193
0.169
0.187
0.192
0.199
0.177
0.212
54
52
59
64
53
49
77
72
86
75
20895
22261
23786
23821
228
169
166
170
6086268
6380356
6221343
6470846
6593099
6931675
9875695
8849592
11973320
9425895
22045
20670
21737
21298
21538
22777
168
200
194
196
188
212
All of the test results of cold-rolled steel sheets produced under the
conditions defined in the present invention were the value of TS x El being
15,000 MPa% or higher, the value of TS x n value being 150 or higher, and the
value of TS'.~x h being 4,500,000 MP~'.~o%r h igher, exhibiting excellent
ductility, work hardening property, and stretch flanging property. In particular,
all of the test results of the metallic structure of hot-rolled steel sheet in which the
average grain size of bcc grains surrounded by a grain boundary having an
orientation difference of 15" or larger was 4.0 pm or smaller, and the cooling
stop temperature after annealing was 340°C or higher were the value of TS x El
being 19,000 MPa% or higher, the value of TS x n value being 160 or higher,
and the value of TS'.' x h being 5,500,000 MP~'-~o%r h igher, exhibiting
especially excellent ductility, work hardening property, and stretch flanging
property.
Example 2
Example 2 describes an example of the case where in the metallic structure
of hot-rolled steel sheet, the average grain size of bcc grains surrounded by a
grain boundary having an orientation difference of 15" or larger is 6.0 pm or
smaller, and the average number density of iron carbides is 1.0 x 10-'iPm2 or
higher.
By using an experimental vacuum melting hrnace, steels each having the
chemical composition given in Table 4 were melted and cast. These ingots
were formed into 30-mm thick billets by hot forging. The billets were heated to
1200°C by using an electric heating furnace and held for 60 minutes, and
thereafter were hot-rolled under the conditions given in Table 5.
Specifically, by using an experimental hot-rolling mill, 6-pass rolling was
performed in the temperature region of Ar3 point or higher to finish each of the
billets into a steel sheet having a thickness of 2 to 3 mm. The draft of the final
one pass was set at 22 to 42% in thickness decrease percentage. After hot
rolling, the steel sheet was cooled to a temperature of 650 to 720°C under various
cooling conditions by using a water spray. Successively, after having been
allowed to cool for 5 to 10 seconds, the steel sheet was cooled to various
temperatures at a cooling rate of 60°C/s, and these temperatures were taken as
coiling temperatures. The steel sheet was charged into an electric heating
hrnace that was held at that temperature, and was held for 30 minutes.
Thereafter, the gradual cooling after coiling was simulated by furnace-cooling
5 the steel sheet to room temperature at a cooling rate of 20°C/h, whereby a hotrolled
steel sheet was obtained.
The obtained hot-rolled steel sheet was heated to various heating
temperatures given in Table 5 at a heating rate of 50°Ck. After being held for
various periods of time or without being held, the steel sheet was cooled to room
10 temperature at a cooling rate of 20°C/h, whereby a hot-rolled and annealed steel
sheet was obtained.
The average grain size of bcc grains of the obtained hot-rolled and
annealed steel sheet was measured by the method described in Example 1. Also,
the average number density of iron carbides of the hot-rolled and annealed steel
15 sheet was determined by the method using the aforementioned SEM and Auger
electron spectroscope.
Next, the obtained hot-rolled and annealed steel sheet was pickled to form
a base metal for cold rolling. The base metal was cold-rolled at a cold rolling
ratio of 50 to 60%, whereby a cold-rolled steel sheet having a thickness of 1.0 to
20 1.2 mm was obtained. By using a continuous annealing simulator, the obtained
cold-rolled steel sheet was heated to 550°C at a heating rate of 10°C/s, thereafter
being heated to various temperatures given in Table 5 at a heating rate of 2"C/s,
and was soaked for 95 seconds. Subsequently, the steel sheet was cooled to
various cooling stop temperatures given in Table 2 with the average cooling rate
25 from 700°C being 60°C/s, being held at that temperature for 330 seconds, and
thereafter was cooled to room temperature, whereby an annealed steel sheet was
obtained.
Note) 1. Ac3 point was determined from thermal expansion change at the time when cold-rolled steel sheet was heated at 2OCIs.
2. Ar, point was determined from thermal expansion change at the time when cold-rolled steel sheet was heated to 900°C and thereafter
was cooled at 0.0 1 'CIS.
Steel
A
B
C
D
E
F
G
H
I
J
K
L
M
N
0
P
Q
R
-
C
0.124
0.145
0.143
0.138
0.149
0.146
0.151
0.166
0.174
0.176
0.175
0.203
0.197
0.197
0.150
0.151
0.149
0.148
Ac3 pint
("(3
792
83 6
849
872
862
86 1
849
856
839
843
848
855
870
848
872
862
864
877
Ar3 point
("c)
698
742
756
757
752
770
760
74 1
742
736
750
768
78 1
786
783
772
775
74 1
Si
0.05*
0.99
1.23
1.49
1.49
1.23
1.52
1.5 1
1.26
1.26
1.25
1.28
1.26
1.28
1.5 1
1 .SO
1.25
1.26
Mn
2.97
2.49
2.50
2.50
2.48
2.45
2.8 1
2.53
2.50
2.5 1
2.50
1.93
1.92
2.24
2.5 1
2.52
2.47
2.48
Chemical
P
0.01 1
0.012
0.009
0.009
0.010
0.009
0.0 10
0.010
0.008
0.008
0.008
0.009
0.009
0.009
0.008
0.009
0.009
0.009
and impurities)
Others
Nb:O.Ol 1
Nb:O.O 1 1
Nb:O.Ol 1
Nb:O.Ol 1
Nb:0.0 13
Nb:O.O 1 1
Tk0.021
Nb:O.Ol 1
Nb:0.010
Nb:O.Ol 1 Cr:0.30
V:O. 1 1 REM:0.0006
Bk0.008
Ca:0.0009 Mg:0.0007
Mo:O. 10 B:0.0015
composition
S
0.003
0.004
0.00 1
0.00 1
0.001
0.001
0.00 1
0.00 1
0.00 1
0.001
0.001
0.001
0.00 1
0.001
0.001
0.00 1
0.00 1
0.00 1
(mass%)
sol.Al
0.031
0.029
0.052
0.053
0.050
0.140
0.045
0.048
0.050
0.051
0.050
0.051
0.140
0.15 1
0.052
0.047
0.152
0.141
(remainder: Fe
N
0.0041
0.0048
0.0028
0.0026
0.0035
0.0031
0.0030
0.0032
0.0032
0.0031
0.0033
0.0027
0.0033
0.0029
0.0034
0.0031
0.0033
0.0030
e
[Table 51
For the obtained annealed steel sheet, the volume fractions of lowtemperature
transformation producing phase, retained austenite, and polygonal
ferrite, the average grain size of retained austenite, the number density (NR) per
unit area of retained austenite grains each having a grain size of 1.2 pm or larger,
5 the yield stress (YS), the tensile strength (TS), the total elongation (El), the work
hardening index (n value), and the bore expanding ratio (h) were measured as
described in Example 1. Table 6 gives the metallic structure observation results
and the performance evaluation results of the cold-rolled steel sheet after being
annealed. In Tables 4 to 6, mark "*" attached to a symbol or numeral indicates
10 that the symbol or numeral is out of the range of the present invention.
[Table 61
I)
All of cold-rolled steel sheets produced pursuant to the method defined in
the present invention had the value of TS x El being 16,000 MPa% or higher, the
value of TS x n value being 155 or higher, and the value of TS'.~x h being
5,000,000 MP~'.~o%r h igher, exhibiting excellent ductility, work hardening
5 property, and stretch flanging property. All of the example in which the
average grain size of bcc grains surrounded by a grain boundary having an
orientation difference of 15" or larger was 4.0 pm or smaller, the average number
density of iron carbides was 8.0 x 10"/pm2 or higher, and the cooling stop
temperature after annealing was 340°C or higher in the metallic structure of hot-
10 rolled steel sheet had the value of TS x El being 19,000 MPa% or higher, the
value of TS x n value being 160 or higher, and the value of TS'.~x h being
5,500,000 ~ a ' . ~or h%igh er, exhibiting especially excellent ductility, work
hardening property, and stretch flanging property.
15 Example 3
Example 3 describes an example of the case where the coiling temperature
in the hot-rolling process using the immediate rapid cooling method is higher
than 400°C.
By using an experimental vacuum melting furnace, steels each having the
20 chemical composition given in Table 7 were melted and cast. These ingots
were formed into 30-mm thick billets by hot forging. The billets were heated to
1200°C by using an electric heating furnace and held for 60 minutes, and
thereafter were hot-rolled under the conditions given in Table 8.
Specifically, by using an experimental hot-rolling mill, 6-pass rolling was
25 performed in the temperature region of Ar3 point or higher to finish each of the
billets into a steel sheet having a thickness of 2 to 3 mm. The draft of the final
one pass was set at 12 to 42% in thickness decrease percentage. After hot
rolling, the steel sheet was cooled to a temperature of 650 to 730°C under various
cooling conditions by using a water spray. Successively, after having been
30 allowed to cool for 5 to 10 seconds, the steel sheet was cooled to various
temperatures at a cooling rate of 60°C/s, and these temperatures were taken as
coiling temperatures. The steel sheet was charged into an electric heating
hrnace that was held at that temperature, and was held for 30 minutes.
Thereafter, the gradual cooling after coiling was simulated by furnace-cooling
the steel sheet to room temperature at a cooling rate of 20°C/h, whereby a hotrolled
steel sheet was obtained.
The average grain size of bcc grains of the obtained hot-rolled steel sheet
was measured by the method described in Example 1.
Next, the obtained hot-rolled steel sheet was pickled to form a base metal
for cold rolling. The base metal was cold-rolled at a cold rolling ratio of 50 to
69%, whereby a cold-rolled steel sheet having a thickness of 0.8 to 1.2 mm was
obtained. By using a continuous annealing simulator, the obtained cold-rolled
steel sheet was heated to 550°C at a heating rate of 10°C/s, thereafter being
heated to various temperatures given in Table 8 at heating rate of 2"C/s, and was
soaked for 95 seconds. Subsequently, the steel sheet was subjected to primary
cooling to various temperatures given in Table 8, and further was subjected to
secondary cooling from the primary cooling temperature to various temperatures
given in Table 8 with the average cooling rate being 60°C/s, being held at that
temperature for 330 seconds, and thereafter was cooled to room temperature,
whereby an annealed steel sheet was obtained.
Note) 1. Ac3 point was determined from thermal expansion change at the time when cold-rolled steel sheet was heated at 2OCIs.
2. Arj point was determined from thennal expansion change at the time when cold-rolled steel sheet was heated to 900°C and
thereafter was cooled at 0.01 'CIS.
Test
No.
ct thickness of hot-rolled steel sheet. 2) Time from rolling completion to rapid cooling stop. 3) Average cooling rate during rapid urnling.
For the obtained annealed steel sheet, the volume fractions of lowtemperature
transformation producing phase, retained austenite, and polygonal
ferrite, the average grain sizes of retained austenite and polygonal ferrite, the
number density (NR) per unit area of retained austenite grains each having a grain
5 size of 1.2 pm or larger, the yield stress (YS), the tensile strength (TS), the total
elongation (El), the work hardening index (n value), and the bore expanding ratio
(h) were measured as described in Example 1. Table 9 gives the metallic
structure observation results and the performance evaluation results of the coldrolled
steel sheet after being annealed. In Tables 7 to 9, mark "*" attached to a
10 symbol or numeral indicates that the symbol or numeral is out of the range of the
present invention.
e
[Table 91
All of cold-rolled steel sheets produced pursuant to the method defined in
the present invention had the value of TS x El being 15,000 MPa% or higher, the
value of TS x n value being 150 or higher, and the value of TS'-x~ h being
4,500,000 MP~'.~o%r h igher, exhibiting excellent ductility, work hardening
property, and stretch flanging property. All of the example in which the roll
draft of the final one pass of hot rolling was higher than 25%, and the secondary
cooling stop temperature after annealing was 340°C or higher had the value of
TS x El being 19,000 MPa% or higher, the value of TS x n value being 160 or
higher, and the value of TS'.~x h being 5,500,000 MP~'.~o%r h igher, exhibiting
further excellent ductility, work hardening property, and stretch flanging property.
All of the example in which the roll draft of the final one pass of hot rolling was
higher than 25%, the soaking treatment temperature in annealing was (Ac3 point -
40°C) or higher and lower than (Ac3 point + 50°C), after soaking treatment, the
steel sheet was cooled by 50°C or more from the soaking temperature at a
cooling rate of lower than 10.O°C/s, and the secondary cooling stop temperature
was 340°C or higher had the value of TS x El being 20,000 MPa% or higher, the
value of TS x n value being 165 or higher, and the value of TS'.~x h being
6,000,000 MP~'.~o%r h igher, exhibiting still hrther excellent ductility, work
hardening property, and stretch flanging property.
Example 4
Example 4 describes an example of the case where a hot-rolled steel sheet
obtained by setting the coiling temperature at 400°C or lower in the hot-rolling
process using the immediate rapid cooling method is subjected to hot-rolled sheet
annealing.
By using an experimental vacuum melting furnace, steels each having the
chemical composition given in Table 10 were melted and cast. These ingots
were formed into 30-mm thick billets by hot forging. The billets were heated to
1200°C by using an electric heating furnace and held for 60 minutes, and
thereafter were hot-rolled under the conditions given in Table 1 1.
Specifically, by using an experimental hot-rolling mill, 6-pass rolling was
performed in the temperature region of Ar3 point or higher to finish each of the
billets into a steel sheet having a thickness of 2 to 3 mm. The draft of the final
one pass was set at 22 to 42% in thickness decrease percentage. After hot
rolling, the steel sheet was cooled to a temperature of 650 to 720°C under various
cooling conditions by using a water spray. Successively, after having been
allowed to cool for 5 to 10 seconds, the steel sheet was cooled to various
temperatures at a cooling rate of 60°C/s, and these temperatures were taken as
coiling temperatures. The steel sheet was charged into an electric heating
furnace that was held at that temperature, and was held for 30 minutes.
Thereafter, the gradual cooling after coiling was simulated by furnace-cooling
the steel sheet to room temperature at a cooling rate of 20°C/h, whereby a hotrolled
steel sheet was obtained.
The obtained hot-rolled steel sheet was heated to various heating
temperatures given in Table 11 at a heating rate of 50°C/h. After being held for
various periods of time or without being held, the steel sheet was cooled to room
temperature at a cooling rate of 20°C/h, whereby a hot-rolled and annealed steel
sheet was obtained.
The average grain size of bcc grains of the obtained hot-rolled and
annealed steel sheet was measured by the method described in Example 1. Also,
the average number density of iron carbides of the hot-rolled and annealed steel
sheet was determined by the method using the aforementioned SEM and Auger
electron spectroscope.
Next, the obtained hot-rolled and annealed steel sheet was pickled to form
a base metal for cold rolling. The base metal was cold-rolled at a cold rolling
ratio of 50 to 69%, whereby a cold-rolled steel sheet having a thickness of 0.8 to
1.2 mm was obtained. By using a continuous annealing simulator, the obtained
cold-rolled steel sheet was heated to 550°C at a heating rate of 10°C/s, thereafter
being heated to various temperatures given in Table 11 at heating rate of 2"C/s,
and was soaked for 95 seconds. Subsequently, the steel sheet was subjected to
primary cooling to various temperatures given in Table 11, and further was
subjected to secondary cooling from the primary cooling temperature to various
temperatures given in Table 1 1 with the average cooling rate being 60°C/s, being
held at that temperature for 330 seconds, and thereafter was cooled to room
i
I temperature, whereby an annealed steel sheet was obtained.
Note) 1. Ac3 point was determined from thermal expansion change at the time when cold-rolled steel sheet was heated at 2OCIs.
2. Ar3 point was determined from thermal expansion change at the time when cold-rolled steel sheet was heated to 900°C and
thereafter was cooled at 0.0 1 OC/s.
0
[Table 111
For the obtained annealed steel sheet, the volume fractions of lowtemperature
transformation producing phase, retained austenite, and polygonal
ferrite, the average grain sizes of retained austenite and polygonal ferrite, the
number density (NR) per unit area of retained austenite grains each having a grain
size of 1.2 pm or larger, the yield stress (YS), the tensile strength (TS), the total
elongation (El), the work hardening index (n value), and the bore expanding ratio
(A) were measured as described in Example 1. Table 12 gives the metallic
structure observation results and the performance evaluation results of the coldrolled
steel sheet after being annealed. In Tables 10 to 12, mark "*" attached to
a symbol or numeral indicates that the symbol or numeral is out of the range of
the present invention.
a
[Table 121
All of cold-rolled steel sheets produced pursuant to the method defined in
the present invention had the value of TS x El being 15,000 MPa% or higher, the
value of TS x n value being 150 or higher, and the value of TS'.~x h being
4,500,000 ~ a ' . o~r h%igh er, exhibiting excellent ductility, work hardening
5 property, and stretch flanging property. All of the example in which the roll
draft of the final one pass of hot rolling was higher than 25%, and the secondary
cooling stop temperature after annealing was 340°C or higher had the value of
TS x El being 19,000 MPa% or higher, the value of TS x n value being 160 or
higher, and the value of TS'.~x h being 5,500,000 MPale7%o r higher, exhibiting
10 further excellent ductility, work hardening property, and stretch flanging property.
All of the example in which the roll draft of the final one pass of hot rolling was
higher than 25%, the total draft of cold rolling was higher than 50%, the soaking
treatment temperature in annealing was (Ac3 point - 40°C) or higher and lower
than (Ac3 point + 50°C), after soaking treatment, the steel sheet was cooled by
15 50°C or more from the soaking temperature at a cooling rate of lower than
10.O°C/s, and the secondary cooling stop temperature was 340°C or higher had
the value of TS x El being 20,000 MPa% or higher, the value of TS x n value
being 165 or higher, and the value of TS'.~x h being 6,000,000 ~ a ' . ~or %
higher, exhibiting still further excellent ductility, work hardening property, and
20 stretch flanging property.

1. A method for manufacuring a cold-rolled steel sheet having a metallic
structure such that the main phase is a low-temperature transformation producing
5 phase, and the secondary phase contains retained austenite, characterized by
comprising the following steps (A) and (B):
(A) a cold-rolling step in which a hot-rolled steel sheet having a chemical
composition consisting, in mass percent, of C: more than 0.020% and less than
0.30%, Si: more than 0.10% and at most 3.00%, Mn: more than 1.00% and at
10 most 3.50%, P: at least 0.10%, S: at most 0.010%, sol.Al: at least 0% and at most
2.00%, N: at most 0.010%, Ti: at least 0% and less than 0.050%, Nb: at least 0%
I
I and less than 0.050%, V: at least 0% and at most 0.50%, Cr: at least 0% and at
I most 1.0%, Mo: at least 0% and at most 0.50%, B: at least 0% and at most I
I
I 0.010%, Ca: at least 0% and at most 0.010%, Mg: at least 0% and at most
I
I 15 0.010%, REM: at least 0% and at most 0.050%, and Bi: at least 0% and at most
0.050%, the remainder of Fe and impurities, wherein the average grain size of the
grains having a bcc structure and the grains having a bct structure surrounded by
I a grain boundary having an orientation difference of 15" or larger is 6.0 pm or
I smaller, is subjected to cold rolling to form a cold-rolled steel sheet; and
20 (B) an annealing step in which the cold-rolled steel sheet is subjected to
soaking treatment in the temperature region of (Ac3 point - 40°C) or higher,
thereafter cooled to the temperature region of 500°C or lower and 300°C or
higher, and is held in that temperature region for 30 seconds or longer.
25 2. The method for manufacuring a cold-rolled steel sheet as set forth in
claim 1, wherein the hot-rolled steel sheet is a steel sheet in which the average
number density of iron carbides existing in the metallic structure is 1 .O x 10-
'lPm2 or higher.
30 3. A method for manufacuring a cold-rolled steel sheet having a metallic
structure such that the main phase is a low-temperature transformation producing
I phase, and the secondary phase contains retained austenite, ~ characterized by
comprising the following steps (C) to (E):
(C) a hot-rolling step in which a slab having a chemical composition
consisting, in mass percent, of C: more than 0.020% and less than 0.30%, Si:
5 more than 0.10% and at most 3.00%, Mn: more than 1.00% and at most 3.50%,
P: at most 0.10%, S: at most 0.010%, sol.Al: at least 0% and at most 2.00%, N: at
most 0.010%, Ti: at least 0% and less than 0.050%, Nb: at least 0% and less than
0.050%, V: at least 0% and at most 0.50%, Cr: at least 0% and at most 1.0%, Mo:
at least 0% and at most 0.50%, B: at least 0% and at most 0.010%, Ca: at least
10 0% and at most 0.010%, Mg: at least 0% and at most 0.010%, REM: at least 0%
and at most 0.050%, and Bi: at least 0% and at most 0.050%, the remainder of Fe
and impurities, is subjected to hot rolling such that the roll draft of the final one
pass is higher than 15%, and rolling is finished in the temperature region of AT3
point or higher to form a hot-rolled steel sheet, and the hot-rolled steel sheet is
15 cooled to the temperature region of 780°C or lower within 0.4 seconds after the
completion of the rolling, and is coiled in the temperature region of higher than
400°C;
(D) a cold-rolling step in which the hot-rolled steel sheet obtained by the
step (C) is subjected to cold rolling to form a cold-rolled steel sheet; and
20 (E) an annealing step in which the cold-rolled steel sheet is subjected to
soaking treatment in the temperature region of (Ac3 point - 40°C) or higher,
thereafter cooled to the temperature region of 500°C or lower and 300°C or
higher, and is held in that temperature region for 30 seconds or longer.
25 4. A method for manufacuring a cold-rolled steel sheet having a metallic
structure such that the main phase is a low-temperature transformation producing
phase, and the secondary phase contains retained austenite, characterized by
comprising the following steps (F) to (I):
(F) a hot-rolling step in which a slab having a chemical composition
30 consisting, in mass percent, of C: more than 0.020% and less than 0.30%, Si:
more than 0.10% and at most 3.00%, Mn: more than 1.00% and at most 3.50%,
P: at most 0.10%, S: at most 0.010%, sol.Al: at least 0% and at most 2.00%, N: at
I most 0.010%, Ti: at least 0% and less than 0.050%, Nb: at least 0% and less than
I
I 0.050%, V: at least 0% and at most 0.50%, Cr: at least 0% and at most 1.0%, Mo:
at least 0% and at most 0.50%, B: at least 0% and at most 0.010%, Ca: at least
0% and at most 0.010%, Mg: at least 0% and at most 0.010%, REM: at least 0%
5 and at most 0.050%, and Bi: at least 0% and at most 0.050%, the remainder of Fe
and impurities, is subjected to hot rolling such that the rolling is finished in the
temperature region of AT3 point or higher to form a hot-rolled steel sheet, and the
hot-rolled steel sheet is cooled to the temperature region of 780°C or lower
within 0.4 seconds after the completion of the rolling, and is coiled in the
10 temperature region of lower than 400°C;
(G) a hot-rolled sheet annealing step in which the hot-rolled steel sheet
obtained by the step (F) is subjected to annealing such that the hot-rolled steel
sheet is heated to the temperature region of 300°C or higher to form a hot-rolled
and annealed steel sheet;
15 (H) a cold-rolling step in which the hot-rolled and annealed steel sheet is
subjected to cold rolling to form a cold-rolled steel sheet; and
(I) an annealing step in which the cold-rolled steel sheet is subjected to
soaking treatment in the temperature region of (Ac3 point - 40°C) or higher,
thereafter cooled to the temperature region of 500°C or lower and 300°C or
20 higher, and is held in that temperature region for 30 seconds or longer.
5. The method for manufacuring a cold-rolled steel sheet as set forth in
any one of claims 1 to 4, wherein, in the metallic structure of the cold-rolled steel
sheet, the secondary phase contains retained austenite and polygonal ferrite.
25
6. The method for manufacuring a cold-rolled steel sheet as set forth in
any one of claims 1 to 5, wherein, in the cold-rolling step (A), (D) or (H), the
cold rolling is performed at a total draft exceeding 50%.
30 7. The method for manufacuring a cold-rolled steel sheet as set forth in
ahy one of claims 1 to 6, wherein, in the annealing step (B), (E) or (I), the
soaking treatment is performed in the
higher and lower than (Ac3 point + 50°C).
8. The method for manufacuring a cold-rolled steel sheet as set forth in
5 any one of claims 1 to 7, wherein, in the annealing step (B), (E) or (I), the
cooling is performed by 50°C or more at a cooling rate of lower than 10.O°C/s
after the soaking treatment.
9. The method for manufacuring a cold-rolled steel sheet as set forth in
10 any one of claims 1 to 8, wherein the chemical composition contains, in mass
percent, one kind or two or more kinds selected from a group consisting of Ti: at
least 0.005% and less than 0.050%, Nb: at least 0.005% and less than 0.050%,
and V: at least 0.010% and at most 0.50%.
15 10. The method for manufacuring a cold-rolled steel sheet as set forth in
any one of claims 1 to 9, wherein the chemical composition contains, in mass
percent, one kind or two or more kinds selected from a group consisting of Cr: at
most 0.20% and at most 1.0%, Mo: at least 0.05% and at most 0.50%, and B: at
least 0.0010% and at most 0.010%.
11. The method for manufacuring a cold-rolled steel sheet as set forth in
any one of claims 1 to 10, wherein the chemical composition contains, in mass
percent, one kind or two or more kinds selected from a group consisting of Ca: at
least 0.0005% and at most 0.010%, Mg: at least 0.0005% and at most 0.010%,
25 REM: at least 0.0005% and at most 0.050%, and Bi: at least 0.0010% and at most
Dated this 1 5'h day of January, 2014.
Sumitorno Metal Corporation
Attorneys for the Applicant