ORIGINAL
-x-
COLD-ROLLED STEEL SHEET
Technical Field
The invention relates to a cold-rolled steel sheet. More particularly, it
5 relates to a high-strength cold-rolled steel sheet that is excellent in ductility, work
hardenability, and stretch flangeability. Background Art
In these days when the industrial technology field is highly fractionalized,
10 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 pressformed
and put in use, more excellent formability has been required with the
diversification of press shapes. In addition, as a high strength has been required,
the use of a high-strength cold-rolled steel sheet has been studied. In particular,
15 concerning an automotive steel sheet, in order to reduce the vehicle body weight
and thereby to improve the fie1 economy from the perspective of global
environments, a demand for a thin high-strength cold-rolled steel sheet having
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.
20 Therefore, a steel sheet further excellent in ductility and stretch flangeability is
required. However, the press formability and the high strengthening of steel
sheet are characteristics contrary to each other, and therefore it is difficult to
satis@ these characteristics at the same time.
So far, as a method for improving the press formability of a high-strength
25 cold-rolled 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
subjected to rolling at a total draft of 80% or higher in a temperature region in the
vicinity of AQ point in the hot-rolling process. Patent Document 2 discloses a
30 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 in hotrolled
steel sheet is improved. However, the above-described Patent
9 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, the crystal grains
5 are liable to be coarsened, and it is difficult to obtain a cold-rolled steel sheet
excellent in press formability. In particular, in the manufacturing of a complex
phase cold-rolled steel sheet containing a low-temperature transformation
product or retained austenite in the structure, which must be annealed in the hightemperature
region of Acl point or higher, the coarsening of crystal grains at the
10 time of annealing is remarkable, and the advantage of complex phase 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 ultrafrne grains, in which method, rolling in the dynamic
recrystallization region is performed with a rolling pass of five or more stands.
15 However, the lowering of temperature at the hot-rolling must be decreased
extremely, and it is difficult to carry out this method in a general hot-rolling
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 hole expansibility (stretch flangeability) is poor, and the
20 press formability 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 crashworthiness and formability, in which retained austenite having
an average crystal grain size of 5 pm or smaller is dispersed in ferrite having an
25 average crystal grain size of 10 pn or smaller. The steel sheet containing
retained austenite in the structure exhibits a large elongation due to
transformation induced plasticity (TRIP) produced by the transformation from
austenite to martensite during working; however, the hole expansibility is
impaired by the formation of hard martensite. For the cold-rolled steel sheet
30 disclosed in Patent Document 4, it is supposed that the ductility and hole
expansibility are improved by making ferrite and retained austenite fine.
However, the limiting hole expansion ratio is at most 1.5, and it is difficult to say
that sufficient press formability is provided. Also, to enhance the work
'C) hardening coefficient and to improve the crashworthiness, it is necessary to make
the main phase a soft ferrite, and it is difficult to obtain a high-strength strength.
Patent Document 5 discloses a high-strength steel sheet excellent in
elongation and stretch flangeability, in which the secondary phase consisting of
5 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 temperature
for a long period of time, so that the rise in production cost and the decrease in
10 productivity are remarkable.
Patent Document 6 discloses a high-strength hot dip galvanized steel sheet
excellent in ductility, stretch flangeability, and fatigue resistance property, in
which retained austenite and low-temperature transformation product are
dispersed in ferrite and tempered martensite having an average crystal grain size
15 of 10 pm or smaller. The tempered martensite is a phase that is effective in
improving the stretch flangeability and fatigue resistance property, and it is
supposed that if grain refinement of tempered martensite is performed, these
properties are M e r improved. However, in order to obtain a metallurgical
structure containing tempered martensite and retained austenite, primary
20 annealing for forming martensite and secondary annealing for tempering
martensite and M e r 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
sheet in which retained austenite is dispersed in fine ferrite, in which method, the
25 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.
30 Citation List
Patent Document
Patent Document 1 : JP 58-123823 A1
Patent Document 2 : JP 59-2294 13 A1
Patent Document 3 : JP 1 1- 152544 A1
Patent Document 4: JP 1 1-6 1326 A1
Patent Document 5: JP 2005- 179703 A1
Patent Document 6: JP 2001-192768 A1
5 Patent Document 7: W02007115541 A1
Summary of Invention
The above-described technique disclosed in Patent Document 7 is
excellent in that a cold-rolled steel sheet in which a fine grain structure is formed
10 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.
However, due to needs for higher performance in recent years, a cold-
15 rolled steel sheet provided with a high strength, good ductility, excellent work
hardenability, and excellent stretch flangeability at the same time has come to be
demanded.
The invention has been made to meet such a demand. Specifically, an
objective of the invention is to provide a high-strength cold-rolled steel sheet
20 having excellent ductility, work hardenability, and stretch flangeability, in which
the tensile strength is 780 MPa or higher.
The present inventors conducted detailed examinations of the influence of
chemical composition and manufacturing conditions exerted on the mechanical
properties of a high-strength cold-rolled steel sheet. In this description, symbol
25 "%" 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
3.00% or less, Mn: more than 1.00% and 3.50% or less, P: 0.10% or less, S:
30 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.
After being hot-rolled, the steel sheets were cooled to the temperature region of
720°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
furnace 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. The hot-rolled steel sheets thus obtained
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.
From each of hot-rolled steel sheets and annealed steel sheets, a test
specimen for micro structure observation was sampled. By using a optical
microscope and scanning electron microscope (SEM) equipped an electron
backscatter diffkaction pattern (EBSP) analyzer, the structure was observed at a
position deep by one-fourth of thickness fiom the surface of steel sheet, and by
using an X-ray diffkactometry (XRD) apparatus, the volume fraction 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 tensile test was conducted,
whereby the ductility was evaluated by total elongation, and the work
hardenability was evaluated by the work hardening coefficient (n-value) in the
strain range of 5 to 10%. Further, from the annealed steel sheet, a 100-mm
square hole expanding test specimen was sampled. By using this test specimen,
a hole expanding on test was conducted, whereby the stretch flangeability was
evaluated. In the hole expanding test, a 10-rnm 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 top angle of 60°, and the expansion ratio
(limiting hole expansion 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 (H) 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
5 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 720°C or lower within 0.40 second after the completion of hot rolling, is coldrolled
and annealed, the ductility and stretch flangeability of annealed steel sheet
are improved with the rise in annealing temperature. However, if the annealing
10 temperature is too high, the austenite grains are coarsened, and the ductility and
stretch flangeability of annealed steel sheet may be deteriorated abruptly.
(B) Increase in final rolling reduction in the hot rolling restrains
coarsening of austenite grains that may occur during the annealing at a high
temperature after the cold rolling. The reason for this is not clear, but this is
15 assumed to result from the fact that: (a) as the final rolling reduction becomes
more increased, ferrite fraction in the structure of the hot-rolled steel sheet
becomes more increased, and refinement of ferrite becomes more encouraged as
well; (b) as the final rolling reduction becomes more increased, a coarse lowtemperature
transformation product in the structure of the hot-rolled steel sheet
20 becomes more decreased; (c) ferrite grain boundaries function as nucleation sites
in transformation from ferrite to austenite during the annealing, and thus as there
exist more refined ferrite grains, a nucleation rte become more increased, so that
austenite becomes more refined; and (d) a coarse low-temperature transformation
product becomes coarse austenite grains during the annealing.
25 (C) If the coiling temperature is increased in a coiling process after rapid
cooling immediately after rolling, coarsening of austenite grains that may occur
during the annealing at a high temperature after the cold rolling is restrained.
The reason for this is not clear, but this is assumed to result from the fact that: (a)
the hot-rolled steel sheet is refined due to the rapid cooling immediately after
30 rolling, and thus increase in coiling temperature significantly increases the
amount of precipitation of iron carbide in the hot-rolled steel sheet; (b) the iron
carbide functions as a nucleation site in transformation from ferrite to austenite
during the annealing, and thus as the amount of precipitation of iron carbide
becomes more increased, the nucleation rate becomes more increased, thereby to
refine austenite; and (c) undissolved iron carbide suppresses growth of austenite
grains, which results in refinement of austenite.
(D) As the Si content becomes greater in the steel, an effect of preventing
5 coarsening of austenite grains becomes stronger. The reason for this is not clear,
but this is assumed to result from the fact that: (a) increase in the Si content
causes refinement of iron carbide, which increases its number density; (b) hence,
the nucleation rate in transformation from ferrite to austenite becomes further
increased; and (c) increase in undissolved iron carbide Wher suppresses growth
10 of austenite grains, which encourages further refinement of austenite.
(E) By soaking the steel at a high temperature while restraining coarsening
of austenite grains and then cooling it, it is possible to obtain metallurgical
structure whose main phase is a refined low-temperature transformation product,
and whose secondary phase contains refined retained austenite, and also contains
15 refined polygonal ferrite in some cases.
Figure 1 is a graph showing the result of examination of grain size
distribution of retained austenite in an annealed steel sheet obtained by hotrolling
under the conditions of the final rolling reduction of 42% in thickness
decrease percentage, the finish rolling temperature of 900°C, the rapid cooling
20 stop temperature of 660°C, and the time of 0.16 seconds fiom rolling completion
to rapid cooling stop, and 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 examination of grain size distribution of retained austenite in an
annealed steel sheet obtained by hot-rolling a slab having the same chemical
25 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 having the grain size of 1.2 pm or larger is
30 restrained, and retained austenite is dispersed finely.
(F) Suppression of generation of coarse retained austenite grains whose
grain size is 1.2 pm or more enhances the stretch flangeability of the steel sheet
whose main phase is a low-temperature transformation product.
Figure 3 is a graph showing a relation between TS'.~x h and the number
density (NR) of coarse retained austenite whose grain size is 1.2 pm or more.
TS denotes a tensile strength, h denotes a limiting hole expansion ratio, and TS'.~
x h denotes an coefficient for evaluating the hole expansiblity based on the
balance between the strength and the limiting hole expansion ratio. As shown
in this drawing, it is understood that TS'.~ x h and NR has a correlation, and as NR
becomes smaller, the hole expansiblity becomes more enhanced. The reason for
this is not clear, but this is assumed to result from the fact that: (a) retained
austenite is changed into hard martensite through working, and if the retained
austenite grains are coarse, the martensite grains also become coarse, and stress
concentration becomes increased, which easily causes void at an interface with a
matrix phase, resulting in initiation of cracking; and (b) coarse retained austenite
grains become martensite at an early stage of working, and thus they more easily
become initiation of cracking than refined retained austenite grains do.
(G) As the annealing temperature becomes more increased, the fraction of
the low-temperature transformation product becomes more increased, so that
work hardenability tends to be deteriorated; however, it is possible to prevent
deterioration of the work hardenability in the steel sheet whose main phase is the
low-temperature transformation product by suppressing generation of coarse
retained austenite grains having a grain size of 1.2 pm or more.
Figure 4 is a graph showing a relation between a TS x n-value and NR.
The TS x n-value is an coefficient for evaluating the work hardenability based on
the balance between the strength and the work hardening coefficient. As shown
in this drawing, it is understood that the TS x n-value has a correlation with NR,
and as NR becomes smaller, the work hardenability becomes more enhanced.
The reason for this is not clear, but it is assumed to result fiom the fact that: (a)
coarse retained austenite grains become martensite at an early stage of working
where strain is less than 5%, and thus they hardly contribute to increase in the nvalue
within the strain range of 5 to 10%; and (b) by suppressing generation of
coarse retained austenite grains, refined retained austenite grains that become
martensite in a high strain range of 5% or more become increased.
(H) As grains having a bcc (body-centered cubic) structure and grains
having a bct (body-centered tetragonal) structure (two kinds of these grains are
also collectively referred to as "bcc grains", hereinafter), which are surrounded
by grain boundaries whose misorientation angle is 15" or more, have smaller
average grain sizes, ductility, work hardenability, and stretch flangeability of the
steel sheet having metallurgical structure whose main phase is the lowtemperature
transformation product, and whose secondary phase contains
retained austenite are enhanced. The reason for this is not clear, but this is
assumed to result from the fact that: (a) the arrangement of retained austenite
becomes more preferable due to the refinement of the bcc grains; and (b) crack
propagation is suppressed by the refinement of the bcc grains.
Based on the above results, it has been found that steel containing certain
amount or more of Si is hot-rolled with greater final rolling reduction, and
thereafter is subjected to rapid cooling immediately after rolling, the steel is
coiled state at a high temperature, and is subjected to cold rolling, and then is
annealed at a high temperature, and thereafter is cooled, thereby to produce a
cold-rolled steel sheet excellent in ductility, work hardenability, and stretch
flangeability, and including metallurgical structure whose main phase is a lowtemperature
transformation product, and whose secondary phase contains
retained austenite and preferably M e r contains polygonal ferrite, wherein the
metallurgical structure contains fewer coarse austenite grains whose grain size is
1.2 pm or more, and preferably contains refined bcc grains.
The present invention provides a cold-rolled steel sheet including a
chemical composition consisting, in mass percent, of C: more than 0.020% to
less than 0.30%; Si: more than 0.10% to 3.00% or less; Mn: more than 1.00% to
3.50% or less; P: 0.10% or less; S: 0.010% or less; sol.Al: 0% or more to 2.00%
or less; N: 0.010% or less; Ti: 0% or more to less than 0.050%; Nb: 0% or more
to less than 0.050%; V: 0% or more to 0.50% or less; Cr: 0% or more to 1 .O% or
less; Mo: 0% or more to 0.50% or less; B: 0% or more to 0.010% or less; Ca: 0%
or more to 0.010% or less; Mg: 0% or more to 0.010% or less; REM: 0% or more
to 0.050% or less; Bi: 0% or more to 0.050% or less; and the remainder being Fe
and impurities, wherein the cold-rolled steel sheet includes metallurgical
structure whose main phase is a low-temperature transformation product, and
whose secondary phase contains retained austenite; the retained austenite has a
volume fraction of more than 4.0% to less than 25.0% relative to overall structure,
iC) and an average grain sire of less than 0.80 pm; and of the retained austenite, a
number density of retained austenite grains whose grain size is 1.2 pm or more is
3.0 x 1o ' ~gr ains/pm2o r less.
The metallurgical structure of the cold-rolled steel sheet according to the
5 present invention preferably satisfies one or both of the followings:
- the average grain size of grains having bcc structure and grains having
bct structure that are surrounded by grain boundaries whose misorientation angle
is 15" or more is 7.0 pm or less; and
- the secondary phase contains retained austenite and polygonal ferrite, and
10 the polygonal ferrite has a volume fraction of more than 2.0% to less than 27.0%
relative to overall structure, and an average grain size of less than 5.0 pm.
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 fiom a group consisting of Ti:
15 0.005% or more and less than 0.050%, Nb: 0.005% or more and less than 0.050%,
and V: 0.010% or more and 0.50% or less; andlor
One kind or two or more kinds selected fiom a group consisting of Cr:
0.20% or more and 1.0% or less, Mo: 0.05% or more and 0.50% or less, and B:
0.00 10% or more and 0.0 10% or less; and/or
20 One kind or two or more kinds selected from a group consisting of Ca:
0.0005% or more and 0.010% or less, Mg: 0.0005% or more and 0.01 0% or less,
REM: 0.0005% or more and 0.050% or less, and Bi: 0.0010% or more and
0.050% or less.
According to the present invention, a high-strength cold-rolled steel sheet
25 having sufficient ductility, work hardenability, and stretch flangeability, which
can be used for working such as press forming, can be obtained. 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 weight reduction of automotive vehicle body.
30
Brief Description of Drawings
[Figure 11 Figure 1 is a graph showing grain size distribution of retained
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
austenite in an annealed steel sheet produced without an immediate rapid cooling
process.
[Figure 31 Figure 3 is a graph showing a relation between TS'.~x h and a number
density (NR) of retained austenite whose grain size is 1.2 pm or more.
[Figure 41 Figure 4 is a graph showing a relation between a TS x n-value and the
number density (NR) of the retained austenite whose grain size is 1.2 pm or more.
Description of Embodiments
The metallurgical structure and chemical composition in a high-strength
cold-rolled steel sheet in accordance with the present invention, and the rolling
and annealing conditions and the like in the method of producing the steel sheet
efficiently, steadily, and economically are described in detail below.
1. Metallurgical structure
The cold-rolled steel sheet of the present invention includes metallurgical
structure whose main phase is a low-temperature transformation product, and
whose secondary phase contains retained austenite and preferably M e r
contains polygonal ferrite, the retained austenite has a volume fraction of more
than 4.0% to less than 25.0% relative to the overall structure, and an average
grain size thereof is less than 0.80 pm, and of the retained austenite, the number
density of retained austenite grains whose grain size is 1.2 pm or more is 3.0 x
10" grains/pm2 or less, and the average grain size of grains having the bcc
structure and grains having the bct structure that are surrounded by grain
boundaries whose misorientation angle is preferably 15" or more is 7.0 pm or
less, andlor the volume fiaction of the polygonal ferrite relative to the overall
structure is more than 2.0% to less than 27.0%, and the average grain size thereof
is less than 5.0 pm.
The main phase means a phase or structure in which the volume fraction is
at the maximum, and the secondary phase means a phase or structure other than
the main phase.
The low-temperature transformation product means a phase and structure
formed by low-temperature transformation, such as martensite and bainite. As a
low-temperature transformation product other than these, bainitic ferrite and
tempered martensite are cited. The bainitic ferrite is distinguished from
5 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 in that iron carbides
do not exist inside and at the interface of grains.
This low-temperature transformation product may contain two or more
kinds of phases and structures, for example, martensite and bainitic ferrite. In
10 the case where the low-temperature transformation product contains two or more
kinds of phases and structures, the sum of volume fractions of these phases and
structures is defrned as the volume fraction of the low-temperature
transformation product.
The bcc phase is a phase having a body-centered cubic lattice (bcc lattice)
15 type crystal structure, and this phase may be exemplified by polygonal ferrite,
bainitic ferrite, bainite, and tempered martensite. Meanwhile, the bct phase is a
phase having a body-centered tetragonal lattice (bct lattice) type crystal structure,
and this phase may be exemplified by martensite. Grains having the bcc
structure are a region surrounded by boundaries whose misorientation angle is
20 15" or more in the bcc phase. Similarly, grains having the bct structure are a
region surrounded by boundaries whose misorientation angle is 15" or more in
the bct phase. Hereinafter, the bcc phase and the bct phase are also collectively
referred to as the bcc phase. This is because no lattice constant is taken into
account in the metallurgical structure evaluation using an EBSP, and thus the bcc
25 phase and the bct phase are detected without being distinguished from each other.
The reason for configuring the structure to include the low-temperature
transformation product as its main phase and retained austenite in its secondary
phase is because this configuration is preferable to enhance ductility, work
hardenability, and stretch flangeability while maintaining tensile strength. If
30 using polygonal ferrite, which is not the low-temperature transformation product,
as main phase, it becomes difficult to secure the tensile strength as well as the
stretch flangeability.
The volume fraction of the retained austenite relative to the overall
structure is defined to be more than 4.0% to less than 25.0%. If the volume
fraction of the retained austenite relative to the overall structure is 4.0% or less,
the ductility becomes insufficient. Hence, the volume fraction of the retained
5 austenite relative to the overall structure is defined to be more than 4.0%.
Preferably, this ratio is more than 6.0 %, more preferably more than 9.0%, and
fixther more preferably more than 12.0%. On the other hand, if the volume
fraction of the retained austenite relative to the overall structure is 25.0% or more,
deterioration of the stretch flangeability becomes significant. Accordingly, the
10 volume fraction of the retained austenite relative to the overall structure is
defined to be less than 25.0%. Preferably, this ratio is less than 18.0 %, more
preferably less than 16.0%, and further more preferably less than 14.0%.
The average grain size of the retained austenite is defined to be less than
0.80 pm. In the cold-rolled steel sheet including the metallurgical structure
15 whose main phase is the low-temperature transformation product, and whose
secondary phase contains the retained austenite, if the average grain size of the
retained austenite is 0.80 pm or more, deterioration of the ductility, the work
hardenability, and the stretch flangeability becomes significant. Preferably, the
average grain size of the retained austenite is less than 0.70 pm, and more
20 preferably less than 0.60 pm. The lower limit of the average grain size of the
retained austenite is not limited to specific one, but it is necessary to set the final
rolling reduction in the hot rolling to be extremely high in order to refine the
retained austenite to be 0.15 pm or less, which results in significant increase in
production load. Accordingly, it is preferable to define the lower limit of the
25 average grain size of the retained austenite to be more than 0.15 pm.
In the cold-rolled steel sheet including the metallurgical structure whose
main phase is the low-temperature transformation product, and whose secondary
phase contains the retained austenite, if the retained austenite whose average
grain size is even less than 0.80 pm contains more coarse retained austenite
30 grains whose grain size is 1.2 pm or more, the work hardenability and the stretch
flangeability are rather deteriorated. Accordingly, the number density of the
retained austenite grains whose grain size is 1.2 pm or more is defined to be 3 -0
x 1o ' ~gr ains/pm2 or less. Preferably, the retained austenite grains whose grain
size is 1.2 pm or more has a number density of 2.0 x 10" grainslpm2 or less,
more preferably 1.5 x grains/pm2 or less, and most preferably 1.0 x 1 o5
grains/pm2 or less.
To further improve the ductility and work hardenability, the secondary
phase preferably contains polygonal ferrite in addition to retained austenite.
The volume fraction of polygonal ferrite relative to the overall structure
preferably exceeds 2.0%. This volume fraction hrther preferably exceeds 8.0%,
still Wher preferably exceeds 13.0%. On the other hand, if the volume
fraction of polygonal ferrite is excessive, the stretch flangeability deteriorates.
Therefore, the volume fraction of polygonal ferrite is preferably lower than
27.0%, further preferably lower than 24.0%, and still W e r preferably lower
than 18.0%.
As the grains of polygonal ferrite are finer, the effect of improving the
ductility and work hardenability increases. Therefore, the average grain size of
polygonal f e ~ ties p referably made smaller than 5.0 pm. This average grain
size is further preferably smaller than 4.0 pm, still further preferably smaller than
3.0 p.m.
To W e r improve the stretch flangeability, the volume fraction of
tempered martensite contained in the low-temperature transformation product
relative to the overall structure is preferably made lower than 50.0%. This
volume fkaction is mher preferably lower than 35.0%, still further preferably
lower than 10.0%.
To enhance the tensile strength, the low-temperature transformation
product preferably contains martensite. In this case, the volume fraction of
martensite relative to overall structure preferably exceeds 4.0%. This volume
fraction M e r preferably exceeds 6.0%, still further preferably exceeds 10.0%.
On the other hand, if the volume fraction of martensite is excessive, the stretch
flangeability deteriorates. Therefore, the volume fraction of martensite relative
to overall structure is preferably made lower than 15.0%.
In order to M e r enhance the ductility, the work hardenability, and the
stretch flangeability, it is preferable that the average grain size of the bcc grains
(as described above, bcc grains collectively denote grains having the bcc
structure and the bct structure that are surrounded by grain boundaries whose
misorientation angle is 15" or more) is '7.0 pm or less. More preferably, the
average grain size of the bcc grains is 6.0 pm or less, and further more preferably
5.0 pm or less.
The metallurgical structure of the cold-rolled steel sheet in accordance
5 with the present invention is measured as described below. The volume
fractions of low-temperature transformation product 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 etched with nital. Thereafter, the metallurgical structure is
10 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 lowtemperature
transformation product and polygonal ferrite are measured.
Assuming that the area fraction is equal to the volume fraction, the volume
fractions of low-temperature transformation product and polygonal ferrite are
15 determined. The average grain size of polygonal ferrite is determined as
described below. A circle equivalent 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 equivalent diameter is defined as
the average grain size.
20 The volume fraction 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.
25 The grain size of retained austenite grains and the average grain size of
retained austenite are measured as described below. A test specimen is sampled
from the steel sheet, and the longitudinal cross sectional surface thereof parallel
to the rolling direction is electropolished. The metallurgical structure is
observed at a position deep by one-fourth of thickness from the surface of steel
30 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 matrix phase is defined as one retained austenite
grain. By image processing, the number density (number of grains per unit
E 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 equivalent diameters of individual
retained austenite grains are determined, and the mean-value thereof is defined as
5 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 irradiated at a pitch of 0.1 pm to make judgment of
phase. Also, among the measured data, the data in which the reliability index
10 (Confidence Index) is 0.1 or more are used for grain size measurement as
effective data. To prevent the grain size of retained austenite from being
undervalued by measurement noise, only the retained austenite grains each
having a circle equivalent diameter of 0.15 pm or larger is taken as effective
grains, whereby the average grain size is calculated.
15 The average grain size of the bcc grains are measured as follows.
Specifically, a test specimen is collected from steel sheets, a longitudinal cross
sectional surface thereof parallel to the rolling direction of each test specimen is
electropolished, and an observation is conducted on metallurgical structure
thereof at a position of deep by one-fourth of the thickness fiom the surface of
20 steel sheet using an SEM equipped with an EBSP. A region that is observed as
a bcc phase, and is surrounded by boundaries whose misorientation angle is 15"
or more is defined as a one bcc grain, and a value calculated in accordance with
the definition of the following Formula (1) is defmed as the average grain size of
the bcc grains. In this formula, N denotes the number of crystal grains
25 contained in the average grain size evaluation region, Ai denotes an area of an ith
(i = 1,2, ..., N) crystal grain, and di denotes a circle equivalent diameter of the
i-th crystal grain, respectively.
[Expression 11
In the present invention, grains having the bcc structure and grains having
the bct structure are integrally treated. This is because no lattice constant is
taken into account in the metallurgical structure evaluation using the EBSP, so
that it becomes difficult to distinguish grains having the bcc structure (such as
polygonal ferrite, bainitic ferrite, bainite, and tempered martensite) from grains
having the bct structure (such as martensite).
In this structure observation using the EBSP, as similar to the above case,
the phase is determined by irradiation with an electron beam with intervals of 0.1
prn in a region of 50 pm in the sheet thickness direction, and of 100 pm in the
rolling direction. Among obtained measurement data, such data having a
Confident Index of 0.1 or more are used as effective data for measurement of the
grain size. In order to prevent underestimation of the grain size caused by
measurement noises, in the evaluation of the bcc phase, which is different from
the case of the aforementioned retained austenite grains, only the bcc grains
whose grain size is 0.47 pm or more are used as effective grains in the above
grain size calculation. In the case of mixed-grain structure in which refined
grains and coarse grains are mixed, if the grain size is evaluated using a intercept
method that is generally used as a crystal grain size evaluation of metallurgical
structure, influence caused by coarse grains may be underestimated. In the
present invention, as a method of calculating the crystal grain size in
consideration of influence caused by coarse grains, the above Formula (1) that
multiplies an area of an individual crystal grain as a weight is used.
In the present invention, the aforementioned metallurgical structure is
defined at a depth position of 1/4 of the sheet thickness from a steel sheet surface
in the case of using a cold-rolled steel sheet, and at a depth position of 1/4 of the
sheet thickness of a steel sheet that is base metal from a boundary between the
steel sheet that is the base metal and a plated layer in the case of using a plated
steel sheet.
In order to secure the impact energy absorbing property as a mechanical
property that can be attained based on the characteristics of the aforementioned
metallurgical structure, the cold-rolled steel sheet according to the present
invention preferably has a tensile strength (TS) of 780 MPa or more in a
direction vertical to the rolling direction, and more preferably has a tensile
strength of 950 MPa or more. On the other hand, TS is preferably less than
1180 MPa in order to secure the ductility.
In the light of the press formability, it is preferable that El that is a value
obtained by converting a total elongation (Elo) in a direction vertical to the
rolling direction into a total stretch corresponding to that of a sheet thickness of
1.2 mrn based on the following Formula (1); an n-value that is a work hardening
coefficient calculated using nominal strains at two points of 5% and 10% where
the strain range is defined to be 5 to lo%, and respective test forces
corresponding to these strains in compliance with Japanese Industrial Standard
JIS 22253; and h that is a limiting hole expansion ratio measured in compliance
with the Japan Iron and Steel Federation standard JFSTlOOl satisfl the following
conditions:
- a value of TS xEl is 19000 MPa% or more, particularly 20000 MPa or more,
- a value of TS x n-value is 160 MPa or more, particularly 165 MPa or more, and
- a value of TS*.' x h is 5500000 MPala7% or more, particularly 6000000
~ a ' . ~or m%or e.
El = Elo x (1.21b)O.~.. . (2)
where Elo in this formula denotes an actual measurement value of the total
elongation that is measured using each JIS No. 5 tensile test specimen, to denotes
a sheet thickness of each JIS No. 5 tensile test specimen that is used for the
measurement, and El denotes a converted value of the total elongation
corresponding to that of a sheet thickness of 1.2 mm.
The work hardening coefficient is represented by an n-value corresponding
to the strain range of 5 to 10% in the tensile test because a strain generated at the
time of press-forming automotive parts is approximately 5 to 10%. If the steel
sheet has a high total elongation, but has a small n-value, propagation property of
the strain becomes insufficient during the press forming of automotive parts,
which is likely to cause forming defects such as local reduction of the sheet
thickness, etc. Preferably, the yield ratio is less than 80%, more preferably less
than 75%, and further more preferably less than 70% in the light of shape
fixability.
2. Chemical composition of steel
C: more than 0.020% to less than 0.30%
-@ The C content of 0.020% or less makes it difficult to attain the
aforementioned metallurgical structure. Accordingly, the C content is defined
to be more than 0.020%. Preferably, the C content is more than 0.070%, more
preferably more than 0.10%, and hrther more preferably more than 0.14%. On
5 the other hand, the C content of 0.30% or more deteriorates not only the stretch
flangeability but also the weldability of the steel sheet. Accordingly, the C
content is defined to be less than 0.30%. Preferably, the C content is less than
0.25%, more preferably less than 0.20%, and further more preferably less than
0.17%.
10 Si: more than 0.10% to 3.00% or less
- -
Si acts to improve the ductility, the work hardenability, and the stretch
flange property through suppression of austenite grain growth during the
annealing. Si is an element acting to enhance stability of austenite, and
effective to attain the aforementioned metallurgical structure. The Si content of
15 0.10% or less makes it difficult to attain effects caused by the above actions.
Accordingly, the Si content is defined to be more than 0.10%. Preferably, the
Si content is more than 0.60%, more preferably more than 0.90%, and further
more preferably more than 1.20%. On the other hand, the Si content of more
than 3.00% deteriorates surface quality of the steel sheet. In addition, the
20 chemical convertibility and the plating property become significantly
deteriorated. Accordingly, the Si content is defined to be 3.00% or less.
Preferably, the Si content is less than 2.00%, more preferably less than 1.80%,
and further more preferably less than 1.60%.
In the case of containing A1 described later, the Si content and the sol.Al
25 content preferably satisfy Formula (3) below, more preferably satis@ Formula
(4) below, and firher more preferably satisfl Formula (5) below.
Si + sol.Al > 0.60 ... (3)
Si + sol.Al > 0.90 ... (4)
Si + sol.Al > 1.20 ... (5)
30 where in the formulas, Si represents the Si content, and sol.Al represents the
acid-soluble A1 content in mass percent in the steel.
Mn: more than 1.00% to 3.50% or less
Mn is an element that acts to enhance hardenability of the steel, and
effective to attain the aforementioned metallurgical structure. The Mn content
of 1.00% or less makes it difficult to attain the aforementioned metallurgical
structure. Accordingly, the Mn content is defined to be more than 1.00%.
5 Preferably, the Mn content is more than 1.50%, more preferably more than
1.80%, and further more preferably more than 2.10%. An excessive Mn content
causes a coarse low-temperature transformation product expanding in the rolling
direction in the metallurgical structure of the hot-rolled steel sheet, and increases
coarse retained austenite grains in the metallurgical structure after the cold
10 rolling and the annealing, resulting in deterioration of the work hardenability and
the stretch flangeability. Accordingly, the Mn content is defined to be 3.50% or
less. Preferably, the Mn content is less than 3.00%, more preferably less than
2.80%, and firher more preferably less than 2.60%.
I
I P: 0.10% or less
I
I 15 P is an element contained as an impurity in the steel, and segregates to
I
grain boundaries, and embrittles the steel. Hence, it is preferable to define the P
content to be as small as possible. Accordingly, the P content is defined to be
0.10% or less. Preferably, the P content is less than 0.050%, more preferably
less than 0.020%, further more preferably less than 0.0 1 5%.
20 S: 0.010% or less
S is an element contained as an impurity in the steel, and generates sulfide
inclusions, and deteriorates the stretch flangeability. Hence, it is preferable to
define the S content to be as small as possible. Accordingly, the S content is
defined to be 0.010% or less. Preferably, the S content is less than 0.005%,
25 more preferably less than 0.003%, and further more preferably less than 0.002%.
sol.Al: 2.00% or less
A1 acts to deoxidize molten steel. The present invention contains Si
having a deoxidization effect, which is the same as Al, and thus A1 is not always
necessary to be contained. In other words, the A1 content may be as close to 0%
30 as possible. In the case of containing A1 for the sake of encouraging
deoxidization, A1 may preferably be contained as sol.Al whose content is
0.0050% or more. More preferably, the sol.Al content is more than 0.020%.
Moreover, A1 is an element acting to enhance stability of austenite as similar to
Si, and effective to attain the aforementioned metallurgical structure, so that A1
may be contained for this purpose. In this case, the sol.Al content is preferably
more than 0.040%, more preferably more than 0.050%, and Wher more
preferably more than 0.060%.
On the other hand, if the sol.Al content is excessively high, not only
surface defects resulting from alumina are likely to be caused, but also the
transformation temperature becomes greatly increased, which makes it difficult
to attain the metallurgical structure whose main phase is the low-temperature
transformation product. Accordingly, the sol.Al content is defined to be 2.00%
or less. Preferably, the sol.Al content is less than 0.60%, more preferably less
than 0.20%, and hrther more preferably less than 0.10%.
N: 0.0 10% or less
N is an element contained as an impurity in the steel, and deteriorates the
ductility. Hence, it is preferable to define the N content to be as small as
possible. Accordingly, the N content is defined to be 0.010% or less.
Preferably, the N content is 0.006% or less, and more preferably 0.005% or less.
The steel sheet according to the present invention may contain the
following elements as optional elements.
One or more types selected from a group of Ti: less than 0.050%, Nb: less than
0.050%, and V: 0.50% or less.
Ti, Nb, and V act to suppress recrystallization in the hot rolling process,
thereby to increase work strain, and refine the metallurgical structure of the hotrolled
steel sheet. They precipitate as carbide or nitride, and act to restrain
coarsening of austenite during the annealing. Accordingly, one or more types
of these elements may be contained. Excessive contents of these elements,
however, rather saturate effects caused by the above actions, which is
uneconomical. Adding to this, the excessive contents thereof increase
recrystallization temperature during the annealing, which makes the metallurgical
structure after the annealing ununiform, and deteriorates the stretch flangeability.
Furthermore, the amount of precipitation of carbide or nitride becomes increased,
the yield ratio becomes increased, and shape fixability becomes deteriorated, as
well.
Accordingly, the Ti content is defined to be less than 0.050%, the Nb
content is defined to be less than 0.050%, and the V content is defined to be
0.50% or less. Preferably, the Ti content is less than 0.040%, and more
preferably less than 0.030%; preferably, the Nb content is less than 0.040%, and
5 more preferably less than 0.030%; and preferably, the V content is 0.30% or less,
and more preferably less than 0.050%. In order to more securely attain effects
caused by the above actions, it is preferable to satis@ any one of Ti: 0.005% or
more, Nb: 0.005% or more, and V: 0.010% or more. In the case of containing
Ti, it is more preferable to define the Ti content to be 0.010% or more; in the
10 case of containing Nb, it is more preferable to define the Nb content to be
0.010% or more; and in the case of containing V, it is more preferable to define
1 the V content to be 0.020% or more.
I
i One or more types selected from a group of Cr: 1.0% or less, Mo: 0.50% or less,
and B: 0.010% or less.
15 Cr, Mo, and B are elements acting to enhance quenching property of the
steel, and effective to attain the aforementioned metallurgical structure.
Accordingly, one or more types of these elements may be contained. Excessive
I contents of these elements, however, rather saturate effect caused by the above
action, which is uneconomical. Accordingly, the Cr content is defined to be
20 1 .O% or less; the Mo content is defined to be 0.50% or less; and the B content is
defined to be 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. In order to more securely attain the effect caused by the above action, it
is preferable to satisfl any one of Cr: 0.20% or more, Mo: 0.05% or more, and B:
25 0.0010%ormore.
One or more types selected from a group of Ca: 0.0 10% or less, Mg: 0.0 10% or
less, REM: 0.050% or less, and Bi: 0.050% or less.
Ca, Mg, REM, and Bi all act to improve the stretch flangeability, by
adjusting shapes of inclusions in the cases of Ca, Mg, and REM, and by refining
30 solidification structure in the case of Bi. Accordingly, one or more types of
I these elements may be contained. Excessive contents thereof, however, rather
saturate effect caused by the above action, which is uneconomical.
23
Accordingly, the Ca content is defined to be 0.0 10% or less; the Mg
content is defined to be 0.010% or less; the REM content is defined to be 0.050%
or less; and the Bi content is defmed to be 0.050% or less. The Ca content is
preferably 0.0020% or less; the Mg content is preferably 0.0020% or less; the
5 REM content is preferably 0.0020% or less; and the Bi content is preferably
0.010% or less. In order to more securely attain the above action, it is
preferable to satisfy any one of Ca: 0.0005% or more, Mg: 0.0005% or more, and
REM: 0.0005% or more, and Bi: 0.0010% or more. REM denotes rare earth
element, and is a general term for 17 elements in total of Sc, Y, and lanthanoid,
10 and the REM content is a total content of these elements.
1 3. Production condition
I The steel having the aforementioned chemical composition is melted with
I
I a well-known method, and thereafter is produced into an ingot through a
continuous casting process, or alternatively is produced into an ingot through any
15 casting process, and thereafter is produced into a billet through a blorning or the
like. In the continuous casting process, in order to suppress generation of
surface defects resulting from inclusions, it is preferable to generate molten steel
agitation using electromagnetic stirring or the like in the molten steel in the mold.
The ingot or billet that is once cooled may be reheated to be hot-rolled; or the
20 ingot in a high-temperature state after the continuous casting, or the billet in a
high-temperature state after the billeting may be hot-rolled as it is, or
alternatively may be held at a high temperature or heated through assist heating
to be hot-rolled. In the present specification, such an ingot and a billet are
collectively referred to as "slabs" as starting material for use in the hot rolling.
25 The temperature of the slab for use in the hot rolling is preferably less than
1250°C for the sake of preventing coarsening of austenite, and more preferably
1200°C or less. The lower limit of the slab for use in the hot rolling is not
limited to a specific one, and any temperature may be used as far as the hot
rolling can be completed at the Ar3 point or more, as described later.
30 The hot rolling is completed in a temperature range at the AT3 point or
more so as to transform austenite after completion of the hot rolling, thereby to
refine the metallurgical structure of the hot-rolled steel sheet. If the finish
rolling temperature is excessively low, a coarse low-temperature transformation
product expanding in the rolling direction is generated, which increases coarse
retained austenite grains in the metallurgical structure after the cold rolling and
the annealing, and thus the work hardenability and the stretch flangeability are
likely to become deteriorated. Hence, the finish rolling temperature is
preferably the Ar3 point or higher and higher than 820°C. More preferably, this
temperature is the AT3 point or higher and higher than 850°C, and further more
preferably the AT3 point or higher and higher than 880°C. On the other hand, if
the finish rolling temperature is excessively high, accumulation of work strain
becomes insufficient, and thus it becomes difficult to refine the metallurgical
structure of the hot-rolled steel sheet. Accordingly, the finish rolling
temperature is preferably lower than 950°C, and more preferably lower than
920°C. For the sake of reducing production load, it is preferable to increase the
finish rolling temperature, thereby to reduce rolling load. From this point of
view, the finish rolling temperature is preferably the AT3 point or higher and
higher than 780°C, and more preferably the AT3 point or higher and higher than
800°C.
In the case of the hot rolling including rough rolling and finish rolling, in
order to complete the finish rolling at the temperatures above, roughly rolled
material may be heated between the rough rolling and the finish rolling. At this
time, it is preferable to heat the roughly rolled material such that a rear end
thereof has a higher temperature than a temperature of a front end thereof,
thereby to reduce variation in temperature in the overall length of the roughly
rolled material at the start of the finish rolling to be 140°C or less. This
configuration enhances uniformity of the product property in the coil.
The heating method of the roughly rolled material may be carried out
using a well-known means. For example, a solenoid-type induction heating
device may be disposed between a rough rolling mill and a finish rolling mill,
thereby to control increase in heating temperature based on the temperature
distribution in the longitudinal direction of the roughly rolled material upstream
of this solenoid-type induction heating device, or the like.
Rolling reduction of the hot rolling is defined such that the rolling
reduction of the final one pass becomes more than 25% in terms of the reduction
rate of the sheet thickness. This is for the purpose of increasing work strain
introduced in austenite, refining the metallurgical structure of the hot-rolled steel
sheet, suppressing generation of coarse retained austenite grains in the
metallurgical structure after the cold rolling and the annealing, and also refining
bcc grains. In the case of the secondary phase containing polygonal ferrite, this
is for the purpose of refining the polygonal ferrite. Preferably, the rolling
reduction in the final one pass is more than 30%, and more preferably more than
40%. Excessively high rolling reduction increases the rolling load, which
makes it difficult to carry out the rolling. Accordingly, the rolling reduction in
the final one pass is preferably defined to be less than 55%, and more preferably
less than 50%. For the sake of reducing the rolling load, so-called lubrication
rolling may be carried out in such a manner that rolling oil is supplied between
rolling rolls and the steel sheet so as to lower the coefficient of friction in the
rolling.
After the hot rolling, the steel sheet is rapidly cooled down to a
temperature range of 720°C or lower within 0.40 seconds after the completion of
the rolling. This is for the purpose of reducing release of the work strain
introduced in austenite through the rolling, transforming the austenite using the
work strain as a driving force, refining the metallurgical structure of the hotrolled
steel sheet, and reducing generation of coarse retained austenite grains in
the metallurgical structure after the cold rolling and the annealing as well as
refining bcc grains. In the case of the secondary phase containing polygonal
ferrite, this is for the purpose of refining the polygonal ferrite. Preferably, the
steel sheet is rapidly cooled down to a temperature range of 720°C or lower
within 0.30 seconds after completion of the rolling, and more preferably, rapidly
cooled down to a temperature range of 720°C or lower within 0.20 seconds after
completion of the rolling. Since release of the work strain is reduced as the
average cooling rate during the rapid cooling becomes more increased, it is
preferable to define the average cooling rate during the rapid cooling to be
300°C/s or more, thereby to further refine the metallurgical structure of the hotrolled
steel sheet. More preferably, the average cooling rate during the rapid
cooling is 400°C/s or more, and further more preferably 600°C/s or more. It is
unnecessary to specifically define a time period from the completion of the
rolling until start of the rapid cooling as well as the cooling rate during this time
period.
Equipment for carrying out the rapid cooling is not limited to a specific
one, and industriall, it is preferable to use water spray equipment having a high
5 water quantity density; and such a method may be exemplified that disposes a
water spray header between rolled-sheet transfer rollers so as to inject highpressure
water with sufficient water quantity density upwardly and downwardly
onto the rolled sheet.
After the rapid cooling is stopped, the steel sheet is coiled in a temperature
10 range of higher than 500°C. This is because iron carbide does not sufficiently
precipitate in the hot-rolled steel sheet if the coiling temperature is 500°C or
lower, and consequently coarse retained austenite grains are generated as well as
bcc grains become coarse in the metallurgical structure after the cold rolling and
the annealing. Preferably, the coiling temperature is more than 550°C, and
15 more preferably more than 580°C. On the other hand, an excessively high
coiling temperature coarsens ferrite in the hot-rolled steel sheet, so that coarse
retained austenite grains are generated in the metallurgical structure after the cold
rolling and the annealing. Accordingly, the coiling temperature is preferably
lower than 650°C, and more preferably lower than 620°C.
20 Conditions from the stop of the rapid cooling until the coiling are not
limited to specific ones, and it is preferable to hold the steel sheet in a
temperature range of 720 to 600°C for one second or more after the rapid cooling
is stopped. This configuration encourages generation of refined ferrite. To the
contrary, excessively longer holding time deteriorates productivity, and thus it is
25 preferable to define the upper limit of the holding time in the temperature range
of 720 to 600°C to be within 10 seconds. After the steel sheet is held in the
temperature range of 720 to 600°C, it is preferable to cool the steel sheet down to
the coiling temperature at the cooling rate of 20°C/s or more for the sake of
preventing coarsening of the generated ferrite.
30 The hot-rolled steel sheet is subjected to descaling with pickling, or the
like, and thereafter is cold-rolled in accordance with a conventional method. In
the cold rolling, in order to encourage recrystallization and uniform the
metallurgical structure after the cold rolling and the annealing, thereby to fbrther
I C enhance the stretch flangeability, it is preferable to define the cold rolling
reduction (total draft in the cold rolling) to be 40% or more. Excessively high
cold rolling reduction increases the rolling load, which makes it difficult to carry
out the rolling, and thus it is preferable to define the upper limit of the cold
5 rolling reduction to be less than 70%, and more preferable to be less than 60%.
The steel sheet after the cold rolling is subjected to treatment such as
degreasing in accordance with a conventional method if necessary, and thereafter
the steel sheet is subjected to the annealing. The lower limit of a soaking
temperature in the annealing is defined to be (Ac3 point - 40°C) or higher. This
10 is for the purpose of attaining the metallurgical structure whose main phase is the
low-temperature transformation product, and whose secondary phase contains the
retained austenite. In order to increase the volume fiaction of the lowtemperature
transformation product, and to enhance the stretch flangeability, it is
preferable to define the soaking temperature to be higher than (Ac3 point - 20°C),
I 15 and more preferable to be higher than the Ac3 point. An excessively high
I I soaking temperature excessively coarsens austenite, so that the metallurgical
structure after the annealing becomes coarse, generation of polygonal ferrite is
reduced, which results in deterioration of the ductility, the work hardenability,
and the stretch flangeability. Accordingly, it is preferable to define the upper
20 limit of the soaking temperature to be lower than (Ac3 point + 100°C), and more
preferable to be lower than (Ac3 point + 50°C), and M e r more preferable to be
lower than (Ac3 point + 20°C). Defming the upper limit of the soaking
temperature to be lower than (Ac3 point + 50°C) makes it possible to refine bcc
grains to the average grain size of 7.0 pm or less, thereby to attain particularly
25 excellent ductility, work hardenability, and stretch flangeability.
The holding time at the soaking temperature (the soaking time) need not
be subject to any special restriction; however, to attain stable mechanical
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
30 is too long, austenite is coarsened excessively, so that the ductility, work
hardenability, and stretch flangeability are liable to deteriorate. Therefore, the
holding time is preferably made shorter than 150 seconds, further 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 further
improve the stretch flangeability, the heating rate from 700°C to the soaking
temperature is preferably made lower than 10.O°C/s. It is further preferably
5 made lower than 8.0°C/s, still further preferably made lower than 5.0°C/s.
In the cooling process after the soaking in the annealing, in order to
encourage generation of refined polygonal ferrite, and to enhance the ductility
and the work hardenability, it is preferable to cool the steel sheet from the
soaking temperature by 50°C or more at a cooling rate of less than 5.0°C/s. The
10 cooling rate at this time is more preferably less than 3 .O°C/s, and fkther more
preferably less than 2.0°C/s. In order to further increase the volume fraction of
polygonal ferrite, the steel sheet is more preferably cooled by 80°C or more, and
further more preferably cooled by 100°C or more, and most preferably cooled by
120°C or more. After the soaking at less than (Ac3 point + 50°C), by cooling
15 the steel sheet at a cooling rate of less than 5.0°C/s from the soaking temperature
by 50°C or more, it is possible to generate polygonal ferrite whose average grain
size is less than 5.0 pm by more than 2.0% in terms of the volume fraction
relative to the overall structure, thereby to attain particularly excellent ductility,
work hardenability, and stretch flangeability.
20 In order to attain the metallurgical structure whose main phase is the lowtemperature
transformation product, it is preferable to cool the steel sheet in a
temperature range of 650 to 500°C at a cooling rate of 15"C/s or more. It is
more preferable to cool the steel sheet in a temperature range of 650 to 450°C at
a cooling rate of 15"C/s or more. As the cooling rate becomes more increased,
25 the volume fiaction of the low-temperature transformation product becomes
more increased, and thus in any of the above temperature ranges, it is more
preferable to define the cooling rate to be more than 30°C/s, and further more
preferable to be more than 50°C/s. On the other hand, an excessively high
cooling rate rather deteriorates the shape of the steel sheet, and thus it is
30 preferable to define the cooling rate to be 200°C/s or less in a temperature range
of 650 to 500°C. The cooling rate is more preferably less than 150°C/s, and
fbrther more preferably less than 130°C/s.
In order to secure an amount of the retained austenite, the steel sheet is
held for 30 seconds or more in a temperature range of 450 to 340°C in the
cooling process. In order to enhance stability of the retained austenite, thereby
to further enhance the ductility, the work hardenability, and the stretch
flangeability, the holding temperature range is preferably 430 to 360°C. As the
holding time is set to be longer, the stability of the retained austenite becomes
more enhanced; therefore, the holding time is preferably defined to be 60 seconds
or more. The holding time is more preferably 120 seconds or more, and further
more preferably more than 300 seconds.
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 clean and condition the surface,
electroplating has only to be performed pursuant to an ordinary method. The
chemical composition and weight of plating film is not subject to any special
restriction. As the kind of electroplating, electro galvanizing, Zn-Ni alloy
electroplating, 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 held in the temperature region of 450 to 340°C for 30 seconds or longer,
the steel sheet is heated as necessary, and is immersed in a plating bath for hot
dip plating. In order to enhance the stability of retained austenite and to fbrther
improve the ductility, work hardenability, and stretch flangeability, the holding
temperature region is 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.
The steel sheet may be reheated after being hot dip plated for alloying treatment.
The chemical composition and weight of deposit of plating film is not subject to
any special restriction. As the kind of hot dip plating, galvanizing, galvanizing,
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 further enhance the corrosion resistance. In place
-D of the conventional chromate treatment, the chemical conversion treatment is
1 preferably performed by using a chromium-free type chemical conversion liquid
I (for example, silicate-based or phosphate-based).
I
The cold-rolled steel sheet and plated steel sheet thus obtained may be
5 subjected to skin-pass rolling pursuant to an ordinary method. However, a large
elongation percentage of skin-pass rolling leads to the deterioration in ductility.
! Therefore, the elongation percentage of skin-pass rolling is preferably made
1.0% or smaller, further preferably made 0.5% or smaller
The present invention will be exemplified by using the following Example.
10 The present invention is not limited to the Example.
Example 1
Using an experimental vacuum melting furnace, steels having the chemical
compositions shown in Table 1 were melted and casted. Each obtained ingot
15 was produced into a billet having a thickness of 30 mm through hot forging.
Each billet was heated to 1200°C using an electric heating furnace, and was held
at this temperature for 60 minutes, and thereafter was hot-rolled under the
conditions shown in Table 2.
Specifically, by using an experimental hot-rolling mill, 6-pass rolling was
20 performed in the temperature region of & 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 terms of the reduction rate of the sheet thickness.
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. After having been
25 cooled naturally 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
30 the steel sheet to room temperature at a cooling rate of 2O0C/h, whereby a hotrolled
steel sheet was obtained.
Each produced hot-rolled steel sheet was subjected to acid pickling so as
to be base metal for cold rolling, and was subjected to cold rolling with cold
rolling reduction of 50 to 60%, thereby to produce a cold-rolled steel sheet
having a thickness of 1.0 to 1.2 mm. Using a continuous annealing simulator,
each produced cold-rolled steel sheet was heated up to 550°C at a heating rate of
10°C/s, and thereafter, was heated up to each temperature shown in Table 2 at a
heating rate of 2"C/s, and was then soaked for 95 seconds. Thereafter, each
cold-rolled steel sheet was subjected to primary cooling down to each
temperature shown in Table 2, was fixther subjected to secondary cooling from
the stop temperature of the primary cooling down to each stop temperature of the
cooling shown in Table 2 at an average cooling rate of 60°C/s, and was held at
this temperature for 330 seconds, and thereafter was cooled down to a room
temperature, thereby to attain an annealed steel sheet.
Note) 1. Ac3 point was determined fiom thermal expansion change at the time when cold-rolled steel sheet was heated at 2OCIs.
2. Ar3 point was determined fiom thermal expansion change at the time when cold-rolled steel sheet was heated to 900°
C and thereafter was cooled at O.Ol°C/s.
I I I Hot Rolling Condition Annealing Condition
I I I I
~ e s t
No. steel soaking
Temp.
CC)
Final Pass
Rolling
Reduction
(%I
primary
Cooling Rate
CC/s)
Sheet
-,-hidmess
~o(l-li,n g"
StopTanp.
of Primary
Cooling CC)
Finish
Rolling
Temp.
(oC)
Stop Temp.
of
Secondary
Cooling Cc)
U
6
N
U
Stop Temp.
ofRapid
Cooling
('C)
Time Until
Rapid Cooling
stop2' (s)
Cooling Rate3'
CCIs)
Coiling
em^.')
CC)
-P A test specimen for SEM observation was sampled from the annealed steel
I sheet, and the longitudinal cross sectional surface thereof parallel to the rolling
I direction was polished. Thereafter, it was etched with nital and the
metallurgical structure was observed at a position deep by one-fourth of
thickness from the surface of steel sheet, and by image processing, the volume
fractions of low-temperature transformation product and polygonal ferrite were
measured. Also, the average grain size (circle equivalent 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.
Also, a test specimen for XRD measurement was sampled from the
annealed steel sheet, and the rolled surface was chemically polished down to a
position deep by one-fourth of thickness fiom the surface of steel sheet.
Thereafter, an X-ray diffraction test was conducted to measure the volume
fraction of retained austenite. Specifically, RINT2500 manufactured by Rigaku
Corporation was used as an X-ray difftactometer, and Co-Ka beams were
applied to measure the integrated intensities of a phase (1 lo), (200), (2 1 1)
diffraction peaks and y phase (1 1 I), (200), (220) diffraction peaks, whereby the
volume fraction of retained austenite was determined.
Furthermore, a test specimen for EBSP measurement was sampled from
the annealed steel sheet, and the longitudinal cross sectional surface thereof
parallel to the rolling direction was electropolished. Thereafter, the
metallurgical structure was observed at a position deep by one-fourth of
thickness fiom the surface of steel sheet, and by image analysis, the average
grain size of bcc grains, the grain size distribution of retained austenite and the
average grain size of retained austenite were measured. Specifically, as an
EBSP measuring device, OIM5 manufactured by TSL Solitions K.K. was used,
electron beams were irradiated 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 Confidence Index was 0.1 or
more was used as effective data to make judgment of bcc phase and fcc phase.
Each region observed as a bcc phase, and surrounded by grain boundaries
whose misorientation angle was 15" or more was treated as a one bcc grain, and a
circle equivalent diameter and an area of each bcc grain were determined so as to
calculate an average grain size in accordance with the definition of the
aforementioned Formula (I). In this average grain size calculation, bcc grains
whose circle equivalent diameter was 0.47 pm or more were treated as effective
bcc grains. Although, strictly speaking, a crystal structure of martensite is a
body-centered tetragonal lattice (bct), no lattice constant is taken into account in
the metallurgical structure evaluation using an EBSP, so that martensite was also
treated as the bcc phase.
With a region that was observed as the fcc phase and was surrounded by a
matrix phase being made one retained austenite grain, the circle equivalent
diameter of individual retained austenite grain was determined. The average
grain size of retained austenite was calculated as the mean-value of circle
equivalent diameters of individual effective retained austenite grains, the
effective retained austenite grains being retained austenite grains each having a
circle equivalent 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 tensile
test at a testing speed of 10 mm/min. The total elongation (El) was determined
as follows: a tensile 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 (2) above. The work hardening coefficient (nvalue)
was determined with the strain range being 5 to 10% by conducting a
tensile 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 flangeability was evaluated by measuring the limiting hole
expansion ratio (A) by the method described below. From the annealed steel
sheet, a 100-mm square hole expanding test specimen was sampled. A 10-rnrn
diameter punched hole was formed with a clearance being 12.5%, the punched
hole was expanded from the sagging side by using a cone-shaped punch having a
top 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 limiting hole expansion ratio.
Table 3 gives the metallurgical 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.
[Table 31
Each steel sheet within the range defined by the present invention had the
following test results: the TS xEl value was 19000 MPa% or more, the value of
TS x n-value was 160 or more, and the TS'.~ x h value was 6000000 MPala7% or
more, which exhibited preferable ductility, work hardenability, and stretch
I 5 flangeability. In particular, in such a steel sheet that had an average grain size
I of bcc grains of 7.0 pm or less, andlor had its secondary phase containing
I retained austenite as well as polygonal ferrite whose volume fraction was more
than 2.0% to less than 27.0%, and whose average grain size was less than 5.0 pm,
the TS xEl value was 20000 MPa% or more, the value of the TS x n-value was
10 165 or more, and the T S ~x. h~ v alue was 6000000 MP~'.~o%r more, which
1 exhibited further enhanced ductility, work hardenability, and stretch flangeability.

1. A cold-rolled steel sheet characterized by having a chemical
composition consisting of, in mass percent, C: more than 0.020% to less than
5 0.30%; Si: more than 0.10% to at most 3.00%; Mn: more than 1.00% to at most
3.50%; P: at most 0.10%; S: at most 0.0 10%; 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
10 0.010%; Ca: at least 0% and at most 0.010%; Mg: at least 0% and at most
0.010%; REM: at least 0% and at most 0.050%; Bi: at least 0% and at most
0.050%; and the remainder being Fe and impurities, and
By having metallurgical structure whose main phase is a lowtemperature
transformation product, and whose secondary phase contains
15 retained austenite, the retained austenite having a volume fraction of more than
4.0% to less than 25.0% relative to overall structure, and an average grain size of
less than 0.80 p,
Wherein of the retained austenite, a number density of retained austenite
grains whose grain size is 1.2 pm or more is 3.0 x 10" grainslPm2 or less.
20
2. The cold-rolled steel sheet as set forth in claim 1, wherein the average
grain size of the grains having a bcc structure and the grains having a bct
structure surrounded by a grain boundary having an misorientation angle of 15"
or larger is 7.0 p or smaller in the metallurgical structure.
25
3. The cold-rolled steel sheet as set forth in claim 1 or claim 2, wherein
in the metallurgical structure, the secondary phase contains the retained
austenite and polygonal ferrite, and
the polygonal ferrite has a volume fraction relative to overall structure of
30 more than 2.0% to less than 27.0%, and an average grain size of less than 5.0 p.
4. The cold-rolled steel sheet as set forth in any of claims 1 - 3, wherein
the chemical composition contains, in mass percent, one kind or two or more
I - 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%.
5 5. The cold-rolled steel sheet as set forth in any of claims 1 - 4, wherein
the chemical composition contains, in mass percent, one kind or two or more
kinds selected from a group consisting of Cr: at least 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%.
10
6. The cold-rolled steel sheet as set forth in any of claims 1 - 5, 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%, REM: at least 0.0005% and
15 at most 0.050%, and Bi: at least 0.0010% and at most 0.050%.
Dated this 1 3th day oofmuary, 201 4.
Nippon Steel &>mitorno Metal Corporation
(Dev or*& Robinson
of Amarchand & Mangaldas &
Suresh A. Shroff & Co.
Attorneys for the Applicant