DESCRIPTION
Title of Invention
STEEL SHEET AND METHOD OF MANUFACTURING STEEL SHEET
5
Technical Field
[0001]
The present invention relates to a steel sheet and a method of manufacturing a
steel sheet. The steel sheet is a high-strength steel sheet which is appropriate for a
10 structural material of a vehicle or the like used mainly by being press worked and has
excellent elongation, V-bendability, and increased press-forming stability.
Priority is claimed on Japanese Patent Application No. 2010-019193, filed on
January 29, 2010, and Japanese Patent Application No. 2010-032667, filed on February
17, 2010, the contents of which are incorporated herein by reference.
15
Background Art
[0002]
Excellent elongation and V-bendability in addition to high strength are required
of a steel sheet used in the vehicle body structure of a vehicle.
20 [0003]
It is known that a TRIP (Transformation Induced Plasticity) steel sheet
containing a retained austenite phase exhibits high strength and high elongation due to
the TRIP effect.
[0004]
25 In Patent Document 1, for the purpose of further increasing the elongation of
2
retained austenite steel, a technique of ensuring a high fraction of a retained austenite
phase thereby controlling two kinds of ferrite phases (bainitic ferrite and polygonal
ferrite phase) is disclosed.
[0005]
5 In Patent Document 2, for the purpose of ensuring elongation and shape
fixability, a technique of specifying the shape of an austenite phase as an aspect ratio is
disclosed.
[0006]
In Patent Document 3, for the purpose of further enhancing elongation, a
10 technique of optimizing the distribution of an austenite phase is disclosed.
[0007]
In addition, in Patent Documents 4 and 5, a technique of enhancing local
ductility through uniformization of the structure is disclosed.
15 Related Art Documents
Patent Documents
[0008]
[Patent Document I] Japanese Unexamined Patent Application, First
Publication No. 2006-274418
20 [Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2007-154283
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2008-56993
[Patent Document 4] Japanese Unexamined Patent Application, First
25 Publication No. 2003-306746
3
[Patent Document 5] Japanese Unexamined Patent Application, First
Publication No. H04-88125
Non-patent Document
5 [0009]
[Non-patent Document 1] M. Takahashi: IS3-2007, (2007), 47-50.
Disclosure of the Invention
Technical Problem
10 [0010]
Retained austenite steel is steel in which a retained austenite phase is contained
in a steel structure by increasing the C concentration of austenite through control of
ferrite transformation and bainite transformation during annealing. However, the
retained austenite steel has a mixed structure and thus may not exhibit high V-bendability
15 (local bendability). Therefore, in the above-mentioned technique, obtaining both higher
elongation and V-bendability required of a current high-strength steel sheet is not
achieved
[0011]
In addition, the TRIP effect has temperature dependence, and in actual press
20 forming, the temperature of a die changes during press forming. Therefore, in a case
where a TRIP steel sheet is subjected to press forming, defects such as cracking may
occur in an initial stage of press forming at, for example, about 25°C and in a late stage
of the press forming at, for example, about 150°C, and thus there is a problem with
press-forming stability.
25 Therefore, in addition to high elongation and V-bendability, realizing excellent
4
press-forming stability without depending on a temperature change during press forming
is an object in practice.
[0012]
An object of the present invention is to provide a steel sheet having higher
5 elongation and V-bendability compared to those of the related art and further having
excellent press-forming stability, and a method of manufacturing the same.
Means for Solving Problem
[0013]
10 The present invention employs the following measures in order to accomplish
the above-mentioned object.
(1) According to a first aspect of the present invention, a steel sheet is provided,
including: as chemical components, by mass%, 0.05% to 0.35% of C; 0.05% to 2.0% of
Si; 0.8% to 3.0% of Mn; 0.01% to 2.0% of Al; equal to or less than 0.1% of P; equal to or
15 less than 0.05% of S; equal to or less than 0.01 % of N; and the balance including iron
and inevitable impurities, wherein an area ratio of equal to or higher than 50% of a total
of aferrite-phase,, a bainiteplmse andiutemperedhrrartensite p}rase isTantained n area
ratio of equal to or higher than 3% of a retained austenite phase is contained, and crystal
grains of the retained austenite phase having a number ratio of equal to or higher than
20 50% satisfy Expression 1, assuming that a carbon concentration at a position of center of
gravity is Cge and a carbon concentration at a grain boundary is Cgb.
Cgb/Cge?1.2. (Expression 1)
(2) The steel sheet described in (1) may further include, in the chemical
components, by mass%, at least one of: 0.01% to 0.5% of Mo; 0.005% to 0.1% of Nb;
25 0.005% to 0.2% of Ti; 0.005% to 0.5% of V; 0.05% to 5.0% of Cr; 0.05% to 5.0% of W;
5
0.0005% to 0.05% of Ca; 0.0005% to 0.05% of Mg; 0.0005% to 0.05% of Zr; 0.0005%
to 0.05% of REM; 0.02% to 2.0% of Cu; 0.02% to 1.0% of Ni; and 0.0003% to 0.007%
of B.
(3) In the steel sheet described in (1), an average grain size of the crystal grains
5 may be equal to or less than 10 fun, and an average carbon concentration in the retained
austenite phase may be equal to or higher than 0.7% and equal to or less than 1.5%.
(4) In the steel sheet described in (1), the crystal grains having a number ratio of
equal to or higher than 40% may be small-diameter crystal grains having an average
grain size of equal to or greater than 1 pm and equal to or less than 2 pm, and the crystal
10 grains having a number ratio of equal to or higher than,20% may be large-diameter
crystal grains having an average grain size of equal to or greater than 2 μm.
(5) In the steel sheet described in (4), the small-diameter crystal grains having a
number ratio of equal to or higher than 50% may satisfy Expression 2, assuming that a
carbon concentration at a position of center of gravity is CgcS and a carbon concentration
15 at a grain boundary is CgbS, and the large-diameter crystal grains having a number ratio
of equal to or higher than 50% may satisfy Expression 3, assuming that a carbon
concentration at a position of center of gravity is CgcL and a carbon concentration at a
grain boundary is CgbL.
CgbS/CgcS> 1.3... (Expression 2)
20 1.3>CgbL/CgcL> 1.1...(Expression 3)
(6) The steel sheet described in any one of (1) to (5) may have a galvanized film
provided to at least one surface.
(7) The steel sheet described in any one of (1) to (5) may have a galvannealed
film provided to at least one surface.
25 (8) According to a second aspect of the present invention, a method of
6
manufacturing a steel sheet is provided, including: a hot-roll ing process of manufacturing
a hot-rolled steel sheet by performing hot rolling on a slab having the chemical
components described in (1) or (2) at a finishing temperature of equal to or higher than
850°C and equal to or less than 970 °C; an air-cooling process of performing air cooling
5 on the hot-rolled steel sheet for a time of equal to or longer than I second and equal to or
shorter than 10 seconds ; a coiling process of cooling the air-cooled hot-rolled steel sheet
to a temperature range of equal to or less than 650°C at an average cooling rate of equal
to or higher than 10 °C/sec and equal to or less than 200°C/sec and thereafter coiling the
steel sheet in a temperature range of equal to or less than 650°C; a cold-rolling process of
10 performing pickling on the coiled hot-rolled steel sheet at a rolling reduction ratio of
equal to or higher than 40% and thereafter performing cold rolling on the steel sheet,
thereby manufacturing a cold-rolled steel sheet; an annealing process of performing
annealing on the cold -rolled steel sheet at a maximum temperature of equal to or higher
than 700° C and equal to or less than 900 °C; a holding process of cooling the annealed
15 cold-rolled steel sheet in a temperature range of equal to or higher than 350°C and equal
to or less than 480°C at an average cooling rate of equal to or higher than 0 . 1 °C/ sec and
equal to or less than 200°C/sec, and holding the steel sheet in this temperature range for a
time of equal to or longer than 1 second and equal to or shorter than 1000 seconds; and a
final cooling process of primarily cooling the cold-rolled steel sheet in a temperature -
20 range from 350°C to 220°C at all average cooling rage of equal to or higher than 5°C/sec
and equal to or less than 25°C/sec, and secondarily cooling the steel sheet in a
temperature range from 120°C to near room temperature at an average cooling rate of
equal to or higher than 100°C/sec and equal to or less than 5°C/sec.
(9) In the method of manufacturing a steel sheet described in (8), rolling may be
7
performed with a strain amount of equal to or less than 20% in each of final two passes in
the hot-rolling process.
(10) In the method of manufacturing a steel sheet described in (8), a slab which
is re-heated to 1100°C or higher after being cooled to 1100°C or less may be used in the
5 hot-rolling process.
(11) The method of manufacturing a steel sheet described in (8) may further
include an immersion process of immersing the steel sheet in a hot-dip galvanizing bath
after the holding process.
(12) The method of manufacturing a steel sheet described in (11) may further
10 include an alloying treatment process,of performing an alloying treatment in a range of
equal to or higher than 500°C and equal to or less than 580°C after the immersion
process.
Advantageous Effects of Invention
15 [0014]
According to the above-described measures, the C concentration gradient in the
retained austenite phase is appropriately controlled, so that an extremely stable retained
austenite phase may be obtained. As a result, due to the TRIP effect of the retained
austenite, extremely high elongation and high V-bendability may be exhibited despite
20 high strength. In addition, in the case where the amounts of the small-diameter crystal
grains and the large-diameter crystal grains are appropriately controlled, the stability of
the TRIP function of the retained austenite may be dispersed. Therefore, excellent
press-forming stability that does not depend on a temperature change during
press-forming may be exhibited. In addition, in a case where the C concentration
25 gradient of the small-diameter crystal grains and the C concentration gradient of the
8
large-diameter crystal grains are appropriately controlled, superior press-forming stability
may be exhibited.
Brief Description of Drawings
5 [0015]
FIG I is a diagram showing the relationship between tensile strength and 25°C
elongation of steel sheets according to Examples and Comparative Examples.
FIG 2 is a diagram showing the relationship between tensile strength and
V-bending minimum radius (V-bendability) of the steel sheets according to the Examples
10 and the Comparative Examples.
FIG. 3 a diagram showing the relationship between tensile strength and 150°C
elongation according to the Examples and the Comparative Examples.
Description of Embodiments
15 [0016]
The inventors found that in order to cause the TRIP effect of retained austenite
to act not only on elongation but also V-bendability, increasing the stability of a retained
austenite phase to a degree of equal to or higher than that until now is effective, and in
order to cause the TRIP effect to act on a wide press-forming temperature range,
20 uniformly dispersing retained austenite phases with different stabilities is effective.
[0017]
However, in a technique of increasing the C concentration in the retained
austenite phases using bainite transformation of the retained austenite steel according to
the related art, the C concentration may not be increased to a concentration of To point or
25 higher described in Non-patent Document 1, and the stability of the retained austenite
9
phase may not be increased.
[0018]
Here, as a result of the intensive examination of the inventors, it was discovered
that an extremely stable retained austenite phase may be obtained by appropriately
5 controlling a C concentration gradient in the retained austenite phase, and austenite
phases with different stabilities may be uniformly dispersed by appropriately controlling
the grain size distribution of austenite grains in the retained austenite phase:
[0019]
Hereinafter, a steel sheet according to an embodiment of the present invention
10 made on the basis of the above-described discovery will be described in detail.
[0020]
First, regarding the steel according to this embodiment and a slab (cast slab)
which is the bulk material thereof, the chemical components of steel will be described.
Here, "0/s" representing the amount of each element means mass%.
15 [0021]
(Basic Elements)
Thu-chemical-components of steel contahrC Sid Mn and-Al as-basic elements-
[0022]
(C: 0.05 to 0.35%)
20 C is an extremely important element for increasing the strength of steel and
ensuring a retained austenite phase. When a C content is less than 0.05%, sufficient
strength may not be ensured, and asufficient retained austenite phase may not be
obtained. On the other hand, when the C content exceeds 0.35%, ductility or spot
weldability is significantly deteriorated. In consideration of the above-described
25 characteristics, the C content may be specified as a narrower range.
10
Therefore, regarding the C content, the lower limit thereof is specified as 0.05%,
preferably 0.08%, and more preferably 0.15%, and the upper limit thereof is specified as
0.35%, preferably 0.26%, and more preferably 0.22%.
[0023]
5 (Si: 0.05 to 2.0%)
Si is an important element in terms of ensuring strength. In a case where a Si
content is equal to or higher than 0.05%, an effect of contributing to the generation of the
retained austenite phase and ensuring ductility is obtained. On the other hand, in a case
where the Si content exceeds 2.0%, such an effect is saturated, and moreover,
10 embrittlement of steel is more likely to occur. In a case where hot-dip galvanizing and
chemical conversion treatments need to be facilitated, the upper limit thereof may be
specified as 1.8%. In consideration of the above-described characteristics, the Si
content may be specified as a narrower range.
Therefore, regarding the Si content, the lower limit thereof is specified as 0.05%,
15 preferably 0.1%, and more preferably 0.5%, and the upper limit thereof is specified as
2.0%, preferably 1.8%, and more preferably 1.6%.
[002}]-
(Mn: 0.8 to 3.0%)
Mn is an important element in terms of ensuring strength. In a case where a
20 Mn content is equal to or higher than 0.8%, an effect of contributing to the generation of
the retained austenite phase and ensuring ductility is obtained. On the other hand, in a
case where the Mn content exceeds 3.0%, hardenability is increased, the retained
austenite phase is transformed into a martensite phase, and thus an excessive increase in
strength is more likely to be caused. As a result, products significantly vary, and
25 ductility becomes insufficient. In consideration of the above-described characteristics,
11
the Mn content may be specified as a narrower range.
Therefore, regarding the Mn content, the lower limit thereof is specified as 0.8%,
preferably 0.9%, and more preferably 1.2%, and the upper limit thereof is specified as
3.0%, preferably 2.8%, and more preferably 2.6%.
5 [0025]
(Al: 0.01 to 2.0%)
In a case where an Al content is equal to or higher than 0.01 %, like Si, an effect
of contributing to the generation of the retained austenite phase and ensuring ductility is
obtained. On the other hand, in a case where the Al content exceeds 2.0%, such an
10 effect is saturated, and steel becomes embrittled. In consideration of the
above-described characteristics, the Si content may be specified as a narrower range.
Therefore, regarding the Al content, the lower limit thereof is specified as 0.01%,
preferably 0.015%, and more preferably higher than 0.04%, and the upper limit thereof is
specified as 2.0%, preferably 1.8%, and more preferably less than 1.4%.
15 In a case where hot-dip galvanizing is performed, Al deteriorates hot-dip
galvanizing properties, and thus it is preferable that the upper limit thereof be 1.8%.
[0026]
In a case where a large amount of the above-mentioned Si and Al having the
same effect is added to the steel, a Si+AI content may be specified.
20 In this case, regarding the Si+AI content, the lower limit thereof is specified as
0.8%, preferably 0.9%, and more preferably higher than 1.0%, and the upper limit thereof
is specified as 4.0%, preferably 3.0%, and more preferably 2.0%.
[0027]
(Limited Elements)
25 In the steel described above, the contents of P, S, and N, which are limited
12
elements, are limited as follows.
[0028]
(P: equal to or less than 0.1%)
A P content is limited depending on a required steel sheet strength. When the P
5 content exceeds 0.1 %, local ductility is deteriorated due to segregation at grain
boundaries, and weldability is deteriorated. Therefore, the P content is limited to be
equal to or less than 0.1 %.
P is inevitably contained in the steel, and thus the lower limit thereof exceeds
0%. However, excessive cost is incurred to limit the P content to be extremely low.
10 Therefore, the lower limit thereof may be specified as 0.001% or 0.006%. In
consideration of the above-described characteristics, the P content may be specified as a
narrower range.
Therefore, the P content is limited to be equal to or less than 0.1%, preferably
equal to or less than 0.05%, and more preferably equal to or less than 0.01%. In
15 addition, the lower limit thereof may be specified as higher than 0%, 0.001%, or 0.006%.
[0029]
(S equal tour leswthan 0.05°f)
S is an element that generates MnS and thus deteriorates local ductility and
weldability. Therefore, a S content is limited to be equal to or less than 0.05%.
20 S is inevitably contained in the steel, and thus the lower limit thereof exceeds
0%. However, excessive cost is incurred to limit the S content to be extremely low.
Therefore, the lower limit thereof may be specified as 0.0005% or higher than 0.001%.
In consideration of the above-described characteristics, the S content may be specified as
a narrower range.
25 Therefore, the S content is limited to be equal to or less than 0.05%, preferably
13
equal to or less than 0.01%, and more preferably less than 0.004%. In addition, the
lower limit thereof may be specified as higher than 0%, 0.0005%, or higher than 0.001%.
[0030]
(N: equal to or less than 0.01%)
5 When a large amount of N is contained, aging characteristics are deteriorated, a
precipitation amount of AIN is increased, and thus an effect of Al addition is reduced.
Therefore, a N content is limited to be equal to or less than 0.01%.
N is inevitably contained in, the steel, and thus the lower limit thereof is
specified as higher than 0%. However, excessive cost is incurred to limit the N content
10 to be extremely low, and thus the lower limit thereof may be specified as 0.001% or
higher than 0.002%. In consideration of the above-described characteristics, the N
content may be specified as a narrower range.
Therefore, the N content is limited to be equal to or less than 0.0 1%, preferably
equal to or less than 0.008%, and more preferably less than 0.005%. In addition, the
15 lower limit thereof may be specified as higher than 0%, 0.001%, or higher than 0.002%.
[0031]
(Fe and inevitable impurities)
The steel described above contains iron and inevitable impurities as the balance.
As the inevitable impurities, there are Sri, As, and the like incorporated from scrap. In
20 addition, other elements may be contained in a range that does not hinder the
characteristics of the present invention.
[0032]
(Selective Elements)
The steel described above may contain at least one of Mo, Nb, Ti, V, Cr, W, Ca,
25 Mg, Zr, REM, Cu, Ni, and B as selective elements.
14
[0033]
(Mo: 0.01 to 0.5%)
In a case where a Mo content is equal to or higher than 0.01%, an effect of
suppressing the generation of a pearlite phase in the steel is obtained. Therefore, Mo is
5 an element that is important in a case where a cooling rate is slow during annealing or in
a case where re-heating is performed due to an alloying treatment or the like of plating.
However, in a case where the Mo content exceeds 0.5%, ductility or chemical conversion
treatment properties may be deteriorated. In order to obtain the balance between higher
strength and ductility, it is preferable that the Mo content be equal to or less than 0.3%.
10 In consideration of the above-described characteristics, the Mo content may be specified
as a narrower range.
Therefore, in a case where Mo is contained in the steel, the lower limit thereof
may be specified as 0.01%, and preferably 0.02%, and the tipper limit thereof may be
specified as 0.5%, preferably 0.3%, and more preferably 0.2%.
15 [0034]
(Nb: 0.005 to 0.1%)
(Ti: 0:005-tcr02%)
(V: 0.005 to 0.5%)
(Cr: 0.05 to 5.0%)
20 (W: 0.05 to 5.0%)
Nb, Ti, V, Cr, and W are elements that generate fine carbides, nitrides, or
carbonitrides and are effective in ensuring strength. In terms of ensuring strength, the
lower limit of Nb may be specified as 0.005%, the lower limit of Ti may be specified as
0.005%, the lower limit of V may be specified as 0.005%, the lower limit of Cr may be
25 specified as 0.05%, and the lower limit of W may be specified as 0.05%,
15
On the other hand, when such elements are excessively added to the steel, the
strength of the steel is excessively increased and thus ductility is degraded. In terms of
ensuring ductility, the upper limit of Nb may be specified as 0.1 %, the upper limit of Ti
may be specified as 0.2%, the upper limit of V may be specified as 0.5%, the tipper limit
5 of Cr may be specified as 5.0%, and the upper limit of W may be specified as 5.0%,
In addition, in consideration of the above-described characteristics, the content
of each of the elements may be specified as a narrower range.
Therefore, in a case where Nb is contained in the steel, the lower limit thereof
may be specified as 0.005%, and preferably 0.01%, and the upper limit thereof may be
10 specified as 0.1 %, preferably 0.05%, and more preferably 0.03%. ,
In addition, in a case where Ti is contained in the steel, the lower limit thereof
may be specified as 0.005%, and preferably 0.01%, and the upper limit thereof may be
specified as 0.2%, preferably 0.1%, and more preferably 0.07%.
In addition, in a case where V is contained in the steel, the lower limit thereof
15 may be specified as 0.005%, and preferably 0.01%, and the upper limit thereof may be
specified as 0.5%, preferably 0.3%, and more preferably 0.1%.
1n addition; wa casnvhere Cris-contained it) the-steel-,the lower lirnit-thereaf
may be specified as 0.05%, and preferably 0.1 %, and the upper limit thereof may be
specified as 5.0%, preferably 3.0%, and more preferably 1.0%.
20 In addition, in a case where W is contained in the steel, the lower limit thereof
may be specified as 0.05%, and preferably 0.1 %, and the upper limit thereof may be
specified as 5.0%, preferably 3.0%, and more preferably 1.0%.
[0035]
(Ca: 0.0005 to 0.05%)
25 (Mg: 0.0005 to 0.05%)
16
(Zr: 0.0005 to 0.05%)
(REM: 0.0005 to 0.05%)
Ca, Mg, Zr, and REM (rare earth elements) control the shapes of sulfides and
oxides and enhance local ductility and hole expandability. Therefore, the lower limit of
5 each of the elements may be specified as 0.0005%.
On the other hand, in a case where the steel excessively contains such elements,
workability is deteriorated. Therefore, the upper limit of each of the elements may be
specified as 0.05%.
In addition, in consideration of the above-described characteristics, the content
10 of each of the elements may be specified as a narrower range.
Therefore, in a case where Ca is contained in the steel, the lower limit thereof
may be specified as 0.0005%, and preferably 0.00 1%, and the tipper limit thereof may be
specified as 0.05%, preferably 0.01%, and more preferably 0.005%.
In addition, in a case where Mg is contained in the steel, the lower limit thereof
15 may be specified as 0.0005%, and preferably 0.001%, and the upper limit thereof may be
specified as 0.05%, preferably 0.01%, and more preferably 0.005%.
In addition i acase-where-Zr is ci ained in-tire steel ,, the loweifiitnit thereof -
may be specified as 0.0005%, and preferably 0.001%, and the upper limit thereof may be
specified as 0.05%, preferably 0.01%, and more preferably 0.005%.
20 In addition, in a case where REM is contained in the steel, the lower limit
thereof may be specified as 0.0005%, and preferably 0.00 1%, and the upper limit thereof
may be specified as 0.05%, preferably 0.01%, and more preferably 0.005%.
[0036]
(Cu: 0.02 to 2.0%)
25 (Ni: 0.02 to 1.0%)
17
(B: 0.0003 to 0.007%)
Cu, Ni, and B may obtain an effect of slowing down transformation and
increasing the strength of the steel. Therefore, the lower limit of Cu may be specified as
0.02%, the lower limit of Ni may be specified as 0.02%, and the lower limit of B may be
5 specified as 0.0003%.
On the other hand, when each of the elements is excessively added,
hardehability is excessively increased, ferrite transformation and bainite transformation
slow down, and thus an increase in the C concentration in the retained austenite phase
slows down. Therefore, the upper limit of Cu may be specified as 2.0%, the upper limit
10 of Ni may be specified as 1.0%, and the upper limit of B maybe specified as 0.007%.
In addition, in consideration of the above-described characteristics, the content
of each of the elements may be specified as a narrower range.
Therefore, in a case where Cu is contained in the steel, the lower limit thereof
may be specified as 0.02%, and preferably 0.04%, and the upper limit thereof may be
15 specified as 2.0%, preferably 1.5%, and more preferably 1.0%.
In addition, in a case where Ni is contained in the steel, the lower limit thereof
---maspecified as 0:02°0, and preferably 0:01%, and-the upper limit tliereofinay be
specified as 1.0%, preferably 0.7%, and more preferably 0.5%.
In addition, in a case where B is contained in the steel, the lower limit thereof
20 may be specified as 0.0003%, and preferably 0.0005%, and the upper limit thereof may
be specified as 0.007%, preferably 0.005%, and more preferably 0.003%.
[0037]
Next, the steel structure of the steel sheet according to this embodiment will be
described. Here, "%" regarding the steel structure means an area ratio, unless otherwise
25 described.
18
[0038]
The steel structure of the steel sheet according to this embodiment contains 50%
or higher, preferably 60%, and more preferably 70% or higher of a total of a ferrite phase,
a bainite phase, and a tempered martensite phase with respect to the entire structure in
5 terms of area ratio. In addition, the steel structure contains 3% or higher, preferably
higher than 5%, and more preferably higher than 10% of a retained austenite phase with
respect to the entire structure. The tempered martensite phase may be contained
depending on a required strength of the steel sheet, and 0% thereof may be contained.
In addition, when 5% or less of the pearlite phase is contained, the pearlite phase does
10, not significantly deteriorate the material quality even though it is contained in the steel
structure, and thus the pearlite phase may be contained in a range of equal to or less than
5%.
[0039]
In a case where less than 50% of a total of the ferrite phase, the bainite phase,
15 and the tempered martensite is contained, the C concentration in the retained austenite
phase may not be increased, and thus it is difficult to ensure the stability of the phases
-even-though the retained austenite phase has Fconcentration gradient. Therefore,
V-bendability is deteriorated. On the other hand, when higher than 95% of a total of the
ferrite phase, the bainite phase, and the tempered martensite is contained, it is difficult to
20 ensure 3% or higher of the retained austenite phase, resulting in the degradation of
elongation. Therefore, 95% or less is preferable.
[0040]
In the steel sheet according to this embodiment, the C concentration distribution
of the crystal grains of the retained austenite phase is appropriately controlled. That is,
25 the C concentration (Cgb) at a phase interface at which the crystal grains of the retained
19
austenite phase border the ferrite phase, the bainite phase, or the tempered martensite
phase is controlled to be higher than the C concentration (Cgc) at a position of the center
of gravity of the crystal grains. Accordingly, the stability of the retained austenite phase
at the phase interface is increased, and thus excellent elongation and V-bendability may
5 be exhibited.
[0041]
More specifically, in a case where the crystal grains of the retained austenite
phase having a number ratio of 50% or higher, preferably 55%, and more preferably 60%
of higher satisfy Expression 1 as follows, an effect of increasing the stability of the entire
10 retained austenite phase is obtained.
Cgb/Cgc_1.2...(Expression 1)
[0042]
Cgb and Cgc (and CgbS, CgcS, CgbL, and CgcL described later) may be
measured by any measurement method as long as the measurement method guarantees
15 accuracy. For example, they may be obtained by measuring a C concentration at a pitch
of 0.5 Ion or less using a FE-SEM-attached EPMA.
[0043]
Here, the C concentration (Cgb) at a phase interface is referred to as the C
concentration at a measurement point which is closest to the grain boundary on the
20 crystal grain side. However, depending on the measurement conditions, there may be
cases where Cgb is measured to be low due to an effect of the outside of the crystal
grains. In this case, the highest C concentration in the vicinity of the grain boundary is
referred to as Cgb.
[0044]
25 Measuring a local C concentration at an interface is impossible in the current
20
technology. However, as a result of intensive examination by the inventors, it was
determined that a sufficient effect is obtained when the condition of Expression 1 is
satisfied during typical measurement.
[0045]
5 The average grain size of the crystal grains of the retained austenite phase may
be equal to or less than 10 ltm, preferably 4 pm, and more preferably equal to or less than
2 pm. The "grain size" mentioned here means an average circle-equivalent diameter,
and the "average grain size" means a number average thereof. When the average grain
size exceeds 10 pm, the dispersion of the retained austenite phase is coarsened, and thus
10 the TRIP effect may not be sufficiently exhibited. Therefore, excellent elongation may
not be obtained. In addition, in a case where the average grain size of the crystal grains
of the retained austenite phase is less than 1 pm, it is difficult to obtain a phase interface
having a predetermined C concentration gradient, and excellent V-bendability may not be
obtained.
15 [0046]
An average carbon concentration in the retained austenite phase significantly
contributes to the stability of the retained austenite, like the C concentration gradient.
When the average C concentration is less than 0.7%, the stability of the retained austenite
is extremely reduced, the.TRIP effect may not be effectively obtained, and thus
20 elongation is degraded. On the other hand, when the average C concentration exceeds
1.5%, an effect of improving elongation is saturated, and thus manufacturing cost is
increased. Therefore, regarding the average carbon concentration in the retained
austenite phase, the upper limit thereof may be specified as 0.7%, preferably 0.8%, and
more preferably 0.9%, and the lower limit thereof may be specified as 1.5%, preferably
21
1.4%, and more preferably 1.3%.
[0047]
In the steel sheet according to this embodiment, retained austenite phases with
different stabilities may be uniformly dispersed by appropriately distributing the grain
5 sizes of the crystal grains of the retained austenite phases. Inthis case, the retained
austenite phase with a high stability contributes to press-formability in an initial stage of
press-forming at, for example, about 25°C, and the retained austenite phase with a low
stability contributes to press-formability in a late stage of the press-forming at, for
example, about 150°C. Therefore, in addition to high elongation and V-bendability,
10 excellent press-forming stability may also be exhibited.
[0048]
In order to ensure press-forming stability, the crystal grains of the retained
austenite phase need to be dispersed so that the TRIP effect is always exhibited even
though a die temperature is changed during a continuous press. Here, in the steel sheet
15 according to this embodiment, it is possible to realize excellent press-formability that
does not depend on the die temperature by uniformly dispersing the crystal grains of the
retained austenite phases having different stabilities.
[0049]
Specifically, it is preferable that the crystal grains of the retained austenite phase
20 in the steel sheet have small-diameter crystal grains having a number ratio of 40% or
higher and grain sizes of equal to or greater than 1 pm and less than 2 pm, and
large-diameter crystal grains having a number ratio of 20% or higher and grain sizes of
equal to or greater than 2 Inn. In this case, austenite grains having different stabilities
are uniformly disposed, and thus excellent press-forming stability may be realized.
22
Grains (crystal grains with extremely small diameters) having sizes of less than
0.5 pm provide a C concentration gradient with extreme difficulty, become the crystal
grains of an extremely unstable retained austenite phase, and thus have a low
contribution to press-formability. Grains having sizes of equal to or greater than 0.5 pin
5 and less than 2 Inn (small-diameter crystal grains) provide a possibility for maintaining a
high concentration gradient in a formed product because a large amount of carbon is
incorporated from adjacent grains. By causing the small-diameter crystal grains to be
present at a number ratio of 40% or higher, this effect may be exhibited. Grains having
sizes of equal to or greater than 2 pm (large-diameter crystal grains) become crystal
10 grains of the retained austenite phase having a relatively low stability, in which an
amount of carbon incorporated from adjacent grains is small and a temperature gradient
is small. Thus retained austenite phase is likely to cause the TRIP effect in a low press
range. By causing the large-diameter crystal grains to be present at a number ratio of
20% or higher, this effect may be exhibited.
15 [0050]
Moreover, in the steel sheet according to this embodiment, an appropriate C
concentration gradient may be provided for each size of the crystal grains of the retained
austenite phase. More specifically, it is preferable that small-diameter crystal grains,
having a number ratio of 50%, preferably 55%, and more preferably 60% or higher
20 satisfy Expression 2 assuming that the carbon concentration at a position of the center of
gravity is CgcS and the carbon concentration at a grain boundary position is CgbS, and
large-diameter crystal grains having a number ratio of 50% or higher, preferably 55%,
and more preferably 60% or higher satisfy Expression 3 assuming that the carbon
concentration at a position of the center of gravity is CgcL and the carbon concentration
23
at a grain boundary position is CgbL.
CgbS/CgcS> 1.3... (Expression 2)
1 .3>CgbL/CgcL> 1.1... (Expression 3)
[0051]
5 As described above, by providing an appropriate C concentration gradient for
each size of the crystal grains of the retained austenite phase, stable and high
press-formability may be exhibited in a relatively low-temperature state at, for example,
about 25°C and in a relatively high-temperature state, for example, about 150°C.
When the small-diameter crystal grains having a value of CgbS/CgcS of higher
10 than 1.3 have a number ratio of equal to or higher than 50% with respect to the entire
small-diameter crystal grains, the small-diameter crystal grains have high stability, and
thus elongation in a low-temperature state in an initial stage of press-forming may be
enhanced. On the other hand, such stable retained austenite has degraded elongation in
a high-temperature state in a late stage of press-forming. In order to compensate for this,
15 when the large-diameter crystal grains having a value of CgbL/CgcL of higher than 1.1
and less than 1.3 have a number ratio of equal to or higher than 50% with respect to the
entire large-diafn crystal grains, tli gc-di`ame er cry tat grains have low stability
which is effective in improving elongation in the high-temperature state in the late stage
of a press. Here, when the value of CgbL/CgcL is less than 1.1, the crystal grains act on
20 elongation at a higher temperature, resulting in the deterioration of elongation at 150°C
or less.
[0052]
When such a concentration ratio is ensured, high press-formability may be
ensured in a range from a low temperature to a high temperature. However, in order to
25 ensure this effect for the entire structure, a number ratio of the small-diameter crystal
24
grains that satisfy Expression 2 of equal to or higher than 50%, preferably 55%, and more
preferably 60% with respect to all the small-diameter crystal grains is needed. When
the number ratio is less than the above value, the TRIP effect thereof is low, and thus
press-formability at a low temperature is deteriorated. On the other hand, when the
5 large-diameter crystal grains satisfy Expression 3, press-formability may be obtained in a
high-temperature region. Even regarding such large-diameter crystal gains, in order to
ensure this effect for the entire structure, a number ratio of the large-diameter grain sizes
that satisfy Expression 3 of equal to or higher than 50%, preferably 55%, and more
preferably 60% with respect to all the large-diameter crystal grains is needed,
10 [0053]
1
The steel sheet according to this embodiment may have a galvanized film or a
galvannealed film on at least one surface.
[0054]
Hereinafter, a method of manufacturing a steel sheet according to the
15 embodiment of the present invention will be described.
[0055]
In the mbodinrent of the presentinvention, a hot-rolling process an-air-cooling
process, a coiling process, a cold-rolling process, an annealing process, a holding process,
and a final cooling process are at least included. Hereinafter, each of the processes will
20 be described in detail.
[0056]
(Hot-rolling Process)
In the hot-rolling process, hot rolling is performed on a cast slab (slab)
immediately after being continuously cast or a cast slab re-heated to 1100°C or higher
25 after being cooled to 1100°C or less, thereby manufacturing a hot-rolled steel sheet. In
25
a case where the re-heated cast slab is used, a homogenization treatment is insufficiently
performed at a re-heating temperature of less than 1100°C, and thus strength and
V-bendability are degraded. A higher finishing temperature in the hot-rolling process is
more preferable in terms of the recrystallization and growth of austenite grains and thus
5 is set to be equal to or higher than 850°C and equal to or less than 970°C. When the
finishing temperature of the hot rolling is less than 850°C, (ferrite+austenite) two-phase
range rolling is caused, resulting in the degradation of ductility. On the other hand,
when the finishing temperature of the hot rolling exceeds 970°C, austenite grains become
coarse, the fraction of a ferrite phase is reduced, and thus ductility is degraded.
10 [0057]
In the case where the C concentration gradient of the crystal grains in the
retained austenite phase is uniformly dispersed, a lower rolling reduction amount is more
preferable in the final two passes (a stage before the final stage and the final stage)
during rolling, and thus the rolling reduction amount in each stage may be set to be equal
15 to or less than 20%. In addition, the rolling reduction ratio in the final one pass (the
final pass) may be set to be equal to or less than 15% or equal to or less than 10%.
Accordingly, the sizes of the crystal grains of the retained austenite phase may be
dispersed, so that the press-forming stability of the steel sheet may be enhanced. When
the rolling reduction amouni in each stage exceeds 20%, recrystallization of austenite
20 grains proceeds, and thus it becomes difficult to obtain crystal grains having grain sizes
(circle-equivalent diameter) of equal to or greater than 2 tm in the final structure.
[0058]
(Air-cooling Process)
In the air-cooling process, cooling (air cooling) is performed on the hot-rolled
26
steel sheet obtained as described above for a time of equal to or longer than 1 second and
equal to or shorter than 10 seconds. When the air-cooling time is shorter than 1 second,
recrystallization and growth of austenite grains are insufficient, and thus the crystal
grains in the retained austenite phase of the final structure are reduced. On the other
5 hand, when the air-cooling time exceeds 10 seconds, austenite grains become coarse,
uniformity is eliminated, and thus elongation is deteriorated. The air-cooling time is set
to, preferably 5 seconds or less, and more preferably 3 seconds or less.
[0059]
(Coiling Process)
10 In the coiling process, after the air-cooled hot-rolled steel sheet is cooled at an
average cooling rate of equal to or higher than 10°C/sec and equal to or less than
200°C/sec to a temperature range of equal to or less than 650°C, the resultant is coiled in
a temperature range of equal to or less than 650°C, preferably equal to or less than 600°C,
and more preferably equal to or less than 400°C. When the average cooling rate is less
15 than 10°C/sec or the coiling temperature exceeds 650°C, a pearlite phase that
significantly deteriorates V-bendability is generated. When the average cooling rate
exceeds 200°C/sec, an effect of suppressing pearlite is saturated, and variations in
cooling end-point temperature become significant.. Therefore, it is difficult to ensure a
stable material.
20 Therefore, regarding the average cooling rate, the lower limit thereof is set to
10°C/sec, preferably 30°C/sec, and more preferably 40°C/sec, and the tipper limit thereof
is set to 200°C/sec, preferably 150°C/sec, and more preferably 120°C/sec. In addition,
regarding the coiling temperature, the lower limit thereof is set to 200°C, preferably
400°C, and more preferably 650°C, and the upper limit thereof is set to 600°C or 550°C.
27
[0060]
(Cold-rolling Process)
In the cold-rolling process, the coiled hot-rolled steel sheet is pickled, and
thereafter the resultant is subjected to cold rolling at a rolling reduction ratio of 40% or
5 higher, thereby manufacturing a cold-rolled steel sheet. In a rolling reduction ratio of
less than 40%, recrystallization or reverse transformation during annealing is suppressed,
resulting in the degradation of elongation. Here, the upper limit of the rolling reduction
ratio is not particularly specified and may be 90% or 70%.
[0061]
10 (Annealing Process)
In the annealing process, annealing is performed on the cold-rolled steel sheet at
a maximum temperature of equal to or higher than 700°C and equal to or less than 900°C.
When the maximum temperature is less than 700°C, the recrystallization of a ferrite
phase during annealing slows down, resulting in the degradation of elongation. When
15 the maximum temperature exceeds 900°C, the fraction of martensite is increased,
resulting in the degradation of elongation.
Therefore, regarding the annealing maximum temperature , the lower limit
thereof is set to 700°C, preferably 720°C, and more preferably 750°C, and the upper limit
thereof is set to 9009C, preferably 880°C, and more preferably less than 850°C.
20 After the annealing process, for the purpose of suppressing yield point
elongation, skin-pass rolling may be performed by about 1%.
[0062]
(Holding Process)
In order to perform an overaging treatment (hereinafter, OA), in the holding
28
process, the annealed cold-rolled steel sheet is cooled in a temperature range of equal to
or higher than 350°C and equal to or less than 480°C at an average cooling rate of equal
to or higher than 0.1°C/sec and equal to or less than 200°C/sec, and is held in this
temperature for a time of equal to or longer than 1 second and equal to or shorter than
5 1000 seconds. During cooling after the annealing, in order to fix the structure and
efficiently cause bainite transformation, the average cooling rate is set to be equal to or
higher than 0.1°C/sec and equal to or less than 200°C/sec. When the average cooling
rate is less than 0.1 °C/sec, transformation may not be controlled. On the other hand,
when the average cooling rate exceeds 200°C/sec, the effect is saturated, and temperature
10 controllability of a cooling end-point temperature that is most important to generate
retained austenite is significantly deteriorated. Therefore, regarding the average cooling
rate, the lower limit thereof is set to 0.1°C/sec, preferably 2°C/see, and more preferably
3°C/sec, and the upper limit thereof is set to 200°C/sec, preferably 150°C/sec, and more
preferably 120°C/sec.
15 [0063]
A cooling end-point temperature and holding thereafter are important to control
the generation of bainite and determine the C concentration of retained austenite. When
the cooling end-point temperature is less than 350°C, a large amount of martensite is
generated, and thus steel strength is excessively increased. Moreover, it is difficult to
20 cause austenite to be retained. Therefore, the degradation of elongation is extremely
increased. When the cooling end-point temperature exceeds 480°C, bainite
transformation slows down and moreover, the generation of cementite occurs during
holding, degrading an increase in the concentration of C in retained austenite.
Therefore, regarding the cooling end-point temperature and the holding temperature, the
29
lower limit thereof is set to 350°C, preferably 380°C, and more preferably 390°C, and the
upper limit thereof is set to 480°C, preferably 470°C, and more preferably 460°C.
[0064]
A holding time is set to be equal to or longer than 1 second and equal to or
5 shorter than 1000 seconds. When the holding time is shorter than 1 second, insufficient
bainite transformation occurs, and an increase in the C concentration in retained austenite
is insufficient. When the holding time exceeds 1000 seconds, cementite is generated in
the austenite phase, and thus a reduction in the C concentration is more likely to occur.
Therefore, regarding the holding time, the lower limit thereof is set to 1 second,
10 preferably 10 seconds, and more preferably 40 seconds, and the upper limit thereof is set
to 1000 seconds, preferably 600 seconds, and more preferably 400 seconds.
[0065]
(Final Cooling Process)
In the final cooling process, the cold-rolled steel sheet after holding is primarily
15 cooled in a temperature range from 350°C to 220°C at an average cooling rate of equal to
or higher than 5°C/sec and equal to or less than 25°C/sec, and is then secondarily cooled
in a temperature range from 120°C to near room temperature at an average cooling rate
of equal to or higher than 100°C/second and equal to or less than 5°C/sec.
Faint transformation that occurs during cooling after OA has an important role to
20 increase a C concentration of the vicinity of the grain boundary in austenite. Therefore,
the steel sheet is cooled during primary cooling in a temperature range from 350°C to
220°C at an average cooling rate of equal to or higher than 5°C/sec and equal to or less
than 25°C/sec. When the cooling rate in tine temperature range from 350°C to 220°C
exceeds 25°C/sec, transformation does not proceed therebetween, and an increase in the
30
C concentration in austenite does not occur. On the other hand, when the cooling rate in
the temperature range from 350°C to 220°C is less than 5°C/sec, the diffusion of C in
austenite proceeds, and thus the concentration gradient of C is reduced.
Therefore, regarding the average cooling rate during primary cooling, the lower
5 limit thereof is set to 5°C/sec, preferably 6°C/sec, and more preferably 7°C/sec, and the
upper limit thereof is set to 20°C/sec, preferably 19°C/sec, and more preferably 18°C/sec.
In addition, in a low-temperature range of equal to or less than 120°C, the
diffusion of C is further restricted, and transformation is not likely to occur. Therefore,
during secondary cooling, the steel sheet is cooled in a temperature range from 120°C to
10 near room temperature at an average cooling rate of equal to or higher than 100°C/sec,
and a C concentration gradient in the austenite phase of from 350°C to 220°C is achieved.
Otherwise, during secondary cooling, the steel sheet is cooled in a temperature range
from 120°C to near room temperature at an average cooling rate of equal to or less than
5°C/sec so as to cause the C concentration gradient in the austenite phase to become
15 more significant. When the average cooling rate is higher than 5°C/sec and less than
100°C/sec during secondary cooling,_transformation does not occur, and a reduction in
the C concentration at the grain boundary occurs.
Therefore, the average cooling rate during secondary cooling is set to be equal to
or less than 5°C/sec, preferably 4°C/sec, and more preferably 3°C/sec, or is set to be
20 equal to or higher than 100°C/sec, preferably 120°C/sec, and more preferably 150°C/sec.
[0066] -
According to the method of manufacturing a steel sheet according to this
embodiment described above, by controlling the cooling condition after the concentration
of C in the retained austenite phase is increased through bainite transformation, it is
31
possible to control the C concentration gradient in the retained austenite phase so as to
increase the C concentration of the grain boundary portion. In addition, with the
increase in the C concentration in the austenite phase during cooling after annealing, it is
possible to increase the stability of the retained austenite phase.
5 In addition, in a case where the C concentration gradient of the retained
austenite phase is uniformly dispersed by dispersing the sizes of the crystal grains of the
retained austenite phase, the press-forming stability of the steel sheet may be enhanced.
[0067]
This technique may be applied to manufacturing of a hot-dip galvanized steel
10 sheet. In this case, after the above-described holding process, the steel sheet is
immersed into a hot-dip galvanizing bath before the final cooling process. Moreover, it
is possible to acid an alloying treatment after immersion. The alloying treatment is
performed in a temperature range of equal to or higher than 500°C and 580°C. At a
temperature of less than 500°C, insufficient alloying occurs, and at a temperature of
15 higher than 580°C, overalloying occurs, and thus corrosion resistance is significantly
deteriorated.
[0068]
In addition , the present invention is not influenced by casting conditions. For
example, an influence of a casting method (continuous casing or ingot casting) and a
20 difference in slab thickness is small , and a special cast such as a thin slab and a
hot-rolling method may be used. In addition, electroplating may be performed on the
steel sheet.
[Examples]
[0069]
25 The present invention will thither be described on the basis of Examples. The
32
conditions of the Examples are condition examples that are employed to confirm the
possibility of embodiment and effects of the present invention, and the present invention
is not limited to the condition examples. The present invention may employ various
conditions without departing from the concept of the present invention as long as the
5 object of the present invention is achieved.
[0070]
First, cast slabs A to V (steel components of Examples) having chemical
components shown in Table 1 and cast slabs a to g (steel components of Comparative
Examples) were manufactured.
10 [0071]
[Table 1]
Steel C Si Mn AI P S N Selective element
mass %
A 0.15 1.9 2.5 0.031 0.006 0.002 0.0 22 Cu: 0.5, Ni: 0.5
B 0.18 1.2 1.7 0.031 0.007 0.003 0.002 Ca: 0.003
C 0.09 1.3 1.5 0.034 0.006 0.001 0.002 REM: 0.005
D 0.22 1.2 2.1 0.041 0.007
- -
0.0 22
-
0.003 -
E 0.19 1.2 1.8 0.04 0007 0.003 0.0 22 -
F 0.30 1.2 19 0.035 0.006
--
0.001
-
0.002
G 0.12 1.3 1.5 0.0 2 6008
-
0.001
- --
0.0 22 -
H 0.23 1.2 2.3 0.035 0.007 6 003 --6-00-3
1 0.30 1.2 2.3 0.035 0.007 0.0 33 0.003
J 0.34 1.0 1.4 0.050 0.006 0.002 -000-2 V: 0.1, W: 0.3
K 0.07 1.5 29 0.015 0.008 0.003 0.009 Nb: 0.05, Mg: 0.004
L 0.15 0.06 1.5 0.600 0.006 0.002 0.003 Mo:0.12
M 0.15 0.11 2.0 1.1 0.007 0.003 0.002
-
Ca: 0.003
N 0.15 0.11 1.3 0.902 0.006 0.0 1 0.0 3 -REM: 0.005
0 0.22 0.10 2.0 1.9 0.007 0.002 0.002 B: 0.005
P 0.22 0.15 1.3 0.903 0.007 0.003 0.002 Mo: 0.15, Ti: 0.02, Nb: 0.02
Q 0.25
-
0.50
- --
1.9
- -
1.0 0.006 0.002 0.002 Mo:0.15
R C30 609 L2 1.0 0.008 0.003 0.002 Ti: 0.07
S 0.30 0.07 1.6 1.4 0.006 0.001 0.003 Mo: 0.15
T 0.25 0.50 1.7 1.4 0.007 0.001 0.004 Mo:0.15
U 0.22 0.09 0.91 1.0 0.006 0.002 0.002 Mo: 0.1, V: 0.1, Cr: 0.3
V 0.22 0.10 1.4 1.0 0.09 0.045 0.003 Mo: 0.2, Zr: 0.005
a 0.40 1.6 2.0 0.030 0.006 0.001 0.002 -
b 0.02 1.2 2.0 0.035 0.007 0.001 0.003 -
33
c 0.22 1.2 1.3 0.041 0.006 0.11 0.003 Me: 0.2
d 0.25 3.0 1.0 0.040 0.006 0.001 0.002 Me: 0.22
e 0.25 1.2 4.0 0.035 0.007 0.001 0.004
f 0.30 0.03 1.4 0.005 0.008 0.001 0.004
g 0.30 0.01 1.2 3.5 0.008 0.003 0.002 Mo:0.6
[0072]
Hot-rolled steel sheets were manufactured by performing hot rolling on these
cast slabs. During hot rolling, rolling reduction ratios in sixth and seventh stages of the
rolling corresponding to the final two passes and finishing temperature were as shown in
5 Table 2. Thereafter, the hot-rolled steel sheet that was subjected to air cooling for a
predetermined time was cooled to about 550°C at an average cooling rate of 60°C/sec,
and was then subjected to coiling at about 540°C. The coiled hot-rolled steel sheet was
subjected to pickling, and was thereafter subjected to cold rolling at a rolling reduction
ratio of 50%, thereby manufacturing a cold-rolled steel sheet.
10 [0073]
In addition, an annealing treatment was performed at a maximum annealing
temperature shown in Table 2. After annealing, for the purpose of suppressing yield
point elongation, skin-pass rolling was performed by about 1%.
_[0074]_
15 Thereafter, in order to perform an averaging treatment, the steel sheet after the
annealing was cooled and held. A cooling rate, a holding temperature, and a holding
time here are shown in Table 2. In addition, regarding some steel sheets, the steel sheets
after holding were immersed into a hot-dip galvanizing bath, and were subjected to an
alloying treatment at a predetermined alloying temperature.
20 [0075]
Lastly, primary cooling (cooling in a range of 350 to 220°C) and secondary
cooling (cooling in a range of 120°C to 20°C) were performed on the cold-rolled steel
34
sheet at a predetermined cooling rate, thereby manufacturing steel sheets Alto V 1 and al
to gl.
[0076]
[Table 2]
.35
Steel
sheet
6th rolling
reduction
ratio
7th rolling
reduction
ratio
Finish
temperature
Air-cool
i^g time
Maximum
annealing
temperature
Cooling
rate
Holding
temperature
Holding
time
Alloying
temperature
Final
primary
cooling
rate
Final
secondary
cooling
rate
% % °C S °C °C/sec °C sec °C °C/sec °C/sec
Al 15 10 879 2.5 850 40 400 400 No plating 14 2
A2 15 10 890 2.5 850 150 400 300 No plating 15 2
A3 40 40 890 2 850 150 400 100 No plating 15 1
A4 25 25 890 2 850 150 400 100 No plating 15 1)
AS 20 15 890 2 850 150 400 100 No plating 15 2
81 12 12 890 4 880 40 400 300 440 20 3
B2 12 12 890 4 850 4 450 40 440. 20 2
B3 12 12 895 4 980 40 425 40 400 15 2
Cl 15 10 901 2.5 850 40 425 300 460 15 1
C2 15 10 895 2.5 850 4 450 40 460 10 2
D1 15 10 892 2.5 775 50 400 300 No plating 10 150
D2 15 10 880 2.5 800 100 425 300 No plating 10 150
D3 15 10 888 2.5 660 100 425 000 No plating 8 150
D4 15 10 888 2.5 660 100 425 300 No plating
g
40 3
El 12 12 883 3 800 40 425 300 No plating 8 150
E2 12 12 900 3 800 100 425 300 No plating 8 150
E3 12 12 900 3 800 100 425 300 No plating 8 50
Fl 15 10 896 3 775 50 400 200 No plating 5 _3
F2 15 10 895 3 780 100 425 300 No latina 15 3
F3 15 10 885 3 780 100 325 300 No plating 10 150
F4 15 10 880 3 780 100 550 300 No plating 10 150
01 10 8 906 2.5 800 40 425 300 No plating 10 150
G2 10 8 900 2.5 800 100 400 300 No plating 10 150
HI 10 8 890 2.5 775 50 400 150 No plating 15 2
H2 10 8 900 2.5 800 100 425 200 No plating 55 2
H3 10 8 000 2.5 800 120 425 1200 No plating 15 2
H4 10 8 890 2.5 800 120 425 200 No plating 2 150
36
11 15 10 886 2.5 775 50 400 300 No plating 15 1
12 15 10 890 2.5 800 100 425 200 No plating 15 2
J1 15 10 887 2.5 800 40 425 300 No plating 15 1^
J2 15 10 892 15.0 800 40 425 300 No plating 15 3
Kl 15 10 881 2.5 800 40 400 400 No plating 15 3
L1 15 10 891 I 2 850 4 450 40 470 15 2
L2 15 10 900 2 775 40 450 400 470 15 3
Ml 15 10 888 2.5 800 4 425 40 500 15 4
M2 15 10 890 I 0.5 800 40 425 300 500 15 2
NI 15 10 905 2.5 800 4 425 40 500 20 3
N2 15 10 900 2.5 800 40 450 300 500 20 3
01 15 10 905 3 800 4 400 40 500 20 2
02 15 10 900 3 800 40 425 300 500 20 2
P1 10 8 902 3 800 4 450 40 520 10 150
P2 10 8 890 3 800 40 450 400 520 10 150
Q 1 10 8 882 2.5 775 4 425 40 520 20 2
Q2 10 8 890 2.5 775 50 450 3 00 520 20 3
R1 10 8 893 2.5 775 4 400 40 500 15 1
R2 10 8 880 2.5 825 40 425 300 500 15 2
Sl 18 15 888 4 825 4 425 40 500 15 3
S2 18 15 895 4 825 40 425 300 500 15 2
TI 18 15 908 4 825 4 425 40 520 15 I
T2 18 15 900 4 775 40 450 350 520 15 2
UI 15 10 909 4 800 4 425 40 520 20 3
VI 15 10 899 4 800 4 425 40 520 20 2
al 15 10 882 2.5 775 40 400 300 No plating 20 2
bl 15 10 907 2.5 775 100 400 300 No plating 20 2
cl 15 10 905 2.5 800 40 400 300 500 20 2
dl 15 10 921 2.5 800 40 400 300 500 20 2
el 15 10 879 2.5 800 4 450 40 No lating 20 2
fl 15 10 891 2.5 775 100 400 300 No Iating 20 2
sl 15 10 913 2.5 800 40 400 300 500 20 2
37
[0077]
The steel structures of the steel sheets obtained as described above and steel
sheet characteristics are shown in Tables 3 and 4. Regarding the steel structures,
"proportion of ferrite+bainite+tempered martensite", "proportion of retained austenite",
5 "proportion of crystal grains that satisfy Expression (1)", "proportion of small-diameter
crystal grains", "proportion of large-diameter crystal grains", "proportion of
small-diameter crystal grains that satisfy Expression (2)", "proportion of large-diameter
crystal grains that satisfy Expression (3)", "average grain size of crystal grains", and
"average C concentration in retained austenite phase" were measured. In addition,
10 , regarding the steel sheet characteristics, "tensile strength", "25°C elongation",
"V-bendability", and "150°C elongation" were evaluated.
[0078]
[Table 3]
38
Steel Proportion of Proportion Proportion of Proportion of Proportion of Proportion of Proportion of
sheet ferrite+bainite+ of retained retained small-diameter large-diameter retained retained
tempered austenite austento grains retained austenite retained austenite austenite grains austenite grains
martensite that satisfy grains grains that satisfy that satisfy
Expression ( 1) Expression (2) Expression (3)
Al 78 20 64 62 23 64 60
A2 79 19 I66 61 24 66 62
A3 77 21 67 85 5 67 63
A4 77 20 68 70 15 68 64
AS 78 21 67 70 22 66 65
B1 89 10 75 57 33 76 72
B2 88 10 74 52 43 76 72
B3 86 2 '64 50 45 65 61
Cl 93 10 67 62 - 23 66 62
C2 92 10 56 60 30 55 52
Dl 83 61 24 56 53 D2 83 62 23 55 52
D3 80 62 23 51 51
D4 81 62 22 _5 25
El 87 58 27 51 51
E2 88 58 27 52 52
E3 88
0
55 26 25 63
FI 82 57 28 66 63
F2 83 59 26 66 62
F3 39 59 55 52
F4 45 68 56 53
G1 93 57 G2 55 52 93 0 ^ 55 35 56 52
ZL 9L tZ 19 SG SI Ot IE
ZL LL Zt £S SL IT 88 ZG IA 9L Zt £S tL 9 £6 Ifl Z9 99 l z t9 99 1 S I t8 Z.I.
Z9 99 0£ 09 99 SI b8 LL £9 99 £Z Z9 L9 91 Z8 ZS
Z9 99 0£ 09 S9' 9I Z8 IS Z9 99 ££ LS 99 ti t8 Z2I
£9 99 6t 9t 59 N S8 Tg
ZG 9L I£ 65 SL ! 9T £8 Zd
ZG 9L Lt £t tL LI I8 I^
£S 95 t£ 9S GSi Oi 68 Zd
ZS 9S OS St SS OI 68 Id IL 9L 9Z 69 tL'I tT 58 ZO
£L LL LE £S cL tt S8 IO ZL LL ZZ £9 SG ! OI £6 ZN
ZL 9L I£ 6S SL II £6 TN Z9 S9 8 GL 99 I T 88 ZY^i
Z9 99 I£ 6S 99 , II 88 ITQ
£9 99 £Z Z9 G9 IT £6 ZI
Z9 99 9Z 6S 99 , IT £6 I7
19 19 £Z Z9 99 OI 6L IN
Z9 S9 Z9 £Z t9 OI 88 Zf
Z9 99 £Z Z9 L9 TI 88 If
£9 99 17( 19 L9 OZ 8L ZI
Z9 99 tZ 19 99 OZ 8G TI
OZ 8£ S£ SS £'' OZ 8L tH
- 0 08 £H
£9 99 S£ SS 99 OZ 8L ZH
Z9 99 8£ ZS S9 8I 08 IH
6 £
IL 9L bZ 19 9L t 6L 10
_ = 0 98 u
ZL LL 0£ 09 9L ZZ S9 IQ
ZL 9L bZ 19 9L bT J78 IP
ZL LL bZ 19 SL' TI L8 h
ZL 9L bZ 19 SL I L6 Ia
01
41
[0079]
[Table 4]
Steel
sheet
Average
grain
size of
crystal
grains
Average C
concentration
in retained
austenite
phase
Tensile
strength
25°C
elongation
V-bendability 150°C
elongation
pill % Nbum-0/0 Mill %
At 1.5 0.8 1170 20 1.7 21
A2 1.6 0.8 1158 20 1.7 21
A3 1.1 0.8 1238 15 3.9 5
A4 1.4 0.8 1190 10 2.7 16
AS 1.5 0.8 1183 20 1.8 24
BI 1.7 1.4 753 40 0.4 44
B2 1.9 1.4 773 37 0.5 45
B3 1.9 1.4 873 21 1.2 23
Cl I 1.5 0.9 1 596 42 No cracking 44
C2 1.7 0.9 636 35 No cracking 41
D1 1.6 1.4 994 28 1.1 32
D2 1.5 1.4 979 28 1.2 32
D3 1.5 1.2 1100 13 2.5 13
D4 1.5 1.3 1110 18 2.5 20
E1 1.6 1.4 817 32 0.6 39
E2 1.6 1.4 790 33 No cracking 40
E3 1.6 1.4 785 25 2.3 30
F1 1.7 1.4 1006 28 1.3 32
F2 1.6 1.4 990 29 1.2 32
F3 1.7 1.4 1220 15 2.9 16
F4 1.5 0.6 880 19 1.6 19
G1 1.7 1.4 584 45 No cracking 55
62 1.8 1.4 592 44 No cracking 55
H1 1.8 1.3 1108 23 1.7 29
H2 1.8 1.2 1188 22 1.9 25
FI3 1090 15 3.4 15
H4 1.8 1.2 1170 17 3.3 16
11 1.6 1.5 1196 25 1.9 27
12 1.6 1.5 1199 25 2.0 27
Ji 1.5 1.4 790 37 0.5 40
J2 2.5 1.1 770 17 1.3 34
Kl 1.5 0.9 1157 21 1.7 23
L1 1.6 1.2 601 45 No cracking 49
L2 1.5 1.2 599 46 No cracking 49
M1 1.7 0.8 777 30 No cracking 36
M2 1.2 0.8 790 25 1.3 15
NI 1.7 1.2 572 50 No crackin 54
42
N2 1.5 1.3 600 51 N ccracking 51
01 1.8 1.0 913 28 0.8 32
02 1.6 1.0 910 30 0.8 31
P1 2.0 1.2 741 31 0.3 43
P2 1.7 1.2 745 33 0.3 40
151 2.0 0.9 1043 24 1.4 28
Q2
-
1.7 1.0 1001 27 1.2 29
RI 2.0 1.2 905 27 0.9 36
R2 1.7 1.2 940 28 1.0 32
S 1 1.7 1:2 1025 27 1.3 30
S2 1.5 1.3 1011 28 1.2 30
Ti 1.7 1.1 951 28 0.9 31
T2 1.5 1.1 960 28 09 29
UI 1.9 1.2 583 47 No cracking 55
V1 1.9 1.2 779 35 No cracking 42
at 1.6 1.2 1519 55 2.9 00
bl 1.6 1.1 426 42 0.3 42
el 1.6 1.2 807 26 2.6 29
d I 1.6 1.2 942 22 2.4 15
el 1.7 0.2 1710 12 3.5 11
171 - 883 20 2.4 21
g1 1.6 1.0 1124 18 3.0 19
For observation of the identification of the structure and positions and
measurement of an average grain size (average circle-equivalent diameter) and
occupancy ratio, a cross-section in a steel sheet rolling direction or a cross-section
5 perpendicular to the rolling direction was corroded by Nital reagent for quantification
through observation using an optical microscope at a magnification of 500x to 1000x.
[0081]
Measurement of "ratio of retained austenite phase" was performed on a surface
that was chemically polished to a 1/4 thickness from the surface layer of the steel sheet,
10 and retained austenite was quantified and obtained from the integrated intensities of the
(200) and (211) planes of ferrite and the integrated intensities of the (200), (220), and
(311) planes of austenite by monochromic MoKu. rays.
[0082]
43
In addition, "average C concentration in retained austenite phase" (Cy) was
calculated by the following Expression A by obtaining a lattice constant (unit: angstroms)
from the angles of reflection of the (200) plane, the (220) plane, and the (311) plane of
austenite through ray analysis using Cu-Ka rays.
5 Cy=(lattice constant-3.572)/0.033... (Expression A)
[0083]
"25°C elongation" and "150°C elongation" were evaluated at the temperatures
of 25°C and 150°C by elongation in the C direction of a JIS #5 tensile test piece.
"V-bendability" was evaluated by a minimum R in which no cracking occurred
10 during a V-bending test. In the V-bending test, a test piece of 30 nunx200 mm was bent
at 90 degrees using V blocks having various R. A distance between the supports was 95
mm, and a wrinkle pressing force (BHF) at the supports was 98 kN. Determination of
cracking was performed through visual observation or observation using a magnifying
glass, and those having cracks or constriction on the surface were determined as
15 cracking.
[0084]
Among the steels a to g of Table 1, the steel a did not satisfy the C upper limit
that is specified by the present invention, and the steel b did not satisfy the C lower limit.
The steels c, d, and e did not satisfy the upper limits of S, Si, and Mn, respectively. The
20 steel f did not satisfy the lower limits of Si and Al. The steel g did not satisfy the lower
limit of Si and the upper limit of Al.
[0085]
The steel sheet A3 and the steel sheet A4 are steel sheets manufactured by
setting the rolling reduction ratios in the final two passes to be high.
44
The steel sheet D3 is a steel sheet manufactured by setting the maximum
temperature during annealing to be low.
The steel sheet D4 is a steel sheet manufactured by setting the final primary
cooling speed to be high.
5 The steel sheet E3 is a steel sheet manufactured by setting the final secondary
cooling speed to 50°C/sea
The steel sheet F3 is a steel sheet manufactured by setting the holding
temperature to be low.
The steel sheet F4 is a steel sheet manufactured by setting the holding
10 temperature to be high.
The steel sheet H3 is a steel sheet manufactured by setting the holding time to be
long.
The steel sheet H4 is a steel sheet manufactured by setting the final primary
cooling speed to be low.
15 The steel sheet J2 is a steel sheet manufactured by setting the air-cooling time to
be long.
The-steel-sheet Iv72is a steel sheet manufactured-by setting Te air cooling-tide
to be short.
[0086]
20 In the steel sheet al, the fraction of ferrite+bainite is out of range, and in the
steel sheet bl, the fraction of austenite is equal to or less than a range. The steel sheet
el has a low average C concentration in austenite. The steel sheet fl and the steel sheet
gl cannot ensure the fractions of austenite.
[0087]
25 FIG. 1 is a diagram showing the relationship between tensile strength and 25°C
45
elongation of the steel sheets according to the Examples and the Comparative Examples,
and FIG. 2 is a diagram showing the relationship between tensile strength and
V-bendability regarding the same steel sheets. From FIGS. 1 and 2, it can be seen that
both high elongation and V-bendability are obtained according to the steel sheet and the
5 method of manufacturing a steel sheet according to the present invention.
In addition, FIG. 3 is a diagram showing the relationship between tensile
strength and 150°C elongation according to the Examples and the Comparative Examples.
From FIGS. 1 and 3, it can be seen that high elongation is realized at both temperatures
of 25°C and 150°C according to the steel sheet and the method of manufacturing a steel
10 sheet according to the present invention.
Industrial Applicability
[0088]
According to the present invention, the present invention may provide a steel
15 sheet having higher elongation and V-bendability compared to that according to the
related art and moreover has excellent press-forming stability, and a method of
manufacturing the same.
46

CLAIMS
1. A steel sheet comprising:
as chemical components, by mass%,
5 0.05% to 0.35% of C;
0.05% to 2.0% of Si;
0.8% to 3.0% of Mn;
0.01% to 2.0% of Al;
equal to or less than 0.1 % of P;
10 equal to or less than 0.05% of S;
equal to or less than 0.01% of N; and
the balance including iron and inevitable impurities,
wherein an area ratio of equal to or higher than 50% of a total of a ferrite phase,
a bainite phase, and a tempered martensite phase is contained,
15 an area ratio of equal to or higher than 3% of a retained austenite phase is
contained, and
crystal grates of the retained austenite phase having a-rumher ratio of-equal-to or
higher than 50% satisfy Expression 1, assuming that a carbon concentration at a position
of center of gravity is Cgc and a carbon concentration at a grain boundary is Cgb:
20 Cgb/Cgc? 1.2... (Expression 1)
2. The steel sheet according to Claim 1, further comprising, in the chemical
components, by mass%, at least one of.-
0.01% to 0.5% of Me;
25 0.005% to0.1%ofNb;
47
0.005% to 0.2% of Ti;
0.005% to 0.5% of V;
0.05% to 5.0% of Cr;
0.05% to 5.0% of W;
5 0.0005% to 0.05% of Ca;
0.0005% to 0.05% of Mg;
0.0005% to 0.05% of Zr;
0.0005% to 0.05% of REM;
0.02% to 2.0% of Cu;
10 0.02% to 1.0% of Ni; and
0.0003% to 0.007% of B.
3. The steel sheet according to Claim 1,
wherein an average grain size of the crystal grains is equal to or less than 10 lun,
15 and
an average carbon concentration in the retained austenite phase is equal to or
higehr than .7%oan -d equal tom less tthan 15° .. -
4. The steel sheet according to Claim 1,
20 wherein the crystal grains having a number ratio of equal to or higher than 40%
are small-diameter crystal grains having an average grain size of equal to or greater than
1 μm and equal to or less than 2 pun, and
the crystal grains having a number ratio of equal to or higher than 20% are
large-diameter crystal grains having an average grain size of equal to or greater than 2
25 pm.
48
5. The steel sheet according to Claim 4,
wherein the small-diameter crystal grains having a number ratio of equal to or
higher than 50% satisfy Expression 2, assuming that a carbon concentration at a position
5 of center of gravity is CgcS and a carbon concentration at a grain boundary is CgbS, and
the large-diameter crystal grains having a number ratio of equal to or higher than
50% satisfy Expression 3, assuming that a carbon concentration at a position of center of
gravity is CgcL and a carbon concentration at a grain boundary is CgbL:
CgbS/CgcS> 1.3... (Expression 2)
10 1.3>CgbL/CgcL> 1.1... (Expression 3)
6. The steel sheet according to any one of Claims I to 5,
wherein the steel sheet has a galvanized film provided to at least one surface.
15 7. The steel sheet according to any one of Claims 1 to 5,
wherein the steel sheet has a galvannealed film provided to at least one surface.
8. A method of manufacturing a steel sheet, the method comprising:
a hot-rolling process of manufacturing a hot-rolled steel sheet by performing hot
20 rolling on a slab having the chemical components according to Claim I or 2 at a finishing
temperature of equal to or higher than 850°C and equal to or less than 970°C;
an air-cooling process of performing air cooling on the hot-rolled steel sheet for
a time of equal to or longer than 1 second and equal to or shorter than 10 seconds;
a coiling process of cooling the air-cooled hot-rolled steel sheet to a temperature
25 range of equal to or less than 650°C at an average cooling rate of equal to or higher than
49
10°C/sec and equal to or less than 200°C/sec and thereafter coiling the steel sheet in a
temperature range of equal to or less than 650°C;
a cold-rolling process of performing pickling on the coiled hot-rolled steel sheet
at a rolling reduction ratio of equal to or higher than 40% and thereafter performing cold
5 rolling on the steel sheet, thereby manufacturing a cold-rolled steel sheet;
an annealing process of performing annealing on the cold-rolled steel sheet at a
maximum temperature of equal to or higher than 700°C and equal to or less than 900°C;
a holding process of cooling the annealed cold'rolled steel sheet in a temperature
range of equal to or higher than 350°C and equal to or less than 480°C at an average
10 cooling rate of equal to or higher than 0.1°C/ sec and equal to or less than 200°C/sec, and
holding the steel sheet in this temperature range for a time of equal to or longer than 1
second and equal to or shorter than 1000 seconds; and
a final cooling process of primarily cooling the cold-rolled steel sheet in a
temperature range from 350°C to 220°C at an average cooling rage of equal to or higher
15 than 5°C/sec and equal to or less than 25°C/sec, and secondarily cooling the steel sheet in
a temperature range from 120°C to near room temperature at an average cooling rate of
equal to or higher than 100°C/sec and equal to or less than 5°C/sec.
9. The method according to Claim 8,
20 wherein rolling is performed with a strain amount of equal to or less than 20% in
each of final two passes in the hot-rolling process.
10. The method according to Claim 8,
wherein a slab which is re-heated to 1100°C or higher after being cooled to
50
1100°C or less is used in the hot-rolling process.
11. The method according to Claim 8, further comprising an immersion
process of immersing the steel sheet in a hot-dip galvanizing bath after the holding
5 process.
12. The method according to Claim I I further comprising an alloying
treatment process of performing an alloying treatment in a range of equal to or higher
than 500°C and equal to or less than 580°C after the immersion process.