SPECIFICATION
5
HIGH-STRENGTH STEEL SHEET HAVING EXCELLENT PROCESSABILITY AND
PAINT BAKE HARDENABILITY, AND METHOD FOR PRODUCING OF
HIGH-STRENGTH STEEL SHEET
Field of the Invention
[0001]
The present invention relates to a high-strength steel sheet having excellent
10 processability and paint bake hardenability which is preferred for use of an automobile
steel sheet, particularly, use of an outer panel, and a method for producing the same.
Priority is claimed on Japanese Patent Application No. 2009-255726, filed
November 9, 2009, the content of which is incorporated herein by reference.
15 Related Art
[0002]
Efforts are being made to reduce the weight-of the body-of-automobiles to
improve the gasoline mileage so as to suppress the amount of carbon dioxide exhausted.
Therefore, a high-strength steel sheet whose thickness can be reduced is being
20 increasingly applied to automobile members. In addition, a high-strength steel sheet has
also become widely used for an automobile body in order to secure passenger safety.
Among them, a steel sheet having a tensile strength of approximately 340 MPa
is in practical use for use in panel members, particularly, outer panels. Recently, for the
purpose of reducing the weight by means of an additional increase in the strength of an
25 outer panel, there is a demand for a steel sheet having a high strength of 390 MPa to 500
2
MPa and having excellent press moldability and surface quality.
[0003]
However, generally, an increase in the tensile strength is followed by an increase
in the yield strength and a decrease in the ductility; therefore the processability,
5 particularly, press moldability is impaired. As a result, as an index for maintaining the
processability while holding the strength, there is a demand for the product of the tensile
strength TS [MPa] and the total elongation EL [%], that is, TSxEI [MPa•%] to be 17000
or more [MPa•%]. It is known that the yield strength and the yield ratio are important
factors in attaining the above demand. The yield strength and the yield ratio have a
10 strong correlation with the processability, particularly, press moldability, and, the yield
strength and the yield ratio need to be 270 MPa or less and 0.55 or less respectively in
order to mold an outer panel. In addition, since the edge portions of an outer panel are
often subjected to a hemming process, the outer panel also needs to be excellent in terms
of tight bending processability.
15 [0004]
As a steel material that satisfies both strength and processability requirements,
dual phase steel (hereinafter referred to as the DP steel) having a complex structure
composed of a hard second phase including ferrite and martensite as the main
components is known. The DP steel has a low yield strength and an excellent ductility.
20 [0005]
On the other hand, increasing the yield strength of a member using a paint
baking treatment after press molding is effective in improving the dent resistance.
Therefore, there is a demand for improvement in the paint bake hardenability (hereinafter
referred to as the BI-I) in order to satisfy both the moldability and the dent resistance.
25 The BH is a characteristic that develops through a so-called strain aging phenomenon in
3
which carbon atoms, nitrogen atoms, and the like are fixed to dislocations introduced by
a heat treatment at a low temperature, such as a paint baking treatment, during molding,
and carbonitrides are precipitated.
[0006]
5 For example, Patent Documents 1 to 3 propose DP steel sheets having excellent
processability. However, the steel sheet as proposed in Patent Document 1 has a low
yield strength, but the product of the tensile strength and the total elongation, that is, the
strength-ductility balance TS x El is not sufficient. In addition, the steel sheet as
proposed in Patent Document 2 has a low yield strength, and also has excellent
10 strength-ductility balance, but there is a problem in that the steel sheet has to be held at a
high temperature for a long time in an annealing process such that degradation of the
productivity is caused. Furthermore, the steel sheets as proposed in Patent Documents 1
and 2 are not produced in consideration of the dent resistance after any of press molding
and a paint baking treatment is carried out. In addition, the steel sheet proposed in
15 Patent Document 3 has excellent dent resistance, and also a low yield strength, but the
product of the tensile strength and the total elongation, that is, the strength-ductility
balance x is not su rcient.
In addition, the steel sheets as proposed in Patent Documents 4 and 5 have a
high yield strength, but the processability is not sufficient.
20 In addition, in the steel sheet as proposed in Patent Document 6, the area
fraction of unrecrystallized ferrite is intentionally increased by adjusting the rate of
temperature rise during annealing.
Reference Documents
25 Patent Documents
4
[0007]
[Patent Document 1] Japanese Unexamined Patent Application, First Publication
No.2001-303184
[Patent Document 2] Japanese Unexamined Patent Application, First Publication
5 No.2000-109965
[Patent Document 3] Japanese Unexamined Patent Application, First Publication
No.2005-28 1 867
[Patent Document 4] Japanese Unexamined Patent Application, First Publication
No.H6-73497
10 [Patent Document 5] Japanese Unexamined Patent Application, First Publication
No.2003-138317
[Patent Document 6] Japanese Unexamined Patent Application, First Publication
No.2008-106350
15 Summary of the Invention
Problems to be Solved by the Invention
_-[0008]
An object of the present invention is to stably provide a steel sheet that has a
tensilestrength of 390 MPa to 500 MPa, and also has excellent processability and,
20 furthermore, paint bake hardenability without impairing the productivity.
Methods for Solving the Problem
[0009]
In order to achieve the above object, the configurations and methods of the
25 respective aspects of the present invention are as follows:
5
[0010]
(1) A high-strength steel sheet according to an aspect of the present invention
includes, by mass%, C: 0.01 % to 0.10%; Si: 0.15% or less; Mn: 0.80% to 1.80%; P:
0.10% or less; S: 0.015% or less; Al: 0.10% to 0.80%; Cr: 0.01 % to 1.50%; and N:
5 0.0100% or less and a balance consisting of iron and unavoidable impurities, in which a
metallic structure is composed of ferrite and a hard second phase, an area fraction of the
ferrite is 80% or more, an area fraction of the hard second phase is I% to 20%, a fraction
of unrecrystallized ferrite in the ferrite is less than 10%, a ferrite grain sizes are 5 pm to
20 μm, and a fraction of the ferrite crystal grains having an aspect ratio of 1.2 or less in
10 entire ferrite crystal grains is 60% or more.
[0011]
(2) In the high-strength steel sheet according to the above (1), a component
composition of the high-strength steel sheet may satisfy that Mn/Cr is 3.0 or less, and
Cr/(Si+Al) is 3.0 or less.
15 [0012]
(3) The high-strength steel sheet according to the above (1) or (2) may further
include, by mass%, one or more of Nb: 0.0005% to 0.0500%; Ti: 0.0005% to 0.0500%;
Mo: 0.005% to t.500%; W: 0.005% to 1.500%; B: 0.0001% to 0.0100%; Ni: 0.005% to
1.500%; Cu: 0.005% to 1.500%; and V: 0.005% to 1.500%.
20 [0013]
(4) In the high-strength steel sheet according to any one of the above (1) to (3), a
molten zinc coat or a molten zinc alloy coat may be provided at the surface of the
high-strength steel sheet.
[0014]
25 (5) The high-strength steel sheet according to the above (4) may further include
6
Cr: 0.20% to 1.50%, and P: less than 0.015%.
[0015]
(6) In a method for producing the high-strength steel sheet according to an
aspect of the present invention, a billet having the chemical components of the
5 high-strength steel sheet according any one of the above (1) to (3) is hot rolled, pickled,
cold rolled at a percentage reduction in thickness of more than 60% so as to obtain a steel
sheet, then, the steel sheet is heated to a temperature range of 720°C to 850°C at a
heating rate controlled to 1°C/s to 10°C/s in a temperature range of an AcI
transformation point to an Ac3 transformation point, subjected to an annealing for a
10 retention time of 10 seconds to 200 seconds during which the temperature of the steel
sheet is 720°C to 850°C, and after the annealing, subjected to a first cooling to 500°C or
lower at a cooling rate of 3°C/s or more, and then subjected to a skin pass rolling of 2.0%
or less.
[0016]
15 (7) In the method for producing the high-strength steel sheet according to the
above (6), a heat treatment may be carried out in a temperature range of 200 °C to 450°C
for 30 seconds or more before the skin pass rolling, and a second cooling may be carried
out at a cooling rate of 1 °C/s to 3°C/s to 100° C or lower after the heat treatment.
[0017]
20 (8) In the method for producing the high-strength steel sheet according to the
above (6), galvanization may be carried out on the steel sheet after the first cooling and
before the skin pass rolling.
[0018]
(9) In the method for producing the high- strength steel sheet according to the
7
above (8), a heat treatment for alloying may be carried out for 10 seconds or longer in a
temperature range of 450°C to 600°C at a timing after the galvanization and before the
skin pass rolling.
[0019]
5 (10) In the method for producing the high-strength steel sheet according to the
above (7), galvanization may be carried out on the steel sheet at a timing after the heat
treatment and before the second cooling.
[0020]
(11) In the method for producing the high-strength steel sheet according to the
10 above (10), a heat treatment for alloying may be carried out for 10 seconds or longer in a
temperature range of 450°C to 600°C at a timing after the galvanization and before the
second cooling.
Effects of the Invention
15 [0021]
According to the respective aspects of the present invention, it is possible to
provide a steel sheet that has a tensile strength of 390 MPa to 500 MPa, has low yield
strength and yield ratio, has excellent strength-ductility balance, and has paint bake
hardenability.
20 In the high-strength steel sheets of the respective aspects of the present invention
and the methods for producing the same, an uneven structure hardness, which is a cause
of occurrence of streaky recess and protrusion surface defects, is suppressed. As a
result, it is possible to stably suppress occurrence of streaky recess and protrusion surface
defects and to significantly improve the yield of high-strength steel sheet production.
25
8
Brief Description of the Drawings
[0022]
FIG. 1 is a view showing the correlation between an Mn/Cr ratio and a yield
ratio YR.
5 FIG. 2 is a view showing the correlation between a Cr/(Si+Al) ratio and the yield
ratio YR.
FIG 3 is a view showing the relationship between a heating rate in a temperature
range of an Ac, transformation point to an Ac3 transformation point and an aspect ratio
of ferrite crystal grains.
10 FIG. 4 is a view showing the heating rate in the temperature range of the Acl
transformation point to the Ac3 transformation point and an appropriate range of
unrecrystallized ferrite fraction.
Detailed Description of the invention
15 [0023]
When the residual percentage of hard structures, such as unrecrystallized ferrite
and martensite, is high, the hardnessof a steelsheet structure becomes uneven:- When
the steel sheet is press-molded, plastic deformation locally starts from portions having a
relatively low hardness. As a result, marks of streaky recess and protrusion are
20 generated on the surface of the steel sheet. For a steel sheet that is used for an
automobile outer sheet and the like, the aesthetic appearance of the surface is important,
and therefore the streaky recess and protrusion surface defects are considered as serious
defects, and a significant decrease in the yield is caused.
As described below, in the high-strength steel sheets according to the respective
25 embodiments of the present invention and methods for producing the same, an uneven
9
structure hardness, which is a cause of occurrence of streaky recess and protrusion
surface defects, is suppressed. As a result, occurrence of streaky recess and protrusion
surface defects can be stably suppressed.
In addition, the present inventors studied the component compositions and
5 microstructures of steel sheets, and, furthermore, methods for producing steel sheets in
order to improve both the processability and the paint bake hardenability of a
high-strength steel sheet. As a result, the inventors paid attention to the ratio of the
added amount between Mn and Cr, which are elements that increase the hardenability,
that is, optimization of a Mn/Cr ratio, and, furthermore, Al, which is an element that
10 promotes generation of ferrite and concentrates C in austenite, and found that both the
processability and the paint bake hardenability of a high-strength steel sheet are
improved by optimizing the ratio of the added amount between Cr and (Si+Al), that is,
the ratio of Cr/(Si+Al).
Hereinafter, the embodiments of the present invention will be described in
15 detail.
[0024]
Firstly, reasons why the steel components are limited in the present invention
will be described. In the following description, the amounts of the components are
indicated by % by weight unless otherwise described.
20 C is an element that promotes generation of martensite, contributes to an
increase in the tensile strength and a decrease in the yield strength, and increases the
yield strength when a paint baking treatment is carried out after molding, thereby
enhancing the dent resistance of a panel member. An appropriate amount of C is added
depending on a target steel strength.
25 The lower limit of the C amount is 0.01% or more in order to secure a sufficient
1.0
amount of martensite, and obtain target tensile strength, yield strength, and BH. The
lower limit of the C amount is more preferably 0.03% or more, and still more preferably
0.05% or more in a case in which a particularly high strength is required.
The upper limit of the C amount is 0.10% in order to suppress the yield strength
5 and secure sufficient press moldability. The upper limit of the C amount is more
preferably 0.08% or less, and still more. preferably 0.07% or less in a case in which
particularly favorable press moldability is required.
[0025]
Si is an element that is, sometimes, added for deoxidization. In order to obtain
10 the effects as described in the present specification, the Si amount may be 0%, and the
lower limit of the Si amount may be undefined. However, production costs are
increased to set the Si amount to less than 0.01%, and therefore the lower limit of the Si
amount is preferably 0.01%. In order to prevent cracking during a tight bending process
and prevent the aspect ratio of ferrite crystal grains from increasing, the upper limit of the
15 Si amount is set to 0.15% or less, preferably 0.10% or less, and more preferably 0.05% or
less.
[0026]
Mn has an action of increasing the strength as an element that contributes to
solid solution strengthening, and is thus effective in obtaining martensite. Therefore, it
20 is necessary to include 0.80% or more of Mn. In order to obtain martensite more stably,
1.0% or more of Mn is preferably included. On the other hand, when the Mn amount
exceeds 1.80%, occurrence of streaky recess and protrusion surface defects is more likely,
both the tensile strength and the yield strength are increased, and deterioration of the
press moldability is caused, and therefore the upper limit is set to 1.80%. In order to
25 further decrease the yield strength and sufficiently secure the press moldability, the upper
it
limit of the Mn amount is preferably set to 1.50%.
[0027]
P is an impurity, and segregated at grain boundaries so as to cause degradation
of the toughness or deterioration of the weldability of a steel sheet. Under ordinary
5 production conditions, 0.0005% or more of Pis included. Furthermore, an alloying
reaction becomes extremely slow during galvanization, and the productivity is degraded.
From the above viewpoint, the upper limit of the P amount is set to 0.10% or less. In
order to further reduce the segregation of P, the P amount is more preferably set to 0.05%
or less, and still more preferably set to 0.015% or less. The lower limit is not
10 particularly limited, but P is an element that can increase the strength at low cost, and
therefore the P amount is preferably set to 0.005% or more.
[0028]
S is included in a steel sheet as an impurity, and, under ordinary production
conditions, 0.0005% or more of S is included. When the S amount exceeds 0.015%, hot
15 cracking is induced, and the processability deteriorates, and therefore the S amount is set
to 0.015% or less. Ina case in which the favorable processability is required, the S
amount is preferably set to 0.012%or less, and rnore preferably set to-0.010% or lesser -
[0029]
Al is an extremely important element in the present invention. Similarly to Si,
20 Al is a ferrite-stabilizing element, but Al is an important element that promotes
generation of ferrite without degrading the coat wetting properties and concentrates C in
austenite, thereby securing martensite. In order to obtain the above effects, the Al
amount needs to be set to 0.10% or more. In order to more stably obtain martensite, the
Al amount preferably exceeds 0.20%. Also, addition of excess Al not only saturates the
25 above effects, but also causes an excessive increase in alloying costs. Due to the above
12
reasons, the Al amount needs to be set to 0.80% or less. In a case in which there is a
strong demand for suppression of the costs, the Al amount is more preferably set to
0.50% or less and still more preferably 0.30% or less.
[0030]
5 Cr is an extremely important element in the present invention. Cr contributes
to solid solution strengthening, and has an action of increasing the strength of a steel
sheet. Also, addition of Cr is also effective in obtaining a sufficient amount of
martensite. Therefore, it is necessary to include 0.01% or more of Cr in a steel sheet.
In order to obtain a sufficient amount of martensite more stably even in a case in which
10 galvanization is carried out, or, furthermore, an alloying treatment is carried out, the Cr
amount is preferably set to 0.10% or more, and the Cr amount is more preferably 0.20%
or more. On the other hand, when the Cr amount exceeds 1.50%, both the tensile
strength and the yield strength are increased, and deterioration of the press moldability is
caused. Therefore, the Cr amount is set to 1.50% or less. In order to further decrease
15 the yield strength and sufficiently secure the press moldability, the Cr amount is
preferably set to 1.00% or less, the Cr amount is more preferably set to 0.40 or less, and
Ehe Cramount is particularly-preferably setto less-thanr0.20% in order to reduce alloying
costs in a case in which galvanization is not carried out.
[0031]
20 N does not necessarily need to be added, but is included in a steel sheet as an
impurity. When the N amount exceeds 0.0100%, the toughness or ductility significantly
deteriorates, and cracking of billets significantly occurs. Therefore, in order to obtain
the sufficient processability of a steel sheet, the N amount is set to 0.0100% or less. In
order to obtain more favorable processability, the N amount is preferably set to 0.0050%
25 or less, and more preferably set to 0.0030% or less. It is not particularly necessary to
13
define the lower limit value of the N amount, but the N amount is 0.0005% or more in an
ordinary steel sheet.
Meanwhile, since addition of N is effective in obtaining martensite, N may be
actively added at an upper limit of the N amount set to 0.0100%.
5 [0032]
Furthermore, either or both of Nb and Ti may be contained. Nb and Ti are
elements that suppress the grain growth of ferrite in an annealing process after cold
rolling so as to contribute to crystal grain refinement strengthening. In order to obtain
such an effect, either or both of Nb and Ti are preferably added at a lower limit set to
10 0.0005% or more respectively. On the other hand, when the contents of either or both
of Nb and Ti exceed 0.0500%, recrystallization of ferrite is significantly suppressed, and
unrecrystallized ferrite remains so as to increase the yield strength, and therefore the
respective upper limits are preferably set to 0.0500% or less. In addition, a preferable
upper limit of the contents of either or both of Nb and Ti is 0.0400% from the viewpoint
15 of alloying costs.
[0033]
All of Mo, W, B, Ni, Cu, and V are elements thatincrease the hardenability, and
one or more of them may be added according to necessity. The effects of the
embodiments of the present invention are not impaired when any of Me, W, Ni, Cu, and
20 V are not actively added, and are inevitably mixed in steel in a range of 0.0000% to
0.0005%. In addition, the effects of the embodiments of the present invention are not
impaired when any of Mo, W, Ni, Cu, and V are added or mixed in steel in a range of
0.0005% to 1.5000%. On the other hand, in order to obtain the effect of strength
improvement through active addition, the respective elements are preferably added at
25 0.100% or more. On the other hand, since excessive addition causes an increase in
14
alloying costs, the upper limits of the amounts of the respective elements added are
preferably set to 1.500% or less.
[0034]
The effects of the embodiments of the present invention are not impaired when
5 B is not actively added, and is inevitably mixed in steel in a range of 0.0000% to
0.0001%. In addition, the effects of the embodiments of the present invention are not
impaired when B is added or mixed in steel in a range of 0.0001% to 0.0100%. On the
other hand, in order to obtain the effect of strength improvement through active addition,
B is preferably added at 0.0001% or more. On the other hand, since excessive addition
10 causes an increase in alloying costs, the upper limits of the added amounts is preferably
set to 0.0100% or less.
[0035]
Next, reasons why the production method is limited will be described.
A billet provided for hot rolling may be produced by an ordinary method, and
15 maybe molten or cast into steel. From the viewpoint of the productivity, continuous
casting is preferred, and steel may be produced using a thin slab caster or the like. In
addition, the production method may be a process of continuous casting and direct rolling -
in which hot rolling is carried out immediately after casting. The hot rolling may be
carried out by an ordinary method, and the conditions, such as the rolling temperature,
20 the percentage reduction in thickness, the cooling rate, and the winding temperature, are
not particularly defined. After the hot rolling, the steel sheet is pickled, cold rolled, and
annealed so as to be made into a cold-rolled steel sheet.
[0036]
In a case in which the percentage reduction in thickness of the cold rolling is
25 60% or less, recrystallization is delayed during the annealing, and unrecrystallized ferrite
15
becomes liable to remain after the annealing such that there are cases in which the yield
strength and the yield ratio are increased, and the press moldability is deteriorated.
Therefore, the percentage reduction in thickness of the cold rolling is set to a range of
more than 60% in the present embodiments . The percentage reduction in thickness of
5 the cold rolling is more preferably more than 65%, still more preferably more than 70%,
and still more preferably more than 75%. On the other hand, when the percentage
reduction in thickness of the cold rolling becomes more than 90%, the load on the rolling
rolls is increased . Therefore, the percentage reduction in thickness of the cold rolling is
preferably 90% or less. In a case in which there is a demand for more economical
10 operation of the rolling rolls, the percentage reduction in thickness of the cold rolling is
preferably 80% or less.
[0037]
The annealing is preferably carried out using a continuous annealing facility in
order to control the heating rate and the heating time. In the continuous annealing, it is
15 important to appropriately adjust the heating rate in a temperature range of the Acl
transformation point to the Ac3 transformation point of a steel sheet . The heating rate
makes the aspect ratio of ferrite crystal grains after the annealing vary as shown in FIG 3:
When the heating rate is 10°C/s or less, the average aspect ratio of ferrite crystal grains is
1.2 or less, and the ferrite crystal grains having an aspect ratio of 1.2 or less accounts for
20 60% or more of the entire crystal grains. As a result, the uneven hardness of the steel
sheet structure is reduced , and a likelihood of streaky recess and protrusion surface
defects occurring during the press molding is decreased . In the above temperature
range, in a case in which the heating rate is less than 1 °C/s during the annealing, the
productivity is degraded, and the ferrite crystal grains grow more than necessary such
25 that the ferrite crystal grains are coarsened , and degradation of the tensile strength is
16
caused. Therefore, in the above temperature range, the heating rate is set to at least
1°C[n or more. In order to more stably suppress the growth of the grain size, the heating
rate is set to more than 3°C/s. On the other hand, when the heating rate becomes 10°C/s
or more in the above temperature range, recrystallization of ferrite is significantly
5 suppressed, and therefore unrecrystallized ferrite remains after the annealing, and the
yield strength is increased. Therefore, the heating rate is set to less than 10°C/s in the
above temperature range. In a case in which more favorable moldability is required, the
heating rate is preferably set to 8°C/s or less, and more preferably set to 6°C/s or less.
Meanwhile, the Act transformation points and the Ac3 transformation points in
10 the respective steel sheets can be estimated by a known method using the component
composition of steel.
[0038]
Furthermore, the lower limit of the peak temperature during the annealing is set
to 720°C or more, and the upper limit is set to 850°C. Ina case in which the peak
15 temperature is lower than 720°C, since ferrite does not transform into austenite, the
amount of martensite is not sufficient, and a decrease in the tensile strength and an
increase in the yield ratio are caused. On the other hand, when the peak temperature
becomes higher than 850°C, since austenite transformation proceeds excessively, the
amount of a hard second phase increases, and the amount of ferrite decreases, thereby
20 causing a decrease in the ductility and an increase in the yield ratio. The range of the
peak temperature that is more preferable to obtain the above effect more stably is 770°C
to 830°C.
[0039]
In addition, the retention time is set to 10 seconds to 200 seconds in a
17
temperature range in which the temperature of the steel sheet is 720°C or higher. When
the time in which the temperature of the steel sheet is 720°C or higher is less than 10
seconds, since transformation from ferrite to austenite does not proceed sufficiently,
martensite cannot be secured sufficiently, and a decrease in the tensile strength and an
5 increase in the yield ratio are caused . On the other hand, when the retention time at
720°C or higher is increased , since degradation of the productivity is caused, the
retention time in the above temperature range is set to 200 seconds or less. The
retention time in a temperature range of 720 °C or higher is preferably 150 seconds or less,
and more preferably 120 seconds or less in order to obtain the above effects more stably.
10 [0040]
In addition, after the annealing, first cooling for cooling the steel to 500°C or
lower is carried out. At this time, in a case in which the cooling rate is less than 3°C/s,
there are cases in which martensite cannot be sufficiently obtained. From the above
viewpoint, the lower limit of the cooling rate is set to 3°C /s. On the other hand, since a
15 special facility needs to be introduced and the like in order to set the cooling rate to
higher than 250°C/s, the upper limit of the cooling rate is preferably set to 250 ° C/s. The
cooling rate after the annealing may be appropriately controlled through spraying of a
coolant, such as water, air blowing, or forcible cooling using mist or the like.
[0041]
20 In a case in which zinc galvanization or zinc alloy galvanization is carried out
after the cooling, the galvanization is carried out after the above first cooling and before
the temper rolling as described below (skin pass rolling). The composition of the zinc
coat is not particularly defined, and, in addition to Zn, Fe, Al, Mn, Cr, Mg, Pb, Sri, Ni,
and the like may be added according to necessary. Meanwhile, the galvanization may
18
be carried out as a separate process from the annealing, but is preferably carried out
through a continuous annealing and galvanization line in which annealing and
galvanization are carried out continuously from the viewpoint of the productivity.
[0042]
5 In a case in which an alloying treatment is carried out on the above
galvanization coat, the alloying treatment is preferably carried out in a temperature range
of 450°C to 600°C at a timing after the galvanization and before the skin pass rolling.
Alloying does not proceed sufficiently at lower than 450°C. In addition, alloying
proceeds excessively at higher than 600°C such that a problem may be induced in which
10 the galvanized layer is embrittled, and the coat is peeled off due to a process of pressing
or the like. The time of the alloying treatment is preferably 10 seconds or more since
alloying does not proceed sufficiently with a time of less than 10 seconds. In addition,
the upper limit of the time of the alloying treatment is not particularly defined, but is
preferably 100 seconds or less from the viewpoint of the production efficiency.
15 In addition, from the viewpoint of the productivity, it is preferable that an
alloying treatment furnace be continuously provided in the continuous annealing and
galvanization line, and the annealing, the galvanization, and the alloying treatment be
continuously carried out.
[0043]
20 The temper rolling (skin pass rolling, SPM) is carried out to correct the shape
and secure the surface properties, and preferably carried out in a range of the elongation
percentage of 2.0% or less. This is because the BH amount is decreased when the
elongation percentage exceeds 2.0%.
A heat treatment (overaging treatment) may be carried out for 30 seconds or
25 more in a temperature range of 200°C to 450°C before the skin pass rolling. In this case,
19
a second cooling is carried out after the overaging treatment.
[0044]
The steel sheet is cooled to 100°C or lower through the second cooling under
conditions of a cooling rate of 1°C/s to 3°C/s. When the secondary cooling rate is less
5 than 1°C/s, since the productivity decreases, the amount of the hard second phase
(particularly martensite) obtained is reduced so as to increase the yield ratio, and there is
a case in which the press moldability is deteriorated, the lower limit is set to 1 °C/s or
more. In addition, when the second cooling rate exceeds 3°C/s, since the amount of the
obtained hard second phase becomes excessive, both the tensile strength and the yield
10 strength are increased so as to increase the yield ratio, and there is a case in which the
press moldability is deteriorated, the upper limit is set to 3°C/s or less.
[0045]
FIGS. 1 and 2 show the numerical values of the yield ratios YR with respect to
the Mn/Cr ratio and the Cr/(Si+Al) ratio respectively. The yield ratio (YR) refers to a
15 value that indicates the ratio of the yield strength (YP) to the tensile strength (TS), and
YR is equal to YP/TS. Meanwhile, the tensile characteristics are measured by a tensile
test as defined in JIS Z 2241, and El [%] refers to the breaking elongation. In addition,
in the tensile test, in a case in which a yield phenomenon is shown, the top yield point is
considered as the yield strength in the evaluation, and, in a case in which a yield
20 phenomenon is not shown, the 0.2% proof test is considered as the yield strength in the
evaluation.
[0046]
As is clear from FIGS. 1 and 2, as the Mn/Cr ratio and the Cr/(Si+Al) ratio are
decreased, the yield strength is degraded, and the processability is improved. The cause
20
of the phenomenon is not clear. Setting the Mn/Cr ratio and the Cr/(Si+Al) ratio in an
appropriate range is extremely important for the reduction of the yield ratio, and the
above described yield ratio becomes 0.55 or less when the Mn/Cr is in a range of 3.0 or
less, and the Cr/(Si+Al) is in a range of 3.0 or less.
5 [0047]
Next, the metallic structure will be described.
The microstructure of the steel sheet obtained by the present invention is
composed of ferrite and a hard second phase (a structure other than ferrite).
When the area fraction of the ferrite is less than 80%, the hard second phase
10 increases such that the yield strength and the yield ratio increases, and the processability,
particularly, the press moldability deteriorates. Therefore, the lower limit of the area
fraction of the ferrite was set to 80% or more.
[0048]
FIG 4 shows the numerical values of the respective yield ratios YR with respect
15 to the ratios of the area fractions of unrecrystallized ferrite to the heating rate from the
Ac1 transformation point to the Ac3 transformation point during the continuous
annealing. It is clear from FIG 4 that setting the heating rate in an appropriate range -
and controlling the area fraction of unrecrystallized ferrite are extremely important for
reduction of the yield ratio. When the area fraction of unrecrystallized ferrite in the
20 ferrite exceeds 10%, the yield strength and the yield ratio are increased, and there is a
case in which the press moldability deteriorates. Therefore, the area fraction of
unrecrystallized ferrite was set to 10% or less. It is clear from FIG. 4 that the heating
rate needs to be set to less than 10°C/s in order to obtain such an area fraction of
unrecrystallized ferrite.
25 [0049]
21
Meanwhile, the unrecrystallized ferrite, ferrite other than the unrecrystallized
ferrite , that is, recrystallized ferrite (ferrite recrystallized during heating for the
annealing), and transformed ferrite (ferrite that is transformed from austenite during
cooling after the annealing ) can be differentiated by analyzing the measurement data of
5 the crystal orientation of an electron back scattering pattern (referred to as the EBSP) by
the Kernel Average Misorientation method (KAM method).
[0050]
In the grains of the unrecrystallized ferrite, dislocations are recovered, but a
continuous change in the crystal orientation, which is caused by plastic deformation
10 during the cold rolling, is present . On the other hand, the change in the crystal
orientation in the grains of the ferrite other than the unrecrystallized ferrite becomes
extremely small. This is because the crystal orientations of adjacent crystal grains
become significantly different due to recrystallization and transformation , but the crystal
orientation becomes constant in a single crystal grain . In the KAM method, it is
15 possible to quantitatively indicate the crystal orientation difference with an adjacent pixel
(measurement point). In the present invention, when a gap between pixels for which the
average crystal orientation difference with an adjacent measurement point is 1° or less
and 2° or more is defined as a grain boundary , grains having a crystal grain size of 3 μm
or more are defined as the ferrite other than the unrecrystallized ferrite, that is, the
20 recrystallized ferrite and the transformed ferrite.
[0051]
The EBSP may be measured at ranges of 100 μm x 100 μm at a position of 1/4
of the thickness of an arbitrary sheet cross section in the thickness direction at
measurement intervals of 1/10 of the average crystal grain size of an annealed sample.
25 As a result of the EBSP measurement , the measurement points obtained are output as
22
pixels. The sample provided for the measurement of the crystal orientation of the EBSP
is produced by reducing the thickness of a steel sheet to a predetermined thickness
through mechanical polishing or the like, subsequently, removing strains through
electrolytic polishing or the like, and, simultaneously, making the surface at a position of
5 1/4 of the sheet thickness into a measurement surface.
[0052]
The total area fraction of the ferrite including the unrecrystallized ferrite is the
remainder of the area fraction of the hard second phase. Therefore, the total area
fraction of the ferrite can be obtained by etching the sample used for the measurement of
10 the crystal orientation of the EBSP using nital, taking an optical microscopic photo of the
same view as used for the measurement at the same magnification, and carrying out an
image analysis of the structure photo obtained. Furthermore, it is also possible to obtain
the sum of the area fractions of the unrecrystallized ferrite and the ferrite other than the
unrecrystallized ferrite, that is, the recrystallized ferrite and the transformed ferrite by
15 comparing the structure photo and the measurement results of the crystal orientation of
the EBSP.
[0053]
When the crystal grain size of the ferrite is less than 5 μm, the yield ratio
increases, and the processability deteriorates. On the other hand, when the crystal grain
20 size of the ferrite exceeds 20 μm, the surface appearance deteriorates after the molding,
and degradation of the strength may be caused. Therefore, the crystal grain size of the
ferrite may be defined in a range of 5 μm to 20 μm.
[0054]
In addition, the ferrite crystal grain size and the aspect ratio of the ferrite crystal
25 grain are measured by image analyses using the above optical microscope photo. The
23
unrecrystallized ferrite crystal grains generally have a flat and approximately ellipsoidal
shape on an optical microscopic photo, and the crystal grains of the recrystallized ferrite
or the transformed ferrite have a shape that is more circular and have a lower aspect ratio
than the unrecrystallized ferrite grains. The quality of a steel sheet can be improved
5 more stably by controlling the aspect ratios (ratio between the long side and the short
side) of the shapes of the crystal grains of the ferrite including the unrecrystallized ferrite,
the recrystallized ferrite, and the transformed ferrite. The aspect ratios of the ferrite
crystal grains are measured by image analyses of the above optical microscopic photo.
That is, a plurality of optical microscopic photos taken at a magnification of 1000 times
10 from ranges of 100 μm x 100 μm at a position of 1 /4 of the thickness of an arbitrary
sheet cross section of a nital-etched sample in the thickness direction is prepared. In
addition, 300 crystal grains are arbitrarily selected from the above photos, and the grain
sizes of the respective crystal grains in the rolling direction and the direction
perpendicular to the rolling direction are obtained by image analyses, the ratios of the
15 lengths (= the grain size in the rolling direction/ the grain size in the direction
perpendicular to the rolling direction) are computed, and used as the aspect ratios. In
addition, the diameters of the equivalent circles are obtained and used as the ferrite
crystal grain sizes.
In the steel sheet according to the embodiment of the present invention, the ratio
20 of the crystal grains having an aspect ratio of 1.2% or less may be defined to be 60% or
more of the total ferrite crystal grains. When the above ratio is maintained, a sufficient
amount of the recrystallized ferrite can be guaranteed in the steel sheet, and favorable
processability can be obtained. The ratio of the crystal grains having an aspect ratio of
1.2% or less is defined to be more preferably 65% or more of the total ferrite crystal
25 grains, and still more preferably 70% or more.
24
[0055]
There is a case in which the hard second phase includes either or both of bainite
and residual austenite as well as martensite. The hard second phase contributes to an
increase in the strength; however, when present in excess, a decrease in the ductility and
5 an increase in the yield ratio are caused, and therefore the lower limit and the upper limit
of the area fraction of the hard second phase are set to 1% and 20% respectively.
In addition, when the area fraction of martensite is less than 3% in the hard
second phase, it becomes difficult to reduce the yield ratio to 0.55 or less, and therefore
the area fraction is preferably 3% or more.
10 [0056]
The microstructure may be observed using an optical microscope after a sample
is taken so that the sheet thickness cross section of the sample in parallel with the rolling
direction is used as an observation surface, and the observation surface is polished,
etched using nital, and etched using a LePera's reagent, if necessary. The total amount
15 of the area fractions of one or more of pearlite, bainite, and martensite can be obtained as
the area fraction of phases other than the ferrite by carrying out image analyses of a
microstructure photo taken using an optical microscope. It is difficult to differentiate
the residual austenite from martensite using an optical microscope, but the volume
fraction can be measured by an X-ray diffraction method. Meanwhile, the area fraction
20 as obtained from the microstructure is equivalent to the volume fraction.
[0057]
When the residual percentage of the hard structures, such as the unrecrystallized
ferrite and martensite, is high, the hardness of a steel sheet structure becomes uneven.
When the steel sheet is press-molded, plastic deformation locally starts from portions
25 having a relatively low hardness, and therefore the sheet thickness in the rolling direction
25
is liable to become uneven. As a result, streaky recess and protrusion marks are
generated on the surface of the steel sheet. For a steel sheet that is used for an
automobile outer sheet and the like, the aesthetic appearance of the surface is important,
and therefore the streaky recess and protrusion surface defects are considered as serious
5 defects, and a significant decrease in the yield is caused.
In the present invention, an uneven structure hardness, which is a cause of
occurrence of streaky recess and protrusion surface defects, is suppressed in order to
suppress occurrence of streaky recess and protrusion surface defects.
In the steel sheet according to the embodiment of the present invention, it is
10 enabled to suppress streaky recess and protrusion surface defects and stably produce an
automobile steel sheet by paying attention to the area fractions of the unrecrystallized
ferrite and the martensite and the aspect ratio of the ferrite crystal grains and controlling
the steel structure.
[Examples]
15 [0058]
Billets obtained by melting and casting steels having the compositions as shown
in Table 1 were reheated at-1150°C to 1250°C and hot-rolled according to an ordinary
method. At this time, the finishing temperature was set to 860°C to 940°C, and the
winding temperature was set to 500°C to 600°C. After that, cold rolling was carried out
20 at the percentage reductions in thickness as shown in Table 2, annealing, and,
furthermore, galvanization were carried out under the conditions as shown in Table 2.
Meanwhile, [-] in Table 1 indicates that the analyzed value of a component fell below the
detection limit. The underlined numerical values in the respective tables indicate that
the numerical values were outside the ranges of the present invention.
25 [0059]
26
[Table 1]
[0060]
[Table 2]
[0061]
5 A tensile test specimen according to JIS Z 2201 No. 5 was taken from a steel
sheet that had undergone cold rolling after being produced considering the width
direction (referred to as the TD direction) as the longitudinal direction, and the tensile
characteristics in the TD direction were evaluated according to JIS Z 2241. In addition,
a tight bending test was carried out by a winding bend method under conditions of the
10 internal radius set to zero and the bending angle set to 180° using a test specimen
according to JIS Z 2248 No. 3, and the surface of the test specimen was visually
observed. In the bending test, in a case in which cracking did not occur in the steel
sheet, and fissures and other defects were not found in the visual observation, the tight
bending processability was evaluated to be favorable, and, in a case in which cracking
15 occurred in the steel sheet, and fissures and other defects were visually found in the
visual observation, the tight bending processability was evaluated to be poor.
[0062]
The microstructure of the sheet thickness cross section of the steel sheet was
observed using an optical microscope after a sample was taken so that the rolling
20 direction was used as an observation surface, and etched by a LePera method. The area
fraction of the hard second phase was obtained as the sum of phases other than the ferrite
by carrying out image analyses of a microstructure photo taken using an optical
microscope. In addition, the area fraction of the unrecrystallized ferrite and the area
fraction of the remainder, that is, the ferrite excluding the unrecrystallized ferrite were
25 obtained by measuring the crystal orientation of the EBSP, checking the measurement
27
results and the optical microscopic structure photo, and carrying out image analyses.
The ferrite grain sizes, the average values of the aspect ratios of the ferrite grains,
and the fractions of the ferrite grains having an aspect ratio of 1.2 or less were obtained
by image analyses of structure photos taken using the above optical microscope.
5 The analysis results are shown in Table 3.
[0063]
[Table 3]
[0064]
The yield strength and the yield ratio have a strong correlation with the
10 processability, particularly, the press moldability, and the processability of a steel sheet
having a yield strength of more than 270 MPa and a yield ratio of more than 0.55 is not
sufficient. Therefore, the favorable upper limits of the yield strength and the yield ratio
are set to 270 MPa or less and 0.55 or less respectively.
The yield strength (hereinafter also referred to simply as the strain aging yield
15 strength) and the BH amount of a steel sheet when resubjected to tension after addition of
a 2% tensile pre-strain and then aging at 170°C for 1200 seconds have a positive
correlation with the dent resistance of a member that has been subjected to molding and,
furthermore, a paint baking treatment, and refer to the strain aging yield load and the BH
amount measured according to the method of a paint bake hardening test as described in
20 the appendix of JIS G 3135. In a case in which the BH amount is less than 50 MPa and
the strain aging yield strength is less than 330 MPa, there is a case in which the thickness
of the steel sheet cannot be sufficiently reduced from the viewpoint of the dent resistance.
Therefore, the BH amount and the strain aging yield strength are favorably set to 50 MPa
or more and 330 MPa or more respectively.
25 [0065]
28
The strength-ductility balance TS x El [MPa•%] is an index of moldability, and,
when the strength-ductility balance TS x El [MPa•%] is less than 17000, there is a case in
which a steel sheet is ruptured during molding, and therefore the strength-ductility
balance TS x El [MPa.%] is preferably 17000 or more. The strength-ductility balance
5 TS x El [MPa.%] is more preferably 17500 or more in order to prevent a steel sheet from
rupturing under stricter molding conditions.
[0066]
As shown in Table 3, the result is that it is possible to obtain a high-strength
galvanized steel sheet that has a low yield strength, a low yield ratio, favorable tight
10 bending processability, and excellent strength-ductility balance by subjecting steel having
the chemical components of the present invention to hot rolling and cold rolling under
appropriate conditions, and, furthermore, annealing under appropriate conditions.
For Steel 0, since the Si amount is large, and the aspect ratio of the ferrite
crystal grains is large, the tight bending processability becomes poor.
15 For Steel Q, since the C amount is large, the ferrite area fraction is low, and the
yield strength and the yield ratio are high.
For Steel R, since the Mn amount is small, the martensite amount is decreased,
and the yield strength and the yield ratio are high.
For Steel S, since the Al amount is small, the ferrite area fraction is low, and the
20 yield strength and the yield ratio are high.
For Steel V, since the Si amount is large, the tight bending processability
becomes poor.
For Steel X, since the Cr amount is small, the martensite amount is decreased,
and the yield strength and the yield ratio are high.
29
Steel AA has substantially the same composition as Steel 1 in Patent Document
6, a large Mn amount, and a low Al amount, and therefore the ferrite area fraction is
small. In addition, in Production No. 57 that was carried out according to the method as
described in Patent Document 6 using Steel AA, since the percentage reduction in
5 thickness of the cold rolling is small, the amount of the unrecrystallized ferrite is large.
Therefore, the yield strength and the yield ratio are high.
For Steel AB, since the Cr amount is large, the martensite amount is increased,
and the tensile strength and the yield strength are high.
For Steel AC, since the N amount is large, the yield strength and the yield ratio
10 are high.
For Production No. 4, the heating rate from the Act transformation point to the
Ac3 transformation point is slow in the annealing, and the crystal grain size of the ferrite
is large. Therefore, the tensile strength is low, and the TS x El is low.
For Production No. 5, since the percentage reduction in thickness of the cold
15 rolling is small, the amount of the unrecrystallized ferrite is large. Therefore, the yield
strength and the yield ratio are high.
For Production No. 6, the cooling rate is temporarily slow. Therefore, the
martensite amount is decreased, and the yield strength and the yield ratio are high.
For Production No. 10, since the heating rate is fast in the annealing, the fraction
20 of the unrecrystallized ferrite is large, the aspect ratio of the ferrite grains is large, and the
crystal grain size of the ferrite is refined. Therefore, the yield strength and the yield
ratio are high.
For Production No. 24, since the peak temperature is low in the annealing, and
the martensite amount is small, the tensile strength is low.
25 For Production No. 25, since the percentage reduction in thickness of the skin
30
pass is large, the BH amount is small.
For Production No. 29, since the peak temperature is high in the annealing, the
fraction of the hard second phase is large, and the yield strength and the yield ratio are
high.
5 For Production No. 34, since the retention time at the peak temperature is short
in the annealing, the martensite amount is small, and the aspect ratio of the ferrite crystal
grains is large, and therefore the yield ratio is large.
For Production No. 43, since the heating rate is fast in the annealing, the fraction
of the unrecrystallized ferrite is large, and the yield strength and the yield ratio are high.
10 For Production No. 44, since the percentage reduction in thickness of the cold
rolling is small, the fraction of the unrecrystallized ferrite is large, and the yield strength
and the yield ratio are high.
Industrial Applicability
15 [0067]
According to the respective aspects of the present invention, it become possible
toprovide a steel sheet that has a tensile strength of 390 MPa-to 500 MPa, has a low------
yield strength and a low yield ratio, has excellent strength-ductility balance, and has paint
bake hardenability, which makes an extremely significant contribution to the industry.
20 Furthermore, the present invention makes it possible to reduce, particularly, the sheet
thickness of an outer panel of an automobile for which excellent processability,
particularly, press moldability, paint bake hardenability, and surface quality are required,
and thus exhibits an extremely noticeable effect that significantly contributes to a
decrease in the weight of an automobile body and the like. In addition, it is possible to
25 stably suppress occurrence of streaky recess and protrusion surface defects and to
31
significantly improve the yield of a high-strength steel sheet.
TABLE 1
Steel Component (by mass%) Mn/ Cr/
R k
No. C Si Mn P S Al Cr N i Nb Ti Mo W B Ni Cu V Cr (Si+AI)
emar s
A 0.07 0.01 1.50 0.015 0.0021 0.45 1.050 0.0022 - - - - - - - - 1.43 2.28 Example
B 0.06 0.10 1.15 0.008 0.0017 0.55 0.950 0.0029 0.010 - - - - - - - 1.21 1.46 Example
C 0.05 0.05 1.28 0.014 0.0022 0.25 0.350 0.0028 0.006 - 0.12 - - - - - 3.66 1.17 Example
D 0.06 0.03 1.35 0.006 0.0037 0.22 0.250 0.0018 - - - - - - - - 5.40 1.00 Example
E 0.05 0.04 1.07 0.011 0.0026 0.27 0.310 0.0025 - 0.003 - - 0.00 - - 3.45 1.00 Example
F 0.07 0.05 1.47 0.014 0.0034 0.26 0.220 0.0026 - - - - - 6.68 0.71 Example
G 0.05 0.06 1,22 0.013 0.0031 0.20 0.300 0.0038 - - - - - - - 0.14 3.45 1.00 Example
H 0.07 0.02 1.28 0.010 0.0030 0.11 0.400 0.0029 - - - - - - - - 3.20 3.08 Example
1 0.05 0.03 1.12 0.012 0.0033 0.12 0.470 0.0030 - - - - - - - - 2.38 3.13 Example
J 0.07 0.04 1.35 0.009 0.0036 0.10 0.430 0.0028 - - - - 0.00 - - - 3.14 3.07 Example
K 0.06 0.05 1.12 0,012 0.0033 0.48 0.070 0.0031 - - - - 0.00 - - 16.00 0.13 Example
L 0.05 0.05 1:45 0.005 0.0029 0.50 0.550 0.0041 - - - 0.50 - - - - 2.64 1.00 Example
M 0.05 0,01 1.30 0.005 0.0021 0.60 0.850 0.0026 - - - - - - 0.20 - 1.53 1.39 Example
N 0.04 0.15 1.25 0.012 0.0028 0.35 0.700 0.0039 - - 0.30 - - - - - 1.79 1.40 Example
O 0.04 0.30 1.35 0.011 0.0025 0.15 0.500 0.0030 - - - - - 0.15 - - 2.70 1.11
Comparative
examp le
P 0.03 0.05 1.50 0.009 0.0022 0,30 0.650 0.0027 - 0.015 - - 0.002 - - - 2.31 1.86 Example
Q 0.15 0.05 0.95 0.014 0.0015 0,50 0.350 0.0032 - - - - - - - 2.71 0.64 Comparative
example
R 0.05 0,10 0.35 0.011 0,0023 0.50 1.200 0.0029 0.010 0,020 - - - 0.30 - 0.29 2,00
Comparative
example
S 0.05 0.10 1.30 0.008 0.0019 0.03
-
0.450 0.0025 - - - 0.30 - - 0.30 - 2.89 3.46 Comparative
examp le
T 0.05 0.15 1.65 0.009 0.0021 0.30 0.050 0.0023 - - 0.15 - 0.001 - - - 3100 0.11 Example
U 0.05 0.15 1.50 0.013 0.0024 0.40 0.300 0.0024 - - - - - - - - 5.00 0.55 Example
V 0.07 0.22 1.40 0.007 0.0034 0.38 0.170 0.0027 0.012 - - - - - - 0.06 8.24 0.28 Comparative
exam p le
W 0.05 0.01 1.20 0.012 0.0018 0.15 0.600 0.0028 - 0.015 0.20 - - - - - 2.00 3.75 Example
X 0.05 0.03 1.18 0.007 0.0022 0.25 0.005 0.0026 - - - - - - - - 236.00 0.02 Comparative
example
Y 0.06 0.02 1.45 0.006 0.0050 0.26 0.200 0.0028 - - - - - - - - 7,25 0.71 Example
Z 0.07 0.03 1.12 0.012 0.0030 0.21 1.100 0.0020 - - - - - - - 0.32 1.02 4.58 Example
AA 0.10 0.10 2.00 0.006 0.0018 0.03 0.180 0.0020 71.11 38 Comparative
exam le
AB 0,07 0.13 1.45 0.009 0.0035 0.43 1,670 I 0.0029 - - - - - - - - 0.87 2.98 Comparative
exam p le
AC 0.07 0.05 1,17 0.011 0.0027 0.26 0.400 0.0120 - - - - - - - - 2-93 1.29
Comparative
exam le
2/9
TABLE 2-1
Percenta e
Production
No.
Steel
No.
g
reduction of
cold rolling
(o%)
Heating
rate from
Act to Ac 3
(°C/S)
1 A 75
2 A 80 5
3 A 80 5
75 0.5
5 A 55 5
6 A 80 5
7 B 75 3
8 B 75 3
9 75 5
10 B 75 30
11 75
12 75 5
13 80 3
14 70 5
15 F 75 5
16 G 75 3
17 H 75 5
18 80
19 J 77
20 K 80--
21 L 75
22 L 70
23 L 70 3
24-- L 75 3
25 L 70 5
26 M 75
27 M 80
28 M 80 5
29 M 75 5
30 M 1 70 1 3
Annealin g process
First
verag ng
Peak I Retention I treatment
temperature time (temperature
(°C) (s) °a^c ' (°C)
800
780
780
810
800
800
790
800
810
800
800
800
810
820
C/s)
790 100 30
790 80 15
820 80 10
820 80 10
870 80 15
810 1 100 20
350
350
350
I 350
3/9
TABLE 2-2
Percentage
Annealing process Overaging
Production
No.
Steel
No.
reduction of
cold rolling
(%)
Heating
rate from
Act to Ac 3
(°C/s)
Peak
temperature
(°C)
Retention
time
(s)
First
cooling
rate
(°C/s)
treatment
temperature
(°C)
31 N 75 3 770 120 25
32 N 75 3 780 100 20 350
33 N 75 3 780 100 20
34 N 75 5 750 5 20
35 N 70 3 800 120 30
36 0 80 5 820 80 5 300
37 0 75 3 790 100 30
38 0 75 3 790 100 30 300
39 0 75 3 800 80 1 300
40 P 70 5 780 120 20
41 75 5 810 100 30 350
42 75 5 810 100 30
43 75 15 800 80 30
44 P 50 800 120 25
45 Q 75 3 800 100 20 350
46 Q 75 3 800 100 20
47 R 70 3 800 100 20
48- S --75--- 5 770 100 20
49 T 75 800 100 20 350
50 T 75 3 800 100 20
51 U 80 5 780 80 20
52 V 70 5 800 100 15
53 W 75 5 790 60 30
54 X 70 5 810 100 15
55 Y 75 5 810 100 30 350
56 Z 75 5 810 100 30
57 AA 55 50 770 10 50 300
58 AB 75 5 810 100 5
59 AC 70 3 800 120 20
4/9
TABLE 2-3
Production
No.
Overaging
time
(s)
Galvanizing
temperature
(°C)
Alloying
treatment
temperature
(°C)
Alloying
time
(s)
Second
cooling
rate
(°C/s)
SPM
elongation
percentage
(%)
emarks
1 460 0.6 Example
200 2.5 0.8 Example
460 2 0.6 Example
460 2 0.6 Comparative
example
5 200 2.5 0.6
Comparative
example
6 200 2.5 0.6 Comparative
example
7 200 2 0.6 Example
460 520 15 2.5 0.6 Example
460 520 15 22®5 0.6 Example
10 460 520 15 2.5 0.6
Comparative
example
460 510 20 0.8 Example
12 460 520 15 2.5 0.8 Example
3 200 2.5 0.8 Example
4 460 500 15 2 0.4 Example
15 460 510 20 2.5 Example
16 460 530 20 2 0.8 Example
17 200 2 0.6 Example
18 460 520 20 2.5 0.8 Example
19 460 520 15 2.5 0.6 Example
20 250---- 2 I -Example
21 460 520 20 2 0.6 Example
22 200 2.5 0.8 Example
23 460 520 20 2 0.6 Example
24 460 520 20 2 0.6
Comparative
example
25 200 2 2.2 Comparative
example
26 200 2.5 0.6 Example
27 460 520 15 2.5 0.6 Example
28 200 2.5 0.6 Example
29 200 2.5 0.6 Comparative
example
30 I - I 460 520 15 5 0.4 Example
5/9
TABLE 2-4
Production
No.
Overaging
time
(s)
Galvanizing
temperature
(°C)
Alloying
treatment
temperature
(°C)
Alloying
time
(s)
Second
cooling
rate
(°C/s)
SPM
elongation
percentage
(%)
emarks
31 460 540 15 2.5 0.6 Example
32 200 2 1 Example
33 460 540 15 2.5 0.6 Example
34 460 540 15 2 0.6 Comparative
example
35 460 520 20 0.5 0.6 Example
36 250 2.5 0.6
Comparative
example
37 460 540 15 2 0.8 Comparative
example
38 250 2.5 0.6 Comparative
example
39 250 2.5 0.6
Comparative
example
40 460 500 20 2.5 0.6 Example
41 200 2 0.4 Example
42 460 500 20 2.5 0.6 Example
43 460 500 20 2.5 0.6
Comparative
example
44 460 500 20 2.5 0.8
Comparative
example
45 200 2 0.6 Comparative
example
46 460 520 15 2.5 0.6 Comparative
example
47 460 520 15 2.5 0.6
Comparative
example _
- 48 460 -520--- --15 - --- 0.6 Comparative
example-
49 200 2 0.6 Example
50 460 520 15 2.5 0.6 Example
51 460 520 15 2.5 0.6 Example
52 460 540 15 2.5 Comparative
example
53 460 520 15 2 0.6 Example
54 460 500 20 2.5 0.6
Comparative
example
55 200 2 0.6 Example
56 460 500 20 2.5 0.4 Example
57 250 2.5 0.6 Comparative
example
58 460 540 20 3 0.6
Comparative
example
59 450 510 20 2.5 0.6 Comparative
example
6/9
TABLE 3-1
Microstructure
Production
No.
Steel
No.
Ferrite
area
fraction (%)
Unrecrystallized
ferrite area
fraction (%)
Ferrite
grain
size
(R m)
Average
aspect
ratio
Aspect
ratio
of 1.2
or less
Hard
second
phase
area
Martensite
are
fraction
(4)
Other
hard
second
phases
1 A 87 0 8 1.1 75 13 7 6
2 A 88 1 8 1.15 75 12 7 5
3 A 88 1 8 1.15 75 12 7 5
4 A 90 0 22 1.05 85 10 - 6 -4
5 A 88 20 5 2 50 12 7 5
6 A 85 1 9 1.15 60 15 2 13
7 B 90 3 7 1.1 70 10 5 5
8 B 90 3 7 1.1 70 10 5 5
9 B 89 4 7 1.15 70 11 6 5
10 B 90 25 4 2.5 50 10 7 3
11 C 90 1 8 1.1 65 10 6 4
12 D 88 0 13 1.15 70 12 5 7
13 D 90 2 11 1.15 70 10 4 6
14 E 91 1 9 1.1 70 9 4 5
15 F 90 2 11 1.15 70 10 4 6
16 G 90 1 7 1.2 65 10 6 4
17 H 89 2 8 1.15 75 11 4 7
18 1 91 0 8 1.2 75 9 3 6
19 J 87 2 7 1.15 75 13 4 9
20 K -- 86 0 -- - 10 1.1 70 14 7 7
21 L 91 0 9 1.1 85 9 5 4
22 L 86 0 8 1.1 75 14 8 6
23 L 86 0 8 1.1 75 14 8 6
24 L 98 0 9 1.1 90 2 2 0
25 L 87 0 8 1.15 85 13 7 6
26 M T9- 1 8 1.15 75 11 6 5
27 M 88 0 7 1.15 75 12 5 7
28 M 88 0 7 1.15 75 12 5 7
29 M 70 0 7 1.15 60 30 9 21
30 M 86 0 8 1.15 75 14 8 6
7/9
TABLE 3-2
Microstructure
Production
No.
Steel
No.
Ferrite
area
fraction
(%)
Unrecrystallized
ferrite area
fraction
(%o)
Ferrite
grain
size
(g m)
Average
aspect
ratio
Aspect
ratio
of 1.2
or less
Hard
second
phase
area
Martensite
are
fraction
(%)
Other
hard
second
phases
31 N 88 1 8 1.1 75 12 6 6
32 N 87 1 7 1 . 1 75 13 7 6
33 N 87 1 7 1.1 75 13 7 6
N 96 2 6 1 . 3 55 4 2 2
N 95 0 7 1.15 75 5 3 2
E
0 87 0 8 1 .55 55 13 4 9
37 0 88 0 8 1.55 55 12 6 6
38 0 88 0 8 1 .55 55 12 6 6
39 0 91 0 9 1.45 60 9 2 7
40 P 89 3 6 1.15 70 11 5 6
41 P 87 1 6 1.15 70 13 6 7
42 P 87 1 6 1.15 70 13 6 7
43 P 89 12 6 1 .8 55 11 6 5
44 P 88 15 5 1.9 50 12 6 6
45 Q 78 0 7 11 60 22 10 12
46 Q 78 0 7 1.1 60 22 10 12
47 R 86 5 5 1 .35 55 14 2 12
48 S 75 1 --8 - t15-_-- 1 -60- 25 4 21
49 T 87 1 8 115 75 13 2 11
50 T 87 1 8 1.15 75 13 2 11
51 U 88 0 7 1.15 75 12 2 10
52 V 91 3 4 1.2 65 9 3 6
53 W 81 1 8 1.15 60 19 3 16
54 X 79 0 10 1 . 15 80 21 4 17
55 Y 88 0 9 1.15 75 12 5 7
56 Z 86 0 8 1 .15 75 14 8 6
57 AA 65 25 4 2.8 35 35 20 15
58 AB 76 1 11 1.25 60 24 15 9
59 AC 85 3 4 1.15 70 15 5 10
8/9
TABLE 3-3
Mechanical characteristics Paint bake hardenabilit
Production
No.
YS
(MPa)
TS
(MPa)
YR El (/
° )
TS x El
(MPa %)
Tight bending
processability
BH
amount
(MPa)
Strain aging
YS
(MPa)
Remarks
1 229 456 0.5 39 17784 Favorable 60 375 Exam le
2 223 452 0.49 40 18080 Favorable 59 370 Example
3 220 453 0.49 41 18573 Favorable 65 380 Exam le
4 201 385 0.52 44 16940 Favorable 58 360 Comparative
example
5 290 475 0.61 34 16150 Favorable 62 360 Comparative
examp le
6 296 448 0.66 36 16128 Favorable 43 378 Comparative
example
7 231 447 0.52 40 17880 Favorable 66 380 Examp le
8 221 447 0.49 41 18327 Favorable 58 373 Examp le
9 236 451 0.52 40 18040 Favorable 57 381 Examp le
10 351 483 033 33 15939 Poor 62 418 Comparative
examp le
11 253 453 0 .56 39 17667 Favorable 51 358 Examp le
12 262 462 0.57 38 17556 Favorable 57 382 Examp le
13 258 464 0.56 39 18096 Favorable 59 386 Examp le
14 254 456 0.56 39 17784 Favorable 53 363 Examp le
15 270 470 0.57 37 17390 Favorable 55 377 Example
16 257 460 0.56 40 18400 Favorable 50 363 Examp le
17 269 483 056 36 17388 Favorable 67 404 Examp le
18 268 456 0.59 38 17328 Favorable 54 371 Examp le
19 269 475 0.57 37 17575 Favorable 56 372 Examp le
20_ 270 477 0.57 38 18126 Favorable 62 389 Examp le
21 234 448 0.52 39 17472 Favorable 60 371 Exam le
22 220 472 0.47 38 17936 Favorable 66 375 Examp le
23 222 471 0.47 37 17427 Favorable 64 370 Examp le
24 245 426 0.58 38 16188 Favorable 51 368 Comparative
example
25 285 455 0.63 35 15925 Favorable 30 320 Comparative
example
26 222 446 0.49 39 17394 Favorable 59 365 Examp le
27 241 455 0.53 38 17290 Favorable 54 363 Examp le
28 236 452 0.52 38 17176 Favorable 60 367 Exam le
29 310 520 0. 6 32 16640 Favorable 65 396
Comparative
exam le
30 252 475 0.53 35 16625 Favorable 51 360 Example
9/9
TABLE 3-4
Mechanical characteristics Paint bake hardenabilit
Production YS TS El TS x El Tight bending
BH Strain aging Remarks
No. (MPa) (MPa) YR (%) (MPa.%) processability
amount YS
(MPa) (MPa)
31 222 449 0.49 39 17511 Favorable 58 372 Exam le
32 233 453 0.51 38 17214 Favorable 63 380 Example
33 225 452 05 38 17176 Favorable 61 378 Example
Comparative
34 253 431 0.59 36 15516 Favorable 48 380 examp le
35 255 435 0.59 36 15660 Favorable 40 320 Example
Comparative
36 241 450 0.54 38 17100 Poor 60 373 examp le -
Comparative
37 230 447 0.51 38 16986 Poor 60 375 exam le
Comparative
38 227 448 0. 51 39 17472 Poor 60 375 exam le
Comparative
39 275 442 0.62 36 15912 Poor 52 391
examp l e
40 238 450 0.53 38 17100 Favorable 58 377 Examp le
41 249 451 0.55 38 17138 Favorable 67 385 Examp le
42 236 452 0.52 38 17176 Favorable 59 374 Examp le
Comparative
43 280 460 0. 61 35 16100 Poor 57 401
examp le
Comparative
44 295 470 0.63 34 15980 Poor 55 360 example
Comparative
45 291 511 0.57 30 15330 Poor 66 407
example
Comparative
46 295 510 0.58 30 15300 Favorable 63 408 exam le
Comparative
47 310 445 0. 7 34 15130 Favorable 50 420
examp le
Comparative
49 285 473 0:6 33 15609 Favorable 58 ---388 examp le
49 284 448 0.63 35 15680 Favorable 381 Exam le
50 289 449 0.64 35 15715 Favorable 387 Exam le
51 279 452 0.62 34 15368 Favorable 392 Exam le
V
Comparative
52 256 447 0.57 35 15645 Poor 55 366 exam le
53 283 469 0. 6 34 15946 Favorable 391 Exam le
Comparative
54 305 442 0.69 35 15470 Favorable 355 examp le
55 295 462 0.64 34 15708 Favorable 64 410 Examp le
56 249 448 0.56 37 16576 Favorable 60 365 Examp le
Comparative
57 510 840 0. 61 18 15120 Poor 72 610 exam le
Comparative
58 296 520 0.57 32 16840 Poor 85 420 examp le
Comparative
59 289 495 0.79 34 16830 Poor 25 484 exam le
32"

CLAIMS
1. A high-strength steel sheet comprising, by mass%:
C: 0.01% to 0.10%;
5 Si: 0.15% or less;
Mn: 0.80% to 1.80%;
P: 0.10% or less;
S: 0.015% or less;
Al: 0.10% to 0.80%;
10 Cr: 0.01 % to 1.50%;
N: 0.0100% or less; and
a balance consisting of iron and unavoidable impurities, wherein:
a metallic structure is composed of ferrite and a hard second phase;
an area fraction of the ferrite is 80% or more;
15 an area fraction of the hard second phase is 1% to 20%;
a fraction of unrecrystallized ferrite in the ferrite is less than 10%;
ferrite grain sizes are 5μm to 20 μm; and
a fraction of ferrite crystal grains having an aspect ratio of 1.2 or less in entire
ferrite crystal grains is 60% or more.
20
2. The high-strength steel sheet according to Claim 1, wherein a component
composition of the high-strength steel sheet satisfies
Mn/Cr is 3.0 or less, and
Cr/(Si+Al) is 3.0 or less.
25
3. The high-strength steel sheet according to Claim 1 or 2, further comprising, by
mass%, one or more of.
Nb: 0.0005% to 0.0500%;
Ti: 0.0005% to 0.0500%;
5 Mo: 0.005% to 1.500%;
W: 0.005% to 1.500%;
B: 0.0001 % to 0.0100%;
Ni: 0.005% to 1.500%;
Cu: 0.005% to 1.500%; and
10 V: 0.005% to 1.500%.
4. The high-strength steel sheet according to any one of Claims 1 to 3, wherein a zinc
galvanization coat or a zinc alloy galvanization coat is provided at a surface of the
high-strength steel sheet.
15
5. The high-strength steel sheet according to Claim 4,
wherein the high-strength-steel sheet further comprises Cr: 0.20% to 1 . 50%, and
P: less than 0.015%.
20 6. A method for producing a high-strength steel sheet,
wherein a billet having a chemical components of the high-strength steel sheet
according any one of Claims 1 to 3 is hot rolled, pickled, cold rolled at a percentage
reduction in a thickness of more than 60% so as to obtain a steel sheet,
then, the steel sheet is heated to a temperature range of 720°C to 850°C at a
25 heating rate controlled to 1 °C/s to 10°C/s in a temperature range of an AcI transformation
34-
point to an Ac3 transformation point, and subjected to an annealing for a retention time
of 10 seconds to 200 seconds during which a temperature of the steel sheet is 720°C to
850°C, and
after the annealing, the steel sheet is subjected to a first cooling to 500°C or
5 lower at a cooling rate of 3°C/s or more, and then subjected to a skin pass rolling of 2.0%
or less.
7. -The method forproducing the high-strength steel sheet according to Claim 6, -
wherein a heat treatment is carried out in a temperature range of 200°C to 450°C for 30
10 seconds or more before the skin pass rolling, and
a second cooling is carried out at a cooling rate of 1°C/s to 3°C/s to 100°C or
lower after the heat treatment.
8. The method for producing the high-strength steel sheet according to Claim 6,
15 wherein galvanization is carried out on the steel sheet after the first cooling and before
the skin pass rolling.
9. The method for producing the high-strength steel sheet according to Claim 8,
wherein a heat treatment for alloying is carried out for 10 seconds or longer in a
20 temperature range of 450°C to 600°C at a timing after the galvanization and before the
skin pass rolling.
10. The method for producing the high-strength steel sheet according to Claim 7,
wherein the galvanization is carried out on the steel sheet at a timing after the heat
treatment and before the second cooling.
11. The method for producing the high-strength steel sheet according to Claim 10,
wherein the heat treatment for alloying is carried out for 10 seconds or longer in a
5 temperature range of 450°C to 600°C at a timing after the galvanization and before the
second cooling.