JP2012237044A - High-strength cold-rolled steel sheet excellent in elongation and stretch flanging property and method for production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet having a tensile strength of 1,180 MPa or more and being excellent in elongation and stretch flanging property.SOLUTION: The high-strength cold-rolled steel sheet has a component composition including, by mass%, 0.15-0.25% C, 1.0-2.0% Si, 2.5-3.5% Mn, 0.030% or less P, 0.0050% or less S, 0.005-0.1% Al, and 0.01% or less N with the balance comprising Fe and unavoidable impurities and has a structure including, by volume fraction, 50-70% bainitic ferrite phase, 15-40% martensite phase, and 5-15% retained austenite phase, wherein the volume fraction of a martensite phase with a long axis length of ≥5 μm accounts for 50% or less (inclusive of 0%) of the total volume fraction of the martensite phase.

Description

  The present invention relates to a high-strength cold-rolled steel sheet suitable for use in automobile parts and the like that are required to be pressed into a complicated shape, and a method for producing the same, and particularly expensive Nb, Cu, Ni, Cr, Mo, Elongation is achieved by utilizing retained austenite without actively adding elements, making the metal structure a uniform structure mainly composed of bainitic ferrite phase, and controlling the grain size of the martensite phase. (El) and stretch flangeability (usually evaluated by hole expansion ratio (λ)), and at the same time, it is intended to achieve a high strength of tensile strength (TS): 1180 MPa or more.

In recent years, steel plates with a tensile strength (TS) of 590 MPa or more have been actively applied to automobile bodies for the purpose of improving fuel economy and collision safety by reducing the weight of automobile bodies. Application of steel sheets is being studied.
Conventionally, high-strength steel sheets of TS: 1180MPa class or higher were often applied to light-worked parts. Recently, however, presses with complex shapes are required to achieve both higher collision safety and improved fuel economy by reducing vehicle weight. Applications to parts are being studied, and there is a great need for steel sheets with excellent workability.

  However, since steel sheets generally tend to have lower workability with higher strength, avoiding cracks during press forming is a major issue in expanding the application of high-strength steel sheets. In particular, when the strength is increased to TS: 1180 MPa class or higher, very expensive rare elements such as Nb, Cu, Ni, Cr and Mo are often positively added from the viewpoint of securing strength.

  As conventional technologies related to high-strength cold-rolled steel sheets with excellent formability, for example, Patent Literatures 1 to 4 utilize mainly bainite phase or retained austenite by limiting steel components and structures, optimizing hot-rolling conditions, and annealing conditions. A technology for manufacturing a high-strength cold-rolled steel sheet is disclosed.

JP 2005-179703 A JP 2005-298964 A JP 2009-256773 A JP 2010-024497 A

However, the technology described in Patent Document 1 not only requires expensive Ni and Cu as an austenite stabilizing element but also has a tempered martensite phase as the main phase, but TS: 1180 MPa or more has sufficient stretch flangeability. Not obtained.
The technology described in Patent Document 2 also requires expensive Mo, and the main phase is a ferrite phase, but the volume fraction of the retained austenite phase that contributes to high elongation characteristics is small, and it has a negative effect on stretch flangeability. Because it contains a martensite phase, it is still impossible to obtain an excellent balance of properties that achieves both excellent elongation and stretch flange characteristics at TS: 1180 MPa or higher. Furthermore, since a large amount of Al is contained, there is a concern about deterioration of weldability and continuous castability.
In the technique described in Patent Document 3, since the Al content is excessive, there is a concern about a decrease in weldability and a concern about a decrease in continuous castability.
Since the technique described in Patent Document 4 is mainly composed of bainite phase and has a small volume fraction of retained austenite phase, sufficient elongation cannot be obtained in the case of high strength of TS: 1180 MPa or more.

  The present invention advantageously solves the above problems, and has improved elongation and stretch flangeability by adjusting the metal structure without containing expensive alloy elements Nb, Cu, Ni, Cr, Mo. An object is to provide a high-strength cold-rolled steel sheet together with its advantageous production method.

Now, as a result of earnest research to solve the above problems, the inventors have found that bainitic ferrite phase and the content of a rare metal that is expensive from the viewpoint of formability, weldability, and continuous castability are not included. By controlling the volume fraction of the retained austenite phase, the elongation can be improved, and the volume fraction of the martensite phase that forms low-temperature transformation from austenite in the metal structure and the size of the martensite phase are strictly controlled. As a result, it was found that the tensile strength (TS): higher than 1180 MPa can be achieved along with the improvement of stretchability and stretch flangeability.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
(1) In mass%,
C: 0.15-0.25%
Si: 1.0-2.0%
Mn: 2.5-3.5%
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005 to 0.1% and N: 0.01% or less, with the balance having a component composition consisting of Fe and inevitable impurities,
Bainitic ferrite phase: 50-70%,
Martensite phase: 15-40% and residual austenite phase: 5-15%
And the ratio of the martensite phase with the major axis length ≧ 5 μm in the total volume fraction of the martensite phase satisfies 50% or less (however, 0% is included) High strength cold rolled steel sheet

(2) The steel sheet is further in mass%,
Ti: 0.005-0.050% and B: 0.0001-0.0050%
The high-strength cold-rolled steel sheet having excellent elongation and stretch flangeability as described in (1) above, comprising one or two selected from the above.

(3) A steel slab having the composition described in (1) or (2) above is hot-rolled and then cold-rolled at a reduction ratio of 20 to 50%, and then at a temperature of 800 to 900 ° C. The first annealing is performed in the temperature range, the cooling rate is 10 to 80 ° C./sec, the cooling stop temperature is 300 to 500 ° C., and the temperature is kept in this temperature range for 100 to 1000 seconds, and then the reduction rate is 20 to A high-strength cold-rolled steel sheet excellent in elongation and stretch flangeability, characterized by performing 50% cold rolling and then performing second annealing in a temperature range of (first annealing temperature ± 50 ° C). Production method.

  According to the present invention, a high-strength cold-rolled steel sheet having excellent elongation and stretch flangeability and having a tensile strength of 1180 MPa or more can be obtained without containing an expensive alloy element. The high-strength cold-rolled steel sheet obtained by the present invention is suitable as an automobile part that is press-formed into a particularly severe shape.

Hereinafter, the present invention will be specifically described.
Now, as a result of intensive studies on the workability of high-strength cold-rolled steel sheets, particularly the improvement of elongation and stretch flangeability, the inventors have found that even in a component system that does not contain Nb, Cu, Ni, Cr, Mo, 50-70% bainitic ferrite phase in fraction, 15-40% martensite phase in volume fraction and 5-15% residual austenite in volume fraction, and long axis length ≧ 5 μm martensite phase It was found that the intended purpose can be advantageously achieved by making the structure satisfying a volume fraction of 50% or less.
Hereinafter, the reasons for limiting the component composition and structure of the present invention will be specifically described. The unit of the element content in the steel sheet is “mass%”, but hereinafter, it is simply indicated by “%” unless otherwise specified.

First, the appropriate range of the component composition of steel in the present invention and the reasons for limitation are as follows.
C: 0.15-0.25%
C is an element that contributes to strength, and effectively contributes to securing strength by solid solution strengthening and structure strengthening by a low-temperature transformation phase. However, if the amount of C is less than 0.15%, it is difficult to obtain a low temperature transformation phase having a required volume fraction. On the other hand, if it exceeds 0.25%, not only the spot weldability is remarkably deteriorated, but the low temperature transformation phase is excessively hardened. Decreasing moldability, especially stretch flangeability. Accordingly, the C content is in the range of 0.15 to 0.25%.

Si: 1.0-2.0%
Si is an important element for promoting C concentration in austenite and stabilizing retained austenite. In order to obtain the above action, it is necessary to contain 1.0% or more, preferably 1.2% or more. Further, in the steel sheet of the present invention containing 15% or more of the martensite phase, the bainitic ferrite phase is solid solution strengthened, and the strength difference between the bainitic ferrite phase and the martensite phase is reduced. Enables uniform deformation of the entire steel sheet and contributes to the improvement of stretch flangeability. On the other hand, if the Si content exceeds 2.0%, the steel sheet becomes brittle, cracks are formed, and formability deteriorates. Therefore, the upper limit of Si content is 2.0% or less. Preferably it is 1.8% or less.

Mn: 2.5-3.5%
Mn is an element that improves hardenability and has an effect of easily ensuring a low-temperature transformation phase that contributes to strength. In order to obtain the above action, it is necessary to contain 2.5% or more. However, if it exceeds 3.5%, it is excessively hardened, resulting in insufficient hot ductility and slab cracking. Therefore, the Mn content is in the range of 2.5 to 3.5%. Preferably it is 2.6 to 3.0% of range.

P: 0.030% or less Since P adversely affects spot weldability, it is preferable to reduce it as much as possible, but 0.030% is acceptable. However, excessively reducing the amount of P lowers the production efficiency in the steelmaking process and increases the cost, so the lower limit of the amount of P is preferably about 0.001%.

S: 0.0050% or less S not only segregates at the grain boundaries to reduce hot brittleness, but also forms sulfide inclusions such as MnS, and this MnS expands by cold rolling, and is deformed. Although it is preferable to reduce as much as possible to reduce the local deformability as a starting point of cracking, it is acceptable up to 0.0050%. However, excessive reduction is industrially difficult and causes an increase in desulfurization cost in the steel making process, so the lower limit of the amount of S is preferably about 0.0001%. Preferably it is 0.0001 to 0.0030% of range.

Al: 0.005-0.1%
Al is mainly added for the purpose of deoxidation. Moreover, it is effective in suppressing the formation of carbides and generating a retained austenite phase, and is a useful element for improving the strength-elongation balance. Addition of 0.005% or more is necessary to achieve the above object. However, if the content exceeds 0.1%, there arises a problem of deterioration of workability due to an increase in inclusions such as alumina. Furthermore, if the Al content exceeds 0.1%, the basicity in the molten slag will increase during continuous casting, making it difficult to ensure lubricity, and the solidified shell and mold will seize and breakout may occur. . Therefore, the Al content is in the range of 0.005 to 0.1%. Preferably it is 0.02 to 0.06% of range.

N: 0.01% or less N is an element that deteriorates aging resistance. When the N content exceeds 0.01%, deterioration of aging resistance becomes remarkable. Moreover, when it contains B, it combines with B, forms BN, consumes B, the hardenability by solid solution B falls, and it becomes difficult to ensure the martensitic phase of a predetermined | prescribed volume fraction. Moreover, since it exists as an impurity element in bainitic ferrite and lowers the ductility by strain aging, it is preferable that the N content is low, but up to 0.01% is acceptable. However, excessive reduction of the amount of N causes an increase in denitrification costs in the steelmaking process, so the lower limit of the amount of N is preferably about 0.0001%. More preferably, it is 0.0010 to 0.0050% of range.

The basic components have been described above. However, in the present invention, the elements described below can be appropriately contained.
Ti: 0.005-0.050%
Ti forms carbonitrides and sulfides and contributes effectively to improving strength. Moreover, when adding B, it is an element effective also in suppressing the formation of BN and fixing the hardenability by B by fixing N as TiN. To obtain these effects, a Ti content of 0.005% or more is required. However, if the Ti content exceeds 0.050%, excessive precipitates are generated in the bainitic ferrite phase, and elongation is caused by excessive precipitation strengthening. Cause a decline. Therefore, the Ti amount is in the range of 0.005 to 0.050%. Preferably it is 0.010 to 0.040% of range.

B: 0.0001-0.0050%
B is an element effective for increasing the hardenability and effectively contributing to securing low-temperature transformation phases such as a martensite phase and a retained austenite phase, and obtaining an excellent strength-elongation balance. In order to acquire this effect, it is necessary to contain B 0.0001% or more. On the other hand, when the amount of B exceeds 0.0050%, the above effect is saturated. Accordingly, the B content is in the range of 0.0001 to 0.0050%. Preferably it is 0.0005 to 0.0020% of range.
In the steel sheet of the present invention, components other than those described above are Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

Next, the appropriate range of the steel structure, which is one of the important requirements for the present invention, and the reason for the limitation will be described.
Volume fraction of bainitic ferrite phase: 50-70%
It is known that in a high-strength steel sheet having a composite structure, stretch flangeability deteriorates due to a hardness difference between a soft phase and a hard phase. On the other hand, it is softer than the martensite phase, which is a low-temperature transformation phase from austenite, and has a structure mainly composed of a bainitic ferrite phase that contributes to ductility, that is, a structure closer to a single phase structure than a composite structure By doing so, the entire steel sheet is uniformly stretched during forming, and stretch flangeability is improved. When the bainitic ferrite phase is less than 50%, the volume fraction of the hard martensite phase is increased to increase the strength excessively, and the elongation and the stretch flange are deteriorated. On the other hand, when the bainitic ferrite phase is present in excess of 70%, it is difficult not only to secure the strength: 1180 MPa, but also to secure a predetermined amount of retained austenite phase that contributes to ductility. Accordingly, the volume fraction of the bainitic ferrite phase is in the range of 50 to 70%.

Martensite volume fraction: 15-40%
The martensite phase contributes to the improvement of strength, and the volume fraction of the martensite phase needs to be 15% or more in order to secure a TS of 1180 MPa or more. However, if the volume fraction of the martensite phase is too large, the strength becomes excessively high and the elongation and stretch flangeability deteriorate, so the volume fraction of the martensite phase needs to be 40% or less. And by adjusting the volume fraction of the martensite phase within a range of 15 to 40%, a material balance with good strength, elongation and stretch flangeability can be obtained.

Volume fraction of retained austenite phase: 5-15%
Residual austenite phase has strain-induced transformation, that is, when the material is deformed, the strained portion transforms into the martensite phase, the deformed portion becomes hard, and has the effect of improving the ductility by preventing strain concentration. In order to increase ductility, it is necessary to contain 5% or more of retained austenite phase. However, since the residual austenite phase has a high C concentration and is hard, if it is excessively present in the steel sheet in excess of 15%, there is a local hard portion, which inhibits uniform deformation of the material during stretch flange forming. Therefore, it becomes difficult to ensure excellent elongation and stretch flangeability. In particular, from the viewpoint of stretch flangeability, it is preferable that the retained austenite is small. Therefore, the volume fraction of the retained austenite phase is set to a range of 5 to 15%.

Ratio of martensite phase with major axis length ≧ 5μm in total volume fraction of martensite phase: 50% or less (including 0%)
The martensite phase is harder than the base structure bainitic ferrite phase, and if the total volume fraction of the martensite phase is the same, coarse martensite with a major axis length of 5 μm or more compared to the martensite phase of less than 5 μm The phase is present in a localized manner, which inhibits uniform deformation and is disadvantageous for stretch flangeability as compared with a fine uniform structure that performs more uniform deformation. In particular, when the ratio of the long axis length martensite exceeds 50%, the martensite phases are adjacent to each other, the non-uniform deformation becomes remarkable, and the stretch and stretch flangeability are adversely affected. Therefore, the ratio of the martensite phase having a major axis length of 5 μm or more is in the range of 50% or less, and the smaller the better. Accordingly, the volume fraction of the martensite phase with the long axis length ≧ 5 μm is set to 50% or less. It may be 0%.

Next, the manufacturing conditions of the high-strength cold-rolled steel sheet according to the present invention and the reasons for limitation will be described.
In the present invention, the process before hot finish rolling may be performed in accordance with a conventional method. For example, a steel slab obtained by melting and casting steel prepared in the above component composition range can be used. In the present invention, not only continuous casting slabs and ingot-splitting slabs, but also thin slabs with a thickness of about 50 to 100 mm can be used. Especially in the case of thin slabs, direct heating without reheating is possible. It can use for a hot rolling process.

The hot rolling is not particularly limited, and may be performed according to a conventionally known method. The preferred conditions are as follows.
The heating temperature during hot rolling is preferably 1100 ° C. or higher. The upper limit is preferably set to 1300 ° C. from the viewpoint of reducing scale generation and reducing fuel consumption. The finishing temperature in hot rolling is preferably 850 ° C. or higher so as to avoid a layered structure of a low-temperature transformation phase such as ferrite and pearlite. In addition, the upper limit is preferably set to 950 ° C. from the viewpoint of reduction of scale formation and fine homogenization of the structure by suppressing coarsening of the crystal grain size.
The winding temperature after the hot rolling is preferably 450 to 600 ° C. from the viewpoint of cold rolling property and surface properties, and the steel sheet after winding is subjected to cold rolling through a pickling process.

Next, cold rolling is performed. In the present invention, the steps after the cold rolling step are important, and two cold rollings and two annealings are performed.
Cold reduction rate (first time): 20-50%
If the reduction ratio in the first cold rolling is less than 20%, the strain introduced into the steel sheet is small, the uniform refinement of the structure does not proceed, and recovery, recrystallization and phase transformation are delayed. As a result, a sufficient austenite phase cannot be obtained during annealing, and finally it becomes difficult to secure a martensite phase having a predetermined volume fraction, making it difficult to secure TS. In addition, since a uniform recovery recrystallized structure cannot be obtained and a non-uniform structure affected by the hot-rolled sheet structure is obtained, the stretch and stretch flangeability are adversely affected. On the other hand, even if the cold rolling reduction exceeds 50%, there is no problem with the material, but the upper limit is set to 50% because the cold rolling load increases.

Annealing temperature (first time): 800 ~ 900 ℃
If the annealing temperature in the first annealing is lower than 800 ° C, the volume fraction of the ferrite phase increases during annealing, and the volume fraction of the bainitic ferrite phase in the final structure increases, so TS: It becomes difficult to secure 1180 MPa. Moreover, C concentration to an austenite phase is accelerated | stimulated during annealing, a martensite phase hardens too much, and stretch flangeability falls. On the other hand, when heated above 900 ° C to the high temperature range of the austenite single phase, the austenite grain size becomes excessively coarse, the crystal grain size of the bainitic ferrite phase and martensite phase becomes coarse, and the stretch flangeability decreases. . Therefore, the annealing temperature in the first annealing is in the range of 800 to 900 ° C.

Cooling rate: 10 to 80 ° C./sec The cooling rate after the first annealing is important for obtaining the desired volume fraction of the low-temperature transformation phase. When the average cooling rate is less than 10 ° C./second, it is difficult to secure the martensite phase and it becomes soft, so that it is difficult to ensure the strength. On the other hand, when it exceeds 80 ° C./second, a martensite phase is excessively generated and is excessively hardened, so that workability such as elongation and stretch flangeability is deteriorated. Accordingly, the cooling rate is in the range of 10 to 80 ° C./second.
The cooling in this case is preferably gas cooling, but other methods such as furnace cooling, mist cooling, roll cooling, and water cooling can be used, or a combination thereof can also be used. .

Cooling stop temperature: 300 ~ 500 ℃
When the cooling stop temperature after the first annealing is less than 300 ° C., the generation of retained austenite is suppressed and the martensite phase is excessively generated, so that the strength becomes too high and it becomes difficult to ensure the elongation. On the other hand, when the temperature exceeds 500 ° C., the formation of bainitic ferrite and retained austenite is suppressed during holding after cooling is stopped, and an excessive martensite phase is generated in the cooling process after holding, thereby obtaining excellent ductility. It becomes difficult. Mainly composed of bainitic ferrite phase, controlling the abundance ratio of martensite phase and residual austenite phase, ensuring the strength of TS: 1180MPa class or more, and at the same time, stop cooling to obtain a good balance between stretch and stretch flangeability. The temperature should be in the range of 300-500 ° C.

Holding time: 100 to 1000 seconds When the holding time in the above-described cooling stop temperature range (which is also the residence temperature range) is less than 100 seconds, the time for the C concentration to proceed to the austenite phase becomes insufficient, and finally It is difficult to obtain a desired retained austenite volume fraction, and a martensite phase is excessively formed to increase the strength, and elongation and stretch flangeability are deteriorated. On the other hand, even if retained for more than 1000 seconds, the amount of retained austenite does not increase, and no significant improvement in elongation is observed. Accordingly, the holding time is in the range of 100 to 1000 seconds.
In addition, as a means for maintaining the steel plate after the cooling stop in the residence temperature range, for example, a means for adjusting a temperature of the steel plate to the residence temperature by providing a heat retaining device or the like in the downstream process of the cooling equipment after annealing, etc. Can be mentioned. Moreover, the steel plate after residence is cooled to a desired temperature by any conventionally known method.

Cold reduction rate (second time): 20-50%
In the present invention, the second cold rolling and the second annealing are carried out for the purpose of further improving the material properties in addition to the building of the material characteristics in the first annealing. The second cold rolling and annealing are important for controlling the structure controlled by the first annealing to a steel sheet structure having more excellent characteristics. Moreover, by carrying out the cold rolling and annealing twice, sufficient diffusion of the additive elements further proceeds, and can be made a more uniform structure than the structure after the cold rolling and annealing once. As a result, excellent elongation and stretch flangeability can be achieved.
When the second cold rolling reduction is less than 20% for the steel sheet having the first cold-rolled and annealed structure, the strain introduced by rolling becomes extremely small, so that the driving force for recovery and recrystallization is Small, nucleation, grain growth is delayed, subsequent transformation is delayed, it becomes difficult to secure a martensite phase having a predetermined volume fraction in the finally obtained steel sheet, softening, it is difficult to ensure strength Become. In addition, since a uniform recovery recrystallized structure cannot be obtained and a non-uniform structure affected by the hot-rolled sheet structure is obtained, the stretch and stretch flangeability are adversely affected. On the other hand, when the cold rolling ratio exceeds 50%, the amount of strain introduced into the steel sheet increases, recovery and recrystallization progress, and transformation progresses. Therefore, the microstructure changes controlled by the first annealing. Since the influence of the second annealing is dominant, it becomes difficult to obtain excellent elongation and stretch flangeability. In addition, the cold rolling load increases and a plate shape defect may occur. Accordingly, the second cold rolling reduction is set in the range of 20 to 50%.

Annealing temperature (second time): First annealing temperature ± 50 ℃
When the second annealing temperature is lower than (first annealing temperature −50 ° C.), C concentration in the austenite phase is promoted during soaking, and the crystal grain size during soaking is finer. In addition, as the bainitic ferrite phase is generated in the cooling and holding steps, C concentration in the austenite phase proceeds, the martensite phase becomes excessively hard, and stretch flangeability decreases. In addition, when the annealing temperature is low, the volume fraction of the ferrite phase becomes high during annealing, and the volume fraction of the ferrite phase in the final structure increases, so it may be difficult to secure TS: 1180 MPa. is there. On the other hand, when the second annealing temperature exceeds (first annealing temperature + 50 ° C.), the austenite grain size becomes excessively large, and the crystal grain size of the bainitic ferrite phase and martensite phase increases accordingly. Therefore, stretch flangeability deteriorates. In addition, when the additive elements are sufficiently diffused by being heated to a high temperature and uniform austenite phase single-phase annealing is performed, the first annealing structure varies, and the residual austenite phase having a desired volume fraction is obtained. Therefore, it is difficult to obtain a steel sheet having excellent elongation. Therefore, the second annealing temperature is in the range of (first annealing temperature ± 50 ° C.).
The cooling after the second annealing described above is not particularly limited, and may be performed according to a conventional method. Preferably, similarly to the first annealing, the cooling rate to the cooling stop temperature is 10 to 80 ° C./second, the cooling stop temperature is 300 to 500 ° C., and the holding time in the cooling stop temperature region is 100 to 1000 seconds. .

  After the second annealing described above, the cold-rolled steel sheet finally obtained may be subjected to temper rolling (skin pass rolling) for the purpose of shape correction or surface roughness adjustment. Since strain is introduced into the steel sheet, the crystal grains are expanded to form a rolled structure, and the ductility may be reduced. Therefore, the rolling reduction of skin pass rolling is preferably about 0.05% to 0.5%.

Steel with the composition shown in Table 1 is melted to form a slab, heated to 1200 ° C, hot rolled at the finish rolling mill temperature: 900 ° C, and after rolling, at a rate of 70 ° C / sec. After cooling and winding at 550 ° C., and then pickling with hydrochloric acid, cold rolling and annealing were performed under the conditions shown in Table 2 to produce a cold-rolled steel sheet having a thickness of 1.6 mm. In addition, the cooling after the second annealing is the preferable condition, the cooling rate to the cooling stop temperature: 10 to 80 ° C./second, the cooling stop temperature: 300 to 500 ° C., the holding time in the cooling stop temperature range: It was set within the range of 100 to 1000 seconds.
About the obtained cold-rolled steel sheet, the material characteristic was investigated by the material test shown below.
The obtained results are shown in Table 3.

(1) Structure of steel plate The cross section in the rolling direction was examined by observing a surface at 1/4 position of the plate thickness with a scanning electron microscope (SEM). The observation was carried out at N = 5 (5 observation fields). For the volume fraction of bainitic ferrite phase, use a cross-sectional structure photograph at a magnification of 2000 times, and by image analysis, obtain the area occupied by the bainitic ferrite phase existing in an arbitrarily set square area of 50 μm x 50 μm. This was defined as the volume fraction of the bainitic ferrite phase. In the above observation, the martensite phase was also observed, but the volume fraction of the martensite phase shown in Table 3 was obtained because the volume fraction was almost the same as the volume fraction of the martensite phase obtained by the method described later. The rate was determined by the method described later. Long axis length: The ratio of the martensite phase of 5 μm or more is a martensite with a long axis length of 5 μm or more existing in a 100 μm × 100 μm square area arbitrarily set by image analysis using a cross-sectional structure photograph at a magnification of 1000 times. The area occupied by the site phase was determined and used as the volume fraction. Here, the extraction of the martensite phase with a major axis length of 5 μm or more is performed when the major axis of the martensite phase, that is, when the maximum diameter is the same as or larger than a circle having a diameter of 5 μm, the major axis length is 5 μm or more. It was said that. Under the present circumstances, what had a comparatively smooth surface and was observed as a lump shape was determined to be martensite.
The amount of residual austenite phase was determined by X-ray diffraction using Mo Kα rays. That is, using a test piece having a surface near a thickness of 1/4 of the steel sheet as a measurement surface, the peaks of the (211) surface and (220) surface of the austenite phase and the (200) surface and (220) surface of the ferrite phase The volume ratio of the retained austenite phase was calculated from the strength.
The volume fraction of each phase is first distinguished from the bainitic ferrite phase and other than the bainitic ferrite phase by visual judgment based on the SEM image described above, and then the volume fraction of the bainitic ferrite phase is determined. The volume fraction of the retained austenite phase was determined by X-ray diffraction, and the remaining volume fraction was determined to be the martensite phase.

(2) Tensile properties Evaluation was performed by conducting a tensile test based on JIS Z 2241 using No. 5 test piece described in JIS Z 2201 with the rolling direction and 90 ° as the longitudinal direction (tensile direction). The evaluation criteria for tensile properties were TS × El ≧ 20,000 MPa ·% or more (TS: tensile strength (MPa), El: total elongation (%)).

(3) Hole expansion rate This was carried out based on the Japan Iron and Steel Federation standard JFST1001. When a hole with an initial diameter of d ₀ = 10 mm was punched and the conical punch with an apex angle of 60 ° was raised to widen the hole, when the crack penetrated the plate thickness, the rise of the punch was stopped. The punching hole diameter d is measured, and the following formula: Hole expansion rate (%) = ((d−d ₀ ) / d ₀ ) × 100
Calculated with Three tests were performed on the same number of steel plates, and the average value (λ) of the hole expansion rate was obtained. The evaluation standard for the hole expansion rate was TS × λ ≧ 36000 MPa ·% or higher.

As is apparent from Table 3, all of the inventive examples No. 1 to 7 are TS ≧ 1180 MPa, TS × El ≧ 20000 MPa ·% or more, and elongation and stretch flangeability satisfying TS × λ ≧ 36000 MPa ·% A high-strength cold-rolled steel sheet having excellent strength is obtained.
On the other hand, No.8 whose steel component is outside the proper range of the present invention has too much volume fraction of the martensite phase and cannot have a desired structure, and therefore is inferior in stretch and stretch flangeability. Yes.
In addition, No. 9 with a low cold rolling reduction, No. 10 with a low annealing temperature, No. 12 with a slow cooling rate after annealing, No. 17 with a low cold rolling reduction, No. 17 with a low second rolling annealing temperature The low No. 18 has a high volume fraction of bainitic ferrite phase and does not satisfy TS: 1180 MPa.
Furthermore, No. 11 with a high annealing temperature, No. 13 with a fast cooling rate, No. 16 with a short holding time, and No. 19 with a high annealing temperature have a high volume fraction of the martensite phase. The strength is excessively high, and the elongation and stretch flangeability are poor.
Furthermore, No. 14 having a low holding temperature after cooling and No. 15 having a high holding temperature after cooling each have a small volume fraction of retained austenite phase and are inferior in elongation.

  In accordance with the present invention, the volume fractions of the bainitic ferrite phase, martensite phase, and retained austenite phase are appropriate without actively containing expensive elements such as Cu, Ni, Cr, Mo, and Nb in the steel sheet. By controlling to, a high-strength cold-rolled steel sheet that is inexpensive, has excellent elongation and stretch flangeability, and has a tensile strength (TS) of 1180 MPa or more can be obtained. Moreover, the high-strength cold-rolled steel sheet of the present invention is suitable as an automobile part, and is also useful for applications that require strict dimensional accuracy and workability, such as in the field of architecture and home appliances.

Claims (3)

% By mass
C: 0.15-0.25%
Si: 1.0-2.0%
Mn: 2.5-3.5%
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005 to 0.1% and N: 0.01% or less, with the balance having a component composition consisting of Fe and inevitable impurities,
Bainitic ferrite phase: 50-70%,
Martensite phase: 15-40% and residual austenite phase: 5-15%
And the ratio of the martensite phase with the major axis length ≧ 5 μm in the total volume fraction of the martensite phase satisfies 50% or less (however, 0% is included) High-strength cold-rolled steel sheet with excellent resistance. The steel sheet is further in mass%,
Ti: 0.005-0.050% and B: 0.0001-0.0050%
The high-strength cold-rolled steel sheet having excellent elongation and stretch flangeability according to claim 1, comprising one or two selected from among them.   A steel slab having the composition according to claim 1 or 2 is hot-rolled, and then cold-rolled at a reduction ratio of 20 to 50%, and then annealed for the first time in a temperature range of 800 to 900 ° C. , Cooling rate: 10-80 ° C / second, cooling stop temperature: 300-500 ° C, hold in this temperature range for 100-1000 seconds, then cold rolling again: reduction rate: 20-50% And then performing a second annealing in a temperature range of (first annealing temperature ± 50 ° C.), a method for producing a high-strength cold-rolled steel sheet excellent in elongation and stretch flangeability.