WO2013179497A1 - Low yield ratio high-strength cold-rolled steel sheet with excellent elongation and stretch flange formability, and manufacturing method thereof - Google Patents

Abstract

A high-strength cold-rolled steel sheet having a low yield ratio and excellent elongation and stretch flange formability, and a manufacturing method thereof are provided. This high-strength cold-rolled steel sheet having a low yield ratio and excellent elongation and stretch flange formability is characterized in that the chemical components of the steel sheet include, in mass%, C: 0.05-0.13%, Si: 0.6-1.2%, Mn: 1.6-2.4%, P: 0.10% or less, S: 0.0050% or less, Al: 0.01-0.10%, and N: less than 0.0050%, and the remainder consists of Fe and unavoidable impurities, and in that the steel sheet has a composite microstructure containing, by volume percentage, 80% or more ferrite, 3-15% martensite, and 0.5-10% pearlite, the yield ratio is 70% or less, and the tensile strength is 590 MPa or greater.

Description

Low yield ratio high strength cold-rolled steel sheet excellent in elongation and stretch flangeability and manufacturing method thereof

The present invention relates to a high-strength cold-rolled steel sheet having a low yield ratio and excellent elongation and stretch flangeability, which is suitable as a member for automobile undercarriage parts and structural parts used by being pressed. The yield ratio (YR) is a value indicating the ratio of the yield strength (YS) to the tensile strength (TS) and is represented by YR (%) = (YS / TS) × 100.

In recent years, CO ₂ emission regulations have become stricter due to increasing environmental problems, and in the automobile field, improvement of fuel consumption by reducing the weight of the vehicle body has become a major issue. For this reason, thinning is being promoted by applying high-strength steel sheets to automobile parts, and steel sheets having a TS of 590 MPa or more are being promoted for parts where steel sheets having a TS of 270 to 440 MPa have been used so far. ing.

The steel sheet having a TS of 590 MPa or more is required to have excellent elongation and stretch flangeability (hole expanding property) from the viewpoint of formability. Furthermore, since it is assembled by arc welding, spot welding or the like after press working and modularized, high dimensional accuracy is required at the time of assembly. For this reason, it is necessary to make it difficult for a springback or the like to occur after processing, and a low yield ratio is required before processing.

As a high-strength steel sheet with a low yield ratio that has both formability and high strength, dual-phase steel (DP steel) with a ferrite-martensite structure is known. Composite structure steel in which the main phase is ferrite and martensite is dispersed has a low yield ratio, a high TS, and an excellent elongation. However, since stress concentrates on the interface between ferrite and martensite, cracks are likely to occur, so that the hole expandability is inferior. Therefore, Patent Document 1 discloses a high-strength steel sheet for automobiles that achieves both collision safety and formability by controlling the space factor and the average crystal grain size of the entire structure of ferrite and martensite.

Patent Document 2 discloses a high-strength steel sheet having improved elongation and stretch flangeability by controlling the space factor of the fine ferrite having an average grain size of 3 μm or less and the martensite having an average grain size of 6 μm or less with respect to the entire structure. It is disclosed. Patent Document 3 discloses a DP steel sheet in which fine inclusions are dispersed in the steel sheet by containing Ce or La in the steel sheet component to improve stretch flangeability.

Also known is a technology for containing bainite and retained austenite in the steel sheet structure in order to improve formability. For example, in Patent Document 4, a composite structure composed of ferrite, residual austenite, the balance being bainite and martensite, the aspect ratio and average particle size of martensite and residual austenite are defined, and martensite and residual per unit area By defining the number of austenite, a composite structure cold-rolled steel sheet having excellent elongation and stretch flangeability is disclosed.

Non-Patent Document 1 will be described in an example.

Japanese Patent No. 3936440 JP 2008-297609 A JP 2009-299149 A Japanese Patent No. 4288364

However, although Patent Document 1 defines the average crystal grain size of ferrite and martensite, it does not ensure sufficient hole expansion property for press molding. In Patent Document 2, since the volume fraction of martensite is remarkably large, the elongation is insufficient. Patent Document 3 is low in production cost because Ce and La are added, and the manufacturing cost is high, and the material variation is large in order to control the size of inclusions.

Also, in Patent Document 4, a steel sheet containing bainite or retained austenite requires a high cooling rate using special equipment to obtain its structure, so that the manufacturing cost is high and the material variation is large. Furthermore, as a characteristic, a high strength steel plate having a steel structure having retained austenite and bainite has a higher YR than DP steel, so it is difficult to stably set the YR to 70% or less.

Thus, it is difficult to ensure elongation and stretch flangeability in a low strength YR high strength steel sheet, and cold rolled steel sheets that satisfy these characteristics (yield ratio, strength, elongation, stretch flangeability) have been developed so far. Not developed.

Therefore, an object of the present invention is to provide a high-strength cold-rolled steel sheet that has solved the above-mentioned problems of the prior art, has excellent elongation and stretch flangeability, and has a low yield ratio and a method for manufacturing the same.

As a result of intensive studies, the inventors have added a suitable amount of Si and controlled the volume fraction of ferrite, martensite and pearlite, and are excellent in elongation and stretch flangeability that ensure high strength at low YR. It was found that a cold-rolled steel sheet can be obtained.

Conventionally, pearlite was thought to degrade stretch flangeability. However, when the present inventors reduce the hardness difference from the hard phase by adding an appropriate amount of Si as a steel plate component and strengthening ferrite by solid solution strengthening in the steel sheet structure in which ferrite, martensite and pearlite are present, voids ( It was discovered that cracks are preferentially generated from the interface between ferrite and martensite, and the generation from the interface with pearlite is suppressed. Further, even if the martensite volume fraction is reduced as compared with the conventional DP steel, strength can be secured by utilizing the solid solution strengthening of ferrite by Si and the presence of pearlite. It was also found that by reducing the volume fraction of martensite, local elongation was improved and elongation and stretch flangeability were improved. Furthermore, by adjusting the volume fraction of martensite and pearlite, it is possible to obtain a low yield ratio high strength cold-rolled steel sheet having a tensile strength of 590 MPa or more while ensuring a low YR.

Specifically, as a steel plate component, Si is added in an amount of 0.6 to 1.2%, the main phase ferrite is 80% or more in terms of volume fraction, martensite is 3 to 15%, and pearlite is 0.5 to 10%. By controlling the steel sheet structure within the range of%, it is possible to obtain a high-strength cold-rolled steel sheet having a yield ratio of 70% or less and a tensile strength of 590 MPa or more and excellent elongation and stretch flangeability.

That is, the present invention provides the following (1) and (2).

(1) The chemical composition of the steel sheet is mass%, C: 0.05 to 0.13%, Si: 0.6 to 1.2%, Mn: 1.6 to 2.4%, P: 0.00. 10% or less, S: 0.0050% or less, Al: 0.01 to 0.10%, N: less than 0.0050%, the balance is made of Fe and inevitable impurities, and the microstructure of the steel sheet is It has a composite structure containing 80% or more of ferrite, 3 to 15% of martensite, and 0.5 to 10% of pearlite in volume fraction, yield ratio is 70% or less, and tensile strength is 590 MPa or more. Low yield ratio high strength cold-rolled steel sheet with excellent elongation and stretch flangeability.

(2) After subjecting the steel slab having the chemical composition described in (1) to hot rolling and cold rolling, the steel slab is heated and held in a temperature range of Ac ₁ to Ac ₃ points. Cooling at an average cooling rate of 1 ° C / s to 25 ° C / s up to a temperature of 500 to 550 ° C, followed by cooling at an average cooling rate of 5 ° C / s or less. Low yield ratio high strength cold rolled steel sheet manufacturing method.

According to the present invention, by controlling the steel plate component, the annealing temperature, and the cooling conditions after annealing, the volume fraction includes ferrite of 80% or more, martensite of 3 to 15%, and pearlite of 0.5 to 10%. High-strength cold-rolled steel sheet with a low yield ratio that has a composite structure, has a tensile strength of 590 MPa or more, a yield ratio of 70% or less, an elongation of 29.0% or more, and an elongation and stretch flangeability of 65% or more. Can be obtained.

Hereinafter, the present invention will be specifically described.

The reason for limiting the chemical composition of the high-strength cold-rolled steel sheet of the present invention will be described. In the following, “%” notation of chemical components means mass%.

C: 0.05 to 0.13%
C is an element effective for increasing the strength of a steel sheet, and contributes to increasing the strength by forming a second phase of pearlite and martensite. In order to obtain this effect, 0.05% or more must be added. Preferably it is 0.08% or more. On the other hand, if it is added excessively, spot weldability is lowered, so the upper limit is made 0.13%.

Si: 0.6-1.2%
Si is an element that contributes to high strength, and since it has a high work-hardening ability, it has a relatively small decrease in elongation with respect to an increase in strength, and is an element that also contributes to an improvement in strength-elongation balance. Furthermore, the solid solution strengthening of the ferrite phase reduces the hardness difference from the hard second phase, thereby contributing to the improvement of stretch flangeability. The addition of an appropriate amount of Si can suppress the generation of voids from the interface between the ferrite phase and the pearlite phase, but in order to obtain the effect, it is necessary to contain 0.6% or more. The upper limit is not particularly defined from the viewpoint of elongation and stretch flangeability, but if it exceeds 1.2%, the chemical conversion treatment property is lowered, so the content is made 1.2% or less. Preferably it is 1.0% or less.

Mn: 1.6 to 2.4%
Mn is an element that contributes to strengthening by forming solid solution strengthening and martensite. To obtain this effect, it is necessary to contain 1.6% or more. On the other hand, when the content is excessive, the moldability is significantly lowered. Therefore, the content is set to 2.4% or less. Preferably it is 2.2% or less.

P: 0.10% or less P contributes to high strength by solid solution strengthening, but when excessively added, segregation to the grain boundary becomes remarkable and the grain boundary becomes brittle, and weldability. Therefore, the content is made 0.10% or less. Preferably it is 0.05% or less.

S: 0.0050% or less When the content of S is large, a large amount of sulfides such as MnS are generated, and the local elongation represented by stretch flangeability is reduced, so the upper limit of the content is 0.0050%. To do. Preferably, it is 0.0030% or less. Although a minimum is not specifically limited, Since ultra-low S raises steelmaking cost, it is preferable to contain 0.0005% or more.

Al: 0.01 to 0.10%
Al is an element necessary for deoxidation. In order to obtain this effect, it is necessary to contain 0.01% or more, but even if it exceeds 0.10%, the effect is saturated. The content is 0.10% or less. Preferably it is 0.05% or less.

N: Less than 0.0050% Since N forms coarse nitrides and deteriorates stretch flangeability, it is necessary to suppress the content. When N is 0.0050% or more, this tendency becomes remarkable, so the N content is less than 0.0050%.

In the present invention, in addition to the above components, one or more of the following components may be added.

V: 0.10% or less V can contribute to an increase in strength by forming fine carbonitrides. In order to exhibit such an effect, it is preferable to contain the addition amount of V 0.01% or more. On the other hand, even if added over 0.10%, the effect of increasing the strength is small, and on the contrary, the cost of the alloy is increased, so the content is preferably 0.10% or less.

Ti: 0.10% or less Ti, like V, can contribute to an increase in strength by forming fine carbonitride, and can be added as necessary. In order to exert such an effect, the Ti content is preferably 0.005% or more. On the other hand, when a large amount of Ti is added, YR increases remarkably, so the content is preferably 0.10% or less.

Nb: 0.10% or less Nb, like V, can contribute to an increase in strength by forming fine carbonitrides, and can be added as necessary. In order to exhibit such an effect, the Nb content is preferably 0.005% or more. On the other hand, when a large amount of Nb is added, YR increases remarkably, so the content is preferably 0.10% or less.

Cr: 0.50% or less Cr is an element that contributes to increasing the strength by improving the hardenability and generating the second phase, and can be added as necessary. In order to exhibit this effect, it is preferable to make it contain 0.10% or more. On the other hand, since the effect is saturated even if the content exceeds 0.50%, the content is preferably 0.50% or less.

Mo: 0.50% or less Mo is an element that improves hardenability and contributes to high strength by generating the second phase, and further contributes to high strength by generating some carbides. Can be added accordingly. In order to exhibit these effects, it is preferable to make it contain 0.05% or more. Since the effect is saturated even if the content exceeds 0.50%, the content is preferably 0.50% or less.

Cu: 0.50% or less Cu is an element that contributes to high strength by solid solution strengthening, improves hardenability, and contributes to high strength by generating a second phase. Can be added. In order to exhibit these effects, it is preferable to make it contain 0.05% or more. On the other hand, even if the content exceeds 0.50%, the effect is saturated and surface defects due to Cu are likely to occur. Therefore, the content is preferably 0.50% or less.

Ni: 0.50% or less Ni, like Cu, is an element that contributes to strengthening by solid solution strengthening, improves hardenability, and generates a second phase to contribute to strengthening. It can be added as necessary. In order to exhibit these effects, it is preferable to make it contain 0.05% or more. Moreover, since it has the effect of suppressing the surface defect resulting from Cu when it adds simultaneously with Cu, it is effective at the time of Cu addition. On the other hand, since the effect is saturated even if the content exceeds 0.50%, the content is preferably 0.50% or less.

The remainder other than the above is Fe and inevitable impurities. Inevitable impurities include, for example, Sb, Sn, Zn, Co, etc. The allowable ranges of these contents are Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0. 01% or less, Co: 0.1% or less. Moreover, in this invention, even if it contains Ta, Mg, Ca, Zr, and REM within the range of a normal steel composition, the effect is not impaired.

Next, the microstructure of the high-strength cold-rolled steel sheet of the present invention and the reason for limitation will be described.

The microstructure of the high-strength cold-rolled steel sheet has a main phase of ferrite with a volume fraction of 80% or more, martensite with a volume fraction of 3-15%, and pearlite with a volume fraction of 0.5-10%. . Here, the volume fraction is the volume fraction with respect to the entire steel sheet.

When the volume fraction of ferrite is less than 80%, since there are many hard second phases, there are many places where the hardness difference from soft ferrite is large, and stretch flangeability is deteriorated. Therefore, the volume fraction of the ferrite phase is 80% or more. Preferably, it is 83% or more.

If the volume fraction of martensite is less than 3%, the effect of increasing the strength is small, sufficient elongation cannot be obtained, and YR exceeds 70%. Therefore, the volume fraction of martensite is 3% or more. On the other hand, when the volume fraction of martensite exceeds 15%, the stretch flangeability is remarkably lowered. Therefore, the volume fraction of martensite is made 15% or less. Preferably it is 12% or less.

When the pearlite volume fraction is less than 0.5%, the effect of increasing the strength is small. Therefore, in order to achieve a good balance between strength and formability, the pearlite volume fraction needs to be 0.5% or more. On the other hand, when the pearlite volume fraction exceeds 10%, YR becomes remarkably high, so the pearlite volume fraction is 10% or less. Preferably it is 8% or less.

The remaining structure other than ferrite, martensite and pearlite may be a structure containing one or more of bainite, residual γ, spherical cementite, etc., but from the viewpoint of stretch flangeability, other than ferrite, martensite and pearlite. The volume fraction of the remaining tissue is preferably 5% or less.

The average crystal grain size of martensite and pearlite is not particularly limited, but if the average crystal grain size is fine, the connection of the generated voids is suppressed and the stretch flangeability is improved. Therefore, the average crystal grain size of martensite is preferably 10 μm or less, and the average crystal grain size of pearlite is preferably 5 μm or less.

Next, a method for producing the high-strength cold-rolled steel sheet of the present invention will be described.

The steel slab having the above component composition (chemical component) is hot-rolled and pickled, then cold-rolled, and then annealed. This will be described in detail below.

The steel slab is preferably manufactured by a continuous casting method in order to prevent macro segregation of components, but can also be manufactured by an ingot forming method or a thin slab casting method.

[Hot rolling process]
The steel slab is subjected to rough rolling and finish rolling to obtain hot rolled sheets. It is preferable to heat the slab before rolling. When the slab heating temperature is less than 1100 ° C., the rolling load is increased, the productivity is lowered, and when it exceeds 1300 ° C., the heating cost is increased. Therefore, the slab heating temperature is preferably 1100 to 1300 ° C. The slab once cooled to room temperature may be reheated in a heating furnace, or the steel slab may be charged into the heating furnace as it is without being cooled to room temperature and reheated. Moreover, you may apply energy saving processes, such as direct feed rolling and direct rolling which carry out hot rolling immediately after heat-retaining steel slab, or hot-rolling as it is after casting, without performing slab heating.

If the finish rolling finish temperature is too low, the structure non-uniformity in the steel sheet and the material anisotropy increase, and the elongation and stretch flangeability after annealing deteriorate, so the hot rolling is finished in the austenite single phase region. Is preferred. Therefore, the finish rolling end temperature is preferably 830 ° C. or higher. On the other hand, when the finish rolling finish temperature exceeds 950 ° C., the hot-rolled structure becomes coarse, and the characteristics after annealing deteriorate. Therefore, the finish rolling finish temperature is preferably 830 to 950 ° C.

The subsequent cooling method is not particularly limited. The coiling temperature is not limited, but when the coiling temperature exceeds 700 ° C., coarse pearlite is remarkably formed, which affects the formability of the steel sheet after annealing. preferable. More preferably, it is 650 degrees C or less. The lower limit of the coiling temperature is not particularly limited, but if the coiling temperature becomes too low, hard bainite and martensite are excessively generated and the cold rolling load increases, so 400 ° C or higher is preferable.

[Pickling process]
After the hot rolling step, it is preferable to carry out an acidic step and remove the scale of the hot rolled sheet surface layer. The pickling step is not particularly limited, and may be performed according to a conventional method.

[Cold rolling process]
The hot-rolled sheet after pickling is subjected to a cold rolling process for rolling into a cold-rolled sheet having a predetermined thickness. A cold rolling process is not specifically limited, What is necessary is just to implement by a conventional method.

[Annealing process]
The annealing process is carried out in order to advance recrystallization and to form a second phase structure of martensite and pearlite for high strength. For this purpose, the annealing step is performed by heating to a temperature range of Ac ₁ to Ac ₃ points (also referred to as a soaking temperature or holding temperature) and then holding the temperature from the soaking temperature to a temperature of 500 to 550 ° C. at 1 ° C./s. Cool at an average cooling rate of ˜25 ° C./s, and then cool at an average cooling rate of 5 ° C./s or less.

Soaking temperature (holding temperature): Ac ₁ to Ac ₃ points Since austenite is not generated when the soaking temperature is less than Ac ₁ point, then martensite cannot be obtained, and if it exceeds Ac ₃ points, coarse austenite is obtained. Therefore, the predetermined martensite and pearlite volume fraction cannot be obtained thereafter. Therefore, the soaking temperature is in the range of Ac ₁ to Ac ₃ points. Ac ₃ points-100 ° C. to Ac ₃ points are preferred. If the heating rate up to the soaking temperature is too high, recrystallization will not proceed easily. If the heating rate is too low, the ferrite grains will become coarse and the strength will decrease, so the average heating rate up to the soaking temperature will be 3-30 ° C. / S is preferable. Further, the soaking time is preferably set to 30 s to 300 s (seconds) in order to sufficiently advance the recrystallization and partially austenite transformation.

Cooling from a soaking temperature to a temperature of 500 to 550 ° C at an average cooling rate of 1 ° C / s to 25 ° C / s (primary cooling)
The microstructure of the steel sheet finally obtained after the annealing process is controlled so that the volume fraction of ferrite is 80% or more, the volume fraction of martensite is 3 to 15%, and the volume fraction of pearlite is 0.5 to 10%. Therefore, primary cooling is performed by cooling from the soaking temperature to a temperature of 500 to 550 ° C. as a primary cooling temperature at an average cooling rate of 1 ° C./s to 25 ° C./s.

When the primary cooling temperature exceeds 550 ° C, martensite is not sufficiently formed, and when it is less than 500 ° C, pearlite is not sufficiently formed. By defining the primary cooling temperature in the range of 500 to 550 ° C., both martensite and pearlite can be formed and the volume fraction thereof can be adjusted. When the average cooling rate to the temperature range of 500 to 550 ° C is less than 1 ° C / s, martensite does not form 3% or more in volume fraction, and when the average cooling rate exceeds 25 ° C / s, pearlite has volume fraction. And not 0.5% or more. Therefore, the average cooling rate from the soaking temperature to the temperature range of 500 to 550 ° C. needs to be 1 ° C./s to 25 ° C./s. A preferable average cooling rate is 15 ° C./s or less.

Cool from the primary cooling temperature at an average cooling rate of 5 ° C / s or less (secondary cooling)
After cooling to the primary cooling temperature (500 to 550 ° C.), secondary cooling is performed at an average cooling rate of 5 ° C./s or less. When the average cooling rate of the secondary cooling exceeds 5 ° C./s, the volume fraction of martensite increases and the predetermined martensite and pearlite volume fraction cannot be obtained, so the average cooling rate from the primary cooling temperature Is 5 ° C./s or less. Preferably it is 3 degrees C / s or less.

Also, temper rolling may be performed after annealing. A preferable range of the elongation rate is 0.3% to 2.0%.

If within the scope of the present invention, in the annealing step, hot dip galvanization may be performed after the primary cooling to obtain a hot dip galvanized steel sheet. It may be a steel plate.

Hereinafter, examples of the present invention will be described.

However, the present invention is not originally limited by the following examples, and can be implemented with appropriate modifications within a range that can be adapted to the gist of the present invention. Included in the scope.

Steels of the chemical components shown in Table 1 (remainder components: Fe and inevitable impurities) are melted and cast to produce 230 mm thick slabs, which are shown in Table 2 after hot rolling, pickling and cold rolling. Annealing was performed under manufacturing conditions, and then skin pass rolling (temper rolling) was performed. In addition, the heating temperature in the case of hot rolling was 1200 degreeC, the finish rolling completion temperature was 890 degreeC, the coiling temperature was 600 degreeC, and the hot rolled sheet (sheet thickness 3.2mm) was manufactured.

Next, pickling and cold rolling were performed to produce a cold-rolled sheet (sheet thickness: 1.4 mm), followed by annealing and temper rolling (elongation rate: 0.7%). The cooling rate 1 in Table 2 indicates the average cooling rate from the soaking temperature during annealing to the primary cooling temperature, and the cooling rate 2 indicates the average cooling rate from the primary cooling temperature to room temperature. The average heating rate up to the soaking temperature was 10 ° C./s.

From the manufactured steel sheet, a JIS No. 5 tensile test piece was sampled so that the direction perpendicular to the rolling direction was the longitudinal direction (tensile direction), and was subjected to a tensile test (JIS Z2241 (1998)) to yield strength (YS), tensile strength ( TS), total elongation (EL), and yield ratio (YR) were measured. Steel plates having good elongation with EL of 29.0% or more and steel plates with low yield ratio of YR of 70% or less were used.

Stretch flangeability is based on the Japan Iron and Steel Federation standard (JFS T1001 (1996)), after punching a hole with a diameter of 10mmφ at a clearance of 12.5%, and setting the burr on the die side after setting The hole expansion rate (λ) was measured by conducting a hole expansion test with a 60 ° conical punch. λ (%) was 65% or more to make a steel sheet having good stretch flangeability.

For the microstructure of the steel sheet, volume fractions of ferrite, martensite and pearlite were determined by the following method.

The microstructure of the steel sheet corrodes the cross section in the rolling direction of the steel sheet (depth position at 1/4 of the plate thickness) using 3% Nital reagent (3% nitric acid + ethanol), and is observed with an optical microscope at 500 to 1000 times. The volume fraction of ferrite, the volume fraction of martensite, and the volume fraction of pearlite were quantified using tissue photographs observed and photographed with an electron microscope (scanning type and transmission type) of 1000 to 100,000 times.

Each 12 visual fields were observed, and the area ratio was measured by the point count method (based on ASTM E562-83 (1988)), and the area ratio was defined as the volume fraction. Ferrite is a region with a slightly black contrast, and martensite has a white contrast. Pearlite is a layered structure in which plate-like ferrite and cementite are alternately arranged.

For structures other than ferrite, martensite, and pearlite, bainite is a plate-like bainite having a higher dislocation density than polygonal ferrite in the observation using the above-described optical microscope or electron microscope (scanning type and transmission type). It is a structure containing tick ferrite and cementite, and spherical cementite is cementite having a spheroidized shape.

As for the presence or absence of retained austenite, the surface polished to 1/4 thickness from the surface layer was used, and the Kα ray of Mo was used as the radiation source at an acceleration voltage of 50 keV by an X-ray diffraction method (apparatus: RINT2200 manufactured by Rigaku). Measure the integrated intensity of the X-ray diffraction lines of the {200}, {211}, {220}, and {200}, {220}, and {311} planes of austenite. Was used to determine the volume fraction of retained austenite from the calculation formula described in Non-Patent Document 1, and the presence or absence of retained austenite was determined.

Table 2 shows the measurement results of tensile properties, stretch flangeability, and steel sheet structure.

From the results shown in Table 2, all of the examples of the present invention have a steel sheet structure in which the volume fraction of ferrite is 80% or more, the volume fraction of martensite is 3 to 15%, and the volume fraction of pearlite is 0.5 to 10%. As a result, good formability with a tensile strength of 590 MPa or more and a yield ratio of 70% or less, and an elongation of 29.0% or more and a hole expansion ratio of 65% or more can be obtained. ing. On the other hand, in the comparative example, the steel sheet structure does not satisfy the scope of the present invention, and as a result, at least one characteristic of tensile strength, yield ratio, elongation, and hole expansion ratio is inferior.

According to the present invention, the volume fraction has a composite structure including ferrite of 80% or more, martensite of 3 to 15%, and pearlite of 0.5 to 10%, a tensile strength of 590 MPa or more, and a yield ratio of 70% or less. Further, a high strength cold-rolled steel sheet having a low yield ratio and excellent elongation and stretch flangeability having an elongation of 29.0% or more and a hole expansion ratio of 65% or more can be obtained.

Claims (2)

1. The chemical composition of the steel sheet is mass%, C: 0.05 to 0.13%, Si: 0.6 to 1.2%, Mn: 1.6 to 2.4%, P: 0.10% or less , S: 0.0050% or less, Al: 0.01 to 0.10%, N: less than 0.0050%, the balance is made of Fe and inevitable impurities, and the microstructure of the steel sheet is volume fraction Characterized by having a composite structure containing 80% or more of ferrite, 3 to 15% of martensite, and 0.5 to 10% of pearlite, a yield ratio of 70% or less, and a tensile strength of 590 MPa or more. Low yield ratio high strength cold-rolled steel sheet with excellent elongation and stretch flangeability.
2. The steel slab having the chemical composition according to claim 1 is hot-rolled and cold-rolled, and then heated and held in a temperature range of Ac ₁ to Ac ₃ points, and then from the holding temperature to 500 to 550 Cooling at an average cooling rate of 1 ° C./s to 25 ° C./s to a temperature of 1 ° C., followed by cooling at an average cooling rate of 5 ° C./s or less, and low yield with excellent elongation and stretch flangeability A method for producing a high strength cold-rolled steel sheet.