KR20150004430A - 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 excellent in elongation and elongation flangeability and having a low resistance ratio and a method for producing the same. The steel sheet according to claim 1, wherein the chemical composition of the steel sheet is 0.05 to 0.13% of C, 0.6 to 1.2% of Si, 1.6 to 2.4% of Mn, 0.10% or less of P, 0.0050% or less of S, N: less than 0.0050%, the balance being Fe and inevitable impurities, the microstructure of the steel sheet having a volume fraction of not less than 80% ferrite, martensite not less than 3 to 15%, pearlite not less than 0.5 to 10 %, A yield ratio of not more than 70%, and a tensile strength of 590 MPa or more, which is excellent in elongation and elongation flangeability.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-resistance high-strength cold-rolled steel sheet having excellent elongation and elongation flangeability,

The present invention relates to a high-strength cold-rolled steel sheet having excellent resistance to elongation and stretch flanging which is suitable as a member of a suspension component, a structural component and the like of an automobile to be used after being press-processed. The yield ratio YR is a value representing the ratio of the yield strength YS to the tensile strength TS and expressed by YR (%) = (YS / TS) x 100.

BACKGROUND ART In recent years, regulations for CO ₂ emissions have been strictly regulated due to an increase in environmental problems. In the field of automobiles, improvement of fuel efficiency by weight reduction of the vehicle body has become a big problem. For this reason, thinner steel sheets for automobile parts are being applied, and steel sheets having a TS of 590 MPa or more have been applied to parts having a steel sheet having a TS of 270 to 440 MPa.

A steel sheet having a TS of 590 MPa or more is required to have excellent elongation and stretch flangeability (hole expandability) from the viewpoint of formability. Further, after being press-processed, it is mounted by arc welding, spot welding, or the like, and is required to have high dimensional accuracy at the time of mounting in order to be modularized. From this point, it is necessary to prevent springback or the like from happening after machining, and it is necessary to have a low resistance before machining.

A dual-phase steel (DP steel) of ferrite-martensite structure is known as a high-strength thin steel plate having low moldability and high strength combined with resistance. Composite structure steel in which martensite is dispersed with ferrite as the main phase is low in resistance, high in TS, and excellent in elongation. However, since stress is concentrated on the interface between the ferrite and the martensite, cracks are likely to occur, which has the drawback that the hole expandability is inferior. Thus, Patent Document 1 discloses a high-strength steel sheet for automobiles that both collision safety and formability are both achieved by controlling the percentage of the whole structure of ferrite and martensite and the average crystal grain size.

Patent Document 2 discloses a high strength steel sheet in which elongation and elongation flangeability are improved by controlling the percentage of the fine ferrite having an average grain size of 3 m or less and the total structure of martensite having an average grain size of 6 m or less. Also, Patent Document 3 discloses a DP steel sheet in which fine inclusions are dispersed in a steel sheet by containing Ce or La in the steel sheet component, thereby improving elongation flangeability.

A technique for containing bainite or retained austenite in a steel sheet structure for improving the formability is also known. For example, Patent Document 4 discloses a composite structure composed of ferrite, retained austenite, and the remaining bainite and martensite, and defines an aspect ratio and an average grain size of martensite and retained austenite, A composite structure cold rolled steel sheet excellent in elongation and stretch flangeability by defining the number of martensite and retained austenite is disclosed.

Non-Patent Document 1 is described in the embodiment.

Japanese Patent No. 3936440 Japanese Patent Application Laid-Open No. 2008-297609 Japanese Patent Application Laid-Open No. 2009-299149 Japanese Patent No. 4288364

&Quot; X-ray Diffraction Handbook ", Rikaku Electric Co., Ltd., 2000, p. 26, 62 - 64

However, Patent Document 1 specifies the average crystal grain size of ferrite and martensite, but does not secure sufficient hole expandability for press forming. Patent Document 2 has insufficient elongation because the volume fraction of martensite is remarkably large. Patent Document 3 has a high productivity because of the addition of Ce and La, and the productivity is low due to large material variation in order to control the size of inclusions.

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

As described above, it is difficult to secure elongation and elongation flangeability in a low-YR high-strength steel sheet, and a cold-rolled steel sheet satisfying these characteristics (yield ratio, strength, elongation, stretch flangeability) has not been developed.

Accordingly, it is an object of the present invention to provide a high-strength cold-rolled steel sheet having excellent elongation and elongation flangeability and having a low resistance, and a method of manufacturing the same.

As a result of intensive investigations, the inventors of the present invention have found that it is possible to obtain a cold rolled steel sheet excellent in elongation and stretch flangeability with low YR and high strength by controlling the volume fraction of ferrite, martensite and pearlite by adding Si in an appropriate amount I found out.

Conventionally, it has been considered that pearlite deteriorates stretch flangeability. However, the present inventors have found that when a hardness difference between a hard phase and a hard phase is reduced by adding a proper amount of Si as a steel sheet component and strengthening the ferrite in the steel sheet structure in which ferrite, martensite and pearlite exist, And martensite, and the occurrence from the interface with pearlite is suppressed. In addition, even if the volume fraction of martensite is reduced as compared with that of the conventional DP steel, strength enhancement of ferrite due to Si can be utilized and strength can be ensured by the presence of pearlite. It was also found that by decreasing the volume fraction of martensite, local elongation was improved and elongation and stretch flangeability were improved. By adjusting the volume fraction of martensite and pearlite, it is possible to obtain a low-resistance, high-strength cold rolled steel sheet having a tensile strength of 590 MPa or more while ensuring low YR.

Concretely, as a steel sheet component, 0.6-1.2% of Si is added, and the steel sheet structure is controlled in the range of 80% or more as the volume fraction of the main phase ferrite, 3 to 15% of the martensite and 0.5 to 10% of the pearlite It is possible to obtain a high strength cold rolled steel sheet excellent in elongation and stretch flangeability with a yield ratio of 70% or less and a tensile strength of 590 MPa or more.

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

(1) The steel sheet according to any one of (1) to (3), wherein the chemical composition of the steel sheet is 0.05 to 0.13% of C, 0.6 to 1.2% of Si, 1.6 to 2.4% of Mn, 0.10% and N: less than 0.0050%, and the balance of Fe and inevitable impurities. The microstructure of the steel sheet has a volume fraction of not less than 80% ferrite, martensite not less than 3-15% And having a yield ratio of 70% or less and a tensile strength of 590 MPa or more, which is excellent in elongation and stretch flangeability.

(2) A steel slab having the chemical composition described in the above item (1) is hot rolled or cold rolled, heated and held at a temperature in the range of Ac ₁ to Ac ₃ , And cooling the steel sheet at an average cooling rate of 1 占 폚 / s to 25 占 폚 / s up to a temperature of 5 占 폚 / s or lower, and thereafter cooling the steel sheet at an average cooling rate of 5 占 폚 / s or less. ≪ / RTI >

According to the present invention, by controlling the steel sheet component, the annealing temperature and the cooling condition after annealing, it is possible to provide a steel sheet having a composite structure containing at least 80% of ferrite, 3 to 15% of martensite and 0.5 to 10% of pearlite, It is possible to obtain a high strength cold rolled steel sheet having a tensile strength of 590 MPa or more, a yield ratio of 70% or less, a elongation of 29.0% or more and a hole expansion ratio of 65% or more and excellent elongation and stretch flangeability.

Hereinafter, the present invention will be described in detail.

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, the "%" of the chemical component means% by mass.

C: 0.05 to 0.13%

C is an effective element for increasing the strength of the steel sheet and contributes to enhancement of strength by forming the second phase of pearlite and martensite. In order to obtain this effect, addition of 0.05% or more is required. It is preferably at least 0.08%. On the other hand, if it is added in excess, the spot weldability decreases, so the upper limit is set to 0.13%.

Si: 0.6 to 1.2%

Si is an element contributing to high strength and has a high work hardening ability, so that the decrease in elongation against the increase in strength is relatively small and contributes to improvement in strength-elongation balance. In addition, since the hardness difference from the hard second phase is reduced by the solid solution strengthening of the ferrite phase, it also contributes to the improvement of the stretch flangeability. By appropriately adding Si, generation of voids from the interface between the ferrite phase and the pearlite phase can be suppressed. However, in order to obtain the effect, it is necessary to contain 0.6% or more of Si. From the viewpoints of elongation and stretch flangeability, the upper limit is not particularly defined, but if it is added in an amount exceeding 1.2%, the chemical conversion property is deteriorated. Therefore, the content thereof should be 1.2% or less. And preferably 1.0% or less.

Mn: 1.6 to 2.4%

Mn is an element contributing to the enhancement of strength by solid solution strengthening and martensite generation. In order to obtain this effect, it is necessary to contain Mn of 1.6% or more. On the other hand, when it is contained in an excessive amount, the lowering of the moldability becomes remarkable, so the content thereof is set to 2.4% or less. Preferably 2.2% or less.

P: not more than 0.10%

P contributes to enhancement of strength by solid solution strengthening. However, when added in excess, segregation to the grain boundary becomes remarkable to cause embrittlement of the grain boundaries or deterioration of weldability. Therefore, the content of P is made 0.10% or less. It is preferably not more than 0.05%.

S: not more than 0.0050%

When the content of S is large, a large amount of sulfides such as MnS is generated and the local elongation represented by stretch flangeability is lowered. Therefore, the upper limit of the content is set to 0.0050%. And preferably 0.0030% or less. Although the lower limit is not particularly limited, since the steelmaking cost increases with the extremely low sodalization, it is preferable that the steel is contained in an amount of 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. However, when the content exceeds 0.10%, the effect is saturated and the content thereof is made 0.10% or less. It is preferably not more than 0.05%.

N: less than 0.0050%

Since N forms a coarse nitride to deteriorate stretch flangeability, it is necessary to suppress the content. When N is 0.0050% or more, this tendency becomes remarkable, so that the content of N is made less than 0.0050%.

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

V: not more than 0.10%

V can contribute to the increase in strength by forming fine carbonitride. In order to exhibit such an effect, the addition amount of V is preferably 0.01% or more. On the other hand, if the content exceeds 0.10%, the effect of increasing the strength is small and the alloy cost is rather increased. Therefore, the content thereof is preferably 0.10% or less.

Ti: not more than 0.10%

Since Ti can contribute to an increase in strength by forming fine carbonitride in the same manner as V, Ti can be added as needed. In order to exhibit such an effect, the content of Ti is preferably 0.005% or more. On the other hand, when Ti is added in a large amount, the YR remarkably increases. Therefore, the content thereof is preferably 0.10% or less.

Nb: not more than 0.10%

Since Nb can also contribute to the increase in strength by forming fine carbonitride in the same manner as V, it can be added as needed. In order to exhibit such an effect, the content of Nb is preferably 0.005% or more. On the other hand, when Nb is added in a large amount, the YR remarkably increases. Therefore, the content thereof is preferably 0.10% or less.

Cr: 0.50% or less

Cr is an element contributing to the enhancement of strength by improving quenching and generating the second phase, and can be added as needed. In order to exhibit this effect, it is preferable to contain 0.10% or more. On the other hand, when the content exceeds 0.50%, the effect is saturated, and therefore, the content thereof is preferably 0.50% or less.

Mo: 0.50% or less

Mo is an element which contributes to the enhancement of strength by producing the second phase by improving the quenching property and contributes to the enhancement of strength by producing some carbides, and can be added as needed. In order to exhibit these effects, it is preferable to contain 0.05% or more. If the content is more than 0.50%, the effect is saturated, and therefore the content thereof is preferably 0.50% or less.

Cu: not more than 0.50%

Cu contributes to enhancement of strength by solid solution strengthening, and further contributes to enhancement of strength by generating second phase by improving quenching, and can be added as needed. In order to exhibit these effects, it is preferable to contain 0.05% or more. On the other hand, if it is contained in an amount exceeding 0.50%, the effect becomes saturated and surface defects attributable to Cu tend to occur. Therefore, the content thereof is preferably 0.50% or less.

Ni: not more than 0.50%

Like Ni, Ni contributes to enhancement of strength by solid solution strengthening. Further, Ni contributes to enhancement of strength by forming second phase by improving quenching, and can be added as needed. In order to exhibit these effects, it is preferable to contain 0.05% or more. In addition, when Cu is added at the same time, it is effective at the time of adding Cu because of the effect of suppressing surface defects caused by Cu. On the other hand, when the content exceeds 0.50%, the effect is saturated, and therefore, the content thereof is preferably 0.50% or less.

The remainder other than the above are Fe and inevitable impurities. Sb: not more than 0.01%, Sn: not more than 0.1%, Zn: not more than 0.01%, Co: not more than 0.01%, and the like. Examples of the inevitable impurities include Sb, Sn, Zn, 0.1% or less. In the present invention, even if Ta, Mg, Ca, Zr and REM are contained in the range of ordinary steel composition, the effect is not inhibited.

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

The microstructure of the high-strength cold-rolled steel sheet has a volume fraction of not less than 80% as a main phase, a volume fraction of martensite of 3 to 15%, and a volume fraction of pearlite of 0.5 to 10%. Here, the volume fraction is the volume fraction of the entire steel sheet.

If the volume fraction of ferrite is less than 80%, a large number of hard second phases exist, so that there are many points having a large difference in hardness from the soft ferrite, and the stretch flangeability is deteriorated. Therefore, the volume fraction of the ferrite phase is 80% or more. It is preferably at least 83%.

When the volume fraction of martensite is less than 3%, the effect of increasing the strength is small and sufficient elongation is not obtained, and YR is more than 70%. Therefore, the volume fraction of the martensite is 3% or more. On the other hand, when the volume fraction of martensite exceeds 15%, the stretch flangeability is remarkably lowered, so that the volume fraction of martensite is 15% or less. It is preferably 12% or less.

When the volume fraction of pearlite is less than 0.5%, the effect of increasing the strength is small. Therefore, in order to balance the strength and formability, the volume fraction of pearlite should be 0.5% or more. On the other hand, if the volume fraction of pearlite exceeds 10%, the volume fraction of pearlite becomes 10% or less because YR becomes remarkably high. And preferably not more than 8%.

The residual structure other than ferrite, martensite and perlite may be a structure containing one or more species of bainite, residual?, Spherical cementite, and the like, but from the viewpoint of stretch flangeability, ferrite, martensite, It is preferable that the volume fraction of the remaining portion other than pearlite is 5% or less.

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

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

A steel slab having the above composition (chemical composition) is subjected to hot rolling and pickling, followed by cold rolling, and then annealing is performed. This will be described in detail below.

Steel slabs are preferably produced by the continuous casting method to prevent macro segregation of the components, but they can also be manufactured by the roughing method and the thin slab casting method.

[Hot rolling process]

The steel slab is subjected to coarse rolling and finish rolling to obtain a hot rolled steel sheet. It is preferable to heat the slab before rolling. When the slab heating temperature is lower than 1100 ° C, the rolling load is increased to lower the productivity. When the slab heating temperature exceeds 1300 ° C, the heating cost is increased. Therefore, the slab heating temperature is preferably 1100 to 1300 ° C. The slabs cooled to room temperature may be reheated in a heating furnace or the steel slabs may be reheated by charging them into a heating furnace without cooling them to room temperature. In addition, an energy saving process such as direct rolling or direct rolling in which the steel slab is hot-rolled immediately after the steel slab is heated, or hot-rolled as it is after casting may be applied.

If the finish rolling finish temperature is too low, the irregularity of the structure in the steel sheet and the anisotropy of the material become large, and the elongation and the stretch flangeability after annealing deteriorate. Therefore, it is preferable to finish the hot rolling in a single phase of austenite. Therefore, the finishing rolling finishing temperature is preferably 830 캜 or higher. On the other hand, if the finishing rolling finish temperature exceeds 950 DEG C, the hot-rolled structure becomes coarse and the properties after annealing deteriorate. Therefore, the finishing rolling finishing temperature is preferably 830 to 950 캜.

Thereafter, the cooling method is not particularly limited. The coiling temperature is not limited. However, since coarse pearlite is remarkably formed when the coiling temperature exceeds 700 DEG C, the coiling temperature is preferably 700 DEG C or less in that it affects the formability of the steel sheet after annealing. More preferably, it is 650 DEG C or less. The lower limit of the coiling temperature is not particularly limited. However, when the coiling temperature becomes too low, the hard bainite or martensite is excessively generated and the cold rolling load is increased.

[Pickling process]

It is preferable to carry out an acidic step after the hot rolling step to remove the scale of the surface layer of the hot rolled sheet. The pickling process is not particularly limited and may be carried out according to a conventional method.

[Cold Rolling Process]

The hot rolled steel sheet after pickling is subjected to a cold rolling step of rolling the cold rolled steel sheet to a predetermined thickness. The cold rolling process is not particularly limited and may be carried out by a conventional method.

[Annealing Process]

The annealing process is carried out to promote recrystallization and to form a second phase structure of martensite and pearlite for high strength. Therefore, after the annealing process is heated and maintained at a temperature range (also referred to as a crack temperature or a holding temperature) of the Ac ₁ to Ac ₃ points, the temperature is raised from 500 ° C to 550 ° C at a rate of 1 ° C / s to 25 ° C / s and then cooled to an average cooling rate of 5 DEG C / s or less.

Crack temperature (holding temperature): Ac ₁ ~ Ac ₃ points

When the crack temperature is lower than Ac ₁ point, no austenite is produced. After that, martensite can not be obtained. When Ac ₃ point is higher than the Ac ₃ point, coarse austenite is formed. Can not be obtained. Therefore, the crack temperature is set in the range of Ac ₁ to Ac ₃ . And preferably Ac ₃ point -100 ° C to Ac ₃ point. If the heating rate to the cracking temperature is too fast, the recrystallization hardly proceeds. If the heating rate is too slow, the ferrite lips become coarse and the strength decreases. Therefore, the average heating rate up to the cracking temperature is in the range of 3 to 30 DEG C / s . It is preferable that the cracking time is 30 s to 300 s (sec) in order to progress the recrystallization and to sufficiently make some austenite transformation.

The cracking temperature is cooled to a temperature of 500 to 550 ° C at an average cooling rate of 1 ° C / s to 25 ° C / s (primary cooling).

In order to control the microstructure of the finally obtained steel sheet after the annealing process so as to control the volume fraction of ferrite to 80% or more, the volume fraction of martensite to 3 to 15% and the volume fraction of pearlite to 0.5 to 10% , And primary cooling is performed by cooling the primary cooling temperature to a temperature of 500 to 550 占 폚 at an average cooling rate of 1 占 폚 / s to 25 占 폚 / s.

When the primary cooling temperature exceeds 550 캜, martensite is not sufficiently formed, and when the primary cooling temperature is lower than 500 캜, pearlite is not sufficiently formed. By defining the primary cooling temperature in the range of 500 to 550 占 폚, both of the martensite and pearlite can be formed and the volume fraction thereof can be adjusted. When the average cooling rate to a temperature range of 500 to 550 ° C is less than 1 ° C / s, martensite is not formed in a volume fraction of 3% or more, and when the average cooling rate exceeds 25 ° C / s, pearlite has a volume fraction No more than 0.5% is formed. Therefore, the average cooling rate from the crack temperature to the temperature range of 500 to 550 占 폚 is required to be 1 占 폚 / s to 25 占 폚 / s. The preferred average cooling rate is 15 占 폚 / s or less.

From the primary cooling temperature to an average cooling rate of 5 DEG C / s or less (secondary cooling).

After cooling to the primary cooling temperature (500 to 550 ° C), secondary cooling is performed by cooling at an average cooling rate of 5 ° C / s or less. When the average cooling rate of the secondary cooling exceeds 5 DEG C / s, the volume fraction of martensite increases and the volume fraction of the predetermined martensite and pearlite can not be obtained. Therefore, the average cooling rate from the primary cooling temperature The speed shall be 5 ° C / s or less. Preferably 3 DEG C / s or less.

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

In addition, as long as it is within the range of the present invention, hot-dip galvanized steel sheet may be formed by performing hot-dip galvanizing after the first cooling in the annealing step, or galvannealed steel sheet may be subjected to alloying treatment after hot- do.

Example

Hereinafter, embodiments of the present invention will be described.

It should be noted that the present invention is not limited to the following embodiments, but may be practiced with appropriate modifications within the scope of the present invention, and these are all included in the technical scope of the present invention.

A steel having the chemical composition shown in Table 1 (the remainder component: Fe and inevitable impurities) was cast and cast to prepare a slab having a thickness of 230 mm. After hot rolling, pickling and cold rolling, annealing was performed under the manufacturing conditions shown in Table 2 And then subjected to skin pass rolling (temper rolling). A hot-rolled sheet (thickness of 3.2 mm) was produced at a hot-rolling temperature of 1200 캜, a finish rolling finish temperature of 890 캜, and a coiling temperature of 600 캜.

Then, pickling and cold rolling were carried out to produce cold-rolled sheets (plate thickness: 1.4 mm), followed by annealing and temper rolling (elongation 0.7%). The cold 1 in Table 2 represents the average cooling rate from the cracking temperature to the first cooling temperature at the time of annealing and the cold rate 2 represents the average cooling rate from the first cooling temperature to the room temperature. In addition, the average heating rate up to the crack temperature was set to 10 ° C / s.

The tensile strength (TS), tensile strength (TS) and tensile strength (JIS) of the JIS No. 5 tensile test specimen were taken from the steel sheet so that the direction perpendicular to the rolling direction was the longitudinal direction The total elongation (EL) and yield ratio (YR) were measured. A steel sheet having a good elongation of EL of 29.0% or more, and a steel sheet having a YR of 70% or less.

With regard to the elongation flangeability, holes were drilled at a clearance of 12.5% in a diameter of 10 mm phi according to the Japanese Steel Association Standard (JFS T1001 (1996)), and the burrs were set on the die side, The hole expansion ratio (?) Was measured by a hole expansion test with a conical punch of a hole. ? (%) of 65% or more was used as a steel sheet having good stretch flangeability.

In the microstructure of the steel sheet, the volume fraction of ferrite, martensite and pearlite was determined by the following method.

The microstructure of the steel sheet was corroded by the use of a 3% detachment reagent (3% nitric acid + ethanol) to corrode a steel sheet in the rolling direction section (depth position at a plate thickness of 1/4) The volume fraction of ferrite, the volume fraction of martensite, and the volume fraction of pearlite were quantified by using a tissue photograph taken and observed with an electron microscope (scanning type and transmission type) at 1,000 to 100,000 times.

The area ratio was measured by the point count method (in accordance with ASTM E562-83 (1988)), and the area ratio was determined as the volume fraction. The ferrite is a region of slightly black contrast, and the martensite has a white contrast. Perlite is a layered structure in which plate-like ferrite and cementite are alternately arranged.

As for the structures other than ferrite, martensite and perlite, in the observation of the above optical microscope or electron microscope (scanning type and transmission type), the bainite has a plate-like bainite having a dislocation density higher than that of polygonal ferrite The structure containing ferrite and cementite, and the spherical cementite is cementite having spherical shape.

With respect to the presence or absence of retained austenite, the surface of the steel sheet was polished to 1/4 thickness from the surface layer, and the surface was polished by the X-ray diffraction method (RINT2200 manufactured by Rigaku) at an accelerating voltage of 50 keV, The integral intensities of the {200}, {211} and {220} planes of the ferrite of iron and the {200} planes, {220} planes and {311} planes of austenite were measured, Using the measured values, the volume fraction of the retained austenite was determined from the calculation formula described in the non-patent document 1 to determine the presence or absence of the retained austenite.

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

From the results shown in Table 2, it can be seen that all of the inventive examples 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, it was possible to obtain a moldability with a stretch ratio of 29.0% or more and a hole expanding ratio of 65% or more while securing a tensile strength of 590 MPa or more and a yield ratio of 70% or less. 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 of the tensile strength, the yield ratio, the elongation and the hole expanding ratio is inferior.

Industrial availability

According to the present invention, there is provided a steel sheet having a composite structure containing a ferrite of 80% or more, martensite of 3-15% and pearlite of 0.5-10% in volume fraction, a tensile strength of 590 MPa or more, a yield ratio of 70% % Or more, and a hole expansion ratio of 65% or more, which is excellent in elongation and extension flangeability.

Claims (2)

The steel sheet according to claim 1, wherein the chemical composition of the steel sheet is 0.05 to 0.13% of C, 0.6 to 1.2% of Si, 1.6 to 2.4% of Mn, 0.10% or less of P, 0.0050% or less of S, N: less than 0.0050%, the balance being Fe and inevitable impurities, the microstructure of the steel sheet having a volume fraction of not less than 80% ferrite, martensite not less than 3 to 15%, pearlite not less than 0.5 to 10 %, A yield ratio of 70% or less, and a tensile strength of 590 MPa or more, which is excellent in elongation and stretch flangeability. A steel slab having the chemical composition according to claim 1 is hot rolled or cold rolled and then heated and maintained at a temperature in the range of Ac ₁ to Ac ₃ and then heated to a temperature of 500 to 550 ° C A method of manufacturing a low-resistance high-strength cold-rolled steel sheet excellent in elongation and elongation flange characteristics, characterized by cooling the steel sheet at an average cooling rate of 5 ° C / s to 25 ° C / s, .