JP2010059452A - Cold-rolled steel sheet and producing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet suitable for a high strength steel sheet for an automobile, for which strict formability and dimensional precision are required, since good ductility and bending property are provided, while this sheet has high strength of ≥780 MPa and sheet thickness of ≥2.0 mm. <P>SOLUTION: The cold-rolled steel sheet has a chemical composition composed of 0.08-0.20% C, ≤1.0% Si, 1.8-3.0% Mn, ≤0.1% P, ≤0.01% S, 0.005-0.5% sol.Al, ≤0.01 N, 0.02-0.2% Ti and the balance Fe with impurities, and the steel structure has, by vol%, ≥10% ferrite, 20-70% bainite, 3-20% retained austenite, 0-20% martensite and also has a structure of ≤10 μm average grain diameter of the ferrite, ≤10 μm average grain diameter of the bainite, ≤3 μm average grain diameter of the retained austenite and ≤3 μm average grain diameter of the martensite. The steel sheet has mechanical characteristics of ≥780 MPa tensile strength TS, ≥14,000 MPa×% product of the tensile strength TS multiplied by total elongation El (TS×El value) and ≤1.5t (t: sheet thickness) the minimum bending radius in the bending test, and has ≥2.0 mm sheet thickness. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a cold-rolled steel sheet and a method for producing the same, and specifically relates to a cold-rolled steel sheet having a tensile strength of 780 MPa or more and a plate thickness of 2.0 mm or more mainly used in industrial fields such as automobiles and a method for producing the same. .

  In recent years, in the field of automotive steel sheets, the application of high-strength steel sheets having a tensile strength of 780 MPa or more has been expanding in order to improve fuel efficiency and impact resistance. In particular, in order to ensure the impact resistance characteristics that are becoming increasingly strict, a high-strength steel sheet having a thickness of 2.0 mm or more is required. And from the viewpoint of the dimensional accuracy of the member after press molding, there is a demand for a cold-rolled steel plate having excellent plate thickness accuracy.

  By the way, many proposals have been made so far for high strength steel sheets having a tensile strength of less than 780 MPa in order to improve ductility which deteriorates with increasing strength. For example, DP steel composed of two phases of ferrite and martensite, which has a reduced yield ratio and improved ductility, has been proposed. Further, a so-called TRIP steel made of ferrite, bainite, and retained austenite with improved elongation characteristics has been proposed.

  However, when the tensile strength is increased to 780 MPa or more, the deterioration of bendability becomes obvious, and it has been difficult to produce automotive steel sheets that require strict formability by extending these techniques. In particular, if the thickness is increased to ensure collision-resistant performance, bending using the same mold R as in the case of thin objects will result in harsh conditions for bending, so the mold R must be made larger than in the case of thin objects. Inevitably, this leads to problems such as a reduction in design freedom.

  On the other hand, for example, Patent Document 1 discloses a method for improving stretch flangeability by making a steel structure mainly composed of bainite, and Patent Document 2 optimizes the fraction of the ferrite phase and the low-temperature transformation phase. A method of improving elongation characteristics, stretch flange characteristics, and bending characteristics by reducing the difference in hardness is disclosed. Further, Patent Document 3 discloses that a bendability and stretch flange can be obtained by having a soft layer on the steel sheet surface layer. A method for improving the performance is disclosed.

  The technology proposed in the above publication does not give consideration to achieving both high ductility and bendability in a thick high-strength cold-rolled steel sheet having a thickness of 2.0 mm or more. For this reason, when it is applied to a thick material, the formability becomes inferior, and in order to require remarkably rapid cooling in the annealing process, the deterioration of flatness becomes remarkable in a thick steel plate having a large thermal stress and a thickness of 2.0 mm or more. Or For this reason, it was difficult to apply to a thick high-strength cold-rolled steel sheet having a thickness of 2.0 mm or more.

On the other hand, as a method for producing a high-strength cold-rolled steel sheet in consideration of sheet thickness, Patent Document 4 discloses a method for producing the sheet under conditions obtained from a relational expression of sheet thickness, component content, temperature condition, and target tensile strength. Has been.
JP-A-7-188767 JP-A-2005-171321 JP 2005-273002 A JP 2003-277832 A

  However, the invention disclosed in Patent Document 4 only considers the tensile strength, and does not take into consideration the ductility and bendability required for a thick high-strength cold-rolled steel sheet. Was insufficient.

  For these reasons, the present invention aims to solve the problems of the conventional techniques as described above, and has a high tensile strength of 780 MPa or more and a thick plate thickness of 2.0 mm or more, while being good. It is to provide a cold-rolled steel sheet having a sufficient elongation and bendability and a method for producing the same.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a high tensile strength of 780 MPa or more and a thick plate thickness of 2.0 mm or more have a good elongation and bendability. The rolled steel sheet can be obtained by adjusting the chemical composition and steel structure of the steel sheet, even under the production conditions where it is easy to ensure flatness without using rapid quenching, and the chemical composition and cold rolling conditions of the steel sheet. The inventors have found that such a steel structure can be realized by adjusting the annealing conditions, and the present invention has been completed based on this finding.

  In the present invention, C: 0.08% or more and 0.20% or less (hereinafter, unless otherwise specified, “%” regarding chemical composition means “mass%”), Si: 1.0% or less, Mn: 1.8% or more and 3.0% or less, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.005% or more and 0.5% or less, N: 0.01% or less, and Ti: 0.02% or more and 0.2% or less, having a chemical composition consisting of Fe and impurities, and volume %, Ferrite: 10% or more, bainite: 20% or more and 70% or less, retained austenite: 3% or more and 20% or less and martensite: 0% or more and 20% or less, and the average grain size of ferrite is 10 μm or less. It has a steel structure having an average particle size of bainite of 10 μm or less, an average particle size of retained austenite of 3 μm or less, and an average particle size of martensite of 3 μm or less, a tensile strength (TS) of 780 MPa or more, and a tensile strength (TS). And the product of total elongation (El) (TS × El value) is 14000 MPa ·% or more and the minimum bending radius in the bending test is 1.5 t or less (t: plate thickness) A cold-rolled steel sheet having characteristics and having a thickness of 2.0 mm or more.

  The cold-rolled steel sheet according to the present invention has a chemical composition of Cr: 1% or less, Mo: 1% or less, Nb: 0.1% or less, V: 1% or less, Cu: It is preferable to contain one or more selected from the group consisting of 1% or less, Ni: 1% or less, and B: 0.005% or less.

These cold-rolled steel sheets according to the present invention preferably have a plating layer on the surface.
From another viewpoint, the present invention provides a cold-rolled steel sheet having a thickness of 2.0 mm or more by subjecting the hot-rolled steel sheet having the above-described chemical composition to cold rolling with a reduction rate of 30% to 60%. The rolled steel sheet is allowed to stay within a temperature range of Ac ₃ points or more (Ac ₃ points + 50 ° C.) within 240 seconds, and then at a temperature range of 680 ° C. or more and 750 ° C. or less at an average cooling rate of 1 ° C./second to 10 ° C./second. And cooling to a temperature range of 400 ° C. or less at an average cooling rate of 20 ° C./second or more and 50 ° C./second or less.

  It is preferable to plate the surface of the cold-rolled steel sheet obtained by the manufacturing method according to the present invention.

  According to the present invention, a cold-rolled steel sheet having good ductility and bendability is obtained while having a high tensile strength of 780 MPa or more and a thick plate thickness of 2.0 mm or more. Is suitable as a high-strength steel sheet for automobiles that requires strict formability and dimensional accuracy, and is extremely useful in industry.

  Hereinafter, the reason why the chemical composition, steel structure, mechanical properties, and production conditions of the steel plate are limited as described above in the present invention will be described together with the operation thereof. In the following description, “%” representing the chemical composition means “mass%”, and “%” representing the ratio of the steel structure means “volume%”.

(1) Chemical composition
C: 0.08% or more and 0.20% or less C has an effect of increasing the strength of the steel sheet, and is an important element in the present invention to ensure a tensile strength of 780 MPa or more. When the C content is less than 0.08%, it is difficult to ensure a tensile strength of 780 MPa or more. Therefore, the C content is 0.08% or more. On the other hand, if the C content is more than 0.20%, the toughness and weldability deteriorate significantly. Therefore, the C content is 0.20% or less. Preferably it is 0.18% or less.

Si: 1.0% or less Si is an element contained as an impurity. However, Si also has an effect of increasing the strength of the steel sheet, so it may be actively contained. However, when the Si content exceeds 1.0%, the surface properties are significantly deteriorated. Therefore, the Si content is 1.0% or less.

Mn: 1.8% or more and 3.0% or less Mn has an effect of enhancing the hardenability and increasing the strength of the steel sheet, and is an extremely effective element in the present invention to ensure a tensile strength of 780 MPa or more. is there. If the Mn content is less than 1.8%, it is difficult to ensure a tensile strength of 780 MPa or more. Therefore, the Mn content is 1.8% or more. Preferably it is 2.0% or more. On the other hand, if the Mn content exceeds 3.0%, the band structure develops and the bendability is significantly reduced. Therefore, the Mn content is 3.0% or less. Preferably it is 2.7% or less.

P: 0.1% or less P is an element generally contained as an impurity, but may also be positively incorporated because it also has an effect of increasing the strength of the steel sheet by solid solution strengthening. However, when the P content exceeds 0.1%, the deterioration of toughness becomes significant. Therefore, the P content is 0.1% or less.

S: 0.01% or less S is an element contained as an impurity and has the effect of degrading bendability by forming MnS in steel. When the S content exceeds 0.01%, the deterioration of bendability becomes remarkable, so the S content is set to 0.01% or less. Preferably it is 0.004% or less, More preferably, it is 0.002% or less.

sol. Al: 0.005% to 0.5% or less Al is added as a deoxidizing agent, is an element having an effect of sound of the steel. sol. When the Al content is less than 0.005%, deoxidation is insufficient. Therefore, sol. The Al content is 0.005% or more. On the other hand, sol. If the Al content exceeds 0.5%, the effect of the above action is saturated, and the cost is unnecessarily high. The Al content is 0.5% or less.

N: 0.01% or less N is an element contained as an impurity, and forms coarse nitrides in steel to deteriorate bendability. When the N content exceeds 0.01%, the deterioration of bendability becomes remarkable, so the N content is set to 0.01% or less.

Ti: 0.02% or more and 0.2% or less Ti is combined with C, N, etc., or further compounded to form fine precipitates, thereby refining the steel structure and thereby improving bendability. It is an important element that has the effect of causing When the Ti content is less than 0.02%, the effect by the above action cannot be sufficiently obtained. Therefore, the Ti content is 0.02% or more. On the other hand, if the Ti content exceeds 0.2%, the effect by the above action is saturated, and the cost increases unnecessarily. Therefore, the Ti content is 0.2% or less.

Selected from the group consisting of Cr: 1% or less, Mo: 1% or less, Nb: 0.1% or less, V: 1% or less, Cu: 1% or less, Ni: 1% or less and B: 0.005% or less Any one or more of these optional elements also have the effect of increasing the strength of the steel sheet. Therefore, in order to ensure a more stable tensile strength of 780 MPa or more, these elements are used alone or Two or more kinds may be combined and actively contained. However, if the content of each element exceeds the above range, the effect due to the above action is saturated, and the cost is unnecessarily high. Therefore, when these elements are contained, the content should be within the above range. Is preferred. Moreover, in order to acquire such an effect reliably, Cr: 0.1% or more, Mo: 0.05% or more, Nb: 0.005% or more, V: 0.01% or more, Cu: 0.00. It is preferable to contain 1% or more, Ni: 0.1% or more, B: 0.0003% or more.

Other than the above are Fe and impurities.
(2) Steel structure The steel structure of the cold-rolled steel sheet according to the present invention is in volume%, ferrite: 10% or more, bainite: 20% or more and 70% or less, retained austenite: 3% or more and 10% or less, and martensite: The average particle size of ferrite is 10 μm or less, the average particle size of bainite is 10 μm or less, the average particle size of retained austenite is 3 μm or less, and the average particle size of martensite is 3 μm or less.

  By making the steel structure as described above, the plate thickness is as thick as 2.0 mm or more, and the TS × El value is 14000 MPa or more and the minimum bending radius is 1.5 t or less while having a high tensile strength of 780 MPa or more. It becomes possible to provide a good ductility and bendability.

  If the volume fraction of ferrite is less than 10%, it is difficult to make the TS × El value 14000 MPa ·% or more. Therefore, the volume ratio of ferrite is 10% or more. In the present invention, the tensile strength of 780 MPa or more is ensured by bainite and retained austenite, which will be described later, and even if the proportion of ferrite that is soft and rich in workability is increased, the workability is not deteriorated. It is not necessary to specify the upper limit of the volume ratio.

  Moreover, when the volume fraction of bainite is less than 20%, it becomes difficult to ensure a tensile strength of 780 MPa or more. In order to secure a tensile strength of 780 MPa or more, it is conceivable to increase the ratio of martensite. However, if this is done, the bendability deteriorates and it becomes difficult to set the minimum bending radius to 1.5 t or less. Therefore, the volume ratio of bainite is 20% or more. On the other hand, when the volume fraction of bainite exceeds 70%, the ductility deteriorates remarkably. Therefore, the volume fraction of bainite is 70% or less.

  If the volume fraction of retained austenite is less than 3%, it may be difficult to ensure good ductility. Therefore, the volume ratio of retained austenite is 3% or more. On the other hand, if the volume fraction of retained austenite exceeds 20%, hard martensite generated by plastic-induced transformation during work deformation significantly deteriorates bendability. Therefore, the volume ratio of retained austenite is 20% or less.

  In the present invention, the tensile strength of 780 MPa or more is ensured by bainite and retained austenite, so the inclusion of martensite is not essential. However, since the strength of the steel sheet can be increased by containing martensite, it may be contained. However, when the volume ratio of martensite exceeds 20%, the bendability is significantly deteriorated. Therefore, the volume ratio of martensite is 20% or less.

  Also, if the ferrite average particle size exceeds 10 μm, the bainite average particle size exceeds 10 μm, the residual austenite average particle size exceeds 3 μm, or the martensite average particle size exceeds 3 μm, bending Since the stress applied to these grain boundaries at the time of deformation increases and causes crack generation and propagation, the bendability deteriorates. Therefore, the average particle size of ferrite is 10 μm or less, the average particle size of bainite is 10 μm or less, the average particle size of retained austenite is 3 μm or less, and the average particle size of martensite is 3 μm or less.

(3) Mechanical properties and sheet thickness The mechanical properties of the cold-rolled steel sheet of the present invention are as follows: tensile strength (TS) is 780 MPa or more, product of tensile strength (TS) and total elongation (El) (TS × El value) is 14000 MPa. The minimum bending radius in the bending test is 1.5 t or less (t: plate thickness), and the plate thickness is 2.0 mm or more.

  If the tensile strength (TS) is less than 780 MPa, the strength is low and the bendability is good in the first place. If the plate thickness is less than 2.0 mm, the level of bendability required to ensure the degree of freedom of design is low in the first place. The present invention has no significance for improving bendability. Therefore, the tensile strength (TS) is 780 MPa or more, and the plate thickness is 2.0 mm or more. When the tensile strength (TS) is 980 MPa or more, the effect of the present invention becomes more remarkable. Therefore, it is preferable to apply the present invention to such a high-strength cold-rolled steel sheet.

  If the product of tensile strength (TS) and total elongation (El) (TS x El value) is less than 14000 MPa ·%, or the minimum bending radius in the bending test is less than 1.5 t (t: thickness) Therefore, it becomes difficult to apply to automobile steel sheets that require strict formability. Therefore, the product (TS × El value) of the tensile strength (TS) and the total elongation (El) is 14000 MPa ·% or more, and the minimum bending radius in the bending test is 1.5 t or less (t: plate thickness).

(4) Manufacturing method The cold-rolled steel sheet of the present invention may be manufactured while satisfying cold rolling conditions and annealing conditions described later, and casting, hot rolling and pickling may be performed by ordinary methods. That is, the hot-rolled steel sheet obtained by hot rolling is subjected to skin pass rolling for flattening as necessary, and pickled to remove scale, and then subjected to cold rolling.

(Cold rolling)
The rolling reduction in this cold rolling is 30% or more and 60% or less. If the rolling reduction in cold rolling is less than 30%, the amount of strain introduced by cold rolling is insufficient, making it difficult to refine the structure after annealing, and the bendability of the steel sheet may deteriorate. Therefore, the rolling reduction in cold rolling is set to 30% or more. On the other hand, if the rolling reduction in cold rolling exceeds 60%, the sheet thickness after cold rolling is in addition to the high strength of the hot-rolled steel sheet that is subjected to cold rolling in order to ensure a tensile strength of 780 MPa or more after annealing. However, since the thickness of the hot-rolled steel sheet used for cold rolling becomes thick, the load on the cold rolling equipment becomes excessive. Therefore, the rolling reduction in cold rolling is set to 60% or less.

(Annealing)
The cold-rolled steel sheet obtained by this cold rolling is allowed to stay within a temperature range of Ac ₃ points or more (Ac ₃ points + 50 ° C.) within 240 seconds, and then an average cooling rate of 1 ° C./second or more and 10 ° C./second or less. And then cooled to a temperature range of 680 ° C. or higher and 750 ° C. or lower, and further cooled to a temperature range of 400 ° C. or lower at an average cooling rate of 20 ° C./second or higher and 50 ° C./second or lower.

If the temperature at which the cold-rolled steel sheet stays is less than Ac _{3, the} bendability deteriorates due to the ferrite grains that remain coarse without being transformed into austenite. Therefore, the temperature is set to Ac ₃ points or more. On the other hand, if the temperature exceeds (Ac ₃ points + 50 ° C.), the austenite becomes coarse, and the grain size of bainite, martensite, and retained austenite in the final product becomes large and the bendability deteriorates. Therefore, the temperature is set to (Ac ₃ points + 50 ° C.) or less.

  In addition, if the time for which the cold-rolled steel sheet stays in the above temperature range exceeds 240 seconds, austenite becomes coarse, and the grain size of bainite, martensite, and retained austenite in the final product becomes large and the bendability deteriorates. Therefore, the above time is 240 seconds or less. The lower limit of the time is not particularly required, but is preferably 10 seconds or more in order to surely eliminate the remaining ferrite grains that remain coarse without being transformed into austenite.

  If the average cooling rate from 680 ° C. to 750 ° C. is less than 1 ° C./second, the ferrite transformation becomes excessively large and the ferrite grains become coarse, and the austenite stabilizes too much to remain in the final product. Since austenite becomes too much, bendability deteriorates. Therefore, the average cooling rate is 1 ° C./second or more. On the other hand, when the average cooling rate exceeds 10 ° C./second, the generation of ferrite is insufficient, and the TS × El value may be less than 14000 MPa ·%. Therefore, the average cooling rate is set to 10 ° C./second or less.

  Furthermore, when the average cooling rate to a temperature range of 400 ° C. or lower is less than 20 ° C./second, it becomes difficult to ensure a tensile strength of 780 MPa or higher. Therefore, the average cooling rate is set to 20 ° C./second or more. On the other hand, if the average cooling rate exceeds 50 ° C./second, a predetermined bainite volume fraction and residual austenite volume fraction cannot be obtained, and it becomes difficult to achieve both good ductility and bendability.

(Other)
There is no problem even if the steel plate obtained by annealing is given a skin pass having an elongation of 4% or less for further flattening as necessary.

  Moreover, there is no problem even if surface treatment such as galvanization is performed on the surface of the steel sheet obtained by annealing. When the surface treatment is hot dip plating, it is preferable to use continuous hot dip plating equipment from the viewpoint of productivity.

  Thus, according to the present embodiment, it is possible to obtain a cold-rolled steel sheet having good ductility and bendability while having a high tensile strength of 780 MPa or more and a thick plate thickness of 2.0 mm or more. it can.

Furthermore, the present invention will be specifically described with reference to examples.
Steels A to I containing the chemical composition shown in Table 1 were made into slabs by continuous casting, charged in a heating furnace and heated, and then hot-rolled under the conditions shown in Table 2 to give a plate thickness of 3.6 mm. A rolled steel sheet was used. The hot-rolled steel sheet was cold-rolled to a sheet thickness of 2.3 mm (rolling ratio: 36.1%) and subjected to continuous annealing under the conditions shown in Table 2. After continuous annealing, temper rolling with an elongation of 0.2% was performed. Further, some test materials were further subjected to electrogalvanization with an adhesion amount of 35 g / m ² per side.

  With respect to the test materials of samples Nos. 1 to 15 thus obtained, JIS No. 5 tensile test pieces and bending test pieces were collected from the direction perpendicular to the rolling direction, and a tensile test and a bending test were performed. The tensile test was carried out according to the JIS method, and the yield point (YP), the tensile strength (TS), and the total elongation (El) were determined, and further, the TS × El value was calculated. Further, the bending test was performed according to the JIS method, and evaluation was performed by “limit bending radius at which a crack occurs × plate thickness”. Furthermore, the steel structure of the plate | board thickness cross section of the rolling direction was observed using the scanning electron microscope. Table 3 shows the results of investigating the steel structure and mechanical properties.

As shown in Table 3, the cold-rolled steel sheets of the inventive examples of trial numbers 1 to 6 have good ductility and bendability while having a high tensile strength of 780 MPa or more and a thick plate thickness of 2.0 mm or more. It has.
On the other hand, trial numbers 7 to 15 are comparative examples that do not satisfy the conditions defined in the present invention.

In the trial No. 7, the C content is less than that of the present invention, and the target tensile strength of 780 MPa or more cannot be obtained.
In the trial No. 8, since the Mn content is less than that of the present invention, the hardenability is insufficient, and the tensile strength of 780 MPa or more which is the object of the present invention cannot be obtained.

  In Test No. 9, since the Mn content is larger than the upper limit of the present invention, the band structure is remarkable, and since S and N are more than the provisions of the present invention, respectively, the formation of MnS and TiN is remarkable. Inferior.

In test No. 10, the soaking temperature in continuous annealing is less than Ac ₃ , and coarse ferrite grains exceeding the provisions of the present invention remain, resulting in poor bendability.
In trial No. 11, it is considered that the austenite grains are coarsened because the stay time in the temperature range of Ac ₃ points or more (Ac ₃ points + 50 ° C.) is longer than the provision of the present invention, and as a result, the final structure remains. The austenite grain size is increased and the bendability is deteriorated.

  In the trial No. 12, since the cooling rate from the soaking temperature to the temperature range of 680 ° C. or more and 750 ° C. or less is slower than that of the present invention, the ferrite transformation becomes too remarkable and coarse ferrite grains are generated. Stabilization proceeds too much, resulting in excessive austenite retained in the final product, resulting in poor bendability.

In the trial No. 13, since the cooling rate from the soaking temperature to the temperature range of 680 ° C. or more and 750 ° C. or less is too high, the generation of ferrite is insufficient, and good ductility is not obtained.
In the trial No. 14, the cooling rate to a temperature range of 400 ° C. or lower is slower than that of the present invention, and the tensile strength of 780 MPa or more which is the object of the present invention cannot be obtained.

  Furthermore, in the trial number 15, the cooling rate to a temperature range of 400 ° C. or lower is faster than that of the present invention, bainite is reduced and martensite is increased, and the bendability is inferior because it deviates from the structure defined by the present invention.

Claims (5)

  In mass%, C: 0.08-0.20%, Si: 1.0% or less, Mn: 1.8-3.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.005 to 0.5%, N: 0.01% or less, and Ti: 0.02 to 0.2%, having a chemical composition consisting of the balance Fe and impurities, and in volume%, ferrite : 10% or more, bainite: 20 to 70%, residual austenite: 3 to 20% and martensite: 0 to 20%, the ferrite has an average particle size of 10 μm or less, and the bainite has an average particle size of 10 μm or less. And having a steel structure having an average particle size of the retained austenite of 3 μm or less and an average particle size of the martensite of 3 μm or less, a tensile strength (TS) of 780 MPa or more, a tensile strength (TS) and a total elongation (El). Product (TS × El value) is 14000 MPa ·% or more, and the minimum bending radius in the bending test is 1.5 t or less (t: plate thickness), and the plate thickness is 2.0. A cold-rolled steel sheet characterized by being not less than mm.   The chemical composition is mass% in place of a part of Fe, Cr: 1% or less, Mo: 1% or less, Nb: 0.1% or less, V: 1% or less, Cu: 1% or less, Ni The cold-rolled steel sheet according to claim 1, comprising one or more selected from the group consisting of: 1% or less and B: 0.005% or less.   The cold-rolled steel sheet according to claim 1, wherein the surface has a plating layer. The hot-rolled steel sheet having the chemical composition according to claim 1 or 2 is subjected to cold rolling with a reduction rate of 30 to 60% to obtain a cold-rolled steel sheet having a thickness of 2.0 mm or more, and the cold-rolled steel sheet is Ac ₃ points ˜ (Ac ₃ points + 50 ° C.) stayed within 240 seconds, then cooled to 680 to 750 ° C. at an average cooling rate of 1 to 10 ° C./second, and an average of 20 to 50 ° C./second A method for producing a cold-rolled steel sheet, characterized by cooling to a temperature range of 400 ° C. or lower at a cooling rate.   A method for producing a cold-rolled steel sheet, wherein the surface of the cold-rolled steel sheet obtained by the production method according to claim 4 is plated.