JP6221424B2 - Cold rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof. More specifically, the present invention relates to an ultra-high-strength cold-rolled steel sheet excellent in workability, which is suitable as an automotive steel sheet formed into various shapes by press working or the like, and a method for producing the same.

In recent years, there has been a demand for improved fuel efficiency of automobiles from the viewpoint of CO ₂ emission regulations associated with global warming countermeasures, and the application of high-strength steel sheets has been expanding more and more to reduce vehicle weight and ensure collision safety. is there. In particular, in recent years, the demand for ultra-high strength cold-rolled steel sheets having a tensile strength of 1180 MPa or more is increasing.

  On the other hand, it goes without saying that steel sheets used for automobile parts are required not only to have strength but also various workability required at the time of part molding, such as press formability and weldability. Specifically, from the viewpoint of press formability, a steel sheet is often required to have excellent elongation (total elongation in a tensile test; El) and stretch flangeability (hole expansion ratio; λ). However, in general, it becomes difficult to secure El and λ at a high level as the strength of the steel plate increases.

  In order to satisfy such a requirement, attempts have been made so far to improve the balance between elongation and stretch flangeability of the 1180 MPa class ultra-high strength cold-rolled steel sheet. Examples are shown below.

  Patent Documents 1 to 3 disclose that the balance of elongation and stretch flangeability is improved by controlling the area ratio of ferrite, tempered martensite, bainite, and retained austenite within a predetermined range. Techniques relating to high-strength cold-rolled steel sheets and hot-dip galvanized steel sheets are disclosed.

  However, in order to build such a steel structure, any of the inventions of Patent Documents 1 to 3 requires a step of re-heating to a predetermined temperature after quenching to the martensite transformation start temperature or lower in the final annealing step. It is said. Therefore, special equipment is required, which is disadvantageous in terms of equipment cost and manufacturing cost.

  In Patent Document 4, a steel structure is made of bainitic ferrite, martensite, and retained austenite, and the form and location of retained austenite are controlled to improve the ductility and hole expandability. Techniques relating to ultra-high strength cold-rolled steel sheets and hot-dip galvanized steel sheets having a strength of 1180 MPa or more are disclosed.

  However, in order to control such a structure, in the present invention, the heating temperature in the final annealing step is set to 925 ° C. to 950 ° C. Such high-temperature heating causes the austenite grains to become coarser and thus deteriorates toughness. In addition, because it inhibits manufacturability during mass production, it is not suitable for mass production.

International Publication No. 2012/036269 Pamphlet JP 2012-31462 A JP 2010-275627 A JP 2009-256773 A

  The present invention has been made in view of the current situation as described above, and an object of the present invention is to provide a tensile strength of 1180 [MPa] or more without requiring special equipment and without inhibiting mass productivity. An ultra-high strength cold-rolled steel sheet having mechanical properties of elongation (EL) of 10 [%] or more, tensile strength and hole expansion ratio (TS × λ): 50 [GPa ·%] or more, and a method for producing the same. There is.

  The present inventors have intensively studied to solve the above problems. First, in order to ensure good overall elongation, it was aimed to contain a certain amount of retained austenite. In addition, the aim was to improve the hole expandability by increasing the stability of retained austenite to the limit and simultaneously reducing martensite as much as possible. Specifically, the steel structure has an area ratio of 40% or less of polygonal ferrite, 5% or more of retained austenite, an average C concentration in the retained austenite of 0.90% by mass or more, and 10% of martensite. It was found that the target strength, total elongation, and hole expansion ratio can be achieved at the same time by making the following.

  The present inventors further examined the chemical composition for obtaining such a steel structure. In the conventional 1180 MPa class cold-rolled steel sheet, the strength is ensured by containing a large amount (2.0% or more) of Mn effective for improving the hardenability of the steel sheet. In the conventional 1180 MPa class cold-rolled steel sheet containing a large amount of Mn, the present inventors have delayed the bainite transformation in the final annealing step, and as a result, to the untransformed austenite, which is the main point of the structure control in TRIP steel. It was ascertained that C enrichment did not proceed sufficiently and that some of the untransformed austenite had undergone martensitic transformation from isothermal holding to cooling to room temperature.

  However, if the Mn content is simply reduced, the hardenability of the steel sheet is lowered and the ferrite transformation proceeds, making it difficult to obtain a tensile strength of 1180 MPa or more. Therefore, the inventors paid attention to B, Mo, and Cr, which have different hardenability improving principles from Mn, as an alternative. It was found that by limiting the content of these alloy elements within a certain range, a remarkable hardenability improving effect was exhibited with respect to the ferrite transformation, while the bainite transformation was hardly delayed. As a result, the C enrichment to untransformed austenite proceeded sufficiently, and the desired steel structure was successfully obtained.

This invention is made | formed based on the said knowledge, The summary is as follows.
(1) By mass%, C: 0.10% or more and 0.30% or less, Si: more than 0.50% and 2.50% or less, Mn: 0.01% or more and less than 2.0%, P: 0.00. 05% or less, S: 0.01% or less, N: 0.01% or less, Al: 0.001% or more and 1.0% or less, Ti: 0.001% or more and 0.20% or less, and B: 0.0. containing 0.010% or less 0,001% or more, further, Cr: 1. 0% or less or Mo: 1 . It has a chemical composition containing any one or two of 0% or less, the contents of B, Mo and Cr satisfy the following formulas (1) and (2), and the balance is Fe and impurities. And
Polygonal ferrite: 40% or less, retained austenite: 5% or more, martensite: 10% or less, balance: bainite, and average C concentration in the retained austenite is 0.90% by mass or more. Has a steel structure,
A cold-rolled steel sheet having mechanical properties of a tensile strength of 1180 MPa or more , a total elongation of 10% or more, and a product of tensile strength and hole expansion rate of 50 GPa ·% or more .

1000 · B + 2 · (Mo + Cr / 2) ≧ 1.0 (1)
1000 · B + 12 · (Mo + Cr / 2) ≦ 9.0 (2)
Here, B 2 , Cr and Mo in the formula represent the content (unit: mass%) of each element.

  (2) The chemical composition contains one or two selected from the group consisting of Ni: 1.0% or less and Cu: 1.0% or less in mass%, instead of a part of the Fe The cold-rolled steel sheet according to item (1), wherein:

  (3) The chemical composition contains one or two selected from the group consisting of V: 0.50% or less and Nb: 0.10% or less in mass% instead of part of the Fe The cold-rolled steel sheet according to (1) or (2).

  (4) The chemical composition is mass% in place of part of the Fe, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Bi: 0.01 The cold-rolled steel sheet according to any one of items (1) to (3), comprising one or more selected from the group consisting of:

(5) The cold-rolled steel sheet according to any one of items (1) to (4), wherein the surface is provided with a hot-dip galvanized layer or an alloyed hot-dip galvanized layer.
(6) A method for producing a cold-rolled steel sheet according to any one of items (1) to (4),
(A) Cold-rolled steel sheet by subjecting a hot-rolled steel sheet having the chemical composition according to any one of items (1) to (4) to cold rolling at a reduction rate of 20% to 80%. rolling; a and (B) the cold-rolled steel sheet, after Zaisa residence below 300 seconds 1 second or more to a temperature region of 800 ° C. or higher 950 ° C. or less, up to a temperature range of below 480 ° C. or higher Ms point from the temperature range 5 ° C. / sec is cooled at a cooling rate higher than than 1000 seconds or 30 seconds to a temperature range of not lower than Ms point 480 ° C. or less residence continuous annealing step of Zaisa.

  (7) A method for producing a cold-rolled steel sheet, comprising subjecting the cold-rolled steel sheet obtained by the production method described in (6) to a hot dip galvanizing treatment.

  (8) A method for producing a cold-rolled steel sheet, comprising subjecting the cold-rolled steel sheet obtained by the production method described in (7) to an alloying treatment in a temperature range of 460 ° C or higher and 550 ° C or lower.

  According to the present invention, an ultra high strength cold-rolled steel sheet having mechanical properties of tensile strength: 1180 [MPa] or more, total elongation: 10 [%] or more, product of tensile strength and hole expansion ratio: 50 [GPa ·%] or more. It can be manufactured stably without any special capital investment. The steel sheet according to the present invention can be used widely in industry, particularly in the automobile field.

FIG. 1 is a graph showing heat treatment conditions in the present invention. FIG. 2 is a graph showing the prescribed ranges of B, Mo and Cr contents in the present invention.

  First, the reason why the chemical composition of the steel sheet according to the present invention is limited as described above will be described. In the present specification, “%” defining the chemical composition is “mass%”.

[C: 0.10% to 0.30%]
C is an essential element for increasing the strength of the steel sheet, and is an essential element for forming retained austenite. When the C content is less than 0.10%, it is difficult to achieve sufficient strength, or it is difficult to ensure sufficient retained austenite. Therefore, the C content is 0.10% or more. Preferably it is 0.15% or more. On the other hand, when the C content exceeds 0.30%, the hole expandability and weldability deteriorate significantly. Therefore, the C content is 0.30% or less. Preferably it is 0.25% or less.

[Si: more than 0.50% and 2.50% or less]
Si is a cementite formation suppressing element and is an essential element for the formation of retained austenite. If the Si content is 0.50% or less, the intended retained austenite may not be ensured. Therefore, the Si content is more than 0.50%. Preferably it is 1.0% or more. On the other hand, when the Si content exceeds 2.50%, the chemical conversion property and the wettability with hot dip galvanizing are significantly deteriorated. Therefore, the Si content is 2.50% or less. Preferably it is 2.0% or less.

[Mn: 0.01% or more and less than 2.0%]
Mn is a strong austenite stabilizing element and is an effective element for improving the hardenability of the steel sheet. However, excessive addition delays the bainite transformation, making it difficult to obtain a desired steel structure. Therefore, the Mn content is less than 2.0%. Preferably it is 1.8% or less. However, it contains 0.01% or more for the purpose of fixing S in steel as MnS. A preferable content is 1.0% or more.

[P: 0.05% or less]
P is generally contained as an impurity, but is also a solid solution strengthening element and is an effective element for increasing the strength of a steel sheet. Therefore, although it may be contained positively, excessive addition deteriorates weldability and toughness. Therefore, the P content is 0.05% or less. More preferably, it is 0.02% or less.

[S: 0.01% or less]
S is an element contained as an impurity, and forms MnS in steel and deteriorates toughness and hole expandability. Therefore, the S content is set to 0.01% or less as a range in which deterioration of toughness and hole expansibility is not remarkable. Preferably it is 0.005% or less, More preferably, it is 0.002% or less.

[N: 0.01% or less]
N is an element contained as an impurity, and when the N content exceeds 0.01%, coarse nitrides are formed in the steel to deteriorate the hole expandability. Therefore, the N content is 0.01% or less. Preferably it is 0.005% or less.

[Al: 0.001% to 1.0%]
Al is contained at least 0.001% for deoxidation of the steel. However, even if it is added excessively, the effect is saturated and the cost is naturally increased, and the transformation temperature of the steel is raised to increase the load during hot rolling. Therefore, the Al content is 1.0% or less.

[Ti: 0.001% to 0.20%]
Ti fixes N as TiN in the steel, thereby suppressing the formation of BN that becomes a hardenability lowering factor. In addition, the austenite grain size during heating is refined to improve toughness. For this reason, it is made to contain at least 0.001%. On the other hand, if added excessively, the ductility of the steel sheet is lowered. Therefore, the Ti content is 0.20% or less.

[B: 0.0001% to 0.010%]
B is an essential element in the present invention because it segregates at the austenite grain boundaries when the steel sheet is heated and stabilizes the austenite grain boundaries to enhance the hardenability of the steel. In order to sufficiently obtain the effect, the content of 0.0001% or more is required. The B content is preferably 0.0005% or more. On the other hand, excessive addition results in a decrease in the hardenability of the steel by forming borides. Therefore, the B content is 0.010% or less. Preferably it is 0.005% or less.

[Cr: 0% to 1.0%, Mo: 0% to 1.0%]
Cr and Mo are both effective elements for increasing the strength of the steel sheet. Therefore, you may contain 1 type or 2 types of these elements. However, if the content exceeds the above upper limit, not only the cost is increased, but the bainite transformation is greatly delayed, C concentration to untransformed austenite does not proceed sufficiently, and the desired steel structure is obtained. It becomes difficult to obtain. Accordingly, the respective contents are as described above. In addition, in order to acquire the effect by the said action more reliably, it is preferable that content of any element shall be 0.001% or more.
[1000 · B + 2 · (Mo + Cr / 2) ≧ 1.0 (1)]
[1000 · B + 12 · (Mo + Cr / 2) ≦ 9.0 ··· (2)]
In the present invention, in order to ensure the hardenability of the steel sheet, the value defined by the formula (1) for the contents of B, Mo and Cr is set as the lower limit value. On the other hand, when these contents exceed the range defined by the formula (2), the bainite transformation is greatly delayed, and the C enrichment to untransformed austenite does not proceed sufficiently, and the desired steel structure is obtained. It becomes difficult to obtain. Therefore, let the value prescribed | regulated by Formula (2) be an upper limit of B, Mo, and Cr content.

[One or two selected from the group consisting of Ni: 1.0% or less and Cu: 1.0% or less]
Ni and Cu are both effective elements for increasing the strength of the steel sheet. Therefore, you may contain 1 type or 2 types of these elements. However, if the content exceeds the above upper limit, not only the cost is increased, but the bainite transformation is greatly delayed, C concentration to untransformed austenite does not proceed sufficiently, and the desired steel structure is obtained. It becomes difficult to obtain. Accordingly, the respective contents are as described above. In addition, in order to acquire the effect by the said action more reliably, it is preferable that content of any element shall be 0.001% or more.

[One or two selected from the group consisting of V: 0.50% or less and Nb: 0.10% or less]
V and Nb are carbide forming elements and are effective elements for increasing the strength of the steel sheet. Therefore, you may contain 1 type or 2 types of these elements. However, even if the content exceeds the upper limit, the effect of the above action is saturated, which is economically disadvantageous. Therefore, the upper limit value of each element is as described above. In addition, in order to acquire the effect by the said action more reliably, it is preferable that content of any element shall be 0.001% or more.

[One or more selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Bi: 0.01% or less]
Ca, Mg, and REM are elements that finely disperse inclusions in the steel, and Bi is an element that reduces microsegregation of substitutional alloy elements such as Mn and Si in the steel. Has the effect of increasing spreadability. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if the content exceeds the upper limit, ductility may be deteriorated. Accordingly, the content of each element is as described above. In addition, in order to acquire the effect by the said action more reliably, it is preferable to make content of any element 0.0001% or more.

Next, the reason which limited the steel structure of the steel plate concerning this invention as mentioned above is demonstrated.
[In area%, polygonal ferrite: 40% or less, retained austenite: 5% or more, martensite: 10% or less, balance: bainite, average C concentration in retained austenite: 0.90% by mass or more]
In order to obtain a tensile strength of 1180 MPa, the area ratio of polygonal ferrite needs to be at least 40% or less. Preferably it is 20% or less, More preferably, it is 10% or less. In order to obtain a total elongation of 10% or more, the area ratio of retained austenite needs to be 5% or more. In order to obtain a hole expansion ratio of 40% or more, it is necessary that the average C concentration in the retained austenite is 0.90% by mass or more, the area ratio of martensite is at least 10% or less, and the balance is bainite. The area ratio of martensite is preferably 5% or less.

  The remainder of the bainite is one that does not contain cementite between and inside the bainitic lath that constitutes the bainite (bainitic ferrite), one that contains cementite between the bainitic lath (upper bainite), and bainity class. Those containing cementite (lower bainite) are included, and are composed of one or more of these.

In addition, the area ratio calculation method of the steel structure in the present invention is as follows.
The area ratio of polygonal ferrite is obtained by cutting a cross section in the rolling direction of a steel sheet, revealing a steel structure with a nital solution, and photographing a 1/4 thickness position using an optical microscope or a scanning electron microscope. The value calculated by the point counting method from the photograph is the area ratio.

  The area ratio of retained austenite is obtained by performing X-ray diffraction using a 1 / 4-thickness surface of the steel sheet as an observation surface, and taking the value calculated from the peak area ratio of bcc and fcc as the area ratio.

  The average C concentration (Cγ) in the retained austenite is obtained by performing X-ray diffraction using the 1 / 4-thickness surface of the steel sheet as an observation surface, obtaining the lattice constant a (Å) from the peak position of fcc, and using the following formula: The converted value.

Cγ = (a−3.578) /0.033 (mass%)
The area ratio of martensite is obtained by cutting a cross section in the rolling direction of the steel sheet, revealing the steel structure with Lepera reagent, and photographing the 1/4 thickness position using an optical microscope. Is calculated by image processing, and a value obtained by subtracting the area ratio of retained austenite from the value is defined as the area ratio.

Next, the reason for limiting the mechanical properties of the cold-rolled steel sheet in the present invention will be described.
[Tensile strength is 1180 (MPa) or more]
The tensile strength of the cold-rolled steel sheet according to the present invention is set to 1180 MPa or more in order to satisfy the weight reduction required for recent cold-rolled steel sheets for automobiles. In addition, the cold-rolled steel sheet according to the present invention has [a total elongation (El) in a tensile test of 10% or more] and [a tensile strength and a hole expansion ratio (λ) measured by the method described in JFS T1001]. The product (TS · λ) is preferably 50 GPa ·% or more].

Next, the reason for limitation of the manufacturing method of the cold rolled steel sheet according to the present invention will be described.
[Cold rolling ratio: 20-80%]
In order to refine the austenite grain size during heating in the subsequent annealing step, which is a subsequent step, the rolling reduction of cold rolling is set to 20% or more. On the other hand, excessive rolling causes excessive rolling load and increases the load on the cold rolling mill, so the rolling reduction is 80% or less.

[Stay for 1 second to 300 seconds in a temperature range of 800 ° C to 950 ° C]
If the heating temperature is less than 800 ° C. or the staying time is less than 1 second, since austenitization does not proceed sufficiently, the polygonal ferrite fraction in the final structure becomes excessive, or the area of residual austenite The rate may be too low. On the other hand, if the heating temperature is higher than 950 ° C. or the staying time is longer than 300 seconds, the austenite grain size becomes coarse, not only the toughness of the steel sheet is deteriorated but also the productivity is hindered. Therefore, the conditions described above are used.

[Cooling at a cooling rate of 5 ° C./second or more from a temperature range of 800 ° C. to 950 ° C. to a temperature range of Ms point to 480 ° C., and 30 seconds to 1000 seconds at a temperature range of Ms point to 480 ° C. stay]
If the cooling rate is less than 5 ° C./second, ferrite transformation proceeds excessively during cooling, and the polygonal ferrite fraction in the final structure becomes excessive. The upper limit of the cooling rate is not particularly defined, but is preferably set to 200 ° C./second or less in order to prevent overcooling from the temperature range of Ms point or higher and 480 ° C. or lower.

  When the cooling end temperature is lower than the Ms point, martensite is generated, so that the hole expandability deteriorates. On the other hand, if the cooling end temperature is higher than 480 ° C. or the staying time is less than 30 seconds, the bainite transformation does not proceed sufficiently, and C concentration to untransformed austenite becomes insufficient. Then, martensite is generated by the subsequent cooling, and the retained austenite becomes excessive. Further, if the staying time exceeds 1000 seconds, the productivity is significantly reduced. Therefore, the conditions described above are used.

[Hot galvanizing]
What is necessary is just to follow a usual method about the immersion conditions of hot dip galvanization, plating bath temperature is 420 degreeC or more and 500 degrees C or less, penetration | invasion board temperature should be 420 degreeC or more and 500 degrees C or less, and immersion time should just be 5 second or less. As a chemical composition in the plating bath, Al is preferably contained in an amount of 0.08% by mass or more and 0.2% by mass or less. In addition, Fe, which are inevitable impurities, and Si, Mg, Mn, Cr, Ti, and Pb are included. Even if it is contained, the present invention is not affected. Further, it is preferable to control the basis weight of plating by a known method such as gas wiping after the dipping step. The basis weight is preferably 25 to 75 g / m ² per side.

[Alloying temperature: 460 ° C or higher and 550 ° C or lower]
When the alloying treatment temperature is less than 460 ° C., the alloying speed is slowed, and productivity is deteriorated, and uneven alloying treatment may occur. On the other hand, when the alloying treatment temperature exceeds 550 ° C., untransformed austenite is decomposed, and residual austenite in the final structure becomes excessive. Furthermore, alloying may proceed excessively and the powdering properties of the steel sheet may deteriorate. Therefore, the alloying treatment temperature is set to 460 ° C. or more and 550 ° C. or less.

  After the above treatment, temper rolling may be performed for flattening the steel sheet and adjusting the surface roughness. In this case, in order to avoid deterioration of ductility, it is preferable that the elongation is 2% or less.

  In addition, in this invention, although other manufacturing conditions are not specifically limited, A suitable example is shown below.

(A) Casting conditions The steel slab used as a raw material for the hot-rolled steel sheet is preferably grown by a continuous casting method from the viewpoint of manufacturability, but may be an ingot casting method or a thin slab casting method. The cast slab may be once cooled to room temperature, or directly sent to the heating furnace without being cooled to room temperature.

(B) Slab heating conditions It is preferable that the slab heating temperature at the time of subjecting to hot rolling shall be 1100 degreeC or more and 1350 degrees C or less. By setting it as 1100 degreeC or more, melt | dissolution of a carbide | carbonized_material can be advanced more reliably. Also, the rolling load can be reduced to make the operation easier. Moreover, by setting it as 1350 degrees C or less, it becomes possible to aim at reduction of a scale loss and reduction of cost.

(C) Hot rolling conditions It is preferable that the finishing temperature of hot rolling shall be 850 degreeC or more and 1050 degrees C or less. By setting the finishing temperature to 850 ° C. or more, rolling resistance can be reduced and rolling can be performed more easily. Further, by setting the finishing temperature to 1050 ° C. or lower, it becomes possible to ensure a more beautiful surface property and to suppress surface defects in the final product. The winding temperature is preferably 300 ° C. or higher and 750 ° C. or lower.

  By setting the coiling temperature to 300 ° C. or higher, the hot-rolled steel sheet can be made softer and subsequent cold rolling can be facilitated. Moreover, the scale thickness of a steel plate surface can be reduced by making winding temperature into 750 degrees C or less, and subsequent pickling property can be improved.

  What is necessary is just to follow the pickling after hot rolling according to a conventional method. Further, before or after pickling, skin pass rolling may be performed for flattening correction and scale peeling promotion. The elongation in the case of performing the skin pass rolling is not particularly specified, but is preferably 0.1% or more and less than 3.0%.

Examples of the present invention will be described below.
Steel having the chemical composition shown in Table 1 was melted in a laboratory to cast a steel ingot, and hot rolled under the conditions shown in Table 2 to obtain a hot rolled steel sheet having a thickness of 3 mm. Then, after pickling, cold rolling was performed at the rolling reduction shown in Table 2 to obtain a cold rolled steel sheet having a thickness of 1.2 mm.

  About the obtained cold-rolled steel plate, the heat processing of the conditions shown in FIG. 1 and Table 2 was performed. A JIS No. 5 tensile test piece was taken from the cold rolled steel sheet thus obtained from a direction perpendicular to the rolling direction, a tensile test was performed, and tensile strength (TS) and total elongation (El) were measured. Moreover, the “JFS T 1001 hole expansion test method” of the Japan Iron and Steel Federation standard was performed, and the hole expansion ratio (λ) was measured. The steel structure was identified according to the method described above.

  The obtained results are shown in Table 3. Moreover, FIG. 2 is a graph which shows the prescription | regulation range of B, Mo, and Cr content in this invention.

  Experiment No. 2 in Table 3 5-7, 9, 11-13, 16-18, 24, 26-29, 31, 33-40 all have the desired chemical composition, manufacturing conditions, and steel structure that meet the scope defined by the present invention. Mechanical properties can be obtained.

  On the other hand, the experiment No. Since 1-4, 8, 10, 14, 15, 19-23, 25, 30, and 32 are out of the range defined by the present invention, either the chemical composition or the production conditions, the steel structure defined by the present invention is The desired mechanical properties cannot be obtained.

Claims (8)

In mass%, C: 0.10% to 0.30%, Si: more than 0.50% to 2.50%, Mn: 0.01% to less than 2.0%, P: 0.05% or less , S: 0.01% or less, N: 0.01% or less, Al: 0.001% or more and 1.0% or less, Ti: 0.001% or more and 0.20% or less, and B: 0.0001% or more containing 0.010% or less, furthermore, Cr: 1. 0% or less or Mo: 1 . It has a chemical composition containing any one or two of 0% or less, the contents of B, Mo and Cr satisfy the following formulas (1) and (2), and the balance is Fe and impurities. And
Polygonal ferrite: 40% or less, retained austenite: 5% or more, martensite: 10% or less, balance: bainite, and average C concentration in the retained austenite is 0.90% by mass or more. Has a steel structure,
A cold-rolled steel sheet having mechanical properties of a tensile strength of 1180 MPa or more , a total elongation of 10% or more, and a product of tensile strength and hole expansion rate of 50 GPa ·% or more .
1000 · B + 2 · (Mo + Cr / 2) ≧ 1.0 (1)
1000 · B + 12 · (Mo + Cr / 2) ≦ 9.0 (2)
Here, B 2 , Cr and Mo in the formula represent the content (unit: mass%) of each element.   The chemical composition contains one or two selected from the group consisting of Ni: 1.0% or less and Cu: 1.0% or less in mass%, instead of a part of the Fe. The cold-rolled steel sheet according to claim 1, characterized in that   The chemical composition contains one or two selected from the group consisting of V: 0.50% or less and Nb: 0.10% or less in mass%, instead of a part of the Fe. The cold-rolled steel sheet according to claim 1 or 2, wherein the cold-rolled steel sheet is characterized.   Instead of a part of the Fe, the chemical composition is, in mass%, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Bi: 0.01% or less. The cold-rolled steel sheet according to any one of claims 1 to 3, comprising one or more selected from the group consisting of:   The cold-rolled steel sheet according to any one of claims 1 to 4, wherein the surface is provided with a hot-dip galvanized layer or an alloyed hot-dip galvanized layer. A method for producing a cold-rolled steel sheet according to any one of claims 1 to 4,
A method for producing a cold-rolled steel sheet comprising the following steps (A) and (B):
(A) A cold rolling step in which a hot rolled steel sheet having the chemical composition according to any one of claims 1 to 4 is subjected to cold rolling at a reduction rate of 20% to 80% to obtain a cold rolled steel sheet. ; and (B) the cold-rolled steel sheet, after residence Zaisa 300 seconds or less than 1 second to a temperature range of 800 ° C. or higher 950 ° C. or less, 5 ° C. from the temperature range to a temperature range of not lower than Ms point 480 ° C. or less / cooled seconds or more cooling rate, than 1000 seconds or 30 seconds to a temperature range of not lower than Ms point 480 ° C. or less residence continuous annealing step of Zaisa.   A method for producing a cold-rolled steel sheet, comprising subjecting the cold-rolled steel sheet obtained by the production method according to claim 6 to a hot dip galvanizing treatment.   A method for producing a cold-rolled steel sheet, comprising subjecting the cold-rolled steel sheet obtained by the production method according to claim 7 to an alloying treatment in a temperature range of 460 ° C or higher and 550 ° C or lower.

Images