TW201812052A - High strength cold rolled steel sheet - Google Patents

Abstract

This high strength cold rolled steel sheet is a steel sheet which includes a predetermined chemical composition. A microstructure of the steel sheet includes, in terms area ration, 10% to 50% of one or two of a ferrite and granular bainite, 10% to 50% of one or two of upper bainite and lower bainite, more than 0% to 30% of a tempered martensite, 5% or more of a retained austenite, and 0% to 10% of a pearlite, a cementite, and martensite. The area ration of the ferrite to the total area of the ferrite and the granular bainite is 25% or less and a tensile strength of the steel sheet is 980 MPa or more.

Description

High strength cold rolled steel sheet

The present invention relates to a high strength cold rolled steel sheet. More particularly, the present invention relates to a high-strength cold-rolled steel sheet which is suitable for use in an extension or hole expandability of an automobile part or the like.

In recent years, in order to suppress the amount of carbon dioxide emitted from automobiles, a method of reducing the weight of an automobile body by application of a high-strength steel sheet is progressing. In addition, in order to ensure the safety of the occupant, the high-strength steel plate is gradually used in the automobile body. In the future, in order to make the car body lighter and the safety improvement progress, it is necessary to increase the strength level of the steel plate to be used.

In addition to the strength, the form factor (processability) of the steel sheet formed into the member for automobiles is also required. For example, steel plates used in skeleton parts are required to extend and ream. However, in general, when the steel sheet is increased in strength, the formability is lowered. Therefore, when the strength of the steel sheet to be applied to the automobile component is increased, the improvement of the formability will become a problem.

In order to solve such a problem, several means have been proposed. For example, Patent Document 1 discloses a high-strength steel sheet which is a carbon concentration gradient in the vicinity of the interface in the residual Worth field and in the grain, and the strength and elongation are improved by processing induced metamorphism. Further, in Patent Document 2, a cold-rolled steel sheet has been disclosed in which the metal structure of the steel sheet is a structure having a medium-hardness toughness or a lower-toughening body as a main body of the fat body and the granule body, and the inter-structure is lowered. The difference in hardness increases the strength and hole expandability. Further, Patent Document 3 discloses a composite structure cold-rolled steel sheet in which the metal structure of the steel sheet is composed of three phases of a fat granule, an upper tempering body or a lower tempering body, and a residual Worth field body, or Further, it contains a matrix of granules and is composed of four phases, whereby it has high workability at a strength level of 45 to 65 kgf/mm ² .

However, these techniques have the following problems. That is, in order to manufacture the steel sheet of Patent Document 1, it is necessary to control the cooling rate after overaging after being generally difficult to control, and to control the carbon concentration gradient in the residual Worth field. Therefore, in the usual equipment, it is not easy to realize the organization disclosed in Patent Document 1. Further, in the technique of Patent Document 2, since the structure of the body of the toughener is formed, although the hole expandability is excellent, it is difficult to obtain sufficient ductility. Further, Patent Document 3 is directed to a steel sheet having a tensile strength of 45 to 65 kgf/mm ² , and the technique of Patent Document 3 is difficult to achieve both tensile strength of 980 MPa or more and sufficient formability. Advanced technical literature

Japanese Unexamined Patent Publication No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei.

Disclosure of the Invention Problems to be Solved by the Invention As described above, in order to continue the development of the weight reduction of the automobile body in the future, it is necessary to increase the strength of use of the steel sheet more than conventionally. In particular, for example, in a high-strength steel sheet used for a skeleton component, it is necessary to improve the hole expandability without deteriorating the elongation. Specifically, in order to form the skeleton member and ensure safety during collision, the steel sheet should satisfy all of the following conditions: tensile strength of 980 MPa or more, strength × full extension (TS × EL) of 12,000 MPa ‧ % or more, strength The hole expandability (TS × λ) was 18,000 MPa‧% or more. However, as shown in Patent Documents 1 to 3, it is extremely difficult to achieve such a high level of strength, elongation, and hole expandability in a high-strength steel sheet. Therefore, the present invention has been made in view of the state of the art, and provides a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more and excellent elongation and hole expandability. Means to solve the problem

The present inventors have made an effort to review the methods for solving the above problems. As a result, the following new insights were obtained: in a steel sheet having a tensile strength of 980 MPa or more, the metal structure includes a fat body and/or a granular tough body, an upper tough body and/or a lower tough body, and a tempered Matian body. And a residual Worth field body, and controlled to have a ratio of the area ratio of the above-mentioned fat granules to 25% or less, thereby ensuring that the ratio of the area ratio of the granules to the granules is 25% or less. Strength and extension, while achieving excellent hole expansion.

Since the invention was constructed based on the above findings, the gist thereof is as follows. (1) The high-strength cold-rolled steel sheet according to one aspect of the present invention contains the chemical composition of the steel sheet in a mass%: C: 0.15% or more, 0.30% or less, P: 0.040% or less, and S: 0.0100% or less. N: 0.0100% or less, O: 0.0060% or less, and one or two kinds of Si and Al: 0.70% or more and 2.50% or less in total, and one or two kinds of Mn and Cr: a total of 1.50% or more and 3.50% or less. Mo: 0% or more and 1.00% or less, Ni: 0% or more and 1.00% or less, Cu: 0% or more and 1.00% or less, Nb: 0% or more and 0.30% or less, and Ti: 0% or more and 0.30% or less. V: 0% or more and 0.30% or less, B: 0% or more and 0.0050% or less, Ca: 0% or more and 0.0400% or less, Mg: 0% or more, 0.0400% or less, and REM: 0% or more and 0.0400% or less The remaining part is made of Fe and impurities; the metal structure of the steel sheet includes, in terms of area ratio, one or two types of fat granules and granular tempered bodies, which are 10% or more and 50% or less. One or two of the body and the lower toughness are 10% or more and 50% or less, and the tempered Matian bulk is more than 0% and 30% or less, and the remaining Worth body is more than 5%, and the Boron body and the Xueming carbon. Body and Ma Tian bulk total 0~10%; The ratio of the area ratio of the above-mentioned fat granules to the total area ratio of the above-mentioned fertilizer granules and the above-mentioned granular squash is 25% or less; and the tensile strength is 980 MPa or more.

(2) The mashage in the metal structure of the cold-rolled steel sheet according to the above (1) may be 3% or less in area ratio.

(3) The chemical composition of the cold-rolled steel sheet according to the above (1) or (2) may be one or more selected from the group consisting of the following: Mo: 0.01% or more and 1.00% or less. Ni: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Nb: 0.005% or more and 0.30% or less, Ti: 0.005% or more and 0.30% or less, and V: 0.005% or more and 0.30% or less. B: 0.0001% or more and 0.0050% or less, Ca: 0.0005% or more and 0.0400% or less, Mg: 0.0005% or more, 0.0400% or less, and REM: 0.0005% or more and 0.0400% or less.

(4) The cold-rolled steel sheet according to any one of (1) to (3) above which may further have a hot-dip galvanized layer on the surface of the steel sheet.

(5) The cold-rolled steel sheet according to any one of the above (1) to (3), which may further have an alloyed hot-dip galvanized layer on the surface of the steel sheet. Effect of the invention

According to the present invention, it is possible to provide a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more which is excellent in both elongation and hole expandability, and is suitable as a structural member for automobiles and the like.

MODE FOR CARRYING OUT THE INVENTION A high-strength cold-rolled steel sheet according to an embodiment of the present invention (hereinafter, referred to as a cold-rolled steel sheet according to the present embodiment) has the following features. (a) Chemical composition is contained in mass%: C: 0.15% or more, 0.30% or less, P: 0.040% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0060% or less, Si and Al: total 0.70% or more and 2.50% or less, Mn and Cr: 1.50% or more and 3.50% or less in total; and further, if necessary, one or more of the following: Mo: 1.00% or less, Ni: 1.00% or less, Cu: 1.00 % or less, Nb: 0.30% or less, Ti: 0.30% or less, V: 0.30% or less, B: 0.0050% or less, Ca: 0.0400% or less, Mg: 0.0400% or less, REM: 0% or more, 0.0400% or less; Part of it is made of Fe and impurities. (b) The metal structure includes, in terms of area ratio, one or two types of the fat body and the granular shape toughness, and the total amount is 10% or more and 50% or less, and one or two kinds of the upper and lower toughening bodies. A total of 10% or more and 50% or less, more than 0% and 30% of the tempered granules, more than 5% of the remaining Worth, and 0 to 10% of the total body of the Borne, smectite, and Matian; The ratio of the total area ratio of the above-mentioned fertilizer body to the above-mentioned granular body toughness is 25% or less. (c) The tensile strength is 980 MPa or more.

The chemical composition (content of each element) of the cold-rolled steel sheet according to the present embodiment will be described. The % related to the content refers to the mass%.

<C: 0.15% or more and 0.30% or less> C is an element effective for increasing the strength of the steel sheet. In addition, C is an essential element for securing a predetermined amount of residual Worth. If the C content is less than 0.15%, the Worthian body will be metamorphosed in the field, so that the residual Worth field cannot be guaranteed to be more than 5%. Therefore, the C content is 0.15% or more. It is preferably 0.18% or more. On the other hand, if the C content exceeds 0.30%, the phase transformation from the Worthfield upper upset or the lower toughness is remarkably suppressed, and the Matian bulk is liable to be formed, and as a result, the remaining Worth field becomes a change. Can't be guaranteed to be above 5%. Therefore, the C content is 0.30% or less. It is preferably 0.22% or less.

<P: 0.040% or less> P is an impurity element. P is an element which segregates in the central portion of the thickness of the steel sheet to lower the toughness and embrittle the welded portion. Although the P content is as small as possible, when the P content is more than 0.040%, the deterioration of the hole expandability is remarkable, so the P content is 0.040% or less. It is preferably 0.015% or less. Since P is as small as possible, it is not necessary to limit the lower limit. However, in practical steel sheets, the P content is less than 0.0001%, which is economically unsatisfactory. Therefore, 0.0001% may be taken as the lower limit.

<S: 0.0100% or less> S is an impurity element. S is an element which lowers the weldability and hinders the manufacturability at the time of casting and hot rolling. Further, S is also an element which forms coarse MnS and lowers hole expandability. In the case where the S content is less than 0.0100%, the weldability is lowered, the manufacturability is lowered, and the hole expandability is lowered. Therefore, the S content is 0.0100% or less. It is preferably 0.0050% or less. Since S is as small as possible, it is not necessary to limit the lower limit. However, in practical steel sheets, the S content is less than 0.0001%, which is economically unsatisfactory. Therefore, 0.0001% may be taken as the lower limit.

<N: 0.0100% or less> N is an element which forms a coarse nitride in a steel sheet and reduces the bendability and hole expandability of the steel sheet. Further, N is an element which causes the pores at the time of welding. Although the N content is as small as possible, when the N content exceeds 0.0100%, the hole expandability is lowered and the generation of pores is remarkable, so the N content is 0.0100%. Since N is as small as possible, it is not necessary to limit the lower limit. However, in practical steel sheets, the N content is less than 0.0005%, which causes a large increase in manufacturing cost. Therefore, 0.0005% can also be taken as the substantial lower limit.

<O: 0.0060% or less> O is an element which forms a coarse oxide in a steel sheet and lowers the bendability and hole expandability of the steel sheet. In addition, O is an element of the cause of the occurrence of pores at the time of welding. Although the O content is as small as possible, when the O content is more than 0.0060%, the hole expandability is lowered and the generation of pores is remarkable. Therefore, the O content is 0.0060% or less. Since O is as small as possible, it is not necessary to limit the lower limit. However, in practical steel sheets, the O content is less than 0.0005%, which causes a large increase in manufacturing cost. Therefore, 0.0005% can also be taken as the lower limit.

<1 or 2 of Si and Al: 0.70% or more and 2.50% or less in total> Both Si and Al are elements effective for obtaining a predetermined amount of the granular tough body and the residual Worth field. The granular tough body is a metal structure which is a poor row of the interface of the toughened fertilizer and granules, which is recovered by heat treatment and becomes a massive toughened fertilizer body. Therefore, if a smectite carbon body is formed at the interface of the toughened fat granule, the granular tempering body cannot be obtained. Si and Al are elements which suppress the formation of sulphur carbon, and are therefore important elements for obtaining a granular tough body. Furthermore, Si and Al are also important elements for obtaining a residual Worth field. When a part of the Worth field changes toward the metamorphic body, the carbon concentration increases toward the untransformed Worth field, and the carbon concentration in the untransformed Worth field increases. By increasing the carbon concentration, the Worthfield body becomes stable. Therefore, the Worth field body which has sufficiently increased the carbon concentration remains even after being cooled to room temperature and does not metamorphose into other tissues. This became a residual Worth field. In the process of increasing the carbon concentration toward the untransformed Worth field, if the snowy carbon precipitates, the carbon concentration in the Worth field will decrease, so it is difficult to maintain the Worth field in the undisturbed state until room temperature. status. That is, since Si and Al are elements which suppress the formation of sulphur carbon, they are important elements for obtaining a residual Worth field. As a result of the inventors' efforts, it has been found that a predetermined amount of the granular tough body and the residual Worth field can be obtained by setting the total content of Si and/or Al to 0.70% or more. Therefore, the total content of Si and Al is 0.70% or more. On the other hand, when the total content (total content) of Si and/or Al is excessive, the steel material itself is embrittled, and the hole expandability is largely deteriorated. Therefore, the total content of Si and/or Al is 2.50% or less. In the cold-rolled steel sheet according to the present embodiment, since Si and Al have the same effect, the total content may be specified, and it is not necessary to define the respective contents. That is, as long as the total content is in the above range, for example, one of them may be 0%.

<1 or 2 of Mn and Cr: 1.50% or more and 3.50% or less in total> Both Mn and Cr are effective elements for improving the strength of the steel sheet. In addition, Mn and Cr are elements which inhibit the metamorphism of the fertilizer body during heat treatment in a continuous annealing apparatus or a continuous hot-dip galvanizing apparatus. If the granule metamorphosis at the time of the above heat treatment is suppressed, it is advantageous in that the ratio of the area ratio of the granules to the area ratio of the total of the granules and the granules is less than 25%. In order to obtain this effect, the total content of Mn and/or Cr is 1.50% or more. On the other hand, if the total content of Mn and/or Cr exceeds 3.50%, the phase transformation state of the upper or lower tough body from the Worthfield body is remarkably suppressed. In this case, as a result, it is impossible to ensure a residual Worth field of 5% or more, and the area ratio of the Mada body is more than 10%. Therefore, the total content of Mn and/or Cr is 3.50% or less. In the cold-rolled steel sheet according to the present embodiment, since Mn and Cr have the same effect, the total content may be specified, and it is not necessary to define the respective contents. That is, as long as the total content is in the above range, for example, one of them may be 0%.

The cold-rolled steel sheet according to the present embodiment basically contains the above elements, and the remainder is made of Fe and impurities. However, one or more selected from the group consisting of Mo, Ni, Cu, Nb, Ti, V, B, Ca, Mg, and REM, which will be described later, may be substituted for one part of Fe. However, since these elements are not necessarily contained, the lower limit is 0%. In addition, the impurity is a component which is mixed in from a raw material such as ore or waste material due to various factors in the production process when the steel material is industrially produced, and is allowed to have a range which does not affect the cold-rolled steel sheet according to the present embodiment. .

<Mo: 0.01% or more and 1.00% or less> Mo is an element effective for improving the strength of the steel sheet. Further, Mo is an element having an effect of suppressing metamorphism of the fertilizer and granules which are generated when the continuous annealing apparatus or the continuous hot-dip galvanizing apparatus performs heat treatment. In order to obtain such effects, if the Mo content is less than 0.01%, a sufficient effect cannot be obtained, so the Mo content is preferably 0.01% or more. On the other hand, when the Mo content exceeds 1.00%, the effect of suppressing the metamorphosis of the fertilizer body is saturated. Therefore, even in the case of inclusion, the Mo content should be 1.00% or less.

<Ni: 0.05% or more and 1.00% or less> Ni is an element effective for improving the strength of the steel sheet. Further, Ni is an element having an effect of suppressing metamorphism of the fertilizer and granules which are generated when the continuous annealing apparatus or the continuous hot-dip galvanizing apparatus performs heat treatment. In order to obtain such effects, if the Ni content is less than 0.05%, a sufficient effect cannot be obtained, so the Ni content is preferably 0.05% or more. On the other hand, if the Ni content exceeds 1.00%, the effect of suppressing the metamorphosis of the fertilizer body is saturated. Therefore, even in the case of inclusion, the Ni content should be 1.00% or less.

<Cu: 0.05% or more and 1.00% or less> Cu is an element effective for improving the strength of the steel sheet. If the Cu content is less than 0.05%, a sufficient strength improving effect cannot be obtained. Therefore, in order to obtain this effect, the Cu content is preferably 0.05% or more. On the other hand, if the Cu content is excessive, it will be brittle during hot rolling, and hot rolling will become impossible. Therefore, even in the case of inclusion, the Cu content should be 1.00% or less.

<Nb: 0.005% or more and 0.30% or less> Nb is an element which makes the crystal grain fine-grained and the strength of the steel plate is improved. Further, Nb is an element having an effect of suppressing metamorphism of the fertilizer and granules which are generated when the continuous annealing apparatus or the continuous hot-dip galvanizing apparatus performs heat treatment. If the Nb content is less than 0.005%, such effects cannot be sufficiently obtained. Therefore, in order to obtain such effects, the Nb content is preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.30%, the phase transformation from the Worthfield upper tough or lower tough body is remarkably suppressed. As a result, the Matian bulk body is excessively generated, and it becomes impossible to secure a residual Worth field body of 5% or more. Therefore, even when Nb is contained, the Nb content is made 0.30% or less.

<Ti: 0.005% or more and 0.30% or less> Ti is an element effective for improving the strength of the steel sheet. Further, Ti is an element having an effect of suppressing metamorphism of the fertilizer and granules which are generated when the continuous annealing apparatus or the continuous hot-dip galvanizing apparatus performs heat treatment. If the Ti content is less than 0.005%, such effects cannot be sufficiently obtained. Therefore, in order to obtain such effects, the Ti content is preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.30%, the phase transformation from the Worthfield upper tough or the lower tough body is remarkably suppressed. As a result, the Matian bulk body is excessively generated, and it becomes impossible to secure a residual Worth field body of 5% or more. Therefore, even in the case where Ti is contained, the Ti content is made 0.30% or less.

<V: 0.005% or more and 0.30% or less> V is an element effective for improving the strength of the steel sheet. In addition, it has an effect of suppressing the effect of metamorphism of the fertilizer and granules which are generated when the continuous annealing apparatus or the continuous hot-dip galvanizing apparatus performs heat treatment. Since the V effect is less than 0.005%, such effects cannot be sufficiently obtained. Therefore, in order to obtain such effects, the V content is preferably 0.005% or more. On the other hand, if the V content exceeds 0.30%, the phase transition state from the Worthfield upper tough or lower tough body is remarkably suppressed. As a result, the Matian bulk body is excessively generated, and it becomes impossible to secure a residual Worth field body of 5% or more. Therefore, even when V is contained, the V content is made 0.30% or less.

<B: 0.0001% or more and 0.0050% or less> B is an element which suppresses metamorphism of the granules by segregation at the grain boundary of the Vostian body in the heat treatment step. In order to obtain this effect, the B content should preferably be 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, the fertilizer granule metamorphosis suppressing effect is saturated, so 0.0050% is preferably the upper limit.

<Ca: 0.0005% or more and 0.0400% or less> <Mg: 0.0005% or more and 0.0400% or less> <REM: 0.0005% or more and 0.0400% or less> Ca, Mg, and REM are all useful for controlling oxides or sulfides. The form and the element that enhances the hole expansion. In order to obtain this effect, the content of any element should be 0.0005% or more. More preferably, it is 0.0010% or more. On the other hand, when the content of Ca, Mg, or REM exceeds 0.0400%, a coarse oxide is formed and the hole expandability is deteriorated. Therefore, the content of any element is 0.0400% or less. More preferably, it is 0.0100% or less. In the case where REM (rare earth element) is contained, a rare earth metal alloy is often added, but in some cases, a lanthanoid compound may be added in addition to La or Ce. Even in such a case, in the case where a metal REM such as metal La or Ce is added, the cold-rolled steel sheet according to the present embodiment can exert an effect.

Next, the metal structure of the cold-rolled steel sheet according to the present embodiment will be described. The present inventors have reviewed the use of the continuous hot rolling equipment and the continuous annealing equipment which are currently used, and the steel sheet having a tensile strength of 980 MPa or more has improved strength and elongation while improving the hole expandability. As described above, the steel sheet which is suitable as a structural member for an automobile or the like is used as a metal structure in order to improve the hole expandability without deteriorating strength and elongation. However, since the fat granules are soft metal structures, the high-strength steel sheets formed of the composite structure become a factor that causes a difference in hardness between the structures, and as a result, deterioration of the hole expandability is a problem. Then, the present inventors paid attention to the use of the granular toughness as a metal structure of the steel sheet. As a result, it is successful to control the ratio of the area ratio of the granules to the granules and the area ratio of the granules to the area ratio of the granules and the granules. While ensuring strength and ductility, one side obtains excellent hole expandability. The metal structure (microstructure) of the cold-rolled steel sheet according to the present embodiment is defined based on the above findings. The details will be described below.

<A total of 10% or more and 50% or less of the area of the fat and granules and the granular toughness are compared with the total area ratio of the granular body and the granular tough body. The ratio of the area ratio of the granules is less than 25%. > The granules contained in the metal structure of the steel sheet are soft and easy to deform. Therefore, the fat granules contribute to the extension of the extension. On the other hand, since the fat granule is a soft structure, the hardness difference between the structure and the hard phase is large, and the composite structural steel sheet is a factor that deteriorates the hole expandability. In contrast, the granular tough body is the same as the fat body, which contributes to the improvement of the extension. On the other hand, the granular tough body is harder than the fat body, and has a hardness intermediate between the fat body and the upper or lower tough body. Therefore, the granular tough body has an effect of lowering the difference in hardness between the microstructures of the composite structural steel sheet as compared with the fat granules. If the total area ratio of the granular body to the granular toughened body is less than 10%, a sufficient extension cannot be obtained. On the other hand, when the area ratio of the total of the granular body and the granular toughened body exceeds 50%, the tensile strength of 980 MPa or more cannot be obtained. In addition, the ratio of the area ratio of the granules to the area ratio of the granules and the granules (the area ratio of the granules and the granules to the granules) When the ratio of the area ratio exceeds 25%, the difference in hardness between the structures becomes large, and the hole expandability deteriorates. That is, the fat granules and the granular tough bodies are contained in a total amount of 10% or more and 50% or less, and the area ratio of the fat granules is compared with the total area ratio of the granules and the granular plastomers. The ratio is 25% or less, whereby the extension and the hole expandability can be improved at the same time. The area ratio of the fat granules is even 0%.

<Residual Worth Field contains 5% or more> Residual Worth Field is a metal structure that contributes to the elongation of elongation in the deformation caused by processing to the Matian bulk metamorphosis (processing induces metamorphosis). In order to obtain a predetermined extension, the residual Worth field in the steel sheet is taken as 5% or more in terms of area ratio. If the area ratio of the remaining Worth field is less than 5%, a sufficient extension cannot be obtained. On the other hand, although the area ratio of the residual Worth field is as high as possible from the viewpoint of the extension, it is necessary to increase the content of C and Mn in order to obtain an area ratio of 20% or more. 20% is the upper limit of the substance.

<1 or 2 types of upper and lower toughening bodies are contained in an area ratio of 10% or more and 50% or less.> Upper and lower toughening bodies are affected and remain in the Worth field. The metal structure that is generated and necessary for obtaining high strength. Often referred to as metamorphic bodies are such tissues. If the area ratio of the upper toughening body and/or the lower toughening body is 10% or more, in the annealing step, when the phase transition state of the upper tough or lower tough body from the Worth field occurs, the carbon The concentration will increase in the untransformed Worth field, and the untransformed Worth field will remain at 5% or more until room temperature. If the area ratio of the upper toughness and/or the lower toughness is less than 10%, the strength of the steel sheet may be insufficient, and the residual Worth field body of 5% or more may not be secured. On the other hand, in the case where the area ratio of the upper tough body and/or the lower tough body exceeds 50%, the metamorphosis state of the upper or lower tough body from the Worth field is excessively promoted, so that the metamorphosis is not metamorphosed. The reduction of the Worth field will eventually ensure that more than 5% of the remaining Worth field is guaranteed. Therefore, the area ratio of the upper toughness and/or the lower toughness is 50% or less in total.

<The tempering Ma Tian bulk body contains more than 0% super and 30% or less in terms of area ratio.> The tempered Matian bulk in the metal structure of the steel sheet affects the formation of the residual Worth field and achieves high strength. A valid metal organization. Therefore, the area ratio of the tempered Ma Tian bulk is more than 0%. It should be 3% or more, more preferably 5% or more. On the other hand, since the tempered granules are a hard structure, the composite structure steel sheet is a metal structure in which the difference in hardness between the structures is increased and the hole expandability is deteriorated as a result. If the area ratio of the tempered granules exceeds 30%, the hole expandability will be significantly deteriorated, so the upper limit of the area ratio of the tempered granules is 30%.

<The total amount of the corrugated body, the smectite, and the granulated body is 0 to 10% in terms of the area ratio.> The cold-rolled steel sheet according to the present embodiment can be used as a structure other than the above, and contains a wave. One or more kinds of body, swarovski carbon, and 麻田散体. However, these structures contain hard iron carbides that form the starting point for pores when reaming. When the total area ratio exceeds 10%, the deterioration of the hole expandability is remarkable, so the total area ratio is limited to 10% or less. In particular, from the viewpoint of the difference in hardness between tissues, the granules of Ma Tian should preferably be 3% or less. The fewer the organizations, the better, that is, the total area ratio of the Borne, the Schönming, and the Matian bulk is 0%. The Ma Tian loose body mentioned here is different from the tempering Ma Tian bulk, which is the so-called primary Ma Tian bulk.

Calculation of the size and area ratio of fat granules, granular metamorphic bodies, Ma Tian loose bodies, upper metamorphic bodies, lower deformed bodies, residual Worth field bodies, wave bodies, smectite carbon bodies, and tempered Matian bulk bodies The steel sheet can be rolled by EBSD (Electron Back Scattering Diffraction), X-ray diffraction, and microstructure observation by scanning electron microscopy using a oxidizing agent or a lining solution. The cross section of the direction section or the direction of the rolling direction is observed and measured at a magnification of 1000 to 50000 times.

Specifically, the area ratio of the granules can be measured by the following method. That is, the EBSD attached to the scanning electron microscope was measured at a pitch of 0.2 μm from a range of 1/8 to 3/8 thick centered on 1/4 of the thickness. Calculate the value of Grain average misorientation from the measured data. Then, the area where the value of Grain average misorientation is less than 0.5° is used as the fat granule, and the area ratio thereof is measured. Here, the Grain average misorientation is a value obtained by calculating the difference in orientation between adjacent pixels in a region surrounded by a grain boundary of 5 or more and averaging all the pixels in the crystal grain.

The area ratio of the residual Worth field can be calculated by using the X-ray measurement. That is, it is removed from the plate surface of the sample by the mechanical polishing and chemical polishing to a depth of 1/4 position. Then, the integrated intensity of the diffraction peaks of (200), (211) and fcc phases (200), (220), and (311) obtained from the rubbed sample using the MoKα line as the characteristic X-ray is obtained. In comparison, the tissue fraction of the residual Worth field is calculated as the area ratio of the residual Worth field.

The area ratio of the granules of Ma Tian can be calculated by the following method: etching the section in the thickness direction with the lining solution, and 1/8 to 3/ centering on the position of 1/4 of the sheet thickness by FE-SEM observation. In the range of 8 thick, the area ratio of the uncorroded area is subtracted from the area ratio of the residual Worth field measured by X-ray. Alternatively, since the tissue of the granules of the genus is high in density and has a structure called a block or a lower part of the granules, it can be distinguished from other metal tissues according to the electron channel contrast image used in the scanning electron microscope. Therefore, the area ratio of the field of Ma Tian can also be obtained by comparing the images with the electronic channel.

The identification of the upper toughness, the lower toughness, and the tempered Matian bulk can be carried out by the following method: the section of the thickness direction is etched by the niobium etchant, and the thickness is 1/1 by FE-SEM observation. 4 is the range of 1/8 to 3/8 thick of the center, and the position and orientation of the stellite carbon contained in the inside of the tissue are observed. Specifically, the upper tough body is a snowy carbon body or a residual Worth field body formed at the interface of the strip-shaped toughened fertilizer body. On the other hand, the lower tough body is a snow-forming carbon body formed inside the strip-shaped toughened fertilizer body, and since the crystal orientation relationship between the toughened fertilizer body and the snow-capped carbon body is one type, the resulting snow Carbon bodies have the same orientation. In addition, although the tempered Matian bulk is a snow-forming carbon body inside the Ma Tian bulk strip, the crystal orientation relationship between the Ma Tian bulk strip and the Xueming carbon body is more than two types, so the generated Xueming carbon body has a complex orientation. . The area was identified by detecting the characteristics of the sulphur carbons and calculating the area ratio.

The identification of the Boron or Schönming carbon can be carried out by etching with a titanium oxide and using a secondary electron image of a scanning electron microscope to observe a quarter of the thickness of the plate. 1/8~3/8 thick range. The area ratio is calculated by using the field of the second electron image in a bright contrast as a corrugated or smectite carbon.

The granular tough body is composed of a hardened spheroidal body having almost no hard spheroidal carbon body and having a low difference in density. Therefore, the conventional etching method or the secondary electron image observation using a scanning electron microscope cannot be distinguished from the fat body. However, the inventors have tried hard to review the results and found that because the granular tough body is composed of a combination of toughened fertilizers and granules, and has a slight crystal orientation difference in the grains, the microcrystals in the grains are detected. It is possible to distinguish the difference from the fat granules. Therefore, the area ratio of the granular tough body can be measured by the following method. That is, using EBSD, the range of 1/8 to 3/8 thick centered on 1/4 of the sheet thickness is measured at intervals of 0.2 μm, and the value of Grain average misorientation is calculated from the measurement data. Then, the value of the area ratio of the upper toughening body, the lower toughening body, the tempering Matian loose body, the wave body, and the Ma Tian bulk body is subtracted as the granularity from the area ratio of the area where the value of the Grain average misorientation is 0.5 or more. The area ratio of the toughened body. The area ratio of each of the above organizations is the ratio of the area of each organization to the overall area of the metal structure.

The cold-rolled steel sheet according to the present embodiment has a tensile strength of 980 MPa or more and is sufficiently high in strength. Therefore, when it is applied to an automobile or the like, it contributes to the weight reduction of the automobile body or the safety of the collision. Although the upper limit of the strength is not limited, if it exceeds 1470 MPa, it will become difficult to achieve strength by using the composite structure as in the present invention, and a structure in which the main body of the field is almost the main phase can be exhibited. Take 1470 MPa as the upper limit of strength.

The cold-rolled steel sheet according to the embodiment may have a hot-dip galvanized layer on the surface. When a hot-dip galvanized layer is formed on the surface, corrosion resistance is improved, so that it is preferable. The hot-dip galvanizing layer should preferably have Fe of 1% or more and less than 7% and the remainder being formed of Zn, Al, and impurities. Further, the cold-rolled steel sheet according to the embodiment may have a alloyed hot-dip galvanized layer on the surface. When the alloyed hot-dip galvanized layer is formed on the surface, the corrosion resistance is improved, which is preferable. The alloyed hot-dip galvanized layer should have a Fe content of 7% or more and 15% or less and the remainder being formed of Zn, Al, and impurities. The hot-dip galvanized layer or the alloyed hot-dip galvanized layer may be formed on one side of the steel sheet or may be formed on both sides.

Next, a description will be given of a method for producing a cold-rolled steel sheet which is suitable for obtaining the present embodiment. The cold-rolled steel sheet according to the present embodiment can obtain the effect regardless of the production method as long as it satisfies the chemical composition and the metal structure described above. However, it is preferable to manufacture the cold-rolled steel sheet according to the present embodiment in accordance with the production method including the following steps (A) to (G). (A) A cast flat embryo having the same composition as that of the cold-rolled steel sheet of the present embodiment is heated to a temperature of 1150 ° C or higher after cooling without cooling to a temperature of less than 1150 ° C or temporarily cooling. (Heating Step) (B) The heated spheroid is supplied to the hot rolling, and the hot rolling is completed at a temperature range equal to or higher than the Ar3 transformation point to obtain a hot rolled steel sheet. (Hot rolling step) (C) The hot-rolled steel sheet is wound in a temperature range of 700 ° C or lower. (Winding step) (D) After pickling the hot-rolled steel sheet to be rolled back, cold rolling is performed to obtain a cold-rolled steel sheet having a cumulative rolling reduction ratio of 30% or more and 80% or less. (Pickling and Cold Rolling Step) (E) The cold rolled steel sheet is continuously annealed in a temperature range of 760 ° C or higher and 900 ° C or lower. (Annealing step) (F) After continuously annealing the cold-rolled steel sheet, it is cooled at an average cooling rate of 15 ° C /sec or more and 100 ° C / sec or less, and is stopped at 180 ° C or more and 400 ° C or less and Ms or less. In this cooling, the cold-rolled steel sheet is allowed to remain in the temperature range of 500 to 650 ° C for 3 seconds or more. (Cooling Step) (G) After the cooling is stopped, the cold-rolled steel sheet is further heated to a temperature range of 300 ° C or more and 460 ° C or less, and held in this temperature range for 15 seconds or more. (Reheating step) The appropriate conditions for each step will be described.

(A) Heating step The cast flat embryo having the same composition as the cold-rolled steel sheet of the present embodiment having a tensile strength of 980 MPa or more may sometimes contain a large amount of alloying elements. Therefore, it is necessary to solidify the alloying elements in the cast flat embryo before the hot rolling. Therefore, in the case where the cast slab is temporarily cooled, it is preferably heated to 1150 ° C and supplied to the hot rolling. When the heating temperature is less than 1150 ° C, not only the coarse alloy carbide remains, but also the deformation resistance of the hot rolling delay becomes high, so it is heated above this temperature. However, in the case where the flat blank is not cooled to less than 1150 ° C after casting and supplied to the hot rolling, it is not necessary to carry out heating. The cast flat embryo to be supplied to the hot rolling is not limited to a specific cast flat embryo as long as it is a cast flat embryo. For example, it is a continuous casting of a flat embryo or a flat embryo made by a thin flat embryo casting machine. As described above, the cast flat embryo is directly supplied to the hot rolling, or is heated after being temporarily cooled and supplied to the hot rolling.

(B) Hot rolling step The flat embryo having a heating step of 1150 ° C or higher is supplied to a hot rolling including coarse rolling and finishing rolling to obtain a hot rolled steel sheet. In the hot rolling, the finishing rolling temperature (the end temperature of the finishing rolling) is important for the structural control of the steel sheet. If the finishing rolling temperature is in the 2-phase temperature range of (Worth field + fat granule), the rolling load of the hot rolling delay becomes large, and cracking may occur in the hot rolling. Therefore, the finishing rolling temperature should be above the Ar3 metamorphic point. It is also possible to continuously carry out hot rolling at the hot rolling delay so that the rough rolling sheets are joined to each other. Here, the Ar3 metamorphic point is a temperature at which the Worth field begins to be metamorphosed during the cooling process, and the present embodiment is calculated by simply using the following formula (1). Ar3=901-325×C+33×Si-92×(Mn+Ni/2+Cr/2+Cu/2+Mo/2) (Formula 1)

(C) Winding step The hot rolled steel sheet after the hot rolling step is preferably wound at a temperature of 700 ° C or lower. If the winding temperature exceeds 700 ° C, there is a possibility that a thick scale is formed on the surface of the steel sheet, and the skin cannot be removed in the pickling step. In this case, it may become difficult to provide the steps after the cold rolling. Further, when it is wound over 700 ° C, the carbide in the hot-rolled steel sheet becomes coarse, and in the subsequent annealing step, the carbide becomes refractory. If the melting of the carbide does not progress during the heating of the annealing step, the predetermined strength may not be obtained. In this case, the fraction of the fertilizer and the granule in the annealing step is increased because of insufficient hardenability, and as a result, To the organization of the predetermined area ratio. The winding temperature is preferably 700 ° C or lower, and the lower limit is not particularly limited. However, since it is technically difficult to wind at a temperature lower than room temperature, room temperature is a substantial lower limit. Since the winding temperature is low, the microstructure of the hot-rolled steel sheet becomes uniform, and the mechanical properties after annealing tend to increase. Therefore, it is preferable to make the winding temperature low in a possible range. On the other hand, the lower the winding temperature, the higher the strength of the hot-rolled steel sheet and the higher the deformation resistance of the cold rolling delay. Therefore, when the winding temperature is low, the hot-rolled steel sheet can be tempered for softening at a temperature of 650 ° C using a box annealing furnace, a continuous annealing facility, or the like. When considering the strength of the hot-rolled steel sheet and the passability in the production line, the winding is preferably 450 ° C or more and 650 ° C or less.

(D) Pickling and cold rolling step The wound hot rolled steel sheet is wound up and subjected to pickling, and then supplied to cold rolling. By pickling, the scale of the surface of the hot-rolled steel sheet can be removed, and the passivation property and the plating property of the cold-rolled steel sheet can be improved. The pickling may be carried out once or may be carried out in plural times. When the pickled hot-rolled steel sheet is cold-rolled and formed into a cold-rolled steel sheet, the cumulative rolling reduction ratio of the cold rolling is preferably 30% or more and 80% or less. If the cumulative reduction ratio is less than 30%, it is difficult to keep the shape of the cold-rolled steel sheet flat and it is not possible to provide the subsequent annealing step, so the cumulative reduction ratio is preferably 30% or more. More preferably 40% or more. On the other hand, when the cumulative rolling reduction ratio exceeds 80%, the rolling load becomes excessively large, and cracking may occur in the cold rolling, which is difficult to provide to the subsequent annealing step, so that the cumulative rolling reduction ratio is preferably 80% or less. More preferably 70% or less. The number of rolling passes and the rolling reduction rate of each pass are not particularly limited. It is sufficient to appropriately set the range in which the cumulative rolling reduction ratio is 30% or more and 80% or less.

(E) Annealing step The cold rolled steel sheet is supplied to a continuous annealing line, heated to an annealing temperature, and annealed. In this case, the annealing temperature is preferably 760 ° C or higher and 900 ° C or lower, and the annealing time is preferably 10 to 600 seconds. If the annealing temperature is less than 760 ° C, the Worth field is not sufficiently formed. In this case, there is a possibility that the area ratio of the fat granules increases and the predetermined strength cannot be achieved. In addition, since the area ratio of the Worth field under the maximum heating temperature (annealing temperature) is also reduced, the metamorphic structure formed in the subsequent cooling, that is, the granular tough body or the tough body, the tempered Matian bulk body The area ratio is reduced. In this case, it is impossible to achieve the concentration of carbon in the Vostian body, which is necessary for obtaining the residual Worth field, and it is impossible to ensure a residual Worth field of 5% or more. On the other hand, when the annealing temperature exceeds 900 ° C, the crystal grain size of the Worth field is coarsened, and the hardenability is excessive, and the area ratio of the predetermined fat granules and the granular tempering body cannot be obtained. In addition, the metamorphosis of the upper or lower tough body from the Worthfield body is suppressed. As a result, it is impossible to ensure a residual Worth field of 5% or more. Therefore, the upper limit of the continuous annealing temperature is preferably 900 °C. The continuous annealing may be carried out in the atmosphere, or may be carried out in the oxidation of the ambient gas for the purpose of improving the adhesion of the plating. Further, when the annealing time is less than 10 seconds, the fraction of the Worthfield body is insufficient, and the melting of the carbide existing before the annealing is insufficient, and the predetermined structure and characteristics are not obtained. Although in terms of characteristics, there is no problem in annealing time of more than 600 seconds, but the length of the line of equipment will become longer, so the 600 second level is the upper limit.

(F) After the cooling step annealing step, it is preferred to cool the cold-rolled steel sheet to 180 ° C or more at an average cooling rate of 15 ° C / sec or more and 100 ° C / sec or less immediately (for example, within 30 seconds, preferably within 10 seconds). Below °C and below the Matian bulk metamorphic start temperature (hereinafter referred to as Ms). This step is a step effective for obtaining a predetermined amount of the granular tough body. Regarding the granular tough body, after the phase change is made by the difference in the amount of the miscellaneous grains contained in the Worthian body particles before the metamorphosis, the difference between the tough and fat body boundaries is maintained by maintaining in a predetermined temperature range. Generated by reply. Therefore, in order to suppress the formation of excess fat and granules and obtain a predetermined amount of the fat and granular bodies and the granular tough body, it is necessary to suppress the fat body and body metamorphosis to some extent by setting the average cooling rate to 15 ° C /sec or more. On the other hand, regarding cooling at an average cooling rate of more than 100 ° C / sec, it is economically disadvantageous in consideration of the ability of the cooling device of the annealing step. Therefore, the upper limit of the substantial average cooling rate is 100 ° C / sec. Further, in the cooling step, it is preferable to stay in the temperature range of 500 to 650 ° C for 3 seconds or more. The toughened fertilizer body formed during cooling is held at the high temperature side of the temperature at which the tough body is formed, whereby the resulting toughened fertilizer body recovers and the granular tough body is obtained. That is, in order to generate the toughened fat granules by the above-described cooling at the average cooling rate, it is effective to ensure the time for the difference to be recovered immediately. When the residence time of 500 to 650 ° C is less than 3 seconds, the recovery of the toughened fertilizer granules is not sufficiently progressed, and it is difficult to obtain the area ratio of the predetermined granular tough body. The retention in the present embodiment is not limited to isothermal holding, but the state in which the steel sheet temperature is commanded at 500 to 650 ° C for 3 seconds or longer. Although there is no upper limit for the residence time, since the average cooling rate is 15 ° C / sec or more, the upper limit of the essence is about 10 to 15 seconds. When the cooling stop temperature is 180 to 400 ° C and the Ms or less, the Ma Tian bulk is formed. The 麻田散体 is tempered in the reheating step of the next step and becomes a tempered granule. Therefore, in the case of obtaining a tempered mass of the field, the cooling stop temperature is preferably 400 ° C or less and Ms or less. In addition, if the cooling stop temperature exceeds 400 ° C or exceeds Ms, the Ma Tian bulk body will not be obtained during cooling, and then the metamorphic metamorphosis state during reheating does not progress sufficiently, and the carbon concentration toward the untransformed Worth field body The increase has not progressed, and a predetermined amount of residual Worth is not available. In this case, since the untransformed Worth field is finally transformed into a granulated loose body during cooling, the hole expandability is remarkably deteriorated. On the other hand, when the cooling stop temperature is less than 180 ° C, the phase transition from the Worthfield to the Ma Tian bulk is excessively promoted, and the amount of the production of the granulated bulk is more than 10%, and the hole expandability is remarkably deteriorated. In the present embodiment, the average cooling rate can be calculated by dividing the difference between the cooling start temperature and the cooling stop temperature by the cooling time (which is also included in the case where the entrapment is performed in the middle). Further, the above Ms is changed by the area ratio of the fertilizer granules and the granular plastomers generated in the annealing step and the cooling step, and it is difficult to calculate by the calculation formula. However, as long as the presence of the tempered granules in the final microstructure is confirmed, the cooling is performed below Ms for cooling, so preliminary tests such as the cooling stop temperature and the area ratio of the tempered granules can be performed in advance. And obtain a predetermined area ratio of the tempered Ma Tian bulk.

(G) Reheating step After cooling is stopped in a temperature range of 180 to 400 ° C and not more than Ms, it is preferred to reheat the cold rolled steel sheet and maintain it in a temperature range of 300 ° C or more and 460 ° C or less for 15 seconds or more. According to this step, carbon diffusion to the Vostian body accompanying the tempering of the Ma Tian bulk body generated in the cooling step occurs, and carbon diffusion to the Vostian body due to the metamorphosis of the metamorphic body occurs. If the temperature is kept below 300 ° C or the holding time is less than 15 seconds, there is a possibility that the deformation of the metamorphic body is insufficient, and the diffusion of carbon into the Vostian body becomes insufficient. On the other hand, if the temperature is maintained above 460 °C, the metamorphosis from the Vostian body to the wave body will progress, the area ratio of the wave body will increase, and the Worth field will become unstable, which may cause the remaining Worthfield. The area ratio of the body is reduced. After the reheating step, the cold rolled steel sheet was allowed to cool to room temperature. Although the cooling rate at this time is not required, it may be 2 ° C / sec or more and 100 ° C / sec or less.

In the case of producing the cold-rolled steel sheet according to the present embodiment, the following steps (H) to (J) may be further performed for the purpose of improving the mechanical properties and improving the corrosion resistance. (H) After the tempering step and the heating step, after the cold-rolled steel sheet is cooled to room temperature or during cooling to room temperature (but not more than Ms), reheating is started, and the temperature is 150° C. or higher and 400° C. or lower. The temperature range is maintained for more than 2 seconds. According to this step, the granulated granules which are generated during the reheating and tempering are tempered to become tempered granules, whereby the difference in hardness between the tissues can be made smaller. Further, as a result, it is possible to ensure excellent hole expandability without deteriorating the ductility. In the case where the tempering step is carried out, if the temperature is kept below 150 ° C or the holding time is less than 2 seconds, the Ma Tian bulk is not sufficiently tempered, and there is almost no change in microstructure and mechanical properties. On the other hand, if the temperature is kept above 400 ° C, there is a possibility that the difference in the discharge density in the tempered granules is lowered, and the tensile strength of 980 MPa or more cannot be obtained. In addition, there is a stagnation of the volcanic body in the untransformed Worth field, which causes the Worth field to be unstable, the volcanic metamorphosis in the Worth field during cooling, and the formation of the granulated body after cooling. Therefore, in the case of tempering, it is preferable to maintain the temperature range of 150 ° C or more and 400 ° C or less for 2 seconds or more. The tempering may be carried out in a continuous annealing apparatus, or may be performed offline after continuous annealing and in another apparatus. At this time, the tempering time varies with the tempering temperature. That is, the lower the temperature, the longer the temperature, and the higher the temperature, the shorter the time. If tempering is performed for a long period of time at a high temperature, the strength is lowered, and it is difficult to obtain a strength of 980 MPa or more. Therefore, regarding the upper limit of the tempering time, it is preferable to confirm the relationship between the tempering temperature and the time-to-strength reduction in the laboratory in advance, and to set it so as not to become a desired intensity or less because of the tempering temperature and composition.

(I) The hot-dip galvanizing step may also be performed on the cold-rolled steel sheet after the reheating step or after the tempering step, if necessary, heating or cooling to (galvanizing bath temperature -40) ° C ~ (galvanizing bath temperature + 50) ° C And the implementation of hot-dip galvanizing. A hot-dip galvanized layer is formed on the surface of the cold-rolled steel sheet by a hot-dip galvanizing step. In this case, the corrosion resistance of the cold-rolled steel sheet is improved, so it is preferable. Even if hot-dip galvanizing is performed, the elongation and hole expandability of the cold-rolled steel sheet can be sufficiently maintained.

(J) Alloying hot-dip galvanizing step The cold-rolled steel sheet having the hot-dip galvanized layer may be subjected to heat treatment in a temperature range of 460 ° C or higher and 600 ° C or lower to be alloyed. If it is alloyed at less than 460 ° C, it is not sufficiently alloyed. Further, when the alloying treatment is performed at a temperature exceeding 600 ° C, the alloying is excessively performed and the corrosion resistance is deteriorated. Therefore, in the case of alloying treatment, it is 460 ° C or more and 600 ° C or less.

Alternatively, plating may be performed by electroplating or vapor deposition plating instead of hot-dip galvanizing. Further, surface treatment such as organic film formation, film lamination, organic salt/inorganic salt treatment, or chromium-free treatment can be applied. Even if the surface treatment described above is carried out, the elongation and hole expandability of the cold rolled steel sheet can be sufficiently maintained. [Examples]

Next, an embodiment of the present invention will be described. The conditions of the examples are those in which the conditions and effects of the present invention are confirmed, and the present invention is not limited to this. Regarding the present invention, various conditions can be employed as long as the gist of the present invention is not exceeded and the object of the invention can be achieved.

The cast flat embryos having the chemical composition (chemical composition) shown in Table 1 were directly or temporarily cooled after casting, and then heated, hot rolled, and wound under the conditions shown in Tables 2 and 3. After pickling the hot-rolled steel sheet, cold rolling and annealing were carried out under the conditions shown in Tables 2 and 3, followed by cooling. Further, after cooling, the overaging belt was reheated under the conditions shown in Tables 4 and 5. An example of this is that tempering, hot-dip galvanizing, and/or alloying treatment are carried out under the conditions shown in Tables 4 and 5. The blank column of Table 1 indicates that it was not intentionally added, and Table 4 and Table 5 "-" indicate that the relevant steps were not performed. However, the "-" of the cooling stop temperature in the reheating step means that cooling is not stopped in the middle and cooling is performed until room temperature.

The metal structure and mechanical properties of the steel sheets after annealing, tempering, or after hot-dip galvanizing and/or alloying were investigated. (Metal organization) Regarding the metal structure, it is to investigate the fat granules, the granular toughness, the upper toughness or the lower toughness, the tempering Matian bulk, the residual Worth field, and the remaining part of the structure (Bolivia, Ma Tian Area ratio of bulk, swarovski carbon. The identification of the fat granules, the granular turf, the tempered granules, the upper tempering, the lower tempering, the residual Worth, the Boron, the smectite, and the granule are calculated. As described above, the range of 1/8 to 3/8 thick centered on the position of 1/4 of the plate thickness is determined by EBSD (Electron Back Scattering Diffraction), X-ray measurement, use of oxidizing agent or lining Corrosion of the drawing liquid and observation of the structure of the scanning electron microscope, and observation and measurement of the cross section of the steel sheet in the rolling direction direction or the direction perpendicular to the rolling direction in the direction of the right angle of 1000 to 50000 times. The results are shown in Tables 6 and 7. (Mechanical properties) Regarding the mechanical properties, tensile strength, total elongation, and hole expandability were evaluated. The tensile strength (TS) and the total elongation (EL) were measured by taking a JIS No. 5 test piece at a right angle to the rolling direction of the steel sheet, and performing a tensile test based on JIS Z2242. The hole expandability (λ) was evaluated in accordance with the hole expansion test method described in Japanese Industrial Standard JIS Z2256. The results are shown in Tables 6 and 7.

【Table 1】

【Table 2】

【table 3】

【Table 4】

【table 5】

[Table 6]

[Table 7]

It can be seen from Tables 1 to 7 that the examples in which the chemical composition and the metal structure are within the scope of the present invention are high strengths of 980 MPa or more, and TS x EL, TS × λ are high, and the elongation and the hole expandability are excellent. Here, it can be seen that the comparative examples in which the chemical composition and the metal structure are outside the range of the present invention are low in strength, TS × EL and/or TS × λ, and poor in elongation and/or hole expandability. Industrial utilization

According to the present invention, it is possible to provide a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more which is excellent in elongation and hole expandability as a structural member of an automobile or the like. The cold-rolled steel sheet according to the present invention contributes to the improvement of the weight of the automobile body or the safety of the collision when it is applied to an automobile or the like, and therefore has high industrial applicability.

Claims (5)

A high-strength cold-rolled steel sheet characterized by having a chemical composition of: C: 0.15% or more and 0.30% or less, P: 0.040% or less, S: 0.0100% or less, and N: 0.0100% or less. O: 0.0060% or less, one or two kinds of Si and Al: 0.70% or more and 2.50% or less in total, and one or two kinds of Mn and Cr: 1.50% or more and 3.50% or less in total, Mo: 0% or more, 1.00% or less, Ni: 0% or more and 1.00% or less, Cu: 0% or more and 1.00% or less, Nb: 0% or more and 0.30% or less, Ti: 0% or more and 0.30% or less, V: 0% or more, 0.30% or less, B: 0% or more and 0.0050% or less, Ca: 0% or more and 0.0400% or less, Mg: 0% or more, 0.0400% or less, and REM: 0% or more and 0.0400% or less, and the remainder is Fe. The metal structure of the steel sheet includes, in terms of area ratio, one or two types of fat granules and granular tempering bodies, and the total amount is 10% or more and 50% or less, and the upper tough body and the lower tough body are formed. One or two of the total amount is 10% or more and 50% or less, and the tempered Ma Tian bulk body is more than 0%, 30% or less, and the remaining Worth field body is more than 5%, and the Borreaux, Xueming carbon, and Ma Tiansan 0 to 10% in total; the relative proportion of fat globules and the total area ratio of the granular variant of the firmware, the area ratio of the fat globules is less than 25%; tensile strength is 980MPa or more. The high-strength cold-rolled steel sheet according to claim 1, wherein in the metal structure, the aforementioned granules have an area ratio of 3% or less. The high-strength cold-rolled steel sheet according to claim 1 or 2, wherein the chemical composition contains one or more selected from the group consisting of the following: Mo: 0.01% or more, 1.00% or less, Ni: 0.05% Above 1.00% or less, Cu: 0.05% or more and 1.00% or less, Nb: 0.005% or more and 0.30% or less, Ti: 0.005% or more and 0.30% or less, V: 0.005% or more and 0.30% or less, B: 0.0001% Above 0.0050%, Ca: 0.0005% or more and 0.0400% or less, Mg: 0.0005% or more, 0.0400% or less, and REM: 0.0005% or more and 0.0400% or less. The high-strength cold-rolled steel sheet according to any one of claims 1 to 3, wherein the surface of the steel sheet has a hot-dip galvanized layer. The high-strength cold-rolled steel sheet according to any one of claims 1 to 3, wherein the surface of the steel sheet further has an alloyed hot-dip galvanized layer.