WO2016158160A1 - HIGH-STRENGTH COLD-ROLLED STEEL SHEET HAVING EXCELLENT WORKABILITY AND COLLISION CHARACTERISTICS AND HAVING TENSILE STRENGTH OF 980 MPa OR MORE, AND METHOD FOR PRODUCING SAME - Google Patents

Abstract

Provided are: a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, having good workability as evaluated by ductility and stretch-flangeability, and having excellent collision characteristics; and a method for producing said steel sheet. In this high-strength cold-rolled steel sheet, the metal structure at a position of ¼ sheet thickness satisfies (1)-(4) below. (1) The area ratio of ferrite is 10-65%, inclusive, and the balance is a hard phase comprising at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite, said hard phase including quenched martensite and retained austenite. (2) The volume ratio V_γ of retained austenite is 5-30%, inclusive. (3) The area ratio V_MA of an MA structure in which quenched martensite and retained austenite are combined is 3-25%, inclusive, and the average equivalent circle diameter of the MA structure is 2.0 μm or less. (4) The ratio V_MA/V_γ of the area ratio V_MA of the MA structure to the volume ratio V_γ of the retained austenite is 0.50-1.50.

Description

High-strength cold-rolled steel sheet with excellent workability and impact properties and a tensile strength of 980 MPa or more, and a method for producing the same

The present invention relates to a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more excellent in workability and impact characteristics, and a method for producing the same. Specifically, the high-strength cold-rolled steel sheet, the high-strength cold-rolled steel sheet, the high-strength electro-galvanized steel sheet formed with the electrogalvanized layer on the surface, and the hot-rolled galvanized layer formed on the surface of the high-strength cold-rolled steel sheet The present invention relates to a high-strength hot-dip galvanized steel sheet, a high-strength galvannealed steel sheet in which an alloyed hot-dip galvanized layer is formed on the surface of the high-strength cold-rolled steel sheet, and methods for producing these.

It is desired to reduce the weight of automobiles, transport aircraft, etc. in order to realize fuel efficiency reduction of automobiles, transport aircraft, etc. In order to reduce the weight, for example, it is effective to use a high-strength steel plate and reduce the plate thickness. However, when the strength of the steel plate is increased, ductility and stretch flangeability are deteriorated, so that workability into a product shape is deteriorated.

Further, from the viewpoint of corrosion resistance, the surface of automobile steel parts is electrogalvanized (hereinafter sometimes referred to as EG), hot dip galvanized (hereinafter sometimes referred to as GI), and alloyed. Steel plates (hereinafter, sometimes collectively referred to as galvanized steel plates) subjected to galvanizing such as hot dip galvanizing (hereinafter sometimes referred to as GA) are often used. These galvanized steel sheets are also required to be strengthened and workable in the same manner as the high-strength steel sheets.

For example, Patent Document 1 discloses an alloyed hot dip galvanized alloy having a metal structure in which martensite and retained austenite are mixed in ferrite, and having a good press workability with a tensile strength TS of 490 to 880 MPa by strengthening the composite structure. The steel plate which gave is disclosed.

Patent Document 2 discloses that TS (Tensile Strength) is 590 MPa or more and excellent in moldability, specifically, TS × EL (EL: Elongation, elongation) is 23000 MPa% or more, a salt warm water test, and a salt spray. A high-strength cold-rolled steel sheet is disclosed that has excellent post-painting corrosion resistance even in harsh environments such as tests and combined cycle corrosion tests. The metal structure of this steel sheet is a structure containing ferrite, retained austenite, bainite and / or martensite. It is described that the retained austenite has a function of improving the ductility of the steel sheet, that is, a so-called TRIP effect.

Incidentally, steel parts for automobiles are also required to have excellent collision characteristics for efficiently absorbing the impact when the automobile collides. As a technique for improving the collision characteristics, for example, Patent Document 3 is known. Patent Document 3 discloses a high-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more, excellent in impact absorption energy, and capable of achieving both a static ratio comparable to that of a 590 MPa class steel plate and a tensile maximum strength of 900 MPa or more, and a method for producing the same. Is disclosed. This manufacturing method is characterized in that after galvanization, cooling is performed, and rolling is performed using a roll having a roughness (Ra) of 3.0 or less.

Japanese Patent No. 3527092 Japanese Patent No. 5076434 Japanese Patent No. 5487916

According to the techniques described in Patent Documents 1 and 2, the workability of the steel sheet can be improved. However, no consideration is given to the collision characteristics. On the other hand, according to the technique described in Patent Document 3, the collision characteristics of the steel sheet can be improved. However, nothing is considered about the workability evaluated by ductility and stretch flangeability.

The present invention has been made paying attention to the above circumstances, and its purpose is a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, and has workability evaluated by ductility and stretch flangeability. An object of the present invention is to provide a high-strength cold-rolled steel sheet that is good and has excellent impact characteristics. Another object of the present invention is to provide a high-strength electrogalvanized steel sheet having an electrogalvanized layer on the surface of the high-strength cold-rolled steel sheet, a high-strength molten metal having a hot-dip galvanized layer on the surface of the high-strength cold-rolled steel sheet. An object of the present invention is to provide a galvanized steel sheet and a high-strength galvannealed steel sheet having an alloyed galvanized layer on the surface of the high-strength cold-rolled steel sheet. Another object of the present invention is to provide a method for producing a high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet, and a high-strength galvannealed steel sheet having the above characteristics.

The high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more according to the present invention that has been able to solve the above-mentioned problems is mass%, C: 0.10% to 0.5%, Si: 1.0% or more. 3% or less, Mn: 1.5% to 7%, P: more than 0% to 0.1%, S: more than 0% to 0.05%, Al: 0.005% to 1%, N: A steel sheet containing more than 0% and 0.01% or less, and O: more than 0% and 0.01% or less, with the balance being iron and inevitable impurities. The gist is that the metal structure at the 1/4 position of the plate thickness satisfies the following (1) to (4). Note that MA is an abbreviation for Martensite-Authentite Constituent.
(1) When the metal structure is observed with a scanning electron microscope, the area ratio of ferrite is greater than 10% and 65% or less with respect to the entire metal structure, and the balance includes quenched martensite and retained austenite, It is a hard phase composed of at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite.
(2) When the metal structure is measured by the X-ray diffraction method, the volume fraction V _{γ of} retained austenite is 5% or more and 30% or less with respect to the entire metal structure.
(3) When the metal structure is observed with an optical microscope, the area ratio V _MA of the MA structure in which quenched martensite and retained austenite are combined with respect to the entire metal structure is 3% or more and 25% or less. The average equivalent circle diameter of the tissue is 2.0 μm or less.
(4) The ratio V _MA / V _γ of the area ratio V _MA of the MA structure to the volume ratio V _γ of the retained austenite satisfies the following formula (i).
0.50 ≦ V _MA / V _γ ≦ 1.50 (i)

The steel sheet, as another element, in mass%,
(A) at least one selected from the group consisting of Cr: more than 0% and 1% or less and Mo: more than 0% and 1% or less,
(B) at least one selected from the group consisting of Ti: more than 0% and 0.15% or less, Nb: more than 0% and 0.15% or less, and V: more than 0% and 0.15% or less,
(C) at least one selected from the group consisting of Cu: more than 0% and 1% or less and Ni: more than 0% and 1% or less,
(D) B: more than 0% and 0.005% or less,
(E) at least one selected from the group consisting of Ca: more than 0% and 0.01% or less, Mg: more than 0% and 0.01% or less, and REM: more than 0% and 0.01% or less,
Etc. may be contained.

In the present invention, a high-strength electrogalvanized steel sheet having an electrogalvanized layer on the surface of the high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the high-strength cold-rolled steel sheet, A high-strength galvannealed steel sheet having an alloyed galvanized layer on the surface of a high-strength cold-rolled steel sheet is also included.

The high-strength cold-rolled steel sheet having excellent workability and impact characteristics according to the present invention and having a tensile strength of 980 MPa or more is a steel that satisfies the above-mentioned composition, and is finished at a rolling rate of 5 to 25% in the final stand of finish rolling. Hot rolling is performed at an Ar ₃ point or higher and 900 ° C. or lower, and the winding temperature is 600 ° C. or lower, winding is performed, cooled to room temperature, cold rolled, and 800 ° C. at an average temperature increase rate of 10 ° C./second or higher. Heated to a temperature range of less than Ac ₃ point, held in the temperature range for 50 seconds or more, soaked, and average cooling rate of 10 ° C./second to any cooling stop temperature T ° C. in the temperature range of 50 ° C. or more and Ms point or less It can be manufactured by cooling and heating to room temperature for 50 seconds or more in the temperature range of above the cooling stop temperature T ° C. and below 550 ° C.

A high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more and excellent workability and impact characteristics according to the present invention is a steel satisfying the above-described component composition, with a rolling rate of 5 to 25% in the final stand of finish rolling, Hot rolling is performed at a finish rolling end temperature of Ar ₃ or higher and 900 ° C. or lower, winding is performed at a winding temperature of 600 ° C. or lower, cooled to room temperature, cold rolled, and 800 ° C./sec or higher at an average temperature increase rate of 10 ° C./second or higher. It is heated to a temperature range of not less than ₃ ° C. and less than Ac ₃ points, soaked in the temperature range for 50 seconds or more, soaked, and an average cooling rate of 10 ° C./temperature until an arbitrary cooling stop temperature T ° C. in the temperature range of 50 ° C. It can be manufactured by cooling for at least 2 seconds, heating and holding for 50 seconds or more in the temperature range above the cooling stop temperature T ° C. and below 550 ° C., and performing hot dip galvanization within the holding time and then cooling to room temperature. The

The high-strength alloyed hot-dip galvanized steel sheet having excellent workability and impact characteristics according to the present invention and having a tensile strength of 980 MPa or more is obtained by using a steel satisfying the above-described component composition at a rolling rate of 5 to 25 in the final stand of finish rolling. %, Hot rolling at a finish rolling finish temperature of Ar ₃ point to 900 ° C., winding at a winding temperature of 600 ° C. or less, cooling to room temperature, cold rolling, average heating rate of 10 ° C./second or more At a temperature range of 800 ° C. or more and less than Ac ₃ point, and held in the temperature range for 50 seconds or more, soaking, and an average cooling rate of 10 to an arbitrary cooling stop temperature T ° C. in a temperature range of 50 ° C. or more and Ms point or less. After cooling at a temperature of ℃ / second or more and heating and holding for 50 seconds or more in the temperature range above the cooling stop temperature T ° C. or more and 550 ° C., hot-dip galvanization is performed within the holding time, and further alloying treatment is performed. To room temperature It can manufacture by cooling with.

According to the present invention, since the component composition and the metal structure are appropriately controlled, high strength cold rolling with a tensile strength of 980 MPa or more excellent in both workability evaluated by ductility and stretch flangeability and impact properties is achieved. Steel sheets, high-strength electrogalvanized steel sheets, high-strength hot-dip galvanized steel sheets, and high-strength galvannealed steel sheets can be provided. The high-strength cold-rolled steel sheet, high-strength electrogalvanized steel sheet, high-strength hot-dip galvanized steel sheet, and high-strength galvannealed steel sheet according to the present invention are particularly excellent in ductility among workability. Moreover, according to this invention, the method of manufacturing the said high-strength cold-rolled steel plate, a high-strength electrogalvanized steel plate, a high-strength hot-dip galvanized steel plate, and a high-strength galvannealed steel plate can be provided. The high-strength cold-rolled steel sheet, high-strength electrogalvanized steel sheet, high-strength hot-dip galvanized steel sheet, and high-strength galvannealed steel sheet according to the present invention are extremely useful particularly in industrial fields such as automobiles.

FIG. 1 is a schematic explanatory diagram illustrating an example of a heat treatment pattern performed in the example.

In order to improve all of ductility, stretch flangeability, and impact characteristics, the present inventors have made extensive studies on a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more. As a result, in order to ensure the tensile strength, the ferrite fraction in the metal structure is within a predetermined range, and the remaining structure is made a hard phase, and in order to improve ductility, a predetermined amount of ferrite is generated. The ratio V _MA / V _γ of the area ratio V _MA of the MA structure in which hardened martensite and retained austenite are combined and the volume ratio V _{γ of} retained austenite with respect to the entire metal structure may be appropriately controlled. In order to improve the flangeability, the MA structure should be made finer, and in order to improve the collision characteristics, the MA structure should be made finer and the ratio V _MA / _Vγ should be controlled appropriately. The present invention has been completed.

First, the metal structure that characterizes the present invention will be described.

The high-strength cold-rolled steel sheet according to the present invention is characterized in that the metal structure at the 1/4 position of the sheet thickness satisfies the following (1) to (4).
(1) When the metal structure is observed with a scanning electron microscope, the area ratio of ferrite is greater than 10% and 65% or less with respect to the entire metal structure, and the balance includes quenched martensite and retained austenite, It is a hard phase composed of at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite.
(2) When the metal structure is measured by the X-ray diffraction method, the volume fraction V _{γ of} retained austenite is 5% or more and 30% or less with respect to the entire metal structure.
(3) When the metal structure is observed with an optical microscope, the area ratio V _MA of the MA structure in which quenched martensite and retained austenite are combined with respect to the entire metal structure is 3% or more and 25% or less. The average equivalent circle diameter of the tissue is 2.0 μm or less.
(4) The volume ratio V _{γ of the} retained austenite and the area ratio V _{MA of the} MA structure satisfy the following formula (i).
0.50 ≦ V _MA / V _γ ≦ 1.50 (i)

The observation of the above metal structure is all 1/4 of the plate thickness on behalf of the steel plate.

Note that the fractions of the metal structure defined in the above (1) to (3) may differ from the measurement method, so the sum of the fractions may exceed 100%. That is, in (1) above, the metal structure is observed with a scanning electron microscope, and the measured area ratio is a ratio when the entire metal structure is 100%. The area ratio measured using a scanning electron microscope includes hardened martensite and retained austenite as the area ratio of the hard phase. On the other hand, in (2) above, the residual austenite fraction of the metal structure is calculated by the X-ray diffraction method, and in (3) above, the area ratio of the MA structure in which quenched martensite and residual austenite are combined is measured with an optical microscope. Observe at. Therefore, the fractions of retained austenite and quenched martensite are measured by a plurality of methods. Hereinafter, retained austenite may be referred to as retained γ. Therefore, the sum of the fractions of the metal structures defined in the above (1) to (3) may exceed 100%. In addition, a structure in which quenched martensite and residual γ are combined may be referred to as an MA structure.

(1) In the present invention, when the metal structure is observed with a scanning electron microscope, the area ratio of ferrite is more than 10% and 65% or less with respect to the entire metal structure. Ferrite is a structure that particularly improves the ductility of the workability of a steel sheet. In order to exert such an effect, in the present invention, the area ratio of ferrite is set to more than 10%. The area ratio of ferrite is preferably 15% or more, and more preferably 20% or more. However, when the ferrite becomes excessive, the strength of the steel sheet is lowered, and a tensile strength of 980 MPa or more cannot be ensured. Therefore, in the present invention, the area ratio of ferrite is set to 65% or less. The area ratio of ferrite is preferably 60% or less, more preferably 50% or less.

The balance of the metal structure includes hardened martensite and residual γ as essential structures, and is a hard phase composed of at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite. These hard phases have a structure harder than ferrite, and the strength of the steel sheet can be increased to 980 MPa or more by forming a predetermined amount of ferrite and using the remaining structure as a hard phase. The reason why hardened martensite and residual γ are included as the essential structure is to generate a predetermined amount of MA structure in which hardened martensite and residual γ are combined, as will be described later.

The metal structure may include at least one selected from the group consisting of pearlite and cementite in addition to ferrite and a hard phase. The total area ratio of pearlite and cementite is not particularly limited as long as the effects of the present invention are not impaired. For example, 20% or less is preferable. The total area ratio is more preferably 15% or less, still more preferably 10% or less.

The area ratio of the metal structure may be calculated by observing a 1/4 position of the plate thickness with nital corrosion and then observing with a scanning electron microscope, and the observation magnification may be 1000 times, for example.

(2) In the present invention, when the metal structure is measured by the X-ray diffraction method, the volume ratio V _γ of the residual γ is set to 5% or more and 30% or less with respect to the entire metal structure. Residual γ has an effect of suppressing the concentration of strain by accelerating hardening of the deformed portion during processing by being deformed by deformation when transformed into martensite when the steel plate is processed. Therefore, the strength-elongation balance of the steel sheet is improved, and the ductility can be improved. In order to exert such an effect, the volume ratio of the residual γ needs to be 5% or more. The volume ratio of the residual γ is preferably 6% or more, more preferably 7% or more. However, if the volume ratio of residual γ increases excessively, stretch flangeability deteriorates. Therefore, in the present invention, the volume ratio of residual γ is set to 30% or less. The volume ratio of the residual γ is preferably 25% or less, more preferably 20% or less.

The volume ratio of the residual γ may be obtained by measuring the 1/4 position of the plate thickness by the X-ray diffraction method. Residual γ is present between the laths of bainitic ferrite or included in the MA structure. Since the effect of the residual γ is exhibited regardless of the presence form, in the present invention, the amount of all residual γ measured by the X-ray diffraction method is summed regardless of the presence form to obtain the volume ratio. .

(3) In the present invention, when the metal structure is observed with an optical microscope, the area ratio V _MA of the _{MA structure} is 3% or more and 25% or less with respect to the entire metal structure. The MA structure is a structure that improves the strength-elongation balance of the steel sheet and can improve the ductility. In order to exert such an effect, the area ratio of the MA structure needs to be 3% or more. The area ratio of the MA structure is preferably 4% or more, more preferably 5% or more. However, when the area ratio of the MA structure increases excessively, the collision characteristics deteriorate. Therefore, in the present invention, the area ratio of the MA structure is 25% or less. The area ratio of the MA structure is preferably 23% or less, more preferably 20% or less.

In the present invention, the average equivalent circle diameter of the MA structure is 2.0 μm or less. By reducing the MA structure, stretch flangeability and impact characteristics can be improved. In order to exert such an effect, it is necessary that the average equivalent circle diameter of the MA structure is 2.0 μm or less. The average equivalent circle diameter of the MA structure is preferably 1.8 μm or less, more preferably 1.5 μm or less. Note that, as the MA structure becomes finer, the stretch flangeability and the impact characteristics become better, so the lower limit of the average equivalent circle diameter of the MA structure is not particularly limited, but about 0.1 μm is the limit industrially. .

The MA structure is a structure in which hardened martensite and residual γ are combined. The hardened martensite is a structure in which untransformed austenite is martensitic transformed in the process of cooling the steel sheet from the heating temperature to room temperature. means. Quenched martensite can be distinguished from tempered martensite tempered by heat treatment by observing with an optical microscope. That is, hardened martensite is observed in white when observed with an optical microscope after repeller corrosion of the metal structure, whereas tempered martensite is observed in gray.

In addition, since hardened martensite and residual γ are difficult to distinguish by observation with an optical microscope, in the present invention, a structure in which hardened martensite and residual γ are combined is measured as an MA structure.

The area ratio of the MA structure is a value measured at a position where the steel sheet has a thickness of 1/4.

The average equivalent circle diameter of the MA structure is a value obtained by calculating the equivalent circle particle diameter based on the area of each MA structure for all the MA structures recognized in the observation field, and averaging these.

(4) In the present invention, it is important that the ratio V _MA / V _γ of the area ratio V _MA of the MA structure to the volume ratio V _γ of the residual γ satisfies the following formula (i).
0.50 ≦ V _MA / V _γ ≦ 1.50 (i)

By controlling the value of the ratio V _MA / V _γ so as to satisfy the above formula (i), both ductility and collision characteristics can be achieved. That is, as described above, in the present invention, the residual γ is positively generated in order to improve the strength-elongation balance that is an index of ductility. As a result, an MA structure is inevitably formed in the steel sheet. Further, when the strength-elongation balance was further examined, when a predetermined amount of residual γ was generated, the area ratio of the MA structure was such that the value of the ratio V _MA / V _γ was 0.50 or more. it has been found that it is sufficient to control the V _MA. The value of the ratio V _MA / V _γ is preferably 0.55 or more, more preferably 0.60 or more. However, if the value of the ratio V _MA / V _γ becomes too large, an excessive amount of MA structure is generated. The hardened martensite present in the MA structure is a very hard structure. Therefore, if the MA structure is excessively formed, cracks are likely to occur at the interface with other structures at the time of collision, and the collision characteristics deteriorate instead. . Therefore, in the present invention, the ratio V _MA / V _γ is set to 1.50 or less in order to reduce the area ratio of the quenched martensite in the MA structure and ensure the collision characteristics. The value of the ratio V _MA / V _γ is preferably 1.40 or less, more preferably 1.30 or less.

The metal structure of the high-strength cold-rolled steel sheet that characterizes the present invention has been described above.

Next, the component composition of the high-strength cold-rolled steel sheet according to the present invention will be described. Hereinafter, “%” means “mass%” for the component composition of the steel sheet.

[C: 0.10% to 0.5%]
C is an element necessary for ensuring a tensile strength of 980 MPa or more, increasing the stability of residual γ, and ensuring a predetermined amount of residual γ. In the present invention, the C amount is 0.10% or more. The amount of C is preferably 0.12% or more, more preferably 0.15% or more. However, when the amount of C becomes excessive, the strength after hot rolling increases, cracking occurs during cold rolling, and the weldability of the final product decreases. Therefore, the C amount is 0.5% or less. The amount of C is preferably 0.40% or less, more preferably 0.30% or less, and still more preferably 0.25% or less.

[Si: 1.0% to 3%]
Si is an element that acts as a solid solution strengthening element and contributes to increasing the strength of steel. Si is an element necessary to suppress the formation of carbides, effectively act on the formation of ferrite and residual γ, and ensure an excellent strength-elongation balance. In the present invention, the Si amount is 1.0% or more. The amount of Si is preferably 1.2% or more, more preferably 1.35% or more, and further preferably 1.5% or more. However, when the amount of Si becomes excessive, a remarkable scale is formed during hot rolling, and scale marks are formed on the surface of the steel sheet, resulting in poor surface properties. Moreover, pickling property also worsens. Therefore, the Si amount is 3% or less. The amount of Si is preferably 2.8% or less, more preferably 2.6% or less.

[Mn: 1.5% to 7%]
Mn is an element that contributes to increasing the strength of the steel sheet by improving the hardenability. Mn is an element necessary for stabilizing γ and generating residual γ. In the present invention, the amount of Mn is 1.5% or more. The amount of Mn is preferably 1.6% or more, more preferably 1.7% or more, further preferably 1.8% or more, and still more preferably 2.0% or more. However, when the amount of Mn becomes excessive, the strength after hot rolling increases, cracking occurs during cold rolling, and the weldability of the final product deteriorates. Moreover, when Mn is added excessively, Mn will segregate and cause ductility and stretch flangeability to deteriorate. Therefore, in the present invention, the amount of Mn is 7% or less. The amount of Mn is preferably 5.0% or less, more preferably 4.0% or less, and still more preferably 3.0% or less.

[P: more than 0% and 0.1% or less]
P is an impurity element inevitably included, and if it is excessively contained, the weldability of the final product deteriorates. Therefore, in the present invention, the P amount is 0.1% or less. The amount of P is preferably 0.08% or less, more preferably 0.05% or less. The amount of P is preferably as small as possible, but it is industrially difficult to reduce it to 0%. Industrially, the lower limit of the amount of P is 0.0005%.

[S: more than 0% and 0.05% or less]
S, like P, is an inevitably contained impurity element, and if contained excessively, the weldability of the final product deteriorates. Further, S forms sulfide inclusions in the steel sheet and causes the ductility and stretch flangeability of the steel sheet to deteriorate. Therefore, in the present invention, the S amount is 0.05% or less. The amount of S is preferably 0.01% or less, more preferably 0.005% or less. The amount of S should be as small as possible, but it is industrially difficult to make it 0%. The lower limit of the amount of S is industrially 0.0001%.

[Al: 0.005% to 1%]
Al is an element that acts as a deoxidizing agent. In order to exert such an effect, the Al content is set to 0.005% or more in the present invention. The amount of Al is more preferably 0.01% or more. However, when the amount of Al becomes excessive, the weldability of the final product is significantly deteriorated. Therefore, in the present invention, the Al amount is 1% or less. The amount of Al is preferably 0.8% or less, more preferably 0.6% or less.

[N: more than 0% and 0.01% or less]
N is an impure element contained inevitably, and when it is contained excessively, a large amount of nitride precipitates and deteriorates ductility, stretch flangeability, and impact characteristics. Therefore, in the present invention, the N content is 0.01% or less. The N amount is preferably 0.008% or less, more preferably 0.005% or less. Note that the amount of N may be 0.001% or more because a small amount of nitride contributes to increasing the strength of the steel sheet.

[O: more than 0% and 0.01% or less]
O is an impure element contained inevitably, and when excessively contained, it is an element that deteriorates ductility and collision characteristics. Therefore, in the present invention, the O amount is 0.01% or less. The amount of O is preferably 0.005% or less, more preferably 0.003% or less. The amount of O is preferably as small as possible, but it is industrially difficult to make it 0%. The lower limit of the amount of O is industrially 0.0001%.

The cold-rolled steel sheet according to the present invention satisfies the above component composition, and the balance is iron and inevitable impurities. The inevitable impurities include, for example, the above-mentioned P, S, N, and O that may be brought into steel depending on the situation of raw materials, materials, manufacturing equipment, and the like, and other trump elements such as Pb, Bi, Sb, Sn. May be included.

The cold-rolled steel sheet according to the present invention further includes other elements,
(A) at least one selected from the group consisting of Cr: more than 0% and 1% or less and Mo: more than 0% and 1% or less,
(B) at least one selected from the group consisting of Ti: more than 0% and 0.15% or less, Nb: more than 0% and 0.15% or less, and V: more than 0% and 0.15% or less,
(C) at least one selected from the group consisting of Cu: more than 0% and 1% or less and Ni: more than 0% and 1% or less,
(D) B: more than 0% and 0.005% or less,
(E) at least one selected from the group consisting of Ca: more than 0% and 0.01% or less, Mg: more than 0% and 0.01% or less, and REM: more than 0% and 0.01% or less,
Etc. may be contained.

These elements (a) to (e) can be contained alone or in any combination. The reason for setting this range is as follows.

[(A) Cr: at least one selected from the group consisting of more than 0% and 1% or less and Mo: more than 0% and 1% or less]
Cr and Mo are both elements that effectively act to improve the hardenability and improve the strength of the steel sheet. In order to effectively exhibit such an action, Cr and Mo are each preferably 0.1% or more, and more preferably 0.3% or more. However, when it contains excessively, ductility and stretch flangeability will fall. Moreover, excessive addition becomes high cost. Therefore, when adding Cr and Mo independently, it is preferable to set it as 1% or less, More preferably, it is 0.8% or less, More preferably, it is 0.5% or less. Cr and Mo can be used alone or in combination. When Cr and Mo are used in combination, it is preferably within the above range when contained alone, and the total amount of Cr and Mo is preferably 1.5% or less.

[(B) At least one selected from the group consisting of Ti: more than 0% and not more than 0.15%, Nb: more than 0% and not more than 0.15%, and V: more than 0% and not more than 0.15%]
Ti, Nb, and V are all elements that have the action of forming carbides and nitrides in the steel sheet, improving the strength of the steel sheet, and refining the old γ grains. In order to exhibit such an action effectively, Ti, Nb, and V are each preferably 0.005% or more, and more preferably 0.010% or more. However, if contained excessively, carbide precipitates at the grain boundaries, and the stretch flangeability and impact characteristics of the steel sheet deteriorate. Therefore, in the present invention, Ti, Nb, and V are each preferably 0.15% or less, more preferably 0.12% or less, and still more preferably 0.10% or less. These elements can be used alone or in combination of two or more selected arbitrarily.

[(C) At least one selected from the group consisting of Cu: more than 0% and 1% or less, and Ni: more than 0% and 1% or less]
Cu and Ni are elements that effectively act to generate and stabilize residual γ. Moreover, Cu and Ni also have the effect | action which improves the corrosion resistance of a steel plate. In order to effectively exhibit such an action, Cu and Ni are each preferably 0.05% or more, and more preferably 0.10% or more. However, since hot workability deteriorates when Cu is contained excessively, when Cu is added alone, it is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5%. % Or less. On the other hand, if Ni is excessively contained, the cost increases, so the Ni content is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. Cu and Ni can be used alone or in combination. When Cu and Ni are used in combination, the above-described action is easily exhibited, and by adding Ni, deterioration of hot workability due to addition of Cu is easily suppressed. When Cu and Ni are used in combination, the total amount is preferably 1.5% or less, and more preferably 1.0% or less.

[(D) B: more than 0% and 0.005% or less]
B is an element that improves hardenability, and is an element that acts to make austenite stably exist up to room temperature. In order to effectively exert such effects, the B content is preferably 0.0005% or more, more preferably 0.0010% or more, and further preferably 0.0015% or more. However, when it contains excessively, a boride may be produced | generated and ductility may be deteriorated. Therefore, the B content is preferably 0.005% or less. The amount of B is more preferably 0.004% or less, and still more preferably 0.0035% or less.

[(E) Ca: at least one selected from the group consisting of more than 0% and less than 0.01%, Mg: more than 0% and less than 0.01%, and REM: more than 0% and less than 0.01%]
Ca, Mg, and REM are elements having an action of finely dispersing inclusions in the steel sheet. In order to exhibit such an action effectively, the Ca, Mg, and REM amounts are each preferably 0.0005% or more, and more preferably 0.0010% or more. However, excessive addition may cause deterioration of castability, hot workability, and the like. Accordingly, the Ca, Mg, and REM amounts are each preferably 0.01% or less, more preferably 0.008% or less, and still more preferably 0.007% or less. These elements can be used alone or in combination of two or more selected arbitrarily. In the present invention, REM is an abbreviation for Rare earth metal (rare earth element), and means to include lanthanoid elements, that is, 15 elements from La to Lu, and Sc and Y.

The high strength cold-rolled steel sheet according to the present invention has been described above.

The high-strength cold-rolled steel sheet may have an electrogalvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer on the surface. That is, the present invention includes a high-strength electrogalvanized steel sheet (hereinafter sometimes referred to as EG steel sheet) having an electrogalvanized layer on the surface of the high-strength cold-rolled steel sheet, and the surface of the high-strength cold-rolled steel sheet. High-strength hot-dip galvanized steel sheet (hereinafter sometimes referred to as GI steel sheet), high-strength galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of the high-strength cold-rolled steel sheet A plated steel plate (hereinafter sometimes referred to as a GA steel plate) is also included.

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

The high-strength cold-rolled steel sheet is hot-rolled with a steel satisfying the above-described component composition, with a rolling rate of 5 to 25% in the final stand of finish rolling, and a finish rolling end temperature of Ar _{3 to} 900 ° C., Winding temperature is set to 600 ° C. or lower, cooled to room temperature, cold-rolled, heated to a temperature range of 800 ° C. or higher and less than Ac ₃ points at an average heating rate of 10 ° C./second or higher, and 50 Hold for at least 2 seconds, soak, and cool at an average cooling rate of 10 ° C./second or higher to an arbitrary cooling stop temperature T ° C. in the temperature range of 50 ° C. or higher and Ms point or lower, and then heated to the above cooling stop temperature T ° C. over 550 ° C. It can be produced by holding for 50 seconds or more in the following temperature range and then cooling to room temperature.

Hereafter, I will explain in order.

[Rolling ratio in final stand of finish rolling: 5-25%]
First, steel satisfying the above component composition is heated according to a conventional method. The heating temperature is not particularly limited, but is preferably 1000 to 1300 ° C., for example. If the heating temperature is less than 1000 ° C., the solid solution of the carbide becomes insufficient, and it is difficult to obtain sufficient strength. On the other hand, when the heating temperature exceeds 1300 ° C., the structure of the hot-rolled steel sheet is coarsened, and the MA structure of the cold-rolled steel sheet is easily coarsened. As a result, the collision characteristics tend to deteriorate.

After the heating, hot rolling is performed. In the present invention, it is important that the rolling reduction in the final stand of finish rolling is 5 to 25%. When the rolling rate is less than 5%, the austenite grain size after hot rolling becomes coarse, and the average equivalent circle diameter of the MA structure in the cold-rolled steel sheet after annealing becomes large. As a result, stretch flangeability deteriorates. Therefore, in the present invention, the rolling ratio needs to be 5% or more. The rolling reduction is preferably 6% or more, more preferably 7% or more, and further preferably 8% or more. However, even if the rolling rate exceeds 25%, the average equivalent circle diameter of the MA structure increases, and the stretch flangeability and the impact characteristics deteriorate. Although this mechanism is unknown, it is thought that the structure after hot rolling is heterogeneous. In the present invention, the rolling ratio needs to be 25% or less. The rolling reduction is preferably 23% or less, more preferably 20% or less.

[Finish rolling finish temperature: Ar ₃ points to 900 ° C]
If the finish rolling finish temperature is lower than the temperature at the Ar ₃ point, the steel sheet structure after hot rolling becomes inhomogeneous, and the stretch flangeability decreases. On the other hand, when the finish rolling finish temperature exceeds 900 ° C., recrystallization of austenite occurs, the crystal grains become coarse, and the average equivalent circle diameter of the MA structure in the cold-rolled steel sheet increases. As a result, stretch flangeability deteriorates. Therefore, in the present invention, the finish rolling end temperature needs to be 900 ° C. or less. The finish rolling end temperature is preferably 890 ° C. or lower, more preferably 880 ° C. or lower.

The temperature at the Ar ₃ point was calculated based on the following formula (ii). In the formula, [] indicates the content (% by mass) of each element, and the content of elements not included in the steel sheet may be calculated as 0% by mass.
Ar ₃ point (° C.) = 910−310 × [C] −80 × [Mn] −20 × [Cu] −15 × [Cr] −55 × [Ni] −80 × [Mo] (ii)

[Winding temperature: 600 ° C or less]
When the coiling temperature exceeds 600 ° C., the crystal grains become coarse, and the average equivalent circle diameter of the MA structure in the cold-rolled steel sheet increases. As a result, stretch flangeability deteriorates. Therefore, in the present invention, the winding temperature is 600 ° C. or less. The winding temperature is preferably 580 ° C. or lower, more preferably 570 ° C. or lower, and further preferably 550 ° C. or lower.

[Cold rolling]
After hot rolling, it may be wound, cooled to room temperature, pickled according to a conventional method if necessary, and then cold rolled according to a conventional method. The cold rolling rate in the cold rolling may be 30 to 80%, for example.

[Annealing]
After the cold rolling, annealing is performed by heating to a temperature range of 800 ° C. or more and less than Ac ₃ point at an average temperature increase rate of 10 ° C./second or more and holding the temperature range for 50 seconds or more and soaking. After the cold rolling, if the average rate of temperature rise to the above temperature range is less than 10 ° C./second, austenite grains grow and become coarse during heating, so the average equivalent circle diameter of the MA structure in the cold-rolled steel sheet is Increases and stretch flangeability decreases. Therefore, in this invention, the said average temperature increase rate shall be 10 degrees C / sec or more. The average temperature rising rate is preferably 12 ° C./second or more, more preferably 15 ° C./second or more. The upper limit of the average heating rate is not particularly limited, but is usually about 100 ° C./second at the maximum.

By setting the soaking temperature to 800 ° C. or higher and lower than Ac ₃ point, a desired amount of ferrite can be secured. When the soaking temperature is lower than 800 ° C., the reverse transformation to austenite becomes insufficient, the work structure remains in the cold-rolled steel sheet, and the workability decreases. Therefore, in the present invention, the soaking temperature is set to 800 ° C. or higher. The soaking temperature is preferably 805 ° C. or higher, more preferably 810 ° C. or higher. However, if the soaking temperature is equal to or higher than the Ac ₃ point temperature, the desired ferrite content cannot be ensured and ductility deteriorates. Therefore, in the present invention, the soaking temperature is set to a temperature lower than the Ac ₃ point. The soaking temperature is preferably Ac ₃ point −10 ° C. or lower, more preferably Ac ₃ point −20 ° C. or lower.

When the soaking time is less than 50 seconds, the processed structure remains in the cold-rolled steel sheet and the ductility deteriorates. Therefore, in the present invention, the soaking time is set to 50 seconds or more. The soaking time is more preferably 60 seconds or longer. The upper limit of the soaking time is not particularly limited, but if the soaking time is too long, the concentration of Mn into the austenite phase proceeds, the Ms point may decrease, and the MA structure may increase and become coarse. Therefore, the soaking time is preferably 3600 seconds or less, more preferably 3000 seconds or less.

The soaking in the above temperature range does not need to be held at the same temperature, and may be varied by heating, cooling and changing in the above temperature range.

The temperature of the Ac ₃ point can be calculated based on the following formula (iii) described in “Leslie Steel Material Chemistry” (Maruzen Co., Ltd., issued May 31, 1985, page 273). In the formula, [] indicates the content (% by mass) of each element, and the content of elements not included in the steel sheet may be calculated as 0% by mass.
Ac ₃ (° C.) = 910−203 × [C] ^1/2 −15.2 × [Ni] + 44.7 × [Si] + 104 × [V] + 31.5 × [Mo] + 13.1 × [W] − (30 × [Mn] + 11 × [Cr] + 20 × [Cu] −700 × [P] −400 × [Al] −120 × [As] −400 × [Ti]) (iii)

[cooling]
After maintaining the soaking, the temperature is cooled to an arbitrary cooling stop temperature T ° C. in a temperature range of 50 ° C. or higher and Ms point or lower. By cooling to this temperature range, untransformed austenite can be transformed into martensite and hard bainite phase, and the MA structure can also be refined. At this time, martensite exists as quenched martensite immediately after the transformation, but is tempered while being reheated and held in a subsequent process, and remains as tempered martensite. This tempered martensite does not adversely affect any of the ductility, stretch flangeability, and impact characteristics of the steel sheet. However, when the cooling stop temperature T exceeds the Ms point, martensite is not generated, the MA structure generated in the reheating and holding process at a high temperature is coarsened, the local deformability is lowered, and the stretch flangeability is improved. Can not. Therefore, in the present invention, the cooling stop temperature T is set to be equal to or lower than the temperature at the Ms point. The cooling stop temperature T is preferably Ms point −20 ° C. or lower, more preferably Ms point −50 ° C. or lower. On the other hand, when the cooling stop temperature T is lower than 50 ° C., the residual γ and the MA structure are hardly generated, so the ductility cannot be improved. Therefore, in the present invention, the lower limit of the cooling stop temperature T is set to 50 ° C. or higher. The cooling stop temperature T is preferably 60 ° C. or higher, more preferably 70 ° C. or higher.

The temperature of the Ms point can be calculated based on the following formula (iv). In the formula, [] indicates the content (% by mass) of each element, and the content of elements not included in the steel sheet may be calculated as 0% by mass. In the formula, Vf indicates the area ratio of ferrite with respect to the entire metal structure.
Ms point (° C.) = 561-474 × [C] / (1-Vf / 100) −33 × [Mn] −17 × [Ni] −17 × [Cr] −21 × [Mo] (iv) )

It is also important that the average cooling rate to the cooling stop temperature T in the above temperature range is 10 ° C./second or more after the above-mentioned soaking is maintained. By appropriately controlling the cooling rate up to the cooling stop temperature T after the soaking is maintained, excessive generation of ferrite can be suppressed. That is, when the average cooling rate is less than 10 ° C./second, ferrite is excessively generated during cooling and the tensile strength is lowered. Therefore, in this invention, the said average cooling rate shall be 10 degrees C / sec or more. The average cooling rate is preferably 15 ° C./second or more, more preferably 20 ° C./second or more. In addition, the upper limit of the said average cooling rate is not specifically limited, You may cool by water cooling or oil cooling.

[Reheating process]
After cooling to an arbitrary cooling stop temperature T ° C. in the temperature range of 50 ° C. or higher and below the Ms point, reheat to a temperature range above the cooling stop temperature T ° C. and below 550 ° C., and hold at this temperature range for 50 seconds or longer. It is important to. By reheating to a temperature range of above the cooling stop temperature T ° C. to 550 ° C., the hard phase such as martensite can be tempered, and the untransformed austenite can be transformed into bainitic ferrite or bainite. If reheating is not performed, the balance between the residual γ and the amount of MA tissue produced becomes poor, and the ratio V _MA / V _γ of the MA tissue area ratio V _MA to the volume ratio V _γ of the residual γ is controlled within an appropriate range. Can not. As a result, the collision characteristics cannot be improved. In addition, the hard phase cannot be tempered, and high-density dislocation occurs. Therefore, in this invention, after cooling to the said cooling stop temperature T, it reheats to the temperature exceeding this cooling stop temperature T. The reheating temperature is preferably T + 20 ° C. or higher, more preferably T + 30 ° C. or higher, and further preferably T + 50 ° C. or higher. However, if the reheating temperature exceeds 550 ° C., residual γ and MA structure are hardly generated, so that the tensile strength is lowered, the value of TS × λ is also reduced, and stretch flangeability cannot be improved. Therefore, in the present invention, the reheating temperature is set to 550 ° C. or lower. The reheating temperature is preferably 520 ° C. or lower, more preferably 500 ° C. or lower, and further preferably 450 ° C. or lower.

In the present invention, “reheating” means heating from the cooling stop temperature T, that is, raising the temperature, as the wording indicates. Therefore, the reheating temperature is higher than the cooling stop temperature T, and the reheating temperature is the same as the cooling stop temperature T even if the reheating temperature is, for example, a temperature range of 50 ° C. or more and 550 ° C. or less. If the reheating temperature is lower than the cooling stop temperature T, it does not correspond to the reheating of the present invention.

After reheating to the above-mentioned cooling stop temperature T ° C. or more and 550 ° C. or less, the temperature is maintained for 50 seconds or more in the temperature range. When the reheat holding time is less than 50 seconds, the MA structure is excessively generated and ductility cannot be improved. Further, since the MA structure becomes coarse and the average equivalent circle diameter cannot be controlled appropriately, the stretch flangeability cannot be improved. Further, since the ratio V _MA / V _γ of the area ratio V _MA of the MA structure to the volume ratio V _γ of the residual γ cannot be appropriately controlled, the collision characteristics cannot be improved. Furthermore, the hard phase cannot be tempered sufficiently, and the transformation of untransformed austenite to bainitic ferrite or bainite does not proceed sufficiently. Therefore, in the present invention, the reheating holding time is 50 seconds or more. The reheating holding time is preferably 80 seconds or longer, more preferably 100 seconds or longer, and further preferably 200 seconds or longer. The upper limit of the reheating holding time is not particularly limited, but when the holding time is increased, productivity is lowered and tensile strength tends to be lowered. From such a viewpoint, the reheating holding time is preferably 1500 seconds or less, and more preferably 1000 seconds or less.

After cooling and holding again, cool to room temperature. Although the average cooling rate at the time of cooling is not specifically limited, For example, it is preferable that it is 0.1 degree-C / second or more, More preferably, it is 0.4 degree-C / second or more. Moreover, it is preferable that an average cooling rate is 200 degrees C / second or less, for example, More preferably, it is 150 degrees C / second or less.

[Plating treatment]
After the reheating and holding, the high-strength cold-rolled steel sheet according to the present invention obtained by cooling to room temperature may be subjected to electrogalvanizing, hot-dip galvanizing, or alloyed hot-dip galvanizing according to a conventional method.

For electrogalvanizing, for example, the high-strength cold-rolled steel sheet may be energized while being immersed in a zinc solution at 50 to 60 ° C. (particularly 55 ° C.) to perform electrogalvanizing treatment. The amount of plating adhesion is not particularly limited, and may be, for example, about 10 to 100 g / m ² per side.

For hot dip galvanization, for example, the high-strength cold-rolled steel sheet may be immersed in a hot dip galvanizing bath at 300 ° C. or higher and 550 ° C. or lower to perform hot dip galvanizing treatment. The plating time may be appropriately adjusted so as to ensure a desired plating adhesion amount, and is preferably set to 1 to 10 seconds, for example.

The alloyed hot dip galvanizing may be performed after the hot dip galvanizing. Although the alloying treatment temperature is not particularly limited, it is preferably 450 ° C. or more, more preferably 460 ° C. or more, and further preferably 480 ° C. or more because alloying does not proceed sufficiently if the alloying treatment temperature is too low. However, if the alloying treatment temperature is too high, alloying proceeds too much, the Fe concentration in the plating layer becomes high, and the plating adhesion deteriorates. From such a viewpoint, the alloying treatment temperature is preferably 550 ° C. or less, more preferably 540 ° C. or less, and further preferably 530 ° C. or less. The alloying treatment time is not particularly limited, and may be adjusted so that hot dip galvanizing is alloyed. The alloying treatment time is, for example, 10 to 60 seconds.

A high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more and excellent workability and impact characteristics according to the present invention is a steel satisfying the above-described component composition, with a rolling rate of 5 to 25% in the final stand of finish rolling, Hot rolling is performed at a finish rolling end temperature of Ar ₃ or higher and 900 ° C. or lower, winding is performed at a winding temperature of 600 ° C. or lower, cooled to room temperature, cold rolled, and 800 ° C./sec or higher at an average temperature increase rate of 10 ° C./second or higher. It is heated to a temperature range of not less than ₃ ° C. and less than Ac ₃ points, soaked in the temperature range for 50 seconds or more, soaked, and an average cooling rate of 10 ° C./temperature until an arbitrary cooling stop temperature T ° C. in the temperature range of 50 ° C. Produced by cooling for at least 2 seconds, heating and holding in the temperature range above the cooling stop temperature T ° C. to 550 ° C. for 50 seconds or more, and after performing hot-dip galvanization within the holding time, cooling to room temperature it can. That is, the process until heating to a temperature range above the cooling stop temperature T ° C. and below 550 ° C. is the same as the above-described method for producing a high-strength cold-rolled steel sheet according to the present invention. What is necessary is just to carry out holding | maintenance for 50 seconds or more performed in the following temperature ranges, and hot dip galvanization.

The hot dip galvanizing may be performed within the holding time in the reheating temperature range, that is, in the temperature range of the cooling stop temperature T ° C. or higher and 550 ° C. or lower, and a specific plating method can be adopted. For example, if a steel sheet heated to a temperature range above the cooling stop temperature T ° C. and below 550 ° C. is immersed in a plating bath adjusted to a temperature in the range above the cooling stop temperature T ° C. and below 550 ° C., hot dip galvanizing treatment is performed. Good. The plating time may be appropriately adjusted so that a desired plating amount can be secured within the reheating and holding time. The plating time is preferably 1 to 10 seconds, for example.

There are the following patterns (I) to (III) as a combination of hot dip galvanizing treatment and reheating without plating treatment.
(I) After only heating, a hot dip galvanizing process is performed.
(II) After the hot dip galvanizing treatment, only heating is performed.
(III) After performing only heating, a hot dip galvanization process is performed, and also only heating is performed in this order.

The reheating temperature when only the heating is performed may be different from the temperature of the plating bath when the hot dip galvanizing is performed. In the present invention, heating or cooling from one temperature to the other may be performed. . Examples of the heating method include furnace heating and induction heating.

The high-strength galvannealed steel sheet having a tensile strength of 980 MPa or more and excellent in workability and impact characteristics according to the present invention is a steel satisfying the above-described component composition, with a rolling rate of 5 to 25 in the final stand of finish rolling. %, Hot rolling at a finish rolling finish temperature of Ar ₃ point to 900 ° C., winding at a winding temperature of 600 ° C. or less, cooling to room temperature, cold rolling, average heating rate of 10 ° C./second or more At a temperature range of 800 ° C. or more and less than Ac ₃ point, and held in the temperature range for 50 seconds or more, soaking, and an average cooling rate of 10 to an arbitrary cooling stop temperature T ° C. in a temperature range of 50 ° C. or more and Ms point or less After cooling at a temperature of ℃ / second or more and heating and holding for 50 seconds or more in the temperature range above the cooling stop temperature T ° C. or more and 550 ° C., hot-dip galvanization is performed within the holding time, and further alloying treatment is performed. To room temperature It can also be manufactured by cooling with. That is, the process until heating to a temperature range above the cooling stop temperature T ° C. and below 550 ° C. is the same as the above-described method for producing a high-strength cold-rolled steel sheet according to the present invention. What is necessary is just to carry out holding | maintenance for 50 seconds or more performed in the following temperature ranges, and hot-dip galvanization, and cooling to room temperature after alloying a hot-dip galvanization layer after that.

Although the alloying treatment temperature is not particularly limited, it is preferably 450 ° C. or higher, more preferably 460 ° C. or higher, and further preferably 480 ° C. or higher because alloying does not proceed sufficiently if the alloying temperature is too low. However, if the alloying treatment temperature is too high, alloying proceeds too much, the Fe concentration in the plating layer becomes high, and the plating adhesion deteriorates. From such a viewpoint, the alloying treatment temperature is preferably 550 ° C. or less, more preferably 540 ° C. or less, and further preferably 530 ° C. or less.

The alloying treatment time is not particularly limited, and may be adjusted so that hot dip galvanizing is alloyed. The alloying treatment time is, for example, 10 to 60 seconds. Since the alloying process is performed after the hot dip galvanizing process is performed for a predetermined time in the temperature range of the above cooling stop temperature T ° C. to 550 ° C., the time required for the alloying process exceeds the above cooling stop temperature T ° C. It is not included in the holding time in the temperature range of 550 ° C. or lower.

After the hot dip galvanization within the holding time in the temperature range above the cooling stop temperature T ° C. and below 550 ° C., and performing the alloying treatment as necessary, it may be cooled to room temperature. Although the average cooling rate at the time of cooling is not specifically limited, For example, it is preferable that it is 0.1 degree-C / second or more, More preferably, it is 0.4 degree-C / second or more. Moreover, it is preferable that an average cooling rate is 200 degrees C / second or less, for example, More preferably, it is 150 degrees C / second or less.

The high-strength cold-rolled steel sheet according to the present invention has a tensile strength of 980 MPa or more. The tensile strength is preferably 1000 MPa or more, more preferably 1010 MPa or more. And the said high intensity | strength cold-rolled steel plate is excellent in workability evaluated by ductility and stretch flangeability, and also is excellent in a collision characteristic.

The ductility can be evaluated by a strength-elongation balance. In the present invention, the product of the tensile strength TS (MPa) and the elongation EL (%) is 17000 MPa ·% or more. The value of TS × EL is preferably 17100 MPa ·% or more, more preferably 17200 MPa ·% or more.

Stretch flangeability can be evaluated by a strength-hole expansion ratio balance. In the present invention, the product of the tensile strength TS (MPa) and the hole expansion ratio λ (%) is 20,000 MPa ·% or more. The value of TS × λ is preferably 21000 MPa ·% or more, more preferably 22000 MPa ·% or more.

The impact characteristics can be evaluated by the strength-VDA bending angle balance. In the present invention, the product of the tensile strength TS (MPa) and the VDA bending angle (°) is 90000 MPa · ° or more. The value of the TS × VDA bending angle is preferably 90500 MPa · ° or more, and more preferably 91000 MPa · ° or more.

The thickness of the high-strength cold-rolled steel sheet according to the present invention is not particularly limited, but is preferably a thin steel sheet of 6 mm or less, for example.

This application claims the benefit of priority based on Japanese Patent Application No. 2015-071438 filed on March 31, 2015 and Japanese Patent Application No. 2015-225507 filed on November 18, 2015. To do. The entire contents of the above Japanese Patent Application No. 2015-071438 and the above Japanese Patent Application No. 2015-225507 are incorporated herein by reference.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and may be implemented with modifications within a range that can meet the above and the gist described below. Of course, these are all possible and are included in the technical scope of the present invention.

The components shown in Table 1 below were contained, and the balance was produced by melting steel composed of iron and inevitable impurities, and subjected to hot rolling, cold rolling, and continuous annealing to produce cold rolled steel sheets. In Table 1 below, “-” means that no element is contained. Table 1 below shows the temperature at the Ar ₃ point calculated based on the above formula (ii) and the temperature at the Ac ₃ point calculated based on the above formula (iii). Moreover, an example of the heat processing pattern performed by continuous annealing is shown in FIG. In FIG. 1, 1 is a heating step, 2 is a soaking step, 3 is a cooling step, 4 is a reheating and holding step, and 5 is a cooling stop temperature.

[Hot rolling]
The slab obtained by melting was heated to 1250 ° C., and the reduction rate in the final stand of finish rolling was set to the reduction rate shown in Table 2-1 or Table 2-2 below. Alternatively, hot rolling was performed to a plate thickness of 2.3 mm as shown in Table 2-2. After hot rolling, the sheet was cooled to the winding temperature shown in Table 2-1 or Table 2-2 below at an average cooling rate of 30 ° C./second and wound up. After winding, the steel sheet was air cooled to room temperature to produce a hot rolled steel sheet.

[Cold rolling]
The obtained hot-rolled steel sheet was pickled to remove the scale on the surface, and then cold-rolled to produce a cold-rolled steel sheet having a thickness of 1.2 mm.

[Continuous annealing]
The obtained cold-rolled steel sheet was continuously annealed based on the heat treatment pattern shown in FIG. That is, the obtained cold-rolled steel sheet was heated to the soaking temperature shown in the following Table 2-1 or Table 2-2 at the average heating rate shown in the following Table 2-1 or Table 2-2 as a heating step. The soaking temperature was maintained as the soaking step. Table 2-1 and Table 2-2 below show soaking times. Tables 2-1 and 2-2 below show values calculated by subtracting the soaking temperature from the temperature at the Ac ₃ point.

After soaking, as a cooling step, cooling was performed at an average cooling rate shown in Table 2-1 or Table 2-2 below to a cooling stop temperature T ° C shown in Table 2-1 or Table 2-2 below.

After cooling, the sample was heated to the reheating temperature shown in Table 2-1 or Table 2-2 below. In the reheating and holding step, the sample was held at the reheating temperature and then cooled to room temperature to produce a test material. Table 2-1 and Table 2-2 below show the reheating holding time. Tables 2-1 and 2-2 below show values calculated by subtracting the cooling stop temperature T from the reheating temperature.

No. shown in Table 2-1 below. Nos. 8 and 11 are examples in which the reheating and holding step is not performed after the cooling is stopped at the cooling stop temperature T shown in Table 2-1. That is, no. No. 8 was cooled at a cooling stop temperature T of 480 ° C., cooled to 350 ° C. lower than this temperature, and held at 350 ° C. for 300 seconds. In Table 2-1 below, for convenience, 350 ° C. is described in the reheating temperature column and 300 seconds are described in the reheating holding time column. No. No. 11 was cooled at a cooling stop temperature T of 330 ° C., then cooled to 300 ° C. lower than this temperature, and held at 300 ° C. for 300 seconds. In Table 2-1 below, for convenience, the reheating temperature column is 300 ° C. and the reheating holding time column is 300 seconds.

[Electrogalvanizing]
No. shown in Table 2-1 below. No. 2 is an example in which an electrogalvanized steel sheet was manufactured by immersing the test material in a galvanizing bath at 55 ° C., performing electrogalvanizing treatment, washing with water and drying. Galvanized treatment was performed with a current density between 40A / dm ^2. The amount of galvanized adhesion was 40 g / m ² per side. In the electrogalvanizing treatment, washing materials such as alkaline aqueous solution degreasing, water washing, and pickling were appropriately performed to produce test materials having an electrogalvanized layer on the surface of the cold rolled steel sheet. In Table 2-1 below, no. In the column of category 2, “EG” is described.

[Hot galvanizing]
No. shown in Table 2-2 below. 36 is an example in which a hot dip galvanized steel sheet was manufactured by immersing the test material in a hot dip galvanizing bath at 460 ° C. and subjecting it to hot dip galvanizing treatment. The amount of hot dip galvanizing was 30 g / m ² per side. In Table 2-2 below, no. In the column of 36 categories, “GI” is described.

[Alloyed hot dip galvanizing]
No. shown in Table 2-1 below. 18 is an example in which the above specimen was immersed in a hot dip galvanizing bath at 460 ° C., subjected to hot dip galvanizing treatment, then heated to 500 ° C. and alloyed to produce an alloyed hot dip galvanized steel sheet. is there. The amount of galvannealed coating was 30 g / m ² per side. In Table 2-1 below, no. In the column of 18 categories, “GA” is described.

For specimens that were not subjected to electrogalvanizing, hot dip galvanizing, or alloying hot dip galvanizing, “cold rolled” is entered in the column of the following Table 2-1 and Table 2-2. did.

For the obtained specimens, the metal structure was observed according to the following procedure.

[Observation of metal structure]
(Area ratio of ferrite and hard phase)
After the cross section of the obtained test material was polished, it was subjected to Nital corrosion, and a 1/4 position of the plate thickness was observed with a scanning electron microscope at a magnification of 1000 times for 3 fields of view and photographed. As for the visual field size, one visual field is 100 μm × 100 μm. The area ratio of the ferrite was measured by a point calculation method with a lattice spacing of 5 μm and a lattice point number of 20 × 20, and an average value Vf of three fields of view was calculated. The calculation results are shown in Table 3-1 and Table 3-2 below. The area ratio of the ferrite was calculated by excluding the area ratio of the hard phase present in the ferrite phase.

Further, based on the component composition shown in Table 1 below and the average area ratio Vf of ferrite shown in Table 3-1 and Table 3-2 below, the Ms point was calculated from the above formula (iv), and the result was It is shown in Table 2-1 and Table 2-2. Tables 2-1 and 2-2 below also show values obtained by subtracting the Ms point temperature from the cooling stop temperature T.

Similarly, the total area ratio of pearlite and cementite was measured by a point calculation method, and the average value of the three fields of view was calculated. The calculation results are shown in Table 3-1 and Table 3-2 below. The total area ratio of pearlite and cementite was expressed as “other structures” in the following Table 3-1 and Table 3-2.

In this example, the structure other than ferrite, pearlite, and cementite calculated by the above point calculation method was used as the hard phase. That is, a value obtained by subtracting the area ratio of ferrite and the total area ratio of pearlite and cementite from 100% was calculated as the area ratio of the hard phase, and the results are shown in Tables 3-1 and 3-2 below.

As a result of observing a specific structure constituting the hard phase, the hard phase contains hardened martensite and residual γ, and is at least selected from the group consisting of bainitic ferrite, bainite, and tempered martensite. There was one.

(Volume ratio V _{γ of} residual γ)
The obtained specimen is polished to 1/4 position of the plate thickness using # 1000 to # 1500 sandpaper, and the surface is further electropolished to a depth of 10 to 20 μm, and then an X-ray diffractometer is used. The volume fraction V _γ of residual γ was measured. Specifically, “RINT 1500” manufactured by Rigaku Corporation was used as an X-ray diffraction apparatus, a Co target was used, 40 kV-200 mA was output, and a range of 40 ° to 130 ° was measured at 2θ. From the obtained diffraction peaks (110), (200), (211) of bcc (α) and diffraction peaks (111), (200), (220), (311) of fcc (γ), the volume of residual γ The rate _Vγ was quantified. The results are shown in Tables 3-1 and 3-2 below.

(MA area ratio V _MA and average equivalent circle diameter)
After polishing the cross section of the obtained test material, it was repeller-corroded, and the 1/4 position of the plate thickness was observed with a light microscope at a magnification of 1000 times for 3 views and photographed. As for the visual field size, one visual field is 100 μm × 100 μm. The area whitened by the repeller corrosion was made into MA structure, the lattice spacing was 5 μm, the area ratio of the MA structure was measured by a point calculation method with 20 × 20 lattice points, and the average value of the three fields of view was calculated. The calculation results are shown in Table 3-1 and Table 3-2 below.

The photograph taken with the optical microscope was subjected to image analysis, the equivalent circle diameter d of each MA tissue was calculated, and the average value was obtained. The results are shown in Tables 3-1 and 3-2 below.

(Ratio of the volume ratio V _gamma and MA tissue area ratio V _MA of residual gamma)
Based on the area ratio V _MA of the volume ratio V _gamma and MA tissues of residual gamma measured by the procedure described above was calculated the ratio V _MA / V _gamma area ratio V _MA of the MA tissue to volume ratio V _gamma of residual gamma . The calculation results are shown in Table 3-1 and Table 3-2 below.

Next, the mechanical properties, ductility, stretch flangeability, and impact properties of the obtained specimens were evaluated according to the following procedures.

[Evaluation of mechanical properties and ductility]
A No. 5 test piece defined in JIS Z2201 was cut out so that the direction perpendicular to the rolling direction of the obtained specimen was the longitudinal direction, and a tensile test was performed using this test piece to obtain a tensile strength TS and Elongation EL was measured. The measurement results are shown in Table 3-1 and Table 3-2 below.

In this example, the case where the tensile strength was 980 MPa or more was evaluated as acceptable with high strength, and the case where the tensile strength was less than 980 MPa was evaluated as unacceptable due to insufficient strength.

Moreover, based on the measured values of tensile strength TS and elongation EL, the value of tensile strength TS × elongation EL was calculated. The calculation results are shown in Table 3-1 and Table 3-2 below. The value of TS × EL indicates a strength-elongation balance and is an index for evaluating ductility.

In this example, when the value of TS × EL was 17000 MPa ·% or more, the ductility was evaluated as excellent and evaluated as acceptable, and when it was less than 17000 MPa ·%, the ductility was poor and evaluated as rejected.

[Evaluation of stretch flangeability]
In order to evaluate the stretch flangeability of the test material, a hole expansion test was performed based on the Steel Federation Standard JFST 1001, and the hole expansion ratio λ was measured. The measurement results are shown in Table 3-1 and Table 3-2 below.

Also, based on the measured tensile strength TS and hole expansion ratio λ, the value of tensile strength TS × hole expansion ratio λ was calculated. The calculation results are shown in Table 3-1 and Table 3-2 below. The value of TS × λ indicates a balance between strength and hole expansion rate, and is an index for evaluating stretch flangeability.

In this example, when the value of TS × λ was 20000 MPa ·% or more, it was evaluated as excellent in stretch flangeability and passed, and when it was less than 20000 MPa ·%, the stretch flangeability was poor and evaluated as rejected.

[Evaluation of collision characteristics]
It is described in the following literature that the impact characteristics correlate with the bending angle.
Literature: P. Larour, H. Pauli, T. Kurz, T. Hebesberger: “Influence of post uniforms and bending properties on the crest of HH”

Therefore, based on the VDA standard (VDA238-100) specified by the German Automobile Manufacturers Association, a bending test is performed under the following conditions, and the displacement at the maximum load measured in the bending test is converted into an angle based on the VDA standard. The bending angle was determined. The conversion results are shown in Table 3-1 and Table 3-2 below.

(Measurement condition)
Test method: roll support, punch push-in roll diameter: φ30mm
Punch shape: Tip R = 0.4mm
Distance between rolls: 2.9 mm
Punch pushing speed: 20 mm / min Test piece size: 60 mm × 60 mm
Bending direction: perpendicular to the rolling direction Testing machine: SIMAZU AUTOGRAPH 20kN

Also, based on the tensile strength TS and VDA bending angle values measured in the tensile test, the value of tensile strength TS × VDA bending angle ° was calculated. The calculation results are shown in Table 3-1 and Table 3-2 below.

In this example, when the value of TS × VDA was 90000 MPa · ° or more, the impact property was evaluated as excellent and evaluated as acceptable, and when it was less than 90000 MPa · °, the impact property was poor and evaluated as rejected.

Based on the above results, TS value is 980 MPa or more, TS × EL value is 17000 MPa ·% or more, TS × λ value is 20000 MPa ·% or more, and TS × VDA value is 90000 MPa · ° or more. When satisfied, the present invention was regarded as an example of the present invention, and the pass was listed in the column for comprehensive evaluation in Table 3-1 and Table 3-2 below. On the other hand, a case where any one of the TS value, TS × EL value, TS × λ value, or TS × VDA value does not satisfy the above acceptance criteria is used as a comparative example. Failures are listed in the column for overall evaluation in Table 3-2.

From Table 1, Table 2-1, Table 2-2, Table 3-1, and Table 3-2, it can be considered as follows.

In Table 3-1 and Table 3-2, examples where “pass” is described in the column of comprehensive evaluation are both steel sheets that satisfy the requirements specified in the present invention, and are in accordance with the tensile strength TS. All of the defined TS × EL value, TS × λ value, and TS × VDA value satisfy the acceptance standard value. It can be seen that these steel sheets have good workability evaluated by ductility and stretch flangeability, particularly excellent ductility, and excellent impact characteristics.

On the other hand, the example described as “Fail” in the column of comprehensive evaluation is a steel sheet that does not satisfy any of the requirements defined in the present invention, and is at least one of ductility, stretch flangeability, and impact characteristics. One could not improve. Details are as follows.

No. 3 is an example in which the MA structure was coarsened because the finish rolling finish temperature was too high. As a result, the value of TS × λ became small and the stretch flangeability could not be improved.

No. No. 4 is an example in which the MA structure is coarsened because the rolling reduction at the final stand at the time of finish rolling is too high beyond the range defined in the present invention. As a result, the value of TS × λ became small and the stretch flangeability could not be improved. Moreover, the value of TS × VDA was reduced, and the collision characteristics could not be improved.

No. No. 5 is an example in which the MA structure is coarsened because the rolling reduction at the final stand during finish rolling is too low below the range defined in the present invention. As a result, the value of TS × λ became small and the stretch flangeability could not be improved.

No. No. 7 is an example in which the amount of ferrite in the range specified in the present invention could not be secured because soaking was performed at a high temperature exceeding 800 ° C. or more and less than the Ac ₃ point temperature range. As a result, the value of TS × EL became small and ductility could not be improved.

No. In No. 8, the cooling stop temperature T after soaking was too high exceeding the temperature range of 50 ° C. or more and the Ms point or less, and the value of V _MA / V _γ was too large because the reheating was not held after cooling. It is an example. As a result, the value of TS × VDA was reduced, and the collision characteristics could not be improved.

No. No. 10 is an example in which the cooling stop temperature T after soaking is less than 50 ° C., so that a predetermined amount of residual γ and MA texture cannot be secured, and the value of V _MA / V _γ falls below the specified range and becomes smaller. It is. As a result, the value of TS × EL became small and ductility could not be improved.

No. No. 11 is that the rolling reduction at the final stand at the time of finish rolling is too high exceeding the range specified in the present invention, and since reheating holding was not performed after cooling, the MA structure becomes coarse and V _MA / V _This is an example in which the value of _γ has become too large. As a result, the value of TS × VDA was reduced, and the collision characteristics could not be improved.

No. No. 14 is an example in which the MA structure becomes coarse because the reheating holding time is too short. As a result, the value of TS × λ became small and the stretch flangeability could not be improved. Moreover, MA structure | tissue produced | generated excessively. As a result, the value of TS × EL became small and ductility could not be improved. Further, the value of V _MA / V _γ became too large. As a result, the value of TS × VDA was reduced and the collision characteristics were deteriorated.

No. 16 and 37 are examples in which the MA structure was coarsened because the average temperature increase rate after winding was too small. As a result, the value of TS × λ became small and the stretch flangeability could not be improved.

No. No. 19 is an example in which the MA structure is coarsened because the cooling stop temperature T after soaking was too high exceeding the temperature range of 50 ° C. or more and the Ms point or less. As a result, the value of TS × λ became small and the stretch flangeability could not be improved.

No. No. 25 is an example in which since the reheating temperature performed after cooling was too high, decomposition of austenite occurred and a predetermined amount of residual γ and MA structure could not be secured. As a result, TS became low. Moreover, the value of TS × λ was small, and the stretch flangeability could not be improved. Moreover, the value of TS × VDA was reduced, and the collision characteristics could not be improved.

No. Nos. 27 and 38 are examples in which the ferrite was excessively generated because the average cooling rate after soaking was too small. As a result, TS became low. Moreover, the value of TS × λ was small, and the stretch flangeability could not be improved.

No. No. 29 is an example in which the MA structure is coarsened because the winding temperature is too high. As a result, the value of TS × λ became small and the stretch flangeability could not be improved.

No. No. 33 is an example in which the amount of C is too small, and the amount of residual γ within the range specified in the present invention could not be secured, and the value of V _MA / V _γ increased beyond the range specified in the present invention. As a result, the value of TS × EL was decreased and ductility was deteriorated.

No. 34 is an example in which the amount of Si is too small, and the amount of ferrite in the range specified in the present invention could not be secured. As a result, the value of TS × EL was decreased and ductility was deteriorated.

No. No. 35 is an example in which the amount of Mn is too small, the hardenability is insufficient, and ferrite is generated excessively, so TS is lowered. Further, the value of TS × λ was decreased, and stretch flangeability was deteriorated.

1 Heating process 2 Soaking process 3 Cooling process 4 Reheating and holding process 5 Cooling stop temperature

Claims (8)

1. % By mass
   C: 0.10% to 0.5%,
   Si: 1.0% or more and 3% or less,
   Mn: 1.5% to 7%,
   P: more than 0% and 0.1% or less,
   S: more than 0% and 0.05% or less,
   Al: 0.005% or more and 1% or less,
   N: more than 0% and 0.01% or less, and O: more than 0% and 0.01% or less,
   The balance is a steel plate made of iron and inevitable impurities,
   A high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more excellent in workability and impact characteristics, characterized in that the metal structure at a 1/4 position of the sheet thickness satisfies the following (1) to (4).
   (1) When the metal structure is observed with a scanning electron microscope,
   For the entire metal structure, the area ratio of ferrite is more than 10% and 65% or less,
   The balance includes hardened martensite and retained austenite, and is a hard phase composed of at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite.
   (2) When the metal structure is measured by the X-ray diffraction method, the volume fraction V _{γ of} retained austenite is 5% or more and 30% or less with respect to the entire metal structure.
   (3) When the metal structure is observed with an optical microscope, the area ratio V _MA of the MA structure in which quenched martensite and retained austenite are combined with respect to the entire metal structure is 3% or more and 25% or less. The average equivalent circle diameter of the tissue is 2.0 μm or less.
   (4) The ratio V _MA / V _γ of the area ratio V _MA of the MA structure to the volume ratio V _γ of the retained austenite satisfies the following formula (i).
   0.50 ≦ V _MA / V _γ ≦ 1.50 (i)
2. The high-strength cold-rolled steel sheet according to claim 1, wherein the steel sheet further contains, as another element, one or more of the following elements (a) to (e) by mass%.
   (A) At least one selected from the group consisting of Cr: more than 0% to 1% and Mo: more than 0% to 1%.
   (B) At least one selected from the group consisting of Ti: more than 0% and not more than 0.15%, Nb: more than 0% and not more than 0.15%, and V: more than 0% and not more than 0.15%.
   (C) At least one selected from the group consisting of Cu: more than 0% and 1% or less, and Ni: more than 0% and 1% or less.
   (D) B: more than 0% and 0.005% or less.
   (E) At least one selected from the group consisting of Ca: more than 0% and 0.01% or less, Mg: more than 0% and 0.01% or less, and REM: more than 0% and 0.01% or less.
3. A high-strength electrogalvanized steel sheet comprising an electrogalvanized layer on the surface of the high-strength cold-rolled steel sheet according to claim 1 or 2.
4. A high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the high-strength cold-rolled steel sheet according to claim 1 or 2.
5. A high-strength alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of the high-strength cold-rolled steel sheet according to claim 1 or 2.
6. Steel satisfying the component composition according to claim 1 or 2,
   Hot rolling at a final rolling final stand of 5 to 25%, finishing rolling finish temperature of Ar ₃ point to 900 ° C., winding at a winding temperature of 600 ° C. or less, cooling to room temperature,
   Cold rolled,
   Heat to a temperature range of 800 ° C. or more and less than Ac ₃ point at an average temperature rising rate of 10 ° C./second or more, hold in the temperature range for 50 seconds or more, and soak,
   Cool at an average cooling rate of 10 ° C./second or more to an arbitrary cooling stop temperature T ° C. in a temperature range of 50 ° C. or more and Ms point or less,
   A high strength cooling with a tensile strength of 980 MPa or more excellent in workability and impact characteristics, characterized by heating and holding for 50 seconds or more in the temperature range above the cooling stop temperature T ° C. to 550 ° C. and then cooling to room temperature A method for producing rolled steel sheets.
7. Steel satisfying the component composition according to claim 1 or 2,
   Hot rolling at a final rolling final stand of 5 to 25%, finishing rolling finish temperature of Ar ₃ point to 900 ° C., winding at a winding temperature of 600 ° C. or less, cooling to room temperature,
   Cold rolled,
   Heat to a temperature range of 800 ° C. or more and less than Ac ₃ point at an average temperature rising rate of 10 ° C./second or more, hold in the temperature range for 50 seconds or more, and soak,
   Cool at an average cooling rate of 10 ° C./second or more to an arbitrary cooling stop temperature T ° C. in a temperature range of 50 ° C. or more and Ms point or less,
   Workability and impact characteristics characterized by heating and holding for 50 seconds or more in the temperature range above the cooling stop temperature T ° C. and below 550 ° C., performing hot dip galvanization within the holding time, and then cooling to room temperature For producing a high-strength hot-dip galvanized steel sheet having an excellent tensile strength of 980 MPa or more.
8. Steel satisfying the component composition according to claim 1 or 2,
   Hot rolling at a final rolling final stand of 5 to 25%, finishing rolling finish temperature of Ar ₃ point to 900 ° C., winding at a winding temperature of 600 ° C. or less, cooling to room temperature,
   Cold rolled,
   Heat to a temperature range of 800 ° C. or more and less than Ac ₃ point at an average temperature rising rate of 10 ° C./second or more, hold in the temperature range for 50 seconds or more, and soak,
   Cool at an average cooling rate of 10 ° C./second or more to an arbitrary cooling stop temperature T ° C. in a temperature range of 50 ° C. or more and Ms point or less,
   Heating and holding for 50 seconds or more in the temperature range above the cooling stop temperature T ° C. and 550 ° C., and after performing hot dip galvanization within the holding time, cooling to room temperature after further alloying treatment A method for producing a high-strength galvannealed steel sheet having a tensile strength of 980 MPa or more and excellent in workability and impact characteristics.
   

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