JP5370104B2 - Manufacturing method of high strength steel plate having high tensile strength of 900 MPa or more excellent in hydrogen embrittlement resistance and high strength cold-rolled steel plate, manufacturing method of high strength galvanized steel plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel sheet having excellent hydrogen embrittlement resistance, and a method for producing the same. <P>SOLUTION: The high strength steel sheet excellent in hydrogen embrittlement resistance and having the maximum tensile strength of &ge;900 MPa is composed of a steel having a composition comprising, by mass, 0.07 to 0.25% C, 0.3 to 2.50% Si, 1.5 to 3.0% Mn, 0.005 to 0.09% Ti, 0.0001 to 0.01% B, 0.001 to 0.03% P, 0.0001 to 0.01% S, &le;2.50% Al, 0.0005 to 0.0100% N and 0.0005 to 0.007% O, and the balance iron with inevitable impurities. The inside of the steel sheet includes a hard structure including ferrite as the main phase and martensite with a block size of &le;1 &mu;m, and the surface layer of the steel sheet is composed of a decarburized layer with a thickness of &ge;0.5 &mu;m in which the volume ratio of the above hard structure is &le;80% the volume ratio of a hard structure included at the position of the 1/4 thickness in the steel sheet. The method for producing the same is also disclosed. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a method for producing a high-strength steel plate excellent in hydrogen embrittlement resistance and a tensile maximum strength of 900 MPa or more, a high-strength cold-rolled steel plate, and a high-strength galvanized steel plate.

In recent years, there has been an increasing demand for higher strength steel sheets used in automobiles and buildings. High-strength cold-rolled steel sheets having a maximum tensile strength of 900 MPa or more have been rapidly applied mainly to reinforcing materials such as bumpers and impact beams. However, it is necessary to solve the problem of delayed fracture when applying high strength steel sheets.
Delayed fracture is a phenomenon in which a member under high stress, such as a PC steel wire or bolt, suddenly breaks, and is known to be closely related to hydrogen entering from the environment.

  Steel sheet strength is known as a factor that greatly affects delayed fracture. This is because a higher strength steel plate is more likely to be used in an environment where high stress acts. That is, if a low-strength material is used for a member to which high stress acts, delayed deformation does not occur because plastic deformation occurs and breaks. On the other hand, high-strength materials are often used in environments where high stress acts because they are difficult to plastically deform and break. Further, in steel materials used as members after forming, such as steel plates for automobiles, residual stress is generated after processing, and the residual stress increases as the strength of the steel plate increases. As a result, the higher the strength of the material, the greater the concern about the occurrence of delayed fracture.

  Conventionally, many developments of steel materials considering delayed fracture resistance have been carried out in the field of strip steel and thick plates. For example, in steel bars and bolt steels, development has been conducted mainly on tempered martensite, and it has been reported that additive elements exhibiting temper softening resistance such as Cr, Mo and V are effective in improving delayed fracture resistance. (For example, see Non-Patent Document 1). This is a technique for shifting the delayed fracture mode from grain boundaries to intragranular fracture by precipitating alloy carbides and utilizing them for hydrogen trap sites.

However, since the steel described in Non-Patent Document 1 has a C content of 0.4% or more and contains a large amount of alloy elements, the workability and weldability required for thin steel sheets are poor. Moreover, since precipitation heat treatment for several hours or more is required to precipitate the alloy carbide, the technique described in Non-Patent Document 1 has a problem in manufacturability.
Patent Document 1 describes an effect of preventing hydrogen defects caused by oxides mainly composed of Ti and Mg. However, this technology is intended for thick steel plates, and is considered for delayed fracture after welding with high heat input, but is not considered at all regarding the compatibility of high formability and delayed fracture resistance required for thin steel plates. Not.

In addition, the thin steel plate has a thin plate thickness, so even if hydrogen enters, it is released in a short time, and from the viewpoint of workability, there was almost no use of a steel plate with a tensile maximum strength of 900 MPa or more. The problem was small. However, since the demand for the application of high-strength steel plates is rapidly increasing, it is necessary to develop high-strength steel plates with excellent hydrogen embrittlement resistance.
However, the technology for improving the hydrogen embrittlement resistance has been developed for steel materials that are used in the form of bolts, strips, and thick plates, and are used below the yield strength or yield stress. That is, it was not a technology that considered steel materials that required hydrogen embrittlement resistance as well as workability such as cutting and member molding (press molding) like automobile members. Further, a stress called residual stress remains in the member after molding. Although the residual stress is local, it may be high enough to exceed the yield stress of the material. For this reason, a steel sheet that does not cause hydrogen embrittlement even under high residual stress is desired.

  Regarding delayed fracture of thin steel sheets, for example, Non-Patent Document 2 reports on the promotion of delayed fracture due to work-induced transformation of the amount of retained austenite. This is in consideration of the forming process of a thin steel sheet, but describes the regulation of the amount of retained austenite which does not deteriorate the delayed fracture resistance. That is, it relates to a high-strength thin steel sheet having a specific structure, and is not a fundamental measure for improving delayed fracture resistance.

  In addition, as a thin steel plate considering hydrogen trapping ability and formability, there is a steel plate for a hollow container described in Patent Document 2 that is excellent in toughness resistance. This is intended to suppress a surface defect called a trap generated after enamelling by trapping hydrogen that enters the steel sheet during production with an oxide contained in the steel sheet. However, with the technique described in Patent Document 2, a large amount of oxide is contained inside the steel plate. If these oxides are dispersed at a high density in the steel sheet, the formability is deteriorated, so that there is a problem in application to a steel sheet for automobiles that require high formability. In addition, these studies do not attempt to achieve both high strength and delayed fracture resistance.

  As a technique for solving this problem, there is a steel plate in which an oxide is precipitated in the steel plate (for example, see Patent Document 3). In such a steel plate, the oxide dispersed in the steel plate functions as a trap site for hydrogen that has penetrated into the steel, thereby suppressing the diffusion and accumulation of hydrogen to the stress concentration site and the site of fear of delayed fracture. However, in order to obtain such an effect by the oxide, it is necessary to disperse the oxide at a high density in the steel sheet, and it is necessary to strictly control the production conditions.

  Patent Document 4 describes a steel sheet that has both delayed fracture resistance and high strength by making the steel sheet interior a martensite or bainite-based structure and the steel sheet surface layer being a ferrite-based structure. . This steel sheet has a structure in which the surface layer of the steel sheet has ferrite as the main phase by making the atmosphere during material control in the annealing equipment a condition that allows decarburization, and the delayed fracture characteristics are improved by reducing the steel sheet strength. Is. For example, a large deformation is applied to a steel sheet surface layer in an automobile member or a building material mainly composed of bending. As a result, the starting point of delayed fracture occurs on the surface layer. From this, softening of the steel sheet surface layer is effective in improving delayed fracture characteristics. However, the steel sheet described in Patent Document 4 has a problem that the internal structure is inferior in ductility because martensite and bainite are the main phases.

Further, as a technique for ensuring a high strength of a tensile maximum strength of 900 MPa or more, a technique for strengthening the structure using martensite is known. However, the martensite structure has a high strength but has a very low uniform elongation and has a problem of poor workability.
As a technique for solving this problem, a composite structure steel plate having a main phase of ferrite and a hard structure of martensite is known. That is, it is a technique of ensuring ductility with soft ferrite and ensuring strength with hard martensite. In this method, since the main phase is made of soft ferrite, the yield stress can be greatly reduced.
However, in order to ensure the maximum tensile strength of a steel sheet of 900 MPa or more, it is necessary to increase the martensite volume ratio, and deterioration of formability is inevitable. As a result, an increase in the volume ratio of martensite was indispensable for increasing the strength of the multiphase steel sheet composed of ferrite and martensite.

In addition, since the strength of the martensite structure strongly depends on the steel plate component (particularly, C), it is difficult to increase the strength without changing the steel plate component. However, when the addition amount of C is increased, weldability including spot welding is deteriorated. For this reason, in the case of a thin steel plate using spot or laser welding as a fastening means for a formed member, or a steel plate that is formed after welding, such as a tailored blank material, the amount of C added is suppressed to ensure sufficient weldability. I had to.
Therefore, in general, when producing a high strength steel sheet having a maximum tensile strength of 900 MPa, a method of adding C in an addition amount suppressed to such an extent that sufficient weldability is obtained and increasing the volume ratio of martensite. Is used. In the case of steel sheets with a maximum tensile strength of 1180 MPa, it is common that nearly 100% of the volume ratio is martensite and bainite. As a result, the maximum tensile strength 1180 MPa class steel sheet was applicable only to simple bending.

  In addition, when performing processes such as continuous annealing and continuous hot dip galvanizing lines, inevitable heat treatment such as annealing, tempering in the overaging zone after cooling, and alloying heat treatment after immersion in the plating bath is additionally provided. By doing so, the martensitic structure is tempered and softened, so that it is difficult to ensure the strength of the steel sheet. From this, when manufacturing a high-strength steel sheet having a tensile maximum strength of 900 MPa, for example, using a manufacturing facility that receives such additional heat treatment, generally, the martensite structure is tempered to cause softening. Even so, the martensite volume fraction is further increased so as to ensure sufficient strength. As a result, the yield stress is further increased. Therefore, it is extremely difficult to develop a high-strength steel sheet that has both delayed fracture resistance and good formability.

  Moreover, as a prior art regarding a high strength steel plate, the technique of patent document 5-patent document 10 is mentioned, for example. Moreover, as a conventional technique regarding the hot dip galvanized steel sheet, for example, a technique described in Patent Document 11 is cited.

JP-A-11-293383 Japanese Patent Laid-Open No. 11-100638 JP 2007-2111279 A Japanese Patent Laid-Open No. 10-130782 Japanese Patent Application Laid-Open No. 11-296991 Japanese Patent Laid-Open No. 9-13147 JP 2002-363695 A JP 2003-105514 A JP 2003-213369 A JP 2003-213370 A JP 2002-97560 A

"New development of hydrogen brittleness elucidation" (Japan Steel Association, published in January 1997) CAMP-ISIJ, vol. 5, no. 6, pp. 1839-1842, Yamazaki et al., October 1992, published by Japan Iron and Steel Institute

  In the conventional technique, when a high strength of a tensile maximum strength of 900 MPa or more is ensured, a martensite or bainite structure has to be the main phase, and thus sufficient ductility cannot be obtained. Moreover, in the conventional high-strength steel sheet having a maximum tensile strength of 900 MPa or more, sufficient hydrogen embrittlement resistance may not be obtained.

  The present invention has been made in view of such circumstances, and provides a high-strength steel sheet capable of obtaining excellent hydrogen embrittlement resistance while securing a high strength of a tensile maximum strength of 900 MPa or more, and a method for producing the same. It is an object to do.

  The present inventor has intensively studied to solve the above problems. As a result, the structure inside the steel sheet includes a hard structure capable of obtaining a high strength of a tensile maximum strength of 900 MPa or more, and the steel sheet surface layer is decarburized to increase the ferrite volume fraction of only the steel sheet surface layer and soften it. The present inventors have found that it is possible to realize a steel sheet having hydrogen embrittlement resistance as if it were a low-strength steel sheet while ensuring high strength. Furthermore, the present inventor has conducted intensive studies, and the structure inside the steel plate is made of ferrite as a main phase and a small amount of martensite having fine blocks dispersed therein, whereby a high tensile strength of 900 MPa or more is obtained. And the present invention has been conceived.

The gist of the present invention is as follows.
(1) By mass%, C: 0.07% to 0.25%, Si: 0.3 to 2.50%, Mn: 1.5 to 3.0%, Ti: 0.005 to 0.09 %, B: 0.0001 to 0.01%, P: 0.001 to 0.03%, S: 0.0001 to 0.01%, Al: 2.50% or less, N: 0.0005 to 0 0.0100%, O: 0.0005 to 0.007%, the balance being steel made of iron and inevitable impurities, and the inside of the steel plate is ferrite as the main phase and martensite with a block size of 1 μm or less And the steel sheet surface layer has a thickness of 0.5 μm or more, wherein the volume ratio of the hard structure is 80% or less of the volume ratio of the hard structure included in the ¼ thickness position of the plate thickness of the steel sheet. High tensile strength with a maximum tensile strength of 900 MPa or more excellent in hydrogen embrittlement resistance, characterized by comprising a coal layer steel sheet.

(2) The high strength steel plate having a tensile maximum strength of 900 MPa or more excellent in hydrogen embrittlement resistance according to (1), further comprising Nb: 0.005 to 0.09% by mass%. .
(3) Further, by mass%, Cr: 0.01-2.0%, Ni: 0.01-2.0%, Cu: 0.01-2.0%, Mo: 0.01-0. The high-strength steel sheet having a tensile maximum strength of 900 MPa or more and excellent in hydrogen embrittlement resistance according to (1) or (2), comprising 8% of one kind or two or more kinds.
(4) Further, the tensile strength excellent in hydrogen embrittlement resistance according to any one of (1) to (3), characterized by containing V: 0.005 to 0.09% in mass%. High strength steel plate with a maximum strength of 900 MPa or more.
(5) Any one of (1) to (4), further comprising 0.0001 to 0.5% in total of one or more of Ca, Ce, Mg, and REM in mass% A high-strength steel sheet having a tensile maximum strength of 900 MPa or more and excellent in hydrogen embrittlement resistance according to claim 1.

(6) The high-strength steel sheet having a tensile maximum strength of 900 MPa or more excellent in hydrogen embrittlement resistance according to any one of (1) to (5), wherein the surface has a galvanized layer.

In addition, the inventor has intensively studied a method for producing a high-strength steel sheet that can increase the strength of the martensite structure without increasing the volume ratio of the martensite that is a hard structure.
As a result, in the annealing process after cold rolling, fine precipitates such as Ti, Nb, and V are deposited to suppress the growth of martensite laths forming individual blocks, and to the steel sheet after annealing. By introducing dislocations as nucleation sites, laths having a plurality of different orientations are generated, whereby the inside of the steel sheet includes ferrite as a main phase and a hard structure containing martensite having a block size of 1 μm or less. It has been found that a high-strength cold-rolled steel sheet or a high-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more excellent in hydrogen embrittlement characteristics can be obtained.

Furthermore, the present inventor has conducted extensive studies, and in the heat treatment in which the continuous annealing line or the continuous hot dip galvanizing line is passed, the atmosphere in the annealing furnace contains 1 to 10% by volume of H ₂ , and the balance N ₂ , H _The atmosphere is composed of ₂ O, O ₂ and inevitable impurities, and the logarithm log (PH ₂ O / PH ₂ ) of the water pressure and the hydrogen partial pressure in the atmosphere is −3 ≦ log (PH ₂ O / PH ₂ ) ≦ By controlling to −0.5, decarburization treatment of the steel sheet surface contributing to improvement of delayed fracture resistance can be performed in a short time, and the steel sheet surface layer has a volume ratio of the hard structure of the steel sheet thickness. A high-strength steel sheet having a tensile maximum strength of 900 MPa or more excellent in hydrogen embrittlement resistance and comprising a decarburized layer having a thickness of 0.5 μm or more, which is 80% or less of the volume fraction of the hard structure contained in the 1/4 thickness position, is obtained. And the inventors have conceived the production method of the present invention.

(7) Casting the slab having the chemical components described in (1) to (5), directly or once cooled, then heated to 1050 ° C or higher, and completed hot rolling above the Ar ₃ transformation point, 400 ° C When the steel sheet that has been rolled in a temperature range of ˜670 ° C., pickled, and cold-rolled with a rolling reduction of 40 to 70% is passed through a continuous annealing line, the inside of the annealing furnace is set to 1 to 10 H ₂ . An atmosphere containing volume% and the balance consisting of N ₂ , H ₂ O, O ₂ and unavoidable impurities, and a logarithm log (PH ₂ O / PH ₂ ) of water pressure and hydrogen partial pressure in the atmosphere is −3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0.5, the temperature is kept between 550 ° C. and 760 ° C. for 30 seconds or more during heating, and annealing is performed at a maximum heating temperature of 760 ° C. to Ac ₃ ° C. Cool between the heating temperature and 630 ° C at an average cooling rate of 10 ° C / second or less, and between 630 ° C and 570 ° C. A method for producing a high-strength cold-rolled steel sheet having a tensile maximum strength of 900 MPa or more excellent in hydrogen embrittlement resistance, characterized by cooling at an average cooling rate of 3 ° C./second or more.

(8) After cooling between 630 ° C. and 570 ° C. at an average cooling rate of 3 ° C./second or more, the temperature is maintained in a temperature range of 450 ° C. to 250 ° C. for 30 seconds or more. A method for producing a high-strength cold-rolled steel sheet having a tensile maximum strength of 900 MPa or more and excellent in hydrogen embrittlement characteristics.
(9) After producing a high-strength cold-rolled steel sheet by the method for producing a high-strength cold-rolled steel sheet according to (7) or (8), it is excellent in hydrogen embrittlement resistance characterized by performing zinc electroplating. A method for producing a high-strength galvanized steel sheet having a maximum tensile strength of 900 MPa or more.

(10) Casting the slab having the chemical components described in (1) to (5), directly or once cooled, and then heating to 1050 ° C or higher, completing the hot rolling above the Ar ₃ transformation point, 400 ° C When the steel sheet which has been rolled in a temperature range of ˜670 ° C., pickled and cold-rolled with a rolling reduction of 40 to 70% is passed through a continuous hot dip galvanizing line, the inside of the annealing furnace is set to 1 H ₂ . 10 volume%, and an atmosphere composed of the balance N ₂ , H ₂ O, O ₂ and inevitable impurities, and the logarithm log (PH ₂ O / PH ₂ ) of the water pressure and the hydrogen partial pressure in the atmosphere is − 3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0.5, the temperature is kept between 550 ° C. and 760 ° C. for 30 seconds or more during heating, and after annealing at the maximum heating temperature of 760 ° C. to Ac ₃ ° C., the maximum Cool between the heating temperature and 630 ° C at an average cooling rate of 10 ° C / second or less, [(Zinc plating bath temperature −40 ° C.) to (Zinc plating bath temperature + 50 ° C.)] After cooling at a mean cooling rate of 3 ° C./second or more, cooling is performed by immersing in a zinc plating bath. A method for producing a high-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more and having excellent hydrogen embrittlement resistance.

(11) After the immersion in the galvanizing bath, the alloy is subjected to an alloying treatment at a temperature of 460 ° C. to 600 ° C. and cooled, and the tensile maximum excellent in hydrogen embrittlement resistance according to (10) A method for producing a high-strength galvanized steel sheet having a strength of 900 MPa or more.

The high-strength steel sheet of the present invention has a predetermined chemical component, and the inside of the steel sheet includes ferrite as a main phase and a hard structure including martensite having a block size of 1 μm or less, and the steel sheet surface layer is formed of the hard structure. Since the volume ratio is composed of a decarburized layer having a thickness of 0.5 μm or more, which is 80% or less of the volume ratio of the hard structure contained in the 1/4 thickness position of the steel sheet thickness, it has excellent formability and hydrogen embrittlement resistance. The tensile strength is 900 MPa or more at which the crystallization characteristics can be obtained.
Further, in the method for producing a steel sheet according to the present invention, a slab having a predetermined chemical component is cast, and a cold-rolled steel sheet is subjected to a continuous annealing line or a continuous hot dip galvanizing line through precipitation of fine precipitates in an annealing process. When controlling the inside of the annealing furnace to a predetermined atmosphere, the steel sheet surface layer is decarburized to soften the steel sheet surface layer, so that excellent formability and hydrogen embrittlement resistance can be obtained. A high-strength cold-rolled steel sheet or a high-strength galvanized steel sheet is obtained.

  The high-strength steel sheet of the present invention has a tensile maximum strength of 900 MPa or more, in which the inside of the steel sheet is composed of a reinforcing layer containing a hard structure and the steel sheet surface layer is composed of a decarburized layer (softening layer). The high-strength steel sheet of the present invention has a high tensile strength of 900 MPa or more, but has a decarburized layer (softening layer) that is softer than the inside of the steel sheet surface layer, as if it had a low strength. Excellent hydrogen embrittlement resistance (delayed fracture resistance) as if it were a steel sheet. Specifically, for example, when the strength of the steel sheet surface layer is 590 MPa class by providing a decarburized layer on the steel sheet surface layer, even if the maximum tensile strength as a steel sheet is 900 MPa or more, it is as if the maximum tensile strength Is a delayed fracture resistance as if it is a 590 MPa class low strength steel sheet.

  In order to sufficiently obtain the effect of providing the decarburized layer on the steel sheet surface layer, the thickness of the decarburized layer is set to 0.5 μm or more. In the present invention, the decarburized layer is a region in which the volume ratio of the hard structure including martensite having a block size of 1 μm or less is 80% or less of the volume ratio of the hard structure included in the ¼ thickness position of the plate thickness of the steel sheet. Means. The hard structure may be only martensite having a block size of 1 μm or less, or may include martensite or bainite having a block size exceeding 1 μm.

  In the present invention, since the volume ratio of the hard structure in the decarburized layer is 80% or less of the volume ratio of the hard structure included in the ¼ thickness position of the steel sheet thickness, by providing the decarburized layer on the steel sheet surface layer. The effect is sufficiently obtained, and excellent hydrogen embrittlement resistance (delayed fracture resistance) is obtained. Specifically, for example, in a steel plate having a maximum tensile strength of 900 MPa as a steel plate, excellent delayed fracture resistance as if the steel plate has a maximum tensile strength of 780 MPa is obtained.

  In the present invention, since the volume ratio of the hard structure of the decarburized layer is defined with reference to the 1/4 thickness position of the plate thickness of the steel sheet, the volume ratio of the hard structure can be accurately defined. For example, when the volume ratio of the hard structure of the decarburized layer is defined on the basis of the 1/8 thickness position of the plate thickness, the thickness of the decarburized layer may reach 1/8 thickness of the plate thickness, and the volume of the hard tissue The rate may not be accurately defined. In addition, the steel plate structure may be greatly different at the plate thickness center position compared to other positions due to Mn center segregation or the like. For this reason, when the volume ratio of the hard structure of the decarburized layer is defined on the basis of the plate thickness center position, an accurate definition may not be possible.

  Moreover, in this embodiment, the decarburization layer is a region where the C concentration is lower than that inside the steel plate. In addition, C density | concentration is not changing from the steel plate surface to the inside of a steel plate, and the ratio of a hard structure is not changing from the steel plate surface to the inside of a steel plate. Moreover, in this embodiment, the volume ratio of a hard structure is so high that the inside of a steel plate, and a hard structure may be hardly contained on the steel plate surface.

The structure of the steel sheet excluding the decarburized layer of the high-strength steel sheet of the present invention (structure inside the steel sheet) is a multiphase structure including ferrite as a main phase and a hard structure containing martensite having a block size of 1 μm or less. . For this reason, in the high-strength steel sheet of the present invention, excellent ductility is obtained simultaneously with high strength.
In the high-strength steel sheet of the present invention, it is preferable to increase the volume fraction of ferrite as much as possible in order to ensure excellent ductility. Even if the martensite is hardened, the strength obtained is limited. For this reason, the volume fraction of ferrite is changed corresponding to the required maximum tensile strength.

  For example, when a high strength steel sheet having a maximum tensile strength of 900 to 1130 MPa is used, the ferrite volume fraction is preferably in the range of 60% to 85%, and more preferably in the range of 65% to 80%. Moreover, when setting it as the high strength steel plate of the tensile maximum strength 1130-1280MPa, it is preferable to make the volume ratio of a ferrite into the range of 55% -80%, and it is more preferable to set it as the range of 60% -75%. In the case of a high-strength steel sheet having a maximum tensile strength of 1280 to 1580 MPa, the ferrite volume fraction is preferably in the range of 50% to 75%, and more preferably in the range of 55% to 70%.

  By setting the volume ratio of ferrite within the above range, the maximum tensile strength can be set to 900 MPa or more, and the strength-ductility balance (product of the maximum tensile strength (TS) and the total elongation in the tensile test (El.)) ( TS × El.) Can be 16000 (MPa ×%) or more, and a steel plate having high strength and excellent ductility can be obtained. In the present invention, a steel sheet having excellent ductility means a steel sheet having a strength-ductility balance of 16000 (MPa ×%) or more.

  Further, the martensite contained in the steel sheet structure of the high-strength steel sheet of the present invention is granular and can be present either in the ferrite grain boundaries or in the grains. The martensite organization is composed of a hierarchical structure called packets, blocks, and laths.

  The packet is composed of a plurality of blocks having the same crystal habit plane of parent phase austenite and different crystal orientations. One packet is composed of a maximum of six blocks having a KS relationship (Kurdjumov-Sachs relationship). In general optical microscope observation, blocks having variants with small crystal orientation differences (brothers with the same crystal structure but different crystal orientations) cannot be distinguished, so two variant pairs with crystal orientation differences of 11 ° or less are used. Often defined as one block. In this case, one packet is composed of three blocks.

A block is a collection of laths having the same crystal orientation. Although the boundaries of the individual laths exist inside the block, the laths within the individual blocks have the same crystal orientation, so that they work as if they were one grain when deformed. Therefore, the strength of martensite depends on the block size.
The lath is needle-shaped and granular depending on the observation direction. Further, the lath has a KS relationship with the austenite of the parent phase, and there are those having 24 orientation relationships with the parent phase.

  Packets, blocks, and laths that make up the martensitic structure can be observed with a transmission electron microscope (TEM), crystal orientation analysis using a scanning electron microscope (SEM) -backscattered electron diffraction (EBSP) method, field emission scanning This can be confirmed by high resolution crystal orientation analysis using an electron microscope (FE (Field Emission) -SEM) -EBSP method.

The present inventor has focused on the relationship between the martensite block size present in the steel sheet and the strength of the steel sheet, and is utilized as a reinforcing structure of the steel sheet by making the martensite block size finer. It has been found that martensite can be further strengthened, and it has been found that it is possible to achieve a high strength exceeding 900 MPa while ensuring a low yield stress and ensuring a volume fraction of ferrite. Specifically, by setting the martensite block size to 1 μm or less, it is possible to utilize martensite as a strengthened structure more than ever.
The martensite constituting the steel sheet structure of the high-strength steel sheet of the present invention is composed of a block size of 1 μm or less. When the block size exceeds 1 μm, the effect of strengthening martensite is not sufficiently obtained, and the strength of the high-strength steel sheet is insufficient.

  For example, in the conventional steel, the block size of martensite having the same crystal orientation was extremely large, from several μm to several tens of μm. For this reason, in the conventional steel, for example, when the steel sheet structure is controlled to be several μm or less, the size of individual martensite grains utilized as the reinforcing structure of the steel sheet is several μm or less, and each martensite grain is single. It consists of blocks. As a result, in conventional steel, martensite could not be fully utilized as a strengthened structure.

  The martensite block size can be measured by TEM observation, SEM-EBSP method, FE-SEM-EBSP method, or the like. The SEM-EBSP method is preferable because a wide area can be measured in a short time. In addition, the FE-SEM-EBSP method is desirable because it can perform tissue observation with high accuracy.

When tissue observation is performed using the FE-SEM-EBSP method, the interval between the measurement points may be in a range in which tissue orientation analysis is possible, but is preferably 10 nm to 200 nm. If the interval between the measurement points exceeds 200 nm, the accuracy may decrease, and an accurate block size may not be measured. Further, if the interval between the measurement points is less than 10 nm, the measurement time is extremely long although it is desirable from the point of accuracy.
In the present invention, when measuring the martensite block size using the FE-SEM-EBSP method, it is possible to analyze the orientation of the structure by measuring the block size of several martensite grains in a preliminary experiment. It is preferable to carry out after determining the interval between the measurement points.

In the present invention, the average crystal grain size (dF) of ferrite is preferably 5 μm or less, more preferably 4.5 μm or less, and most preferably 4 μm or less.
The reason why the average crystal grain size (dF) of the ferrite is set in the above range is that by reducing the crystal grain size of the ferrite, the maximum tensile strength is increased without significantly reducing the ductility. When the average crystal grain size of ferrite is outside the above range, the contribution to strength increase due to finer grain becomes small, so the strength must be compensated for by increasing the volume fraction of martensite. It is not preferable because it causes a reduction and a significant ductile deterioration associated therewith. In addition, by reducing the average grain size of ferrite, it becomes difficult for localization of deformation and crack propagation to occur.In the case of tensile deformation, local ductility is improved. The expansion rate is improved. However, if the average crystal grain size of ferrite is reduced, the yield stress increases, so it is not preferable to make the average crystal grain size of ferrite extremely small, and the lower limit is preferably 1 μm or more.

The average crystal grain size (dM) of martensite is preferably 1/3 or less of the average crystal grain size (dF) of ferrite, specifically 1.5 μm or less. More preferably, it is set to 2 μm or less, and most preferably 0.9 μm or less.
The reason why the average crystal grain size (dM) of martensite is in the above range is to suppress the concentration of deformation at the ferrite and martensite interface during deformation, and to suppress the formation of microvoids and cracks at the interface. . That is, soft ferrite and hard martensite differ greatly in deformability, so if they are tensile deformation under large deformation after necking, if bending molding, during bending with small R, or during hole expansion molding Deformation concentrates at the interface between ferrite and martensite, leading to destruction. Therefore, the martensite is refined to suppress the concentration of deformation at individual interfaces, and these characteristics can be greatly improved. In addition, when machining such as shear cutting or punching, if coarse martensite is present in the cut part, flaws and fine cracks originating from martensite are generated, which is subsequently performed. This greatly reduces workability during processing. For this reason, it is necessary to make the average crystal grain size of martensite as small as possible.

Next, the chemical component (composition) of the high-strength steel sheet of the present invention will be described. In the following description, [%] is [% by mass].
“C: 0.07 to 0.25%”
C increases the strength of martensite and is added to increase the strength of the high-strength steel plate. However, when the C content exceeds 0.25%, weldability and workability become insufficient. On the other hand, if the C content is less than 0.07%, the strength is insufficient. The C content is preferably in the range of 0.08 to 0.23%, and more preferably in the range of 0.09 to 0.21%.

“Si: 0.3-2.50%” “Al: 2.5% or less”
Si and Al are ferrite stabilizing elements, and increase the Ac ₃ transformation point. Therefore, a large amount of ferrite can be formed at a wide annealing temperature, and are added from the viewpoint of improving the structure controllability. Moreover, since Si and Al contribute to solid solution strengthening, it is desirable to add positively. Since ferrite is a bcc phase containing almost no C, it is possible to concentrate C in austenite by forming a large amount of ferrite. Concentrating C in austenite contributes to increasing the strength of martensite. Moreover, Si and Al have the effect of increasing the activity of C in ferrite and austenite and allowing the decarburization reaction to proceed in a short time, and in a short time such as continuous annealing equipment and continuous hot dip galvanizing equipment. In the production using production equipment, it is necessary to add 0.3% or more.

  If the Si content is less than 0.3%, the volume fraction of ferrite becomes insufficient, and the strengthening of martensite becomes insufficient, resulting in poor ductility, delayed fracture resistance and strength of the high-strength steel sheet. It will be enough. When Al is contained, the same effect as that obtained when Si is contained can be obtained. However, when the above effect can be sufficiently obtained by containing only Si, Al may not be contained. . Further, if the Si content exceeds 2.50% or the Al content exceeds 2.5%, weldability and workability become insufficient. The Si content is preferably in the range of 0.5 to 2.2%, more preferably in the range of 0.7 to 2.0%. The content of Al is preferably in the range of 0.005 to 1.5%, and more preferably in the range of 0.01 to 1.0%.

Moreover, in the manufacturing method of the high-strength steel sheet of the present invention, when performing annealing at a low temperature of 760 ° C. to Ac ₃ ° C. during heating when passing through a continuous annealing line or a continuous hot dip galvanizing line, Since it takes a long time to dissolve the cementite generated during heating in the continuous annealing line or the continuous hot dip galvanizing line, there is a possibility that a sufficient volume ratio of martensite cannot be secured. For this reason, when producing the high-strength steel sheet of the present invention using a continuous annealing line or a continuous hot dip galvanizing line, a large amount of Si and Al is contained within the above range to increase the Ac ₃ transformation point, While securing a sufficient volume fraction of martensite, it is preferable to increase the volume fraction of ferrite to concentrate C in the austenite to increase the strength of the martensite.

“Mn: 1.5 to 3.0%”
Mn is added to increase the strength of the high-strength steel plate. However, if the Mn content exceeds 3.0%, the volume ratio of martensite increases too much, the volume ratio of ferrite contributing to securing ductility becomes insufficient, and the ductility and bendability become insufficient. Further, the hardness distribution of the steel sheet surface layer due to the segregation of Mn is also increased. On the other hand, if the Mn content is less than 1.5%, the pearlite transformation that occurs during the cooling process cannot be suppressed, and the steel sheet structure becomes a ferrite and pearlite structure, resulting in insufficient strength. The Mn content is preferably in the range of 1.6 to 2.8%, more preferably in the range of 1.7 to 2.6%.

“Ti: 0.005 to 0.09%”
Ti is a strengthening element. It contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. It is also added to suppress B from becoming a nitride. Although B contributes to the structure controllability at the time of hot rolling and the structure control and high strength in continuous annealing equipment and continuous hot dip galvanizing equipment, this effect cannot be obtained when B becomes a nitride. , B must be added to prevent nitrides from becoming nitrides.
However, if the Ti content exceeds 0.09%, the precipitation of carbonitride increases and the formability deteriorates. In addition, when the addition amount is large, recrystallization of ferrite is greatly delayed during continuous annealing or continuous hot dip galvanizing production, so that unrecrystallized ferrite tends to remain after annealing, which causes a significant decrease in ductility. For this reason, the upper limit of the Ti content needs to be 0.09% or less.
Moreover, the said effect obtained by containing Ti will become inadequate that content of Ti is less than 0.005%. The Ti content is preferably in the range of 0.01 to 0.08%, and more preferably in the range of 0.015 to 0.07%.

“B: 0.0001 to 0.01%”
Since B delays the ferrite transformation from austenite, it can be utilized for increasing the strength of the steel sheet. In addition, B delays the ferrite transformation from austenite even during hot rolling, so by making the hot rolled sheet a bainite single phase structure, the homogeneity of the hot rolled sheet can be improved and the bendability can be improved. is there. Therefore, it is necessary to add. If the addition amount is less than 0.0001%, a sufficient effect cannot be obtained by adding B. Therefore, it is necessary to add 0.0001% or more. Moreover, when the addition amount of B exceeds 0.01 mass%, not only the effect is saturated, but also the production at the time of hot rolling is lowered, so the upper limit was made 0.01%. The B content is preferably in the range of 0.0003 to 0.007%, and more preferably in the range of 0.0005 to 0.0050%.

“P: 0.001 to 0.03%”
P tends to segregate in the central part of the plate thickness of the steel sheet, causing the weld to become brittle. If it exceeds 0.03%, embrittlement of the weld becomes significant, so the appropriate range is limited to 0.03% or less. Since it is economically disadvantageous to make the P content less than 0.001%, this value is the lower limit.

“S: 0.0001 to 0.01%”
S adversely affects weldability and manufacturability during casting and hot rolling. Therefore, the upper limit is set to 0.01% or less. Since it is economically disadvantageous to make the S content less than 0.0001%, this value is the lower limit. Further, since S is combined with Mn to form coarse MnS, the bendability is lowered. For this reason, it is necessary to reduce as much as possible in order to improve bendability.

“N: 0.0005 to 0.0100%”
N forms coarse nitrides and degrades bendability and hole expansibility, so it is necessary to suppress the addition amount. This is because when N exceeds 0.0100%, this tendency becomes remarkable. Therefore, the range of N content is set to 0.0100% or less. In addition, N is better because it causes blowholes during welding. The lower limit of the N content is not particularly defined, and the effects of the present invention are exhibited. However, if the N content is less than 0.0005%, this leads to a significant increase in manufacturing costs. This is a practical lower limit.

“O: 0.0005 to 0.007%”
O forms an oxide and degrades bendability and hole expansibility, so it is necessary to suppress the addition amount. In particular, oxides often exist as inclusions, and if they are present on the punched end face or cut face, they form notched scratches and coarse dimples on the end face, which causes stress during bending and strong processing. Concentration is the starting point for crack formation, which leads to significant hole expandability or bendability degradation. When the O content exceeds 0.007%, this tendency becomes remarkable, so the upper limit of the O content is set to 0.007% or less. Setting the content of O to less than 0.0005% causes excessive cost and is not economically preferable, so this was set as the lower limit. However, even if the O content is less than 0.0005%, it is possible to ensure a TS of 900 MPa or more and excellent delayed fracture resistance, which are the effects of the present invention.

The high-strength steel sheet of the present invention may further contain the following elements as necessary.
“Nb: 0.005 to 0.09%”
Nb is a strengthening element. Similar to Ti, it contributes to an increase in the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. However, if the Nb content exceeds 0.09%, carbonitride precipitation increases and the formability deteriorates. In addition, when the amount of Nb added is large, recrystallization of ferrite is significantly delayed during continuous annealing and continuous hot dip galvanizing production, so unrecrystallized ferrite tends to remain after annealing, resulting in a significant reduction in ductility. Bring. For this reason, it is preferable that the upper limit of the amount of Nb added be 0.09% or less.
Moreover, the said effect acquired by containing Nb will become inadequate that content of Nb is less than 0.005%. The Nb content is preferably in the range of 0.01 to 0.08%, and more preferably in the range of 0.015 to 0.07%.

1 of “Cr: 0.01 to 2.0%”, “Ni: 0.01 to 2.0%”, “Cu: 0.01 to 2.0%”, “Mo: 0.01 to 0.8%” Species or two or more types Cr, Ni, Cu, and Mo are elements that contribute to the improvement of strength, and can be used in place of part of Mn. It is preferable that Cr, Ni, Cu, and Mo contain 0.01% or more of one or more of each. On the other hand, if the content of each element is too large, pickling properties, weldability, hot workability and the like may deteriorate, so the content of Cr, Ni, and Cu may be 2.0% or less. The Mo content is preferably 0.8% or less.

"V: 0.005-0.09%"
V is a strengthening element. Similar to Ti and Nb, it contributes to an increase in the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. Moreover, the delayed fracture characteristic can be improved by containing V. For this reason, it is desirable to add it in the production of a steel sheet having a maximum tensile strength of a steel sheet such as the steel sheet of the present invention exceeding 900 MPa. However, if the V content exceeds 0.09%, carbonitride precipitation increases and the formability deteriorates. In addition, if the V content is large, the recrystallization of the ferrite is significantly delayed during continuous annealing or continuous hot dip galvanizing equipment, so that unrecrystallized ferrite tends to remain after annealing, resulting in a significant reduction in ductility. Bring. For this reason, the upper limit of the V content is preferably 0.09% or less.
Moreover, the said effect obtained by containing V will become inadequate that content of V is less than 0.005%. The content of V is preferably in the range of 0.01 to 0.08%, and more preferably in the range of 0.015 to 0.07%.

“Total of 0.0001 to 0.5% of one or more of Ca, Ce, Mg, and REM”
Ca, Ce, Mg, and REM contribute to improvement in strength and material. If the content of one or more of Ca, Ce, Mg, and REM is less than 0.0001%, a sufficient effect may not be obtained. If it exceeds 0.5%, ductility may be impaired. is there. Moreover, when these content exceeds 0.5% in total, it will become the cause of a deterioration of molding processability. REM is an abbreviation for Rare Earth Metal and refers to an element belonging to the lanthanoid series. In the present invention, REM and Ce are often added by misch metal and may contain a lanthanoid series element in combination with La and Ce. Even if these lanthanoid series elements other than La and Ce are included as inevitable impurities, the effect of the present invention is exhibited. Even if the metal La or Ce is added, the effect of the present invention is exhibited.

  The high-strength steel plate of the present invention may be a high-strength galvanized steel plate by forming a galvanized layer or an alloyed galvanized layer on the surface. Since the galvanized layer is formed on the surface of the high-strength steel plate, the steel sheet has excellent corrosion resistance. In addition, since the alloyed galvanized layer is formed on the surface of the high-strength steel sheet, it has excellent corrosion resistance and excellent paint adhesion.

Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.
In order to manufacture the high-strength steel sheet of the present invention, first, a slab having the above-described chemical component (composition) is cast.
As the slab used for hot rolling, a slab produced by a continuous casting slab, a thin slab caster or the like can be used. The manufacturing method of the high-strength steel sheet of the present invention is compatible with a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.

The hot-rolled slab heating temperature needs to be 1050 ° C. or higher. If the slab heating temperature is too low, the finish rolling temperature will fall below the Ar3 point, resulting in a two-phase rolling of ferrite and austenite, and the hot-rolled sheet structure will be a heterogeneous mixed grain structure that has undergone cold rolling and annealing processes. However, the inhomogeneous structure is not eliminated and the ductility and bendability are poor. Moreover, since this steel plate has added a large amount of alloy elements in order to ensure the maximum tensile strength of 900 MPa or more after annealing, the strength at the time of finish rolling tends to be high. The decrease in the slab heating temperature causes a decrease in the finish rolling temperature, further increases the rolling load, and there is a concern that rolling may become difficult or the shape of the steel sheet after rolling may be poor. It is necessary to set it to 1050 ° C. or higher.
The upper limit of the slab heating temperature is not particularly defined, and the effect of the present invention is exhibited. However, since it is economically undesirable to make the heating temperature too high, the upper limit of the heating temperature should be less than 1300 ° C. Is desirable.

The Ar ₃ temperature is calculated by the following formula.
Ar ₃ = 901-325 × C + 33 × Si-92 × (Mn + Ni / 2 + Cr / 2 + Cu / 2 + Mo / 2)
In said formula, C, Si, Mn, Ni, Cr, Cu, and Mo are content [mass%] of each element.

  On the other hand, the upper limit of the finishing temperature is not particularly defined, and the effect of the present invention is exhibited. However, when the finishing rolling temperature is excessively high, the slab heating temperature must be excessively high in order to secure the temperature. Don't be. For this reason, the upper limit temperature of the finish rolling temperature is desirably 1000 ° C. or less.

  The winding temperature needs to be 400-670 ° C. When the coiling temperature exceeds 670 ° C., coarse ferrite and pearlite structure exists in the hot rolled structure, so that the structure heterogeneity after annealing becomes large and the bendability of the final product is deteriorated. The structure after annealing is refined to improve the strength ductility balance. In addition, winding at a temperature exceeding 670 ° C. is not preferable because the thickness of the oxide formed on the steel sheet surface is excessively increased, and the pickling property is poor. The winding temperature is preferably 630 ° C. or less from the viewpoint of homogeneously dispersing the second phase and improving bendability. Further, when the coiling temperature is less than 400 ° C., the hot rolled sheet strength is extremely increased, so that troubles such as sheet breakage and shape defects are easily induced during cold rolling. Therefore, the lower limit value of the winding temperature needs to be 400 ° C.

Note that rough rolling sheets may be joined to each other during hot rolling to continuously perform finish rolling. Moreover, you may wind up a rough rolling board once.
The hot-rolled steel sheet thus manufactured is pickled. Since pickling can remove oxides on the surface of steel sheets, it can improve the chemical conversion properties of cold-rolled high-strength steel sheets as final products, and improve the hot-plating properties of cold-rolled steel sheets for hot-dip galvanized or galvannealed steel sheets It is important for that. Moreover, pickling may be performed once or may be performed in a plurality of times.

  The pickled hot-rolled steel sheet is cold-rolled at a rolling reduction of 40 to 70% and passed through a continuous annealing line or a continuous hot-dip galvanizing line. If the rolling reduction is less than 40%, it is difficult to keep the shape flat. Moreover, since the ductility of the final product becomes poor, the rolling reduction is 40% or more as the lower limit. On the other hand, when the rolling reduction exceeds 70%, the cold rolling load becomes too large and it becomes difficult to cold rolling. For this reason, the rolling reduction is 70% or less. The rolling reduction is more preferably 45 to 65%. In addition, the effect of the present invention is exhibited without particularly defining the number of rolling passes and the rolling reduction for each pass.

Thereafter, the obtained cold-rolled steel sheet is passed through a continuous annealing line or a continuous hot-dip galvanizing line to produce a high-strength cold-rolled steel sheet or a high-strength galvanized steel sheet.
In the present invention, when passing through a continuous annealing line (equipment) or a continuous hot dip galvanizing line (equipment), the inside of the annealing furnace contains 1 to 10% by volume of H ₂ , and the balance is N ₂ , H ₂ O, The atmosphere is composed of O ₂ and inevitable impurities, and the logarithm log (PH ₂ O / PH ₂ ) of the water pressure and the hydrogen partial pressure in the atmosphere is −3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0. Control to 5.
Thereby, in a continuous annealing line or a continuous hot-dip galvanizing line, a cold-rolled steel sheet is annealed (heat treatment) in the above atmosphere, and C contained in the steel sheet surface layer is reduced (decarburization process) in a short time. As a result, the surface layer of the steel sheet is softened, has high strength exceeding the maximum tensile strength of 900 MPa, and excellent hydrogen embrittlement resistance (lag resistance) as if it is a low-strength steel plate having a maximum tensile strength of less than 900 MPa. Steel sheet having fracture characteristics).

In addition, the ratio of the water pressure and the hydrogen partial pressure in the atmosphere in the annealing furnace of a continuous annealing line or a continuous hot dip galvanizing line can be adjusted by the method of blowing water vapor into the annealing furnace. Thus, the method of adjusting the ratio between the water pressure and the hydrogen partial pressure in the atmosphere in the annealing furnace is simple and preferable.
Further, if the H ₂ concentration exceeds 10% by volume in the atmosphere in the annealing furnace, it is not preferable because the cost is increased. On the other hand, when the H ₂ concentration is less than 1% by volume, Fe contained in the steel plate is oxidized, so that wettability and plating adhesion are insufficient.

In addition, the logarithm log (PH ₂ O / PH ₂ ) of the moisture pressure and the hydrogen partial pressure in the atmosphere in the annealing furnace is set to −3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0.5. In an annealing furnace of a continuous annealing line or a continuous hot dip galvanizing line, which is heat treatment equipment for a long time, a decarburized layer can be formed on the surface layer of the steel sheet, and the delayed fracture resistance can be improved. Note that the lower limit of the logarithm log (PH ₂ O / PH ₂ ) of the water pressure and the hydrogen partial pressure is set to -3 or more. If it is less than -3, it is used for a short time process such as continuous annealing or continuous galvanizing equipment. This is because a decarburized layer having a thickness that can improve the delayed fracture characteristics cannot be formed. On the other hand, the upper limit of the logarithm log (PH ₂ O / PH ₂ ) of the water pressure and the hydrogen partial pressure was set to −0.5 or less because if it exceeds −0.5, the thickness of the decarburized layer becomes too thick and the tensile maximum This is because it is difficult to ensure high strength exceeding 900 MPa.

  Further, in order to obtain a decarburized layer having a thickness of 0.5 μm or more in the annealing furnace, not only the atmosphere in the annealing furnace is controlled to the above atmosphere, but also the slab for obtaining a cold-rolled steel sheet within the above range. It is essential to contain Al and / or Al to promote the decarburization reaction. When the content of Si and Al is less than the above range, even if the atmosphere in the annealing furnace is the above atmosphere, a decarburized layer having a thickness of 0.5 μm or more that can sufficiently improve delayed fracture resistance is formed. In order to achieve this, a long heat treatment is required. For this reason, when the Si and Al contents are less than the above range, even if the atmosphere in the annealing furnace is the above, the inside of the annealing furnace of a continuous annealing line or a continuous hot dip galvanizing line which is a short-time heat treatment facility is used. And a decarburized layer having a sufficient thickness cannot be formed.

  Other manufacturing methods for forming a decarburized layer by softening the surface of the steel plate from the inside of the steel plate include, for example, a method of decarburizing a slab or a hot-rolled steel plate, or joining plates having different compositions only in the steel plate surface layer at the casting stage. A method can be considered. However, when the steel sheet surface layer is softened using these methods, C contained in the steel sheet diffuses into the steel sheet surface layer by annealing by passing a continuous annealing line or a continuous hot dip galvanizing line. Therefore, there is a problem that it is difficult to form a softened layer on the steel sheet surface layer. This problem is that when annealing is performed under the conventional manufacturing conditions, the atmosphere in the annealing furnace is an atmosphere in which decarburization reaction does not occur. This occurs because C diffuses into a low region of the carbon and the decarburized layer disappears. Therefore, when annealing is performed under conventional manufacturing conditions, even if the decarburized layer is formed by softening the steel sheet surface layer from the inside of the steel plate by the above method, the decarburized layer is likely to disappear due to diffusion, greatly increasing the thickness of the decarburized layer. It is difficult to ensure a strength exceeding the maximum tensile strength of 900 MPa.

  Moreover, in this embodiment, when letting a continuous annealing line (equipment) or a continuous hot dip galvanizing line (equipment) pass through, the inside of the annealing furnace is controlled to the above atmosphere, so that oxidation of Si, Mn and Al on the surface of the steel sheet is performed. In addition to being able to improve the plating properties, it is possible to promote the alloying reaction.

The present invention is not limited to the above example.
For example, in the above-described method for producing a high-strength cold-rolled steel sheet or a high-strength galvanized steel sheet, the moisture pressure and hydrogen partial pressure are controlled to control the atmosphere in the annealing furnace. You may control the atmosphere in an annealing furnace using the method of controlling a pressure, the method of controlling the partial pressure of nitrogen dioxide and nitric oxide, or the method of blowing oxygen directly in a furnace. Even in this case, similarly to the case where the atmosphere in the annealing furnace is controlled by controlling the moisture pressure and the hydrogen partial pressure, Si oxide (or Si oxide and Al oxide) is formed inside the steel plate near the surface layer. Can be deposited, and the same effect as described above can be obtained.

In this embodiment, when manufacturing a high-strength cold-rolled steel sheet by letting a cold-rolled steel sheet pass through a continuous annealing line, it is performed under the first condition or the second condition shown below.
"First condition"
When passing through a continuous annealing line, the temperature between 550 ° C. and 760 ° C. is retained for 30 seconds or more during heating, and annealing is performed at a maximum heating temperature of 760 ° C. to Ac ₃ ° C., and then an average cooling is performed between the maximum heating temperature and 630 ° C. Cooling is performed at a rate of 10 ° C./second or less, and cooling is performed between 630 ° C. and 570 ° C. to room temperature at an average cooling rate of 3 ° C./second or more.

"Second condition"
When passing through the continuous annealing line, after performing annealing in the same manner as the first condition described above, the maximum heating temperature between 630 ° C and 630 ° C to 570 ° C is cooled in the same manner as the first condition, After maintaining at a temperature range of 450 ° C. to 250 ° C. for 30 seconds or more, it is cooled to room temperature at an average cooling rate of 3 ° C./second or more.
Furthermore, in this invention, it is good also as a high intensity | strength galvanized steel sheet by giving zinc electroplating to the high intensity | strength cold-rolled steel sheet obtained by letting a continuous annealing line pass in 1st condition or 2nd condition. .

In the present invention, the cold-rolled steel sheet obtained by the above method may be passed through a continuous hot dip galvanizing line to produce a high-strength galvanized steel sheet. In this case, the third condition or the fourth condition shown below is performed.
"Third condition"
When passing through the continuous hot dip galvanizing line, after annealing in the same manner as the above-mentioned first condition, cooling between the highest heating temperature and 630 ° C. is performed in the same manner as in the first condition, and then from 630 ° C. to [(Zinc plating bath temperature−40 ° C.) to (Zinc plating bath temperature + 50 ° C.)] After cooling at an average cooling rate of 3 ° C./second or more, the sample is immersed in a zinc plating bath and cooled.
Thus, cooling between 630 ° C. to [(Zinc plating bath temperature −40 ° C.) to (Zinc plating bath temperature + 50 ° C.)] ° C. at a suitable temperature and immersing in the galvanizing bath results in zinc on the surface. A high-strength galvanized steel sheet with a plated layer is obtained.

"4th condition"
When passing through the continuous hot dip galvanizing line, after performing the steps up to immersing in the galvanizing bath in the same manner as the third condition described above, the alloying treatment is performed at a temperature of 460 ° C. to 600 ° C. and cooling is performed. I do.
Here, when the alloying treatment is performed, a Zn-Fe alloy formed by alloying a galvanized layer on the surface is formed, and a high-strength galvanized steel sheet having the alloyed galvanized layer on the surface is obtained.

  In addition, when passing through the continuous hot dip galvanizing line, the average cooling rate is between 630 ° C. and [(Zinc plating bath temperature−40 ° C.) − (Zinc plating bath temperature + 50 ° C.)] ° C. in the same manner as the third condition. You may hold | maintain for 30 seconds or more in the temperature range of 450 to 250 degreeC, after performing the process until it cools at 3 degrees C / second or more, and before immersing in a galvanization bath.

In the method of manufacturing a steel sheet according to the present embodiment, the above-described chemical components include Ti, which is an element that precipitates fine precipitates, and Nb, V, etc., elements that precipitate fine precipitates as necessary, as a slab. A slab having (composition) is used. Therefore, when a cold-rolled steel sheet is manufactured by the above-described manufacturing method and a high-strength cold-rolled steel sheet or a high-strength galvanized steel sheet is manufactured under any of the first condition to the fourth condition, an annealing process after cold-rolling In this case, fine precipitates such as Ti are deposited. More specifically, when the cold-rolled steel sheet obtained after cold rolling is annealed in the vicinity of the maximum heating temperature when passing through a continuous annealing line or a continuous hot-dip galvanizing line, fine precipitates such as Ti precipitate. Is done. In the present invention, when letting a continuous annealing line or a continuous hot dip galvanizing line pass through, the temperature is kept between 550 ° C. and 760 ° C. for 30 seconds or more during heating, and annealing is performed at a maximum heating temperature of 760 ° C. to Ac ₃ ° C. Therefore, fine fine precipitates such as Ti are deposited.

In the present embodiment, the maximum heating temperature is set to 760 ° C. to Ac ₃ ° C. because the cementite precipitated in the hot-rolled sheet or precipitated during heating in the continuous annealing equipment and the continuous hot-dip galvanizing equipment after cold rolling. This is because the cementite is dissolved and a sufficient volume ratio of austenite is secured. If annealing is performed at a maximum heating temperature of less than 760 ° C, it takes a long time to dissolve cementite, resulting in a decrease in productivity or a decrease in the martensite volume fraction after cooling due to the fact that cementite remains undissolved. The strength of 900 MPa or more cannot be secured.

  In the present invention, after annealing, between the maximum heating temperature and 630 ° C. is cooled at an average cooling rate of 10 ° C./second or less, the formation of ferrite is promoted, and the volume fraction of ferrite contributing to securing ductility is set. It can be secured sufficiently. On the other hand, when cooling between the maximum heating temperature and 630 ° C. at an average cooling rate exceeding 10 ° C./sec, the volume ratio of ferrite is insufficient, and the ductility and bending of high-strength cold-rolled steel plate or high-strength galvanized steel plate May be insufficient.

  In the present invention, the average cooling rate between 630 ° C. and 570 ° C. or between 630 ° C. and [(Zinc plating bath temperature−40 ° C.) − (Zinc plating bath temperature + 50 ° C.)] ° C. is 3 ° C./second or more. Therefore, pearlite and bainite transformation that can occur in this temperature range can be suppressed, and austenite can be efficiently transformed into martensite. On the other hand, when the average cooling rate in the above temperature range is less than 3 ° C./second, the austenite transforms into pearlite, so the volume ratio of martensite is insufficient, and high strength cold-rolled steel sheet or high-strength galvanizing is performed. The strength of the steel sheet may be insufficient. Further, an extreme decrease in the cooling rate in the above temperature range is not preferable because it causes a decrease in productivity.

  In the method for producing a high-strength galvanized steel sheet according to the present invention, in order to improve plating adhesion, the steel sheet before annealing is plated with one or more kinds selected from Ni, Cu, Co and Fe. May be.

Moreover, when manufacturing the high-strength galvanized steel sheet of the present invention, as a process from annealing to dipping in a galvanizing bath, “after degreasing and pickling, heating in a non-oxidizing atmosphere, including H ₂ and N ₂ . After annealing in a reducing atmosphere, cool it down to near the galvanizing bath temperature and immerse it in the galvanizing bath ”or“ Adjust the atmosphere during annealing, first oxidize the steel plate surface, then reduce After cleaning the surface of the steel sheet before plating, it is immersed in a galvanizing bath, or the total reduction furnace method, or "after the steel sheet is degreased and pickled, and flux treatment is performed using ammonium chloride, A flux method or the like that is then immersed in a galvanizing bath may be used.

  Cast slabs having chemical components (compositions) A to Z shown in Tables 1 and 2 and hot rolling immediately after casting under the conditions shown in Tables 3 and 4 (slab heating temperature and hot rolling completion temperature). Then, after winding in the winding temperature range shown in Table 3 and Table 4, pickling, and cold rolling at the rolling reduction shown in Table 3 and Table 4, the experiment shown in Table 3 and Table 4 having a thickness of 1.6 mm The cold-rolled steel sheet of Example 1 to Experimental Example 71 was used.

Thereafter, the cold-rolled steel sheets of Experimental Examples 1 to 71 are contained in the annealing furnace in a volume% of H ₂ shown in Tables 5 to 7, and the moisture pressures and hydrogen partial pressures shown in Tables 5 to 7 are included. It is controlled to an atmosphere in the furnace which is logarithmic log (PH ₂ O / PH ₂ ), and a continuous annealing line or a continuous hot dip galvanizing line is passed under the conditions shown in Tables 5 to 7, and Experimental Examples 1 to 71 Steel plates (cold rolled steel plate (CR), electrogalvanized steel plate (EG), hot dip galvanized steel plate (GI), alloyed hot dip galvanized steel plate (GA) shown in Tables 3 to 10).

  When passing through the continuous annealing line, during heating, between 550 ° C. and 760 ° C. is retained for the residence time shown in Tables 5 to 7, and after annealing at the maximum heating temperature shown in Tables 5 to 7, maximum heating is performed. The temperature was cooled between 630 ° C. and the average cooling rate shown in Table 5 to Table 7, and the cooling was performed between 630 ° C. and 570 ° C. at the average cooling rate shown in Table 5 to Table 7.

Thereafter, in some experimental examples, the samples were held at the holding temperatures shown in Tables 5 to 7 and the holding times shown in Tables 5 to 7 and then cooled to room temperature, and in other experimental examples, Tables 5 to 7 were used. Cooled to room temperature at the indicated average cooling rate.
Then, about the one part of the experimental example which let the continuous annealing line pass, the zinc system electroplating was given by the method shown below, and the electrogalvanized steel plate (EG) was manufactured. First, as a pretreatment for plating, alkaline degreasing, water washing, pickling, and water washing were sequentially performed on a steel plate that was passed through a continuous continuous annealing line. Thereafter, a liquid circulation type electroplating apparatus is used for the pretreated steel sheet, a plating bath made of zinc sulfate, sodium sulfate, and sulfuric acid is used, and the current density becomes a predetermined plating thickness at 100 A / dm ^2. Electrolytic treatment was performed until Zn plating was performed.

Moreover, when letting a continuous hot dip galvanizing line pass, it was made to retain at the residence time shown in Table 5-Table 7 between 550 degreeC-760 degreeC at the time of heating, and it annealed with the maximum heating temperature shown in Table 5-Table 7. After cooling between the maximum heating temperature and 630 ° C. at the average cooling rate shown in Table 5 to Table 7, and after cooling between 630 ° C. and the galvanizing bath temperature at the average cooling rate shown in Table 5 to Table 7, It cooled by immersing in the zinc plating bath of the temperature shown in Table 5-7.
Moreover, in some experimental examples, after performing the process until it was immersed in a zinc plating bath, the alloying process was performed at the temperature shown in Table 5-Table 7, and it cooled.

  Moreover, when passing through a continuous hot dip galvanizing line, in some examples, after performing the process until it cools between 630 degreeC-galvanizing bath temperature with the average cooling rate shown in Table 5-Table 7. Before being immersed in the galvanizing bath, it was held at the holding temperatures shown in Tables 5 to 7 and the holding times shown in Tables 5 to 7.

  With respect to the steel plates of Experimental Examples 1 to 71 ((CR) (EG) (GI) (GA) shown in Tables 3 to 10) thus obtained, the steel plates inside the steel plates were formed by the FE-SEM-EBSP method. The tissue was observed. The volume ratio inside the steel sheet was specified by obtaining the area ratio of each structure by image analysis. The results are shown in Tables 8-10.

As shown in Tables 8 to 10, Experimental Examples 1, 7 to 10, 14, 18, 19, 25, 29, 30, 32, 35 to 38, 42, 43, 45, 49, which are examples of the present invention. -64, it has confirmed that the steel plate structure was mainly ferrite and contained martensite.
On the other hand, in Experimental Examples 3, 5, 16, 17, 27, 28, 47, 66, and 69 which are comparative examples of the present invention, the volume ratio of ferrite is 50% or less, and the steel sheet contains mainly ferrite. It wasn't something to do. Moreover, in Experimental Examples 6, 13, 21, 22, 34, 40, 48, and 68, which are comparative examples of the present invention, the inside of the steel sheet did not contain martensite.

Moreover, about the steel plate of Experimental example 1-Experimental example 71, the block size of the martensite inside a steel plate was measured with the method shown below.
Measure the block size of several martensite grains by measuring the block size of several martensite grains by conducting a preliminary experiment using the FE-SEM-EBSP method to measure the block size of martensite. Was determined after measuring 25 nm.

The results are shown in Tables 8-10. From Table 8 to Table 10, in the examples of the present invention, the block size was 1 μm or less.
In contrast, in Experimental Examples 5, 17, 27, 47, and 70, which are comparative examples of the present invention, the block size exceeded 1 μm.

Moreover, about the steel plate of Experimental example 1-Experimental example 71, the steel plate structure inside a steel plate was observed by FE-SEM-EBSP method, the average crystal grain diameter of a ferrite and a martensite was measured, and the ratio was computed.
The results are shown in Tables 8-10.

Moreover, about the steel plate of Experimental example 1-Experimental example 71, the hydrogen embrittlement resistance characteristic was investigated with the method shown below.
First, the obtained steel plate was shear-cut to make a test piece of 1.2 mm × 30 mm × 100 mm whose longitudinal direction is the direction perpendicular to the rolling direction, and the end face was mechanically ground. In order to evaluate the effect of improving delayed fracture resistance by the softened layer of the steel sheet surface, the end surface was subjected to mechanical grinding in order to prevent delayed fracture occurring from the defects introduced during shear cutting. Thereafter, the test piece was bent by a push bending method to produce a bending test piece having a radius of 5R. The opening amount of the bending test piece after stress unloading was 40 mm. Thereafter, a strain gauge was attached to the surface and tightened with a bolt to elastically deform the bending test piece, and the amount of strain at that time was read to calculate the load stress. After that, the bending test piece is immersed in an aqueous solution of ammonium thiocyanate containing 3% NaCl and subjected to electrolytic charging under the condition of a current density of 1.0 mA / cm ² , thereby causing delayed fracture that causes hydrogen to penetrate into the steel sheet. An accelerated test was conducted.
Then, even when the electrolytic charging time was 100 hours, a steel plate having no cracks was evaluated as a steel plate having good (O) delayed fracture resistance, and a steel plate having cracks was evaluated as defective (x).

The results are shown in Tables 8-10. As shown in Tables 8 to 10, in the examples of the present invention, the evaluation was good and the hydrogen embrittlement resistance was excellent.
On the other hand, in Experimental Examples 3, 11, 15, 20, 24, 26, 31, 33, 39, 44, 46, 66, and 67 which are comparative examples of the present invention, the evaluation is x and the hydrogen embrittlement resistance is low. It was insufficient.

  In addition, a tensile test piece based on JIS Z 2201 was taken from the steel plates of Experimental Examples 1 to 71, and a tensile test was performed based on JIS Z 2241. The maximum tensile strength (TS) and total elongation (El.) Were measured. Was measured, and a strength-ductility balance (TS × El.), Which was the product of the maximum tensile strength (TS) and the total elongation (El.), Was calculated.

The results are shown in Tables 8-10. As shown in Tables 8 to 10, in Examples of the present invention, the maximum tensile strength was 900 MPa or more, and the strength-ductility balance (TS × El.) Was 16000 (MPa ×%) or more.
In contrast, in Experimental Examples 3 to 5, 16, 17, 27, 28, 40, 47, 48, 66, and 69, which are comparative examples of the present invention, the maximum tensile strength and / or the strength-ductility balance (TS × El) .) Was insufficient.

Claims (11)

% By mass
C: 0.07% to 0.25%,
Si: 0.3-2.50%,
Mn: 1.5-3.0%
Ti: 0.005 to 0.09%,
B: 0.0001 to 0.01%
P: 0.001 to 0.03%,
S: 0.0001 to 0.01%,
Al: 2.5% or less,
N: 0.0005 to 0.0100%,
O: 0.0005 to 0.007%,
The balance is iron and inevitable impurities, and the inside of the steel sheet contains ferrite as a main phase and a hard structure containing martensite with a block size of 1 μm or less, and the steel sheet surface layer is the hard structure It is excellent in hydrogen embrittlement resistance, characterized by comprising a decarburized layer having a thickness of 0.5 μm or more, which is 80% or less of the volume ratio of the hard structure contained in the 1/4 thickness position of the steel sheet thickness. High strength steel plate with a maximum tensile strength of 900 MPa or more. Furthermore, in mass%,
The high-strength steel sheet having a tensile maximum strength of 900 MPa or more excellent in hydrogen embrittlement resistance according to claim 1, comprising Nb: 0.005 to 0.09%. Furthermore, in mass%,
Cr: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
Cu: 0.01 to 2.0%,
Mo: 0.01-0.8% of 1 type or 2 types or more, The high tensile maximum strength of 900 MPa or more excellent in hydrogen embrittlement resistance according to claim 1 or 2 Strength steel plate. Furthermore, in mass%,
The high-strength steel plate having a tensile maximum strength of 900 MPa or more and excellent in hydrogen embrittlement resistance according to any one of claims 1 to 3, wherein V: 0.005 to 0.09%. Furthermore, in mass%,
The hydrogen embrittlement resistance according to any one of claims 1 to 4, comprising 0.0001 to 0.5% in total of one or more of Ca, Ce, Mg, and REM. High-strength steel sheet with a maximum tensile strength of 900 MPa or more with excellent crystallization properties.   The high-strength steel sheet having a tensile maximum strength of 900 MPa or more excellent in hydrogen embrittlement resistance according to any one of claims 1 to 5, wherein the surface has a galvanized layer. Casting the slab having the chemical component according to any one of claims 1 to 5, directly or once cooled, and then heated to 1050 ° C or higher, completing the hot rolling above the Ar ₃ transformation point, When the steel sheet subjected to cold rolling with a rolling reduction of 40 to 70% is passed through a continuous annealing line after winding in a temperature range of 400 ° C to 670 ° C and pickling.
In the annealing furnace, an atmosphere containing 1 to 10% by volume of H ₂ and the balance N ₂ , H ₂ O, O ₂ and unavoidable impurities is used, and the logarithm log of the moisture pressure and the hydrogen partial pressure in the atmosphere ( PH ₂ O / PH ₂ ) is controlled to −3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0.5, and during heating, the temperature is kept between 550 ° C. and 760 ° C. for 30 seconds or more, and the maximum heating temperature is from 760 ° C. After annealing at Ac ₃ ° C., cooling between the maximum heating temperature and 630 ° C. at an average cooling rate of 10 ° C./second or less, and cooling between 630 ° C. and 570 ° C. at an average cooling rate of 3 ° C./second or more. A method for producing a high-strength cold-rolled steel sheet having a tensile maximum strength of 900 MPa or more and excellent in hydrogen embrittlement resistance.   8. The hydrogen embrittlement resistance according to claim 7, wherein after cooling between 630 ° C. and 570 ° C. at an average cooling rate of 3 ° C./second or more, the temperature is maintained at 450 ° C. to 250 ° C. for 30 seconds or more. A method for producing a high-strength cold-rolled steel sheet having excellent tensile properties and a maximum tensile strength of 900 MPa or more.   Tensile maximum strength excellent in hydrogen embrittlement resistance characterized by performing zinc electroplating after producing a high-strength cold-rolled steel sheet by the method for producing a high-strength cold-rolled steel sheet according to claim 7 or claim 8. A method for producing a high-strength galvanized steel sheet of 900 MPa or more. Casting the slab having the chemical component according to any one of claims 1 to 5, directly or once cooled, and then heated to 1050 ° C or higher, completing the hot rolling above the Ar ₃ transformation point, When winding a steel sheet which has been rolled in a temperature range of 400 ° C. to 670 ° C., pickled, and cold-rolled with a rolling reduction of 40 to 70%, through a continuous hot-dip galvanizing line,
In the annealing furnace, an atmosphere containing 1 to 10% by volume of H ₂ and the balance N ₂ , H ₂ O, O ₂ and unavoidable impurities is used, and the logarithm log of the moisture pressure and the hydrogen partial pressure in the atmosphere ( PH ₂ O / PH ₂ ) is controlled to −3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0.5, and during heating, the temperature is kept between 550 ° C. and 760 ° C. for 30 seconds or more, and the maximum heating temperature is from 760 ° C. After annealing at Ac ₃ ° C., cooling between the maximum heating temperature and 630 ° C. at an average cooling rate of 10 ° C./second or less, 630 ° C. to [(Zinc plating bath temperature −40 ° C.) to (Zinc plating bath temperature + 50 ° C.) ] After cooling at an average cooling rate of 3 ° C / second or more at a temperature of 0 ° C, it is immersed in a galvanizing bath and cooled, and has a high tensile strength of 900 MPa or more with excellent hydrogen embrittlement resistance Manufacturing method of galvanized steel sheet.   The maximum tensile strength with excellent hydrogen embrittlement resistance according to claim 10 or more, wherein the alloy is subjected to an alloying treatment at a temperature of 460 ° C to 600 ° C after being immersed in the galvanizing bath. Manufacturing method of high strength galvanized steel sheet.