JP5644094B2 - High-strength steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability, method for producing high-strength cold-rolled steel sheet, and method for producing high-strength galvanized steel sheet - Google Patents

Description

  The present invention relates to a high-strength steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability, a method for producing a high-strength cold-rolled steel sheet, and a method for producing a high-strength galvanized steel sheet.

  In recent years, demands for increasing the strength of steel sheets used in automobiles and the like have increased, and high-strength cold-rolled steel sheets having a maximum tensile stress of 900 MPa or more are also being used. Usually, when the strength of the steel sheet is improved, workability such as ductility, bendability and hole expansibility deteriorates, but it is desirable that a high-strength steel sheet has sufficient workability.

  In general, when a part such as an automobile is manufactured using a steel plate, the steel plate is machined or punched, such as mechanically cutting or punching, and thereafter, pressing or the like is performed. When the workability of the steel sheet is insufficient, when the vicinity of the end face of the steel sheet is greatly deformed during machining or forming of the steel sheet, there is a risk that a crack will occur at the end of the steel sheet. It is known that the workability of a steel sheet is related to defects such as microvoids generated at the end of the steel sheet during machining or forming of the steel sheet. In particular, in bending and stretch flange processing with strain concentration on specific parts, the deformation mechanism is the deformation concentration, crack formation, and propagation to specific parts. Often take.

  For example, if the steel sheet is made of ferrite and martensite, the hardness is different between hard martensite and soft ferrite, so that deformation concentrates on the interface between ferrite and martensite during machining or forming. Defects such as microvoids occur. It is known that microvoids are more likely to occur as the difference in deformability (hardness difference) between ferrite and martensite increases. As a result, bendability and hole expandability deteriorate.

  In addition, when casting molten steel, a dendrite structure is formed near the meniscus from the slab surface toward the slab center. The dendrite has a so-called dendritic shape with a primary or secondary arm. For this reason, inclusions and defects are easily trapped between the dendrite arms. Further, since dendrite grows while discharging Mn into the molten steel in the formation process, the Mn concentration at the center of the dendrite trunk and the slab as the final solidification position is considerably higher than that of the initial solidification structure (dendrites). As a result, the Mn concentration region exists inhomogeneously on the slab surface layer. In general, since Mn has a slower diffusion rate than C, even if it undergoes subsequent hot rolling, cold rolling, and annealing, the Mn concentration cannot be homogenized. Therefore, the inhomogeneous region of Mn concentration generated in the solidification process affects the material even after cold rolling-annealing.

  On the other hand, in order to make the steel sheet structure composed of ferrite and martensite in order to improve ductility, the cooling process after continuous annealing or continuous hot dip galvanizing equipment, two-phase region annealing, or once annealing with austenite single phase Heat treatment is performed to form ferrite. In this case, since inclusions can become nucleation sites for ferrite transformation, when inclusions are trapped unevenly on the slab surface during solidification, the structure develops unevenly on the steel sheet surface starting from these inclusions. It will be. In particular, in bending, the surface and the vicinity thereof are greatly deformed, so that they are easily affected by the uneven structure of the steel sheet surface. In addition, since the region enriched in Mn, which is an austenite stabilizing element, remains as austenite and transforms into martensite during the cooling process, the steel sheet surface layer has an inhomogeneous hardness distribution along the Mn concentration distribution. Will exist. In addition, an inclusion or a defect is used as each generation site, and an inhomogeneous structure may occur in the surface layer, which deteriorates the bendability closely related to the surface layer characteristics of the steel sheet.

As a result, the bendability of the steel having an inhomogeneous solidified structure, in particular, the high-strength steel sheet having a multiphase structure composed of ferrite and martensite for improving ductility was inferior.
For this reason, conventionally, the bendability has been improved by using a steel sheet structure that is hardly affected by the solidified structure. Specifically, as a method for ensuring high strength and bendability at the same time, attempts have been made to change the steel sheet structure to a martensite single phase structure or a precipitation strengthened ferrite single phase structure (for example, Non-Patent Document 1). 2, 3, and Patent Document 1). In a steel sheet composed of these single-phase structures, there is no change in the structure of the surface layer due to inclusions, defects, or segregation.

  For example, a steel sheet composed of a martensite single phase structure is manufactured by using a continuous annealing facility or a continuous hot dip galvanizing facility to heat the austenite single phase region, and then ferrite, pearlite and bainite in the subsequent cooling process. It is obtained by suppressing the transformation. Since Mn hardly contributes to solid solution strengthening, even if there is a concentration distribution of Mn in the steel sheet surface layer, there is no hardness distribution in the steel sheet surface layer. In addition, such a steel sheet is homogeneous because the structure is a martensite single phase, and can have high strength with a maximum tensile stress of 900 MPa or more as well as bendability. However, since martensite is a structure containing many dislocations, a steel sheet composed of a martensite single-phase structure has extremely low ductility. For this reason, steel sheets with a martensite single-phase structure can be applied to members that perform only simple bending work, but they have a complicated shape that mixes bending and overhanging and deep drawing like automotive parts. It was difficult to apply to members having

Moreover, since the ferrite single phase structure steel utilizing precipitation strengthening by Nb or Ti does not include martensite as a hard structure, the hardness distribution of the steel sheet surface layer does not exist and is excellent in homogeneity. However, as shown below, it is difficult to secure high strength of ferrite single-phase structure steel with a tensile maximum stress of 900 MPa or more.
That is, the precipitation strengthening including NbC and TiC exhibits the strongest effect by coherent precipitation in the ferrite as the parent phase. Therefore, for example, when using this strengthening mechanism in a hot rolling facility, NbC and TiC are once dissolved at an ultrahigh temperature (austenite single phase region) exceeding 1200 ° C., and then cooled to about 600 ° C., By holding in this temperature range, ferrite is formed, and NbC and TiC are coherently precipitated in the formed ferrite, whereby the strength of the steel sheet can be increased.

  However, when manufacturing cold-rolled steel sheets and continuous hot-dip galvanized steel sheets using steel sheets with precipitation-strengthened NbC or TiC by hot rolling, the hot-rolled steel sheets are cold-rolled (processed) and then annealed. Since the ferrite is recrystallized, the consistency between these precipitates and the ferrite is lowered, and the steel sheet strength is greatly lowered. Further, since these precipitates are coarsened during annealing, the allowance for increasing the strength due to precipitation strengthening is also greatly reduced. Furthermore, Nb and Ti greatly suppress recrystallization, so if a large amount of Nb or Ti is added to compensate for strength reduction, even after annealing in continuous annealing equipment or continuous hot dip galvanizing equipment, As-processed non-recrystallized ferrite tends to remain, and the ductility is greatly reduced. Further, non-recrystallized ferrite is an as-processed structure and is extremely hard, so that the steel sheet structure becomes inhomogeneous and the bendability is greatly reduced. Moreover, in a continuous annealing facility or a continuous hot dip galvanizing facility, it is difficult to heat to a temperature exceeding 1200 ° C., and productivity is extremely inferior. As described above, it has been difficult to produce a high-strength cold-rolled steel sheet and hot-dip galvanized steel sheet having a tensile maximum stress of 900 MPa or more and having a ferrite single-phase structure utilizing precipitation strengthening.

  The homogeneity of the structure of the steel sheet surface layer is, for example, a method for measuring Rockwell hardness from the surface of the steel sheet, or a structure observation in the width direction, length direction, or width direction of the steel sheet once possible. After embedding and polishing, the surface hardness of the steel sheet can be measured by a method of measuring by a Vickers hardness test. However, since steel sheets used as automotive parts are assumed to be bent in various directions, Rockwell hardness measurement is possible, which can simultaneously measure the hardness distribution of the steel sheet surface layer in the length and width directions. It is desirable to measure the homogeneity of the structure of the steel sheet surface layer.

  In addition, while making the structure of the steel sheet a structure consisting of ferrite and martensite, as a technique to achieve both elongation and bendability, after making the ferrite and martensite structure once, by tempering to soften the martensite, Methods for improving bendability and hole expandability are known (see, for example, Non-Patent Documents 2 and 4 and Patent Documents 2 to 4). When martensite is softened, the difference in deformability (hardness difference) between ferrite and martensite is reduced, and it becomes difficult for strain to concentrate on the interface between ferrite and martensite during machining or molding. This suppresses the formation of microvoids and improves the workability of the steel sheet. However, when martensite is softened, the strength of the steel sheet is reduced, so that a steel sheet having sufficient strength may not be obtained.

JP 2002-161336 A JP-A-6-108152 JP 2006-104532 A Japanese Patent Laying-Open No. 2005-256044

Plasticity and processing, 36-416 (1995) p973 CAMP-ISIJ Vol. 20 (2007) p437 CAMP-ISIJ Vol. 12 (1999) p1219 CAMP-ISIJ Vol. 13 (2000) p391

  As described above, conventionally, in order to achieve both strength and bendability, the steel sheet structure has been changed to a martensite single phase structure that is not easily affected by a heterogeneous solidified structure or a precipitation strengthened ferrite single phase structure. However, the elongation (homogeneous elongation) of these single-phase steel sheets is compared to the elongation (homogeneous elongation) of steels having a two-phase structure composed of ferrite and martensite, and multi-phase structure steel sheets composed of ferrite, bainite, and retained austenite. And had the problem of being significantly inferior. In other words, conventionally, bendability has been ensured at the expense of elongation (homogeneous elongation) required for thin steel sheets for automobiles. Moreover, it is difficult to achieve ultra high strength exceeding the maximum tensile strength of 900 MPa with precipitation strengthened ferritic single phase steel.

  In steels with a conventional ferrite and martensite structure, the steel sheet structure is composed of soft ferrite and hard martensite. As a starting point, bending workability was easy to deteriorate. In particular, in a bending process in which a large deformation is applied to the surface layer of the steel plate or the vicinity thereof, it is difficult to improve the characteristics only by improving the microstructure inside the steel plate. For example, if the internal structure of the steel sheet is composed of ferrite and tempered martensite, the structure of the steel sheet surface layer is non-uniform, so there is a risk that deformation will concentrate on the steel sheet surface layer during bending and cracks may be formed. there were. Once a crack is formed, stress concentration occurs at the crack tip, and it is difficult to suppress the propagation of the crack even if the structure inside the steel sheet is excellent in bendability.

  The present invention has been made in view of such circumstances, and has sufficient ductility and bendability obtained by improving the inhomogeneity of the solidified structure and homogenizing the hardness distribution of the steel sheet surface layer. And a method for producing a high-strength steel plate, a high-strength steel plate, and a high-strength galvanized steel plate having a high tensile strength of 900 MPa or more.

  In order to solve the above-mentioned problems, the present inventor has made extensive studies. As a result, a slab having a predetermined chemical component is cast under the condition that the molten steel flow velocity at the solidification interface near the mold meniscus is 15 cm / second or more using an in-mold electromagnetic stirrer and the like, and is in the solidification process due to the flow of the molten steel. By stirring the molten steel on the surface of the slab, inclusions and defects are prevented from being trapped between the dendrite arms, resulting in the development of a heterogeneous solidified structure formed near the surface of the slab during casting. Defects in the slab and steel sheet surface layer, segregation of Mn, and non-uniform structure distribution and hardness fluctuations on the steel sheet surface can be reduced, which is given by the standard deviation of the Rockwell hardness of the steel sheet surface. The structural homogeneity index, which is an index indicating the homogeneity of the steel sheet, is 0.4 or less, and the bendability of the multiphase steel sheet composed of ferrite and martensite is greatly improved. Headings, a method of manufacturing a high strength steel sheet and high strength cold rolled steel sheet of the present invention was conceived a method of manufacturing a high strength galvanized steel sheet.

  In the present invention, the steel sheet surface layer does not indicate only the surface of the steel sheet, but if it is a cold-rolled steel sheet, it is from the steel sheet surface to 1/8 thickness, and if it is a plated steel sheet, the thickness from the interface between the plating layer and the steel sheet. It means the position up to 1/8 thickness. In the present invention, that the structure of the steel sheet surface layer is homogenized means that the structure is not inhomogeneous in the width direction of the steel sheet surface layer or in the rolling direction.

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: 0.0001-0.01%, P: 0.001-0.03%, S: 0.0001-0.01%, Al: 2.5% or less, N: 0.0005-0. 0100%, O: 0.0005-0.007%, the balance is steel consisting of iron and inevitable impurities,
The steel sheet structure is mainly composed of ferrite and contains martensite, and the surface of the steel sheet is 10 × 10 points (in total in the direction perpendicular to the rolling direction and the direction parallel to the rolling direction) with an indentation interval of 2.0 mm on the Rockwell C scale. 100 points) The texture homogeneity index, which is an index showing the homogeneity of the steel sheet given by giving indentation, measuring the Rockwell hardness and calculating the standard deviation, is 0.4 or less , the strength-ductility balance ( TS × El.) is a high-strength steel sheet having 16000 (MPa ×%) or der Rukoto good tensile or maximum stress 900MPa ductility and bendability characterized by.

(2) The high-strength steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability 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 stress of 900 MPa or more with good ductility and bendability according to (1) or (2), comprising 8% of one kind or two or more kinds.
(4) Further, the tensile maximum with good ductility and bendability according to any one of (1) to (3), characterized by containing, in mass%, V: 0.005 to 0.09% A high-strength steel sheet having a stress 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 stress of 900 MPa or more with good ductility and bendability according to item 1.

(6) A high-strength steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability according to any one of (1) to (5), wherein the surface has a galvanized layer.

(7) (1) to (5) A method of producing a high strength steel sheet according to any one of (1) to a slab having a chemical composition according to any one of the mold meniscus proximity (5) Casting under the condition that the molten steel flow velocity at the solidification interface is 15 cm / sec or more, directly or once cooled and then heated to 1050 ° C. or higher, and hot rolling is completed at the Ar ₃ transformation point or higher, and the temperature is 400 ° C. to 670 ° C. Winding in the region, pickling, cold rolling with a rolling reduction of 40-70%, and passing through the continuous annealing line, at the time of heating, after annealing at a maximum heating temperature of 760 ° C ~ Ac ₃ ° C, The temperature between the maximum heating temperature and 630 ° C. is cooled at an average cooling rate of 10 ° C./second or less, and the temperature between 630 ° C. and 570 ° C. is cooled at an average cooling rate of 3 ° C./second or more. Good ductility and bendability characterized by holding for more than a second Method for producing a high strength cold rolled steel sheet having the above tensile maximum stress 900 MPa.

(8) The ductility and bendability according to (7) , wherein the slab is cast under the condition that the molten steel flow velocity at the solidification interface near the mold meniscus is 15 cm / second or more using an in-mold electromagnetic stirring device. A method for producing a high-strength cold-rolled steel sheet having a good tensile maximum stress of 900 MPa or more.
(9) The inside of the annealing furnace of the continuous annealing line is an atmosphere containing ₁ to 60% by volume of H ₂ and the balance consisting of the balance N ₂ , H ₂ O, O ₂ and unavoidable impurities, and the moisture pressure in the atmosphere The logarithm log (PH ₂ O / PH ₂ ) of the hydrogen partial pressure and -3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0.5 is described in (7) or (8) A method for producing a high-strength cold-rolled steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability.
(10) Ductility and bending characterized in that after producing a high strength cold-rolled steel sheet by the method for producing a high-strength cold-rolled steel sheet according to any one of (7) to (9) , zinc electroplating is performed. For producing a high-strength galvanized steel sheet having good tensile maximum stress of 900 MPa or more.

(11) A method for producing a high-strength steel sheet according to (6), wherein the slab having the chemical component according to any one of (1) to (5) is melted at a molten steel flow velocity of 15 cm at the solidification interface near the mold meniscus. Casted under the conditions of more than 1 second / second, directly or once cooled, then heated to 1050 ° C. or higher, completed hot rolling at the Ar ₃ transformation point or higher, and wound up in the temperature range of 400 ° C. to 670 ° C. After washing, when cold rolling with a rolling reduction of 40 to 70% and passing through a continuous hot dip galvanizing line, annealing is performed at a maximum heating temperature of 760 ° C. to Ac ₃ ° C., and then between a maximum heating temperature of 630 ° C. Is cooled at an average cooling rate of 10 ° C./second or less, and is cooled at an average cooling rate of 3 ° C./second or more between 630 ° C. and [(Zinc plating bath temperature−40 ° C.) − (Zinc plating bath temperature + 50 ° C.)] ° C. It is immersed in a galvanizing bath and cooled A method for producing a high-strength galvanized steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability.

(12) A method for producing a high-strength steel sheet according to (6), wherein the slab having the chemical component according to any one of (1) to (5) is melted at a molten steel flow velocity of 15 cm at the solidification interface near the mold meniscus. Casted under the conditions of more than 1 second / second, directly or once cooled, then heated to 1050 ° C. or higher, completed hot rolling at the Ar ₃ transformation point or higher, and wound up in the temperature range of 400 ° C. to 670 ° C. After washing, when cold rolling with a rolling reduction of 40 to 70% and passing through a continuous hot dip galvanizing line, annealing is performed at a maximum heating temperature of 760 ° C. to Ac ₃ ° C., and then between a maximum heating temperature of 630 ° C. Is cooled at an average cooling rate of 10 ° C./second or less, and is cooled at an average cooling rate of 3 ° C./second or more between 630 ° C. and [(Zinc plating bath temperature−40 ° C.) − (Zinc plating bath temperature + 50 ° C.)] ° C. After being immersed in a galvanizing bath, 460 ° C to 600 ° C A method for producing a high-strength galvanized steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability, characterized by performing an alloying treatment at a temperature of ° C and cooling.

(13) according to the slab, the molten steel flow speed of the solidification interface of the mold meniscus proximity with the mold in the electromagnetic stirring apparatus is characterized in that the casting under the condition that a 15cm / sec or more (11) or (12) A method for producing a high-strength galvanized steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability.
(14) the said annealing furnace of a continuous galvanizing line, and _{H 2} contains 1 to 60 vol%, the remainder _{N 2,} H 2 O, an atmosphere consisting of _{O 2} and inevitable impurities, in the atmosphere The logarithm log (PH ₂ O / PH ₂ ) of the water pressure and the hydrogen partial pressure is set to −3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0.5, (11) to (13) A method for producing a high-strength galvanized steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability according to any one of the items.

  ADVANTAGE OF THE INVENTION According to this invention, the high intensity | strength of 900 MPa or more of maximum tensile stress can be ensured, the manufacturing method of the high strength steel plate which has sufficient ductility and bendability, and a high strength cold-rolled steel plate, manufacture of a high strength galvanized steel plate Can provide a method.

The high-strength steel sheet of the present invention has a tensile maximum stress of 900 MPa or more with good ductility and bendability.
The steel sheet structure of the high-strength steel sheet of the present invention is a multi-phase structure steel sheet mainly composed of ferrite and containing martensite, and particularly excellent ductility can be secured by making the volume fraction of ferrite contributing to ensuring ductility within the following range. .

Since the maximum tensile stress of the high-strength steel sheet of the present invention depends on the volume ratio of martensite, which is a strengthened structure, it is desirable to change the volume ratio of martensite in accordance with the target steel sheet strength.
For example, if the maximum tensile stress of the steel sheet is in the range of 900 to 1130 MPa, the ferrite volume fraction is preferably in the range of 60% to 85%, and more preferably in the range of 65% to 80%. .
Further, if the tensile maximum stress of the steel sheet is in the range of 1130 to 1280 MPa, the ferrite volume fraction is preferably in the range of 55% to 80%, and more preferably in the range of 60% to 75%. .
If the maximum tensile stress of the steel sheet is in the range of 1280 to 1430 MPa, the volume ratio of ferrite is preferably in the range of 50% to 75%, and more preferably in the range of 55% to 70%.

  By controlling the volume fraction of ferrite in the above range according to the tensile maximum stress of the steel sheet, the tensile strength (TS) is higher than 900 MPa and the strength-ductility balance (TS × El.) 16000 (MPa ×%). ) The above excellent ductility can be obtained, and a steel sheet having both excellent strength and ductility can be obtained. The strength-ductility balance (TS × El.) Is the product of the maximum tensile stress (TS) and total elongation (El.) In a tensile test, and changes according to the maximum tensile stress.

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, the yield stress and the maximum tensile strength are increased without significantly reducing the ductility. Also, if the average crystal grain size (dF) of ferrite is 5 μm or less, localization of deformation and crack propagation are less likely to occur, and if it is tensile deformation, local ductility is improved, and bending and hole expansion molding are performed. If there is, the bendability and the hole expansion rate will be improved. However, if the average crystal grain size (dF) of the ferrite exceeds the above range, the contribution to the strength increase due to the refinement becomes small. For this reason, it is necessary to supplement the strength by increasing the volume ratio of martensite contained in the steel, and the volume ratio of ferrite may be insufficient and the ductility may be insufficient.

Further, the average crystal grain size (dM) of martensite is preferably 1/3 or less of the average crystal grain size (dF) of ferrite, and is preferably as small as possible. Specifically, the average crystal grain size (dM) of martensite is preferably 1.5 μm or less, more preferably 1.2 μm or less, and even more preferably 0.9 μm or less.
Hard martensite is significantly different in deformability from soft ferrite. For this reason, deformation is concentrated at the ferrite and martensite interface of the steel plate during bending with a small radius or hole expansion forming under large deformation after necking if it is tensile deformation, leading to destruction. End up. In addition, if coarse martensite is present in the cut part, when machining such as shear cutting or punching, flaws or fine cracks originating from martensite are likely to occur, and subsequent processing is performed. Will greatly reduce sex.
When martensite is refined and the average crystal grain size (dM) is 1.5 μm or less, the concentration of deformation at the individual interface between ferrite and martensite is suppressed, so microvoids and cracks are formed at the interface. Can be suppressed.

  In addition, the martensite contained in the steel 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 may have a network structure connected to the grain boundaries. Further, martensite may have a hierarchical structure such as a packet, a block, and a lath, but the martensite may be either one crystal grain or martensite having these hierarchical structures, and the average crystal The effect of the present invention is exhibited as long as the particle size (dM) is satisfied.

  Further, the steel plate structure mainly contains ferrite and contains martensite, but may contain residual austenite, bainite, cementite, pearlite, and the like as a hard structure other than ferrite and martensite.

  In addition, the ferrite, martensite, pearlite, cementite, bainite, austenite, and the remaining structure, the observation of the existing position, and the measurement of the area ratio were performed by using the Nital reagent and the reagent disclosed in JP-A-59-219473. It can be quantified by corroding the rolling direction cross section or the perpendicular direction of the rolling direction and performing 1000 times optical microscope observation and 1000 to 100000 times scanning and transmission electron microscope observations. The structure can also be discriminated from crystal orientation analysis using FESEM (Field Emission Scanning Electron Microscope) -EBSP (Backscattered Electron Diffraction) method and micro region hardness measurement such as micro Vickers hardness measurement.

In the high-strength steel sheet of the present invention, the texture homogeneity index, which is an index indicating the homogeneity of the steel sheet given by the standard deviation of the Rockwell hardness of the steel sheet surface, is 0.4 or less. If the texture homogeneity index exceeds 0.4, the hardness of the surface layer of the high-strength steel sheet will vary greatly, resulting in insufficient bendability.
On the other hand, when the structure homogeneity index is 0.4 or less, the balance of ductility, bendability and strength of the high-strength steel sheet is extremely excellent. Specifically, after preparing a test piece so that the axis of the bending test is parallel to the rolling direction, a crack occurs on the surface even when the inner radius R is 1 mm or less in the V bending test at a bending angle of 90 degrees. There will be no good bendability. Here, the axis of the V-bending test was made parallel to the rolling direction because the defects and segregation in the surface layer of the slab formed during solidification and the inhomogeneity of the structure caused by this occurred through hot rolling and cold rolling. This is because it extends in the rolling direction and exists in a streak shape along the rolling direction to deteriorate the bending workability. However, the steel plate of the present invention can obtain good bendability even when a test piece is produced with the axis of the bending test perpendicular to the rolling direction.

  The tissue homogeneity index in the present invention is a standard deviation of Rockwell hardness, and is determined by the method shown below. That is, 10 × 10 points (a total of 100 points) of indentations were applied to the steel plate surface in the direction perpendicular to the rolling direction and in the direction parallel to the rolling direction at an indentation interval of 2.0 mm on the Rockwell C scale. It is obtained by measuring the hardness and calculating the standard deviation.

  The indentation interval at the time of measuring Rockwell hardness was determined by conducting the following preliminary test. First, 10 × 10 points (total of 100 points) of indentations in the direction perpendicular to the rolling direction and the direction parallel to the rolling direction are provided on the steel plate surface at various indentation intervals of 0.05 to 20 mm on the Rockwell C scale. And Rockwell hardness was measured. Thereafter, the standard deviation at each indentation interval was calculated, and the correlation between the standard deviation at each indentation interval and bendability was examined. As a result, the correlation between the standard deviation and the bendability when the indentation interval was 2.0 mm was the highest. For this reason, in this invention, the indentation space | interval at the time of measuring Rockwell hardness was 2.0 mm.

In addition, it is thought that it was based on the following reasons that the standard deviation and bendability were the highest when the indentation interval was 2.0 mm.
For example, when the indentation interval is too small, such as when the indentation interval is 0.05 mm, it is difficult to remove the influence of the indentation previously applied, and it is considered that accurate calculation is difficult. Further, if the indentation interval is too large, it is considered difficult to capture the influence of the structure non-uniformity caused by the solidified structure at the time of casting that affects the bendability. Specifically, for example, a region where a plate having a thickness of 1.0 mm is bent with an inner radius R of 1.0 mm is limited to a narrow region, and the hardness distribution in the bent region is a bending deformation. It is thought that it causes inhomogeneous deformation and characteristic deterioration of time. Therefore, when the indent interval is too large, such as when the indent interval is 10 to 20 mm, it is considered difficult to obtain a correlation between the standard deviation and the bendability.

  Moreover, in the result of the preliminary test, the change in the Rockwell hardness along the direction perpendicular to the rolling direction was larger than the change in the Rockwell hardness along the direction parallel to the rolling direction. This is presumably because the non-uniform structure formed during casting was extended in the rolling direction by subsequent hot rolling and cold rolling.

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 content of C is preferably in the range of 0.075 to 0.23%, and more preferably in the range of 0.08 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 it contributes also to solid solution strengthening, adding positively is desirable. 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.

When the Si content is less than 0.3%, the ferrite volume fraction becomes insufficient, and the ductility, bendability, and strength of the high-strength steel sheet become insufficient. 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 2.25%, and more preferably in the range of 0.7 to 2.0%. Further, the Al content is preferably in the range of 0.005 to 1.6%, and more preferably in the range of 0.01 to 0.6%. Further, since Al can be used as a deoxidizing material, it may be added not only for the structure control of the steel sheet but also for deoxidation.

Moreover, in the manufacturing method of the high-strength steel sheet of the present invention, when annealing is performed 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, ferrite during annealing is used. The volume fraction of austenite varies depending on the annealing temperature. That is, when the annealing temperature is less than 760 ° C., it takes a long time for the cementite to dissolve, so that the volume ratio of austenite (martensite after cooling) becomes too small to secure a strength of 900 MPa or more. On the other hand, when the annealing temperature exceeds Ac ₃ ° C, it becomes an austenite single-phase region annealing, and when cooled as it is, a martensite single-phase structure is formed, so that the structure of the present invention cannot be obtained. For this reason, in order to manufacture a steel sheet with small material variations, it is desirable that the Ac ₃ transformation point is sufficiently high and the steel sheet structure does not change much even if the annealing temperature changes. Si and Al, since increasing the Ac ₃ transformation point, and a large amount by containing Si and Al in the above-mentioned range by increasing the Ac ₃ transformation point, it is desirable to hard to rely on steel sheet structure to the annealing temperature .

“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, and 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. Ti is also added to suppress B from becoming a nitride. B contributes to the structure controllability at the time of hot rolling, the structure control in the continuous annealing equipment and the continuous hot dip galvanizing equipment, and high strength, but this effect cannot be obtained when B becomes a nitride. By containing Ti, B can be prevented from becoming a nitride.

  However, if the Ti content exceeds 0.09%, the precipitation of carbonitride increases and the formability deteriorates. Also, if the Ti content is too large, the recrystallization of ferrite is greatly delayed during continuous annealing and continuous hot dip galvanizing equipment production, so that unrecrystallized ferrite tends to remain after annealing. Reduces ductility. For this reason, the upper limit of the Ti content is set to 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, since B delays the ferrite transformation from austenite even during hot rolling, inclusion of B improves the homogeneity of the hot-rolled steel sheet as a bainite single-phase structure and improves bendability. it can. However, if the B content is less than 0.0001%, sufficient effects cannot be obtained. Moreover, when content of B exceeds 0.01 mass%, the effect by containing B will not only be saturated, but the productivity at the time of hot rolling will be reduced. The content of B is preferably in the range of 0.0003 to 0.007%, and more preferably in the range of 0.0005 to 0.005%.

“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. When the P content exceeds 0.03%, embrittlement of the welded portion becomes remarkable, so the appropriate range is limited to 0.03% or less. The lower limit of the content of P is not particularly defined, but it is economically disadvantageous to make it less than 0.001%, so this value is set as the lower limit.
“S: 0.0001 to 0.01%”
S adversely affects weldability and manufacturability during casting and hot rolling. For this reason, the upper limit of the S content is set to 0.01% or less. The lower limit value of the S content is not particularly defined, but if it is less than 0.0001%, it is economically disadvantageous, so this value is the lower limit value. Further, since S is combined with Mn to form coarse MnS, the bendability is lowered. For this reason, it is preferable that the content of S is as small as possible.

“N: 0.0005 to 0.0100%”
N forms coarse nitrides and degrades bendability and hole expansibility, so the content needs to be suppressed. When the N content exceeds 0.0100%, this tendency becomes remarkable, so the N content range is set to 0.0100% or less. In addition, N is better because it causes blowholes during welding. Although the lower limit of the N content is not particularly defined, the effects of the present invention are exhibited. However, if the N content is less than 0.0005%, the manufacturing cost is significantly increased. This is a reasonable lower limit.

“O: 0.0005 to 0.007%”
Since O forms an oxide and degrades bendability and hole expandability, the content needs to be suppressed. In particular, oxides are often present as inclusions, and if they are present on the punched end face or cut face, notch scratches and coarse dimples are formed on the end face, so stress concentration occurs during bending and during strong processing. Invited, it becomes the starting point of crack formation, resulting in significant hole expandability or bendability deterioration. 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. Further, setting the content of O to less than 0.0005% causes excessive cost and is not economically preferable. Therefore, this is set as the lower limit. However, even if the O content is less than 0.0005%, the tensile maximum stress of 900 MPa or more and excellent ductility and bendability, which are the effects of the present invention, can be ensured.

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. Nb, like Ti, 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, if the Nb content is large, recrystallization of ferrite is greatly delayed during production in continuous annealing and continuous hot dip galvanizing equipment, so that unrecrystallized ferrite tends to remain after annealing, and significant ductility Bring about a decline. For this reason, it is preferable to make the upper limit of Nb content 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. V, like Ti and Nb, contributes to increasing 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. From this, it is desirable to contain V in the production of a steel sheet having a maximum tensile strength of more than 900 MPa in the present invention.
However, when the content of V exceeds 0.09%, the precipitation of carbonitrides increases and the formability deteriorates. In addition, when the V content is large, recrystallization of ferrite is greatly delayed during production in continuous annealing and continuous hot dip galvanizing equipment, so that unrecrystallized ferrite tends to remain after annealing, resulting in significant ductility. In order to bring about a reduction, the upper limit is preferably made 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”
One or two or more selected from Ca, Ce, Mg, and REM can be added in a total amount of 0.0001 to 0.5%. Ca, Ce, Mg, and REM are elements used for controlling the form of oxides and sulfides. By containing one or two or more in total of 0.0001% or more, the oxide size after deoxidation and The size of the sulfide existing in the hot-rolled sheet can be reduced, which contributes to bendability. However, when the content exceeds 0.5% of these in total, causing moldability deteriorates. Therefore, these contents are preferably in the range of 0.0001 to 0.5% in total.
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 can be added by misch metal, and in addition to La and Ce, a lanthanoid series element may be contained in combination.

  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. In the present invention, it is necessary to cast the slab under the condition that the molten steel flow velocity at the solidification interface near the mold meniscus is 15 cm / second or more using an in-mold electromagnetic stirring device or the like. The molten steel flow velocity in the vicinity of the meniscus was set to 15 cm / second or more. Inclusion and defects were trapped between the dendrite arms by increasing the molten steel flow velocity and stirring the molten steel on the surface of the slab in the solidification process by the molten steel flow. This hinders the development of an inhomogeneous solidified structure in the vicinity of the slab surface during casting, and causes nonuniform fluctuations in the structure of the steel sheet surface after cold rolling and annealing due to the inhomogeneity of these solidified structures. This is to reduce the deterioration of bendability due to the above.

  When the molten steel flow rate is less than 15 cm / sec, the formation of a heterogeneous solidified structure during casting cannot be sufficiently prevented, and the steel plate homogeneity given by the standard deviation of the Rockwell hardness of the steel plate surface is shown. Since the tissue homogeneity index serving as an index exceeds 0.4, the bendability is greatly deteriorated. Therefore, the molten steel flow rate needs to be 15 cm / second or more, preferably 18 cm / second or more, and more preferably 20 cm / second or more.

The manufacturing method of the slab used for hot rolling is not particularly limited. That is, what was manufactured with the continuous casting slab, the thin slab caster, etc. can be used. The production method of the present invention can also be adapted to a continuous casting-direct rolling (CC-DR) process in which hot rolling is performed immediately after casting.
Next, the cast slab is directly or once cooled and then heated to 1050 ° C. or higher, and the hot rolling is completed at the finish rolling temperature not lower than the Ar ₃ transformation point. In this embodiment, the heating temperature of the slab in hot rolling needs to be 1050 ° C. or higher. The upper limit of the heating temperature of the slab in hot rolling is not particularly defined, and the effect of the present invention is exhibited. However, it is economically undesirable to raise the heating temperature of the slab too high. For this reason, it is desirable that the upper limit of the slab heating temperature be less than 1300 ° C.

When the heating temperature of the slab in the hot rolling is less than 1050 ° C., the finish rolling temperature becomes less than the Ar ₃ transformation point, and two-phase rolling of ferrite and austenite occurs, and the mixed grain structure in which the hot rolled sheet structure is inhomogeneous Therefore, even if it goes through a cold rolling and annealing process, a heterogeneous structure | tissue will not be eliminated, but it will be inferior to ductility and bendability. Moreover, in the manufacturing method of the high strength steel plate of this embodiment, in order to obtain the high strength steel plate which has a maximum tensile stress of 900 MPa or more after annealing, a large amount of alloy elements are contained in the slab. For this reason, the rolling load at the time of finish rolling of hot rolling tends to be large. If the heating temperature of the slab in hot rolling is less than 1050 ° C., the finish rolling temperature is lowered, and the rolling load is further increased, so that finishing rolling in hot rolling becomes difficult or obtained after hot rolling. There is a concern of causing a defective shape of the steel sheet.

In the present embodiment, the Ar ₃ transformation point is calculated by the following formula.
Ar ₃ transformation point (° C.) = 901-325 × C + 33 × Si-92 × (Mn + Ni / 2 + Cr / 2 + Cu / 2 + Mo / 2)
(C, Si, Mn, Ni, Cr, Cu, and Mo in the formula are the content [% by mass] of each component in the steel.)
Further, the finish rolling temperature of the hot rolling may be any Ar ₃ transformation point or higher, the upper limit without specifically defined, the effect of the present invention is exhibited. However, when the hot rolling finish rolling temperature is excessively high, the heating temperature of the slab must be excessively high in order to secure the temperature, which is not preferable. From this, it is desirable that the upper limit temperature of the finish rolling temperature of hot rolling be 1000 ° C. or less.

Next, the steel plate that has been hot-rolled is wound up in a temperature range of 400 ° C to 670 ° C. When the coiling temperature here exceeds 670 ° C., coarse ferrite and pearlite structure exist in the hot-rolled structure. For this reason, the structure heterogeneity after annealing becomes large, and the bendability of the final product deteriorates. On the other hand, when the coiling temperature exceeds 670 ° C., the thickness of the oxide formed on the surface of the steel sheet is excessively increased.
Moreover, it is more preferable that the coiling temperature is 630 ° C. or less because the structure after annealing can be made fine to improve the strength ductility balance, and the structure after annealing can be uniformly dispersed to improve the bendability. However, 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 of the winding temperature needs to be 400 ° C.

  Note that the finish rolling may be continuously performed by joining the rough rolled plates during hot rolling. The rough rolled plate may be wound up once.

  Next, hot rolling is completed and the wound steel sheet is pickled. By pickling, the oxide on the steel sheet surface can be removed. For this reason, pickling is important in order to improve the chemical conversion property of the cold-rolled high-strength steel sheet as the final product and the hot-plating property of the cold-rolled steel sheet for hot-dip zinc or alloyed hot-dip galvanized steel sheet. Pickling may be performed once or may be performed in a plurality of times.

  Next, the hot-rolled steel sheet after pickling is cold-rolled with a rolling reduction of 40 to 70%. When the rolling reduction here is less than 40%, it becomes difficult to keep the shape of the cold-rolled steel sheet obtained after cold rolling flat, and the ductility of the final product becomes poor. On the other hand, if the rolling reduction exceeds 70%, the cold rolling load becomes too large, and cold rolling becomes difficult. The rolling reduction is more preferably 45 to 65%. In addition, the effect of the present invention is exhibited without any particular limitation on the number of rolling passes and the rolling reduction for each rolling pass.

Thereafter, the obtained cold-rolled steel sheet is passed through a continuous annealing line to produce a high-strength cold-rolled steel sheet. At this time, it is performed under the first condition or the second condition described below.
"First condition"
When passing through the continuous annealing line, during the heating, after annealing at a maximum heating temperature of 760 ° C. to Ac ₃ ° C., cooling between the maximum heating temperature to 630 ° C. at an average cooling rate of 10 ° C./second or less, and 630 ° C. between to 570 ° C. at average cooling rate 3 ° C. / sec or more to cool to room temperature.

In the present invention, Ac ₃ means the lower limit temperature at which an austenite single phase is obtained, and can be measured by investigating the temperature dependence of the length change. That is, in steel, ferrite that is stable at room temperature and austenite that is stable at high temperature have different densities and thermal expansion coefficients. As a result, the Ac ₃ transformation point can be easily measured by measuring the temperature dependence of the change in length of the test piece.

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 facility or the continuous hot-dip galvanizing facility after cold rolling. This is because the cementite is dissolved and a sufficient volume ratio of austenite is secured. When the maximum heating temperature is less than 760 ° C., the time for dissolving cementite becomes long, and the productivity is lowered. On the other hand, if the maximum heating temperature is less than 760 ° C., cementite does not dissolve and remains undissolved, the volume ratio of martensite after cooling decreases, and a strength of 900 MPa or more may not be ensured.

Moreover, in this embodiment, since it cools between maximum heating temperature -630 degreeC with an average cooling rate of 10 degrees C / second or less, formation of a ferrite is accelerated | stimulated and sufficient volume fraction of the ferrite which contributes to ductility ensuring is fully provided. Can be secured. In particular, when increasing the volume fraction of ferrite for the purpose of improving workability, it is desirable to cool at an average cooling rate of 5 ° C./second or less.
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.

"Second condition"
After passing through the continuous annealing line, after annealing in the same manner as the first condition described above, after cooling the maximum heating temperature to between 630 ° C and between 630 ° C to 570 ° C in the same manner as the first condition , Maintained at 450 ° C. to 250 ° C. for 30 seconds or more, and 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. .

Examples of the method of applying zinc electroplating include the following methods. First, alkaline degreasing, water washing, pickling, and water washing are sequentially performed as a pretreatment for plating on a steel plate that has been passed through a continuous continuous annealing line. Thereafter, a liquid circulation type electroplating apparatus is used for the pre-treated steel sheet, and a plating bath made of zinc sulfate, sodium sulfate, or sulfuric acid is used, for example, with a current density of 100 A / dm ² to a predetermined plating thickness. Electrolytic treatment is carried out until a Zn plating is applied.

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.

In the present embodiment, 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 is transformed into pearlite, so that the martensite volume ratio is insufficient, and the high-strength cold-rolled steel sheet or high-strength zinc The strength of the plated steel sheet may be insufficient. Moreover, when the average cooling rate in the said temperature range shall be less than 3 degree-C / sec, since a fall of productivity will be caused, it is unpreferable.

"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.
By performing such alloying treatment, a Zn-Fe alloy formed by alloying the zinc plating layer on the surface is formed, and a high-strength galvanized steel sheet having the alloyed zinc plating layer on the surface is obtained.
The lower limit of the alloying temperature is set to 460 ° C. or more because if the alloying temperature is less than 460 ° C., the alloying reaction takes a long time and the productivity is poor. On the other hand, when the alloying temperature exceeds 600 ° C., carbides are formed in the austenite, so that the volume ratio of martensite obtained after cooling decreases, and it becomes difficult to ensure strength. For this reason, the alloying temperature needs to be 600 ° C. or less.

  In this embodiment, a cold-rolled steel sheet is manufactured by the above method using a slab having the above-described chemical component (composition), and a high-strength cold-rolled steel sheet or a high-strength steel is produced under any of the first to fourth conditions. Since we manufacture high-strength galvanized steel sheets, even if the steel sheet structure is a multiphase steel sheet composed of ferrite and martensite, the index indicates the homogeneity of the steel sheet given by the standard deviation of the Rockwell hardness of the steel sheet surface A high-strength cold-rolled steel sheet or a high-strength galvanized steel sheet having a small texture homogeneity index is obtained.

In the present embodiment, as described below, the atmosphere in the annealing furnace of a continuous annealing line or continuous galvanizing line during manufacture of high strength cold rolled steel sheet or high-strength galvanized steel sheet, the H ₂ 1 It is made into an atmosphere containing ˜60% by volume and the balance being 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 − It is preferable that 3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0.5.

  When the atmosphere in the annealing furnace is the atmosphere described above, before Si, Mn, and Al contained in the steel sheet diffuse into the steel sheet surface, O diffused into the steel sheet and Si, Mn, and Al contained in the steel sheet are contained. As a result of the reaction, oxides are formed inside the steel sheet, and the formation of oxides composed of these elements on the steel sheet surface is suppressed. Therefore, by setting the atmosphere in the annealing furnace to the atmosphere described above, non-plating due to the formation of oxide on the steel sheet surface can be suppressed and the alloying reaction can be promoted. It is possible to prevent deterioration of chemical conversion properties due to the formation of oxides.

  In addition, the ratio of the water pressure and the hydrogen partial pressure in the atmosphere in the annealing furnace can be adjusted by a method of blowing water vapor into the 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, in an atmosphere of the annealing furnace, the concentration of H ₂ exceeds 60 vol% is not preferable because it increases the cost. On the other hand, when the H ₂ concentration is less than 1% by volume, Fe contained in the steel plate is oxidized, and the wettability and plating adhesion of the steel plate may be insufficient.
In addition, by setting the logarithm log (PH ₂ O / PH ₂ ) of moisture pressure and hydrogen partial pressure in the atmosphere in the furnace to −3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0.5, Even if it is a steel containing a large amount, sufficient plating properties can be secured. Note that the lower limit of the logarithm log (PH ₂ O / PH ₂ ) of the water pressure and the hydrogen partial pressure was set to −3 or more. This is because there is a risk that the wettability and plating adhesion may be reduced. 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 is set to −0.5 because the effect is saturated.

On the other hand, when manufacturing the high-strength cold-rolled steel sheet or high-strength galvanized steel sheet according to the present invention using the conventional steel sheet manufacturing method, the atmosphere in the annealing furnace at the time of manufacturing is set as the atmosphere in the annealing furnace. As a result, the following problems may occur.
That is, in the present invention, a slab having the above-described chemical component (composition) containing Mn containing Si (or Si and Al) for improving the ferrite volume fraction and ensuring ductility and enhancing the strength of the high-strength steel sheet. Is used. Since Si, Mn, and Al are elements that are very easily oxidized compared to Fe, even in a reducing atmosphere of Fe, the surface of the steel sheet containing Si (or Si and Al) and Mn has Si Oxides (or Si oxides and Al oxides) and Mn oxides are formed.

Oxides containing Si, Mn, and Al alone or in combination formed on the surface of the steel sheet cause deterioration of the chemical conversion property in the high-strength cold-rolled steel sheet. In addition, these oxides have poor wettability with molten metals such as zinc, and therefore cause non-plating when forming a galvanized layer on the surface of high-strength steel sheets containing Si (or Si and Al). It becomes. Si and Al may cause problems such as delaying alloying when producing a high-strength galvanized steel sheet subjected to alloying treatment.
Here, as a method for suppressing the formation of oxides on the steel sheet surface, a method in which the atmosphere in the annealing furnace is made a reducing atmosphere of each element is conceivable. Although it was a reducing atmosphere of Fe, an atmosphere such as Si, Mn, and Al was very easily oxidized.

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, or the method of blowing oxygen directly in a furnace. Even in this case, Si, Mn, and Al are contained alone or in combination in the steel plate near the surface layer, as in the case where the atmosphere in the annealing furnace is controlled by controlling the moisture pressure and the hydrogen partial pressure. An oxide can be deposited, and the same effect as described above can be obtained.

  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.

  A slab having the chemical composition (composition) shown in Tables 1 and 2 was cast at a molten steel flow velocity at the solidification interface in the vicinity of the mold meniscus shown in Tables 3 to 5 using an in-mold electromagnetic stirrer. The temperature range shown in Tables 3 to 5 is directly heated to the temperatures shown in Tables 3 to 5 (slab heating temperature), and hot rolling is completed at the temperatures shown in Tables 3 to 5 (hot rolling completion temperatures). After winding up at (winding temperature) and pickling, cold rolling at the rolling reduction shown in Tables 3 to 5 was performed to obtain cold rolled steel sheets of Experimental Examples 1 to 69.

  In Tables 3 to 5, CR indicating the product plate type is a cold-rolled steel plate manufactured with a continuous annealing facility, GI is a galvanized steel plate manufactured with a continuous hot-dip galvanizing facility, and GA is a continuous melt. It is an alloyed galvanized steel sheet manufactured by galvanizing equipment.

Subsequently, a part of the experimental examples of the cold-rolled steel sheets of Example 1 to Experiment Example 69, was prepared cold-rolled steel sheet C R by Tsuban a continuous annealing line. The atmosphere in the annealing furnace of the continuous annealing line is an N ₂ atmosphere containing 1% by volume of H _2, and the logarithm log (PH ₂ O / PH ₂ ) of the moisture pressure and the hydrogen partial pressure in the atmosphere in the furnace is used. -2.8.
Moreover, when letting a continuous annealing line pass, after performing annealing at the maximum heating temperature shown in Tables 6 to 8 at the time of heating, the average cooling rate shown in Tables 6 to 8 is between the maximum heating temperature and 630 ° C. Cool, cool between 630 ° C. and 570 ° C. at the average cooling rate shown in Table 6 to Table 8, hold at the holding temperature shown in Table 6 to Table 8, and holding time shown in Table 6 to Table 8, and then to room temperature Cooled down.

Moreover, the cold-rolled steel plate which did not let the continuous annealing line pass through the cold-rolled steel plates of Experimental Examples 1 to 69 was passed through the continuous hot-dip galvanizing line to produce a galvanized steel plate GI. The atmosphere in the annealing furnace of the continuous hot dip galvanizing line is an N ₂ atmosphere containing 1% by volume of H _2, and the logarithm log (PH ₂ O / PH ₂₎ of the water pressure and the hydrogen partial pressure in the atmosphere in the furnace. ) Was set to -1.2.

When passing through a continuous hot dip galvanizing line, after annealing at the maximum heating temperature shown in Tables 6 to 8, during heating, the average cooling rate shown in Tables 6 to 8 is between the maximum heating temperature and 630 ° C. It cooled, it cooled by the average cooling rate shown in Table 6-Table 8 between 630 degreeC-zinc plating bath temperature, and it immersed in the zinc plating bath of the temperature shown in Table 6-Table 8, and cooled.
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 Tables 6 to 8, and cooling was performed.

  For the cold rolled steel sheets or galvanized steel sheets of Experimental Examples 1 to 69 thus obtained, the steel sheet structure was observed by the FE-SEM-EBSP method, and the volume fraction of ferrite, martensite, and remaining structure, and ferrite And the average crystal grain size of martensite was examined. The results are shown in Tables 9-11.

In Tables 9 to 11, P described in the remaining structure means pearlite, B means bainite, RA means retained austenite, and C means cementite.
From Tables 9 to 11, Experimental Examples 1, 7, 8, 13 to 15, 20, 21, 23, 24, 26 to 29, 32 to 40, 42, 43, 45 to 47, which are examples of the present invention, In 49, 51, and 53, it was confirmed that the steel sheet structure was mainly composed of ferrite and contained martensite.

Moreover, about the cold rolled steel plate or the galvanized steel plate of Experimental Example 1 to Experimental Example 69, a structure homogeneity index serving as an index indicating the homogeneity of the steel sheet given by the standard deviation of the Rockwell hardness of the steel sheet surface was determined.
The results are shown in Tables 9-11. From Tables 9 to 11, Experimental Examples 1, 7, 8, 13 to 15, 20, 21, 23, 24, 26 to 29, 32 to 40, 42, 43, 45 to 47, which are examples of the present invention, In 49, 51, and 53, the tissue homogeneity index was 0.4 or less.

Moreover, about the cold-rolled steel plate or galvanized steel plate of Experimental example 1-Experimental example 69, the V-bending test was performed at a bending angle of 90 degrees, and the inner radius R was measured.
The results are shown in Tables 9-11. From Tables 9 to 11, Experimental Examples 1, 7, 8, 13 to 15, 20, 21, 23, 24, 26 to 29, 32 to 40, 42, 43, 45 to 47, which are examples of the present invention, In 49, 51, and 53, the inner radius R was 1 mm or less.

Furthermore, a tensile test piece based on JIS Z 2201 was taken from the high-strength cold-rolled steel sheet or the high-strength galvanized steel sheet of Experimental Examples 1 to 69, and the tensile test was performed based on JIS Z 2241. (TS) and elongation (El) were measured.
The results are shown in Tables 9-11. From Tables 9 to 11, Experimental Examples 1, 7, 8, 13 to 15, 20, 21, 23, 24, 26 to 29, 32 to 40, 42, 43, 45 to 47, which are examples of the present invention, In 49, 51 and 53, the maximum tensile stress and elongation were good.

In Experimental Examples 2 to 4, 9, 16, 17, 22, 25, 30, 31, 41, 44, 48, 50, and 52, which are comparative examples of the present invention, the molten steel flow velocity at the solidification interface near the mold meniscus is Since it is less than the range of the present invention, the tissue homogeneity index exceeds 0.4, the inner radius R exceeds 1 mm, and the bendability is insufficient.
In Experimental Example 5, which is a comparative example, the molten steel flow velocity at the solidification interface near the mold meniscus is less than the range of the present invention, the maximum heating temperature exceeds the range of the present invention, and the average cooling between the maximum heating temperature and 630 ° C. Since the speed exceeds 10 ° C./second, the tissue homogeneity index exceeds 0.4, the inner radius R exceeds 1 mm, the bendability is insufficient, and the maximum tensile stress is less than 900 MPa. There was also insufficient elongation.
In Experimental Examples 6 and 11, which are comparative examples of the present invention, the average cooling rate between 630 and 570 ° C. is less than the range of the present invention, and thus the maximum tensile stress was less than 900 MPa.

In Experimental Examples 10 and 18 where the maximum heating temperature was less than the range of the present invention, martensite was hardly contained, and the maximum tensile stress was less than 900 MPa.
In Experimental Example 12, where the alloying temperature was higher than the preferred range, the maximum tensile stress was less than 900 MPa.

  Moreover, in Experimental example 19 which is a comparative example of this invention, since the average cooling rate between 630-570 degreeC was less than the range of this invention, elongation was inadequate.

In Experimental Examples 54 and 55, which are comparative examples of the present invention, the maximum tensile stress was less than 900 MPa because the C content was small.
In Experimental Examples 56 and 57, which are comparative examples of the present invention, since Ti and B were not included, the maximum tensile stress was less than 900 MPa.
In Experimental Examples 58 and 59, which are comparative examples of the present invention, since the Si content is small, the tissue homogeneity index exceeds 0.4 and the inner radius R exceeds 1 mm, and the bendability is poor. It was enough.

In Experimental Examples 60 and 61, which are comparative examples of the present invention, the maximum tensile stress was less than 900 MPa because the Mn content was small. In Experimental Example 60, martensite was hardly contained.
In Experimental Examples 62 and 63, which are comparative examples of the present invention, since the Mn content is large, the volume ratio of martensite is large, the volume ratio of ferrite is insufficient, and ferrite is not mainly used. For this reason, in Experimental Examples 62 and 63, the ductility was insufficient. In Experimental Example 63, the tissue homogeneity index exceeded 0.4 and the inner radius R exceeded 1 mm, and the bendability was insufficient.

In Experimental Examples 64 and 65, which are comparative examples of the present invention, since Ti was not included, the maximum tensile stress was less than 900 MPa.
In Experimental Examples 66 and 67, which are comparative examples of the present invention, since B was not included, the maximum tensile stress was less than 900 MPa.
In Experimental Examples 68 and 69, which are comparative examples of the present invention, since the C content is large, the volume ratio of martensite is large, the volume ratio of ferrite is insufficient, and ferrite is not mainly used. In Experimental Examples 68 and 69, since the C content is large and the Si content is small, the texture homogeneity index exceeds 0.4 and the inner radius R exceeds 1 mm, and the bendability is poor. It was enough.

Claims (14)

% 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%,
And the balance is steel consisting of iron and inevitable impurities,
The steel sheet structure is mainly composed of ferrite and contains martensite, and the surface of the steel sheet is 10 × 10 points (in total in the direction perpendicular to the rolling direction and the direction parallel to the rolling direction) with an indentation interval of 2.0 mm on the Rockwell C scale. 100 points) The texture homogeneity index, which is an index showing the homogeneity of the steel sheet given by giving indentation, measuring the Rockwell hardness and calculating the standard deviation, is 0.4 or less, the strength-ductility balance ( A high-strength steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability, wherein TS × El.) Is 16000 (MPa ×%) or more. Furthermore, in mass%,
Nb: 0.005 to 0.09%
The high-strength steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability according to claim 1. Furthermore, in mass%,
Cr: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
Cu: 0.01 to 2.0%,
Mo: 0.01 to 0.8%
The high-strength steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability according to claim 1 or 2, characterized by containing one or more of the following. Furthermore, in mass%,
V: 0.005-0.09%
A high-strength steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability according to any one of claims 1 to 3.   Furthermore, 0.0001-0.5% of 1 type or 2 types in total of Ca, Ce, Mg, and REM is contained by mass%, The any one of Claims 1-4 characterized by the above-mentioned. A high-strength steel sheet having a maximum tensile stress of 900 MPa or more with good ductility and bendability.   The high-strength steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability according to any one of claims 1 to 5, wherein the surface has a galvanized layer. It is a manufacturing method of the high intensity steel plate according to any one of claims 1 to 5 ,
A slab having the chemical component according to any one of claims 1 to 5 is cast under a condition that a molten steel flow velocity at a solidification interface in the vicinity of a mold meniscus is 15 cm / sec or more, and directly or once cooled, and then 1050 ° C or more To complete the hot rolling at the Ar ₃ transformation point or higher, winding in a temperature range of 400 ° C. to 670 ° C., pickling, cold rolling with a rolling reduction of 40 to 70%, and continuous annealing line When letting the plate pass, after annealing at a maximum heating temperature of 760 ° C. to Ac ₃ ° C., cooling between the maximum heating temperature of 630 ° C. and an average cooling rate of 10 ° C./second or less, and between 630 ° C. and 570 ° C. Is cooled at an average cooling rate of 3 ° C./second or higher, and then held at a temperature range of 450 ° C. to 250 ° C. for 30 seconds or longer. A method for producing rolled steel sheets.   8. The ductility and bendability according to claim 7, wherein the slab is cast under the condition that the molten steel flow velocity at the solidification interface near the mold meniscus is 15 cm / second or more using an electromagnetic stirring device in the mold. A method for producing a high-strength cold-rolled steel sheet having a maximum tensile stress of 900 MPa or more. In the annealing furnace of the continuous annealing line, an atmosphere containing ₁ to 60% by volume of H ₂ and the balance N ₂ , H ₂ O, O ₂ and unavoidable impurities is used, and the moisture pressure and hydrogen content in the atmosphere The logarithm log of the pressure (PH ₂ O / PH ₂ ) is set to −3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0.5, and the ductility and bendability according to claim 7 or 8, A method for producing a high-strength cold-rolled steel sheet having a good tensile maximum stress of 900 MPa or more.   A high strength cold-rolled steel sheet manufactured by the method for manufacturing a high-strength cold-rolled steel sheet according to any one of claims 7 to 9, and then zinc electroplating is performed, and a tensile with good ductility and bendability is provided. A method for producing a high-strength galvanized steel sheet having a maximum stress of 900 MPa or more. It is a manufacturing method of the high strength steel plate according to claim 6,
A slab having the chemical component according to any one of claims 1 to 5 is cast under a condition that a molten steel flow velocity at a solidification interface in the vicinity of a mold meniscus is 15 cm / sec or more, and directly or once cooled, and then 1050 ° C or more To complete the hot rolling at the Ar ₃ transformation point or higher, winding in a temperature range of 400 ° C. to 670 ° C., pickling, cold rolling with a rolling reduction of 40 to 70%, continuous hot dip galvanization When letting through the line,
At the time of heating, after annealing at a maximum heating temperature of 760 ° C. to Ac ₃ ° C., cooling between the maximum heating temperature of 630 ° C. at an average cooling rate of 10 ° C./second or less, and from 630 ° C. to [(zinc plating bath temperature −40 ° C.) ~ (Zinc plating bath temperature + 50 ° C)] Cooling at an average cooling rate of 3 ° C / second or more between ° C, immersed in a galvanizing bath and cooling, and having a maximum tensile stress with good ductility and bendability A method for producing a high-strength galvanized steel sheet having 900 MPa or more. It is a manufacturing method of the high strength steel plate according to claim 6,
A slab having the chemical component according to any one of claims 1 to 5 is cast under a condition that a molten steel flow velocity at a solidification interface in the vicinity of a mold meniscus is 15 cm / sec or more, and directly or once cooled, and then 1050 ° C or more To complete the hot rolling at the Ar ₃ transformation point or higher, winding in a temperature range of 400 ° C. to 670 ° C., pickling, cold rolling with a rolling reduction of 40 to 70%, continuous hot dip galvanization When letting through the line,
At the time of heating, after annealing at a maximum heating temperature of 760 ° C. to Ac ₃ ° C., cooling between the maximum heating temperature of 630 ° C. at an average cooling rate of 10 ° C./second or less, and from 630 ° C. to [(zinc plating bath temperature −40 ° C.) ~ (Zinc plating bath temperature + 50 ° C)] After cooling at a mean cooling rate of 3 ° C / second or more and submerging in a galvanizing bath, alloying treatment is performed at a temperature of 460 ° C to 600 ° C and cooling is performed. A method for producing a high-strength galvanized steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability.   13. The ductility and bendability according to claim 11 or 12, wherein the slab is cast under the condition that the molten steel flow velocity at the solidification interface near the mold meniscus is 15 cm / second or more using an electromagnetic stirring device in the mold. A method for producing a high-strength galvanized steel sheet having a good tensile maximum stress of 900 MPa or more. In the annealing furnace of the continuous hot dip galvanizing line, an atmosphere containing ₁ to 60% by volume of H ₂ and the balance consisting of the balance N ₂ , H ₂ O, O ₂ and unavoidable impurities, and the moisture pressure in the atmosphere, The logarithm log (PH ₂ O / PH ₂ ) of the hydrogen partial pressure is set to -3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0.5, according to any one of claims 11 to 13 A method for producing a high-strength galvanized steel sheet having a tensile maximum stress of 900 MPa or more with good ductility and bendability as described.