Steel sheet and member, and methods for producing same

Abstract

Provided is a steel sheet having a TS of 980 MPa to less than 1180 MPa, high yield stress, excellent ductility, and excellent bending fracture resistance and fatigue characteristics. This steel sheet has: a predetermined component composition; a predetermined steel structure at a position 1 / 4 of the sheet thickness; a surface layer soft layer having a Vickers hardness of 84% or less of the Vickers hardness 84% at a position 1 / 4 of the sheet thickness, and a thickness of 20-140 μm; a steel structure in which the area ratio of ferrite in the surface layer soft layer is 50.0-100.0%, the number ratio of ferrite grains having an area of 50 μm2 or less among the ferrite grains of the surface layer soft layer is 0.01-0.99, and the number ratio of ferrite grains for which the ratio of the length in the rolling direction to the length in the sheet thickness direction is greater than 1.00 is 0.55 or more; and a tensile strength of 980 MPa to less than 1180 MPa.

Description

Steel plates and components, and methods for manufacturing them.

[0001] This invention relates to steel plates and components, as well as methods for manufacturing them.

[0002] From the standpoint of protecting the global environment, improving the fuel efficiency of automobiles has become a crucial issue. Therefore, there is a growing movement to lighten automobile bodies by increasing the strength and thinning the steel sheets used as materials for automobile components.

[0003] Furthermore, there is a growing societal demand for improved collision safety in automobiles. Therefore, in addition to high strength, there is a need for steel sheets with excellent impact resistance (hereinafter simply referred to as impact resistance) in the event of a collision while the vehicle is in motion. In particular, from the perspective of vehicle body corrosion prevention, steel sheets used as materials for automobile components are often zinc-plated. Therefore, there is a demand for zinc-plated steel sheets that possess both high strength and excellent impact resistance.

[0004] Patent Document 1 describes a high-strength hot-dip galvanized steel sheet with a thickness of 0.6 to 5.0 mm having a plating layer on the surface of the steel sheet, wherein the steel sheet structure contains 40 to 90% ferrite phase and 3% to 25% retained austenite phase by volume fraction, the retained austenite phase having a solid solution carbon content of 0.70 to 1.00%, an average particle diameter of 2.0 μm or less, an average distance between particles of 0.1 to 5.0 μm, a decarburized layer thickness in the surface layer of the steel sheet of 0.01 to 10.0 μm, an average particle diameter of oxides contained in the surface layer of the steel sheet of 30 to 120 nm, and an average density of 1.0 × 10⁻¹⁶. １２ pieces / m ２ Furthermore, a high-strength hot-dip galvanized steel sheet with excellent mechanical cutting characteristics is disclosed, characterized in that the work hardening coefficient (n value) during plastic deformation of 3-7% is 0.080 or higher on average.

[0005] Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet with excellent delayed fracture resistance, having a volume fraction of 40-90% ferrite phase and 5% or less retained austenite phase, wherein the proportion of unrecrystallized ferrite in the entire ferrite phase is 50% or less, the grain size ratio, which is the value obtained by dividing the average grain size in the rolling direction of the crystal grains of the ferrite phase by the average grain size in the sheet width direction, is 0.75-1.33, the length ratio, which is the value obtained by dividing the average length in the rolling direction of the hard structure dispersed in island-like structures within the ferrite phase by the average length in the sheet width direction, is 0.75-1.33, and the average aspect ratio of inclusions is 1.0-5.0.

[0006] Patent Document 3 describes a steel sheet comprising a hot-dip galvanized layer, wherein the steel sheet comprises a base material and a decarburized ferrite layer, and the microstructure at a depth of 1 / 4 of the sheet thickness contains 5.0 volume% or more of tempered martensite and 0.5 volume% or more but less than 7.0 volume% of retained austenite, with the remainder consisting mainly of 4 to 70 volume% of ferrite and bainite, and some or all of the tempered martensite and retained austenite form M-A, and the microstructure of the decarburized ferrite layer contains 120% or more of ferrite relative to the ferrite content of the microstructure at a depth of 1 / 4 of the sheet thickness, the average grain size of the ferrite is 20 μm or less, the thickness is 5 μm or more but less than 200 μm, it contains 1.0 volume% or more of tempered martensite and the number density is 0.01 particles / μm ２ The above describes a hot-dip galvanized steel sheet having good elongation characteristics and bendability.

[0007] Patent Document 4 describes a case where the segregation width of Mn occurring in the center of the plate thickness is 1 μm or less, and the number density of carbides containing Nb is 2.3 × 10⁻¹⁶. ２２ pieces / m ３ The cold-rolled steel sheet disclosed is characterized by having a combined martensite content and retained austenite content of 1.5% or less, a yield strength of 1000 MPa or more, and a fatigue limit of 650 MPa or more.

[0008] International Publication No. 2013 / 047739, International Publication No. 2013 / 047760, Japanese Patent Publication No. 2017-48412, Japanese Patent Publication No. 2017-66454

[0009] Incidentally, in recent years, the practical application of steel plates with a tensile strength (hereinafter also referred to as TS) of 780 MPa or higher has been progressing for impact energy absorbing members in automobiles, such as front side members and rear side members. This is because improving the yield strength (hereinafter also referred to as YS) is effective in increasing the energy absorbed during impact (hereinafter also referred to as impact absorption energy).

[0010] However, increasing the TS and YS of a steel sheet generally reduces its press formability, particularly its ductility, hole-expandability, and bendability. Therefore, applying such a steel sheet with increased TS and YS to an automobile's impact energy absorbing member makes press forming difficult, and it is expected that variations during forming will reduce the yield. In particular, the reduced press formability at the edges of the steel sheet can lead to end-face cracking in the actual member. Patent documents 1 to 4 did not consider these issues, and there was room for improvement.

[0011] In view of the above problems, the present invention aims to provide a steel sheet having a TS of 980 MPa or more and less than 1180 MPa, and possessing high yield strength, excellent ductility, excellent bending fracture resistance and fatigue properties, along with an advantageous manufacturing method thereof. Furthermore, the present invention aims to provide a component made using the above-mentioned steel sheet and a method for manufacturing the same.

[0012] The inventors of the present invention have diligently studied to solve the above problems and have obtained the following findings: A steel sheet has a predetermined component composition, a predetermined steel structure at the 1 / 4 position of the sheet thickness, a Vickers hardness of 84% or less of the Vickers hardness at the 1 / 4 position of the sheet thickness, a soft surface layer with a thickness of 20 μm to 140 μm, and a ferrite area ratio of 50.0% to 100.0% in the soft surface layer, and among the ferrite grains of the soft surface layer, the area is 50 μm ２A steel sheet can be obtained having a steel structure in which the number ratio of ferrite grains is 0.01 or more and 0.99 or less, and the number ratio of ferrite grains with a rolling direction length / thickness direction length of more than 1.00 is 0.55 or more, thereby having a TS of 980 MPa or more and less than 1180 MPa, and possessing high yield strength, excellent ductility, excellent bending fracture resistance and fatigue properties. Furthermore, when manufacturing the steel sheet, a steel sheet like the above can be obtained by setting the dew point, annealing temperature, holding time, and surface soft layer formation coefficient of the annealing process, the average cooling rate and cooling stop temperature of the cooling process, and the tempering temperature and holding time of the tempering process within predetermined ranges.

[0013] In other words, the gist of the present invention is as follows:

[0014] [1] A composition comprising, by mass%, C: more than 0.160% and 0.300%, Si: 0.20% to 3.00%, Mn: 1.00% to 3.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% to 2.000%, and N: 0.0100% or less, with the remainder being Fe and unavoidable impurities; a surface soft layer with a thickness of 20 μm to 140 μm having a Vickers hardness of 84% or less of the Vickers hardness at the 1 / 4 position of the plate thickness; and at the 1 / 4 position of the plate thickness, the area ratio of ferrite is 0.0% or more and less than 42.0%, and the area ratio of bainitic ferrite is more than 10.0% and 70.0% or less. The total area ratio of ferrite and bainitic ferrite is 40.0% or more and less than 75.0%, the area ratio of retained austenite is 10.0% or more and 20.0%, the area ratio of fresh martensite is 0.0% or more and 30.0%, and the area ratio of tempered martensite is 8.0% or more and less than 40.0%, the standard deviation of the grain area of ​​fresh martensite and retained austenite is 0.40 or more and 2.60 or less, in the soft surface layer, the area ratio of ferrite is 50.0% or more and 100.0%, and among the ferrite grains, the area is 50 μm ２A steel sheet having a structure in which the number ratio of ferrite grains is 0.01 or more and 0.99 or less, and of the ferrite grains, the number ratio of ferrite grains with a length in the rolling direction / length in the thickness direction exceeding 1.00 is 0.55 or more, and a tensile strength of 980 MPa or more and less than 1180 MPa.

[0015] [2] The above component composition is further, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less A steel sheet as described in [1], containing at least one selected from the group consisting of Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

[0016] [3] The steel sheet according to [1] above, having a zinc plating layer on at least one surface.

[0017] [4] The steel sheet according to [2] above, having a zinc plating layer on at least one surface.

[0018] [5] A member made of a steel plate as described in any one of the above items [1] to [4].

[0019] [6] A hot rolling step of subjecting the steel slab having the component composition described in [1] or [2] above to hot rolling to obtain a hot rolled steel sheet;Next, a pickling step of pickling the hot rolled steel sheet; Next, an optional cold rolling step of subjecting the hot rolled steel sheet to cold rolling to obtain a cold rolled steel sheet; Next, the hot rolled steel sheet or the cold rolled steel sheet is annealed in an atmosphere where the dew point D is -10°C or higher, the annealing temperature T is 780°C or higher and 940°C or lower, and the holding time t in the temperature range of (T - 40)°C or higher and T°C or lower is 30 seconds or more and 600 seconds or less. The surface softening layer formation coefficient DC = 0.005t×(D + 55), and the annealing step of annealing under the condition that DC is 10°C·sec or more and 200°C·sec or less to obtain an annealed steel sheet; Next, a first cooling step of cooling the annealed steel sheet with an average cooling rate CR of 2.0°C / sec or more in the temperature range from the annealing temperature T to 700°C; Next, a first holding step of holding the annealed steel sheet at a first holding temperature T １ of 360°C or higher and 550°C or lower for a first holding time t of 10 seconds or more and 400 seconds or less １ ; Next, a second cooling step of cooling the annealed steel sheet to a cooling stop temperature of less than 100°C; Next, the annealed steel sheet is heated to a second holding temperature T １ such that the absolute value of the difference from the first holding temperature T is 5°C or higher and 270°C or lower, and the second holding temperature T is 200°C or higher and 520°C or lower ２ and held for a second holding time t of 10 seconds or more and 800 seconds or less in the temperature range of (T ２ - 20°C) or higher and T ２ °C or lower. The manufacturing method of a steel sheet having an index A defined by the following formula (1) of 8.0 or more and 800.0 or less. ２

[0020] [7] After the first holding step and before the second holding step, or after the second holding step, a hot dip galvanizing step of dipping the annealed steel sheet in a hot dip galvanizing bath to form a zinc plating layer on the surface of the annealed steel sheet is provided. The manufacturing method of the steel sheet according to [6] above.

[0021] [8] Immediately after the hot dip galvanizing step, the annealed steel sheet is alloyed at an alloying temperature T Ａ for an alloying time t ＡA method for manufacturing a steel sheet according to [7] above, comprising an alloying step of heating and alloying the zinc plating layer by holding it for a certain period of time, wherein the index B defined by the following formula (2) is 8.0 or more and 800.0 or less.

[0022] [9] The method for manufacturing a steel sheet according to [6], further comprising an electro-zinc plating step of immersing the annealed steel sheet in an electro-zinc plating bath after the second holding step to form a zinc plating layer on the surface of the annealed steel sheet.

[0023]

[10] A method for manufacturing a component, comprising the step of applying one or both of the forming process and joining process to a steel plate described in any one of the above items [1] to [4] to form a component.

[0024] According to the present invention, a steel sheet having a TS of 980 MPa or more and less than 1180 MPa, and possessing high yield strength, excellent ductility, excellent bending fracture resistance, and fatigue properties, can be provided along with an advantageous manufacturing method thereof. Furthermore, according to the present invention, a component made using the above-mentioned steel sheet and a method for manufacturing the same can be provided.

[0025] This is a microstructure image obtained by SEM used for identifying the steel structure in one embodiment of the present invention. This is a grain map obtained by EBSD used for grain analysis in one embodiment of the present invention. This is (a) a microstructure image of the surface cross-section of a steel sheet according to one embodiment of the present invention obtained by optical microscope, and (b) a graph showing the relationship between the distance from the surface of the steel sheet and the carbon content. This is a schematic diagram of the outer surface of a VDA bending sample, and (b) an example of a load-stroke curve and a maximum principal strain-stroke curve graph.

[0026] The following describes embodiments of steel plates and components according to one embodiment of the present invention, as well as embodiments for manufacturing them. Note that the embodiments described below are examples that embody the present invention, and these specific examples do not limit the configuration of the present invention.

[0027] (Steel Plate) First, let me describe a steel plate according to one embodiment of the present invention.

[0028] [Component Composition] First, we will explain the component composition of the steel sheet. In the explanation of component composition, the "%" representing the content of each element means "mass percent" unless otherwise specified.

[0029] C: Over 0.160% and under 0.300% C is an effective element for generating appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite, thereby ensuring a TS of 980 MPa to under 1180 MPa and a high YR. If the C content is 0.160% or less, the hardness of the hard phase decreases, making it difficult to achieve a TS of 980 MPa or higher. It also leads to a decrease in YS. Therefore, the C content should be over 0.160%, preferably 0.180% or higher. On the other hand, if the C content exceeds 0.300%, the hardness of fresh martensite, tempered martensite, and retained austenite increases excessively, making it difficult to achieve a TS of under 1180 MPa. Furthermore, ferrite formation on the surface is suppressed, and the desired fracture strain and crack propagation rate cannot be achieved during bending deformation, resulting in a decrease in bending fracture resistance. Therefore, the C content should be 0.300% or less, and preferably 0.260% or less.

[0030] Si: 0.20% to 3.00% Si is an effective element for suppressing carbide formation during cooling and holding after annealing, controlling the decomposition of austenite (carbide formation) and the degree of tempering of martensite, and ensuring the desired amount of retained austenite. Here, if the Si content is less than 0.20%, excessive carbides are formed from austenite and martensite in the second holding step, reducing the amount of retained austenite. Therefore, the Si content should be 0.20% or more, preferably 0.80% or more. On the other hand, if the Si content exceeds 3.00%, the YS decreases due to an excessive increase in retained austenite. Therefore, the Si content should be 3.00% or less, preferably 2.00% or less.

[0031] Mn: 1.00% or more and 3.00% or less. Mn is an element that adjusts the area ratio of the steel structure. If the Mn content is less than 1.00%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 980 MPa or more. Also, an excessive soft surface layer is formed, reducing fatigue properties. Therefore, the Mn content should be 1.00% or more, preferably 1.40% or more. On the other hand, if the Mn content exceeds 3.00%, the formation of ferrite and bainitic ferrite after the annealing process is suppressed, resulting in insufficient carbon enrichment in the austenite. As a result, the desired area ratio of retained austenite cannot be obtained, and ductility decreases. Furthermore, ferrite formation on the surface is suppressed, and the desired fracture strain and crack propagation rate cannot be achieved during bending deformation, thus reducing bending fracture resistance. Therefore, the Mn content should be 3.00% or less, preferably 2.60% or less.

[0032] P: 0.100% or less. P is an element that has a solid solution strengthening effect and increases the TS and YS of steel sheets. To obtain this effect, a P content of 0.001% or more is preferable. On the other hand, if the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries, embrittles the grain boundaries, and may cause grain boundary cracking during fatigue testing, potentially preventing the achievement of the desired fatigue properties. Therefore, the P content should be 0.100% or less, and preferably 0.030% or less.

[0033] S: 0.0200% or less. S exists in steel as inclusions such as MnS. These inclusions become wedge-shaped during rolling, causing large stress concentrations around them, which negatively affects fatigue properties. In particular, if the S content exceeds 0.0200%, the desired fatigue properties may not be achieved. Therefore, the S content should be 0.0200% or less, preferably 0.0080% or less. Furthermore, due to production technology constraints, an S content of 0.0001% or more is preferable.

[0034] Al: 0.010% to 2.000% Al is an element that affects the area ratio of retained austenite because it suppresses carbide formation during cooling and holding after annealing and promotes the formation of retained austenite. To obtain such effects, the Al content should be 0.010% or more, preferably 0.015% or more. On the other hand, if the Al content exceeds 2.000%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 980 MPa or more. It also leads to a decrease in YS. Therefore, the Al content should be 2.000% or less, preferably 1.000% or less, and more preferably 0.050% or less.

[0035] N: 0.0100% or less. N is an element that combines with Nb to form coarse carbonitrides. Such coarse carbonitrides reduce fatigue properties. In particular, if the N content exceeds 0.0100%, the desired fatigue properties may not be achieved. Therefore, the N content should be 0.0100% or less, and preferably 0.0050% or less. There is no particular lower limit to the N content, but due to production technology constraints, an N content of 0.0005% or more is preferred.

[0036] The remaining component steel sheet contains the above elements, with the remainder being Fe and unavoidable impurities. Furthermore, it is preferable that the steel sheet contains the above elements, with the remainder being Fe and unavoidable impurities.

[0037] The composition of steel sheets, in addition to the basic components listed above, includes: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0. It may contain at least one element selected from the group consisting of 200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less. Note that the lower limit of the content of these optional components is not particularly limited, as the effects of the present invention can be obtained as long as they are included below the upper limit. Note that if an optional element is included below the preferred lower limit value described later, that element shall be considered an unavoidable impurity.

[0038] Ti: 0.200% or less. Ti is an element that increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. To obtain such an effect, the Ti content is preferably 0.001% or more, and more preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the desired fatigue properties may not be achieved. Therefore, when Ti is included, the Ti content should be 0.200% or less, and preferably 0.060% or less.

[0039] Nb: 0.200% or less. Like Ti, Nb is an element that increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. To obtain such effects, the Nb content is preferably 0.001% or more, and more preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the desired fatigue properties may not be achieved. Therefore, when Nb is included, the Nb content should be 0.200% or less, and preferably 0.060% or less.

[0040] V: 0.200% or less V, like Nb or Ti, is an element that increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. To obtain such an effect, the V content is preferably 0.001% or more, more preferably 0.005% or more, even more preferably 0.010% or more, and even more preferably 0.030% or more. On the other hand, if the V content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the desired fatigue properties may not be achieved. Therefore, when V is included, the V content should be 0.200% or less, and preferably 0.060% or less.

[0041] B: 0.0100% or less B is an element that enhances hardenability by segregating at austenite grain boundaries. B is also an element that suppresses ferrite formation and grain growth during cooling after annealing. To obtain these effects, the B content is preferably 0.0001% or more, more preferably 0.0002% or more, even more preferably 0.0005% or more, and even more preferably 0.0007% or more. On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling. In addition, ferrite formation and grain growth in the soft surface layer are suppressed, and the desired fracture strain and crack propagation rate cannot be achieved during bending deformation, which may reduce bending fracture resistance. Therefore, when B is included, the B content should be 0.0100% or less, and preferably 0.0050% or less.

[0042] Cu: 1.000% or less. Since Cu is an element that enhances hardenability, the addition of Cu generates an appropriate amount of martensite, ensuring a TS of 980 MPa or higher and a high YS. To obtain these effects, the Cu content is preferably 0.005% or more, more preferably 0.008% or more, even more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, if the Cu content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and a large amount of coarse precipitates and inclusions may be generated. In such cases, the generation of a large amount of coarse precipitates and inclusions, and the suppression of ferrite formation on the surface, may prevent the achievement of the desired YS and fatigue characteristics. Therefore, when Cu is included, the Cu content should be 1.000% or less, and preferably 0.200% or less.

[0043] Cr: 1.000% or less. Since Cr is an element that enhances hardenability, the addition of Cr generates a large amount of tempered martensite, ensuring a TS of 980 MPa or higher and a high YS. To obtain these effects, the Cr content is preferably 0.0005% or more, more preferably 0.010% or more, even more preferably 0.030% or more, and even more preferably 0.050% or more. On the other hand, if the Cr content exceeds 1.000%, ferrite formation on the surface is suppressed, and the desired fracture strain and crack propagation rate cannot be achieved during bending deformation, which may reduce the bending fracture resistance. Therefore, when Cr is included, the Cr content should be 1.000% or less, preferably 0.800% or less, and more preferably 0.700% or less.

[0044] Ni: 1.000% or less. Since Ni is an element that enhances hardenability, the addition of Ni generates a large amount of tempered martensite, ensuring a TS of 980 MPa or higher and a high YS. To obtain such effects, the Ni content is preferably 0.005% or more, more preferably 0.020% or more, even more preferably 0.040% or more, and even more preferably 0.060% or more. On the other hand, if the Ni content exceeds 1.000%, ferrite formation on the surface is suppressed, and the desired fracture strain and crack propagation rate cannot be achieved during bending deformation, which may reduce the bending fracture resistance. Therefore, when Ni is included, the Ni content should be 1.000% or less, preferably 0.800% or less, more preferably 0.600% or less, and even more preferably 0.400% or less.

[0045] Mo: 1.000% or less. Since Mo is an element that enhances hardenability, the addition of Mo generates an appropriate amount of martensite, ensuring a TS of 980 MPa or higher and a high YS. To obtain these effects, the Mo content is preferably 0.010% or more, and more preferably 0.030% or more. On the other hand, if the Mo content exceeds 1.000%, ferrite formation on the surface is suppressed, and the desired fracture strain and crack propagation rate cannot be achieved during bending deformation, which may reduce the bending fracture resistance. Therefore, when Mo is included, the Mo content should be 1.000% or less, preferably 0.500% or less, more preferably 0.450% or less, even more preferably 0.400% or less, even more preferably 0.350% or less, and even more preferably 0.300% or less.

[0046] Sb: 0.200% or less. Sb is an effective element for suppressing the diffusion of carbon near the surface of the steel sheet during annealing and controlling the formation of a soft layer near the surface of the steel sheet. However, if the soft layer near the surface of the steel sheet increases excessively, it becomes difficult to achieve a TS of 980 MPa or higher. It also leads to a decrease in YS. Therefore, the Sb content is preferably 0.002% or more, and more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.200%, a soft layer is not formed on the surface of the steel sheet, and the desired fracture strain and crack propagation rate cannot be achieved during bending deformation, which may reduce the bending fracture resistance. Therefore, when Sb is included, the Sb content should be 0.200% or less, and preferably 0.020% or less.

[0047] Sn: 0.200% or less. Like Sb, Sn is an effective element for suppressing the diffusion of C near the surface of the steel sheet during annealing and controlling the formation of a soft layer near the surface of the steel sheet. However, if the soft layer near the surface of the steel sheet increases excessively, it becomes difficult to achieve a TS of 980 MPa or higher. It also leads to a decrease in YS. Therefore, the Sn content is preferably 0.002% or more, and more preferably 0.005% or more. On the other hand, if the Sn content exceeds 0.200%, a soft layer is not formed on the surface of the steel sheet, and the desired fracture strain and crack propagation rate cannot be achieved during bending deformation, which may reduce the bending fracture resistance. Therefore, when Sn is included, the Sn content should be 0.200% or less, and preferably 0.020% or less.

[0048] Ta: 0.100% or less. Like Ti, Nb, and V, Ta is an element that increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. In addition, Ta partially dissolves in Nb carbides and Nb carbonitrides, generating composite precipitates such as (Nb,Ta)(C,N). This suppresses the coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS and YS. To obtain such effects, the Ta content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.004% or more. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the desired fatigue properties may not be achieved. Therefore, when Ta is included, the Ta content should be 0.100% or less, more preferably 0.090% or less, and even more preferably 0.080% or less.

[0049] W: 0.500% or less. Since W is an element that enhances hardenability, the addition of W generates an appropriate amount of martensite, ensuring a TS of 980 MPa or higher and a high YS. To obtain these effects, the W content is preferably 0.001% or more, and more preferably 0.030% or more. On the other hand, if the W content exceeds 0.500%, ferrite formation on the surface is suppressed, and the desired fracture strain and crack propagation rate cannot be achieved during bending deformation, which may reduce the bending fracture resistance. Therefore, when W is included, the W content should be 0.500% or less, preferably 0.450% or less, more preferably 0.400% or less, and even more preferably 0.300% or less.

[0050] Mg: 0.0200% or less. Mg is an effective element for spheroidizing the shape of inclusions such as sulfides and oxides, thereby improving the fracture strain and further the bending fracture resistance of steel sheets. To obtain such effects, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more. On the other hand, if the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the desired fatigue characteristics may not be achieved. Therefore, when Mg is included, the Mg content should be 0.0200% or less, preferably 0.0180% or less, and more preferably 0.0150% or less.

[0051] Zn: 0.0200% or less. Zn is an effective element for spheroidizing the shape of inclusions, improving the fracture strain of steel sheets, and further, the bending fracture resistance. To obtain such effects, the Zn content is preferably 0.0010% or more, more preferably 0.0020% or more, and even more preferably 0.0030% or more. On the other hand, if the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the desired fatigue characteristics may not be achieved. Therefore, when Zn is included, the Zn content should be 0.0200% or less, preferably 0.0180% or less, and more preferably 0.0150% or less.

[0052] Co: 0.200% or less. Co, like Zn, is an effective element for spheroidizing the shape of inclusions and improving the fracture strain and even the bending fracture resistance of steel sheets. To obtain such effects, the Co content is preferably 0.0010% or more, more preferably 0.0020% or more, and even more preferably 0.0030% or more. On the other hand, if the Co content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the desired fatigue characteristics may not be achieved. Therefore, when Co is included, the Co content should be 0.200% or less, preferably 0.100% or less, and more preferably 0.020% or less.

[0053] Zr: 0.1000% or less. Zr, like Zn and Co, is an effective element for spheroidizing the shape of inclusions and improving the fracture strain and further bending fracture resistance of steel sheets. To obtain such effects, a Zr content of 0.0010% or more is preferable. On the other hand, if the Zr content exceeds 0.1000%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the desired fatigue characteristics may not be achieved. Therefore, when Zr is included, the Zr content should be 0.1000% or less, preferably 0.0300% or less, and more preferably 0.0100% or less.

[0054] Ca: 0.0200% or less. Ca is an element that exists as an inclusion in steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be formed. In such cases, the desired fatigue properties may not be achieved. Therefore, when Ca is included, the Ca content should be 0.0200% or less, and preferably 0.0020% or less. There is no particular lower limit to the Ca content, but due to production technology constraints, the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0055] Se: 0.0200% or less Te: 0.0200% or less Ge: 0.0200% or less Sr: 0.0200% or less Cs: 0.0200% or less Hf: 0.0200% or less Pb: 0.0200% or less Bi: 0.0200% or less REM: 0.0200% or less Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM are all effective elements for improving the fracture strain and further bending fracture resistance of steel sheets. To obtain such effects, the content of each of these elements is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0008% or more. On the other hand, if the content of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM exceeds 0.0200% each, a large amount of coarse precipitates and inclusions may be formed. In such cases, the desired fatigue properties may not be achieved. Therefore, when at least one of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM is included, the content of each of these elements should be 0.0200% or less, preferably 0.0180% or less, and more preferably 0.0150% or less. Here, REM refers to the 17 elements that make up the 15 lanthanide elements from La (lanthanum) with atomic number 57 to Lu (lutetium) with atomic number 71, along with Sc (scandium) with atomic number 21 and Y (yttrium) with atomic number 39. These 17 elements can be included individually or in combination. Furthermore, the REM content in this invention refers to the total content of these 17 elements. Among the REMs, it is particularly preferable that they contain La.

[0056] As: 0.0500% or less. As is an effective element for improving the fracture strain and, furthermore, the bending fracture resistance of steel sheets. To obtain such effects, the As content is preferably 0.0001% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more. On the other hand, if the As content exceeds 0.0500%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the desired fatigue characteristics may not be achieved. Therefore, when As is included, the As content should be 0.0500% or less, preferably 0.0400% or less, and more preferably 0.0300% or less.

[0057] The remainder of the mixture, other than the elements listed above, consists of Fe and unavoidable impurities. Note that any of the optional additive elements may be present at 0%. Unavoidable impurities are those inevitably introduced from raw materials, manufacturing processes, or manufacturing equipment, and their presence is permissible as long as it does not hinder the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap. Examples of impurities include H (hydrogen) and O (oxygen). Furthermore, if the content of each of the optional additive elements listed above is below the preferred lower limit, that element may be included as an unavoidable impurity.

[0058] [Steel structure] Next, the steel structure of a steel plate will be described. The steel structure of a steel plate will be described as being at the 1 / 4 position of the plate thickness, which is the position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate.

[0059] Ferrite area ratio: 0.0% or more and less than 42.0%. Soft ferrite is a phase that improves ductility. However, if the ferrite area ratio increases excessively, it becomes difficult to achieve a TS of 980 MPa or more. It also leads to a decrease in YS. Therefore, the ferrite area ratio should be less than 42.0%, preferably 40.0% or less, and more preferably 35.0% or less. There is no particular lower limit to the ferrite area ratio, and the area ratio may be 0.0%. Therefore, the ferrite area ratio should be 0.0% or more, preferably 5.0% or more, and more preferably 10.0% or more.

[0060] Area ratio of bainitic ferrite: greater than 10.0% and less than or equal to 70.0%. Bainitic ferrite is a phase that has a hardness intermediate between soft ferrite and hard fresh martensite, and can also ensure good ductility and high strain dispersion ability. Retained austenite is also effective in ensuring ductility, but if its amount is large, there is a risk that the YS will decrease. In this invention, the ductility-enhancing effect of bainitic ferrite itself is utilized to balance YS and ductility. Therefore, the area ratio of bainitic ferrite is greater than 10.0%, preferably 15.0% or more, and more preferably 20.0% or more. On the other hand, if the area ratio of bainitic ferrite increases excessively, it becomes difficult to make the TS 980 MPa or more. Therefore, the area ratio of bainitic ferrite is 70.0% or less, preferably 60.0% or less, and more preferably 50.0% or less. In this invention, bainite ferrite refers to upper bainite with a low carbide content that is formed in a relatively high-temperature range.

[0061] Total area ratio of ferrite and bainitic ferrite: 40.0% or more and less than 75.0%. Ferrite and bainitic ferrite are relatively soft phases and are important for improving ductility. Furthermore, the formation of ferrite or bainitic ferrite promotes the formation of retained austenite by enriching the surrounding austenite with carbon, resulting in good ductility. Therefore, the total area ratio of ferrite and bainitic ferrite should be 40.0% or more, preferably 45.0% or more, and more preferably 50.0% or more. On the other hand, if the total area ratio of ferrite and bainitic ferrite increases excessively, it becomes difficult to achieve a TS of 980 MPa or more. Therefore, the total area ratio of ferrite and bainitic ferrite should be less than 75.0%, preferably 73.0% or less, and more preferably 70.0% or less.

[0062] Area ratio of retained austenite: 10.0% or more and 20.0% or less. From the viewpoint of obtaining good ductility and high strain dispersion ability, the area ratio of retained austenite should be 10.0% or more, preferably 11.0% or more, and more preferably 12.0% or more. On the other hand, if the area ratio of retained austenite increases excessively, the amount of carbon in the retained austenite decreases and becomes unstable. As a result, the desired YR cannot be achieved. Therefore, the area ratio of retained austenite should be 20.0% or less, preferably 17.0% or less, and more preferably 15.0% or less.

[0063] Area ratio of fresh martensite: 0.0% or more and 30.0% or less. If the area ratio of fresh martensite increases excessively, mobile dislocations are introduced into the ferrite and bainitic ferrite, making it impossible to achieve the desired YS (Yield Strength). Furthermore, since mobile dislocations adversely affect fatigue properties, there is a risk that the desired fatigue properties cannot be achieved. Therefore, from the viewpoint of ensuring high YS and fatigue properties, the area ratio of fresh martensite should be 30.0% or less, preferably 25.0% or less, and more preferably 20.0% or less. On the other hand, the lower limit of the area ratio of fresh martensite is not particularly limited, and the area ratio may be 0.0%. Therefore, the area ratio of fresh martensite should be 0.0% or more, preferably 3.0% or more, and more preferably 6.0% or more. In this invention, fresh martensite refers to martensite in the as-quenched (untempered) state.

[0064] Area ratio of tempered martensite: 8.0% or more and less than 40.0% Tempered martensite has a hardness intermediate between soft ferrite and hard fresh martensite, and is an effective phase for improving fracture strain, TS, YR, and λ by reducing the hardness difference between microstructures. Therefore, the area ratio of tempered martensite should be 8.0% or more, preferably 10.0% or more, and more preferably 12.0% or more. On the other hand, if the area ratio of tempered martensite increases excessively, ductility decreases. Therefore, the area ratio of tempered martensite should be less than 40.0%, preferably 30.0% or less, and more preferably 20.0% or less.

[0065] The steel structure of the residual structure steel sheet may include residual structures other than those described above. The residual structures are not particularly limited, but examples include carbides such as lower bainite, pearlite, or cementite, or internal oxides. The total area percentage of the residual structures is preferably 10.0% or less, and more preferably 5.0% or less. Alternatively, the total area percentage of the residual structures may be 0.0%. The type of residual structure can be confirmed, for example, by observation using a Scanning Electron Microscope (SEM).

[0066] Here, the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard second phase (a phase consisting of retained austenite and fresh martensite) in the steel sheet microstructure can be determined as follows. A sample is cut from the steel sheet so that the cross-section parallel to the rolling direction of the steel sheet becomes the observation surface. Next, the observation surface of the sample is mirror-polished using diamond paste. Then, the observation surface of the sample is finished polished with colloidal silica, and the microstructure is revealed by etching with 3 vol. % nital. Three fields of view of 25.6 μm × 17.6 μm are captured on the revealed microstructure using a SEM under the conditions of acceleration voltage: 15 kV and magnification: 5000x, at a position 1 / 4 of the thickness of the observation surface of the sample.

[0067] Figure 1 shows an example of a tissue image taken by SEM. In the tissue shown in Figure 1, ferrite, bainitic ferrite, hard secondary phase, carbide, and tempered martensite can be identified as follows.

[0068] Ferrite is observed as a black region and has a nodular shape closed by linear grain boundaries. Furthermore, ferrite contains very few carbides. However, if carbides are present, the area of ​​the iron-based carbides is included in the area of ​​the ferrite. The same applies to bainitic ferrite and tempered martensite, which will be discussed later.

[0069] Bainitic ferrite is a region that appears black to dark gray, has wavy grain boundaries, and is irregular in shape. It also contains little to no carbides.

[0070] Tempered martensite is a gray region with an irregular shape. It also contains a relatively large number of carbides.

[0071] The hard second phase is a region that appears white to light gray, and its shape is irregular. Furthermore, the hard second phase does not contain carbides. In cases where the size is relatively large, the color gradually darkens as it moves away from the interface with other tissues, and the interior may appear dark gray.

[0072] The remaining structure includes carbides such as the lower bainite, pearlite, or cementite, or internal oxides, as described above, and their forms are well known. In particular, the carbides are white regions and appear as dots or lines. The carbides may be embedded in ferrite, bainitic ferrite, and tempered martensite.

[0073] Specifically, in the black regions, structures with wavy grain boundaries and irregular shapes are classified as ferrite, while structures with clearly defined grain boundaries and a nodular structure are classified as bainitic ferrite. In the gray regions, areas containing white carbides are classified as tempered martensite, while areas without carbides are classified as hard secondary phase.

[0074] Next, the area ratio of each phase region identified in the tissue image is calculated using the following method. In the tissue image obtained by SEM using the method described above, a 20x20 grid with equal spacing is placed on a region of actual length 25.6 μm x 19.2 μm, and the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard second phase are measured by point counting, which counts the intersections of the grid on each phase. The average value is calculated from the measurement results in the three observed fields and is used as the area ratio of each phase.

[0075] Furthermore, the area fraction of retained austenite can be determined as follows: After mechanically grinding the steel plate in the thickness direction (depth direction) to a position 1 / 4 of the plate thickness, chemical polishing with oxalic acid is performed to create the observation surface. Next, the observation surface is observed by X-ray diffraction. Using MoKα rays as the incident X-rays, the ratio of the diffraction intensity of the (200), (211), and (220) surfaces of fcc iron (austenite) to the diffraction intensity of the (200), (220), and (311) surfaces of bcc iron is determined, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensity of each surface. Then, assuming that the retained austenite is three-dimensionally homogeneous, the calculated volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0076] Furthermore, the area percentage of fresh martensite can be determined by the following formula (3). That is, it can be determined by subtracting the area percentage of retained austenite from the area percentage of the hard second phase determined as described above. [Area percentage of fresh martensite (%)] = [Area percentage of hard second phase (%)] - [Area percentage of retained austenite (%)] ... (3)

[0077] Furthermore, the area percentage of the remaining structure can be determined by the following formula (4). That is, it can be determined by subtracting the area percentages of ferrite, bainitic ferrite, tempered martensite, and hard second phase, which were determined as described above, from 100.0%. [Area percentage of the remaining structure (%)] = 100.0 - [Area percentage of ferrite (%)] - [Area percentage of bainitic ferrite (%)] - [Area percentage of tempered martensite (%)] - [Area percentage of hard second phase (%)] ... (4)

[0078] Standard deviation of grain area of ​​fresh martensite and retained austenite: 0.40 or more and 2.60 or less. Fresh martensite and retained austenite (hard phase) contribute to crack propagation during bending deformation. Cracks during bending deformation propagate by the formation and connection of voids between the hard phase and the surrounding phase. When hard phases of various sizes are formed, carbon enrichment progresses more as the hard phase is finer and less as it is coarser. Therefore, by forming hard phases of various hardness and size, the stress of void formation around each hard phase differs during crack propagation, which can suppress crack propagation. Accordingly, the standard deviation of grain area of ​​fresh martensite and retained austenite should be 0.40 or more, preferably 0.50 or more, and more preferably 0.60 or more. On the other hand, if the standard deviation of grain area of ​​the hard phase is excessively high, the number of relatively coarse hard phases increases, the number of crack initiation points increases, and the fracture strain during bending deformation decreases. Therefore, the standard deviation of the grain area of ​​fresh martensite and retained austenite should be 2.60 or less, preferably 2.40 or less, and more preferably 2.20 or less.

[0079] The grain area and standard deviation of fresh martensite and retained austenite can be measured using three or more fields of view with the aforementioned 5000x magnification SEM image. A transparent sheet is placed over the printed SEM image, the portion of the sheet overlapping the hard phase region is colored, and the grain area of ​​the hard phase is measured from the scanned image using image analysis software. The standard deviation of the grain area is calculated using the following formula (5), where n is the total number of grains in the hard phase and x ｋ x is the grain area of ​​the k-th grain. 平均 This represents the average particle area. Note that the method for measuring particle area and number is not limited to the above method, as long as it allows for accurate measurement.

[0080] [Soft Surface Layer] The steel sheet shall have a soft surface layer. The soft surface layer suppresses cracking during bending deformation and further suppresses crack propagation after cracking occurs, thereby significantly improving bending fracture resistance. In this invention, the soft surface layer refers to the decarburized layer and is a surface region having a Vickers hardness of 84% or less of the Vickers hardness at the 1 / 4 position of the steel sheet thickness.

[0081] Thickness of the soft surface layer: 20 μm or more and 140 μm or less. When the thickness of the soft surface layer is 20 μm or more, the effect of improving bending fracture resistance can be obtained. Therefore, the thickness of the soft surface layer should be 20 μm or more, preferably 22 μm or more, and more preferably 25 μm or more. On the other hand, if the soft surface layer is formed excessively, the strength and fatigue properties will decrease. Therefore, the thickness of the soft surface layer should be 140 μm or less, preferably 120 μm or less, and more preferably 100 μm or less.

[0082] The thickness of the soft surface layer can be determined as follows: For the sample subjected to the above-mentioned SEM microstructure observation, the observation surface of the sample is mirror-polished using diamond paste. Based on JIS Z 2244-1 (2020), a microhardness measuring device is used to press a square pyramidal Vickers indenter with a vertex angle of 136° into the steel plate at 10 μm intervals in the thickness direction, starting from a depth of 10 μm from the outermost layer and extending to a position 1 / 4 of the plate thickness from the surface, with a load of 10 gf. The Vickers hardness is measured at five points for each thickness position, and the average value is taken as the hardness at that thickness position. By storing the data points in a straight line, a hardness profile in the depth direction is obtained. The thickness of the soft layer is determined by reading the depth position from the hardness profile where the hardness is 84% ​​or less of the hardness at the 1 / 4 thickness position.

[0083] Area ratio of ferrite in the soft surface layer: 50.0% or more and 100.0% or less. When subjected to bending, the surface layer deforms more than the interior. Therefore, voids are likely to occur in the surface layer during bending. When the area ratio of ferrite in the soft surface layer is 50.0% or more, the occurrence of voids that serve as crack initiation points in the surface layer is suppressed, and crack propagation is inhibited. Therefore, the area ratio of ferrite in the soft surface layer should be 50.0% or more, preferably 55.0% or more, and more preferably 60.0% or more. On the other hand, there is no particular upper limit to the area ratio of ferrite in the soft surface layer, and the area ratio may be 100.0%.

[0084] Furthermore, if the area ratio of ferrite in the surface soft layer is less than 100.0%, the surface soft layer may also contain the retained austenite, fresh martensite, bainitic ferrite, tempered martensite, and the remaining structure as described above.

[0085] Furthermore, the area ratio of ferrite in the soft surface layer can be determined by observing the microstructure of the cross-section of the soft surface layer of the steel plate using the method described above.

[0086] Area 50 μm ２ The following is the ratio of ferrite grains: 0.01 to 0.99. In the soft surface layer, the area of ​​ferrite grains, i.e., the particle size of ferrite grains, greatly affects the fatigue properties. Area of ​​50 μm ２ When the number ratio of ferrite grains that meet the following criteria is 0.01 or higher, fatigue crack initiation and growth are inhibited, and fatigue properties are improved. Therefore, in the surface soft layer, among the ferrite grains, those with an area of ​​50 μm ２ The ratio of ferrite grains is 0.01 or higher, preferably 0.05 or higher, and more preferably 0.10 or higher. On the other hand, if there are too many fine ferrite grains, the surface hardness increases due to refinement, the strain dispersion ability decreases, and the bending fracture resistance decreases. Therefore, in the soft surface layer, among the ferrite grains, the area of ​​50 μm ２ The number ratio of ferrite grains is 0.99 or less, preferably 0.95 or less, and more preferably 0.90 or less. The area of ​​the ferrite grains can be controlled by the annealing dew point and annealing time, as will be described later.

[0087] The inventors investigated the influence of shape on the deformability of ferrite grains and found that, when the area of ​​the ferrite grains is the same, ferrite grains that are elongated in the rolling direction have superior deformability in the rolling direction compared to grains with a smaller ratio of rolling direction length to thickness direction length. Therefore, in the soft surface layer, the proportion of ferrite grains in which the rolling direction length to thickness direction length is greater than 1.00 should be 0.55 or more, preferably 0.60 or more, and more preferably 0.70 or more. On the other hand, in the soft surface layer, there is no particular upper limit to the proportion of ferrite grains in which the rolling direction length to thickness direction length is greater than 1.00, but this proportion is generally 0.95 or less.

[0088] In the soft surface layer, as the annealing time increases, two or more adjacent ferrite grains merge to form a single ferrite grain. This merging is particularly frequent in the rolling direction. Specifically, in the soft surface layer, the ratio of the length in the rolling direction to the length in the thickness direction of ferrite grains increases with increasing annealing time. Furthermore, the area ratio of ferrite in the soft surface layer also significantly influences ferrite grain merging. The ratio of the length in the rolling direction to the length in the thickness direction of ferrite grains can be controlled by the annealing dew point and time, as will be discussed later.

[0089] In the surface soft layer, the area is 50 μm ２The following can be determined: the number ratio of ferrite grains and the number ratio of ferrite grains where the length in the rolling direction / length in the plate thickness direction exceeds 1.00. For the sample on which the steel structure observation described above was performed, the observation surface of the sample is repolished using diamond paste, and then the observation surface of the sample is further polished using colloidal silica. Next, in the observation field of view of the outermost layer of the steel plate on the observation surface, the range of vertical: thickness of the soft surface layer × horizontal: 300 μm or more is analyzed for crystal structure and orientation by EBSD. At that time, the gauge length (step) is set to 0.03 to 0.50 μm. For the analysis of the data obtained by the EBSD method, TSL's "OIM Analysis 6.0" or a newer version is used. A grain boundary map is obtained by defining the boundary where the crystal orientation difference is 15 degrees or more as a grain boundary. An example of a grain boundary map is shown in Figure 2. In Figure 2, the boundary where the crystal orientation difference is 15 degrees or more is a grain boundary. In the obtained grain boundary map, ImageJ (open source) was used to determine the area of ​​10 μm ２ The above grains are defined as ferrite grains, and the area, length in the rolling direction, and length in the thickness direction of each ferrite grain are determined. From the area of ​​each ferrite grain, the area is 50 μm ２ The following percentage of ferrite grains is determined. Additionally, the ratio of the length in the rolling direction to the length in the thickness direction is calculated, and the percentage of ferrite grains where this ratio exceeds 1.00 is determined.

[0090] [Mechanical Properties] Next, we will explain the mechanical properties of steel plates.

[0091] Tensile strength (TS): 980 MPa or more and less than 1180 MPa. Steel plates shall have a tensile strength (TS) of 980 MPa or more and less than 1180 MPa. Tensile strength TS shall be measured by a tensile test in accordance with JIS Z 2241.

[0092] Yield Strength and Ductile Properties Furthermore, the steel sheet obtained by the present invention has high yield strength and excellent ductility. Here, high yield strength means that the yield strength YS measured in a tensile test in accordance with JIS Z 2241 is 600 MPa or more. Excellent ductility means that the total elongation (El) measured in a tensile test in accordance with JIS Z 2241 is 18.0% or more, and the critical hole expansion ratio (λ) measured in a hole expansion test in accordance with JIS Z 2256 is 20.0% or more.

[0093] TS, YS, and El can be determined as follows: A JIS No. 5 test piece is taken from the steel plate so that its longitudinal direction is perpendicular to the rolling direction of the steel plate. Using the taken test piece, a tensile test is performed in accordance with JIS Z 2241 at a crosshead speed of 10 mm / min, and TS, YS, and El are measured. From the obtained results, if TS is 980 MPa or more and less than 1180 MPa, it is judged as a pass; otherwise, it is judged as a fail. Similarly, if YS is 600 MPa or more, it is judged as a pass; if it is less than 600 MPa, it is judged as a fail. In addition, if El is 18.0% or more, it is judged as a pass; if it is less than 18.0%, it is judged as a fail.

[0094] λ can be determined as follows. A hole expansion test is performed in accordance with JIS Z 2256. A 100 mm x 100 mm test piece is taken from the steel plate by shearing. A hole with a diameter of 10 mm is punched into the obtained test piece with a clearance of 12.5%. Next, a wrinkle-holding force of 9 tons (88.26 kN) is applied around the hole using a die with an inner diameter of 75 mm to hold the test piece in place, and a conical punch with an apex angle of 60° is pressed into the hole in the test piece, and the diameter of the hole in the test piece at the crack initiation limit (when the crack initiates) is measured. From the measurement results, D ｆ : Diameter of the hole in the test specimen at the time of crack initiation (mm), D ０ λ (%) = {(D ｆ -D ０ ) / D ０ The limit hole expansion ratio: λ (%) is calculated by multiplying} by 100. If λ is 20.0% or more, it is judged as a pass; if it is less than 20.0%, it is judged as a fail.

[0095] [Bending Fracture Resistance] The steel sheet obtained by the present invention exhibits excellent bending fracture resistance. In the present invention, excellent bending fracture resistance means that the strain distribution capacity and fracture strain of the bent deformation area are high, and the bending crack propagation rate is slow. Here, high strain distribution capacity means that the strain increase rate of the bent deformation area, measured in a bending test that partially conforms to the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry, is 0.075 mm. －１ This means the following: A high fracture strain means that the strain at fracture in the bent deformation section measured in the VDA bending test is 0.20 or higher. A slow bending crack propagation rate means that, in the load-stroke curve of the VDA bending test, the stroke from crack initiation to a 50% reduction in load is greater than 0.3 mm.

[0096] The bending fracture resistance can be evaluated as follows: A VDA bending test is performed using the digital image correlation method. The VDA bending test is a bending test that conforms to the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry, except for the test speed. First, a 70 mm x 60 mm test piece is taken from the obtained steel plate by shearing. Here, the 60 mm side is taken so that it is parallel to the rolling direction L of the steel plate. Figure 4(a) shows a schematic diagram of the outer surface of the bent VDA bending test piece. In order to use the image correlation method, a random pattern is applied to a 30 mm x 30 mm area in the center of the outer surface of the bend as a grid transfer area.

[0097] Next, a VDA bending test is performed on the test specimen under the following conditions: Test method: Roll support, punch pressing Roll diameter: φ30 mm Punch tip radius: 0.4 mm Distance between rolls: (plate thickness × 2) + 0.5 mm Bending direction: Perpendicular to rolling (C) direction

[0098] To capture images of the bending ridge, the stroke speed is set to 5 mm / min. After the bending test starts and the maximum load is exceeded, images of the bending ridge are continuously captured until the load decreases to 50% of the maximum load. From the captured results, the maximum principal strain-stroke curve is obtained using the image correlation method. Figure 4(b) shows an example of the load-stroke curve and the maximum principal strain-stroke curve. From the obtained load-stroke curve, the stroke at which the load is maximum is determined as the critical bending stroke (S). ＶＤＡ ) Let the maximum principal strain-stroke curve be S ＶＤＡ The strain in is the fracture strain F. Ｓ F Ｓ / S ＶＤＡ The strain increase rate V Ｓ Let's assume that. V Ｓ 0.065 mm －１ The following is a passing grade: 0.065 mm －１ If it exceeds this, it will be judged as a failure. Similarly, F Ｓ A score of 0.20 or higher is considered a pass, while a score below 0.20 is considered a fail.

[0099] Furthermore, in the above VDA bending test, the stroke S from the stroke at maximum load until the load decreases by 50% ｓ Evaluate S ｓ If the size is 0.30 mm or less, it will be judged as a failure, S ｓ If the measurement exceeds 0.30 mm, it will be judged as passing.

[0100] [Fatigue Characteristics] The steel plate obtained by the present invention has excellent fatigue characteristics. In the present invention, excellent fatigue characteristics mean that the FR (durability ratio), which is the value obtained by dividing the fatigue limit measured in a fatigue test in accordance with JIS Z 2275 by the TS measured in the tensile test, is greater than 0.23.

[0101] Fatigue characteristics can be evaluated as follows: A No. 1 test specimen (20 mm wide, 1-20 shape symbol) as defined in JIS Z 2275 is prepared so that the longitudinal direction of the test specimen is perpendicular to the rolling direction. Frequency: 25 Hz, Maximum number of cycles: 10 ７In a double-swing fatigue test with a stress ratio of -1, the fatigue limit is determined. Fracture is judged to have occurred when the stress decreases by 80% from the set stress. The value obtained by dividing the determined fatigue limit by the total stress (TS) is defined as the fatigue ratio (FR). If the FR is greater than 0.23, it is judged as a pass; if it is 0.23 or less, it is judged as a fail.

[0102] [Plate Thickness] The thickness of the steel plate is not particularly limited, but is preferably 0.8 mm or more, more preferably 0.9 mm or more, even more preferably 1.0 mm or more, and most preferably 1.2 mm or more. On the other hand, the thickness of the steel plate is preferably 3.5 mm or less, and more preferably 2.3 mm or less. The width of the steel plate is not particularly limited, but is preferably 500 mm or more, and more preferably 750 mm or more. On the other hand, the width of the steel plate is preferably 1600 mm or less, and more preferably 1450 mm or less.

[0103] [Zinc Plating Layer] The steel sheet may have a zinc plating layer on one or both sides. The zinc plating layer referred to here refers to a plating layer whose main component is Zn (Zn content of 50.0% or more), and examples include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electro-galvanized layer.

[0104] The hot-dip galvanized layer preferably has a component composition consisting of 20.0% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al, with the remainder being Zn and unavoidable impurities. The hot-dip galvanized layer may also optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% by mass or more and 3.5% by mass or less. Furthermore, the Fe content of the hot-dip galvanized layer is more preferably less than 7.0% by mass.

[0105] The alloyed hot-dip galvanized layer preferably has a component composition consisting of, for example, 20% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al, with the remainder being Zn and unavoidable impurities. The alloyed hot-dip galvanized layer may also optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, in a total amount of 0.0% by mass or more and 3.5% by mass or less. The Fe content of the alloyed hot-dip galvanized layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. Furthermore, the Fe content of the alloyed hot-dip galvanized layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less.

[0106] The electro-zinc plating layer preferably has a component composition containing 9.0 to 25.0% by mass of Ni, with the remainder being Zn and unavoidable impurities. The electro-zinc plating layer may also optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, in a total amount of 0.0% to 3.5% by mass.

[0107] In addition, the amount of plating deposited on one side of the zinc plating layer is not particularly limited, but is 20 g / m². ２ It is preferable to have the above. Furthermore, the amount of plating deposited on one side of the zinc plating layer is 80 g / m². ２ The following is preferable.

[0108] The amount of zinc plating deposited can be measured as follows: Prepare a treatment solution by adding 0.6 g of a corrosion inhibitor for Fe ("Ibit 700BK" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10% hydrochloric acid aqueous solution. Immerse the steel plate to be tested in the prepared treatment solution to dissolve the zinc plating layer. Then, measure the amount of mass loss of the test material before and after dissolution, and divide this value by the surface area of ​​the steel plate (the surface area of ​​the part covered by plating) to obtain the amount of plating deposited (g / m²). ２ Calculate the result.

[0109] (Method for manufacturing steel sheets) Next, a method for manufacturing steel sheets according to one embodiment of the present invention will be described.

[0110] First, a steel slab having the above-described component composition is prepared. For example, the steel material is melted to obtain molten steel having the above-described component composition. The melting method is not particularly limited, and known melting methods such as converter melting or electric furnace melting can be used. Next, the obtained molten steel is solidified to form a steel slab. The method for obtaining a steel slab from molten steel is not particularly limited, and for example, continuous casting, ingot casting, or thin slab casting can be used. From the viewpoint of preventing macrosegregation, it is preferable to use continuous casting.

[0111] [Hot Rolling Process] In the hot rolling process, the steel slab is subjected to hot rolling to produce hot-rolled steel sheet. Hot rolling may be carried out using energy-saving processes. Energy-saving processes include direct rolling (a method in which the steel slab is charged into the heating furnace while still hot without being cooled to room temperature, and then hot-rolled) or direct rolling (a method in which the steel slab is rolled immediately after being given a small amount of heat). Rough rolling and finish rolling can be performed as part of the hot rolling process.

[0112] The hot rolling conditions are not particularly limited and can be carried out under the following conditions, for example: The steel slab is cooled to room temperature, then reheated, and then rolled. The slab heating temperature is preferably 1100°C or higher from the viewpoint of dissolving carbides and reducing the rolling load. Furthermore, to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. The slab heating temperature is based on the temperature of the steel slab surface.

[0113] Next, the steel slab is subjected to rough rolling according to a conventional method to obtain a rough-rolled sheet (hereinafter also referred to as a sheet bar). Then, the sheet bar is subjected to finish rolling to obtain a hot-rolled steel sheet. If the slab heating temperature is kept low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling to prevent problems during finish rolling. From the viewpoint of reducing the rolling load, it is preferable to set the finish rolling temperature to 800°C or higher. Furthermore, if the reduction ratio in the unrecrystallized state of austenite is high, an abnormal structure that is elongated in the rolling direction may develop, which may reduce the workability of the annealed sheet. In addition, by setting the finish rolling temperature to 800°C or higher, the steel structure at the hot-rolled steel sheet stage, and consequently the steel structure of the final product, tends to become more uniform. If the steel structure is non-uniform, the bendability tends to decrease. On the other hand, if the finish rolling temperature exceeds 950°C, the amount of oxide (scale) formation increases. As a result, the interface between the base metal and the oxide may become rough, potentially degrading the surface quality of the steel sheet after pickling and cold rolling. Furthermore, the coarsening of the crystal grains may reduce the strength and bendability of the steel sheet. Therefore, it is preferable to keep the finishing rolling temperature below 950°C.

[0114] After finish rolling, the hot-rolled steel sheet is wound up. The winding temperature is preferably 450°C or higher. It is also preferable that the winding temperature be 750°C or lower.

[0115] Furthermore, sheet bars may be joined together during hot rolling and continuous finish rolling may be performed. Alternatively, the sheet bars may be wound up before finish rolling. In addition, to reduce the rolling load during hot rolling, part or all of the finish rolling may be lubricated rolling. Lubricated rolling is effective from the viewpoint of uniformizing the shape and material of the steel sheet. The coefficient of friction during lubricated rolling is preferably in the range of 0.10 to 0.25. In the hot rolling process, steel slabs generally become sheet bars through rough rolling and then become hot-rolled steel sheets through finish rolling. However, depending on the mill capacity, it may be acceptable not to adhere to such a division as long as the desired size is achieved.

[0116] [Pickling Process] Next, the hot-rolled steel sheet after the hot-rolling process is pickled. Pickling removes oxides from the surface of the steel sheet, ensuring good chemical conversion treatment properties and plating quality. Pickling may be performed once or in multiple stages. There are no particular limitations on the pickling conditions, and conventional methods should be followed.

[0117] [Cold Rolling Process] Next, an optional cold rolling process may be performed to cold-roll the hot-rolled steel sheet to obtain a cold-rolled steel sheet. Cold rolling can be performed by multi-pass rolling, such as tandem multi-stand rolling or reverse rolling, which requires two or more passes. When the reduction ratio (cumulative reduction ratio) of cold rolling is 20% or more, coarsening and non-uniformity of the steel structure can be suitably prevented in the annealing process, and suitable TS and bendability can be obtained in the final product. Therefore, when performing a cold rolling process, it is preferable to set the reduction ratio of cold rolling to 20% or more. On the other hand, when the reduction ratio of cold rolling is 80% or less, shape defects of the steel sheet can be suitably prevented, and non-uniformity of the zinc plating adhesion can be suitably prevented. Therefore, it is preferable to set the reduction ratio of cold rolling to 80% or less. In addition, the cold-rolled steel sheet obtained after cold rolling may optionally be pickled.

[0118] [Annealing Process] Next, an annealing process is carried out to anneal the hot-rolled or cold-rolled steel sheet to produce an annealed steel sheet. The annealing process may be carried out two or more times, but one time is preferred from the viewpoint of energy efficiency. A radiant tube furnace is generally used as the heat treatment furnace for the annealing process.

[0119] Annealing atmosphere dew point D: -10°C or higher If the dew point D of the annealing atmosphere is below -10°C, the diffusion of carbon near the surface of the steel sheet becomes insufficient, and the desired soft surface layer is not formed. By performing annealing with a dew point D of -10°C or higher, the decarburization reaction is promoted, and the soft surface layer can be formed more deeply. Therefore, the dew point D should be -10°C or higher, preferably -5°C or higher, more preferably 0°C or higher, and even more preferably above 5°C. On the other hand, there is no particular upper limit to the dew point D, but from the viewpoint of improving the plating adhesion when applying the zinc plating layer, it is preferable that the dew point D be 30°C or lower.

[0120] Annealing temperature T: 780°C or higher and 940°C or lower. If the maximum temperature reached during the annealing process (annealing temperature T) is less than 780°C, the proportion of austenite formed during heating in the ferrite and austenite two-phase region becomes insufficient. As a result, the area ratio of ferrite increases excessively after annealing, and in addition, the amount of fresh martensite increases due to excessive concentration of carbon in the austenite during annealing, making it impossible to achieve the desired YS. Furthermore, it becomes difficult to achieve a TS of 980 MPa or higher. Moreover, an excessive soft surface layer is formed, reducing fatigue properties. Therefore, the annealing temperature T should be 780°C or higher, preferably 800°C or higher. On the other hand, if the annealing temperature T exceeds 940°C, the total area ratio of ferrite and bainite decreases due to the reduction of prior austenite grain boundaries that become ferrite and bainite formation sites, resulting in reduced ductility. Furthermore, the proportion of ferrite near the surface of the steel sheet during annealing becomes insufficient, and the desired soft surface layer is not formed. Therefore, the annealing temperature T should be 940°C or lower, preferably 900°C or lower. Note that the annealing temperature is the highest temperature reached during the annealing process.

[0121] The annealing temperature is based on the surface temperature of the steel plate. The surface temperature of the steel plate can be measured using a thermometer. The method of temperature measurement is not particularly limited, but a radiation thermometer that measures the temperature by sensing the infrared radiation emitted by the steel plate is preferred. When using a radiation thermometer, it may be affected by reflected infrared radiation emitted by the surrounding furnace body, so a cover may be provided between the measuring part of the radiation thermometer and the detection part of the steel plate. Also, since it may be affected by the emissivity of the surface of the steel plate, a multi-reflection type measurement method that utilizes the wedge-shaped space between the furnace conveying roll and the steel plate may be adopted.

[0122] Holding time t in the temperature range of (T-40)°C to T°C: 30 seconds to 600 seconds In the annealing process, if the holding time t in the temperature range of (T-40)°C to T°C is less than 30 seconds, the reverse transformation to austenite and grain growth will be insufficient. As a result, the area ratio of ferrite will increase excessively after annealing, and the YS will decrease. Therefore, the holding time t should be 30 seconds or more, preferably 40 seconds or more, and more preferably 50 seconds or more. On the other hand, if the holding time t exceeds 600 seconds, the ferrite grains in the soft surface layer will grow excessively, and the strength and fatigue properties will decrease. Therefore, the holding time t should be 600 seconds or less, preferably 575 seconds or less, and more preferably 550 seconds or less. Note that the holding time t includes not only the isothermal holding time at the annealing temperature T, but also the residence time in the temperature range of (T-40°C) to T°C during heating and cooling before and after reaching the annealing temperature.

[0123] Surface soft layer formation coefficient DC: 10°C·sec to 200°C·sec. The balance between the dew point D and holding time t is extremely important in controlling the area of ​​ferrite grains in the surface soft layer and the ratio of rolling direction length to plate thickness direction length. Assuming the surface soft layer formation coefficient DC = 0.005t × (D + 55), if DC is less than 10°C·sec, the growth of ferrite grains in the surface soft layer becomes insufficient, resulting in an area of ​​50 μm². ２ The proportion of ferrite grains that meet the following criteria exceeds 0.99. Furthermore, the coalescence of ferrite grains in the soft surface layer does not progress, and the proportion of ferrite grains with a rolling direction length / thickness direction length exceeding 1.00 is less than 0.55. Therefore, DC should be 10°C·sec or higher, preferably 30°C·sec or higher. On the other hand, if the soft surface layer formation coefficient DC exceeds 200°C·sec, the ferrite grains in the soft surface layer grow excessively, and the area becomes 50 μm². ２ The proportion of ferrite will be less than 0.01. Therefore, DC should be 200°C·second or less, and preferably 120°C·second or less.

[0124] [First Cooling Process] Next, the annealed steel sheet is cooled in a first cooling process, in which the average cooling rate CR over the temperature range from the annealing temperature T to 700°C is 2.0°C / second or more.

[0125] The average cooling rate CR in the temperature range from the annealing temperature T to 700°C is 2.0°C / second or higher. During the annealing process, carbon (C) may diffuse from the inside of the steel sheet towards the surface, resulting in regions after annealing where the C content is lower than that inside the steel sheet. Figure 3(a) shows an optical microscope image of the surface cross-section of the steel sheet, and Figure 3(b) shows a graph illustrating the relationship between the distance from the steel sheet surface and the C content. As shown in Figure 3(b), the region with a lower C content and ferrite transformation during annealing is defined as the low-C region, and the region with a higher C content and no ferrite transformation during annealing is defined as the medium-C region. In the medium-C region, ferrite transformation does not occur during annealing, but there is a risk of ferrite transformation during cooling from the annealing temperature T to 700°C. Since there is no time for grain growth and coalescence of the ferrite generated during cooling, it is difficult to control the desired ferrite grain area and aspect ratio. Therefore, in order to suppress ferrite transformation in the medium C region during cooling, the average cooling rate CR in the temperature range from the annealing temperature T to 700°C should be 2.0°C / second or more, preferably 5.0°C / second or more. On the other hand, there is no particular upper limit to the average cooling rate CR in the temperature range from the annealing temperature T to 700°C, but the average cooling rate should generally be 50°C / second or less.

[0126] [First holding process] After that, a first holding temperature T of 360°C to 550°C is set. １ The first holding time t is between 10 seconds and 400 seconds. １ A first holding process is performed, during which the item is held in place.

[0127] First holding temperature T １ The first holding step, performed at a temperature of 360°C to 550°C, is necessary to control the bainitic ferrite or fresh martensite within a predetermined range and ensure ductility. If the holding temperature is below 360°C, the area ratio of the bainitic ferrite produced in the holding step may be insufficient, in which case the ductility cannot be sufficiently improved. Also, there may be an excess of fresh martensite, in which case the desired YS and fatigue properties cannot be obtained. Therefore, the first holding temperature T １ The temperature should be 360°C or higher, preferably 400°C or higher. On the other hand, the first holding temperature T １If the temperature exceeds 550°C, the area ratio of bainitic ferrite generated in the first holding process may be insufficient, in which case the ductility cannot be sufficiently improved. Also, there may be an excess of fresh martensite, in which case the desired YS and fatigue properties cannot be obtained. Therefore, the first holding temperature T １ The temperature should be 550°C or lower, preferably 520°C or lower.

[0128] First retention time t １ : 10 seconds or more and 400 seconds or less First holding time t １ If the first holding time t is less than 10 seconds, the bainite transformation in the first holding step will be insufficient, and bainite ferrite with an area ratio of more than 10.0% or fresh martensite with an area ratio of less than 50.0% cannot be obtained. Therefore, the first holding time t １ The first holding time t is 10 seconds or more, preferably 20 seconds or more. １ If the first holding time t exceeds 400 seconds, the bainite transformation in the first holding process will proceed excessively, resulting in a bainite ferrite area ratio exceeding 70.0%, or a total ferrite and bainite ferrite area ratio exceeding 75.0%, or a fresh martensite area ratio falling below 13.0%, making it difficult to achieve a TS of 980 MPa or higher. Therefore, the first holding time t １ The time should be 400 seconds or less, preferably 360 seconds or less.

[0129] [Second Cooling Process] Next, a second cooling process is performed to cool the annealed steel sheet to a cooling stop temperature of less than 100°C.

[0130] Cooling stop temperature: Below 100°C The second cooling step is necessary to control the area ratio of tempered martensite and retained austenite generated in the subsequent reheating step to a predetermined range. If the cooling stop temperature exceeds 100°C, the untransformed austenite, which has become carbon-enriched by bainite transformation, does not undergo martensitic transformation, and ultimately the area ratio of tempered martensite decreases. As a result, the desired λ cannot be achieved. Therefore, the cooling stop temperature should be below 100°C, preferably 80°C or lower. The inventors have found that tempered martensite formed at a cooling stop temperature below 100°C contributes less to the reduction of El compared to tempered martensite formed at a cooling stop temperature of 100°C or higher. The reason is not clear, but it is possible that because the martensite formed at a cooling stop temperature below 100°C has a relatively high carbon concentration, carbon is distributed to the surrounding austenite in the subsequent second holding step, stabilizing the retained austenite. On the other hand, there is no particular lower limit to the cooling stop temperature, but the cooling stop temperature will generally be 3°C or higher.

[0131] [Second holding process] Next, the annealed steel sheet is held at the first holding temperature T １ The absolute value of the difference between the two temperatures is 5°C or more and 270°C or less, and the second holding temperature T is 200°C or more and 520°C or less. ２ Heat until (T ２ -20℃) or more T ２ Second holding temperature t: 10 seconds to 800 seconds in a temperature range below °C ２ A second holding process is performed to maintain the material for a certain period. This allows for bainite transformation at a different temperature range than the first holding process, controlling the standard deviation of the grain area of ​​the hard phase to a desired range and ensuring bending fracture resistance. Furthermore, the martensite present in the steel at the end of the second cooling process can be tempered. In addition, by diffusing the supersaturated dissolved carbon in the martensite into the untransformed austenite, stable austenite at room temperature, i.e., retained austenite, can be produced.

[0132] Second holding temperature T ２ :First holding temperature T １ The absolute value of the difference between this and the second holding temperature T is between 5°C and 270°C. ２ The first holding temperature T１ If the absolute value of the difference is less than 5°C, a hard phase with a grain area equivalent to that formed in the first holding process is formed, making it difficult to make the standard deviation of the grain area of ​​fresh martensite and retained austenite 0.40 or higher. Therefore, the second holding temperature T ２ The first holding temperature T １ The absolute value of the difference between the two temperatures should be 5°C or more, preferably 10°C or more, and more preferably 15°C or more. On the other hand, the second holding temperature T ２ The first holding temperature T １ If the absolute value of the difference exceeds 270°C, the standard deviation of the grain area of ​​fresh martensite and retained austenite exceeds the desired value. Therefore, the second holding temperature T ２ The first holding temperature T １ The absolute value of the difference between the two values ​​should be 270°C or less, preferably 260°C or less, and more preferably 250°C or less.

[0133] Second holding temperature T ２ : 200℃ or more and 520℃ or less Second holding temperature T ２ If the temperature is below 200°C, bainite transformation does not occur sufficiently, and hard phases of different sizes are not formed, making it difficult to obtain the standard deviation of the desired hard phase area. Therefore, the second holding temperature T ２ The temperature should be 200°C or higher, preferably 220°C or higher, and more preferably 240°C or higher. On the other hand, the second holding temperature T ２ If the temperature exceeds 520°C, the untransformed austenite formed in the first holding step decomposes as carbide (pearlite), resulting in a failure to obtain the desired amount of residual γ and a decrease in ductility. Furthermore, the bainite transformation in the second holding step does not occur sufficiently, making it difficult to obtain the desired standard deviation of the hard phase area. Therefore, the second holding temperature T ２ The temperature should be 520°C or lower, preferably 500°C or lower, and more preferably 480°C or lower.

[0134] Second retention time t ２ : 10 seconds or more and 800 seconds or less Second holding time t ２If it is less than 10 seconds, during the second holding process, the bainite transformation does not proceed sufficiently, and a hard phase with a size different from the hard phase formed in the first holding process cannot be formed. As a result, it becomes difficult to make the standard deviation of the area of the hard phase within a desired range, and the flexural fracture resistance property deteriorates. Therefore, the second holding time t ２ should be 10 seconds or more, preferably 20 seconds or more, and more preferably 30 seconds or more. On the other hand, when the second holding time t ２ exceeds 800 seconds, in the second holding process, the bainite transformation proceeds excessively, the area ratio of bainitic ferrite exceeds 70.0%, or the total area ratio of ferrite and bainitic ferrite becomes 75.0% or more, and it becomes difficult to make TS 980 MPa or more. Also, since the untransformed austenite present in the steel at the start of the second holding process decomposes into carbide (pearlite), the ductility deteriorates. Therefore, the second holding time t ２ should be 800 seconds or less, preferably 600 seconds or less, and more preferably 400 seconds or less. Note that the second holding time t ２ includes, in addition to the holding time at the second holding temperature T ２ , the residence time in the temperature range of (T ２ - 20°C) or higher and T ２ °C or lower during heating and cooling before and after reaching the second holding temperature T ２ .

[0135] Heat treatment conditions: A in formula (1): 8.0 or more and 800.0 or less The heat treatment conditions are such that the index A defined by the following formula (1) is 8.0 or more and 800.0 or less. Thereby, while realizing the standard deviation of the areas of the desired surface soft phase and the desired hard phase, even when the reduction of TS and YS due to the formation of the surface soft phase occurs, the desired TS and YS can be obtained.

[0136] In formula (1), if A is less than 8.0, the standard deviation of the hard phase area exceeds 2.60, and the fracture strain decreases. Therefore, A in formula (1) should be 8.0 or greater, preferably 15.0 or greater. On the other hand, if A exceeds 800.0, the formation of both the surface soft layer and bainite is promoted, making it difficult to obtain a TS of 980 MPa or greater. Therefore, A in formula (1) should be 800.0 or less, preferably 700.0 or less.

[0137] [Hot-dip galvanizing process] After the first holding process and before the second cooling process, or after the second holding process, a hot-dip galvanizing process may be performed in which the annealed steel sheet is immersed in a hot-dip galvanizing bath to form a galvanized layer on the surface of the annealed steel sheet.

[0138] When performing a hot-dip galvanizing process, it is preferable to immerse the steel sheet in a zinc plating bath at 440°C to 500°C, and then adjust the amount of plating by gas wiping or the like. The hot-dip galvanizing bath is not particularly limited as long as it has the composition of the zinc plating layer described above, but it is preferable to use a plating bath with a composition in which the Al content is 0.10% by mass or more, and the remainder consists of Zn and unavoidable impurities. The Al content is preferably 0.23% by mass or less. The preferred range for the amount of plating is as described above.

[0139] [Alloying Process] Furthermore, an alloying process may be performed immediately after the hot-dip galvanizing process to heat and alloy the zinc plating layer. Here, immediately after the hot-dip galvanizing process means that the alloying process is performed at the same timing as the hot-dip galvanizing process (either during the cooling process, after the cooling process and before the tempering process, or after the tempering process). If the alloying temperature is less than 400°C, the Zn-Fe alloying rate will be slow, and alloying may become difficult. Therefore, in the alloying process, the alloying temperature is preferably 400°C or higher, and preferably 420°C or higher. On the other hand, if the alloying temperature exceeds 520°C, the untransformed austenite will transform into pearlite, making it difficult to achieve a TS of 980 MPa or higher, and the ductility may decrease. Therefore, the alloying temperature is preferably 520°C or lower, and more preferably 500°C or lower. The holding time at the alloying temperature is preferably 5 to 150 seconds.

[0140] When performing the alloying process, in addition to the index A described above, it is preferable that index B, defined by the following formula (2), be between 8.0 and 800.0. When B in formula (2) is 8.0 or higher, a suitable standard deviation of the hard phase area can be obtained, and a decrease in fracture strain can be suitably prevented. Therefore, B is preferably 8.0 or higher, and more preferably 15.0 or higher. On the other hand, when B is 800.0 or lower, the formation of the surface soft layer and bainite is suitably suppressed, and a TS of 980 MPa or higher can be suitably obtained. Therefore, B is preferably 800.0 or lower, and more preferably 700.0 or lower.

[0141] The cooling conditions after the second holding step are not particularly limited and can be carried out according to conventional methods. For example, gas jet cooling, mist cooling, roll cooling, water cooling, and air cooling can be applied as cooling methods. Furthermore, from the viewpoint of preventing surface oxidation, it is preferable to cool to 50°C or below after holding in the reheating temperature range, and more preferably to room temperature. The average cooling rate during cooling after holding in the reheating temperature range is preferably, for example, 1°C / second or more and 50°C / second or less.

[0142] [Electro-galvanizing process] After the second holding process, the annealed steel sheet may be cooled to room temperature, and then immersed in an electro-galvanizing bath to form a zinc plating layer on the surface of the annealed steel sheet in an electro-galvanizing process. When performing the electro-galvanizing process, the processing conditions for the electro-galvanizing treatment are not particularly limited and should be in accordance with conventional methods.

[0143] [Temper Rolling Process] The steel sheet obtained as described above may be subjected to further temper rolling. If the reduction ratio of temper rolling exceeds 2.00%, the yield stress will increase, and there is a risk that the dimensional accuracy when forming the steel sheet into a component will decrease. For this reason, a reduction ratio of 2.00% or less is preferable. There is no particular lower limit to the reduction ratio of temper rolling, but from the viewpoint of productivity, a reduction ratio of 0.05% or more is preferable. Furthermore, temper rolling may be performed on equipment continuous with the annealing equipment for each of the above processes (online), or on equipment discontinuous with the annealing equipment for each of the above processes (offline). Furthermore, the number of times temper rolling is performed may be one or two or more. Note that rolling by a leveler or the like is also acceptable as long as it can provide an elongation rate equivalent to that of temper rolling.

[0144] Other than the conditions mentioned above, there are no particular limitations; you may follow the usual law.

[0145] (Component) Next, a component according to one embodiment of the present invention will be described. The component according to one embodiment of the present invention is a component made using the steel plate described above (used as the material). For example, the steel plate, which is the material, is subjected to forming and joining processes, or both, to form the component.

[0146] Here, the steel plate has a TS of 980 MPa or more and less than 1180 MPa, and possesses high yield strength, excellent ductility, excellent bending fracture resistance, and fatigue properties. Therefore, the member has high strength and excellent impact resistance. Accordingly, the member is particularly suitable for use as an impact energy absorbing member in the automotive sector.

[0147] (Method for Manufacturing a Member) Next, a method for manufacturing a member according to one embodiment of the present invention will be described. The method for manufacturing a member according to one embodiment of the present invention includes the step of forming the steel plate described above and performing one or both of the forming process and joining process to form a member. Here, the forming process method is not particularly limited, and for example, a general processing method such as press working can be used. Similarly, the joining process method is not particularly limited, and for example, a general welding method such as spot welding, laser welding, or arc welding, or a riveting or crimping can be used. The forming conditions and joining conditions are not particularly limited and can be followed according to conventional methods.

[0148] For processes and conditions not described in this specification, conventional methods may be used.

[0149] Steel material having the component composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was melted in a converter and formed into steel slabs by continuous casting. In Table 1, "-" indicates the content level of unavoidable impurities.

[0150]

[0151] The obtained steel slab was heated to 1200°C, and then subjected to hot rolling consisting of rough rolling and finish rolling at a finishing temperature of 900°C to obtain a hot-rolled steel sheet. Next, the obtained hot-rolled steel sheet was subjected to pickling and cold rolling (reduction ratio: 50%) to obtain a cold-rolled steel sheet with the thickness shown in Table 2. Then, the obtained cold-rolled steel sheet was subjected to annealing, cooling, galvanizing, and tempering processes under the conditions shown in Table 2 to obtain a steel sheet (galvanized steel sheet).

[0152] Here, in the zinc plating process, hot dip galvanizing treatment, alloyed hot dip galvanizing treatment, or electro-galvanizing treatment was performed to obtain a hot dip galvanized steel sheet, an alloyed hot dip galvanized steel sheet, or an electro-galvanized steel sheet. For the hot dip galvanizing treatment and the alloyed hot dip galvanizing treatment, they were carried out at any timing during the cooling process, after the cooling process and before the tempering process, or after the tempering process. The electro-galvanizing treatment was carried out after the reheating and holding process. In the column of the type of zinc plating process in Table 2, they were described as "GI", "GA", or "EG" respectively. In the examples of GI and EG, since no alloying treatment was performed, the alloying temperature was described as "-".

[0153] The zinc plating bath temperature was set at 470°C in all examples. The zinc plating adhesion amount was 10 - 60 g / m per side when manufacturing EG ２ , 45 - 72 g / m per side when manufacturing GI ２ , and 45 g / m per side when manufacturing GA ２ . The composition of the hot dip galvanized layer in GI contained Fe: 0.1 - 1.0% by mass and Al: 0.2 - 0.33% by mass, and the balance was Zn and unavoidable impurities. The composition of the alloyed hot dip galvanized layer in GA contained Fe: 8.0 - 12.0% by mass and Al: 0.1 - 0.23% by mass, and the balance was Zn and unavoidable impurities. The composition of the electro-galvanized layer in EG contained Ni: 9.0 - 25.0% by mass, and the balance was Zn and unavoidable impurities. Also, the zinc plating layer was formed on both sides of the steel sheet.

[0154] Using the obtained steel sheet, the steel structure of the steel sheet was identified and the grain areas of fresh martensite and retained austenite were measured by the method described above. The measurement results are shown in Table 3. In Table 3, the proportion of ferrite grains with an area of 50 μm ２ or less is F Ａ≦５０ , and the proportion of ferrite grains with a rolling direction length / plate thickness direction length exceeding 1.00 is F ＡＳ＞１As shown in the table below, F represents ferrite, BF represents bainitic ferrite, RA represents retained austenite, FM represents fresh martensite, and TM represents tempered martensite. In addition, the remaining structure observed in some examples was carbide.

[0155]

[0156]

[0157] Tensile tests, VDA bending tests, and fatigue tests were performed using the methods described above. The measurement results are shown in Table 4.

[0158]

[0159] As shown in Table 4, all of the invention examples are measured by tensile strength (TS), yield stress (YS), total elongation (El), critical hole expansion ratio (λ), and strain increase rate (V) in the VDA bending test. Ｓ ), fracture strain (F) in the VDA bending test Ｓ ), the stroke change (S) from the stroke at maximum load in the VDA bending test until the load decreases by 50%. Ｓ All of the following passed. On the other hand, in the comparative example, tensile strength (TS), yield stress (YS), total elongation (El), and strain increase rate (V) in the VDA bending test all passed. Ｓ ), fracture strain (F) in the VDA bending test Ｓ ), the stroke change (S) from the stroke at maximum load in the VDA bending test until the load decreases by 50%. Ｓ At least one of the following was not sufficient.

[0160] Furthermore, a member obtained by forming or joining a steel plate according to the present invention may have the following properties: tensile strength (TS), yield stress (YS), total elongation (El), critical hole expansion ratio (λ), and strain increase rate (V) in the VDA bending test. Ｓ ), fracture strain (F) in the VDA bending test Ｓ ), the stroke change (S) from the stroke at maximum load in the VDA bending test until the load decreases by 50%.Ｓ It was found that all of the ) possess the excellent properties that characterize this invention.

[0161] According to the present invention, a steel sheet having a TS of 980 MPa or more and less than 1180 MPa, and possessing high yield strength, excellent ductility, excellent bending fracture resistance, and fatigue properties, can be provided along with an advantageous manufacturing method thereof. Furthermore, according to the present invention, a component made using the above-mentioned steel sheet and a method for manufacturing the same can be provided.

Claims

1. A composition comprising, by mass%, C: over 0.160% and up to 0.300%, Si: 0.20% to 3.00%, Mn: 1.00% to 3.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% to 2.000%, and N: 0.0100% or less, with the remainder being Fe and unavoidable impurities; a surface soft layer with a thickness of 20 μm to 140 μm having a Vickers hardness of 84% or less of the Vickers hardness at the 1 / 4 position of the plate thickness; and at the 1 / 4 position of the plate thickness, the area ratio of ferrite is 0.0% or more and less than 42.0%, and the area ratio of bainitic ferrite is over 10.0% and 70.0% or less. The total area ratio of ferrite and bainitic ferrite is 40.0% or more and less than 75.0%, the area ratio of retained austenite is 10.0% or more and 20.0%, the area ratio of fresh martensite is 0.0% or more and 30.0%, and the area ratio of tempered martensite is 8.0% or more and less than 40.0%, the standard deviation of the grain area of ​​fresh martensite and retained austenite is 0.40 or more and 2.60 or less, in the soft surface layer, the area ratio of ferrite is 50.0% or more and 100.0%, and among the ferrite grains, the area is 50 μm ２ A steel sheet having a structure in which the number ratio of ferrite grains is 0.01 or more and 0.99 or less, and of the ferrite grains, the number ratio of ferrite grains with a length in the rolling direction / length in the thickness direction exceeding 1.00 is 0.55 or more, and a tensile strength of 980 MPa or more and less than 1180 MPa.

2. The above component composition is further, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less The steel sheet according to claim 1, comprising at least one selected from the group consisting of Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

3. The steel sheet according to claim 1, having a zinc plating layer on at least one surface.

4. The steel sheet according to claim 2, having a zinc plating layer on at least one surface.

5. A member made using the steel plate described in any one of claims 1 to 4.

6. A hot rolling step of hot rolling a steel slab having the component composition described in claim 1 or 2 to obtain a hot-rolled steel sheet; a pickling step of pickling the hot-rolled steel sheet; an optional cold rolling step of cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; and an annealing step of annealing the hot-rolled steel sheet or the cold-rolled steel sheet in an atmosphere with a dew point D of -10°C or higher, an annealing temperature T of 780°C or higher and 940°C or lower, a holding time t in a temperature range of (T-40)°C or higher and T°C or lower for 30 seconds or higher and 600 seconds or lower, and a surface soft layer formation coefficient DC = 0.005t × (D + 55), such that DC is 10°C·second or higher and 200°C·second or lower to obtain an annealed steel sheet. Next, a first cooling step is performed in which the annealed steel sheet is cooled at an average cooling rate CR of 2.0°C / second or more in the temperature range from the annealing temperature T to 700°C, and then the annealed steel sheet is cooled at a first holding temperature T of 360°C to 550°C. １ The first holding time t is between 10 seconds and 400 seconds. １ A first holding step involves holding the annealed steel sheet for a period of time, followed by a second cooling step of cooling the annealed steel sheet to a cooling stop temperature of less than 100°C, and then cooling the annealed steel sheet to the first holding temperature T １ The absolute value of the difference between the two temperatures is 5°C or more and 270°C or less, and the second holding temperature T is 200°C or more and 520°C or less. ２ Heat until (T ２ -20℃) or more T ２ Second holding time t: 10 seconds to 800 seconds in a temperature range below °C ２ A method for manufacturing a steel sheet, comprising a second holding step of holding for a certain period, wherein the index A defined by the following formula (1) is 8.0 or more and 800.0 or less.

7. The method for manufacturing a steel sheet according to claim 6, further comprising a hot-dip galvanizing step, after the first holding step and before the second holding step, or after the second holding step, of immersing the annealed steel sheet in a hot-dip galvanizing bath to form a galvanized layer on the surface of the annealed steel sheet.

8. Immediately after the hot-dip galvanizing process, the annealed steel sheet is held at an alloying temperature T Ａ for an alloying time t Ａ to heat-treat and alloy the galvanized layer, and the method for manufacturing a steel sheet according to claim 7, wherein an index B defined by the following formula (2) is 8.0 or more and 800.0 or less.

9. The method for manufacturing a steel sheet according to claim 6, further comprising an electro-zinc plating step, after the second holding step, of immersing the annealed steel sheet in an electro-zinc plating bath to form a zinc plating layer on the surface of the annealed steel sheet.

10. A method for manufacturing a component, comprising the step of forming and joining a steel plate according to any one of claims 1 to 4 to form a component.

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