Steel plates and components, and methods for manufacturing them.

Abstract

The present invention provides a steel sheet having a TS of 780 MPa or more and less than 1180 MPa, and possessing a high yield ratio, excellent ductility, excellent bending fracture resistance, and fatigue properties. The steel sheet of the present invention 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 surface soft layer with a thickness of 20 μm to 100 μm, and a ferrite area ratio of 50.0% to 100.0% in the surface soft layer, with an area of ​​50 μm among the ferrite grains in the surface soft layer. 2 The steel structure has a number ratio of ferrite grains that is between 0.20 and 0.95, and of the ferrite grains, the number ratio of ferrite grains with a rolling direction length / plate thickness direction length exceeding 1.00 is 0.70 or more, and a tensile strength of 780 MPa or more.

Description

[Technical Field]

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

[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 of 0.01 to 10.0 μm in the surface layer of the steel sheet, and an average particle diameter of oxides contained in the surface layer of the steel sheet with 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, 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, the remainder consisting mainly of 4 to 70 volume% ferrite and bainite, part or all of the tempered martensite and retained austenite forming MA, 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 ferrite grain size 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 × 10⁻⁶. 22 pieces / m 3 The disclosed cold-rolled steel sheet 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. [Prior art documents] [Patent Documents]

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0009] By the way, in recent years, regarding impact energy absorption members of automobiles typified by front side members and rear side members, the practical application of steel plates with a tensile strength (hereinafter, also referred to as TS) of 780 MPa or more has been progressing. This is because in order to increase the absorbed energy during impact (hereinafter, also referred to as impact absorption energy), it is effective to improve the yield stress (hereinafter, also referred to as YS).

[0010] However, when the TS and YS of the steel plate are increased, generally, characteristics such as press formability, particularly ductility, hole expansion property, and bending property deteriorate. Therefore, when applying such a steel plate with increased TS and YS to an impact energy absorption member of an automobile, press forming becomes difficult, and it is assumed that the yield will decrease due to variations during forming. In particular, the deterioration of the press formability at the end of the steel plate leads to the occurrence of end face cracking of the actual member. In Patent Documents 1 to 4, such problems are not considered and there is room for improvement.

[0011] In view of the above problems, an object of the present invention is to provide a steel plate having a TS of 780 MPa or more and less than 1180 MPa, and having a high yield stress, excellent ductility, excellent bend fracture resistance characteristics, and fatigue characteristics, together with its advantageous manufacturing method. Another object of the present invention is to provide a member using the above steel plate and its manufacturing method.

Means for Solving the Problems

[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 2 A steel sheet can be obtained having a steel structure in which the number ratio of ferrite grains is 0.20 or more and 0.95 or less, and the number ratio of ferrite grains with a rolling direction length / thickness direction length of more than 1.00 is 0.70 or more, thereby having a TS of 780 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] In mass%, C: 0.050% or more and 0.160% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.00% or more and 3.50% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% to 2.000%, and N: 0.0100% or less The component composition contains, with the remainder being Fe and unavoidable impurities, A soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at the 1 / 4 position of the plate thickness, and a thickness of 20 μm to 140 μm, At the position where the plate thickness is 1 / 4, The area ratio of ferrite is 0.0% or more and less than 35.0%. The area ratio of bayite ferrite is between 30.0% and 70.0%. The area ratio of retained austenite is between 0.0% and 10.0%. The area ratio of fresh martensite is more than 5.0% and less than or equal to 30.0%, and The area ratio of tempered martensite is between 10.0% and 50.0%. The standard deviation of the grain area of ​​fresh martensite and retained austenite is between 0.05 and 1.80. In the aforementioned soft surface layer, The ferrite area ratio is between 50.0% and 100.0%. Among the ferrite grains, those with an area of ​​50 μm 2 The number ratio of ferrite grains is between 0.20 and 0.95, Of the aforementioned ferrite grains, the proportion of ferrite grains whose length in the rolling direction / length in the thickness direction exceeds 1.00 is 0.70 or more. Steel structure and, Tensile strength of 780 MPa or more and less than 1180 MPa, A steel plate having [something].

[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, 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, REM: 0.0200% or less A steel plate as described in [1], comprising at least one selected from the group consisting of the following.

[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 component made of a steel plate as described in any one of the above items [1] to [4].

[0019] [6] A hot rolling step of hot rolling a steel slab having the component composition described in [1] or [2] above to obtain a hot-rolled steel sheet, Next, a pickling process is performed to pickle the hot-rolled steel sheet, Next, the hot-rolled steel sheet is subjected to a cold-rolling process to obtain a cold-rolled steel sheet, which is an optional cold-rolling process. Next, the hot-rolled steel sheet or the cold-rolled steel sheet is annealed in an atmosphere with a dew point D of -10°C or higher, with an annealing temperature T of 780°C or higher and 920°C or lower, a holding time t in the temperature range of (T-40)°C or higher and T°C or lower of 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 130°C·second or lower, in an annealing step to obtain an annealed steel sheet. Next, a first cooling step of cooling the annealed steel sheet with an average cooling rate CR in the temperature range from the annealing temperature T to 700°C of 3.0°C / second or more; Next, a first holding step of holding the annealed steel sheet at a first holding temperature T1 of 380°C or more and 550°C or less for a first holding time t1 of 10 seconds or more and 300 seconds or less; Next, a second cooling step of cooling the annealed steel sheet to a cooling stop temperature of 100°C or more and 300°C or less; Next, the annealed steel sheet is heated to a second holding temperature T2 where the absolute value of the difference from the first holding temperature T1 is 5°C or more and 270°C or less, and 250°C or more and 520°C or less, and held in the temperature range of (T2 - 20°C) or more and T2°C or less for a second holding time t2 of 10 seconds or more and 2000 seconds or less; A method for manufacturing a steel sheet, comprising the above steps and having an index A defined by the following formula (1) of 8.0 or more and 380.0 or less.

Number

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

[0021] [8] Immediately after the hot-dip galvanizing step, the annealed steel sheet is held at an alloying temperature T A for an alloying time t A to have an alloying step of heat alloying the zinc plating layer, A method for manufacturing a steel sheet according to [7] above, wherein an index B defined by the following formula (2) is 8.0 or more and 380.0 or less.

Number

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

[0024] According to the present invention, a steel sheet having a TS of 780 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. [Brief explanation of the drawing]

[0025] [Figure 1] This is a microstructure image obtained by SEM used to identify the steel structure in one embodiment of the present invention. [Figure 2] This is a grain map obtained by EBSD used for grain analysis in one embodiment of the present invention. [Figure 3] (a) Microscopic image of the surface cross-section of a steel sheet according to one embodiment of the present invention, and (b) a graph showing the relationship between the distance from the surface of the steel sheet and the carbon content. [Figure 4] (a) A schematic diagram of the outer surface of a VDA bending sample, and (b) Examples of graphs of the load-stroke curve and the maximum principal strain-stroke curve. [Figure 5] (a) A schematic diagram of the sample after VDA bending, (b) an example of 3D shape measurement results of the bent ridge, and (c) an example of a height profile obtained by 3D shape measurement. [Modes for carrying out the invention]

[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, let's explain the composition of steel sheets. In the explanation of composition, unless otherwise specified, the "%" indicating the content of each element means "mass percent".

[0029] C: 0.050% or more and 0.160% or less Carbon (C) is an effective element for ensuring a TS of 780 MPa to less than 1180 MPa and a high YR by generating appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite. If the C content is less than 0.050%, the hardness of the hard phase decreases, making it difficult to achieve a TS of 780 MPa or higher. It also leads to a decrease in YS. Therefore, the C content should be 0.050% or higher, preferably 0.070% or higher. On the other hand, if the C content exceeds 0.160%, the hardness of fresh martensite, tempered martensite, and retained austenite increases excessively, making it difficult to achieve a TS of less than 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 0.160% or less, preferably 0.150% or less.

[0030] Si: 0.20% or more and 2.00% or less 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 a TS of 780 MPa to less than 1180 MPa and a 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 0.50% or more. On the other hand, if the Si content exceeds 2.00%, the YS decreases due to an excessive increase in retained austenite. Therefore, the Si content should be 2.00% or less, preferably 1.80% or less.

[0031] Mn: 1.00% or more and 3.50% or less Mn is an element that adjusts the area ratio of the steel microstructure. If the Mn content is less than 1.00%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or higher. In addition, an excessive soft surface layer is formed, reducing fatigue properties. Therefore, the Mn content should be 1.00% or higher, preferably 1.40% or higher. On the other hand, if the Mn content exceeds 3.50%, 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 3.50% or less, preferably 3.00% 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, 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% or more and 2.000% or less Al is an element that affects the area fraction of retained austenite because it suppresses carbide formation during cooling and holding after annealing, and promotes the formation of retained austenite. To obtain these 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 fraction of ferrite increases excessively, making it difficult to achieve a TS of 780 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, preferably 0.0050% or less. Although there is no particular lower limit to the N content, due to production technology constraints, an N content of 0.0005% or more is preferred.

[0036] remaining components The steel sheet has a composition containing the above elements, with the remainder being Fe and unavoidable impurities. Preferably, the steel sheet has a composition containing 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 and annealing. To obtain this 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 this effect, 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. Furthermore, B suppresses ferrite formation and grain growth during cooling after annealing. To achieve 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. Also, 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, potentially reducing bending fracture resistance. Therefore, when B is included, the B content should be 0.0100% or less, preferably 0.0050% or less.

[0042] Cu:1.000% or less Since Cu is an element that enhances hardenability, the addition of Cu ensures the formation of an appropriate amount of martensite, thereby securing a total stiffness (TS) of 780 MPa or higher and a high yield strength (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 may increase excessively, and a large amount of coarse precipitates and inclusions may be formed. In such cases, the formation 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 properties. 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 780 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%, the area ratio of hard fresh martensite increases excessively, and mobile dislocations may be introduced into the ferrite and bainitic ferrite, potentially reducing YS and fatigue properties. 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 780 MPa or higher and a high YS. To obtain these 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%, the area ratio of fresh martensite increases excessively, and mobile dislocations are introduced into the ferrite and bainitic ferrite, which may reduce the YS and fatigue properties. 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 total stiffness (TS) of 780 MPa or higher and a high yield strength (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%, the area ratio of fresh martensite increases excessively, and mobile dislocations may be introduced into the ferrite and bainitic ferrite, potentially reducing YS and fatigue properties. 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 C near the surface of the steel sheet during annealing and for 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 780 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 780 MPa or higher. It also leads to a decrease in YS. Therefore, a Sn content of 0.002% or more is preferable, and 0.005% or more is more preferable. On the other hand, if the Sn content exceeds 0.200%, a soft layer will not form 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 Ta, like Ti, Nb, and V, 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, further improving TS and YS. To obtain these 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 780 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 improving the fracture strain and further the bending fracture resistance of steel sheets by spheroidizing the shape of inclusions such as sulfides and oxides. 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, thereby improving the fracture strain and, furthermore, the bending fracture resistance of steel sheets. 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 formed. 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 inclusions and improving the fracture strain and even 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 formed. In such cases, the desired fatigue properties 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 inclusions and improving the fracture strain and, furthermore, the 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 formed. In such cases, the desired fatigue properties may not be achieved. Therefore, when including Zr, 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. When 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, a Ca content of 0.0005% or more is preferred, and 0.0010% or more is more preferred.

[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, furthermore, the 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. In this context, REM refers to the 17 elements comprising 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 REM elements, the inclusion of La is particularly preferred.

[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 formed. 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 composition, 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, we will explain the microstructure of the steel plate. The microstructure of the 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 35.0% Soft ferrite is a phase that improves ductility. However, if the area fraction of ferrite increases excessively, it becomes difficult to achieve a TS of 780 MPa or higher. It also leads to a decrease in YS. Therefore, the area fraction of ferrite should be less than 35.0%, preferably 30.0% or less, and more preferably 25.0% or less. There is no particular lower limit to the area fraction of ferrite, and it may be 0.0%. Therefore, the area fraction of ferrite should be 0.0% or more, preferably 2.0% or more, and more preferably 5.0% or more.

[0060] Area ratio of bayite ferrite: 30.0% to 70.0% Bainitic ferrite is a phase with a hardness intermediate between soft ferrite and hard fresh martensite, and because it is relatively soft, it can ensure good ductility and high strain dispersion capacity. While retained austenite is also effective in ensuring ductility, if its amount is large, the yield strength (YS) may 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 set to 30.0% or more, preferably 35.0% or more, and more preferably 40.0% or more. On the other hand, if the area ratio of bainitic ferrite increases excessively, it becomes difficult to achieve a TS of 780 MPa or more. Therefore, the area ratio of bainitic ferrite is set to 70.0% or less, preferably 65.0% or less, and more preferably 60.0% or less. In this invention, bainitic ferrite refers to upper bainite with few carbides that is formed in a relatively high-temperature range.

[0061] Area percentage of retained austenite: 0.0% to 10.0% If the area ratio of retained austenite increases excessively, the amount of carbon in the retained austenite decreases, leading to instability. As a result, the desired YS cannot be achieved. Therefore, the area ratio of retained austenite should be 10.0% or less, preferably 9.0% or less, and more preferably 8.0% or less. On the other hand, there is no particular lower limit to the area ratio of retained austenite, but the inclusion of retained austenite allows for favorable ductility and strain dispersion. Therefore, the area ratio of retained austenite should be 0.0% or more, preferably 3.0% or more, and more preferably 5.0% or more.

[0062] Area percentage of fresh martensite: over 5.0% and under 30.0% 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 Saturation). 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, fresh martensite is a phase that improves TS (Total Saturation). Therefore, the area ratio of fresh martensite should exceed 5.0%, preferably 8.0% or more, and more preferably 10.0% or more. In this invention, fresh martensite refers to martensite in the as-quenched (untempered) state.

[0063] Area ratio of tempered martensite: 10.0% to 50.0% Tempered martensite has a hardness intermediate between soft ferrite and hard fresh martensite, and is an effective phase for improving fracture strain, TS, and YR by reducing the hardness difference between microstructures. Therefore, the area ratio of tempered martensite should be 10.0% or more, preferably 15.0% or more, and more preferably 20.0% 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 50.0% or less, preferably 45.0% or less, and more preferably 40.0% or less.

[0064] remnant tissue The steel structure of the steel sheet may include residual structures in addition to the structures described above. These 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).

[0065] 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 then etched with 3 vol.% nital to reveal the microstructure. Using a scanning electron microscope (SEM), under the conditions of acceleration voltage: 15 kV and magnification: 5000x, three fields of view of 25.6 μm × 17.6 μm are captured at a position 1 / 4 of the thickness of the observation surface of the sample.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] Tempered martensite is a gray region with an irregular shape. It also contains a relatively large number of carbides.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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), (220), and (311) surfaces of fcc iron (austenite) to the diffraction intensity of the (200), (211), and (220) 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.

[0075] Furthermore, the area ratio of fresh martensite can be determined by the following equation (3). That is, it can be determined by subtracting the area ratio of retained austenite from the area ratio of the hard second phase obtained as described above. [Area percentage of fresh martensite (%)] = [Area percentage of hard secondary phase (%)] - [Area percentage of retained austenite (%)] ... (3)

[0076] Furthermore, the area ratio of the remaining structure can be determined by the following formula (4). That is, it can be determined by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of hard second phase, which were determined as described above, from 100.0%. [Area percentage of remaining tissue (%)] = 100.0 - [Area percentage of ferrite (%)] - [Area percentage of bainitic ferrite (%)] - [Area percentage of tempered martensite (%)] - [Area percentage of hard secondary phase (%)] ... (4)

[0077] Standard deviation of grain area of ​​fresh martensite and retained austenite: 0.05 to 1.80 Fresh martensite and retained austenite (hard phases) contribute to crack propagation during bending deformation. Cracks during bending deformation propagate through 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 phases become finer and less as they become coarser. Therefore, by forming hard phases of various hardnesses and sizes, the stress of void formation around each hard phase differs during crack propagation, thereby suppressing crack propagation. Accordingly, the standard deviation of the grain area of ​​fresh martensite and retained austenite should be 0.05 or higher, preferably 0.07 or higher, and more preferably 0.10 or higher. On the other hand, if the standard deviation of the grain area of ​​the hard phases 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 1.80 or less, preferably 1.60 or less, and more preferably 1.40 or less.

[0078] 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.

number

[0079] [Surface soft 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, which is the surface region having a Vickers hardness of 84% or less of the Vickers hardness at the 1 / 4 position of the steel sheet thickness.

[0080] Thickness of the surface soft layer: 20 μm to 140 μm When the thickness of the soft surface layer is 20 μm or more, the bending fracture resistance is improved. 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 excessively formed, 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.

[0081] The thickness of the soft surface layer can be determined as follows: For the sample subjected to the microstructure observation by SEM described above, 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. The Vickers hardness is measured at 5 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.

[0082] Area ratio of ferrite in the surface soft layer: 50.0% to 100.0% When bending, the surface deforms more significantly than the interior. Therefore, voids are likely to form in the surface layer during bending. When the area ratio of ferrite in the soft surface layer is 50.0% or more, the formation 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%.

[0083] 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.

[0084] 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.

[0085] Area 50 μm 2 The following is the percentage of ferrite grains: 0.20 to 0.95. In the soft surface layer, the area of ​​ferrite grains, i.e., the particle size of ferrite grains, greatly affects the fatigue properties. (Area: 50 μm) 2 When the number ratio of ferrite grains that meet the following criteria is 0.01 or higher, the initiation and growth of fatigue cracks are inhibited, and fatigue properties are improved. Therefore, in the soft surface layer, among the ferrite grains, those with an area of ​​50 μm 2 The ratio of ferrite grains is 0.20 or higher, preferably 0.30 or higher, and more preferably 0.40 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, those with an area of ​​50 μm 2The number ratio of ferrite grains is 0.95 or less, preferably 0.93 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.

[0086] Percentage of ferrite grains where the ratio of rolling direction length to plate thickness direction length exceeds 1.00: 0.70 or higher The inventors investigated the effect of shape on the deformability of ferrite grains and found that, when the area of ​​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 with a rolling direction length / thickness direction length ratio exceeding 1.00 should be 0.70 or higher, preferably 0.72 or higher, and more preferably 0.75 or higher. On the other hand, in the soft surface layer, there is no particular upper limit to the proportion of ferrite grains with a rolling direction length / thickness direction length ratio exceeding 1.00, but this proportion is generally 0.95 or lower.

[0087] 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. That is, 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. Note that 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.

[0088] 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 Analysys 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 define an area of ​​10 μm. 2 The above grains are considered 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.

[0089] [Mechanical properties] Next, we will explain the mechanical properties of steel plates.

[0090] Tensile strength (TS): 780 MPa or more and less than 1180 MPa The steel plate shall have a tensile strength (TS) of 780 MPa or more and less than 1180 MPa. The tensile strength TS shall be measured by a tensile test in accordance with JIS Z 2241.

[0091] Yield stress and ductility Furthermore, the steel sheet obtained by the present invention has high yield strength and excellent ductility. Here, high yield strength means that the YS (yield strength) measured in a tensile test in accordance with JIS Z 2241 satisfies either equation (A) or (B) below, depending on the TS measured in the tensile test. (A) If 780MPa ≤ TS < 980MPa, then 500MPa ≤ YS (B) If 980MPa ≤ TS < 1180MPa, then 740MPa ≤ YS

[0092] Furthermore, excellent ductility means that the total elongation (El) measured in a tensile test in accordance with JIS Z 2241 is 18.0% or higher. Depending on the TS measured in the tensile test, it means that the following equation (A) or (B) is satisfied. (A) If 780MPa ≤ TS < 980MPa, then 17.0% ≤ El (B) If 980MPa ≤ TS < 1180MPa, then 11.0% ≤ El

[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 to measure TS, YS, and El. From the results obtained, if TS is 780 MPa or more and less than 1180 MPa, it is judged as a pass; otherwise, it is judged as a fail. Similarly, if YS satisfies either (A) or (B) above, it is judged as a pass; otherwise, it is judged as a fail. Similarly, for El, if it satisfies either (A) or (B) above, it is judged as a pass; otherwise, it is judged as a fail.

[0094] [Bending fracture resistance] The steel sheet obtained by this invention exhibits excellent bending fracture resistance. In this 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, as measured in a bending test partially conforming to the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry, is 0.070 mm -1 This means the following: High fracture strain means that the strain at fracture in the bent deformation section measured in the VDA bending test is 0.30 or higher. Slow bending crack propagation rate means that in the load-stroke curve of the VDA bending test, the stroke from crack initiation at the bending ridge until the load decreases to 50% is more than 1.0 mm, and the stroke elapsed from crack initiation at the bending ridge until the crack penetrates the ridge direction, as measured in the VDA bending stop test described later, is 0.5 mm or higher.

[0095] 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 × 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 the central 30 mm × 30 mm area of ​​the outer surface of the bend as a grid transfer area.

[0096] Next, a VDA bending test is performed on the test specimen under the following conditions. Test method: Roll support, punch press Roll diameter: φ30mm Punch tip radius: 0.4mm Roll spacing: (plate thickness × 2) + 0.5 mm Bending direction: Perpendicular to rolling (C) direction

[0097] 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 0.070mm -1 The following is a passing grade: 0.070mm -1 If it exceeds this, it will be judged as a failure. Similarly, F S A score of 0.30 or higher is considered a pass, while a score below 0.30 is considered a fail.

[0098] Furthermore, in the VDA bending test described above, the stroke S from the stroke at maximum load until the load decreases by 50% s Evaluate S s If the size is 1.00 mm or less, it will be judged as unacceptable. s If the measurement exceeds 1.00 mm, it will be judged as passing.

[0099] Next, to measure the crack propagation rate during bending deformation, a VDA bending stop test utilizing 3D shape measurement will be performed. The stroke speed will be set to 20 mm / min in accordance with the standard, and the stroke will be S VDAThe load is removed when the bending pressure reaches +0.5mm ± 0.2mm. Figure 5(a) shows a schematic diagram of the bent specimen after the VDA bending stop test. 3D shape measurement is performed on the bent specimen in Figure 5(a). 3D shape measurement is performed at 40x magnification using a one-shot 3D shape measuring machine (Keyence VR6000 series or a newer model). The obtained data is analyzed using the analysis software included with the one-shot 3D shape measuring machine. During analysis, shape changes due to bending can be corrected (flattened) to make it easier to observe cracks. In the corrected height data, the height profile perpendicular to the bent ridge is confirmed using the profile measurement function in the software. Figure 5(b) shows the 3D shape measurement results of the bent ridge. In Figure 5(b), the height profile is confirmed at 10 or more uniform height profile measurement locations in the L direction. Figure 5(c) shows an example of a height profile. In the height profile, a crack is determined to have occurred if the difference X between the maximum and minimum heights is 15 μm or more.

[0100] If crack formation is observed in all measured height profiles, it is determined that the bending crack has penetrated the bending ridge. In this case, the crack propagation stroke (S) in the VDA bending stop test is determined. P Since the crack propagation stroke (S) is 0.5 mm or less, it is judged as a failure. On the other hand, if no cracks are found in one or more of the measured height profiles, the crack propagation stroke (S) in the VDA bending stop test is determined to be 0.5 mm or less. P Since the measurement exceeds 0.5 mm, it is judged to be acceptable.

[0101] [Fatigue characteristics] The steel plate obtained by the present invention has excellent fatigue properties. In the present invention, excellent fatigue properties 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 described above, is 0.30 or higher.

[0102] 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 7 In 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 factor (TS) is defined as the fatigue ratio (FR). An FR of 0.30 or higher is judged as a pass, and an FR less than 0.30 is judged as a fail.

[0103] [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.

[0104] [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 electroplated zinc layer.

[0105] The hot-dip galvanized layer preferably has a composition 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.

[0106] The alloyed hot-dip galvanized layer preferably has a 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.

[0107] The electroplated zinc layer preferably has a composition containing 9.0 to 25.0% by mass of Ni, with the remainder being Zn and unavoidable impurities. The electroplated zinc 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.

[0108] 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.

[0109] 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 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 ).

[0110] (Method of manufacturing steel plates) Next, a method for manufacturing steel sheets according to one embodiment of the present invention will be described.

[0111] 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.

[0112] [Hot rolling process] In the hot rolling process, steel slabs are subjected to hot rolling to produce hot-rolled steel sheets. Hot rolling may be carried out using energy-saving processes. Energy-saving processes include direct rolling (a method in which the steel slabs are 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 slabs are rolled immediately after being given a short period of heat retention). Rough rolling and finish rolling can be performed as part of the hot rolling process.

[0113] 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 surface temperature of the steel slab.

[0114] 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.

[0115] 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.

[0116] 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, etc., such divisions may not be strictly followed, and it is acceptable as long as the specified size is achieved.

[0117] [Pickling process] Next, the hot-rolled steel sheet is pickled after the hot-rolling process. 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; standard methods should be followed.

[0118] [Cold rolling process] Next, any cold rolling process may be performed to obtain cold-rolled steel sheets by cold rolling the hot-rolled steel sheets. Cold rolling can be performed by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling. 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 sheets 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 sheets obtained after cold rolling may optionally be pickled.

[0119] [Annealing process] Next, an annealing process is performed to anneal the hot-rolled or cold-rolled steel sheet to produce an annealed steel sheet. While the annealing process may be repeated two or more times, one annealing is preferred from an energy efficiency standpoint. A radiant tube furnace is generally used as the heat treatment furnace for the annealing process.

[0120] Dew point D in annealing atmosphere: -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.

[0121] Annealing temperature T: 780℃ or higher and 920℃ or lower If the maximum temperature reached during the annealing process (annealing temperature T) is less than 780°C, the proportion of austenite generated during heating in the ferrite-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 carbon concentration in the austenite during annealing, making it impossible to achieve the desired YS. Furthermore, it becomes difficult to achieve a TS of 780 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 920°C, the number of prior austenite grain boundaries that serve as ferrite and bainite generation sites decreases, reducing the total area ratio of ferrite and bainite ferrite, and thus reducing 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 920°C or lower, preferably 900°C or lower. The annealing temperature is the highest temperature reached during the annealing process.

[0122] 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.

[0123] Holding time t in the temperature range of (T-40)℃ to T℃: 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 yield strength (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.

[0124] Surface soft layer formation coefficient DC: 10°C·sec to 130°C·sec To control the area of ​​ferrite grains in the soft surface layer and the ratio of rolling direction length to plate thickness direction length, the balance between the dew point D and holding time t is extremely important. Assuming the soft surface layer formation coefficient DC = 0.005t × (D + 55), if DC is less than 10°C·seconds, the growth of ferrite grains in the soft surface layer becomes insufficient, resulting in an area of ​​50 μm². 2The proportion of ferrite grains that meet the following criteria exceeds 0.70. 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 ratio 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 130°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 0.20. Therefore, the DC should be 130°C·second or less, preferably 120°C·second or less.

[0125] [First cooling process] Next, the annealed steel sheet is cooled in a first cooling process, where the average cooling rate CR over the temperature range from the annealing temperature T to 700°C is 3.0°C / second or higher.

[0126] Average cooling rate CR: 3.0°C / sec or higher in the temperature range from annealing temperature T to 700°C. During the annealing process, carbon (C) may diffuse from the interior 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 where ferrite transformation occurs during annealing is defined as the low-C region, and the region with a higher C content and where ferrite transformation does not occur 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 occurring 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, 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 3.0°C / second or higher, preferably 5.0°C / second or higher. 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 is generally 50°C / second or less.

[0127] [First holding process] Subsequently, a first holding process is performed in which the material is held at a first holding temperature T1 of 380°C to 550°C for a first holding time t1 of 10 seconds to 300 seconds.

[0128] First holding temperature T1: 380℃ or more and 550℃ or less The first holding step is necessary to control the bainitic ferrite or fresh martensite within a predetermined range and ensure ductility. If the holding temperature is below 380°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 T1 should be 380°C or higher, preferably 400°C or higher. On the other hand, if the first holding temperature T1 exceeds 550°C, the area ratio of the bainitic ferrite produced in the first 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 T1 should be 550°C or lower, preferably 520°C or lower.

[0129] First holding time t1: 10 seconds or more and 300 seconds or less If the first holding time t1 is less than 10 seconds, the bainite transformation in the first holding step will be insufficient, and bainite ferrite with an area ratio of 30.0% or more cannot be obtained. Therefore, the first holding time t1 should be 10 seconds or more, preferably 20 seconds or more. On the other hand, if the first holding time t1 exceeds 400 seconds, the bainite transformation in the first holding step will proceed excessively, the area ratio of bainite ferrite will exceed 70.0%, and it will be difficult to achieve a TS of 780 MPa or more. Therefore, the first holding time t1 should be 300 seconds or less, preferably 290 seconds or less.

[0130] [Second cooling process] Next, a second cooling process is performed to cool the annealed steel sheet to a cooling stop temperature of 100°C to 300°C.

[0131] Cooling stop temperature: 100℃ or more and 300℃ or less 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 300°C, the untransformed austenite, which has become carbon-enriched by bainite transformation, will not undergo martensitic transformation, and the area ratio of tempered martensite will ultimately decrease. As a result, the desired YS cannot be achieved. Therefore, the cooling stop temperature should be 300°C or lower, preferably 250°C or lower. On the other hand, if the cooling stop temperature is below 100°C, TS and El will decrease due to an excessive increase in tempered martensite or an excessive decrease in fresh martensite. Therefore, the cooling stop temperature should be 100°C or higher, preferably 150°C or higher.

[0132] [Second holding process] Next, the annealed steel sheet is heated to a second holding temperature T2, where the absolute difference from the first holding temperature T1 is between 5°C and 270°C, and between 250°C and 520°C. A second holding process is then performed, in which the sheet is held in a temperature range of (T2-20°C) to T2°C for a second holding time t2 of 10 seconds to 2000 seconds. This causes bainite transformation in 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 untransformed austenite, stable austenite at room temperature, i.e., retained austenite, can be produced.

[0133] Second holding temperature T2: The absolute value of the difference between this temperature and the first holding temperature T1 is between 5°C and 270°C. If the absolute value of the difference between the second holding temperature T2 and the first holding temperature T1 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 achieve a standard deviation of 0.40 or more for the grain area of ​​fresh martensite and retained austenite. Therefore, the absolute value of the difference between the second holding temperature T2 and the first holding temperature T1 should be 5°C or more, preferably 10°C or more, and more preferably 15°C or more. On the other hand, if the absolute value of the difference between the second holding temperature T2 and the first holding temperature T1 exceeds 270°C, the standard deviation of the grain area of ​​fresh martensite and retained austenite will exceed the desired value. Therefore, the absolute value of the difference between the second holding temperature T2 and the first holding temperature T1 should be 270°C or less, preferably 260°C or less, and more preferably 250°C or less.

[0134] Second holding temperature T2: 250℃ or higher and 520℃ or lower If the second holding temperature T2 is less than 250°C, bainite transformation will not occur sufficiently, and hard phases of different sizes will not be formed, making it difficult to obtain the desired standard deviation of the hard phase area. Therefore, the second holding temperature T2 should be 250°C or higher, preferably 280°C or higher, and more preferably 310°C or higher. On the other hand, if the second holding temperature T2 exceeds 520°C, the untransformed austenite formed in the first holding step will decompose as carbides (pearlite), and the desired amount of residual γ will not be obtained, resulting in a decrease in ductility. Furthermore, bainite transformation will not occur sufficiently in the second holding step, making it difficult to obtain the desired standard deviation of the hard phase area. Therefore, the second holding temperature T2 should be 520°C or lower, preferably 500°C or lower, and more preferably 480°C or lower.

[0135] Second holding time t2: 10 seconds or more and 2000 seconds or less If the second holding time t2 is less than 10 seconds, the bainite transformation does not proceed sufficiently during the second holding process, and a hard phase of a different size from the hard phase formed in the first holding process cannot be formed. As a result, it becomes difficult to keep the standard deviation of the hard phase area within the desired range, and the bending fracture resistance decreases. Therefore, the second holding time t2 should be 10 seconds or more, preferably 20 seconds or more, and more preferably 30 seconds or more. On the other hand, if the second holding time t2 exceeds 2000 seconds, the bainite transformation proceeds excessively during the second holding process, and the area ratio of bainite ferrite exceeds 70.0%, making it difficult to achieve a TS of 780 MPa or more. In addition, the untransformed austenite present in the steel at the start of the second holding process decomposes as carbides (pearlite), resulting in a decrease in ductility. Therefore, the second holding time t2 should be 2000 seconds or less, preferably 800 seconds or less, and more preferably 400 seconds or less. Furthermore, the second holding time t2 includes not only the holding time at the second holding temperature T2, but also the residence time in the temperature range of (T2-20°C) to T2°C during heating and cooling before and after reaching the second holding temperature T2.

[0136] Heat treatment conditions: A in equation (1): 8.0 to 380.0 The heat treatment conditions shall be such that index A, defined by the following formula (1), is between 8.0 and 380.0. This allows for achieving the desired standard deviation of the area of ​​the desired soft surface phase and the desired hard phase, while also obtaining the desired TS and YS even if a decrease in TS and YS occurs due to the formation of the soft surface phase.

number

[0137] In equation (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 equation (1) should be 8.0 or greater, preferably 15.0 or greater. On the other hand, if A exceeds 380.0, the formation of both the surface soft layer and bainite is promoted, making it difficult to obtain a TS of 780 MPa or greater. Therefore, A in equation (1) should be 380.0 or less, preferably 300.0 or less.

[0138] [Hot-dip galvanizing process] After the first holding step and before the second cooling step, or after the second holding step, a hot-dip galvanizing step 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.

[0139] 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.

[0140] [Alloying process] Furthermore, an alloying step may be performed immediately after the hot-dip galvanizing step to heat-alloy the zinc plating layer. Here, "immediately after the hot-dip galvanizing step" means that the alloying step is performed at the same timing as the hot-dip galvanizing step (either during the cooling step, after the cooling step and before the tempering step, or after the tempering step). 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 step, 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 780 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.

[0141] 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 380.0. 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 380.0 or lower, the formation of the surface soft layer and bainite is suitably suppressed, and a TS of 780 MPa or higher can be suitably obtained. Therefore, B is preferably 380.0 or lower, and more preferably 300.0 or lower.

number

[0142] 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.

[0143] [Electro-galvanizing process] After the second holding step, the annealed steel sheet may be cooled to room temperature, and then immersed in an electro-zinc plating bath to form a zinc plating layer on the surface of the annealed steel sheet in an electro-zinc plating step. If an electro-zinc plating step is performed, the processing conditions for the electro-zinc plating treatment are not particularly limited and should be in accordance with conventional methods.

[0144] [Temper rolling process] Furthermore, 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. Also, 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). Also, the number of times temper rolling is performed may be one or two or more. 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.

[0145] Other than the conditions mentioned above, there are no particular limitations; you may follow the usual law.

[0146] (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 (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.

[0147] Here, the steel plate has a TS of 780 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.

[0148] (Method of manufacturing components) 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 / or joining it to form a member. Here, the forming method is not particularly limited, and for example, a general processing method such as press working can be used. Similarly, the joining method is not particularly limited, and for example, a general welding method such as spot welding, laser welding, or arc welding, or a riveted joint or crimped joint can be used. The forming conditions and joining conditions are not particularly limited and can be followed according to conventional methods.

[0149] For processes and conditions not described in this specification, conventional methods may be used. [Examples]

[0150] 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.

[0151] [Table 1] TIFF0007878605000007.tif233140TIFF0007878605000008.tif233146

[0152] 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 pickled and cold-rolled (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).

[0153] In the zinc plating process, hot-dip galvanizing, alloyed zinc plating, or electro-galvanizing was performed to obtain hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, or electro-galvanized steel sheets. Hot-dip galvanizing and alloyed zinc plating were performed during the cooling process, after the cooling process and before the tempering process, or after the tempering process. Electro-galvanizing was performed after the reheating and holding process. In Table 2, the column for the type of zinc plating process is indicated as "GI," "GA," or "EG," respectively. In the examples of GI and EG, the alloying temperature is indicated as "-" because no alloying treatment was performed.

[0154] The zinc plating bath temperature was 470°C in all cases. The zinc plating amount was 10-60 g / m² per side when manufacturing EG. 2 When manufacturing GI, the amount is 45-72g / m² per side. 2 When manufacturing GA, use 45g / m² per side. 2 The composition of the hot-dip galvanized layer in GI was 0.1-1.0 mass% Fe and 0.2-0.33 mass% Al, with the remainder being Zn and unavoidable impurities. The composition of the alloyed hot-dip galvanized layer in GA was 8.0-12.0 mass% Fe and 0.1-0.23 mass% Al, with the remainder being Zn and unavoidable impurities. The composition of the electroplated galvanized layer in EG was 9.0-25.0 mass% Ni, with the remainder being Zn and unavoidable impurities. In all cases, the galvanized layer was formed on both sides of the steel sheet.

[0155] Using the obtained steel sheets, the microstructure of the steel sheets was identified and the grain areas of fresh martensite and retained austenite were measured using the method described above. The measurement results are shown in Table 3. Note that in Table 3, the area is 50 μm². 2 The number ratio of ferrite grains is F A≦50 The ratio of ferrite grains whose length in the rolling direction / length in the thickness direction exceeds 1.00 is F AS>1As shown in the table below, F represents ferrite, BF represents bainitic ferrite, TM represents tempered martensite, RA represents retained austenite, and FM represents fresh martensite. In addition, the remaining tissue observed in some cases was carbide.

[0156] [Table 2] TIFF0007878605000010.tif233123TIFF0007878605000011.tif233102

[0157] [Table 3] TIFF0007878605000013.tif233138TIFF0007878605000014.tif233113

[0158] Tensile tests, VDA bending tests, VDA bending stop tests, and fatigue tests were performed using the method described above. The measurement results are shown in Table 4.

[0159] [Table 4] TIFF0007878605000016.tif227170TIFF0007878605000017.tif233144

[0160] As shown in Table 4, all of the invention examples are based on tensile strength (TS), yield stress (YS), total elongation (El), and strain increase rate (V) in the VDA bending test. S ), fracture strain (F) in 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 ), crack propagation stroke (S) in VDA bending stop test P All of the following passed: tensile strength (TS), yield stress (YS), total elongation (El), and strain increase rate (VDA bending test). S), fracture strain (F) in 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 ), crack propagation stroke (S) in VDA bending stop test P ), at least one of them was not sufficient.

[0161] Furthermore, members obtained by forming or joining using the steel plate of the present invention are subject to the following criteria: tensile strength (TS), yield stress (YS), total elongation (El), and strain increase rate (VDA bending test). S ), fracture strain (F) in 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 ), crack propagation stroke (S) in VDA bending stop test P It was found that all of these possess the excellent properties that characterize this invention. [Industrial applicability]

[0162] According to the present invention, a steel sheet having a TS of 780 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. [Explanation of Symbols]

[0163] X Difference between maximum and minimum height

Claims

1. In mass percent, C: 0.050% or more and 0.160% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.00% or more and 3.50% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% to 2.000%, N: 0.0100% or less The component composition contains, with the remainder being Fe and unavoidable impurities, A soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at the 1 / 4 position of the plate thickness, and a thickness of 20 μm to 140 μm, At the position where the plate thickness is 1 / 4, The area ratio of ferrite is 0.0% or more and less than 35.0%. The area ratio of bayite ferrite is 30.0% or more and 70.0% or less. The area ratio of retained austenite is 0.0% or more and 10.0% or less. The area ratio of fresh martensite is more than 5.0% and 30.0% or less, The area ratio of tempered martensite is 10.0% or more and 50.0% or less. The standard deviation of the grain area of ​​fresh martensite and retained austenite is between 0.05 and 1.

80. In the aforementioned soft surface layer, The ferrite area ratio is 50.0% or more and 100.0% or less. Among the ferrite grains, those with an area of ​​50 μm ２ The number ratio of ferrite grains is 0.20 or more and 0.95 or less, Of the aforementioned ferrite grains, the proportion of ferrite grains whose length in the rolling direction / length in the thickness direction exceeds 1.00 is 0.70 or more. Steel structure and, Tensile strength of 780 MPa or more and less than 1180 MPa, A steel plate having [something].

2. The aforementioned component composition is further expressed 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, 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, REM: 0.0200% or less The steel plate according to claim 1, comprising at least one selected from the group consisting of the following.

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 soft surface layer having a thickness of 20 μm to 140 μm and having a Vickers hardness of 84% or less of the Vickers hardness at the 1 / 4 position of the plate thickness, At the position where the plate thickness is 1 / 4, The area ratio of ferrite is 0.0% or more and less than 35.0%. The area ratio of bayite ferrite is 30.0% or more and 70.0% or less. The area ratio of retained austenite is 0.0% or more and 10.0% or less. The area ratio of fresh martensite is more than 5.0% and 30.0% or less, The area ratio of tempered martensite is 10.0% or more and 50.0% or less. The standard deviation of the grain area of ​​fresh martensite and retained austenite is between 0.05 and 1.

80. In the aforementioned soft surface layer, The ferrite area ratio is 50.0% or more and 100.0% or less. Among the ferrite grains, the proportion of ferrite grains with an area of ​​50 μm² or less is 0.20 or more and 0.95 or less. Of the aforementioned ferrite grains, the proportion of ferrite grains whose length in the rolling direction / length in the thickness direction exceeds 1.00 is 0.70 or more. Steel structure and, A method for manufacturing a steel sheet having a tensile strength of 780 MPa or more and less than 1180 MPa, The method described above is 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, Next, a pickling process is performed to pickle the hot-rolled steel sheet, Next, the hot-rolled steel sheet is subjected to a cold-rolling process to obtain a cold-rolled steel sheet, which is an optional cold-rolling process. Next, the hot-rolled steel sheet or the cold-rolled steel sheet is annealed in an atmosphere with a dew point D of -10°C or higher, with an annealing temperature T of 780°C or higher and 920°C or lower, a holding time t in the temperature range of (T-40)°C or higher and T°C or lower of 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 130°C·second or lower, in an annealing step to obtain an annealed steel sheet. Next, the annealed steel sheet is cooled in a first cooling step, in which the average cooling rate CR over a temperature range from the annealing temperature T to 700°C is 3.0°C / second or more. Next, the annealed steel sheet is subjected to a first holding temperature T of 380°C to 550°C. １ The first holding time t is between 10 seconds and 300 seconds. １ A first holding step of holding for a certain period, Next, a second cooling step is performed to cool the annealed steel sheet to a cooling stop temperature of 100°C or higher and 300°C or lower. Next, the annealed steel sheet is subjected 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 250°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 2000 seconds in a temperature range below °C ２ A second holding step involves holding the item for a certain period of time, A method for manufacturing steel plates, wherein the index A defined by the following formula (1) is 8.0 or more and 380.0 or less. [Math 1]

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

8. Immediately after the molten zinc plating step, the annealed steel sheet is held at an alloying temperature T Ａ for an alloying time t Ａ to perform an alloying step of heat alloying the zinc plating layer. A method for manufacturing a steel plate according to claim 7, wherein the index B, defined by the following formula (2), is 8.0 or more and 380.0 or less. [Math 2]

9. The method for manufacturing a steel sheet according to claim 6, further comprising an electro-zinc plating step after the second holding step, in which the annealed steel sheet is immersed 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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