Steel sheet and electrogalvanized steel sheet

Abstract

A high-strength steel sheet according to the present invention is composed of 0.30-0.50 mass% C, 0-1.50 mass% Si, 0.10-3.50 mass% Mn, 0-0.020 mass% P, 0-0.010 mass% S, 0.001-1.000 mass% Al, 0-0.30 mass% Cr, and 0.08%-0.15 mass% Ti, the remainder being Fe and unavoidable impurities. The proportion of martensite structure in the total metal structure is 95 area% or more, and the tensile strength is 1700 MPa or more.

Description

Steel sheets and electro-galvanized steel sheets

[0001] This disclosure relates to steel sheets and electro-galvanized steel sheets using the same.

[0002] Traditionally, there has been a demand for improved occupant safety in vehicles, and to this end, the strength of vehicle bodies has been increased. On the other hand, against the backdrop of worsening global warming and other issues, the movement to improve the fuel efficiency of automobiles is accelerating. It is known that reducing the weight of the vehicle body is effective in improving fuel efficiency.

[0003] In recent years, there has been a growing trend towards increasing the strength of automotive steel sheets in order to achieve both weight reduction and collision safety in automobiles. Martensitic steel, manufactured by water quenching, is suitable for automotive steel sheets because it can achieve high strength. Furthermore, to ensure the dimensional accuracy of the automotive parts being manufactured, automotive steel sheets must have good flatness. In addition, since complex-shaped automotive frame parts are press-formed, excellent press formability is also desired. Automotive parts such as bumpers are mainly formed by bending, so automotive steel sheets must have particularly excellent bendability among press formability properties.

[0004] Patent Document 1 proposes a steel sheet that achieves both strength, flexibility, and delayed fracture resistance by controlling the morphology of carbides in martensite. Patent Document 2 proposes a 1700 MPa class steel sheet with excellent delayed fracture resistance, and Patent Document 3 proposes a 1310 MPa class steel sheet with excellent joint strength after welding. Patent Document 4 proposes a high-strength steel sheet with excellent flexibility, flatness, and resistance to work-induced embrittlement, and a tensile strength (TS) of 1180 MPa or higher.

[0005] Japanese Patent Publication No. 2018-109222, Japanese Patent Publication No. 2021-113353, International Publication No. 2023 / 063288, Japanese Patent No. 7323096

[0006] High-strength steel sheets with a tensile strength of 1700 MPa or higher can exhibit significant deterioration in flatness and warping. Warped steel sheets require flattening with a leveler before press forming or other processes, leading to increased manufacturing costs for automotive parts. Therefore, there is a demand for high-strength steel sheets with excellent flatness that do not require flattening with a leveler before press forming or other processes.

[0007] However, Patent Documents 1 to 3 do not address improving the flatness of ultra-high-strength steel sheets with a tensile strength of 1700 MPa or more. Although Patent Document 4 mentions the flatness of steel sheets, it deals with steel sheets with a tensile strength of approximately 1180 MPa and does not provide any insights into how to eliminate the warping that occurs in ultra-high-strength steel sheets with a tensile strength of 1700 MPa or more.

[0008] This disclosure is made in view of the above circumstances and aims to provide a high-strength steel sheet with a tensile strength of 1700 MPa or more and good flatness, and to provide an electro-galvanized steel sheet using said high-strength steel sheet.

[0009] One aspect of the present invention is a high-strength steel sheet containing C: 0.30% by mass or more and 0.50% by mass or less, Si: 0% by mass or more and 1.50% by mass or less, Mn: greater than 0.10% by mass and 3.50% by mass or less, P: 0% by mass or more and 0.020% by mass or less, S: 0% by mass or more and 0.010% by mass or less, Al: 0.001% by mass or more and 1.000% by mass or less, Cr: greater than 0% by mass and 0.30% by mass or less, Ti: 0.08% by mass or more and 0.15% by mass or less, B: 0.0001% by mass or more and 0.0050% by mass or less, with the remainder being Fe and unavoidable impurities, the proportion of martensitic structure in the total metallic structure being 95% by area or more, and a tensile strength of 1700 MPa or more.

[0010] Embodiment 2 of the present invention is a high-strength steel plate according to Embodiment 1, wherein the flatness is less than 40 mm.

[0011] Embodiment 3 of the present invention is a high-strength steel sheet according to Embodiment 1 or 2, satisfying one or more of the following (a) to (d): (a) Ca: further containing more than 0% by mass and 0.0010% by mass or less, and in a cross-sectional view, the number density of inclusions with an equivalent circular diameter of 3.35 μm or more in the surface layer from the surface to a depth of (plate thickness × 0.1) is 7.0 pieces / mm ２ The following further contain: (b) at least one selected from the group consisting of Cu: greater than 0% by mass and 1.00% by mass or less and Ni: greater than 0% by mass and 1.00% by mass or less; (c) at least one selected from the group consisting of V: ​​greater than 0% by mass and 0.1% by mass or less, Nb: greater than 0% by mass and 0.1% by mass or less, and Mo: greater than 0% by mass and 0.5% by mass or less; (d) at least one selected from the group consisting of Mg: greater than 0% by mass and 0.005% by mass or less, and REM: greater than 0% by mass and 0.005% by mass or less.

[0012] Aspect 4 of the present invention is an electro-galvanized steel sheet comprising a high-strength steel sheet according to any one of aspects 1 to 3 and an electro-galvanized layer covering the surface of the high-strength steel sheet.

[0013] According to one embodiment of the present invention, it is possible to provide a high-strength steel sheet having a tensile strength of 1700 MPa or more and good flatness, and to provide an electro-galvanized steel sheet using the high-strength steel sheet.

[0014] To solve the aforementioned problems, the inventors diligently conducted research to improve the flatness of steel sheets with a martensitic structure that can achieve high strength of 1700 MPa or more. First, they investigated the cause of warping in high-strength steel sheets with a tensile strength of 1700 MPa or more from various perspectives. As a result, although this does not limit the technical scope of the present invention, they have come to estimate that the mechanism is as follows. In water quenching, which is performed to increase strength, the steel shrinks due to the rapid temperature change when the high-temperature steel sheet comes into contact with water (occurrence of thermal strain), and subsequently, volume expansion of the steel occurs due to martensitic transformation (occurrence of transformation strain). It is thought that the flatness of high-strength steel sheets deteriorates significantly when the time between the occurrence of thermal strain and the occurrence of transformation strain is extremely short, that is, when shrinkage and expansion occur almost simultaneously within the steel sheet. The reason for this is unknown, but it is assumed that when shrinkage and expansion occur almost simultaneously within the steel sheet, the stress distribution within the steel sheet becomes disordered and stress uniformity cannot be achieved. If this estimated mechanism is correct, lowering the temperature at which transformation strain occurs (the Ms point) would increase the time it takes to cool down to the Ms point. This would extend the time between the onset of thermal strain and the onset of transformation strain, potentially reducing the warping of the steel plate.

[0015] The inventors further researched methods to lower the Ms point while maintaining high tensile strength and concluded that controlling the component composition (particularly increasing the C and Ti content) is effective. In particular, Ti has the effect of not only lowering the Ms point but also increasing the strength at high temperatures (high-temperature strength). Improved high-temperature strength allows for a certain degree of strength to be imparted to the steel sheet in the high-temperature state immediately before water quenching, thus improving resistance to deformation caused by shrinkage during water cooling and deformation caused by volume expansion due to martensitic transformation. In other words, a steel sheet containing an appropriate amount of Ti (0.08 mass% to 0.15 mass%) can be expected to have two effects: a lower Ms point that suppresses warping of the steel sheet, and increased high-temperature strength that also suppresses warping of the steel sheet. Based on these findings, the inventors completed a high-strength steel sheet according to an embodiment of the present invention.

[0016] Furthermore, the inventors also investigated another characteristic required for automotive steel sheets: good bendability (hereinafter sometimes simply referred to as "bendability"). They found that bendability could be improved by controlling the number density of inclusion particles present near the surface (surface layer) of the metal structure, along with controlling the component composition.

[0017] The requirements specified in the embodiments of the present invention will be described in detail below.

[0018] 1. Composition The steel sheet according to the embodiment of the present invention has the composition described below.

[0019] [C: 0.30% by mass or more and 0.50% by mass or less] Carbon (C) is an element necessary to obtain a tensile strength of 1700 MPa or more, and is required in an amount of at least 0.30% by mass. On the other hand, if the amount of C is excessive, the strength of the martensitic structure will increase excessively and the flexibility will decrease. Therefore, the amount of C should be 0.50% by mass or less. Preferably, the amount of C is 0.45% by mass or less, and more preferably 0.40% by mass or less.

[0020] [Si: 0% by mass or more and 1.50% by mass or less] Si is an effective element for improving tempering softening resistance. It is also an effective element for improving strength through solid solution strengthening. The amount of Si may be 0% by mass, but in order to exhibit the above effects, it is preferable to contain 0.02% by mass or more of Si. On the other hand, since Si is a ferrite-forming element, if the amount of Si is excessive, the hardenability will be impaired and it will be difficult to ensure high strength. Therefore, the amount of Si should be 1.50% by mass or less. The amount of Si is preferably 0.20% by mass or less, more preferably 0.10% by mass or less, and even more preferably 0.05% by mass or less.

[0021] In this specification, "0% by mass or more" means the content in embodiments in which the element is not intentionally added, for example, the content at the level of an unavoidable impurity (this does not exclude cases in which the element is intentionally added within a predetermined range). On the other hand, in this specification, "greater than 0% by mass" means that the element is intentionally added.

[0022] [Mn: greater than 0.10 mass% and 3.50 mass% or less] Mn is an effective element for improving hardenability and increasing strength. To effectively exert these effects, the amount of Mn should be greater than 0.10 mass%. The amount of Mn is preferably 0.20 mass% or more, and more preferably 0.50 mass% or more. On the other hand, if the amount of Mn is excessive, the delayed fracture resistance and weldability will deteriorate. Therefore, the amount of Mn should be 3.50 mass% or less. The amount of Mn is preferably 2.00 mass% or less, more preferably 1.80 mass% or less, and even more preferably 1.50 mass% or less.

[0023] [P: 0% by mass or more and 0.020% by mass or less] Although phosphorus (P) has the effect of strengthening steel, it also reduces toughness and ductility, so its content should be 0.020% by mass or less. Preferably, the amount of P is 0.010% by mass or less, and more preferably 0.006% by mass or less. The amount of P may be 0% by mass, but a small amount (for example, about 0.001% by mass or more) may be present due to constraints in the manufacturing process.

[0024] [S: 0% by mass or more and 0.010% by mass or less] S generates sulfide-based inclusions, which degrade the workability and weldability of the base material. Therefore, the amount of S should be 0.010% by mass or less. Preferably, the amount of S is 0.005% by mass or less, and more preferably 0.003% by mass or less. The less S there is, the better, so the lower limit is 0% by mass, but due to constraints in the manufacturing process, a small amount (for example, about 0.001% by mass or more) may be present.

[0025] [Al: 0.001% by mass or more and 1.000% by mass or less] Al is useful as a deoxidizing element and is also useful for fixing solid-solution N present in the steel as AlN. In order to effectively exert these effects, the amount of Al should be 0.001% by mass or more. Preferably, the amount of Al is 0.035% by mass or more, and more preferably 0.040% by mass or more. However, if the amount of Al is excessive, a large amount of inclusions will be generated and the flexibility will deteriorate. Therefore, the amount of Al should be 1.000% by mass or less. Preferably, the amount of Al is 0.100% by mass or less, more preferably 0.070% by mass or less, and even more preferably 0.055% by mass or less.

[0026] [Cr: greater than 0 mass% and less than or equal to 0.30 mass%] Cr is an effective element for increasing strength by improving hardenability. Cr is also an effective element for increasing the tempering softening resistance of martensitic steel. To fully exert these effects, the amount of Cr is greater than 0 mass%. Preferably, the amount of Cr is 0.01 mass% or more. However, if the amount of Cr is excessive, the delayed fracture resistance deteriorates. Therefore, the amount of Cr is 0.30 mass% or less. Preferably, the amount of Cr is 0.15 mass% or less.

[0027] [Ti: 0.08% by mass or more and 0.15% by mass or less] Ti is an important element in the embodiments of the present invention and is effective in improving strength and delayed fracture resistance by refining γ grains. It is also effective in improving flatness after annealing and rapid cooling. The detailed mechanism of flatness improvement is not limited to the technical scope of the embodiments of the present invention, but is thought to be as follows. By including Ti, fine precipitates such as TiC are formed, which can suppress the coarsening of austenite grains, thereby shifting the Ms point to a lower temperature. In other words, including Ti can delay the timing of martensitic transformation, thus suppressing deformation of the steel sheet. In addition, precipitation strengthening by fine precipitates such as TiC can increase the high-temperature strength of the steel sheet immediately before rapid cooling, thus suppressing deformation of the steel sheet after rapid cooling. These effects are thought to suppress warping of the steel sheet. To achieve these effects, the amount of Ti is 0.08% by mass or more. The amount of Ti is preferably 0.09% by mass or more. However, if Ti is present in excess, the precipitation of carbonitrides and other materials increases, reducing processability. Therefore, the amount of Ti should be 0.15% by mass or less, preferably 0.12% by mass or less.

[0028] [B: 0.0001% by mass or more, or 0.0050% by mass or less] B is an effective element for improving hardenability. To fully exert this effect, the amount of B should be 0.0001% by mass or more. Preferably, the amount of B is 0.0005% by mass or more. However, if B is present in excess, the ductility will decrease, so the amount of B should be 0.0050% by mass or less.

[0029] The basic components of the steel sheet according to the embodiment of the present invention are as described above, and in one preferred embodiment, the remainder consists of Fe and unavoidable impurities. As unavoidable impurities, the inclusion of elements (e.g., As, Pb, Bi, Sb, Sn, N, O, H, etc.) introduced depending on the conditions of the raw materials, materials, manufacturing equipment, etc. is permissible. Note that, for example, elements such as S are generally preferable in smaller amounts and are therefore unavoidable impurities, but their composition range is separately defined as described above. For this reason, in this specification, "unavoidable impurities" constituting the remainder is a concept that excludes elements whose composition range is separately defined.

[0030] • Other Selective Elements In addition, in another preferred embodiment of the present invention, elements other than those described above may be included as needed, as long as they do not impair the function of the embodiment of the present invention. Examples of such selective elements are shown below.

[0031] [Ca: greater than 0 mass% and less than or equal to 0.0010 mass%] Ca is an element that can improve delayed fracture resistance by bonding with S instead of Mn and controlling the form of MnS that stretches in the rolling direction. Ca may be added to achieve this effect. That is, the amount of Ca may be greater than 0 mass%. Preferably, the amount of Ca is 0.0001 mass% or more. On the other hand, if Ca is included in excess, the workability deteriorates, so the amount of Ca may be 0.0010 mass% or less, and more preferably 0.0005 mass% or less.

[0032] [At least one element selected from the group consisting of Cu: greater than 0 mass% and 1.00 mass% or less, and Ni: greater than 0 mass% and 1.00 mass% or less] Cu and Ni are effective elements for improving delayed fracture resistance by improving the corrosion resistance of steel sheets. To achieve this effect, at least one element selected from the group consisting of Cu and Ni may be added. That is, the content of each element, Cu and Ni, may be greater than 0 mass%. When Cu is added, the amount of Cu is preferably 0.01 mass% or more, more preferably 0.05 mass% or more, and even more preferably 0.08 mass% or more. On the other hand, if Cu is included in excess, the pickling resistance and chemical treatment resistance will deteriorate, so the amount of Cu may be 1.00 mass% or less, preferably 0.50 mass% or less, and even more preferably 0.20 mass% or less. When Ni is added, the amount of Ni is preferably 0.01 mass% or more, more preferably 0.05 mass% or more, and even more preferably 0.08 mass% or more. On the other hand, if Ni is present in excess, the ductility and workability of the base material will decrease, so the amount of Ni may be 1.00 mass% or less, preferably 0.50 mass% or less, and more preferably 0.20 mass% or less.

[0033] [At least one element selected from the group consisting of V: ​​greater than 0% by mass and 0.1% by mass or less, Nb: greater than 0% by mass and 0.1% by mass or less, and Mo: greater than 0% by mass and 0.5% by mass or less] V, Nb, and Mo are all elements that are effective in improving strength and improving toughness after quenching by refining γ grains. In order to fully exert these effects, at least one element selected from the group consisting of V, Nb, and Mo may be added. That is, the content of each element V, Nb, and Mo may be greater than 0% by mass. When V, Nb, and Mo are added, their content is preferably 0.003% by mass or more, more preferably 0.02% by mass or more. On the other hand, if V, Nb, and Mo are present in excess, the precipitation of carbonitrides and the like increases, and the workability of the base material decreases. Therefore, when adding each of the elements V, Nb, and Mo, the amounts of V and Nb should be 0.1% by mass or less, preferably 0.05% by mass or less, and the amount of Mo should be 0.5% by mass or less.

[0034] [Mg: greater than 0% by mass and 0.005% by mass or less, and REM: at least one selected from the group consisting of greater than 0% by mass and 0.005% by mass or less] Mg and REM are elements that are effective in refining inclusions and improving processability. To achieve this effect, at least one selected from the group consisting of Mg and REM may be added. That is, the content of each element, Mg and REM, may be greater than 0% by mass. When Mg and REM are added, their content is preferably 0.0010% by mass or more, more preferably 0.0015% by mass or more. On the other hand, if Mg and REM are included in excess, ductility will decrease. Therefore, when Mg and REM are added, their content is preferably 0.005% by mass or less, more preferably 0.003% by mass or less.

[0035] 2. Metal structure The metal structure of the high-strength steel sheet according to the embodiment of the present invention is characterized in that the proportion of martensitic structure is 95 area % or more. Furthermore, the number density of inclusions with an equivalent circular diameter of 3.35 μm or more in the surface layer of the high-strength steel sheet is 7.0 inclusions / mm². ２ The following is preferable:

[0036] [Martensitic structure proportion of 95 area % or more] In the embodiments of the present invention, in order to ensure a strength of 1700 MPa, the proportion of martensitic structure in the total metallic structure is set to 95 area % or more. The proportion of martensitic structure is preferably 97 area % or more, and may be 100 area %. In addition to the martensitic structure described above, the high-strength steel sheet according to the embodiments of the present invention may also include structures that are inevitably included in the manufacturing process, such as ferrite structure, bainite structure, retained austenite structure, etc.

[0037] The area ratio of the martensitic structure is determined from a cross-sectional SEM image. An L-shaped section (a perpendicular section parallel to the rolling direction) passing approximately through the center of the width direction of the steel plate is polished, and after Nital etching, a section t / 4 (where t is the plate thickness) is observed with a scanning electron microscope (SEM) at a magnification of 1000 to 2000x to obtain a cross-sectional SEM image. In the cross-sectional SEM image, the areas observed as white are defined as the martensitic structure, and the areas observed as black are defined as other structures (e.g., ferrite structure). In any single field of view (the size of one field of view is, for example, 90 μm × 120 μm), 10 lines are drawn at equal intervals both vertically and horizontally, and the number of intersections on the martensitic structure is divided by the total number of intersections to obtain the area ratio of the martensitic structure.

[0038] [In a cross-sectional view, the number density of inclusions with an equivalent circular diameter of 3.35 μm or larger in the surface layer from the surface to a depth of (plate thickness × 0.1) is 7.0 inclusions / mm².] ２ [The following is a separate explanation] If there are many relatively coarse inclusions near the surface of a steel sheet, cracks are more likely to occur during bending, originating from these inclusions, thus degrading the bendability of the steel sheet. From the viewpoint of improving bendability, the number density of inclusions with an equivalent circular diameter of 3.35 μm or larger in the range from the surface to a depth of (sheet thickness × 0.1) (referred to as the "surface layer") (referred to as the "inclusion density") should be 7.0 inclusions / mm ２ The following is preferable. The measurement range is set to the surface layer because the influence of inclusions present within the thickness of the plate on the bendability is small. The inclusion density in the surface layer is more preferably 5.0 pieces / mm ２ The following, and more preferably 3.0 pieces / mm ２ The following, and particularly preferably 1.0 pieces / mm ２ The following applies:

[0039] The measurement of the inclusion density in the surface layer region is carried out according to the following procedure. After polishing an L-section (a vertical section parallel to the rolling direction) passing through approximately the center in the width direction of the steel plate and without corrosion, a range (surface layer region) from the steel plate surface to a position in the plate thickness direction of (plate thickness × 0.1) is subjected to cross-sectional SEM observation at a magnification of 400 times. Cross-sectional SEM images are obtained from 10 fields by changing the shooting position. When the cross-sectional SEM images are processed by image processing and binarized, the inclusions can be distinguished as black and the matrix phase (steel) as white. By image processing, the equivalent circle diameter of each inclusion and the number of inclusions are calculated, and the number of inclusions with an equivalent circle diameter of 3.0 μm or more is divided by the field area where the inclusions are measured to obtain the inclusion density. The same operation is performed on the cross-sectional SEM images of 10 fields, and the arithmetic mean of the inclusion densities obtained for each field is evaluated.

[0040] 3. Tensile strength The high-strength steel plate according to the embodiment of the present invention has a tensile strength of 1700 MPa or more. The tensile properties are measured according to the method specified in JIS Z 2241:2011 by taking a JIS No. 5 tensile test piece from the steel plate so that the direction perpendicular to the rolling direction of the steel plate is the longitudinal direction. In the examples, those with a tensile strength of 1700 MPa or more were evaluated as having high strength.

[0041] 4. Flatness By satisfying the above characteristics, a high-strength steel plate with a flatness of less than 40 mm can be obtained. The "flatness" in this specification is obtained by cutting out a plate material with a length of 500 mm in the rolling direction from the coil of the steel plate, installing it on a surface plate so that the warp of the end face faces upward, and measuring the warp height from the surface plate to the steel plate surface at the location where the warp height is maximum with a scale. The smaller the value of the flatness (mm), the smaller the warp of the steel plate (the better the flatness). The flatness is preferably 35 mm or less, and more preferably 32 mm or less.

[0042] 5. Electrogalvanized Steel Sheet By applying electrogalvanization to the high-strength steel sheet according to the embodiment of the present invention, a galvanized steel sheet with excellent flatness can be obtained. When performing galvanization, the electrogalvanization method is used instead of the hot-dip galvanization method. In hot-dip galvanization, when passing the steel sheet through the hot-dip galvanizing bath, it is necessary to heat the steel sheet to a temperature of about 460 °C, for example. In order to manufacture a steel sheet with a tensile strength of about 1700 MPa, quenching is performed as described later and cooled to room temperature to about 100 °C. If hot-dip galvanization is then performed on the steel sheet, it is difficult to maintain high tensile strength and excellent flatness. On the other hand, the temperature of the plating bath in the electrogalvanization method is usually about several tens of °C to 100 °C, for example, 50 to 60 °C. That is, since electrogalvanization can be performed at a relatively low temperature, deterioration of the flatness of the steel sheet and a decrease in tensile strength are unlikely to occur. As a result, an electrogalvanized steel sheet with excellent flatness and a high strength of 1700 MPa or more can be obtained.

[0043] 4. Manufacturing Method Next, a recommended manufacturing method for the high-strength steel sheet and the electrogalvanized steel sheet according to the embodiment of the present invention will be described. The inventors have found that by performing the annealing treatment described in detail below on rolled materials such as hot-rolled steel sheets and cold-rolled steel sheets having the above component compositions, a high-strength steel sheet having the above-described desired metal structure and exhibiting desired properties can be obtained. Hereinafter, the recommended manufacturing method will be described in detail.

[0044] In the manufacture of the steel sheet, general conditions can be adopted except for the above annealing treatment. When using a cold-rolled steel sheet as the steel sheet to be heat-treated, the cold-rolled steel sheet can be obtained by melting according to a conventional method, obtaining a steel slab or the like by continuous casting, heating to about 1100 °C to 1250 °C, performing hot rolling, winding, pickling, and then cold rolling. Next, the heat treatment to be performed will be described in detail below."

[0045] [Annealing Treatment] The steel sheet is heated on a continuous annealing line at an annealing temperature T1 between the Ac3 transformation point and 950°C. By completely reversing the structure of the steel sheet through heating, a structure with a martensitic structure of 95% or more area can be reliably obtained in the subsequent quenching treatment. The holding time t1 in the temperature range between the Ac3 transformation point and 950°C can be appropriately determined according to the annealing temperature T1, for example, between 30 seconds and 1000 seconds.

[0046] If the annealing temperature T1 is below the Ac3 point, or if the holding time t1 is too short (e.g., less than 30 seconds), the microstructure (e.g., ferrite-pearlite) of the rolled material subjected to heat treatment, such as hot-rolled steel sheet, may remain. As a result, even if a quenching process is performed afterward, a martensite-dominant microstructure may not be obtained, making it difficult to reliably obtain a tensile strength of 1700 MPa or more. If the annealing temperature T1 exceeds 950°C, or if the holding time t1 is too long (e.g., more than 1000 seconds), the grain size becomes coarse, which is disadvantageous in terms of strength and toughness, and furthermore, the equipment load increases, making it economically inferior.

[0047] The annealing temperature T1 is preferably between (Ac3 transformation point + 30°C) and 930°C. The holding time t1 is preferably between 100 seconds and 900 seconds, and more preferably between 100 seconds and 800 seconds.

[0048] The Ac3 point is calculated using the following formula (1) (see formula (VII-20) on page 273 of William C. Leslie's Iron and Steel Materials Science (1985): Ac3 (°C) = 910 - 203 × [°C] １／２ -15.2 × [Ni] + 44.7 × [Si] + 10⁴ × [V] + 31.5 × [Mo] + 13.1 × [W] - 30 × [Mn] - 11 × [Cr] - 20 × [Cu] + 700 × [P] + 400 × [Al] + 120 × [As] + 400 × [Ti] ... (1) In equation (1), [element name] indicates the mass percentage content of each element in the steel, and elements that are not present are calculated as zero.

[0049] [Quenching Treatment] Next, the material is cooled from the annealing temperature T1 to the quenching start temperature T2, and then quenching is performed by rapid cooling (water cooling) from the quenching start temperature T2 to the cooling stop temperature T3. A martensite-dominant structure, i.e., a metal structure in which the proportion of martensite structure is 95 area % or more, is obtained. The quenching start temperature T2 should be 600°C or higher. The cooling stop temperature T3 is generally 100°C or lower when the cooling method is water cooling. The effects of the present invention will be achieved even if the lower limit of the cooling stop temperature T3 is not specifically defined, but since setting the cooling stop temperature T3 below room temperature would be economically burdensome, room temperature is practically the lower limit.

[0050] During water quenching, tension is applied to the steel plate. This improves the flatness of the steel plate. The tension applied should be 10 MPa or more, preferably 15 MPa. If the tension is too low, the steel plate will bend while water-cooled, making it difficult to stably ensure the flatness of the steel plate. The upper limit of the tension applied is, for example, 40 MPa.

[0051] The average cooling rate during water cooling is generally 50°C / second or higher. If the cooling rate is slower than this, ferrite will precipitate during cooling, preventing the acquisition of a single-phase martensite structure and making it impossible to secure a tensile strength of 1700 MPa or higher. Preferably, the average cooling rate during water cooling is 100°C / second or higher.

[0052] [Tempering Treatment] After the quenching treatment, a tempering treatment is performed. By appropriately setting the temperature range for this tempering treatment, both tensile strength (TS) and bendability can be improved. In the tempering treatment, first, the material is heated (reheated) to the tempering temperature T4. In order to actively create nuclei for carbide precipitation, the tempering temperature T4 is set to 180°C or higher and 250°C or lower, and the holding time t4 in this temperature range is set to 50 seconds or higher. If the tempering temperature T4 is excessively high, excessive carbide precipitation will occur, and the bendability of the steel sheet will decrease. The tempering temperature T4 is preferably 200°C or higher and 240°C or lower.

[0053] From the cooling stop temperature T3 to the tempering temperature T4, heating is performed at an average reheating rate of 1.0 °C / second or more. When the average reheating rate is less than 1.0 °C / second, the carbides precipitated during tempering become coarsened, which is not preferable. The average reheating rate is preferably 5.0 °C / second or more. Although the upper limit of the average reheating rate is not particularly provided, for example, it can be 250 °C / second.

[0054] When the holding time t4 is less than 70 seconds, the diffusion of C becomes insufficient, so a sufficient effect cannot be obtained. Therefore, the holding time t4 is set to be 70 seconds or more, preferably more than 100 seconds, more preferably more than 240 seconds, and even more preferably more than 360 seconds. Also, since long-time holding at the tempering temperature T4 is economically disadvantageous, the holding time t4 is less than 1000 seconds, preferably less than 800 seconds, and more preferably less than 600 seconds. Then, it is cooled to a temperature below 100 °C (for example, room temperature). The average cooling rate during this cooling is preferably 20 °C / second or less, and for example, it can be set to 10 °C / second

[0055] In this way, the steel sheet according to the embodiment of the present invention can be manufactured.

[0056] [Plating treatment] By subjecting the obtained steel sheet to electro-galvanizing according to a conventional method, an electro-galvanized steel sheet can be manufactured. When performing electro-galvanizing, the steel sheet after quenching and tempering treatment is immersed in a zinc plating solution at 50 to - 60 °C and energized to perform the electro-galvanizing treatment. The plating adhesion amount is not particularly limited, and for example, it may be about 10 to 100 g / m per side. ２ The corrosion resistance of the steel sheet is improved by performing the electro-galvanizing treatment.

[0057] Steel grades A to M having the component compositions shown in Table 1 were melted. Specifically, after primary refining in a converter, desulfurization was carried out in a ladle. Also, if necessary, after ladle refining, a vacuum degassing treatment by the RH method was carried out. Then, continuous casting was carried out by a conventional method to obtain a slab. After hot rolling, pickling and cold rolling were sequentially carried out by a conventional method to obtain a cold-rolled steel sheet. Next, continuous annealing was carried out.

[0058] Using steel grades A to M, annealing was performed at the annealing temperature T1 shown for steel plates No. 1 to 41 and 43 to 49 in Table 2, and steel plate No. 42 in Table 3, with a holding time t1 of 100 to 200 seconds. After annealing, the steel plates were cooled to the quenching start temperature T2 at an average cooling rate of 10°C / second. Next, the steel plates were quenched by rapid cooling (water cooling) from the quenching start temperature T2 to the cooling stop temperature T3 (room temperature) at an average cooling rate of 200 to 400°C / second. The tension applied to the steel plates during the quenching process was in the range of 17 to 22 MPa. Furthermore, the steel plates were heated from the cooling stop temperature T3 to the tempering temperature T4 at an average reheating rate of 2°C / second, and tempered by holding time t4 at the tempering temperature T4 for 300 to 500 seconds to produce the steel plates.

[0059] The obtained steel plates were used to evaluate various properties under the conditions shown below.

[0060] [Tensile Strength] Tensile properties were measured by taking JIS No. 5 tensile test specimens from the steel sheet so that the longitudinal direction was perpendicular to the rolling direction of the steel sheet, and measuring according to the method specified in JIS Z 2241:2011. The results are shown in Table 2 below.

[0061] [Flatness] A sheet of steel with a length of 500 mm in the rolling direction was cut from a coil of steel plate. It was placed on a surface plate with the end face curvature facing upwards, and the point where the curvature height from the surface plate to the surface of the steel plate was maximum was measured with a scale and is shown in Table 2. If the curvature height (mm) was less than 40 mm, it was evaluated as having excellent flatness, and if it was 40 mm or more, it was evaluated as having poor flatness. Note that flatness measurement was not performed for No. 42.

[0062] Next, the inclusion density and bendability were evaluated for steel plates No. 2, 6, 13, and 42-49.

[0063] [Inclusion Density] After polishing an L-shaped section (a perpendicular section parallel to the rolling direction) passing approximately through the center of the width direction of the steel plate, the area from the surface of the steel plate to a position (plate thickness × 0.1) in the thickness direction (surface layer) was observed at a magnification of 400x using cross-sectional SEM. Ten fields of view were acquired by changing the imaging position. The cross-sectional SEM images were image-processed and binarized to distinguish between inclusions (black) and matrix (white). The equivalent circle diameter and number of inclusions for each inclusion were calculated, and the number of inclusions with an equivalent circle diameter of 3.35 μm or more was divided by the measured field of view area to determine the inclusion density. The same operation was performed for the cross-sectional SEM images of the ten fields of view, and the arithmetic mean of the inclusion densities obtained for each field of view was calculated.

[0064] [Bendability] The bendability of the steel plate was evaluated according to the following procedure. A test specimen with a width of 40 mm and a length of 100 mm was prepared with the long axis perpendicular to the rolling direction. Bending tests were performed using the V-block method in accordance with JIS Z 2248:2014, and the bending radius was varied. The minimum bending radius at which the test specimen could be bent without fracture was determined and defined as the limit bending radius R (mm). The limit bending radius R (mm) / plate thickness t (mm) was calculated and used as the index of bendability (R / t). When R / t was 3.3 or less, it was evaluated as having excellent bendability, and when it exceeded 3.3, it was evaluated as having poor bendability.

[0065] These measurement results are discussed below. Steel plates No. 1-8 and 43-49 used steel grade A or M (Table 1) and were examples that satisfied all the requirements (composition and manufacturing conditions) of the embodiment of the present invention (Table 2). As a result, as shown in Table 2, they achieved high tensile strength and good flatness. Furthermore, as shown in Table 3, steel plates No. 2, 6, and 43-49, which used steel grade A, had low inclusion density in the surface layer and good bendability.

[0066] Steel sheets No. 9 to 42 were comparative examples that did not satisfy the requirements of the embodiments of the present invention. Steel type B had a high Ca content. Therefore, as shown in Table 3, the obtained steel sheets No. 13 and 42 had a high inclusion density in the surface layer and poor bendability. It is presumed that other steel sheets using steel type B, as well as steel sheets using steel types C to D with high Ca content, would similarly have poor bendability. In addition, steel sheets No. 11, 12, and 15 had a high tempering temperature T4 and were excessively tempered, resulting in low tensile strength.

[0067] Steel grades E to L had low carbon and titanium content, resulting in steel plates No. 24 to 41 made from these grades having low tensile strength, and some of these steel plates also exhibited poor flatness.

[0068]

[0069]

[0070]

[0071] This application is based on a priority claim to Japanese Patent Application No. 2024-159380, filed on 13 September 2024, and Japanese Patent Application No. 2025-119070, filed on 15 July 2025. Japanese Patent Applications No. 2024-159380 and No. 2025-119070 are incorporated herein by reference.

Claims

1. A high-strength steel sheet containing C: 0.30% by mass or more and 0.50% by mass or less, Si: 0% by mass or more and 1.50% by mass or less, Mn: greater than 0.10% by mass and 3.50% by mass or less, P: 0% by mass or more and 0.020% by mass or less, S: 0% by mass or more and 0.010% by mass or less, Al: 0.001% by mass or more and 1.000% by mass or less, Cr: greater than 0% by mass and 0.30% by mass or less, Ti: 0.08% by mass or more and 0.15% by mass or less, B: 0.0001% by mass or more and 0.0050% by mass or less, with the remainder being Fe and unavoidable impurities, the proportion of martensitic structure in the total metallic structure being 95% by area or more, and a tensile strength of 1700 MPa or more.

2. The high-strength steel plate according to claim 1, wherein the flatness is less than 40 mm.

3. A high-strength steel plate according to claim 1, satisfying one or more of the following (a) to (d): (a) Ca: further containing more than 0% by mass and 0.0010% by mass or less, and in a cross-sectional view, the number density of inclusions with an equivalent circular diameter of 3.35 μm or more in the surface layer from the surface to a depth of (plate thickness × 0.1) is 7.0 pieces / mm ２ The following further contain: (b) at least one selected from the group consisting of Cu: greater than 0% by mass and 1.00% by mass or less and Ni: greater than 0% by mass and 1.00% by mass or less; (c) at least one selected from the group consisting of V: ​​greater than 0% by mass and 0.1% by mass or less, Nb: greater than 0% by mass and 0.1% by mass or less, and Mo: greater than 0% by mass and 0.5% by mass or less; (d) at least one selected from the group consisting of Mg: greater than 0% by mass and 0.005% by mass or less, and REM: greater than 0% by mass and 0.005% by mass or less.

4. An electro-galvanized steel sheet comprising a high-strength steel sheet according to any one of claims 1 to 3 and an electro-galvanized layer covering the surface of the high-strength steel sheet.

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