Steel sheets and electro-galvanized steel sheets

Abstract

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

Description

[Technical Field]

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

[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 the worsening problem of global warming, 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. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2018-109222 [Patent Document 2] Japanese Patent Publication No. 2021-113353 [Patent Document 3] International Publication No. 2023 / 063288 [Patent Document 4] Patent No. 7323096 [Overview of the project] [Problems that the invention aims to solve]

[0006] In high-strength steel sheets with a tensile strength of 1700 MPa, the flatness of the steel sheet deteriorates significantly, and warping can occur. Warped steel sheets require flattening with a leveler before press forming or other processes, which increases the manufacturing cost of 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. [Means for solving the problem]

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

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

[0011] A third aspect of the present invention is: The high-strength steel plate is described in embodiment 1 or 2, satisfying one or more of the following conditions (a) to (d). (a) Ca: Further contains more than 0% by mass and 0.0010% by mass or less, 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 below (b) Further containing 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) Further containing 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) Further containing 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 This is an electro-galvanized steel sheet comprising a high-strength steel sheet according to any one of embodiments 1 to 3 and an electro-galvanized layer covering the surface of the high-strength steel sheet.

Advantages of the Invention

[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.

Modes for Carrying Out the Invention

[0014] In order to solve the above problems, the inventors intensively studied to improve the flatness of a steel sheet mainly composed of a martensite structure capable of achieving a high strength of 1700 MPa or more. First, as a result of examining the causes of warpage in high-strength steel sheets with a tensile strength of 1700 MPa or more from various viewpoints, although it does not limit the technical scope of the present invention, it was estimated that the mechanism would be as follows. In water quenching carried out to increase the strength, the steel contracts due to a rapid temperature change when the high-temperature steel sheet comes into contact with water (generation of thermal strain), and subsequently volume expansion of the steel occurs due to martensite transformation (generation of transformation strain). When the time from the occurrence of thermal strain to the occurrence of transformation strain is extremely short, that is, when contraction and expansion occur almost simultaneously in the steel sheet, it is considered that the flatness of the high-strength steel sheet deteriorates significantly. Although the reason is unknown, it is assumed that when contraction and expansion occur almost simultaneously in the steel sheet, the stress distribution in the steel sheet becomes disordered and stress homogenization cannot be achieved. If this estimated mechanism is correct, by lowering the temperature (Ms point) at which transformation strain occurs, the time required to cool to the Ms point becomes longer, so the time from the occurrence of thermal strain to the occurrence of transformation strain can be extended, and as a result, it is considered that the warpage of the steel sheet can be reduced.

[0015] The inventors further researched methods to lower the Ms point while maintaining high tensile strength and concluded that controlling the composition (especially increasing the amounts of carbon and titanium) is effective. In particular, titanium not only lowers the Ms point but also increases 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 its 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, steel sheets containing an appropriate amount of Ti (0.08 mass% to 0.15 mass%) can be expected to have two effects: a lower Ms point which suppresses warping of the steel sheet, and increased high-temperature strength which also suppresses warping of the steel sheet. Based on these findings, the inventors have completed a high-strength steel plate 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, as well as controlling the component composition.

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

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

[0019] [C: 0.30 mass% or more and 0.50 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 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 mass% or less. Preferably, the amount of C is 0.45 mass% or less, and more preferably 0.40 mass% or less.

[0020] [Si: 0 mass% or more and 1.50 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 can be 0 mass%, but in order to achieve the above effects, it is preferable to contain 0.02 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 mass% or less. The amount of Si is preferably 0.20 mass% or less, more preferably 0.10 mass% or less, and even more preferably 0.05 mass% or less.

[0021] In this specification, "0% by mass or more" means that the content of the element in question is included in embodiments in which the element is not intentionally added, for example, in the case of content at an unavoidable impurity level (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 in question has been intentionally added.

[0022] [Mn: more than 0.10 mass% and 3.50 mass% or less] Mn is an effective element for improving hardenability and increasing strength. To effectively achieve these effects, the amount of Mn should be greater than 0.10 mass%. Preferably, the amount of Mn is 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. Preferably, the amount of Mn is 2.00 mass% or less, more preferably 1.80 mass% or less, and even more preferably 1.50 mass% or less.

[0023] [P: 0 mass% or more and 0.020 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 due to constraints in the manufacturing process, a trace amount (for example, about 0.001% by mass or more) may be present.

[0024] [S: 0 mass% or more and 0.010 mass% or less] S generates sulfide-based inclusions, degrading the workability and weldability of the base material. Therefore, the amount of S should be 0.010 mass% or less. Preferably, the amount of S is 0.005 mass% or less, and more preferably 0.003 mass% or less. Since the amount of S is preferable as low as possible, the lower limit is 0 mass%, but due to constraints in the manufacturing process, trace amounts (for example, about 0.001 mass% or more) may be present.

[0025] [Al: 0.001 mass% or more and 1.000 mass% or less] Al is useful as a deoxidizing element and also useful for fixing solid-solution nitrogen present in steel as AlN. 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: more than 0 mass% and 0.30 mass% or less] Cr is an effective element for increasing strength by improving hardenability. Furthermore, Cr is an effective element for increasing the tempering softening resistance of martensitic steel. To fully realize these effects, the Cr content should be greater than 0 mass%. Preferably, the Cr content is 0.01 mass% or more. However, excessive Cr content degrades delayed fracture resistance. Therefore, the Cr content should be 0.30 mass% or less. Preferably, the Cr content is 0.15 mass% or less.

[0027] [Ti: 0.08 mass% or more and 0.15 mass% or less] Ti is an important element in this invention, and is effective in improving strength and delayed fracture resistance through γ grain refinement. 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 this 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, by including Ti, the timing of martensitic transformation can be delayed, 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 should be 0.08 mass% or more. Preferably, the amount of Ti should be 0.09 mass% or more. However, if Ti is included in excess, the precipitation of carbonitrides and the like increases, reducing workability. Therefore, the amount of Ti is set to 0.15% by mass or less, preferably 0.12% by mass or less.

[0028] [B: 0.0001 mass% or more and 0.0050 mass% or less] B is an effective element for improving hardenability. To fully realize this effect, the amount of B should be 0.0001% by mass or more. Preferably, the amount of B should be 0.0005% by mass or more. However, since an excess of B reduces ductility, 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 is 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. Furthermore, for example, elements like sulfur (S) are generally preferable in lower amounts and are therefore unavoidable impurities, but their composition range is specified separately as described above. For this reason, in this specification, the "unavoidable impurities" that constitute the remainder are a concept that excludes elements whose composition range is specified separately.

[0030] Other selective elements Furthermore, in another preferred embodiment of the present invention, elements other than those described above may be included as needed, provided that the effects of the embodiment of the present invention are not impaired. Examples of such selective elements are shown below.

[0031] [Ca: more than 0 mass% and 0.0010 mass% or less] 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 present 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 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] Cu and Ni are effective elements for improving delayed fracture resistance by enhancing 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 adding Cu, the amount of Cu is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.08% by mass or more. On the other hand, if Cu is present in excess, the pickling properties and chemical treatment properties will deteriorate, so the amount of Cu may be 1.00% by mass or less, preferably 0.50% by mass or less, and more preferably 0.20% by mass or less. When Ni is added, the amount of Ni is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.08% by 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% by mass or less, preferably 0.50% by mass or less, and more preferably 0.20% by mass or less.

[0033] [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] V, Nb, and Mo are all elements that are effective in improving strength and toughness after quenching by refining γ grains. 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 V, Nb, and Mo are added, the amount 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] [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] Mg and REM are elements that are effective in refining inclusions and improving processability. To achieve this effect, at least one element 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 adding Mg and REM, their respective content is preferably 0.0010% by mass or more, and more preferably 0.0015% by mass or more. On the other hand, if Mg and REM are present in excess, the ductility decreases. Therefore, when adding Mg and REM, their respective content is preferably 0.005% by mass or less, and more preferably 0.003% by mass or less.

[0035] 2.Metal structure The microstructure of the high-strength steel sheet of the present invention is characterized by having a martensitic structure proportion of 95% or more by area. 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 tissue accounts for 95% or more of the area] In this 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. Preferably, the proportion of martensitic structure is 97 area % or more, and may be 100 area %. In addition to the martensitic structure described above, the high-strength steel sheet of the present invention may also include structures that are inevitably present during the manufacturing process, such as ferrite structures, bainite structures, and retained austenite structures.

[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 Below] If there are many relatively coarse inclusions near the surface of a steel sheet, cracks are more likely to occur during bending, starting from these inclusions, thus degrading the bendability of the steel sheet. From the perspective 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 / mm². 2 The following is preferable. The measurement range was set to the surface layer because the influence of inclusions present within the thickness of the plate on bendability is small. The inclusion density in the surface layer is more preferably 5.0 particles / mm³. 2 The following, and more preferably 3.0 pieces / mm 2 The following, and especially preferably 1.0 pieces / mm 2 The following applies:

[0039] The density of inclusions in the surface layer is measured using the following procedure. After polishing an L-shaped section (a perpendicular section parallel to the rolling direction) passing approximately through the center of the steel plate in the width direction, the area from the surface of the steel plate to a position (plate thickness × 0.1) in the thickness direction (surface layer) is observed at a magnification of 400x using a cross-sectional SEM. Ten cross-sectional SEM images are acquired by changing the imaging position. When the cross-sectional SEM images are processed and binarized, inclusions can be distinguished as black and the matrix (steel) as white. The equivalent circle diameter and number of inclusions are calculated through image processing, and the number of inclusions with an equivalent circle diameter of 3.0 μm or more is divided by the measured field of view area to determine the inclusion density. The same operation is 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 is evaluated.

[0040] 3. Tensile strength The high-strength steel plate according to an embodiment of the present invention has a tensile strength of 1700 MPa or more. Tensile properties are measured by taking a JIS No. 5 tensile test specimen from a steel sheet so that the longitudinal direction is perpendicular to the rolling direction of the steel sheet, and measuring it according to the method specified in JIS Z 2241:2011. In the examples, specimens with a tensile strength of 1700 MPa or higher were evaluated as 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. In this specification, "flatness" is determined by cutting a 500mm long sheet of steel from a coil, placing it on a surface plate with the end face curvature facing upwards, and measuring the point where the curvature height from the surface plate to the steel plate surface is maximum using a scale. A smaller flatness (mm) value indicates less curvature of the steel plate (better flatness). The flatness is preferably 35 mm or less, and more preferably 32 mm or less.

[0042] 5. Electro-galvanized steel sheet By electro-galvanizing a high-strength steel sheet according to an embodiment of the present invention, a galvanized steel sheet with excellent flatness can be obtained. Furthermore, when performing zinc plating, electro-galvanizing is used instead of hot-dip galvanizing. In hot-dip galvanizing, the steel sheet needs to be heated to a temperature of, for example, 460°C before being passed through the hot-dip galvanizing bath. To produce a steel sheet with a tensile strength of 1700 MPa, water quenching is performed as described later, and the sheet is cooled to room temperature to about 100°C. If hot-dip galvanizing 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 electrogalvanizing method is typically around several tens of degrees Celsius to 100 degrees Celsius, for example, 50 to 60 degrees Celsius. In other words, because electrogalvanizing can be carried out at relatively low temperatures, deterioration of the flatness of the steel sheet and a decrease in tensile strength are unlikely to occur. As a result, electrogalvanized steel sheets with excellent flatness and high strength of 1700 MPa or more can be obtained.

[0043] 4. Manufacturing method Next, a recommended manufacturing method for high-strength steel sheets and electro-galvanized steel sheets according to the embodiments 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-mentioned component composition, a high-strength steel sheet having the desired metallic structure and exhibiting the desired properties can be obtained. The recommended manufacturing method will be described in detail below.

[0044] In the manufacture of steel sheets, general conditions can be adopted, except for the annealing treatment described above. When cold-rolled steel sheets are used as the steel sheets to be subjected to heat treatment, these cold-rolled steel sheets can be obtained by melting them according to conventional methods, obtaining steel billets such as slabs by continuous casting, heating them to approximately 1100°C to 1250°C, hot rolling, coiling, pickling, and cold rolling. The subsequent heat treatment will be described in detail below.

[0045] [Annealing treatment] In a continuous annealing line, the steel sheet is heated at an annealing temperature T1 between the Ac3 transformation point and 950°C. By completely reversing the microstructure of the steel sheet through heating, a microstructure with a martensitic structure of 95% or more area can be reliably obtained in the subsequent quenching process. The holding time t1 in the temperature range above the Ac3 transformation point and below 950°C can be appropriately determined according to the annealing temperature T1, for example, 30 seconds to 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 higher. If the annealing temperature T1 exceeds 950°C, or if the holding time t1 is too long (for example, more than 1000 seconds), the grain size becomes coarser, which is disadvantageous in terms of strength and toughness, and also increases the equipment load, making it economically uneconomical.

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

[0048] The Ac3 point is calculated using the following formula (1) (refer to 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] [Heat 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 with a martensite structure of 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 applied tension 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 applied tension 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 quenching, 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, the material is first heated (reheated) to the tempering temperature T4. In order to actively create carbide precipitation nuclei, the tempering temperature T4 should be between 180°C and 250°C, and the holding time t4 in this temperature range should be 50 seconds or more. 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 between 200°C and 240°C.

[0053] The material is heated from the cooling stop temperature T3 to the tempering temperature T4 at an average reheating rate of 1.0°C / second or higher. An average reheating rate of less than 1.0°C / second is undesirable because it causes the carbides precipitated during tempering to become coarser. The average reheating rate is preferably 5.0°C / second or higher. There is no particular upper limit set for the average reheating rate, but it can be, for example, 250°C / second.

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

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

[0056] [Plating process] By applying electro-galvanizing to the obtained steel sheet in accordance with 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 galvanizing solution at 50 to 60 °C and energized to perform electro-galvanizing treatment. The plating adhesion amount is not particularly limited. For example, it may be about 10 to 100 g / m per side. 2 That's fine. By performing electro-galvanizing treatment, the corrosion resistance of the steel sheet is improved.

Example

[0057] Steel grades A to M with 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, 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 carried out at the annealing temperature T1 shown in Steel Sheet Nos. 1 to 41 and 43 to 49 in Table 2 and Steel Sheet No. 42 in Table 3 with a holding time t1 of 100 to 200 seconds. Then, it was cooled at an average cooling rate of 10 °C / second to the quenching start temperature T2. Next, it was quenched (water-cooled) at an average cooling rate of 200 to 400 °C / second from the quenching start temperature T2 to the cooling stop temperature T3 (room temperature) for quenching treatment. The tension applied to the steel sheet during the quenching treatment was in the range of 17 to 22 MPa. Further, it was heated at an average reheating rate of 2 °C / second from the cooling stop temperature T3 to the tempering temperature T4, and tempering treatment was carried out with a holding time t4 of 300 to 500 seconds at the tempering temperature T4 to produce a steel sheet.

[0059] Using the obtained steel sheets, evaluations of various properties were carried out under the following conditions.

[0060] [Tensile strength] For the tensile properties, a JIS No. 5 tensile test piece was sampled from the steel sheet such that the direction perpendicular to the rolling direction of the steel sheet was the longitudinal direction, and it was measured 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 steel plate surface 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. 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 steel plate's width, the surface layer (from the surface of the steel plate to a position (plate thickness × 0.1) was observed at a magnification of 400x using a cross-sectional SEM. Ten cross-sectional SEM images 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 were calculated for each inclusion. 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 procedure was performed for the cross-sectional SEM images from all 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 using 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 will be 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 embodiments 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 plates 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 calcium content. As a result, as shown in Table 3, steel plates No. 13 and 42 had a high density of inclusions in the surface layer and poor bendability. It is presumed that other steel plates using steel type B, as well as steel plates using steel types C to D with high calcium content, will similarly have poor bendability. Furthermore, steel plates No. 11, 12, and 15 had low tensile strengths because they were excessively tempered due to a high tempering temperature T4.

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

[0068] [Table 1]

[0069] [Table 2]

[0070] [Table 3]

Claims

1. 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: more than 0.10% by mass and not more than 3.50% by mass, 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: more than 0% by mass and not more than 0.30% by mass, 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, It contains, with the remainder being Fe and unavoidable impurities. The proportion of martensitic structure in the total metallic structure is 95% or more by area. High-strength steel plate with 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, 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 below (b) Further containing 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) Further containing 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) Further containing 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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