High-strength cold-rolled steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized steel sheet, and methods for producing the same

Abstract

A high-strength cold-rolled steel sheet according to one aspect of the present invention contains, by mass%, 0.070-0.140% C, 0.80-1.80% Si, and 1.80-2.80% Mn, with the remainder being iron and inevitable impurities. The total of bainite, tempered martensite, and hard phases, as measured by area ratio in the structure observed using a scanning electron microscope, accounts for 85% or more. The hard phase comprises a composite structure of quenched martensite and retained austenite. The present invention relates to a composite material comprising at least one of MA and cementite, wherein the hard phase has a short axis of less than 0.4 μm and a long axis of greater than 1.2 μm, accounting for more than 0.5% of the hard phase, the hard phase has a short axis of greater than 1.2 μm, accounting for less than 5.0% of the hard phase, the bainite, the tempered martensite and the structure other than the hard phase, accounting for less than 15%, and the retained austenite, measured by volume fraction using an X-ray diffraction method, is greater than 3.0% and less than 7.0%.

Description

Technical Field

[0001] The present invention relates to a high-strength cold-rolled steel sheet, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet using the same, and a method for producing these steel sheets. Background Art

[0002] In recent years, as automotive and transportation machinery parts have become increasingly stronger, the steel sheets used in these parts have also become stronger. However, with these high-strength steel sheets, workability, such as ductility and stretch-flangeability, decreases as the strength increases. Consequently, it has become difficult to press-form complex-shaped parts using high-strength steel sheets, such as those for automotive and transportation machinery.

[0003] Furthermore, these high-strength steel sheets have reduced local ductility, such as bendability and stretch-flangeability. Consequently, if these high-strength steel sheets are used in vehicle body structural components that absorb impact energy during collisions, they may break during collision deformation.

[0004] In order to solve these problems, it is necessary to develop high-strength steel sheets that are excellent in workability and resistance to cracking during collision deformation (hereinafter referred to as “collision characteristics”).

[0005] When automotive steel sheets are used in vehicle body structural components to absorb impact energy, they must be resistant to cracking when subjected to high loads and high-speed deformation caused by collisions. Such components experience severe bending deformation due to buckling during deformation. Therefore, high-strength steel sheets with excellent local ductility, such as bendability and stretch-flangeability, are considered suitable for use in such components.

[0006] As documents that disclose technologies related to the above-mentioned properties of steel sheets, for example, Patent Document 1 and Patent Document 2 can be cited.

[0007] Patent Document 1 discloses a high-strength plated steel sheet having excellent workability, including elongation, bendability, and hole expandability (stretch-flange formability), wherein, when the metallographic structure is observed using a scanning electron microscope, the sheet comprises a low-temperature transformation-generated phase accounting for 70% by area or more of the entire metallographic structure, and a pearlite, quenched martensite, and MA mixed phase (a composite phase of quenched martensite and retained austenite) accounting for 15% by area or less of the entire metallographic structure.

[0008] Patent Document 2 discloses a high-strength cold-rolled steel sheet having excellent workability (evaluated by ductility and stretch flangeability) and excellent crash characteristics, wherein the MA structure composed of quenched martensite and retained austenite has an equivalent circle diameter of 2.0 μm or less and an area ratio of the MA structure of V MA Volume fraction V relative to retained austenite γ V MA / Vγ Satisfy 0.50≦V MA / V γ ≦1.50 high strength cold rolled steel plate.

[0009] However, the inventors of the present application have conducted studies and have concluded that the steel sheets disclosed in Patent Documents 1 and 2 have room for further improvement in stretch flange formability and bendability.

[0010] The present invention has been made in view of the above-mentioned circumstances, and its object is to provide a high-strength cold-rolled steel sheet having excellent stretch-flange formability and bendability, as well as a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the high-strength cold-rolled steel sheet. Furthermore, the present invention also aims to provide a method for manufacturing these high-strength cold-rolled steel sheets, hot-dip galvanized steel sheets, and alloyed hot-dip galvanized steel sheets.

[0011] Prior art literature

[0012] Patent Literature

[0013] Patent Document 1: Japanese Patent No. 6085348

[0014] Patent Document 2: Japanese Patent No. 6554396 Summary of the Invention

[0015] The present inventors have conducted various studies and found that the above-mentioned objects can be achieved by the following invention.

[0016] A high-strength cold-rolled steel sheet according to one aspect of the present invention contains, by mass%, 0.070% to 0.140% C, 0.80% to 1.80% Si, and 1.80% to 2.80% Mn, with the balance being iron and inevitable impurities. The steel sheet comprises 85% or more of bainite, tempered martensite, and a hard phase, in terms of area ratio, in a structure observed using a scanning electron microscope. The hard phase comprises at least one of MA (a composite of quenched martensite and retained austenite), and cementite. The hard phase comprises at least 0.5% of a hard phase having a minor axis of 0.4 μm or less and a major axis of 1.2 μm or greater, and at most 5.0% of a hard phase having a minor axis of 1.2 μm or greater. The structure other than the bainite, tempered martensite, and hard phase comprises at most 15%. The retained austenite comprises at most 3.0% and at most 7.0%, in terms of volume ratio, as measured by X-ray diffraction.

[0017] Another aspect of the present invention relates to a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet comprising: the high-strength cold-rolled steel sheet described above; and a hot-dip galvanized layer or an alloyed hot-dip galvanized layer provided on a surface of the high-strength cold-rolled steel sheet.

[0018] In addition, another aspect of the present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet, which includes the following steps in sequence: a rolling step of sequentially hot-rolling and cold-rolling a steel slab having the above-mentioned composition; a heating step of heating a steel sheet obtained by cold-rolling the steel slab to a temperature range of (Ac3 point + 200°C) or lower at a heating rate of 1.5°C / s to 30°C / s at a temperature of 700°C or higher; a soaking step of holding the steel sheet after the heating step for 10 seconds to 100 seconds; a first cooling step of cooling the steel sheet after the soaking step at a cooling rate of 10°C / s to 50°C / s to a first cooling temperature of 100°C to 410°C; a holding step of holding the steel sheet cooled to the first cooling temperature at a holding temperature of 100°C to 410°C for 10 seconds to 80 seconds; and a second cooling step of cooling the steel sheet after the holding step to room temperature at a cooling rate of 15°C / s or higher.

[0019] Another aspect of the present invention relates to a method for manufacturing a hot-dip galvanized steel sheet and a method for manufacturing an alloyed hot-dip galvanized steel sheet, wherein in the method for manufacturing a high-strength cold-rolled steel sheet, the steel sheet is subjected to a galvanizing treatment before the second cooling step, or is subjected to a galvanizing treatment and then an alloying treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram for explaining the size and shape of the hard phase specified in this embodiment.

[0021] Figure 2 This is a schematic diagram of the heating pattern of the annealing process, where (A) is the basic pattern, (B) is the pattern with a reheating process, (C) is the pattern with a plating (alloying) process, and (D) is the pattern with a reheating process and a plating (alloying) process. DETAILED DESCRIPTION

[0022] To provide a high-strength cold-rolled steel sheet with a tensile strength of 900 MPa or greater and excellent stretch-flange formability and bendability, the present inventors have conducted extensive research focusing on the steel sheet's chemical composition, structural structure, retained austenite volume fraction, and the size and shape of MA and cementite. Specifically, they believe that the structural structure of the steel sheet and the size and shape of MA and cementite dispersed within the structure affect stretch-flange formability and bendability, and conducted detailed investigations on these structures.

[0023] As a result, it was discovered that if the chemical composition, structural structure, volume ratio of retained austenite, and area ratio of MA and cementite of specified size and shape in the structure of the steel sheet respectively meet the specified necessary conditions described below, good stretch flange formability and bendability can be successfully achieved in high-strength cold-rolled steel sheet, thus completing the present invention.

[0024] According to the present invention, a high-strength cold-rolled steel sheet having excellent stretch-flange formability and bendability, a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the high-strength cold-rolled steel sheet, and an alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of the high-strength cold-rolled steel sheet can be provided.

[0025] Furthermore, according to the present invention, a method for producing these high-strength cold-rolled steel sheets, hot-dip galvanized steel sheets, and alloyed hot-dip galvannealed steel sheets can be provided.

[0026] In the following description, regarding steel sheets, hot-dip galvanized steel sheets, and alloyed hot-dip galvanized steel sheets, "high strength" means "tensile strength of 900 MPa or greater." Furthermore, "MA," as defined in this invention, stands for "Martensite-Austenite Constituent," which refers to a composite structure of quenched martensite and retained austenite.

[0027] Hereinafter, a high-strength cold-rolled steel sheet and a method for producing the same according to an embodiment of the present invention will be described.

[0028] (Metallographic structure of high-strength cold-rolled steel sheet)

[0029] The high-strength cold-rolled steel sheet according to this embodiment has a combined area ratio of bainite, tempered martensite, and a hard phase of 85% or more in the microstructure observed using a scanning electron microscope. The hard phase comprises at least one of MA (a composite of quenched martensite and retained austenite) and cementite. The hard phase has a minor axis of 0.4 μm or less and a major axis of 1.2 μm or greater, accounting for 0.5% or more, and a minor axis of 1.2 μm or greater, accounting for 5.0% or less. Microstructures other than the bainite, tempered martensite, and hard phases account for 15% or less. Retained austenite, measured by volume as measured by X-ray diffraction, is 3.0% or more and 7.0% or less. This provides a high-strength cold-rolled steel sheet with excellent stretch-flange formability and bendability.

[0030] (Hard phase)

[0031] The hard phase specified in this embodiment includes at least one of MA and cementite. Furthermore, MA is a composite of quenched martensite and retained austenite. The high-strength cold-rolled steel sheet according to this embodiment comprises, in terms of area ratio in the microstructure observed using a scanning electron microscope (SEM), a hard phase having a minor axis of 0.4 μm or less and a major axis of 1.2 μm or greater (hereinafter referred to as the "first hard phase") of 0.5% or greater, and a hard phase having a minor axis of 1.2 μm or greater (hereinafter referred to as the "second hard phase") of 5.0% or greater.

[0032] Figure 1 This is a schematic diagram illustrating the size and shape of the hard phases specified in this embodiment. In the figures below, reference numeral 1 represents a hard phase (first hard phase), and reference 2 represents a hard phase (second hard phase). In this embodiment, computer-processed images of a steel plate cross section taken using a SEM were used to measure the minor axis, major axis, and area of each hard phase in the cross section.

[0033] like Figure 1 As shown, in the structure of the high-strength cold-rolled steel sheet according to the present embodiment, a hard phase exists together with bainite or tempered martensite. Figure 1 The hard phase 1 shown has a major axis of 2.5 μm and a minor axis of 0.3 μm, and belongs to the first hard phase. The hard phase 2 has a minor axis of 1.4 μm, and belongs to the second hard phase. Figure 1 The hard phase 3 shown has a major axis of 0.9 μm and a minor axis of 0.3 μm, belonging to neither the first nor the second hard phase. The hard phase 4 has a minor axis of 0.7 μm, belonging to neither the first nor the second hard phase.

[0034] If the area ratio of the first hard phase is less than 0.5%, the stretch-flange formability and bendability of the steel sheet will decrease. The reason for this is not entirely clear, but it is generally considered as follows. Specifically, it is speculated that if the area ratio of the first hard phase is less than 0.5%, microcracks and cracks are more likely to propagate within the steel sheet structure when the steel sheet is subjected to stretch-flange deformation or bending deformation, resulting in deterioration of the steel sheet's stretch-flange formability and bendability. The area ratio of the first hard phase is preferably 0.55% or greater, and more preferably 0.60% or greater.

[0035] The reason for specifying the area ratio of the first hard phase, defined as "a hard phase with a minor axis of 0.4 μm or less and a major axis of 1.2 μm or greater," is as follows. It is believed that a hard phase with a minor axis of 0.4 μm or less can deform similarly to the surrounding tissue when the steel sheet is deformed. Furthermore, a major axis of 1.2 μm or greater is believed to be sufficiently large to act as a barrier to the propagation of microcracks and cracks within the tissue. Therefore, the area ratio of the hard phase with a minor axis of 0.4 μm or less and a major axis of 1.2 μm or greater can be used to evaluate the stretch flangeability and bendability of a steel sheet.

[0036] Furthermore, if the area ratio of the second hard phase exceeds 5.0%, the stretch-flange formability and bendability of the steel sheet decrease. The reason for this is not entirely clear, but it is generally considered as follows. Specifically, it is speculated that if the area ratio of the second hard phase exceeds 5.0%, microcracks are more likely to form when the steel sheet is subjected to stretch-flange deformation or bending deformation, resulting in deterioration of the stretch-flange formability and bendability of the steel sheet. The area ratio of the second hard phase is preferably 4.8% or less, and more preferably 4.6% or less.

[0037] The reason for specifying the area ratio of the second hard phase, or hard phase with a "minor axis of 1.2 μm or greater," is as follows. It is believed that hard phases with a minor axis of 1.2 μm or greater are unable to deform in the same manner as the surrounding structure during material deformation, thus becoming a source of microcracks. Therefore, the area ratio of the hard phase with a "minor axis of 1.2 μm or greater" can be used to evaluate the stretch flangeability and bendability of a steel sheet.

[0038] (Retained austenite)

[0039] The impact of retained austenite in the microstructure on the properties of high-strength cold-rolled steel sheets is clear. Therefore, in the high-strength cold-rolled steel sheets of this embodiment, the volume fraction of retained austenite relative to the total structure, as measured by X-ray diffraction, is set to 3.0% or more and 7.0% or less. The volume fraction of retained austenite is preferably 3.5% or more, more preferably 4.0% or more. Furthermore, the volume fraction of retained austenite is preferably 6.5% or less, more preferably 6.0% or less.

[0040] If the volume fraction of retained austenite is less than 3.0%, the effect of plasticity on ductility improvement due to retained austenite transformation during steel sheet processing is reduced, resulting in decreased ductility of the steel sheet. On the other hand, if the volume fraction of retained austenite exceeds 7.0%, the retained austenite transforms into martensite due to stress-induced working transformation, becoming a source of microcracks and cracks, thereby deteriorating the stretch flangeability and bendability of the steel sheet.

[0041] In the structure observed by SEM, retained austenite is contained in bainite and cannot be observed independently. Therefore, in the high-strength cold-rolled steel sheet according to this embodiment, the area ratio of retained austenite in the structure observed by SEM is not specified.

[0042] (Bainite, tempered martensite)

[0043] In the high-strength cold-rolled steel sheet according to this embodiment, bainite and tempered martensite constitute the basic structure of the high-strength cold-rolled steel sheet. To meet the aforementioned hard phase area ratio and retained austenite volume ratio, the combined area ratio of bainite and tempered martensite in the structure observed by SEM is 85% or greater. The combined area ratio of bainite and tempered martensite is preferably 90% or greater, more preferably 95% or greater. The high-strength steel sheet according to this embodiment may not contain both bainite and tempered martensite, but may contain only one of them.

[0044] (Other organizations)

[0045] In the high-strength cold-rolled steel sheet according to this embodiment, the total area ratio of the components other than bainite, tempered martensite, and hard phase (hereinafter referred to as the "remaining structure") in the microstructure observed by SEM is 15% or less. Examples of the main components of the remaining structure include as-quenched (As-Quenched) martensite (in this embodiment, as-quenched martensite includes autotempered martensite), as well as inevitably formed ferrite and pearlite.

[0046] The reason for setting the total area ratio of the residual structure to 15% or less is that if the steel plate contains a large amount of structures with different hardnesses, such as hard as-quenched martensite or soft ferrite, and pearlite with intermediate hardness, the hardness difference between the structures will lead to deterioration in stretch flange formability and bendability. The total area ratio of the residual structure is preferably 10% or less, and more preferably 5% or less.

[0047] (Chemical composition)

[0048] The chemical composition of the high-strength cold-rolled steel sheet according to this embodiment is not particularly limited as long as the aforementioned structural structure, the area ratio of the hard phase of the specified size and shape, and the volume ratio of retained austenite are achieved. However, since C, Si, and Mn significantly affect the strength of the steel sheet and the amount of retained austenite, the following composition ranges are specified. The reasons for specifying the composition ranges for each element are explained below. The "%" in the following description of the chemical composition represents "mass %."

[0049] (C: 0.070~0.140%)

[0050] C is an essential element for ensuring the strength of steel sheets. If the C content is insufficient, the tensile strength of the steel sheet decreases, so the C content is set to 0.070% or higher. The lower limit of the C content is preferably 0.080% or higher, more preferably 0.090% or higher. However, if the C content is excessive, the area ratio of coarse MA and the volume fraction of retained austenite increase, reducing the stretch flangeability and bendability of the steel sheet. Therefore, the upper limit of the C content is 0.140% or lower. The upper limit of the C content is preferably 0.130% or lower, more preferably 0.120% or lower, and even more preferably 0.110% or lower.

[0051] (Si: 0.80-1.80%)

[0052] Si is known as a solid solution strengthening element, effectively suppressing the decrease in ductility of steel sheets and increasing their tensile strength. It is also an element that effectively ensures the volume fraction of retained austenite. To effectively exert this effect, the Si content must be at least 0.80%. The lower limit of the Si content is preferably at least 1.10%, and more preferably at least 1.40%.

[0053] However, if the Si content is excessive, the volume fraction of retained austenite also increases, and the stretch flangeability and bendability of the steel sheet decrease. Therefore, the upper limit of the Si content is 1.80% or less. The upper limit of the Si content is preferably 1.70% or less, and more preferably 1.60% or less.

[0054] (Mn: 1.80-2.80%)

[0055] Mn is an element that contributes to the high strength of steel sheets. To effectively achieve this effect, the Mn content must be at least 1.80%. The lower limit of the Mn content is preferably at least 1.9%, more preferably at least 2.0%. However, excessive Mn content reduces the ductility and stretch flangeability of the steel sheet. Therefore, the upper limit of the Mn content is 2.80% or less. The upper limit of the Mn content is preferably at most 2.70%, more preferably at most 2.60%.

[0056] The basic composition of the high-strength cold-rolled steel sheet according to this embodiment is as described above, with the remainder being essentially iron. However, the presence of impurities inevitably introduced by the raw materials, materials, manufacturing equipment, and other factors is permitted. Examples of such inevitable impurities include phosphorus and sulfur, as described below.

[0057] (P: more than 0% and less than 0.015%)

[0058] P is an unavoidable element that segregates at grain boundaries, promoting grain boundary embrittlement and deteriorating the bendability of steel sheets. Therefore, it is recommended to minimize the P content. Therefore, the P content is preferably 0.015% or less, more preferably 0.013% or less, and even more preferably 0.010% or less. Furthermore, P is an unavoidable impurity in steel, and it is impossible to reduce its content to 0% in industrial production.

[0059] (S: more than 0% and less than 0.0050%)

[0060] Like P, S is an unavoidable element. Since it forms inclusions and reduces the stretch-flangeability of steel sheets, it is recommended to minimize the S content. Therefore, the S content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less. Furthermore, S is an impurity that inevitably enters steel, and it is impossible to reduce its content to 0% in industrial production.

[0061] In addition, as inevitable impurities contained in steel, for example, N, O, etc. are included in addition to P and S. The contents of N and O are preferably within the following ranges, respectively.

[0062] (N: more than 0% and less than 0.0100%)

[0063] Nitrogen is inevitably present as an impurity element and degrades the bendability of steel sheets. Therefore, the Nitrogen content is preferably 0.0100% or less, more preferably 0.0060% or less, and even more preferably 0.0050% or less. The lower the Nitrogen content, the better. However, achieving a 0% Nitrogen content is difficult in industrial production.

[0064] (O: more than 0% and less than 0.0020%)

[0065] O is inevitably present as an impurity element, which degrades the bendability of the steel sheet. Therefore, the O content is preferably 0.0020% or less, more preferably 0.0015% or less, and even more preferably 0.0010% or less. The lower the O content, the better. However, achieving a 0% O content is difficult in industrial production.

[0066] The high-strength cold-rolled steel sheet according to the present embodiment may contain one or more elements selected from the group consisting of Al, Cr, Ti, B, Cu, Ni, Cr, Mo, V, Nb, and Ca in the following ranges as needed. By containing these elements alone or in combination, the properties of the steel sheet can be further improved depending on the types of elements contained.

[0067] (Al: 0.015~0.60%)

[0068] Al is an element that acts as a deoxidizer. In order to effectively exert this effect, the Al content is preferably 0.015% or more, more preferably 0.025% or more, and further preferably 0.030% or more. In addition, Al is an element that effectively ensures the volume fraction of retained austenite. However, if the Al content is excessive, it may cause the Ac3 point to rise excessively and increase manufacturing costs. Therefore, the upper limit of the Al content is preferably 0.60% or less, more preferably 0.55% or less, and further preferably 0.50% or less.

[0069] (Cr: more than 0% and less than 0.60%)

[0070] Cr is an element that contributes to the high strength of the steel sheet and can be contained as needed. Its effect increases with increasing content. In order to effectively exert the above-mentioned effect, the Cr content is preferably 0.05% or more, more preferably 0.10% or more, and further preferably 0.15% or more. However, if the Cr content is excessive, unplated steel sheets or alloyed galvanized steel sheets will be left uncoated. Therefore, the upper limit of the Cr content is preferably 0.60% or less, more preferably 0.55% or less, further preferably 0.50% or less, and further more preferably 0.45% or less.

[0071] (Ti: 0.010~0.040%)

[0072] Ti is an element that forms carbides or nitrides, thereby increasing the strength of steel sheets. Furthermore, it effectively utilizes the hardenability-enhancing effect of B, described later. Specifically, Ti reduces the nitrogen content in the steel by forming nitrides. This suppresses the formation of B nitrides in the steel, maintaining B in a solid solution state, effectively utilizing the hardenability-enhancing effect of B. Thus, Ti improves hardenability, contributing to the increased strength of the steel sheet. To effectively utilize this effect, the Ti content is preferably 0.010% or greater, more preferably 0.013% or greater, and even more preferably 0.015% or greater.

[0073] However, if the Ti content is excessive, Ti carbides or Ti nitrides become excessive, which degrades the stretch flangeability of the steel sheet. Therefore, the upper limit of the Ti content is preferably 0.040% or less, more preferably 0.035% or less, and even more preferably 0.030% or less.

[0074] (B: 0.0015~0.0040%)

[0075] B is an element that improves hardenability and contributes to the high strength of steel sheets. To effectively exert this effect, the B content is preferably 0.0015% or more, more preferably 0.0020% or more, and even more preferably 0.0025% or more. However, if the B content is excessive, the effect becomes saturated, resulting in increased costs. Therefore, the B content is preferably 0.0040% or less, and more preferably 0.0035% or less.

[0076] (Cu: more than 0% and less than 0.30%)

[0077] Cu is an element that effectively improves the corrosion resistance of steel sheets and can be contained as needed. Its effect increases with increasing content. In order to effectively exert the above-mentioned effect, the Cu content is preferably 0.03% or more, more preferably 0.05% or more. However, if the Cu content is excessive, its effect is saturated and the cost increases. Therefore, the upper limit of the Cu content is preferably 0.30% or less, more preferably 0.20% or less, and further preferably 0.15% or less.

[0078] (Ni: more than 0% and less than 0.30%)

[0079] Ni is an element that effectively improves the corrosion resistance of steel sheets and can be contained as needed. Its effect increases with increasing content. In order to effectively exert the above-mentioned effect, the Ni content is preferably 0.03% or more, more preferably 0.05% or more. However, if the Ni content is excessive, its effect is saturated and the cost increases. Therefore, the upper limit of the Ni content is preferably 0.30% or less, more preferably 0.20% or less, and further preferably 0.15% or less.

[0080] (Mo: more than 0% and less than 0.30%)

[0081] Mo is an element that contributes to the high strength of steel sheets and can be contained as needed. Its effect increases with increasing content. To effectively achieve these effects, the Mo content is preferably 0.03% or more, more preferably 0.05% or more. However, if the Mo content is excessive, the effect becomes saturated and costs increase. Therefore, the upper limit of the Mo content is preferably 0.30% or less, more preferably 0.25% or less, and even more preferably 0.20% or less.

[0082] (V: more than 0% and less than 0.30%)

[0083] V is an element that contributes to the high strength of steel sheets and can be contained as needed. Its effect increases with increasing content. To effectively exert the above effects, the V content is preferably 0.005% or more, more preferably 0.010% or more. However, if the V content is excessive, the effect is saturated and costs increase. Therefore, the upper limit of the V content is preferably 0.30% or less, more preferably 0.25% or less, further preferably 0.20% or less, and even more preferably 0.15% or less.

[0084] (Nb: more than 0% and less than 0.040%)

[0085] Nb is an element that contributes to the high strength of steel sheets and can be included as needed. Its effect increases with increasing Nb content. To effectively achieve these effects, the Nb content is preferably 0.003% or higher, more preferably 0.005% or higher. However, excessive Nb content can degrade bendability. Therefore, the upper limit of the Nb content is preferably 0.040% or lower, more preferably 0.035% or lower, and even more preferably 0.030% or lower.

[0086] (Ca: more than 0% and less than 0.0050%)

[0087] Ca is an element that spheroidizes sulfides in steel, effectively improving bendability. It can be included as needed. Its effect increases with increasing content. To effectively exert these effects, the Ca content is preferably 0.0005% or higher, more preferably 0.0010% or higher. However, excessive Ca content saturates the effect and increases costs. Therefore, the upper limit of the Ca content is preferably 0.0050% or lower, more preferably 0.0030% or lower, and even more preferably 0.0025% or lower.

[0088] (Characteristics of high-strength cold-rolled steel sheets)

[0089] The high-strength cold-rolled steel sheet according to this embodiment, which satisfies the chemical composition, structural structure, volume ratio of retained austenite, and area ratio of hard phase of specified size and shape described above, has a tensile strength of 900 MPa or more and, depending on its strength grade, also has excellent ductility and stretch-flange formability.

[0090] The high-strength cold-rolled steel sheet according to the present embodiment preferably satisfies a tensile strength of 900 MPa or more and further satisfies any of the following characteristics, for example.

[0091] The elongation EL (Elongation) is preferably 12% or higher, more preferably 13% or higher. The stretch-flange formability (hole expansion ratio λ) is preferably 60% or higher, more preferably 70% or higher. The VDA bend angle is preferably 100° or higher, more preferably 105° or higher. The upper limit of the elongation EL is not particularly specified, but is typically around 18%. The higher the hole expansion ratio λ, the better the stretch-flange formability. The upper limit is not particularly specified, but is typically around 100%.

[0092] (hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets)

[0093] The hot-dip galvanized steel sheet according to this embodiment comprises: the high-strength cold-rolled steel sheet described above; and a hot-dip galvanized layer provided on the surface of the high-strength cold-rolled steel sheet. Furthermore, the alloyed hot-dip galvanized steel sheet according to this embodiment comprises: the high-strength cold-rolled steel sheet described above; and an alloyed hot-dip galvanized layer provided on the surface of the high-strength cold-rolled steel sheet.

[0094] These hot-dip galvanized steel sheets and alloyed hot-dip galvannealed steel sheets also have a tensile strength of 900 MPa or more, and excellent ductility and stretch-flangeability, similar to the high-strength cold-rolled steel sheets according to the present embodiment.

[0095] Hereinafter, the high-strength cold-rolled steel sheet, the hot-dip galvanized steel sheet, and the alloyed hot-dip galvanized steel sheet according to the present embodiment are collectively referred to as "high-strength steel sheet."

[0096] (Method for manufacturing high-strength cold-rolled steel sheet)

[0097] Next, a method for producing a high-strength cold-rolled steel sheet according to this embodiment will be described.

[0098] The method for manufacturing a high-strength cold-rolled steel sheet according to this embodiment includes a rolling step in which a steel slab is sequentially hot-rolled and cold-rolled; and an annealing step in which the steel sheet obtained by cold-rolling the steel slab is heated to a predetermined temperature range and then cooled at a predetermined rate. The high-strength cold-rolled steel sheet according to this embodiment, which meets the aforementioned requirements, is manufactured in this manufacturing method by appropriately controlling the annealing step after cold rolling.

[0099] The following describes the method for producing a high-strength cold-rolled steel sheet according to the present embodiment in the order of the rolling process (hot rolling, cold rolling) and the annealing process. Note that hot rolling and cold rolling do not necessarily need to be performed under the following conditions.

[0100] (Rolling process)

[0101] In the rolling process, the steel slab satisfying the above-mentioned composition is subjected to hot rolling and cold rolling in sequence. The conditions for hot rolling are as follows, for example.

[0102] [Hot rolling conditions]

[0103] In hot rolling, a steel slab heated to a predetermined temperature is rolled once or more until it reaches a predetermined thickness. If the heating temperature before hot rolling is low, carbides such as TiC may have difficulty dissolving in austenite. Therefore, the heating temperature before hot rolling is preferably 1200°C or higher, more preferably 1250°C or higher. However, if the heating temperature before hot rolling is too high, the cost will increase. Therefore, the upper limit of the heating temperature before hot rolling is preferably 1350°C or lower, more preferably 1300°C or lower.

[0104] In this embodiment, the slab temperature during the final hot rolling pass (finishing rolling) is referred to as the "finishing temperature." If the hot rolling finishing temperature is too low, rolling in the austenite single-phase region cannot be achieved, and deformation resistance during rolling may increase, making operation difficult. Therefore, the finishing temperature is preferably 850°C or higher, more preferably 870°C or higher. However, if the finishing temperature is too high, crystals may coarsen. Therefore, the finishing temperature is preferably 980°C or lower, more preferably 950°C or lower.

[0105] The average cooling rate from the end of finish rolling to the start of coiling during hot rolling is preferably 10°C / second or higher, and more preferably 20°C / second or higher, considering productivity. On the other hand, if the average cooling rate is too high, equipment costs increase. Therefore, the average cooling rate is preferably 100°C / second or lower, and more preferably 50°C / second or lower.

[0106] Next, the conditions of the steps after hot rolling will be described.

[0107] [Coiling temperature after hot rolling]

[0108] The hot-rolled steel sheet (hot-rolled steel sheet) is coiled. If the coiling temperature after hot rolling is lower than 570°C, the strength of the hot-rolled steel sheet increases, making it difficult to reduce during cold rolling. Therefore, the coiling temperature after hot rolling is preferably 570°C or higher, more preferably 580°C or higher, and even more preferably 590°C or higher. On the other hand, if the coiling temperature after hot rolling is too high, the pickling performance for descaling will deteriorate. Therefore, the coiling temperature is preferably 700°C or lower, more preferably 690°C or lower, and even more preferably 680°C or lower.

[0109] [Reduction rate during cold rolling]

[0110] The hot-rolled steel sheet, which is coiled in a coil, is unwound, pickled to remove scale, and then cold-rolled. The rolling reduction during cold rolling (synonymous with "reduction") is preferably 20% or more and 60% or less. In order to obtain a steel sheet of a specified thickness with a rolling reduction of less than 20% during cold rolling, the thickness of the hot-rolled steel sheet must be reduced during the hot rolling process. If the thickness of the hot-rolled steel sheet is thin, the length of the steel sheet becomes longer, so pickling takes time and productivity decreases. Therefore, the rolling reduction during cold rolling is preferably 20% or more, more preferably 25% or more. On the other hand, if the rolling reduction during cold rolling exceeds 60%, a higher-capacity cold rolling mill is required. Therefore, the upper limit of the rolling reduction during cold rolling is preferably 60% or less, more preferably 55% or less, and even more preferably 50% or less.

[0111] (Annealing process)

[0112] In the annealing process, the steel plate obtained by hot rolling and cold rolling the steel billet is annealed using an annealing furnace. In order to obtain the high-strength cold-rolled steel plate involved in the present embodiment, it is important to appropriately adjust the conditions in each process included in the annealing process after cold rolling. In the manufacturing method of the high-strength cold-rolled steel plate involved in the present embodiment, the annealing process includes: (a) a heating process, (b) a soaking process performed after the heating process, (c) a first cooling process performed after the soaking process, (d) a holding process performed after the first cooling process, and (e) a second cooling process performed after the holding process. The heating pattern (temporal change of the steel plate temperature) of the annealing process including each of these (a) to (e) processes is schematically shown in Figure 2 (A) Hereinafter, each step (a) to (e) will be described in order.

[0113] (a) Heating process

[0114] In the heating process of this embodiment, the cold-rolled steel sheet obtained by cold rolling the steel slab is heated to a temperature range below (Ac3 point + 200°C), referred to as the soaking temperature, at a heating rate (i.e., temperature increase rate) of 1.5°C / s to 30°C / s at or above 700°C. The lower limit of the heating temperature range is preferably the lower of the Ac3 point or 950°C. Specifically, when the Ac3 point is below 950°C, heating is preferably performed to a temperature range between the Ac3 point and (Ac3 point + 200°C). When the Ac3 point is above 950°C, heating is preferably performed to a temperature range between 950°C and (Ac3 point + 200°C). If the temperature increase rate at or above 700°C is less than 1.5°C / s, the time required to reach the soaking temperature increases, leading to higher equipment costs. Therefore, the lower limit of the temperature increase rate at or above 700°C is 1.5°C / s or higher.

[0115] On the other hand, if the heating rate exceeds 30°C / second at temperatures above 700°C, it becomes difficult to control the steel plate temperature, increasing equipment costs. Therefore, the upper limit of the heating rate at temperatures above 700°C is 30°C / second or less. The upper limit of the heating rate at temperatures above 700°C is preferably 25°C / second or less, and more preferably 20°C / second or less. Furthermore, in the heating process according to this embodiment, the heating rate in the temperature range below 700°C can be any rate.

[0116] (b) Soaking process

[0117] In the soaking process of this embodiment, the steel plate, having undergone the heating process, is held at a predetermined temperature for a predetermined time. Specifically, after heating the steel plate to a temperature range below (Ac3 point + 200°C) in the heating process, the soaking process is performed by holding the steel plate at this temperature for a predetermined time. Hereinafter, the soaking temperature of the steel plate is referred to as the "soaking temperature," and the soaking time is referred to as the "soaking time." By setting the soaking temperature below (Ac3 point + 200°C) and maintaining the soaking time for the predetermined time, steel plates with excellent stretch flange formability can be produced in subsequent processes without excessive energy consumption.

[0118] If the soaking temperature is lower than the Ac3 point when the Ac3 point is lower than 950°C, or if the soaking temperature is lower than 950°C when the Ac3 point is higher than 950°C, ferrite may form in the structure during the soaking process, making it difficult to ensure the required stretch flangeability. Therefore, the lower limit of the soaking temperature is preferably at least the lower of the Ac3 point or 950°C. By setting the lower limit of the soaking temperature at least the lower of the Ac3 point or 950°C and maintaining the soaking time for a specified period, the area fraction of austenite in the entire structure can be made 85% or higher. The lower limit of the soaking temperature is more preferably at least the lower of the Ac3 point + 10°C or 960°C.

[0119] On the other hand, if the soaking temperature exceeds Ac3 point + 200°C, excessive energy is required for industrial production of high-strength cold-rolled steel sheets. Therefore, the upper limit of the soaking temperature is Ac3 point + 200°C or lower. The upper limit of the soaking temperature is preferably Ac3 point + 150°C or lower.

[0120] Here, the Ac3 point of the steel plate can be calculated based on the following formula (1). In the formula, (% [element name]) represents the content (mass %) of each element, and elements not contained in the steel plate are calculated as 0%. The following formula (1) is quoted from "Leslie's Steel Materials" (published by Maruzen Co., Ltd., authored by William C. Leslie, page 273).

[0121] Ac3 point = 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)

[0122] In the soaking process of this embodiment, the soaking time is 10 seconds to 100 seconds. By setting the soaking time to 10 seconds to 100 seconds and maintaining the soaking temperature, the area fraction of austenite in the entire microstructure can be increased to 85% or more, allowing the subsequent process to produce steel plates with excellent stretch flange formability with high productivity. If the soaking time is shorter than 10 seconds, excessive ferrite may form, making it difficult to ensure the required stretch flange formability. Therefore, the lower limit of the soaking time is 10 seconds or more. The lower limit of the soaking time is preferably 13 seconds or more. On the other hand, if the soaking time exceeds 100 seconds, productivity decreases. Therefore, the upper limit of the soaking time is 100 seconds or less. The upper limit of the soaking time is preferably 80 seconds or less.

[0123] In the soaking process, the soaking temperature is preferably fixed to the temperature at the end of the heating process, that is, the temperature at which the steel plate stops heating. However, if it is in the temperature range below (Ac3 point + 200°C) (preferably above the lower temperature of Ac3 point or 950°C), and the heating rate is slower than that above 700°C, it can be changed, and the temperature at the start and end of soaking can be different. Figure 2 The gradual increase in temperature during the soaking step (A) means that even if the temperature increase of the annealing furnace is stopped, the temperature inside the furnace and the steel sheet will continue to rise slightly.

[0124] (c) First cooling step

[0125] In the first cooling step of this embodiment, the steel plate that has undergone the soaking step is cooled at a predetermined cooling rate to a predetermined cooling stop temperature (hereinafter referred to as the "first cooling temperature"). The cooling rate from the soaking temperature to the first cooling temperature (hereinafter referred to as the "first cooling rate") is 10°C / second or higher and 50°C / second or lower.

[0126] If the first cooling rate is less than 10°C / second, the area ratio of the first hard phase decreases and the area ratio of the second hard phase increases, which is believed to reduce stretch flangeability. Therefore, the lower limit of the first cooling rate is 10°C / second or higher. The lower limit of the first cooling rate is preferably 15°C / second or higher, and more preferably 18°C / second or higher.

[0127] On the other hand, if the first cooling rate exceeds 50°C / second, it becomes difficult to control the steel plate temperature, and equipment costs increase. Therefore, the upper limit of the first cooling rate is 50°C / second or less. The upper limit of the first cooling rate is preferably 40°C / second or less, and more preferably 30°C / second or less.

[0128] The first cooling temperature is 100° C. to 410° C. By cooling to a temperature range of 100° C. to 410° C. at the first cooling rate, the steel sheet structure is mainly composed of bainite or prior-quenched martensite.

[0129] If the first cooling temperature is lower than 100° C., the volume fraction of retained austenite decreases, and ductility deteriorates. Therefore, the lower limit of the first cooling temperature is 100° C. or higher. The lower limit of the first cooling temperature is preferably 150° C. or higher, and more preferably 200° C. or higher.

[0130] On the other hand, if the first cooling temperature exceeds 410°C, the area ratio of the first hard phase becomes less than 0.5%, or the area ratio of the second hard phase becomes more than 5.0%, which reduces the stretch flange formability and bendability of the steel sheet. Therefore, the upper limit of the first cooling temperature is 410°C or lower. The upper limit of the first cooling temperature is preferably 400°C or lower.

[0131] During the first cooling step, the steel plate microstructure can be controlled by controlling the first cooling temperature. Specifically, by setting the first cooling temperature to between 350°C and 410°C, the steel plate microstructure can be primarily composed of bainite (with an area ratio of 85% or more). Furthermore, by setting the first cooling temperature to between 100°C and 350°C, the steel plate microstructure can be primarily composed of as-quenched martensite. By further performing a reheating step (described later) after the first cooling step, the as-quenched martensite can be tempered, resulting in a steel plate microstructure primarily composed of tempered martensite (with an area ratio of 85% or more).

[0132] (d) Maintaining process

[0133] In the holding step after the first cooling step, the holding temperature is 100° C. or higher and 410° C. or lower, and the holding time is 80 seconds or less.

[0134] If the holding temperature in the holding step is lower than 100°C, the volume fraction of retained austenite decreases, and ductility deteriorates. Therefore, the lower limit of the holding temperature in the holding step is 100°C or higher, preferably 150°C or higher, and more preferably 200°C or higher.

[0135] On the other hand, if the holding temperature in the holding step exceeds 410°C, the area ratio of the first hard phase becomes less than 0.5%, or the area ratio of the second hard phase becomes more than 5.0%, resulting in a decrease in the stretch flange formability and bendability of the steel sheet. Therefore, the upper limit of the holding temperature in the holding step is 410°C or lower. The upper limit of the holding temperature in the holding step is preferably 400°C or lower.

[0136] Furthermore, it is believed that if the holding time in the holding process is shorter than 10 seconds, carbon enrichment from the bainite or martensite generated in the first cooling process into the untransformed austenite by diffusion does not proceed sufficiently, and the transformation of the untransformed austenite to martensite in the second cooling process proceeds excessively. Consequently, the volume fraction of retained austenite decreases, and ductility is reduced. On the other hand, if the holding time in the holding process exceeds 80 seconds, the transformation of the untransformed austenite to bainite or martensite proceeds excessively. Consequently, the volume fraction of retained austenite decreases, and ductility is reduced. Furthermore, if the holding time exceeds 80 seconds, productivity decreases. Therefore, the upper limit of the holding time is 80 seconds or less.

[0137] In the holding step, the holding temperature is preferably fixed to the temperature at the end of the first cooling step. However, the holding temperature may be different at the beginning and end of the holding step. Specifically, if the holding temperature is in the temperature range of 100°C to 410°C and the cooling rate is slower than the first cooling rate, it may be changed. Figure 2 The gradual decrease in temperature during the holding step (A) means that even if the temperature drop of the annealing furnace is stopped, the temperature inside the furnace and the steel sheet will continue to decrease slightly.

[0138] Furthermore, when the first cooling temperature is 350°C to 410°C, the holding temperature is preferably 350°C to 410°C. This allows the steel sheet structure, primarily composed of bainite, formed during the first cooling step, to be maintained during the holding step. Similarly, when the first cooling temperature is 100°C to 350°C, the holding temperature is preferably 100°C to 350°C. This allows the steel sheet structure, primarily composed of prior-quenched martensite, formed during the first cooling step, to be maintained during the holding step.

[0139] (e) Second cooling step

[0140] In the second cooling process, after the holding process, cooling is performed at a cooling rate of 15°C / second or more from the holding temperature at the end of the holding process. In the high-strength cold-rolled steel sheet involved in this embodiment, the C content is as low as 0.140% or less, so it is difficult to obtain retained austenite. Therefore, it is believed that if the cooling rate in the second cooling process (hereinafter referred to as the "second cooling rate") is less than 15°C / second, the volume fraction of retained austenite decreases and the ductility decreases. Therefore, the lower limit of the second cooling rate is 15°C / second or more. The lower limit of the second cooling rate is preferably 18°C / second or more. The upper limit of the second cooling rate is not particularly limited, for example, it is 30°C / second or less. In addition, the cooling stop temperature in the second cooling process is not particularly limited, and it can usually be cooled to room temperature.

[0141] In the annealing step of the method for producing high-strength steel sheet according to this embodiment, in addition to the steps (a) to (e) described above, either or both of the steps (f) of reheating and (g) of plating may be added. Furthermore, the step (h) of alloying may be added to the step (g). Each of the steps (f) to (h) will be described below.

[0142] (f) Reheating process

[0143] In the method for manufacturing the high-strength steel sheet according to this embodiment, Figure 2 As shown in (B), a reheating step (f) is provided between the holding step (d) and the second cooling step (e), and the steel sheet is reheated to a temperature of 400° C. to 500° C. and held for 10 seconds or more.

[0144] During the reheating step, the previously quenched martensite generated during the first cooling step and retained during the holding step can be tempered, resulting in a steel sheet structure primarily composed of tempered martensite (with an area ratio of 85% or more). When a reheating step is included, previously quenched martensite is generated during the first cooling step. Therefore, the first cooling temperature of the first cooling step is set to 100°C or higher and lower than 350°C, and the holding temperature of the holding step is set to 100°C or higher and lower than 350°C. The aforementioned second cooling step is performed after the reheating step.

[0145] (g) Plating process, (h) Alloying process

[0146] In the method for manufacturing a high-strength steel sheet according to the present embodiment, by providing a step of performing a galvanizing treatment on the steel sheet (plating step) before the second cooling step, a hot-dip galvanized steel sheet according to the present embodiment can be manufactured. In addition, in the method for manufacturing a high-strength steel sheet according to the present embodiment, by providing a step of performing a galvanizing treatment and an alloying treatment on the steel sheet in sequence (plating step and alloying step) before the second cooling step, an alloyed hot-dip galvanized steel sheet according to the present embodiment can be manufactured. Specifically, Figure 2 As shown in (C), the plating step (g) is performed before the second cooling step (e), or the plating step (g) and the alloying step (h) are performed.

[0147] The hot-dip galvanized steel sheet according to the present embodiment can be manufactured by subjecting the steel sheet that has passed the holding step (d) to a conventional method of galvanizing (immersing in a galvanizing bath at approximately 460°C for approximately 1 to 5 seconds) (plating step (g)), followed by cooling in the second cooling step (e).

[0148] The alloyed hot-dip galvanized steel sheet according to the present embodiment can be manufactured as follows: the steel sheet that has passed the holding step (d) is subjected to a galvanizing treatment using a conventional method (immersing in a galvanizing bath at approximately 460°C for approximately 1 to 5 seconds) (plating step (g)), and further subjected to a hot-dip galvanizing and steel alloying treatment (heating to 430 to 550°C and holding for 20 to 40 seconds) (alloying step (h)), followed by cooling using the second cooling step (e).

[0149] When the plating step (g) is provided, by setting the first cooling temperature of the first cooling step (c) to 350°C or higher and 410°C or lower, and setting the holding temperature of the holding step (d) to 350°C or higher and 410°C or lower, a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having a steel sheet structure mainly composed of bainite can be obtained.

[0150] In addition, in the method for manufacturing a hot-dip galvanized steel sheet and the method for manufacturing an alloyed hot-dip galvanized steel sheet according to this embodiment, the reheating step (f) may be provided before the plating step (g). Specifically, Figure 2 As shown in (D), after the holding step (d) and before the second cooling step (e), the reheating step (f) is performed, and the plating step (g) is performed after the reheating step (f). Alternatively, the alloying step (h) may be performed after the plating step (g).

[0151] When the reheating step (f) and the plating step (g) are provided, by setting the first cooling temperature of the first cooling step to 100°C or higher and lower than 350°C and the holding temperature of the holding step to 100°C or higher and lower than 350°C, a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having a steel sheet structure mainly composed of tempered martensite can be obtained.

[0152] The high-strength steel sheet according to the present embodiment is not limited to the one obtained by the above-mentioned manufacturing method, but may be obtained by other manufacturing methods as long as the required conditions specified in the present invention are satisfied.

[0153] This specification discloses the above-mentioned various aspects of technology, among which the main technologies are summarized as follows.

[0154] As described above, the high-strength cold-rolled steel sheet according to one aspect of the present invention contains, by mass%, 0.070-0.140% C, 0.80-1.80% Si, and 1.80-2.80% Mn, with the remainder being iron and inevitable impurities. In terms of area ratio in the structure observed by a scanning electron microscope, the total of bainite, tempered martensite, and hard phase accounts for 85% or more, and the hard phase comprises quenched martensite and retained austenite. The present invention relates to a composite material comprising at least one of MA and cementite, the hard phase having a short axis of 0.4 μm or less and a long axis of 1.2 μm or greater in the hard phase accounts for 0.5% or greater, the hard phase having a short axis of 1.2 μm or greater in the hard phase accounts for 5.0% or less, the bainite, the tempered martensite, and the structure other than the hard phase accounts for 15% or less, and the retained austenite, in terms of volume fraction measured by X-ray diffraction, is 3.0% or greater and 7.0% or less.

[0155] According to this configuration, a high-strength cold-rolled steel sheet having excellent stretch-flange formability and bendability can be obtained.

[0156] The high-strength cold-rolled steel sheet having the above-described structure may further contain, in mass %, one or more elements selected from the group consisting of Al: 0.015% to 0.60%, Cr: more than 0% and less than 0.60%, Ti: 0.010% to 0.040%, B: 0.0015% to 0.0040%, Cu: more than 0% and less than 0.30%, Ni: more than 0% and less than 0.30%, Mo: more than 0% and less than 0.30%, V: more than 0% and less than 0.30%, Nb: more than 0% and less than 0.040%, and Ca: more than 0% and less than 0.0050%.

[0157] According to this configuration, a high-strength cold-rolled steel sheet can be obtained that is excellent not only in stretch-flange formability and bendability but also in other properties.

[0158] The high-strength cold-rolled steel sheet having the above-described structure may further contain, in mass %, P exceeding 0% and 0.015% or less, and S exceeding 0% and 0.0050% or less as the unavoidable impurities.

[0159] According to this configuration, a high-strength cold-rolled steel sheet can be obtained that is excellent in stretch-flange formability and bendability and in which the influence of inevitable impurities is suppressed.

[0160] Another aspect of the present invention relates to a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet comprising: the high-strength cold-rolled steel sheet described above; and a hot-dip galvanized layer or an alloyed hot-dip galvanized layer provided on a surface of the high-strength cold-rolled steel sheet.

[0161] According to this configuration, a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having excellent stretch-flange formability and bendability can be obtained.

[0162] In addition, a method for manufacturing a high-strength cold-rolled steel sheet according to another aspect of the present invention includes the following steps in sequence: a rolling step of sequentially hot-rolling and cold-rolling a steel slab having the above-mentioned composition; a heating step of heating a steel slab obtained by cold-rolling the steel slab to a temperature range of (Ac3 point + 200°C) or lower at a heating rate of 1.5°C / s to 30°C / s at a temperature of 700°C or higher; a soaking step of holding the steel slab after the heating step for 10 seconds to 100 seconds; a first cooling step of cooling the steel slab after the soaking step at a cooling rate of 10°C / s to 50°C / s to a first cooling temperature of 100°C to 410°C; a holding step of holding the steel slab cooled to the first cooling temperature at a holding temperature of 100°C to 410°C for 10 seconds to 80 seconds; and a second cooling step of cooling the steel slab after the holding step to room temperature at a cooling rate of 15°C / s or higher.

[0163] According to this configuration, a high-strength cold-rolled steel sheet having excellent stretch-flange formability and bendability can be obtained.

[0164] In the method for producing a high-strength cold-rolled steel sheet having the above configuration, the first cooling temperature in the first cooling step may be 350° C. to 410° C., and the holding temperature in the holding step may be 350° C. to 410° C.

[0165] According to this configuration, a high-strength cold-rolled steel sheet having a steel sheet structure mainly composed of bainite can be produced.

[0166] In the manufacturing method of the high-strength cold-rolled steel sheet having the above-mentioned structure, the first cooling temperature of the first cooling process can be higher than 100°C and lower than 350°C, and the holding temperature of the holding process can be higher than 100°C and lower than 350°C. The manufacturing method may further include: a reheating process, between the holding process and the second cooling process, reheating the steel sheet to a temperature of higher than 400°C and lower than 500°C and holding it for more than 10 seconds.

[0167] According to this configuration, a high-strength cold-rolled steel sheet having a steel sheet structure mainly composed of tempered martensite can be produced.

[0168] Another aspect of the present invention relates to a method for manufacturing a hot-dip galvanized steel sheet and a method for manufacturing an alloyed hot-dip galvanized steel sheet. In the method for manufacturing a high-strength cold-rolled steel sheet, the steel sheet may be subjected to a galvanizing treatment before the second cooling step, or may be subjected to a galvanizing treatment and then an alloying treatment.

[0169] According to this configuration, a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having excellent stretch-flange formability and bendability can be obtained.

[0170] The present invention will be described in more detail below with reference to the following embodiments. However, the present invention is not limited to the following embodiments and can be implemented with modifications within the scope of the above and following gist. Such implementations are all included in the technical scope of the present invention.

[0171] Example

[0172] Steel slabs having the chemical compositions shown in Table 1 (steel types: steels 1 to 8) were produced. The steel slabs were hot rolled at a coiling temperature of 660° C. The resulting hot-rolled steel sheets were pickled and then cold rolled.

[0173] Afterwards, based on Figure 2 The heat treatment was carried out under the conditions shown in Table 2 (heat treatments 1 to 14) in the heating mode shown. In Table 1, the "-" column indicates that no addition was made. In Table 1, the contents of V, Nb, and Ca are shown with the measured values rounded to the third decimal place, so "0.00" for these contents means that the measured value is less than 0.004%. In addition, "N / A" means below the detection limit. In addition, as mentioned above, P, S, N, and O are unavoidable impurities, and the values shown in the P, S, N, and O columns indicate the amounts inevitably contained.

[0174]

[0175]

[0176] Under the heat treatment conditions 1 to 14 shown in Table 2, the same thermal history as that of the alloyed hot-dip galvannealed steel sheet was provided. In Table 2, the "holding temperature" is the temperature at the end of the holding process, hereinafter referred to as the "holding stop temperature". The "soaking temperature" is the average value of the temperature range in the soaking process that can be estimated based on the measured value. In Table 2, the "-" column indicates that the reheating process was not performed.

[0177] For each cold-rolled steel sheet thus obtained, the microstructure fraction (area ratio), the volume ratio of retained austenite, the size and shape of the hard phase, and various properties (tensile properties, stretch-flange formability, bendability) were measured according to the following procedures.

[0178] [Organizational fraction]

[0179] The metallographic structure of the green steel sheet constituting the steel sheet was observed according to the following procedure. The area ratios of ferrite, pearlite, bainite, tempered martensite, and prior-quenched martensite (including autotempered martensite) in the metallographic structure were calculated based on the results of observation using a scanning electron microscope (SEM).

[0180] SEM observation was conducted as follows. A cross-section of the green steel plate parallel to the rolling direction was polished, electrolytically polished, and then etched with nitric acid. Three fields of view at a position equal to one-quarter of the plate thickness were observed using an SEM at 5000x magnification. The observation field measured approximately 30 μm x 30 μm.

[0181] In the image thus observed, the area ratio is not calculated for a steel plate clearly consisting of only bainite or tempered martensite. The area ratio is calculated by image processing only when ferrite or prior quenched martensite exists.

[0182] SEM observation revealed that bainite and tempered martensite are structures in which MA and cementite (a hard phase) are dispersed within the grains of bainite or tempered martensite. Bainite or tempered martensite is primarily gray, while MA is white or light gray and cementite is white. The retained austenite constituting MA is indistinguishable from the quenched martensite and is observed as a single entity.

[0183] In the case of SEM observation, retained austenite and carbides are also included in bainite, so the area ratio of bainite is calculated as the area ratio including retained austenite and carbides.

[0184] Furthermore, ferrite was observed to be primarily gray, nearly white, with no MA or cementite present within the ferrite grains. Pearlite was observed to be a structure in which carbides and ferrite formed layers. As-quenched martensite (including autotempered martensite) was observed to be primarily gray, with extremely fine carbides dispersed within the grains of the as-quenched martensite.

[0185] [Volume fraction of retained austenite]

[0186] Regarding retained austenite, test pieces measuring 10 to 20 mm in thickness by 10 to 20 mm in thickness were cut from the annealed cold-rolled steel sheets. These pieces were ground to a portion t / 4 of the sheet thickness t and then chemically polished. The volume fraction of retained austenite was then measured by X-ray diffraction (ISIJ Int. Vol. 33. (1993), No. 7, p. 776).

[0187] [Measurement of the size and shape of the hard phase]

[0188] The SEM observation images obtained for the above-mentioned determination of the microstructure fraction were processed and analyzed to calculate the minor axis, major axis, and area of the hard phase. The term "major axis" refers to the longest diameter of the hard phase, while the "minor axis" refers to the longest diameter perpendicular to the major axis. Using the calculated minor axis, major axis, and area of the hard phase, the area ratios of the hard phase with a minor axis of 0.4 μm or less and a major axis of 1.2 μm or greater (first hard phase) and the hard phase with a minor axis of 1.2 μm or greater (second hard phase) were calculated.

[0189] [Tensile properties]

[0190] JIS No. 5 test pieces (plate-shaped test pieces) were prepared parallel to the cold-rolled surface of the steel sheet, with the longitudinal direction perpendicular to the rolling direction. These test pieces were used in a tensile test to measure the tensile strength (TS) and elongation (EL). The ductility of the steel sheet was evaluated based on the measured elongation (EL).

[0191] [Stretch flange properties]

[0192] Test pieces measuring 90 mm thick x 90 mm were collected from the cold-rolled steel sheets. Hole expansion tests were performed using these test pieces in accordance with JIS Z 2256:2010 to measure the hole expansion ratio λ. The stretch flangeability of the steel sheets was evaluated based on the measured hole expansion ratio λ.

[0193] [VDA bending angle]

[0194] Test pieces measuring 60 mm thick x 60 mm were taken from the cold-rolled steel sheets and subjected to bending tests under the following conditions, in accordance with the VDA standard (VDA 238-100) specified by the German Association of the Automotive Industry. The bending angle was calculated by converting the displacement under maximum load measured during the bending test into an angle in accordance with the VDA standard.

[0195] (Measurement conditions)

[0196] Test method: roller support, punch press

[0197] Pressure roller diameter (diameter): φ30mm

[0198] Punch shape: Tip R = 0.4mm

[0199] Distance between rollers: plate thickness × 2 + 0.5 mm

[0200] Punch pressing speed: 20mm / min

[0201] Test piece size: 60mm×60mm

[0202] Bending direction: The direction at right angles to the rolling direction

[0203] Testing machine: Shimadzu AUTOGRAPH 20kN

[0204] The eligibility criteria for each test item are as follows.

[0205] A tensile strength of 900 MPa or greater, a ductility (elongation EL) of 12% or greater, a stretch-flange formability (hole expansion ratio λ) of 60% or greater, and a VDA bend angle of 100° or greater were considered acceptable, while all other conditions were considered unacceptable. A higher elongation EL indicates better ductility, and a higher hole expansion ratio λ indicates better stretch-flange formability. Furthermore, a higher VDA bend angle indicates better bendability.

[0206] These results are shown in Table 3 along with the applicable steel grades and heat treatment conditions. In Table 3, the "-" column indicates that no measurement was performed for that item. The "remaining structure" in Table 3 refers to the structure other than bainite, tempered martensite, and hard phases, primarily pearlite, ferrite, and as-quenched martensite (including autotempered martensite).

[0207]

[0208] According to the results shown in Table 3, the following can be observed.

[0209] Test Nos. 1, 3, 5 to 8, 13, and 14 are examples (inventive examples) of steel sheets produced using steel grades satisfying the chemical composition specified in the present invention (steels 1 to 4, 7, and 8 in Table 1) and under appropriate heat treatment conditions (heat treatment Nos. 1, 3, 5 to 8, 13, and 14 in Table 2. All of these are thermal histories when producing alloyed hot-dip galvannealed steel sheets).

[0210] All of these examples met the following acceptance criteria: a tensile strength of 900 MPa or greater, an elongation EL of 12% or greater, a hole expansion ratio λ of 60% or greater, and a VDA bend angle of 100° or greater. The microstructures of the high-strength steel plates of Test Nos. 1, 3, 5-8, 13, and 14 met the requirements specified in the present invention for the area ratios of bainite and tempered martensite, the volume ratio of retained austenite, the area ratios of the first and second hard phases, and the area ratio of the residual structure.

[0211] In contrast, Test Nos. 2, 4, and 9 are examples (comparative examples) of steels produced using the chemical composition specified in the present invention (steels 1 to 4 in Table 1) under heat treatment conditions outside the appropriate range (heat treatments 2, 4, and 9 in Table 2). These steels failed to achieve the desired properties and did not meet the acceptance criteria.

[0212] Specifically, in test No. 2, the first cooling temperature and holding temperature (holding stop temperature) were high (heat treatment 2), the area ratio of the first hard phase was less than 0.5%, the area ratio of the second hard phase was greater than 5.0%, the hole expansion ratio λ was less than 60%, and the VDA bending angle was less than 100°.

[0213] In Test No. 4, the first cooling temperature and the holding stop temperature were high (heat treatment 4). The area ratio of the second hard phase in the obtained steel plate exceeded 5.0%, and the hole expansion ratio λ was less than 60%.

[0214] In Test No. 9, the first cooling temperature and the holding stop temperature were high (heat treatment 9). The resulting steel plate had an area ratio of residual microstructure exceeding 15%, an area ratio of the first hard phase less than 0.5%, a volume fraction of retained austenite less than 3%, a hole expansion ratio λ less than 60%, and a VDA bend angle less than 100°.

[0215] On the other hand, Test No. 12, an example (comparative example) produced using a steel grade (Steel 6 in Table 1) that does not meet the chemical composition specified in the present invention, under appropriate heat treatment conditions (Heat Treatment 12 in Table 2), failed to achieve the desired properties and failed to meet the acceptance criteria. Specifically, Test No. 12 had a high C content (Steel Grade 6), a volume fraction of retained austenite exceeding 7%, and a hole expansion ratio λ of less than 60%.

[0216] Test Nos. 10 and 11 are examples (comparative examples) of steel produced using a steel grade that does not satisfy the chemical composition specified in the present invention (Steel 5 in Table 1) and under heat treatment conditions outside the appropriate range (Heat Treatments 10 and 11 in Table 2). The desired properties could not be obtained and the steel did not meet the acceptance criteria.

[0217] Specifically, in Test No. 10, the Si content was low (steel grade 5), the heating rate and first cooling rate above 700°C were low, and the first cooling temperature and holding temperature were high (heat treatment 10). The resulting steel plate had a hole expansion ratio λ of less than 60% and an elongation of less than 12%.

[0218] In Test No. 11, the Si content was low (steel type 5), and the heating rate and first cooling rate above 700°C were low, while the first cooling temperature and holding temperature were high (heat treatment 11). The resulting steel plate had a hole expansion ratio λ of less than 60%.

[0219] This application is based on Japanese patent application No. 2020-169102 filed on October 6, 2020, the contents of which are incorporated herein by reference.

[0220] To illustrate the present invention, the present invention has been appropriately and fully described above with reference to specific examples and embodiments. However, it should be understood that those skilled in the art can easily modify and / or improve the above-described embodiments. Therefore, as long as the modified or improved embodiments implemented by those skilled in the art do not depart from the scope of protection of the claims set forth in the appended claims, such modified or improved embodiments should be construed as being included within the scope of protection of the claims.

[0221] Industrial applicability

[0222] The present invention has broad industrial applicability in the technical fields related to high-strength cold-rolled steel sheets, hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, and methods for producing these steel sheets.

Claims

1. A high-strength cold-rolled steel sheet, characterized in that: In mass%, contains C：0.070～0.140%、 Si: 0.80~1.80%, Mn: 1.80~2.80%, Al：0.015～0.60%、 Ti: 0.010~0.040%, and B: 0.0015~0.0040%, the rest is iron and inevitable impurities, As measured by area ratio in the tissue observed using a scanning electron microscope, The total of bainite, tempered martensite, and hard phase is 85% or more, and the hard phase includes at least one of MA composed of quenched martensite and retained austenite, and cementite. The hard phases having a short axis of 0.4 μm or less and a long axis of 1.2 μm or more account for 0.5% or more. The hard phases having a short axis of 1.2 μm or more account for 5.0% or less of the hard phases. The proportion of the structures other than the bainite, the tempered martensite and the hard phase is 15% or less. The retained austenite is 3.0% or more and 7.0% or less in terms of volume fraction measured by X-ray diffraction.

2. The high-strength cold-rolled steel sheet according to claim 1, characterized in that: In mass %, it also contains Cr: more than 0% and less than 0.60%, Cu: more than 0% and less than 0.30%, Ni: more than 0% and less than 0.30%, Mo: more than 0% and less than 0.30%, V: more than 0% and less than 0.30%, Nb: more than 0% and less than 0.040%, and Ca: one or more kinds of elements selected from the group consisting of more than 0% and 0.0050% or less.

3. The high-strength cold-rolled steel sheet according to claim 1, characterized in that: In terms of mass %, the unavoidable impurities also include P: more than 0% and less than 0.015%, and S: more than 0% and 0.0050% or less.

4. A hot-dip galvanized steel sheet, characterized in that have: The high-strength cold-rolled steel sheet according to any one of claims 1 to 3; and A hot-dip galvanized layer is provided on the surface of the high-strength cold-rolled steel sheet.

5. An alloyed hot-dip galvanized steel sheet, characterized in that have: The high-strength cold-rolled steel sheet according to any one of claims 1 to 3; and An alloyed hot-dip galvannealed layer is provided on the surface of the high-strength cold-rolled steel sheet.

6. A method for manufacturing high-strength cold-rolled steel sheet, characterized in that The method for manufacturing a high-strength cold-rolled steel sheet according to any one of claims 1 to 3, comprising: A rolling process, sequentially hot rolling and cold rolling the steel slab having the composition satisfying any one of claims 1 to 3; a heating step of heating the steel sheet obtained by cold rolling the steel slab to a temperature range of (Ac3 point + 200°C) or lower at a heating rate of 1.5°C / s to 30°C / s at a temperature of 700°C or higher; a soaking step of holding the steel plate after the heating step for 10 seconds to 100 seconds; a first cooling step of cooling the steel plate after the soaking step to a first cooling temperature of 100° C. to 410° C. at a cooling rate of 10° C. / s to 50° C. / s; a holding step of holding the steel plate cooled to the first cooling temperature at a holding temperature of 100° C. to 410° C. for 10 seconds to 80 seconds; and The second cooling step is to cool the steel plate that has undergone the holding step to room temperature at a cooling rate of 15° C. / second or higher.

7. The method for manufacturing a high-strength cold-rolled steel sheet according to claim 6, wherein: The first cooling temperature in the first cooling step is 350° C. or higher and 410° C. or lower, and the holding temperature in the holding step is 350° C. or higher and 410° C. or lower.

8. The method for manufacturing a high-strength cold-rolled steel sheet according to claim 6, wherein: The first cooling temperature of the first cooling step is 100° C. or higher and lower than 350° C. The holding temperature in the holding step is 100°C or higher and lower than 350°C. The method for manufacturing the high-strength cold-rolled steel sheet further includes: The reheating step is between the holding step and the second cooling step, and reheats the steel plate to a temperature of 400° C. or higher and 500° C. or lower and holds the temperature for 10 seconds or longer.

9. A method for manufacturing a hot-dip galvanized steel sheet, characterized in that ， In the method for producing a high-strength cold-rolled steel sheet according to claim 6, the steel sheet is further subjected to a galvanizing treatment before the second cooling step.

10. A method for manufacturing alloyed hot-dip galvanized steel sheet, characterized in that ， In the method for producing a high-strength cold-rolled steel sheet according to claim 6, the steel sheet is further subjected to a galvanizing treatment and an alloying treatment in sequence before the second cooling step.

Images