Cold-rolled steel sheet and method for manufacturing the same

Abstract

The cold-rolled steel sheet of the present invention has a specified chemical composition. The microstructure at a position (t / 4 portion) on the thickness direction, at a distance of 1 / 4 of the sheet thickness t from the surface, contains, by volume percentage, 2.0% to 8.0% retained austenite, 80.0% to 98.0% tempered martensite, 0.0% to 15.0% ferrite and bainite combined, and 0.0% to 5.0% martensite. This composition is present at both the central portion and at a distance of 50 mm from the end in the width direction. In which part of the edge, the metal microstructure at a position 20 μm from the surface in the thickness direction of the plate (i.e., the 20 μm portion) contains, by volume percentage, 75.0% or more and 100.0% or less of ferrite and bainite, and 0.0% or more and 25.0% or less of martensite and tempered martensite. Furthermore, the metal microstructure at a position 75 μm from the surface in the thickness direction of the plate (i.e., the 75 μm portion) contains, by volume percentage, 0.0% or more and 15.0% or less of ferrite and bainite.

Description

Technical Field

[0001] This invention relates to cold-rolled steel sheets and their manufacturing methods.

[0002] This application claims priority based on Japanese Patent Application No. 2021-038717, filed on March 10, 2021, the contents of which are incorporated herein by reference. Background Technology

[0003] In today's highly specialized industrial technology landscape, the materials used in various technological fields consistently demand specific and high performance characteristics. Particularly concerning automotive steel sheets, from an environmental perspective, the demand for thin-walled, highly formable, high-strength cold-rolled steel sheets has significantly increased to achieve lightweighting and improved fuel efficiency. Even in automotive steel sheets, especially cold-rolled steel sheets used in body frame components, high strength is required, which in turn demands high formability for wider applications. For example, essential properties for automotive steel sheets include a tensile strength (TS) of 1400 MPa or higher and a uniform elongation of at least 5.0%. Alternatively, depending on the processing method and the component being applied, a minimum limiting bending radius R (R / t) standardized to the sheet thickness t at a 90° V-bending is required to be 5.0 or less.

[0004] While forming a ferrite-containing microstructure is effective in ensuring uniform tensile strength and other ductility, it is necessary to harden the second phase to achieve a strength of over 1400 MPa through the ferrite-containing microstructure. However, a hard second phase degrades flexural properties.

[0005] On the other hand, as a technology to achieve high strength, steel plates with tempered martensite as the main phase have been proposed (for example, Patent Documents 1 and 2). Patent Documents 1 and 2 describe a microstructure that forms a single phase of tempered martensite, resulting in excellent bending properties, and a microstructure with finely dispersed carbides serving as hydrogen trapping sites, thus exhibiting excellent resistance to hydrogen embrittlement.

[0006] In addition, Patent Document 3 proposes a steel plate that utilizes the TRIP effect based on retained austenite as a technology that balances high strength and high formability.

[0007] In addition, Patent Document 4 proposes an alloyed hot-dip galvanized steel sheet with a tensile strength of up to 1470 MPa or more, and excellent uniform deformation (uniform elongation) and local deformation (local elongation).

[0008] However, the tensile strength of the steel plate in Patent Document 1 is as low as less than 1400 MPa. Therefore, in order to achieve higher strength, it is necessary to further improve the workability, bending properties, and resistance to hydrogen embrittlement, which deteriorate with the steel plate.

[0009] In addition, although the steel plate of Patent Document 2 can achieve a high strength of over 1400 MPa, it is cooled to near room temperature during quenching, resulting in a reduction in the volume fraction of retained austenite and an inability to obtain a high uniform tensile strength.

[0010] Furthermore, in the steel plate of Patent Document 3, it is difficult to obtain a high strength of over 1400 MPa due to the presence of a ferrite phase, and the bending performance is poor due to the strength difference within the microstructure.

[0011] Furthermore, patent document 4 does not consider resistance to hydrogen embrittlement.

[0012] As an invention to solve these technical problems, Patent Document 5 discloses a cold-rolled steel sheet that, by forming a tempered martensite structure containing retained austenite at a representative position of the steel sheet, i.e., a position 1 / 4 of the sheet thickness from the surface, and then by softening the surface layer based on dew point control and refining the hard phase in the surface layer during annealing, can achieve a high level of balance between formability and resistance to hydrogen embrittlement, which are problems in high-strength steel sheets.

[0013] Existing technical documents

[0014] Patent documents

[0015] Patent Document 1: Japanese Patent Application Publication No. 2009-30091

[0016] Patent Document 2: Japanese Patent Application Publication No. 2010-215958

[0017] Patent Document 3: Japanese Patent Application Publication No. 2006-104532

[0018] Patent Document 4: Japanese Patent No. 6187710

[0019] Patent Document 5: Japanese Patent No. 6635236 Summary of the Invention

[0020] The problem that the invention aims to solve

[0021] However, the inventors conducted repeated studies and found that in high-strength steel with a tensile strength of 1400 MPa or higher, when the surface softening layer is thick, the yield strength (applied component yield strength) of the component formed from the steel sheet through processing sometimes does not increase with the strength of the steel sheet. In other words, it was found that even if the steel sheet is made stronger, the design load as a component cannot always be increased.

[0022] This invention was made to solve such problems, with the aim of providing cold-rolled steel sheets with high strength, excellent formability, and yield strength of application components, as well as a method for manufacturing the same.

[0023] Methods for solving problems

[0024] The inventors have studied a method for obtaining good component yield strength while improving bending properties in cold-rolled steel sheets with a tempered martensite-based microstructure that can achieve high strength and excellent resistance to hydrogen embrittlement.

[0025] As a result, it was found that if, during the manufacturing process, especially by adjusting the dew point and heat treatment conditions during annealing, the proportion of a relatively soft microstructure is increased at a position 20 μm from the surface in the thickness direction, thereby reducing the hardness, and at the same time, a microstructure identical to that inside the steel plate is formed at a position 75 μm from the surface in the thickness direction, it is possible to improve the flexibility while suppressing the decrease in the yield strength of the component.

[0026] This invention is based on the above-mentioned insights. The main points of this invention are as follows.

[0027] [1] One aspect of the present invention relates to a cold-rolled steel sheet, wherein the chemical composition, by mass%, contains: C: 0.180% or more and 0.350% or less, Mn: 2.00% or more and 4.00% or less, P: 0% or more and 0.100% or less, S: 0% or more and 0.010% or less, Al: 0% or more and 0.100% or less, N: 0% or more and 0.0100% or less, Si: 0% or more and 1.00% or less, Ti: 0% or more and 0.050% or less, and Nb: 0% or more. The following are the concentrations of the following compounds: V: 0% and 0.50%; Cu: 0% and 1.00%; Ni: 0% and 1.00%; Cr: 0% and 1.00%; Mo: 0% and 0.50%; B: 0% and 0.0100%; Ca: 0% and 0.010%; Mg: 0% and 0.0100%; REM: 0% and 0.0500%; and Bi: 0% and 0.050%. The remaining portion includes F. e and impurities; the metal microstructure at a position 1 / 4 of the plate thickness t from the surface in the thickness direction, i.e., the t / 4 portion, contains, by volume percentage, 2.0% or more and 8.0% retained austenite, 80.0% or more and 98.0% tempered martensite, 0.0% or more and 15.0% ferrite and bainite combined, and 0.0% or more and 5.0% martensite; regardless of whether it is at a position 50 mm from the end in the width direction (i.e., the edge portion) or the center portion in the width direction, at a position 20 μm from the surface in the thickness direction. The metal microstructure at position m (i.e., 20 μm) contains, by volume percentage, 75.0% to 100.0% ferrite and bainite, and 0.0% to 25.0% martensite and tempered martensite. In the metal microstructure at position m (i.e., 75 μm) from the surface, the average grain size of the martensite and tempered martensite is 5.0 μm or less. The metal microstructure at position m (i.e., 75 μm) from the surface in the thickness direction of the plate contains, by volume percentage, 0.0% to 15.0% ferrite and bainite.

[0028] [2] According to the cold-rolled steel sheet described in [1] above, the chemical composition, in mass %, may also contain the following: Si: 0.005% or more and 1.00% or less; Ti: 0.001% or more and 0.050% or less; Nb: 0.001% or more and 0.050% or less; V: 0.01% or more and 0.50% or less; Cu: 0.01% or more and 1.00% or less; Ni: 0.01% or more and 1.00% or less; C r: 0.01% or more and 1.00% or less, Mo: 0.01% or more and 0.50% or less, B: 0.0001% or more and 0.0100% or less, Ca: 0.0001% or more and 0.010% or less, Mg: 0.0001% or more and 0.0100% or less, REM: 0.0005% or more and 0.0500% or less, and Bi: 0.0005% or more and 0.050% or less, one or more of the following:

[0029] [3] According to the cold-rolled steel sheet described in [1] or [2] above, the sheet may also have a tensile strength of 1400 MPa or more, a uniform elongation of 5.0% or more, and the value obtained by dividing the ultimate bending radius R of 90° V-bending by the sheet thickness t, i.e., R / t, is 5.0 or less.

[0030] [4] The cold-rolled steel sheet according to any one of [1] to [3] above may also have a hot-dip galvanized layer formed on the surface.

[0031] [5] According to the cold-rolled steel sheet described in [4] above, the hot-dip galvanized layer may also be an alloyed hot-dip galvanized layer.

[0032] [6] Another aspect of the present invention relates to a method for manufacturing a cold-rolled steel sheet, comprising the following steps: a hot rolling step, wherein a cast slab is heated as needed and then hot-rolled to form a hot-rolled steel sheet, wherein the cast slab has the following chemical composition (in mass%): C: 0.180% or more and 0.350% or less, Mn: 2.00% or more and 4.00% or less, P: 0% or more and 0.100% or less, S: 0% or more and 0.010% or less, Al: 0% or more and 0.100% or less, N: 0% or more and 0.0100% or less, Si: 0% or more and 1.00% or less, Ti: 0% or more and 0.050% or less, Nb: 0% or more and 0.050% or less. The composition is as follows: 0.50% or less; V: 0% or more and 0.50% or less; Cu: 0% or more and 1.00% or less; Ni: 0% or more and 1.00% or less; Cr: 0% or more and 1.00% or less; Mo: 0% or more and 0.50% or less; B: 0% or more and 0.0100% or less; Ca: 0% or more and 0.010% or less; Mg: 0% or more and 0.0100% or less; REM: 0% or more and 0.0500% or less; and Bi: 0% or more and 0.050% or less, with the remainder including Fe and impurities; a coiling process in which the hot-rolled steel sheet is cooled to a coiling temperature below 550°C and coiled at said coiling temperature; a cold rolling process in which the hot-rolled steel sheet is subjected to a cold rolling process... The hot-rolled steel sheet is pickled and cold-rolled to form a cold-rolled steel sheet; in the annealing process, the furnace atmosphere during heating is specified as a nitrogen-hydrogen mixed atmosphere with a dew point of -20°C to 20°C and containing 1.0% to 20% by volume of hydrogen, and the cold-rolled steel sheet after the cold rolling process is heated to 820°C or higher, i.e., the homogenization temperature, at an average heating rate of less than 10.0°C / second from 700°C to the homogenization temperature, and annealed at the homogenization temperature for more than 30 seconds and less than 200 seconds; in the first cooling process, the cold-rolled steel sheet after the annealing process is cooled to a temperature range of more than 425°C and less than 600°C; in the holding process, after the first cooling process, the cold-rolled steel sheet is... The steel sheet is held in a temperature range of 425°C to 600°C for more than 200 seconds and less than 750 seconds; a second cooling process, after the holding process, cools the cold-rolled steel sheet to a temperature of 50°C to 250°C; a tempering process, after the second cooling process, tempers the cold-rolled steel sheet at a temperature of 200°C to 350°C for more than 1 second; a third cooling process, after the tempering process, cools the sheet to a temperature suitable for surface finishing; and a surface finishing process, performs surface finishing on the cold-rolled steel sheet after the third cooling process; and within 10 hours from the end of the hot rolling process, the temperature of the hot-rolled steel sheet is brought up to 500°C or less.

[0033] [7] According to the method for manufacturing cold-rolled steel sheet described in [6] above, the chemical composition of the cast slab, in mass %, may also contain the following: Si: 0.005% or more and 1.00% or less; Ti: 0.001% or more and 0.050% or less; Nb: 0.001% or more and 0.050% or less; V: 0.01% or more and 0.50% or less; Cu: 0.01% or more and 1.00% or less; Ni: 0.01% or more and 1.00% or less. One or more of the following: less than 0%, Cr: more than 0.01% and less than 1.00%, Mo: more than 0.01% and less than 0.50%, B: more than 0.0001% and less than 0.0100%, Ca: more than 0.0001% and less than 0.010%, Mg: more than 0.0001% and less than 0.0100%, REM: more than 0.0005% and less than 0.0500%, and Bi: more than 0.0005% and less than 0.050%.

[0034] [8] In the method of manufacturing cold-rolled steel sheet described in [6] or [7] above, the cold-rolled steel sheet may also be immersed in a plating bath at a temperature exceeding 425°C and below 600°C during the holding process to form a hot-dip galvanized layer on the surface.

[0035] [9] In the method for manufacturing cold-rolled steel sheet described above [8], alloying treatment may also be performed in the holding process to alloy the hot-dip galvanized layer.

[0036] Invention Effects

[0037] According to the above-described solution of the present invention, it is possible to provide a cold-rolled steel sheet with high strength, excellent formability, and yield strength suitable for use in components, and a method for manufacturing the same. Detailed Implementation

[0038] A method for manufacturing a cold-rolled steel sheet (the cold-rolled steel sheet according to this embodiment) according to one embodiment of the present invention will be described.

[0039] Regarding the cold-rolled steel sheet involved in this embodiment, (a) it has a specified chemical composition, (b) the metal structure at a position 1 / 4 of the sheet thickness t from the surface in the thickness direction, i.e., the t / 4 portion, is controlled within a specified range, and (c) regardless of whether it is at a position 50 mm from the end in the width direction, i.e., the edge portion, or the central portion in the width direction, the metal structure at a position 20 μm from the surface in the thickness direction, i.e., the 20 μm portion, and at a position 75 μm from the surface in the thickness direction, i.e., the 75 μm portion, is controlled within a specified range.

[0040] The cold-rolled steel sheet involved in this embodiment includes not only cold-rolled steel sheets without a coating on the surface, but also hot-dip galvanized steel sheets with a hot-dip galvanized coating on the surface, or alloyed hot-dip galvanized steel sheets with an alloyed hot-dip galvanized coating on the surface. Its main conditions are also the same as those of hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets.

[0041] However, when cladding steel sheets, the surface that serves as the reference for indicating the location of the specified metal structure refers to the surface of the base steel sheet from which the coating has been removed.

[0042] The following sections will explain each of them.

[0043] <Chemical Composition>

[0044] The chemical composition of the cold-rolled steel sheet involved in this embodiment will be described. Hereinafter, the percentage (%) of each element in the chemical composition refers to mass (%) unless otherwise specified.

[0045] C: Above 0.180% and below 0.350%

[0046] If the carbon content is below 0.180%, it is difficult to obtain the aforementioned metallic microstructure, and the target tensile strength cannot be achieved. Furthermore, the ratio (R / t) of the ultimate bending radius R to the plate thickness t during a 90° V-bending deteriorates. Therefore, the carbon content is specified to be 0.180% or more. The carbon content is preferably more than 0.180%, and more preferably 0.200% or more.

[0047] On the other hand, if the carbon content exceeds 0.350%, weldability and bending properties deteriorate. Furthermore, resistance to hydrogen embrittlement also deteriorates. Therefore, the carbon content is specified to be 0.350% or less. The carbon content is preferably less than 0.350%, and more preferably less than 0.300%.

[0048] Mn: 2.00% or more and 4.00% or less

[0049] Mn improves the hardenability of steel and is an effective element for obtaining the desired microstructure described later. If the Mn content is below 2.00%, it is difficult to obtain the desired microstructure. In this case, sufficient tensile strength cannot be obtained. Therefore, the Mn content is specified to be 2.00% or more. The Mn content is preferably more than 2.00%, more preferably 2.20% or more, and even more preferably 2.40% or more.

[0050] On the other hand, if the Mn content exceeds 4.00%, not only will the effect of improving hardenability be reduced due to Mn segregation, but it will also lead to an increase in raw material costs. Therefore, the Mn content is specified to be 4.00% or less. The Mn content is preferably less than 4.00%, more preferably less than 3.50%, and even more preferably less than 3.25%.

[0051] P: Above 0% and below 0.100%

[0052] Phosphorus (P) is an element contained in steel as an impurity, and it causes embrittlement of steel by segregating towards grain boundaries. Therefore, a lower P content is preferred, and it can be 0%, but considering the time and cost of P removal, the P content is specified as 0.100% or less. The P content is preferably 0.020% or less, and more preferably 0.015% or less. Considering the cost of refining, etc., the P content can also be specified as 0.005% or more.

[0053] S: 0% or more and less than 0.010%

[0054] Sulfur (S) is an element contained in steel as an impurity, and it deteriorates the bending properties by forming sulfide inclusions. Therefore, a lower S content is preferred, and it can be 0%, but considering the time and cost of S removal, the S content is specified as 0.010% or less. The S content is preferably 0.005% or less, more preferably 0.003% or less, and even more preferably 0.001% or less. Considering the costs of refining, etc., the S content can also be specified as 0.0001% or more.

[0055] Al: Above 0% and below 0.100%

[0056] Al is an element that deoxidizes molten steel. When Al is included for deoxidation purposes, a content of 0.005% or more, and more preferably 0.010% or more, is preferred to ensure effective deoxidation. Furthermore, Al, like Si, improves the stability of austenite and is effective for obtaining the aforementioned metallic structure; therefore, it may also be included. When included, the Al content may, for example, be 0.010% or more.

[0057] On the other hand, if the Al content is too high, not only are surface defects originating from alumina more likely to occur, but the phase transformation point also rises significantly, and the volume fraction of ferrite increases. In this case, it is difficult to obtain the aforementioned metallic structure, and sufficient tensile strength cannot be obtained. Therefore, the Al content is specified to be 0.100% or less. The Al content is preferably 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less. In the cold-rolled steel sheet according to this embodiment, it is not necessary to contain Al; the Al content can also be 0%.

[0058] N: 0% or more and less than 0.0100%

[0059] Nitrogen (N) is an element that can be contained in steel as an impurity, and it deteriorates the bending properties by forming coarse precipitates. Therefore, the N content is specified to be 0.0100% or less. The N content is preferably 0.0060% or less, and more preferably 0.0050% or less. The lower the N content, the more preferred, and it can also be 0%. Considering the costs of refining, etc., the N content can also be specified to be 0.0010% or more, or 0.0020% or more.

[0060] The cold-rolled steel sheet involved in this embodiment may contain the above-mentioned elements, with the remainder including Fe and impurities. It may also contain one or more of the elements listed below that affect strength and bending properties as arbitrary elements. However, since it is not necessary to contain any arbitrary element, the lower limit is 0%.

[0061] Si: 0% or more and less than 1.00%

[0062] Si is a useful element for increasing the strength of steel plates through solid solution strengthening. Furthermore, since Si inhibits the formation of cementite, it promotes the enrichment of carbon into austenite, making it an effective element for the formation of retained austenite after annealing. Therefore, Si can also be included. To fully obtain the above effects, it is preferable to specify the Si content as 0.005% or more. With a Si content of 0.005% or more, a uniform tensile strength can be sufficiently obtained, resulting in excellent resistance to hydrogen embrittlement. More preferably, the Si content is 0.01% or more, 0.03% or more, 0.05% or more, 0.10% or more, or 0.30% or more.

[0063] On the other hand, if the Si content exceeds 1.00%, the austenitic phase transformation during annealing is delayed, and sometimes the phase transformation from ferrite to austenite cannot occur sufficiently. In this case, excessive ferrite remains in the microstructure after annealing, and the target tensile strength cannot be achieved. Furthermore, bending properties, surface properties of the steel sheet, chemical conversion treatment properties, and plating properties are sometimes significantly deteriorated. Therefore, the Si content is specified to be 1.00% or less. The Si content is preferably less than 1.00%, more preferably 0.90% or less or 0.85% or less.

[0064] Ti: 0% or more and less than 0.050%

[0065] Nb: 0% or more and less than 0.050%

[0066] V: Above 0% and below 0.50%

[0067] Cu: 0% or more and less than 1.00%

[0068] Ti, Nb, V, and Cu are elements that improve the strength of steel plates through precipitation hardening. Therefore, these elements can also be included. To fully obtain the above-mentioned effects, it is preferable to specify the Ti content and Nb content as 0.001% or more, and preferably the V content and Cu content as 0.01% or more. More preferably, the Ti content and Nb content are 0.005% or more, and more preferably the V content and Cu content are 0.05% or more. Obtaining the above-mentioned effects is not mandatory. Therefore, it is not necessary to particularly limit the lower limits of the Ti content, Nb content, V content, and Cu content; their lower limits are 0%.

[0069] On the other hand, if these elements are present in excess, the recrystallization temperature rises, the metal structure of the cold-rolled steel sheet becomes inhomogeneous, and thus impairs its flexibility. Therefore, when these elements are present, the Ti content is specified to be 0.050% or less, the Nb content to be 0.050% or less, the V content to be 0.50% or less, and the Cu content to be 1.00% or less. The Ti content is preferably less than 0.050%, more preferably less than 0.030%, and even more preferably less than 0.020%. The Nb content is preferably less than 0.050%, more preferably less than 0.030%, and even more preferably less than 0.020%. The V content is preferably less than 0.30%. The Cu content is preferably less than 0.50%.

[0070] Ni: 0% or more and less than 1.00%

[0071] Cr: 0% or more and less than 1.00%

[0072] Mo: 0% or more and 0.50% or less

[0073] B: Above 0% and below 0.0100%

[0074] Ni, Cr, Mo, and B are elements that improve hardenability and contribute to the high strength of steel sheets, and are effective in obtaining the aforementioned metallic structure. Therefore, these elements may also be included. To fully obtain the above effects, it is preferable to specify the Ni content, Cr content, and Mo content as 0.01% or more and / or the B content as 0.0001% or more. More preferably, the Ni content, Cr content, and Mo content are 0.05% or more, and the B content is 0.0010% or more. Obtaining the above effects is not mandatory. Therefore, it is not necessary to specifically limit the lower limits of the Ni content, Cr content, Mo content, and B content; their lower limits are 0%.

[0075] On the other hand, even if these elements are present in excess, the effects described above become saturated and uneconomical. Therefore, when these elements are present, the Ni and Cr contents are specified to be 1.00% or less, the Mo content to be 0.50% or less, and the B content to be 0.0100% or less. The Ni and Cr contents are preferably 0.50% or less, the Mo content is preferably 0.20% or less, and the B content is preferably 0.0030% or less.

[0076] Ca: 0% or more and 0.010% or less

[0077] Mg: ≥0% and ≤0.0100%

[0078] REM: Above 0% and below 0.0500%

[0079] Bi: 0% or more and less than 0.050%

[0080] Ca, Mg, and REM are elements that can improve the strength and flexibility of steel plates by adjusting the shape of inclusions. Bi is an element that can improve strength and flexibility by refining the solidification structure. Therefore, these elements may also be included. To fully obtain the above effects, it is preferable to specify the Ca content as 0.0001% or more, the Mg content as 0.0001% or more, and the REM and Bi contents as 0.0005% or more respectively. More preferably, the Ca content is 0.0008% or more or 0.0010% or more, the Mg content is 0.0008% or more, and the REM and Bi contents are 0.0007% or more respectively. Obtaining the above effects is not mandatory. Therefore, it is not necessary to particularly limit the lower limits of the Ca, Mg, Bi, and REM contents; their lower limits are 0%.

[0081] On the other hand, even with an excess of these elements, the effects described above become saturated and uneconomical. Therefore, when these elements are present, the Ca content is specified to be 0.010% or less, the Mg content to be 0.0100% or less, the REM content to be 0.050% or less, and the Bi content to be 0.050% or less. Preferably, the Ca content is 0.009% or less or 0.002% or less, the Mg content is 0.0020% or less, the REM content is 0.0020% or less, and the Bi content is 0.010% or less. REM refers to rare earth elements, a collective term for 17 elements including Sc, Y, and the lanthanides; the REM content is the total content of these elements.

[0082] <Metal structure at a position (t / 4 part) on the plate thickness t from the surface along the thickness direction>

[0083] In the description of the metal structure of the cold-rolled steel sheet according to this embodiment, the structure fraction is expressed as volume fraction. Therefore, unless otherwise specified, "%" means "volume %".

[0084] [Retained austenite: ≥2.0% and ≤8.0%]

[0085] Retained austenite improves the ductility of steel sheets through the TRIP effect, which helps to increase the uniform elongation. Therefore, the volume fraction of retained austenite is specified to be 2.0% or more. The volume fraction of retained austenite is preferably more than 2.0%, more preferably 2.5% or more, and even more preferably 3.5% or more.

[0086] On the other hand, if the volume fraction of retained austenite is excessive, the grain size of the retained austenite increases. Such large-grained retained austenite becomes coarse and hard martensite after deformation. In this case, crack initiation is more likely to occur, leading to deterioration of bending properties. Therefore, the volume fraction of retained austenite is specified to be 8.0% or less. The volume fraction of retained austenite is preferably less than 8.0%, more preferably less than 7.0%, and even more preferably less than 6.0%.

[0087] [Tempered martensite: ≥80.0% and ≤98.0%]

[0088] Tempered martensite, like primary martensite, is an aggregate of lath-shaped grains. However, unlike primary martensite, it is a hard structure containing fine iron-based carbides formed through tempering. Tempered martensite can be obtained by tempering martensite formed after annealing and subsequent cooling, or by heat treatment.

[0089] Compared to martensite, tempered martensite is a non-brittle and ductile microstructure. In the cold-rolled steel sheet according to this embodiment, in order to improve strength, bending properties, and resistance to hydrogen embrittlement, the volume fraction of tempered martensite is specified to be 80.0% or more. The volume fraction of tempered martensite is preferably 85.0% or more. Since the volume fraction of retained austenite is specified to be 2.0% or more, the volume fraction of tempered martensite is 98.0% or less.

[0090] [Ferrite and bainite: Total ≥0.0% and ≤15.0%]

[0091] Ferrite is a soft phase obtained through two-phase zone annealing or slow cooling after annealing. Ferrite can improve the ductility of steel sheets when mixed with hard phases such as martensite, but to achieve high strengths above 1400 MPa, the volume fraction of ferrite needs to be limited.

[0092] Furthermore, bainite is a phase obtained after annealing by holding it at a temperature above 350°C and below 450°C for a certain period of time. Bainite is softer than martensite, thus improving ductility, but in order to achieve high strengths of over 1400 MPa, its volume fraction needs to be limited, just like ferrite.

[0093] Therefore, the combined volume fraction of ferrite and bainite is specified to be 15.0% or less. Preferably, it is 10.0% or less. Since ferrite and bainite can also be absent, their lower limit is 0.0%.

[0094] Furthermore, since ferrite is softer than bainite, when the total volume fraction of ferrite and bainite is less than 15.0%, in order to achieve a high strength of 1400 MPa or more, it is preferable that the volume fraction of ferrite is less than 10.0%.

[0095] [Martensite: ≥0.0% and ≤5.0%]

[0096] Martensite (primary martensite) is an aggregate of lath-shaped grains formed by a phase transformation from austenite during final cooling after tempering. Because martensite is hard and brittle, it easily becomes the initiation point for cracks during deformation; therefore, a high martensite volume fraction deteriorates flexibility. Thus, the martensite volume fraction is specified to be 5.0% or less. Preferably, the martensite volume fraction is 3.0% or less, more preferably 2.0% or less. Since martensite can also be absent, the lower limit for the martensite volume fraction is 0.0%.

[0097] In the metal microstructure located at a position (t / 4 portion) at a distance of 1 / 4 of the plate thickness t from the surface in the thickness direction, pearlite may also be present as a remaining portion of the microstructure, in addition to the above-mentioned components. However, pearlite is a microstructure containing cementite, which consumes carbon (C) in the steel that contributes to increased strength. If the volume fraction of pearlite is 5.0% or less, the strength of the steel plate can be increased. Therefore, it is preferable to specify the volume fraction of pearlite as 5.0% or less. More preferably, the volume fraction of pearlite is 3.0% or less, and even more preferably, 1.0% or less.

[0098] The volume fraction in the t / 4 portion of the cold-rolled steel sheet involved in this embodiment can be determined as follows.

[0099] In other words, the volume fractions of ferrite, bainite, martensite, tempered martensite, and pearlite can be determined by collecting test pieces from any position relative to the rolling direction of the steel plate and from the center position relative to the width direction. A longitudinal section parallel to the rolling direction (i.e., a section parallel to both the rolling and thickness directions) is ground. The microstructure revealed by nitric acid-ethanol etching is observed using SEM at a position 1 / 4 of the plate thickness t from the surface along the thickness direction. In SEM observation, at 3000x magnification, the viewer is positioned at 1 / 4 of the plate thickness t from the surface along the thickness direction. Five fields of view (30 μm) are observed in the thickness direction, and five fields of view (50 μm) are observed in the rolling direction. The area fraction of each microstructure is measured from the observed images, and its average value is calculated. Since there is no microstructure change in the vertical direction (steel plate width direction) relative to the rolling direction, the area fraction and volume fraction of the longitudinal section parallel to the rolling direction are equal. Therefore, the area fraction obtained through microstructure observation is taken as the respective volume fraction.

[0100] When determining the area ratio of each microstructure, regions with low brightness and no visible lower microstructure were classified as ferrite. Furthermore, regions with layered ferrite and cementite were classified as pearlite. Additionally, regions with high brightness and no visible lower microstructure were classified as martensite or retained austenite. Finally, regions with visible lower microstructure were classified as tempered martensite or bainite.

[0101] Bainite and tempered martensite can be further distinguished by careful observation of the carbides within the grains.

[0102] Specifically, tempered martensite consists of martensite laths and cementite formed within the laths. Since there are more than two possible crystal orientation relationships between the martensite laths and cementite, the cementite constituting tempered martensite has multiple varieties. On the other hand, bainite can be classified into upper bainite and lower bainite. Upper bainite is easily distinguished from tempered martensite because it consists of lath-shaped bainitic ferrite and cementite formed at the lath interfaces. Lower bainite consists of lath-shaped bainitic ferrite and cementite formed within the laths. In this case, the crystal orientation relationship between the bainitic ferrite and cementite is different from that of tempered martensite, being only one type, and the cementite constituting lower bainite has the same variety. Therefore, lower bainite and tempered martensite can be distinguished based on the variety of cementite.

[0103] On the other hand, martensite and retained austenite cannot be clearly distinguished in SEM observation. Therefore, the volume fraction of martensite can be calculated by subtracting the volume fraction of retained austenite, calculated by the method described later, from the volume fraction of the microstructure identified as martensite or retained austenite.

[0104] The volume fraction of retained austenite can be quantified by taking test pieces from any position relative to the rolling direction of the steel plate and from the center position relative to the width direction, from the chemically ground rolled surface of the steel plate up to 1 / 4 of the plate thickness, and from the (200), (210) area fraction intensity of ferrite and the (200), (220) and (311) area fraction intensity of austenite based on MoKα rays.

[0105] In the cold-rolled steel sheet according to this embodiment, regardless of whether it is the edge portion or the central portion in the width direction at a position 50 mm from the end, the metal structure at a position 20 μm from the surface in the thickness direction (20 μm portion) and the metal structure at a position 75 μm from the surface in the thickness direction (75 μm portion) are controlled as follows.

[0106] <Metal structure at a position 20 μm from the surface along the thickness direction of the plate (20 μm portion)>

[0107] The metal microstructure at a position 20 μm away from the surface in the thickness direction of the plate, i.e., the 20 μm portion, contains, by volume percentage, 75.0% or more and 100.0% or less of ferrite and bainite, and 0.0% or more and 25.0% or less of martensite and tempered martensite, with an average grain size of 5.0 μm or less for the martensite and tempered martensite.

[0108] By forming the surface layer into a ferrite and bainite matrix that is softer than tempered martensite, the flexibility is improved. Therefore, the total volume fraction of ferrite and bainite is specified to be 75.0% or more. Preferably, the total volume fraction of ferrite and bainite is 77.0% or more, more preferably 80.0% or more. On the other hand, the upper limit of the total volume fraction of ferrite and bainite can also be 100.0%.

[0109] Furthermore, the hard martensite and tempered martensite in the surface layer not only harden the surface layer but also increase the initiation point of cracks, thus deteriorating the bending properties and resistance to hydrogen embrittlement. In other words, the martensite and tempered martensite in the surface layer need to have a low volume fraction and be finely textured. Therefore, in the 20 μm section, the total volume fraction of martensite and tempered martensite is specified to be 25.0% or less, and the average grain size of martensite and tempered martensite is specified to be 5.0 μm or less. The total volume fraction of martensite and tempered martensite is preferably 22.0% or less, more preferably 20.0% or less. Since martensite and tempered martensite may also be absent, the lower limit of its total volume fraction can also be specified as 0.0%.

[0110] Furthermore, the average grain size of martensite and tempered martensite is preferably 4.5 μm or less. On the other hand, there is no specified lower limit for the average grain size of martensite and tempered martensite; the average grain size may also be specified as 0.1 μm or more, or as 0.5 μm or more.

[0111] The remaining portion of the metallic microstructure in the 20 μm section may also contain retained austenite and pearlite. Their combined volume fraction is preferably 3.0% or less.

[0112] <Metal structure at a position 75 μm from the surface along the thickness direction of the plate (75 μm portion)>

[0113] The metal microstructure at a position 75 μm away from the surface in the thickness direction of the plate, i.e., the 75 μm portion, contains a total of 0.0% to 15.0% ferrite and bainite by volume.

[0114] The flexibility is improved by increasing the proportion of soft tissue in the surface layer and reducing its hardness. However, if the thickness of the layer containing more soft tissue (soft layer) from the surface increases, the yield strength of the steel sheet as a component (component yield strength) may sometimes decrease, even if the strength of the steel sheet itself does not decrease.

[0115] The inventors conducted research and found that even when the surface layer (e.g., the aforementioned 20μm section) is mainly composed of soft tissue, as long as the 75μm section has the same tissue composition as the interior of the steel plate (e.g., the t / 4 section) (i.e., less soft tissue), the yield strength of the component will not decrease.

[0116] Therefore, in the cold-rolled steel sheet of this embodiment, the total volume fraction of ferrite and bainite in the 75μm portion is specified to be 15.0% or less. Preferably, the total volume fraction of ferrite and bainite is 10.0% or less. On the other hand, since ferrite and bainite may also be absent, the lower limit of the total volume fraction can also be specified as 0.0%.

[0117] Even at a position 60 μm from the surface in the thickness direction of the plate (60 μm portion), it is preferable that the total volume fraction of ferrite and bainite is 0.0% or more and 15.0% or less.

[0118] In the metal microstructure within the 75μm region, the remaining portion may also be retained austenite, tempered martensite, and martensite. For example, it may contain retained austenite of 2.0% or more and 8.0% or less, tempered martensite of 80.0% or more and 98.0% or less, and martensite of 0.0% or more and 5.0% or less.

[0119] Previously, as described in Patent Document 4 above, the formation of a surface soft layer was studied in a portion of the study. The average fraction of the hard structure other than ferrite and pearlite in this surface soft layer was 0.9 times that of the region from 1 / 4 to 1 / 2 of the plate thickness. However, Patent Document 4 does not focus on the total volume fraction of ferrite and bainite in the surface soft layer, and its approach differs from the metal structure relationship of the 20μm and 75μm portions of the cold-rolled steel sheet involved in this embodiment.

[0120] Regarding the volume fractions of ferrite, bainite, martensite, tempered martensite, and pearlite in the microstructure of the 20μm, 60μm, and 75μm portions, test pieces were collected from any position relative to the rolling direction of the steel plate, and from the center position and 50mm from the end position relative to the width direction. The longitudinal section parallel to the rolling direction of each test piece was ground. At positions 20μm from the surface in the thickness direction (range of 5-35μm from the surface × area of ​​50μm in the rolling direction), 60μm (range of 45-75μm from the surface × area of ​​50μm in the rolling direction), or 75μm (range of 60-90μm from the surface × area of ​​50μm in the rolling direction), the microstructure revealed by nitric acid ethanol etching was observed using SEM following the same procedure as for the t / 4 portion.

[0121] Furthermore, regarding the volume fraction of retained austenite in the 20μm and 75μm portions of the metal microstructure, test pieces were collected from any position relative to the rolling direction of the steel plate and from the center position and 50mm from the end position relative to the width direction. The rolled surface was chemically ground from the steel plate surface to the 20μm or 75μm position, and the surface fraction intensity of ferrite (200), (210) and austenite (200), (220) and (311) based on MoKα rays was quantified.

[0122] The average grain size of the martensite and tempered martensite in the 20μm section was determined by the following method.

[0123] Test pieces were collected from arbitrary positions relative to the rolling direction of the steel plate, and from the center and 50 mm from the end in the width direction. The longitudinal section of each test piece, parallel to the rolling direction, was ground. The metal microstructure revealed by nitric acid-ethanol etching was observed using SEM at a position 20 μm from the steel plate surface in the thickness direction. The equivalent circle average diameter of the microstructure identified as martensite or tempered martensite was calculated using the cutting method described in JIS G0551 (2013) and used as the average grain size of the martensite and tempered martensite.

[0124] The cold-rolled steel sheet involved in this embodiment, as described above, has the same volume fraction and average grain size for the 20μm and 75μm portions of the metal structure not only in the central portion in the width direction, but also in the edge portion 50mm from the end in the width direction.

[0125] In this case, the steel plate does not need to be trimmed when used in components, which can improve the yield of the steel plate.

[0126] <Mechanical Properties>

[0127] [Tensile strength: ≥1400MPa]

[0128] [Uniform elongation: ≥5.0%]

[0129] [The value obtained by dividing the ultimate bending radius R of a 90° V-bend by the plate thickness t (R / t): 5.0 or less]

[0130] In the cold-rolled steel sheet according to this embodiment, the tensile strength (TS) is preferably specified to be 1400 MPa or more, which is beneficial for the lightweighting of the automobile body. From the viewpoint of impact absorption, the tensile strength of the steel sheet is more preferably 1470 MPa or more. It is not necessary to limit the upper limit of the tensile strength, but if the tensile strength is increased, the formability may sometimes decrease, so the tensile strength may also be specified to be 1900 MPa or less.

[0131] Furthermore, from the viewpoint of formability, it is preferable to specify the uniform elongation (uEl) as 5.0% or more. To further improve formability, the uniform elongation (uEl) is more preferably 5.5% or more. There is no upper limit to the uniform elongation, but it may also be specified as 30.0% or less, or 20.0% or less.

[0132] Furthermore, from the viewpoint of bendability, the value obtained by dividing the ultimate bending radius R of a 90° V-bend by the plate thickness t (i.e., the ultimate bending radius R standardized by dividing by the plate thickness t) (R / t) is preferably specified to be 5.0 or less. To further improve bendability, (R / t) is more preferably 4.0 or less, and even more preferably 3.0 or less. (R / t) may also be specified to be 0.5 or more, or 1.0 or more.

[0133] Tensile strength (TS) and uniform elongation (uEl) are determined by taking JIS No. 5 tensile test specimens from the steel plate in a direction perpendicular to the rolling direction and conducting tensile tests in accordance with JIS Z2241:2011.

[0134] Furthermore, the standardized limit bending radius (R / t) based on the plate thickness is obtained by using a 90° V-shaped bending die, varying the radius R at 0.5mm intervals, calculating the minimum bending radius (limit bending radius) R without cracking, and then dividing it by the plate thickness t.

[0135] In the cold-rolled steel sheet of this embodiment, a hot-dip galvanized layer can also be provided on the surface. This coating improves corrosion resistance. When there are concerns about perforation caused by corrosion in automotive steel sheets, even with high strength, it is sometimes impossible to reduce the sheet thickness below a certain level. One of the goals of increasing the strength of steel sheets is to achieve lightweighting through thinning; therefore, even if high-strength steel sheets are developed, low corrosion resistance limits their applications. As a solution to these problems, applying a coating such as hot-dip galvanizing, which provides higher corrosion resistance, to the steel sheet is considered. Because the cold-rolled steel sheet of this embodiment controls the steel sheet composition as described above, hot-dip galvanizing can be performed on it.

[0136] Hot-dip galvanized coatings can also be alloyed hot-dip galvanized coatings.

[0137] <Manufacturing Conditions>

[0138] Specifically, the cold-rolled steel sheet involved in this embodiment can be manufactured by a manufacturing method comprising the following (Ⅰ) to (XI).

[0139] (I) The hot rolling process involves heating a cast slab with the aforementioned chemical composition as needed, and then hot rolling it to form a hot-rolled steel plate.

[0140] (II) The coiling process involves cooling the hot-rolled steel sheet to a coiling temperature below 550°C and then coiling it at that temperature.

[0141] (III) Cold rolling process, which involves pickling and cold rolling the above-mentioned hot-rolled steel sheet to form a cold-rolled steel sheet.

[0142] (IV) Annealing process: The furnace atmosphere during heating is specified as a nitrogen-hydrogen mixed atmosphere with a dew point of -20°C to 20°C and containing 1.0% to 20% by volume of hydrogen. The cold-rolled steel sheet after the above-mentioned cold rolling process is heated to 820°C or higher, i.e., the above-mentioned soaking temperature, at an average heating rate of less than 10.0°C / second from 700°C to the soaking temperature. Annealing is then carried out at the above-mentioned soaking temperature for 30 seconds to 200 seconds.

[0143] (V) The first cooling process cools the cold-rolled steel sheet after the annealing process to a temperature range exceeding 425°C but below 600°C.

[0144] (VI) A holding process, which, after the first cooling process described above, involves holding the cold-rolled steel sheet in a temperature range of more than 425°C and less than 600°C for more than 200 seconds and less than 750 seconds.

[0145] (VII) The second cooling process, after the aforementioned holding process, cools the cold-rolled steel sheet to a temperature of 50°C or higher and 250°C or lower.

[0146] (VIII) Tempering process, which involves tempering the cold-rolled steel sheet at a temperature of 200°C or higher and 350°C or lower for more than 1 second after the second cooling process described above.

[0147] (IX) The third cooling process, after the above tempering process, cools to a temperature at which surface finishing rolling can be performed;

[0148] (X) Skin smoothing process, which performs skin smoothing on the above-mentioned cold-rolled steel sheet after the third cooling process;

[0149] (XI) Within 10 hours after the completion of the above hot rolling process, the temperature of the above hot rolled steel plate shall be brought up to 500°C or below.

[0150] The following is a description of each process.

[0151] [Hot rolling process]

[0152] In the hot rolling process, a cast slab with the above-mentioned chemical composition is heated and hot rolled to form a hot-rolled steel sheet. If the temperature of the cast slab is high, it can be supplied directly for hot rolling without cooling it to near room temperature.

[0153] There are no specific limitations on the hot rolling conditions, but it is preferable to hot roll at a temperature above 1100°C, with the temperature at the finish roll exit side reaching or above the Ar3 phase transformation point. By specifying the heating temperature as above 1100°C, insufficient homogenization of the material can be avoided. Furthermore, as long as the temperature at the finish roll exit side is specified as above the Ar3 phase transformation point, ferrite processing structures are not generated, and a uniform microstructure is easily obtained, which is beneficial for improving flexibility.

[0154] The Ar3 phase transformation point (°C) is simply shown by the following formula based on its relationship with the element content in the steel. That is, it is described as in the following formula (1).

[0155] Ar3=910-310×[C]+25×{[Si]+2×[Al]}-80×[Mn eq Equation (1)

[0156] Here, without B, [Mn eq It is represented by the following formula (2a).

[0157] [Mneq ]=[Mn]+[Cr]+[Cu]+[Mo]+[Ni] / 2+10([Nb]-0.02) Formula (2a)

[0158] Furthermore, when B is present, [Mn] eq It is represented by the following formula (2b).

[0159] [Mn eq ]=[Mn]+[Cr]+[Cu]+[Mo]+[Ni] / 2+10([Nb]-0.02)+1 Formula (2b)

[0160] In the formula, [C] represents the C content (mass%), [Si] represents the Si content (mass%), [Al] represents the Al content (mass%), [Mn] represents the Mn content (mass%), [Cr] represents the Cr content (mass%), [Cu] represents the Cu content (mass%), [Mo] represents the Mo content (mass%), [Ni] represents the Ni content (mass%), and [Nb] represents the Nb content (mass%).

[0161] [Winding process]

[0162] After hot rolling, the steel sheet is coiled after cooling to the coiling temperature. The coiling temperature is specified to be below 550°C. If the coiling temperature exceeds 550°C, the microstructure of the hot-rolled steel sheet becomes a coarse ferrite-pearlite structure, resulting in an inhomogeneous microstructure and deteriorated bending properties after annealing. Furthermore, surface decarburization increases during coiling, making it impossible to control the microstructure of the annealed 75μm portion within the aforementioned range. Moreover, the accelerated cooling rate at the edges increases the microstructure difference in the width direction of the hot-rolled steel sheet, leading to inhomogeneity of the annealed microstructure in the width direction. A coiling temperature of 525°C or below is preferred.

[0163] There is no lower limit to the winding temperature, but if the winding temperature is low, it can be difficult to control the temperature in the width direction. Therefore, the winding temperature can also be specified as above 450°C, above 500°C, or above 510°C.

[0164] When the strength of hot-rolled steel plates is high, softening heat treatments such as BAF can be performed before cold rolling.

[0165] In the cold-rolled steel sheet manufacturing method of this embodiment, the steel sheet temperature is raised to below 500°C within 10 hours from the end of the hot rolling process described above. By raising the steel sheet temperature to below 500°C within 10 hours, the metal structure of the central portion and the edge portion of the sheet width is made uniform.

[0166] The time from the end of the hot rolling process to the steel plate temperature reaching below 500°C can be controlled by adjusting the cooling during the coiling process and the cooling after coiling.

[0167] The optimal time from the end of the hot rolling process to the temperature of the steel plate reaching below 500°C is within 5 hours.

[0168] Furthermore, it is preferable to achieve a steel plate temperature of 450°C or below within 10 hours from the end of the hot rolling process, and more preferably within 8 hours from the end of the hot rolling process.

[0169] [Cold rolling process]

[0170] In the cold rolling process, after hot-rolled steel sheets are pickled or otherwise descaled, they are cold-rolled to form cold-rolled steel. While there are no particular limitations on the cold rolling conditions, promoting recrystallization and homogenizing the metal structure after cold rolling and annealing improves flexibility. Therefore, it is preferable to specify a cold rolling yield (cumulative reduction) of 40% or more. More preferably, it is 45% or more, and even more preferably 50% or more.

[0171] If the cold rolling ratio is too high, the rolling load increases, making rolling difficult. Therefore, a cold rolling ratio of less than 70% is preferred. More preferably, it is less than 65%, and even more preferably, it is less than 60%.

[0172] [Annealing process]

[0173] In the annealing process, after the cold-rolled steel sheet has been degreased and treated according to known methods as needed, the furnace atmosphere during heating is specified as a nitrogen-hydrogen mixed atmosphere with a dew point of -20°C to 20°C and containing 1.0% to 20% hydrogen by volume. The sheet is heated to 820°C or higher, i.e., the soaking temperature, at an average heating rate of less than 10.0°C / second from 700°C to the soaking temperature. Annealing is then performed at the soaking temperature for 30 to 200 seconds.

[0174] The furnace atmosphere (heating zone and soaking zone) is specified as a nitrogen-hydrogen mixture with a dew point of -20°C to 20°C, containing 1.0% to 20% hydrogen by volume, and the remainder being nitrogen and impurities. Annealing under this atmosphere produces moderate decarburization in the surface layer of the steel plate. As a result, the desired metallic structure can be formed in the 20μm and 75μm sections. In other words, the surface layer, with its low C content due to decarburization, undergoes ferrite and bainite phase transformations before the phase transformation begins in the C-rich central layer. Therefore, only the surface layer is soft, and by suppressing the necessary amount of decarburization, the metallic structure of the 75μm section can be made equivalent to that of the t / 4 section.

[0175] However, since decarburization is prone to occur in temperature ranges above 700°C, heating to the homogenization temperature at an average heating rate of less than 10.0°C / second from 700°C promotes decarburization. The average heating rate in this temperature range is preferably less than 8.0°C / second, and more preferably less than 5.0°C / second. While there is no lower limit to the average heating rate from 700°C to the homogenization temperature, from an operational perspective, an average heating rate of 1.0°C / second or higher may also be specified.

[0176] The soaking temperature in the annealing process is specified to be 820°C or higher. If the soaking temperature is lower than 820°C, the volume fraction of ferrite in the portion t / 4 from the surface increases, resulting in insufficient tempered martensite and making it difficult to ensure adequate flexibility. The soaking temperature is preferably 840°C or higher, more preferably 850°C or higher. Higher soaking temperatures make it easier to ensure strength, but if the soaking temperature is too high, the manufacturing cost increases. Therefore, the soaking temperature is preferably 900°C or lower, more preferably 880°C or lower, and even more preferably 870°C or lower.

[0177] The soaking time is specified to be between 30 and 200 seconds. By specifying a soaking time of 30 seconds or more, sufficient austenitization can be achieved. On the other hand, from a productivity point of view, the soaking time is specified to be 200 seconds or less.

[0178] [First Cooling Process]

[0179] [Maintaining process]

[0180] To obtain the gradient microstructure described above (i.e., a microstructure with a difference in the total volume fraction of ferrite and bainite in the 20μm and 75μm portions), the cold-rolled steel sheet after the annealing process is cooled to a temperature range exceeding 425°C and falling below 600°C (first cooling process), and held in this temperature range (above 425°C and below 600°C) for a dwell time of 200 seconds to 750 seconds (holding process). If the cooling stop temperature and subsequent holding temperature are below 425°C, the volume fraction of bainite in the t / 4 portion increases, while the volume fraction of tempered martensite decreases. As a result, tensile strength decreases, and bending performance deteriorates.

[0181] On the other hand, if the cooling stop temperature and subsequent holding temperature are above 600°C, the ferrite fraction in the central part of the steel plate increases, while the volume fraction of tempered martensite decreases. As a result, tensile strength decreases, and bending performance deteriorates.

[0182] Therefore, the cooling stop temperature and holding temperature are specified to be above 425°C and below 600°C. The holding temperature is preferably above 440°C and below 580°C, and more preferably above 450°C and below 560°C. As long as it is within this temperature range, temperature changes during the dwell time are not a problem.

[0183] In the first cooling process, in order to suppress the ferrite phase transformation during cooling, it is preferable to perform cooling at an average cooling rate of 5.0°C / second or higher. More preferably, the average cooling rate is 10.0°C / second or higher.

[0184] If the residence time at temperatures exceeding 425°C but below 600°C is less than 200 seconds, the ferrite and bainite phase transformations in the surface layer (e.g., the 20 μm layer) will not progress. The untransformed austenite will become martensite and tempered martensite after final cooling. Therefore, not only does the volume fraction of martensite and tempered martensite increase, but its grain size also increases. Therefore, the residence time at temperatures exceeding 425°C but below 600°C in the holding process is specified to be 200 seconds or more. The residence time is preferably 300 seconds or more, and more preferably 350 seconds or more.

[0185] On the other hand, if the dwell time is too long, ferrite and bainite phase transformations will occur even in the 75μm and t / 4 sections, failing to obtain the desired microstructure, reducing the strength of the steel sheet, and deteriorating its bending properties. Therefore, the upper limit for the dwell time above 425°C and below 600°C is specified as 750 seconds or less. The dwell time above 425°C and below 600°C is preferably 650 seconds or less, and more preferably 550 seconds or less.

[0186] In the holding process, from the viewpoint of the chemical conversion treatment of the steel sheet or the adhesion of the coating, it is preferable to specify the furnace atmosphere as a reducing atmosphere.

[0187] When manufacturing cold-rolled steel sheets with a hot-dip galvanized surface (hot-dip galvanized steel sheet), hot-dip galvanizing can be performed by immersing the cold-rolled steel sheet in a hot-dip galvanizing bath during the holding process (hot-dip galvanizing process). Furthermore, when manufacturing cold-rolled steel sheets with an alloyed hot-dip galvanized surface (alloyed hot-dip galvanized steel sheet), alloying treatment can be performed in a subsequent hot-dip galvanizing process to form an alloyed hot-dip galvanized coating (alloying process).

[0188] [Second Cooling Process]

[0189] [Tempering process]

[0190] After holding the cold-rolled steel sheet, it is cooled to a temperature of 50°C or higher and 250°C or lower (second cooling process) to transform the untransformed austenite into martensite. In the second cooling process, to suppress the bainite transformation during cooling, it is preferable to cool at an average cooling rate of 5.0°C / second or higher. An average cooling rate of 10.0°C / second or higher is more preferable. Then, by tempering the cold-rolled steel sheet at a temperature of 200°C or higher and 350°C or lower for more than 1 second (tempering process), a microstructure dominated by tempered martensite can be obtained at a position one-quarter of the sheet thickness from the surface.

[0191] In the case of hot-dip galvanizing and / or alloying processes, after the cold-rolled steel sheet after the hot-dip galvanizing process, or the cold-rolled steel sheet after the hot-dip galvanizing and alloying processes, is cooled to a temperature of 50°C or higher and 250°C or lower, it is tempered at a temperature of 200°C or higher and 350°C or lower for more than 1 second.

[0192] If the cooling stop temperature of the second cooling process exceeds 250°C, the martensitic transformation is insufficient, the volume fraction of untempered martensite increases, and flexibility deteriorates. On the other hand, if the cooling stop temperature of the second cooling process is below 50°C, retained austenite will not remain, leading to deterioration of ductility. Therefore, the cooling stop temperature is specified to be 50°C or higher and 250°C or lower. Preferably, the cooling stop temperature is 75°C or higher and 225°C or lower, more preferably 100°C or higher and 200°C or lower.

[0193] In the subsequent tempering process, if the tempering temperature exceeds 350°C, the strength of the steel plate will decrease. Therefore, the tempering temperature is specified to be below 350°C. Preferably, the tempering temperature is below 330°C, and more preferably below 310°C.

[0194] On the other hand, if the tempering temperature is below 200°C, the tempering is insufficient, and the flexibility deteriorates. Therefore, the tempering temperature is specified to be 200°C or higher. The tempering temperature is preferably 250°C or higher, more preferably 260°C or higher, and even more preferably 270°C or higher.

[0195] The tempering time only needs to be 1 second or more, but it is preferable to be 5 seconds or more, and more preferably 10 seconds or more, in order to ensure stable tempering. On the other hand, in order to avoid a decrease in the strength of the steel plate, the tempering time is preferably 90 seconds or less, and more preferably 60 seconds or less.

[0196] In this embodiment, tempering refers to holding the temperature at which the material has been cooled to the tempering temperature during the second cooling process, or holding the material at the tempering temperature after cooling to a temperature below the tempering temperature during the second cooling process. Furthermore, holding the material during the tempering process not only means maintaining it at a fixed temperature, but also allows for temperature variations of less than 1.0°C / second within the tempering temperature range (i.e., 200°C or higher and 350°C or lower).

[0197] [Third Cooling Process]

[0198] [Surface smoothing process]

[0199] After the tempering process, the cold-rolled steel sheet is cooled to a temperature suitable for surface finishing (third cooling process) and then subjected to surface finishing (surface finishing process). When cooling after annealing (first cooling process) using water spray cooling, immersion cooling, or air-water cooling, pickling and the formation of trace amounts of one or more of Ni, Fe, Co, Sn, and Cu coatings can be performed before surface finishing to remove the oxide film formed by contact with water at high temperatures and to improve the chemical conversion treatment properties of the steel sheet. Here, "trace amounts" refers to a concentration of 3–30 mg / m² on the steel sheet surface. 2 The amount of plating on the left and right.

[0200] By surface rolling, the shape of the steel sheet can be adjusted. Preferably, the elongation rate of the surface rolled sheet is 0.1% or more, more preferably 0.2% or more, and even more preferably 0.3% or more. On the other hand, if the elongation rate of the surface rolled sheet is high, the volume fraction of retained austenite decreases, and the ductility deteriorates. Therefore, it is preferable to specify the elongation rate as 1.0% or less. More preferably, the elongation rate is 0.8% or less, even more preferably 0.6% or less, and most preferably 0.5% or less.

[0201] Example

[0202] The present invention will be described in more detail with reference to the embodiments.

[0203] Cast slabs with the chemical composition shown in Table 1 were produced. The cast slabs were heated to above 1100°C and hot-rolled to 2.8 mm in diameter, with the temperature at the finishing mill exit side reaching above the Ar3 phase transformation point. After coiling at the temperatures shown in Tables 2A and 2B, the slabs were cooled to room temperature. The time from the end of hot rolling until the steel plate temperature reached below 500°C and below 450°C are shown in Tables 2A and 2B.

[0204] Then, the oxide scale is removed by pickling, and after cold rolling to 1.4 mm, annealing is performed according to the conditions shown in Tables 2A and 2B. The holding time at the homogenization temperature is 120 seconds. In addition, the furnace atmosphere is specified as a nitrogen-hydrogen mixed atmosphere with a dew point of -20°C to 20°C and containing 1.0% to 20% hydrogen by volume.

[0205] After annealing, the temperature is cooled at 10°C / second to the holding temperature (first cooling) in Tables 2A and 2B, and then held at that temperature for the time shown in Tables 2A and 2B.

[0206] For some examples, hot-dip galvanizing and alloying were performed during the process. In Table 6, CR represents ungalvanized cold-rolled steel sheet, GI represents hot-dip galvanized steel sheet, and GA represents alloyed hot-dip galvanized steel sheet. For hot-dip galvanized steel sheet, a coating thickness of 35–65 g / m² was applied. 2 The hot-dip galvanizing is approximately [value missing]. Regarding alloyed hot-dip galvanized steel sheets, [the following is unclear due to incomplete sentence fragments]: 35–65 g / m [value missing]. 2 After hot-dip galvanizing, the metal is alloyed at a temperature below 600°C. In this embodiment, the temperature during the dwell time exceeding 425°C but below 600°C is specified as fixed, but as mentioned above, as long as it is within this temperature range, temperature variations during the dwell time are not a problem.

[0207] Furthermore, after holding, the material is cooled at a rate of 10.0°C / second or higher to a cooling stop temperature of 50°C or higher and 250°C or lower (second cooling), followed by tempering heat treatment at a tempering temperature of 250°C or higher and 350°C or lower for at least 1 second. If the cooling stop temperature is lower than the tempering temperature, tempering is performed by heating to the tempering temperature and holding at that temperature; if the cooling stop temperature is the same as the tempering temperature, tempering is performed by cooling and then holding at that temperature.

[0208] Then, it is cooled to 50°C (third cooling) and subjected to surface finishing with a stretching ratio of 0.1 to 1.0%.

[0209] As described above, SEM specimens were collected from the obtained annealed steel sheet (cold-rolled steel sheet). After grinding the longitudinal section parallel to the rolling direction, the microstructure in the 20μm, 60μm, 75μm, and t / 4 sections was observed, and the volume fraction of each microstructure was determined according to the above-mentioned procedures. In addition, the average grain size of martensite and tempered martensite in the 20μm section was also determined.

[0210] In addition, X-ray diffraction test pieces were collected, and as mentioned above, the volume fraction of retained austenite was determined by X-ray diffraction on surfaces from chemically ground to depths of 20 μm, 75 μm, and 1 / 4 of the plate thickness.

[0211] For the central portion in the width direction, the volume fraction of each microstructure in the t / 4 section was determined. On the other hand, for the edge portion and the central portion in the width direction of the steel plate, respectively, at a position 50 mm from the end (i.e., the edge portion) and the central portion in the width direction, the volume fraction of each microstructure in the 20 μm, 60 μm, and 75 μm portions, as well as the average grain size of martensite and tempered martensite in the 20 μm portion, were determined.

[0212] The results are shown in Tables 3, 4A, 4B, 5A, and 5B.

[0213] Tensile strength (TS) and uniform elongation (uEl) were determined by taking JIS No. 5 tensile test specimens from the center of the width direction of the obtained cold-rolled steel sheet, perpendicular to the rolling direction, and conducting tensile tests according to JIS Z2241 (2011). The results are shown in Table 6.

[0214] The limiting bending radius (R / t) was determined by using a 90° V-shaped bending die at 0.5 mm intervals along the central part of the width direction of the obtained cold-rolled steel sheet, varying the radius R, to find the minimum bending radius R without cracking, and then dividing it by the sheet thickness (1.4 mm). The results are shown in Table 6.

[0215] In addition, the following test was conducted to evaluate hydrogen embrittlement resistance. Specifically, a test piece with its mechanically ground end faces was bent into a U-shape using a bending method to create a U-shaped test piece with a radius of 5R. This U-shaped test piece was then bolted in a manner where the non-bent parts were parallel. After elastic deformation, it was immersed in hydrochloric acid at pH 1 to conduct a delayed fracture promotion test that allowed hydrogen to penetrate the steel plate. Steel plates that did not develop cracks even after 100 hours of immersion were evaluated as having good (OK) delayed fracture resistance, while those that developed cracks were evaluated as poor (NG). To remove the influence of the coating, the coating was removed from the coated material with hydrochloric acid containing a corrosion inhibitor before the test, and then the hydrogen embrittlement resistance was evaluated. The results are shown in Table 6.

[0216] The yield strength of the component is determined by the following method.

[0217] The obtained cold-rolled steel sheet was bent at R5 to form a hat shape with a height of 50mm, a top edge of 70mm, a bottom edge of 120mm, and a length of 900mm. A sample component was fabricated by merging steel sheets of the same size on the bottom side and spot-welding the flange. Furthermore, a comparative component was fabricated using a steel sheet with a surface layer (i.e., the 20μm portion) having the same microstructure as the t / 4 portion, similarly to the sample component. Both the sample component and the comparative component were bent by pressing a circular indenter along the central portion, and the maximum load at this point was taken as the component's yield strength. The sample component's yield strength was considered acceptable if it met or exceeded 95% of the comparative component's yield strength.

[0218] However, the yield strength tests were only conducted on cold-rolled steel sheets with a tensile strength of 1400 MPa or higher and an ultimate bending radius (R / t) of 5.0 or lower. The results are shown in Table 6.

[0219]

[0220]

[0221]

[0222] Table 3

[0223]

[0224]

[0225]

[0226]

[0227]

[0228] Table 6

[0229]

[0230] 1) The ultimate bending radius is the value obtained by dividing the crack initiation bending radius R during a 90° V-bend by the plate thickness t.

[0231] 2) CR is non-coated material, GA is alloyed hot-dip galvanized material, and GI is hot-dip galvanized material.

[0232] 3) When the surface strength is 95% or higher compared to the strength of a component made of the same material as the central component, it is set as OK.

[0233] As shown in Tables 1 to 6, in the inventive examples within the scope of this invention, the chemical composition and the metal microstructure of the t / 4, 20μm, and 75μm portions exhibit high strength, excellent flexibility, and sufficient yield strength of the components.

[0234] In contrast, in comparative examples where one or more of the chemical composition and the metal structure of the t / 4 section, 20μm section and 75μm section are outside the scope of the present invention, the target is not met for any one of the strength, bending properties and component yield strength.

[0235] In Test No. 11 (Comparative Example), where the "total percentage of ferrite and bainite", "total percentage of martensite and tempered martensite" and "average grain size of martensite and tempered martensite (μm)" at a position 20 μm from the surface in the width direction edge were outside the scope of this invention, although not shown in the table, the R / t (the value obtained by dividing the ultimate bending radius R during a 90° V-bend by the plate thickness t) at the width direction edge (a position 50 mm from the end in the width direction of the steel plate) was measured, and the result was as high as 5.0, indicating that the overall quality of the steel plate did not meet the requirements. As a result, the yield rate was significantly reduced.

Claims

1. A cold-rolled steel sheet, wherein, Its chemical composition, expressed as a percentage by mass, contains: C: Above 0.180% and below 0.350% Mn: Above 2.00% and below 4.00% P: Above 0% and below 0.100% S: 0% or more and less than 0.010% Al: Above 0% and below 0.100% N: 0% or more and less than 0.0100% Si: 0% or more and less than 1.00% Ti: 0% or more and 0.050% or less Nb: 0% or more and 0.050% or less V: Above 0% and below 0.50% Cu: 0% or more and less than 1.00% Ni: 0% or more and less than 1.00% Cr: 0% or more and less than 1.00% Mo: 0% or more and 0.50% or less B: Above 0% and below 0.0100% Ca: 0% or more and less than 0.010% Mg: ≥0% and ≤0.0100% REM: 0% or higher and 0.0500% or lower, and Bi: 0% or more and 0.050% or less The remaining portion includes Fe and impurities; The metal microstructure at a position (t / 4) on the plate thickness direction, at a distance of 1 / 4 from the surface, contains, by volume fraction: Residual austenite: 2.0% or more but less than 8.0% Tempered martensite: 80.0% or more and 98.0% or less Ferrite and bainite: totaling 0.0% to 15.0%, and Martensite: ≥0.0% and ≤5.0% Regardless of whether it is at a position 50mm from the end in the width direction, i.e., the edge portion or the center portion in the width direction, The metal microstructure at a position (20 μm) 20 μm away from the surface along the thickness direction of the plate contains, by volume fraction: Ferrite and bainite: Total ≥75.0% and ≤100.0% Martensite and tempered martensite: Total ≥0.0% and ≤25.0%, In the 20 μm portion of the metal microstructure, the average grain size of the martensite and the tempered martensite is 5.0 μm or less. The metal microstructure at a position 75 μm away from the surface along the thickness direction of the plate, i.e., the 75 μm portion, contains, by volume fraction: Ferrite and bainite: Total ≥0.0% and ≤15.0%.

2. The cold-rolled steel sheet according to claim 1, wherein, The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the following: Si: ≥0.005% and ≤1.00% Ti: ≥0.001% and ≤0.050% Nb: ≥0.001% and ≤0.050% V: Above 0.01% and below 0.50% Cu: ≥0.01% and ≤1.00% Ni: ≥0.01% and ≤1.00% Cr: ≥0.01% and ≤1.00% Mo: ≥0.01% and ≤0.50% B: Above 0.0001% and below 0.0100% Ca: ≥0.0001% and ≤0.010% Mg: ≥0.0001% and ≤0.0100% REM: 0.0005% or more and 0.0500% or less, and Bi: Above 0.0005% and below 0.050%.

3. The cold-rolled steel sheet according to claim 1 or 2, wherein, Tensile strength is above 1400MPa. The uniform elongation is above 5.0%. The value obtained by dividing the ultimate bending radius R of a 90° V-bend by the plate thickness t, i.e., R / t, is less than 5.

0.

4. The cold-rolled steel sheet according to any one of claims 1 to 3, wherein, A hot-dip galvanized layer is formed on the surface.

5. The cold-rolled steel sheet according to claim 4, wherein, The hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

6. A method for manufacturing cold-rolled steel sheet, wherein, It includes the following processes: The hot rolling process involves heating a cast slab as needed and then hot rolling it to form a hot-rolled steel sheet. The chemical composition of the cast slab, in mass percent, contains: C: ≥0.180% and ≤0.350%, Mn: ≥2.00% and ≤4.00%, P: ≥0% and ≤0.100%, S: ≥0% and ≤0.010%, Al: ≥0% and ≤0.100%, N: ≥0% and ≤0.0100%, Si: ≥0% and ≤1.00%, Ti: ≥0% and ≤0.050%, Nb... The following components are present in the following concentrations: 0% or more and 0.050% or less; V: 0% or more and 0.50% or less; Cu: 0% or more and 1.00% or less; Ni: 0% or more and 1.00% or less; Cr: 0% or more and 1.00% or less; Mo: 0% or more and 0.50% or less; B: 0% or more and 0.0100% or less; Ca: 0% or more and 0.010% or less; Mg: 0% or more and 0.0100% or less; REM: 0% or more and 0.0500% or less; and Bi: 0% or more and 0.050% or less. The remainder includes Fe and impurities. The coiling process involves cooling the hot-rolled steel sheet to a coiling temperature below 550°C and then coiling it at that temperature. The cold rolling process involves pickling and cold rolling the hot-rolled steel sheet to form a cold-rolled steel sheet. In the annealing process, the furnace atmosphere during heating is specified as a nitrogen-hydrogen mixed atmosphere with a dew point of -20°C to 20°C and containing 1.0% to 20% hydrogen by volume. The cold-rolled steel sheet after the cold rolling process is heated to 820°C or higher, i.e., the homogenization temperature, at an average heating rate of less than 10.0°C / second from 700°C to the homogenization temperature, and then annealed at the homogenization temperature for more than 30 seconds and less than 200 seconds. The first cooling process cools the cold-rolled steel sheet after the annealing process to a temperature range exceeding 425°C and below 600°C. The holding process, after the first cooling process, involves holding the cold-rolled steel sheet in the temperature range of more than 425°C and less than 600°C for more than 200 seconds and less than 750 seconds. The second cooling process, after the holding process, cools the cold-rolled steel sheet to a temperature of 50°C or higher and 250°C or lower. The tempering process involves tempering the cold-rolled steel sheet at a temperature of 200°C or higher and 350°C or lower for more than one second after the second cooling process. The third cooling process involves cooling the material to a temperature suitable for surface finishing after the tempering process. as well as The surface finishing rolling process involves performing surface finishing rolling on the cold-rolled steel sheet after the third cooling process. The temperature of the hot-rolled steel plate reaches 500°C within a time period of 3 to 5 hours after the end of the hot rolling process.

7. The method for manufacturing cold-rolled steel sheet according to claim 6, wherein, The chemical composition of the cast slab, expressed as a percentage by mass, contains one or more elements selected from the following: Si: ≥0.005% and ≤1.00% Ti: ≥0.001% and ≤0.050% Nb: ≥0.001% and ≤0.050% V: Above 0.01% and below 0.50% Cu: ≥0.01% and ≤1.00% Ni: ≥0.01% and ≤1.00% Cr: ≥0.01% and ≤1.00% Mo: ≥0.01% and ≤0.50% B: Above 0.0001% and below 0.0100% Ca: ≥0.0001% and ≤0.010% Mg: ≥0.0001% and ≤0.0100% REM: 0.0005% or more and 0.0500% or less, and Bi: Above 0.0005% and below 0.050%.

8. The method for manufacturing cold-rolled steel sheet according to claim 6 or 7, wherein, In the holding process, the cold-rolled steel sheet is immersed in a plating bath at a temperature exceeding 425°C but below 600°C to form a hot-dip galvanized layer on the surface.

9. The method for manufacturing cold-rolled steel sheet according to claim 8, wherein, In the holding process, an alloying treatment is performed to alloy the hot-dip galvanized layer.

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