Hot-rolled steel sheet and manufacturing method thereof

Abstract

A hot-rolled steel sheet according to the present invention has a specific chemical composition, while having a metal structure that contains, in area%, 25.0% or more of bainite, a total of 70.0% or less of fresh martensite and tempered martensite, 4.0% to 12.0% of residual austenite, 5.0% or less of ferrite and 3.0% or less of pearlite, with the average prior austenite grain size being 25.0 µm or less. With respect to the metal structure, if θ is the angle between the rolling direction and the respective major axis directions of the crystal grains of residual austenite, fresh martensite and tempered martensite, the proportion of the crystal grains having θ of 0° to 15°, the proportion of the crystal grains having θ of more than 15° but not more than 30°, the proportion of the crystal grains having θ of more than 30° but not more than 45°, the proportion of the crystal grains having θ of more than 45° but not more than 60°, the proportion of the crystal grains having θ of more than 60° but not more than 75° and the proportion of the crystal grains having θ of more than 75° but not more than 90° are 40% or less in percentage.

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet and a method for producing the same. This application claims priority based on Japanese Patent Application No. 2022-001424, filed on January 7, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, efforts have been made to reduce the weight of automobiles and machine parts. By ensuring rigidity through optimal part design, it is possible to reduce the weight of automobiles and machine parts. Furthermore, for blank-formed parts such as press-formed parts, weight can be reduced by reducing the thickness of the part material.

[0003] However, if we want to maintain the strength properties of parts, such as static fracture strength and yield strength, while reducing the plate thickness, it becomes necessary to use high-strength materials. In particular, the use of steel sheets with a strength of over 780 MPa is being considered for automotive suspension parts, such as lower arms, trail links, and knuckles. These suspension parts are manufactured by subjecting steel sheets to burring, stretch flange forming, bending, and other processes. Therefore, the steel sheets used for these suspension parts must have excellent formability, particularly ductility and hole expandability.

[0004] For example, Patent Document 1 discloses a high-strength steel sheet characterized in that the metal structure is essentially a two-phase structure of ferrite and bainite, and carbides containing Ti and Mo are dispersed and precipitated in the ferrite phase. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2003-321725 Summary of the Invention [Problem to be solved by the invention]

[0006] However, Patent Document 1 does not take into consideration ductility and hole expandability.

[0007] Automotive undercarriage parts such as those described above are manufactured by subjecting steel sheets to multiple forming processes. Therefore, steel sheets used in automotive undercarriage parts are required to have excellent formability even after being subjected to a certain degree of pre-strain in the upstream process. In the process of multiple forming processes, if a rapid change in strain path occurs during the transition from the upstream process to the downstream process, the strain-induced transformation of retained austenite tends to progress rapidly, resulting in a decrease in the retained austenite fraction. This tends to reduce the work hardening rate in the later stage of deformation in the downstream forming process, and the inherent formability of the hot-rolled steel sheet may not be exhibited.

[0008] The present invention has been made in view of the above-mentioned circumstances, and aims to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent formability after pre-straining, and a manufacturing method thereof. [Means for solving the problem]

[0009] The present inventors have conducted creative research into the above-mentioned hot-rolled steel sheet and a method for producing the same, and as a result have come to the following findings.

[0010] In order to improve formability after pre-straining, it is important that uniform elongation does not decrease significantly, especially when the deformation path changes suddenly. We have found that in order to prevent a decrease in uniform elongation when the deformation path changes, it is important that the angle θ between the major axis direction of the crystal grains of retained austenite, fresh martensite, and tempered martensite and the rolling direction is not concentrated in a specific direction but is appropriately dispersed.

[0011] In order to obtain the above-mentioned metal structure, it is effective to strictly control the rough rolling conditions and finish rolling conditions.

[0012] The gist of the present invention, which was made based on the above findings, is as follows. (1) A hot-rolled steel sheet according to one aspect of the present invention has a chemical composition, in mass%, C: 0.11 to 0.23%, Si: 0.70 to 1.80% Mn: 1.95-3.10% P: 0.060% or less, S: 0.005% or less, Al: 0.010~0.430%, N: 0.0070% or less, Ti: 0.006 to 0.055% Nb: 0.006 to 0.040%, B: 0.0001 to 0.0030%, Cr:0 .050 ~0.470%, Mo:0 .005 ~0.120%, V: 0~0.10%, Cu: 0-0.40% Ni: 0 to 0.30% Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, REM: 0 to 0.1000%, Bi: 0 to 0.020% The sum of Zr, Co, Zn and W: 0 to 1.00%, and Sn: 0 to 0.05% the balance being Fe and impurities, The metal structure is, in area%, Bainite: 25.0% or more, Sum of fresh martensite and tempered martensite: 70.0% or less, Retained austenite: 4.0 to 12.0% Ferrite: 5.0% or less, and Pearlite: 3.0% or less, The average grain size of prior austenite grains is 25.0 μm or less, When the angle θ is defined as the angle between the long axis direction of the crystal grains of the retained austenite, the fresh martensite, and the tempered martensite and the rolling direction, the percentage of the crystal grains having an angle of θ of 0 to 15°, the percentage of the crystal grains having an angle of more than 15° and not more than 30°, the percentage of the crystal grains having an angle of more than 30° and not more than 45°, the percentage of the crystal grains having an angle of more than 45° and not more than 60°, the percentage of the crystal grains having an angle of more than 60° and not more than 75°, and the percentage of the crystal grains having an angle of more than 75° and not more than 90° are each 40% or less. (2) In the hot-rolled steel sheet described in (1) above, the area ratio of the tempered martensite may be 80.0% or more in terms of percentage, based on the total area ratio of the fresh martensite and the tempered martensite. (3) The hot-rolled steel sheet according to (1) or (2) above, wherein the chemical composition is, in mass%, V: 0.01 to 0.10%, Cu: 0.01 to 0.40% Ni: 0.02 to 0.30% Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, REM: 0.0005 to 0.1000%, and Bi: 0.0005 to 0.020% The compound may contain one or more of the group consisting of: (4) A method for producing a hot-rolled steel sheet according to another aspect of the present invention is the method for producing a hot-rolled steel sheet according to the above (1), A heating step of holding a slab having the chemical composition described in (1) above in a temperature range of 1220 to 1300 ° C for 40 minutes or more; a rough rolling process in which rough rolling is performed so that the reduction rates for each of the first to third passes are 10 to 30%, and the reduction rates for each of the fourth and subsequent passes are 15 to 50%; A finish rolling process in which the final pass is rolled in a temperature range of 940 to 1020 ° C. at a reduction rate of more than 25%, or the rolling one pass before the final pass is rolled in a temperature range of 940 to 1020 ° C. at a reduction rate of more than 25%, and the final pass is rolled at a reduction rate of less than 15%; a cooling step in which cooling is started within 2.0 seconds after the completion of finish rolling, and cooling is performed to a temperature range of 300 to 480°C within 17.0 seconds; a winding step of winding in the temperature range; a reheating step of heating the film within 30 minutes after the winding step so that the heating amount ΔT satisfies the following formula (1); An air-cooling step is performed to a temperature range of 200°C or less. 245 - 0.942 × CT + 0.00092 × CT 2 <ΔT<686-2.38×CT+0.0022×CT 2 …(1) In the above formula (1), CT is the cooling stop temperature. [Effects of the Invention]

[0013] According to the above aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent formability after pre-straining, and a manufacturing method thereof. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 10 is a diagram for explaining a method of approximating a crystal grain with an ellipsoid. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, the hot-rolled steel sheet according to this embodiment will be described in detail. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present invention.

[0016] In the following, numerical ranges separated by "to" include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" are not included in the numerical range. All "%" in chemical composition refers to "mass %."

[0017] The chemical composition of the hot-rolled steel sheet according to this embodiment is, in mass%, C: 0.11 to 0.23%, Si: 0.70 to 1.80%, Mn: 1.95 to 3.10%, P: 0.060% or less, S: 0.005% or less, Al: 0.010 to 0.430%, N: 0.0070% or less, Ti: 0.006 to 0.055%, Nb: 0.006 to 0.040%, B: 0.0001 to 0.0030%, and the balance: Fe and impurities. Each element will be described in detail below.

[0018] C: 0.11 to 0.23% C is an element necessary for obtaining the desired tensile strength of the hot-rolled steel sheet. If the C content is less than 0.11%, the desired tensile strength cannot be obtained. Therefore, the C content is set to 0.11% or more. The C content is preferably 0.12% or more, 0.13% or more, or 0.15% or more. On the other hand, if the C content exceeds 0.23%, the area ratio of martensite becomes excessive, and the ductility and / or hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the C content is set to 0.23% or less. The C content is preferably 0.21% or less, and more preferably 0.19% or less.

[0019] Si: 0.70 to 1.80% Si is an element that stabilizes retained austenite. If the Si content is less than 0.70%, the desired amount of retained austenite cannot be obtained, and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 0.70% or more. The Si content is preferably 0.90% or more or 1.00% or more, and more preferably 1.10% or more. On the other hand, if the Si content exceeds 1.80%, the amount of retained austenite becomes too large, which deteriorates the hole expandability of the hot-rolled steel sheet. Therefore, the Si content is set to 1.80% or less. The Si content is preferably 1.60% or less, and more preferably 1.50% or less.

[0020] Mn: 1.95 to 3.10% Mn is an element necessary for improving the strength of hot-rolled steel sheets. If the Mn content is less than 1.95%, the area ratio of ferrite becomes too high, making it impossible to obtain the desired tensile strength. Therefore, the Mn content is set to 1.95% or more. The Mn content is preferably 2.00% or more, and more preferably 2.10% or more. On the other hand, if the Mn content exceeds 3.10%, the strength of the hot-rolled steel sheet becomes too high, and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Mn content is set to 3.10% or less. The Mn content is preferably 3.00% or less, 2.80% or less, and more preferably 2.60% or less.

[0021] P:0.060% or less P is an element that segregates in the center of the thickness of a hot-rolled steel sheet. P is also an element that embrittles welded joints. If the P content exceeds 0.060%, slab cracking is likely to occur, making casting difficult. Therefore, the P content is set to 0.060% or less. The P content is preferably 0.020% or less, and more preferably 0.010% or less. The lower the P content, the better, and 0% is preferable, but if the P content is reduced too much, the cost of dephosphorization increases significantly, so the P content may be set to 0.0005% or more.

[0022] S: 0.005% or less S is an element that embrittles slabs when present as sulfides. S also deteriorates the formability of hot-rolled steel sheets. If the S content exceeds 0.005%, the hole expandability of the hot-rolled steel sheets deteriorates. Therefore, the S content is set to 0.005% or less. The S content is preferably 0.004% or less, and more preferably 0.003% or less. The lower the S content, the better, and 0% is preferable, but if the S content is reduced too much, the cost of desulfurization increases significantly, so the S content may be set to 0.0005% or more.

[0023] Al: 0.010 to 0.430% Al acts as a deoxidizer and is an element that improves the cleanliness of steel. If the Al content is less than 0.010%, a sufficient deoxidizing effect cannot be obtained, and a large amount of inclusions (oxides) is formed in the steel sheet. Such inclusions deteriorate the formability of the hot-rolled steel sheet. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, if the Al content exceeds 0.430%, casting becomes difficult. Therefore, the Al content is set to 0.430% or less. The Al content is preferably 0.400% or less, 0.300% or less, and more preferably 0.100% or less.

[0024] N: 0.0070% or less N is an element that forms coarse nitrides in steel and deteriorates the hole expandability of hot-rolled steel sheets. If the N content exceeds 0.0070%, the hole expandability of hot-rolled steel sheets deteriorates. Furthermore, if a large amount of N is contained, the risk of slab cracking increases. Therefore, the N content is set to 0.0070% or less. The N content is preferably 0.0050% or less or 0.0040% or less, and more preferably 0.0035% or less. The lower the N content, the better, and 0% is preferable, but if the N content is reduced too much, the cost of denitrification increases significantly, so the N content may be 0.0005% or more.

[0025] Ti: 0.006 to 0.055% Ti is an element that increases the strength of hot-rolled steel sheets by forming fine nitrides in the steel. If the Ti content is less than 0.006%, the desired tensile strength cannot be obtained. Therefore, the Ti content is set to 0.006% or more. The Ti content is preferably 0.010% or more, 0.020% or more, or 0.025% or more. On the other hand, if the Ti content exceeds 0.055%, the hole expandability of the hot-rolled steel sheet deteriorates. Also, if a large amount of Ti is contained, the risk of slab cracking increases. Therefore, the Ti content is set to 0.055% or less. The Ti content is preferably 0.050% or less, and more preferably 0.045% or less.

[0026] Nb: 0.006 to 0.040% Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Nb also forms fine carbides, thereby increasing the strength of the hot-rolled steel sheet. If the Nb content is less than 0.006%, prior austenite grains cannot be refined, and the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the Nb content is set to 0.006% or more. The Nb content is preferably 0.010% or more or 0.013% or more, and more preferably 0.015% or more. On the other hand, if the Nb content exceeds 0.040%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the Nb content is set to 0.040% or less. The Nb content is preferably 0.035% or less, and more preferably 0.030% or less.

[0027] B: 0.0001 to 0.0030% B is an element that suppresses the formation of ferrite during the cooling process and increases the strength of the hot-rolled steel sheet. If the B content is less than 0.0001%, the desired tensile strength cannot be obtained. Therefore, the B content is set to 0.0001% or more. The B content is preferably 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the B content exceeds 0.0030%, the hot deformation resistance increases, making it difficult to perform hot rolling. Therefore, the B content is set to 0.0030% or less. The B content is preferably 0.0025% or less.

[0028] The balance of the chemical composition of the steel sheet according to this embodiment may be Fe and impurities. In this embodiment, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, or substances that are allowed to exist within a range that does not adversely affect the steel sheet according to this embodiment.

[0029] The steel sheet according to this embodiment may contain the following optional elements instead of part of Fe. When no optional elements are contained, the lower limit of the content is 0%. Each optional element will be described below.

[0030] Cr: 0 to 0.470% Cr is an element that exhibits effects similar to those of Mn. To ensure that the Cr content enhances the strength of the hot-rolled steel sheet, the Cr content is preferably 0.001% or more. On the other hand, if the Cr content exceeds 0.470%, the martensite fraction increases and the ductility of the hot-rolled steel sheet deteriorates, so the Cr content is set to 0.470% or less.

[0031] Mo: 0 to 0.120% Mo is an element that increases the strength of a hot-rolled steel sheet by forming fine carbides in the steel. To ensure this effect, the Mo content is preferably 0.001% or more. On the other hand, if the Mo content exceeds 0.120%, the hole expandability of the hot-rolled steel sheet deteriorates, so the Mo content is set to 0.120% or less.

[0032] V: 0 to 0.10% V is an element that increases the strength of the hot-rolled steel sheet by forming fine carbides in the steel. To ensure this effect, the V content is preferably 0.01% or more. On the other hand, if the V content exceeds 0.10%, the hole expandability of the hot-rolled steel sheet deteriorates, so the V content is set to 0.10% or less.

[0033] Cu: 0 to 0.40% Cu has the effect of improving the hardenability of steel sheet and the effect of precipitating as carbides in steel at low temperatures to increase the strength of hot-rolled steel sheet. To more reliably obtain the effects of these actions, the Cu content is preferably 0.01% or more. However, if the Cu content exceeds 0.40%, intergranular cracking may occur in the slab, so the Cu content is set to 0.40% or less.

[0034] Ni: 0 to 0.30% Ni has the effect of improving the hardenability of the steel sheet and increasing the strength of the hot-rolled steel sheet. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing grain boundary cracking of the slab caused by Cu. To more reliably obtain the above effects, the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is not economically preferable to add a large amount of Ni, so the Ni content is set to 0.30% or less.

[0035] Ca: 0 to 0.0200% Mg: 0 to 0.0200% REM: 0 to 0.1000% Ca, Mg, and REM all have the effect of improving the formability of hot-rolled steel sheets by controlling the shape of inclusions to a preferred shape. Therefore, one or more of these elements may be added. To more reliably obtain the effects of these actions, it is preferable to set the content of any one or more of Ca, Mg, and REM to 0.0005% or more. However, if the Ca content or Mg content exceeds 0.0200%, or if the REM content exceeds 0.1000%, excessive inclusions may be formed in the steel, which may deteriorate the ductility of the hot-rolled steel sheets. Therefore, the Ca content and Mg content are set to 0.0200% or less, and the REM content is set to 0.1000% or less.

[0036] Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of misch metal.

[0037] Bi: 0 to 0.020% Bi has the effect of improving the formability of hot-rolled steel sheets by refining the solidification structure. To ensure this effect, the Bi content is preferably 0.0005% or more. However, if the Bi content exceeds 0.020%, the effect of this effect saturates, which is economically undesirable. Therefore, the Bi content is set to 0.020% or less. The Bi content is preferably 0.010% or less.

[0038] Total of Zr, Co, Zn and W: 0 to 1.00% Sn: 0 to 0.05% The inventors have confirmed that the effects of the hot-rolled steel sheet according to this embodiment are not impaired even if the total content of Zr, Co, Zn, and W is 1.00% or less. Therefore, one or more of Zr, Co, Zn, and W may be contained in a total content of 1.00% or less. Furthermore, the inventors have confirmed that the effect of the hot-rolled steel sheet according to this embodiment is not impaired even if a small amount of Sn is added. However, since defects may occur during hot rolling, the Sn content is set to 0.05% or less.

[0039] The chemical composition of the hot-rolled steel sheet can be analyzed using a spark discharge optical emission spectrometer or similar. The values ​​of C and S are determined by burning the steel in an oxygen stream using a gas composition analyzer or similar, and measuring the results using an infrared absorption method. The values ​​of N are determined by melting a test piece taken from the hot-rolled steel sheet in a helium stream and measuring the results using a thermal conductivity method.

[0040] Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described. The hot-rolled steel sheet according to this embodiment has a metal structure, in area %, of bainite: 25.0% or more, the sum of fresh martensite and tempered martensite: 70.0% or less, retained austenite: 4.0 to 12.0%, ferrite: 5.0% or less, and pearlite: 3.0% or less, the average grain size of prior austenite grains is 25.0 μm or less, and when the angle θ is defined as the angle between the long axis direction of the crystal grains of the retained austenite, the fresh martensite, and the tempered martensite and the rolling direction, the proportion of the crystal grains whose angle θ is 0 to 15°, the proportion of the crystal grains whose angle θ is more than 15° and less than 30°, the proportion of the crystal grains whose angle θ is more than 30° and less than 45°, the proportion of the crystal grains whose angle θ is more than 45° and less than 60°, the proportion of the crystal grains whose angle θ is more than 60° and less than 75°, and the proportion of the crystal grains whose angle θ is more than 75° and less than 90° are each 40% or less.

[0041] In this embodiment, the metallographic structure at the 1 / 4 position in the thickness direction of the hot-rolled steel sheet (the region from the surface to the depth of 1 / 8 of the thickness direction of the sheet) is specified because the metallographic structure at this position represents a typical metallographic structure of the steel sheet.

[0042] Bainite area ratio: 25.0% or more Bainite is a structure that increases the strength, ductility, and hole expandability of hot-rolled steel sheets. If the area fraction of bainite is less than 25.0%, the desired strength, ductility, and / or hole expandability cannot be obtained. Therefore, the area fraction of bainite is set to 25.0% or more. Preferably, the area fraction is 30.0% or more, 40.0% or more, 55.0% or more, 60.0% or more, or 65.0% or more. The upper limit of the area fraction of bainite is not particularly limited, but may be set to 96.0% or less in relation to the area fraction of retained austenite. The area fraction of bainite may also be set to 90.0% or less or 85.0% or less.

[0043] Total area ratio of fresh martensite and tempered martensite: 70.0% or less Fresh martensite and tempered martensite are effective structures for increasing the strength of hot-rolled steel sheets. However, if the total area ratio of these structures exceeds 70.0%, the ductility and hole expandability of the hot-rolled steel sheet deteriorate. Therefore, the total area ratio of fresh martensite and tempered martensite is set to 70.0% or less. Preferably, it is 40.0% or less, 20.0% or less, or 10.0% or less. It is not necessary to include both fresh martensite and tempered martensite, and only one of them may be included. The total area ratio of fresh martensite and tempered martensite may be 0%, or may be 1.0% or more, or 3.0% or more.

[0044] The percentage of the area ratio of tempered martensite out of the total area ratio of fresh martensite and tempered martensite: 80.0% or more The area ratio of tempered martensite is preferably 80.0% or more of the total area ratio of fresh martensite and tempered martensite. By making the area ratio of tempered martensite 80.0% or more, fresh martensite adjacent to retained austenite can be prevented from reducing the stability of the retained austenite. As a result, even when the deformation path is changed, the reduction in uniform elongation can be further reduced. There is no particular upper limit, but it may be set to 100%.

[0045] Area ratio of retained austenite: 4.0 to 12.0% Retained austenite is a structure that increases the ductility of a hot-rolled steel sheet. If the area fraction of retained austenite is less than 4.0%, it may be impossible to obtain the desired ductility or excellent formability after pre-straining. Therefore, the area fraction of retained austenite is set to 4.0% or more. Preferably, it is 5.0% or more or 6.0% or more. On the other hand, if the area fraction of retained austenite exceeds 12.0%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the area fraction of retained austenite is set to 12.0% or less, preferably 11.0% or less, 10.0% or less, or 8.0% or less.

[0046] Ferrite area ratio: 5.0% or less If the area ratio of ferrite exceeds 5.0%, the strength of the hot-rolled steel sheet will deteriorate. Therefore, the area ratio of ferrite is set to 5.0% or less, preferably 3.0% or less or 2.0% or less, and may be 0.0%.

[0047] Perlite area ratio: 3.0% or less If the area fraction of pearlite exceeds 3.0%, the strength, ductility, and / or hole expandability of the hot-rolled steel sheet may deteriorate, or excellent formability may not be obtained after pre-straining. Therefore, the area fraction of pearlite is set to 3.0% or less. It is preferably 2.0% or less or 1.0% or less, and may be 0.0%.

[0048] The method for measuring the area ratio of each texture will be explained below. Test pieces are taken from the hot-rolled steel sheet in a cross section parallel to the rolling direction so that the metal structure can be observed at a position 1 / 4 of the sheet thickness from the surface (the region from 1 / 8 depth of the sheet thickness from the surface to 3 / 8 depth of the sheet thickness from the surface) and at the center position in the sheet width direction.

[0049] The cross section of the specimen was polished using #600 to #1500 silicon carbide paper, then polished to a mirror finish using a liquid in which diamond powder with a grain size of 1 to 6 μm was dispersed in a diluted solution such as alcohol or pure water. Next, the specimen was polished for 8 minutes at room temperature using colloidal silica with a grain size of 0.25 μm and no alkaline solution to remove the strain introduced into the surface layer of the specimen. At any position along the longitudinal direction of the specimen, a region extending 50 μm in the longitudinal direction of the specimen, from 1 / 8 of the plate thickness depth to 3 / 8 of the plate thickness depth from the surface, was measured using electron backscatter diffraction at 0.1 μm measurement intervals to obtain crystal orientation information.

[0050] For the measurements, an EBSD device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) was used. The vacuum level inside the EBSD device was 9.6 × 10 -5 The conditions are: 1. Vapor pressure (Pa) or less, 2. Acceleration voltage (kV), 3. Probe current level (13), and electron beam irradiation level (62). From the obtained crystal orientation information, the "Phase Map" function in the "OIM Analysis (registered trademark)" software attached to the EBSD analyzer is used to identify regions with an fcc crystal structure and calculate the area fraction of these regions. This gives the area fraction of retained austenite.

[0051] Next, regions with a bcc crystal structure are determined to be bainite, ferrite, pearlite, fresh martensite, and tempered martensite. For these regions, the "Grain Orientation Spread" function in the "OIM Analysis®" software provided with the EBSD analyzer is used to extract regions with a "Grain Orientation Spread" of 1° or less as ferrite, under the condition that 15° grain boundaries are considered to be crystal grain boundaries. The area fraction of the extracted ferrite is then calculated to obtain the ferrite area fraction.

[0052] Next, within the remaining region (the region where "Grain Orientation Spread" exceeds 1°), under the condition that the 15° grain boundary is defined as the crystal grain boundary, when the maximum value of "Grain Average IQ" in the ferrite region is Iα, the region where it exceeds Iα / 2 is extracted as bainite, and the region where it is Iα / 2 or less is extracted as "pearlite, fresh martensite, and tempered martensite." The area fraction of bainite is obtained by calculating the area fraction of the extracted bainite. If ferrite is not extracted from the observation field, the GAM "Grain Average Misorientation" function is used for the same field, and under the condition that 5° grain boundaries are considered to be crystal grain boundaries, areas where "Grain Average Misorientation" is greater than 0.50° and less than 0.75° are extracted as bainite, and areas where it is greater than 0.75° are extracted as "pearlite, fresh martensite, and tempered martensite." The area fraction of bainite is obtained by calculating the area fraction of the extracted bainite.

[0053] The extracted "pearlite, fresh martensite, and tempered martensite" are distinguished into pearlite, fresh martensite, and tempered martensite by the following method.

[0054] To observe the same area as the EBSD measurement area with an SEM, a Vickers indentation is made near the observation position. Surface contamination is then polished away, leaving the structure of the observation surface, and the specimen is then etched with nital. The same field of view as the EBSD observation surface is then observed with an SEM at 3000x magnification. Of the areas identified as "pearlite, fresh martensite, and tempered martensite" in the EBSD measurement, areas with intragranular substructure and where cementite precipitates in multiple variants are deemed to be tempered martensite. Areas where cementite precipitates in a lamellar form are deemed to be pearlite. Areas with high brightness and where the substructure is not revealed by etching are deemed to be fresh martensite. The area fractions of tempered martensite, pearlite, and fresh martensite are determined by calculating the area fractions of each.

[0055] Contamination on the surface of the observation surface can be removed by buffing using alumina particles with a particle size of 0.1 μm or less, or by Ar ion sputtering.

[0056] Average grain size of prior austenite grains: 25.0 μm or less If the grain size of the prior austenite grains is large, local elongation deteriorates, resulting in deterioration of the hole expandability of the hot-rolled steel sheet. If the average grain size of the prior austenite grains exceeds 25.0 μm, the hole expandability of the hot-rolled steel sheet deteriorates significantly. Therefore, the average grain size of the prior austenite grains is set to 25.0 μm or less, preferably 20.0 μm or less, or 15.0 μm or less. The smaller the average grain size of the prior austenite grains, the more improved the hole expandability of the hot-rolled steel sheet, but this effect saturates if the average grain size is less than 7.0 μm. Therefore, the average grain size of the prior austenite grains may be set to 7.0 μm or more.

[0057] Method for measuring the grain size of prior austenite grains A sample is taken from a thickness cross section of a hot-rolled steel plate perpendicular to the rolling direction, allowing observation of a position 1 / 4 of the thickness from the surface (a region from 1 / 8 to 3 / 8 of the thickness from the surface). The structure of the thickness cross section is revealed using an etching solution made by adding a saturated aqueous solution of picric acid to an etching solution of sodium dodecylbenzenesulfonate. The grain size of prior austenite grains is measured from metallographic photographs taken at 500x magnification at three locations within a range of 200 μm in the thickness direction and 200 μm in the direction perpendicular to the rolling direction, from the position 1 / 4 of the thickness from the surface (a region from 1 / 8 to 3 / 8 of the thickness from the surface). The circle equivalent diameter of one of the prior austenite grains included in each observation field is calculated. The above operation is performed for all prior austenite grains included in each observation field, excluding prior austenite grains that are not entirely included in the field of view, such as those at the edges of the field of view, to determine the circle-equivalent diameters of all prior austenite grains in each field of view. The average circle-equivalent diameters of the prior austenite grains obtained in each field of view are calculated to obtain the average grain size of the prior austenite grains.

[0058] When the angle between the long axis direction of the crystal grains of retained austenite, fresh martensite, and tempered martensite and the rolling direction is θ, the proportion of the crystal grains having an angle of θ of 0 to 15°, the proportion of the crystal grains having an angle of more than 15° and not more than 30°, the proportion of the crystal grains having an angle of more than 30° and not more than 45°, the proportion of the crystal grains having an angle of more than 45° and not more than 60°, the proportion of the crystal grains having an angle of more than 60° and not more than 75°, and the proportion of the crystal grains having an angle of more than 75° and not more than 90° are each 40% or less. If the long axis directions of the crystal grains of retained austenite, fresh martensite, and tempered martensite are concentrated in a specific direction, the deformation-induced transformation of the retained austenite is likely to progress due to deformation in multiple processes, and the retained austenite fraction is likely to decrease. As a result, the formability of the hot-rolled steel sheet after pre-straining is deteriorated. Therefore, in this embodiment, the long axis directions of the crystal grains are appropriately dispersed. In this embodiment, when the angle θ is defined as the angle between the long axis direction of the crystal grains of retained austenite, fresh martensite, and tempered martensite and the rolling direction, the percentage of the crystal grains whose θ is 0 to 15°, the percentage of the crystal grains whose θ is more than 15° and less than 30°, the percentage of the crystal grains whose θ is more than 30° and less than 45°, the percentage of the crystal grains whose θ is more than 45° and less than 60°, the percentage of the crystal grains whose θ is more than 60° and less than 75°, and the percentage of the crystal grains whose θ is more than 75° and less than 90° (hereinafter, sometimes simply referred to as crystal grain percentage) is each set to 40% or less. Preferably, it is 38% or less, 35% or less, or 30% or less. The lower limit is not particularly limited, but may be 0% or more, 10% or more, 15% or more, or 20% or more.

[0059] The above-mentioned crystal grain ratio is measured by the following method. Using the same method as when measuring the area ratio of each structure, retained austenite, fresh martensite, and tempered martensite are identified in the metal structure at the 1 / 4 position from the surface to the center of the sheet width (the region from 1 / 8 depth of the sheet thickness from the surface to 3 / 8 depth of the sheet thickness from the surface). Next, the crystal grains of retained austenite, fresh martensite, and tempered martensite are approximated by ellipsoids, and the direction of their major axes is determined. The angle θ (0 to 90°) between this major axis direction and the rolling direction of the hot-rolled steel sheet is calculated. The obtained θ is divided into 15° intervals: 0 to 15°, greater than 15° and less than 30°, greater than 30° and less than 45°, greater than 45° and less than 60°, greater than 60° and less than 75°, and greater than 75° and less than 90°, to create a histogram. The proportion of crystal grains in each interval is calculated as a percentage.

[0060] A method for approximating the crystal grains of retained austenite, fresh martensite, and tempered martensite with ellipsoids will be described. As shown in Figure 1, the area S of the grain region not included in the ellipsoid out and the area of ​​the non-grain region within the ellipsoid, S in x0, y0, a, b, and θ are calculated by approximating the grain with an ellipsoid so that the sum of the above is minimized. If the length of the major axis of the ellipsoid, a, is 1 μm or less, the grain is not included in the measurement. Also, if the value (a / b) obtained by dividing the length of the major axis of the ellipsoid, a, by the length of the minor axis, b, is less than 1.1, the grain is not included in the measurement.

[0061] The rolling direction of the hot-rolled steel sheet can be determined by the following method. First, a test piece is taken so that the thickness cross section of the hot-rolled steel sheet can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed using an optical microscope. The observation range is the entire thickness of the sheet, and areas with dark brightness are judged to be inclusions. For inclusions with a major axis length of 5 μm or more, the direction parallel to the extension direction of the inclusion is judged to be the rolling direction.

[0062] Tensile strength: 1100 MPa or more The hot-rolled steel sheet according to this embodiment may have a tensile strength of 1100 MPa or more. By setting the tensile strength to 1100 MPa or more, the steel sheet can be suitably applied to various automobile suspension parts. The tensile strength may be 1200 MPa or more, or 1300 MPa or more. The higher the tensile strength, the better, but it may be 1400 MPa or less.

[0063] Uniform elongation: 5.0% or more The hot-rolled steel sheet according to this embodiment may have a uniform elongation of 5.0% or more. By setting the uniform elongation to 5.0% or more, the steel sheet can be suitably applied to automobile suspension parts. Preferably, the uniform elongation is 5.5% or more, 6.0% or more, or 7.0% or more. There is no particular upper limit, but it may be 20.0% or less.

[0064] The tensile strength and uniform elongation are measured by conducting a tensile test in accordance with JIS Z 2241: 2011 using a No. 5 test piece of JIS Z 2241: 2011. The tensile test piece is taken from the center position in the sheet width direction, with the direction perpendicular to the rolling direction as the longitudinal direction. The uniform elongation is the "total elongation at maximum test force" as defined in JIS Z 2241:2011.

[0065] Hole expansion ratio: 30% or more The hot-rolled steel sheet according to this embodiment may have a hole expansion ratio of 30% or more. By setting the hole expansion ratio to 30% or more, the sheet can be suitably applied to automobile suspension parts. Preferably, the hole expansion ratio is 35% or more, 40% or more, 45% or more, or 50% or more. There is no particular upper limit, but the hole expansion ratio may be 80% or less. The hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.

[0066] Decrease in uniform elongation after pre-straining: 2.0% or less In order to improve formability even after pre-straining, it is important that uniform elongation does not decrease significantly even during deformation in which the deformation path changes suddenly. In this embodiment, the decrease in uniform elongation when the deformation path changes suddenly is evaluated by the following method. A JIS Z 2241:2011 No. 5 test piece was taken from the hot-rolled steel sheet. This test piece was subjected to a tensile prestrain of 0.03 true strain in a direction parallel to the rolling direction. Small tensile test pieces were then taken from the test piece so that the tensile direction was 0°, 45°, and 90° relative to the tensile direction. Tensile tests were conducted using these tensile test pieces to obtain the uniform elongation when the tensile direction was 0°, 45°, and 90°. The reduction in uniform elongation when the tensile direction was 45° and 90° relative to the uniform elongation when the tensile direction was 0° (uniform elongation in the 0° direction minus uniform elongation in the 45° direction, and uniform elongation in the 0° direction minus uniform elongation in the 90° direction) was calculated. If the reduction in uniform elongation was 2.0% or less, it was determined that the uniform elongation did not decrease significantly even when the deformation path suddenly changed. In other words, it was determined that the material had excellent formability even after prestraining. The tensile test is performed in accordance with JIS Z 2241:2011.

[0067] The hot-rolled steel sheet according to this embodiment may be provided with a plating layer on the surface to improve corrosion resistance or the like, thereby forming a surface-treated steel sheet. The plating layer may be an electroplated layer or a hot-dip plated layer. Examples of electroplated layers include electrogalvanized plating and electrolytic Zn-Ni alloy plating. Examples of hot-dip plated layers include hot-dip galvanized plating, alloyed hot-dip galvanized plating, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. The coating weight is not particularly limited and may be the same as conventional coating weights. Furthermore, it is also possible to further improve corrosion resistance by performing an appropriate chemical conversion treatment after plating (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

[0068] Next, a preferred method for manufacturing the hot-rolled steel sheet according to this embodiment will be described. Unless otherwise specified, the temperatures described below refer to the surface temperatures of the slab or steel plate.

[0069] A preferred method for producing a hot-rolled steel sheet according to this embodiment is as follows: a heating step of holding the slab having the above-mentioned chemical composition at a temperature range of 1220 to 1300°C for 40 minutes or more; a rough rolling process in which rough rolling is performed so that the reduction rates for each of the first to third passes are 10 to 30%, and the reduction rates for each of the fourth and subsequent passes are 15 to 50%; A finish rolling process in which the final pass is rolled in a temperature range of 940 to 1020 ° C. at a reduction rate of more than 25%, or the rolling one pass before the final pass is rolled in a temperature range of 940 to 1020 ° C. at a reduction rate of more than 25%, and the final pass is rolled at a reduction rate of less than 15%; a cooling step in which cooling is started within 2.0 seconds after the completion of finish rolling, and cooling is performed to a temperature range of 300 to 480°C within 17.0 seconds; a winding step of winding in the temperature range; a reheating step of heating the film within 30 minutes after the winding step so that the heating amount ΔT satisfies the following formula (1); An air-cooling step is performed to a temperature range of 200°C or less. 245 - 0.942 × CT + 0.00092 × CT 2 <ΔT<686-2.38×CT+0.0022×CT 2 …(1) In the above formula (1), CT is the cooling stop temperature. Each step will be described below.

[0070] heating process If the heating temperature is less than 1220°C, the solution treatment does not proceed well, the amount of ferrite increases, and the strength of the hot-rolled steel sheet deteriorates. Therefore, the heating temperature is set to 1220°C or higher, preferably 1240°C or higher. On the other hand, if the heating temperature exceeds 1300°C, the austenite grains become coarse during heating, resulting in a deterioration in the hole expandability of the hot-rolled steel sheet. Therefore, the heating temperature is set to 1300°C or less. From the viewpoint of energy costs, the heating temperature is preferably 1280°C or less.

[0071] If the holding time in the temperature range of 1220 to 1300°C is less than 40 minutes, the solution treatment does not proceed, the amount of ferrite increases, and the strength of the hot-rolled steel sheet deteriorates. Therefore, the holding time in the above temperature range is set to 40 minutes or more, preferably 60 minutes or more, or 80 minutes or more. There is no particular upper limit to the retention time, but it may be set to 200 minutes or less.

[0072] The slab to be heated is not particularly limited except that it has the above-mentioned chemical composition. For example, a slab produced by melting molten steel having the above-mentioned chemical composition using a converter or an electric furnace and then continuous casting the slab can be used. Instead of continuous casting, an ingot casting method, a thin slab casting method, or the like may also be used.

[0073] Rough rolling process If rolling is performed with a reduction rate of less than 10% in the first to third passes, or if rolling is performed with a reduction rate of less than 15% in the fourth pass and thereafter, the prior austenite grains will become coarse. Therefore, the reduction rates for each of the first to third passes are set to 10% or more, and for each of the fourth pass and thereafter, the reduction rates are set to 15% or more. Preferably, the reduction rates for each of the first to third passes are 15% or more or 20% or more, and for each of the fourth pass and thereafter, the reduction rates are 20% or more or 25% or more. Furthermore, if rolling is performed with a reduction rate of more than 30% in the first to third passes, or if rolling is performed with a reduction rate of more than 50% in the fourth pass or later, finish rolling is performed with non-uniform prior austenite grains, and prior austenite grains elongated in a specific direction are likely to form after finish rolling. As a result of the tendency for crystal grains to concentrate in a specific direction and the crystal grain ratio increasing, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the reduction rates for each of the first to third passes are set to 30% or less, and for each of the fourth and subsequent passes are set to 50% or less. Preferably, the reduction rates for each of the first to third passes are set to 25% or less, and for each of the fourth and subsequent passes are set to 40% or less.

[0074] The reduction rate of each pass can be expressed as {1-(t1 / t0)}×100(%), where t0 is the thickness at the entrance of each pass and t1 is the thickness at the exit of each pass.

[0075] The rough rolling completion temperature (the temperature at the outlet of the final pass of rough rolling) is not particularly limited, but is preferably 1070°C or higher from the viewpoint of hot deformation resistance, and is preferably 1200°C or lower from the viewpoint of reducing defects due to scale bite.

[0076] Finishing rolling process In the finish rolling process, if the reduction ratio and / or rolling temperature in the following condition I or II is too low, recrystallization does not proceed sufficiently, and the average grain size of the prior austenite grains cannot be reduced. Furthermore, if the rolling temperature in the following condition I or II is too high, the prior austenite grains become coarse, and the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, in the finish rolling process, finish rolling is performed to satisfy either condition I or condition II. Condition I: The final rolling pass is carried out in a temperature range of 940 to 1020°C at a reduction rate of more than 25%. Condition II: The rolling pass one pass before the final pass is carried out in a temperature range of 940 to 1020°C at a reduction rate of more than 25%, and the rolling pass of the final pass is carried out at a reduction rate of less than 15%.

[0077] Under condition I, the rolling one pass before the final pass is preferably carried out at 960°C or higher, and preferably at 1020°C or lower, and is preferably carried out at a reduction ratio of 30% or higher. The reduction ratio may be 60% or lower. Under condition II, the rolling one pass before the final pass is preferably carried out at a reduction ratio of 30% or higher, and the upper limit may be 60% or lower. The rolling of the final pass is preferably carried out at a reduction ratio of less than 10%, and the lower limit may be 5% or higher.

[0078] cooling process If the time from the completion of finish rolling to the start of cooling exceeds 2.0 seconds, the grain growth of austenite recrystallized grains progresses, and the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, cooling is started within 2.0 seconds after the completion of finish rolling. The time until the start of cooling here refers to the time from the final pass of finish rolling to the start of water cooling.

[0079] Although cooling may be started immediately after the completion of finish rolling, this requires the injection of cooling water directly below the finish rolling mill, which impairs productivity. The time until the start of cooling is preferably 1.0 second or longer.

[0080] If the cooling time from the completion of finish rolling to the temperature range of 300 to 480°C exceeds 17.0 seconds, the area ratio of ferrite increases, and the ductility and hole expandability of the hot-rolled steel sheet deteriorate. Therefore, after cooling begins, the sheet is cooled to the temperature range of 300 to 480°C within 14.0 seconds after the completion of finish rolling. The shorter the cooling time, the better. However, to cool to the desired temperature range in a short time, the mass density must be increased, which increases the load on the cooling nozzle and reduces economic efficiency. Therefore, the cooling time from the completion of finish rolling to the temperature range of 300 to 480°C is preferably 5.0 seconds or more or 7.0 seconds or more.

[0081] Cooling is stopped after cooling to a temperature range of 300 to 480°C. If the cooling stop temperature is less than 300°C, the amount of martensite increases and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the cooling stop temperature is set to 300°C or higher, preferably 320°C or higher. On the other hand, if the cooling stop temperature exceeds 480°C, the amount of pearlite increases and the amount of retained austenite decreases, resulting in deterioration of the strength, ductility, and hole expandability of the hot-rolled steel sheet. Therefore, the cooling stop temperature is set to 480°C or less, preferably 460°C or less.

[0082] Winding process After the cooling is stopped, the wire is immediately wound into a coil, i.e., the winding temperature and the cooling stop temperature are the same.

[0083] Reheating process After coiling, the steel sheet is heated within 30 minutes so that the heating amount ΔT in °C satisfies the above formula (1). Note that the temperature here refers to the surface temperature of the steel sheet at the outermost periphery of the coil.

[0084] If the heating amount ΔT is less than the left side of the formula (1), the retained austenite cannot be stabilized, and the desired amount of retained austenite cannot be obtained. Therefore, the heating amount ΔT is set to be greater than the left side of the formula (1). On the other hand, if the heating amount ΔT is equal to or greater than the right side of the above formula (1), a large amount of pearlite will be produced. Therefore, the heating amount ΔT is set to be less than the right side of the above formula (1).

[0085] If the time required for ΔT heating exceeds 30 minutes, a carbide reaction occurs near the bainite interface, reducing the amount of retained austenite. Therefore, it is preferable to limit the time required for ΔT heating to 30 minutes or less.

[0086] After heating so that the heating amount ΔT satisfies the above formula (1), the material is air-cooled. Taking safety into consideration when transporting the coil, air-cooling is preferably performed to a temperature range of 100°C or less.

[0087] The hot-rolled steel sheet according to the present embodiment can be manufactured by the manufacturing method including the steps described above. The manufacturing method of the hot-rolled steel sheet according to the present embodiment may further include the following steps. [Example]

[0088] Next, the effects of one embodiment of the present invention will be explained in more detail using examples, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0089] Slabs having the chemical compositions shown in Tables 1 and 2 were produced by continuous casting. Steel plates having a thickness of 3.0 mm were produced using the obtained slabs under the conditions shown in Tables 3A to 4B. After the reheating process, the plates were air-cooled to a temperature range of 100°C or less. In addition, for Test No. 54, production after finish rolling was discontinued due to high deformation resistance in the hot state. Blank cells in Tables 1 and 2 indicate that the element in question is not intentionally included.

[0090] For the obtained steel sheets, the area ratio of each structure, the average grain size of the prior austenite grains, the angle between the long axis direction of the crystal grains of the retained austenite, fresh martensite, and tempered martensite and the rolling direction was defined as θ, and the proportion of the crystal grains having an angle of θ of 0 to 15°, the proportion of the crystal grains having an angle of more than 15° and not more than 30°, the proportion of the crystal grains having an angle of more than 30° and not more than 45°, the proportion of the crystal grains having an angle of more than 45° and not more than 60°, the proportion of the crystal grains having an angle of more than 60° and not more than 75°, and the proportion of the crystal grains having an angle of more than 75° and not more than 90°, tensile strength, uniform elongation, hole expansion ratio, and the decrease in uniform elongation after pre-straining were determined using the above-mentioned methods. The results obtained are shown in Tables 5A to 5C.

[0091] In addition, TM, P, etc. in Tables 5A to 5C respectively represent the following. TM: Tempered martensite P: Perlite α: Ferrite B: Bainite FM: Fresh martensite γr: Retained austenite Maximum value of crystal grain ratio: When the angle between the long axis direction of the crystal grains of retained austenite, fresh martensite, and tempered martensite and the rolling direction is θ, the maximum value among the ratio of the crystal grains having θ of 0 to 15°, the ratio of the crystal grains having θ of more than 15° and not more than 30°, the ratio of the crystal grains having θ of more than 30° and not more than 45°, the ratio of the crystal grains having θ of more than 45° and not more than 60°, the ratio of the crystal grains having θ of more than 60° and not more than 75°, and the ratio of the crystal grains having θ of more than 75° and not more than 90° TM ratio: The ratio of the area ratio of tempered martensite to the total area ratio of fresh martensite and tempered martensite

[0092] If the tensile strength was 1100 MPa or more, the hot-rolled steel sheet was judged to have high strength and to have passed the test, whereas if the tensile strength was less than 1100 MPa, the hot-rolled steel sheet was judged to have low strength and to have failed the test.

[0093] When the uniform elongation was 5.0% or more, the hot-rolled steel sheet was judged to have excellent ductility and to have passed the test, whereas when the uniform elongation was less than 5.0%, the hot-rolled steel sheet was judged to have poor ductility and to have failed the test.

[0094] When the hole expansion ratio was 30% or more, the hot-rolled steel sheet was judged to have excellent hole expandability and to have passed the test. On the other hand, when the hole expansion ratio was less than 30%, the hot-rolled steel sheet was judged to have no excellent hole expandability and to have passed the test.

[0095] When the decrease in uniform elongation after pre-straining was 2.0% or less, the hot-rolled steel sheet was judged to have excellent formability after pre-straining and to have passed the test. On the other hand, when the decrease in uniform elongation after pre-straining was more than 2.0%, the hot-rolled steel sheet was judged to have poor formability after pre-straining and to have passed the test.

[0096] [Table 1]

[0097] [Table 2]

[0098] [Table 3A]

[0099] [Table 3B]

[0100] [Table 4A]

[0101] [Table 4B]

[0102] [Table 5A]

[0103] [Table 5B]

[0104] It can be seen from Tables 5A and 5B that the steel sheets according to the examples of the present invention have high strength, excellent ductility and hole expandability, and excellent formability after pre-straining. Among the examples of the present invention, it is clear that hot-rolled steel sheets in which the area ratio of tempered martensite out of the total area ratio of fresh martensite and tempered martensite is 80.0% or more have better hole expandability. On the other hand, it is clear that the steel sheets according to the comparative examples are inferior in at least one of the above properties. [Industrial Applicability]

[0105] According to the above aspects of the present invention, it is possible to provide a steel sheet having high strength, excellent ductility and hole expandability, and excellent formability after pre-straining, and a manufacturing method thereof.

Claims

1. The chemical composition, in mass%, is C: 0.11-0.23%, Si: 0.70-1.80%, Mn: 1.95-3.10%, P: 0.060% or less, S: 0.005% or less, Al: 0.010-0.430%, N: 0.0070% or less, Ti: 0.006 to 0.055%, Nb: 0.006-0.040%, B: 0.0001 to 0.0030%, Cr: 0.050-0.470%, Mo: 0.005-0.120%, V: 0 to 0.10%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, Ca: 0-0.0200%, Mg: 0 to 0.0200%, REM: 0-0.1000%, Bi: 0 to 0.020%, The sum of Zr, Co, Zn, and W: 0 to 1.00%, and Sn: 0 to 0.05%; the balance being Fe and impurities; The metal structure is, in area%, Bainite: 25.0% or more, Sum of fresh martensite and tempered martensite: 70.0% or less, Retained austenite: 4.0 to 12.0% Ferrite: 5.0% or less; and Pearlite: 3.0% or less, The average grain size of prior austenite grains is 25.0 μm or less, a ratio of the crystal grains having an angle of θ between the rolling direction and the long axis direction of the crystal grains of the retained austenite, the fresh martensite, and the tempered martensite, the ratio of the crystal grains having an angle of θ of 0 to 15°, the ratio of the crystal grains having an angle of θ greater than 15° and not greater than 30°, the ratio of the crystal grains having an angle of θ greater than 30° and not greater than 45°, the ratio of the crystal grains having an angle of θ greater than 45° and not greater than 60°, the ratio of the crystal grains having an angle of θ greater than 60° and not greater than 75°, and the ratio of the crystal grains having an angle greater than 75° and not greater than 90° are each 40% or less.

2. 2. The hot-rolled steel sheet according to claim 1, wherein the area ratio of the tempered martensite is 80.0% or more in terms of percentage of the total area ratio of the fresh martensite and the tempered martensite.

3. The chemical composition is, in mass %, V: 0.01 to 0.10%, Cu: 0.01-0.40%, Ni: 0.02-0.30%, Ca: 0.0005-0.0200%, Mg: 0.0005-0.0200%, REM: 0.0005 to 0.1000%, and Bi: 0.0005-0.020%, The hot-rolled steel sheet according to claim 1 or 2, characterized in that it contains one or more of the group consisting of

4. The method for producing a hot-rolled steel sheet according to claim 1, A heating step of holding a slab having the chemical composition according to claim 1 in a temperature range of 1220 to 1300 ° C. for 40 minutes or more; a rough rolling process in which rough rolling is performed so that the reduction rates for each of the first to third passes are 10 to 30% and the reduction rates for each of the fourth and subsequent passes are 15 to 50%; A finish rolling process in which the final pass of rolling is performed in a temperature range of 940 to 1020 ° C. and at a reduction rate of more than 25%, or the rolling one pass before the final pass is performed in a temperature range of 940 to 1020 ° C. and at a reduction rate of more than 25%, and the final pass of rolling is performed at a reduction rate of less than 15%; a cooling step in which cooling is started within 2.0 seconds after the completion of finish rolling, and cooling to a temperature range of 300 to 480°C within 17.0 seconds; a winding step of winding in the temperature range; a reheating step of heating the film within 30 minutes after the winding step so that the heating amount ΔT satisfies the following formula (1); and an air-cooling step of air-cooling the steel sheet to a temperature range of 200°C or less. 245-0.942×CT+0.00092×CT ２ <ΔT<686-2.38×CT+0.0022×CT ２ …(1) In the above formula (1), CT is the cooling stop temperature.

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