Hot-rolled steel sheet

Abstract

This hot-rolled steel sheet has a specific chemical composition; and the metal structure of the inner region of this hot-rolled steel sheet contains, in area ratios, a total of 40% to 80% of martensite and / or bainite, and 20% to 60% of ferrite, with the remainder structure being less than 5% by area. The ratio αs / αc of the ferrite area ratio αs in the surface region to the ferrite area ratio αc in the inner region is from 1.15 to 2.50; the hardness difference ratio (1 – Hvs / Hvc) of the Vickers hardness Hvs in the surface region to the Vickers hardness Hvc in the inner region is 0.20 or less; and the tensile strength is 980 MPa or more.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet. Specifically, it relates to a hot-rolled steel sheet having high strength, as well as excellent fatigue strength, toughness and ductility.

Background Art

[0002] In recent years, for the purpose of improving the durability and collision safety of automobiles, the application of high-strength steel sheets to automobile members has been actively studied. However, when the steel sheet is strengthened, the toughness generally deteriorates. Therefore, in the development of high-strength steel sheets, it is an important issue to increase the strength without deteriorating the material properties. In particular, for high-strength steel sheets applied to automobile members, it is important to ensure the fatigue durability of the parts. When processed into parts, cracks progress from the punched surface or the like, and even when a high-strength steel sheet is used, the fatigue durability of the parts does not necessarily improve.

[0003] On the other hand, in Patent Document 1, a high-strength hot-rolled steel sheet having excellent bendability is proposed, in which the metal structure has a surface layer region having a ferrite phase as the main phase and an internal region having a bainite phase as the main phase, and the ratio of the surface layer region in the thickness direction of the steel sheet is 1.0 to 5.0% of the total thickness of each of the front and back surfaces of the steel sheet.

[0004] In Patent Document 2, a high-strength hot-rolled steel sheet having excellent workability is proposed, which has a central portion mainly composed of bainite and a surface layer portion mainly composed of polygonal ferrite, and the surface layer portion is formed in a region at least 0.2 mm deep from both surfaces of the steel sheet.

[0005] In Patent Document 3, a high-strength steel sheet having excellent bendability is proposed, in which the average Vickers hardness and the standard deviation of the hardness from the surface to the 1 / 2 thickness position of the plate thickness are suppressed low.

[0006] Patent Document 4 proposes a hot-rolled steel sheet in which the area fraction and Vickers hardness of martensite are controlled within a predetermined range for each depth direction of the plate thickness, thereby improving the fatigue characteristics and surface machinability.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, in the hot-rolled steel sheets described in Patent Documents 1 to 3, the surface layer has ferrite as the main phase and is softened, and there is still room for further improvement in fatigue characteristics.

[0009] In addition, in the invention described in Patent Document 4, the surface layer is softened, and there is still room for further improvement in fatigue strength. Furthermore, since precipitation strengthening is performed inside the plate thickness, the dislocation movement in ferrite is inhibited, and there is still room for further improvement in toughness from this viewpoint.

[0010] In recent years, against the background of further requirements for weight reduction of automobiles and the complication of component shapes, etc., high-strength hot-rolled steel sheets having higher fatigue strength and toughness have been demanded. The present invention has been made in view of the above problems, and an object thereof is to provide a hot-rolled steel sheet having high strength, excellent fatigue strength and toughness. Further, the present invention aims to provide a hot-rolled steel sheet having excellent ductility, which is a property generally required for hot-rolled steel sheets applied to automobile members, while having the above-described various properties.

Means for Solving the Problems

[0011] The structure strengthened by precipitation inhibits dislocation movement and thus has excellent fatigue strength. Therefore, the structure strengthened by precipitation is widely used in automotive underbody parts. On the other hand, when dislocation movement is suppressed, plastic deformation is less likely to occur, resulting in deterioration of impact properties (especially toughness). Therefore, it is presumed that there is an inverse relationship between fatigue strength and impact properties. The inventors of the present invention analyzed in detail the deformation mechanisms of fatigue strength and impact strength respectively in order to improve both fatigue strength and toughness. As a result, the inventors considered that the metal structure and hardness in the surface layer region of the hot-rolled steel sheet greatly affect the fatigue strength, and the metal structure and hardness in the internal region of the hot-rolled steel sheet greatly affect the crack propagation.

[0012] The gist of the present invention made based on the above findings is as follows. (1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition in mass%, C: 0.02 to 0.30%, Si: 0.10 to 2.00%, Mn: 0.5 to 3.0%, sol.Al: 0.10 to 1.00%, Ti: 0.06 to 0.20%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0100% or less, Nb: 0 to 0.100%, Ca: 0 to 0.0060%, Mo: 0 to 0.50%, Cr: 0 to 1.00%, V: 0 to 0.40%, Ni: 0 to 0.40%, Cu: 0 to 0.40%, B: 0 to 0.0020%, and Sn: 0 to 0.20% and contains the balance consists of Fe and impurities, the metal structure in the internal region contains, in area ratio, a total of 40 to 80% of one or two of martensite and bainite, 20 to 60% of ferrite, and the area ratio of the remaining structure is less than 5%, The ratio αs / αc, which is the ratio of the ferrite area ratio αs of the surface region to the ferrite area ratio αc of the internal region, is 1.15 to 2.50, The ratio βs / βc, which is the ratio of the total area fraction βs of martensite and bainite in the surface region to the total area fraction βc of martensite and bainite in the internal region, is 0.30 to 0.90, the hardness difference ratio (1 - Hvs / Hvc), which is the hardness difference ratio between the Vickers hardness Hvs of the surface region and the Vickers hardness Hvc of the internal region, is 0.20 or less, and the tensile strength is 980 MPa or more. (2) In the hot-rolled steel sheet according to the above (1), the chemical composition is in mass%, Nb: 0.010 to 0.100%, Ca: 0.0005 to 0.0060%, Mo: 0.02 to 0.50%, Cr: 0.02 to 1.00%, V: 0.01 to 0.40%, Ni: 0.01 to 0.40%, Cu: 0.01 to 0.40%, B: 0.0001 to 0.0020%, and Sn: 0.01 to 0.20% may contain one or more selected from the group consisting of.

Advantages of the Invention

[0013] According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, excellent fatigue strength, toughness, and ductility. According to this hot-rolled steel sheet, it is possible to reduce the weight of the vehicle body of an automobile or the like and improve the durability, so it has high industrial value.

Embodiments for Carrying Out the Invention

[0014] A hot-rolled steel sheet according to an embodiment of the present invention (hereinafter, may sometimes be referred to as the hot-rolled steel sheet according to the present embodiment.) will be described. However, the present invention is not limited only to the configuration disclosed in the present embodiment, and various modifications are possible without departing from the spirit of the present invention.

[0015] The individual components of the present invention will be described in detail below. First, the reasons for limiting the chemical composition of the hot-rolled steel sheet according to this embodiment will be described. In the numerical limit ranges described with "~" in between below, the lower limit value and the upper limit value are included in the range. The numerical values indicated as "less than" or "exceeding" are not included in the numerical range. In the following description, % regarding the chemical composition is mass % unless otherwise specified.

[0016] The hot-rolled steel sheet according to this embodiment has a chemical composition containing, by mass %, C: 0.02 to 0.30%, Si: 0.10 to 2.00%, Mn: 0.5 to 3.0%, sol.Al: 0.10 to 1.00%, Ti: 0.06 to 0.20%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0100% or less, and the balance: Fe and impurities. Each element will be described in detail below.

[0017] <C: 0.02 to 0.30%> C is an important element for improving the strength of the hot-rolled steel sheet. In order to obtain the desired strength, the C content is set to 0.02% or more. Preferably it is 0.04% or more. On the other hand, when the C content exceeds 0.30%, the toughness of the hot-rolled steel sheet deteriorates. Therefore, the C content is set to 0.30% or less. Preferably it is 0.20% or less.

[0018] <Si: 0.10 to 2.00%> Si is an element that suppresses the formation of carbides during ferrite transformation and has the effect of improving the toughness of the hot-rolled steel sheet. In order to obtain this effect, the Si content is set to 0.10% or more. Preferably it is 0.20% or more or 0.50% or more. On the other hand, when the Si content exceeds 2.00%, the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 2.00% or less. Preferably it is 1.50% or less.

[0019] <Mn: 0.5 to 3.0%> Mn is an element effective in improving the strength of hot-rolled steel sheets by enhancing hardenability and solid-solution strengthening. To obtain this effect, the Mn content is set to 0.5% or more. Preferably, it is 1.0% or more. On the other hand, when the Mn content exceeds 3.0%, MnS, which is harmful to toughness and fatigue strength, is generated. Therefore, the Mn content is set to 3.0% or less. Preferably, it is 2.5% or less or 2.0% or less.

[0020] <sol.Al: 0.10 - 1.00%> Al is an important element for controlling ferrite transformation. To obtain this effect, the sol.Al content is set to 0.10% or more. Preferably, it is 0.15% or more or 0.20% or more. On the other hand, when the sol.Al content exceeds 1.00%, alumina precipitated in clusters is generated, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the sol.Al content is set to 1.00% or less. Preferably, it is 0.80% or less or 0.50% or less. Note that sol.Al means acid-soluble Al and indicates the solid-solution Al present in the steel in a solid-solution state.

[0021] <Ti: 0.06 - 0.20%> Ti is an element that precipitates and strengthens ferrite and is also an important element for controlling ferrite transformation to obtain a desired amount of ferrite. To obtain excellent fatigue strength by precipitation strengthening and control of ferrite transformation, the Ti content is set to 0.06% or more. Preferably, it is 0.08% or more. On the other hand, when the Ti content exceeds 0.20%, inclusions caused by TiN are generated, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Ti content is set to 0.20% or less. Preferably, it is 0.16% or less or 0.13% or less.

[0022] <P: 0.1000% or less> P is an impurity, and the lower the P content, the more preferable. In particular, when the P content exceeds 0.1000%, the workability and weldability of the hot-rolled steel sheet are significantly deteriorated, and the fatigue strength also decreases. Therefore, the P content is set to 0.1000% or less. Preferably, it is 0.0500% or less or 0.0200% or less. The lower limit of the P content does not particularly need to be specified, but from the viewpoint of refining cost, it is preferably 0.0010% or more.

[0023] <S: 0.0100% or less> S is an impurity, and the lower the S content, the more preferable. In particular, when the S content exceeds 0.0100%, a large amount of inclusions such as MnS, which is harmful to the isotropy of toughness, are generated. Therefore, the S content is set to 0.0100% or less. When more excellent toughness is required, the S content is preferably 0.0060% or less. More preferably, it is 0.0050% or less. The lower limit of the S content does not particularly need to be specified, but from the viewpoint of refining cost, it is preferably 0.0001% or more.

[0024] <N: 0.0100% or less> N is an impurity. When the N content exceeds 0.0100%, coarse Ti nitrides are formed in the high-temperature range, so the toughness of the hot-rolled steel sheet deteriorates. Therefore, the N content is set to 0.0100% or less. Preferably, it is 0.0060% or less or 0.0050% or less. The lower limit of the N content does not particularly need to be specified, but from the viewpoint of refining cost, it is preferably 0.0001% or more.

[0025] The hot-rolled steel sheet according to the present embodiment contains the above chemical components, and the balance may consist of Fe and impurities. In the present embodiment, the impurities mean those mixed from ores, scraps as raw materials, or the manufacturing environment, etc., and / or those allowed within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment.

[0026] Although not essential for imparting the desired properties, the following optional elements may be included to reduce manufacturing variations or further improve the strength of the hot-rolled steel sheet. However, since it is not essential to include these elements, the lower limit of the content of these elements is 0%. Note that if the content of each optional element is less than the lower limit value of the content described below, it can be regarded as an impurity.

[0027] <Nb: 0.010~0.100%> Nb is an element that has the effect of increasing the strength of the hot-rolled steel sheet by refining the crystal grain size of the hot-rolled steel sheet and precipitation strengthening of NbC. When this effect is to be obtained reliably, it is preferable that the Nb content be 0.010% or more. On the other hand, when the Nb content exceeds 0.100%, the above effect saturates. Therefore, even when Nb is included, the Nb content is set to 0.100% or less. Preferably, it is 0.060% or less.

[0028] <Ca: 0.0005~0.0060%> Ca is an element that has the effect of dispersing a large number of fine oxides during deoxidation of the molten steel and refining the structure of the hot-rolled steel sheet. Also, Ca is an element that fixes S in the steel as spherical CaS and suppresses the formation of elongated inclusions such as MnS, thereby improving the hole expansion property of the hot-rolled steel sheet. When these effects are to be obtained reliably, it is preferable that the Ca content be 0.0005% or more. On the other hand, when the Ca content exceeds 0.0060%, the above effect saturates. Therefore, even when Ca is included, the Ca content is set to 0.0060% or less. Preferably, it is 0.0040% or less.

[0029] <Mo: 0.02~0.50%> Mo is an element effective for precipitation strengthening of ferrite. When this effect is to be obtained reliably, it is preferable that the Mo content be 0.02% or more. More preferably, it is 0.10% or more. On the one hand, when the Mo content becomes excessive, the cracking susceptibility of the slab increases, making it difficult to handle the slab. Therefore, even when Mo is contained, the Mo content should be 0.50% or less. Preferably, it is 0.30% or less.

[0030] <Cr: 0.02~1.00%> Cr is an element effective in improving the strength of the hot-rolled steel sheet. When this effect is to be obtained reliably, the Cr content is preferably 0.02% or more. More preferably, it is 0.10% or more. On the other hand, when the Cr content becomes excessive, the ductility of the hot-rolled steel sheet decreases. Therefore, even when Cr is contained, the Cr content should be 1.00% or less. Preferably, it is 0.80% or less.

[0031] <V: 0.01~0.40%> V improves the strength of the hot-rolled steel sheet by precipitation strengthening, grain refinement strengthening by suppressing the growth of ferrite crystal grains, and dislocation strengthening by suppressing recrystallization. When these effects are to be obtained reliably, the V content is preferably 0.01% or more. On the other hand, when the V content is excessive, a large amount of carbonitrides precipitate, reducing the formability of the hot-rolled steel sheet. Therefore, the V content should be 0.40% or less. Preferably, it is 0.20% or less.

[0032] <Ni: 0.01~0.40%> Ni suppresses the phase transformation at high temperatures and improves the strength of the hot-rolled steel sheet. When this effect is to be obtained reliably, the Ni content is preferably 0.01% or more. On the other hand, when the Ni content is excessive, the weldability of the hot-rolled steel sheet decreases. Therefore, the Ni content should be 0.40% or less. Preferably, it is 0.20% or less.

[0033] <Cu: 0.01~0.40%> Cu exists in the steel in the form of fine particles and improves the strength of the hot-rolled steel sheet. When this effect is to be obtained reliably, the Cu content is preferably 0.01% or more. On the one hand, if the Cu content is excessive, the weldability of the hot-rolled steel sheet will deteriorate. Therefore, the Cu content should be 0.40% or less. Preferably, it is 0.20% or less.

[0034] <B: 0.0001 - 0.0020%> B suppresses the phase transformation at high temperatures and improves the strength of the hot-rolled steel sheet. When this effect is to be obtained reliably, it is preferable that the B content is 0.0001% or more. On the other hand, if the B content is excessive, B precipitates will form and the strength of the hot-rolled steel sheet will decrease. Therefore, the B content should be 0.0020% or less. Preferably, it is 0.0005% or less.

[0035] <Sn: 0.01 - 0.20%> Sn is an element that suppresses the coarsening of crystal grains and improves the strength of the hot-rolled steel sheet. When this effect is to be obtained reliably, it is preferable that the Sn content is 0.01% or more. On the other hand, if the Sn content becomes excessive, the steel will become brittle and be prone to breakage during rolling. Therefore, the Sn content should be 0.20% or less. Preferably, it is 0.10% or less.

[0036] The chemical composition of the hot-rolled steel sheet described above may be measured by a general analysis method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that sol.Al may be measured by ICP-AES using the filtrate after heating and decomposing the sample with acid. C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method.

[0037] Next, the metallographic 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 the internal region that contains, by area ratio, one or both of martensite and bainite in a total amount of 40 to 80%, ferrite in an amount of 20 to 60%, and the area ratio of the remaining structure is less than 5%. The ratio αs / αc, which is the ratio of the ferrite area ratio αs in the surface layer region to the ferrite area ratio αc in the internal region, is 1.15 to 2.50, and the hardness difference ratio (1 - Hvs / Hvc), which is the ratio of the Vickers hardness Hvs in the surface layer region to the Vickers hardness Hvc in the internal region, is 0.20 or less.

[0038] Note that the internal region refers to the region from 1 / 8 of the plate thickness depth to 3 / 8 of the plate thickness depth from the surface of the hot-rolled steel sheet, centered at a depth of 1 / 4 of the plate thickness from the surface of the hot-rolled steel sheet. The surface layer region refers to the region from the surface of the hot-rolled steel sheet to a depth of 20 μm from the surface.

[0039] A structure mainly composed of martensite and bainite has a fine structure and excellent toughness. Although there are many unclear points about its mechanism, it is known that steel having a structure mainly composed of martensite and bainite has inferior fatigue strength compared with precipitation hardening steel and dual-phase (DP) steel having a composite structure of ferrite and martensite. On the other hand, in precipitation hardening steel and DP steel, high-speed dislocation movement in ferrite is inhibited, resulting in inferior fatigue strength and toughness. Conventionally, in automotive parts, the steel sheet structure has been fabricated according to the required properties, but as the demand for further high strength progresses, it has become difficult to obtain both high fatigue strength and toughness. Therefore, different from the prior art, the hot-rolled steel sheet according to this embodiment enhances the amount of ferrite in the surface layer region to utilize a composite structure of ferrite and martensite with excellent fatigue strength and precipitation strengthening in the surface layer region, and utilizes a metal structure mainly composed of one or both of martensite and bainite with excellent toughness in the internal region. Thereby, high strength of 980 MPa or more, as well as excellent fatigue strength, toughness, and ductility, can be obtained.

[0040] The metal structure of the internal region The metallographic structure of the internal region of the hot-rolled steel sheet has a great influence on the toughness of the hot-rolled steel sheet. Therefore, the metallographic structure of the internal region is mainly composed of a low-temperature transformation structure. The low-temperature transformation structure refers to martensite and bainite. If the total area ratio of these structures is less than 40%, the toughness of the hot-rolled steel sheet will be inferior. Therefore, the total area ratio of martensite and bainite should be 40% or more. Preferably, it is 45% or more, and more preferably, it is 50% or more.

[0041] On the other hand, when the total area ratio of martensite and bainite exceeds 80%, the hardness difference from the metallographic structure of the surface layer region becomes large, resulting in inferior fatigue strength of the hot-rolled steel sheet. Therefore, the total area ratio of martensite and bainite should be 80% or less. Preferably, it is 75% or less, and more preferably, it is 70% or less.

[0042] In this embodiment, when the metallographic structure of the internal region contains only one of martensite or bainite, the content of only one of martensite or bainite needs to be within the above-mentioned range. When both martensite and bainite are included, the total content of martensite and bainite needs to be within the above-mentioned range.

[0043] In the metallographic structure of the internal region, if the area ratio of ferrite is less than 20%, the hardness difference from the metallographic structure of the surface layer region becomes large, resulting in inferior fatigue strength of the hot-rolled steel sheet. Therefore, the area ratio of ferrite should be 20% or more. Preferably, it is 25% or more, and more preferably, it is 30% or more.

[0044] On the other hand, when the area ratio of ferrite exceeds 60%, there may be cases where the strain cannot be relaxed by the precipitated and strengthened ferrite grains and the workability cannot be ensured, resulting in deterioration of the toughness of the hot-rolled steel sheet. Therefore, the area ratio of ferrite should be 60% or less. Preferably, it is 55% or less, and more preferably, it is 50% or less.

[0045] The metallographic structure of the inner region, in terms of area ratio, has a remaining structure of less than 5%. The remaining structure is one or more of pearlite and retained austenite. The remaining structure is preferably less than 3%, more preferably 2.5% or less, and even more preferably 2% or less.

[0046] Metallographic structure of the surface layer region In the metallographic structure of the surface layer region of the hot-rolled steel sheet, when the ratio αs / αc of the ferrite area ratio αs in the surface layer region to the ferrite area ratio αc in the inner region is less than 1.15, the suppression of dislocation movement in ferrite becomes insufficient, and the fatigue strength of the hot-rolled steel sheet becomes inferior. Therefore, αs / αc should be 1.15 or more. Preferably it is 1.20 or more or 1.30 or more, and more preferably 1.50 or more.

[0047] On the other hand, when αs / αc exceeds 2.50, carbon concentrates inside the plate thickness during ferrite transformation, and the hardness difference from the metallographic structure of the inner region increases, resulting in inferior toughness and / or fatigue strength of the hot-rolled steel sheet. Therefore, αs / αc should be 2.50 or less. Preferably it is 2.20 or less, and more preferably 2.00 or less.

[0048] In the metallographic structure of the surface layer region of the hot-rolled steel sheet, the ratio βs / βc of the total area ratio βs of martensite and bainite in the surface layer region to the total area ratio βc of martensite and bainite in the inner region is preferably 0.30 to 0.90. When βs / βc is 0.90 or less, the dislocation movement in martensite and bainite is sufficiently suppressed, and the fatigue strength of the hot-rolled steel sheet is increased. βs / βc is more preferably 0.85 or less, and even more preferably 0.80 or less.

[0049] On the other hand, when βs / βc is 0.30 or more, the concentration of carbon inside the plate thickness during the transformation of martensite and bainite and the increase in the hardness difference from the metallographic structure of the inner region are suppressed, and the toughness and fatigue strength of the hot-rolled steel sheet are increased. βs / βc is more preferably 0.40 or more, even more preferably 0.45 or more, and even more preferably 0.50 or more.

[0050] The metal structure of the surface region may contain 30 to 80% ferrite by area ratio. Further, the metal structure of the surface region may contain, as the remaining structure other than ferrite, a total of 20 to 70% of one or more of bainite, martensite, pearlite, and retained austenite by area ratio.

[0051] Method for measuring metal structure A sample is cut out from the hot-rolled steel sheet so that a plate thickness cross-section perpendicular to the surface can be observed. After polishing the plate thickness cross-section of this sample using silicon carbide paper from #600 to #1500, a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water is used to finish it to a mirror surface, and nital etching is performed. Next, at an arbitrary position in the longitudinal direction of the sample cross-section, photographs of a plurality of fields are taken using a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL). Grids at equal intervals are drawn on the photographed images, and the structure at the grid points is identified. The number of grid points corresponding to each structure is obtained and divided by the total number of grid points to obtain the area ratio of each structure. The larger the total number of grid points, the more accurately the area ratio can be obtained. In this embodiment, the grid interval is 2 μm × 2 μm, and the total number of grid points is 1500 points.

[0052] A region where cementite is precipitated in a lamellar shape within the grains is judged as pearlite. A region with low brightness and no underlying structure visible is judged as ferrite. A region with high brightness and no underlying structure revealed by etching is judged as martensite and retained austenite. A region not corresponding to any of the above is judged as bainite. The area ratio of martensite is obtained by subtracting the area ratio of retained austenite obtained by EBSD analysis described later from the area ratio of martensite and retained austenite obtained from the photographed images.

[0053] Cut out a sample from the same position as the above measurement so that a plate thickness cross-section perpendicular to the surface can be observed. After polishing the plate thickness cross-section of this sample using silicon carbide paper from #600 to #1500, finish it to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Next, polish for 8 minutes using colloidal silica that does not contain an alkaline solution at room temperature to remove the strain introduced into the surface layer of the sample. At an arbitrary position in the longitudinal direction of the sample cross-section, measure by the electron backscatter diffraction method at a measurement interval of 0.1 μm to obtain crystal orientation information. For the measurement, use an EBSD device composed of a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL). At this time, the vacuum degree in the EBSD device is 9.6×10 -5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. Calculate the area ratio of retained austenite using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device for the obtained crystal orientation information. Note that those with an fcc crystal structure are judged as retained austenite.

[0054] Perform each of the above measurements in the region from 1 / 8 of the plate thickness from the surface to 3 / 8 of the plate thickness from the surface, and in the region from the surface of the hot-rolled steel sheet to 20 μm deep from the surface, to obtain the area ratio of the metallographic structure in each of the internal region and the surface layer region.

[0055] Hardness difference ratio between the Vickers hardness of the surface layer region and the Vickers hardness of the internal region: 0.20 or less If the hardness difference ratio (1 - Hvs / Hvc), which is the ratio of the Vickers hardness Hvs of the surface layer region to the Vickers hardness Hvc of the internal region, exceeds 0.20, the surface layer region is softened and the fatigue strength of the hot-rolled steel sheet becomes inferior. Therefore, the hardness difference ratio (1 - Hvs / Hvc) between Hvs and Hvc shall be 0.20 or less. Preferably it is 0.15 or less, and more preferably 0.10 or less. The hardness difference ratio (1 - Hvs / Hvc) between Hvs and Hvc is preferably small, but from the perspective of manufacturing, it may be -0.10 or more, 0.00 or more, or 0.01 or more.

[0056] Method for Measuring Vickers Hardness Cut out a test piece from the hot-rolled steel sheet so that a cross-section perpendicular to the surface can be observed. After polishing the cross-section of the test piece using silicon carbide paper from #600 to #1500, finish it to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. This cross-section is used as the measurement surface. Using a micro Vickers hardness tester, measure the Vickers hardness at intervals of more than 3 times the indentation depth with a load of 1 kgf in the region from 1 / 8 of the sheet thickness from the surface to 3 / 8 of the sheet thickness from the surface on the measurement surface. Measure a total of 20 points and calculate their average value to obtain the Vickers hardness Hvc of the metal structure in the internal region. Similarly, measure the Vickers hardness in the region from the surface to 20 μm deep from the surface on the measurement surface and calculate the average value of 20 points to obtain the Vickers hardness Hvs of the metal structure in the surface layer region. By calculating (1 - Hvs / Hvc) using the obtained Hvs and Hvc, the Hardness difference ratio of Vickers hardness is obtained.

[0057] The hot-rolled steel sheet according to this embodiment has a tensile (maximum) strength of 980 MPa or more. Preferably, it is 1000 MPa or more. If the tensile strength is less than 980 MPa, the applicable parts are limited and the contribution to vehicle body weight reduction is small. The upper limit does not particularly need to be limited, but from the perspective of suppressing die wear, it may be 1500 MPa or less or 1300 MPa or less. Also, the hot-rolled steel sheet according to this embodiment may have an elongation of 10% or more, an absorbed energy at -20 °C of 80 J / cm 2 or more, and a fatigue limit ratio (fatigue strength / tensile strength) of 0.48 or more.

[0058] Tensile strength and total elongation are evaluated by conducting a tensile test in accordance with JIS Z 2241:2011. The test piece shall be No. 5 test piece of JIS Z 2241:2011. The sampling position of the tensile test piece shall be the 1 / 4 part from the end in the plate width direction, and the direction perpendicular to the rolling direction may be taken as the longitudinal direction. For toughness, first, a V-notch test piece with a 2.5 mm sub-size defined in JIS Z 2242:2018 is sampled from a position close to the sampling position of the test piece used in the tensile test. Using this test piece, the absorbed energy is measured by conducting a Charpy impact test of C-direction notch at -20 °C. For hot-rolled steel sheets with a thickness of less than 2.5 mm, the test shall be conducted on the full thickness. Fatigue strength is measured in accordance with JIS Z 2275:1978 using a Schenck-type plane bending fatigue testing machine. When measuring, the stress load shall be set with a two-way oscillation at a test speed of 30 Hz, and the fatigue strength at 107 cycles shall be measured. Then, the fatigue limit ratio (fatigue strength / tensile strength) is calculated by dividing the fatigue strength at 107 cycles by the tensile strength measured by the above-described tensile test.

[0059] The thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but it may be 1.2 to 8.0 mm. When the thickness of the hot-rolled steel sheet is less than 1.2 mm, it becomes difficult to ensure the rolling completion temperature and the rolling load becomes excessive, which may make hot rolling difficult. Therefore, the thickness of the hot-rolled steel sheet according to this embodiment may be 1.2 mm or more. Preferably it is 1.4 mm or more. On the other hand, when the thickness exceeds 8.0 mm, it may be difficult to obtain the above-described metal structure after hot rolling. Therefore, the thickness may be 8.0 mm or less. Preferably it is 6.0 mm or less.

[0060] The hot-rolled steel sheet according to this embodiment having the above-described chemical composition and metal structure may be a surface-treated steel sheet provided with a plating layer on the surface for the purpose of improving corrosion resistance and the like. The plating layer may be an electroplated layer or a hot-dip plating layer. Examples of the electroplated layer include electrogalvanizing and electro Zn-Ni alloy plating. Examples of the hot-dip plating layer include hot-dip galvanizing, alloyed hot-dip galvanizing, 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 plating adhesion amount is not particularly limited and may be the same as in the prior art. Further, it is also possible to further enhance the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.

[0061] Regardless of the manufacturing method, the hot-rolled steel sheet according to this embodiment has the above-described chemical composition and metal structure, and thus its effects can be obtained. However, according to the manufacturing method shown below, it is preferable because the hot-rolled steel sheet according to this embodiment can be stably obtained.

[0062] In a preferable manufacturing method of the hot-rolled steel sheet according to this embodiment, by performing bending during finish rolling in hot rolling, strain is imparted to the surface layer region, and ferrite transformation in the surface layer region is promoted. After precipitated and strengthened ferrite is crystallized in the surface layer region, by rapid cooling, martensite and bainite are generated in the internal region in addition to ferrite. Therefore, the hardness difference between the surface layer region strengthened by precipitation and the internal region where low-temperature transformation structure is formed without precipitation strengthening can be reduced.

[0063] Hot rolling The heating temperature of the slab has a great influence on solution treatment and elimination of element segregation. By setting the heating temperature of the slab to 1100 °C or higher, it is possible to suppress insufficient solution treatment and elimination of element segregation, and as a result, deterioration of the tensile properties and toughness of the product can be suppressed. Further, by setting the heating temperature of the slab to 1350 °C or lower, the effect of solution treatment and elimination of element segregation can be saturated. Therefore, the heating temperature of the slab is preferably 1100 to 1350 °C, more preferably 1150 to 1300 °C. Note that the temperature of the slab and the temperature of the steel sheet in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.

[0064] In finish rolling, rolling is performed by continuously passing the slab through a rolling stand for finish rolling a plurality of times. In finish rolling, it is preferable that the temperature of the hot-rolled steel sheet after the final pass (finish temperature) is equal to or higher than the Ar3 point, and the reduction ratio of the final pass is 12 to 45%. Note that the temperature of the hot-rolled steel sheet after the final pass is the lowest temperature in finish rolling rolled by a plurality of stands. The reduction ratio after the final pass can be expressed as {(t0 - t1) / t0} × 100 (%) where t0 is the inlet plate thickness before the final pass and t1 is the outlet plate thickness after the final pass. The Ar3 point is represented by the following formula (1).

[0065] Ar3 point = 901 - 325 × C + 33 × Si - 92 × Mn + 287 × P + 40 × sol.Al ··· Formula (1) Each element symbol in the above formula (1) indicates the content (% by mass) of each element. When the element is not contained, 0 is substituted.

[0066] By setting the temperature of the hot-rolled steel sheet after the final pass (finish temperature) of finish rolling to be equal to or higher than the Ar3 point, it is possible to suppress the formation of ferrite during finish rolling, and as a result, a desired metal structure and properties can be obtained.

[0067] By setting the reduction ratio in the final pass of finish rolling to 12% or more, recrystallization can be promoted in finish rolling, the metallographic structures in the internal region and the surface layer region can be favorably controlled, and excellent fatigue strength can be obtained. Also, by setting the reduction ratio in the final pass to 45% or less, it is possible to suppress an increase in the load on the rolling stand and deterioration of the shape of the hot-rolled steel sheet after finish rolling. Therefore, the reduction ratio in the final pass in finish rolling is preferably set to 12 - 45%. More preferably, it is 15 - 45%.

[0068] Between the final pass of finish rolling and the pass one stage before it, by performing bending, it is preferable to impart a strain of 0.002 - 0.020 to the surface layer region (region from the surface to a depth of 20 μm from the surface) of the hot-rolled steel sheet. By setting the strain during bending to 0.002 or more, a desired metallographic structure can be created in the surface layer region. Therefore, the strain during bending is preferably set to 0.002 or more. More preferably, it is 0.003 or more or 0.004 or more. Also, by setting the strain during bending to 0.020 or less, it is possible to suppress the occurrence of buckling during finish rolling and the loss of manufacturing stability. Also, by setting the strain during bending to 0.020 or less, the metallographic structures in the surface layer region and the internal region can be favorably controlled. Therefore, the strain during bending is preferably set to 0.020 or less. More preferably, it is 0.015 or less or 0.010 or less.

[0069] Note that the bending is performed by a method such as a method of pushing up the steel sheet from below with a roll between stands, and the strain during bending can be controlled by adjusting the bending angle with the amount of pushing up and the diameter of the roll. For example, when performing bending by a method of pushing up the steel sheet from below with a roll between stands, the amount of strain during bending can be obtained by the following formula (2).

[0070] Amount of strain = 1.5 × (sheet thickness) × (amount of pushing up) / (diameter of the tip of the pushing-up device) 2 ···Formula (2)

[0071] After finish of finish rolling, the elapsed time until start of cooling is preferably 1.6 seconds or less. By setting the elapsed time from completion of finish rolling to start of cooling to 1.6 seconds or less, it is possible to suppress recovery of bending and rolling strain, and to preferably control the metal structure of the surface layer region.

[0072] After completion of finish rolling, as primary cooling, it is preferable to cool to a temperature range of 600 to 750°C at an average cooling rate of 40°C / second or more, and then to perform air cooling for 2 to 6 seconds. In general, the cooling rate during air cooling is 2 to 10°C / second. By setting the stop temperature of cooling at an average cooling rate of 40°C / second or more to a temperature range of 600 to 750°C and then performing air cooling, it is possible to promote ferrite transformation and obtain a desired amount of ferrite.

[0073] After air cooling, as secondary cooling, it is preferable to cool to a temperature range of 200°C or less at an average cooling rate of 60°C / second or more, and then to wind it up in a coil shape. By setting the average cooling rate to 200°C or less to 60°C / second or more, it is possible to promote martensite transformation and obtain a desired amount of martensite and bainite.

[0074] Here, the average cooling rate is defined as the value obtained by dividing the temperature drop width of the steel sheet from the start of cooling to the end of cooling by the required time from the start of cooling to the end of cooling.

[0075] In addition, among cooling facilities, there are facilities without an air cooling section in the middle and facilities having one or more air cooling sections in the middle. In this embodiment, any cooling facility may be used. Even when using a cooling facility having an air cooling section, the average cooling rate from the start of cooling to the end of cooling may be within the range described above.

[0076] Since coiling of the hot-rolled steel sheet is performed immediately after secondary cooling, the coiling temperature is approximately equal to the cooling stop temperature of secondary cooling. By setting the coiling temperature to 200°C or less, it is possible to suppress the generation of a large amount of polygonal ferrite or bainite, and to obtain a desired metal structure and properties.

[0077] In addition, after coiling, the hot-rolled steel sheet may be subjected to temper rolling according to a conventional method, or pickling may be performed to remove the scale formed on the surface. Alternatively, plating such as the above-described hot-dip galvanizing or electro-galvanizing may be formed, or further chemical conversion treatment may be performed.

[0078] According to the above manufacturing method, a hot-rolled steel sheet having the above-described metal structure can be stably manufactured. Therefore, it is possible to stably manufacture a hot-rolled steel sheet having high strength, excellent fatigue strength, and toughness.

Example

[0079] Next, the effects of one aspect of the present invention will be described more specifically by way of examples. The conditions in the examples are one set of conditions adopted for confirming the feasibility and effects of the present invention, and the present invention is not limited to this one set of conditions. The present invention can adopt various conditions as long as it does not deviate from the gist of the present invention and achieves the object of the present invention.

[0080] Steel having the chemical composition shown in Table 1 was melted and a slab having a thickness of 240 to 300 mm was produced by continuous casting. Using the obtained slab, hot-rolled steel sheets shown in Tables 4 and 5 were obtained according to the manufacturing conditions shown in Tables 2 and 3. Note that the bending process was performed by pushing up from below the steel sheet with a roll between stands. The amount of strain during the bending process was controlled by adjusting the bending angle with the amount of pushing up and the diameter of the roll. At this time, the amount of strain during the bending process was obtained by the above formula (2).

[0081] For the obtained hot-rolled steel sheet, the area fraction and Vickers hardness, tensile strength, total elongation, absorbed energy at -20°C, and fatigue limit ratio of the metal structure in the internal region and the surface layer region were determined by the above-described method. The obtained measurement results are shown in Tables 4 and 5.

[0082] Evaluation method for characteristics of hot-rolled steel sheet When the tensile strength TS was 980 MPa or more, it was judged as qualified as a hot-rolled steel sheet with excellent strength. On the other hand, when the tensile strength TS was less than 980 MPa, it was judged as unqualified as not being a hot-rolled steel sheet with excellent strength.

[0083] When the total elongation was 10% or more, it was judged as qualified as a hot-rolled steel sheet with excellent ductility. On the other hand, when the total elongation was less than 10%, it was judged as unqualified as not being a hot-rolled steel sheet with excellent ductility. - When the absorbed energy at -20 °C was 80 J / cm 2 or more, it was judged as qualified as a hot-rolled steel sheet with excellent toughness. On the other hand, when the absorbed energy at -20 °C was 80 J / cm 2 less than that, it was judged as unqualified as not being a hot-rolled steel sheet with excellent toughness. When the fatigue limit ratio was 0.48 or more, it was judged as qualified as a hot-rolled steel sheet with excellent fatigue strength. On the other hand, when the fatigue limit ratio was less than 0.48, it was judged as unqualified as not being a hot-rolled steel sheet with excellent fatigue strength.

[0084]

Table 1

[0085]

Table 2

[0086]

Table 3

[0087]

Table 4

[0088]

Table 5

[0089] Referring to Tables 4 and 5, it can be seen that the hot-rolled steel sheet according to the example of the present invention has high strength, as well as excellent toughness, fatigue strength and ductility. On the other hand, it can be seen that the hot-rolled steel sheet according to the comparative example is inferior in one or more of strength, toughness and fatigue strength.

Industrial Applicability

[0090] According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, as well as excellent fatigue strength, toughness and ductility. According to this hot-rolled steel sheet, it is possible to reduce the weight of the vehicle body of an automobile or the like, integrally mold parts, and shorten the processing process, etc., and it is possible to improve fuel efficiency and reduce manufacturing costs, so the industrial value is high.

Claims

1. The chemical composition is by mass%, C: 0.02 to 0.30%, Si: 0.10 to 2.00%, Mn: 0.5 to 3.0%, sol. Al: 0.10 to 1.00%, Ti: 0.06 to 0.20%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0100% or less, Nb: 0 to 0.100%, Ca: 0 to 0.0060%, Mo: 0 to 0.50%, Cr: 0 to 1.00%, V: 0 to 0.40%, Ni: 0 to 0.40%, Cu: 0 to 0.40%, B: 0 to 0.0020%, and Sn: 0 to 0.20%, and the balance consists of Fe and impurities, The metallographic structure of the internal region contains, by area ratio, one or both of martensite and bainite totaling 40 to 80%, ferrite 20 to 60%, and the area ratio of the remaining structure is less than 5%, The ratio αs / αc, which is the ratio of the ferrite area ratio αs in the surface layer region to the ferrite area ratio αc in the internal region, is 1.15 to 2.50, The ratio βs / βc, which is the ratio of the total area ratio βs of martensite and bainite in the surface layer region to the total area ratio βc of martensite and bainite in the internal region, is 0.30 to 0.90, The hardness difference ratio (1 - Hvs / Hvc), which is the hardness difference ratio between the Vickers hardness Hvs in the surface layer region and the Vickers hardness Hvc in the internal region, is 0.20 or less, The tensile strength is 980 MPa or more A hot-rolled steel sheet characterized by the above.

2. The chemical composition contains, by mass%, Nb: 0.010 to 0.100%, Ca: 0.0005 to 0.0060%, Mo: 0.02 to 0.50%, Cr: 0.02 to 1.00%, V: 0.01 to 0.40%, Ni: 0.01 to 0.40%, Cu: 0.01 to 0.40%, B: 0.0001 to 0.0020%, and Sn: 0.01 to 0.20%, and contains one or more selected from the group consisting of A hot-rolled steel sheet according to Claim 1, characterized by the above. ​

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