Hot-rolled steel sheet

Abstract

The hot-rolled steel sheet has a prescribed chemical composition and a metal structure, and when the pole density of the {001} plane of ferrite and bainite in a central region is set to P in a region obtained by dividing a sheet thickness section parallel to a rolling direction into three in a sheet thickness direction i , the pole density of the {001} plane of ferrite and bainite in a surface layer region is set to P s , and P i / P s is 1.2 to 2.0, the hot-rolled steel sheet has a tensile strength of 950 MPa or more.

Description

Technical Field

[0001] This invention relates to hot-rolled steel sheets. More specifically, it relates to hot-rolled steel sheets having high strength and excellent fatigue properties and shear workability.

[0002] This application claims priority based on Japanese Patent Application No. 2021-122173 filed on July 27, 2021, the contents of which are incorporated herein by reference. Background Technology

[0003] In recent years, research has been actively conducted on the application of high-strength steel sheets in automotive components with the aim of improving vehicle durability and crash safety. In particular, ensuring the fatigue durability of components using high-strength steel sheets has become crucial.

[0004] When steel plates are processed into components, blanks are made by punching the steel plates to create the shape of the components. If cracks occur at the punching and shearing surfaces, even if high-strength steel plates are used, the fatigue durability of the components may not necessarily be improved.

[0005] For example, Patent Document 1 proposes a hot-rolled steel sheet that exhibits excellent fatigue characteristics at the shear end face by increasing the volume fraction of martensite and decreasing the volume fraction of pearlite.

[0006] Patent document 2 proposes a steel plate that achieves excellent fatigue characteristics of the punching and shearing section by using ferrite and bainite structures as the main body and reducing the maximum height of the steel plate surface.

[0007] However, with respect to the technology described in Patent Documents 1 and 2, the fatigue characteristics are insufficient when the part is formed by processing because the cracking of the punching and shearing surface cannot be sufficiently suppressed.

[0008] Patent document 3 proposes a steel plate that ensures the balance of ferrite and martensite. <011> and <111> And inhibit <001> This extends the lifespan of fatigue-induced cracking.

[0009] Patent document 4 proposes a method that controls the crystal orientation of the ferrite or bainite main phase structure by controlling the shape ratio up to the final pass during finishing rolling.

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: Japanese Patent Application Publication No. 2001-40450

[0013] Patent Document 2: Japanese Patent Application Publication No. 2001-172745

[0014] Patent Document 3: International Publication No. 2016 / 010005

[0015] Patent Document 4: Japanese Patent Application Publication No. 2009-19265 Summary of the Invention

[0016] The problem that the invention aims to solve

[0017] However, from the viewpoint of not only improving fatigue properties but also improving shear workability, the technologies described in Patent Documents 3 and 4 have room for improvement.

[0018] Against the backdrop of increasing demands for lightweight automobiles and more complex component shapes in recent years, there is a need for high-strength hot-rolled steel sheets with superior fatigue properties and shear processability.

[0019] The present invention was made in view of the above-mentioned problems, and the object of the present invention is to provide a hot-rolled steel sheet with high strength and excellent fatigue properties and shear workability.

[0020] Methods for solving problems

[0021] As a microstructure exhibiting excellent fatigue properties, a multiphase structure in which hard primary martensite is dispersed within ferrite that hinders dislocation movement is known to be effective. Steel sheets with this multiphase structure are, for example, frequently used in automobile wheel disc components.

[0022] On the other hand, since the hard primary martensite also hinders plastic deformation, during intense processing such as blanking, voids are formed around the primary martensite, making cracking prone to occur on the blanking shear surface. Therefore, hot-rolled steel sheets with a multiphase structure utilizing ferrite and primary martensite generally exhibit deteriorated shearing workability.

[0023] To break down these interrelationships, the inventors of this invention conducted a detailed analysis of each deformation mechanism. As a result, they discovered that by precisely controlling the crystal orientation of ferrite and bainite in the desired region, it is possible to ensure the fatigue characteristics of hot-rolled steel sheets and improve their shear workability. Specifically, they found that by controlling the area ratio of hard primary martensite and tempered martensite as the main phase, fatigue characteristics are ensured, and by appropriately producing a crystal orientation of ferrite and bainite with greater crystal rotation through punching in the desired region, thereby achieving a balance between the fatigue characteristics and shear workability of hot-rolled steel sheets in a high-dimensional manner.

[0024] This invention is based on the above-mentioned knowledge, and the main points of this invention are as follows.

[0025] (1) The chemical composition of the hot-rolled steel sheet of one embodiment of the present invention, in mass percent, contains:

[0026] C: 0.02~0.30%

[0027] Si: 0.10~2.00%

[0028] Mn: 0.5-3.0%

[0029] P: below 0.100%

[0030] S: less than 0.010%

[0031] Al: 0.10–1.00%

[0032] N: below 0.0100%

[0033] Ti: 0.06~0.20%

[0034] Nb: 0–0.10%

[0035] Ca: 0–0.0060%

[0036] Mo: 0–1.00%

[0037] Cr: 0–1.00%

[0038] V: 0~0.40%

[0039] Ni: 0-0.40%

[0040] B: 0~0.0020%

[0041] Cu: 0–1.00%

[0042] Sn: 0–0.50%, and

[0043] Zr: 0~0.050%,

[0044] The remainder consists of Fe and impurities.

[0045] The metal structure, expressed as an area ratio, is as follows:

[0046] Total ferrite and bainite: 30-47%

[0047] Tempered martensite: 50-70%

[0048] Primary martensite: 3-10%,

[0049] In a region obtained by dividing a plate section parallel to the rolling direction into three parts along the plate thickness direction, the extreme density of the {001} planes in the ferrite and bainite in the central region is set to P.i The extreme density of the {001} plane in the ferrite and bainite of the surface region is set to P. s At that time, P i / P s The value is 1.2 to 2.0.

[0050] The tensile strength of the hot-rolled steel plate is above 950 MPa.

[0051] (2) The hot-rolled steel plate according to (1) above, wherein the above chemical composition may also contain one or more of the following elements in mass percent:

[0052] Nb: 0.01~0.10%

[0053] Ca: 0.0005~0.0060%

[0054] Mo: 0.02–1.00%

[0055] Cr: 0.02~1.00%

[0056] V: 0.01~0.40%

[0057] Ni: 0.01~0.40%

[0058] B: 0.0001~0.0020%

[0059] Cu: 0.02–1.00%

[0060] Sn: 0.01–0.50%, and

[0061] Zr: 0.001~0.050%.

[0062] Invention Effects

[0063] According to the above-described solution of the present invention, a hot-rolled steel sheet with high strength, excellent fatigue properties, and shear workability can be provided. The hot-rolled steel sheet of the present invention enables the integral forming of lightweight components for automobile bodies, shortens processing steps, and achieves improved fuel efficiency and reduced manufacturing costs. Detailed Implementation

[0064] A hot-rolled steel sheet (sometimes referred to as the hot-rolled steel sheet of this embodiment) according to one embodiment of the present invention will be described. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the spirit of the present invention.

[0065] The essential constituent conditions of the present invention will be described in detail below. First, the reasons for limiting the chemical composition of the hot-rolled steel sheet in this embodiment will be explained.

[0066] For the numerical ranges specified by the following separator “~”, the lower and upper limits are included within the range. Values ​​expressed as “below” or “above” are not included in the range. Unless otherwise specified, “%” for chemical composition refers to “mass %”.

[0067] The hot-rolled steel sheet of this embodiment has the following chemical composition by mass %: C: 0.02-0.30%, Si: 0.10-2.00%, Mn: 0.5-3.0%, P: less than 0.100%, S: less than 0.010%, Al: 0.10-1.00%, N: less than 0.0100%, Ti: 0.06-0.20%, and the remainder: Fe and impurities.

[0068] <C:0.02~0.30%>

[0069] Carbon (C) is an important element for improving the strength of hot-rolled steel sheets. If the C content is below 0.02%, the desired strength cannot be obtained. Therefore, the C content is set to 0.02% or more. Preferably, it is 0.04% or more, 0.06% or more, or 0.10% or more.

[0070] On the other hand, if the carbon content exceeds 0.30%, the shearing processability of the hot-rolled steel sheet deteriorates. Therefore, the carbon content is set to 0.30% or less. Preferably, it is 0.25% or less or 0.20% or less.

[0071] <Si:0.10~2.00%>

[0072] Si is an element that inhibits the formation of carbides during ferrite phase transformation and improves the fatigue properties of hot-rolled steel sheets. This effect cannot be achieved if the Si content is below 0.10%. Therefore, the Si content is set to 0.10% or more. Preferably, it is 0.20% or more, 0.30% or more, or 0.50% or more.

[0073] On the other hand, if the Si content exceeds 2.00%, the shearing processability of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 2.00% or less. Preferably, it is 1.80% or less, 1.60% or less, or 1.50% or less.

[0074] <Mn:0.5~3.0%>

[0075] Mn is an effective element for improving the strength of hot-rolled steel sheets through enhanced hardenability and solid solution strengthening. This effect cannot be achieved if the Mn content is below 0.5%. Therefore, the Mn content is set to 0.5% or more, preferably 0.7% or more or 1.0% or more.

[0076] On the other hand, if the Mn content exceeds 3.0%, the fatigue characteristics of the hot-rolled steel sheet deteriorate due to the formation of MnS. Therefore, the Mn content is set to 3.0% or less. It is preferably 2.8% or less, 2.5% or less, 2.3% or less, or 2.0% or less.

[0077] <P: 0.100% or less>

[0078] P is an impurity, and the lower the P content, the more preferable it is, and 0% is preferred. If the P content exceeds 0.100%, the workability and weldability of the hot-rolled steel sheet deteriorate significantly, and the fatigue characteristics also deteriorate. Therefore, the P content is set to 0.100% or less. It is preferably 0.070% or less, 0.050% or less, or 0.030% or less.

[0079] From the perspective of refining cost, the P content can also be set to 0.001% or more.

[0080] <S: 0.010% or less>

[0081] S is an impurity, and the lower the S content, the more preferable it is, and 0% is preferred. If the S content exceeds 0.010%, a large amount of inclusions such as MnS are generated, and the shearing workability of the hot-rolled steel sheet deteriorates. Therefore, the S content is set to 0.010% or less. It is preferably 0.008% or less, 0.007% or less. In the case where more excellent shearing workability is required, the S content is preferably set to 0.006% or less.

[0082] From the perspective of refining cost, the S content can also be set to 0.001% or more.

[0083] <Al: 0.10 - 1.00%>

[0084] Al is an element important for controlling the ferrite phase transformation. If the Al content is less than 0.10%, the area ratio of ferrite cannot be preferably controlled. Therefore, the Al content is set to 0.10% or more. It is preferably 0.20% or more, 0.30% or more, or 0.40% or more.

[0085] On the other hand, if the Al content exceeds 1.00%, alumina precipitates in clusters, and the shearing workability of the hot-rolled steel sheet deteriorates. Therefore, the Al content is set to 1.00% or less. It is preferably 0.90% or less, 0.80% or less, 0.70% or less, or 0.60% or less.

[0086] <N: 0.0100% or less>

[0087] Nitrogen (N) is an impurity, and a lower N content is preferred, ideally 0%. If the N content exceeds 0.0100%, coarse Ti nitrides will form at high temperatures, deteriorating the shearing processability of the hot-rolled steel sheet. Therefore, the N content is set to 0.0100% or less, preferably 0.0080%, 0.0060%, or 0.0050% or less.

[0088] From the perspective of refining costs, the nitrogen content can also be set to 0.0001% or higher.

[0089] <Ti:0.06~0.20%>

[0090] Ti is an element that strengthens ferrite precipitation and is crucial for obtaining the desired amount of ferrite by controlling the ferrite phase transformation. If the Ti content is below 0.06%, the effects of precipitation strengthening and ferrite phase transformation control cannot be achieved. Therefore, the Ti content is set to 0.06% or more, preferably 0.08% or more or 0.10% or more.

[0091] On the other hand, if the Ti content exceeds 0.20%, inclusions caused by TiN are formed, deteriorating the shear workability of the hot-rolled steel sheet. Therefore, the Ti content is set to 0.20% or less. Preferably, it is 0.18% or less or 0.16% or less.

[0092] The hot-rolled steel sheet of this embodiment may also have the above-described chemical composition, with the remainder containing Fe and impurities. Here, impurities refer to components that are mixed in during the industrial manufacturing of steel due to raw materials such as ore and waste, or other factors, and / or components that are permissible within a range that do not adversely affect the hot-rolled steel sheet of this embodiment.

[0093] Regarding the chemical composition of the hot-rolled steel sheet of this embodiment, although it is not necessary to meet the required properties, it may contain the following optional elements to reduce manufacturing inconsistencies or further improve strength. However, none of the following optional elements are necessary to meet the required properties, therefore their content is limited to 0%.

[0094] <Nb:0.01~0.10%>

[0095] Nitrogen (Nb) is an element that can improve the strength of hot-rolled steel sheets by refining the crystal grain size and by strengthening the precipitation of NbC. To achieve this effect, the Nb content is preferably set to 0.01% or more.

[0096] On the other hand, the above-mentioned effect saturates when the Nb content exceeds 0.10%. Therefore, even when Nb is present, the Nb content is set to 0.10% or less. Preferably, it is 0.06% or less.

[0097] <Ca:0.0005~0.0060%>

[0098] Ca is an element that improves the porosity of hot-rolled steel sheets by fixing sulfur in steel in the form of spherical CaS and inhibiting the formation of elongated inclusions such as MnS. To achieve these effects, the Ca content is preferably set to 0.0005% or more.

[0099] On the other hand, the above-mentioned effect saturates when the Ca content exceeds 0.0060%. Therefore, even when Ca is present, the Ca content is set to 0.0060% or less. Preferably, it is 0.0040% or less.

[0100] <Mo:0.02~1.00%>

[0101] Mo is an effective element for improving the strength of hot-rolled steel sheets through ferrite precipitation strengthening. To achieve this effect, the Mo content is preferably set to 0.02% or more, more preferably 0.10% or more.

[0102] On the other hand, when the Mo content exceeds 1.00%, the slab's slab's susceptibility to cracking increases, making slab handling difficult. Therefore, even when Mo is present, the Mo content is set to 1.00% or less. Preferably, it is 0.60% or less, 0.50% or less, or 0.30% or less.

[0103] <Cr:0.02~1.00%>

[0104] Cr is an effective element for improving the strength of hot-rolled steel sheets. To achieve this effect, the Cr content is preferably set to 0.02% or more, more preferably 0.10% or more.

[0105] On the other hand, the ductility of hot-rolled steel sheets deteriorates when the Cr content exceeds 1.00%. Therefore, even when Cr is present, the Cr content is set to 1.00% or less. Preferably, it is 0.80% or less.

[0106] <V:0.01~0.40%>

[0107] V is an element that enhances the strength of hot-rolled steel sheets through precipitation strengthening and dislocation strengthening by inhibiting recrystallization. To achieve these effects, the V content is preferably set to 0.01% or more.

[0108] On the other hand, when the V content exceeds 0.40%, a large amount of carbonitrides precipitate, thus reducing the formability of the hot-rolled steel sheet. Therefore, the V content is set to 0.40% or less, preferably 0.20% or less.

[0109] <Ni:0.01~0.40%>

[0110] Ni is an element that suppresses phase transformation at high temperatures and improves the strength of hot-rolled steel sheets. To achieve this effect, the Ni content is preferably set to 0.01% or more.

[0111] On the other hand, the weldability of hot-rolled steel sheets decreases when the Ni content exceeds 0.40%. Therefore, the Ni content is set to 0.40% or less, preferably 0.20% or less.

[0112] <B:0.0001~0.0020%>

[0113] Boron (B) is an element that inhibits phase transformation at high temperatures and improves the strength of hot-rolled steel sheets. To achieve this effect, the B content is preferably set to 0.0001% or higher.

[0114] On the other hand, when the boron content exceeds 0.0020%, boron precipitates are formed, thereby reducing the strength of the hot-rolled steel sheet. Therefore, the boron content is set to 0.0020% or less, preferably 0.0005% or less.

[0115] <Cu:0.02~1.00%>

[0116] Cu is an element that exists in steel in the form of fine particles and increases the strength of hot-rolled steel sheets. To achieve this effect, the Cu content is preferably set to 0.02% or more.

[0117] On the other hand, the weldability of hot-rolled steel sheets deteriorates when the Cu content exceeds 1.00%. Therefore, the Cu content is set to 1.00% or less, preferably 0.80% or less.

[0118] <Sn:0.01~0.50%>

[0119] Sn is an element that inhibits grain coarsening and improves the strength of hot-rolled steel sheets. To achieve this effect, the Sn content is preferably set to 0.01% or more.

[0120] On the other hand, when the Sn content exceeds 0.50%, the steel becomes embrittled and thus prone to fracture during rolling. Therefore, the Sn content is set to 0.50% or less, preferably 0.30% or less.

[0121] <Zr:0.001~0.050%>

[0122] Zr is an element that helps improve the formability of hot-rolled steel sheets. To achieve this effect, the Zr content is preferably set to 0.001% or more.

[0123] On the other hand, the ductility of hot-rolled steel sheets deteriorates when the Zr content exceeds 0.050%. Therefore, the Zr content is set to 0.050% or less, preferably 0.030% or less.

[0124] The chemical composition of the hot-rolled steel sheet described above can be determined using general analytical methods. For example, it can be determined using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). It should be noted that C and S can be determined using the combustion-infrared absorption method, and N can be determined using the inert gas melting-thermal conductivity method.

[0125] Next, the metal structure of the hot-rolled steel sheet of this embodiment will be described.

[0126] The microstructure of the hot-rolled steel sheet in this embodiment, by area ratio, comprises: 30-47% ferrite and bainite, 50-70% tempered martensite, and 3-10% primary martensite. In a region obtained by dividing a section parallel to the rolling direction into three parts along the thickness direction, the maximum density of the {001} planes in the ferrite and bainite of the central region is set to P. i The extreme density of the {001} plane in the ferrite and bainite of the surface region is set to P. s At that time, P i / P s It ranges from 1.2 to 2.0.

[0127] The hot-rolled steel sheet of this embodiment preferably has a microstructure consisting only of ferrite, bainite, tempered martensite, and primary martensite. That is, the hot-rolled steel sheet of this embodiment preferably has a microstructure consisting, in terms of area ratio, only of a total of ferrite and bainite: 30-47%, tempered martensite: 50-70%, and primary martensite: 3-10%.

[0128] It should be noted that, in this embodiment, the area ratios of ferrite, bainite, tempered martensite, and primary martensite in the region from 1 / 8 to 3 / 8 of the depth from the surface are specified. This is because the microstructure in this region represents the representative microstructure of hot-rolled steel sheet.

[0129] The total percentage of ferrite and bainite is 30-47%.

[0130] Ferrite and bainite improve the shear workability of hot-rolled steel sheets. If the combined area ratio of ferrite and bainite is less than 30%, the shear workability or fatigue strength of the hot-rolled steel sheet may deteriorate. Therefore, the combined area ratio of ferrite and bainite is set to 30% or more. Preferably, it is 33% or more, 35% or more, or 37% or more.

[0131] On the other hand, if the combined area ratio of ferrite and bainite exceeds 47%, the strength and fatigue properties of the hot-rolled steel sheet may deteriorate, or its shear workability may deteriorate. Therefore, the combined area ratio of ferrite and bainite is set to 47% or less. Preferably, it is 45% or less or 43% or less.

[0132] It should be noted that in this embodiment, it is not necessary to contain both ferrite and bainite; it may contain only one of ferrite and bainite, and its area ratio may be within the range described above.

[0133] Tempered martensite: 50-70%

[0134] To improve the fatigue properties of hot-rolled steel sheets, including primary martensite is effective. However, to balance shear workability and fatigue properties, the martensite formation temperature is high, and including tempered martensite formed by tempering during cooling is effective.

[0135] If the area ratio of tempered martensite is less than 50%, the strength and fatigue properties of the hot-rolled steel sheet deteriorate. Therefore, the area ratio of tempered martensite is set to 50% or more. Preferably, it is 53% or more or 55% or more.

[0136] On the other hand, if the area ratio of tempered martensite exceeds 70%, the shear workability of the hot-rolled steel sheet may deteriorate, or the strength of the hot-rolled steel sheet may deteriorate. Therefore, the area ratio of tempered martensite is set to 70% or less. Preferably, it is 65% or less or 60% or less.

[0137] Primary martensite: 3–10%

[0138] Primary martensite improves the fatigue strength of hot-rolled steel sheets. If the area fraction of primary martensite is less than 3%, the fatigue strength and / or overall strength of the hot-rolled steel sheet may deteriorate. Therefore, the area fraction of primary martensite is set to 3% or more, preferably 4% or more or 5% or more.

[0139] On the other hand, if the area ratio of primary martensite exceeds 10%, the shear workability of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of primary martensite is set to 10% or less. Preferably, it is 9% or less or 8% or less.

[0140] The area ratio of each organization was obtained using the following methods.

[0141] First, test pieces were collected from hot-rolled steel sheets in a manner that allowed observation of the metal structure at a depth of 1 / 4 of the sheet thickness (1 / 8 to 3 / 8 of the sheet thickness) and at the center of the sheet width in a section of the sheet thickness parallel to the rolling direction.

[0142] The cross-section of the above-mentioned test piece was ground using #600 to #1500 silicon carbide paper, and then refined into a mirror finish using a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water. Next, it was ground at room temperature using colloidal silica without an alkaline solution to remove the strain introduced into the surface layer of the sample. At any position along the length of the sample cross-section, crystal orientation information was obtained by measuring a region 50 μm in length and ranging from 1 / 8 to 3 / 8 of the plate thickness from the surface at measurement intervals of 0.1 μm using electron backscatter diffraction.

[0143] For the measurements, an EBSD analysis apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) was used. The vacuum level within the EBSD analysis apparatus was set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage was set to 15 kV, the irradiation current level to 13, and the electron beam irradiation level to 62. Using the obtained crystal orientation information, the "Grain Orientation Spread" function of the "OIM Analysis" software (registered trademark) included in the EBSD analysis device was used to extract regions with a "Grain Orientation Spread" of 1° or less as ferrite, under the condition that 15° grain boundaries were considered as crystal grain boundaries. The area fraction of the extracted ferrite was then calculated to obtain the ferrite area fraction.

[0144] Next, in the remaining region (the region where "Grain Orientation Spread" exceeds 1°), under the condition that the 5° grain boundary is considered as a crystal grain boundary, when the maximum value of "Grain Average IQ" of the ferrite region is set to Iα, the region exceeding Iα / 2 is extracted as bainite, and the region below Iα / 2 is extracted as "primary martensite and tempered martensite". The area ratio of bainite is obtained by calculating the area ratio of the extracted bainite.

[0145] The extracted primary martensite and tempered martensite are distinguished by the following method.

[0146] To observe the same area as the EBSD measurement area using SEM, Vickers indentations were made near the observation location. Afterwards, the tissue of the observation surface was preserved, surface contaminants were removed by grinding, and nitric acid ethanol etching was performed. Subsequently, the same field of view as the EBSD observation surface was observed using SEM at 3000x magnification.

[0147] In EBSD measurements, regions classified as "primary martensite and tempered martensite" that contain substructure within the grains and where cementite precipitates in multiple variants are identified as tempered martensite. Regions with high brightness and for which the substructure is not visible after etching are identified as primary martensite. The area ratios of tempered martensite and primary martensite are calculated.

[0148] It should be noted that for removing dirt from the surface of the observation surface, polishing with alumina particles with a particle size of less than 0.1 μm or Ar ion sputtering can be used.

[0149] P i / P s : 1.2~2.0

[0150] If the rolling surface is parallel to the {001} plane, there are fewer dislocation slip systems, and crystal rotation is not caused during shearing. This makes cracking more likely to occur on the punching shear surface, thus deteriorating the shearing workability of the hot-rolled steel sheet. The inventors of this invention have discovered that cracking during shearing tends to occur in the central region of the area obtained by dividing the plate thickness section parallel to the rolling direction into three parts along the plate thickness direction. In this embodiment, the shearing workability of the hot-rolled steel sheet is improved by preferably controlling the extreme density of the {001} plane in the ferrite and bainite in the central region and the surface region.

[0151] In a region obtained by dividing a plate section parallel to the rolling direction into three parts along the plate thickness direction, the extreme density of the {001} planes in the ferrite and bainite in the central region is set to P. i The extreme density of the {001} plane in the ferrite and bainite of the surface region is set to P. s At that time, P i / P s A value below 1.2 indicates a uniform distribution from the surface of the hot-rolled steel sheet. In this case, crystal rotation occurs from the shearing plane during punching, leading to increased edge collapse during shearing and making the shearing plane more prone to cracking. Consequently, the shearing workability of the hot-rolled steel sheet deteriorates. Therefore, P i / P s Set to 1.2 or higher. Preferably, it is 1.3 or higher, 1.4 or higher, or 1.5 or higher.

[0152] On the other hand, P i / P s A value exceeding 2.0 indicates excessive concentration of the {001} facets in the central region. In this case, the {001} facets, which are brittle fracture surfaces, become more abundant in the fracture surface, making it more prone to cracking in the punching and shearing planes. Consequently, the shearing workability of the hot-rolled steel sheet deteriorates. Therefore, P i / P s Set to 2.0 or lower. Preferably, it should be 1.9 or lower, 1.8 or lower, or 1.7 or lower.

[0153] It should be noted that the so-called central region refers to the region at a depth of 1 / 3 to 2 / 3 of the plate thickness from the surface in the region obtained by dividing the plate thickness section parallel to the rolling direction into 3 parts along the plate thickness direction. Furthermore, the so-called surface region refers to the region at a depth of 1 / 3 of the plate thickness from the surface in the region obtained by dividing the plate thickness section parallel to the rolling direction into 3 parts along the plate thickness direction, or the region at a depth of 2 / 3 of the plate thickness from the surface to the back side (the surface on the other side different from the aforementioned surface). In this embodiment, there is no particular limitation on which region it refers to.

[0154] Furthermore, {hkl} represents a crystal plane parallel to the rolling plane. That is, {hkl} indicates that the rolling direction is parallel to the {hkl} plane.

[0155] The polar density of the {001} planes of ferrite and bainite was determined using a combination of a scanning electron microscope and an EBSD analysis system, along with an OIM Analysis (registered trademark) device manufactured by TSL Corporation. The polar density was calculated using orientation data determined by EBSD (Electron Back Scattering Diffraction) and a spherical harmonic function, which represents the crystal orientation distribution function (ODF) of the three-dimensional texture. It should be noted that the measurement interval was set to 5 μm per step.

[0156] The measurement range was set as follows: the central region (the region obtained by dividing the plate thickness section parallel to the rolling direction into three parts along the thickness direction, with a depth of 1 / 3 to 2 / 3 of the plate thickness from the surface), and the surface region (the region obtained by dividing the plate thickness section parallel to the rolling direction into three parts along the thickness direction, with a depth of 1 / 3 of the plate thickness from the surface, or a region obtained by dividing the plate thickness section parallel to the rolling direction into three parts along the thickness direction, with a depth of 2 / 3 of the plate thickness from the surface to the back side (the surface on the other side different from the aforementioned surface)). Furthermore, for regions considered as ferrite and bainite using the same method as the EBSD measurement described above, extreme density was measured.

[0157] Tensile strength: above 950MPa

[0158] The hot-rolled steel sheet of this embodiment has a tensile strength of 950 MPa or higher. Preferably, it has a tensile strength of 1000 MPa or higher. If the tensile strength is lower than 950 MPa, the applicable parts are limited, and the contribution to vehicle body weight reduction is small. There is no need to specifically limit the upper limit, but from the viewpoint of suppressing die wear, it can also be set to 1500 MPa or lower or 1300 MPa or lower.

[0159] Furthermore, the fatigue limit ratio (fatigue strength / tensile strength) of the hot-rolled steel plate in this embodiment can also be 0.35 or higher.

[0160] Tensile strength was evaluated by tensile testing according to JIS Z 2241:2011. The test piece was set as test piece No. 5 of JIS Z2241:2011. The tensile test piece was collected at a distance of 1 / 4 from the end in the width direction of the plate, and the direction perpendicular to the rolling direction was taken as the length direction.

[0161] Fatigue strength was determined according to JIS Z 2275:1978. Test specimen No. 1 was collected from hot-rolled steel plate and tested using a Schenker plane bending fatigue testing machine. The stress load during testing was alternating, and the test speed was set to 30 Hz. Tests were conducted over 10 days. 7 Fatigue strength during 10 cycles. Then, by 10 7 The fatigue strength during each cycle is divided by the tensile strength determined by the tensile test described above, and the fatigue limit ratio (fatigue strength / tensile strength) is calculated.

[0162] The thickness of the hot-rolled steel sheet in this embodiment is not particularly limited, but it can be set to 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 too large, which may make hot rolling difficult. On the other hand, when the thickness exceeds 8.0 mm, it may become difficult to obtain the above-mentioned metal structure after hot rolling.

[0163] The hot-rolled steel sheet of this embodiment, having the aforementioned chemical composition and metallic structure, can also be surface-treated to produce a steel sheet by applying a coating to its surface for purposes such as improving corrosion resistance. The coating can be an electroplated coating or a hot-dip galvanized coating. Examples of electroplated coatings include electroplated zinc layers and electroplated Zn-Ni alloy layers. Examples of hot-dip galvanized coatings include hot-dip galvanized layers, alloyed hot-dip galvanized layers, hot-dip aluminized layers, hot-dip Zn-Al alloy layers, hot-dip Zn-Al-Mg alloy layers, and hot-dip Zn-Al-Mg-Si alloy layers. The coating amount is not particularly limited and can be set as before. Furthermore, appropriate chemical conversion treatment (e.g., coating and drying with a silicate-based chromium-free chemical conversion solution) can be performed after plating to further improve corrosion resistance.

[0164] Regardless of the manufacturing method, the hot-rolled steel sheet of this embodiment achieves the desired effect by having the aforementioned chemical composition and metallic structure. However, the hot-rolled steel sheet of this embodiment can be stably obtained using the manufacturing method shown below, and is therefore preferred.

[0165] In the preferred manufacturing method of the hot-rolled steel sheet of this embodiment, the hot rolling conditions and subsequent cooling conditions are strictly controlled. A detailed description will follow.

[0166] The heating temperature of the slab has a significant impact on solution formation and the elimination of elemental segregation. When the slab heating temperature is below 1100°C, solution formation and the elimination of elemental segregation are insufficient, failing to achieve adequate precipitation strengthening in the final product and resulting in decreased tensile strength. Furthermore, if the slab heating temperature exceeds 1350°C, not only does the effect of solution formation and the elimination of elemental segregation become saturated, but the average grain size of austenite also coarsens, leading to inhomogeneity in crystal rotation during rolling and making it difficult to obtain the desired texture. Therefore, the slab heating temperature is preferably set between 1100 and 1350°C, and more preferably between 1150 and 1300°C.

[0167] It should be noted that the temperatures of the slab and the steel plate in this embodiment refer to the surface temperatures of the slab and the steel plate, respectively.

[0168] In finishing rolling, the slab is rolled multiple times consecutively through a finishing mill stand. Preferably, the rolling conditions in the last three stands (the final rolling stand, the stand preceding the final stand, and the two stands preceding the final stand) satisfy equations (1) and (2) below. In this embodiment, the average value of the last three stands preferably satisfies equations (1) and (2) below.

[0169] 2.0 ≤ 2 × {R(H1 - H2)} 0.5 / (H1+H2)≤10.0 Equation (1)

[0170] The symbols in equation (1) above are as follows.

[0171] R: Roller radius (mm)

[0172] H1: Thickness of the steel plate on the inlet side (mm)

[0173] H2: Thickness of the steel plate on the outward side (mm)

[0174] 5≤ΔT≤35 Equation (2)

[0175] In equation (2) above, ΔT is the difference between the temperature of the steel plate entering the mill and the temperature of the steel plate exiting the mill in each mill stand.

[0176] The middle edge of equation (1) above is the formula for calculating the rolling shape ratio. By controlling the rolling shape ratio, it is possible to control the crystal rotation caused by rolling and obtain the desired crystal orientation in the desired region. If the average rolling shape ratio of the final three stands is less than 2.0, the compressive strain inside the steel plate increases due to rolling, and the formation of recrystallization texture by rolling leads to a decrease in the extreme density of the {001} plane in the central region. As a result, P i / P s It became lower than 1.2.

[0177] Furthermore, if the average rolling shape ratio of the final three stands exceeds 10.0, enhanced shear deformation is applied to the steel plate surface, resulting in an extreme increase in the density of the {001} facet in the surface region. As a result, P... i / P s It became lower than 1.2.

[0178] Therefore, the average value of the rolling shape ratio of the final three stands is preferably set to 2.0 to 10.0. That is, the average value of the rolling shape ratio in the final rolling stand, the rolling shape ratio in the rolling stand preceding the final one, and the rolling shape ratio in the rolling stands preceding the final two is preferably set to 2.0 to 10.0.

[0179] Regarding equation (2) above, controlling the temperature difference ΔT between the steel plate's entry and exit sides at each rolling stand is effective for controlling the internal temperature of the steel plate. During hot rolling, heat is generated simultaneously from the contact with the rolls (de-heating) and from within the steel plate (heating from processing energy or frictional heat with the rolls). In the later stages of finishing rolling, especially as the plate thickness decreases and the rolling speed increases, de-heating decreases while the effect of processing heat increases. Therefore, it becomes important to manufacture the steel plate using an appropriate pass speed based on the roll diameter, surface condition, and the thickness of the plate being manufactured.

[0180] If the average ΔT of the three frames is less than 5, the temperature difference along the thickness of the steel plate decreases. As a result, the difference in polar density between the surface region and the central region's {001} plane is small, and P... i / P s It became lower than 1.2.

[0181] Furthermore, if the average ΔT of the final three racks exceeds 35, the heat removal from the steel plate surface increases, thus increasing the shear deformation of the steel plate surface. As a result, the extreme density of the {001} plane in the surface region is drastically reduced, and P... i / P s It has become more than 2.0.

[0182] Therefore, the average value of ΔT for the final three stands is preferably set to 5 to 35. That is, the average value of ΔT in the final rolling stand, ΔT in the rolling stand preceding the final one, and ΔT in the rolling stands preceding the final two are preferably set to 5 to 35.

[0183] After finishing rolling, cooling should preferably begin within 1.6 seconds. If the time until cooling begins exceeds 1.6 seconds, strain generated during rolling will recover, potentially making it impossible to optimally control the extreme density of the {001} facet in the surface region. As a result, it is possible that P... i / P s It becomes below 1.2. The time until cooling begins is more preferably within 0.6 seconds.

[0184] After finishing rolling, as a primary cooling process, it is preferable to cool to a temperature range of 600–750°C at an average cooling rate of 50°C / s or higher. Subsequently, air cooling is preferably performed in this temperature range for 2.0–6.0 seconds. When the temperature range for air cooling is below 600°C or above 750°C, the ferrite phase transformation will not occur sufficiently, and therefore the desired amount of ferrite may not be obtained. As a result, the combined area ratio of ferrite and bainite may not reach the desired amount. From the viewpoint of suppressing the addition of cooling equipment, the average cooling rate for the primary cooling can also be set to 250°C / s or less.

[0185] Furthermore, if the air cooling time in the temperature range of 600–750°C exceeds 6.0 seconds, a large amount of ferrite may be generated, and the combined area ratio of ferrite and bainite will not be desirable. If the air cooling time in this temperature range is less than 2.0 seconds, the area ratio of tempered martensite may increase, and the area ratio of primary martensite will not be desirable.

[0186] After the aforementioned air cooling, secondary cooling is preferably performed at an average cooling rate of 40°C / s or higher, cooling to a temperature range below 200°C. When the average cooling rate of the secondary cooling is below 40°C / s, it falls below the critical cooling rate required for martensitic transformation, and therefore it may not be possible to obtain the desired amount of primary martensite and / or tempered martensite. From the viewpoint of suppressing the addition of additional cooling equipment, the average cooling rate of the secondary cooling can also be set to 250°C / s or lower.

[0187] Here, in this embodiment, the average cooling rate is set as the value obtained by dividing the temperature drop of the steel plate from the start of cooling to the end of cooling by the required time from the start of cooling to the end of cooling. The start of cooling is defined as when the cooling medium is first sprayed onto the steel plate using the cooling equipment, and the end of cooling is defined as when the steel plate is removed from the cooling equipment.

[0188] Furthermore, there are cooling devices that have no air cooling section in the middle and devices that have one or more air cooling sections in the middle. In this embodiment, either type of cooling device can be used.

[0189] After being cooled to a temperature range below 200°C through secondary cooling, the steel sheet is rolled into a coil. Since the rolling of the steel sheet occurs immediately after secondary cooling, the rolling temperature is approximately equal to the cooling stop temperature of the secondary cooling. If the rolling temperature exceeds 200°C, a large amount of ferrite or bainite may form, making it impossible to obtain the desired metallic structure. Therefore, the rolling temperature, which becomes the cooling stop temperature, is preferably set below 200°C.

[0190] It should be noted that after coiling, the hot-rolled steel sheet can also be subjected to temper rolling using conventional methods. In addition, pickling can be performed to remove the oxide scale formed on the surface. Alternatively, further coating treatments such as hot-dip galvanizing, electroplating, or chemical conversion treatment can be carried out.

[0191] According to the manufacturing method described above, the hot-rolled steel sheet of this embodiment can be manufactured stably.

[0192] Example

[0193] Next, the effects of one aspect of the present invention will be described in more detail through embodiments. However, the conditions in the embodiments are examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these one example. Various conditions can be adopted by the present invention as long as they do not depart from the spirit of the present invention and achieve the purpose of the present invention.

[0194] Steels with the chemical compositions shown in Tables 1A and 1B were smelted and continuously cast to produce slabs with a thickness of 240–300 mm. Using the obtained slabs, hot-rolled steel plates as shown in Table 3 were obtained under the manufacturing conditions shown in Tables 2A and 2B. It should be noted that since finishing rolling was performed using a finishing mill with seven stands, the rolling shape ratios and ΔT for F5 (the two stands preceding the final stand), F6 (the one stand preceding the final stand), and F7 (the final stand) are recorded in the table.

[0195] For the obtained hot-rolled steel sheet, the area ratio and P of the metal structure are calculated using the method described above. i / P s Tensile strength and fatigue limit ratio. The test results are shown in Table 3.

[0196] When the tensile strength (TS) is 950 MPa or higher, it is deemed acceptable as a hot-rolled steel sheet with high strength. On the other hand, when the tensile strength (TS) is lower than 950 MPa, it is deemed unacceptable as a hot-rolled steel sheet without high strength.

[0197] When the fatigue limit ratio is 0.35 or higher, the hot-rolled steel sheet is deemed acceptable as having excellent fatigue strength. On the other hand, when the fatigue limit ratio is lower than 0.35, the hot-rolled steel sheet is deemed unacceptable as not having excellent fatigue strength.

[0198] In addition, the shearing workability of hot-rolled steel sheets was evaluated using the following methods.

[0199] According to JIS Z 2256:2020, three punched holes were produced using a φ10mm punch with a clearance of 15% and a punching speed of 3m / s. For each of the three punched holes, the maximum length of cracks in the sheared surface (the section perpendicular to the sheet surface) was measured. If the maximum crack length was 300μm or more, the sheet was deemed unqualified as hot-rolled steel with poor shear workability. Conversely, if the maximum crack length was less than 300μm, the sheet was deemed qualified as hot-rolled steel with excellent shear workability.

[0200] [Table 1A]

[0201]

[0202] The underlined part indicates that it is outside the scope of this invention.

[0203] [Table 1B]

[0204]

[0205] [Table 2A]

[0206]

[0207] An underline indicates that the manufacturing conditions are not optimal.

[0208] [Table 2B]

[0209]

[0210] An underline indicates that the manufacturing conditions are not optimal.

[0211] [Table 3]

[0212]

[0213] The underlined part indicates that the invention is outside the scope of the invention or that the characteristics are not preferred.

[0214] Observing Table 3, it can be seen that the hot-rolled steel sheet of the present invention has high strength and excellent fatigue characteristics and shear workability. On the other hand, it can be seen that one or more of the above-mentioned characteristics of the hot-rolled steel sheet of the comparative example are inferior.

[0215] Industrial availability

[0216] According to the present invention, a hot-rolled steel sheet with high strength, excellent fatigue properties, and shear workability can be provided. The hot-rolled steel sheet according to the present invention enables lightweighting of vehicle bodies, integral forming of components, and shortening of processing steps, thereby improving fuel efficiency and reducing manufacturing costs.

Claims

1. A hot-rolled steel plate, characterized in that, Chemical composition, by mass%, contains: C：0.02～0.30%、 Si: 0.10~2.00% Mn: 0.5–3.0%, P: Below 0.100% S: Below 0.010% Al：0.10～1.00%、 N: below 0.0100% Ti: 0.06~0.20%, Nb: 0~0.10%, Ca: 0~0.0060%, Mo: 0~1.00%, Cr：0～1.00%、 V：0～0.40%、 Ni: 0~0.40%, B：0～0.0020%、 Cu: 0~1.00%, Sn: 0-0.50%, and Zr：0～0.050%， The remainder consists of Fe and impurities. The metal structure, expressed as an area ratio, is as follows: The total percentage of ferrite and bainite is 30-47%. Tempered martensite: 50-70% Primary martensite: 3-10%, In a region obtained by dividing a plate section parallel to the rolling direction into three parts along the plate thickness direction, the extreme density of the {001} planes in the ferrite and bainite in the central region is set to P. i The extreme density of the {001} plane in the ferrite and bainite of the surface region is set to P. s At that time, P i / P s The value is 1.2 to 2.

0. The hot-rolled steel plate has a tensile strength of 950 MPa or higher. The central region refers to the area within the region obtained by dividing the plate thickness section parallel to the rolling direction into three parts along the plate thickness direction, with a depth from 1 / 3 of the plate thickness to 2 / 3 of the plate thickness from the surface. The surface region refers to the region obtained by dividing a plate thickness section parallel to the rolling direction into three parts along the plate thickness direction, from the surface to a depth of 1 / 3 of the plate thickness, or from the surface to the back side at a depth of 2 / 3 of the plate thickness. The extreme density is obtained by calculating the crystal orientation distribution function (ODF) representing the three-dimensional texture using orientation data determined by the EBSD method and the spherical harmonic function.

2. The hot-rolled steel plate according to claim 1, characterized in that, The chemical composition, expressed as a percentage by mass, contains one or more of the following elements: Nb: 0.01~0.10%, Ca: 0.0005~0.0060% Mo: 0.02–1.00% Cr：0.02～1.00%、 V：0.01～0.40%、 Ni: 0.01~0.40%, B：0.0001～0.0020%、 Cu: 0.02–1.00% Sn: 0.01–0.50%, and Zr：0.001～0.050%。

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