Hot-rolled steel plate

Abstract

The hot-rolled steel sheet has a desired chemical composition, and the metal structure comprises, by area%, 10-30% ferrite, 40-85% bainite, 5-30% retained austenite, 5% or less fresh martensite, and 5% or less pearlite. The average grain size of the ferrite is 5.00 μm or less, the difference between the average nanoindentation hardness of the ferrite and the average nanoindentation hardness of the bainite is 1000 MPa or less, and the tensile strength of the hot-rolled steel sheet is 980 MPa or greater.

Description

Technical Field

[0001] The present invention relates to hot rolled steel sheets.

[0002] This application claims priority based on Japanese Patent Application No. 2021-002859 filed in Japan on January 12, 2021, the contents of which are incorporated herein by reference. Background Art

[0003] To protect the global environment, efforts are underway to reduce the weight of automobile bodies and improve fuel efficiency. To further reduce the weight of automobile bodies, the strength of steel sheets suitable for automobile bodies needs to be increased. However, generally speaking, increasing the strength of steel sheets reduces their formability.

[0004] One method for improving the formability of steel sheets is to include retained austenite in the metallurgical structure of the steel sheet. However, while the inclusion of retained austenite in the metallurgical structure of the steel sheet improves ductility, it may also reduce hole expandability and bendability. Bending, hole expansion, and flanging require not only excellent ductility but also excellent hole expandability and bendability.

[0005] Patent Document 1 discloses a hot-rolled steel sheet with excellent local deformability, low orientation dependence of formability, and excellent ductility, and a method for producing the same. The present inventors have recognized that the hot-rolled steel sheet described in Patent Document 1 needs to have further improvements in strength, ductility, hole expandability, and bendability.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent No. 5533729 Summary of the Invention

[0009] Problems to be solved by the invention

[0010] An object of the present invention is to provide a hot-rolled steel sheet having excellent strength, ductility, hole expandability, and bendability.

[0011] Means for solving problems

[0012] In view of the above problems, the present inventors have conducted intensive studies on the relationship between the chemical composition and metal structure of hot-rolled steel sheets and mechanical properties. As a result, they obtained the following findings (a) to (d), and completed the present invention.

[0013] (a) In order to obtain excellent strength, it is necessary to include a desired amount of bainite in the metal structure and to include a desired amount of Ti so that Ti carbides are precipitated in ferrite to increase the strength of ferrite.

[0014] (b) To achieve excellent ductility, the metal structure must contain a desired amount of ferrite and retained austenite. However, if ferrite and retained austenite are contained, the hole expandability and bendability of the hot-rolled steel sheet will be reduced.

[0015] (c) By controlling the average grain size of ferrite to a desired range, the strength can be further increased, and the hole expandability and bendability can be improved.

[0016] (d) By reducing the hardness difference between ferrite and bainite, the hole expandability and bendability can be further improved.

[0017] The gist of the present invention made based on the above findings is as follows.

[0018] (1) The chemical composition of the hot-rolled steel sheet according to one embodiment of the present invention contains, in mass %, the following:

[0019] C: 0.100~0.350%,

[0020] Si: 0.01-3.00%,

[0021] Mn: 1.00~4.00%,

[0022] sol.Al: 0.001~2.000%,

[0023] Si+sol.Al: 1.00% or more,

[0024] Ti: 0.010~0.380%,

[0025] P: 0.100% or less,

[0026] S: 0.0300% or less,

[0027] N: 0.1000% or less,

[0028] O: 0.0100% or less,

[0029] Nb: 0~0.100%,

[0030] V: 0~0.500%,

[0031] Cu: 0-2.00%,

[0032] Cr: 0-2.00%,

[0033] Mo: 0-1.00%,

[0034] Ni: 0-2.00%,

[0035] B: 0~0.0100%,

[0036] Ca: 0~0.0200%,

[0037] Mg: 0~0.0200%,

[0038] REM: 0~0.1000%

[0039] Bi: 0~0.020%,

[0040] One or more of Zr, Co, Zn and W: 0 to 1.00% in total, and

[0041] Sn: 0~0.050%,

[0042] Tief represented by the following formula (a) is 0.010 to 0.300%,

[0043] The rest contains Fe and impurities.

[0044] The metal structure contains in area %:

[0045] Ferrite: 10-30%,

[0046] Bainite: 40~85%,

[0047] Retained austenite: 5-30%,

[0048] New martensite: less than 5%, and

[0049] Pearlite: less than 5%,

[0050] The average particle size of the ferrite is 5.00 μm or less.

[0051] The difference between the average nanoindentation hardness of the ferrite and the average nanoindentation hardness of the bainite is 1000 MPa or less.

[0052] The tensile strength of the hot-rolled steel plate is greater than 980 MPa.

[0053] Tief=Ti-48 / 14×N-48 / 32×S (a)

[0054] The symbols of the elements in the above formula (a) represent the content in mass %.

[0055] (2) The chemical composition of the hot-rolled steel sheet described in (1) above may contain, in terms of mass %, one or more elements selected from the group consisting of the following elements:

[0056] Nb: 0.005-0.100%,

[0057] V: 0.005~0.500%,

[0058] Cu: 0.01-2.00%,

[0059] Cr: 0.01~2.00%,

[0060] Mo: 0.01~1.00%,

[0061] Ni: 0.02-2.00%,

[0062] B: 0.0001~0.0100%,

[0063] Ca: 0.0005~0.0200%,

[0064] Mg: 0.0005~0.0200%,

[0065] REM: 0.0005~0.1000%, and

[0066] Bi: 0.0005~0.020%.

[0067] Effects of the Invention

[0068] According to the above aspects of the present invention, a hot-rolled steel sheet having excellent strength, ductility, hole expandability, and bendability can be provided. DETAILED DESCRIPTION

[0069] The chemical composition and metal structure of the hot-rolled steel sheet of this embodiment will be described in more detail below. 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.

[0070] For numerical ranges enclosed by "to," the lower and upper limits are included in the range. Numerical values ​​expressed as "less than" or "exceeding" are not included in the numerical range. In the following description, % in the chemical composition of steel sheets is mass % unless otherwise specified.

[0071] Chemical composition

[0072] The chemical composition of the hot-rolled steel sheet according to the present embodiment includes, in mass%, C: 0.100-0.350%, Si: 0.01-3.00%, Mn: 1.00-4.00%, sol.Al: 0.001-2.000%, Si+sol.Al: 1.00% or more, Ti: 0.010-0.380%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and the remainder: Fe and impurities.

[0073] Hereinafter, each element will be described in detail.

[0074] C: 0.100~0.350%

[0075] C is an element necessary to achieve the desired strength. If the C content is less than 0.100%, it becomes difficult to achieve the desired strength. Therefore, the C content is set to 0.100% or more. The C content is preferably 0.120% or more, or 0.150% or more.

[0076] On the other hand, if the C content exceeds 0.350%, the transformation rate slows down, making it easier to form MA (a mixed phase of martensite and retained austenite), making it difficult to achieve excellent hole expandability and bendability. Therefore, the C content is set to 0.350% or less. The C content is preferably 0.330% or less, 0.310% or less, 0.300% or less, or 0.280% or less.

[0077] Si: 0.01~3.00%

[0078] Si has the effect of delaying the precipitation of cementite. Through this effect, it is possible to increase the amount of austenite that remains without phase transformation, i.e., the area ratio of retained austenite. In addition, it is possible to ensure that the amount of solid solution C in the hard phase is relatively large, and to improve strength by preventing the coarsening of cementite. In addition, Si itself also has the effect of improving the strength of the hot-rolled steel sheet by solid solution strengthening. In addition, Si has the effect of (suppressing defects such as pores in steel) that improves the steel by deoxidation. When Si content is less than 0.01%, the effect brought by the above-mentioned effect cannot be obtained. Therefore, Si content is set to more than 0.01%. Si content is preferably more than 0.50%, more than 1.00%, more than 1.20%, more than 1.50%.

[0079] On the other hand, a Si content exceeding 3.00% significantly delays cementite precipitation, leading to excessive retained austenite, which is undesirable. Furthermore, the surface properties and chemical conversion treatability of the hot-rolled steel sheet, and consequently, its ductility and weldability, significantly deteriorate, and the A3 transformation point significantly increases. Consequently, stable hot rolling becomes difficult. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.70% or less, or 2.50% or less.

[0080] Mn: 1.00~4.00%

[0081] Mn suppresses ferrite transformation and increases the strength of hot-rolled steel sheets. If the Mn content is less than 1.00%, the desired strength cannot be achieved. Therefore, the Mn content is set to 1.00% or higher. The Mn content is preferably 1.50% or higher, 1.80% or higher, 2.00% or higher, or 2.40% or higher.

[0082] On the other hand, if the Mn content exceeds 4.00%, the ductility, hole expandability, and bendability of the hot-rolled steel sheet deteriorate. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less, 3.50% or less, 3.30% or less, or 3.00% or less.

[0083] sol.Al: 0.001~2.000%

[0084] Sol-Al, like Si, deoxidizes steel, strengthening the steel sheet, and inhibits the precipitation of cementite from austenite, promoting the formation of retained austenite. A sol-Al content below 0.001% does not achieve these benefits. Therefore, the sol-Al content is set to 0.001% or higher. A sol-Al content of 0.010% or higher is preferred.

[0085] On the other hand, if the sol.Al content exceeds 2.000%, the above effects are saturated and are not economically preferable. Furthermore, the A3 transformation point rises significantly, making stable hot rolling difficult. Therefore, the sol.Al content is set to 2.000% or less. The sol.Al content is preferably 1.500% or less, or 1.300% or less.

[0086] It should be noted that in the present embodiment, sol.Al refers to acid-soluble Al, and indicates solid-solution Al present in the steel in a solid-solution state.

[0087] Si+sol.Al: 1.00% or more

[0088] Both Si and sol.Al have the effect of delaying the precipitation of cementite. This effect can increase the amount of austenite remaining untransformed, i.e., the area ratio of retained austenite. If the combined Si and sol.Al content is less than 1.00%, the effects of this effect cannot be achieved. Therefore, the combined Si and sol.Al content is set to 1.00% or more. Preferably, it is 1.20% or more, or 1.50% or more.

[0089] The total content of Si and sol.Al may be set to 5.00% or less, 3.00% or less, or 2.60% or less.

[0090] In addition, Si in "Si+sol.Al" represents the content of Si in mass %, and sol.Al represents the content of sol.Al in mass %.

[0091] Ti: 0.010~0.380%

[0092] Ti precipitates in steel as carbides or nitrides (primarily Ti carbides), miniaturizing the metal structure through the pinning effect and further increasing the strength of ferrite through precipitation strengthening. As a result, the hardness difference between ferrite and bainite can be reduced. This effect cannot be achieved if the Ti content is less than 0.010%. Therefore, the Ti content is set to 0.010% or more. Preferably, it is 0.050% or more, 0.070% or more, 0.090% or more, or 0.120% or more.

[0093] On the other hand, even if the Ti content exceeds 0.380%, the above-mentioned effect is saturated. Therefore, the Ti content is set to 0.380% or less, preferably 0.350% or less, 0.320% or less, or 0.300% or less.

[0094] P: 0.100% or less

[0095] P is an element generally contained in steel as an impurity, but it has the effect of increasing the strength of hot-rolled steel sheets through solid solution strengthening. Therefore, P can be actively contained. However, P is an element that tends to segregate. If the P content exceeds 0.100%, the reduction in ductility due to grain boundary segregation becomes significant. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less.

[0096] The lower limit of the P content does not need to be particularly specified, but is preferably set to 0.001% from the viewpoint of refining cost.

[0097] S: 0.0300% or less

[0098] S is an element contained in steel as an impurity. It forms sulfide-based inclusions in the steel, reducing the ductility of the hot-rolled steel sheet. If the S content exceeds 0.0300%, the ductility of the hot-rolled steel sheet decreases significantly. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0050% or less.

[0099] The lower limit of the S content does not need to be particularly specified, but is preferably set to 0.0001% from the viewpoint of refining cost.

[0100] N: 0.1000% or less

[0101] Nitrogen is an element contained in steel as an impurity that reduces the ductility of hot-rolled steel sheets. When the N content exceeds 0.1000%, the ductility of the hot-rolled steel sheet decreases significantly. Therefore, the N content is set to 0.1000% or less. The N content is preferably set to 0.0800% or less, or 0.0700% or less. While there is no specific lower limit for the N content, it is preferably set to 0.0010% or more, and more preferably 0.0020% or more, to promote the precipitation of carbonitrides.

[0102] O: 0.0100% or less

[0103] If O is contained in large amounts in steel, it forms coarse oxides that serve as fracture starting points, causing brittle fracture or hydrogen-induced cracking. Therefore, the O content is set to 0.0100% or less. The O content is preferably set to 0.0080% or less, or 0.0050% or less.

[0104] In order to disperse a large amount of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more, or 0.0010% or more.

[0105] Tief: 0.010~0.300%

[0106] Tief, represented by the following formula (a), is an indicator related to the formation of Ti carbides. Ti nitrides and Ti sulfides form at higher temperatures than Ti carbides. Therefore, when there are high amounts of N and S in the steel, Ti carbides cannot be fully formed. If Tief is less than 0.010%, the amount of Ti carbide precipitation is small, and the effect of improving the strength of ferrite due to Ti carbides cannot be achieved. As a result, the hardness difference between ferrite and bainite cannot be reduced. Therefore, Tief is set to 0.010% or higher. Preferably, it is 0.050% or higher, or 0.100% or higher.

[0107] On the other hand, even if Tief is set to more than 0.300%, the above-mentioned effect is saturated, which is not economically preferable. Therefore, Tief is set to 0.300% or less, preferably 0.270% or less, or 0.250% or less.

[0108] Tief=Ti-48 / 14×N-48 / 32×S (a)

[0109] The symbols of the elements in the above formula (a) represent the content in mass %.

[0110] The remainder of the chemical composition of the hot-rolled steel sheet of this embodiment includes Fe and impurities. In this embodiment, impurities refer to elements that have been introduced from raw materials such as ore and scrap iron, or from the manufacturing environment, or elements that have been intentionally added in trace amounts. These elements are permitted within a range that does not adversely affect the hot-rolled steel sheet of this embodiment.

[0111] The hot-rolled steel sheet of this embodiment may contain the following elements as optional elements in addition to the above elements. When the above optional elements are not contained, the lower limit of the content is 0%. Each optional element will be described in detail below.

[0112] Nb: 0.005-0.100% and V: 0.005-0.500%

[0113] Both Nb and V precipitate as carbides or nitrides in steel, refining the metal structure through a pinning effect. Therefore, one or more of these elements may be contained. To more reliably achieve the effects of these actions, the Nb content is preferably set to 0.005% or more, or the V content is preferably set to 0.005% or more.

[0114] However, even if these elements are contained in excess, the effects of the above-mentioned actions are saturated, which is not economically preferable. Therefore, the Nb content is set to 0.100% or less, and the V content is set to 0.500% or less.

[0115] Cu: 0.01~2.00%, Cr: 0.01~2.00%, Mo: 0.01~1.00%, Ni: 0.02~2.00% and B: 0.0001~0.0100%

[0116] Cu, Cr, Mo, Ni, and B all improve the hardenability of hot-rolled steel sheets. Cr and Ni also stabilize retained austenite, while Cu and Mo precipitate carbides in the steel, increasing the strength of the hot-rolled steel sheet. Furthermore, Ni, when Cu is included, effectively suppresses grain boundary cracking in the slab caused by Cu. Therefore, one or more of these elements may be included.

[0117] Cu has the function of improving the hardenability of steel sheets and precipitating as carbides in steel at low temperatures to increase the strength of hot-rolled steel sheets. In order to more reliably obtain the effects brought about by these functions, the Cu content is preferably set to 0.01% or more.

[0118] However, if the Cu content exceeds 2.00%, grain boundary cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less.

[0119] As described above, Cr has the function of improving the hardenability of the steel sheet and stabilizing retained austenite. In order to more reliably obtain the effects of the above functions, the Cr content is preferably set to 0.01% or more.

[0120] However, if the Cr content exceeds 2.00%, the chemical conversion treatability of the hot-rolled steel sheet will be significantly reduced. Therefore, the Cr content is set to 2.00% or less.

[0121] As described above, Mo has the function of improving the hardenability of the steel sheet and the function of precipitating carbides in the steel to increase the strength. In order to more reliably obtain the effects of the above functions, the Mo content is preferably set to 0.01% or more.

[0122] However, even if the Mo content exceeds 1.00%, the effect due to the above-mentioned action is saturated, which is not economically preferable. Therefore, the Mo content is set to 1.00% or less.

[0123] As mentioned above, Ni improves the hardenability of steel sheets. Furthermore, when Ni contains Cu, it effectively suppresses grain boundary cracking in the slab caused by Cu. To more reliably achieve these effects, the Ni content is preferably set to 0.02% or more.

[0124] Ni is an expensive element, so containing a large amount is not economically preferable. Therefore, the Ni content is set to 2.00% or less.

[0125] As described above, B has the effect of improving the hardenability of the steel sheet. In order to more reliably obtain the effect of this effect, the B content is preferably set to 0.0001% or more.

[0126] However, if the B content exceeds 0.0100%, the ductility of the hot-rolled steel sheet decreases significantly, so the B content is made 0.0100% or less.

[0127] Ca: 0.0005-0.0200%, Mg: 0.0005-0.0200%, REM: 0.0005-0.1000%, and Bi: 0.0005-0.020%

[0128] Ca, Mg, and REM all have the effect of improving the formability of hot-rolled steel sheets by controlling the shape of inclusions to a preferred shape. Furthermore, Bi has the effect of improving the formability of hot-rolled steel sheets by refining the solidification structure. Therefore, one or more of these elements may be contained. To more reliably achieve the effects brought about by these effects, it is preferred that the content of one or more of Ca, Mg, REM, and Bi be set to 0.0005% or higher. However, if the Ca content or Mg content exceeds 0.0200%, or the REM content exceeds 0.1000%, excessive inclusions will form in the steel, sometimes even reducing the ductility of the hot-rolled steel sheet. Furthermore, even if the Bi content is set to over 0.020%, the effects brought about by these effects are saturated, making it economically undesirable. Therefore, the Ca and Mg contents are set to 0.0200% or lower, the REM content is set to 0.1000% or lower, and the Bi content is set to 0.020% or lower. The Bi content is preferably 0.010% or lower.

[0129] Here, REM refers to a total of 17 elements including Sc, Y and lanthanoid elements, and the above-mentioned REM content refers to the total content of these elements. In the case of lanthanoid elements, they are added in the form of mixed rare earth metals in industry.

[0130] One or more of Zr, Co, Zn, and W: 0 to 1.00% in total, and Sn: 0 to 0.050%

[0131] Regarding Zr, Co, Zn, and W, the present inventors have confirmed that even if these elements are contained in a total amount of 1.00% or less, the effects of the hot-rolled steel sheet of this embodiment are not impaired. Therefore, one or more of Zr, Co, Zn, and W may be contained in a total amount of 1.00% or less.

[0132] The present inventors have confirmed that the effects of the hot-rolled steel sheet of the present embodiment are not impaired even if a small amount of Sn is contained. However, defects may occur during hot rolling, so the Sn content is set to 0.050% or less.

[0133] The chemical composition of the hot-rolled steel sheet can be determined by general analytical methods. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) can be used for determination. It should be noted that sol.Al can be determined by ICP-AES using the filtrate after the sample is heated and decomposed with acid. C and S can be determined using the combustion-infrared absorption method, N can be determined using the inert gas fusion-thermal conductivity method, and O can be determined using the inert gas fusion-non-dispersive infrared absorption method.

[0134] Metal structure of hot-rolled steel plate

[0135] Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described.

[0136] In the hot-rolled steel sheet of this embodiment, the metal structure includes, in area %, ferrite: 10-30%, bainite: 40-85%, retained austenite: 5-30%, fresh martensite: 5% or less, and pearlite: 5% or less, the average grain size of the ferrite is 5.00 μm or less, and the difference between the average nanoindentation hardness of the ferrite and the average nanoindentation hardness of the bainite is 1000 MPa or less.

[0137] It should be noted that in this embodiment, the metal structure at a depth of 1 / 4 of the plate thickness from the surface (the region from 1 / 8 to 3 / 8 of the plate thickness from the surface) in a plate thickness cross section parallel to the rolling direction is defined. This is because the metal structure at this position represents the representative metal structure of a hot-rolled steel plate.

[0138] Ferrite: 10-30%

[0139] Ferrite is a structure that has low strength but improves the ductility of hot-rolled steel sheets. If the area fraction of ferrite is less than 10%, the desired ductility cannot be achieved. Therefore, the area fraction of ferrite is set to 10% or higher, preferably 12% or higher, or 15% or higher.

[0140] On the other hand, if the area ratio of ferrite exceeds 30%, the desired strength cannot be obtained. Therefore, the area ratio of ferrite is set to 30% or less, preferably 27% or less, or 25% or less.

[0141] Bainite: 40~85%

[0142] Bainite is a microstructure that improves the strength and ductility of hot-rolled steel sheets. If the area fraction of bainite is less than 40%, the desired strength and ductility cannot be achieved. Therefore, the area fraction of bainite is set to 40% or higher. Preferably, it is 50% or higher, 55% or higher, or 60% or higher.

[0143] On the other hand, if the area ratio of bainite exceeds 85%, desired ductility cannot be obtained. Therefore, the area ratio of bainite is set to 85% or less, preferably 82% or less, or 80% or less.

[0144] Retained austenite: 5-30%

[0145] Retained austenite is a structure that improves the ductility of hot-rolled steel sheets. If the area fraction of retained austenite is less than 5%, desired ductility cannot be achieved. Therefore, the area fraction of retained austenite is set to 5% or higher. Preferably, it is 7% or higher, 10% or higher, 12% or higher, 13% or higher, 14% or higher, or 15% or higher.

[0146] On the other hand, if the area ratio of retained austenite exceeds 30%, the desired strength cannot be obtained. Therefore, the area ratio of retained austenite is set to 30% or less, preferably 25% or less, or 23% or less.

[0147] New martensite: less than 5%

[0148] Fresh martensite is a hard structure, thus contributing to improved strength of hot-rolled steel sheets. However, fresh martensite also lacks ductility. If the area fraction of fresh martensite exceeds 5%, the desired ductility cannot be achieved. Therefore, the area fraction of fresh martensite is set to 5% or less. Preferably, it is set to 4%, 3%, or 2%. The area fraction of fresh martensite may also be 0%.

[0149] Pearlite: less than 5%

[0150] If the area fraction of pearlite is too high, the desired amount of retained austenite cannot be obtained. Therefore, the area fraction of pearlite is set to 5% or less. Preferably, it is 4% or less, 3% or less, or 2% or less. The area fraction of pearlite may also be 0%.

[0151] In each of the above-mentioned structures, the area ratio of structures other than retained austenite was measured by the following method.

[0152] Test pieces are collected from hot-rolled steel plates in such a way that the metal structure in the section of the plate thickness parallel to the rolling direction at a depth of 1 / 4 of the plate thickness from the surface (the area at a depth of 1 / 8 of the plate thickness from the surface to a depth of 3 / 8 of the plate thickness from the surface) can be observed. Next, after grinding the section of the plate thickness, the ground surface is corroded with nitric acid, and an optical microscope and a scanning electron microscope (SEM) are used to observe the structure of an area of ​​30 μm × 30 μm. The observation area is set to at least 3 areas. By performing image analysis on the tissue photograph obtained by using the tissue observation, the area ratios of ferrite, pearlite and bainite are obtained. Thereafter, LePera corrosion is performed on the same observation position, and the tissue is observed using an optical microscope and a scanning electron microscope. The area ratio of the newly formed martensite is obtained by performing image analysis on the obtained tissue photograph.

[0153] In the above tissue observation, each tissue was identified by the following method.

[0154] Since fresh martensite has a high dislocation density and has underlying structures such as lath blocks and lath bundles within grains, it can be distinguished from other metal structures by electron channel contrast images using a scanning electron microscope.

[0155] Bainite refers to a structure that is an aggregation of lath-shaped grains, does not contain Fe-based carbides with a long diameter of 20 nm or more within the structure, is not fresh martensite, or contains Fe-based carbides with a long diameter of 20 nm or more within the structure and has a single modification, that is, Fe-based carbides extending in the same direction. Here, Fe-based carbides extending in the same direction refer to carbides whose extension directions differ by within 5°.

[0156] Ferrite is defined as a structure having massive grains and no underlying structure such as laths within the structure.

[0157] Pearlite is a structure in which plate-like ferrite and Fe-based carbides overlap in layers.

[0158] The area ratio of retained austenite is measured by the following method.

[0159] In this embodiment, the area fraction of retained austenite is measured by X-ray diffraction. First, using Co-Kα radiation, the integrated intensities of six peaks, namely α(110), α(200), α(211), γ(111), γ(200), and γ(220), are determined at a depth of 1 / 4 of the plate thickness from the surface (the region from 1 / 8 to 3 / 8 of the plate thickness from the surface) in a cross-section of the plate thickness parallel to the rolling direction of the hot-rolled steel plate. The intensity is then averaged and calculated. This yields the area fraction of retained austenite.

[0160] Average particle size of ferrite: 5.00 μm or less

[0161] The size of ferrite significantly affects the strength, hole expandability, and bendability of hot-rolled steel sheets. If the average ferrite grain size exceeds 5.00 μm, the strength, hole expandability, and / or bendability of the hot-rolled steel sheet cannot be improved. Therefore, the average ferrite grain size is set to 5.00 μm or less. Preferably, it is 4.00 μm or less, 3.50 μm or less, or 3.00 μm or less.

[0162] The lower limit is not particularly specified, but the average grain size of ferrite may be set to 0.50 μm or more, or 1.00 μm or more.

[0163] The average particle size of ferrite is measured by the following method.

[0164] The average crystal grain size of ferrite is obtained by performing the following measurements on the same area as the area observed with the optical microscope and the scanning electron microscope mentioned above. After the plate thickness section is polished using silicon carbide paper of #600 to #1500, it is polished into a mirror surface 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, the strain introduced into the surface layer of the sample is removed by electrolytic polishing. At any position in the longitudinal direction of the sample cross section, an area with a length of 50 μm and a depth of 1 / 8 of the plate thickness from the surface to a depth of 3 / 8 of the plate thickness from the surface is measured by electron backscatter diffraction at a measurement interval of 0.1 μm to obtain crystal orientation information. In the measurement, an EBSD device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the vacuum degree in the EBSD device is set to 9.6×10 -5 Pa or less, the acceleration voltage was set to 15 kV, the irradiation current level was set to 13, and the electron beam irradiation level was set to 62.

[0165] The obtained crystal orientation data group was analyzed by analysis software (TSLOIM Analysis), and the interface with an orientation difference of 15° or more was defined as a crystal grain boundary. The area of ​​the region surrounded by the crystal grain boundary was used as the equivalent circle diameter to calculate the crystal grain size. Among them, for the crystal grains identified as ferrite by the above-mentioned optical microscope and scanning electron microscope (SEM), the histogram of the crystal grain size was used as the median diameter (D 50 ) to calculate the average crystal grain size.

[0166] Difference between the average nanoindentation hardness of ferrite and the average nanoindentation hardness of bainite: less than 1000 MPa

[0167] If the difference between the average nanoindentation hardness of ferrite and the average nanoindentation hardness of bainite exceeds 1000 MPa, hole expandability and / or bendability cannot be improved. Therefore, the difference between the average nanoindentation hardness of ferrite and the average nanoindentation hardness of bainite is set to 1000 MPa or less. Preferably, it is 950 MPa or less, 900 MPa or less, or 850 MPa or less.

[0168] The lower limit is not particularly specified, but the difference between the average nanoindentation hardness of ferrite and the average nanoindentation hardness of bainite may be set to 500 MPa or more, 600 MPa or more, or 700 MPa or more.

[0169] The average nanoindentation hardness of ferrite and the average nanoindentation hardness of bainite were measured by the following method.

[0170] Within the field of view where the area ratio of the metal structure was measured, nanoindentation hardness was measured for areas identified as ferrite. The average nanoindentation hardness of the ferrite was obtained by measuring the martensite hardness at at least 20 points and calculating the average. The same procedure was performed for bainite to obtain the average nanoindentation hardness of the bainite.

[0171] In addition, for the measurement, TriboScope / Tribo Indenter manufactured by Hysitron Corporation was used, and the measurement load was set to 1 mN.

[0172] Mechanical properties

[0173] The hot-rolled steel sheet of this embodiment has a maximum tensile strength of 980 MPa or greater. Setting the tensile strength to 980 MPa or greater can contribute to vehicle weight reduction. More preferably, the tensile strength is 1180 MPa or greater. The upper limit does not need to be specifically set, but may be 1470 MPa.

[0174] The product of tensile strength and uniform elongation (TS×uEl), which is an indicator of ductility, may be 8260 MPa·% or more.

[0175] The hole expansion ratio, which is an indicator of hole expansion properties, may be 45% or more.

[0176] The maximum bending angle, which is an indicator of bendability, may be 60° or greater.

[0177] Tensile strength TS and uniform elongation uEl were measured using JIS Z 2241: 2011 No. 5 test specimens. The tensile test specimens were collected at a point 1 / 4 of the width from the end of the sheet, with the longitudinal direction perpendicular to the rolling direction.

[0178] The hole expansion ratio λ is measured in accordance with JIS Z 2256: 2020. The hole expansion test piece may be collected at a position 1 / 4 of the distance from the end in the sheet width direction of the hot-rolled steel sheet.

[0179] The maximum bending angle α was evaluated based on the VDA standard (VDA 238-100) specified by the German Association of the Automotive Industry. The maximum bending angle α was determined by converting the displacement at the maximum load obtained in the bending test into an angle based on the VDA standard.

[0180] Plate thickness

[0181] The plate thickness of the hot-rolled steel plate of the present embodiment is not particularly limited and can also be set to 0.5 to 8.0 mm. By setting the plate thickness of the hot-rolled steel plate to 0.5 mm or more, it becomes easier to ensure the rolling completion temperature, and the rolling load can be reduced, making it easy to perform hot rolling. Therefore, the plate thickness of the hot-rolled steel plate of the present embodiment can also be set to 0.5 mm or more. Preferably, it is 1.2 mm or more and 1.4 mm or more. In addition, by setting the plate thickness to 8.0 mm or less, it becomes easier to refine the metal structure and the above-mentioned metal structure can be easily ensured. Therefore, the plate thickness can also be set to 8.0 mm or less. Preferably, it is 6.0 mm or less.

[0182] coating

[0183] The hot rolled steel sheet of the present embodiment with the above-mentioned chemical composition and metal structure can also make the surface have coating and make surface treated steel sheet with the purpose of improving corrosion resistance etc. The coating can be an electroplated layer or a hot dipped layer. As the electroplated layer, electrogalvanized layer, electroplated Zn-Ni alloy layer etc. can be exemplified. As the hot dipped layer, hot dipped galvanized layer, alloyed hot dipped galvanized layer, hot dipped aluminized layer, hot dipped Zn-Al alloy layer, hot dipped Zn-Al-Mg alloy layer, hot dipped Zn-Al-Mg-Si alloy layer etc. can be exemplified. The coating adhesion amount is not particularly limited, and can be the same as in the past. In addition, it is also possible to implement suitable chemical conversion treatment (such as coating and drying of the chromium-free chemical conversion treatment solution of silicate system) after plating and further improve corrosion resistance.

[0184] Manufacturing conditions

[0185] In the preferred method for manufacturing a hot-rolled steel sheet according to the present embodiment, the following steps (1) to (7) are performed in sequence. It should be noted 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. Regarding the temperature of the hot-rolled steel sheet in the present embodiment, if it is at the extreme end in the width direction of the sheet, it is measured with a contact or non-contact thermometer. If it is at a point other than the extreme end in the width direction of the hot-rolled steel sheet, it is measured with a thermocouple or calculated by heat transfer analysis.

[0186] (1) The slab is heated to a temperature range of T0°C or higher represented by the following formula (1), maintained in this temperature range for 6000 seconds or longer, and then subjected to rough rolling.

[0187] (2) After the rough rolling is completed, the finishing rolling is performed within 150 seconds.

[0188] (3) The cumulative reduction ratio in the temperature range of T1 (°C) to T1 + 30°C is set to more than 30%, the cumulative reduction ratio of the finish rolling is set to 90% or more, and the final reduction ratio of the finish rolling is set to 15% or more. Note that T1 (°C) is represented by the following formula (2).

[0189] (4) Cooling begins within 1.0 second after the finish rolling is completed, and the steel is cooled to a temperature range of 600 to 700°C at an average cooling rate of 20°C / s or more.

[0190] (5) After air cooling for 1.0 to 3.0 seconds in a temperature range of 600 to 700°C, cooling is performed at an average cooling rate of 40°C / s or higher.

[0191] (6) Coil in a temperature range of T2 (°C) to 500°C.

[0192] (7) The average cooling rate to the temperature range of 150°C or lower is set to 15 to 40°C / h.

[0193] T0(℃)=7000 / {2.75-log(Ti×C)}-273 (1)

[0194] T1(℃)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V(2)

[0195] T2(℃)=591-474×C-33×Mn-17×Ni-17×Cr-21×Mo (3)

[0196] It should be noted that the element symbols in the above formulae (1) to (3) represent the content of each element in mass %, and when the element is not contained, 0 is substituted.

[0197] Slab temperature and holding time during hot rolling

[0198] The slabs for hot rolling can be those obtained by continuous casting or those obtained by casting and blooming. Slabs obtained by hot working or cold working can be used as needed. In order to fully dissolve Ti carbides, the slabs for hot rolling are preferably heated to a temperature range above T0 (°C) and maintained in this temperature range for more than 6000 seconds. If Ti carbides cannot be fully dissolved, it will result in an inability to precipitate a sufficient amount of Ti carbides in ferrite, and sometimes it will be impossible to reduce the hardness difference between ferrite and bainite.

[0199] Hot rolling is preferably performed using a reversing mill or a tandem mill as a multi-pass rolling process. In particular, from the perspective of industrial productivity, it is more preferable to perform hot rolling using a tandem mill for at least the final several stages.

[0200] Rough rolling

[0201] After maintaining the temperature in the T0 (° C.) or higher range for 6000 seconds or longer, rough rolling is performed. The conditions for rough rolling are not particularly limited, and the rough rolling may be performed by a conventional method.

[0202] Finishing

[0203] Finish rolling is preferably performed within 150 seconds after the completion of rough rolling. Specifically, the first pass of finish rolling is preferably performed within 150 seconds after the completion of the final pass of rough rolling. Performing finish rolling within 150 seconds of the completion of rough rolling prevents excessive precipitation of Ti carbides in the retained austenite during the secondary cooling process described later, allowing sufficient Ti carbides to precipitate in the ferrite. This results in a reduced hardness difference between ferrite and bainite.

[0204] Furthermore, for finish rolling, the cumulative reduction ratio in the temperature range of T1°C to T1+30°C is preferably set to over 30%, the cumulative reduction ratio during finish rolling is set to over 90%, and the final reduction ratio during finish rolling is set to over 15%. By performing finish rolling under these conditions, the desired amount of ferrite can be obtained. The finish rolling completion temperature is preferably set to over 830°C.

[0205] It should be noted that the so-called cumulative reduction rate in the temperature zone of T1 (℃) to T1+30℃ can be expressed as (t0-t1) / t0×100(%) when the entrance plate thickness before the first pass in rolling in this temperature zone is set to t0 and the exit plate thickness after the final pass in rolling in this temperature zone is set to t1.

[0206] The cumulative reduction rate of finishing rolling is defined as the thickness of the plate before the first pass of finishing rolling is set to t i , the outlet plate thickness after the final pass of finishing rolling is set to t f When (t i -t f ) / t i ×100(%).

[0207] The so-called final reduction rate of finishing rolling can be expressed as (t2-t3) / t2×100(%) when the entrance plate thickness before the final pass of finishing rolling is set to t2 and the exit plate thickness after the final pass of finishing rolling is set to t3.

[0208] First cooling after finishing rolling

[0209] It is preferred that cooling be initiated within 1.0 second after the completion of finish rolling, and cooling be performed at an average cooling rate of 20°C / s or more to a temperature range of 600-700°C. In other words, it is preferred that cooling be initiated within 1.0 second after the completion of finish rolling at an average cooling rate of 20°C / s or more, and the cooling be performed to a temperature range of 600-700°C. By performing a primary cooling within 1.0 second after the completion of finish rolling, the average grain size of ferrite can be preferably controlled. In addition, by performing the primary cooling to a temperature range of 600-700°C, the hardness difference between ferrite and bainite can be reduced.

[0210] In addition, the average cooling rate in this embodiment is a value obtained by dividing the temperature difference between the start of cooling and the end of cooling by the elapsed time from the start of cooling to the end of cooling.

[0211] Intermediate air cooling and secondary cooling

[0212] After cooling to a temperature range of 600-700°C, air cooling is performed in this temperature range for 1.0-3.0 seconds, and then cooling is performed at an average cooling rate of 40°C / s or more. The so-called air cooling here refers to cooling with an average cooling rate of 10°C / s or less. As long as no external heat input is performed using a heating device, the cooling rate during air cooling is about 3°C / s even for a plate thickness of about half an inch. By performing two cooling operations under such conditions, the desired amount of ferrite and retained austenite can be obtained, and a sufficient amount of Ti carbide can be precipitated in the ferrite. As a result, the hardness difference between ferrite and bainite can be reduced.

[0213] Cooling at an average cooling rate of 40°C / s or higher is preferably performed to a temperature range of T2°C to 500°C, so that the steel can be coiled at the coiling temperature described below. In other words, the cooling stop temperature for cooling at an average cooling rate of 40°C / s or higher is preferably set to a temperature range of T2°C to 500°C.

[0214] Coil

[0215] The coiling temperature is preferably set to a temperature range of T2°C to 500°C. Coiling within this temperature range can suppress excessive precipitation of fresh martensite, allowing the desired amount of bainite to be obtained. Coiling temperatures exceeding 500°C may promote the formation of cementite accompanying the bainite transformation, making it impossible to obtain the desired amount of retained austenite. Coiling temperatures below T2°C may result in the formation of tempered martensite.

[0216] 3 times cooling after coiling

[0217] After coiling, the average cooling rate to a temperature range of 150°C or less is preferably set to 15-40°C / h. By performing three cooling cycles under these conditions, carbon can be concentrated in the retained austenite, stabilizing the retained austenite. As a result, the desired amount of retained austenite can be obtained. The average cooling rate is more preferably 20°C / h or higher. Furthermore, the average cooling rate is more preferably less than 30°C / h.

[0218] In addition, the average cooling rate after coiling should be controlled by using a heat-insulating hood, edge shield, spray cooling, etc.

[0219] Example

[0220] Next, the effects of one embodiment of the present invention will be described in more detail using examples. However, the conditions in the examples are merely examples of conditions employed to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions may be employed within the scope of the present invention and as long as they do not deviate from the spirit and purpose of the present invention.

[0221] Steels having the chemical compositions shown in Tables 1 and 2 were melted and continuously cast to produce slabs having a thickness of 240 to 300 mm. Hot-rolled steel sheets were obtained using the obtained slabs under the production conditions shown in Tables 3 and 4.

[0222] It should be noted that before hot rolling, the slabs were heated to the slab heating temperature listed in Table 3 and held for 6000 seconds or longer. Production No. 10 in Table 4 was air-cooled in a temperature range of 530°C or lower for the air-cooling time listed in Table 4 after the primary cooling. Production No. 11 was air-cooled in a temperature range of over 700°C and 723°C or lower for the air-cooling time listed in Table 4 after the primary cooling. In all cases, the third cooling was performed to a temperature range of 150°C or lower.

[0223] The hot-rolled steel sheets obtained were measured using the above-described methods for the area ratio of each microstructure, the average ferrite grain size, the difference between the average nanoindentation hardness of ferrite and the average nanoindentation hardness of bainite, the tensile strength TS, the uniform elongation uEl, the hole expansion ratio λ, and the maximum bend angle α. It should be noted that the total elongation E1 (referred to as the elongation at break in JIS Z 2241:2011) was obtained through a tensile test that measured the tensile strength TS and the uniform elongation uEl.

[0224] The obtained measurement results are shown in Table 5. In addition to the structures shown in Table 5, Production No. 15 also produced 40 area % of tempered martensite (a structure not identified as any structure by the above-mentioned structure observation method).

[0225] Evaluation Benchmarks

[0226] When the tensile strength TS is 980 MPa or more, it is determined to have excellent strength and judged as acceptable. On the other hand, when the tensile strength TS is less than 980 MPa, it is determined to have insufficient strength and judged as unacceptable.

[0227] If the product of the tensile strength TS and the uniform elongation uEl (TS×uEl) is 8260 MPa·% or more, the sample is deemed to have excellent ductility and is judged as passing. On the other hand, if TS×uEl is less than 8260 MPa·%, the sample is deemed to have poor ductility and is judged as failing.

[0228] When the hole expansion ratio λ is 45% or more, the sample is judged as having excellent hole expandability and acceptable. On the other hand, when the hole expansion ratio λ is less than 45%, the sample is judged as not having excellent hole expandability and unacceptable.

[0229] If the maximum bending angle is 60° or more, the bendability is considered to be excellent and the test is judged as acceptable. On the other hand, if the maximum bending angle is less than 60°, the bendability is considered to be poor and the test is judged as unacceptable.

[0230]

[0231]

[0232] Table 3

[0233]

[0234] Underlined conditions indicate less preferred conditions.

[0235] Table 4

[0236]

[0237] Underlined conditions indicate less preferred conditions.

[0238] Table 5

[0239]

[0240] The underlined characters indicate values ​​outside the scope of the present invention or undesirable property values.

[0241] Table 6

[0242]

[0243] The underlined characters indicate values ​​outside the scope of the present invention or undesirable property values.

[0244] As can be seen from Table 6, in the examples of the present invention, hot-rolled steel sheets having excellent strength, ductility, hole expandability, and bendability were obtained.

[0245] On the other hand, the comparative examples, whose chemical compositions and / or metal structures were not within the ranges specified in the present invention, were inferior in one or more of the above-mentioned properties. It should be noted that in Production No. 15, due to insufficient bainite, tempered martensite was formed, resulting in poor ductility. Furthermore, in Production No. 16, due to the high amount of newly formed martensite, the overall hardness difference between the microstructures increased, resulting in poor hole expandability and bendability.

[0246] Industrial applicability

[0247] According to the above aspects of the present invention, a hot-rolled steel sheet having excellent strength, ductility, hole expandability, and bendability can be provided.

Claims

1. A hot-rolled steel plate, characterized in that: Its chemical composition in mass % contains: C：0.100～0.350％、 Si: 0.01-3.00%, Mn: 1.00~4.00%, sol.Al: 0.001~2.000%, Si+sol.Al: 1.00% or more, Ti: 0.010~0.380%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Nb: 0~0.100%, V：0～0.500％、 Cu: 0-2.00%, Cr：0～2.00％、 Mo: 0-1.00%, Ni: 0-2.00%, B：0～0.0100％、 Ca: 0~0.0200%, Mg: 0~0.0200%, REM: 0~0.1000% Bi: 0~0.020%, One or more of Zr, Co, Zn and W: 0 to 1.00% in total, and Sn: 0 to 0.050%, Tief represented by the following formula (a) is 0.010 to 0.300%, The rest is composed of Fe and impurities. The metal structure contains in area %: Ferrite: 10-30%, Bainite: 40~78%, Retained austenite: 12-30%, New martensite: less than 5%, and Pearlite: less than 5%, The average particle size of the ferrite is 5.00 μm or less, The difference between the average nanoindentation hardness of the ferrite and the average nanoindentation hardness of the bainite is 1000 MPa or less, The tensile strength of the hot-rolled steel plate is above 980 MPa. Tief=Ti-48 / 14×N-48 / 32×S (a) The symbols of the elements in the formula (a) represent the content in mass %.

2. The hot-rolled steel sheet according to claim 1, wherein The chemical composition contains, in mass %, one or more elements selected from the group consisting of the following elements: Nb: 0.005-0.100%, V：0.005～0.500％、 Cu: 0.01-2.00%, Cr：0.01～2.00％、 Mo: 0.01~1.00%, Ni: 0.02-2.00%, B：0.0001～0.0100％、 Ca: 0.0005~0.0200%, Mg: 0.0005~0.0200%, REM: 0.0005~0.1000%, and Bi: 0.0005~0.020%.

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