Hot rolled steel sheet and method for manufacturing the same

Abstract

The hot-rolled steel sheet has a prescribed chemical composition, a metal structure comprising 53.0% to 76.0% by area of ​​ferrite, 3.0% to 10.0% by area of ​​martensite, 14.0% to 39.0% by area of ​​bainite, and 2.6% or less by area of ​​pearlite, the average diameter of the martensite being 0.26 μm to 0.70 μm, and the total length of the interfaces between the martensite and the bainite, among all interfaces of the martensite, being 75.0% or more relative to the total length of all interfaces of the martensite.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel plate and a method for manufacturing the same. Background Art

[0002] In recent years, from the perspective of global environmental protection, the lightweighting of automobile bodies and parts has been progressing in order to improve the fuel efficiency of automobiles. In order to further reduce the weight of automobile bodies and parts, it is necessary to increase the strength of steel sheets used in the bodies and parts.

[0003] Traveling parts in automobiles, such as lower arms, often have complex shapes to fulfill their functions. Therefore, to ensure both strength and workability, high-strength hot-rolled steel sheets with a thickness of 2.0 to 6.0 mm are sometimes used for these traveling parts. Furthermore, since hot-rolled steel sheets used in automobile parts are processed into complex shapes as described above, they are particularly required to have high ductility and hole expandability. In recent years, hot-rolled steel sheets used in automobile parts have been required to have a tensile strength of 780 MPa or higher, as well as excellent ductility and hole expandability.

[0004] As a method for improving the strength of hot-rolled steel sheets, there is a method using Ti and Nb. Ti and Nb are elements that cause fine alloy carbides to precipitate in ferrite, which contributes to the improvement of strength. In order to achieve such precipitation strengthening of ferrite brought about by Ti and Nb, Si is sometimes added to the hot-rolled steel sheet. In particular, since the line length of the section from finish rolling to coiling in a hot rolling line is limited, Si is often added when ferrite is formed in this section and Ti and Nb carbides are precipitated. On the other hand, if Si is contained in the hot-rolled steel sheet, it is possible that not only will oxide scale patterns be generated on the surface of the steel sheet, thereby damaging the appearance, but fatigue properties will also be reduced.

[0005] In order to solve such a problem, Patent Document 1 discloses a steel sheet having a main structure of ferrite-bainite to which Si and Al are added in an amount of 0.25% by mass or less, and a method for producing the same.

[0006] Patent Document 2 discloses a high-strength steel sheet having a metal structure mainly composed of ferrite and a low area ratio of martensite, thereby achieving both elongation (ductility) and hole expandability and improving fatigue strength.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 2004-204326

[0010] Patent Document 2: International Publication No. 2014 / 051005 Summary of the Invention

[0011] Problems to be solved by the invention

[0012] However, hot-rolled steel sheets containing large amounts of Ti and Nb, such as those described in Patent Document 1, while maintaining ductility and hole expandability, present a problem: during shearing, delamination of the sheared end surface (hereinafter also referred to as sheared end surface microcracks) occurs, and forming cracks occur in the stretch flange portion, originating from these sheared end surface microcracks. In other words, conventional hot-rolled steel sheets have the problem of not being able to achieve sufficient stretch flangeability due to the microcracks generated in the sheared end surface during shearing.

[0013] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a hot-rolled steel sheet having excellent strength, ductility, hole expandability, and stretch flangeability, and a method for producing the same.

[0014] Means for solving problems

[0015] In view of the above problems, the inventors of the present invention have conducted intensive research on the relationship between the chemical composition and metal structure of hot-rolled steel sheets and the above characteristics. As a result, they have obtained the following findings (a) to (e), and have completed the present invention.

[0016] (a) In order to obtain excellent strength, it is effective to include a desired amount of martensite in the metal structure.

[0017] (b) In order to obtain excellent ductility, it is necessary to include a desired amount of ferrite in the metal structure and to control the amount of bainite in the metal structure to a desired range.

[0018] (c) In order to obtain excellent hole expandability, it is important to control the amount of pearlite in the metal structure to a desired range.

[0019] (d) To achieve excellent stretch flange formability, it is important to prevent microcracking on the sheared end face during shearing. To prevent microcracking on the sheared end face, it is effective to arrange bainite, which has a deformability intermediate between ferrite and martensite, so that it covers the martensite, i.e., adjacent to the martensite, while simultaneously controlling the average diameter of the martensite within a desired range. By arranging the various metal structures in an appropriate arrangement and controlling the shape of the martensite, microcracking on the sheared end face during shearing can be prevented, resulting in excellent stretch flange formability.

[0020] (e) In order to achieve the above-mentioned optimal arrangement of the metal structure and the average grain size of martensite within the desired range, it is important to carefully control the cooling conditions after hot rolling. In particular, it is important to control the cooling conditions in the bainite transformation temperature range after hot rolling within the desired range to thereby generate a uniform and large amount of bainite.

[0021] The gist of the present invention made based on the above findings is as follows.

[0022] [1] The chemical composition of the hot-rolled steel sheet according to one embodiment of the present invention comprises, in mass %, the following:

[0023] C: 0.035% to 0.085%,

[0024] Si: 0.001% to 0.15%,

[0025] Mn: 0.70% to 1.80%,

[0026] P: 0.020% or less,

[0027] S: 0.0050% or less,

[0028] Ti: 0.075% to 0.170%,

[0029] Nb: 0.003% to 0.050%,

[0030] Al: 0.10% to 0.40%,

[0031] N: 0.0080% or less,

[0032] Cr: 0% to 0.27%,

[0033] B: 0% to 0.0050%,

[0034] Ca: 0% to 0.0050%,

[0035] Mo: 0% to 0.40%,

[0036] Ni: 0% to 0.50%,

[0037] Cu: 0% to 0.50%, and

[0038] REM: 0%~0.0300%,

[0039] The rest contains Fe and impurities.

[0040] Metallic tissue contains:

[0041] Ferrite with an area ratio of 53.0% to 76.0%,

[0042] 3.0% to 10.0% of martensite in terms of area ratio,

[0043] Bainite exceeding 14.0% and not more than 39.0% by area ratio,

[0044] Pearlite with an area ratio of 2.6% or less,

[0045] The average diameter of martensite is 0.26μm~0.70μm,

[0046] Among all interfaces of the martensite, the total length of interfaces between the martensite and the bainite accounts for 75.0% or more of the total length of all interfaces of the martensite.

[0047] [2] The hot-rolled steel sheet according to [1], wherein the chemical composition may contain one or more of the following elements by mass:

[0048] Cr: 0.06% to 0.27%,

[0049] B: 0.0003% to 0.0050%,

[0050] Ca: 0.0003% to 0.0050%,

[0051] Mo: 0.01% to 0.40%,

[0052] Ni: 0.01% to 0.50%,

[0053] Cu: 0.01% to 0.50%, and

[0054] REM: 0.0003% ~ 0.0300%.

[0055] [3] A method for producing a hot-rolled steel sheet according to one embodiment of the present invention comprises the following steps:

[0056] A hot rolling process in which a slab having the chemical composition described in [1] or [2] is subjected to rolling at a finishing temperature of 880°C to 950°C.

[0057] After the hot rolling step, a primary cooling step is performed to cool the steel sheet to a primary cooling stop temperature of 680°C to 760°C at an average cooling rate of 60°C / s or higher;

[0058] After the primary cooling step, a secondary cooling step is performed at an average cooling rate of 20°C / second or less for a period of 1.6 seconds to 6.3 seconds;

[0059] After the secondary cooling step, a tertiary cooling step is performed at an average cooling rate of 60°C / s to 130°C / s to a tertiary cooling stop temperature of 195°C to 440°C;

[0060] After the above three cooling processes, 3 / min / mm 2 ~7.2m 3 / min / mm 2 The water density is 0.33 seconds to 1.50 seconds during the four cooling processes;

[0061] After the above four cooling steps, five cooling steps of air cooling are performed during 3.0 seconds to 5.0 seconds;

[0062] After the five cooling steps, the coiling step is performed at a temperature lower than 180°C.

[0063] Effects of the Invention

[0064] According to the above aspects of the present invention, a hot-rolled steel sheet having excellent strength, ductility, hole expandability, and stretch flangeability, and a method for producing the same can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 This is a graph showing the relationship between the forming height of a stretch flange formed portion in a conventional hot-rolled steel sheet having a tensile strength of 340 to 780 MPa and the fracture limit strain obtained by a side bending test.

[0066] Figure 2 This is a schematic diagram of a test piece used in a lateral bending test.

[0067] Figure 3 This is a photo taken with a microscope of cracks occurring on the shear end face of a stretched flange.

[0068] Figure 4A This is a cross-sectional photograph of the sheared end face before the flange is formed by stretching.

[0069] Figure 4B yes Figure 4A SEM photograph of the vicinity of the crack formed at the shear end surface shown in FIG.

[0070] Figure 5 This is a graph showing the relationship between the fracture limit strain and the coverage in this example.

[0071] Figure 6 This is a graph showing the relationship between the fracture limit strain and the average diameter dM of martensite in this example.

[0072] Figure 7This is a graph showing the relationship between the coverage ratio and the water density in the four cooling steps in this example.

[0073] Figure 8 This is a graph showing the relationship between the average diameter dM of martensite and the cooling time of the four cooling steps in this example.

[0074] Figure 9 This is a graph showing the relationship between the coverage rate and the air cooling time in this example.

[0075] Figure 10A This is a structural photograph (SEM photograph) of Test No. 21 in this Example.

[0076] Figure 10B This is a microstructure photograph (SEM photograph) of the vicinity of the sheared end surface after shearing was performed on Test No. 21 in this example.

[0077] Figure 11A This is a structural photograph (SEM photograph) of Test No. 17 in this Example.

[0078] Figure 11B yes Figure 11A Magnified view of area A shown in . DETAILED DESCRIPTION

[0079] First, the inventors' research results on factors affecting stretch flange formability in hot-rolled steel sheets and their new findings on the relationship between stretch flange formability and metal structure will be described.

[0080] Conventional hot-rolled steel sheets are known to be effective in improving ductility and hole expandability by appropriately designing their chemical compositions, particularly by actively utilizing Ti and Nb. However, further in-depth research by the present inventors revealed that when hot-rolled steel sheets with excellent ductility and hole expandability are sheared, micro-peeling occurs at the sheared end surfaces. Further stretch-flange forming of hot-rolled steel sheets that have experienced these micro-cracks at the sheared end surfaces can lead to forming cracks in the stretch-flange portion. Specifically, the present inventors discovered that, even with conventional hot-rolled steel sheets with excellent ductility and hole expandability, micro-cracks at the sheared end surfaces during shearing, and forming cracks in the stretch-flange portion caused by these micro-cracks, can result in insufficient stretch-flangeability. Therefore, the present inventors investigated the factors that cause forming cracks in the stretch-flange portion and the indicators that indicate stretch-flangeability.

[0081] First, using a slab having the steel composition B shown in Table 1 of the examples described later, various changes were made to the cooling conditions after the hot rolling process to produce hot-rolled steel sheets. Samples were cut out from the 1 / 4 position in the width direction of the produced hot-rolled steel sheets and various characteristics were investigated. As a result, the strength, ductility (elongation at break) and hole expandability in the direction perpendicular to the rolling direction were good. However, for the hot-rolled steel sheets produced under some cooling conditions, shearing processing was performed and further tensile flange forming (side bending test) was performed. As a result, cracks penetrated the plate thickness direction and broke. The observation results of the cross section of the broken part are shown in FIG. Figure 3 middle.

[0082] Figure 3 This is a photo taken with a microscope of the cracks on the shear end face of the stretched flange. Figure 3 As shown in the figure, cracks parallel to the plate surface developed on the sheared end surface after shearing, which was subjected to stretch flange formation in a direction perpendicular to the rolling direction. Specifically, it is presumed that the aforementioned fracture of the hot-rolled steel plate produced under certain cooling conditions occurred because the cracks parallel to the plate surface served as starting points and then penetrated the plate thickness direction.

[0083] Cracks parallel to the plate surface were investigated in detail. Figure 4A The cross-sectional photograph of the sheared end face before the stretch flange is formed is shown in FIG. Figure 4A As shown in , it is known that cracks parallel to the plate surface are formed on the sheared end surface during the shearing process.

[0084] Furthermore, the results of the investigation on the relationship between the cracks parallel to the plate surface and the metal structure are shown in FIG. Figure 4B middle. Figure 4B yes Figure 4A The SEM photograph of the crack formed near the shear end surface shown in FIG. Figure 4B This is an SEM photo taken after Lepera corrosion. The white part represents martensite, the gray part represents ferrite, and the black part represents the voids generated by the cracking of martensite. The results of the investigation are as follows. Figure 4B As shown in the figure, near the crack formed at the shear end, a portion where martensite deformation and peeling occurred (arrow (1)) and cracking of the martensite itself (arrow (2)) were observed. Figure 4B The peeling as shown by the arrow (1) and the forming cracking of the martensite itself are the main factors that lead to the cracks parallel to the plate surface during the shear surface processing. It should be noted that in this specification, Figure 4B The portion where martensite deformation and separation occurs (arrow (1)) and the cracking of the martensite itself (arrow (2)) are collectively referred to as "martensite forming cracking".

[0085] Based on the above findings, the inventors next investigated the effective metal structure morphology for preventing martensite cracking, focusing on bainite, which has a deformability intermediate between ferrite and martensite. As a result, they discovered that, rather than simply specifying the area ratio of bainite, the presence of bainite surrounding martensite is effective for preventing martensite cracking. Specifically, they discovered that increasing the proportion of interfaces with bainite among all martensite interfaces prevents martensite cracking, resulting in improved stretch flange formability.

[0086] The mechanism by which bainite effectively acts on the forming cracking of martensite is not clear, but it is considered to play a role similar to a buffer.

[0087] In addition, the researchers investigated whether there were other factors that caused cracking during shear surface processing besides martensite forming cracking, and found that the diameter of the martensite also had an impact.

[0088] Cracks generated during shearing processing can also be caused by voids, and research has been conducted to suppress the formation of these voids. The results have revealed the following: To suppress void formation and prevent cracks, it is important to set the martensite diameter within a desired range.

[0089] Specifically, the inventors of the present invention have determined that the following are important for preventing cracks parallel to the sheet surface during shearing. Specifically, if the diameter of martensite is too large, even if martensite is surrounded by bainite, the difference in hardness between the two causes deformation to concentrate at the interface between bainite and martensite, potentially leading to the formation of voids. Therefore, to prevent cracks parallel to the sheet surface during shearing and to improve stretch flange formability, it is important to achieve a desired arrangement of bainite and martensite and to set the martensite diameter within a desired range.

[0090] Next, the stretch flange formability in the present invention will be described.

[0091] Conventionally, methods for evaluating stretch flangeability have generally been hole expansion tests and notch tensile tests. However, these tests may not accurately evaluate stretch flangeability in automotive parts, particularly those subject to predominant tensile deformation, such as frame and chassis components.

[0092] For example, while the hole expansion test method exhibits a dramatic change in strain distribution along the circumferential direction, the strain distribution during stretch flange deformation exhibits a gentle strain gradient along the circumferential radial direction. Consequently, the conventional hole expansion test cannot evaluate stretch flangeability, which reflects the influence of the actual part shape and the properties of the steel sheet used as the raw material during forming. This makes it difficult to accurately evaluate stretch flangeability in the context of actual cut forming.

[0093] Therefore, the present invention uses the ultimate strain at break evaluated by the side bend test as an indicator of stretch flangeability. Specifically, the term "stretch flangeability" as used herein refers to the ultimate strain at break (hereinafter referred to as "ultimate strain") at which a crack forms through the thickness of a steel plate by in-plane deformation of the open cross-section. In the present invention, stretch flangeability is evaluated using the ultimate strain at break obtained by the side bend test.

[0094] Next, the chemical composition and metal structure of the hot-rolled steel sheet (hereinafter sometimes simply referred to as the steel sheet) according to this embodiment will be specifically described below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications may be made without departing from the scope of the present invention.

[0095] For numerical ranges indicated by "to" below, the lower and upper limits are included in the range. Numerical values ​​indicated as "less than" or "exceeding" are not included in the numerical range. In the following description, "%" regarding the chemical composition of steel sheets refers to "mass %" unless otherwise specified.

[0096] (Chemical Composition)

[0097] The chemical composition of the hot-rolled steel sheet according to the present embodiment includes, by mass%, C: 0.035% to 0.085%, Si: 0.001% to 0.15%, Mn: 0.70% to 1.80%, P: 0.020% or less, S: 0.005% or less, Ti: 0.075% to 0.170%, Nb: 0.003% to 0.050%, Al: 0.10% to 0.40%, N: 0.008% or less, and the remainder: Fe and impurities. Each element is described in detail below.

[0098] C: 0.035%~0.085%

[0099] The C (carbon) content is set to 0.035% to 0.085%. When the C content is lower than 0.035%, it becomes difficult to fully ensure the area ratio of martensite. Therefore, the C content is set to 0.035% or more. In addition, from the perspective of steelmaking cost, it is not preferable to reduce the C content excessively. Due to these factors, the C content is preferably 0.037% or more, and more preferably 0.040% or more. On the other hand, when the C content exceeds 0.085%, there is a possibility that the area ratio of martensite will become excessively large. Therefore, the C content is set to 0.085% or less. In addition, in order to reduce the incidence of cracking of the slab during the casting process, it is preferable to suppress the C content. Due to these factors, the C content is preferably set to 0.065% or less.

[0100] Si: 0.001% to 0.15%

[0101] The Si (silicon) content is preferably low in terms of the appearance of the steel sheet. Furthermore, if the Si content exceeds 0.15%, the area ratio of ferrite may become excessively large. Therefore, the Si content is set to 0.15% or less. Furthermore, to reduce the cost of the pickling process for removing the oxide scale generated during the hot rolling process, the lower the Si content, the better. Due to these considerations, the Si content is preferably 0.07% or less. On the other hand, if the Si content is excessively reduced, not only will the manufacturing cost in the steelmaking process increase significantly, but the aforementioned effects will also saturate. Therefore, the Si content is set to 0.001% or more. The Si content is preferably 0.003% or more.

[0102] Mn: 0.70% to 1.80%

[0103] Mn (manganese) suppresses ferrite transformation, thereby increasing the strength of hot-rolled steel sheets. If the Mn content is less than 0.70%, the area fraction of ferrite increases, and the desired strength cannot be achieved. Therefore, the Mn content is set to 0.70% or higher. The Mn content is preferably 0.80% or higher, and more preferably 0.90% or higher. On the other hand, if the Mn content exceeds 1.80%, the area fraction of ferrite decreases excessively, the area fraction of bainite increases, and ductility deteriorates. Therefore, the Mn content is set to 1.80% or lower. The Mn content is preferably 1.75% or lower, or 1.70% or lower. It should be noted that if the Mn content is less than 1.20%, edge wrinkles may form at the widthwise ends of the steel sheet or steel sheet coil. Typically, such edge wrinkles are trimmed, resulting in a reduction in yield. Therefore, the Mn content is preferably set to 1.20% or higher.

[0104] P: 0.020% or less

[0105] P (phosphorus) is an element generally contained as an impurity, but has the effect of increasing the strength of hot-rolled steel sheets through solid solution strengthening. Therefore, P may be intentionally contained, but P is an element that segregates at grain boundaries and also has the effect of causing a decrease in ductility. In addition, when the P content exceeds 0.020%, it is possible to reduce the toughness of the hot-rolled slab and cause cracking during the rolling process, especially cracking at the corners of the ingot (corner cracking). Therefore, the P content is set to 0.020% or less. The P content is preferably 0.015% or less. There is no need to specifically specify the lower limit of the P content, and the lower the P content, the better. The lower limit of the P content may include 0%. However, if the P content is set to less than 0.001%, the refining cost in the steelmaking process becomes extremely high. Therefore, the P content is preferably set to 0.001% or more.

[0106] S: 0.0050% or less

[0107] S (sulfur) is an element contained as an impurity, and is an element that forms non-metallic inclusions and reduces the ductility of hot-rolled steel sheets. In addition, if the S content exceeds 0.0050%, the ductility of the hot-rolled steel sheet will be significantly reduced, or defects and fractures will be generated during the hot rolling process. Therefore, the S content is set to 0.0050% or less. The S content is preferably 0.0040% or less. There is no need to specifically specify the lower limit of the S content, and the lower the S content, the better. The lower limit of the S content may include 0%. However, if the S content is set to less than 0.0001%, the refining cost in the steelmaking process becomes high. From the perspective of refining cost, the S content is preferably set to 0.0001% or more.

[0108] Ti: 0.075%~0.170%

[0109] Ti (titanium) is an element that precipitates fine alloy carbides in ferrite, which has the effect of increasing strength. If the Ti content is less than 0.075%, sufficient strength cannot be achieved. Therefore, the Ti content is set to 0.075% or more. In addition, Ti is also an element that is effective in improving hole expandability. To further achieve these effects, the Ti content is preferably set to 0.090% or more. On the other hand, if the Ti content exceeds 0.170%, there is a possibility of cracking in the slab after cold blanking. Therefore, the Ti content is set to 0.170% or less. The Ti content is preferably 0.150% or less.

[0110] Nb: 0.003% to 0.050%

[0111] Nb (niobium) is an element that precipitates fine alloy carbides. Furthermore, Nb has the following effects: by suppressing the grain growth of austenite during hot rolling, it suppresses the coarsening of the crystal grain size of ferrite generated by subsequent phase transformation. By exerting these effects, the strength of the steel plate can be improved. To achieve this effect, the Nb content is set to 0.003% or more. Furthermore, Nb has the effect of suppressing the coarsening of grains in the heat-affected zone during arc welding and suppressing the softening of the heat-affected zone. To achieve these effects, the Nb content is preferably set to 0.010% or more. On the other hand, if the Nb content exceeds 0.050%, the toughness of the hot-rolled slab decreases, resulting in the possibility of cracking and flaws during the rolling process. Therefore, the Nb content is set to 0.050% or less. The Nb content is preferably 0.045% or less.

[0112] Al: 0.10% to 0.40%

[0113] Al (aluminum) is an element that deoxidizes steel and is effective in improving the soundness of the steel plate. In addition, Al is an element that effectively plays a role in increasing the area ratio of ferrite. When the Al content is less than 0.10%, the area ratio of ferrite becomes insufficient. Therefore, the Al content is set to 0.10% or more. In addition, Al also has the effect of lowering the melting point of the oxide scale formed on the surface of the steel plate during the hot rolling process, so the oxide scale can be easily removed when hot. In order to further obtain these effects, the Al content is preferably set to 0.20% or more. On the other hand, when the Al content exceeds 0.40%, the area ratio of ferrite becomes excessively large and the strength is insufficient. Therefore, the Al content is set to 0.40% or less. The Al content is preferably set to 0.35% or less.

[0114] N: 0.0080% or less

[0115] N (nitrogen) is an element that forms nitrides of Ti, Nb, Al, and the like. These nitrides reduce the toughness of hot-rolled slabs, causing flaws and cracks during the rolling process, particularly cracks at the corners of the slab (corner cracks). Therefore, in manufacturing, the lower the N content, the better, and the N content is set to 0.0080% or less. The lower limit of the N content may include 0%. On the other hand, if the N content is less than 0.0005%, the steelmaking cost may become extremely high. Therefore, the N content is preferably set to 0.0005% or more, and more preferably to 0.0010% or more.

[0116] The remainder of the chemical composition of the hot-rolled steel sheet of this embodiment may be Fe and impurities. In this embodiment, impurities refer to elements that have been introduced from raw materials such as ore and scrap, or from the manufacturing environment, or elements that have been intentionally added in trace amounts, and are permitted within a range that does not adversely affect the hot-rolled steel sheet of this embodiment.

[0117] In addition to the above-mentioned elements, the hot-rolled steel sheet of this embodiment may also contain the following elements as optional elements to improve strength, ductility, or other properties. Specifically, in place of a portion of Fe, one or more of Cr, B, Ca, Mo, Ni, Cu, and REM may be contained within the ranges described below. If these optional elements are not present, the lower limit of their content is 0%. Each optional element is described in detail below.

[0118] Cr: 0.06%~0.27%

[0119] Cr (chromium) increases the tensile strength of steel sheets. To achieve this effect, the Cr content is preferably set to 0.06% or higher. A more preferred Cr content is 0.10%. On the other hand, a Cr content exceeding 0.27% may excessively increase the area fraction of bainite. Therefore, the Cr content is preferably set to 0.27% or lower. A more preferred Cr content is 0.25% or lower.

[0120] B: 0.0003%~0.0050%

[0121] Boron (B) increases the tensile strength of steel sheets. To achieve this effect, the B content is preferably set to 0.0003% or higher. A B content of 0.0005% or higher is more preferred. On the other hand, a B content exceeding 0.0050% may not only make it difficult to achieve a sufficient ferrite area ratio, but also increase the bainite area ratio excessively. Therefore, the B content is preferably set to 0.0050% or lower. A B content of 0.0040% or lower is more preferred.

[0122] Ca: 0.0003% to 0.0050%

[0123] Ca (calcium) spheroidizes non-metallic inclusions, thereby improving ductility. To achieve this effect, the Ca content is preferably set to 0.0003% or higher. A more preferred Ca content is 0.0005% or higher. On the other hand, a Ca content exceeding 0.0050% may reduce the toughness of the slab, potentially causing cracks and flaws in the slab during the rolling process. Therefore, the Ca content is preferably set to 0.0050% or lower. A more preferred Ca content is 0.0040% or lower.

[0124] Mo: 0.01%~0.40%

[0125] Mo increases the tensile strength of steel sheets. To achieve this effect, the Mo content is preferably set to 0.01% or higher. A more preferred Mo content is 0.03% or higher. On the other hand, if the Mo content exceeds 0.40%, the area fraction of bainite may increase excessively. Therefore, the Mo content is preferably set to 0.40% or lower. A more preferred Mo content is 0.35% or lower.

[0126] Ni: 0.01% to 0.50%

[0127] Nickel increases the tensile strength of steel sheets. To achieve this effect, the Ni content is preferably set to 0.01% or higher. A more preferred Ni content is 0.08% or higher. On the other hand, a Ni content exceeding 0.50% may increase the area fraction of bainite excessively. Therefore, the Ni content is preferably set to 0.50% or lower. A more preferred Ni content is 0.40% or lower.

[0128] Cu: 0.01% to 0.50%

[0129] Cu has the effect of increasing the tensile strength of steel sheets. To achieve this effect, the Cu content is preferably set to 0.01% or more. The Cu content is more preferably set to 0.08% or more. On the other hand, if the Cu content exceeds 0.50%, the area ratio of bainite may increase excessively. Therefore, the Cu content is preferably set to 0.50% or less. The Cu content is more preferably set to 0.40% or less.

[0130] REM: 0.0003%~0.0300%

[0131] REM (rare earth metal) is an element that has the effect of reducing the size of inclusions and is an element that helps improve hole expandability and ductility (elongation at break). If the REM content is less than 0.0003%, the effects brought about by these effects cannot be fully obtained. Therefore, it is preferable to set the REM content to 0.0003% or more. The REM content is more preferably 0.0005% or more. On the other hand, if the REM content exceeds 0.0300%, there is a possibility that castability and hot workability will deteriorate, so the REM content is preferably set to 0.0300% or less.

[0132] Here, REM refers to a total of 17 elements including Sc, Y and lanthanoid elements, and the above 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 alloys in industry.

[0133] The chemical composition of the hot-rolled steel sheet can be measured using ICP emission spectrometry (ICP) of cut steel sheets in accordance with JIS G 1201:2014. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) can be used. C and S can be measured using the combustion-infrared absorption method, while N can be measured using the inert gas fusion-thermal conductivity method.

[0134] (Metallic Structure)

[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 metallographic structure comprises, by area percentage, 53.0% to 76.0% ferrite, 3.0% to 10.0% martensite, 14.0% to 39.0% bainite, and 2.6% or less pearlite. Furthermore, in the hot-rolled steel sheet of this embodiment, the average diameter of the martensite is 0.26 μm to 0.70 μm, and the total length of the martensite-bainite interface, among all martensite interfaces, accounts for at least 75.0% of the total length of all martensite interfaces.

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

[0138] Ferrite: 53.0% to 76.0%

[0139] Since ferrite is a soft structure, it is the metal structure that mainly bears deformation. By making a dual-phase steel sheet containing martensite, such as the hot-rolled steel sheet of this embodiment, the effect of improving elongation, that is, the effect of improving ductility, can be obtained with the increase in the area ratio of ferrite. However, when the area ratio of ferrite is less than 53.0%, ductility decreases. Therefore, the area ratio of ferrite is set to 53.0% or more. The area ratio of ferrite is preferably 57.0% or more, and more preferably 60.0% or more. On the other hand, if the area ratio of ferrite exceeds 76.0%, the desired strength may not be obtained. Therefore, the area ratio of ferrite is set to 76.0% or less. The area ratio of ferrite is preferably 73.0% or less, and more preferably 70.0% or less.

[0140] Martensite: 3.0% to 10.0%

[0141] Since martensite is a hard structure, it helps to improve the strength of hot-rolled steel sheets. When the area ratio of martensite is less than 3.0%, the desired strength may not be obtained. Therefore, the area ratio of martensite is set to 3.0% or more. The area ratio of martensite is preferably 4.0% or more. On the other hand, when the area ratio of martensite exceeds 10.0%, the hole expandability may deteriorate significantly. Therefore, the area ratio of martensite is set to 10.0% or less. The area ratio of martensite is preferably set to 9.0% or less, more preferably to 8.0% or less, and even more preferably to 7.0% or less.

[0142] Bainite: 14.0%~39.0%

[0143] Bainite is a structure that improves the strength and ductility of hot-rolled steel sheets. In addition, by setting the metal structure configuration such that bainite surrounds martensite, stretch flange formability can be improved. When the area ratio of bainite is less than 14.0%, it becomes difficult to set the above-mentioned metal structure configuration, and the desired stretch flange formability cannot be obtained. Therefore, the area ratio of bainite is set to 14.0% or more. The area ratio of bainite is preferably 17.0% or more, more preferably 20.0% or more, and further preferably 25.0% or more. On the other hand, when the area ratio of bainite exceeds 39.0%, there is a possibility that ductility (elongation at break) will deteriorate significantly. Therefore, the area ratio of bainite is set to 39.0% or less. The area ratio of bainite is preferably 35.0% or less, more preferably 30.0% or less, and further preferably 28.0% or less.

[0144] Pearlite: less than 2.6%

[0145] If the area ratio of pearlite exceeds 2.6%, the hole expandability may deteriorate. Therefore, the area ratio of pearlite is set to 2.6% or less. It is preferably 1.7% or less, and more preferably 1.2% or less. The area ratio of pearlite may also be 0%.

[0146] In addition to the above-mentioned metal structures, retained austenite may also be included. However, if the area fraction of retained austenite exceeds 4.0%, toughness may be reduced. Therefore, when retained austenite is included, the area fraction is preferably set to 4.0% or less, and more preferably 3.0% or less. The area fraction of retained austenite may also be 0%.

[0147] Next, regarding the metal structure of the steel sheet according to the present embodiment, the ratio (area %) of each structure was measured by the following method.

[0148] The area ratio of the metal structure of the hot-rolled steel plate can be determined by using the value of the metal structure information such as the metal structure photograph obtained by using a scanning electron microscope in a cross section parallel to the rolling direction of the hot-rolled steel plate. Regarding the metal structure information of the metal structure photograph, it is sufficient to cut out the central position of the width in the direction perpendicular to the rolling direction and the plate thickness direction in parallel with the rolling direction, and set it to an observation field with a depth of 3 / 8 of the plate thickness from the surface in the plate thickness direction at the center. The observation field is set to more than 3 places, and the average value of the area ratio of the metal structure measured in each field of view can be used as the value of the area ratio of the representative metal structure of the steel plate. It should be noted that the area ratio of ferrite, pearlite, bainite, martensite, retained austenite, the average diameter of martensite, and the coverage are measured in the same field of view.

[0149] In the present embodiment, the area ratio of ferrite (hereinafter sometimes referred to as Vα) refers to the area ratio of the ferrite structure determined by the electron backscatter diffraction (EBSD) method. In order to measure the area ratio of ferrite, first, the EBSD method is used to obtain crystal orientation information (crystal orientation mapping data). For the measurement, a device consisting of a thermal field emission scanning electron microscope ("JSM-7001F" manufactured by JEOL) and an EBSD detector ("Hikari detector" manufactured by TSL) is used. The crystal orientation mapping data can be obtained using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. During the measurement, the vacuum degree in the device can be set to 9.6×10 -5 Below Pa, the acceleration voltage can be set to 20kV.

[0150] The procedure for determining the area ratio of ferrite from the crystal orientation mapping data is divided into the following three steps.

[0151] The first step is to define crystal grains from the crystal orientation mapping data. A crystal grain is defined as a region surrounded by a grain boundary where a crystal orientation difference of 15° or more is observed between any measurement point and an adjacent measurement point in the crystal orientation mapping data.

[0152] The second step is to determine whether the grains defined in the first step are ferrite grains. The determination method for ferrite is to use the local misorientation average (GAM value). The GAM value is a value indicating the misorientation of the grains. If the GAM value of the grain to be determined is within 0.35°, the grain is determined to be ferrite.

[0153] The third step is to perform the determination in the second step on all grains recorded in the crystal orientation mapping data. The ratio of the number of measurement points belonging to grains determined to be ferrite relative to the total number of measurement points in the crystal orientation mapping data is calculated. This ratio is defined as the area ratio of ferrite.

[0154] In order to fully reduce the error caused by the measurement position of the area ratio of ferrite, it is sufficient to make the crystal orientation mapping data include a total of 1000 grains. The measurement magnification when obtaining the crystal orientation mapping data by EBSD analysis is set in such a way that the field of view contains 1000 grains. In addition, for analysis performed at a low magnification of less than 200 times, the measurement accuracy is reduced due to the influence of the deflection of the electron beam, etc. Therefore, the measurement magnification is set to 250 times. In this embodiment, a 500μm×500μm area is measured at a magnification of 250 times.

[0155] The measurement range of the area ratio of ferrite is set to a quadrilateral consisting of edges in the plate thickness direction and the rolling direction. The edge in the plate thickness direction is set to 500μm, and the edge in the rolling direction is set to be equal to the edge in the plate thickness direction. The measurement of the area ratio of ferrite is carried out within a range included in a position 3 / 8 of the plate thickness away from the surface in the plate thickness direction. In addition, the measurement interval of the crystal orientation within the measurement range is set to 0.03μm. When the measurement interval is less than 0.03μm, there is a possibility that the interference range of the electron beam will be repeated. On the other hand, when the measurement interval exceeds 0.03μm, the number of measurement points of the crystal orientation contained in the grain is insufficient, and measurement errors are likely to occur.

[0156] The sample for ferrite measurement may be cut out parallel to the rolling direction at right angles to the rolling direction and the plate thickness direction at the center of the width, and observed in a direction perpendicular to the rolling direction and the plate thickness direction.

[0157] In this embodiment, the area ratio of martensite (hereinafter sometimes referred to as VM) is a value measured from the metal structure revealed by Lepera corrosion. The metal structure observed with white contrast in the metal structure revealed by Lepera corrosion is identified as martensite. The ratio of the area of ​​the metal structure identified as martensite, that is, the ratio of the white contrast to the entire area of ​​the observation field, is the area ratio of martensite VM.

[0158] Hereinafter, a method for measuring the area ratio VM of martensite will be described.

[0159] First, a scanning electron microscope is used to capture the field of view used in the measurement of the martensite area ratio VM. To improve the measurement accuracy, the metal structure can be captured at 5000 times. In addition, if the magnification is set to 5000 times, at least one martensite grain can be captured within one field of view. Therefore, the magnification is preferably set to 5000 times. In this embodiment, the martensite area ratio is measured by observing an area of ​​500 μm × 500 μm at a magnification of 5000 times.

[0160] When measuring the area fraction VM of martensite, the accelerating voltage during electron beam irradiation is set to a range of 10.0 kV to 15.0 kV. If the accelerating voltage is set above 15.0 kV, the grain boundaries may become unclear. On the other hand, if the accelerating voltage is lower than 10.0 kV, the resolution is reduced, making it unsuitable for observation.

[0161] The reflected electron image obtained by these observation samples and observation conditions is used to measure the area ratio VM of martensite. Specifically, in order to reduce the error between the observation fields, the area ratio VM of martensite is obtained by a measurement range in which the total number of grains is more than 600. The total number of grains within the measurement range is set to 1000. The measurement range is carried out within a range including a position of 3 / 8 of the thickness of the plate from the surface in the thickness direction. In addition, the measurement range is set to a range of 500 μm in the thickness direction and 500 μm in the rolling direction.

[0162] Regarding the area ratio of bainite (hereinafter sometimes referred to as VB), the remainder obtained by subtracting the total area ratios of ferrite, martensite, retained austenite described below, and pearlite obtained by the above method from 100% is the area ratio VB of the bainite structure.

[0163] The area ratio of pearlite (hereinafter sometimes referred to as VP) is a value measured from a metal structure revealed by nital etching.

[0164] The pearlite measurement sample may be cut out parallel to the rolling direction at right angles to the rolling direction and the plate thickness direction at the center of the width, and observed from the direction at right angles to the rolling direction and the plate thickness direction.

[0165] In the sample collected for pearlite measurement, a metal structure photograph was obtained within the measurement range centered at a position 3 / 8 from the steel plate surface in the plate thickness direction. Note that the sample for pearlite measurement was set to be the same as the sample for measuring the area ratio of ferrite and martensite.

[0166] In this embodiment, a metal structure photograph for measuring the area ratio of pearlite is obtained by scanning electron microscopy. In the photography performed by scanning electron microscopy, the acceleration voltage when irradiating electron beams is set to a range of 10.0kV to 15.0kV. If the acceleration voltage is set to more than 15.0kV, the grain boundaries may become unclear. On the other hand, when the acceleration voltage is lower than 10.0kV, the resolution is reduced and it is not suitable for observation. In addition, in order to fully improve the measurement accuracy, the metal structure can be photographed at a magnification of 2000 times or more. It should be noted that if the magnification is set to 10000 times or less, it is possible to photograph more than one pearlite grain in one field of view. Therefore, the magnification is set to 10000 times or less. It should be noted that in order to reduce the number of fields of view and obtain measurement accuracy, the magnification is preferably 5000 times. In addition, the measurement range is set to a range of 10μm to 40μm in the plate thickness direction and 10μm to 55μm in the rolling direction.

[0167] The area fraction of retained austenite (hereinafter sometimes referred to as Vγ) is the value obtained by dividing the number of measurement points in the crystal orientation mapping data used to determine the area fraction of ferrite (Vα) described above, where the crystal structure was determined to have an fcc crystal orientation, by the total number of measurement points in the crystal orientation mapping data. The crystal orientation mapping data used to determine the area fraction of retained austenite (Vγ) is the same as that used to determine the area fraction of ferrite (Vα). That is, the measurement range, measurement magnification, and field of view can be set similarly to those used for determining the area fraction of ferrite.

[0168] (Morphology of Metal Structure)

[0169] Next, the metal structure of the hot-rolled steel sheet according to the present embodiment will be described.

[0170] In order to obtain excellent stretch flange formability in the hot-rolled steel sheet of this embodiment, it is important to set the metal structure composition as described above and the area ratio within the desired range. On this basis, it is important to set the ratio of the interface length between martensite and bainite to the total interface length of martensite and the average diameter dM of martensite within the desired range.

[0171] The ratio of the interface length between martensite and bainite to the total interface length of martensite (hereinafter sometimes referred to as coverage) is set to 75.0% or more. It is believed that since bainite is a metal structure with intermediate strength between ferrite and martensite, it plays a role in alleviating the deformation difference between ferrite and martensite, that is, a buffering role. If the coverage of martensite by bainite is less than 75.0%, the buffering effect becomes insufficient, and micro cracking of the shear end face occurs. Then, as a result, it becomes difficult to obtain excellent stretch flangeability. In addition, if the coverage is less than 75.0%, the fracture limit strain described later is reduced. Therefore, the higher the ratio of the interface length between martensite and bainite to the total interface length of martensite, the better, and it is preferably set to 78% or more. On the other hand, the upper limit of the ratio of the interface length between martensite and bainite to the total interface length of martensite is not particularly specified and can also be 100%.

[0172] As described above, in order to improve stretch flange formability, it is effective not only to increase the area ratio of bainite but also to arrange bainite so as to surround martensite, that is, to increase the ratio of the interface length between martensite and bainite to the total interface length of martensite.

[0173] From the perspective of suppressing voids, the average diameter dM of martensite is set to 0.26μm to 0.70μm. By setting the average diameter dM within this range, microcracking of the shear end face can be suppressed, resulting in high stretch flange formability. When the average diameter dM of martensite exceeds 0.70μm, due to the difference in hardness between martensite and bainite, deformation is concentrated at the interface between the two. As a result, even if the coverage rate of bainite is satisfied, voids may form near the interface between martensite and bainite. In addition, if voids are formed, microcracking of the shear end face may occur, and the fracture limit strain described later may be reduced. Therefore, the average diameter dM of martensite is set to 0.70μm or less. The average diameter dM of martensite is preferably 0.65μm or less, and more preferably 0.60μm or less. On the other hand, if the average diameter dM of martensite is less than 0.26μm, there is a possibility that the martensite will no longer contribute to strength. If the average diameter dM of martensite is less than 0.26 μm, the coverage may be reduced. Therefore, the average diameter dM of martensite is set to 0.26 μm or more. Preferably, the average diameter dM of martensite is 0.30 μm or more.

[0174] Here, the ratio of the interface length between martensite and bainite to the total interface length of martensite represents the ratio of the total boundary length (interface length) between martensite and bainite to the total boundary length (interface length) between martensite and other adjacent metal structures. The method for calculating this ratio is described below.

[0175] First, the total interface lengths of martensite, i.e. the total lengths of the boundaries of martensite and other metal structures adjacent thereto, are set to the total value obtained by measuring the length (interface length) of the boundaries of the martensite and other metal structures adjacent thereto in the martensite identified by the above-mentioned method. The total interface lengths of the martensite are obtained using a metal structure photograph taken using the same method as the determination method of the average diameter dM of the martensite described later. Specifically, 300 martensites are selected from the metal structure photographs taken to obtain the interface lengths of these grains. For each martensite, the boundary lengths of martensite and other metal structures adjacent thereto are measured, and the value obtained by all of the totals is the total lengths of the boundaries of martensite and other metal structures adjacent thereto, i.e. the total interface lengths of martensite.

[0176] Next, the total length of the boundary between martensite and bainite is calculated. The total length of the boundary between martensite and bainite refers to the total value obtained by measuring the length of the boundary between martensite and bainite identified by the above method. This value is measured using the same martensite as the martensite used as the measurement object when measuring the total interface length of martensite, and the number of measurements is also set to be the same. That is, the "martensite-bainite boundary" refers to the boundary between martensite and bainite in the boundary between martensite and other metal structures adjacent to it obtained by the above method, and the total length of this boundary is the "martensite-bainite boundary length".

[0177] The value obtained by dividing the total length of the boundary between martensite and bainite obtained by the above method by the total length of the boundary between martensite and other adjacent metal structures is the coverage rate of martensite with bainite, that is, the ratio of the interface length between martensite and bainite to the total interface length of martensite.

[0178] Next, a method for determining the average diameter dM of martensite will be described.

[0179] The martensite in the hot-rolled steel sheet of this embodiment has a plate-like morphology. Therefore, the martensite grains are approximated as ellipsoids, their major and minor diameters are measured, and the average of these values ​​is taken as the average diameter of the martensite. The average of the diameters of all the measured martensite is then calculated, and the average of these values ​​is taken as the average diameter dM of the martensite in the hot-rolled steel sheet.

[0180] The number of measurements of the average diameter dM of martensite is set to 300. It should be noted that most of the martensite to be measured is fine (with a diameter of several μm or less). Therefore, it is preferred to use a metal structure photograph taken at a magnification of 5000 times for measurement. The field of view of the metal structure photograph used when measuring the average diameter dM and the measurement sample are set to the same field of view used when measuring the area ratio of martensite as described above.

[0181] (characteristic)

[0182] Next, the characteristics of the hot-rolled steel sheet according to this embodiment will be described.

[0183] The hot-rolled steel sheet of this embodiment also meets the requirements of a tensile strength of 780 MPa or greater, a ductility (elongation at break) of 15.0% or greater, and a hole expansion (hole expansion ratio) of 60% or greater. Furthermore, the ultimate strain at break, as determined by the lateral bending test described below, can be 0.5 or greater. By meeting these requirements in the hot-rolled steel sheet, it is possible to obtain a hot-rolled steel sheet suitable not only for automobile bodies but also for use as raw material for complexly shaped automotive parts (particularly chassis components).

[0184] <Tensile Strength>

[0185] The tensile strength of the hot-rolled steel sheet of this embodiment can also be 780 MPa or higher. Setting the tensile strength to 780 MPa or higher can further contribute to lightweighting of vehicle bodies and components. The upper limit does not need to be specifically set, but can also be set to 950 MPa or lower.

[0186] <Ductility (elongation at break)>

[0187] The hot-rolled steel sheet of the present embodiment may also have a breaking elongation of 15.0% or more.

[0188] <Hole Expandability>

[0189] The hot-rolled steel sheet of the present embodiment may also have a hole expandability (hole expansion ratio) of 60% or more.

[0190] The tensile strength and elongation at break were measured using the No. 5 test piece of JIS Z 2241:2011 in accordance with JIS Z 2241:2011. The tensile test piece was collected in a direction perpendicular to the rolling direction and the plate thickness direction (plate width direction) in such a way that the distance from the end of the steel plate to 1 / 4 was included. At this time, the tensile test piece was collected with the direction perpendicular to the rolling direction as the length direction. The crosshead speed in the tensile test can be such that the strain rate becomes 0.005s -1 Keep the conditions constant.

[0191] Hole expansion properties (hole expansion ratio) were evaluated using the hole expansion ratio (λ) specified in JIS Z 2256:2010. Specifically, a hole was punched using a 10 mm φ punch with a die diameter selected to achieve a clearance of 12.5%. A hole expansion test was then conducted at a stroke speed of 10 mm / minute using a conical die with a 60° tip angle, with the fins facing outward. The test was terminated when cracks formed around the hole penetrated the plate thickness, and the hole diameters before and after the hole expansion test were compared.

[0192] <Tensile flangeability (fracture limit strain)>

[0193] In the hot-rolled steel sheet of this embodiment, the fracture limit strain evaluated by the side bending test described below is used as an index of stretch flangeability. The fracture limit strain of the hot-rolled steel sheet of this embodiment can be set to 0.5 or more.

[0194] Among them, when forming automobile parts, in order to ensure the rigidity of the parts, it is preferred to form them in a manner that can fully ensure the vertical wall height (forming height) of the stretched flange part. That is, the raw material steel plate used for the parts is preferably a raw material steel plate that can also withstand the increase in the forming height. Generally speaking, for example, a raw material steel plate that can ensure a forming height of 18 mm or more is preferred. Therefore, the inventors of the present invention used a conventional hot-rolled steel plate with a tensile strength of 340 to 780 MPa to investigate the relationship between the forming height of the stretched flange forming part and the fracture limit strain obtained based on the side bending test. The results are shown in Figure 1 It should be noted that Figure 1 In the diagram shown in , white symbols indicate that the mold was possible, while black symbols indicate that cracks occurred. In the diagram, for example, "780 material" means 780 MPa material.

[0195] Depend on Figure 1 The graph shown in Figure 2 shows that 340 MPa, 440 MPa, and 590 MPa materials with a limit strain of 0.5 or greater can be formed without fracture or cracking up to a forming height of 18 mm or greater. However, 780 MPa materials with a limit strain of 0.35 can be formed up to a forming height of approximately 15 mm, but cracking occurs at heights above this. In other words, it was found that by setting the fracture limit strain of the raw steel sheet to 0.5 or greater, forming of the stretch flange forming portion to a forming height of 18 mm or greater is also possible.

[0196] As can be seen from the above, the fracture limit strain of the hot-rolled steel sheet according to the present embodiment obtained by the following side bending test can be set to 0.5 or more, and preferably set to 0.6 or more.

[0197] (Side bending test method)

[0198] The fracture limit strain, which serves as an indicator of stretch flange formability, is set to a value measured by the following side bend test. The side bend test in this embodiment uses the method described in "Nippon Steel Technical Report No. 393 (2012) pp. 18-24" and "Japanese Patent Application Publication No. 2009-145138."

[0199] The test piece shape for the lateral bending test is set to Figure 2 The semicircular portion of the test piece can be any semicircular portion manufactured by shearing. Specifically, first, a 35mm×100mm plate is cut out from the steel plate. Then, a φ30mm punch is used to punch out a semicircular hole in the plate with a plate thickness clearance (the value obtained by dividing the gap between the punch and the die by the plate thickness) of 12.5%. Through these processes, a semicircular hole is made. Figure 2 The test piece shown in FIG. The radius of the semicircular portion of the test piece is set to 15 mm. It should be noted that before the lateral bending test, a 2 mm checkered pattern is preferably drawn on the surface of the test piece in order to measure the fracture limit strain.

[0200] Using the aforementioned lateral bending test piece, the test was conducted using the apparatus and method described in "Nippon Steel Technical Report No. 393 (2012) pp. 18-24" and "Japanese Patent Publication No. 2009-145138." Specifically, the test piece was deformed at a stroke speed of 10 mm / minute, and the formation of cracks at the hole edge was defined as "fracture." After fracture, the fracture limit strain was measured using a total of three elements: the identified element and its adjacent elements, with a gauge length of 6.0 mm.

[0201] It should be noted that three or more side bending test specimens are prepared from the hot-rolled steel sheet to be evaluated, and the aforementioned ultimate fracture strain is measured on each test specimen. The average value of these ultimate fracture strains is then defined as the ultimate fracture strain obtained from the side bending test for the hot-rolled steel sheet.

[0202] (plate thickness)

[0203] The thickness of the hot-rolled steel sheet of this embodiment is not particularly limited, but can be set to 1.6 to 8.0 mm. In particular, when considering applications in automotive running parts, the thickness is often 1.6 mm or more. Therefore, the thickness of the hot-rolled steel sheet of this embodiment can also be set to 1.6 mm or more. Preferably, it is 1.8 mm to 2.0 mm. In addition, by setting the thickness to 8.0 mm or less, the metal structure becomes more refined, and the above-mentioned metal structure can be easily ensured. Therefore, the thickness can also be set to 8.0 mm or less. Preferably, it is 7.0 mm or less.

[0204] (Plating)

[0205] With regard to the hot-rolled steel sheet of the present embodiment with the above-mentioned chemical composition and metal structure, for the purpose of improving corrosion resistance, the surface can also be provided with a coating and made into a surface-treated steel sheet. The coating can be an electroplated layer or a hot-dip coated layer. As the electroplated layer, examples include electrogalvanized layers, electroplated Zn-Ni alloy layers, etc. As the hot-dip coated layer, examples 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, hot-dip Zn-Al-Mg-Si alloy layers, etc. The coating adhesion is not particularly limited and can be set to 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) to further improve corrosion resistance after plating.

[0206] (Manufacturing conditions)

[0207] Next, the method for producing a hot-rolled steel sheet according to this embodiment will be described. It should be noted that the slab temperature and steel sheet temperature in this embodiment refer to the surface temperature of the slab and steel sheet. In this embodiment, the temperature of the hot-rolled steel sheet is measured at the extreme width ends using a contact or non-contact thermometer. Temperatures other than the extreme width ends of the hot-rolled steel sheet are measured using thermocouples or calculated using heat transfer analysis.

[0208] The method for manufacturing a hot-rolled steel plate according to the present embodiment comprises the following steps: a hot rolling step in which a slab having the above-mentioned chemical composition is rolled under the condition that the finishing rolling temperature is 880°C to 950°C; a primary cooling step in which, after the hot rolling step, the slab is cooled at an average cooling rate of 60°C / s or more to a primary cooling stop temperature of 680°C to 760°C; a secondary cooling step in which, after the primary cooling step, the slab is cooled at an average cooling rate of 20°C / s or less for a period of 1.6 seconds to 6.3 seconds; a tertiary cooling step in which, after the secondary cooling step, the slab is cooled at an average cooling rate of 60°C / s to 130°C / s to a tertiary cooling stop temperature of 195°C to 440°C; and a tertiary cooling step in which, after the tertiary cooling step, the slab is cooled at an average cooling rate of 2.0 m / s to a tertiary cooling stop temperature of 195°C to 440°C. 3 / min / mm 2 ~7.2m 3 / min / mm 2 The water density is 0.33 seconds to 1.50 seconds during the four cooling processes, and the air cooling is performed five times during the 3.0 seconds to 5.0 seconds after the four cooling processes. After the five cooling processes, the coiling process is performed at a temperature below 180°C.

[0209] (Hot rolling process)

[0210] First, the hot slab having the above-mentioned chemical composition is rough-rolled, and then finish-rolled under the condition that the final rolling outlet temperature (finishing rolling temperature) is 880°C to 950°C. By finishing rolling under such conditions, the area ratio of ferrite can be set to an appropriate range. When the finishing rolling temperature is lower than 880°C, the area ratio of ferrite becomes excessively large. In addition, when the finishing rolling temperature exceeds 950°C, it becomes difficult to fully ensure the area ratio of ferrite. Therefore, the finishing rolling temperature is set to 880°C to 950°C, and preferably the finishing rolling temperature is set to 890°C to 940°C.

[0211] (Primary cooling process)

[0212] After the hot rolling process, the steel is cooled at an average cooling rate of 60°C / second or more to a primary cooling stop temperature of 680°C to 760°C (primary cooling process). In this primary cooling process, when the average cooling rate is lower than 60°C / second, pearlite is excessively generated, making it difficult to improve the hole expandability. Therefore, the average cooling rate in the primary cooling process is preferably set to 65°C / second or more. It should be noted that the upper limit of the average cooling rate in the primary cooling process is not particularly specified, but it can also be set to 150°C / second or less, or it can be set to 110°C / second or less. The cooling stop temperature in the primary cooling process (primary cooling stop temperature) can be set to 680°C to 760°C. When the primary cooling stop temperature is lower than 680°C, the area ratio of ferrite may be insufficient. In addition, even if the primary cooling stop temperature exceeds 760°C, the area ratio of ferrite may be insufficient, and the area ratio of bainite may increase.

[0213] (Secondary cooling process)

[0214] After the primary cooling process, cooling is performed at an average cooling rate of less than 20°C / second during a period of 1.6 seconds to 6.3 seconds (secondary cooling process). When the average cooling rate of the secondary cooling process exceeds 20°C / second, the area ratio of ferrite may become insufficient. Therefore, the average cooling rate of the secondary cooling process is set to less than 20°C / second, preferably less than 18°C / second. In addition, when the cooling time in the secondary cooling process is less than 1.6 seconds, the area ratio of ferrite may become insufficient, and the area ratio of bainite may increase. On the other hand, when the cooling time in the secondary cooling process exceeds 6.3 seconds, the area ratio of ferrite may increase excessively, making it difficult to improve the strength. In addition, if the cooling time in the secondary cooling process is too long, the area ratio of bainite may also be insufficient. Therefore, the cooling time in the secondary cooling process is preferably set to 1.8 seconds to 6.1 seconds.

[0215] (Three cooling steps)

[0216] After the secondary cooling process, the steel is cooled at an average cooling rate of 60°C / s to 130°C / s to a tertiary cooling stop temperature of 195°C to 440°C. In this embodiment, the desired metal structure morphology is obtained by finely controlling the fourth cooling process and the third cooling process described later. Therefore, the third cooling process and the fourth cooling process are important processes from the perspective of ensuring stretch flange formability. That is, by increasing the average cooling rate of the third cooling process, a large amount of bainite can be formed from the interface between ferrite and austenite, or the austenite / austenite grain boundary, generated in the primary cooling process and the secondary cooling process, and the bainite coverage of the remaining austenite can be increased. Then, in the fourth cooling process, the remaining austenite is transformed into martensite, thereby increasing the bainite coverage of the present embodiment.

[0217] If the average cooling rate in the three cooling steps is set to 60°C / s to 130°C / s, the desired amount of bainite can be ensured. If the average cooling rate in the three cooling steps is less than 60°C / s, sufficient undercooling cannot be ensured, and a large amount of bainite is formed only at specific grain boundaries. As a result, it becomes difficult to achieve sufficient coverage of martensite with bainite after the four cooling steps. Therefore, in the three cooling steps, the average cooling rate is set to 60°C / s or higher, preferably 65°C / s or higher, and more preferably 70°C / s or higher. On the other hand, if the average cooling rate in the three cooling steps exceeds 130°C / s, bainite formation does not proceed sufficiently, and it becomes difficult to achieve sufficient coverage of martensite with bainite after the four cooling steps. Therefore, in the three cooling steps, the average cooling rate is set to 130°C / s or lower, preferably 125°C / s or lower, and more preferably 120°C / s or lower.

[0218] The temperature at which the tertiary cooling process ends (tertiary cooling stop temperature) can be set to 195°C to 440°C. If the tertiary cooling stop temperature is lower than 195°C, the area ratio of bainite becomes insufficient. Therefore, the tertiary cooling stop temperature is preferably set to 220°C or higher, and more preferably set to 250°C or higher. On the other hand, if the tertiary cooling stop temperature exceeds 440°C, the area ratio of bainite increases, making it difficult to obtain a good elongation at break. Therefore, the tertiary cooling stop temperature is preferably set to 420°C or lower, and more preferably set to 400°C or lower.

[0219] (Four cooling steps)

[0220] After three cooling processes, the 3 / min / mm 2 ~7.2m 3 / min / mm 2The water density is 0.33 seconds to 1.50 seconds during which water cooling is performed.

[0221] This fourth cooling step is an important step in terms of controlling the coverage of martensite with bainite, the average diameter of martensite, and the area ratio of martensite, similarly to the third cooling step.

[0222] In the four cooling processes, when the water density is less than 2.0m 3 / min / mm 2 In this case, it may not be possible to ensure the coverage of martensite by bainite. According to the investigation of the inventors of the present invention, it is known that the coverage of martensite by bainite increases as the water density increases in the four cooling processes. Although the detailed mechanism is not clear, it is believed that the reduction in water density will lead to a reduction in the driving force of bainite transformation, resulting in a delay in the bainite transformation around the martensite. In this embodiment, by setting the water density in the four cooling processes to 2.0m 3 / min / mm 2 The above can improve the coverage of martensite with bainite. It should be noted that, from the perspective of the coverage of martensite with bainite, the upper limit of the water density is not particularly specified, but is 7.2 m 3 / min / mm 2 If the water pressure exceeds 7.2m, the plate may be deformed due to water pressure. Therefore, the water density is set to be less than 7.2m 3 / min / mm 2 , preferably set to 7.0m 3 / min / mm 2 Below, more preferably 6.8m 3 / min / mm 2 the following.

[0223] In the four cooling steps, the water density is set within the above range, and the cooling time is set to 0.33 seconds to 1.50 seconds. The inventors of the present invention have discovered that the average diameter dM of martensite changes depending on the cooling time of the four cooling steps. Specifically, if the cooling time is less than 0.33 seconds, the average diameter dM of martensite becomes excessively small. On the other hand, if the cooling time exceeds 1.50 seconds, the average diameter dM of martensite becomes excessively large. Therefore, the cooling time of the four cooling steps is set to 0.33 seconds to 1.50 seconds, and preferably to 0.40 seconds to 1.40 seconds.

[0224] (Five cooling steps)

[0225] After the fourth cooling step, air cooling was performed for 3.0 seconds to 5.0 seconds until the temperature dropped below 180° C. (fifth cooling step).

[0226] In the five cooling steps, after the fourth cooling step, air cooling is performed for 3.0 to 5.0 seconds instead of water cooling. This time without water cooling, i.e., the air cooling time, affects the coverage of martensite with bainite. The detailed mechanism is not clear, but it is presumed that the coverage of martensite is improved by forming bainite during the air cooling of the five cooling steps. If the air cooling time is less than 3.0 seconds, the coverage may become insufficient. On the other hand, if the air cooling time exceeds 5.0 seconds, the area ratio of bainite may increase excessively.

[0227] The inventors of the present invention have investigated and found that when the air cooling time is less than 3.0 seconds or exceeds 5.0 seconds, the coverage of martensite with bainite becomes less than 75.0%. In order to obtain a more sufficient effect, the air cooling time should be set to 4.0 seconds or more, and preferably 4.8 seconds.

[0228] In the fifth cooling step, the steel sheet is air-cooled to a coiling temperature of less than 180° C., and then coiled.

[0229] When the coiling temperature is 180°C or higher, the area ratio of martensite becomes insufficient, making it difficult to obtain excellent strength. Therefore, the coiling temperature is preferably set to lower than 180°C.

[0230] The hot-rolled steel sheet according to the present embodiment can be manufactured by the method described above.

[0231] In addition, the steel plate passing speed in the fourth cooling step can be set to 360 to 790 mpm (meters per minute).

[0232] Furthermore, after the hot-rolled steel sheet is coiled to form a hot-rolled coil, the hot-rolled coil may be uncoiled and pickled for the purpose of removing the oxide film. Alternatively, skin pass rolling may be performed within a range that does not degrade ductility.

[0233] In this embodiment, the equipment used to perform the cooling steps described above is not limited. Industrially, it is preferable to use a water spray device that can precisely control the water density. For example, a water spray device can be placed between the transport rollers that transport the steel plate, spraying a predetermined amount of water from above and below the steel plate for cooling. Furthermore, by controlling the sprayed water density or varying the valve opening and closing positions, the thermal history of the supercooling step described above can be achieved.

[0234] Example

[0235] Next, the embodiments of the present invention will be described. However, the conditions in the embodiments are merely examples of conditions employed to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to this single example. Various conditions may be employed in the present invention as long as they do not deviate from the spirit of the present invention and achieve the purpose of the present invention.

[0236] Using the ingot slabs with the chemical composition shown in Table 1A and Table 1B, steel coils (hot-rolled steel plates) with a width of 800 mm to 1080 mm were manufactured under the conditions shown in Table 2A to Table 2C. It should be noted that in the four cooling processes, the plate passing speed was set to a range of 360 to 780 mpm (meters per minute). In addition, in each cooling process, the opening and closing position of the valve in the output roller table (ROT) was changed to set a prescribed thermal history (cooling rate). In addition, the plate thickness of the hot-rolled steel plate was set to a range of 2.0 mm to 6.0 mm. It should be noted that "FT" in Table 1 refers to the final rolling temperature of the finishing rolling in the hot rolling process.

[0237] The microstructure of the produced hot-rolled steel sheets was measured using the aforementioned measurement methods to determine the area ratio of each structure, the average grain size of martensite, and the coverage of martensite by bainite. Furthermore, the various properties of the hot-rolled steel sheets were evaluated using the following methods. The evaluation results are shown in Tables 3A to 3C.

[0238] Tensile Strength (TS)

[0239] The tensile strength (TS) of the hot-rolled steel plate is determined using the No. 5 test piece of JIS Z 2241: 2011 according to the test method described in JIS Z 2241: 2011. The tensile test piece is collected in a direction perpendicular to the rolling direction and the plate thickness direction (plate width direction) in such a way that a portion 1 / 4 of the distance from the end of the steel plate is included. At this time, the tensile test piece is collected with the direction perpendicular to the rolling direction as the length direction. The crosshead speed in the tensile test is such that the strain rate becomes 0.005s -1 The test was carried out under such constant conditions. A tensile strength of 780 MPa or higher was judged as acceptable, and a tensile strength of less than 780 MPa was judged as unacceptable.

[0240] Elongation at break

[0241] The elongation at break, an indicator of ductility, was determined similarly to the tensile strength (TS) evaluation method described above, according to the test method described in JIS Z2241: 2011. Elongation at break (%) of 15.0% or more was considered acceptable, while less than 15.0% was considered unacceptable.

[0242] "pore expansion"

[0243] The hole expansion property is evaluated by the hole expansion ratio λ (%) measured in accordance with JIS Z 2256:2010. Specifically, a punch of φ10 mm is used, and the die diameter is selected so that the clearance becomes 12.5%, and a hole is punched. Then, a hole expansion test is carried out at a stroke speed of 10 mm / min using a conical die with a front end angle of 60°, with the fins on the outside. The test is terminated when the cracks formed around the hole penetrate the plate thickness. The hole diameters are compared before and after the hole expansion test to calculate the hole expansion ratio (%). A hole expansion ratio (%) of 60% or more is judged as acceptable, and a hole expansion ratio of less than 60% is judged as unacceptable.

[0244] "Microcracking of shear end faces"

[0245] Hot-rolled steel sheets were sheared, and the occurrence of microcracks in the sheared end surfaces was visually observed. Specifically, the shearing process involved observing the semicircular end portions of the side bending test specimens obtained by punching out the semicircular portion at 50x magnification. Microcracks were defined as cracks that occurred only at the punched-out end portions and did not penetrate the sheet thickness. In this test, no cracks penetrated the sheet thickness. The blanking clearance was set at 12.5%.

[0246] "Side bending test"

[0247] As an indicator of stretch flangeability, the fracture limit strain evaluated by the side bending test was used. The side bending test was conducted using the method described in "Nippon Steel Technical Report No. 393 (2012) pp. 18-24" and "Japanese Patent Application Publication No. 2009-145138."

[0248] Specifically, first, a 35mm x 100mm plate is cut out from the steel plate. Then, a φ30mm punch is used to punch a semicircular hole in the plate with a plate thickness clearance (the value obtained by dividing the gap between the punch and the die by the plate thickness) of 12.5%. Through these steps, a semicircular hole is formed. Figure 2 The test piece shown in .

[0249] Next, the test piece was deformed at a stroke speed of 10 mm / min, and the formation of cracks at the edge of the hole was defined as "fracture". After the fracture, the fracture limit strain with a gauge length of 6.0 mm was measured using a total of three elements, namely, the judged element and the elements adjacent thereto. It should be noted that in the present embodiment, three side bending test pieces were made, and the above-mentioned fracture limit strain was measured in each test piece, and the average value of these fracture limit strains was used for evaluation. The case where the fracture limit strain was 0.5 or more was judged to be qualified, and the case where it was less than 0.5 was judged to be unqualified.

[0250] [Table 1A]

[0251]

[0252] [Table 1B]

[0253]

[0254] [Table 2A]

[0255]

[0256] [Table 2B]

[0257]

[0258] [Table 2C]

[0259]

[0260] [Table 3A]

[0261]

[0262] [Table 3B]

[0263]

[0264] [Table 3C]

[0265]

[0266] According to Tables 3A to 3C, it is understood that Test Nos. 11 to 25, 37 to 41, and Test Nos. 56 to 68, which are inventive examples, have high strength and are excellent in ductility, hole expandability, and stretch flangeability.

[0267] Figure 5 The relationship between the fracture limit strain and the coverage rate in test numbers 2, 4, 5, 8, 9, and 11 to 25 is shown in FIG. It should be noted that test numbers 11 to 25 are examples in which the average diameter of martensite and the area ratio of bainite are within the range of the present invention. Figure 5 As shown in , it was found that by producing a hot-rolled steel sheet in which the average diameter of martensite and the area ratio of bainite are within the ranges of the present invention, and the coverage of martensite with bainite is 75.0% or more, the fracture limit strain obtained by side bending can be set to 0.5 or more. In other words, it was found that even if the average diameter of martensite and the area ratio of bainite are within the ranges of the present invention, it is difficult to increase the fracture limit strain if the coverage of martensite with bainite is less than 75.0%.

[0268] also, Figure 6The relationship between the fracture limit strain and the average diameter dM of martensite in test numbers 6, 7, and 11 to 25 is shown in FIG. It should be noted that test numbers 11 to 25 are examples in which the area ratio of bainite and the coverage ratio of martensite by bainite are within the range of the present invention. Figure 6 As shown in , it was found that by producing a hot-rolled steel sheet in which the area ratio and coverage of bainite are within the ranges of the present invention and the average diameter dM of martensite is within the ranges of the present invention, the fracture limit strain obtained by side bending can be set to 0.5 or greater. In other words, it was found that even if the area ratio and coverage of bainite are within the ranges of the present invention, it is difficult to increase the fracture limit strain if the average diameter dM of martensite is outside the range.

[0269] also, Figure 7 The relationship between the coverage rate and the water density of the four cooling processes in test numbers 4, 5, 11 to 25 is shown in FIG. It should be noted that test numbers 11 to 25 are examples in which the manufacturing conditions other than the water density of the four cooling processes are within the scope of the present invention. Figure 7 As shown in , it was found that by manufacturing a hot-rolled steel sheet under all manufacturing conditions, including the water density in the fourth cooling step, within the range of the present invention, the coverage ratio of the hot-rolled steel sheet can be sufficiently increased. In other words, it was found that even if the manufacturing conditions other than the water density in the fourth cooling step are within the range of the present invention, it is difficult to increase the coverage ratio if the water density in the fourth cooling step is outside the range.

[0270] also, Figure 8 The relationship between the average diameter dM of martensite and the cooling time of the four cooling steps in test numbers 6, 7, 11 to 25 is shown in FIG. It should be noted that test numbers 11 to 25 are examples in which the manufacturing conditions other than the cooling time of the four cooling steps are within the scope of the present invention. Figure 8 As shown in , it was found that by manufacturing a hot-rolled steel sheet under all manufacturing conditions, including the cooling time of the fourth cooling step, within the range of the present invention, the average diameter dM of martensite can be sufficiently increased. In other words, it was found that even if the manufacturing conditions other than the cooling time of the fourth cooling step are within the range of the present invention, it is difficult to increase the average diameter dM of martensite if the cooling time of the fourth cooling step is outside the range.

[0271] also, Figure 9 The relationship between the coverage of martensite by bainite and the air cooling time in test numbers 8, 9, 11 to 25 is shown in FIG. It should be noted that test numbers 11 to 25 are examples in which the manufacturing conditions other than the air cooling time are within the scope of the present invention. Figure 9As shown in , it was found that by producing hot-rolled steel sheets with all production conditions, including the air cooling time, within the range of the present invention, the coverage ratio can be sufficiently increased. In other words, it was found that even if the production conditions other than the air cooling time were within the range of the present invention, it would be difficult to increase the coverage ratio of martensite with bainite if the air cooling time was outside the range.

[0272] As described above, it was found that all of Test Nos. 11 to 25, 37 to 41, and Test Nos. 56 to 68, which are invention examples within the scope of the present invention, were excellent in properties.

[0273] For example, the microstructure of test number 21, which is an example of the invention, was observed and the results showed that Figure 10A The microstructure shown in Figure 10A As also shown, in Test No. 21 which is an inventive example, it was found that the martensite was sufficiently covered with the bainite (the dotted line region in the figure).

[0274] In addition, in order to subject Test No. 21 to the scoliosis test, shearing was performed on Test No. 21, and the structure near the sheared end surface was observed. Figure 10B According to the tissue photo shown in Figure 10B , it was found that no cracking was observed in the deformed martensite itself or at the interface between martensite and other structures, but rather that voids were generated within the heavily deformed ferrite. In other words, it is believed that the bainite, arranged so as to cover the martensite, acted as a buffer, mitigating the difference in deformation between the ferrite and martensite, thereby suppressing cracking in the martensite.

[0275] As described above, it was found that at least one of the comparative examples, namely, test numbers 1 to 10, 26 to 36, and test numbers 42 to 55, had inferior characteristics.

[0276] For example, in the comparative examples of test numbers 4 and 5, the water density in the four cooling steps was lower than 2.0 m 3 / min / mm 2 , so the coverage of martensite with bainite fell below 75.0%. In addition, microcracks were observed at the shear end faces, and the fracture limit strain obtained from the side bending test did not meet the target.

[0277] In comparative examples, Test Nos. 26 to 36, while the coverage of martensite with bainite and the average martensite diameter dM fell within the inventive range, they failed to meet other metallographic requirements and, therefore, failed to satisfy one or more of the properties. For example, in Test No. 27, which used steel grade G with an appropriate chemical composition, the area ratio of ferrite fell below 53.0% due to the finish rolling temperature exceeding 950°C. As a result, ductility decreased.

[0278] Furthermore, in the comparative example, Test No. 50, although manufactured under manufacturing conditions within the scope of the present invention, the high Al content in the chemical composition increased the area ratio of ferrite. As a result, the tensile strength fell below 780 MPa. It should be noted that the conditions for the fourth cooling step, such as Test No. 50, were appropriate, and the examples in which the area ratio of bainite exceeded 14.0% achieved the target ultimate strain in lateral bending. This demonstrates that controlling the conditions of the fourth cooling step within the scope of the present invention is extremely important in order to suppress microcracking on the shear end surface and thereby improve the ultimate strain in lateral bending.

[0279] In addition, Figure 11A The tissue photograph (SEM photograph) of the comparative example, test number 6, in which the holding time is shorter than 0.33 seconds is shown in FIG. Figure 11B yes Figure 11A An enlarged view of area A is shown in FIG. Figure 11B In the portion indicated by the arrow, linear contrast, distinct from the grain boundary, is observed. This contrast is believed to be due to the interface between bainite and austenite after the tertiary cooling process. It is known that martensite transformation progresses at this austenite-ferrite interface and in the adjacent portion, resulting in a decrease in the coverage of martensite by bainite.

Claims

1. A hot-rolled steel plate, characterized in that: The chemical composition includes in mass %: C：0.035％～0.085％、 Si: 0.001% to 0.15%, Mn: 0.70% to 1.80%, P: 0.020% or less, S: 0.0050% or less, Ti: 0.075% to 0.170%, Nb: 0.003% to 0.050%, Al：0.10％～0.40％、 N: 0.0080% or less, Cr：0％～0.27％、 B：0％～0.0050％、 Ca: 0% to 0.0050%, Mo: 0% to 0.40%, Ni: 0% to 0.50%, Cu: 0% to 0.50%, and REM: 0%~0.0300%, The rest contains Fe and impurities. The metal structure at a depth of 1 / 4 of the plate thickness from the surface and in the center of the plate width includes: Ferrite with an area ratio of 53.0% to 76.0%, 3.0% to 10.0% of martensite in terms of area ratio, Bainite with an area ratio of 14.0% to 39.0%, Pearlite with an area ratio of 2.6% or less, and Less than 4.0% retained austenite, The average diameter of martensite is 0.26μm~0.70μm, Among all interfaces of the martensite, the total length of the interfaces between the martensite and the bainite accounts for 75.0% or more of the total length of all interfaces of the martensite. The total length of all interfaces of the martensite is the total length of the boundaries between the martensite and other metal structures adjacent thereto.

2. The hot-rolled steel sheet according to claim 1, wherein The chemical composition contains one or more of the following elements by mass: Cr：0.06％～0.27％、 B：0.0003％～0.0050％、 Ca: 0.0003% to 0.0050%, Mo: 0.01% to 0.40%, Ni: 0.01% to 0.50%, Cu: 0.01% to 0.50%, and REM: 0.0003% ~ 0.0300%.

3. A method for manufacturing a hot-rolled steel plate, characterized in that: Has the following processes: A hot rolling process in which a slab having the chemical composition according to claim 1 or 2 is rolled at a finishing temperature of 880° C. to 950° C.; After the hot rolling step, a primary cooling step is performed to cool the steel sheet to a primary cooling stop temperature of 680°C to 760°C at an average cooling rate of 60°C / s or higher; After the primary cooling step, a secondary cooling step is performed at an average cooling rate of 20°C / second or less for a period of 1.6 seconds to 6.3 seconds; After the secondary cooling step, a tertiary cooling step is performed at an average cooling rate of 60°C / s to 130°C / s to a tertiary cooling stop temperature of 195°C to 440°C; After the three cooling steps, 3 / min / mm 2 ~7.2m 3 / min / mm 2 The water density is 0.33 seconds to 1.50 seconds during the four cooling processes; After the four cooling steps, five cooling steps of air cooling are performed for a period of 3.0 seconds to 5.0 seconds; After the five cooling steps, a coiling step is performed at a temperature lower than 180°C.

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