Steel sheet for hot stamping, method for manufacturing the same, hot-stamped component, and method for manufacturing the same

Abstract

The hot stamping steel sheet has the following chemical composition: by mass%, it contains C: 0.060 to 0.120%, Si: 0 to 0.70%, Mn: 1.60 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, Al: 0.001 to 0.100%, Ti: 0.005 to 0.050%, B: 0.0005 to 0.0100%, and further contains one or more of Nb, V, W, Ni, Cu, C, Mo, Sn, Ca, Mg and REM as required, with the balance containing Fe and impurities, and the microstructure contains 70% or more of upper bainite by area ratio, and the number density of iron carbide with a major axis of 0.1 μm or more contained in the upper bainite is 4 per μm 2 or more.

Description

Technical Field

[0001] The present invention relates to a steel sheet for hot stamping, a method for manufacturing the same, a hot stamping member, and a method for manufacturing the same.

[0002] This application claims priority based on Japanese Patent Application No. 2020-174457 filed on October 16, 2020, and incorporates its content herein. Background Art

[0003] In recent years, from the viewpoint of restricting greenhouse gas emissions in response to global warming countermeasures, further improvement in fuel efficiency of automobiles has been required. Most of the structures of automobiles are formed of iron, particularly steel sheets. Therefore, by thinning the steel sheets, the weight is reduced, the vehicle body is lightened, and the fuel efficiency is improved. However, if the thickness of the steel sheet is simply thinned to reduce the weight of the steel sheet, there is a concern that the strength of the structure and the collision safety are reduced. Therefore, in order to thin the steel sheet, it is required to improve the mechanical strength of the steel sheet used in such a manner that the strength of the structure is not reduced. Thus, in recent years, in order to lighten the vehicle body and ensure collision safety, the application of high-strength steel sheets in automotive parts has been gradually expanding.

[0004] In addition, similarly, for automobiles, from the viewpoint of global warming countermeasures, the electrification of EVs (electric vehicles) is progressing. Compared with gasoline vehicles and the like, the vehicle weight of EVs generally increases significantly. Therefore, the requirement for the application of high-strength steel sheets is high. In addition, in recent years, collision regulations have also been strengthened. Therefore, for EVs, the application of ultra-high-strength steel (steel having a tensile strength of 980 MPa or more) in collision deformation members has been studied.

[0005] For example, in Patent Document 1, a method for manufacturing a high-strength steel sheet having an excellent strength-ductility balance and strength-hole expansion balance and having extremely excellent stretch flangeability is disclosed. Patent Document 1 also discloses that the tensile strength becomes 980 MPa or more.

[0006] However, in the case of EVs, in some vehicle models, due to the heavy vehicle weight, even if ultra-high-strength steel is used, from the viewpoints of rigidity and the like, a plate thickness of 3 mm or more may be required. Ultra-high-strength steel of such a thickness cannot be cold press formed.

[0007] Then, as a method for reducing the load during press forming, hot stamping in which hot forming and die quenching are performed simultaneously has attracted attention. In hot stamping, the material to be formed is temporarily heated to a high temperature, and after being press-formed by pressing the softened material by heating, it is cooled, or cooled simultaneously with the forming. That is, since the material is temporarily heated to a high temperature to be softened and press-formed in a softened state of the material, the material can be easily press-formed.

[0008] However, the cold-rolled steel sheet of Patent Document 1 does not consider subsequent hot stamping.

[0009] For example, Patent Document 2 describes the following: By hot stamping a steel sheet with a tensile strength of 500 to 600 MPa and a thickness of 1.0 to 1.8 mm, a member with a tensile strength of 1400 MPa or more is obtained.

[0010] However, in the case of forming a steel sheet with a relatively thick thickness by hot stamping as described above, in the case of conventional hot stamping such as Patent Document 2, the steel sheet is not sufficiently quenched, and sufficient strength cannot be obtained in the steel sheet (hot-stamped member) after hot stamping. In addition, even in the case of a thin steel sheet, when joining steel sheets with different thicknesses to form a TWB (tailor welded blank), it may be difficult to manage the clearance of the die at the portion where the thickness changes, and sufficient quenching may not be performed.

[0011] Due to such a situation, for the steel sheet to be hot-stamped, it is required to obtain sufficient strength even when the cooling rate during hot stamping is slow.

[0012] However, such research has not been conducted in the past, typified by Patent Document 2.

[0013] In addition, conventional hot-stamped members are mainly applied to non-deformable parts (members that ensure the safety of passengers by suppressing deformation), and their application to deformable parts has not been taken into consideration. That is, as described above, in the case of considering application to a collision-deformable member, a characteristic of suppressing cracking during large deformation is required, but for conventional hot-stamped members, ensuring such a characteristic has basically not been considered.

[0014] Prior Art Documents

[0015] Patent Documents

[0016] Patent Document 1: Japanese Patent No. 5407168

[0017] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-102980 Summary of the Invention

[0018] Problems to be Solved by the Invention

[0019] The present invention has been made in view of the above problems. The problems of the present invention are to provide a hot stamping steel sheet having excellent hardenability during hot stamping (even when the cooling rate during hot stamping is relatively low, high strength can be obtained after hot stamping) and excellent collision resistance characteristics after hot stamping (characteristics capable of suppressing cracking during large deformation), and a manufacturing method thereof. In addition, the problems of the present invention are to provide a hot stamping member obtained using the above hot stamping steel sheet as a raw material and a manufacturing method of a hot stamping member using the above hot stamping steel sheet.

[0020] Means for Solving the Problems

[0021] The inventors of the present invention conducted in-depth research in order to obtain a steel sheet (hot stamping steel sheet) having high strength and excellent collision resistance characteristics after hot stamping. As a result, it was found that by setting the microstructure of the steel sheet to mainly consist of upper bainite and having carbide of a specified size present therein, the hardenability of the steel sheet is improved, and by hot stamping this steel sheet, a hot stamping member having high strength and excellent collision resistance characteristics (excellent deformability, not easily broken during collision) can be obtained.

[0022] Based on the above findings, the present invention is made with the following as the main theme.

[0023] [1] A hot stamping steel sheet according to one aspect of the present invention has the following chemical composition: by mass%, it contains C: 0.060 to 0.120%, Si: 0 to 0.70%, Mn: 1.60 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, Al: 0.001 to 0.100%, Ti: 0.005 to 0.050%, B: 0.0005 to 0.0100%, Nb: 0 to 0.100%, V: 0 to 0.100%, W: 0 to 0.100%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Cr: 0 to 2.00%, Mo: 0 to 2.00%, Sn: 0 to 0.200%, Ca: 0 to 0.0500%, Mg: 0 to 0.0500%, and REM: 0 to 0.0500%, the balance contains Fe and impurities, and the microstructure contains 70% or more of upper bainite by area ratio, and the number density of iron-based carbides having a major axis of 0.1 μm or more contained in the above upper bainite is 4 pieces / μm 2 or more.

[0024] [2]The hot stamping steel sheet according to [1], wherein the above chemical composition may also contain, by mass%, one or more elements selected from the following elements: Nb: 0.005 to 0.100%, V: 0.005 to 0.100%, W: 0.005 to 0.100%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, Sn: 0.005 to 0.200%, Ca: 0.0003 to 0.0500%, Mg: 0.0003 to 0.0500%, and REM: 0.0003 to 0.0500%.

[0025] [3]The hot stamping steel sheet according to [1] or [2], the tensile strength of which may also be less than 980 MPa.

[0026] [4]The hot stamping steel sheet according to any one of [1] to [3] may also have a coating on the surface.

[0027] [5]The hot stamping steel sheet according to [4], wherein the above coating may also be a hot dip galvanized layer, an alloyed hot dip galvanized layer, an electrogalvanized layer, or an Al coating.

[0028] [6]The method for manufacturing a hot stamping steel sheet according to another aspect of the present invention includes the following steps: a heating step of directly heating a steel billet or a slab having the above chemical composition described in [1] to 1150 to 1300°C, or heating it to 1150 to 1300°C after being temporarily cooled; a hot rolling step of hot rolling the steel billet or the slab after the heating step so that the finished product temperature becomes 850°C or higher to produce a hot rolled steel sheet; a coiling step of coiling the hot rolled steel sheet after the hot rolling step at 640 to 450°C; a holding step of holding the hot rolled steel sheet after the coiling step in a temperature range of 500 to 450°C for 1.0 hour or more; and a cooling step of cooling the hot rolled steel sheet after the holding step to room temperature.

[0029] [7]The method for manufacturing a hot stamping steel sheet according to [6] may further include a cold rolling step of cold rolling the hot rolled steel sheet after the holding step with a cumulative reduction ratio of 30 to 70% to produce a cold rolled steel sheet.

[0030] [8] The manufacturing method of the hot stamping steel sheet according to another aspect of the present invention includes the following steps: a heating step of directly heating a steel billet or slab having the above chemical composition described in [1] to 1150 - 1300 °C, or heating it to 1150 - 1300 °C after temporary cooling; a hot rolling step of hot rolling the steel billet or slab after the above heating step to produce a hot rolled steel sheet in such a manner that the finished temperature is 850 °C or higher; a coiling step of coiling the hot rolled steel sheet after the above hot rolling step at 700 - 500 °C; a cooling step of cooling the hot rolled steel sheet after the above coiling step to room temperature; a pickling step of pickling the hot rolled steel sheet after the above cooling step; a cold rolling step of cold rolling the hot rolled steel sheet after the above pickling step with a cumulative reduction ratio of 30 - 70% to produce a cold rolled steel sheet; an annealing step of heating the cold rolled steel sheet to an annealing temperature range of 840 - 900 °C and holding it in the above annealing temperature range for 10 - 2000 seconds; and a heat treatment step of cooling the cold rolled steel sheet after the above annealing step to a temperature range of 400 - 600 °C, holding it in the above temperature range for 100 - 1000 seconds, and then cooling it to room temperature.

[0031] [9] According to the manufacturing method of the hot stamping steel sheet described in [8], it may further include a plating step of immersing the cold rolled steel sheet after the above heat treatment step in a plating bath to form a plating layer on the surface.

[0032]

[10] According to the manufacturing method of the hot stamping steel sheet described in [9], it may further include an alloying step of holding the cold rolled steel sheet after the above plating step in an alloying temperature range of 450 - 600 °C to alloy the above plating layer.

[0033]

[11] According to the manufacturing method of the hot stamping steel sheet described in [8], it may further include a plating step of immersing the cold rolled steel sheet after the above annealing step and before the above heat treatment step in a plating bath to form a plating layer on the surface.

[0034]

[12] According to the manufacturing method of the hot stamping steel sheet described in

[11] , it may further include an alloying step of holding the cold rolled steel sheet after the above plating step and before the above heat treatment step in an alloying temperature range of 450 - 600 °C to alloy the above plating layer.

[0035]

[13] Another embodiment of the hot stamping member of the present invention has the following chemical composition: by mass%, containing C: 0.060 - 0.120%, Si: 0 - 0.70%, Mn: 1.60 - 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, Al: 0.001 - 0.100%, Ti: 0.005 - 0.050%, B: 0.0005 - 0.0100%, Nb: 0 - 0.100%, V: 0 - 0.100%, W: 0 - 0.100%, Ni: 0 - 2.00%, Cu: 0 - 2.00%, Cr: 0 - 2.00%, Mo: 0 - 2.00%, Sn: 0 - 0.200%, Ca: 0 - 0.0500%, Mg: 0 - 0.0500% and REM: 0 - 0.0500%, the balance containing Fe and impurities, and the microstructure containing 90% or more tempered martensite by area ratio.

[0036]

[14] For the hot stamping member according to

[13] , wherein the above chemical composition may also contain, by mass%, one or more selected from the following elements: Nb: 0.005 - 0.100%, V: 0.005 - 0.100%, W: 0.005 - 0.100%, Ni: 0.01 - 2.00%, Cu: 0.01 - 2.00%, Cr: 0.01 - 2.00%, Mo: 0.01 - 2.00%, Sn: 0.005 - 0.200%, Ca: 0.0003 - 0.0500%, Mg: 0.0003 - 0.0500% and REM: 0.0003 - 0.0500%.

[0037]

[15] The manufacturing method of another embodiment of the hot stamping member of the present invention comprises the following hot stamping process: after heating the hot stamping steel sheet described in any one of [1] - [5] in a heating furnace with an atmosphere temperature of 850 - 950°C for 3 minutes or more, cooling it at a cooling rate of 10°C / second or more to below the martensite transformation start temperature.

[0038]

[16] For the manufacturing method of the hot stamping member according to

[15] , wherein the cooling rate in the above hot stamping process may also be 10 - 20°C / second.

[0039] Advantages of the Invention

[0040] According to the present invention, it is possible to provide a hot stamping steel sheet and its manufacturing method with excellent hardenability during hot stamping and excellent collision resistance characteristics after hot stamping, as well as a hot stamping member using the above hot stamping steel sheet and its manufacturing method. Detailed Embodiments

[0041] A hot stamping steel sheet according to an embodiment of the present invention (hereinafter referred to as the steel sheet of the present embodiment), a method for manufacturing the same, a hot stamping member obtained by hot stamping the steel sheet of the present embodiment (the hot stamping member of the present embodiment), and a method for manufacturing a hot stamping member using the steel sheet of the present embodiment will be described.

[0042] First, the steel sheet of the present embodiment will be described. The steel sheet of the present embodiment has a prescribed chemical composition, and the microstructure contains 70% or more of upper bainite in terms of area ratio. The number density of iron-based carbides having a major axis of 0.1 μm or more contained in the upper bainite is 4 per μm 2 or more.

[0043] [Microstructure]

[0044] [The microstructure contains 70% or more of upper bainite in terms of area ratio]

[0045] [The number density of iron-based carbides having a major axis of 0.1 μm or more contained in the upper bainite is 4 per μm 2 or more]

[0046] Generally, hot stamping steel sheets are designed to be easily processed during cutting or the like and to become high-strength after hot stamping. Therefore, the microstructure of a hot stamping steel sheet (before hot stamping) is mostly a structure containing a large amount of soft ferrite (for example, a structure of ferrite and pearlite). On the other hand, if a large amount of alloying elements are added to improve the hardenability during hot stamping heat treatment, problems such as an increase in the strength of the hot stamping steel sheet, poor cuttability, and difficulty in straightening with a leveling machine will occur. In addition, the addition of a large amount of alloying elements increases the hardenability of the steel sheet and lowers the Ms point. Therefore, the microstructure of the formed body after hot stamping becomes mainly primary martensite (martensite without carbides). In this case, the deformation ability during collision is poor.

[0047] In contrast, the inventors of the present invention have found that by setting the area ratio of upper bainite in the microstructure of the hot stamping steel sheet to 70% or more and setting the number density of iron-based carbides having a major axis of 0.1 μm or more contained in the upper bainite to 4 per μm 2 or more, even in the case of containing a large amount of alloying elements, it is possible to suppress the hardness of the hot stamping steel sheet in order to ensure workability represented by cutting, and to balance the hardenability that can obtain sufficient strength after hot stamping and the collision resistance characteristics after hot stamping.

[0048] In the microstructure, in addition to upper bainite, it may also contain ferrite, pearlite, martensite, etc. However, if the area ratio of upper bainite becomes lower than 70% and the total area ratio of ferrite and pearlite becomes more than 30%, excellent collision resistance characteristics (reduction of VDA bendability described later) cannot be obtained after hot stamping, and it is difficult to suppress fracture during collision. On the other hand, if the area ratio of upper bainite is lower than 70% and the area ratio of martensite becomes more than 30%, the strength of the hot stamping steel sheet becomes too high and the workability is poor. Therefore, the area ratio of upper bainite is set to 70% or more.

[0049] The number density of iron carbide with a major axis of 0.1 μm or more is set to 1 μm 2 The range of 4 or more is to promote the dissolution of carbides during hot stamping, improve hardenability, and improve bendability during collision by uniformly dispersing the carbides. The carbides contained in upper bainite are in a needle-like form, exist between the laths of upper bainite, and are elongated in one direction. In the present embodiment, the major axis is defined as the size of the iron carbide.

[0050] When the iron carbide with a major axis of 0.1 μm or more is 1 μm 2 In the case where the range is less than 4, if the chemical composition and microstructure of the steel sheet of the present embodiment (the area percentage of upper bainite is 70% or more) are taken as a premise, a large amount of iron carbide with a major axis of less than 0.1 μm will precipitate, or the iron carbide will become excessively coarsened. In the case where a large amount of iron carbide with a major axis of less than 0.1 μm precipitates, the strength of the steel sheet increases excessively due to precipitation strengthening, and the workability decreases. In addition, the oversized carbides do not dissolve during hot stamping, resulting in a decrease in hardenability, or become the starting point for crack formation during collision deformation. Therefore, the number density of iron carbide with a major axis of 0.1 μm or more is set to 1 μm 2 The range is 4 or more. The size of the carbide is preferably 0.1 to 0.5 μm. In addition, by setting the size of the carbide to 0.1 to 0.5 μm, it is also helpful to set the strength of the hot stamping steel sheet to 980 MPa or less. Furthermore, although the detailed mechanism is unknown, by setting the carbide within the above range, it is also helpful to increase the area ratio of tempered martensite in the hot stamping formed body (hot stamping component).

[0051] The area ratio of the microstructure and the number density of iron carbide in upper bainite can be obtained by the following method.

[0052] First, after cutting out the steel plate parallel to the rolling direction, grind it in such a way that the surface in the plate thickness direction becomes the observation surface, and etch it with a nitric acid ethanol reagent. After that, use SEM to observe at a magnification of 1000 - 30000 times the position that is 1 / 4 of the plate thickness from the surface in the plate thickness direction (as long as it is within the range of 1 / 8 - 3 / 8 of the plate thickness from the surface), whereby the identification of ferrite, upper bainite, lower bainite, pearlite, and martensite can be carried out.

[0053] During the identification, it can be judged from the following tissue morphologies: Ferrite is equiaxed grains without iron-based carbides; Pearlite is a lamellar structure of ferrite and cementite; Upper bainite refers to a structure in the form of laths and contains cementite and retained austenite between the laths; Lower bainite contains carbides within the laths. Calculate the area ratio of each tissue identified from the SEM observation image.

[0054] For martensite, there are both tempered martensite containing carbides within the laths and quenched martensite (primary martensite) without carbides, and they can be identified by observing with SEM and TEM and confirming the presence or absence of carbides. For example, observe a range of 30μm × 25μm (field of view) in 10 fields of view at a magnification of 3000 times, and take the average value as the area ratio.

[0055] The carbides in upper bainite can also be quantified by the above observation. However, since the carbides in upper bainite are of the order of 0.1μm and are fine, it is preferably observed with a high-resolution FE (field emission type; Field Emision type)-SEM. For example, observe a range of 10μm × 8μm (field of view) in 20 fields of view at a magnification of 10000 times, and take the average value as the number density.

[0056] Measure the position that is 1 / 4 of the plate thickness from the surface in the plate thickness direction because generally the tissue at this position represents the most representative tissue of the steel plate. However, in the steel plate of the present embodiment, since the tissue is substantially uniform in the plate thickness direction, the same tissue can be obtained even if the measurement is carried out at other positions.

[0057] The steel sheet of the present embodiment can make the microstructure of the component after hot stamping (hot-stamped component) contain 90% or more tempered martensite within a wide range where the cooling rate during hot stamping is 10°C / second or more by controlling the microstructure as described above and controlling the chemical composition as described later, and sufficient strength can be obtained. In particular, the hardenability in the range of 10 to 30°C / second of the cooling rate, which is a problem in conventional hot-stamping steel sheets, is greatly improved. Therefore, of course, high strength can be obtained after hot stamping when the thickness of the hot-stamping steel sheet is thin, and even when the thickness is relatively thick (for example, about 3 to 6 mm), high strength can be obtained after hot stamping.

[0058] <Chemical composition>

[0059] Next, the chemical composition of the steel sheet of the present embodiment will be described. Hereinafter, "%" regarding the chemical composition means "mass%".

[0060] C: 0.060 to 0.120%

[0061] C is the following element: It increases the strength of the steel sheet and contributes to the improvement of VDA bendability by controlling the martensite transformation start temperature during cooling in the hot stamping process. If the C content is less than 0.060%, a tensile strength (maximum tensile strength) of 980 MPa or more cannot be ensured after hot stamping. Therefore, the C content is set to 0.060% or more. The C content is preferably 0.070% or more.

[0062] On the other hand, if the C content exceeds 0.120%, the martensite transformation start temperature becomes too high, and a sufficient area ratio of tempered martensite cannot be obtained in the hot stamping process. In this case, fracture suppression during collision becomes insufficient in the steel sheet (hot-stamped component) after hot stamping. Therefore, the C content is set to 0.120% or less. The C content is preferably 0.110% or less.

[0063] Si: 0 to 0.70%

[0064] Si is an element that increases the Ae3 point and increases the heating temperature required to make tempered martensite the main phase after hot stamping. Therefore, if the Si content is excessive, productivity and economy deteriorate. Thus, the Si content is set to 0.70% or less.

[0065] On the other hand, the Si content can also be 0%, but Si is an element that increases the strength of the steel sheet. In addition, regarding non-coated steel sheets, Si is an element that improves the adhesion of the scale. Therefore, Si can also be contained. In the case of obtaining this effect, the Si content is preferably set to 0.05% or more.

[0066] Mn: 1.60 to 3.00%

[0067] Mn is an element that improves the hardenability of the steel sheet. In order to delay the ferrite phase transformation during the cooling process in hot stamping, the microstructure of the hot-stamped formed body is made mainly of tempered martensite at a cooling rate of 10°C / second or more. Therefore, the Mn content is set to 1.60% or more.

[0068] On the other hand, if the Mn content exceeds 3.00%, not only does the effect saturate, but the steel sheet becomes brittle, or cracking occurs during casting, cold rolling, or hot rolling. Therefore, the Mn content is set to 3.00% or less.

[0069] P: 0.100% or less

[0070] P is an element that segregates at the center of the steel sheet thickness and also makes the welded part brittle. When the P content exceeds 0.100%, the embrittlement of the welded part becomes significant. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less. There is no particular regulation for the lower limit of the P content, and it can also be 0%. Reducing the P content to less than 0.001% is economically disadvantageous. Therefore, the P content can also be set to 0.001% or more.

[0071] S: 0.0100% or less

[0072] S is an element that exists as inclusions such as MnS and deteriorates the collision resistance characteristics. Therefore, it is preferable to reduce the S content. When the S content exceeds 0.0100%, the deterioration of the collision resistance characteristics becomes significant. Therefore, the S content is set to 0.0100% or less. There is no particular regulation for the lower limit of the S content, and it can also be 0%. However, reducing the S content to less than 0.0001% is economically disadvantageous. Therefore, the S content can also be set to 0.0001% or more.

[0073] N: 0.0100% or less

[0074] N is an element that forms coarse nitrides and becomes the starting point of cracking during collision, deteriorating the collision resistance characteristics. If the N content exceeds 0.0100%, the collision resistance characteristics deteriorate significantly. Therefore, the N content is set to 0.0100% or less. There is no particular need to regulate the lower limit of the N content, and it can also be 0%. However, if the N content is reduced to less than 0.0001%, the manufacturing cost increases significantly. Therefore, the N content can also be set to 0.0001% or more or 0.0005% or more.

[0075] Al: 0.001 - 0.100%

[0076] Al is an element that functions as a deoxidizer. When the Al content is less than 0.001%, sufficient deoxidation effect cannot be obtained, resulting in a large amount of inclusions (oxides) in the steel plate. These inclusions become the starting points of cracks during collision and cause fracture, so they are not preferred. Therefore, the Al content is set to 0.001% or more.

[0077] On the other hand, if the Al content exceeds 0.100%, the Ae3 point increases, and the heating temperature during hot stamping needs to be increased, so it is not preferred. Therefore, the Al content is set to 0.100% or less.

[0078] Ti: 0.005 - 0.050%

[0079] Ti is an element that forms TiN by combining with N to inhibit B from becoming a nitride and improve hardenability. To obtain this effect, the Ti content is set to 0.005% or more. The Ti content is preferably 0.007% or more.

[0080] On the other hand, if the Ti content exceeds 0.050%, the amount of C that contributes to the strengthening of martensite decreases due to the formation of Ti carbide, and the strength of the steel plate (hot stamping component) after hot stamping cannot be obtained sufficiently. Therefore, the Ti content is set to 0.050% or less. The Ti content is preferably 0.040% or less.

[0081] B: 0.0005 - 0.0100%

[0082] B is an element effective for improving the hardenability of the steel plate and making the main phase of the microstructure of the steel plate (hot stamping component) after hot stamping into tempered martensite. This effect becomes significant when the content is 0.0005% or more, so the B content is set to 0.0005% or more.

[0083] On the other hand, if the B content exceeds 0.0100%, not only does its effect saturate, but iron-based borides precipitate, and the effect of improving hardenability brought by B decreases. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, more preferably 0.0050% or less.

[0084] In the steel plate of this embodiment, the remaining part of the above elements can also be Fe and impurities. Impurities refer to elements that may be mixed in during the manufacturing process of the steel plate, etc., and are elements that are tolerated within the range that does not cause obvious adverse effects on the steel plate of this embodiment. As impurities, for example, there are the above-mentioned P, S, O, etc. When O is contained, it mostly forms oxides and exists as inclusions.

[0085] On the other hand, the steel plate of this embodiment can also further contain the following elements as needed.

[0086] Ni: 0 to 2.00%

[0087] Cu: 0 to 2.00%

[0088] Cr: 0 to 2.00%

[0089] Mo: 0 to 2.00%

[0090] Ni, Cu, Cr, and Mo are the following elements: By increasing the hardenability of the steel sheet and making the main phase of the microstructure of the steel sheet after hot stamping into tempered martensite, it contributes to high strength. This effect becomes significant by containing one or more of Ni, Cu, Cr, and Mo at 0.01% or more. Therefore, in the case of obtaining the above effect, it is preferable to set the total or individual content of these elements to 0.01% or more.

[0091] On the other hand, if the amount of each element becomes excessive, the weldability, hot workability, etc. deteriorate, or the strength of the hot stamping steel sheet becomes too high, resulting in manufacturing failures. Therefore, when contained, the content of Ni, Cu, Cr, and Mo is set to 2.00% or less respectively.

[0092] Nb: 0 to 0.100%

[0093] V: 0 to 0.100%

[0094] W: 0 to 0.100%

[0095] Nb, V, and W are the following elements: During hot stamping, they suppress the growth of austenite and contribute to an increase in the strength and toughness of hot stamping components through grain refinement strengthening. This effect becomes significant by containing one or more of Nb, V, and W at 0.005% or more. Therefore, in the case of obtaining the above effect, it is preferable to set the total or individual content of these elements to 0.005% or more.

[0096] On the other hand, if the content of each element exceeds 0.100%, the amount of C that contributes to the strengthening of martensite is reduced through the formation of carbides of Nb, V, and W, resulting in a decrease in strength. Therefore, when contained, the content of Nb, V, and W is set to 0.100% or less respectively. Preferably, it is 0.090% or less.

[0097] REM: 0 to 0.0500%

[0098] Ca: 0 to 0.0500%

[0099] Mg: 0 to 0.0500%

[0100] REM, Ca, and Mg are elements that contribute to the improvement of the strength of the steel plate and the improvement of the material properties. If the total of one or more of REM and Ca is less than 0.0003%, sufficient effects cannot be obtained. Therefore, when the above effects are to be obtained, it is preferable to set the total content of REM, Ca, and Mg to 0.0003% or more.

[0101] On the other hand, if the total content of one or more of REM, Ca, and Mg exceeds 0.0500%, the castability and hot workability deteriorate. Therefore, when contained, the content of these elements is set to 0.0500% or less.

[0102] REM is an abbreviation for Rare Earth Metal, and refers to elements belonging to the Sc, Y, and lanthanide element series. As REM, La and Ce are mostly contained, and mostly added as a mixed rare earth alloy. However, in addition to La and Ce, elements of the lanthanide element series can also be contained in combination, and even when a metal is added, effects are exhibited.

[0103] Sn: 0 to 0.200%

[0104] Sn is an element that improves corrosion resistance. This effect becomes significant when the content is 0.005% or more. Therefore, when this effect is to be obtained, it is preferable to set the Sn content to 0.005% or more.

[0105] On the other hand, if the Sn content exceeds 0.200%, the slab becomes brittle, causing cracking during casting and cracking on the surface of the hot-rolled steel plate. Therefore, when contained, the Sn content is set to 0.200% or less.

[0106] The above chemical composition can be determined by general analytical methods. For example, it can be determined using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S can be determined using combustion-infrared absorption method, and N can be determined using inert gas fusion-thermal conductivity method.

[0107] The steel plate of the present embodiment may also have a coating on the surface. According to the coating, generation of scale during hot stamping can be prevented and corrosion resistance can be improved, so it is preferable.

[0108] The coating is not limited, and for example, it is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electrogalvanized layer, or an Al coating. They can be formed according to the use.

[0109] <Mechanical properties, plate thickness>

[0110] Tensile strength (TS): preferably less than 980 MPa

[0111] For the hot stamping steel sheet, it is preferable that it is easy to process during processing such as cutting, and becomes high-strength after hot stamping. Therefore, for example, if the tensile strength is 980 MPa or more, the loss of the blade of the shearing machine becomes severe, or it becomes difficult to correct the shape of the steel sheet. Therefore, one of the purposes of applying hot stamping, such as the ease of cutting and the ease of shape correction, disappears, so it is not preferable. Therefore, the tensile strength of the steel sheet of the present embodiment is preferably less than 980 MPa.

[0112] The tensile strength of the steel sheet is obtained by the following method: A tensile test piece in the shape of JIS No. 5 is collected from the steel sheet in such a way that the direction perpendicular to the rolling direction becomes the tensile direction, and a tensile test is carried out in accordance with JIS Z2241:2011.

[0113] The thickness of the steel sheet of the present embodiment is not limited. For example, it is 1.0 to 6.0 mm, but the thickness at which the effect of the steel sheet of the present embodiment becomes significant is 3.0 mm or more. However, even for thin steel sheets of 1.0 to 3.0 mm, stable hardness can be obtained even under the conditions of a large gap with the mold and a small cooling rate. Therefore, even when used for applications with a small thickness, the effect can be obtained.

[0114] The steel sheet of the present embodiment has excellent hardenability during hot stamping and excellent collision resistance characteristics after hot stamping. Therefore, for example, in the case of performing the following hot stamping, a tensile strength of 980 MPa or more and excellent collision resistance characteristics such that the VDA maximum bending angle becomes 80° or more in the case of performing the following VDA bending test can be obtained: After heating in a heating furnace at an atmosphere temperature of 850 to 950 °C for 3 minutes or more, it is cooled to below the martensite transformation start temperature at a wide range of cooling rates of 10 °C / second or more, including cases where the cooling rate is slow, such as 20 °C / second or less.

[0115] <Manufacturing method>

[0116] The steel sheet of the present embodiment can be manufactured by the manufacturing method described in <1> or <2> below.

[0117] <1> A manufacturing method, which includes the following steps:

[0118] A heating step of directly heating a steel billet or a slab having a prescribed chemical composition to 1150 to 1300 °C, or heating it to 1150 to 1300 °C after temporarily cooling;

[0119] A hot rolling step of hot rolling the steel billet or the slab after the above heating step so that the finished product temperature becomes 850 °C or more to form a hot rolled steel sheet;

[0120] A coiling step of coiling the hot-rolled steel sheet after the above hot-rolling step at 640 to 450°C;

[0121] A holding step of holding the hot-rolled steel sheet after the above coiling step in a temperature range of 500 to 450°C for 1.0 hour or more; and

[0122] A cooling step of cooling the hot-rolled steel sheet after the above holding step to room temperature.

[0123] <2>A manufacturing method, which includes the following steps:

[0124] A heating step of directly heating a steel billet or slab having a specified chemical composition to 1150 to 1300°C, or heating it to 1150 to 1300°C after temporary cooling;

[0125] A hot-rolling step of hot-rolling the steel billet or slab after the above heating step so that the finished product temperature is 850°C or higher to produce a hot-rolled steel sheet;

[0126] A coiling step of coiling the hot-rolled steel sheet after the above hot-rolling step at 700 to 500°C;

[0127] A cooling step of cooling the hot-rolled steel sheet after the above coiling step to room temperature;

[0128] A pickling step of pickling the hot-rolled steel sheet after the above cooling step;

[0129] A cold-rolling step of cold-rolling the hot-rolled steel sheet after the above pickling step at a cumulative reduction ratio of 30 to 70% to produce a cold-rolled steel sheet;

[0130] An annealing step of heating the above cold-rolled steel sheet to an annealing temperature range of 840 to 900°C and holding it in the above annealing temperature range for 10 to 2000 seconds; and

[0131] A heat treatment step of cooling the cold-rolled steel sheet after the above annealing step to a temperature range of 400 to 600°C, holding it in the above temperature range for 100 to 1000 seconds, and then cooling it to room temperature.

[0132] Hereinafter, the preferred conditions for each step will be described. The preferred conditions up to the hot-rolling step are common in the manufacturing methods of <1> and <2>, so they will be described together. On the other hand, since the preferred condition ranges of the coiling step migration are different, <1> and <2> will be described separately. Hereinafter, for the conditions not described, known conditions can be adopted.

[0133] [Heating step]<1><2>Common

[0134] In the heating process, a steel billet or slab having the same chemical composition as the steel plate of the present embodiment described above, which is manufactured by casting, is heated before hot rolling. As the slab to be subjected to hot rolling, a continuous casting slab, a slab manufactured by a thin slab caster, etc. can be used. In addition, the slab can be heated after the temperature is temporarily lowered to near room temperature after casting, or reheated during the temperature drop.

[0135] The heating temperature of the steel billet or slab is set to 1150 - 1300 °C. If the heating temperature is lower than 1150 °C, in the subsequent hot rolling process, it will cause a decrease in the finish rolling temperature and become a cause of an increase in the mill load. In this case, rolling becomes difficult, or it becomes a factor in the poor shape of the steel plate after rolling. On the other hand, if the heating temperature exceeds 1300 °C, it will not only result in high costs but also cause surface cracking, etc. In this case, the collision resistance characteristics of the steel plate (hot stamping member) after hot stamping deteriorate.

[0136] [Hot Rolling Process]<1><2>Common

[0137] In the hot rolling process, the heated steel billet or slab is rolled in such a way that the finish rolling temperature (the temperature on the exit side of the final pass of finish rolling) becomes 850 °C or higher to produce a hot rolled steel plate. If the finish rolling temperature becomes lower than 850 °C, the rolling load becomes high, rolling becomes difficult, or it causes a poor shape of the steel plate after rolling. The upper limit of the finish rolling temperature does not need to be particularly specified, but if the finish rolling temperature is excessively increased, in order to ensure its temperature, the heating temperature in the heating process must be excessively increased. Therefore, it is preferable to set the finish rolling temperature to 1000 °C or lower.

[0138] Hereinafter, the processes after the coiling process of the manufacturing method of <1> will be described.

[0139] [Coiling Process]<1>

[0140] In the coiling process, the hot rolled steel plate after the hot rolling process is coiled at 640 - 450 °C.

[0141] If the coiling temperature exceeds 640 °C, structures such as ferrite and pearlite are formed, and in the microstructure of the hot rolled steel plate, the upper bainite area ratio cannot be set to 70% or more. In addition, the carbides present in the upper bainite become coarse, and the number density of iron-based carbides cannot be sufficiently obtained. The coarse carbides are also difficult to dissolve during the heating in hot stamping, becoming a cause of a decrease in hardenability, and the undissolved remaining coarse carbides become the starting points of cracking, so they become a cause of a decrease in the collision resistance characteristics after hot stamping. Therefore, the coiling temperature is set to 640 °C or lower.

[0142] On the other hand, when the coiling temperature is lower than 450°C, the structure of the steel plate becomes mainly martensite, the strength becomes too high, and the workability decreases. Therefore, the coiling temperature is set to 450°C or higher.

[0143] [Holding process]<1>

[0144] In the holding process, the hot-rolled steel plate after the coiling process is held in the temperature range of 500 - 450°C for 1.0 hour or more. Through this holding, the area ratio of upper bainite in the microstructure is set to 70% or more, and in this upper bainite, 4 or more iron-based carbides with a major axis of 0.1 μm or more precipitate in the range of 1 μm 2 range.

[0145] When the holding temperature exceeds 500°C, the carbides in the upper bainite coarsen, and the number density of iron-based carbides with a major axis of 0.1 μm or more cannot be set to 4 or more in the range of 1 μm 2 range.

[0146] On the other hand, when the holding temperature is lower than 450°C, a large amount of carbides with a major axis lower than 0.1 μm precipitate, and the strength of the steel plate increases excessively due to strengthening, and the number density of iron-based carbides with a major axis of 0.1 μm or more cannot be set to 4 or more in the range of 1 μm 2 range.

[0147] In addition, even if the holding time in the temperature range of 500 - 450°C is less than 1.0 hour, the number density of iron-based carbides with a major axis of 0.1 μm or more cannot be set to 4 or more in the range of 1 μm 2 range.

[0148] The holding does not necessarily have to be at a constant temperature. As long as it is in the temperature range of 500 - 450°C, there can also be temperature variations. In addition, when placed after normal coiling, it is difficult to hold for 1.0 hour or more at 500 - 450°C. Therefore, it is sufficient to hold for 1.0 hour or more by performing heat preservation or heating.

[0149] On the premise of the manufacturing method of <1>, it is difficult to substitute the holding process with a BAF (box-type annealing furnace). Since the steel plate of this embodiment has high hardenability, if it is cooled without holding, it is likely to become mainly martensite. Therefore, even if it is cooled temporarily and then heat-treated using a BAF or the like in subsequent processes, it becomes mainly tempered martensite, and it is difficult to set the area ratio of upper bainite to 70% or more. In addition, there may be precipitation of coarse carbides through the BAF, and the hardenability during hot stamping is poor.

[0150] [Cooling process]<1>

[0151] In the cooling process, the hot-rolled steel sheet after the holding process is cooled to room temperature. The cooling conditions are not particularly limited.

[0152] [Pickling process]<1>

[0153] After the cooling process, pickling may be performed to remove the scale formed on the surface of the steel sheet. Any method can be used as long as it can remove the scale, and hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, or a mixture thereof can also be used. In addition, the dissolution of the base metal can be inhibited by adding an inhibitor. In addition, it can be carried out once or in multiple steps.

[0154] [Cold rolling process]<1>

[0155] The hot-rolled steel sheet after the pickling process can also be further cold-rolled to produce a cold-rolled steel sheet. In the case of cold rolling, if the cumulative reduction ratio is less than 30%, it becomes difficult to keep the shape of the steel sheet flat, and in addition, the ductility of the final product deteriorates. On the other hand, if the cumulative reduction ratio exceeds 70%, the rolling load becomes too large and cold rolling becomes difficult. Therefore, it is preferable to set the cumulative reduction ratio to 30 - 70%. The cumulative reduction ratio is more preferably 40 - 70%.

[0156] The number of rolling passes and the reduction ratio per pass may not be particularly specified.

[0157] Hereinafter, the process after the coiling process of the manufacturing method of <2> will be described.

[0158] [Coiling process]<2>

[0159] In the coiling process, the hot-rolled steel sheet after the hot rolling process is coiled at 700 - 500°C.

[0160] If the coiling temperature exceeds 700°C, the structure of the steel sheet becomes ferrite and pearlite structures. Therefore, the coiling temperature is set to 700°C or lower.

[0161] On the other hand, when the coiling temperature is lower than 500°C, the structure of the steel sheet becomes a structure mainly composed of martensite, the strength becomes too high, and the workability decreases. Therefore, the coiling temperature is set to 500°C or higher.

[0162] [Cooling process]<2>

[0163] In the cooling process, the hot-rolled steel sheet after the coiling process is cooled to room temperature. The cooling conditions are not particularly limited.

[0164] [Pickling process]<2>

[0165] In the pickling process, the scale formed on the surface of the steel sheet after the cooling process is removed. The pickling can be any method as long as it can remove the scale, and hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, or their mixture can also be used. In addition, the dissolution of the base metal can be inhibited by adding an inhibitor. In addition, the pickling can be carried out once or in multiple steps.

[0166] [Cold rolling process]<2>

[0167] The hot-rolled steel sheet after the pickling process is cold-rolled to produce a cold-rolled steel sheet. In the case of cold rolling, if the cumulative reduction ratio is less than 30%, it becomes difficult to keep the shape of the steel sheet flat, and in addition, the ductility of the final product deteriorates. On the other hand, if the cumulative reduction ratio exceeds 70%, the rolling load becomes too large and cold rolling becomes difficult. Therefore, the cumulative reduction ratio is set to 30 - 70%. The cumulative reduction ratio is preferably 40 - 70%. The number of rolling passes and the reduction ratio per pass may not be particularly specified.

[0168] [Annealing process]<2>

[0169] In the annealing process, the cold-rolled steel sheet obtained by cold rolling is heated to an annealing temperature of 840 - 900 °C and held at this annealing temperature for 10 - 2000 seconds.

[0170] If the annealing temperature is lower than 840 °C, a single-phase austenite cannot be formed during annealing, and in the subsequent heat treatment, the upper bainite area ratio cannot be set to 70% or more. On the other hand, when the annealing temperature exceeds 950 °C, not only does its effect saturate and the economy deteriorate, but the bainite phase transformation is delayed due to the coarsening of the austenite grain size. Therefore, 70% or more of the upper bainite cannot be ensured by holding for 100 - 1000 seconds in the temperature range of 400 - 600 °C in the subsequent process.

[0171] In addition, when the annealing time (holding time) is less than 10 seconds, the carbides formed during hot rolling cannot be dissolved, a single-phase austenite cannot be formed during annealing, and the upper bainite area ratio cannot be set to 70% or more in the subsequent heat treatment. On the other hand, when the annealing time exceeds 2000 seconds, the austenite coarsens and the bainite phase transformation is delayed. Therefore, 70% or more of the upper bainite cannot be ensured by holding for 100 - 1000 seconds in the temperature range of 400 - 600 °C in the subsequent process.

[0172] [Heat treatment process]<2>

[0173] In the heat treatment process, the cold-rolled steel sheet after the annealing process (after holding for a specified time) is cooled to the temperature range of 400 - 600 °C, held in this temperature range for 100 - 1000 seconds, and then cooled to room temperature.

[0174] Through this heat treatment, the area ratio of upper bainite in the microstructure is set to 70% or more, and in the upper bainite, four or more iron-based carbides with a major axis of 0.1 μm or more precipitate in the range of 1 μm 2 range.

[0175] If the holding temperature exceeds 600 °C, ferrite and pearlite are formed, and the area ratio of upper bainite cannot be set to 70% or more. On the other hand, when the holding temperature is lower than 400 °C, the structure is mainly martensite.

[0176] In addition, if the holding time is less than 100 seconds, the area ratio of upper bainite cannot be obtained sufficiently, and martensite is formed during the subsequent cooling. Therefore, in the cooled steel plate, the area ratio of upper bainite cannot be set to 70% or more. On the other hand, if the holding time exceeds 1000 seconds, due to the coarsening of carbides, the number density of iron-based carbides with a major axis of 0.1 μm or more becomes less than 4 in the range of 1 μm 2 range.

[0177] After holding, it is cooled to room temperature. The cooling conditions are not particularly limited as long as the holding time at 400 - 600 °C does not exceed 1000 seconds.

[0178] [Plating process]<2>

[0179] When forming a plating layer on the surface of the steel plate, the cold-rolled steel plate after the annealing process and before the heat treatment process, or after the heat treatment process, can also be immersed in the plating bath. The bath temperature of the plating bath is usually 400 - 600 °C even in the case of galvanizing or plating containing other elements. By immersing in the plating bath, a plating layer is formed on the surface and upper bainite is formed in the steel plate. However, for a general plating device, the immersion time does not exceed 100 seconds. Therefore, even if the plating process is performed, the heat treatment process cannot be omitted.

[0180] When performing the plating process, the upper limit of the holding time of the heat treatment process is preferably set to the time obtained by subtracting the immersion time in the plating bath from 1000 seconds.

[0181] [Alloying process]<2>

[0182] The cold-rolled steel plate with a plating layer formed on the surface after the plating process (when the plating process is performed before the heat treatment process, it is after the plating process and before the heat treatment process) can also be held in the temperature range of 450 - 600 °C (alloying temperature range) to alloy the plating layer.

[0183] In the case of performing the plating process and the alloying process, the upper limit of the holding time in the heat treatment process is preferably set to the time obtained by subtracting the immersion time in the plating bath and the time held at 450 to 600 °C for alloying from 1000 seconds.

[0184] Next, the hot stamping member of the present embodiment will be described.

[0185] <Chemical composition>

[0186] The hot stamping member of the present embodiment is obtained by hot stamping the above-described steel sheet (hot stamping steel sheet) of the present embodiment. Since the chemical composition does not substantially change by hot stamping, the range of the chemical composition of the hot stamping member of the present embodiment is the same as that of the steel sheet of the present embodiment described above. Therefore, the description is omitted here.

[0187] <Microstructure>

[0188] [Tempered martensite: 90% or more by area]

[0189] The tempered martensite is set to 90% or more in order to balance the tensile strength and bendability. If the area ratio of ferrite, upper bainite, etc. exceeds 10%, the strength of 980 MPa or more cannot be ensured, or if the primary martensite exceeds 10%, the angle at the time of crack generation in the VDA test is lower than 80°, so the collision resistance characteristics are poor. Therefore, the tempered martensite is set to 90% or more.

[0190] Tempered martensite refers to martensite containing carbides in the martensite.

[0191] In the present embodiment, regarding ensuring the tempered martensite in the hot stamping process, instead of performing a special heat treatment, it is achieved by controlling the chemical composition of the hot stamping steel sheet and the microstructure of the hot stamping steel sheet. That is, by limiting the C content to 0.060 to 0.120% to increase the martensite transformation start temperature, and by making the microstructure of the hot stamping steel sheet mainly composed of upper bainite, martensite is formed during cooling in the hot stamping process, and tempered martensite is formed by precipitating carbides in the martensite.

[0192] As the microstructure other than the tempered martensite, as long as it is less than 10% by area ratio, it may also contain ferrite, pearlite, upper bainite, lower bainite, primary martensite (martensite without carbides).

[0193] The area ratio of each phase of the microstructure can be obtained by the following method.

[0194] First, after cutting out the hot-stamped member (formed body) in a direction parallel to the rolling direction, it is polished and etched with a nitric acid ethanol reagent. Then, using SEM, an observation is made at a magnification of 1000 to 30000 times for a range of 8000 μm 2 or more, at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction, whereby identification of ferrite, upper bainite, lower bainite, pearlite, tempered martensite, and primary martensite can be performed. Regarding the microstructure of the hot-stamped member of the present embodiment, as long as it is at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction, the same microstructure is obtained regardless of whether it is in a direction parallel to the rolling direction of the steel plate or in a direction perpendicular to the rolling direction of the steel plate.

[0195] At the time of identification, similar to the case of the hot-stamping steel sheet, judgment can be made based on the following tissue morphologies: ferrite is equiaxed grains without iron-based carbides; pearlite is a layered structure of ferrite and cementite; upper bainite refers to a structure in the form of laths and is a structure containing cementite and retained austenite between the laths; lower bainite contains carbides within the laths. The area ratio of each tissue identified from the SEM observation image is obtained.

[0196] For martensite, there are both tempered martensite containing carbides within the laths and martensite in the quenched state (primary martensite) without carbides, and they can be identified by observing with SEM and TEM and confirming the presence or absence of carbides. For example, a range of 30 μm × 25 μm (field of view) of 10 fields of view is observed at a magnification of 3000 times, and the average value thereof is used as the area ratio.

[0197] <Characteristic>

[0198] The hot-stamped member of the present embodiment has high strength and excellent collision resistance characteristics. In the present embodiment, the excellent collision resistance characteristics mean that it is not easily broken even when deformed during a collision, and it means that the VDA maximum bending angle becomes 80° or more in the method described in VDA238-100. In addition, the high strength means that the tensile strength is 980 MPa or more.

[0199] The VDA maximum bending angle (bendability) is measured according to the method described in VDA238-100. Specifically, after reducing the thickness of the steel plate from one side to 1.2 mm, the VDA bending test is performed in such a way that the hot-stamped surface that has not been thickness-reduced becomes the outer side of the bend.

[0200] In VDA238-100, the test piece size is 60 mm in width × 60 mm in length. However, depending on the component, the plane is limited, and it is difficult to collect test pieces of the above dimensions. Therefore, in order to measure the bendability of the formed body, for example, the bendability can also be evaluated by changing the width, such as changing the test piece size to 30 mm in width × 60 mm in length.

[0201] <Manufacturing method of component>

[0202] Next, the manufacturing method of the hot stamping component of the present embodiment will be described.

[0203] The manufacturing method of the hot stamping component of the present embodiment includes the following hot stamping process: After heating the steel sheet (hot stamping steel sheet) of the present embodiment obtained by the above method in a heating furnace at an atmosphere temperature of 850 to 950°C for 3 minutes or more, it is cooled at a cooling rate of 10°C / second or more to below the martensite transformation start temperature. Here, the cooling rate is a value obtained by dividing the difference between the temperature taken out from the furnace and the mold cooling end temperature by the time from taking out from the furnace to the mold cooling end. The mold take-out temperature (mold cooling end temperature) can obtain the effects of the present invention without particular limitation, but when the mold take-out temperature exceeds 200°C, safety issues such as burns will occur, so it is preferably set to 200°C or less.

[0204] According to this manufacturing method, a hot stamping component with a tempered martensite area ratio of 90% or more, a tensile strength of 980 MPa or more, and excellent collision resistance can be obtained.

[0205] If the atmosphere temperature is lower than 850°C and the heating time is less than 3 minutes, the austenite phase transformation will not occur sufficiently, and in the hot stamping component, the area ratio of tempered martensite will not reach 90% or more, and sufficient strength cannot be obtained.

[0206] In addition, if the atmosphere temperature exceeds 950°C, not only will its effect saturate, but the austenite grains will be overly coarsened, resulting in a decrease in TS, so it is not preferred.

[0207] Examples

[0208] (Example 1)

[0209] A slab having the chemical composition shown in Table 1 was cast, and after heating the slab to 1150 to 1300°C, hot rolling was performed so as to achieve the finish rolling temperature shown in Table 2A, obtaining a hot rolled steel sheet (HR) with a thickness of 4.0 mm.

[0210] After that, coiling was performed at the coiling temperature shown in Table 2A, and holding was performed at the time shown in Table 2A and at 500 to 450°C.

[0211] Thereafter, it is cooled to room temperature by air cooling.

[0212] In addition, thereafter, for a part of the steel plates, pickling is performed to remove the scale formed on the surface, and cold rolling is performed at a cumulative reduction ratio of 50% to produce a cold-rolled steel plate (FH) with a thickness of 2.0 mm.

[0213] For the obtained steel plate (hot-rolled steel plate or cold-rolled steel plate), by the above method, the microstructure at a position 1 / 4 of the plate thickness from the surface is observed, and the upper bainite area ratio and the number density of iron-based carbides with a major axis of 0.1 μm or more are determined.

[0214] The results are shown in Table 2A and Table 2B.

[0215] In Table 2A and Table 2B, A to N and a to m of the first character of the steel number respectively represent steels with the chemical compositions of A to N and a to m in Table 1.

[0216] In addition, the tensile strength of the obtained steel plate is determined by the above method.

[0217] In addition, a sample of 350 mm × 650 mm is collected from the obtained steel plate, and this sample is used to simulate hot stamping with a die in the shape of a hat-shaped member. After heating in a furnace at a temperature of 910°C for 6 minutes, cooling is immediately performed using the die at the cooling rate shown in Table 2B. The value obtained by dividing the difference between the temperature at which it is taken out from the furnace and the temperature at which it is taken out from the die by the time from when it is taken out from the furnace until the die cooling is completed is used as the cooling rate.

[0218] For the sample (hot-stamped member) after die cooling, the microstructure at a position 1 / 4 of the plate thickness from the surface is observed by the above method.

[0219] In addition, a tensile test piece in the shape of JIS No. 5 is collected from the sample (hot-stamped member (formed body)) after die cooling, and a tensile test is performed in accordance with JIS Z 2241:2011 to determine the tensile strength. If the tensile strength (TS) is 980 MPa or more, it is judged that the hardenability of the steel plate is sufficient.

[0220] The results are shown in Table 2B.

[0221] In addition, a bending test piece in the shape of 50 × 50 mm is collected from the sample (formed body) after die cooling. At this time, grinding is performed unidirectionally until the plate thickness becomes 1.2 mm. Thereafter, in accordance with the manner in which the ground surface is on the punch side, a VDA bending test is performed in accordance with VDA238-100. If the VDA maximum bending angle is 80° or more, it is judged that the collision resistance characteristics are excellent (cracking can be suppressed during large deformation).

[0222] The results are shown in Table 2B.

[0223] [Table 1]

[0224]

[0225] The underlined part indicates outside the scope of the present invention.

[0226] "-" indicates that each element is not added.

[0227] [Table 2A]

[0228]

[0229] *1 indicates HR: hot-rolled steel sheet, FH: cold-rolled state, CR: cold-rolled steel sheet annealed after cold rolling, GI: hot-dip galvanized steel sheet, GA: alloyed hot-dip galvanized steel sheet.

[0230] [Table 2B]

[0231]

[0232] As can be seen from Tables 1, 2A to 2B, in the examples of the present invention, the tensile strength (TS) after hot stamping is 980 MPa or more, and the maximum VDA bending angle is 80° or more.

[0233] In contrast, for the comparative examples, at least one of the tensile strength (TS) and the maximum VDA bending angle after hot stamping is inferior.

[0234] (Example 2)

[0235] The slab having the chemical composition described in Table 1 was heated to 1180 - 1250 °C, and then hot-rolled so that the finished temperature became 880 - 960 °C to obtain a hot-rolled steel sheet with a thickness of 4.0 mm. Then, it was coiled at a coiling temperature of 700 - 500 °C, cooled to room temperature by air cooling, and pickled to remove the scale formed on the surface. In addition, this hot-rolled steel sheet was cold-rolled at a cumulative reduction rate of 50% to produce a cold-rolled steel sheet with a thickness of 2.0 mm.

[0236] Furthermore, for this cold-rolled steel sheet, annealing and heat treatment were performed under the conditions shown in Table 3A to obtain a cold-rolled steel sheet (CR).

[0237] For the obtained steel sheet, in the same manner as in Example 1, the microstructure was observed, and the area ratio of upper bainite and the number density of iron-based carbides with a major axis of 0.1 μm or more were determined. In addition, a tensile test was performed to determine the tensile strength.

[0238] The results are shown in Tables 3A and 3B.

[0239] In addition, a sample of 350×650 mm was collected from the obtained steel sheet. The sample was simulated for hot stamping using a die in the shape of a cap-shaped member, heated in a furnace at a temperature of 910°C for 4 minutes, and then immediately cooled using the die. The cooling rate was as shown in Table 3C. Regarding the cooling rate, the value obtained by dividing the difference between the temperature taken out from the furnace and the temperature taken out from the die by the time from taking out from the furnace to the end of die cooling was defined as the cooling rate.

[0240] From the sample (formed body) after the die cooling was completed, the microstructure was observed in the same manner as in Example 1. In addition, a tensile test was conducted in the same manner as in Example 1, and the tensile strength was obtained. If the tensile strength (TS) was 980 MPa or more, it was judged that the hardenability of the steel sheet was sufficient.

[0241] The results are shown in Table 3B.

[0242] In addition, from the sample (formed body) after the die cooling was completed, a VDA bending test was conducted in the same manner as in Example 1. If the maximum VDA bending angle was 80° or more, it was judged that the collision resistance characteristics were excellent (cracking could be suppressed during large deformation).

[0243] The results are shown in Table 3B.

[0244] In Tables 3A to 3B, A to N and a to m for the first character of the steel number respectively indicate steels having the chemical compositions of A to N and a to m in Table 1.

[0245] [Table 3A]

[0246]

[0247] *1 means HR: hot-rolled steel sheet, FH: cold-rolled state, CR: cold-rolled steel sheet annealed after cold rolling, GI: hot-dip galvanized steel sheet, GA: alloyed hot-dip galvanized steel sheet.

[0248] [Table 3B]

[0249]

[0250] As can be seen from Tables 1, 3A to 3B, in the examples of the present invention, the tensile strength (TS) after hot stamping was 980 MPa or more, and the maximum VDA bending angle was 80° or more.

[0251] On the other hand, for the comparative examples, at least one of the tensile strength (TS) and the maximum VDA bending angle after hot stamping was inferior.

[0252] (Example 3)

[0253] The slab having the chemical composition shown in Table 1 is heated to 1180 - 1250 °C, and then hot-rolled in such a way that the finish temperature becomes 880 - 960 °C to obtain a hot-rolled steel sheet with a thickness of 4.0 mm. Then, it is coiled at a coiling temperature of 680 - 500 °C, cooled to room temperature by air cooling, and pickled to remove the scale formed on the surface. In addition, this hot-rolled steel sheet is cold-rolled with a cumulative reduction ratio of 50% to produce a cold-rolled steel sheet with a thickness of 2.0 mm.

[0254] Furthermore, this cold-rolled steel sheet is annealed and heat-treated under the conditions shown in Table 4A to obtain a cold-rolled steel sheet (CR).

[0255] After that, using a hot-dip galvanizing facility, an annealing process, a heat treatment process, a plating process, and an alloying process as required are carried out. The plating process and the alloying process are carried out at a certain timing after the annealing process and before the heat treatment process, or after the heat treatment process.

[0256] Thus, a hot-dip galvanized steel sheet (GI) or an alloyed hot-dip galvanized steel sheet (GA) is obtained.

[0257] For the obtained steel sheet, the microstructure is observed in the same manner as in Example 1, and the area ratio of upper bainite and the number density of iron-based carbides with a major axis of 0.1 μm or more are determined. In addition, a tensile test is carried out to determine the tensile strength.

[0258] The results are shown in Table 4B and Table 4C.

[0259] In addition, a sample of 350×650 mm is collected from the obtained steel sheet, and the sample is simulated for hot stamping. After heating in a furnace at a temperature of 910 °C for 4 minutes, cooling is immediately carried out using a die. The cooling rate is as shown in Table 4C. Regarding the cooling rate, the value obtained by dividing the difference between the temperature taken out from the furnace and the temperature taken out from the die by the time from taking out from the furnace to the end of die cooling is used as the cooling rate.

[0260] From the sample (formed body) after die cooling, the microstructure is observed in the same manner as in Example 1. In addition, a tensile test is carried out in the same manner as in Example 1 to determine the tensile strength. If the tensile strength (TS) is 980 MPa or more, it is judged that the hardenability of the steel sheet is sufficient.

[0261] The results are shown in Table 4C.

[0262] In addition, from the sample (formed body) after die cooling, a VDA bending test is carried out in the same manner as in Example 1. If the VDA maximum bending angle is 80° or more, it is judged that the collision resistance characteristics are excellent (cracking can be suppressed during large deformation).

[0263] The results are shown in Table 4C.

[0264] In Tables 4A to 4C, A to N and a to m of the first character of the steel number respectively represent steels having the chemical compositions of A to N and a to m in Table 1.

[0265] [Table 4A]

[0266]

[0267] *1 means HR: hot-rolled steel sheet, FH: cold-rolled state, CR: cold-rolled steel sheet annealed after cold rolling, GI: hot-dip galvanized steel sheet, GA: alloyed hot-dip galvanized steel sheet.

[0268] *2 means that holding is not performed at 400 to 600°C.

[0269] [Table 4B]

[0270]

[0271] *1 means HR: hot-rolled steel sheet, FH: cold-rolled state, CR: cold-rolled steel sheet annealed after cold rolling, GI: hot-dip galvanized steel sheet, GA: alloyed hot-dip galvanized steel sheet.

[0272] *2 means that holding is not performed at 400 to 600°C.

[0273] [Table 4C]

[0274]

[0275] As can be seen from Table 1 and Tables 4A to 4C, in the examples of the present invention, the tensile strength (TS) after hot stamping is 980 MPa or more, and the maximum VDA bending angle is 80° or more.

[0276] In contrast, in the comparative examples, at least one of the tensile strength (TS) and the maximum VDA bending angle after hot stamping is inferior.

Claims

1. A steel sheet for hot stamping, having the following chemical composition by mass%: C：0.060～0.120％、 Si: 0 - 0.70%, Mn: 1.60 - 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, Al：0.001～0.100％、 Ti: 0.005 - 0.050%, B：0.0005～0.0100％、 Nb: 0 - 0.100%, V：0～0.100％、 W：0～0.100％、 Ni: 0 - 2.00%, Cu: 0 - 2.00%, Cr：0～2.00％、 Mo: 0 - 2.00%, Sn: 0 - 0.200%, Ca: 0 - 0.0500%, Mg: 0 - 0.0500%, and REM: 0 - 0.0500%, the balance being Fe and impurities, the microstructure contains 70% or more of upper bainite by area ratio, The number density of iron carbide with a major axis of 0.1 μm or more contained in the upper bainite is 4 per μm 2 or more the upper bainite is a lath-shaped structure and contains cementite between the laths.

2. The hot stamping steel sheet according to claim 1, wherein, The chemical composition contains one or more selected from the following elements by mass%: Nb: 0.005 - 0.100%, V：0.005～0.100％、 W：0.005～0.100％、 Ni: 0.01 - 2.00%, Cu: 0.01 - 2.00%, Cr：0.01～2.00％、 Mo: 0.01 - 2.00%, Sn: 0.005 - 0.200%, Ca: 0.0003 - 0.0500%, Mg: 0.0003 - 0.0500%, and REM: 0.0003 - 0.0500%.

3. The steel sheet for hot stamping according to claim 1 or 2, having a tensile strength lower than 980 MPa.

4. The steel sheet for hot stamping according to claim 1 or 2, having a coating on the surface.

5. The steel sheet for hot stamping according to claim 3, having a coating on the surface.

6. The hot stamping steel sheet according to claim 4, wherein, The coating is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electrogalvanized layer or an Al coating.

7. The hot stamping steel sheet according to claim 5, wherein, The coating is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electrogalvanized layer or an Al coating.

8. A method for manufacturing a steel sheet for hot stamping, comprising the following steps: a heating step of directly heating a steel billet or slab having the chemical composition according to claim 1 to 1150 - 1300 °C, or heating it to 1150 - 1300 °C after temporary cooling; a hot rolling step of hot rolling the steel billet or slab after the heating step to form a hot-rolled steel sheet such that the final temperature is 850 °C or higher; a coiling step of coiling the hot-rolled steel sheet after the hot rolling step at 640 - 450 °C; a holding step of holding the hot-rolled steel sheet after the coiling step in the temperature range of 500 - 450 °C for 1.0 hour or more; and a cooling step of cooling the hot-rolled steel sheet after the holding step to room temperature.

9. The method for manufacturing a steel sheet for hot stamping according to claim 8, further comprising a cold rolling step of cold rolling the hot-rolled steel sheet after the holding step at a cumulative reduction ratio of 30 - 70% to form a cold-rolled steel sheet.

10. The method for manufacturing a steel sheet for hot stamping according to claim 1, comprising the following steps: a heating step of directly heating a steel billet or slab having the chemical composition according to claim 1 to 1150 - 1300 °C, or heating it to 1150 - 1300 °C after temporary cooling; A hot rolling process of hot rolling the steel billet or slab after the heating process in such a way that the finished product temperature becomes 850 °C or higher to produce a hot rolled steel sheet; A coiling process of coiling the hot rolled steel sheet after the hot rolling process at 700 - 500 °C; A cooling process of cooling the hot rolled steel sheet after the coiling process to room temperature; A pickling process of pickling the hot rolled steel sheet after the cooling process; A cold rolling process of cold rolling the hot rolled steel sheet after the pickling process at a cumulative reduction rate of 30 - 70% to produce a cold rolled steel sheet; An annealing process of heating the cold rolled steel sheet to an annealing temperature range of 850 - 900 °C and holding it for 10 - 2000 seconds in the annealing temperature range to ensure upper bainite of 70% or more by area ratio; and A heat treatment process of cooling the cold rolled steel sheet after the annealing process to a temperature range of 400 - 600 °C, holding it for 100 - 1000 seconds in the temperature range, and then cooling it to room temperature.

11. The method for manufacturing a hot stamping steel sheet according to claim 10, further comprising a plating process of immersing the cold rolled steel sheet after the heat treatment process in a plating bath to form a plating layer on the surface.

12. The method for manufacturing a hot stamping steel sheet according to claim 11, further comprising an alloying process of holding the cold rolled steel sheet after the plating process in an alloying temperature range of 450 - 600 °C to alloy the plating layer.

13. The method for manufacturing a hot stamping steel sheet according to claim 10, further comprising a plating process of immersing the cold rolled steel sheet after the annealing process and before the heat treatment process in a plating bath to form a plating layer on the surface.

14. The method for manufacturing a hot stamping steel sheet according to claim 13, further comprising an alloying process of holding the cold rolled steel sheet after the plating process and before the heat treatment process in an alloying temperature range of 450 - 600 °C to alloy the plating layer.

15. A hot stamping component, characterized in that, Having the following chemical composition: by mass%: C：0.060～0.120％、 Si: 0 - 0.70%, Mn: 1.60 - 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, Al：0.001～0.100％、 Ti: 0.005 - 0.050%, B：0.0005～0.0100％、 Nb: 0 - 0.100%, V：0～0.100％、 W：0～0.100％、 Ni: 0 - 2.00%, Cu: 0 - 2.00%, Cr：0～2.00％、 Mo: 0 - 2.00%, Sn: 0 - 0.200%, Ca: 0 - 0.0500%, Mg: 0 - 0.0500%, and REM: 0 - 0.0500%, The balance being Fe and impurities, The microstructure contains 90% or more tempered martensite by area ratio.

16. The hot stamping component according to claim 15, wherein, The chemical composition contains one or more of the following elements by mass%: Nb: 0.005 - 0.100%, V：0.005～0.100％、 W：0.005～0.100％、 Ni: 0.01 - 2.00%, Cu: 0.01 - 2.00%, Cr：0.01～2.00％、 Mo: 0.01 - 2.00%, Sn: 0.005 - 0.200%, Ca: 0.0003 - 0.0500% Mg: 0.0003 to 0.0500% and REM: 0.0003 to 0.0500%.

17. The method for manufacturing a hot stamping member according to claim 15, comprising the following hot stamping process: After heating the hot stamping steel sheet according to any one of claims 1 to 7 in a heating furnace with an atmosphere temperature of 850 to 950°C for 3 minutes or more, it is cooled at a cooling rate of 10 to 20°C / second to below the martensite transformation start temperature.

Images