Hot stamping steel sheet, method for manufacturing hot stamping steel sheet, and hot stamped product

Abstract

A steel sheet for hot stamping which has a base material steel sheet having a prescribed chemical composition and a zinc plating layer formed on the surface of the base material steel sheet, wherein: the microstructure at a 1 / 4-depth position within the range of 1 / 8-3 / 8 of the plate thickness from the surface of the base material steel sheet in the plate thickness direction contains ferrite in the amount of 20-95% by volume and pearlite in the amount of 5-80% by volume; the remaining structure comprises bainite; the basis weight of the zinc plating layer is 90g / m2 or higher; and Bps is at least 1.2 times Bqs, if Bps is the maximum value of the B content in the zinc plating layer and Bqs is the B content at the 1 / 4-depth position in the base material steel sheet.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet for hot stamping, a manufacturing method thereof, and a hot-stamped steel. This application claims priority based on Japanese Patent Application No. 2022-001024, filed on January 6, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] In response to the recent trend toward fuel economy and lightweighting, weight reduction has been considered for components such as automobile door guard bars and side members. From a material perspective, thinning is an effective way to reduce weight. However, simply thinning the material reduces the load-bearing capacity of the component. Therefore, high strength is required for steel sheets used as materials for automobile parts so that strength and collision safety can be maintained even when the material is thinned. However, the formability of a material generally deteriorates as its strength increases. Therefore, to achieve weight reduction in the above-mentioned components, the steel sheet used as the material (raw material) is required to have both excellent formability and high strength.

[0003] As a steel sheet that has both high strength and excellent formability, there is TRIP (transformation induced plasticity) steel, which utilizes the martensitic transformation of retained austenite, as disclosed in Patent Documents 1 and 2, for example, and the applications of this TRIP steel have been expanding in recent years. However, although TRIP steel improves deep drawability and elongation during forming, its high steel sheet strength poses a problem in that the shape fixability of the component after press forming is poor.

[0004] Furthermore, Patent Documents 3 and 4 disclose ultra-high tensile steel sheets for cold pressing with a tensile strength of 1470 MPa or more. However, although these steel sheets are designed to prevent hydrogen embrittlement due to cold working, they suffer from significant blade wear during shear cutting and trimming. Alternatively, they have the problem that the press load is too high to allow for cold pressing. For these reasons, there has been a significant challenge in further increasing the strength of steel sheets for automotive parts.

[0005] To address these issues, there is a method known as hot stamping, as disclosed in Patent Document 5, in which a soft steel sheet is cut to a predetermined size, heated to an austenite single-phase region of 800°C or higher, press-formed in the austenite single-phase region, and then quenched, as disclosed in Patent Document 5. By applying this hot stamping, it is possible to manufacture components with high strength of 980 MPa or higher and excellent shape fixability, while suppressing wear on the shear and trim and reducing the press load. However, in hot stamping, steel sheets are inserted into a heating furnace or heated to high temperatures exceeding 800°C in the atmosphere using electrical heating or far-infrared heating. Therefore, even if a plating is applied to the surface of steel sheets for hot stamping, the plating reacts with the base steel, resulting in the loss of the plating's properties. Therefore, while plating steel sheets for hot stamping can be useful in terms of preventing scale buildup, there is also the issue that this may not sufficiently contribute to ensuring the rust resistance of the formed body after hot stamping.

[0006] To address these issues, Patent Documents 6 and 7 disclose that zinc plating containing 9% to 30% by mass of Fe can be applied to a steel sheet at a coating density of 30 g / m by incorporating Si or Al into the coating layer to suppress the volatilization of zinc. 2 A hot stamped product formed with the above deposition amount is disclosed. The hot-stamped steel sheets described in Patent Documents 6 and 7 can be galvanized by suppressing zinc volatilization. However, even if zinc volatilization is suppressed, the alloying reaction between Fe and zinc progresses during hot stamping, making it difficult to stably control the Fe content in the galvanized layer within the range of 9 to 30% by mass. In particular, when the heating temperature is high or the heating time is long, the alloying reaction between Fe and zinc progresses excessively, resulting in the Fe content in the galvanized layer exceeding 30% by mass, resulting in a loss of corrosion resistance. Additionally, because the alloying reaction between Fe and Zn occurs nonuniformly from the interface between the plated layer and the base metal, the alloying reaction forms irregularities on the surface of the plated layer, resulting in nonuniform thickness of the electrodeposition coating, which is essential for rust prevention treatment of automotive parts. In other words, if the surface after electrodeposition coating is flat, the electrodeposition coating thickness at the convex parts of the plated layer becomes thin, resulting in peeling of the coating and reduced corrosion resistance. Therefore, a solution to this problem was needed. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 1-230715 [Patent Document 2] Japanese Patent Publication No. 2-217425 [Patent Document 3] Japanese Patent No. 6354921 [Patent Document 4] Japanese Patent No. 5365216 [Patent Document 5] Japanese Patent No. 3582512 [Patent Document 6] Japanese Patent No. 4671634 [Patent Document 7] Japanese Patent No. 4733522 Summary of the Invention [Problem to be solved by the invention]

[0008] As described above, conventional techniques have had problems in stably producing hot-stamped steel sheets having excellent rust prevention properties, including corrosion resistance and paint film adhesion. In view of the above circumstances, an object of the present invention is to provide a hot-stamped product that is less affected by heating conditions during hot stamping (i.e., can be manufactured under a wide range of heating conditions) and has excellent rust prevention properties, a steel sheet for hot stamping that is suitable as a raw material for the hot-stamped product, and a method for manufacturing the steel sheet for hot stamping. [Means for solving the problem]

[0009] The present inventors have conducted extensive research into methods for solving the above problems, and as a result, have found that by concentrating B in the zinc-based coating layer of a steel sheet for hot stamping, it is possible to delay the alloying reaction between Fe (iron) and Zn (zinc), thereby making it possible to obtain hot stamp formability with a coating layer having a Γ phase containing 9 to 30% by mass of Fe over a wide range of hot stamping heat treatment conditions, and that this hot-stamped steel has high corrosion resistance and can suppress paint peeling when subjected to chemical conversion treatment and electrodeposition treatment.

[0010] The present invention has been made based on the above findings, and the gist of the present invention is as follows. [1] A steel sheet for hot stamping according to one aspect of the present invention comprises a base steel sheet and a zinc-based coating layer formed on a surface of the base steel sheet, wherein the base steel sheet contains, in mass%, C: 0.030 to 0.600%, Si: 0.01 to 1.50%, Mn: 0.10 to 2.50%, Al: 0.001 to 0.100%, Ti: 0.010 to 0.100%, B: 0.0005 to 0.0100%, P: 0.100% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V: 0 to 0.500%, W: 0 to 0.500%, Cr: 0 to 1.00%, and Mo: 0 to 0.50%. %, Co: 0 to 1.000%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, REM: 0 to 0.0100%, Zr: 0 to 0.0500%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, As: 0 to 0.100%, Sn: 0 to 0.50%, Sb: 0 to 0.50%, and the balance: Fe and impurities; and the microstructure at a 1 / 4 depth position, which is within a range of 1 / 8 to 3 / 8 of the sheet thickness from the surface of the base steel sheet in the sheet thickness direction, contains, by volume, 20 to 95% ferrite and 5 to 80% pearlite, with the balance being bainite; and the zinc-based coating layer has a coating weight of 90 g / m 2 or more, and when the maximum value of the B content in the zinc-based plating layer is Bps and the B content at the 1 / 4 depth position of the base steel sheet is Bqs, Bps is 1.2 times or more of Bqs. [2] In the steel sheet for hot stamping described in [1], the chemical composition of the zinc-based plating layer may contain less than 9.0% Fe by mass. [3] The steel sheet for hot stamping according to [1] or [2], wherein the chemical composition of the base steel sheet is, in mass%, Nb: 0.005 to 0.050%, V: 0.005 to 0.500%, W: 0.005 to 0.500%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, Co: 0.01 to 1.000%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, REM: 0.0003 to 0.0100%, Zr: 0.0003 to 0.0500%, Ca: 0.0003 to 0.0100%, Mg: 0.0003 to 0.0100%, As: 0.001 to 0.100%, It may contain one or more elements selected from the group consisting of Sn: 0.01 to 0.50% and Sb: 0.01 to 0.50%.

[0011] [4] A method for producing a steel sheet for hot stamping according to another aspect of the present invention comprises: A method for producing a steel sheet for hot stamping according to [1], In mass%, C: 0.030 to 0.600%, Si: 0.01 to 1.50%, Mn: 0.10 to 2.50%, Al: 0.001 to 0.100%, Ti: 0.010 to 0.100%, B: 0.0005 to 0.0100%, P: 0.100% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V: 0 to 0.500%, W: 0-0.500%, Cr: 0-1.00%, Mo: 0-0.50%, Co: 0-1.000%, Ni: 0-1.00%, Cu: 0-1.00%, REM: 0-0.0100%, Zr: 0-0.0500%, Ca: 0-0.0100%, Mg: 0-0.0100%, As: 0-0.100%, Sn: 0-0.50%, Sb: 0-0.50%, and the balance: Fe and impurities. a hot rolling process in which the hot rolled steel sheet obtained by the hot rolling process is coiled in a temperature range of 600°C or less; a cold rolling process in which the hot rolled steel sheet obtained by the hot rolling process is pickled and then cold rolled at a cumulative reduction of 30 to 80% to obtain a cold rolled steel sheet; an annealing process in which the cold rolled steel sheet is annealed in an annealing furnace in which the oxygen potential of the atmosphere is -1.20 to -0.50 and the temperature of the atmosphere is 650 to 800°C for 30 seconds or more, and cooled to a surface temperature of 500 to 400°C; and a galvanizing process in which the cold rolled steel sheet obtained by the annealing process is immersed in a galvanizing bath to obtain a coating weight of 90 g / m. 2 The method includes a plating step of forming the above zinc-based plating layer, and a cooling step of cooling the cold-rolled steel sheet after the plating step to 50°C or less.

[0012] [5] A hot-stamped steel according to another aspect of the present invention includes a base steel sheet and a coating layer containing Zn and Fe formed on a surface of the base steel sheet, wherein the base steel sheet contains, in mass%, C: 0.030 to 0.600%, Si: 0.01 to 1.50%, Mn: 0.10 to 2.50%, Al: 0.001 to 0.100%, Ti: 0.010 to 0.100%, B: 0.0005 to 0.0100%, P: 0.100% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V: 0 to 0.500%, W: 0 to 0.500%, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Co: 0 to 1.000%, The base steel sheet has a chemical composition consisting of Ni: 0-1.00%, Cu: 0-1.00%, REM: 0-0.0100%, Zr: 0-0.0500%, Ca: 0-0.0100%, Mg: 0-0.0100%, As: 0-0.100%, Sn: 0-0.50%, Sb: 0-0.50%, and the balance: Fe and impurities, and the microstructure at a 1 / 4 depth position that is within a range of 1 / 8 to 3 / 8 of the sheet thickness from the surface in the sheet thickness direction contains, by volume, 5-100% martensite and 0-95% ferrite, with the balance consisting of one or both of bainite and pearlite, and the coating weight of the Γ layer, which has an Fe content of 9-30% by mass, is 40 g / m 2 or more, and when the maximum B content in mass% in the coating layer is Bp and the B content in mass% at the 1 / 4 depth position of the base steel sheet is Bq, the Bp is 1.2 times or more of the Bq.

[0013] [6] In the hot-stamped steel according to [5], the chemical composition of the base steel sheet is, in mass%, Nb: 0.005 to 0.050%, V: 0.005 to 0.500%, W: 0.005 to 0.500%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, Co: 0.01 to 1.000%, Ni: 0.01 to 1.00%, Cu: 0.01 to It may contain one or more selected from the group consisting of: Cr: 0.001 to 0.100%, REM: 0.0003 to 0.0100%, Zr: 0.0003 to 0.0500%, Ca: 0.0003 to 0.0100%, Mg: 0.0003 to 0.0100%, As: 0.001 to 0.100%, Sn: 0.01 to 0.50%, and Sb: 0.01 to 0.50%. [Effects of the Invention]

[0014] According to the above-described aspects of the present invention, it is possible to provide a hot-stamped product having excellent rust prevention properties (corrosion resistance and coating adhesion), a steel sheet for hot stamping suitable as a material for the hot-stamped product, and a method for manufacturing the steel sheet for hot stamping. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, a hot stamping steel sheet according to one embodiment of the present invention (hot stamping steel sheet according to the present embodiment), a hot stamped product according to one embodiment of the present invention (hot stamped product according to the present embodiment), and methods for manufacturing them will be described in this order.

[0016] [Steel sheets for hot stamping] The steel sheet for hot stamping according to this embodiment has a base steel sheet and a zinc-based plating layer formed on the surface of the base steel sheet. The zinc-based plating layer may be formed on one side or both sides of the base steel sheet.

[0017] <Base material steel plate> <Microstructure> In the steel sheet for hot stamping according to this embodiment, the microstructure at a 1 / 4 depth position, which is in the range of 1 / 8 to 3 / 8 of the sheet thickness from the surface of the base steel sheet in the sheet thickness direction, contains, by volume fraction, 20 to 95% ferrite and 5 to 80% pearlite, with the remaining structure being bainite. The reason why the microstructure at the 1 / 4 depth position is specified is that the microstructure at this position is a typical microstructure of the steel sheet and has a strong correlation with the properties.

[0018] If the ferrite volume fraction is less than 20%, the proportion of hard structures such as martensite and bainite becomes too high, resulting in poor workability. On the other hand, if the ferrite volume fraction exceeds 95%, it is not possible to ensure a volume fraction of 5% or more of pearlite, which becomes austenite during hot stamping heat treatment and martensite during die quenching. In this case, the strength obtained after hot stamping decreases, and sufficient strength cannot be obtained as a hot-stamped body. Therefore, the ferrite volume fraction is set to 20 to 95%.

[0019] Since pearlite contains carbides, it contributes to increasing strength through hot stamping (heating and quenching). If the volume fraction of pearlite is less than 5%, the increase in strength through hot stamping is insufficient, resulting in a decrease in the strength of the hot-stamped body. Therefore, the volume fraction of pearlite is set to 5% or more. The volume fraction of pearlite is preferably 9% or more, and more preferably 13% or more. On the other hand, if the volume fraction of pearlite exceeds 80%, the volume fraction of ferrite will fall below 20%. Therefore, the volume fraction of pearlite is set to 80% or less. Here, pearlite refers to a structure in which ferrite and cementite are layered. In this embodiment, even if part of the cementite is fragmented or spheroidized, it is considered to be pearlite. The total volume fraction of ferrite and pearlite is preferably 95% or more.

[0020] The microstructure may contain bainite in a volume fraction of 5% or less as the remainder other than ferrite and pearlite. Bainite is a hard microstructure second only to ferrite and pearlite, and may be contained within a range that does not significantly increase the strength of the steel sheet for hot stamping. If the volume fraction of bainite exceeds 5%, the strength of the steel sheet increases more than necessary. The volume fraction of bainite is preferably 2% or less, and may be 0%.

[0021] In the case of the steel sheet for hot stamping according to this embodiment, a sample is taken with a thickness cross section parallel to the rolling direction of the steel sheet as the observation surface, the observation surface is polished and etched with nital, and a range from the surface to 1 / 8 to 3 / 8 of the sheet thickness, centered at a position of 1 / 4 of the sheet thickness in the sheet thickness direction from the surface, is observed with a field emission scanning electron microscope (FE-SEM) to measure the area ratio, which is taken as the volume ratio. When measuring the area ratio, the field size was set to 1000 μm at a magnification of 3000 times. 2 The above 10 fields of view are observed, and the average value is taken as the area ratio.Ferrite, pearlite, and bainite can be determined by the following characteristics. Ferrite is a bcc phase with equiaxed particles that does not contain carbides. Pearlite is a layered structure of bcc phase and cementite, and can be distinguished by observation with an electron microscope. Bainite can be classified into upper bainite, which contains cementite between laths of the bcc phase with a lath-like morphology, and lower bainite, which contains cementite with a single orientation relationship (same variant) within the laths. Either microstructure can be distinguished by observation with an electron microscope. Both upper and lower bainite are hard and contribute to high strength of steel sheets for hot stamping, so in this embodiment, both are referred to as bainite. Martensite can be classified into fresh martensite consisting of a bcc or bct phase with a lath-like morphology that does not contain carbides, and tempered martensite containing cementite (iron-based carbide) with multiple orientation relationships (lower bainite also contains cementite within the laths, but with a single orientation relationship). Both can be distinguished by observation with an electron microscope. Although it is not necessary to measure the volume fraction of martensite here, this method can also be used to measure the volume fraction of martensite.

[0022] ≪Chemical composition≫ The chemical composition of the base steel sheet of the steel sheet for hot stamping according to this embodiment will be described below. Hereinafter, % regarding the content of each element means % by mass.

[0023] C: 0.030 to 0.600% C is an element effective for increasing the strength of steel. If the C content is less than 0.030%, the maximum tensile strength (tensile strength) of the hot stamped steel cannot be sufficiently ensured. Therefore, the C content is set to 0.030% or more. The C content is preferably 0.050% or more, and more preferably 0.080% or more. On the other hand, if the C content exceeds 0.600%, the weldability and workability become insufficient. Therefore, the C content is set to 0.600% or less. The C content is preferably 0.550% or less, and more preferably 0.500% or less.

[0024] Si: 0.01 to 1.50% Si is a solid-solution strengthening element. When the Si content is 0.01% or more, the strength increases significantly, so the Si content is set to 0.01% or more. On the other hand, if the Si content exceeds 1.50%, not only will the effect saturate, but the Ac3 point will rise, making it necessary to increase the heating temperature during hot stamping, which is undesirable. Therefore, the Si content is set to 1.50% or less.

[0025] Mn: 0.10 to 2.50% Mn is an element that improves hardenability. By setting the Mn content to 0.10% or more, the ferrite transformation during the cooling process during hot stamping can be delayed, and the martensite volume fraction of the hot stamped steel can be set to 5% or more. Therefore, the Mn content is set to 0.10% or more. On the other hand, if the Mn content exceeds 2.50%, the hardenability becomes too high, and the strength of the hot-rolled steel sheet becomes too high, which may cause the sheet to break when cold-rolled. Therefore, the Mn content is set to 2.50% or less.

[0026] Al: 0.001 to 0.100% Al is an element that acts as a deoxidizer. If the Al content is less than 0.001%, a sufficient deoxidizing effect cannot be obtained, and a large amount of inclusions (oxides) will be present in the steel sheet. These inclusions are undesirable because they become the starting point of fracture during hot stamping and cause breakage. Therefore, the Al content is set to 0.001% or more. The Al content is preferably 0.005% or more. On the other hand, if the Al content exceeds 0.100%, the Ac3 point rises, making it necessary to increase the heating temperature during hot stamping, which reduces productivity. Therefore, the Al content is set to 0.100% or less.

[0027] Ti: 0.010 to 0.100% Ti is an element that contributes to increasing the strength and toughness of hot-stamped steel by strengthening the grains by suppressing the grain growth of austenite during the heating process of hot stamping. In addition, Ti is an element that inhibits B from becoming a nitride by bonding with N to form TiN. To achieve this effect, the Ti content is set to 0.010% or more. The Ti content is preferably 0.012% or more, and more preferably 0.015% or more. On the other hand, if the Ti content exceeds 0.100%, Ti carbides are formed, reducing the amount of C that contributes to strengthening martensite, which may result in a decrease in the strength of the hot-stamped steel. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably 0.080% or less, and more preferably 0.060% or less.

[0028] B: 0.0005 to 0.0100% B is an element that diffuses into the coating layer during hot stamping, delays the reaction between Fe and Zn during hot stamping, and is effective in improving corrosion resistance and coating adhesion. To achieve these effects, the B content is set to 0.0005% or more. The B content is preferably 0.0007% or more, and more preferably 0.0009% or more. In the steel sheet for hot stamping according to this embodiment, even if B is added to the coating bath, oxidation and other factors occur, making it impossible to stably incorporate B into the coating layer. For this reason, B is incorporated into the base steel sheet, and then concentrated on the surface of the steel sheet in an annealing process as described below, before plating, thereby incorporating B into the coating layer. On the other hand, if the B content exceeds 0.0100%, not only will the effect saturate, but iron-based borides will precipitate, making it impossible to obtain the hardenability-improving effect of B. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, more preferably 0.0060% or less, and even more preferably 0.0030% or less.

[0029] P:0.100% or less P is an element that segregates near the center of the steel plate thickness and also embrittles welds. Therefore, a lower P content is preferable, but if the P content exceeds 0.100%, the embrittlement of welds becomes significant, so the P content is set to 0.100% or less. The P content is preferably 0.080% or less, and more preferably 0.050% or less. There is no need to particularly set a lower limit for the P content (0% is acceptable), but reducing the P content to less than 0.001% is economically disadvantageous, so the P content may be set to 0.001% or more.

[0030] S: 0.0100% or less S is an element that adversely affects weldability and manufacturability during casting and hot rolling. A lower S content is preferable, but if the S content exceeds 0.0100%, the above adverse effects become significant, so the S content is set to 0.0100% or less. There is no need to set a lower limit for the S content (it may be 0%), but reducing the S content to less than 0.0001% is economically disadvantageous, so the S content may be set to 0.0001% or more.

[0031] N: 0.0150% or less N is an element that forms coarse nitrides and deteriorates bendability and hole expandability. N is also an element that can cause blowholes during welding. A lower N content is preferable, but if the N content exceeds 0.0150%, bendability and hole expandability will deteriorate significantly. Therefore, the N content is set to 0.0150% or less. There is no need to particularly set a lower limit for the N content (it can be 0%), but reducing the N content to less than 0.0001% significantly increases manufacturing costs, so the N content may be set to 0.0001% or more, or 0.0005% or more.

[0032] O: 0.0100% or less O is an element that forms oxides and exists as inclusions in steel. These inclusions cause deterioration of the properties of steel sheets for hot stamping. A low O content is preferable (it may be 0%), but if the O content exceeds 0.0100%, the above tendency becomes significant. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0050% or less. On the other hand, reducing the O content to less than 0.0001% leads to an excessive increase in costs and is economically undesirable, so the O content may be set to 0.0001% or more. The oxides referred to here are oxides that exist as inclusions in the steel sheet and are different from scale formed during hot stamping. Furthermore, since the steel sheet for hot stamping according to this embodiment has a zinc-based coating layer, it is possible to suppress the formation of iron oxides on the surface of the base steel sheet.

[0033] In the chemical composition of the base steel sheet of the hot stamping steel sheet according to this embodiment, the balance other than the above elements may be Fe and impurities. On the other hand, the following elements (optional elements) may also be contained as necessary. Since the optional elements do not have to be contained, the lower limit of each is 0%. Furthermore, even if the content is below the range in which a clear effect is obtained, it does not adversely affect the properties of the hot stamping steel sheet according to this embodiment.

[0034] Nb: 0 to 0.050% V: 0 to 0.500% W: 0 to 0.500% Nb, V, and W are elements that contribute to increasing strength and toughness by strengthening grains by suppressing the grain growth of austenite during the heating process of hot stamping. For this reason, they may be added. To obtain this effect, the content of Nb, V, and / or W is preferably 0.005% or more. More preferably, the Nb content, V content, and W content are each 0.010% or more. On the other hand, if the Nb content exceeds 0.050%, or if the V content or W content exceeds 0.500%, carbides of these elements are formed, reducing the amount of C that contributes to strengthening martensite and causing a decrease in the strength of the hot-stamped body. Therefore, the Nb content is set to 0.050% or less, and the V content and W content are each set to 0.500% or less.

[0035] Cr: 0 to 1.00% Mo: 0 to 0.50% Co: 0 to 1.000% Ni: 0 to 1.00% Cu: 0 to 1.00% Cr, Mo, Co, Ni, and Cu are elements that improve hardenability during hot stamping, promote martensite formation, and contribute to increasing the strength of the hot-stamped body. This effect becomes more pronounced when Co is contained in an amount of 0.001% or more and / or one or more of Cr, Mo, Ni, and Cu is contained in an amount of 0.01% or more. Therefore, it is preferable that the Co content be 0.001% or more, and that the Cr content, Mo content, Ni content, and Cu content be each 0.01% or more. On the other hand, excessive contents of these elements deteriorate weldability and hot workability, or the strength of the steel sheet for hot stamping becomes too high, leading to manufacturing problems. Therefore, the Cr content, Cu content, and Ni content are each set to 1.00% or less, the Co content to 1.000% or less, and the Mo content to 0.50% or less.

[0036] REM: 0 to 0.0100% Zr: 0 to 0.0500% Ca: 0 to 0.0100% Mg: 0 to 0.0100% To improve the toughness of the hot-stamped steel sheet, one or more of REM, Zr, Ca, and Mg may be further added. To obtain a sufficient effect, it is preferable to add 0.0003% or more of one or more of REM, Zr, Ca, and Mg. These elements have strong bonding strength with oxygen and S, and convert coarse oxides and sulfides formed in the steel sheet into fine oxides, thereby contributing to improving the toughness of the hot-stamped steel sheet and suppressing fracture during collision deformation. On the other hand, if the REM content, Ca content, and Mg content each exceed 0.0100%, or if the Zr content exceeds 0.0500%, castability and hot workability deteriorate, so the REM content, Ca content, and Mg content each are set to 0.0100% or less, and the Zr content to 0.0500% or less. In this embodiment, REM is an abbreviation for Rare Earth Metal, and refers to elements belonging to the lanthanoid series, including La and Ce, and Y. REM is often added to misch metal, and may contain a combination of elements of the lanthanoid series in addition to La and Ce.

[0037] As: 0 to 0.100% Sn: 0 to 0.50% Sb: 0 to 0.50% To further improve rust prevention, one or more of As, Sn, and Sb may be added. To obtain this effect, it is preferable that the As content be 0.020% or more, and / or the Sn content and Sb content be 0.02% or more, respectively. On the other hand, if the As content exceeds 0.100%, the Sn content exceeds 0.50%, or the Sb content exceeds 0.50%, castability and hot workability deteriorate. Therefore, the As content is set to 0.100% or less, and the Sn content and Sb content are each set to 0.50% or less.

[0038] The above-mentioned chemical compositions may be measured by a general analytical method. For example, they may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. In the hot stamping steel sheet according to this embodiment, B in the surface layer of the steel sheet is concentrated in the coating layer or at the interface with the coating layer by controlling the atmosphere during annealing. Therefore, the chemical composition (element concentration) of the surface layer of the base steel sheet differs from the value at the 1 / 4 depth position or the average value over the entire thickness. For this reason, a region 150 μm deep from the surface of the base steel sheet is removed by polishing, and then the component analysis is performed. When the above region is removed, either the 1 / 4 depth position or the average value over the entire thickness may be used.

[0039] In the steel sheet for hot stamping according to this embodiment, as will be described later, B in the surface layer portion of the base steel sheet is concentrated on the surface, and the B concentrated on the surface is diffused into the coating layer. Therefore, in the surface layer portion of the steel sheet for hot stamping, there may be a region where the B content (concentration) is reduced. Specifically, the B content at a position 10 μm from the surface of the base steel plate in the depth direction (plate thickness direction) may be 0.80 times (80%) or less of the B content at a depth position 1 / 4 of the plate thickness of the base steel plate. In this case, B is sufficiently diffused in the coating layer, and the alloying reaction between the zinc coating layer and the base steel sheet during hot stamping heat treatment is suppressed, resulting in a coating weight of 40 g / m2 of the Γ layer with an Fe content of 9 to 30 mass%. 2 This is preferable because it is possible to obtain a hot stamped product having excellent rust resistance. On the other hand, adding B to the coating bath is one method of incorporating B into the coating bath. However, as a result of investigations by the present inventors, it was found that because the temperature of the coating bath is high, B turns into compounds such as oxides, and therefore B cannot be concentrated in the coating layer as B, and therefore the alloying reaction between the galvanized layer and the base steel sheet during hot stamping heat treatment cannot be suppressed.

[0040] The B content in mass% at a depth of 10 μm from the surface of the base steel sheet can be determined by cutting a test piece 50 mm wide x 50 mm long from the hot stamping steel sheet and measuring the B content in mass% using GDS.

[0041] <Zinc-based plating layer> <Basis weight> The zinc-based coating layer formed on the surface of the base steel sheet has a coating weight of 90 g / m on each side. 2 That's all. In particular, when used on visible parts such as B-pillar outer panels, high rust prevention is required, as it is necessary to suppress not only the swelling of the normal coating but also the red rust caused by corrosion. In order to ensure the rust prevention properties of the hot-stamped product obtained by hot stamping a steel sheet for hot stamping when the above-mentioned applications are assumed, the coating weight of the Γ layer, which is a layer made of a Γ phase containing 9 to 30% Fe by mass out of the Zn and Fe alloy layer, in the coating layer of the hot-stamped product must be 40 g / m 2 It is necessary to have more than this. In the steel sheet for hot stamping according to this embodiment, as will be described later, B is contained in the zinc-based coating layer to delay the reaction between Fe and zinc (Zn). However, even so, the coating weight of the zinc-based coating layer is 90 g / m 2 If the alloying strength is less than 40 g / m, the alloying reaction between iron and zinc will proceed excessively during the hot stamping heat treatment, and the hot stamped product will have a hardness of 40 g / m 2 Therefore, the weight of the zinc-based plating layer should be set to 90 g / m 2 The basis weight is preferably 95 g / m 2 That's all.

[0042] ≪Chemical composition≫ When the maximum B content in the zinc-based coating layer is Bps and the B content at 1 / 4 depth of the base steel sheet is Bqs, Bps is 1.2 times or more of Bqs. B is concentrated in the coating layer to delay the alloying reaction between Fe and Zn, and after hot stamping (hot stamped body), the deposition weight of the Γ layer is increased to 40 g / m 2This contributes to ensuring the above. This is because the formation of a B-enriched layer on the steel sheet surface during the heat treatment process delays the alloying reaction between iron and zinc during hot stamping heat treatment. In the steel sheet for hot stamping according to this embodiment, B in the zinc-based coating layer needs to diffuse from the steel sheet to the zinc-based coating layer during heat treatment. However, excessive B in the steel sheet leads to the formation of borides in the steel, which prevents B from concentrating in the zinc-based coating layer during heat treatment. In addition, excessive B in the steel excessively strengthens the hot-rolled steel sheet and embrittles it, making subsequent cold rolling and pickling difficult. By setting the ratio of the maximum value (peak concentration) of B in the zinc-based coating layer, Bps, to the B content Bqs at 1 / 4 depth of the base steel sheet, at least 1.2 times, a sufficient alloying delay effect can be obtained, and in the hot stamped product, the coating weight of the Γ layer can be increased to 40 g / m 2 If Bps / Bqs is less than 1.2, sufficient effect cannot be obtained. Although there is no upper limit, excessive thickening not only saturates the effect but also lengthens the heat treatment time required for thickening, so from the standpoint of economic rationality, it is preferable that the value be 5.0 or less. As a result of investigations by the present inventors, it was found that even if B is added to the plating bath, it is not possible to stably incorporate B into the plating layer due to the occurrence of oxidation, etc. Therefore, B is incorporated into the plating layer by concentrating B contained in the steel sheet on the surface of the steel sheet in the annealing process and then plating in that state.

[0043] Bps / Bqs is calculated using the following method. To measure Bps, a 50 mm x 50 mm test piece is cut from the hot stamping steel sheet and subjected to elemental analysis from the surface layer using GDS (glow discharge optical emission spectroscopy), which allows the measurement of the maximum B content Bps in mass% in the zinc-based coating layer. To identify whether it is a zinc-based coating layer or a base steel sheet, the region where the Zn concentration is 70 mass% or more can be determined to be the zinc-based coating layer. The B content in mass% at a 1 / 4 depth position in the base steel sheet was measured by cutting a test piece 50 mm wide x 50 mm long from the hot stamped body and polishing it to a 1 / 4 depth position in the base steel sheet, and then measuring the B content in mass% using GDS. During the measurement, the average B content in the range of 10 μm to 20 μm from the polished surface in the thickness direction is taken as Bqs. The reason for measuring the B content at this position is that polished test pieces may have surface deposits, which can lead to a lack of quantitative accuracy. However, if the deposits can be removed, the average B content in the range of 0 to 10 μm from the polished surface can also be taken as Bqs. The reason for taking the average B content in the 10 μm range (10 to 20 μm or 0 to 10 μm) in the thickness direction as Bqs is to eliminate the influence of slight segregation formed during casting. On the other hand, although expanding the measurement range in the thickness direction is advantageous from the perspective of accuracy, expanding the measurement range in the thickness direction results in longer measurement times. Since the longer the measurement area in the depth direction, the longer the measurement time required for element distribution measurement in the thickness direction using GDS, it is desirable to limit the measurement to 100 μm.

[0044] The chemical composition of the zinc-based coating layer of the steel sheet for hot stamping according to this embodiment is not limited as long as it satisfies the above-mentioned Bps / Bqs and contains zinc, but can be, for example, in mass %, B: 0.0005-0.0500%, Al: 0-1.00%, Fe: 0.1-20.0%, Si: 0-1.0%, Mg: 0-0.5%, Mn: 0-0.5%, Pb: 0-0.5%, Sb: 0-0.5%, and the balance: Zn and impurities. The Zn content is preferably 80.0% or more, and more preferably 85.0% or more. In particular, Al forms an oxide on the surface during hot stamping heating, preventing Zn from evaporating during hot stamping heat treatment. This allows the Γ layer in the hot stamped compact to be formed at a temperature of 40 g / m 2This contributes to ensuring the above. Therefore, although Al does not necessarily need to be contained, if the aim is to ensure a stable Γ phase in the hot-stamped steel, it is preferable that the Al content be 0.01% or more. On the other hand, if the Al content exceeds 1.00%, the formation of Al oxides during hot stamping becomes significant, increasing the risk of problems such as adverse effects on weldability due to the formed oxides and wear on the press die. For this reason, it is preferable that the Al content be 1.00% or less. The Al content is more preferably 0.40% or less, and even more preferably 0.35% or less. Furthermore, if the Fe content in the chemical composition of the zinc-based coating layer is less than 9.0%, the Fe content in the zinc-based coating layer of the hot stamping steel sheet is low, and even if the alloying reaction between Fe and Zn progresses during hot stamping heat treatment, the time during which the Fe content in the coating layer remains at 30% or less will be extended. This is preferable because it is easier to ensure a large amount of the Γ layer after hot stamping heat treatment. The Fe content in the coating layer can be controlled by alloying heat treatment after immersion in the coating bath. However, in the case of the hot stamping steel sheet according to this embodiment, it is desirable to reduce the Fe content in the coating layer in order to form a Γ layer after hot stamping heat treatment. Therefore, it is preferable not to perform alloying heat treatment or to perform alloying heat treatment at a temperature of 490°C or less.

[0045] The chemical composition and coating weight of the zinc-based plating layer can be determined by the following method. The coating weight can be determined by immersing the steel sheet in a 5% HCl aqueous solution containing 0.02% of an inhibitor (Ibit 700A, Asahi Chemical Industry Co., Ltd.) that suppresses the dissolution of Fe in the steel sheet for 10 minutes at room temperature to dissolve the entire zinc-based plating layer, and then calculating the weight change before and after dissolution. The completion of dissolution of the zinc-based plating layer is determined based on the completion of foaming caused by hydrogen generation during dissolution. The chemical composition of the zinc-based plating layer is measured by ICP analysis of the solution in which the zinc-based plating layer has been dissolved. The chemical composition measured by this method is the average chemical composition of the zinc-based plating layer.

[0046] <Strength of steel sheets for hot stamping> The steel sheet for hot stamping according to this embodiment can ensure excellent rust prevention after hot stamping regardless of strength. However, although high strength is required for the formed body after hot stamping, the steel sheet for hot stamping is also required to be easy to cut and press-form. Therefore, it is not preferable for the strength of the steel sheet for hot stamping to be subjected to cutting and press-forming to be high. For this reason, it is desirable for the tensile strength of the steel sheet for hot stamping to be 980 MPa or less.

[0047] [Method of manufacturing steel sheets for hot stamping] Next, a method for manufacturing a steel sheet for hot stamping according to this embodiment will be described. Known conditions can be applied to conditions not described below. The steel sheet for hot stamping according to this embodiment can be manufactured by a manufacturing method including the following steps. (I) a casting process for casting a steel ingot or slab having a predetermined chemical composition; (II) a hot rolling step of heating the steel ingot or the slab to 1100°C or higher and hot rolling the steel ingot or the slab at a finish rolling temperature of 800°C or higher to obtain a hot-rolled steel sheet; (III) a coiling step of coiling the hot-rolled steel sheet after the hot rolling step in a temperature range of 600°C or less; (IV) a cold rolling step of pickling the hot-rolled steel sheet after the coiling step and then cold-rolling it at a cumulative reduction rate of 30 to 80% to obtain a cold-rolled steel sheet; (V) an annealing step of holding the cold-rolled steel sheet in an annealing furnace having an atmosphere with an oxygen potential of −1.20 to −0.50 and an atmosphere temperature of 650 to 800° C. for 30 seconds or more, and after holding, cooling the steel sheet so that the surface temperature of the steel sheet becomes 500 to 400° C.; (VI) The cold-rolled steel sheet after the annealing step is immersed in a plating bath to have a coating weight of 90 g / m per side. 2 a plating step of forming the above zinc-based plating layer; (VII) A cooling step of cooling the cold-rolled steel sheet after the plating step to 50°C or less. The preferred conditions for each step will be explained below.

[0048] <Casting process> In the casting process, a steel ingot or slab having the same chemical composition as the steel sheet for hot stamping according to this embodiment is cast. The slab may be a continuous cast slab or one produced by a thin slab caster. This manufacturing method is also suitable for processes such as continuous casting-direct rolling (CC-DR), in which hot rolling is performed immediately after casting.

[0049] <Hot rolling process> In the hot rolling process, the cast steel ingot or slab is heated to 1100°C or higher and hot rolled to a finish rolling temperature of 800°C or higher to obtain a hot-rolled steel sheet. If the heating temperature is less than 1100°C, the finish rolling temperature will be lowered. This can make rolling difficult and cause the steel sheet to have a poor shape after rolling. Therefore, the heating temperature should be 1100°C or higher. However, if the temperature of the steel ingot or slab after casting is 1100°C or higher, hot rolling can be performed without heating. Furthermore, if the finish rolling temperature is below 800°C, the rolling load becomes high, making rolling difficult and causing defects in the shape of the steel sheet after rolling. Therefore, the finish rolling temperature is set to 800°C or higher. There is no particular need to set an upper limit for the finish rolling temperature, but if the finish rolling temperature is set too high, the heating temperature must be set too high to maintain that temperature, so the finish rolling temperature is preferably 1100°C or lower.

[0050] <Winding process> In the hot rolling process, the hot-rolled steel sheet after the hot rolling process is coiled at a temperature of 600°C or lower. If the coiling temperature exceeds 600°C, the thickness of the oxide formed on the steel sheet surface increases excessively, which is undesirable as it deteriorates the pickling properties. In addition, B concentrates in the iron oxide formed during coiling, which reduces the B content (concentration) in the surface layer of the steel sheet, which is undesirable as it results in insufficient B concentration in the surface layer of the steel sheet in subsequent processes. On the other hand, if the coiling temperature is less than 400°C, the strength of the hot-rolled steel sheet increases extremely, which is likely to induce sheet breakage or defective shape during cold rolling, so if cold rolling is to be performed thereafter, the coiling temperature is desirably 400°C or higher. However, if the coiled hot-rolled steel sheet is to be softened by heating it in a box annealing furnace or continuous annealing equipment, it may be coiled at a low temperature of less than 400°C. During hot rolling, the roughly rolled sheets may be joined together and continuously subjected to finish rolling. Alternatively, the roughly rolled sheets may be temporarily wound up.

[0051] <Cold rolling process> In the cold rolling process, the hot-rolled steel sheet after the coiling process is pickled and then cold-rolled at a rolling reduction (cumulative rolling reduction) of 30 to 80% to obtain a cold-rolled steel sheet. The purpose of pickling is to remove scale formed during hot rolling. Pickling is preferably carried out using hydrochloric acid containing an inhibitor, but other acids such as hydrochloric acid, sulfuric acid, and nitric acid without an inhibitor, or a combination of these, may also be used as long as surface scale can be removed by pickling. If the reduction is less than 30%, it becomes difficult to maintain the shape of the steel sheet flat, and the ductility of the final product deteriorates. Therefore, the reduction is set to 30% or more. On the other hand, if the rolling reduction exceeds 80%, the rolling load becomes too large, making cold rolling difficult. Therefore, the rolling reduction is set to 80% or less. The rolling reduction is preferably 40 to 70%. There is no need to particularly specify the number of rolling passes or the rolling reduction per pass.

[0052] <Annealing process> In the annealing step, the cold-rolled steel sheet is held for 30 seconds or more in an annealing furnace in which the oxygen potential of the atmosphere is -1.20 to -0.50 and the temperature of the atmosphere is 650 to 800°C, and after holding, the steel sheet is cooled so that the surface temperature of the steel sheet becomes 500 to 400°C. The annealing step is preferably carried out by passing the sheet through a continuous hot-dip galvanizing line so as to be carried out continuously with the subsequent galvanizing step. The annealing temperature (atmosphere temperature) is set to 650 to 800°C in order to soften the steel sheet and enrich B on the surface of the steel sheet. If the annealing temperature is in the range of 650 to 800°C, dislocations introduced during cold rolling are released by recovery, recrystallization, or phase transformation, so annealing is preferably performed within this temperature range. If the annealing temperature (atmosphere temperature) exceeds 750°C, austenite is formed during annealing, and the austenite tends to transform into bainite or martensite during subsequent cooling, resulting in hardening. Furthermore, since the diffusion of B is slower in austenite than in ferrite, it takes a long time for B to enrich in the surface layer, which tends to result in a decrease in B enrichment in the zinc-based coating layer during the subsequent plating process. For this reason, the annealing temperature (atmosphere temperature) is preferably set to 750°C or less. However, if the annealing temperature (atmosphere temperature) is higher than 750°C and not higher than 800°C, the hot stamping steel sheet tends to be harder than when the annealing temperature (atmosphere temperature) is set to 650 to 750°C, and the heat treatment time (annealing time) needs to be extended to ensure rust resistance, but both are limited. For this reason, even when the annealing temperature (atmosphere temperature) is set to higher than 750°C and not higher than 800°C, the hot stamping steel sheet according to this embodiment can be obtained. On the other hand, if the annealing temperature is less than 650°C, it becomes difficult to recrystallize the cold-rolled steel sheet, and the softening that is the purpose of annealing cannot be achieved.Therefore, the annealing temperature is set to 650°C or higher.

[0053] The oxygen potential of the atmosphere during annealing is set to -0.50 to -1.20. If the oxygen potential exceeds -0.50, not only will the effect of concentrating B in the surface layer saturate, but the oxygen potential in the furnace will become too high, causing deterioration of the refractories in the furnace and hearth rolls. If the oxygen potential is less than -1.20, B cannot be concentrated on the steel sheet surface during annealing, and as a result, sufficient B cannot be contained in the zinc-based coating layer. The oxygen potential in the furnace is defined as log(PH2O / PH2), where PH2O is the partial pressure of water vapor in the atmosphere and PH2 is the partial pressure of hydrogen in the atmosphere. When the oxygen potential is set as above, for example, in an atmosphere containing about 10% hydrogen, the dew point will be about 0 to +20°C.

[0054] If the holding time at 650 to 800°C is less than 30 seconds, the concentration of B on the steel sheet surface will be insufficient. On the other hand, if the holding time exceeds 6000 seconds, not only will the effect saturate, but it will also be less economically viable, which is undesirable. This annealing process concentrates B on the surface of the steel sheet. The reason for concentrating B in the annealing process is that even if B is concentrated on the surface of the steel sheet in, for example, the hot rolling process, the B-concentrated layer is removed by pickling or the like before the subsequent cold rolling, and B cannot be diffused into the coating layer in the coating process. In addition, if B concentrates in the iron oxide formed during hot rolling and coiling, the B concentration in the steel sheet surface layer decreases, and therefore B concentration in the steel sheet surface layer in subsequent processes becomes insufficient.

[0055] After holding, the steel sheet is cooled so that the surface temperature falls to 500 to 400°C in preparation for the subsequent plating process. Generally, the temperature of the coating bath is about 450°C, so if a steel sheet at a temperature below 400°C is immersed in the coating bath, problems such as solidification of the coating occur, resulting in non-plating. On the other hand, if a steel sheet at a temperature above 500°C is immersed in the coating bath, zinc oxide is formed during the coating, resulting in non-plating. For this reason, steel sheets to be subjected to the coating process are cooled to 500 to 400°C. During cooling, reducing the temperature difference within the sheet makes the immersion temperature in the coating bath constant, which contributes to suppressing non-plating and uneven alloying, so the average cooling rate is preferably less than 2.0°C / second.

[0056] <Plating process> In the plating process, the cold-rolled steel sheet after the annealing process is immersed in a plating bath to achieve a coating weight of 90 g / m 2 The above zinc-based plating layer is formed. The plating bath temperature may be adjusted according to the chemical composition of the desired plating layer, and the plating bath temperature may be within a known range. The coating weight of the zinc-based plating layer can be adjusted by wiping or the like after removing it from the plating bath.

[0057] <Alloying process> In the steel sheet for hot stamping according to this embodiment, since it is desirable to reduce the Fe content in the coating layer in order to form a Γ layer after hot stamping heat treatment, alloying heat treatment is not performed, or the cold-rolled steel sheet on which the zinc-based coating layer has been formed is heated to 460 to 490°C to perform limited alloying. Here, limited alloying means that the alloying heat treatment is performed so that the Fe content in the chemical composition of the zinc-based coating layer is less than 9.0%. Heat treatment at a temperature exceeding 490°C is not preferred because the Fe content in the steel sheet for hot stamping becomes 9.0% or more. Alloying heat treatment at a temperature below 460°C requires a long time for alloying, resulting in reduced thermal efficiency and time efficiency. However, for the purpose of producing a hot-dip galvanized steel sheet, heat treatment may be performed in a temperature range that does not promote the alloying reaction.

[0058] <Cooling process> After the plating step or the alloying step, the cold-rolled steel sheet is cooled to 50° C. or below, for example, to room temperature. The cooling conditions are not limited.

[0059] [Hot stamped compact] Next, the hot-stamped product according to this embodiment will be described. The hot-stamped steel according to this embodiment has a base steel sheet and a coating layer containing Zn and Fe formed on the surface of the base steel sheet. The base steel sheet referred to here does not necessarily have to be a flat sheet, but also includes steel sheets formed into various shapes by press working or the like. The hot-stamped steel according to this embodiment is obtained by hot stamping the steel sheet for hot stamping according to this embodiment, as will be described later.

[0060] <Plating layer> The plating layer of the hot-stamped body according to this embodiment is a plating layer containing Zn and Fe, which is produced when the Zn in the zinc-based plating layer on the surface of the steel sheet for hot stamping is alloyed with the Fe in the steel sheet by hot stamping. In a coating layer containing Zn and Fe, multiple layers may be formed depending on the ratio of Zn and Fe. However, the coating layer of the hot stamped steel according to this embodiment has a Γ layer made of a Γ phase with an Fe content of 9 to 30 mass % at a coating weight of 40 g / m 2 That's all. This Γ layer has a chemical composition containing, for example, in mass %, Al: 0-1.00%, Fe: 9.0-30.0%, Si: 0-1.0%, Mg: 0-0.5%, Mn: 0-0.5%, Pb: 0-0.5%, Sb: 0-0.5%, Ni: 0-5.0%, Co: 0-5.0%, Mn: 0-5.0%, P: 0-0.5%, B: 0.0005-0.0500%, with the remainder being Zn and impurities. Furthermore, the coating layer of the hot-stamped steel according to this embodiment may have an outermost layer including one or more oxides of Al, Si, Mn, and Zn, a Γ layer on the base steel sheet side thereof, and a zinc solid solution of Fe on the base steel sheet side thereof. The deposition weight of the Γ layer with an Fe content of 9 to 30 mass% is 40 g / m 2 If the thickness is less than 40g / m, sufficient rust prevention cannot be ensured. 2 The amount of adhesion of the Γ layer is preferably 45 g / m 2 Although there is no upper limit to the amount of adhesion, the amount of adhesion of the Γ layer is set to 100 g / m 2 It may be less than 80 g / m 2 or less than 60g / m 2 The following may also be used. In a plating layer containing Zn and Fe, if the Fe content in either layer exceeds 30% by mass, sacrificial corrosion protection is lost.On the other hand, if the Fe content is less than 9% by mass, the Fe content in the zinc plating is too low, making the plating soft and easily scratched, which is undesirable. In addition to the Γ layer, a solid solution layer mainly composed of Fe was also formed at 5 g / m 2 However, there is no particular restriction on the presence of this layer.

[0061] After quenching by hot stamping, even if the oxide film on the surface is removed with an alkaline or acidic solution to improve paint adhesion and chemical conversion treatment properties, a Zn-Fe alloy layer (Γ layer) with 9 to 30 mass% Fe, mainly Zn, remains at 40 g / m. 2 As long as the above-mentioned properties are present, the hot-stamped steel according to the present embodiment is within the scope of the present invention. The plating layer may contain elements such as Ni, Co, Mn, P, and B to further improve corrosion resistance and chemical conversion treatability, but even if it contains these elements, it is a Γ layer if Zn is the main component (for example, 60 mass% or more) and Fe: 9 to 30 mass%.

[0062] The deposition amount of the Γ layer is determined by the following method. The Γ layer, which is mainly composed of Zn and contains 9 to 30 mass % of Fe, is measured by the following method: the hot stamped body is immersed in an aqueous solution of NH4Cl:150 g / l and subjected to a current of 4 mA / cm 2 Using a saturated calomel electrode as the reference electrode, electrolysis is performed up to the Γ layer at the point where the voltage changes significantly to -800 mV vs. SCE or less by constant current electrolysis, and the electrolyte is measured by ICP to identify the amount of adhesion and chemical composition. To measure the Zn-Fe alloy layer containing more than 30% by mass of Fe, after electrolysis up to the Γ layer, the electrolyte is replaced with a new solution and electrolysis is continued up to the iron potential (approximately -560 mV vs. SCE), and the electrolyte is similarly measured by ICP.

[0063] Furthermore, in the hot stamped steel according to this embodiment, when the maximum B content in mass% in the coating layer is Bp and the B content in mass% at the 1 / 4 depth position of the base steel sheet is Bq, Bp is 1.2 times or more of Bq (Bp / Bq≧1.2). By making Bp 1.2 times or more of Bq, the rust resistance of the hot stamped body can be improved. Although the detailed mechanism is unknown, the presence of B in the coating layer suppresses the alloying reaction between Fe and Zn during hot stamping, and the Γ layer can be made 40 g / m 2This is thought to be because it is possible to ensure a Bp / Bq of 1.2 or more, and by suppressing the non-uniform reaction of Fe and Zn in the plating layer, the formation of unevenness in the plating layer is suppressed. This effect is significant when Bp / Bq is 1.2 or more, so Bp / Bq is set to 1.2 or more. Bp / Bq is preferably 1.3 or more, and more preferably 1.4 or more.

[0064] Bp / Bq is calculated using the following method. To measure Bp, a 50mm x 50mm test piece is cut from the molded body and subjected to elemental analysis from the surface layer using GDS (glow discharge optical emission spectroscopy), which allows the maximum value of B in the coating layer, Bp, to be measured. Identification of whether it is a coating layer or a base material can be done by referring to the Zn and Fe contents. The region where the Fe content in Zn is 9 to 30% by mass is defined as the Γ layer, and the maximum value of the B content in mass% in the coating layer is defined as Bp. The B content in mass% at a 1 / 4 depth position in the base steel sheet was measured by cutting a test piece 50 mm wide x 50 mm long from the hot stamped body and polishing it to a 1 / 4 depth position in the base steel sheet, and then measuring the B content in mass% using GDS. In the measurement, the average B content in the range of 10 μm to 20 μm from the surface of the polished compact in the thickness direction is taken as Bq. The reason for measuring the B content at this position is that polished test pieces may have surface deposits, which can lead to a lack of quantitative accuracy. However, if the deposits can be removed, the average B content in the range of 0 to 10 μm from the polished surface can also be taken as Bq. The reason for taking the average B content in the 10 μm range (10 to 20 μm or 0 to 10 μm) in the thickness direction as Bq is to eliminate the influence of slight segregation formed during casting. On the other hand, although expanding the measurement range in the thickness direction is advantageous from the perspective of accuracy, expanding the measurement range in the thickness direction results in longer measurement times. Since the longer the measurement area in the depth direction, the longer the measurement time required for element distribution measurement in the thickness direction using GDS, it is desirable to limit the measurement to 100 μm.

[0065] <Base material steel plate> ≪Chemical composition≫ The hot-stamped steel according to this embodiment is obtained by hot stamping the steel sheet for hot stamping according to the embodiment described above. Since the chemical composition of the base steel sheet does not substantially change due to hot stamping, the chemical composition of the base steel sheet of the hot-stamped steel according to this embodiment has the same range and reasons for limitation as the chemical composition of the base steel sheet of the steel sheet for hot stamping according to this embodiment.

[0066] <Microstructure> The microstructure of the hot-stamped steel according to this embodiment needs to be controlled in accordance with the strength of the desired steel. In order to achieve a tensile strength of 800 MPa or more for the hot-stamped steel, the microstructure should contain, in volume fractions, 5 to 100% martensite, 0 to 95% ferrite, and the remainder bainite and / or pearlite at a 1 / 4 depth position, which is 1 / 8 to 3 / 8 of the thickness from the surface of the base steel sheet in the thickness direction. If the volume fraction of martensite is less than 5%, sufficient strength cannot be obtained. If the tensile strength of the hot stamped steel is to be in the range of 1000 MPa or more (for example, 1200 MPa or less), the martensite volume fraction is preferably 10% or more. If the tensile strength of the hot stamped steel sheet is to be in the range of more than 1200 MPa, the martensite volume fraction is preferably 80% or more. The balance other than martensite and ferrite is one or more (one or two) of bainite and pearlite. The balance may not be included.

[0067] The chemical composition and microstructure of the base steel sheet of the hot-stamped steel according to this embodiment can be measured in the same manner as the chemical composition and microstructure of the base steel sheet of the steel sheet for hot stamping according to this embodiment. As described above, the hot-stamped steel according to this embodiment is obtained by hot-stamping a steel sheet for hot stamping in which B in the surface layer portion of the base steel sheet is concentrated on the surface and the concentrated B on the surface is diffused into the coating layer. Therefore, in the surface layer portion of the base steel sheet of the hot-stamped steel, there may be a region where the B content (concentration) is reduced. Specifically, the B content at a depth of 10 μm from the surface of the base steel plate may be 80% or less of the B content at a depth of ¼ of the plate thickness of the base steel plate.

[0068] [Characteristics] The strength of the hot stamped body is not particularly specified, and excellent rust prevention properties can be ensured. However, since the main purpose of using a hot-stamped steel is to obtain a high-strength steel, the tensile strength of the hot-stamped steel is desirably 800 MPa or more. Depending on the application, the tensile strength of the hot-stamped steel may be 1000 MPa or more, or 1200 MPa or more.

[0069] [Method of manufacturing hot stamped compact] The hot-stamped steel according to this embodiment is obtained by hot stamping the steel sheet for hot stamping according to this embodiment. The hot stamping conditions may be within known ranges, but the steel sheet for hot stamping according to this embodiment is less affected by the heating conditions during hot stamping. For example, the steel sheet can be heated at a heating temperature of 850 to 920°C for a heating time of 180 to 600 seconds, and then cooled at an average cooling rate of 30°C / second or more to a martensitic transformation start temperature or lower while being formed. The hot stamped compact may be cooled in a die and produced to have a uniform strength as a compact, or may be gradually cooled in a die equipped with an air cooling or heater or the like simultaneously with die cooling, thereby producing different strengths depending on the part. [Example]

[0070] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0071] Slabs having the chemical compositions shown in Table 1 (units are mass %, the remainder being Fe and impurities) were cast. These slabs were hot-rolled under the conditions shown in Tables 2-1 and 2-2 to produce hot-rolled steel sheets with a thickness of 4.0 mm. The hot-rolled steel sheets were coiled and uncoiled, then pickled, and cold-rolled to a thickness of 2.0 mm to produce cold-rolled steel sheets. These cold-rolled steel sheets were then passed through a continuous hot-dip galvanizing line, where they were annealed and plated to obtain steel sheets for hot stamping. Specifically, the steel sheets were held for 120 seconds in the atmospheres shown in Tables 2-1 and 2-2, and cooled at an average cooling rate of 1.6°C / second until the surface temperature reached 500 to 400°C. After that, they were immersed in a plating bath at a bath temperature of 460°C to form a plating layer on the surface. They were then cooled to 50°C or below. Some of the steel sheets were subjected to limited alloying at the temperatures shown in the tables.

[0072] The microstructure of the base steel sheet and the chemical composition of the coating layer of the obtained steel sheet for hot stamping were measured by the methods described above. Bps / Bqs was also measured. The results are shown in Tables 2-3 and 2-4. The remainder of the chemical composition of the coating layer, other than B, Fe, and Al, was Zn and impurities including Mn, Si, Cr, Ti, etc. that had been mixed in from the steel sheet.

[0073] [Table 1]

[0074] [Table 2-1]

[0075] [Table 2-2]

[0076] [Table 2-3]

[0077] [Table 2-4]

[0078] These steel sheets for hot stamping (A1 to m1) were subjected to hot stamping. As shown in Tables 3-1 and 3-2, two samples were prepared for each example and inserted into an atmospheric furnace at a furnace temperature of 860 to 950°C. After two heating periods, one for 6 minutes and the other for 9 minutes, each sample was removed from the furnace and quenched using a mold (cooled to the martensitic transformation start temperature at an average cooling rate of 30°C / sec or more). This resulted in a hot-stamped compact.

[0079] The microstructure of the base steel sheet and the coating weight of the Γ layer of these hot-stamped steels were measured using the methods described above. The maximum B content of the coating layer was also measured using the method described above, and Bp / Bq was calculated. The results are shown in Tables 3-1 to 3-4. Note that, except for the coating weight of the Γ layer, the tables show the results of a test specimen heated for 9 minutes as an example.

[0080] Furthermore, the hot stamped steel sheets heated for 9 minutes, which is a more stringent evaluation, were evaluated for tensile strength and rust resistance in the following manner. The results are shown in Tables 3-3 and 3-4.

[0081] [Tensile strength] From the obtained hot stamped compact, a No. 5 test piece according to JIS Z 2241:2011 was cut out and subjected to a tensile test in accordance with JIS Z 2241:2011. The crosshead speed in the tensile test was a strain rate of 0.005 s -1 The experiment was carried out under conditions where the temperature was constant. It was determined that a tensile strength of 800 MPa or more would satisfy the expected target value.

[0082] [Rust prevention] (Corrosion resistance - Paint film swelling) A 70mm x 150mm sample was taken from the hot stamped body after production, and the surface was degreased, then chemically treated with Palbond LA35 (manufactured by Parkerizing Japan Co., Ltd.), and then coated with 15μm of cationic electrodeposition coating (Powernics 110: manufactured by Nippon Paint Co., Ltd.), after which cross-cutting was performed. This test piece was subjected to 300 cycles of corrosion testing under the conditions specified in the Society of Automobile Manufacturers Standard SAE-J2334, and the width of the paint film blister (on one side) from the cross-cut portion was measured. Those with a blister width of 10 mm or less were judged to have excellent corrosion resistance.

[0083] [Rust prevention] (Corrosion resistance - rust resistance) A 65mm x 120mm sample was taken from the hot stamped body after production, and the surface was degreased. Afterwards, a chemical conversion treatment was carried out using Palbond LA35 (manufactured by Nippon Parkerizing Co., Ltd.), followed by cationic electrodeposition coating (Powernics 110: manufactured by Nippon Paint Co., Ltd.) to form a coating film 15μm thick. The periphery and back of the sample were then sealed with resin tape, and a combined cyclic corrosion test using the following low-concentration salt water was carried out in accordance with JIS G 0594:2019. This cyclic corrosion test consisted of 1) salt spray (1 hour), 2) dry conditions (4 hours), and 3) wet conditions (3 hours), with a total of 8 hours per cycle, and the state of red rust formation was examined after 50 cycles. 1) Salt spray (1 hour) Sodium chloride concentration: 1g / L, pH: 6-7, temperature: 35°C, spray volume: 80cm 2 1.5ml / h 2) Under drying (4 hours) Temperature: 50℃, Humidity: 30% or less 3) Humidity (3 hours) Temperature: 40℃, Humidity: 90% Test piece placement angle: 20° from vertical The corrosion resistance was evaluated according to the following criteria. ◎: No red rust occurs ○: The total area ratio of red rust (including minute particles) is less than 5% △: The area ratio of red rust is 5 to 20% in total ×: The total area ratio of red rust is more than 20% A rating of ◯ was determined to be within the acceptable range, and a rating of ⊚ was determined to be excellent in red rust resistance.

[0084] [Rust prevention] (paint adhesion) A 70mm x 150mm sample was taken from the hot stamped body after production, and the surface was degreased. After that, a chemical conversion treatment was performed using Palbond LA35 (manufactured by Nippon Parkerizing Co., Ltd.), and then cationic electrodeposition coating (Powernics 110: manufactured by Nippon Paint Co., Ltd.) was performed to form a coating film with a thickness of 15μm. The test pieces were subjected to a hot saltwater immersion test to evaluate the paint film adhesion. In the hot saltwater immersion test, a 45mm cross-cut was made with a cutter on the chemically treated and electrodeposition coated samples, and the samples were then immersed in a 5% NaCl solution at 60°C for 240 hours, after which they were rinsed with water and dried. After drying, the sample was subjected to tape peeling in which adhesive tape was applied to the cut defect and then peeled off, and the maximum total peeled width, which was the maximum width of the coating peeled in the direction perpendicular to the length of the cut defect, was measured. Those with a maximum peel width of 5 mm or less were judged to have good adhesion to hot and salt water (coating adhesion).

[0085] [Table 3-1]

[0086] [Table 3-2]

[0087] [Table 3-3]

[0088] [Table 3-4]

[0089] As can be seen from Tables 1 to 3-4, in the invention examples A-1 to C-6, C-10, C-11, C-13, C-14, C-17 to D-6, D-10, D-11, E-1 to E-2, and F-1 to T-1, the steel sheets had a predetermined chemical composition and a coating weight of 90 g / m 2 As described above, Bps was 1.2 times or more of Bqs, and the hot stamped compacts obtained by hot stamping these steels, YA-1 to YC-6, YC-10, YC-11, YC-13, YC-14, YC-17 to YD-6, YD-10, YD-11, YE-1 to YE-2, and YF-1 to YT-1, had high strength and excellent rust prevention properties. On the other hand, the comparative examples C-7 to C-9, C-12, C-15, C-16, D7 to D-9, D-12, E3 to E-5, a-1 to c-1, e-1 to f-1, and j-1 to m-1 had chemical compositions outside the specified range, or Bps was less than 1.2 times Bqs. The comparative examples YC-7 to YC-9, YC-12, YC-15, C-16, YD7 to YD-9, YD-12, YE3 to YE-5, Ya-1 to Yc-1, Ye-1 to Yf-1, and Yj-1 to Ym-1, which were hot stamped, did not have sufficient tensile strength and rust resistance. In addition, in d-1 and g-1 to i-1, the strength of the hot-rolled steel sheet was too high and it broke during cold rolling, or the P and S were too high and it broke during cold rolling, and it could not be passed through to the subsequent process. [Industrial Applicability]

[0090] According to the present invention, it is possible to provide a hot-stamped product that can be produced under a wide range of heating conditions and has excellent rust prevention properties (corrosion resistance and paint film adhesion), a steel sheet for hot stamping that is suitable as a material for the hot-stamped product, and a method for producing the steel sheet for hot stamping.

Claims

1. Base steel plate and a zinc-based plating layer formed on the surface of the base steel sheet; and The base steel plate is, in mass%, C: 0.030-0.600%, Si: 0.01-1.50%, Mn: 0.10 to 2.50%, Al: 0.001-0.100%, Ti: 0.010 to 0.100%, B: 0.0005-0.0100%, P: 0.100% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V: 0 to 0.500%, W: 0-0.500%, Cr: 0-1.00%, Mo: 0 to 0.50%, Co: 0-1.000%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, REM: 0-0.0100%, Zr: 0 to 0.0500%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, As: 0 to 0.100%, Sn: 0 to 0.50%, Sb: 0 to 0.50%, and Remainder: Fe and impurities and a chemical composition consisting of The microstructure at a 1 / 4 depth position in the range of 1 / 8 to 3 / 8 of the plate thickness from the surface of the base steel plate in the plate thickness direction is, in terms of volume fraction, Ferrite: 20 to 95%, Perlite: 5 to 80%, The remaining structure is bainite, The zinc-based plating layer has a coating weight of 90 g / m ２ That's all, When the maximum value of the B content in the zinc-based plating layer is defined as Bps and the B content at the 1 / 4 depth position of the base steel sheet is defined as Bqs, Bps is 1.2 times or more of Bqs. A steel sheet for hot stamping, characterized in that:

2. The chemical composition of the zinc-based plating layer is, by mass, Fe: less than 9.0% The steel sheet for hot stamping according to claim 1 ,

3. The chemical composition of the base steel plate is, in mass%, Nb: 0.005-0.050%, V: 0.005-0.500%, W: 0.005-0.500%, Cr: 0.01-1.00%, Mo: 0.01-0.50%, Co: 0.01-1.000%, Ni: 0.01-1.00%, Cu: 0.01 to 1.00%, REM: 0.0003-0.0100%, Zr: 0.0003 to 0.0500%, Ca: 0.0003-0.0100%, Mg: 0.0003 to 0.0100%, As: 0.001 to 0.100%, Sn: 0.01 to 0.50%, and Sb: 0.01 to 0.50%, Contains one or more selected from the group consisting of The steel sheet for hot stamping according to claim 1 or 2.

4. A method for manufacturing a steel sheet for hot stamping according to claim 1, comprising: In mass%, C: 0.030 to 0.600%, Si: 0.01 to 1.50%, Mn: 0.10 to 2.50%, Al: 0.001 to 0.100%, Ti: 0.010 to 0.100%, B: 0.0005 to 0.0100%, P: 0.100% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V: 0 to 0.500%, W: 0 to 0.500%, Cr: 0 to a casting step of casting a steel ingot or slab having a chemical composition consisting of: 1.00%, Mo: 0-0.50%, Co: 0-1.000%, Ni: 0-1.00%, Cu: 0-1.00%, REM: 0-0.0100%, Zr: 0-0.0500%, Ca: 0-0.0100%, Mg: 0-0.0100%, As: 0-0.100%, Sn: 0-0.50%, Sb: 0-0.50%, and the balance: Fe and impurities; a hot rolling step of heating the steel ingot or the slab to 1100°C or higher and hot rolling the steel ingot or the slab at a finish rolling temperature of 800°C or higher to obtain a hot-rolled steel sheet; a coiling step of coiling the hot-rolled steel sheet after the hot rolling step in a temperature range of 600°C or less; a cold rolling step of pickling the hot-rolled steel sheet after the coiling step and then cold-rolling it at a cumulative reduction rate of 30 to 80% to obtain a cold-rolled steel sheet; an annealing step in which the cold-rolled steel sheet is held in an annealing furnace in which the oxygen potential of the atmosphere is −1.20 to −0.50 and the temperature of the atmosphere is 650 to 800° C. for 30 seconds or more, and annealed by cooling the steel sheet so that the surface temperature of the steel sheet becomes 500 to 400° C.; The cold-rolled steel sheet after the annealing step was immersed in a plating bath to have a coating weight of 90 g / m ２ a plating step of forming the above zinc-based plating layer; a cooling step of cooling the cold-rolled steel sheet after the plating step to 50°C or less; Equipped with A method for producing a steel sheet for hot stamping, comprising:

5. A base steel plate; a plating layer containing Zn and Fe formed on a surface of the base steel sheet; and The base steel plate is, in mass%, C: 0.030-0.600%, Si: 0.01-1.50%, Mn: 0.10 to 2.50%, Al: 0.001-0.100%, Ti: 0.010 to 0.100%, B: 0.0005-0.0100%, P: 0.100% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V: 0 to 0.500%, W: 0-0.500%, Cr: 0-1.00%, Mo: 0 to 0.50%, Co: 0-1.000%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, REM: 0-0.0100%, Zr: 0 to 0.0500%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, As: 0 to 0.100%, Sn: 0 to 0.50%, Sb: 0 to 0.50%, and Remainder: Fe and impurities and a chemical composition consisting of The microstructure at a 1 / 4 depth position in the range of 1 / 8 to 3 / 8 of the plate thickness from the surface of the base steel plate in the plate thickness direction is, in terms of volume fraction, Martensite: 5 to 100%, Ferrite: 0 to 95%; The remaining structure is composed of one or both of bainite and pearlite, Of the plating layers, the coating weight of the Γ layer having an Fe content of 9 to 30 mass % is 40 g / m ２ That's all, a hot-stamped steel, characterized in that, when a maximum value of the B content in mass% in the plating layer is defined as Bp and a B content in mass% at the 1 / 4 depth position of the base steel sheet is defined as Bq, the Bp is 1.2 times or more of the Bq.

6. The chemical composition of the base steel plate is, in mass%, Nb: 0.005-0.050%, V: 0.005-0.500%, W: 0.005-0.500%, Cr: 0.01-1.00%, Mo: 0.01-0.50%, Co: 0.01-1.000%, Ni: 0.01-1.00%, Cu: 0.01 to 1.00%, REM: 0.0003-0.0100%, Zr: 0.0003 to 0.0500%, Ca: 0.0003-0.0100%, Mg: 0.0003 to 0.0100%, As: 0.001 to 0.100%, Sn: 0.01 to 0.50%, and Sb: 0.01 to 0.50%, Contains one or more selected from the group consisting of The hot stamped product according to claim 5 .

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