BODY FORMED BY HOT STAMPING

Abstract

A hot-stamped body includes a steel sheet and a zinc-plated layer provided over the steel sheet. The steel sheet has a predetermined chemical composition, and a certain percentage of the martensitic area has a microstructure of 90% or more at a position corresponding to 1 / 4 of the steel sheet thickness from a surface of the steel sheet in the thickness direction. The zinc-plated layer includes a G phase and a Fe-Zn solid solution, and the cross-sectional area ratio of the voids present in the zinc-plated layer is 15.0% or less.

Description

BODY FORMED BY HOT STAMPING Technical field of the invention [1] The present invention relates to a body formed by hot stamping. Priority is claimed in Japanese Patent Application No. 2020-057272, filed on March 27, 2020, the contents of which are incorporated herein by reference. Background of the invention [2] In recent years, there has been an increasing need to increase the strength of vehicle members from the perspective of stricter vehicle crash safety criteria and improved fuel efficiency. The application of hot stampings has become widespread to achieve an increase in the strength of vehicle members. Hot stamping is a technique of pressing a blank that is heated to a temperature (point A) at which the single-phase austenite region is formed, or higher (e.g., heated to about 900°C), and then rapidly quenching the blank in a die while forming to perform quenching. According to this technique, it is possible to manufacture a press-formed product that has both high shape-holding ability and high strength. [3] Since a zinc component remains in the surface layer of a shaped product obtained after hot stamping when hot stamping is applied to a zinc-plated steel sheet, an effect of improving corrosion resistance is obtained compared with a shaped product obtained by hot stamping an unplated steel sheet. Therefore, the application of hot stamping on zinc-plated steel sheet is becoming more widespread. [4] Patent Document 1 describes a hot-pressed formed steel member manufactured by a method including a heating step of heating a galvanized steel sheet to a temperature equal to or higher than a point Here and a hot-press forming step of performing hot-press forming at least twice after the heating step and wherein all of the hot-press forming performed in the hot-press forming step is performed to satisfy a predetermined equation. [5] In the case where the zinc-plated steel sheet is subjected to hot stamping, electrode sticking (a phenomenon in which a copper electrode and the plating provided on the surface of the formed product melt and adhere to each other) may occur during spot welding in a formed product obtained after hot stamping. It is not preferable for electrodes to stick during spot welding because it may cause poor welding or will inevitably cause manufacturing downtime for replacing the copper electrode. Electrode sticking during spot welding is not considered in Patent Document 1. Prior art document Patent document [6] Patent Document 1: International Publication of the PCT No. WO2013 / 147228 Description of the invention Problems to be solved by invention [7] The present invention has been made taking into account the above-mentioned circumstances, and an object of the present invention is to provide a hot-stamping formed body that is excellent in spot weldability. Furthermore, an object of the present invention is to provide a hot-stamping formed body that has the above-mentioned property and has the strength generally required for a hot-stamping formed body. Means to solve the problem [8] The present inventor investigated the cause of electrode sticking during spot welding. As a result, the present inventor found that electrode sticking during spot welding is further suppressed since the number of voids present in a zinc-plated layer is smaller, since electrode sticking during spot welding is greatly affected by voids (voids) present in the zinc-plated layer. The present inventor thought that overcurrent is caused by a narrow electric current path caused by the voids in the zinc-plated layer, and the overcurrent causes overheating which makes it easy for the electrode to stick between an electrode and the zinc plating. [9] Furthermore, although the detailed mechanism is uncertain, the present inventor thought that the occurrence of voids is caused by a difference in thermal contraction between a base metal and a zinc-plated layer and a difference in thermal contraction between different phases (a Γ phase and a Fe-Zn solid solution) present in the zinc-plated layer during hot stamping forming. As a result of researching a method for suppressing the occurrence of voids, the present inventor discovered that a predetermined contact pressure during hot stamping forming can flatten voids (i.e., the pressure can reduce the density of the number of voids present in the zinc-plated layer), and that results in improved spot weldability. The excellent spot weldability means that sticking of electrodes during spot welding can be suppressed.Furthermore, the tensile strength (maximum) generally required for a body formed by hot stamping is in a range of 1500 MPa to 2500 MPa.

[10] The essence of the present invention made on the basis of the above-mentioned knowledge is as follows. [1] A hot stamping formed body according to an aspect of the present invention includes a steel sheet and a zinc plated layer provided on the steel sheet. The steel sheet has a chemical composition containing, in mass %, C: 0.18% to 0.50%, Si: 0.10% to 1.50%, Mn: 1.5% to 2.5%, Al sol.: 0.001% to 0.100%, Ti: 0.010% to 0.100%, S: 0.0100% or less, P: 0.100% or less, N: 0.010% or less, Nb: 0% to 0.05%, V: 0% to 0.50%, Cr: 0% to 0.50%, Mo: 0% to 0.50%, B: 0% to 0.010%, Ni: 0% to 2.00%, and a total of REM, Ca, Co, and Mg: 0% to 0.030%.A remainder consists of Fe and impurities, a martensite area % has a microstructure of 90% or more at a position corresponding to 1 / 4 of the sheet thickness of the steel sheet from a surface of the steel sheet in the sheet thickness direction, the zinc-plated layer includes a Γ phase and a Fe-Zn solid solution, and a cross-sectional area ratio of voids present in the zinc-plated layer is 15.0% or less. [2] In the body formed by hot stamping according to [1], the chemical composition may contain, in % by mass, one or two or more selected from the group consisting of Nb: 0.02% to 0.05%, V: 0.005% to 0.50%, Cr: 0.10% to 0.50%, Mo: 0.005% to 0.50%, B: 0.0001% to 0.010%, Ni: 0.01% to 2.00%, and a total of REM, Ca, Co and Mg: 0.0003% to 0.030%. [3] In the body formed by hot stamping in accordance with [1] or [2], the chemical composition may contain, in % by mass, C: 0.24% to 0.50%. Effects of the invention

[11] In accordance with the aspect of the present invention, it is possible to provide a hot stamping formed body which is excellent in spot weldability and has the strength generally required for a hot stamping formed body. Modalities of the invention

[12] A hot-stamped formed body according to this method will be described in detail below. First, the reason for limiting the chemical composition of a steel sheet of the hot-stamped formed body according to this method will be described. All percentages (%) related to the chemical composition represent % by mass.

[13] A steel sheet of the body formed by hot stamping in accordance with this method contains, in % by mass: C: 0.18% to 0.50%; Si: 0.10% to 1.50%; Mn: 1.5% to 2.5%; Al sol.: 0.001% to 0.100%; Ti: 0.010% to 0.100%; S: 0.0100% or less; P: 0.100% or less; N: 0.010% or less; and a remainder consisting of Fe and impurities. Each element will be described below.

[14] C: 0.18% to 0.50% C is an element that improves the strength of the hot-stamped formed body. The C content is set to 0.18% or more to achieve the desired strength. The C content is preferably 0.20% or more, and most preferably 0.24% or more. On the other hand, if the C content exceeds 0.50%, the strength is excessively high, deteriorating the ductility and toughness of the hot-stamped formed body. For this reason, the C content is set to 0.50% or less. The C content is preferably 0.40% or less.

[15] Si: 0.10% and 1.50% Si is an element that improves fatigue properties. Furthermore, Si also improves hot-dip galvanizing properties, particularly the wettability of the plating, by forming a stable oxide film on the surface of the steel sheet during recrystallization annealing. To achieve these effects, the Si content is set to 0.10% or more. The Si content is preferably 0.15% or more. Furthermore, if the Si content is excessively high, the Si content in the steel diffuses during heating during hot stamping and forms oxide on the surface of the steel sheet. The oxide formed on the surface of the steel sheet impairs the properties of phosphate treatment. Furthermore, Si is also an element that raises the Acs point of the steel sheet.If the A point is elevated, the heating temperature must be increased to sufficiently austenitize the steel sheet, and the heating temperature during hot stamping exceeds the evaporation temperature of galvanizing. For this reason, the Si content is set to 1.50% or less. The Si content is preferably 1.40% or less.

[16] Mn: 1.5% to 2.5% Mn is an element that improves the hardenability of steel. The Mn content is adjusted to 1.5% or more to improve hardenability and obtain the desired amount of martensite. The Mn content is preferably 1.8% or more. On the other hand, even if the Mn content exceeds 2.5%, the hardenability-enhancing effect is saturated and the steel becomes embrittled, making quenching cracks more likely to occur during casting, hot rolling, and cold rolling. For this reason, the Mn content is adjusted to 2.5% or less. The Mn content is preferably 2.1% or less.

[17] In the sun: 0.001% to 0.100% Al is an element that deoxidizes molten steel to suppress the formation of oxide, which causes fracture. Furthermore, Al is also an element that suppresses an alloying reaction between Zn and Fe and improves corrosion resistance. To achieve these effects, the sol-Al content is set to 0.001% or more. The sol-Al content is preferably 0.005% or more. On the other hand, if the sol-Al content is excessive, the AC3 point of the steel sheet is raised, the heating temperature must be increased to sufficiently austenitize the steel sheet, and the heating temperature during stamping exceeds the evaporation temperature of zinc plating. For this reason, the sol-Al content is set to 0.100% or less. The sol-Al content is preferably 0.090% or less. In this embodiment, Al sol. stands for acid-soluble Al and indicates the Al solute present in the steel in a solid solution state.

[18] Ti: 0.010% to 0.100% Ti is an element that increases oxidation resistance after galvanizing. Furthermore, Ti is also an element that improves hardenability by combining with N to form nitride (TiN) and suppressing the formation of nitride (BN) from B. To achieve these effects, the Ti content is adjusted to 0.010% or more. The Ti content is preferably 0.020% or more. On the other hand, if the Ti content is excessive, the Acg point is raised and the heating temperature is increased during hot stamping. For this reason, productivity may deteriorate, and it may be difficult to ensure a Γ phase since the formation of a Fe-Zn solid solution may be facilitated. Furthermore, if the Ti content is excessive, a large amount of Ti carbide is formed and the amount of solute C is reduced, thereby reducing strength.Furthermore, the wettability of the coating may deteriorate, and its toughness may be impaired due to excessive precipitation of Ti carbide. For this reason, the Ti content is adjusted to 0.100% or less. The Ti content is preferably 0.070% or less.

[19] S: 0.0100% or less S is an impurity element that forms sulfide in steel, causing toughness deterioration and impairing delayed fracture resistance. For this reason, the S content is set to 0.0100% or less. The S content is preferably 0.0050% or less. A 0% S content is preferable. However, since the cost of removing S increases if the S content is reduced excessively, the S content can be set to 0.0001% or more.

[20] P: 0.100% or less P is an impurity element that segregates at grain boundaries, impairing toughness and resisting delayed fracture. For this reason, the P content is set to 0.100% or less. The P content is preferably 0.050% or less. A P content of 0% is preferable. However, since the cost of removing P increases if the P content is reduced excessively, the P content can be set to 0.001% or more.

[21] N: 0.010% or less N is an impurity element that forms coarse nitride in steel, impairing its toughness. Furthermore, N is also an element that facilitates the formation of blow holes during spot welding. Furthermore, when B is present, N combines with B to reduce the amount of solute B and impairs hardenability. For this reason, the N content is adjusted to 0.010% or less. The N content is preferably 0.007% or less. A N content of 0% is preferable. However, since manufacturing costs increase if the N content is reduced excessively, the N content can be adjusted to 0.0001% or more.

[22] The remaining chemical composition of the steel sheet of the body formed by hot stamping in accordance with this method is Fe and impurities. The elements, which are inevitably mixed from a raw material or steel scrap and / or during steelmaking and are allowed in a range where the properties of the body formed by hot stamping in accordance with this method are not deteriorated, are illustrated as impurities.

[23] The steel sheet of the body formed by hot stamping in accordance with this embodiment may contain the following elements as arbitrary elements instead of a part of Fe. The content of the following arbitrary elements in a case where the following arbitrary elements are not contained is 0%.

[24] Nb: 0% to 0.05% Nb is a carbide-forming element in steel, which refines crystal grains during hot stamping and improves the toughness of the hot stamping body. To achieve this effect reliably, it is preferable to set the Nb content at 0.02% or more. On the other hand, even if the Nb content exceeds 0.05%, the aforementioned effect saturates and hardenability deteriorates. For this reason, the Nb content is set at 0.05% or less.

[25] V: 0% to 0.50% V is an element that finely forms carbonitride in steel to improve strength. To achieve this effect reliably, it is preferable to set the V content at 0.005% or more. On the other hand, if the V content exceeds 0.50%, the steel's toughness deteriorates during spot welding, and cracking is likely to occur. For this reason, the V content is set at 0.50% or less.

[26] Cr: 0% to 0.50% Cr is an element that improves the hardenability of steel. To achieve this effect reliably, it is preferable to set the Cr content at 0.10% or more. On the other hand, if the Cr content exceeds 0.50%, Cr carbide forms in the steel, and it is difficult for the Cr carbide to dissolve during hot stamping heating, thus deteriorating hardenability. For this reason, the Cr content is set at 0.50% or less.

[27] Mo: 0% to 0.50% Molecular weight is an element that improves the hardenability of steel. To achieve this effect reliably, it is preferable to set the Mole content at 0.005% or higher. On the other hand, even if the Mole content exceeds 0.50%, the hardenability improvement effect is saturated. For this reason, the Mole content is set at 0.50% or lower.

[28] B: 0% to 0.010% B is an element that improves the hardenability of steel. To achieve this effect reliably, it is preferable to set the B content at 0.0001% or more. On the other hand, even if the B content exceeds 0.010%, the hardenability improvement effect is saturated. For this reason, the B content is set at 0.010% or less.

[29] Ni: 0% to 2.00% Ni is an element that improves the toughness of steel, suppresses the embrittlement of steel caused by liquid Zn during hot stamping, and improves the hardenability of steel. To reliably achieve these effects, it is preferable to set the Ni content at 0.01% or more. On the other hand, even if the Ni content exceeds 2.00%, the aforementioned effects are saturated. For this reason, the Ni content is set at 2.00% or less.

[30] Total REM, Ca, Co and Mg: 0% to 0.030% REM, Ca, Co, and Mg are elements that suppress the appearance of cracks during spot welding by controlling sulfide and oxide in a preferred manner and suppressing the formation of coarse inclusions. To reliably achieve this effect, it is preferable to set the total content of REM, Ca, Co, and Mg to 0.0003% or more. To reliably achieve the aforementioned effect, the content of any of REM, Ca, Co, and Mg can be 0.0003% or more. On the other hand, if the total content of REM, Ca, Co, and Mg exceeds 0.030%, excessive inclusions will be generated and cracks are likely to occur during spot welding. For this reason, the total content of REM, Ca, Co, and Mg is set to 0.030% or less.

[31] In this embodiment, REM refers to a total of 17 elements which are composed of Se, Y and lanthanoid and REM content refers to the total content of these elements.

[32] The chemical composition of the steel sheet described above can be measured by a general analysis method. For example, the chemical composition of the steel sheet described above can be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S can be measured using an infrared absorption-combustion method, and N can be measured using an inert gas fusion thermal conductivity method. In addition, Al sol. can be measured by ICP-AES using a filtrate obtained in the case where a sample is decomposed with an acid by heating. The chemical composition can be analyzed after the zinc-plated layer provided on the surface of the hot-stamping body can be removed by mechanical grinding.

[33] The microstructure of the steel sheet of the body formed by hot stamping in accordance with this method will be described below. In the microstructure of the steel sheet of the body formed by hot stamping according to this method at a position corresponding to 1 / 4 of the sheet thickness from the surface of the steel sheet in the sheet thickness direction, the area % of martensite is 90% or more. In this embodiment, the microstructure at a position corresponding to 1 / 4 of the sheet thickness from the steel sheet surface in the sheet thickness direction (a region between a position corresponding to 1 / 8 of the sheet thickness from the steel sheet surface and a position corresponding to 3 / 8 of the sheet thickness from the steel sheet surface) is shown. The reason for this is that this depth position is midway between the steel sheet surface and a center position of the sheet thickness, and the microstructure at this depth position typifies the steel structure of the steel sheet (it shows the average microstructure of the entire steel sheet).

[34] Martensite: 90% or more Martensite is a structure that improves the strength of steel sheets. If the area ratio of martensite is less than 90%, the desired strength cannot be achieved in the hot-formed body. For this reason, the martensite area ratio is set to 90% or more. The martensite area ratio is preferably 95% or more or 96% or more. Since a higher martensite area ratio is more preferable, the upper limit of the martensite area ratio can be set to 100%.

[35] Ferrite, pearlite, bainite, and residual austenite are included in the microstructure of the steel sheet of the hot-stamping formed body according to this embodiment as the remainder of the microstructure. Since a desired strength cannot be obtained in a case where the area ratio of the remainder in the microstructure is high, the area ratio of the remainder in the microstructure may be set to 10% or less. The area ratio of the remainder in the microstructure is preferably 5% or less, more preferably 4%, and still most preferably 0%.

[36] The area ratio of martensite is measured by the following method. A sample is cut from an arbitrary position away from an end surface of the hot-stamped formed body at a distance of 50 mm or more (a position avoiding an end portion in case the sample cannot be taken from this position) such that a cross-section (cross-section of the sheet thickness) perpendicular to the surface can be observed. The sample size also depends on a measuring device, but is set to a size that allows the size to be observed within about 10 mm in the rolling direction. In the case where the hot-stamped formed body includes a welded portion, a sample is taken from a position avoiding the welded portion and the proximity of the welded portion.

[37] The cross-section of the sample is etched using LePera's reagent.A position corresponding to t / 4 (t denotes the thickness of the sheet) of the cross section etched with LePera's reagent (a region between a position corresponding to 1 / 8 of a sheet thickness from the sample surface and a position corresponding to 3 / 8 of a sheet thickness from the sample surface) is observed in 10 visual fields at a magnification of 500, and image analysis is performed on an obtained optical microscope image using image analysis software Photoshop CS5 manufactured by Adobe Systems Inc. to obtain the area ratio of martensite. As an image analysis method, the maximum brightness value Lmax and the minimum brightness value Lmin of an image are obtained from the image, a portion including pixels having a brightness in a range of Lmax-0.3 (Lmax-Lmin) to Lmax is defined as a white region, a portion including pixels having a brightness in a range of Lmin to Lmin+0.3 (Lmax-Lmin) is defined as a black region, other portions are defined as a gray region, and the martensite area ratio is calculated, which is a white region. Image analysis is performed in the same manner as described above for a total of 10 observed fields of view to measure martensite area ratios, and these area ratios are averaged to calculate an average value. The obtained average value is considered as the martensite area ratio. As a result, the martensite area ratio is obtained. Furthermore, the area ratio of martensite is subtracted from 100% to obtain the area ratio of the remainder in the microstructure.

[38] Next, a zinc-plated layer forming the hot-stamping formed body according to this embodiment will be described. The zinc-plated layer is provided on the aforementioned steel sheet and includes a Γ phase and a Fe-Zn solid solution, and the cross-sectional area ratio of voids present in the zinc-plated layer is 15.0% or less. The zinc-plated layer may be provided on both surfaces or on either surface of the aforementioned steel sheet. Furthermore, the zinc-plated layer refers to a layer in which the Γ phase and the Fe-Zn solid solution are present. The Γ phase and the Fe-Zn solid solution will be described later. The zinc plated layer will be described below.

[39] Including the Γ phase and Fe-Zn solid solution The zinc-plated layer includes the Γ phase and the Fe-Zn solid solution. The Γ phase is a layer having a Zn concentration close to the Zn concentration of a zinc bath. The Fe-Zn solid solution is a phase generated when the zinc in the zinc bath and the Fe contained in the steel sheet are alloyed with each other. For this reason, the Fe concentration of the Γ phase is lower than that of the Fe-Zn solid solution. In this embodiment, a phase with an Fe concentration in a range of 10 mass % to 30 mass % is defined as the Γ phase, and a phase with an Fe concentration in a range of 50 mass % to 80 mass % is defined as the Fe-Zn solid solution. In addition to the Γ phase and the Fe-Zn solid solution, a δι phase and an ξ phase can be included in the zinc-plated layer. The Fe concentration of each of the δι and ξ phases is less than 10% by mass.

[40] In the case where the Γ phase is not included in the zinc-plated layer due to over-alloying of the zinc-plated layer, the corrosion resistance deteriorates compared to a zinc-plated layer that includes the Γ phase. Furthermore, the fact that the Γ phase is not included in the zinc-plated layer means that the zinc-plated layer is being alloyed. As the zinc-plated layer is alloyed (in the case where the zinc-plated layer is an alloyed zinc-plated layer), an oxide film (ZnO) forms and grows on the surface of the plating, which increases the contact resistance during spot welding. As a result, extrusion is likely to occur. For this reason, it is important that the Γ phase is included in the zinc-plated layer.

[41] Since the properties of the body formed by hot stamping according to this embodiment can be exhibited as long as even a small amount of the Γ phase is included in the zinc-plated layer, the ratio of the Γ phase in the zinc-plated layer is not particularly limited. The adhesion amount of the zinc-plated layer depends on the desired corrosion resistance target, but can be set within a range of, for example, 5 g / m2 to 150 g / m2 per side. However, for example, to ensure corrosion resistance equal to or greater than the corrosion resistance of a cold-formed article of galvanized steel sheet, the amount of the Γ phase can be set to 30 g / m2 or more per side.The reason for this is that since the Fe-Zn solid solution generated by heating during cooling causes oxidation of Fe and increases in volume in the case where the amount of Γ phase is less than 30 g / m2, the oxidized Zn during corrosion does not form a dense protective film and the corrosion resistance equal to or higher than the corrosion resistance of a cold-formed article of zinc-plated steel sheet cannot be obtained.

[42] The analysis of Fe concentration in the zinc-plated layer is performed using the following method. A sample is cut from an arbitrary position away from an end surface of the hot-stamping formed body at a distance of 50 mm or more (a position avoiding an end portion in case the sample cannot be taken from this position) such that a cross-section (sheet thickness cross-section) perpendicular to the surface can be observed. The sample size also depends on a measuring device, but is set to a size that allows the size to be observed within about 10 mm in the rolling direction. In the case where the hot-stamping formed body includes a welded portion, a sample is taken from a position avoiding the welded portion and the proximity of the welded portion.

[43] After the sample is embedded in a resin and polished, the layer structure of a sheet thickness cross-section is observed with a scanning electron microscope (SEM).Specifically, the layer structure of the cross-section of the sheet thickness is observed by SEM at a magnification where the steel sheet and the zinc-plated layer are included in an observed field of view. Then, a linear analysis is performed in the sheet thickness direction from the surface using SEM-energy dispersive X-ray spectroscopy (SEM-EDS) to specify each layer in the layer structure of the cross-section of the sheet thickness, and the quantitative analysis of the Fe concentration of each layer is performed. The linear analysis is performed on the observation cross-section of the sample at 100 points arranged at intervals of 0.1 pm in a direction parallel to the surface.In linear analysis, quantitative analysis is performed at 1 nm intervals in the film thickness direction by energy-dispersive X-ray spectroscopy (EDS) in which the diameter of an electron beam is set to 10 nm. A phase whose Fe concentration is in a range of 10 mass% to 30 mass% is defined as the Γ phase, and a phase in which the Fe concentration is in a range of 50 mass% to 80 mass% is defined as the Fe-Zn solid solution. Phases in which the Fe concentration is less than 10 mass% are defined as the 51 phase and the ξ phase.

[44] Next, a linear analysis is performed in the thickness direction of the film using SEM-energy dispersive X-ray spectroscopy (EDS), and quantitative analysis of the Fe concentration of each layer is performed. The device to be used is not particularly limited, but, for example, SEM (NB5000 manufactured by Hitachi High-Tech Corporation), EDS (XFlash (r)6|30 manufactured by Bruker AXS Inc.), and EDS analysis software (ESPRIT1.9 manufactured by Bruker AXS Inc.) can be used in this mode. From the COMBO image observation results and the SEM-EDS quantitative analysis results described above, a region, which is present at the deepest position in the thickness direction of the film and in which the Fe content exceeds 80 mass% excluding measurement noise, is determined to be a steel film.Furthermore, a phase whose Fe content is in a range of 10 mass% to 30 mass% excluding measurement noise is determined as the Γ phase, and a region in which the Fe concentration is in a range of 50 mass% to 80 mass% is determined as Fe-Zn solid solution.

[45] Next, a method for measuring the Γ phase included in the zinc-plated layer will be described. A test piece is taken from the hot-stamping formed body, and this test piece is immersed in an aqueous solution of NH4Cl:150g / L. Constant current electrolysis is performed at 4 mA / cm2 using a saturated calomel electrode as a reference electrode, and a range in which the electric potential is -800 mV versus SCE or less is regarded as the Γ phase. The reason for this is that this range can be regarded as the Γ phase containing Zn as the main component and in which the Fe content is 30% by mass or less. An electrolyte obtained from electrolysis of the Γ phase is measured using inductively coupled plasma (ICP), and the sum of the amount of Fe and the amount of Zn is regarded as the amount of the Γ phase.

[46] Ratio of cross-sectional area of ​​voids present in the zinc-plated layer: 15.0% or less In a case where the cross-sectional area ratio of the voids present in the zinc-plated layer is set to 15.0% or less, it is possible to suppress electrode sticking during spot welding of the body formed by hot stamping. The inventor of the present invention reasoned that electrode sticking is likely to occur in a case where the cross-sectional area ratio of the voids exceeds 15.0%, which means that the electric current path is locally narrow enough to cause an overcurrent that causes overheating during spot welding. For this reason, in this embodiment, the cross-sectional area ratio of the voids present in the zinc-plated layer of at least one region serving as the portion to be welded is set to 15.0%. The cross-sectional area ratio of the voids present in the zinc-plated layer is preferably 13.0% or less, most preferably 10.0% or less, and most preferably 5.0% or less. Since a lower cross-sectional area ratio of the voids present in the zinc-plated layer is more preferable, the lower limit of the cross-sectional area ratio of the voids present in the zinc-plated layer can be set to 0%.

[47] The cross-sectional area ratio of voids present in the zinc plated layer is measured using the following method. First, a sample is cut from an arbitrary position away from an end surface of the hot-formed body at a distance of 50 mm or more (a position that avoids an end portion in case the sample cannot be taken from this position) so that a cross-section (a cross-section of the sheet thickness) perpendicular to the surface can be observed. The sample size also depends on a measuring device, but it is set to a size that allows the size to be observed within about 10 mm in the rolling direction.

[48] ​​Next, after an observation cross-section is polished and imaged with a scanning electron microscope (SEM) at a magnification of 300, the void area ratio of the cross-section is calculated by binarization image processing. A device used for calculating the void area ratio of the cross-section is not particularly limited, but, for example, the built-in software of a VHX-5000 digital microscope manufactured by Keyence Corporation can be used to determine voids using luminance and to automatically measure the void area. The cross-sectional area ratio of voids present in the zinc plated layer is obtained using the above method.

[49] Sheet thickness and tensile strength. The sheet thickness of the body formed by hot stamping in accordance with this method is not particularly limited. However, in terms of reducing the weight of a vehicle body, it is preferable for the sheet thickness of the body formed by hot stamping in accordance with this method to be within a range of 0.5 mm to 3.5 mm. Furthermore, in terms of reducing the weight of a vehicle body, it is preferable to set the tensile strength of the hot-stamped body at 1500 MPa or higher. On the other hand, if the tensile strength exceeds 2500 MPa, the strength is excessively high, and the toughness and ductility of the hot-stamped body may deteriorate. For this reason, it is preferable to set the tensile strength at 2500 MPa or lower.

[50] Next, a method for manufacturing the hot stamping formed body in accordance with this embodiment will be described. A steel sheet (zinc-plated steel sheet) including a zinc-plated layer on its surface is subjected to hot stamping to apply a predetermined contact pressure and then cooled, thereby producing a hot-stamped formed body in accordance with this method. Since the Γ phase included in a zinc-plated layer disappears (during the alloying of zinc plating), when a hot-dip galvanized annealed layer is used, no corrosion resistance improvement effect is obtained. For this reason, a hot-dip galvanized steel sheet is preferable.

[51] First, a method of manufacturing zinc-plated steel sheet will be described. After a casting is heated to 1200°C or more and held for 20 minutes or more, hot rolling is performed so that the finishing temperature of the finish rolling is 810°C or more. Further, cold rolling is performed to manufacture a steel sheet having the above-mentioned chemical composition, and then a zinc-plated layer is formed on the surface of the steel sheet by a continuous hot-dip galvanizing line, so that the zinc-plated steel sheet is manufactured. A cumulative rolling reduction during cold rolling can be set in a range of 30% to 90%. In the method of manufacturing galvanized steel sheet, annealing of a hot-rolled sheet can be performed between hot rolling and cold rolling.Pickling can also be performed. Cold rolling can be omitted, and a hot-rolled steel sheet can be introduced into the continuous hot-dip galvanizing line. If cold rolling is omitted, annealing of a hot-rolled sheet and pickling can be omitted.

[52] In the continuous hot-dip galvanizing line, the steel sheet is heated and clamped and then immersed in a hot-dip galvanizing bath, so that a zinc-plated layer is formed on the surface of the steel sheet. The adhesion amount of the zinc-plated layer can be adjusted within a range of 5 g / m2 to 150 g / m2 per side. Since electrogalvanizing requires additional elements to delay the alloying, which increases the manufacturing cost, electrogalvanizing is not desirable.

[53] Then, the zinc-plated steel sheet is heated so that the heating temperature is in an upper range of the Acg point and 800°C to 950°C. In addition, a heating time (a time elapsed until the zinc-plated steel sheet comes out of the heating furnace after being put into the heating furnace and then maintained at the heating temperature (a time elapsed between transporting the zinc-plated steel sheet in the heating furnace and taking the zinc-plated steel sheet out of the heating furnace)) is set in a range of 60 seconds to 600 seconds. The AC3 point is represented by the following equation (1). In case the heating temperature is lower than the upper of the Acs point and 800°C or the heating time is less than 60 seconds, the galvanized steel sheet cannot be sufficiently austenitized.As a result, the desired amount of martensite cannot be obtained. If the heating temperature exceeds 950°C or the heating time exceeds 600 seconds, the Γ phase included in a zinc-plated layer disappears due to excessive alloying of the zinc-plated steel sheet. An average heating rate during heating can be set within a range of 0.1°C / s to 200°C / s. The average heating rate mentioned here is a value obtained when the temperature difference between the steel sheet surface temperature at the start of heating and the heating temperature is divided by a time difference from the start of heating to a time when the temperature reaches the heating temperature.In the case that the steel sheet is kept in a temperature range above the point Here and 800°C to 950°C, the temperature of the steel sheet may change or be constant.

[54] Examples of a heating method to be performed before hot stamping include heating using an electric furnace, a gas furnace or the like, flame heating, electric resistance heating, high frequency heating, induction heating and the like.

[55] Ac3(°C) = 910-203 χ C0·5- 30 χ Μη + 44.7χ Si + 400 χ Ti. . . (1) An element symbol in Equation (1) represents the content of each element in % by mass.

[56] Hot stamping is performed after the heating and holding described above. After hot stamping, cooling is preferably performed at an average cooling rate of 20°C / s or more to a temperature range of 250°C or less. In the case where the cooling stop temperature is higher than 250°C or the average cooling rate is lower than 20°C / s in the cooling to be performed after hot stamping, the desired amount of martensite cannot be obtained. Since the addition of cooling equipment is required and the manufacturing cost increases in the case where the average cooling rate is set to more than 500°C / s, the average cooling rate may be set to 500°C / s or less.The average cooling rate mentioned here is a value obtained in a case where the temperature difference between the surface temperature of the steel sheet at the time of cooling start and the cooling end temperature is divided by a time difference from the start of cooling to a time when cooling stops. A cooling rate after performing cooling to a temperature range of 250°C or less is not particularly limited, and examples of a cooling method include air cooling.

[57] In this embodiment, a contact pressure of 50 MPa or more is applied to the galvanized steel sheet during hot stamping. Since a contact pressure of 50 MPa or more is applied, the voids in the zinc-plated layer are flattened, and the cross-sectional area ratio of the voids can be reduced. Consequently, it is possible to suppress the occurrence of sticking electrodes during spot welding of the hot-stamped formed body. If the contact pressure is less than 50 MPa, the cross-sectional area ratio of the voids in the zinc-plated layer cannot be sufficiently reduced. It is preferable that the contact pressure to be applied during hot stamping be set to 100 MPa or more.

[58] In case the contact pressure is excessively high, many irregularities are formed on the surface of the hot stamping formed body due to surface abrasion between a die and oxide scale formed on the surface of a hot stamping steel sheet. For this reason, there is a concern that weldability will deteriorate. From this point of view, it is preferable that the contact pressure be set to 500 MPa or less. In addition, in case the contact pressure exceeds 200 MPa, the hot stamping equipment may be damaged due to excessively high contact pressure. For this reason, it is preferable that the contact pressure be set to 200 MPa or less.

[59] In order to reduce the cross-sectional area ratio of the voids present in the zinc-plated layer, it is important to control the contact pressure during hot stamping as in this embodiment. For example, although pressing is performed again after hot stamping and the contact pressure to be applied during this pressing is controlled, the cross-sectional area ratio of the voids present in the zinc-plated layer cannot be reduced.

[60] It is desirable that a region of the galvanized steel sheet to which a contact pressure of 50 MPa or more is to be applied be fitted to a region to be welded (a region serving as a portion to be welded) after the formation of the hot-stamping formed body. The reason for this is that it is possible to reduce the occurrence of sticking electrodes during spot welding in case the cross-sectional area ratio of voids is reduced in a region, to be welded, of the hot-stamping formed body. Examples

[61] Next, examples of the present invention will be described. The conditions in the examples are an example of a condition used to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of a condition. The present invention may employ various conditions to achieve the object of the present invention without departing from the scope of the present invention.

[62] After the steel pieces manufactured from the molten steel casting with the chemical composition shown in Table 1 were heated to 1200°C or more and held for 20 minutes or more, hot rolling was performed so that the finish rolling end temperature was 810°C or more and cold rolling was performed. As a result, steel sheets were obtained. A cumulative rolling reduction during cold rolling was set in a range of 30% to 90%. Zinc plating was applied to the steel sheets obtained by a continuous hot-dip galvanizing line, so that zinc-plated steel sheets (hot-dip zinc-plated steel sheets) were obtained. The adhesion amount of a zinc-plated layer was set in a range of 5 g / m2 to 150 g / m2 per side.The hot stamping formed bodies shown in Tables 2 and 3 were manufactured using the galvanized steel sheets obtained under the conditions shown in Tables 2 and 3. An average heating rate during the heating performed before hot stamping was set within a range of 0.1°C / s to 200°C / s.

[63] An underline in the tables represents that a condition is outside the range of the present invention, a condition is outside a preferred manufacturing condition, or a property value is not preferred.

[64] Regarding the microstructure of the steel sheet of each body formed by hot stamping, the area ratio measurement of each structure was carried out by the above-mentioned measurement method. Ferrite, pearlite and bainite were observed as the remainder in a microstructure different from martensite.

[65] Furthermore, the structure of the zinc-plated layer of the hot-stamping formed body was analyzed using the aforementioned method, and the cross-sectional area ratio of voids present in the zinc-plated layer was measured using the aforementioned method. In the case where a Γ phase was observed in the zinc-plated layer, presence was written in the tables. In the case where the Γ phase was not observed in the zinc-plated layer, absence was written in the tables. The zinc-plated layer of the example where presence was written in the tables with respect to the zinc-plated layer included a Fe-Zn solid solution in addition to the Γ phase, and the adhesion amount of the Γ phase was in a range of 5 g / m2 to 150 g / m2 per side.

[66] The mechanical properties (tensile strength and spot weldability) of each hot stamping formed body were evaluated using the following methods.

[67] Tensile strength The No. 5 test pieces described in JIS Z 2241:2011 were prepared from an arbitrary position of the hot-stamping formed body, and the tensile strength of the hot-stamping formed body was obtained according to a test method described in JIS Z 2241:2011. In a case where the tensile strength was in a range of 1500 MPa to 2500 MPa, the test piece was determined to be acceptable as it had the strength generally required for a hot-stamping formed body. Furthermore, in a case where the tensile strength was less than 1500 MPa, the test piece was determined to be unacceptable due to insufficient strength. Furthermore, in a case where the tensile strength exceeded 2500 MPa, the test piece was determined to be unacceptable for being insufficient in toughness and ductility due to excessively high strength.

[68] Spot welding Regarding the body formed by hot stamping, two test pieces having a size of 100 mm χ 30 mm were taken from a position excluding a region within 10 mm from an end surface, these test pieces were superimposed on each other, and spot welding was performed while changing the current under the following conditions. Electrode force: 400 kgf Welding time: 15 cycles Maintenance time: 9 cycles Electrode tip shape: DR type, tip φ6mm-radius of curvature of R40mm

[69] Spot welding was performed while increasing the current from a current lo at which the nugget diameter was 4Vt (t denotes the sheet thickness of the test piece) to obtain a current at which blowout occurred and a current at which electrode sticking occurred (electrode sticking current Is). An interval between lo and the current at which blowout occurred was set as an appropriate current range. In one case where molten metal was splashed, it was determined that blowout occurred. In one case where blowout occurred when spot welding was performed at the current lo at which the nugget diameter was 4Vt, it was judged that there was no appropriate current range and it was written in Table 3.One example where there was no appropriate current range was determined to be unacceptable as it was unsuitable as zinc-plated steel sheet for spot welding.

[70] Furthermore, spot weldability was evaluated with respect to the electrode sticking current Is obtained 5 on the basis of the following criteria. Here, Io(kA) is the current at which the diameter of a nugget was 4it (t denotes the sheet thickness of the test piece), and Ia(kA) is lox1.4. Examples evaluated as good and fair were determined to be acceptable as they were excellent in spot weldability. On the other hand, an example evaluated as poor was determined to be unacceptable as it was insufficient in spot weldability. Good: Is> Iax1.15 Regular: Iaχ1.10 < Is^Iax1.15 Bad: IsIax1.10

[71] Table 1 Steel No. Chemical Composition (mass %) Remainder Fe & Impurities AC3 (°C) Note c Si Mn Al sol. Ti SPN Others 1 0.19 0.20 2.4 0.030 0.025 0.0020 0.004 0.003 768 Present Invention Steel 2 0.49 C . 15 2.0 0.030 0.033 0.0020 0 . o:o 0.003 729 Present Invention Steel 3 0.31 0.10 1.8 0.040 0.025 0.0020 0.010 0.003 757 Present Invention Steel 4 0.31 1.40 1.8 0.040 0.025 0.0020 0.0-0 0.003 816 Present Invention Steel 5 0.33 0.20 1.5 0.040 0.025 0.0020 0.015 0.005 767 Present Invention Steel 6 0.33 0.20 2.5 0.040 0.025 0.0020 0.015 0.005 737 Present Invention Steel 7 0.33 0.15 1.8 0.050 0.050 0.0002 0.090 0.005 766 Present steel of the invention 8 0.33 0.15 1.8 0.050 0.050 0.0100 0.090 0.005 766 Present steel of the invention 9 0.34 0.18 1.5 0.040 0.010 0.0030 0.010 0.005 759 Steel of the present invention 10 0.34 0.18 1.5 0.040 0 . 100 0.0030 0.010 0.005 795 Steel of the present invention 11 0.33 0.15 1.5 0.040 0.025 0.0020 0.090 0.005 765 Steel of the present invention 12 0.33 0.15 1.5 0.040 0.025 0.0020 0.001 0.005 765 Steel of the present invention 13 0.22 0.20 1.8 0.090 0.025 0.0030 0.010 0.005 780 Steel of the present invention 14 0.22 0.20 1.8 0.005 0.025 0.0030 0.010 0.005 780 Steel of the present invention 15 0.22 0.18 1.8 0.040 0.030 0.0020 0.010 0.010 781 Steel of the present invention 16 0.22 0.18 1.8 0.040 0.030 0.0020 0.010 0.003 781 Steel of the present invention 17 0.33 0.20 2.0 0.040 0.024 0.0020 0.010 0.005 752 Steel of the present invention 18 0.33 0.21 2.0 0.040 0.024 0.0020 0.010 0.005 Nb:0.0 5 752 Steel of the present invention 19 0.33 0.20 2.0 0.040 0.025 0.0020 0.010 0.005 V:0.20 752 Steel of the present invention 20 0.33 0.20 2.0 0.040 0.025 0.0020 0.010 0.005 Cr:0.20 752 Steel of the present invention 21 0.33 C.22 1 . 9 0.040 0.022 0.0020 0.010 0.005 Mo:0.02 755 Steel of the present invention 22 0.34 0.25 1 . 9 0.040 0.023 0.0020 0.020 0.005 B:0.003 755 Steel of the present invention 23 0.33 0.25 2.0 0.040 0.022 0.0020 0.020 0.005 Ni:0.04 753 Steel of the present invention 24 0.31 0.20 2.1 0.040 0.024 0.0020 0.010 0.005 Mg:0.001 753 Steel of the present invention 25 0.31 0.20 2.1 0.040 0.024 0.0020 0.010 0.005 Ca:0.001,Mg:0.001 753 Steel of the present invention 26 0.31 0.20 2.0 0.040 0.024 0.0020 0.010 0.005 REM:0.001,Co:0.005 756 Steel of the present invention 27 0.17 0.20 2.0 0.040 0.022 0.0020 0.010 0.005 784 Steel of the present invention 28 0.52 0.20 2.0 0.040 0.022 0.0020 0.010 0.005 721 Comparative steel 29 0.23 1.60 1.8 0.050 0.023 0.0020 0.010 0.005 839 Comparative Steel 30 0.22 0.20 1.3 0.050 0.025 0.0040 0.009 0.005 795 Comparative Steel 31 0.49 0.20 2.6 0.050 0.025 0.0040 0.009 0.005 709 Comparative Steel 32 0.22 0.20 2.0 0.040 0.020 0_.013_0 0.010 0.005 772 Comparative steel 33 0.22 0.20 1. 6 0.040 0.008 0.0010 0.010 0.005 779 Comparative steel 34 0.22 0.20 1 . 6 0.040 0.110 0.0010 0.010 0.005 820 Comparative steel 35 0.22 Ü.2Ü 2.0 0.040 0.020 ü . Oülü 0 . llü 0.005 772 Comparative steel 36 0.33 0.20 1.8 0.110 0.020 0.0030 0.020 0.005 756 Comparative steel 37 0.22 0.20 1.8 0.020 0.030 0.0030 0.020 0.015 782 Comparative steel 38 0.33 1.20 1. 9 0.040 0.025 0.0020 0.010 0.005 800 Steel of the present invention 39 0.33 1.00 2.0 0.040 0.025 0.0020 0.010 0.005 788 Steel of the present invention. An underline represents that a condition is outside the scope of the present invention.

[72] Table 2 ML / a / ZUZZ / U 11 zzu c 'O 8 -Q ,pj Φ 2 No. of steel. | Hot Stamping Steel Sheet Galvanized Layer Mechanical Properties Note Heating Temperature (°C) Heating Time (s) Average Cooling Rate up to 250°C ('C / s) Contact Pressure P (MPa) Mart ensite (% Area) Presence / Absence of Γ Phase Cross Sectional Void Area Ratio (%) Tensile Strength (MPa) Appropriate Current Range (kA) Current Io for 4\t (kA) -1 lax 1.10 (kA) Iax1.15 (kA) Current Is for Electrode Adhesion (kA) Determination of Spot Weldability 1 1 880 120 130 100 100 Presence 7.0 1505 1.40 5.80 8.12 8.93 9.34 10.00 Good Steel of the Present Invention 2 2 820 90 110 100 100 Presence 3.0 2480 1.20 5.70 7.98 8.78 9.18 10.00 Good Steel of the present invention 3 3 850 60 130 100 100 Presence 5.1 1950 1.50 5.80 8.12 8.93 9.34 10.00 Good Steel of the present invention 4 4 850 60 130 100 100 Presence 5.5 1920 1.50 5.80 8.12 8.93 9.34 9.50 Good Steel of the present invention 5 5 880 120 130 100 100 Presence 7.5 2045 1.30 6.00 8.40 9.24 9.66 10.00 Good Steel of the present invention 6 6 830 90 110 100 100 Presence 4.0 2030 1.80 5.70 7.98 8.78 9.18 10.00 Good Steel of the present invention 7 7 850 60 120 100 100 Presence 5.1 1980 1.40 5.60 7.84 8.62 9.02 10.00 Good Steel of the present invention 8 8 850 60 120 100 100 Presence 5.0 1975 1.40 5.80 8.12 8.93 9.34 10.00 Good Steel of the present invention 9 9 920 60 150 100 100 Presence 14.2 2100 0.90 6.00 8.40 9.24 9.66 9.50 Fair Steel of the present invention 10 10 950 60 150 100 100 Presence 13.5 1980 0.20 6.40 8.96 9.86 10.30 10.00 Fair Steel of the present invention 11 11 880 90 130 100 100 Presence 8.3 2025 1.20 5.90 8.26 9.09 9.50 10.00 Good Steel of the present invention 12 12 880 90 130 100 100 Presence 7.9 2035 1.20 5.80 8.12 8.93 9.34 9.50 Good Steel of the present invention 13 13 830 60 100 100 100 Presence 3.3 1520 1.60 5.60 7.84 8.62 9.02 10.40 Good Steel of the present invention 14 14 830 60 110 100 100 Presence 2.9 1545 1.70 5.80 8.12 8.93 9.34 10.00 Good Steel of the present invention 15 15 830 90 100 100 95 Presence 3.5 1540 1.60 5.80 8.12 8.93 9.34 10.00 Good Steel of the present invention 16 16 830 120 100 100 100 Presence 3.0 1550 1.50 6.00 8.40 9.24 9.66 10.50 Good Steel of the present invention 17 17 830 60 100 100 100 Presence 3.9 2030 1.80 5.70 7.98 8.78 9.18 10.50 Good Steel of the present invention 18 18 830 120 110 100 100 Presence 3.3 2020 1.70 5.80 8.12 8.93 9.34 10.50 Good Steel of the present invention 19 19 830 120 120 100 100 Presence 4.0 2050 1.70 5.60 7.84 8.62 9.02 9.50 Good Steel of the present invention 20 20 830 120 100 100 100 Presence 3.3 1980 1.70 5.90 8.26 9.09 9.50 10.00 Good Steel of the present invention 21 21 830 120 100 100 100 Presence 4.0 2010 1.70 6.00 8.40 9.24 9.66 10.00 Good Steel of the present invention 22 22 830 120 120 100 100 Presence 4.4 2040 1.80 5.70 7.98 8.78 9.18 10.00 Good Steel of the present invention. 23 23 830 120 100 100 100 Presence 4.1 2050 1.90 5.70 7.98 8.78 9.18 9.90 Good Steel of the present invention 24 24 830 120 100 100 100 Presence 3.9 1995 1.70 5.90 8.26 9.09 9.50 10.00 Good Steel of the present invention 25 25 830 120 100 100 100 Presence 4.0 2010 1.70 5.80 8.12 8.93 9.34 10.00 Good Steel of the present invention ML / a / ZUZZ / U 11 zzu

[73] Table 3 Manufacturing No. Steel No. Hot stamping Steel sheet Galvanized layer Mechanical properties Note Heating temperature (°C) Heating time (s) Average cooling rate up to 250°C (°C / s) Contact pressure P (MPa) Mart ensite (% area) Presence / absence of Γ phase Area ratio of cross-sectional voids (%) Tensile strength (MPa) Appropriate current range (kA) Current lo for 4\t (kA) la (kA) lax 1.10 (kA) la* 1.15 (kA) Current ls for electrode adhesion (kA) Determination of spot weldability 26 26 830 120 100 100 100 Presence 4.4 2000 1.70 5.80 8.12 8.93 9.34 10.00 Good Example of the present invention 27 27 850 120 120 100 100 Presence 5.2 1475 1.30 6.00 8.40 9.24 9.66 10.00 Good Comparative example 28 28 850 120 130 100 100 Presence 6.1 2550 1.10 5.90 8.26 9.09 9.50 9.50 Fair Comparative example 29 29 820 120 110 100 30 Presence 3.2 1380 1.60 5.90 8.26 9.09 9.50 10.50 Good Comparative Example 30 30 820 90 120 100 90 Presence 2.7 1480 1.70 5.70 7.98 8.78 9.18 10.50 Good Comparative Example 31 31 920 60 120 100 100 Presence 15.5 2570 0.20 6.20 8.68 9.55 9.98 9.00 Bad Comparative Example 32 32 810 90 110 100 90 Presence 2.8 1480 1.50 6.00 8.40 9.24 9.66 10.00 Good Comparative Example 33 33 810 90 110 100 85 Presence 1.2 1310 1.70 6.00 8.40 9.24 9.66 9.90 Good Comparative example 34 34 810 90 120 100 30 Presence 1.5 1150 1.70 5.50 7.70 8.47 8.86 9.50 Good Comparative example 35 35 810 90 110 100 85 Presence 1.9 1340 1.70 5.70 7.98 8.78 9.18 10.00 Good Comparative example 36 36 900 180 110 100 80 Absence 3.3 1790 - 6.00 8.40 9.24 9.66 9.40 Regular Comparative example 37 37 810 90 100 100 95 Presence 2.4 1490 130 5.70 7.98 8.78 9.18 10.00 Good Comparative example 38 18 780 120 100 100 10 Presence 3.2 1000 1.80 5.80 8.12 8.93 9.34 11.00 Good Comparative example 39 18 960 60 140 100 100 Presence 15.4 2150 - 6.30 8.82 9.70 10.14 8.50 Bad Comparative example 40 18 800 30 100 100 30 Presence 1.2 1380 1.70 5.70 7.98 8.78 9.18 10.00 Good Comparative example 41 18 830 630 100 100 100 Absence 4.5 2120 - 6.10 8.54 9.39 9.82 9.50 Fair Comparative example 42 17 850 120 15 100 20 Presence 6.0 1240 1.30 5.60 7.84 8.62 9.02 9.50 Good Comparative example 43 17 920 90 120 40 100 Presence 18.0 2110 - 6.00 8.40 9.24 9.66 8.50 Bad Comparative example 44 38 850 60 100 100 100 Presence 9.0 2020 1.40 5.80 8.12 8.93 934 10.00 Good Example of the present invention 45 39 850 60 100 100 100 Presence 8.2 2040 1.50 5.80 8.12 8.93 934 10.00 Good Example of the present invention 46 17 850 60 50 100 97 Presence 4.0 2000 1.60 5.80 8.12 8.93 9.34 10.50 Good Example of the present invention 47 18 850 60 50 100 98 Presence 5.3 2010 1.70 5.90 8.26 9.09 9.50 10.00 Good Example of the present invention. 48 17 850 60 30 100 96 Presence 4.4 1980 1.60 6.00 8.40 9.24 9.66 10.00 Good Example of the present invention 49 18 850 60 30 100 96 Presence 4.5 1990 1.60 5.80 8.12 8.93 9.34 10.00 Good Example of the present invention An underline represents that a condition is outside the scope of the present invention, a manufacturing condition is not preferred, or the properties are not preferred.

[74] It is found from Tables 2 and 3 that a hot-stamped formed body whose chemical composition and microstructure of a steel sheet and the layer structure and cross-sectional area ratio of voids of a zinc-plated layer are in the range of the present invention has a tensile strength in a range of 1500 MPa to 2500 MPa and is excellent in spot weldability.

[75] On the other hand, it is found that a hot stamping formed body of which one or more of the chemical composition and microstructure of a steel sheet and the layer structure and cross-sectional area ratio of voids of a zinc-plated layer are outside the present invention has a tensile strength in a range of 1500 MPa to 2500 MPa and / or is inferior in spot weldability. Manufacturing Nos. 36, 39, 41, and 43 are examples wherein ejection occurred before the current reached current lo at which the nugget diameter was 4v / t. Industrial applicability

[76] In accordance with the aspect of the present invention, it is possible to obtain a hot stamping formed body which is excellent in spot weldability and has the strength generally required for a hot stamping formed body.

Claims

CLAIMS 1. A body formed by hot stamping comprising: a steel sheet; and a zinc-plated layer provided over the steel sheet, wherein the steel sheet has a chemical composition containing, in % by mass, C: 0.18% to 0.50%, Si: 0.10% to 1.50%, Mn: 1.5% to 2.5%, Al: 0.001% to 0.100%, Ti: 0.010% to 0.100%, S: 0.0100% or less, P: 0.100% or less, N: 0.010% or less, Nb: 0% to 0.05%, V: 0% to 0.50%, Cr: 0% to 0.50%, Mo: 0% to 0.50%, B: 0% to 0.010%, Ni: 0% to 2.00%, and a total of REM, Ca, Co and Mg: 0% to 0.030%, a residue consisting of Fe and impurities, a % area of ​​martensite has a microstructure of 90% or more at a position corresponding to 1 / 4 of the sheet thickness of the steel sheet from a surface of the steel sheet in the direction of the sheet thickness, the zinc-plated layer includes a Γ phase and a solid solution of Fe-Zn, and a cross-sectional area ratio of the voids present in the zinc-plated layer is 15.0% or less.

2. The body formed by hot stamping according to claim 1, wherein the chemical composition contains, in % by mass, one or two or more selected from the group consisting of Nb: 0.02% to 0.05%, V: 0.005% to 0.50%, Cr: 0.10% to 0.50%, Mo: 0.005% to 0.50%, B: 0.0001% to 0.010%, Ni: 0.01% to 2.00%, and a total of REM, Ca, Co and Mg: 0.0003% to 0.030%.

3. The body formed by hot stamping according to claim 1 or 2, wherein the chemical composition contains, in % by mass, C: 0.24% to 0.50%.