HOT-FORMED STEEL

Abstract

A hot-stamped steel includes a base material that is made of steel, a plated layer that is formed on a surface of the base material, and a phosphate coating that is formed on a surface of the plated layer; The chemical composition of the plated layer contains 20.00 to 45.00% by mass of Al, 10.00 to 45.00% by mass of Fe, 4.50 to 15.00% by mass of Mg, 0.10 to 3.00% by mass of Si, 0.05 to 3.00% by mass of Ca, 0 to 0.50% by mass of Sb, 0 to 0.50% by mass of Pb, 0 to 1.00% by mass of Cu, 0 to 1.00% by mass of Sn, 0 to 1.00% by mass of Ti, 0 to 0.50% by mass of Sr, 0 to 1.00% by mass of Cr, 0 to 1.00% by mass of Ni, and 0 to 1.00% by mass of Mn with a remainder of Zn and impurities; The phosphate coating consists of zinc phosphate crystals containing 5.0 to 50.0% by mass of Mg and 0.5 to 5.0% by mass of Ca; and the amount of adhesion of the phosphate coating per surface is in the range of 0.1 to 10.0 g / m2.

Description

HOT-FORMED STEEL Technical field of the invention [1] The present invention relates to a hot-stamped steel. Priority is claimed to Japanese Patent Application No. 2019-157205, filed on August 29, 2019, the contents of which are incorporated herein by reference. Background of the invention [2] In recent years, there have been calls to curb the consumption of chemical fuels for environmental protection and to prevent global warming. This call is no exception for, for example, vehicles that are essential for daily life and activities such as transportation. Regarding this call, improvements in fuel efficiency and similar benefits for vehicles have been examined, particularly those resulting from a reduction in the vehicle body's weight. Since most of a vehicle's structure is made of iron, specifically steel sheets, reducing the thickness of the steel sheets to decrease weight is highly effective in reducing the vehicle body's weight.However, since the strength of a structure decreases when the thickness of the steel sheet is reduced simply to decrease its weight, there is concern that safety will be compromised. Therefore, to reduce the thickness of the steel sheet, it is necessary to increase the mechanical strength of the steel sheet used to avoid reducing the structure's strength. Therefore, a steel sheet has been researched and developed whose mechanical strength can be maintained or increased, even when made thinner than previously used steel sheets. There is a demand for this type of steel sheet not only in the vehicle manufacturing industry but also in various other manufacturing industries. [3] In general, a material with high mechanical strength tends to have low shape retention capacity during forming, such as bending. For this reason, if this material needs to be worked into a complex shape, working the material is difficult. Examples of methods for overcoming forming problems include the so-called hot pressing method (a method of hot stamping, a high-temperature pressing method, and a die tempering method). In this hot pressing method, the material to be formed is heated to a high temperature once, and the softened material is cooled after being formed by pressing or is cooled simultaneously with the forming process. [4] According to this hot pressing method, since a material is heated to a high temperature once it softens, and the pressing is carried out in a softened state, the pressing can be easily performed on the material. Consequently, this hot pressing process yields a pressure-formed article that has both good shape retention and high mechanical strength. In particular, when the material is steel, the mechanical strength of the pressure-formed article can be increased by a tempering effect obtained through cooling after forming. [5] However, if this hot pressing method is applied to a steel sheet, the steel sheet is heated to a high temperature of, for example, 800°C or more, so that the iron and similar materials on the surface of the steel sheet oxidize and scale (oxides) forms. Consequently, since a scale removal step is required after the hot pressing, productivity is reduced. Furthermore, for members and similar materials requiring corrosion resistance, an anti-corrosion treatment or metal coating must be applied to the surface of the member after the pressing. For this reason, a surface cleaning step and a surface treatment step are required, which also reduces productivity. [6] A method for applying coating, such as plating, to a steel sheet, which is not yet subjected to hot stamping, to improve corrosion resistance and omit a scale removal step is considered an example of a method for inhibiting such a reduction in productivity. [7] A hot-pressed plated steel sheet, comprising an Al-Zn-based alloy plating layer formed on the surface of a steel sheet and containing 20 to 95% by mass of Al, 0.01 to 10% by mass of Ca+Mg and Si, is described, for example, in Patent Document 1 as such a plated steel material. According to Patent Document 1, scale formation is inhibited during heating prior to hot pressing, the plating does not adhere to the dies during hot pressing, and the resulting hot-pressed member has a good appearance, excellent plating adhesion, and excellent corrosion resistance. [8] In addition, Patent Document 2 describes a zezznn / zznz / E / YiAi plated steel material comprising a plated layer formed on the surface of a steel material and comprising an Al-Fe alloy layer and a Zn-MgA1 alloy layer and whose corrosion resistance is drastically improved. [9] The members obtained by hot stamping from the aforementioned plated steel are often applied to vehicle components, and a phosphate treatment is applied to these members when they are used as vehicle components. However, the properties obtained when a phosphate coating is formed are not examined at all in Patent Documents 1 and 2. As a result of the examination carried out by the inventors, it has been discovered that there is a case in which a zinc phosphate film is not easily formed, even though a member obtained by hot stamping from a sheet of steel that includes a plated layer containing Al or Zn is subjected to zinc phosphate treatment which is a general phosphating treatment for vehicles, or the adhesion of the coating film is not sufficient in a case in which a coating film is formed by electrodeposition or similar on the surface of a zinc phosphate film even if the zinc phosphate film is formed.

[10] Patent Document 3 describes a method for treating the surface of a zinc-aluminum-magnesium alloy-plated steel sheet with a metal surface treatment agent. Patent Document 3 describes a method in which a fluorine compound is contained in a chemical conversion solution to form a reaction layer, containing aluminum fluoride and magnesium fluoride, on the surface of a plated layer of the steel sheet plated by phosphating treatment, and the adhesion between a phosphate coating and the surface of the plated layer can be further enhanced.

[11] However, the adhesion of the coating film in the case of a coating film being formed by electrodeposition or similar means on the phosphate coating is not examined in Patent Document 3. zezznn / zznz / E / YiAi Previous technique document Patent document

[12] Patent Document 1: Japanese Patent Application Not Examined, First Publication No. 2012-112010 Patent Document 2: Japanese Patent Application Not Examined, First Publication No. 2017-66459 Patent Document 3: Japanese Patent Application Not Examined, First Publication No. 2016-89232 Description of the invention Problems that must be solved by the invention

[13] The invention has been made taking into account the aforementioned problem. An object of the invention is to provide a hot-stamped steel that includes a phosphate coating in which the phosphate coating defect is inhibited and which is excellent in coating film adhesion. Means to solve the problem

[14] The inventors discovered that a hot-stamped steel including a phosphate coating, in which the phosphate coating defect is inhibited and which is excellent in coating film adhesion, is obtained in a case in which the balance of the composition of a plated layer of a hot-dip ZnAl-Mg plated steel sheet and the conditions during its manufacture are controlled. The invention has been completed on the basis of the aforementioned knowledge, and its essence is as follows.

[15] (1) A hot-stamped steel according to an aspect of the invention includes a base material made of steel, a plated layer formed on a surface of the base material, and a phosphate coating formed on a surface of the plated layer. The chemical composition of the veneered layer contains 20.00 to 45.00% by mass of Al, 10.00 to 45.00% by mass of Fe, 4.50 to 15.00% by mass of Mg, 0.10 to 3.00% by mass of Si, 0.05 to 3.00% by mass of Ca, 0 to 0.50% by mass of Sb, 0 to 0.50% by mass of Pb, 0 to 1.00% by mass of Cu, 0 to 1.00% by mass of Sn, 0 to 1.00% by mass of Ti, 0 to 0.50% by mass of Sr, 0 to 1.00% by mass of Cr, 0 to 1.00% by mass of Ni, and 0 to 1.00% by mass of Mn with a remainder of Zn and impurities. The phosphate coating consists of zinc phosphate crystals containing 5.0 to 50.0% by mass of Mg and 0.5 to 5.0% by mass of Ca, and the amount of adhesion of the phosphate coating per surface is in the range of 0.1 to 10.0 g / m2. (2) In hot-stamped steel in accordance with (1), the chemical composition of the plated layer may contain one or both of 25.00 to 35.00% by mass of Al and 6.00 to 10.00% by mass of Mg. (3) In hot-stamped steel in accordance with (1) or (2), the amount of phosphate coating adhesion per surface may be in the range of 1.5 to 8.0 g / m2. Effects of the invention

[16] According to the aspect of the invention, it is possible to provide a hot-stamped steel that includes a phosphate coating in which the phosphate coating defect is inhibited and which is excellent in coating film adhesion. Brief description of the dioujos

[17] Figure 1 is a schematic diagram showing a hot-stamped steel in accordance with a modality. Figure 2 is a diagram showing an example of the structure of a phosphate coating of hot-stamped steel in accordance with this modality. Figure 3 is a diagram showing an example of the structure of a phosphate coating on a hot-stamped steel from Example No. 12 (Comparative Example). Modalities of the invention

[18] A hot-stamped steel according to an embodiment of the invention (a hot-stamped steel according to this embodiment) shall be described with reference to the drawings. In Figure 1, the hot-stamped steel 1 according to this modality includes a base material 2 that is formed of steel, a plated layer 3 that is formed on the surface of the base material 2, and a phosphate coating 4 that is formed on the surface of the plated layer 3. The plated layer 3 and the phosphate coating 4 are formed on only one surface of the base material 2 in Figure 1, but can be formed on both surfaces of the same. zezznn / zznz / E / YiAi

[19] Base material Base material 2 is made of steel. Base material 2 is, for example, a hot-stamped component obtained by hot-stamping a steel sheet. For this reason, base material 2 is sheet-shaped in Figure 1, but the shape of base material 2 is not limited. Furthermore, in hot-formed steel 1 according to this modality, the plating layer 3 and the phosphate coating 4 are important, and the chemical composition and similar properties of the base material 2 are not particularly restricted. With respect to the base material 2, the steel to be subjected to plating, hot-forming, and phosphate coating can be determined based on the product to which the base material 2 is to be applied, or the required strength, sheet thickness, and similar characteristics of the base material 2. For example, a hot-rolled steel sheet described in JIS G3193:2008 or a cold-rolled steel sheet described in JIS G3141:2017 can be used as the base material.

[20] Veneered layer Chemical composition From here on, the % related to the chemical composition of the plated layer and phosphate coating is % by mass unless otherwise stated.

[21] Al: 20.00 to 45.00% Aluminum (Al) is essential for improving the corrosion resistance of plating layer 3. Furthermore, if the Al content is less than 20.00%, an intermetallic compound, composed primarily of calcium (Ca) and aluminum (Al), which inhibits the evaporation of zinc (Zn) and molten metal (Mq) and serves as a source of calcium for the phosphate coating layer during hot stamping, is not sufficiently produced on the plating layer surface. As a result, the phosphate coating defect in layer 4, formed on plating layer 3, becomes significant. For this reason, the Al content is adjusted to 20.00% or higher, preferably 25.00% or higher. Furthermore, even if the Al content exceeds 45.00%, it is unlikely that an intermetallic compound, composed primarily of Ca and Al, will form on the surface of the plated layer 3 during hot stamping. As a result, the defect in the phosphate coating of phosphate coating 4 becomes significant. For this reason, the Al content is adjusted to 45.00% or less. The Al content is preferably 35.00% or less.

[22] Fe: 10.00 to 45.00% Since Fe diffuses into the plated layer 3 from the base material 2 in the case of a plated steel sheet being heated during hot stamping, Fe is necessarily contained in the plated layer 3 of the hot stamped steel 1. If the Fe content is less than 10.0%, spot weldability and deposit properties tend to deteriorate. Therefore, the Fe content is adjusted to 10.00% or higher. On the other hand, if the iron content is excessively high, corrosion resistance tends to deteriorate. For this reason, the iron content is adjusted to 45.00% or less.

[23] Mg: 4.50 to 15.00% Magnesium (Mg) is an element that contributes to improving the corrosion resistance of plating layer 3. Furthermore, since Mg combines with a zinc component contained in plating layer 3 and prevents the generation of liquid zinc during hot stamping heating, Mg also inhibits liquid metal effect (LME) cracking. Additionally, in hot-stamped steel 1 produced according to this method, the zezznn / zznz / E / YiAi Magnesium (Mg) is an element that diffuses into the phosphate coating and improves the adhesion of the coating film. To achieve these effects, the Mg content is adjusted to 4.50% or higher. If the Mg content is lower than 4.50%, the adhesion of the coating film is reduced. The Mg content is preferably 6.0% or higher. On the other hand, when the Mg content exceeds 15.00%, the sacrificial protection becomes excessive. As a result, the corrosion resistance of plated layer 3 tends to decrease. Furthermore, a large amount of Mg-based intermetallic compound forms in plated layer 3, impairing its phosphate treatment capabilities. This leads to a larger defect in the phosphate coating of layer 4 and reduces the adhesion of the coating film. For this reason, the Mg content is adjusted to 15.00% or less. Ideally, the Mg content should be adjusted to 10.00% or less.

[24] Yes: 0.10 to 3.00% Silicon (Si) is an element that forms a compound with magnesium (Mg) and contributes to improved corrosion resistance. Furthermore, Si also prevents the alloy layer that forms between the steel sheet surface and the plated layer from becoming excessively thick, thus improving adhesion between the steel sheet and the plated layer when the plated layer is applied to the steel sheet. If the Si content is less than 0.10%, these effects are not sufficiently achieved. For this reason, the Si content is adjusted to 0.10% or higher. Furthermore, if the Si content exceeds 3.00%, Mq₂Si, Mq₂Ca, or (Mq,Ca)₂Si are formed. Consequently, the Mg and Ca content of the phosphate coating 4 is insufficient, reducing the adhesion of the coating film. Additionally, if an excess Si phase occurs, both spalling resistance and shock resistance are reduced. Therefore, the Si content is adjusted to 3.00% or less.

[25] Ca: 0.05 to 3.00% In hot-stamped steel 1 according to this method, calcium is an element that diffuses into the phosphate coating 4 and improves the adhesion of the coating film. If the calcium content is less than 0.05%, the defect in the phosphate coating 4 becomes large. For this reason, the calcium content is adjusted to 0.05% or more. On the other hand, if the Ca content exceeds 3.00%, an intermetallic compound composed primarily of Ca forms, which impairs the treatability of the phosphate. For this reason, the Mq and Ca content of the phosphate coating are insufficient, and the zrzznn / zznz / E / YiAi defect in the phosphate coating becomes large, thus reducing the adhesion of the coating film. Therefore, the Ca content is adjusted to 3.00% or less.

[26] Basically, the plated layer 3 of the hot-stamped steel 1 in accordance with this modality contains the elements mentioned above with a remainder of Zn and impurities. However, veneered layer 3 may contain Sb, Pb, Cu, Sn, Ti, Sr, Cr, Ni, and Mn in the following ranges in addition to the elements mentioned above. Since these elements are not required, the lower limits for their quantities are 0%. Furthermore, it is preferable that the total quantity of these elements be 5.00% or less.

[27] Sb: 0 to 0.50% Pb: 0 to 0.50% Cu: 0 to 1.00% Sn: 0 to 1.00% Ti: 0 to 1.00% Sb, Pb, Cu, Sn, and Ti are replaced with Zn in the plated layer 3 and form a solid solution in the MgZn2 phase. However, as long as Sb, Pb, Cu, Sn, and Ti are within predetermined ranges, they do not adversely affect the properties of the hot-stamped steel 1. Consequently, these elements can be contained in the plated layer 3. However, if the amounts of the respective elements are excessive, their oxides precipitate during the heating of the hot stamping, and the surface properties of the hot-stamped steel 1 deteriorate, thus reducing its phosphate treatability. Furthermore, if the Pb and Sn content are excessive, the deposit properties and LME resistance also deteriorate. For this reason, the Sb and Pb content is adjusted to 0.50% or less, and the Cu, Sn, and Ti content is adjusted to 1.00% or less. It is preferable that the Sb and Pb content be adjusted to 0.20% or less. The Cu, Sn, and Ti content is preferably 0.80% or less, and very preferably 0.50% or less.

[28] Mr: 0 to 0.50% Sr is an element that effectively inhibits the formation of upper slag in a plating bath during manufacturing. Furthermore, since Sr inhibits atmospheric oxidation during hot stamping heat treatment, it inhibits color change in the plated steel sheet subjected to this heat treatment. For this reason, Sr may be included in the plating layer. To achieve the aforementioned effects, it is preferable that the Sr content be adjusted to 0.05% or higher. On the other hand, when the sprue content is excessive, it negatively affects the blistering width of the coating film and the flux oxidation in a corrosion test. For this reason, the sprue content is adjusted to 0.50% or less. The sprue content is preferably adjusted to 0.30% or less, and very preferably to 0.10% or less.

[29] Cr: 0 to 1.00% Ni: 0 to 1.00% Mn: 0 to 1.00% Cr, Ni, and Mn are elements that concentrate near the interface between the plated layer and the base material in the plated steel sheet, and they have the effect of eliminating flake formation on the surface of the plated layer and similar defects. Therefore, one or more of these selected elements, Cr, Ni, and Mn, may be included in the plated layer. If these effects are to be achieved, it is preferable that the Cr, Ni, and Mn content be limited to 0.01% or less. On the other hand, if the quantities of these elements are excessive, the blistering width of the coating film and flux oxidation increase, so the corrosion resistance tends to deteriorate. Therefore, the Cr, Ni, and Mn content is adjusted to 1.00% or less. The Cr, Ni, and Mn content is preferably adjusted to 0.50% or less, and very preferably to 0.10% or less.

[30] The plating layer dissolves and exfoliates, and the quantities of elements contained in the exfoliated plating layer are analyzed by inductively coupled plasma atomic emission spectrometry (ICP), thus measuring the average composition of the plating layer. Regarding the exfoliation of the plating layer, for example, hot-stamped steel is immersed in 10% hydrochloric acid to which an inhibitor (pickling inhibitor: manufactured by ASAHI Chemicals Co., Ltd.) is added to inhibit corrosion of the material, and dissolution can be determined to be complete when foaming ceases.

[31] The structure of the veneered layer is not limited, but includes, for example, an Fe-Al phase, a ZnMg phase, and a Zn-Al-Mg phase. Furthermore, the adhesion of the veneered layer is not limited, but is preferably in the range of 10 to 120 g / m². The adhesion of the veneered layer can be obtained from the weight change before and after the veneered layer is dissolved at room temperature using the method described above.

[32] Phosphate coating The phosphate coating consists of zinc phosphate crystals containing 5.0 to 50.0% by mass of Mg and 0.5 to 5.0% by mass of Ca Typically, an oxide consisting primarily of Al₂O₃ forms on the surface (the surface of a plated layer) of hot-stamped steel obtained by hot-stamping a steel containing an Al-containing plated layer. The Al₂O₃ formed on the surface of the plated layer inhibits the formation of the phosphate coating. Consequently, if phosphate treatment is applied to the plated steel containing the Al₂O₃ formed on its surface, the proportion of defects in the phosphate coating becomes high in the hot-stamped steel. In contrast, in hot-stamped steel produced according to this modality, the plating is carried out using a method that will be described later. Consequently, an oxide film composed mainly of Ca and Mg forms on the surface of the hot-stamped steel that is to be phosphated. If zinc phosphate treatment (phosphating) is performed on this steel, the Mg and Ca diffuse into the phosphate coating, forming phosphate coating 4, which consists of zinc phosphate crystals containing 5.0 to 50.0% by mass of Mg and 0.5 to 5.0% by mass of Ca. Figure 2 shows a typical photograph of the phosphate coating 4 structure of hot-stamped steel produced according to this modality. The phosphate coating 4 consists of zinc phosphate crystals 11 containing 5.0 to 50.0% Mg and 0.5 to 5.0% Ca, and the zinc phosphate 11 crystals are flaky with a grain size of 10 µm or less along the major axis and 5 µm or less along the minor axis. Furthermore, if zinc phosphate 11 crystals form, the defect rate in the phosphate coating is 10% or less. Additionally, since the magnesium-containing phosphate coating itself offers excellent corrosion resistance, it contributes to improved adhesion (coating film bonding) after coating compared to a non-magnesium-containing phosphate coating. If a sufficient oxide film containing Ca and Mg does not form on the surface of the steel to be phosphated, an Al oxide is generated, thus increasing the defect in the phosphate coating 4. If the Mg and Ca content of the phosphate coating is low, the corrosion resistance of the phosphate coating is reduced, thereby decreasing the adhesion of the coating film. On the other hand, if the Mg and Ca content are high, the corrosion resistance of the phosphate coating is significantly reduced, thus reducing the adhesion of the coating film. Depending on the hot stamping conditions or the thickness of the plated layer, the Fe, which diffuses into the plated layer when alloyed, may be present in the phosphate coating consisting of zinc phosphate crystals.

[33] The phosphate coating is dissolved with a chemical that dissolves only the phosphate coating without dissolving the plated layer, for example, 20% chromic acid. A solution in which the phosphate coating is dissolved is analyzed by ICP atomic emission spectrometry, and the average composition of the phosphate coating is measured, thus obtaining the chemical composition of the phosphate coating. Alternatively, the chemical composition of the phosphate coating can be measured by preparing a calibration curve in advance and performing a quantitative analysis using X-ray fluorescence.

[34] The amount of adhesion of the phosphate coating per surface is 0.1 to 10 q / m2 In cases where the adhesion content of the phosphate coating 4 per surface is less than 0.1 g / m², the coating film adhesion improvement effect (adhesion when a coating film forms on the phosphate coating surface) is insufficient. Furthermore, if the adhesion content of the phosphate coating 4 per surface exceeds 10 g / m², cracking of the phosphate coating is likely to occur during bending or similar stresses, leading to delamination of the coating film. For this reason, the adhesion content of the phosphate coating 4 per surface is typically between 0.1 and 10 g / m². Ideally, the adhesion content of the phosphate coating 4 per surface should be between 1.5 and 8.0 g / m².

[35] If the amount of phosphate coating adhesion to a surface is to be obtained using a dissolution method, an opposing surface and the end surfaces of the hot-stamped steel other than the surface to be measured are sealed with tape, and the hot-stamped steel is immersed in a solvent solution to obtain a solution in which the phosphate coating formed on the single surface to be measured exfoliates. The amount of phosphate coating adhesion per surface can then be obtained from the change in weight before and after dissolution. If the amount of adhesion of the phosphate coating to a surface is measured using fluorescent X-rays, the amount of adhesion of the phosphate coating to a single surface may be required.

[36] In hot-stamped steel 1 in accordance with this embodiment, an oxide film may also be provided between the plated layer 3 and the phosphate coating 4. This oxide film is made, for example, of calcium or magnesium oxide. zezznn / zznz / E / YiAi

[37] Manufacturing method As long as hot-stamped steel produced according to this method possesses the aforementioned characteristics without relying on a manufacturing method, the effects of those characteristics are achieved. However, hot-stamped steel can be reliably produced according to a manufacturing method that includes the following steps. Therefore, this manufacturing method is preferable. In other words, hot-stamped steel in accordance with this method can be obtained by a manufacturing process that includes: (I) a plating step of obtaining a plated steel that includes a plated layer by immersing a steel in a plating bath; (II) a hot stamping step of performing hot stamping on the plated steel subject to the plating step; and (III) a chemical conversion step of performing a phosphating treatment on the plated steel subject to hot stamping, (IV) in the plating step, the steel is cooled to ambient temperature during quenching after immersion of the steel in the plating bath, such that the average cooling rate in a temperature range from the bath temperature to 450°C is set to 10°C / sec or higher, the average cooling rate in a temperature range from 450 to 350°C is set to 7°C / sec or less, and the average cooling rate in a temperature range from 350 to 150°C is set to 4°C / sec or less.

[38] Veneering step Immersion in plating bath In the plating step, the steel, such as a steel sheet, which is used as the original sheet, is immersed in the plating bath to form a plating layer on the surface of the steel. With regard to the conditions for immersion in the plating bath, for example, the surface of an original sheet to be galvanized is subjected to a heat reduction treatment in a temperature range of 600 to 940°C, and the steel is immersed in a plating bath having a bath temperature in the range of 500 to 750°C for approximately 0.2 to 6 seconds after the steel is cooled with air using N2 gas so that the temperature of the steel reaches the bath temperature of +20°C. If the immersion time is less than 0.2 seconds, the plated layer may not form sufficiently. Conversely, if the immersion time exceeds 6 seconds, the plated layer and the steel are excessively alloyed, and the plated layer contains a high amount of iron (Fe). If the plated layer contains excess Fe, it is difficult to inhibit the evaporation of zinc (Zn) and magnesium (Mg) during the hot stamping process. For this reason, if the immersion time exceeds 6 seconds, a phosphate coating with a predetermined composition is not obtained through the subsequent chemical conversion step, thus reducing the adhesion of the coating film to the hot stamped steel. The plating bath can be configured to include Zn, Al, Mg, and other elements according to the composition of the target plating layer 3. For example, the plating bath composition includes 30.00 to 75.00% Al, 4.00 to zrzznn / zznz / E / YiAi 17.00% Mg and 0.20 to 2.00% Si, and includes optional elements that are desired to be contained in the plated layer as required with a remainder of Zn and impurities.

[39] Cooling The average cooling rate over a temperature range from a bath temperature to 450°C: 10°C / sec or higher In a hot-stamped steelmaking method according to this method, the plated steel is lifted from the plating bath and then cooled such that the average cooling rate at temperatures up to 450°C is 10°C / sec or higher. If the average cooling rate in this temperature range is set at 10°C / sec or higher, the formation of aluminum oxide on the surface of the plated steel can be inhibited.

[40] Average cooling rate in a temperature range of 450 to 350°C: 7°C / sec or less After the above cooling, the plated steel is cooled in such a way that the average cooling rate in a temperature range of 450 to 350°C is adjusted to 7°C / sec or less. If the cooling rate is reduced within this temperature range to control the solidification structure, magnesium and calcium oxides with low aluminum content (e.g., 10% or less) form on the surface of the hot-stamped steel in a subsequent hot-stamping step. As a result, if phosphating is performed, a phosphate coating is obtained, which has a smaller defect and contains magnesium and calcium. Furthermore, there is concern that zinc (Zn) may evaporate during hot stamping in the case of a zinc-based plated layer. However, although a detailed mechanism is not readily apparent, an intermetallic compound containing aluminum, zinc, calcium, and / or silicon (Al), which inhibits the evaporation of zinc and magnesium (Mg), elements with high vapor pressures, preferentially forms near the surface of the plated layer in the controlled solidification structure described above. Consequently, the evaporation of Zn and Mg can be inhibited during the subsequent hot stamping heating process. zezznn / zznz / E / YiAi

[41] The average cooling rate in a temperature range of 350 to 150°C: 4°C / sec or less In a case where the average cooling rate over a temperature range of 350 to 150°C is set to 4°C / sec or lower after the initial cooling, a solid solution of Al and Zn contained within the solidification structure separates into an Al phase and a Zn phase. Consequently, the melting point of the plated layer is reduced, and an intermetallic compound containing Al, Zn, Ca, and / or Si readily migrates to the surface of the molten plated layer during hot stamping. As a result, since the evaporation of Zn and Mg can be more effectively inhibited, a phosphate coating containing Mg and Ca can form. However, if the cooling rate is high in part of the temperature range, even if the average cooling rate over a temperature range of 350 to 150°C is 4°C / s or less, a preferable metallographic structure is not obtained. Therefore, it is preferable for the average cooling rate over a temperature range of 350 to 150°C to be 4°C / s or less, the average cooling rate over a temperature range of 350 to 250°C to be 4°C / s or less, and the average cooling rate over a temperature range of 250 to 150°C to be 4°C / s or less.

[42] Hot stamping step Hot stamping is performed on plated steel (steel that includes the base material and the plated layer formed on the surface of the base material) subjected to the plating process. The conditions for hot stamping are not limited. However, for example, one method can be used to heat a plated steel sheet to a temperature range of 750 to 1200°C, hold the plated steel sheet at that temperature for 0 to 8 minutes, and then place the plated steel sheet between flat dies, which are at approximately room temperature, and rapidly cool the plated steel sheet. zezznn / zznz / E / YiAi

[43] Phosphating treatment step The phosphating treatment is performed on a hot-stamped, formed article. There are no restrictions on the phosphating treatment, and any publicly available zinc phosphate treatment may be used.

[44] According to the manufacturing method, hot stamped steel can be obtained in accordance with this method. Examples

[45] The invention will be described more specifically below using examples, but is not limited to these examples.

[46] Tables 1 to 3 show the examples described in the invention. Several batches of Zn-Al-Mg-based coating were prepared and then used for heating the hot stamping process. A steel sheet (containing 0.2% C and 1.3% Mn) with a sheet thickness of 1.6 mm was used as the original sheet to be coated. After cutting an original sheet to 100 mm x 200 mm, the plating was carried out using an in-house manufactured, discontinuous-type hot-dip plating test fixture. The temperature of the sheet was measured using a thermocouple spot-welded to the center portion of the original sheet to be plated. The surface of the original sheet to be coated was subjected to a heat reduction treatment before immersion in the plating bath at a temperature of 800°C in an atmosphere of N2-5% H2 gas in a furnace where the oxygen concentration was 20 ppm or less. The original sheet to be plated was then immersed in a plating bath at the temperature shown in Table 2 for approximately 3 seconds. After plating, the sheet was cooled with air using N2 gas so that its temperature reached the bath temperature +20°C. After immersion in the plating bath, the sheet was lifted at a rate of 20 to 200 mm / sec. During lifting, the amount of adhesion of the plating layer was controlled by the N2 cleaning gas as shown in Table 2. After lifting from the plating bath, the steel sheet was cooled to room temperature from the plating bath temperature under the conditions shown in Table 2. The hot stamping process involved heating and rapid cooling using dies on the produced plated steel sheet. For the heating conditions, the plated steel sheet was placed in a heating oven at 900°C and held there for 0 to 8 minutes after reaching a temperature of 10°C. The plated steel sheet was then placed between flat dies, which were at approximately room temperature, and rapidly cooled to produce a shaped article. As a result of the investigation using the method mentioned above, the chemical composition of the plated layer subjected to hot stamping was as shown in Table 1. In No. 31, hot stamping was performed on a commercially available galvanized annealed steel sheet.

[47] After that, a sample with a size of 50 x 100 mm (x sheet thickness) was collected from the hot-stamped steel, and zinc phosphate treatment was performed on this sample according to the SD5350 system (Standard of Nipponpaint Industrial Coatings Co., LTD.) to form a phosphate coating. zezznn / zznz / E / YiAi

[48] ​​Table 1 No. Classification Manufacturing Method Chemical Composition of the Plating Layer (% by Mass) Zn Al Mg Si Ca Fe Others Type Total 1 Comparative Example 47.80 19.40 12.10 0.40 0.80 19.50 -0.00 2 Comparative Example 53.40 20.60 4.20 0.20 1.00 20.60 -0.00 3 Example of the Invention 52.00 20.00 6.00 0.40 1.50 20.00 N1:0.10 0.10 4 Comparative Example 38.50 20.00 20.00 0.40 1.00 20.10 -0.00 5 Example of the Invention 47.00 22.50 6.50 0.50 1.00 22.50 - 0.00 6 Example of the invention 35.45 25.90 11.00 0.60 1.00 26.00 Sb:0.05 0.05 7 Example of the invention 38.05 27.50 6.50 0.40 0.05 27.50 - 0.00 8 Example of the invention 26.70 27.50 14.50 0.40 3.00 27.90 - 0.00 9 Example of the invention 31.74 29.50 8.50 0.15 0.10 30.00 T¡:0.01 0.01 10 Example of the invention 29.50 29.30 8.50 0.60 1.00 30.10 Cu:1.00 1.00 11 Comparative Example 30.37 30.00 8.50 0.60 0.03 30.50 - 0.00 12 Comparative Example 27.90 31.00 8.30 0.60 1.00 31.20 - 0.00 13 Comparative Example 28.60 31.00 8.30 0.60 1.00 30.50 - 0.00 14 Comparative Example 29.70 31.00 8.30 0.00 1.00 30.00 - 0.00 15 Comparative Example 4.70 31.30 6.20 0.30 0.50 57.00 - 0.00 16 Comparative Example 28.00 31.00 8.30 0.50 1.00 31.20 - 0.00 17 Example of the Invention 20.08 31.00 14.00 0.40 3.00 31.50 Pb:0.02 0.02 18 Example of the Invention 28.10 31.00 8.30 0.40 1.00 31.20 - 0.00 19 Example of the Invention 30.30 31.00 5.00 1.10 1.00 31.50 Mn:0.10 0.10 20 Example of the invention 21.58 33.00 8.00 2.00 1.20 34.20 Sn:0.02 0.02 21 Comparative example 6.80 29.00 6.00 0.20 0.50 57.50 - 0.00 22 Comparative example 18.10 33.30 8.00 0.60 6.00 34.00 - 0.00 23 Example of the invention 19.29 34.50 7.80 2.00 1.40 35.00 Cr:0.01 0.01 24 Example of the invention 12.90 37.00 7.50 2.00 2.10 38.00 N¡:0.50 0.50 25 Example of the 12.30 38.30 6.20 2.00 2.30 38.90 - 0.00. Invention 26 Example of the invention 9.50 39.40 6.10 2.00 2.50 40.10 Mn:0.40 0.40 27 Comparative example 8.90 37.10 9.00 5.50 1.00 38.50 - 0.00 28 Example of the invention 14.90 38.30 4.80 2.00 1.00 39.00 - 0.00 29 Example of the invention 2.48 45.00 4.50 2.00 1.00 45.00 Sr:0.02 0.02 30 Comparative example 4.00 45.50 8.50 1.00 1.00 40.00 - 0.00 31 Comparative example Steel sheet galvanized-annealed

[49] Table 2 No. Manufacturing Method Bath Temperature Immersion Time Average Cooling Rate in the temperature range of bath temperature to 450°C Average Cooling Rate in the temperature range of 450 to 350°C Average Cooling Rate in the temperature range of 350 to 250°C Average Cooling Rate in the temperature range of 250 to 150°C Average Cooling Rate in the temperature rangeFrom 350 to 150°C Adhesion quantity of phosphate coating per surface (°C) (sec) (°C / sec) (°C / sec) (°C / sec) (°C / sec) (°C / sec) (g / m2) 1 520 3 15 5 2 2 2 40 2 530 3 15 5 2 2 2 45 3 530 3 15 5 2 2 2 42 4 600 3 15 5 2 2 2 48 5 580 3 15 5 2 2 2 42 6 580 3 15 5 2 2 2 25 7 550 3 15 5 2 2 2 44 8 600 3 15 5 2 2 2 45 9 570 3 15 5 2 2 2 46 10 570 3 15 5 2 2 2 120 11 570 3 15 5 2 2 2 40 12 570 3 15 15 2 2 2 41 13 570 3 5 5 2 2 2 41 14 570 3 15 5 2 2 2 41 15 570 16 15 5 2 2 2 190 16 570 3 15 5 5 5 5 44 17 600 3 15 5 2 2 2 44 18 570 3 15 5 2 2 22 19 570 3 10 5 2 2 2 42 20 590 3 15 5 2 2 2 31 21 570 3 15 5 20 2 4 36 22 590 3 15 5 2 2 2 41 23 600 3 15 5 2 2 2 35 24 630 3 15 5 2 2 2 34 25 650 3 15 5 2 2 2 31 26 650 3 15 5 2 2 2 35 27 650 3 15 5 2 2 2 30 28 680 3 15 5 2 2 2 35 29 680 3 15 5 2 2 2 36 30 690 3 15 5 2 2 2 30 31 Galvanized-annealed steel sheet.

[50] Phosphate coating evaluation To investigate the phosphate coating, the produced sample was dissolved in 20% chromic acid. The resulting solution containing the phosphate coating was analyzed using inductively coupled plasma atomic emission spectrometry (ICP-AES), and the average composition of the phosphate coating was measured. Additionally, the degree of adhesion of the phosphate coating to each surface was determined by measuring the weight change before and after dissolution. Furthermore, the surface of the plated steel sheet on which the phosphate coating was formed was observed using SEM, and the proportion (% of the area) of the defect in the phosphate coating was measured. In this case, the ratio of the area of ​​a region of the steel sheet exposed in a field of view observed by SEM was defined as the proportion of the defect in the phosphate coating. In cases where the proportion of the defect in the phosphate coating was 10% of the area or less, the defect in the phosphate coating was determined to be inhibited. The sample subjected to phosphating treatment was cut at 25 mm (C direction) x 15 mm (L direction), and an SEM image of the sample surface was obtained. The shapes of the zinc phosphate crystals were observed based on this SEM image. As a result, it was confirmed in example zezznn / zznz / E / YiAi of the invention that the zinc phosphate crystals are flake-shaped and have a grain size of 10 µm or less on the major axis and 5 µm or less on the minor axis. zezznn / zznz / E / YiAi

[51] Coating film adhesion (SDT resistance) As described above, the zinc phosphate treatment was performed on a sample that was 50 x 100 mm in size according to the SD5350 system (standard of Nipponpaint Industrial Coatings Co., LTD.), the electrodeposition coating (PN110 Powernics Gray: standard of Nipponpaint Industrial Coatings Co., LTD.) was performed so that the thickness of a coating film was 20 pm, the sample was baked at a baking temperature of 150°C for 20 minutes, a steel sheet coated with cross-sectional portions (40 × V2 mm, two) that reached the base material was immersed in salt water with 5% NaCl at a temperature of 55°C for 1000 hours, an electrodeposition coating film was exfoliated by exfoliation using a tape, and the adhesion of the coating film to the hot-stamped steel was then evaluated from the ratio of the area of ​​a portion where exfoliation occurred and the red oxide generation state. Cases where the exfoliated area was 5% or less were rated AA, cases where the exfoliated area was greater than 5% and 10% or less were rated A, and cases where the exfoliated area exceeded 10% or a case where red oxide was generated were rated B.

[52] The results are shown in Table 3.

[53] Table 3 No. Phosphate Coating (Zinc Phosphate Crystallines) SDT Resistance Phosphate Coating Adhesion Amount Mg Content Ca Content Phosphate Coating Defect (g / m2) (% by mass) (% by mass) (% of area) 1 7.2 6.0 0.6 15.0 B 2 6.5 11.5 B 3 2.3 18.6 0.6 2.0 A 4 2.4 51.5 1.2 11.0 B 5 2.5 20.1 1.6 1.0 A 6 3.1 32.9 1.2 0.0 AA 7 3.5 18.9 1.0 1.0 AA 8 4.1 46.0 5.0 0.0 A 9 2.6 31.2 0.5 0.0 AA 10 3.5 33.5 2.6 0.0 AA 11 2.1 5.5 0.4 10.1 B 12 1.5 4.1 0.3 10.5 B 13 1.9 4.6 0.5 11.6 B 14 2.1 4.9 0.7 15.2 B 15 2.1 2.1 0.1 22.9 B 16 1.6 4.0 0.2 10.1 B 17 2.1 49.9 2.6 0.0 A 18 3.2 31.5 2.8 0.0 AA 19 3.1 11.1 2.0 0.0 A 20 3.5 30.5 4.5 0.0 AA 21 2.2 1.5 0.2 21.0 B 22 3.6 2.0 0.2 15.3 B 23 4.0 31.5 4.2 0.0 AA 24 1.0 36.7 4.6 1.0 AA 25 2.5 20.1 4.1 2.0 A 26 3.5 18.9 4.8 2.0 A 27 2.6 0.0 0.0 10.1 B 28 4.1 5.2 4.8 3.2 A 29 3.2 5.1 4.9 3.5 A 30 2.5 5.1 4.1 18.5 B 31 Galvanized-annealed steel sheet B zezznn / zznz / E / YiA

[54] In each of the numbers 3, 5 to 10, 17 to 20, 23 to 26, 28, and 29, which were examples of the invention, a plated steel sheet was produced under appropriate manufacturing conditions and hot-stamped, so that a phosphate coating containing Mg and Ca was formed in a plated layer. As a result, a hot-stamped steel had excellent adhesion of the coating film. Figure 2 shows the SEM image (BSE image) of No. 10 from Tables 1 to 3. Since zinc phosphate crystals formed on the surface of one sample, the defect in the phosphate coating where zinc phosphate crystals did not form was not observed. As a result of ICP atomic emission spectrometry, 33.5% Mg and 2.6% Ca were contained in the zinc phosphate crystals. On the other hand, in comparative examples where the chemical composition of the plated layer was not within the range of the invention or a manufacturing method was not preferred, the composition of the phosphate coating and / or the defect area ratio in the phosphate coating was large, resulting in low adhesion of the coating film. For example, in No. 12, Ca and Mg were not present in a zinc phosphate film 13, and numerous defects were also observed in the phosphate coating 12, as shown in Figure 3. In addition, the adhesion of the coating film was lower in No. 28 where hot stamping was performed on a commercially available galvanized-annealed steel sheet. Brief description of the reference symbols

[55] 1: hot stamped steel 2: base material 3: veneered layer 4: phosphate coating 11: Zinc phosphate crystals containing Mg and Ca 12: defect in the phosphate coating 13: Zinc phosphate crystals that do not contain Mg or Ca

Claims

1. A hot-stamped steel comprising: a base material formed of steel; a plated layer formed on a surface of the base material; and a phosphate coating that forms on a surface of the plated layer, wherein the chemical composition of the plated layer contains 20.00 to 45.00% by mass of Al, 10.00 to 45.00% by mass of Fe, 4.50 to 15.00% by mass of Mg, 0.10 to 3.00% by mass of Si, 0.05 to 3.00% by mass of Ca, 0 to 0.50% by mass of Sb, 0 to 0.50% by mass of Pb, 0 to 1.00% by mass of Cu, 0 to 1.00% by mass of Sn, 0 to 1.00% by mass of Ti, 0 to 0.50% by mass of Sr, 0 to 1.00% by mass of Cr, 0 to 1.00% by mass of Ni, and 0 to 1.00% by mass of Mn with a remainder of Zn and impurities, the phosphate coating consists of zinc phosphate crystals containing 5.0 to 50.0% by mass of Mq and 0.5 to 5.0% by mass of Ca, and the amount of adhesion of the phosphate coating per surface is in the range of 0.1 to 10.0 g / m2.

2. Hot-stamped steel according to claim 1, wherein the chemical composition of the plated layer contains one or both of 25.00 to 35.00% by mass of Al and 6.00 to 10.00% by mass of Mg.

3. Hot-stamped steel according to claim 1 or 2, wherein the amount of adhesion of the phosphate coating 5 per surface is in the range of 1.5 to 8.0 g / m2.