Hot-formed member and method for manufacturing the same

Abstract

The present invention provides a hot-formed component and a method for manufacturing the same, which reduce wear on the hot-forming die during hot-forming. [Solution] A hot-formable member comprising a base steel sheet and an aluminum alloy plating layer formed on the base steel sheet, wherein the aluminum alloy plating layer comprises an alloying layer (I) formed on the base steel sheet and containing Al: 5-30% by weight, an alloying layer (II) formed on the alloying layer (I) and containing Al: 30-60% by weight, an alloying layer (III) formed on the alloying layer (II) and containing Al: 20-50% and Si: 5-20% by weight, and an alloying layer (IV) formed continuously or discontinuously on at least a part of the surface of the alloying layer (III) and containing Al: 30-60%, wherein the proportion of the alloying layer (III) exposed on the outermost surface of the aluminum alloy plating layer is 10% or more.

Description

Technical Field

[0001] The present invention relates to a hot-formed member and a method for manufacturing the same.

Background Art

[0002] In recent years, due to the depletion of petroleum energy resources and the high concern about the environment, regulations on improving the fuel efficiency of automobiles have been increasingly strengthened. From the perspective of materials, one of the methods for improving the fuel efficiency of automobiles is to reduce the thickness of the steel sheets used in automobiles. However, when reducing the thickness, there is a possibility of problems with the safety of automobiles. Therefore, it is necessary to ensure an improvement in the strength of the steel sheets.

[0003] For these reasons, there is a continuous demand for high-strength steel sheets, and various steel sheets have been developed. By the way, these steel sheets have a problem in that their workability is poor because they themselves have high strength. That is, for each grade of steel sheet, the product of strength and elongation always tends to have a constant value. Therefore, when the strength of the steel sheet increases, there is a problem that the elongation, which is an index of workability, decreases.

[0004] In order to solve such problems, a hot press forming method has been proposed. The hot press forming method is a method of forming a low-temperature structure such as martensite in a steel sheet by processing the steel sheet at a high temperature where it is easy to process and then rapidly cooling it to a low temperature to increase the strength of the final product. In this case, there is an advantage that the problem of workability can be minimized when manufacturing a member having high strength.

[0005] However, when using the hot press forming method described above, there was a problem in that the surface of the steel sheet oxidizes when the steel sheet is heated to a high temperature, requiring an additional step to remove the oxide from the surface of the steel sheet after press forming. As a method to solve this problem, the technology described in U.S. Patent No. 6,296,805 was proposed. In U.S. Patent No. 6,296,805, an aluminum-plated steel sheet is heated and rapidly cooled after hot press forming or room temperature forming (simply put, "post-heat treatment"), and in this case, since the aluminum plating layer is present on the surface of the steel sheet, the steel sheet does not oxidize when heated.

[0006] However, when hot-press forming aluminum-plated steel sheets, a problem arises in that severe die wear occurs during hot forming, even though the material's strength is very low at high temperatures. This is because, during the heating process for hot forming of the plated steel sheet, the base iron diffuses into the aluminum plating layer, forming a hard Fe and Al alloy layer on the surface of the steel sheet. Since the hardness of this alloy layer is usually higher than that of the die material, which is typically made of tool steel, severe die wear occurs during press forming. As a result, when hot-press forming aluminum-plated steel sheets, the die must be ground or replaced at short intervals, which significantly increases the manufacturing cost of the hot-formed parts. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The present invention aims to provide a hot-formed member and a method for manufacturing the same that reduce wear on the hot-forming die during hot-forming.

[0008] The problems addressed by the present invention are not limited to those described above. Anyone with ordinary skill in the art to which the present invention pertains will have no difficulty understanding the additional problems addressed by the present invention from the overall content of this specification. [Means for solving the problem]

[0009] One aspect of the present invention is a hot-formable member comprising a base steel sheet and an aluminum alloy plating layer formed on the base steel sheet, wherein the aluminum alloy plating layer comprises an alloying layer (I) formed on the base steel sheet and containing Al: 5-30% by weight, an alloying layer (II) formed on the alloying layer (I) and containing Al: 30-60% by weight, an alloying layer (III) formed on the alloying layer (II) and containing Al: 20-50% and Si: 5-20% by weight, and an alloying layer (IV) formed continuously or discontinuously on at least a part of the surface of the alloying layer (III) and containing Al: 30-60%, wherein the proportion of alloying layer (III) exposed on the outermost surface of the aluminum alloy plating layer is 10% or more.

[0010] Multiple pores are formed in the alloyed layer (III), and the porosity of the alloyed layer (III) can be 5 to 50%.

[0011] The above-mentioned base steel sheet may contain, by weight %, C: 0.04-0.5%, Si: 0.01-2%, Mn: 0.1-5%, P: 0.001-0.05%, S: 0.0001-0.02%, Al: 0.001-1%, N: 0.001-0.02%, the remainder being Fe, and other impurities.

[0012] The above-mentioned base steel sheet may further contain one or more of the following in weight percent: B: 0.001-0.01%, Cr: 0.01-1%, and Ti: 0.001-0.2%.

[0013] Another aspect of the present invention is a method for manufacturing a hot-formed member, comprising the steps of: aluminum plating the surface of a base steel sheet and winding it to obtain an aluminum-plated steel sheet; annealing the aluminum-plated steel sheet to obtain an aluminum-iron alloy plated steel sheet; and hot press forming the aluminum-iron alloy plated steel sheet, wherein the amount of aluminum plating is 30 to 200 g / m2 based on one side of the steel sheet, the cooling rate to 250°C after aluminum plating is 20°C / second or less, the winding tension during winding is 0.5 to 5 kg / mm2, and the annealing is performed in a box annealing furnace. This is a method for manufacturing hot-formed members, which involves heating in a heating temperature range of 550 to 750°C for 30 minutes to 50 hours, with an average heating rate of 10 to 100°C / h when heating from room temperature to the heating temperature during the above annealing process, and an average heating rate of 1 to 15°C / h in the 400 to 500°C section, with a difference of 5 to 80°C between the ambient temperature in the box annealing furnace and the steel plate temperature, and then performing heat treatment in a temperature range of Ac3 to 950°C during hot press forming, with a heating rate of 3 to 18°C / s from 200°C to the Ac3 to 950°C range, and a total heating time of 1 to 15 minutes before hot press forming.

[0014] When hot-formed members are produced 500 times using the above-described method for manufacturing hot-formed members, the average wear depth of the hot-forming die at 10 points can be 15 μm or less. [Effects of the Invention]

[0015] According to the present invention, when manufacturing hot-formed members, the surface hardness of the plating layer is lower than that of the hot-forming die, and die wear is reduced. This allows for an increase in the grinding or replacement cycle of the hot-forming die, resulting in improved manufacturing costs and production efficiency for hot-formed members.

[0016] The diverse and beneficial advantages and effects of the present invention are not limited to those described above and can be more easily understood in the process of describing specific embodiments of the present invention. [Brief explanation of the drawing]

[0017] [Figure 1]This is a scanning electron microscope image showing a cross-section of the plating layer of a hot-formed member manufactured according to Invention Example 1. [Figure 2] This is a scanning electron microscope image of a cross-section of the plating layer of a hot-formed member manufactured according to Comparative Example 1. [Modes for carrying out the invention]

[0018] The following describes in detail a hot-formed member according to one aspect of the present invention. In the present invention, it should be noted that when expressing the content of each element, unless otherwise specified, it means weight percent. Also, unless otherwise specified, the ratio of crystals and structures is based on area.

[0019] [Hot-formed components] First, a hot-formed member according to one aspect of the present invention comprises a base steel sheet and an aluminum alloy plating layer formed on the base steel sheet, wherein the aluminum alloy plating layer comprises an alloying layer (I) formed on the base steel sheet and containing Al: 5-30% by weight, an alloying layer (II) formed on the alloying layer (I) and containing Al: 30-60% by weight, an alloying layer (III) formed on the alloying layer (II) and containing Al: 20-50% and Si: 5-20% by weight, and an alloying layer (IV) formed continuously or discontinuously on at least a part of the surface of the alloying layer (III) and containing Al: 30-60%.

[0020] Preferably, each of the above alloy layers can have the following component ranges. The alloying layer (I) can contain, by weight %, Al: 5 to 30%, Si: 0 to 10%, the balance being Fe, and inevitable impurities due to other alloying. The alloying layer (II) can contain, by weight %, Al: 30 to 60%, Si: 0 to 5%, the balance being Fe, and inevitable impurities due to other alloying. The alloying layer (III) can contain, by weight %, Al: 20 to 50%, Si: 5 to 20%, the balance being Fe, and inevitable impurities due to other alloying. And the alloying layer (IV) can contain, by weight %, Al: 30 to 60%, Si: 0 to 5%, the balance being Fe, and inevitable impurities due to other alloying.

[0021] When aluminum is plated on the base steel sheet and then heat-treated, Fe in the base steel sheet diffuses into the aluminum plating layer with a high Al content. In the hot-formed member of the present invention, alloying of Al and Fe is performed in the plating layer by annealing treatment for alloying and heat treatment during hot press forming, and a layer structure composed of alloying layers (I) to (IV) is formed according to the degree of alloying of Fe.

[0022] The alloying layer (IV) can be formed continuously or discontinuously on at least a part of the surface of the alloying layer (III). That is, the alloying layer (IV) may be formed on a part of the surface instead of on the entire surface of the alloying layer (III).

[0023] Further, by forming the alloying layer (IV) on at least a part of the surface of the alloying layer (III), a part of the surface of the alloying layer (III) can be exposed on the outermost surface of the aluminum alloy plating layer. Here, the outermost surface means the outermost surface of the aluminum alloy plating layer on the side opposite to the base steel sheet. When an oxide layer is formed on the surface of the aluminum alloy plating layer, it means the uppermost surface among the remaining layers excluding the oxide layer.

[0024] At this time, the ratio of the alloying layer (III) exposed on the outermost surface of the aluminum alloy plating layer is preferably 10% or more. Here, the ratio of the alloying layer (III) exposed on the outermost surface can be defined as the ratio of the length of the region where the alloying layer (III) is exposed to the total length of the outermost surface portion when observing the cross-section of the alloy plating layer. In some cases, it can also be defined as the area ratio of the surface area of the alloying layer (III) exposed on the outermost surface to the surface area of the outermost surface of the aluminum alloy plating layer. Among the above alloying layers, the hardness of the alloying layer (II) and the alloying layer (IV) is very high at about 900 Hv level, while the hardness of the alloying layer (I) and the alloying layer (III) is at about 300 - 700 Hv level, which is relatively low compared with the alloying layer (II) and the alloying layer (IV). Therefore, during hot press forming, when the exposed area of the alloying layer (III) with relatively low hardness on the outermost surface of the aluminum alloy plating layer in contact with the mold becomes wider, the overall average hardness of the outermost surface becomes lower, and the wear of the mold is reduced. When the ratio of the alloying layer (III) exposed on the outermost surface is less than 10%, the difference between the average hardness of the outermost surface and the mold hardness becomes smaller, and the mold wear cannot be effectively suppressed. From the point of suppressing mold wear, since the lower the hardness of the outermost surface of the aluminum alloy plating layer is, the more preferable it is, there is no need to specifically limit the upper limit of the above ratio. Preferably, the above ratio is 15% or more, and in some cases, it can be 20% or more.

[0025]

[0026] ​On the other hand, multiple pores can be formed in the alloyed layer (III). When an aluminum-plated steel sheet is subjected to alloying heat treatment in a box annealing furnace under predetermined conditions to produce an aluminum alloy-plated steel sheet, multiple alloy layers are formed in the aluminum alloy-plated steel sheet. Due to the difference in the interdiffusion coefficients of Fe, Al, Si, etc., between alloy layers of different components, multiple pores are formed in the upper alloy layer. In this case, more pores are formed towards the upper end of the alloy layer, resulting in a higher porosity. When the aluminum alloy-plated steel sheet is heated and then hot-press-formed, the uppermost alloy layer with high-density pores is crushed by the press-forming process, forming small particles. The rolling friction generated by these small particles rolling is smaller than the sliding friction between the steel sheet and the die, thus increasing the lubricity between the die and the steel sheet. Here, the porosity can be defined as the ratio of the pore area to the area of ​​each alloy layer (or alloyed layer) when observing the cross-section of the alloy layer (or alloyed layer).

[0027] However, as shown in Figure 1, during hot press forming, most of the alloying layer (IV) is crushed by the press forming process, making it difficult to measure the porosity of the alloying layer (IV) in hot-formed members. Therefore, the properties of the present invention were demonstrated using the porosity of alloying layer (III), which is less affected by press forming and shows a close correlation with the porosity of alloying layer (IV) before press forming.

[0028] As a result, the porosity of the alloyed layer (III) of the hot-formed member according to one aspect of the present invention can be 5 to 50%. If the porosity is less than 5%, it is difficult to expect a lubricating effect due to rolling friction during hot press forming. On the other hand, if the porosity exceeds 50%, the structure of the alloyed layer (III) of the hot-formed member is too weak, and there is a risk that the mold will become severely contaminated by particles that detach from the plating layer in the mold during continuous hot forming. Therefore, in the present invention, the porosity is preferably 5 to 50%, and in some cases it can be 7 to 50%.

[0029] On the other hand, the base steel sheet of the present invention is a steel sheet for hot press forming, and its composition is not particularly limited as long as it is used for hot press forming. However, according to one aspect of the present invention, it contains, by weight percent, C: 0.04~0.5%, Si: 0.01~2%, Mn: 0.1~5%, P: 0.001~0.05%, S: 0.0001~0.02%, Al: 0.001~1%, N: 0.001~0.02%, the remainder being Fe, and other impurities. Each component system will be described in detail below.

[0030] C: 0.04~0.5% The above-mentioned C is an essential element for improving the strength of heat-treated members and can be added in an appropriate amount. That is, in order to ensure sufficient strength of heat-treated members, C can be added in an amount of 0.04% or more. Preferably, the lower limit of the C content can be 0.1% or more. However, if the content is too high, when cold rolling of hot-rolled material is performed during the production of cold-rolled material, the strength of the hot-rolled material will be excessively high, resulting in significantly poor cold-rollability and a significant decrease in spot-weldability. Therefore, in order to ensure sufficient cold-rollability and spot-weldability, it can be added in an amount of 0.5% or less. Furthermore, the C content may be limited to 0.45% or less, more preferably to 0.4% or less.

[0031] Si: 0.01~2% The above-mentioned Si is necessary to be added as a deoxidizing agent in steelmaking and plays a role in suppressing the formation of carbides, which have the greatest impact on the strength of hot-press-formed members. In the present invention, in hot-press forming, Si can be added at a content of 0.01% or more in order to secure retained austenite by concentrating carbon at the lath grain boundaries of martensite after the formation of martensite. Furthermore, in order to ensure sufficient plating properties when aluminum plating is performed on the steel sheet after rolling, the upper limit of the Si content can be set to 2%. Preferably, the Si content may be limited to 1.5% or less.

[0032] Mn: 0.1~5% The above-mentioned Mn can be added in a content of 0.1% or more in order to not only ensure the effect of solid solution strengthening, but also to lower the critical cooling rate required to secure martensite in hot press-formed members. Furthermore, the content of the above-mentioned Mn can be limited to 5% or less in order to ensure the workability of the hot press-forming process, reduce manufacturing costs, and improve spot weldability by appropriately maintaining the strength of the steel plate.

[0033] P: 0.001~0.05% The above-mentioned P exists as an impurity in steel, and it is advantageous to have as low a content as possible. Therefore, in the present invention, the P content can be limited to 0.05% or less, and preferably to 0.03% or less. Since P is an impurity element that is advantageous in smaller amounts, there is no need to specifically determine an upper limit for its content. However, since excessively reducing the P content may increase manufacturing costs, considering this, the lower limit can be set to 0.001%.

[0034] S: 0.0001~0.02% Since S is an element present as an impurity in steel that inhibits the ductility, impact properties, and weldability of the component, its maximum content can be limited to 0.02%, and preferably to 0.01% or less. Furthermore, since a minimum content of less than 0.0001% may increase manufacturing costs, the lower limit of its content can be set to 0.0001%.

[0035] Al: 0.001~1% The above-mentioned Al, together with Si, can perform a deoxidizing action in steelmaking to improve the cleanliness of the steel, and to obtain the above effect, it can be added in a content of 0.001% or more. Furthermore, in order to prevent the Ac3 temperature from becoming excessively high and to allow the heating required during hot press forming to be performed within an appropriate temperature range, the content of the above-mentioned Al can be limited to 1% or less.

[0036] N: 0.001~0.02% The above-mentioned N is an element contained as an impurity in steel. Since a lower content is advantageous in order to reduce sensitivity to crack formation during continuous casting of slabs and to ensure impact properties, it can be included at a concentration of 0.02% or less. There is no need to specifically determine a lower limit, but considering factors such as the increase in manufacturing costs, the N content may be set at 0.001% or higher.

[0037] In addition to the alloy composition described above, an aluminum-iron alloy plated steel sheet according to one aspect of the present invention may further contain one or more of the following: B: 0.001-0.01%, Cr: 0.01-1%, and Ti: 0.001-0.2%.

[0038] B: 0.001~0.01% The element B, as described above, not only improves hardening ability even when added in small amounts, but also segregates at the prior austenite grain boundaries, suppressing the brittleness of hot-press-formed members caused by grain boundary segregation of P and / or S. Therefore, B can be added in amounts of 0.0001% or more. However, if it exceeds 0.01%, not only does its effect saturate, but it also causes brittleness during hot rolling, so the upper limit is set to 0.01%, and preferably the content of B can be reduced to 0.005% or less.

[0039] Cr: 0.01~1% The above-mentioned Cr, like Mn, is an element added for the effect of solid solution strengthening and improvement of hardening ability during hot forming, and can be added in amounts of 0.01% or more to obtain the above effects. However, in order to ensure the weldability of the component, its content can be limited to 1% or less. Furthermore, if it exceeds 1%, the effect of improving hardening ability is negligible compared to the amount added, which is also disadvantageous from a cost standpoint.

[0040] Ti: 0.001~0.2% The above-mentioned Ti is effective not only in increasing the strength of the heat-treated component by forming fine precipitates and improving the collision performance of the component by refining the crystal grains, but also, when B is added, it reacts with N first, maximizing the effect of adding B. To obtain the above effects, Ti can be added in an amount of 0.001% or more. However, the formation of coarse TiN due to the increase in Ti content degrades the collision performance of the component, so its content can be limited to 0.2% or less.

[0041] Other components besides those mentioned above include iron (Fe) and unavoidable impurities. Furthermore, there are no particular restrictions on the addition of any components that may be present in hot-press-formed steel sheets.

[0042] When manufacturing hot-formed members having the alloy composition and layer structure described above, the proportion of the low-hardness alloyed layer (III) on the surface of the aluminum alloy plated steel sheet increases during hot press forming, resulting in a lower average surface hardness. This effectively reduces mold wear caused by hardness differences. In particular, even after producing hot-formed members more than 500 times, the average wear depth of the hot-forming die at 10 points can be kept below 15 μm.

[0043] Furthermore, during hot press forming, multiple voids are formed in the alloying layers (III) and (IV), which are the upper layers of the aluminum alloy plating layer. These voids cause the alloying layer (IV) to break during press forming, resulting in a lubricating effect through rolling friction, which further reduces damage to the mold.

[0044] The following describes in detail a method for manufacturing a hot-formed member according to another aspect of the present invention. However, it should be noted that the following method for manufacturing a hot-formed member is merely an example, and the hot-formed member of the present invention does not necessarily have to be manufactured by this method. Any manufacturing method that satisfies the claims of the present invention will pose no problem in realizing each example of the present invention.

[0045] [Method for manufacturing hot-formed members] The hot-formed member of the present invention is prepared by preparing a hot-rolled or cold-rolled base steel sheet, and the base This can be obtained by applying aluminum plating to the surface of a steel sheet, performing alloying heat treatment in a box annealing furnace to obtain an aluminum alloy plated steel sheet, and then hot press forming it under predetermined conditions.

[0046] First, a base steel sheet having the alloy composition described above is prepared, and an aluminum-plated steel sheet (coil) is obtained by applying aluminum plating to the surface of the base steel sheet under appropriate conditions and then winding it up.

[0047] First, the surface of the rolled steel sheet can be plated with aluminum at a plating rate of 30 to 200 g / m2 per side. For aluminum plating, either type I AlSi plating (containing 80% or more Al and 5-20% Si, with additional elements as needed) or type II plating (containing 90% or more Al, with additional elements as needed) can be used. Hot-dip aluminum plating can be performed to form the plating layer, and annealing treatment may be applied to the steel sheet before plating. The appropriate plating amount is 30 to 200 g / m2 per side. If the plating amount is too high, alloying to the surface takes an excessively long time; conversely, if the plating amount is too low, it is difficult to obtain sufficient corrosion resistance.

[0048] Next, after aluminum plating, the cooling rate to 250°C can be set to 20°C / second or less. The cooling rate after aluminum plating affects the formation of a diffusion-inhibiting layer between the plating layer and the base iron. If the cooling rate after aluminum plating is too fast, the diffusion-inhibiting layer may not be formed uniformly, which may lead to uneven alloying behavior of the coil during the subsequent annealing treatment. Therefore, the cooling rate to 250°C after aluminum plating can be set to 20°C / second or less.

[0049] When winding the steel sheet after plating to obtain a coil, the winding tension of the coil can be adjusted from 0.5 to 5 kg / mm². Adjusting the winding tension of the coil can alter the alloying behavior and surface quality of the coil during the subsequent annealing process.

[0050] Subsequently, an aluminum-iron alloy plated steel sheet can be obtained by performing an annealing treatment on the aluminum-plated steel sheet under the following conditions.

[0051] Aluminum-plated steel sheets (coils) are heated in a batch annealing furnace (BAF). When heating the steel sheets, the target heat treatment temperature and maintenance time are preferably maintained within a range of 550 to 750°C (in this invention, the highest temperature the material reaches within this temperature range is called the heating temperature) for 30 minutes to 50 hours, based on the steel sheet temperature. Here, the maintenance time is the time from when the coil temperature reaches the target temperature until cooling begins. If sufficient alloying is not achieved, the plating layer may peel off during roll leveling; therefore, the heating temperature can be raised to 550°C or higher to ensure sufficient alloying. Furthermore, in order to prevent excessive oxide formation on the surface and ensure spot weldability, the heating temperature can be set to 750°C or lower. Also, in order to ensure a sufficient plating layer and prevent a decrease in productivity, the maintenance time can be set to 30 minutes to 50 hours. In some cases, the steel plate temperature may have a heating pattern in which the temperature continues to rise without a cooling process until it reaches the heating temperature, or a heating pattern in which the temperature is maintained at a temperature below the target temperature for a certain period of time before being raised may be applied.

[0052] When heating the steel plates to the above-mentioned heating temperatures, in order to ensure sufficient productivity and uniformly alloy the plating layer across all steel plates (coils), the average heating rate based on the steel plate (coil) temperature throughout the entire temperature range (from room temperature to the heating temperature) can be set to 10-100°C / h. While the overall average heating rate can be controlled within the above numerical range, in one implementation example of the present invention, in order to prevent the rolling oil mixed in during rolling from remaining in the above-mentioned temperature range where it vaporizes, causing surface unevenness, and to ensure sufficient productivity, the heating can be performed with an average heating rate of 1-15°C / h in the 400-500°C range.

[0053] Furthermore, the temperature difference between the ambient temperature inside the box annealing furnace and the steel plate temperature can be set to 5-80°C. In general box annealing furnaces, the method of heating the steel plate (coil) by raising the ambient temperature inside the annealing furnace is used, rather than directly heating the steel plate (coil). In this case, a temperature difference between the ambient temperature and the coil temperature is unavoidable, but in order to minimize variations in material properties and plating quality at different locations within the steel plate, the temperature difference between the ambient temperature and the steel plate temperature can be set to 80°C or less, based on the point at which the target temperature for heat treatment is reached. Ideally, the temperature difference should be as small as possible, but in this case, the heating rate must be slowed down, and it may become difficult to meet the condition of the overall average heating rate. Considering this, the temperature difference can be set to 5°C or more. Here, the temperature of the steel plate refers to the temperature measured at the bottom of the charged steel plate (coil) (meaning the lowest part of the coil), and the ambient temperature refers to the temperature measured at the center of the internal space of the heating furnace.

[0054] After manufacturing an aluminum alloy plated steel sheet using the manufacturing method described above, a hot-formed member can be manufactured by hot-press forming the aluminum alloy plated steel sheet. In this case, the hot-press forming can be carried out using a method commonly used in the art, and as an unrestricted example, the material can be heat-treated in a temperature range of Ac3 to 950°C, heated from 200°C to the Ac3 to 950°C temperature range at a heating rate of 3 to 18°C / s, and then hot-press-formed after a total heating time of 1 to 15 minutes. Here, the total heating time can be defined as the heating time including the heating time in the heating temperature range and the heating time in the Ac3 to 950°C temperature range.

[0055] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are merely illustrative and concretize the present invention and are not intended to limit the scope of the rights of the present invention. The scope of the rights of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom. [Examples]

[0056] First, a cold-rolled steel sheet for hot press forming, having the composition shown in Table 1 below, was prepared as the base steel sheet. The surface of the steel sheet was then plated with a type I plating bath having the composition Al-9%Si-1.5%Fe. The plating amount was adjusted to 75 g / m2 per side. After aluminum plating, the material was cooled to 250°C at a cooling rate of 10°C / second, and then wound up with a winding tension of 3 kg / mm² to obtain an aluminum-plated steel sheet.

[0057] [Table 1]

[0058] Subsequently, the plated steel sheets were subjected to alloying heat treatment in a box annealing furnace under the conditions shown in Table 2 below, and then each was subjected to 500 hot press forming cycles to obtain hot-formed members. However, in Comparative Example 1, the above-mentioned aluminum-plated steel sheets were not subjected to alloying heat treatment, and hot-formed members were obtained by hot press forming under the conditions shown in Table 2 below.

[0059] [Table 2]

[0060] Subsequently, after 500 production cycles, the wear depth was randomly measured at 10 points on the molds used in each inventive example and comparative example, and the average values ​​are shown in Table 3 below. Meanwhile, for each example, 10 samples were randomly taken from the 500 produced items, and their cross-sections were observed with a scanning electron microscope to confirm the surface occupancy rate of the alloying layer (III). The average value of this occupancy rate is shown in Table 3 below. In addition, the porosity of the alloying layer (III) was measured, and the results are shown in Table 3 below. It was confirmed that the deviation between the surface occupancy rate and the porosity of the alloying layer (III) was not large in the same example (inventive example or comparative example).

[0061] [Table 3]

[0062] As shown in Table 3 above, in Invention Examples 1 to 3, where the area ratio of the alloyed layer (III) exposed on the outermost surface is 10% or more and the porosity is 5% or more, it was confirmed that even after producing hot-formed members according to Invention Examples 1 to 3 500 times, the average wear depth of the mold was 15 μm or less, confirming that the wear of the hot-forming mold was effectively suppressed. In contrast, Comparative Example 1 was a hot-press-formed ordinary Al-Si plated steel sheet, and the area ratio of the alloyed layer (III) exposed on the outermost surface was less than 10%, and the porosity was low, so it was confirmed that the wear of the mold was significantly increased compared to the Invention Examples.

[0063] Furthermore, in Comparative Example 2, although the aluminum plating layer underwent alloying heat treatment, the alloying heat treatment temperature was too low, resulting in insufficient alloying. Consequently, the area ratio of the alloyed layer (III) exposed on the outermost surface was less than 10%, and due to the low porosity, it can be confirmed that mold wear increased significantly, similar to Comparative Example 1.

[0064] As described above with reference to examples, those skilled in the art will understand that the present invention can be modified and altered in various ways without departing from the spirit and scope of the invention as set forth in the appended claims. This disclosure includes the following embodiments. Embodiment 1 A hot-formed member comprising a base steel sheet and an aluminum alloy plating layer formed on the base steel sheet, The aforementioned aluminum alloy plating layer is An alloying layer (I) formed on the aforementioned base steel sheet, containing Al: 5-30% by weight, An alloying layer (II) is formed on the alloying layer (I) and contains Al: 30-60% by weight, An alloying layer (III) is formed on the alloying layer (II) and contains Al: 20-50% and Si: 5-20% by weight, Formed continuously or discontinuously on at least a portion of the surface of the alloyed layer (III), It contains an alloying layer (IV) containing Al:30-60%, A hot-formable member in which the ratio of the alloyed layer (III) exposed on the outermost surface of the aluminum alloy plating layer is 10% or more. Embodiment 2 Multiple voids (pores) are formed in the alloyed layer (III), The hot-formed member according to Embodiment 1, characterized in that the porosity of the alloyed layer (III) is 5 to 50%. Embodiment 3 The hot-formed member according to Embodiment 1, characterized in that the base steel sheet contains, by weight percent, C: 0.04-0.5%, Si: 0.01-2%, Mn: 0.1-5%, P: 0.001-0.05%, S: 0.0001-0.02%, Al: 0.001-1%, N: 0.001-0.02%, the remainder being Fe, and other impurities. Embodiment 4 The hot-formed member according to Embodiment 3, characterized in that the base steel sheet further contains one or more of the following in weight percent: B: 0.001 to 0.01%, Cr: 0.01 to 1%, and Ti: 0.001 to 0.2%. Embodiment 5 The steps include: aluminum plating the surface of a raw steel sheet and winding it up to obtain an aluminum-plated steel sheet; The steps include: annealing an aluminum-plated steel sheet to obtain an aluminum-iron alloy plated steel sheet; A method for manufacturing a hot-formed member, comprising the step of hot-press forming the aluminum-iron alloy plated steel sheet, The amount of aluminum plating is 30 to 200 g / m2 based on one side of the steel plate. The cooling rate from aluminum plating to 250°C should be 20°C / second or less. The winding tension during winding is set to 0.5 to 5 kg / mm². The aforementioned annealing is carried out in a box annealing furnace at a heating temperature range of 550 to 750°C for 30 minutes to 50 hours. During the annealing process, when heating from room temperature to the heating temperature, the average heating rate is set to 10-100°C / h, and the average heating rate in the 400-500°C range is set to 1-15°C / h. The difference between the ambient temperature inside the box annealing furnace and the steel plate temperature is set to 5 to 80°C. A method for manufacturing a hot-formed component, comprising: heat treatment in the temperature range of Ac3 to 950°C during hot press forming, where the material is heated from 200°C to the Ac3 to 950°C range at a heating rate of 3 to 18°C / s, with a total heating time of 1 to 15 minutes, followed by hot press forming. Embodiment 6 The method for manufacturing a hot-formed member according to Embodiment 5, characterized in that when a hot-formed member is produced 500 times by the method described above, the average wear depth of 10 points of the hot-forming die is 15 μm or less.

Claims

1. A hot-formed member comprising a base steel sheet and an aluminum alloy plating layer formed on the base steel sheet, The aforementioned aluminum alloy plating layer is An alloying layer (I) is formed on the aforementioned base steel sheet, consisting of, by weight, Al: 5-30%, the remainder being Fe and unavoidable impurities, An alloying layer (II) is formed on the alloying layer (I), and consists of, by weight, Al: 30-60%, with the remainder being Fe and unavoidable impurities. An alloying layer (III) is formed on the alloying layer (II), and consists of, by weight %, Al: 20-50%, Si: 5-20%, with the remainder being Fe flowing in from the base steel sheet and unavoidable impurities. The alloying layer (IV) is formed continuously or discontinuously on at least a portion of the surface of the alloying layer (III), and comprises, by weight, Al: 30-60%, the remainder being Fe and unavoidable impurities. A hot-formable member in which the ratio of the alloyed layer (III) exposed on the outermost surface of the aluminum alloy plating layer is 10% or more (excluding 100%).

2. The hot-formable member according to claim 1, wherein the ratio of the alloyed layer (III) exposed on the outermost surface of the aluminum alloy plating layer is 15% or more.

3. The hot-formable member according to claim 2, wherein the ratio of the alloyed layer (III) exposed on the outermost surface of the aluminum alloy plating layer is 20% or more.

4. The hot-formed member according to claim 1, wherein a plurality of pores are formed in the alloyed layer (III).

5. The hot-formable member according to claim 1, wherein the porosity of the alloyed layer (III) is 5 to 50%.

6. The hot-formable member according to claim 5, wherein the porosity of the alloyed layer (III) is 7 to 50%.

7. The hot-formed member according to claim 1, wherein the base steel sheet consists of, by weight percent, C: 0.04-0.5%, Si: 0.01-2%, Mn: 0.1-5%, P: 0.001-0.05%, S: 0.0001-0.02%, Al: 0.001-1%, N: 0.001-0.02%, the remainder being Fe and other impurities.

8. The hot-formed member according to claim 7, wherein the base steel sheet further comprises one or more of the following in weight percent: B: 0.001 to 0.01%, Cr: 0.01 to 1%, and Ti: 0.001 to 0.2%.

9. The hot-formable member according to claim 1, wherein an oxide layer is formed on the surface of the aluminum alloy plating layer.

10. The hot-formable member according to claim 1, wherein the hardness of the alloyed layer (I) is lower than the hardness of the alloyed layer (II).

11. The hot-formable member according to claim 1, wherein the hardness of the alloyed layer (I) is 300 to 700 Hv.

12. The hot-formed member according to claim 1, wherein the hardness of the alloyed layer (III) is lower than the hardness of the alloyed layer (II).

13. The hot-formable member according to claim 1, wherein the hardness of the alloyed layer (III) is 300 to 700 Hv.

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