Plated steel sheet for hot press forming and its manufacturing method

Abstract

A plated steel sheet for hot press forming having improved hydrogen delayed fracture properties is disclosed. The plated steel sheet of the present disclosure comprises a base steel sheet; and a plating layer formed on the surface of the base steel sheet and containing, by weight, 5.0-15.0% Si, 10.0-30.0% Zn, 4.0-12.0% Fe, with the remainder being Al and unavoidable impurities.

Description

[Technical Field]

[0001] The present invention relates to a plated steel sheet for hot press forming and a manufacturing method thereof, and more particularly to a plated steel sheet for hot press forming having improved hydrogen delayed fracture properties by optimizing the component conditions and heat treatment conditions, and a manufacturing method thereof. [Background technology]

[0002] Recently, as demands for safety and fuel efficiency have increased, the use of high-strength steel to reduce the weight of automobiles has increased. However, high-strength steel has problems in that it is difficult to form products with complex and precise shapes due to the tendency for the material to break during processing or for springback to occur after processing.

[0003] Hot press forming (HPF) is a method that has recently been gaining popularity as a solution to these problems. Hot press steel sheets are typically heated to 800-950°C and hot processed, making them easy to form. They also have the advantage of being able to increase the strength of the formed product by rapidly cooling them through a mold.

[0004] However, when steel is heated at high temperatures, oxidation occurs on the surface of the steel, which necessitates an additional process of removing the oxidation products from the steel sheet surface after press forming.To address this issue, a method of applying an aluminum or zinc plating layer to the surface of a steel sheet used as a material for hot press forming has been proposed.

[0005] However, the use of high-strength plated steel sheets is limited due to the problem of delayed fracture caused by hydrogen. Specifically, delayed fracture is a delayed fracture phenomenon in which hydrogen diffuses into the steel after the part has been formed, reducing the ductility and toughness of the steel, causing fracture even without plastic deformation.

[0006] In particular, in aluminum-plated steel sheets based on high-strength steel, the hydrogen diffusion coefficient of the Al and Al-Fe-based interfacial alloy layer is extremely low, making the problem of delayed fracture more serious than in steel sheets without a metal coating.

[0007] Therefore, there is a need to develop a hot press-formable plated steel sheet that can be used for automobile parts by ensuring not only strength but also hydrogen delayed fracture properties. Summary of the Invention [Problem to be solved by the invention]

[0008] An embodiment of the present invention provides a plated steel sheet for hot press forming having improved hydrogen delayed fracture resistance and a method for manufacturing the same. [Means for solving the problem]

[0009] A plated steel sheet for hot press forming according to one embodiment of the present invention comprises: a base steel sheet; and a coating layer formed on the surface of the base steel sheet, the coating layer containing, by weight, 5.0 to 15.0% Si, 10.0 to 30.0% Zn, 4.0 to 12.0% Fe, and the remainder being Al and unavoidable impurities.

[0010] Furthermore, according to one embodiment of the present invention, the base steel sheet contains, by weight percent, C: 0.1 to 0.4%, Mn: 0.5 to 3.0%, Si: 0.1 to 0.8%, B: 0.01% or less (excluding 0), Ti: 0.1% or less (excluding 0), and the remainder being Fe and other unavoidable impurities.

[0011] According to one embodiment of the present invention, the coating weight of the coating layer applied to the surface of the base steel sheet is 40 to 120 g / m 2 may be.

[0012] According to one aspect of the present invention, the plating layer may have a thickness of 5 to 40 μm.

[0013] According to one aspect of the present invention, an Al-Fe-based interfacial alloy layer may be formed between the base steel sheet and the coating layer, the Al-Fe-based interfacial alloy layer containing, by weight %, 30.0 to 60.0% Al and 5.0 to 20.0% Si.

[0014] According to one aspect of the present invention, the plating layer may include silicon crystals having a silicon concentration of 60% or more, a zinc concentration of less than 20%, and a major axis length of 1 μm or more.

[0015] A method for manufacturing a plated steel sheet for hot press forming according to another aspect of the present invention includes the steps of: forming a base steel sheet; and immersing the base steel sheet in a plating solution containing, by weight, 5.0 to 15.0% Si, 10.0 to 30.0% Zn, and the remainder being Al and unavoidable impurities to manufacture a plated steel sheet; heating the plated steel sheet to above 800°C and below 1,100°C; hot press forming the plated steel sheet; and cooling the hot press formed plated steel sheet.

[0016] Also, according to one embodiment of the present invention, the plated steel sheet may be heated for more than 2 minutes and less than 15 minutes.

[0017] Furthermore, according to one embodiment of the present invention, the plated steel sheet can be heated at a rate of 2°C / sec to 15°C / sec.

[0018] According to one embodiment of the present invention, in the step of cooling the hot press-formed plated steel sheet, the sheet can be cooled at a rate of 15°C / sec or more in a temperature range of 700°C to 350°C. [Effects of the Invention]

[0019] According to an embodiment of the present invention, a plated steel sheet for hot press forming having improved hydrogen delayed fracture properties and a manufacturing method thereof can be provided, which can be applied to automobile parts. [Brief explanation of the drawings]

[0020] FIG. 1 is a photograph of the cross section of the plating layer of Example 1 observed with a scanning electron microscope. DETAILED DESCRIPTION OF THE INVENTION

[0021] A plated steel sheet for hot press forming according to one embodiment of the present invention comprises: a base steel sheet; and a coating layer formed on the surface of the base steel sheet and containing, by weight, 5.0 to 15.0% Si, 10.0 to 30.0% Zn, 4.0 to 12.0% Fe, and the remainder being Al and unavoidable impurities.

[0022] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are presented to fully convey the concept of the present invention to those skilled in the art to which the present invention pertains. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. In the drawings, parts not relevant to the description are omitted to clarify the present invention, and the sizes of components are somewhat exaggerated to facilitate understanding.

[0023] Throughout the specification, when a part is described as "comprising" a certain element, this does not mean that other elements are excluded, but that other elements may also be included, unless otherwise specified to the contrary. The singular expression includes the plural expression unless the context clearly dictates otherwise.

[0024] Hot press forming (HPF) is a method of processing steel sheets into complex shapes at high temperatures by taking advantage of their tendency to soften as the temperature increases. More specifically, HPF involves heating the steel sheet above the austenite region, i.e., to a state where a phase transition is possible, and then rapidly cooling it while processing, thereby transforming the steel sheet structure into martensite, thereby enabling the production of high-strength, precisely shaped products.

[0025] When high-strength steel is heated at high temperatures, surface defects such as corrosion and decarburization can occur on the steel surface. To prevent this, the surface is plated with zinc or aluminum before hot press forming. The zinc (Zn) or aluminum (Al) used in the plated layer protects the steel sheet from the external environment, thereby improving the corrosion resistance of the steel sheet.

[0026] Compared to zinc-plated steel sheets, aluminum-plated steel sheets have the advantage of not forming a thick oxide film on the plating layer even at high temperatures due to the high melting point of aluminum and the dense, thin aluminum oxide film that forms on top of the plating layer.

[0027] However, when aluminum-plated steel sheets are hot-pressed, moisture in the air is adsorbed onto the surface of the steel sheet during heating and dissociated into hydrogen, easily causing delayed fracture. In steel sheets without a metal coating on the surface of the base material, hydrogen can diffuse into the atmosphere on the surface of the base material, but in aluminum-plated steel sheets, hydrogen does not easily escape from the base material to the outside, making them prone to delayed fracture.

[0028] Therefore, the inventors of the present invention have confirmed that it is possible to improve the hydrogen-delayed fracture properties of aluminum-plated steel sheets by introducing a coating layer structure that allows hydrogen adsorbed to the base metal during the plating process to easily escape to the outside and by suppressing hydrogen adsorption during the manufacturing process, and have thereby devised the present invention.

[0029] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0030] A plated steel sheet for hot press forming according to one embodiment of the present invention includes a base steel sheet and an aluminum-based plating layer formed on the surface of the base steel sheet.

[0031] The base steel sheet of the present invention is a base steel sheet used to produce plated steel sheets for hot press forming, and can be applied to any steel type that undergoes martensitic transformation during hot press forming. For example, the base steel sheet contains, by weight, 0.1-0.4% C, 0.5-3.0% Mn, 0.1-0.8% Si, 0.01% or less (excluding 0) B, 0.1% or less Ti (excluding 0), and the remainder being Fe and other unavoidable impurities.

[0032] The reasons for limiting the contents of alloying elements will be explained below. Unless otherwise specified, the unit is % by weight.

[0033] The C content is 0.1 to 0.4%.

[0034] Carbon (C) is an element that stabilizes the austenite phase and can be added at 0.1% or more to improve hydrogen delayed fracture resistance. However, considering that excessive C content increases electrical resistivity and deteriorates weldability, the upper limit can be limited to 0.4%.

[0035] The Mn content is 0.5 to 3.0%.

[0036] Manganese (Mn) is an element that stabilizes the austenite structure together with carbon and can be added in an amount of 0.5% or more to suppress the formation of α'-martensite structure during processing. However, considering that excessive Mn content not only increases costs but also saturates the effect of improving strength, the upper limit can be limited to 3.0%.

[0037] The Si content is 0.1 to 0.8%.

[0038] Silicon (Si) is an element used as a deoxidizer and can be added in an amount of 0.1% or more to improve the yield strength and tensile strength of steel through solid solution strengthening. However, if the content is excessive, a large amount of silicon oxide is formed on the surface during hot rolling, which reduces pickling properties and increases electrical resistivity, resulting in poor weldability. Therefore, the upper limit of Si can be limited to 0.8%.

[0039] The B content is 0.01% or less (excluding 0).

[0040] Boron (B) is an element that is effective in delaying the ferrite transformation from austenite. However, if its content is excessive, it can cause a problem of deteriorating hot workability. Therefore, in the present invention, the boron content is limited to 0.01% or less.

[0041] The Ti content is 0.1% or less (0 is excluded).

[0042] Titanium (Ti) is an element that improves the formability of steel by reacting with nitrogen in steel to form nitrides and improves the strength of steel by reacting with carbon in steel to form carbides. However, excessive Ti content can lead to excessive formation of precipitates, which can degrade the fatigue properties of steel. Therefore, the present invention aims to limit the titanium content to 0.1% or less.

[0043] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintentional impurities may be inevitably mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in normal manufacturing processes, the contents of all of them will not be specifically mentioned in this specification.

[0044] Next, in order to apply the above-described base steel sheet to a steel sheet for hot press forming, it is preferable to form a plating layer on the base steel sheet. The plating layer may be a conventional aluminum-based plating layer, and more specifically, may be a hot-dip aluminum plating layer or a hot-dip aluminum alloy plating layer.

[0045] Pure Al and Al-Fe interfacial alloy layers present in the aluminum plating layer are prone to delayed fracture due to their low hydrogen diffusion coefficient.

[0046] In the present invention, Si is introduced into the aluminum plating layer to suppress the formation of an Al-Fe alloy phase, and Zn is dissolved in the aluminum plating layer to facilitate the diffusion of hydrogen in the plating layer into the external atmospheric environment.

[0047] The aluminum-based plating layer of the present invention contains, by weight, 5.0 to 15.0% Si, 10.0 to 30.0% Zn, 4.0 to 12.0% Fe, and the remainder being Al and unavoidable impurities.

[0048] Hereinafter, the reasons for limiting the alloying element contents of the coating layer formed on the surface of the base steel sheet in accordance with an embodiment of the present invention, which contributes to improving the corrosion resistance and hydrogen delayed fracture characteristics of the steel sheet, will be described.

[0049] The Si content is 5.0 to 15.0%.

[0050] Silicon (Si) is an element added to control the alloying between the Al in the coating layer and the Fe in the base steel. Low Si content can result in excessive reaction between the Fe in the base steel and the Al in the coating bath, forming a highly brittle Al-Fe phase. A coating layer containing an excessive amount of this brittle phase can easily peel off during blanking and handling of the coated steel sheet. Furthermore, when Si is added to pure Al, it lowers the melting point of the coating bath, thereby lowering the bath temperature. Adding 5.0% or more is recommended to effectively suppress defects such as zinc ash. However, excessive Si content beyond the process composition can cause a rapid rise in the coating bath temperature. This can lead to the need to maintain a high coating bath temperature, which can cause deterioration of high-temperature structures such as sink rolls in the coating bath during operation and reduce operational efficiency. Therefore, the upper limit is limited to 15.0%.

[0051] The Zn content is 10.0 to 30.0%.

[0052] Zinc (Zn) is an element with sacrificial corrosion protection properties, improving the basic corrosion resistance of the coating layer and promoting hydrogen diffusion within the coating layer relative to the aluminum coating layer. Furthermore, zinc forms zinc oxide on the surface during hot forming, which has a porous structure with more open spaces than dense aluminum oxide. In the case of steel sheets containing only aluminum, the surface structure is dense and hard, and when zinc oxide is formed on the surface, it forms a structure through which hydrogen can diffuse out of the coating layer, thereby facilitating the release of hydrogen from the coating layer. To this end, the zinc content in the coating layer may be 15.0% or more, preferably 18% or more, and more preferably 22% or more. However, since excessive zinc content can cause liquid metal embrittlement (LME) during welding, the upper limit of the zinc content in the coating layer may be limited to 35.0%, preferably 30%, and more preferably 27%.

[0053] The Fe content is 4.0 to 12.0%.

[0054] Iron (Fe) is an element that forms an Al-Fe-based interfacial alloy layer. The Al in the coating layer and the Fe in the base iron diffuse to form an Al-Fe-based alloy phase uniformly on the surface of the base metal, which is present in the coating layer. Without such an alloy layer, the continuous diffusion of Fe can thicken the interfacial layer and increase the brittleness of the coating layer, so a content of 4% or more is preferred. However, excessive Fe content in the coating layer can lead to the formation of alloy phases other than FeAl2, FeAl3, and Fe2Al5 among the Fe-Al-based intermetallic compounds, so the upper limit can be set to 12.0%.

[0055] The thickness of the coating layer is preferably 5 to 40 μm. If the average thickness of the coating layer is less than 5 μm, the corrosion resistance of the plated steel sheet cannot be sufficiently ensured. On the other hand, if the average thickness of the coating layer exceeds 40 μm, although it is advantageous in terms of ensuring corrosion resistance, there is a problem in that the amount of coating increases excessively, thereby increasing the manufacturing cost of the steel sheet.

[0056] The plated steel sheet for hot press forming according to the present invention may further include an Al-Fe-based interfacial alloy layer formed at the interface between the base steel sheet and the aluminum-based coating layer, and in this case, the Al-Fe-based interfacial alloy layer may contain, by weight %, 30.0 to 60.0% Al and 5.0 to 20.0% Si. The Fe and Mn contained in the alloy layer may originate from the coating bath components during hot-dip coating, or may be diffused from the base steel sheet.

[0057] The plating layer may contain silicon crystals having a silicon concentration of 60% or more, a zinc concentration of less than 20%, and a major axis length of 1 μm or more.

[0058] The silicon crystals may appear in the plating layer in the shape of a polygon, and the major axis of the polygon refers to the longest distance between the vertices of the polygon.

[0059] In addition, the upper limit of the length of the major axis of the plating layer is preferably 4 / 5 or less of the thickness of the plating layer, because if the length of the silicon crystal is similar to the thickness of the plating layer, it may cause surface defects or peeling of the plating layer.

[0060] Next, a method for manufacturing a plated steel sheet for hot press forming according to another aspect of the present invention will be described.

[0061] A method for manufacturing a plated steel sheet for hot press forming according to one embodiment of the present invention includes the steps of: forming a base steel sheet; and immersing the base steel sheet in a plating solution containing, by weight, 5.0 to 15.0% Si, 10.0 to 30.0% Zn, and the remainder being Al and unavoidable impurities to manufacture a plated steel sheet; heating the plated steel sheet to a temperature above 800°C and below 1,100°C; hot press forming the plated steel sheet; and cooling the hot press formed plated steel sheet.

[0062] The reasons for limiting the alloying element contents are as described above.

[0063] The prepared base steel sheet is immersed in an aluminum-silicon, zinc alloy plating bath to form a plating layer on at least one surface of the base steel sheet.

[0064] The steel sheet then passes through a sink roll and exits the zinc-plating bath. The amount of coating on the surface of the steel sheet can be adjusted by controlling the flow rate and speed of the gas sprayed from the air knife. The coating amount on the surface of the steel sheet is 40 to 120 g / m. 2 can be controlled.

[0065] In addition, the step of producing the plated steel sheet may include a step of cooling from 600°C to 450°C at a cooling rate of 10 to 35°C / s to grow silicon crystals. The gas injected from the air knife alone is not sufficient to induce supercooling of the coating layer. Therefore, the cooling step induces supercooling of the coating layer, allowing the growth of the initially deposited silicon crystals.

[0066] Hereinafter, the heat treatment conditions during heating, hot press forming, and cooling in the method for manufacturing a plated steel sheet for hot press forming will be described in detail.

[0067] The heating conditions during the hot press forming process are the main factor that causes hydrogen to diffuse out of the steel sheet. In the present invention, in order to reduce hydrogen delayed fracture, the heating conditions during the hot press forming process are optimized to release hydrogen absorbed in the steel sheet to the outside.

[0068] If the temperature range during heat treatment is too low, austenite transformation will be insufficient to ensure strength, and hydrogen will not diffuse sufficiently to the outside, so in the present invention, the heating temperature is controlled to exceed 800°C. In contrast, if the temperature range during heat treatment is too high, the hydrogen solid solubility of the steel sheet will increase, so the heating temperature is controlled to less than 1,100°C. The heating temperature can be controlled preferably between 850 and 1,000°C, and more preferably between 870 and 970°C.

[0069] On the other hand, if the heating time is long, the crystal size continues to grow, making the steel sheet vulnerable to hydrogen-induced delayed fracture. Furthermore, when moisture is adsorbed on the surface to form oxides, oxygen remains in the oxide form, but dissociated hydrogen remains within the steel sheet, making it susceptible to delayed fracture. Taking the above into consideration, the present invention aims to control the heating time to more than 2 minutes and less than 15 minutes. The heating time can be controlled preferably between 2 and 12 minutes, and more preferably between 3 and 10 minutes.

[0070] On the other hand, in the hot pressing process, it is necessary to reduce the heating time of the material in order to improve productivity, and to achieve this, electrical heating or high-frequency induction heating is used.

[0071] However, all rapid heating methods use the principle that the temperature rises due to the resistance of the steel sheet when an electric current flows through it, and the electric current also generates a magnetic field, which causes the molten metal on the surface of the base material to move.

[0072] Even when rapid heating is performed using IR instead of electric current, the principle of IR heating, which starts from the surface, means that the surface of the plating layer melts first, causing the plating layer to flow, resulting in an uneven surface.

[0073] In the present invention, as a method for reducing hydrogen-induced delayed fracture, the temperature rise conditions during heating of the plated steel sheet are optimized to a rate of 2°C / sec to 15°C / sec, thereby attempting to release hydrogen from within the steel sheet to the outside.

[0074] Finally, the product is hot-pressed and then cooled.

[0075] At this time, the plated steel sheet can be cooled at a rate of 15°C / sec or more in the temperature range of 700°C to 350°C so as to ensure a martensite structure of 60% or more.

[0076] In this way, when the alloy components of the plating layer and the heating conditions during hot press forming are controlled, the hydrogen delayed fracture properties of the aluminum-plated steel sheet can be improved. [Example]

[0077] The present invention will be described in more detail below through examples.

[0078] Cold-rolled steel sheets containing, by weight, 0.2% C, 2.0% Mn, 0.5% Si, 0.005% B, and 0.05% Ti were each placed in a coating bath having the composition shown in Table 1 below, and the temperature of the coating bath was maintained at 40°C higher than the melting point to perform coating. The coating amount was controlled by continuously removing excess coating layer from the test pieces immersed in the coating bath through an air knife.

[0079] The plating amount and the components of the plating layer were measured by removing the surface oxides from the plating layer with NaOH solution, dissolving the plating layer with hydrochloric acid, and then analyzing with an ICP (Inductively Coupled Plasma Spectrometer). The results are shown in Table 1.

[0080] Meanwhile, the Si crystals contained in the plating layer were analyzed using energy dispersive X-ray spectroscopy (EDS) at 3000 magnification using a scanning electron microscope (SEM).

[0081] Next, the plated test specimens were processed into 200mm x 120mm test specimens for hot press forming, and then placed in a box heating furnace set at an ambient temperature of 900°C and heated for 5 minutes. The atmosphere in the box heating furnace was air, and the ambient temperature was maintained uniformly by measuring with a thermocouple and adjusting the amount of electricity. The heated test specimens were quenched using a mold with a flow of cooling water to complete the martensitic transformation.

[0082] The delayed fracture properties were then evaluated. Specifically, the quenched test specimens were cut to a length of 180 mm and a width of 30 mm, bent to a displacement of approximately 80% of the yield strength, and immersed in a 0.2N hydrochloric acid solution for 120 hours. The specimens were then visually inspected for cracks, and the results are listed in Table 1 below. In Table 1, the presence of cracks was indicated by "○", and the absence of cracks was indicated by "×".

[0083] When cracks occurred, they occurred within the yield strength, resulting in delayed fracture.

[0084] [Table 1]

[0085] Referring to Table 1, different delayed fracture properties were derived when the composition of the coating layer was changed under the same heat treatment conditions. Figure 1 is a photograph of a cross section of the coating layer of Example 1 observed with a scanning electron microscope. Specifically, referring to Table 1 and Figure 1, in the case of Inventive Steels 1 to 4, in which the composition of the coating layer satisfied the alloy composition proposed by the present invention and cracks did not occur, the coating layer existed in the form of zinc being dissolved in aluminum, and silicon partially formed crystals and precipitated in the coating layer with a major axis length of 1 μm or more.

[0086] The researchers concluded that the presence of various crystals in the coating layer ensures various grain boundaries between the surface and the base metal, and that these grain boundaries provide paths through which hydrogen can diffuse during the heating process, improving hydrogen-induced delayed fracture characteristics. Furthermore, zinc-doped aluminum distorts the aluminum crystal lattice, helping hydrogen, which has a small atomic size, to move between the lattice, allowing it to diffuse to the surface and be released.

[0087] Comparative steel 1 did not contain Zn, and Zn could not be solid-dissolved in the aluminum plating layer. Comparative steels 2 to 4 did not contain Si at a content of 5.0%, and the formation of the Al-Fe alloy phase could not be suppressed, resulting in cracks during the delayed fracture test.

[0088] Next, 22MnB5 cold rolled steel sheets were plated to have the composition shown in Table 1 above, and heated under the conditions shown in Table 2 below during the hot press forming process to evaluate the surface quality and delayed fracture properties.

[0089] In Table 2 below, samples with a tensile strength of less than 85% after a typical hot pressing process were judged as unacceptable, with pass being indicated by ○ and fail being indicated by ×.

[0090] [Table 2]

[0091] To confirm the process conditions, the coating layers used in the experiments were plated steel sheets whose main components were aluminum and silicon, and plated steel sheets with aluminum, zinc, and silicon additives. The heating temperature refers to the ambient temperature of the furnace, and the heating time refers to the time elapsed after placing the test specimen in a furnace at a specified heating temperature, with the plated steel sheet at room temperature. Referring to Table 2, both types of plated steel sheets showed good delayed fracture resistance when the ambient temperature of the heating furnace was low and the heating time was short. This was due to the material not achieving the target value, rather than the influence of hydrogen in the steel, and the material not being able to maintain sufficient tensile strength.

[0092] In the example where silicon and zinc were introduced into the coating layer and the heating conditions were met during the hot press forming process, hydrogen in the steel sheet was released to the outside, ensuring not only strength but also delayed fracture properties.

[0093] In contrast, in Comparative Examples 7 to 9, the heating temperature was less than 850° C., and in Comparative Example 10, the heating time was 2 minutes, which was insufficient, and delayed fracture did not occur due to low tensile strength upon rapid cooling.

[0094] On the other hand, in Comparative Example 11, the heating time was long, and the crystal size continued to grow, causing delayed fracture.

[0095] Next, 22MnB5 cold rolled steel sheets were plated in the same manner as shown in Table 3 below, and a thermocouple was attached to a 200*100mm test piece for hot press forming, and the test piece was heated using an IR heater. The power of the IR heater was adjusted according to the temperature of the thermocouple, and the heating rate was controlled according to the conditions shown in Table 3 below.

[0096] Specifically, the temperature of the steel plate was measured using a thermocouple, and when the steel plate temperature reached 900°C, the temperature increase was stopped and maintained at 900°C. The heating time was measured based on the time when the temperature of the test piece began to increase. In Table 3 below, the average heating rate of the steel plate is shown based on the time it took to reach 900°C.

[0097] Next, the specimens were quenched by pressing them between water-cooled dies for forming, and the surface properties, plating surface quality, and delayed fracture properties were evaluated.

[0098] [Table 3]

[0099] Referring to Table 3, in Examples 2 to 4 where the heating rate was 3 to 10°C / s, hydrogen in the steel sheet was released to the outside, ensuring not only strength but also delayed fracture properties. In contrast, in Comparative Example 12 where the heating rate was low at 2°C / s, no cracks occurred during the delayed fracture test, but the austenite transformation temperature of the steel sheet was not reached even after heating for 5 minutes, so the tensile strength of the material could not be ensured.

[0100] In addition, in the case of Comparative Example 13 where the heating rate was 15° C. / s, the melted plating layer on the surface flowed, resulting in unevenness on the surface of the plating layer and uneven color.

[0101] According to the embodiments of the present disclosure, the hydrogen delayed fracture properties of the aluminum-plated steel sheet can be improved by optimizing the alloy components of the coating layer and the heating conditions during hot press forming.

[0102] While exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and it should be understood that a person having ordinary skill in the art can make various changes and modifications within the scope of the following claims. [Industrial Applicability]

[0103] According to one embodiment of the present invention, it is possible to provide a plated steel sheet for hot press forming having improved hydrogen delayed fracture properties and a method for manufacturing the same.

Claims

1. Base steel sheet; and a coating layer formed on the surface of the base steel sheet, the coating layer containing, by weight, Si: 5.0 to 15.0%, Zn: 10.0 to 30.0%, Fe: 4.0 to 12.0%, and the remainder being Al and unavoidable impurities; The plating layer contains silicon crystals having a silicon concentration of 60% or more, a zinc concentration of less than 20%, and a major axis length of 1 μm or more.

2. 2. The plated steel sheet for hot press forming according to claim 1, characterized in that the base steel sheet contains, by weight%, C: 0.1 to 0.4%, Mn: 0.5 to 3.0%, Si: 0.1 to 0.8%, B: 0.01% or less (excluding 0), Ti: 0.1% or less (excluding 0), and the remainder being Fe and other unavoidable impurities.

3. The coating weight of the coating layer adhered to the surface of the base steel sheet is 40 to 120 g / m ２ The plated steel sheet for hot press forming according to claim 1, characterized in that

4. The plated steel sheet for hot press forming according to claim 1, wherein the thickness of the plated layer is 5 to 40 μm.

5. formed between the base steel sheet and the coating layer, 2. The plated steel sheet for hot press forming according to claim 1, wherein an Al-Fe-based interface alloy layer containing, by weight %, 30.0 to 60.0% Al and 5.0 to 20.0% Si is formed.

Images