Hot-formed member and manufacturing method thereof

Abstract

The present invention relates to a hot-formed member used for automobile parts, etc., and relates to a hot-formed member produced by hot forming an aluminum-based plated steel material, and a method for producing the same.

Description

[Technical Field]

[0001] The present invention relates to a hot-formed member used for automobile parts and the like, and relates to a hot-formed member produced by hot forming an aluminum-based plated steel material, and a method for producing the same. [Background technology]

[0002] Recently, the use of high-strength steel has been increasing to reduce the weight of automobiles, but high-strength steel has problems such as material breakage during processing and spring back after processing, making it difficult to form products with complex and precise shapes.To solve these problems, methods called hot forming and hot press forming (HPF) have recently become popular.

[0003] The hot forming method has the advantage that the steel material is easily formed by heating it to a temperature of typically 800 to 900°C and then processing (pressing) it in a heated state, and the strength of the formed product can be increased by rapidly cooling it through a mold. However, when the steel material is heated to a high temperature, surface oxidation of the steel material is unavoidable. Therefore, an additional process of removing the oxides on the steel material surface after press forming is required, which increases costs. To prevent this, the raw material used for hot forming is one that has an aluminum plating layer on the surface of the steel material (Patent Documents 1 and 2), a zinc plating layer (Patent Document 3), or a plated steel material that combines these two layers (Patent Document 4).

[0004] However, during the heating process for hot forming of the plated steel, the low-melting-point coating layer transforms into a liquid phase. When this liquid phase comes into contact with the atmosphere, the moisture (H2O) in the air dissociates into oxygen (O2) and hydrogen (H). The oxygen remains as an oxide on the surface, and the hydrogen accumulates as a solid solution in the coating layer. When the coating layer is rapidly cooled after forming in this state, a large amount of hydrogen remains in the solid phase. However, because the solid phase coating layer has a lower solubility of hydrogen than the liquid phase, hydrogen diffuses from the coating layer to the surrounding area, and some of this hydrogen enters the steel. The hydrogen that has entered the steel accumulates in the joints, causing hydrogen-delayed fracture and resulting in hydrogen embrittlement.

[0005] Therefore, there is a need for a solution to the hydrogen embrittlement problem. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] U.S. Patent No. 6,296,805 [Patent Document 2] Patent No. 3845271 [Patent Document 3] Japanese Patent Application Laid-Open No. 2003-147499 [Patent Document 4] Japanese Patent Application Laid-Open No. 2006-051543 Summary of the Invention [Problem to be solved by the invention]

[0007] One aspect of the present invention is to provide a hot-formed member having excellent resistance to hydrogen embrittlement and a method for producing the same.

[0008] The object of the present invention is not limited to the above-mentioned matters. Further object of the present invention is described in the entire contents of the specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the further object of the present invention from the contents described in the specification of the present invention. [Means for solving the problem]

[0009] One aspect of the present invention includes a base iron and an aluminum-based alloy layer formed on the base iron, The aluminum-based alloy layer includes a diffusion layer a, a first alloy layer b, a second alloy layer c, and a third alloy layer d, which are successively formed on the base steel based on a cross section, The thickness t1 of the first alloy layer b is 8.5 μm or less, The thickness t1 is for a hot-formed part and is the average of measurements taken at 45 or more points at equal intervals of 10 μm in the horizontal direction on three images of the cross section taken with an optical microscope at 500x magnification.

[0010] The first alloy layer b may contain 37 to 52 weight % Al, 38 to 51 weight % Fe, 1.5 to 5 weight % Si, and 2 to 20 weight % Zn, the second alloy layer c may contain 19 to 47 weight % Al, 45 to 69 weight % Fe, and 7.2 to 15 weight % Si, and the third alloy layer d may contain 35 to 52 weight % Al, 38 to 51 weight % Fe, 1.5 to 5.5 weight % Si, and 5 to 26 weight % Zn.

[0011] The total thickness of the first alloy layer b, the second alloy layer c, and the third alloy layer d may be 30 μm or less.

[0012] The thickness t1 of the first alloy layer b may be smaller than the thickness t2 of the third alloy layer.

[0013] The second alloy layer c may include a discontinuous region in the longitudinal direction of the cross section.

[0014] The Zn content of the first alloy layer b may be less than the Zn content of the third alloy layer d.

[0015] The Si content of the second alloy layer c may be at least twice the Si content of the first alloy layer b or the third alloy layer d.

[0016] The number of voids having a major axis of 500 nm or more contained in the first alloy layer (b) may be less than the number of voids having a major axis of 500 nm or more contained in the third alloy layer (d).

[0017] The major axis of the voids having a major axis of 500 nm or more contained in the first alloy layer (b) may be smaller than the major axis of the voids having a major axis of 500 nm or more contained in the third alloy layer (d).

[0018] It may contain Mg oxide formed on the third alloy layer d.

[0019] The upper portion of the diffusion layer a may further include a Si-enriched layer in which the Si composition is enriched 1.5 times or more the average of the diffusion layer a.

[0020] The amount of diffusible hydrogen in the base iron may be 0.1 ppm by weight or less.

[0021] The above base iron contains, by weight, C: 0.02 to 0.6%, Si: 0.001 to 2%, Al: 0.001 to 1%, Mn: 0.1 to 4%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, Ti: 0 to 0.1%, B: 0.0001 to 0.01%, Cu: 0 to 1.00%, Mo: 0 to 1.00%, Cr: 0 to 1.00%. 0%, Ni: 0-1.00%, V: 0-1.00%, Ca: 0-0.01%, Nb: 0-0.1%, Sn: 0-1%, W: 0-1%, Sb: 0-1%, Mg: 0-0.1%, Co: 0-1%, As: 0-1%, Zr: 0-1%, Bi: 0-1%, REM: 0-0.3%, and the remainder can include Fe and unavoidable impurities.

[0022] Another aspect of the present invention is a method for manufacturing a steel sheet comprising the steps of: providing an aluminum-based plated steel sheet including a base steel sheet and an aluminum-based plating layer formed on a surface of the base steel sheet; producing a blank using the aluminum-plated steel sheet and heating the blank; and forming and cooling the heated blank; The step of providing the aluminum-based plated steel sheet comprises: forming the aluminum-based coating layer by immersing the base steel sheet in a coating bath containing 10 to 35 wt % of Zn, 15 wt % or less of Si, and the remainder being Al and unavoidable impurities; cooling the aluminum-based plating layer to its solidification point at an average rate of 20°C / s or more; and The present invention relates to a method for producing a hot-formed part, which comprises a step of cooling from the solidification point to 350°C at an average rate of less than 20°C / s.

[0023] The plating bath may contain up to 4% by weight of Fe.

[0024] The plating bath may contain one or more of Mg, Mn, Cr and Ca in an amount of 4.5 wt % or less.

[0025] After the immersion and before cooling, the K value defined by the following [Equation 1] can be 4 to 39. [Formula 1]K=a*(c*d / b) Here, a is the Zn content of the plating bath (wt.%), b is the line speed (mpm), c is the air knife (A / K) spacing (mm), and d is the A / K pressure (kPa).

[0026] In the step of heating the blank, the blank may be heated at a heating rate of 5 to 12°C / s in a first heating section where the blank temperature is less than 600°C, and at a heating rate of 1.2 to 3.5°C / s in a second heating section where the blank temperature is 600°C or higher.

[0027] The duration of the first temperature increase section may be shorter than the duration of the second temperature increase section. [Effects of the Invention]

[0028] According to the present invention, the inhibition of hydrogen diffusion into the plating layer during hot forming is minimized, and hydrogen infiltration into the steel is suppressed, thereby improving resistance to hydrogen embrittlement. When applied to automobile parts, this can improve resistance to crash properties.

[0029] The various yet significant advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0030] [Figure 1] 1 is a cross-sectional photograph of Comparative Example 7 of the test pieces of the hot-formed members produced in the examples of the present invention. [Figure 2] 1 is a cross-sectional photograph of Inventive Example 2 of the test pieces of the hot-formed members produced in the examples of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0031] The terminology used herein is for the purpose of describing the invention and is not intended to limit the invention. Furthermore, as used herein, the singular forms "a," "an," and "the" also include the plural forms unless the relevant definition clearly indicates otherwise.

[0032] The meaning of "comprises" as used in the specification is to specify features and does not exclude the presence or addition of other features.

[0033] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content.

[0034] Hydrogen adsorbed in the steel of hot-formed components can cause serious problems with material properties such as strength, ductility, stiffness, and bendability. Hot-formed components are composed mostly of the martensite phase, which is highly susceptible to hydrogen-induced embrittlement. As higher strength steels are used to reduce the weight of automobiles, issues such as hydrogen-induced delayed fracture and hydrogen-induced ductility loss become more serious as the strength of hot-formed components increases.

[0035] The present inventors discovered that when plated steel is hot-formed, the state of hydrogen adsorption changes due to a physical phase change in the plated layer that occurs during the heat treatment process, and arrived at the present invention.

[0036] Specifically, when manufacturing hot-formed components, plated steel is placed in a heating furnace at room temperature and heated for a certain period of time until it undergoes austenite transformation. Over time, the state of the plated layer changes from solid to liquid, then transforms again into an intermetallic compound due to Fe diffusion from the base material, and finally transforms back into a solid phase. In the section where the plated layer is initially in the solid phase after heating, if the plated layer is an aluminum-based plated layer, the diffusion rate of hydrogen is very slow, so it acts as a physical barrier that prevents external hydrogen from entering the steel.

[0037] As the hot-forming heat treatment progresses, the temperature of the coating layer rises above its melting point, causing the coating layer to transform into a liquid phase. The hydrogen solubility of the molten coating layer, although varying depending on the coating layer's composition, is much higher than that of the solid phase. Eventually, the Fe in the base material and the coating layer form an intermetallic compound, transforming the coating layer into a solid phase. Because hydrogen solubility in the solid phase is low, excess hydrogen dissolved in the liquid phase diffuses into the base material of the hot-formed component to maintain hydrogen equilibrium in the solid phase. It was recognized that the hydrogen diffusion is affected by the coating layer's structure, composition, etc. This led to the present invention.

[0038] First, a hot-formed member will be described in detail as one embodiment of the present invention. Referring to Fig. 2, which is a cross-sectional photograph of Example 2 among the examples described later, the hot-formed member includes a base steel and an aluminum-based alloy layer formed on the base steel. The aluminum-based alloy layer may include a diffusion layer a, a first alloy layer b, a second alloy layer c, and a third alloy layer d formed in this order on the cross-section of the base steel, and the thickness t1 of the first alloy layer b may be 8.5 μm or less. In the present invention, the thickness of the aluminum-based alloy layer may be an average of values ​​measured at 45 or more points at equal intervals of 10 μm in the horizontal direction on three images of the cross-section of the hot-formed member taken with an optical microscope at 500x magnification.

[0039] The base steel refers to the base material of a part obtained by hot forming a steel material for hot forming, and indicates the majority of the thickness in the cross section of the hot-formed part. The base steel is the base material of a hot-formed part obtained after hot forming a base steel plate, which is the base material of the steel material for hot forming. In this case, the base steel can usually have a mainly martensite structure through hot forming and cooling.

[0040] The compositions of the base iron and base steel sheet are not particularly limited, and any composition usable as a hot-formed member is sufficient. In a preferred example, the base iron contains, by weight, 0.02 to 0.6% C, 0.001 to 2% Si, 0.001 to 1% Al, 0.1 to 4% Mn, 0.05% or less P, 0.02% or less S, 0.02% or less N, 0 to 0.1% Ti, 0.0001 to 0.01% B, 0 to 1.00% Cu, 0 to 1.00% Mo, and 0 to 1.00% Cr. 0-1.00%, Ni: 0-1.00%, V: 0-1.00%, Ca: 0-0.01%, Nb: 0-0.1%, Sn: 0-1%, W: 0-1%, Sb: 0-1%, Mg: 0-0.1%, Co: 0-1%, As: 0-1%, Zr: 0-1%, Bi: 0-1%, REM: 0-0.3%, and the remainder can include Fe and unavoidable impurities.

[0041] The aluminum-based alloy layer refers to an alloyed plating layer formed by alloying the aluminum-based plating layer of an aluminum-based plated steel material for hot forming with the base material during the hot forming process. The aluminum-based plating layer refers to a layer containing aluminum (Al) as a main component and a portion (up to 15 wt %) of silicon (Si) to prevent excessive alloying of the aluminum with Fe in the steel material during plating. Other components may also be included as necessary.

[0042] In the aluminum-based alloy layer, the diffusion layer a may be a ferrite layer containing a portion of Al formed by combining Fe of the base steel sheet with a portion of Al, which is the main component of the aluminum-based plating layer. In this case, the Al content may be 3 to 9 wt %, with the remainder being Fe and unavoidable impurities.

[0043] On the other hand, the upper part of the diffusion layer a may include a concentrated layer in which Si is partially concentrated. The Si content of the concentrated layer may be 1.5 times or more the average of the diffusion layer a.

[0044] The first alloy layer (b) and the third alloy layer (d) are formed as a solid phase during the heat treatment process for the hot-formed component, when Fe diffused from the base material and the aluminum plating layer formed an intermetallic compound. The first alloy layer (b) and the third Al-Fe alloy layer (d) are intermetallic compounds of Al and Fe, and the alloy phase formed relatively quickly during the heat treatment process can form many voids between the atomic arrangements. These voids act as pathways for hydrogen diffusion, providing a path for supersaturated hydrogen in the plating layer to diffuse into the base material. However, in alloy phases containing Si, such as the second alloy layer (c), Si atoms preferentially occupy vacancies, filling the vacancies that serve as hydrogen diffusion paths.

[0045] On the other hand, the first alloy layer (b) may preferably have a composition of 37-52 wt% Al, 38-51 wt% Fe, 1.5-5 wt% Si, and 2-20 wt% Zn, the third alloy layer (d) may preferably have a composition of 35-52 wt% Al, 38-51 wt% Fe, 1.5-5.5 wt% Si, and 5-26 wt% Zn, the first alloy layer (b) may more preferably have a composition of 37.0-52.0 wt% Al, 38.0-51.0 wt% Fe, 1.50-5.0 wt% Si, and 2.0-20.0 wt%, and the third alloy layer (d) may more preferably have a composition of 35.0-52.0 wt% Al, 38.0-51.0 wt% Fe, 1.50-5.50 wt% Si, and 5.0-26.0 wt% Zn.

[0046] As described above, the second alloy layer c is located between the first alloy layer b and the third alloy layer d, and contains a large amount of Si to prevent hydrogen from diffusing into the base material. To this end, the Si content in the second alloy layer c may be at least twice that of the first alloy layer b and the third alloy layer d. A preferred composition of the second alloy layer c may include 19 to 47 wt% Al, 45 to 69 wt% Fe, and 7.2 to 15 wt% Si. A more preferred composition of the second alloy layer c may include 19.0 to 47.0 wt% Al, 45.0 to 69.0 wt% Fe, and 7.20 to 15.0 wt% Si.

[0047] From this perspective, the closer the second alloy layer c is to the diffusion layer a, the lower the probability that hydrogen contained in the plating layer will diffuse into the base material, which is advantageous for reducing hydrogen embrittlement. Therefore, since reducing the distance between the diffusion layer a and the second alloy layer c is advantageous for reducing hydrogen embrittlement, in the present invention, the thickness t1 of the first alloy layer b can be 8.5 μm or less.

[0048] On the other hand, the second alloy layer c may include a region (discontinuous region) that is not continuous in the longitudinal direction on the cross section of the aluminum-based alloy layer. However, if a sufficient three-dimensional surface is formed, the effect of reducing hydrogen embrittlement can be sufficiently exhibited from a macroscopic perspective. If the second alloy layer c is excessively discontinuous, its role as a hydrogen diffusion barrier film will be reduced. However, when observed with an optical microscope at a magnification of 200x or less, a continuous band shape of 5 μm or more in the horizontal direction may be present, and preferably a continuous region of 10 μm or more may be present. It is preferable that the total length cut on the cross section be kept to 40% or less in the horizontal direction.

[0049] On the other hand, the absolute amount of hydrogen that flows into the aluminum-based plating layer during the manufacturing process of the hot-formed member is also important. Therefore, if the aluminum-based plating layer for manufacturing the hot-formed member contains elements such as zinc and magnesium, this has the effect of reducing the amount of hydrogen that flows into the plating layer during the manufacturing process of the hot-formed member.

[0050] Aluminum-based plating layers containing zinc or magnesium have low hydrogen solubility in the molten state, so the absolute amount of hydrogen that dissociates from the surface and enters the plating layer is small. Atmospheric moisture preferentially forms zinc or magnesium oxide on the surface, preventing the surface adsorption of moisture to the plating layer and suppressing hydrogen injection.

[0051] However, if the zinc content in the coating layer is high, it can cause liquid metal embrittlement (LME), which can lead to cracks in the base steel. Furthermore, if the magnesium content is high, excessive magnesium oxides can form, which can cause the coating layer to adhere to the mold and fall off. This process can result in defects in hot-formed parts. Furthermore, magnesium can cause spatter during welding, which can degrade weldability.

[0052] As the Fe alloying of the base steel continues in a molten state, the zinc content of the coating layer increases overall, and the proportion of zinc begins to decrease. The zinc content of the hot-formed member may be 5 to 35 wt %. The zinc is mainly distributed in the first alloy layer (b) and the third alloy layer (d). According to an embodiment, the Zn content of the first alloy layer (b) is lower on average than the Zn content of the third alloy layer (d).

[0053] Magnesium is highly oxidizable and tends to migrate to the surface during heat treatment. It is present in a relatively larger amount in the third alloy layer (d) than in the first alloy layer (b), with the remainder present as Mg oxide in the surface layer. The magnesium content must be controlled to 5 wt. % or less in the molded member to ensure weldability and prevent seizure. According to an embodiment, the magnesium content can be controlled to an average of 3 wt. % or less when observed from the center of the third alloy layer (d) using a scanning electron microscope.

[0054] According to an embodiment, the thickness of the aluminum-based alloy layer of the hot-formed part, excluding the diffusion layer a, is preferably 30 μm or less. The thickness of the diffusion layer can be varied depending on the heat treatment time for manufacturing the hot-formed part. If the remaining layer after heat treatment excluding the diffusion layer is thick, the effect of naturally releasing hydrogen after manufacturing the hot-formed part may be reduced. As the thickness of the alloy layer increases, the absolute amount of hydrogen contained in the alloy layer increases, which may result in poor hydrogen embrittlement resistance.

[0055] On the other hand, according to the embodiment, the thickness t1 of the first alloy layer b may be smaller than the thickness t2 of the third alloy layer d.

[0056] Unlike the first alloy layer (b), which is close to the base metal, the third alloy layer (d) is relatively far from the base metal, which may result in locally uneven amounts of diffused Fe. The liquid coating layer shrinks as it reacts with Fe and transforms into a solid phase. This uneven Fe diffusion causes irregular shrinkage of the solid phase, which can lead to the formation of voids in the alloy phase. These voids generally act as hydrogen traps, but if there are many of them, the hydrogen in the coating layer will collect in the voids, reducing its ability to diffuse into the base metal.

[0057] While the second alloy layer c acts as a hydrogen diffusion prevention layer, the voids present in the third alloy layer d have the advantage of positioning the amount of hydrogen in the plating layer far away from the base material, thereby reducing the amount of hydrogen in the base material.

[0058] According to an embodiment, the number of voids with a major axis of 500 nm or more contained in the first alloy layer b may be smaller than the number of voids with a major axis of 500 nm or more contained in the third alloy layer d. Meanwhile, the major axis of the voids with a major axis of 500 nm or more contained in the first alloy layer b may be smaller than the major axis of the voids with a major axis of 500 nm or more contained in the third alloy layer d.

[0059] The amount of diffusible hydrogen contained in the base steel of the hot-formed part is highest immediately after the initial formation of the part, and decreases over time as the hydrogen is naturally released to the outside through the front cross section or plating layer of the part. To prevent hydrogen embrittlement, a phenomenon in which a formed part spontaneously breaks due to internal stress before an external impact is applied after the formed part is fabricated, the diffusible hydrogen content is preferably 0.1 wt ppm or less when measured within one week after the formed part is fabricated. This measurement is performed on test specimens of hot-formed parts fabricated in a heating furnace without a separate dew point control device.

[0060] Next, a method for producing a hot-formed member according to another embodiment of the present invention will be described in detail.

[0061] The manufacturing method includes the steps of providing an aluminum-based plated steel sheet including a base steel sheet and an aluminum-based plating layer formed on a surface of the base steel sheet; manufacturing a blank using the aluminum-plated steel sheet and heating the blank; and forming and cooling the heated aluminum-plated steel sheet.

[0062] The present invention does not particularly limit the above-mentioned base steel sheet, and there are no particular limitations on the type, composition, etc., as long as it can be used in the technical field to which the present invention pertains.

[0063] The method for forming the aluminum-based plating layer is not particularly limited and may be hot-dip plating, electroplating, or the like.

[0064] For example, a plated steel sheet can be manufactured by annealing a cold-rolled steel sheet for hot-formed members, immersing it in a hot-dip galvanizing bath, adjusting the coating amount, and cooling it. Specifically, the aluminum-based coating layer is formed by immersing the steel sheet in a coating bath containing Al as the main component, and the coating bath contains 10 to 35 wt% of Zn, 15 wt% or less of Si, and the remainder being Al and unavoidable impurities.

[0065] Preferably, it can contain 15 to 30 wt% Zn, 12 wt% or less Si, and the remainder Al and unavoidable impurities, and more preferably, it can contain 20 to 28 wt% Zn, 9 wt% or less Si, and the remainder Al and unavoidable impurities.

[0066] On the other hand, it may further contain 4 wt % or less of Fe. Also, in order to improve corrosion resistance, it may contain 4.5 wt % or less of one or more of Mg, Mn, Cr and Ca.

[0067] Among the components of the coating bath, Si prevents the formation of an Al-Fe alloy layer due to bonding between aluminum and the Fe in the base steel sheet, and also prevents corrosion of the coating equipment immersed in the coating bath. However, if the content is excessive, exceeding 15 wt%, the growth of the Al-Fe alloy layer may be excessively suppressed, which may cause weld liquor embrittlement.

[0068] The Zn content can be 10 wt% or more to prevent Si from diffusing into the surface layer due to solid phase growth during heat treatment to produce a hot-formed part, thereby controlling the position of the second alloy layer, and to ensure the corrosion resistance of the hot-formed part. However, if the Zn content exceeds 35 wt%, the melting point of the coating layer decreases, the fluidity increases, and liquid embrittlement is likely to occur. On the other hand, the Zn content can be preferably 15 to 30 wt%.

[0069] On the other hand, Fe in the coating bath accumulates as it dissolves from the base steel sheet over time during the continuous coating process, and must be controlled to 4 wt % or less to prevent dross defects.

[0070] The plating amount can be adjusted using an air knife (A / K), and the gas used is basically air, but if defects occur due to surface oxidation of the plating layer, nitrogen or a gas containing nitrogen can be used. The plating amount is 10 g / m on one side. 2 More than 90g / m 2 If the plating amount is too small, problems with corrosion resistance may occur and surface quality may deteriorate, whereas if the plating amount is too large, hydrogen absorbed in the plating layer during hot forming may be inhibited from being released to the outside, resulting in burning onto the mold and reducing productivity, which may cause component defects.

[0071] To control the coating weight, the air knife gas pressure and the distance between the workpiece and the air knife must be adjusted. Coating weight control involves the removal of excess coating layer through the shear stress of the gas emitted from the air knife. Even if the same shear stress is applied to the coating layer, the coating weight, surface condition, and coating layer structure can vary depending on the line speed (mpm), which is the speed at which the steel sheet passes through the air knife, and the viscosity of the coating bath. For example, the chemical composition of the coating bath can include aluminum, zinc, and silicon. In the case of magnesium, these elements affect the viscosity of the liquid coating bath. The Fe and aluminum components of the workpiece react more quickly than the other components in the coating bath, forming Fe-Al alloy phases less than 10 μm in size. These alloy phases have many irregularities on the surface, and the liquid coating layer that adheres to these phases cannot be uniformly removed by the air knife, resulting in poor surface morphology. Such uneven plating weight deviation can hinder the formation of a continuous phase on the second alloy layer c, which is a diffusion barrier layer, during heat treatment for the heat-treated molded part, so K, defined by the following [Equation 1], can be in the range of 4 to 39 after the above immersion and before cooling. [Formula 1]K=a*(c*d / b) Here, a is the Zn content of the plating bath (wt.%), b is the line speed (mpm), c is the air knife (A / K) spacing (mm), and d is the A / K pressure (kPa).

[0072] When cooling after adjusting the coating amount, the material can be cooled to the solidification point at an average rate of 20°C / s or more, and then cooled from the solidification point to 350°C at an average rate of less than 20°C / s.

[0073] Immediately after passing through the air knife, the coating layer remains in a liquid state above its melting point, and the strip temperature remains similar to that of the coating bath, allowing an Al-Fe alloy layer to continue growing between the coating layer and the base steel sheet. Excessive growth of the Al-Fe alloy layer increases the brittleness of the coating layer, potentially leading to coating peeling during coiling or blank shearing. To prevent this, cooling is preferably performed at a rate of 20°C / s or greater before the freezing point is reached. After the freezing point, cooling is preferably performed at a rate of less than 20°C / s to suppress strip vibration and warping caused by air generated in the cooling tower and thereby reduce coating weight deviations.

[0074] The aluminum-based plated steel sheet provided as above can be manufactured by processing it into a blank form having a shape close to the shape of a part for forming.

[0075] The blank is heated for hot forming. The heating can be performed at a temperature of Ac3°C or higher and 970°C or lower, preferably 850°C to 950°C. As an example, the aluminum-based plated steel sheet can be placed in a heat treatment furnace set at the above temperature and heated for 180 to 600 seconds so that the steel structure is completely transformed into austenite. If the temperature is too low, it is difficult for the steel material to completely transform into austenite single phase. If the temperature is too high, the surface may oxidize, the hydrogen content in the steel may increase, and in the case of a coating layer, evaporation may be so severe in the liquid phase that the coating layer may not remain sufficiently, resulting in a deterioration of corrosion resistance.

[0076] For example, during the blank heating, the heating rate may be 5 to 12°C / s in the first heating section where the blank temperature is less than 600°C, and the heating rate may be 1.2 to 3.5°C / s in the second heating section where the blank temperature is 600°C or higher. In this case, the duration of the first heating section may be shorter than the duration of the second heating section.

[0077] On the other hand, the atmosphere in the heating furnace is usually air, but to actively reduce hydrogen embrittlement of the components, the moisture concentration in the air can be controlled. The moisture concentration is measured via the dew point, and the heating can be carried out in an atmosphere with a dew point of -25°C to 15°C.

[0078] The heating method is not particularly limited, and may be a radiant tube, a radiation heating method, induction heating, current heating, etc. The heating rate during heating may be adjusted depending on the sheet thickness and the type of coating layer.

[0079] The aluminum-based plated steel sheet that has been transformed into austenite single phase by heating is transferred from the heating furnace and placed in a mold, where it is cooled while being processed. The transfer time from the heating furnace to the mold is preferably within 15 seconds; if it is longer than this, ferrite may form during natural cooling of the material before being cooled in the mold, which may result in deterioration of the material.

[0080] The critical cooling rate for obtaining martensitic transformation from a mold may differ depending on the steel composition, but considering that cooling generally occurs from the time the material is transferred from the heating furnace to the mold, it is effective to start from the initial temperature of the material placed in the mold, which is 700°C, and then cool it down to 350°C at an average cooling rate of 20°C / s to 50°C / s. [Example]

[0081] Examples of the present invention will now be described. It goes without saying that various modifications of the following examples are possible within the scope of the present invention, as understood by those skilled in the art. The following examples are provided for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following examples, but should be defined by the claims below as well as equivalents thereof.

[0082] (Example) A cold-rolled steel sheet having the composition shown in Table 1 below (the remainder being Fe and unavoidable impurities) was prepared, and for plating purposes, it was heated from room temperature to 780°C in an annealing furnace in a nitrogen atmosphere containing 5% hydrogen, and after holding at the annealing temperature of 780°C for 80 seconds, the base steel sheet was cooled to a temperature 10°C higher than the temperature of the hot-dip plating bath, and then immersed in a plating bath to perform aluminum-based alloy plating.

[0083] The plating bath composition (wt %) is as follows: Plating bath 1: Si 9%, Fe 2%, the remainder being Al and unavoidable impurities Plating bath 2: Si 8%, Zn 27%, Fe 1.5%, the remainder being Al and unavoidable impurities Plating bath 3: Si 6%, Zn 25%, Mg 1%, the remainder being Al and unavoidable impurities Plating bath 4: Si 6%, Zn 25%, Mg 5%, the remainder being Al and unavoidable impurities Plating bath 5: Si 6%, Zn 38%, Mg 5%, the remainder being Al and unavoidable impurities

[0084] Plating bath 1 was used to prepare Comparative Examples 1 to 7, Plating bath 2 was used to prepare Invention Examples 1 to 4, Plating bath 3 was used to prepare Invention Example 5, Plating bath 4 was used to prepare Comparative Example 8, and Plating bath 5 was used to prepare Comparative Example 9.

[0085] After the coating, the coating weight was adjusted using an air knife. The coating weight at this time is shown in Table 2 below. After adjusting the coating weight, the steel sheet was cooled to the solidification point at a cooling rate of 20°C / s, and from the solidification point to 350°C at a cooling rate of 10°C / s, and then naturally cooled to produce a coated steel sheet.

[0086] [Table 1]

[0087] The plated steel sheet was sheared into a blank of 400 x 380 mm size, and the blank was placed in a box furnace in an air atmosphere at an atmospheric temperature of 900°C, where it was subjected to heat treatment.

[0088] The heat treatment was broadly divided into two temperature sections, with the heating rate adjusted accordingly. The first heating section began at room temperature and continued to below 600°C, just before the coating surface liquefied. During this period, the blank was heated at an average heating rate of 6–12°C / s. The second heating section, during which the liquefied coating re-alloyed with the Fe in the steel substrate and solidified, was maintained at a heating rate of 1.2–3.5°C / s from 600°C to 900°C. After reaching the target heat treatment temperature, the temperature deviation across the blank was kept within 10°C. The duration of the first heating section was kept shorter than that of the second heating section to minimize the destruction of the layered structure due to rapid alloying.

[0089] The total heat treatment time was measured from the time the sample was placed in the box furnace at room temperature until it was finally removed from the box furnace. Hot press forming was performed using a water-cooled mold at a cooling rate of 20°C / s to obtain a hot-formed part.

[0090] The cross section of each hot-formed test piece was then observed using a scanning electron microscope (SEM) to analyze the structure of the alloy layer. Furthermore, the die adhesion, base iron cracks, spot weldability, and diffusible hydrogen content of each test piece were measured, and the results are shown in Table 2 below.

[0091] Specifically, the cross-sections of each phase were identified at 500x magnification, and the composition was analyzed using a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) at 2000x magnification, with the center of each phase measured three times at different locations, and the average of the three values ​​was used. The thicknesses t1 and t2 of the first alloy layer b and the third alloy layer d were calculated as the average of measurements taken at 45 or more locations horizontally at equal intervals of 10 μm on three images taken with an optical microscope at 500x magnification. In this case, where the c layer was discontinuous, a virtual line segment was drawn connecting the discontinuous portion to the next c layer, and the distance to the a layer was measured using this line segment as the reference.

[0092] To evaluate mold adhesion, after a part was made, the mold surface was blown with air and inspected visually. If any irregularities were found that felt like foreign matter was in the surface, the foreign matter was removed and a new part was made using the same material. If the plating layer that had fallen off the part again adhered to the mold, the mold adhesion was judged to be poor.

[0093] To observe cracks in the base steel, hot-formed components were fabricated using an omega-shaped die. The cracks in the base steel were divided into LME and microcracks. LME is a type of liquid brittle fracture caused by molten metal in the liquid state. It is observed hundreds of micrometers deep in the base steel and occurs in large numbers in areas where tension is greatest at temperatures above the melting point of the coating layer. Microcracks are caused by weakening of the grain boundaries of the base steel due to solid-state diffusion in the coating layer. They occur in areas where friction between the die and the material is greatest, and are tens of micrometers deep. To observe these two cracks, formed components were fabricated using a 10 cm high, centrally protruding die, and the cross section was sheared using a laser. Test specimens were fabricated at the same location on the component for each experiment, and the cross section of the component was observed at 200x magnification using an optical microscope. At least three locations were observed in each area, and a component was deemed defective if cracks in the base steel were observed in one or more locations.

[0094] The amount of diffusible hydrogen was evaluated by quantifying the amount of hydrogen released while heating test pieces from room temperature to 400°C using Bruker's G8 GALILEO product. Because hydrogen contained in components naturally releases to the outside over time, measurements were performed on test pieces that had been manufactured the same amount of time ago. The greater the amount of diffusible hydrogen contained in the steel, the more hydrogen concentrates around potentials and defects within the steel, potentially causing sudden fracture of the material due to internal stress or external force. Component fractures have been observed in general HPF materials with diffusible hydrogen levels of 0.1 ppm or higher, and test pieces with hydrogen levels of 0.1 ppm or higher were deemed to be at risk of hydrogen embrittlement.

[0095] The weldability evaluation method used ISO 18278. A 6mm electrode, 4kN pressure, and 320ms welding time were used. The current required to reach the specified button size was measured, and then the current was increased by 0.5kA or more and welding was performed five times. If sputtering was observed two or more times, the product was rated as defective. All sputtering was evaluated, including sputtering that occurred on the surface of the component and between the welded test pieces after they were separated. Regardless of the current range, a product was also rated as defective if the minimum button diameter specified in the standard could not be achieved.

[0096] [Table 2]

[0097] [Table 3]

[0098] FIG. 1 is a cross-sectional view of Comparative Example 7, and FIG. 2 is a photograph of the cross-section of Invention Example 2.

[0099] As can be seen from the results in Tables 2 and 3, the examples satisfying the conditions of the present invention ensure excellent spot weldability and hydrogen embrittlement resistance, while the comparative examples show poor hydrogen embrittlement resistance and, in some cases, poor spot weldability. On the other hand, Figure 2 shows that in example 2, t2 is greater than t1.

[0100] In the cases of Comparative Examples 1 to 7, the thickness t1 of the first alloy layer b in the aluminum-based alloy layer of the manufactured hot-formed member was excessive, making it difficult to ensure hydrogen embrittlement resistance. In particular, it can be seen from Figure 1 that t1 is larger than t2.

[0101] On the other hand, in Comparative Example 8, the excessive Mg content resulted in magnesium in the coating layer being present in greater amounts in the form of magnesium oxide on the surface of the coating layer than in the coating layer during the heat treatment process for producing the molded part. The adhesive magnesium oxide strongly adheres to the mold, causing a sticking phenomenon. This requires mold cleaning after each production run, reducing productivity. Furthermore, the oxides result in poor weldability.

[0102] In Comparative Example 9, the Zn and Mg contents were excessive during plating, and the hot-formed parts produced suffered from severe liquation embrittlement fracture and cracks in the base iron, such as microcracks, and the weldability was also deteriorated due to various oxides.

Claims

1. The steel sheet includes a base steel and an aluminum-based alloy layer formed on the base steel, The aluminum-based alloy layer includes a diffusion layer a, a first alloy layer b, a second alloy layer c, and a third alloy layer d, which are formed in sequence from the base steel on a cross-section basis, the thickness t1 of the first alloy layer b is 8.5 μm or less, The thickness t1 of the hot-formed member is an average of measurements taken at 45 or more points at equal intervals of 10 μm in the horizontal direction from three images of the cross section taken with an optical microscope at 500x magnification.

2. the first alloy layer b contains 37 to 52 wt % Al, 38 to 51 wt % Fe, 1.5 to 5 wt % Si, and 2 to 20 wt % Zn, the second alloy layer c contains 19 to 47 wt % Al, 45 to 69 wt % Fe, and 7.2 to 15 wt % Si, 2. The hot-formed member according to claim 1, wherein the third alloy layer d contains 35 to 52 wt% Al, 38 to 51 wt% Fe, 1.5 to 5.5 wt% Si, and 5 to 26 wt% Zn.

3. 2. The hot-formed member according to claim 1, wherein the total thickness of the first alloy layer (b), the second alloy layer (c), and the third alloy layer (d) is 30 μm or less.

4. The hot-formed member according to claim 1 , wherein the thickness t1 of the first alloy layer b is smaller than the thickness t2 of the third alloy layer b.

5. The hot-formed member according to claim 1 , wherein the second alloy layer c includes discontinuous regions in the longitudinal direction of the cross section.

6. The hot-formed member according to claim 1 , wherein the second alloy layer c includes a continuous region of 5 μm or more in the longitudinal direction of the cross section.

7. 2. The hot-formed member according to claim 1, wherein the Zn content of the first alloy layer (b) is less than the Zn content of the third alloy layer (d).

8. 2. The hot-formed member according to claim 1, wherein the Si content of the second alloy layer (c) is at least twice the Si content of the first alloy layer (b) or the third alloy layer (d).

9. 2. The hot-formed member according to claim 1, wherein the number of voids having a major axis of 500 nm or more contained in the first alloy layer (b) is less than the number of voids having a major axis of 500 nm or more contained in the third alloy layer (d).

10. 2. The hot-formed member according to claim 1, wherein the major axis of voids having a major axis of 500 nm or more contained in the first alloy layer (b) is smaller than the major axis of voids having a major axis of 500 nm or more contained in the third alloy layer (d).

11. The hot-formed part according to claim 1 , further comprising Mg oxide formed on said third alloy layer d.

12. 2. The hot-formed member according to claim 1, further comprising a Si-enriched layer in an upper portion of the diffusion layer a, the Si composition of which is 1.5 times or more the average of the diffusion layer a.

13. 2. The hot-formed member according to claim 1, wherein the amount of diffusible hydrogen in the base steel is 0.1 ppm by weight or less.

14. The base iron contains, by weight, C: 0.02 to 0.6%, Si: 0.001 to 2%, Al: 0.001 to 1%, Mn: 0.1 to 4%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, Ti: 0 to 0.1%, B: 0.0001 to 0.01%, Cu: 0 to 1.00%, Mo: 0 to 1.00%, Cr: 0 to 1.00%, Ni: : 0-1.00%, V: 0-1.00%, Ca: 0-0.01%, Nb: 0-0.1%, Sn: 0-1%, W: 0-1%, Sb: 0-1%, Mg: 0-0.1%, Co: 0-1%, As: 0-1%, Zr: 0-1%, Bi: 0-1%, REM: 0-0.3%, the balance being Fe and unavoidable impurities. The hot-formed member according to claim 1,

15. providing an aluminum-based plated steel sheet including a base steel sheet and an aluminum-based plating layer formed on a surface of the base steel sheet; manufacturing a blank using the aluminum-plated steel sheet and heating the blank; and forming and cooling the heated blank; The step of providing the aluminum-based plated steel sheet comprises: forming the aluminum-based plating layer by immersing the base steel sheet in a plating bath containing 10 to 35 wt % of Zn, 15 wt % or less of Si, and the remainder being Al and unavoidable impurities; cooling the aluminum-based plating layer to its solidification point at an average rate of 20°C / s or more; and 3. A method for producing a hot-formed part, comprising the step of cooling from said solidification point to 350°C at an average rate of less than 20°C / s.

16. The method for producing a hot-formed part according to claim 15, wherein the plating bath contains 4 wt% or less of Fe.

17. 17. The method for producing a hot-formed part according to claim 15 or 16, wherein the plating bath contains one or more of Mg, Mn, Cr and Ca in an amount of 4.5 wt. % or less.

18. The method for producing a hot-formed member according to claim 15, wherein the K value defined by the following [Equation 1] satisfies 4 to 39 before cooling after the immersion. [Formula 1] K=a*(c*d / b) In this case, a is the Zn content (wt. %) of the plating bath, b is the line speed (mpm), c is the air knife (A / K) interval (mm), and d is the A / K pressure (kPa).

19. 16. The method for manufacturing a hot-formed part according to claim 15, wherein in the step of heating the blank, the blank is heated at a heating rate of 5 to 12°C / s in a first heating section where the temperature of the blank is less than 600°C, and heated at a heating rate of 1.2 to 3.5°C / s in a second heating section where the temperature of the blank is 600°C or higher.

20. The method for manufacturing a hot-formed part according to claim 19, wherein the time of the first temperature-raising section is shorter than the time of the second temperature-raising section.

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