Thermoformed component and method for manufacturing the same

Abstract

The present invention relates to a hot-formed member for use in an automobile member or the like, and relates to a hot-formed member manufactured by hot-forming an aluminum-plated steel material, and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a hot-formed part for automotive parts and the like, and to a hot-formed part manufactured by hot-forming aluminized steel and a manufacturing method thereof. Background Art

[0002] In recent years, in order to lighten the weight of automobiles, the use of high-strength steel is increasing. However, since high-strength steel has problems such as material fracture during processing or spring back after processing, it is difficult to form products with complex and precise shapes. To solve this problem, a method called hot forming or hot press forming (HPF) has recently been expanded.

[0003] The advantages of the hot forming are that by heating the steel to usually 800 - 900 °C and then performing processing (stamping) in the heated state, it is easy to form, and the strength of the formed product can be increased by rapid cooling with a mold. However, when the steel is heated to a high temperature, surface oxidation of the steel surface may be inevitable. This requires an additional process of removing the oxide on the steel surface after stamping, which has a problem of increased cost. To prevent this, as a material for hot forming, aluminized steel (Patent Document 1 and Patent Document 2), galvanized steel (Patent Document 3), or a plated steel in a form of a mixture of both (Patent Document 4) is used on the steel surface.

[0004] However, in the heating process for hot forming of the plated steel, the coating layer with a low melting point becomes a liquid phase. When the liquid phase contacts the atmosphere, the water (H2O) contained in the atmosphere dissociates into oxygen (O2) and hydrogen (H), and the oxygen remains in the surface layer in the form of an oxide while the hydrogen dissolves in the coating layer and accumulates. In this state, a large amount of hydrogen remains in the solid-phase coating layer after the forming process and rapid cooling. However, the solubility of hydrogen in the solid-phase coating layer is lower than that in the liquid-phase coating layer. Therefore, hydrogen diffuses and moves from the coating layer to the surroundings, and a part of this hydrogen enters the steel, and the hydrogen introduced into the steel accumulates in the bonds, causing hydrogen-induced delayed fracture, and thus causing a hydrogen embrittlement problem.

[0005] Therefore, a solution for eliminating the above hydrogen embrittlement problem is needed.

[0006] (Patent Document 1) U.S. Patent No. 6,296,805

[0007] (Patent Document 2) Japanese Patent No. 3845271

[0008] (Patent Document 3) Japanese Unexamined Patent Application Publication No. 2003 - 147499

[0009] (Patent Document 4) Japanese Unexamined Patent Application Publication No. 2006-051543 Summary of the Invention

[0010] (1) Technical Problem to be Solved

[0011] An object of one aspect of the present invention is to provide a thermoformed part having excellent resistance to hydrogen embrittlement and a method for manufacturing the same.

[0012] The technical problem of the present invention is not limited to the above. Additional technical problems of the present invention are described throughout the specification, and those skilled in the art to which the present invention pertains can easily understand the additional technical problems of the present invention from the content described in the specification of the present invention.

[0013] (2) Technical Solution

[0014] One embodiment of the present invention relates to a thermoformed part, the thermoformed part including a base iron and an aluminum-based alloy layer formed on the base iron, the aluminum-based alloy layer including a diffusion layer (a), a first alloy layer (b), a second alloy layer (c), and a third alloy layer (d) formed in sequence from the base iron based on a cross section, the thickness (t1) of the first alloy layer (b) being 8.5 μm or less, the thickness (t1) being an average of values measured at 45 or more positions at 10-μm intervals in the horizontal direction in 3 images of the cross section taken at 500-fold magnification in an optical microscope.

[0015] The first alloy layer (b) may contain: Al: 37-52% by weight, Fe: 38-51% by weight, Si: 1.5-5% by weight, and Zn: 2-20% by weight, the second alloy layer (c) may contain: Al: 19-47% by weight, Fe: 45-69% by weight, and Si: 7.2-15% by weight, and the third alloy layer (d) may contain: Al: 35-52% by weight, Fe: 38-51% by weight, Si: 1.5-5.5% by weight, and Zn: 5-26% by weight.

[0016] 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.

[0017] The thickness (t1) of the first alloy layer (b) may be less than the thickness (t2) of the third alloy layer.

[0018] The second alloy layer (c) may include a discontinuous region along the length direction of the cross section.

[0019] The content of Zn in the first alloy layer (b) may be less than the content of Zn in the third alloy layer (d).

[0020] The Si content of the second alloy layer (c) can be greater than or equal to twice the Si content of the first alloy layer (b) or the third alloy layer (d).

[0021] The number of voids with a major axis of 500 nm or more contained in the first alloy layer (b) can be less than the number of voids with a major axis of 500 nm or more contained in the third alloy layer (d).

[0022] The major axis of the voids with a major axis of 500 nm or more contained in the first alloy layer (b) can 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).

[0023] The above thermoformed part can include Mg oxide formed on the third alloy layer (d).

[0024] A Si-rich layer can be further included on the upper part of the diffusion layer (a), and the composition of Si enriched in the Si-rich layer is greater than or equal to 1.5 times the average value of the Si composition of the diffusion layer (a).

[0025] The diffusible hydrogen amount of the base iron can be 0.1 weight ppm or less.

[0026] By weight%, the base iron can include: C: 0.02 - 0.6%, Si: 0.001 - 2%, Al: 0.001 - 1%, Mn: 0.1 - 4%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, Ti: 0 - 0.1%, B: 0.0001 - 0.01%, Cu: 0 - 1.00%, Mo: 0 - 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%, the balance being Fe and inevitable impurities.

[0027] Another embodiment of the present invention relates to a method for manufacturing a thermoformed part, the manufacturing method comprising the following steps: providing an aluminized steel sheet, the aluminized steel sheet comprising a base steel sheet and an aluminum-based coating formed on the surface of the base steel sheet; manufacturing a slab using the aluminized steel sheet and heating the slab; and forming and cooling the heated slab, wherein the step of providing the aluminized steel sheet comprises the following steps: immersing the base steel sheet in a plating bath to form the aluminum-based coating, the plating bath comprising 10-35% by weight of Zn, 15% by weight or less of Si, the balance being Al and inevitable impurities; cooling the aluminum-based coating to the solidification point at an average rate of 20 °C / second or more; and cooling from the solidification point to 350 °C at an average rate of less than 20 °C / second.

[0028] The plating bath may comprise 4% by weight or less of Fe.

[0029] The plating bath may comprise one or more of 4.5% by weight or less of Mg, Mn, Cr, and Ca.

[0030] The K value defined by the following [Equation 1] before cooling after the immersion may satisfy 4 to 39.

[0031] [Equation 1] K = a (c d / b)

[0032] wherein a is the content of Zn in the plating bath (% by weight), b is the linear velocity (meters per minute (mpm)), c is the air knife (A / K) interval (mm), and d is the A / K pressure (kPa).

[0033] In the step of heating the slab, in the first heating section where the temperature of the slab is lower than 600 °C, heating may be performed at a heating rate of 5-12 °C / second, and in the second heating section where the temperature is above 600 °C, heating may be performed at a heating rate of 1.2-3.5 °C / second.

[0034] The time of the first heating section may be shorter than the time of the second heating section.

[0035] (III) Beneficial effects

[0036] According to the present invention, the inhibition of hydrogen diffusion into the coating during the thermoforming process can be minimized, and the hydrogen introduced into the steel can be inhibited, thereby improving the hydrogen embrittlement resistance. When applied to automotive parts, the anti-collision characteristics can be improved.

[0037] The various beneficial advantages and effects of the present invention are not limited to the above content, and the various beneficial advantages and effects of the present invention will be more easily understood during the process of describing the specific embodiments of the present invention. Brief description of the drawings

[0038] Figure 1 Cross-sectional photograph of Comparative Example 7 in the thermoformed part test piece manufactured in the embodiment of the present invention.

[0039] Figure 2 Cross-sectional photograph of Invention Example 2 in the thermoformed part test piece manufactured in the embodiment of the present invention. Best Mode

[0040] The terms used in this specification are for describing the present invention and are not intended to limit the present invention. In addition, unless the relevant definitions show a clearly contrary meaning, the singular forms used in this specification also include the plural forms.

[0041] The meaning of "comprising" used in the specification is to specify the composition and does not exclude the existence or addition of other compositions.

[0042] Unless otherwise defined, all terms, including technical terms and scientific terms, used in this specification have the same meaning as commonly understood by those skilled in the art. Terms defined in the dictionary are interpreted to conform to the meaning of the relevant technical literature and the currently disclosed content.

[0043] Hydrogen adsorbed in the steel of thermoformed parts may cause serious problems for materials such as strength, ductility, rigidity, and bendability. Most thermoformed parts are composed of a martensite phase, which is extremely susceptible to hydrogen embrittlement. For the lightweighting of automobiles, when using higher-strength steel, as the strength of the thermoformed parts increases, problems such as hydrogen-induced delayed fracture and reduced ductility due to hydrogen become more important.

[0044] The inventors of the present invention found that when thermoforming plated steel, the adsorption of hydrogen changes according to the physical phase change of the coating layer generated during the heat treatment process, thereby completing the present invention.

[0045] That is, when manufacturing thermoformed parts, the plated steel enters a heating furnace at room temperature and is heated for a certain time to undergo austenite phase transformation. As time passes, the state of the coating layer changes from a solid phase to a liquid phase, and then through the diffusion of Fe in the base material, it becomes an intermetallic compound again and finally becomes a solid phase. In the interval where the coating layer exists in a solid phase at the initial stage of heating, the diffusion rate of hydrogen in the aluminum-based coating layer is very slow, so it acts as a physical barrier to prevent external hydrogen from entering the steel.

[0046] As the thermoforming heat treatment progresses, when the temperature of the coating layer rises above the melting point, the coating layer turns into a liquid phase. The hydrogen solubility of the molten coating layer varies according to the composition of the coating layer, but is significantly higher than that of the solid phase. Eventually, Fe of the base material forms an intermetallic compound with the coating layer, and the coating layer turns into a solid phase. The hydrogen solubility in the solid phase is low, so the excessive hydrogen dissolved in the liquid phase diffuses into the base material of the thermoformed part to maintain the hydrogen equilibrium state in the solid phase. It is recognized that the diffusion of the hydrogen is affected by the structure and composition of the coating layer, etc. Based on this, the present invention has been completed.

[0047] First, as a specific embodiment of the present invention, the thermoformed part will be described in detail. Referring to Figure 2 which is a cross-sectional photograph of Invention Example 2 in the following embodiments, the thermoformed part includes a base material iron and an aluminum-based alloy layer formed on the base material iron.

[0048] 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) sequentially formed from the base material iron with the cross-section as a reference. The thickness (t1) of the first alloy layer (b) can be 8.5 μm or less. In the present invention, the thickness of the aluminum-based alloy layer can be the average of values measured at 45 or more positions at 10-μm intervals in the horizontal direction in 3 images taken at 500 times magnification of the cross-section of the thermoformed part in an optical microscope.

[0049] The base material iron refers to the base material in the part obtained by thermoforming a thermoforming steel, which occupies most of the thickness in the cross-section of the above thermoformed part. The base material iron is the base material of the thermoformed part obtained by thermoforming a base steel which is the base material of the thermoforming steel. At this time, the base material iron can mainly have a martensite structure through ordinary thermoforming and cooling.

[0050] The composition of the base material iron or the base steel is not particularly limited as long as it can be used as a thermoformed part. As a preferred example, in terms of weight%, the base material iron may contain: C: 0.02 - 0.6%, Si: 0.001 - 2%, Al: 0.001 - 1%, Mn: 0.1 - 4%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, Ti: 0 - 0.1%, B: 0.0001 - 0.01%, Cu: 0 - 1.00%, Mo: 0 - 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%, the balance being Fe and unavoidable impurities.

[0051] The aluminum-based alloy layer refers to an alloyed coating formed by alloying of the aluminum-based coating of aluminized steel for hot forming with the base material during the hot forming process. The aluminum-based coating refers to a layer mainly composed of aluminum (Al) and containing a part (within 15% by weight) of silicon (Si) to prevent excessive alloying of Al with Fe in the steel during plating. In addition, other components can be included as needed.

[0052] In the aluminum-based alloy layer, the diffusion layer (a) can be a ferrite layer containing a part of Al formed by combining with a part of Al which is the main component of the base steel (Fe) and the aluminum-based coating. At this time, the content of Al can be 3-9% by weight, and the balance can be Fe and inevitable impurities.

[0053] In addition, the upper part of the diffusion layer (a) can include an enrichment layer enriched with a part of Si. The Si in the enrichment layer can be greater than or equal to 1.5 times the average value of Si in the diffusion layer (a).

[0054] The first alloy layer (b) and the third alloy layer (d) form intermetallic compounds from Fe diffused from the base material and the aluminized layer and turn into a solid phase during the heat treatment for hot forming parts. The first alloy layer (b) and the third Al-Fe alloy layer (d) are intermetallic compounds of Al and Fe, and the alloy phase forms relatively fast during the heat treatment, so that many voids can be formed between the atomic arrangements. These voids serve as channels for hydrogen diffusion and as channels for supersaturated hydrogen in the coating to diffuse to the base material. However, like the second alloy layer (c), in the alloy phase containing Si, Si atoms are preferentially located in the pores, so that the hydrogen diffusion channels play a role in filling the pores.

[0055] In addition, the preferred composition of the first alloy layer (b) can include: Al: 37-52% by weight, Fe: 38-51% by weight, Si: 1.5-5% by weight, and Zn: 2-20% by weight. The preferred composition of the third alloy layer (d) can include: Al: 35-52% by weight, Fe: 38-51% by weight, Si: 1.5-5.5% by weight, and Zn: 5-26% by weight. The more preferred composition of the first alloy layer (b) can include: Al: 37.0-52.0% by weight, Fe: 38.0-51.0% by weight, Si: 1.50-5.0% by weight, and Zn: 2.0-20.0% by weight. The more preferred composition of the third alloy layer (d) can include: Al: 35.0-52.0% by weight, Fe: 38.0-51.0% by weight, Si: 1.50-5.50% by weight, and Zn: 5.0-26.0% by weight.

[0056] As described above, the second alloy layer (c) is present between the first alloy layer (b) and the third alloy layer (d) and contains a large amount of Si, thereby acting to prevent hydrogen from diffusing into the base material. For this purpose, the content of Si in the second alloy layer (c) can be greater than or equal to twice that of the first alloy layer (b) and the third alloy layer (d). The preferred composition of the second alloy layer (c) can include: Al: 19 - 47 wt%, Fe: 45 - 69 wt%, and Si: 7.2 - 15 wt%. The more preferred composition of the second alloy layer (c) can include Al: 19.0 - 47.0 wt%, Fe: 45.0 - 69.0 wt%, and Si: 7.20 - 15.0 wt%.

[0057] From this perspective, the closer the second alloy layer (c) is to the diffusion layer (a), the lower the possibility that the hydrogen contained in the coating diffuses into the base material, and thus it can be beneficial to hydrogen embrittlement. Therefore, since reducing the interval between the diffusion layer (a) and the second alloy layer (c) is beneficial to reducing hydrogen embrittlement, the thickness (t1) of the first alloy layer (b) in the present invention can be 8.5 μm or less.

[0058] In addition, the second alloy layer (c) may include a region (discontinuous region) that is discontinuous in the length direction on the cross-section of the aluminum-based alloy layer. However, if a sufficient surface is formed in three dimensions, the effect of reducing hydrogen embrittlement can be fully exerted from a macroscopic perspective. When the second alloy layer (c) is too discontinuous, its function as a hydrogen diffusion barrier will be reduced. However, when observed with an optical microscope at a magnification of 200 times or less, there can be a continuous band shape of 5 μm or more in the horizontal direction, and preferably a continuous region of 10 μm or more. The total length of fractures in the cross-section is preferably kept below 40% in the horizontal direction.

[0059] In addition, during the process of manufacturing a hot-forming part, the absolute amount of hydrogen introduced into the aluminum-based coating is also important. Therefore, when elements such as zinc and magnesium are contained in the aluminum-based coating used for manufacturing the hot-forming part, it has the effect of reducing the hydrogen introduced into the coating during the manufacturing process of the hot-forming part.

[0060] The aluminum-based coating containing zinc or magnesium has a lower solubility of hydrogen in the molten state. Therefore, the absolute amount of hydrogen that dissociates on the surface and enters the coating is reduced, and zinc or magnesium oxides are first formed from the moisture in the atmosphere on the surface layer, hindering the surface adsorption of the coating and moisture, thereby suppressing the injection of hydrogen.

[0061] However, when the content of zinc contained in the plating layer increases, liquid brittle fracture caused by zinc (Liquid Metal Embrittlement, LME) may occur, and thus cracks may be generated in the base material iron. In addition, when the content of magnesium increases, due to the excessive formation of magnesium oxide, the adhesiveness increases, which may cause the plating layer to adhere to the mold and fall off. During this process, defects may occur in the hot-formed part. In addition, magnesium causes spatter during welding, so the weldability may deteriorate.

[0062] While the zinc contained in the plating layer continuously undergoes Fe alloying of the base material iron in a molten state and the whole plating layer increases, the proportion of zinc begins to decrease. The content of zinc in the hot-formed part can be 5-35% by weight. The zinc is mainly distributed in the first alloy layer (b) and the third alloy layer. According to an embodiment, the content of Zn contained in the first alloy layer (b) is on average less than the content of Zn contained in the third alloy layer (d).

[0063] Magnesium has high oxidability and easily moves to the surface layer during the heat treatment process. Therefore, compared with the first alloy layer (b), there is a larger amount of magnesium in the third alloy layer (d), and the rest may exist in the surface layer in the form of Mg oxide. The content of magnesium in the formed part must be controlled below 5% by weight to prevent weldability and adhesion. According to an embodiment, when observing the center of the third alloy layer (d) with an electron scanning microscope, it can be controlled below 3% by weight on average.

[0064] According to an embodiment, effectively, the thickness of the aluminum-based alloy layer of the hot-formed part, except for the diffusion layer (a), the thickness of the remaining layers is 30 μm or less. The diffusion layer can vary according to the heat treatment time used to manufacture the hot-formed part. When the remaining layers except the diffusion layer are very thick after heat treatment, the effect of hydrogen release that naturally escapes after manufacturing the hot-formed part will decrease. As the thickness of the alloy layer increases, the absolute amount of hydrogen contained in the alloy layer increases, and the hydrogen embrittlement resistance may deteriorate.

[0065] In addition, according to an embodiment, the thickness (t1) of the first alloy layer (b) can be less than the thickness (t2) of the third alloy layer (d).

[0066] The third alloy layer (d) is different from the first alloy layer (b) close to the base material and is relatively far from the base material. Therefore, the amount of diffused Fe may be locally non-uniform. The liquid-phase plating layer reacts with Fe and becomes solid-phase while shrinking. Due to this non-uniform diffusion of Fe, irregular shrinkage of the solid-phase occurs, and thus voids can be formed in the alloy phase. These voids usually act as traps for hydrogen. When there are many such voids, the hydrogen present in the plating layer accumulates in the voids, reducing the possibility of diffusion to the base material.

[0067] The second alloy layer (c) functions 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 coating away from the base material and bring the effect of reducing the amount of hydrogen in the base material.

[0068] According to an embodiment, the number of voids having a major axis of 500 nm or more included in the first alloy layer (b) can be less than the number of voids having a major axis of 500 nm or more included in the third alloy layer (d). Additionally, the major axis of the voids having a major axis of 500 nm or more included in the first alloy layer (b) can be smaller than the major axis of the voids having a major axis of 500 nm or more included in the third alloy layer (d).

[0069] The amount of diffusible hydrogen contained in the base iron in the hot-formed part has the highest hydrogen content immediately after the initial formed part is made, and the content decreases while being naturally released to the outside through the cross-section of the part or the coating over time. To prevent hydrogen embrittlement, when measured within one week after manufacturing the formed part, it is preferably 0.1 weight ppm or less. Hydrogen embrittlement is a phenomenon in which the formed part spontaneously breaks due to internal stress before an external impact is applied after manufacturing the formed part. This measurement is obtained by measuring a test piece of the hot-formed part made in a heating furnace without a separate dew point adjustment device.

[0070] Next, as another specific embodiment of the present invention, a method for manufacturing a hot-formed part will be described in detail.

[0071] The manufacturing method includes the following steps: providing an aluminized steel sheet, the aluminized steel sheet including a base steel sheet and an aluminum-based coating formed on the surface of the base steel sheet; manufacturing a slab using the aluminized steel sheet and heating the slab; and forming and cooling the heated aluminized steel sheet.

[0072] In the present invention, there is no particular limitation on the base steel sheet as long as it can be used in the technical field to which the present invention pertains, and there is no particular limitation on its type, composition, etc.

[0073] There is no particular limitation on the method for forming the aluminum-based coating, including methods such as hot dip plating and electroplating.

[0074] As an example, a cold-rolled steel sheet for a hot-formed part can be annealed, adjusted for the plating amount after entering a molten plating bath, and cooled to manufacture a plated steel sheet. Specifically, the aluminum-based coating is immersed in a plating bath mainly composed of Al, and the composition of the plating bath can include 10 - 35 wt% of Zn, 15 wt% or less of Si, the balance of Al, and inevitable impurities.

[0075] Preferably, it may contain 15-30% by weight of Zn, 12% by weight or less of Si, the balance of Al and inevitable impurities. More preferably, it may contain 20-28% by weight of Zn, 9% by weight or less of Si, the balance of Al and inevitable impurities.

[0076] In addition, it may further contain 4% by weight or less of Fe. Moreover, in order to improve corrosion resistance, it may contain one or more of Mg, Mn, Cr, and Ca at 4.5% by weight or less.

[0077] In the plating bath composition, Si combines with aluminum and the base steel plate Fe, preventing the formation of the Al-Fe alloy layer and playing a role in preventing the erosion of the plating equipment immersed in the plating bath. However, when Si is excessive and exceeds 15% by weight, the growth of the Al-Fe alloy layer is overly inhibited, which may cause welding liquation brittleness.

[0078] The Zn plays a role in adjusting the position of the second alloy layer by suppressing the diffusion of Si to the surface layer as the solid phase grows during the heat treatment process for manufacturing hot forming parts. At the same time, in order to ensure the corrosion resistance of the hot forming parts, it may contain 10% by weight or more of Zn. However, when the Zn exceeds 35% by weight, the melting point of the coating decreases and the fluidity increases, resulting in a high possibility of liquid phase brittleness. Additionally, it is preferably 15-30% by weight.

[0079] In addition, Fe in the plating bath dissolves and accumulates from the base steel plate over time during continuous plating, so it is necessary to control Fe below 4% by weight to prevent dross defects.

[0080] The plating amount can be adjusted by an air knife (A / K). At this time, the gas used is basically air, but when defects are caused by surface oxidation of the coating, nitrogen or a gas containing part of nitrogen can be used. The plating amount is 10 g / m 2 or more and 90 g / m 2 or less is effective. When the plating amount is too small, there are corrosion resistance problems and the surface quality may deteriorate. When the plating amount is too large, it can inhibit the release of hydrogen adsorbed in the coating during hot forming to the outside, adhere to the mold, resulting in poor productivity, and may cause part defects.

[0081] In order to adjust the plating amount, it is necessary to adjust the gas pressure of the air knife and the distance between the material and the air knife. The adjustment of the adhesion amount is a process of removing the excessive plating layer by the shear stress of the gas from the air knife. Even when the same shear stress is applied to the plating layer, the plating adhesion amount, the surface state, and the plating layer structure will vary depending on the linear velocity (mpm) which is the speed of the steel sheet passing through the air knife and the viscosity of the plating bath. As an example, when the chemical composition of the plating bath contains aluminum, zinc, silicon, and also contains magnesium, etc., such elements entering the plating bath affect the viscosity of the liquid plating bath. Fe, which is a component of the material, reacts faster with other components of the aluminum plating bath to form an Fe-Al alloy phase with a thickness of less than 10 μm. This alloy phase contains many irregularities on the surface layer, and the plating layer of the liquid phase attached above it may not be evenly cut by the air knife, so the surface shape may not be excellent. Such uneven plating adhesion amount is a factor that hinders the formation of a continuous phase on the second alloy layer (c) as a diffusion prevention layer during the heat treatment of the deviation heat treatment forming part. Therefore, K defined by the following [Equation 1] can be in the range of 4 to 39 before cooling after the impregnation.

[0082] [Equation 1] K = a (c d / b)

[0083] At this time, a is the content (weight %) of Zn in the plating bath, b is the linear velocity (mpm), c is the distance (mm) between the air knife (A / K), and d is the A / K pressure (kPa).

[0084] After adjusting the plating amount, it can be cooled to the freezing point at an average speed of 20 °C / second or more during cooling, and cooled from the freezing point to 350 °C at an average speed of less than 20 °C / second.

[0085] Even immediately after passing through the air knife, the plating layer is still in a liquid phase state above the melting point, the strip temperature is similar to that of the plating bath, and the Al-Fe alloy layer can continue to grow between the plating layer and the base steel sheet. The excessive growth of the Al-Fe alloy layer increases the brittleness of the plating layer, and plating layer peeling may occur during coiling or slab shearing. To prevent this, it is preferably cooled at a speed of 20 °C / second or more before reaching the freezing point to suppress the vibration and bending of the strip caused by the gas generated in the cooling tower after the freezing point. To suppress the deviation of the plating adhesion amount caused thereby, it is preferably cooled at a speed of less than 20 °C / second.

[0086] The aluminized steel sheet provided as above can be processed and formed into a blank in a form close to the shape of the part for forming.

[0087] Heat the slab for hot forming. The heating can be carried out at a temperature above Ac3℃ and below 970℃, preferably at a temperature of 850℃ to 950℃. As an example, place the aluminized steel sheet in a heat treatment furnace set at the above temperature and heat it for 180 - 600 seconds so that the structure of the steel can be completely transformed into austenite. When the temperature is too low, it is difficult for the steel material to be completely transformed into a single-phase austenite. When the temperature is too high, the surface is oxidized or the hydrogen content in the steel increases. In the case of the coating, the volatilization in the liquid phase is severe, making it impossible to fully retain the coating, resulting in poor corrosion resistance.

[0088] As an example, when heating the slab, in the first heating section where the temperature of the slab is below 600℃, the heating rate can be 5 - 12℃ / s, and in the second heating section where the temperature is above 600℃, the heating rate can be 1.2 - 3.5℃ / s. At this time, the time of the first heating section can be shorter than the time of the second heating section.

[0089] In addition, the atmosphere in the heating furnace is usually an air atmosphere, but the moisture concentration in the air can be controlled to actively reduce the hydrogen embrittlement of the components. The moisture concentration is measured by the dew point, and the dew point can be carried out in an atmosphere of -25℃ to 15℃.

[0090] The heating method is not particularly limited. The radiant tube or radiant heating method can be used, and induction heating and electric heating can also be used. When heating, the heating rate can be adjusted according to the plate thickness and the type of coating.

[0091] The aluminized steel sheet transformed into a single-phase austenite by heating is transported out of the heating furnace and placed in a mold, and cooling is achieved while processing with the mold. Preferably, the time from the heating furnace to the mold can be within 15 seconds. When the time is longer than the above range, ferrite is formed while the material naturally cools before rapid cooling in the mold, and the material properties may deteriorate.

[0092] The critical cooling rate for obtaining martensite transformation in the mold may vary due to the composition of the steel. However, considering the cooling that generally occurs when transferring from the heating furnace to the mold, starting from 700℃, which is the initial temperature of the material placed in the mold, until 350℃, an average cooling rate of 20℃ / s to 50℃ / s is effective. Detailed implementation

[0093] Hereinafter, embodiments of the present invention will be described. Of course, those skilled in the art to which the present invention pertains can make various changes to the following embodiments without departing from the scope of the present invention. The following embodiments are for understanding the present invention, and the scope of the rights of the present invention is not limited to the following embodiments. The scope of the rights of the present invention shall be determined by the claims and their equivalents.

[0094] (Embodiment)

[0095] Prepare a cold-rolled steel sheet having the composition shown in Table 1 below (the balance being Fe and inevitable impurities). For plating, in an annealing furnace with a nitrogen atmosphere containing 5% hydrogen, heat from room temperature to 780 °C, hold at the annealing temperature of 780 °C for 80 seconds, then cool the base steel sheet to a temperature 10 °C higher than the temperature of the molten plating bath, and then immerse it in the plating bath to perform aluminum-based alloy plating.

[0096] The plating bath composition (by weight %) is as follows.

[0097] Plating bath 1: 9% Si, 2% Fe, the balance Al and inevitable impurities

[0098] Plating bath 2: 8% Si, 27% Zn, 1.5% Fe, the balance Al and inevitable impurities

[0099] Plating bath 3: 6% Si, 25% Zn, 1% Mg, the balance Al and inevitable impurities

[0100] Plating bath 4: 6% Si, 25% Zn, 5% Mg, the balance Al and inevitable impurities

[0101] Plating bath 5; 6% Si, 38% Zn, 5% Mg, the balance Al and inevitable impurities

[0102] The said plating bath 1 is used to manufacture Comparative Examples 1 to 7, plating bath 2 is used to manufacture Invention Examples 1 to 4, plating bath 3 is used to manufacture Invention Example 5, plating bath 4 is used to manufacture Comparative Example 8, and plating bath 5 is used to manufacture Comparative Example 9.

[0103] After the said plating, the plating amount is adjusted by an air knife. At this time, the plating amounts are shown in Table 2 below. After adjusting the said plating amount, cool to the solidification point at a cooling rate of 20 °C / second, and after the solidification point, start to cool to 350 °C at a cooling rate of 10 °C / second and then cool naturally to manufacture a plated steel sheet.

[0104] [Table 1]

[0105]

[0106] Cut the said plated steel sheet to a size of 400 A slab of 380 mm is loaded into a box furnace in an air atmosphere with an atmosphere temperature of 900 °C and heat-treated.

[0107] The heat treatment is roughly divided into two steps according to temperature ranges to adjust the heating rate separately. The first heating range starts at room temperature and is heated to below 600 °C before the surface of the coating is liquefied. At this time, the slab is heated while maintaining an average heating rate of 6 - 12 °C / second. The second heating range is from 600 °C to 900 °C. The range from 600 °C to 900 °C is the range where the liquefied coating alloyizes with Fe of the base iron again to become a solid phase, and the heating rate is maintained at 1.2 - 3.5 °C / second. After that, after reaching the target heat treatment temperature, the temperature deviation at each position of the slab does not deviate from 10 °C. At this time, the time of the first heating range is kept shorter than that of the second heating range to minimize the damage to the layered structure caused by rapid alloying.

[0108] The total heat treatment time is the time measured from when it is placed in the box furnace at room temperature until it is finally taken out of the box furnace. Hot stamping is to obtain a hot-formed part using a water-cooled mold under the condition of a cooling rate of 20 °C / second.

[0109] After that, the cross-section of the hot-formed part obtained from each specimen is observed with a scanning electron microscope (SEM) to analyze the structure in the alloy layer. In addition, the mold adhesiveness, base iron crack, spot weldability, and diffusible hydrogen amount of the part of each specimen are measured, and the results are shown in Table 2 below.

[0110] Specifically, the cross-section of each phase is distinguished at a magnification of ×500. For composition analysis, an electron scanning microscope (SEM) is used, and at a magnification of ×2000, energy dispersive X-ray spectroscopy (EDS) is used to measure the center points of each phase three times at different positions and then use the average value of the three points. The thicknesses t1 and t2 of the first alloy layer (b) and the third alloy layer (d) are the averages of the values measured at more than 45 positions at equal intervals of 10 μm in the horizontal direction in 3 images taken at 500 times magnification in an optical microscope. At this time, the c layer is discontinuous. For the interrupted part, a hypothetical connecting line segment is drawn from the interrupted part to the next c layer, and the distance from the a layer is measured based on the line segment.

[0111] For mold adhesiveness, after making the part, the surface of the mold is blown with air, and then the mold adhesiveness is visually confirmed. When there are irregularities that can be felt by hand, the foreign matter is removed, and then when making the part again with the same material, when the coating that has peeled off from the part adheres to the mold, it is determined that the adhesiveness is poor.

[0112] To observe the cracks in the base iron, a hot-formed part was made using a die in the shape of the Greek letter omega (Ω). The cracks in the base iron were observed by classifying them into LME and microcracks. LME is liquid-phase brittle fracture caused by molten metal in the liquid phase. The LME can be observed hundreds of micrometers deep in the base iron, and a large amount of the LME appears at the location where the maximum tensile stress is applied at a high temperature above the melting point of the coating. Microcracks are cracks generated due to the weakening of the grain boundaries of the base iron caused by the solid-phase diffusion of the coating, and occur at the location where the friction between the die and the material is the most intense, with a depth of up to dozens of micrometers. To observe both of them, a forming part was made using a die that protruded upward to a height of 10 cm in the center, and then the cross-section was sheared with a laser. Test pieces were made at the same positions of the parts in each experiment, and in each experiment, the cracks in the base iron of the cross-section of the part were observed at 200 times magnification through an optical microscope. More than 3 positions were observed at each part, and when cracks in the base iron were observed at more than 1 position, it was judged as poor.

[0113] For the amount of diffusible hydrogen, a product of G8 GALILEO from Bruker was used. The test piece was heated to 400 °C at room temperature while quantifying the amount of hydrogen released and evaluating it. The hydrogen contained in the part naturally releases to the outside over time, so test pieces that had gone through the same amount of time after the part was made were used in the measurement. As the amount of diffusible hydrogen contained in the steel increases, hydrogen accumulates around the dislocations or defects in the steel, and due to internal stress or external force, sudden fracture of the material may occur. When the amount of diffusible hydrogen in a normal HPF material is 0.1 ppm or more, it was found that the part broke, and test pieces with a hydrogen amount of 0.1 ppm or more were judged to be at risk of hydrogen embrittlement.

[0114] The evaluation method of weldability was evaluated using the standard of ISO18278. An electrode of 6 mm was used, an electrode pressure of 4 kN was used, a welding time of 320 milliseconds (ms) was used, and the current reaching the specified button size was measured. Then, the current was increased by more than 0.5 kA from this current and welded 5 times. At this time, when spatter was observed more than 2 times, it was evaluated as poor. At this time, for spatter, the situation where spatter occurred on the surface of the part and the situation where spatter occurred between the test pieces after separating the welded test pieces were evaluated. Regardless of the current range, even if the minimum button diameter specified by the standard was not ensured, it was judged as poor.

[0115] [Table 2]

[0116]

[0117] [Table 3]

[0118]

[0119] Figure 1For observing the photograph of the cross-section of Comparative Example 7, Figure 2 For observing the photograph of the cross-section of Invention Example 2.

[0120] From the results in Table 2 and Table 3, it can be seen that the invention examples satisfying the conditions of the present invention can ensure excellent spot weldability and hydrogen embrittlement resistance, but the hydrogen embrittlement resistance of the comparative examples is poor, and sometimes the spot weldability is also poor. In addition, by observing Figure 2 it can be seen that in Invention Example 2, t2 is greater than t1.

[0121] In the case of Comparative Examples 1 to 7, the thickness (t1) of the first alloy layer (b) in the aluminum-based alloy layer of the produced hot-formed part is too thick, and it is difficult to ensure hydrogen embrittlement resistance. In particular, from Figure 1 it can be confirmed that t1 is greater than t2.

[0122] In addition, in Comparative Example 8, due to the excessive content of Mg, for the magnesium in the coating, during the heat treatment process for manufacturing the formed part, compared with the coating, the magnesium exists more in the form of magnesium oxide on the surface of the coating, causing the adhesion phenomenon that the strongly adhesive magnesium oxide strongly adheres to the mold. This results in the need to clean the mold every time production is carried out, thus deteriorating the productivity. In addition, due to the influence of this oxide, the weldability deteriorates.

[0123] In Comparative Example 9, it can be seen that due to the excessive content of Zn and Mg during plating, the occurrence of substrate iron cracks such as liquation brittle fracture and microcracks in the produced hot-formed part is very serious, and the weldability is also poor due to various oxides.

Claims

1. A hot - formed component, which comprises 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) formed in sequence from the base iron based on the cross - section, The thickness (t1) of the first alloy layer (b) is 8.5 μm or less, The thickness (t1) is the average of values measured at 45 or more positions at 10 - μm intervals in the horizontal direction in 3 images of the cross - section taken at 500 times magnification in an optical microscope.

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

3. The thermoformed part 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 thermoformed part according to claim 1, wherein, The thickness (t1) of the first alloy layer (b) is less than the thickness (t2) of the third alloy layer.

5. The thermoformed part according to claim 1, wherein, The second alloy layer (c) includes a discontinuous region along the length direction of the cross - section.

6. The thermoformed part according to claim 1, wherein, The second alloy layer (c) includes a continuous region of 5 μm or more along the length direction of the cross - section.

7. The thermoformed part according to claim 1, wherein, The content of Zn in the first alloy layer (b) is less than the content of Zn in the third alloy layer (d).

8. The thermoformed part according to claim 1, wherein The content of Si in the second alloy layer (c) is greater than or equal to 2 times the content of Si in the first alloy layer (b) or the third alloy layer (d).

9. The thermoformed part according to claim 1, wherein, The number of voids with a major axis of 500 nm or more contained in the first alloy layer (b) is less than the number of voids with a major axis of 500 nm or more contained in the third alloy layer (d).

10. The thermoformed part according to claim 1, wherein, The major axis of the voids with a major axis of 500 nm or more contained in the first alloy layer (b) is less than the major axis of the voids with a major axis of 500 nm or more contained in the third alloy layer (d).

11. The thermoformed part according to claim 1, wherein, The hot - formed component contains Mg oxide formed on the third alloy layer (d).

12. The thermoformed part according to claim 1, wherein, Further includes a Si - enriched layer on the upper part of the diffusion layer (a), and the composition of Si enriched in the Si - enriched layer is greater than or equal to 1.5 times the average value of the Si composition of the diffusion layer (a).

13. The thermoformed component according to claim 1, wherein The diffusible hydrogen content of the base iron is 0.1 weight ppm or less.

14. The thermoformed part according to claim 1, wherein, By weight, the base iron contains: C: 0.02 - 0.6%, Si: 0.001 - 2%, Al: 0.001 - 1%, Mn: 0.1 - 4%, P: 0.05% or less, S: C: below 0.02%, N: below 0.02%, Ti: 0 - 0.1%, B: 0.0001 - 0.01%, Cu: 0 - 1.00%, Mo: 0 - 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%, the balance being Fe and inevitable impurities.

15. A method for manufacturing a hot - formed part, comprising the following steps: Providing an aluminized steel sheet, the aluminized steel sheet comprising a base steel sheet and an aluminum - based coating formed on the surface of the base steel sheet; Manufacturing a slab using the aluminized steel sheet and heating the slab; and Forming and cooling the heated slab, wherein the step of providing the aluminized steel sheet comprises the following steps: Immersing the base steel sheet in a plating bath to form the aluminum - based coating, the plating bath containing 10 - 35 wt% of Zn, below 15 wt% of Si, the balance being Al and inevitable impurities; Cooling the aluminum - based coating to the solidification point at an average speed of 20°C / second or more; and Cooling from the solidification point to 350°C at an average speed of less than 20°C / second.

16. The manufacturing method of the thermoformed part according to claim 15, wherein, The plating bath contains 4 wt% or less of Fe.

17. The method for manufacturing a thermoformed part according to claim 15 or 16, wherein, The plating bath contains one or more of 4.5 wt% or less of Mg, Mn, Cr, and Ca.

18. The manufacturing method of the thermoformed part according to claim 15, wherein, The K value defined by the following [Equation 1] before cooling after immersion satisfies 4 to 39, [Formula 1] K = a (c d / b) where a is the content of Zn in the plating bath in wt%, b is the linear speed in m / minute, c is the air knife (A / K) interval in mm, and d is the A / K pressure in kPa.

19. The manufacturing method of the thermoformed part according to claim 15, wherein, In the step of heating the slab, the slab is heated at a heating rate of 5 - 12°C / second in a first heating range where the temperature of the slab is below 600°C, and at a heating rate of 1.2 - 3.5°C / second in a second heating range where the temperature is above 600°C.

20. The manufacturing method of the thermoformed part according to claim 19, wherein, The time of the first heating range is shorter than the time of the second heating range.

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