Aluminum-plated steel sheet, hot-formed member using the same, and manufacturing method thereof

Abstract

The present invention relates to an aluminum-plated steel sheet used in automobiles and the like, a hot-formed member produced using the same, and a method for producing the same.

Description

[Technical Field]

[0001] The present invention relates to an aluminum-plated steel sheet used in automobiles and the like, a hot-formed member produced using the same, and a method for producing the same. [Background technology]

[0002] Recently, efforts have been made to reduce the weight of automobiles to improve fuel efficiency. While reducing the thickness of steel materials is one way to achieve this, it is necessary to increase the strength of the steel materials, as this may cause problems with the stability of the automobile. For this reason, there has been a continuous demand for high-strength steel materials, and various types of steel materials have been developed. However, due to the high strength of such steel materials, problems arise in terms of poor workability, such as springback.

[0003] To solve these problems, a hot press forming process has been proposed. This hot forming process is also called hot press forming or hot working. The hot forming process involves processing steel at a high temperature (800°C or higher) where it is easy to process, and then press-forming it in a mold while rapidly cooling it to a low temperature, thereby forming low-temperature structures such as martensite in the steel and increasing the strength of the final product. This method can minimize workability issues when manufacturing high-strength components.

[0004] Hot forming has recently become more widely used due to its advantages of being able to easily form complex shapes and ensuring high strength for the manufactured parts (hot-formed components). In particular, high corrosion resistance can be ensured by using plated steel material with aluminum, zinc, etc. on the surface of the steel material. As an example, Patent Document 1 discloses the use of aluminum-plated steel sheet in a hot forming process.

[0005] When aluminum- or zinc-plated steel is heated for hot forming, the Fe in the base steel diffuses into the plating layer, causing an alloying reaction with the aluminum or zinc plating layer. This can cause the plating layer to become brittle, resulting in the generation of alloy powder on the surface. This alloy powder can accumulate on the die during press forming, ultimately causing die pollution.

[0006] When powder from the alloy layer accumulates on the die, causing metal burn-off, dents form on the hot-formed part formed with the die. The dents caused by die burn-off are different from the surrounding areas, where the steel is relatively thin. This causes stress concentration and increases the risk of breakage when applied to car bodies, which require high strength. This issue directly impacts safety when applied to automobiles, and therefore needs to be resolved.

[0007] Since the die sticking phenomenon is an unavoidable situation for auto parts manufacturers, auto parts manufacturers check the condition of the die and remove the alloy layer powder accumulated on the die by blowing it off with an air blower before die sticking occurs, or when die sticking occurs, they stop the hot forming line and remove the powder using a grinder or the like.

[0008] The problems and inconveniences caused by mold burning result in a decrease in product quality and productivity, and there is a constant demand for improvements to this problem. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 6,296,805 Summary of the Invention [Problem to be solved by the invention]

[0010] An embodiment of the present invention relates to an aluminum-plated steel sheet that can be used for hot forming or the like, and relates to an aluminum-plated steel sheet having excellent mold adhesion resistance, a hot-formed member using the same, and a method for producing the same.

[0011] The object of the present invention is not limited to the above-mentioned content, and a person having ordinary skill in the art to which the present invention pertains will have no problem in understanding further object of the present invention from the overall matters of the specification of the present invention. [Means for solving the problem]

[0012] One embodiment of the present invention includes a base steel sheet and an Al plating layer formed on the base steel sheet, The results of GDOES analysis from the surface of the aluminium coating layer in the thickness direction showed that the Fe concentration gradient in the aluminium coating layer in the section where the Fe content was 40 to 90 wt.% was 13 to 26 wt.% / μm, The aluminum-plated steel sheet has an Fe content of 8 to 24 wt.% in the Al-plated layer.

[0013] The Al plating layer may include an alloy layer formed at the interface with the base steel sheet, and the thickness of the alloy layer may be 2 to 6 μm.

[0014] The Fe concentration gradient may be 13.5 to 23 wt.%.

[0015] The Al plating layer may have an Fe content of 8.5 to 23 wt. %.

[0016] In order to ensure the above-mentioned sufficient corrosion resistance and mold burn resistance, the thickness of the Al plating layer may be 3 to 30 μm.

[0017] The base steel sheet contains, in wt.%, 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. 0.00%, 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 remainder may contain Fe and impurities.

[0018] Another embodiment of the present invention is a method for manufacturing a steel sheet, comprising the steps of: immersing a steel sheet in an aluminum (Al) plating bath containing silicon (Si) to deposit a plating solution on a surface of the steel sheet; adjusting the coating weight on the surface of the base steel sheet using an air knife (A / K); and transferring the base steel sheet with the plating solution adhered thereto to a cooling means and cooling it, wherein the K value in the following (Equation 2) is 200 to 400.

[0019] (Formula 2)K=10 6 *(a*d) / (b*c*e)

[0020] where a: Si content (wt.%) of plating bath, b: line speed (mpm), c: A / K interval (mm), d: A / K pressure (kPa), A / K height (mm).

[0021] In order to ensure coating quality and suppress the generation of dross, the temperature at which the base steel sheet is drawn into the coating bath may be 600 to 680°C.

[0022] The coating weight is 8 to 80 g / m 2 may be.

[0023] The plating bath may contain 6 to 12 wt. % silicon (Si), 1 to 4 wt. % iron (Fe), and the remainder aluminum (Al) and unavoidable impurities.

[0024] The plating bath may contain 5 to 13 wt.% silicon (Si), 15 to 35 wt.% zinc (Zn), 0.01 to 5 wt.% magnesium (Mg), 0 to 5 wt.% iron (Fe), 0.01 to 5 wt.% manganese (Mn), 0.01 to 7 wt.% chromium (Cr), with the remainder being aluminum (Al) and unavoidable impurities.

[0025] In order to ensure the fluidity of the coating bath and to suppress the generation of dross, the temperature of the coating bath may be 600 to 680°C.

[0026] Yet another embodiment of the present invention provides a steel sheet including a base steel, an Fe-Al alloy plating layer formed on the base steel, and a diffusion layer formed between the base steel and the Fe-Al alloy plating layer, The present invention relates to a hot-formed member having a hard layer ratio defined by the following (Equation 3) of 50 to 75%.

[0027]

number

[0028] Yet another embodiment of the present invention provides a method for manufacturing a blank using the aluminum-plated steel sheet, comprising the steps of: heating the blank; forming the heated blank in a mold and cooling it. [Effects of the Invention]

[0029] When hot forming the aluminum-plated steel sheet according to one embodiment of the present invention, it is possible to reduce die burn-in caused by powder of the plating layer due to alloying of the plating layer. This prevents deterioration in the quality of the hot-formed part and ensures product stability. Furthermore, it is possible to omit the work of removing the powder of the plating layer, thereby reducing the productivity and cost of the product.

[0030] The various beneficial 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]

[0031] [Figure 1] 1 shows the results of Fe depth profile of GDOES in Example 2 of the Examples. [Figure 2] 1 is a photograph showing an observation of a test piece after a V-bending experiment in an example. [Figure 3] 1 shows the results of measuring the amount of powder generated after a V-bending experiment in an example. [Figure 4] 1 shows the results of measuring the hard layer ratio in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0032] The present invention will be described in detail below.

[0033] The terms used in the present invention are for the purpose of describing the present invention and are not intended to limit the present invention. Also, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the relevant definition clearly indicates otherwise.

[0034] As used herein, the meaning of "comprises" embodies features and does not exclude the presence or addition of other features.

[0035] 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 as having a meaning consistent with the relevant technical literature and the presently disclosed content.

[0036] In the present invention, the term "member" refers to a part or material for a part manufactured by hot forming. The term "steel plate" refers to a material before hot forming, and includes forms such as coils.

[0037] When hot forming is performed using plated steel, powder from the plated layer due to alloying of the plated layer is the main cause of die adhesion, so it has become clear that it is important to develop materials that minimize the powder from the plated layer during hot forming.

[0038] When aluminum-plated steel is heated, Fe in the base steel diffuses into the aluminum plating layer, forming an Fe-Al alloy plating layer, and a diffusion layer is formed between the base steel and the Fe-Al alloy plating layer. The diffusion layer and the Fe-Al alloy plating layer form an Fe-Al alloy as Fe diffuses from the base steel, starting with α-Fe, which has a high Fe content, in the following order: Fe3Al → FeAl → Fe2Al5 → FeAl3, depending on the Fe content. The portion where high Fe content α-Fe, Fe3Al, and FeAl are formed is called the diffusion layer, while the other portion where relatively high Al content Fe2Al5 and FeAl3 are formed is called the Fe-Al alloy plating layer. Among the Fe-Al alloys, Fe2Al5 and FeAl3 are harder and more brittle than α-Fe, Fe3Al, and FeAl, resulting in the generation of large amounts of powder during hot press forming, which is a major cause of die seizure problems during hot press forming.

[0039] Therefore, in order to alleviate the problem of die adhesion, the present inventors have conducted extensive research into a method for increasing the content of α-Fe, Fe3Al, and FeAl, which have high Fe contents among Fe-Al alloys, and conversely, decreasing the proportions of Fe2Al5 and FeAl3, in alloying the coating layer during hot forming of aluminum-plated steel sheet, and have arrived at the present invention.

[0040] First, one embodiment of the aluminum-plated steel sheet of the present invention will be described in detail.

[0041] The aluminum-plated steel sheet includes a base steel sheet and an Al plating layer formed on the base steel sheet, and the Al plating layer includes an alloy layer formed at the interface with the base steel sheet. The alloy layer refers to a layer formed by reaction between the base steel sheet and the Al plating layer. As an example of how the alloy layer is formed, when the base steel sheet is immersed in an Al plating bath, the alloy layer may be formed by first reacting the base steel sheet with components of the Al plating bath.

[0042] Glow Discharge Optical Emission Spectrometry (GDOES) analysis results from the surface of the aluminized layer to the interior of the base steel sheet in the thickness direction show that the Fe concentration gradient in the aluminized layer in the section where the Fe content is 40 to 90 wt.% is 13 to 26 wt.% / μm. Preferably, the Fe concentration gradient in the aluminized layer in the section where the Fe content is 40.0 to 90.0 wt.% is 13.0 to 26.0 wt.% / μm.

[0043] As mentioned above, to improve die adhesion, the formation of Fe2Al5 and FeAl3, which have low Fe content and relatively brittle characteristics, must be suppressed during the alloying process of the aluminum coating layer during heating for hot forming, and the content of α-Fe, Fe3Al, and FeAl, which have relatively high Fe content, must be increased. To achieve this, Fe must be sufficiently diffused into the aluminum coating layer in a short period of time during the hot forming process to achieve alloying.

[0044] According to Fick's first law of diffusion (Equation 1) below, diffusion occurs from an area of ​​high concentration to an area of ​​low concentration, and the greater the concentration gradient, the greater the amount of diffusion.

[0045]

number

[0046] J B Flux: The number of elements passing through a unit area per unit time D B : Diffusion coefficient of B atoms C: Concentration x: direction dC B / dx: density change rate in the x direction

[0047] That is, according to the above formula (1), the method of increasing the amount of Fe diffusion or flux in the base steel sheet within a short time during hot forming becomes possible as the Fe concentration gradient formed between the aluminum-coated layer and the base steel sheet in the aluminized steel sheet becomes larger. The steeper the Fe concentration gradient, the more Fe diffusion can be increased within a short time during hot forming, and more alloy phases with high Fe content are formed in the diffusion layer and alloy layer of the hot-formed part, reducing the proportion of brittle alloy phases.

[0048] In the results of GDOES analysis from the surface of the aluminized layer to the interior of the base steel sheet in the thickness direction, it is effective that the Fe concentration gradient in the section of the aluminized layer where the Fe content is 40 to 90 wt.% is 13 to 26 wt.% / μm, preferably 13.5 to 23 wt.% / μm. This will be explained in detail below with reference to Figure 1. Figure 1 is a graph showing the depth profile of the Fe concentration measured by GDOES (Glow Discharge Optical Emission Spectroscopy) from the surface of the plated steel sheet of Invention Example 2, which was produced in the Examples described later.

[0049] As shown in Figure 1, when the Fe concentration gradient was analyzed using a GDOES depth profile from the surface of the aluminized layer to the interior of the base steel sheet in the thickness direction, the Fe content gradient observed from the surface of the aluminized layer gradually increased, then gradually decreased again. This confirmed the formation of an alloyed layer at the interface with the base steel sheet, and confirmed that the thickness of this alloyed layer was approximately 2 to 6 μm. In other words, an inflection point (see arrows in Figure 1) where the Fe gradient in the alloyed layer and the Fe gradient within the aluminized layer change was observed on the GDOES profile. This inflection point can be considered an indicator for distinguishing between the AlSi coating layer and the alloyed layer (AlSiFe) within the aluminized layer, and the alloyed layer can be distinguished by this inflection point. In the Fe content (wt.%) profile graph on the GDOES profile, the inflection point where the Fe gradient changes is located mainly between 30 wt.% and 60 wt.%. Therefore, in order to define the Fe concentration gradient in more detail, the Fe concentration gradient value is limited to the range of 40 wt.% to 90 wt.%, and the concentration gradient value in the 40 to 90 wt.% Fe section is measured.

[0050] If the Fe concentration gradient in the above-mentioned aluminized layer in the range of 40 to 90 wt.% Fe content is less than 13 wt.% / μm, this is the characteristic of a normal aluminized steel sheet and is not an aluminized steel sheet with improved die adhesion that is the objective of the present invention. If it exceeds 26 wt.% / μm, the thickness of the alloy layer in the aluminized steel sheet is too thin, making it easy for the Fe in the base steel to destroy the thin alloy layer during hot forming of the aluminized steel sheet, causing rapid diffusion. This results in the formation of a non-uniform alloy layer and potentially reducing corrosion resistance.

[0051] It is effective that the Fe content of the Al plating layer is 8 to 24 wt.%, preferably 8.0 to 24.0 wt.%, and more preferably 8.5 to 23 wt.%.

[0052] As an example of a method for measuring the Fe content of the Al plating layer, a circular piece of the plating layer with a radius of 25 mm is dissolved and measured using ICP (Inductively Coupled Plasma). Specifically, a circular piece of the Al plating layer with a radius of 25 mm is primarily dissolved in a 20% NaOH solution, and then the entire plating layer is secondarily dissolved in a solution containing HCl and an inhibitor in a ratio of 1:3. The combined solution of the primary and secondary solutions is then analyzed using ICP to measure the Al, Si, and Fe contents. In this case, the Fe content is effective only when it is 8 to 24 wt.%, more preferably 8.5 to 23 wt.%.

[0053] In the GDOES Fe profile, the Fe content can be determined by integrating the Fe profile, but the GDOES profile indicates the Fe content at the corresponding position in the thickness direction, making it somewhat difficult to observe the content of components contained in the entire coating layer.In contrast, ICP analysis is a component analysis of a sample obtained by dissolving the coating layer, and can determine the Fe content contained in the entire coating layer, including the Al alloy layer, so the content measurement data is highly reliable.

[0054] On the other hand, if the Fe content of the aluminized layer is less than 8 wt.%, this is the characteristic of a normal aluminized steel sheet and not the aluminized steel sheet with improved die adhesion that is the objective of the present invention. If the Fe content exceeds 24 wt.%, the greater the difference between the Fe content in the coating layer and the Fe content in the base steel sheet, the more Fe diffusion can be promoted during hot forming. However, if the Fe content in the coating layer exceeds 24 wt.%, there is no longer a large difference between the Fe content in the base steel sheet and the Fe content in the coating layer, which may cause a problem of slowing down the rate at which Fe in the base steel diffuses into the aluminized steel sheet during hot forming.

[0055] The Al plating layer may be any Al-based plating widely used in hot-forming coated steel sheets by ordinary engineers in the technical field of hot-forming coated steel sheets to which the present invention pertains, and the content and type of components thereof are not particularly limited. This may include not only pure Al plating, but also plating formed by incorporating a portion of Si in Al, plating formed by incorporating a portion of Zn in Al, and plating formed by incorporating a portion of Si, Mg, Zn, Fe, etc.

[0056] It is effective that the thickness of the Al plating layer is 3 μm to 30 μm, preferably 3.0 to 30.0 μm, and more preferably 4 μm to 26 μm.

[0057] If the thickness of the aluminized layer is less than 3 μm, the thickness of the coating layer is too thin, resulting in a large amount of uncoated area on the surface of the aluminized steel sheet, which may affect the corrosion resistance of the part after hot forming. If the thickness exceeds 30 μm, even if the Fe in the base steel rapidly diffuses during hot forming, the thickness of the coating layer is so thick that the proportion of brittle phases is high, which may make it difficult to improve die adhesion.

[0058] On the other hand, the base steel sheet is not particularly limited as long as it is a steel sheet that can be used for hot forming. There are no particular limitations on the steel type or alloy composition system, including not only manufacturing process classifications such as hot-rolled steel sheet and cold-rolled steel sheet, but also dual-phase steel (DP), multi-phase steel, TRIP steel, TWIP steel, etc.

[0059] A typical specific example is 22MnB5 steel, which may be a steel containing, in wt. % (weight %), carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.0%, silicon (Si): 0.02 to 0.30%, boron (B): 5 to 45 ppm, with the remainder being unavoidable impurities and iron (Fe).

[0060] The base steel sheet has a chemical composition, in wt.%, of carbon (C): 0.02 to 0.6%, silicon (Si): 0.001 to 2%, aluminum (Al): 0.001 to 1%, manganese (Mn): 0.1 to 4%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, titanium (Ti): 0 to 0.1%, boron (B): 0.0001 to 0.01%, copper (Cu): 0 to 1.00%, molybdenum (Mo): 0 to 1.00%, chromium (Cr): 0 to 1.00%, nickel (Ni): 0.0001 to 0.01%, and chromium (Cr): 0.0001 to 0.01%. The steel may be composed of nickel (Ni): 0-1.00%, vanadium (V): 0-1.00%, calcium (Ca): 0-0.01%, niobium (Nb): 0-0.1%, tin (Sn): 0-1%, tungsten (W): 0-1%, antimony (Sb): 0-1%, magnesium (Mg): 0-0.1%, cobalt (Co): 0-1%, arsenic (As): 0-1%, zirconium (Zr): 0-1%, bismuth (Bi): 0-1%, rare earth metals (REM): 0-0.3%, with the remainder being Fe and impurities.

[0061] Next, a detailed description will be given of one embodiment of a method for manufacturing an aluminum-plated steel sheet according to the present invention. The method for manufacturing an aluminum-plated steel sheet includes the steps of immersing a base steel sheet in a coating bath to cause a coating solution to adhere to the surface of the base steel sheet, adjusting the coating weight on the surface of the base steel sheet, and transporting the base steel sheet with the coating solution adhered thereto to a cooling means. Each step will be described in detail below.

[0062] First, the base steel sheet is immersed in a plating bath to allow the plating solution to adhere to the surface of the base steel sheet.

[0063] As described above, the base steel sheet is not particularly limited in terms of its intended use or steel type, as long as it can be used as a hot-formed member. Before immersing the base steel sheet in the coating bath, the base steel sheet can be heated to a certain temperature (drawing temperature). In this case, a drawing temperature of 600 to 680°C is effective. A drawing temperature of 600.0 to 680.0°C is more effective. Heating the base steel sheet to the drawing temperature prevents problems such as rapid solidification of molten aluminum on the surface of the steel sheet, resulting in reduced fluidity and uneven coating weight or uncoated areas. However, excessive heating may actually promote the dissolution of the steel sheet, accelerating the generation of dross.

[0064] The plating bath may be an Al-based plating bath, and the Al-based plating bath can be applied to the present invention without any restrictions as long as it is for forming an Al plating layer and is applicable to plated steel sheets for hot forming.

[0065] As an example, the composition of the plating bath may include 6 to 12 wt.% silicon (Si), 1 to 4 wt.% iron (Fe), and the remainder being aluminum (Al) and unavoidable impurities. As another example, the composition of the plating bath may include 5 to 13 wt.% silicon (Si), 15 to 35 wt.% zinc (Zn), 0.01 to 5 wt.% magnesium (Mg), 0 to 5 wt.% iron (Fe), 0.01 to 5 wt.% manganese (Mn), and 0.01 to 7 wt.% chromium (Cr), and the remainder being aluminum (Al) and unavoidable impurities.

[0066] The temperature of the plating bath is effectively 600 to 680° C. More preferably, it is more effectively 600.0 to 680.0° C. If the temperature of the plating bath is too low, the fluidity of the plating solution in the plating bath may decrease, whereas if the temperature is too high, the generation of dross in the plating bath may increase.

[0067] Next, the coating weight of the base steel sheet is adjusted. The means or method for adjusting the coating weight is not particularly limited, and can be a method commonly used in the technical field to which the present invention pertains. For example, an air knife (A / K) is used.

[0068] The above plating coverage is 8 to 80 g / m on one side. 2 It is effective that the thickness of the plating layer is 3 to 30 μm. Preferably, the plating coating weight is 8.0 to 80.0 g / m on one side. 2 It is more effective for the thickness of the plating layer to be 3.0 to 30.0 μm. The plating weight of Al plating can usually be converted to the plating weight when the thickness of the plating layer is multiplied by 2.7, and when a plating layer thickness of 3 to 30 μm is converted to the plating weight, it is about 8 to 80 g / m 2 The more preferable thickness is 4 to 26 μm, and the plating weight is 10 to 70 g / m 2 is.

[0069] The base steel sheet with the coating weight adjusted is transferred to a cooling means for cooling. When the base steel sheet with the coating solution adhered thereto enters the cooling means, rapid cooling begins, which stops the reaction of the alloy layer in the Al coating layer. An example of the cooling means is a cooling tower.

[0070] In the above production method, it is effective that the K value defined by the following formula 2 is 200 to 400.

[0071] The above equation 2 can be calculated as follows:

[0072] (Formula 2)K=10 6 *(a*d) / (b*c*e)

[0073] where a: Si content of the coating bath (wt.%), b: line speed (mpm), c: A / K spacing (mm), d: A / K pressure (kPa), A / K height (mm). Line speed refers to the speed at which the steel sheet is transported through the coating bath, air knife (A / K) spacing for adjusting the coating weight refers to the distance between the steel sheet and the air knife, and A / K height refers to the distance from the coating bath to the A / K.

[0074] The above formula (2) takes into consideration the respective characteristics of various variables that determine the properties of a coating layer in order to manufacture a coated steel sheet that improves the anti-sticking properties of a mold, and is technically significant in that it takes into consideration factors that affect the manufacture of a coated steel sheet and their correlations in order to improve the anti-sticking properties of a mold.

[0075] In order to manufacture a steel sheet for improving the anti-sticking properties of a mold, various process factors such as the Si content of the coating bath, the coating bath temperature, the drawing temperature, the line speed, the A / K interval, the A / K pressure, the A / K height, and the cooling rate of the cooling tower must be considered, and the Si content of the coating bath, the line speed, the A / K interval, the A / K pressure, and the A / K height must be optimized. This is derived using the above formula (2), and when the K value of the above formula (2) is 200 to 400, the anti-sticking properties of the mold can be improved. Preferably, the K value is 200.0 to 400.0, which is even more effective.

[0076] Specifically, when the K value is less than 200, it is difficult to improve the adhesion resistance because the A / K pressure is not high enough to improve the mold adhesion resistance, or the line speed, A / K interval, and A / K height are low. When the K value is more than 400, it is difficult to improve the adhesion resistance because the A / K pressure is too high, or the line speed, A / K interval, and A / K height are higher than the appropriate values.

[0077] The base steel sheet may be prepared in various ways before being immersed in a coating bath. For example, the base steel sheet may be manufactured by heating a steel slab, followed by hot rolling, coiling, cold rolling, annealing, etc. The specific process conditions required for the heating, hot rolling, coiling, cold rolling, annealing, etc. of the steel slab are not particularly limited, as they may vary depending on the properties required of the base steel sheet.

[0078] Next, one embodiment of the hot-formed member of the present invention will be described in detail.

[0079] The hot-formed member of the present invention includes a base steel, an Fe—Al alloy plating layer formed on the base steel, and a diffusion layer formed between the Fe—Al alloy plating layer and the base steel.

[0080] The Fe-Al alloy plating layer and diffusion layer refer to the alloy formed by interdiffusion and reaction between the plating layer components and the base iron components during the heating process of aluminum-plated steel sheet. The diffusion layer and Fe-Al alloy plating layer are referred to as the portion with a high Fe content, consisting mainly of α-Fe, Fe3Al, and FeAl, while the remaining portion with a relatively high Al content, consisting mainly of Fe2Al5 and FeAl3, is referred to as the Fe-Al alloy plating layer. The diffusion layer and Fe-Al alloy plating layer can be optically confirmed by Nital etching the cross section of a hot-formed part. The diffusion layer is the first layer located directly above the base iron transformed into martensite, and the subsequent layers can be considered to be the Fe-Al alloy plating layer.

[0081] It is effective that the hot-formed member has a hard layer ratio of 50 to 75% as defined by the following formula (3), and it is more effective that the hard layer ratio is preferably 50.0 to 75.0%.

[0082]

number

[0083] If the hard layer ratio exceeds 75%, the heating time during hot forming must be further extended, which may result in a decrease in the productivity of the part-formed material, and therefore it is effective to set the hard layer ratio to 75% or less, which is the lowest possible hard layer ratio within a range that does not decrease productivity with a normal heating time.On the other hand, if the hard layer ratio is less than 50%, this is a hard layer ratio that can normally be achieved in a hot-formed aluminized steel sheet part, and it is difficult to consider the part to be a hot-formed aluminized steel sheet part with improved mold adhesion resistance.

[0084] The base steel of a hot-formed member does not differ significantly in alloy composition from the base steel of the above-mentioned plated steel sheet for hot forming, but there may be differences in microstructure, so it is preferable to distinguish between them. Therefore, in the present invention, the base steel sheet is used in the plated steel sheet for hot forming, while the base steel is used in the hot-formed member. On the other hand, the base steel sheet of the plated steel sheet for hot forming has a ferrite and pearlite structure, but the base steel of the hot-formed member formed through hot forming forms martensite as the main phase, and there is a possibility that bainite will form in part.

[0085] Next, one embodiment of the method for manufacturing a hot-formed member of the present invention will be described in detail. For this purpose, a blank is provided. As an example of the blank, the aluminum-plated steel sheet described above can be used for manufacturing the blank.

[0086] The blank is preferably heated to a temperature above the austenite single-phase region, more specifically, in the Ac3 to 975°C temperature range. If the heating temperature is below the Ac3 temperature, the presence of untransformed ferrite in the two-phase region makes it difficult to ensure strength and crash resistance. On the other hand, if the heating temperature exceeds 975°C, excessive oxides are formed on the surface of the part, making it difficult to ensure spot weldability and increasing manufacturing costs for maintaining the high temperature. The blank thus heated is preferably held at the above temperature range for 1 to 1,000 seconds. Holding times of less than 1 second make it difficult to achieve uniform temperature distribution across the entire blank, potentially resulting in material variations at different locations. Holding times of more than 1,000 seconds not only make it difficult to ensure spot weldability due to the excessive oxides formed on the surface, but also increase manufacturing costs for the part.

[0087] The heated blank is transferred to a press where it is formed and cooled. The cooling rate is preferably 20°C / s or more. The cooling rate is more preferably 20.0°C / s or more. If the cooling rate is less than 20°C / s, ferrite phases may be introduced and formed at grain boundaries during cooling, potentially deteriorating physical properties such as strength and impact resistance. The blank transfer, forming, and cooling processes are not particularly limited, and processes commonly used in hot forming processes may be used. [Example]

[0088] 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 to aid in 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 set forth below as well as equivalents thereof.

[0089] (Example) In the examples of the present invention, a conventional 22MnB5 base steel sheet was prepared and immersed in a coating bath containing approximately 9-10 wt.% Si, with the remainder being Al and unavoidable impurities, to produce an aluminum-coated steel sheet. The temperature of the coating bath was 640-670°C, and the drawing temperature of the base steel sheet was 650-680°C.

[0090] In order to produce the above-mentioned plated steel sheet, the Si content in the plating bath, the transport speed (line speed) of the steel sheet, and the thickness of the steel sheet are set to 10 to 80 g / m 2 The coating weight on one side was adjusted using an air knife (A / K) and then transferred to a cooling tower for cooling. The process conditions are shown in Table 1.

[0091] [Table 1]

[0092] The above equation 2 can be calculated as follows:

[0093] (Formula 2)K=10 6 *(a*d) / (b*c*e)

[0094] where a: Si content (wt.%) of plating bath, b: line speed (mpm), c: A / K interval (mm), d: A / K pressure (kPa), A / K height (mm).

[0095] For the aluminum-plated steel sheets produced as described above, the Fe concentration gradient in the Al-plated layer and the alloying components of the Al-plated layer were measured, and the results are also shown in Table 2.

[0096] The Fe concentration gradient was determined in the 40-90 wt.% Fe content range from the surface to the depth direction of the Al plating layer of the manufactured aluminum-plated steel sheet through GDOES analysis. The alloy components were determined by first dissolving the Al plating layer in a 20% NaOH solution, then dissolving the entire plating layer in a solution containing HCl and an inhibitor in a 1:3 ratio (secondary dissolution), and then analyzing the combined solution of the first and second dissolutions by ICP to measure the Al, Si, and Fe contents.

[0097] [Table 2]

[0098] FIG. 1 is a graph showing the Fe concentration profile of the GDOES of Example 2 in Table 2 above.

[0099] The aluminum-plated steel sheets manufactured as shown in Tables 1 and 2 above were evaluated for mold adhesion resistance, and the results are shown in Figs. 2 to 3 and Table 3 below. In particular, photographs evaluating the mold adhesion resistance of Comparative Examples 1 and 4 and Invention Examples 1 and 3 among the above examples are shown in Fig. 2 and Table 3. Fig. 2(a) and (b) are photographs of Comparative Examples 1 and 4, and Fig. 2(c) and (d) are photographs of Invention Examples 1 and 3.

[0100] To confirm the mold burn-in resistance, each test piece was heat-treated at 900°C for 5 minutes to produce a component, and the heat-treated component was evaluated for powdering using the V-bending test method (conditions: 20°, 2R). Burn-in occurs when a large amount of powder peels off from the plating layer during mold pressing and accumulates, and this powder cannot be removed each time. Therefore, mold burn-in resistance can be confirmed by evaluating powdering.

[0101] To evaluate the powdering properties, the heat-treated test pieces were sheared to 60 x 30 mm and then subjected to a V-bending experiment. The test conditions were 20° and 2R, and the weights were measured before and after the experiment, with the results shown in Table 3. Figure 2 shows photographs of some of the test pieces in Table 3 after the V-bending test, and Figure 3 shows a graph of the amount of powder generated (average weight loss) for each test piece in Table 2.

[0102] [Table 3]

[0103] As can be seen from the results in Table 3 and Figures 2 and 3, the aluminum-plated steel sheets corresponding to the invention examples that satisfy the ranges set forth in the present invention significantly reduced the generation of plating powder during hot forming.

[0104] Meanwhile, for each heat-treated test piece in Table 2, it was confirmed that the alloyed plating layer formed a diffusion layer and an Fe-Al alloy plating layer, and the hard layer ratio was measured. The results are shown in Figure 4. To measure the hard layer ratio, the cross section of each test piece was optically observed and the thickness of the Fe-Al alloy plating layer and the diffusion layer were measured. The hard layer ratio was calculated using the following equation (3).

[0105]

number

[0106] The results shown in FIG. 4 above show that the hard layer ratio is higher in the comparative example than in the invention example.

[0107] As described above, in the inventive examples according to the present invention, not only was a large amount of Fe present in the coating layer of the plated steel sheet, but the Fe gradient in the alloy layer was large, allowing a large amount of Fe to diffuse. As a result, the ratio of the diffusion layer after heat treatment was high and the ratio of the hard layer was low, resulting in a reduction in the amount of powdering.

Claims

1. The steel sheet includes a base steel sheet and an Al plating layer formed on the base steel sheet, GDOES analysis of the Al plating layer from the surface to the thickness direction showed that the Fe concentration gradient in the Al plating layer in the section where the Fe content was 40 to 90 wt. % was 13 to 26 wt. % / μm, The aluminum-plated steel sheet has an Fe content of 8 to 24 wt. % in the aluminum-plated layer.

2. 2. The aluminum-plated steel sheet according to claim 1, wherein the Al plating layer includes an alloy layer formed at the interface with the base steel sheet, and the alloy layer has a thickness of 2 to 6 μm.

3. 3. The aluminum-plated steel sheet according to claim 1, wherein the Fe concentration gradient is 13.5 to 23 wt. %.

4. The aluminum-plated steel sheet according to any one of claims 1 to 3, wherein the Al-plated layer has an Fe content of 8.5 to 23 wt.%.

5. The aluminum-plated steel sheet according to any one of claims 1 to 4, wherein the Al plating layer has a thickness of 3 to 30 µm.

6. The base steel sheet contains, in wt. %, 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 to 1.00%, V:

6. The aluminum-plated steel sheet according to claim 1, wherein the aluminum-plated steel sheet comprises 0 to 1.00%, Ca: 0 to 0.01%, Nb: 0 to 0.1%, Sn: 0 to 1%, W: 0 to 1%, Sb: 0 to 1%, Mg: 0 to 0.1%, Co: 0 to 1%, As: 0 to 1%, Zr: 0 to 1%, Bi: 0 to 1%, REM: 0 to 0.3%, the balance being Fe and impurities.

7. immersing the base steel sheet in an aluminum (Al) plating bath containing silicon (Si) to deposit a plating solution on the surface of the base steel sheet; adjusting the coating weight of the surface of the base steel sheet using an air knife (A / K); and transferring the base steel sheet having the plating solution adhered thereto to a cooling means and cooling it, wherein the K value of the following (Equation 2) is 200 to 400. (Equation 2) K = 10 ６ *(a*d) / (b*c*e) Here, a: Si content (wt.%) of the plating bath, b: line speed (mpm), c: A / K interval (mm), d: A / K pressure (kPa), A / K height (mm).

8. The method for producing an aluminum-plated steel sheet according to claim 7, wherein the base steel sheet is drawn into the plating bath at a temperature of 600 to 680°C.

9. The plating coating weight is 8 to 80 g / m ２ The method for producing an aluminum-plated steel sheet according to claim 7 or 8,

10. 10. The method for producing an aluminum-plated steel sheet according to any one of claims 7 to 9, wherein the plating bath contains 6 to 12 wt. % silicon (Si), 1 to 4 wt. % iron (Fe), and the remainder being aluminum (Al) and inevitable impurities.

11. 10. The method for producing an aluminum-plated steel sheet according to any one of claims 7 to 9, wherein the coating bath contains 5 to 13 wt.% silicon (Si), 15 to 35 wt.% zinc (Zn), 0.01 to 5 wt.% magnesium (Mg), 0 to 5 wt.% iron (Fe), 0.01 to 5 wt.% manganese (Mn), 0.01 to 5 wt.% chromium (Cr), and the remainder being aluminum (Al) and inevitable impurities.

12. The method for producing an aluminum-plated steel sheet according to any one of claims 7 to 11, wherein the temperature of the plating bath is 600 to 680°C.

13. The steel sheet includes a base steel, an Fe-Al alloy plating layer formed on the base steel, and a diffusion layer formed between the base steel and the Fe-Al alloy plating layer, A hot-formed member having a hard layer ratio defined by the following (Equation 3) of 50 to 75%. [Equation 1]

14. A step of manufacturing a blank using the aluminum-plated steel sheet according to any one of claims 1 to 6; heating the blank; forming the heated blank in a die and cooling it.

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