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 manufactured using the same, and a method for manufacturing 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 manufactured using the same, and a method for manufacturing the same. [Background technology]

[0002] Recently, efforts have been made to reduce the weight of automobiles to improve fuel efficiency. To achieve this, the thickness of steel materials can be reduced, but reducing the thickness can cause problems with the stability of automobiles, so the strength of the steel must be improved. For this reason, there is a continuous demand for high-strength steel materials, and various types of steel materials have been developed. However, because such steel materials have high strength, they suffer from 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 a steel material at a high temperature (800°C or higher) where it is easy to process, and then rapidly cooling it to a low temperature while press-forming it in a mold. This forms a low-temperature structure such as martensite in the steel material, thereby increasing the strength of the final product. This method can minimize workability issues when manufacturing high-strength components.

[0004] Hot forming has recently become increasingly popular because it not only makes it possible to easily form complex shapes but also ensures high strength in the manufactured parts (hot-formed components). In particular, the use of plated steel material with aluminum, zinc, etc. on the surface of the steel material ensures corrosion resistance. As an example, Patent Document 1 discloses the use of aluminum-plated steel sheet in a hot forming process.

[0005] The hot-formed member using the aluminum-plated steel material can ensure corrosion resistance by performing aluminum plating, but there is a problem that roll seizure may occur as the high-temperature aluminum-plated steel material moves on rolls during die forming.

[0006] When roll seizure occurs during the process of being transferred to the mold, the high-temperature aluminum-plated steel material gradually tilts in the direction of movement as it is transferred to the mold, causing it to move at an angle rather than in a straight line to the mold. This stops the process, which should be automatic up to mold formation, causing a major problem that reduces productivity. The coating layer of the high-temperature aluminum-plated steel material traveling on the rolls can exist in a molten aluminum state. As it moves while in contact with the roll in this state, the molten aluminum coating layer adheres to the roll, causing roll seizure. As roll seizure is an inevitable problem, much research is being conducted to solve this problem.

[0007] The problems and inconveniences caused by roll baking result in a decrease in product quality and productivity, and there is a constant demand for improvements therein. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] U.S. Patent No. 6,296,805 Summary of the Invention [Problem to be solved by the invention]

[0009] One aspect of the present invention relates to an aluminum-plated steel sheet that can be used for hot forming and the like, and more particularly to an aluminum-plated steel sheet having excellent roll seizure resistance, a hot-formed member produced using the same, and methods for producing the same.

[0010] 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 difficulty in understanding further object of the present invention from the entire content of the specification of the present invention. [Means for solving the problem]

[0011] One aspect 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.

[0012] Another aspect of the present invention is a method for manufacturing a steel sheet by a method comprising the steps of: immersing a base steel sheet in an aluminum (Al) plating bath containing silicon (Si) to allow a plating solution to adhere to a surface of the base steel sheet; adjusting the coating weight on the surface of the base steel sheet using an air knife (A / K); and The present invention relates to a method for producing an aluminum-plated steel sheet, which includes a step of transferring a base steel sheet having a plating solution attached thereto to a cooling means and cooling the same, and satisfies the condition of the following (Equation 2). (Formula 2)K=10 6 *(a*d) / (b*c*e) where a: Si content in plating bath (wt.%), b: line speed (mpm), c: A / K spacing (mm), d: A / K pressure (kPa), A / K height (mm).

[0013] Another aspect of the present invention is a steel sheet comprising a base iron, an Fe-Al alloy plating layer formed on the base iron, and a diffusion layer formed between the base iron and the Fe-Al alloy plating layer, In the GDOES analysis graph of the Fe and Al contents observed from the surface to the thickness direction of the Fe-Al alloy plating layer, Fe-humps and Al-humps are present. The hot-formed member satisfies the following relational expression 1. [Equation 1] Maximum value of Fe in Fe-hump (wt.%) > Maximum value of Al in Al-hump (wt.%) The maximum value (wt.%) of Fe in the Fe-hump may be 50 wt.% or more. The maximum value (wt.%) of Al in the Al-hump may be 50 wt.% or less. The hot-formed member can satisfy the following relational expression 2. [Equation 2] The minimum Fe content (wt.%) observed between the maximum value of the Fe hump and the diffusion layer > the maximum value of the Al hump (wt.%) There can be two or more Al-humps. The hot-formed member can satisfy the following relational expression 3. [Equation 3]

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[0014] The above base steel contains, in wt.%, 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 remainder may contain Fe and impurities.

[0015] Yet another aspect of the present invention is a method for manufacturing a blank using the aluminum-plated steel sheet; heating the blank; and The present invention relates to a method for manufacturing a hot-formed part, which includes forming a heated blank in a mold and cooling it. [Effects of the Invention]

[0016] The hot-formed part of the present invention can reduce roll contamination caused by molten aluminum on the surface layer due to slow alloying of the coating layer during transport through a heating furnace, thereby ensuring the quality of the hot-formed part and improving work efficiency.

[0017] The various yet significant advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0018] [Figure 1] 1 shows the results of a GDOES Fe Depth profile for the aluminum-plated steel sheet of Inventive Example 2 among the Examples. [Figure 2] 1 shows the profile results of Fe and Al obtained by GDOES analysis of the hot-formed member of Example 3 of the present invention. [Figure 3] 1 is a photograph showing an observation of a test piece after a V-bending experiment in an example. [Figure 4] 1 shows the results of measuring the amount of powder generated after a V-bending experiment in an example. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be described in detail below.

[0020] The terms used herein are for the purpose of describing the present invention and are not intended to limit the present invention. Furthermore, as used herein, the singular forms also include the plural forms unless the relevant definition clearly indicates otherwise.

[0021] The meaning of "comprises" as used in the specification is to specify features and does not exclude the presence or addition of other features.

[0022] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content.

[0023] In the present invention, the term "member" refers to a part or material for a part manufactured by hot forming. Note that the term "steel plate" refers to a steel plate or steel material that is a material before hot forming, and includes forms such as coils.

[0024] When hot forming is performed using plated steel, the slower the alloying of the coating layer, the more the molten coating layer adheres to the roll, causing roll seizure. Therefore, it has become recognized that it is important to develop a material that can maximize the speed of alloying of the coating layer during hot forming.

[0025] When aluminum-plated steel is heated, Fe from the base steel diffuses into the aluminum coating, forming an Fe-Al alloy coating layer. This diffusion layer and the Fe-Al alloy coating layer form between the base steel and the Fe-Al alloy coating layer. The diffusion layer and the Fe-Al alloy coating layer form an Fe-Al alloy as Fe diffuses from the base steel, starting with high-Fe α-Fe, followed by Fe3Al → FeAl → Fe2Al5 → FeAl3, depending on the Fe content. The portion containing high-Fe α-Fe and Fe3Al is called the diffusion layer, while the remaining portion containing relatively high-Al content FeAl, Fe2Al5, and FeAl3 is called the Fe-Al alloy coating layer. Aluminum has a melting point of 660°C, and when it travels through a 900°C furnace on rolls, the aluminum adheres to the rolls. However, if Fe rapidly diffuses from the furnace in the form of FeAl3 to the surface in a short time, the melting point of FeAl3 is 1160°C, delaying roll seizure. Therefore, it is important to develop materials that can maximize the alloying rate of Fe.

[0026] Therefore, in order to alleviate the roll seizure problem, the present inventors have conducted in-depth research into a method for increasing the diffusion rate of Fe in the alloying of a coating layer during hot forming of an aluminum-plated steel sheet, and have arrived at the present invention.

[0027] First, an example of an aluminum-plated steel sheet according to the present invention will be described in detail.

[0028] 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 alloying the base steel sheet and the Al plating layer through a 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.

[0029] Glow Discharge Optical Emission Spectrometry (GDOES) analysis results from the surface of the aluminized layer to the thickness direction of the base steel sheet showed that the Fe concentration gradient in the aluminized layer in the range of 40 to 90 wt.% Fe content was 13 to 26 wt.% / μm.

[0030] As described above, to improve roll seizure, it is necessary to increase the diffusion rate of Fe during the alloying process of the aluminized layer when the workpiece is moved through the heating furnace. To achieve this, it is necessary to sufficiently diffuse Fe into the aluminized layer and perform alloying in a short period of time during the movement through the heating furnace.

[0031] 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. (Formula 1)

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[0032] That is, according to the above formula (1), the method of increasing the amount of Fe diffusion and flux in the base steel sheet within a short time when moving through a heating furnace becomes more feasible as the Fe concentration gradient formed between the aluminized layer and the base steel sheet in the aluminized steel sheet increases. The steeper the Fe concentration gradient, the more Fe can be diffused within a short time when moving through a heating furnace, and Fe can diffuse to the surface layer within a short time in the initial heating stage, thereby forming the FeAl3 phase, which has a high melting point.

[0033] It is effective that, 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, the Fe concentration gradient in the aluminized layer within the 40-90 wt.% Fe content range is 13-26 wt.% / μm, preferably 13.5-23 wt.% / μm. This will be explained in detail below with reference to Figure 1. Figure 1 is a graph showing the Fe concentration depth profile of 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.

[0034] As shown in Figure 1, when the Fe concentration gradient is 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 begins as a gradual gradient, then increases sharply, before gradually decreasing again. This confirms the formation of an alloy layer at the interface with the base steel sheet, and the thickness of this alloy layer can be determined to be approximately 2 to 6 μm. In other words, an inflection point where the Fe gradient in the alloy layer changes from the Fe gradient within the aluminized layer is observed on the GDOES profile. The inflection point on the GDOES profile can be considered an indicator for distinguishing between the AlSi coating layer and the AlSiFe alloy layer, and the alloy layer can be distinguished around the inflection point. Specifically, the Fe content (wt.%) on the GDOES profile and the location of the inflection point where the Fe gradient changes on the profile graph are 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 range is measured.

[0035] If the Fe concentration gradient in the 40 to 90 wt.% Fe content range in the aluminized layer is less than 13 wt.% / μm, the aluminized steel sheet does not have the characteristics of a typical aluminized steel sheet, and does not have the improved roll seizure resistance that is sought in the present invention. However, if the Fe concentration gradient exceeds 26 wt.% / μm, the thickness of the alloy layer in the aluminized steel sheet is so thin that the Fe in the base steel can easily destroy the thin alloy layer and cause rapid diffusion when the aluminized steel sheet is moved through a heating furnace, resulting in the formation of a non-uniform alloy layer and a decrease in corrosion resistance.

[0036] It is effective that the Fe content in the Al plating layer is 8 to 24 wt.%, more preferably 8.5 to 23 wt.%.

[0037] 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 elevated and measured using ICP (Inductively Coupled Plasma). Specifically, the circular piece of the Al plating layer with a radius of 25 mm is first 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 first and second dissolutions is analyzed using ICP to measure the Al, Si, and Fe contents. In this case, it is effective if the Fe content is 8 to 24 wt.%, and more preferably 8.5 to 23 wt.%.

[0038] The Fe content in a GDOES Fe profile can be determined by integrating the Fe profile, but the GDOES profile only shows the Fe content at a given position in the thickness direction, and cannot accurately indicate the content of elements contained in the entire plating layer.In contrast, ICP analysis is a component analysis of a sample in which only the plating layer has been dissolved, and can determine the Fe content contained in the entire plating layer, including the Al alloy layer, so the data is highly reliable.

[0039] On the other hand, if the Fe content of the aluminized layer is less than 8 wt.%, the result is not a roll-basis aluminized steel sheet with the properties of a typical aluminized steel sheet, as is the case with the present invention. However, if the Fe content exceeds 24 wt.%, the difference between the Fe content in the coating layer and the Fe content in the base steel sheet is large enough to promote Fe diffusion during hot forming. However, if the Fe content in the coating layer exceeds 24 wt.%, there is no significant difference between the Fe content in the base steel sheet and the Fe content in the coating layer, which can cause a problem of slow diffusion of Fe from the base steel into the aluminized steel sheet when it is moved through a heating furnace.

[0040] The Al plating layer is not particularly limited in content or type of components, and may be any Al-based plating widely used in hot-forming coated steel sheets by ordinary technicians in the technical field of hot-forming coated steel sheets to which the present invention pertains. This may include not only pure Al plating, but also plating with Al partially containing Si, plating with Al partially containing Zn, and plating with Al partially containing one or more of Si, Mg, Zn, etc.

[0041] It is effective that the thickness of the Al plating layer is 3 μm to 30 μm, and more preferably 4 μm to 26 μm.

[0042] 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 Fe in the base steel rapidly diffuses during transport through the heating furnace, the thickness of the coating layer is so thick that molten Al is likely to be present on the surface, making it difficult to alleviate roll seizure.

[0043] Meanwhile, the base steel sheet is not particularly limited as long as it can be used for hot forming. There are no particular limitations on the manufacturing process classification such as hot-rolled steel sheet or cold-rolled steel sheet, as well as on the steel type and alloy composition such as dual phase steel (DP), multi-phase steel, TRIP steel, TWIP steel, etc.

[0044] A typical example is 22MnB5 steel, which may contain, in wt.%, 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).

[0045] Next, a detailed description will be given of an example of a method for manufacturing an aluminum-plated steel sheet according to one embodiment of 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 transferring the base steel sheet with the coating solution adhered thereto to a cooling means. Each step will be described in detail below.

[0046] First, the base steel sheet is immersed in a plating bath to adhere the plating solution to the surface of the base steel sheet.

[0047] As described above, the base steel sheet is not particularly limited in terms of its target 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 at a certain temperature (drawing temperature). A drawing temperature of 620 to 680°C is effective. By heating the base steel sheet at the drawing temperature, the molten aluminum rapidly solidifies on the surface of the steel sheet, reducing fluidity and preventing problems such as deviations in coating weight or missing coating. However, excessive heating may actually promote the melting of the steel sheet, accelerating the generation of dross.

[0048] The plating bath may be an Al-based plating bath, and the Al-based plating bath is for forming an Al plating layer, and as long as it is a plating that can be applied to a plated steel sheet for hot forming, it can be applied to the present invention without any restrictions. As a preferred example, the plating bath composition includes 6 to 12 wt% silicon (Si), 1 to 4 wt% iron (Fe), and the remainder aluminum (Al) and inevitable impurities.

[0049] The temperature of the plating bath is effectively 630 to 680° C. If the temperature of the plating bath is too low, the fluidity of the plating solution in the plating bath decreases, whereas if the temperature is too high, the generation of dross in the plating bath may increase.

[0050] Next, the coating weight of the base steel sheet is adjusted. The means and method for adjusting the coating weight are not particularly limited, and may be a method commonly used in the technical field to which the present invention pertains. For example, an air knife (A / K) may be used.

[0051] The above coating weight is 8 to 80 g / m on one side. 2 The effective thickness of the plating layer is 3 to 30 μm. The coating weight of Al plating can usually be calculated by multiplying the coating layer thickness by 2.7, and when converting a coating layer thickness of 3 to 30 μm into a coating weight, it is approximately 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.

[0052] 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 may stop the reaction of the alloy layer in the Al coating layer. One example of the cooling means is a cooling tower.

[0053] In the above production method, it is effective that the K value defined by the following formula 2 is 200 to 400.

[0054] Equation 2 above can be calculated as follows: (Formula 2)K=10 6 *(a*d) / (b*c*e) 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.

[0055] The above (Equation 2) takes into consideration the respective characteristics of various variables that determine the properties of the coating layer in order to produce a coated steel sheet that has improved roll seizure resistance, and is technically significant in that it takes into consideration factors that affect the production of a coated steel sheet and their correlations in order to improve roll seizure resistance.

[0056] In order to manufacture steel sheet with improved roll seizure resistance, 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 tower cooling rate must be taken into consideration. As a result, it is necessary to optimize the Si content of the coating bath, the line speed, the A / K interval, the A / K pressure, and the A / K height. This is derived using the above (Equation 2). When the K value in the above (Equation 2) is 200 to 400, roll seizure resistance can be improved.

[0057] Specifically, when the K value is less than 200, the A / K pressure is not high enough to improve roll adhesion, or the line speed, A / K interval, and A / K height are low, making it difficult to improve roll adhesion. When the K value exceeds 400, the A / K pressure is too high, or the line speed, A / K interval, and A / K height are higher than the appropriate values, making it difficult to improve roll adhesion.

[0058] 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. Here, the specific process conditions required for the heating, hot rolling, coiling, cold rolling, annealing, etc. of the steel slab may vary depending on the properties required of the base steel sheet, and are not particularly limited.

[0059] Next, an embodiment of the hot-formed member of the present invention will be described in detail.

[0060] 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.

[0061] The Fe-Al alloy plating layer and diffusion layer are formed by the interdiffusion and reaction of the plating layer components with the base iron components during the heating process of aluminum-plated steel sheet. The diffusion layer refers to the portion of the diffusion layer where high-Fe content α-Fe and Fe3Al are abundant, while the remaining portion where relatively high-Al content FeAl, Fe2Al5, and FeAl3 are abundant is called 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 Fe-Al alloy layers.

[0062] The compositions of the base iron and base steel sheet are not particularly limited, and any composition usable as a hot-formed member is sufficient. In a preferred example, the base iron contains, in wt.%, 0.02 to 0.6% C, 0.001 to 2% Si, 0.001 to 1% Al, 0.1 to 4% Mn, 0.05% or less P, 0.02% or less S, 0.02% or less N, 0 to 0.1% Ti, 0.0001 to 0.01% B, 0 to 1.00% Cu, 0 to 1.00% Mo, and 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 remainder may contain Fe and impurities.

[0063] The Fe-Al alloy plating layer refers to an alloyed plating layer formed by alloying the aluminum-based plating layer of a plated steel sheet for hot forming having an aluminum-based plating layer with the base material during the hot forming process. The aluminum-based plating layer refers to a layer containing aluminum (Al) as a main component and a portion (up to 15 wt %) of silicon (Si) to prevent excessive alloying of the aluminum with the Fe in the steel during plating. Other components may also be included as necessary.

[0064] On the other hand, the diffusion layer refers to a layer formed by bonding Fe in the base steel sheet with part of Al, which is the main component of the aluminum-based plating layer.

[0065] In the hot-formed part, in a Glow Discharge Optical Emission Spectrometry (GDOES) analysis graph of the Fe and Al contents observed from the surface of the Fe-Al alloy plating layer in the thickness direction, an Fe hump and an Al hump exist, and the following Relational Formula 1 can be satisfied. [Equation 1] Maximum value of Fe in Fe-hump (wt.%) > Maximum value of Al in Al-hump (wt.%)

[0066] Hereinafter, a detailed description will be given with reference to FIG. 2. FIG. 2 is a graph showing a GDOES analysis performed on the surface of the hot-formed part of Example 3 of the following Examples. From FIG. 2, it can be seen that one Fe hump and two to three Al humps are present. The Fe hump can be observed between the Al humps.

[0067] The Fe-Al alloy plating layer of the hot-formed part is divided into 1 to 4 layers, any one of which may be an AlFeSi layer. In the present invention, the AlFeSi layer may be modified to have excellent roll seizure properties. In particular, the higher the Fe content, the higher the melting point of the alloy layer formed. The AlFeSi layer with a high Fe content can delay roll seizure when the aluminum hot-formed part is transported through a heating furnace.

[0068] To increase the melting point of the Fe-Al alloy coating layer, it is easy to understand by referring to the Fe-Al binary phase diagram. In the Fe-Al binary phase diagram, as the Fe content in Al increases, the melting point increases from 660°C to 1538°C, and when the Al and Fe content is 1:1, the melting point is around 1100°C. However, when the Fe content in Al increases from 0 wt.% to 50 wt.%, there is a section where the melting point increases sharply. In other words, the melting point may easily decrease due to changes in the Fe content depending on the atmosphere in the heating furnace, which may cause roll seizure.

[0069] Therefore, in the Fe-Al alloy coating layer, the maximum Fe value of the Fe-hump must be higher than the maximum Al value of the Al-hump. In other words, there is a portion where the Fe-hump is higher than the Al-hump, which indicates that the diffusion rate of Fe is fast when the coated steel sheet moves through the heating furnace. Therefore, the hot-formed member of the present invention can stably form an Fe-Al alloy coating layer with a high melting point.

[0070] On the other hand, the maximum value (wt.%) of Fe in the Fe-hump may be 50 wt.% or more, and the maximum value of Al in the Al-hump may be 50 wt.% or less.

[0071] If the maximum value of Fe in the Fe-hump is less than 50 wt.%, an Fe-Al alloy plating layer in a range where the melting point changes suddenly may be formed as described above, which may cause roll seizure problems. Similarly, if the maximum value of Al in the Al-hump is more than 50 wt.%, an Fe-Al alloy plating layer with a low melting point may be formed, which may cause roll seizure.

[0072] Meanwhile, the hot-formed member can satisfy the following relational expression 2 or 3 with respect to the Fe and Al contents measured in the depth direction in the GDOES analysis graph. [Equation 2] The minimum Fe content (wt.%) observed between the maximum value of the Fe hump and the diffusion layer > the maximum value of the Al hump (wt.%) [Equation 3]

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[0073] The above-mentioned Relational Formula 2 and Relational Formula 3 can be considered in conjunction with the above-mentioned Fe-Al Binary Phase Diagram. When the minimum value of the Fe content observed between the maximum value of the Fe hump and the diffusion layer is greater than the maximum value of the Al hump in the Fe-Al alloy coating layer, roll seizure can be sufficiently delayed, thereby bringing about favorable results.

[0074] The above-mentioned relational expression 3 is a relaxed condition of relational expression 2, and it can delay roll seizure even when the minimum Fe content observed between the maximum value of the Fe hump and the diffusion layer is equal to or greater than the average value of the maximum and minimum values ​​of the Al hump in the Fe-Al alloy coating layer.

[0075] As explained above in the Fe-Al Binary Phase Diagram, when the Fe content increases from 0 wt.% to 50 wt.%, the melting point of the Fe-Al alloy layer increases rapidly. However, if the Fe content is at least equal to or greater than the average of the maximum and minimum values ​​of the Al hump in the Fe-Al alloy plating layer, the melting point of the alloy plating layer can be maintained at 1000°C or higher, which is higher than the temperature condition of the heating furnace (850 to 950°C).

[0076] Meanwhile, in the GDOES analysis graph, there may be two or more Al-humps, and the Fe-humps may be observed between the Al-humps in the thickness direction.

[0077] Meanwhile, the thickness of the diffusion layer may be 3 to 10 μm, and the thickness of the diffusion layer and the Fe—Al alloy plating layer may be 10 to 30 μm.

[0078] Next, an embodiment of the method for producing a hot-formed part according to the present invention will be described in detail.

[0079] The manufacturing method can be carried out by providing a base steel sheet having an aluminum-based plating layer, heating the provided base steel sheet, hot forming it, and then cooling it.

[0080] For example, the base steel sheet having the aluminum-based plating layer may be sheared and provided as a blank. The provided blank is preferably heated above the austenite single-phase temperature, more specifically, within a temperature range of Ac3 to 975°C. If the heating temperature is below Ac3, 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 heated blank is preferably held within 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, which can lead to 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 formation of excessive oxides on the surface, but also increase manufacturing costs for the part. The heated blank is then transferred to a press for forming and cooling. The cooling rate is preferably 20°C / s or higher. If the cooling rate is less than 20°C / s, a ferrite phase may be introduced and generated at the grain boundaries during cooling, which may deteriorate physical properties such as strength and impact resistance. The processes of transferring, forming, and cooling the blank are not particularly limited, and the processes commonly used in hot forming methods may be applied as they are. [Example]

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

[0082] (Example) In the present example, 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.

[0083] In order to produce the above-mentioned plated steel sheet, the Si content in the plating bath, the steel sheet transport speed (line speed) and the coating weight on one side (10 to 80 g / m 2 The coating weight was adjusted using an air knife (A / K) and the product was transferred to a cooling tower for cooling. The process conditions are shown in Table 1.

[0084] [Table 1]

[0085] Equation 2 above can be calculated as follows: (Formula 2)K=10 6 *(a*d) / (b*c*e) where a: Si content of plating bath (wt%), b: line speed (mpm), c: A / K interval (mm), d: A / K pressure (kPa), A / K height (mm).

[0086] 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.

[0087] 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 determine the Al, Si, and Fe contents.

[0088] [Table 2]

[0089] FIG. 1 is a graph showing the GDOES Fe concentration profile of Inventive Example 2 in Table 2 above.

[0090] The roll seizure properties of the aluminum-plated steel sheets manufactured as shown in Tables 1 and 2 above were evaluated, and the results are shown in Figs. 3 and 4 and Table 3 below.

[0091] The aluminum-plated steel sheets manufactured as shown in Tables 1 and 2 above were heat-treated at 900°C for 5 minutes, and then GDOES was performed to confirm Fe-hump and Al-hump in the Fe-Al alloy coating layer. Roll seizure is caused by slow alloying due to slow Fe diffusion as the sheet moves through the heating furnace. Therefore, Fe-hump and Al-hump were measured to observe fast alloying through GDOES, and the results are shown in Table 3.

[0092] [Table 3]

[0093] To evaluate the roll sticking characteristics, each test piece was heat-treated at 900°C for 5 minutes, and then the powdering property evaluation was carried out. In the above powdering property evaluation, the test piece heat-treated as above was subjected to a V-bending experiment after shearing at 60×30 mm. At this time, the experimental conditions were carried out at 20° and 2R, and the average powder generation amount was shown in Table 3. In addition, in Figure 3, photographs after the V-bending test results of some test pieces in Table 3 below are shown, and Figure 4 shows the powder generation amount in each test piece of Table 3 in a graph.

[0094] The cause of roll sticking is that when the content of Al with a low melting point increases on the surface due to the slow alloying of the base iron Fe during the movement of the heating furnace, the molten aluminum alloy layer adheres to the roll. However, it is difficult to evaluate this in an actual line.

[0095] Therefore, in order to compare those in which α-Fe and Fe3Al with a high melting point, that is, a high Fe content, are formed in the state where the heat treatment is completed, it is possible to perform a predictive evaluation of roll sticking at room temperature in consideration of these characteristics.

[0096] That is, the faster the Fe diffusion rate during the movement in the heating furnace, the order of the content of the Al alloy plating layer in the plating steel material to be invented is Al < FeAl3 < Fe2Al5 < FeAl < Fe3Al < α-Fe. When it exists in the direction of increasing α-Fe, it means that the alloying rate is high, and it also means that a phase with a high melting point is formed.

[0097] The characteristics of the above phases are that α-Fe and Fe3Al with a relatively high Fe content show ductile characteristics at room temperature, and in addition, FeAl, Fe2Al5, and FeAl3 with a relatively high Al content show brittle characteristics. Therefore, when comparing the powdering generation amount of the members after heat treatment in consideration of the ductile and brittle characteristics of these phases at room temperature, the Fe diffusion rate can be relatively compared, and the powdering property can be evaluated to confirm the roll sticking characteristics.

[0098] As can be seen from the results in Table 3 and Figures 3 and 4, the aluminum-plated steel sheets corresponding to the examples of the present invention, which satisfy the ranges set forth in the present invention, significantly reduced the generation of plating powder during hot forming.

[0099] Therefore, in the inventive examples of the plated steel sheets according to the present invention, not only was a large amount of Fe present in the coating layer, but the Fe gradient in the alloy layer was large, allowing a large amount of Fe to diffuse, resulting in a high diffusion layer ratio and a low hard layer ratio after heat treatment, and a reduced amount of powdering.

Claims

1. 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, In the GDOES analysis graph of the Fe and Al contents observed from the surface to the thickness direction of the Fe-Al alloy plating layer, Fe-humps and Al-humps are present. A hot-formed member that satisfies the following relational expression 1. [Relationship 1] Maximum value of Fe (wt.%) in Fe-hump > Maximum value of Al (wt.%) in Al-hump

2. 2. The hot-formed member according to claim 1, wherein the maximum value (wt.%) of Fe in the Fe-hump is 50 wt.% or more.

3. 2. The hot-formed member according to claim 1, wherein the maximum value (wt.%) of Al in the Al-hump is 50 wt.% or less.

4. The hot-formed member according to claim 1, which satisfies the following relational expression 2: [Relationship 2] The minimum value (wt.%) of the Fe content observed between the maximum value of the Fe hump and the diffusion layer > the maximum value (wt.%) of the Al hump

5. The hot-formed part according to claim 1, wherein the number of said Al-humps is two or more.

6. The hot-formed member according to claim 5, which satisfies the following relational expression 3: [Relationship 3] [Equation 1]

7. The hot-formed part according to claim 5 , wherein the Fe humps are present between the Al humps in the thickness direction.

8. The base steel 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: 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 being Fe and impurities.

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