Aluminized steel sheet, thermal-molded component using same, and method for producing same

Abstract

The present invention relates to an aluminized steel sheet for automobiles and the like, a hot-formed member manufactured using the aluminized steel sheet, and methods for manufacturing the aluminized steel sheet and the hot-formed member.

Description

Technical Field

[0001] The present invention relates to an aluminized steel sheet for automobiles and the like, a hot-formed part manufactured using the aluminized steel sheet, and methods thereof. Background Art

[0002] In recent years, fuel efficiency has been improved by reducing the weight of automobiles. For this purpose, the thickness of steel can be reduced, but when the thickness is reduced, problems with the stability of the automobile may occur. Therefore, it is necessary to increase the strength of the steel as a support. For this reason, the demand for high-strength steel has been increasing, and various types of steel have been developed. However, since such steel has high strength, there are problems with poor workability such as springback.

[0003] To solve such problems, a hot press forming process has been proposed. The hot press forming process is also called hot stamping, hot working, etc. The hot press forming process is a method of processing steel at a high temperature suitable for processing (800 °C or higher), then performing stamping in a mold, and rapidly cooling to a low temperature, thereby forming a low-temperature structure such as martensite in the steel, thereby increasing the strength of the final product. In the above situation, problems with workability can be minimized when manufacturing parts with high strength.

[0004] When using such hot forming, not only can complex shapes be easily formed, but the manufactured parts (hot-formed parts) have the advantage of ensuring high strength. Therefore, its usability has been increasing in recent years. In particular, by using a coated steel sheet with aluminum, zinc, etc. on the surface of the steel, an attempt is made to ensure corrosion resistance. As an example, Patent Document 1 discloses the use of an aluminized steel sheet in a hot press forming process.

[0005] The hot-formed part using the aluminized steel can ensure corrosion resistance by aluminizing, but there is a problem that roll sticking may occur when the high-temperature aluminized steel moves along the roll during the mold forming process.

[0006] If roll sticking occurs during the conveyance to the mold, the moving direction of the high-temperature coated steel sheet conveyed to the mold will be slightly tilted, so that it cannot be conveyed straight to the mold but moves in a diagonal direction. That is, the process that must be automatically performed until the mold forming stops, which causes a significant reduction in productivity. The coating of the high-temperature aluminized steel moving on the roll can exist in a molten aluminum state. Since it moves while contacting the roll in this state, the molten aluminum coating adheres to the roll, resulting in roll sticking. Since roll sticking is an inevitable problem, a lot of research is being conducted to solve the problems caused thereby.

[0007] As described above, the problems and inconveniences caused by roll sticking have the problem of reducing the quality and productivity of products, and thus there has always been a need to improve this problem.

[0008] (Patent Document 1) U.S. Patent No. 6,296,805 Summary of the Invention

[0009] (I) Technical Problem to be Solved

[0010] One aspect of the present invention relates to an aluminized steel sheet that can be used for thermoforming and the like, an aluminized steel sheet having excellent anti-roll sticking properties, a thermoformed part manufactured using the aluminized steel sheet, and a manufacturing method thereof.

[0011] The technical problems of the present invention are not limited to the above. Those skilled in the technical field to which the present invention pertains can easily understand the additional technical problems of the present invention based on the entire text of the specification of the present invention.

[0012] (II) Technical Solution

[0013] One embodiment of the present invention relates to an aluminized steel sheet including a base steel sheet and an Al plating layer formed on the base steel sheet. From the GDOES analysis result in the thickness direction from the surface of the Al plating layer, the concentration gradient of Fe in the range of 40 - 90% by weight of Fe in the Al plating layer is 13 - 26% by weight / μm, and the content of Fe in the Al plating layer is 8 - 24% by weight.

[0014] Another embodiment of the present invention relates to a manufacturing method of an aluminized steel sheet, which includes the following steps: immersing a base steel sheet in an aluminum (Al) plating bath containing silicon (Si) to attach a plating solution to the surface of the base steel sheet; adjusting the plating attachment amount on the surface of the base steel sheet using an air knife (A / K); and transporting the base steel sheet with the attached plating solution to a cooling device for cooling, wherein the following condition of (Equation 2) is satisfied.

[0015] (Equation 2)

[0016] Where a: the content of Si in the plating bath (% by weight), b: the linear velocity (meters per minute (mpm)), c: the A / K interval (mm), d: the A / K pressure (kPa), A / K height (mm)

[0017] Another embodiment of the present invention relates to a thermoformed part, which comprises: a base iron, a diffusion layer formed between the base iron and an Fe-Al alloy coating, and an Fe-Al alloy coating formed on the base iron. In the GDOES analysis diagram of the Fe and Al contents observed along the thickness direction from the surface of the Fe-Al alloy coating, there are Fe-peaks and Al-peaks, and the following relational expression 1 is satisfied.

[0018] [Relational expression 1]

[0019] The maximum value of Fe (wt%) of the Fe-peak > the maximum value of Al (wt%) of the Al-peak

[0020] The maximum value of Fe (wt%) of the Fe-peak can be 50 wt% or more.

[0021] The maximum value of Al (wt%) of the Al-peak can be 50 wt% or less.

[0022] The thermoformed part can satisfy the following relational expression 2.

[0023] [Relational expression 2]

[0024] The maximum value of the Fe-peak and the minimum value of the Fe content (wt%) observed between the diffusion layers > the maximum value of the Al-peak

[0025] There can be two or more Al-peaks.

[0026] The thermoformed part can satisfy the following relational expression 3.

[0027] [Relational expression 3]

[0028]

[0029] The Fe-peak can exist between the Al-peaks in the thickness direction.

[0030] The base iron can contain, by weight: C: 0.02 to 0.6%, Si: 0.001 to 2%, Al: 0.001 to 1%, Mn: 0.1 to 4%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, Ti: 0 to 0.1%, B: 0.0001 to 0.01%, Cu: 0 to 1.00%, Mo: 0 to 1.00%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, V: 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.

[0031] Another embodiment of the present invention relates to a method for manufacturing a thermoformed part, which includes the following steps: manufacturing a blank using the aluminized steel sheet; heating the blank; and forming and cooling the heated blank in a mold.

[0032] (III) Beneficial effects

[0033] When the thermoformed part as an example of the present invention moves to the heating furnace, due to the slow alloying rate of the coating, the roll pollution caused by the molten aluminum on the surface layer can be reduced. Therefore, the quality of the thermoformed part can be ensured and the working efficiency can be improved.

[0034] The multiple beneficial advantages and effects of the present invention are not limited to the above content and can be more easily understood during the description of the specific embodiments of the present invention. Description of the drawings

[0035] Figure 1 It is the Fe depth profile result of GDOES of the aluminized steel sheet of Invention Example 2 in the examples.

[0036] Figure 2 It is the profile result of Fe and Al of GDOES analysis of the thermoformed part of Invention Example 3 in the examples.

[0037] Figure 3 It is a photo of the test piece after the V-bending experiment in the examples.

[0038] Figure 4 It is the result of measuring the amount of powder generated after the V-bending experiment in the examples. Detailed implementation manners

[0039] Hereinafter, the present invention will be described in detail.

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

[0041] The meaning of "including" used in the specification is used to specifically illustrate the components and does not exclude the existence or addition of other components.

[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. The terms defined in the dictionary are interpreted to conform to the meaning of the relevant technical literature and the currently disclosed content.

[0043] In the present invention, a component refers to a component manufactured by hot forming or a material for a component. In addition, a steel sheet refers to a steel sheet or steel material as a material before hot forming, and also includes forms such as coil sheets.

[0044] When hot forming is carried out using a plated steel material, the later the alloying of the plating layer is delayed, the easier it is for the molten plating layer to adhere to the roll, thereby causing roll sticking. Therefore, it is recognized that it is crucial to develop a material that can alloy the plating layer as soon as possible during hot forming.

[0045] In aluminized steel, Fe in the base iron diffuses into the Al plating layer during heating to form an Fe-Al alloy plating layer, and a diffusion layer is formed between the base iron and the Fe-Al alloy plating layer. In the diffusion layer and the Fe-Al alloy plating layer, according to the content of Fe, starting from α-Fe with a high Fe content in the base iron part, Fe diffuses in the form of Fe3Al → FeAl → Fe2Al5 → FeAl3 to form an Fe-Al alloy. The part where a large amount of α-Fe and Fe3Al with a high Fe content are formed is called the diffusion layer. In addition, the part where a large amount of FeAl, Fe2Al5, and FeAl3 with a relatively high Al content are formed is called the Fe-Al alloy plating layer. Regarding Al, the melting point is 660 °C, and when moving along the roll in a 900 °C heating furnace, Al will adhere to the roll. However, if Fe rapidly diffuses to the surface in the form of FeAl3 in a short time in the heating furnace, the melting point of FeAl3 is 1160 °C, which can delay roll sticking. Therefore, it is crucial to develop a material that can accelerate the alloying speed of Fe as much as possible.

[0046] Therefore, in order to improve the roll sticking problem, the inventor of the present invention has conducted in-depth research on a solution that can increase the diffusion speed of Fe during the alloying process of the plating layer during the hot forming of aluminized steel sheets, and thus completed the present invention.

[0047] First, a specific embodiment of the aluminized steel sheet of the present invention will be described in detail.

[0048] The aluminized 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 between the Al plating layer and the base steel sheet. The alloy layer refers to a layer alloyed by the reaction of the base steel sheet and the Al plating layer. As an example of forming the alloy layer, when the base steel sheet is immersed in an Al plating bath, the base steel sheet and the components of the Al plating bath can react first to form an alloy layer.

[0049] In the glow discharge optical emission spectrometry (GDOES) analysis results along the thickness direction inside the base steel plate from the surface of the Al plating layer, the concentration gradient of Fe in the range where the Fe content in the Al plating layer is 40 - 90 wt% includes 13 - 26 wt% / μm.

[0050] As described above, in order to improve roll sticking, during the alloying process of the Al coating when moving to the heating furnace, it is necessary to increase the diffusion rate of Fe. For this purpose, when moving to the heating furnace, Fe must be fully diffused into the Al plating layer in a short time and alloyed.

[0051] According to the first diffusion law (Fick's first law) of the following (Equation 1), diffusion occurs from a region of high concentration to a region of low concentration, and the higher the concentration gradient, the greater the amount of diffusion.

[0052] (Equation 1)

[0053]

[0054] J B : Flux representing the number of atoms passing through a unit area per unit time

[0055] D B : Diffusion coefficient of B atoms

[0056] C: Concentration

[0057] x: Direction

[0058] dC B / dx: Rate of change of concentration along the x direction

[0059] That is, according to the (Equation 1), the greater the concentration gradient of Fe formed between the Al plating layer and the base steel plate in the Al-plated steel plate, the more feasible the method of increasing the Fe diffusion amount or flux of the base steel plate in a short time when moving to the heating furnace. The steeper the concentration gradient of Fe, the more the Fe diffusion amount can be increased in a short time when moving to the heating furnace. In the initial heating stage, Fe can diffuse to the surface layer in a shorter time to form a FeAl3 phase with a higher melting point.

[0060] In the GDOES analysis results along the thickness direction inside the base steel plate from the surface of the Al plating layer, it is effective when the concentration gradient of Fe in the range where the Fe content in the Al plating layer is 40 - 90 wt% is 13 - 26 wt% / μm, preferably 13.5 - 23 wt% / μm. Hereinafter, through reference to Figure 1 , a detailed description will be given. Figure 1It is a diagram showing the Fe concentration depth profile by glow discharge optical emission spectrometry (GDOES) starting from the surface of the aluminized steel sheet of Invention Example 2 manufactured in the following embodiments.

[0061] As Figure 1 shown, for the concentration gradient of the Fe, when analyzed by GDOES depth profiling along the thickness direction inside the base steel sheet from the surface of the Al plating layer, the content of Fe changes in a pattern of first being gentle, then the gradient increasing rapidly, and then being gentle again. Thus, it is confirmed that an alloy layer is formed at the interface with the base steel sheet, and it can be confirmed that the thickness of the alloy layer is about 2 - 6 μm. That is, the Fe gradient of the alloy layer and the inflection point where the Fe gradient in the plating layer changes are confirmed in the GDOES distribution. The inflection point in the GDOES distribution can be regarded as an index for distinguishing the AlSi plating layer and the AlSiFe alloy layer, and the alloy layer can be distinguished with the inflection point as the center. Specifically, in the GDOES distribution, the content (weight %) of Fe and the position of the inflection point where the Fe gradient changes in the distribution diagram mainly exist between 30 weight % and 60 weight %. Therefore, in order to more precisely define the concentration gradient of the Fe, the concentration gradient value of Fe is limited to between 40 weight % and 90 weight %, and the concentration gradient value in the Fe range of 40 - 90 weight % is measured.

[0062] When the concentration gradient of Fe in the interval of 40 - 90 weight % in the Al plating layer is less than 13 weight % / μm, it is the characteristic of a conventional aluminized steel sheet and is not the aluminized steel sheet with improved anti-roll adhesion developed in the present invention. When the Fe concentration gradient exceeds 26 weight % / μm, since the thickness of the alloy layer in the aluminized steel sheet is too thin, Fe of the base iron in the aluminized steel sheet easily breaks the thin alloy layer when moving to the heating furnace, resulting in rapid diffusion, so an uneven alloy layer is formed, which may lead to a decrease in corrosion resistance.

[0063] It is effective that the Fe content of the Al plating layer is 8 - 24 weight %, more preferably 8.5 - 23 weight %.

[0064] As an example of the method for measuring the Fe content in the Al-coated layer, a circular coating layer with a radius of 25 mm is melted and measured by Inductively Coupled Plasma (ICP). As a specific method, the Al-coated layer with a radius of 25 mm is dissolved once in a 20% NaOH solution, and then the coating layer is completely dissolved twice in a solution in which HCl and an inhibitor are added at a ratio of 1:3. Then, the solution containing all of the first dissolution solution and the second dissolution solution is analyzed by ICP to measure the contents of Al, Si, and Fe. At this time, the Fe content in the present invention should be 8-24 wt%, more preferably 8.5-23 wt% to be effective.

[0065] The Fe content in the Fe distribution (profile) of GDOES can also be obtained by integrating the Fe distribution. However, the GDOES distribution represents the Fe content at the corresponding position in the thickness direction, so it cannot accurately show the content of the components contained in the entire coating layer. In contrast, ICP analysis only analyzes the components of the sample in which the coating layer is dissolved, and can obtain the Fe content contained in the entire coating layer including the Al alloy layer. Therefore, the reliability of the data is high.

[0066] In addition, when the Fe content in the Al-coated layer is less than 8 wt%, it has the characteristics of a conventional Al-coated steel sheet, and it is not the Al-coated steel sheet with improved roll adhesion developed in the present invention. When the Fe content in the Al-coated layer exceeds 24 wt%, when the difference between the Fe content in the coating layer and the Fe content in the base steel sheet is large enough, Fe diffusion during hot forming can be promoted. However, when the Fe content in the coating layer exceeds 24 wt%, instead, there is no large difference between the Fe content in the base steel sheet and the Fe content in the coating layer, which may cause the problem that the diffusion of Fe in the base iron to the Al-coated steel sheet slows down when moving to the heating furnace.

[0067] The Al-coated layer may be any Al-based coating widely used in the hot-forming coated steel sheets carried out by those skilled in the art of the hot-forming coated steel sheets to which the present invention pertains. There are no particular restrictions on the content or type of its components. It includes not only pure Al coating, but also coatings containing partial Si in Al, coatings containing partial Zn in Al, coatings containing any one or more of partial Si, Mg, Zn, etc. in Al.

[0068] The thickness of the Al-coated layer is effective when it is 3 μm to 30 μm, more preferably 4 μm to 26 μm.

[0069] When the thickness of the Al coating layer is less than 3 μm, due to the excessively thin coating layer, a large amount of uncoated phenomenon occurs on the surface of the Al-coated steel plate, which may affect the corrosion resistance of the components after hot forming. When the thickness of the Al coating layer exceeds 30 μm, even if the Fe of the base iron diffuses rapidly when moving to the heating furnace, due to the thick coating layer, there is a high possibility that molten Al exists on the surface layer, and there may be a problem that it is difficult to improve roll sticking.

[0070] In addition, as long as the base steel plate is a steel plate that can be used for hot forming, there is no particular limitation on the object. There is no particular limitation not only on the classification according to the manufacturing process such as hot-rolled steel plates and cold-rolled steel plates, but also on the steel grades and alloy composition systems such as dual-phase steel (DP), complex structure steel, TRIP steel, TWIP steel, etc.

[0071] As a specific example, taking the 22MnB5 steel grade as a representative, it can be a steel grade containing by weight % (wt. %): carbon (C): 0.1 - 0.3%, manganese (Mn): 1.0 - 2.0%, silicon (Si): 0.02 - 0.30%, boron (B): 5 - 45 ppm, and the rest contains inevitable impurities and iron (Fe).

[0072] Next, a specific implementation of the manufacturing method of the aluminized steel plate according to an example of the present invention will be described in detail. The manufacturing method of the aluminized steel plate includes the following steps: immersing the base steel plate in a plating bath and attaching a plating solution to the surface of the base steel plate; adjusting the plating attachment amount on the surface of the base steel plate; and transferring the base steel plate with the attached plating solution to a cooling device. Hereinafter, each step will be described in detail.

[0073] First, immerse the base steel plate in a plating bath and attach a plating solution to the surface of the base steel plate.

[0074] As described above, as long as the base steel plate can be used as a hot-forming component, there is no particular limitation on its object, steel grade, etc. Before immersing the base steel plate in the plating bath, the base steel plate can be heated to a certain temperature (introduction temperature). At this time, it is effective that the introduction temperature is 620 - 680°C. Heating the base steel plate to the introduction temperature can prevent the deviation of the plating attachment amount or the problem of uncoating caused by the reduction in fluidity, etc. However, when overheated, it promotes the melting of the steel plate instead and may accelerate the generation of dross.

[0075] The plating bath can be an Al-based plating bath, which is used to form an Al plating layer. As long as it can be applied to the plating of hot-formed plated steel sheets, it can be applied without limitation in the present invention. As a preferred example, the composition of the plating bath can include: silicon (Si): 6-12% by weight, iron (Fe): 1-4% by weight, the balance aluminum (Al) and inevitable impurities.

[0076] The temperature of the plating bath is effective at 630-680°C. When the temperature of the plating bath is too low, the fluidity of the plating solution in the plating bath may decrease. On the other hand, when the temperature of the plating bath is too high, the generation of scum in the plating bath may increase.

[0077] Next, adjust the plating adhesion amount of the base steel sheet. The device or method for adjusting the plating adhesion amount is not particularly limited and can be carried out by a method commonly used in the technical field to which the present invention pertains. As an example, an air knife (A / K) is used.

[0078] The plating adhesion amount is 8-80 g / m based on one side 2 , and the coating thickness is effective at 3-30 μm. The plating adhesion amount of Al plating can generally be converted to the plating adhesion amount when the coating thickness is multiplied by 2.7. When the coating thickness of 3-30 μm is converted to the plating adhesion amount, it is about 8-80 g / m 2 . A more preferred thickness is 4-26 μm, and the plating adhesion amount is 10-70 g / m 2 .

[0079] Transfer the base steel sheet with the adjusted plating adhesion amount to a cooling device and cool it. When the base steel sheet with the plating solution attached enters the cooling device, rapid cooling starts, and the reaction of the alloy layer in the Al plating layer may stop. As an example of the cooling device, it includes a cooling tower.

[0080] In the manufacturing method, the K value defined by the following formula 2 is effective at 200 to 400.

[0081] The formula 2 can be calculated as follows.

[0082] (Formula 2)

[0083] Where a: the content of Si in the plating bath (% by weight), b: the linear speed (m / min), c: the A / K interval (mm), d: the A / K pressure (kPa), the A / K height (mm). The linear speed refers to the speed at which it passes through the plating bath and is transferred. The interval of the air knife (A / K) used to adjust the plating adhesion amount refers to the interval between the steel sheet and the air knife. The A / K height refers to the distance from the plating bath to the A / K.

[0084] In the case of the (Formula 2), for manufacturing a plated steel sheet with improved roll adhesiveness, the respective characteristics of various variables that determine the characteristics of the plating layer are considered, and the technical significance lies in considering the influencing factors and their correlations during the manufacturing of the plated steel sheet for the purpose of improving roll adhesiveness.

[0085] For manufacturing a steel sheet with improved roll adhesiveness, considering the results of various process factors such as the Si content in the plating bath, the plating bath temperature, the introduction temperature, the line speed, the A / K interval, the A / K pressure, the A / K height, and the cooling rate of the cooling tower, it is necessary to optimize the Si content, the line speed, the A / K interval, the A / K pressure, and the A / K height in the plating bath, thereby obtaining the (Formula 2). When the K value of the (Formula 2) is from 200 to 400, the roll adhesiveness can be improved.

[0086] Specifically, when the K value is less than 200, the A / K pressure is insufficient to improve the roll adhesiveness, or the line speed, the A / K interval, and the A / K height are low, so it is difficult to improve the roll adhesiveness. When the K value exceeds 400, the A / K pressure is too high or the line speed, the A / K interval, and the A / K height are higher than the appropriate values, so it is difficult to improve the roll adhesiveness.

[0087] Before immersing the base steel sheet in the plating bath, it can be provided in various ways. As an example, after heating the billet, it can be manufactured through processes including hot rolling, coiling, cold rolling, annealing, etc. The specific process conditions such as the heating, hot rolling, coiling, cold rolling, annealing, etc. of the required billet can vary according to the characteristics required for the base steel sheet, so there is no particular limitation.

[0088] Next, a specific embodiment of the hot forming part of the present invention will be described in detail.

[0089] The hot forming part of the present invention includes a base iron, an Fe-Al alloy plating layer formed on the base iron, and a diffusion layer formed between the Fe-Al alloy plating layer and the base iron.

[0090] The Fe-Al alloy plating layer and the diffusion layer refer to those alloyed and formed through the mutual diffusion and reaction of the plating layer components and the base iron components during the heating process of the aluminized steel sheet. In the diffusion layer and the Fe-Al alloy plating layer, the part where a large amount of α-Fe and Fe3Al with a high Fe content is formed is called the diffusion layer, and in addition, the part where a relatively high content of Al, such as FeAl, Fe2Al5, and FeAl3, is formed in a large amount is called the Fe-Al alloy plating layer. The diffusion layer and the Fe-Al alloy plating layer can be optically confirmed by performing Nital etching on the cross-section of the hot forming part. The diffusion layer is the first layer directly above the base iron that has transformed into martensite, and starting from the second layer thereafter, it can be regarded as the Fe-Al alloy layer.

[0091] The composition of the base material iron or the base steel plate is not particularly limited as long as it can be used as a hot forming part. As a preferred example, the base material iron may contain, by weight%: C: 0.02 to 0.6%, Si: 0.001 to 2%, Al: 0.001 to 1%, Mn: 0.1 to 4%, P: less than 0.05%, S: less than 0.02%, N: less than 0.02%, 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 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.

[0092] The Fe-Al alloy coating refers to an alloyed coating formed by alloying the aluminum-based coating of a hot forming coated steel plate with an aluminum-based coating 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 aluminum and Fe of the steel during plating. In addition, other components may be included as needed.

[0093] In addition, the diffusion layer refers to a layer formed by the combination of a part of Fe of the base steel plate and the main component Al of the aluminum-based coating.

[0094] In the glow discharge optical emission spectrometry (GDOES) analysis chart of the Fe and Al contents observed along the thickness direction from the surface of the Fe-Al alloy coating, an Fe-peak and an Al-peak may exist in the hot forming part, and the following relational expression 1 can be satisfied.

[0095] [Relational expression 1]

[0096] The maximum value of Fe (weight%) of the Fe-peak > the maximum value of Al (weight%) of the Al-peak

[0097] Hereinafter, refer to Figure 2 for a detailed description. Figure 2 This is a diagram of GDOES analysis of the surface of the hot forming part of Invention Example 3 in the following examples. Refer to the Figure 2 , it can be confirmed that there is 1 Fe-peak and 2 to 3 Al-peaks. The Fe-peak can be observed between the Al-peaks.

[0098] The Fe-Al alloy coating of the hot forming part can be divided into 1 to 4 layers, and any one of them can be an AlFeSi layer. In the present invention, the AlFeSi layer can be changed to form a layer with excellent roll adhesion. In particular, as the Fe content increases, an alloy layer with a high melting point is formed, and the AlFeSi layer with a high Fe content can delay roll adhesion when the aluminum hot forming part moves in the heating furnace.

[0099] To increase the melting point in the Fe-Al alloy coating, it is easy to understand by referring to the binary phase diagram of Fe-Al (Binary Phase Diagram). In the Fe-Al binary phase diagram, as the Fe content in Al increases, the melting point rises from 660 °C to 1538 °C. When the contents of Al and Fe are 1:1, the melting point is about 1100 °C. However, when the Fe content in Al increases from 0 wt% to 50 wt%, it is the interval where the melting point rises rapidly. In other words, the melting point may decrease due to the change in Fe content caused by the atmosphere in the heating furnace, which may cause roll adhesion.

[0100] Therefore, the maximum value of Fe at the Fe-peak in the Fe-Al alloy coating should be higher than the maximum value of Al at the Al-peak. In other words, this means that there is a part where the Fe-peak is higher than the Al-peak, indicating that the diffusion rate of Fe is faster when the plated steel sheet moves into the heating furnace. Therefore, the hot forming part of the present invention can stably form an Fe-Al alloy coating with a high melting point.

[0101] In addition, the maximum value of Fe at the Fe-peak (wt%) can be 50 wt% or more, and the maximum value of Al at the Al-peak can be 50 wt% or less.

[0102] When the maximum value of Fe at the Fe-peak is less than 50 wt%, as described above, the Fe-Al alloy coating may be formed in the interval where the melting point changes rapidly, so roll adhesion problems may occur. When the maximum value of Al at the Al-peak exceeds 50 wt%, similarly, the formation of an Fe-Al alloy coating with a low melting point may also cause roll adhesion.

[0103] In addition, for the hot forming part, the Fe and Al contents measured along the depth direction in the GDOES analysis diagram can satisfy the following relational expression 2 or relational expression 3.

[0104] [Relational expression 2]

[0105] The maximum value at the Fe-peak and the minimum value of Fe content observed between the diffusion layers (wt%) > the maximum value at the Al-peak (wt%)

[0106] [Relational expression 3]

[0107]

[0108] The above-mentioned relational expressions 2 and 3 can be considered in combination with the above Fe-Al binary phase diagram. When the maximum value of the Fe-peak is greater than the minimum value of the Fe content observed between the diffusion layers in the Fe-Al alloy coating, the roll sticking can be sufficiently delayed, and thus excellent results can be obtained.

[0109] The relational expression 3 is a relaxed condition of the relational expression 2. Even when the minimum value of the Fe content observed between the maximum value of the Fe-peak and the diffusion layer in the Fe-Al alloy coating is greater than or equal to the average value of the maximum and minimum values of the Al-peak, the roll sticking will be delayed.

[0110] As described in the above 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 rapidly increases. At this time, when at least the content of Fe is greater than or equal to the average value of the maximum and minimum values of the Al-peak in the Fe-Al alloy coating, the melting point of the alloy coating can be sufficiently maintained above 1000 °C, so that the melting point is higher than the temperature condition (850 - 950 °C) of the heating furnace.

[0111] In addition, in the GDOES analysis chart, there may be two or more Al-peaks, and the Fe-peak can be observed between the Al-peaks based on the thickness direction.

[0112] In addition, the thickness of the diffusion layer can be 3 - 10 μm, and the thickness of the diffusion layer and the Fe-Al alloy coating can be 10 - 30 μm.

[0113] Next, a specific implementation of the manufacturing method of the hot forming part of the present invention will be described in detail.

[0114] The manufacturing method can be manufactured by providing a base steel plate with an aluminum-based coating, heating the provided base steel plate, hot forming it, and then cooling it.

[0115] For example, a blank can be provided by shearing a base steel plate with the aluminum-based coating. It is preferable to heat the provided blank to a temperature above the austenite single-phase region temperature, more specifically, to a temperature range of Ac3 to 975°C. When the heating temperature is lower than the Ac3 temperature, it is difficult to ensure strength and collision resistance due to the presence of untransformed ferrite according to the two-phase region interval. On the other hand, when the heating temperature exceeds 975°C, excessive oxides are generated on the surface of the component, so it is difficult to ensure spot weldability, and the manufacturing cost for maintaining high temperature increases. It is preferable to hold the blank heated as described above within the above temperature range for 1 - 1000 seconds. When the holding time is less than 1 second, it is difficult to achieve a uniform temperature distribution throughout the blank, so material deviation at each position may be caused. When the holding time exceeds 1000 seconds, it is difficult to ensure spot weldability due to excessive oxides generated on the surface, and an increase in the manufacturing cost of the component may also be caused. Transfer the heated blank to a press and perform the processes of forming and cooling. At this time, the cooling rate is preferably 20°C / second or more. When the cooling rate is less than 20°C / second, a ferrite phase is introduced during cooling and generated at the grain boundaries, and physical properties such as strength and collision resistance may deteriorate. The processes of transferring, forming, and cooling the blank are not particularly limited, and for the hot forming process, the commonly used processes can be directly applied. Detailed Description of the Invention

[0116] 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 rights of the present invention is not limited to the following embodiments. The scope of rights of the present invention should be determined by the claims and their equivalents.

[0117] (Embodiment)

[0118] In an embodiment of the present invention, a conventional 22MnB5 base steel plate is prepared and immersed in a plating bath containing about 9 - 10 wt% of Si and the rest containing Al and inevitable impurities to manufacture an aluminized steel plate. At this time, the temperature of the plating bath is 640 - 670°C, and the introduction temperature of the base steel plate is 650 - 680°C.

[0119] To manufacture the coated steel plate, at this time, the content of Si in the plating bath, the steel plate transfer speed (linear speed), and the single-sided plating adhesion amount (10 - 80 g / m 2 ) are adjusted. The plating adhesion amount is adjusted with an air knife (A / K) and transferred to a cooling tower as a cooling device for cooling. At this time, the process conditions are shown in Table 1.

[0120] [Table 1]

[0121]

[0122] The formula 2 is calculated as follows.

[0123] (Formula 2)

[0124] Wherein, a: the content of Si in the plating bath (wt%), b: the linear velocity (m / min), c: the A / K interval (mm), d: the A / K pressure (kPa), and the A / K height (mm).

[0125] For the aluminized steel sheet manufactured as described above, the concentration gradient of Fe in the Al plating layer and the alloy composition of the Al plating layer are measured and shown together in Table 2.

[0126] For the concentration gradient of Fe, in the results of GDOES analysis performed along the depth direction from the surface of the Al plating layer of the manufactured aluminized steel sheet, the concentration gradient of Fe in the range where the content of Fe is 40 - 90 wt% is obtained. For the alloy composition, the Al plating layer is dissolved once in a 20% NaOH solution, and the plating layer is completely dissolved (second time) in a solution in which HCl and an inhibitor are added at a ratio of 1:3, and then the solution containing all of the first dissolution solution and the second dissolution solution is analyzed by ICP to measure the contents of Al, Si, and Fe.

[0127] [Table 2]

[0128]

[0129] Figure 1 is a graph showing the Fe concentration distribution of GDOES in Invention Example 2 in Table 2 above.

[0130] For the aluminized steel sheets manufactured as described in Table 1 and Table 2 above, the roll sticking characteristics are evaluated, and the results are shown below Figures 3 to 4 and in Table 3 below.

[0131] As described in Table 1 and Table 2 above, the manufactured aluminized steel sheet is heat-treated at 900 °C for 5 minutes, and then the Fe-peak and Al-peak in the Fe-Al alloy plating layer are confirmed by GDOES. The reason for roll sticking is that Fe diffuses slowly during movement in the heating furnace, resulting in slow alloying. Therefore, in order to observe rapid alloying by GDOES, the Fe-peak and Al-peak are measured, and the results are shown in Table 3.

[0132] [Table 3]

[0133]

[0134] In order to evaluate the roll sticking characteristics, each test piece was heat-treated at 900 °C for 5 minutes, and then the pulverization property was evaluated. The evaluation of the pulverization property was carried out by cutting the heat-treated test piece into 60×30 mm and then performing a V-bending test. At this time, the test conditions were 20° and 2R, and the average powder generation amount is shown in Table 3. In addition, Figure 3 The photograph after showing the V-bending test results of some test pieces in Table 3 below, Figure 4 The powder generation amount in each test piece of Table 3 is shown by a graph.

[0135] The reason for roll sticking is that the slow alloying of the base iron Fe when moving to the heating furnace causes an increase in the low melting point Al content on the surface layer, and the molten aluminum alloy layer adheres to the roll, but it is difficult to evaluate this on an actual production line.

[0136] Therefore, in order to compare the formation of a large amount of high melting points, that is, α-Fe and Fe3Al phases with a high Fe content, in the state after heat treatment, considering these characteristics, a roll sticking prediction evaluation can be carried out at room temperature.

[0137] That is, when moving to the heating furnace, the faster the diffusion rate of Fe, the order of the content of the Al alloy coating in the expected invention of the plated steel is Al < FeAl3 < Fe2Al5 < FeAl < Fe3Al < α-Fe. When it exists in the direction of increasing α-Fe, it means a fast alloying rate and also means the formation of a high melting point phase.

[0138] The characteristics of the above phases are that α-Fe and Fe3Al with a relatively high Fe content exhibit ductility at room temperature, while FeAl, Fe2Al5, and FeAl3 with a relatively high Al content exhibit brittleness. Therefore, considering the ductility and brittleness of these phases at room temperature, if the pulverization generation amount of the components after heat treatment is compared, the diffusion rate of Fe can be relatively compared, and the roll sticking characteristics can be confirmed by evaluating the pulverization property.

[0139] From the above Table 3 and Figures 3 to 4 the results, it can be seen that for the aluminized steel sheet corresponding to the inventive example that satisfies the range proposed by the present invention, the generation of plating powder during the hot forming process is significantly reduced.

[0140] Therefore, in the inventive example according to the present invention, not only a large amount of Fe exists in the coating of the plated steel sheet, but also the Fe gradient in the alloy layer is large, so a large amount of Fe can diffuse, so the proportion of the diffusion layer after heat treatment is high and the hard layer rate is reduced, and thus the pulverization generation amount is reduced.

Claims

1. A thermoformed part, the thermoformed part comprising: A base material of iron, an Fe-Al alloy coating formed on the base material of iron, and a diffusion layer formed between the base material of iron and the Fe-Al alloy coating In the GDOES analysis chart of the Fe and Al contents observed in the thickness direction from the surface of the Fe-Al alloy coating, there are Fe-peaks and Al-peaks And the following relational expression 1 is satisfied [Relational expression 1] The maximum value of Fe (wt%) of the Fe-peak > the maximum value of Al (wt%) of the Al-peak 2. The thermoformed part according to claim 1, wherein, The maximum value of Fe of the Fe-peak is 50 wt% or more, in wt% units 3. The thermoformed part according to claim 1, wherein, The maximum value of Al of the Al-peak is 50 wt% or less, in wt% units 4. The thermoformed part according to claim 1, wherein, The hot forming part satisfies the following relational expression 2 [Relational expression 2] The maximum value of the Fe-peak and the minimum value of the Fe content (wt%) observed between the diffusion layers > the maximum value of the Al-peak 5. The thermoformed part according to claim 1, wherein, There are 2 or more of the Al-peaks 6. The thermoformed part according to claim 5, wherein, The hot forming part satisfies the following relational expression 3 [Relational expression 3] 。 7. The thermoformed part according to claim 5, wherein, The Fe-peak exists between the Al-peaks in the thickness direction 8. The thermoformed part according to claim 1, wherein, By weight, the base material of iron consists of 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 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%, and the balance of Fe and impurities

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