Coated steel sheet with thin aluminum alloy coating and coating method thereof

Abstract

The present invention relates to a coated steel sheet with a thin aluminum alloy coating and a coating method thereof. The coated steel sheet of the present invention is used for hot stamping, and the coating thickness of the coated steel sheet is 5 to 14 μm, wherein the aluminum alloy coating includes a FeAlSi inhibition layer close to a base steel sheet and an Al alloy layer on its outer side, wherein the thickness of the FeAlSi inhibition layer is not more than 60% of the coating thickness and is 1.5 to 6.0 μm, and the diameter of the Kirkendall holes within 2 μm from the interface between the FeAlSi inhibition layer and the base steel to the base steel is less than 2.5 μm, wherein the number of Kirkendall holes with a diameter of more than 0.5 μm and less than 2.5 μm does not exceed 15 / 35 μm. The present invention also discloses a coating method for coating a thin aluminum alloy coating on a base steel sheet for hot stamping, which can eliminate missed plating and make the hot stamping formed component obtained from the coated steel sheet have excellent resistance spot welding performance.

Description

Technical Field

[0001] The present invention relates to a coated steel sheet with a thin aluminum alloy coating and a coating method thereof. Background Art

[0002] In recent years, with the increasingly stringent requirements on energy and environmental protection, collision regulations, and passive safety of passengers, hot stamping processes have been increasingly applied to the design and production of automotive components. In order to improve the toughness of hot stamping components of aluminum-silicon coated plates, CN108588612A proposes to use thin-coated plates to obtain hot stamping components, wherein the initial coating thickness of the coated steel plate before hot stamping is 3 to 19 μm. However, when producing thin-coated plates, it was found that the production process is prone to problems such as large fluctuations in coating thickness, unstable production, and localized plating leakage.

[0003] In addition, during the hot stamping process, the coated steel sheet blank is first heated to 880-960°C for heat preservation, so that the Fe in the base steel and the Al in the initial coating diffuse mutually to form Fe-Al intermetallic compounds on one side of the coating, and an interdiffusion layer with a high Al content is formed between the substrate and the Fe-Al intermetallic compound layer. Since the diffusion rate of Fe into the coating to form Fe-Al intermetallic compounds is much higher than the diffusion rate of Al into the substrate, as the diffusion continues, a large amount of Fe diffuses into the coating, and the vacancies in the interdiffusion layer are not replenished in time, thereby forming Kirkendall holes. The resistance of the area where these Kirkendall holes are located is relatively high, so sparks are prone to spatter during welding, and the welding performance is reduced. CN108588612A has found that the use of a thin initial coating can inhibit the mutual diffusion during the hot stamping process to a certain extent. Therefore, compared with a thick initial coating, it can be expected that a thin initial coating will inhibit the formation and growth of Kirkendall voids during the hot stamping process to a certain extent, which is beneficial to improving the resistance spot welding performance of hot stamping components.

[0004] However, the inventors of the present application have found that even with a thin initial coating, there is still room for further improvement in resistance spot welding performance. Summary of the invention

[0005] The present invention is made in view of the above-mentioned problems existing in the prior art, and one of its purposes is to provide a coated steel plate for hot stamping having a thin aluminum alloy coating, which can eliminate missed plating and make the hot stamping formed components obtained from the coated steel plate have excellent resistance spot welding performance.

[0006] To achieve the above-mentioned object, the coating thickness of the aluminum alloy coating on at least one surface of the coated steel plate of the present invention is 5 to 14 μm, wherein the aluminum alloy coating includes a FeAlSi inhibition layer close to the base steel plate and an Al alloy layer on its outer side, wherein the thickness of the FeAlSi inhibition layer is not more than 60% of the coating thickness and is 1.5 to 6.0 μm, and the diameter of the Kirkendall holes within 2 μm from the interface between the FeAlSi inhibition layer and the base steel to the base steel is less than 2.5 μm, wherein the number of Kirkendall holes with a diameter of more than 0.5 μm and less than 2.5 μm does not exceed 15 / 35 μm, preferably does not exceed 10 / 35 μm, and more preferably does not exceed 5 / 35 μm.

[0007] By reducing the thickness of the FeAlSi inhibition layer, the plating leakage condition is eliminated and the fluctuation of the coating thickness is reduced, thereby improving the production stability. In addition, since there are fewer and smaller Kirkendall holes in the matrix steel near the above interface, it is beneficial to further inhibit the formation of large-sized holes during hot stamping, thereby improving the resistance spot welding performance of subsequent hot stamping formed components.

[0008] Preferably, the flat thickness of the aluminum alloy coating on at least one surface is 6 to 13 μm, wherein the thickness of the FeAlSi inhibition layer is not more than 50% of the coating thickness and is 1.5 to 5.0 μm, the diameter of the Kirkendall pores within 2 μm from the interface to the base steel is less than 2.5 μm, and the number of Kirkendall pores with a diameter of more than 0.5 μm and less than 2.5 μm does not exceed 13 / 35 μm; and further preferably, the diameter of the Kirkendall pores is less than 2.0 μm, and the number of Kirkendall pores with a diameter of more than 0.5 μm and less than 2.0 μm does not exceed 15 / 35 μm, and preferably does not exceed 10 / 35 μm, and more preferably does not exceed 5 / 35 μm.

[0009] Preferably, the coating thickness of the aluminum alloy coating on at least one surface is 7 to 12 μm, wherein the thickness of the FeAlSi inhibition layer is not more than 40% of the coating thickness and is 2.45 to 3.95 μm, the diameter of the Kirkendall pores within 2 μm from the interface to the base steel is less than 2.5 μm, and the number of Kirkendall pores with a diameter of more than 0.5 μm and less than 2.5 μm does not exceed 13 / 35 μm; and further preferably, the diameter of the Kirkendall pores is less than 2.0 μm, and the number of Kirkendall pores with a diameter of more than 0.5 μm and less than 2.0 μm does not exceed 15 / 35 μm, preferably does not exceed 10 / 35 μm, and more preferably does not exceed 5 / 35 μm.

[0010] Smaller and fewer Kirkendall voids further improve the resistance spot welding performance of subsequent hot stamping formed components.

[0011] Herein, unless otherwise specified, the thickness of the coating layer, the thickness of the FeAlSi inhibition layer and the thickness of the Al alloy layer are respectively the average values ​​of at least 3 corresponding measured values.

[0012] In order to meet the requirements of the hot stamping process for the hardenability of the steel plate, a microstructure dominated by martensite is formed in the hot stamping component and a strength of 900MPa to 2200MPa is achieved. The base steel plate contains the following components by weight percentage: 0.05 to 0.45% C, 0.5 to 10% Mn, 0 to 0.01% B, 0 to 0.4% Nb+Ti+V, 0.01 to 2% Si, 0.01 to 2% Al, 0.01 to 5% Cr+Ni+Mo+Cu and 0 to 2% Cr, 0 to 2% Ni, 0 to 2% Mo and 0 to 2% Cu, and the balance is Fe and unavoidable impurity elements.

[0013] Preferably, the base steel plate comprises the following components by weight percentage: 0.09-0.39% C, 0.6-3.5% Mn, 0-0.004% B, 0-0.4% Nb+Ti+V, 0.01-2% Si, 0.01-2% Al, 0.01-5% Cr+Mo+Ni+Cu, 0-2% Cr, 0-2% Ni, 0-2% Mo and 0-2% Cu, and the balance is Fe and unavoidable impurity elements.

[0014] More preferably, in order to form a martensite-based microstructure in the hot stamping component and achieve a strength of 1400MPa to 2100MPa, the base steel plate contains the following components by weight percentage: 0.18 to 0.39% C, 0.6 to 3.5% Mn, 0 to 0.004% B, 0.05 to 0.3% Nb+Ti+V, 0.01 to 2% Si, 0.01 to 2% Al, 0.01 to 5% Cr+Mo+Ni+Cu and 0 to 2% Cr, 0 to 2% Ni, 0 to 2% Mo and 0 to 2% Cu, and the balance is Fe and unavoidable impurity elements.

[0015] Preferably, the thickness of the base steel plate is 0.5-3.0 mm.

[0016] Another object of the present invention is to provide a coating method for coating a thin aluminum alloy coating on a base steel sheet for hot stamping, which can eliminate missed plating and make the hot stamping formed component obtained from the coated steel sheet have excellent resistance spot welding performance.

[0017] To achieve the above object, in the coating method of the present invention, the plating solution composition comprises by weight: 9% to 12% Si, less than 4% Fe, and the balance Al and inevitable impurities.

[0018] Preferably, the Si content in the plating solution is 9.2% to 11.2% by weight.

[0019] The coating method according to the present invention comprises:

[0020] a) Pre-treat the base steel plate before coating;

[0021] b) heating the pretreated base steel plate and then cooling it to a predetermined temperature in the range of 610 to 650° C., preferably in the range of 620 to 645° C., more preferably in the range of 625 to 639° C., and further preferably in the range of 625 to 635° C.;

[0022] c) immersing the base steel sheet cooled to the predetermined temperature in b) in a heated plating solution for 2 to 7 seconds for hot dip plating, during which the plating solution temperature is higher than the predetermined temperature and maintained at 630 to 670° C., preferably 640 to 660° C.;

[0023] d) after the base steel sheet leaves the plating solution and before the plating solution on at least one surface solidifies, removing excess plating solution on at least one surface by air knife blowing to control the thickness of the coating on the at least one surface; and

[0024] e) cooling the steel sheet to room temperature to obtain a coated steel sheet having a thin aluminum alloy coating layer.

[0025] The above-mentioned coating method can be completed in a continuous hot-dip coating process. Pretreatment of the base steel plate includes, for example, degreasing, water washing, descaling, warm water washing, plating assistance and drying. In the process of hot-dip aluminum coating of the above-mentioned steel plate, the heating of the base steel plate can be achieved by induction heating, heating furnace and other methods. Preferably, the plating solution temperature is 5 to 20°C higher than the predetermined temperature when the steel plate enters the plating solution (i.e., the temperature of the steel plate entering the pot), and more preferably, 7 to 15°C. In step e), the cooling rate of the steel plate is preferably not less than 5°C / s. In addition, those skilled in the art will understand that any range or any value within the above-mentioned intervals is applicable to the present invention. For example, the predetermined temperature may be any range or specific value within the range of 610-650°C, such as any range of 610-620°C, 635-650°C, 635-645°C, or any value such as 612°C, 614°C, 616°C, 618°C, 620°C, 622°C, 624°C, 626°C, 628°C, 630°C, 632°C, 634°C, 636°C, 638°C, 640°C, 642°C, 644°C, 646°C, 648°C.

[0026] The coated steel sheet obtained by the coating method of the present invention has a coating thickness of 5 to 14 μm, preferably 6 to 13 μm, and more preferably 7 to 12 μm, wherein the thickness of the FeAlSi inhibition layer in the coating is not more than 60% of the coating thickness and is in the range of 1.5 to 6 μm, preferably, not more than 50% of the coating thickness and is in the range of 1.5 to 5.0 μm, and more preferably, not more than 40% of the coating thickness and is in the range of 2.45 to 3.95 μm; wherein, the thickness from the interface between the FeAlSi inhibition layer and the base steel to 2 μm in the base steel The diameter of the Kirkendall pores is below 2.5 μm, wherein the number of Kirkendall pores with a diameter of at least 0.5 μm and below 2.5 μm does not exceed 15 / 35 μm, preferably does not exceed 13 / 35 μm, and more preferably does not exceed 5 / 35 μm; and further preferably, the diameter of the Kirkendall pores is below 2.0 μm, and the number of Kirkendall pores with a diameter of at least 0.5 μm and below 2.0 μm does not exceed 15 / 35 μm, preferably does not exceed 10 / 35 μm, and more preferably does not exceed 5 / 35 μm.

[0027] In the method of the present invention, the plating solution temperature in the aluminum pot and the temperature of the steel plate when entering the aluminum pot are lowered, the Si content in the plating solution is increased, and the residence time of the steel plate in the plating solution is shortened. The combined effect of these influencing factors inhibits the mutual diffusion between Fe in the substrate and Al in the coating, so that the obtained coating has a stable coating thickness and eliminates plating leakage on the one hand; on the other hand, it inhibits the formation of Kirkendall holes in the substrate steel near the interface between the FeAlSi inhibition layer and the substrate steel, so that the holes are fewer and smaller in diameter, thereby improving the resistance spot welding performance of the hot stamping formed component made of the coated steel plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a SEM photograph of the local coating morphology of the coated steel plate according to Example 5 of the present invention;

[0029] Figure 2 This is a SEM photograph of the local coating morphology of the coated steel plate of Comparative Example 4;

[0030] Figure 3 This is a photo of a typical non-plating defect of the coated steel plate of Comparative Example 4;

[0031] Figure 4 are metallographic photographs of the local coating morphology of the coating samples of Example 5 and Comparative Example 4 after hot stamping;

[0032] Figure 5 These are the spot welding performance test results of the coated samples of Example 5 and Comparative Example 4 after hot stamping. DETAILED DESCRIPTION

[0033] The present invention will be described in more detail below with reference to exemplary embodiments. The following examples or experimental data are intended to illustrate the present invention by way of example, and it should be clear to those skilled in the art that the present invention is not limited to these examples or experimental data.

[0034] The invention provides a coated steel plate for hot stamping and a coating method thereof.

[0035] During the hot-dip aluminum plating process, the Fe on the surface of the base steel and the Al and Si in the plating solution will undergo an alloying reaction, thereby forming a FeSiAl intermetallic alloy compound layer on the surface of the base steel, namely the FeAlSi inhibition layer. With the formation of the FeAlSi inhibition layer on the surface of the base steel, the mutual diffusion of Fe and Al is significantly reduced. The outer side of the FeAlSi inhibition layer is an Al alloy layer, and its thickness is adjusted according to the air knife blowing. When producing thin-coated plates, if the coating thickness is determined, if the Al alloy layer in the coating is too thin, it will lead to problems such as unstable coating thickness of the steel plate during continuous production and frequent local leakage. Therefore, the Al alloy layer should not be too thin. Therefore, it is necessary to obtain a thin FeAlSi inhibition layer on the surface of the steel plate during coating to ensure sufficient Al alloy layer thickness.

[0036] In addition, in the prior art, compared with the heating process of hot stamping, since the temperature of the steel plate before entering the plating solution is below 700°C and the hot dip plating time is only a few seconds, it is generally believed that the diffusion of alloy elements during hot dip plating is slow and the Kirkendall effect will not occur. However, after in-depth research by the inventors of the present invention, it was found that since the outside of the steel substrate is liquid aluminum during hot dip plating, Fe atoms can still quickly react with Al and Si in the liquid aluminum to form intermetallic compounds (FeAlSi inhibition layer). The essence of the Kirkendall effect is that the rate of Fe diffusion to the outside is much greater than the rate of Al diffusion into the iron matrix, and the presence of a few microns of FeAlSi inhibition layer in the hot dip plating layer also fully demonstrates that there is indeed a phenomenon of Fe diffusion to the outside during hot dip plating, that is, there is the possibility of forming Kirkendall holes. Through a large number of microscopic observations by the inventors, it was found that there are indeed a large number of Kirkendall holes within 2μm from the interface between the FeAlSi inhibition layer and the substrate steel to the substrate steel, and their size is much smaller than the size of the hole after hot stamping, so it is not easy to find. The present invention believes that the thicker the FeAlSi inhibition layer is, the more Fe diffuses outward, and the easier it is to form Kirkendall holes. Reducing the thickness of the FeAlSi inhibition layer can reduce the diffusion of Fe atoms in the base steel to the outside, thereby reducing the formation of Kirkendall holes.

[0037] At the same time, the present invention finds that in the subsequent hot stamping process, the Kirkendall effect holes formed in the hot dip plating process are very likely to grow rapidly, significantly increasing the resistance of the coating during spot welding, thereby causing sparks to easily occur during welding, seriously affecting the resistance spot welding performance of the hot stamping formed component. Therefore, in order to ensure the resistance spot welding performance of the final part, the present invention hopes to achieve the purpose of suppressing the formation of Kirkendall holes by controlling the hot dip plating conditions.

[0038] To this end, the method of the present invention aims to obtain a thin FeAlSi inhibition layer and inhibit the formation of Kirkendall voids in the base steel near the interface between the FeAlSi inhibition layer and the base steel, so as to improve the stability of the coating thickness, eliminate the plating skipping condition and improve the resistance spot welding performance of the components subsequently hot stamped by the coated steel sheet.

[0039] The plating solution used in the present invention contains, by weight, 9% to 12% Si, 4% or less Fe, and the balance being Al or Al alloy and inevitable impurities.

[0040] Preferably, the Si content in the plating solution is 9.2% to 11.2% by weight.

[0041] The coating method of the coated steel sheet for hot stamping forming according to the present invention specifically comprises:

[0042] a) Pre-treat the base steel plate before coating;

[0043] b) heating the pretreated base steel plate and then cooling it to a predetermined temperature in the range of 610-650° C., preferably 620-645° C., more preferably 625-639° C., and further preferably 625-635° C.;

[0044] c) immersing the base steel sheet cooled to the predetermined temperature in b) in a heated plating solution for 2 to 7 seconds for hot dip plating, during which the plating solution temperature is higher than the predetermined temperature and maintained at 630 to 670° C., preferably 640 to 660° C.;

[0045] d) after the base steel sheet leaves the plating solution and before the plating solution on at least one surface solidifies, removing excess plating solution on at least one surface by air knife blowing to control the thickness of the coating on the at least one surface; and

[0046] e) cooling the steel sheet to room temperature to obtain a coated steel sheet having a thin aluminum alloy coating layer.

[0047] In the above method, pretreatment of the base steel plate includes, for example, degreasing, washing, descaling, warm water washing, plating assistance and drying. In step c), preferably, the plating solution temperature is 5 to 20°C higher than the predetermined temperature of the steel plate entering the plating solution, more preferably, 7 to 20°C. In step e), the cooling rate of the steel plate is preferably not less than 5°C / s.

[0048] In the method of the present invention, the plating solution is selected to have a high Si content. As the Si content in the plating solution increases, the melting point of the plating solution becomes lower, which is conducive to reducing the plating solution temperature, thereby inhibiting the mutual diffusion of Al and Fe atoms to obtain a reduced FeAlSi inhibition layer thickness, slowing down the formation and growth of Kirkendall holes near the surface of the base steel plate during the hot dip plating process and the subsequent hot stamping forming process of the coated steel plate. Therefore, the Si content is not less than 9%. However, the Si content should not be too high. Excessive Si content will increase the resistivity of the alloyed layer in the steel plate coating after hot stamping of the coated steel plate, and reduce the welding performance of its hot stamping formed components. Therefore, the Si content cannot exceed 12%. Preferably, the Si content of the present invention is 9.2% to 11.2%.

[0049] Secondly, the present invention proposes to reduce the temperature of the plating solution and the predetermined temperature at which the steel plate enters the plating solution (i.e., the temperature at which the steel plate enters the pot) to inhibit the formation of Kirkendall holes. As mentioned above, in the process of forming the FeSiAl inhibition layer, the Fe atoms in the base steel diffuse into the plating solution to form FeSiAl intermetallic compounds, and at the same time, the Al atoms diffuse into the Fe matrix. The diffusion of Fe atoms and Al atoms in the matrix is ​​carried out by the vacancy mechanism, that is, the metal atoms exchange positions with the vacancies to achieve diffusion. When the rate at which Al atoms enter the matrix is ​​not enough to make up for the number of Fe atoms diffused from the matrix, holes will be formed in the matrix due to the aggregation of vacancies. Therefore, inhibiting the thickness and growth rate of the FeAlSi inhibition layer can essentially inhibit the formation of Kirkendall effect holes. It is well known that temperature has a significant effect on the diffusion rate. Therefore, reducing the plating solution temperature and the predetermined temperature at which the steel plate enters the plating solution can inhibit the formation of Kirkendall holes. On the one hand, it is considered to reduce the predetermined temperature at which the steel plate enters the plating solution. At high temperatures, the difference in the diffusion rates of Fe and Al atoms increases, resulting in the formation of more large-sized Kirkendall holes. Experimental data show that when the predetermined temperature of the steel plate entering the plating solution is higher than 655°C, more large-sized Kirkendall holes are obviously formed in the base steel near the above-mentioned interface. Relatively speaking, in hot-dip plating, in order to ensure the plating property of the steel plate and prevent the occurrence of problems such as surface leakage plating, the predetermined temperature of the steel plate entering the plating solution should not be too low. Experimental data show that when the predetermined temperature of the steel plate entering the plating solution is lower than 610°C, leakage plating is serious. Therefore, according to the present invention, the predetermined temperature of the steel plate entering the plating solution is designed to be 610-650°C, preferably 620-645°C, more preferably 620-639°C, and further preferably 625-635°C. On the other hand, it is considered that lowering the plating solution temperature is conducive to inhibiting the alloying reaction between Fe, Al and Si atoms to form a thin inhibition layer, but, relatively, in order to ensure the fluidity and uniformity of the plating solution, the plating solution temperature should not be too low. Therefore, the plating solution temperature is designed to be higher than the predetermined temperature and be 630-670°C, preferably 640-660°C.

[0050] Furthermore, the present invention proposes to shorten the residence time of the steel plate in the plating solution. First, too long a residence time will promote the continued mutual diffusion of Fe and Al, resulting in the thickening of the FeAlSi inhibition layer and the formation of Kirkendall pores. Second, the length of the production line is limited, and if the residence time is too long, the production line will inevitably need to reduce the operating speed, which affects production efficiency and increases costs. Therefore, the residence time of the steel plate in the plating solution needs to be controlled within 2 to 7 seconds.

[0051] Finally, the thickness of the Al alloy layer is controlled by maintaining high-intensity blowing of the air knife to obtain a thin aluminum alloy coated steel sheet. Therefore, after the base steel sheet leaves the plating solution and before the plating solution on at least one surface solidifies, the excess plating solution on at least one surface is removed by air knife blowing to control the coating thickness on the at least one surface. Subsequently, the steel sheet is cooled to room temperature at a cooling rate preferably not less than 5°C / s to obtain a coated steel sheet with a thin aluminum alloy coating.

[0052] In addition, preferably, a relatively high bath temperature is taken into account to ensure coating performance and a low predetermined temperature of the steel plate entering the bath is taken into account to ensure a low interface reaction rate and reduce the formation of Kirkendall holes. The present invention specifically points out that the coating is carried out in a manner where the predetermined temperature is lower than the bath temperature. Preferably, the predetermined temperature is lower than the bath temperature by more than 5°C, which is beneficial for reducing the interface reaction rate while ensuring coating performance to reduce Kirkendall holes. At the same time, since an excessively large temperature difference between the steel plate and the bath will lead to an unstable bath temperature, the present invention designs the temperature difference to be no more than 20°C, preferably, the temperature difference is 7°C to 15°C.

[0053] The method of the present invention provides a coated steel sheet for hot stamping with a thin aluminum alloy coating having a thickness of 0.5 to 3.0 mm. On any surface of the steel sheet, the aluminum alloy coating has a coating thickness of 5 to 14 μm, preferably 6 to 13 μm, and more preferably 7 to 12 μm.

[0054] The above aluminum alloy coating has a unique coating structure, which includes:

[0055] A FeAlSi inhibition layer close to a base steel, wherein the thickness of the FeAlSi inhibition layer is not more than 60% of the coating thickness and is 1.5-6 μm, preferably not more than 50% of the coating thickness and is 1.5-5 μm, and more preferably not more than 40% of the coating thickness and is 2.45-3.95 μm; from the interface between the FeAlSi inhibition layer and the base steel to within 2 μm in the base steel, the diameter of the Kirkendall pores is less than 2.5 μm, wherein the number of Kirkendall pores with a diameter of more than 0.5 μm and less than 2.5 μm does not exceed 15 / 35 μm, preferably does not exceed 13 / 35 μm, and more preferably does not exceed 5 / 35 μm. Further preferably, the diameter of the Kirkendall pores is less than 2.0 μm, and the number of Kirkendall pores with a diameter of more than 0.5 μm and less than 2.0 μm is not more than 13 / 35 μm, preferably not more than 10 / 35 μm, and more preferably not more than 5 / 35 μm; and

[0056] Al alloy layer outside the FeAlSi inhibition layer.

[0057] The FeAlSi inhibition layer is a FeSiAl alloy compound layer formed by the reaction of Al atoms and Si atoms in the plating solution with Fe atoms on the surface of the steel plate when the steel plate is immersed in the plating solution. The main components include Fe2SiAl7, in which the mass ratio of Si element to the sum of Si and Al elements is greater than 0.12, which is higher than the Si content in the plating solution. The thickness of the Al alloy layer is adjusted by an air knife to achieve different aluminum-silicon coating thicknesses.

[0058] In order to meet the requirements of the hot stamping process for the hardenability of the steel plate, a martensite-based microstructure is formed in the hot stamping component and a strength of 900MPa to 2200MPa is achieved. The base steel plate contains the following components by weight: 0.05 to 0.45% C, 0.5 to 10% Mn, 0 to 0.01% B, 0 to 0.4% Nb+Ti+V, 0.01 to 2% Si, 0.01 to 2% Al, 0.01 to 5% Cr+Ni+Mo+Cu and 0 to 2% Cr, 0 to 2% Ni, 0 to 2% Mo and 0 to 2% Cu, as well as unavoidable impurity elements.

[0059] In the coated steel sheet of the present invention, the Kirkendall holes in the base steel near the interface between the FeAlSi inhibition layer and the base steel are small in diameter and small in number, which helps to reduce the formation of large-sized holes in the coating of the hot stamping formed component during the hot stamping process, thereby ensuring that the component has good resistance spot welding performance. When the coating thickness is determined, a thin FeAlSi inhibition layer means a thicker Al alloy layer, which is beneficial to air knife control, improves the stability of the coating thickness and prevents the occurrence of missed plating.

[0060] As an example, the test base steel plate has the composition shown in Table 1, and its corresponding manufacturing process is as follows:

[0061] a) Steelmaking: According to the composition in Table 1, steel is smelted in a vacuum induction furnace, an electric furnace or a converter, and cast ingots are produced by continuous casting technology, or thin slab continuous casting and rolling process is directly adopted;

[0062] b) Hot rolling: the steel billet is heated to 1120-1280°C for hot rolling, the total reduction of hot rolling is more than 50%, the final rolling temperature is above 800°C to obtain hot rolled steel sheet, and the hot rolled steel sheet is coiled at a temperature below 700°C to form hot rolled steel coil, and the hot rolled coil is pickled to remove the oxide scale generated during the hot rolling process; and

[0063] c) Cold rolling: The pickled hot-rolled coil is cold rolled with a cold rolling reduction of 30% to 70% to obtain a 1.4 mm cold-rolled steel coil.

[0064] Table 1 Chemical composition of base steel plate

[0065] (wt%, the balance is Fe and other inevitable impurity elements)

[0066] Base steel plate C Si Mn B Al Cr Nb Ti V Base steel 1 0.10 0.20 2.5 0.0031 0.04 0.22 / 0.04 / Base steel 2 0.21 0.25 1.4 0.0022 0.04 0.25 / 0.04 / Base steel 3 0.34 0.61 1.9 0.0025 0.65 0.15 0.04 ～ 0.06

[0067] The obtained base steel sheet is plated according to the coating process in Table 2, and the target coating thickness is 8 to 12 μm, wherein the plating solution contains by weight: 9% to 12% Si, less than 4% Fe, and the balance is Al or Al alloy and inevitable impurities. The coating process in Table 2 comprehensively considers the influence of hot dip plating process parameters such as plating solution temperature, the predetermined temperature of the steel sheet entering the plating solution (i.e., the steel sheet entering the pot temperature), the temperature difference between the plating solution and the steel sheet, the hot dip plating time, and the Si content in the plating solution.

[0068] Table 2 List of coating process parameters

[0069]

[0070] After the above coating and plating process, the steel coil is subjected to a macroscopic surface quality inspection to detect surface plating leakage. It should be pointed out that the surface plating leakage mentioned in this article includes any of the conditions of exposed base steel plate and exposed FeAlSi inhibition layer. At the same time, the thickness of the coating and the thickness of the FeAlSi inhibition layer are determined by selecting five positions at 1 / 6, 1 / 3, 1 / 2, 2 / 3 and 5 / 6 of the width of the steel coil, and measuring the thickness of the FeAlSi inhibition layer and the coating thickness under a scanning electron microscope (SEM), and averaging the measurement results of the five positions and giving the deviation.

[0071] Method for determining the number of Kirkendall holes: Count the Kirkendall holes within a length of 35 μm along the surface of the substrate steel in the field of view of the SEM and measure their diameter. Method for determining the diameter of the Kirkendall holes: Under the same field of view, measure the longest diameter and the shortest diameter of the hole, and take half of the sum of the two as the hole diameter.

[0072] The statistical results of the coating structure, macroscopic surface and number of Kirkendall holes are shown in Table 3.

[0073] Table 3 Coating structure, macroscopic surface and number of Kirkendall holes

[0074]

[0075] * The plating leakage in Comparative Examples 3 and 5 is too serious. The above values ​​are taken from part of the measurable area and cannot reflect the overall state of the coating.

[0076] It can be seen from Examples 1 to 8 that when the target coating thickness is 8 to 12 μm, the thickness of the FeAlSi inhibition layer obtained according to the method of the present invention is controlled at about 2.9 to 4.1 μm, so that the thickness of the Al alloy coating is controlled at about 5.1 to 8 μm, wherein the FeAlSi inhibition layer accounts for about 29% to 45% of the coating thickness. In this case, although the coating thickness of the steel plate is relatively thin, during the production process, due to the relatively thin thickness of the FeAlSi inhibition layer, the thickness of the Al alloy layer can still be adjusted by air knife blowing to more easily achieve the control of the target coating thickness. Therefore, the thickness fluctuation of the coating obtained in the end is small and there is no leakage. In addition, the maximum diameter of the Kirkendall holes near the interface between the base steel and the coating does not exceed 2 μm, and the number is generally no more than 13 / 35 μm, which is conducive to improving the resistance spot welding performance of the coated steel plate after hot stamping. For example, comparing the data of Example 5 and Example 8, the difference between the plating solution temperature and the steel plate entering the pot in Example 5 is 7°C, and the difference in Example 8 is 5°C. The number of Kirkendall pores in Example 8 is 8 / 35 μm, and the number of Kirkendall pores in Example 5 is 5 / 35 μm. It can be seen that an appropriate temperature difference is beneficial to further reduce the formation of Kirkendall pores.

[0077] Figure 1 This is a SEM photograph of the local coating morphology of the coated steel plate according to Example 5 of the present invention, the coating thickness is about 9.0 μm, wherein the thickness of the FeAlSi inhibition layer is about 3.2 μm, the diameter of the Kirkendall holes does not exceed 2.5 μm, wherein the number of Kirkendall holes with a diameter in the range of 0.5 μm to 2.5 μm is about 5 / 35 μm.

[0078] Figure 2 This is a SEM photograph of the local coating morphology of the coated steel plate of Example 4. The coating thickness is about 8.6 μm, wherein the thickness of the FeAlSi inhibition layer is about 6.7 μm, and the number of Kirkendall holes with a diameter of 0.5 μm to 2.5 μm is about 29 / 35 μm.

[0079] The coating process parameters of Example 5 and Comparative Example 4 differ only in the steel plate entering the pot temperature, where Comparative Example 4 has a significantly higher steel plate entering the pot temperature. Therefore, more Kirkendall holes and thicker FeAlSi inhibition layer in Comparative Example 4 are caused by the high steel plate entering the pot temperature. It can be seen that a high steel plate entering the pot temperature is undesirable.

[0080] Figure 3 The following is a typical photo of the plating defect of the coated steel sheet of Comparative Example 4. It can be clearly seen that the plating is serious in some areas. This is because compared with Example 5, the higher steel sheet entering the pot temperature of Comparative Example 4 accelerates the diffusion, resulting in a thicker FeAlSi inhibition layer and a thinner corresponding Al alloy layer, so the air knife purging requirements are high and the control is difficult, so the plating is missed.

[0081] Each comparative example shows different degrees of plating leakage defects and has large and numerous Kirkendall holes for the following reasons: in comparative example 1, the Si content in the aluminum plating solution is too low; in comparative example 2, the steel plate stays in the aluminum plating solution for too long; in comparative example 4, the temperature of the steel plate entering the pot is too high; and in comparative example 6, the plating solution temperature is too high. The above four situations all lead to the thickness of the FeAlSi inhibition layer finally obtained being thicker, reaching 6.6-7.5 μm, making the thickness of the Al alloy layer thinner and the thickness measurement results at different positions extremely deviate, and the thickness uniformity is poor, resulting in obvious fluctuations in the thickness of the final coating and local plating leakage, which affects the production stability of the steel plate. In addition, in the above four cases, the number of Kirkendall holes with a diameter of 0.5 μm to 2.5 μm in the base steel near the interface between the base steel and the FeAlSi inhibition layer is relatively large, reaching 17-29 / 35 μm. These large Kirkendall holes weaken the resistance spot welding performance of the hot stamping formed components obtained later. Therefore, it can be seen that low Si content, long residence time, high steel plate entry temperature and high bath temperature will all be conducive to diffusion, leading to the formation of more and larger Kirkendall holes. Therefore, these four factors must be controlled simultaneously to inhibit the formation of Kirkendall holes under their synergistic effect.

[0082] In addition, in Comparative Example 3, since the temperature of the steel plate entering the pot is low, the surface temperature of the steel plate is close to the solidification point of the Al-Si alloy, and therefore, the steel plate is poor in plating performance, and there are plating leakage problems in many areas. The large deviation also shows that the thickness of the FeAlSi inhibition layer obtained and the coating thickness control are extremely uneven. In Comparative Example 5, since the plating solution temperature is too low, the fluidity and uniformity of the plating solution are poor. This also leads to poor coating quality, uneven coating thickness (large deviation), and local plating leakage.

[0083] Combining the above data and the data in Table 2 and Table 3, it can be seen that the Si content in the plating solution, the temperature of the steel plate entering the pot, the plating solution temperature and the hot-dip plating time all have a significant impact on the thickness uniformity of the coating, the formation of skipped plating and Kirkendall holes. Any condition beyond the predetermined range will lead to uneven coating thickness, skipped plating or the formation and growth of more Kirkendall holes, weakening the performance of the product. The combined effect of the Si content in the plating solution, the temperature of the steel plate entering the pot, the plating solution temperature and the hot-dip plating time selected by the present invention not only eliminates the skipped plating condition but also reduces the number of large-sized Kirkendall holes, thereby improving the yield rate of the coated steel plate.

[0084] Correspondingly, the resistance spot welding performance of the subsequent hot stamping formed components is also affected by the combined effects of the Si content in the plating solution, the steel plate entry temperature, the plating solution temperature and the hot dip plating time. The following only takes Example 5 and Comparative Example 4 as examples to illustrate the influence of the coating process on the resistance spot welding performance of hot stamping formed components. The thin-coated plates of Example 5 and Comparative Example 4 were subjected to hot stamping simulation. The heating process was completed in a laboratory tubular furnace with a heating temperature of 930°C and a holding time of 240s. Subsequently, the heated sample was taken out and placed in a hot stamping simulation device and cooled to below 100°C within 8 to 10s. The coating morphology of the obtained hot stamping formed sample was observed, and the results are shown in FIG. Figure 4 shown.

[0085] Depend on Figure 4 It can be seen that under the same hot stamping conditions, the final coating thickness of Example 5 is about 20μm, and the thickness of the interdiffusion layer is about 8.55μm; while the final coating thickness of Comparative Example 4 is about 16.42μm, and the thickness of the interdiffusion layer is about 9.83μm. In addition, the Kirkendall holes of Comparative Example 4 have basically formed a linear distribution. This corresponds to the data in Table 3, in which the number of Kirkendall holes with an initial diameter of 0.5μm to 2.5μm in Example 5 and Comparative Example 4 is 5 / 35μm and 29 / 35μm, respectively, and the maximum diameter of Example 5 is 0.65μm, while the maximum diameter of the initial holes in Comparative Example 4 is 1.71μm. Comparative Example 4 has relatively more large-sized Kirkendall holes at the beginning, so the holes in Comparative Example 4 are obviously more serious after the same hot stamping forming process.

[0086] The resistance spot welding experiment was carried out on the obtained hot stamping flat plate. The welding method and evaluation criteria were based on the AWS D8.9M:2012 standard. Single pulse welding was selected and the welding parameters were as follows: electrode cap end face diameter 7 mm, electrode pressure 5.5 kN, electrode pre-pressing time 400 ms, welding time 360 ​​ms, and post-weld holding time 200 ms. Figure 5 It is the spot welding evaluation result after hot stamping of two coated steel plates. It can be seen from the figure that the spot welding weldable current range of the hot stamping sample of Example 5 is 1.2kA, and the minimum welding current for spatter is 7.8kA. In contrast, the spot welding weldable current range of the hot stamping sample of Comparative Example 4 is 0.8kA, and the minimum welding current for spatter is 7.4kA. Obviously, the hot stamping sample of Comparative Example 4 has a narrow weldable current range and a small spatter current. The experimental results show that the significant Kirkendall holes in Comparative Example 4 increase the contact resistance of the coating, making it easy to generate spark spatter even at a smaller welding current during spot welding, thereby reducing the weldable current range of the steel plate. In contrast, the coated steel plate (Example 5) with few and small Kirkendall holes obtained by the present invention improves the resistance spot welding performance of hot stamping formed components.

[0087] In summary, the coating thickness of the aluminum alloy coating of the coated steel plate of the present invention is 5 to 14 μm, wherein the thickness of the FeAlSi inhibition layer is 1.5 to 6 μm and not more than 60% of the coating thickness, and the diameter of the Kirkendall holes from the interface between the FeAlSi inhibition layer and the base steel to the 2 μm in the base steel is less than 2.5 μm, wherein the number of Kirkendall holes with a diameter of more than 0.5 μm and less than 2.5 μm does not exceed 15 / 35 μm. The aluminum alloy coated steel plate having the above-mentioned coating characteristics can be made into a hot stamping formed component with excellent resistance spot welding performance. The coating method for producing the coated steel plate according to the present invention ensures the uniformity of the coating thickness, avoids the occurrence of surface leakage plating, and at the same time suppresses the formation of large-sized Kirkendall holes, ensuring the good resistance spot welding performance of the hot stamping formed component.

[0088] The above embodiments and experimental data are intended to illustrate the present invention by way of example. It should be clear to those skilled in the art that the present invention is not limited to these embodiments, and various modifications may be made without departing from the scope of protection of the present invention.

Claims

1. A coated steel sheet for hot stamping with an aluminum alloy coating, comprising a base steel sheet and an aluminum alloy coating coated on at least one surface thereof, The base steel plate comprises the following components by weight percentage: 0.05-0.45% C, 0.5-10% Mn, 0-0.01% B, 0-0.4% Nb+Ti+V, 0.01-2% Si, 0.01-2% Al, 0.01-5% Cr+Ni+Mo+Cu, 0-2% Cr, 0-2% Ni, 0-2% Mo and 0-2% Cu, and the balance is Fe and unavoidable impurity elements; in, The aluminum alloy coating comprises: a FeAlSi inhibition layer close to the base steel; and an Al alloy layer outside the FeAlSi inhibition layer; Wherein, the thickness of the FeAlSi inhibition layer is not greater than 60% of the thickness of the coating, and the thickness of the FeAlSi inhibition layer is 1.5 to 6 μm; Among them, the diameter of the Kirkendall holes from the interface between the FeAlSi inhibition layer and the base steel to within 2 μm in the base steel is less than 2.5 μm, and the number of Kirkendall holes with a diameter of more than 0.5 μm and less than 2.5 μm does not exceed 15 / 35 μm.

2. The coated steel sheet according to claim 1, wherein: The base steel plate contains the following components by weight percentage: 0.09-0.39% C, 0.6-3.5% Mn, 0-0.004% B, 0-0.4% Nb+Ti+V, 0.01-2% Si, 0.01-2% Al, 0.01-5% Cr+Mo+Ni+Cu with 0-2% Cr, 0-2% Ni, 0-2% Mo and 0-2% Cu, and the balance is Fe and unavoidable impurity elements.

3. The coated steel sheet according to claim 1, wherein: The base steel plate comprises the following components by weight percentage: 0.18-0.39% C, 0.6-3.5% Mn, 0-0.004% B, 0.05-0.25% Nb+Ti+V, 0.01-2% Si, 0.01-2% Al, 0.01-5% Cr+Mo+Ni+Cu, 0-2% Cr, 0-2% Ni, 0-2% Mo and 0-2% Cu, and the balance is Fe and unavoidable impurity elements.

4. The coated steel sheet according to claim 1, wherein: The coating thickness of the aluminum alloy coating is 6-13 μm, and the thickness of the FeAlSi inhibition layer is not greater than 50% of the coating thickness.

5. The coated steel sheet according to claim 1, wherein: The coating thickness of the aluminum alloy coating is 7-12 μm, and the thickness of the FeAlSi inhibition layer is not greater than 40% of the coating thickness.

6. The coated steel sheet according to any one of claims 1 to 5, wherein the number of Kirkendall voids having a diameter of 0.5 μm or more and 2.5 μm or less within 2 μm from the interface between the FeAlSi inhibition layer and the base steel is not more than 13 / 35 μm.

7. The coated steel sheet according to any one of claims 1 to 5, wherein the number of Kirkendall voids having a diameter of 0.5 μm or more and 2.5 μm or less within 2 μm from the interface between the FeAlSi inhibition layer and the base steel is not more than 10 / 35 μm.

8. The coated steel sheet according to any one of claims 1 to 5, wherein the diameter of the Kirkendall pores from the interface between the FeAlSi inhibition layer and the base steel to within 2 μm in the base steel is less than 2.0 μm, and the number of Kirkendall pores with a diameter of more than 0.5 μm and less than 2.0 μm does not exceed 10 / 35 μm.

9. The coated steel sheet according to any one of claims 1 to 5, wherein: The thickness of the base steel plate is 0.5-3.0 mm.

10. The coated steel sheet according to claim 9, wherein: During the hot stamping process, the coated steel sheet is first heated to 880-960° C. for heat preservation.

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