Zinc-aluminum-magnesium coated steel plate and production process therefor

Abstract

Disclosed are a zinc-aluminum-magnesium coated steel plate and a production process therefor, belonging to the technical field of alloys. A plating solution for the coating consists of the following components in percentage by mass: 1.5-2.0% of Mg, 0.2-3.5% of Al and the balance being Zn and inevitable impurities. The production process for the zinc-aluminum-magnesium coated steel plate comprises the procedures of smelting, continuous casting, hot rolling, acid rolling, annealing, hot dipping, coiling and high-temperature forming, wherein in the hot dipping process, the temperature of the plating solution is controlled to be 440-480ºC, and the time period for dipping is controlled to be 5-10 s, and after the hot dipping, the steel plate is cooled to room temperature at more than 30ºC / s. In view of the problem of liquid metal embrittlement resulting from the hot forming procedure of the zinc-based coated steel plate, the microstructure composition of the coating is improved by changing the composition ratio of chemical elements in the plating solution and process optimization, the reaction of the zinc-aluminum-magnesium coated steel plate and Zn is inhibited, and a cracking problem on the surface of the steel plate during high-temperature forming is alleviated.

Description

Zinc-aluminum-magnesium coated steel plate and production process thereof Technical Field

[0001] The invention relates to a zinc-aluminum-magnesium coated steel plate and a production process thereof, belonging to the technical field of alloys. Background Art

[0002] Under the global climate of energy conservation and emission reduction, the automotive industry has also undergone significant changes. Reducing the weight of the entire vehicle to achieve energy conservation and emission reduction is of great significance. The application of hot forming technology can reduce vehicle weight, improve fuel economy, achieve energy conservation and emission reduction goals, and at the same time ensure a certain degree of safety. Commonly used hot-formed steels include bare plate, aluminum-silicon coating, and zinc-based coating. Bare plate is prone to surface oxidation and decarburization during the hot forming process, and suffers from poor corrosion resistance and reduced surface fatigue strength during use. The application of aluminum-silicon coating can prevent surface oxidation and decarburization of steel plates, but its production cost is high and its corrosion resistance during perforation and incision is poor. Research has begun to focus on zinc-based coatings, which have lower production costs and provide excellent cathodic protection. However, zinc-based coatings face the problems of volatilization of the zinc-based coating during the hot forming process and brittle fracture caused by liquid metal in the coating penetrating into the substrate. These problems have limited their use in the automotive field.

[0003] The cause of liquid metal cracks, according to existing literature 1 (Bi Wenzhen, Hong Jiyao, Wang Li. Research Progress of Hot Stamping Steel with Zinc-Based Coating [J]. Baosteel Technology, 2019(06):38-44.), shows that during the high-temperature forming process, the coating of pure Zn-coated steel sheets consists of a solid α-Fe(Zn) phase and liquid Zn, with the liquid Zn existing between the α-Fe(Zn). Cracks initiate at the interface between solid α-Fe(Zn) and liquid Zn. The liquid Zn continuously diffuses along the austenite grain boundaries and continuously reacts with the substrate to form an α-Fe(Zn) layer. Under the action of external stress, the cracks continuously extend along the interface between α-Fe(Zn) and liquid Zn into the substrate, causing the substrate to break. Based on the above reasons, alloy elements such as Al and Ni can be added to the pure zinc plating solution to generate alloy inhibition layers such as Fe2Al5 at the interface between the coating and the substrate to hinder the diffusion of zinc into the substrate, that is, to inhibit the reaction of zinc with Fe in the matrix and hinder the formation of Fe-Zn brittle phase.

[0004] According to existing literature 2 (Qiao Changle. Microstructure Evolution and Crack Control of Zinc-Based Coatings on 22MnB5 Hot-Formed Steel [D]. Northeastern University, 2018. DOI: 10.27007 / d.cnki.gdbeu.2018.000250.), the Al element added to the pure zinc plating bath reacts with the Fe element before the Zn element during the hot-dip plating process to form an Fe2Al5 alloy inhibition layer, which hinders the reaction between Fe and the zinc solution. The presence of this inhibition layer effectively suppresses the formation of brittle phases. However, as the temperature of the coated plate increases during the austenitization process, the Al element in the alloy layer migrates to the coating surface, destroying the Fe2Al5Fe2Al5 alloy inhibition layer and losing its inhibitory effect. A large amount of Zn in the coating diffuses into the substrate, forming an α-Fe(Zn) phase, which causes the substrate to fracture.

[0005] To address the above-mentioned issues, existing document 3 (Ultra-high-strength zinc-aluminum-magnesium coated steel sheet for high-temperature forming and its manufacturing method, CN107099748B[P].2019) discloses an ultra-high-strength zinc-aluminum-magnesium coated steel sheet for high-temperature forming and its manufacturing method. The coating composition is: 1.5% to 20% Al element, 1.3% to 7% Mg element, and the remainder is Zn element and unavoidable impurities. The coating before high-temperature forming includes three structures: Zn / Al eutectic structure, Zn / Al / Zn2Mg ternary eutectic structure and pure Zn particles. However, this technology still has the following problems: the content of Zn / Al / Zn2Mg ternary eutectic structure and the enrichment of zinc in the coating alloy are difficult to control, and the high-temperature reaction of locally enriched Zn with the steel substrate is difficult to suppress, resulting in high-temperature cracking of the hot-dip galvanized steel sheet. In addition, in order to form the eutectic of Zn / Al and Zn / Al / Zn2Mg, a large amount of Al and Mg needs to be added. For hot-formed steel, the volatilization of Mg and Zn during the high-temperature austenitization process can easily lead to the formation of pores and cracks on the coating surface, resulting in a decrease in the surface quality of the steel plate.

[0006] Summary of the Invention

[0007] The purpose of the present invention is to provide a low-aluminum, low-magnesium zinc-aluminum-magnesium coated steel plate for high-temperature forming and a preparation method thereof. In order to solve the problem of liquid metal embrittlement caused by the zinc-based coated plate during the hot forming process, the microstructure of the coating is improved by changing the chemical element composition ratio in the plating solution and optimizing the process, thereby inhibiting the reaction between the zinc-aluminum-magnesium coated steel plate and Zn, and reducing the problem of cracking on the surface of the steel plate during the high-temperature forming process. The present invention also elaborates on the chemical composition design of the steel plate, the full-process production process and the preparation method of the low-aluminum, low-magnesium zinc-aluminum-magnesium coated steel plate to ensure smooth production.

[0008] A zinc-aluminum-magnesium coated steel plate. The plating solution used for the coating consists of the following components by mass percentage: Mg: 1.5-2.0%, Al: 0.2-3.5%, and the rest is Zn and inevitable impurities.

[0009] In the zinc-aluminum-magnesium coated steel sheet of the present invention, the structure of the coating before high-temperature forming varies slightly according to the Al content in the plating solution. A large amount of dendrite structure exists in the coating, and Mg / Zn eutectic is distributed in the gaps between the dendrite structure. When the Al content is 0.2% to 1.5%, the Mg / Zn eutectic region mainly contains Mg2Zn 11 When the Al content is between 1.5% and 3.5%, the Mg / Zn eutectic region primarily consists of the MgZn2 phase. In the intermediate layer at the interface between the coating and the substrate, Al and Fe form the Fe2Al5 phase. As the Al content increases, its enrichment in the intermediate layer becomes more pronounced, making the intermediate layer denser. Furthermore, increasing Al content refines the grain size of the coating. When the Al content is between 2.5% and 3.5%, the steel plate exhibits the best corrosion and liquid metal crack resistance.

[0010] Preferably, the plating solution used for the coating layer is composed of the following components by mass percentage: Mg: 1.5-2.0%, Al: 2.5-3.5%, and the rest is Zn and unavoidable impurities.

[0011] There are a lot of dendrites in the coating of the present invention, and Mg / Zn eutectics are distributed in the gaps between the dendrites. The Mg / Zn eutectic of the present invention is MgZn2 phase eutectic or Mg2Zn 11 Phase eutectic.

[0012] Furthermore, when the Al content is between 0.2% and 1.5%, the Mg / Zn eutectic region mainly contains Mg2Zn 11 phase; when the Al element content is between 1.5% and 3.5% (excluding 1.5%), the Mg / Zn eutectic region mainly contains MgZn2 phase.

[0013] The structure of the coating before high temperature forming varies according to the Al content in the plating solution. When the Al content is 0.2% to 1.5%, the main phase in the Mg / Zn eutectic region is Mg2Zn 11 phase, a small amount of MgZn2 phase; when the Al content exceeds 1.5%, the main phase in the Mg / Zn eutectic region is MgZn2 phase, a small amount of Mg2Zn 11 Mutually.

[0014] In the coating of the present invention, in the intermediate layer at the junction of the coating and the substrate, Al elements and Fe elements form Fe2Al5 phase. As the Al content increases, its enrichment in the intermediate layer becomes more obvious, the intermediate layer becomes denser, and the grains in the coating become finer.

[0015] In the zinc-aluminum-magnesium coated steel plate of the present invention, the steel plate substrate is composed of the following components by mass percentage: C: 0.30-0.36%, Si: 0.05-0.5%, Mn: 1.0-2.0%, P not more than 0.050%, S not more than 0.008%, Al ≥ 0.080%, Ti: 0.020-0.040%, V: 0.1-0.3%, O not more than 0.005%, N not more than 0.005%, RE: 0.01-0.05%, B: 0.0005-0.0040%, and the rest is Fe and unavoidable impurities.

[0016] The zinc-aluminum-magnesium coated steel plate of the present invention has a thickness specification of 1.0-2.5 mm and a width specification of 850 mm-1800 mm. After high-temperature forming, the tensile strength is ≥1900 MPa and the elongation is ≥6%.

[0017] Another object of the present invention is to provide a production process for the above-mentioned zinc-aluminum-magnesium coated steel sheet.

[0018] A production process for zinc-aluminum-magnesium coated steel plates, comprising smelting, continuous casting, hot rolling, pickling, annealing, hot-dip plating, coiling, and high-temperature forming steps, wherein:

[0019] In the hot-dip plating process, the plating solution temperature is controlled at 440-480° C., the plating time is controlled at 5-10 seconds, and after hot-dip plating, the plating solution is cooled to room temperature at a speed of more than 30° C. / s.

[0020] Furthermore, in the annealing process, the dew point is controlled at -50°C to -20°C, the heating temperature is controlled at 700°C to 850°C, and the cooling is carried out at a cooling rate of 30°C to 50°C / s to 420°C to 500°C.

[0021] Furthermore, the high-temperature forming process is direct forming, the heating temperature of the steel plate is 850-900°C, the holding time is 4-10 minutes, and then the steel plate is quickly transferred to the mold for stamping, and the mold closing temperature is controlled at 700-750°C. After stamping, the sheet is quickly cooled to room temperature in the mold at a cooling rate of 100-140°C / S.

[0022] Furthermore, the steel plate substrate is made by the following smelting method: the pretreated molten iron is smelted in a converter and refined in an LF furnace and an RH furnace. The molten iron pretreatment ensures that the S content of the finished product meets the internal control requirements, the P element content of the molten iron entering the converter is ≤0.1%, the temperature out of the converter is ≥1600°C, the LF N increase is controlled, the N increase amount is ≤20ppm, and the standing time after RH is ≥15min.

[0023] Furthermore, during the continuous casting process, the pulling speed is within the range of 1.6 to 2.4 m / min and is constant.

[0024] Furthermore, in the hot rolling process, the slab is heated to 1250-1300°C, the finishing temperature is 780-900°C, and the coiling temperature is 600-720°C.

[0025] The beneficial effects of the present invention are as follows: after the technology of the present invention has been tested, in the salt spray test of Zn-Al-Mg coating, it was found through SEM and EDS characterization that the MgZn2 phase in the Mg-containing coating was corroded first, protecting the Zn in the dendrites, and the Mg-Zn phase improved the corrosion resistance of the coating. 11 The two Mg-Zn phases have a lower anodic dissolution current than the pure zinc phase, which also improves the corrosion resistance of the coating. After hot-dip plating is completed, the Al element in the coating is enriched at the interface between the coating and the substrate and reacts with the Fe element to form a Fe-Al intermediate alloy layer, which hinders the diffusion of the Zn element into the substrate and prevents the generation of small cracks. After the coating plate is austenitized, the coating is continuous and complete, without cracks that diffuse into the substrate; due to its easy oxidation when heated, the Mg element quickly diffuses to the surface of the coating at high temperatures to form an oxide layer to prevent the volatilization of the zinc liquid, and the position of the Fe-Al alloy layer moves toward the surface of the coating. At the same time, compared with the traditional process, the hot stamping process of the steel plate in the present invention reduces the heating temperature and energy consumption while meeting the performance requirements, which meets the requirements of green and low-carbon development. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 shows the microstructure observation photos of the steel plate after continuous annealing and hot stamping in Example 1, where (a) and (b) are microstructure observation photos of sample 1 at 100 times and 500 times, respectively; (c) and (d) are microstructure observation photos of sample 2 at 100 times and 500 times, respectively; (e) and (f) are microstructure observation photos of samples 3 and 4 at 500 times, respectively.

[0027] FIG2 is a photograph showing the microstructure of the Zn-Al-Mg alloy coating on the steel plate after hot-dip coating in Example 1.

[0028] FIG3 is a microstructure photograph of the cross section of the coating after austenitization of the steel plate in Example 1.

[0029] FIG4 is a microstructure observation photograph of the coating cross section of the steel plate after hot-dip coating in Comparative Example 1.

[0030] FIG5 is a microstructure photograph of the cross section of the coating after austenitization of the steel plate in Comparative Example 1.

[0031] Figure 6 shows the crack morphology at the bending part of the steel plates in Example 1 and Comparative Example 1, where (a) is the morphology at the bending part of the Zn-3.5Al-1.8Mg coated plate after hot stamping; (b) is the morphology at the bending part of the pure Zn coated plate after hot stamping.

[0032] FIG7 is a microstructure photograph of the cross section of the coating after austenitization of the steel plate in Example 2.

[0033] FIG8 is a microstructure photograph of the cross section of the coating after austenitization of the steel plate in Example 2.

[0034] FIG9 is a microstructure observation photograph of the steel plate after hot stamping in Example 3, wherein (a) and (b) are microstructure observation photographs of samples 5 and 6 at 500 times magnification, respectively.

[0035] FIG10 is an EDS analysis of the cross section of the steel plate after hot stamping and coating in Example 4.

[0036] FIG11 is a microstructure observation photograph of the steel plate after hot stamping in Example 4, wherein (a) and (b) are microstructure observation photographs of samples 7 and 8 at 500 times magnification, respectively.

[0037] FIG12 is an EDS analysis of the cross section of the coating on the steel plate after hot stamping in Example 5.

[0038] FIG13 is a microstructure observation photograph of the steel plate after hot stamping in Example 5, wherein (a) and (b) are microstructure observation photographs of samples 9 and 10 at 500 times magnification, respectively.

[0039] Figure 14 shows the salt spray test results of steel plates with different coatings, among which (a) to (c) are the 81-hour test results of Zn-3.5Al-1.8Mg coating, pure Zn coating, and Al-Si coating, respectively; (d) and (e) are the 157-hour test results of Zn-3.5Al-1.8Mg coating and Al-Si coating, respectively; (f) and (g) are the 229-hour test results of Zn-3.5Al-1.8Mg coating and Al-Si coating, respectively; (h) and (i) are the 283-hour test results of Zn-3.5Al-1.8Mg coating and Al-Si coating, respectively; (j) and (k) are the 396-hour test results of Zn-3.5Al-1.8Mg coating and Al-Si coating, respectively.

[0040] Figure 15 shows the polarization curve test results of steel plates with different coatings, where (a) and (b) are the test results of Zn-3.5Al-1.8Mg coating; (c) and (d) are the test results of pure Zn coating; (e) and (f) are the test results of Al-Si coating. DETAILED DESCRIPTION

[0041] The following non-limiting examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0042] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are all commercially available unless otherwise specified.

[0043] Chinese invention patent number ZL 2016 10535069.3 discloses a bare-sheet hot-stamped steel product. This 2GPa hot-stamped steel was developed by Benxi Iron and Steel in 2017 and industrialized for use in the door anti-collision beams of BAIC New Energy vehicles, marking its global debut. To further improve oxidation resistance, break free from the constraints of foreign patents, and advance the localization of hot-stamped steel, this study, based on this bare-sheet hot-stamped steel, modified the process to develop a low-aluminum, low-magnesium zinc-aluminum-magnesium-coated automotive hot-stamped steel and its preparation method.

[0044] A method for preparing a low-aluminum, low-magnesium zinc-aluminum-magnesium coated steel plate for high-temperature forming.

[0045] The mass percentages of the steel plate are: C: 0.30-0.36%, Si: 0.05-0.5%, Mn: 1.0-2.0%, P not more than 0.050%, S not more than 0.008%, Al≥0.080%, Ti: 0.020-0.040%, V: 0.1-0.3%, O not more than 0.005%, N not more than 0.005%, RE: 0.01-0.05%, B: 0.0005-0.0040%, and the rest are Fe and unavoidable impurities.

[0046] The plating solution composition should contain the following chemical elements and their mass percentages: Mg: 1.5-2.0%, Al: 0.2-3.5%, and the rest being Zn and unavoidable impurities.

[0047] The preparation method of the low-aluminum and low-magnesium zinc-aluminum-magnesium coated steel plate for high-temperature forming includes smelting, continuous casting, hot rolling, pickling, annealing, hot-dip plating, coiling, high-temperature forming and other processes, wherein:

[0048] In the annealing process, the dew point is controlled at -50°C to -20°C, and the heating temperature is controlled at 700°C to 850°C; in the hot-dip plating process, the plating solution temperature is controlled at 440°C to 480°C, and the dipping time is controlled at 5 to 10 seconds; in the high-temperature direct forming process, the heating temperature of the steel plate is 850 to 900°C, the holding time is 4 to 10 minutes, and then it is quickly cooled to room temperature.

[0049] The steel plate is made by the following smelting method: after pretreatment, the molten iron is smelted in a converter and refined in an LF furnace and an RH furnace. The molten iron pretreatment ensures that the S content of the finished product meets the internal control requirements, the P content of the molten iron entering the converter is ≤0.1%, the temperature out of the converter is ≥1600°C, the LF N increase is controlled to be ≤20ppm, and the standing time after RH is ≥15min.

[0050] During the continuous casting process, the pulling speed is within the range of 1.6 to 2.4 m / min, the pulling speed is constant, and the continuously cast billet is required to be free of burrs.

[0051] The slab is heated to 1250-1300°C, the final rolling temperature is 780-900°C, and the coiling temperature is 600-720°C.

[0052] The steel billet must be in the heating furnace for more than 110 minutes, and the temperature of the steel billet must be less than 1150° C. when it undergoes finish rolling.

[0053] During the annealing process, the annealing temperature needs to be controlled at 700-850°C, and cooled to 420-500°C at a cooling rate of 30-50°C / s. After hot-dip coating, the temperature needs to be cooled to room temperature at a rate greater than 30°C / s.

[0054] The coated steel plate obtained by the above method has a thickness specification of 1.0-2.5 mm and a width specification of 850 mm-1800 mm. After high-temperature forming, the tensile strength is ≥1900 MPa and the elongation is ≥6%.

[0055] Example 1

[0056] The preparation of a low-aluminum, low-magnesium zinc-aluminum-magnesium coated steel plate for high-temperature forming comprises the following steps:

[0057] Step 1, billet production: All raw materials are smelted into molten iron according to the designed chemical composition. This process includes hot metal pretreatment, converter smelting, refining outside the furnace, and continuous casting. The steel plate comprises the following chemical elements by weight: C 0.33%, Si 0.2%, Mn 1.45%, P no more than 0.03%, S no more than 0.006%, Al 0.08%, Ti 0.02%, V 0.18%, N no more than 0.005%, Re 0.03%, B 0.002%, and the remainder being Fe and unavoidable impurities. The casting speed during continuous casting is controlled at 2 m / min.

[0058] Step 2: heating, rolling, controlled cooling and coiling the steel billet after continuous casting, with the final rolling temperature being 880°C and the coiling temperature being 680°C.

[0059] Step 3: The hot-rolled sheet is pickled, rolled and continuously annealed at a pickling speed of 200 mpm, a rolling reduction of 65%, a continuous annealing temperature of 780°C and a dew point of -30°C.

[0060] Step 4, galvanizing process, the plating solution composition is 3.5% Al element, 1.8% Mg element, and the rest is Zn element and inevitable impurities, the immersion time is 10 seconds, the plating solution temperature is 460°C, and after the immersion plating is completed, it is cooled to room temperature at a cooling rate greater than 30°C / s to finally obtain a zinc-aluminum-magnesium coated steel plate.

[0061] Step 5, high temperature forming process, heat the steel plate to 875℃ to make it completely austenitized, keep it warm for 5 minutes, then stamp it in a mold, and then quickly cool it to room temperature to obtain a zinc-aluminum-magnesium coated steel plate based on hot dip coating and high temperature forming technology.

[0062] The metallographic structure of the steel plate after continuous annealing prepared according to the process in step 3 was tested, and two samples were taken, numbered 1# and 2#; the metallographic structure of the steel plate after hot stamping was tested, and two samples were taken, numbered 3# and 4#. The test results are shown in Figure 1.

[0063] The mechanical properties of the steel plates in three states, namely hot rolling, continuous annealing and hot stamping, were tested respectively. Three specimens were taken for tensile tests in each process. The mechanical properties results after hot rolling are shown in Table 1, the mechanical properties results after continuous annealing are shown in Table 2, and the mechanical properties results after hot stamping are shown in Table 3.

[0064] Table 1 Mechanical properties after hot rolling

[0065] Table 2 Mechanical properties test after continuous retreat

[0066] Table 3 Mechanical properties after hot stamping

[0067] According to the test results of the microstructure (Figure 1) and mechanical properties of the steel plates produced by the process of this embodiment after each process, it can be found that the microstructure of the steel plates after continuous annealing is composed of F+P, with low overall strength and high elongation, meeting the standard requirements and meeting the customer's usage conditions; and during the hot stamping process, since the steel plates are completely austenitized at high temperatures, the original microstructure is transformed from F+P to a fully martensitic structure during cooling to room temperature, the strength is greatly improved, the tensile strength reaches more than 2 GPa, and the elongation is also greater than 6%, with good performance.

[0068] The microstructure of a cross-section of a Zn-Al-Mg-coated plate after hot-dip coating and before stamping was observed, as shown in Figure 2. The coating thickness is approximately 30 to 40 μm, and the alloy layer is thin and almost invisible. The coating contains a large amount of dendrites, with a dense Zn+MgZn₂ eutectic distributed in the gaps between the dendrites. The MgZn₂ phase contributes to the corrosion resistance of the coating.

[0069] The microstructure of the Zn-Al-Mg coating after austenitization at 910°C was observed in a cross-section. A microstructure photograph of the alloy coating after austenitization of the steel plate is shown in Figure 3. The cross-section consists of a uniform alloyed coating with even distribution of Zn, Al, and Mg. No delamination was observed, and the coating was dense, free of voids and cracks.

[0070] Comparative Example 1

[0071] In order to demonstrate the influence of Al and Mg elements in the plating solution on the coating performance, Comparative Example 1 is provided. Comparative Example 1 is a pure Zn-plated plate, that is, the plating solution is completely composed of zinc elements and does not contain Al and Mg elements. The preparation steps not specifically described are the same as those in Example 1. Since other process parameters remain unchanged, the microstructure and performance tests of the steel plate in Comparative Example 1 are not performed.

[0072] The microstructure of the pure zinc coating cross-section of the hot-dip coated steel sheet was observed. Figure 4 shows the microstructure of the cross-section of the hot-dip coated layer. The coating thickness is approximately 30 μm and consists of two layers: an upper Zn phase and a lower Fe-Zn phase. No eutectic structure is present. Numerous microcracks exist in the lower Fe-Zn phase, but they do not extend into the substrate.

[0073] The microstructure of the pure Zn coating after austenitization was observed. A microstructure photograph of the coating after austenitization is shown in Figure 5. The coating is approximately 30 μm thick and contains a large amount of single-phase structure. Numerous cracks are present at the coating-substrate interface and extend into the steel substrate. Due to the continuous diffusion of Fe from the substrate into the coating, the coating is primarily composed of α-Fe(Zn) and the FeZn7 phase at the interface between the steel substrate and the coating.

[0074] Compared with Comparative Example 1, the Mg element added in Example 1 produces Mg / Zn eutectic in the coating, thereby improving the corrosion resistance of the coating; the added Al and Mg elements make the coating more uniform and dense after austenitization, thereby reducing the generation of microcracks.

[0075] Traditional pure Zn-coated steel sheets face the problem of LEM during hot stamping. To verify whether the Zn-Al-Mg-coated steel sheets designed in the present invention can effectively address this LEM issue, U-die stamping experiments were conducted on the steel sheets from Example 1 and Comparative Example 1. Post-test crack analysis at the bend was performed, and the results are shown in Figure 6. Microstructural observations reveal that cracks in the Zn-Al-Mg-coated sheet terminate at the substrate and do not extend into it, while a small number of cracks extend into the substrate in the pure Zn-coated sheet. This design of the present invention can help reduce the LEM problem.

[0076] Example 2

[0077] In order to explore the effect of adjusting the Al content on the liquid metal embrittlement problem in high-strength steel coated with a Zn-Al-Mg alloy coating, Example 2 is provided. Example 2 is a Zn-Al-Mg coated steel plate in which the mass percentage of each chemical element contained in the plating solution is different from that in Example 1. The preparation steps not specifically described are the same as those in Example 1, except that: the plating solution composition: 0.2% Al element, 1.8% Mg element, and the rest is Zn element and unavoidable impurities. Since other process parameters remain unchanged, the microstructure and performance testing of the steel plate in Example 2 is not performed.

[0078] The microstructure of the Zn / Al / Mg coating cross section of the hot-dip coated steel sheet was observed. The microstructure observation picture of the coating cross section after hot-dip coating is shown in Figure 6. A large number of dendrites appeared in the coating, and the dendrite size was larger than that in Example 1. This was mainly due to the deviation from the eutectic composition caused by the reduction of Al element. Dense Mg / Zn eutectics were distributed in the gaps between the dendrites. The eutectic region was composed of Mg2Zn 11 and MgZn2 phase, of which Mg2Zn 11 The content of the phase is higher, and both phases contribute to improving the corrosion resistance of the coating. In addition, the intermetallic compound layer at the interface between the coating and the substrate is relatively continuous, and the Mg element is enriched in the top layer of the coating due to oxidation in contact with air.

[0079] The microstructure of the cross section of the Zn-Al-Mg coating after austenitization was observed, and a microstructure photograph of the cross section of the coating after austenitization is shown in Figure 7. The backscattered image shows that the Fe, Zn, Al, and Mg elements are evenly distributed.

[0080] By comparing Example 1 and Example 2, it can be seen that the increase in the Al content in the coating leads to finer grains in the coating, an increase in the amount of dendrite structure, and a thinner thickness of the diffusion layer after austenitization, indicating that increasing the Al content in the coating can inhibit the diffusion of Zn elements in the coating into the substrate.

[0081] Example 3

[0082] A low-aluminum, low-magnesium zinc-aluminum-magnesium coated steel sheet for high-temperature forming should contain the following chemical elements: 0.35% C, 0.4% Si, 1.8% Mn, no more than 0.03% P, no more than 0.006% S, 0.1% Al, 0.04% Ti, 0.28% V, no more than 0.005% N, 0.045% Re, 0.004% B, and the remainder being Fe and unavoidable impurities. The chemical element composition of the plating solution is the same as that of Example 1.

[0083] The hot rolling process maintained a finishing temperature of 850°C, coiling at 700°C, and annealing at 750°C with a dew point of -40°C. The immersion time was 10 seconds, and the bath temperature was 440°C. During hot forming, the temperature was heated to 870°C, held for 3 minutes, and then stamped and cooled to room temperature. Metallographic and mechanical property testing was performed on the hot stamped specimens. The metallographic results for two specimens, numbered 5# and 6#, are shown in Figure 8. The mechanical property results for two specimens are shown in Table 4.

[0084] Table 4. Mechanical properties after hot stamping

[0085] The plating bath composition remained unchanged from Example 1, so the sample coatings were not tested. Metallographic results showed that the prepared steel plate had a completely martensite structure after high-temperature forming, resulting in high strength. Mechanical properties revealed a tensile strength exceeding 1800 MPa and an elongation exceeding 6%, demonstrating excellent performance and meeting application requirements.

[0086] Example 4

[0087] A low-aluminum, low-magnesium zinc-aluminum-magnesium coated steel sheet for high-temperature forming should contain the following chemical elements: C 0.31%, Si 0.1%, Mn 1.2%, P not exceeding 0.03%, S not exceeding 0.006%, Al 0.09%, Ti 0.02%, V 0.1%, N not exceeding 0.005%, Re 0.015%, B 0.001%, with the remainder being Fe and unavoidable impurities. The chemical composition of the plating bath is: Al 1.5%, Mg 1.8%, with the remainder being Zn and unavoidable impurities.

[0088] The hot rolling process maintained a finishing temperature of 800°C, coiling at 660°C, and annealing at 820°C during the continuous annealing process, with a dew point of -30°C. The galvanizing process was the same as in Example 1, with heating to 890°C for high-temperature forming, followed by a 10-minute hold, followed by stamping and cooling to room temperature. The microstructure of the cross-section of the hot-dip galvanized coating was observed, as shown in Figure 9. EDS analysis of the cross-section of the coating was performed, as shown in Figure 10. Metallographic and mechanical property testing was performed on the hot-stamped specimens. The metallographic results of two specimens, numbered 7 and 8, are shown in Figure 11. The mechanical property results of two specimens are shown in Table 5.

[0089] Table 5. Mechanical properties after hot stamping

[0090] The morphology of the coating after hot-dip coating is similar to that of Example 1. A large number of fine dendrites appear in the coating, and dense Mg / Zn eutectics are distributed in the gaps between the dendrites. Mg2Zn exists in the eutectic area. 11 MgZn2 phase, in which Mg2Zn 11 A higher phase content helps improve corrosion resistance. In the intermediate layer at the junction of the coating and the substrate, the ratio of Fe and Al atoms approaches that of the Fe2Al5 phase, preventing liquid metal embrittlement. The microstructure after hot stamping is fully martensite, and the mechanical properties are excellent, similar to those of Examples 1, 2, and 3.

[0091] Example 5

[0092] A low-aluminum, low-magnesium zinc-aluminum-magnesium coated steel sheet for high-temperature forming should contain the following chemical elements: C 0.32%, Si 0.3%, Mn 1.6%, P not exceeding 0.03%, S not exceeding 0.006%, Al 0.09%, Ti 0.03%, V 0.15%, N not exceeding 0.005%, Re 0.02%, B 0.0025%, with the remainder being Fe and unavoidable impurities. The chemical composition of the plating bath is: Al 2.5%, Mg 1.8%, with the remainder being Zn and unavoidable impurities.

[0093] The hot rolling process maintained a finishing temperature of 900°C, coiling at 620°C, and annealing at 800°C during the continuous annealing process, with a dew point of -30°C. The immersion time was 10 seconds, and the bath temperature was 480°C. During hot-dip forming, the zinc coating was heated to 910°C, held at that temperature for 10 minutes, and then stamped and cooled to room temperature. The microstructure of the zinc coating after hot-dip coating was observed, as shown in Figure 12. EDS analysis of the coating cross-section was performed, as shown in Figure 13. Metallographic and mechanical property tests were performed on the hot-stamped specimens. The metallographic results of two specimens, numbered 9# and 10#, are shown in Figure 14. The mechanical property results of two specimens are shown in Table 6.

[0094] Table 6. Mechanical properties after hot stamping

[0095] With the increase of Al content in the plating solution, the grains are refined after hot dip plating, and Mg2Zn exists in the eutectic region. 11 Phase and MgZn2 phase, MgZn2 phase content is higher, the two phases help to improve the corrosion resistance of the material; there is Fe2Al5 phase in the middle layer at the junction of the coating and the substrate. As the Al element content of the plating solution increases, Al enrichment in the middle layer becomes more obvious, the middle layer becomes denser, and the ability to resist liquid metal cracks is stronger. After high-temperature stamping, the mechanical properties are good and the elongation is high.

[0096] To more directly demonstrate the improved corrosion resistance of the newly designed low-aluminum, low-magnesium zinc-aluminum-magnesium coating, salt spray tests and polarization curve tests were conducted on Zn-3.5Al-1.8Mg-coated steel sheets, pure Zn-coated steel sheets, and the more common Al-Si-coated steel sheets on the market. The salt spray test results are shown in Figure 14. Analysis of the surface conditions of the differently coated steel sheets at the same time reveals that red rust has begun to appear on the surface of the pure Zn-coated steel sheet after 81 hours, while the surfaces of the zinc-aluminum-magnesium and aluminum-silicon coatings remain in good condition. Therefore, the corrosion resistance of the pure Zn coating is not as good as that of the other two coatings. Subsequent observations will be conducted on the surfaces of the zinc-aluminum-magnesium and aluminum-silicon-coated steel sheets. As time goes by, the surface quality of the Al-Si coated steel plate decreases significantly after 157h. After 229h, the surface quality of the Al-Si coated steel plate is obviously not as good as that of the Zn-3.5Al-1.8Mg coated steel plate. After 283h, red rust can be vaguely seen on the surface of the Al-Si coated plate. After 396h, the red rust on the surface of the Al-Si coated plate is very obvious. At this time, the surface of the Zn-3.5Al-1.8Mg coated plate is still in good condition and no red rust is seen. This shows that the corrosion resistance of the designed zinc-aluminum-magnesium coated plate is greatly improved compared with the pure Zn coated plate and the aluminum-silicon coated plate.

[0097] The polarization curve test results are shown in Figure 15. According to the polarization curve test results, it can be seen that the potential of the Zn-3.5Al-1.8Mg plate is much lower than that of the Al-Si plate, indicating that the Zn-3.5Al-1.8Mg plate can play a cathodic protection role compared with the Al-Si plate, which is beneficial to improving the corrosion resistance of the material.

Claims

1. A zinc-aluminum-magnesium coated steel sheet, characterized in that: The plating solution used for the coating consists of the following components by mass percentage: Mg: 1.5 - 2.0%, Al: 0.2 - 3.5%, and the balance is Zn and inevitable impurities.

2. The steel sheet according to claim 1, characterized in that: The plating solution used for the coating consists of the following components by mass percentage: Mg: 1.5 - 2.0%, Al: 2.5 - 3.5%, and the balance is Zn and inevitable impurities.

3. The steel sheet according to claim 1, characterized in that: A large number of dendritic structures exist in the coating, and Mg / Zn eutectic is distributed in the gaps between the dendritic structures.

4. The steel sheet according to claim 3, characterized in that: When the Al element is in the range of 0.2% to 1.5%, the Mg / Zn eutectic region mainly contains Mg2Zn 11 phase; when the Al element content is in the range of 1.5% to 3.5%, the Mg / Zn eutectic region mainly contains MgZn2 phase.

5. The steel sheet according to claim 1, characterized in that: In the intermediate layer at the junction of the coating and the substrate, Al element and Fe element form Fe2Al5 phase. With the increase of the content of Al element, its enrichment in the intermediate layer becomes more obvious, the intermediate layer becomes denser, and the grains in the coating become finer.

6. The steel sheet according to claim 1, characterized in that: The steel plate substrate consists of the following components by mass percentage: C: 0.30 - 0.36%, Si: 0.05 - 0.5%, Mn: 1.0 - 2.0%, P not more than 0.050%, S not more than 0.008%, Al ≥ 0.080%, Ti: 0.020 - 0.040%, V: 0.1 - 0.3%, O not more than 0.005%, N not more than 0.005%, RE: 0.01 - 0.05%, B: 0.0005 - 0.0040%, and the balance is Fe and inevitable impurities.

7. The steel sheet according to claim 1, characterized in that: The thickness specification of the steel plate is 1.0 - 2.5 mm, the width specification is 850 mm - 1800 mm, the tensile strength after hot forming is ≥ 1900 MPa, and the elongation is ≥ 6%.

8. The production process of the zinc-aluminum-magnesium coated steel sheet according to any one of claims 1 to 7, characterized in that: The process includes smelting, continuous casting, hot rolling, pickling and rolling, annealing, hot dip coating, coiling, and hot forming processes. Among them, In the hot dip coating process, the temperature of the plating solution is controlled at 440 - 480 °C, the dipping time is controlled at 5 - 10 s, and after hot dip coating, it needs to be cooled to room temperature at a rate greater than 30 °C / s.

9. The process according to claim 8, characterized in that: In the annealing process, the dew point is controlled at -50 °C - -20 °C, the heating temperature is controlled at 700 - 850 °C, and it is cooled to 420 - 500 °C at a cooling rate of 30 - 50 °C / S.

10. The process according to claim 8, characterized in that: The hot forming process is direct forming. The heating temperature of the steel plate is 850 - 900 °C, the holding time is 4 - 10 min, then the steel plate is quickly transferred to the mold for stamping, the mold closing temperature is controlled at 700 - 750 °C, and after stamping, the blank is quickly cooled to room temperature in the mold at a cooling rate of 100 - 140 °C / S; In the hot rolling process, the slab is heated to 1250 - 1300 °C, the finish rolling temperature is 780 - 900 °C, and the coiling temperature is 600 - 720 °C; During the continuous casting process, the drawing speed is in the range of 1.6 - 2.4 m / min and the drawing speed is constant.

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