Production method of aluminum-silicon coating thermal forming steel plate with excellent coating binding force and corrosion resistance

Abstract

The invention discloses a production method of an aluminum-silicon coating hot-formed steel plate with excellent coating binding force and corrosion resistance. The method comprises the following steps: (1) carrying out low-temperature annealing on a hot-formed steel hot-rolled black coil at 400-500 DEG C by adopting a bell-type furnace in a high-purity nitrogen protective atmosphere, and carrying out acid pickling and cold rolling to obtain a clean base material of which the surface Mn is less than or equal to 0.2%, the surface Si is less than or equal to 0.1% and the surface Cr is less than or equal to 0.05%; (2) the base material is annealed for 2-4 min in a two-phase region of Ac1 + 40-80 DEG C, controlled cooling is conducted to 550-570 DEG C, a complex-phase structure of bainite + ferrite + dispersed carbide is formed, and the hot forming austenitizing time is remarkably shortened; and (3) hot dipping is conducted in an Al-based plating solution containing 9%-11% of Si, 2%-4% of Mg and 0.1%-0.5% of Mn, a plating layer containing MgSi dendritic crystals and an interface AlMn phase is formed, the potential of the plating layer is-1.1 V to-1.0 V, and the cathode protection function is achieved. Through the synergistic effect of substrate purification, structure optimization and coating modification, the problems that a traditional aluminum-silicon coating is poor in binding force, and the corrosion resistance depends on a passive film are solved, and the aluminum-silicon coating is suitable for light-weight high-strength parts of automobiles.

Description

Technical Field

[0001] The present invention relates to the technical field of pre-coated hot-formed steel manufacturing, and specifically to a method for producing aluminum-silicon coated hot-formed steel plates that synergistically improves coating adhesion and corrosion resistance through substrate pretreatment-structure regulation-coating modification. Background Art

[0002] Aluminum-silicon coated hot-formed steel is widely used in the automotive manufacturing, construction, home appliances, and new energy sectors due to its lightweight, high-strength, and corrosion-resistant properties. Driven in particular by the demand for lightweight new energy vehicles, structural energy-absorbing materials with higher strength, higher toughness, and crack resistance will become a key choice for industry upgrades, which will bring a steady incremental market for high-strength aluminum-silicon coated hot-formed steel. Currently, the production of commercial aluminum-silicon coated hot-formed steel primarily relies on a pre-oxidation process to enhance the immersion performance of hot-formed steel sheets. The coating composition is also primarily based on traditional Al-10%Si coatings, and the microstructure of semi-finished steel sheets is primarily based on the traditional spheroidizing recrystallization annealing structure of ferrite + granular carbides.

[0003] As market demands for both cost and performance of aluminum-silicon-coated hot-dip steel materials increase, adopting a simpler pretreatment process to improve the hot-dip coating performance of hot-dip steel and achieve superior quality aluminum-silicon coatings, as well as shortening the austenitization heating time before hot-dip forming through microstructure control, will be effective ways to control the cost of aluminum-silicon-coated hot-dip steel while simultaneously improving its bending and forming properties and coating crack resistance. Furthermore, due to the inherent properties of traditional aluminum-silicon coatings, their corrosion resistance is heavily dependent on the integrity of the Al2O3 passivation film and coating on the surface. This leads to poor corrosion resistance in highly corrosive acidic industrial atmospheres and high relative humidity coastal salt spray atmospheres. Furthermore, the aluminum-silicon coating acts as a cathode at a higher potential relative to the substrate in corrosive environments. Therefore, the presence of surface defects and exposed substrates in traditional aluminum-silicon coatings significantly accelerates corrosion of the substrate. Therefore, a aluminum-silicon-coated hot-dip steel product that balances improved corrosion resistance, superior coating quality, and low manufacturing costs is a pressing need for most customers.

[0004] In summary, the current problems in the aluminum-silicon coated hot-formed steel industry are as follows: 1. Process bottleneck: Traditional production relies on pre-oxidation to improve immersion plating properties. However, the enrichment of easily oxidizable elements (Cr / Si / Mn) on the substrate surface makes pre-oxidation control difficult, resulting in unstable coating adhesion and the need for additional equipment investment. 2. Structural defects: The existing technology uses a single spheroidized annealing structure of ferrite + spheroidal carbides. The carbon diffusion efficiency is low during the austenitizing heat treatment, and a long holding time (usually >8 minutes) is required. This leads to the formation of an excessively thick Al-Si-Fe alloy layer (>5μm) and carbon enrichment at the coating / substrate interface, which is prone to cracking during bending. 3. Corrosion resistance limitations: The potential of traditional Al-10%Si coatings is higher than that of the substrate (approximately -0.7V vs SCE), which accelerates substrate corrosion when the coating is damaged. Furthermore, the corrosion resistance relies solely on the Al2O3 passivation film, leading to a high risk of failure in acidic industrial atmospheres or high-humidity salt spray environments.

[0005] These pain points directly restrict the application of aluminum-silicon coated hot-formed steel in high-end scenarios such as lightweighting of new energy vehicles, and the existing technology fails to systematically solve the problem of coordinated optimization of coating bonding strength, hot forming efficiency and corrosion resistance. Summary of the Invention

[0006] In order to overcome the shortcomings of traditional aluminum-silicon coatings in hot-dip coating quality, microstructure control and overly simple coating corrosion resistance mechanism, the present invention provides a method for producing aluminum-silicon coated hot-formed steel plates with excellent coating bonding strength, short austenitization time before the hot forming process, and corrosion resistance significantly exceeding that of conventional pre-coated hot-formed steel plates.

[0007] In order to achieve the above-mentioned purpose, the production method of the aluminum-silicon coated hot-formed steel plate described in the present invention has excellent coating adhesion, short austenitization time before the hot forming process, and corrosion resistance significantly exceeding that of conventional pre-coated hot-formed steel plates, including a steel plate pretreatment method for enhancing the hot-dip coating performance and coating adhesion of hot-formed steel, a microstructure control method for the steel plate before hot-dip coating to reduce the austenitization holding time in the hot forming process, and a production method for aluminum-silicon coated hot-formed steel with better comprehensive corrosion resistance by adding Mg and Mn elements.

[0008] (1) Steel plate pretreatment to enhance the hot-dip coating performance and coating adhesion of hot-formed steel: Use 99.9% high-purity nitrogen as a bell-type furnace annealing process for the protective atmosphere. According to the charging plan of the predetermined full hydrogen bell-type furnace heat treatment process, the hot-rolled black coil of the hot-formed steel is heat-treated at a holding temperature of 400-500°C for 1-10 hours. When cooling, it is cooled with a cooling hood. When the temperature is below 150°C, the cooling hood is removed and cooled to room temperature. Then, it is pickled with hydrochloric acid and cold rolled in the acid continuous rolling unit to obtain a clean base material with a surface Mn≤0.2%, Si≤0.1%, and Cr≤0.05%.

[0009] When the hot-formed steel cold-rolled sheets that have undergone the above pretreatment are produced by a continuous hot-dip coating unit, H2 or H2+N2 mixed gas with a dew point of -45 to -30°C is used for pre-oxidation in the preheating section.

[0010] Using a bell-type annealing process with high-purity nitrogen as the protective atmosphere, hot-rolled black coils of hot-formed steel undergo low-temperature heat treatment to eliminate easily oxidized alloying elements such as Cr, Si, and Mn from the surface matrix. The steel is then pickled and cold-rolled through a conventional acid-continuous rolling process to serve as the base material for hot-dip plating. This hot-formed steel base material, devoid of easily oxidized alloying elements on the surface, allows a simple pre-oxidation step prior to hot-dip plating to significantly improve the plateability and coating adhesion of the steel sheet.

[0011] (2) Microstructure control of the steel plate before hot-dip galvanizing to reduce the austenitizing holding time in the hot-dip galvanizing process: the base material treated in step (1) is annealed in the two-phase region of Ac1+40-80°C for 2-4 min in a continuous hot-dip galvanizing unit, and then cooled to 550-570°C at a cooling rate of 10-15°C / s to obtain a hot-formed steel substrate.

[0012] The substrate structure obtained by the above continuous annealing is a complex phase structure of bainite + ferrite + fine dispersed spherical carbides, wherein bainite accounts for 15-35%, ferrite accounts for 65-85%, and the diameter of the spherical carbides is less than 1 μm.

[0013] After the hot-formed steel coil is processed into a galvanized base material through a steel plate pretreatment process, it is continuously annealed in a continuous hot-dip galvanizing unit at a two-phase temperature of Ac1+40~80℃ and kept warm for 2~4min, and then cooled to 550~570℃ at a cooling rate of 10~15℃ / s for hot-dip galvanizing. In this way, a complex phase structure of bainite + ferrite + fine dispersed spherical carbides can be obtained. When the complex phase structure is austenitized, the average diffusion distance required for C atoms is significantly shorter than that of the traditional ferrite + granular carbide structure. Therefore, the kinetic conditions of the structure during austenitization heat treatment are better, which can significantly shorten the heating time, thereby reducing the thickness of the Al-Si-Fe alloy layer, which is beneficial to improving the bending forming performance of the hot-formed parts.

[0014] (3) Production of aluminum-silicon coated hot-formed steel with better comprehensive corrosion resistance by adding Mg and Mn elements: The hot-formed steel substrate obtained in step (2) is hot-dip plated in a plating solution with a chemical composition of 9-11% Si, 2-4% Mg and 0.1-0.5% Mn by mass, and the balance is Al, and then the hot-dip plated strip is cooled to room temperature at an average cooling rate of 5-20°C / s.

[0015] In the coating obtained by the above hot-dip coating process, in addition to the conventional Al-Si phase and Al-Si-Fe alloy phase, there are also dispersed Mg2Si phases and Al6Mn phases enriched in the coating near the coating / substrate interface.

[0016] The Al-Si coating obtained above with added Mg and Mn elements has a coating thickness of 10 to 40 μm and a potential of -1.1 to -1.0 V in a 3.5% NaCl solution, which is more negative than the potential of the hot-formed steel substrate (-0.8 to -0.6 V). Therefore, the coating has cathodic protection performance.

[0017] After the heat treatment in step (2) and cooling to 550-570°C, the hot-dip plating of the hot-formed steel substrate is carried out on a continuous hot-dip plating unit with a plating solution composition of 9-11% Si, 2-4% Mg and 0.1-0.5% Mn by mass, and the balance Al. The hot-dip-plated strip is then cooled to room temperature at an average cooling rate of 5-20°C / s. The hot-formed steel aluminum-silicon coating produced by this process contains a dispersed Mg2Si phase, which refines the coating structure and improves the mechanical properties of the coating. It also significantly reduces the electrode potential of the coating, so that the coating obtains cathodic protection performance similar to that of GI plate. The Al6Mn phase formed by the Mn element will be enriched in the coating near the coating / substrate interface, forming a synergistic effect with the Al-Si-Fe alloy layer, further enhancing the density of the coating. After adding Mg (2-4%) and Mn (0.1-0.5%), the coating potential dropped to -1.1-1.0V, which is significantly lower than that of the substrate (-0.8-0.6V), and has cathodic protection performance.

[0018] The unique control method for the matrix structure of the hot-formed steel disclosed in this invention significantly improves the kinetic conditions of austenite transformation, reducing the furnace holding time during austenitization prior to hot forming. This results in a thinner continuous Al-Si-Fe alloy layer at the coating / substrate interface, suppressing carbon enrichment in the matrix at the coating / substrate interface and helping to improve the bending performance of the steel sheet during hot forming. The unique composition design of this invention refines the coating structure and enhances its mechanical properties by forming a Mg2Si phase in the coating. It also significantly reduces the electrode potential of the coating, providing it with cathodic protection for the substrate. Furthermore, the Al6Mn phase formed by the Mn element inhibits corrosion expansion, synergizing with the coating's cathodic protection to further enhance the corrosion resistance of the coated plate disclosed in this invention.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention utilizes conventional bell-type furnaces and acid continuous rolling equipment, taking advantage of the selective oxidation of easily oxidizable elements under low oxygen partial pressure and slow diffusion at low temperatures to reduce the concentration of easily oxidizable elements on the surface of hot-formed steel used as a base material for hot-dip plating to the level of ordinary carbon steel, significantly improving its galvanizability. The strip treated by this technology can even achieve ideal surface galvanizability without the need for pre-oxidation treatment in the preheating section of the annealing furnace of a continuous hot-dip plating line, without any equipment modification.

[0020] 2. The present invention utilizes the annealing furnace of a continuous hot-dip plating unit for microstructure control. By obtaining a bainite + ferrite + fine dispersed spherical carbide structure with uniform C element distribution and partial non-diffusion undercooled austenite transformation, the efficiency of the austenitic heat treatment before the hot forming process can be significantly increased. At the same time, the problems of the graying of the coating surface caused by a long austenitizing heat treatment, the formation of an excessively thick Al-Si-Fe alloy layer in the coating, and the significant C element enrichment at the substrate / coating interface can be reduced. The toughness of the coating during forming is improved, and the bending cracking rate is reduced by 40%.

[0021] Through two-phase annealing (Ac1+40~80℃) and controlled cooling, a complex phase structure of bainite (15~35%) + ferrite (65~85%) + dispersed carbides (<1μm) is formed, the carbon diffusion path is shortened by 50%, and the austenitization time is reduced to less than 4 minutes.

[0022] 3. The Al-Si coating designed by the present invention with the addition of Mg and Mn elements completely overcomes the problems of traditional aluminum-silicon coatings lacking cathodic protection ability, and its corrosion resistance relying on the surface passivation effect and coating integrity, which makes it difficult to meet the corrosion resistance requirements of harsh environments, without significantly increasing the production cost. While broadening the use environment and scenarios of aluminum-silicon coated hot-formed steel, it basically eliminates the problem of plate surface corrosion caused by coating damage during transportation and processing.

[0023] 4. Applicable to acidic industrial atmosphere and high-humidity salt spray environment (such as new energy vehicle chassis parts), solving the failure problem of traditional coatings in harsh environments; suitable for hot-dip plating pretreatment of steel plates with high alloy content of easily oxidized elements such as hot-formed steel, DP steel, Q&P steel of various strength levels, as well as microstructure control of hot-formed steel of various strength levels and production of aluminum-silicon coated hot-formed steel plates with excellent corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the EDS surface distribution diagram of Si, Mn and Cr elements in the oxide scale cross section after the hot-rolled blackboard of hot-formed steel is pretreated by the bell furnace annealing process described in the present invention.

[0025] Figure 2 The metallographic structure of the hot-formed steel cold-rolled plate after heat treatment by the continuous hot-dip coating unit is used in the present invention, wherein the white one is ferrite and the black one is bainite.

[0026] Figure 3 This is an EDS profile of a cross-section of an Al-Si-Mg-Mn coating produced using the continuous hot-dip coating system described in the present invention on a hot-formed cold-rolled steel sheet. The Mg2Si phase is dispersed in a dendritic pattern, while Al6Mn is concentrated at the coating / substrate interface, forming a continuous layered structure that synergistically enhances density with the Al-Si-Fe alloy layer. DETAILED DESCRIPTION

[0027] The following examples are used to illustrate the specific application of the present invention, but are not intended to limit the scope of the present invention. Unless otherwise specified, the examples are all based on conventional experimental conditions. In addition, for those skilled in the art, without departing from the essence and scope of the present invention, various modifications or improvements to the material composition and dosage involved in these embodiments are within the scope of protection claimed in the present invention.

[0028] The selected hot-formed steel hot-rolled black coil has a thickness of 3.5 mm, a width of 1260 mm, and a coil weight of 24 tons. The specific chemical composition (mass percentage) is C: 0.34%, Si: 0.46%, Mn: 1.1%, P: 0.012%, S: 0.0035%; Al: 0.03%, B: 0.0035%, Cr: 0.33%, Ti: 0.019%, N: 0.0048.

[0029] The present invention is implemented by three steps: pre-treatment of the steel plate to enhance the hot-dip coating performance and coating adhesion of the hot-dip steel; microstructure control of the steel plate before hot-dip coating to reduce the austenitization holding time in the hot-forming process; and hot-dip coating of the Al-Si coating with added Mg and Mn elements. The steps are as follows: (1) Steel plate pretreatment to enhance hot-dip coating performance and coating adhesion of hot-formed steel The hot-rolled black coil was heat treated in a bell furnace according to the charging plan of the full hydrogen bell furnace heat treatment process. The heat treatment temperature was 450°C and the holding time was 8 hours. The protective atmosphere in the inner bell was 99.9% high-purity nitrogen. The coil was cooled to 200°C using a cooling hood. When cooling, the coil was cooled with a cooling hood. When the temperature was below 150°C, the cooling hood was removed and cooled to room temperature.

[0030] After heat treatment in a bell furnace, the base material is treated in an acid continuous rolling mill (the acid solution is hydrochloric acid, the concentration is 10-15%, and the temperature is 50-70°C) to remove residual oxide scale and obtain a clean base material with surface Mn, Si, and Cr contents ≤0.2%, ≤0.1%, and ≤0.05%, respectively. The base material is then cold rolled to 1.5 mm.

[0031] The base material is subjected to the next step of production in a continuous hot-dip coating unit, and is heated in a preheating section using an H2 atmosphere with a dew point of -45°C.

[0032] (2) Microstructure control of steel sheets before hot-dip galvanizing to reduce the austenitization holding time in the hot forming process The base material obtained in step (1) was subjected to continuous annealing at 790°C for 2.5 minutes on a continuous hot-dip coating unit, and then cooled to 550-570°C at a cooling rate of 12°C / s in preparation for hot-dip coating. The resulting hot-formed steel structure consisted of 75% ferrite + 25% bainite and spherical carbides with an average size of 0.8 μm uniformly distributed in the ferrite structure.

[0033] (3) Hot dip production of Al-Si coating with added Mg and Mn elements The hot-formed steel substrate obtained in step (2) is plated in a bath with a composition of Al-10%Si The coating was hot-dip plated in a plating solution of -3%Mg-0.3%Mn, and the final coating thickness was 20μm. The coating contained a dendritic dispersed Mg2Si phase and a continuous layered Al6Mn phase enriched near the coating / substrate interface. The potential measured in 3.5%NaCl solution was -1.05V (relative to SCE).

[0034] Figure 1 This is an EDS plot of Si, Mn, and Cr in the cross-section of the oxide scale after bell-type annealing pretreatment of hot-rolled blackboard using the present invention. The plot shows that after bell-type pre-annealing, the interface between the substrate and the oxide scale is enriched with Si, Mn, and Cr, with Si and Cr being particularly enriched.

[0035] Figure 2 The metallographic structure of the hot-formed steel cold-rolled sheet after heat treatment in the continuous hot-dip galvanizing unit described in the present invention is shown in Figure 1, where white represents ferrite and black represents bainite. Bainite accounts for 30% and ferrite accounts for 69%, and the diameter of the spherical carbides is less than 1 μm.

[0036] Figure 3 This is an EDS profile of a cross-section of an Al-Si-Mg-Mn coating produced using the continuous hot-dip coating system described in the present invention on a hot-formed steel cold-rolled sheet. In addition to the Fe-Al-Si alloy layer and Al-Si layer present in conventional coatings, the coating also contains a dendritic, dispersed Mg2Si phase and a continuous, layered Al6Mn phase concentrated near the coating / substrate interface.

[0037] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for producing an aluminum-silicon coated hot-formed steel sheet with excellent coating adhesion and corrosion resistance, characterized in that: The following steps are involved: (1) Steel plate pretreatment: The hot-rolled black coil of hot-formed steel is heat-treated at 400-500°C for 1-10 hours using a bell-type furnace annealing process in a high-purity nitrogen protective atmosphere. After cooling, it is pickled with hydrochloric acid and cold-rolled to obtain a clean base material with a surface Mn ≤ 0.2%, Si ≤ 0.1%, and Cr ≤ 0.05%. (2) Microstructure control: the base material treated in step (1) is annealed in a continuous hot-dip coating unit at a two-phase region of Ac1+40-80°C for 2-4 min, and cooled to 550-570°C at a rate of 10-15°C / s to obtain a composite microstructure substrate of bainite + ferrite + dispersed spherical carbides; (3) Hot dip plating: The substrate obtained in step (2) is hot dipped in a plating solution having a composition of 9-11% Si, 2-4% Mg, 0.1-0.5% Mn, and the balance Al, and cooled to room temperature at a rate of 5-20°C / s to form a coating containing a Mg2Si phase and an interfacial Al6Mn phase, with a coating potential of -1.1 to -1.0 V.

2. The production method according to claim 1, characterized in that The protective atmosphere of the bell-type furnace annealing in step (1) is 99.9% high-purity nitrogen. During cooling, the bell-type furnace is cooled to below 150° C. with a cooling hood and then naturally cooled to room temperature.

3. The production method according to claim 1, characterized in that In step (1), the pickling is carried out using a 10-15% hydrochloric acid solution at a temperature of 50-70°C.

4. The production method according to claim 1, characterized in that Step (1) The cold-rolled base material is pre-oxidized in a preheating section of a continuous hot-dip coating unit using H2 or H2+N2 mixed gas with a dew point of -45 to -30°C.

5. The production method according to claim 1, characterized in that The two-phase region annealing temperature in step (2) is 780-800°C, and the holding time is 2.5 minutes.

6. The production method according to claim 1, characterized in that The mass fraction of bainite in the complex phase structure obtained in step (2) is 15-35%, the mass fraction of ferrite is 65-85%, and the diameter of spherical carbides is less than 1 μm.

7. The production method according to claim 1, characterized in that The coating thickness in step (3) is 10 to 40 μm, the Mg2Si phase is dispersed in a dendritic manner, and Al6Mn is enriched at the coating / substrate interface to form a continuous layered structure, which synergistically enhances the density with the Al-Si-Fe alloy layer.

8. The method according to claim 1, wherein: The hot-rolled black coil of hot-formed steel in step (1) contains the following chemical components by mass fraction: C 0.2-0.4%, Si 0.2-0.6%, Mn 0.8-1.5%, P 0.01-0.02%, S 0.002-0.005%, Al 0.03%, B 0.002-0.005%, Cr 0.2-0.5%, Ti 0.01-0.03%, and N 0.004-0.005%.

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