Zinc coating hot stamping forming steel with excellent surface performance and preparation method thereof

Abstract

The invention provides zinc coating hot stamping forming steel with excellent surface performance and a preparation method thereof, and relates to the technical field of metal material machining. The zinc coating hot stamping forming steel comprises a base material steel plate and an optimized zinc coating, and the base material steel plate comprises, by mass, 0.15-0.3 wt% of C, 0.5-1.5 wt% of Si, 0.5-3.0 wt% of Mn, less than or equal to 2.0% of Cr, more than or equal to 0 and less than or equal to 3 wt% of Mn + Cr, less than or equal to 0.05 wt% of Ni, less than or equal to 0.05 wt% of Mo and the balance Fe and inevitable impurity elements; the optimized zinc plating layer is obtained by coating the surface of the base material steel plate with zinc liquid through a hot-dip galvanizing process and then carrying out salt water bath treatment on the surface. By controlling the types and contents of a phosphating agent, a passivating agent and an organic adsorbent in a solution, an organic adsorption film, a passivating layer and a phosphating layer are sequentially generated on the surface of the galvanized steel sheet from the outer surface to a base body. And thus, the zinc coating hot stamping forming steel which inhibits the generation of LME cracks and has excellent surface performance is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material processing, and in particular to a zinc-coated hot stamping steel with excellent surface properties and a preparation method thereof. Background Art

[0002] With the rapid development of the automotive industry, international and domestic requirements for automobile safety, performance, and environmental protection have become increasingly stringent, and relevant laws and regulations have been promulgated to limit automobile carbon emissions, making lightweighting the primary method of energy conservation and emission reduction in the automotive field. This requires an increase in the strength of automotive steel plates, and the proportion of ultra-high-strength steel plates used is increasing. In addition, in order to improve the corrosion resistance of steel plates, plating technology is usually applied to the surface of the steel plates, which means forming a corrosion-resistant coating on the surface. Coated steel plates are generally divided into Al-plated and Zn-plated, but Zn is widely used in plating due to its excellent cathodic protection ability, good processing performance and low cost.

[0003] Hot stamping, a key processing method for high-strength steel, has become an indispensable process in automotive manufacturing. This technique heats steel sheets to a high temperature, causing them to fully austenitize before forming them. The process then rapidly cools them to below the martensitic transformation temperature, ultimately yielding parts with ultra-high strength and hardness. However, because the heating temperature is above the melting point of zinc (419.5°C), the application of hot stamping on galvanized steel faces a technical bottleneck: liquid metal embrittlement (LME).

[0004] Liquid metal embrittlement refers to the melting of zinc in the coating during the hot stamping process of galvanized steel sheets, which penetrates into the grain boundaries of the substrate by the capillary principle, resulting in a decrease in the cohesion of the grain boundaries and continuous penetration into the interior of the substrate by the stress-assisted diffusion mechanism. Eventually, cracks are formed under the action of tensile stress, resulting in a significant reduction in the ductility and fracture strength of the galvanized steel sheets.

[0005] Currently, researchers have conducted a series of studies to address the LME (Long-Mean-Term Evolution) problem in galvanized hot-stamped steel sheets. Chinese Patent 201611069267.1 discloses a method for manufacturing galvanized steel sheets for hot stamping. The steel sheet contains the following chemical elements and their mass percentages: C: 0.10-0.5%, Si: 0.7-2.5%, Mn: 1.0-3%, Al: 0.01-0.5%, with the balance being iron and unavoidable impurities. The resulting hot-stamped galvanized steel sheet produces an alloyed molten zinc layer at the interface between the coating and the base steel sheet, suppressing the occurrence of LME cracks. While this patent promotes the formation of an oxide layer within the subsurface of the substrate, it does not significantly reduce the liquid zinc content in the galvanized layer. This method places certain requirements on the steel substrate composition, limiting its scope of application.

[0006] Chinese Patent 202410349013.3 discloses a hot forming method for zinc-based hot-formed steel, a hot-formed part, and a preparation method thereof. The steel plate contains the following chemical elements and their mass percentages: C: 0.1-0.4%, Si: 0.2-2.0%, Mn: 1.0-4.0%, Al: 0.05-0.5%, Ti: 0.01-0.1%, V: 0.01-0.2%, Cr: 0.2-2.0%, B: 0.001-0.005%, P: ≤ 0.005%, S ≤ 0.005%, N ≤ 0.005%, and the balance is iron. The thickness of the zinc coating of the steel plate is 8-40 μm, and the overall coating includes Γ(Fe3Zn 10 ) phase and α-Fe(Zn) phase. The invention studies the liquid zinc content and proposes a segmented heating + liquid pre-cooling process before stamping, which reduces the liquid zinc content during the stamping process, thereby reducing the cracks caused by the embrittlement of the liquid metal, and at the same time adjusts the cooling rate to ≥30°C / s to ensure that more than 90% of the substrate structure is austenite, thereby ensuring the mechanical properties of the material after hot forming. However, this patent uses a water bath for pre-cooling. When the heated galvanized steel is placed in water, it will cause the liquid to boil, thereby generating bubbles. There is a large amount of air in the bubbles. When the bubbles enter the molten zinc liquid, they will cause cavities. At the same time, rapid cooling is not enough to allow the zinc liquid enough time to flow freely to a uniform state, thus causing a large number of pores. Therefore, although this method has a high cooling rate, it may cause uneven local surface temperature, resulting in a decrease in the density of the surface coating. At the same time, boiling water cooling cannot improve the surface properties of galvanized steel.

[0007] Therefore, how to inhibit LME while improving the surface quality of galvanized steel sheets and enhancing the quality of subsequent phosphating, electrophoresis and other processes is a technical problem that needs to be solved urgently. Summary of the Invention

[0008] In order to solve the above technical problems, the present invention provides a zinc-coated hot-stamped steel with excellent surface properties and a preparation method thereof. By performing a salt bath pre-cooling treatment on the hot-dip galvanized steel after annealing, the liquid zinc content in the coating is reduced, thereby suppressing the occurrence of LME cracks and improving the quality of subsequent hot-stamped parts in the phosphating and electrophoresis processes.

[0009] To this end, the technical solution adopted in the present invention is as follows:

[0010] The present invention provides a zinc-coated hot-stamped steel with excellent surface properties, comprising a base steel sheet and an optimized zinc coating. The base steel sheet contains, by weight percentage (wt%), the following: C: 0.15-0.3wt%, Si: 0.5-1.5wt%, Mn: 0.5-3.0wt%, Cr: ≤2.0% and 0≤Mn+Cr≤3wt%, Ni: ≤0.05wt%, and Mo: ≤0.05wt%, with the remainder being Fe and unavoidable impurity elements. The present invention has no specific requirements for the elements in the steel sheet; the types and contents of the elements involved cover those of conventional galvanized steel.

[0011] The optimized zinc coating is first obtained by hot-dip galvanizing process, in which zinc liquid is coated on the surface of the substrate steel plate, and then the surface is subjected to salt water bath treatment.

[0012] Preferably, in the preparation process of the zinc-coated hot stamping steel, annealing, salt water bath treatment and stamping are performed in sequence.

[0013] Preferably, the optimized zinc coating comprises an organic adsorption film, a passivation layer, a phosphating layer and a zinc coating layer in order from the outer surface of the coating to the base steel plate.

[0014] Preferably, the thickness of the organic adsorption film is ≤10 nm, the thickness of the passivation layer is 0.1-0.2 μm, and the thickness of the phosphating layer is 0.3-0.8 μm. The organic adsorption film and passivation layer should not be too thick to avoid affecting the subsequent electrophoresis and phosphating effects.

[0015] Preferably, the galvanized layer phase composition includes 60% to 90% of α-Fe(Zn) phase and the balance of Γ(Fe3Zn 10 ) phase and other phases.

[0016] The present invention also provides a method for preparing the zinc-coated hot stamping steel having excellent surface properties, comprising the following steps:

[0017] S1. Smelting and continuously casting steel raw materials to obtain slabs;

[0018] S2, performing sheet rolling on the slab to obtain a steel strip;

[0019] S3, sequentially annealing and hot-dip galvanizing the steel strip to obtain a zinc-coated steel coil;

[0020] S4. Hot stamping the zinc-coated steel coil, wherein the hot stamping process comprises heating, pre-cooling and stamping in sequence, to obtain zinc-coated hot stamped steel with excellent surface properties.

[0021] Preferably, the continuous casting process involved in S1 is well known to those skilled in the art, and the specific continuous casting parameters are: a casting speed of 0.5-2.0 m / min, and a billet drawing speed of 1.0-2.5 m / min.

[0022] Preferably, the sheet rolling process involved in S2 is well known to those skilled in the art, and specifically includes hot rolling, pickling, and cold rolling. Hot rolling parameters include a heating temperature of 1100-1300°C, a finishing temperature of 800-930°C, and a coiling temperature of 500-700°C. Pickling is performed using a 15% by mass HCl solution at a temperature of 50-70°C for 60-120 seconds. Cold rolling parameters include a total reduction of 60-90%.

[0023] Preferably, in step S2, the thickness h of the steel strip is 1-20 mm, more preferably 1 mm ≤ h < 9 mm or 9 mm ≤ h < 20 mm.

[0024] Preferably, the annealing and hot-dip galvanizing processes involved in S3 are well known to those skilled in the art. Annealing parameters include an atmosphere of 5-20% H₂ + 80-95% N₂, an annealing temperature of 700-850°C, and a holding time of 30-60 seconds. Hot-dip galvanizing parameters include a zinc bath containing 0.1wt%-0.3wt% Al, the balance Zn, a zinc bath temperature of 440-460°C, an immersion time of 1-8 minutes, a distance between the air knife and the strip of 5-15 mm, an air knife pressure of 1-3 bar, and an air knife angle of 30-60°.

[0025] Preferably, in step S4, different brine bath pre-cooling rates are selected for different plate thicknesses. When the plate thickness h is 9 mm ≤ h < 20 mm, the cooling rate is 25°C / s to 40°C / s; when the plate thickness h is 1 mm ≤ h < 9 mm, the cooling rate is 40°C / s to 50°C / s.

[0026] Preferably, in step S4, the salt water bath solution includes a base liquid, a main solute, an auxiliary component, and a stabilizer. The base liquid is water, the main solute is selected from at least one of zinc phosphate (Zn(H2PH4)2), ammonium dihydrogen phosphate (NH4H2PO4), and sodium metasilicate (Na2SiO3), the auxiliary component is selected from at least one of sodium manganate (Na2MoO4) and benzotriazole, and the stabilizer is selected from at least one of disodium ethylenediaminetetraacetic acid and sodium nitrite (NaNO2).

[0027] Preferably, in step S4, the base liquid of the salt water bath solution is water, the main solutes are Zn(H2PH4)2, NH4H2PO4 and Na2SiO3, the auxiliary components are Na2MoO4 and benzotriazole, and the stabilizers are disodium edetate and sodium nitrite. In terms of concentration and percentage of solution occupied, the water content accounts for 90-95% of the total volume of the solution, the Zn(H2PH4)2 is 1-2.5 g / L, the NH4H2PO4 is 0.5-2 g / L, the Na2SiO3 is 0.5-0.4 g / L, the Na2MoO4 is 0.3-1.5 g / L, the benzotriazole is 0.05-0.3 g / L, the disodium edetate is ≤0.5 g / L, and the NaNO2 is ≤0.5 g / L.

[0028] Preferably, in step S4, the pH value of the salt water bath solution is in the range of 5.5 to 6. If the pH value of the solution is not within the above range, phosphoric acid (H3PO4) and sodium hydroxide (NaOH) are used to adjust the overall pH value of the solution to between 5.5 and 6.

[0029] Preferably, in step S4, the pre-cooling time is 5 to 20 seconds.

[0030] Preferably, the heating and stamping processes involved in S4 are well known to those skilled in the art, and the hot stamping process parameters are: heating temperature of 850-1000°C, holding time of 5-20 minutes, and stamping speed of 10-25 mm / s.

[0031] The present invention has the following beneficial effects:

[0032] The present invention pre-cools the hot-dip galvanized steel by finding a suitable salt bath solution, so that the surface of the material obtains a multi-layer optimized adsorption layer, thereby improving the quality of subsequent phosphating and electrophoresis processes. 10 ) phase and liquid zinc transform into α-Fe(Zn) phase by peritectic transformation, reducing the liquid zinc content and thus inhibiting the occurrence of LME cracks during hot stamping.

[0033] When a galvanized sheet is immersed in a saltwater bath, the phosphating agents (such as Zn(H2PH4)2 and NH4H2PO4) in the solution, due to their high concentration and pH value of 5.5-6, preferentially react with the zinc in the galvanized layer, forming dense, insoluble zinc phosphate (Zn3(PO4)2) that is deposited on the surface. Subsequently, limited by the formation of the phosphate layer and the relatively low reaction rate of the passivating agents, passivating agents (such as Na2MoO4 and NaNO2) further react on the phosphate layer, forming a stable passive oxide layer. Finally, organic adsorbents (such as benzotriazole) coat the surface of the passivation layer through adsorption, forming the outermost, dense organic protective film.

[0034] The main reaction of the phosphating layer is: ; The reaction that promotes the refinement of zinc phosphate crystals is: .

[0035] The main reaction of the passivation layer formation is: ; The reaction of strengthening the passivation layer is: .

[0036] The reaction of the organic adsorption film formation is: ,in It is a stable organic protective film.

[0037] The present invention utilizes a water bath pretreatment process to reduce the liquid zinc content in the zinc coating through a high cooling rate, thereby suppressing the formation of LME cracks during hot stamping. Simultaneously, by controlling the types and contents of the phosphating agent, passivating agent, and organic adsorbent in the solution, an organic adsorption film, a passivation layer, and a phosphating layer are formed on the surface of the galvanized steel sheet, sequentially from the outer surface to the substrate. This allows for the production of zinc-coated hot-stamped steel with excellent surface properties, while suppressing LME cracks under relatively few process conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0039] Figure 1 Schematic diagram of the reaction in the zinc coating;

[0040] Figure 2 Schematic diagram of the surface structure layer for optimizing zinc-coated steel plate;

[0041] Figure 3 This is the microstructure diagram of the cross section of the optimized zinc-coated steel sheet of Comparative Example 2. DETAILED DESCRIPTION

[0042] In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0043] The present invention provides a zinc-coated hot-stamped steel with excellent surface properties, comprising a base steel sheet and an optimized zinc coating. The base steel sheet contains, by weight percentage (wt%), the following: C: 0.15-0.3wt%, Si: 0.5-1.5wt%, Mn: 0.5-3.0wt%, Cr: ≤2.0% (where 0≤Mn+Cr≤3wt%), Ni: ≤0.05wt%, and Mo: ≤0.05wt%, with the remainder being Fe and unavoidable impurities. The optimized zinc coating is obtained by hot-dip galvanizing, where zinc liquid is applied to the surface of the base steel sheet, followed by a salt water bath treatment.

[0044] The present invention also provides a method for preparing the zinc-coated hot stamping steel having excellent surface properties, comprising the following steps:

[0045] S1. Smelting and continuously casting steel raw materials to obtain slabs. The continuous casting parameters are: casting speed of 0.5-2.0 m / min, and slab drawing speed of 1.0-2.5 m / min;

[0046] S2. Thin-plate rolling is performed on the slab to obtain a steel strip. The thin-plate rolling process includes hot rolling, pickling, and cold rolling. Hot rolling parameters are: heating temperature of 1100-1300°C, finishing temperature of 800-930°C, coiling temperature of 500-700°C, pickling using 15% HCl solution at a temperature of 50-70°C for 60-120 seconds. Cold rolling parameters are: total reduction of 60-90%.

[0047] S3. The steel strip is sequentially annealed and hot-dip galvanized to produce a zinc-coated steel coil. Annealing parameters are: atmosphere of 5-20% H₂ + 80-95% N₂, annealing temperature of 700-850°C, and holding time of 30-60 seconds. Hot-dip galvanizing parameters are: zinc bath containing 0.1wt%-0.3wt% Al, balance Zn, bath temperature of 440-460°C, immersion time of 1-8 minutes, air knife-strip distance of 5-15 mm, air knife pressure of 1-3 bar, and air knife angle of 30-60°.

[0048] S4. Hot stamping the zinc-coated steel coil. The hot stamping process includes heating, pre-cooling, and stamping, thereby obtaining zinc-coated hot-stamped steel with excellent surface properties. The hot stamping process parameters are: heating temperature of 850-1000°C, holding time of 5-20 minutes, and stamping speed of 10-25 mm / s.

[0049] Figure 1 Schematic diagram of the reaction in the galvanized layer. When no salt water bath cooling is used, the galvanized steel layer contains α-Fe(Zn) phase and a large amount of Γ(Fe3Zn10 ) phase and the remaining pure zinc phase, cracks are generated due to the embrittlement of liquid metal. The cracks are deep and extend into the steel matrix. When cooling in a salt water bath, the peritectic reaction occurs due to the rapid cooling effect. Γ(Fe3Zn 10 ) phase reacts with pure zinc to form α-Fe(Zn) phase. During the stamping process, the liquid zinc content decreases rapidly, and fewer zinc atoms penetrate into the steel matrix. After the stamping is completed, only a small number of tiny cracks are generated, which will not extend into the steel matrix.

[0050] Figure 2 Schematic diagram of the surface structure of zinc-coated steel sheets. After cooling in a salt water bath, the coating surface forms an organic adsorption film ≤10nm, a passivation layer of 0.1-0.2μm, and a phosphating layer of 0.3-1μm, from the outer surface to the zinc coating.

[0051] The steel elements and contents and salt bath cooling parameter control of Examples 1-5 and Comparative Examples 1-2 are shown in Table 1-2. Specific parameters not listed in the table are:

[0052] S1. Smelt and continuously cast steel raw materials to obtain slabs. The continuous casting parameters are: pouring speed of 1.5m / min, slab pulling speed of 2m / min;

[0053] S2. Thin-plate rolling is performed on the slab to obtain a steel strip. The thin-plate rolling process includes hot rolling, pickling, and cold rolling. Hot rolling parameters are: heating temperature of 1200°C, finishing temperature of 900°C, and coiling temperature of 650°C; pickling parameters are: pickling solution: 15% (volume ratio) HCl, pickling time of 90 seconds; cold rolling parameters are: total reduction ratio of 60%;

[0054] S3. The steel strip is sequentially annealed and hot-dip galvanized to obtain a zinc-coated steel coil. Annealing parameters are: continuous annealing (CAL), 10% H2 + 90% N2 atmosphere, annealing temperature: 800°C, holding time: 60 seconds; hot-dip galvanizing process parameters are: zinc bath Al content: 0.3 wt%, zinc bath temperature: 450°C, immersion time: 3 minutes, air knife-strip distance: 12 mm, air knife pressure: 1 bar, air knife angle: 45°; hot stamping parameters are: heating temperature: 900°C, holding time: 8 minutes, and stamping speed: 20 mm / s.

[0055] The process parameters used in Comparative Example 1 are the same as those in Example 4, except that the steel matrix has a higher Si content. The steel matrix used in Comparative Example 2 has the same composition as that in Example 4, but pre-cooling is performed using the water bath method described in 202410349013.3 of the background technology.

[0056] Table 1 shows the chemical composition of the steel materials of various examples and comparative examples.

[0057] Table 1

[0058]

[0059] Table 2 shows the salt water bath solution formula and pre-cooling process parameters of each embodiment and comparative example.

[0060] Table 2

[0061]

[0062] The coating composition and phase volume fraction, as well as the LME crack length after the hot stamping process, were measured for the finished products of the above-mentioned embodiments and comparative examples. Specific results are shown in Table 3.

[0063] Table 3

[0064]

[0065] In Examples 1-5, when the steel substrates with elemental compositions within the specified range are used, within the range of salt bath solution compositions defined in the present invention, as the corresponding solution compositions increase, the thicknesses of the organic adsorption film, the passivation layer, and the phosphating layer show an increasing trend, and more Γ(Fe3Zn 10 ) and zinc liquid produce peritectic reaction to generate α-Fe(Zn), which makes the crack length show a decreasing trend after stamping.

[0066] The process parameters used in Comparative Example 1 are the same as those in Example 4, but the steel matrix element Si content is relatively high, which causes oxides to segregate on the surface during the hot stamping process, resulting in an uneven surface. Therefore, in the subsequent pre-cooling process, the thickness of the surface organic adsorption layer, passivation layer and phosphating layer are uneven, and even some surfaces are unable to produce the above protective layers, ultimately resulting in an increase in the maximum crack length exceeding the specified range.

[0067] The steel matrix composition used in Comparative Example 2 is the same as that in Example 4, but pre-cooling in a boiling water bath is adopted. The crack depth after the final hot stamping increases but is still within the allowable range. However, the surface quality of the coating is poor, the coating is relatively loose and has holes. Figure 3 This is the microstructure of the cross section of the optimized zinc-coated steel plate of Comparative Example 2. It can be seen that the coating surface is loose and has relatively dense holes, which affects the subsequent use quality of the coating.

[0068] 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 modifications 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 zinc-coated hot stamping steel with excellent surface properties, characterized in that: The invention comprises a base steel sheet and an optimized zinc coating, wherein the base steel sheet contains, by mass percentage, the following: C: 0.15-0.3wt%, Si: 0.5-1.5wt%, Mn: 0.5-3.0wt%, Cr: ≤2.0% and 0≤Mn+Cr≤3wt%, Ni: ≤0.05wt% and Mo: ≤0.05wt%, with the balance being Fe and unavoidable impurity elements; The optimized zinc coating is first obtained by hot-dip galvanizing process, in which zinc liquid is coated on the surface of the substrate steel plate, and then the surface is subjected to salt water bath treatment.

2. The zinc-coated hot stamping steel with excellent surface properties according to claim 1, characterized in that: The optimized zinc coating comprises an organic adsorption film, a passivation layer, a phosphating layer and a zinc coating layer in sequence from the outer surface of the coating to the base steel plate. The thickness of the organic adsorption film is ≤10 nm, the thickness of the passivation layer is 0.1-0.2 μm, and the thickness of the phosphating layer is 0.3-0.8 μm.

3. The zinc-coated hot stamping steel with excellent surface properties according to claim 2, characterized in that: The zinc coating phase composition includes 60% to 90% of α-Fe(Zn) phase and the balance of Γ(Fe3Zn 10 ) phase and other phases.

4. The method for preparing zinc-coated hot stamping steel with excellent surface properties according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1. Smelting and continuously casting steel raw materials to obtain slabs; S2, performing sheet rolling on the slab to obtain a steel strip; S3, sequentially annealing and hot-dip galvanizing the steel strip to obtain a zinc-coated steel coil; S4. Hot stamping the zinc-coated steel coil, wherein the hot stamping process comprises heating, pre-cooling and stamping in sequence, to obtain zinc-coated hot stamped steel with excellent surface properties.

5. The method according to claim 4, characterized in that In step S2, the thickness h of the steel strip is 1-20 mm, preferably 1 mm ≤ h < 9 mm or 9 mm ≤ h < 20 mm.

6. The method according to claim 5, characterized in that In step S4, different brine bath pre-cooling rates are selected for different plate thicknesses. When the plate thickness h is 9 mm ≤ h < 20 mm, the cooling rate is 25°C / s ~ 40°C / s; when the plate thickness h is 1 mm ≤ h < 9 mm, the cooling rate is 40°C / s ~ 50°C / s.

7. The method according to claim 4, characterized in that In step S4, the brine bath solution includes a base liquid, a main solute, an auxiliary component and a stabilizer. The base liquid of the brine bath solution is water, the main solutes are Zn(H2PH4)2, NH4H2PO4 and Na2SiO3, the auxiliary components are Na2MoO4 and benzotriazole, and the stabilizers are disodium ethylenediaminetetraacetic acid and sodium nitrite.

8. The method according to claim 7, characterized in that In terms of concentration and percentage of the solution, the water content accounts for 90-95% of the total volume of the solution, the Zn(H2PH4)2 is 1-2.5 g / L, the NH4H2PO4 is 0.5-2 g / L, the Na2SiO3 is 0.5-0.4 g / L, the Na2MoO4 is 0.3-1.5 g / L, the benzotriazole is 0.05-0.3 g / L, the disodium ethylenediaminetetraacetic acid is ≤0.5 g / L, and the NaNO2 is ≤0.5 g / L.

9. The method according to claim 8, characterized in that In step S4, the pH value of the salt water bath solution ranges from 5.5 to 6.

10. The method according to claim 4, characterized in that In step S4, the pre-cooling time is 5 to 20 seconds.

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