Method for producing hot-dip galvannealed steel sheet for automobile outer panel

Abstract

The application discloses a preparation method of a hot-dip galvanizing zinc-aluminum-magnesium plated steel plate for automobile outer plates. The method controls the uniformity of the substrate organization composition, the surface structure and the organization, strictly controls the thickness of the substrate surface oxide film and the crystallization orientation, and enhances the wettability, adhesion, reactivity and solubility between the plating solution and the steel base. The method overcomes the quality defects such as shrinkage, stripes and small particles caused by the local shrinkage of the plating solution due to the surface tension and wettability difference in the hot-dip process of the prior art. The method not only realizes the 'zero defect' of the zinc-aluminum-magnesium plated steel plate surface, but also has high surface quality, compactness and uniformity, good adhesion, excellent forming performance and coating performance, and can meet the requirements of automobile outer plates, has a wide application prospect in the automobile industry, and will produce very significant social and economic benefits.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of steel metallurgical production, and particularly relates to a preparation method of a hot-dip galvanizing zinc-aluminum-magnesium plated steel plate for automobile outer plates. BACKGROUND

[0002] As a new type of zinc-based alloy, zinc-aluminum-magnesium alloy has excellent corrosion resistance and forming performance, and the development of the new type of zinc-aluminum-magnesium plated steel plate has become a hot issue in current research. In recent years, the zinc-aluminum-magnesium plated steel plate has developed rapidly abroad, and the product quality and surface quality have been greatly improved, and the product has been applied to the automobile industry. Since the core technology of this plated material has not been disclosed at the present stage, the relevant research, production and application in this regard are relatively backward in China. Although a lot of research work has been done on zinc-aluminum-magnesium products in China in recent years, the products are mainly used in household appliances and photovoltaic supports; and the overall surface quality of the zinc-aluminum-magnesium plated products at home and abroad is poor, and there are serious defects such as zinc particles, zinc scars, zinc strips and shrinkage holes, which cannot meet the "zero defect" requirement of automobile outer plates. SUMMARY

[0003] In order to overcome the defects of the prior art, the present application controls the uniformity of the base plate organization composition, surface structure and organization, strictly controls the thickness of the base plate surface oxide film and the crystallization direction, and enhances the wettability, adhesion, reactivity and solubility between the plating solution and the steel base, so as to not only realize the "zero defect" of the zinc-aluminum-magnesium plated steel plate surface, but also have excellent forming performance, which can meet the requirements of automobile outer plates.

[0004] In order to achieve the above-mentioned purpose, the present application provides a preparation method of a hot-dip galvanizing zinc-aluminum-magnesium plated steel plate for automobile outer plates, which comprises the following steps:

[0005] ①The raw steel of the hot-dip galvanizing zinc-aluminum-magnesium plated steel plate for automobile outer plates is an ultra-deep drawing steel, and the chemical composition of the raw steel is as follows in terms of mass percentage: C≤0.003%, Si≤0.020%, Mn: 0.10-0.20%, P≤0.015%, S≤0.010%, Als: 0.015-0.045%, N≤0.004%. The steel base is rolled by a cold rolling finishing mill work roll after laser texturing and polishing to improve the uniformity of the surface geometry of the steel strip; wherein the surface roughness of the finishing mill work roll is: 3.0um≤the arithmetic average value of the roll surface roughness Ra≤4.0um, 120≤the peak number Rpc≤150, and the maximum surface roughness Rmax≤3Ra. The roughness of the cold hard coil base obtained after rolling should be: 1.0um≤the arithmetic average value of the roll surface roughness Ra≤1.5um, 100≤the peak number Rpc≤120, and the maximum surface roughness Rmax≤3Ra.

[0006] ②The cold hard roll obtained in step 1 is subjected to hot-dip galvanizing with zinc-aluminum-magnesium alloy coating on a continuous production line, and the strip steel is subjected to degreasing treatment by cathode electrolytic degreasing. After degreasing and cleaning, the reflectivity of the strip steel is above 85%. Full radiation heating is used in the on-line continuous annealing furnace. The temperature in the soaking section of the annealing furnace is 750-850 DEG C, and the soaking time is 50-100 s. A mixture of 5-25% hydrogen and 75-95% nitrogen is used as the weak reducing atmosphere in the annealing furnace, and the oxygen content is less than or equal to 5 ppm, and the dew point is -50 DEG C to -30 DEG C.

[0007] ③A vacuum device is installed in the hot-dip galvanizing nose, and the vacuum degree is less than or equal to 10 -2 Pa, which can remove the zinc ash in the hot-dip galvanizing nose and inhibit the formation of zinc ash in the hot-dip galvanizing nose, thereby avoiding the attachment of zinc vapor on the surface of the strip steel due to the supersaturation of zinc vapor in the nose, and causing surface quality defects.

[0008] ④The hot-dip plating temperature is 450-520 DEG C, the coating thickness is 60-120 g / m 2 , and the hot-dip plating time is 3-5 s. Under the above conditions, the strip steel has good wettability in the zinc liquid, and the coating has good ductility and forming performance, and the contact angle should be less than or equal to 10 DEG. In addition, an automatic slag removal device is also installed in the zinc pot.

[0009] ⑤The steel plate after hot-dip plating in step 4 is rapidly cooled by air cooling at a cooling speed of 10-20 DEG C / s. The reaction between magnesium and water vapor under high temperature conditions is avoided, and the surface quality is affected.

[0010] In the above technical solution, further, in step 1, the steel base is an ultra-deep drawing IF steel, and the average content of inclusions is less than or equal to 3 per mm 2 , which is dispersedly distributed in the steel in the form of particles with a size of less than or equal to 10 μm. The inclusions exist in the form of independent phases, which destroy the continuity of the steel base and cause the organization of the steel to be uneven. Due to the uneven organization of the steel base, the surface tension and wettability are uneven. Due to the difference in surface tension, the plating liquid is locally contracted during hot plating, forming shrinkage, stripes and small particle defects. Therefore, the content, distribution and size of inclusions need to be strictly controlled to reduce their influence on the surface quality of the coating.

[0011] Further, in step 1, the finishing mill work roll is polished after laser texturing. The surface roughness of the base plate after rolling should be: 1.0 um≤Ra≤1.5 um, 100≤Rpc≤120, and the maximum surface roughness Rmax≤3Ra, which ensures that the distribution of micro-pits on the surface of the steel base plate after rolling is highly uniform in all directions. Not only can it avoid the wettability gradient between the plating liquid and the steel base plate, but also can ensure the uniformity in all directions during hot plating, and can avoid the orange peel phenomenon after product coating.

[0012] Further, in step 2, the thickness of the surface oxide film of the steel plate after continuous annealing is ≤ 16 nm, and the proportion of the (111) crystal plane is ≥ 85%. If the thickness of the surface oxide film is too thick, it will not only affect the wettability between the plating solution and the steel base, but also affect the adhesion between the steel base and the plating layer, which seriously affects the quality of the plating layer. The (111) crystal plane not only has high reactivity with the plating solution, but also has greater solubility than other crystal orientations, which is beneficial to the entire hot plating process.

[0013] Further, in step 4, the composition of the plating solution is 1.0%≤Al≤3.0%, 1.0%≤Mg≤3.0%, 0.1%≤RE≤0.2%, and the rest is Zn and unavoidable impurities, by mass percent. Under this condition, the zinc solution has good fluidity.

[0014] Compared with the prior art, the present application has the following advantages:

[0015] By adjusting the composition of the substrate, the surface structure and the uniformity of the substrate, strictly controlling the thickness of the surface oxide film and the crystal orientation, and enhancing the wettability, adhesion, reactivity and solubility between the plating solution and the steel base, the present application overcomes the quality defects such as shrinkage, stripes and small particle defects caused by the difference in surface tension and wettability during the hot plating process of the prior art. Not only does it achieve "zero defects" on the surface of the zinc-aluminum-magnesium plated steel plate, but also significantly improves the surface quality, density and uniformity of the zinc-aluminum-magnesium plated steel plate, and has good adhesion, excellent forming performance and coating performance, which can meet the requirements of automobile outer plates and has wide application prospects in the automobile industry, thus producing very significant social and economic benefits. DETAILED DESCRIPTION

[0016] The present application will be further described below in conjunction with specific examples, but in no way limits the present application. For the sake of brevity, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0017] The raw steel for the hot-dip zinc-aluminum-magnesium plated steel plate for automobile outer plates described in the following examples is an ultra-deep drawing IF steel, and the chemical composition of the raw steel is as follows by mass: C: 0.002%, Si: 0.010%, Mn: 0.15%, P: 0.010%, S: 0.005%, Als: 0.025%, N: 0.002%; the average content of inclusions is ≤ 3 / mm 2 , which is dispersedly distributed in the steel in the form of particles with a size of ≤ 10 μm.

[0018] Example 1

[0019] The preparation method of the hot-dip zinc-aluminum-magnesium plated steel plate for automobile outer plates is as follows:

[0020] IF steel was rolled using cold-rolled finishing mill work rolls that had undergone laser texturing and polishing. The arithmetic mean surface roughness Ra of the finishing mill work rolls was 3.0 μm, the peak value Rpc was 130, and the maximum surface roughness Rmax was 8.5 μm. The resulting cold-rolled sheet had an arithmetic mean surface roughness Ra of 1.0 μm, a peak value Rpc of 100, and a maximum surface roughness Rmax of 2.8 μm. The thickness of the rolled steel sheet was 0.8 mm.

[0021] The obtained cold-rolled coils were hot-dip galvanized aluminum-magnesium alloy coated on a continuous production line. The strip steel was first degreased by cathodic electrolysis for 30 seconds. After degreasing and cleaning, the reflectivity of the strip steel was about 90%. The strip steel after degreasing and cleaning was annealed at a temperature of 800℃. The atmosphere in the annealing furnace was a mixture of 5% hydrogen and 95% nitrogen, with an oxygen content of 2ppm and a dew point of -50℃. After continuous annealing, the oxide film thickness on the steel plate surface was about 10nm, and the (11 1) crystal plane ratio was about 90%.

[0022] After annealing, the strip steel enters the zinc pot through the furnace nose for hot-dip galvanizing. A vacuum device is installed at the hot-dip galvanizing furnace nose, maintaining a vacuum level of 10. -2 Pa; the strip temperature in the zinc bath is 520℃, the hot-dip galvanizing bath temperature is 500℃, and the bath composition is Al 2.0%, Mg 2.0%, RE 0.1%, with the remainder being Zn and unavoidable impurities. Under these conditions, the contact angle between the bath and the steel substrate is approximately 10°. The hot-dip galvanizing time is 5 seconds, the air knife pressure is 200 bar, and the coating thickness is 100 g / m. 2 .

[0023] The hot-dip galvanized samples were rapidly cooled by air at a rate of 20°C / s. The hot-dip galvanized aluminum-magnesium product exhibited no cracking or peeling of the coating after a 180° bend (0T), with a cupping value of 10.0 cm. The coating surface was free of any zinc particles, streaks, or shrinkage cavities that could affect surface quality. After phosphating, the phosphating film weight was 2–3 g / m³. 2 The grain size of the phosphating film is 3-5 μm, and the phosphorus content of the phosphating film is above 85%, which meets the requirements of automotive outer panels.

[0024] Example 2

[0025] The preparation method of hot-dip galvanized aluminum-magnesium coated steel sheet for automotive outer panels, the specific process flow is as follows:

[0026] The cold hard coil is obtained by rolling the IF steel with the laser roughened and polished work roll of the cold rolling finishing mill, wherein the arithmetic average value of the surface roughness of the work roll of the finishing mill is Ra=4.0 um, the peak value of the surface roughness is Rpc=140, and the maximum value of the surface roughness is Rmax=10 um. The arithmetic average value of the surface roughness of the cold hard coil after rolling is Ra=1.2 um, the peak value of the surface roughness is Rpc=108, and the maximum value of the surface roughness is Rmax=3.4 um. The thickness of the steel plate after rolling is 0.6 mm.

[0027] The obtained cold hard coil is hot-dip plated on a continuous production line. The strip steel is first subjected to cathode electrolytic degreasing for 30 s. After degreasing and cleaning, the reflectivity of the strip steel is 95%. The degreasing and cleaned strip steel is subjected to annealing treatment. The annealing temperature is 850 DEG C. The atmosphere in the annealing furnace is a mixed gas of 10% hydrogen and 90% nitrogen. The oxygen content is 3 ppm. The dew point is -40 DEG C. The thickness of the oxide film on the surface of the steel plate after continuous annealing is 8 nm. The proportion of the (111) crystal face is 95%.

[0028] The annealed strip steel is subjected to hot-dip plating by entering a zinc pot through a furnace nose. The temperature of the strip steel entering the zinc pot is 500 DEG C. The temperature of the plating solution for hot-dip plating is 480 DEG C. The composition of the plating solution is Al 1.5%, Mg 1.5%, RE 0.2%, and the rest is Zn and unavoidable impurities. The contact angle between the plating solution and the steel base under the condition is about 15 DEG. The hot-dip plating time is 4 s. The pressure of the air knife is 230 bar. The thickness of the plating layer is 60 g / m 2 .

[0029] The sample after hot-dip plating is subjected to rapid cooling. The cooling mode is air cooling. The cooling speed is 20 DEG C / s. The hot-dip galvanized aluminum magnesium product obtained has no cracking and peeling of the plating layer in 180 DEG bending (0T). The cupping value is 9.0 cm. The plating layer surface has no defects such as zinc particles, stripes and shrinkage holes affecting the surface quality. After phosphating, the film weight of the phosphating film is 2-3 g / m 2 . The grain size of the phosphating film is 3-5 um. The P ratio of the phosphating film is above 85%, meeting the needs of automobile outer plates.

[0030] For any person skilled in the art, many possible changes and modifications or equivalent embodiments of the technical solutions of the present application can be made to the technical contents disclosed above without departing from the scope of the technical solutions of the present application. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present application should still belong to the scope of protection of the technical solutions of the present application.

Claims

1. A method for preparing hot-dip galvanized aluminum-magnesium coated steel sheet for automotive outer panels, characterized in that, The method includes the following steps: ① The raw material steel for the hot-dip galvanized aluminum-magnesium coated steel sheet used for automotive outer panels is ultra-deep drawing steel. The chemical composition of the raw material steel, by mass, is: C≤0.003%, Si≤0.020%, Mn: 0.10%~0.20%, P≤0.015%, S≤0.010%, Als: 0.015%~0.045%, N≤0.004%. The steel base is rolled by the cold-rolled finishing mill work rolls after laser texturing and polishing. The surface roughness of the finishing mill work rolls is: 3.0μm≤ the arithmetic mean of the roll surface roughness Ra≤4.0μm, 120≤ the number of peaks Rpc≤150, and the maximum surface roughness Rmax≤3Ra. The roughness of the cold-rolled hardened coil substrate obtained after rolling is: 1.0μm≤ the arithmetic mean of the roll surface roughness Ra≤1.5μm, 100≤ the number of peaks Rpc≤120, and the maximum surface roughness Rmax≤3Ra. The steel base is ultra-deep drawing IF steel, with an average inclusion content of ≤3 inclusions / mm. 2 It is dispersed in the steel in the form of granules with a size ≤10μm; ② The cold-rolled coil obtained in step ① is hot-dip galvanized aluminum-magnesium alloy coating on a continuous production line. The strip steel is decontaminated by cathodic electrolytic degreasing. After degreasing and cleaning, the reflectivity of the strip steel reaches more than 85%. The online continuous annealing furnace adopts full radiation heating. The temperature of the soaking section of the annealing furnace is 750℃~850℃, and the soaking time is 50~100s. The annealing furnace uses a mixed gas of 5%~25% hydrogen and 75%~95% nitrogen as a weak reducing atmosphere. The oxygen content is ≤5ppm and the dew point is -50℃~-30℃. The oxide film thickness on the surface of the steel plate after continuous annealing is ≤16nm, and the proportion of (1 1 1) crystal plane is ≥85%. ③ A vacuum device is installed at the nose of the hot-dip galvanizing furnace, with a vacuum degree ≤10. -2 Pa; ④ The hot-dip galvanizing temperature is 450℃~520℃, and the coating thickness is 60~120g / m. 2 The hot-dip plating time is 3-5 seconds; The composition of the hot-dip galvanizing solution, by mass percentage, is 1.0% ≤ Al ≤ 2.0%, 1.0% ≤ Mg ≤ 1.5%, 0.1% ≤ RE ≤ 0.2%, with the remainder being Zn and unavoidable impurities; an automatic slag removal device is installed in the zinc pot for hot-dip galvanizing; the contact angle of the coating on the hot-dip galvanized strip is ≤ 10°; ⑤ The steel plate after hot-dip galvanizing in step ④ is rapidly cooled by air cooling at a rate of 10-20℃ / s.