A method for strengthening the coating adhesion of 780mpa grade hot-dip galvanized dual-phase steel

Abstract

The present application relates to the technical field of metal material surface treatment, and particularly relates to a 780MPa-grade hot-dip galvanized dual-phase steel coating bonding force strengthening method, which comprises the following steps: soaking a base material in a plating solution to perform three-stage temperature rising continuous annealing, pre-oxidation, hydrogen reduction, two-stage temperature lowering, post-plating cooling, and obtaining the strengthened hot-dip galvanized dual-phase steel coating; the base material composition is as follows: C<=0.18%, Mn 1.75-2%, Si 0.35-0.45%, P 0.02-0.035%, Cr 0.35-0.45wt%, Nb 0.015-0.03%, Als 0.02-0.06%, Y 0.01-0.014%, and Nb / Y is 1.1-3, according to 100% mass percentage. The scheme solves the problems of uneven structure, poor interface bonding force, poor continuity of the inhibition layer, unstable corrosion resistance, and poor surface of the existing hot-dip galvanized DP780 steel coating.

Description

Technical Field

[0001] This invention belongs to the field of metal material surface treatment technology, specifically involving a method for strengthening the adhesion of hot-dip galvanized duplex steel coatings at 780MPa. Background Technology

[0002] With the increasing emphasis on lightweighting and safety standards in the automotive industry, 780MPa grade hot-dip galvanized duplex steel, due to its high strength, good formability, and corrosion resistance, has become a core material for vehicle body structural components (such as anti-collision beams and pillar reinforcement plates). However, this grade of steel faces a severe problem of insufficient coating adhesion during production, directly affecting the reliability of components and the safety of the entire vehicle. Specific bottlenecks are as follows: 1. To achieve high strength, a relatively high content of alloying elements such as Si (0.1%~0.3%) and Mn (1.8%~2.1%) is required. However, during continuous annealing, Si and Mn readily diffuse to the surface and form an inner oxide layer (such as Si / Mn oxide). These oxides hinder the wetting reaction between the steel substrate and the zinc bath, resulting in an inhibitory layer (Fe2Al5Zn). x 1. Uneven growth: The formation of the inhibition layer in the oxide-covered area is limited, and some areas may even be missing; Decreased coating adhesion: The coating is prone to peeling (powdering) during bending or stamping, and the ball punch test grade is difficult to consistently reach level 1. 2. Conflicts in composition design: Although reducing the Si content can alleviate oxidation, it will sacrifice the hardenability and strength of the steel (e.g., the strength of conventional DP600 steel with Si content <0.1% is difficult to exceed 780MPa); Sensitive annealing control: Traditional annealing processes (e.g., dew point -30℃~-40℃) are not effective in inhibiting oxides on the surface of high-strength steel, the process window is narrow, and parameter fluctuations can easily cause batch defects; Limited coating strengthening methods: Post-treatment such as finishing can only improve surface roughness and cannot fundamentally solve the problem of weakened interface bonding. 3. Industry demand and technology gap: High-end cars require galvanized parts to simultaneously meet the following requirements: tensile strength ≥780MPa, elongation ≥17%; coating quality: continuous and dense inhibition layer, zinc layer adhesion reaching ball punch grade 1 (no peeling); however, due to the above problems, the current high-strength galvanized duplex steel (such as DP780) has a pass rate of less than 90%, and there is an urgent need to develop a systematic solution that takes into account both composition design and process optimization. Summary of the Invention

[0003] The present invention aims to solve the problems of uneven microstructure, poor interfacial bonding, poor continuity of the inhibition layer, unstable corrosion resistance, and poor surface in the existing hot-dip galvanized DP780 steel coating.

[0004] According to one aspect of the present invention, a method for strengthening the adhesion of a 780MPa grade hot-dip galvanized duplex steel coating is provided, comprising the following steps:

[0005] The substrate is immersed in the plating solution and subjected to a three-stage heating and annealing process, pre-oxidation, hydrogen reduction, two-stage cooling, and post-plating cooling to obtain a reinforced 780MPa grade hot-dip galvanized duplex steel coating. The composition of the substrate is as follows: C: ≤0.18wt%, Mn: 1.75wt%-2.00wt%, Si: 0.35wt%-0.45wt%, P: 0.020wt%-0.035wt%, Cr: 0.35wt%-0.45wt%, Nb: 0.015wt%-0.030wt%, Als: 0.020wt%-0.060wt%, Y: 0.010wt%-0.014wt%, with the balance being Fe, wherein the Nb / Y ratio is 1.1-3.

[0006] Based on the above technical solution, the plating solution uses zinc (Zn) as the base and the aluminum (Al) content is controlled at 0.18wt%-0.22wt%.

[0007] Based on the above technical solution, the three-stage heating and annealing process is as follows: using 0.7-2.0mm low-carbon alloy steel, heating at 5-6℃ / s to relieve stress to 690-695℃, increasing the heating rate to 10-12℃ / s to 720-730℃, suppressing the diffusion of silicon and manganese to the surface by shortening the time, and rapidly heating to 830-850℃ at a heating rate of 20-25℃ / s, and holding at that temperature for 2-4s.

[0008] Based on the above technical solution, the pre-oxidation specifically involves: using a mixture of air with a dew point of -35℃ to -30℃ and nitrogen containing water vapor with a dew point of -5℃ to 0℃ for pre-oxidation. The mixing ratio of the two gases is approximately 5:1, wherein the water vapor content in the nitrogen mixture with a dew point of -5℃ to 0℃ is 20-25%, and the gas flow rate is 20-40 m³ / h. 3 / min, with the pre-oxidized layer thickness controlled within 0.5μm.

[0009] Based on the above technical solution, the hydrogen reduction specifically involves reducing the hydrogen at 800-850℃ for 50-85 seconds in a hydrogen atmosphere with a mass fraction of 13-15% to obtain an ideal surface and improve the adhesion of the coating.

[0010] Based on the above technical solution, the two-stage cooling process is as follows: first, the temperature is reduced to 720-730℃ at a cooling rate of 5-10℃ / s, and then a rapid cooling mode is entered, with a cooling rate of 25-30℃ / s to rapidly cool down to 480-485℃, to obtain a martensitic structure, and the zinc liquid temperature is controlled at 450-455℃ to ensure the continuous release of the latent heat of the strip.

[0011] Based on the above technical solution, the post-plating cooling is a three-stage variable temperature cooling, specifically: the first stage is from 465-470℃ after exiting the zinc pot, using an air knife distance of 23mm-30mm, an air knife pressure of 250-280mbar, and a cooling rate of 5-10℃ / s to cool to 370-380℃; the second stage is the air cooling stage, cooling to 230-240℃ at a cooling rate of 35-40℃ / s to eliminate the influence of latent heat release of the strip steel, refine the coating grain structure, and optimize the coating structure; the third stage is the water cooling stage, cooling to room temperature (25℃) at a cooling rate of 25-35℃ / s to reduce residual stress in the coating, quickly solidify the structure, and improve the adhesion of the coating structure.

[0012] Based on the above technical solution, the substrate of the coating has a continuous and dense inhibition layer structure, and the inhibition layer exhibits a single-layer inhibition layer grain morphology, wherein the size of the inhibition layer grain is 0.5-1.0 micrometers.

[0013] Beneficial effects The technical solution disclosed in this invention solves the problems of uneven microstructure, poor interfacial adhesion, poor continuity of the inhibition layer, unstable corrosion resistance, and poor surface finish in existing hot-dip galvanized DP780 steel coatings. To achieve high strength, DP780 steel requires the addition of relatively high contents of alloying elements such as Si (0.1%~0.3%) and Mn (1.8%~2.1%). However, during continuous annealing, Si and Mn readily diffuse to the surface and form an inner oxide layer (such as Si / Mn oxide). These oxides hinder the wetting reaction between the steel substrate and the zinc bath, leading to the formation of an inhibition layer (Fe2Al5Zn). x Uneven growth: The formation of the inhibition layer in the oxide-covered area is limited, and some areas may even be missing; Decreased coating adhesion: The coating is prone to peeling (powdering) during bending or stamping, and the ball punch test grade is difficult to consistently reach the level 1 standard; Contradictory design of the steel composition: Although reducing the Si content can alleviate oxidation, it will sacrifice the hardenability and strength of the steel (e.g., conventional DP600 steel with Si content <0.1% has difficulty exceeding 780MPa in strength); Sensitive to annealing control: Traditional annealing processes (e.g., dew point -30℃~-40℃) are not effective in inhibiting oxides on the surface of high-strength steel, the process window is narrow, and parameter fluctuations can easily cause batch defects; Limited coating strengthening methods: Post-treatment such as finishing can only improve surface roughness and cannot fundamentally solve the problem of weakened interface bonding. Therefore, this invention adds rare earth elements, limiting the Nb / Y ratio to 1.1-3, so that Nb and Y elements have a synergistic effect, improving the corrosion resistance of steel. Combined with microstructure regulation and pre-oxidation control technology, it inhibits the enrichment of Si and Mn elements on the surface of the strip steel, reduces the formation of silicon and manganese oxides, strengthens the oxidation capacity of the pre-oxidation section, reduces the thickening of the brittle phase, reduces strip steel loss, and reduces the production of bottom slag in the zinc pot. Attached Figure Description

[0014] Figure 1 This is the morphology of the lower inhibition layer of the conventional hot-dip galvanized duplex steel coating structure described in the comparative example of the present invention; Figure 2 The morphology of the lower inhibition layer of the reinforced hot-dip galvanized duplex steel coating structure described in Embodiment 1 of the present invention; Figure 3 The microstructure of the single-layer structure of the reinforced hot-dip galvanized duplex steel suppression layer described in Example 1 of this invention; Figure 4 This is a line scan diagram of the elemental distribution in the lower suppression layer region of the conventional hot-dip galvanized duplex steel coating structure described in the comparative example of this invention. Figure 5 This is a line scan diagram of the elemental distribution of the suppression layer region of the reinforced hot-dip galvanized duplex steel described in Embodiment 1 of the present invention. Detailed Implementation

[0015] To make the objectives and technical solutions of this invention clearer, the following embodiments are provided for further explanation. However, the scope of protection of this invention is not limited to these embodiments; the embodiments are merely for illustrative purposes. Those skilled in the art should understand that any changes or equivalent substitutions that do not depart from the concept of this invention are included within the scope of protection of this invention.

[0016] Unless otherwise specified, all reagents and raw materials used in this invention are obtained through purchase.

[0017] Example 1 Plating solution composition: Zinc (Zn) as the base, aluminum (Al) content controlled at 0.22 wt%.

[0018] Substrate composition: C: 0.11wt%, Mn: 1.95wt%, Si: 0.42wt%, P: 0.021wt%, Cr: 0.40wt%, Nb: 0.017wt%, Als: 0.040wt%, Y: 0.010wt%, where Nb / Y is 1.7, and the balance is Fe.

[0019] The continuous annealing process employs a three-stage heating process, using 1.7mm low-carbon alloy steel. Stress relief is achieved by heating at 6℃ / s to 695℃, followed by increasing the heating rate to 11℃ / s to 722℃. Shortening the heating time inhibits the diffusion of silicon and manganese to the surface. Rapid heating (25℃ / s) raises the temperature to 845℃, holding for 3 seconds. Pre-oxidation is performed using a mixture of air (dew point -35℃) and nitrogen containing water vapor (dew point -4℃, water vapor content 22%), with a gas volume ratio of 5:1 and a gas flow rate of 25m³ / s. 3The pre-oxidized layer thickness is controlled within 0.5 μm at a rate of 15% by mass at 824℃ for 58 seconds to obtain an ideal surface and improve coating adhesion. Then, the temperature is reduced in two stages: first at a cooling rate of 8℃ / s to 724℃, and then in a rapid cooling mode at a cooling rate of 27℃ / s to 480℃ to obtain a martensitic structure. The zinc bath temperature is 455℃ to ensure the continuous release of latent heat of the strip. The post-plating cooling process employs a three-stage variable-temperature cooling process. The first stage involves cooling from 465℃ (after exiting the zinc bath) to 380℃ using a 25mm air knife distance, 275mbar air knife pressure, and a cooling rate of 7℃ / s. The second stage is air cooling, where the temperature is reduced to 235℃ at a rate of 35℃ / s to eliminate the influence of latent heat release from the strip, refine the coating grain size, and optimize the coating structure. The third stage is water cooling, where the temperature is reduced to room temperature at a rate of 28℃ / s to reduce residual stress in the coating, rapidly solidify the structure, improve coating adhesion, and suppress the grain size of the lower suppressive layer to 0.7 micrometers. The morphology of the lower suppressive layer in the strengthened hot-dip galvanized duplex steel coating structure is shown in [Figure number missing]. Figure 2 The microstructure of the single-layer structure of the inhibition layer of the reinforced hot-dip galvanized duplex steel is shown in the figure. Figure 3 (Red line: Fe-Al phase grains in the suppression layer; Blue line: primary Fe-Al phase grains). See the scanned image of the elemental distribution in the suppression layer region of the strengthened hot-dip galvanized duplex steel. Figure 5 (a: zinc layer, b: inhibition layer, c: steel base).

[0020] Example 2 Plating solution composition: Zinc (Zn) as the base, aluminum (Al) content controlled at 0.22 wt%.

[0021] Substrate composition: C: 0.13wt%, Mn: 1.85wt%, Si: 0.42wt%, P: 0.020wt%, Cr: 0.45wt%, Nb: 0.019wt%, Als: 0.025wt%, Y: 0.014wt%, where Nb / Y is 1.36, and the balance is Fe.

[0022] The continuous annealing process employs a three-stage heating process, using 2.0mm low-carbon alloy steel. Stress relief is achieved by heating at 6℃ / s to 692℃, followed by increasing the heating rate to 11℃ / s to 725℃. Shortening the heating time inhibits the diffusion of silicon and manganese to the surface. Rapid heating (25℃ / s) raises the temperature to 845℃, holding for 4 seconds. Pre-oxidation is performed using a mixture of air (dew point -32℃) and nitrogen containing water vapor (dew point -3℃, water vapor content 25%), with a volume ratio of approximately 5:1 and a gas flow rate of 35m³. 3The pre-oxidized layer thickness is controlled within 0.5 μm at a rate of [per unit weight], followed by a reduction section where reduction is performed at 800℃ for 80 seconds in 15% hydrogen to obtain an ideal surface and improve coating adhesion. The cooling process is then divided into two stages: first, cooling at a rate of 8℃ / s to 725℃, followed by rapid cooling at 30℃ / s to 480℃ to obtain a martensitic structure. The zinc bath temperature is 455℃ to ensure continuous release of latent heat from the strip. The post-plating cooling process employs a three-stage variable-temperature cooling process. In the first stage, after exiting the zinc bath at 469℃, cooling is performed at a rate of 8℃ / s to 380℃ using a 30mm air knife distance and 277mbar air knife pressure. The second stage is the air cooling stage, which cools the strip to 235°C at a cooling rate of 38°C / s to eliminate the influence of latent heat release and refine the coating grain structure. The third stage is the water cooling stage, which cools the strip to room temperature at a cooling rate of 25°C / s to reduce residual stress in the coating, rapidly solidify the structure, improve the adhesion of the coating, and suppress the grain size to 0.8 micrometers.

[0023] Comparative Example Steel plates were manufactured according to the manufacturing method of 780MPa grade hot-dip galvanized DH steel disclosed in CN 119710452 A. The composition of the steel plates is as follows: C 0.16wt%, Mn 1.6wt%, Si 0.5wt%, Al 0.9wt%, Nb 0.016wt%, Ti 0.015wt%, Cr 0.10wt%, with the balance being iron. The hot rolling process parameters are as follows: furnace inlet temperature 587℃, heating temperature 1250℃, initial rolling temperature 1062℃, final rolling temperature 889℃, coiling temperature 624℃, and descaling water pressure 18MPa. The cold rolling and hot-dip galvanizing process parameters are as follows: annealing temperature 830℃, slow cooling outlet temperature 750℃, rapid cooling outlet temperature 420℃, dew point in the annealing furnace -14℃, hydrogen content 4%, furnace nose dew point -50℃. The morphology of the lower layer of the hot-dip galvanized duplex steel coating structure is shown in [reference needed]. Figure 1 It can be seen that the poor reaction interface resulted in the absence of an inhibitory layer on the coating substrate; the elemental distribution line scan diagram of the lower inhibitory layer region of the hot-dip galvanized duplex steel coating structure is shown in [reference needed]. Figure 4 (No clear iron-aluminum inhibition layer, a: zinc layer, b: steel base, iron-aluminum inhibition layer area exists).

[0024] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for strengthening the adhesion of a 780MPa grade hot-dip galvanized duplex steel coating, characterized in that, Includes the following steps: Plating solution composition: Zinc as the base, aluminum content controlled at 0.22 wt%; Substrate composition: C: 0.11wt%, Mn: 1.95wt%, Si: 0.42wt%, P: 0.021wt%, Cr: 0.40wt%, Nb: 0.017wt%, Als: 0.040wt%, Y: 0.010wt%, where Nb / Y is 1.7, and the balance is Fe; The continuous annealing process employs a three-stage heating process, using 1.7mm low-carbon alloy steel. Stress relief is achieved by heating at 6℃ / s to 695℃, then the heating rate is increased to 11℃ / s to 722℃. A rapid heating process at 25℃ / s further raises the temperature to 845℃, followed by a 3-second holding period. Pre-oxidation is performed using air mixed with nitrogen containing water vapor at a volume ratio of 5:1 and a gas flow rate of 25m³ / s. 3 The pre-oxidized layer thickness is controlled within 0.5 μm. Then, it enters the reduction section, where it is reduced at 824℃ for 58 seconds using 15% hydrogen by mass. The cooling process is then divided into two stages: first, cooling at 8℃ / s to 724℃, followed by rapid cooling at 27℃ / s to 480℃, resulting in a martensitic structure. The zinc bath temperature is 455℃. The post-plating cooling process employs a three-stage variable-temperature cooling process. In the first stage, after exiting the zinc bath at 465℃, a 25mm air knife distance and 275mbar air knife pressure are used to cool to 380℃ at a rate of 7℃ / s. The second stage is an air-cooling stage, cooling to 235℃ at a rate of 35℃ / s. The third stage is a water-cooling stage, cooling to room temperature at a rate of 28℃ / s to reduce residual stress in the coating, resulting in a strengthened 780MPa grade hot-dip galvanized duplex steel coating with a suppressed layer grain size of 0.7 micrometers. The dew point of the air is -35°C, the water vapor content in the nitrogen gas containing water vapor is 22%, and the dew point of the nitrogen gas containing water vapor is -4°C.

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