A manufacturing method of rare earth weathering steel cold rolling annealing and color coating

Abstract

The application relates to the field of metal material processing, and particularly discloses a rare earth weather-resistant steel cold-rolling annealing and color coating manufacturing method, which comprises the following steps: taking weather-resistant steel containing rare earth elements and Cu, Cr, Ni and P as a base material, performing pickling, cold rolling, annealing, flatting and straightening, chemical conversion and two-coating and two-baking processes to obtain a finished product. Through the matching design of the annealing system and the cold rolling reduction rate, a uniform and fine recrystallized structure is obtained, and the yield platform is eliminated. Meanwhile, the chemical conversion and the coating system are matched with the surface characteristics of the rare earth weather-resistant steel, so that the color coated steel plate has excellent forming performance and coating adhesion, and is suitable for the industrialized stable production in the fields of buildings, rail transit and outdoor equipment.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing, specifically a method for cold rolling and annealing of rare earth weathering steel and its color coating manufacturing. Background Technology

[0002] Rare earth weathering steel is a type of low-alloy structural steel formed by adding small amounts of alloying elements such as copper, chromium, nickel, and phosphorus to ordinary carbon steel. Its significant characteristic, distinguishing it from ordinary structural steel, is its ability to form a dense, firmly bonded protective rust layer in natural atmospheric conditions. This rust layer effectively hinders further penetration by corrosive media, thus extending the service life of the steel. Rare earth elements (lanthanum, cerium, etc.) have deoxidizing and desulfurizing properties, purify grain boundaries, refine grains, and modify inclusions. In recent years, they have been explored for introduction into weathering steel systems to further enhance the steel's atmospheric corrosion resistance. Based on this, rare earth weathering steel color-coated sheets have broad application prospects in fields with high requirements for durability and appearance, such as building envelopes, rail transit vehicles, outdoor equipment shells, and photovoltaic brackets.

[0003] Currently, the production processes for rare earth weathering steel, including hot rolling, pickling, cold rolling, annealing, and color coating, are often relatively independent in actual industrial production, lacking a systematic process plan adapted to the characteristics of rare earth weathering steel. In particular, the annealing process after cold rolling, if mismatched with the microalloying characteristics of rare earths and the cold rolling reduction rate, can easily lead to insufficient recrystallization or abnormal grain growth, resulting in uneven microstructure and yield plateaus, which in turn affect the sheet shape and coating adhesion in subsequent color coating processes. This disconnect between the substrate microstructure and the color coating process limits the full realization of the corrosion resistance and formability of rare earth weathering steel in color-coated products. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a cold-rolled annealing method for rare earth weathering steel and its color coating manufacturing method. This solves the problem in the prior art where the cold-rolled annealing process is mismatched with the microalloying characteristics and cold-rolling reduction rate of rare earth weathering steel, resulting in uneven substrate structure and yield plateau, which affects the shape of the color-coated sheet and the adhesion of the coating.

[0005] This invention provides a method for cold-rolled annealing and color-coating of rare earth weathering steel, comprising the following steps:

[0006] A hot-rolled substrate of rare earth weathering steel is provided, wherein the chemical composition of the rare earth weathering steel is as follows (by mass percentage): C 0.06-0.12%, Si 0.10-0.40%, Mn 0.30-0.80%, P 0.05-0.12%, S≤0.008%, Cu 0.20-0.40%, Cr 0.30-0.60%, Ni 0.10-0.30%, RE(La+Ce) 0.005-0.035%, Als 0.010-0.030%, with the balance being Fe and unavoidable impurities;

[0007] The hot-rolled substrate is subjected to hydrochloric acid pickling, cold rolling, electrolytic degreasing, annealing, leveling and straightening, pretreatment for color coating, chemical conversion, primer coating and baking, topcoat coating and baking, cooling and finishing to obtain the finished product.

[0008] Preferably, the hydrochloric acid concentration in the hydrochloric acid pickling process is 10% to 18%, and the pickling temperature is 70 to 85°C.

[0009] Preferably, in the cold rolling process, the thickness of the incoming material is 2.0 to 4.0 mm, the total reduction rate is 50% to 75%, the thickness of the finished product is 0.35 to 2.0 mm, the thickness accuracy is ±0.01 mm, and the surface roughness Ra is 0.8 to 1.2 μm.

[0010] Preferably, the annealing process adopts a continuous annealing method: heating rate of 5-15℃ / s, soaking temperature of 780-820℃, holding temperature for 60-120s; then slow cooling at 670-710℃ at a slow cooling rate of 3-8℃ / s, and then rapid cooling to below 200℃ at a rate of 15-30℃ / s, with the protective atmosphere being a mixture of N2 and H2, and the H2 volume content being 3%-8%.

[0011] Preferably, the annealing process adopts a bell-type annealing method: the protective atmosphere is a pure hydrogen atmosphere with an H2 volume content of ≥95%, the hot spot temperature is 670~700℃, the cold spot temperature is 610~640℃, the holding time is 8~15h, and then the furnace is cooled to below 200℃ before being taken out of the furnace.

[0012] Preferably, the leveling elongation rate in the leveling and straightening process is 0.6% to 1.5%, and the straightening elongation rate is 0.3% to 0.8%.

[0013] Preferably, the pretreatment for color coating includes alkaline degreasing and multi-stage rinsing. The pH of the alkaline degreasing solution is 11.5-12.5, the temperature is 50-65℃, and the time is 15-30s. After degreasing, multi-stage rinsing and hot air drying are performed.

[0014] Preferably, the chemical conversion employs chromium-free passivation or zirconium salt conversion treatment, and the conversion membrane weight is 30–80 mg / m³. 2 .

[0015] Preferably, the primer coating and topcoat coating are applied using a two-coat, two-bake method: the primer is an epoxy primer or a polyester primer, with a wet film thickness of 5–10 μm, a peak curing temperature of 180–205°C, and a curing time of 25–40 s; the topcoat is a high-weather-resistant polyester SMP or fluorocarbon PVDF, with a wet film thickness of 10–20 μm, a peak curing temperature of 210–240°C, and a curing time of 30–50 s.

[0016] The rare earth weather-resistant color-coated steel sheet prepared according to the above method has an elongation A≥30%, T-bend ≤2T, neutral salt spray test time≥500h, and atmospheric corrosion resistance is 3 to 6 times that of Q235 steel.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] By adding trace amounts of rare earth elements and synergistically combining them with weather-resistant elements such as Cu, Cr, Ni, and P, the grain boundaries in the steel are purified, the microstructure is refined, and the morphology of non-metallic inclusions is modified. This enables the steel matrix to form a dense and stable inner rust layer in an atmospheric environment, thereby significantly delaying the penetration and spread of corrosive media and achieving a fundamental improvement in the weather resistance of the steel itself.

[0019] This invention eliminates the yield plateau caused by cold rolling work hardening by matching the cold rolling reduction rate, annealing temperature and holding time, and controlling the elongation rate of flattening and tension leveling. This results in a uniform and fine recrystallized grain structure, enabling the steel plate to maintain excellent plasticity reserves while possessing high strength, and achieving stable forming and crack-free forming during the stamping and bending process.

[0020] This invention combines chemical conversion treatment adapted to the surface characteristics of rare earth weathering steel with a two-coat, two-bake coating system, forming good interlayer adhesion and synergistic protective effect between the conversion film and the primer, and between the primer and the topcoat. This enables the coating to effectively resist ultraviolet aging, humid environments and salt spray corrosion during long-term outdoor service, achieving durable protection and appearance maintenance throughout the entire life cycle of the coating system. At the same time, the stable connection of the entire process ensures the continuity of industrial production and the consistency of product quality. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the process routes for continuous annealing and bell-type annealing in the embodiments of the present invention. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1: This example provides a method for cold rolling and annealing rare earth weathering steel and its color coating manufacturing process, such as... Figure 1 As shown, a continuous annealing process is used in conjunction with a high weather-resistant polyester topcoat (SMP) color coating scheme.

[0024] The chemical composition of the rare earth weathering steel hot-rolled substrate used in this embodiment, by mass percentage, is as follows: C 0.09%, Si 0.25%, Mn 0.55%, P 0.08%, S 0.005%, Cu 0.30%, Cr 0.45%, Ni 0.20%, RE(La+Ce) 0.018%, Als 0.020%, with the balance being Fe and unavoidable impurities.

[0025] The hot-rolled substrate was pickled with hydrochloric acid at a concentration of 14% and a pickling temperature of 78°C. After removing the surface iron oxide scale, it was ensured that there was no residual acid or over-corrosion on the surface.

[0026] After pickling, the steel strip enters the cold rolling process. The incoming material thickness is 3.0mm. It is rolled into a finished product thickness of 1.2mm through multiple cold rolling passes. The total reduction rate is 70%. The finished product thickness accuracy is controlled within ±0.01mm, and the surface roughness Ra is controlled at 1.0μm.

[0027] After cold rolling, the steel strip undergoes electrolytic degreasing to remove surface rolling oil and residual contaminants.

[0028] The degreased steel strip is then fed into a continuous annealing furnace for annealing.

[0029] The annealing process is as follows: the steel strip is heated to a uniform temperature of 800℃ at a heating rate of 10℃ / s and held at 800℃ for 90s; then it enters the slow cooling section and is slowly cooled to 690℃ at a cooling rate of 5℃ / s; then it is rapidly cooled to below 180℃ at a cooling rate of 20℃ / s.

[0030] The protective atmosphere throughout the annealing process is a mixture of N2 and H2, with H2 accounting for 5% by volume and the balance being N2.

[0031] The annealed steel strip undergoes leveling and tension straightening. The leveling elongation is controlled at 1.0%, and the tension straightening elongation is controlled at 0.5% to eliminate any yield plateaus that may exist after annealing, while also improving the flatness of the strip.

[0032] After leveling and straightening, the steel strip enters the pretreatment process for color coating: it is degreased with alkaline solution at a pH of 12.0 and a temperature of 60℃ for 20 seconds; after degreasing, it undergoes multi-stage rinsing and is finally dried with hot air.

[0033] Subsequently, a chemical conversion treatment was performed using a chromium-free passivation solution, with the conversion membrane weight controlled at 50 mg / m³. 2 .

[0034] After chemical conversion, the steel strip enters the color coating process, which uses a two-coat, two-bake method.

[0035] First, apply an epoxy primer with a wet film thickness of 8 μm and cure at a peak temperature of 190℃ for 30 seconds. Then, apply a high-weather-resistant polyester topcoat (SMP) with a wet film thickness of 15 μm and cure at a peak temperature of 225℃ for 40 seconds.

[0036] After the color coating is completed, the steel strip is cooled to 45°C and then rolled up. After finishing, the finished color-coated steel sheet is obtained.

[0037] The finished color-coated steel sheet obtained in this embodiment was subjected to performance testing: the yield strength was 310 MPa, the elongation was 34%; the coating adhesion T-bend was ≤2T; no blistering was observed in the neutral salt spray test for 750 h; no obvious powdering was observed in the QUV aging test for 1200 h; and the atmospheric corrosion resistance was 4.5 times that of Q235 steel.

[0038] Example 2: This example provides a method for cold rolling and annealing rare earth weathering steel and its color coating manufacturing process, such as... Figure 1 As shown, the process of using a hood annealing method combined with a fluorocarbon topcoat (PVDF) color coating solution is suitable for outdoor high weather resistance applications.

[0039] The chemical composition of the rare earth weathering steel hot-rolled substrate used in this embodiment, by mass percentage, is as follows: C 0.08%, Si 0.25%, Mn 0.55%, P 0.08%, S 0.005%, Cu 0.30%, Cr 0.45%, Ni 0.20%, RE(La+Ce) 0.025%, Als 0.020%, with the balance being Fe and unavoidable impurities.

[0040] The hot-rolled substrate was pickled with hydrochloric acid at a concentration of 15% and a pickling temperature of 80°C. After removing the surface iron oxide scale, it was ensured that there was no residual acid or over-corrosion on the surface.

[0041] After pickling, the steel strip enters the cold rolling process. The incoming material thickness is 3.0 mm. It is rolled into a finished product thickness of 0.8 mm through multiple cold rolling passes. The total reduction rate is 73%. The finished product thickness accuracy is controlled within ±0.01 mm, and the surface roughness Ra is controlled at 1.1 μm.

[0042] After cold rolling, the steel strip undergoes electrolytic degreasing to remove surface rolling oil and residual contaminants.

[0043] The degreased steel strip is loaded into a bell-type annealing furnace in the form of a steel coil for annealing treatment.

[0044] The annealing process is as follows: the protective atmosphere is a pure hydrogen atmosphere with an H2 volume content of 98%; the hot spot temperature is controlled at 680℃ and the cold spot temperature at 625℃; the furnace is held at the holding temperature for 12 hours; after the holding period, the furnace is cooled to below 200℃ and then removed from the furnace.

[0045] The annealed steel strip undergoes leveling and tension straightening.

[0046] The elongation rate of the leveling process is controlled at 1.2%, and the elongation rate of the tension leveling process is controlled at 0.6%.

[0047] After leveling and straightening, the steel strip enters the pretreatment process for color coating: it is degreased with alkaline solution at a pH of 12.0 and a temperature of 60℃ for 25 seconds; after degreasing, it undergoes multi-stage rinsing and is finally dried with hot air.

[0048] Subsequently, a chemical conversion treatment was performed using a zirconium salt conversion solution, with the conversion membrane weight controlled at 60 mg / m³. 2 .

[0049] After chemical conversion, the steel strip enters the color coating process, employing a two-coat, two-bake method. First, an epoxy primer is applied, with a wet film thickness controlled at 8 μm, and cured at a peak temperature of 195℃ for 35 seconds. Then, a fluorocarbon topcoat (PVDF) is applied, with a wet film thickness controlled at 18 μm, and cured at a peak temperature of 230℃ for 45 seconds. After color coating, the steel strip is cooled to 45℃ and then wound up. After finishing, the finished color-coated steel sheet is obtained.

[0050] The finished color-coated steel sheet obtained in this embodiment was subjected to performance testing: the yield strength was 295MPa, the elongation was 32%; the coating adhesion T-bend was ≤2T; there was no failure after 1000h of neutral salt spray test; there was no obvious powdering after 1500h of QUV aging test; and the atmospheric corrosion resistance was 5 times that of Q235 steel.

[0051] Example 3: This example provides a method for cold rolling and annealing rare earth weathering steel and its color coating manufacturing. It adopts a continuous annealing process combined with a high weather-resistant polyester topcoat (SMP) color coating scheme. The difference from Example 1 is that the cold rolling reduction rate, annealing temperature and color coating process parameters are different.

[0052] The chemical composition of the rare earth weathering steel hot-rolled substrate used in this embodiment, by mass percentage, is as follows: C 0.11%, Si 0.15%, Mn 0.70%, P 0.10%, S 0.006%, Cu 0.35%, Cr 0.50%, Ni 0.25%, RE(La+Ce) 0.030%, Als 0.025%, with the balance being Fe and unavoidable impurities.

[0053] The hot-rolled substrate was pickled with hydrochloric acid at a concentration of 12% and a pickling temperature of 75°C.

[0054] After pickling, the steel strip enters the cold rolling process. The incoming material thickness is 3.5mm, and it is rolled through multiple cold rolling passes to a finished thickness of 1.5mm. The total reduction rate is 57%, and the finished thickness accuracy is controlled within ±0.01mm, with a surface roughness Ra controlled at 0.9μm. The cold-rolled steel strip undergoes electrolytic degreasing.

[0055] The degreased steel strip is then fed into a continuous annealing furnace for annealing.

[0056] The annealing process is as follows: the steel strip is heated to a uniform temperature of 810℃ at a heating rate of 8℃ / s and held at 810℃ for 100s; then it enters the slow cooling section and is slowly cooled to 700℃ at a cooling rate of 6℃ / s; then it is rapidly cooled to below 180℃ at a cooling rate of 25℃ / s.

[0057] The protective atmosphere throughout the annealing process is a mixture of N2 and H2, with H2 accounting for 6% by volume.

[0058] The annealed steel strip undergoes leveling and tension straightening. The elongation during leveling is controlled at 1.3%, and the elongation during tension straightening is controlled at 0.7%.

[0059] After leveling and straightening, the steel strip undergoes pretreatment before color coating (alkaline degreasing at pH 11.8, temperature 55℃, time 25s, multi-stage rinsing, and hot air drying) and chromium-free passivation treatment (film weight 55mg / m³). 2 ).

[0060] Then, a two-coat, two-bake process was performed: the primer was a polyester primer with a wet film thickness of 7μm, a peak curing temperature of 195℃, and a curing time of 35s; the topcoat was a high weather-resistant polyester topcoat (SMP) with a wet film thickness of 12μm, a peak curing temperature of 220℃, and a curing time of 45s.

[0061] After the color coating is completed, the steel strip is cooled to 45°C and then rolled up. After finishing, the finished color-coated steel sheet is obtained.

[0062] The finished color-coated steel sheet obtained in this embodiment was subjected to performance testing: the yield strength was 335MPa, the elongation was 31%; the coating adhesion T-bend was ≤2T; no blistering occurred in the neutral salt spray test for 650h; and the atmospheric corrosion resistance was 4 times that of Q235 steel.

[0063] Example 4: This example provides a method for cold rolling and annealing rare earth weathering steel and its color coating manufacturing. It adopts a bell-type annealing process combined with a fluorocarbon topcoat (PVDF) color coating scheme. The difference from Example 2 is that the cold rolling reduction rate, annealing temperature and holding time are different.

[0064] The chemical composition of the rare earth weathering steel hot-rolled substrate used in this embodiment, by mass percentage, is as follows: C 0.07%, Si 0.35%, Mn 0.40%, P 0.06%, S 0.004%, Cu 0.25%, Cr 0.35%, Ni 0.15%, RE(La+Ce) 0.010%, Als 0.015%, with the balance being Fe and unavoidable impurities.

[0065] The hot-rolled substrate was pickled with hydrochloric acid at a concentration of 16% and a pickling temperature of 82°C.

[0066] After pickling, the steel strip enters the cold rolling process. The incoming material thickness is 2.5mm, and it is rolled through multiple cold rolling passes to a finished thickness of 0.675mm, with a total reduction rate of 73%. The finished thickness accuracy is controlled within ±0.01mm, and the surface roughness Ra is controlled at 1.2μm. The cold-rolled steel strip undergoes electrolytic degreasing.

[0067] The degreased steel strip is loaded into a bell-type annealing furnace in the form of a steel coil for annealing treatment: the protective atmosphere is a full hydrogen atmosphere with an H2 volume content of 97%; the hot spot temperature is 695℃ and the cold spot temperature is 635℃; the holding time is 10h; after the holding time is completed, the strip is cooled to below 200℃ in the furnace before being taken out.

[0068] The annealed steel strip undergoes leveling and tension straightening. The elongation during leveling is controlled at 0.8%, and the elongation during tension straightening is controlled at 0.4%.

[0069] After leveling and straightening, the steel strip undergoes pretreatment before color coating (alkaline degreasing at pH 12.3, temperature 62℃, time 18s, multi-stage rinsing, and hot air drying) and zirconium salt conversion treatment (film weight 40mg / m³). 2 ).

[0070] Then, a two-coat, two-bake process was performed: the primer was an epoxy primer with a wet film thickness of 6μm, a peak curing temperature of 185℃, and a curing time of 30s; the topcoat was a fluorocarbon topcoat (PVDF) with a wet film thickness of 14μm, a peak curing temperature of 235℃, and a curing time of 35s.

[0071] After the color coating is completed, the steel strip is cooled to 45°C and then rolled up. After finishing, the finished color-coated steel sheet is obtained.

[0072] The finished color-coated steel sheet obtained in this embodiment was subjected to performance testing: the yield strength was 280MPa, the elongation was 33%; the coating adhesion T-bend was ≤2T; there was no failure in the neutral salt spray test for 850h; and the atmospheric corrosion resistance was 4.8 times that of Q235 steel.

[0073] The embodiments of the present invention are given for the purposes of illustration and description. Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Any changes, modifications, substitutions and variations made by those skilled in the art to the above embodiments within the scope of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for cold-rolling and annealing rare earth weathering steel and its color coating manufacturing process, characterized in that, Includes the following steps: A hot-rolled substrate of rare earth weathering steel is provided, wherein the chemical composition of the rare earth weathering steel is as follows (by mass percentage): C 0.06-0.12%, Si 0.10-0.40%, Mn 0.30-0.80%, P 0.05-0.12%, S≤0.008%, Cu 0.20-0.40%, Cr 0.30-0.60%, Ni 0.10-0.30%, RE(La+Ce) 0.005-0.035%, Als 0.010-0.030%, with the balance being Fe and unavoidable impurities; The hot-rolled substrate is subjected to hydrochloric acid pickling, cold rolling, electrolytic degreasing, annealing, leveling and straightening, pretreatment for color coating, chemical conversion, primer coating and baking, topcoat coating and baking, cooling and finishing to obtain the finished product.

2. The manufacturing method according to claim 1, characterized in that, The hydrochloric acid pickling process uses hydrochloric acid with a concentration of 10% to 18% and a pickling temperature of 70 to 85°C.

3. The manufacturing method according to claim 1, characterized in that, In the cold rolling process, the incoming material thickness is 2.0–4.0 mm, the total reduction rate is 50%–75%, the finished product thickness is 0.35–2.0 mm, the thickness accuracy is ±0.01 mm, and the surface roughness Ra is 0.8–1.2 μm.

4. The manufacturing method according to claim 1, characterized in that, The annealing process adopts a continuous annealing method: heating rate of 5-15℃ / s, uniform heating temperature of 780-820℃, holding temperature for 60-120s; then slow cooling at 670-710℃ at a slow cooling rate of 3-8℃ / s, and then rapid cooling to below 200℃ at a rate of 15-30℃ / s. The protective atmosphere is a mixture of N2 and H2, with an H2 volume content of 3%-8%.

5. The manufacturing method according to claim 1, characterized in that, The annealing process adopts a bell-type annealing method: the protective atmosphere is a pure hydrogen atmosphere with an H2 volume content of ≥95%, the hot spot temperature is 670~700℃, the cold spot temperature is 610~640℃, the holding time is 8~15h, and then the furnace is cooled to below 200℃ before being taken out of the furnace.

6. The manufacturing method according to claim 1, characterized in that, The leveling and straightening process has a leveling elongation of 0.6% to 1.5% and a straightening elongation of 0.3% to 0.8%.

7. The manufacturing method according to claim 1, characterized in that, The pretreatment for color coating includes alkaline degreasing and multi-stage rinsing. The pH of the alkaline degreasing solution is 11.5-12.5, the temperature is 50-65℃, and the time is 15-30s. After degreasing, multi-stage rinsing and hot air drying are performed.

8. The manufacturing method according to claim 1, characterized in that, The chemical conversion employs chromium-free passivation or zirconium salt conversion treatment, with a conversion membrane weight of 30–80 mg / m³. 2 .

9. The manufacturing method according to claim 1, characterized in that, The primer and topcoat are applied using a two-coat, two-bake method: the primer is an epoxy primer or a polyester primer, with a wet film thickness of 5–10 μm, a peak curing temperature of 180–205℃, and a curing time of 25–40 s; the topcoat is a high-weather-resistant polyester SMP or fluorocarbon PVDF, with a wet film thickness of 10–20 μm, a peak curing temperature of 210–240℃, and a curing time of 30–50 s.

10. The rare earth weathering-resistant color-coated steel sheet prepared by the method according to any one of claims 1 to 9, characterized in that, The steel plate has an elongation A ≥ 30%, a T-bend ≤ 2T, a neutral salt spray test time ≥ 500h, and atmospheric corrosion resistance 3 to 6 times that of Q235 steel.

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