Cold-rolled hot-dip aluminum-zinc-plated steel plate with yield strength of 500 MPa

Abstract

The invention discloses a cold-rolled hot-dip aluminum-zinc-plated steel plate with yield strength of 500MPa. The technical problems that an existing cold-rolled hot-dip aluminum-zinc-plated steel plate with yield strength of 500MPa is high in cold rolling load, high in production cost and low in production efficiency are mainly solved. According to the technical scheme, a base plate of the cold-rolled hot-dip aluminum-zinc-plated steel plate with the yield strength of 500 MPa comprises the following chemical components in percentage by weight: 0.14%-0.16% of C, 0.001%-0.050% of Si, 0.81%-0.90% of Mn, less than or equal to 0.015% of P, less than or equal to 0.015% of S, 0.060%-0.075% of Ti, 0.0015%-0.0025% of B, 0.010%-0.030% of Al, 0.25% < = (C + Mn / 6) < = 0.35%, 0.12% < = (C-Ti / 4) < = 0.15% and the balance of Fe and inevitable impurities.

Description

Technical Field

[0001] The invention relates to a cold-rolled hot-dip aluminum-zinc coated steel sheet, in particular to a cold-rolled hot-dip aluminum-zinc coated steel sheet with a yield strength of 500 MPa and a manufacturing method thereof, belonging to the technical field of iron-based alloys. Background Art

[0002] Cold-rolled hot-dip galvanized steel sheets have excellent corrosion resistance, heat resistance, formability and beautiful surface. After 2000, galvanized products have been widely used, mainly in the construction, home appliances and electrical cabinet industries.

[0003] Mainstream construction plants use galvanized aluminum products as roof panels. In recent years, a large number of plants have been built along the coastal frontline. The coastal climate is accompanied by strong winds, which requires the plants to have strong wind resistance and a wind resistance strength greater than 3MPa. The existing steel plate yield strength level is 350MPa, and the wind resistance is still insufficient. The use of 500MPa-grade steel plates has better wind resistance, but the existing 500MPa-grade steel plates have high costs and cannot meet the construction industry's requirements for reducing steel costs.

[0004] The Chinese patent document with application publication number CN102094149A discloses a niobium-containing high-strength hot-dip galvanized steel sheet and its production method, and provides a high-value and low Δγ value structural niobium-containing high-strength hot-dip galvanized steel sheet and its manufacturing method. The niobium-containing high-strength hot-dip galvanized steel sheet has the following composition: by mass%, C: 0.03-0.1%, Si: ≤0.05%, Mn: 0.5-0.9%, P: ≤0.025%, S: ≤0.015%, N: ≤0.005%, Ti: ≤0.005%, Nb: 0.03-0.06%, Al: 0.01%-0.08%, and the balance is Fe and unavoidable impurities. Its preparation method is: subjecting the steel billet to hot rolling, coiling, pickling, cold rolling and hot-dip galvanizing processes. The invention provides a high-strength hot-dip galvanized steel sheet containing Nb with Δγ≤0, yield strength≥460MPa, tensile strength≥570MPa, and elongation at break≥28%, that is, it has excellent forming performance while ensuring high strength. The patent provides a design method for high-strength steel, with high alloy addition, high cost, and yield strength below 500MPa.

[0005] The Chinese patent document with application publication number CN102199723A discloses a high-strength cold-rolled hot-dip aluminum-zinc precipitation-strengthening steel and a manufacturing method thereof, wherein the chemical composition of the substrate is: C: 0.09-0.12wt%, Si≤0.05wt%, Mn: 0.8-1.8wt%, P≤0.02wt%, S≤0.01wt%, N≤0.008wt%, Al: 0.02-0.06wt%, Nb+Ti: 0.05-0.08wt%, and the rest is Fe and unavoidable impurities. The microstructure is ferrite plus second phase particles, the particle diameter in the second phase is about less than 6um, the yield strength is 500-600MPa, the tensile strength is 570-710MPa, and the total elongation is greater than 14% (80 gauge length, transverse stretching). It adds more alloy elements Nb+Ti in the composition, and the production cost is high.

[0006] A Chinese patent document with application publication number CN105039848A discloses a method for producing 500-600MPa grade cold-rolled annealed low-alloy high-strength steel. The tensile strength of the product produced by this method is between 500 and 600MPa, and the yield ratio varies in the range of 0.55 to 0.85. The tensile strength of the final product obtained is 500 to 600MPa. Expensive metals such as Mn and Nb are added. It is produced in a vertical annealing furnace, with a long heating time and high production cost.

[0007] The Chinese patent document with application publication number CN101348880A discloses a method for producing a high-strength galvanized sheet for structure, characterized in that the chemical composition mass percentage of the galvanized sheet is: C, 0.16-0.20%; Mn, 0.9-1.1%; Si, ≤0.05%; P, ≤0.020%; S, ≤0.020%; Als, 0.02-0.06%, and the balance is Fe; hot rolling process parameters: heating temperature, 1250±20℃; rough rolling outlet temperature, 1070±20℃; final rolling temperature, 850±20℃; coiling temperature, 560±20℃; cold rolling reduction rate: controlled at 60-75% according to different specifications; galvanizing annealing temperature: 680-720℃; soaking time: 30-45s. It only meets the yield strength requirement of more than 350MPa. It adopts high carbon and high manganese design, the rolling production process is difficult, the production efficiency is low, and the material yield strength is lower than 500MPa.

[0008] The application publication number CN108866428B discloses a hot-dip aluminum-zinc steel plate with a yield strength of 550 MPa and a method for manufacturing the same. The chemical composition of the substrate is C: 0.05-0.06%, Si: 0-0.05%, Mn: 1.0-1.2%, P: 0-0.015%, Nb: 0.061-0.08%, and its metallographic structure is fibrous ferrite-cementite and fine niobium carbide precipitates. The elongation after fracture is 10-18%. The hot-dip aluminum-zinc steel plate proposed in the patent has a large amount of Mn and Nb added, and the production cost is relatively high.

[0009] Application publication number CN107760999A discloses a 550MPa grade hot-dip aluminum-zinc steel sheet for structure and a preparation method thereof, with a chemical composition of C: 0.02-0.07%, Si≤0.03%, Mn: 0.15-0.30%, P≤0.020%, Si≤0.020%, Nb: 0.015-0.030%, Als: 0.020-0.070%, and cold rolling is performed at a low cold rolling reduction rate of 55-60%. The yield strength is above 550MPa, the tensile strength is 560MPa, and the elongation is about 10%. The steel sheet proposed in this patent has a low elongation, and the expensive Nb element is added, resulting in high production cost.

[0010] In order to obtain high-strength properties, existing cold-rolled hot-dip aluminum-zinc coated steel sheets with a yield strength of 500 MPa usually use carbon-manganese strengthening and micro-alloying with niobium elements to increase the yield strength. This brings about technical problems such as high product design alloy cost, high cold rolling load, and low production efficiency. Summary of the invention

[0011] The purpose of the present invention is to provide a cold-rolled hot-dip aluminum-zinc coated steel plate with a yield strength of 500 MPa and a manufacturing method thereof, mainly to solve the technical problems of high cold rolling load, high production cost and low production efficiency of the existing cold-rolled hot-dip aluminum-zinc coated steel plate with a yield strength of 500 MPa; the cold-rolled hot-dip aluminum-zinc coated steel plate of the present invention meets the construction industry's demand for roll forming plastic processing of cold-rolled hot-dip aluminum-zinc coated steel plates.

[0012] The technical idea adopted by the present invention is to add low-cost C, Ti, and B elements without adding expensive alloying elements Nb and Mn; to use hot rolling controlled rolling and cooling technology to obtain hot-rolled steel plates with low yield strength, to achieve the low-load cold rolling requirements of the acid rolling process, to reduce rolling force, to improve product manufacturing capabilities, and to reduce energy consumption; and then through a rapid annealing process and rapid cooling in a horizontal continuous annealing furnace, phase transformation strengthening and precipitation strengthening are adopted to obtain a metallographic structure including recrystallized fibrous ferrite + 3% to 4% bainite + cementite + fine titanium carbide precipitates in terms of area percentage, to achieve high yield strength and cold rolling low-load rolling of cold-rolled hot-dip aluminum-zinc-plated steel plates, to reduce manufacturing costs and improve production efficiency, and to meet the industry's high wind resistance requirements.

[0013] The technical idea of ​​the present invention is based on the following research findings of the applicant:

[0014] The yield strength of the existing 500MPa grade cold-rolled steel sheet after annealing designed with Nb and Mn strengthening elements is 50-60MPa lower than that of the hot-rolled steel sheet; in order to achieve the requirement that the yield strength of the final cold-rolled product is greater than 500MPa, the yield strength of the hot-rolled steel sheet must be greater than 550MPa. The addition of Nb and Mn strengthening elements can significantly improve the yield strength of the hot-rolled steel sheet, which will lead to high cold rolling load, high energy consumption and low production efficiency. The applicant has found through experimental research and production practice that the requirement of cold rolling yield strength greater than 500MPa can be achieved by adding C, Ti and B elements to replace the existing Nb and Mn strengthening elements, and the yield strength of the hot-rolled steel sheet after adding C, Ti and B elements is 50-60MPa lower than the yield strength of the traditional design, which can solve the problem of high cold rolling load.

[0015] The applicant further discovered through hot rolling process research that by using the strengthening method of C, Ti, and B elements, the existence of precipitates can be controlled by controlling the hot rolling process, thereby achieving the performance regulation of the hot-rolled plate and achieving the purpose of low-yield hot-rolled plate.

[0016] The wind resistance of the roof panels made of galvanized steel sheets with a yield strength of 500 MPa and a metallographic structure characterized by recrystallized fibrous ferrite + 3% to 4% bainite + cementite + fine titanium carbide precipitates is above 6.0 MPa, while the wind resistance of the roof panels made of ordinary galvanized steel sheets with a yield strength of 350 MPa is 3.5 MPa. The cold-rolled and hot-dip galvanized steel sheets of the present invention have strong wind resistance.

[0017] The technical solution adopted by the present invention is a cold-rolled hot-dip aluminum-zinc-coated steel plate with a yield strength of 500MPa, wherein the chemical composition weight percentage of the substrate is: C: 0.14% to 0.16%, Si: 0.001% to 0.050%, Mn: 0.81% to 0.90%, P≤0.015%, S≤0.015%, Ti: 0.060% to 0.075%, B: 0.0015% to 0.0025%, Al: 0.010% to 0.030%, and satisfies 0.25%≤(C+Mn / 6)≤0.35%, 0.12%≤(C-Ti / 4)≤0.15%, and the rest is Fe and unavoidable inclusions.

[0018] The metallographic structure of the cold-rolled hot-dip aluminum-zinc-coated steel sheet of the present invention comprises 3% to 4% of bainite + recrystallized fibrous ferrite + cementite + fine titanium carbide precipitates in terms of area percentage, and the grain size of the ferrite in the metallographic structure is II12 to II13; the yield strength R of the cold-rolled hot-dip aluminum-zinc-coated steel sheet with a thickness of 0.50 to 0.80 mm isP0.2 520~620MPa, tensile strength R m 610~730MPa, elongation after fracture A 80mm 18% to 24%, 180° bending test, d = 0a qualified, no zinc layer shedding and substrate cracking; single-sided zinc layer weight of cold-rolled hot-dip aluminum-zinc steel sheet is 75 to 90g / m 2 .

[0019] The wind resistance strength of the roof panel made of the cold-rolled hot-dip aluminum-zinc steel sheet of the present invention is above 6.0 MPa.

[0020] The reasons why the chemical composition of the substrate of the cold-rolled hot-dip galvanized steel sheet with a yield strength of 500 MPa described in the present invention is limited to the above range are as follows:

[0021] Carbon: Carbon is the most economical solid solution strengthening element. Its main function is to form carbides with the economic titanium element in steel to control the cold rolling annealing recrystallization process for matrix strengthening. The design of carbon content takes into account the tensile strength and low cost requirements of the material. A carbon content of 0.14-0.16% can achieve a tensile strength greater than 610MPa. Within this range, the composition control of the steelmaking process and the surface quality of the slab are not prone to surface cracks. Through a large number of tests, it was found that when the carbon content exceeds 0.16%, the yield of the hot-rolled plate will increase significantly, which will lead to a large cold rolling load, a decrease in production efficiency, and the elongation of the material after annealing will not meet the requirement of 18%; this design fully considers that the addition of low-cost titanium needs to be combined with carbon to form carbides for strengthening. After a large number of tests, (C-Ti / 4) When (C-Ti / 4) is less than 0.12%, the tensile strength of the cold-rolled sheet will not meet the requirement of 610MPa. When (C-Ti / 4) is greater than 0.15%, excessive pearlite aggregation will occur in the hot-rolled structure, resulting in the elongation of the cold-rolled annealed galvanized steel sheet failing to meet the requirement of 18%. The applicant has found that when the C and Ti element contents in the cold-rolled hot-dip galvanized steel sheet satisfy the following relationship: by weight percentage, 0.12%≤(C-Ti / 4)≤0.15%, more economical cost and mechanical properties can be obtained. Taking into account the low-cost characteristics, the present invention limits the C content to 0.14% to 0.16%.

[0022] Silicon: Silicon is a residual element in steel. No silicon alloy is added during the production process. The present invention limits Si to 0.001% to 0.05%.

[0023] Manganese: An important solid solution strengthening element. Adding appropriate Mn can effectively improve the strength of steel plates. However, manganese alloys are expensive. Adding too much manganese does not meet the low cost requirements. It also leads to solid solution strengthening, which increases the cold rolling load. This patent takes into account the economic benefits of strengthening. The applicant has found that when 0.25% ≤ (C + Mn / 6) ≤ 0.35%, the performance requirements of the yield strength 500MPa grade steel plate can be achieved, and the addition of low-cost manganese alloys can be achieved. Therefore, through repeated tests and verifications, the present invention limits the Mn content to 0.81% to 0.90%.

[0024] Titanium: Titanium is an economical strengthening element. It plays a role of precipitation strengthening in steel. During the annealing process, fine titanium carbides are precipitated, which plays a role of grain refinement and precipitation strengthening. The grain refinement and precipitation of fine titanium carbides in the metallographic structure of the material will increase the interface area between ferrite grains and between ferrite and cementite and bainite in the structure, making it difficult for cracks to expand during the wind resistance test, thereby improving the wind resistance of the steel plate. The setting of the hot rolling coiling temperature and the annealing temperature combined with the process will be more conducive to the strengthening effect of the titanium element. A titanium content greater than 0.075% will cause the hot-rolled plate to have too high strength, resulting in instability in cold rolling and increased composition. A titanium content less than 0.060% will cause the yield strength of the steel plate to fail to reach 500MPa. The applicant has found that when the C and Ti element contents in the cold-rolled hot-dip aluminum-zinc coated steel sheet satisfy the following relationship: 0.12%≤(C-Ti / 4)≤0.15% by weight, relatively economical cost and mechanical properties can be obtained; the present invention limits the Ti content to 0.060% to 0.075%.

[0025] Sulfur: S in the present invention is an impurity element. According to the actual steelmaking production capacity, the present invention limits S to ≤ 0.015%.

[0026] Phosphorus: P in the present invention is an impurity element. According to the actual steelmaking production capacity, the present invention limits P to ≤ 0.015%.

[0027] Aluminum: In addition to deoxidation, adding Al to steel can combine with N in steel to produce AlN. When the Al content is greater than 0.030%, excessive aluminum will increase costs and is not conducive to the strengthening effect of nitrogen in steel. When the Al content is less than 0.10%, it is easy to cause insufficient deoxidation of molten steel, resulting in inclusion problems and cracking in bending. Therefore, the addition range of aluminum in steel is controlled to ensure deoxidation while exerting the strengthening effect of nitrogen. The present invention sets Al: 0.010% to 0.030%.

[0028] Boron: Boron combines with carbon and nitrogen in steel and precipitates at grain boundaries, stabilizing the austenite phase. It is one of the most economical elements for obtaining bainite structure. Combined with the cooling process capabilities after annealing and plating, and considering the economy of addition, B is designed to be 0.0015% to 0.0025%.

[0029] A method for manufacturing a cold-rolled hot-dip aluminum-zinc coated steel sheet with a yield strength of 500 MPa, the method comprising:

[0030] The molten steel is continuously cast to obtain a continuous casting slab, wherein the chemical composition of the molten steel is as follows: C: 0.14% to 0.16%, Si: 0.001% to 0.050%, Mn: 0.81% to 0.90%, P≤0.015%, S≤0.015%, Ti: 0.060% to 0.075%, B: 0.0015% to 0.0025%, Al: 0.010% to 0.030%, and the following conditions are satisfied: 0.25%≤(C+Mn / 6)≤0.35%, 0.12%≤(C-Ti / 4)≤0.15%, and the rest is Fe and unavoidable inclusions;

[0031] The continuous casting slab is heated to 1220°C-1260°C in a heating furnace and then hot rolled. The hot rolling is a two-stage rolling process. The rough rolling is 6 passes, and the rough rolling end temperature is 1040°C-1080°C; the finishing rolling is 7 passes, and the finishing temperature is 800°C-830°C. The finishing rolling reduction rate is 92%-93%; after finishing rolling, the thickness of the hot rolled steel plate is 2.0-2.3mm, and the laminar cooling adopts the front stage cooling; the coiling temperature is 550°C-580°C to obtain the hot rolled steel coil;

[0032] After the hot-rolled steel coil is unrolled, it is pickled, cold rolled, annealed in a horizontal continuous annealing furnace, and hot-dip galvanized to obtain a finished cold-rolled hot-dip galvanized steel sheet with a thickness of 0.50-0.80 mm. The cold rolling reduction rate is 65%-80%. The heating temperature of the hardened strip after cold rolling in the horizontal continuous annealing furnace is 630-650° C., the annealing temperature in the soaking section is 790-815° C., and the annealing time of the strip in the soaking section is 22-25 seconds. The annealed strip is put into a zinc pot for hot-dip galvanizing, and the weight of the single-side zinc layer of the cold-rolled hot-dip galvanized steel sheet is controlled to be 75-90 g / m 2 ; The temperature of the strip entering the aluminum-zinc pot is 590-600℃; the strip is air-cooled after leaving the aluminum-zinc pot, the air-cooling speed of the strip is 40-60℃ / s, and the end temperature of the strip air-cooling is 350-380℃.

[0033] The reasons for the production process adopted by the present invention are as follows:

[0034] 1. Setting of heating temperature of continuous casting slab

[0035] In order to fully dissolve the second phase particles such as TiC and give full play to the grain refinement effect of Ti element, the heating process of continuous casting slab requires a higher heating temperature. Through experiments, it is found that when the heating temperature is lower than 1220℃, the strengthening effect of Ti element cannot be well exerted, which will lead to the yield strength not meeting the requirement of 500MPa. The heating temperature is too high, the energy cost is high, and the economy is not good. After repeated tests, the heating temperature is set at 1220~1260℃.

[0036] 2. Setting of finishing temperature

[0037] Due to the addition of carbon, manganese, titanium, and boron microalloying elements in steel, the phase transition point A3 of the material decreases. According to material tests and calculations, the phase transition point A3 ranges from 764°C to 783°C. In order to achieve the purpose of austenite rolling and ensure the stability of the material structure, it is found through experimental research that when the finishing temperature is less than 800°C, the finishing rolling process is rolled in the ferrite + austenite two-phase region, which easily causes uneven ferrite structure, resulting in the elongation of the cold-rolled steel sheet after fracture failing to meet the requirement of 18%, and cracking is prone to occur when bending; if the finishing temperature is greater than 830°C, the austenite grains before the phase transition will be too large, and the yield strength of the material will be unstable. Therefore, the present invention sets the finishing temperature of the finishing rolling to 800°C to 830°C.

[0038] 3. Setting of laminar cooling method and hot rolling coiling temperature

[0039] The hot rolling coiling temperature is particularly critical for the control of hot rolling grain size, pearlite formation and precipitation of TiC second phase particles. In order to achieve the purpose of low yield strength of hot rolled materials and prevent the aggregation of pearlite in hot rolled plates, realize low rolling load of cold rolling and improve the elongation of aluminum zinc coated steel plates, low temperature coiling temperature is used for experiments. The study found that when the coiling temperature is greater than 580℃, the titanium element added to the steel will precipitate in large quantities as TiC, thereby strengthening the matrix and significantly improving the yield strength, which will cause the hot rolled steel plate to be too strong and difficult to cold roll, and fail to achieve the purpose of reducing the cold rolling load. At the same time, the aggregation of pearlite will cause the problem of insufficient elongation. When the coiling temperature is less than 550℃, the cooling rate will be too fast, the hot rolled steel plate will have fine grains, the hot rolled steel plate will have high yield strength, and the difficulty of controlling the hot rolling coiling temperature will also increase, resulting in large performance fluctuations. The laminar cooling adopts the front cooling to reduce the precipitation of TiC in the coiling area from the end of hot rolling. The steel plate needs to be cooled to the coiling temperature quickly. After comprehensive consideration, the laminar cooling adopts the front cooling, the coiling temperature is 550℃~580℃, and the hot-rolled steel coil is obtained by coiling.

[0040] 4. Setting of cold rolling reduction rate

[0041] The present invention provides a cold rolling process for the cold-rolled hot-dip aluminum-zinc-coated steel sheet subjected to the above rapid heating treatment. After the hot-rolled steel coil with the above composition and subjected to hot rolling is unrolled and pickled to remove the surface iron oxide scale, it is subjected to multiple cold rolling on a cold rolling mill or a reciprocating single-stand rolling mill. The cold rolling reduction rate is less than 65%, the recrystallization annealing is insufficient, and the elongation rate does not meet the requirement of 18%. The cold rolling reduction rate is greater than 80%, which will significantly increase the cold rolling load. Due to the limitation of rolling capacity, the cold rolling reduction rate is determined to be 65% to 80% after comprehensive consideration.

[0042] 5. Setting of annealing temperature and annealing time

[0043] Through the research of material annealing process, the annealing recrystallization temperature under the composition and hot rolling process conditions is 750-760℃. Combined with the rapid heating capacity of the horizontal continuous annealing furnace, the advantage of short-time rapid annealing is used to promote the austenitization of part of the ferrite, creating conditions for obtaining a complex phase structure. In order to obtain the metallographic structure of the cold-rolled hot-dip aluminum-zinc-coated steel sheet, including 3% to 4% bainite + recrystallized fibrous ferrite + cementite + fine titanium carbide precipitates in terms of area percentage, too high or too low temperature will lead to insufficient annealing of the recrystallized ferrite, and the ferrite grain size cannot reach the Ⅱ12-Ⅱ13 level, thus failing to reach the yield strength R of the steel plate. P0.2 520~620MPa, tensile strength R m 610~730MPa, elongation after fracture A 80mm The annealing temperature is 18% to 24%. Under this temperature condition, the recrystallized fibrous ferrite after annealing is beneficial to increase the interface area between the grains, thereby improving the wind resistance. Therefore, the annealing temperature of the strip steel in the horizontal continuous annealing furnace is designed to be 630 to 650°C, the annealing temperature in the soaking section is 790 to 815°C, and the annealing time of the strip steel in the soaking section is 22 to 25s.

[0044] 6. Setting the temperature of the strip entering the aluminum-zinc pot

[0045] In order to achieve the effect of phase transformation strengthening and give full play to the role of adding B element in steel, after the steel plate completes recrystallization annealing and partial ferrite austenitization at a soaking section temperature of 790-815°C, it is necessary to implement rapid cooling to maintain the stable state of the residual austenite in order to obtain a partial phase transformation bainite structure. Therefore, combined with the actual hot dip coating process test results, the temperature of the strip entering the aluminum-zinc pot is set to 590°C-600°C.

[0046] 7. Setting of cooling speed of strip steel after leaving aluminum-zinc pot

[0047] The air cooling speed of the strip after the aluminum-zinc pot and the cooling end temperature determine the content of residual bainite in the steel plate. In order to adjust the matching of material performance and organization, it is found that the cooling speed of the strip after the aluminum-zinc pot is less than 40℃ / s, and the cooling end temperature is greater than 400℃. The area percentage of bainite in the strip does not reach 3-4%, and the cooling strengthening effect cannot be achieved. Combined with the actual capacity of the blowing air cooling device, the strip is air-cooled after leaving the aluminum-zinc pot. The strip air-cooling speed is 40℃ / s~60℃ / s, and the strip air-cooling end temperature is 350~380℃, and then normal cooling is carried out.

[0048] The metallographic structure of the hot-rolled steel plate produced by the method of the present invention is ferrite + pearlite, and the ferrite grain size level in the metallographic structure is 11 to 12 levels; the yield strength R of the 2.0 to 2.3 mm thick hot-rolled steel plate is P0.2 485~555MPa, tensile strength R m 570~659MPa, elongation after fracture A 80mm It is 20% to 25%.

[0049] The metallographic structure of the cold-rolled hot-dip aluminum-zinc-coated steel sheet produced by the method of the present invention comprises 3% to 4% of bainite + recrystallized fibrous ferrite + cementite + fine titanium carbide precipitates in terms of area percentage, and the grain size of the ferrite in the metallographic structure is II12 to II13; the yield strength R of the cold-rolled hot-dip aluminum-zinc-coated steel sheet with a thickness of 0.50 to 0.80 mm is P0.2 520~620MPa, tensile strength R m 610~730MPa, elongation after fracture A 80mm 18% to 24%, 180° bending test, d = 0a qualified, no zinc layer shedding and substrate cracking; single-sided zinc layer weight of cold-rolled hot-dip aluminum-zinc steel sheet is 75 to 90g / m 2 .

[0050] Compared with the prior art, the present invention has the following positive effects: 1. The method of the present invention is a research and development of an economical low-alloy element-added cold-rolled hot-dip aluminum-zinc steel plate. The economical C, Ti, and B elements are used to replace the current Nb and Mn elements for strengthening. The alloy addition amount is reduced by 0.5%, and the cost per ton of steel is reduced by 160 yuan. It is a low-cost product design method with a yield strength of 500MPa. 2. The present invention is easy to control the steelmaking and smelting components through the design of C, Ti, and B alloy components, the surface quality of the continuous casting slab is good, the hot rolling process is easy to implement, and the hot-rolled plate obtains a low-yield strength soft hot-rolled steel plate, which can solve the technical problem of high cold rolling load, bring about a significant increase in production efficiency, and is a low-cost and reliable manufacturing technology. 3. The present invention fully utilizes the advantages and characteristics of the horizontal continuous annealing furnace equipment through the low-cost alloy component design, gives full play to the characteristics of the annealing furnace for rapid annealing, rapid cooling and dipping, and rapid cooling after plating, gives full play to the role of process heat treatment strengthening, has high production efficiency, and is an economical production method for obtaining high-strength steel by multi-mode strengthening. 4. The present invention achieves the purpose of low cost through the design of low-cost alloy components, and obtains the mechanical properties and bending performance stability of the aluminum-zinc-coated steel plate with a yield strength of 500MPa, and realizes a wind resistance that is 71% higher than that of the conventional yield 350MPa product. The wind resistance of the roof panel made of the cold-rolled hot-dip aluminum-zinc-coated steel plate of the present invention is above 6.0MPa, and the reliability of use at the user end is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 This is a metallographic structure photograph of the hot-rolled steel plate of Example 4 of the present invention.

[0052] Figure 2 This is a metallographic structure photograph of the cold-rolled hot-dip aluminum-zinc coated steel sheet according to Example 4 of the present invention. DETAILED DESCRIPTION

[0053] The present invention is further described below in conjunction with Examples 1-6, as shown in Tables 1-5.

[0054] Table 1 shows the chemical composition (by weight percentage) of the substrate of the cold-rolled hot-dip aluminum-zinc coated steel sheet according to the embodiment of the present invention, with the remainder being Fe and unavoidable impurities.

[0055] Table 1 Chemical composition of the substrate of the cold-rolled hot-dip aluminum-zinc coated steel sheet according to the embodiment of the present invention, unit: weight percentage.

[0056]

[0057]

[0058] The contents of C, Ti and Mn elements in the substrate of the cold-rolled hot-dip aluminum-zinc-coated steel sheet of the present invention satisfy the following relationships at the same time: by weight percentage, (C-Ti / 4) is 0.12% to 0.15%, and (C+Mn / 6) is 0.25% to 0.35%; in Examples 1-6, (C-Ti / 4) is 0.125%, 0.135%, 0.127%, 0.138%, 0.126% and 0.130%, respectively; (C+Mn / 6) is 0.283%, 0.300%, 0.293%, 0.302%, 0.293% and 0.286%, respectively.

[0059] Molten steel with required chemical composition is obtained by converter smelting, and the molten steel is continuously cast to obtain continuous casting slabs. The thickness of the continuous casting slabs is 210mm-230mm, the width is 800mm-1300mm, and the length is 5000mm-10000mm.

[0060] The continuously cast slab is sent to a heating furnace for reheating, and after being removed from the furnace and dephosphorized, it is sent to a continuous hot rolling mill for rolling; the rolling is controlled by a rough rolling mill and a finishing rolling mill, and the laminar cooling adopts front-stage cooling, and then coiling is performed to produce hot-rolled steel coils, and the thickness of the hot-rolled steel plate is 2.0 to 2.3 mm; the hot rolling process control is shown in Table 2.

[0061] Table 2 Hot rolling process control parameters of the embodiment of the present invention

[0062]

[0063] The hot rolled steel sheet obtained by the above method is Figure 1 The metallographic structure of the hot-rolled steel plate is ferrite + pearlite, and the ferrite grain size in the metallographic structure is 11 to 12 levels; the yield strength R of the 2.0 to 2.3 mm thick hot-rolled steel plate P0.2 485~555MPa, tensile strength R m 570~659MPa, elongation after fracture A 80mm It is 20% to 25%.

[0064] The hot-rolled steel plate obtained by the present invention was subjected to a tensile test according to "GB / T228.1-2010 Tensile test of metallic materials Part 1: Room temperature test method", and the mechanical properties of the hot-rolled steel plate are shown in Table 3.

[0065] Table 3 Mechanical properties of hot-rolled steel sheets according to the present invention

[0066]

[0067]

[0068] After the hot-rolled steel coil is unrolled, it is pickled, cold rolled, annealed in a horizontal continuous annealing furnace, and hot-dip galvanized to obtain a finished cold-rolled hot-dip galvanized steel sheet with a thickness of 0.50-0.80 mm. The cold rolling reduction rate is 65%-80%. The heating temperature of the hardened strip after cold rolling in the horizontal continuous annealing furnace is 630-650° C., the annealing temperature in the soaking section is 790-815° C., and the annealing time of the strip in the soaking section is 22-25 seconds. The annealed strip is put into a zinc pot for hot-dip galvanizing, and the weight of the single-side zinc layer of the cold-rolled hot-dip galvanized steel sheet is controlled to be 75-90 g / m 2 ; The temperature of the strip entering the aluminum-zinc pot is 590-600℃; the strip is air-cooled after exiting the aluminum-zinc pot, the air-cooling speed of the strip is 40-60℃ / s, and the end temperature of the air-cooling of the strip is 350-380℃; then normal cooling is carried out; the cold-rolled hot-dip aluminum-zinc steel sheet is not leveled. The process control parameters of cold rolling, annealing and air cooling of the strip are shown in Table 4.

[0069] Table 4 Process control parameters for cold rolling, annealing and air cooling of steel strip according to the embodiment of the present invention

[0070]

[0071] The cold-rolled hot-dip aluminum-zinc coated steel sheet obtained by the above method is shown in Figure 2 The metallographic structure of the cold-rolled hot-dip aluminum-zinc coated steel sheet includes 3% to 4% bainite + recrystallized fibrous ferrite + cementite + fine titanium carbide precipitates in terms of area percentage, and the grain size of the ferrite in the metallographic structure is Ⅱ12 to Ⅱ13; the yield strength R of the 0.50 to 0.80 mm thick cold-rolled hot-dip aluminum-zinc coated steel sheet is P0.2 520~620MPa, tensile strength R m 610~730MPa, elongation after fracture A 80mm 18% to 24%, 180° bending test, d = 0a qualified, no zinc layer shedding and substrate cracking; single-sided zinc layer weight of cold-rolled hot-dip aluminum-zinc steel sheet is 75 to 90g / m 2 .

[0072] The cold-rolled hot-dip aluminum-zinc coated steel sheet obtained by the present invention was subjected to a tensile test according to "GB / T228.1-2010 Metallic Material Tensile Test Part 1: Room Temperature Test Method", and a bending test was performed with reference to "GB / T232-2010 Metallic Material Bending Test Method". The performance indicators of the steel sheet are shown in Table 5.

[0073] Table 5 Performance indexes of cold-rolled hot-dip aluminum-zinc coated steel sheets according to the present invention

[0074]

[0075]

[0076] In the 180° bending test of the cold-rolled hot-dip aluminum-zinc coated steel sheets of Examples 1-6 of the present invention, d=0a, no zinc layer fell off the coating surface, and the substrate of the cold-rolled hot-dip aluminum-zinc coated steel sheets was cracked.

[0077] In addition to the above embodiments, the present invention may also have other implementation modes. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present invention.

Claims

1. A cold-rolled hot-dip galvanized steel sheet with a yield strength of 500 MPa, wherein the chemical composition of the substrate is as follows: C: 0.14% to 0.16%, Si: 0.001% to 0.050%, Mn: 0.81% to 0.90%, P≤0.015%, S≤0.015%, Ti: 0.060% to 0.075%, B: 0.0015% to 0.0025%, Al: 0.010% ~0.030%, and satisfy 0.25%≤(C+Mn / 6)≤0.35%, 0.12%≤(C-Ti / 4)≤0.15%, and the rest are Fe and unavoidable inclusions; the metallographic structure of the cold-rolled hot-dip aluminum-zinc-coated steel sheet includes 3% to 4% of bainite + recrystallized fibrous ferrite + cementite + fine titanium carbide precipitates in terms of area percentage, and the grain size grade of ferrite in the metallographic structure is Ⅱ12 to Ⅱ13.

2. The high surface quality cold-rolled hot-dip aluminum-zinc coated steel sheet according to claim 1, characterized in that: The weight of the single-sided zinc layer of the cold-rolled hot-dip aluminum-zinc steel sheet is 75 to 90 g / m 2 ; Yield strength R of 0.50~0.80mm thick cold-rolled hot-dip galvanized steel sheet P0.2 520~620MPa, tensile strength R m 610~730MPa, elongation after fracture A 80mm 18% to 24%, 180° bending test, d=0a passed.

3. The high surface quality cold-rolled hot-dip aluminum-zinc coated steel sheet according to claim 1, characterized in that: The wind resistance strength of the roof panel made of the cold-rolled and hot-dip aluminum-zinc steel sheet is above 6.0 MPa.

4. A method for manufacturing a cold-rolled hot-dip aluminum-zinc coated steel sheet with a yield strength of 500 MPa, characterized in that: The method comprises the following steps: The molten steel is continuously cast to obtain a continuous casting slab, wherein the chemical composition of the molten steel is as follows: C: 0.14% to 0.16%, Si: 0.001% to 0.050%, Mn: 0.81% to 0.90%, P≤0.015%, S≤0.015%, Ti: 0.060% to 0.075%, B: 0.0015% to 0.0025%, Al: 0.010% to 0.030%, and the following conditions are satisfied: 0.25%≤(C+Mn / 6)≤0.35%, 0.12%≤(C-Ti / 4)≤0.15%, and the rest is Fe and unavoidable inclusions; The continuous casting slab is heated to 1220°C-1260°C in a heating furnace and then hot rolled. The hot rolling is a two-stage rolling process. The rough rolling is 6 passes, and the rough rolling end temperature is 1040°C-1080°C; the finishing rolling is 7 passes, and the finishing temperature is 800°C-830°C. The finishing rolling reduction rate is 92%-93%; after finishing rolling, the thickness of the hot rolled steel plate is 2.0-2.3mm, and the laminar cooling adopts the front stage cooling; the coiling temperature is 550°C-580°C to obtain the hot rolled steel coil; After the hot-rolled steel coil is unrolled, it is pickled, cold rolled, annealed in a horizontal continuous annealing furnace, and hot-dip galvanized to obtain a finished cold-rolled hot-dip galvanized steel sheet with a thickness of 0.50-0.80 mm. The cold rolling reduction rate is 65%-80%. The heating temperature of the hardened strip after cold rolling in the horizontal continuous annealing furnace is 630-650° C., the annealing temperature in the soaking section is 790-815° C., and the annealing time of the strip in the soaking section is 22-25 seconds. The annealed strip is put into a zinc pot for hot-dip galvanizing, and the weight of the single-side zinc layer of the cold-rolled hot-dip galvanized steel sheet is controlled to be 75-90 g / m 2 ; The temperature of the strip entering the aluminum-zinc pot is 590-600℃; the strip is air-cooled after leaving the aluminum-zinc pot, the air-cooling speed of the strip is 40-60℃ / s, and the end temperature of the strip air-cooling is 350-380℃.

5. The method for manufacturing a cold-rolled hot-dip aluminum-zinc coated steel sheet with a yield strength of 500 MPa as claimed in claim 4, characterized in that: The metallographic structure of the hot-rolled steel plate is ferrite + pearlite, and the ferrite grain size level in the metallographic structure is 11 to 12; the yield strength R of the 2.0 to 2.3 mm thick hot-rolled steel plate is P0.2 485~555MPa, tensile strength R m 570~659MPa, elongation after fracture A 80mm It is 20% to 25%.

6. The method for manufacturing a cold-rolled hot-dip aluminum-zinc coated steel sheet with a yield strength of 500 MPa as claimed in claim 4, characterized in that: The metallographic structure of the cold-rolled hot-dip aluminum-zinc-coated steel sheet includes 3% to 4% bainite + recrystallized fibrous ferrite + cementite + fine titanium carbide precipitates in terms of area percentage, and the grain size of the ferrite in the metallographic structure is II12 to II13; the yield strength R of the 0.50 to 0.80 mm thick cold-rolled hot-dip aluminum-zinc-coated steel sheet is P0.2 520~620MPa, tensile strength R m 610~730MPa, elongation after fracture A 80mm 18% to 24%, 180° bending test, d=0a passed.

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