A high-surface-quality 980mpa-grade hot-dip galvanizing dh steel and a preparation method thereof

Abstract

The application discloses a high-surface-quality 980MPa-grade hot-dip galvanizing DH steel and a preparation method thereof. The main chemical component mass content of the hot-dip galvanizing DH steel is as follows: C: 0.17-0.21%, Si: 0.4-0.7%, Mn: 1.8-2.2%, Nb: 0.01-0.02%, Ti: 0.01-0.02%, Cr: 0.4-0.5%, and Als: 0.14-0.9%. The method adopts low-carbon design and high-dew-point annealing process to produce the hot-dip galvanizing steel plate which meets the low-carbon environmental protection requirement and has high surface quality, so as to meet the urgent needs of the market. The high-dew-point annealing process is a new type of annealing technology, the oxygen partial pressure is increased by increasing the dew point, the Si and Mn element internal oxidation is realized, and thus the plating leakage defect is effectively improved, and the surface smoothness and flatness of the steel plate are improved.

Description

Technical Field

[0001] This invention belongs to the field of high-strength steel technology, specifically relating to a high surface quality 980MPa grade hot-dip galvanized DH steel and its preparation method. Background Technology

[0002] The steel industry is facing unprecedented pressure to transform, and developing low-carbon, environmentally friendly, and high-performance steel has become an urgent need for industry development. In the field of hot-dip galvanized steel sheets, surface quality is one of the key indicators for measuring product quality. Traditional hot-dip galvanizing processes often struggle to simultaneously meet the requirements of high strength and good surface quality, especially when producing high-strength steel sheets, such as 980MPa grade hot-dip galvanized DH steel. Due to its high strength, it is prone to surface cracks, roughness, and incomplete galvanizing defects during processing, which seriously affect the application range and market value of the product.

[0003] To address this challenge, the industry has conducted extensive research and practice. Among these efforts, low-carbon design, as an effective strategy, has been widely applied in the development of various types of steel. By optimizing chemical composition and reducing carbon content, not only can carbon emissions during production be reduced, but the processing and welding properties of the steel can also be improved, making it possible to produce hot-dip galvanized steel sheets with high surface quality. Furthermore, the annealing process, as a crucial step in the production of hot-dip galvanized steel sheets, has a particularly significant impact on the surface quality of the steel sheets. Traditional annealing processes often struggle to precisely control the atmosphere and temperature during annealing, leading to problems such as oxidation and decarburization on the steel sheet surface. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention employs a low-carbon design and a high dew point annealing process to produce hot-dip galvanized steel sheets that meet both low-carbon and environmental protection requirements and possess high surface quality, thus satisfying urgent market demands. The high dew point annealing process, as a novel annealing technology, increases oxygen partial pressure by raising the dew point, causing internal oxidation of Si and Mn elements, thereby effectively improving plating defects and enhancing the surface finish and smoothness of the steel sheet.

[0005] To achieve the above-mentioned objective, this invention provides a high surface quality 980MPa grade hot-dip galvanized DH steel. The chemical composition of the hot-dip galvanized DH steel is as follows: C: 0.17%–0.21%, Si: 0.4%–0.7%, Mn: 1.8%–2.2%, Nb: 0.01%–0.02%, Ti: 0.01%–0.02%, Cr: 0.4%–0.5%, Als: 0.14%–0.9%, with the balance being iron and unavoidable impurities.

[0006] In the above technical solution, the microstructure of the hot-dip galvanized DH steel contains 20% to 40% ferrite, 40% to 65% martensite, 3% to 12% retained austenite, and 3% to 15% bainite by volume percentage.

[0007] Furthermore, the hot-dip galvanized DH steel has a yield strength ≥550MPa and a tensile strength ≥980MPa along the direction perpendicular to the rolling direction. 80 The elongation after fracture is 15%–22%, the thickness is 1.0–1.6 mm, and the porosity is ≥23%.

[0008] A method for preparing the aforementioned high surface quality 980MPa grade hot-dip galvanized DH steel, the method comprising the steps of converter smelting, slab continuous casting, removal of edge defects in the cast slab, hot rolling, pickling and cold rolling, preheating, heating, annealing, slow cooling, rapid cooling, and galvanizing after rapid cooling; wherein...

[0009] Converter smelting: The sulfur content of the pretreated molten iron entering the furnace is ≤0.003%, and the slag removal rate is ≥94%.

[0010] Scrap steel is added to the converter, accounting for 15%–18% of the total steel content, while molten iron accounts for 82%–85%. The converter bottom blowing adopts a continuous argon blowing mode, with the final oxygen level controlled to ≤650ppm. A refining LF path is employed: 30–40 kg of slag is added; vacuum decarburization is performed, with a vacuum degree ≤150Pa and a decarburization time of 18–25 min; vacuum deoxidation and alloying is then performed, adding ferrosilicon, ferromanganese, aluminum granules, and ferroniobium, in the Al-Mn-Nb alloying sequence.

[0011] Slab continuous casting: Superheat control during continuous casting: 17℃~27℃; Maintain a constant casting speed of 1.0~1.3m / min during continuous casting, and adopt dynamic light pressure. After continuous casting, the slab needs to have its edge defects removed.

[0012] Hot rolling: billet heating temperature 1230℃~1250℃, initial rolling temperature 1050℃~1150℃, final rolling temperature 870℃~900℃, coiling temperature 550℃~590℃; descaling water pressure 18~20MPa.

[0013] Galvanizing: Nitrogen humidification equipment is used to precisely control the dew point in the furnace. The dew point in the heating section of the annealing furnace is -10℃ to -5℃, the dew point in the annealing section and slow cooling section is -40℃ to -30℃, and the dew point in the furnace nose is -50℃ to -40℃. The hydrogen content (by volume) in the annealing furnace is 3% to 5%.

[0014] Annealing temperature: 830℃~850℃; slow cooling outlet temperature: 690℃~740℃; rapid cooling outlet temperature: 400℃~430℃; after galvanizing, the strip is first air-cooled to 400℃~420℃; the finishing elongation during the finishing process is controlled within the range of 0.5%~1.0%.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0016] ① Achieving a harmonious balance between high strength and excellent formability: Thanks to precise material proportions and optimized process flow, this invention successfully integrates the high strength and excellent formability of steel, fully meeting the stringent standards of modern industry for material performance.

[0017] ② High cost-effectiveness: While pursuing high performance, this invention also takes cost control into account. By optimizing the material ratio and process flow, especially the ingenious use of chromium and manganese, effective cost reduction is achieved while ensuring performance.

[0018] ③ Excellent surface quality: Through the precise design of Si and Mn content and the combination of high dew point annealing process, the surface quality of the product of this invention has been significantly improved.

[0019] ④ Balancing Environmental Protection and Sustainability: While pursuing high performance, this invention also places great emphasis on environmental protection and sustainability. By optimizing heating temperature and rolling process, we have effectively reduced energy consumption and significantly reduced carbon emissions during the production process. Attached Figure Description

[0020] Figure 1 The stress-strain curve of the high surface quality 980MPa grade hot-dip galvanized DH steel prepared in Example 1 is shown.

[0021] Figure 2 The image shows the surface of the galvanized sheet of the high surface quality 980MPa grade hot-dip galvanized DH steel prepared in Example 1. Detailed Implementation

[0022] The present invention will be further described below with reference to specific embodiments, but this does not limit the invention in any way. To avoid redundancy, unless otherwise specified, the raw materials used in the following embodiments are all commercially available products, and the methods used are all conventional methods unless otherwise specified.

[0023] A high surface quality 980MPa grade hot-dip galvanized DH steel, wherein the chemical composition of the hot-dip galvanized DH steel is as follows (mass content): C: 0.17%–0.21%, Si: 0.4%–0.7%, Mn: 1.8%–2.2%, Nb: 0.01%–0.02%, Ti: 0.01%–0.02%, Cr: 0.4%–0.5%, Als: 0.14%–0.9%, with the balance being iron and unavoidable impurities.

[0024] A method for preparing the above-mentioned high surface quality 980MPa grade hot-dip galvanized DH steel includes the following steps: converter smelting, slab continuous casting, removal of edge defects of the slab, hot rolling, pickling and cold rolling, preheating, heating, annealing, slow cooling, rapid cooling, and galvanizing after rapid cooling.

[0025] Any aspects not described in the following embodiments are the same as those described in the specific embodiments above.

[0026] Example

[0027] A high surface quality 980MPa grade hot-dip galvanized DH steel, the specific chemical composition of the hot-dip galvanized DH steel described in Examples 1-10 is shown in Table 1.

[0028] Table 1. Chemical composition (wt%) of hot-dip galvanized DH steel in the examples.

[0029] Example C Mn Si Als Nb Ti Cr 1 0.190 1.9 0.5 0.90 0.020 0.017 0.50 2 0.205 1.8 0.6 0.44 0.018 0.010 0.45 3 0.190 2.1 0.4 0.73 0.016 0.016 0.46 4 0.195 2.0 0.7 0.35 0.019 0.019 0.43 5 0.174 2.2 0.4 0.50 0.020 0.015 0.46 6 0.184 1.9 0.7 0.58 0.016 0.011 0.50 7 0.186 2.2 0.5 0.43 0.013 0.013 0.42 8 0.171 1.8 0.6 0.55 0.019 0.013 0.43 9 0.185 2.1 0.5 0.67 0.020 0.013 0.42 10 0.202 2.0 0.6 0.52 0.016 0.014 0.40

[0030] A method for preparing high surface quality 980MPa grade hot-dip galvanized DH steel, the specific process flow of Examples 1-10 is as follows, Table 2 shows the hot rolling process parameters; billet heating temperature 1230℃~1250℃, initial rolling temperature 1050℃~1150℃, final rolling temperature 870℃~900℃, coiling temperature 550℃~590℃; descaling water pressure 18~20MPa.

[0031] Table 2 Hot rolling process parameters of the preparation method in the examples

[0032] Example Heating temperature / ℃ Rolling temperature / ℃ Final rolling temperature / ℃ Winding temperature / ℃ Descaling water pressure / MPa 1 1237 1093 897 588 18 2 1234 1139 888 557 18 3 1234 1092 879 574 19 4 1246 1130 883 587 19 5 1233 1065 883 580 19 6 1246 1121 877 576 18 7 1250 1130 899 585 18 8 1246 1116 878 580 18 9 1237 1093 897 588 18 10 1234 1139 888 557 18

[0033] Table 3 lists the process parameters for cold rolling and hot-dip galvanizing of the steel in the examples. Nitrogen humidification equipment is used to precisely control the dew point in the furnace. The dew point in the annealing furnace is -10℃ to -5℃, and the hydrogen (volume) content is 3% to 5%. The dew point at the furnace nose is -50℃ to -40℃.

[0034] Galvanizing: Annealing temperature 830℃~850℃, slow cooling exit temperature 690℃~740℃, fast cooling exit temperature 400℃~430℃, after galvanizing, the strip steel is first air-cooled to 400℃~420℃; the finishing elongation rate during the finishing process is controlled within the range of 0.5%~1.0%.

[0035] Table 3. Cold rolling and hot-dip galvanizing process parameters for the preparation methods in the examples.

[0036]

[0037] The mechanical properties and key parameters of the high surface quality 980MPa grade hot-dip galvanized DH steel obtained in Examples 1-10 are shown in Table 4. The yield strength of the hot-dip galvanized DH steel perpendicular to the rolling direction is ≥550MPa, and the tensile strength is ≥980MPa. 80 The elongation after fracture is 15%–22%, the thickness is 1.0–1.6 mm, and the porosity is ≥23%.

[0038] Table 4. Performance parameters of the high surface quality 980MPa grade hot-dip galvanized DH steel prepared in the examples.

[0039]

[0040] The microstructure of the hot-dip galvanized DH steel, by volume percentage, contains 20%–40% ferrite, 40%–65% martensite, 3%–12% retained austenite, and 3%–15% bainite. The stress-strain curve of the high surface quality 980-grade hot-dip galvanized DH steel obtained in Example 1 is shown below. Figure 1 As shown; the surface condition of the galvanized sheet, as follows. Figure 2 As shown, the surface quality is good, with no precipitation defects.

[0041] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention, or equivalent embodiments can be modified based on the disclosed technical content, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. A high surface quality 980MPa grade hot-dip galvanized DH steel, characterized in that, The hot-dip galvanized DH steel has the following chemical composition by mass: C: 0.17%–0.21%, Si: 0.4%–0.7%, Mn: 1.8%–2.2%, Nb: 0.01%–0.02%, Ti: 0.01%–0.02%, Cr: 0.4%–0.5%, Als: 0.14%–0.9%, with the balance being iron and unavoidable impurities. The microstructure of the hot-dip galvanized DH steel contains, by volume percentage, 20%–40% ferrite, 40%–65% martensite, 3%–12% retained austenite, and 3%–15% bainite. The hot-dip galvanized DH steel has a yield strength ≥550MPa and a tensile strength ≥980MPa along the direction perpendicular to the rolling direction. 80 Elongation after fracture is 15%–22%, thickness is 1.0–1.6 mm, and porosity is ≥23%; The preparation method of the 980MPa grade hot-dip galvanized DH steel includes the following steps: converter smelting, slab continuous casting, hot rolling, pickling and cold rolling, preheating, heating, annealing, slow cooling, rapid cooling, and galvanizing. in, Converter smelting: The sulfur content of the pretreated molten iron entering the furnace is ≤0.003%, and the slag removal rate is ≥94%; during the converter smelting process, the proportion of scrap steel is 15%~18%, and the proportion of molten iron is 82%~85%; the converter bottom blowing adopts the full-process argon blowing mode, and the oxygen control at the end point is ≤650ppm; the refining LF path is adopted, and the slag addition amount is 30~40kg; vacuum decarburization treatment is carried out, with a vacuum degree ≤150Pa and a decarburization time of 18~25min; During the continuous casting of slabs, the superheat of continuous casting is controlled at 17℃~27℃; the continuous casting speed is kept constant at 1.0~1.3m / min, and dynamic light pressure is adopted. After continuous casting, edge defects of the slab are removed. Nitrogen humidification equipment is used to precisely control the dew point inside the furnace. The dew point in the heating section of the annealing furnace is -10℃ to -5℃, the dew point in the annealing section and slow cooling section is -40℃ to -30℃, and the dew point in the furnace nose is -50℃ to -40℃. The hydrogen content in the annealing furnace is 3% to 5%. Hot rolling: billet heating temperature 1230℃~1250℃, initial rolling temperature 1050℃~1150℃, final rolling temperature 870℃~900℃, coiling temperature 550℃~590℃; descaling water pressure 18~20MPa; Galvanizing: Annealing temperature 830℃~850℃, slow cooling exit temperature 690℃~740℃, fast cooling exit temperature 400℃~430℃, after galvanizing, the strip steel is first air-cooled to 400℃~420℃; the finishing elongation rate during the finishing process is controlled within the range of 0.5%~1.0%.

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