High-surface-quality 780MPa-grade hot-dip galvanized DH steel and production process

Abstract

The invention discloses high-surface-quality 780MPa-grade hot-dip galvanized DH steel and a production process thereof. The hot-dip galvanized DH steel comprises the following main chemical components in percentage by mass: 0.155 to 0.19 percent of C, 1.6 to 2.0 percent of Mn, 0.2 to 0.6 percent of Si, 0.1 to 0.15 percent of Cr, 0.4 to 1.0 percent of Al, 0.015 to 0.030 percent of Nb, less than or equal to 0.021 percent of Ti, 0.7 to 1.5 percent of Si and Al, and the balance of Fe and inevitable impurities. By accurately controlling the contents of Si and Mn in the steel and assisting with pre-oxidation-reduction treatment, external oxidation of Si and Mn elements is reduced to the maximum extent, the wettability and adhesive force of the surface of the steel are improved, and therefore high-quality hot galvanizing treatment is achieved, and the problem of skip plating is effectively solved. According to the comprehensive technical means, the excellent performance of the steel is reserved, meanwhile, the completeness and uniformity of a plating layer are guaranteed, the corrosion resistance of a product is improved, and the service life of the product is prolonged.

Description

Technical Field

[0001] The invention belongs to the technical field of high-strength steel, and in particular relates to a high-surface-quality 780MPa-grade hot-dip galvanized DH steel and a production process. Background Art

[0002] In the field of hot-dip galvanizing, especially in the hot-dip galvanizing process for 780MPa grade high-strength steel, the problem of non-plating has always been a major problem that has plagued the industry. Among them, the external oxidation of Si and Mn elements is one of the main factors leading to non-plating; these elements are easily oxidized during the hot-dip galvanizing process, forming an oxide layer that is difficult to be wetted by zinc liquid, which causes the steel surface to be unable to be evenly covered by the zinc layer, resulting in non-plating. Although the external oxidation and non-plating phenomenon can be reduced to a certain extent by adjusting the chemical composition of the steel alone, such as reducing the Si and Mn content, the effect of this method is limited, because Si and Mn elements play an important role in improving the strength and other properties of steel, and excessive reduction of their content may damage the overall performance of the steel.

[0003] On the other hand, although the pre-oxidation process can improve the wettability and adhesion of the steel surface and improve the uniformity and integrity of the coating, it cannot completely solve the problem of missed plating caused by the external oxidation of Si and Mn elements. This is because the pre-oxidation treatment can only improve the chemical properties of the steel surface to a certain extent, but cannot directly reduce the external oxidation of Si and Mn elements. Summary of the invention

[0004] In order to overcome the defects of the above-mentioned prior art, the present invention combines the method of reducing Si and Mn content and pre-oxidation-reduction process to solve the problem of missed plating. By accurately controlling the Si and Mn content in the steel, and supplemented by pre-oxidation-reduction treatment, it is intended to minimize the external oxidation of Si and Mn elements, improve the wettability and adhesion of the steel surface, thereby achieving high-quality hot-dip galvanizing and effectively solving the problem of missed plating. This comprehensive technical means not only retains the excellent performance of the steel, but also ensures the integrity and uniformity of the coating, and improves the corrosion resistance and service life of the product.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides a high surface quality 780MPa grade hot-dip galvanized DH steel, the chemical composition mass content of the hot-dip galvanized DH steel is C: 0.155% ~ 0.19%, Mn: 1.6% ~ 2.0%, Si: 0.2% ~ 0.6%, Cr: 0.10% ~ 0.15%, Al: 0.4% ~ 1.0%, Nb: 0.015% ~ 0.030%, P ≤ 0.01%, S ≤ 0.01%, B ≤ 0.005%, V ≤ 0.05%, Ti ≤ 0.021%, and Si + Al: 0.7% ~ 1.5%, the balance is iron and unavoidable impurities.

[0006] In the above technical solution, further, the plating solution composition of the hot-dip galvanized DH steel contains 0.16% to 0.25% Al by mass, and the rest is Zn and unavoidable impurities; the weight of the zinc layer per unit area of ​​the hot-dip galvanized steel sheet is 60 to 200 g / m 2 .

[0007] Furthermore, the microstructure of the hot-dip galvanized DH steel contains, by volume percentage, 34.5% to 55.8% ferrite, 35.3% to 51.5% martensite, 3% to 12% retained austenite, and 3% to 15% bainite.

[0008] Furthermore, the yield strength of the hot-dip galvanized DH steel perpendicular to the rolling direction is 440-540 MPa, the tensile strength is 780-860 MPa, and the A 80 The elongation after fracture is 20% to 24%, the thickness is 1.0 to 2.5 mm, the hole expansion rate is ≥ 25%. The surface rating of the steel plate reaches C grade.

[0009] A production process of the above-mentioned high surface quality 780MPa grade hot-dip galvanized DH steel, the production process comprising the steps of converter smelting, slab continuous casting, hot rolling, pickling and cold rolling, preheating, preoxidation, heating, slow cooling, rapid cooling and galvanizing after rapid cooling; wherein,

[0010] Hot rolling: billet entering furnace temperature 400℃~650℃, heating temperature 1150℃~1280℃, starting rolling temperature 1020℃~1160℃, final rolling temperature 850℃~950℃, coiling temperature 600℃~700℃.

[0011] Pickling cold rolling: pickling speed 100-150m / s, pickling temperature 75℃-85℃;

[0012] Galvanizing: The annealing furnace is equipped with a pre-oxidation chamber, the pre-oxidation temperature is 500℃~600℃, the dew point of the pre-oxidation chamber is -30℃~-15℃, the oxygen (volume) content is 0.8%~1.2%, and the air flow rate is 30~50Nm 3 / h. The dew point in the annealing furnace is -40℃~-30℃, the dew point of the furnace nose is -50~-40℃, and the hydrogen (volume) content is 3%~5%. The annealing temperature is 820℃~850℃, the slow cooling outlet temperature is 680℃~750℃, the rapid cooling rate is greater than 20℃ / s, the rapid cooling outlet temperature is 400℃~430℃, the galvanizing temperature is 450℃~460℃, and the strip is first air-knife cooled to 400℃~420℃ after galvanizing; the skin-finishing elongation of the skin-finishing process is controlled in the range of 0.5%~1.0%.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] The present invention provides a pre-oxidation chamber and a reasonable pre-oxidation temperature to oxidize the outer layer Fe first, and then reduces the surface iron oxide under the action of the subsequent reducing atmosphere in the furnace. At the same time, the Si and Mn contents are reduced as much as possible in the composition design, thereby solving the problem of leakage plating of 780MPa grade DH steel with high Si and Mn contents and improving the surface quality of the steel plate. Combined with the reduction of Si and Mn contents and the pre-oxidation-reduction process, while ensuring the overall performance of the steel, the integrity and uniformity of the coating are effectively improved, and the corrosion resistance and service life of the product are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a microstructural morphology of the high surface quality 780MPa grade hot-dip galvanized DH steel prepared in Example 1;

[0016] Figure 2 The tensile curve of the 780MPa grade hot-dip galvanized DH steel with high surface quality obtained in Example 1;

[0017] Figure 3 This is the surface diagram of the galvanized sheet of 780MPa grade hot-dip galvanized DH steel with high surface quality obtained in Example 1. DETAILED DESCRIPTION

[0018] The present invention is further described below in conjunction with specific examples, but the present invention is not limited in any way. To avoid redundant description, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0019] The invention discloses a high-surface-quality 780MPa-grade hot-dip galvanized DH steel. The chemical composition mass content of the hot-dip galvanized DH steel is C: 0.155%-0.19%, Mn: 1.6%-2.0%, Si: 0.2%-0.6%, Cr: 0.1%-0.15%, Al: 0.4%-1.0%, Nb: 0.015%-0.030%, P≤0.01%, S≤0.01%, B≤0.005%, V≤0.05%, Ti≤0.021%, Si+Al: 0.7%-1.5%, and the balance is iron and unavoidable impurities.

[0020] The plating solution of the hot-dip galvanized DH steel contains 0.16% to 0.25% Al by mass, and the rest is Zn and unavoidable impurities; the weight of the zinc layer per unit area of ​​the hot-dip galvanized steel sheet is 60 to 200 g / m 2 .

[0021] A production process of the above-mentioned high surface quality 780MPa grade hot-dip galvanized DH steel includes the steps of converter smelting, slab continuous casting, hot rolling, pickling and cold rolling, preheating, preoxidation, heating, slow cooling, rapid cooling and galvanizing after rapid cooling.

[0022] The matters not described in the following embodiments are the same as the description of the above specific implementation methods.

[0023] Example

[0024] A high surface quality 780MPa 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.

[0025] Table 1 Chemical composition of hot-dip galvanized DH steel in the embodiment (wt%)

[0026] Example C Mn Si Al Nb Ti Cr V B 1 0.175 2.0 0.4 1.0 0.021 0.013 0.13 0.03 0.003 2 0.164 1.9 0.5 0.9 0.021 0.021 0.12 0.04 0.003 3 0.17 1.7 0.5 0.5 0.019 0.011 0.13 0.03 0.003 4 0.161 1.7 0.4 0.8 0.018 0.018 0.12 0.05 0.005 5 0.167 1.6 0.5 0.9 0.016 0.021 0.15 0.03 0.003 6 0.155 2.0 0.4 1.0 0.025 0.015 0.10 0.05 0.003 7 0.161 2.0 0.2 0.5 0.017 0.015 0.11 0.04 0.003 8 0.167 1.9 0.6 0.7 0.022 0.018 0.12 0.03 0.004 9 0.175 1.7 0.5 0.4 0.023 0.021 0.11 0.04 0.003 10 0.178 1.8 0.6 0.7 0.015 0.013 0.12 0.03 0.003

[0027] A production process of a high surface quality 780MPa grade hot-dip galvanized DH steel. The specific process flow of Examples 1-10 is as follows, and Table 2 is the hot rolling process parameters; the billet furnace entry temperature is 400°C to 650°C, the heating temperature is 1150°C to 1280°C, the start rolling temperature is 1020°C to 1160°C, the final rolling temperature is 850°C to 950°C, and the coiling temperature is 600°C to 700°C.

[0028] Table 2 Hot rolling process parameters of the production process of the embodiment

[0029]

[0030] Table 3 lists the process parameters of cold rolling and hot-dip galvanizing of the example steel. Pickling cold rolling: pickling speed 100-150 m / s, pickling temperature 75°C-85°C. Galvanizing: the annealing furnace is provided with a pre-oxidation chamber, the pre-oxidation temperature is 500°C-600°C, the dew point of the pre-oxidation chamber is -30°C--15°C, the oxygen content is 0.8%-1.2%, and the air flow rate is 30-50 Nm 3 / h; the dew point in the annealing furnace is -40℃~-30℃, the dew point of the furnace nose is -50~-40℃, and the hydrogen content is 3%~5%; the annealing temperature is 820℃~850℃, the slow cooling outlet temperature is 680℃~750℃, the rapid cooling rate is greater than 20℃ / s, the rapid cooling outlet temperature is 400℃~430℃, the galvanizing temperature is 450℃~460℃, and the strip is first air-knife cooled to 400℃~420℃ after galvanizing; the skin-finishing elongation of the skin-finishing process is controlled in the range of 0.5%~1.0%.

[0031] Table 3 Cold rolling and hot dip galvanizing process parameters of the production process of the embodiment

[0032]

[0033] The mechanical properties and key parameters of the hot-dip galvanized DH steel with high surface quality of 780 MPa obtained in Example 1-10 are shown in Table 4. The yield strength of the hot-dip galvanized DH steel perpendicular to the rolling direction is 440-540 MPa, the tensile strength is 780-860 MPa, and the A 80The elongation after fracture is 20% to 24%, the thickness is 1.0 to 2.5 mm, the hole expansion rate is ≥ 25%. The surface rating of the steel plate reaches C grade.

[0034] Table 4 Performance parameters of 780MPa grade hot-dip galvanized DH steel obtained in Example

[0035] Example Thickness / mm Yield strength / MPa Tensile strength / MPa <![CDATA[A 80 Elongation %]]> Hole expansion rate % 1 1.6 461 804 23 28 2 2.3 477 849 20 30 3 1.6 442 801 24 26 4 1.6 510 823 20 28 5 2.4 532 856 22 27 6 1.6 525 834 24 28 7 1.6 465 841 23 29 8 2.1 479 809 23 30 9 2.1 464 808 20 27 10 2.5 512 785 24 26

[0036] The microstructure of the 780MPa grade hot-dip galvanized DH steel with high surface quality obtained in Examples 1-10 is 34.5% to 55.8% ferrite, 35.3% to 51.5% martensite, 3% to 12% retained austenite, and 3% to 15% bainite by volume percentage. The stress-strain curve of the 780 grade hot-dip galvanized DH steel with high surface quality obtained in Example 1 is shown in FIG. Figure 1 As shown; microstructure morphology, such as Figure 2 As shown; the surface condition of the galvanized sheet, such as Figure 3 shown.

[0037] For any technician familiar with the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention should still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A high surface quality 780 MPa grade hot-dip galvanized DH steel, characterized in that: The chemical composition mass content of the hot-dip galvanized DH steel is C: 0.155%-0.19%, Mn: 1.6%-2.0%, Si: 0.2%-0.6%, Cr: 0.1%-0.15%, Al: 0.4%-1.0%, Nb: 0.015%-0.030%, P≤0.01%, S≤0.01%, Ti≤0.021%, and Si+Al: 0.7%-1.5%, and the balance is iron and unavoidable impurities.

2. The hot-dip galvanized DH steel according to claim 1, characterized in that: The plating solution of the hot-dip galvanized DH steel contains 0.16% to 0.25% Al, and the rest is Zn and unavoidable impurities; the weight of the zinc layer per unit area of ​​the hot-dip galvanized steel sheet is 60 to 200 g / m 2 .

3. The hot-dip galvanized DH steel according to claim 1, characterized in that: The microstructure of the hot-dip galvanized DH steel contains, by volume percentage, 34.5% to 55.8% ferrite, 35.3% to 51.5% martensite, 3% to 12% residual austenite, and 3% to 15% bainite.

4. The hot-dip galvanized DH steel according to claim 1, characterized in that: The hot-dip galvanized DH steel has a yield strength of 440-540 MPa and a tensile strength of 780-860 MPa perpendicular to the rolling direction. 80 The elongation after fracture is 20% to 24%, the thickness is 1.0 to 2.5 mm, and the hole expansion rate is ≥25%.

5. A production process for 780 MPa grade hot-dip galvanized DH steel as claimed in claim 1, characterized in that: The production process includes the following steps: converter smelting, slab continuous casting, hot rolling, pickling and cold rolling, preheating, preoxidation, heating, slow cooling, fast cooling, and galvanizing; in, Hot rolling: billet furnace temperature 400℃~650℃, heating temperature 1150℃~1280℃, rolling start temperature 1020℃~1160℃, final rolling temperature 850℃~950℃, coiling temperature 600℃~700℃; Pickling cold rolling: pickling speed 100-150 m / s, pickling temperature 75℃-85℃; Galvanizing: The annealing furnace is equipped with a pre-oxidation chamber, the pre-oxidation temperature is 500℃~600℃, the dew point of the pre-oxidation chamber is -30℃~-15℃, the oxygen content is 0.8%~1.2%, and the air flow rate is 30~50 Nm 3 / h; the dew point in the annealing furnace is -40℃~-30℃, the dew point of the furnace nose is -50~-40℃, and the hydrogen content is 3%~5%; the annealing temperature is 820℃~850℃, the slow cooling outlet temperature is 680℃~750℃, the rapid cooling rate is greater than 20℃ / s, the rapid cooling outlet temperature is 400℃~430℃, the galvanizing temperature is 450℃~460℃, and the strip is first air-knife cooled to 400℃~420℃ after galvanizing; the skin-finishing elongation of the skin-finishing process is controlled in the range of 0.5%~1.0%.

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