Atmospheric-corrosion-resistant medium-wide strip steel star-shaped crack control method

Abstract

The invention belongs to the technical field of metallurgical production, and particularly relates to an atmospheric corrosion-resistant medium-wide strip steel star-shaped crack control method which comprises the following chemical components in steel according to design requirements: 0.25 wt%-0.35 wt% of Cu, 0.15 wt%-0.25 wt% of Ni, 0.02 wt%-0.03 wt% of Nb and 0.015 wt%-0.035 wt% of Ti; the continuous casting process is controlled: the alkalinity of casting powder is as follows: R1 = CaO / SiO2 = 1.0-1.2; and the large reduction rate design is adopted for the first two passes of rolling, wherein the reduction rate ranges from 35% to 55%. By optimizing the component design, the continuous casting process and the rolling process, generation of star-shaped cracks is effectively inhibited, and the problem of contradiction between the corrosion resistance of the atmospheric corrosion resistant steel and the crack sensitivity is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical production, and in particular relates to a method for controlling star-shaped cracks in medium-width steel strips resistant to atmospheric corrosion. Background Art

[0002] Atmospheric corrosion-resistant steel is widely used in bridges, buildings, vehicles and other fields due to its excellent atmospheric corrosion resistance. However, in the production process of atmospheric corrosion-resistant wide strip steel, the addition of elements such as Cu, Cr, and Ni in the steel easily forms microcracks under the skin of the continuous casting billet. After hot rolling, it usually manifests as radial fine cracks on the surface of the strip, seriously affecting the surface quality and mechanical properties of the strip. In the existing technology, cracks are controlled by reducing the pulling speed or adjusting the secondary cooling intensity, but it is easy to cause the center segregation of the billet to worsen (segregation index ≥1.25); the existing hot rolling mostly adopts a single-pass large reduction rate (≥15%) to improve efficiency, but it leads to insufficient deformation penetration, aggravates surface stress concentration, and provides a path for the expansion of star-shaped cracks. The generation of some cracks can be reduced by adjusting the rolling process parameters, optimizing the cooling method, etc., but it is still impossible to completely eliminate star-shaped cracks. Patent document CN202410249996.3 discloses a control method for reducing surface cracks in corrosion-resistant steel continuous casting ingots by optimizing the composition, reducing the C and S content, adjusting the Mn and Cu ranges, reducing the water flow rate in the crystallizer, reducing the secondary cooling water ratio in continuous casting, and increasing the basicity of the protective slag. This method reduces the surface crack rate of corrosion-resistant steel ingots from 2.3% to less than 0.3%, but reducing elements such as C and Cu may sacrifice material strength or corrosion resistance, and the optimization of continuous casting related processes relies on high-precision equipment support or increases maintenance costs. The existing technology either focuses on corrosion resistance and ignores crack control, or achieves local improvements by sacrificing production efficiency and cost, and fails to systematically solve the problem of star-shaped cracks caused by Cu segregation, sulfide inclusions and the synergistic effect of thermal stress in atmospheric corrosion-resistant steel. Summary of the Invention

[0003] To solve the above problems, the present invention proposes a method for controlling star-shaped cracks in wide-band steel resistant to atmospheric corrosion. By optimizing the composition design, continuous casting process and rolling process, the generation of star-shaped cracks is effectively suppressed, solving the contradiction between the corrosion resistance and crack sensitivity of atmospheric corrosion-resistant steel.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] A method for controlling star-shaped cracks in wide-band steel resistant to atmospheric corrosion, comprising:

[0006] 1) Design requirements for chemical composition of steel: 0.25wt% to 0.35wt% Cu, 0.15wt% to 0.25wt% Ni, 0.02wt% to 0.03wt% Nb, 0.015wt% to 0.035wt% Ti, and control 1≤Cu / Ni≤2, P content ≤0.015%.

[0007] The reasons for the chemical composition design of the present invention are as follows:

[0008] Control of Cu, P, and Ni elements: Cu, P, and Ni are all elements that are beneficial to improving corrosion resistance in atmospheric corrosion-resistant steel, but elements such as Cu and P are prone to segregation during the solidification process, forming local brittle areas. Excessive Cu content (≥0.4%) can easily induce hot brittle cracks, and excessive P will aggravate grain boundary segregation and promote crack propagation. Ni can inhibit the grain boundary segregation of Cu, but a high Cu / Ni ratio may lead to Cu segregation, so it is necessary to control 1≤Cu / Ni≤2. In summary, the present invention controls the Cu content in atmospheric corrosion-resistant steel to 0.25%~0.35%, the Ni content to 0.15%~0.25%, and the P content to ≤0.015%.

[0009] In the microalloying design of the present invention, a small amount of Nb and Ti is added to refine the grains and improve the toughness and crack growth resistance of the steel. The present invention controls the Nb content to 0.02% to 0.03% and the Ti content to 0.015% to 0.035%.

[0010] In the chemical composition design of the steel of the present invention, the C content and carbon equivalent are optimized: a high carbon equivalent increases the tendency of welding cold cracks and may indirectly cause star-shaped cracks. The C content is controlled to be ≤0.10% and the carbon equivalent is ≤0.40%, and the strengthening effect is achieved by partially replacing C with Ti, Nb, etc.

[0011] Control of impurity elements such as S and O: If the content of impurity elements such as S and O is too high, non-metallic inclusions will accumulate and become the source of cracks. The content of harmful elements needs to be reduced. The present invention controls the chemical composition of the steel to have an S content of ≤0.008% and an O content of ≤20ppm.

[0012] 2) Continuous casting process control: The viscosity of the mold powder is 0.10-0.15 Pa·s, the basicity is R1=CaO / SiO2=1.0-1.2; the water content in the secondary cooling zone of continuous casting is 0.6-0.9 L / kg, and the straightening operation temperature during continuous casting is 850-950°C.

[0013] The mold protection slag requires low viscosity and high alkalinity to improve the lubricity of the slag film, avoid uneven heat transfer leading to stress concentration in the solidification shell, control the slag layer thickness to 10-15mm, and ensure the continuity and stability of the liquid slag layer.

[0014] The secondary cooling zone of continuous casting adopts a weak cooling system to reduce the repeated rise of the longitudinal temperature of the ingot and the uniformity of the transverse temperature, avoid the precipitation of second phase particles at the austenite grain boundaries, and reduce the probability of intergranular cracks forming on the surface of the ingot.

[0015] According to the thermoplasticity curve of atmospheric corrosion resistant steel, the straightening operation in the continuous casting process is selected at 850-950℃, at which time the thermoplasticity of the casting is optimal.

[0016] 3) Rolling process control: The hot rolling start temperature is 1015-1145°C; the finishing rolling temperature is controlled at 860-890°C, avoiding the temperature range (below 850°C) where the thermoplasticity of atmospheric corrosion-resistant steel is poor; the coiling temperature is set at 640-680°C. A computer automatic control system is used to adjust the rolling temperature online throughout the rolling process to avoid residual stress caused by rapid cooling.

[0017] Before rolling, high-pressure water descaling (pressure ≥ 25MPa) is used to thoroughly remove the oxide scale on the surface of the ingot to prevent the oxide scale from being pressed into the surface of the steel strip during rolling to form a crack source.

[0018] The first two rolling passes have a single-pass reduction of 35% to 55%. This intense deformation promotes dynamic recrystallization of austenite, refines grains, improves material uniformity, and reduces local stress concentration. This ensures mill roll gap accuracy (tolerance ≤ ±0.5mm) to avoid uneven thickness or local overpressure caused by roll gap fluctuations, which can induce surface cracks.

[0019] Laminar cooling technology is used to control the uniformity of the cooling rate and avoid the internal stress caused by local cooling rate differences. After rolling, the steel coils are slowly cooled in a pit for 48 hours to avoid the accumulation of shrinkage stress caused by rapid cooling.

[0020] The finished steel plate of the present invention has a width of 500-800 mm and a thickness of 2.5-14 mm.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] By optimizing the composition design and improving the continuous casting and rolling processes, this method effectively reduces the probability of star-shaped cracks on the steel surface without sacrificing corrosion resistance and mechanical properties. The atmospheric corrosion-resistant medium and wide strip steel produced using this method exhibits excellent surface quality, no obvious defects, stable chemical composition, and excellent mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The surface quality of atmospheric corrosion resistant wide strip steel Q355NH produced by the method of the present invention is shown.

[0024] Figure 2 The surface quality of atmospheric corrosion resistant wide strip steel Q550NHD produced by the method of the present invention is shown.

[0025] Figure 3 The surface quality of atmospheric corrosion resistant wide strip steel Q500NHE produced by the method of the present invention is shown.

[0026] Figure 4 This is the surface quality of Q355NH steel strip from a foreign factory (conventional technology). DETAILED DESCRIPTION

[0027] The specific embodiments of the present invention will be further described below in conjunction with examples. The following examples are used to specifically illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0028] Example 1:

[0029] The method of the present invention is used to produce Q355NH steel, with a continuous casting slab cross-sectional size of 180 mm × 705 mm and a rolling specification of 3.5 mm × 700 mm. The specific implementation method is as follows:

[0030] 1. The total charge of the converter is 143.2 tons, including 126.6 tons of molten iron and 18.6 tons of cold material. The temperature of the molten iron is 1320℃.

[0031] 2. The final tapping temperature of the converter is 1619℃, the final C content is 0.05%, and the final P content is 0.018%.

[0032] 3. Aluminum particles are added during refining, and high-aluminum refining agent ensures deoxidation and desulfurization effects.

[0033] 4. Continuous casting: The mold powder viscosity is 0.12 Pa·s, the basicity is R1=CaO / SiO2=1.2, and the powder thickness is 12 mm. The water content in the secondary cooling zone of continuous casting is 0.89 L / kg. The straightening operating temperature during continuous casting is 900°C. The continuous casting speed is 1.52 m / min, and the superheat is 24°C.

[0034] 5. Rolling temperature control: heating temperature 1266℃, dephosphorization temperature 1151℃, high-pressure water descaling before rolling, pressure 30MPa; rolling start temperature 1125℃, first rolling reduction rate 40%, second rolling reduction rate 35%, rolling mill roll gap accuracy control tolerance is ±0.5mm; finishing rolling temperature 889℃, coiling temperature 667℃, finished product enters the pit for slow cooling for 48h.

[0035] 6. Chemical composition of finished steel: C: 0.08%, Si: 0.15%, Mn: 0.79%, P: 0.012%, S: 0.001%, Cu: 0.28%, Nb: 0.022%, Ti: 0.028%, Cr: 0.50%, Ni: 0.19%, Als: 0.022%, O content: 9.7ppm, carbon equivalent: 0.34.

[0036] The atmospheric corrosion resistant wide strip steel Q355NH ingot produced by the above technical solution has no obvious defects in low-magnification inspection, the slab quality is good, the steel strip surface is smooth, and there are no defects such as star-shaped cracks.

[0037] Example 2:

[0038] The method of the present invention is used to produce Q550NHD steel, with a continuous casting slab cross-sectional size of 180 mm × 705 mm and a rolling specification of 4.0 mm × 700 mm. The specific implementation method is as follows:

[0039] 1. The total charge of the converter is 143.9 tons, including 121.2 tons of molten iron and 22.7 tons of cold material. The temperature of the molten iron is 1336℃.

[0040] 2. The final tapping temperature of the converter is 1626℃, the final C content is 0.07%, and the final P content is 0.013%.

[0041] 3. Aluminum particles are added during refining, and high-aluminum refining agent ensures deoxidation and desulfurization effects.

[0042] 4. Continuous casting: The mold powder viscosity is 0.10 Pa·s, the basicity is R1=CaO / SiO2=1.2, and the powder thickness is 12 mm. The water content in the secondary cooling zone of continuous casting is 0.75 L / kg. The straightening operating temperature during continuous casting is 930°C. The continuous casting speed is 1.62 m / min, and the superheat is 25°C.

[0043] 5. Rolling temperature control: heating temperature 1260℃, dephosphorization temperature 1146℃, high-pressure water descaling before rolling, pressure 30MPa; rolling start temperature 1130℃, first rolling reduction rate 40%, second rolling reduction rate 35%, rolling mill roll gap accuracy control tolerance is ±0.5mm; finishing rolling temperature 870℃, coiling temperature 650℃, finished product enters the pit for slow cooling for 48h.

[0044] 6. Chemical composition of finished steel: C: 0.08%, Si: 0.25%, Mn: 1.20%, P: 0.012%, S: 0.003%, Cu: 0.27%, Nb: 0.030%, Ti: 0.033%, Cr: 0.44%, Ni: 0.19%, Als: 0.032%, O content: 9.5ppm, carbon equivalent: 0.39.

[0045] The atmospheric corrosion resistant wide strip steel Q550NHD produced by the above technical solution has no obvious defects in low-magnification inspection, the slab quality is good, the steel strip surface is smooth, and there are no defects such as star-shaped cracks.

[0046] Example 3:

[0047] The method of the present invention is used to produce Q500NHE steel, with a continuous casting slab cross-sectional size of 180 mm × 705 mm and a rolling specification of 8.0 mm × 700 mm. The specific implementation method is as follows:

[0048] 1. The total charge of the converter is 143.4 tons, including 124.5 tons of molten iron and 18.9 tons of cold material. The temperature of the molten iron is 1332℃.

[0049] 2. The final tapping temperature of the converter is 1624℃, the final C content is 0.06%, and the final P content is 0.015%.

[0050] 3. Aluminum particles are added during refining, and high-aluminum refining agent ensures deoxidation and desulfurization effects.

[0051] 4. Continuous casting: The mold powder viscosity is 0.12 Pa·s, the basicity is R1=CaO / SiO2=1.2, and the powder thickness is 12 mm. The water content in the secondary cooling zone of continuous casting is 0.75 L / kg. The straightening operating temperature during continuous casting is 923°C. The continuous casting speed is 1.62 m / min, and the superheat is 26°C.

[0052] 5. Rolling temperature control: heating temperature 1265℃, dephosphorization temperature 1150℃, high-pressure water descaling before rolling, pressure 30MPa; rolling start temperature 1125℃, first pass reduction rate 40%, second pass reduction rate 35%, rolling mill roll gap accuracy control tolerance is ±0.5mm; finishing rolling temperature 875℃, coiling temperature 670℃, finished product enters the pit for slow cooling for 48h.

[0053] 6. Chemical composition of finished steel: C: 0.08%, Si: 0.22%, Mn: 1.14%, P: 0.011%, S: 0.003%, Cu: 0.28%, Nb: 0.023%, Ti: 0.028%, Cr: 0.46%, Ni: 0.19%, Als: 0.030%, O content: 9.5ppm, carbon equivalent: 0.38.

[0054] The atmospheric corrosion resistant wide strip steel Q500NHE ingot produced by the above technical solution has no obvious defects in low-magnification inspection, the slab quality is good, the steel strip surface is smooth, and there are no defects such as star-shaped cracks.

[0055] The performance of all the finished products of the embodiments is shown in Table 1:

[0056] Table 1 Finished product performance data

[0057]

[0058] *Ds: Sum of ratings of non-metallic inclusions (GB / T 10561).

Claims

1. A method for controlling star-shaped cracks in wide-band steel with atmospheric corrosion resistance, characterized in that: Specifically include: 1) Chemical composition design requirements for steel: 0.25wt% to 0.35wt% Cu, 0.15wt% to 0.25wt% Ni, 0.02wt% to 0.03wt% Nb, 0.015wt% to 0.035wt% Ti, with 1≤Cu / Ni≤2, and P content ≤0.015%; 2) Continuous casting process control: The viscosity of the mold powder is 0.10-0.15 Pa·s, the basicity is R1=CaO / SiO2=1.0-1.2; the water content in the secondary cooling zone of continuous casting is 0.6-0.9 L / kg, and the straightening operating temperature during continuous casting is 850-950°C; 3) Rolling process control: hot rolling start temperature is 1015~1145℃; finishing rolling temperature is controlled at 860~890℃; coiling temperature is set at 640~680℃; the single pass reduction rate of the first two passes of rolling is 35%~55%.

2. The method for controlling star-shaped cracks in wide steel strips resistant to atmospheric corrosion according to claim 1, characterized in that: In the chemical composition design of the steel, the C content is controlled to be ≤0.10% and the carbon equivalent is controlled to be ≤0.40%.

3. A method for controlling star-shaped cracks in wide-band steel resistant to atmospheric corrosion according to claim 1 or 2, characterized in that: Control the chemical composition of steel to S content ≤ 0.008%, O content ≤ 20ppm.

4. The method for controlling star-shaped cracks in wide-band steel resistant to atmospheric corrosion according to claim 1, characterized in that: The thickness of the mold protective slag layer is controlled to be 10 to 15 mm.

5. The method for controlling star-shaped cracks in wide steel strips resistant to atmospheric corrosion according to claim 1, characterized in that: High-pressure water is used for descaling before rolling, with a pressure of ≥25MPa.

6. The method for controlling star-shaped cracks in wide and medium-width atmospheric corrosion-resistant steel according to claim 1, characterized in that: The rolling mill roll gap precision control tolerance is ≤±0.5mm.

7. The method for controlling star-shaped cracks in wide steel strips resistant to atmospheric corrosion according to claim 1, characterized in that: After rolling, the steel coils are put into the pit for slow cooling for more than 48 hours.

8. The method for controlling star-shaped cracks in wide and medium-width atmospheric corrosion-resistant steel according to claim 1, characterized in that: The width of the finished steel plate is 500-800mm and the thickness is 2.5-14mm.

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