Method for controlling star crack of middle wide band steel against atmospheric corrosion

Abstract

The present application belongs to the technical field of metallurgical production, and particularly relates to a method for controlling star crack of wide band steel resistant to atmospheric corrosion, which comprises the following steps: designing the chemical composition of the steel, i.e., containing 0.25wt%-0.35wt% of Cu, 0.15wt%-0.25wt% of Ni, 0.02wt%-0.03wt% of Nb and 0.015wt%-0.035wt% of Ti; controlling the continuous casting process, i.e., the basicity of the protective slag is R1=CaO / SiO2=1.0-1.2; and adopting large reduction ratio design of 35%-55% in the first two passes before rolling. The present application effectively inhibits the generation of star crack by optimizing the component design, the continuous casting process and the rolling process, and solves the contradiction between the corrosion resistance and crack sensitivity of the steel resistant to atmospheric corrosion.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metallurgical production, and particularly relates to a method for controlling star-shaped cracks of atmospheric corrosion-resistant medium wide strip steel. BACKGROUND

[0002] Atmospheric corrosion-resistant steel is widely used in the fields of bridges, buildings, vehicles and the like due to its excellent atmospheric corrosion resistance. However, in the production process of the atmospheric corrosion-resistant medium wide strip steel, micro-cracks are easily formed under the skin of the continuous casting billet due to the addition of Cu, Cr, Ni and other elements in the steel, and after hot rolling, the strip steel surface usually presents a fine crack in a radial pattern, which seriously affects the surface quality and mechanical properties of the strip steel. In the prior art, the cracks are controlled by reducing the pulling speed or adjusting the secondary cooling intensity, but this easily leads to deterioration of the center segregation of the billet (segregation index >= 1.25); in the prior art, single-pass large reduction (>= 15%) is usually adopted in hot rolling to improve the efficiency, but this leads to insufficient deformation penetration, aggravates the stress concentration of the surface layer, and provides a path for the expansion of the star-shaped cracks. The generation of part of the cracks can be reduced by adjusting the rolling process parameters and optimizing the cooling mode, but the star-shaped cracks cannot be completely eliminated. Patent document CN202410249996.3 discloses a method for controlling the surface cracks of the corrosion-resistant steel continuous casting billet by means of composition optimization, reduction of C and S contents, adjustment of Mn and Cu ranges, reduction of the mold water flow rate, reduction of the secondary cooling water quantity of the continuous casting, and increase of the basicity of the protective slag. According to the method, the surface crack rate of the corrosion-resistant steel billet is reduced from 2.3% to less than or equal to 0.3%, but the reduction of C, Cu and other elements may sacrifice the material strength or corrosion resistance, and the optimization of the continuous casting related procedures depends on the support of high-precision equipment or increases the maintenance cost. The prior art either focuses on corrosion resistance and ignores crack control, or realizes local improvement by sacrificing production efficiency and cost, and fails to systematically solve the problem of the star-shaped cracks caused by the synergistic effect of Cu segregation, sulfide inclusions and thermal stress in the atmospheric corrosion-resistant steel. SUMMARY

[0003] To solve the above problems, the application provides a method for controlling star-shaped cracks of atmospheric corrosion-resistant medium wide strip steel, which effectively suppresses the generation of star-shaped cracks by optimizing the composition design, continuous casting process and rolling process, and solves the contradiction between corrosion resistance and crack sensitivity of the atmospheric corrosion-resistant steel.

[0004] To achieve the above purpose, the application adopts the following technical scheme:

[0005] A method for controlling star-shaped cracks of atmospheric corrosion-resistant medium wide strip steel, specifically comprising the following steps.

[0006] 1) Steel chemical composition design requirements: containing 0.25wt%-0.35wt% of Cu, 0.15wt%-0.25wt% of Ni, 0.02wt%-0.03wt% of Nb, 0.015wt%-0.035wt% of Ti, and controlling 1≤Cu / Ni≤2, P content≤0.015%.

[0007] The chemical composition design reasons of the application are as follows:

[0008] Cu, P, Ni element control: in the atmospheric corrosion resistant steel, Cu, P, Ni elements are beneficial to improve the corrosion resistance, but Cu, P and other elements are easy to segregate in the solidification process, forming local brittle area. Cu element content is too high (≥0.4%) to induce hot brittleness crack, P element excess will aggravate grain boundary segregation, promote crack propagation, Ni element can inhibit the grain boundary segregation of Cu element, but the high Cu / Ni ratio may lead to Cu segregation, so it is necessary to control 1≤Cu / Ni≤2. In summary, the Cu content in the atmospheric corrosion resistant steel is controlled at 0.25%-0.35%, the Ni content is controlled at 0.15%-0.25%, and the P content is≤0.015%.

[0009] In the micro-alloying design of the application: a small amount of Nb and Ti is added to refine the grain, improve the toughness and crack propagation resistance of the steel, the Nb content is controlled at 0.02%-0.03%, and the Ti content is controlled at 0.015%-0.035%.

[0010] In the steel chemical composition design of the application, the C content and carbon equivalent are optimized: high carbon equivalent increases the tendency of welding cold crack, which may indirectly cause star-shaped crack, the C content is controlled at≤0.10%, the carbon equivalent is controlled at≤0.40%, and part of the strengthening effect of C is replaced by Ti, Nb, etc.

[0011] S, O and other impurity element control: too high content of S, O and other impurity elements leads to the aggregation of non-metallic inclusions, which becomes a crack source, the harmful element content needs to be reduced, the S content in the steel is controlled at≤0.008%, and the O content is controlled at≤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 specific water consumption of the continuous casting secondary cooling zone is 0.6-0.9 L / kg, and the straightening operation temperature during continuous casting is 850-950℃.

[0013] The mold powder requires low viscosity and high basicity to improve the lubricity of the slag film, avoid stress concentration caused by uneven heat transfer, control the thickness of the slag layer to 10-15mm, and ensure the continuity and stability of the liquid slag layer.

[0014] The weak cooling system is used in the secondary cooling zone of the continuous casting to reduce the repeated rising of the longitudinal temperature of the casting blank and the uniformity of the transverse temperature, avoid the second phase particles from being precipitated at the austenite grain boundaries, and reduce the probability of the formation of the surface intergranular cracks of the casting blank.

[0015] According to the hot plasticity curve of the atmospheric corrosion resistant steel, the straightening operation is selected at 850-950 DEG C in the continuous casting process, and the hot plasticity of the casting blank is optimal.

[0016] 3) Rolling process control: the hot rolling starting temperature is 1015-1145 DEG C; the finish rolling temperature is controlled at 860-890 DEG C, and the temperature interval (below 850 DEG C) in which the hot plasticity of the atmospheric corrosion resistant steel is poor is avoided; the coiling temperature is set at 640-680 DEG C; the rolling temperature is adjusted on line by using the computer automatic control system in the whole rolling process, and the residual stress caused by the rapid cooling is avoided.

[0017] The high-pressure water descaling (pressure is greater than or equal to 25 MPa) is used before rolling, and the casting blank surface oxide is thoroughly removed to prevent the oxide from being pressed into the surface of the steel strip to form a crack source in the rolling process.

[0018] The single pass reduction rate of the first two passes is 35%-55% before rolling, the austenite dynamic recrystallization is promoted by strong deformation, the grain is refined, the material uniformity is improved, and the local stress concentration is reduced. The rolling mill roll gap precision (tolerance is less than or equal to ± 0.5 mm) is ensured, and the surface cracks caused by the thickness unevenness or local overpressure due to the roll gap fluctuation is avoided.

[0019] The laminar cooling technology is used to control the uniformity of the cooling speed, and the internal stress caused by the local cooling speed difference is avoided. The steel coil is slowly cooled in the pit for 48 hours after rolling to avoid the superposition of the shrinkage stress caused by the rapid cooling.

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

[0021] Compared with the prior art, the present application has the beneficial effects that:

[0022] By optimizing the component design and improving the continuous casting and rolling process, the probability of the formation of the surface star-shaped cracks of the steel is effectively reduced without sacrificing the corrosion resistance and mechanical properties. The atmospheric corrosion resistant medium wide strip steel produced by the method of the present application has good surface quality, no obvious defects, stable chemical composition and excellent mechanical properties. BRIEF DESCRIPTION OF DRAWINGS

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

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

[0025] Figure 3 Surface quality of the atmospheric corrosion resistant medium wide strip steel Q500NHE produced by the method of the present application.

[0026] Figure 4 Surface quality of the Q355NH steel strip produced by a certain factory (conventional technology). DETAILED DESCRIPTION

[0027] The specific embodiments of the present application will be further described below in conjunction with examples, which are used to specifically illustrate the content of the present application, and these examples are only a general description of the content of the present application, and do not limit the content of the present application.

[0028] Example 1:

[0029] The Q355NH steel is produced by the method of the present application, the continuous casting slab section size is 180mmx705mm, the rolling specification is 3.5mmx700mm, and the specific embodiment is as follows:

[0030] 1. The total charge of the converter is 143.2 tons, of which 126.6 tons is hot metal and 18.6 tons is cold material, and the hot metal temperature is 1320℃.

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

[0032] 3. Aluminum particles are added in the refining, and high-aluminum refining agent is used to ensure the deoxidation and desulfurization effect.

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

[0034] 5. Rolling temperature control: heating temperature 1266℃, dephosphorization temperature 1151℃, high-pressure water descaling before rolling, pressure 30MPa; opening rolling temperature 1125℃, first pass reduction rate 40%, second pass reduction rate 35%, rolling mill roll gap precision control tolerance ±0.5mm; finish rolling final rolling temperature 889℃, coiling temperature 667℃, finished product into pit slow cooling 48h.

[0035] 6. The chemical composition of the finished steel is: 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 Q355NH cast slab produced by the technical scheme has no obvious defects in macroscopic inspection, and the slab quality is good, and the steel strip surface is smooth without defects such as star-shaped cracks.

[0037] Example 2

[0038] The Q550NHD steel is produced by the method of the application, the continuous casting slab section size is 180mmx705mm, and the rolling specification is 4.0mmx700mm, and the specific implementation manner is as follows:

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

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

[0041] 3, the refining is added with aluminum particles, and the high-aluminum refining agent is used to ensure the deoxidation and desulfurization effect.

[0042] 4, continuous casting: the viscosity of the mold powder is 0.10Pa·s, the basicity is R1=CaO / SiO2=1.2, the mold powder thickness is 12mm; the specific water consumption of the continuous casting secondary cooling zone is 0.75L / kg, the straightening operation temperature in the continuous casting process is 930℃; the continuous casting speed is 1.62m / min, and the superheat is 25℃.

[0043] 5, the rolling temperature control: the heating temperature is 1260℃, the phosphorus removal temperature is 1146℃, the high-pressure water is used for descaling before rolling, and the pressure is 30MPa; the opening rolling temperature is 1130℃, the first pass reduction rate is 40%, the second pass reduction rate is 35%, the rolling mill roll gap precision control tolerance is ±0.5mm; the finish rolling final rolling temperature is 870℃, the coiling temperature is 650℃, and the finished product is put into the pit for slow cooling for 48h.

[0044] 6, the chemical composition of the 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 Q550NHD cast slab produced by the technical scheme has no obvious defects in macroscopic inspection, and the slab quality is good, and the steel strip surface is smooth without defects such as star-shaped cracks.

[0046] Example 3

[0047] The Q500NHE steel is produced by the method of the present application, the cross section size of the continuous casting slab is 180mmx705mm, the rolling specification is 8.0mmx700mm, and the specific implementation is as follows:

[0048] 1. The total charging amount of the converter is 143.4 tons, of which 124.5 tons is hot metal and 18.9 tons is cold material, and the hot metal temperature is 1332℃.

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

[0050] 3. Aluminum particles are added in the refining, and high-aluminum refining agent is used to ensure the deoxidation and desulfurization effect.

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

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

[0053] 6. The chemical composition of the finished steel is: 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 Q500NHE steel produced by the above technical scheme has no obvious defects in macroscopic examination, the slab quality is good, and the steel strip surface is smooth without star-shaped cracks and other defects.

[0055] The performance of the finished product of all examples is shown in Table 1:

[0056] Table 1 Performance data of finished product

[0057]

[0058] “*Ds: Non-metallic inclusion rating total (GB / T 10561)”.

Claims

1. A method for controlling star-shaped cracks in medium-wide strip steel resistant to atmospheric corrosion, characterized in that, Specifically, it includes: 1) The chemical composition of the steel includes: C, Si, Mn, P, S, Cu, Nb, Ti, Cr, Ni, Als, and O, with the balance being Fe and unavoidable impurities; the design requirements for the chemical composition of the steel are: containing 0.25wt%~0.35wt% Cu, 0.15wt%~0.25wt% Ni, 0.02wt%~0.03wt% Nb, and 0.015wt%~0.035wt% Ti, and controlling 1.42≤Cu / Ni≤2, and P content≤0.015%; In the design of the chemical composition of steel, the C content should be controlled to be ≤0.10% and the carbon equivalent to be ≤0.40%. The chemical composition of the steel should be controlled with S content ≤0.008% and O content ≤20ppm; 2) Continuous casting process control: The viscosity of the mold flux is 0.12~0.15Pa·s, and the basicity is R1=CaO / SiO2=1.0~1.2; the specific water volume in the secondary cooling zone of continuous casting is 0.6~0.9L / kg, and the straightening operation temperature during continuous casting is 850~950℃; 3) Rolling process control: The initial hot rolling temperature is 1015~1145℃; the final finishing rolling temperature is controlled at 860~890℃; the coiling temperature is set at 640~680℃; the single-pass reduction rate for the first two rolling passes is 35%~55%; The precision control tolerance of the rolling mill roll gap is ≤ ±0.5mm.

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

3. The method for controlling star-shaped cracks in atmospheric corrosion-resistant wide strip steel according to claim 1, characterized in that, Before rolling, high-pressure water descaling is performed with a pressure ≥25MPa.

4. The method for controlling star-shaped cracks in atmospheric corrosion-resistant wide strip steel according to claim 1, characterized in that, After rolling, the steel coil is placed in a pit for slow cooling for more than 48 hours.

5. The method for controlling star-shaped cracks in atmospheric corrosion-resistant wide-band steel according to claim 1, characterized in that, The finished steel plates have a width of 500-800mm and a thickness of 2.5-14mm.

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