The porosity and shrinkage of round billets are key indicators that need to be controlled for the internal quality of round billets. Porosity is the aggregation of internal micropores, and shrinkage is the void formed by volume shrinkage during solidification. Both will reduce the density of the round billet and may form cracks or voids after rolling. It is mainly controlled and improved through process optimization. The following is a specific introduction:
Secondary cooling water distribution optimization:
Large-section billets use weak cooling process and appropriately reduce the secondary cooling water ratio, which can increase the central equiaxed crystal rate and reduce porosity and shrinkage. For example, Wuhu Xinxing Casting Pipe optimizes the secondary cooling water ratio to 0.15-0.20L/kg, and adjusts the water distribution ratio of each section to stabilize the flaw detection pass rate at more than 98%.
Electromagnetic stirring process optimization:
Mold electromagnetic stirring (M-EMS): By adjusting the current, frequency and stirring time of M-EMS, the solidification structure of the billet can be improved and the central segregation and porosity can be reduced. For example, Baosteel uses M-EMS on round billet casting machines to increase the equiaxed crystal rate from 10.65% to 16.02% and reduce the center C segregation index from 1.07 to 0.97.
Final electromagnetic stirring (F-EMS): Electromagnetic stirring is applied at the end of solidification to promote the flow of molten steel, supplement shrinkage, and reduce shrinkage and porosity. When the end electric stirring current is 300A, the center porosity level of the billet is 1.0 and the shrinkage is 0.5; but if the current is too large (such as 400A), regular porosity points will appear on the periphery of the white bright band.
Soft reduction process
Soft reduction at the end of solidification: When the solid phase ratio is about 90%, the billet is lightly pressed down by the straightening roller to compensate for solidification shrinkage and improve center porosity and shrinkage. For example, Wuhu Xinxing Casting Pipe uses soft reduction technology to make the core of the billet free of center cracks, and the flaw detection pass rate is significantly improved.
Precisely control the amount of reduction: According to the cross section and solidification characteristics of the ingot, the amount of reduction and the position of the light reduction are precisely controlled. By calculating the solid phase ratio at the straightening roller, the appropriate light reduction area is determined, such as the first 3 sets of straightening rollers (13.4m, 14.9m, and 19.0m from the meniscus) are suitable for light reduction.
Molten steel superheat control
Low superheat casting: Reducing the superheat of molten steel can reduce the growth of columnar crystals and increase the proportion of equiaxed crystals, thereby reducing porosity and shrinkage. For example, when Baosteel produces Φ178mm round billets, the superheat is controlled below 30℃, and the proportion of billets without shrinkage cavities increases significantly. Stable superheat: Keep the superheat stable to avoid the influence of superheat fluctuations on the solidification process. Control the temperature of the molten steel through the refining process to ensure that the superheat is within the target range.
Drawing speed control
Low drawing speed casting: Reducing the drawing speed can extend the solidification time of the molten steel in the crystallizer, make the solidification more uniform, and reduce porosity and shrinkage. For example, a study by Sumitomo Metal of Japan showed that when the drawing speed is low, the degree of central porosity is significantly reduced. Constant drawing speed: Avoid drawing speed fluctuations to maintain the stability of the casting process. Drawing speed fluctuations will lead to unstable solidification process and increase the probability of defects.
Composition control
Reduce S content: Too high S content will increase the hot brittleness of the molten steel, leading to central cracks and porosity. General round tube billets require P≤0.020%, S≤0.015%, and special oil pipes and special round tube billets require P≤0.015%, S≤0.010%. Increase Mn/S ratio: Increasing the Mn/S ratio can make S and Mn combine to form MnS, avoid grain boundary brittleness, and reduce the sensitivity of cracks and porosity. For example, Baosteel research shows that with the increase of Mn/S, the average levels of center cracks, intermediate cracks, subcutaneous cracks and inclusions all decrease.
