The center segregation of round billets is a key indicator of the internal quality of round billets that needs to be controlled. It will seriously affect its mechanical properties and subsequent processing performance. It can be controlled and improved by optimizing process parameters, improving equipment and adopting advanced technologies. The following is a detailed introduction:
Optimize molten steel composition and pouring process
Control carbon content: When the carbon content is in the peritectic reaction zone (0.13%-0.17%), the linear shrinkage is large, which is easy to cause central segregation. Within the allowable range of steel composition, try to avoid this interval and ensure that the composition fluctuation between continuous casting furnaces is small.
Control the purity of molten steel: Reduce the content of harmful elements such as S and P in steel through refining process, increase the Mn/S ratio (such as ≥50), and reduce sulfide segregation. Increasing the Mn/S ratio can reduce the sensitivity of central cracks and segregation.
Reduce the superheat of molten steel: High superheat will cause excessive growth of columnar crystals and aggravate central segregation. Controlling the superheat of molten steel in the tundish at 15-25℃ can reduce the columnar crystal area, promote the formation of equiaxed crystals, and make the composition distribution more uniform.
Stabilize the drawing speed: Drawing speed fluctuations will destroy the stability of the solidification process and aggravate central segregation. According to the steel type and section, the pulling speed is controlled within the appropriate range, such as the pulling speed of Φ350mm round billet is controlled at 0.50±0.05m/min, avoiding too fast or too slow.
Optimize the insertion depth of the submerged nozzle: inserting too deep or too shallow will affect the flow field of the molten steel and the melting of the protective slag, resulting in segregation. Adjusting the insertion depth of the Φ350mm round billet submerged nozzle to 90-130mm can improve the flow state of the molten steel in the crystallizer and reduce the segregation of inclusions.
Full protection casting: adopt large ladle long nozzle + argon blowing protection, tundish covering agent, crystallizer protective slag and other measures to prevent the secondary oxidation of molten steel and the involvement of inclusions, and reduce the source of segregation.
Strengthening electromagnetic stirring technology
Mold electromagnetic stirring (M-EMS): By generating a rotating magnetic field, it promotes the flow of molten steel, breaks up dendrites, increases the core of equiaxed crystals, reduces the bridging of columnar crystals, and makes the segregation elements evenly distributed. For example, in the continuous casting of round billets at Ma Steel, when the M-EMS current is 300A, the central equiaxed crystal rate can reach 72.2%, and the degree of segregation is significantly reduced.
Final electromagnetic stirring (F-EMS): acts on the end of solidification, promotes the flow of molten steel to compensate for shrinkage, and reduces central segregation. The current and frequency need to be reasonably controlled. For example, when Wuhu Xinxing Casting Pipe produces 4145H steel, the central looseness level is significantly improved when the final stirring current is 300A, but excessive current (such as 400A) may lead to new loose points.
Improve the secondary cooling process
Optimize cooling intensity: adopt weak cooling to avoid local solidification differences caused by uneven cooling. For example, Panzhihua Steel uses weak cooling technology for high carbon steel in the continuous casting of round billets, and the secondary cooling water ratio is controlled at 0.15-0.20L/kg, which reduces segregation caused by improper cooling.
Reasonable distribution of water in each section: adjust the water ratio of each cooling zone according to the solidification characteristics of the billet. For example, after optimization, the water distribution ratios of section 0, section 1, and section 2 are 55%, 28%, and 17%, so that the surface temperature of the ingot is uniformly reduced, reducing the segregation caused by thermal stress.
Apply light reduction technology
Light reduction at the end of solidification: When the solid phase rate is about 90%, the ingot is pressed by the straightening roller to compensate for the solidification shrinkage and suppress the center segregation. For example, Wuhu Xinxing Casting Pipe conducted a light reduction test on a Φ600mm round billet, which made the central crack disappear and the flaw detection pass rate increased to more than 98%.
Accurately control the reduction amount: Calculate the appropriate reduction amount and reduction position according to the section of the ingot and the type of steel. For example, the study by Nakatsubo Xiuyi shows that light reduction with a solid phase rate of about 90% has a significant effect on improving the internal quality.
Improve the arc accuracy of the casting machine:
Ensure that the arc error of the runner composed of the guide rollers such as the copper mould tube, foot roller, and fan-shaped segment is no more than 0.2mm to avoid uneven solidification and segregation caused by equipment deviation. For example, Laigang Special Steel has improved the center quality of the ingot by using offline and online arc alignment technologies.
