15CrMoG钢棒材表面缺陷机理分析和工艺改进

分析得出,棒材表面细小纵裂纹和表面裂口缺陷产生于铸坯加热之前,且与结晶器弯月面保护渣有关。利用Thermo-Calc热力学软件计算15CrMoG钢凝固相变过程,结合亚包晶钢连铸凝固特点综合分析15CrMoG钢棒材表面缺陷的产生原因和产生机理。结果表明:15CrMoG钢在固相线温度附近发生包晶反应L+δ→γ和包晶转变δ→γ,不仅导致初生坯壳生长不均匀,而且加剧P、S元素在凝固前沿的偏析。而初生坯壳不均匀是导致棒材表面缺陷根本原因。棒材表面细小纵裂纹产生于结晶器内坯壳薄弱处,经过二冷和轧制工序在夹杂物和硫偏聚处扩展长大。棒材表面裂口缺陷是初生坯壳不均匀导致结晶器内液面波动大,造成铸坯夹渣所致。通过控制C]0.16%～0.17%、S]≤0.005%、保护渣碱度1.2、熔点≥1200℃、粘度≥1.0Pa·s,260 mm×30mm铸坯水量150 m³/h,拉速0.5 m/min等措施,裂纹合格探伤合格率由原45%提高至98%。

Surface Defects Analysis and Mechanism Research of 15CrMoG Steel Bars and Process Improvement

It is obtained by analysis that the surface defects of the bars are formed before heating the continuous casting billet and related to the mold meniscus fluxes. Thermo-Calc software is used to calculate the solidification phase transition of 15CrMoG steel and the formation mechanism of surface defects for 15CrMoG steel bars are analyzed. The occurrence of peritectic reaction( L+δ→γ) and peritectic transformation ( δ→γ) near the solidus temperature of 15CrMoG steels lead to the uneven growth of the initial solidified shell and intensified the segregation of P and S elements at the solidification front. Uneven growth of the initial solidified shell is the primary cause of surface defects for the bars. Fine longitudinal cracks is generated in the weakness of the shell, and grow in the place where inclusions and sulphur gathered. Surface clefts is caused by slag inclusions in casting blanks which caused by mold level fluctuation rooted in uneven growth of the initial solidified shell. By the measures including to control C]0.16% ~0.17% ,S] ≤0.005% ,shielding slag basicity 1.2, slag melting point 1200℃ and viscosity 1.0Pa·s, water rate of 260 mm x 300 mm cast bloom 150 m³/h, and casting speed 0. 5 m/min, the non -destructive test qualified rate increases from original 45% to 98% .