直上钢Q235B板坯高拉速下中间裂纹的形成与调控

Formation and control of midway crack in LD-CC Q235B steel slab with high casting speed

基于凝固数值模拟和板坯应变计算分析了直上钢Q235B连铸中间裂纹的形成机理和影响因素,并根据实际生产条件提出了铸坯高拉速质量控制策略。研究发现,辊缝和对中精度为Q235B铸坯中间裂纹形成的主要因素,两者应变之和占凝固前沿总应变60%以上;而表面回温高于50℃/m的铸机前段冷却也有重要影响,其应变占比最高约40%。当辊缝的邻辊正偏差为1.5mm时,通过强化冷却和控制回温可使铸坯凝固前沿总拉应变整体降低约20%;当辊缝的邻辊正偏差为0.5mm时,冷却优化对凝固前沿拉应变的影响较小。随着拉速增大,辊缝精度对铸坯中间裂纹的影响愈加显著。当前工况下,将锰硫比提高到25,坯壳可最多承受1.4m/min时邻辊正偏差1.5mm、对中偏差±0.5mm和回温不超过50℃/m带来的附加应变。为抑制1.5m/min以上高拉速下直上钢Q235B铸坯的中间裂纹,建议将辊缝整体精度控制在±0.5m且邻辊正偏差控制在0.5mm以内。

The formation mechanism and influencing factors for midway crack in continuously cast Q235 Bslab by LD-CC route were studied based on numerical modeling and strain calculation,and the strategy to control the slab quality at high casting speed was also developed according to the plant condition.It was found that the precision of roll gap and alignment was the key factor affecting the occurring of midway crack in Q235 Bslab,as the tensile strain induced by both roll gap fluctuation and roll misalignment was larger than 60%.The secondary cooling in initial zones of the caster also has significant influence.The strain by surface temperature rebound was about 40%at most.The total tensile strain at the solidification front was lowered by 20%when the cooling was strengthened and homogenized with positive deviation of the roll gap between neighboring rolls along pulling direction about 1.5 mm,while that effect was less obvious when the deviation decreases to 0.5 mm.The influence of roll gap precision became more prominent as the casting speed increased.Increasing the Mn/S to higher than 25 enable the shell to resist the additional strain from 1.5 mm positive deviation between neighboring rolls,±0.5 mm roll misalignment and50℃/m surface temperature rebounding.It was suggested that the total precision of the roll gap should be controlled to±0.5 mm and the positive deviation of the roll gap between neighboring rolls should be within 0.5 mm to prevent the midway crack in Q235 Bsteel by LD-CC route with casting speed higher than 1.5 m/min.