中碳低合金钢薄板坯中间裂纹的形成原因

Formation cause of thin slab intermediate cracks of low alloy steel with medium carbon

中碳低合金钢薄板坯易出现中间裂纹,严重降低了连铸连轧产品的质量与效益。采用凝固定律公式及纵截面中间裂纹的始、末位置特征计算裂纹发生凝固区间,并对内、外弧侧中间裂纹的特征差异及裂纹周围的元素偏析、析出物进行了分析,旨在解析CSP生产薄板坯中间裂纹的形成原因。结果表明:横、纵截面上中间裂纹特征存在明显差异,这是由于铸坯试样的纵截面与横截面相交于一条线,导致了纵截面上间隔、倾斜分布的线状中间裂纹在横截面上显示为点状缺陷或者没有显示。而且,纵截面上内、外弧侧中间裂纹的起始、结束、间隔距离的凝固位置特征也存在明显差异。因此,以纵截面上中间裂纹的起始、结束的凝固位置计算裂纹形成的凝固区间,能为二冷设备精准维修提供理论依据。中碳低合金钢中间裂纹处[S]、[P]、[C]、[N]富集及Ti(CN)、Nb(CN)、MnS析出导致严重的晶界脆化,造成枝晶间临界应力降低是中间裂纹产生和扩展的内因;扇形段冷却喷嘴堵塞、鼓肚严重造成铸坯在凝固前沿所受应力增加是中间裂纹形成的主要外因。内、外弧侧扇形段的冷却效果、辊缝精度存在差异是导致内、外弧侧中间裂纹特征存在差异的根本原因。避免扇形段冷却喷嘴堵塞、确保辊缝精度以抑制铸坯鼓肚是控制CSP薄板坯中间裂纹的关键。通过铸坯内在质量的定期检测与扇形段的精准维修,有效降低了中间裂纹发生率75%。

Medium carbon low alloy steel casting billet is prone to intermediate cracks,which seriously reduce the quality and benefit of continuous casting and rolling products.The solidification sections of crack occurrence was calculated by using solidification law formula with the beginning and end position characteristics of the middle crack in longitudinal section of the thin slab,and the difference in the characteristics of intermediate cracks on the inner and outer arc layers of the thin slab and the elemental segregation and precipitates around the cracks were analyzed,in order to reveal the formation causes of intermediate cracks in thin slabs produced by CSP process.The results show that the characteristics of intermediate cracks on the cross and longitudinal sections has obvious differences,which is because the longitudinal section of the casting billet specimen intersects the cross section in a line,as a result,the interval and inclined linear intermediate cracks on the longitudinal section is shown as point defects or not shown on the cross section.Moreover,the difference in the solidification position characteristics of the beginning,end and interval of intermediate cracks on the inner and outer arc layer of the casting slab in the longitudinal cross-section is also clearly visible.Therefore,calculating the solidification section according to position of the beginning and end of intermediate cracks on the longitudinal cross-section can provide a theoretical basis for the precision maintenance of the second cooling equipment.The enrichment of elements such as[S],[P],[C]and[N]and the precipitation of Ti(CN),Nb(CN)and MnS at the intermediate cracks in casting slabs of low alloy steel with medium carbon lead to severe grain boundary embrittlement,resulting in the reduction of the critical stress between the dendrites,which is the internal cause of the intermediate crack generation and expansion;the blockage of the cooling nozzle in the secondary cooling sections and the increase of stress on the solidification front of the cast billet caused by the severe bulging are the main external causes of the intermediate crack formation.The difference in cooling effect and roll gap accuracy between the inner and outer arc side sector is the root cause of the difference in the characteristics of intermediate cracks between the inner and outer arc layer of the thin slab.Avoiding the blockage of cooling nozzles in the sector section and ensuring the accuracy of the roll gap to suppress the bulging of the cast slab are the keys to control the intermediate cracking of CSP thin slabs.By regular inspection of the intrinsic quality of the cast billets and precise maintenance of the secondary cooling sections,the incidence rate of intermediate cracks has been effectively reduced by 75%.