超高强钢铸坯裂纹观察及成因分析

Crack Observation and Cause Analysis of Ultra-high Strength Steel Slab

采用金相显微镜和扫描电镜观察与分析了980MPa级超高强钢铸坯裂纹断口形貌及组织,并利用Gleeble-2000D研究了铸坯高温热塑性。结果表明:裂纹铸坯原始奥氏体晶界处有大量的铁素体析出,并在晶界处连接成片形成膜状铁素体带,膜状铁素体上分布着白色链状的Nb、Ti碳氮化物。980 MPa级超高强钢铸坯的第Ⅲ脆性区为650℃-950℃,其断面收缩率均低于40%,而在775℃时热塑性最差,断面收缩率仅为12%。处于低塑性温度区间的铸坯在热应力和机械应力在强度较低的奥氏体晶界上的膜状铁素体集中促进裂纹萌生和扩展。

The crack morphology and microstructure of 980MPa grade ultra-high strength steel slab were observed and analyzed by metallographic microscope and scanning electron microscope(SEM),and the high temperature thermoplastic of the slab was studied by Gleeble-2000D.The results showed that there was a large amount of ferrite precipitated at the initial austenite grain boundary of the crack casting blank,which was connected into a sheet to form a film-like ferrite belt with white chain-like Nb and Ti carbonitride precipitates on it.The thirdⅢbrittle zone of the steel slab is 650-950℃,where their section shrinkage Z is lower than 40%,the worst thermoplastic at 775℃,and the Z is only 12%.The thermal stress and mechanical stress of the cast slab in the low plastic temperature range are concentrated on the austenite grain boundary with lower strength,which promotes the initiation and propagation of cracks.