27SiMn大尺寸棒材内裂原因及冷却控制技术

The Genesis of Inner Cracks in Large Section Forging 27SiMn and Improvement of Slow Cooling Process

利用光学显微镜和扫描电子显微镜观察了大截面锻材27SiMn内部裂纹的显微形貌和组织特征,利用电子背散射衍射(electron backseatter diffraction,EBSD)技术分析了裂纹两侧晶粒取向关系,采用拉伸断口法观察了裂纹分离面微观断口特征,对裂纹区域宏观异常组织进行了化学成分湿法分析,在Gleeble-1500热模拟试验机上测定了27SiMn材料在连续冷却过程中的相变温度,判定了裂纹形成时段和原因,在国内某生产线上进行了缓冷技术改进。结果表明:裂纹主要在点状偏析区或带状珠光体内萌生和扩展,裂纹两侧晶粒取向相同,裂纹面具有钿窝和解理两种断口特征,判定裂纹形成于奥氏体向铁素体-珠光体转变(A→F+P)完成后,化学成分的宏观和微观偏析是裂纹形成的成分基础,成分偏析导致相变不同步从而产生较大组织应力是裂纹形成的基本原因,采用新型缓冷技术可基本消除内部裂纹,使其四级探伤合格率由15%提高至75%以上。

The macro morphology and microstructure of inner cracks of the large section forging 27SiMn were researched by means of OM and SEM,the oriention of grains on both sides of the cracks was investigated by EBSD,the characteristic of the fracture of the crack was surveyed by SEM,the abnormal microstructure in the crack area was analyzed by wet chemical method,the phase transformation temperature during continuous cooling was tested by Gleeble thermal simulation testing machine,the genesis and initiation period of the crack was determined,the low cooling process were improved on a domestic production line.The results show that the inner crack initiated after A→F+P transformation,segregation of chemical and structure stress caused by non-synchronization of transformation are the basic reason of the crack initiation,cracks can be basically eliminated by low cooling in 400℃ condition and the qualified ratio of flaw inspection was increased from 15%(prior to research)to above 75%.