超快冷工艺对钢板心部异常带状组织的抑制作用

采用OM、TEM和EMPA方法对比研究了超快冷工艺及终轧温度对355 MPa级钢板心部异常带状组织的影响.结果表明,950℃高温终轧及超快冷钢板(UC1钢)心部带状组织完全消失,900℃终轧及超快冷钢板(UC2钢)心部则形成了微弱带状组织,而轧后15℃/s层流冷却钢板(LC钢)心部则形成了包括马氏体/奥氏体低温相的严重带状组织.热力学计算显示,钢板心部偏析降低铁素体相变温度144℃,从而提高消除带状组织所需临界冷速到8℃/s.温度计算得到UC1和UC2钢板心部冷速分别达12.1和13.4℃/s,而LC钢板心部冷速只有5.5℃/s,表明超快冷足以抑制心部带状组织,但降低终轧温度削弱了这个效果,而层流冷速则无法抑制带状组织.

Effect of Ultrafast Cooling on Preventing Abnormal Microstructural Banding at Centerline of Steel Plate

The abnormal microstructural banding at centerline was compared among three steel plates of 355 MPa grade using OM， TEM and EPMA， and the effects of ultrafast cooling (UFC) and finish rolling temperature (FRT) were discussed. It was found that UFC was effective to suppress banding in centerline segregation zones， as the bands were entirely disappeared in UFC steel with FRT at 950℃ (UC1 steel)， comparing with the feeble bands in UFC steel with FRT at 900℃(UC2 steel). In contrast， severe bands consisting of martensite/austenite constituent (M/A) were observed in a conventional laminar-water cooled steel (LC steel)with a cooling rates of 15℃/s. The thermodynamic calculation showed that the equilibrium temperature of austenite-to-ferrite transformation could be reduced by 144℃ in elements-rich centerline zone， increasing the critical cooling rate to 8℃/s for avoidance of banding. The simulated temperature indicated the cooling rate of 12.1 and 13.4℃/s at the mid-thickness of UC1 and UC2 steels， respectively， comparing with that of 5.5℃/s for LC steel. Therefore， UFC provides sufficient cooling capability for suppressing microstructural banding at plate centerline， while the lower FRT could offset the effect. In contrast， banding cant be prevented by exploring laminar-water cooling because of its relatively low cooling rate.