微合金化2000 MPa热成形钢的析出相热力学计算与强韧性

Thermodynamic calculation of precipitates in microalloyed 2000 MPa hot stamping steel and its strength and toughness

通过热力学计算软件Thermo-Calc计算了2000 MPa热成形钢的平衡相图、各相的析出温度、相中的元素含量、碳化物在不同温度下的长大规律以及不同Nb、V含量对其碳化物析出温度和析出量的影响规律。选定特定成分,利用50 kg真空炉进行了熔炼,并进行热轧和冷轧,利用平板模具淬火的方式模拟热成形工艺并进行了力学性能检测和三点弯曲性能检测。利用场发射扫描电镜和EBSD对组织进行了表征。结果表明,Nb、V微合金化2000 MPa热成形钢中的碳化物主要有NbC和VC,析出温度分别在1150℃以上及880℃以上,且其析出温度分别随着Nb和V含量的升高而升高。平板模具淬火后热成形钢板的抗拉强度超过2000 MPa,伸长率超过8%,拉伸断口为韧性断口,且三点弯曲角度超过66°。SEM和EBSD的结果表明,马氏体组织由马氏体束(packet)、马氏体块(block)和马氏体板条(lath)组成,原奥氏体晶粒约为10μm,且马氏体块的尺寸<5μm,马氏体块内部由马氏体板条组成,马氏体板条间为不连续的小角度晶界,晶界的取向差大部分小于5°。细小的原奥氏体晶粒和马氏体块组织是微合金化2000 MPa热成形钢具有高强度、高塑韧性的主要原因。

Thermodynamic software Thermo-Calc was used to calculate the equilibrium phase diagram,precipitation temperature of each phase,element content in the phase,growth law of carbides at different temperatures and the influence of Nb and V content on precipitation temperature and amount of carbides in the microalloyed 2000 MPa hot stamping steel.A steel with selected composition was smelted by 50 kg vacuum furnace,and hot rolling and cold rolling were carried out.The hot forming process was simulated by plate die quenching,the mechanical properties and three-point bending properties were tested,and the field emission scanning electron microscope and EBSD were used to characterize the microstructure.The results show that the main carbides in Nb and V microalloyed 2000 MPa hot stamping steel are NbC and VC,and the precipitation temperatures are above 1150℃ and 880℃,respectively,and the precipitation temperatures increase with the increase of Nb and V content,respectively.The tensile strength and elongation of hot stamping steel plate after plate die quenching are more than 2000 MPa and 8%,respectively.The tensile fracture is ductile fracture and the three-point bending angle is more than 66°.The results of SEM and EBSD show that the martensite structure is composed of packet,block and lath.The prior austenite grain size is about 10μm,and the size of martensite block is<5μm.The martensite block is composed of martensite lath,and the small angle grain boundary between the martensite lath is discontinuous,and most of the grain boundary orientations are less than 5°.Fine prior austenite grain and martensite block structure are the main reasons for the high strength and high plasticity of microalloyed 2000 MPa hot stamping steel.