回火温度对两相区退火海工钢组织和性能的影响

Effect of Tempering Temperature on Microstructure and Properties of Intercritical Annealing Marine Steel

对690 MPa级海工钢进行"淬火+两相区退火+回火"三步热处理,研究了回火温度对其组织和性能的影响、分析了力学性能变化与组织演变和残余奥氏体体积分数之间的关系。结果表明:回火后实验钢的显微组织为回火贝氏体/马氏体、临界铁素体和残余奥氏体的混合组织。随着回火温度的提高贝氏体/马氏体和临界铁素体逐渐分解成小尺寸晶粒,而残余奥氏体的体积分数逐渐增加;屈服强度由787 MPa降低到716 MPa,塑性和低温韧性明显增强,断后伸长率由20.30%增至29.24%,-40℃下的冲击功由77 J提升至150 J。残余奥氏体体积分数的增加引起裂纹扩展功增大,是低温韧性提高的主要原因。贝氏体/马氏体的分解和残余奥氏体的生成,引起组织细化、晶粒内低KAM值位错的比例逐渐提高和小角度晶界峰值的频率增大,使材料的塑性和韧性显著提高。

The influence of tempering temperature on microstructure and mechanical properties of 690 MPa grade marine steel after quenching+intercritical annealing+tempering three-step heat treatment was investigated in terms of the microstructure evolution,the volume fraction of retained austenite and the change in mechanical properties.The results show that the microstructure of the steel after tempering is a mixture of tempered bainite/martensite,intercritical ferrite and retained austenite.With the increasing tempering temperature,bainite/martensite and intercritical ferrite gradually decomposed into small grains,while the volume fraction of retained austenite gradually increased.Yield strength decreased from 787 MPa to 716 MPa.Plasticity and low temperature toughness were significantly enhanced,elongation after fracture increased from 20.30%to 29.24%,and impact energy at-40℃increased from 77 J to 150 J.The increase of the volume fraction of retained austenite lead to the increase of crack propagation work,which was the main cause responsible to the improvement of low temperature toughness.The decomposition of bainite/martensite and the formation of retained austenite lead to grain refinement,the number of dislocations with low KAM values in the grains increased,and the frequency of low angle grain boundary peaks increased,which may be beneficial to the significant increment of plasticity and toughness of the steel.