采用热膨胀法测定了 Nb-V-Ti微合金化 Q345E钢的相变临界温度Ac1和Ac3,使用45 kW箱式电阻炉对实验钢进行了正火处理,利用光学显微镜(OM)、扫描电镜(SEM)、电子背散射衍射(EBSD)和透射电镜(TEM)观察了正火后试样的组织演变规律,分析了正火温度对组织与低温韧性的影响。结果表明,Q345E钢的Ac1约732℃、Ac3约871℃；当正火温度为820℃和850℃时,组织由针状珠光体和未发生重结晶的粗大原始铁素体构成,针状珠光体由针状奥氏体转变而来,相邻针状珠光体中的铁素体相具有相同的晶体取向特征；当正火温度不低于880℃时,组织由重结晶后的铁素体和珠光体构成,随正火温度的提高组织尺寸逐渐增大；当正火温度为820~940℃时,随正火温度的提高冲击功呈现先增加后降低的变化规律,组织类型、尺寸、形态和均匀性是影响大截面锻材 Q345E 低温韧性的主要因素。
Phase transition critical temperature of Q345E steel was measured by thermal expansion methods. The test steel was normalized in 45 kW chamber electric furnace,and its microstructure evolvement induced by nor-malization were observed by OM,SEM,EBSD (electron back-scatter diffraction)and TEM.The effects of normali-zing temperature on microstructure and low-temperature toughness of the steel were also analyzed.The results showed that Ac1 and Ac3 of Q345E steel is 732 ℃ and 871 ℃,respectively.When the normalizing temperature is 820 ℃and 850 ℃,the microstructure of the steel is composed of acicular pearlite and coarse original ferrite.The adjacent acicular pearlites which were transformed from acicular austenite have the same crystal orientation.When the normalizing tem-perature is above 880 ℃,the microstructure is composed of pearlite and ferrite,and the microstructure size increases with the increasing of the normalizing temperature.When the normalizing temperature increases within 820 ℃ and 940℃,the impact energy increases at first and then decreases.Microstructure type,size,shape and uniformity are the main factors affecting the low-temperature toughness of large section Q345E steel forging.
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