焊接热循环对P460NL1高强正火容器钢微观组织及低温冲击韧性的影响

Effects of welding thermal cycle on microstructure and low-temperature impact toughness of P460NL1 high-strength normalized container steel

采用焊接热模拟技术,结合OM、SEM、TEM及-40℃低温冲击韧性实验,研究了焊接热循环(不同峰值温度和t8/5参数)对P460NL1高强正火容器钢热影响区组织和低温韧性的影响,重点分析了不同焊接热循环中强化相V(C,N)粒子的演变。结果显示,当t8/5同为45 s时,P460NL1钢模拟热影响区的低温冲击韧性随峰值温度的升高大致呈降低趋势,且温度超过1200℃后冲击韧性急剧降低。峰值温度为1350、1200℃且t8/5在15~100 s范围时,模拟的是P460NL1钢焊接热影响区粗晶区,组织主要为铁素体和贝氏体混合组织,此条件下P460NL1钢的低温冲击韧性较低且基本不随t8/5的变化而变化;t8/5为45 s时,峰值温度1100、950℃对应的是焊接热影响区细晶区,此时组织为铁素体+贝氏体+珠光体混合组织,峰值温度870℃模拟的是两相区,主要为铁素体和珠光体组织。利用Thermal-Calc软件计算得到P460NL1钢中V(C,N)溶解温度为1160℃,故当峰值温度超过1200℃时,V(C,N)粒子完全溶解且未再析出,基体中存在的游离N会降低P460NL1钢的低温冲击韧性,且当峰值温度为1350℃时,随着t8/5增加,晶粒尺寸逐渐增大,但冲击韧性却没有因此而降低,表明游离氮是热影响区粗晶区冲击韧性的关键因素。

By using welding thermal simulation approach,combined with OM,SEM,TEM and low-temperature(-40℃)impact toughness test,the effects of welding thermal cycle(different peak temperatures and t8/5)on the microstructure and low-temperature toughness of the heat-affected zone of P460NL1 high-strength normalized container steel were studied with the emphasis on the evolution of strengthening phase V(C,N)particles in different welding thermal cycles.The results show that when t8/5 is 45 s,the low-temperature impact toughness of the simulated heat-affected zone of P460NL1 steel generally decreases with the increase of peak temperature,and the impact toughness decreases sharply when the temperature exceeds 1200℃.When the peak temperature is 1350,1200℃and t8/5 is in the range of 15~100 s,what is simulated is the coarse-grained heat affected zone of P460NL1 steel,and the microstructure is mainly a mixture of ferrite and bainite.Under this condition,the low-temperature impact toughness of P460NL1 steel remains low and basically does not change with the change of t8/5.When t8/5 is 45 s,the fine-grain heat affected zone is obtained after welding thermal test at the peak temperature of 1100℃and 950℃,and the microstructure is the mix of ferrite,bainite and pearlite;the intercritically heat affected zone is obtained after simulating at the peak temperature of 870℃,and the microstructure composes of ferrite and pearlite.Moreover,by using Thermal-Calc software,the dissolution temperature of V(C,N)particles in P460NL1 steel is calculated to be 1160℃.Therefore,when the peak temperature exceeds 1200℃,V(C,N)particles are completely dissolved without re-precipitation.The presence of free nitrogen will reduce the low-temperature impact toughness of P460NL1 steel.When the peak temperature is 1350℃,with the increase of t8/5,the grain size gradually increases,but the impact toughness does not decrease as a result,indicating that the free nitrogen is the key factor affecting the impact toughness of the coarse-grained heat affected zone.