荒管再加热前冷速对E470钢管在线常化工艺的影响

Effect of pierced billet cooling rate before reheating on online normalization process of E470 steel pipes

通过分析在线常化工艺生产的E470钢管在两种不同的荒管冷却速度下出现的性能异常情况,结果发现:当在719~600℃温度区间荒管冷却速度超过贝氏体临界冷却速度时,因在奥氏体冷却过程中未完全形成铁素体+珠光体而存在部分过冷奥氏体组织,在随后的再加热过程中未转变的过冷奥氏体继续长大形成粗大晶粒,并在随后的冷却中形成了贝氏体组织,最终组织为粗大的贝氏体+铁素体+珠光体,从而大大地降低了材料的冲击韧性。但当在719~600℃温度区间荒管冷却速度未超过贝氏体临界冷却速度时,最终组织为细化的铁素体+珠光体组织,获得了良好的强韧性匹配。据此提出为了确保在线常化的效果,应把荒管脱管后的冷却速度作为在线常化工艺的一个重要工艺参数进行控制。

When the pierced billets were cooled by two different rates before reheating,the E470 steel pipes manufactured by online normalization process were found to have performance anomaly.The analysis results show that,when the pierced billet cooling rate exceeds the critical cooling rate of bainite during 719-600℃,the austenite does not transform into ferrite and pearlite completely during the cooling process but partial undercooled austenite can be found.When the undercooled austenite is reheated,it continues to grow into coarse grains and transforms into bainite during the consequential cooling process,the final microstructure is composed of coarse bainite,ferrite,and pearlite,so that the impact toughness of the material reduces greatly.When the cooling rate of pierced billet does not exceed the critical cooling rate of bainite during 719-600℃,the final microstructure is composed of refined ferrite and pearlite,so the good match of strength and toughness can be achieved.In order to ensure the online normalization effect,it is proposed that the cooling rate of the pierced billet after piercing rolling shall be controlled as one important process parameter.