预应变与烘烤温度对低碳钢和双相钢组织性能的影响

Effects of Pre-strain and Baking Temperature on the Microstructure and Bake-hardening Behavior of Mild Steel and Dual Phase Steel

研究了预应变和烘烤温度对低碳钢和双相钢微观组织演变和烘烤硬化性能的影响。将低碳钢和双相钢加热至800℃保温90 s后再缓冷至680℃,随即快速冷却至300℃进行过时效处理以模拟连续退火工艺,最终分别获得铁素体—渗碳体和铁素体—马氏体组织,然后进行预应变和烘烤处理以测量其BH值。结果表明,对于低碳钢,在0%~8%范围内,随着预应变增加,BH值明显提高;预应变为8%时,BH值达到最大值65 MPa。对于双相钢,随着预应变增加,BH值先提高后降低;预应变为1%时,BH值达到最大值79 MPa。在140~300℃范围内,随着烘烤温度提高,在铁素体中形成的Cottrell气团和碳化物数量增加,低碳钢和双相钢的BH值均明显提高;烘烤温度为300℃时,低碳钢和双相钢BH值分别达到最大值71 MPa和148 MPa。对于低碳钢,烘烤后拉伸曲线均出现明显的屈服平台。对于双相钢,在140~170℃范围内,烘烤后工程应力—应变曲线仍保持连续屈服状态;在210~300℃范围内,烘烤后拉伸曲线均出现明显的屈服平台。

In a typical process,mild steel and dual phase steel were respectively annealed at 800 ℃ for 90 s,cooled slowly to 680 ℃ and then cooled rapidly to 300 ℃ to carry out the overaging treatment.As a result,the ferrite-cementite mild steel and ferrite-martensite dual phase steel were obtained after the continuous annealing process.After pre-straining,the specimens were baked to measure the bake-hardening(BH) values.The influences of pre-strain and baking temperature on the microstructure evolution and bake-hardening behavior of mild steel and dual phase steel were investigated.It was found out that the bake-hardening(BH) value gradually increased with an increase in pre-strain from 0% to 8%.The BH value reached a maximum value of 65 MPa at the pre-strain of 8% in the mild steel.In the dual phase steel,the BH value apparently increased with an increase in pre-strain from 0% to 1%;however,increasing pre-strain from 1% to 8% led to a decrease in the BH value.The BH value reached a maximum value of 79 MPa at the pre-strain of 1%.Furthermore,an increase in the baking temperature from 140 ℃ to 300 ℃ favored a gradual improvement in the BH value in both of mild steel and dual phase steel.And the maximum BH values at the baking temperature of 300 ℃ were 71 MPa and 148 MPa in the mild steel and dual phase steel,respectively.The yielding platform appeared in the engineering stress-strain curve in the mild steel after baking.While the yielding platform was not observed in the engineering stress-strain curves of dual phase steel after baking at 140~170 ℃.However,the yielding platform appeared in the engineering stress-strain curve due to the martensite tempering in the dual phase steel baked at 210~300 ℃.