不同冷速对700 MPa级汽车大梁钢过冷奥氏体转变的影响

在DIL805L型淬火膨胀仪上测定了一种700 MPa级汽车大梁钢(700L)的连续冷却转变曲线(CCT曲线),结合金相-硬度法分析其相变规律、相变组织及影响因素。结果表明:试验钢的临界点Ac₃=898 ℃,Ac₁=772 ℃;当冷速小于0.5 ℃/s时,过冷奥氏体的转变产物为铁素体、珠光体和少量贝氏体;当冷速大于0.5 ℃/s时,珠光体消失;当冷速为5 ℃/s时,铁素体消失,转变产物全部为贝氏体;当冷速为10 ℃/s时,转变产物中出现少量马氏体组织,随着冷速增大,马氏体逐渐增多,贝氏体逐渐减少直至完全消失;随着冷速的提高,试验钢的硬度逐渐增大,当冷速为10 ℃/s时,硬度值有明显大幅度的增加。

Effect of cooling rate on undercooling austenite transformation of 700 MPa automotive beam steel

The continuous cooling transformation(CCT)curves of 700 MPa automotive beam steel 700 L were determined by using DIL805 L quenching dilatometer.Combined with metallography-hardness method,the phase transformation rules and microstructure as well as the influence factors were analyzed.The results show that the critical point of 700 L steel is A_c3=898℃,A_c1=772℃.The transformation product of undercooling austenite is ferrite,pearlite and bainite when the cooling rate is less than 0.5℃/s,then pearlite disappears when the cooling rate is more than 0.5℃/s,ferrite disappears and the transformation product is all bainite when the cooling rate is 5℃/s.A few martensite appears in the transformation products when the cooling rate is 10℃/s,and with the increase of cooling rate,martensite increases and bainite decreases gradually until totally disappears.The hardness of the steel gradually increases with the increase of cooling rate,and increases significantly at 10℃/s.