Ti-Mo-V微合金化超高强钢相变行为

Transformation behavior of Ti-Mo-V microalloyed ultra-high strength steel

Ti-Mo-V微合金化超高强钢具有良好的塑韧性和成型性,且生产工艺简单,在汽车制造、工程机械等领域具有广阔的应用前景。利用Gleeble3 500热模拟试验机、金相显微镜(OM)、透射电子显微镜(TEM)和显微硬度计等设备系统研究了Ti-Mo-V微合金化超高强钢的连续冷却相变和等温相变行为,获得了试验钢的连续冷却转变(CCT)曲线和时间-温度转变(TTT)曲线,探究了(Ti,Mo,V)C粒子的析出行为对硬度的影响。研究结果表明,当冷却速率低于1℃/s时,试验钢中的组织由铁素体和珠光体组成;当冷却速率为5℃/s时,基体中出现粒状贝氏体,铁素体和珠光体含量均显著减少;当冷却速率增大至10~20℃/s时,试验钢中珠光体组织消失,室温组织转变为铁素体和贝氏体;当冷却速率达到30℃/s时,过冷奥氏体全部转变为贝氏体,硬度达到最大,约为269.2HV;然而,当冷却速率进一步增大至50℃/s时,基体中(Ti,Mo,V)C的析出受到抑制,沉淀强化效果减弱,硬度下降至256.9HV。试验钢在620~730℃等温时仅发生铁素体相变,其相变动力学随温度的降低呈先增快后减缓的变化规律。当等温温度为700℃时,相变孕育期最短,约为25 s;而当等温温度由730℃降低至620℃时,铁素体的平均晶粒尺寸由29.8μm细化至12.3μm,基体中(Ti,Mo,V)C的平均粒径由9.6 nm减小至3.0 nm,此时试验钢中铁素体的显微硬度最大,约为348.7HV,主要是晶粒细化和沉淀强化所致。

The microalloyed ultra-high strength steel has broad prospect of application in the fields of automobile manufacturing and machinery manufacturing with excellent plasticity,toughness,molding and simple production process.The continuous cooling transformation and isothermal transformation behavior of Ti-Mo-V microalloyed ultra-high strength steel were investigated by the Gleeble 3500 thermal simulation testing machine,OM,TEM and Vickers hardness tester.The Continuous Cooling Transformation(CCT)curve and Time-Temperature Transformation(TTT)curve of the tested steel were obtained.The effect of precipitation behavior of(Ti,Mo,V)C particles on hardness was also explored.The results of study show that the microstructure of tested steel is composed of ferrite and pearlite when the cooling rate is lower than 1℃/s.When the cooling rate is 5℃/s,a large number of granular bainite appear and the content of ferrite and pearlite decreases significantly.When the cooling rate increases to 10-20℃/s,pearlite disappears from the matrix and the microstructure consists of ferrite and bainite.When the cooling rate reaches 30℃/s,the austenite transfers to fully bainite and the matrix hardness reaches a maximum of approximately 269.2 HV.However,when the cooling rate further increases to 50℃/s,the precipitation of(Ti,Mo,V)C particles in the matrix is restrained,leading to a weakened precipitation strengthening effect and consequently a decrease in hardness to 256.9HV.The isothermal transformation of austenite to ferrite will occur when the tested steel is held at the temperature in the range of 620-730℃.With the decrease of temperature,the ferrite transformation kinetics firstly accelerates and then slows.The incubation period of transformation is the shortest(about 25 s)when the isothermal temperature is held at 700℃.When the isothermal temperature decreases from 730℃to 620℃,the average grain size of ferrite is gradually refined from 29.8μm to 12.3μm and the average particle size of(Ti,Mo,V)C in the matrix decreases from 9.6 nm to 3.0 nm.In this case,the hardness of ferrite reaches a maximum of approximately,mainly due to grain refinement and precipitation strengthening.