硼对Mn-Si-Cr系贝氏体型非调质钢过冷奥氏体连续转变行为的影响

Effect of boron on continuous cooling transformation behavior of Mn-Si-Cr type bainitic non-quenched and tempered steels

本文采用热膨胀法结合硬度法及金相法研究了添加0.0016%(质量分数)的B与未加B的Mn⁃Si⁃Cr系贝氏体型非调质钢在不同冷却速度(0.02~44℃/s)下过冷奥氏体的连续冷却转变行为。结果表明,B元素的添加使得高温铁素体+珠光体转变受到明显抑制而促进了中温贝氏体转变,降低了贝氏体开始转变温度和形成贝氏体的冷却速度,使得实验钢获得最大贝氏体体积分数(约80%)的冷却速度由未加B钢的~0.17℃/s降到添加B钢的~0.13℃/s。B的添加能够提高实验钢的硬度,硬度增量随着冷却速度的增加呈现先增大后减小的趋势,并在0.28℃/s时硬度增量达到峰值为55 HV5。在目前的成分体系下,很难获得体积分数接近100%的贝氏体组织,需要进一步优化实验钢的成分体系,以便在尽可能宽的冷却速度范围内获得尽可能多的贝氏体组织。

In this paper,we explore the continuous cooling transformation behavior of supercooled aus⁃tenite of Mn-Si-Cr bainitic non-quenched and tempered steels with 0.0016%(mass fraction)B addi⁃tion and without B addition for reference at different cooling speeds(0.02~44°C/s)by using thermal expansion,hardness and metallography testing.The results show that the addition of B element signifi⁃cantly inhibits the ferrite+pearlite transformation at high temperature and promotes the bainite trans⁃formation at medium temperature,and reduces the bainite start transformation temperature and the cooling rate for obtaining bainite.As a result,the cooling rate to obtain the maximum bainite content(~80 vol.%)is reduced from~0.17℃/s for the steel without B addition to~0.13℃/s for the steel with B addition.The addition of B enhances the hardness of the tested steel,and the hardness incre⁃ment shows a trend of increasing and then decreasing tendency with increasing cooling rate,and the peak value of hardness increment as high as 55 HV5 is obtained at 0.28℃/s.It is difficult to obtain a complete bainite with it’s volume fraction up to 100%under the current chemical composition system.