【作者】 韩赟；

【导师】 董瀚； 时捷；

【作者基本信息】 钢铁研究总院 ， 材料学， 2013， 博士

【摘要】 低碳马氏体钢由于碳的固溶强化、位错强化等作用通常具有较高的强度。碳化物析出强化可以进一步提高低碳马氏体钢的强度,多年来Mo2C、VC等碳化物的强化作用已经得到充分研究。但是,直接淬火及回火时低碳马氏体钢中TiC类碳化物的析出行为及其强化作用研究较少。本文通过SEM、TEM、EBSD以及萃取相分析等试验方法,研究低碳马氏体钢中(Ti,Mo)C纳米析出相在不同工艺中的析出行为及其对力学性能的影响,为提高低碳马氏体钢的强韧性提供理论依据。利用TEM、相分析并结合热力学、动力学计算等方法,定量研究了(Ti,Mo)C在不同工艺过程中各个阶段析出尺寸、数量及控制因素。研究发现,低碳马氏体钢中(Ti,Mo)C碳化物析出包括两个主导性的阶段：在奥氏体区析出鼻尖温度至大概850℃之间进行变形,可以获得大量5-36nm的纳米析出相,Ti析出量大概达到47.0%；再加热过程进入奥氏体区及随后的奥氏体区保温过程中,奥氏体中会继续析出大量5-18nm的析出相,Ti析出大概为33.0%到46.6%之间。利用SEM、TEM、EBSD等方法,研究了组织演变及(Ti,Mo)C对晶粒细化的作用。研究发现,试验钢中(Ti,Mo)C对a/γ界面移动阻碍作用很小,但在奥氏体长大过程中钉扎晶界,成为晶粒细化的主要原因。添加Ti、Mo试验钢中马氏体板条明显变短,相邻板条块位向差增大。低温奥氏体未再结晶区轧制后马氏体组织扁平化,试样中储存的形变能可以显著增加奥氏体形核驱动力及形核率,且变形后亚结构位向关系的改变促进了再加热后位向关系的遗传,再加热后晶粒尺寸得到更进一步的细化。系统研究了(Ti,Mo)C析出相及组织遗传对低碳马氏体钢强韧性的影响。利用(Ti,Mo)C及组织遗传对再加热后晶粒细化、强化方式改变,获得了1000-1700MPa高强度低碳马氏体钢,其冲击吸收能量较常用低碳马氏体钢有明显提高,(Ti,Mo)C析出相的晶粒细化作用使低碳马氏体钢的韧脆转变温度降低。更多还原

【Abstract】 Low carbon martensitic steel has high strength because of the effect of carbon solution strengthening and dislocation strengthening. Moreover, the strength of low carbon martensitic steel can be further improved by the precipitation strengthening of carbides. The strengthening effects of carbides, such as Mo2C and VC, have been widely researched these years. However, the precipitation behaviour of TiC during the direct quenching and tempering processes and their strengthening effects in low carbon martensitic steel have hardly investigated. In the present study, the precipitation behaviours of (Ti,Mo)C during different processes and their effects on mechanical properties in low carbon martensitic steel were investigated by SEM, TEM, EBSD and phase analysis method. These results can provide the theoretic foundation for the mechanical properties improvement of low carbon martensitic steel.To elaborate the precipitation behaviors of (Ti,Mo)C nano-precipitates in steel, the size, amount and controlling factors of precipitates during different stages were investigated by performing TEM and phase analysis method combining with the calculation of thermodynamics and kinetics. There are two dominant stages as follows: large amount of nano-precipitates with the diameter in5-36nm were formed during the the austenite deformation between the nose temperature and almost850℃, the whole mass fraction of Ti precipitation can reach47.0%. Furthermore, lots of nano-precipitates with the diameter in5-18nm were also formed during the reheating process, and the whole mass fraction of Ti precipitation can be33.0%to46.6%.To describe the microstructures evolution and the effect of (Ti,Mo)C on the grain refinement, the SEM, TEM, EBSD were performed. The results show that the (Ti,Mo)C can hardly inhibit the migration of y/a interface during reverse transformation. However, the (Ti,Mo)C can refine the grain size by the pinning effect on the grain boundary. Meanwhile, obviously shorter martensite lath, increment of high angle grain boundaries and increment of orientation difference between adjacent blocks were found in Ti-Mo steel after reheating process. The martensite became pancake and the austenite nucleation driving force was enhanced by the stored energy after being rolled at low temperature in unrecrytallized austenite region. Therefore, the orientation of sub-structure was inherited and the grain size was further refined after reheating. Effects of (Ti,Mo)C and microstructure heredity on the mechanical properties of low carbon martensitic steel were systematically investigated.1000MPa-1700MPa high strength martensitic steels had been obtained by the grain refinement and the modification of strengthening mechanisms induced by the (Ti,Mo)C. The impact energies of martensitic steels above had obvious improvement comparing to the usual low carbon martensitic steels. Furthermore, the ductile-brittle transition temperatures of low carbon martensitic steels were also decreased by the grain refinement induced by the (Ti,Mo)C.更多还原