Study on the mechanism of bainitic transformation in steels.

采用Fe-9%Ni-C和Fe-2%Si-I%Mn-C合金对贝氏体进行了相变动力学测量和显微组织观察，以阐明钢中贝氏体铁素体成分的形成机理。主要研究结果如下：(1)在碳含量为0.1% ~ 0.3%的三种Fe-9%Ni-C合金中，TTT图均由两条c曲线组成；较高的对应于前共析铁素体+珠光体，较低的对应于贝氏体。这清楚地表明，即使在没有强碳化物形成的合金中，贝氏体反应也有自己的C曲线。此外，证实了Bs温度几乎与碳含量无关，与Fe-9%Ni二元合金的Ms温度相对应。(2) 2%Si-I - %Mn-0.34%C亚共析钢在823K时主要形成无碳化物贝氏体铁素体，但随着分解温度的降低，原共析铁素体的数量减少，在873K时组织完全变成珠光体。2%Si-I%Mn0.59%C共晶钢在823K ~ 723K温度下形成无碳化物贝氏体铁素体，而在973K ~ 873K温度下形成完全珠光体。结果表明，贝氏体铁素体的形成机制与原共析铁素体的形成机制不同。(3)对Fe-2%Si-I%Mn-0.59%C合金中加工硬化奥氏体形成的贝氏体的透射电镜观察表明，贝氏体铁素体继承了加工硬化奥氏体的位错胞组织。另一方面，在由加工硬化奥氏体形成的珠光体中几乎没有观察到位错。以上结果表明，贝氏体铁素体的形成是由位移机制而不是扩散机制引起的。

The measurement of transformation kinetics and microstructural observation of bainite have been performed using Fe-9%Ni-C and Fe-2%Si-I%Mn-C alloys in order to clarify the formation mechanism of the ferrite component of bainite in steels. The main findings are as follows.(1) In all of three Fe-9%Ni-C alloys with different carbon contents from 0.1% to 0.3%, the TTT diagram consisted of two C-curves ; a higher one corresponded to pro-eutectoid ferrite + pearlite and a lower one corresponded to bainite. This clearly indicates that the bainite reaction has its own C -curve even in the alloys without strong carbide formers. Moereover, it was confirmed that the Bs temperature was almost independent on the carbon content and corresponded to the Ms temperature of the Fe-9%Ni binary alloy.(2) In the 2%Si-I%Mn-0.34%C hypoeutectoid steel, carbide-free bainitic ferrite formed predominantly at 823K, even though the amount of proeutectoid ferrite decreased with decreasing the decomposition temperature and the structure became fully pearlitic at 873K. Moreover, in the 2%Si-I%Mn0.59%C eutectoid steel, carbide-free bainitic ferrite formed at temperatures from 823K to 723K, whereas the structure was fully pearlitic at higher temperatures between 973K and 873K. These results indicate that the formation mechanism of bainitic ferrite is different from that of the proeutectoid ferrite.(3) TEM observation of bainite formed from work-hardened austenite in the Fe-2%Si-I%Mn-0.59%C alloy showed that the bainitic ferrite inherited the dislocation cell structure from the work-hardened austenite. On the other hand, dislocations were scarcely observed in pealite formed from the work-hardened austenite.It was concluded from the above results that the bainitic ferrite forms by the displacive mechanism rather than the diffusional one.