Study on the mechanism of bainitic transformation in steels.

本文对Fe-9% Ni-C和Fe-2% Si-I%Mn-C两种合金进行了贝氏体相变动力学测定和显微组织观察，以阐明钢中贝氏体铁素体组元的形成机理。主要调查结果如下。(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，较高的一条对应于先共析铁素体+珠光体，较低的一条对应于贝氏体。这清楚地表明，即使在没有强碳化物形成体的合金中，贝氏体反应也有其自身的C曲线。此外，还证实了Bs温度与Fe-9%Ni二元合金的Ms温度基本一致，与碳含量无关。(2)在2% Si-I%Mn-0.34% C亚共析钢中，虽然先共析铁素体的数量随着分解温度的降低而减少，但在823 K时主要形成无碳化物的贝氏体铁素体，在873 K时组织完全转变为珠光体。在823 ~ 723 K温度范围内，2% Si-I% Mn 0. 59% C共析钢组织为无碳化物贝氏体铁素体，而在973 ~ 873 K温度范围内，组织为完全珠光体。结果表明，贝氏体铁素体的形成机制不同于先共析铁素体。(3)对Fe-2% Si-I%Mn-0.59%C合金中由加工硬化奥氏体形成的贝氏体的TEM观察表明，贝氏体铁素体继承了加工硬化奥氏体的位错胞结构。而在加工硬化奥氏体形成的珠光体中几乎没有观察到位错，由此推断贝氏体铁素体的形成机制是位移机制而不是扩散机制。

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.