低碳钢中晶界铁素体/原奥氏体界面对贝氏体转变的影响

INFLUENCE OF INTERFACE BETWEEN GRAIN BOUNDARY FERRITE AND PRIOR AUSTENITE ON BAINITE TRANSFORMATION IN A LOW CARBON STEEL

采用电子背散射衍射(EBSD)研究了低碳Fe-C-Mn-Si钢中晶界铁素体/原奥氏体界面对贝氏体形核的影响。通过两阶段等温热处理,获得了晶界铁素体和贝氏体的混合组织。结合金相观察和取向测量,发现晶界铁素体与贝氏铁素体之间的界面分为两种,一种界面不清晰,一种界面清晰。分析表明,在晶界铁素体/贝氏体界面不清晰一侧,晶界铁素体与原奥氏体保持取向关系,贝氏体在这类界面形核生长,且取向与晶界铁素体保持一致;在晶界铁素体/贝氏体界面清晰一侧,晶界铁素体与原奥氏体无取向关系,且贝氏体与晶界铁素体之间取向差较大。

The low carbon bainitic steels gain increasing attention due to their high strength, high toughness, and good weldability. To improve the toughness and weldability of this kind of steel the carbon concentration is usually deduced to below 0.06% （mass fraction）. As a result the hardenability of the steel is decreased and the ferrite usually becomes the first phase formed during the cooling process before the austenite transforms to the bainite. To decrease the nucleation activation barrier the grain boundary ferrite prefers to nucleate at the prior austenite grain boundaries, which are also potential nucleation sites for the bainite. The prior austenite grain boundaries are occupied by the ferrite, meanwhile ferrite/austenite interfaces are formed, which may influence the following nucleation of bainite. To understand the effect of grain boundary ferrite/prior austenite interface on the nucleation of bainite, a low carbon Fe-C-Mn-Si steel was investigated using optical microscope and electron back-scattering diffraction （EBSD）. The grain boundary ferrite and bainite were formed during the two-step isothermal holding. By combining metallographic observation with orientation measurement, two kinds of interfaces were found between grain boundary ferrite and bainitic ferrite： one is non-clear interface, and another is clear interface. The analyses show that grain boundary ferrite has nearly the K-S orientation relationship with the prior austenite on the non-clear interface side, at which bainite nucleates and grow with an orientation similar to the grain boundary ferrite, while the grain boundary ferrite has a random orientation relationship with the prior austenite on the clear interface side, and large misorientation exists between bainite and grain boundary ferrite.