Effect of deformation on precipitation-strengthened Ti bearing high strength dual phase steels

To reduce fuel consumption, CO2 emissions and ensure passengers’ safety, there is an attempt to develop advanced high strength, lightweight, and affordable steels used for automobiles. Titanium is one of the most commonly used alloy addition in steels. The major effect of titanium is to form titanium carbide in steels. Titanium carbides precipitation formed in austenite state can hinder austenite grain boundary movement and thus refines prior austenite grain size. On the other hand, precipitation occurring during phase transformation was found to strengthen ferrite matrix by interphase precipitation mechanism, which will be discussed in part I. Also, the role of Ti element on transformation kinetics during austenite decomposition will be studied using time-temperature-transformation (TTT) diagram. The second part of the experiment discusses the effect of deformation on ferrite phase in titanium strengthened dual phase steel. Pancaked austenite can be obtained by hot deformation in non-crystalizing region, providing much more nucleation site for ferrite in two phase region, and result in grain refinement. 20% hot deformation can slightly increase ferrite hardness due to smaller sheet spacing of interphase precipitation, while 50% heavy hot deformation generating bimodal distributed ferrite grain with lower hardness. In the 50% deformation condition, when austenite/ferrite interface moves rapidly, which usually happens in small ferrite grains, interphase precipitation could not be able to happen. Instead, randomly distributed titanium carbides formed in a form of supersaturated carbide. Vickers hardness test shows that different morphology of ferrite would result in different hardness. By TEM observation, supersaturated random precipitation and interphase precipitation show different pairs of B-N OR under [100] zone of ferrite, 3 and 1 for each. Furthermore, strain induced precipitation occurs during hot deformation, consuming alloy element (titanium or carbon) for later precipitation. Statistic taken in SEM BSE images shows that precipitates containing titanium ranged from 100nm to 1µm don''t show much difference in area fraction between each hot deformation condition. By HRTEM observation, size of interphase precipitated and randomly precipitated titanium carbides are smaller than 3nm, so 3nm is a dividing line for statistic to differentiate the phase state in which titanium carbides precipitate. By statistic, size of precipitates ranged from 3~50nm drastically increase under 50% hot deformation. Using selected area diffraction, precipitates smaller than 50nm and larger than 3nm is observed without B-N orientation, which can match the statistic result. So lower Vickers hardness in 50% deformed samples can somewhat attribute to strain induced precipitates formed during hot deformation.