Effects of ausforming on strength and toughness in low alloy banitic/martensitic steels

The effect of ausforming in the unrecrystallized austenite region on strength and toughness of low-alloy bainitic/martensitic steels was studied. Sample plates were prepared through two types of processing, interrupted direct quenching and direct quenching - tempering, under various reheating, hot rolling, interruption of accelerated cooling and tempering conditions. Microstructural observations for the samples by optical, scanning electron and transmission electron microscopy were carried out to understand microstructure - property relationships in the steels. In plates with interrupted direct quenching, the tensile strength was generally increased by ausforming, especially in the low interrupted temperatures, mainly due to strengthening of bainite or martensite by retained dislocations. The tensile strength rapidly dropped by increasing the interrupted temperature over 573K by recovering, and the effect of ausforming on strength was diminished. The toughness was generally improved by ausforming in plates with interrupted direct quenching. However, the extent of improvement was varied depending on the interrupted temperature. Ausforming reduced the facet size through microstructural refinement of martensite, such as block or packet, below 523K, and suppressed the temper embrittlement between 523 and 673K. Above 673K, marked improvement in toughness was obtained by ausforming, due to fine dispersed carbides, in addition to the fine microstructural characteristics of bainite. Microstructure-property relationships in the ausformed low-alloyed steels will be discussed in detail, comparing those in plates with interrupted direct quenching and direct quenching - tempering.