Microstructure and Mechanical Behavior of Micro-Alloyed Steel

Abstract

Based on perfect candidacy of micro-alloyed steel to be classified, after being treated, as one of the Advanced High Strength Steel (AHSS) grades, to effectively fulfill the growing need in industry for furtherly enhanced combination of mechanical properties, the current study has been made to evaluate the viability of applying the significantly recommended (results-wise) novel one step Quenching and Partitioning heat treatment on the investigated low carbon Nb-based micro-alloyed steel alloy, and to determine the extent of its efficiency in achieving the main target of attaining the enhanced set of properties as a third-generation Q&P steel grade. The investigated 0.2 C-1.62 Mn- 0.69Si- 1.47 Al- 0.021 Nb, (Wt.%) micro-alloyed steel alloy, in the highly-suited form of 2mm thickness sheets, has been submitted to different cycles of the 1-step Q&P heat treatment scheme. Four different temperatures (175°C, 200°C, 225°C, and 275°C) were used as quenching and partitioning temperatures, along with eight different soaking periods (30, 100, 200, 300, 500, 1000, 1200, and 1800 Seconds) as partitioning times at each temperature. Means of metallographic examination, mechanical testing, scanning electron microscopy, and work hardening behavior analysis, were used to analyze the results. The findings were discussed in the light of different involved factors such as the effect of Partitioning temperature and time, alloying effects, Niobium alloying, Retained Austenite stability, microstructure-property relationship, partitioning kinetics, along with other factors. By taking the combination of the attained strength and ductility, besides other mechanical properties, as a reference, the applied conditions of 225°C as Q&P temperature and 500 Seconds as partitioning time at that temperature, attained the optimum combination.