Abstract
An experimental investigation was conducted using laboratory-processed low carbon Mn-Mo-Nb-Ti-B dual phase steel with additions of 0.2Cr or 0.6%Cr. This study investigates the effect of Cr additions on ferrite recrystallization and austenite formation during continuous annealing in the intercritical temperature range. It was found that moderate additions of Cr (0.2%) accelerates the ferrite recrystallization (nucleation and growth) resulting in coarse ferrite grains. An increase in Cr content up to 0.6% also accelerates the ferrite recrystallization which in addition to the presence of Cr-rich carbides facilitates the austenitization predominantly through the nucleation process resulted in finer austenite grains. Thereby, the higher Cr content increases the volume of homogenized and refined martensite in the final quenched structure.
Highlights
Increased emphasis on crash worthiness in combination with weight reduction has prompted automotive manufacturers to search for steels with much better mechanical properties, what pushes the steels producers in the direction of the development of the Advanced High Strength Steels (AHSS) of low cost
Hereafter discussion addressing ferrite recrystallization and phase transformation will be developed based on the microstructures of steels presenting (Fig. 3 to 10), related to steels containing different concentration of Cr annealed at several different temperatures
The comparison of microstructures of steels annealed at 720oC (Fig. 3 a - e) with cold rolled microstructures presented in Fig. 1 shows that carbon containing constituents such as martensite and pearlite are mostly dissolved during heating treatment but carbides dissolve differently depending on chromium concentrations
Summary
Increased emphasis on crash worthiness in combination with weight reduction has prompted automotive manufacturers to search for steels with much better mechanical properties, what pushes the steels producers in the direction of the development of the Advanced High Strength Steels (AHSS) of low cost. It has been shown that the addition of alloy elements such as Si, Mo and Cr in high strength steels is very effective in improving the strength ductility balance 1-3, through microstructure optimization. The detailed mechanism of these alloying elements, Cr effect in particular on final structure, is not fully understood. As the final microstructure is strongly dependent on the austenite formation during annealing the evaluation of austenite formation affected by Cr addition are of fundamental importance 4-8. The austenite formation depends on several factors such as an initial microstructure, degree of ferrite recrystallization, carbon distribution and homogeneity of individual phases. 8,11,12 In this way, the present study attempts to investigate the effect of Cr addition on the final microstructure of Mn-Mo-Nb-Ti-B dual-phase A complex interaction between recrystallization, grain growth, and phase transformation occurs . 9,10 The degree of such interplay depends, first of all, on the chemical composition of the steel (e.g., microalloying additions) and processing parameters . 8,11,12 In this way, the present study attempts to investigate the effect of Cr addition on the final microstructure of Mn-Mo-Nb-Ti-B dual-phase
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