Abstract
Hot plastic working behavior and microstructure evolution were investigated during a production process of four medium-Mn steels, which differed in Mn (3 and 5%) and Nb contents. The production process started with casting, followed by hot forging, rough hot-rolling and concluded with final thermomechanical processing, which was performed to obtain multiphase bainite-based alloys with some fractions of retained austenite. The rough rolling was composed of four passes with total true strain of 0.99 and finishing rolling temperature of 850 °C, whereas thermomechanical processing contained five passes and total true strain of 0.95 at a finishing rolling temperature of 750 °C. During the process, the force parameters were recorded, which showed that the rolling forces for steels containing 3% Mn are higher compared to the 5% Mn alloys. There was no significant influence of Nb on the rolling parameters. The produced as-cast microstructures were composed of dendritic bainitic-martensitic phases. A positive effect of Nb micro-addition on a refinement of the as-cast structure was noticed. The thermomechanical processed steels showed fine multiphase microstructures with some fractions of retained austenite, the fraction of which depended on the Mn content in steel. The steels containing 3% Mn generated higher forces both during rough and thermomechanical rolling, which is related to slower recrystallization softening in these alloys compared to the steels containing 5% Mn.
Highlights
The modern automotive industry uses mainly AHSS (Advanced High Strength Steels) steels for car body constructions [1,2]
AHSS steels are divided into three generations: the first generation consists of low-alloy multi-phase steels, such as TRIP (Transformation Induced Plasticity), DP etc. [4,5] They guarantee good properties at a relatively low price of the material and its production
A major part of all research is focused on heat treatment parameters’ optimalizations [13], studies of its various variants [14,15], and explaining the complex mechanisms occurring in these steels [16,17,18]. Many studies on these steels are the result of testing the material from the beginning to the end conducted on a laboratory scale [16,19]. This type of research is very efficient in terms of the abovementioned investigations, but the results usually differ from those obtained during final industrial production processes
Summary
The modern automotive industry uses mainly AHSS (Advanced High Strength Steels) steels for car body constructions [1,2]. Many studies on these steels are the result of testing the material from the beginning (melting and casting) to the end (heat treatment and property testing) conducted on a laboratory scale [16,19] This type of research is very efficient in terms of the abovementioned investigations, but the results usually differ from those obtained during final industrial production processes. Due to the shortage of data from the semi-industrial producing process of MMn steels, a detailed analysis concerning the force parameters of the hot rolling and microstructure evolution of MMn steels with the use of semi-industrial equipment was conducted These results provide a broader view of the behavior of MMn steels under conditions similar to industrial production, which may facilitate the adaptation of rolling lines to their production, and design pilot tests on such lines. This is a necessary step in bringing any new material to the market
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