Thermo-Mechanical Treatment of 42sicr and 42mnsi Steels

Abstract

Two high strength low alloyed steels with 0.4%C, 0.6%Mn, 2%Si and either 1.3% of chromium or without chromium were used in this work to evaluate the effect of chromium on the final microstructure obtained by thermo-mechanical processing. Various heating temperatures, cooling rates and bainitic hold temperatures were tested. High strengths around 1700 MPa were achieved for the chromium alloyed steel, however total elongation reached only 9%. Chromiumfree steel turned out to be better suited for TRIP (transformation induced plasticity) processing. The relatively high strengths around 900 MPa were in this case accompanied by very high total elongations exceeding 30%. The final microstructure of chromium-free steel was also more typical for TRIP steel, as it consisted mainly of the mixture of bainite and polygonal ferrite.Keywords: TRIP steel, chromium, thermomechanical processing, scanning electron microscopy 1 Introduction TRIP (transformation induced plasticity) steels are advanced steel grades with high strength and enhanced total elongation and formability. Their good mechanical properties result from a complex microstructure containing ferritic matrix, carbide-free bainite and retained austenite [1,2]. Suitable microstructures with proper volume fractions, distributions and morphologies of individual phases and structural components are produced usually by thermo-mechanical treatment [1,3]. Ideal processing parameters vary according to the particular chemical composition of steel. The most conventional concept is based on C-Mn-Si alloying [1,[4][5]. Further TRIP steel variations were investigated in last decades, testing other alloying elements and their combinations. Silicon was fully or partially replaced by aluminium to improve galvanizing properties of thin sheets for automotive industry [6,7]. Micro alloying by Niobium was designed to refine the final microstructure and increase retained austenite content [8][9][10]. Chromium alloying of medium carbon TRIP steels has not been investigated very thoughtfully [11], probably due to its reputation of a carbide-forming element with a potential to increase the incubation time and decrease the transformation rate of isothermal bainite. The effect of chromium on higher hardenability is usually used in martensite-based steel, primarily to increase the strength [11][12][13]. However, chromium also retards pearlite growth rate and refines the microstructure by reducing the growth rate of prior austenite grain in steels [14][15] and improves corrosion resistance [16], which can be convenient for TRIP steels as well.

Archives of Metallurgy and Materials

“…A higher carbon content affects also the possibility of obtaining a higher amount of retained austenite. It is related to the fact that carbon is an austenite stabilizer [18][19][20][21][22]. The disadvantage of this solution is that with increasing C content there is a significant deterioration in the weldability of steel [22,23].…”

Use of Multi-Phase Trip Steel for Press-Hardening Technology

“…TRIP (TRansformation Induced Plasticity) steels, which possess a good combination of strength and ductility, fall into this group [8,9]. Their microstructures consist of ferrite, carbide-free bainite and retained austenite (RA) [8][9][10][11][12][13][14][15]. Upon cold deformation, RA transforms into high-carbon martensite which substantially contributes to work hardening [9,14].…”