Abstract
Medium-carbon steel 42SiMn (0.4C-0.6Mn-2Si-0.03Nb) was used for a two-step heat treatment consisting of a soaking hold and an annealing hold at bainite transformation temperature. Various heating temperatures, cooling rates, and bainitic hold temperatures were applied to the steel to obtain microstructures typical for TRIP (Transformation Induced Plasticity) steels. TRIP steels utilize the positive effects of a multiphase microstructure with retained austenite, creating a good combination of strength and total elongation and an excellent deep-drawing ability. Typical microstructures consist of ferrite, bainite, and 10–15% of retained austenite. In this work, tensile strengths in the region of 887–1063 MPa were achieved with total elongation A5mm of 26–47%, and the final microstructures contained 4–16% of retained austenite. The most suitable microstructure and the best combination of high strength and total elongation were achieved for the processing with intercritical heating temperature of 850 °C and cooling at 30 °C/s to the bainitic hold of 400 °C. Very fine pearlite persisted in the microstructures, even after applying a cooling rate of 50 °C/s, however these small areas with extremely fine laths did not prevent the retention of up to 16% of retained austenite, and high total elongation A5mm above 40% was still reached for these microstructures.
Highlights
Increasing demands from the automotive industry have sustained the development of new steel grades with improved mechanical properties and lower amounts of alloying elements, in order to produce lighter vehicles at lower costs
To evaluate the effect of different heating temperatures, the same schedules with a cooling rate of 30 ◦ C/s and bainitic hold at 400 ◦ C were repeated for three soaking temperatures of 850, 900, and 1000 ◦ C
The carbon content of 42SiMn steel was at the upper boundary of the limit that was usually used for TRIP steels and its manganese content of 0.6% was relatively low
Summary
Increasing demands from the automotive industry have sustained the development of new steel grades with improved mechanical properties and lower amounts of alloying elements, in order to produce lighter vehicles at lower costs. Plasticity) steel, which generally possesses high strengths above 800 MPa when combined with good total elongation of around 30%. These TRIP steels are typically low to medium carbon low alloyed materials with 0.2–0.4% carbon and 1–2% of manganese and silicon [1,2,3,4,5,6]. Depending on the selected processing parameters, various amounts and Metals 2017, 7, 537; doi:10.3390/met7120537 www.mdpi.com/journal/metals
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