Alloy & process design of low carbon bainitic steel formachinery industry

In this study, alloy and process design of a low-cost bainitic steel grade that can be used as an alternative to high-cost quench and temper (QT) treated steels in the production of semi-finished long products has been carried out. It is aimed that the mechanical properties and weldability of the new low-cost bainitic steel grade designed for use in the machinary industry illustrate a similar or better performance compared to those of the QT treated steels. In this study, mechanical properties of 42CrMo4+QT were taken as the reference values. The chemical composition of low carbon bainitic (LCB) steel alloy was selected with the support of JMatPro – Computational Materials Engineering program used in material modeling simulations, calculations of the weldability and recrystallization temperature followed by the laboratory-scale production and finally microstructure and mechanical characterization studies carried out in our laboratory. Following theoretical calculations four different steel grades with different manganese and chromium content were produced in a 100 kg capacity vacuum induction melting (VIM) furnace. In order to perform a physical simulation of the standart rolling process (SRP) along with the thermomechanical rolling process (TMHP), the produced ingots were forged to 30 mm round pieces at two deformation temperature (Tdef) which are above (SRP: Tdef > TNR) and below (TMHP: Tdef < TNR) the austenite recrystallization temperature (TNR) respectively and subsequently they were left to cool down in still air. Charpy notch impact tests at -40°C, -20°C, 0°C and room temperature and tensile test were carried out on the mechanical test specimens machined from forged pieces. While the phase structure of the materials were investigated by optical microscope (OM), the thickness of the ferritic bainite plates and cementite were measured by scanning electron microscope (SEM), and the formation of thin bainite packages were observed. The crystallographic orientation and grain size distribution of austenite grains of the final steel grade were investigated for both SHP and TMHP conditions. After theoretical calculations, physical casting and shaping activities, the final steel grade was determined as 0.1C-3Mn-0.2Cr-1Si with microalloying elements (MAE). The mechanical properties of the final steel grade were determined as follows: yield strength (Rp0,2) 785 MPa, tensile strength (Rm) 1095 MPa and elongation 7.2% for standart rolling conditions and Rp0,2 879 MPa, Rm 1215 MPa and elongation 9.5% for TMHP. Charpy notch impact test results at -40°C, -20°C, 0°C and room temperature were measured as 18 J, 27 J, 30 J and 45 J for SHP condition and 37 J, 39 J, 39 J and 46 J for TMHP condition, respectively. Microstructural investigations revealed that the structure consisted of fine lower bainite and granular bainite with some amount of martensite phase. Consequently, in this study an economical novel bainitic steel grade has been designed that meets the mechanical properties of 42CrMo4+QT steels and has a lower cost in terms of not containing high cost alloying elements and not requiring additional heat treatment. This novel steel grade can be used instead of standart QT steel grades.