The low-carbon high-Mn austenitic steel microalloyed with titanium was investigated in this work. The steel was solution heat-treated at different temperatures in a range from 900 to 1200 °C. The aim was to receive a different grain size before the static tensile test performed at room temperature. The samples of different grain sizes showed the different strain hardening behavior and resulting mechanical properties. The size of grain diameter below 19 μm was stable up to 1000 °C. Above this temperature, the very enhanced grain growth took place with the grain diameter higher than 220 μm at 1200 °C. This huge grain size at the highest temperature resulted in the premature failure of the sample showing the lowest strength properties at the same time. Correlations between the grain size, the major strengthening mechanism, and fracture behavior were addressed. The relationships were assessed based on microstructural investigations and fractography tests performed for the deformed samples. The best combination of strength and ductility was found for the samples treated at 1000–1100 °C.
Tests were carried out on two high-Mn steels: 27Mn-4Si-2Al-Nb with Nb microaddition and 24Mn-3Si-1.5Al-Nb-Ti with Nb and Ti microadditions. High-manganese austenitic steels, due to their good strength and plastic properties belong to the AHSS (Advanced High-Strength Steel) group and are used in the automotive industry. The main difficulties faced during the casting of the steel and hot working are hot cracks, which can appear in the surface of the ingot. Cracks on the edges of the sheet after hot rolling are the reason for cutting the edges of the sheet and increasing production costs and material losses. The main reason for the formation of hot cracks is the decrease in metal ductility in the high-temperature brittleness range (HTBR). The width of the HTBR depends on mechanical properties and microstructural factors, i.e., non-metallic inclusions or intermetallic phases at austenite grain boundaries. In this paper, a hot tensile test was performed. The research was performed on the GLEEBLE 3800 thermomechanical simulator. This test allows us to determine the width of the high-temperature brittleness range (HTBR), the Nil Strength Temperature (NST), the Nil Ductility Temperature (NDT), and the Ductility Recovery Temperature (DRT). Hot ductility was determined from the value of the reduction in area R(A). The obtained results make it possible to determine the temperature of the beginning of hot working from the tested high-Mn steels. Fractographic research enabled us to define mechanisms of hot cracking. It was found that hot cracks form as a result of disruptions in the liquid film on crystals’ boundaries.
"Hot ductility tests were carried out on Fe-0.05C24Mn-3.5Si-1.6Al high-manganese austenitic steel with Nb and Ti microadditions at a concentration of 0.029% and 0.075%, respectively. Hot tensile tests were performed using Gleeble 3800 thermomechanical simulator on specimens with a diameter of 6 mm and 116.5 mm in length. Deformation was carried out in a temperature range from 1050 °C to 1200 °C with a strain rate of 2.5 · 10-3 s -1 and 5.0 · 10-3 s -1 . Hot ductility was determined on the basis on evaluation of reduction in area (RA). Higher values of stress on the work-hardening curves were observed during tension carried out at higher strain rate. An analysis of the form and course of curves obtained in the tensile test allows to state that, in the studied range of hot plastic deformation parameters, the decrease of strain hardening is caused by the process of dynamic recrystallization (DRX). The reduction in area in a temperature range from 1050 °C to 1200 °C decreases from 90% to approx. 58% for the strain rate of 2.5 · 10-3 s -1 and from 82% to 48% – for the strain rate of 5.0 · 10-3 s -1 ."
The aim of the work was to determine the effect of non-metallic inclusions on the hot ductility of two newly . For this purpose, a hot tensile test was carried out in the temperature range from 1050°C to 1200°C with a constant strain rate of 2.5⸱10 -3 s -1 . The tests were performed on the Gleeble 3800 thermomechanical simulator. Hot ductility of tested steels was defined by determining the reduction in area (% RA). Examined steels demonstrate diversified hot ductility. Clearly higher hot ductility was noted for the 24Mn-3Si-1.5Al-Ti steel. The reduction in area of this steel in the temperature range from 1050°C to 1200°C decreases from approx. 90% to about 58%, while the reduction in area of the 27Mn-4Si-2Al steel, in the same temperature range, decreases from approx. 66% to about 34%. The presence of single, regular-shaped AlN particles and complex MnS-AlN-type non-metallic inclusions was revealed in the 27Mn-4Si-2Al steel. Whereas fine (Ce, La, Nd)S-type sulphides, properly modified with rare earth elements, were identified in the 24Mn-3Si-1.5Al-Ti steel. The AlN-type inclusions and complex MnS-AlN-type inclusions were not revealed in the 24Mn-3Si-1.5Al-Ti steel. This is due to the presence of Ti microaddition, the concentration of which guaranteed binding of the whole nitrogen into stable TiN-type nitrides. Sulphides, disclosed in the 24Mn-3Si-1.5Al-Ti steel, are globular or slightly elongated in the direction of plastic deformation, as confirmed by a very low value of the elongation factor equal 1.48. This creates the opportunity to produce sheets of high strength and ductility and low anisotropy of mechanical properties.
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