EFFECTS OF ROLLING PROCESS PARAMETERS ON THE MECHANICAL AND MICROSTRUCTURAL PROPERTIES OF HOT –ROLLED ST60Mn STEEL

Abstract

Hot-rolled St60Mn steel is commonly used in the construction of bridges and houses. Reduced quality characterising hot-rolled steel has become a major research concern. Previous studies focused on the effects of chemical composition on the mechanical and microstructural properties of hot-rolled St60Mn steel. However, there is still the limitation of lower yield strength and coarsegrain size. Literature is scarce on the effects of rolling process parameters on the mechanical and microstructural properties of St60Mn steel. This study was designed to investigate the effects of Percentage Total Deformation (PTD), Rolling Strain Rate (RSR) and Finish Rolling Temperature (FRT) on the mechanical and microstructural properties of hot-rolled St60Mn steel. Three hundred and twenty-four St60Mn steel billets of dimension 120 x 120 x 12000 mm were used for the experiment. These were charged into a reheating furnace at 1230ºC and hot-rolled to standard rebar sizes at PTD of 96.0, 96.5, 97.0, 97.5, 98.0, 98.5, 99.0 and 99.5; RSR of 4000, 4500, 5000, 5500, 6000, 6500,7000 ,7500 s-1 and FRT of 915, 917, 919, 921, 923, 925, 927 and 929ºC. Samples were prepared according to BS 4449. Mechanical tests were performed on the rolled samples at 27ºC to obtain the Tensile Strength (TS), Yield Strength (YS) and toughness, which were compared to those in literature. The samples were sectioned using machine fitted with abrasives and the resulting samples were mounted with bakelite and acrylic. Thereafter , these were ground with silicon carbides and aluminium oxide, polished and etched with 2.0% nittal for metallographic examination using optical microscopy. Their microstructural grain sizes were obtained and further analysed via the edge detection method. Optimum parameters were determined using response surface methodology. Data were analysed using ANOVA at α0.05. The TS (MPa), YS (MPa) and toughness (J/mm2) at PTD ranged from 611.2 (96.0%) to 620 (99.5%), 432 (96.0%) to 432.2 (99.5%) and 0.48 (96.0%) to 0.41 (99.5%), respectively. For RSR, the samples ranged from 611.3 (4000 s-1) to 697.2 (7500 s-1), 432.1 (4000 s-1) to 455.8 (7500 s-1) and 0.48 (4000 s-1) to 0.42 (7500 s-1), respectively. At FRT, the samples also ranged from 611 (915oC) to 605 (929oC), 432 (915oC) to 428 (929oC) and 0.48 (915oC) to 0.59 (929oC), respectively; while that existing in literature were 500, 420 and 0.40,respectively. The microstructural grain sizes (μm) ranged from 48 (96.0%) to 41 (99.5% PTD), 48 (4000 s-1) to 40 (7500 s-1 RSR), and from 47 (915oC) to 55 (929oC FRT), while that in literature was 56. Higher TS and YS were observed as PTD and RSR increased, but decreased with increasing FRT. However, increasing PTD, RSR and FRT improved the toughness and grain sizes of the hot-rolled products. Optimum RSR, FRT and PTD were obtained as 7000s-1,920.30°C and 99%, respectively. The effects of PTD, RSR and FRT on mechanical and microstructural properties were significant when compared with literature values. Mechanical and microstructural properties of hot-rolled St60Mn steel were improved using higher Percentage Total Deformation and Rolling Strain Rate, with lower Finish Rolling Temperature.