Effect of Low Temperature Controlled Rolling and Online Quenching Process on Microstructure and Toughness of NM500 Quenc

    
    Recent advancements in steel manufacturing technology have enabled the production of high-performance steels with higher strength, improved ductility and improved levels of toughness. One such material is NM500 Quenched and Tempered (Q&T) Steel which is a hot-rolled medium-alloy steel with a strength level of 500MPa. In this study, the effect of Low Temperature Controlled Rolling (LTCR) and Online Quenching (OQ) process on the microstructure and toughness of NM500 Q&T steel was investigated.
    
    The investigated material was subjected to LTCR and OQ process separately or combined. In the LTCR process, the thickness of the rolled sheet was decreased by passing it through a pair of smooth rolls kept at a temperature slightly higher than the recrystallisation temperature. This process was carried out at a decreasing rate of about 6 to 8 per cent per rolling pass until the desired sheet thickness was achieved. In the OQ process, cold compressed air was used to reduce the temperature of the sheet above the martensitic transformation temperature. It was then quenched and tempered to obtain the desired properties.
    
    Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the microstructures of the samples. The results indicated that the LTCR process led to finer microstructures which were characterized by increased dislocation density. In this process, a high density of dislocations helps to reduce the yield strength of the material without a significant loss of the tensile strength. On the other hand, the OQ process resulted in an increase in the volume fraction of ferrite in the microstructure and an overall enhancement in the toughness of the steel.
    
    The Vickers microhardness test was used to measure the hardness of the samples. The hardness of the samples after LTCR and OQ were found to be significantly lower than the hardness of the as-received material. This indicates that the LTCR and OQ processes resulted in reduced internal stresses and improved the ductility of the material.
    
    Tensile tests were conducted on the samples in order to evaluate their strength and ductility. The results showed that the combination of LTCR and OQ processes resulted in an increase in the yield strength and ultimate tensile strength of the material, while the ductility was improved. The toughness of the material was also increased. The through-thickness toughness was found to be significantly higher for the combined LTCR and OQ samples compared to the other samples.
    
    The effects of the LTCR and OQ process on the microstructure and toughness of NM500 Q&T steel have been successfully studied. It was found that the combination of the LTCR and OQ processes resulted in finer microstructures, improved yield strength and improved toughness of the steel, as compared to the as-received material. The results of this study have implications for the manufacturing of this steel in the automotive, engineering and construction industries.

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