Effect of Processing Path on Microstructure and Mechanical Properties of a Low Carbon, Low Alloy Advanced High Strength Steel

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To address the issue of safety, reliability, and greenhouse gas emissions, there is a need for stronger, tougher, and yet cheaper material. In response to this requirement, several variants of advanced high strength steels (AHSS) have been developed with the latest focus on 3rd generation AHSS. In this work, microstructural evolution and mechanical properties of an automotive grade low carbon, low alloy steel under air-cooled, water-quenched, and tempered conditions have been studied. The microstructural characterization has been carried out using an optical microscope, scanning electron microscope, energy-dispersive spectroscope, and electron backscattered diffraction tools. Mechanical properties were determined by subjecting the steel samples to quasi-static uniaxial tensile testing at room-temperature. The microstructure of the steel in air-cooled condition was a mixture of ferrite, MA microconstituents, and cementite. In water-quenched condition, the microstructure mostly consisted of martensitic lath with a very small fraction of retained austenite and carbides (probably due to auto-tempering). In tempered state, the steel sample comprised of tempered martensite and (probably metastable) carbides. A significant increase in yield strength, YS, (1010 MPa) and ultimate tensile strength, UTS, (1109 MPa) was noted in water-quenched condition in comparison with the YS of 426 MPa and UTS of 612 MPa in air-cooled condition. The total elongation (TE) in air-cooled and water-quenched conditions was 33.5% and 21.7%, respectively. However, the majority of the plastic deformation in water-quenched state was non-uniform in nature. The mechanical properties and plastic deformation behavior of the steel in tempered condition was similar to that in water-quenched condition. Fractography revealed the scale and nature of fractographic features associated with the fractured surface to be dependent on the processing conditions. In this work, a strong influence of processing on microstructural evolution and mechanical properties were noted.

Electronic Thesis or Dissertation
Advanced high strength steel, MA Microconstituent, martensite, processing