The Evolution of Microstructure of an High Ni HSLA X100 Forged Steel Slab by Thermomechanical Controlled Processing

An HSLA X100 steel was studied. After casting and forging to slab condition, thermomechanical controlled process was employed to produce steel sheet. Heat treatment was then employed to control the microstructure and optimize mechanical properties. Austenitization was performed at 950 °C followed by different quenching media. Samples were tempered from 500 to 750 °C. Mechanical testing and microstructural analyses were performed by Optical Microscopy, Scanning and Transmission Electron Microscopes. Results showed a complex microstructure of bainite, ferrite, martensite, retained austenite, and various carbides. High temperature tempering removed retained austenite from martensite lath providing a relatively high amount of toughness. A uniform distribution of carbides was detected in the tempered situations. Retained austenite was present in the quenched only samples. Low temperature tempering removed part of this retained austenite. Increasing the tempering temperature decreased the tensile properties and increased the toughness. Quenched samples showed inferior mechanical properties compared to the tempered ones.