Effect of tempering temperature on microstructure properties of 1300 MPa ultra-high strength steel

Ultra high strength steel has high strength and good comprehensive mechanical properties, which is mainly used in large-scale mechanical engineering, bridges and other fields. It is generally treated with quenching and low-temperature tempering, tempering process is the decisive process that affects its final structure and properties. This article determines the optimal quenching temperature window of the test steel through austenite grain growth tests, elucidates the effect of microalloying element carbonitride on the original austenite grain size, and focuses on studying the influence of different tempering temperatures on the microstructure and properties of the test steel. Research has found that abnormal grain growth and mixed crystal phenomena occur when the experimental steel is quenched above 950 ℃. Within the quenching temperature range of 850～950 ℃, due to the carbon nitride of microalloying elements pinning grain boundaries and hindering grain boundary migration, the original austenite grains of the test steel did not show significant growth, with grain sizes only ranging from 11～13 μm and a wide quenching temperature window. After low-temperature tempering, the experimental steel forms a tempered martensitic structure. As the tempering temperature increases, the tensile strength and elongation monotonically decrease, while the yield strength and impact toughness first increase and then decrease. When tempered at 300 ℃, it has entered the brittle temperature range. When tempered at 180 ℃, the best mechanical properties can be obtained, which is yield strength of 1 316 MPa, tensile strength of 1 653 MPa, elongation at break of 13%, and impact toughness of 24.1 J at-40 ℃.