Study On The Low-Carbon Martensite Of Non-Quenched And Tempered Steels With High Strength

Non-quenched and tempered steels have been widely used in industry because of their advantages of saving energy, simplifying processes, decreasing distortion and cracks during quenching and so on. However, the conventional pearlitic and bainitic non-quenched-tempered steels are difficult to meet the needs for both high strength and good toughness. Therefore, low-carbon martensitic non-quenched and tempered steels were investigated for this purpose.In this paper some non-quenched and tempered martensitic steels were prepared by using Mn, Si and Cr as main alloying elements under low carbon of 0.10⁰.25wt%, and their content was 2.0³.5wt%Mn, 1.0².0wt%Si and 0.5wt%Cr. The lath martensite, between which there exist some stable retained austenite films, was obtained through air cooling or quenching in water after forging. The steels containing 0.16⁰.25wt%C, 2.0³.5wt%Mn, 1.5².0wt%Si and 0.5wt%Cr obtained martensite microstructure through air cooling after forging. Moreover, the steel with 0.2C-3.5Mn-1.6Si — 0.5Cr steel showed high strength of σ_b= 1550 1650MPa and good plasticity of δ₅ = 8.8—12.6% after forging. On the other hand, the steels containing 0.10⁰.16wt%C, 2.5³.5wt%Mn, 1.5².0wt%Si and 0.5wt%Cr may also obtain the martensite microstructure through quenching in water after forging, and the steels also presented high strength σ b>1300MPa and good plasticity δ₅>H %. Measurements on mechanical properties revealed that they were almost unchanged at the temperature less than 400℃ under the action of Si, which showed a good resistance to tempering.Moreover, measurements on retained austenite showed that the non-quenched and tempered martensitic steels contained the retained austenite less than 9%. The content of retained austenite were increased with an increase of the carbon content in the steel. Therefore,the low-carbon martensite of non-quenched and tempered steels could have both high strength and good plasticity because the retained austenite had high stability due to lath martensites around and high Si content.