The Microstructure Of An Ultra-high Strength Steel And Its Effect On Mechanical Properties

Based on the analysis of strengthening and toughening mechanism, a new type of medium carbon ultra-high strength steel was designed. It is of a good comprehensive performance:a tensile strength higher than2000MPa, a yield strength greater than1800MPa, an impact toughness up to47.5J/cm2. It can be widely used in areas such as high pressure die, bulletproof armor, mechanical structure materials and others. Main contents of this article are as following:1. Analyzed compositional character of super-strength steels produced at home and abroad, analyzed the roles of various alloy elements played in such steels, and designed the chemical composition of the steel to be used in present study.2. Used thermal expansion method to measure the critical transformation points and the CCT curve of the steel at L78RITA thermal expansion phase change machine.3. The tensile and yield strengths of the heat-treated steel are measured by the MTS universal tensile testing machine; the hardness was tested by HR-150A; the impact toughness was measured by JBW-500screen display impact machine.4. The effects of heat treatment on microstructure and mechanical properties were studied by metallurgical microscope、SEM、X-ray diffraction and mechanical testing.It was found that after directly and isothermal quenching, the tensile strength of the tested steel is more than2000MPa, the yield strength more than1800MPa, the highest impact toughness is11.75J/cm2; the tempering, however, has no strong influence on the strength of the steel, after250℃tempering, the tensile and the yield strength still remains the high level of2000MPa and1850MPa, respectively. On the other hand, the impact toughness was improved significantly to a level of47.5J/cm2by the tempering at250℃. After direct and isothermal quenching, the microstructure of the steel is consists of martensite, lower bainite and retained austenite. A small amount of Ti (CN) was also observed. The martensite observed is mostly the twin type, The small amount of retained austenite observed around the martensite is more than5.4%based on the XRD measurement. The best comprehensive mechanical properties of ultra high strength steel were obtained by the following treatment condition:austenitized at900℃, and then isothermal quenched at250℃for1h, and finally tempered at250℃for1h.