Low Alloy High Strength Steel Organizational Performance

A review was carried out on the research and application of Low-alloying Ultrahigh-Strength Steel, and new steels were designed on the basis of conventional Ultrahigh-Strength Steel. Phases at different temperature were calculated by Thermo-Calc. Optimization of alloying elements was carried out and new Ultrahigh-Strength Steel was refined by experimental melts according to thermodynamic calculation. Optimized heat treatment processes were obtained according to the strength and toughness of the steels.UHS1-Steel and UHS2-Steel were designed by Thermo-Calc thermodynamic calculation. there is only MC undissolved carbides in UHS1-Steel from 850℃to 1232℃. However, there are MC, M6C and MC_SHP undissolved carbides in UHS2-Steel, and MC_SHP dissolved at 864℃, M6C dissolved at 927℃, MC dissolved at 1245℃. Based on the effect of undissolved carbides on grain refining and minimizing embrittlement of undissolved carbides, the austenitizing temperature should be above 950℃.Strength of UHS1, oil quenched and tempered at 260℃, decrease lightly as the austenitizing temperature increases from 890℃to 1040℃, and Charpy impact toughness reached peak value at 1010℃. According to physical-chemical phase analysis, there are two types of undissolved carbides from 890℃to 950℃:MC and M6C. There is only MC carbides above 950℃, and the amount of MC decreases as the temperature is increased. The optimized quench temperature is 1010℃according to thermodynamic calculation, microstructure and mechanical properties. There are also some undissolved M6C carbides in UHS2-Steel at 950℃, but only MC carbides existed at 1000℃, which were almost the same as 950℃, this result was consistant with thermodynamic calculation.The study concluded:Thermo-Calc software is effective on transformation temperature and phase calculation, especially undissolved carbides, and it becomes the major method on optimization of alloying and heat treatment. Investigation of experimental steels showed that effect of quenching temperature is the key to amount, size and distribution of undissolved carbides,and to the toughness of ultrahigh strength steel.