The effects of gas nitriding temperature on the cross section morphology, element nitrogen distribution, and surface layer compositions of a cold rolled and pre-strained high manganese austenitic TWIP steel 25Mn-3Cr-3Al-0.3C-0.01N and the corresponding anti-corrosion ability have been studied. The results show that, depending on nitriding temperature, the distribution of element nitrogen and main phase compositions are significantly different in the nitriding layers. At a temperature lower than 500 °C, the main composition in the modified layer is S-phase and the nitrogen concentration linearly decreases from the surface to the center, while Fe₄N forms with S-phase and a step-like distribution of nitrogen content is present at nitriding temperature of 600 °C. Caused by the increasing of modified layer thickness and the formation of S-phase and Fe₄N, the surface hardness was obviously enhanced. Anodic polarization curves in 3.5 wt. % NaCl solution indicate that the nitrided processes have a tremendous modification effect on anti-corrosion ability. Moreover, the increase of (111) oriented grain, caused by the elevated nitriding temperature, has a positive effect on the corrosion resistance.

The microstructure, mechanical properties, and hydrogen permeation behavior of hot rolled enamel steel were investigated. Three coiling temperatures were adopted to gain different sizes of ferrite grain and TiC precipitates. The results show that a large number of interphase precipitates of nano-sized TiC can be obtained at coiling temperatures of 650 and 700 °C, while a few precipitates are found in experimental steel when coiling temperature is 600 °C. The yield strength and ultimate tensile strength decrease with increasing coiling temperature, while elongation increases. The experimental steel has the best resistance to fish-scaling at coiling temperature of 700 °C, due to the large quantities of nano-sized interphase precipitates of TiC.

In this research, a new type of spring steel with ultra-high strength and toughness was designed, and its mechanical properties and microstructure under different heat treatment processes were studied. The results show that the optimal heat treatment process for the steel is oil quenching at 890 °C for 40 min, followed by tempering at 400 °C for 1 h. Its mechanical properties have an optimal combination of 1865MPa tensile strength, a yield strength of 1662 MPa, an elongation of 11.5%, a cross-sectional shrinkage of 51.5%, and a Charpy impact energy of 43.7 J at room temperature. With increasing austenitizing temperature, the austenite grain size increases, the martensite lath becomes thicker, and the strength decreases. With increasing tempering temperature, the lath boundary of martensite becomes blurred, the strength decreases, and the plasticity improves. In addition, it was found that during tempering at higher temperature (450 °C), large particle inclusions and secondary cracks appeared in the fractured surface, and a large number of carbides precipitated, leading to the brittleness of tempered martensite.