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In order to research the effect of microalloying Nb, the microstructure and heat treatment distortion of case carburizing steels with and without Nb addition are compared. Results show that a uniform and fine prior austenite grain size was obtained for the steel with the addition of 0.03 wt.% Nb even after being carburized at 980 °C for 37 h, resulting in a very deep hardened layer of about 4 mm. Nb is an effective microalloying element to hinder austenite grain growth of gears during high-temperature carburizing. Theoretical calculation and experimental observation of NbC precipitates indicate that fine NbC precipitates have not evidently dissolved at 980 °C, and thus they can act as grain refiners due to pinning effect. Heat treatment distortion of Nb-added steel is much lower than that of the steel without Nb addition. It may be contributed to its fine and uniform grain size, which presumably influences stress during martensitic transformation.

Hot deformation behavior of an Fe–20Mn–19Cr–0.5C–0.6N high-nitrogen austenitic steel has been studied by isothermal compression tests in deformation temperature range of 800–1200 °C and strain rate range of 0.01–10 s−1. Results indicate that the Fe–20Mn–19Cr–0.5C–0.6N steel has high deformation resistance due to strong hindering effect on dislocation moving by nitrogen-induced lattice misfit. The twinning-induced plasticity effect is gradually suppressed with the increase in deformation temperature, and high-temperature plastic deformation mechanism by twinning is gradually replaced by dislocation planar slip. The deformation resistance is up to 343 MPa at deformation conditions of (1000 °C, 0.01 s−1), which is over 100 MPa higher than that in martensitic steel and 50 MPa higher than that in austenitic steel. Besides, value of deformation activation energy for the Fe–20Mn–19Cr–0.6N steel is up to 784 kJ mol−1. Power dissipation efficiency is lower than 0.13, while hot processing map exhibits a very wide range of working area. The optimum hot working process obtains at deformation temperature range of 950–1200 °C and strain rate range of 0.01–10 s−1, when deformation activation energy is less than 662.6 kJ mol−1, power dissipation efficiency exceeds 0.22, dynamic recrystallization fraction is over 46.1% and microstructures are without instable characteristics.

Rolling contact fatigue (RCF) properties of SAE 8620 steel after case carburizing have been investigated under two contact stresses of 4.0 and 5.5 GPa. Results show that the RCF life ranges from 2.5 × 106 to 3 × 107 cycles under the contact stress of 5.5 GPa, while it can be more than 1 × 108 cycles under the contact stress of 4.0 GPa. The rated fatigue life L10 (lives with the 10% failure) is also drastically shortened from 9.8 × 106 to 5.4 × 105 cycles when the contact stress is increased from 4.0 to 5.5 GPa. Theoretical calculations and fractographs show that the maximum shear stress and the contact area increase with increasing the contact stress, making RCF tend to occur earlier.

Hydrogen plays an important role in the formation of quench cracks of structural steels. To clarify hydrogen absorption and desorption during heat treatment of AISI 4140 steel, thermal desorption spectrometry (TDS) analysis was carried out for the specimens in the as-rolled, as-quenched, and quenched and tempered conditions. Results show that hydrogen content increased from 0. 127 × 10−6 in the as-rolled specimen to 0. 316 × 10−6 in the as-oil-quenched specimen. After tempering at 200 °C, the hydrogen content in the oil-quenched specimen decreased to 0. 155 × 10−6, and the peak temperature of hydrogen desorption increased from 200 to 360 °C. From the dependence of hydrogen content in the as-quenched specimens on austenitizing time, it can be deduced that hydrogen absorption occurs during austenitizing. The simulation of hydrogen absorption contributes to a better understanding on the distribution of hydrogen during the heat treatment in structural steels.

High temperature carburization is becoming more and more attractive because it can remarkably reduce processing time and increase productivity. However, the commonly used gear steels which are microalloyed by Al are not suitable for high temperature carburization due to abnormal grain coarsening. The gear steel 20CrMnTiNb, which is microalloyed with 0. 048% Nb and 0. 038% Ti, has been compared with the gear steel 20CrMn in terms of microstructure in the case of hardened layer and in the core after carburizing at 1000 °C for 4 h and mechanical properties after carburizing and pseudo-carburizing. The results indicate that the fine austenite grains exist in the carburized case of 20CrMnTiNb steel, while there is abnormal coarsening and duplex grain structure in the case and core of steel 20CrMn. The average prior austenite grain sizes are 19.5 and 34.2 μm for the steels 20CrMnTiNb and 20CrMn, respectively. In addition, the mechanical properties of 20CrMnTiNb steel are superior to those of 20CrMn steel. In particular, the HV hardness of the former is higher than that of the latter by about 40–70 in the range of less than 0.7 mm in depth. Therefore, the steel 20CrMnTiNb is suitable for high temperature carburization.

The mechanical properties dependence on the microstructure was reviewed and analyzed, and the ultrafine grained duplex microstructure of BCC matrix and large fractioned austenite was given as one of the optimum structures to develop the third generation steel with high strength and high ductility. The medium-Mn steels with different carbon contents processed by austenite reverted transformation (ART-annealing) were studied thoroughly to fabricate the ultrafine duplex steels with large fractioned austenite. The lamellar typed ultrafine structure, the granular typed ultrafine duplex structure and the corresponding mechanical properties of the medium-Mn steels processed by ART-annealing were demonstrated in this paper. It was revealed that the duplex structure with large fraction of austenite and ultrafine grain structure is capable of producing steels with excellent combination of strength and ductility, i.e., Rm×A about 30–50 GPa%, which is about two times of that of the conventional automobile steels and close to that of the TWIP steels. It was concluded that the ART-annealing of the medium-Mn steels would be at least one of the promising ways to fabricate the third generation automobile steels in the near future.

The microstructure of steels treated by QP the fresh martensite formed at the final quenching step looks like ‘blocky’ type phase, and the retained austenite is mainly located on the packet boundary and initial austenite grain boundary. The inhomogeneous microstructure causes the experimental optimum quenching temperature corresponding to maximum retained austenite fraction to be higher than the calculation based on CPE (constrained paraequilibrium) model. The product of tensile strength and total tensile elongation is 47. 5 GPa%, and tensile strength of 1760 MPa was obtained for the steel with carbon content of 0.51 wt%. The TRIP (transformation induced plasticity) effects of the large fractioned metastable austenite make a main contribution to the high ductility improvement, and the martensitic matrix provides high strength.

In order to locate a reasonable heating system, the austenite grain growth behavior of Nb microalloyed medium carbon steel has been experimentally studied at various austenitizing temperatures and for different holding times. It is indicated that austenite grain growth increases with increasing austenitizing temperatures and holding times. Particularly when the austenitizing temperature was above 1100 °C, austenite grains grew rapidly, and an abnormal austenite grain growth was observed. When the austenitizing temperature was lower than 1100 °C, austenite grain size and growth rate were small. The activation energy of grain growth in the tested steel is 397 679.5 J/mol. To ensure an absence of coarse grains in microstructures, the heating technology of the tested steel should be controlled for 1 h at 1100 °C. The relationships of austenite average grain size with soaking temperature and time of tested steel were obtained by mathematical calculation, and austenite average grain size was found to be in agreement with the measured size for different holding times.

The microstructural evolution of Fe–0.2C–5Mn steel during intercritical annealing with holding time for up to 144 hours was examined by TEM and STEM. It was demonstrated by TEM that the martensite lath structure gradually transformed into a lamellar ferrite and austenite duplex structure. The partitioning of manganese from ferrite to austenite was found by STEM. Typical Kurdjumov-Sachs orientation relationship between austenite lath and ferrite lath was observed by electron back scattered diffraction (EBSD). Based on the analysis of the austenite lath thckening behavior, it was proposed that the Mn-partitioning in austenite dominated the microstructure evolution of the ultrafine lamellar ferrite and austenite duplex structure during annealing process.

The influence of direct quenching (DQ) on microstructure and mechanical properties of 0.19C-1.7Si-1.0 Mn-0.05Nb steel was studied. The microstructure and mechanical properties of reheat quenched and tempered (RQ&T) steel plate were compared with those of direct quenched and tempered (DQ&T) steel plates which were hot rolled at different finish rolling temperatures (1173 K and 1123 K), i.e., recrystallization-controlled-rolled direct-quenched (RCR&DQ) and controlled-rolled direct-quenched (CR&DQ), respectively. The strengths generally increased in the following order: RQ&T

We examine the hydrogen embrittlement susceptibility of a high-strength AISI 4135 steel by means of a slow strain-rate test (SSRT) using notched round bar specimens. Hydrogen was introduced into the specimens by electrochemical charging and its content was measured by thermal desorption spectrometry (TDS). It was found that the maximum tensile stress decreased in a power law manner with increasing diffusible hydrogen content. Finite element method (FEM) calculations demonstrated that the peak value of the maximum principal stress and the peak value of the locally accumulated hydrogen concentration at the maximum tensile stress were in good agreement with one power law relationship for the specimens with different stress concentration factors.

The effect of cold deformation on microstructure and mechanical properties of a Fe–20Mn–19Cr–0.5C–0.6N high nitrogen austenitic steel has been studied. Microstructural observations show that the low cold deformation is fully dominated by the dislocation structures, namely, pile-ups, Taylor lattice, and dislocation cells. With further cold deformation, the effect of twinning induced plasticity and grains fragmentation play an important role during cold deformation process. At critical cold deformation reduction range of 26–36%, the microstructural transitions by dislocation planar slip and multiplication gradually transits to twinning. For mechanical properties, with the increase of cold deformation reduction, the yield strength increases from 0.52 to 1.60 GPa, being increased by three times. The flow stress–strain model of the solution-treated specimen can be described as σ = 1974 ϵ 0.402 + e − 0.3625 ⋅ e − 33.114 ϵ . In addition, the strength-elongation product and the cold deformation reduction present a linear relationship with a correlation coefficient of −0.27 to estimate the variation rate of the strength-elongation product during cold deformation.

In recent years, economic and environmental considerations have increased the need to safely extend the service life of components and structures beyond their original design life. It well known that fatigue and delayed fracture are the two important mechanisms for the failure of steel components and structures in service. Based on our systematic studies of the fatigue failure and delayed fracture behaviours of high strength steels in the last 10 years, we proposed a new kind of concept to improve both fatigue failure resistance and delayed fracture resistance of high strength steels, which comprises the combination of inclusion modification, structure controlling, hydrogen trap controlling and grain boundary strengthening, and it is called IST & GST technology in this paper. Firstly, basic considerations for the improvement of service life of high strength steels are introduced. Secondly, methods such as inclusion modification, structure controlling and hydrogen trapping controlling are discussed for improve the fatigue failure and delayed fracture resistance of high strength steels. Finally, based on IST & GST technology, examples of development of long life high strength steels such as a newly developed 2,000 MPa grade high strength spring steel with excellent fatigue failure resistance and 1,500 MPa grade high strength bolt steel with superior delayed fracture resistance are introduced.

In this study, research and development on the 3rd generation automobile steel, with targets of R m × A no less than 30 GPa% at R m level of 1–1.5 GPa, was carried out to fabricate high strength and high ductility steel by two methodologies, one is the medium manganese steels fabricated by intercritical annealing through austenite reverted transformation (ART-annealing) and another is the conventional carbon steels processed by quenching and partitioning(Q&P). The ultrafine grain sized austenite-ferrite duplex microstructure and the tempered martensite-fresh martensite-austenite multiphase microstructure were demonstrated based on the microstructure characterization in ART-annealed medium manganese steels and Q&P processed conventional carbon steels. In both heat treatment conditions, substantially enhanced ductility (30–40%) at ultrahigh tensile strength level (1–1.5 GPa) was obtained, which results in a significant improvement of the product of tensile strength to total elongation about 30–40 GPa%. Analysis on the work hardening behaviors and the relationship between microstructures and mechanical properties of the studied steels indicates that the greatly improved ductility results from the aid of the phase transformation induced plasticity (TRIP effects) and the ultrahigh strength stems from the hard matrix, such as the ultrafine grained duplex structure in ART-annealed steels and the martensite matrix in Q&P processed carbon steels. It is interesting to find that a strong dependence of the product of tensile strength to total elongation on the fraction of retained austenite phase of steels produced by both ART-annealing and Q&P processing techniques. It was proved that both ART-annealing and Q&P processes can be applied to fabricate the third generation automobile sheet steels offering ultrahigh strength and high ductility.