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Wear-resistant Fe-based coating was prepared by high-speed laser cladding on bainitic steel. The influence of laser scanning speed on microstructure, microhardness, and wear resistance of cladding coating was investigated. Scanning electron microscopy results showed that the coating was mainly composed of dendrite and interdendrite. Scanning electron microscope images were converted by binary algorithm to facilitate statistics of dendrite and interdendrite area. Statistical results indicate that by accelerating the scanning speed, the interdendrite eutectic structure increased. According to energy-dispersive X-ray spectroscopy and X-ray diffraction results, the interdendrite was enriched with elements Cr, Mo, and B, and main structures in the coating were α-Fe, γ-Fe, and M23C6. The hardness of the coating was much higher than that of the substrate. Elements diffused from coating to substrate, resulting in a transition zone of hardness. Moreover, with an increase in the scanning speed, the diffusion of elements at the coating–matrix interface decreased, while the hardness and wear resistance of the coating increased. Grain refinement and interdendrite (γ-Fe, M23C6) increasing due to high scanning speed were the major contributors to the increase in hardness and wear resistance.

In order to clarify the austenite reverse transformation of a 0.2C-3Mn-1.7Si steel during intercritical region with an ultra-slow heating rate of 0.05°C/s, DIL (dilatometer), SEM, XRD and EPMA were carried out. The experimental results indicated that the dilatometric cures exhibited the shape of two “valleys” and the growth of austenite from quenched martensite was composed of three consecutive sub-stages. This phenomenon is associated with Mn-rich and Mn-poor regions in martensite matrix. During the heat treatment,the austenite forms firstly in region enriched in Mn. However, with increasing temperature, the growth of γ is restrained owing to the concentration of Si. Upon the further process of heating, the free energy for the reverse transformation increases, and the residual Mn-depleted regions start to transform at a higher temperature.

There is a currently desirable demand for high strength steels with good ductility reduce the weight of steel parts for automobile and train applications. Retained austenite in steels can improve the toughness and plasticity. The austenite reverse transformation + quenching and partitioning (ART + Q&P) process was treated on a 0.2C-Mn-Si-Cr low alloyed steel, a multiphase microstructure composed of intercritical ferrite (IF), martensite, bainite and retained austenite (RA) can be obtained in the low carbon steel. Microstructures of the steel treated by different heat treatments were characterized by SEM and XRD. Results show that the formation of RA in low alloy steel depends on the following: (1) the enrichment of the carbon and manganese in the reversed austenite during the ART step; (2) the secondary enrichment of carbon in retained austenite during the following Q&P step. High fraction of RA (14 vol.%) was obtained through the two-step element enrichment treatment (ART + Q&P). Due to continuous TRIP effect of RA during the deformation, a good combination of strength and plasticity was achieved in our works: the product of strength and elongation is greater than 35 GPa•%, the tensile strength is more than 1230 MPa, the yield strength greater than 890 MPa, the total elongation is about 28.6%.

Retained austenite has an important effect on strength and toughness of 20Mn2SiCrMo bainitic rail steel. In this work, the stability of retained austenite and mechanical properties have been studied. The results show that impact toughness of experimental steel has close relationship with the stability of retained austenite. When tempered at lower than 350°C, retained austenite owns good thermal stability, corresponding to relatively high impact toughness.

A new type of high strength and low cost bainitic steel with ultra-low carbon content and high Si content has been developed on the basis of Mn-series air-cooling bainitic steels. The tensile properties of YS>690MPa and the impact toughness of AKV>60J at-40°C were obtained by controlling the processing parameters. This was attributed to the formation of the grain boundary allotriomorphic ferrite (FGBA) and the granular bainite (GB) with different shape of M/A islands. The high strength due to the inter-lath lamellar M/A islands or retained austenite companying with high dislocated bainitic ferrite laths of average 300nm width. The effect of microstructure on the impact crack initiation and propagation was studied. The results showed that crack initiation occurred in two different types of sites: at interphase boundaries of bainite ferrite (BF) and M/A islands, at grain boundaries. The FGBA and bainite ferrite (BF) both had blunting effect on microcrack tip to reduce the crack propagation path. Because of the presence of FGBA, the unit crack path was short, at less than 5μm. The blunting effect of BF could be enhanced by the M/A islands, which force the cracks change the propagation path and reduce the unit crack path to less than the size of bainite packets. The mechanism of low temperature microcrack origin of the ultra-low carbon bainitic (ULCB) steel with the microstructure of the FGBA and GB was also discussed.

Quenching-parttioning-tempering (Q-P-T) process were applied to a quenched carbid-free bainite/martensite CFB/M) multiphase steel 40Mn2Si2Cr and its effect on the mechanical properties was studied. The results showed that the partitioning time of Q-P-T treatment can be increased greatly to 90 min or even longer. The tensle strength, elongation and product of strength and elongation (PSE) of the experimental steel can achieve 1650 MPa, about 20% and over 33 GPa · %, respectively. The significant increase of partitioning time and excellent mechanical properties could be related to the CFB microstructure introduced in the quenching process during the Q-P-T treatment.

Quenching-partitioning-tempering (Q-P-T) process were applied to a quenched carbide-free bainite/martensite (CFB/M) multiphase steel 40Mn2Si2Cr and its effect on the mechanical properties was studied. The results showed that the partitioning time of Q-P-T treatment can be increased greatly to 90 min or even longer. The tensile strength, elongation and product of strength and elongation (PSE) of the experimental steel can achieve 1650 MPa, about 20% and over 33 GPa · %, respectively. The significant increase of partitioning time and excellent mechanical properties could be related to the CFB microstructure introduced in the quenching process during the Q-P-T treatment.

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The relationship between the toughness and silicon content of high strength Mn-Si-Cr series bainitic steels has been investigated. The results show that with increasing in silicon content, the onset temperature of the steel’s tempered martensite embrittlement (TME) rises; moreover, the minimum value of tested toughness decreases and the tempering temperature corresponding to the minimum value of toughness increases. This phenomenon results from the effect of silicon on the stability of filmy carbon-enriched retained austenite in carbide-free bainite/martensite (CFB/M) microstructure.