B.Z. Bai

The mechanism of bainite transformation in steels

Abstract Sympathetic nucleation and ledgewise growth are competitive with each other throughout the bainite formation in steels. Many evidences are provided to support this mechanism as follows: ultra-fine structures of bainite and its surface relief, and carbide nucleation inside austenite at the interface of ferrite/austenite, etc.

Creation of Air-Cooled Mn Series Bainitic Steels

The development and mechanical performances of new type air-cooled Mn series bainitie steels including granular bainitie steels, FGBA/BG duplex steels, CFB/M duplex steels, medium carbon bainite/martensite steels, cast bainitie steels invented by the authors are summarized. The novel series of bainitie steels are alloyed with Mn, and several series bainitie duplex microstructures can be easily obtained under the condition of air cooling through unique composition design. The invented idea, the principle of alloying design, the strengthening mechanism, and the evolution of the microstructure of new type air-cooled Mn series bainitie steels are presented. Furthermore, the applications in different fields of these Mn series air-cooled bainitie steels with different strength level are also introduced. It is suggested that the significance of the development of the air-cooled Mn series bainitie steel can be summarized as follows: reducing costs of both raw materials and production; good combination of strength and toughness; self-hardening with high bainitie hardenability by air cooling from hot working without additional quenching-tempering treatment or quenching procedure; large savings in energy resources; and reduced environmental pollution.

Observation of bainite surface reliefs in Fe–C alloy by atomic force microscopy

Abstract Atomic force microscopy was employed to study the reliefs (undulations) at the pre-polished surface resulting from the transformation of austenite to bainite in an Fe–0.33C–3.0Mn–2.15Si–1.25Ni (wt.%) alloy. All the surface reliefs observed were shown to be tent-shaped, which was confirmed by the observation of corresponding microstructure of surface relief region after etching. The widths of surface reliefs are close to those of the bainite plates, indicating no significant plastic deformation of adjacent matrix. The measured surface relief angles agree well with the prediction of diffusion controlled ledgewise growth theory.

Study on Very High Cycle Fatigue Fracture of High Strength Steel with CFB/M Complex Microstructure

The influence of production conditions and Nb microalloy on the very high cycle fatigue (VHCF) behaviors of different Carbide-free bainite/martensite (CFB/M) steels was studied, which include steels produced in laboratory and steelworks. Results show the Nb-microalloy can strengthen the microstructure including the soft bainite plate and improve the VHCF property evidently. In addition, the high purity of steels can avoid inclusion induced crack initiation, but can not improve the VHCF property unless the microstructure possesses favorable uniformity. The critical inclusion size is suggested to be 10-15μm and the CFB/M complex microstructure may be less sensitive to inclusions than other high strength steels.

Vertical Short Crack Initiation in Medium Carbon Bainitic Steel Under Mild Tractive Rolling Contact

To improve the current grinding procedure of the back-up roll of CVC hot rolling mills so that the back-up roll service life can be extended, the crack initiation and propagation behavior of medium carbon bainitic back-up roll steel was investigated, a kind of asperity-scale, surface originated vertical short cracks occurred at 5 × 102 − 1 × 104 cycles. Theoretical analysis indicated that the maximum tensile stress occurring at the back edge of the contact of asperities keeps at above 1 347.97 MPa, and ratcheting and cyclic plastic deformation take place at such sites within 1 × 104 cycles. The early initiation of the vertical short cracks is caused by the asperity contact. According to the crack initiation mechanism, short crack behavior and preventive grinding strategy, steel consumption can be reduced considerably by decreasing the surface roughness and removing the asperity influenced surface thin layer at about 70%–80% of the surface distress life.

Refinement of CFB/M High Strength Steel Micro-Alloyed with Nb

A novel carbide free bainite/martensite (CFB/M) high strength steel of 1500MPa grade is micro-alloyed with Nb in order to refine the microstructure and improve the toughness. After the steel containing Nb was forged, coarse microstructure and strong transmissibility of structure were found. It is suggested that the large numbers of precipitates of Nb(CN) in the steel makes the structure transmissibility more serious. After solution treatment at 1200°C, both complete annealing twice and isothermal treatment at 600°C twice can eliminate the structure transmissibility and refine the structure. Investigation on refinement of cycle phase transformation shows that excellent effect of the grain refinement is obtained when the heating velocity exceeds some threshold. In addition, the temperature of austenitizing is of little influences on the size of refined austenite grain.