The microstructure and properties of a medium‐carbon high‐strength bainitic steel treated by integrated technology of austempering and boriding treatments (boro‐austempering) are investigated by metallographic method, electrochemical impedance spectroscopy, and so on. The results show that the carbide‐free bainite of substrate and the borided layers on the surface are successfully obtained by integrated boro‐austempering treatment. The hardness of surface is up to 2.5–3.0 times that of substrate and the surface corrosion resistance is significantly enhanced by boro‐austempering treatment, proving that boro‐austempering treatment is an effective method for increasing corrosion resistance and hardness of high‐strength bainitic steels. The present study provides a new and effective processing technology for the production of medium‐carbon high‐strength steels with better corrosion resistance and higher hardness.

Niobium (Nb), a microalloy element, is purposefully added to improve the thermal stability of ferrite grains in a low-carbon ultrafine grain (UFG) steel. Results manifest the excellent thermal stability in the Nb-UFG steel with 0.028 wt% Nb addition by providing favorable kinetics and thermodynamic stabilization effects. The almost unchanged grain size and mechanical properties of Nb-UFG steels annealed for 45 and 180 min at 500 C are simultaneously obtained by the heterogeneous ferrite grains, NbC precipitates, and geometrically necessary dislocations (GNDs) mainly formed during tensile tests. The imperceptible change in size and volume fraction of NbC particles of the two Nb-UFG steels annealed at different time are attributed to the inherent thermal stability of NbC and main precipitation pattern of NbC particles from the α phase.

No abstract

No abstract

The work hardening behavior of α + θ UFG steel related to α + θ two phase microstructure is more complicated than that of single-phase materials. Very few studies have been conducted on the work hardening of α + θ UFG steels. Therefore, it is necessary to study the correlation between the work hardening and α + θ microstructure. In this study, the work hardening behavior of low-carbon ultrafine grain (UFG) steels with different grain size of ferrite and cementite particles, fabricated by rolling and annealing process, was studied. The α grain size was decreased to 132 ± 11 and 200 ± 19 nm in specimens cryorolled and annealed at 450 and 550 °C, which were smaller than that in specimen cold-rolled and annealed at 550 °C. However, the specimen cryorolled and annealed at 550 °C had a tensile strength of 740.3 MPa, which was lower than that in the other specimens. Results indicate that the work hardening is affected by ferrite and cementite in the UFG steels. The relatively coarse ferrite phase and the large number of fine intragranular cementite particles contribute to better work hardening. The intragranular cementite particles play a significant role in the improvement of work hardening, because the geometrically necessary dislocations are apt to form and store around intragranular cementite particles, while the intergranular cementite particles result in the decreased dislocation accumulation ability of ferrite and impair the strength of grain boundaries and work hardening of ferrite + cementite ultrafine grain steels.