Study Of Retained Austenite Regulation And Mechanical Properties In Low Carbon Low Alloy High Performance Steels

In the present work, the control of retained austenite in two different series low carbon low alloy steels of 0.23C-1.32Si-1.82Mn and 0.08C-2-3Mn-0.5Ni was studied for developing the third generation steels with high strength, high ductility, high toughness and good weldability. The stabilization mechanism of retained austenite during microstructure controlling and their effects on properties were focused on.By one-step isothermal holding at 300-500℃ after intercritical annealing at 760℃, a multi-phase microstructure of intercritical ferrite, bainite and retained austenite was obtained in a 0.23C-1.32Si-1.82Mn steel. Results showed that the volume fraction of retained austenite in the steel was related to the fraction of bainitic transformation during isothermal holding. When isothermal holding at a low temperature of 300℃, austenite transformed to carbide-free bainite continuously, then only little of retained austenite can be obtained. When isothermal holding at an intermediate temperature of 400℃, partial of austenite transformed to carbide-free bainite, and sufficient carbon rejected to residual austenite, then 8% of retained austenite can be obtained. When isothermal holding at a high temperature of 400℃, only little of austenite transformed to carbide-free bainite, there is insufficient carbon for stabilizing residual austenite, so these unstable residual austenite transformed to martensite furtherly and no retained austenite can be obtained. Moreover,6-9% of retained austenite can be reached by the two-step processing of fisrt quenching to 300℃ from 760℃ and second holding at 400℃ for 60-900s in the experimental steel.The control of retained austenite and properties in 0.08C-2-3Mn-0.5Ni low alloy steels were investigated by intercritical treatments. Results showed that about 13% retained austenite was obtained in low carbon high manganess (3%) steel by intercritcal annealing at 680℃ for 30 min, also, about 8% retained austenite was reached in low carbon low manganess (2%) steel by the two-step intercritical treatment. Therefore, excellent combination properties of 700 MPa grade yield strength, uniform elongation> 10%, total elongation ＞25% and good low temperature toughness were achieved in both high Mn and low Mn steels by one-step annealing and two-step intercritical treatment, respectively.Retained austenite obtained by the two-step intercritical treatment in a 0.08C- 2.4Mn-0.5Ni-Mo-Nb-Ti steel was carefully characterized to study its stabilization mechanism. Results showed that the stabilization of retained austenite during the two-step intercritical treatment benefited from the following there aspects:firstly, as alloying elements was preliminary enriched in reversed austenite during the first step of intercritical annealing, those reversed austenite transformed to martensite/bainite, these alloying elements enriched martensite/bainite had a lower Ac1 temperature, thus they acted as nuclei for the second reversion transformation during the second step of intercritical tempering. Secondly, the further highly enrichment of alloying elements (C ＞ 0.4%, Mn＞6%, Ni ＞ 0.6) in the second reversed austenite improved the stability of austenite furtherly. Thirdly, the second reversed austenite was well dispersed, film-like and ultra-fine (average size of about 300 nm), the nano-size effect played an important role in austenite stabilization.Results from in-situ EBSD observation after different stain stages showed retained austenite occurred strain-induced-martensite-transformation gradually during tensile deformation, resulting TRIP effect and increased work hardening rate. The work hardening behavior of retained austenite exhibited a three-stage process such that necking was delayed. This was the underlying reason for high uniform elongation in the low alloy steel. On the other hand, retained austenite was also helpful for toughness, and the improvement of toughness by retained austenite became more obvious when testing temperature was lower. Results from instrument impact test indicated that retained austenite contributed to enhance plasticity before crack initiation at low temperature, leading to the improvement of crack initiation energy, resulting in good low temperature toughness. High performance steel with yield strength greater than 500 MPa, uniform elongation higher than 20%, Charpy v-notch energy at-100℃ higher than 60 J was achieved by stable retained austenite.