Cunyu Wang

Application of Quenching and Partitioning to Improve Ductility of Ultrahigh Strength Low Alloy Steel

In this study Quenching and Partitioning (Q&P) as proposed by Speer was applied to improve the ductility of C-Mn high strength Low Alloy steel (HSLAs). Microstructural observations revealed a multiphase microstructure including first martensite, fresh martensite and retained austenite in the Q&P processed steel. During tensile process, the austenite volume fraction gradually decreased with strain increasing, suggesting the phase transformation induced plasticity for the Q&P processed steel. Ultrahigh strength about 1300-1800MPa and tensile elongation about 20% were obtained after Q&P processing at specific conditions, which is significant higher than that of ~10% of conventional martensitic steel. The the product of tensile strength to total elongation increased from 25 to 35GPa% with increasing carbon content in studied steel. This improved mechanical properties were related to the ductility contribution from TRIP effects of the retained austenite and strength contribution from the hard martensitic matrix. At last it was turned out that the Q&P process is a promising way to produce ultrahigh strength steel with relative high ductility under tailored heat treatment conditions for different micro-alloyed carbon steel.

Ultrafine Grained Duplex Structure Developed by ART-annealing in Cold Rolled Medium-Mn Steels

The microstructural evolutons of the cold rolled Fe-0. 1C-5Mn steel during intercrical annealing were examined using combined advanced techniques. It was demonstrated that intercrtical annealing results in an ultrafine granular ferrite and austenite duplex structure in cold rolled 0.1C-5Mn steel. The strong partitioning of manganese and carbon elements from ferrite to austenite was found during intercritical annealing by scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD). Strong effects of boundary characters on the austenite formation were indicated by austenite fast nucleation and growth in the high angle boundaries but sluggish nucleation and growth in the low angle boundaries. The ultrafine grained duplex structure in 0.1C-5Mn was resulted from the the sluggish Mn-diffusion and the extra high Gibbs free energy of ferrite phase. Based on the analysis of the microstructure evolution, it was pointed out that the intercritical annealing of the medium-Mn steels could be applied to fabricate an ultrafine duplex grained microstructure, which would be a promising approach to develop the 3rd generation austomobile steels with excellent combination of strength and ductility.

The 3rd Generation Automobile Sheet Steels Presenting with Ultrahigh Strength and High Ductility

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.

Work Hardening Behavior and Stability of Retained Austenite for Quenched and Partitioned Steels

Both microstructure and mechanical properties of low alloy steels treated by quenching and partitioning (Q&P) process were examined. The mixed microstructure of martensite and large-fractioned retained austenite (about 27. 3%) was characterized and analyzed, excellent combinations of total elongation of 19% and tensile strength of 1835 MPa were obtained, and three-stage work hardening behavior was demonstrated during tensile test. The enhanced mechanical properties and work hardening behavior were explained based on the transformation-induced plasticity effect of large-fractioned austenite.

Dislocation Pipe Diffusion of Mn during Annealing of 5Mn Steel

The dislocation pipe diffusion of Mn during annealing of 5Mn steel was experimentally investigated using transmission electron microscopy (TEM). Many dislocations existed inside the ferrite and terminated at the α/γ interface of the sample after intercritical annealing at 650 °C for 1 min. Line scans of Mn distribution demonstrated a high Mn concentration in austenite and Mn enrichment at dislocations, indicating that the dislocation pipe diffusion of Mn during intercritical annealing occurred in addition to the γ/α interface diffusion. In-situ TEM observations at 500 °C revealed that due to Ostwald ripening, large cementite precipitates grew while small cementite precipitates dissolved via Mn diffusion along the dislocations between them.

Influences of Austenization Temperature and Annealing Time on Duplex Ultrafine Microstructure and Mechanical Properties of Medium Mn Steel

A duplex ultrafine microstructure in a medium manganese steel (0.2C-5Mn) was produced by austenite reverted transformation annealing (ART-annealing). The microstructural evolution during annealing was examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Based on the microstructure examination, it was found that some M3C type carbides appeared in the martensitic matrix at the beginning of the ART-annealing. But with further increasing annealing time, these carbides would be dissolved and finally disappeared. Meanwhile, the austenite lath was developed in the ART-annealing process and the volume fraction of austenite increased with the increase of the annealing time, which resulted in a duplex microstructure consisting of ultrafine-grained ferrite and large fraction of reverted austenite after long time annealing. The mechanical property examinations by uniaxial tensile tests showed that ART-annealing (6 h, 650 °C) resulted in a superhigh product of strength to elongation up to 42 GPa · %.

In-situ microstructural evolutions of 5Mn steel at elevated temperature in a transmission electron microscope

The microstructural evolutions of 5Mn steel during various heat treatments have been investigated by in-situ transmission electron microscopy (TEM). The specimen of 5Mn steel was prepared using focused ion beam (FIB) milling, which allowed the selection of specific morphology of interest prior to the in-situ observation. The complete austenization at 800 °C was verified at the atomic scale by minimizing thermal expansion and sample drift in a heating holder based on micro-electro-mechanical-systems. During annealing at 650 °C, the formation of reverted austenite was dynamically observed, while the morphologies of austenite laths of 5Mn steel after in-situ heating were quite similar to that after ex-situ intercritical annealing. During annealing at 500 °C, the morphological evolution of cementite and associated Mn diffusion were investigated. It was demonstrated that a combination of FIB sampling and high temperature in-situ TEM enables us to probe the morphological evolution and elemental diffusion of specific areas of interest in steel at high spatial resolution.

Microstructure and mechanical properties of 20Si2CrNi3MoV steel treated by HDQP process

A combined process of hot-deformation plus two-step quenching and partitioning (HDQP) treatment was applied to a low-carbon 20Si2CrNi3MoV steel, and transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Vickers hardness and tension test were used to characterize the microstructure and mechanical properties. More stable retained austenite due to fine microstructures and typical curved micromorphology is obtained, and the newly-treated steel obtains more retained austenite because of the effect of hot deformation. The retained austenite fraction increases and then decreases with the increasing quenching temperature from 200 to 350 °C. The maximum retained austenite fraction (18.3%) and elongation (15%) are obtained to enhance the ductility.