Preparation And Mechanical Properties Of Dual Phase Steel With Ultrafine Ferrite And Low Temperature Bainite

The cold-rolled medium-carbon high-silicon alloy steel with tempered troostite microstructure was intercritically annealed in ?+? two-phase region and quickly immerged into salt bath with the temperature slightly higher than the martensite starting point for austempering and air cooled to room temperature. Eventually, the ultrafine ferrite and low temperature bainite was obtained, which leads to high strength and good ductility.The steel was subjected to oil quenching after austenitized at 900 oC for 30 min and tempering at 500 oC for 60 min, followed by air cooling to obtain tempered troostite. After that, the steel was cold rolled with a reduction of 40% and intercritically annealed at 760?790 oC(?+? region) to obtain ultrafine ferrite and austenite, and then isothermally transformed in a salt bath furnace until the bainite transformation accomplished. The temperatures of the salt were about 10 oC, 25 oC, 40 oC above the MS point. Mechanical properties such as hardness, tensile and subsized U–notch impact energy of the treated samples were measured. The phase composition, microstructures and fracture morphology were analyzed by optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction.Results show that the microstructures of the steel after thermomechanical approach was consist of ultrafine ferrite and low temperature bainite. The average size of ferrite grains was about 0.5–2 μm. The reasons why the ferrite was refined are:(i) the initial microstructure was refined by martensite transformation;(ii) ferrite grains were refined through ferrite recrystallization during intercritical annealing. During austempering, the austenite was transformed into low temperature bainite, i.e. caibide-free nanostructured bainite, which is mainly because the higher carbon content in austenite(much higher than that in the steel) and higher Si content. With the austempered temperature increasing, the yield strength and hardness are decreased, whereas the retained austenite fraction, size of bainite ferrite lath, impact energy and elongation are increased. The lath size and hardness increase and the contents of ultrafine ferrite and retained austenite decrease with the austenitizing temperature increasing. The tensile strength, yield strength, elongation and formability reach to 1 300–1 600 MPa, 750–950 MPa, 13.5–18.5% and 21 000–25 000 MPa?%, respectively.