Preparation Of Ultrafine Grained Low Carbon Steel With Bimodal Grain Size Distribution And Its Microstructure And Mechanical Properties

This dissertation aims to improve the plasticity of ultra-fine grain/nano-crystalline low-carbon steel by bimodal sized ferrite coupled of nano-scale carbides which is obtained by plastic deformation and subsequent recrystallization annealing of M/F dual phase structure.The recrystallization and tensile deformation behavior, and the relationship between structure and tensile mechanical properties were studied by using optical microscopy, scanning electron microscopy, transmission electron microscopy and tensile tests.Results show that the microstructure composed of bimodal ferrite and nano-scale carbides can be formed in 14MnNb steel by cold rolling and recrystallization annealing of M/F structure which was gained through subcritical quenching. The processes are (a) 1000 oC×30 min austenitizing furnace cooling to 760 oC quenching + 65%-reduction cold rolling + 560°C×90 min annealing. and (b) 1000°C×30 min brine quenching + 760°C×30 min brine quenching + 70%-reduction cold rolling + 560°C×90 min annealing. The coarse grain size of ferrite in specimen treated by process (a) is about 3.79μm and the fine grain size is about 1.25μm. The coarse grain size of ferrite in specimen treated by process (b) is about 5.11μm and the fine grain size is about 0.35μm.The results of the uniaxial tensile tests at room temperature and the microhardness tests show the tensile strength 788.5 MPa, the yield strength 663.0 MPa, the uniform elongation 17.8% and the total elongation 21.6% in specimen processed by (a), and the tensile strength 785.8 MPa, the yield strength 624.0 MPa, the uniform elongation 13.6% and the total elongation 17.5% in specimen processed by (b). The mode of the tensile fracture is dimple fracture. The recrystallization dynamics of cold-rolled 14MnNb steel was studied by the method of remained work hardening. The recrystallization activation energy was measured to be about 169.2 kJmol-1 for the specimen processed by 1000 oC×30 min austenitizing furnace cooling to 760 oC quenching + 65% cold rolling, and about 147.9 kJ mol-1 for the specimen processed by twice brine quenching at 1000°C and 760°C +70% cold rolling.