Research On Solidification Behavior Of 1Cr18Ni9Ti Steel

The prevailing columnar crystals and all kinds of cracks are the main problems in the 1Cr18Ni9Ti austenitic stainless steel continuous casting production, which is inseparable from the solidification process. With the development of near-net-shape continuous casting, solidification rate increasing makes the solidification structure more complex. The solidification structure is variable due to the component, the cooling condition and the subsequent solid-state phase transition. Therefore, systematic research of the solidification process of 1Cr18Ni9Ti austenitic stainless steel is particularly necessary.A biconical type body contraction measuring instrument and Gleeble 3800 thermal physical simulation system are employed to investigate the body contraction and the high-temperature mechanical properties during the solidification. The results show that the improvement the cooling rate increases the tendency of the dispersion contraction and the mechanical property above 1302～1305℃, but decreases below 1302～1305℃.Solidification process of 1Cr18Ni9Ti steel is investigated by directional solidification device. There is a transition from FA to AF solidification mode under 50μm/s growth velocity. However, FA solidification mode is maintainted until the end of solidification above 50μm/s growth velocity. With the increasing of growth velocity, the width of initial transition zone becomes narrower and narrower. During steady transition zone, with the increasing of growth velocity, the primary phase and solid-liquid interface change at the same time. The primary phase is austenite when solidi-liquid interface present planar, cellular morphology, but ferrite at dendritic and ultra-fine dendritic interface. The island banding structure appears during the transition from planar to cellular morphology. The nucleation of island banding ferrite happens in the primary austenite rather than at the solid-liquid interface. Full equiaxed structure is found at 100μm/s. In ultra-fine dendrite growth stage, the austenite phase grows competely with ferrites and eventually reachs the symbiotic growth state at 1000μm/s.Considering the non-equilibrium solidification, interface response function of different phases is calculated and the phase and morphology selection is verified by the maximum growth temperature criterion. The calculation results agree well with the experimental results. The variation of constitutional undercooling in front of solid-liquid interface is also calculated to explain the full equiaxed structure at 100μm/s. Constitutional undercooling degree of ferrite is more than austenite at all growth velocities. The maximum constitutional undercooling degree turns into positive at 50μm/s and reaches the nucleation condition of ferrite at 100μm/s. The results show that the greater growth velocity the greater constitutional undercooling degree, but narrower the width of the undercooling zone. The maximum constitutional undercooling degree decreases slightly, and the width of the undercooling zone becomes narrower and narrower with the increase of temperature gradient.The Bridgeman setup with real time controlling of draw speed and heating temperature is employed to thermally physically simulate the solidification process of slab continuous casting. Effects of continuous casting process parameters on the solidification structure are studied. Low central temperature of slabs is benefitial to the solidification structure refinement. Improvement of casting speed and solidification coefficient is in favor of the dendrite refinement and also conducives to promoting probability of solidification mode changes between FA and AF solidification mode.