Sudipta Patra

Effect of Rolling and Subsequent Annealing on Microstructure, Microtexture, and Properties of an Experimental Duplex Stainless Steel

A lean duplex stainless steel (LDSS) has been prepared with low-N content and processed by different thermo-mechanical schedules, similar to the industrial processing that comprised hot-rolling, cold-rolling, and annealing treatments. The microstructure developed in the present study on low-N LDSS has been compared to that of high-N LDSS as reported in the literature. As N is an austenite stabilizer, lower-N content reduced the stability of austenite and the austenite content in low-N LDSS with respect to the conventional LDSS. Due to low stability of austenite in low-N LDSS, cold rolling resulted in strain-induced martensitic transformation and the reversion of martensite to austenite during subsequent annealing contributed to significant grain refinement within the austenite regions. δ-ferrite grains in low-N LDSS, on the other hand, are refined by extended recovery mechanism. Initial solidification texture (mainly cube texture) within the δ-ferrite region finally converted into gamma-fiber texture after cold rolling and annealing. Although MS-brass component dominated the austenite texture in low-N LDSS after hot rolling and cold rolling, that even transformed into alpha-fiber texture after the final annealing. Due to the significant grain refinement and formation of beneficial texture within both austenite and ferrite, good combination of strength and ductility has been achieved in cold-rolled and annealed sample of low-N LDSS steel.

Energy Conservation Potential in Stainless Steel Making by use of Molten Pig Iron and Liquid Ferro-Chrome

Stainless steel is mostly manufactured by Electric Arc Furnace Argon Oxygen Decarburization route utilising 100% solid charge of steel scrap/DRI/stainless steel scrap and ferroalloys.The process consumes around 415/450 KWH of power per ton of stainless steel.In the state of Odisha, we are endowed with both iron and chromite ores,which offer opportunity for use of molten pig iron and liquid ferrochrome for energy efficient stainless steel making for a plant located there. This paper analyses potential for energy conservation in EAF for three different grades of stainless steel belonging to AISI 200,300,400 series. In this concept, melting is performed in two different furnaces,one EBT type and the other Spout type.In EBT furnace only high ferrous melt with liquid pig iron and DRI/Scrap are charged and dephosphorized. In the spout furnace high carbon ferrochrome which has a tendancy to solidify rapidly due to its high melting point of nearly 1575°C is diluted with scrap which brings down its melting point drastically to enable it to be kept molten.Silicon in the high carbon ferrochrome is gainfully utilised to raise the temperature of molten pool.The diluted ferrochrome is taken in ladle in appropriate portion to which dephosphorised decarburized molten steel is added from EBT furnace for charging into AOD. The material and energy balance have been carried out for the entire charge mix using classical approach of energy calculation. Both variants of DRI and scrap addition to hot metal have been considered for the EBT furnace. Calculations for EBT furnace have been validated using data from an existing carbon steel plant using similar practice of steel making. Calculated values for such energy efficient stainless steel making have been compared with data on existing stainless steel plant using solid charge. Energy usage can be reduced by more than 60% with this approach. Keywords: Electric arc furnace, Stainless steel melting, liquid ferrochrome, energy conservation.

Effect of Starting As-cast Structure on the Microstructure–Texture Evolution During Subsequent Processing and Finally Ridging Behavior of Ferritic Stainless Steel

Effect of the initial as-cast structure on the microstructure–texture evolution during thermomechanical processing of 409L grade ferritic stainless steel was studied. Samples from the regions of cast slab having ‘columnar,’ ‘equiaxed,’ and a mixture of ‘columnar’ and ‘equiaxed’ grains were subjected to two different processing schedules: one with intermediate hot-band annealing before cold-rolling followed by final annealing, and another without any hot-band annealing. EBSD study reveals that large columnar crystals with cube orientation are very difficult to deform and recrystallize uniformly. Resultant variations in ferrite grain structure and retention of cube-textured band in cold-rolled and annealed sheet contribute to ridging behavior during stretch forming. Initial equiaxed grain structure is certainly beneficial to reduce or even eliminate ridging defect by producing uniform ferrite grain structure, free from any texture banding. Application of hot-band annealing treatment is also advantageous as it can maximize the evolution of beneficial gamma-fiber texture and eliminate the ridging defect in case of completely ‘equiaxed’ starting structure. Such treatment reduces the severity of ridging even if the initial structure contains typically mixed ‘columnar-equiaxed’ grains.

Production of High Nitrogen Stainless Steel X8CrMnN18-18 through EAF-AOD-LRF-CC-Steckel Mill Route and its Hot Deformation Study by Gleeble Thermomechanical Simulator

High nitrogen containing austenitic stainless steel X8CrMnN18-18 exhibits attractive combination of high strength, toughness and corrosion resistance.This grade containing more than 5000 ppm of nitrogen was produced commercially through EAF-AOD-LRF-CC-Steckel mill route and its microstructure and mechanical properties were studied. Excellent combination of strength,ductility and toughness is achieved in the entire range of 6mm to 50mm thick hot rolled plates. Uniaxial compression tests were carried out to understand the hot deformation behavior by varying temperature and strain rate. Softening behavior during deformation was analyzed from flow stress strain curves and microstructural analysis. Dynamic recrystallization (DRX) behavior of the material was observed during thermo mechanical processing. Critical strain related to DRX and Avrami kinetics of DRX was calculated by analyzing the flow curve data. Microstructural characterization was done by optical microscopy and EBSD analysis. Extensive grain refinement can be achieved by thermo-mechanical processing controlled by DRX. Keywords: High nitrogen stainless steel, Strength and toughness, DRX, Grain refinement, TMCP

Effect of Hot-Deformation on Micro-Texture in Ultra-Fine Grained HSLA Steel

Plane-strain compression testing on HSLA steel samples using single-pass and multi-pass hot-deformation schedules showed that heavy deformation of metastable austenite below Ae3, developed ultra-fine ferrite grains (<3 µm), increased the fraction of high-angle (>15º misorientation) boundaries (>70%), and encouraged the formation of beneficial g-fibre ((ND//<111>) components along with {332}<113> and {554}<225> texture components, minimising the intensity of harmful ‘cube’ texture.