Development of Special Quality Steels at SAIL for Automotive Industry

B.K. Jha, Anjana Deva, B. Sarkar and S.K. Chaudhuri
Steel Authority of India Limited

During 2005-06, the Indian automobile industry witnessed an impressive growth and the trend is likely to continue with the upsurge in the national economy. Steel Authority of India Limited (SAIL), the leading steel producer in the country, has been responding to the needs of superior quality steels from auto industry. The paper highlights the efforts made by SAIL towards development of varieties of formable quality products. Hot rolled grades namely, high strength ferrite-pearlite steels (YS: 350-500MPa min.), dual phase steel (YS: ~350 MPa min.), LPG cylinder grade steel for auto-cylinders, medium carbon steel and cold rolled products like, low carbon extra deep drawing (EDD) and high strength cold rolled steel (HSCR 26 / 35) were commercialized for varieties of automotive applications.

The guiding principle in developing high strength steels used for fabrication of auto chassis and wheels had been to produce ferritepearlite microstructure with reduced pearlite content and strengthening the ferrite matrix by fine dispersion of niobium carbonitrides. Dual phase steel with low YS / UTS ratio (~0.65) and continuous yielding characteristics was developed by maintaining high and low coiling temperatures i.e. 620 & 400oC respectively. The demand for normalized LPG cylinders (YS: 295 Mpa min.) both from indigenous as well as export market, especially for automotive application, hot rolled (HR) coil property of carbon -manganese steel was balanced with respect to normalizing heat treatment of cold formed cylinders. Further, medium carbon (C: 0.6% max) HR coils considered as prime input material for cold reducers and used for manufacturing various auto-components after heat treatment was developed by introducing new casting and hot rolling practices.

In case of cold rolled product, an improved quality low carbon (C: 0.03% max.) EDD steel was developed through combined blowing technology to attain superior formability properties ( r: 1.8 min., n: 0.22 min., El: 40% min.). This was possible by adhering to specially designed annealing cycle which favoured development of pancake type ferrite grains, dispersed with fine spherical cementite particles. High strength cold rolled grades; namely HSCR 26 / 35 for moderate forming applications with high dent resistance were developed to promote use of thinner gauge material without adversely affecting rigidity of the formed component. Promotion of such product will help in enhancing fuel economy of car. Indian automobile industries have been engaged in partnership role with the steel producers for the development of newer car models through Early Vendor Involvement (EVI) system. The assessment of performance of newly developed sheet metal was pursued by evaluating forming limit diagram, based on accurate determination of strain paths during in plane as well as out of plane stretching of steel sheets. Such plot was then used as reference in comparing with the localized strain distribution in the industrially formed component. This helped in understanding cold forming behaviour of steel sheet during manufacturing of actual component in the industry.

Research on development of transformation induced plasticity (TRIP) and ultra-fine grained steels aimed at development of cost effective alloy design and processing of steel to achieve excellent formability property combined with higher strength. The tensile strength of TRIP steel was in excess of 900 MPa with total elongation of 30% min. Development of ultra fine-grained steel of less than 1-micron ferrite grain size had been possible by deploying unique thermo-mechanical processing of plain C-Mn steel to achieve high strength combined with adequate toughness. On completion of massive modernisation of steelmaking technology and mill facilities, SAIL will be soon poised to undertake commercial production of such type of advanced class of materials for automotive industry.

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