S.V.S. Narayana Murty

Mechanical Properties of Ultrafine Grained Steels Processed by Large Strain-High Z Deformation - A Review

Ultrafine-grained steels with a grain size of about one micron offer the prospect of high strength coupled with high toughness among conventional steel compositions and are attracting the attention of researchers worldwide. Application of these ultrafine-grained steels to potential engineering structures demands extensive study of their mechanical properties. While there are many studies on the development of ultrafine-grained microstructures through various deformation processing techniques on a spectrum of compositions, fewer studies were reported on the more important aspect of evaluating their mechanical properties. This is to verify the basic assumption that the microstructural refinement at bulk level indeed improves the mechanical properties offering the prospect of a realistic replacement of the existing conventional steels in the near future. As we move towards the ultimate goal of applying these advanced high strength materials, this review article attempts to present a comprehensive picture on the mechanical properties of ultrafine-grained steels with varying carbon contents fabricated by large strain warm deformation. Finally, it is believed that time is ripe for exploring the possible applications of these materials for structural applications.

Process Design Concepts for the Production of Ultrafine Grained Steels through Multi-Pass Warm Rolling: Bridging Science and Technology

Steel bars having a cross section of 18mm square with uniform distribution of ultrafine ferrite grains were produced through a multi-pass warm caliber rolling process in a 0.15%C-0.3%Si-1.5%Mn steel. The average ferrite grain sizes of 0.43μ m, 0.70μ m and 1.2 μ m were obtained in the isothermal warm caliber rolling processes at 773K, 823K and 873K respectively. Even though caliber rolling results in inhomogeneous strain distribution, multi-pass caliber rolling to large cumulated strains of 2 or 3 can be uniformly introduced in to the bar samples. Strain accumulation due to the multi-pass warm deformations was confirmed by comparing microstructural evolution through the multi-pass deformations with that of single pass deformation. The size of ultrafine grains formed through warm deformation was found to depend on the Zener-Hollomon parameter. The similarity of the microstructural evolution with single pass deformation reveals that the multi-pass warm deformation is an effective method to obtain ultrafine grained ferrite structure in bulk materials. It is proposed that compressive strain-Z parameter plots along with grain size-Z parameter plots help in establishing the processing conditions for obtaining products with a desired microstructure and grain size. Finally, such “processing maps” developed for a variety of materials serve useful purpose in bridging the science and technology of developing bulk ultrafine grained materials in semi-finished / finished products.

Microstructural Evolution during Simple Heavy Warm Deformation of a Low-Carbon Steel

We examined the microstructure development in low carbon steel (0.15% C) during heavy warm deformation (HWD) using field emission scanning electron microscopy (FESEM) and electron back-scattering diffraction (EBSD). Plane strain compression tests have been conducted in the temperature range of 773-923 K at strain rates of 0.01 s-1 and 1 s-1 with the specimens deformed to 25% of their original thickness. We summarize the strain rate and temperature into the Zener-Hollomon parameter and investigate its variation with plastic strain on the basis of the evolved microstructures and grain boundary character with a view to understanding the critical conditions for forming ultrafine grains and classifying them. Once established, these compressive strain-Z parameter plots simplify the selection of processing parameters (such as strain, strain rate, and temperature), towards achieving tailor-made microstructures in industrial components.