Study on Improvement in the Quality of Rolled Products by Innovation in Rolling Process

Abstract

The New Plate Mill (NPM) at Rourkela Steel Plant (RSP) features advanced facilities such as roll bending, roll shifting, and an Accelerated Cooling Control (ACC) unit, with a notable high roll separating force of 90 MN. This mill leverages an innovative TMCP route, integrating Thermo-Mechanical Control Processing (TMCP) with Deformation Induced Ferrite Transformation (DIFT). This combination enables the production of steel plates with exceptional tensile strength and toughness by precisely controlling microstructure and grain size. This study examines the industrial-scale integration of Deformation Induced Ferrite Transformation (DIFT) with conventional Thermo-Mechanical Controlled Processing (TMCP) and the challenges involved. Scaling up from the lab, the combined process achieved yield strengths of 360-424 MPa (from 250 MPa) in plain carbon steel, with Charpy Impact Energy (CIE) of 104-120 Joules at 0°C (up from 60 J) and 64-98 Joules at -20°C (up from 27 J), and elongation values of 35-46%. For low-carbon steel, yield strengths were 360-397 MPa, with CIE values of 132-143 Joules at 0°C and 100-108 Joules at -20°C, and elongation from 45-61%. The TMCP-DIFT integration eliminated pearlitic banding, refining the microstructure with acicular ferrite and ferritic bainite. However, high deformation energy raised the final rolling temperature to 884°C, presenting a challenge for optimal DIFT processing, which relies on lower temperatures for fine grain structures. This method significantly reduces the need for costly micro-alloying elements, offering a more economical solution. Furthermore, innovative TMCP regimes were deployed with minimal micro-alloying elements—Nb and Ti—to augment yield strength while maintaining high impact toughness. Leveraging precipitation hardening and grain refinement, the steel achieved enhanced yield strength (510–550 MPa) and Charpy Impact energy for toughness (189–230 J at 0ºC and 185–226 J at -20 ºC). Notably, a substantial increase in yield strength and Charpy Impact energy was attributed to high cumulative reductions in the last three finishing passes (>65%). Optical micrographs demonstrated the grain refinement of ferrite due to very high reductions in finishing passes, especially at lower finishing temperatures, contributing to the enhanced properties. Additionally, EBSD maps highlighted the presence of deformation-induced ferrite in plates with more than 65% cumulative reductions in the last three finishing passes. Moreover, the new TMCP process coupled with DIFT processing is extensively employed for producing modern pipeline steel (API X 70) with extremely lean chemistry, involving controlled rolling along with ACC post-finish rolling. Despite past trials, producing API X-70 plates commercially within SAIL remained elusive, accomplished only by a handful of global steel industries. This milestone has been achieved by optimizing TMCP and DIFT processing with very lean chemistry, deftly utilizing the Plate Mill's full potential. However, post-rolling, waviness appeared in the plates after passing through hot plate leveller (HPL), later resolved by employing proper descaling practices and maximizing temperature homogenization (<450 ºC) before entering the hot plate leveller. This solution eliminated waviness, facilitating the commercial production of the esteemed grade. The study demonstrated that incorporating Deformation Induced Ferrite Transformation (DIFT) within the TMCP route greatly enhanced the mechanical properties of plain carbon, plain low-carbon, low microalloyed, and API X70 steel plates. Utilizing DIFT mechanisms during processing resulted in superior toughness and strength, transforming the microstructure from a pearlite-polygonal ferrite mix to refined ferrite and acicular ferrite. This innovative integration of DIFT with TMCP offers a promising advancement in steel production, enabling the development of high strength plates with outstanding mechanical properties.