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Ray S.K.,Research and Development Center for Iron and Steel
Advanced Materials Research

Recent advances in continuous casting have facilitated remarkable improvement in the quality of cast steel. The focus of the present paper is directed towards understanding of the influence of chemistry and intrinsic solidification behaviour on the different quality issues of stainless steel. An attempt has been made, wherever possible in quantitative terms, to correlate the chemistry and the process parameters with the quality factors. The relevant theoretical background has been touched upon. Understanding has been developed on the role of the ratio of nickel equivalent and chromium equivalent. This factor can be used to represent the chemistry of any stainless steel grade. Under similar casting conditions, specific problem of mould sticking, strand bulging, or depression formation has been explained invoking this chemistry factor. Each quality issue owes its genesis to interaction between the high-temperature strength and ductility of the solidifying shell during casting. The important roles of micro-segregation, the effective shell thickness during the early stage of casting, and the temperature range of transformation for δ-ferrite to austenite have been highlighted. An approach of grade-specific casting practice has been suggested to formulate the specific casting parameters. The developed understanding has been utilised to address specific quality problems in continuously cast slabs and rolled products of different stainless steel grades. This integrated understanding can be useful for ensuring the quality of stainless steel of any chemistry. © (2013) Trans Tech Publications, Switzerland. Source

Shukla R.,Research and Development Center for Iron and Steel | Shukla R.,Steel Processing Unit | Das S.K.,Indian National Metallurgical Laboratory | Ravi Kumar B.,Indian National Metallurgical Laboratory | And 3 more authors.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

In the current study, a novel ultra-low carbon, high-molybdenum-bearing microalloyed steel has been thermomechanically processed. Transformation of this steel during continuous cooling has been assessed. Variation in the microstructure and mechanical properties at different finish rolling temperatures has been studied. The average grain size, misorientation of grain boundary, and distribution of ferrite grains have been analyzed by using electron backscatter diffraction. The lower yield strength (251 to 377 MPa) with moderate tensile strength (406 to 506 MPa) along with high ductility (30 to 47 pct) has been achieved in the selected range of finish rolling temperatures. Superior impact toughness value in the range of 153 to 162 J is obtained in the subsize specimen even at subzero temperatures (233 K [-40 °C]), which is attributed to fine average ferrite grain size. The acicular ferrite dominated microstructure obtained at the 1023 K (750 °C) finish rolling temperature is the most attractive microstructure for pipeline applications due to its excellent combination of strength and toughness. © 2012 The Minerals, Metals & Materials Society and ASM International. Source

Patra S.,Indian Institute of Technology Kharagpur | Patra S.,Jindal Stainless Ltd | Ghosh A.,Indian Institute of Technology Kharagpur | Kumar V.,Research and Development Center for Iron and Steel | And 2 more authors.
Materials Science and Engineering A

The microstructural evolution during hot deformation of 2101 grade lean duplex stainless steel and its effect on hot-workability have been investigated by hot-compression testing using Gleeble® simulator over the range of deformation temperature of 800-1100 °C. Besides the dynamic recovery of δ-ferrite matrix and deformation of large austenite (γ) regions, fine γ-islands (<8 μm in size) were observed to form inside the δ-matrix. The density of those islands reached the highest value at deformation temperature of 900-1000 °C and increased significantly with the increase in applied true strain from 0.25 to 0.8. Such γ-islands are expected to form either by dynamic strain induced δ to γ transformation or by γ to δ strain-induced transformation followed by rapid precipitation of γ on heterogeneous nucleation sites (sub-grain boundaries, deformation bands etc.) present in the δ-matrix. The average size of the islands decreased with the decrease in deformation temperature. As the precipitated γ-islands follow Kurdjumov-Sachs orientation relationship with the δ-ferrite matrix, formation of such islands is detrimental to the hot-workability of the duplex stainless steel. Those islands not only restrict the plastic flow in δ-matrix but also provide favourable path for crack propagation through the δ. © 2016 Elsevier B.V. Source

Patwari K.,Research and Development Center for Iron and Steel | Pradhan N.,Research and Development Center for Iron and Steel | Banerjee N.,Research and Development Center for Iron and Steel | Pal B.R.,Steel Authority of India Ltd | And 4 more authors.
Materials and Manufacturing Processes

Substantial market exists for high carbon grades having 0.50-0.65 wt%C for agricultural, automobile, and wire drawing application. However, attributes of cast product in terms of internal and surface/subsurface quality have to be achieved for the required end applications. For production of high carbon grades, the process route followed at Durgapur Steel Plant is through BOF-LTS/LF-Billet Caster. In order to control total oxygen (O) and inclusion level, process modifications were implemented to deoxidize the slag and metal during tapping and at LF/LTS. For proper castability, control of [Mn] and [Si] to achieve proper Mn/Si ratio and O level in steel were standardized. Caster parameters like casting speed, Mn/S ratio, electromagnetic stirring (EMS), lubricating oil, mould, and secondary cooling were optimized to consistently achieve acceptable billet quality. Quality of cast billets with respect to surface defects (laps/bleeds)/subsurface defects (off-corner crack and pinholes/blowholes) were evaluated. Evaluation of internal quality like reheating crack, equiaxed zone (EAZ), and porosity were also carried out. Nitrogen (N) mapping at different stages of steelmaking and refining has revealed that a sharp increase in nitrogen level occurs at ladle stage for high carbon grades as compared to normal medium carbon grades. The surface quality is occasionally marred by the presence of laps and bleeds. It has also been established that EMS current lower than 250A is inadequate, while higher current on the other hand creates laps and bleeds on billet surface. In order to achieve the requisite product attributes, various recommendations have been implemented. Laps and bleeds are minimized when S < 0.030%, Mn > 0.8%, and casting speed <2.2 m/min. Sound billet quality in terms of central looseness and porosity is possible to achieve when the EMS current is maintained in the range of 270-300 A. Thus, with use of appropriate measures in chemistry control, secondary refining, and continuous casting, high carbon billet of acceptable quality can be produced. © 2010 Taylor & Francis Group, LLC. Source

Rath S.,Research and Development Center for Iron and Steel | Singh A.P.,Research and Development Center for Iron and Steel | Bhaskar U.,Research and Development Center for Iron and Steel | Krishna B.,Research and Development Center for Iron and Steel | And 3 more authors.
Materials and Manufacturing Processes

Accurate prediction of roll force during hot rolling process is very important for model based automation (Level-2) of plate mills. Exit thickness of plate for each pass is calculated from roll gap, mill spring, and predicted roll force. The response of gauge control hardware is highly dependant on the accuracy of prediction of roll force. Traditionally, mathematical models based on plane homogeneous plastic deformation theory are used for prediction of roll force. This method is based on many simplified assumptions which are not valid for actual industrial application. An artificial neural network (ANN)-based data driven model has been developed for prediction of roll force during plate rolling process. A very accurate data acquisition system has been installed in Plate Mill of Bhilai Steel Plant through which input and output parameters have been recorded. For a particular grade of steel, inputs to the ANN model are roll gap of previous pass, roll gap of current pass, rolling temperature, rolling speed, plate width, and pass number (6 inputs). The model output is roll force (1 output). In this article, the methodologies of development, training, and validation of ANN model has been discussed. Feed forward network has been chosen as ANN structure. Back propagation algorithm with variable learning rate and conjugate gradient optimization of cost function has been chosen as network training methodology. The model was found to be highly accurate with r-square value about 0.94. © 2010 Taylor & Francis Group, LLC. Source

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