THERMOMECHANICAL PROCESSING OF PLAIN LOW CARBON AND MICROALLOYED STEELS

Abstract

The usage and importance of structural applications of steels continue to lead in critical areas even when the availability of ceramics, polymeric and composite materials with the improved properties exists for many decades, now. There is ever increasing demand of high performance steels in many critical strategic applications such as in military applications, ship building, nuclear power, aerospace, automobiles etc. The manufacturers are always trying to reduce the cost of raw materials and production without the cost of quality. Thus, to produce quality products at low cost any scientific approach should be explored and employed to optimize the processing parameters. Further, the mechanical properties can be enhanced by the alloy design and heat treatment. Thermomechanical processing (TMP) is a technique that can control the hot deformation process parameters, which result in improved mechanical properties of the materials. Thermomechanical processing is commonly related to the hot working operations. Hot working involves controlling various processing parameters such as reheating temperatures, deformation temperature, applied strain, strain rate, and the post deformation-cooling rate, which decides the final mechanical properties of the products. The review of literature shows that there is a need to perform more hot deformation studies in the widest range of processing parameters (deformation temperature, strain rate), and also to compare the different dynamic materials models (DMM). A new instability parameter κj can also be employed in the processing maps and compared with the other parameters, for the study of plain low carbon and microalloyed steels. There are some reports regarding the use of flow localization parameter,  in non-ferrous alloys but its use for plain low carbon and microalloyed steels is rarely reported. In addition, significance of Zener-Hollomon parameter, Z is not evaluated along with that of the processing maps and flow curves. Low carbon steels are important common structural steel in which high strength and good ductility is desired. Using DIFT technique in safe workable region, low carbon steels are expected to produce defect free grain refinement. This will obviate the need for expensive alloying, and improve the quality of the product at low cost. In the present study, hot deformation behavior of three steels has been studied using thermomechanical simulator Gleeble 3800. These are AISI 1010, AISI 1016 and low carbon Ti-Nb microalloyed steels. The microstructural evolution after hot deformation at different iii deformation temperatures and strain rates is studied. Processing maps using different models, flow localization parameter, Zener-Hollomon parameter, and constitutive equations are studied and correlated. In addition, the different processing maps are compared and the safe workability region is defined. The effect of carbon content in the plain carbon steel and effect of micro alloying elements are also studied on the various processing maps, microstructural evolution and apparent activation energy of hot deformation. The grain refinement of plain low carbon (AISI 1010) and low C Ti-Nb microalloyed steel using DIFT is investigated in the safe workable region. The investigations of fine-grained metals have been mainly concerned with structural characterization, micro hardness and tensile behaviour. This thesis consists of the eight different chapters. The first chapter consists of introduction to the topic and gives a brief idea about the hot deformation behavior, grain refinement using DIFT technique, overview of the work included in the thesis and the importance of this work. The literature review is given in chapter two along with the history of the hot deformation, and the hot deformation behaviour of plain carbon and microalloyed steel. Various processing maps with different power dissipation efficiency and instability criterion have also been explained in the review. Significance of the present study is explained based on the research gaps found in the literature about the hot deformation processing parameters of plain low carbon steel as well as microalloyed steel. Moreover, grain refinement using the different hot deformation routes is studied. The details of all the experimentation performed right from the determination of phase transformation temperatures up to the development of the fine-grained steel and their characterization is given in chapter three. Microstructural analysis was performed in the mid-section of the deformed specimens using optical microscopy, SEM, and EBSD. Macro and micro hardness tests were performed to determine the hardness of specimens. Chapter four consists of theory and analysis in which the various processing maps are discussed using different criteria. Processing maps are shown in the processing space i.e. on the axes of temperature and strain rate for a given strain. Prasad et al. developed processing maps using the DMM, in which no storage of energy in the material is assumed. The modified DMM developed by Murty and Rao are also described. A new instability criterion κj developed by Poletti et al. is explained along with the instability criterion of Prasad et al. and Murty and Rao. The flow localization parameter and constitutive equations are explained in this chapter. iv Chapter five consists of results and discussion of hot deformation behaviour and processing map of plain low carbon and Ti-Nb microalloyed steels. In all three steels the hot deformation behavior were studied using dynamic materials model (DMM) and modified DMM using different instability parameters given by Prasad el al. and Murty and Rao. A new instability criterion κj developed by Poletti et al. was also studied. Also the effect of increase in carbon content, and micro alloying on hot deformation behaviour were studied. This study show that as the amount of carbon content increased the values of strain rate sensitivity, m, and power dissipation efficiency, η increased and also the workability region increased. The instability parameter, κj predict the more accurate deformation behaviour. The value of strain rate sensitivity parameter, m, power dissipation efficiency, ηPrasad and ηMurty & Rao is reduced with the addition of Ti-Nb microalloying and the region of the high value of m is shifted towards the higher temperature zone. The safe and unsafe regions of workability using the processing maps were determined and verified with the microstructural study. The apparent activation energy is increased with increase in carbon contents and also increased with Ti-Nb micro alloying. This is because that in Ti-Nb microalloyed steel Nb can be present either in dissolved form where it exhibits a strong solute drag effect, or as NbC precipitates which effectively pin the grain boundaries. Drag exerted on the migrating grain boundaries by Zener pinning effect of the NbC/TiC precipitates or by the presence of dissolved Nb solute can retard the progress of recrystallization. The Chapter six consists of results and discussion of grain refinement by thermomechanical processing of plain low C AISI 1010 and Ti-Nb microalloyed steels. In this chapter, the hot deformation was performed in order to make use of the deformation induced ferrite transformation (DIFT) technique. The effect of different processing parameters such as strain, prior austenite grain size, deformation temperatures etc. on DIFT are analyzed. In addition, the processing conditions are optimized in the safe working zone to obtain the ultra-fine ferritic (UFF) microstructure. In this study, the strain rate is kept constant and is 1 s-1 as reflected in the results of hot deformation (finer ferritic grains). The effects of deformation temperatures and the amount of applied strain are discussed in this chapter. Hot deformation was performed in the two steps at different temperatures with different amount of strain, and the average grain size of 3.2μm for AISI 1010 steel and 1.1μm for Ti-Nb microalloyed steel were obtained. The mechanical properties are determined using the Tabor’s relation. Chapters seven and eight deal with conclusions and suggestions for future work, respectively.