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Title: An investigation into the effect of carbon and niobium on microstructure development during reheating of steel
Author: Khalid, Omar
ISNI:       0000 0004 6060 7533
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2017
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Thermo-mechanical rolling processes can be divided into three stages, reheating, rolling, and cooling. An important parameter in obtaining a sound microstructure with good mechanical properties is to start with a uniform austenite structure. It is of paramount importance that the prior austenite grain size is kept small and as uniform as possible during the reheating stage. There has been a considerable amount of work done on the prior austenite grain size and its effects on the mechanical properties of the steel. However there is a gap in the literature, when the niobium content is less than 0.02 wt. % in low, medium or high carbon steels. There is also a gap when it comes to the effect of varying carbon on the prior austenite grain size. Research on the effect of carbon and niobium on the austenite grain growth during different reheating temperatures, varying hold times at different reheating temperatures, and varying reheating rates is insufficient. The outputs from this thesis will be used to increase the accuracy of predictions for the austenite grain size during different reheating temperatures using empirical models, and to gain knowledge on the T_GC (Grain coarsening temperature) with respect to each composition. In this work a detailed study of the effect of temperature, hold time and reheat rate on both the prior austenite grain size microstructure and precipitation evolution, of microalloyed steels in the as reheated condition is investigated. Five different carbon steels (0.08 wt. %, 0.2 wt. %, 0.4 wt. %, 0.6 wt. %, and 0.8 wt. %) each consisting of a plain C-Mn composition (with no niobium element addition) and three different niobium concentrations (0.005 wt. %, 0.01 wt. %, and 0.02 wt. %) were investigated. The reheat temperatures ranged from 950°C to 1250°C in 50°C increments, after which the specimens were held for an hour and then quenched in ice water. The average prior austenite grain size was determined using ASTM E112 as a function of reheat temperature, hold time and reheat rate, Standard deviation analysis has been used to measure the bimodal distribuation of the grain size. Second phase particles were analysed using TEM (transmission electron microscopy). Results from this investigation indicate, that the niobium increases the formation of pearlite in the as received material. The micro hardness tests indicate that in the low carbon (0.08 wt. %) steel the ferrite phase and pearlite is not affected by the niobium wt. % as the hardness values remain constant with increasing niobium. However a decrease in ferrite grain size is observed with increasing niobium. Hardness increases with an increase in niobium content for medium carbon (0.4 wt. %) steel and for high carbon (0.8 wt. %) steel. Reheating temperature results indicate that with an increase in carbon content for plain C-Mn steel there is a decrease in the average prior austenite grain size. Similar trends of decreasing austenite grain size can be seen in all the range of carbon contents microalloyed with niobium. The standard deviation results have shown that the amount of niobium addition in the steel determines the T_GC. It has also been shown that as the carbon content increases so does the T_GC , the temperature at which the abnormal grain growth starts to occur. During different hold times and for different carbon contents at a reheat temperature of 1050°C and 1150°C it was indicated that in plain C-Mn steel, the prior austenite grain growth was linear as a function of increasing time. When niobium is present in the specimen at 1050°C the growth trend is logarithmic. As the reheat temperature is raised to 1150°C the growth trend for plain C-Mn and 0.005 wt. % Nb steel are linear with increasing time. Both the 0.01 wt. % Nb and 0.02 wt. % Nb steels experience a logarithmic growth trend. Standard deviation analysis was used to measure the bimodality of the prior austenite grains and showed that other than hold time, reheat temperature also plays a major part in normal/abnormal grain structure together with the wt. % of Nb. It was found that the time exponent n values are higher for 1150°C and lower for 1050°C. This is because of the reduction in solute drag which is attributable to the Zener pinning imposed by NbC particles at grain boundaries. The n value decreases dramatically for 0.8wt% carbon steel for the same niobium content. At the higher temperature of 1150°C the n value remains relatively constant for all the carbon contents. However for the 0.8 wt. % carbon steel there is a decrease in the n value for the 0.01 and 0.02 wt. % Nb microalloyed steels. The effect of reheating rate on the decrease in the prior austenite grain size was more prominent for heating rates of 2.5°C s-1 and 5°C s-1 at lower temperatures of 1000°C. When the heating rate was 15°C s-1 the decrease in the prior austenite grain size was not as significant. The effect of precipitates indicated that with increasing heating rate the precipitation number density decreases, the average precipitate size decreases and the precipitates are much finer at higher heating rates.
Supervisor: Khalid, Omar Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available