Date of Award

2018-01-01

Degree Name

Doctor of Philosophy

Department

Material Science and Engineering

Advisor(s)

Devesh Misra

Abstract

The process parameters related to thermo-mechanical controlled processing (TMCP) of steels play a vital role in affecting the ultimate mechanical properties. We illustrate here the impact of coiling temperature, the impact of finish cooling temperature on interrupted cooling and compare with continuous cooling, and the impact of carbon and niobium on the microstructure and precipitation behavior and corresponding changes in mechanical properties, in an ultrahigh strength titanium-niobium microalloyed steel. The microstructural advancement was contemplated via transmission electron microscopy and electron backscattered diffraction (EBSD). The goal was to underscore the impact of coiling temperature on the nature and distribution of microstructural constituents that essentially added to differences in the yield and tensile strength of these steels. Depending on the coiling temperature, the microstructure comprised of either a combination of fine lath-type bainite and polygonal ferrite or polygonal ferrite together with the precipitation of carbides of size less than 10 nm in the matrix. The microstructure of steel coiled at lower temperature predominantly comprised of bainitic ferrite with lower yield strength contrasted with the steel coiled at a higher temperature, and the yield to tensile strength ratio was found to be around 0.76. The microstructure of the steel coiled at a higher temperature comprised of polygonal ferrite and extensive precipitation of carbides and was characterized by higher yield strength and with yield strength/tensile strength ratio of 0.94. The microstructure of continuously cooled and interrupted cooled steels with various finish exit temperatures comprised of polygonal ferrite, bainite and martensite/austenite constituent. However, the fraction of various microstructural constituents was diverse in each of the trial steels. Similarly, there were differences in the distribution and average size of (Nb, Ti)C precipitates. The previously mentioned differences in the microstructure and precipitation introduced differences in mechanical properties. Moreover, electron backscattered diffraction studies showed particular variation in average grain area and high angle boundaries between continuously cooled and interrupted cooled steels. Carbon and niobium (Nb) play a vital role in influencing the ultimate microstructure and tensile properties. The increase of carbon weight percent in steel increased the precipitation temperature of (Nb, Ti)(C, N), which prompted relatively larger size precipitates in the matrix. Besides, high carbon content added to stabilization of austenite and delayed the transformation temperature of ferrite and bainite, such that martensite/austenite constituent (M/A) was obtained. This affected the properties by slightly increasing the strength of the steel. Polygonal ferrite microstructure in steel with high Nb-content was responsible for relatively low strength in comparison with steels with higher carbon content. Granular bainite and lath bainite in steel were characterized by the best combination of strength and elongation. The outcomes of the thermodynamic simulations were consistent with the experimentally observed microstructure.

Language

en

Provenance

Received from ProQuest

File Size

220 pages

File Format

application/pdf

Rights Holder

Venkata Vignesh Natarajan

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