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Influence of high temperature mechanisms on microstructural evolution of Dual Phase steels
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Bandi, Bharath (2022) Influence of high temperature mechanisms on microstructural evolution of Dual Phase steels. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3755557
Abstract
Dual phase steels are one of the prominent advanced high strength steels used as structural and safety components in the present day automotive industries. The microstructure of these steels is significantly affected by various processing steps involved in the manufacturing process. During the heating, and soaking steps of the continuous annealing process, the microstructure of the initial cold rolled material undergoes various mechanisms such as ferrite recovery and recrystallization, cementite spheroidization and dissolution, and austenite formation. These processes generally occur consecutively, and the final microstructural features are highly sensitive to the progress of these processes. However, to fulfil the ever rising demand for higher strength materials, increasing the amount of alloying content in order to induce precipitate hardening and solid solution strengthening is becoming a common practice. The higher percentage of alloying content can potentially retard the high temperature processes, especially ferrite recrystallization process, and thereby shift their progress into the austenite formation region. This research work was done to understand the effect of simultaneous happening of these high temperature processes on the microstructural evolution of DP steels by application of high heating rates during the heating step of annealing process.
In this research work, Boron-Vanadium micro-alloyed steels with manganese segregation were used to evaluate the effect of overlap of ferrite recrystallization, cementite spheroidization and austenite formation processes on the microstructural evolution of DP steels. The activation energy for recrystallization of 50% cold reduced steel was calculated using the Johnson-Mehl-Avrami- Kolmogorov equation and was found to be 339 kJ/mole. This is much higher than the self-diffusion activation energy for bcc iron (251 kJ/mole) which indicates the retarding effect of alloying elements on the progress of recrystallization process. The experimentally obtained recrystallization fraction values prove that increase in the cold reduction increases the kinetics of the recrystallization process. This was evidenced by the increase in the Avrami exponent from 0.87 for 50% cold reduced steel to 0.98 for 75 % cold reduced steel. It was found from the dilatometric measurements that increase in heating rate increased both the recrystallization start temperature and the austenite start temperature. A continuous heating rate model was developed using the Johnson-Mehl-Avrami- Kolmogorov xi
constants and process starting temperatures to predict the heating rates required to obtain a specific amount of overlap between ferrite recrystallization and austenite formation processes. It was found that with increase in cold reduction the amount of overlap possible for a given heating rate decreases. The model predicted that heating rates of 0.2 0C/s, 0.9 0C/s, 1.8 0C/s, 7 0C/s, 50.5 0C/s, and 511 0C/s are required for a predefined overlap of 1%, 15%, 34%, 67%, 88% and 99% respectively.
Using the heating rates obtained from the model, heat treatments were conducted on 50% cold reduced steels at increasing inter-critical temperatures and soaking times. It was found that with the increase in overlap of high temperature processes the amount of martensite formed decreased for the samples directly quenched without soaking time (0 sec). However, for higher soaking times, increase in overlap increased the austenite kinetics and thereby increased the final martensite fraction. For instance, sample annealed at 0.2 0C/s at 750 0C for 900 sec produced 51% of martensite, whereas sample at 50.5 0C/s at 750 0C for 900 sec produced 57% of martensite in the final microstructure. Moreover, increase in heating rates transformed martensite with necklace morphology to banded morphology. Using SEM microstructural images, hardness measurements, EDX scans, and EBSD results different austenite formation mechanisms were proposed for different percentages of overlaps, inter-critical temperatures and soaking times. SEM microstructural images and EBSD scans show that the austenite preferentially nucleates and grows on recrystallized ferrite grain boundaries during slow heating rate condition thereby developing necklace martensite morphology. The stability of spheroidized cementite and potentially long distances between grain boundary nucleated austenite was found to be responsible for slower austenite kinetics at slower heating rate condition. For higher heating rate condition, the presence of partially spheroidized cementite and dislocation rich recovered ferrite region enabled faster austenite formation kinetics. Increase in heating rate in hot rolled steels also increased the austenite formation kinetics. Moreover, hot rolled steels developed through thickness martensite morphological anisotropy and texture inhomogeneity. In both hot rolled and cold rolled steels, the manganese segregated region found to have at least 3 times higher manganese concentration. ThermoCalc simulations show that the manganese segregated region has a 70 0C lower equilibrium austenite formation temperature than the non-segregated region which can potentially lead to martensite bands at the centre of the sheets.
Item Type: | Thesis (PhD) | ||||
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Subjects: | T Technology > TA Engineering (General). Civil engineering (General) T Technology > TN Mining engineering. Metallurgy T Technology > TS Manufactures |
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Library of Congress Subject Headings (LCSH): | Steel, High strength, Steel -- Microstructure, Steel -- Heat treatment, Steel -- Effect of temperature on, Steel -- Metallurgy | ||||
Official Date: | 2022 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Warwick Manufacturing Group | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Srirangam, Prakash ; Williams, Mark A. | ||||
Format of File: | |||||
Extent: | xxv, 164 leaves : illustrations (mostly colour) | ||||
Language: | eng |
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