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A Study of the Microstructural Basis for the Strength and Toughness Properties of Water-Quenched and Air-Cooled HSLA-100, HSLA-100 with Increased Copper, and a ULCB Steel

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Master's thesis

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The microstructural basis for strength of the water-quenched and air- cooled HSLA-100, HSLA-100 with increased copper, and a ULCB steel was investigated by conducting an inclusion study and characterizing the microstructure of each of the steels. For the inclusion study, the SEM was used to examine a large number of fields and determine inclusion morphology while the EDX was used to determine inclusion composition. The microstructure of both the water-quenched and air-cooled steels was characterized using the optical microscope, SEM, and TEM. The HSLA-100 with increased copper steel was adequately calcium treated and aluminum deoxidized as evidenced by the low sulfur content, few MnS stringers, and lack of large oxide arrays. The ULCB steel was not calcium treated or Al-killed nor was it thermo-mechanically processed as shown by the lack of lipped, broken, or elongated stringers. Both MnS ans oxide inclusions were present, and consequently, ladle metallurgy would have to be used before the ULCB steel was hot-worked. The microstructural basis for the strength of the as-quenched HSLA-100, HSLA-100 with increased copper, and ULCB steels is the transformation product packet size, small lath width, and high dislocation density. The as-quenched increased copper steel also contained e-copper precipitates which added to its high level of strength and toughness. The as-quenched ULCB steel did not reach the desired strength level because its transformation product packet size was too large approximately the same size as the prior-austenite grains. Thermo-mechanical processing of the steel would result in a finer packet size.

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  • Properties of Metals and Alloys

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