Nucor Corporation

Paul Kelly

James W Watson

Walter N Blejde

Christopher Ronald Killmore

Mike Schueren

David C Van Aken

This application discloses thin metal strips and methods of making thin metal strip. Particular embodiments of such methods include cooling the thin metal strip to a temperature equal to or less than a bainite or a martensite start transformation temperature Bor Mto thereby form bainite and/or martensite, respectively, within the thin metal strip, reheating the thin metal strip to a reheat temperature equal to or greater than transformation temperature Acand holding the thin metal strip at the reheat temperature for at least 2 seconds and thereby forming austenite within the thin metal strip with at least 75% of austenite grains having a grain size equal to or less than 15 &#x3bc;m, and rapidly recooling the thin metal strip to a temperature equal to or less than the martensite start transformation temperature Mand thereby providing finer martensite within the thin metal strip from a finer prior austenite.

Thermal cycling for austenite grain refinement

A hot rolled light-gauge martensitic steel sheet made by the steps comprising: (a) preparing a molten steel melt comprising: (i) by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than 0.05% niobium, less than 0.5% molybdenum, and silicon killed containing less than 0.01% aluminum, and (ii) the remainder iron and impurities resulting from melting; (b) solidifying at a heat flux greater than 10.0 MW/mand cooling the molten melt into a steel sheet less than 2.0 mm in thickness in a non-oxidizing atmosphere to below 1080&#xb0; C. and above Artemperature at a cooling rate greater than 15&#xb0; C./s; and (c) hot rolling the steel sheet to between 15% and 50% reduction and rapidly cooling.

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Charlotte, NC, United States of America

Paul Kelly