Evaluation of DQ&P Processing Route for the Development of Ultra-high Strength Tough Ductile Steels

Abstract

A novel concept of direct quenching and partitioning (DQ&P) processing route has been exp lored with the specific aim of producing ultra -high strength structural steels with yield strengths ≈1100 M Pa co mb ined with good uniform and total elongations and impact toughness. The approach used was to design suitable compositions based on high silicon and/or aluminiu m content, establish the DQ&P parameters with the aid of physical simulation on a Gleeble simu lator and finally, design the DQ&P p rocessing route for trials on a laboratory ro lling mill. Dilatation experiments were made on a Gleeb le simulator to determine the appropriate cooling rates and quench stop temperatures for obtaining martensite fractions in the range 60-90%. Two types of dilatation tests were conducted starting with either strained or unstrained austenite prior to quenching to roughly simulate industrial rolling with low and high fin ish rolling temperatures, respectively. Laboratory rolled samples were direct quenched in water to desired quench stop temperatures followed by partit ioning in a furnace held at the quench stop temperature. Some samples were cooled very slowly thus simu lating the cooling of the coiled strips in industrial rolling. Metallographic studies showed that the desired martensite-austenite microstructures were ach ieved. Besides high strengths, the ductility (including uniform elongation) and impact toughness were significantly improved. St rain ing of austenite prior to DQ&P led to extensive refinement of martensite packets/blocks and laths and hence resulted in imp roved mechanical p roperties, particu larly impact toughness. No adverse effect o f pro longed partitioning simu lating co iling has been noticed suggesting new possibilities for developing tough ductile structural steels both for strip and plate products.