Abstract
A new microalloy steel has been developed which relies on 0·5%Ni to depress the austenite-ferrite transformation temperature, and on 0·10–0·17%Nb to give large amounts of Nb(C, N) in the austenite during hot processing to enhance grain refinement. It is found that a significantly higher rolling temperature can be used to achieve the properties of conventional microalloy steels, with the implied benefits of relaxed rolling schedules, reduced anisotropy, and the elimination of fracture separations. Various combinations of soaking temperature, thermomechanical processing, plate thickness, and finish rolling temperature have been investigated in full-scale industrial trials on 150 t heats conducted by seven European steel companies. Strength levels of X65 to X75 (yield strengths of 450 to 515 MN m−2) can be obtained, along with excellent toughness. Measurements of dissolved Nb and electron microscopy have indicated that up to 95% of the Nb can be precipitated out in the austenite in a wide range of particle sizes and distributions, depending on rolling conditions. The high strength is a result of grain refinement and precipitation strengthening of both the austenite and the ferrite by Nb(C, N) particles of 2–10 nm dia. The toughness is achieved by ferrite grain refinement, avoidance of ferrite rolling, and a reduction in the level of dissolved N by strain-induced precipitation of Nb(C, N).
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