Synopsis:The effects of morphology, size and stability of reversed austenite (rrev .) on the mechanical properties have been investigated in very low C-high Ni-Steels with particular reference to the Widmanstatten-like Trev. (rw). The main results are summerized as follows:(1) Precipitation of rw caused increase of tensile strength and yield strength of steels investigated.(2) Strengthening by the precipitation of relatively stable rw was detrimental for the increase of ductility at low temperature, which is the characteristic of low C-Ni-Steels.(3) When the stability rw was relatively low, the high ductility at low temperature was obtained together with remarkable strengthening by rw.(4) Increase of absorbed energy together with some strengthening was observed in the steels in which small amounts of very fine rw precipitated.(5) The marked strengthening by the precipitation of a large amount of rw decreased absorbed energy. Precipitation of (rLB) along the lath boundaries did not cause remarkable increase of hardness but the decrease of absorbed energy by large amounts of rLB was smaller than that by rw .
A process for producing one-side galvannealed steel sheet by masking coat has been developed. The functions of masking coat consisting of sodium silicate, sodium borate, magnesia, titania and alumina are in masking and repelling from molten zinc and easily exfoliating by light bending after galvannealing heat treatment. The ungalvannealed side surface has an excellent cosmetic corrosion resistance due to fine phosphate coating originating from the very small quantity of zinc deposition of several mg/m2. The galvannealed layer can be free from powdering by reducing the coating weight to as low as 20 g/m2 on one side with the aid of flame wiping and air knife. Deep drawable steel sheet can be made in a continuous galvanizing line without pre-or post-annealing by adopting an extra low carbon steel stabilized with niobium. Thus produced galvannealed steel sheet is considered to be an excellent material against both perforation and cosmetic corrosion for car body exposed panels.
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