A study was made of the effect of manganese and sulfur on the austenite grain size and recrystallized ferrite grain size after cold-rolling in hot-rolled 0.035% carbon steels. No element for refining austenite grains, such as aluminium, niobium etc., has been added in the steels, whose compositions were similar to that of rimmed steels for cold strip.
Manganese and sulfur contents were varied within the range of 0.1 to 0.4% and 0.01 to 0.03% respectively. Decrease of the manganese content or increase of the sulfur content reduced austenite grain size of ASTM No 1-2 to No 7-8 of the hot-rolled steels which was measured after carburizing at 925°C for 6 hr, and raised the austenite grain coarsening temperature. It was found that the hot-rolled steel with densely dispersed fine particles of 100 to 500Å in diameter showed fine austenite grains, but the steel with sparsely dispersed particles showed coarse grains. It was also found that the recrystallized ferrite grains became finer, when the hot-rolled steel which showed fine austenite grains by carburizing at 925°C was cold rolled and fully annealed. It was thus concluded that the refinement of austenite grains as well as recrystallized ferrite grains results from the fine particles precipitated during hot-roll process, most of which could be identified as β type MnS.