Rapidly spheroidized medium carbon steel bar and its manufacturing method

Abstract

The present invention relates to a rapid spheroidized medium-carbon steel bar and its manufacturing method. The chemical composition of the steel is, by mass percentage, 0.30-0.60% C, ≦0.40% Si, 0.50-0.90% Mn, ≦0.020% P, ≦0.030% S, ≦0.30% Cr, and typically ≦0.25% Cu and Ni. Microalloying elements such as B can be added as needed to adjust performance. To ensure grain size, one or more grain-refining elements such as Al, Nb, and N can be added, with the balance consisting of Fe and unavoidable impurities. The steel undergoes smelting, continuous casting, heating, ultra-low-temperature rolling, and rapid spheroidization. The total spheroidization time is 5.5-6.5 hours, which is similar to soft annealing but significantly shorter than conventional spheroidization annealing processes. The resulting material structure consists of dotted spheroids, ferrite, and a small amount of lamellar cementite. When the hardness is ≦160HBW, the elongation rate is ≧30%, and the rolling rate is 80%, there are no cracks during cold forging, making it suitable for cold forming processes with large deformation amounts, and realizing modern high-precision, energy-saving, emission-reducing, eco-friendly, and high-quality production.

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