Preparation method for manufacturing low-cost ultrafine grain transformation-induced plastic steel seamless tube online

The invention relates to a preparation method for manufacturing a low-cost ultrafine grain transformation-induced plastic steel seamless tube online. The method comprises the following steps: forging, turning, heating, perforating and coldly drawing a cast ingot to obtain a seamless cold-drawn steel tube; then carrying out circulating heating and quenching treatment for over three times by adopting a medium-frequency heating induction coil; finally, air cooling, water cooling or naturally cooling to room temperature, so as to obtain the low-cost ultrafine grain transformation-induced plastic steel seamless tube. Chemical components of common low-carbon steel are utilized, so that the cost is relatively low, the tensile strength is greater than or equal to 900 MPa, the yield ratio is smaller than or equal to 0.64, the ductility is greater than or equal to 24.0%, the work hardening index is greater than or equal to 0.23, the anisotropic index is greater than or equal to 1.01, the average grain size of a microscopic structure is smaller than or equal to 3mum, and the volume fraction of the retained austenite is 11.5-19.5%.