The Research On Low-carbon 800MPa Cold Heading Steel

Manufactures (above 8.8 levels) the fastener to use the high strength steel, as a result of operational strength's request, must be finally the medium carbon steel or the carbon alloy steel. At present the microstructure of the steel produced by the metallurgy factory is not very ideal, and is difficult to directly cold heading. Therefore, fastener before production carries on the spheroidizing is an essential link, however, this processing not only increasing the cost, but also the polluting the environment.Specially regarding massive small and medium-sized enterprises, which does not have the protective atmosphere heat-treatment furnace, it will cause the superficial oxidation and the decarbonization frequently and serious influence the surface quality. The bars which can be directly cold headed without annealing is very required.The use of controlled rolling and controlled cooling and TRIP on the production of cold heading steel could significantly increase the strength and plastic property of the steel. Controlled rolling and controlled cooling could not only improve the strength and toughness properties of hot rolling steel products which can be directly fabricated without heat-treating, but also save alloy, simplify working operation, reduce energy consumption, increase labor productivity and enterprise's economic efficiency.In this article, the chemical composition of the high-strength cold heading steel has been designed. And then the steel has been heated to high temperatures on the thermal simulation machine, and through analyzing dynamic CCT curves and the observation of microscopic structures of the steel, the transmutation products of the deformed austenite at different cooling rates have been determined which would provide a theoretical basis for the follow-up experiment--controlled rolling and controlled cooling. OnΦ450 mill in the laboratory different kinds of experiments were conducted such as conventional rolling and controlled cooling. Tensile tests has been carried out on the sample of the steel, and through analyzing the microscopic structures and mechanical properties, the effects on the high strength cold heading steel of the final rolling temperature, the ending temperature and the way of the cooling have been studied. It is approved that the mechanical properties of the high strength cold heading steel after being rolled at low temperature with controlled cooling are better than that of conventional rolling with controlled cooling and multiphase microstructuers containing ferrite, bainite and retained austenite were obtained by controlled rolilng and cooling of steel.By using controlled rolling and controlled cooling technology, the tensile strength of the cold heading steel reached above 800MPa, and the elongation reached more than 16%, all the mechanical properties reached the requirement of the fastener above 8.8 levels. A theoretical basis was offered for the actual development of the low-carbon cold heading steel instead of themiddle-carbon steel.