Research On Microstructure And Mechanical Properties Of TRIP Steel With Nb

The properties and technology development of modern vehicles tends to less weight, save energy, reduce emissions and improve safety. High-strength steel of car's body panels can be used to reduce body weight, but also can save resources and reduce emissions to protect the environment. In a series of high-strength low-alloy steel, TRJP(Transformation Induced plasticity )steel is a new type of high strength structural steel for automobiles.TRIP (Transformation Induced plasticity)steel combine attractive strength and formability properties. They composed of ferrite, bainite and retained austenite. They simultaneously have both strengths and elongation because of the martensitic transformation of metastable retained austenite during plastic deformation, they have been receiving increasing attention for automobile bodies. The microstructure was observed under a series of cooling rates by thermal simulation, and the rule of transformation of TRIP steel was also studied; The study of high-temperature thermoplastic technology can be used to choice hot-rolled steel thermoplastic area, Niobium has also been found at high temperature area makes intensity raising and thermoplastic descending; The effection of Nb on microstructure and mechanical properties of TRIP steel was researched; and the Nb precipitation state was researched too. The microstructures of test steel were examined and analyzed by colour metallogragh, SEM, TEM, in situ tension test and so on. The main conclusions are drawn as follows:(1)With the metal alloy chemical element Nb in the TRIP steel, the microstructure of TRIP steel were refined. Added Nb, which increases the stability of overcooling austenite and inhibits the occurrence of transformation, therefore the whole CCT curve was moved downward(2)The study of high-temperature thermoplastic technology can be used to choice hot-rolled steel thermoplastic area, Niobium has also been found at high temperature area makes intensity raising and thermoplastic descending. At room temperature,niobium precipitates also makes cold-rolled steel tensile and yield strength higher than TRIP Steel without niobium. However, due to the precipitation of niobium makes austenitic phase transitions obstructed, and as a result, retained austenite more stable. Maintaining small grains for containing niobium, thus plastic of the cold rolled steel TRIP which containing niobium is not decreased.(3)Experiments for thermal annealing simulation for TRIP Steel which containing niobium is been on by Gleeble-3800 machine, the regulation of the microstructure changing and properties of niobium are found with the temperature rising in two-phase area, it is found that the precipitates niobium aggregation grows up with the content of retained austenite raising. Meanwhile bainite content is increased and ferrite content is declined, making intensity increased and plastic decreased for niobium TRIP steel. The regulation of the structure changing with different temperature is relatively more complicated.