Development Of Rolling And Heat Treatment Process Of 9Ni Steel

With the enhancement of protecting the environment consciousness, the LNG (Liquefied Natural Gas) is used more and more because of being a variety of clean energy. Consequently, it is a critical thing to develop 9Ni constructional material with self-owned intellectual property rights. The mechanical property of 9Ni steel was focused. And the influence of each process on the microstructure and mechanical property was investigated by means of component design, thermal simulation, rolling process and heat treatment process. Moreover, parameters optimization was achieved. The main experiments and conclusions were as follows:1. Referred to domestic and overseas standards of 9Ni steel, effective component scheme was provided though optimizing component parameters.2. The hyperthermia deformation experiments were done by use of the thermal simulation machine. The relationship of austenitic stress-strain under different temperatures and speeds was established by single-pass compression experiment. In addition, model of the resistance to deformation was deduced. And the static and dynamic CCT curves of 9Ni steel were drawn by continuous cooling transformation experiment. The conclusions were as follows:there are only bainite transformation zone. Compared with static CCT curve, the temperature of bainitic starting transformation is lower, conversely its finishing transformation is higher. So it means that the speed of dynamic bainite transformation is higher.3. The die-casting and continuous casting-rolling process were repeated. In order to get 4R(Reduce, Reuse, Recycle, Remanufacture) target, rolling process of finishing temperature at two-phase region and heat treatment online was tried. And mechanical properties of 9Ni steel under both processes were excellent.4. The influences of component of 9Ni steel and finishing rolling temperature on mechanical property were analyzed. The conclusions were as follows:the content of nickel is significant to its mechanical property. Neither lower nor higher of the nickel content is benefit to its mechanical property. Additionally, the microstructure is fine when the finishing rolling temperature is 730℃. 5. The parameters of heat treatment processes were optimized by changing the temperature parameters in every stage. Moreover, the relationship among temperature, microstructure and hypothermia impact toughness was studied. The conclusions were as follows:in the process of quenching and tempering (QT) heat treatment, the optimized temperature parameters are quenching at 800℃and tempering at 580℃. Additionally, the temperature of quenching has a great effect upon grain size, and the temperature of tempering has a great influence on the precipitation and stabilization of reverted austenite. Then in the process of quenching, intermediate quenching and tempering (QLT) heat treatment, the optimized temperature parameters are quenching at 800℃, intermediate quenching at 670℃and tempering at 580℃. But the temperature of quenching has no obvious influence in QLT heat treatment. Similarly, the temperature of tempering has the same effect. Compared two heat treatment process, the hypothermia impact toughness of 9Ni steel is improved by the QLT heat treatment.