Effect of Nb on Austenitic Grain Growth Behavior on Nanoscale Under Different Manufacturing Deformation Temperatures and Holding Time

With the increasing demand for oil and natural energy, northwest area gradually became an important area for oil and gas development. This facilitates the raising of the transportation pressure and efficiency. Top quality toughness property under low temperature of pipeline steel demand is increasing, which also forced pipeline steel to have higher requirements. For thick gauge pipeline steel, austenite grain size was a critical element. In this chapter, the impact of niobium (Nb) on austenite grains growth tendency of pipeline steel under different heating processes was researched by using thermal simulation experiments, measurement of austenite crystal size, and observation of precipitates morphology and distribution. The results showed that at the same deformation temperature and soaking time, the austenite crystal size of the steel containing 0.03% Nb is tremendously smaller than that of the steel with 0% Nb, and the gap between the two types of steel is about 5–7 μm. Along with climb of shaping temperature and soaking duration, volume fraction of Nb precipitated phase went down and the size went up; when shaping temperature was above 1020 °C or soaking time was longer than 1h, the diameter of the nanoscale Nb precipitations became larger and the number decreased. For 0.03% Nb steel, the heating temperature should be lower than 1020 °C and holding stage was below 1 h to purchase a smaller uniform austenite grains. Under this condition, precipitation can effectively inhibit pellet boundary trek, and austenite spark dimension can be controlled. Based on the change pattern of austenite grain expansion at different manufacturing deformation degrees and soaking extension, the kinetic modeling of austenite grain growing characteristics was formed. It can help predict how nanoscale precipitates influence austenite grain growth condition. And the kinetic modeling can effectively provide guidance for on-site manufacturing.