The test results of 650 semi continuous casting machine equipped with nitrogen protection device are analyzed, which provides evidence for the study of the relationship between nitrogen content and slab surface quality and oxygen content. Take samples from the middle part of the cast red copper ingot. First, remove the oxide scale of about 1 mm thick on the surface of the red copper ingot with pliers. Cut a sample of about 1 g in the inner part of the red copper ingot with a clean steel drill. Clean the sample with the mixed solution of ethanol and hydrochloric acid. After drying the sample surface with a dryer, weigh it, and put the sample into the oxygen content analyzer for oxygen content analysis And records. By observing the test data, it can be found that with the continuous increase of nitrogen pressure, the oxygen content in the sample presents a downward trend. When the oxygen content drops to 27 ~ 29ppm, it basically maintains a stable value, and the nitrogen pressure of the key change node is 1.4pa. At the same time, with the continuous increase of nitrogen filling amount and pressure, the fluctuation range of oxygen content is small. When the nitrogen pressure is greater than 1.4pa, the slab surface cracking can be obviously observed, which can not support the rolling requirements of subsequent processing.
Next, samples were taken from several pieces of red copper ingots with different oxygen content to analyze the macro metallographic structure of the samples. It can be observed that with the continuous increase of nitrogen charging amount and pressure, the grain size gradually shrinks and the number of columnar crystals synchronously decreases, but the number of equiaxed crystals gradually increases and the structure is dense. Therefore, it can be preliminarily judged that there is a certain correlation between nitrogen pressure and the grain size of the slab. The samples with nitrogen pressure of 1.0pa and 1.4pa were selected for macro metallographic analysis. It can be observed that when nitrogen pressure is 1.0pa, the grain volume of the slab is smaller and the number of equiaxed grains is more; when nitrogen pressure is 1.4pa, equiaxed grains and columnar grains are distributed from the center to the periphery, and the volume of equiaxed grains is larger.
In view of the requirement to minimize the grain volume of rolled copper strip, if there are coarse equiaxed and columnar grains distributed on the surface of the slab, the porosity of the slab will be increased, and the pitting and linear peeling defects will be easily formed at the grain boundary in the later rolling process. Therefore, the nitrogen charging pressure should be controlled at about 1.4pa, so as to effectively control the oxygen content.