Analysis and Control of Black Streak Defects on the Surface of 304 Stainless Steel Rolled Strips

The black streak defect on the surface of 304 stainless steel is one of the key issues affecting the quality of stainless-steel products and the economic benefits of the industry. Cold-rolled plates were selected, and the microscopic images and composition of the sample surface were analyzed using a super-depth optical microscope, SEM, and EDS for the areas without black streaks and black stripe defects. There are significant differences in morphology and composition between areas without black streaks and areas with black streak defects. The mold flux particles will form uneven protrusions and pits on the surface of stainless steel. The height difference between the protrusion and the stainless-steel surface is greater than 20 μm. The black streak defect area on the surface of stainless steel clearly contains elements such as C, O, Si, Na, and Al, which are related to the composition of the mold flux particles. Therefore, the reasons and mechanisms for the formation of black streak defects were analyzed. The root cause of black streak defects is the inclusion of carbonaceous mold flux particles in the molten steel during the continuous casting process. In the continuous casting process and equipment, by adjusting the mold taper, controlling the insertion depth of the submerged entry nozzle, selecting the mold flux suitable for melting characteristics, stabilizing the casting speed, and other measures, the liquid level in the mold during the molten steel casting process was stabilized, and the production conditions were effectively improved for the working condition with a casting speed of 1.45 m/min. The occurrence rate of black streak and degradation rate continued to decrease and were stabilized below 3% and 1%, respectively.