The Study On Mechanism Of Cold Bending Crack Formation For Container Plate

The leading product of Guangzhou Zhujiang steel company Limited is container plate; its production and sale are more than 520 thousand tons, accounting for 1/6 of the world's total production capacity, so as to create a huge economic benefits. However, there have been some quality problems in part of the container plate, in the cold bending tests, microcrack is found in the outside cambered surface of thicker container plate, which seriously affect the product quality and business efficiency. This subject is starting with the morphology and texture of microcrack, analyzing their chemical composition, production technology, equipment and other factors to carry out the formation mechanism of cold bending crack for container plate of thin slab continuous casting and rolling, and provide a theoretical basis for the plant to improve production processes and reduce the crack.SPA-H Container plate is the research object in this paper, we adopt metaloscope and SEM test to observe and analyze a large number of microcrack specimen. The main conclusions are as follows:1) There are mainly 3 kinds of defects for cold bending microcrack: subcutaneous microcrack 10～30μm away from surface; Line inclusion; segregation of residual elements (Cu, Ni, As) in the area between surface oxide and matrix. And subcutaneous microcrack occurs more than the other two defects.2) There are a large number of irregularly shaped cracks without certain directions on the surface of the casting blank and the crack deep into matrix up to several hundred microns to 1 mm. EDS spectrum analysis indicate that there is obvious non-matrix material in the crack, These non-matrix material includes segregation of Cu, Ni, P, As and other elements except oxide. Moreover, there are Cu-Ni precipitates between scale and the matrix near the crack and there is segregation of residual elements in some regions.Although part of the slab cracks can be welded together by rolling, because of the use of thin slab casting and rolling technology in Zhujiang steel, the total compression ratio is relatively small ,and the bonding capacity is limited to the slab cracks ,which make the slab cracks inherit to the finished product and destroy the continuity of the matrix. When the plate is bended or punched, it results in stress concentration in the defects; cold bending cracks don't arise until allowed tensile stress is reached.