Research On Surface Micro-cracks Of Medium Plate In Chongqing Iron & Steel Co.

In order to solve the problem of micro-cracks appeared on slab of CC and medium plate of CHONG STEEL, the secondary electron microscopy technology, metallographic analyzing, and electron probe technology were used to examine the morphosis and forming mechanism of micro-cracks on slab and steel plate, the mold fluxes used in CC of medium carbon steel and the effect of Nb, Ti on mechanic properties at high temperature were also studied, the key processing parameters of inducing the micro-cracks were understood. Through the study mentioned above, effective programs were proposed, the programs were successfully applied in plant practice.The studies and plant practices can be briefly described as follows:(1) The micro-cracks appeared on the steel plate have the following characters: the out appearance of the cracks is slim and curving, but the subsurface cracks is long and remarkable. The micro-cracks usually happened on medium thick plate of low alloy steel, especially on low alloy steel containing trace Nb and Ti. The micro-cracks on plate originally appeared along with the grain boundary at surface and subsurface of strand during CC of steel. The study reveals that the precipitated inclusion will boost the forming of micro-cracks.(2) Surface micro-cracks appeared on plate can be defined as star-cracks, net-shaped cracks, transverse cracks, and longitudinal cracks, which are matched with the cracks on strands. Among these cracks, the star and net-shaped cracks are much more harmful than others. The forming reasons of micro-cracks are complex, the high temperature mechanic properties of steel determined by chemical composition of steel is the interior factors of cracks forming, processing parameters and condition are the exterior factors.(3) By adopting of Ni-Fe coated mold, optimizing and increasing lubrication ability of mold fluxes used in CC of low alloy steel, the friction enforced on strands was greatly reduced, and the star and net-shaped cracks were remarkably minimized. The oscillation depth and transverse cracks were also reduced by controlling viscosity and consumption of mold fluxes. Reducing cooling strength of strands, and keeping stable casting condition are all helpful for minimizing the surface cracks appeared on medium plate.