High strength ship plate tension stratification control
In recent years, with the rapid development of China's shipbuilding industry, many domestic steel mills are in the development and production of high-strength ship plate steel, such as Baosteel, Wisco, etc., but high-strength ship plate will appear due to tensile test after the fracture center split, that is, tension stratification and scrap phenomenon. The results show that unrolled loose holes, segregation of C and Mn elements in the center, strip MnS inclusions, and composite inclusions of MnS, Al2O3 and TiO2 in the center of high strength ship plate thickness are the causes of tension delamination.
The relevant control measures are:
1. Reduce S content
Ship plate steel is a kind of high manganese steel, when the Mn content is about 1.0%, S is easy to form MnS with Mn in the steel, so the S content in the steel should be controlled, generally controlled below 0.005%.
2. Feed the silicon calcium wire
The sulfide inclusions, especially the MnS inclusions, can be denatured by feeding the silicon-calcium wire, and their shape can be controlled to be spherical, which is less harmful to steel, so as to reduce the tension delamination phenomenon.
3. Optimize continuous casting
1) Superheat
In the case of no freezing of the water nozzle, the thick plate casting machine should be poured with low superheat as far as possible, and the superheat is controlled at 15-25℃.
2) Pull speed
Under the premise of not affecting the output, the pulling speed should not be too large, and the pulling speed should be controlled at 0.9-1.1m/min.
3) Secondary cold
The strong cooling mode can slow down the growth of columnar crystals, increase the equiaxed crystal rate, and reduce the occurrence of center segregation and center porosity.
4) Electromagnetic stirring
The use of secondary cooling electromagnetic stirring can improve the internal quality of the billet, reduce the segregation tendency of each element and reduce the center loose of the billet.
4. Optimize heating and controlled rolling and cooling
1) Heating temperature
The higher soaking temperature and longer soaking time are conducive to the diffusion of segregation elements, and the opening rolling temperature can be controlled at 1100-1180℃.
2) Control rolling and cooling
By increasing the reduction rate, the stress-strain permeability can be increased, the defects produced in the casting billet can be welded, and the lamination defects of the steel plate can be reduced.
During the cooling process, the C atom in the Mn poor zone can complete the transformation from austenite to ferrite before it is too late to diffuse to the Mn rich zone, and the banded distribution of ferrite and perlite does not appear, and the formation of banded structure is inhibited.
