The banded structure is a kind of defect in steel, which can be divided into primary banded structure and secondary banded structure. The existence of banded structure makes the structure of steel uneven, and seriously affects the properties of square tube, reduces its plasticity, impact toughness, fracture toughness and section shrinkage, resulting in cold bending incompatibility and high scrap rate of stamping. The square pipe is easy to deform and crack during quenching. To improve this problem, we can adopt the following four methods:
No.1 rolling process
The primary strip structure occurs in the continuous casting process, but the secondary strip structure can be effectively restrained by adopting reasonable rolling process. The parameters such as heating system, start-up temperature, deformation, final rolling temperature and cooling rate are particularly important in rolling process. By heating the billet, the composition segregation of as-cast structure can be homogenized, and the deformation resistance of square tube during rolling can also be reduced. Under permissible conditions, higher heating temperature should be adopted as far as possible, and enough heating time should be ensured.
Composition of No.2 Steel
In the aspect of steel grade composition design, the relationship between elements that increase transformation temperature and elements that decrease transformation temperature should be properly handled. The effect of alloying elements on transformation temperature is interaction and interaction. If the influence of several elements can be counteracted and the difference of transformation temperature between dendrites and within dendrites can be reduced, the banded structure of square tubes can be alleviated.
THREE HEAT TREATMENT TECHNOLOGY
In the annealing process of square tubes, the pre-eutectoid ferrite precipitates sufficiently and the banded structure grade is aggravated due to furnace cooling. In the normalizing process, the cooling rate is faster, which can reduce the banded structure. When isothermal normalizing process is adopted, the steel is heated and heated for a period of time, and rapidly cooled to a certain temperature in the pearlite transition zone. Then the uniform transformation of ferrite and pearlite is completed by thermal insulation, and then cooled in air.
Four Continuous Casting Process
The strip structure of steel after rolling mainly comes from dendrite segregation in continuous casting slab. Controlling dendrite segregation in continuous casting slab and promoting the uniform distribution of elements are effective ways to reduce or eliminate strip structure. From the point of view of continuous casting technology, the dendrite segregation can be effectively controlled by enlarging the equiaxed zone and obtaining fine secondary dendrites. By controlling the pouring temperature reasonably and keeping constant pouring speed, the equiaxed grain region can be effectively enlarged. In addition, reasonable secondary cooling technology is adopted to control the cooling water volume in each section of the secondary cooling zone. The surface temperature of square tubes can be controlled to make the cooling uniform, and the equiaxed crystals in large areas can also be obtained.