Method of eliminating banded structure of seamless rectangular tube

The banded structure makes the part anisotropic, which reduces the elongation, area shrinkage and impact toughness values in the direction perpendicular to the banded structure. Banded tissue serious parts, heat treatment is easy to produce deformation, the same part of the hardness is not uniform, low carbon content of the parts of the quenching soft point.


The reason for the formation of banded structure is the segregation of alloying elements in steel, especially the segregation of sulfur and phosphorus and various non-metallic inclusions, such as MnS and silicate inclusions, which are easy to form banded structure. The dendrite segregation produced during solidification of steel can not be eliminated in the later pressure processing process, which often keeps the banded structure. In addition, the hot tie and forging process is improper, especially without repeated forging, change direction flip forging, will also make the raw material banded structure preserved.



The specific elimination methods of seamless rectangular tube are as follows:


(1) The banded structure caused by component segregation, that is, when the steel contains harmful impurities such as phosphorus and is pressed, the impurities stretch along the rolling direction. When the steel is cooled to less than 4, these impurities become the core of ferrite so that the ferrite morphology is banded, and then the pearlite is banded. This banded structure can be eliminated by electroslag remelting and increasing the crystallization rate.


(2) Banded structure caused by improper hot working temperature, I .e. when the hot working stop forging temperature is in the two-phase zone (between/4, and 40), ferrite is precipitated from austenite in a banded form along the metal flow direction, austenite that has not yet decomposed is cut into banded form, and when cooled to A,, banded austenite is transformed into banded pearlite. This structure can be improved or eliminated by increasing the final rolling temperature, increasing the forging ratio or diffusion annealing and normalizing.


(3) The banded structure caused by component segregation is difficult to eliminate by heat treatment. This segregation is usually mitigated to some extent by normalizing, which normally corrects the segregation to an allowable level. If the banded tissue is serious, it can be improved by normalizing many times. The most reliable method is the first high-temperature diffusion annealing, followed by a normalizing, which can achieve the effect of completely eliminating the banded structure, but the cost will be very high, for the steel plate is limited by the surface quality, it is difficult to achieve.