Formation Causes of 15Mn Seamless Steel Tube Banded Structure

In the production of 15Mn seamless steel tube with hot rolling process, the pipe body has obvious banded structure．

15Mn seamless steel pipe, in the metallographic examination found that the matrix exists ribbon structure. Why does this happen?

Steel tube production process: Converter smelting → LF furnace refining → Round billet continuous casting → Tube heating – Fungus perforation → MPM Continuous rolling tube → Alignment → Cooling bed cooling.

During the cooling process, the molten steel solidifies in a dendritic manner. Segregation exists between dendrites and dendrites. Dendritic segregation of continuous casting slabs is the root cause of banded structure. The nucleation process is a process in which a floating atomic group in a molten steel gradually grows to a certain size, forming a solid particle (a stable atomic group), enabling the surrounding atoms to be stacked upward. The preconditions for nucleation are structural fluctuations, energy fluctuations and a certain degree of supercooling. There are mainly two forms of homogeneous nucleation and heterogeneous nucleation.

Improvement measures:

1. Using low temperature casting method, properly reducing the superheat of molten steel, can inhibit the growth of columnar crystals, increase the proportion of equiaxed crystals, thereby increasing the width of equiaxed crystal ribbons, refining crystal grains, and improving round billet The purpose of internal quality.

2. Add a small amount of iron powder, aluminum iron or ferrotitanium alloy powder to the molten steel during pouring to reduce the temperature of the surrounding molten steel. On the other hand, it can react with molten steel to form particles such as Al2O3 and TiN to promote heterogeneous nucleation. Increase the number of crystal grains to achieve grain refinement and improve segregation of components.

3. Properly increase the strength of the electromagnetic stirring process of the crystallizer to ensure that the molten steel can effectively break the dendrite and increase the number of crystal grains, so as to refine the grains and improve the segregation of the components.

4. The soft pressing process at the solidification end can effectively break the dendrites, increase the fluidity of the molten steel, and dilute the C and Mn elements enriched in the mushy zone to alleviate the segregation of the constituents of the round billet.

During the process of cross-rolling and perforation, the tube blank is subjected to the rolling torque. The metal is plastically deformed under the action of stress. The band structure presents a certain spiral shape. During the rolling process, the steel tube longitudinally extends and the spiral angle becomes smaller. The banded structure makes the steel pipe anisotropy and reduces transverse mechanical properties.