Delamination Defects and Control Measures of Structural Seamless Steel Tubes

Definition and characteristics of delamination defects:
Delamination is a layered separation phenomenon parallel to the tube wall inside a seamless steel tube, usually caused by non-metallic inclusions, pores, shrinkage residuals or improper rolling process.

Its characteristics include:
Macroscopic manifestation: layered cracking on the cross section, abnormal reflection waves during ultrasonic testing.
Microscopic manifestation: inclusion aggregation, banded structure segregation or hydrogen-induced cracking.

The main causes of delamination defects in structural seamless steel tubes:

From the perspective of mechanism, it is generally believed that non-metallic inclusions in the tube blank will destroy the continuity and compactness of the 16mn structural seamless steel tube, and serious inclusions may even cause delamination inside the 16mn structural seamless steel tube. The other is considered to be hydrogen-induced cracking, that is, due to the accumulation of hydrogen in the steel, the partial pressure of the gas inside the metal is too high, white spots are formed in the round tube billet, and the cracks expand during the rolling process, eventually forming delamination defects. In addition, the stress generated by the uneven deformation of the two-roll cross-rolling piercing exceeds the plastic strength and will also cause delamination.

In the case of strict smelting control, the third situation often occurs, and its control measures are:

1. Improve the plastic toughness of 16mn structural seamless steel tube
Improve the cleanliness of molten steel and reduce harmful inclusions; increase the proportion of equiaxed crystals of continuous casting slabs to reduce center segregation and center porosity; adopt a reasonable cooling system to avoid internal cracks inside the slab; The slow cooling process reduces internal stress, thereby ensuring that the microstructure and mechanical properties of the pipe and the finished 16mn structural steel pipe meet the requirements of technical standards.

2. Reasonably control the heating temperature
By determining the thermoplastic curve, choose the best heating temperature. It is also necessary to pay attention to sufficient heat preservation time when heating the tube blank, so as to reduce the deformation resistance and improve the plastic toughness of the 16mn structural steel pipe.

3. Reduce the roll speed

The roll speed is a key parameter of the piercing process. During the change of the roll speed from low to high, there is a critical roll speed at which stratification begins to appear. When the roll speed is low, the tube billet is easy to form cavities; when the roll speed is high, the tube billet and 16mn structural steel pipe are easy to form delamination defects. In order to eliminate the delamination defects of the tube blank and 16mn structural steel pipe, the roll speed should be reduced below the critical roll speed at which delamination begins to appear.

Read more: Seamless Tube Production Equipment