Causes of Delamination of 07Cr18Ni11Nb Stainless Steelduring Diagonal Rolling Piercing Process

A batch of 07Cr18Ni11Nb stainless steel exhibited delamination during the piercing process. The causes of delamination were analyzed through macroscopic and microscopic inspection, chemical composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the chain-like and rod-like primary Nb(C,N) segregation phase and non-uniform deformation during the piercing process were the main causes of delamination. During the piercing process, the stress concentration caused by the deformation incompatibility between primary Nb(C,N) phase and austenite matrix reduced the local fracture strength of the tube billet. Under the effect of additional tensile stress caused by non-uniform deformation, cracks occurred in the stress concentration area and continued to expand in the subsequent rolling process, resulting in the delamination. It was recommended to control the content of carbon and niobium in smelting process, improve the remelting and solidification conditions, adjust the piercing process parameters and reduce the roll speed appropriately to effectively prevent the occurrence of piercing delamination.