Y.sawaragi et al. found that due to the precipitation hardening effect of fine TiC and NbC, adding a small amount of titanium and niobium increased the durability of stainless steel. Through the analysis of niobium-containing steel solution treatment and stainless steel pipes produced by different manufacturers, the results show that the carbonitride content in carbonitrides produced by stainless steel tubes containing 0.2%-0.4% niobium is almost the same. Therefore, they believe that when the solution treatment temperature is 1150, the best niobium content to improve durability is 0.2%.
However, the permanent plasticity of stainless steel pipes containing titanium and niobium varies greatly between 650 and 750. The plasticity of copper and titanium-containing steel is as low as copper-containing steel. However, adding niobium to copper-containing steel will increase the permanent plasticity of stainless steel pipes, especially adding 0.4% ~ 0.8% niobium to steel with 3% copper content, which will still have good permanent plasticity of stainless steel pipes even after long-term aging. Some experts believe that the improvement of the permanent plasticity of steel containing copper and niobium is achieved by refining the structure by retaining undissolved NbC in the state of solution treatment.