Special steel is needed, because the submarine's function is to dive into the water to perform tasks, so according to the operational needs, it must conform to its diving depth, and its external compressive layer must be firm, and it must be hidden and easy for sonar detection. Otherwise, it is easy to be found by the other party. Therefore, its technical content is very high, which is beyond the competence of ordinary steel. The water pressure of several hundred meters is very strong. You also need to consider tactical requirements such as drifting and hitting. , so it is not easy to produce, and the cost and technology are very high, and the average country can't afford it. India is also a big country, but the damaged nuclear submarine still needs Russia to repair it. He can't produce such steel.