The reason of armor-piercing bullet could shot through the thick armored target is the head of the bullet. It is made of tungsten carbide alloy. But at early stage. It is made of casted iron mostly.
The early 1850’s the wrought iron was considered thick armor, but a Major Sir W. Palliser found a solution to it. He invented a method of hardening the head of the pointed cast-iron shot. This allowed the metal to cool slowly and the body of the shot to be made tough.
Then it is 1880s. Forged steel rounds with points hardened by water took the place of the cast-iron shot.. At first, these forged-steel rounds were made of ordinary carbon steel. But as the armor improved fast. It need new method to keep up with.
Time comes to 1890s. Cemented steel armor became more and more common. A steel contains chromium and nickel became the new material to make the hard corn bullet. A Russian admiral introduce a soft metal cap over the point of shell. This made the bullet increased the penetration by cushioning the impact and avoid the bullet be damaged.
Since then, the bullet made by more and more tough, high hardness and density material. And thanks to that. It made tungsten carbide more useful. And we will change the world we known by our courage and spirit of innovation.