The solid solution strengthening and cold work hardening of P in steel are extremely strong. It increases the strength and hardness of the steel, and decreases the plasticity and toughness (especially the low temperature toughness). This is the cold brittleness of steel. Phosphorus segregates in the steel, increasing the tempering brittleness of the steel (Cr-Mo steel is more obvious).

Controlling or reducing the S and P content in the steel not only improves the plasticity and toughness of the steel, improves the welding performance, reduces hot and cold brittleness and tempering brittleness, but also improves the cleanliness of the steel, reduces the brittleness transition temperature, and increases the thickness of the thick plate along the thickness direction Z direction performance. This is one of the most important ways for steel to improve toughness.