H is the most harmful element in general steel. Hydrogen dissolved in steel can cause defects such as hydrogen embrittlement and white spots in steel. Like oxygen and nitrogen, hydrogen has very little solubility in solid steel. It dissolves in molten steel at high temperatures. When it cools, it does not reach out and accumulates in the structure to form high-pressure fine pores, which makes the steel's plasticity, toughness and fatigue strength drop sharply. In severe cases, it will cause cracks and brittle fracture. "Hydrogen embrittlement" mainly occurs in martensitic steel, which is not very prominent in ferrite steel, and generally increases with hardness and carbon content.
On the other hand, H can increase the magnetic permeability of steel, but it also increases the coercive force and iron loss (the coercive force can be increased by 0.5 to 2 times after adding H).