When direct current was passed through a coil with a steel core, a magnetic field appeared

When direct current was passed through a coil with a steel core, a magnetic field appeared around it in some way to reduce it.

A core coil is called a solenoid. When direct current is passed through this coil, a magnetic field is generated. The induction of this field is determined in the formula:
B = μ * μ0 * n * I, where μ is the magnetic permeability of the core, μ0 is the magnetic constant, n is the number of turns per 1m, n = N / 2πr, I is the current in the coil.
The formula shows that the induction directly depends on the current strength, the number of coil turns and the magnetic permeability of the core.
It turns out that in order to reduce the magnetic field, it is necessary: to reduce the current strength, to reduce the number of turns, or to pull the core out of the coil.