Just like the type of magnetic steel rod you usually see, a permanent magnet is an object that can retain a certain residual magnetization after removing the external magnetic field. To achieve zero residual magnetization and complete elimination of magnetism for such an object, a reverse magnetic field must be applied. The reverse magnetic field required for complete demagnetization of ferromagnetic materials is called the Coercivity of ferromagnetic materials. Both steel and iron are ferromagnetic, but their Coercivity is different. Steel has a larger Coercivity, while iron has a smaller Coercivity. This is because during the steelmaking process, elements such as carbon, tungsten, and chromium are added to iron, resulting in the production of carbon steel, tungsten steel, chromium steel, etc. The addition of elements such as carbon, tungsten, and chromium results in various non-uniformities within steel at room temperature, such as uneven crystal structure, internal stress, and magnetic strength. The inhomogeneity of these physical properties increases the Coercivity of steel. Moreover, the greater the degree of unevenness within a certain range, the greater the Coercivity. But these non-uniformities are not the best state that steel can possess or have achieved in any situation. In order to achieve the best internal non-uniformity of steel, appropriate heat treatment or mechanical processing must be carried out. For example, in the melting state, carbon steel has similar magnetic properties to ordinary iron; After being quenched at high temperature, its unevenness rapidly increases and becomes a permanent magnet material. If the steel is cooled slowly from high temperature, or the quenched steel is melted at 600 or 700 ℃, its internal atoms have enough time to arrange into a stable structure, and various unevenness decreases, so the Coercivity decreases, and it no longer becomes a permanent magnetic material