Studies of the interrelation of metallurgical and magnetic properties of high strength pearlitic steels are described. High voltage Lorentz electron microscopy has been used to identify directly the pinning sites for domain boundaries. The results lead via a physical model to a formula which permits the prediction of coercivity from the metallurgical microstructure. The empirical relationship suggested by Kneppo is found to describe well the virgin magnetization curve for a range of steels including those heat treated. It is shown that weld renormalization leads to magnetic parameters almost indistinguishable from the parent plate. Heat input variations do not significantly affect the magnetic properties.