Grain size and precipitates have a significant effect on the mechanical properties of steels and it is desirable to be able to characterise these in a non-destructive manner. Grain and precipitate sizes and their spatial distributions in both an extra-low-carbon steel and a laboratory model steel have been individually varied and compared with a variety of characteristic magnetic parameters measured from major and minor magnetisation loops. These magnetic parameters are shown to be very sensitive to grain size distribution when there are no precipitates within the grains. However, the magnetic parameters exhibit complex behaviours with precipitate size distribution, which is linked to a critical precipitate size for effective pinning and another critical precipitate size for strongest pinning to domain walls. The interaction between grain size and precipitate distribution effects on the minor loop properties in the studied steels are discussed.