The results of the course of martensite transformation in steel under the action of the magnetic field are presented. These results indicate that the formation of stress-assisted martensite in the temperature interval Md-Ms (where superplasticity was observed) is possible. It was found that during hardening of steel products in magnetic field, martensite can be formed not only below Ms, but higher than this point (in the temperature range). The significant structural transformations and steel properties improvement under thermal treatment in a magnetic field depend on the catalytic effect of a field on the transformation development. This effect occurs when the initial phase is paramagnetic, and the product of the transformation is ferromagnetic. Stress-assisted martensite and cooling-induced martensite are formed in the superplastic temperature range of the austenite. This makes it possible for the undeformation hardening of steel products in a magnetic field. The undeformation hardening is caused by a magnetic field holding the product and the formation of martensitic structure oriented predominantly towards the magnetic field vector.