New methods in steel design and basic understanding of the novel materials require large scale ab initio calculations of ground state and finite temperature properties of transition metal alloys. In this contribution we present ab initio modeling of the structural and magnetic properties of XYZ compounds and alloys where X,Y=Mn,Fe,Co,Ni and Z=C,Si with emphasis on the Fe-Mn steels. The optimization of structural and magnetic properties is performed by using different simulation tools. In particular, the finite-temperature magnetic properties are simulated using a Heisenberg model with magnetic exchange interactions from first-principles calculations. Part of the calculations are extended to the nanoparticle range showing how ferromagnetic and antiferromagnetic trends influence the nucleation, morphologies and growth of Fe-Mn-based nanoparticles. (C) 2011 Materials Research Society.