This Finite Element Analysis shows the development of buckling in a HEA 500 steel beam. The buckling analysis takes into account geometric nonlinearity (large deformations) according to the formulation of Simo J.C. and Miehe C. The material nonlinearity of steel after yielding (elastoplasticity) is simulated by taking into account the Von Mises constitutive model with isotropic linear hardening. In particular, it is possible to detect several instability modes developing at different critical loads.