The invention presents method and apparatus for forming of large thin panels and similar parts having integral stiffeners at one side of a board. The method includes progressive forming of preheated billets into sculptured dies of corresponding length and width by a forging die of substantially identical width and short length with slightly inclined and flat forging surfaces providing a smoothly convergent working zone, and a periodical transfer of the sculptured die into the working zone between successive strokes of the forging die. Control of the material flow is performed by selection of a ratio of the working zone length to the billet thickness, variable contact friction at both dies and application of additional compressive forces to the billet at ends of the working zone. For semi-continuous processing of very long parts, the sculptured die is composed by plurality of sectioned elements. Embodiments of the method are conditions of isothermal and super plastic forming of light alloys with sub-micron grained structures.