The paper d e a l s with t h e formation o f g r a i n boundaries during hot deformat i o n of a u s t e n i t i c , f e r r i t i c and a u s t e n i t i c -f e r r i t i c s t e e l s . It i s shown t h a t i n a l l m a t e r i a l s under i n v e s t i g a t i o n t h e formation o f high angle boundaries occurs a f t e r c o n s i derable s t r a i n i n g . A t t h e e a r l i e r s t a g e s o f p l a s t i c flow s t r u c t u r a l changes a r e defined by m a t e r i a l and s t r a i

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... n temperature. A t low temperatures i n a u s t e n i t i c s t e e l and i n t h e a u s t e n i t i c phase of a two-phase m a t e r i a l t h e formation o f fragmented s t r u c t u r e occurs. Developed s u b s t r u c t u r e is formed i n f e r r i t e i n a l l t h e temperature-strain r a t e i n t e r v a l under study, while i n a u s t e n i t e only a t considerably high temperatures. Mechanisms of s t r u c t u r e formation i n p l a s t i c flow i n s t a i n l e s s s t e e l s a r e discussed.