By the start of the16th century Spanish fashions influenced Italian and English ladies. An iron hinged armour like corset was worn to flatten the body giving a smooth outline beneath gowns.
The iron corset must have been exceptionally uncomfortable and heavy to wear and could only have been worn by Elizabethan ladies not doing any form of heavy work. Their only benefit seems to be that they produced the incredibly small waisted, elongated flat chested smooth line torso.
This was illustrated in paintings of great Elizabethan ladies wearing fabulous structured bejewelled gowns. Left - Elongated boyish flattened torso of Queen Elizabeth 1 in the long Elizabethan era - 1592/3. Held at National Portrait Gallery London.
Corsets of the late 16th century would be more recognizable to us today than the iron version. These later corsets incorporated materials such as whalebone, bone, wood and flexible steel. The patterns on the corsets showed the placement of the chosen support and were elongated after a fashion trend set by the boyish figure of Queen Elizabeth I. 
Due to large amount of metal used, such corsets were heavy and more uncomfortable that ordinary fabric corsets. They were also padded underneath like armour. The metal corsets also worked as a bulletproof vests, because assassinations by knife in heart were a common risk at the time.
Some sources attribute the introduction of the iron corset to Catherine de Medici, wife of Henry II of France (1579 - 1589). It was said that she frowned upon anyone having a thick waist and introduced a rigid hardened stiff corset as well as the iron corset. Other sources believe that the 'iron' corset was only intended for remedial wear or for women with physical deformities.
Fontanel, B.,(1992) Support and Seduction, The History of Corsets and Bras: New York, Harry N. Abrams, Inc. Ewing, E. (1978) Dress and Undress, A History of women's Underwear: New York, Drama Book Specialists.