As the previous post noted at length, steel has been used in armor from ancient times. It is still the world’s most ubiquitous armor material, on account of its excellent mechanical properties, coupled with the fact that it is an extremely low-cost solution.
Steel body armor, as a means of defeating high velocity aimed projectiles, was extensively used in WWI. Its use was generally experimental, and the experiments generally received poor-to-middling feedback, thus at no point in the war was steel body armor common. Soldiers generally found it burdensome, and not worth the effort of wearing, which is little surprise, as the steel body armor of those days was both thick — regularly weighing in excess of 30lbs — and it offered imperfect protection, being roughly equivalent to the flak jacket of later eras. For these reasons, taken together, the use of steel armor was generally relegated to soldiers in more-or-less stationary positions: Sentries, machine gun crews, and sappers. Even so, the use of steel body armor was judged a failure, and its use all but disappeared in the days following WWI. 
To launch a blog about armor, I believe that it may be best to start with a review of its historical uses, from antiquity to World War I. The author of the following historical account, the very eminent armor expert and historian Bashford Dean, has already written such a review. It can hardly be improved upon, so I shall incorporate it in its entirety.
The author, in his own words, has sought to answer the following questions:
(A) What kinds of armor were early used?
(B) Was armor actually an important means of saving life and limb?
(C) How was it made?
(D) How was it tested?
(E) How heavy, irksome and even dangerous was it to wear?
(F) What in summary was its use in later times but prior to the Great War?
It should go without saying that the answers to these questions set the stage for the discussion of modern armor — and, what’s more, that the reasons for the disappearance of armor from the battlefield has much to teach the modern armorer. So although it might seem unusual to launch a technology blog with a thorough review of the now-distant past, Mr. Dean’s fine work should make it clear that our forefathers still have much to teach us, and their techniques and lessons are worth careful study.
I have taken the liberty of italicizing passages which I deem to be especially relevant.