It's impressive that post-WW1 Germany could produce sufficient armour for its warships and vehicles. When Britain was rearming in the late 1930s they had to import armour plates from Czechoslovakia due to lack of British steelmaking capacity. For example, "Britain was obliged to procure 12,500 tons of armour plate from Czechoslovakia in 1938-39 to complete the cruisers Kenya and Trinidad, and the aircraft carriers Illustrious, Victorious and Formidable." Brown, 1998, Armor from Czechoslovakia, Warship International, 35 (1), pp9-10.
Firstly, steel plate does not equal armour plate. It will have different elements added to it to make it harder to begin with (Chromium, Nickel & Molybdenum for example|). It will then be subjected to other processes to harden its faces to varying depths. Different types of armour were subjected to different processes. Post WW1 with the battleship holiday, there was much less demand for armour plate from the Royal Navy as the principal customer (not much demand from Army for tanks or the RAF for aircraft inter-war) and the Admiralty, in a time of financial stringency, refused to subsidise the manufacturers to maintain the plant so they turned the plant to other uses or closed it.
Around 1936 the 3 main armour suppliers had a capacity of around 18,000 tons per year. Demand in 1938-39 for the Admiralty was expected to be 44,000 tons. So with the expanded rearmament programme from 1936 demand increased again and the Admiralty persuaded the Treasury to invest at least £2.25m into expanding the armour production capacity of a number of companies. But those facilities took time to come on line and that did not match the demand of the shipbuilding programmes. In 1936 alone the Admiralty ordered 34,361 tons of armour plate. Problems were then encountered in 1937 with too many armour plates failing quality control tests. So in Jan 1938 it was decided imports would be required and the Admiralty went shopping. After approaching the USA, France, Sweden & Germany (yes the future enemy!) only Czechoslovakia was willing to assist, which was fortunate as the Skoda Works at Vitkovice (owned by the Rothschilds) had produced some of the best armour in the world pre-WW1. Of the 12,500 tons ordered from Czechoslovakia only 10,000 tons had been delivered by the end of Aug 1939. The shortage caused delays to the completion of Victorious. Another odd thing is that although Germany had completed its occupation of Czechoslovakia in March 1939, deliveries via the German rail system continued until the end of August 1939.
Secondly the scale of production required by each nation was completely different.
Steel and armour plate was generally taken to weigh 40lbs for a 1ft x 1ft x 1in plate (length / width / thickness). The thicker the plate generally the greater the difficulty in making armour plate.
As built a King George V class battleship had 12,460 tons devoted to its protection. Plates up to 14-15" for the side armour and 5-6in for the deck armour. 5 built = 62,300 tons.
An Illustrious class carrier had 4,941 tons devoted to its protection (only half the weight of the flight deck armour is in that figure due to their design using it as structural steel as well as armour protection - Friedman). Plates up to 3" thick on the flight deck and 4.5" thick for the side armour. 3 built 14,823+tons.
Indomitable 4,299 tons for protection (as above)
Implacable 3,645 tons for protection (as above). 2 built. 7,290 tons
Fiji class cruisers 1,310 tons plates up to 3.5" thick. 11 built. 14,410 tons
So that lot adds up to 103,122 tons and only covers that part of the 1936-39 programmes that I can quickly put my hands on.
In comparison how many big ships did Germany produce from the beginning of 1937?
2 Bismarcks - plates up to 14.2in thick; total armour weight 18,780 tons = 37,560 tons,
Graf Zeppelin,
5 Hipper class cruisers.
The vast majority of the tanks Germany produced prior to WW2 were small
Panzer I weight 5.3 tons; armour thickness 7-13mm (0.25-0.5") 2,435 produced mostly 1934-38
Panzer II weight 8.8 tons; armour thickness 5-15mm (0.2-0.6") 1,856 produced 1935-44.
Panzer III weight 14.8-19.2 tons; armour thickness 15-30mm (0.6-1.2") on the pre-WW2 variants of which c250 produced
Panzer IV weight 18.1-19.7 tons; armour thickness 15-30mm (0.6-1.2") on pre-WW2 variants of which c260 produced.
And those weight are the fully loaded weight including all the non-armour items like engnes, guns etc.
In the run up to WW2 one problem the KM faced was that it was in competition with the Wehrmacht for steel generally and armour in particular. So it was constrained as to just how many ships it could build.