These Carbon levels are very high, compared to typical naval homogenious armours. This illustrates how tank RHA reqiures slightly different properties than naval armours to deal with different threats. Carbon is of course the most important alloy used in steel for both hardness and strength. Carbon content also determines the reponse to heat treatments. Very low carbon steels don't respond to heat treatments. Were max hardness and strength are at a premium, cheap and easy to obtain carbon, gives the required results, and the properties can be more readily manipulated by heat treatments. Were the Cr levels may get reduced (pre war steels of similar type had uip to 5.5% Cr) an increase in C, can compensate to a degree too. In naval homogenious armour, excellant ductilty must be retained. Great toughness requires both great strength and ductilty.
Wh used much less C, and a more defined mix of alloying elements. By using greater concentrations of carefuly measured micro alloys, to obtain a specific fine grained micro structure, Wh was able to obtain greater strength, without unduly compromising ductility.
Welding requires a material with great toughness. Generally, steels with more than ~.30-.40% actual C, are considered unweldable, or at least extremely difficult to weld. Additionally, alloying agents (particularly Mn, and V) can intensify the effect of carbon. Usually the preheating reqiured for welding is determined by taking into account both the actual C content, and the micro alloys used. This is called the carbon eqivilency. One the common formulas used to determine C eqivilency is(all percentages by weight): C+ (Mn/06)+(Si/30)+(Ni/60)+(Cr/20)+(Mo/15)+(V/10). Weldable homogenious armours have a total C eqivilency, of no more than about .4-.6%. Incidently, Wh and St52 end up with virtually the same Ce. This is different from the simlar Schaffler nickel and Cr eqivilencies for micro structures of chromium and nickel alloys, but these can be of critical importance in determining dilution effects during welding of armour plates.
Since the 30's, more and more steels are produced from re-melting scrap steel. Scrap is also used more readily by electric arc furnace smelting. This is probably particulary true of wartime German tank armour. From the scrap, residual amounts of potential alloying elements (Ni, Cu, V, Cr...ect..) can find their way into the composition. However, Vanadium is so powerful, that what would be considered only trace amounts during WWII, can have significant effect.