Jim - the common Euro method for casting steel was developed by Benjamin Huntsman in 1745 - as I noted it was originally used to cast ingots which were then forged into parts such as knife blades. This was also known as crucible steel and was originally developed for use in clock springs. Cast steel, although not cast into shape, was after widely used for knife blades, axe heads, and even gun barrels in the 1830's.
Cast steel was made by taking chunks of blister steel (wrought iron turned into steel via the cementation process), melting them down in a puddle type furnace, and that was then poured into special clay crucibles to form ingots of varying sizes. It was one of the three major methods used to make steel (blister, shear, and cast or crucible being the three), especially in England, prior to the 1860's when the Bessemer process was developed. Serendipitously Sheffield, England is one of the areas of the world that has natural clay deposits suitable for high temp casting of iron and steel.
The casting of steel itself though is much older - wootz steel for instance is just one type of cast steel.
Investment aka lost wax casting was developed much earlier and was widely used for sculptures, jewelry and such, but a similar system was developed in Europe for casting iron gun gun barrels by the mid-16th Century. Later steel parts may have been cast in iron and then case hardened thus creating a product at least by the early 1800's called steel rather than iron (early 1830's Henry rifle mounts for instance were ordered as steel not iron). On the other hand both wrought iron and the simple unalloyed steels of the day melt at the same temps about 2600° F, so I'm not sure why you would think that simple steel would be harder to cast since there was no problem bringing wrought iron up to temp.
FWIW - I've cast a few mild steel and WI parts using simple back yard equipment and using a simple wax cast type molds using high temp ceramic type clay. As for the cast iron parts needing to be bulky - as I noted late 1840's and later Hawken rifle parts such as thin butt plates and trigger guards were cast in iron or steel - the parts I cast were copies of the Carson rifle butt plate and trigger guard whihc date from the early 1850's. The casting method and molds of that time were fine enough to show unmelted bits of brass used to braze the two parts of the butt plates together which were then used as patterns - that fineness is generally more indicative of a lost wax method than it is of sand casting.
It's been a while but IIRC some of the info I garnered on 19th century gun parts casting was in the book(s) on the Mantons, Feltwad my be able to offer more info. The info on the cast iron/steel Hawken parts came via John Baird and Tom Dawson.
