Siemens-Martin
William Siemens's greatest single invention was the regenerative furnace.
1865 the French engineer Pierre Blaise Emile Martin took out a licence from William Siemens for the regenerative furnace and was the first to apply it to making steel[1]
The Siemens-Martin open-hearth furnace converted steel in a bath of liquid pig-iron[2]
The most appealing characteristic of the Siemens regenerative furnace is the rapid production of large quantities of basic steel. The usual size of furnaces is 50 to 100 tons.
The Siemens-Martin process was slower, and thus easier to control, than the Bessemer Process, so it complemented rather than replaced it.