There were a number of ways to make steel from iron. Some, like blister steel or shear steel, involved casehardening iron bars, then folding and re-working them at a forge until the carbon was mixed through the metal. They did not know the chemistry of the iron they started with, but they did know what worked and what did not ( Swedish iron was known to be very good) and if they had some decent iron to begin with, the resulting steel could be hardened to around Rockwell C58, and could be strong enough to be used for things like watch springs.

It was hard to get homogeneity with these methods, however. Not a problem if you are making small stuff, like clock springs: if one breaks from a hidden defect, another spring can be made that likely won't have a defect. But it is a problem for bigger objects, where defects are harder to avoid. More homogeneity could be achieved with getting the carbon into the iron by melting it in a covered crucible- or, to be more precise, a number of them, because they were rather small. In the 19th c., it's common to see "finest crucible steel" on good chisels, plane irons, and other cutting tools.

So, steel was made in small batches. But this is all high-carbon steel that we're talking about, above 0.7% carbon. That's what most people of, say, 1790, would think was meant by steel. The low-carbon or mild steel that is meant for structural uses, architecture, would be something that came later, with the advent of inventions like the Bessemer Converter, and Rolling Mills. Steel production was another thing that benefitted from the Industrial Revolution, and like a lot of other things, steel then became available in quantity. There would be some pretty impressive things done with cast iron in the 1700's, however. That's simpler to make than steel. The famous Iron Bridge in Shropshire, England , for example, built in 1779 from a number of very large castings.

And there would be very little known of alloys, nothing known of vanadium, cobalt, molybdenum...those metals were discovered later. It's those that can make a steel very tough, or make it resist corrosion, or make it about as hard at 1,500 degrees F. as it is when it's at room temperature. So, the S1 shock steel that makes the chisel on that jack-hammer keep breaking into the rock without cracking: no, they would not have that. Instead, there would be a high-carbon steel star-bit, the edges hardened and then tempered to a light blue, that would be pounded into the rock with a hammer until dull, then gathered up with the other dull star-bits and taken to the blacksmith, who would anneal, sharpen, re-harden and re-temper them, and send them back.

I should say that an enormous amount of structural steel these days has enough carbon that it should be called medium-carbon steel...but it's still called mild steel, anyway.

TL:DR Steel could be made quite hard, but it could not be made in vast enough quantities to make buildings, and not enough chemistry was known to be able to make exotic alloy steels.

Ede, George. (1866) The Management of Steel