When I first started forging, I knew there were different types of steel, but my knowledge was limited to 'high-carbon' and 'low-carbon'. And that's only the beginning.
In the beginning there was iron. Iron is one of the only elements that is sometimes found in a pure state in nature, but pure iron isn't very useful. Somewhere someone found that if they refined ore certain ways they got wrought iron. Very tough, useful for many things, but with a drawback; except for work-hardening it cannot be made harder, so it won't hold a cutting edge well, and work-hardening makes it brittle. Another rough form of iron has a different structure, very course, and is brittle, but works wonderfully for casting cooking pots for instance, known- amazing!- as cast iron.
Further along someone found that if you treated iron a certain way, it changed into something else. We know it's adding carbon, God knows what they thought it was. And it makes BIG changes. Wrought iron has a fibrous structure(find a piece of badly corroded wrought and you can see it), steel structure is crystilline. And it's stronger than wrought iron, both harder to bend and more resistant to wear. And if you have the right amount of carbon, you have something wondrous. Not only is it much stronger and more wear-resistant- better for cutting tools- it can be hardened.
Take a steel with about .5 percent carbon content, just half of one percent, and heat it to about 1500 degrees F, then quench it in water, and it gets even harder and tougher. From .6 percent up, remarkably so, so much that it's brittle. Then you can heat it up to a lower temperature, roughly 400 to 600 F depending on use, you remove some of the hardness, but not all. This makes a sword blade that can flex like a spring, and hold an edge through a fight. Or a knife or razor that will make many cuts before it becomes dull. And springs, that made clocks work.
Mild steel- with low carbon content- is used all over the place; building framing, vehicle frames, sheet metal for roofs and panels and boxes and cases. It was more expensive to make than wrought iron for a long time, so wasn't used for some things. Railroad rails, for instance. Then someone did a test; when the wrought iron rails along a stretch of track wore out, replace with mild steel rails and see how long they last. About 3-5 times longer than iron rails depending on straight track or curves; add in replacement transport and labor costs and it's now more cost-efficient than iron, and safer.
Medium carbon, makes springs, makes tool frames of many kinds, things that have to be stronger than mild but don't need the hardness of high carbon.
High carbon gives knives, axe edges, swords, plow points, files razors, cutting tools that will cut other metals, scissors, and other things.
It took a long time to find reliable, repeatable ways to make this newer stuff, but our ancestors were not stupid. They found ways and used them. They couldn't see into the structure, but they found how to handle it to make this new stuff, over and over, whatever it was about these treatments that made the change.
Now? Tool steels that can get so hot they have a low red glow, and still be hard enough to cut. Stainless and stain-resistant steels that, in some cases, you have to work at to make them rust. Absolutely marvelous stuff, and much of it made in such quantities and consistent quality that it's cheap. Oil-quench hardening steel of a quality the ancients would have traded gold for, can be bought in three-foot bars for a few dollars, delivered to your house.
But not every good stuff is new. In India, and maybe Persia, they found a way to make a stuff we know as wootz steel. Legends came down of the blades that could be made from this stuff. A few years ago a man named Al Pendray, a blacksmith & knifemaker, got together with a metallurgist to see about making the stuff, and succeeded. And it can be almost magical; hard to forge, very touchy to shape, and cuts and holds an edge incredibly. One odd thing about it, carbon content runs about 2.5 percent, some batches a little higher. With modern tool steels, anything above maybe 2.2 percent becomes cast iron, rough structure. Yet this stuff works. When some samples were sent for analysis, at least one analyst where it came from because you can't have a structure like this!
But you can.