plain-carbon steel
Steel is iron with a pinch of carbon dissolved in it, usually less than 1 percent by weight, and that pinch changes everything. Plain-carbon steel is the plainest recipe: mostly iron, a controlled amount of carbon, and only the small amounts of manganese, silicon, and impurities that come along for the ride. It is the metal of car bodies, beams, nails, wrenches, and railway track, by far the most-used engineering metal on Earth.
The carbon content sorts plain-carbon steels into three families. Low-carbon steel (under about 0.25 percent carbon, also called mild steel) is soft, cheap, and easy to bend, weld, and press, think car panels and structural beams. Medium-carbon steel (about 0.25 to 0.6 percent carbon) can be quenched and tempered to a useful mix of strength and toughness, think gears, axles, and rails. High-carbon steel (about 0.6 to 1.4 percent carbon) hardens the most and holds a sharp edge, think springs, knives, and hand tools. More carbon means more of the hard iron-carbide phase (cementite), so more strength and hardness but less ductility.
Plain-carbon steel is cheap and strong, but it has two honest limits. First, it rusts readily; bare steel needs paint, zinc, or oil to survive weather. Second, it does not harden deeply: quench a thick bar of plain-carbon steel and only the surface goes hard while the core stays soft, because it has poor hardenability. When a design needs deep hardening or corrosion resistance, engineers add alloying elements and move to alloy or stainless steels.
A common mild steel is AISI 1018 (about 0.18 percent carbon): easy to machine and weld, with a yield strength around 300 to 370 MPa. A high-carbon 1080 steel (about 0.80 percent carbon) can be hardened to over 60 on the Rockwell C scale for a spring or a chisel.
In the AISI 10xx plain-carbon grades, the two digits after 10 are the carbon content in hundredths of a percent, so 1018 is 0.18 percent carbon.
Plain-carbon means no deliberate alloying beyond carbon and residual manganese and silicon; it does not mean pure iron. Pure iron is actually soft and weak, and it is the carbon (and later heat treatment) that makes steel useful.