High-strength carbon and low-alloy steels have yield strengths greater than 275 MPa and can be more or less divided into four classes:

* As-rolled carbon-manganese steels
* As-rolled high-strength low-alloy (HSLA) steels (which are also known as micro alloyed steels)
* Heat-treated (normalized or quenched and tempered) carbon steels
* Heat-treated low-alloy steels.

These four types of steels have higher yield strengths than mild carbon steel in the as-hot-rolled condition. The heat-treated low-alloy steels and the as-rolled HSLA steels also provide lower ductile-to-brittle transition temperatures than do carbon steels.

These four types of high-strength steels have some basic differences in mechanical properties and available product forms. In terms of mechanical properties, the heat-treated (quenched and tempered) low-alloy steels offer the best combination of strength and toughness.

Structural Carbon Steels
Structural carbon steels include mild steels, hot-rolled carbon-manganese steels, and heat-treated carbon steels. Mild steels and carbon-manganese steels are available in all the standard wrought forms: sheet, strip, plate, structural shapes, bar, bar-size shapes, and special sections. The heat-treated grades are available as plate, bar and. occasionally, sheet and structural shapes.

Mild (low-carbon) steels are normally considered to have carbon contents up to 0.25% C with about 0.4 to 0.7% Mn, 0.1 to 0.5% Si and some residuals of sulfur, phosphorus, and other elements. These steels are not deliberately strengthened by alloying elements other than carbon; they contain some manganese for sulfur stabilization and silicon for deoxidation. Mild steels are mostly used in the as-rolled, forged, or annealed condition and are seldom quenched and tempered.

The largest category of mild steels is the low-carbon (<0.08%>C, with <0.4%>Mn) mild steels used for forming and packaging. Mild steels with higher carbon and manganese contents have also been used for structural products such as plate, sheet, bar, and structural sections.

High-strength structural carbon steels have yield strengths greater than 275 MPa and are available in various product forms:

* Cold-rolled structural sheet
* Hot-rolled carbon-manganese steels in the form of sheet, plate, bar, and structural shapes
* Heat-treated (normalized or quenched and tempered) carbon steels in the form of plate, bar, and occasionally, sheet and structural shapes.

The heat treatment of carbon steels, which typically attain yield strengths of 290 to 690 MPa, consists of either normalizing or quenching and tempering. These heat treatments can be used to improve the mechanical properties of structural plate, bar, and occasionally, structural shapes.

Normalizing involves air-cooling from austenitizing temperatures and produces essentially the same ferrite-pearlite microstructure as that of hot-rolled carbon steel, except that the heat treatment produces a finer grain size. This grain refinement makes the steel stronger, tougher, and more uniform throughout.

Quenching and tempering, that is heating to about 900oC, water quenching, and tempering at temperatures of 480 to 600oC or higher, can provide a tempered martensitic or bainitic microstructure that results in better combinations of strength and toughness. An increase in the carbon content to about 0.5%, usually accompanied by an increase in manganese, allows the steels to be used in the quenched and tempered condition.