Development of ferrite-based high strength steels
The stable form of iron at low temperatures is a body centred cubic (bcc) structure called ferrite. This has extremely high ductility but is relatively weak. Steels with the dissolved carbon as low as possible are called interstitial free (IF). A development of these, bake hardening (BH) steels, contain very carefully controlled dissolved carbon, in the order of parts per million, which produce strength increases during the heating cycle used for paint curing.
Strength can also be increased by using faster cooling rates from high temperature which result in the formation of bainitic microstructures (ferrite strengthened with iron carbide precipitates) and, at still faster cooling rates, martensite (ferrite with carbon in solid solution).
As an alternative to carbon, higher strengths can also be obtaining by refining the microstructure, or grain size. This is achieved using microalloying elements such as Nb, Ti and V, and finishing hot rolling at lower temperatures. This is the approach used in the High Strength low alloy (HSLA) steels.
Attractive combinations of strength and toughness can be achieved by combining ferrite, bainite and martensite microstructural constituents to form a range of dual phase (DP) steels based on mixtures of ferrite and martensite and complex phase (CP) steels based on ferrite/bainite/martensite mixtures. By varying the amounts of the different phases a wide range of strengths can be obtained. For example dual phase steels with strengths of 600-1000 MPa are available commercially. Complex phase steels can readily exceed these strength levels.
Still higher strengths can be obtained by the use of fully martensitic microstructures. These require the use of very high cooling rates to develop this microstructure and, because of its limited ductility at room temperature, they must be hot formed and then cooled rapidly by the forming dies. These can produce strength levels up to 2,000 MPa which are useful for components where high crash resistance is needed, for example in door pillars.
As illustrated in the Banana diagram, shown here, the tensile strength/ductility combinations for all these bcc based steels lie on a single curve with the ductility falling as the strength increases. To improve the ductility at a given strength level it is necessary to introduce a new phase into the microstructure called austenite which has a face-centred cubic structure.