Bainite directly from the rolling heat in the world's most modern wire rod mill

16 February 2022 |

Bainite is a microstructure which is formed during the heat treatment of carbon steel by isothermal or slowly continuous cooling. Bainitic microstructures are formed at temperatures or cooling rates that are intermediate between those for pearlite or martensite formation.

Bainitic steels possess many positive properties and are suitable for application in screw manufacturing for the automotive industry (e.g. chassis screws or engine bolts) or for the higher-end fastener industry (flow-drill screws or earthquake-resistant fasteners), among others.

Matthew Galler, R&D voestalpine Wire Technology

The advantages of the bainitic microstructure, compared to the martensitic microstructure, result from the resistance of bainite with respect to hydrogen-induced stress corrosion cracking.

For example, the use of bainitic microstructures in bolts allows higher strength classes to be achieved when compared to the standard values (12.9 or 10.9 in corrosive environments). The higher strength classes of up to 1800MPa tensile strength for certain bolts, enable weight savings in lightweight automotive construction and promote the concept of downsizing.

Currently unique: bainite directly from hot rolling in the most modern rolling mill

In addition to the classic production routes for bainitic steels, voestalpine Wire Technology's state-of-the-art rolling mill is currently pursuing a unique and successful concept that makes the expensive and CO₂-intensive production steps of final quenching and tempering or GKZ annealing obsolete: the formation of the bainitic microstructure directly in the forming heat. Thus, our customers benefit from a reduction of the CO₂ footprint while gaining ductility and hydrogen resistance in high-strength materials.

Our R&D expertise enables these customized wire solutions. In cooperation with the University of Leoben and a number of core customers, high-quality alloy concepts have been developed at voestalpine's research facilities. voestalpine's own metallurgy laboratory (MetLab) and the metallurgy pilot plant (TechMet) provide research facilities that are unique worldwide and can be used to test and innovate new alloys on a very small scale.

The material properties can be precisely defined through further testing in the wire pilot plant. Small series production and close coordination with the process technology in the wire rolling mill allow for this efficient material development.

Prototypes on the verge of series production

Currently, three alloys with bainitic microstructure, directly from the forming heat, are being tested as prototypes. Another alloy, for which a final isothermal salt bath is necessary on the customer's side, is about to make the leap to series production.