Special Process and Evolving AHSS Types


 

Special Process Steels

The microstructures of DP, TRIP and other AHSS have islands or particles of different phases. One example is the martensite islands. When forming steels, these particles cause localization of strain during edge stretching, stretch-bending over tight radii, or other deformation modes whereby strain can localize to generate high peak strains. These steels then are subject to failure at deformations less than that predicted by the properties. Steel companies have been conducting extensive research to develop specialized steels for such applications that minimize the negative effect of these phases or particles. More information is contained in Section 3.B.2 Deformation Limits.


 

Evolving AHSS Types

In response to automotive demands for additional AHSS capabilities, research laboratories in the steel industry and academic institutions continue to search for new types of steel or modification of existing types. The goal of the primary research (Enhanced Multiphase Products) is improving formability for a given strength range while reducing the cost and welding problems associated with steels having a high percent of austenite. Other examples of these developing steels areultrafine (nano) grain, low density, and high Young’s modulus steels.

Three-Phase Steel With Nano-Precipitation

A new family of high-performance cold forming steels consists of a ferritic matrix containing bainite, residual austenite and nano-precipitations. During the forming, retained austenite is converted to a great extent to martensite. This work-induced transformation has a positive effect on the hardening behavior of the material to create a good combination of high formability and high strength. Due to their high resistance to local thinning, these grades allow component geometries that are very difficult to achieve with other high-strength steels of the same strength class. The following nano-precipitation grades are produced for automotive applications.

680/780: HR660Y760T-TP        B-pillar, Crossmember, longitudinal members, side panel reinforcement

750/900: HR730Y880T-TP        B-pillar, Crossmember, longitudinal members, bumper, side panel reinforcement

Q&P – Quenching and Partitioning

Q&P steels are a series of C-Si-Mn, C-Si-Mn-Al or other likely compositions subjected to the quenching and partitioning (Q&P) heat treatment process. With a final microstructure of ferrite (in the case of partial austenitization), martensite and retained austenite, Q&P steels exhibit an excellent combination of strength and ductility, which permitted their use in a new generation of advanced high strength steels (AHSS) for automobiles. Q&P steels are suitable for cold stamping for those structure and safety parts of automotive with relatively complicated shape o improve fuel economy while promoting passenger safety.