Metallurgy and processing of ultralow carbon bake hardening steels

Abstract

The development of bake hardening steels is reviewed. Classical strain aging theories still have relevance to modern ultralow carbon chemistries, although the drastically reduced carbon contents of these steels can in¯uence the kinetics observed during bake hardening. High solute carbon levels increase the bake hardening response but reduce room temperature aging resistance, hence excess carbon levels are kept within the range 15 ± 25 ppm. The strength increase appears to be enhanced by a ® ne ferrite grain size; the published evidence is contradictory and further research is considered necessary. Bake hardening steels can be produced from interstitial free chemistries either by adjusting the chemistry during steelmaking to leave carbon in solution, or by annealing at high temperature to take carbides formed during processing into solid solution. Care must be taken during coiling, annealing, and temper rolling to ensure that suitable amounts of carbon remain in solution to produce a bake hardening product. Further research into the in¯uence of processing conditions on the metallurgy of these steels is required if they are to be successfully produced in industrial conditions.MST/5160

Observations of decomposition of martensite during heat treatment of steels using atom probe tomography

“…Transformation-induced plasticity (TRIP) steels are promising candidates for applications in the automotive industry [6][7][8][9]. After forming, the automotive body is subjected to a paint baking process (typically for 20 minutes at 175-180 o C), which results in an additional strength increase [7][8][9].…”

Understanding the Behavior of Advanced High-Strength Steels Using Atom Probe Tomography

“…[34,[37][38][39] It was proposed that the bake hardening behavior is associated with the changes of the dislocation substructure in the polygonal ferrite matrix. [30][31][32][33][34]37,38] However, the information on the bake hardening mechanism for the TRIP steels having a predominantly bainite matrix with a high dislocation density rather than a polygonal ferrite one is limited. [39] Therefore, in-depth studies of solute segregation resulting from the prestrain and bake hardening (PS/BH) treatment of such TRIP steels are warranted.…”

“…[30][31][32][33][34] This is an important contribution to in-service properties, such as a low springback and improved dent and crash resistance. [32] Extensive investigations have established the mechanisms and stages of the bake hardening behavior of low carbon steels, which are controlled by the amount of interstitial carbon atoms and the formation of Cottrell atmosphere and precipitations. [35,36] Compared with the low carbon steels, the bake hardening behavior in TRIP steels is expected to be more complex in nature due to their complex multiphase microstructure.…”

A Simple Method for Prediction of Shelf Life of Bake Hardening Steels

“…The factors affecting the bake hardening and aging behaviour have been dealt in various literature. [1][2][3][4][5][6] Ultra low carbon (ULC) steel is the prime material for bake hardening (BH) grade, since it possesses excellent formability and it is easier to control the solute carbon to a desired level. However, automakers are concerned about the room temperature aging problem in these steels which, causes a large rejection due to stretcher strain.…”

“…[1][2][3][4][5] Bake hardening is primarily achieved by the presence of controlled amount of solute carbon in steel matrix. Due to presence of solute C/N in this steel, it is prone to ambient temperature strain aging causing defect such as stretcher strain during forming of automobile component.…”