Nanometric Scale Investigation of Phase Transformations in Advanced Steels for Automotive Application
- PDF / 1,113,107 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 12 Downloads / 202 Views
CTION
IN a constant quest to reduce carbon dioxide emissions, the automotive industry is seeking to produce lighter vehicles. Producing higher strength steels allows manufacturers to reduce the weight of the finished vehicle as sheet thickness is greatly reduced. However, increasing strength can cause forming difficulties. This is why the strength/formability balance is now a big challenge for steel manufacturers. The development of multiphase steels mainly composed of ferrite, bainite, and martensite allows this balance to be achieved. However, if the resistance levels are easy to obtain by introducing high fractions of martensite, it is more difficult to obtain the optimum forming properties. These properties require a careful control of the martensite characteristics: carbon content and distribution. One of the main causes of damage in multiphase steels comes from the differences in hardness, which exist between the martensite and softer ferritic or bainitic matrix.[1,2] A reduction in the carbon concentration of the martensite softens this phase and limits the gap of hardness between the different phases. Two solutions are possible in order to minimize the carbon concentration of the martensite: (1) to reduce the nominal carbon JOSE´E DRILLET, Research Engineer, and THIERRY IUNG, Program Leader, are with ArcelorMittal Maizie`res R&D, BP30320, 57283 Maizie`res-les-Metz Cedex, France. Contact e-mail: josee. drillet@ arcelormittal.com NATHALIE VALLE, Research Engineer, is with the SAM Department, CRP-Gabriel Lippmann, 41 rue du Brill, 4422 Belvaux, Luxembourg. Manuscript submitted April 4, 2011. Article published online November 6, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
concentration and (2) to carry out tempering to induce carbon precipitation. The development of high-strength steels with low carbon concentration necessitates the addition of hardening elements to limit the formation of ferrite during the cooling stage. An interesting solution is to add boron. This element has the advantage of being effective in very low concentrations (few parts per million).[3] This small quantity of boron has no effect on the thermodynamic properties of austenite or ferrite. Its effect on the hardenability is due to its diffusion toward the austenitic grain boundaries. The presence of boron decreases the interface energy and in this way delays the nucleation of the ferrite.[4] However, the boron diffusion coincides with a diffusion of carbon. In sufficient quantities, the codiffusion of these two elements can lead to boron carbide precipitation. These boron carbides lower the quantity of boron in the solid solution at the austenitic grain boundaries and cause a loss of hardenability.[5] The effectiveness of boron as a hardening element therefore is strongly linked to its distribution and its state. In steels containing boron, the investigation of the hardenability mechanisms requires the analysis of boron and carbon at a local scale in very low concentrations. The atom probe, which has seen an increase in use over the last few years, a
Data Loading...
