BASEFORM

Bainite and second-phase engineering for improved formability

Financed by

Número de contrato: RFSR-CT-2014-00017
Programa: RFCS

Partners

• Materials innovation institute (M2i)

• Tata Steel Nederland Technology BV

• ArcelorMittal Maizières Research SA

• ThyssenKrupp Steel Europe AG

• Technische Universiteit Delft

• Ceit-IK4

Summary

BaseForm is the research project funded by the European Union's Research Fund for Coal and Steel (RFCS) research programme. The aim of the project is to improve the cold formability of Advanced High Strength bainitic steels for automotive application by controlling the presence of secondary phases and optimizing the microstructure. This means to combine high yield strength with favourable ductility in tension and hole expansion tests.

In bainitic microstructures, a crucial, but poorly understood role is played by second-phase constituents like martensite and retained austenite. The understanding of the mechanisms of microstructure formation, deformation and damage processes can lead to a better control of these properties. Guidelines for producing bainitic advance high strength steels with these favourable properties will be delivered.

Project objectives

The project goal is to optimize the microstructures with a bainitic matrix in order to improve the cold formability and edge-cracking behavior of advanced high-strength steels with high yield strength. This will enable more efficient steel applications, like weight reduction without loss of formability, enhanced energy absorption due to high yield strength, higher elongation and favorable crash folding behavior due to improved bending properties. The main objectives are to:

• Investigate the very fine microstructure evolution during processing and in particular, the formation mechanisms of the morphology (size, shape and distribution) of second-phase constituents (martensite, retained austenite or martensite/austenite islands and cementite) in the bainitic matrix. Develop or improve models to describe the relationship between process condition and microstructural parameters.

• Investigate the relationship between microstructure and mechanical properties in a variety of mechanical tests that are of relevance to automotive applications (tensile tests, hole expansion tests or bending tests).

• Investigate the impact of microstructural features in bainitic AHSS and to identify the critical microstructural parameters that play a role in damage initiation processes.

• Design and validate a workbench for industrial product development of high-strength–high-formability bainitic steels with constrictions of annealing plants.

• Provide data suitable for the development of improved predictive models of mechanical properties and damage properties for bainitic matrix steels.

• Develop guidelines for producing bainitic, high strength AHSS with high elongation and high forming capacity (bending, hole expansion).

Ceit's role in the project

The role of Ceit in this project tackles several aspects. The first one is to develop improved microstructural characterization techniques to be applied to these highly complex bainite based microstructures. A combination of various techniques including color etching for optical microscopy, FEG-SEM, EBSD, X-ray diffraction and nano-indentation has revealed to be the most suitable manner to characterise and quantify all the relevant microstructural features required for a deep understanding of these complex mixed microstructures.

This quantitative information has been necessary for two different reasons:

• On the one hand, to gain knowledge of the processing, microstructure and mechanical properties relationships of bainitic matrix multiphase steels. The results have shown the relevance of the temperature/time of the bainite transformation along with the martensite and retained austenite contents in the tensile curves. Instead, the hole expansion coefficient has much more complex relationships with both processing and microstructural features.

• On the other hand, to run a mixed-microstructure model for predicting the tensile properties of the different microstructures produced by varying the processing conditions (continuous annealing after cold rolling). Initially, the available model at Ceit was suited for application to dual-phase steels but needed to be adapted to address combinations of bainite with martensite, or bainite and retained austenite transfoming into martensite during deformation (TRIP effect). The new microstructure-based model that also incorporates the TRIP effect has shown a good predictive capability for the microstructures in this project.

Ceit has also worked on the investigation of the evolution of the damage voids on a selection of microstructures. This part of the work aimed at relating the damage formation with the observed microstructural features.