New developments and optimisation of high strength boron treated steels through the application of advanced boron monitoring techniques
Número de contrato: RFSR-CT-2012-00018
CENTRE DE RECHERCHES METALLURGIQUES ASBL (BELGIQUE )
MAX-PLANCK-INSTITUT FÜR EISENFORSCHUNG GmbH (DEUTSCHLAND)
ONDERZOEKSCENTRUM VOOR AANWENDING VAN STAAL N.V. (BELGIQUE)
TATA STEEL UK LIMITED (UNITED KINGDON)
This research is aimed at improving the consistency of properties in B-treated ultrahigh strength plate and investigating the applicability of B additions for producing advanced multiphase cold rolled and annealed strip with high strength, formability, wear resistance and weldability at reduced cost.
An important part of the work is the investigation of the sensitivity of different characterization techniques and the definition of the guidelines for improved monitoring of B in its different forms.
This will provide data to support investigation of including B in an existing model of recrystallisation and assessment of the accuracy of existing precipitation and transformation models.
The present work is directed to high (>600MPa) and ultrahigh-strength (>900MPa) boron treated plate/strip and cold rolled and annealed sheet steels. Boron is a widely used alloying element in the production of high strength steels and is commonly added on account of its dramatic effect in improving hardenability thereby allowing bainitic or martensitic microstructures to be obtained using lower levels of alloying elements such as Mn, Cr and Mo.
Although a lot of past research exists on B-steels, many questions remain and need clarification and this project aims at obtaining a better understanding of the behaviour of boron, through the use of advanced detection techniques and electron microscopy.
The main objectives of present work are:
To investigate the efficiency of available state of the art characterisation techniques to monitor B in its different forms (free B, precipitated, segregated), to define guidelines about capabilities and detection limits and to adapt laboratory methodologies to B-rich species.
Helped by these improved detection techniques, to progress in the understanding of B interactions with other additions (AlN versus BN and competition between different carbide species in Nb, V and Mo added grades) in order to improve consistency of properties in quenched (and tempered) high and ultrahigh strength plate/strip.
To compare direct quenching versus air cooling plus subsequent Q&T treatment as a precursor for the elimination of a costly and energy consuming reheating treatment after hot rolling, with a particular focus on optimum boron protection methods for each route and control of recrystallisation in hot rolling.
To explore the applicability of B in conjunction with other alloying elements (Mn, Cu+Ni) for a consistent production of multi phase cold rolled and annealed strip combining high strength, abrasion resistance, formability and weldability at a reduced cost with respect to present DP and TRIP steel grades.
To test the sensitivity of current precipitation and phase transformation models and to incorporate B effects into existing models for recrystallization that already include Nb, Ti and Mo.
Ceit's role in the project
The main role of Ceit in this project was to investigate the effect of composition on Boron precipitation and steel hardenability by applying a series of thermal and thermomechanical cycles, followed by cooling at different rates. One important part of the work was the application of different characterization techniques allowing the monitoring of boron state and location. Ceit mainly concentrated on optical, FEG-SEM+microanalysis, EBSD and transmission electron microscopy (TEM) to identify the different precipitate species and their relation to the steel thermomechanical history. Several steel variants of Boron-treated steels were considered: Ti-protected classical compositons, Al-added steels, Al+Nb, Al+V and Al+Mo, among others.
The obtained results from precipitate direct observations in FEG-SEM and TEM and indirect data related to B in solution affecting the hardenability were used to assess the accuracy of current precipitation models.
Another important part of the work performed by Ceit was to incorporate B effects into existing models for austenite recrystallization kinetics during hot rolling that already included Nb, Ti and Mo. It was concluded that the addition of boron has a strong delaying effect on the recrystallization kinetics, particularly in presence of Nb. The improved model was applied to design hot rolling sequences for optimized compositions, considering two scenarios: 1) accumulation of strain in austenite before phase transformation and 2) transformation from a recrystallized austenite. One of the industrial partners involved in the project applied these two defined thermomechanical sequences in a pilot plant rolling mill. The obtained results of these trials were used for the validation of both, the optimized composition and the model.