Serge Claessens

Galvanisability of silicon free CMnAl TRIP steels

Abstract Transformation induced plasticity (TRIP) steels are a promising solution for the production of cars with low body mass because of the combination of high strength and large uniform elongation that they offer. However, conventional CMnSi TRIP steels with more than 1 wt-%Si have the drawback of poor Zn coating quality after continuous galvanising. This problem is due to the presence of complex Si–Mn oxides on the strip surface. The present research work therefore focused on the full substitution of Si by Al in TRIP steels and the detailed analysis of the galvanising behaviours of these Si free CMnAl TRIP steels. If the hot dipping is done after a combination of intercritical annealing and isothermal bainitic transformation in a furnace atmosphere with a high dewpoint, the wetting of the strip by the liquid Zn is improved significantly. However, the improvement is limited and not enough to avoid bare spots and coating defects cannot be avoided on conventional CMnSi TRIP steel. In contrast, the Si free CMnAl TRIP steel has a much better wettability when annealed at a low dewpoint. The surface segregation of the elements, which have a high affinity for oxygen, i.e. Si, Al, and Mn, was studied in detail and this revealed that Si was much more readily enriched on the surface than Al during the annealing in the low dewpoint atmosphere. The difference in the surface segregation between Si and Al resulted in a clear difference in the galvanisability. The limited presence of Al on the strip surface is due to the fact that Al can be oxidised internally during hot rolling. As a result, an Al depleted surface region is formed owing to selective internal oxidation of Al before the continuous galvanising.

Hot dip galvanising and galvannealing of P and Mn strengthened TiNb IF steels

Abstract Phosphorus, Mn, or Si solid solution strengthened sheet steels stabilised with Ti and Nb are known to have poor galvanising and galvannealing properties. When these steels are continuously annealed in higher dewpoint atmospheres, selective oxidation processes can occur in the subsurface region of the sheet. A laboratory simulated continuous galvannealing process was therefore used to study this effect in P and P+Mn solid solution strengthened sheet steels stabilised with Ti and Nb, by making a detailed analysis of the fundamental surface and interface processes that control the wetting of the surface by the liquid Zn, the inhibition layer formation, and the Fe-Zn reaction.

Selective Oxidation during the Austenitic Annealing of a CMnSi Steel

High strength multiphase CMnSi steel is increasingly used in passenger cars. Si and Mn alloying levels are typically in the range of 1-2% in mass. While Si improves the mechanical properties, it considerably deteriorates the galvanisability of steel. Residual water vapour in the reducing gas atmosphere during the intercritical or austenitic annealing results in the selective oxidation of Si and Mn at the steel surface. Besides Mn and Si, C is oxidized as well at the steel surface, leading to the formation of CO gas and decarburisation of the steel surface. This decarburisation has a major influence on the phase composition in the steel surface region: it shifts the ferrite to austenite transformation to higher annealing temperatures, leading to differences in surface and bulk microstructure. The phase composition influences the solubility and diffusivity of all alloying elements near the surface. The evolution with temperature of the selective oxidation at the steel surface has been studied by interrupted annealing in a protective atmosphere containing residual water vapour. The influence of the annealing temperature on the selective oxidation of Mn and Si is characterized by XPS (X-ray Photo-electron Spectroscopy) analysis.

Galvanising and galvannealing behaviour of B added TiNb IF high strength steel

Abstract Solid solution strengthened steels stabilised with Ti and Nb are known to have poor galvanising and galvannealing properties. When these steels are alloyed with boron to prevent cold work embrittlement, the selective oxidation processes are influenced by the presence of B. The continuous galvannealing process has therefore been simulated in a laboratory in order to study the effect of B on the fundamental surface and interface processes that control the wetting of the surface by liquid Zn, the inhibition layer formation, and the Fe-Zn reaction.

Transformation Behaviour and Related Mechanical Properties of B-Added ULC- and IF Steels

Boron may be added to modern ULC- and IF steels for various reasons. It is well known that the presence of this element has a strong effect on the kinetics of the γ/α transformation, which in its turn has a major effect on the resulting microstructures. However, the origin and the nature of the resulting microstructures in ULC-and IF steels is poorly understood. In principle, the addition of boron offers the possibility of obtaining an excellent combination of strength and ductility in deep-drawable ULC-steels. In P-bearing high strength IF-steels, B is mainly added to avoid Cold-Work-Embrittlement. In this case, the effect of B on the transformation behaviour and the resulting microstructure may easily become detrimental to the mechanical properties and further processing through cold-rolling. For both ULC-steels and IF-steels, the optimisation of properties requires a trade-off in steel composition and processing conditions. This requires knowledge of the prevailing transformation mechanism. The transformation behaviour of various B-added ULC- and IF steels was studied by dilatometric and microstructural analysis, combined with hardness testing of hot-rolled material. Identification of the various constituents of the microstructure was accomplished by comparing the results of microstructural analysis and hardness tests with literature data. Tuning of the mechanical properties also requires knowledge of the recovery and recrystallisation behaviour, which is investigated in combination with the relevant mechanical properties.