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 signi® cantly. 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 af® nity 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.MST/5293

Design of Composition in (Al/Si)-alloyed TRIP Steels

“…18) In fact, it has been found that steels with contents of up to 1.5 % Al can be successfully galvanized. 10,19,20) Mahieu et al found that Al, like Si, form ironaluminum-oxides, but these do not affect the coatability of low-Si steels. 11) On the contrary, Al has the ability to prevent Si from segregating to the surface.…”

Fusion Zone Microstructure Evolution of Al-Alloyed TRIP Steel in Diode Laser Welding

“…8) According to Meyer,9) the partial replacement of Si by Al in TRIP steels results in a much improved galvanisability. Maki 10) also conducted research on the galvanisability of Sifree CMnAl and Al-free CMnSi TRIP steels respectively and similar results were obtained. Furthermore, the mechanical properties of cold rolled Si free CMnAl TRIP steel were comparable to those of conventional CMnSi TRIP steels.…”

Metallurgical and Mechanical Properties of Fusion Zones of TRIP Steels in Laser Welding

“…Maki also conducted research on the galvanizability of Si-free CMnAl and Al-free CMnSi TRIP steels respectively and similar results were obtained. 9) Furthermore, the mechanical properties of cold rolled Si free CMnAl TRIP steel were comparable to those of conventional CMnSi TRIP steels.…”