In order to reduce the weight of automobiles, there is an increasing demand for high-strength formable sheet steel for automobiles. For this reason, most of the current automotive sheet research focuses on the development of high-strength steels with both high tensile strength and high elongation. Since alloying elements affect the microstructure and strength of the steel, the correct selection of these elements is extremely important to obtain such high-strength steels. However, large amounts of alloying elements can cause difficulties in zinc coating steel because they segregate to the surface to form oxides.
Composite phase (CP) steels have high ultimate tensile strengths, and their microstructures contain ferrite, bainite, and martensite, possibly with a small amount of retained austenite. Recently, Mesplont has developed a Cold rolled CP sheet. The tensile strength of this steel is about 1000MPa, and the elongation is about 1o. The alloying elements used are Cr, Mo and Si, wherein Cr and Mo are hardenability additives and Si is used to inhibit the formation of carbides. In the current literature, galvanizing of 1000 MPa CP steel simulated by laboratory hot dip plating is reported. The tests used two steels of different compositions. The surface condition before coating as well as the quality and adhesion of the coating were investigated. The production parameters, especially the annealing temperature and the dew point of the annealing atmosphere, were analyzed.
Production of 1000MPaCP steel. The standard annealing cycle for producing 1000MPa CP cold rolled steel is reheating to the critical zone annealing temperature, followed by cooling and isothermal holding at around 400°C, above the martensite start temperature (Ms). The ferrite volume percentage and its carbon content are controlled by the critical zone annealing temperature and annealing time. During cooling and holding, part of the critical region austenite transforms into bainite. The volume percent of austenite transformed to bainite is a function of the isothermal bainite transformation time and temperature and TI which determines the C content in the austenite and the bainite onset temperature (Bs). During the subsequent cooling to room temperature, martensite forms, but some austenite may remain untransformed.
Galvanizing and Surface Studies. Surfaces of 0.3%Cr and 0.5%Cr steels were investigated by XPS before immersion plating. The effects of two annealing temperatures and two annealing furnace dew points were also investigated. The alloying elements Si, Mn and Cr segregate to the surface to form oxides at low dew points. The surface composition is the same at both annealing temperatures. The temperature difference is so small that it has no significant effect on the amount of segregation of alloying elements. When the Cr content is 0.3%, the amount of Cr on the surface is small, but it has no effect on other elements. Based on the peak energy of Cr2p3, Cr appears on the surface in the form of Cr2O3.
Quantitative analysis was performed on these spectra, and in all cases analyzed, the Mn/Si ratio was equal to 2. This indicates that Si and Mn appear on the surface in the form of composite 2MnO?SiO2 oxides.
When the dew point is high, the surface Si, Mn and Cr are less when the dew point is low. This situation can be explained by the higher the dew point, the higher the oxygen partial pressure, and more oxygen penetrates into the steel. Since the alloying elements are oxidized before segregating to the surface, this leads to the formation of internal oxides. This internal oxidation results in much less preferential oxides on the surface. This phenomenon is thought to improve the wettability of steel. Both CP steel specimens can be galvanized in a laboratory hot-dip galvanizing simulator. There was no significant difference in the coating quality of the annealed sheets at the two annealing temperatures or the two dew points. The visual appearance of Zn surface of 0.3%Cr steel is slightly better.
Both 0.5%Cr and 0.3%Cr CP steels can be galvanized. But the appearance of 0.3%Cr steel and the adhesion of Zn layer are slightly better. In the low dew point annealing furnace atmosphere at -30°C, Cr, Mn and Si segregate to the surface and form composite Mn2SiO4 and Cr2O3. At a high dew point of +10°C, there is less oxide on the surface. When the annealer cycle starts with a high dew point and the surface is clean, wettability is no problem even at low dew points. Annealing temperature also has no effect on continuous galvanizing.