Investigation of aging behaviour of cold rolled dual phase DP600 steels

During the recent years, the necessity of light materials that provides low fuel consumption and superior mechanical properties in automotive applications enables advanced high strength steels to improve. Advanced high strength steels (AHSS) are widely preferred in automotive applications due to their superiority in strength and formability in comparison with high strength low alloy (HSLA) steels. Nowadays, automotive industry has an important role in research, development and production of new steel types, as well as in steel consumption. Automotive design and production take its shape in accordance with the automotive market requirement, requirements of the era and the international agreements. International environmental regulations on greenhouse emission, preservation of natural resource and awareness of the prevention of the environment stimulate low exhaust emission, low fuel consumption and, in parallel with these, reducing vehicle weight. At the same time, the necessity of materials having high strength in low thicknesses, high deformability and impact-absorbance property is the main motivation in searching for advanced steels for automotive industry. Herein, even if conventional high strength low alloy steels meet the requirements of automotive producers, advanced high strength steels are primarily preferred due to their higher deformability properties. Advanced high strength steels, which are widely used in automotive industry lately, contribute to yielding in vehicle weight without decrease in strength in the case of small thicknesses and hence reducing fuel consumption and decreasing greenhouse emission. By comparison with conventional high strength steels, these steels are greatly preferred in places requiring high security in automotive sector due to their higher impact-absorbance property. New generation advanced high strength steels are considered in third generation according to their technological development. First generation advanced high strength steels can be classified as dual-phase (DP) steels, complex-phase (CP) steels, transformation-induced plasticity (TRIP) steels, martensitic steels (MS) and press hardened steels (PHS); second generation advanced high strength steels can be divided as austenitic stainless steels and twinning- induced plasticity (TWIP) steels and third generation advanced high strength steels can be classified as nano steels, quenched and partitioned (QP) steels and TRIP aided bainitic ferrite (TBF) steels. Being in the new generation high strength steels family and preferred in automotive components, dual phase (DP) steels are characterized by a microstructure in a ferrite matrix that consists of dispersions of 10-20% martensite islands. In addition to this, since they contain low small amount of retained austenite, dual phase steels have a great application area in automotive applications with their high formability. Dual phase steels have become a solution for highly deformable high strength steel need, in automotive industry while they are used in cold rolled automobile components such as chassis, wheel, car bumper, seat frame and door panels. Deformation areas in different intensities form on dual phase steels when they are deformed according to the part and function where they will serve. Later, they are subjected to painting for visualization and corrosion protection. Painting requires curing and it provides additional rise in strength of steel. This phenomenon, which is called as "Bake Hardening" (BH), occurs in dual phase steels. While at low temperatures bake hardening of carbon in ferrite has advantages for dual phase steels in terms of strength; higher temperatures cause the second main phase, martensite to transform into tempered martensite and decreases the strength. Welding process of cold rolled dual phase components is a thermal process; therefore, it changes the mechanical properties and the microstructure. When look at the aging behavior of dual phase steels from physical metallurgical aspect; several stages must be considered in both the ferrite phase during the static strain ageing processes and the martensite structure in the course of tempering processes: (1) the Cottrell atmosphere formation stage that can be seen pinning, (2) in the ferrite phase the carbon- clustering stage,(3) the precipitation stage in the ferrite phase and (4) the tempering of the martensite structure stage, such as the volume contraction of martensite via tempering effects, the changes in the martensite strength and the additional precipitation carbide or carbon clustering near the martensite-ferrite interfaces.The strengthening stages are expected to overlap at higher aging temperatures for example 400°C and 600°C. In this study, it was investigated how microstructure and mechanical properties change during a heat treatment like painting or welding after cold rolling process of dual phase DP600 steels. Since different deformations form in reality, pre-deformed 0%, 1%, 2%, 4% and 8% cold rolled dual phase steels were aged at different temperatures such as 100°C, 200°C, 400°C, 600°C for different time durations 10 min., 20 min., 40 min. and 120 min. and the experimental study and later, microstructural and mechanical property changes were examined. All of the experiments carried out at the room temperature. At the end of the experimental studies, specific results are observed. As a result of experimental studies; in hardness measurements, it was confirmed that there is no effect of ageing time on hardness; but, deformation amount increases hardness. In yield and tensile strengths, there is an increase with increasing temperature (up to 600°C) and increasing deformation in general. It was observed that there is a decrease in total elongation and homogenous elongation depending on pre-deformation at all temperatures and time durations. It was observed that as ageing temperature rises up, yield strength gets closer to tensile strength. From this point of view, material can be aged by applying deformation at low temperatures in order to raise yield strength. It was observed that BH values increase with increasing deformation and ageing time duration at temperatures 100°C and 200°C; however, it decreases after 400°C. The experiments performed at 600°C were done to reveal how the material behaves under heat in welding application and designs. At this temperature and long ageing durations, material becomes too soft and martensite structure transforms into totally different structure. When all experiments are taken into account, it was seen that the highest strength, hardness and BH values were achieved providing that there was %2 deformation-200°C-120 minutes (8-2-12).