Forming / production processes of third generation high strength automotive steels combined by welding

In recent years, automobile manufacturers have focused on weight reduction efforts in line with environmental problems and the goal of reducing fuel consumption. Increasing safety regulations with weight reduction are visibly increasing the use of high-strength steels in vehicle body parts day by day. This situation has required steel manufacturers to develop steel types containing innovative and complex microstructures suitable for the industry's tool-mold-process investment and can be used with cold or hot forming processes. Among these, hot-forming steels (Press Hardening Steels- PHS) stand out, allowing the design of structural parts with tensile strength up to 1500 MPa. This technology, which has matured in the last decade, is far from being economic and competitive compared to the traditional cold-forming method, considering both the high material costs and the investment costs of special presses, ovens, molds, and automation. This situation has led steel manufacturers to develop 3rd generation advanced high strength steels (Advanced High Strength Steels-AHSS) cold work steels, which are high strength and suitable for the use of traditional tool-molds. These new generation steels, called TBF (TRIP-Aided Bainitic Ferrite) and QP (Quenching and Partitioning), can be defined as cold work steels with formability properties similar to the 1st generation advanced high strength steels (Dual Phase (DP) DP600, TRIP780) and with tensile strengths reaching 1300 MPa. The main purpose of this thesis is to obtain information about the formability and mechanical properties of automotive parts, whose impact resistance is critical after the high-strength 3rd generation AHSS steels are combined with laser welding and turned into sheet metal. In the thesis study, it is aimed to experimentally reveal the relationships between micro-macro internal structure, mechanics, formability and fracture behaviors after laser welding of high-strength DP600, QP980, and QP1180 steels of different thicknesses for the automotive industry and to determine the design parameters for applications in the automotive industry.