STUDY OF DUAL PHASE STEEL THROUGH CYCLIC INTERCRITICAL ANNEALING AND QUENCHING ON SS400 STEEL

• Main Author: Bonfilio Kerta Jaya, Delvin
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/85286
• Institution: Institut Teknologi Bandung

Dual Phase (DP) steel is a type of steel commonly used in the automotive industry, particularly for car frames, due to its high strength and toughness. The steel typically used to produce DP steel is High Strength Low Alloy (HSLA) steel, specifically the VDA 239-100 type, which is expensive due to the addition of alloying elements such as Si, Mn, and Ti to modify its mechanical properties. To date, there has been no research on the production of DP steel using plain low carbon steel, which is cheaper than HSLA steel. Therefore, this study aims to produce DP steel using plain low carbon steel (SS400). The cyclic intercritical annealing method was chosen for producing DP steel in this study. This method was selected because it can reduce the strength difference between the martensitic and ferritic phases, resulting in higher elongation compared to conventional methods. A series of cyclic intercritical annealing experiments at various intercritical annealing temperatures and holding durations were conducted to study the effects of temperature and holding time on the mechanical properties and microstructure of the resulting DP steel. The experiment began with normalizing at 870 °C for 10 minutes. This was followed by three cycles of intercritical annealing at temperatures of 775 °C, 800 °C, and 825 °C with holding times of 3, 6, and 9 minutes, followed by water quenching. Microstructural characterization, hardness, strength, and elongation were performed on tensile test samples. Fracture type characterization and toughness tests were performed on impact test samples. The data obtained from the tensile tests were processed into stress-strain curves to determine yield strength and ultimate tensile strength values. The microstructure from the experiments was observed using optical microscopy and Scanning Electron Microscope (SEM). The experimental results showed that the resulting microstructure consisted of island-like martensite and equiaxed ferrite. There was a trend of increasing strength and decreasing % elongation with increasing intercritical annealing temperature and duration. The optimum DP steel was obtained at 800 °C with a 9-minute duration, with YS, UTS, and elongation values of 370 MPa, 638.44 MPa, and 20.40%, respectively. Hardness increased with the strength of the DP steel. The hardness and impact energy values at the optimum variation were 192.75 HV and 91.18 J/cm², respectively. The DP steel in this experiment exhibited higher % elongation than conventional methods. The DP steel produced at 800 °C for 9 minutes had mechanical properties similar to DP 300/500 and DP 350/600, which are more expensive.