Study On Microstructure And Mechanical Properties Of TWIP Steel Welded Joints By Laser

As a kind of Twinning Induced Plasticity steel, TWIP steel has a higher elongation (>60%), high tensile strength (～600MPa), and high energy absorption capacity. Therefore, the use of TWIP steel as automobile steel plate has the advantages of high strength steel does not have has become a direction of automotive steel in recent years. TWIP steel has become a direction of automotive steel in recent years. Domestic and foreign researchers pay more and more attention to research of TWIP steel in recent years, along with the industrialization of TWIP steel production gradually formed, the automobile body covering parts and automotive structures will be replaced by the TWIP steel.Laser welding has the advantage of high energy, small welding, fast welding speed and the small parent metal heating effect. Considering the application of the laser welding and the unique advantages of the TWIP steel. In this paper, two kinds of different TWIP steel with thickness of1mm and3mm material and laser welding joint organization and mechanical properties were studied. Using observation of the microstructure, determination of chemical composition, X-ray diffraction test, microhardness test, impact test, static tensile test and fatigue test, the main conclusions are as follows:1)The microstructure of BM is a typical structure of austenitic steels, But HAZ near fusion boundary is almost invisible. HAZ is composed of the columnar grains, In the center of the weld place formed the isometric. Spectrum analysis showed that TWIP steel base metal and laser welded joint are not appear new elements.In the process of welding, part of Mn and Si element are evaporating and loss. Through X-ray diffraction analysis, no new phase occurs after laser welding. Compared with the parent metal, diffraction peaks of the laser welding joint has moved to the right, and X-ray diffraction peak strength is weakened in a degree.2)TWIP steel with thickness of1mm material and welded joint have the tensile strength of580MPa and465MPa respectively, elongation were63.5%and55%respectively, strong plastic product reached36830MPa·%and25575MPa·%. The tensile fracture of base metal and welding joint were ductile fracture. The hardness of laser welded joint along the horizontal distribution was saddle shaped curve, in the depth direction, hardness curve was "U" shape. The fatigue strength was211.1MPa and119.1MPa for the base and the joints with thickness of1mm when the cycles are5×106. Fatigue striations, secondary crack and dimple were observed in the fracture, and the micro-fracture surface consists of quasi-cleavage patterns.3)TWIP steel with thickness of3mm material and welded joint have the tensile strength of564.2MPa and605MPa respectively, elongation were62.7%and50%respectively, strong plastic product reached37632.1MPa·%and30260MPa·%. The tensile fracture of base metal and welding joint were ductile fracture. TWIP steel welded joint specimen impact toughness lower than that of the parent metal, as the temperature is lower and lower. At temperature from-117℃to-196℃, cold short phenomenon often occurred in the welded joint. At temperature from-117℃to-25℃, the parent metal and welded joint impact fracture are ductile fracture, At temperature from-117℃to-196℃, the parent metal is still the ductile fracture, and the welded joint is brittle fracture. The fatigue strength was216.4MPa and186.5MPa for the base and the joints with thickness of1mm when the cycles are5×106. A large number of fatigue striations and secondary cracks were observed by scanning electron microscopy (SEM), There is a large number of toughening nest in transient fault zone, and the fatigue fracture is ductile fracture.