Microstructure-Properties Control Of Low-Carbon Cold-Rolled TRIP Steel And Study On Its Resistance Spot Welding Properties

Due to high content of Si,the traditional TRIP steel are easy to form a stable Mn2SiO4 oxide in the production process,adversely affecting the surface quality and performance of coated steel plate.Considering that A1 has a similar effect to Si in TRIP steel,therefore,the Al is used to instead of the Si and a new composition of high aluminum low silicon TRIP steel was designed.Furthermore,the continuous cooling transformation behavior,the heat treatment,welding performance of tested steel were studied.The main the works are present as follows:(1)According to the composition design idea of "the substitution of Al for Si",a new composition of high aluminum low silicon TRIP steel was designed on the basis of the traditional TRIP steel,which was composited of 0.15C-1.2Al-2.0Mn-0.06Si-0.003P-0.003S.First of all,the continuous cooling transformation behavior of tested steel was studied.The Ac1 was 671? and Ac3 was 946?.The static CCT curve of the tested steel was plotted.The variation of austenite volume fraction under different intercritical annealing temperatures was investigated by using Thermo-Calc thermodynamic software and thermal simulation test machine.(2)By using the Thermo-Calc software and the empirical formula,the carbide precipitation and martensitic transformation temperature were calculated.The isothermal temperature range of bainite is determined to be 400??460? During the isothermal process,it is ensured that a large number of martensite or cementite will not be formed which is unfavourable to the microstructure and properties of TRIP steel(3)The effect of heat treatment process on the microstructure and properties of the tested steel was investigated.The comprehensive performance achieved the best when the intercritical annealing temperature was 840??845?,bainite isothermal temperature was 415?425?,the isothermal time was 300s.The tensile strength,elongation and strength ductility reached 671 MPa,33%and 22.14GPa.%respectively(4)The effect of spot welding parameters,such as welding current and welding time,on the micro structure and properties of resistance spot welded joints of tested steel was investigated.The results show that,in the absence of welding defects,martensitic structure in the nugget area is continuously roughened with the increase of welding heat input and the welding indicators are gradually improved Under the best welding parameters,the welding performance of the tested steel reached the best.The nugget diameter,the tensile shear strength,penetration rate and the indentation rate is 7.26mm,23.76kN,74.7%and 6.6%respectively.