چكيده به لاتين
In this research, the effects of nickel and manganese were studied in order to produce suitable steels for quench and partitioning (Q&P) heat treatment. The Q&P heat treatment is designed to produce AHSS steels. Although, a standard composition is not presented for such steels. Here, three low-alloy medium-carbon steels with 0.4wt.%C were designed, which alloy1 contains 1Ni-4.5M, alloy 2, 2Ni-3Mn and alloy3, 3Ni-1Mn (in wt.%). Q&P heat treatment parameters were calculated by dilatometry data and TTT diagram for each alloy. At first, partitioning time and temperature were studied and then the effect of interrupted quench was investigated using nominated previous parameters. The Q&P was carried out in both dilatometer and laboratory furnaces. OM, SEM, EBSD, TKD and TEM were utilized to study the microstructure. In order to quantify the amount of retained austenite (RA), Rietveld refinement applied to XRD data, and to investigate the mechanical properties of the alloys, hardness and uniaxial tensile tests were used.
Results showed that RA grain size was in nano-scale order and had a lath and blocky-like morphologies. 12vol.% RA was stabilized in alloy1 and 2 which was the highest amount reached in this study. The increase in partitioning time and temperature caused the increase in austenite decomposition to bainitic ferrite and bainite which resulted in the reduction of RA. It is shown that RA was more influential on instantaneous n and uniform elongation. Carbides (in the bainite and tempered martensite) increased the yield strength, however it decreased the tensile strength of the steels. The tensile test results showed that all Q&P specimens had tensile strengths higher than 1500 MPa and yield strengths over 1000MPa, and the elongation obtained was up to 11% which categorize these steels as ultrahigh strength steels.
