| 23MnNiCrMo54 steel, which referred to as 54 steel, is widely used in coal mining equipments and transportation machineries as a transmission chain because of its excellent mechanical properties. However, the spheroidization-annealed pretreatment of 54 steel shows a poor effect of carbide and the long production period. Thus, the mass production and wide application of large size chain steel were restricted by the carbide morphology.In this study, the organization properties and mechanical properties under different spheroidization-annealed processes of hot-rolled 54 steel bars were emphatically studied.The diameter of 54 steel bars were 26 mm, which was thermal treated in different spheroidization-annealed. This will provide the theoretical and technical support for the large size chain steel.The thermal dilation method, OM, SEM were used to study the phase transition properties of 54 steel and the isothermal transformation kinetics of bainite. The spheroidizing results and the effect on the mechanical behavior of 54 steel under different spheroidization-annealed processes are also be studied. The main conclusions are as follows:The main microstructure of hot rolled 54 steel was bainite which has obvious segregation bands on it, and it was mostly assembled by granular bainite and martensite.The phase transition temperature of Ac1, Ac3 and Ms were 715 ℃, 807 ℃, 370 ℃,respectively.The isothermal bainitic transformation kinetics of 54 steel was studied in the temperature range of 400~500 ℃. The results showed that the amounts of transformed bainite reduced as the increase of isthermal temperature. When the isothermal temperature was 400℃, the quantity of transformation reached the maximum.The effects on microstructure of 23 MnNi CrMo54 steel under different temperatures and periods of subcritical annealing were systematically analysised. After spheroidizing annealing at different temperatures(660~690℃) for 5 hours, the incompletely decomposed M-A island structure was observed at a low anneal temperature of 600℃. With the annealing temperature increasing, some thick and long strip carbides appeared on thesegregation band at 680℃ and 690℃. And the best effect of carbide spheroidization was at 670℃. After testing different annealing hours of 54 steel at 670℃, We find that the M-A island structure decomposed incompletely at short annealing time of 2 hours. With the annealing time increasing, the carbides in small size were partly dissolved while the big carbides were concentrated and spheroidized. The aggregation and growth of carbides was obvious after 11 hours’ annealing.The two-stage spheroidization-annealed process was studied(The second stage of spheroidization-annealed process is 670 ℃ ×5h). The temperature of the initial stage of annealing process is 450~500℃ and the time is 30 min. It was found that the 54 steel has the best cabide morphology, which is ball, uniform distribution, besides, there is a very small amount of short rod carbide when the temperature was 400 ℃ during the initial stage.The mechanical properties of 54 steel in the subcritical annealing and two-stage spheroidization-annealed pretreatment process were compared and analyzed. The result showed that under the pretreatment process of 400℃×30 min+670℃×5h, comparing to other spheroidizing heat treatment process, the yield strength, tensile strength, absorbed energy and reduction in area of 54 steel achieved the maximum value of 1149 MPa, 1260 MPa, 139 J and 68.3%, respectively. The elongation also achieved a larger value which was 16.1%. The impact energy reached 139 J. The spheroidizing pretreatment has an important effects on the final performance of the tested steel, which means the better the result of spheroidization of carbide is, the better ultimate mechanical properties it will has. |
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