Sweden's Cr-V high-quality spring steel wire belongs to this category. After steel contact heating and oil quenching, the grain size of the steel wire reaches ASTM1112 (4Lm5Lm). This method of refining the grains not only improves the quality of the spring, but also facilitates mechanized production. In this study, the 65Mn flat steel strip was subjected to electrical contact heating and oil quenching and tempering production test. The purpose was to find a reasonable steel strip refining process and improve the toughness of the 65Mn flat steel strip. The flattened steel strip after the grain processing is directly rolled into a snap ring spring, which improves the product quality and efficiency and meets the production requirements. Test method, the test material test steel is the 65Mn steel wire rod with diameter <615mm provided by Tianjin Steel Works Metal Products Factory, its chemical composition (mass fraction,%) 01670169C, 1107Mn, 01260129Si, 0102701028P, 0101001013S. According to GB/T3530 1983 flat steel strip size performance requirements, using two methods of hot rolling and cold rolling. The hot rolling forming production process is: <615mm wire rod cold drawing <516mm cold drawing <417mm cold drawing <410mm. The area shrinkage rate is about 62%. The <410mm steel wire is heated to 800e, hot rolled to 119mm@512mm and then hot pickled and shaped. The final size is 117mm@5mm, the deformation of the shaping is 1315%, and the grain size after hot rolling is 10 grade. It is a fine-grained and flaky mixed sorbite. The cold rolling forming production process is <615 mm (wire rod) cold drawing <512 mm cold drawing <414 mm, the area shrinkage is about 54%, and after recrystallization at 730e, two cold rollings are performed. It is cold-rolled by 414mm@418mm cold-rolled 2mm@512mm, and its area shrinkage is 36127%. Then it is recrystallized and aged at 700e, and the final size of the steel strip is 1195mm@5mm. The steel strip has a grain size of (67) and is organized into fine-grained sorbite.
The hot-rolled steel strip of the test process was rapidly contacted by a cold-rolled steel strip at a speed of 2617 e/s at a speed of 29 e/s and 54 e/s, and the heating temperatures were 800, 820, 840, 860, 880 and 900 e, respectively. After oil quenching, the grain size and hardness were measured, and the effect of quenching heating temperature on grain refinement was obtained. Hot-rolling and cold-rolling 65Mn spring steel strips obtained ultra-fine grain after deformation and rapid heating, and the test showed that with heating The temperature increases the grain size has a tendency to grow, the effect of the original structure on the grain refinement. The grain recrystallization of the 700e recrystallization annealing after cold rolling is (67) grade, and the microstructure is uniform fine soosite. The storage energy is low, the grain size reaches 15 grades after one refinement treatment, and the cold-formed spring steel strip after hot-rolling forming has the original grain size of 10 grades, and the structure is sheet-like and fine-grained sorbite, which is rapidly heated. After processing, its grain size reached 14 grades. This shows that the former obtains finer grains than the latter. Effect of heating rate on grain refinement For cold-rolled steel strips, the heating rate is increased from 29e/s to 54e/s under electrical contact heating, and the austenite formation temperature is raised from 820e to 860e. Further grain refinement has little effect.
The formation of austenite during rapid heating is also a process of nucleation and core growth. Ultra-fine crystal grains can be obtained after the nucleation rate is high and the growth rate is small. Austenitic nucleation requires nucleation work. The new phase nucleation of solid phase transformation is $F*=4nC327($fT+Es)2, where n is the shape factor, Es is the strain energy of each atom in the nucleus, and $fT is the new and old two-phase unit volume free energy. Poor, C is the phase interface energy. The smaller the nucleation work under the same conditions, the greater the nucleation rate. There are two conditions for lowering the nucleation work$F. One is to lower the C and nucleate at the ready-made interface to reduce the interfacial energy. For example, austenite is formed at the interface between ferrite and cementite, which can lower the interface energy. <68>. The cold-rolled 65Mn steel strip is cold-deformed four times, so that the eutectoid carbide is broken and refined, and evenly distributed on the ferrite matrix, which increases the austenite nucleation position and contributes to grain refinement. The second is to increase $fT (negative value), the faster the heating rate, the larger the $fT, the smaller the nucleation work and the larger the number of nucleation, which contributes to grain refinement. The 65Mn steel strip formed by hot rolling has a grain size of 10 grades and a large number of grain boundary subgrain boundaries. During rapid heating, austenite is formed both at the grain boundary subgrain boundary and also at the phase boundary. Austenite nucleation and core growth are controlled by the diffusion of carbon. The nucleation and core growth rate of austenite on the ferrite grain boundary is lower than that at the interface between granular carbide and ferrite. At the same heating rate and at the same heating temperature (860e), the austenite grains formed on the grain boundaries are finer. When the heating rate is similar, the cold-rolled steel strip is finely dispersed due to cementite, and the phase interface is large, and the degree of grain refinement is better than that of the hot-rolled 65Mn steel strip.
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