| In recent years, with the improvement of rock drilling machine power and theincreasing market competition of hollow drilling steel, the mechanized drillingmanipulated by people becomes dominant and gradually replaces the pneumatic rock drillmachine operated by manual; the requirements on the quality of hollow drilling steelproduct becomes more higher, especially for the light drilling rod product withtremendous demand. Drilling work with a light drill rod is under the severe wear of the orerock, the erosion corrosion of the high pressure water, water&air mixture and mine water,and also under the tension pressure, bending, torsion and the high-frequency impact of therock drilling machine. Due to the complex working and stress conditions, rock drill tooloften occur fatigue crack. The average drilling depth of domestic light drilling rod is only100metres, which has a big gap with the same products abroad.One important way to increase the drilling depth of the drilling rod is to improve thequality of the hollow drilling steel. The quality issues of the light drilling rod mainly focuson5aspects: materials, billet internal porosity, finish goods physical dimension accuracy,and microstructure evolution control in both rolling and cooling process. Therefore, takingthe hollow drilling steel rolling and cooling process as study objects, combining thenonlinear finite element technology with the material microstructure evolution theory, thispaper studies the workpiece physical dimension accuracy, density, and microstructureevolution rules by numerical simulation analysis and uses the experimental data to verifythe numerical simulation results.Using Gleeble3500thermal simulated test machine, the high temperature flow stresscurve of55SiMnMo medium carbon alloy steel was obtained through one-way thermalcompression test, the high temperature flow stress behavior of55SiMnMo steel wasanalyzed, and also the flow stress prediction model of55SiMnMo steel was established byusing the Arrhenius equation and artificial neural network theory. By comparing theprediction results of artificial neural network model and regression model, advantages anddisadvantages of two flow stress models were analyzed, which establishes the foundation for the future finite element calculation of the hollow steel rolling.Based on the hollow steel rolling process with Mechanical Drilling Method of thedomestic manufacturer, finite element model about the thermal, stress, microstructureevolution and density variation during hollow steel rolling was established by therigid-plastic finite element software; macroscopic fields (temperature field, effective strainand effective stain rate), microscopic fields (austenite grain size) and density distributionand variation of the workpiece were obtained by numerical simulation. With comparisonof the simulated results and experimental data of austenite grain size, the accuracy of finiteelement model during hollow steel rolling was verified.Adopting the Orthogonal Test method, the rolling parameters’ impact on theeccentricity and ovality of workpiece hole was analyzed base on the numerical modelingwith finite element software, and the relation model of influence factors and hole sizeaccuracy was obtained by nonlinear regression. With the relation model, the objectivefunction of hole size dimensional accuracy was established. Meanwhile combining withthe revised broadening formula, hollow steel rolling pass system of the domesticmanufacturer was optimized by using the genetic algorithm to improve the workpiece sizeaccuracy. In addition, the numerical simulation results were verified through the leadrolling experiment.In order to study the microstructure phase transformation during hollow steel coolingprocess after rolling, and analyze the cooling process of hollow steel, the isothermalexpansion test was conducted by using the Formaster-F type dilatometer, and the timetemperature transformation (TTT) curve of55SiMnMo steel was measured. Base onJMAK phase transformation kinetics model and finite element method, the continuouscooling process of55SiMnMo steel was simulated numerically, the microstructure phasetransformation with different cooling rates was analyzed, and the numerical simulationaccuracy was verified by comparison of the experimental result and simulated data.Hollow steel cooling process after rolling was also simulated numerically, and thetemperature field and microstructure field during hollow steel cooling process wereobtained; different cooling conditions’ impacts on the microstructure of final workpiecewere also analyzed, which provides the theoretical foundation for the optimization of cooling process. |
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