Iron & Steel Technology
Process Metallurgy & Product Applications
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72
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The effects of vanadium and aluminum grain refining treatment on the machinability of hot-rolled engineering plain-carbon and low-alloy 4140 steels were investigated using the critical tool wear method during turning of several bars. The flank wear of the machining tool was accurately measured after each machined bar using a scanning electron microscope (SEM). The microstructure and non-metallic inclusions in steel bars and machining chips were characterized using microhardness measurements, optical microscopy and automated SEM/energy-dispersive x-ray spectroscopy (EDX) inclusion analysis. High-resolution transmission electron microscopy was used to characterize MnS/metal matrix boundary in severely deformed chips. Worn machining tools were SEM/EDX analyzed to characterize deposits which were located on the tool surface. The investigation showed that aluminum-treated steels exhibit better machinability than vanadium-treated steels in both plain-carbon and 4140 steel grades. The combined effects of hardness, microstructure and precipitates on the steel machinability are presented and discussed.
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80
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Along with research aimed at understanding and improving liquid metal embrittlement (LME) resistance, the effects of LME cracks on mechanical performance should also be considered. Literature studies of tensile-shear and cross-tension tests with LME cracks did not indicate significant property degradation due to cracks smaller than 300 µm at room temperature. In the present work, the quasi-static strength of spot-welded specimens from a galvanized advanced high-strength steel was tested using tensile-shear and cross-tension with weld crack lengths near 480 µm. Each configuration was tested at room temperature and temperatures as low as –10°C. The peak load decreased with decreasing temperature for both test configurations, but there was no correlation of peak load with crack size or location. The failure modes of the cross-tension tests were consistent between test temperatures. However, tensile-shear tests showed predominantly button pullout failure at low temperatures and interfacial failure at room temperature. Finite element models were also developed for a 2D tensile-shear configuration to provide insight on critical crack geometries that affect performance. The highest stress near the crack tip exists for crack angles of approximately 60° with respect to the surface sheet surface, and stresses increase with increasing crack length. However, these stresses near the crack tip are significantly smaller than those at the weld edge; therefore, the models predict that LME cracks would have no influence on the tensile-shear failure mode.
This paper is the recipient of the 2020 AIST Jerry Silver Award.
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86
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Temperature variation of molten steel in the tundish seriously affects the quality of slab. In order to solve the phenomenon of temperature drop of molten steel in tundish, tundish plasma heating is proposed. In this paper, an industrial test of plasma heating in tundish was analyzed and studied. The influence of temperature rising with plasma heating on molten steel in tundish, the composition change of molten steel and tundish slag were analyzed. The results showed that the temperature of plasma heating was 10.58°C℃ higher than that without heating. And the temperature could be maintained in a stable range. When the heating time became longer, the temperature of molten steel in tundish rose greatly, the viscosity of slag became small and the fluidity of slag was improved, which was conducive to absorbing inclusions and improving the quality of the slab.
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92
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In recent decades, environmental and safety regulations have encouraged the production of stronger and lighter steels. For this reason, this paper explores the associative action of grain refinement and carbide precipitation induced by warm forging and subcritical annealing in two automotive steels: C-Mn and high-strength, low-alloy (HSLA). To do so, several samples were warm forged at 550°C, quenched in water, and subcritically annealed between 1 and 10 minutes. Then, the microstructure and hardness of both steels were assessed in each processing stage through optical microscopy, scanning electron microscopy, transmission electron microscopy and Vickers hardness. These tests showed that the thermomechanical treatment significantly refined the grain size in both steels by means of controlled ferrite recrystallization. Furthermore, the recovery process in the HSLA was slowed by the precipitation of fine carbides in the grain boundaries. Finally, the thermomechanical treatment improved the hardness of both steels.
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108
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Oxidation reduction is an effective way to improve the galvanizability of Si- and Mn-added advanced high-strength steel (AHSS). Due to the suppression of external oxidation by reduced iron, the wettability of molten zinc can be significantly improved, while the control of coating adhesion is still a challenge. An industrial galvanized 980 MPa AHSS having poor coating adhesion was tested by 0T bending and investigated. It was found that the adhesion between the coating and the reduced iron was strong due to the formation of enough Fe-Al inhibition layer. However, the adhesion between the reduced iron and matrix was poor due to the formation of a continuous layer of external Si and Mn oxides at the interface of reduced iron and matrix. It is suggested to precisely control both the oxidation and reduction process to achieve a good coating adhesion.