• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.05a
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• pp.313-316
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• 2005

In the present study, Bauschinger effect was investigated for the micro-alloying forging steel which has been developed for about 30 years ago to save energy consumption by eliminating the heat treatment processes in the forging industry. The micro-alloying steels used fur cold forging industry mainly aim to replace the usual carbon steel. With the conventional carbon steels, all the deformation history can be eliminated after the final heat treatment(quenching and tempering). In the case of micro-alloying forging steels, however, the prior deformation history should be taken into consideration to meet the mechanical property requirement since the microstructure of micro-alloying steels might exhibit the Bauschinger effect, which was not needed to consider in the case of conventional carbon steel having quenching and tempering treatment. In the present study, the reverse loading tests were carried out to determine the Bauschinger effect of micro-alloying steel which composed of ferrite and cementite phases.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.05a
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• pp.317-320
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• 2005

The micro-alloying forging steels have been developed to save energy consumption during forging and subsequent heat treatment stages. The work hardening ability of micro-alloying forging steels is one of major hardening component while it gives severe die damage if the forging process design is poorly set up on the other hand. In the present study, it was tried to characterize three types of micro-alloying forging steels to understand the differences with the conventional low carbon steels used fur cold forging with a spherodizing heat treatment. After forging of a certain forging part with both micro-alloying forging steels and conventional low carbon steel, several mechanical tests were carried out.

• Journal of the Korean Society for Heat Treatment
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• v.30 no.3
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• pp.106-112
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• 2017

In this study dual-phase steels with different ferrite grain size and martensite fraction were fabricated by varying micro-alloying elements and intercritical anneling temperatures, and then the tensile properties were investigated in terms of yield and tensile strengths, elongation, and yield ratio. The addition of micro-alloying elements reduced ferrite grain size, and the increased intercritial transformation tempeature increased the martensite fracton. The tensile test results showed that yield and tensile strengths of all the steel specimens increased with increasing the martensite fraction. However, the elongation and yield ratio were differently changed according to variations in the morphology and carbon content of martensite, ferrite grain size, and precipitates resulting from the addition of micro-alloying elements and intercritical annealing.

• Journal of the Korean Society for Heat Treatment
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• v.30 no.1
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• pp.6-12
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• 2017

In this study tensile and impact properties of three hypo-eutectoid steels containing different micro-alloying elements were investigated in terms of microstructural factors such as pro-eutectoid ferrite grain size, pearlite fraction, interlamellar spacing, and cementite thickness. Yield point phenomenon appeared in all the steel specimens during tensile testing, and ultimate tensile stress was mainly dependent on pearlite fraction. On the other hand, the refinement of austenite grain size caused by the addition of micro-alloying elements resulted in the increment of ferrite volume fraction and carbon contents in pearlite because of the refinement of pro-eutectoid ferrite grain size. As a result, cementite thickness in pearlite increased and had an effect on deteriorating the low temperature impact toughness.

• Korean Journal of Materials Research
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• v.26 no.10
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• pp.535-541
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• 2016

The present study deals with the effects of micro-alloying elements such as Ni, V, and Ti on the recrystallization behavior of carbon steels at different strain rates. Eight steel specimens were fabricated by varying the chemical composition and reheating temperature; then, a high-temperature compressive deformation test was conducted in order to investigate the relationship of the microstructure and the recrystallization behavior. The specimens containing micro-alloying elements had smaller prior austenite grain sizes than those of the other specimens, presumably due to the pinning effect of the formation of carbonitrides and AlN precipitates at the austenite grain boundaries. The high-temperature compressive deformation test results indicate that dynamic recrystallization behavior was suppressed in the specimens with micro-alloying elements, particularly at increased strain rate, because of the pinning effect of precipitates, grain boundary dragging and lattice misfit effects of solute atoms, although the strength increased with increasing strain rate.

• Korean Journal of Materials Research
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• v.27 no.4
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• pp.184-191
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• 2017

This present study deals with the effect of micro-alloying elements and transformation temperature on the correlation of microstructure and tensile properties of low-carbon steels with ferrite-pearlite microstructure. Six kinds of low-carbon steel specimens were fabricated by adding micro-alloying elements of Nb, Ti and V, and by varying isothermal transformation temperature. Ferrite grain size of the specimens containing mirco-alloying elements was smaller than that of the Base specimens because of pinning effect by the precipitates of carbonitrides at austenite grain boundaries. The pearlite interlamellar spacing and cementite thickness decreased with decreasing transformation temperature, while the pearlite volume fraction was hardly affected by micro-alloying elements and transformation temperature. The room-temperature tensile test results showed that the yield strength increased mostly with decreasing ferrite grain size and elongation was slightly improved as the ferrite grain size and pearlite interlamellar spacing decreased. All the specimens exhibited a discontinuous yielding behavior and the yield point elongation of the Nb4 and TiNbV specimens containing micro-alloying elements was larger than that of the Base specimens, presumably due to repetitive pinning and release of dislocation by the fine precipitates of carbonitrides.

• Korean Journal of Materials Research
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• v.26 no.11
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• pp.583-589
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• 2016

In this study, recrystallization behaviors in the two-phase (${\alpha}+{\gamma}$) region of micro-alloyed steels such as Base, Nb, TiNbV and CAlN were investigated in terms of flow stress, microstructure and associated grain boundary characteristics. The flow stress of all specimens reached peak stress and gradually decreased, which means that recrystallization or recovery of proeutectoid deformed ferrite and recovery or transformation to ferrite of deformed austenite occurred by thermal activation. The precipitation of carbide or nitride via the addition of micro-alloying elements, because it reduced prior austenite grain size upon austenitization, promoted transformation of austenite to ferrite and increased flow stress. The strain-induced precipitation under deformation in the two-phase region, on the other hand, increased the flow stress when the micro-alloying elements were dissolved during austenitization. The recrystallization of the Nb specimen was more effectively retarded than that of the TiNbV specimen during deformation in the two-phase region.

• Transactions of Materials Processing
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• v.15 no.8 s.89
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• pp.609-614
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• 2006

Micro-alloyed steels(MA steels) for cold forging was developed to replace the usual quenched and tempered steel. MA steels have several advantages over the conventional quenched and tempered carbon steels. First of all, energy consumption could be lowered due to the elimination of spherodizing annealing and quenching/tempering heat treatment. Also, bending during quenching could be avoided when MA steels are applied for manufacturing of long fastener parts. However, larger amount of load is exerted on the dies compared than in the case of conventional mild steels, which might lead to the earlier fracture of dies, when MA forging steels are applied in forging practice. Therefore, die lift could be a critical factor to determine whether HA forging steels could be widely applied in cold forging practice. In the present study, authors have investigated the forging characteristics of non-heat treated micro-alloyed steel by using a series of experimental and numerical analyses. Firstly, microstructural features and its effect on the deformation behavior have been studied. Numerical analysis has been done on the forging of guide rod pin to investigate for the optimization of forging process and die stress prediction.

• Corrosion Science and Technology
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• v.18 no.3
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• pp.73-77
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• 2019

Hot-dip aluminized coating has been widely used to protect steel substrate against corrosion. In this study, the corrosion behavior of hot-dip aluminized type 409L (11% Cr) stainless steel (SS) was investigated using macro- and micro-scale polarization tests. An Al-Fe-Si alloy layer that was formed due to inter-diffusion of alloying elements between Al coating and SS substrate was observed between Al coating and 409L SS substrate. In both macro- and micro-scale polarization tests, the corrosion potential ($E_{corr}$) of the 409L SS substrate was much nobler than that of the Al coating and alloy layer. $E_{corr}$ of the alloy layer was between that of Al coating and 409L SS substrate. This indicates that the alloy layer can act as a buffer between the more active Al coating and the nobler SS substrate for pit growth in aluminized SS. The presence of the alloy layer appears to be helpful in hindering pitting corrosion of aluminized SS.

• Korean Journal of Materials Research
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• v.22 no.10
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• pp.513-518
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• 2012

Recently, automobile parts have been required to have high strength and toughness to allow for weight lightening or improved stability. But, traditional micro-alloyed steel cannot be applied in automobile parts. In this study, we considered the influence of quenching temperature and cooling rate for specimens fabricated by vacuum induction furnace. Directly quenched micro-alloyed steel for hot forging can be controlled according to its micro structure and the heat-treatment process. Low carbon steel, as well as alloying elements for improvement of strength and toughness, was used to obtain optimized conditions. After hot forging at $1,200^{\circ}C$, the ideal mechanical properties (tensile strength ${\geq}$ 1,000 MPa, Charpy impact value ${\geq}\;100\;J/cm^2$) can be achieved by using optimized conditions (quenching temperature : 925 to $1,050^{\circ}C$, cooling rate : ${\geq}\;5^{\circ}C/sec$). The difference of impact value according to cooling rate can be influenced by the microstructure. A fine lath martensite micro structure is formed at a cooling rate of over $5^{\circ}C/sec$. On the other hand, the second phase of the M-A constituent microstructure is the cause of crack initiation under the cooling rate of $5^{\circ}C/sec$.