The resistance of metallic materials to ballistic penetration generally depends on a number of parameters related to projectile, impact, and armor plate. Recently, armor materials have been required to have various properties such as hardness, strength, and impact toughness in order to maintain an excellent ballistic resistance even after impact. In the present study, the influence of tempering on the microstructure and mechanical properties of an ARMOX 500T armor steel plate was investigated and then compared with those of S45C and SCM440 steels. As the tempering temperature increased, the hardness and strength gradually decreased, whereas the ductility and impact toughness clearly increased because the hardness, tensile, and impact properties were affected by the microstructural evolution and precipitation occurring during tempering. On the other hand, temper embrittlement appeared at tempering temperatures of 300 to 400 °C for the impact specimens tested at low temperature.

In this study, two Fe-30Mn-0.2C-(1.5Al) high-manganese steels with different surface conditions were hydrogencharged under high temperature and pressure; then, tensile testing was performed at room temperature in air. The yield strength of the 30Mn-0.2C specimen increased with decreasing surface roughness(achieved via polishing), but that of the 30Mn-0.2C- 1.5Al specimen was hardly affected by the surface conditions. On the other hand, the tendency of hydrogen embrittlement of the two high-manganese steels was not sensitive to hydrogen charging or surface conditions from the standpoints of elongation and fracture behavior. Based on the EBSD analysis results, the small decrease in elongation of the charged specimens for the Fe-30Mn-0.2C-(1.5Al) high-manganese steels was attributed to the enhanced dislocation pile-up around grain boundaries, caused by hydrogen

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 lowcarbon 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.

In this study, six kinds of low-carbon steel specimens with different ferrite-pearlite microstructures were fabricated by varying the Nb content and the transformation temperature. The microstructural factors of ferrite grain size, pearlite fraction, interlamellar spacing, and cementite thickness were quantitatively measured based on optical and scanning electron micrographs; then, Charpy impact tests were conducted in order to investigate the correlation of the microstructural factors with the impact toughness and the ductile-brittle transition temperature (DBTT). The microstructural analysis results showed that the Nb4 specimens had ferrite grain size smaller than that of the Nb0 specimens due to the pinning effect resulting from the formation of carbonitrides. The pearlite interlamellar spacing and the cementite thickness also decreased as the transformation temperature decreased. The Charpy impact test results indicated that the impact-absorbed energy increased and the ductile-brittle transition temperature decreased with addition of Nb content and decreasing transformation temperature, although all specimens showed ductile-brittle transition behaviour.

In this study, recrystallization behaviors in the two-phase (α+γ) 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.

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.

The hydrogen embrittlement of two austenitic high-manganese steels was investigated using tensile testing under high-pressure gaseous hydrogen. The test results were compared with those of different kinds of austenitic alloys containing Ni, Mn, and N in terms of stress and ductility. It was found that the ultimate tensile stress and ductility were more remarkably decreased under high-pressure gaseous hydrogen than under high-pressure gaseous argon, unlike the yield stress. In the specimens tested under high-pressure gaseous hydrogen, transgranular fractures were usually observed together with intergranular cracking near the fracture surface, whereas in those samples tested under high-pressure gaseous argon, ductile fractures mostly occurred. The austenitic high-manganese steels showed a relatively lower resistance to hydrogen embrittlement than did those with larger amounts of Ni because the formation of deformation twins or microbands in austenitic highmanganese steels probably promoted planar slip, which is associated with localized deformation due to gaseous hydrogen.

This paper presents a study of the tensile properties of austenitic high-manganese steel specimens with different grain sizes. Although the stacking fault energy, calculated using a modified thermodynamic model, slightly decreased with increasing grain size, it was found to vary in a range of 23.4 mJ/m2 to 27.1 mJ/m2. Room-temperature tensile test results indicated that the yield and tensile strengths increased; the ductility also improved as the grain size decreased. The increase in the yield and tensile strengths was primarily attributed to the occurrence of mechanical twinning, as well as to the grain refinement effect. On the other hand, the improvement of the ductility is because the formation of deformation-induced martensite is suppressed in the high-manganese steel specimen with small grain size during tensile testing. The deformationinduced martensite transformation resulting from the increased grain size can be explained by the decrease in stacking fault energy or in shear stress required to generate deformation-induced martensite transformation.

In the present study, the tensile properties and dynamic strain aging of an Fe-24.5Mn-4Cr-0.45C alloy were investigated in terms of strain rate. During tensile testing at room temperature, all the stress-strain curves exhibited serrated plastic flows related to dynamic strain aging, regardless of the strain rate. Serration appeared right after yield stress at lower strain rates, while it was hardly observed at high strain rates. On the other hand, strain-rate sensitivity, indicating a general relationship between flow stress and strain rate at constant strain and temperature, changed from positive to negative as the strain increased. The negative strain-rate sensitivity can be explained by the Portevin Le Chatelier effect, which is associated with dynamic strain aging and is dependent on the strain rate because it is very likely that the dynamic strain aging phenomenon in high-manganese steels is involved in the interaction between moving dislocations and point-defect complexes.

This paper presents a study on the room- and low-temperature impact toughness of hypoeutectoid steels with ferritepearlite structures. Six kinds of hypoeutectoid steel specimens were fabricated by varying the carbon content and austenitizing temperature to investigate the effect of microstructural factors such as pearlite volume fraction, interlamellar spacing, and cementite thickness on the impact toughness. The pearlite volume fraction usually increased with increasing carbon content and austenitizing temperature, while the pearlite interlamellar spacing and cementite thickness mostly decreased with increasing carbon content and austenitizing temperature. The 30C steel with medium pearlite volume fraction and higher manganese content, on the other hand, even though it had a higher volume fraction of pearlite than did the 20C steel, showed a better low-temperature toughness due to its having the lowest ductile-brittle transition temperature. This is because various microstructural factors in addition to the pearlite volume fraction largely affect the ductile-brittle transition temperature and lowtemperature toughness of hypoeutectoid steels with ferrite-pearlite structure. In order to improve the room- and low-temperature impact toughness of hypoeutectoid steels with different ferrite-pearlite structures, therefore, more systematic studies are required to understand the effects of various microstructural factors on impact toughness, with a viewpoint of ductile-brittle transition temperature.

The present study is concerned with the influence of niobium(Nb) addition and austenitizing temperature on the hardenability of low-carbon boron steels. The steel specimens were austenitized at different temperatures and cooled with different cooling rates using dilatometry; their microstructures and hardness were analyzed to estimate the hardenability. The addition of Nb hardly affected the transformation start and finish temperatures at lower austenitizing temperatures, whereas it significantly decreased the transformation finish temperature at higher austenitizing temperatures. This could be explained by the non-equilibrium segregation mechanism of boron atoms. When the Nb-added boron steel specimens were austenitized at higher temperatures, it is possible that Nb and carbon atoms present in the austenite phase retarded the diffusion of carbon towards the austenite grain boundaries during cooling due to the formation of NbC precipitate and Nb-C clusters, thus preventing the precipitation of M23(C,B)6 along the austenite grain boundaries and thereby improving the hardenability of the boron steels. As a result, because it considerably decreases the transformation finish temperature and prohibits the nucleation of proeutectoid ferrite even at the slow cooling rate of 3 oC/s, irrespective of the austenitizing temperature, the addition of 0.05 wt.% Nb had nearly the same hardenability-enhancing effect as did the addition of 0.2 wt.% Mo.

The ductile-brittle transition behavior of two austenitic Fe-18Cr-10Mn-N-C alloys with different grain sizes was investigated in this study. The alloys exhibited a ductile-brittle transition behavior because of an unusual brittle fracture at low temperatures unlike conventional austenitic alloys. The alloy specimens with a smaller grain size had a higher yield and tensile strengths than those with a larger grain size due to grain refinement strengthening. However, a decrease in the grain size deteriorated the low-temperature toughness by increasing the ductile-brittle transition temperature because nitrogen or carbon could enhance the effectiveness of the grain boundaries to overcome the thermal energy. It could be explained by the temperature dependence of the yield stress based on low-temperature tensile tests. In order to improve both the strength and toughness of austenitic Fe-Cr-Mn-N-C alloys with different chemical compositions and grain sizes, more systematic studies are required to understand the effect of the grain size on the mechanical properties in relation to the temperature sensitivity of yield and fracture stresses.

The hardenability of boron steel specimens with different molybdenum and chromium contents was investigated using dilatometry and microstructural observations, and then was quantitatively measured at a critical cooling rate corresponding to 90 % martensite hardness obtained from a hardness distribution plotted as a function of cooling rate. Based on the results, the effect of an austenitizing temperature on the hardenability and tensile properties was discussed in terms of segregation and precipitation behavior of boron atoms at austenite grain boundaries. The molybdenum addition completely suppressed the formation of pro-eutectoid ferrite even at the slowest cooling rate of 0.2 oC/s, while the chromium addition did at the cooling rates above 3 oC/s. On the other hand, the hardenability of the molybdenum-added boron steel specimens decreased with an increasing austenitizing temperature. This is associated with the preferred precipitation of boron atoms since a considerable number of boron atoms could be concentrated along austenite grain boundaries by a non-equilibrium segregation mechanism. The secondary ion mass spectroscopy results showed that boron atoms were mostly segregated at austenite grain boundaries without noticeable precipitation at higher austenitization temperatures, while they formed as precipitates at lower austenitization temperatures, particularly in the molybdenum-added boron steel specimens.

In this study, low-carbon hypoeutectoid steels with different ferrite-pearlite microstructures were fabricated byvarying transformation temperature. The microstructural factors such as pearlite fraction and interlamellar spacing, and cementitethickness were quantitatively measured and then Charpy impact tests conducted on the specimens in order to investigate thecorrelation of the microstructural factors with impact toughness and ductile-brittle transition temperature. The microstructuralanalysis results showed that the pearlite interlamellar spacing and cementite thickness decreases while the pearlite fractionincreases as the transformation temperature decreases. Although the specimens with higher pearlite fractions have low absorbedenergy, on the other hand, the absorbed energy is higher in room temperature than in low temperature. The upper-shelf energyslightly increases with decreasing the pearlite interlamellar spacing. However, the ductile-brittle transition temperature is hardlyaffected by the pearlite interlamellar spacing because there is an optimum interlamellar spacing dependent on lamellar ferriteand cementite thickness and because the increase in pearlite fraction and the decrease in interlamellar spacing with decreasingtransformation temperature have a contradictory role on absorbed energy.

The present study deals with the effects of tempering treatment on the microstructure and mechanical properties of Cu-bearing high-strength steels. Three kinds of steel specimens with different levels of Cu content were fabricated by controlled rolling and accelerated cooling, ; some of these steel specimen were tempered at temperatures ranging from 350˚C to 650˚C for 30 min. Hardness, tensile, and Charpy impact tests were conducted in order to investigate the relationship of microstructure and mechanical properties. The hardness of the Cu-added specimens is much higher than that of Cu-free specimen, presumably due to the enhanced solid solution hardening and precipitation hardening, result from the formation of very-fine Cu precipitates. Tensile test results indicated that the yield strength increased and then slightly decreased, while the tensile strength gradually decreased with increasing tempering temperature. On the other hand, the energy absorbed at room and lower temperatures remarkably increased after tempering at 350˚C; and after this, the energy absorbed then did not change much. Suitable tempering treatment remarkably improved both the strength and the impact toughness. In the 1.5 Cu steel specimen tempered at 550˚C, the yield strength reached 1.2 GPa and the absorbed energy at -20˚C showed a level above 200 J, which was the best combination of high strength and good toughness.

Effects of Cu and B on effective grain size and low-temperature toughness of thermo-mechanically processed high-strength bainitic steels were investigated in this study. The microstructure of the steel specimens was analyzed using optical, scanning, and transmission electron microscopy; their effective grain size was also characterized by electron back-scattered diffraction. To evaluate the strength and low-temperature toughness, tensile and Charpy impact tests were carried out. The specimens were composed of various low-temperature transformation products such as granular bainite (GB), degenerated upper bainite (DUB), lower bainite (LB), and lath marteniste (LM), dependent on the addition of Cu and B. The addition of Cu slightly increased the yield and tensile strength, but substantially deteriorated the low-temperature toughness because of the higher volume fraction of DUB with a large effective grain size. The specimen containing both Cu and B had the highest strength, but showed worse low-temperature toughness of higher ductile-brittle transition temperature (DBTT) and lower absorbed energy because it mostly consisted of LB and LM. In the B-added specimen, on the other hand, it was possible to obtain the best combination of high strength and good low-temperature toughness by decreasing the overall effective grain size via the appropriate formation of different low-temperature transformation products containing GB, DUB, and LB/LM.

The effect of tungsten (W) addition on the hardenability of low-carbon boron steels was investigated using dilatometry, microstructural observations and secondary ion mass spectroscopy. The hardenability was discussed with respect to transformation behaviour aspects depending on the segregation and precipitation of boron at austenite grain boundaries. A critical cooling rate producing a hardness corresponding to 90 % martensite structure was measured from a hardness distribution plot, and was used as a criterion to estimate hardenability at faster cooling rates. In the low-carbon boron steel, the addition of 0.50 wt.% W was comparable to that of 0.20 wt.% molybdenum in terms of critical cooling rate, indicating hardenability at faster cooling rates. However, the addition of 0.50 wt.% W was not more effective than the addition of .0.20 wt.% molybdenum at slower cooling rates. The addition of 0.20 wt.% molybdenum completely suppressed the formation of eutectoid ferrite even at the slow cooling rate of 0.2˚C/s, while the addition of 0.50 wt.% W did not, even at the cooling rate of 1.0˚C/s. Therefore, it was found that the effect of alloying elements on the hardenability of low-carbon boron steels can be differently evaluated according to cooling rate.

The effect of interstitial elements on the ductile-brittle transition behavior of austenitic Fe-18Cr-10Mn-2Ni alloys with different nitrogen and carbon contents was investigated in this study. All the alloys exhibited ductile-brittle transition behavior because of unusual low-temperature brittle fracture, even though they have a faced-centered cubic structure. With the same interstitial content, the combined addition of nitrogen and carbon, compared to the sole addition of nitrogen, improved the low-temperature toughness and thus decreased the ductile-brittle transition temperature (DBTT) because this combined addition effectively enhances the metallic component of the interatomic bonds and is accompanied by good plasticity and toughness due to the increased free electron concentration. The increase in carbon content or of the carbon-to-nitrogen ratio, however, could increase the DBTT since either of these causes the occurrence of intergranular fracture that lead to the deterioration of the toughness at low temperatures. The secondary ion mass spectroscopy analysis results for the observation of carbon and nitrogen distributions confirms that the carbon and nitrogen atoms were significantly segregated to the austenite grain boundaries and then caused grain boundary embrittlement. In order to successfully develop austenitic Fe-Cr-Mn alloys for low-temperature application, therefore, more systematic study is required to determine the optimum content and ratio of carbon and nitrogen in terms of free electron concentration and grain boundary embrittlement.

The hardenability of low-carbon boron steels with different molybdenum and chromium contents was investigated using dilatometry, microstructural observations and secondary ion mass spectroscopy (SIMS), and then discussed in terms of the segregation and precipitation behaviors of boron. The hardenability was quantitatively evaluated by a critical cooling rate obtained from the hardness distribution plotted as a function of cooling rate. It was found that the molybdenum addition was more effective than the chromium addition to increase the hardenability of boron steels, in contrast to boron-free steels. The addition of 0.2 wt.% molybdenum completely suppressed the formation of eutectoid ferrite, even at the slow cooling rate of 0.2˚C/s, while the addition of 0.5 wt.% chromium did this at cooling rates above 3˚C/s. The SIMS analysis results to observe the boron distribution at the austenite grain boundaries confirmed that the addition of 0.2 wt.% molybdenum effectively increased the hardenability of boron steels, as the boron atoms were significantly segregated to the austenite grain boundaries without the precipitation of borocarbide, thus retarding the austenite-to-ferrite transformation compared to the addition of 0.5 wt.% chromium. On the other hand, the synergistic effect of molybdenum and boron on the hardenability of boron steels could be explained from thermodynamic and kinetic perspectives.