• Journal of the Korean Society for Heat Treatment
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• v.16 no.3
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• pp.127-133
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• 2003

The effect of chemical composition on the microstructural change and tensile property in TRIP-assisted steels with different chemical composition was investigated by using SEM, TEM, XRD and UTM. As a result of microscopic observation, the morphology of retained austenite could be identified as two types; a granular type in a steel containing higher Si and a film type in a steel having higher C. For the case of higher C-containing steel with a tensile strength of 860 MPa and a total elongation of 38%, film-typed retained austenite could be observed between lath bainitic ferrite. Actually, metastable retained austenite was a requisite for the good formability, which means that chemical composition plays a significant role in the microstructure and tensile property of TRIP-assisted steels. With respect to tensile property, the steels containing suitable Si and Mn, respectively, showed a typical TRIP effect in stress-strain curve, while a steel containing higher Mn content exhibited the similar behavior shown in dual phase steel.

• Transactions of the Korean Society of Automotive Engineers
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• v.15 no.3
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• pp.106-113
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• 2007

The effect of chemical composition on the microstructural change and tensile property in TRIP-assisted steels with different chemical composition was investigated by using SEM, TEM, XRD and UTM. As a result of microscopic observation, the morphology of retained austenite could be identified as two types : a granular type in a steel containing higher sillicon and a film type in a steel having higher carbon. For the case of higher carbon-containing steel with a tensile strength of 860 MPa and a total elongation of 38%, film-typed retained austenite could be observed between lath bainitic ferrite. Actually, metastable retained austenite was a requisite for the good formability, which means that chemical composition plays a significant role in the microstructure and tensile property of TRIP-assisted steels. With respect to tensile property, the steels containing suitable silicon and manganese, respectively, showed a typical TRIP effect in stress-strain curve, while a steel containing higher manganese content exhibited the assimilar behavior shown in dual phase steel.

• Korean Journal of Metals and Materials
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• v.50 no.3
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• pp.224-232
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• 2012

In this study, the effect of hot-stamping heat treatment on the microstructure and hardness of TWB(Tailor Welded Blank) laser joints in Al-Si-coated boron steel and Zn-coated DP(Dual Phase)590 steel was investigated. In the TWB joints without heat treatment, hardness profiles showed local hardness deviation near the fusion zone. However, there was no hardness deviation in the heat treated specimen and its hardness was higher than that of the one without the heat treatment, due to a fully martensite microstructure. In the TWB joints of both the boron and DP steels, the maximum hardnesses were observed at the HAZ(Heat Affected Zone) near the base metal, and the hardness decreased gradually to the base metal. In the heat treated joints, the hardnesses of the HAZ and the base metal of the boron steel side were similar to the maximum hardness of the weld, while those of the HAZ and the base metal of the DP steel side were higher than the maximum hardness.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.4
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• pp.747-754
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• 1988

High strength steels (HS) of dual phase and monogalvanized steel sheets (GA ; plated plane, GAB ; bare plane0 were used to investigate the fatigue strengths of four kinds of single-spot-welded joint specimen under tensile-shear repeated load. The specimen is classified as the same mating metal specimen (HS*HS, GA*GAB) and different mating metal specimen (HS*GA, HS*GAB). Some of the results are ; (1) Static tensile load of single-spot-welded joint specimens is proportional to tensile strength of metal and rigidity of mating metal sheet. (2) Fatigue life of HS*HS specimens increased about 20% longer than that of GA*GAB specimens in low cycle range. (3) In different mating metal specimens, the fatigue life of HS*GA specimens increased about 84% more that of HS*HS specimens in high cycle range.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.6
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• pp.867-876
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• 1987

Corrosion fatigue fracture with change in the grain size of M.E.F. dual phase steel is investigated in 3.5% NaCI aqueous solution at pH 2, 4, 6, 9, and 11. Generally speaking, decrease in corrosion fatigue life is strongly dependent on decrease in pH and slightly on the grain size. For the B material with the big grain size, the fatigue life is small due to its large reduction ratio of corrosion fatigue life. The influence of grain size on the reduction ratio of corrosion fatigue life is large at pH 11-6. Whi9le at pH 4-2 the reduction ratio of corrosion fatigue life only depends on the corrosion effect. The larger grain size and the lass pH result in the greater influence on corrosion fatigue crack propagation rate. As pH decreases, the plateau portion in the crack propagation rate curves of the B material are distinct. Crack propagation rate curves become slow down at high .DELTA.K range because crack closure effect by minute corrosion products inside crack causes the oxidation corrosion action less effective for a certain period of time. In A material with small grain size, fatigue life is increased in proportion with increase of martensite intergranular which brings forth restraining the crack propagation decreases crack propagation rate. Corrosion pit which is created in the surface of specimen is found at pH 6,4 and 2 which is noticeable and the unevenness of the surface of the specimen becomes severe as pH decreases. The unevenness of corrosion fatigue fracture surface is severe as the effect of pH increases i.e. as pH decreases. In proportion with increase in the grain size and decrease in pH, the aspect of brittle fracture becomes evident.

• Journal of the Korean Institute of Gas
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• v.18 no.1
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• pp.61-67
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• 2014

Small punch(SP) tests were performed on high strength Dual Phase(DP) steels in order to evaluate the behavior of hydrogen embrittlement. For this purpose, three different kinds of DP steel specimens were charged with hydrogen by electochemical hydrogen charging experiment. After charging with hydrogen, the amount of charged hydrogen was measured. The measurement results showed that amounts of charged hydrogen were largely dependent on the martensite volume fraction of DP steel. The hydrogen charging time of 25 hrs with current densities of 150 and $200mA/cm^2$ was investigated as saturation condition with hydrogen. The analysis results on the SP energy and height of SP bulbs after SP tests showed that those were decreased as the amount of charged hydrogen increased. Fractographs of SP bulbs were observed a brittle fracture mixed with quasi-cleavage fractures, layered structures and clear facets.

• Journal of the Korean Society for Heat Treatment
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• v.27 no.2
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• pp.79-89
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• 2014

The effect of carbon on the microstructure and texture of low carbon steels was investigated in a series of 1.6 Mn-0.3Cr-0.2Mo-0.001B steels with carbon ranging from 0.021 to 0.048%. Intensity of {111} orientation increased with decreasing the carbon content, resulting in the increase in $r_m$ value. The highest $r_m$ value of 1.30 was obtained in 0.021%C steel annealed at $820{\sim}850^{\circ}C$ according to the typical galvannealing heat cycle. Martensite volume fraction was not substantially affected by the annealing temperature. It was found that the fine and uniformly distributed martensite particles which were present in amounts of about 5% volume fraction were desirable for the highest $r_m$ value. The other factor affecting the high $r_m$ value was the preferred epitaxial growth of retained ferrite with {111} orientation into austenite during cooling.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.10a
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• pp.126-129
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• 2005

Microstructures and tensile properties of low carbon steels, 5083 Al alloy and Ti-6Al-4V alloy fabricated by equal channel angular pressing (ECAP) were examined in order to understand their deformation response associated with a formation of an ultrafine grained (UFG) structure. Room temperature tensile properties of UFG low carbon ferrite/pearlite steels and UFG ferrite/martensite dual phase steel were compared for exploring a feasibility enhancing the strain hardening capability of UFG materials. In addition, low temperature and high strain rate superplasticity of the two grades of the UFG 5083 Al alloy, and Ti-6Al-4V alloy were presented. From the analysis of a series of experiments, it was found that UFG materials exhibited the enhanced mechanical properties compared to coarse grained counterparts.

• Journal of Welding and Joining
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• v.33 no.6
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• pp.21-26
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• 2015

Recently, application of UHSS(Ultra High Strength Steels) whose tensile strength is over 1000MPa to car body structure are growing due to great needs for light weighting and improved crash worthiness. However, their poor weldability is one of obstacles to expand selecting to car body. In this study, effect of Phosphorus contents on resistance spot weldability of high elongation DP980 steel whose Si content is over 1% was investigated. The cross tension strength (CTS) was decreased showing partial interface fracture as Phosphorus content increase because of solidification segregation of Phosphorus. In order to improve resistance spot weldability by modification of welding condition, in-situ post-weld heating pulse was introduced after main pulse. The optimum cooling time between main and post pulse and post-pulse current condtion were determined through FEM welding simulation and DOE tests. The CTS was increased about 1.5 time showing plug fracture. The decrease of Phosphorus segregation was found to be a major reason for weld ductility and CTS improvement.

• Transactions of Materials Processing
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• v.15 no.2 s.83
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• pp.105-111
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• 2006

A study was made to investigate the microstructure and the mechanical properties of low-carbon steel, Al-Mg alloy and Ti-6Al-4V alloy each representing bcc, fcc and hcp crystal structures, respectively fabricated by equal-channel angular(ECA) pressing. After a series of ECA pressings was performed, most grains were significantly refined below ${\mu}m$ in diameter with high mis-orientation of grain boundaries irrespective of different crystal structure used. Regarding the strain hardening capability, tensile tests of ultrafine grain (UFG) dual-phase (ferrite/martensite) steel which was different from UFG ferrite-pearlite steel were carried out at ambient temperature, and corresponding mechanical properties were discussed in relation to modified C-J analysis. Low-temperature and/or high strain-rate superplasticity of the UFG Al-Mg alloy and UFG Ti-6Al-4V alloy were also studied. Based on the analysis used in this study, it was concluded that UFG alloys exhibited the enhanced mechanical properties as compared to coarse-grained (CG) counterparts.