• Journal of the Korean Society for Heat Treatment
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• v.12 no.3
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• pp.231-239
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• 1999

Volume fraction and morphology of retained austenite, tensile properties of TRIP type high strength steel sheet with Fe-C-Si-Mn-Cu chemical composition have been investigated. The retained austenite of granular, bar and film type existing in specimen was obtained after intercritical annealing and austempering. The granular type retained austenite increased with increase of intercritical annealing and austempering temperature. With increase of intercritical annealing temperature, retained austenite and carbon contents increased. Maximum contents of retained austenite was obtained by austempering at $400^{\circ}C$. The maximum tensile strength was obtained by austempering at $450^{\circ}C$ and maximum elongation was obtained at $400^{\circ}C$. T.S${\times}$E.L value increased with increase of retained austenite contents due to the elongation strongly controlled by contents of retained austenite, but tensile strength was affected with various factors such as bainitic structure etc.

• Korean Journal of Materials Research
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• v.30 no.1
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• pp.44-49
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• 2020

The effect of intercritical annealing temperature on the microstructure and mechanical properties of Fe-9Mn-0.2C-3Al-0.5Si medium manganese steels containing Cu and Ni is investigated in this study. Six kinds of medium manganese steels are fabricated by varying the chemical composition and intercritical annealing temperature. Hardness and tensile tests are performed to examine the correlation of microstructure and mechanical properties for the intercritical annealed medium manganese steels containing Cu and Ni. The microstructures of all the steels are composed mostly of lath ferrite, reverted austenite and cementite, regardless of annealing temperature. The room-temperature tensile test results show that the yield and tensile strengths decrease with increasing intercritical annealing temperature due to higher volume fraction and larger thickness of reverted austenite. On the other hand, total and uniform elongations, and strain hardening exponent increase due to higher dislocation density because transformation-induced plasticity is promoted with increasing annealing temperature by reduction in reverted austenite stability.

• Corrosion Science and Technology
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• v.7 no.1
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• pp.1-5
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• 2008

The gas atmosphere in continuous annealing and galvanizing lines alters both composition and microstructure of the surface and sub-surface of sheet steel. The formation and morphology of the oxides of alloying elements in High Strength Interstitial Free (HS-IF), Dual Phase (DP) and Transformation-Induced Plasticity (TRIP) steels are strongly influenced by the furnace dew point, and the presence of specific oxide may result in surface defects and bare areas on galvanized sheet products. The present contribution reviews the progress made recently in understanding the selective formation of surface and subsurface oxides during annealing in hot dip galvanizing and conventional continuous annealing lines. It is believed that the surface and sub-surface composition and microstructure have a pronounced influence on galvanized sheet product surface quality. In the present study, it is shown that the understanding of the relevant phenomena requires a combination of precise laboratory-scale simulations of the relevant technological processes and the use of advanced surface analytical tools.

• Korean Journal of Metals and Materials
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• v.46 no.11
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• pp.713-724
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• 2008

The need to lower the weights of automotive vehicle and to improve the safety of cars has resulted in the development of high strength steels such as TRIP(Transformation Induced Plasticity) and DP (Dual Phase) steel. It is well known that the higher strength of steel shows the poorer press formability. Among the high strength steels, DP steel shows several good characteristics such as low yield ratio, high initial n value, high elongation, high bake hardenability and anti-aging property. However, there's a certain limit in application of DP steels to the automotive panel parts because their poor deep drawbility caused by martensite. In this study, the effect of alloying elements on the deep drawability and recrystallization texture in TS 440MPa grade DP steel with 0.015~0.02% carbon has been investigated on the base of SEM, TEM, XRD and EBSD analysis.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.3
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• pp.52-60
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• 2018

In the automobile industry, in order to increase the fuel efficiency and conform to the safety regulations, it is necessary to make the vehicles as light as possible. Therefore, it is crucial to manufacture dual phase steels, complex phases steels, MS steels, TRIP steels, and TWIP from high strength steels with a tensile strength of 700Mpa or more. In order to apply ultra-high tensile strength steel to the body, the welding process is essential. Resistance spot welding, which is advantageous in terms of its cost, is used in more than 80% of cases in body welding. It is generally accepted that ultra-high tensile strength steel has poor weldability, because its alloy element content is increased to improve its strength. In the case of the resistance spot welding of ultra-high tensile steel, it has been reported that the proper welding condition area is reduced and interfacial fracture and partial interfacial fracture occur in the weld zone. Therefore, research into the welding quality judgment that can predict the defect and quality in real time is being actively conducted. In this study, the dynamic resistance of the weld was monitored using the secondary circuit process variables detected during resistance spot welding, and the factors necessary for the determination of the welding quality were extracted from the dynamic resistance pattern. The correlations between the extracted factors and the weld quality were analyzed and a regression analysis was carried out using highly correlated pendulums. Based on this research, a regression model that can be applied to the field was proposed.

• Journal of Powder Materials
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• v.18 no.1
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• pp.64-72
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• 2011

Nondestructive instrumented indentation test is the method to evaluate the mechanical properties by analyzing load - displacement curve when forming indentation on the surface of the specimen within hundreds of micro-indentation depth. Resistance spot welded samples are known to difficult to measure the local mechanical properties due to the combination of microstructural changes with heat input. Particularly, more difficulties arise to evaluate local mechanical properties of resistance spot welds because of having narrow HAZ, as well as dramatic changed in microstructure and hardness properties across the welds. In this study, evaluation of the local mechanical properties of resistance spot welds was carried out using the characterization of Instrumented Indentation testing. Resistance spot welding were performed for 590MPa DP (Dual Phase) steels and 780MPa TRIP (Transformation Induced Plasticity) steels following ISO 18278-2 condition. Mechanical properties of base metal using tensile test and Instrumented Indentation test showed similar results. Also it is possible to measure local mechanical properties of the center of fusion zone, edge of fusion zone, HAZ and base metal regions by using instrumented indentation test. Therefore, measurement of local mechanical properties using instrumented indentation test is efficient, reliable and relatively simple technique to evaluate the tensile strength, yield strength and hardening exponent.

• Journal of Korea Foundry Society
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• v.32 no.2
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• pp.104-108
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• 2012

The effect of heat treatment on mechanical properties of 0.4C-2.3Si(wt%) steel with bainitic ferrite matrix were investigated. This steel has been synthesized intergrating concepts from TRIP(Transformation Induced Plasticity) steel & Austempered Ductile Cast Iron(ADI) technology. The low alloy medium carbon (0.4 %C) steel with high silicon (2.3 %Si) was initially annealed for 60 min at $800^{\circ}C$, $820^{\circ}C$ and $840^{\circ}C$ respectively in the intercritical region and then subsequently austempered at various temperatures at $260^{\circ}C$, $320^{\circ}C$ and $380^{\circ}C$ for 30 min in a salt bath. The mechanical properties were measured by using a tensile test. A detailed study of the microstructure of this steel after heat treatment was carried out by means of electron back scattering diffraction (EBSD) technic. In this study, a new low alloy steel with high strength (780~1,050MPa) and exceptionally high ductility (20~40%) was obtained.

• Journal of Welding and Joining
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• v.32 no.5
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• pp.44-49
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• 2014

The effect of weldbond hybrid joining process on the mechanical behavior of single lap and L-tensile joints was investigated for the newly developed 1.2GPa grade ultra high strength TRIP(transformation induced plasticity) steel. In the case of single lap shear behavior, the weldbond joint of 1.2GPa TRIP steel showed lower maximum tensile load and elongation than that of the adhesive bonding only. It was considered to be due to the reduction of real adhesion area, which was caused by the degradation of adhesive near the spot weld, and the brittle fracture behavior of the spot weld joint. In the case of L-tensile behavior, however, the maximum tensile load of the weldbond joint of 1.2GPa TRIP steel was dramatically increased and the fracture mode was change to the base metal fracture which is desirable for the spot weld joint. These synergic effect of the weldbond hybrid joining process in 1.2GPa TRIP steel was considered to be due to the stress dissipation around the spot weld joint by the presence of adhesive which resulted in the change of crack propagation path.

• Journal of Welding and Joining
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• v.31 no.1
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• pp.43-50
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• 2013

The spot welds of Transformation Induced Plasticity (TRIP) steels are prone to interfacial failure and narrow welding current range. Hard microstructures in weld metal and heat affected zone arenormally considered as one of the main reason to accelerate the interfacial failure mode. There fore, detailed observation of weld microstructure for TRIP steels should be made to ensure better weld quality. However, it is difficult to characterize the microstructure, which has similar color, size, and shape using the optical or electron microscopy. The atomic force microscope (AFM) can help to analyze microstructure by using different energy levels for different surface roughness. In this study, the microstructures of resistance spot welds for AHSS are analyzed by using AFM with measuring the differences in average surface roughness. It has been possible to identify the different phases and their topographic characteristics and to study their morphology using atomic force microscopy in resistance spot weld TRIP steels. The systematic topographic study for each region of weldments confirmed the presence of different microstructures with height of 350nm for martensite, 250nm for bainite, and 150nm for ferrite, respectively.

• Journal of Welding and Joining
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• v.34 no.6
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• pp.28-34
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• 2016

Galvannealed(GA) steels are now generally used in car body manufacturing for corrosion resistance. In this study, the weldability and joint mechanical behavior of a newly developed 1.2GPa grade GA ultra high strength TRIP(transformation induced plasticity) steel was investigated for three joining processes, such as adhesive bonding, resistance spot welding and weldbonding. Under both shear and peel stress conditions, the failure mode of the adhesive joints were the mixture of the adhesive cohesive failure, adhesive interface failure and coating layer failure. It means that the adhesion strength of GA coating onto the base metal was similar to that of adhesive bonding onto the GA coating. Under the shear stress condition, the weldbonding exerted to expand the optimal spot welding condition of 1.2GPa GA TRIP steel because the strength of adhesive bond overwhelmed that of the resistance spot weld. Under the peel stress condition, the weldbonding also exerted to expand the optimal spot welding condition of 1.2GPa GA TRIP steel by inducing the tear fracture mode rather than the partial plug fracture mode.