Comparative Analysis of Strengthening with Respect to Microstructural Evolution for 0.2 Carbon DP, TRIP, Q&P Steels

Jin, Jong-Won,Park, Yeong-Do,Nam, Dae-Geun,Lee, Seung-Bok,Kim, Sung-Il,Kang, Nam-Hyun,Cho, Kyung-Mox 한국재료학회 2009 한국재료학회지 Vol.19 No.6

The microstructures and mechanical properties of Dual Phase (DP), Transformation-Induced Plasticity (TRIP), and Quenching & Partitioning (Q&P) steels were investigated in order to define the strengthening mechanism of 0.2 C steel. An intercritical annealing between Ac1 and Ac3 was conducted to produce DP and TRIP steel, followed by quenching the DP and TRIP steel being quenched at to room temperature and by the TRIP steel being austemperingaustempered-air cooling cooled the steel toat room temperature, respectively. The Q&P steel was produced from full austenization, followed by quenching to the temperature between $M_s$ and $M_f$, and then enriching the carbon to stabilize the austenite throughout the heat treatment. For the DP and TRIP steels, as the intercritical annealing temperature increased, the tensile strength increased and the elongation decreased. The strength variation was due to the amount of hard phases, i.e., martensite and bainite, respectively in the DP and TRIP steels. It was also found that the elongation also decreased with the amount of soft ferrite in the DP and TRIP steels and with the amount of the that was retained in the austenite phasein the TRIP steel, respectively for the DP and TRIP steels. For the Q&P steel, as the partitioning time increased, the elongation and the tensile strength increased slightly. This was due to the stabilized austenite that was enriched with carbon, even when the amount of retained austenite decreased as the partitioning time increased from 30 seconds to 100 seconds.

Strain rate dependent tensile behavior of advanced high strength steels: Experiment and constitutive modeling

Kim, J.H.,Kim, D.,Han, H.N.,Barlat, F.,Lee, M.G. Elsevier Sequoia 2013 Materials science & engineering. properties, micro Vol.559 No.-

High strain rate tensile tests were conducted for three advanced high strength steels: DP780, DP980 and TRIP780. A high strain rate tensile test machine was used for applying the strain rate ranging from 0.1/s to 500/s. Details of the measured stress-strain responses were comparatively analyzed for the DP780 and TRIP780 steels which show similar microstructural feature and ultimate tensile strength, but different strengthening mechanisms. The experimental observations included: usual strain rate dependent plastic flow stress behavior in terms of the yield stress (YS), the ultimate tensile strength (UTS), the uniform elongation (UE) and the total elongation (TE) which were observed for the three materials. But, higher strain hardening rate at early plastic strain under quasi-static condition than that of some increased strain rates was featured for TRIP780 steel, which might result from more active transformation during deformation with lower velocity. The uniform elongation that explains the onset of instability and the total elongation were larger in case of TRIP steel than the DP steel for the whole strain rate range, but interestingly the fracture strain measured by the reduction of area (RA) method showed that the TRIP steel has lower values than DP steel. The fractographs using scanning electron microscopy (SEM) at the fractured surfaces were analyzed to relate measured fracture strain and the microstructural difference of the two materials during the process of fracture under various strain rates. Finally, constitutive modeling for the plastic flow stresses under various strain rates was provided in this study. The proposed constitutive law could represent both Hollomon-like and Voce-like hardening laws and the ratio between the two hardening types was efficiently controlled as a function of strain rate. The new strength model was validated successfully under various strain rates for several grades of steels such as mild steels, DP780, TRIP780, DP980 steels.

자동차 차체용 TRIP강판의 저항 점용접부 Partial Interfacial Fracture 특성에 관한 연구

최철영 ( Chul Young Choi ),김인배 ( In Bae Kim ),김양도 ( Yang Do Kim ),박영도 ( Yeong Do Park ) 대한금속재료학회 ( 구 대한금속학회 ) 2012 대한금속·재료학회지 Vol.50 No.2

Resistance spot welding of TRIP780 steels was investigated to enhance understanding of weld fracture mode after tensile shear testing (TST) and L-shape tensile testing (LTT). The main failure mode for spot welds of TRIP780 steels was partial interfacial fracture (PIF). Although PIF does not satisfy the minimum button diameter (4√t) for acceptable welds, it shows enough load carrying capacity of resistance spot welds for advanced high strength steels. In the analysis of displacement controlled L-shape tensile test results, cracks initiated at the notch of the faying surface and propagated through the interface of weldments, and finally, cracks change path into the sheet thickness direction. Use of the ductility ratio and CE analysis suggested that the occurrence of PIF is closely related to high hardness and brittle welds, which are caused by fast cooling rates and high chemical compositions of TRIP steels. Analysis of the hold time and weld time in a welding schedule demonstrated that careful control of the cooling rate and the size of a weld nugget and the HAZ zone can reduce the occurrence of PIF, which leads to sound welds with button fractures (BFs).

인장전단시험을 이용한 TRIP1180강의 계면파단특성 평가

박상순,최영민,남대근,김영석,유지훈,박영도,Park, Sang-Soon,Choi, Young-Min,Nam, Dae-Geun,Kim, Young-Seok,Yu, Ji-Hun,Park, Yeong-Do 대한용접접합학회 2008 대한용접·접합학회지 Vol.26 No.6

The weldability of resistance spot welding of TRIP1180 steels for automobile components investigated enhance in order to achieve understanding of weld fracture during tensile-shear strength (TSS) test. The main failure modes for spot welds of TRIP1180 steels were nugget pullout and interfacial fracture. The peak load to cause a weld interfacial failure was found to be related to fracture toughness of the weld and the weld diameter. Although interfacial fracture occurred in the spot welded samples, the load-carrying capacity of the weld was high and not significantly affected by the fracture mode. Substantial part of the weld exhibits the characteristic dimple (or elongated dimple) fractures on interfacial fractured surface also, dimple fracture areas were drawmatically increased with heat input which is propotional to the applied weld current. In spite of the high hardness values associated with the martensite microstructures due to high cooling rate. The high load-carrying ability of the weld is directly associated with the area of ductile fracture occurred in weld. Therefore, the judgment of the quality of resistance spot welds in TRIP1180 steels, the load-carrying capacity of the weld should be considered as an important factor than fracture mode.

논문 : 저항 점 용접된 TRIP590강의 계면파단특성에 관한 평가

박상순 ( Sang Soon Park ),이상민 ( Sang Min Lee ),조용준 ( Yong Joon Cho ),강남현 ( Nam Hyun Kang ),유지훈 ( Ji Hun Yu ),김영석 ( Young Seok Kim ),박영도 ( Yeong Do Park ) 대한금속재료학회 ( 구 대한금속학회 ) 2008 대한금속·재료학회지 Vol.46 No.10

The resistance spot welding of TRIP590 steels was investigated to enhance understanding of weld fracture during tensile-shear strength (TSS) test. The main failure modes for spot welds of TRIP590 steels were nugget pullout and interfacial fracture. The peak load to cause a weld interfacial failure was found to be related to fracture toughness of the weld and the weld diameter. Although interfacial fracture occurred in the samples, the load carrying capacity of the weld was high and not significantly affected by the fracture mode. Substantial part of the weld exhibits the characteristic dimple (or elongated dimple) fractures on interfacial fractured surface, in spite of the high hardness values associated with the martensite microstructures. The high load-bearing ability of the weld is directly associated with the area of ductile fracture occurred in weld. Therefore, the judgment of the quality of resistance spot welds in TRIP590 steels, the load carrying capacity of the weld should be considered as an important factor than fracture mode.

Predicting Ductility and Failure Modes of TRIP Steels under Different Loading Conditions

K.S. Choi,W.N. Liu,X. Sun,M.A. Khaleel 한국소성가공학회 2010 기타자료 Vol.2010 No.6

In this paper, we examine the ultimate ductility and failure modes of a TRIP (TRansformation-Induced Plasticity) 800 steel with an advanced micromechanics-based finite element analysis. The representative volume element (RVE) for the TRIP800 under examination is developed based on an actual microstructure obtained from scanning electron microscopy (SEM). The evolution of retained austenite during deformation process and the mechanical properties of the constituent phases of the TRIP800 steel are obtained from the synchrotron-based in-situ high-energy Xray diffraction (HEXRD) experiments and a self-consistent (SC) model. The ductile failure of the TRIP800 under different loading conditions is predicted in the form of plastic strain localization without any prescribed failure criteria for the individual phases. Comparisons of the computational results with experimental measurements suggest that the microstructure-based finite element analysis can well capture the overall macroscopic behavior of the TRIP800 steel under different loading conditions. The methodology described in this study may be extended for studying the ultimate ductile failure mechanisms of TRIP steels as well as the effects of the various processing parameters on the macroscopic behaviors of TRIP steels.

Geometric accuracy of incremental sheet forming for TRIP590

Lihua Li,Jin Wang,Baoping Wang 대한기계학회 2017 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.31 No.11

TRIP steels have both high strength and good ductility, and they are typically used in automotive industry for the purpose of safety and energy saving. Deforming TRIP steel by ISF is practically valuable, but the application is significantly restricted by its geometric accuracy. At present, there is not enough research on geometric accuracy for TRIP steels. In this study, we explored the geometric accuracy of ISF for TRIP590 by referring to 1060Al steel and 08F steel. We found that the accuracy of TRIP590 steel was the lowest, which was because of the phase transformation of the material which occurred during deformation. To verify this, X-ray diffraction was employed to figure out the volume fraction of retained austenite. The results have shown that there is always TRIP effect during ISF process, and the relationship between natural logarithm of austenite volume fraction and the equivalent strain is negative linear. Lastly, it is proved that multiple reverse compensations could improve the geometric accuracy for TRIP steels.

원자력간 현미경을 이용한 TRIP강 저항 점용접부의 미세조직 분석에 관한 연구

최철영,지창욱,남대근,장재호,김순국,박영도,Choi, Chul Young,Ji, ChangWook,Nam, Dae-Geun,Jang, Jaeho,Kim, Soon Kook,Park, Yeong-Do 대한용접접합학회 2013 대한용접·접합학회지 Vol.31 No.1

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.

Design method for TRIP-aided multiphase steel based on a microstructure-based modelling for transformation-induced plasticity and mechanically induced martensitic transformation

Han, H.N.,Oh, C.S.,Kim, G.,Kwon, O. Elsevier Sequoia 2009 Materials science & engineering. properties, micro Vol.499 No.1

In recent years, for automotive applications, the need for new advanced high-strength sheet steels (AHSSs) with high ductility has rapidly increased. This is mainly related to the need for more fuel-efficient (and therefore more environmentally friendly) cars, and the increasing consumer demand for safer vehicles. In this research, the transformation-induced plasticity (TRIP) that accompanies the mechanically induced martensitic transformation (MIMT) in TRIP-aided multiphase steel was analyzed. The analysis was performed using a computational model that takes the ductile fracture during tensile deformation into account. The TRIP and MIMT phenomena were calculated using the concept of variant selection, which is based on the Kurdjumov-Sachs (K-S) orientation relationship. To consider the localization of the plastic flow in the deforming material, the increase in void nucleation due to the martensitic transformation and the void growth based on the yield criterion for porous material were studied. The feasibility of the extra advanced high-strength sheet steel (X-AHSS) was assessed by analyzing the results obtained using various initial volume fractions and various stabilities of the retained austenite in the TRIP-aided multiphase steel. Subsequently, the optimum volume fraction and stability of the retained austenite in the TRIP-aided multiphase steel could be determined.

차체용 1.2GPa급 초고장력 TRIP강의 Weldbond 접합부의 기계적 거동

이종대,이소정,방정환,김동철,강문진,김목순,김준기,Lee, Jong-Dae,Lee, So-Jeong,Bang, Jung-Hwan,Kim, Dong-Cheol,Kang, Mun-Jin,Kim, Mok-Soon,Kim, Jun-Ki 대한용접접합학회 2014 대한용접·접합학회지 Vol.32 No.5

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.