• Title/Summary/Keyword: DP steel

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Lobe Curve Characteristic Analysis of Resistance Spot Welding for Sheet Combination of 780MPa Steel Sheet Using Simulation (시뮬레이션을 이용한 780MPa급 강재의 판재 조합에 따른 저항 점 용접의 로브곡선 특성 분석)

  • Son, Chang-Seok;Park, Young Whan
    • Journal of Welding and Joining
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    • v.30 no.6
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    • pp.68-73
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    • 2012
  • Nowadays, car manufacturers tried to improve automotive fuel efficiency, and applied many high strength steels such as AHSS or UHSS to car bodies. Therefore, the number of steel combinations for the resistance spot welding are dramatically increased and the need for weldability evaluation of these combinations are also required. In this study, we suggest the lobe curve using FEM simulations for DP780 steel with 1.0t, 1.4t. The lobe curves which could expressed weldablity and optimal welding condition were obtained according to 6 steel combinations. There were two combinations for same steel sheet which were DP780 1.0t, DP780 1.4t. Dissimilar steel sheet combination with different thickness was 1.0t and 1.4t of DP780. Different steel combinations were DP780 1.0t and SPRC440 1.0t, and DP780 1.0t and DP590 1.0t. Finally dissimilar combinations was and DP780 1.0t and DP590 1.4t. The trend of low boundary and high boundary variation of lobe curve were analyzed with a viewpoint of the contact resistance and the heat input.

Effect of Prior Deformation on the Sliding Wear of Ultra-fine Grained Ferrite-Martensite Dual Phase Steel (초기 소성변형이 초미세 결정립 페라이트-마르텐사이트 이상조직 탄소강의 건식 미끄럼마멸 특성에 미치는 영향)

  • Park, J.K.;Yi, S.K.;Shin, D.H.;Kim, Y.S.
    • Proceedings of the Korean Society for Technology of Plasticity Conference
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    • 2008.05a
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    • pp.521-524
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    • 2008
  • Effect of prior deformation on the sliding wear of the ultra-fine grained (UFG) ferrite-martensite dual phase (DP) steel was investigated. The UFG DP steel was fabricated by the ECAP and subsequent intercritical annealing. The steel was cold rolled before the wear test, and the effect of the prior deformation on the wear was examined. The wear tests were carried out at various loads against a bearing steel ball. The wear rate of the UFG DP steel that did not experience the prior deformation was higher than that of the coarse-grained (CG) DP steel, because of more severe surface shear deformation. The wear rate of the specimens with prior deformation was much higher than that of the specimen without prior deformation. The deformed CG DP specimen showed higher rate than the deformed UFG DP specimen, and the rate-variation of the CG DP steel was much bigger under the same test condition.

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Microstructural and Mechanical Analysis of a Friction Stir Welded Joint of Dissimilar Advanced High-Strength Steels (초고강도 합금강의 이종마찰교반 접합부에서의 미세조직 특성 및 기계적 물성 연구)

  • Lee, J.W.;Cho, H.H.;Mondal, Mounarik;Das, Hrishikesh;Hong, S.T.
    • Transactions of Materials Processing
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    • v.29 no.1
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    • pp.11-19
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    • 2020
  • For microstructural analysis of a friction stir welded (FSWed) joint of advanced high-strength steels, dual phase (DP) and complex phase (CP) steels, are studied. FSWed joints are successfully fabricated in the following four cases: (i) DP/DP; (ii) CP/CP; (iii) DP/CP, where the advancing side is DP and the retreating side is CP; (iv) CP/DP, where the advancing side is CP and the retreating side is DP. The stir zone (SZ) of (i) the DP/DP joint mainly consists of lath martensite, while the stir zone of (ii) the CP/CP joint consists not only of lath martensite but also of bainite. In the case of (iii) DP/CP and (iv) CP/DP, they exhibit a similar microstructure including acicular-shaped phases in the joints; however, cross-sections of the joints show differences in material mixing in each case. In (iv) the CP/DP joint, temperature towards the CP steel is sufficient to cause softening, thus leading to better mixing than that in (iii) DP/CP. The phases of the SZ in each of the four cases are formed by phase transformation during the FSWed process; however, the transformed phase volume fraction of CP steel is lower than that of DP steel, indicating that dynamic recrystallization occurs mainly in CP steel. The hardness values of the SZ are significantly higher than those of the base materials, especially, the SZ of (iii) the DP/CP joint has the highest value due to highest fraction of lath martensite.

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
    • Korean Journal of Materials Research
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    • v.19 no.6
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    • pp.293-299
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    • 2009
  • 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.

Prediction of Fracture Strains for DP980 Steel Sheets for a Wide Range of Loading Paths (다양한 하중경로에서의 DP980 강판의 파단변형률 예측에 관한 연구)

  • Park, N.;Huh, H.
    • Transactions of Materials Processing
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    • v.24 no.3
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    • pp.176-180
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    • 2015
  • The current study is concerned with the prediction of fracture strains for DP980 steel sheets over a wide range of loading paths. The use of DP980 steel is increasing significantly in automotive industries for enhanced safety and higher fuel efficiency. The material behavior of advanced high-strength steels (AHSSs) sheets sometimes show unpredictable and sudden fracture during sheet metal forming. A modified Lou-Huh ductile fracture criterion is utilized to predict the formability of AHSSs because the conventional forming limit diagram (FLD) constructed based on necking is unable to evaluate the formability of AHSSs sheets. Fracture loci were extracted from three dimensional fracture envelopes by assuming the plane-stress condition to evaluate equivalent plastic strains at the onset of fracture for a wide range of loading paths. Three different types of specimens -- pure shear, dog-bone and plane strain grooved -- were utilized for tensile testing to calibrate the fracture model of DP980 steel sheets. Fracture strains of each loading path were evaluated such that there shows little deviation between fracture strains predicted from the fracture model and the experimental measurements. From the comparison, it is clearly shown that the three dimensional fracture envelopes can accurately predict the onset of the fracture of DP980 steel sheets for complicated loading conditions from compressive loading to shear loading and to equibiaxial tensile loading.

Microstructures and Hardness of DISK Laser Welds in Al-Si Coated Boron Steel and Zn Coated DP Steel (Al-Si Coated Boron Steel과 Zn Coated DP Steel 이종금속의 DISK Laser 용접부 미세조직과 경도)

  • An, Yong-Gyu;Kang, Chung-Yun;Kim, Young-Su;Kim, Cheol-Hee;Han, Tae-Kyo
    • Journal of Welding and Joining
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    • v.29 no.1
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    • pp.90-98
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    • 2011
  • Al-Si coated Boron steel and Zn coated DP steel were welded using DISK laser and the microstructure and hardness of the weld were investigated. Full penetration was obtained, when the welding speed was lower than 4m/min. In the specimen welded with laser power of 3 kW and welding speed of 2 m/min, the hardness was the highest in the heat affect zone in the boron steel (HAZ-B) and that of the heat affect zone in the DP steel (HAZ-D) was lower than HAZ-B. The hardness of fusion zone was in between those of HAZ-B and HAZ-D. The decreased hardness from each HAZ to base metal(BM) could be explained that ferrite contents increases when access to the BM. The variation of hardness in the welds could be explained by the difference of microstructure, that is, full martensite in HAZ-B, mixture of martensite and bainite in the fusion zone, and the mixture of martensite, ferrite and bainite in HAZ-D.

A Study on the Development of Electric Resistance Welding of DP780 Grade Steel for Hydroforming Tube (하이드로포밍용 DP780MPa급 강판의 전기저항용접 강관 개발에 관한 연구)

  • Park, Sungpill;Kwon, Yongjai
    • Transactions of the Korean Society of Automotive Engineers
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    • v.23 no.3
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    • pp.279-286
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    • 2015
  • To achieve lightweight design, research & development of various lightweighting technologies such as hydroforming are underway worldwide. In the case of hydroforming, application of ultra high strength steel is essential for weight reduction of the car. However, considering common high-strength carbon steel, it is not suitable to the actual hydroformed parts since the lack of formability. DP steel offers an outstanding combination of strength and formability as a result of their microstructure. DP steel has high strength and good formability but it's difficult to secure stable quality of welding section because of softening of weld section and chemical composition. Therefore, most of companies use LASER welding when making high strength tube. Electric resistance welding is excellent production method for steel tube manufacturing considering the productivity. Optimum electric resistance welding technology is needed to be developed for application of high strength hydroformed parts using DP steel. This study is comprehensive research & development from electric resistance welding to actual formabililty evaluation.

Hydrogen Embrittlement Evaluation of Subsurface Zone in 590DP Steel by Micro-Vickers Hardness Measurement (미소경도 측정에 의한 590DP강 Subsurface Zone 내 수소취성 평가)

  • Choi, Jong-Un;Park, Jae-Woo;Kang, Kae-Myung
    • Korean Journal of Materials Research
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    • v.21 no.11
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    • pp.581-586
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    • 2011
  • This study describes a hydrogen embrittlement evaluation of the subsurface zone in 590DP steel by micro-Vickers hardness measurement. The 590DP steel was designed to use in high-strength thin steel sheets as automotive materials. The test specimens were fabricated to 5 series varying the chemical composition through the process of casting and rolling. Electrochemical hydrogen charging was conducted on each specimen with varying current densities and charging times. The relationship between the embrittlement and hydrogen charging conditions was established by investigating the metallography. The micro-Vickers hardness was measured to evaluate the hydrogen embrittlement of the subsurface zone in addition to the microscopic investigation. The micro-Vickers hardness increased with the charging time at the surface. However, the changing ratio and maximum variation of hardness with depth were nearly the same value for each test specimen under the current density of 150 mA/$cm^2$ and charging time of 50 hours. Consequently, it appears that hydrogen embrittlement in 590DP steel can be evaluated by micro-Vickers hardness measurement.

Effect of Hot-stamping Heat Treatment on Microstructure and Hardness in TWB Laser Joints of Al-Si-coated Boron Steel and Zn-coated DP Steel (Al-Si 도금된 보론강과 Zn 도금된 DP강의 TWB 레이저 용접부 미세조직과 경도에 미치는 핫 스탬핑 열처리의 영향)

  • Jung, Byung-Hun;Kong, Jong-Pan;Kang, Chung-Yun
    • 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.

The Effect of Mo and Cr addition on the Deep Drawability of Dual Phase Steel Sheets (이상조직강판의 성형특성에 미치는 Mo와 Cr첨가의 영향)

  • Han, Seong Ho;Ahn, Yeon Sang;Chin, Kwang Geun;Kim, In Bae
    • 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.