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.

텐션 레벨러 인터메쉬 레벨링과 스트립 웨이브와의 상관 관계 연구

나영종(Young-Jong Na) 한국산학기술학회 2023 한국산학기술학회논문지 Vol.24 No.3

근래 들어 초 고장력 강판이 개발 되면서, 자동차 외판재의 경량화와 고강도 강판의 생산이 늘어나고 있다. AHSS(Advanced High Strength Steel) 소재는 인장 강도가 높아 일반적인 스킨패스 설비는 평탄도 교정에 한계가 있고, 고장력으로 인한 불량 발생 및 설비 부하로 인한 설비 변형과 파손으로 이어질수 있으며,이 부분을 보완하고자 텐션 레벨러가 추가된 스킨패스 설비가 도입되는 추세이다.종래의 스킨패스 설비 시스템으로는 목표 연신의 한계가 있으며,최근수요는 연강부터 고장력 강판까지 안정된 재화 능력을 필요로 한다.텐션 레벨러는 1차 스킨패스밀에 이어 추가로 2차 소성변형으로 이어지는 효과적인 형상교정 방법으로 업계에서 품질 향상을 인정받고 있으며, 그에 따른 텐션 레벨러의 중요성과 인터메쉬 레벨링 기술이 요구되고 있다.텐션 레벨러는 연속식의 스트립 평탄도 교정 장치로서 각각 3~5개의 Roll로 구성된 텐션 브라이들 롤이 스킨패스 밀(Mill)과 레벨러를 기준으로 좌우에 한쌍씩 있어 그 사이를 스트립이 통과 하는 동안에 인장력에 의해 평탄도가 교정된다.또한 이상적인 인터메쉬와 스트립 표면 웨이브와의 파장은 직접적인 상관관계를 바탕으로 형상 교정에 영향을 미친다.따라서 본 논문에서는 이 텐션 레벨러의 적정 인터메쉬 설정으로 스트립 웨이브의 변화를 고찰하고,효율적인 형상 교정에 대한 경도와 변형률의 상관 관계 이론을 설명하여 교정 기술의 설계에 방향성을 제시하고자 한다. With the development of ultra-high-tension steel plates in recent years, the weight reduction of exterior plates for automobiles and the production of high-strength steel plates are increasing. Advanced high-strength steel (AHSS) has high tensile strength, so general skin pass facilities have limitations in flatness correction and can lead to facility deformation and damage due to high tension and equipment load. Conventional skin pass equipment has limitations in target elongation, and recent demands require stable goods from mild steel to high-tension steel plates. Tension levelers are recognized in industry for quality improvement as an effective shape correction method that leads to secondary plastic deformation following the primary skin pass mill. Accordingly, tension levelers and intermesh leveling technology are required. A tension leveler is a continuous-strip flatness-calibration device, and there are pairs of tension bridle rolls consisting of three to five rolls each on the left and right sides of the skin pass mill and leveler. The flatness is corrected by the tensile force while the strip passes between them. In addition, the wavelength of the ideal intermesh and strip surface wave affects the shape calibration based on direct correlation. Therefore, in this paper, we consider the variation of strip waves with a proper intermesh setting of this tension leveler and explain the correlation theory of hardness and strain for efficient shape correction to give direction in the design of calibration techniques.

고장력강판(DP980) 적용 자동차 roof frame 제작 금형 개발을 위한 성형 해석

손종윤(J. Y. Son),백경윤(K. Y. Baeck),이은미(E. M. Lee),노경보(K. B. No),심도식(D. S. Shim) 한국소성가공학회 2014 한국소성가공학회 학술대회 논문집 Vol.2014 No.10

Advanced high strength steels are widely used for lightweight automobile parts. With advanced high strength steels, automobile parts are formed thinner and lighter than normal steels keeping structural strength. However, the advanced high strength steel causes serious problems in forming process, such as springback and low formability. In this study, forming simulation for the automobile roof frame, applied advanced high strength steel (DP980), is conducted by using the commercial code PAM-STAMP, based on Finite Element Method (FEM). Results of the simulation are quantitatively analyzed, and shape deviations caused by springback phenomenon are mainly observed. To reduce the deviation, restriking and bending processes are added after drawing process. Through the FE simulations, it is found that the newly designed forming process is more effective to improve the shape accuracy.

Fusion Weldabilities of Advanced High Manganese Steels: A Review

Gitae Park,Seonghoon Jeong,Changhee Lee 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.7

A large amount of manganese has been added to next-generation advanced high strength steels for automotive applications. The increased manganese content changes the microstructural and mechanical characteristics by varying both the stackingfault energy and the austenite stability, but it is known to deteriorate weldability. The current review provides a general strategyto address several problems in fusion welds of advanced high manganese steels from the viewpoint of microstructuraltransitions, plasticity mechanism, and mechanical properties based on the welding metallurgy. The importance of researchon the weldabilities of advanced Mn steels was highlighted. Among the representative types of advanced manganese steels,this review focuses on high Mn twinning-induced plasticity steel, high Mn austenitic lightweight steel, and medium Mntransformation-induced plasticity steel. Specifically, this review suggests fundamental concepts for designing each steelwith improved weld characteristics by presenting many research results on the correlations between the microstructure andmechanical properties of welds and heat-affected zones for these three different steel groups.

Pitting corrosion behavior in advanced high strength steels

Park, Il-Jeong,Lee, Sang-Min,Kang, Minwoo,Lee, Sukjin,Lee, Young-Kook Elsevier 2015 Journal of Alloys and Compounds Vol.619 No.-

<P><B>Abstract</B></P> <P>The type and size of inclusions and their pitting corrosion behavior in dual-phase (DP), transformation-induced plasticity (TRIP), and twinning-induced plasticity (TWIP) steels were investigated. Whereas DP steel had single MnS inclusions, TRIP and TWIP steels had the complex inclusions of MnS and Al<SUB>2</SUB>O<SUB>3</SUB>, and MnS and Mn oxy-sulfide, respectively. TWIP steel had the most wide size distribution and the largest average size of inclusions. DP steel exhibited the lowest resistance to pitting corrosion due to the high density of inclusions. MnS inclusions in TRIP steel and Mn oxy-sulfide inclusions in TWIP steel acted as a galvanic anode in complex inclusions.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The density of inclusions was high in an order of DP, TWIP, and TRIP steels. </LI> <LI> TRIP and TWIP steels exhibited complex inclusions as well as single inclusions. </LI> <LI> DP steel had the lowest resistance to pitting corrosion. </LI> <LI> Initial pits occurred at the MnS particles of complex inclusions in TRIP steel. </LI> <LI> Initial pits formed at the Mn oxy-sulfide of complex inclusions in TWIP steel. </LI> </UL> </P>

Zn-Flash 코팅 처리가 전기아연도금 시 초고강도 강재의 수소 발생, 유입 및 취화 거동에 미치는 영향

방혜린,김상헌,김성진,Hye Rin Bang,Sang Heon Kim,Sung Jin Kim 한국부식방식학회 2023 Corrosion Science and Technology Vol.22 No.5

In the present study, effects of a thin Zn-flash coating on hydrogen evolution, infusion, and embrittlement of advanced high strength steel during electro-galvanizing were examined. The electrochemical permeation technique in conjunction with impedance spectroscopy was employed under applied cathodic polarization. Moreover, a slow-strain rate test was conducted to evaluate loss of elongation (i.e., indicative of hydrogen embrittlement (HE)) and examine fracture surfaces. Results showed that the presence of a thin Zn-flash coating, even when it was not distributed uniformly, reduced hydrogen evolution rate and substantially impeded infusion of hydrogen into the steel substrate. This was primarily due to a hydrogen overvoltage on Zn coating and trapping of hydrogen at the interface of Zn coating/flash coating/steel substrate. Consequently, the sample with flash coating had a smaller HE index than the sample without flash coating. These results suggest that a thin Zn-flash coating could be an effective technical strategy for mitigating HE in advanced high-strength steels.

Effect of a Conically Shaped Hollow Electrode on Advanced High Strength Steel in Three-Sheet Resistance Spot Welding

김동현,유지영,이세헌 한국정밀공학회 2016 International Journal of Precision Engineering and Vol.17 No.3

Application of high strength steel is one of the most promising solutions to the problem of vehicle weight reduction. However, weldability problems occur when three sheet metals of the high strength steel are welded. This is because under such situation, the suitable welding range in the lobe diagram is too narrow and the strength between the contact zones is lower than the required value, and also, expulsions between the sheets occur. In this work, a conically shaped hollow electrode was applied at the lower electrode to improve resistance spot weldability of three-sheet welding including high strength steel. Shear strength test and analysis of welding signal and weld cross section were conducted. It was found that the proposed lower electrode has some advantages in welding the three-sheets welding compared with conventional electrodes or hollow electrodes. Consequently, a welding process using the proposed electrode was studied and verified with numerical analysis.

Effect of process parameters on optimum welding condition of DP590 steel by friction stir welding

김영곤,김지선,김인주 대한기계학회 2014 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.28 No.12

In the automotive industry, vehicle weight reduction techniques have been actively studied to improve the rate of fuel consumption andto cope with the regulation restricting exhaust gas. For this reason, advanced high-strength steel (AHSS) is preferred in the automobileindustry as its tensile strength is 590 MPa and over. In this study, to obtain the optimum welding condition, the friction stir welding(FSW) process applied to AHSS was considered. The FSW experiment was performed on a stir plate using a Si3N4 tool and a 1.4-mmthickDP590 steel sheet manufactured by cold rolling. In addition, to investigate the temperature distribution of the advancing and retreatingsides in the welding state, the tool rotation speed of 800 rpm, and the welding speed of 180 mm/min, a K-type thermocouple wasinserted in the backing plate, and the peak temperature was evaluated at each point. Especially, the correlation between the heat input perunit length and the formation of the FSW zone was minutely analyzed.

Predicting Shear Failure of Dual-Phase Steels

Ji Hoon Kim,Ji Hyun Sung,D. K. Matlock,Daeyong Kim,R. H. Wagoner 한국소성가공학회 2010 기타자료 Vol.2010 No.6

Dual-phase (DP) steels are being used increasingly to make automotive panels because of their advantageous combinations of high ductility (for forming) and high strength (for service). However, their adoption has been limited because of failures during die tryout that are unpredicted by the usual methods of finite element modeling and forming limit diagrams. The failures, often called “shear failures” occur at regions of high curvature (low R/t) where sheet of thickness t is drawn over a tool radius R. Recent work revealed that the type of failure and the formability of DP steels depend not only on R/t, but also on strain rate, an effect derived from the propensity of these steels to locally heat in areas of high strain when strain rates are sufficiently high to limit heat transfer. The formability is reduced significantly by the thermal effect for rates greater than approximately 0.1/s. This result explains at least partially why forming limit diagrams, which are measured quasi-statically (and thus isothermally) do not reflect the behavior of DP steels formed industrially (at typical strain rates of approximately 10/s). In order to apply laboratory test results of draw-bend formability to industrial forming operations, the inputs to commercial finite element codes (constitutive equations, forming limits) must be adapted to the reality of the material (DP steel) and underlying physics (thermal effects on constitutive behavior). Toward this end, two procedures have been developed and tested, one numerical and one analytical. Together they predict similar forming limits and provide a path for understanding the applied formability of DP steels.

고강도 강판 저항 점용접부 강도 및 파단에 미치는 Paint Baking의 영향

최철영 ( Chul Young Choi ),이동윤 ( Dong Yun Lee ),김인배 ( In Bae Kim ),김양도 ( Yang Do Kim ),박영도 ( Yeong Do Park ) 대한금속재료학회(구 대한금속학회) 2011 대한금속·재료학회지 Vol.49 No.12

Conventional fracture tests of resistance spot welds have been performed without consideration of the paint baking process in the automobile manufacturing line. The aim of this paper is to investigate the effect of the paint baking process on load carrying capacity and fracture mode for resistance spot welded 590 dual phase (DP), 780DP, 980DP, 590 transformation in duced plasticity (TRIP), 780TRIP and 1180 complex phase (CP) steels. With paint baking after resistance spot welding, the l-shape tensile test (LTT) and nano-indentation test were conducted on the as-welded and paint baked samples. Paint baking increased the load-carrying capacity of the resistance spot welded samples and improved the fracture appearance from partial interfacial fracture (PIF) to button fracture (BF). Improvement in fracture appearance after LTT is observed on weldments of 780 MPa grade TRIP steels, especially in the low welding current range with paint baking conditions. The higher carbon contents (or carbon equivalent) are attributed to the low weldability of the resistance spot welding of high strength steels. Improvement of the fracture mode and load carrying ability has been achieved with ferrite hardening and carbide formation during the paint baking process. The average nano-indentation hardness profile for each weld zone shows hardening of the base metal and softening of the heat affected zone (HAZ) and the weld metal, which proves that microstructural changes occur during low temperature heat treatment.