• Title, Summary, Keyword: accelerated cooling

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Analysis of stress and distortion that develop during accelerated cooling of plate (가속냉각시 강판에 발생하는 응력 및 변형에 대한 연구)

  • 김호영;김창영;주웅용;장래웅
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.12 no.5
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    • pp.952-958
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    • 1988
  • To analyze the nonflatness and residual stress in accelerated cooled plate, a numerical analysis model has been developed. Two factors, i.e. temperature and phase transformation, are considered in calculating the stress distribution that develops during cooling. The plastic strain and plate-buckling, which are often shown in accelerated cooled plate, were determined from this stress. Mean temperature in through thickness direction and temperature difference in width direction are considered in the model to simplify the calculation. The temperature and stress distribution changes caused by phase transformation are involved in terms of the effective specific heat and the effective thermal expansion coefficient. With the model, accelerated cooling of 10mm(t) $^{*}$3000mm(w) plate was simulated. The condition of accelerated cooling was .deg. C/sec from just after hot rolling to 500.deg. C. The initial temperature-difference ratio, .DELTA.Tr, in width direction is an important factor in evaluating the stress distribution. When .DELTA.Tr is 0.08, buckling occurs during cooling and 7kgf/m $m^{2}$ of residual stress develops at the edge of plate. To secure the flatness, .DELTA.Tr should be less than 0.07. Small scaled cooling test was conducted to verify the exactness of the model and the results proved the usefulness of this numerical analysis model.l.

Microstructure Characteristics and Identification of Low-Carbon Steels Fabricated by Controlled Rolling and Accelerated Cooling Processes (제어 압연과 가속 냉각에 의해 저탄소강에서 형성되는 미세조직의 특징과 구분)

  • Lee, Sang-In;Hong, Tae-Woon;Hwang, Byoungchul
    • Korean Journal of Materials Research
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    • v.27 no.11
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    • pp.636-642
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    • 2017
  • In the present study the microstructure of low-carbon steels fabricated by controlled rolling and accelerated cooling processes was characterized and identified based on various microstructure analysis methods including optical and scanning electron microscopy, and electron backscatter diffraction(EBSD). Although low-carbon steels are usually composed of ${\alpha}-ferrite$ and cementite($Fe_3C$) phases, they can have complex microstructures consisting of ferrites with different size, morphology, and dislocation density, and secondary phases dependent on rolling and accelerated cooling conditions. The microstructure of low-carbon steels investigated in this study was basically classified into polygonal ferrite, acicular ferrite, granular bainite, and bainitic ferrite based on the inverse pole figure, image quality, grain boundary, kernel average misorientation(KAM), and grain orientation spread(GOS) maps, obtained from EBSD analysis. From these results, it can be said that the EBSD analysis provides a valuable tool to identify and quantify the complex microstructure of low-carbon steels fabricated by controlled rolling and accelerated cooling processes.

The characteristics of Near-thrshold fatigue crack propagation for welding zone in TMCP high strength steels (TMCP 고장력강 용접부의 하한계 피로균열진전 특성평가)

  • 이택순;오대석;이휘원
    • Journal of Ocean Engineering and Technology
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    • v.11 no.3
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    • pp.39-47
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    • 1997
  • Recently developed TMCP steels, which were manufactured by controlled rolling followed by accelerated cooling process, were examined to study their characteristics and weldability. Accelerated cooling type TMCP steel's hardness test result exhibited high value on weld zone. On the contrary, base metal and HAZ exhibited comparatively the similar value. On this experiment result Softening of HAZ is not occurred. in the-heat affected zone, grain size repression be caused by chemical composition properties which a small quantity Al-Ti-B-N. Changing stress ratio near-threshold fatigue crack propagation experiments were carried out. According to this result, crack propagation velocity of the HAZ exhibited slower than the base metal and near-threshold value had increased at the HAZ. Finally accelerated cooling type TMCP steels were exhibited excellent mechanical properties in both strength and toughness.

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Effect of Cooling Rate on DC Accelerated Aging Characteristics of ZPCCY-Based Varistor Ceramics (ZPCCY계 바리스터 세라믹스의 DC 가속열화 특성에 미치는 냉각속도의 영향)

  • 남춘우;김향숙
    • Journal of the Korean Institute of Electrical and Electronic Material Engineers
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    • v.15 no.9
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    • pp.776-782
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    • 2002
  • The microstructure, V-Ι characteristics, and stability of ZnO-P $r_{6}$ $O_{11}$ CoO-C $r_2$ $O_3$- $Y_2$ $O_3$-based varistor ceramics were investigated with cooling rate in the range of 2~8$^{\circ}C$/min. The cooling rate relatively weakly affected the microstructure, the varistor voltage, and the leakage current in the V-Ι characteristics. But the nonlinear exponent relatively strongly affected by cooling rate. The cooling rate also greatly affected the stability of V-Ιand dielectric characteristics for DC accelerated aging stress. On the whole, the varistors cooled with 4$^{\circ}C$/fin exhibited the highest performance in the densification, nonlinearity, and stability. Especially, they exhibited a high stability, in which the variation rate of the varistor voltage( $V_{1㎃}$), the nonlinear exponent($\alpha$), and the dissipation factor(tan $\delta$) is -1.4%, -4.9%, and +60.0%, respectively, under DC accelerated aging stress such as 0.95 $V_{1㎃}$15$0^{\circ}C$/12 h)

Effects of Controlled Cooling on Microstructures and Mechanical Properties of a Steel for Cold Forming (냉간성형용 강의 미세조직과 기계적성질에 미치는 제어냉각의 영향)

  • Kim N. G.;Park S. D.;Kim B. O.;Choi H. J.
    • Proceedings of the Korean Society for Technology of Plasticity Conference
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    • pp.391-394
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    • 2004
  • The main purpose of the present study has been placed on investigating the effects of controlled cooling on the microstructures and mechanical properties of 0.2C-0.2Si-0.8Mn-B steel for cold forming. The steel was processed in steel making factory(EAF, VD) and casted to $\Box160$ billet then reheated in walking beam furnace and rolled to coil, rolling stock was acceleratly cooled before coiling. Microstructual observation, tensile test and charpy impact tests were conducted. The mechanical properties and microsture of the steel were changed by cooling condition. The grain size of rolled product decreased with increasing cooling rate, resulting in increase of impact toughness and tensile strength, elongation and reduction of area . From the result of this study, it is conformed that mechanical properties and microstructure of 0.2C-0.2Si-0.8Mn-B steel for cold forming were enhanced by accelerated cooling.

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Continuous Cooling Transformation, Microstructure and Mechanical Properties of High-Strength Low-Alloy Steels Containing B and Cu (B과 Cu가 포함된 고강도 저합금강의 연속냉각 변태와 미세조직 및 기계적 특성)

  • Hwang, Byoungchul
    • Korean Journal of Materials Research
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    • v.23 no.9
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    • pp.525-530
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    • 2013
  • This study investigated the continuous cooling transformation, microstructure, and mechanical properties of highstrength low-alloy steels containing B and Cu. Continuous cooling transformation diagrams under non-deformed and deformed conditions were constructed by means of dilatometry, metallographic methods, and hardness data. Based on the continuous cooling transformation behaviors, six kinds of steel specimens with different B and Cu contents were fabricated by a thermomechanical control process comprising controlled rolling and accelerated cooling. Then, tensile and Charpy impact tests were conducted to examine the correlation of the microstructure with mechanical properties. Deformation in the austenite region promoted the formation of quasi-polygonal ferrite and granular bainite with a significant increase in transformation start temperatures. The mechanical test results indicate that the B-added steel specimens had higher strength and lower upper-shelf energy than the B-free steel specimens without deterioration in low-temperature toughness because their microstructures were mostly composed of lower bainite and lath martensite with a small amount of degenerate upper bainite. On the other hand, the increase of Cu content from 0.5 wt.% to 1.5 wt.% noticeably increased yield and tensile strengths by 100 MPa without loss of ductility, which may be attributed to the enhanced solid solution hardening and precipitation hardening resulting from veryfine Cu precipitates formed during accelerated cooling.

A Study on the Stress Corrosion Cracking Evaluation for Weld Joint of TMCP steel by SP-SSRT Method (SP-SSRT법에 의한 TMCP강 용접부의 응력부식균열 평가에 관한 연구)

  • 유효선;정희돈;정세희
    • Journal of Welding and Joining
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    • v.15 no.1
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    • pp.46-54
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    • 1997
  • The object of this paper is to evaluate SCC(stress corrosion cracking) susceptibility for parent metal and bond line region of weld joints which have the various weld heat input condtions in TMCP(thermo-mechanical control process) steel by SP-SSRT(small punch-slow strain rate test) method. And the SCC test results of TMCP steel are compared with those of the conventional HT50 steel which has te almost same tensile strength level like TMCP steel. The loading rate used was $3\times10^{-4}$mm/min and the corrosive environment was synthetic sea water. According to the test results, in the case of parent metal, TMCP steel showed higher SCC susceptibility than HT50 steel because of the high plastic strain level of ferrite microstructure obtained by accelerated cooling. And in the case of bond line, the both TMCP steel and HT50 steel showed low load-displacement behaviors and higher SCC susceptibility above 0.6. These results may be caused by theembrittled martensite structure on HT50 steel and by the coarsened grain and the proeutectoid ferrite structure obtained by the impart of accelerated cooling effect on TMCP steel.

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TMCP 강의 용접열영향부 인성에 관한 연구

  • 신민태;윤중근;김희진
    • Journal of Welding and Joining
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    • v.4 no.3
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    • pp.43-49
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    • 1986
  • Weldability of the TMCP steel manufactured by controlled rolling followed by accelerated cooling process was investigated. For comparison, two other steel plates produced by different manufacturing processes were selected; normalized and controlled rolled. Tandem submerged arc welding with both side one run technique was carried out. The results of this study can be summarized as follows; TMCP steel having the lowest carbon equivalent shows the best combination of mechanical properties, not only in the base metal but also in the heat affected zone. In the HAZ, the accelerated colling effect imarted on the trengthis releved by the weld thermal cycles, and thus the strength of the welded joint decrease substantially accompanied with the fracture in the HAZ. On the other hand, not only the softening but the fine microstructure can preserve the high toughness of TMCP steel in the HAZ.

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An Experimental Study of the Air-side Particulate Fouling of Finned-Tube Heat Exchangers of Air Conditioners by using Accelerated Particle-Loading System (파울링 형성 가속장치를 이용한 공기조화기용 열교환기의 공기측 파울링 특성에 대한 실험적 연구)

  • 안영철;조재민;이재근;이현욱;안승표;윤덕현;하삼철;강태욱;옥주호
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.15 no.10
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    • pp.802-808
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    • 2003
  • The air-side particulate fouling of the HVAC heat exchangers degrades the performance of cooling capacity, pressure drop across a heat exchanger, and indoor air quality. The purpose of this study is to investigate the fouling characteristics using accelerated particle loading system. The fouling characteristics are analyzed as functions of a dust concentration, a face velocity and a wet or dry surface condition. The pressure drop increases with increasing test operation and reaches constant asymptotic level. For the saturated condition due to particle loading, the pressure drop across the slitted finned-tube heat exchangers at the face velocity of 1 m/sec increases up to 57% and the cooling capacity decreases about 2%. The cooling capacities are not affected greatly by the presence of the fouling deposits if the thickness of the fouling deposits can not change substantially the flow pattern through the fins.

Effects of Ni addition on continuous cooling transformation behavior of low carbon HSLA steels (저탄소${\cdot}$저합금 강의 연속 냉각 변태에 미치는 Ni의 영향)

  • Kang J. S.;Jun J. H.;Park C. G.
    • Proceedings of the Korean Society for Technology of Plasticity Conference
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    • pp.456-459
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    • 2005
  • Continuous cooling transformation behaviors were studied fur low carbon HSLA steels containing three different level $(1\~3\;wt\%)$ of Ni addition. Thermo-mechanical processing (TMP) simulations to construct continuous cooling (CCT) diagram were conducted by using Gleeble system. As cooling rate increased, pearlite, granular bainite, acicular ferrite, bainitic ferrite and lath martensite were transformed from deformed austenite. Fully bainitic microstructure were developed at all cooling rate condition in high Ni containing steel due to hardenability increasing effects of Ni. Ni also influenced the transformation kinetics. At the slowest cooling rate of $0.3^{\circ}C/s$, transformation delayed with decreasing Ni contents because of the diffusion of substitutional alloy elements. However, cooling rate slightly increased to $1^{\circ}C/s$, transformation kinetics accelerated with decreasing Ni contents because nucleation of bainite was sluggish due to hardening of residual austenite.

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