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논문 : 응고 ; 연속주조중 Strand 변형이 슬라브 품질에 미치는 영향
이주동 ( Joo Dong Lee ),임창희 ( Chang Hee Yim ) 대한금속재료학회 ( 구 대한금속학회 ) 1998 대한금속·재료학회지 Vol.36 No.7
Strand deformation is induced by bulging, roll misalignment and strand passline condition(roll gap profile during casting) and has a major role in formation of various defects such as crack and segregation. In order to prevent excessive slab deformation during casting, it is essential to predict how strand is deformed through on-line measuring of displacement. In the present paper, bulging measuring system has been developed to diagnose strand deformation during casting and its effects on slab qualities has been discussed, Excessive pressing by pinch roll, which was concluded to be a main reason of band shape centerline-segregation, was successfully predicted by on-line displacement measuring. And band segregation could be removed by reducing pinch roll pressure. Diagnosing of strand stoppage reveals that large bulging is induced with time by high temperature creep and causes large fluctuation of displacement after casting restart. In-cast roll gap profile in the casting direction can also be predicted by measuring roll displacement in the straightener and horizontal zones. In addition, the frequency of transversal corner crack has been reduced by improvement of in-cast roll gap profile.
탄소강의 열간 연성에 미치는 저주기 피로 시기와 냉각속도의 영향
박태은 ( Tae Eun Park ),이운해 ( Un Hae Lee ),손광석 ( Kwang Suk Son ),이성근 ( Sung Keun Lee ),김인수 ( In Soo Kim ),임창희 ( Chang Hee Yim ),김동규 ( Dong Gyu Kim ) 대한금속·재료학회 2010 대한금속·재료학회지 Vol.48 No.3
The effects of cooling rate and onset time of fatigue test on hot ductility of plain carbon steels containing 0.06~0.8 wt.% carbon were investigated at various temperatures. The tensile tested specimen after fatigue strain during cooling showed higher hot ductility than the specimen fatigued and tensile-strained after cooling. With a decreasing cooling rate from solutionizing temperature to test temperature, hot ductility was increased in all temperature ranges, typically in the trough region, and the minimum point of ductility moved to a lower temperature. Also, the depth and width of the trough decreased with lowering the cooling rate.
Si 첨가강의 연속주조공정 신뢰성 향상을 위한 고온변형특성에 관한 연구
옥명렬(Myoung-Ryul Ok),서진유(Jin-Yoo Suh),심재혁(Jae-Hyeok Shim),홍경태(Kyung-Tae Hong),이주동(Joo-Dong Lee),임창희(Chang-hee Yim) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.8
A study was made on the high temperature deformation behavior of Si steel to increase the reliability of continuous casting process. In continuous casting process, the cast slab experiences cooling and deformation simultaneously, and various sources for defect formation exist. Of all these defects, cracking has the most deleterious effect on the property of the cast slab. To avoid or minimize transverse cracking, unbending temperature should be chosen carefully. In general, transverse cracks can easily be formed when unbending temperature lies within the ductility minimum temperature range of hot tensile test. Relative ratio of austenite-ferrite and their distribution or morphology have significant effect on the reliability of unbending process. Moreover, inhomogeneous deformation can be the source of defect formation. So, to produce Si steel by continuous casting, deformation behavior of Si steel as well as the relative ratio of austenite-ferrite or phase transformation temperature(Ar₃) should be known. We carried out hot tensile and hot compression test to evaluate the mechanical properties of the Si steel at high temperature. Hot ductility minimum temperature was not found with all Si contents, but inhomogeneous deformation occurred in high Si(1.17% and 3.21%) samples. It can be concluded that to minimize inhomogeneous deformation is the key to the successful unbending process of silicon steel. To analyze the effect of the silicon on the strength of silicon steel, a model was established, by using rule of mixture.
박태은 ( Tae Eun Park ),이운해 ( Un Hae Lee ),손광석 ( Kwang Suk Sohn ),이성근 ( Sung Keun Lee ),김인수 ( In Soo Kim ),임창희 ( Chang Hee Yim ),김동규 ( Dong Gyu Kim ) 대한금속재료학회 ( 구 대한금속학회 ) 2010 대한금속·재료학회지 Vol.48 No.5
The effects of oxidation behavior on the hot ductility of plain carbon steels were investigated at various temperatures in order to simulate the continuous casting process more precisely, in which the process undergoes in air atmosphere rather than Ar atmosphere. The high temperature oxidation behavior and scale morphology of the carbon steels exposed to the air and Ar atmosphere at various temperatures were also investigated in order to assess the mechanism of the RA value decreasing in an air atmosphere. The RA values obtained from the air atmosphere were marked below 45% by the test temperature, except for over 1000℃, with the RA values remaining in low values in both the low and high temperature region, at which the RA values generally recovered in the Ar atmosphere. The surface roughness of the specimen was developed by external and internal oxidation when the specimen was deformed in an air atmosphere at high temperature, with the result being the stress concentrated at the roughness of the specimen surface, resulting in low RA values. The hot ductility in the air atmosphere was found to be likely controlled by the oxidation rate instead of the microstructures corresponding to test temperatures.
탄소강의 연속주조시 strand 변형과 비정상벌징의 측정 및 해석
이주동,임창희 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.7
A driven roll measuring system has been developed to diagnose strand deformation and bulging during continuous casting and their effect on slab qualities has been discussed. In-cast roll gap profile in the casting direction could be predicted by driven roll displacement monitoring and was discussed regarding the frequency of transversal corner crack. Unsteady bulging, which is generally accompanied by steel level variation within mold and casting speed variation, has a tendency to occur for the steels with 0.09-0.12% C and to increase along downstream side. Frequency analysis which was performed on unsteady bulging showed that high frequency of 0.2-0.25 ㎐ as well as roll rotation and roll pitch frequencies occurred. These particular frequencies were discussed regarding the relation of unevenness of solidified shell. A mathematical model has been developed to simulate the unsteady bulging phenomena based on FEM analysis on the case of periodical casting speed change. Finally, the effect of unsteady bulging on slab qualities also has been discussed.
실연주기에서 Strand 변형 측정 및 동적크리프를 고려한 벌징변형 해석
이주동,임창희 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.3
Strand bulging is presumed to play a major role in the formation of internal cracks and centerline segregation in continuous casting. During the solidification process, the shell is subjected to cyclic mechanical and thermal loads which can cause various defects when the strains exceed critical value. In the present study, multi roll pitch and dynamic creep model has been developed for bulging analysis. Bulging strain was reasonably predicted to increase by repetitative bulging when considering creep damage accumulation by the model. Also cumulative creep bulging strain could be calculated by the regressive equation which has been derived based on the present model. Bulging increase was measured during strand stoppage and compared with the calculated results. Finally we considered the effect of roll misalignment on bulging strain. The model can be used for the caster design and optimum operation for soft reduction, high speed casting, etc.
Fe-Al-C 계 합금의 고온 기계적 성질에 미치는 Al 의 영향
김동규,김병곤,임창희,나형용 대한금속재료학회(대한금속학회) 1989 대한금속·재료학회지 Vol.27 No.2
For the purpose of investigating the effects of the Al content on the elevated temperature mechanical properties of Fe-Al-C alloys, high temperature tensile test and thermal fatigue test were performed using a series of alloys containing various amounts of Al ranging up to ∼4.5 wt%. Thermal conductivity was also measured. At low temperatures, the thermal conductivity of flake graphite cast iron was higher than that of C/V graphite east iron, but both cast irons exhibited a similar value of conductivity at high temperature near 300℃. The thermal conductivity was decreased with the increase in the Al content and the decreasing rate was reduced with the increase in the temperature. The tensile strength at 700℃ was increased with the increase in the Al content in bath cast irons, but the tensile strength was higher in the case of the C/V graphite cast iron as compared to the flake graphite cant iron. The thermal fatigue life of C/V graphite cast iron was, at temperatures from 300℃ to 700℃, superior to t at of flake graphite cast iron, particular1y in the case of low Al content. The thermal fatigue life of the C/V graphite cast iron deteriorated with increasing Al content, but that of the flake graphite cast iron improved with increasing Al content.
이상헌,오규환,임창희,박중길,한흥남,최주,이동녕 대한금속재료학회(대한금속학회) 1997 대한금속·재료학회지 Vol.35 No.1
Thermal deformation of slab mold during continuous casting was analysed by the coupled finite element method with heat and thermo-elasto-plastic deformation. In order to consider the effect of air gap on the heat transfer coefficient between strand and mold, thermal deformation of strand and temperature distributions in strand and mold were calculated. The effects of mold thickness, temperature of cooling water, velocity of cooling water and mold taper on deformation of mold was investigated. The deformation of mold was due to the difference of thermal expansion between mold and supporting plate of mold. During heating in continuous casting process, mold was deformed toward the strand, while mold was deformed toward the supporting plate of mold during cooling. The amount of deformation increased with increasing the hot face temperature in mold.
元泳穆,申容昌,任昌熙 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.12
The optimum conditions of soft reduction were determined to minimize centerline segregation in the continuously cast slabs using a solidification model and a series of tests at Pohang Works of POSCO. A solidification model reflecting microsegregation model that can precisely predict surface temperature and the crater end, was developed on the basis of surface temperature measured by thermocouple and solid shell thickness measured by pin shooting method. Optimum soft reduction conditions such as reduction zones and casting speeds for various carbon steel grades were determined on the basis of the simulation results. Changing test of casting speed was carried out to validate optimum casting speed predicted by a solidification model. In a series of tests for soft reduction, the center segregation degree of manganese was reduced from 1.08 to 1.04 and the area fraction of phosphorus segregation was reduced from 0.77 to 0.18%.