페라이트-펄라이트 조직 아공석강의 상온 및 저온 충격 인성에 미치는 미세조직적 인자의 영향

이승용,정상우,황병철,Lee, Seung-Yong,Jeong, Sang-Woo,Hwang, Byoungchul 한국재료학회 2015 한국재료학회지 Vol.25 No.11

This paper presents a study on the room- and low-temperature impact toughness of hypoeutectoid steels with ferrite-pearlite structures. Six kinds of hypoeutectoid steel specimens were fabricated by varying the carbon content and austenitizing temperature to investigate the effect of microstructural factors such as pearlite volume fraction, interlamellar spacing, and cementite thickness on the impact toughness. The pearlite volume fraction usually increased with increasing carbon content and austenitizing temperature, while the pearlite interlamellar spacing and cementite thickness mostly decreased with increasing carbon content and austenitizing temperature. The 30C steel with medium pearlite volume fraction and higher manganese content, on the other hand, even though it had a higher volume fraction of pearlite than did the 20C steel, showed a better low-temperature toughness due to its having the lowest ductile-brittle transition temperature. This is because various microstructural factors in addition to the pearlite volume fraction largely affect the ductile-brittle transition temperature and low-temperature toughness of hypoeutectoid steels with ferrite-pearlite structure. In order to improve the room- and low-temperature impact toughness of hypoeutectoid steels with different ferrite-pearlite structures, therefore, more systematic studies are required to understand the effects of various microstructural factors on impact toughness, with a viewpoint of ductile-brittle transition temperature.

Mn 및 V 함량이 다른 페라이트-펄라이트 조직강의 강도와 변형능에 미치는 미세조직 인자의 영향

홍태운,이상인,심재혁,이준호,이명규,황병철,Hong, Tae-Woon,Lee, Sang-In,Shim, Jae-Hyeok,Lee, Junho,Lee, Myoung-Gyu,Hwang, Byoungchul 한국재료학회 2018 한국재료학회지 Vol.28 No.10

This study examines the effect of microstructural factors on the strength and deformability of ferrite-pearlite steels. Six kinds of ferrite-pearlite steel specimens are fabricated with the addition of different amounst of Mn and V and with varying the isothermal transformation temperature. The Mn steel specimen with a highest Mn content has the highest pearlite volume fraction because Mn addition inhibits the formation of ferrite. The V steel specimen with a highest V content has the finest ferrite grain size and lowest pearlite volume fraction because a large amount of ferrite forms in fine austenite grain boundaries that are generated by the pinning effect of many VC precipitates. On the other hand, the room-temperature tensile test results show that the V steel specimen has a longer yield point elongation than other specimens due to the highest ferrite volume fraction. The V specimen has the highest yield strength because of a larger amount of VC precipitates and grain refinement strengthening, while the Mn specimen has the highest tensile strength because the highest pearlite volume fraction largely enhances work hardening. Furthermore, the tensile strength increases with a higher transformation temperature because increasing the precipitate fraction with a higher transformation temperature improves work hardening. The results reveal that an increasing transformation temperature decreases the yield ratio. Meanwhile, the yield ratio decreases with an increasing ferrite grain size because ferrite grain size refinement largely increases the yield strength. However, the uniform elongation shows no significant changes of the microstructural factors.

Pilot Plant를 이용한 600 MPa급 내진용 철근들의 제조, 미세조직과 기계적 특성 비교

홍태운,황병철,Hong, Tae-Woon,Hwang, Byoungchul 한국재료학회 2019 한국재료학회지 Vol.29 No.6

This study deals with the microstructure and tensile properties of 600 MPa-grade seismic reinforced steel bars fabricated by a pilot plant. The steel bar specimens are composed of a fully ferrite-pearlite structure because they were air-cooled after hot-rolling. The volume fraction and interlamellar spacing of the pearlite and the ferrite grain size decrease from the center region to the surface region because the surface region is more rapidly cooled than the center region. The A steel bar specimenwith a relatively high carbon content generally has a higher pearlite volume fraction and interlamellar spacing of pearlite and a finer ferrite grain size because increasing the carbon content promotes the formation of pearlite. As a result, the A steel bar specimen has a higher hardness than the B steel bar in all the regions. The hardness shows a tendency to decrease from the center region to the surface region due to the decreased pearlite volume fraction. On the other hand, the tensile-to-yield strength ratio and the tensile strength of the A steel bar specimen are higher than those of the B steel bar with a relatively low carbon content because a higher pearlite volume fraction enhances work hardening. In addition, the B steel bar specimen has higher uniform and total elongations because a lower pearlite volume fraction facilitates plastic deformation caused by dislocation slip.

Nb 첨가에 따른 저탄소강의 충격 특성에 미치는 변태 온도의 영향

이상인,강준영,황병철,Lee, Sang-In,Kang, Jun-Young,Hwang, Byoungchul 한국재료학회 2016 한국재료학회지 Vol.26 No.11

In this study, six kinds of low-carbon steel specimens with different ferrite-pearlite microstructures were fabricated by varying the Nb content and the transformation temperature. The microstructural factors of ferrite grain size, pearlite fraction, interlamellar spacing, and cementite thickness were quantitatively measured based on optical and scanning electron micrographs; then, Charpy impact tests were conducted in order to investigate the correlation of the microstructural factors with the impact toughness and the ductile-brittle transition temperature (DBTT). The microstructural analysis results showed that the Nb4 specimens had ferrite grain size smaller than that of the Nb0 specimens due to the pinning effect resulting from the formation of carbonitrides. The pearlite interlamellar spacing and the cementite thickness also decreased as the transformation temperature decreased. The Charpy impact test results indicated that the impact-absorbed energy increased and the ductile-brittle transition temperature decreased with addition of Nb content and decreasing transformation temperature, although all specimens showed ductile-brittle transition behaviour.

Artificial Neural Network for Modeling the Tensile Properties of Ferrite-Pearlite Steels: Relative Importance of Alloying Elements and Microstructural Factors

Tae‑Woon Hong,Sang‑In Lee,Jae‑Hyeok Shim,Myoung‑Gyu Lee,Joonho Lee,Byoungchul Hwang 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.10

An artificial neural network (ANN) model was developed to predict the tensile properties as a function of alloying elementand microstructural factor of ferrite-pearlite steels. The input parameters of the model were composed of alloying elements(Mn, Si, Al, Nb, Ti, and V) and microstructural factors (pearlite fraction, ferrite grain size, interlamellar spacing, and cementitethickness), while the output parameters of the model were yield strength and tensile strength. Although the ferrite-pearlitesteels have complex relationships among the alloying elements, microstructural factors, and tensile properties, the ANNmodel predictions were found to be more accurate with experimental results than the existing equation model. In the presentstudy the individual effect of input parameters on the tensile properties was quantitatively estimated with the help of theaverage index of the relative importance for alloying elements as well as microstructural factors. The ANN model attemptedfrom the metallurgical points of view is expected to be useful for designing new steels having required mechanical properties.

페라이트-펄라이트 조직 아공석강의 강도와 연성에 미치는 미세조직적 인자의 영향

이상인 ( Sang-in Lee ),강준영 ( Jun-young Kang ),이상윤 ( Sang-yoon Lee ),황병철 ( Byoung Chul Hwang ) 한국열처리공학회 2016 熱處理工學會誌 Vol.29 No.1

This article presents a study on the tensile properties of hypoeutectoid steels with different ferritepearlite microstructures. Nine kinds of hypoeutectoid steel specimens were fabricated by varying carbon content and isothermal transformation temperature. The microstructural factors such as ferrite & pearlite fraction, interlamellar spacing, and cementite thickness were quantitatively measured and then tensile tests were carried out on the specimens in order to investigate the correlation of the microstructural factors with strength and ductility. The pearlite volume fraction usually increased with decreasing transformation temperature, while the pearlite interlamellar spacing and cementite thickness decreased mostly with decreasing transformation temperature, irrespective of carbon content. The tensile test results showed that the yield and tensile strengths of all the steel specimens increased and their ductility was also improved as the transformation temperature decreased. For the steel specimens investigated, the difference in the transformation temperature dependence of strength and ductility could be explained by the fact that the variation in pearlite fraction with transformation temperature noticeably affected various microstructural factors such as pearlite interlamellar spacing and cementite thickness associated with pearlite fracture mechanism such as void initiation, cementite necking, and cracking. (Received December 17, 2015; Revised December 24, 2015; Accepted December 30, 2015)

Microstructural control of hot rolled strips and their tensile strengths after hot stamping process

하원,이창훈,박철재 대한기계학회 2015 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.29 No.1

The influence of microstructure of hot rolled strips on tensile property after hot stamping was investigated using boron-bearing steel of1470 MPa grade for hot stamping. Hot rolled strips with two kinds of microstructures were produced by mill trial based on a timetemperaturetransformation (TTT) diagram that resulted from a dilatometric experiment. The first had a ferrite-pearlite banded microstructureand the other had a bainitic microstructure. The strip with bainite had a higher tensile strength after hot stamping compared withthe tensile strength of the strip with ferrite-pearlite banded microstructure after hot stamping. The former almost fully transformed tomartensite after hot stamping, while the latter transformed to partial martensite and ferrite that originated from the inhomogeneous hardenabilityof the ferrite-pearlite banded structure due to segregation of alloying elements such as carbon and manganese.

고대 철확(철솥)의 1천년 반복 가열 및 열화현상

고형순 ( Hyeong Sun Go ),한민수 ( Min Su Han ),최병학 ( Byung Hak Choe ),민두식 ( Doo Sik Min ),심윤임 ( Yun Im Shim ),정효태 ( Hyo Tae Jeong ),조남철 ( Nam Chul Cho ) 대한금속재료학회 ( 구 대한금속학회 ) 2012 대한금속·재료학회지 Vol.50 No.4

The microstructural changes of three pieces from an ancient iron pot were studied in order to identify present the material degradation due to repeated heating for one-thousand years. The microstructures of the pieces were divided into the areas of ferrite/graphite, ferrite/pearlite, and corroded oxidation. The area of ferrite/graphite was undergone by severe Galvanic corrosion, but that of ferrite/pearlite was not even during a thousand years` using . The shape of the graphites was coexisted with types of A, B, and C of as modern graphite classification. In the ferrite/pearlite area, abnormal acicula precipitates with a high aspect ratio of 0.2㎛thickness and several hundreds ㎛length were presented. They might be a kind of carbide in the ferrite matrix with its special precipitate plane.

다중회귀분석을 이용한 페라이트-펄라이트 조직 아공석강의 충격인성에 미치는 미세조직적 인자의 민감도 해석

이승용 ( Seung-yong Lee ),이상인 ( Sang-in Lee ),황병철 ( Byoung Chul Hwang ) 대한금속재료학회(구 대한금속학회) 2016 대한금속·재료학회지 Vol.54 No.9

In this study, the effect of microstructural factors on the impact toughness of hypoeutectoid steels with ferrite-pearlite structure was quantitatively investigated using multiple regression analysis. Microstructural analysis results showed that the pearlite fraction increased with increasing austenitizing temperature and decreasing transformation temperature which substantially decreased the pearlite interlamellar spacing and cementite thickness depending on carbon content. The impact toughness of hypoeutectoid steels usually increased as interlamellar spacing or cementite thickness decreased, although the impact toughness was largely associated with pearlite fraction. Based on these results, multiple regression analysis was performed to understand the individual effect of pearlite fraction, interlamellar spacing, and cementite thickness on the impact toughness. The regression analysis results revealed that pearlite fraction significantly affected impact toughness at room temperature, while cementite thickness did at low temperature.(Received March 2, 2016; Accepted April 8, 2016)

페라이트-펄라이트 조직 저탄소강의 수소 취성에 미치는 Nb 첨가와 예비 변형의 영향

고석우,이지민,황병철 대한금속·재료학회 2020 대한금속·재료학회지 Vol.58 No.11

The effect of pre-strain on the hydrogen embrittlement of Nb-free and Nb-added low-carbon steels with ferrite-pearlite structure was investigated in this study. After the steels were electrochemically charged with hydrogen, slow-strain rate tensile (SSRT) tests were conducted on them to examine hydrogen embrittlement behavior. The SSRT test results revealed that the Nb-added steel had a lesser decrease of elongation and reduction of area than the Nb-free steel. The formation of NbC carbide and grain refinement caused by the Nb addition improved resistance to hydrogen embrittlement. The loss of elongation and the reduction of area after hydrogen charging occurs when pre-strain is increased. The pre-strain increases dislocation density and thus increases the amount of reversible hydrogen trap sites associated with hydrogen embrittlement. 10% pre-strained specimens exhibited a significant loss in elongation and reduction of area, regardless of Nb addition. Based on the results of electron back-scatter diffraction, fractographic, and silver decoration analyses for Nb-free and Nb-added steels, the hydrogen embrittlement mechanism in low-carbon steels with different amounts of pre-strain is discussed in terms of dislocation density and hydrogen distribution.