Effect of Thermal Cycle and Nitrogen Content on the Hot Ductility of Boron-bearing Steel

Cho, Kyung Chul,Mun, Dong Jun,Kang, Myeong Hun,Lee, Jae Sang,Park, Joong Kil,Koo, Yang Mo The Iron and Steel Institute of Japan 2010 ISIJ international Vol.50 No.6

<P>Hot ductility of Boron (B)-bearing steel has been examined in view of slab corner cracking problem. Addition of B to the low carbon steel reduced its hot ductility under a thermal cycle in which samples were cooled directly to the test temperature before straining. The change in hot ductility of B-bearing steel with deformation temperature showed one trough in the temperature range of 800–1000°C, which covered the lower temperature region of austenite single phase (region (I)), and near the austenite/ferrite transformation temperature (Ae<SUB>3</SUB>) (region (II)). An abrupt temperature decrease and reheating before straining heavily deteriorated the hot ductility of B-bearing steel in the region (I). In all steels, the strain concentration in the film-like ferrite primarily reduced hot ductility in region (II) regardless of the addition of B and the thermal cycles before straining. The ductility reduction of B-bearing steel is caused by the distribution and amount of BN precipitation, which is determined by the thermal cycles and the N content. Increase in the N content remarkably reduced hot ductility of B-bearing steel in region (I), where the behavior of BN precipitates controlled hot ductility. The results shows that the improvement of hot ductility in B-bearing steel can be attained by decreasing the N content and by avoiding an abrupt temperature decrease in the secondary cooling stage of the slab after solidification.</P>

열간가공된 TiAl 합금의 미세조직 제어가 기계적 특성에 미치는 영향

김종훈,김재권,김성웅,박용호,김승언 대한금속·재료학회 2020 대한금속·재료학회지 Vol.58 No.7

The microstructure and mechanical properties of a newly developed, β-phase containing TiAl alloy have been studied through hot working and post heat treatment to enhance room temperature ductility and strength. The controlled microstructures hadthree types of structure, fully lamellar, nearly lamellar and duplex, and were produced by cyclic heat-treatment in a single α region and (α+γ) region after a hot-forging process in high temperature (α+β) region. As a result of the room temperature tensile test, the fully lamellar structure exhibited a tensile strength of 622 MPa and ductility of 0.62%. The duplex structure had a tensile strength of 787 MPa and ductility of 1.22%, while the nearly lamellar structure showed a tensile strength of 880 MPa and ductility of 1.76%. In the room temperature tensile test, the nearly lamellar structure exhibited excellent tensile strength and ductility. The strength and ductility were increased by decreasing grain size and β / B2 phase fraction. The newly developed TiAl alloy showed higher tensile values compared with the previous TiAl alloys. The relationship between microstructure and room temperature tensile properties of the newly developed β-phase containing TiAl alloy was examined, and the best approach for hot working and post heat-treatment to obtain the most balanced mechanical properties was proposed.

연주크랙 저감을 위한 API X-50 강의 고온연성에 대한 미세조직의 영향

金成圭,金俊星,金洛준 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.2

The rapid cooling in continuous casting process produces large thermal stress in the slabs, due to steep thermal gradient between the interior and the exterior of the slabs and a mechanically induced stress caused by friction in the mold, roll pressure, etc. There is also an occurrence of tensile stress at the upper region and compressive stress at the lower region of the slabs at the straightening zone at around 700℃-900℃ in the case of vertical continuous casters. In the case of steels with poor hot ductility, the combination of the stresses mentioned above quite often results in the formation of cracks. Among various types of steels, it is known that steels containing strong carbide formers such as Nb are particularly susceptible to cracking during continuous casting. The present study is concerned with microstructural analyses of the Nb containing steel (A steel) and Nb+B containing steels (B steel) to reduce the transverse corner crack at the straightening zone, with emphasis on elucidating the mechanism of hot ductility. There are differences in the microstructures between the A steel and the B steel. The B steel has a smaller amount of ferrite films along the prior austenite grain boundaries than the A steel. Also, the B steel contains proeutectoid ferrite within the prior austenite grains and 5-㎛-sized coarse Fe_23(B, C)_6 along the prior austenite grain boundaries as well as within the prior austenite grains. The ductility trough is virtually non-existent in the B steel, although variation of values of reduction of area (RA) with temperature shows that the A steel has a trough of RA between 700℃ to 850℃. It is believed that the presence of intraganular ferrite improves the homogeneity of deformation and accordingly the hot ductility of the B steel.

Ni-Cr-Fe계 합금의 고온균열

남상우(Sangwoo Nam),김철희(Cheolhee Kim),김영민(Young-Min Kim) 대한용접·접합학회 2017 대한용접·접합학회지 Vol.35 No.5

Ni-Cr-Fe alloys have excellent corrosion resistance and strength at high-temperature, thus these alloys are used as basic structural materials in high-temperature parts such as aerospace industry, nuclear power generators, and ultra-supercritical power plants. Also, the materials are widely utilized for similar and dissimilar welding of austenitic alloys, however, various hot cracking such as solidification cracking, liquation cracking and ductility-dip cracking occur. Various types of hot cracking test methods have been developed to reproduce the thermal/mechanical deformation of the actual welding, and the evaluation of cracking susceptibility employed this methods was discussed. In this paper, previous studies to understand the mechanisms of crack formation and to reduce hot cracking were introduced. In addition, the effect of the various carbides (MC, M23C6) and intermetallic compounds (γ", γ"", δ, σ and Laves) caused by solidification and reheating during multi-pass welding and the influence of metallurgical changes on crack sensitivity were described.

Understanding dual precipitation strengthening in ultra-high strength low carbon steel containing nano-sized copper precipitates and carbides

Phaniraj M. P.,Shin Young-Min,Jung Woo-Sang,김만호,Choi In-Suk 나노기술연구협의회 2017 Nano Convergence Vol.4 No.16

Low carbon ferritic steel alloyed with Ti, Mo and Cu was hot rolled and interrupt cooled to produce nano-sized precipitates of copper and (Ti,Mo)C carbides. The steel had a tensile strength of 840 MPa, an increase in yield strength of 380 MPa over that of the plain carbon steel and reasonable ductility. Transmission electron microscopy and small angle neutron scattering were used to characterize size and volume fraction of the precipitates in the steels designed to form only copper precipitates and only (Ti,Mo)C carbides. The individual and combined precipitation strengthening contributions was calculated using the size and volume fraction of precipitates and compared with the measured values.

주조 및 열간 압연된 Fe-0.7wt%C-2.3wt%Si-0.3wt%Mn 강의 프로세싱 윈도우와 기계적 성질에 미치는 오스템퍼링 조건의 영향 비교

손제영 ( Je Young Son ),황동찬 ( Dong Chan Hwang ),최재주 ( Jae Joo Choi ),송준환 ( June Hwan Song ),김지훈 ( Ji Hun Kim ),김원배 ( Won Bae Kim ),예병준 ( Byung Joon Ye ) 한국주조공학회 2010 한국주조공학회지 Vol.30 No.2

HY계 고강도 고인성강의 용접성

이종봉,안상곤,심인옥 대한용접접합학회 1995 대한용접·접합학회지 Vol.13 No.3

Weldability of DS100 and HY type high strength-toughness steel plates, tentatively produced as domestic production, was investigated. DS100 and DS130A had nearly same hardenability in HAZ in spite of its difference in Ceq. Based upon the y-groove test results, cold cracking susceptibility of DS130 was superior to that of DS100 because of its lower hydrogen level in weld metal. Solidification cracking tested by the Trans-Varestraint test was occured in all of the weld metals, and its susceptibility was high in the row of DS100, DS130A and DS130B. However, no liquation cracking and ductility-dip cracking tested by the Longi-Varestraint test with 6.0% augmented strain were detected in base metal and reheated weld metal. Toughness in the GMA welding joint was satisfied with the relative Mill Spec, even though welded joint of DS100 had relatively low impact energy especially at the weld metal.

Hot working behavior of a nitrogen-alloyed Fe-18Mn-18Cr-N austenitic stainless steel

Moon, J.,Lee, T.H.,Shin, J.H.,Lee, J.W. Elsevier Sequoia 2014 Materials science & engineering. properties, micro Vol.594 No.-

The hot workability of a nitrogen-alloyed Fe-18Mn-18Cr-N austenitic stainless steel was investigated through hot compression tests using a Gleeble simulator in a temperature range of 700-1100<SUP>o</SUP>C and at strain rates of 0.01-5s<SUP>-1</SUP>. From stress(σ)-strain(ε) curves obtained in the hot compression tests, a processing map was established to estimate the hot workability, based on the dynamic material model (DMM). The processing map predicted that the optimum hot working regimes are in the temperature range from 1050<SUP>o</SUP>C to 1100<SUP>o</SUP>C at strain rate of 0.01s<SUP>-1</SUP>. After the hot compression tests, intergranular cracking arose under most conditions, except for the optimum hot working condition predicted by the processing map. This study discusses the cause of the intergranular cracking in terms of the precipitation of intergranular Cr<SUB>2</SUB>N particles. In addition, hot ductility tests were carried out for a better understanding of the decrease in the hot workability, and a significant loss of ductility was observed with intergranular Cr<SUB>2</SUB>N precipitation.

저탄소강의 열간 연성 거동에 미치는 Cu의 영향

손광석 ( Kwang Suk Son ),박태은 ( Tae Eun Park ),박병호 ( Byung Ho Park ),김동규 ( Dong Gyu Kim ) 대한금속재료학회 ( 구 대한금속학회 ) 2009 대한금속·재료학회지 Vol.47 No.4

Cu as a tramp element has been reported to encourage transverse cracking upon straightening operation during continuous casting or mini-mill processing. Therefore, the hot workability of steels containing Cu should be investigated. The purpose of the present study was to examine the effect of Cu contents on the hot ductility of low carbon steels by using hot compression test. Hot compression test was carried out using a Gleeble. The specimens were heated to 1300℃ for solution treatment and then held for 300s before cooling at a rate of 1℃/s to test temperatures in the range of 650~1150℃ (50℃ intervals) with strain rate of 5×10(-3)/s. In Cu containing steels, the hot ductility was decreased with increasing Cu content at high temperature region which is to be attributed to copper enriched phase formed at scale/steel interface, and low hot ductility with increasing Cu content at low temperature region is attributable to the strengthening of matrix by the formation of ε-Cu. The width of ductility trough region was decreased with increasing Cu content.

Ti-Nb 합금강에서 합금성분의 변화에 따른 석출물거동이 고온연성에 미치는 영향

한원배 ( Won Bae Han ),이종호 ( Jong Ho Lee ),김희수 ( Hee Soo Kim ),안현환 ( Hyeun Hwan An ),이승재 ( Seung Jae Lee ),김성우 ( Seong Woo Kim ),서석종 ( Seok Jong Seo ),윤종승 ( Chong Seung Yoon ) 대한금속재료학회 ( 구 대한금속학회 ) 2012 대한금속·재료학회지 Vol.50 No.4

Hot ductility behavior of precipitation-hardened low-carbon iron alloys containing 0.02 wt% Ti and 0.05 wt% Nb was characterized by a hot tensile stress test. Carbon (0.05, 0.1, 0.25 wt%) and boron (0.002 wt%) contents were varied to study the effect of precipitates on the high-temperature embrittlement of the alloys in the temperature range of 600~800℃. Ductility loss was observed at 700℃ for the tested alloys. The cause of the ductility loss was mainly attributed to the carbides and ferrite films formed at the grain boundaries during deformation. Although the carbon content tended to raise the total fraction of Nb (C, N), the precipitates were formed mostly in the grain interior as the precipitation temperature was raised above the deformation temperature by the high carbon content. Hence, carbon in excess suppressed the hot ductility loss. Meanwhile, boron addition improved the hot ductility of the alloys. The improvement is likely due to the boron atoms capturing carbon atoms and thus retarding the carbide formation.