Effect of Ausforming Strain on the Microstructural Evolution of Lath Martensite in Low Carbon Steel

B. B. He,Q. W. Guan 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.10

Dislocation engineering is a novel alloy design strategy to produce advanced high strength steels with both high strength andgood ductility, which can be accomplished through an ausforming process. The present work studies the effect of ausformingstrain on the evolution of lath martensite microstructure in low carbon steel by detailed transmission electron microscopyobservation. Ausforming strain determines the length of the martensite blocks and the substructure in the lath martensite. The large ausforming strain (20%–45%) reduces the martensite blocks and leads to the development of dislocation cellstructure, both of which are absent in the ausformed martensite with a small ausforming strain (5%–10%). The formationof the dislocation cell structure in the prior austenite grains after the large plastic deformation inhibits the propagation ofmartensite and thus reduces the length of martensite blocks. Irrespective of the ausforming strain, the plastic deformation ofaustenite grains does not lead to an obvious change of martensite lath width. The large ausforming strain slightly enhancesthe auto-tempering of martensite on the aspect of precipitates density. The coarse precipitates formed close to the lathboundaries after small ausforming strain of 5% is absent in the martensite with large ausforming strain (20%–45%), whichis ascribed to the presence of intensive dislocations generated by elevated ausforming strain can facilitate the pipe diffusionof carbon. The present finding serves as the microstructural basis for the application of the dislocation engineering conceptin the design of advanced high-strength steel with the martensite as the intrinsic component.

Computational Modelling and Experimental Analysis of Hardness and Microstructure of Reinforcing Bars Produced by Tempcore Process

Woonam Choi,Sungwook Choi,Sehwook Bae,Namhyun Kang 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.9

The reinforcing bar (rebar) produced by Tempcore process had tempered martensite structure on its surface, owing to quenchingand tempering, and ferrite–pearlite structure at the core owing to slow cooling rate. Bainite, a low-temperature transformationphase, was observed in the transition area. Mechanical properties of the rebar are significantly influenced by arearatio and hardness of the tempered martensite. In this study, microstructure and area ratio of the tempered martensite of therebar produced by the Tempcore process were predicted using finite volume method and a continuous cooling transformationdiagram. The area ratio of the tempered martensite was predicted with error range of ± 6%. Furthermore, association betweenthe microstructure and hardness was determined by a non-isothermal tempering experiment. The hardness of the temperedmartensite was successfully predicted using the tempering temperature with R-squared 98%. The proposed methodologycan be effectively used to control the mechanical properties of the rebars.

전기화학적 및 고압 수소 분위기하 템퍼드 마르텐사이트강의 수소취화 특성 비교

Sang-Gyu Kim,Jae-Yun Kim,Hyun-Joo Seo,Hwan-Gyo Jung,Jaeyoung Park,Un-Bong Baek,황병철 대한금속·재료학회 2023 대한금속·재료학회지 Vol.61 No.11

The effect of hydrogen charging methods on the hydrogen embrittlement characteristics of tempered martensitic steels were discussed in terms of hydrogen diffusion behavior. Two tempered martensitic steels with different Si content were fabricated by quenching and tempering. The steel with high Si content had a lower cementite fraction because the addition of Si changed the morphology of cementite from a long film-like shape to a short-rod shape by suppressing the precipitation and growth of the cementite. To evaluate the hydrogen embrittlement resistance of the two tempered martensitic steels with different Si content, slow strain-rate tensile testing was employed after introducing hydrogen using three types of hydrogen charging methods (ex-situ electrochemical hydrogen charging, in-situ electrochemical hydrogen charging, and in-situ high-pressure gaseous hydrogen environment). For the hydrogen pre-charged tensile specimens using the ex-situ electrochemical charging method, the steel with high Si content had a better hydrogen embrittlement resistance, with a higher relative reduction in area. On the other hand, there was no significant difference in the relative notch tensile strength of the two tempered martensitic steels with different Si content, regardless of the hydrogen charging methods. In addition, the ex-situ hydrogen charging method exhibited higher relative notch tensile strength compared to the in-situ hydrogen charging method due to the release of hydrogen during the tensile test, after exsitu hydrogen charging. This implies that hydrogen embrittlement resistance can be differently estimated depending on the kind of hydrogen charging methods.

템퍼링된 2상 조직강의 준정적 및 동적 변형거동 연구

이한상,황병철,이성학,이창길,김성준 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.2

Effects of martensite morphology and tempering on quasi-static and dynamic deformation behavior of dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens, which had different martensite morphology and tempering effect, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and fracture mode. In the intermediate annealed (IA) specimens, small martensites were well distributed in the ferrite matrix, but bulky martensites were mixed with ferrites in the step quenched (SQ) specimens. Under a dynamic loading condition, the fracture mode of SQ-specimens was changed from cleavage to ductile fracture as the tempering temperature increased, whereas IA-specimens had a ductile fracture mode, irrespective of tempering. These phenomena were analyzed using a rule of mixtures, phase continuity, and interfacial strength between martensite and ferrite.

구상화 열처리된 고탄소강의 미끄럼 마멸 거동에 미치는 시멘타이트 형상과 페라이트 기지조직의 영향

허하리(H. L. Hur),권혁우(H. Gown),구본우(B. Gu),김용석(Y. -S. Kim) 한국소성가공학회 2015 한국소성가공학회 학술대회 논문집 Vol.2015 No.10

This study was conducted to elucidate the effect of cementite morphology and matrix ferrite microstructure on sliding wear behavior of spheroidized high carbon (1 wt. %C) steel. The high carbon steel was primarily heat treated to obtain full pearlite and martensite microstructure before the spheroidization. Spheroidizing heat treatment was performed on the full pearlite steel for 100 hours at 700 ℃ and tempering was performed on the martensite steel for 3 hours at 650 ℃ to get spheroidized cementite in ferrite matrix in both full pearlite and martensite microstructure. Sliding wear tests were carried out using pin-on-disk wear tester with the steel as a disk specimen. An alumina(Al₂O₃) ball was used as a pin counterpart. The sliding wear tests were carried out at room temperature in the air with humidity of 402%. Adapted sliding distance and applied load was 300 m and 100 N, respectively. Sliding speed was 0.1m/s and the wear-track radius was 9mm. Worn surfaces and cross-sections of wear track were examined using an SEM. Micro Vickers hardness of the wear track subsurface was measure as a function of depth from the worn surface. After the spheroidizing heat treatment and the tempering, both pearlite and martensite exhibited similar microstructure of spheroidized cementite in ferrite matrix. Spheroidized pearlite specimens showed lower hardness than the tempered martensite, but wear resistance of the spheroidized pearlite was superior to that of the tempered martensite.

Enhanced Cross-Tension Property of the Resistance Spot Welded Medium-Mn Steel by In Situ Microstructure Tailoring

Bingge Zhao,Yuanfang Wang,Kai Ding,Guanzhi Wu,Tao Wei,Hua Pan,Yulai Gao 한국강구조학회 2021 International Journal of Steel Structures Vol.21 No.2

Medium Mn steels, one of the most promising 3rd generation advanced high strength steels (AHSS), achieve an encouraging trade-off between the outstanding mechanical property and the production cost. As a typical medium Mn steel, 7Mn steels have superior mechanical property but their poor cross-tension property becomes the Achilles’ heel, hindering the application in the automotive industry. The current study focuses on the cross-tension property of the resistance spot weld of 7Mn steel. Generally, martensite is produced in the nugget during the resistance spot welding (RSW). However, the microstructure in the weld nugget can be correspondingly tuned by directly optimizing the welding parameters. With post-weld pulses, in situ tempering occurs, which can decrease the segregation of Mn existing along martensite lath boundaries and facilitate the microstructure transition from martensite to tempered martensite. The tuning on the nugget microstructure facilitates the increase of cross-tension strength (CTS) from 1.5 to 3.7 kN. Although both cases fail in an interfacial fracture mode, a partial ductile fracture is demonstrated in the specimen with post-weld treatment, which is attributed to the occurrence of low-carbon α phase and second phase particles. This study elucidates that the decrease of segregation and the microstructure transition in the nugget are the dominant factor determining the CTS. It is therefore demonstrated that the reduction of Mn segregation and the formation of tempered martensite can increase the weldability of RSW joints of the medium Mn steels.

Mod. 440A 마르텐사이트계 스텐인리스강의 공식에 미치는 탄화물의 영향

권순두,허성화,강창룡,Kwon, Soon-Doo,Heo, Sung-Hwa,Kang, Chang-Yong 한국재료학회 2015 한국재료학회지 Vol.25 No.12

In this study, we investigated the effect of the residual carbides and tempered carbides precipitated by tempering treatment after quenching on the pitting corrosion of mod. 440A martensitic stainless steel. In quenched specimens and tempered specimens after quenching of mod. 440A martensitic stainless steel, the volume fraction of the residual carbides and total carbides decreased with the increase of the austenitizing temperature. Pitting resistance increased with the increase of austenitizing temperature. With the increase of the volume fraction of the residual and total carbides, the pitting resistance of mod. 440A martensitic stainless steel was decreased. The pitting resistance of mod. 0.5C-17Cr-0.5Ni 440A martensitic stainless steel had stronger affected by residual carbides than precipitated carbides produced by tempering.

Kinetics Study on Low-Temperature Tempering of Martensitic Phase in Medium Mn Steel Weldment During Paint-Baking Heat Treatment

Gitae Park,Kwangjoong Kim,Youngchai Lee,Sangho Uhm,Changhee Lee 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.5

The kinetics of low-temperature tempering in a dissimilar resistance spot weldment between medium Mn transformationinducedplasticity (MT) steel and dual-phase (DP) steel were investigated to interpret the change of failure mode after apaint-baking process. After the isothermal heat treatments with various soaking conditions, the Johnson–Mehl–Avrami (JMA)model describing nucleation and growth kinetics was adopted. The results indicated that the tempering kinetics parameterswere different for each area of the welds. Especially in the coarse-grained heat-affected zone (CGHAZ) in the MT steel,carbon diffusion from martensite to austenite acts as a tempering mechanism at an earlier stage of tempering. Thereafter,the tempering mechanism changes to epsilon carbide precipitation and coarsening. In comparison, the change was relativelyslow or absent in the DP steel CGHAZ and fusion zone because of its lower kinetics parameters. Based on the results ofthis study, ways to improve the cross-tension strength of entire welds in automotive applications of MT steel are discussed.

핫스탬핑용 30MnB5강의 템퍼링 조건에 따른 미세조직 및 기계적 물성 연구

정준영,박상천,신가영,이창욱,김태정,최민수 대한금속·재료학회 2018 대한금속·재료학회지 Vol.56 No.11

The effects of tempering condition on the microstructure and mechanical properties of 30MnB5 hot stamping steel were investigated in this study. Before the tempering, hot-stamped 30MnB5 steel was composed of only α′-martensite microstructure without precipitates. After the tempering at 180 oC for 120 min, nano-sized ε-carbides were precipitated in the α′-martensite laths. After tempering at 250 oC for 60 min, cementite was precipitated along the α′-martensite lath boundaries. The cementite was also observed in the specimens tempered at 350 oC for 30 min and 450 oC for 6 min, respectively. The globular α-ferrite appeared at 350 oC-30 min tempering, and the volume fraction of α-ferrite increased when the tempering temperature was increased. The yield strength increased after tempering, and it reached a peak with the tempering condition of 180 oC-120 min, due to the nano-sized precipitates in the α′- martensite lath. After the tempering, the steel’s ultimate tensile strength (UTS) was decreased due to the reduction in dislocation density and C segregation to lath boundaries. The highest elongation was observed at the 180 oC-120 min tempering condition, due to the reduction of residual stress, and the lack of precipitates along the lath boundaries. The 180 oC-120 min tempering condition was considered to have outstanding crash performance, according to toughness and anti-intrusion calculation results. In drop tower crash tests, the 30MnB5 door impact beam tempered at 180 oC for 120 min showed better crash performance compared to a 22MnB5 door impact beam.

이중조직을 가지는 Ti-6Al-4V 합금의 동적 변형특성에 미치는 템퍼드 마르텐사이트 분율의 영향

이동근,이유환,이종수,이성학 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.6

The effects of volume fraction of tempered martensite on quasi-static tensile and dynamic deformation properties of Ti-6Al-4V alloy with bimodal microstructure were investigated in this study. Five microstructures with various tempered martensite volume fractions were obtained by varying heat-treatment temperatures and times. Dynamic torsional tests were conducted on them by using a torsional Kolsky bar. The test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear band formation. Under a dynamic loading condition, maximum shear stress increased with increasing tempered martensite volume fraction, whereas fracture shear strain decreased. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that a number of voids initiated mainly at a/martensite interfaces, and that the number of voids increased with increasing martensite volume fraction. Adiabatic shear bands of 6-10 lam in width were formed in the specimens having lower martensite volume fractions, while they were not formed in those having higher martensite volume fractions. The possibility of the adiabatic shear band formation was explained by concepts of absorbed deformation energy and void initiation. (Received April 7, 2004)