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김새암 ( Sea Arm Kim ),이상우 ( Sang Woo Lee ) 한국열처리공학회 2003 熱處理工學會誌 Vol.16 No.2
N/A As a research for developing fine-grained high strength low carbon steels, the effects of austenitization temperature and hot deformation on microstructure was investigated in 0.15 wt% carbon steels with microalloying elements such as Nb and Ti. When the steels were reheated at 1250℃, Nb containing steel showed very coarse austenite grain size of 200㎛ whereas Nb-Ti steel did fine one of 70㎛ because Ti carbonitrides could suppress the austenite grain growth. In case of 50% reduction at 850℃, the austenite grains in the Nb steel partially recrystallized while those in the Nb-Ti steel fully recrystallized probably due to finer prior austenite grains. For the Nb-Ti steel, ferrite grain size was not sensitively changed with austenitization temperature and compression strain and, severe deformation of 80% reduction was not essentially necessary to refine ferrite grains to about 3㎛ which could be obtained through lighter deformation of 40% reduction.
Solidification Structure Refining of 409L Ferritic Stainless Steel Using Fe-Ti-N Master Alloy
Chao Wang,Haiyan Gao,Yongbing Dai,Jun Wang,Baode Sun 대한금속·재료학회 2012 METALS AND MATERIALS International Vol.18 No.1
A novel in-situ synthesized Fe-Ti-N master alloy was employed as a grain refiner for refining of the 409L ferritic stainless steel solidification structure. Two groups of experiments have been carried out to test the grain refinement performance of the Fe-Ti-N master alloy. In one group, 2 wt% of the Fe-Ti-N master alloy has been added into 409L ferritic stainless steel melts with various temperatures just before casting. In the other group, 409L ferritic stainless steel melts with 2 wt% Fe-Ti-N master alloy added at 1803K were held for various times before casting. It was found that high melt temperature resulted in a lower proportion of equiaxed grains and coarser structures. As the holding time increased, the proportion of the equiaxed grain zone decreased quickly. However, the proportion of the equiaxed grain zone re-increased when the holding time was extended longer than extreme points. The mechanisms of these experimental phenomena have been analyzed in terms of thermodynamics.
Bowen Zhou,Guangqiang Li,Xiangliang Wan,Yu Li,Kaiming Wu 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.2
The effect of Zr-Ti combined deoxidation on the grain refinement in the simulated coarse-grained heat-affected zone of a high-strength low-alloy steel was investigated by means of analytical characterization techniques such as in-situ microscopy, transmission electron microscopy, and electron backscattered diffraction analysis. Owing to the Zr-Ti combined deoxidation, a large amount of fine Zr-Ti oxide particles were formed in the steel and retarded the austenite grain growth during simulated welding thermal cycle. The austenite grains were small and uniform. The Mn can diffuse spontaneously from austenite to Zr-Ti oxide inclusion and MnS precipitated on ZrO2, which can form Mn depleted zone in the vicinity of inclusion. The acicular ferrite grains nucleated on intragranular Zr-Ti oxide inclusions in austenite grains grew in different directions and effectively divided the austenite grain into several finer and separate regions at intermediate temperature. The crystallographic grain size became small in the simulated coarse-grained heat-affected zone of Zr-Ti-killed steel due to the effective pinning effect by Zr-Ti oxide particles and acicular ferrite formation.
He, Yinsheng,Yang, Cheol-Woong,Lee, Je-Hyun,Shin, Keesam Korean Society of Microscopy 2015 Applied microscopy Vol.20 No.1
In this work, various electron microscopy and analysis techniques were used to investigate the microstructural evolution of a 9% Cr tempered martensite ferritic (TMF) steel T91 upon ultrasonic nanocrystalline surface modification (UNSM) treatment. The micro-dimpled surface was analyzed by scanning electron microscopy. The characteristics of plastic deformation and gradient microstructure of the UNSM treated specimens were clearly revealed by crystal orientation mapping of electron backscatter diffraction (EBSD), with flexible use of the inverse pole figure, image quality, and grain boundary misorientation images. Transmission electron microscope (TEM) observation of the specimens at different depths showed the formation of dislocations, dense dislocation walls, subgrains, and grains in the lower, middle, upper, and top layers of the treated specimens. Refinement of the $M_{23}C_6$ precipitates was also observed, the size and the number density of which were found to decrease as depth from the top surface decreased. The complex microstructure and microstructural evolution of the TMF steel samples upon the UNSM treatment were well-characterized by combined use of EBSD and TEM techniques.
A. Zargaran,김한수,곽재현,김낙준 대한금속·재료학회 2015 METALS AND MATERIALS International Vol.21 No.1
Microstructure and tensile properties of ferritic Fe-8Al-5Mn-0.1Nb lightweight steels with different C contents(0.005, 0.02, and 0.05 wt%) have been investigated in the present study. It shows that the microstructurebecomes more elongated along the rolling direction, i.e., increasing propensity towards unrecrystallizationwith an increase in C content. This is mainly due to the effect of NbC on retarding the dynamic recrystallizationof ferrite during hot rolling, which is active for higher C (0.02C and 0.05C) containing alloys. Inthe case of the 0.05C alloy, there is an additional precipitation of κ-carbide particles, which also retard thedynamic recrystallization of ferrite during hot rolling, resulting in a much more elongated structure in the 0.05Ccontaining alloy than in the 0.02C alloy in as-hot rolled condition. Although κ-carbide particles retard thedynamic recrystallization of ferrite during hot rolling, they play an opposite role during final annealing,i.e., promoting static recrystallization by the operation of particle-stimulated nucleation mechanism, resulting inthe development of homogeneously distributed fine grains in the 0.05C alloy. As a result, the 0.05C alloy showshigher strength and larger elongation than the lower C containing alloys.
Dynamic Strain-Induced Ferrite Transformation during Hot Compression of Low Carbon Si-Mn Steels
Cai, Ming-Hui,Ding, Hua,Lee, Young-Kook The Japan Institute of Metals 2011 Materials transactions Vol.52 No.9
<P>The dynamic strain-induced transformation (DSIT) of austenite to ferrite was investigated under different undercooling conditions using three low carbon Si-Mn steels. The undercooling of austenite (Δ<I>T</I>) was controlled by varying the cooling rate between austenitization and deformation temperatures. Uniform DSIT ferrite grains (∼2.3 μm) were produced at a relatively high deformation temperature above 840°C using a low carbon high Si steel (0.077C-0.97Mn-1.35Si, mass%) in connection with a larger Δ<I>T</I>. The critical conditions for DSIT were determined based on the flow stress-strain curves measured during hot compression tests. Influence of deformation temperature on DSIT of low carbon Si-added steel was also discussed.</P>
Microstructure and Tensile Properties of Ferritic Lightweight Steel Produced by Twin-Roll Casting
Yunik Kwon,Ji Hyun Hwang,Hee Chae Choi,T. T. T. Trang,Byoungkoo Kim,A. Zargaran,Nack J. Kim 대한금속·재료학회 2020 METALS AND MATERIALS International Vol.26 No.1
Ferritic Fe–8Al–5Mn–0.1Nb–0.1C lightweight steel has been produced by twin-roll casting (TRC) process. The microstructures,in as-cast condition and during subsequent thermomechanical treatments, and tensile properties of the steel have beeninvestigated and compared to those of the similar steel produced with conventional ingot casting (IC). TRC significantly refinesthe microstructure of the steel in as-cast condition, with a formation of NbC and κ-carbide particles within matrix and alonggrain boundaries. NbC particles inhibit the grain coarsening during subsequent homogenization treatment, while κ-carbideparticles promote recrystallization during final annealing by particle-stimulated nucleation mechanism. Tensile properties ofthe TRC steel are comparable to those of the IC steel; however, such properties of the TRC steel have been achieved under amuch smaller number of processing steps, making TRC as an ideal alternative for the production of ferritic lightweight steels.