Effects of Fine Precipitates on Austenite Grain Refinement of Micro-alloyed Steel during Cyclic Heat Treatment

Saito, Genya; Sakaguchi, Norihito; Ohno, Munekazu; Matsuura, Kiyotaka; Takeuchi, Misaichi; Sano, Taichi; Minoguchi, Koki; Yamaoka, Takuya

doi:10.2355/isijinternational.isijint-2019-153

Cited by 14 publications

(7 citation statements)

References 24 publications

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“…The volume fraction of the precipitates was measured by quantifying the sample thickness of each observed area using electron energy loss spectroscopy. 22) The precipitate was observed using the extraction replica method. 23) The steel was electrically etched in an acetylacetone (10%) and tetramethylammonium (1%) methanol solution, and the exposed precipitates on the sample surface were transferred into a replica film.…”

Section: Methodsmentioning

confidence: 99%

Effects of Cooling Rate after Hot Forging on Precipitation of Fine Particles during Subsequent Normalizing and Austenite Grain Growth during Carburization of Al- and Nb-microalloyed Case-hardening Steel

et al. 2021

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This study deals with austenite grain growth during high-temperature carburization of an Al-and Nb-microalloyed case-hardening steel. The grain size after carburization-simulated heating for 5 h at 1 050°C decreased with the increase in the cooling rate from hot forging-simulated heating for 1 h at 1 250°C. The increase in cooling rate led to the decreases in the volume fractions and sizes of AlN and Nb(C,N) particles precipitated during cooling, and AlN disappeared when the cooling rate increased to 16°C/min, while Nb(C,N) still slightly exited at 16°C/min. Because of oversaturation caused by cooling within a finite time, further precipitation occurred during the subsequent normalization for 3 h at 1 070°C, resulting in the formation of AlN-Nb(C,N) combined particles. When the cooling rate increased, the volume fraction and number density of these combined particles increased while their size decreased. Therefore, a higher cooling rate causes a larger pinning effect on grain growth during carburization; thus grain size after carburization decreased with the increase in cooling rate. Transmission electron microscopy confirmed the formation of a coherent AlN-Nb(C,N) interface due to good lattice matching between the crystal planes of AlN (1120) and Nb(C,N) (220). This led to the preferential nucleation of AlN on the Nb(C,N) particles, thereby forming stable AlN-Nb(C,N) particles.

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Section: Methodsmentioning

confidence: 99%

Effects of Cooling Rate after Hot Forging on Precipitation of Fine Particles during Subsequent Normalizing and Austenite Grain Growth during Carburization of Al- and Nb-microalloyed Case-hardening Steel

et al. 2021

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“…To measure the volume fraction of the precipitates, the sample thickness at each observed area was quantitatively evaluated via electron energy loss spectroscopy (EELS). 22) Large precipitates with a size of more than 100 nm were observed using the scanning electron microscopy technique (SEM, ProX, Phenom-World B.V.) for each sample. Thermodynamic calculations were performed using the Thermo-Calc software (version 2018b), using the TCFE9 database.…”

Section: Methodsmentioning

confidence: 99%

Effects of Concentrations of Micro-alloying Elements and Hot-forging Temperature on Austenite Grain Structure Formed during Carburization of Case-hardening Steel

et al. 2020

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Effects of fine precipitates on the austenite (γ) grain structures were investigated in JIS SCM420-based case-hardening steels with several different concentrations of the micro-alloying elements and hot-forging temperatures. Micro-alloyed steels of 18Al (0.018 mass% Al) and 35Al-32Nb (0.035 mass% Al, 0.032 mass% Nb) were forging-simulated at 1 150°C or 1 250°C, normalized at 1 070°C, and carburized at 1 050°C. When the as-received 18Al steel was normalized and carburized without forging-simulated heating, a uniform γ grain structure was observed with the distribution of fine AlN precipitates. However, coarsening of AlN occurred when the forging-simulated temperature was 1 150°C and it caused abnormal grain growth during carburization. In 35Al-32Nb steel, the same heating did not induce the abnormal grain growth owing to the AlN-Nb(C,N) combined particles. The size of these particles increase with an increase in the forging-simulated temperature. The high forging-simulated temperature caused the dissolution of the fine precipitates, followed by reformation and coarsening of the precipitates during the subsequent cooling and the normalization heating, which resulted in a decreased pinning force and γ grain coarsening. Furthermore, TEM observations revealed that a considerable amount of Nb(C,N) particles exist near large eutectic MnS particles. Thermodynamic calculations based on the Scheil's condition showed that the formation of these Nb(C,N) particles was due to segregation during solidification. It was suggested that such local concentration of the precipitate particles in the last solidifying region leads to ununiform distribution of the pinning force that may induce the abnormal grain growth.

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“…As it was difficult to distinguish the Nb(C, N) particles from the Fe matrix using high-angle annular dark field (HAADF) imaging, energy dispersive spectroscopy (EDS) mapping was used to analyze the precipitates. The volume fraction of the precipitates was measured by quantifying the sample thickness of the observed area using electron energy loss spectroscopy 20) .…”

Section: Effects Of the Normalizing Temperaturementioning

confidence: 99%

Effects of Normalizing Temperature on the Precipitation of Fine Particles and Austenite Grain Growth during Carburization of Al- and Nb-Microalloyed Case-Hardening Steel

et al. 2023

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This paper deals with the precipitation behaviors of AlN and Nb(C, N) particles during normalizing of hot-forged Al-and Nb-microalloyed case-hardening steel, and investigates the effects of the number density and volume fraction of these particles on the grain growth behavior of austenite (γ) during the subsequent high-temperature carburization. When the sample was cooled from the hot forging temperature at 16 °C/min, AlN did not precipitate at all and was completely supersaturated in the matrix, although fine Nb(C, N) particles precipitated during cooling. When reheated to the normalizing temperature, AlN particles started precipitation at approximately 620-640 °C on the grain boundaries of the bainitic ferrite matrix and the interfaces between the cementite particles and the matrix, and the AlN particles contained Cr and Mn, and had an NaCl structure and Bain orientation relationship with the matrix. The AlN structure changed from NaCl type into wurtzite type at approximately 800 °C. AlN particles grew during reheating to the normalizing temperature and holding at the temperature. Both the number density and volume fraction of the AlN particles were dramatically increased by reducing the normalizing temperature from 1070 to 900 °C. The distribution of these particles strongly affected the  grain structure formed during carburization. As the normalizing temperature was reduced from 1070 to 950 and 900 °C, the  grain structure became finer. Some abnormally large grains were formed near the carburized surface, when normalized at 1070 °C. However, no abnormal grain was found when normalized below 950 °C.

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Effects of Fine Precipitates on Austenite Grain Refinement of Micro-alloyed Steel during Cyclic Heat Treatment

Cited by 14 publications

References 24 publications

Effects of Cooling Rate after Hot Forging on Precipitation of Fine Particles during Subsequent Normalizing and Austenite Grain Growth during Carburization of Al- and Nb-microalloyed Case-hardening Steel

Effects of Cooling Rate after Hot Forging on Precipitation of Fine Particles during Subsequent Normalizing and Austenite Grain Growth during Carburization of Al- and Nb-microalloyed Case-hardening Steel

Effects of Concentrations of Micro-alloying Elements and Hot-forging Temperature on Austenite Grain Structure Formed during Carburization of Case-hardening Steel

Effects of Normalizing Temperature on the Precipitation of Fine Particles and Austenite Grain Growth during Carburization of Al- and Nb-Microalloyed Case-Hardening Steel

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