The effect of finish rolling temperature and tempering on the microstructure, mechanical properties and dislocation density of direct-quenched steel
- University of Oulu, Centre for Advanced Steels Research, PL 8000, 90014 Oulu (Finland)
- SSAB Europe Oy, Rautaruukintie 155, 92100 Raahe (Finland)
- Department of Materials Science and Engineering, National Taiwan University. No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan, ROC (China)
Highlights: • The effect of thermomechanical treatment and tempering on direct-quenched steel has been studied. • Dislocation density of direct-quenched and tempered martensite is evaluated using both TEM and Williamson-Hall method. • Statistical analysis has been performed to study the effect of different factors on toughness of direct-quenched martensite. - Abstract: A unique batch tempering treatment for industrial scale direct-quenched steel coils has been studied using laboratory simulations. The tempering treatment was non-isothermal with slow heating to 570 °C and slow cooling to simulate the tempering of large steel coils. The paper presents the effect of finishing rolling temperature (FRT) relative to the non-recrystallization temperature (T{sub NR}) and the effect of long time tempering on the microstructure, dislocation density and mechanical properties of direct-quenched coiled strips. Conditioning austenite below the recrystallization stop temperature resulted in a finer effective grain size distribution, which correlated strongly with the impact toughness of the final product. Furthermore low finish rolling temperature resulted in partially ferritic microstructures while higher finishing rolling temperatures led to mixtures of bainite and martensite. Dislocation densities determined with TEM and XRD showed somewhat different trends regarding the effect of tempering: intra-lath dislocation density, as measured with TEM, showed a statistically significant drop in only one case, while XRD analysis indicated a drop in all cases. Furthermore, no significant correlation between finishing rolling temperature and dislocation density existed in XRD studies. The XRD results indicate that the decrease in dislocation density corresponds to about 100 MPa lower dislocation strengthening. However, precipitation hardening and potential internal micro stress relief compensates this as yield strength remains unchanged or even increases during tempering.
- OSTI ID:
- 22804961
- Journal Information:
- Materials Characterization, Vol. 139; Other Information: Copyright (c) 2017 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA); ISSN 1044-5803
- Country of Publication:
- United States
- Language:
- English
Similar Records
Correlation Between Structure and Properties of Low-Carbon Cu-Ni-Mo-Ti-Nb Ultrahigh-Strength Steel
Structure and mechanical properties of Fe--Cr--Mo--C alloys with and without boron