Effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region

Abstract

The effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region was investigated using a Gleeble 1500 thermomechanical simulator and a continuous annealing thermomechanical simulator. The results showed that the microstructure consisted of ferrite and bainite as the main phases with a small amount of retained austenite and martensite islands at cooling rate of 5 and 50 °/s, respectively. Fast cooling after continuous annealing affected all constituents of the microstructure. The mechanical properties were improved considerably. Ultimate tensile strength (UTS) increased and total elongation (TEL) decreased with increasing cooling rate in all specimens. The specimen 1 at a cooling rate of 5 °/s exhibited the maximum TEL and UTS×TEL (20% and 27 200 MPa%, respectively) because of the competition between weakening by presence of the retained austenite plus the carbon indigence by carbide precipitation, and strengthening by martensitic islands and precipitation. The maximum UTS and YS (1 450 and 951 MPa, respectively) were obtained for specimen 2 at a cooling rate of 50 °/s. This is attributed to the effect of dispersion strengthening of finer martensite islands and the effect of precipitation strengthening of carbide precipitates.

Keywords

cooling rate / low-carbon high-strength steel / intercritical annealing / microstructure / mechanical properties

References

[1] Ahmad E, Manzoor T, Ziai M M A Effect of Martensite Morphology on Tensile Deformation of Dual-Phase Steel [J]. J. Mater. Eng. Perform., 2012, 21(3): 382-387.

[2] Rocha R O, Melo T M F, Pereloma E V Microstructural Evolution at the Initial Stages of Continuous Annealing of Cold Rolled Dual-Phase Steel [J]. Metall. Mater. Trans. A, 2005, 391(1–2): 296-304.

[3] Bakhtiari R, Ekrami A The Effect of Bainite Morphology on the Mechanical Properties of a High Bainite Dual Phase (HBDP) Steel [J]. Mater. Sci. Eng. A, 2009, 525(1–2): 159-165.

[4] Kumar A, Singh S B, Ray K K Influence of Bainite/Martensite-Content on the Tensile Properties of Low Carbon Dual-Phase Steels [J]. Mater. Sci. Eng. A, 2008, 474(1–2): 270-282.

[5] Akbarpour M R Effect of Ferrite Volume Fraction on Work Hardening Behavior of High Bainite Dual Phase (DP) Steels [J]. Metall. Mater. Trans. A, 2008, 477(1–2): 306-310.

[6] Mohanty R R, Girina O A, Fonstein N M Effect of Heating Rate on the Austenite Formation in Low-Carbon High-Strength Steels Annealed in the Intercritical Region [J]. Metall. Mater. Trans. A, 2011, 42(12): 3 680-3 690.

[7] Meyer M D, Vanderschueren D, Cooman B C D, . The Influence of the Substitution of Si by Al on the Properties of Cold Rolled C-Mn-Si TRIP Steels [J]. ISIJ Int., 1999, 39(8): 813-822.

[8] Ryu H B, Speer J G Effect of Thermomechanical Processing on the Retained Austenite Content in a Si-Mn Transformation-Induced-Plasticity [J]. Metall. Trans., 2002, 33A(9): 2 811-2 816.

[9] Headley T J, Brooks J A A New Bcc-Fcc Orientation Relationship Observed between Ferrite and Austenite in Solidification Structures of Steels [J]. Metall. Mater. Trans. A, 2002, 33(1): 5-15.

[10] Calcagnotto M, Ponge D, Raabe D On the Effect of Manganese on Grain Size Stability and Hardenability in Ultrafine-Grained Ferrite/Martensite Dual-Phase Steels [J]. Metall. Mater. Trans. A, 2012, 43(1): 37-46.

[11] Suh D W, Park S J, Lee T H, . Influence of Al on the Microstructural Evolution and Mechanical Behavior of Low-Carbon, Manganese Transformation-Induced-Plasticity Steel [J]. Metall. Mater. Trans. A, 2010, 41(2): 397-408.

[12] Colla V, Desanctis M, Dimatteo A, . Prediction of Continuous Cooling Transformation Diagrams for Dual-Phase Steels from the Intercritical Region[J]. Metall. Mater. Trans. A, 2011, 42(9): 2 781-2 793.

[13] Sakuma Y, Matsumura O, Takechi H Mechanical Properties and Retained Austenite on Intercritically Heat-Treated Bainite Transformed Steel and Their Variation with Si and Mn Additions[J]. Metall. Trans. A, 1991, 22(2): 489-498.

[14] Mahieu J, Maki J, De C B C, . Phase Transformation and Mechanical Properties of Si-Free CMnAl Transformation-Induced Plasticity-Aided Steel [J]. Metall. Trans. A, 2002, 33(8): 2 573-2 580.

[15] Okamoto R, Borgenstam A, Ågren J Interphase Precipitation in Niobium-Microalloyed Steels[J]. Acta Mater., 2010, 58(14): 4 783-4 790.

[16] Sugimoto K I, Misu M, Kobayashi M, . Effects of Second Phase Morphology on Retained Austenite Morphology and Tensile Properties in a TRIP-Aided Dual-Phase Steel Sheet [J]. ISIJ Int., 1993, 33(7): 775-782.

[17] Sun S, Pugh M Properties of Thermomechanically Processed Dual-Phase Steels Containing Fibrous Martensite [J]. Mater. Sci. Eng. A, 2002, 335(1–2): 298-308.

[18] Timokhina I B, Hodgson P D, Pereloma E V Effect of Deformation Schedule on the Microstructure and Mechanical Properties of a Thermomechanically Processed C-Mn-Si Transformation-Induced Plasticity Steel [J]. Metall. Mater. Trans. A, 2003, 34(8): 1 599-1 609.