High Strength-Ductility Nb-microalloyed Low Martensitic Carbon Steel: Novel Process and Mechanism

doi:10.1007/s40195-015-0321-x

Abstract

Abstract:

A low-carbon Nb-microalloyed Fe-Mn-Si-based steel was treated by a novel quenching-partitioning-tempering (Q-P-T) process as a modified quenching and partitioning (Q&P) process. After processed by Q-P-T treatment, this steel exhibits excellent mechanical properties such as high product of strength and elongation. The addition of Nb markedly raises both the yield strength and tensile strength of Q-P-T martensitic steel, especially the yield strength, which can be attributed to the strong grain refinement strengthening and precipitation strengthening of Nb. The Nb addition can also lead to a little increase in ductility. The Nb-microalloyed steel treated by Q-P-T process displays much higher ductility than that treated by traditional quenching and tempering (Q&T) process. The mechanisms of Q-P-T process on ductility enhancement were fully analyzed and can be attributed to high quenching temperature and considerable amount of retained austenite.

Key words: Heat treatment, Mechanical properties, Microstructure, Microalloyed steel, Martensitic transformation

Ke Zhang, Ping Liu, Wei Li, Zheng-Hong Guo, Yong-Hua Rong. High Strength-Ductility Nb-microalloyed Low Martensitic Carbon Steel: Novel Process and Mechanism[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(10): 1264-1271.

Figures/Tables 8

Table 1 Chemical compositions of experimental steels (in wt pct)

Steel	C	Mn	Si	Nb	Fe	Ac₃ (°C)	M_s (°C)	M_f (°C)
Nb00	0.2	1.54	1.58	-	Bal.	904 ± 5	415 ± 5	173 ± 3
Nb03	0.19	1.52	1.57	0.029	Bal.	911 ± 5	395 ± 5	171 ± 3

Fig. 1 Schematic heat treatment diagram of the Q-P-T, Q&P and Q&T processes, where the abbreviation of M.S.Q. and W.Q. represent molten salt quenching and water quenching, respectively

Fig. 2 Tensile engineering stress and strain curves of the Nb00 steel and Nb03 steel treated by Q-P-T, Q&P and Q&T processes

Table 2 Mechanical properties of the steels treated by Q&T, Q&P and Q-P-T processes

Steel	Process	σ_0.2 (MPa)	σ_b (MPa)	δ_u (%)	δ_b (%)	σ_b × δ_b (GPa%)
Nb00	Q&T	1149 ± 7	1266 ± 9	3.8 ± 0.3	11.7 ± 0.8	14.8
Nb03	Q&T	1227 ± 13	1337 ± 14	4.0 ± 0.1	12.2 ± 0.5	16.3
Nb00	Q&P	1094 ± 17	1236 ± 9	5.4 ± 0.8	13.9 ± 0.8	17.2
Nb03	Q-P-T	1146 ± 15	1247 ± 15	6.0 ± 0.3	15.0 ± 0.5	18.7

Fig. 3 XRD patterns of the Nb00 steel and Nb03 steel treated by Q-P-T, Q&P and Q&T processes

Fig. 4 TEM micrographs of the Nb00 steel treated by Q&T and Q&P processes, respectively: a Q&T, bright-field image; b Q&T, dark-field image and \(\bar{1}\bar{1}1_{\gamma }\) and \(1\bar{1}1_{\gamma }\) diffraction spots of retained austenite; c Q&P, bright-field images; d Q&P, dark-field image and \(\bar{1}\bar{1}1_{\gamma }\) and \(1\bar{1}1_{\gamma }\) diffraction spots of retained austenite

Fig. 5 TEM micrographs of the Nb03 steel treated by Q&T and Q&P processes: a Q&T, bright-field image; b Q&T, dark-field image and \(11\bar{1}_{\text{NbC}}\) diffraction spots of retained austenite; c Q&P, bright-field images; d Q&P, dark-field image and \(11\bar{1}_{\text{NbC}}\) diffraction spots of retained austenite

Fig. 6 SEM fractographs of the Nb03 steel treated by Q&T a, Q-P-T b processes

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