Microstructure, Mechanical Properties, and Strengthening Mechanism of Cr-Mo Microalloy Cold Heading Steel
CHEN Jilin1,2,3(), FENG Guanghong1, MA Honglei2, YANG Dong2, LIU Wei2
1.Metallurgical Technology Institute, Central Iron and Steel Research Institute, Beijing 100081, China 2.Xingtai Iron and Steel Co., Ltd., Xingtai 054027, China 3.Hebei Engineering Research Center for Wire Rod, Xingtai 054027, China
Cite this article:
CHEN Jilin, FENG Guanghong, MA Honglei, YANG Dong, LIU Wei. Microstructure, Mechanical Properties, and Strengthening Mechanism of Cr-Mo Microalloy Cold Heading Steel. Acta Metall Sin, 2022, 58(9): 1189-1198.
Lightweight and safety of automobiles is the development trend of high-strength automobile fasteners, which is conducive to saving resources and protecting the environment. High-strength fasteners connect parts of engine, and their strength affects the overall life of the engine, thereby affecting the safety performance of the vehicle. Presently, high-strength fastener steels for cars are mainly 35CrMo, and the fastener strength reaches 10.9/12.9 level, which must fulfill sufficient delayed fracture and fatigue properties. Therefore, it is important to develop cold heading steel with high fatigue and strong plasticity matching. In this study, the microstructure, mechanical properties, and strengthening mechanism of Cr-Mo microalloyed cold heading steel, under different thermomechanical control processes (TMCPs), were investigated using OM, SEM, and TEM. The results show that the TMCP parameters affect the structure and mechanical properties of the experimental steel. With an increase in the finish rolling temperature and acceleration of the cooling rate, the ferrite-pearlite composite structure in the steel gradually changes to bainitic, dislocation density gradually increases, tensile strength monotonously increases, and elongation fluctuates. When the finish rolling temperature is 935oC, the microstructure is mainly uniformly distributed bainite phase, which is in the form of short rod and granular, and there is dislocation entanglement. The experimental results show that this process has the best strength and toughness matching. Its tensile strength and elongation reach 925 MPa and 20%, respectively, and the hardness at 7 mm from the quenched end (J7) is 53.1 HRC. When the finish rolling temperature is 900oC, grain refinement strengthening is the main strengthening mechanism, accounting for 31%-36% of the yield strength; when the finish rolling temperature is more than 935oC, dislocation strengthening is the main strengthening mechanism, accounting for the total strength of 35%-38%. The hardenability results show that the hardenability of the experimental steel is unaffected by the microstructure and mechanical properties, and it maintains high-quality hardenability. In addition, a model of the end quenching curve of the Cr-Mo microalloyed steel is established to predict hardenability.
Table 1 Rolling technology of Cr-Mo microalloyed steels
Fig.1 OM (a, c, e, g) and SEM (b, d, f, h) images of Cr-Mo microalloyed steels under processes 1 (a, b), 2 (c, d), 3 (e, f), and 4 (g, h)
Fig.2 TEM images of Cr-Mo microalloyed steels under processes 1 (a), 2 (b), 3 (c), and 4 (d)
Fig.3 TEM images of dislocation (a) and twin (b) in Cr-Mo microalloyed steels
Fig.4 Tensile engineering stress-strain curves of Cr-Mo microalloyed steels at room temperature
Process
Tensile strength
Elongation
Reduction of
MPa
%
area / %
1
713
26.5
62
2
845
18.5
51
3
925
20.0
56
4
979
14.5
47
Table 2 Mechanical properties of Cr-Mo microalloyed steels at room temperature
Fig.5 Tensile fracture SEM images of Cr-Mo microalloyed steel at room temperature (process 2) (a) macroscopic feature (b) shear lip zone (c) edge of fiber zone (d) center of fiber zone (e) fiber zone
Fig.6 Hardenability curves of the Cr-Mo microalloyed steel by Jominy method
Fig.7 OM image of Cr-Mo microalloyed steel 7 mm from quenching end
Process
σ0
σs
σG
σDis
σy
1
48
134
153
86
421
2
48
134
138
113
433
3
48
134
130
169
481
4
48
134
126
191
499
Table 3 Calculated values of yield strength (σy) and its component of Cr-Mo microalloyed steels
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