Microstructure and Mechanical Properties of HSLA Steel Containing 1.4%Cu
DU Yubin1,2, HU Xiaofeng1, ZHANG Shouqing1,2, SONG Yuanyuan1, JIANG Haichang1, RONG Lijian1()
1 CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
Cite this article:
DU Yubin, HU Xiaofeng, ZHANG Shouqing, SONG Yuanyuan, JIANG Haichang, RONG Lijian. Microstructure and Mechanical Properties of HSLA Steel Containing 1.4%Cu. Acta Metall Sin, 2020, 56(10): 1343-1354.
The Cu bearing high strength low alloy (HSLA) steels exhibit high-strength, high toughness and good weldability, which have been widely used in shipbuilding, offshore structures etc. Due to the extremely poor impact energy when attained peak strength, the Cu bearing HSLA steels are usually used at overaged state, which have a good combination of impact energy and strength. In order to clarify the effect of Cu on mechanical properties especially on the impact energy for HSLA steels at peak ageing state, two HSLA steels without Cu (0Cu) and with 1.4%Cu (1.4Cu), were prepared by vacuum induction melting in this study. The influence of Cu on the microstructure of HSLA steel was investigated by OM, SEM and EBSD. Meanwhile, the Cu-riched clusters were characterized by APT and the mechanical properties were measured by tensile test and impact test. The results show that the Cu is completely solid-solutioned into the matrix after quenching, and there are a great number of Cu-riched clusters precipitated in the matrix and boundaries after tempering. Cu element has no obvious effect on the prior austenite grain size, microstructure and effective grain size of tempered HSLA steel, but has significant influence on the strength and impact energy for tempered HSLA steel. After tempered at 450 ℃, the 1.4Cu steel attained the maximum yield strength (1053 MPa), higher than that of 0Cu steel. It is worth noting that the impact energy of 1.4Cu steel tempered at 450 ℃ is only 24 J at room temperature and the impact fracture is a quasi-cleavage brittle fracture mode dominated by river patterns. However, 0Cu steel exhibits a completely ductile fracture mode dominated by dimples at room temperature and the impact energy is 127 J. The APT results show that both 0Cu and 1.4Cu tempered steels have the segregation of C, Cr, Ni, Mn elements at the lath boundary. Compared with 0Cu steel, there precipitate a great number of Cu-riched clusters at the lath boundary for 1.4Cu steel, which will result in the stress concentration and then promote the crack initiation at the lath boundary. In addition, the Cu-rich clusters precipitated at the lath boundary could prevent the Mo segregated at the lath boundary, which will decrease the bonding energy and then promote the crack propagation along the lath boundary. Besides, the negative effect of strengthening due to the Cu-riched clusters at matrix will also accelerate the crack propagation in the matrix, which will decrease the impact energy of 1.4Cu steel. Therefore, the impact energy of 1.4Cu steel is much lower than that of 0Cu steel at room temperature.
Fund: National Key Research and Development Program of China(2016YFB0300601);National Key Research and Development Program of China(2017YFB1201302);Liaoning Revitalization Talents Program(XLYC1907143)
Fig.1 Yield strengths (a) and elongations (b) of 0Cu and 1.4Cu steels tempered at different temperatures (Q—as quenching)
Fig.2 The temperature dependent impact energy curves of 0Cu and 1.4Cu steels tempered at 450 ℃ (RT—room temperature)
Fig.3 SEM images of prior austenite grains (a, b) and microstructures (c, d) of 0Cu (a, c) and 1.4Cu (b, d) steels tempered at 450 ℃
Fig.4 Misorientation distributions of grain boundaries (a, b) and misorientation angle distribution maps (c, d) of 0Cu (a, c) and 1.4Cu (b, d) steels tempered at 450 ℃ (The black lines and red lines denote the high angle grain boundaries (misorientation angle>15°) and low angle grain boundaries (misorientation angle 2°~15°), respectively) Color online
Fig.5 The Cu-riched clusters (a), C element distribution (b), Cu element distributions at matrix (c) and interface (d), and the proxigram composition profiles of 5%Cu (atomic fraction) isoconcentration surfaces at matrix (e) and interface (f) in APT 3D reconstruction for 1.4Cu steel tempered at 450 ℃ Color online
Fig.6 C atoms distribution maps (a, b) and one-dimensional (1-D) composition profiles, obtained with a diameter of 30 nm cylindrical area, across the interfaces delineated with C atoms in an APT 3D reconstruction (c, d) of 0Cu (a, c) and 1.4Cu (b, d) steels tempered at 450 ℃ Color online
Fig.7 The distributions of Mo (a), Cu (b) elements at the interface and the 1-D composition profiles of Cu-riched cluster at the interface (c) for 1.4Cu steel tempered at 450 ℃ Color online
Fig.8 Fractographs of 0Cu (a) and 1.4Cu (b) steels tempered at 450 ℃ for 120 min after tensile test at room temperature
Fig.9 Fractographs of 0Cu (a) and 1.4Cu (b) steels tempered at 450 ℃ for 120 min after impact test at room temperature
Fig.10 SEM image of the area adjacent to the fracture (a), the strain distribution map (b) and the schematics illustrating the crack propagation initiated at lath boundary (c) of 1.4Cu steel tempered at 450 ℃ for 120 min after Charpy impact test Color online
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