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Acta Metall Sin  2024, Vol. 60 Issue (6): 789-801    DOI: 10.11900/0412.1961.2022.00297
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Crystallographic Understanding of the Effect of Ni Content on the Hardenability of High-Strength Low-Alloy Steel
SU Shuai1, HAN Peng1,2, YANG Shanwu1, WANG Hua2, JIN Yaohui2, SHANG Chengjia1,2()
1 Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China
2 State Key Laboratory of Metal Material for Marine Equipment and Application, Ansteel Group Corporation, Anshan 114000, China
Cite this article: 

SU Shuai, HAN Peng, YANG Shanwu, WANG Hua, JIN Yaohui, SHANG Chengjia. Crystallographic Understanding of the Effect of Ni Content on the Hardenability of High-Strength Low-Alloy Steel. Acta Metall Sin, 2024, 60(6): 789-801.

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Abstract  

A matrix structure with high strength, such as lath martensite/bainite is created via quenching to achieve conventional high-strength low-alloy ultra-heavy plates. Subsequently, this structure is tempered to improve its toughness. However, it is usually impossible to avoid the low cooling rate in the center of the ultra-heavy plates during cooling, causing inhomogeneous microstructure and mechanical properties along the normal direction. Therefore, it is necessary to enhance the hardenability of the alloy. At lower cooling rates, granular bainite/ferrites are formed in the center of the plates with low hardenability. While this leads to the incompletely transformed martensite/austenite islands (M/A islands), which often cause cracks, fewer high angle grain boundaries (HAGBs) are also formed, which can effectively impede crack propagation. Therefore, improving the strength, toughness, and hardenability is crucial for the development of high-strength low-alloy steel. The addition of nickel can improve the hardenability as well as the toughness of the heavy plates. In this study, two high-strength low-alloy steels with different nickel contents are designed. In addition, the effect of nickel content on hardenability and phase transition temperature is tested using end quenching and thermal mechanical simulation testing. The effects of nickel content on the microstructure and crystallographic characteristics of coherent phase-transformed products are characterized using SEM and EBSD. The results reveal that the increased nickel content greatly improves the hardenability and significantly reduces the phase transition temperature. At a low cooling rate of 0.5oC/s, the microstructure of 2.94Ni steel is lath bainite, and the M/A islands are dispersed on a thin film, forming a phase transformation mode with higher HAGB density, block boundary density and V1/V2 variant pair content, and high hardness. This mode is dominated by the close-packed plane group. While the microstructure of 0.92Ni steel is granular bainite and the M/A islands are distributed in coarse blocks, forming a phase transformation mode with lower HAGB density, block boundary density and V1/V2 variant pair content, and significantly low hardness. Moreover, this mode is dominated by the Bain group. Additionally, the results demonstrate that at the cooling rate of 0.5oC/s, as nickel content increases, the driving force of phase transformation is greatly improved to obtain a higher transformation rate than the steel with low nickel content. The maximum carbon content of untransformed austenite is higher, which promotes the complete transformation of bainite and produces fewer M/A islands. Therefore, this research possesses great potential for the composition design and process control of high-strength low-alloy steel.

Key words:  high-strength low-alloy steel      hardenability      bainite      variant pair      martensite/austenite island     
Received:  15 June 2022     
ZTFLH:  TG142  
Fund: Liaoning Revitalization Talents Program(XLYC1907186)
Corresponding Authors:  SHANG Chengjia, professor, Tel: (010)62332428, E-mail: cjshang@ustb.edu.cn

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2022.00297     OR     https://www.ams.org.cn/EN/Y2024/V60/I6/789

SteelCSi + MnNiCr + MoV + TiBSPFe
0.92Ni0.161.400.921.220.1070.00150.00810.0170Bal.
2.94Ni0.161.412.941.190.1050.00140.00820.0171Bal.
Table 1  Chemical compositions of high-strength low-alloy steel (mass fraction / %)
Fig.1  Hardenability curves of 0.92Ni steel and 2.94Ni steel
Fig.2  Continuous cooling transition (CCT) curves of 0.92Ni steel and 2.94Ni steel (Ms—starting temperature of martensitic transformation, Ts—starting temperature of phase transformation at each cooling rate, Tf—finishing temperature of phase transformation at each cooling rate)
Fig.3  Vickers hardnesses and hardness decrease rates of 0.92Ni steel and 2.94Ni steel at different cooling rates
Fig.4  SEM images (a, b) and band contrast (BC) (c, d) images with grain boundaries of 0.92Ni steel (a, c) and 2.94Ni steel (b, d) at a cooling rate of 0.5℃/s (M/A—martensite/austenite; G1 and G2: representative grains of two steels; white lines indicate low angle grain boundaries (LAGBs): 5° < θ < 15° (θ—misorientation angle), black lines (15° ≤ θ ≤ 45°) and yellow lines (θ > 45°) indicate high angle grain boundaries (HAGBs))
Steel5° < θ < 15°15° ≤ θ ≤ 45°θ > 45°
0.92Ni0.190.050.48
2.94Ni0.220.091.38
Table 2  Grain boundary densities of 0.92Ni steel and 2.94Ni steel at a cooling rate of 0.5oC/s (μm-1)
Fig.5  Inverse pole figures (a, c) and pole figures (b, d) of the representative grain G1 of 0.92Ni steel (a, b) and representative grain G2 of 2.94Ni steel (c, d) at a cooling rate of 0.5oC/s (Black dots: theoretical pole figure based on K-S relationship; color plots: experimental pole figure)
Fig.6  Microstructures of representative grain G1 of 0.92Ni steel (a-c) and representative grain G2 of 2.94Ni steel (d-f) depicted by grain boundary (GB) (a, d), closed-packed plane (CP) group (b, e), and Bain group (c, f)

Variant

Plane parallel

Direction

parallel

Rotation angle/axis from V1

CP

group

Bain

group

Boundary

type

Exact K-S OR0.92Ni steel2.94Ni steel
V1(111) γ //(011) α[1¯01] γ //[1¯1¯1] α--CP1Bain1-
V2[1¯01] γ //[1¯11¯] α60.0°/[111¯]60.360.2Bain 2Block
V3[011¯] γ //[1¯1¯1] α60.0°/[011]59.960.0Bain 3Block
V4[011¯] γ //[1¯11¯] α10.5°/[01¯1¯]5.05.2Bain 1Sub-block
V5[11¯0] γ //[1¯1¯1] α60.0°/[01¯1¯]59.960.0Bain 2Block
V6[11¯0] γ //[1¯11¯] α49.5°/[011]55.254.9Bain 3Block
V7(11¯1) γ //(011) α[101¯] γ //[1¯1¯1] α49.5°/[1¯1¯1]52.351.2CP2Bain 2Packet
V8[101¯] γ //[1¯11¯] α10.5°/[111¯]8.89.9Bain 1Packet
V9[1¯1¯0] γ //[1¯1¯1] α50.5°/[10313]53.152.4Bain 3Packet
V10[1¯1¯0] γ //[1¯11¯] α50.5°/[7¯5¯5]52.051.0Bain 2Packet
V11[011] γ //[1¯1¯1] α14.9°/[13 5 1]12.113.1Bain 1Packet
V12[011] γ //[1¯11¯] α57.2°/[3¯56]57.957.5Bain 3Packet
V13(1¯11) γ //(011) α[01¯1] γ //[1¯1¯1] α14.9°/[513 1]12.113.1CP3Bain 1Packet
V14[01¯1] γ //[1¯11¯] α50.5°/[5¯57¯]52.051.0Bain 3Packet
V15[1¯01¯] γ //[1¯1¯1] α57.2°/[6¯2¯5]57.056.3Bain 2Packet
V16[1¯01¯] γ //[1¯11¯] α20.6°/[1111 6]15.016.2Bain 1Packet
V17[110] γ //[1¯1¯1] α51.7°/[11611]51.851.0Bain 3Packet
V18[110] γ //[1¯11¯] α47.1°/[24 1021]52.451.4Bain 2Packet
V19(111¯) γ //(011) α[1¯10] γ //[1¯1¯1] α50.5°/[3¯ 13 10]53.152.4CP4Bain 3Packet
V20[1¯10] γ //[1¯11¯] α57.2°/[365¯]57.957.5Bain 2Packet
V21[01¯1¯] γ //[1¯1¯1] α20.6°/[301¯]16.718.2Bain 1Packet
V22[01¯1¯] γ //[1¯11¯] α47.1°/[102124]52.451.4Bain 3Packet
V23[101] γ //[1¯1¯1] α57.2°/[2¯5¯6¯]57.056.3Bain 2Packet
V24[101] γ //[1¯11¯] α21.1°/[94¯0]17.118.6Bain 1Packet
Table 3  Misorientation axes and angles between V1 and the other variants calculated from the experimentally determined orientation relationship (actual OR), and the inter-variant boundary characteristics
Fig.7  Densities of inter-variant boundaries in 0.92Ni steel and 2.94Ni steel cooled at 0.5oC/s
SteelBoundary density / μm-1Vickers
BlockSub-blockPacket(θ < 15°)

Packet

(θ > 15°)

hardness

HV

0.92Ni0.340.010.020.12325 ± 11
2.94Ni1.130.020.030.25407 ± 5
Table 4  Boundary densities of blocks, sub-blocks, and packets; and Vickers hardnesses of 0.92Ni steel and 2.94Ni steel cooled at 0.5oC/s
Fig.8  Curves of phase transition driving force (ΔG) versus temperature for 0.92Ni steel and 2.94Ni steel (Black line: phase transition driving force value corresponding to Ms; green line: phase transition driving force value corresponding to the phase transition start temperature when the cooling rate is 0.5oC/s)
Fig.9  Curves of transformation fraction of austenite vs temperature for 0.92Ni steel and 2.94Ni steel cooled at 0.5oC/s and corresponding fitting curves obtained by BiDoseResp function
Fig.10  Curves of transformation rate of austenite vs temperature for 0.92Ni steel and 2.94Ni steel cooled at 0.5oC/s (a) and ΔG corresponding to the temperature of the fastest transition rate (b)
FunctionabTemperature range
ΔGNMγα = a + bT (J·mol-1)-66607900 K > T > 300 K
ΔGNMγα = a + bT (J·mol-1)650-1900 K > T > 620 K
ΔGMγα = a + bT (J·mol-1)00T < 620 K
Table 5  Approximate representations of the free energy components for the γα transformation in pure iron[33]
Alloying elementΔTM / (K·%-1)ΔTNM / (K·%-1)
Si-30
Mn-37.5-39.5
Ni-6-18
Mo-26-17
Cr-19-18
V-44-32
Table 6  Values of ΔTM and ΔTNM for a variety of substitutional solutes[34]
Fig.11  T0' curves of 0.92Ni steel and 2.94Ni steel and corresponding fitting curves obtained by BiDoseResp function (T0'—equilibrium transformation temperature in stored energy of bainite)
Fig.12  Curves of maximum carbon composition of untransformed austenite vs austenite transformation fraction for 0.92Ni steel and 2.94Ni steel cooled at 0.5oC/s
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