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.
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.
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))
Steel
5° < θ < 15°
15° ≤ θ ≤ 45°
θ > 45°
0.92Ni
0.19
0.05
0.48
2.94Ni
0.22
0.09
1.38
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 OR
0.92Ni steel
2.94Ni steel
V1
(111) γ //(011) α
[01] γ //[1] α
-
-
CP1
Bain1
-
V2
[01] γ //[1] α
60.0°/[11]
60.3
60.2
Bain 2
Block
V3
[01] γ //[1] α
60.0°/[011]
59.9
60.0
Bain 3
Block
V4
[01] γ //[1] α
10.5°/[0]
5.0
5.2
Bain 1
Sub-block
V5
[10] γ //[1] α
60.0°/[0]
59.9
60.0
Bain 2
Block
V6
[10] γ //[1] α
49.5°/[011]
55.2
54.9
Bain 3
Block
V7
(11) γ //(011) α
[10] γ //[1] α
49.5°/[1]
52.3
51.2
CP2
Bain 2
Packet
V8
[10] γ //[1] α
10.5°/[11]
8.8
9.9
Bain 1
Packet
V9
[0] γ //[1] α
50.5°/[10313]
53.1
52.4
Bain 3
Packet
V10
[0] γ //[1] α
50.5°/[5]
52.0
51.0
Bain 2
Packet
V11
[011] γ //[1] α
14.9°/[13 5 1]
12.1
13.1
Bain 1
Packet
V12
[011] γ //[1] α
57.2°/[56]
57.9
57.5
Bain 3
Packet
V13
(11) γ //(011) α
[01] γ //[1] α
14.9°/[513 1]
12.1
13.1
CP3
Bain 1
Packet
V14
[01] γ //[1] α
50.5°/[5]
52.0
51.0
Bain 3
Packet
V15
[0] γ //[1] α
57.2°/[5]
57.0
56.3
Bain 2
Packet
V16
[0] γ //[1] α
20.6°/[1111 6]
15.0
16.2
Bain 1
Packet
V17
[110] γ //[1] α
51.7°/[11611]
51.8
51.0
Bain 3
Packet
V18
[110] γ //[1] α
47.1°/[24 1021]
52.4
51.4
Bain 2
Packet
V19
(11) γ //(011) α
[10] γ //[1] α
50.5°/[ 13 10]
53.1
52.4
CP4
Bain 3
Packet
V20
[10] γ //[1] α
57.2°/[36]
57.9
57.5
Bain 2
Packet
V21
[0] γ //[1] α
20.6°/[30]
16.7
18.2
Bain 1
Packet
V22
[0] γ //[1] α
47.1°/[102124]
52.4
51.4
Bain 3
Packet
V23
[101] γ //[1] α
57.2°/[]
57.0
56.3
Bain 2
Packet
V24
[101] γ //[1] α
21.1°/[90]
17.1
18.6
Bain 1
Packet
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
Steel
Boundary density / μm-1
Vickers
Block
Sub-block
Packet(θ < 15°)
Packet
(θ > 15°)
hardness
HV
0.92Ni
0.34
0.01
0.02
0.12
325 ± 11
2.94Ni
1.13
0.02
0.03
0.25
407 ± 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)
Function
Temperature range
= a + bT (J·mol-1)
-6660
7
900 K > T > 300 K
= a + bT (J·mol-1)
650
-1
900 K > T > 620 K
= a + bT (J·mol-1)
0
0
T < 620 K
Table 5 Approximate representations of the free energy components for the γ→α transformation in pure iron[33]
Alloying element
TM / (K·%-1)
TNM / (K·%-1)
Si
-3
0
Mn
-37.5
-39.5
Ni
-6
-18
Mo
-26
-17
Cr
-19
-18
V
-44
-32
Table 6 Values of TM and TNM for a variety of substitutional solutes[34]
Fig.11 curves of 0.92Ni steel and 2.94Ni steel and corresponding fitting curves obtained by BiDoseResp function (—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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