Flatness Defect Evolution of Cold-Rolled High Strength Steel Strip During Quenching Process
Qingdong ZHANG,Xiao LIN(),Qiang CAO,Xingfu LU,Boyang ZHANG,Shushan HU
School of Mechanical and Engineering, University of Science and Technology Beijing, Beijing 100083, China
Cite this article:
Qingdong ZHANG,Xiao LIN,Qiang CAO,Xingfu LU,Boyang ZHANG,Shushan HU. Flatness Defect Evolution of Cold-Rolled High Strength Steel Strip During Quenching Process. Acta Metall Sin, 2017, 53(4): 385-396.
Quenching is a key process in cold-rolled high strength steel manufacturing for the improvement of the material strength and plasticity. The quenching, however, may bring initial flatness defects of the steel strips, which causes problems for subsequent production process. It is thus necessary to study the flatness defects evolution during the quenching process. Using the secondary development of ABAQUS subroutine UMAT, this work establishes a temperature-microstructure-stress coupling finite element modeling (FEM) model to simulate the quenching process of the high strength steel with initial buckling defects. Thermal simulation experiments are further conducted to verify the present FEM model. Then, the elastic-plastic deformation behavior of the steel plates and its effects on flatness buckling during the quenching process is investigated using the FEM model. As a consequence, the buckling defect evolution mechanism in heat treatment process is obtained for the cold-rolled high strength steel. The flatness change or the forming of new flatness defect is mainly caused by the longitudinal extension arising from temperature gradient and the sequential phase transformation different in width and transverse directions. Change rates of the wave height, width, and length are used to describe the flatness change degree, quantifying the influence of the tension and initial transverse temperature difference on flatness change. The simulation shows that the tension has a positive correlation with the improvement of initial bucking defects. The initial edge waves become more severe after quenching along with the appearance of the new quarter waves, when the initial temperature of strip center is higher than that of the edge. On the contrary, the initial central waves become more serious when the initial temperature of strip center is lower. Meanwhile, joint impact of the tension and the initial transverse temperature difference on wave height is revealed for the application of industrial practice. Furthermore, quenching experiments of the high strength steel plates with initial single edge wave buckling defects are carried out using the experiment system in lab. Different sides of the plates quench into the water tank to reproduce the sequence of the phase change. The simulation and experiments produce consistent results qualitatively. This work makes connections between technological parameters and flatness change during quenching process, which can provide support to industrial heat treatment of high strength steel.
Fund: Supported by National Natural Science Foundation of China (No.51575040) and National Key Technologies Research & Development Program of China (No.2011BAE13B05)
Fig.1 Transverse displacement versus temperature during quenching process of cold-rolled high strength steel strip under different tensions
Fig.2 Models of steel sheet with initial edge wave (a) and initial central wave (b)
Fig.3 Martensite volume fraction of steel strip during quenching process
Fig.4 Changes of edge wave height during quenching process
Fig.5 Differences of strains between edge region and central region of steel strip in stages I (a), II (b) and III (c)
Fig.6 Displacement in thickness direction of steel strip after quenching
Tension / MPa
Wave height decrease / %
Wave length increase / %
Wave width decrease / %
5
8.58
1.10
0.40
10
15.15
1.06
0.80
15
21.29
1.03
1.20
20
28.86
1.01
1.63
25
31.97
0.97
2.00
30
36.64
0.94
2.40
Table 1 Influences of tension on change rate of edge wave after quenching
Fig.7 Displacement in thickness direction with an initial transverse temperature difference of 10 ℃
Fig.8 Distributions of new-formed quarter wave peak and valley displacements in thickness direction after quenching with an initial transverse temperature difference of 10 ℃
Fig.9 Displacement in thickness direction along longitudinal direction after quenching with an initial transverse temperature difference of 10 ℃
Fig.10 Change of strip longitudinal stress distribution over time
ΔT / ℃
Wave height decrease / %
Wave length increase / %
Wave width decrease / %
-20
48.86
1.15
2.50
-15
47.46
1.14
2.40
-10
32.07
1.12
1.88
-5
28.08
1.07
1.75
0
26.86
1.01
1.63
5
17.71
1.07
1.00
10
-1.75
1.12
0.13
15
-7.82
1.14
-0.13
20
-19.39
1.15
-1.25
Table 2 Influences of initial transverse temperature difference on change rate of edge wave after quenching
ΔT / ℃
Wave shape
Wave height of quarter wave / mm
Wave width of quarter wave / mm
Wave length of quarter wave / mm
-20
Edge wave
-
-
-
-15
Edge wave
-
-
-
-10
Edge wave
-
-
-
-5
Edge wave
-
-
-
0
Edge wave
-
-
-
5
Edge wave + quarter wave
0.11
90.36
494.51
10
Edge wave + quarter wave
0.25
164.82
494.40
15
Edge wave + quarter wave
0.41
233.33
494.33
20
Edge wave + quarter wave
0.53
308.54
494.25
Table 3 Influences of initial transverse temperature difference on new-formed quarter wave after quenching
Tension / MPa
ΔT=0
ΔT= -10 ℃
ΔT = -20 ℃
10
15.15%
23.28%
41.32%
20
26.86%
32.07%
48.86%
30
36.64%
40.10%
55.53%
Table 4 Decreases of edge wave height in different quenching conditions
Tension / MPa
Wave height decrease / %
Wave length increase / %
Wave width decrease / %
5
4.23
1.10
0.13
10
7.55
1.08
0.44
15
10.80
1.05
0.96
20
13.99
1.03
1.67
25
17.29
1.00
2.08
30
20.76
0.98
2.29
Table 5 Influences of tension on change rate of central wave after quenching
Fig.11 Influences of initial transverse temperature difference on central wave height (a), central wave length and width (b) after quenching
Fig.12 Displacement in thickness direction with an initial transverse temperature difference of -20 ℃
Fig.13 Displacements in thickness direction of the quarter buckle peak and valley along width direction (a) and displacement in thickness direction along longitudinal direction (b) (ΔT=-20 ℃)
ΔT ℃
Wave height of quarter wave mm
Wave width of quarter wave mm
Wave length of quarter wave mm
-20
0.08
151.28
494.35
-15
0.05
137.64
494.37
-10
0.03
127.81
494.43
-5
0.02
96.84
494.63
Table 6 Influences of initial transverse temperature difference on wave shape after quenching
Fig.14 Changes of strip longitudinal stress distribution over time
Tension / MPa
ΔT=0
ΔT=10 ℃
ΔT=20 ℃
10
7.55%
1.43%
8.77%
20
13.99%
8.18%
15.57%
30
20.76%
12.88%
22.03%
Table 7 Change rates of central wave height in different conditions
Fig.15 Edge wave height (a), edge wave length (b) and edge wave width (c) of steel sheet with different quenching methods
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