Effects of Cooling Paths on Through-Thickness Microstructure and Mechanical Properties of Heavy Gauge X80 Pipeline Steel

doi:10.1007/s40195-017-0557-8

Abstract

Abstract:

The effects of various cooling paths on uniformity of through-thickness microstructure and mechanical properties of X80 pipeline steel of 22.0 mm in thickness were studied. The finite difference method was employed to calculate the temperature field during cooling. It was confirmed by the experimental result and temperature field calculation that the optimizing process was achieved by the ultra-fast cooling with medium cooling capacity (cooling rate of ~23 K/s) followed by ultimate cooling capacity (cooling rate of ~50 K/s). After optimization, the experimental steel displayed much uniform microstructure and the deviation of through-thickness hardness was controlled within 20 HV. In addition, the yield strength, tensile strength and elongation of the experimental steel were 621, 728 MPa and 21.5%, respectively, meeting the requirements of the API standard for X80 pipeline steels.

Key words: X80 pipeline steel, Ultra-fast cooling, Cooling path, Finite difference method, Microstructure, Mechanical properties

Xu-Dong Li, Cheng-Ning Li, Guo Yuan, Guo-Dong Wang. Effects of Cooling Paths on Through-Thickness Microstructure and Mechanical Properties of Heavy Gauge X80 Pipeline Steel[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(5): 483-492.

Figures/Tables 14

Table 1 Chemical composition of the experimental steel (wt%)

C	Si	Mn	P	S	Nb	V	Ti	Cr	Mo	Fe
0.056	0.18	1.8	0.009	0.001	0.07	0.03	0.015	0.35	0.15	Bal.

Fig. 1 Schematic illustration of the rolling and cooling conditions

Table 2 Thermo-physical properties of the experimental steels [14]

Temperature (K)	c_p (J kg^-1 K^-1)	λ (W m^-1 K^-1)
273	48.31	65.5074
373	48.34	60.4884
473	52.32	55.0746
573	57.12	49.5558
673	62.16	45.3432
773	70.14	41.1306
873	78.96	36.5148
973	86.94	33.2052
1073	91.14	28.5894
1173	61.74	27.9864

Fig. 2 Schematic of region discretization

Fig. 3 Temperature field simulation results: a LC, b Ult-Med UFC, c Med-Ult UFC

Fig. 4 Through-thickness microstructures obtained by LC: a surface, b thickness of 1/8, c thickness of 1/4, d thickness of 3/8, e thickness of 1/2

Fig. 5 Through-thickness microstructures obtained by Ult-Med UFC: a surface, b thickness of 1/8, c thickness of 1/4, d thickness of 3/8, e thickness of 1/2

Fig. 6 Through-thickness microstructures obtained by Med-Ult UFC: a surface, b thickness of 1/8, c thickness of 1/4, d thickness of 3/8, e thickness of 1/2

Fig. 7 TEM micrographs of steels: a surface of LC process, b center of LC process, c surface of Ult-Med UFC process, d center of Ult-Med UFC process, e surface of Med-Ult UFC process, f center of Med-Ult UFC process

Table 3 Mechanical properties of the experimental steels cooled at various paths

Cooling process	Rt_0.5 (MPa)	R_m (MPa)	A₅₀ (%)	vE at 253 K (J)
LC	542	677	23	235, 220, 263
Ult-Med UFC	610	719	21	282, 289, 280
Med-Ult UFC	621	728	21.5	289, 294, 291
API X80	552-690	621-827	15	≥145 (268 K)

Fig. 8 Impact specimen fractographs obtained by LC: a low-magnification fractographs, b high-magnification fractographs of region A, c high-magnification fractographs of region B, d low-magnification fractographs of delamination, e high-magnification fractographs of delamination surface (region C)

Fig. 9 Impact specimen fractographs obtained by Ult-Med UFC: a low-magnification fractographs, b high-magnification fractographs of region A, c high-magnification fractographs of region B

Fig. 10 Impact specimen fractographs obtained by Med-Ult UFC: a low-magnification fractographs, b high-magnification fractographs of region A, c high-magnification fractographs of region B

Fig. 11 Through-thickness HV distributions during various cooling paths

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