Microstructure Control of Non-quenched and Tempered CT80 Grade Coiled Tubing Steel

doi:10.1007/s40195-014-0063-1

Abstract

Abstract:

Complex working condition of coiled tube (CT) steel demands high strength and low ratio of yield strength (YS) to tensile strength (TS). Reasonable microstructure control is a key problem of CT steel. Controlled rolling and controlled cooling experiments were carried out by two kinds of tested steels with different chemical compositions to develop the non-quenched and tempered CT80 steel. Mechanical properties of the tested steels at different processes are all in good agreement with the properties requirement of CT80. Increasing of Mo and Nb contents improves transformation of acicular ferrite and martensite–austenite (M–A) islands. 4 vol% of fine M/A islands increase TS and decreases ratio of YS/TS. Cooling rate increasing also improves acicular ferrite transformation and enhances TS, but has little effect on ratio of YS/TS. To meet the requirement of high strength and low ratio of YS/TS, optimized complex microstructure of the CT80 steel is composed of acicular ferrite, about 30 vol% of bainite and 4 vol% of M–A islands.

Key words: Coiled tube, M–A islands, Acicular ferrite, Mechanical properties

Leyu Zhou, Bo Jiang, Mingyang Li, Fuchun Yuan, Chaolei Zhang, Yazheng Liu. Microstructure Control of Non-quenched and Tempered CT80 Grade Coiled Tubing Steel[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(3): 464-468.

Figures/Tables 8

Table 1 Chemical compositions of tested steels (wt%)

Steel	C	Si	Mn	Cr	P	S	Cu	Ni	Mo	Nb	Ti	V	Al	Fe
No. 1	0.07–0.12	≤0.45	0.70–1.10	0.45–0.65	≤0.015	≤0.002	≤0.30	≤0.25	0.24	0.049	0.010–0.020	0.015–0.040	0.07–0.12	Bal
No. 2	0.07–0.12	≤0.45	0.70–1.10	0.45–0.65	≤0.015	≤0.002	≤0.30	≤0.25	0.15	0.017	0.010–0.020	0.015–0.040	0.07–0.12	Bal

Table 2 Actual production process parameters of tested steels

Steel	Heating temperature (°C)	Finish rolling temperature (°C)	Cooling rate (°C/s)	Coiling temperature (°C)
No. 1–1	1,200	818	46	450
No. 1–2	1,200	830	50	450
No. 2–1	1,200	810	52	450
No. 2–2	1,200	810	69	450

Fig. 1 Microstructures of No. 1–1 a, No. 1–2 b, No. 2–1 c, No. 2–2 d steels

Fig. 2 EBSD results of No. 1–1 a, No. 1–2 b, No. 2–1 c, No. 2–2 d steel

Fig. 3 Color metallographies of No. 1–1 a, No. 1–2 b steels etched by Lepera reagent

Table 3 Mechanical properties of the tested steels

Steel	YS (MPa)	TS (MPa)	Ratio of YS/TS	Elongation (%)
No. 1–1	620	754	0.82	29.2
No. 1–2	650	770	0.84	23.2
No. 2–1	580	650	0.89	33
No. 2–2	605	680	0.89	30

Fig. 4 TEM micrographs of polygonal ferrite in No. 2–1 steel a, acicular ferrite in No. 2–2 steel b, dislocation in the acicular ferrite in No. 2–2 steel c

Fig. 5 TEM micrographs of mass M–A land abar shape M–A land b in No. 1 tested steel

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