Correlation of Microstructure Feature with Impact Fracture Behavior in a TMCP Processed High Strength Low Alloy Construction Steel

doi:10.1007/s40195-021-01250-0

Abstract

Abstract:

The present article aims at elucidating the effect of thermo-mechanical controlled processing (TMCP), especially the finish cooling temperature, on microstructure and mechanical properties of high strength low alloy steels for developing superior low temperature toughness construction steel. The microstructural features were characterized by scanning electron microscope equipped with electron backscatter diffraction, and the mechanical behaviors in terms of tensile properties and impact toughness were analyzed in correlation with microstructural evolution. The results showed that the lower finish cooling temperature could lead to a considerable increase in impact toughness for this steel. A mixed microstructure was obtained by TMCP at lower finish cooling temperature, which contained much fine lath-like bainite with dot-shaped M/A constituent and less granular bainite and bainite ferrite. In this case, this steel possesses yield and ultimate tensile strengths of ~ 885 MPa and 1089 MPa, respectively, and a total elongation of ~ 15.3%, while it has a lower yield ratio of ~ 0.81. The superior impact toughness of ~ 89 J at -20 °C was obtained, and this was resulted from the multi-phase microstructure including grain refinement, preferred grain boundaries misorientation, fine lath-like bainite with dot-shaped M/A constituent.

Key words: High strength low alloy steel, Thermo-mechanical controlled processing (TMCP), Finish cooling temperature, Microstructure, Mechanical properties, Impact toughness

Wen-Ting Zhu, Jun-Jun Cui, Zhen-Ye Chen, Yang Zhao, Li-Qing Chen. Correlation of Microstructure Feature with Impact Fracture Behavior in a TMCP Processed High Strength Low Alloy Construction Steel[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(4): 527-536.

Figures/Tables 15

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

C	Si	Mn	Nb + V + Ti	Cr + Cu + Ni	Mo	N	B	P	S	Fe
0.08	0.3	1.1	0.12	1.1	0.4	0.004	0.002	0.008	0.0016	Bal

Fig. 1 Schematic illustration of thermo-mechanical controlled processing for the experimental steels (RRST - rough rolling start temperature; RRFT - rough rolling finish temperature; FRST - finish rolling start temperature; FRFT - finish rolling finish temperature; SCT - start cooling temperature; FCT - finish cooling temperature)

Table 2 Processing parameters used in two-stage controlled rolling and controlled cooling

Steel	Rough rolling		Finish rolling		Accelerated cooling		Cooling rate from SCT to FCT(°C·s^-1)
Steel	RRST (°C)	RRFT (°C)	FRST (°C)	FRFT (°C)	SCT (°C)	FCT (°C)	Cooling rate from SCT to FCT(°C·s^-1)
FCT580	1127	1078	930	893	780	578	15
FCT480	1086	1052	930	882	780	484	20
FCT380	1091	1065	930	905	780	381	20

Fig. 2 SEM micrographs of the experimental steels with different finish cooling temperatures:a FCT580, the inset is the magnified image of pearlite; b FCT480; c FCT380

Table 3 Microstructural features of the experimental steels with different finish cooling temperatures

Steel	Volume percentage (%)					Effective grain size (μm)
Steel	GB	LB	P	BF	M/A	Effective grain size (μm)
FCT580	77.1 ± 9.3	8.5 ± 2.1	1.9 ± 0.7	12.5 ± 3.3	2.6 ± 1.1	4.1 ± 1.1
FCT480	12.2 ± 2.4	48.3 ± 7.5	0	39.5 ± 6.1	4.1 ± 0.7	2.7 ± 0.4
FCT380	8.5 ± 1.8	63.2 ± 8.6	0	28.3 ± 4.4	9.7 ± 2.2	1.5 ± 0.9

Fig. 3 Orientation image maps of the experimental steels with different finish cooling temperatures: a FCT580; b FCT480; c FCT380

Fig. 4 Relative frequency of different misorientation angles for the experimental steels with different finish cooling temperatures: a misorientation angle < 15°; b misorientation angle between 15° and 35°; c misorientation angle between 35° and 50°; d misorientation angle > 50°

Fig. 5 Tensile stress-strain curves for the experimental steels with different finish cooling temperatures

Table 4 Tensile properties of the experimental steel at different finish cooling temperatures

Steel	Yield strength (MPa)	Ultimate tensile strength (MPa)	Total elongation (%)	Yield ratio
FCT580	550 ± 7	827 ± 11	19.2 ± 1.4	0.67 ± 0.02
FCT480	700 ± 8	843 ± 9	16.7 ± 0.8	0.83 ± 0.01
FCT380	885 ± 4	1089 ± 12	15.3 ± 1.1	0.81 ± 0.01

Fig. 6 Variation of strain hardening rate with true strain for the experimental steel at different finish cooling temperatures

Fig. 7 Typical schematic of instrumented CVN impact test

Fig. 8 Variation in finish cooling temperatures on total impact absorbed energy of the experimental steel at -20 °C

Fig. 9 Variations of impact load and impact absorbed energy vs deflection for the experimental steels with different finish cooling temperatures: a FCT580; b FCT480; c FCT380

Table 5 Experimental data of the instrumented CVN impact test for the experimental steels with various finish cooling temperatures

Steel	P_m (kN)	P_f (kN)	P_a (kN)	P_m - P_f (kN)	P_a/P_f	E (J)	E₁ (J)	E₂ (J)	E₃ (J)
FCT580	6.35	6.25	1.97	0.10	0.32	7.51	1.92	0.32	5.27
FCT480	20.27	7.27	3.30	13.00	0.45	59.35	26.08	11.60	21.67
FCT380	21.00	4.78	3.00	16.22	0.63	88.66	27.73	37.26	23.67

Fig. 10 Schematic diagram and SEM morphologies of the impact fractured surfaces for the experimental steels with different finish cooling temperatures: a schematic diagram; b and c FCT580; d-f FCT480; g-i FCT380

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