Microstructures and Mechanical Properties of a New Multi-functional 460 MPa Grade Construction Structural Steel

doi:10.1007/s40195-021-01359-2

Abstract

Abstract:

This article reports a new generation of Q460 multi-functional construction structural steel, which has high strength (yield strength larger than 460 MPa), excellent toughness (higher than 110 J/cm² at - 60 °C), lower yield ratio (lower than 0.8), good fire resistance (yield strength at 600 °C larger than two-thirds of its room-temperature yield strength) and better corrosion resistance. The effects of finish cooling temperature (FCT) on the microstructure and properties were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), emission electron probe micro-analysis (EPMA), electron backscattering diffraction (EBSD), tensile tester, impact tester, periodic immersion cycle acceleration test and electrochemical experiment. The results show that the strength and toughness are simultaneously improved by decreasing the FCT due to more lath-like bainite with large number of dislocations, refined martensite/austenite (M/A) with higher carbon concentration and increased high angle boundaries. In addition, the fire resistance of the newly developed Q460 steel is obviously better than the conventional one, which is mainly due to non-recrystallized lath-like bainite with high dislocation density at elevated temperature. The corrosion resistance of the new Q460 steel is also improved due to the addition of Cu and Cr.

Key words: Construction structural steel, Thermo-mechanical controlled processing (TMCP), Finish rolling temperature, Microstructure, Mechanical properties

Zhenye Chen, Zhangguo Lin, Jianjun Qi, Yang Feng, Liqing Chen, Guodong Wang. Microstructures and Mechanical Properties of a New Multi-functional 460 MPa Grade Construction Structural Steel[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(7): 1131-1142.

Figures/Tables 17

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

Steels	C	Mn	Si	Cr + Mo	Ni + Cu	Nb + B	Fe
New Q460	≤ 0.07	≤ 1.10	≤ 0.15	≤ 0.55	≤ 0.75	≤ 0.03	Bal.
Conventional Q460	0.174	1.534	0.277	-	-	≤ 0.03	Bal.

Fig. 1 Schematic of hot rolling process with different FCTs

Fig. 2 Typical micrographs of different specimens: a-c OM images, d-f SEM images, g-i BC maps of the specimens with FCTs of 580 °C a, d, g, 480 °C b, e, h. 380 °C c, f, i. The red area in BC maps represents RA

Fig. 3 TTT curves of the experimental steel calculated by JMatPro software

Fig. 4 Typical TEM micrographs of the experimental steel with FCTs of 580 °C a, 480 °C b, 380 °C c

Fig. 5 Grain boundary distributions a-c and corresponding statistical data d-f of FCT-580, FCT-480 and FCT-380, respectively

Table 2 Room temperature tensile properties of the experimental steels

Specimens	YS (MPa)	TS (MPa)	TE (%)	YR	TS·TE (GPa⋅%)
FCT-580	456 ± 7	623 ± 10	18.9 ± 0.8	0.730 ± 0.005	11.8 ± 0.7
FCT-480	515 ± 7	645 ± 5	21.1 ± 0.8	0.798 ± 0.009	13.6 ± 0.6
FCT-380	521 ± 24	654 ± 19	20.1 ± 1.4	0.797 ± 0.036	13.1 ± 1.2
Conventional Q460	512 ± 9	618 ± 11	20.0 ± 0.9	0.828 ± 0.029	12.4 ± 0.7
Performance Standards	≥ 460	570-720	≥ 18%	≤ 0.83	-

Table 3 Impact toughness (J/cm2) of the experimental steels with different FCTs

Temperature (°C)	FCT-580	FCT-480	FCT-380
0	118 ± 19	171 ± 7	127 ± 2
- 20	61 ± 16	170 ± 5	116 ± 8
- 40	33 ± 1	149 ± 1	107 ± 3
- 60	22 ± 1	114 ± 14	111 ± 10

Fig. 6 Impact toughness a and impact fractured morphologies in FCT-380 b, FCT-480 c, FCT-580 d

Fig. 7 Carbon distribution in the different specimens: a, d FCT-580; b, e FCT-480; c, f FCT-380

Table 4 Tensile properties of the experimental steels at 600 °C

Specimens	YS (MPa)	TS (MPa)	TE (%)	YS (600 °C)/YS(RT)	TS (600 °C)/TS(RT)
FCT-580	312 ± 5	366 ± 7	20.4 ± 1.1	0.684	0.587
FCT-480	359 ± 11	441 ± 13	20.0 ± 1.3	0.704	0.684
FCT-380	366 ± 6	436 ± 11	15.2 ± 0.8	0.702	0.667
Conventional Q460	212 ± 2	256 ± 5	42.6 ± 2.1	0.414	0.414

Fig. 8 SEM morphologies of the specimens held at 600 °C for 2 h followed by quenching to the room temperature: a FCT-580; b FCT-480; c FCT-380; d conventional Q460

Fig. 9 TEM morphologies of the specimens held at 600 °C for 2 h followed by quenching to the room temperature: a, d FCT-480; b, e FCT-380; c, f conventional Q460 steel

Fig. 10 Thermodynamic calculations using JMatPro: a phase constitutions; b enlarged graph of the square area a; c distribution of Mo in different phases

Fig. 11 Weight loss of the experimental steel

Fig. 12 Polarization curves of different specimens

Table 5 Corrosion current density and potential for the experimental steels

Specimens	Corrosion current density (μA/cm²)	Corrosion potential (V)
FCT-580	6.880	- 0.3899
FCT-480	7.814	- 0.4034
FCT-380	2.229	- 0.3837
Conventional Q460	11.468	- 0.4521

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