Effect of Heterogeneous Microstructural Morphology on Tensile Behavior in a Series of High-Strength Wind Power Steels

doi:10.1007/s40195-025-01926-x

Abstract

Abstract:

A series of high-strength wind power steels with various microstructural morphologies was produced by hot-rolled and thermo-mechanical controlled processes. The microstructure, microhardness, and tensile behavior observed using in-situ techniques in various types of steels were investigated. The experimental results demonstrated that the 3 microstructural morphologies (band-, net-, and fiber-structures) can be clarified and categorized; each type possesses different tensile strengths, yield behaviors, and strain hardening behaviors. This can be attributed to different strain distribution caused by the structural morphology; band-structure steels exhibit a yield plateau primarily attributed to the relatively weak constraint effect of pearlite on ferrite; net-structure steels display 3 strain hardening stages due to the staged plastic deformation; fiber-structure steels achieve superior strength through their uniform stress distribution. Furthermore, the initial strain hardening rate, transition strain, and uniform elongation were influenced by the features of the constituent phases. Based on these findings, methods for estimating the yield strength and tensile strength of the steels with two phases were discussed and experimentally validated.

Key words: Wind power steels, Microstructural morphology, Tensile behavior, In-situ tensile technique, Strain hardening behavior

Chong Gao, Zi-Hao Chen, Zhi-Zhi Liang, Li-Xi Xiong, Jian-Chao Pang, Heng Ma, Kang He, Shou-Xin Li, Xiao-Wu Li, Zhe-Feng Zhang. Effect of Heterogeneous Microstructural Morphology on Tensile Behavior in a Series of High-Strength Wind Power Steels[J]. Acta Metallurgica Sinica (English Letters), 2025, 38(11): 2047-2062.

Figures/Tables 20

Table 1 Chemical compositions of the tested steels (wt%)

Steel	C	Si	Mn	P	S	Cu	Ni	Cr	Nb	Fe
Q390-25	0.14	0.23	1.50	0.011	0.003	0.01	0.01	0.02	0.027	Bal.
Q420-10	0.16	0.22	1.48	0.011	0.002	0.02	0.01	0.02	0.029	Bal.
Q420-25	0.15	0.22	1.48	0.008	0.003	0.02	0.01	0.02	0.027	Bal.
Q420-40	0.15	0.25	1.48	0.013	0.003	0.03	0.01	0.02	0.027	Bal.
Q460-25	0.09	0.19	1.52	0.010	0.002	0.01	0.17	0.32	0.039	Bal.
Q500-25	0.09	0.22	1.60	0.008	0.002	0.01	0.20	0.35	0.040	Bal.

Table 2 Ac1 and Ac3 temperatures of the tested steels

Steel	$A_{\mathrm{c} 1}\left({ }^{\circ} \mathrm{C}\right)$	$A_{\mathrm{c} 3}\left({ }^{\circ} \mathrm{C}\right)$
Q390-25	681	821
Q420-10	682	815
Q420-25	700	855
Q420-40	682	819
Q460-25	676	824
Q500-25	650	826

Fig. 1 Shapes and sizes of conventional tensile a and in-situ tensile specimens b (unit: mm)

Fig. 2 Microstructure of Q390-25 a, Q420-10 b, Q420-25 c, Q420-40 d, Q460-25 e and Q500-25 f, (ND: Normal direction, TD: Transverse direction; RD: Rolling direction)

Table 3 Microhardness and volume fraction of the steels

Steel	HV_H	HV_S	∆HV	V_H (%)	V_S (%)
Q390-25	233	170	63	26	74
Q420-10	255	216	39	30	70
Q420-25	327	185	142	27	73
Q460-25	252	216	36	30	70

Fig. 3 KAM maps of Q390-25 a, Q420-10 b, Q420-25 c, Q420-40 d, Q460-25 e, and Q500-25 f

Fig. 4 Mechanical properties of experimental steels: a engineering stress-strain curves; b true stress-strain curves; c relation of tensile strength and yield strength; d relation of tensile strength and elongation to fracture

Table 4 Mechanical properties of the steels

Steel	σ_y (MPa)	σ_b (MPa)	Z (%)	A (%)	σ_y/σ_b
Q390-25	420	549	74.61	37.32	0.77
Q420-10	477	624	62.72	28.69	0.76
Q420-25	454	577	72.85	32.96	0.79
Q420-40	582	693	75.82	24.88	0.84
Q460-25	476	649	71.82	27.68	0.73
Q500-25	622	734	47.28	20.12	0.85

Fig. 5 a Relation between tensile strength and hardness of mixtures; b relation between yield strength and microhardness of soft phase

Fig. 6 Relations among microhardness, strength and plasticity of the steels

Fig. 7 Micrographs of tensile fractured surfaces and fibrous zones of the steels: Q390-25 a, Q420-10 b, Q420-25 c, Q420-40 d, Q460-25 e, and Q500-25 f

Fig. 8 Variation in strain hardening rate as a function of true strain a, and true stress b

Fig. 9 The lnΘ vs. lnσt plots of different types of steels a and the strain hardening behaviors by MC-J analysis method for BS steels b, NS steels c, and FS steels d

Table 5 Values of strain hardening exponent and transition train of the steels in MC-J analysis

Materials	1/m₁	1/m₂	1/m₃	ε_tr1	ε_tr2
Q390-25	0.249	0.175	/	0.0753	/
Q420-10	0.243	0.165	/	0.0789	/
Q420-25	0.234	0.168	/	0.0732	/
Q420-40	0.098	/	/	/	/
Q460-25	0.090	0.152	0.121	0.0145	0.0946
Q500-25	0.081	/	/	/	/
35CrMo-HR	0.072	0.159	0.091	0.0130	0.0637

Fig. 10 Comparison of strain hardening exponent a and transition strain b of the steels

Fig. 11 Relations between microhardness with strain hardening rate of BS steels in yield point Θy a, transition strain εtr1 b, and uniform elongation Au c

Fig. 12 In-situ observation on the tensile behaviors at three stages for Q390-25: yield point a, transition strain b, and tensile strength c

Fig. 13 In-situ observation on the tensile behaviors of Q460-25 at yield point a, transition strain εtr1 b, transition strain εtr2 c, and tensile strength d

Fig. 14 In-situ observation on the tensile behaviors of Q500-25 at yield point a, and at tensile strength b

Fig. 15 Summary of the tensile deformation mechanisms and dominant factors for the BS, NS, and FS steels

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