Achieving Fine-Grained Microstructure and Superior Mechanical Property in a Plain Low-Carbon Steel Using Heavy Cold Rolling Combined with Short-Time Heat Treatment

doi:10.1007/s40195-023-01579-8

Abstract

Abstract:

A new thermomechanical process consisting of heavy cold rolling (HCR) and short-time heat treatment (STH) is developed to fabricate fine-grained martensite microstructure in a low-cost plain low-carbon steel. To achieve the optimal mechanical properties after STH, three different ferrite-pearlite (F-P) dual-phase microstructures are prepared via hot rolling (HR), HR and austenitizing, and HR and HCR. The microstructure evolution and the comprehensive mechanical properties of the alloy during STH are then investigated. We find that the volume fractions of transformed martensite after STH increase with decreasing grain sizes of the pre-STH F-P dual phases. The rapid heating and short-time holding of STH promote grain nucleation and inhibit grain growth, resulting in microstructure refinement. The formation of martensites with different morphologies and different carbon concentrations in the HR and HCR + STH alloy is identified, owing to the inhomogeneous carbon distribution by STH. Tensile experiments demonstrate that STH greatly improves the comprehension mechanical properties of the alloy. Excellent mechanical properties, with a yield strength of 1224 MPa, a tensile strength of 1583 MPa, a uniform elongation of 4.0% and a total elongation of 7.3% are achieved in the HR and HCR + STH alloy. These excellent mechanical properties are principally attributed to the microstructure refinement and martensite formation induced by STH, with a yield strength improvement of 134% and a tensile strength improvement of 150% relative to the HR alloy.

Key words: Heat treatment, Microstructure, Plain low-carbon steel, Mechanical properties

Yuxuan Liu, Shichang Liu, Liming Fu, Huanrong Wang, Wei Wang, Mao Wen, Aidang Shan. Achieving Fine-Grained Microstructure and Superior Mechanical Property in a Plain Low-Carbon Steel Using Heavy Cold Rolling Combined with Short-Time Heat Treatment[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(10): 1719-1734.

Figures/Tables 17

Table 1 Chemical compositions of the plain low-carbon steel (wt%)

C	Si	Mn	P	S	Fe
0.17	0.27	0.42	0.03	0.027	Bal.

Fig. 1 a CCT diagram and b the equilibrium phase diagram of the plain low-carbon steel calculated by JmatPro software

Fig. 2 a XRD patterns of all samples without and with STH; b the magnified XRD profiles of (211) diffraction peaks after normalizing

Table 2 Full width at half maxima (β) and dislocation density (ρ) of the plain low-carbon steel in different states

State	β (rad)	ρ (10¹² m⁻²)
RACed	0.152	2.9
IHRed	0.236	7.1
HCRed	0.422	22.6
RACed-STH	0.224	6.4
IHRed-STH	0.371	17.4
HCRed-STH	0.829	87.0

Fig. 3 Optical images of all the samples without and with STH: a RACed; b IHRed; c HCRed; d RACed-STH; e IHRed-STH; f HCRed-STH

Fig. 4 SEM images of all the samples without and with STH: a, d RACed; b, e IHRed; c, f HCRed; g, j RACed-STH; h, k IHRed-STH; i, l HCRed-STH

Fig. 5 High magnification SEM images of the samples after STH: a, b RACed-STH; c, d IHRed-STH; e, f HCRed-STH

Fig. 6 EBSD characterization of the samples without STH. The constituent phases and grain boundaries: a RACed; d IHRed; g HCRed. The grain size images: b RACed; e IHRed; h HCRed. IPF images: c RACed; f IHRed; i HCRed. Grain boundary misorientation distributions and grain size distributions are shown in insets of corresponding images

Fig. 7 EBSD characterization of the samples with STH. The constituent phases and grain boundaries: a RACed-STH; d IHRed-STH; g HCRed-STH. The grain size images: b RACed-STH; e IHRed-STH; h HCRed-STH. The IPF color images: c RACed-STH; f IHRed-STH; i HCRed-STH. Grain boundary misorientation distributions and grain size distributions are shown in insets of corresponding images

Table 3 Measured fractions of ferrite (F), average grains sizes (davg) and fractions of HAGBs (fHAGB)

State	Phase	F (%)	d_avg (μm)	f_HAGB (%)
RACed	F-P	86.0	12.2	58.3
IHRed	F-P	84.0	4.1	34.8
HCRed	F-P	85.0	0.25	23.8
RACed-STH	F-M	29.5	3.3	48.2
IHRed-STH	F-M	10.9	2.4	43.8
HCRed-STH	M	0	1.9	38.9

Fig. 8 a, c, e High magnification of SEM images and b, d, f corresponding carbon distribution maps of the HCRed-STH sample

Fig. 9 a, b TEM bright field images of the HCRed-STH sample showing the typical lath martensite, c TEM bright field image and d TEM dark field image with corresponding diffraction patterns of the HCRed-STH showing the typical twins

Fig. 10 TEM bright field images of the HCRed-STH: a, b showing spherical cementite; c, d showing super-fined carbides in martensitic matrix with corresponding diffraction patterns

Fig. 11 a, b Room temperature tensile engineering stress-strain curves and c, d strain-hardening rates as a function of true strain

Table 4 Tensile properties including yield strength (YS), tensile strength (TS), yield strength/tensile strength ratio (Y/T), uniform elongation (UE) and total elongation (TE)

State	YS (MPa)	TS (MPa)	Y/T	UE (%)	TE (%)
RACed	306	471	0.65	18.0	28.5
IHRed	523	633	0.83	11.0	22.0
HCRed	1053	1072	0.98	0.86	3.1
RACed-STH	975	1413	0.69	2.7	5.9
IHRed-STH	1135	1511	0.75	2.9	7.2
HCRed-STH	1224	1583	0.77	4.0	7.3

Table 5 Comparison of YS, TS and TE of plain low-carbon steels fabricated by different processing techniques

Processing techniques	YS (MPa)	TS (MPa)	TE (%)	Reference
Intercritical annealing	220	400	33	Jamei et al. [23]
Intercritical annealing	390	520	27	Kundu et al. [24]
Intercritical annealing	400	670	29	Balbi et al. [25]
Intercritical annealing	720	812	15.1	Molaei and Ekrami [26]
Intercritical annealing + Aging	390	520	27	Zamani et al. [27]
Cold roll bonding	361	540	20	Saeidi et al. [28]
Cold rolling + Intercritical annealing	300	520	29	Nikkhah et al. [29]
Cold rolling + Intercritical annealing	458	947	12.5	Park et al. [30]
Cold rolling + Intercritical annealing	556	1048	4.2	Sodjit and Uthaisangsuk [31]
Warm rolling + Intercritical annealing	525	1037	7.3	Calcagnotto et al. [32]
Asymmetric rolling + Intercritical annealing	1067	1172	13.5	Yaghoobi et al. [33]
HCR + STH	1224	1583	7.3	Present work

Fig. 12 Calculated contributions from four strengthening mechanisms and comparison with experimental results

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