Abstract
In this study, the evolution of high-strength HSLA steel microstructure was studied using high-temperature laser confocal microscopy and SEM, TEM, and EPMA techniques. The effect of precipitates on grain boundary migration of austenite during high-temperature heating and the effect of inclusions in undercooled austenite on AF phase transformation were studied. The effect of multiphase microstructure on impact toughness was studied by Gleeble thermal simulation at 550, 600, and 650 °C. The results show that the austenite grain is refined by TiN pinning at high temperatures, and a large number of NbC and VCN are precipitated in ferrite for precipitation strengthening. The (Ti-Mn-O) + (Al + Si + Mn-O) + MnS composite inclusions with smaller sizes have a greater promoting effect on the nucleation of acicular ferrite than single-phase MnS. With a decrease in isothermal temperature, the content of acicular ferrite increases. When the isothermal temperature is 550 °C, an increase in the maximum impact toughness of acicular ferrite with large-angle grain boundary is clearly observable.
Highlights
A current trend in the rapid development of the automobile industry is high-strength and lightweight materials [1,2]; the automobile industry has established higher requirements for the material properties of automobile parts [3]
In the traditional metallurgical field, the influence of inclusions on steel is complex; it is well known that large nonmetallic inclusions are harmful to the mechanical properties of steel because they act as nucleation sites for pores and cracks during deformation and service, thereby becoming the source of cracks [6]
In the field of welding, several studies have confirmed that acicular ferrite (AF) structure is important for improving mechanical properties and impact toughness [7,8,9]
Summary
A current trend in the rapid development of the automobile industry is high-strength and lightweight materials [1,2]; the automobile industry has established higher requirements for the material properties of automobile parts [3]. High-Strength LowAlloy steel (HSLA) has the advantages of fewer generation and manufacturing links, no heat treatment process, low generation cost, good environmental protection performance, and excellent mechanical properties; it has attracted the attention of many studies [4,5]. In the traditional metallurgical field, the influence of inclusions on steel is complex; it is well known that large nonmetallic inclusions are harmful to the mechanical properties of steel because they act as nucleation sites for pores and cracks during deformation and service, thereby becoming the source of cracks [6]. In the field of welding, several studies have confirmed that AF structure is important for improving mechanical properties and impact toughness [7,8,9]. In a study by J M Dowling [18]
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