Influence of Microstructure on Yield Strength of Ferrite–Pearlite Steels

Abstract

The mechanical properties of two-phase steels are mainly controlled with the volume fraction of the harder phase. In the previous reports, however, some different relationships between the mechanical properties and pearlite volume fraction have been demonstrated for ferrite–pearlite steels. This means that the relationship must be changed sensitively to experimental conditions so that the influence of the volume fraction of pearlite on the mechanical properties of hot-rolled ferrite–pearlite steels were experimentally investigated using Fe–C alloy with variation of carbon content.
Yield strength of the air-cooled samples after hot-rolling gradually increased with the increase of pearlite volume in lower volume fraction range while dramatically in higher range. The water-cooled samples exhibited higher yield strength than that of the air-cooled samples especially in the lower pearlite volume fractions. These results indicated that the mechanical properties of the ferrite–pearlite steels having the lower pearlite fraction are determined by properties in ferrite area. This is attributed to sparse distribution of pearlite; almost all ferrite grains are surrounded by the other ferrite grains.
The yield strength of ferritic steels consisted of grain refinement strengthening, precipitation strengthening by carbides and solid-solution strengthening of carbon. The solid-solution strengthening varied with change in the amount of carbon in solution and grain refinement strengthening also raised with increase in Hall–Petch coefficient by grain boundary segregation of carbon during aging.
These results confirmed that the aging condition of ferrite is important for alloy designing of ferrite–pearlite steels.