Precipitation and Mechanical Property of V‐Alloyed Steel: Role of Cooling Rate

Abstract

Herein, the effect of cooling rate on the precipitation behavior and mechanical properties of V-added microalloyed steel is systematically investigated by nanoindentation test, X-ray diffraction, and transmission electron microscopy. The present observation reveals that random precipitates and interphase precipitates are simultaneously observed at the low cooling rate of 0.5 C s À1 , whereas only random precipitates are obtained at the high cooling rate of 2 C s À1 . The contribution of the nanoscale precipitates to yield strength is evaluated to be 199 MPa at the low cooling rate of 0.5 C s À1 , which is almost 4 times higher than that of 2 C s À1 . The predicted yield strength by semiempirical model based on the structural parameters is consistent with the result from tensile testing. In addition, the greater difference between upper and lower yield points in sample B is caused by dislocations motion, which is related to the smaller interparticle spacing of nanosized precipitates within ferrite matrix.

Evaluation of Strengthening Mechanisms in Novel Fully Ferritic Advanced High‐Strength Steels

“…Therefore, the classic Williamson–Hall method was recently utilized in several studies which evaluated the dislocation density of the tempered microstructure of ultrahigh strength steels. [ 45–52 ] No martensitic microstructures with the high dislocation density exist in the present steels, and their microstructure is comparable with the tempered steel consisting of fully ferritic matrix‐containing precipitates; thus, this study calculated the dislocation densities using the classic Williamson–Hall method (Equation (3))where ρ s is the dislocation density calculated from strain broadening and ρ p is the dislocation density calculated from crystallite size. The following Equation (4) and (5) were used to evaluate ρ s and ρ p of the body‐centered cubic metalwhere ε is microstrain, b is Burgers vector, F is an interaction factor assumed to be 1, and D is crystallite size.…”

“…Therefore, the classic Williamson-Hall method was recently utilized in several studies which evaluated the dislocation density of the tempered microstructure of ultrahigh strength steels. [45][46][47][48][49][50][51][52] No martensitic microstructures with the high dislocation density exist in the present steels, and their microstructure is comparable with the tempered steel consisting of fully ferritic matrix-containing precipitates; thus, this study calculated the dislocation densities using the classic Williamson-Hall method (Equation (3))…”

“…Their effects are generally considered independent factors, as seen in Equation ( 6), although a simple addition of the strengthening from dislocations and precipitates may result in an overestimation of their contribution, as pointed out in the literature. [32,33,48]…”

“…where k is the grain boundary resistance to the dislocation movement. The value of the constant k varies slightly in literature; [15,19,20,48] nevertheless, the value of 18.1 MPa mm À0.5 used in ref. [19] was applied in this study.…”

Study on Diversified Carbide Precipitation in High‐Strength Low‐Alloy Steel during Tempering

“…When HSLA steels contain a ternary system of the abovementioned three alloy elements or a binary system in which two elements are combined, the relationship among the tempering process parameters, carbide precipitation, and performance becomes complex. [5,7,8] MX-type carbon (nitrogen) compounds with the NaCl (B1) cubic crystal structure are precipitated, such as NbC, Nb (C, N), (Ti, Nb) C, and (Ti, Nb, V)C, in the binary or ternary system of microalloy steels. In addition to MX-type precipitation, M 3 C, M 2 C, M 23 C 6 , and M 7 C 3 precipitates [7,9,10] cause the carbides in HSLA steels to exhibit the characteristics of diversified precipitation and "multiscale precipitation."…”