B. S. Levy

Failure During Sheared Edge Stretching

Failure during sheared edge stretching of sheet steels is a serious concern, especially in advanced high-strength steel (AHSS) grades. The shearing process produces a shear face and a zone of deformation behind the shear face, which is the shear-affected zone (SAZ). A failure during sheared edge stretching depends on prior deformation in the sheet, the shearing process, and the subsequent strain path in the SAZ during stretching. Data from laboratory hole expansion tests and hole extrusion tests for multiple lots of fourteen grades of steel were analyzed. The forming limit curve (FLC), regression equations, measurement uncertainty calculations, and difference calculations were used in the analyses. From these analyses, an assessment of the primary factors that contribute to the fracture during sheared edge stretching was made. It was found that the forming limit strain with consideration of strain path in the SAZ is a major factor that contributes to the failure of a sheared edge during stretching. Although metallurgical factors are important, they appear to play a somewhat lesser role.

Review of the Shearing Process for Sheet Steels and Its Effect on Sheared-Edge Stretching

Failure in sheared-edge stretching often limits the use of advanced high-strength steel sheets in automotive applications. The present study analyzes data in the literature from laboratory experiments on both the shearing process and the characteristics of sheared edges. Shearing produces a surface with regions of rollover, burnish, fracture, and burr. The effect of clearance and tensile strength on the shear face characteristics is quantified. Higher strength, lower ductility steels exhibit an increase in percent fracture region. The shearing process also creates a zone of deformation adjacent to the shear face called the shear-affected zone (SAZ). From an analysis of data in the literature, it is concluded that deformation in the SAZ is the dominant factor in controlling failure during sheared-edge stretching. The characteristics of the shear face are generally important for failures during sheared-edge stretching only as there is a correlation between the characteristics of the shear face and the characteristics of the SAZ. The effect of the shear burr on shear-edge stretching is also related to a correlation with the characteristics of the SAZ. In reviewing the literature, many shearing variables that could affect sheared-edge stretching limits are not identified or if identified, not quantified. It is likely that some of these variables could affect subsequent sheared-edge stretching limits.

Effect of a Strain-Hardening Rate at Uniform Elongation on Sheared Edge Stretching

Edge failure during stretching of sheared edges limits the use of sheet steels in a number of product applications. The shearing process causes a highly strained region adjacent to the shear face, called the shear-affected zone. In the present study, the strain-hardening rate at uniform elongation, Z, is used as an empirical measure of cohesive strength at the interface of the various phases in steel microstructures. The higher the value of Z, the lower the macro strain when voids begin to form that lead to decohesion of the interface and subsequent failure. The data from four different studies are used to show that the true circumferential strain at failure in a hole expansion is a direct function of Z for most microstructural conditions. Sheet steels that exhibit better performance than that which would be expected for their Z values have one or more of the following characteristics—an increase in ferrite strength, lower carbon martensite in DP steels, or TRIP steels. A hot-rolled ferrite/pearlite microstructure is the only case of decreased true circumferential strain at failure for a given value of Z.

Failure during Sheared Edge Stretching of Dual-Phase Steels

The use of dual-phase steels has been limited in a number of applications, due to failure during sheared edge stretching. Previous investigations have studied the properties of dual-phase steels, especially regarding the mechanical properties of the individual phases or constituents, the strain partitioning to the microconstituents during loading, and the decohesion at the interface during loading. On the basis of the literature review, a hypothesis is developed in which failure in sheared edge stretching is the result of a sequence of events. Cracking first develops in the hard constituent, cracks grow in the interface between the hard constituent and ferrite, and relative movement of ferrite relative to the hard constituent increases the rate of cracking. In the present study, a single steel was heat treated to produce different amounts of hard constituent within the ferrite matrix in order to better understand the behavior of dual-phase steels during sheared edge stretching. The results of the study are consistent with the proposed hypothesis. It was found that in contrast to other studies, increased strength of the hard constituent retards crack initiation. Crack growth increased with increasing surface area of hard constituent–ferrite interfaces and increasing movement of ferrite relative to the hard constituent.

A Method for Measuring the Hardness of the Surface Layer on Hot Forging Dies Using a Nanoindenter

The properties and characteristics of the surface layer of forging dies are critical for understanding and controlling wear. However, the surface layer is very thin, and appropriate property measurements are difficult to obtain. The objective of the present study is to determine if nanoindenter testing provides a reliable method, which could be used to measure the surface hardness in forging die steels. To test the reliability of nanoindenter testing, nanoindenter values for two quenched and tempered steels (FX and H13) are compared to microhardness and macrohardness values. These steels were heat treated for various times to produce specimens with different values of hardness. The heat-treated specimens were tested using three different instruments—a Rockwell hardness tester for macrohardness, a Vickers hardness tester for microhardness, and a nanoindenter tester for fine scale evaluation of hardness. The results of this study indicate that nanoindenter values obtained using a Nanoindenter XP Machine with a Berkovich indenter reliably correlate with Rockwell C macrohardness values, and with Vickers HV microhardness values. Consequently, nanoindenter testing can provide reliable results for analyzing the surface layer of hot forging dies.

Bauschinger effect response of automotive sheet steels

In a study of the Bauschinger effect, data were collected from three sources in the published literature. Quantitative stress-strain data were taken from these papers, and the results re-analyzed. The resulting database has 44 lots of sheet steels, including drawing quality, interstitial free, bake hardening, HSLA (and related grades), dual phase, TRIP, recovery annealed, and martensitic grades. In analyzing the data, it is found that use of the 0.05% yield strength on reversal instead of the conventional 0.2% yield strength provides more generality in explaining the results. In this analysis, the Bauschinger effect is characterized by a term (BE), which is the difference between the steel strength just prior to reversal and the 0.05% yield strength on reversal normalized by the strength just prior to reversal. An initial prestrain of 2% is needed to establish a dislocation morphology that can be generalized across many of the steel grades. For steels with a predominantly ferrite microstructure and no added interstitial elements, BE exhibits a single monotonic trend line with respect to the strength just prior to reversal. For the bake hardenable grades, the 0.05% yield strength on reversal is slightly greater than the trend line. For the DP980, TRIP, and martensite grades, the 0.05% yield strength on reversal is considerably greater than the trend line. These steel grades either initially or after deformation have a greater volume fraction of martensite. The recovery annealed steels, which possess a much greater dislocation density, also exhibit 0.05% yield strengths that are significantly greater than the trend line. For steels with a predominantly ferrite microstructure over a wide range of strength, the single characteristic curve indicates that the initial reverse yield strength can be determined from the strength at the point just prior to the reversal. For modelers of sheet forming processes, the relationship between strength and yield on reversal can be used in material constitutive equations.