Introduction to Mechanical Properties
Tensile property characterization of mild and High Strength Low Alloy steel (HSLA) traditionally was tested only in the rolling direction and included only yield strength, tensile strength, and total elongation. Properties vary as a function of orientation relative to the rolling (grain) direction, so testing in the longitudinal (0°), transverse (90°), and diagonal (45°) orientations relative to the rolling direction is done to obtain a better understanding of metal properties (Figure 1).
A more complete perspective of forming characteristics is obtained by also considering work hardening exponents (n-values) and anisotropy ratios (r-values), both of which are important to achieve improved and consistent formability.
Hardness readings are sometimes included in this characterization, but hardness readings are of little use in assessing formability requirements for sheet steel. Hardness testing is best used to assess the heat treatment quality and durability of the tools used to roll, stamp, and cut sheet metal.
The formability limits of different grades of conventional mild and HSLA steels were learned by correlating press performance with as-received mechanical properties. This information can be fed into computer forming simulation packages to run tryouts and troubleshooting in a virtual environment. Many important parameters can be measured in a tensile test, where the output is a stress-strain curve (Figure 2).
Press shop behavior of Advanced High-Strength Steels is more complex. AHSS properties are modified by changing chemistry, annealing temperature, amount of deformation, time, and even deformation path. With new microstructures, these steels become “Designer Steels” with properties tailored not only for initial forming of the stamping but in-service performance requirements for crash resistance, energy absorption, fatigue life, and other needs. An extended list of properties beyond a conventional tensile test is now needed to evaluate total performance with virtual forming prior to cutting the first die, to ensure ordering and receipt of the correct steel, and to enable successful troubleshooting if problems occur.
With increasing use of advanced steels for value-added applications, combined with the natural flow of more manufacturing occurring down the supply chain, it is critical that all levels of suppliers and users understand both how to measure the parameters and how they affect the forming process.
- The multiphase microstructure in Advanced High Strength Steels results in properties that change as the steel is deformed. An in-depth understanding of formability properties is necessary for proper application of these steels.
- Tensile test data characterizes the ability of a steel grade to perform with respect to global (tensile and necking) formability. Different tests like hole expansion and bending characterize performance at cut edges or bend radii.
- DP steels have higher n-values in the initial stages of deformation compared to conventional HSLA grades. These higher n-values help distribute deformation more uniformly in the presence of a stress gradient and thereby help minimize strain localization that would otherwise reduce the local thickness of the formed part.
- The n-value of certain AHSS grades, including dual phase steels, is not constant: there is a higher n-value at lower strains followed by a drop as strain increases.
- TRIP steels have a smaller initial increase in n-value than DP steels during forming but sustain the increase throughout the entire deformation process. Part designers can use these steels to achieve more complex geometries or further reduce part thickness for weight savings.
- TRIP steels have retained austenite after forming that transforms into martensite during a crash event, enabling improved crash performance.
- Normal anisotropy values (rm) approximately equal to 1 are a characteristic of all hot-rolled steels and most cold-rolled and coated AHSS and conventional HSLA steels.
- AHSS work hardens with increasing strain rate, but the effect is less than observed with mild steel. The n-value changes very little over a 105 (100,000x) increase in strain rate.
- As-received AHSS does not age-harden in storage.
- DP and TRIP steels have substantial increase in YS due to a bake hardening effect, while conventional HSLA steels have almost none.