Ultrasonic measurement of drawability (r-values) of low carbon steel sheets

The present research was designed to evaluate an ultrasonic technique for measuring the drawability of low carbon steel sheets nondestructively. This technique, which uses electro magnetic acoustic transducers (EMAT's) is embodied in a fully automatic ultrasonic device called Ultra-Form, which was developed at Iowa State University (by the Center For Nondestructive Evaluation) in 1990. EMAT transducers generate the ultrasonic pulses in the sheet (without any need for a coupling medium) whose velocities are then related to the microstructural factors (crystallographic texture) that control drawability in rolled sheets. Drawability (usually interpreted by measuring r-values of low carbon steels) is an important property indicative of the formability of steel sheets used in the auto and appliance indus­ tries. It is measured by the destructive tension test which is sensitive to the experimental er­ rors and is time consuming. From a quality control perspective, there is a considerable in­ dustrial interest in further development of nondestructive on-line measurements of r-values during production (at the steel mill, or during the stamping operations). Towards this end, a collaborative research between LTV Steel Company, National Steel Corporation, Ford Mo­ tor Company and Ames Laboratory was conducted. This included theoretical and experi­ mental research on a total of 260 steel samples cut from 26 lifts (-10-12 samples of 20 in. by 20 in. (50 by 50 cm) per lift) of 5 types of commercially produced steels of Interstitially Free (IF), Aluminum-Killed Drawing Quality (AKDQ), Bake Hardenable (BH), High Strength Low Alloy (HSLA) and Phosphorus High Strength (HS) grades. These investigations also included tensile measurements on the steels (one per lift) and neutron diffraction (one per lift for 10 selected lifts). This research evaluated the ultrasonic technique with respect to its ac­ curacy and reproducibility, sensitivity to the microstructure of the steel sheets (texture), and the factors that may affect the relative value of its predictions of plastic properties. The ex­ perimental correlations and the repeatability data (obtained from sequential ultrasonic meas-