Dynamic Materials Testing, Texture, and Yield-Surface Calculation of an Automotive Sheet Steel CARL M. CADY, SHUH RONG CHEN, GEORGE T. GRAY III, DAVID A. KORZEKWA, and JOHN F. BINGERT The relationships between the stress state and anisotropic mechanical response for a drawing-quality, special-killed (DQSK) mild sheet steel has been analyzed. The strain rate and temperature sensitivity of the flow stress and the insensitivity of the strain hardening to strain rate are shown to be consistent with thermal activation over a Peierls barrier as the rate-controlling mechanism for deformation in DQSK. A calculated yield surface, using the quadratic Hill criterion, is shown to produce an accurate correlation with the experimental results as a function of stress state. Annealing of the DQSK sheet steel at 773 K for 1 hour reduced some of the residual stresses developed during the forming process, but had little effect on the texture. The R values derived from computed yield surfaces suggested very little difference between in-plane (IP) and transverse tensile tests, consistent with the experimental results. A comparison of the stress-strain response with the calculated yield-surface and texture measurements correlates well with the relationships between the IP and through-thickness (TT) deformation. I. INTRODUCTION plane) and a fiber (^110& direction parallel to the rolling direction). Since ^111& represents a strong crystallographic STEEL sheet, possessing a high in-plane isotropy (low direction in bcc metals, the formation of a sharp g-fiber Dr) and high plastic-strain ratio (r), is widely used in the texture leads to an increase in the through-thickness (TT) automotive industry to meet the requirement for good drawa- strength as compared to the in-plane (IP) strength. [12,13] The bility when forming panels and possessing complex shapes. anisotropic behavior of steels is usually characterized by In recent years, it has become increasingly important to means of the coefficient of normal plastic anisotropy, con- reduce the thickness and weight of the sheet steel used in ventionally reported as the r value (Lankford parameter), automobiles. This reduction has been driven by growing an average of the r values measured in the plane of the global environmental concerns pressuring automobile manu- sheet. Several industrially important mechanical properties facturers to produce ever-increasingly more efficient, cleaner of rolled steel sheets are determined by the nature and inten- products while simultaneously improving vehicle safety. [1–4] sity of its texture, and the r value is one of these. It has One obvious way to increase fuel efficiency is to reduce the been shown to be an effective tool to characterize the drawa- overall weight of the steel used in cars and trucks. However, bility of steel sheet. [14–22] designers must also maintain strength and damage-resistance In this study, emphases were placed on investigating (1) criteria in the final product. Significant research has gone the influence of strain rate, temperature, and heat treatment into the compositional and production development of sheet on the stress-strain response of an as-manufactured drawing- steels that will improve the formability of thinner gages of quality, special-killed (DQSK) mild steel, (2) the develop- steel without sacrificing either the yield or flow strength of ment of yield-surface predictions, and (3) the texture and r the steel sheets. [5,6] The UltraLight Steel Auto Body value for a DQSK steel. (ULSAB) program initiated by AISI and IISI was created to design just such a vehicle. [7] The divergence of plastic-strain ratios from unity is a II. EXPERIMENTAL PROCEDURE consequence of plastic anisotropy, and this anisotropy is The material examined in this study was DQSK mild derived from polycrystalline textures evolved during ther- sheet steel, a low-carbon steel commonly used in the manu- momechanical processing. There are several factors affect- facture of automobiles, produced by National Steel Corpora- ing texture development in DQSK steel. Deformation tion (Livonia, MI). The analyzed composition of the DQSK textures, especially their sharpness, are most strongly influ- steel studied was (in wt pct): 99.6 Fe, 0.23 Mn, 0.049 Cr, enced by total reduction, while recrystallization textures are 0.04 Al, 0.023 C, and ,0.006 P, Si, Ni, Mo, and Sn. [23] The controlled by carbon content and intragranular heterogenei- as-received cold-rolled sheet stock had a gage of 2.8 mm ties, such as aluminum nitride precipitation during anneal- and exhibited a “pancake” grain structure with a nominally ing. [8–11] The primary texture components in rolled steel equiaxed shape in the IP direction of 80 and 30 mm in the consist of the g fiber ({111} planes parallel to the rolling TT direction. Tensile specimens and compression cylinders were wire electrodischarge-machined from the as-received steel plates, CARL M. CADY, SHUH RONG CHEN, DAVID A. KORZEKWA, and as shown schematically in Figure 1. During the machining JOHN F. BINGERT, Staff Members, and GEORGE T. GRAY III, Team process, the galvanized surface coating on the as-received Leader, are with the Materials Science and Technology Division, Los steel plates was removed. Tensile specimens were machined Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted July 8, 1998. both axially and transverse to the rolling direction. Two METALLURGICAL AND MATERIALS TRANSACTIONS A U.S. GOVERNMENT WORK VOLUME 31A, OCTOBER 2000—2439 NOT PROTECTED BY U.S. COPYRIGHT