Correlations between nanoindentation hardness and macroscopic mechanical properties in DP980 steels

Correlations between nanoindentation hardness and macroscopic mechanical properties in DP980 steels M.D. Taylor b , K.S. Choi a , X. Sun a,n , D.K. Matlock b , C.E. Packard b , L. Xu c , F. Barlat d a Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, United States b Colorado School of Mines, Golden, CO 80401, United States c Leongjin Special Steel Co Ltd., Jintan City, Jiangsu Province, China d GIFT-POSTECH, Pohang, Gyeongbuk 790-784, Republic of Korea article info Article history: Received 5 September 2013 Received in revised form 27 December 2013 Accepted 28 December 2013 Available online 6 January 2014 Keywords: Nanoindentation Dual phase Constituent hardness Hole expansion ratio abstract Nanoindentation measurements were obtained on eight commercially-produced DP980 dual-phase steels to quantify the hardness of the individual constituents, ferrite and martensite, in each steel. Each microstructure was also evaluated to determine grain size, martensite volume fraction (MVF), and retained austenite content. Nanoindentation hardnesses and quantitative microstructural measurements were correlated with tensile properties and performance in hole expansion tests to assess the importance of the individual constituent properties. Hole expansion samples were prepared with both sheared edges produced by mechanical punching, and non-deformed edges produced by electric discharge machining (EDM). Average material hardness based on nanoindentation data correlated directly to Vickers hardness measurements, verifying the capability of the nanoindentation technique to produce data consistent with traditional hardness measurements. Yield strength (YS) correlated directly to ferrite hardness indicating that, for a similar MVF and microstructural morphology, the YS is controlled by the strength of the softer matrix phase (ferrite). Hole expansion ratios (HER) on EDM samples decreased with an increase in both martensite and ferrite hardness, indicating that EDM HER values can be enhanced by softening both constituents. Punched-hole HER values decreased with increasing martensite hardness and martensite-to-ferrite hardness ratio, but were independent of ferrite hardness, indicating that softening the martensite while increasing the ferrite hardness could produce a higher HER. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Advanced High Strength Steels (AHSS) are being increasingly used by the global automotive industry to cost-effectively reduce vehicle weight. AHSS micro-constituents can include retained austenite, bainite, ferrite, and martensite, combinations of which produce higher strengths compared to mild steels while still maintaining sufficient elongation [1,2]. Most AHSS products available are performance-based steels, e.g., user-defined criteria are specified by some basic mechanical property such as the minimum ultimate tensile strength (UTS), or yield ratio [3]. Steel makers can produce steels that meet a specific strength class by multiple different alloying and thermo-mechanical processing strategies, which correspond to potentially different microstructures. These variations in microstructure could affect formability performance in automotive manufacturing operations. Based on current and planned usage, dual-phase (DP) steels represent the most important AHSS grade. DP steels contain primarily martensite and ferrite, and multiple DP grades can be produced by controlling the martensite volume fraction (MVF) [4]. DP steels are commercially available with UTS values up to 980 MPa (designated as DP980), and higher strength grades are under development. Within the DP980 strength class, steel producers offer focused modifications with enhanced specific char- acteristics, e.g. improved bending, stretch flangeability, or high yield ratio [3]. As a result, two steels of the same strength class can exhibit different performances during forming, believed to be in response to their respective microstructural constituent properties [2,5–10]. An area of interest for both steel producers and users is to develop a more complete understanding of the microstructural properties that influence local formability and fracture of AHSS. Dual-phase steels are known to be sensitive to localized fracture [11], and traditional measures of ductility, such as total elongation (TE) obtained from a uni-axial tensile test, are often inaccurate indicators of local formability [8,12,13]. Laboratory measurements of hole expansion ratios (HER) are commonly utilized to represent Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.12.084 n Corresponding author. Tel.: þ1 509 372 6489; fax: þ1 509 372 6099. E-mail address: [email protected] (X. Sun). Materials Science & Engineering A 597 (2014) 431–439

Correlations between nanoindentation hardness and macroscopic mechanical properties in DP980 steels

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