Mechanisms of 3rd Generation Advanced High-Strength Steel Slab Embrittlement

Abstract

This project links properties and performance in as-cast condition for advanced high strength steel (AHSS) by comparing silicon content and microstructure to mechanical properties. Levels of C, Mn and Si in new AHSS grades lead to a complex evolution of microstructure upon solidification and cooling and can lead to slab cracking. This cracking phenomenon happens in high silicon containing industrial slabs, so microstructure and properties of production scale slabs were studied through impact test (Charpy V-notch). It was found that microstructure of cracked and non-cracked slabs are different, with cracked slabs presenting ferriteallotriomorphs at prior austenite grain boundaries (PAGB) that were correlated with toughness loss on impact test. Cracks propagate along ferrite allotriomorphs preferentially.Two thermodynamic/kinetic models were developed in Thermo-Calc and DICTRA to analyze the effects of Mn and Si on microstructure development with emphasis on ferrite growth. These models served as baseline to define the levels of Mn and Si for lab scale studies. Nine ingots with compositions of 0.2wt.%C, 2.5-3.0wt.%Mn and 0.5-3.0wt.%Si were lab cast and characterized by their microstructure and mechanical properties in the as-cast state. Light Optical Microscopy (LOM) and Scanning Electron Microscopy (SEM) confirmed that granular bainite/acicular ferrite were the major constituents of matrixmicrostructure. The amount of allotriomorphic ferrite, when present, increased with increasing silicon anddecreasing manganese. The mechanical behavior of all steels in the as-cast state was characterized byhardness tests and Charpy V-Notch (CVN) tests at 100°C. For a subset of compositions, tensile tests wereperformed at room temperature and ductile to brittle transition temperature (DBTT) curves were built up to400°C based on CVN tests. Allotriomorphic ferrite, when present, changed crack propagation path and increased DBTT. Hardness was found to increase with silicon and manganese contents. The effect of silicon on microstructure evolution and properties was studied by submitting as cast steels of varying chemical composition to two temperature profiles designed to approximate cooling of industrial slabs after casting. CVN tests were used to assess the effect of cooling profile on toughness. The amount of ferrite was increased at slower cooling rates and higher silicon contents. A relationship between allotriomorphic ferrite thickness and CVN toughness was investigated bysubmitting samples from the nine lab cast ingots to three heat treatments designed to yield different ferritequantities. It was found that toughness decreased with increasing silicon and ferrite thickness. Lower energyabsorption was observed when the allotriomorphic ferrite network is connected throughout the microstructure. A simple statistical model based on matrix correlation and linear regression was used to determine which variables influenced toughness the most – silicon content and heat treatment were found to be the most determining ones.