Killed steel, or steel that has had all of the oxygen removed from it, is achieved through a deoxidizing process in order to make an ideal steel for forged products. Oxygen can become incorporated into the steel as bubbles during the steelmaking process, and can combine with carbon to make carbon monoxide bubbles. These bubbles become trapped in the casting, causing failure points in the steel.
In order to eliminate these carbon monoxide bubbles, manufacturers add deoxidizing agents, such as silicon, aluminum. If the manufacturer chooses to use aluminum, the oxygen will combine with it to from alumina (Al2O3), which can prevent the grain size from increasing during the heat treatment process. Since the oxygen has been removed, the steel has a more homogeneous quality than steel with oxygen, as well as freedom from porosity, segregation, and blow holes.
If all the oxygen is not removed, the steel is referred to as semi-killed steel. This steel has around 0.15% to 0.25% carbon, slightly less than the killed steel carbon content. These steels are used more for structural purposes, since they have the majority of the killed steel properties, such as the uniformity. The goal in creating semi-killed steel is to produce steel free of surface blowhole and pipe, and close monitoring is needed to prevent additional oxygen inclusions or oxides from incorporating into the steel.
Obtaining killed steel requires close monitoring of the oxides quantity during the steelmaking process. For this analysis, Aspex has equipped its Personal Scanning Electron Microscope (PSEM) with an up-to-date inclusion analyzer software platform, called Metal Quality Analyzer (MQA). Since the goal of killed steel is maintaining more than 0.25% carbon, and the elimination of oxygen, the MQA software can be set up on the steelmaking line. This allows the scanning of samples before they reach the end product, and allows the manufacturer to adjust the steel mixture, if needed.
The MQA is capable of scanning and categorizing upwards of 10,000 inclusions per hour, giving manufacturers quick, real-time results. In order to have steel properly evolved into killed, semi-killed, or another grade, the steelmaking process must have a way to monitor the oxygen inclusions within the steelmaking line. The Aspex PSEM with MQA software can size, count, and analyze the oxides within the steel sample, saving the manufacturer costly re-melt time and resources and producing a high quality product with a lower chance for failure due to trapped carbon monoxide bubbles.
Reference:
Aspex, http://www.aspexcorp.com/industries/metals.html
AzoMaterials, http://www.azom.com/details.asp?ArticleID=1697
Steeltalk, http://www.steeltalk.com/classification_other.php
