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Open Access Dissertation

Keywords

Steel founding; Steel--Inclusions--Prevention;

Abstract

A low level of non-metallic inclusions in steel is the key requirement in the concept of clean steel. Although the concept has received its name quite recently, it has been implemented at the large-scale steel mills for more than 40 years. Production of clean steel became possible due active development and adoption of the post-melting treatment of molten metal in the teeming ladle. Ladle metallurgy, an umbrella term for various methods of treatment, includes operations that cannot be performed during primary metallurgical processes such as melting in an electric arc, basic oxygen, or air induction furnace. Initially, the most common operations of ladle treatment were deoxidation and alloying. More sophisticated methods like vacuum degassing, chemical reheating, or injection of powdered refining mixtures have also been successfully implemented into steelmaking practice. Treatment of large quantities of steel at once made the ladle metallurgy operations economically sustainable.

Most of the steel foundries operate with much smaller quantities of metal than the steel mills. This makes most types of ladle treatment not cost effective. Also smaller quantities of molten metal would lose temperature quicker, endangering successful pouring of the castings. Also, the foundries typically use acid-based or neutral refractory materials that do not allow to form the basic slags necessary for refining.

On the other hand, the steel founders are striving to keep up with constantly rising quality standards for steel casting by the manufacturing industry. Therefore, the foundries are limited in selection of ladle metallurgy methods, and, in the same time, are facing the necessity of advancing their processing technology. Given such conditions and considering the experience from the steelmaking industry, a few methods of liquid metal treatment are potentially available to most of the steel foundries: (a) control of metal-slag reactions, (b) homogenization of the steel melt by inert gas injection; and (c) complex deoxidation of metal.

The major premise of the current study was to demonstrate experimentally how the extremes of the treatment practices affect certain indicators of steel cleanliness and to make conclusions on which practice yields better results from the metal quality standpoint. For that purpose, the experimental conditions were organized to replicates real industrial settings as close as possible. This affected the choice of melting method, the selection of raw materials and master alloys, as well as the choice of molding practice.

The experimental plan for the melting operations was designed to test how combinations of commercial and returning scrap, stirring of molten metal by argon, and removal of slag from metal surface affect certain response variables. The experiment on steel treatment in the ladle was conducted using four different combinations of deoxidizing alloys: a single element addition and three complex additions of two and three deoxidizers. The response variables in this study were the total concentrations of oxygen and nitrogen concentration in metal, chemical and physical specifications of nonmetallic inclusions, and impact toughness of the steel.

The analysis of experimental data on melting revealed that selected process variables impose a limited influence on the total content of nitrogen and oxygen in the metal. Statistical analysis of the experimental data determined that only the total nitrogen concentration was affected by the use of returning scrap steel and argon stirring, while the total oxygen content was found independent from all three selected process variables. Particularly, use of returning foundry steel scrap in combination with additional mixing created by argon bubbles would favor the increase of nitrogen concentration in the metal by the end of melting operation.

The results of the ladle treatment experiment confirmed that method of deoxidation affects the total oxygen content but does not significantly alter the parameters of inclusions and the impact toughness. The most significant decrease in total oxygen content was observed when a combination of strong and moderate deoxidizers was added. It was experimentally determined that prolonged holding of the metal in the ladle facilitates the removal of inclusions by floatation.

Variations in the impact toughness values were found to be in correspondence with the inclusions spacing, an average distance between inclusions. Also, the increase in the concentration of aluminum and nitrogen produced a negative effect on the impact toughness values. The investigation of inclusions morphology revealed that the samples bear a significant amount of large nitrides in plain and complex form. The growth of the nitride inclusions was attributed to the additions of titanium, a strong nitride-forming element.

Concluding the findings of the study, it can be said that the treatment of steel in the ladle, even in its simplified form, provided a set of effective tools for improving the cleanliness of steel. An appropriately selected combination of deoxidation additions, ladle holding time, and temperature could lead to a lower level of inclusions, and, hence, the greater values of impact toughness. On the practical side, despite the technological differences between steel processing at steel mills and foundries, the ladle metallurgy methods are applicable to the latter and have a great potential to the cast steel quality improvement.

Date of Award

2017

Degree Name

Doctor of Technology

Department

Department of Technology

First Advisor

Scott R. Giese

Second Advisor

Nageswara Rao Posinasetti

Date Original

2017

Object Description

1 PDF file (ix, 119 pages)

Language

EN

File Format

application/pdf

Included in

Metallurgy Commons

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