Abstract
Super-white spots (SWS) on steel billets are one of the most important surface defects in steel production. The causes and effects of the formation of SWS are very complex and are closely related to the quality of the steel products. It is therefore important for steel producers to understand the mechanism of SWS formation and the factors influencing its formation. In this paper, the mechanism of SWS formation, the influence of various factors on its formation, and preventive measures are discussed with an aim to improve the production process of steel billets to avoid its formation.
Introduction
Steel billets are semi-finished products used in steel manufacturing. A defect that may occur during the production process is the formation of super-white spots (SWS) on the surface of the steel billets. SWS appears as white dots on the surface of the billets. It is a very common defect and is a matter of concern to steel producers, because it affects the aesthetic appeal of the steel products and is potentially detrimental to the further manufacturing process.
SWS is a surface defect that is formed during solidification. Its formation has complex mechanisms and is affected by various factors, such as the chemical composition of the steel, the process parameters, and the environment which surrounds the steel billets. On the surface of the steel billets, the SWS is mostly composed of oxygen, carbon and nitrogen, which are mainly dissolved into the steel during the casting process.
The formation of SWS has an adverse effect on the appearance of the steel products, leading to possible complaints from customers, lower sales, and losses for steel producers. Therefore, it is important for steel producers to understand the mechanism of SWS formation and the factors that contribute to its formation. The aim of this research is to analyze the mechanism of SWS formation, the factors influencing its formation, and preventive measures that can be taken to avoid its formation.
Mechanism of SWS Formation
SWS formation is a complex phenomenon that is caused by a combination of factors, such as chemical composition, heat transfer, and local air flow. The process of SWS formation can be divided into three stages: 1) oxygen dissolving; 2) solidification; and 3) nucleation.
In the first stage, oxygen is dissolved in the steel due to the effects of heat and pressure gradient, which increases the partial pressure of oxygen in the environment. Depending on the chemical composition of the steel, the rate of oxygen dissolution will be different.
In the second stage, solidification takes place in the molten steel due to the effect of cooling. The solidification process of steel is a heterogeneous nucleation process. As the molten steel cools, oxygen is also released from solution creating nucleation sites on the surface of the billet.
In the third stage, nucleation takes place when enough oxygen is released from solution on the surface of the billet. SWS is an oxygen-rich segregation that has a high oxygen content and a very low melting point, which means it can form on the surface of the steel billet.
Factors Influencing the Formation of SWS
``There are several factors that can affect the formation and growth of SWS, these include
1. Chemical Composition: The chemical composition of the steel has a great influence on the formation of SWS. The most influential elements are oxygen and nitrogen, and their content in the steel can significantly affect SWS formation.
2. Process Parameters: The process parameters, such as casting speed, pouring temperature, surface cooling rate and mould sand quality, can greatly influence SWS formation, thus the process parameters must be strictly controlled to avoid SWS formation.
3. Environment: The environment around the steel billets, such as the atmosphere and the local air, can also affect the formation of SWS. The atmosphere near the steel billets must be kept clean and free of any impurities to avoid SWS formation.
Preventive Measures
In order to prevent the formation of SWS on steel billets, measures must be taken to control the various factors that can affect its formation.
1. Chemical Composition Control: The content of oxygen and nitrogen in the steel must be strictly controlled to avoid the formation of SWS.
2. Process Parameter Control: The process parameters, such as the casting speed, pouring temperature, and mould sand quality must be strictly controlled.
3. Clean Environment: The environment around the steel billets must be kept clean and free of any impurities to prevent the formation of SWS.
Conclusion
SWS is a common defect in steel production, and its formation has adverse effects on the steel product quality. While it is difficult to completely eliminate SWS, it is possible to reduce its occurrence by controlling the various factors that influence its formation, such as chemical composition, process parameters, and environment. Understanding the mechanism of SWS formation, the factors influencing its formation, and preventive measures are the key to successfully reducing or eliminating SWS.
