An Overview of the Solidification Structure of Austenitic Steel, Martensitic Steel and Continuously Cast Bloom
Introduction
When steel is produced and passed from a liquid state to a solid state, it goes through a process of solidification. This transformation is governed by the laws of thermodynamics, and ultimately, the solidification structure of the steel is directly related to its cooling rate. Depending on the rate at which steel cools, different solidification structures can form. This paper will provide an overview of the solidification structure found in three of the most common types of steel: Austenitic Steel, Martensitic Steel and Continuously Cast Bloom (or Continuous-Casting Steel).
Austenitic Steel
Austenitic Steel is an iron-chromium-nickel alloy, known for its excellent malleability and ductility. This steel must be cooled quickly and evenly, as an improper cooling rate can cause the formation of martensite, which will significantly decrease the ductility and malleability of the steel.
The proper cooling rate can be achieved by introducing a uniform cooling medium, often water, to the heated steel material. As the steel cools, a columnar dendritic structure is formed, composed of austenite. As each austenitic grain cools, a small amount of ferrite or pearlite is formed.
Martensitic Steel
Martensitic Steel is another type of iron-chromium-nickel alloy, known for its strength and hardness. While Austenitic Steel is cooled at a fairly rapid rate to form austenite, the solidification of a Martensitic Steel requires a much slower cooling, or quenching, process in order to create a martensitic structure.
In this process, the steel is cooled from an elevated temperature, at a rate that is slower than austenitic cooling. As it cools, the previously existing austenite undergoes a transformation, creating smaller grains of martensite. The number of martensite grains created is directly related to the length of time the steel is quenched, as well as its starting temperature. The more time and temperature it is exposed to, the more martensite transformation occurs.
Continuously Cast Bloom
Continuously Cast Bloom (or Continuous-Casting Steel) is a steel production process whereby steel is melted and is continuously cast into a shape, typically in the form of a bloom. It is a process commonly used for steel production that is both cost-effective and efficient.
During the process of casting, the molten steel is quenched rapidly in order to maintain its shape and give it added solidity. As the steel cools, it forms a columnar structure, composed of primarily pearlite, as well as some ferrite and a small amount of martensite. The columnar structure is beneficial, as it is much better suited for downstream processing than other structures.
Conclusion
The solidification structure of steel is greatly dependent on its rate of cooling, as well as its starting temperature. Austenitic Steel must be cooled quickly, in order to form an austenitic structure composed primarily of austenite. Martensitic Steel must be cooled much slower, so that its austenite grains can be transformed into martensite. And lastly, Continuously Cast Bloom must be cooled quickly so that a columnar structure composed of mostly pearlite, with a small amount of ferrite and martensite, is formed. All of these processes occur to maintain the structural integrity, as well as the mechanical properties, of the steel.
