Hydrogen is a small interstitial atom in steel. Thus it has high mobility and it can easily diffuse through the lattice causing problems such as hydrogen embrittlement (badly affecting mechanical properties) and internal defects especially in high
strength steels. The detrimental effect of hydrogen is due to its solubility behaviour. Hydrogen solubility in liquid steel is considerably higher than in solid steel. As a result, hydrogen absorbed in liquid steel and/or in solid steel just after solidification can be retained in the structure.
During cooling, the supersatured hydrogen can be accumulate in structural defects (traps). If the hydrogen concentration is high enough and the local site is susceptible, hydrogen atoms can recombine in gaseous one increasing the internal pressure at levels that can exceed the mechanical strength of the steel, inducing cracks. Since the formation of defects depends on the hydrogen content, it is directly related to the steel production process, semiproducts and products thickness and cooling conditions. Moreover, hydrogen detection on final products is not an easy task, being a destructive test. Thus it is necessary to have a reliable tool to predict hydrogen content in steel products based on their thermal history and on the knowledge of the mechanism of hydrogen diffusion.
Therefore, the objectives of this RFCS project are:
- developing a mathematical model able to minimize the hydrogen content in liquid steel by assessing the best steelmaking practice to the best vacuum degassing conditions;
- developing a mathematical model able to minimize the final hydrogen content in final products by assessing thebest cooling strategy both after casting and after thermo-mechanical treatments for all the investigated steels.
In order to achieve these main objectives, other intermediate objectives will be gained, that are summarized in the following:
- analysis of all the possible hydrogen sources in liquid steel and during casting;
- determination of the best method to control hydrogen pick up in liquid steel and during casting;
- determination of hydrogen diffusion coefficients at high temperature in order to use these data as inputs for the mathematical model that will be developed;
- assessment of hydrogen critical concentrations for different steel grades investigated during the project.