Building the Backbone of MOWSES: Defining the Test Matrix for Green Structural Steels

A central milestone in the MOWSES project has been reached with the definition of the project’s test matrix, a set of steel compositions that will underpin all experimental investigations in the coming years.

Why a test matrix is needed

As Europe moves towards more sustainable steel production, the use of recycled scrap in Electric Arc Furnace (EAF) steelmaking is increasing. While this significantly reduces CO₂ emissions, it also introduces so-called residual elements, such as copper, tin, arsenic, antimony, lead, bismuth, or zinc. These elements cannot easily be removed during steelmaking. Even in very small amounts, they can influence strength, toughness, weldability, and processing behaviour, especially in demanding applications like bridges or offshore structures. To understand and control these effects, MOWSES does not test random steels. Instead, it relies on a carefully designed test matrix: a defined set of steel compositions where “pure” steel is alloyed with individual elements and combinations of elements. This approach allows researchers to link changes in the mechanical behaviour of the steel directly to specific chemical features.

Defining realistic reference steels

The starting point of the test matrix is the definition of reference steel grades that represent widely used structural steels in Europe. The consortium agreed on three grades to cover a broad strength range and different delivery conditions:

• S355N (normalised),
• S550M (thermomechanically rolled)
• S690QT (quenched and tempered)

Together, these grades span minimum yield strengths from 355 MPa to 690 MPa and reflect typical steels used in critical infrastructure. Their chemical compositions and delivery conditions were chosen to ensure industrial relevance while allowing meaningful scientific comparison.

From single elements to complex combinations

Building on these reference steels, a working group lead by Dillinger defined a single-element variation matrix, in which individual residual elements such as Sn, As, Sb, Pb, Bi, Zn, or Co are added in separate heats at defined concentrations. This approach makes it possible to isolate the specific effect of each element on mechanical properties, phase formation, and weldability. However, real scrap-based steels rarely contain only one residual element. Therefore, the working group also developed a multi-element combination matrix. Here, binary, ternary, and multi-element systems, up to seven residual elements at once, are defined to simulate realistic contamination scenarios from mixed scrap. The matrix is structured in two stages, starting with lower-risk combinations and progressively moving towards more challenging systems that include low-melting elements such as lead and zinc.

A foundation for experiments and modelling

The test matrix is not only a list of compositions, but it will also guide thermodynamic modelling (CALPHAD), laboratory steel production, and experimental testing such as Gleeble simulations, welding trials, and mechanical characterisation. By defining clear reference points and controlled variations, the matrix ensures that results from different partners and test methods can be compared and combined into a coherent picture.

Enabling greener and safer steels

With the test matrix in place, MOWSES is now equipped to address a core question: how much scrap-related residual content structural steels can tolerate without compromising safety and performance. Led by Dillinger, with key contributions from TU Delft (CALPHAD simulations) and OCAS (definition of the S355 reference steel), this work lays the foundation for defining safe limits and enabling greener, contamination-tolerant steels for European infrastructure.

Some examples of residual elements tested within MOWSES: