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Structural steel has become one of the most prevalent construction materials of the century, often seen as an extremely important component of modern buildings and housing. According to the World Steel Association, over 1,600 million tonnes were produced in 2016, 197 million more than the previous year. It’s become viable for any kind of project and offers several benefits, which many building plans rely on for structural safety.

Availability

The widespread adoption of steel has made it easy to find, both as a raw alloy and pre-made components. Fabricated parts will often be openly sold by suppliers (with many factories selling both locally and overseas), allowing beams and frames to be purchased directly. Thanks to this, companies can work under tighter deadlines and access a supply of steel parts anywhere in the world.

Steel parts can be ordered as soon as the architectural plan is agreed on, saving time that would be spent waiting for them to arrive at the site. This provides extra time to check measurements and find suitable storage, issues that could normally delay construction by several hours.

Weight

Its lightweight makes steel easy to transport over land and lift via a crane, reducing the amount of fuel wasted getting it to the site. In addition, this can make buildings far easier to take down: a prototype ProLogic warehouse was built at Heathrow to demonstrate how over 80% of the entire structure was reusable, which could be disassembled in a fraction of the time an average warehouse would take.

Low weight can aid in moving and rebuilding structures, as shown with the 9 Cambridge Avenue warehouse relocation: the warehouse itself was dismantled and rebuilt 1 mile away, using almost no steel except the existing components. This added mobility and versatility makes steel a very desirable building material for structures that have extra land for expansion.

Sustainability

As the desire for eco-friendly buildings increases, steel will become more convenient for construction projects. It can easily be recycled and doesn’t need to be permanently disposed of, so old buildings or temporary supports can be repurposed into new projects as needed. Roughly 97.5% of all steel from UK demolition sites is recovered and reused, according to data gathered by Steel Construction.

Recovered steel components that haven’t been damaged can be re-used in other projects, removing the cost of getting the alloy melted down and re-cut as a new part. If a building is being demolished and rebuilt, existing parts could be stripped out and repurposed to save money kept in storage for future projects or simply sold to another company as components (or raw alloy, if sold back to a steel fabrication company).

Strength

Due to its high strength-to-weight ratio, less steel is needed in a single support or beam, reducing material costs and improving its sustainable nature. It can withstand strong physical impacts and forces, keeping building occupants safe, but won’t wear away or need to be replaced afterwards. This extra strength can be retained through the design, rather than the amount of steel used. Steel I-beams are often used in modern construction since they’re lighter, stronger and less wasteful than any wooden beam of the same size.

The natural fire and rust resistance of alloy steel makes it viable for exterior structures, such as fire escapes or balcony supports – MIMA also suggest possible use as external walls to contain insulating materials.

Many industries place strict requirements on the acceptable level of surface defects and imperfections that may appear on a plastic molded part. Such conditions often apply to critical components for the medical and pharmaceutical industries as well as for the manufacturers of lenses and other optical devices. However, for aesthetic reasons, many other consumer goods have similar restrictions. After all, any defect that appears on the surface of the mold steel is likely to be replicated onto the molded part.

Problems associated with the texturing or polishability of a mold cavity can often be traced back to the mold steelmaking process. The material properties that have shown the greatest influence on obtaining a good surface finish are the microcleanliness level, the severity of chemical segregation and the appearance of primary carbides.

Many grades of tool steel that are used for building molding components also are used for other industrial applications. For example, AISI S7 and H13 are commonly used for plastic injection molds; however, S7 also is used for metalforming operations and H13 for forging. The properties that are important for forging or stamping are quite different from the properties that are important to a moldmaker or plastics molder. Therefore, one must take precautions to ensure that they are using a mold quality steel. To do that one must consider the microcleanliness level of the mold steel, the degree of micro and macrosegregation and the restrictions on the number and size of large, primary carbides.

Standard Mold Steel Production

From the standpoint of the steel manufacturer, there are basically two means for improving existing steels used for molding applications:(1) Adjusting the chemical composition. Adding specific alloying elements and balancing their levels can significantly influence the characteristics of the material.(2) The actual steelmaking process. With the use of specialized melting techniques, mold steels can be produced which possess a very high microcleanliness level and homogeneous microstructure. These are two extremely important properties with regard to the steel's polishability and etching/texturing characteristics.

The Mold Steelmaking Process

In order to distinguish the different quality levels that are available for mold steels it is important to first have a fundamental understanding of the mold steelmaking process.

Mold steels are manufactured by melting starting material in an electric arc furnace (EAF). The starting material is comprised of carefully selected, low alloy scrap steel with the lowest possible level of impurities. Typically, the EAF units can melt as much as 50 tons of starting material per heat. Once the initial melting is complete, the molten steel is transferred to a ladle or refining vessel, where the composition of the steel is adjusted to provide the final chemistry. Additional steps such as slag treatments and degassing procedures also are involved to remove undesirable elements.

Following the refining stage, the molten steel is poured into large molds where it solidifies into a simple form called an ingot. These ingots will take several hours to completely solidify. This relatively long period of time will lead to significant amounts of chemical segregation, resulting in a variation in composition throughout the ingot's cross-section.

Segregation and Banding

During solidification of an ingot, the steel production process involves unavoidable segregation of the alloying elements. On a grain-size scale one talks of microsegregations; on an ingot-size scale they are referred to as macrosegregations. These inhomogeneities will exert a negative effect on the polishability and texturing characteristics of the mold, as previously discussed. In addition, the toughness properties will be degraded, particularly in the direction that is transverse to the primary hot forming direction.

The solidification process begins with the formation of crystals within the melt. These crystals have a tree-like, branching appearance and are referred to as dendrites. The first dendrites that form in the molten steel have a relatively low carbon content. As the freezing continues, these dendrites will become surrounded with the remaining liquid steel that is comprised of a higher carbon level. The melt that then freezes around the original dendrites will therefore have a different chemical composition.

If for example we examine an H13 steel – a material commonly used for molding applications – it is shown that segregation also occurs with regard to some of the other alloying elements. This variation in chemical composition will take place with respect to the chromium, molybdenum and vanadium additions. The variation in alloying element concentrations across a sample of steel can be measured with the help of an electron beam microprobe analyzer. This laboratory technique uses an electron beam to scan the surface of a sample of mold steel and determine the distribution of alloying elements.

However, it also is helpful to use a much simpler method to visually show these variations. For example, when polished and treated with an acidic solution samples of an H13 steel will reveal their relative levels of chemical segregation. These alloying inhomogeneities appear in the form of bandings. That is, the banded areas or zones within the ingot are comprised of various concentrations of the steel's alloying elements.