High-strength steel: strong, stronger, strongest
Over the past five years, ArcelorMittal Gent has worked intensively with ArcelorMittal Global Research and Development Gent (also known as OCAS) on the development of a range of steels with high-strength properties in both the hot rolled and the cold rolled and zinc-plated states. These steels are primarily used in the automotive industry. For the ultra-high-strength types, however, applications certainly exist in other markets, such as energy transport (gas or oil pipelines).
High-strength steel: a development rather than an invention
Figure 1: overview of cold rolled high-strength steels
The development of high-strength steels involves proper application and the best possible combination of a number of fundamental metallurgical principles.
- The most efficient way of increasing the strength is by refining the grain of the microstructure. This is achieved by controlling the treatment of the steel in the hot strip mill and during annealing, and by adjustment of the steel's chemical composition.
- A second method is solid-solution hardening. This method entails the addition of alloying elements such as Mn, Si and P. The last one in particular has a great influence upon the strength, even in small quantities.
- A third method involves the creation of precipitates such as titanium carbide (TiC) and niobium carbide (NbC) in the steel. These particles, which may measure as little as a few nanometres (1 nm = 0.000000001 m), have a major effect upon the final mechanical properties. The first steels to make major use of this method are the microalloyed steels, which have a strength in the order of 500 MPa.
Figure 2: MnS precipitate as a feeding-ground for TiN precipitates
- In order to attain even higher strengths, the various phases in the microstructure have to be modified. Steel normally consists of a ferrite matrix in which Fe3C particles (cementite) are distributed. By optimization of the process parameters, however, metastable phases such as bainite, martensite and austenite can be obtained. They substantially enhance the strength, and in some cases other special properties such as toughness.
High-strength steels are typically grouped as follows:
Figure 3: Typical microstructure of an IF high-strength steel (a), a dual-phase steel (b) and a TRIP steel (c)
ArcelorMittal Gent was the first ArcelorMittal plant to develop the following new products:
- High-strength interstitial-free (IF) steels: these are degassed products with a complete ferritic matrix to which Mn, Si and P are added, on order to attain strengths of typically up to 260 MPa. These steels may also have a bake-hardening effect, that causes a little residual free carbon to give rise to a further increase in strength during the paint baking at the (automotive) customer. The great advantage of these steels is that they can be used for external car body parts.
- High-strength low-alloy (HSLA) steels or microalloyed steels in which strength is attained by the precipitation of Nb(C,N) and Ti(C,N). These steels have already formed part of ArcelorMittal Gent's product range for some time; they are, however, continually being improved.
- Dual-phase steels consisting of a ferrite matrix containing small islands of martensite. These steels are characterized by a favourable deformation behaviour and by the absence of a fluid threshold without skinpass, which makes them unsusceptible to ageing. Improved hardenability of the steel is necessary to obtain the martensite phase. VCr and Mo are added for this purpose.
- TRIP (transformation induced plasticity) steels are characterized by a ferrite matrix containing a fine bainite phase and restaustenite. These steels owe their excellent formability to the last phase, and typically have high Al and Si contents.
- ULC BH 260, hot-dip galvanized, for external automotive components
- TRIP 590, 700, 780, hot-dip galvanized
- DP1000, hot-dip galvanized
- DP500, hot-dip galvanized, for external automotive components
The use of transformation induced plasticity (TRIP) steel, which combines high strength with exceptional formability, enables our automotive customers to make important progress in the field of passenger safety and weight savings (environmental responsibility).
Since the beginning, in the late 1990s, the developments progressed particularly in the attained tensile-strength level, reaching 700MPa (TRIP 700). Thanks to close cooperation between researchers from a number of disciplines and representatives from the various production divisions, ArcelorMittal Gent was able to produce the first hot-dip galvanized TRIP coil in 2001. That was a first for the ArcelorMittal group.
Application tests performed by certain leading automobile manufacturers indicated, however, that scope still remained for improvement, particularly in the area of spot-weldability. In the course of 2002, we then developed an adapted concept involving less carbon, which resulted in the ArcelorMittal product attaining the same standard as that of our chief competitors. At the same time, we continued laboratory-scale research into ways of reducing the carbon content even further, by adding phosphor (P). These efforts were rewarded in mid-2003 by industrial-scale implementation at ArcelorMittal Gent of a hot-dip galvanized TRIP 700 type with the lowest carbon content (best spot-weldability) anywhere on the European market. Within one year, we had therefore achieved an exceptionally important breakthrough in the area of weldable and hot-dip galvanized TRIP steels.
At the end of 2003, we expanded this patented concept further, so that ArcelorMittal Gent is now able to offer a complete range (590-780 MPa) of hot-dip galvanized TRIP steels. This achievement would not have been possible without the close and sound relationship between the ArcelorMittal Global Research and Development Gent and ArcelorMittal Gent.
Parallel to these efforts, a concept for galvannealed TRIP - initially developed by Nippon Steel in conjunction with ArcelorMittal's Ledepp laboratory - was produced at ArcelorMittal Gent, resulting in full-hard material. This material was then zinc-coated in Bregal and Mardyck. This industrial test was completed so rapidly that it caught the attention of the senior management at ArcelorMittal Auto.
At the "Journées Sidérurgiques Internationales", accolades are awarded each year to a number of projects that are the product of an expressly multidisciplinary approach. ArcelorMittal Gent / ArcelorMittal Global Research and Development Gent was nominated for the award and received the ATS prize for the development of hot-dip galvanized TRIP steel. The award is a recognition of strong and sustained teamwork.