Steel for a better engineering future

Tom Shelley reports on developments in affordable high tensile

steels that should have a marked effect on product design

A new formable and weldable high tensile steel sheet is just now coming available, nearly 50% stronger than its immediate predecessors and like to be very reasonably priced.

Its underlying technology was discovered way back in the nineteenth century, but it is only now that it has become possible to produce it in an easy to use form.

Immediate intended applications are in cranes and construction equipment, but its technology is quite generally applicable, and points the way to future developments likely to be two to three times stronger still.

The vast bulk of steel used today derives its strength from the presence of small amounts of carbon in various forms. The basic principles of enhancing strength by heating steel up and cooling it down quickly, usually followed by heating it again carefully, have been known for thousands of years. Patented cold drawn wire for mechanical springs, BS 5216, with carbon contents from 0.6% to 1% can be produced with tensile strengths up to more than 3,500MPa (226tsi), which, as a matter of interest, is similar to that of the strongest commercially available polymer, 'Kevlar' while being considerably less expensive. It is, however, almost impossible to either form or weld. On the other hand, mild steel, which contains little carbon, is very widely used because it is easy to form and weld, but has a tensile strength of only 430MPa (28tsi).

One of the main goals of steel metallurgists for more than a century, has been to try to develops steels with strengths in the right direction towards 3,500MPa, which can still be bent, cut and welded, and which can be made at an acceptably low cost.

SSAB Swedish steel has for some years been selling two ranges of higher tensile strength steel sheet products that can indeed be formed and welded. The company's 'Domex' range is hot rolled, with yield strengths (the point where the material starts to deform) up to 750 MPa or so, thanks to the presence of very small amounts of niobium or other elements. These combine with some of the carbon present to form a fine dispersion of very small, hard, particles. The other range of products, sold as, 'Docol' are hot rolled then cold rolled, heated up again so that precipitated iron carbide dissolves again, water quenched and then tempered (carefully heated again). The whole process can be undertaken on a continuous production line, so prices are not excessive, and ultimate tensile strengths (the point when the material breaks) are up to 1400MPa. Strength with formability results from their being a mixture of ferrite (essentially soft iron) interspersed with martensite, a hard phase in which the carbon atoms get stuck in the iron crystals, but are then released to form a very fine dispersion of carbide particles by the tempering process.

The new steel is quite different in that it is based on a phase called bainite. If steel is heated up until the carbon dissolves (austenite) and then allowed to cool slowly, the carbon comes out of solution as plates of iron carbide (pearlite). If the steel is cooled fairly quickly, and drawn as it cools, it is possible to ensure that all the plates are very thin, and aligned in the direction of drawing, which is how the 3,500 MPa wire is obtained. However, if the steel is cooled quickly to a few hundred degrees C, and then cooled slowly, it forms a phase called bainite, which looks like feathers under the microscope, but consists of ferrite, mainly iron, and a semi-regular dispersion of fine carbide particles. Researchers led by Professor Harry Bhadeshia working in the Department of Materials Science and Metallurgy at the University of Cambridge have been able to make bainitic steel with a tensile strength in excess of 2300MPa and a toughness of some 30MPa m1/2 . The bainite plates are only 20 to 40nm thick with very thin layers of austenite between the plates, the carbon being retained in the austenite by the presence of a "judicious" amount of silicon in the steel. The only downside of their development is that heat treatment processes require considerable amounts of time. In particular, the steels have to be carefully held at temperatures in the range 125 to 325 deg C for periods of 2 to 60 days.

The new product from SSAB, on the other hand, is hot rolled on a continuous line and the bainite produced by "Cooling the steel a bit differently" in the words of Borje Sundell, the company's senior technical adviser on R&D. The result is what is expected to be a very reasonably priced range of steel sheets with yield strengths in the range 800 to 900 MPa. "At these strength levels, design limitations are set by onset of buckling and not by tensile failure so engineers may need to re-think their designs to make use of it," Sundell cautioned.

Very small amounts of alloy additions (microalloying) help control the properties. Elongation is about 10%, which although not very great, does allow the steels to be formed into shapes. They may also be welded, although care has to be taken with weld design and process, as with all high tensile steels, so as to avoid cracking or just ruining the high tensile properties in the heat affected weld zones. Sheet thicknesses to be offered are 3mm and above, with first samples to be made available about now. Target markets are cranes and construction equipment, although the company has in the past achieved great commercial successes for customers seeking to reduce weight in truck bodies, railway wagons and transport containers while maintaining or enhancing life and reducing cost of ownership.

Commercial steels, with tensile strengths properties approaching 2500 MPa are still on the horizon. Dr Staffan Ekelund, managing director of the Swedish Institute for Metals Research told Eureka, "We are far from the possible physical limits of what can be done. We should still see a lot of progress. There are still things we can do to take advantage of time temperature transformation [heat treatment] curves. Unfortunately, steel is not a very fashionable field of research right now, so it is difficult to obtain government money." When asked about possible limitations that might arise from 'tramp' elements resulting from recycling scrap, Dr Ekelund replied confidently, "These problems can all be solved."

SSAB Swedish Steel
Phase Transformations & Complex Properties Research Group, University of Cambridge Department of Materials Science and Metallurgy
Dr Staffan Ekelund

" We are far from the possible physical limits of what can be done. We should still see a lot of progress."

Eureka says:

It is remarkable what can still be done to improve the properties of steel for general engineering use, despite its long history

Pointers

* New steel will be formable, reasonably priced, and have yield strengths of 800 to 900 MPa

* Elongation is around 10% so it can be formed. It can also be welded with care

* Low alloys steels with strengths of up to 2500 MPa are on the horizon

Author
Tom Shelley

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