Steel advances

Tom Shelley reports on some of the dramatic advances now being made in very high strength steels that are low enough in cost to be used in mass market products

Some of the biggest advances being made in metallurgy at the moment are in steels, with TWIP steels and super bainite steels poised to go commercial for automotive applications and armour plate, and many advances being made in stainless steels.

Novel diamond hard coatings on steels have been shown to give a new dimension to stainless steel, and have potential applications in putting hard surfaces on other steels and most other kinds of materials, but some of the most striking advances are now being made in China, which currently produces nearly half the world's total output of steel and is putting a major effort into research and development aimed at producing new and improved products.

Corus is heavily involved in developing both TWIP and super bainite steels, which are a British invention, arising from work by Corus and a team led by Professor Harry Bhadeshia at the University of Cambridge Department of Materials Science and Metallurgy. Super bainite steels have an ultimate tensile strength of 2500 MPa, whereas ordinary 'mild' steel comes out around 400 MPa and carbon fibre offers 5650 MPa. They have hardnesses of 600 to 670 HV and a toughness of 30 to 40 MPa m1/2. A typical composition is around 0.8% carbon, 1½% silicon, 2% manganese, 0.25% molybdenum with small amounts of other metals. The steel is solution treated at 920°C for 30 minutes, plunged into a molten salt bath to cool it to a temperature below that at which the iron carbide will separate, but not so cold that the material will form the brittle martensite phase, then placed in an oven at 125° to 250°C for periods that can range from hours to days.

Lower temperatures produce finer structures, but require much longer times, adding to cost. The heat treatment temperatures are so low that carbon cannot diffuse far, so that the resulting plates of iron, separated by carbides of iron and other metals are only 20 to 40nm thick. The current big interest in this material is to make armour plate with holes in it – the idea is that the edges of the holes interfere with incoming projectiles, and DSTL and QinetiQ have been heavily involved in the development, but it potentially has many other applications, since it is not fundamentally very expensive, being potentially about one ninetieth the cost of maraging steels. It has reached the stage of being fabricated in tonnage quantities as plate, rolled strip and bars, although Corus is currently being coy about exactly when they intend to bring it to market on a commercial scale.

TWIP or TWinning Induced Plasticity steels, are said to be the invention of Professor Georg Frommeyer, head of the Department of Materials Technology at the Max Planck Institute for Iron Research in Dusseldorf. These contain about 20% manganese and small amounts of carbon, aluminium and silicon. The large amount of manganese means that even at room temperature these steels have the face centre cubic structure of austenite – the normal high temperature structure of plain carbon steels. When they are deformed, they harden by forming twins – areas of the crystal structure which form a mirror image to the rest of the structure. Dislocations – imperfections within the crystal structure - have difficulty traversing the boundaries of the twins, making the metal stronger, but do not so much difficulty that the material becomes brittle. Metals forming lots of twins work harden at a high rate, absorbing large amounts of energy as they do so. The big interest is in using them to improve the crash absorption of the structural parts of cars, while reducing weight, or improving crash resistance at the same rate. The only down side is that they also require more energy to form than normal steels, but there are ways round this.

Patrick Kelder, business development manager Corus Automotive told us that the company's current top commercial offering for the automotive market is 'DP800HyPerform', which is a steel with a ferrite-martensite structure and a minimum UTS of 780MPa and an elongation of 20%. This makes the material more formable than is usually the case for steels of this strength, where elongations are in most cases only 14% to 16%. It has a hot dipped, zinc coating and is easily weldable. It is said to be less expensive than TRIP (TRansformation Induced Plasticity) steels.

Deliberately forming nano sized twins at an early stage in face centered cubic structured metals has been found to result in dramatically greater strengthening effects according to a Chinese research group. Professor K Lu from the Shenyang National Laboratory for Materials Science was invited to give the Kelly lecture to the Department of Materials Science at Metallurgy at Cambridge on June 15th 2010, and introduced by Professor Kelly himself, author of the book, "Strong solids" and generally considered to be the father of modern fibre reinforced composites.

Professor Lu described how he and his colleagues had succeeded in making copper more than ten times as strong as it is normally, without adversely affecting its electrical conductivity. He then revealed, for the first time outside China, forgetting that we were present, that he had just succeeded in applying his methods to making AISI 316L stainless steel up to five times stronger, by a commercially viable manufacturing technique.

The trick is to come up with cost effective methods of producing twins on a 15nm or smaller scale in usable pieces of metal. The team started by using pulsed electrodeposition to produce foils up to 30µm thick. However, they then moved on to producing the effect by dropping a hammer onto copper samples in liquid nitrogen from a height of 7m. The top strength performance achieved so far is a yield stress of more than 600MPa, as compared with 70MPa for conventional annealed conductivity copper, while retaining 97% of the electrical conductivity. When applied to stainless steel, the low temperature forming process produces nano sized grains as well as nano twins, which make the material brittle, but this problem can be overcome by annealing the metal in such a way that the grains recrystallise to larger size, without making the twins thicken significantly. If this is done, ultimate tensile strength can be raised to 900 to 1400 MPa instead of 260 MPa. His latest work is to apply the same methodology to iron 25% manganese steel, at slightly higher manganese contents than the TWIP steels described earlier.

Leader of Chinese steel companies is Baosteel, headquartered in Shanghai, which has recently integrated its stainless steel R&D with its sales and market development operations in a new Stainless Steel Technology Center, adding to and improving the more than 60 grades of stainless that it currently offers. Latest developments are new ferritic grades for automotive exhaust systems and solar water heaters. The company produces more than 30m tonnes of steel per annum, three times the entire output of the UK, has a partnership with Thyssen Krupp and is fully ISO 4001 and 9001 compliant.

Stainless steel is also the demonstration substrate for extremely hard amorphous sp3 diamond coatings invented by Sergey Aleksandrov, now chief technology officer of Diamond Hard Surfaces in Towcester, which is engaged in commercialising his discoveries. Not only do these give steel and other substrates hardnesses of 4000 HV, they also result in friction coefficients lower than the 0.025 associated with polytetrafluorethylene. The original demonstration coating on stainless steel was only 12µm thick but coatings 40 to 50µm thick are now produced regularly. Applications abound in the chemical, industrial processing, electronics, medical, aerospace and oil and gas industries, where they are applied to oil drilling mud pumps and drill tools, increasing useful lives from 300h to 1200h, according the company's Chris Walker. Process temperatures are only around 100°C, so the process can be applied to coating infra red optics and suitable plastics.

Pointers

* Super bainite steel is the strongest low alloy steel ever produced, more than six times stronger than 'mild' steel yet should sell at a very acceptable price when it goes into commercial production. It has currently been produced as slab, bar and strip in tonnage quantities

* TWIP steels are designed to absorb exceptionally large amounts of deformation energy during vehicle crashes by twinning

* The best performing high strength steel that is currently available commercially for automotive use has a UTS of 800MPa and an elongation of 20%

* Deliberately inducing nano sized twins in copper, stainless steel and steels close to the compositions of TWIP steels has a dramatic effect on strength, increasing that of pure copper for electrical devices by ten times and AISI 316L stainless steel by five times

* A new low temperature diamond coating process raises surface hardnesses of steels and other materials to 4000V while imparting coefficients of friction of less than 0.025

Author
Tom Shelley

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