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Materials Engineering

Innovations in steels

Researchers develop advanced materials with better electrical properties and more resistance to corrosion

EDUARDO CESARSheet steel at Brasmetal: passing on results of research guarantees better production process EDUARDO CESAR

Steel is gaining some innovations that are going to make it more efficient. The technological innovations proposed for this traditional family of materials, fundamental for making a wide range of industrial products  from knives and forks to motors and bone prostheses -, arose from an extensive study by a group of São Paulo researchers that is dedicating itself to researching and perfecting the microstructure and the properties of these materials.Made up of researchers from the Polytechnic School of the University of São Paulo (Poly-USP), from the State of São Paulo Technological Researches Institute (IPT) and from the Institute of Nuclear Energy and Research (Ipen), the team has, for example, succeeded in producing in the laboratory stainless steels that are 15 to 20 times more resistant to cavitation (the appearance of small holes in the surface of the material) than the current ones.

It has also developed sheets more suited to the process of cold stamping (shaping, by pressing, to get bodies for cars, household appliances, etc.), besides more effective electrical steels, proper for use in refrigerator motors, in air conditioning apparatuses and in the transformers for televisions sets and computers. “We have improved the quality of these steels, modifying their crystallographic texture (the distribution of the directions of the grains that form the material’s microstructure) in such a way as to raise the mechanical, electrical and magnetic properties and resistance to wear and to corrosion”, explains materials engineer Angelo Fernando Padilha, from the Poly-USP, who is coordinating the study, which is financed by FAPESP, through a thematic project started in May 2000.

One of the main advances achieved by the team relates to stainless steels, a group of steel and chrome alloys that accounts for a world-wide production of 12 million tons a year. They have as their main characteristic their resistance to corrosive and oxidating agents, and they are used in household utensils, hospital equipment, and even in hydroelectric power stations turbines. The studies offer solutions for combating cavitation, a flaw common in stainless steels, particularly damaging for submerged equipment and those that undergo great variations in pressure in this environment, such as ships’ screw propellers and hydroelectric turbines. “To avoid this problem, we developed a special stainless steel with a given quantity of nitrogen – between 0.5 and 0.6% of the mass – on the surface of the metal. With the addition of nitrogen, the metal becomes harder and highly resistant to cavitation”, explains engineer André Paulo Tschiptschin, from the Poly-USP, who is coordinating this line of research.

To receive the nitrogen, the piece of steel is put in a reactor under a high temperature, over 1,000º Celsius, where there is a gaseous atmosphere that penetrates into the material’s crystalline structure. “In laboratory tests, these new steels are from 8 to 25 times more durable than the current ones. We are now preparing a patent to cover the use of this new alloy in turbines submitted to great pressure variations in water”, the researcher says. According to Tschiptschin, the studies proved that the texture of the material is very important for increasing resistance to cavitation. “That is why we are also working on the distribution of the directions of the grains on the surface of the metal, trying to create more efficient textures. We observed that cavitation occurs preferably in certain kinds of shapes of grains, and we used the strategy of processing the material to avoid these formations.”

Biocompatible steels
The group is also working on the improvement of stainless steels with a view to their use in making permanent bone prostheses. This material is still used only in temporary prostheses, such as pins, plates and screws. Its use in permanent prostheses, such as those in the hips, is not recommendable, for not being sufficiently biocompatible, nor resistant to some types of corrosion. The materials most employed in permanent prostheses are titanium alloys and alloys of chrome-cobalt-molybdenum, which have the disadvantage of being expensive. In Brazil, they cost about US$ 4,500, while the stainless steel ones work out at US$ 600. “We are researching into how to improve the stainless steel prosthesis by adding high levels of nitrogen, in the range of from 0.2% to 1.2% of their mass”, Tschiptschin says. “This brings a considerable reduction to the problems of biocompatibility and corrosion. We have now improved the chemical composition of the prostheses and carried out all the tests specified in the standards fortheir acceptance. We are now perfecting the texture of the steel, because the deterioration of the material is also related to this”, says the researcher.

Also with regard to stainless steels, the group has advanced in the study of ferritic-austenitic steels, also called duplex steel. Created in the decade of the 1970’s, this material is much used in environments that call for great resistance to corrosion, such as centrifuges for producing soap in chemical industries and hydraulic pumps that work in the oil industry and in the mining industry, in contact with muddy surroundings. “Amongst other aspects, we studied the directional relationship between the two phases (ferritic and austenitic) of these steels. When this material has to be shaped (or cold stamped, in other words) for it to take on its final form, texture is a fundamental factor in its behavior when cold stamped. That is why we need to select the thermomechanical processes that are going to induce the texture that is suitable for shaping the material”, explains physicist Nelson Batista de Lima, from Ipen’s X-Ray Diffraction Laboratory.

Electric motors
Besides the advances in the area of stainless steels, the researchers have made headway in perfecting the characteristics of the electric steel with silicon used in electric motors and transformers for various items of electronic equipment. These steels, which contain in their composition about 2% of silicon, play a crucial role in the world’s energy matrix, because roughly 50% of the electricity produced is consumed by electric motors. “Our work has a single objective: to develop steels for electromagnetic purposes that have a lower dissipation of energy when in operation. This means that the overheating of these motors has to be reduced”, explains metallurgical engineer Fernando Landgraf, a researcher at the IPT and one of the project’s sub-coordinators.

The function of steel in an electric motor is to expand the magnetic field. Accordingly, the forces of magnetization are amplified, and consequently the power of the motor. As the motor is magnetized and demagnetized 60 times a second, the steel heats up and energy is dissipated. This is an undesirable side effect of the workings of electric motors, called hysteresis loss. “This is what has to be brought down. Our big challenge is to produce steels of a better quality and thus reduce this kind of loss”, the researcher says.The quality of electric steels, in turn, is related to five factors: the average size of the crystals that make up the alloy, the number of crystalline defects, the number and the size of microcrystals of impurity (called inclusions), and the crystallographic texture, the spatial direction of the crystals. “Steel manufacturers now have good recipes for controlling the first three variables and for reducing hysteresis losses. The problem lies in controlling the texture”, says Landgraf. “At least 30 groups world-wide are trying to discover a method that makes it possible to make the ideal texture for minimizing these losses.”

In a steel with a perfect texture, the cube-shaped crystals ought to be distributed in such a way that they all have one of their sides parallel to the surface of the plate. Moreover, they should be spread out at random, in such a way that when a magnetic field is applied from any direction, a large quantity of crystals always prevails with an edge parallel to the direction of this field. “We are pursuing this dream by two different paths: one based on a new technology for making steels, called continuous strip casting, and the other in the conventional technology of continuous casting”, says Landgraf.

According to this researcher from the IPT, the new method offers a great economic advantage, because using liquid steel it is possible to produce sheets 2 millimeters (mm) thick, compared with the 250 mm of the traditional process. The sheets for making electric steels should have a thickness of 0.5 millimeters. “The new technology provides for great savings in resources in the casting process and, in addition, it produces electric steels with the ideal texture. Except that, when the steel undergoes a further rolling to reduce its thickness from 2 to 0.5 millimeters, the texture, which was perfect, suffers a drastic deterioration”, explains Landgraf. “Our efforts are directed towards recovering the ideal texture, equal to that existing at the end of the process of continuous strip casting, by controlling the rolling and the heating.”

The good news is that the group from the IPT achieved, one year ago, some encouraging results. “After much research, we managed to get 20% of the crystals to keep the side of the cube parallel to the surface. In the course of 2003, we repeated the process and confirmed it four times. It was an important discovery, which should have an impact at the world level. That is why we are at the moment of finalizing the deposit of a patent for this new technique”, the researcher says. To get a notion of the progress that this represents, suffice it to know that the electric steels produced today have only 5% of their crystals properly orientated. Putting 20% of the grains in order, the researchers reduced losses with the dissipation of energy by 20%. This made the motors become 3% more efficient.

The other path pursued by the team consists of an improvement in the conventional process of continuous casting. In this, the steel is hot-rolled until reaching a thickness of 2 millimeters and, next, undergoes a cold rolling that leaves it 0.54 millimeters thick. At a later stage, the steel is heated and once again cold rolled, to reach the ideal thickness of 0.5 millimeters. Afterwards, it is reheated, to increase the size of the crystals, to eliminate some flaws, and to develop texture. “In this technology, we studied some alterations, for there to be, at the end of the processing of the steel, a larger number of crystals arranged in an ideal manner”, the researcher explains.

Carbon steels
Another of the project’s lines of research is linked to a better performance of the ultra-low carbon steels (with a percentage of carbon lower than 0.005) also used in cold stamping processes. These steels are made up, almost totally, of iron, because the concentration of carbon is very small. They are employed in making the casings of electric irons, grill covers, locks, hinges, fenders, automobile bodies etc. Getting a more advanced texture than the current one is important because it will bring about fewer losses in production and making better use of the material. The information produced by the group is being passed on to the reroller Brasmetal-Waelzholz, a joint venture made up of 51% from the Brazilian Vidigal group and 49% from the German parent. Set up in Diadema (SP), the company has been carrying out work in partnership with the Poly-USP for more than ten years.

“Thanks to research, we have managed to improve the process of making some of our products, such as casings for electric irons and air and oil filter cases, raising considerably the approval rate. The production of these materials is complex, because their shaping, or molding, is very difficult, which jeopardizes the success of the cold stamping. With the knowledge generated by the project, the pass rate is now 100%”, explains metallurgical engineer Antenor Ferreira Filho, Brasmetal-Waelzholz’s industrial director. The company does not disclose what the pass rate was previously, and has still not calculated the economic gains coming from the use of the optimized process. According to Ferreira Filho, the optimization of the project was only possible as a result of a better understanding of the aspects involving the texture of low carbon steels.

To do so, the researchers were able to count on the assistance of modern equipment, such as a scanning electron microscope, installed in the Polytechnic School, which makes it possible to carry out microtexture analyses by means of a sophisticated technique called Electron Back Scattered Diffraction – EBSD.The fourth subject for study for the group are the iron aluminates formed between atoms that correspond to iron and aluminum alloys that show a well-defined chemical composition and have as a characteristic the fact of being hard and, at the same time, fragile. “The materials based on these compounds are given the name of ordered intermetallic materials”, explains physicist Cláudio Geraldo Schön, from USP’s Polytechnic School, who is also taking part in the thematic project.

New compounds
The researches aiming at the development of these compounds have advanced in several parts of the world, because the intermetallic materials based on iron, nickel and titanium aluminates offer a series of advantages, when used for structural applications, like the turbine blades used in the generation of thermoelectric power. “As iron aluminates have high levels of aluminum, they are lighter than steel, and their resistance to corrosion and oxidation is very high. Moreover, these materials are characterized by an ordered structure, which gives them excellent structural stability and, as a consequence, good mechanical resistance to high temperatures”, Schön explains.

The problem not yet solved by the researchers is the mechanical fragility of this compound at room temperature, induced by the introduction of a great quantity of aluminum. “These new materials have a low resistance to impacts and a loss of malleability. While in traction tests steel can be stretched up to 40% over its original size, the intermetallic compounds only reach 4%”, says the researcher from the Polytechnic School. To control this limitation, two approaches can be adopted. The first, based on a chemical route, provides for the addition to the alloy of chrome and boron, amongst other elements, to increase its ductility, that is, is capacity for deforming without breaking. The second route consists of submitting the material to a thermomechanical processing, by means of controlled rolling. The researchers believe that the increase in ductility has a correlation with a special texture developed in the course of rolling.

used this method to increase the lengthening A group of scientists from the University of Science and Technology Beijing has successfully to 17%. The team coordinated by Schön is investigating the causes of this phenomenon. “Our research group is working on the characterization of these textures, during all the stages of thermomechanical processing, to make it possible to understand the mechanisms that cause the improvement in the properties of the iron aluminates”, explains Schön. “We are doing basic research, but there is a great potential for a technological application in the medium term, for example, in the automobile industry, in the manufacture of parts for internal combustion engines that work in contact with high temperature exhaust gases”, the researcher says.

Workshop of weight
The success of the project, which has now given rise to a book and a CD about crystallographic texture, can be measured by the group’s scientific production, which includes the reproduction of 35 articles in foreign periodicals, the publication of 71 works in annals of Brazilian and international events, and the production of nine dissertations for master’s degrees and three doctoral theses. In almost four years of work, about 60 people took part in the project, including researchers, postgraduate and scientific initiation students, and technicians with secondary or higher qualifications, besides the coordinators and another seven subcoordinators of the thematic project. “At least a dozen young researchers have been trained and given solid knowledge and independence to carry out researches in the areas of crystallography and texture, known as Grain Shape Engineering”, explains engineer Angelo Padilha.

Furthermore, the team that is coordinating the thematic project, organized, at the beginning of December, the 2nd Workshop on Texture and Relations of Orientation, held at the Ipen, which enjoyed the presence of over a hundred participants, amongst researchers from the main research groups in the area and engineers from companies producing and processing flat metallic materials, where controlling texture is particularly important. “It is not just any country that can bring together over one hundred persons interested in discussing such technically elaborate subjects”, the researcher concludes.

The project
Optimization of the Microstructure and Mesotexture of Advanced Ferrous Materials (nº 99/10796-8); Modality Thematic Project; Coordinator
Angelo Fernando Padilha – USP; Investment R$ 345,075.08 and US$ 447,946.00