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New production technologies for thin-walled high strength steel components

Utilisation of new steel qualities and technologies will be applied for thin-walled high strength tube production. Trip effect assisted steels combined with bake hardening treatment will be utilised to achieve extraordinary properties.

Utilization of steels in the production of transportation vehicles, agricultural machinery, sports equipment, construction, power industry and other industrial branches increasingly requires reductions in weight and material consumption of structural parts, together with maintaining, or even enhancing, their static and dynamic parameters during loading. Besides utility features, technological properties are also significant to steel component manufacturers. Considering the fact that more than 90% of steel output is processed as wrought products and semi-products, both formability at higher temperatures and cold formability rank among the parameters that are investigated most frequently. Cold formability has received increased attention in the past two decades, when production of cold-drawn extrusions has been increasing significantly. The growth in production of this type is related to extensive development of technologies of drawing, hydro-forming and to the need to obtain final properties through forming. Satisfying the joint requirements of high cold formability and high strength instigated a new course of steel development: steels with controlled-formed functional microstructure. Among these rank the following: "bake hardening steels", two-phased steels, duplex steels, complex phase steels, and now also steels exhibiting the Transformation Induced Plasticity phenomenon - TRIP steels. TRIP steels, despite the short period of their development so far, possess extraordinary prerequisites for successful applicability. It is their advantageous combination of cold formability and resultant strength achieved through straining, which has no match among steels with ferrite-carbide structures. Currently, TRIP steels are used almost exclusively for production of sheets and pressings made of sheets. In the proposed project, it is expected that these materials would be also utilized in production of thin-walled high-strength tubes, pressings made of tubes and in production of sections. The project will be carried out on products, whose manufacturing requires high cold formability, and in which high utility strength is required. High cold formability through transformation-induced plasticity will be aided by attaining the following microstructure features: * Controlled formation of large amount of pro-eutectoid ferrite * Suppressed formation of pearlite * Enhanced formation of bainitic matrix * Stabilization of highest possible fraction of retained austenite * Achievement of very fine-grained microstructure. Fine-grained complex phase microstructure would be achieved either by means of thermo-mechanical treatment in the course of forming and controlled cooling, or through subsequent heat treatment, which would be specially adapted in order to produce the above-mentioned micro-structural features. Ultra-fine microstructure and proper composition of the phases form the first two essential prerequisites for high cold formability. Application of the increased plasticity phenomenon is the third prerequisite. In the case of TRIP steels, it is the transformation of retained austenite to strain-induced martensite. Through joint operation of the mentioned phenomena one can attain cold formability of up to 50%. Overall strengthening of the material will be produced above all in the last forming operation: the cold deformation. In this step, the work hardening will be joined by transformation strengthening, when retained austenite transforms to strain-induced martensite. Further strengthening can be achieved if the "bake hardening effect" is successfully applied to the experimental material. This phenomenon is based on precipitation of fine particles during low-temperature annealing (200 degrees C). In general it is possible to achieve strengths of up to 1,000 MPa in complex phase TRIP steels containing 0.2% C. Development of the material for individual applications will, above all, obey the following aspects: * Strength of individual products will comply with requirements of the end-user. * It will be possible to induce high stability in the austenite phase even at ambient temperatures. * Under certain conditions to be adhered to, the transformation-induced plasticity phenomenon will occur in the material during transformation of retained austenite to martensite. * The material's weldability must be good; therefore the carbon content will be significantly reduced. * The steel must be electroplated without problems. Production of high-strength thin-walled tubes will consist of hot rolling - mandrel rolling and subsequent sizing without mandrel - and cold drawing. Recrystallization annealing will be carried out between individual draughts in cold drawing. Heat treatment aimed at attaining a suitable microstructure for the TRIP effect will be performed prior to the last reduction. Thus, the TRIP effect will be induced in the last cold drawing operation, allowing for large strain in the last reduction and resulting in significant overall strengthening (*). Production of pressings from tubes will include pressing with a pressurized liquid or bending and pressing by means of solid tools. Rolled or drawn tubes, heat-treated to induce the TRIP effect in final reduction, will serve as initial semi-products (*). Production of sections will entail bending of rolled sheets, which were thermo-mechanically treated or heat-treated to achieve the TRIP effect (*). (*) Description of technologies and final products will be specified upon consulting individual production companies. Keywords: plasticity, strength, steel.
Acronym: 
TRIPFORM
Project ID: 
2 927
Start date: 
01-01-2003
Project Duration: 
36months
Project costs: 
880 000.00€
Technological Area: 
Iron and Steel, Steelworks
Market Area: 

Raising the productivity and competitiveness of European businesses through technology. Boosting national economies on the international market, and strengthening the basis for sustainable prosperity and employment.