María José Quintana

Ultrafine Grained HSLA Steels for Cold Forming

The industrial level production of ultrafine grained (or ultrafine ferrite) ferrous alloys was investigated through three examples of steels that complied with the EN 10149-2 Euronorm and were produced by advanced controlled hot rolling techniques. The steel samples were tension tested and chemically analyzed, and the microstructure was evaluated through quantitative metallographic techniques to determine parameters such as yield stress, amount of microalloying elements, strain hardening coefficient, grain size, and grain size distribution. These steels were micro-alloyed with Ti, Nb, and Mn with ASTM grain sizes of approximately 13–15. The careful control of chemical composition and deformation during production, giving a specific attention to the deformation sequences, austenite non-recrystallization temperatures and allotropic transformations during cooling, are indispensable to obtain steels with an adequate strain hardening coefficient that allows cold working operations such as bending, stretching or drawing.

Mechanical behaviour of thermomechanically produced ultrafine grained dual-phase steels

Abstract Dual-phase (DP) steels are an excellent alternative in the production of automotive parts that require high mechanical resistance, high impact strength and elevated elongation. These materials are produced using low alloy steels as a basis, reducing costs and resulting in a combination of martensite and ferrite structures with ultrafine grain size. These characteristics are achieved through strict control of rolling conditions, strain rate, cooling rate and coiling temperature. This work presents the results of tension testing of two types of DP steels, along with microstructural characterisation, in order to understand the effect of the advanced thermomechanical controlled rolling processes on the formation of the microstructure and resulting mechanical properties. Les aciers biphasés sont une excellente substitution pour la production de composantes automobiles qui requièrent une résistance mécanique élevée, une haute résistance à l’impact et une grande élongation. Ces matériaux sont produits en utilisant des aciers faiblement alliés comme base, réduisant les coûts et engendrant des combinaisons de structures martensitiques et ferritiques à granularité ultrafine. On obtient ces caractéristiques grâce à un contrôle strict des conditions de laminage, de la vitesse de déformation, de la vitesse de refroidissement et de la température d’enroulement. Cet article présente les résultats des essais de traction de deux nuances d’acier biphasé, de même que la caractérisation de la microstructure, afin de comprendre l’effet des procédés avancés de laminage thermomécanique contrôlé sur la formation de la microstructure et les propriétés mécaniques qui en déroulent.

Tribo-corrosion protection of valves and rotors using cermet layers applied with HVOF

Protection of conventional steel parts in equipment that must withstand wear of the tribo-corrosion type (combination of heat and an aqueous medium) can be achieved by High Velocity Oxy-Fuel (HVOF) techniques, applying carbide-based cermet layers which decrease tribological and corrosion mechanisms. Three different carbide layers were applied to a ferritic-pearlitic steel in order to characterize properties such as sliding friction coefficient, phase identification, adhesion to the substrate, porosity, layer thickness and wear mechanisms. An example of a real rotatory equipment after a working campaign, both with and without protective layer is presented. The capacity of the protective layer to extend the life of parts like the one analyzed is evident even if zones of the layer are detached during the campaign.

Structural Ultrafine Grained Steels Obtained by Advanced Controlled Rolling

Steels with ultrafine grains (lower than 5 μpa), which usually known as ultrafine ferrite or ultrafine grained materials, are presently the object of intense research, because of the improvement in resistance and fracture toughness they may reach compared to conventional steels (with grain sizes above this value). It is shown that the forenamed steels designated in the Euronorm EN 10149-2, which are manufactured by advanced techniques of controlled rolling and mainly used in automotive industry, have an ultrafine grain size in the range of 2. 5 to 3. 5 μm, and with elastic yield stresses higher than 400 MPa. Based on the Morrison-Miller criterion, it is shown that values of the strain-hardening coefficient lower than 0. 08 would make the industrial application of these steels unfeasible.