Strain induced transformation (SIT) of austenite into ferrite has been frequently used as a
powerful ferrite grain refinement mechanism. Ordinarily ferrite grain sizes of the order of 1-3μm
are achieved via mechanical testing such as compression and torsion. Nonetheless, most of the work
done so far employed continuous deformation in the range of 0.8 for compression experiments and
in excess of this for torsion. SIT is a promising technique which may be used during actual hot
rolling processing. However, in this case, not only deformations are applied with time interrupts
between them but also the amount of total deformation allowable is relatively low, in order to attend
to flatness and final gauges requirements. This work explores the consequences on SIT
microstructure of deformation given in multiple passes as opposite to the usual continuous
deformation presented in the literature. Multiple pass deformation at high temperature led to partial
dynamic recrystallization and to a mixture of coarse and fine ferrite grains. Multiple pass
deformation at the vicinity of Ar3 produced, on the hand, finer ferrite grains indicating that SIT took
place. In this case, ferrite grains in the range of 1-3μm were produced and a much more
homogeneous distribution of these grains was present.
Wire rods are supplied for end users manufactures of products with increasing complexity and mechanical properties requirements. Demanding end users field applications must provide high yield stress, ductility and, in certain cases, toughness. These properties can be improved simultaneously if appropriate rolling technique and alloy design are used. It has been known for decades in the flat rolling industry that microalloyed steels can be produced achieving requirements just mentioned. The technology, however, is less often seen applied at the long products industry.
In order to obtain metallurgical sound products, steelmakers need to run industry scale trials. These, however, are costly, time consuming and results are not always easy to analyze given the number of variables involved. Mathematical models are frequently used in order to reduce plant trials reducing costs. There models already published in the literature suitable for wire rod applications. They are however more concerned with the thermomechanical process than with alloy design. This paper presents a model for wire rod rolling of microalloyed steels in which some consideration to alloy design is taken into account. Predicted and mill trial results were compared. Reasonable agreement was found proving the model to be a valuable tool in both schedule and alloy design.
ResumoMelhorar a qualidade dos aços e dos processos tem sido uma rotina dos engenheiros metalurgistas e das siderúrgicas em um mercado que exige produtos com qualidade e preço competitivos. No processo de refino dos aços são feitos os ajustes de composição química e temperatura, além das modificações inclusionárias necessárias às características de qualidade dos produtos e as exigências para uma perfeita lingotabilidade. O processo de solidificação mais comum é o lingotamento contínuo, onde a continuidade de fluxo de aço líquido através das válvulas é assegurada através da temperatura adequada e presença de inclusões no estado líquido nas temperaturas de lingotamento. Neste trabalho foram utilizados os softwares Thermocalc e CEQCSI com o objetivo de avaliar o efeito dos elementos químicos carbono, silício e enxofre na janela de lingotabilidade de diagramas de fases alumínio vs cálcio. Os softwares se mostraram bastante apropriados e eficazes neste estudo, e os resultados indicam que os elementos estudados afetam diretamente a posição da janela de lingotabilidade dos aços carbono. A análise de amostra de tarugo de um aço 0,2%C via MEV mostrou que há grande heterogenidade de inclusões no aço acalmado ao alumínio e tratado com cálcio. Palavras-chave: Diagrama de fases; Thermocalc; Lingotamento contínuo; Cálcio.
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