Comparative study of austenite-ferrita transformation in two if steels with different chemical compositions.

Os a?os IF s?o livres de elementos intersticiais, como o C e o N, na matriz ferr?tica. Estes elementos s?o controlados no processo de refino na aciaria, a partir da aplica??o de t?cnicas de desgaseifica??o a v?cuo, juntamente com a adi??o de elementos estabilizantes, como Ti e Nb. Durante a fabrica??o e em algumas aplica??es dos a?os IF, como por exemplo soldagem, a transforma??o de fase austenita?ferrita e o efeito dos elementos de liga na sua cin?tica t?m forte influ?ncia na microestrutura final.Neste contexto, este trabalho caracterizou microestruturalmente e comparou as cin?ticas de transforma??o de fase de dois tipos de a?os IF com composi??es qu?micas diferentes, sendo eles o IF-Ti e o IF-TiNb. Por meio dos resultados obtidos, foi poss?vel concluir que o a?o IF-TiNb, como fabricado, possui uma microestrutura mais refinada devido ? presen?a do Nb e seu papel na lamina??o controlada do a?o. Al?m disso, observou-se que as temperaturas cr?ticas Ar3 e Ar1 do a?o IF-TiNb s?o sistematicamente menores do que as temperaturas medidas para o a?o IF-Ti. Como a fra??o dos elementos microligantes em solu??o s?lida ? muito pequena, atribui-se este efeito ? diferen?a no teor de Mn dos a?os estudados, que parece potencializar o refino de gr?o na etapa final de resfriamento da chapa de a?o laminada.The IF steels are free of interstitial elements, such as C and N, in the ferritic matrix, which are controlled during the steel refining process using vacuum degassing techniques, andstabilizing element additions (Ti and Nb). During the steel manufacturing and in some applications, welding for example, the austenite?ferrite phase transformation and the effect of the alloying elements on the phase transformation kinetics have strong influence on steel final microstructure. In this context, this work characterized the steel microstructures and compared the phase transformation kinetics of two types of IF steels with different chemical compositions: the IF-Ti and the IF-TiNb. The obtained results showed that the IF-TiNb steel has the finer microstructure due to the presence of Nb and its effect on controlled rolling. Besides that, it was observed that the critical temperatures Ar3 and Ar1 for the IF-TiNb steel are systematically smaller than the obtained for the IF-Ti steel. As the fraction of microalloying elements in solid solution is too low, the difference on studied steel Mn contents, possibly is the main cause of this. The Mn content seems to potentiate the ferritic grain refinement during the final steel cooling

Proposition of an empirical model for determination of critical temperatures during continuous cooling in heat affected zones of IF steels welded by the TIG process.

A?os IF s?o amplamente empregados na ind?stria automobil?stica. No decorrer das aplica??es de soldagem, eles passam pela transforma??o de fase austenita (?) ? ferrita (?) que ocorre por nuclea??o e crescimento de gr?o e, em condi??es isot?rmicas, ? descrita pela equa??o JMAK. Todavia, as transforma??es de fases p?s-soldagem acontecem a partir de resfriamento cont?nuo. Assim, o presente trabalho caracterizou a cin?tica de transforma??o ? ? ? e prop?s um modelo emp?rico para prever as temperaturas cr?ticas durante o resfriamento cont?nuo em zonas termicamente afetadas de tr?s tipos de a?os IF. Amostras dos a?os foram submetidas a ensaios dilatom?tricos com diferentes taxas de resfriamento cont?nuo. Determinaram-se as temperaturas cr?ticas de transforma??o de fase (???) e obteve-se uma equa??o emp?rica que correlacionou a temperatura cr?tica com a taxa de resfriamento e que viabilizou a previsibilidade da cin?tica de transforma??o de fases. Processo de soldagem TIG mecanizado foi aplicado nos a?os IF com monitoramento de ciclos t?rmicos e, em regi?es da ZTA com condi??es de austenitiza??o e taxa de resfriamento similares ?s utilizadas nos ensaios dilatom?tricos, verificou-se que a equa??o emp?rica obtida ? uma alternativa vi?vel para prever as temperaturas cr?ticas e a microestrutura durante o resfriamento cont?nuo na ZTA destes materiais.IF steels are widely used in the automotive industry. In the course of the welding applications, they undergo the transformation of austenite (?) ? ferrite (?) phase that occurs by nucleation and grain growth and, under isothermal conditions, is described by the JMAK equation. However, post-weld phase transformations occur under continuous cooling conditions. Thus, the present work characterized the transformation kinetics ? ? ? and proposed an empirical model to predict critical temperatures during continuous cooling in heat affected zones of three types of IF steels. Samples of the steels were submitted to dilatometric tests with different rates of continuous cooling. The critical phase transformation temperatures (? ? ?) were determined and an empirical equation was obtained which correlated the critical temperature with the cooling rate, which enabled the predictability of phase transformation kinetics. Mechanized TIG process was applied to IF steels with thermal cycling monitoring and, in regions of the HAZ with austenitizing conditions and cooling rates similar to those used in the dilatometric tests, it was verified that the obtained empirical equation is a viable alternative to predict the critical temperatures and the microstructure during the continuous cooling in the HAZ of these steels

Caracteriza??o e avalia??o de modelos de previsibilidade da cin?tica de transforma??o de fases austenita/ferrita de tr?s a?os IF.

Programa de P?s-Gradua??o em Engenharia de Materiais. Departamento de Engenharia Metal?rgica, Escola de Minas, Universidade Federal de Ouro Preto.Os a?os IF s?o amplamente empregados na ind?stria automobil?stica, sendo usualmente conformados e soldados. No decorrer do processamento, os a?os IF passam pela transforma??o de fase austenita (?) ? ferrita (?) que, em fun??o do tamanho de gr?o das fases, parti??es qu?micas e outros aspectos, tem um impacto significativo na metalurgia do produto. A transforma??o ? ? ? ocorre por nuclea??o e crescimento de gr?o, sendo que em condi??es isot?rmicas ela pode ser descrita pela equa??o Johnson-Mehl-Avrami-Kolmogorov (JMAK). Todavia, as transforma??es de fase na grande maioria dos processos metal?rgicos acontecem a partir da aplica??o de resfriamento cont?nuo. Tendo em vista que existem poucos trabalhos na literatura que exploram quantitativamente a cin?tica de transforma??o de fases em processos n?o isot?rmicos em a?os IF, t?o pouco que apresentam modelos de previsibilidade que possam ser empregados na melhoria do processamento destes a?os, o presente trabalho teve como principal objetivo caracterizar a cin?tica de transforma??o ? ? ? para tr?s tipos de a?os IF que possuem composi??es qu?micas diferentes, sendo eles nomeados: IF-Ti, IF-Nb e IF-TiNb. Amostras dos a?os foram qu?mica e microestruturalmente caracterizadas no estado de entrega. Ensaios de dilatometria foram executados considerando 1100?C como temperatura de austenitiza??o e as amostras foram submetidas a diferentes taxas de resfriamento. As amostras ensaiadas foram microestruturalmente caracterizadas e os dados de dilatometria foram devidamente tratados com o objetivo de se caracterizar as temperaturas cr?ticas e a cin?tica de decomposi??o austen?tica. Foi feita a proposi??o de um novo modelo matem?tico de previsibilidade identificado como Ferrita-Tempo. A efici?ncia deste modelo foi avaliada por meio de compara??es com resultados obtidos a partir da simula??o da cin?tica considerando a adapta??o do modelo cl?ssico JMAK para resfriamento cont?nuo. Al?m disso, foi proposta uma rela??o funcional emp?rica para prever as temperaturas cr?ticas de transforma??o de fases em fun??o da taxa de resfriamento. Verificou-se que tanto as temperaturas cr?ticas determinadas experimentalmente como a simula??o para obten??o do diagrama TRC calculado considerando a rela??o funcional emp?rica foram bem ajustadas umas com as outras e que, apesar dos ensaios terem sido realizados para taxas espec?ficas, ? poss?vel obter uma previsibilidade da cin?tica para uma taxa qualquer dentro do intervalo de 0,5 a 100oC/s, considerando o diagrama calculado. Observou-se tamb?m, que as temperaturas cr?ticas determinadas experimentalmente e as determinadas a partir do modelo proposto Ferrita-Tempo no diagrama TRC dos a?os IF se ajustaram de maneira satisfat?ria e que, apesar da adapta??o matem?tica do modelo cl?ssico JMAK, at? os dias atuais, ser muito utilizada para previs?o da cin?tica de transforma??o de xii fases mesmo em processo n?o isot?rmicos, esse modelo cl?ssico n?o prev? de maneira satisfat?ria a cin?tica de todo tipo de a?o da classe IF, como ? o caso do a?o IF-TiNb.The IF steels are widely used in the automotive industry, being usually shaped and welded. In the course of their processing, the IF steels pass through the transformation of austenite (?) ? ferrite (?) phase which, due to the grain size of the phases, chemical partitions and other aspects, has a significant impact on the metallurgy of the product. The ? ? ? transformation occurs by nucleation and grain growth, and under isothermal conditions it can be described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. However, the phase transformations in the great majority of the metallurgical processes happen from the application of continuous cooling. Considering that there are few works in the literature that quantitatively explore the kinetics of phase transformation in non-isothermal processes in IF steels, which do not have predictability models that can be used to improve the processing of these steels, the objective of this work was to characterize the transformation kinetics ? ? ? for three types of IF steels having different chemical compositions, named IF-Ti, IF-Nb and IF-TiNb. Samples of the steels were chemically and microstructurally characterized in initial state. Dilatometric tests were performed considering 1100?C as austenitization temperature and the samples were submitted to different cooling rates. The tested samples were microstructurally characterized and the dilatometry data were properly treated in order to characterize the critical temperatures and austenitic decomposition kinetics. A new mathematical model of predictability, identified as Ferrite-Time, was proposed. The efficiency of this model was evaluated through comparisons with results obtained from the simulation of kinetics considering the adaptation of the classic JMAK model for continuous cooling. In addition, an empirical functional relationship was proposed to predict the critical phase transformation temperatures as a function of the cooling rate. It was found that both the experimentally determined critical temperatures and the simulation to obtain the CCT calculated considering the empirical functional relationship were well fitted with each other and that despite the tests being performed for specific rates, it is possible to obtain kinetic predictability for any rate within the range of 0.5 to 100oC/s, taking into account the calculated diagram. It was observed that the critical temperatures, experimentally determined, and those determined from the proposed Ferrite-Time model in the CCT diagram of the IF steels were satisfactorily fitted and that despite the mathematical adaptation of the classic JMAK model to the present day, used to predict the kinetics of transformation of non-isothermal phases in process, this classic model does not predict satisfactorily the kinetics of all type of steel of class IF, as is the case of the IF-TiNb steel