Título

EFEITO DO Nb NAS PROPRIEDADES MECÂNICAS E "BAKE HARDENABILITY" EM AÇOS DE CARBONO EXTRA- BAIXO

THE EFFECT OF NIOBIUM ON THE MECHANICAL PROPERTIES AND BAKE HARDENABILITY OF EXTRA LOW CARBON STEELS

Autoria

Pereira, João Francisco Batista; Marra, Kleiner Marques

DOI

10.5151/2594-5327-0007

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Resumo

A influência do Nb nas propriedades mecânicas à tração, cinética de recristalização e "bake hardenability" de aços extra-baixo carbono, laminados a frio e recozidos continuamente, foi avaliada em grupos distintos de aços. No primeiro, o carbono total (Ct,), foi mantido constante e o teor de Nb variado de forma a serem obtidos diferentes valores de carbono em solução sólida (Css), calculado por Css = Ct – CNbC. No segundo, foram variados Ct e Nb, mantendo-se fixo o teor de Css. Comportamentos distintos foram observados para os conjuntos avaliados. A dutilidade e o valor r (coeficiente de anisotropia normal), aumentam com o aumento da relação Nb/C em aços com Ct constante e diminuem em aços com Css constante. Já a propriedade de “bake hardenability” (valor BH) e o índice de envelhecimento (AI) , tendem a diminuir com o aumento da relação Nb/C, sendo essa redução bem mais acentuada nos aços com Ct constante. A temperatura de recristalização é sensivelmente aumentada com a adição de Nb a um aço de carbono extra baixo. Contudo, em aços que já contêm Nb, esse aumento é menos intenso.

 

The influence of Nb on the tensile properties, recristallyzation kinetics and bake hardenability of cold rolled, continuous annealed extra low carbon steels was evaluated in different groups of steels. In the first one, the total carbon amount, Ct, was kept constant and the Nb amount was changed in a such way where different stoiquiometric carbon in solid solution, Css, were obtained. In the second one, Ct and Nb were changed keeping constant Css. The results showed that the influence of Nb amount, characterized by the Nb/C relationship, change in distinct ways dependent on the steel group. The ductility, in steels where Ct was kept constant, is enlarged when the Nb/C relationship is increased, as a result of reduction in Css. However, in the group where Css was kept constant, it decreases as a result of the both factor, carbon in solid solution and grain refining. "Bake hardenability" and Aging Index decrease in both group of steels with the increase of Nb/C relationship. However the reduction is more severe in the steels group where Ct was kept constant as a result of decrease in carbon in solid solution. The r-value increases with the increase of Nb/C relationship in steels of Ct constant and decreases in that one of Css constant. Recrystallization temperature is very sensitive to Nb addition in steels where originally any Nb was present. However, if some Nb is already present, the post addition of this element does not introduce sensible changes in this temperature.

Palavras-chave

Aços Bake Hardening; Envelhecimento por deformação; Nb.

Bake hardening steels; Strain aging; Nb