Model Refinement is a dependency relationship that relates two lements that represent the same concept at different levels of abstraction. In the UML specification document this relationship, like others concepts, is still described in an ambiguous, informal way. In order to avoid inconsistencies and wrong model interpretations, in this article we propose, in first instance, a formalization of the Use Case specification, represented by a conversation between an actor and the system. The Use Case conversation does not have representation in the UML metamodel. In second instance we propose to formalize the refinement relation between model elements of the same kind, such as refinement relation between Use Cases and between Collaborations. Then on top of these formalizations, we discuss refinement relation between models of different kind (use case models and collaboration models realizing them) This work provides an enhancement to the UML metamodel specification. The formalization proposed should be used as a formal foundation for the construction of case tools performing consistency checking of models. Support offered by tools will improve the quality of software development process.