删除mvc4中实体框架表字段中的主键

I have created a model in the name of Sample with an Id and DocumentId property.In this I don't mention primary key for Id property.But it forms the primary key when I created the Sample as table in entity framework.I want to remove the primary key for Id. What do I have to do. Please help me. I am very new to mvc4. I have created a model in the name of Sample wi