[SNelson]

The properties that defines a materials stiffness are the modulus elasticity or the modulus of rigidity. Which property (mod. of elasticity or mod. of rigidity) matters depends on your particular loading scheme.

Also, a material's hardness generally refers to a material's ability to resist indentations, scratching, etc.

A good resource for material properties is matweb.com. However, most steels have the same modulus of elasticity (~200 GPa). You may consider using a different material, possibly a carbon composite.

If you give me a few more details about how the steel part is being loaded, I might be able to make suggest an alternative material.