**12. Conclusion**

Strengthening mechanism can be done by precipitation forming and grain refining. Micro-alloy element hinders grain growth that causes grain refinement [13]. Precipitates forming on ferrite or austenite cause improvement of hardening or strengthening of steel. Phase transformation in some cases also causes strengthening of steels. In phase transformation, different micro-alloying elements appear to contribute considerably. Strengthening of steels can be done by different heat

treatment techniques as well in addition to alloying. Besides, the most economical alternative way of improving the mechanical properties of steel is adding smaller amounts of some special elements.
