Achieving a trade-off of high strength and low thermal expansion in invar steel via synergy of dislocations and vanadium carbonitrides

Invar steels possess excellent thermal expansion properties, making them suitable as materials for manufacturing precision instruments. However, conventional invar steels lack sufficient strength for engineering applications, and various strengthening methods are urgently needed to enhance their strength. In this work, the possibility of enhancing the strength and maintaining low coefficient of thermal expansion (CTE) of the steel through mechanical heat treatment and the introduction of vanadium carbonitride is demonstrated. V–N microalloying and various heat treatment processes enable invar steel to enhance its strength while maintaining low thermal expansion properties. The strength of low-nitrogen addition invar steel measured 593 MPa during direct aging, representing a 44.6% increase compared to invar steel. After undergoing cold-deformation aging, the strength of low-nitrogen invar steel increased to 790 MPa, indicating a substantial improvement in strength relative to the direct aging condition. Notably, the coefficient of thermal expansion remained at 0.98 × 10⁻⁶ °C⁻¹. By further increasing N content to introduce more vanadium carbonitride, the strength of high-nitrogen invar steel reached 927 MPa under cold-deformation process while maintaining a low CTE value of 1.02 × 10⁻⁶ °C⁻¹. This achieved an extraordinary balance of high strength and low CTE, which is due to a well combination of various strengthening mechanisms, especially the Orowan strengthening where dislocations continuously bypass vanadium carbonitride to achieve the strengthening effect. The resulting findings are important for future preparation of excellent properties invar steel in industrial applications.