The Influence of Regimes of Thermomechanical Treatment on the Structural-Phase State, Mechanical Properties, and Fracture Toughness of Low-Alloy Steel API 5L Grade X52

Possible improvement of mechanical properties and fracture toughness of low-alloy steel API 5L X52 after helical rolling under the conditions of cooling rate variations is analyzed. Upon helical rolling and air cooling (Regime I), the average ferrite grain size in this steel is observed to decrease from 13.5 μm in the initial state to 3.5 μm, the volume fraction of pearlite also decreases, the banded orientation disappears, and the structure becomes more homogeneous. The structural changes provide an increase in the yield stress value from 270 to 430 MPa and strength from 480 to 630 MPa, with the plasticity remaining at the initial level (ε = 25%). Simultaneously, the impact toughness value (KCV) increases in the entire temperature interval by about 30%, compared to that in the initial state, and the critical temperature of brittleness Т 50 is shifted towards the region below –70°С. After rolling of low-alloy steel X52 and water quenching (Regime II), a ferrite-bainite structure is formed. Regime II favors an increase in the yield strength of the steel by a factor of 1.6, while the level of KCV slightly decreases.