Managing Structure and Properties of Pipe Steel Alloyed with Chromium in the Process of Cooling After Thermomechanical Rolling

Abstract

A study was conducted concerning the kinetics of phase transformations and structure of grade K60 (X70) pipe steels alloyed with small amounts of chromium (K60Cr) and molybdenum (K60Mo). It was shown that K60Mo steel differs by a higher structural content of M/A components. The laboratory tests and microstructure studies of the obtained samples by color etching, TEM, and XRD methods have shown that by using the improved cooling conditions after thermomechanical rolling of K60Cr steel, it becomes possible to obtain samples having different matrix microstructure (polygonal, quasi-polygonal, or bainitic ferrite) and various secondary phases (M/A or cementite), which yields different mechanical properties ranging from 467 to 633 MPa (yield point – σ_y ); 681 to 804 MPa (ultimate strength – σ_t ); 22 to 31.8% (percent elongation – δ_e); 8.8 to 17.4% (proportional elongation – δ_p); and 0.68 to 0.88 (σ_y /σ_t ratio). It was established that the presence of M/A component instead of cementite in the structure contributes to an increase in strength and decrease in σ_y /σ_t ratio without a considerable decline in cold resistance. At JSC “Vyksa Metallurgical Plant” (VMZ), K60Cr steel, which is more economical compared to K60Mo, and newly developed technology were used to produce commercial batches of rolled products and large diameter pipes, which demonstrated the specified σ_y /σ_t ratio and higher impact toughness and cold resistance values during a drop weight tear test (DWTT).