Structure and properties of H-beams after accelerated water cooling

Abstract

The structure and properties of the surface of DP155 H-beams made of 09G2S low-carbon steel are determined on the basis of materials physics, before and after thermomechanical strengthening—that is, accelerated water cooling. Such H-beams are used in monorail tracks. Highly defective structure in the surface layer is created by accelerated cooling of the beam in the line of the 450 bar mill at AO EVRAZ Zapadno-Sibirskii Metallurgicheskii Kombinat, in the following conditions: rolling speed 6 m/s; water pressure in the crosspiece-cooling section 0.22–0.28 MPa; temperature before cooling about 800°C. As a result, the hardness, wear resistance, and scalar dislocation density are higher than in the steel without strengthening. Without thermal strengthening, the microhardness of the samples is 2.70 ± 0.33 GPa, while the Young’s modulus is 269.2 ± 27.1 GPa. Thermomechanical strengthening increases its microhardness to 3.30 ± 0.29 GPa, and decreases the Young’s modulus to 228.2 ± 25.7 GPa. In addition, the microhardness range is increased from 2.20–3.80 GPa to 2.64–4.60 GPa, while the Young’s modulus range is reduced from 208.0–403.0 GPa to 184.1–278.2 GPa on thermomechanical strengthening. It is found that thermomechanical strengthening increases the wear resistance of the steel’s surface layer by a factor of ~1.36 (decrease in wear rate from 5.3 × 10^–5 to 2.9 × 10^–5 mm³/N m) and increases the frictional coefficient by a factor of 1.36 (from 0.36 to 0.49). Without thermal strengthening, the structure observed is dislocational chaos; the scalar density of the dislocations is (0.9–1.0) × 10¹⁰ cm^–2. High-temperature rolling and subsequent accelerated cooling of the samples produces dislocational substructure of band type in the ferrite grains and of reticular type in the martensite grains: the mean scalar density of the dislocations in the surface layer is 4.5 × 10¹⁰ cm^–2. Possible explanations for such behavior are discussed.