STRUCTURE-PHASE STATES, MECHANICAL AND TRIBOLOGICAL PROPERTIES OF THERMOMECHANICALLY STRENGTHENED BEAM

Using the methods of modern materials science the researches and the comparative analysis are performed for the structural and phase states, dislocation substructure, mechanical and tribological properties of the surface of the thermo-mechanically strengthened and non-strengthened I-beam DP155 from low carbon steel used for mine monorails. It was found that accelerated cooling of the beam section in line 450 of the mill at “EVRAZ – Consolidated West-Siberian Metallurgical Plant” (rolling speed is 6 m/s, the water pressure on the sections of shelf cooling is 0.22 – 0.28 MPa, the temperature before the refrigerator is approximately 800 °С) forms a high defect surface layer structure characterized by higher (relatively to the unhardened state) values of hardness, wear resistance and scalar dislocation density. In the non-heat-resistant state, the microhardness of the samples is 2.70 ± 0.33 GPa, and Young’s modulus is 269.6 ± 27.1 GPa. Thermo-mechanical hardening of the material leads to a decrease in its microhardness up to 3.30 ± 0.29 GPa and to an increase in the Young’s modulus up to 228.2 ± 25.7 GPa, respectively. In addition, an increase in the range of microhardness values from 2.20 – 3.80 GPa to 2.64 – 4.60 GPa and a decrease in the Young’s modulus range from 208.0 to 403.0 GPa to 184.1 to 278.2 GPa is established during thermomechanical hardening of steel. It is shown that the thermomechanical strengthening of steel leads to an increase in the wear resistance of the surface layer in approximately 1.36 times (the wear rate varies from 5.3·10^–5 mm³/N·m  to 2.9·10^–5 mm³/N·m) and an increase in the friction coefficient by 1.36 times (from 0.36 to 0.49). In the non-strengthened state the dislocation chaos structure is observed (the dislocation scalar density is (0.9 . 1.0)·10¹⁰ cm^–2). High-temperature rolling and subsequent accelerated cooling of the samples lead to the formation of a strip dislocation substructure in the grains of ferrite and a reticular dislocation substructure in martensite grains (the average scalar dislocation density in the surface layer is 4.5·10¹⁰ cm^–2). The possible reasons for the observed regularities are discussed. 

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