The structural state of structural steels with various strengths influences their corrosion under both hydrogen and oxygen depolarization [1].For example, with increase in the amount of pearlite, i.e., with increase in the carbon content of a hot-rolled metal from 0.5 to 0.8%, the corrosion activity of rolled metals in acidic media sharply increases [2]. In such a medium, at the same strength, steel with laminated carbides is characterized by a higher corrosion wear than steel with globular carbides [3].Modem carbon and low-alloy structural steel can be produced on rolling mills at various (in the range from 800 to 1100~ temperatures at the stage of rolling termination. Hence, for the same steel, the grain size in the rolled steel can vary significantly.It is known [4,5] that the grain growth negatively influences the resistance of the metal to brittle failure, though the corresponding experimental data are absent.The aim of the present work is to estimate the influence of the actual size of grains in ferrite-pearlite nonstrengthened steel on its corrosion in acidic media.We study hot-rolled St3Gps steel which contains (in mass. %) 0.17 C, 0.99 Mn, 0.07 Si, 0.024 S, and 0.007 P. Samples 62 x 35 x 4mm in size were produced from a No. 16 channel beam. To obtain a distinct difference in grain sizes at a more or less dispersed state of pearlite, the samples were subjected to thermal treatment in the initial state: heating at 900~ (for 15 min), cooling in air (normalization), heating at 1200~ (for 15 min, with surrounding cuttings of cast iron to avoid decarbonization), transfer of the samples to the furnace heated to 900~ holding for 15 min, and cooling in air.The microstructure of the metal was studied with the use of a "Neofot-2" optical microscope. The average size of actual ferrite grains was determined by the intersecting method [6]. At least five sections of the structure were analyzed at each processing. After various thermal treatments, the bulk part of the phase components (ferrite and pearlite) was estimated with the use of an "l~pikvant" structure analyzer. The samples were ground, degreased, and then tested in a 1 N (0.5 M) solution of H2SO 4 at ambient temperature for 72 hours: 4 to 5 samples were processed for each variant.
Results of Experiments and DiscussionA fourfold difference in the size of ferrite grains (from 15.8 to 68 ~un) was obtained by thermal treatment. In this case, the normalization from 900~ caused a slight decrease in the size of ferrite grains relative to the initial state (from 18.3 to 15.8 lam), whereas heating to 1200~ with subsequent normalization from 900~ led to its pronounced growth (up to 68 I.tm). Note that grinding did not significantly change the volume of the pearlite component, whereas the roughening treatment, despite the final cooling of identical samples in air from the same temperature (900~ resulted in a pronounced increase in the pearlite component. In this case, though the size of ferrite grains (a factor responsible for the weakening of the strength of the steel) ...
