Existing Form Of Nitrogen And Carbon In SWRH82B Steel And The Effect On The Microstructure And Properties

High-strength low-relaxation prestressed steel strands are used widely in construction of highway, railway, buildings, bridges and some venues. The quality of SWRH82B wire rod, a raw material for steel strands, has a direct influence on steel strands, so studying on SWRH82B wire rod is of great significance. What is more, test materials were produced by Shuicheng iron and steel company.In the paper the existing form of carbon and nitrogen elements and their effect on the microstructure and properties of SWRH82B steel were studied. According to Fe-C phase diagram, Fe-N phase diagram and chemical composition of SWRH82B steel, the existing form of carbon and nitrogen elements was identified. TEM analysis indicated that the size and amount of precipitations in wire rod with different nitrogen contents had some differences, and most of the precipitations were located at austenite grain boundaries and dislocation lines. OM analysis indicated that secondary cementite and slight shrinkage were exsited in the center of two kinds of castings with different nitrogen contents, but bad microstructures were not found in wire rod.The tensile test indicated that high nitrogen steel has higher yield strength, tensile strength and lower elongation and reduction of area; there were more inclusions on the tensile fracture surfaces of the high nitrogen steel, and some large inclusions rich in Al,Si were presented on the fracture of the two kinds of wire rods. The chemical analysis and XRF results showed that the segregation of elements carbon, manganese, silicon, vanadium and chromium was within the normal range.Austenite was not homogeneous and the secondary cementite in the center of the castings remained undissolved under conditions of heating temperature 800℃, holding time for 30 minutes. When the heating temperature was 900℃, holding time was 2 hours, austenite was fully homogeneous. The cooling rate had a very important influence on the steel microstructure, and a reasonable cooling rate can be inferred from the thermal simulation test. The hardness results of the thermal simulation test samples indicated that hardness values growed as the increasing of the cooling rate, and the hardness value of the low nitrogen casting was larger that that of a high nitrogen casting in the same cooling rate.