Effect of boron addition on microstructure and mechanical properties of low carbon steels

In this study, two casts were carried out to ceramic molds by adding the boron at various ratios to the Ç1020 material melted at 1635 ºC. In the first casting process boron-free, 7, 15, 26, 29 and 42 ppm boron were obtained, whereas 10, 17, 19, 23, 32, 42, 49 and 63 ppm boron were obtained in the second casting process. Square boron materials at 30x30 mm thickness obtained at various ratios were rolled at 2 pass, and 70 to 80 % deformation was provided. 2nd pass exit temperature was considered to be in the austenite region.The rolled specimens were examined in optical microscope and it was observed that there were heterogeneous global points ranging between 0.1 to 1 ?m in the grain. In addition, it was observed that from place to place there were diagonal orange (usually rectangular) particles at 1 to 5 ?m in size. A significant difference was not observed in grain structure of various boron ratios. Point analyses of black dots observed with optical microscope were examined in analyses performed with Scanning Electron Microscope (SEM), and they were determined to be cementite particles. The diagonal orange particles observed with optical microscope were also understood to be titanium nitride particles (TiN). Boron could not be determined in analyses made with SEM. On the other hand, in analyses made with Electron Probe Micro Analysis (EPMA), the dark black dots observed particularly in ferrite grains were determined to be boron nitride (BN) with point analysis. In addition, it was observed with the EPMA mapping method that the boron distributed homogenously within the grain.In hardness tests conducted to Ç1020 steel containing various ratios of boron, an increase at 20 HV5 value was observed in Ç1020 steel containing 15 ppm boron. In notch impact test, an increase of 2 joule was observed in Ç1020 steel containing 10 ppm boron however it showed a falling tendency with an increase in boron ratio. As a result of hardness and impact tests, it was concluded that optimum boron ratio was between 10 and 15 ppm. On the other hand, according to the results of tensile tests, it was observed that while there was a 17 MPa increase in yield strength in Ç1020 steel containing 26 ppm boron, there was a 20 MPa increase in tensile strength. Boron increased the strength by creating a solid solution in the steel. However, conducting the casting process in a non-vacuum induction furnace made it difficult to control the processes. As a result, boron nitride formation could not be prevented. While yield strength and tensile strength increased, no decrease was observed in ductility.The Ç1020 material containing rolled boron-free, 15 and 26 ppm boron were kept at 875 ºC for 1 hour, and the heat treatment was conducted by cooling it in air and water. While martensite was observed in all specimens cooled in water, it was observed that specimens cooled in air contained homogenous ferrite and perlite. A significant difference could not be observed in hardness values of boron at various ratios after the heat treatment. In addition, aging treatment was applied by keeping it at 200 ºC for 2, 4, 6 and 8 hours, but it was understood that the available produced steel did not have an aging tendency.Key Words: Boron, Boron Steels, Microstructure, Mechanical Testing.