© 2022 Walter de Gruyter GmbH, Berlin/Boston.In this work, the change on the microstructure and mechanical properties of high chromium steel investigated by adding ultra-high boron contents. Specimens produced by casting method. A total of 430 type martensitic stainless steel scraps melted in an induction furnace where the chemical composition of melt controlled (by adding ferro alloys, de-oxidation, removing nitrogen, etc.). At 1550 °C, melt was poured into sand molds with the shape of Y blocks. Wet chemical analysis was used for boron analysis. The change on the microstructure and mechanical properties (hardness test, Charpy impact test and pin on disc abrasive wear test) determined. Experimental results revealed that, martensitic matrix obtained in both boron free steel and ultra-high boron steels. Chromium carbides and borides were embedded each other (M2B and M7(C, B)5 type of carbide/borides) according to X-ray diffraction results. With the change of boron content, carbide/boride volume fraction, distribution and shapes were changed as well. Hardness and abrasion resistance improved for the steels having higher boron contents however impact toughness deteriorated significantly.