THE DESIGN OF THE LOW-CARBON MARTENSITIC STRUCTURAL STEEL FOR HIGH STRENGTH AND TOUGHNESS AND ITS STRUCTURE-PROPERTY RELATIONS

Abstract
Some design giudelines and technological processes for improving strength-toughness combinations are discussed and compared for medium and low-carbon martensitic structural steels. The structure-property relations for low-carbon martensitic structural steels based on Fe-Si-Mn-Mo-V with Cr and Ni additions have been investigated systematically by means of the statistical regression analysis techniques. The regression equations were obtained, related the composition of these steels to their properties. The contributing factors of carbon and alloy elements on strength and toughness were studied quantitatively and the optimization of the range of compositions for these steels was suggested. In the quenched condition, the microstructures of steel consist of dislocated lath martensite, thin interlath retained austenite films and ε -carbides. The effects of various microstructural parameters on strength and toughness are discussed. The investigation shows that when one of the experimental steels is quenched from 950°C and tempered at 300-350°C, an optimum combination of strength and toughness can be obtained. The tensile strength will be greater than 1765 Mpa(180 kg/mm²), the yield strength greater than 1422 Mpa (145 kg/mm²) and the fracture toughness greater than 127.7 MN/m (410 kg/mm^3/2). This shows that for developing an ultra-high strength steel, further strengthening a low-carbon martensiticsteel with high toughness has an advantage over improving the toughness of a medium carbon martensitic steel. Furthermore, for the same combination of strength and toughness, the suggested steel only contains about 7% of cheap-alloy elements, while the maraging steel contains around 30% of more expensive elements.