Interrelationships between yield strength, low-temperature impact toughness, and microstructure in low-carbon, copper-precipitation-strengthened, high-strength low-alloy plate steels

Interrelationships between room-temperature yield strength and low-temperature impact toughness are examined for low-carbon, copper-precipitation-strengthened, high-strength low-alloy (HSLA) plate steels. Three steels, designated as HSLA-80, HSLA-80/100, and HSLA-100, are compared based on plots of yield strength versus 50% shear fracture-appearance transition temperature, followed by comparison of yield strength versus energy absorbed during Charpy V-notch testing at −84 °C. Analysis of both approaches produced similar outcomes, indicating that either is acceptable for predicting the influence of microstructure on the combination of strength and toughness. Data from over 15 studies including over 160 data points are amassed into a single master plot. Strengthening for the highest-strength steels is associated with a strength-toughness vector with slope equal to −0.67 J/MPa. A grain-refinement vector is associated with a slope of approximately +0.18 J/MPa. Since austenite grain size variation was virtually nonexistent in this study, variation of effective grain size was related to the differences in crystal size and/or packet size for low-carbon martensite (finest), low-carbon bainite, and polygonal ferrite (coarsest). A detrimental effect of untempered, brittle, medium-carbon martensite islands was hypothesized. Tempering of this microconstituent during aging heat treatment reduces, but does not eliminate, the negative effect of these islands. Base microstructures of low-carbon martensite show a superior combination of strength and impact toughness, followed by low-carbon martensite with islands of stable austenite, low-carbon bainite, and polygonal ferrite. A vector approach to strength-toughness in HSLA-100 steels is used to clarify property differences from previous studies. Future developments for this class of steels should address grain refinement and changes in processing or alloying that avoid islands of medium-carbon martensite.