[20] A sample of low-carbon steel is found to have a yield strength of 622 [MPa] for an average grain size of 180m, and a yield strength of 663 [MPa] for an average grain size of 22.5m. Given that the steel obeys the Hall-Petch relationship, what is the grain size needed for the steel to have a yield strength of 689 [MPa]? [20] A sample of low-carbon steel is found to have a yield strength of 622 [MPa] for an average grain size of 180m, and a yield strength of 663 [MPa] for an average grain size of 22.5m. Given that the steel obeys the Hall-Petch relationship, what is the grain size needed for the steel to have a yield strength of 689 [MPa]