The effect of increasing Cu and Ni on a significant enhancement of mechanical properties of high strength low alloy, low carbon steels of HSLA-100 type

Abstract

High strength low alloy steels (HSLAs) containing copper, nickel and low amount of Carbon are a new generation of high strength low alloy steels that are widely used in shipbuilding. Strengthening elements such as Cu, Ni, and to some extend C, play a significant role in this family of steels. By increasing the amount of Cu, Ni, and by selection of proper final temperature of rolling, austenitization and aging, it is possible to enhance the yield strength of this steel up to 160 ksi (1102 MPa) while at the same time toughness remains at an acceptable level. In this investigation, the effect of increasing Cu and Ni on mechanical properties of these steels was elaborated. Four steels with varying amounts of Cu and Ni were cast, and hot rolled by a thermomechanical controlled processing. The steels were then austenitized at 920 °C for 45 min followed by quenching in water. Finally, the steels were aged at 650 °C for one hour. Optical microscope, and Filed Emission Scanning Electron Microscope (FE-SEM) were used to study microstructural evolution. Mechanical properties variation was evaluated by tensile and microhardness tests. Results showed that by increasing the amount of these two elements, hardness and tensile strength were increased in thermomechanically controlled processed, austenitized and aged specimens. Yield strength values of the rolled samples were somehow scattered which was due to a slight difference in final rolling temperature and the subsequent rapid cooling. Yield strength under austenitization and quenching conditions increased by increasing the amount of Cu and Ni which could be due to the presence of Nb (C, N) precipitates. Increasing the amount of Cu precipitates after quench could also be another reason for yield strength increment. Increasing the amount of Cu and Ni in samples containing 4.16 and 7.65 wt% for each of these alloys led to the reduction of Ac1 (austenite formation temperature). Selecting aging temperature (650 °C) higher than the austenite formation temperature could probably lead to the growth of the carbides, softening of martensite layers and lower amount of dislocations density hence for the reduction of yield strength of the aged steels. As well, with the addition of the mentioned amounts of Cu and Ni, yield strength of 705 MPa (102.2 ksi), tensile strength of 1072 MPa (155.4 ksi), elongation of 14% and area reduction of 63% were obtained in the aged steels of this investigation.