Research on low yield strength ratio of low carbon bainite steel Q690D

Q690D steel with low yield to strength ratio and low cost was developed using low-carbon microalloying composition design and TMCP+low-temperature tempering process. By comparing the changes in yield ratio and microstructure of low-carbon bainitic steel Q690D at different final rolling temperatures, final cooling temperatures, and tempering temperatures, the main factors and mechanisms affecting its yield ratio were studied, providing a reference for the development of low-carbon high-strength steel. Research has found that when the final rolling temperature is between 810～830 ℃ and the final cooling temperature is between 300～400 ℃, the hot-rolled yield to strength ratio of the steel plate is the lowest. In addition, by controlling the bainite morphology and the proportion of precipitated phases through appropriate tempering processes, the soft and hard phases can be reasonably matched, and the steel plate can achieve good yield to strength ratio and comprehensive mechanical properties. The yield-tensile strength ratio of steel plates shows a trend of first increasing and then decreasing with the increase of tempering temperature. The yield strength increases significantly compared to the hot-rolled state, while the tensile strength decreases. The toughness and plasticity are significantly improved, but when it reaches a certain level, the strength decreases rapidly. This process reduces the addition of a large amount of expensive alloys, lowers costs, and can now be produced in large quantities, effectively enhancing the market competitiveness of high-strength steel products.