Quality Evaluation And Improvement Research Of Q345B Hot-rolled Mid-wide Strip

Q345B steel, a typical kind of the HSLA steels, not only has the advantages in chemical composition, strength, ductility and formability of HSLA steel, but also have the preponderances of economical investment、low cost of production、wide product range and various application of mid-wide stripe. Q345B steel has been widely used in shipbuilding industry, bridge construction、container、boiler、engineering construction、petrochemical industry and national defense construction, and had a critical place in iron and steel making industry, and play a tremendous role in the national economic construction and development.With detects and records of the chemical composition、mechanical properties、 microstructure morphology and nonmetallic inclusion,the influencing factors of bad and unstable mechanical properties of Q345B steel are found. At first, the chemical composition and mechanical properties of Q345B steel are detected and recorded to have a knowledge of the most important reasons to reduce the mechanical properties from chemical composition; then, with the help of Optical microscope, the microstructure morphologies of different specifications and different positions of Q345B steel are recorded to research the influences of hot-rolling process on microstructure of Q345B steel; at last, influences of types and distribution of the nonmetallic inclusion on mechanical properties of Q345B steels are researched by means of SEM and EDS.After tests and researches mentioned above, the result can come to the following conclusions. High level and unstable content of Sulfur and Phosphorus in Q345B steel have an obvious correlation with the unstable mechanical properties of Q345B steel. The thicker strip has large and unevenly grain size which will become small and even with the rolling reduction increasing. The thinner strip has small and unevenly grain size. Serious banded structure has been found and has unapparent connection with the rolling reduction. During continuous casting process, dendritic crystal growing excessively, which result in obvious elemental segregation. Serious banded structure forms along with the segregation boundary. Plenty and large MnS inclusions occupy the main part of all the inclusions with a lot of complex inclusions coming from outside, which is caused by the high level of sulfur and slag inclusion coming from errors in operation during steel smelting stage.With a meticulous analysis, the suggestive improvement project is brought up as followed, optimized chemical composition strict LF smelting process、lower rolling temperature、increased rolling reduction and accelerated and evened after-rolling cooling speed have been used in the improved Q345B steel, which has a higher and more stable mechanical properties, the suggestive improvement project provide many references for properties improvement and production methods optimization of Q345B mid-wide stripe.