Improvement Of Delayed Fracture Of30MnSi Steel Bar For Pre-stressed Concrete

Several years since the introduction of high-speed wire rod mill from MorganStelmor Inc., Tianjin Steel Plant has been put much effort in the research andtechnological improvement of the production line. The researchers have acquired thekey techniques, on the basis of which developed a series of new products, withvarying specifications of Ф8.0-Ф13.0. However, problems of delayed fracture areoccasionally reported in the products’ service life, causing inconvenience to thecustomers.To solve these problems, it is imperative that quality of products be improvedthrough intensive investigation into the delayed fracture of30MnSi hot-rolled wirerod. In the present study, the problem products were analyzed through industrialproduction and laboratory thermal simulation methods of, and the followingconclusions were achieved.(1) In the smelting process, by use of high quality molteniron, strict control of raw material quality, and adoption of LF refining, the number ofnon-metallic inclusions could be effectively reduced and the content of elements inthe steel could be precisely controlled.(2) Adopting whole process protection, lowsuperheat, electromagnetic-stirred casting, combined with lower pulling rate, andcontrol of liquid surface fluctuation, the surface quality of the slab, and thecompositional segregation of the slab could be improved.(3) Strict inspection of therolling blank, rational development, appropriate planning and execution of rollingprocess, and timely replacement of worn parts can effectively reduce surface defectsof the rolling materials; controlling rolling tension to achieve negligible-tension ornon-tension rolling could effectively improve the rate of qualified products;developing an appropriate cooling process could refine the grains, improve toughnessand avoid the formation of martensite, bainite, and Widmannstatten structures.(4)Proper deep processing techniques could reduce the probability of delayed fracture;acid pickling and phosphorization before drawing, and lubrication during drawingwould influence the performance of the finished wires.