Research Of Reduced Steel Rolling Technology And Yield

Supported by the eleventh-five-years national technological support plan"new generation recyclable iron and steel flow process technology" project, based on the background of one domestic heavy steel plates production line construction and new products development, and with the production of high toughness ultra thickness steel plates from continuous cast slabs and the improvement of heavy steel plates general yield as the study topic, research work was conducted in the area of heavy steel plates direct rolling technology, rolling technology of high toughness ultra thick steel plates from continuous cast slabs, and the improvement of heavy steel plates general yield. The developed technology of high efficiency heavy steel plates rolling and different shearing guidelines according to different rolling thickness have been applied successfully to one domestic heavy steel plate mill production line. The production efficiency was significantly improved and the yield was gradually uplifted after the process technologies put into use. The main study contents and the achievements are as follow:1. With the triangular velocity field and the upper-bound power, obtain the critical mechanics conditions for slab rolling internal defect closing for the first time, and the corresponding deformation geometry condition is l/h≥0.518defect closing.2. Take the simulation rolling of320mm thickness slab with internal defect in the lab as example, and the simulation ratio is2:1. When the reduction is<10%, the critical conditions of defect closing are not reached, so the ultrasonic test(UT) test is not qualified and thorough thickness reduction of cross section is low; as the increasing of the reduction, the thorough thickness reduction increased accordingly, when the critical conditions of defect closing are reached, the defects are closing, and the UT test is qualified.3. After simulation test in the lab, trial production was carried out in the production line which testifies that when the rolling pass reduction is low and the roughing mill cross rolling stage can’t fulfill the critical conditions for defect closing, the internal defects are not pressed fit so the UT is not qualified; when the roughing mill cross rolling l/h is over0.518,150mm thickness plates internal defects are closing, and the UT is qualified. 4. Through post-weld heat treatment(PWHT), research work concerning C-Mn high temperature dynamic recrystallization and static recrystallization behavior during austenite deformation process was conducted, and the hot working equation was established, and predict the austenite recrystallization deformation activation energy is273kJ/mol. And based on this,60-85mm thickness Q345steel plates was rolled successfully on the production line by high temperature austenite direct rolling plus water cooling, so the reduction rolling target was realized.5. Research work was conducted concerning ultra thick plate quality control, heating schedule, reduction arrangement and rolling&cooling control impact on the high toughness ultra thick steel plates, which indicate that the harmful elements like P and S have to be controlled less than0.014%and0.004%, and the content of hydrogen shall be controlled within1.0ppm, and the central segregation shall be within class C0.5; The heating of the slabs shall make the slab fully and evenly heat up, and the rolling is low speed large reduction(△hmax=38mm), the slab shall be slow cooling to promote the diffuse of hydrogen, and after900℃normalizing, the target for high toughness plates grade Q345E-Z35thinner than130mm produced from400mm thickness slabs are realized, which enhanced this domestic heavy plate production line to be the first mill able to produce high toughness steel plates with yield up to89.14%. The new process increases the yield by8-15%compared with the ingot rolling.6. With the ultra fast cooling(UFC) technology, the cooling speed for the17.5mm thickness pipeline plates was increased from25℃/s to35℃/s. The X70pipeline plate microstructure is bainite+acicular ferrite and small amount of MA islands, and fine grain distributed at the top surface and the1/4position is typical bainite+acicular ferrite, and the core grains are different to the top surface, and the core acicular ferrite is coarse. And the yield strength is higher by60MPa compared with accelerate cooling(ACC) in use, and the tensile strength is increased with45.8MPa. Alloy content is decreased with17.5%, and the alloy content is substantially reduced.7. For high strength steel plates, UFC and pile cooling were adapted, so Q550D steel plate was produced by means of ther thermo-mechanical control process (TMCP) rolling, and the original tempering after ACC was canceled, and the process was simplified.8. The key factors impacting on the yield was investigated, and based on large amount of data on site, the thickness tolerances, edge cutting and head and tail cutting were optimized, and the rational slab dimensions guidelines was set up. Human machine friendly interface application software was developed with Power Builder program software. Compared with manual calculation, this software has avoided the possible manual calculation mistakes effectively, contributing to the guidelines optimization. After putting into use in the4300mm production line, the yield of ship plate has been increased with2.74%comparing with the initial phase of production, and the target to reduce geometry dimensions of the slabs were achieved.