| Hot-rolled seamless steel pipe as an economic cross-section and efficient steel,has been very widely used in almost all industrial sectors,the main countries of hot-rolled seamless steel pipe are all pay attention to the production and development seriously.In the process of hot-rolled seamless steel pipe,the pipes are usually cooling in air after sizing in China currently,lacking of effective means to control the microstructure and properties.Depending on heat treatment and alloying elements to improve mechanical properties.Control cooling as an important means to enhance the material properties,has been widely promoted and applied in hot rolled strip.However in the field of hot-rolled seamless,controlled rolling and cooling technology has not yet been applied.This paper combined Northeastern University in cooperation with Baosteel to carry out"PQF460 seamless steel pipe on-line cooling equipment and automation control systems"project,to develop controlling cooling after rolling process for the L360 line pipe in this study,The main contents and conclusions are as follows:(1)Austenite microstructure evolution behavior of L360 line pipe under continuous coolingThe main issue of this part is microstructure evolution behavior of L360 line pipe deformed and undeformed austenite under continuous cooling,and draw its continuous cooling transformation curve.The results show that:Deformation can improve the starting transformation temperature of austenite,and expandferrite phase transformation region,simultaneously the Widmanstatten can form in a certain cooling zone during continuous cooling,so we can take controll deformation and cooling rate to inhibite the Widmanstatten forming.The cooling zones of forming Widmanstatten ferrite are respectively less than 1 ?/s in condition of undeformed and less than 1?/s and more than 0.2?/s in condition of 30%deformed.(2)Effect of process parameters on microstructure evolution behavior for L360 line pipeMicrostructure evolution behavior and mechanical properties with the austenitizing temperature,holding time,deformation temperature,cooling path(cooling rate,cooling temperature point)were studied by thermal simulation testing machine.Different heating temperature and time significantly affect composition of steel.The higher austenitizing temperature and the holding time,the higher the percentage of Widmanstatten,while the lower the macrohardness of tested steel.Different pocesses lead to different transformation phase,ferrite+pearlite,ferrite+pearlite+bainite,bainite,etc.This series of experimental results can be used to provide theoretical basis for achieve controlling L360 line pipe microstructure and mechanical propertiesbased on controlled cooling.(3)During hot rolling process L360 microstructure evolution behavior and its impact on the mechanical propertiesThe main research is finishing rolling temperature,cooling rate and cooling path and its impact on microstructure evolution behavior and mechanical properties.When the finishing rolling temperature is higher than 950 ? and then air cooling,the microstructure with Widmanstatten severely reducing steel toughness,so that the tested steel at-273K impact energy less than 10J.The rate of cooling after rolling also has a significant impact on L360 pipeline microstructure.The larger the cooling rate,strength of steel increased significantly,but for Widmanstatten ferrite,leading poor toughness of tested steel.Controlling cooling path after rolling,help obtain finer microstructure than air cooling,also can obtain small,quantity of precipitated particles,which contribute to strength and toughness.When the finish rolling temperature is 950 ? cooling to 725 ?,the experimental steel has good properties:yield strength 481Mpa,tensile strength 651Mpa,yield ratio 0.74,elongation 30.7%,impact Gong reached 38J at 0?.(4)Widmanstatten and its influence on mechanical propertiesStudy the main factors affect Widmanstatten generated.By the impact of the sample macrostructure/microstructure fracture surface and crack extension,with observation by TEM of Widmanstaten finer structure,analyze the influence of Widmanstatten on tested steel strength and toughness.When the austenitizing temperature is 1250?,the keep time is 10-60min,the percent of Widmanstatten is 1.65%?5.24%?13.49%?25.90%?34.70%?44.60%.Indicating that coarse austenite grains,resulting in the wide cooling zone of Widmanstatten.Smaller austenite grains,the wide small Widmanstatten cooling zone.When the microstructure with Widmanstatten segmentation matrix,the steel has poor plastic,it may benot release the stress by plastic deformation.The widmanstatten plate keep the K-S relation with austenite,so that the plate grow along the<111>?upon(110)?.the face(110)is the close packed plane,so the widmanstatten structure is to the disadvantage the toughness... |
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