Study On Soft Reduction Process Theory And Central Segregation Of Pressure Vessel Steel Wide Slab

With the development of domestic petrochemical and nuclear power industries,pressure equipment such as pressure vessels confronts increasingly harsh environments such as high temperature,high pressure and extended service life,and the internal quality requirements for these steel plates are more stringent.The central segregation of a slab could be inherited by the manufactured plate,which seriously affects its flaw detection pass rate and impact toughness property.Due to the uneven cooling in the width direction of the continuous cast wide slabs,the shape of the solidification end is complex,which not only makes the soft reduction intervals and efficiencies at different width positions different,but also causes uneven solidification microstructures,significantly increasing the center segregation control difficulty.Aiming at alleviating the wide slab center segregation control problem of a container steel SA516Gr70,research on wide slab solidification end shape and shrinkage amount,soft reduction interval and reduction efficiency,simulation and control of solidification microstructure were conducted.A segment roll gap correction model was developed,and industry trials of soft reduction was performed as well.This research provides theoretical and practical basis for the center segregation control of wide slabs.A slab solidification heat transfer model under non-uniform cooling conditions was established,and the effect of continuous casting process parameters on the shape of the solidification end and the reduction interval of the wide slab was studied.The results show that for slabs with a dimension of 200 mm × 2300 mm,with the increase of casting speed and superheat degree,the non-uniformity values at the beginning and the end of solidification end(SI1 and SI2),the length of solidification end(SI3)and the non-uniformity of solidification end(SI4)all rise.Among them,the casting speed has the greatest impact on SI3.The increase of the secondary cooling specific water volume has little effect on SI1 and SI2,but can reduce SI3.Given the matching between the continuous casting process parameters and the reduction interval,the combination of low casting speeds,low superheat degrees and high secondary cooling intensities are beneficial to improve the uniformity of the reduction interval.According to the temperature control requirements of slab corners,the optimum main technological parameters are theoretically determined:the casting speed is 1.05 m·min-1,the secondary cooling specific water volume is 0.43 L·kg-1,and the superheat degree range is 15～35 K.A model predicting the shrinkage amount at the wide slab solidification end and a thermal-mechanical coupling model for the soft reduction process under the non-uniform cooling conditions were established.The effects of continuous casting process parameters on the solidification end shrinkage and soft reduction efficiency were studied.The results show that for slabs with a dimension of 200 mm × 2300 mm,the reduction efficiency at the 1/2 width position is the largest,followed by the 1/8 width position,and the smallest at the 1/4 width position.The process conditions that can improve the uniformity of solidification end are conducive to improve the uniformity of reduction efficiency.Therefore,low casting speed,low superheat and high specific water volume of secondary cooling can reduce the difference between the reduction efficiency of 1/4 and 1/8 width positions and 1/2 width,and improve the uniformity of the reduction efficiency of wide slabs,among which the influence of casting speed is the greatest.At the same position,the solidification end shrinkage decreases with the increase of casting speed and specific water volume of secondary cooling,and increases with the increase of superheat.With the same continuous casting parameters,the solidification end shrinkage at the 1/2 width position is the smallest,while that at the 1/8 width position is the largest.There is a close relationship between the continuous cast slab solidification microstructure and the formation of central segregation.Based on the CAFE model,the characteristics of wide slab solidification microstructure were studied through the macro-micro coupling simulation.The results show that under non-uniform cooling conditions,the secondary dendrite arm spacing(SDAS)and the thickness of the central equiaxed grain region at the 1/8 width position both are larger than those at the 1/2 width position.In the range of 60～90 mm,the average grain radius at the 1/8 width position is smaller than that at the 1/2 width position.With the increase of casting speed,the SDAS and the thickness of the central equiaxed grain region increase little,and the average grain radius tends to decrease.With the increase of superheat degree,the SDAS and the thickness of the central equiaxed grain region both are reduced,and the average grain radius tends to increase.The SDAS,the thickness of the central equiaxed grain region and the average grain radius are all decreased with the increase of the secondary cooling specific water volume.When the secondary cooling specific water volume is 0.49 L·kg-1,the average thickness of the central equiaxed grain area of the slab is small,and columnar grain impingement appears in some areas,which has the risk of forming centerline segregation.The accurate implementation of soft reduction process requires good equipment precision conditions.For this reason,the test for the relationship between the segment roll gap and the pressure was carried out,and the segment roll gap correction model was established.A continuous casting machine roll gap real-time correction system was developed.Combined with the above numerical simulation research on the solidification and soft reduction process of SA516Gr70 wide slabs,the main continuous casting process parameters were optimized.On the basis of ensuring the precision of the casting equipment,the influence of different soft reduction process parameters on the slab center segregation was investigated through industrial tests.The results show that when the casting speed is 1.05 m·min-1,the optimal reduction interval is fsc=0.19～0.71.When the total reduction is 5 mm in the optimal reduction interval,the maximum value of the central carbon segregation index at the 1/4 width decreases from 1.29 to 1.18.When the average carbon segregation index of the slab decreases by 0.15～0.22,the elongation of the steel plate increases by 3.6%～5.0%.