| Among all the Medium and Heavy Plate Product of Jigang, the output proportion of Jigang Q345 grade steel plate is higher than 50%, the non-conformity in flaw detection of plate is the important influence factor to the quality of Q345 grade steel plate. By making a contrast analysis about the forming of microstructure mechanism of non-conformity in flaw detection of the plate in different technologies systematically and comprehensively, the causes of the non-conformity in flaw detection of plate are got and the main technologic measure to raise the percent of pass in flaw detection of plate is understanded more clearly.The experiment methods such as ultrasonic morphology, metallographic examination, inclusion analysis, SEM observation, micro-hardness test, Z-directional test and in-situ analysis are used. Moreover, the flaw detection in different processes and technologies (refining, cast slab stacking slow-cooling, heating, rolling, cooling method after rolled and normalizing etc)are systematically studied.The research on defect in flow detection of Q345 grade steel plates has been studied in laboratory and on the pot, the results show that:(1)The non-conformity in flaw detection of the Q345 plate is mainly located three places:including inclusion,segregation and centre crack. (2)The non-conformity in flaw detection of steel plates caused by inclusion are less, and the steel plates in this case are most less than 20mm, inclusions are most come from crystallizer slag entrapment etc. (3)The main reason for the non-conformity in flaw detection of the Q345 grade steel plates which are more than 40mm is centre crack in steel plate. And the crack includes two types:one is macroscopic crack,the other is microcrack. (4)The character of macroscopic crack in the centre of steel plate is that the two sides of the crack is discontinuous, and the composition of the crack most has been oxidated, the cause of the crack is mainly due to the severe central segregation and shrinkage, the central segregation and the crack structure can not be welded because of the reduction lack when single pass rolling. (5)The character of microcrack in the centre of steel plate is that the two sides of the crack is continuous, and the composition of the crack has not been oxidized, the severe central segregation of the original casting slab is the generating reason for the cracks, the central segregation,which can not be cooled and diffused sufficiently during the control cooling before or after rolling,heating, translates to hard structure such as Bainite and Martensite the during the control cooling after rolling, the crack was resulted in the cracking of the hard structures under the action of structure stress, thermo stress and central MnS cracks. (6)Improving the central segregation of the casting slab,reducing the grade of the central shrinkage for casting slab,extending the time of the stacking slow-cooling to promote the diffusing of C. Mn,S and the escaping of H, By this means,the qualified rate of flaw detection for Q345 grade steel plate can be increased obviously. (7)Make sure that the heating time of the casting slab in heating furnace is 09-1.1min/mm, raising the single reduction ratio, increasing the thickness of the intermediate billet, by adopting above measures, the qualified rate of flow detection for Q345 grade steel plate can be improved greatly. (8)The qualified rate of flow detection for steel plate can be also improved by means of measures: adding a temperature-keeping cover over the cooling bed and stacking slow-cooling compared to nature air cooling or only stacking slow-cooling. (9)The central segregation and banded structure can be improved after normalizing, the non-conformity in flaw detection of steel plates caused by this reason can be resaved, and the effect of normalizing to the disqualification in flaw detection is reducing as the thickness increasing of the steel plate... |
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