Fundamental Research On The Control Of Residual Element Sn In Q245R Steel For Boilers And Pressure Vessels

With the rapid development of society and economy,the demand for the output and quality of steel continues to increase,and high-quality iron ore resources are gradually consumed.At the same time,the consumption of low-grade iron ore is increasing.Scrap steel is a direct iron resource,and the recycling rate of it increases gradually.Low-grade iron ore and scrap steel both contain residual element tin,but conventional steelmaking processes can't remove it effectively,resulting in the continuous accumulation of residual element tin in steel.But the residual element tin existing in steel will seriously deteriorate the performance of steel.Therefore,how to effectively control the residual element tin in the smelting process of pure steel is particularly critical for the development of high-quality steel.In response to the above problems,this thesis selects the boilers and pressure vessels steel Q245R as the research steel to study the micro-segregation law of the residual element tin during the solidification of molten steel,and explore the existence form of the residual element tin in the steel and its effect on the mechanical properties of steel.In addition,the use of calcium alloy or CaO-SiO₂-Al₂O₃ slag system to remove tin from molten steel was calculated and analyzed by thermodynamic calculations separately.Based on the results of thermodynamic research,experiments on detinning of molten steel were carried out in the laboratory and the influence of different process parameters on the detinning of molten steel was explored.Finally,the existing forms of tin removal products in molten steel and slag were observed,and a kinetic model for the removal of residual tin in molten steel by calcium-based alloys was established.The main conclusions obtained from the above research work are as follows:(1)By establishing the micro-segregation model of tin during the solidification of molten steel,the micro-segregation law of tin under different cooling conditions and different initial contents of solute elements was studied.The results show that the range of secondary dendrite arm spacing is 73.44?m?340.89?m under the condition of the cooling rate is 0.1?/s?10?/s.With the increase of cooling rate,the secondary dendrite arm spacing decreases correspondingly and the degree of tin micro-segregation reduces obviously.The micro-segregation of tin mainly occurs at the end of solidification(fs>0.9).When the initial carbon content in the molten steel increases from 0.1%to 0.12%,the micro-segregation degree of tin suddenly increases from 2.86 to 55.33 due to the change of the solidification method of the molten steel.But the initial content changes of Si,Mn,P,and S in the molten steel have no significant effect on the micro-segregation of tin.As the initial tin content in the molten steel increases,the micro-segregation degree of tin at the end of solidification decreases significantly,but the zero strength temperature(ZST),zero plasticity temperature(ZDT)and the brittle temperature zone between the above two are less affected.(2)The steel samples with different tin contents were smelted using a vacuum induction furnace.The metallographic structure of the steel samples and the existing form of residual element tin in the steel were studied.It is found that segregation phenomenon of Sn occur both inside grain and at grain boundary.Meanwhile,part of tin is present in the composite inclusions composed of Mn S and oxides.As the tin content increases from0%to 0.08%,the average size of ferrite and pearlite in the steel increases and decreases respectively.With the further increase of tin content to 0.12%,the average size of ferrite decreases,while the average size of pearlite increases accordingly.The hardness test,the room temperature(25?)tensile test and the Charpy impact test(at 0°C)of different tin-containing steel samples were carried out.The results show that when the tin content increases from 0%to 0.12%,the Vickers hardness,the impact toughness,the elongation and the reduction of area of the steel gradually decrease,while the tensile strength and the yield strength of the tin-containing steel samples are significantly lower than that of the non-tin steel samples.When the tin content is 0.027%,both the tensile strength and the yield strength reach the lowest value.Besides,when the tin content is higher than 0.059%,the tensile strength and the yield strength tend to be unchanged.(3)Using the thermodynamic database software Factsage7.0 and existing related thermodynamic data,a thermodynamic study was carried out on the detinning of molten steel.The results show that the use of Ca for detinning needs to be carried out under reducing conditions and there is a critical Ca content at the same time.When the Ca content is lower than the critical calcium content,the detinning reaction cannot proceed.As the O content and S content in the molten steel increase,the critical Ca content increases.However,the increase of the tin content in the molten steel will reduce the critical Ca content.To ensure the detinning effect,it is necessary to first reduce the O content in the molten steel to a lower level,and then further reduce the S content in the molten steel.Also,a lower temperature is conducive to the removal of tin.If only use CaO-SiO₂-Al₂O₃slag system to detin without combining other detinning methods,it is difficult to achieve detinning of molten steel.The slag can only adsorb the detin product.With the increase of the content of SiO₂ in the slag,it is beneficial for the slag to adsorb the detinning product.But the increase of Al₂O₃ or CaO content will cause the tin capacity of slag to decrease significantly.(4)Through carrying out the experimental research work on the detinning of molten steel,it is found that whether it is a direct detin process with a detin agent or a synergistic detinning process with the CaO-SiO₂-Al₂O₃ slag system and a detin agent,the detin effect of silicon-calcium alloy particles is significantly better than that of pure calcium wire as the detin agent under the condition of equal calcium equivalent.But when one of them is used as the detin agent,the reaction time is controlled within 5 mins?10mins is appropriate.With the increase of the amount of silicon-calcium alloy,the detinning rate under the same reaction time is higher,but when the amount of addition is increased from 10%to 15%,the increase of the detinning rate is significantly higher than that of the amount of addition from 5%to 10%.When the calcium-based alloy is used for the detinning of the molten steel,no matter whether the slag is added,a certain degree of“returning tin”will occur.Because the CaO-SiO₂-Al₂O₃ slag system has a certain adsorption effect on the detinning product,the detin ratio in the later stage of the reaction has risen.Besides,when the synergistic detinning process with the slag system and the detin agent are used,the additional amount of slag has a significant effect on the tin removal from molten steel.(5)Combined with experimental analysis methods such as SEM-EDS,XRD,etc.,the existing forms of detinning products in molten steel and molten slag were studied.The results show that when silicon-calcium alloy is used as detinning agent,the detinning products in steel are mainly three types of composite inclusions,including Ca-Sn-O-S type,Ca-Sn-O type and Ca-Sn-S-Si-Al-O type.All of the above inclusions are about 1?m in size.When pure calcium wire is used for detinning,the detin products are mainly Mn-S-Ca-Sn type and Mn-S-Al-Ca-Sn type composite inclusions,and the contents of Ca and Sn in the inclusions are significantly lower than that of using silicon-calcium alloy as a detinning agent.In the process of solidification of molten steel,some detinning products that have not floated to the interface of steel slag will nucleate heterogeneously with CaO and Ca S as the core,while tin that has not reacted with calcium will have Mn S as the crystallization core.The phase of calcium detinning product in the steel slag is Ca₂Sn,and the high alkalinity region rich in CaO in the slag has limited adsorption capacity for the detinning product,while the lower alkalinity region composed of CaO-Al₂O₃ has higher tin capacity.The analysis results based on the established kinetic model indicate that when silicon-calcium alloy particles are used to detin molten steel comparing with pure calcium wire as a detin agent,the diameter of the calcium bubbles initially formed is smaller and the residence time of calcium bubbles in molten steel is longer,therefore the calcium recovery rate is higher.