Theoretical Calculation And Experimental Study Of Inclusion Precipitation During Solidification Process Of Wear-resistant Steel Containing Titanium

Titanium-containing wear-resistant steel is a kind of reinforced low-alloy wear-resistant steel,whose wear-resistant mechanism is mainly enhanced by the precipitation of some wear-resistant particles during the the production process of the steel.However,due to the addition of alloying element [Ti],inclusions are easy to be produced in the production process,which will have some influence on the casting process and properties of the steel.In order to obtain the control conditions of molten steel composition to promote the performance improvement of wear-resistant steel and provide theoretical basis and ideas for inclusion control of titanium-containing wear-resistant steel,based on the composition of wear-resistant steel produced by a domestic steel mill,a proper amount of Ti element is added to the steel,The inclusion precipitation coupling calculation model was established to calculate the inclusion precipitation theory,the solidification process of molten steel was observed in situ by using LSCM,and the precipitation of inclusions was characterized by the laboratory research of cooling rapid solidification of molten steel combined with relevant material detection methods.The main research results are as follows:(1)Based on comprehensive consideration of thermodynamic and kinetic conditions of inclusion precipitation and solute element segregation,combined with a kind of thermodynamic calculation software called Factsage,the inclusion precipitation coupling calculation model was established.The model includes a calculation module for inclusion precipitation during solidification and a calculation module for inclusion precipitation growth size.The Macroprocessing macro command is used in series to calculate and predict the type,quantity and precipitation growth size of inclusions precipitated during solidification.At the same time,by comparing with the existing experimental data,it is verified that the calculation results of this model are accurate.(2)Based on the basic principle of thermodynamic calculation of solute equilibrium partition coefficient,the influence of temperature,phase composition and composition on equilibrium partition coefficient ki of molten steel during solidification is calculated and discussed,and the relationship between ki and temperature T is fitted according to the solidification process of molten steel.The calculation results show that the solute equilibrium distribution coefficient ki is not a fixed value during solidification,but should be calculated according to the change of specific steel grade composition.Compared with the commonly used fixed value ki,it also proves that the calculated ki value is reasonable and more targeted.At the same time,through calculation and experimental verification,it is proved that the calcutated ki value is more in line with the actual inclusion precipitation situation.(3)The inclusion precipitation coupling model established in this paper is used to calculate that the inclusions precipitated in titanium-bearing wear-resistant steel during continuous casting mainly include tin,Ti C,Ti4S2C2,Ti2O3 and Al2O3.Among them,Ti N precipitates in the initial stage of solidification or before solidification,which depends on the content of [Ti] and [N] in steel Ti4S2C2 and Ti C generally precipitate in the later stage of solidification,and Ti4S2C2 precipitates earlier than Ti C.The precipitation amount of Ti2O3 and Al2O3 is very small,only a few ppm,and the precipitation type of oxide inclusions will change from Al2O3 to Ti2O3 with the increase of [Ti] content in steel.Therefore,in order to avoid the precipitation of large-scale Ti N inclusions in molten steel before solidification,when the molten steel condition [C]=0.17wt%,and [Ti]?0.4wt%,the content of [N] in steel should be controlled to be less than 30 ppm.In order to reduce the precipitation of Ti4S2C2 during solidification,according to the actual production level,[S] in steel should be controlled to be less than 20 ppm,at which time Ti4S2C2 precipitates.At the same time,in order to reduce the precipitation of Al2O3 inclusions during solidification and ensure a certain amount of Ti C precipitation,[Ti] in steel should not be less than 0.5wt%,and Al2O3 inclusions will no longer be generated at this time.The actual production control should guarantee these target conditions as much as possible.The precipitation of inclusions after solidification is mainly Ti C,and the kinetic calculation results show that Ti C is easier to nucleate and precipitate at the interface.At the same time,with the decrease of temperature,the critical nucleation size and critical nucleation work of Ti C decrease,the nucleation rate increases,and the coarsening rate decreases.Therefore,in order to obtain more fine Ti C particles,the temperature in the subsequent heat treatment process should not be too high.(4)The results of laboratory research show that the precipitated inclusions are mainly Ti N,Ti C,Ti4S2C2,Ti2O3 and Al2O3,as well as Ti N,Ti C and Ti(C,N)composite inclusions formed with Al2O3 or Ti2O3 as the core.Meanwhile,EDS surface scanning results show that Ti(C,N)precipita Tion generally occurs at the position with higher ti and c content.When [Ti] in steel is more than 0.3 wt%,Ti C will precipitate near the grain boundary in the late solidification stage,and in the subsequent temperature drop process,part of Ti C will cross the grain boundary and continue to grow inside the grain.Comparing the precipitation laws of inclusions in steel under different cooling rates,it shows that with the increase of solidification cooling rate of molten steel,the number density of inclusions in steel increases and the size of inclusions decreases,which is in line with the calculation results of coupling calculation model of inclusion precipitation.Therefore,in order to achieve the reinforcement effect of wear-resistant steel,the precipitation size of other titanium-containing inclusions should be reduced while ensuring a large number of micron-sized Ti C inclusions precipitated.According to the calculation results of the model,the cooling rate of Ti C precipitation stage should be no higher than 10K/s and no lower than 0.5 K/s.The above experimental results also accord with the calculation results of inclusion precipitation coupling calculation model,which further illustrates the accuracy of the model and calculation method in this paper.