Mechanism Of Titanium Inclusions Precipitated In Hypereutectoid Tire Cord Steel And Its Control

Tire cord steel is the ideal material for production of car radial tire skeleton-steel cord, and the representative products of super clean steel. It has special requirements on smelting, rolling, processing and so on during the production, and is the genuine top-quality steel of whole process. The steel cord wire rod production technology is of great level, honoured as “the best in wire rod”, “wire crown pearl”, the production process contains a very high technical content.The development of tire cord steel products are from the ordinary strength SWRH62 A, SWRH67 A since the 1990 s, to the high strength SWRH72 A at the end of the twentieth century, and developing to the ultra high strength SWRH82 A, SWRH 90 A and so on in the 21 st century. Under the manufacturing background of low carbon,energy saving, environmental protection and low cost, the users omit the lead bath quenching during wire drawing process, making the stress produced in the process of cold working cannot be released fully, and then it must bear bending and torsional stress,thus easiest to cause wire break when drawing and stranding. With the development of plasticization technology of oxide inclusion, the wire break problem which caused by brittle oxide inclusions during process of drawing for tire cord steel are solved. But with the improvement of tire cord steel strength level, and the change of straight drawing process, the drawing wire break phenomenon caused by titanium inclusion increased significantly. Although the content of Ti and N can be controlled in a very low range via secondary refining technology, the titanium inclusions in tire cord steel products could not be able to control in optimal level. As for the high level hypereutectoid tire cord steel with super strength, the titanium inclusions larger than 4.0μm will increase the times of broken wires significantly.On the basis of fully literatures investigation and combined with actual production data and thermodynamic theory, the precipitated conditions of titanium inclusion in hypereutectoid steel during solidification, influence of C N Ti on the behavior of precipitated titanium inclusion, influencing factors and changing law of Ti and N which formed the titanium inclusion during the tire cord steel production process and its controllment are studied in this paper. The decomposition solid solution behavior of tire cord steel slab during heating process are researched and explored. Based on these, the process optimization study is developed comined with the controllment of titanium inclusions during the production process for hypereutectoid tire cord steel in a certain plant. The conclusion are as follows:(1) Thermodynamic analysis shows that:When the carbon content is the same, the decisive factor of TiN inclusion precipitated in the solidification front is the initial product of Ti and N content w[N]0×w[Ti]0, unrelated with single element content of Ti or N. Cooling rate has obvious influence on the final size of precipitated TiN inclusion.When cooling rate and carbon content are constant, the determinant of final TiN inclusion size is the initial N content rather than Ti in the liquid steel.(2) The higher the cord steel strength, the higher the carbon content is, and the lower solidifying front temperature titanium inclusion precipitated, the supersaturation of titanium inclusion formed elements is also greater, thus the driving force of “titanium inclusion nucleation-precipitation-grow up” becomes greater, and more helpful for the precipitation and grow up of titanium inclusion. In order to control the titanium inclusion size in ultra high strength grade hypereutectoid tire cord steel, more strict measures of steelmaking and continuous casting process must be implemented, to further reduce the initial Ti and N content in the molten steel, especially the N content.(3) The titanium content in BF has a good linear relationship with silicon content,and It(furnace temperature) has a significant impact on the titanium content. The titanium content at BOF endpoint in liquid steel is effected by carbon content and temperature, of which the temperature is especially striking. In order to reduce the titanium content at BOF endpoint, It is essential to reduce the converter tapping temperature and control the carbon content at 0.06%~0.12%. For the sake of reducing the situation of titanium pick-up during refining and the subsequent process, the BOF tapping slag amount as well as the TiO2 content in converter slag must be strictly controlled, and the TiO2 content in slag material added to furnace should be also reduced. Adopting the low titanium alloy,using low TiO2 content refining slag, ladle and tundish covering flux and continuous casting mould fluxes.(4) During the metaphase stages of converter smelting, the reaction of C-O is very fierce and decarburization rate is very high. So it is helpful to denitrification in molten steel, in order to control the nitrogen content, the carbon content at BOF endpoint should be increased properly. At the late stage of BOF blowing process, adding ore or dolomite into the furnace to make slag foaming, thus to avoid the molten steel exposing in“fire zone”, and effectively prevent nitrogen absorption in molten steel. The amount of nitrogen absorption mainly related with the oxygen content of bof endpoint. When the w[O] in the range of 0%~0.03%, △w[N]E or ηE decreases obviously with the increase of oxygen content; When the w[O]>0.03%, the △w[N]E or ηE changes slightly with the increase of oxygen content, at this moment the △w[N]E is less than 0.0004%.(5) The original position analysis of 72 A and 82 A slab show that the center segregation of tire cord steel slab is obvious, titanium inclusions are mainly distributed in the center and inner arc area of the slab. As for the slab samplings, the nitrogen content of 72 A slab is significantly higher than that of 82 A. The OPA results show that the size and number of titanium inclusions for 72 A slab of high N content are more than that for 82 A slab of low N content. It also proves the correctness of the thermodynamic calculation results.(6) The dissolution behavior of foreign titanium inclusion particles under the condition of converter and refining process analog show that if the foreign titanium inclusions exist in earlier smelting process, it can be completely dissolved in the liquid steel in a short period of time. The large titanium inclusion existed in wire rod, is most likely to come from continuous casting processes. It has important guiding significance to control the large foreign titanium inclusion that may occur in cord steel wire rod via improving performance of continuous casting mould fluxes and casting operation process.(7) The decomposition thermodynamic environment of titanium inclusions in the tire cord steel slab when heating is different from the precipitation thermodynamic environment of titanium s in liquid steel. The higher strength level of tire cord steel, the higher decomposition temperature of titanium inclusion. For the same strength level of tire cord steel, when the Ti or N content in steel is high, the initial decomposition temperature of titanium inclusion is also high. In 1150°C~1250°C temperature range,with the increase of heating temperature and holding time, the titanium inclusions can dissolve continuously, and the size and amount of titanium inclusion are effectively controlled.(8) Combining with the actual production process of a certain plant, through the optimization of steelmaking and rolling process, at the adverse condition of abolishing VD vacuum refining, via improving the temperature of steel rolling heating furnace, the decomposition of titanium inclusions in slab are accelerated, the penalty qualified rate of titanium inclusions for 82 A steel wire rod increase significantly.Innovation of this article is mainly in the following aspects:(1) Through the thermodynamic theory calculation, the concept of "iso-product line of nitrogen and titanium content" is put forward in this paper, combined with the original position analysis of titanium inclusions in slab, find that the final size of titanium inclusions influenced by N content is more greater than that by Ti content in the molten steel. Putting forward the key control of the size and number of titanium inclusions in the tire cord steel, are the initial N content and the initial w[N]0×w[Ti]0 in the molten steel.(2) The TiC mole fraction of titanium inclusion precipitated in tire cord steel during solidification, and the size of titanium inclusion depend on the C content. The higher C content in tire cord steel, the lower temperature titanium inclusion precipitated, and the greater driving force of “titanium inclusion nucleation-precipitation-grow up”. The TiC mole fraction of titanium inclusion precipitated is also higher, and the earlier the titanium inclusion precipitated, the more chance to grow up.(3) Exploring the the solution law of titanium inclusion in tire cord steel slab when heating at high temperature, the higher strength level of tire cord steel, the higher decomposition temperature of titanium inclusion; For the same strength level of tire cord steel, when the Ti or N content in steel is high, the initial decomposition temperature of titanium inclusion is also high. Based on which the technological measures by increasing heating temperature of tire cord steel slab before rolling to control the size and quantity of titanium inclusions in the wire rod after rolling are put forward.