Research On The Hot Rolling Structure Control And Application Of TA1 Slab Cast In EB Furnac

China introduced a vacuum self-consumption electric arc melting furnace（VAR）from Japan to repeatedly melt titanium ingots.These ingots were then forged into slabs which were then rolled into industrial pure titanium（TA1）strips using the existing rolling equipment at the steel plant.The above process is a realizable case of the combination between the steel and titanium industries,thus reducing the production costs of titanium products.Subsequently,a breakthrough in TA1 slab casting technology appeared in China,resulting from applying the electron beam cold hearth melting furnace（EB furnace）.As a result,there is an attempt to use the EB furnace to melt and cast TA1 slabs,which can go through a direct hot roll without the forging blanking process.However,the slabs produced by EB exhibited a coarse structure at the centimeter level in the subsequent forging and rolling processes.This was in stark contrast to the micron-level fine forging structure achieved by the traditional“2-3 times VAR furnace melt casting+forging”method.As a result,the mechanical properties couldn’t be ensured.Furthermore,the hot rolled strips performed multiple issues and displayed a low success rate in rolling,unstable product performance,and poor surface quality.This paper studied the microstructure and properties of single pass TA1 slab produced by the EB furnace during the hot deformation process.The purpose is to overcome the adverse factors of coarse original microstructure of EB furnace single melt cast slab,and optimize the process parameters suitable for hot deformation of EB furnace single melt cast slab.Finally,it is possible to realize the combination of steel and titanium industries,and realize the production of qualified TA1 strips on the existing rolling equipment of the steel plant.The main research results are as follows:（1）By conducting single-pass hot deformation simulation experiments on EB furnace single melt cast TA1,the hot deformation constitutive model of EB furnace single melt cast TA1 considering strain effect was obtained.Combined with constructing a hot processing map based on EB furnace single melt cast TA1,the results show that the suitable processing area of EB furnace single melt cast TA1 is:deformation temperature range840～900℃,strain rate range 0.1～0.01s^-1.Further multi-pass deformation experiments were carried out to aim at the characteristics of multiple deformation of the existing rolling mill.The results show that the homogenization temperature of EB furnace single melt cast TA1should be controlled at 850～900℃,the whole deformation should maintain high temperature rolling,but it should be controlled below the phase transition temperature of890℃.（2）Based on the process conditions obtained from the hot deformation simulation experiment,trial rolling was done at the test mill,the existing Steckel mill and hot continuous rolling mill in the steel plant were tested and optimized industrially.The optimal process scheme was obtained as follows:the homogenization section temperature of the heating furnace was set at 900～920℃,and the heating time of the EB furnace melt cast blank with a thickness of 150～200mm was 190～250 minutes;the rough rolling start rolling temperature range was 860～890℃,and the finish rolling start rolling temperature was guaranteed to be above 800℃.The strip produced by rolling EB furnace melt cast TA1slab on two kinds of rolling mills was compared and analyzed.The results show that the strip rolled by both rolling mills can meet the national standard requirements,among which the furnace coil rolling mill is suitable for rolling multi-specification and small batch titanium strips,the hot coil plate convexity control is better,and the grain structure is relatively fine;the hot continuous rolling mill is suitable for rolling large batch and single specification strip,the powerful roughing mill and 7-frame finishing mill have stronger adaptability to the thickness of the blank,and can complete the target thickness at high speed,and the rolled titanium coil has a better plate shape and better surface quality.（3）The microstructure and mechanical properties analysis of three different specifications of the TA1 strip shows that the original coarse grain structure of EB furnace single melt cast TA1 slab was refined from centimeter level to micron level after rolling deformation using an optimized process.During the hot deformation process,the grains were twisted,and the rolling deformation amount led to the preferred orientation of the grains after rolling.With the heat treatment temperature increase,the basal texture disappeared,and the maximum pole density decreased.The preferred orientation of the plate was weakened,and the heat treatment temperature had little effect on the room temperature tensile properties of various specifications of the titanium plate.Their performance at room temperature was relatively stable,but after high temperature tensile decreased,especially the high-temperature tensile yield strength of 8mm TA1 strip after750℃heat treatment decreased by about 60MPa compared with that without heat treatment.The technical indicators of TA1 strip met the national standard requirements,and could replace the“2-3 times VAR furnace melt casting+forging”slab rolling process,realizing the new process of“steel-titanium”combined short process rolling TA1 strip production using the equipment in steel plant.（4）By studying the thickening kinetics of the oxide layer of EB furnace melt cast TA1,the thickening kinetics model of the oxygen-rich layer under high-temperature air environment of 750-850℃was obtained.The results show that the oxygen atom diffusion activation energy（Q）of EB furnace single melt cast TA1 was 203.473k J/mol,the thickness of the oxygen-rich layer x was proportional to the holding time t^0.5,and the thickness of the oxygen-rich layer increased exponentially with the increase of temperature.The annealing test confirmed that when the annealing temperature reached above 650℃,even if only holding for 5 minutes,theαphase grains had been fully recovered and recrystallized.With the increase of annealing temperature,TA1 strip was softened,the tensile strength decreased by about 100MPa,the elongation increased from below 10%to above 30%,and the yield strength decreased faster than the tensile strength;the suitable annealing temperature was 650～700℃,and the specific annealing process should be different according to the different specifications of the raw materials.