WO2008004906A1

WO2008004906A1 - Method for producing sheet semifinished product from a titanium alloy

Info

Publication number: WO2008004906A1
Application number: PCT/RU2007/000123
Authority: WO
Inventors: Vladimir Vasilievich Astanin; Oskar Akramovich Kaibyshev
Original assignee: Institut Problem Sverkhplastichnosti Metallov Ran
Priority date: 2006-07-06
Filing date: 2007-03-14
Publication date: 2008-01-10
Also published as: EP2048260A4; RU2320771C1; US20080302451A1; EP2048260A1

Abstract

The invention relates to a method for producing sheet semifinished product which is made of a titanium alloy having a submicrocrystalline structure and is suitable for low-temperature siuperplastic deformation. In a more efficient way, said invention can be used for producing sheet semiproduct, including foil, from a low-ductile titanium alloy. The inventive method consists in rolling a blank having a prepared structure at a temperature lower than a polymorphic transformation temperature in isothermal or quasi-isothermal conditions formed by hot rolls. The rolling is carried out in a low-temperature superplastic deformation mode. Preferably, during the first operation, deformation is carried out with a degree of reduction εmin, wherein εmin is the minimum degree, at which the structural state of alloy for obtaining a cooperative grain-boundary slipping during deformation is formed in selected temperature-speed conditions. After each successive rolling operation, the blank is cooled directly at the exit thereof from a deformation zone for fixing the thus obtained by deformation structural state. When the blank is heated in a furnace for a successive rolling operation, a heating time is limited in such a way that the alloy structural state obtained at the previous rolling operation is preserved. Said invention makes it possible to improve the quality of a sheer semiproduct to be exposed to following low-temperature siuperplastic deformation.

Description

METHOD FOR PRODUCING A SHEET SEMI-FINISHED PRODUCT FROM

TITANIUM ALLOY

The invention relates to the field of metal forming, and more particularly to sheet rolling, and relates to a method for manufacturing a semi-finished sheet of titanium alloy with a submicrocrystalline structure suitable for low-temperature superplastic deformation. With the greatest efficiency, the invention can be used in the manufacture of thin-sheet, including foil, semi-finished product from low-plastic biphasic titanium alloy. The quality of the semi-finished sheet is determined by the characteristics interconnected by means of modes and methods of rolling: surface condition, accuracy of geometric dimensions and shape, mechanical properties due to structure, incl. grain size, semi-finished product, as well as anisotropy or isotropy of mechanical properties, due to the type of metallographic texture formed during rolling.

The tendency to improve the accuracy of the size and shape of the semi-finished sheet and to increase the cleanliness of its surface has always been decisive. On the one hand, this is dictated by the problem of saving metal, and on the other, toughening consumer requirements in connection with the specifics of a number of industries. So, for example, it is impossible to stretch, and even more deeply stretch, a semi-finished sheet product having different thicknesses, since irreversible rejection by the wall thickness of the product is inevitable due to the localization of the deformation, until the sheet semi-finished product ruptures during the drawing process. It is also impossible to diffuse weld sheets having a rough surface and breaking flatness.

With a decrease in the thickness of the semi-finished sheet, the tolerance field has even narrower limits. The thickness variation of the sheet semi-finished product can be transverse and longitudinal, respectively, along and across the direction of rolling. Violation of the form, as a rule, is expressed in the occurrence of waviness, warpage of the sheet prefabricated. Any change in the rolling conditions and characteristics of the rolled material is reflected in the final thickness and shape of the semi-finished sheet. In this case, the main factor affecting the formation and change of final dimensions in thickness is the rolling force. It is assumed that the elastic deformation of the working stand is directly proportional to the rolling force [1]. During hot and warm rolling, fluctuations in the rolling force are also caused by a change in the deformation resistance of the rolled material, which in turn is caused by a fluctuation in the rolling temperature. In addition, the rolling force depends on the tension, runout of the rolls, the thickness of the rolled billet (rolled). Due to the technological inheritance that exists in sheet rolling, the resulting thickness of the semi-finished product does not disappear completely, despite the leveling effect of subsequent rolling processes, and on mills without control systems it can even get worse. In addition to the aforementioned, the thickness and surface quality of a semi-finished sheet of titanium alloy due to intense oxidation and saturation of the surface of the titanium billet with nitrogen and oxygen depend to a large extent on the thickness of the solid, saturated with gases, and the loose oxidized scale layer. Therefore, it is impossible to obtain hot-rolled sheet semi-finished products with high dimensional accuracy and surface cleanliness without the use of vacuum or a special protective environment. But even when used at elevated temperatures, the stiffness of the roll system decreases and the elastic deformation of the rolls increases. As a result, hot rolling generally fails to obtain a prefabricated sheet or foil.

Hot rolled strips are usually semi-finished products for subsequent cold rolling.

Cold rolling of hot rolled pickled strips is carried out using intermediate annealing [1]. A particularly large number of intermediate anneals are required by slightly ductile two-phase titanium alloys. During cold rolling, the deformation resistance increases, while the forces and moments of rolling should not exceed the maximum allowable values for this mill. The cold-rolled semi-finished product is characterized by the presence of a pronounced metallographic texture, leading to anisotropy of its properties. Anisotropy of the properties of the semi-finished sheet in most cases is undesirable. An exception may be attributed, perhaps, to electrical steel. If the sheet semi-finished product is subsequently subjected to deformation to obtain a product, such as a shell, then the anisotropy of its properties is not only undesirable, but even unacceptable.

To some extent, the solution to the problem is the use of warm rolling, in which the disadvantages inherent in both hot and cold rolling are not so clearly manifested.

In addition, during warm rolling, it becomes possible to maintain a submicrocrystalline (CMK) or nanocrystalline (HK) structure in a semi-finished sheet from a two-phase titanium alloy — a fine-grained structure, respectively, with a grain size of less than 1 μm and less than 0.1 μm, if such a structure had place in the original workpiece. Of course, there is a need for special preparation of such a structure in the initial procurement, but many methods are known for this. Even high-quality industrial sheet metal today has a grain size of the order of 3-5 microns. The presence of a structure in a CMK and HK sheet semi-finished product is very relevant, since it further ensures the manufacture of a complex-shaped product by superplastic molding or in a superplastic molding process combined with diffusion welding (SPF / DS) using the effect of low-temperature superplasticity. Lowering the temperature of molding and / or diffusion welding can increase the durability of the tooling used in these processes and make them generally more economical. There is an opportunity to get a better connection of sheet titanium semi-finished products by reducing the saturation with gases of the surfaces to be joined. It is also possible to preserve the structure of the rolled semi-finished product in the finished product CMK or HK, which allows to fully realize the unique set of mechanical properties inherent in CMK and HK materials, manifested in increased strength and fatigue characteristics. And finally, there is a unique opportunity to carry out the process of joint deformation of the workpiece from aluminum or its alloy, and a semi-finished sheet of titanium alloy to obtain a connection between them by pressure welding at a temperature of the order of 400-450 ° C and below, at which a brittle intermetallic layer is not formed in the connection zone. Products in which such a combination can be implemented are of interest to modern science and technology. To date, in the reference literature [2, 3], the possibility of joining a billet of aluminum or its alloy and a billet of technically pure enough titanium or low alloyed titanium alloys at indicated temperatures is noted. A known method of manufacturing a semi-finished sheet of a two-phase titanium alloy by warm rolling [4] at a temperature of the onset of deformation by an amount from 400 to 550 ° C below the temperature of the polymorphic transformation at a speed of from 10 ^'4 to 10 ^"2 s ^{" 1} and the degree of deformation from 5 to 15% followed by annealing at a temperature of 400 to 550 ° C below the polymorphic transformation temperature and repeating this treatment cycle until a total degree of deformation of 75 to 95% is achieved. To prepare the structure, it is envisaged to pre-treat the billet in β and (α + β) regions before rolling. After such preliminary processing, the preform has a coarse-grained lamellar or partially recrystallized globular-lamellar structure.

During warm rolling, recrystallization and globularization processes go through the indicated modes. However, these processes for the cross section of a semi-finished product are hereditarily developing unevenly and not completely. As a result, the resulting semi-finished product has an inhomogeneous microstructure and, therefore, inhomogeneous mechanical properties. The heterogeneity of the microstructure is unacceptable for further superplastic deformation. But even with the exclusion of heterogeneity of the microstructure, the insufficiently small grain size does not allow using the obtained semi-finished product for low-temperature superplastic deformation. In addition, fluctuations in the rolling force associated with the heterogeneity of the structure cause fluctuations in the thickness of the rolled billet.

For the implementation of warm rolling used rolling mill DUO- 200. An electric resistance furnace KS-300 is used to heat the workpieces.

A known method of manufacturing a sheet of a semi-finished product from a titanium alloy [5], including pre-processing the workpiece for a structure with a submicron grain size and subsequent rolling. Rolling starts from a temperature selected in the temperature range from 150 to 500 ° C below the polymorphic transformation temperature, which is regulated by the required submicron grain size in the finished semi-finished product. As a result, a semi-finished sheet product suitable for further processing in the low-temperature superplasticity mode can be obtained. In this case, self-rolling directly, although it involves the use of a temperature range, which, as part of the temperature range, which is characteristic of low-temperature superplasticity, is not carried out in the superplasticity mode. The superplasticity regime requires strict compliance with such deformation parameters as the deformation temperature, strain rate, and grain size in the workpiece under isothermal conditions in the deformation zone.

In the known method [5] at reduced temperatures, rolling to the required thickness of the semi-finished product is carried out in several passes with partial reductions of 5-20%, and after reaching a total degree of deformation of 40-65%, intermediate annealing is carried out at a temperature below the polymorphic transformation temperature of the alloy by a value of 150 up to 500 ° C.

The pretreatment of the preform to a structure with a submicron grain size is carried out by methods other than rolling by pressure treatment, since it is believed that it is not possible to carry out intensive and uniform plastic deformation of the material over the cross section of the preform to obtain a homogeneous submicrocrystalline structure in the sheet prefabricated. As one of the methods, it is recommended to use multilateral deformation, which includes a set of operations of upsetting and drawing with the change of deformation axes.

In the known method [5] rolling pre-processed on a structure with a submicron grain size of the workpiece is carried out in the range the existence of CMK material structure. The upper limit of the temperature range of rolling is regulated by the required submicron grain size in the finished sheet semi-finished product. The lower boundary is limited by the technological ductility of the material processed by CMK. Thus, during the rolling process, either the initial microstructure of the pre-processed billet is preserved, or the grains are slightly crushed, or coarsened within the submicron range to the required grain size in the finished semi-finished product. But as already noted, any change in grain size leads to a change in the ductility of the rolled material, the rolling force, and as a result, fluctuations in the thickness of the semi-finished sheet.

Also, in the known method, a basic texture can be created that provides isotropy of the mechanical characteristics in two directions in the plane of the sheet, due to the use of longitudinal-transverse rolling. However, this technique can only be used for square cards.

Also in the known method, to reduce the undercooking of the workpiece during the rolling process in non-isothermal conditions and to stabilize the rolling conditions, roll heating is used. If the rolls are heated to a deformation temperature, rolling is carried out in isothermal conditions. However, this technique is not required when implementing the method. Although if rolling according to the known method starts from a temperature lower than the polymorphic transformation temperature by 500 ° C, as a result of cold-rolling, especially during the rolling of semi-finished products, it may happen that rolling becomes impossible due to insufficient ductility of the alloy.

Despite the fact that the grain size remains within the submicron range, when using temperatures in the range of 150 to 200 ° C below the polymorphic transformation temperature, oxygen and nitrogen are actively dissolved in titanium with the formation of a gas-saturated layer and scale. To obtain a semi-finished product with a given thickness, it is necessary either to carry out rolling in a vacuum or to protect the original billet with a shell. In the case of using the last trick in the manufacture thin-sheet semi-finished product or foil, it may be difficult to separate them from the shell, up to a breach of integrity. In addition, both methods seem economically unprofitable. It also seems economically unprofitable that, using fairly laborious methods, it is possible to obtain CMK structures and even HK structures with a specific grain size on the order of fractions of a micron, and then rolling in the range from 150 to 200 ° C below the temperature polymorphic transformation (T _pp ), lose this specific grain size.

The objective of the invention is to improve the quality of a semi-finished sheet of titanium alloy, intended for further low-temperature superplastic deformation by stabilizing grain size, achieving a more complete isotropy of properties, and also by reducing the thickness variation of the semi-finished product, improving the quality of its surface, while reducing the economic cost of the technological cycle receiving a sheet semi-finished product.

Another objective of the invention is to expand the technological capabilities of the method by producing particularly thin-sheet semi-finished products, including foil with predetermined geometric dimensions, surface cleanliness and grain size. An additional object of the invention is to reduce the possible transverse thickness difference of the sheet semi-finished product and increase the degree of flatness.

Another additional objective of the invention is to further reduce the cost of implementing the technological cycle of obtaining a sheet of semi-finished product, including the stage of preparation of the structure in the original workpiece.

The tasks are solved in the case when the method of manufacturing a semi-finished sheet of titanium alloy suitable for low-temperature superplastic deformation, including rolling a workpiece with a prepared structure at a temperature below the polymorphic transformation temperature in isothermal or quasi-isothermal conditions provided by heating the rolls, differs from the known that rolling is carried out in a low-temperature superplastic mode deformation, in this case, predominantly, in the first pass, the deformation is carried out with the degree ε> ε _m j _n , where ε _m j _{n is the} minimum degree at which the structural state of the alloy is formed in the selected temperature-speed rolling mode, which is necessary to ensure the cooperative grain-boundary slippage (KZGP), in addition, after each subsequent rolling pass, immediately upon exiting the deformation zone, the workpiece is cooled to fix the structural state obtained during deformation, except for t On the other hand, during furnace heating of the billet for the next rolling pass, the heating time is limited in order to avoid disturbing the structural state of the alloy obtained in the previous rolling pass.

The tasks set are also solved if:

- rolling is carried out at a temperature selected in the temperature range from T _PP -450 to T _PP -350 ° C; - rolling is carried out with a strain rate selected in the speed range from 10 ^"3 to 10 ^'1 s ^'1;

- when rolling to achieve a compression ratio of 30-60%, after every three to five longitudinal passes, turn the workpiece by 90 ° and conduct a transverse pass, and the rest of the deformation is gained during rolling in one direction;

- in the manufacture of a semi-finished sheet with a thickness of not more than 1 mm, the workpiece is heated by contact with the work rolls;

- for rolling use a workpiece with a prepared globular structure, with a grain size of less than 1 micron; - for rolling use a workpiece with a prepared lamellar structure, with a grain size in the cross section of less than 1 μm;

- preparation of the structure in the billet for rolling is carried out by pre-rolling the initial billet with a grain size of at least one cross section, not more than 10 microns, until a degree of deformation of at least 80% is reached, while rolling starts at a temperature selected in the temperature range from T _pp - 300 ° C to T _pp - 200 ° C, and finish at a temperature not lower than the temperature of the main rolling with the strain rate selected in the range from 10 ² to 10 ° c ^"ι ;

- preparation of the structure in the billet for rolling is carried out by preliminary rolling of the initial billet with a grain size of 10 to 80 μm, carried out in two stages, and at the first stage, the initial billet is rolled to a degree of deformation of not more than 60%, while rolling starts at a temperature , selected in the temperature range of T _pp - 200 ° c to T _pp - 5O ⁰ c and finished at a temperature not lower than the temperature of the primary rolling at a strain rate is selected in the range of from 10 ^"2 to ^10" is rolled on the second stage for otovku under isothermal or quasi-isothermal conditions at a temperature and strain rate with the main rolling to achieve the degree of strain 20-30%.

- rolling is carried out using a rolling mill having two workers and at least four backup rolls;

- change the magnitude of the deflection of the backup rolls in direct contact with the work rolls by changing the cooling intensity of the bearing assemblies of the backup rolls;

- heating of the work rolls is carried out by means of electric resistance heaters located inside the rolls.

One of the main distinguishing features of the invention is that the method is not only designed to produce a semi-finished sheet suitable for low-temperature superplastic deformation, but also the rolling itself is carried out in the low-temperature superplasticity mode. At the same time, efforts aimed at preparing the structure in the billet for rolling are spent more strictly for their intended purpose. However, to achieve the objectives it is more important to change, compared with the known solution, the technical essence of the rolling method.

To clarify the technical nature of the proposed method, we consider in more detail the physics of the deformation process in superplastic conditions.

It is known that deformation under conditions of superplasticity occurs to a significant degree without the accumulation of residual stresses, and with significantly lower deformation forces, which is especially important when processing low plastic materials, including biphasic titanium alloy.

The main mechanism of alloy deformation under conditions of superplasticity is grain boundary slippage (CGP). Upon reaching a certain degree of deformation, i.e. Involvement in the process of deformation of the entire or most of the volume of pre-fabricated ZGP takes on a cooperative character - KZGP [6, 7]. Unlike intragranular glide, which plays a secondary role in deformation under superplastic conditions, in the process of slipping along the boundaries, the grains do not stretch and remain equiaxed or in other words globular. As a result, the formation of both a metallographic and crystallographic texture occurs to a much lesser extent. Moreover, if the texture took place in the initial billet, then during deformation under conditions of superplasticity, due to KZGP, the texture is scattered. Thus, the development of KZGP during rolling provides an isotropy of the properties of the semi-finished sheet in any arbitrary direction on the sheet plane.

When a deforming force is applied, KZGP does not begin to develop immediately. First, a shear band is formed, combining a large number of consecutively conjugated grain boundaries. This process occurs according to the principle of self-organization and is associated with an increase in angles in triple joints (straightening of boundaries). At this stage, corresponding to 3-15% deformation, an intensive increase in the flow stress is observed, which entails an increase in the deformation force (Fig. 7, 8). After the formation of shear bands, the flow stress is established at a stationary level or gradually decreases. Moreover, the specific value of the degree of deformation required to establish the process depends on the grain size in the deformable workpiece, the smaller the grain size, the smaller the degree of deformation required to form the necessary structural state. The stage of stable flow corresponds to the superplasticity regime, when KZGP acts as the main mechanism of deformation. The presence of formed shear bands, that is, KZGP, is judged, as noted above, by the flow stress - strain diagram (hereinafter simplified stress - strain), which is removed for the simplest and most obvious case of sample loading - uniaxial tension (continuous line in Fig. 7 ) It should be noted that with small degrees of deformation (10-15%), the values of relative strains during tension and during rolling (compression) are close, and the comparison is valid.

However, this material behavior is valid for monotonic deformation. During rolling, the deformation is fractional. The deformation zone is constantly moving, rolling is carried out in several passes, and the degree of deformation on the passage is, as noted, only 10-15%.

Therefore, it becomes important to form shear bands, mainly in the first pass, so that in subsequent passes, when the workpiece section again falls into the deformation zone, already have formed shear bands, and as a result, if possible, in all subsequent passes to stabilize the plastic characteristics of the rolled material and the force rolling.

If a workpiece with a prepared, globular CMK or HK structure is subjected to rolling, already after 5-7% deformation, the structural state necessary for the implementation of KZGP is achieved. If a workpiece with a prepared, lamellar structure with a plate size in the cross section of less than 1 μm is subjected to rolling, the degree value can be larger than in the previous case and amount to 10-15%.

Another necessary technique for rolling under superplasticity is cooling the workpiece upon exiting the deformation zone after the next pass, which allows maintaining grain size and formed shear bands. Exposure of the material between passes at a temperature close to the deformation temperature, which does not even lead to grain growth, leads to a change in the state of grain boundaries and a partial restoration of the initial structure. Moreover, annealing between passes leads to a complete restoration of the equilibrium structure and enlargement of the grains. In both cases, i.e. after holding and annealing, an increase in stress is observed compared with stresses under continuous loading for the same degrees of deformation (Fig. 7).

In FIG. Figure 8 shows a diagram taken for a continuous and fractional process of loading a sample with partial (at 100 ° C) cooling during removal of the load, which shows the convergence of both graphs.

To preserve the formed shear bands limit the time heating the workpiece for the next rolling pass when using furnace heating. The last condition does not require compliance when heating the sheet preform due to contact with the work rolls.

The implementation of a new and non-obvious technique, which consists in influencing the plastic characteristics of the rolled material in order to stabilize them by maintaining the state of grain boundaries in the process of fractional non-monotonic deformation of rolling, becomes effective, namely, during warm rolling. During hot rolling, the magnitude of this effect is lost against the background of intense temperature effects on the stiffness of the roll system. Therefore, another necessary technique is to carry out rolling under conditions of low-temperature superplasticity.

In addition, warm rolling provides semi-finished sheet products with high accuracy and surface finish due to the almost complete elimination of the formation of a gas-saturated layer and scale. As a result, it becomes possible to obtain a wide range of thin-sheet semi-finished products, including foils of various thicknesses, and without the use of vacuum or protective media. The reduction in rolling forces characteristic of superplasticity, and the elimination of the further processing of the sheet prefabricated product to remove the gas-saturated layer, as well as the absence of the need for vacuum, result in a significant reduction in the economic costs of implementing the technological cycle of producing high-quality sheet prefabricated products, despite the need to heat the rolls.

As already noted, a single temperature range characteristic of low-temperature superplasticity is an integral part of the known temperature range. However, this technique is used in conjunction with other methods of the method. Moreover, the new combination offers numerous advantages. Therefore, considering the technique of rolling in a temperature range that is part of the known as one of the terms, one can judge the claimed technical solution as having an extra-cumulative effect compared to the known one.

To comply with the regime of low-temperature superplasticity in general the presence of a homogeneous, equiaxial and fine-grained structure of the alloy being processed, and isothermal conditions for the implementation of the deformation are also necessary.

Each particular deformation temperature carried out in the superplasticity mode corresponds to its specific grain size. For low-temperature superplasticity, this is a grain size of less than 1 μm. This implies the need for special preparation of the structure in the procurement, since industrial steel today does not meet the specified requirements.

During rolling, isothermal conditions mean a constant temperature in the deformation zone. Therefore, the technique of heating the rolls during the implementation of the method becomes necessary to solve the tasks. Deformation heating due to the occurrence of deformation at a rate characteristic of low-temperature superplastic deformation can be neglected. Or, the deformation heating of the billet can be fully compensated by the choice of the corresponding lower roll temperature. For example, if the isothermal conditions are strictly followed at a strain rate έ = 10 ^{"with ^2"} ¹ during the rolling the preform is heated at 30 ° C, then for selecting a roll temperature equal to 470 ° C against the required 500 ° C, in the deformation zone.

It should be noted that when rolling thin sheets, a large total degree of deformation is gained, which, in combination with the conditions of low-temperature superplasticity, affects the final structure, namely, leads to additional grinding of grains, which can be regarded as an advantage of the method.

Thus, the tasks are solved by the totality of the features of the claimed invention.

Further, the invention is concretized and supplemented. Experimentally verified optimal temperature and speed strain intervals under conditions of low-temperature superplasticity are presented, which are suitable for most titanium alloys. The technique, which is that when rolling to achieve a compression ratio of 30-60%, after every three to five longitudinal passes, turn the workpiece through 90 ° and conduct a transverse passage, and the rest of the deformation typed during rolling in one direction, allows you to increase the degree of flatness of the card. After 60% reduction, platelet disturbance is not significant. The difference from the known method [5] is that there a similar technique is used only to ensure anisotropy of the properties of the card in the corresponding directions on its plane.

In the manufacture of prefabricated sheets with a thickness of not more than 1 mm, the workpiece is heated directly by contact with the work rolls. This process can be considered quasi-isothermal. Due to the small thickness of the sheet and the slow rolling speed in the deformation zone rather quickly, already at the initial stage of rolling, the required temperature is established. A workpiece of a greater thickness warms up slowly, or may not have time to warm up to a predetermined temperature during rolling, therefore, such a workpiece is heated in a furnace immediately before rolling. In this case, as a rule, use a continuous furnace. To implement low-temperature superplasticity and KZGP during rolling use a workpiece with a prepared structure. It should be a homogeneous structure with equiaxed (globular) grains, with a size of less than 1 micron. Moreover, the structure may be such in the initial procurement, that is, prepared in advance by known methods [4, 5]. In this case, in order for the KZGP to "work)), it is enough to form shear bands between the grains, which corresponds, as was noted, approximately 5-10% of the deformation.

Or the structure can be prepared in such a way as to transform into the necessary structure during rolling, mainly in the first pass. This requirement “meets the lamellar structure with elongated grains, with a cross-sectional size of from 0.9 to 1.5 microns. The low rolling temperature and the slow deformation rate during rolling provide the dynamic recrystallization process with the division of the plates and the formation of very small, about 0.2 μm, equiaxed grains. Isothermal conditions ensure the uniformity of the course of this process and its smooth transition into the process of formation of shear bands. In this case, approximately 10-15% deformation is required. Subsequently, when KZGP develops, that is, in the absence of dynamic recrystallization, the grains retain their shape and size. Such a lamellar structure can be obtained by preliminary rolling the initial billet.

If the grain size in the initial billet does not exceed 10 μm, the billet is subjected to preliminary rolling, which is started at a temperature that is lower than the polymorphic transformation temperature by 200 to 300 ⁰ C, and injected at a temperature not lower than the temperature of the main rolling with a strain rate selected in velocities ranging from 10 ^"2 to 10 ° ^{sec" 1.} The strain rate selected in this interval contributes to the active process of dynamic recrystallization. The degree of deformation exceeds the degree, of the order of 70%, necessary for the development of dynamic recrystallization. The latter means that in the process of deformation, the grains acquire an equiaxed shape, and then they lose it again, that is, are elongated. As a result, the preform acquires a lamellar structure, with a cross-sectional size of the plates from 0.9 to 1.5 μm. If the grain size in the original billet exceeds 10 μm, the billet is subjected to preliminary rolling in two stages. Moreover, at the first stage, the initial billet is rolled to a degree of deformation of not more than 60%, while rolling starts in the temperature range from T _pp -300 ⁰ C to T _pp -200 ° C and is completed at a temperature not lower than the rolling temperature with a strain rate selected in the range from ^10'2 to 10 s ^"1. At the second stage, the workpiece is rolled under isothermal conditions at a temperature and with a strain rate of the main rolling until a strain of 20-30% is reached. A feature of such a two-stage rolling is that the degree of deformation at at the first stage, the degree of deformation should be less than that leading to the formation of equiaxed (globular) grains in the workpiece as a result of dynamic recrystallization. Deformation with a degree of less than 60% only leads to elongation and thinning of the plates. If the grains become equiaxed and are large enough, then Subsequently, significant deformation will be needed to give them a plate shape.To prevent the development of the process of dynamic recrystallization, thin plates can be obtained. Then, deforming the thin plates at a lower temperature, namely, at the temperature of the main rolling, you can get smaller plates with the required cross-sectional size. As in the previous case, but only at the second stage of preliminary rolling, when the total degree of deformation of about 70% is reached, the process of dynamic recrystallization undergoes the formation of equiaxed grains. In this case, the grains are obtained quite small, due to the lower deformation temperature. Then, with continued deformation, the grains are extended again. As a result, as in the previous case, the preform acquires a plate structure, with a cross-sectional size of the plates of less than 1 μm.

In both cases, heating for the first pass of the main rolling may be accompanied by static recrystallization, which contributes to some leveling of the structure and globularization of the grains. The structure is completely leveled, and the grains acquire equiaxed shape already during the main rolling process.

The highest dimensional accuracy of a semi-finished sheet product can be achieved if rolling is carried out using a rolling mill having two workers and at least four backup rolls, for example a six-roll mill (Fig. L).

The cooling of the backup rolls in direct contact with the work rolls allows, firstly, to increase the rigidity of the roll system. Secondly, the uneven cooling of the backup rolls directly in contact with the work rolls allows you to create a slight gradient along the length of the barrel of the work roll, sufficient to reduce the transverse thickness variation of the sheet. The most optimal is the cooling of the backup rolls by cooling the respective bearing units. The cooling intensity of the bearing units in this case depends on the required value of the roll barrel

To heat the work rolls, it is recommended to use an adjustable electric resistance heater integrated in the roll, which allows you to set the optimum temperature of the roll depending on the grade of alloy and grain size in the workpiece for rolling.

The invention is illustrated by the following graphic materials:

FIG. 1 is a diagram of a method;

FIG. 2 - microstructure (a) and electron diffraction pattern (b) of the initial billet, prepared by methods other than rolling (comprehensive forging);

Fig.Z is the microstructure and electron diffraction pattern of the sheet semi-finished product obtained after rolling the initial billet with a structure prepared by methods other than rolling; Figure 4 - the microstructure of the initial billet prepared by rolling in one stage, x500;

Figure 5 - the microstructure of the initial billet prepared by rolling in two stages, x500;

6 is a microstructure of a sheet of semi-finished product obtained after rolling the initial billet with the structure prepared by rolling;

FIG. 7 - stress-strain diagrams for a continuous and fractional process with intermediate annealing without load for 1 min;

Fig. 8 is a stress-strain diagram for a continuous and fractional process with partial (at 100 ° C) cooling during unloading; Fig.9 - the calculated deflection of the backup roll under the action of the experimentally obtained rolling force. The maximum difference in the deflection of the middle part and the edge of the barrel is 0.054 mm. At KTP = 18x10 ^"6 and a wolf diameter of 150 mm, it is compensated by a temperature difference of 40 ° C.

In FIG. 1 shows: 1 - rolled billet; 2 - work rolls with built-in heaters (the latter are not shown in the figure); 3 - backup rolls (number - four); 4 - feedthrough preheating furnace. Examples of specific performance of the method:

Examples of specific performance do not cover all possible embodiments of the proposed method, in terms of used titanium alloys and sizes of sheet semi-finished products.

Examples are given on methods for manufacturing a prefabricated sheet 0.3 mm thick and a 0.05 mm thick foil from titanium alloys BT-6 and BT-22.

Two-phase titanium alloys BT22 and BT6 were processed. The temperature of the polymorphic transformation and the chemical composition of the alloys in mass. % are given in table 1. Examples are given for the manufacture of strips with a thickness of 0.1; 0.5 and 0.7 mm. Table 1

Example 1. It is necessary to obtain by rolling a sheet of two-phase titanium alloy BT6 0.5 mm thick. The initial billet with a thickness of 14 mm and a size of 6O x 100 mm, with a grain size of 0.4 μm (Fig. 2) was obtained by multiaxial precipitation with decreasing temperature to 600 ⁰ C [5].

The rolling temperature was chosen 56O ⁰ C, which is 43O ⁰ C lower than the temperature of the polymorphic transformation. The linear speed of rotation of the rolls 1 mm / s, which corresponded to the strain rate in the center 5xlO ^"3 s ^{" 1} .

Before rolling, tensile samples were cut from the initial billet to determine the minimum degree (ε _m j _n ) at which the structural state of the alloy is achieved in the selected temperature-speed rolling mode, which is necessary to ensure the KZGP during deformation. According to the maximum value of the flow stress, after which it gradually decreases (Fig. 8), ε _m i _n ⁼ 9% was determined.

The initial billet was rolled on a six-roll mill LIS-6/200, with heated work rolls with a diameter of 65 mm (Fig. 1). The heating temperature of the work rolls is 560 ⁰ C. The rolls were heated from the inside by built-in resistance electric heaters. The heating of the backup rolls occurs as a result of contact with the work rolls and reaches 120-180 ^° C in the middle of the roll barrel. The cooling of the backup rolls is carried out by pumping liquid lubricant through the bearing units. The cooling rate is selected so that the temperature difference between the middle and the edge of the barrel of the backup rolls is 40 ± 5 ° C, which provides compensation for the deflection of the rolls and obtain a uniform sheet thickness. The temperature of the backup rolls is controlled by a thermal imager. A feed-through furnace was installed at the mill inlet (Fig. 1), the heating temperature in which was 560 ⁰ C. The degree of deformation in the first pass was 15%. As we approach the final thickness, the degree of deformation per passage decreases. Total number of passes 32. After each pass at the exit from the deformation zone, the preform was cooled in air to a temperature of 400 to 45O ⁰ C. Before each subsequent pass, the preform was placed in a continuous furnace. The heating time was determined from the calculation of 1 min per 1 mm of thickness, which is sufficient only for heating the workpiece and does not imply holding the workpiece at a temperature. After reaching a strip thickness of 5 mm, the length of the workpiece becomes greater than the length of the continuous furnace. Then, before rolling, the beginning of the billet is placed in the continuous furnace and heated for a time t = 0.9 / z min, where h is the thickness of the billet. After that, the workpiece is fed to the rolls. The rest of the workpiece is heated by a continuous furnace as the strip enters the deformation zone. To ensure heating of the workpiece, the length of the heated zone of the furnace is defined as /> 54v- / g, where v is the linear speed of rotation of the rolls. So, with v = 1 mm / s and a workpiece thickness h = 5 mm, the length of the heated zone of the furnace should be at least 250 mm. At the same time, the long billet cannot be heated entirely, since at a low rolling speed this will lead to exposure at the rolling temperature. In this example, the length of the heated zone of the kiln was 300 mm. This technique provides heating, but limits the residence time of the billet at rolling temperature, which allows you to maintain the structural state of the material between passes, which is necessary for the implementation of the main mechanism of superplasticity.

Upon reaching a strip thickness of 2 mm, in order to prevent annealing before being fed to the rolls, the furnace temperature was set equal to 400 to 450 ° C, and the final heating was carried out directly from the work rolls when the strip entered the contact zone. The resulting sheets were thoroughly examined. Deviations of the sheet thickness from the specified 0.5 mm did not exceed 0.02 mm. The surface was covered with a dense thin oxide film of a dark blue color with no signs of scale. The methods of microstructural analysis and microhardness measurements showed the absence of gas saturation of the surface layer at a depth of more than 1 μm. The microstructure of the strip is uniform globular, with a grain size of 0.2 μm and an elongation coefficient of not more than 1.45 (Fig. 3), while the total degree of strip elongation was e = 13. The intensity of the texture maxima, characterizing the degree of anisotropy did not exceed two units of pole density.

Example 2. The initial billet of a two-phase titanium alloy BT22 with a grain size of 0.6 μm, obtained by multiaxial precipitation with decreasing temperature [5], taken a thickness of 15 mm and a size of 60 x 80 mm. A tensile specimen was cut from it on an electric spark machine, which was used to determine the minimum degree of deformation required to reach a stationary superplastic flow at a given temperature, ε _m j _n = 11%. The billet was rolled to a thickness of 0.7 mm on a six-roll mill LIS-6/200, with heated work rolls with a diameter of 65 mm. The roll heating temperature is 550 ^° C, which is 310 ^° C lower than the temperature of polymorphic transformation. The temperature of the feed-through furnace at the inlet of the mill was 550 ^° C. The linear speed of the rolls was 1 mm / s, which, at 10% compression during the passage, ensured a deformation rate in the center of 6x10 ^"3 s ^{" 1} . This corresponds to the conditions of low temperature superplasticity for a given alloy. The use of a feed-through furnace eliminates the undercoating of the workpiece at such a low feed rate. At the exit from the deformation zone, the workpiece was cooled in air. The resulting stripes were coated with a dense dark blue oxide film. Microstructural analysis and microhardness measurements showed the absence of gas saturation of the surface layer, at least at a depth whiter than 1 μm. Deviations of the sheet thickness from the specified 0.7 mm did not exceed 0.01 mm. The microstructure in the longitudinal section of the strip is homogeneous, with a grain size of 0.3 μm and an elongation coefficient of not more than 1.4. In this case, the total degree of strip stretching was e = 20.4. No signs of intense crystallographic texture were detected by X-ray diffraction.

Example 3. Similar to example 1 except that the rolling temperature was chosen equal to 600 ⁰ C, and the deformation per pass at the initial stage was 20%. At the same roll rotation speed (1 mm / s), the deformation rate in the center was 1, IxIO ^-2 C ^"1. This also corresponds to the conditions of low-temperature superplasticity for an alloy with a given grain size at a given temperature. As a result, the number of passes is reduced to 23 while maintaining the geometric parameters of the resulting sheet. In this case the grain size of the original workpiece is preserved. This technique has achieved a significant increase in process productivity. The temperature in the continuous furnace, taking into account the deformation heating, was selected 58O ⁰ C.

Example 4. Similar to example 1 except that at the initial stage of rolling, after every 3 passes, the workpiece was rotated 90 ° and a transverse pass was made. This technique was performed until a reduction ratio of 60% was achieved, while the workpiece width became equal to the roll barrel length (200 mm). To achieve the necessary combination of the degree of compression and the width of the workpiece, the dimensions of the original workpiece, in contrast to example 1, were selected 16 x 60 x 80 mm, This method has achieved the following goals:

1) increase the flatness of the workpiece, providing a more uniform deformation in subsequent passes;

2) an increase in the width of the rolled strip, if the original billet had limited dimensions, for example, in the form of a standard bar; 3) a decrease in the coefficient of elongation of grains to 1.2 (in the plane of the sheet) and a decrease in the intensity of texture maxima.

Further rolling was carried out in one direction until the specified strip sizes were reached.

Example 5. Similar to example 1, but aimed at obtaining sheets with a thickness of less than 0.5 mm. When the strip reaches the specified value, it is fed to the hot rolls without preliminary heating, or the temperature of the input device is set to not more than Vi T rolling. This technique limits as much as possible the residence time of the workpiece at the rolling temperature, which helps to maintain the state of superplasticity of the material between the passes and increases the accuracy of the final rolling in thickness. Using this technique, it is necessary to slightly increase the heating power of the work rolls in order to compensate for the heat consumption for heating the workpiece. If the roll heaters have a power reserve and are equipped with a thermostat with feedback, then the heating power is compensated automatically. The operation of heaters in automatic mode requires a margin of power of about 30% above the calculated value. Samples of fouls with a thickness of 0.1 ± 0.0 lmm were obtained.

Example 6. As the initial billet taken industrial bar BT22 alloy with a diameter of 60 mm with a plate structure with an average plate size of 80 x 6 microns. The billet is heated to a temperature of 850 ° C, which is 30 ° C lower than the temperature of polymorphic transformation, and 300 ⁰ C higher than the temperature of the main rolling. The workpiece is rolled on a DUO 300 mill, on cold rolls, at a speed of 200 mm / s. With a compression ratio of 20% per pass, this corresponds to a strain rate of 1.2 s ^{"1. The} rolling is carried out in several passes to a thickness of 10 mm, which is 83% reduction. After the first three passes, the workpiece is turned and one transverse pass is performed. In the process of preliminary rolling, the workpiece temperature decreases in each pass by 10-15 ^° C up to 700 ^° C. As the temperature decreases, the degree of compression per pass is reduced, which leads to a gradual decrease in the deformation rate in the center to values below 10 s ^"1 . Then the scale is cleaned from the preform and the gas-saturated layer of 0.12 mm on each side is removed. As a result, a lamellar structure is formed with thin grains elongated in the rolling direction, the average size of which in the transverse direction is 1.3 μm (Fig. 4). The final rolling was carried out analogously to example 2. But in the first pass, an increased pressing force of the rolling stand was required. Further, the lamellar structure is gradually transformed into a globular submicrocrystalline and the process goes into a low-temperature superplasticity mode. The structure of the obtained sheet is less uniform than in the case of a submicrocrystalline preform, but also submicrocrystalline. With a grain size of 0.4-0.5 microns and an elongation coefficient in the longitudinal section of the strip 1.4. The crystallographic texture is poorly expressed.

Example 7. As the initial billet, a BT22 alloy billet with a size of 100x60x60 mm and an average grain size of 50 μm was taken. The workpiece is heated to a temperature of 820 ⁰ C. The workpiece is rolled on a DUO 300 mill, on cold rolls, at a speed of 100 mm / s. With a compression ratio of 20% per pass, this corresponds to a strain rate of 0.7 s ^{"1. The} rolling is carried out in two stages, with several passes at each. The first stage is rolling to a thickness of 27 mm, which is 55% compression. In the preliminary process rolling temperature of the workpiece decreases at each pass by 10-15 ⁰ C up to 65O ⁰ C. As the temperature decreases, the degree of compression per pass is reduced, which leads to a gradual decrease in the deformation rate in the focus to values below 10 ^"1 C ^{" 1} . Then the scale is cleaned from the preform and the gas-saturated layer of 0.12 mm on each side is removed. As a result, elongated grains are formed, the average size of which in the transverse direction is 1.9 μm. The second stage of rolling was carried out in isothermal conditions at a temperature of 550 ° C in several transitions with a total degree of deformation of 28%. The resulting structure is shown in Fig.5. The average grain size in the cross section of 0.9 μm. The main rolling was carried out analogously to example 6. Sources of information taken into account:

1. "Technology of pressure processing of non-ferrous metals and alloys" / Zinoviev A.B., Kolpashnikov A.I., Polukhin P.I., and others - M .; Metallurgy, 1992, 512s.

2. Metallurgy and welding technology of titanium and its alloys / Gurevich SM., Zamkov B.H., Blaschuk V.E. et al. - 2nd ed., ext. and reslave. - Kiev: Science. Dumka, 1986. - 24O c.

3. Welding and materials to be welded: In 3 tons. T.l Weldability of materials. Reference, ed. / Ed. E.L. Makarov- M .: Metallurgy, 1991, - 528 s.

4. Patent RU 2058418, IPC C22F 1/18, 1996. 5. Patent RU 2224047, IPC C22F 1/18, 2004.

6. Cooperative group baipdaru slidip apd superlastiс flow pature. V.V. Astapip, O. A. Kaibushev / Materiels Сiepse Forum VoIs. 170-172, Edit. b. T. Lapgdop, Traps Tes Publistiop, Switzerlapd, (1994) pp. 23-28

7. Superrlastitresultiprom from cooperative baipdaru slidip OA Kaibushev, A.I. Rshepishpiuk apd V.V. Astapip. ASTA Mater., Vol. 46, No. 14, pp. 4911-

4916, 1998.

Claims

CLAIM

1. A method of manufacturing a semi-finished sheet of a titanium alloy suitable for low-temperature superplastic deformation, comprising rolling a billet with a prepared structure at a temperature below the polymorphic transformation temperature in isothermal or quasi-isothermal conditions provided by heating the rolls, characterized in that the rolling is carried out in the low-temperature superplastic mode strain, wherein, preferably, the first pass is carried out with a degree of deformation ε> ε _m j _n, rD ε _m i _n - minimum degree at which the selected temperature-speed rolling operation is formed structural condition of an alloy required for maintenance at the deformation cooperative grain boundary slip, in addition, after each successive pass rolling, directly at the exit of the deformation zone, the billet is cooled to fixing the structural state obtained during deformation, in addition, during furnace heating of the billet for the next rolling pass, the heating time is limited to avoid disturbance structural state of the alloy obtained in the previous rolling pass.

2. The method according to p. 1, characterized in that the rolling is carried out at a temperature selected in the temperature range from T _PP -450 to ° C to T _PP -350 ° C.

3. The method according to p. 1, characterized in that the rolling is carried out with a strain rate selected in the speed range from 10 ^"3 to 10 ^{" 1} s ^"1 .

4. The method according to p. 1, characterized in that when rolling to achieve a compression ratio of 30-60%, after every three to five longitudinal passes, turn the workpiece through 90 ° and conduct a transverse passage, and the remaining part of the deformation is gained during rolling in one direction.

5. The method according to p. 1, characterized in that in the manufacture of a semi-finished sheet with a thickness of not more than 1 mm, the workpiece is heated by contact with the work rolls.

6. The method according to claim 1, characterized in that a preform with a prepared globular structure with a grain size of less than 1 μm is used for rolling.

7. The method according to p. I ₅ characterized in that for rolling use a workpiece with a prepared plate structure, with a grain size in the cross section of about 1 μm.

8. The method according to claim 7, characterized in that the preparation of the structure in the billet for rolling is carried out by pre-rolling the original billet with a grain size of at least one section, not more than 10 microns, to achieve a degree of deformation of at least 80%, wherein rolling starts in the temperature range from T _PP -ZOO ° C to T _pp -200 ⁰ C and ends at a temperature not lower than the temperature of the main rolling with a strain rate selected in the range from 10 ^"2 to 10 ° s ^{" 1} .

9. The method according to claim 7, characterized in that the preparation of the structure in the billet for rolling is carried out by pre-rolling the initial billet with a grain size of 10 to 80 μm, carried out in two stages, and at the first stage, the initial billet is rolled until the degree of deformation is reached no more than 60%, while rolling starts in the temperature range from Tpp-200 ⁰ C to T _pp -50 ⁰ C and ends at a temperature not lower than the temperature of the main rolling with a strain rate selected in the range from 10 ^"2 to 10 s ^" \ at the second stage of rolling t preform under isothermal conditions at a temperature and strain rate with the main rolling to achieve the degree of strain 20-30%.

10. The method according to p. 1, characterized in that the rolling is carried out using a rolling mill having two workers and at least four backup rolls.

11. The method according to claim 10, characterized in that the deflection of the backup rolls directly in contact with the work rolls is changed by changing the cooling intensity of the bearing assemblies of the backup rolls.

12. The method according to claim 1, characterized in that the heating of the work rolls is carried out by means of electrical resistance heaters located inside the rolls.