Creep-annealing of titanium rolled sheets

FIELD: process engineering.

SUBSTANCE: invention relates to metal forming and is intended for straightening of rolled sheet in annealing at constant load, primarily, large-size sheets and boards from titanium alloys. Proposed creep annealing comprises setting of batch composed of one or several sheets onto steel heated plate of vacuum straightening plant. Plant inner space is evacuated at simultaneous loading of the batch outer side, heating to annealing temperature is performed as well as holding and cooling. Cooling is executed with intermediate stage at temperature of 220±20°C with holding of 1 to 5 hours.

EFFECT: stable sheet surface shape.

2 cl

 

The invention relates to the processing of metals by pressure, and is intended to edit the sheet metal in the annealing process under constant load (creep-annealing) mainly large-sized sheets and plates of titanium alloys used, for example, in aviation, shipbuilding, mechanical engineering and chemical industries.

High requirements on geometry in the production of a sheet of semi-finished products, in particular the requirements for flatness, are realized in the invention, using the effect of creep (creep) slow rise time of plastic deformation of the material when a force impacts are lower than those that can cause residual strain during testing normal duration. Creep is accompanied by stress relaxation. It is common to almost all construction materials in the entire temperature range.

Optimum is a combination of simulated annealing creep-edit, which is held either in bell-type electric furnace under a load of steel plate, or in a vacuum creep-edits. This radically reduces the time of process operation changes. The process creep-annealing allows for slow cooling to avoid large internal stresses that it is very important to avoid distortion of the plates during their subsequent fur the technical processing.

As is well known, titanium exists in two stable allotropic modifications - high temperature with volume-centered cubic lattice of β and low-temperature hexagonal α. In titanium alloys, depending on the doping may be present both phase even at room temperature. During heating from room temperature to the temperature of polymorphic transformation CCI is an increase in the number of β-phase to 100%. When cooled from the temperature of polymorphic transformation Chamber to room temperature in the alloy, the reverse change in the ratio of these phases. Creep-annealing, usually performed at a temperature of about 800°C. depending on the type of alloy, the percentage of β - phase at a given temperature can be from 10% to 100%.

These structural transformations are accompanied by volume changes of titanium alloy. In addition, in the cooling process, especially in the temperature range 300-400°C during the transformation β→α nabludaetsa the emergence of unstable intermediate ω-phase. If the polymorphic transformation of β↔α is accompanied by a small volume effect, which, according to various authors, is about 0.15%, while the transformation β→ω surround effect is of the order of 0.9 to 1.2%. It should be noted that even small localized changes the volume of the mA metal give rise to internal stresses, which can sum up randomly in the cooling process and cause recurrent deformation, leading to distortion of the outer geometric shape of the product.

Deflection at a particular point in time is a constant and represents the sum consisting of the deformation of return (elastic deformation and creep. The creep strain is increasing all the time, and the total deformation remains constant; hence, the rst term decreases strain of return, which is associated with the voltage according to Hooke's law (voltage occurring in the body during its deformation is directly proportional to the magnitude of this strain). This means that the growth of creep decreases the voltage. In particular, when the creep-annealing titanium sheet metal, in the cooling process, it is necessary to create conditions under which the relaxation of internal stresses due to the increase of creep and deformation decrease return.

There is a method of creep-annealing titanium sheet products, including installation charge, consisting of one or more sheet articles on the steel heated plate, creating pressure in the working space with simultaneous uniform loading of the upper outer surface of the charge, heated to the temperature the s annealing, restraint and forced cooling in conditions of exhaustion (RF Patent No. 2357827, IPC B21D 1/00, publ. 10.06.2009) prototype.

The use of this invention allows high-quality editing leaves at the stage of annealing, which allows you to exclude or minimize machining surface of the sheet of titanium alloys products.

A significant disadvantage of this method is that it does not include phase transformations during cooling characteristic of titanium alloys, which result in dimensional changes of metal and the occurrence of internal stresses. Technology not provided the necessary time and temperature cooling, allow at least sufficient to transform emerging recurrent deformation in the strain relaxation. Therefore, the process is not stable and in the process of creep-editing, in this way, observed individual cases exceeding the value field tolerance prefabricated sheet flatness in 2 and more times.

The task to be solved by the invention is the stability of the surface forms of sheet metal of titanium alloys.

Technical result achieved in the implementation of the present invention is in the regulation of thermal and temporal modes to the Fe-annealing stage of cooling, in which the ratio of creep to the deformation of return will increase, and the internal stress in the products sheet metal is reduced to values that guarantee the production of products with variations of flatness within tolerance.

This technical result is achieved by the fact that the way the creep-annealing titanium sheet products, including installation charge, consisting of one or more sheet articles on the steel heated plate, creating pressure in the working space with simultaneous uniform loading of the external outer surface of the charge, heated to the annealing temperature, the shutter speed and cooling, cooling is performed with an intermediate stage at a temperature in stage 220±20°C with a holding time of from 1 to 5 hours.

Form error products rolled sheet can be diminished by the introduction of the intermediate cooling stages with the temperature range of 150-400°C. between steps, starting with the annealing temperature, and exposure at each step from 1 to 5 hours.

In the process of heating metal sheet before annealing temperature in the region of 800°C or more in the structure of the occurring phase changes depending on the type of alloy, the percentage of β - phase at a given temperature can be from 10% to 100%. Accordingly, local changes in grain size, include the data from the polymorphic transformations of β↔α and β↔ω, can be 0.15 to 1.2 percent. As a consequence, the resulting voltage can be summarized randomly and warp consisting of recurrent deformation and creep. Significant return deformation after removal of the load can cause changes in the shape of products, larger than the allowable. Introduction intermediate stage cooling with shutter speeds from 1 to 5 hours at a temperature of 220±20°C allows to transform a return deformation in creep strain, which guarantees the preservation of the shape of the product subjected to creep-annealing. At this temperature the phase changes are minor and do not create significant stresses, while the accumulated value of the creep of metal at a given temperature and aging can reduce the amount of recoverable deformation to an acceptable level.

When a significant component of the β-phase in the alloy, and also depending on the geometric dimensions of the product you want to enter additional stage cooling with a temperature range of 150-400°C. between steps, starting with the annealing temperature, and exposure at each step from 1 to 5 hours

Temperature intervals between stages, as well as their temperature is chosen empirically.

Industrial applicability of the invention is confirmed by the specific embodiments thereof.

The results of control of flatness to the creep-annealing:

Plate No. 1 maximum variation of flatness of 16.0 mm;

Plate No. 2 maximum variation of flatness of 19.5 mm;

Plate No. 3 maximum variation of flatness 7.0 mm;

Plate No. 4 maximum variation of flatness 15,0 mm

Creep-annealing was performed in vacuum straightening plates and sheets, the dimensions of the back plate 1880x3900 mm Temperature creep-annealing to 720°C, holding for 10 hours, cooling oven for 48 hours to a temperature of 200±10°C., aged for 5 hours. Further, the metal was cooled to 120°C for 40 hours.

The results of control of flatness after creep-annealing:

Plate No. 1 maximum variation of flatness 5.0 mm;

Plate No. 2 maximum variation of flatness 3.5mm;

Plate No. 3 maximum variation of flatness 4.0 mm;

Plate No. 4 maximum variation of flatness 5,0 mm

Variation of flatness was concentrated at the ends of the plates. After cutting the boards to length variation of flatness does not exceed 1,8 mm This variation of flatness was sufficient for further grinding plates on the customer's equipment.

1. The way the creep-annealing titanium sheet products, including installation charge, consisting of one or more sheet articles on the steel heated plate vacuum editing, creating pressure in the working Pro is transto installation with simultaneous uniform loading of the external outer surface of the cages, heating to the annealing temperature, the shutter speed and cooling, wherein the cooling is carried out with an intermediate stage at a temperature in stage 220±20°C with a holding time of from 1 to 5 hours.

2. The method according to p. 1, characterized in that it further introduce an intermediate step of cooling the temperature range of 150-400°C., since the temperature of annealing, aged from 1 to 5 hours.



 

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