RU2243833C1 - Method for making thin sheets of high strength titanium alloys - Google Patents

Abstract

FIELD: plastic working of metals, namely manufacture of thin sheets of high strength titanium alloys by pack rolling process.

SUBSTANCE: method comprises steps of heating billets before first rolling stage until temperature exceeding by 50 - 150 C temperature of polymorphous conversion; seasoning for 15 - 50 min and quenching by cooling in water; hot rolling of pack in case heated up to 650-750 C initially lengthwise or crosswise relative to direction of rolling initial billet at total deformation degree 61 - 70 %; hot rolling of pack in case in direction normal relative to direction of first rolling of pack at the same temperature-deformation parameters; subjecting case to annealing. Invention allows to make sheets with ultra-fine grain structure suitable for super-plastic deformation at temperature lower than 800 C.

EFFECT: enhanced strength of material during super-plastic forming process, lowered power consumption.

2 dwg, 1 tbl, 2 ex

Description

The invention relates to the field of metal forming, and in particular to a method for manufacturing thin sheets of high-strength titanium alloys by the method of batch rolling.

A known method of manufacturing thin sheets of strong and high-strength alloys (RF patent No. 2179899, CL 21 V 1/38, publ. 02.27.02). The method includes preparing a card blank, assembling a bag in a steel case, heating the case to a temperature of 880 ° C and hot rolling with a degree of deformation of 60%, annealing the package at 770 ° C for 30 minutes, dressing, cutting the package and finishing sheet operations.

This method allows to obtain sheets with dimensions of the α-phase in the microstructure of 2-4 microns, which is quite enough for the manufacture of parts from sheets under conditions of superplastic deformation (SPD) at temperatures of 900-960 ° C. This is the optimal temperature to obtain the required values of flow stresses and elongation at strain rates of 10 ^-3 -10 ^-4 sec ^-1 .

Lowering the temperature of the SPD below 800 ° C leads to a sharp increase in the flow stress to 75 MPa, and the sheets obtained by the known method are not suitable for SPD at temperatures below 800 ° C.

The closest in technical essence to the claimed invention is a method of batch rolling of thin sheets (0.076-1.0 mm) of strong and high-strength metals such as titanium, zirconium and their alloys (US patent No. 2985945, publ. 30.05.61). The method includes preparing a card blank, assembling a bag in a steel case, heating the bag to 727-759 ° C, hot rolling the bag, annealing, cold rolling with a degree of deformation of 10-60%, heat treatment, cutting the bag, finishing sheet operations.

The processing of high-strength alloys in the proposed temperature range is difficult, leading to the formation of microcracks and gaps in the processed material.

The sheets obtained by the prototype method can be used for molding parts of complex shape by the method of superplastic deformation (SPD) only at high temperatures (900-960 ° C). A decrease in temperature below 800 ° C leads to a sharp increase in stress during deformation.

The manufacturing process of parts under SPD conditions is carried out in special furnaces, inside which dies are placed and heated to a deformation temperature of 900-960 ° С. Through channels made in the upper die, a heated inert gas is supplied to the workpiece under pressure, which creates the force necessary for molding the part. Due to the high temperatures of the SPD, the tool (stamp) life is extremely small, and the power consumption is extremely high. Therefore, there was a need to reduce the temperature of the manufacturing process of parts in the SPD conditions to 800 ° C and below.

It is known that to expand the temperature-speed range of SPD, it is necessary to reduce the grain size of the α phase (O.A. Kaibyshev. Superplasticity of industrial alloys. M: Metallurgy, 1984). Therefore, the problem to which the invention is directed is to produce sheets with an ultrafine-grained structure suitable for superplastic deformation at temperatures below 800 ° C.

The problem is solved in that in the method of manufacturing thin sheets of high-strength titanium alloys, including the preparation of card blanks, assembling the bag in a steel case, heating and hot rolling of the bag, heat treatment, cutting the bag and finishing operations of the sheets, before the first rolling, the card blanks are heated to a temperature 50-150 ° C higher than the polymorphic transformation temperature, incubated for 15-50 min and quenched with cooling in water, hot rolling of the bag in a case heated to a temperature of 650-750 ° C, initial It is carried out in the longitudinal or transverse direction relative to the rolling direction of the initial billet with a total degree of deformation of 61-70%, and the subsequent hot rolling of the packages in the case is carried out in the direction perpendicular to the direction of the first rolling of the package at the same temperature and deformation parameters, after rolling the case is annealed at a temperature of 650-700 ° C with a shutter speed of 30-60 minutes

Heating card blanks to a temperature of 50-150 ° C above the temperature of polymorphic transformation (to the temperature of existence of the β-phase) with subsequent quenching in water makes it possible to obtain needle-shaped α'-martensite with a thickness of not more than 1 μm in the alloy structure. Subsequent heating to 650-750 ° C and hot rolling with a degree of deformation of 61-70%, the needle-shaped α'-martensite is destroyed, turning into the α-phase, which is deformed with the formation of line inclusions consisting of fine grains, providing a very fine-grained microstructure, which improves the superplasticity of the alloy.

The direction of rolling the packages is essential in the formation of the crystallographic texture of the sheets. By changing the sequence of longitudinal and transverse rolling of the package (rotation of 90 °) relative to the direction of rolling of the original billet (slab), you can get a different crystallographic texture in the sheets and reduce the anisotropy of the mechanical properties.

For experimental testing of the proposed method of manufacturing sheets suitable for SPD at temperatures below 800 ° C, the chemical composition of the Ti-6Al-4V alloy was selected within the requirements of the ASM-T-9046 specification with the following components, wt.%: 5.5- 6.0 Al; 4.0-4.5 V; 0.08-0.16 About ₂ ; 0.2-0.3 Fe; 0.06-0.1 Ni; 0.06-0.1 Cr; not more than 0.005 C; not more than 0.005 N, Ti - the rest.

The purpose of selecting the chemical composition of the alloy was to maximize the content of the β phase in the alloy by increasing the content of alloying elements that stabilize the β phase, which leads to a decrease in the temperature of the polymorphic transformation of the β phase into the α phase, and, as a result, a decrease in the temperature at which an equal number of phases (50% of the α phase and 50% of the β phase) is established, which is necessary to obtain the best values of the properties of superplasticity in the alloy, i.e. to reduce the flow stress during SPD.

Sheets with dimensions of 2.23 × 915 × 1650 mm (Example 1) and 2.032 × 1219 × 3658 mm (Example 2) were made from an ingot with such a chemical composition. The polymorphic transformation temperature (TPP) of the alloy is 940 ° C.

Example 1

A strip 20 mm thick was cut into 1380 × 1120 mm cards. Cards were heated to a temperature of 1050 ° C (Tp + 110 ° C), held for 30 minutes and quenched in water. After removal of the gas-saturated layer and defects from the surface, the cards were placed in a case made of carbon steel. The assembled case was heated to a temperature of 700 ° C and rolled in the transverse direction relative to the rolling direction of the initial billet (slab) with a total degree of deformation of 63% for a sheet thickness of 7.2 mm. Then the cards were transferred to the case to get the finished sheet, again heated to a temperature of 700 ° C and rolled in the direction perpendicular to the direction of the first rolling of the package with a total degree of deformation of 63%, to obtain sheets with a thickness of 2.4 mm. Next, the case was annealed at a temperature of 650 ° C with holding at this temperature for 60 minutes.

Then, the usual finishing operations were carried out, including straightening the sheets on a straightening machine, grinding, etching, cutting out test specimens, and cutting the sheets to the finished size. As a result, sheets with dimensions of 2.23 × 915 × 1650 mm were obtained.

Example 2

Sheets with dimensions of 2.032 × 1219 × 3658 mm were produced by analogy with example 1 using double batch rolling. The difference was in a change in the direction of rolling after quenching of card blanks to α'-martensite (first rolling). The package was first rolled in the longitudinal direction relative to the direction of rolling of the original billet (slab), and then in the direction perpendicular to the direction of the first rolling of the package.

Mechanical tests were carried out on samples of sheets obtained in Example 1 and Example 2. The results of the mechanical tests are shown in the table.

Table Dimensions of sheets, mm along the rolling direction across rolling direction σ _0.2 , MPa σ _in , MPa relative
elongation,% σ _0.2 , MPa σ _in , MPa Relative extension, % 2.23 × 915 × 1650 978 1049 12.0 1071 1073 8.0 2.032 × 1219 × 3658 876 903 15.6 888 916 10.6

The microstructure of the obtained sheets is shown in figure 1, where:

a) the microstructure of the sheets obtained in example 1;

C) - the microstructure of the sheets obtained in example 2.

An analysis of the microstructure showed that the average grain size of the α phase is less than 1 μm, which is significantly (3-5 times) smaller than the grain size of commercially available sheets.

Samples of the sheets were tested under conditions of superplastic deformation (SPD) at a temperature of 760 ° C at a strain rate of 3 · 10 ^-4 sec ^-1 . The test results are shown in figure 2.

Analysis of the test results showed that the flow stress of the samples of serial sheets with a grain size of 6.0 microns, tested at a temperature of 900 ° C, does not practically differ from the flow stress of the material of sheets with a grain size of 1.0 microns, but tested at a temperature of 760 ° C (with true strain = 1.1, the flow stress does not exceed 35 MPa). But at the same time, the true deformation before fracture of samples with a grain size of 1.0 μm is 2.0 versus 1.7 for samples from commercially available sheets. Thus, the resulting sheets are suitable for SPD at a temperature of 760 ° C.

Lowering the temperature of the SPD will significantly increase the resistance of the dies during the superplastic stamping process and reduce energy consumption during operation of the furnaces. In addition, reducing the heating temperature of the sheets before superplastic stamping will reduce the cost of irretrievable metal losses associated with cleaning the surface of the part after the stamping process from scale and gas-saturated layer. Irrecoverable metal losses will decrease by 3-10 times depending on the conditions of the SPD.

Claims (1)

A method of manufacturing thin sheets of high-strength titanium alloys, including preparing card blanks, assembling the bag in a steel case, heating and hot rolling the bag, heat treatment, cutting the bag and finishing the sheet, characterized in that before the first rolling, the card blanks are heated to a temperature of 50- 150 ° C above the polymorphic transformation temperature, incubated for 15-50 minutes and quenched with cooling in water, hot rolling of the bag in a case heated to a temperature of 650-750 ° C, initially carried out in longitudinal or transverse direction relative to the direction of rolling of the initial billet with a total degree of deformation of 61-70%, and the subsequent hot rolling of the bag in the case is carried out in the direction perpendicular to the direction of the first rolling of the bag at the same temperature and deformation parameters, after rolling the case is annealed at a temperature of 650 -700 ° C with a shutter speed of 30-60 minutes.

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