RU2484176C2 - Method of making thin sheets from pseudo-beta-titanium alloys - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: proposed method comprises smelting of alloy, making slab, machining its surface, hot, warm, and cold rolling, sintering and ageing. Smelted is pseudo-beta-titanium alloy with aluminium content not higher than 5.0 wt % and molybdenum equivalent No eq. ≥ 12 wt %, calculated by the following formula: Mo eq. wt % = %Mo + %Ta/4 + %Nb/3.3 + %W/2 + %V/1.4 + %Cr/0.6 + +%Fe/0.5 + %Ni/0.8 + %Mn/0.6 + %Co/0.9. Semi-finished 8-2 mm-thick rolled stock produced in hot and cold rolling is subjected, prior to cold rolling, to quenching at T_pt+(20-50°C) for 0.1-0.5 h with cooling. Cold rolling is performed to sheet thickness of 6-1 mm in signal-phase beta-state in two and more steps in several passes with 1-6%-reduction in one pass and total reduction at every step of 30-50%. Note here that intermediate quenching is carried out between said steps in conditions identical to quenching of semi-finished rolled stock before cold rolling.

EFFECT: high-quality rolled thin sheets.

5 dwg, 2 tbl

Description

The invention relates to the field of metal forming, and in particular to methods of manufacturing thin sheets by cold rolling from high-strength pseudo-β-titanium alloys that can be used in aerospace, chemical industries, mechanical engineering, medicine and other areas of the national economy.

Cold rolling compared with hot rolling has two big advantages. Firstly, it allows the production of sheets and strips with a thickness of less than 1.0-0.8 mm up to several microns, which is unattainable by hot rolling. Secondly, it provides higher quality products in all respects - dimensional accuracy, surface finish, physical and mechanical properties.

Titanium alloys are quite laborious in processing, so the cost of processing them is much higher in comparison with most other structural metals. In particular, most titanium alloys are difficult to deform at room temperature, as a result of this, industry prefers hot deformation processing to produce semi-finished products, including sheet metal.

For example, there is a known method of manufacturing thin sheets of alloys primarily based on titanium by rolling in a bag, including preparing the workpiece, assembling the bag using a steel case, hot rolling the bag, heat treating the bag, separating the sheets, heat treating, laying, dressing and surface finishing of the sheets, wherein hot rolling of the package is carried out at thermo-deformation parameters that implement the scheme of deformation of uniform compression of the material of the case and sheets, the sheets are heat treated and adjusted mainly in a vacuum furnace in creep conditions (RF Patent No. 2179899, IPC B21B 1/38).

The process previously requires careful scrupulous preparation, it is costly and inefficient in comparison with cold rolling. In addition, the implementation of the technology at high temperatures in itself significantly complicates the process itself and requires the availability of expensive heating equipment.

Known conditions under which it is possible to significantly increase technological ductility and reduce the resistance to deformation of titanium alloys at room temperature to an acceptable level that allows for cold rolling.

With practically identical impurity contents, the following factors have a critical effect on the value of technological plasticity at room temperature:

- increased content of the β-phase with a body-centered cubic lattice, which by its nature is more plastic than the hexagonal α-phase;

- low aluminum content, because with an increase in its content in the alloy, the process ductility decreases, and with a content of more than 6% (by weight) Al, the process ductility of the alloys becomes insignificant.

Under certain circumstances, these requirements correspond to high-alloy pseudo-β-titanium alloys with a low aluminum content and a high β-phase content, which is fixed by quenching. These alloys in the hardened state have high ductility and are capable of cold deformation.

However, in the final product, in order to achieve high mechanical properties with high fracture toughness, it is necessary to age the alloys. In the process of aging, the β-phase undergoes dispersive decomposition with the formation of a thin layer of the α-phase along the grain boundaries, which reduces the technological plasticity of the alloy and makes it impossible to cold roll titanium alloys.

A known method of manufacturing sheets of β-titanium alloys, including machining the surface of the slab, hot, warm, cold rolling, annealing and aging (RF patent No. 2318913, IPC C22F 1/18, B21B 3/00).

The method does not provide the production of sheets from pseudo-β-titanium alloys, since it does not guarantee the absence of the α phase during cold rolling and does not limit the critical aluminum content in titanium alloys that can be processed by this method.

The problem to which the claimed invention is directed, is to obtain a high-quality semi-finished sheet of high-strength pseudo-β-titanium alloys with a thickness of up to 1 mm or less with an increased yield with minimal labor and energy costs.

The technical result achieved by the implementation of the invention is to obtain high-quality sheet metal, including sheet from high-alloy pseudo-β-titanium alloys, by cold rolling, which is performed on a workpiece with a prepared single-phase β-state of the alloy with a regulated aluminum content in it.

The technical result is achieved by the fact that in the method of manufacturing thin sheets of pseudo-β-titanium alloys, including smelting the alloy, obtaining a slab, machining the surface of the slab, hot, warm, cold rolling, annealing and aging, the pseudo-β-titanium alloy is melted with Al content in the alloy is not more than 5.0 wt.% and molybdenum equivalent Mo eq, wt.% ≥12 wt.%, calculated by the formula:

Mo eq, wt.% =% Mo +% Ta / 4 +% Nb / 3.3 +% W / 2 +% V / 1.4 +% Cr / 0.6 +% Fe / 0.5 +% Ni / 0, 8 +% Mn / 0.6 +% Co / 0.9,

at the same time, a tackle obtained after hot and heat rolling, with a thickness of 8–2 mm, is subjected to hardening at T _pp + (20–50 ° C) for 0.1–0.5 hours before cold rolling, followed by cooling, and cold rolling, respectively, to a thickness a sheet of 6-1 mm in a single-phase β-state in two or more stages in several passes with a degree of deformation of 1-6% in one pass and a total deformation of 30-50% at each stage, while between the stages intermediate hardening is carried out according to the identical mode hardening of the rolling before cold rolling.

The method is applicable when rolling pseudo-β-titanium alloys, the composition of which corresponds to the following conditions:

1. The molybdenum equivalent (Mo eq.) Should be at least 12 wt.%. This makes it possible to fix the metastable β-phase during air quenching of sheets with a thickness of up to 8 mm and thereby, in subsequent operations, it is guaranteed to increase technological ductility to an acceptable level.

2. The Al content should not exceed 5.0 wt.%, Because exceeding this value reduces the technological ductility of pseudo-β-titanium alloys to the level that makes sheet cold rolling problematic.

The entire process chain, from the processing of the ingot to the manufacture of rolled products, is based on well-known methods of hot and warm processing, because they are the most technologically and economically profitable and fully meet the requirements of today.

Before cold rolling, the rolling is hardened at a temperature at T _pp + (20-50 ° C), holding for 0.1-0.5 hours, followed by cooling, the time and temperature intervals during hardening are selected from the following considerations:

- quenching modes below the lower boundaries do not guarantee the formation of a structure consisting entirely of β-phase;

- exceeding the upper boundaries leads to an intensive growth of β-grains, which is inherited by the metal to the final product and leads to a significant decrease in the mechanical properties of the alloy, especially in the aged state.

Hardening allows in alloys with Mo eq. ≥12 wt.% Transfer 100% alloy structure to a single-phase β-state.

The aluminum content in the alloy should not exceed 5%, because this value is critical and its excess reduces technological plasticity to a level that impedes the cold rolling.

The ability of pseudo-β-titanium alloys quenched to a metastable β-phase to decompose upon heating to form second phases makes it possible to apply hardening heat treatment to obtain the required level of mechanical properties in the final product. For this, it is necessary to introduce stress energies sufficient for recrystallization processes into the material before annealing and aging.

Cold rolling is performed with degrees:

- the total degree of deformation is carried out within 30-50% in one stage, which is carried out in several passes;

- deformation in one pass - 1-6%.

To preserve the single-phase β-state during the cold rolling process, intermediate hardening is carried out between the stages according to the mode identical to the first hardening of the tackle.

Deformation within the limits indicated below does not allow creating a sufficient hardening of the material, sufficient for the return processes to occur during annealing by the recrystallization mechanism.

Deformation of more than these creates the prerequisites for the mechanical decay of the β-phase, the formation of the α-phase at the grain boundaries and, as a consequence, the reduction of technological plasticity and cracking during rolling.

The proposed method was tested under the production conditions of a sheet rolling workshop in the manufacture of sheets of pseudo-β-titanium alloy VST3553 with a thickness of H = 1.6 mm.

The sheets are made of hot-rolled blanks of pseudo-β-titanium alloy VST3553 with a thickness of Ho = 20 mm. The chemical composition is shown in table 1.

Table 1 The chemical composition of the alloy Mass fraction of elements,% Al Mo V Cr Zr Fe Si C N 3.18 4.82 5.20 2.62 <0.003 0.334 0.049 0.009 0.014

The temperature of the polymorphic transformation T _PP determined by the method of trial hardening, it amounted to 795 ° C.

The molybdenum equivalent was calculated using the following formula

Mo eq. =% Mo ++% V / 1.4 +% + Cr / 0.6 +% Fe / 0.5 = 4.82 + 3.71 + 4.36 + 0.66 = 13.56 wt.% .

Manufacturing technology of 1.6 mm thick sheets

1. Heating of the workpieces in an electric furnace at an installation temperature of 750 ° C, the duration of 30 minutes

2. Rolling billets to a thickness of 5 mm: Hi = 20 → 5 mm with intermediate heatings with a duration of 10 minutes with intermediate thicknesses of the tack 15 mm, 10 mm. The total degree of deformation ε = 75%.

2. Quenching 820 ° C, 20 minutes, cooling in water.

3. The first stage of cold rolling Hi = 5 mm → 2.55 mm, ε = 49%, in 10 passes.

4. The second rolling stage Hi = 2.55 mm → 1.6 mm, ε = 37%, in 7 passes.

5. Heat treatment: hardening + aging.

The invention is illustrated by photographs.

Figure 1 shows the microstructure of the hot-rolled tackle H = 5 mm, the average transverse size of the β-grain is in the center of 180-230 μm, on the periphery of 150 μm, which indicates the heterogeneity of the deformation over the cross section of the sheet. The primary α phase, mainly globular, its size is 1-2 microns. It forms clusters of a darker color, which indicates the heterogeneity of the deformation.

Figure 2 shows the microstructure of the hot-rolled tackle H = 5 mm after quenching, consisting of equilibrium recrystallized grain with an average size of 65 ± 13 μm, consisting of β-phase.

Figure 3 shows the microstructure of a cold-rolled sheet H = 2.55 mm after the first stage of cold rolling of the rolled product. The average β-grain size decreased to 45 ± 3 μm with anisotropy increasing to 2. A large number of slip lines and twins are observed in the grain body. In a longitudinal section, slip lines passing through several grains are visible.

Figure 4 shows the microstructure of a cold-rolled sheet H = 1.6 mm after the second stage of cold rolling of the rolled product. There is a decrease in the average transverse size of the initial β-grain to D _β ≈30-40 μm with anisotropy 3-4, as well as an increase in the number and density of slip lines and twins.

Figure 5 shows the microstructure of a cold-rolled sheet H = 1.6 mm, quenching from a temperature of 815 ° C after holding for 15 minutes and aging at a temperature of 550 ° C, holding for 2 hours. Exposure for 15 minutes at the temperature of the β-region provides a fine-grained recrystallized structure of the β-phase with an average grain size of 55 ± 3 μm. Subsequent aging leads to the decomposition of the supersaturated solid solution with the formation of a finely lamellar α-phase, leading to a significant hardening of the alloy.

The mechanical properties of the obtained sheets h = 1.6 mm from VST3553 alloy after various aging conditions are shown in table 2.

table 2 Mechanical properties of cold-rolled sheets h = 1.6 mm from VST3553 alloy after various aging conditions Along across σ _0.2 , MPa σ _in , MPa δ,% σ _0.2 , MPa σ _in , MPa δ,% sheet No. 4 - h = 1.6 mm 742 823 fourteen 772 818 8.6 815 ° C 15 min water 759 813 11,4 753 810 10.8 (source) wed 750.5 818 12.7 762.5 814 9.7 815 ° С 15 min water + 530 ° С 6 hours air 1243 1322 3.4 1383 1438 one 815 ° С 15 min water +
550 ° C 6 hours air 1149 1210 5,6 1189 1246 4.4 1134 1228 6.8 1217 1306 4.9 wed 1141.5 1219 6.2 1203 1276 4.65 815 ° С 15 min water +
580 ° C 6 hours air 1067 1130 6.5 1149 1205 5,6 1066 1158 8.4 1167 1233 4.6 wed 1066 1144 7.45 1158 1219 5.1

This method allows to obtain thin high-quality sheets from high-strength pseudo-β-titanium alloys with low anisotropy of mechanical properties using standard technological equipment.

Claims (1)

A method of manufacturing thin sheets of pseudo-β-titanium alloys, including alloy smelting, obtaining a slab, machining the surface of a slab, hot, warm, cold rolling, annealing and aging, characterized in that the pseudo-β-titanium alloy is melted with Al content alloy not more than 5.0 wt.% and molybdenum equivalent Mo eq.≥12 wt.%, calculated by the formula:
Mo eq. wt% =% Mo +% Ta / 4 +% Nb / 3.3 +% W / 2 +% V / 1.4 +% Cr / 0.6 + +% Fe / 0.5 +% Ni / 0.8 +% Mn / 0.6 +% Co / 0.9,
at the same time, a tackle obtained after hot and warm rolling, with a thickness of 8-2 mm, is subjected to hardening at T _pp + (20-50 ° C) for 0.1-0.5 hours, followed by cooling, cold rolling, respectively, to a thickness a sheet of 6-1 mm in a single-phase β-state in two or more stages in several passes with a degree of deformation in one pass of 1-6% and a total degree of deformation at each stage of 30-50%, with intermediate hardening between the stages, identical rolling hardening before cold rolling.

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