RU2492275C1 - Method of producing plates from two-phase titanium alloys - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: proposed method comprises hot forming of slab, hot rolling and teat treatment of plate, whereat hot forming if carried out in one step. Immediately after reaching required thickness in slab forming it is quickly cooled to the depth of 20-30 mm at the rate of at least 50°C/min. Subsequent hot lengthwise rolling at performed at first step in α+β-area by partial reduction with deformation degree ε_i varying from 3% to 5% to total deformation ε=25…30% with breaks between passes of 8 to 12 s. At second step, it is performed in β-area from heating temperature determined by definite formula. At the next step rolling is performed in α+β-are with breaks and heating in lengthwise or transverse directions with total degree of deformation e after every break to 60%.

EFFECT: homogeneous fine-grain microstructure, high and stable mechanical properties, high precision, no surface defects.

Description

The invention relates to the processing of metals by pressure, in particular to thermomechanical processing of two-phase titanium alloys in the process of producing thick sheets and plates

It is known that the structure and its homogeneity have a decisive influence on the level and stability of the mechanical properties of thick sheets and plates (hereinafter plates). Therefore, when developing the technological process of plate manufacturing, it is critical to select the conditions for the formation of the structure during hot deformation of pseudo α-, (α + β) -titanium alloys.

A well-known standard scheme of the technology for the production of hot-rolled plates, including heating the slab, hot rolling, cutting to length, annealing and finishing operations (Titanium alloys. Semi-finished products from titanium alloys. Responsible editors: NF Anoshkin, MZ Yermanok. M ., ONTI WILS, 1996, p.207-210).

A known method of manufacturing plates of titanium alloys (RF Patent No. 2169791, IPC C22F 1/18), comprising heating the slab to a rolling temperature, preliminary rolling in two stages, heating the roll to a rolling temperature and final rolling, while rolling is carried out with regulated temperature and deformation modes

The use of a patent (No. 2169791, IPC C22F 1/18) in the manufacture of plates from α + β-alloys of the VT23, VT22 type with a higher β-phase coefficient (so-called “critical” alloys) than that of the VT6 alloy, yields heterogeneous structure, impact on properties, increased rejection of metal.

In these schemes for manufacturing hot-rolled plates, the temporal factors of the technological process and the influence of the chemical composition of titanium alloys are not regulated, which leads to heterogeneity of the structure, instability and anisotropy of mechanical properties.

The prototype selected a method of manufacturing plates of two-phase titanium alloys (RF Patent No. 2378410, IPC C22F 1/18), which includes hot deformation of the ingot into a slab in three stages, hot rolling of plates in several stages with intermediate cooling between stages to room temperature and subsequent heat treatment of the plates . The duration of the process of deformation of the ingot into a slab is due to the need to give the metal the necessary structural and plastic properties for subsequent rolling already at this stage. The resulting plates are characterized by a homogeneous fine-grained macrostructure, an increased level and stability of mechanical properties.

The disadvantages of the prototype is that the process of deformation of the ingot into a slab is carried out in three stages, which is associated with significant time and material costs for heating, basic and auxiliary technological operations, as well as large losses of metal associated with fumes and surface defects.

Attempts to reduce the complexity of the deformation of the ingot, namely, to limit the pressing of the slab in one step, while maintaining the quality of the plates, did not give positive results due to the heterogeneous structure of the alloy over the cross section, since in the future this leads to the fact that:

- when the ingot is deformed in one step at the boundary of large β-grains that are stored in the surface layers, the rim of the α-phase is intensively developed, which can exceed the critical dimensions and cause intensive crack development during subsequent rolling;

- in case of insufficient study of the alloy structure (lack of ductility) during the first rolling operation, an unregulated rolling rhythm leads to unacceptable cooling of the surface of the slab in contact with cold rolls (due to the low thermal conductivity of titanium alloys, the temperature of the surface layers does not have time to recover due to the heat from the central regions) and there is a deficit in their ductility.

Both factors reduce the plasticity resource and lead to the appearance of deep surface cracks, which can be classified as correctable, and in unfavorable circumstances, as well as incorrigible marriage. Fixable plates require additional machining of the surface of the plates until cracks are removed. This operation is laborious, leading to large losses of metal, as well as additional time and material costs.

The objective of the invention is to increase the competitiveness of thick sheets and plates by reducing the cost while maintaining a high level and stability of mechanical properties, as well as surface quality.

The technical result in the implementation of this invention is to reduce the complexity of the process, increase the yield, prevent the appearance of surface cracks in the process of thermal deformation processing of plates, with guaranteed ensuring a uniform macro - and microstructure.

This technical result is ensured by the fact that in the method of manufacturing plates from two-phase titanium alloys, including hot deformation of the ingot into a slab, hot rolling and subsequent heat treatment of the plates, hot deformation of the ingot into a slab is carried out in one step and immediately after reaching the final thickness in the deformation process slab, it is quickly cooled to a depth from the surface of 20 to 30 mm at a speed of at least 50 ° C / min, the subsequent hot longitudinal rolling is carried out at the first stage in the α + β-region from the temperature of T eva = _H (T _PP -20 ... 40) ° C small partial crimping with a degree of deformation ε _i from 3% to 5%, up to a total degree of deformation ε = 25 ... 30%, with pauses between passes lasting from 8 to 12 s, and the second stage of longitudinal rolling is carried out in the β-region of the heating temperature, which is determined by the formula:

$$T_N\ge {(T_{PP}+(\mathrm{one}0\times M{o}_{E\mathrm{TO}}+80))}^{\circ }C,(\mathrm{one})$$

where T _N - the temperature of the slab, ° C;

T _PP - the temperature of the polymorphic transformation, ° C;

Mo _EC is the molybdenum equivalent, which is calculated by the formula:

$$M{o}_{E\mathrm{TO}}=\left[Mo\right]+\left[V\right]/\mathrm{one},5+\left[Cr\right]*\mathrm{one},25+\left[Fe\right]*2,5+\left[Ni\right]/0,8,m\mathrm{but}\mathrm{from}\mathrm{from}.\%,(2)$$

subsequent rolling stages are performed in the α + β region with interruptions and heating in the longitudinal or transverse directions with a total degree of deformation after each interruption ε of up to 60%.

In the process of cooling the slabs deformed at one stage at the β-grain boundary, the rim of the α-phase develops, which reduces the plasticity resource of the metal and leads to the formation of surface cracks during subsequent rolling in the α + β-region. This becomes a critical factor for the proposed invention, since the ingot is deformed into a slab in one step and the structure of the slab still largely inherits the insufficiently plastic cast structure of the ingot, this is especially noticeable in the surface layers. To stop this process, namely the formation of an acceptable thickness of the rim of the α-phase at the grain boundary, it is possible while ensuring the necessary cooling rate of the ingot after the final stage of the deformation of the ingot into a slab, while the cooling rate should be at least 50 ° C / min and extend to a depth of 20 to 30 mm. As practice has shown, in many cases it is quite sufficient to maintain contact between the surfaces of the slab and the tool after deformation or to cool the slab in water, while the contact time and the location of the slab in water are determined empirically.

At the first stage of rolling, due to the fact that the structure of the slab deformed in one stage is not yet sufficiently developed and its plasticity resource is limited, therefore, an unregulated rhythm of rolling leads to a critical cooling of the surface of the slab in contact with cold rolls. This is due to the fact that in order to increase the productivity of the rolling process, rolling mill operators traditionally focus on reducing the duration of pauses between passes to 3-5 s. Due to the low thermal conductivity of titanium alloys, the temperature of the surface layers does not have time to recover due to the arrival of heat from the central regions , which leads to the appearance on the cool surface of the slab of cracks originating at the triple junctions of β-grains. The interval between passes from 8 to 12 s lasts allows you to align the temperature field along the slab cross section to an acceptable level that does not provoke the formation of surface cracks.

When choosing the temperature of the first α + β-rolling, it was taken into account that a sufficient degree of “semi-hot hardening” is only communicated to the metal if there is an appropriate stability of the interphase boundaries. In order to prevent the development of dynamic and spontaneous recrystallization processes in the α + β-region at the first stage of rolling, rolling was carried out by small partial reductions ε _i = 3 ... 5% to a total degree of deformation ε from 20% to 30% at a temperature T _{PP of} -20 ... 40 ° C, while the metal is given a sufficient degree of "semi-hot hardening", which provides a higher energy level in the metal to provide the effect of recrystallization of the β-grain structure in the subsequent second rolling stage in the β-region. The heating temperature of the subsequent second stage of rolling in the β-region is determined by formula (1), which is obtained on the basis of experimental data.

The heating temperature of the slab in the second stage of rolling, which provides the desired effect of β-phase recrystallization, depends on the chemical composition of the alloy, namely, on its molybdenum equivalent Mo _EC , formula (2).

To ensure a given level of mechanical properties, the subsequent stages of rolling are performed in the α + β region with interruptions and heating in longitudinal or transverse directions with a total degree of deformation after each interruption of 8 to 60%. Metal at this stage and attached modes has a margin of plasticity, which does not require strict regulation of the rolling rhythm.

In the following specific examples, the heating temperature before the second rolling was determined in accordance with formulas 1 and 2.

Example 1

The proposed method was tested in the manufacture of plates with dimensions of 16 × 900 × 2000 mm from a two-phase titanium alloy Ti-6Al4V. The temperature of the polymorphic transformation of the alloy TPP = 980 ° C. The slab was made from an ingot with a diameter of 740 mm. The ingot was heated to a temperature of 1180 ° C (210 ° C higher than the TPP), pressed in a closed die to a size of 276 × 1080 × 1600 mm, and the resulting slab was kept in the die without raising the upper engraving for 3 minutes, which ensured the cooling rate of the surface layer to a depth of 20 mm at least 50 ° C / min. Further cooling of the slab was carried out separately on adjuzhage without contact with other hot slabs. A slab machined to a size of 266 × 1080 × 1700 mm was heated to a temperature of 940 ° C (40 ° C lower than the TPP) and rolled on a quarto-2000 mill with small reductions ε _i = 3-4% with an interval between passes of 8 to 12 s to the total degree of deformation ε = 25%, followed by cooling to the temperature of the workshop. On visual inspection, surface cracks were not found. Next, the roll was heated to a temperature of 1090 ° C, rolled with a degree of deformation ε = 55% and cooled to a workshop temperature. Then the roll was heated to a temperature of 940 ° C (40 ° C lower than the TPP) and rolling was carried out with a degree of deformation ε = 50%. After cooling, the strip was cut into edges and the final rolling with a degree of deformation ε = 50% was carried out in the transverse direction.

The resulting plates were subjected to heat treatment processing, as well as subsequent testing of mechanical properties and structural control.

The test results met the requirements of domestic TU-1-805-391-79 and international standards A3304, AMS4911H.

Example 2

The proposed method was tested in the manufacture of plates with dimensions of 50 × 1000 × 2000 mm from a two-phase titanium alloy VT 23. The temperature of the polymorphic transformation of the alloy Tpp = 890 ° C. The slab was made from an ingot with a diameter of 740 mm and a mass of 3200 kg. The ingot was heated to a temperature of 1150 ° C (260 ° C higher than the TPP), forged in one step to a size of 300 × 1100 × 1700 mm, and the resulting slab was cooled in a water tank for 3 minutes to ensure the cooling rate of the surface layer to a depth 20 mm at least 50 ° C / min. Further cooling of the slab was carried out separately on adjuzh without contact with other hot slabs. A slab machined to a size of 280 × 1080 × 1730 mm was heated to a temperature of 860 ° C and rolled in a quarto2000 mill with small reductions ε _i = 3-4% with an interval between passes of 8 to 12 s to a total degree of deformation of 8 = 25% followed by cooling to workshop temperature. On visual inspection, surface cracks were not found. Next, the roll was heated to a temperature of 1050 ° C, rolled with a degree of deformation ε = 65% and cooled to the temperature of the workshop. Then the roll was heated to a temperature of 860 ° C (30 ° C below T _PP ) and the final rolling was carried out with a degree of deformation ε = 50%.

The resulting plates were subjected to heat treatment processing, as well as subsequent testing of mechanical properties and structural control.

The test results met the requirements of domestic TY1-805-103-8L

The mechanical properties are given in table 1.

Table 1 Mechanical properties The method of manufacturing
plate The condition of the test samples temporary resistance □, kg / mm ² elongation δ,% relative narrowing Ψ,% impact strength KCU, MJ / m ² Aged 132 9.6 28.0 0.35 Proposed Hardened and aged 130.5 10.6 31,0 0.33 Aged 124.3 7.8 13.9 0.34 Famous Hardened and aged 128.4 8.4 18,4 0.31 TU1-805-103-81 Heat strengthened 110-130 8.0 11.0 0.30

Example 3

The proposed method was used in the manufacture of plates with dimensions of 45 × 1000 × 2000 mm from a two-phase titanium alloy VT 22. The temperature of the polymorphic transformation of the alloy T _PP = 870 ° C. The slab was made from an ingot with a diameter of 740 mm. The ingot was heated to a temperature of 1200 ° C (330 ° C above T _PP ) and forged in one step to 300 × 1100 × 1700 mm and the resulting slab was cooled in a water tank for 3 minutes to provide a cooling rate of the surface layer at a depth of 20 mm is not less than 50 ° C / min. Further cooling of the slab was carried out separately on adjuzh without contact with other hot slabs. A slab machined to a size of 280 × 1080 × 1700 mm was heated to a temperature of 840 ° C (30 ° C below T _PP ) and rolling was performed on a quarto 2000 mill with small reductions ε _i = 3-4% to a total degree of deformation ε = 25% , with intervals between passes of 8-12 s and subsequent cooling to the temperature of the workshop. On visual inspection, surface cracks were not found. Next, the roll was heated to a temperature of 1075 ° C, rolled with a degree of deformation ε = 58% and cooled to the temperature of the workshop. Then the roll was heated to a temperature of 900 ° C (30 ° C below T _PP ) and the final rolling was carried out with a degree of deformation ε = 50%.

The resulting plates were subjected to heat treatment, as well as subsequent tests of mechanical properties and structural control. The test results of the mechanical properties are given in table.2.

The macrostructure of the plates is 5-6 points, sections of 7 points allowed in TU 1-92-31-74 and I1054-76 for serial plates are absent.

The microstructure of plates 1-3 types, sections 4 types allowed in TU 1-92-31-74 and I1054-76 for serial plates

table 2 Manufacturing Method
plate The condition of the test samples Mechanical properties temporary resistance □, kg / mm ² elongation δ,% relative narrowing Ψ,% impact strength KCU, MJ / m ² Offer
my annealed 120.1 12.8 30.9 0.30 annealed 122.8 12.5 29.1 0.32 Famous annealed 124.3 7.2 12,4 0.34 annealed 118.4 8.0 18,4 0.31 TU1-92-31-74 annealed 110-130 6 16,0 0.25

The resulting plates are characterized by a homogeneous fine-grained macrostructure, an increased level and stability of mechanical properties, as well as high accuracy of geometric dimensions and the absence of surface defects.

Claims (1)

A method of manufacturing plates from two-phase titanium alloys, including hot deformation of the ingot into a slab, hot rolling and subsequent heat treatment of the plates, characterized in that they produce a one-stage hot deformation of the ingot into a slab and, immediately after reaching the final thickness of the slab, carry out rapid cooling to the depth of the slab from the surface from 20 mm to 30 mm with a speed of at least 50 ° C / min, then the subsequent hot longitudinal rolling is carried out, moreover, in the first stage, in the α + β-region of the heating temperature wa T _N = (T _PP -20 ... 40) ° C by partial compressions with the degree of deformation ε _i , from 3% to 5% to the total degree of deformation ε = 25 ... 30% and with pauses between passes from 8 s to 12 s , and in the second stage - in the β-region of the heating temperature, determined by the formula:
T _N ≥ (T _PP + (10 × Mo _EK +80)) ° С,
where T _N - the temperature of the slab, ° C;
T _PP - the temperature of the polymorphic transformation, ° C;
Mo _EC - molybdenum equivalent, calculated by the formula:
Mo _EC = [Mo] + [V] / 1.5 + [Cr] · 1.25 + [Fe] · 2.5 + [Ni] / 0.8, wt.%,
at subsequent stages, rolling is carried out in the α + β region with interruptions and heating in the longitudinal or transverse directions with a total degree of deformation ε after each interruption of up to 60%.