A preparation method of Ti35 titanium alloy strip

Abstract

The present invention discloses a method for preparing a Ti35 titanium alloy strip, which method comprises the following steps: Step 1, heating a Ti35 titanium alloy slab to a temperature below the phase transition point, performing multi-pass hot rolling and annealing, and then air cooling to obtain a hot-rolled slab; Step 2, after shot blasting, pickling and surface grinding of the hot-rolled slab, performing first cold rolling and vacuum annealing to obtain a cold-rolled blank; Step 3, performing second cold rolling on the cold-rolled blank to obtain a titanium alloy strip coil; Step 4, performing recrystallization annealing on the titanium alloy strip coil to obtain a high-plasticity titanium alloy strip. The method of the present invention can ensure the synchronism and uniformity of the metal flow on the surface and inside of the titanium alloy material by adopting a preparation process of multi-pass hot rolling + multi-pass cold rolling + recrystallization annealing, thereby making the grain distribution and size of the Ti35 titanium alloy strip uniform, improving both strength and plasticity simultaneously, meeting the preparation and use requirements of gaskets, and being suitable for the technical field of titanium alloy material processing.

Description

Technical Field

[0001] The present invention belongs to the technical field of titanium alloy material processing, and in particular relates to a method for preparing Ti35 titanium alloy strip. Background Art

[0002] Due to its excellent resilience, the spiral wound gasket has the self-adjusting ability for the thermal cycle and vibration of pressure vessel equipment and pipelines, and is suitable for occasions with uneven loads and the joint force changing periodically with temperature and pressure; the spiral wound gasket is a static seal for the flange connections of valves, pumps, heat exchangers, towers, manholes, handholes, etc., and is widely used in the fields of petrochemical, machinery, electric power, metallurgy, shipbuilding, medicine, atomic energy and aerospace. The Ti-Ta alloy with 6% Ta element added is a new type of material for nuclear spent fuel reprocessing equipment, which has the characteristics of corrosion resistance to nitric acid, low density, high specific strength and excellent process performance, and can be used to prepare spiral wound gaskets.

[0003] However, the presence of Ta element in Ti35 titanium alloy makes the alloy structure and properties very sensitive to process factors. The implementation of the actual process, the control of the processing process and the heat treatment of the product have a great influence on the performance and quality of the final product.

[0004] Therefore, there is an urgent need to develop a Ti35 alloy strip that can meet the strength and plasticity indexes of spiral wound gaskets for spent fuel reprocessing. Summary of the Invention

[0005] The purpose of the present invention is to overcome the deficiencies in the above-mentioned prior art and provide a method for preparing Ti35 titanium alloy strip. By adopting the preparation process of multi-pass hot rolling + multi-pass cold rolling + recrystallization annealing, this preparation method can ensure the synchronism and uniformity of the metal flow on the surface and inside of the titanium alloy material, and then ensure the uniformity of the grain distribution and size of the Ti35 titanium alloy strip, improve both strength and plasticity while taking them into account, meet the preparation and use requirements of spiral wound gaskets, and solve the problem that the Ti35 alloy strip in the prior art cannot meet the strength and plasticity indexes of spiral wound gaskets.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is: a method for preparing Ti35 titanium alloy strip, characterized in that the method includes the following steps:

[0007] Step 1: Heat the Ti35 titanium alloy slab to below the phase transformation point for multi-pass hot rolling, and then air cool after annealing to obtain a hot-rolled slab;

[0008] Step 2: Shot blast, pickling and surface grinding are carried out on the hot-rolled slab obtained in Step 1, and then the first cold rolling and vacuum annealing are carried out in sequence to obtain a cold-rolled blank;

[0009] Step 3: Carry out the second cold rolling on the cold-rolled blank obtained in Step 2 to obtain a titanium alloy strip coil;

[0010] Step 4: Subject the titanium alloy strip coil obtained in Step 3 to recrystallization annealing to obtain a high-strength and high-ductility titanium alloy strip.

[0011] In the present invention, shot peening, pickling, and surface grinding are carried out to fully remove the oxide film and defects on the surface of the hot-rolled slab, reducing stress concentration.

[0012] The above-mentioned method for preparing a Ti35 titanium alloy strip is characterized in that the mass percentage content of Ta element in the Ti35 titanium alloy slab in Step 1 is 5.5% - 6.5%.

[0013] The above-mentioned method for preparing a Ti35 titanium alloy strip is characterized in that the multi-pass hot-rolling regime in Step 1 is: heating to 10°C - 70°C below the phase transition point, holding for a period of time, and then performing rolling; the phase transition point is the temperature point at which α-phase transforms to β-phase, in °C; the holding time t1 is calculated by the following formula:

[0014] ;

[0015] where t1 is the holding time, in min; a has a value of 1, in min / mm; H is the thickness of the Ti35 titanium alloy slab in Step 1, in mm.

[0016] In the present invention, by controlling the temperature and the holding time according to the thickness of the hot-rolled slab, the slab can be fully heated, improving the workability of the slab and reducing the rolling resistance.

[0017] The above-mentioned method for preparing a Ti35 titanium alloy strip is characterized in that the annealing temperature in Step 1 is 680°C - 720°C.

[0018] In the present invention, by controlling the annealing temperature at 680°C - 720°C, the work hardening generated during the rolling process of the titanium alloy can be eliminated, reducing the rolling resistance and facilitating subsequent rolling.

[0019] The above-mentioned method for preparing a Ti35 titanium alloy strip is characterized in that the first cold rolling in Step 2 is carried out using a reversible cold rolling mill, and the regime of the first cold rolling is: performing cold rolling with a cumulative deformation amount not greater than 60% for multiple times, and heating to 680°C - 720°C for intermediate vacuum annealing between two cold rolling treatments; the deformation amount of each pass of the first cold rolling is 2% - 8%.

[0020] The above-mentioned method for preparing a Ti35 titanium alloy strip is characterized in that the temperature of the vacuum annealing in Step 2 is 680°C - 720°C, and pickling and degreasing are carried out after the vacuum annealing.

[0021] The above-mentioned method for preparing a Ti35 titanium alloy strip is characterized in that in step three, the second cold rolling is carried out by a twenty-high cold rolling mill, and the deformation per pass is 10% - 20%.

[0022] The present invention uses a reversible cold rolling mill to perform the first cold rolling on a hot-rolled slab. This reversible cold rolling mill can improve the rolling efficiency and reach the working range of a twenty-high cold rolling mill after fewer passes of rolling, facilitating subsequent rolling by a twenty-high rolling mill. At this time, the hot-rolled slab is relatively thick. By controlling the deformation amount to a relatively small 2% - 8%, the rolling resistance can be reduced, and the strength and toughness of the hot-rolled slab can be made more uniform, stable, and the internal tissue defects can be reduced. Then, the second cold rolling is carried out by a twenty-high cold rolling mill to obtain a high-precision and high-quality titanium alloy strip coil. At this time, the thickness of the slab after the first cold rolling is reduced, which can meet the rolling requirements of the twenty-high cold rolling mill. Moreover, after increasing the deformation per pass to 10% - 20%, the actual deformation resistance is still relatively small, which can effectively shorten the rolling cycle, thereby reducing the service life of the twenty-high cold rolling mill and reducing equipment wear.

[0023] The above-mentioned method for preparing a Ti35 titanium alloy strip is characterized in that during the second cold rolling in step three, intermediate vacuum annealing is carried out. The temperature of the intermediate vacuum annealing is 680°C - 720°C, and the time t2 of the intermediate vacuum annealing is calculated by the following formula:

[0024] ;

[0025] where t2 is the time of the intermediate vacuum annealing, in minutes; the value of b is 2, in minutes / mm; R is the radius of the titanium alloy strip coil in step three, in mm.

[0026] The above-mentioned method for preparing a Ti35 titanium alloy strip is characterized in that degreasing and pre-shearing are carried out before the recrystallization annealing in step four. The system of the recrystallization annealing is as follows: evacuate to a vacuum degree not greater than 5.0×10 -2 Pa and then heat up to 550°C - 610°C for heat preservation and carry out recrystallization annealing; the heat preservation time t3 is calculated by the following formula:

[0027] ;

[0028] where t3 is the heat preservation time, in minutes; the value of c is 2, in minutes / mm; R is the radius of the titanium alloy strip coil in step three, in mm.

[0029] By adopting the above multi-annealing system, the present invention can refine the microstructure of the Ti35 titanium alloy strip, form equiaxed α grains with a grain size of 3.04 μm to 5.53 μm, and effectively increase the elongation after fracture to reach 30%; the pre-shearing refers to removing the irregular edges generated during the rolling process to reach the finished product target size.

[0030] The present invention controls the vacuum annealing time and the recrystallization annealing holding time according to the thickness of the titanium alloy strip coil, avoiding the problem that the titanium alloy strip coil is not thermally penetrated due to too short holding time, which affects the performance of the titanium alloy strip coil and subsequent rolling, and can also avoid the problems of overburning, thickening of the oxide layer and increased loss of the titanium alloy strip coil due to too long time; at the same time, the holding time of the heat treatment can be calculated according to the formula, improving the versatility of the preparation method, and the holding time can be directly obtained by measuring the material size, avoiding the need for multiple tests for materials with different thicknesses and facilitating large-scale promotion.

[0031] In the above-mentioned preparation method of a Ti35 titanium alloy strip, it is characterized in that in step four, the tensile strength Rm of the high-strength and high-plasticity titanium alloy strip is not less than 350 MPa, the yield strength is not less than 250 MPa, the elongation after fracture A is not less than 28%, and the thickness of the high-strength and high-plasticity titanium alloy strip is 0.1 mm to 0.3 mm.

[0032] The present invention has the following advantages compared with the prior art:

[0033] 1. By adopting the preparation process of multi-pass hot rolling + multi-pass cold rolling + recrystallization annealing, the present invention can ensure the synchronism and uniformity of the metal flow on the surface and inside of the titanium alloy material, and then improve the uniformity of the grain distribution and size of the Ti35 titanium alloy strip, improve both strength and plasticity at the same time to meet the preparation and use requirements of the spiral wound gasket, and then extend the service life of the spiral wound gasket and greatly reduce the operation and maintenance cost of the equipment; the elongation after fracture and strength of the Ti35 titanium alloy strip prepared by this method can meet the preparation requirements of the bellows, expanding its application working conditions; compared with the single hot rolling or cold rolling processing method, the present invention adopts the combination of multi-pass hot rolling and two-pass cold rolling processes, which can greatly shorten the production process of the strip coil and obtain a Ti35 titanium alloy strip with high surface quality and dimensional accuracy.

[0034] 2. The present invention strictly designs and controls the processing parameters and annealing system of hot rolling and cold rolling of the Ti35 alloy slab according to the use working conditions, so as to obtain a strip coil for Ti35 alloy spiral wound gaskets with good high-strength and high-plasticity matching.

[0035] 3. The present invention uses a Ti35 titanium alloy slab with a Ta element mass percentage of 5.5% - 6.5% as the raw material. The Ta element and the Ti element can form an infinite solid solution, greatly enhancing the corrosion resistance of the titanium alloy in an oxidizing medium. Compared with traditional pure titanium materials, it can effectively improve the corrosion resistance to concentrated nitric acid.

[0036] 4. The Ti35 titanium alloy strip with a thickness of 0.1 mm - 0.3 mm prepared by the present invention has the characteristics of uniform structure, refined recrystallized grains, good strength-plasticity matching, and excellent corrosion resistance.

[0037] The technical solution of the present invention will be further described in detail below through the accompanying drawings and embodiments. Description of the Drawings

[0038] Figure 1 It is a flowchart of the preparation method of the present invention.

[0039] Figure 2 It is a corrosion rate diagram of the cold-rolled sheet blank with a thickness of 2 mm in step two of Example 1 of the present invention.

[0040] Figure 3 It is a metallographic structure diagram of the high-strength and high-plasticity titanium alloy strip obtained in Example 1 of the present invention.

[0041] Figure 4 It is a metallographic structure diagram of the high-strength and high-plasticity titanium alloy strip obtained in Example 2 of the present invention.

[0042] Figure 5 It is a metallographic structure diagram of the high-strength and high-plasticity titanium alloy strip obtained in Example 3 of the present invention. Detailed Embodiments

[0043] Example 1

[0044] As Figure 1 shown, the method of this embodiment includes the following steps:

[0045] Step 1. Heat a Ti35 titanium alloy slab with a thickness of 204 mm to 860 °C and hold for 220 min, then perform three-pass hot rolling, and then heat up to 700 °C for annealing, and obtain a hot-rolled slab with a thickness of 4 mm after air cooling; the mass percentage of the Ta element in the Ti35 titanium alloy slab is 6.2%; the hot rolling regime for the three passes is as follows: after the first pass of hot rolling, the temperature of the Ti35 titanium alloy slab is not lower than 700 °C, and the total deformation is 55% - 90%; after the second and third passes of hot rolling, the temperature of the Ti35 titanium alloy slab is not lower than 400 °C, and the total deformation is 20% - 85% for both. And before the second pass of hot rolling, the Ti35 titanium alloy slab is reversed, and for larger-sized slabs, material separation is performed after the first pass of hot rolling;

[0046] Step 2: Shot blast, pickling, and surface grinding are performed on the hot-rolled slab obtained in Step 1. Then, the first cold rolling is carried out using a six-high reversing cold rolling mill to obtain a cold-rolled sheet blank with a thickness of 2 mm. It is heated to 700 °C for vacuum annealing, followed by pickling and degreasing, and surface quality inspection. If there are surface defects, grinding continues. If the grinding amount on the sheet surface is large, pickling continues. Subsequently, a cold-rolled blank is obtained. The grinding is carried out along the rolling direction, and the deformation amount per pass of the first cold rolling is 2% - 8%.

[0047] Step 3: The cold-rolled blank obtained in Step 2 is subjected to the second cold rolling using a twenty-high cold rolling mill to obtain a titanium alloy strip coil with a thickness of 0.2 mm and a radius of 45 mm. The deformation amount per pass of the second cold rolling is 10% - 20%.

[0048] Step 4: The titanium alloy strip coil obtained in Step 3 is degreased and pre-sheared, evacuated to a vacuum degree not greater than 5.0×10 -2 Pa, then heated to 550 °C and held for 90 min for recrystallization annealing, and then subjected to a flattening straightening treatment to obtain a high-strength and ductile titanium alloy strip with a thickness of 0.2 mm.

[0049] The corrosion rate of the cold-rolled sheet blank with a thickness of 2 mm obtained in Step 2 of this example is tested. The results are as Figure 2 shown. The measured corrosion rates of the cold-rolled sheet blanks are all not higher than 0.03 mm / a, and the corrosion rate decreases with the increase of the corrosion time. The corrosion rate test includes the following 4 corrosion conditions:

[0050] (1) The temperature is 108 °C, and the corrosion medium is a simulated dissolution solution (configured according to the actual service environment of the spiral wound gasket). The concentration of HNO3 in this simulated dissolution solution is 6 mol / L, the concentration of V element is 1.70 g / L, the concentration of Ce element is 2.09 g / L, the concentration of Ru element is 2.24 g / L, and the concentration of Cr element is 0.125 g / L; (2) The temperature is 108 °C, and the corrosion medium is a 6 mol / L nitric acid solution; (3) The temperature is 111 °C, and the corrosion medium is a 7.5 mol / L nitric acid solution; (4) The temperature is 112 °C, and the corrosion medium is an 8 mol / L nitric acid solution.

[0051] The microstructure of the high-strength and ductile titanium alloy strip obtained in this example is analyzed. As Figure 3 shown, the grains of the high-strength and ductile titanium alloy strip are all α equiaxed grains, and the particle size range is 10 μm - 25 μm. After testing, the tensile strength of the high-strength and ductile titanium alloy strip is 438 MPa, the yield strength is 366 MPa, the elongation after fracture A is 31.0%, and the Vickers hardness HV is 182.

[0052] Example 2

[0053] The difference between this embodiment and Embodiment 1 lies in that: in Step 2, the system of the first cold rolling is: two cold rolling treatments with a cumulative deformation amount not greater than 60% are carried out, and an intermediate vacuum annealing is carried out between the two cold rolling treatments. The temperature of this intermediate vacuum annealing is 700°C, and the holding time of the intermediate vacuum annealing is 15 min; the temperature of the recrystallization annealing in Step 4 is 610°C.

[0054] In this embodiment, by adding an intermediate vacuum annealing during the first cold rolling in Step 2, the internal stress can be reduced, enabling the grains to recover.

[0055] Microstructural analysis was performed on the high-strength and high-ductility titanium alloy strip obtained in this embodiment, as Figure 4 shown. The grains of the high-strength and high-ductility titanium alloy strip are all α equiaxed grains, and the particle size range is 10 μm to 25 μm; after testing, the tensile strength of the high-strength and high-ductility titanium alloy strip is 420 MPa, the yield strength is 349 MPa, the elongation after fracture A is 32%, and the Vickers hardness HV is 180.5.

[0056] Embodiment 3

[0057] The difference between this embodiment and Embodiment 1 lies in that: the annealing temperature in Step 1 and the vacuum annealing temperature in Step 2 are both 680°C; in the second cold rolling process in Step 3, an intermediate vacuum annealing at a temperature of 680°C and a holding time of 90 min is carried out. Four passes of rolling are carried out for each heat, the cumulative deformation amount for each heat is not higher than 55%, and the deformation amount for each pass is 10% to 20%; the temperature of the recrystallization annealing in Step 4 is 600°C.

[0058] Microstructural analysis was performed on the high-strength and high-ductility titanium alloy strip obtained in this embodiment, as Figure 5 shown. The grains of the high-strength and high-ductility titanium alloy strip are all α equiaxed grains, and the particle size range is 10 μm to 25 μm; after testing, the tensile strength of the high-strength and high-ductility titanium alloy strip is 436 MPa, the yield strength is 364 MPa, the elongation after fracture A is 31.5%, and the Vickers hardness HV is 183.

[0059] Embodiment 4

[0060] The difference between this embodiment and Embodiment 3 lies in that: in Step 1, a Ti35 titanium alloy slab with a thickness of 204 mm is heated to 880°C and held for 254 min, and then hot rolled in three heats. The mass percentage content of Ta element in the Ti35 titanium alloy slab is 5.5%; the annealing temperature in Step 1, the vacuum annealing temperature in Step 2, and the intermediate vacuum annealing temperature in Step 3 are all 720°C.

[0061] After testing, the thickness of the high-strength and high-ductility titanium alloy strip is 0.15 mm, the tensile strength is 416 MPa, the yield strength is 338 MPa, the elongation after fracture A is 33.5%, and the Vickers hardness HV is 176.

[0062] Example 5

[0063] The difference between this example and Example 4 is that in Step 1, a Ti35 titanium alloy slab with a thickness of 204 mm is heated to 820 °C and held for 214 min, and then hot-rolled in three passes. The mass percentage content of Ta element in the Ti35 titanium alloy slab is 6.5%.

[0064] After testing, the thickness of the high-strength and high-ductility titanium alloy strip is 0.3 mm, the tensile strength is 420 MPa, the yield strength is 342 MPa, the elongation after fracture A is 30.5%, and the Vickers hardness HV is 193.

[0065] The above are only the preferred embodiments of the present invention, and do not impose any limitations on the present invention. Any simple modifications, changes, and equivalent structural transformations made to the above embodiments according to the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing a Ti35 titanium alloy strip, characterized in that, The method comprises the following steps: Step 1: Heating a Ti35 titanium alloy slab to a temperature below the phase transformation point, performing multi-pass hot rolling, air cooling after annealing, to obtain a hot-rolled slab; Step 2: Shot peening, pickling, and surface grinding the hot-rolled slab obtained in Step 1, sequentially performing first cold rolling and vacuum annealing, to obtain a cold-rolled blank; the first cold rolling is carried out using a reversible cold rolling mill, and the regime of the first cold rolling is: performing cold rolling with a cumulative deformation amount not greater than 60% for multiple times, and heating to 680°C - 720°C for intermediate vacuum annealing treatment between two cold rolling treatments; the deformation amount of each pass of the first cold rolling is 2% - 8%; Step 3: Performing second cold rolling on the cold-rolled blank obtained in Step 2, to obtain a titanium alloy strip coil; the second cold rolling is carried out using a twenty-high cold rolling mill, and the deformation amount of each pass is 10% - 20%; Step 4. Perform recrystallization annealing on the titanium alloy strip coil obtained in Step 3 to obtain a high-strength and high-formability titanium alloy strip; degreasing and pre-shearing are carried out before the recrystallization annealing, and the recrystallization annealing regime is as follows: evacuate to a vacuum degree not greater than 5.0×10 -2 Pa, then heat up to 550°C - 610°C and hold for recrystallization annealing; the holding time t3 is calculated by the following formula: ； Wherein, t3 is the holding time, with the unit of min; the value of c is 2, with the unit of min / mm; R is the radius of the titanium alloy strip coil described in Step 3, with the unit of mm.

2. The preparation method of a Ti35 titanium alloy strip according to claim 1, characterized in that, The mass percentage content of Ta element in the Ti35 titanium alloy slab described in Step 1 is 5.5% - 6.5%.

3. A method for preparing a Ti35 titanium alloy strip according to claim 1, characterized in that, The regime of the multi-pass hot rolling described in Step 1 is: heating to a temperature 10°C - 70°C below the phase transformation point, holding for rolling; the phase transformation point is the temperature point of α-phase to β-phase transformation, with the unit of °C; the holding time t1 is calculated by the following formula: ； Wherein, t1 is the holding time, with the unit of min; the value of a is 1, with the unit of min / mm; H is the thickness of the Ti35 titanium alloy slab described in Step 1, with the unit of mm.

4. A method for preparing a Ti35 titanium alloy strip according to claim 1, characterized in that, The annealing temperature described in Step 1 is 680°C - 720°C.

5. A method for preparing a Ti35 titanium alloy strip according to claim 1, characterized in that, The vacuum annealing temperature described in Step 2 is 680°C - 720°C, and pickling and degreasing are carried out after the vacuum annealing.

6. A method for preparing a Ti35 titanium alloy strip according to claim 1, characterized in that, Intermediate vacuum annealing is carried out during the second cold rolling described in Step 3, the temperature of the intermediate vacuum annealing is 680°C - 720°C, and the time t2 of the intermediate vacuum annealing is calculated by the following formula: ； Wherein, t2 is the time of the intermediate vacuum annealing, with the unit of min; the value of b is 2, with the unit of min / mm; R is the radius of the titanium alloy strip coil described in Step 3, with the unit of mm.

7. A method for preparing a Ti35 titanium alloy strip according to claim 1, characterized in that, The tensile strength Rm of the high-strength and high-ductility titanium alloy strip described in Step 4 is not less than 350 MPa, the yield strength is not less than 250 MPa, the elongation after fracture A is not less than 28%, and the thickness of the high-strength and high-ductility titanium alloy strip is 0.1 mm - 0.3 mm.

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