CN115647106A - A kind of production method of small-diameter thick-walled titanium alloy seamless pipe - Google Patents

Abstract

The invention provides a production method of a small-caliber thick-wall titanium alloy seamless pipe, which comprises the following steps: the method comprises the following steps: carrying out VAR smelting for the third time to obtain a titanium alloy ingot; step two: cogging, drawing and radially forging a titanium alloy ingot into a round bar, and processing the round bar into a hollow pipe blank by deep hole drilling, inner honing and outer turning; step three: carrying out three times of cold rolling on the hollow pipe blank to prepare a finished product of the titanium alloy seamless pipe; sampling the titanium alloy pipe after each cold rolling, observing the crack defect of the titanium alloy pipe, removing the crack defect, and performing intermediate vacuum annealing after removing the crack defect; step four: and (4) carrying out vacuum annealing, straightening, acid washing, ultra-probing and packaging on the titanium alloy finished pipe obtained in the step three to obtain the small-caliber thick-wall titanium alloy seamless pipe. The invention comprehensively sets measures from the aspects of specification design of the tube blank, processing technology of the tube blank, cold rolling technology design of the titanium alloy tube, design of the vacuum annealing technology, defect treatment technology of the inner surface and the outer surface of the tube blank and the like, and solves the problem of the inner surface cracks of the cold rolling of the small-caliber thick-wall titanium alloy seamless tube.

Description

A kind of production method of small-diameter thick-walled titanium alloy seamless pipe

technical field

The invention relates to the technical field of preparation of nonferrous metal seamless pipes, in particular to a production method of small-diameter thick-walled titanium alloy seamless pipes.

Background technique

Due to poor plasticity, cold-rolled texture and strong anisotropy of titanium alloy, cold-rolled titanium alloy tubes are prone to poor dimensional accuracy and internal and external surface defects.

For small-diameter thick-walled titanium alloy seamless pipes with an outer diameter of ≤10mm, a wall thickness of ≥1mm, and a ratio of outer diameter to wall thickness of ≤5.5, due to the influence of specifications, cracks will easily appear on the inner surface of the titanium alloy pipe during cold rolling. , causing the product to be scrapped, and the existing preparation method does not have a perfect solution.

Contents of the invention

According to the above technical problems, a method for producing a small-diameter thick-walled titanium alloy seamless pipe is provided. The invention comprehensively formulates measures from the aspects of tube blank specification design, tube blank processing technology, titanium alloy tube cold rolling process design, vacuum annealing process design, tube blank inner and outer surface defect treatment process, etc., to solve the problem of small-diameter thick-walled titanium alloy seamless Cracks on the inner surface of the cold-rolled tube.

The technical means adopted in the present invention are as follows:

A method for producing a small-diameter thick-walled titanium alloy seamless pipe, comprising the steps of:

Step 1: three times of VAR melting to obtain titanium alloy ingot;

Step 2: The titanium alloy ingot is subjected to 5 fires, three piers and three pulls to form a billet, 1 fire to grow a long rod, and 3 fires to forge into a round bar. The radial forging temperature is 840°C to 850°C. The round bar meets the AA level of the GB/T5193 standard for flaw detection. The round bar is made into a hollow tube blank with an outer diameter of D and a wall thickness of S through deep hole drilling, inner honing, and outer machining. The outer diameter and wall thickness ratio ( D/S)≤5.5; the internal honing is made of green silicon carbide oilstone of W40 particle size bonded by resin binder, the working pressure is 1.3Mpa, the honing speed is 18-21 meters per minute, and the roughness after honing of the hollow tube blank is ≤ 0.4 microns.

Step 3: The hollow tube blank is subjected to three passes of cold rolling to produce a titanium alloy seamless tube product with an outer diameter of D1 and a wall thickness of S1; the wall thickness S of the hollow tube blank is equal to that of the finished titanium alloy seamless tube wall thickness S1 3 times;

In the cold rolling process, the deformation rate ε and K value of each pass of cold rolling are smaller than those of the previous pass, and the K value of each pass of cold rolling is ≥ 1; a titanium alloy tube with an outer diameter of Da and a wall thickness of Sa is cold rolled once The formula for calculating the deformation rate ε of this cold rolling pass is as follows:

ε=((Da-Sa)×Sa-(Db-Sb)×Sb)/((Da-Sa)×Sa);

The formula for calculating the K value of this cold rolling pass is: K=(Sa-Sb)×Da/(Da-Db)×Sa.

Before each pass of cold rolling, apply rolling lubricating oil with a fineness of 900 mesh MoS ₂ , paraffin wax and No. 32 anti-wear machine oil in a weight ratio of 1:1:8 to the inner surface of the titanium alloy tube and the surface of the mandrel. , Filter the rolling mill lubricating oil before and during each cold rolling pass, and the filtration accuracy reaches 3 microns.

After each pass of cold rolling, the titanium alloy pipe is sampled, the crack defects are observed, and the crack defects are removed. The specific method is: after each pass of cold rolling of the titanium alloy pipe, a 100mm long pipe section sample is taken, and the wall thickness of the pipe section sample is removed by pickling. After mm, cut open and observe the inner and outer surfaces, confirm the distribution, orientation and maximum defect position of crack defects, take longitudinal and transverse metallographic samples at the maximum defect position, determine the maximum depth of crack defects as h and observe the opening distance of crack defects, And remove the crack defect according to the opening distance and the maximum depth h. After removal, take a 100mm long pipe section sample, use an imager and a stereo microscope to observe and confirm that there are no defects on the inner and outer surfaces, and then perform intermediate vacuum annealing and the next pass on the titanium alloy pipe Cold rolling;

For inner surface defects whose opening distance of crack defect is ≥0.02mm, choose internal honing to remove the inner surface wall thickness h+0.02mm of titanium alloy tube; inner honing uses green silicon carbide oilstone with W40 grain size bonded by resin bond, working pressure 1.3Mpa , the honing speed is 18-21 meters per minute;

For the outer surface defects whose opening distance of the crack defect is ≥0.02mm, select the abrasive belt to remove the outer surface wall thickness h+0.02mm of the titanium alloy pipe;

For inner and outer surface defects whose opening distance of crack defects is less than 0.02mm, the inner and outer surfaces should be pickled by flowing pickling to remove the titanium alloy pipe wall thickness h+0.02mm. Plastic bags are used to cover the outer surface of the titanium alloy pipe for flow pickling on the inner surface alone, and plastic is used to block both ends of the titanium alloy pipe for flow pickling on the outer surface alone; the pickling solution uses HF acid: HNO ₃ acid: water at 4:15 : It is made by mixing 81% by weight, and flows through the inner surface and/or outer surface of the titanium alloy tube at a speed greater than or equal to 3 m/min.

After each pass of cold rolling, intermediate vacuum annealing is carried out after removing crack defects. The intermediate vacuum annealing process for titanium alloy tubes with an outer diameter of Db and a wall thickness of Sb cold-rolled with a deformation rate of ε is: heating rate 2°C/ minutes, the intermediate annealing temperature is (600+300×ε)°C, the holding time is 55×Sb minutes, and the elongation rate of the titanium alloy tube after annealing is ≥30%.

Step 4: Vacuum anneal, straighten, pickle, super-explore, and package the titanium alloy finished tube in step 3 to make a small-diameter thickness with an outer diameter D2≤10mm, a wall thickness S2≧1mm, and an outer diameter wall thickness ratio≤5.5 Wall titanium alloy seamless pipe. The vacuum annealing in step four is adjusted according to the performance requirements of the preset product.

Compared with the prior art, the present invention has the following advantages:

The three times of VAR smelting in the present invention ensure the uniform composition of the titanium alloy; the thermal deformation process combination of 5 fires, 3 piers, 3 pullouts, 1 fire drawing length, and 840°C-850°C 3 fire diameter forging ensures the titanium alloy round bar Uniform structure and performance, good plasticity, elongation ≥ 28%, round bar flaw detection can reach GB/T5193 standard AA level, reducing the probability of defects in titanium alloy tube cold rolling; adopt deep hole drilling, internal honing, external lathe The hollow tube blank is prepared by machining, which completely avoids the inner and outer surface defects of the tube blank; the optimized internal honing process ensures that the inner surface roughness of the hollow tube blank is less than or equal to 0.4 microns, reducing the probability of defects on the inner surface of the cold-rolled titanium alloy tube; at the same time, machining The tube billet can be flexibly processed according to the size of the finished tube to facilitate the design of the cold rolling process. It is machined into a hollow tube with an outer diameter and wall thickness ratio ≤ 5.5, and the wall thickness S is the thickness of the finished titanium alloy tube S1. 3 times, to ensure that the K value of each pass from the tube blank to the finished tube is ≥ 1, and the K value of cold rolling ≥ 1 is a necessary condition to ensure that there are no cracks on the inner surface of the titanium alloy tube; The inner surface of the tube and the surface of the mandrel are coated with lubricating oil to strengthen the lubrication of the inner surface of the titanium tube during the cold rolling process and reduce the probability of cold rolling defects; keep the rolling mill lubricating oil clean by filtering to avoid cold rolling defects caused by impurities in the oil; according to the titanium alloy tube cold rolling The vacuum annealing temperature is determined by the amount of rolling deformation, and the holding time is determined according to the wall thickness of the titanium alloy tube. The probability of rolling defects; the observation method of internal and external surface defects of titanium tubes and the combined treatment process of internal and external surface defects completely eliminate the defects that may occur in the previous cold rolling, and prevent the defects from inheriting to the next pass to continue to expand and cause the titanium tube to be scrapped; Crack defects with large distances cannot be removed by flow pickling, and can only be removed by inner buffing and outer abrasive belt polishing. For inner and outer surface defects whose opening distance is less than 0.02mm, the acid solution will be removed due to the small opening distance. It is impossible to enter the defect, so the defect can be removed by flow pickling.

Based on the above reasons, the present invention can be widely promoted in fields such as the production of small-diameter thick-walled titanium alloy seamless pipes.

Detailed ways

It should be noted that, in the case of no conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below in conjunction with examples.

Apparently, the described embodiments are only some of the embodiments of the present invention, but not all of them. The following description of at least one exemplary embodiment is merely illustrative in nature and in no way taken as limiting the invention, its application or uses. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

The invention provides a production method of a small-diameter thick-walled titanium alloy seamless pipe, comprising the following steps:

Step 1: three times of VAR melting to obtain titanium alloy ingot;

Step 2: The titanium alloy ingot is subjected to 5 fires, three piers and three pulls to form a billet, 1 fire to grow a long rod, and 3 fires to forge into a round bar. The radial forging temperature is 840°C to 850°C. The round bar meets the AA level of the GB/T5193 standard for flaw detection. The round bar is made into a hollow tube blank with an outer diameter of D and a wall thickness of S through deep hole drilling, inner honing, and outer machining. The ratio of outer diameter to wall thickness is ≤ 5.5; The inner honing is made of green silicon carbide oilstone of W40 particle size bonded by resin binder, the working pressure is 1.3Mpa, the honing speed is 18-21 meters per minute, and the roughness after honing of the hollow tube blank is ≤0.4 microns.

Step 3: The hollow tube blank is subjected to three passes of cold rolling to produce a titanium alloy seamless tube product with an outer diameter of D1 and a wall thickness of S1; the wall thickness S of the hollow tube blank is equal to that of the finished titanium alloy seamless tube wall thickness S1 3 times;

In the cold rolling process, the deformation rate ε and K value of each pass of cold rolling are smaller than those of the previous pass, and the K value of each pass of cold rolling is ≥ 1; a titanium alloy tube with an outer diameter of Da and a wall thickness of Sa is cold rolled once The formula for calculating the deformation rate ε of this cold rolling pass is as follows:

ε=((Da-Sa)×Sa-(Db-Sb)×Sb)/((Da-Sa)×Sa);

The formula for calculating the K value of this cold rolling pass is: K=(Sa-Sb)×Da/(Da-Db)×Sa.

Before each pass of cold rolling, apply rolling lubricating oil with a fineness of 900 mesh MoS ₂ , paraffin wax and No. 32 anti-wear machine oil in a weight ratio of 1:1:8 to the inner surface of the titanium alloy tube and the surface of the mandrel. , Filter the rolling mill lubricating oil before and during each cold rolling pass, and the filtration accuracy reaches 3 microns.

After each pass of cold rolling, the titanium alloy pipe is sampled, the crack defects are observed, and the crack defects are removed. The specific method is: after each pass of cold rolling of the titanium alloy pipe, a 100mm long pipe section sample is taken, and the wall thickness of the pipe section sample is removed by pickling. After mm, cut open and observe the inner and outer surfaces, confirm the distribution, orientation and maximum defect position of crack defects, take longitudinal and transverse metallographic samples at the maximum defect position, determine the maximum depth of crack defects as h and observe the opening distance of crack defects, And remove the crack defect according to the opening distance and the maximum depth h. After removal, take a 100mm long pipe section sample, use an imager and a stereo microscope to observe and confirm that there are no defects on the inner and outer surfaces, and then perform intermediate vacuum annealing and the next pass on the titanium alloy pipe Cold rolling;

For inner surface defects whose opening distance of crack defect is ≥0.02mm, choose internal honing to remove the inner surface wall thickness h+0.02mm of titanium alloy tube; inner honing uses green silicon carbide oilstone with W40 grain size bonded by resin bond, working pressure 1.3Mpa , the honing speed is 18-21 meters per minute;

For the outer surface defects whose opening distance of the crack defect is ≥0.02mm, select the abrasive belt to remove the outer surface wall thickness h+0.02mm of the titanium alloy pipe;

For inner and outer surface defects whose opening distance of crack defects is less than 0.02mm, the inner and outer surfaces should be pickled by flowing pickling to remove the titanium alloy pipe wall thickness h+0.02mm. Plastic bags are used to cover the outer surface of the titanium alloy pipe for flow pickling on the inner surface alone, and plastic is used to block both ends of the titanium alloy pipe for flow pickling on the outer surface alone; the pickling solution uses HF acid: HNO ₃ acid: water at 4:15 : It is made by mixing 81% by weight, and flows through the inner surface and/or outer surface of the titanium alloy tube at a speed greater than or equal to 3 m/min.

After each pass of cold rolling, intermediate vacuum annealing is carried out after removing crack defects. The intermediate vacuum annealing process for titanium alloy tubes with an outer diameter of Db and a wall thickness of Sb cold-rolled with a deformation rate of ε is: heating rate 2°C / minutes, the intermediate annealing temperature is (600+300×ε)°C, the holding time is 55×Sb minutes, and the elongation rate of the titanium alloy tube after annealing is ≥30%.

Step 4: Vacuum anneal, straighten, pickle, super-explore, and package the titanium alloy finished tube in step 3 to make a small-diameter thickness with an outer diameter D2≤10mm, a wall thickness S2≧1mm, and an outer diameter wall thickness ratio≤5.5 Wall titanium alloy seamless pipe.

Example 1

To produce TA16 titanium alloy seamless pipes with specifications of Φ9×1.7mm, the ratio of outer diameter to wall thickness is 9/1.7=5.294.

The production process used is as follows: three times of vacuum self-consumption VAR smelting into Φ490 round TA16 titanium alloy ingot → forging machine 5 times of fire, three piers and three pulls free forging into 210 square billet → forging machine 1 fire pull to grow into Φ160 long round rod → 3. The second diameter is forged into a Φ29 black leather round bar → deep hole drilling, inner honing, and outer machining to make a Φ22×5.1 hollow tube blank (outer diameter wall thickness ratio 4.3, wall thickness 1.7×3) → LG15 two Roller cold rolling machine cold rolled into Φ16×3.2 titanium tube (ε is 52%, K value is 1.37) → take a 100mm pipe section sample and find that there are microcracks on the inner surface with a maximum depth of 0.02mm, and the crack opening distance is 0.025mm. Honing (green silicon carbide oilstone with W40 grain size bonded by resin binder, working pressure 1.3Mpa, honing speed 19 meters per minute) to remove Φ16×3.2 titanium alloy pipe wall thickness 0.04mm→756℃(600+300× 0.52), heat preservation for 176 (55×3.2) minutes, vacuum annealing, straightening→LG10 two-roll cold rolling mill cold rolled into Φ12×2.3 titanium alloy pipe (ε is 46%, K value is 1.13)→Take a 100mm pipe section sample and find There are micro-cracks with a maximum depth of 0.02mm on the inner surface, and the crack opening distance is 0.01mm. The Φ12×2.3 titanium alloy pipe wall thickness is 0.04mm→738°C (600+300×0.46), and the heat preservation is 126.5 (55× 2.3) Minute vacuum annealing, straightening → LG10 two-roll cold rolling mill cold rolling into Φ9 × 1.7 titanium tube (ε is 44%, K value is 1.04) → 732 ° C (600 + 300 × 0.44), heat preservation 93.5 (55 ×1.7) minutes vacuum annealing, straightening→pickling→ultra-exploration→packaging.

The Φ9×1.7mm specification TA16 titanium alloy seamless pipe prepared in this embodiment has a yield strength of 470Mpa, a tensile strength of 590Mpa, and an elongation of 28%.

Example 2:

To produce TA18 titanium alloy seamless pipe with a specification of Φ10×1.9mm, the ratio of outer diameter to wall thickness is 10/1.9=5.263.

The production process used is as follows: three times of vacuum self-consumption to form a Φ490 round TA18 titanium alloy ingot → forging machine 5 fires, three pier and three pulls free forging into a 210 square billet → forging machine 1 fire to grow into a Φ160 long round rod → 3 fires The secondary diameter is forged into a Φ29 black leather round bar→deep hole drilling, inner honing, and outer turning machining to make a Φ24×5.7 hollow tube billet (outer diameter wall thickness ratio 4.21, wall thickness 1.9×3)→LG15 two-roll cooling The pipe mill was cold-rolled into Φ17×3.7 titanium pipe (ε is 53%, K value is 1.2) → take a 100mm pipe section sample and find that there are microcracks on the outer surface with a maximum depth of 0.03mm, and the crack opening distance is 0.02mm. The outer surface abrasive belt is used Polishing and removal of Φ17×3.7 titanium tube wall thickness 0.05mm→759℃(600+300×0.53), heat preservation for 203.5(55×3.7) minutes, vacuum annealing, straightening→LG10 two-roll cold rolling tube machine cold rolling into Φ12×2.5 titanium Pipe (ε is 52%, K value is 1.1) → Take a 100mm pipe section and find that there are microcracks on the outer surface with a maximum depth of 0.02mm and a crack opening distance of 0.01mm. mm→756°C (600+300×0.52), heat preservation for 137.5(55×2.5) minutes, vacuum annealing, straightening→LG10 two-roll cold rolling tube mill cold rolled into Φ10×1.9 titanium tube (ε is 41%, K value 1.04) → 723°C (600+300×0.41), heat preservation for 104.5 (55×1.9) minutes, vacuum annealing, straightening → pickling → super probe → packaging.

The yield strength of the TA18 titanium alloy seamless pipe with the specification of Φ10×1.9mm obtained in this embodiment is 570Mpa, the tensile strength is 680Mpa, and the elongation is 25%.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: It is still possible to modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the technical solutions of the various embodiments of the present invention. scope.

Claims (8)

1. The production method of the small-caliber thick-wall titanium alloy seamless pipe is characterized by comprising the following steps of:

the method comprises the following steps: carrying out VAR smelting for the third time to obtain a titanium alloy ingot;

step two: cogging, drawing and radial forging a titanium alloy ingot into a round bar, wherein the round bar meets the AA level of the GB/T5193 standard for flaw detection, the round bar is processed into a hollow tube blank with the outer diameter of D and the wall thickness of S through deep hole drilling, inner honing and outer turning, and the ratio of the outer diameter to the wall thickness is less than or equal to 5.5;

step three: carrying out three times of cold rolling on the hollow pipe blank to prepare a finished titanium alloy seamless pipe product with the outer diameter of D1 and the wall thickness of S1;

sampling the titanium alloy pipe after each cold rolling, observing the crack defect of the titanium alloy pipe, removing the crack defect, and performing intermediate vacuum annealing after removing the crack defect;

the wall thickness S of the hollow pipe blank is 3 times of the wall thickness S1 of the finished titanium alloy seamless pipe;

step four: and (3) carrying out vacuum annealing, straightening, acid washing, ultra-probing and packaging on the titanium alloy finished tube obtained in the step three to obtain the small-diameter thick-wall titanium alloy seamless tube with the outer diameter D2 of not more than 10mm, the wall thickness S2 of not less than 1mm and the outer diameter-wall thickness ratio of not more than 5.5.

2. The production method of a small-caliber thick-wall titanium alloy seamless tube according to claim 1, characterized in that: in the second step, the titanium alloy ingot is subjected to three-pier three-drawing cogging for 5 times of fire to form a square billet, is subjected to drawing growth for 1 time of fire to form a long rod, and is subjected to radial forging for 3 times of fire to form a round rod, wherein the radial forging temperature is 840-850 ℃.

3. The production method of a small-caliber thick-wall titanium alloy seamless tube according to claim 1 or 2, characterized in that: and step two, internally honing the green silicon carbide oilstone with the granularity of W40 bonded by using a resin bonding agent, wherein the working pressure is 1.3Mpa, the honing speed is 18-21 meters per minute, and the roughness of the hollow pipe blank after internal honing is less than or equal to 0.4 micrometer.

4. The production method of a small-caliber thick-wall titanium alloy seamless tube according to claim 1, characterized in that: in the third step, before each cold rolling pass, the MoS with the fineness of 900 meshes is obtained ₂ Paraffin and No. 32 antiwear engine oil are mixed according to the proportion of 1:1: the rolling lubricating oil mixed in the weight ratio of 8 is coated on the inner surface of the titanium alloy pipe and the surface of the core rod, the lubricating oil of a rolling mill is filtered before and during each cold rolling, and the filtering precision reaches 3 microns.

5. The production method of a small-caliber thick-wall titanium alloy seamless tube according to claim 1, characterized in that: in the third step, after each pass of cold rolling of the titanium alloy pipe, taking a 100mm long pipe section sample, removing the wall thickness of the pipe section sample by acid washing by 0.01mm, cutting open and observing the inner and outer surfaces, confirming the distribution, the trend and the maximum defect position of crack defects, taking longitudinal and transverse metallographic structure samples at the maximum defect position, determining the maximum depth of the crack defects to be h and the opening distance for observing the crack defects, removing the crack defects according to the opening distance and the maximum depth h, taking the 100mm long pipe section sample after removing, observing by an imager and a stereomicroscope to confirm that the inner and outer surfaces have no defects, and then carrying out intermediate vacuum annealing and next pass of cold rolling on the titanium alloy pipe;

the inner surface defect with the opening distance of the crack defect being more than or equal to 0.02mm is subjected to inner honing to remove the inner surface wall thickness h +0.02mm of the titanium alloy pipe;

selecting an abrasive belt to remove the wall thickness h +0.02mm of the outer surface of the titanium alloy pipe by outwards polishing the outer surface defect with the opening distance of the crack defect being more than or equal to 0.02mm;

and selecting inner and outer surface flowing acid cleaning for the inner and outer surface defects with the opening distance of the crack defect less than 0.02mm, and removing the wall thickness h +0.02mm of the titanium alloy pipe.

6. The production method of a small-caliber thick-wall titanium alloy seamless tube according to claim 5, characterized in that: in the third step, the inner honing adopts green carborundum oilstone with W40 granularity bonded by resin bonding agent, the working pressure is 1.3Mpa, and the honing speed is 18-21 m per minute; the single inner surface flow pickling adopts plastic bags to cover the outer surface of the titanium alloy pipe in a bundling manner, and the single outer surface flow pickling adopts plastic to block two end heads of the titanium alloy pipe;

the pickling solution adopts HF acid: HNO ₃ Acid: water is added according to the weight ratio of 4:15:81 weight percent, and flowing through the inner surface and/or the outer surface of the titanium alloy pipe at a speed of more than or equal to 3 m/min.

7. The production method of a small-caliber thick-wall titanium alloy seamless tube according to claim 1, characterized in that: in the third step, the deformation rate epsilon and the K value of each cold rolling pass in the cold rolling process are smaller than those of the previous cold rolling pass, and the K value of each cold rolling pass is more than or equal to 1;

the titanium alloy pipe with the outer diameter Da and the wall thickness Sa is cold-rolled into a titanium alloy pipe with the outer diameter Db and the wall thickness Sb for one pass, and the deformation rate epsilon of the cold rolling for the pass is calculated by the following formula:

ε＝((Da-Sa)×Sa-(Db-Sb)×Sb)/((Da-Sa)×Sa)；

the calculation formula of the K value of the pass cold rolling is as follows: k = (Sa-Sb) × Da/(Da-Db) × Sa.

8. The production method of a small-caliber thick-wall titanium alloy seamless tube according to claim 7, characterized in that: in the third step, the intermediate vacuum annealing process of the titanium alloy pipe with the deformation rate of epsilon and the outer diameter of Db and the wall thickness of Sb is as follows: the temperature rising speed is 2 ℃/min, the intermediate annealing temperature is (600 +300 multiplied by epsilon) DEG C, the heat preservation time is 55 multiplied by Sb minutes, and the elongation of the annealed titanium alloy tube is more than or equal to 30 percent.