Cold-working hardened pipe and preparation method thereof

Abstract

The invention provides a cold work hardening state pipe and a preparation method thereof, the preparation method comprises the following steps: (1) an austenitic stainless steel bar is made into a pierced billet, the austenitic stainless steel bar contains N and Nb, and the N accounts for 0.25-0.36 wt% of the austenitic stainless steel bar; (2) the pierced billet is subjected to one-pass or multi-pass cold rolling or cold drawing treatment, and an intermediate tube is obtained; the deformation amount of the intermediate pipe obtained through the last cold rolling or cold drawing treatment is controlled to be 20-40%; and (3) carrying out cold drawing or cold rolling on the intermediate pipe to obtain a finished pipe with a required specification, so as to obtain the cold hardened pipe. According to the method, the content of N in the austenitic stainless steel bar is limited to be 0.25-0.36%, and the deformation of the last intermediate pipe is controlled to be 20-40%, so that the austenitic stainless steel bar is in a small deformation range, dislocation and distortion are reduced technologically, the influence caused by dislocation and lattice distortion accumulation of the previous pass of finished products is reduced as much as possible, and the yield of finished products is improved. And the mechanical property of the finished pipe is kept stable within a certain cold deformation range.

Description

Technical Field

[0001] The invention relates to the technical field of alloy materials, in particular to a cold-worked hardened pipe and a preparation method thereof. Background Art

[0002] In addition to meeting technical standards, aviation products also have high requirements for material performance batch stability. Only when the material can maintain consistent performance in different batches can the overall performance and quality of the aircraft be ensured.

[0003] The performance of stainless steel solid solution pipes is the result of the combined effect of its chemical composition, heat treatment process and other factors. When the chemical composition is similar, the performance of different batches of products can be adjusted to a comparable level through heat treatment, making it easier to control the stability between different batches.

[0004] The performance of stainless steel cold-hardened pipes is determined by its chemical composition, cold working degree and other factors. When the chemical composition and cold working degree are similar, the mechanical properties of the finished product are also affected by the performance of the intermediate pipe before the finished product. Although the performance of the finished product can meet the technical standard requirements by adjusting the cold working degree, its stability between different batches will be greatly affected.

[0005] The Chinese patent document discloses "a method for improving the stress corrosion resistance of iron-nickel-based corrosion-resistant alloy oil well pipes", and its publication number is CN 109234500A. This patent can significantly improve its stress corrosion cracking resistance in H2S medium environment without changing the cold plastic deformation optical microstructure and corresponding strength level of iron-nickel-based corrosion-resistant alloy oil well pipes required by international standards and design specifications, and replace the more expensive nickel-based corrosion-resistant alloy oil well pipes with more economical low-cost iron-nickel-based corrosion-resistant alloy oil well pipes, thereby reducing the development cost of highly acidic oil and gas resources. However, the process of this patent is not suitable for the production of cold-worked hardened pipes, and the stability of cold-worked hardened pipes between different batches cannot be guaranteed. Summary of the invention

[0006] In view of the above-mentioned shortcomings of the prior art, an object of the present invention is to provide a cold-worked hardened pipe and a preparation method thereof, so as to solve the problem that the stability of different batches of the existing cold-worked hardened pipe is difficult to control.

[0007] To achieve the above-mentioned object and other related objects, the present invention provides a method for preparing a cold-worked hardened pipe, comprising the following steps: (1) forming an austenitic stainless steel bar into a rough tube, wherein the austenitic stainless steel bar contains N and Nb, wherein the proportion of N in the austenitic stainless steel bar is 0.25-0.36 wt %; (2) subjecting the rough pipe to one or more cold rolling or cold drawing processes to obtain an intermediate pipe; controlling the deformation of the intermediate pipe obtained by the last cold rolling or cold drawing process to be 20 to 40%; (3) The intermediate tube is cold drawn or cold rolled to a finished tube of required specifications to obtain a cold-worked hardened tube.

[0008] During the cold working process, stainless steel forms a large number of dislocations and lattice distortions, which significantly improves its strength. As the amount of cold deformation increases, the strength of stainless steel will increase accordingly, because the deformation-induced martensitic phase transformation and the increase in dislocations lead to an increase in the hardness of the material. Solution heat treatment can improve the internal stress and dislocations generated during cold working, but it cannot completely eliminate its dislocations and lattice distortions, and these dislocations and lattice distortions will accumulate with the plastic deformation of the material, thereby affecting the mechanical properties of the material.

[0009] The austenitic stainless steel bar formula in the prior art contains N, but does not contain Nb, and the proportion of N is generally 0.15-0.40%. There is a problem that the finished product strength fluctuates greatly and even does not meet the standard requirements. The applicant of this application unexpectedly discovered that N can play an important role in solid solution strengthening and stabilizing the structure in austenitic stainless steel, and Nb can play a role in refining grains in austenitic stainless steel. This application creatively adds Nb elements to austenitic stainless steel. Nb elements can form carbides or nitrides in austenitic stainless steel. These compounds prevent grain growth during heating, thereby refining grains. In addition, Nb restricts the climb movement of dislocations through the solute drag mechanism, inhibits the recovery and recrystallization of deformed austenite, increases the driving force of phase transformation, and leads to an increase in the nucleation rate of ferrite, and finally refines the structure. The grain size of austenitic stainless steel pipes has a significant effect on ultrasonic flaw detection. Coarse grains will lead to increased attenuation and an increase in grass-like echoes, thereby reducing the detectability and accuracy of flaw detection, and can improve ultrasonic bottom waves.

[0010] The applicant has found that when the N content is less than 0.25%, the effect of improving strength and ductility is not obvious; when the N content is higher than 0.36%, the flow stress is not conducive to obtaining efficient hot workability. However, simply increasing Nb and limiting the N content in austenitic stainless steel bars to 0.25-0.36% cannot solve the problem of the difficulty in controlling the stability of different batches of existing cold-worked hardened pipes. This application improves the process and controls the deformation of the intermediate pipe obtained by the last cold rolling or cold drawing process to 20-40%, so that it is in a small deformation range, reduces the formation of dislocations and distortions from the process, and minimizes the impact of the accumulation of dislocations and lattice distortions in the previous pass of the finished product, so that the mechanical properties of the finished pipe remain stable within a certain cold deformation range. The process improvement and formula improvement work together to solve the problem of the difficulty in controlling the stability of different batches of existing cold-worked hardened pipes.

[0011] Preferably, in step (1), the austenitic stainless steel bar is composed of the following components in weight percentage: C≤0.04%, Mn: 8.00-10.00%, Si≤1.00%, P≤0.030%, S≤0.030%, Cr: 19.00-21.50%, Ni: 5.50-7.50%, N: 0.25-0.36%, Mo≤0.75%, Cu≤0.75%, Nb: 0.02-0.10%, and the balance is Fe.

[0012] Preferably, in step (1), the austenitic stainless steel rod is prepared according to the above-mentioned formula, smelted by vacuum induction and electroslag remelting process, and then forged.

[0013] Preferably, in step (1), before the austenitic stainless steel bar is made into a rough pipe, a heat treatment process before forging the austenitic stainless steel bar is also included, the heat treatment temperature is 1180-1220° C., and the heat treatment time is ≥4h.

[0014] Preferably, in step (1), the outer diameter of the rough pipe is 68-108 mm, the wall thickness is 7-15 mm, and the unevenness of the wall thickness is no more than 10%.

[0015] Preferably, in step (2), the deformation amount of the intermediate tube obtained by the last cold rolling or cold drawing treatment is controlled to be 20-70%.

[0016] Preferably, in step (3), the intermediate tube is cold rolled to a cold deformation amount of 15 to 30% for a finished tube of required specifications.

[0017] Preferably, in step (2), each cold rolling or cold drawing process further includes degreasing, heat treatment, straightening and finishing steps.

[0018] More preferably, the degreasing process includes: using a mixed solution of nitric acid and hydrofluoric acid or an alkaline degreasing solution to remove the lubricating oil on the surface of the intermediate pipe.

[0019] Preferably, in step (2), the heat treatment time is 3 to 10 minutes.

[0020] Preferably, in step (2), the heat treatment temperature after the last cold rolling or cold drawing is controlled to be 1000-1100°C, and the heat treatment temperature after other cold rolling or cold drawing steps except the last cold rolling or cold drawing is controlled to be 1040-1160°C.

[0021] In the above technical solution of the present application, the grain size of the final finished pipe is controlled by lowering the heat treatment temperature after the last cold rolling or cold drawing process, thereby achieving the effect of grain refinement.

[0022] The present invention also provides a cold-worked hardened pipe made by the above-mentioned preparation method.

[0023] Preferably, the cold-worked hardened pipe is composed of the following components in weight percentage: C≤0.04%, Mn: 8.00-10.00%, Si≤1.00%, P≤0.030%, S≤0.030%, Cr: 19.00-21.50%, Ni: 5.50-7.50%, N: 0.25-0.36%, Mo≤0.75%, Cu≤0.75%, Nb: 0.02-0.10%, and the balance is Fe.

[0024] Preferably, the tensile strength of the cold-worked hardened pipe is 979-1117 MPa, the yield strength is ≥827 MPa, the elongation after fracture is ≥20%, the grain size is level 7-9, and the bottom wave of ultrasonic detection is ≤25%.

[0025] As described above, the cold-worked hardened pipe and the preparation method thereof of the present invention have the following beneficial effects: (1) By adding Nb elements, carbides or nitrides can be formed in austenitic stainless steel, which prevents grain growth and refines grains during heating. Nb restricts the climb motion of dislocations through the solute drag mechanism, inhibits the recovery and recrystallization of deformed austenite, increases the driving force of phase transformation, and leads to an increase in the ferrite nucleation rate and ultimately refinement of the structure.

[0026] (2) By limiting the nitrogen content in the austenitic stainless steel bar to 0.25-0.36%, and controlling the deformation of the intermediate tube obtained by the last cold rolling or cold drawing process to 20-40%, so that it is in a small deformation range, the formation of dislocation and distortion is reduced from the process, and the influence of the accumulation of dislocation and lattice distortion before the finished product is minimized, so that the mechanical properties of the finished tube remain stable within a certain cold deformation range; (3) The cold-worked hardened pipes prepared by the formula and process of the present application have a tensile strength of 979-1117 MPa, a yield strength of ≥827 MPa, an elongation after fracture of ≥20%, a grain size of 7-9, an ultrasonic bottom wave of ≤25%, and good stability control between different batches. DETAILED DESCRIPTION

[0027] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0028] It should be noted that the process equipment or devices not specifically specified in the following embodiments are all conventional equipment or devices in the art.

[0029] In addition, it should be understood that one or more method steps mentioned in the present invention do not exclude the existence of other method steps before or after the combination step or the insertion of other method steps between these explicitly mentioned steps, unless otherwise specified; it should also be understood that the combination connection relationship between one or more devices / apparatuses mentioned in the present invention does not exclude the existence of other devices / apparatuses before or after the combination device / apparatus or the insertion of other devices / apparatuses between these explicitly mentioned two devices / apparatuses, unless otherwise specified. Moreover, unless otherwise specified, the numbering of each method step is only a convenient tool for identifying each method step, and is not intended to limit the order of arrangement of each method step or the scope of the present invention. Changes or adjustments in their relative relationships should also be regarded as the scope of the present invention without substantially changing the technical content.

[0030] Example 1 The present application provides a method for preparing a cold-worked hardened pipe, comprising the following steps: (1) According to the following formula, vacuum induction and electroslag remelting processes are used to smelt an austenitic stainless steel ingot, the ingot is heat treated at 1220° C. for 4 hours, and then forged to obtain an austenitic stainless steel bar, wherein the austenitic stainless steel bar is composed of the following components in weight percentage: C: 0.02%, Mn: 8.53%, Si: 0.30%, P: 0.01%, S: 0.001%, Cr: 20.50%, Ni: 6.90%, N: 0.33%, Mo: 0.42%, Cu: 0.03%, Nb: 0.04%, and the balance is Fe; The austenitic stainless steel rod is made into a rough tube by hot extrusion, and the extrusion temperature is controlled to be 1180°C, the extrusion speed is 130mm / s, and the extrusion ratio is 7.5; the outer diameter of the obtained rough tube is 108mm, the wall thickness is 13mm, and the wall thickness unevenness is not greater than 10%; (2) The rough pipe is subjected to 5 cold rolling processes to obtain an intermediate pipe; the deformation amount of the intermediate pipe obtained by the last intermediate cold rolling process is controlled to be 24%; the deformation amount of the intermediate pipe obtained by the cold rolling process other than the last intermediate cold rolling process is controlled to be 20-70%; each cold rolling process further includes degreasing, heat treatment, straightening and finishing processes; the degreasing process includes: removing lubricating oil on the surface of the intermediate pipe by using a mixed solution of nitric acid and hydrofluoric acid; the heat treatment temperature after the last cold rolling process is controlled to be 1050° C., and the heat treatment temperature after the cold rolling process other than the last cold rolling process is controlled to be 1040-1080° C.

[0031] (3) Cold rolling the intermediate tube to a finished tube of required specifications, controlling the cold deformation amount to 25%, and obtaining a cold-worked hardened tube.

[0032] Example 2 The present application provides a method for preparing a cold-worked hardened pipe, comprising the following steps: (1) According to the following formula, vacuum induction and electroslag remelting processes are used to smelt an austenitic stainless steel ingot, the ingot is heat treated at 1220° C. for 4 hours, and then forged to obtain an austenitic stainless steel bar, wherein the austenitic stainless steel bar is composed of the following components in weight percentage: C: 0.02%, Mn: 8.53%, Si: 0.30%, P: 0.01%, S: 0.001%, Cr: 20.50%, Ni: 6.90%, N: 0.33%, Mo: 0.42%, Cu: 0.03%, Nb: 0.04%, and the balance is Fe; The austenitic stainless steel rod is made into a rough pipe by hot perforation, and the perforation temperature is controlled to be 1190° C. The outer diameter of the obtained rough pipe is 68 mm, the wall thickness is 7 mm, and the unevenness of the wall thickness is not greater than 10%; (2) The rough tube is subjected to four cold drawing processes to obtain an intermediate tube; the deformation of the intermediate tube obtained by the last cold drawing process is controlled to be 20%; the deformation of the intermediate tube obtained by the last cold drawing process is controlled to be about 60%; each cold drawing process includes degreasing, heat treatment, straightening and finishing processes; the degreasing process includes: removing lubricating oil on the surface of the intermediate tube by using a mixed solution of nitric acid and hydrofluoric acid; the heat treatment temperature after the last cold drawing process is controlled to be 1020°C, and the heat treatment temperature after other cold drawing processes except the last cold drawing process is controlled to be 1100-1160°C.

[0033] (3) The intermediate pipe is cold drawn to the finished pipe of the required specification, and the cold deformation is controlled to be 15% to obtain the cold-worked hardened pipe.

[0034] Example 3 The present application provides a method for preparing a cold-worked hardened pipe, comprising the following steps: (1) According to the following formula, vacuum induction and electroslag remelting processes are used to smelt an austenitic stainless steel ingot, and the ingot is heat treated at 1180° C. for 4 hours, and then forged to obtain an austenitic stainless steel bar, wherein the austenitic stainless steel bar is composed of the following components in weight percentage: C: 0.032%, Mn: 9.20%, Si: %, P: 0.006%, S: 0.0005%, Cr: 20.46%, Ni: 5.84%, N: 0.26%, Mo: 0.35%, Cu: 0.01%, Nb: 0.10%, and the balance is Fe; The austenitic stainless steel rod is made into a rough tube by hot extrusion, and the extrusion speed is controlled to be 146 mm / s and the extrusion ratio is 24; the outer diameter of the obtained rough tube is 68 mm, the wall thickness is 7 mm, and the unevenness of the wall thickness is not greater than 10%; (2) The rough pipe is subjected to 4 cold rolling processes to obtain an intermediate pipe; the deformation amount of the intermediate pipe obtained by the last cold rolling process is controlled to be 40%; the deformation amount of the intermediate pipe obtained by the last cold rolling process is controlled to be about 60%; each cold rolling process includes degreasing, heat treatment, straightening and finishing processes; the degreasing process includes: removing lubricating oil on the surface of the intermediate pipe by using a mixed solution of nitric acid and hydrofluoric acid; the heat treatment temperature after the last cold rolling process is controlled to be 1000°C, and the heat treatment temperature after other cold rolling processes except the last cold rolling process is controlled to be 1060-1120°C.

[0035] (3) The intermediate tube is cold rolled to a finished tube of required specifications, and the cold deformation is controlled to be 29% to obtain a cold-worked hardened tube.

[0036] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the formula of the austenitic stainless steel bar is different, specifically: C: 0.02%, Mn: 8.45%, Si: 0.33%, P: 0.006%, S: 0.001%, Cr: 20.61%, Ni: 6.74%, N: 0.33%, Mo: 0.40%, Cu: 0.03%, and the balance is Fe. The rest of the processes are exactly the same.

[0037] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the formula of the austenitic stainless steel bar is different, specifically: C: 0.02%, Mn: 9.61%, Si: 0.24%, P: 0.01%, S: 0.001%, Cr: 20.20%, Ni: 6.40%, N: 0.17%, Mo: 0.01%, Cu: 0.01%, Nb: 0.03%, and the balance is Fe. The rest of the processes are exactly the same.

[0038] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that in step (2), the deformation amount of the intermediate tube obtained by the last cold rolling treatment of the intermediate product is different, specifically 78%, and the other processes are exactly the same.

[0039] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that in step (2), the deformation amount of the intermediate tube obtained by the last cold rolling treatment is different, specifically 15%, and the rest of the processes are exactly the same.

[0040] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is that in step (2), the heat treatment temperature after the last cold rolling treatment is different, specifically 1120° C., and the rest of the processes are exactly the same.

[0041] The properties of the cold-worked hardened pipes obtained in Examples 1-3 and Comparative Examples 1-5 were characterized, and the results are shown in Table 1: Table 1. Performance test results of cold-hardened pipes obtained in Examples 1-3 and Comparative Examples 1 to 5 As can be seen from Table 1, by comparing Example 1 with Comparative Examples 1 and 2, it can be seen that the formula has a great influence on the strength and grain size; by comparing Example 1 with Comparative Examples 3 and 4, it can be seen that the deformation of the intermediate pipe obtained by the last cold rolling treatment has a great influence on the strength; the intermediate product with a small deformation (24%) of Example 1 of the present application can make the cold deformation control range of the finished product wider, and the strength fluctuation range of the finished product is smaller, and the stability is better; the intermediate product with a large deformation (78%) of Comparative Example 1 makes the cold deformation control range of the finished product very narrow, and the strength of the finished product is high, which is very easy to exceed the standard requirement range. The intermediate product with too small deformation (15%) of Comparative Example 2 can make the cold deformation of the finished product reach a large level, so that the strength of the finished product can meet the standard requirements. Therefore, for the pipes delivered in the cold-worked hardened state, the control of the deformation of the previous pass of the finished product is very important; by comparing Example 1 with Comparative Example 5, it can be seen that the heat treatment temperature after the last cold rolling treatment has a great influence on the grain size of the cold-worked hardened pipe.

[0042] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A method for preparing a cold-hardened pipe, characterized in that: The following steps are involved: (1) forming an austenitic stainless steel bar into a rough tube, wherein the austenitic stainless steel bar contains N and Nb, wherein the proportion of N in the austenitic stainless steel bar is 0.25-0.36 wt %; (2) subjecting the rough pipe to one or more cold rolling or cold drawing processes to obtain an intermediate pipe; controlling the deformation of the intermediate pipe obtained by the last cold rolling or cold drawing process to be 20 to 40%; (3) The intermediate tube is cold drawn or cold rolled to a finished tube of required specifications to obtain a cold-worked hardened tube.

2. The preparation method according to claim 1, characterized in that: In step (1), the austenitic stainless steel bar is composed of the following components in weight percentage: C≤0.04%, Mn: 8.00-10.00%, Si≤1.00%, P≤0.030%, S≤0.030%, Cr: 19.00-21.50%, Ni: 5.50-7.50%, N: 0.25-0.36%, Mo≤0.75%, Cu≤0.75%, Nb: 0.02-0.10%, and the balance is Fe.

3. The preparation method according to claim 2, characterized in that: In step (1), the austenitic stainless steel rod is prepared according to the above-mentioned formula, smelted by vacuum induction and electroslag remelting process, and then forged.

4. The preparation method according to claim 1, characterized in that: In step (1), the austenitic stainless steel rod is made into a rough tube by hot extrusion or hot perforation.

5. The preparation method according to claim 1, characterized in that: In step (1), before the austenitic stainless steel bar is made into a rough pipe, a heat treatment process before forging the austenitic stainless steel bar is also included, the heat treatment temperature is 1180-1220° C., and the heat treatment time is ≥4h.

6. The preparation method according to claim 1, characterized in that: In step (1), the outer diameter of the rough pipe is 68-108 mm, the wall thickness is 7-15 mm, and the unevenness of the wall thickness is not greater than 10%.

7. The preparation method according to claim 1, characterized in that: In step (2), the deformation of the intermediate tube obtained by the last cold rolling or cold drawing process is controlled to be 20-70%; in step (3), the intermediate tube is cold rolled to a finished tube of required specifications, and the cold deformation is 15-30%.

8. The preparation method according to claim 1, characterized in that: In step (2), each cold rolling or cold drawing process further includes degreasing, heat treatment, straightening and finishing processes; the heat treatment temperature after the last cold rolling or cold drawing process is controlled to be 1000-1100°C, and the heat treatment temperature after other cold rolling or cold drawing processes except the last cold rolling or cold drawing process is controlled to be 1040-1160°C.

9. A cold-worked hardened pipe obtained by the preparation method according to any one of claims 1 to 8.

10. The cold-worked hardened pipe according to claim 9, characterized in that: The cold-worked hardened pipe is composed of the following components in weight percentage: C≤0.04%, Mn: 8.00-10.00%, Si≤1.00%, P≤0.030%, S≤0.030%, Cr: 19.00-21.50%, Ni: 5.50-7.50%, N: 0.25-0.36%, Mo≤0.75%, Cu≤0.75%, Nb: 0.02-0.10%, and the balance is Fe; The cold-worked hardened pipe has a tensile strength of 979-1117 MPa, a yield strength of ≥827 MPa, an elongation after fracture of ≥20%, a grain size of 7-9, and an ultrasonic detection bottom wave of ≤25%.

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