A hot extrusion forming process for stainless steel pipes

Abstract

The present invention discloses a hot extrusion forming process for stainless steel pipes, which belongs to the technical field of metal material preparation, and includes the following steps: blank preparation, preheating, primary heating, primary lubrication, hole expansion, cleaning, secondary heating, secondary lubrication, and hot extrusion; wherein the secondary lubrication is to put the blank after the secondary heating into an extruder for a second lubrication. The present invention produces the required stainless steel pipes by the combination of hole expansion and hot extrusion; by setting the secondary lubrication process in the extruder, the time consumed from the induction furnace to the lubrication workbench and then to the extruder is shortened, the lubrication workbench is omitted and the lubrication process of the blank is arranged in the extruder, so that the temperature of the blank is in the plastic temperature range, the quality of the finished product of hot extrusion forming is improved, and the performance requirements of the finished stainless steel pipe forming product can be met.

Description

Technical Field

[0001] The invention belongs to the technical field of stainless steel pipe production, in particular to a hot extrusion forming process for a stainless steel pipe. Background Art

[0002] Stainless steel refers to steel that is resistant to corrosion by weakly corrosive media such as air, steam, and water, and chemically corrosive media such as acids, alkalis, and salts. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. The basic alloying elements of stainless steel include nickel, molybdenum, nitrogen, etc., to meet the requirements of stainless steel structure and performance for various uses. Stainless steel is easily corroded by chloride ions because chromium, nickel, and chlorine are isotopic elements, and isotopic elements will interchange and assimilate to form stainless steel corrosion. In addition, the strength of stainless steel is also very high, so it is widely used in transportation pipelines, that is, stainless steel pipelines.

[0003] Existing stainless steel pipe forming processes include extrusion and cold drawing. In the extrusion process, the stainless steel tube blank is first perforated by a puncher, and then the perforated stainless steel tube blank is extruded into the required stainless steel tube by an extruder; in the cold drawing process, the stainless steel tube blank is pulled through a tube drawing die to gradually thin and elongate the steel tube to form the required stainless steel tube. Compared with extrusion forming, the cold drawing forming method has unstable quality, and quality defects such as cracks are very likely to appear on the surface, and the flaw detection pass rate is low. On the other hand, the material utilization rate is low, there is a lot of material waste, and there are more processes and higher costs.

[0004] The traditional hot extrusion process of stainless steel pipes includes hole expansion and hot extrusion. Before these two processes, the blank needs to be lubricated, otherwise it will cause mold wear and surface defects of the finished product. The lubrication of stainless steel pipe blanks requires glass as a lubricant. When applying glass lubricant, the glass powder needs to be evenly applied to the inner and outer surfaces of the stainless steel pipe blank. This process is usually carried out manually on a special workbench, so it takes a long time. When the lubricated blank is loaded into the extrusion device, the temperature of the blank will also decrease, that is, the temperature is no longer in the plastic temperature range. The finished product of hot extrusion is often of low quality and does not meet the performance requirements of stainless steel pipes. Summary of the invention

[0005] The present invention provides a hot extrusion forming process for a stainless steel pipe, which is used to solve the technical problems raised in the above background technology.

[0006] The technical solution adopted by the present invention is: a hot extrusion forming process of a stainless steel pipe, comprising the following steps:

[0007] S1. Blank preparation: a through inner hole is machined in the center of the cylindrical blank, a bell mouth is machined at the top of the inner hole of the blank, and a fillet is machined at the bottom of the outer circle of the blank;

[0008] S2, preheating: placing the blank into a preheating device for preheating;

[0009] S3, primary heating: placing the preheated blank into a heating device for the first heating, the heating temperature is: 1000-1100°C;

[0010] S4, primary lubrication: lubricate the blank for the first time through glass powder;

[0011] S5, hole expansion: put the lubricated blank into the hole expansion machine for hole expansion;

[0012] S6, cleaning: after the expanded blank is taken out, the glass residues on the inner and outer surfaces are cleaned;

[0013] S7, secondary heating: the cleaned blank is placed in a heating device for secondary heating, the heating temperature is: 1100-1250°C;

[0014] S8, secondary lubrication: put the billet after secondary heating into the extruder for secondary lubrication;

[0015] S9, hot extrusion: put the billet into the extruder for extrusion molding.

[0016] As a preferred embodiment of the present invention, the blank composition of the stainless steel pipe is as follows by mass percentage: C: ≤0.02%, Si: 0.75-0.90%, Mn: 0.55-1.00%, P: ≤0.02%, S: ≤0.001%, Cr: 20.0-25.0%, Ni: 15.5-18.5%, Mo: 4.0-6.5%, Cu: 0.75-0.95%, N: 0.24-0.32%, B: 0.003-0.005%, Al: 0.005-0.020%, and the rest is Fe.

[0017] As a preferred embodiment of the present invention, the second lubrication step comprises:

[0018] ① Preheat the extrusion die, core rod, extrusion cylinder and extrusion pad in the extruder;

[0019] ② Apply graphite lubricant on the inner wall of the extrusion cylinder and the surface of the core rod;

[0020] ③Put the glass lubricating pad that matches the inner wall of the extrusion barrel into the barrel, and then put the glass lubricating pad that matches the inner hole of the blank on the core rod;

[0021] ④ Place the blank into the extrusion cylinder and place a glass lubrication pad between the blank and the extrusion die.

[0022] As a preferred embodiment of the present invention, the preheating device in the S2 preheating is a ring furnace, and the preheating temperature is 700-800°C.

[0023] As a preferred solution of the present invention, the heating device in the S3 primary heating and the S7 secondary heating is an induction furnace, and the heating frequency should be set so that the heat penetration depth reaches 1 / 2 of the billet thickness.

[0024] As a preferred solution of the present invention, in the cleaning step S6, the blank is placed in a vibration dephosphorization device, and the glass residue on the blank is cleaned by mechanical high-speed vibration.

[0025] As a preferred embodiment of the present invention, the first lubrication step comprises:

[0026] ① Load the blank after primary heating into the rolling machine;

[0027] ② Start the rolling machine to press out annular patterns on the inner and outer surfaces of the blank;

[0028] ③ When the blank is rotating in the rolling machine, glass powder is evenly sprayed on the inner and outer surfaces of the blank.

[0029] As a preferred solution of the present invention, the pressure of the rolling machine is 150 MPa, the pressing speed is 2 m / min, and the pressed texture depth is 2 mm.

[0030] As a preferred solution of the present invention, a glass powder spraying device is also installed on one side of the rolling machine, and the glass powder is evenly sprayed on the inner and outer surfaces of the blank through the glass powder spraying device in coordination with the rotation of the blank in the rolling machine.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] The invention discloses a hot extrusion forming process for a stainless steel pipe, and the required stainless steel pipe is produced by the cooperation of hole expansion and hot extrusion; the time consumed from an induction furnace to a lubrication workbench and then to the extruder is shortened by arranging the secondary lubrication process in an extruder, and the lubrication process of the blank is arranged to be completed in the extruder without the lubrication workbench, so that the temperature of the blank is in a plastic temperature range, the quality of the finished product of the hot extrusion forming is improved, and the performance requirements of the finished stainless steel pipe forming product can be met. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of the process flow of the present invention.

[0034] Figure 2 It is a structural schematic diagram of the rolling machine of the present invention.

[0035] Figure 3It is a structural schematic diagram of the working process of the extruder of the present invention.

[0036] The text markings in the figure indicate: 1. rolling machine; 2. roller; 3. blank; 4. glass powder spraying device; 5. spraying pipe; 6. spraying port; 7. silo; 8. die base; 9. extrusion die; 10. extrusion cylinder; 11. glass lubrication pad; 12. extrusion rod; 13. extrusion pad; 14. core rod. DETAILED DESCRIPTION

[0037] In order to enable those skilled in the art to better understand the technical solution, the technical solution is described in detail below in conjunction with the embodiments. The description in this section is only exemplary and explanatory and should not have any limiting effect on the scope of protection of the present application.

[0038] like Figure 1-3 As shown, a hot extrusion forming process for a stainless steel pipe comprises the following steps:

[0039] S1. Blank preparation: a through inner hole is machined in the center of the cylindrical blank, a bell mouth is machined at the top of the inner hole of the blank, and a fillet is machined at the bottom of the outer circle of the blank;

[0040] S2, preheating: placing the blank into a preheating device for preheating;

[0041] S3, primary heating: placing the preheated blank into a heating device for the first heating, the heating temperature is: 1000-1100°C;

[0042] S4, primary lubrication: lubricate the blank for the first time through glass powder;

[0043] S5, hole expansion: put the lubricated blank into the hole expansion machine for hole expansion;

[0044] S6, cleaning: after the expanded blank is taken out, the glass residues on the inner and outer surfaces are cleaned;

[0045] S7, secondary heating: the cleaned blank is placed in a heating device for secondary heating, the heating temperature is: 1100-1250°C;

[0046] S8, secondary lubrication: put the billet after secondary heating into the extruder for secondary lubrication;

[0047] S9, hot extrusion: put the billet into the extruder for extrusion molding.

[0048] Specifically, during S1 blank preparation, the blank shape is as follows Figure 3As shown in , specifically in the preparation process of this embodiment, the surface roughness Ra of the inner and outer surfaces and one side end face is not greater than 1.6μm. When there are defects on the outer surface, the outer surface should be polished to eliminate them, and the inner surface defects should be removed by boring and other methods. The deviation of the inner and outer surface centers of the blank should be less than 0.3mm, and the edges should be smoothly transitioned with arcs. The taper of the bell mouth should match the expansion cone or be slightly larger than 1 / 2 of the cone angle of the expansion cone. The outer chamfer is determined according to the different specifications of the blank and the requirements. The drilling diameter of the inner hole needs to be determined according to the expansion diameter and the outer diameter of the blank. Selecting a smaller drilling diameter without significantly increasing the difficulty of expansion can improve the metal utilization rate. At the same time, if the hole is directly drilled to the inner diameter of the blank required by the extruder without significantly reducing the metal utilization rate, the hole can be directly drilled to the inner diameter of the blank required by the extruder, eliminating the subsequent heating, lubrication and expansion steps, saving time and processes, and improving production efficiency.

[0049] Specifically, in S5 hole expansion, the hole pre-drilled in the preparation step is expanded into a through hole required for extrusion, with an inner hole 7-20mm larger than the core rod diameter. In this process, if the hole expansion diameter is too small, the billet will hang on the core rod when entering the extruder, or the glass lubricant inside it will be scraped off by the core rod, resulting in tearing due to the local lack of lubricant during extrusion. At the same time, in order to facilitate the ejection of the expanded billet, a certain taper is set in the expanding barrel of the expanding machine, that is, the inner diameter of the upper end is set to be 2mm larger than the lower end. In order not to scrape off the glass lubricant on the outer surface of the billet when loading the billet, a chamfer is set at the upper edge of the expanding barrel. The expanding barrel should be preheated to 380-430℃ before use, and the taper of the expanding cone should reach more than 50°, and the taper should be increased as much as possible within the range where hole expansion is easy to carry out, which can not only reduce its cost, but also reduce the amount of chamfering processing of the billet and reduce metal consumption. The expansion cone should also be preheated to about 100°C before use. Since general mold steel is prone to cracks after being immediately cooled with water at high temperatures, the expansion cone should be cooled naturally after use.

[0050] Specifically, Figure 3As shown, in S9 hot extrusion, extrusion is to extrude the expanded tube into a steel pipe of the required specifications. In this embodiment, a horizontal extruder is used to extrude the billet in the extrusion barrel of the horizontal extruder. When the billet enters the extrusion barrel of the extruder, the end face of the bell mouth of the billet should face the extrusion die. All positions in the extruder that are in direct contact with the billet need to be preheated. The components in the extruder are naturally cooled to a suitable temperature during the extrusion interval, and water cooling and other methods are not used. In this embodiment, the extrusion speed of the extruder should not be too low, and hot extrusion is performed at a speed of 200-400mm / s. Because the billet temperature is high, if the extrusion speed is very low and the extrusion time is long, the surface temperature of the extrusion die may exceed the working temperature, and the outer surface of the product will become rough due to the thick glass film. Within the capacity of the equipment, try to use a larger extrusion barrel, because the larger the specification of the extrusion barrel, the larger the extrusion ratio, the better the uniformity of the product structure and performance, and the production efficiency and yield rate can be improved.

[0051] Specifically, after hot extrusion forming in S9, the formed stainless steel pipe needs to be heat treated, straightened, sawed, pickled, ground, inspected and other steps to finally obtain the required stainless steel pipe. Among them, heat treatment is to evenly heat the stainless steel pipe to 1040-1080°C and water-cool it after it comes out of the furnace; straightening is to use roller straightening tools to correct the stainless steel pipe after heat treatment so that it has no obvious bending and twisting; sawing is to cut off the unqualified parts of the two ends of the stainless steel pipe; pickling is to put the stainless steel pipe into the pickling tank to remove oil from the surface; after pickling, fine grinding and inspection are carried out to remove some cracks, scratches and other defects on the surface of the stainless steel pipe, and finally physical and chemical performance tests such as size detection, ultrasonic flaw detection, and liquid penetration detection are carried out.

[0052] Furthermore, the blank composition of the stainless steel pipe is as follows by mass percentage: C: ≤0.02%, Si: 0.75-0.90%, Mn: 0.55-1.00%, P: ≤0.02%, S: ≤0.001%, Cr: 20.0-25.0%, Ni: 15.5-18.5%, Mo: 4.0-6.5%, Cu: 0.75-0.95%, N: 0.24-0.32%, B: 0.003-0.005%, Al: 0.005-0.020%, and the rest is Fe.

[0053] Specifically, in this embodiment, the blank composition of the stainless steel pipe is: C: 0.015%, Cr: 21.5%, Ni: 18.0%, Mo: 6.0%, Mn: 0.95%, Si: 0.84%, P: 0.012%, S: 0.001%, N: 0.27%, Cu: 0.79%, B: 0.004%, Al: 0.013%, and the balance is Fe.

[0054] Further, such as Figure 3As shown, the second lubrication step includes:

[0055] ① Preheat the extrusion die, core rod, extrusion cylinder and extrusion pad in the extruder that will contact the billet. The preheating temperatures are 100-250°C, 200-300°C, 350-450°C and 200-300°C respectively;

[0056] ②Evenly apply graphite lubricant on the inner wall of the extrusion cylinder and the surface of the core rod;

[0057] ③Put the glass lubricating pad that matches the inner wall of the extrusion barrel into the barrel, and then put the glass lubricating pad that matches the inner hole of the blank on the core rod;

[0058] ④ Place the blank into the extrusion cylinder and place a glass lubrication pad between the blank and the extrusion die.

[0059] Specifically, in this step, glass powder is no longer used as a glass lubricant, but a glass mat adapted to the blank and the extrusion barrel is used. The glass mat in this embodiment has a thickness of about 2 mm. When it is directly placed in the extruder, the temperature in the extruder is not enough to melt it, but the glass mat is kept in its corresponding state. Only after the blank is loaded into the extruder after secondary heating, it will be melted by high temperature and evenly attached to the surface of the blank; that is, in this embodiment, the characteristic of glass melting at high temperature is utilized. When the blank is loaded into the extrusion barrel and the core rod is inserted into its inner hole, the glass mat will be melted by high temperature and adhere to the corresponding inner and outer surfaces of the blank, and play a lubricating role as a glass lubricant when the extruder is working, and also wrap the blank as an insulation layer to keep the blank warm. At the same time, the above operations are all completed in the extruder, and there is no need to use other places or devices to lubricate the blank. The lubrication step is directly combined with the process of loading the blank into the extruder, which saves time and processes, greatly reduces the temperature loss of the blank, and further improves the final hot extrusion molding effect of the blank.

[0060] Furthermore, the preheating device in the S2 preheating is a ring furnace, the preheating temperature is: 700-800°C, and a sufficient insulation time is set; through the preheating operation, cracks are not likely to occur in the subsequent heating process, thereby improving the forming effect of the blank.

[0061] Furthermore, the heating device in the primary heating in S3 and the secondary heating in S7 is an induction furnace, and the value of the heating frequency should be set so that the heat penetration depth reaches 1 / 2 of the thickness of the blank.

[0062] Specifically, the billet is preheated in the annular furnace and then placed in the induction furnace. The induction furnace heats the billet quickly, reduces the oxidation of the billet surface, and improves the utilization efficiency of the induction furnace. When the billet is placed in the induction furnace, it should be in the center of the furnace and maintain a good parallelism with the center line of the furnace, so that the induction furnace can better control the heating process of the billet. In order to reduce the internal stress generated by the billet during the rapid heating process, in addition to the uniform preheating of the billet in the annular furnace, a lower induction heating frequency should be used in this step to make the heat penetration depth deeper and the temperature gradient inside the billet smaller. The selection of the heating frequency should make the heat penetration depth reach about 1 / 2 of the billet thickness. If the frequency of the induction furnace is set too high, a large temperature gradient will be generated or the heating time will be extended, reducing the thermal efficiency; if the frequency is set too low, the electric heating efficiency will be reduced and the production efficiency will be reduced. During induction heating, a shorter holding time is used after the billet is heated to the target temperature, so that the internal temperature of the billet is lower than the external temperature. Because if the billet is heated evenly, the outside of the billet will dissipate heat quickly after it comes out of the furnace, while the inside will dissipate heat very slowly. In addition, due to the heat absorption of the billet by the expansion tube and the extrusion tube, the outside temperature of the billet will be much lower than the inside temperature, and the billet temperature is seriously uneven. Due to the temperature difference, the deformation resistance of the inner and outer layers is very different. During expansion and hot extrusion, the flow speed of the inner and outer layers of the billet is very different, which is easy to cause cracks. In order to make up for the above defects, a higher heating temperature is used when heating in the induction furnace, and the insulation time is shortened, so that the outside temperature of the billet is higher than the inside temperature. In this way, the inside and outside temperatures of the billet will be basically the same during expansion and hot extrusion.

[0063] Furthermore, in the cleaning step S6, the blank is placed in a vibration dephosphorization device, and the glass residue on the blank is cleaned by mechanical high-speed vibration. Conventional dephosphorization uses high-pressure water to impact the surface of the blank to remove the glass residue, but the high-pressure water will quickly cool the blank, increase the internal gravity of the blank, and easily cause cracks in the blank. Therefore, in this embodiment, a vibration dephosphorization device is used, that is, a vibration source is set in the box, the vibration is transmitted to the blank, and the glass residue on the surface of the blank is cleaned by high-speed mechanical vibration, thereby avoiding rapid cooling of the blank.

[0064] Further, such as Figure 2 As shown, the first lubrication step includes:

[0065] ① Load the blank after primary heating into the rolling machine;

[0066] ② Start the rolling machine to press out annular patterns on the inner and outer surfaces of the blank;

[0067] ③ When the blank is rotating in the rolling machine, glass powder is evenly sprayed on the inner and outer surfaces of the blank.

[0068] Specifically, rollers adapted to the blank are installed in the rolling machine at intervals and rotated therein. The blank loaded into the device can be clamped by the rollers, and the blank can be driven to roll by the rotation of the rollers. During the clamping and rotation of the rollers, the annular patterns on the rollers are imprinted on the inner and outer surfaces of the blank.

[0069] In the subsequent hole expansion step, a vertical hole expander is used to expand the hole in its hole expansion cylinder; therefore, after the blank is placed in the hole expansion cylinder, the glass lubricant will be lost due to gravity, so lubricant should be added again. The adding method is: first use a glass fiber cotton ball to block the inner hole of the blank, and then manually sprinkle glass powder on the chamfer of the inner hole at the upper end of the blank. The entire chamfered surface should be sprinkled with glass powder. If the opening diameter of the chamfered end face is smaller than the hole expansion cone, the glass powder should also be sprinkled within the diameter range of the hole expansion cone on the upper end face of the blank.

[0070] In the present embodiment, when the inner and outer surfaces of the blank have annular patterns, since the blank is placed vertically in the subsequent hole expanding step to facilitate hole expanding by the hole expanding cone, the annular patterns can block the glass lubricant on its surface and prevent the glass lubricant from being lost quickly due to gravity. There is no need to add glass powder to the blank after the blank is loaded into the hole expanding machine to fill the lost part, thereby reducing steps and improving efficiency. At the same time, the annular patterns on the blank can also make the glass lubricant on the surface of the blank more uniform and improve the lubrication effect.

[0071] At the same time, the annular patterns on the blank can cooperate with the glass pad in the subsequent secondary lubrication process, that is, when the glass pad is melted by the high-temperature blank, the filling effect of the patterns can also make the glass lubricant more uniform on the surface of the blank; and the regular annular patterns will not affect the molding during hot extrusion in the extruder, because during the hot extrusion process, the patterns will be squeezed first and filled.

[0072] Furthermore, the pressure of the rolling machine is 150MPa, the pressing speed is 2m / min, and the pressed texture depth is 2mm; the pressure can be controlled to gradually increase to 150MPa to avoid excessive embossing causing material deformation or damage; while ensuring the quality of the texture, the pressing speed can be increased as much as possible, but not too fast, otherwise it will easily cause unclear textures or damage to the material surface.

[0073] Furthermore, a glass powder spraying device is installed on one side of the rolling machine, and the glass powder spraying device cooperates with the rotation of the blank in the rolling machine to evenly spray the glass powder on the inner and outer surfaces of the blank.

[0074] Specifically, the glass powder spraying device is provided with spraying tubes adapted to the thickness of the blank at intervals, one end of which is used for controlling the spraying of glass powder inside the blank, and the other end of which is used for spraying glass powder on the outer surface of the blank. The spraying tube is provided with a spraying port to make the sprayed glass powder more uniform. At the same time, the glass powder spraying device also includes a silo arranged on one side, and the silo is filled with glass powder, which makes it more convenient to use and reduces the waste of glass powder.

[0075] According to the above embodiments, the present invention discloses a hot extrusion forming process for stainless steel pipes, and the required stainless steel pipes are produced by the cooperation of hole expansion and hot extrusion; by setting the secondary lubrication process in the extruder, the time consumed from the induction furnace to the lubrication workbench and then to the extruder is shortened, and the lubrication workbench is omitted to arrange the lubrication process of the blank in the extruder, which will not cause a large loss of the temperature of the blank, so that the temperature of the blank is in the plastic temperature range, and the quality of the finished product of hot extrusion forming is improved, and the performance requirements of the finished product of the stainless steel pipe forming can be met. Through the cooperation of the rolling machine and the glass powder spraying device, when the blank is sprayed with glass powder, the annular pattern is embossed on the inner and outer surfaces of the blank. When the inner and outer surfaces of the blank have annular patterns, the glass lubricant on the surface can be blocked to prevent the glass lubricant from being lost rapidly due to gravity; there is no need to add glass powder to the blank after the blank is loaded into the hole expansion machine to fill the lost part, reduce steps, and improve efficiency; at the same time, the annular pattern on the blank can also make the glass lubricant on the surface of the blank more uniform, and improve the lubrication effect. At the same time, the annular patterns on the blank can cooperate with the glass pad in the subsequent secondary lubrication process, that is, when the glass pad is melted by the high-temperature blank, the filling effect of the patterns can also make the glass lubricant more uniform on the surface of the blank; and the regular annular patterns will not affect the molding during hot extrusion in the extruder, because during the hot extrusion process, the patterns will be squeezed first and filled.

[0076] It should be noted that, in this article, the terms "include", "comprises" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. Specific examples are used in this article to illustrate the principles and implementation methods of the technical solution of the present application. The above examples are only used to help understand the method of the present application and its core idea. The above is only a preferred implementation method of the present application. It should be pointed out that due to the limitations of textual expression and the objective existence of infinite specific structures, for ordinary technicians in this technical field, without departing from the principles of the present application, several improvements, modifications or changes can be made, and the above technical features can also be combined in an appropriate manner; these improvements, modifications, changes or combinations, or the direct application of the patent concept and technical solution to other occasions without improvement, should be regarded as the scope of protection of the present application.

Claims

1. A hot extrusion forming process for a stainless steel pipe, characterized in that: The steps include: S1. Blank preparation: a through inner hole is machined in the center of the cylindrical blank, a bell mouth is machined at the top of the inner hole of the blank, and a fillet is machined at the bottom of the outer circle of the blank; S2, preheating: placing the blank into a preheating device for preheating; S3, primary heating: placing the preheated blank into a heating device for the first heating, the heating temperature is: 1000-1100°C; S4, primary lubrication: lubricate the blank for the first time through glass powder; S5, hole expansion: put the lubricated blank into the hole expansion machine for hole expansion; S6, cleaning: after the expanded blank is taken out, the glass residues on the inner and outer surfaces are cleaned; S7, secondary heating: the cleaned blank is placed in a heating device for secondary heating, the heating temperature is: 1100-1250°C; S8, secondary lubrication: put the billet after secondary heating into the extruder for secondary lubrication; S9, hot extrusion: put the billet into the extruder for extrusion molding; Wherein, the one-time lubrication step comprises: ① Load the blank after primary heating into the rolling machine; ② Start the rolling machine to press out annular patterns on the inner and outer surfaces of the blank; ③ When the blank is rotating in the rolling machine, the glass powder is evenly sprayed on the inner and outer surfaces of the blank; Wherein, the secondary lubrication step comprises: ① Preheat the extrusion die, core rod, extrusion cylinder and extrusion pad in the extruder; ② Apply graphite lubricant on the inner wall of the extrusion cylinder and the surface of the core rod; ③Put the glass lubricating pad that matches the inner wall of the extrusion barrel into the barrel, and then put the glass lubricating pad that matches the inner hole of the blank on the core rod; ④ Place the blank into the extrusion cylinder and place a glass lubrication pad between the blank and the extrusion die.

2. The hot extrusion forming process of a stainless steel pipe according to claim 1, characterized in that: The blank composition of the stainless steel pipe is as follows by mass percentage: C: ≤0.02%, Si: 0.75-0.90%, Mn: 0.55-1.00%, P: ≤0.02%, S: ≤0.001%, Cr: 20.0-25.0%, Ni: 15.5-18.5%, Mo: 4.0-6.5%, Cu: 0.75-0.95%, N: 0.24-0.32%, B: 0.003-0.005%, Al: 0.005-0.020%, and the rest is Fe.

3. The hot extrusion forming process of a stainless steel pipe according to claim 1, characterized in that: The preheating device in the S2 preheating is a ring furnace, and the preheating temperature is: 700-800°C.

4. The hot extrusion forming process of a stainless steel pipe according to claim 1, characterized in that: The heating device in the S3 primary heating and the S7 secondary heating is an induction furnace, and the heating frequency should be set so that the heat penetration depth reaches 1 / 2 of the blank thickness.

5. The hot extrusion forming process of a stainless steel pipe according to claim 1, characterized in that: In the cleaning step S6, the blank is placed in a vibration dephosphorization device, and the glass residue on the blank is cleaned by mechanical high-speed vibration.

6. The hot extrusion forming process of a stainless steel pipe according to claim 1, characterized in that: The pressure of the rolling machine is 150 MPa, the pressing speed is 2 m / min, and the pressed texture depth is 2 mm.

7. A hot extrusion forming process for a stainless steel pipe according to claim 6, characterized in that: A glass powder spraying device is also installed on one side of the rolling machine, and the glass powder is evenly sprayed on the inner and outer surfaces of the blank through the glass powder spraying device in coordination with the rotation of the blank in the rolling machine.

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