Method for producing large-diameter duplex stainless steel seamless tube through periodic rolling mill

Abstract

The invention discloses a method for producing a large-diameter duplex stainless steel seamless tube by a periodic rolling mill. The method comprises the following steps: processing a through hole by a bar billet machine to prepare a hollow tube blank; carrying out cross piercing treatment on the hollow tube blank to prepare a tubular billet; rolling the tubular billet by a periodic rolling mill in a forging manner to prepare a pierced billet, carrying out solid solution heat treatment on the pierced billet, and processing the pierced billet by a pierced billet machine to prepare a seamless steel tube. The mechanical property and the metallographic structure of the material of the duplex stainless steel seamless pipe are obviously superior to those of a conventional production process, so that the production process difficulty of the S22053 large-diameter duplex stainless steel seamless pipe is greatly reduced, the lasting stability, especially the strength and the impact toughness, of the product quality is improved, the metal yield of the large-diameter S22053 duplex stainless steel pipe is greatly increased, and the production cost is reduced. The production cost is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of seamless steel pipe manufacturing, and in particular to a method for producing large-diameter duplex stainless steel seamless pipes by a cycle rolling mill. Background Art

[0002] S22053 is a widely used duplex stainless steel, which is composed of 22%Cr, 2.5%Mo and 4.5%Ni-N alloy. Its standard grade is 022Cr23Ni5Mo3N. It has high strength, good impact toughness and excellent stress corrosion resistance. It has good corrosion resistance in seawater, chemicals and high temperature environments. It is widely used in high-corrosion environments such as petroleum, chemical industry, vacuum salt production, flue gas desulfurization, papermaking, etc. However, with the increase of the content of elements such as Cr, Mo, and N in steel, the thermoplasticity of S22053 duplex stainless steel deteriorates, and it is easy to crack during hot working, forming serious defects such as cracks, internal and external folding, and causing waste. At present, the S22053 duplex stainless steel seamless pipe produced by forging, perforation or perforation + cold rolling has many disadvantages, such as complex process, long production process, low production efficiency, low geometric dimensional accuracy (large machining allowance and large metal loss), limited caliber, low final material utilization rate (yield rate of 12% to 18%), and high processing cost. It is impossible to form large-scale industrial production and meet the demand for high-precision, large-caliber, low-cost S22053 duplex stainless steel seamless steel in the fields of petrochemical industry, which restricts the development of related industries.

[0003] In particular, in order to further improve the material's resistance to pitting and crevice corrosion and reliability while maintaining sufficiently high strength, the industry generally requires that the ferrite content in the metallographic structure be controlled between 45% and 55% as much as possible. The metallographic structure of S22053 duplex stainless steel seamless pipes produced by forging, perforation or perforation + cold rolling can only meet the requirements of 40% to 60% specified in GB / T21833-2020 "Austenitic-Ferritic Duplex Stainless Steel Seamless Pipe", but it is difficult to meet the industry's requirements of 45% to 55%. Summary of the invention

[0004] The purpose of the present invention is to provide a method for producing large-diameter duplex stainless steel seamless pipes by a cycle rolling mill, so as to meet all technical requirements of the target product, greatly reduce the production cost of large-diameter S22053 duplex stainless steel seamless pipes, greatly improve the production efficiency of large-diameter S22053 duplex stainless steel seamless pipes, greatly reduce the production process difficulty of large-diameter S22053 duplex stainless steel seamless pipes, and greatly improve the metal recovery rate of duplex stainless steel seamless pipes.

[0005] To achieve the above object, the present invention provides the following technical solutions: The present invention provides a method for producing a large-diameter duplex stainless steel seamless pipe by a cycle rolling mill, comprising: processing a through-hole by a rod billet machine to produce a hollow pipe billet, performing oblique rolling and perforation treatment on the hollow pipe billet to produce a rough pipe, forging rolling the rough pipe by a cycle rolling mill to produce a rough pipe, performing solid solution heat treatment on the rough pipe, and processing the rough pipe by a machine to produce a seamless steel pipe, and the method is characterized in that the method specifically comprises the following steps: S1. The hollow tube is made by machining the through hole of the rod blank. The size of the through hole of the rod blank is determined according to the deformation amount of the oblique rolling and piercing. S2. Hollow tube billets are processed by oblique rolling and piercing to form rough tubes. After precise calculation and three-dimensional simulation, combined with the rolling characteristics of large-diameter billets, the oblique rolling and piercing are slightly expanded and deformed, the longitudinal grooves at the entrance of the piercing roller are lengthened and deepened, the friction between the bar and the roller surface is increased, the roller drag force is increased, and the total piercing reduction rate, top front reduction rate and roller speed are set to be smaller, which effectively improves the stability of the piercing process, reduces the uneven deformation during the piercing process, and effectively inhibits the occurrence of internal and external surface defects of the product; S3, the rough pipe is rolled into a rough pipe by a forging rolling mill. In one cycle, the rough pipe is forged to a diameter reduction of 100mm to 200mm by the forging function, and then the rolled piece is rolled to the designed size by finishing rolling and final rolling; S4, solution heat treatment of rough pipe; S5. The rough pipe is machined into seamless steel pipe. Large lathes and deep hole boring machines are used to process the forged rough pipe into finished seamless steel pipe by internal boring and external boring technology.

[0006] Furthermore, the diameter of the through hole in S1 is between Φ80 mm and Φ120 mm.

[0007] Furthermore, the cyclic tube mill in S3 adopts a continuous segmented longitudinal forging process, and performs forging rolling on the rolled piece in a concentrated deformation zone. The process parameters of the cyclic forging rolling are as follows: hole type = outer diameter of rough tube - 0 to 15 mm; mandrel = outer diameter of rough tube - wall thickness of rough tube × 2; roller speed = 33 to 45 rpm; feed amount = 30 to 50 mm / time.

[0008] Furthermore, in S4, the process steps of charging furnace temperature ≤ 400°C, heating time ≥ 3h; heating to 1080°C and keeping warm for 3h; and rapidly hoisting into quenching pool for strong cooling after being taken out of the furnace are carried out to achieve solution annealing of the steel pipe.

[0009] Furthermore, the rough pipe machining in S5 includes the following processes: frame positioning, rough boring, semi-finishing boring, fine boring, rough turning, semi-finishing turning, fine turning, outer surface polishing, flat end face chamfering and inner surface polishing.

[0010] Furthermore, after the rough pipe is processed into a seamless steel pipe by machining, the finished product inspection also includes the inspection of the chemical composition of the seamless steel pipe, dimensional deviation and error, mechanical property inspection, metallographic structure inspection, grain size inspection and non-destructive inspection.

[0011] Furthermore, the duplex stainless steel is S22053 duplex stainless steel, which is a complex stainless steel composed of 22% Cr, 2.5% Mo and 4.5% Ni-N alloy.

[0012] Based on the above technical solution, the embodiments of the present invention can at least produce the following technical effects: The cyclic rolling mill forging tube rolling process adopted by the present invention not only meets the characteristics of uniform structure and stable performance of forged products due to its forging and rolling combination characteristics, but also achieves the purpose of high geometric dimensional accuracy of products and high final material utilization rate (the yield rate can reach 38% to 55%). At the same time, due to the rolling method, a steel pipe with a length of 6 to 10 meters can be formed every 3 to 5 minutes. Its extremely high production efficiency makes large-scale industrial production a reality, providing a strong guarantee for the rapid development of the petrochemical industry.

[0013] The metallographic structure of the product produced by the present invention has met the industry requirements after inspection, and the ferrite content in the metallographic structure is controlled as much as possible between 45% and 55%, which further improves the material's ability to resist pitting corrosion and crevice corrosion and its reliability and has sufficiently high strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0015] Figure 1 is a process diagram of the surface of a punching machine roll according to an embodiment of the present invention; Figure 2 This is a metallographic structure diagram of a large-diameter S22053 duplex stainless steel seamless pipe according to an embodiment of the present invention; In the figure: the white phase is austenite, the dark phase is ferrite, the volume percentage of austenite is about 46%, and the volume percentage of ferrite is about 54%; the average grain size of austenite is 6.5. DETAILED DESCRIPTION

[0016] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0017] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0018] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0019] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to the skilled artisan. The present invention description and examples are exemplary only.

[0020] like Figure 1 As shown in the surface process diagram of the punching machine roll of the present invention, the rod billet is processed through holes by the rod billet machine, the rod billet is obliquely rolled and perforated, the rough pipe periodic rolling mill is forged, the rough pipe is solid solution heat treated, and the rough pipe machine is processed to obtain the S22053 duplex stainless steel seamless pipe, including the rod billet processing through hole treatment to make a hollow pipe billet, the hollow pipe billet is obliquely rolled and perforated to make a rough pipe, the rough pipe periodic rolling mill is forged to make a rough pipe, the rough pipe is solid solution heat treated to reduce the hardness, and the rough pipe machine is processed to make a seamless steel pipe. The details are as follows: S1. The rod blank is processed by machine through hole to make a hollow tube blank: S22053 duplex stainless steel has a high alloy element content and is a duplex stainless steel composed of 22% Cr, 2.5% Mo and 4.5% Ni-N alloy. This type of steel has the characteristics of two phases and still maintains a two-phase structure of ferrite and austenite within the hot working temperature range. During high-temperature deformation, due to the difference in deformation behavior of the two-phase structure, the stress distribution is uneven, and cracks are easily formed and extended on the grain boundaries, especially on large-diameter billets, resulting in a decrease in the thermoplasticity of the steel and deterioration in the processing performance. Cracks or inward folding are easily formed during the oblique rolling and piercing process, resulting in pierced scrap.

[0021] The rod billet is machined through a hole so that the inside and outside of the rod billet are heated simultaneously, which improves the heating efficiency of the duplex steel, reduces the heating temperature difference between the inside and outside of the large-diameter rod billet, and improves the uneven stress distribution of the duplex steel. At the same time, it can reduce the axial resistance of the tool during punching deformation, promote the metal to flow and deform in the longitudinal direction, and improve the plastic processing performance of the duplex steel.

[0022] Secondly, the rod billet is machined through the hole, so that the deformation of the rod billet in the process of being made into a rough tube is reduced, so that the harsh deformation conditions of oblique rolling and piercing can be fully improved, and the conditions for the occurrence of product defects such as external folding, external surface cracks, internal cracks, and internal folding are reduced, thereby reducing the rolling scrap rate.

[0023] During implementation, the size of the through hole machined by the rod billet is determined by calculation based on the deformation amount of the oblique rolling and piercing, and the diameter range of the through hole is generally determined to be between Φ80mm and Φ120mm.

[0024] S2. Hollow tube billet is processed by oblique rolling and piercing to make rough tube: Oblique rolling and piercing is one of the main deformation processes of S22053 duplex stainless steel seamless pipe. During the oblique rolling and piercing process, the metal flow speeds of the inner and outer layers of the tube billet are different, and the metal is subjected to various tensile, compressive, shearing and torsion stresses, resulting in poor deformation conditions, which easily lead to product defects such as external folding, external surface cracks, internal cracks, and internal folding; at the same time, due to the differences in strength / plasticity and deformation behavior of the two phases in duplex stainless steel, its thermoplasticity is reduced, and its hot processing performance is worse than that of ordinary austenitic stainless steel. During the oblique rolling and piercing process, due to the large deformation amount, the temperature rise of the deformation thermal effect (usually reaching about 100°C) makes the material enter the temperature range where the plasticity decreases sharply, resulting in fracture or partial fracture of the material to form folding defects, and even rolling jam accidents in severe cases.

[0025] During implementation, after precise calculation and three-dimensional simulation, combined with the rolling characteristics of large-diameter billet, the oblique rolling piercing process is optimized from conventional diameter reduction deformation to micro-diameter expansion deformation, the longitudinal groove is lengthened and deepened at the entrance of the piercing roller, the friction between the bar and the roller surface is increased, the roller drag force is increased, and a smaller total piercing reduction rate, top front reduction rate and roller speed are set, which effectively improves the stability of the piercing process and reduces the uneven deformation in the piercing process. It can greatly improve the deformation conditions of the billet during the piercing process, avoid accidents such as front and back jams, and effectively inhibit the occurrence of internal and external surface defects of the product.

[0026] The perforation process parameters are generally set according to the following principles: Total reduction rate: 8%~9%; front reduction rate: 4.5%~5.5%; ovality coefficient: 1.08; rolling angle 0.2~0.3°.

[0027] S3. The rough tube is processed into rough tube by forging-type rolling of periodic mill: the periodic mill is mainly composed of a feeder and a periodic mill body. Among them, the feeder mainly completes the work of reciprocating feeding, billet rotation, and synchronization with the mill; the periodic mill body is designed based on the idea of ​​concentrating the deformation segments of each pass in the tube rolling in the same rolling groove. When a pair of relatively rotating rollers placed in the periodic mill body rotates one circle, the tube billet passes through the rolling groove to achieve the three processes of forging, finishing rolling, and final rolling. A tube is made through several passes of periodic forging rolling.

[0028] The cyclic tube mill adopts a continuous segmented longitudinal forging and rolling process, and implements forging rolling on the rolled piece in the concentrated deformation zone (forging and rolling section). In addition, the feed amount of the feeder can be adjusted each time, which creates conditions for rolling difficult-to-deform and high-alloy tubes. The diameter reduction of the cyclic forging tube mill can reach more than 200mm, which is incomparable to other tube mills. This large compression ratio and large elongation deformation process provides a strong guarantee for directly using large-section tube billets, further refining grains, improving the longitudinal and transverse organization and performance differences of the products, and improving the comprehensive performance of tube products. When producing S22053 duplex stainless steel seamless pipes, it can greatly improve the volume ratio of ferrite and austenite in the metallographic structure, reduce the difference in the volume fraction of the two phases, optimize the grain size, and fundamentally improve the performance indicators of production.

[0029] As the cyclic forging tube mill has the process characteristics of forging, rolling and large deformation, the maximum forging ratio can reach 20, the product grains are fully refined, the longitudinal and transverse properties are uniform, and the final rolling temperature is reasonable. The comprehensive performance of the products is excellent compared with the same period last year.

[0030] During implementation, within one cycle, the periodic forging tube mill uses its forging function to forge the rough tube to a diameter reduction of 100mm to 200mm, and then rolls the rolled piece to the designed size through fine rolling and final rolling.

[0031] The process parameters of periodic forging rolling are generally set according to the following principles: Hole type = outer diameter of rough pipe - 0~15mm; mandrel = outer diameter of rough pipe - wall thickness of rough pipe × 2; roller speed = 33~45 rpm; feed amount = 30~50mm / time.

[0032] Step 4: Solution heat treatment of rough pipe to reduce hardness: S22053 duplex stainless steel has high alloy content, high yield strength, tensile strength and hardness indexes, which brings turning difficulties to subsequent machining processes and residual stress caused by excessive turning additional stress. Therefore, solution heat treatment must be carried out to significantly reduce the hardness of the product to eliminate the above unfavorable factors.

[0033] During implementation, the charging temperature is ≤400℃, the heating time is ≥3h; heating to 1080℃ and keeping warm for 3h; after being taken out of the furnace, it is quickly hoisted into the quenching pool for strong cooling to achieve solid solution annealing of the steel pipe, which can achieve the indicators of grain size of grade 5 or finer, hardness ≤290HBW, and retain the uniformity of the volume ratio of ferrite and austenite caused by the large deformation of the cyclic rolling mill, providing guarantee for the performance and organization of subsequent products to meet the requirements.

[0034] S5. Processing of rough pipe into seamless steel pipe: Large lathes and deep hole boring machines are used to process forged rough pipe into finished seamless steel pipe by internal boring and external boring. This machining technology is quite mature in China.

[0035] Actual test results of large diameter S22053 duplex stainless steel seamless pipe 1. Chemical composition of seamless pipe: The chemical composition, macrostructure and non-metallic inclusion requirements of S22053 duplex steel specified in GB / T21833-2020 "Austenitic-ferritic duplex stainless steel seamless steel pipe".

[0036] 2. Allowable deviation of dimensions and appearance: outer diameter D±0.4~0.8%, wall thickness S±5~9%, ovality = 40~60% of the allowable deviation of outer diameter, curvature 0.4~0.95mm / m, surface roughness Ra=3.2~6.3µm.

[0037] 3. Mechanical properties: tensile strength Rm = 705 ~ 720MPa, yield strength Rp0.2 = 510 ~ 550MPa, section shrinkage Z = 35 ~ 40%, Brinell hardness 210 ~ 220HBW.

[0038] 4. Macrostructure: There are no shrinkage holes, cracks, bubbles, inclusions, white spots and folds visible on the cross-section acid-immersed macrostructure test piece of steel. The macrostructure of acid-immersed macrostructure is generally loose at level 0.

[0039] 5. Grain size: 6.0~7.0.

[0040] 6. Nondestructive testing: meet GB / T6402 Level 3.

[0041] 7. Intergranular corrosion: The intergranular corrosion test was carried out according to GB / T4334, and there was no tendency of intergranular corrosion.

[0042] 8. Pitting corrosion: Pitting corrosion test is carried out according to GB / T21883.1. The corrosion rate at room temperature for 24 hours is 8mg / (dm2•d).

[0043] 9. Metallographic structure: The volume percentage of ferrite is about 54%, and the volume percentage of austenite is about 46%.

[0044] like Figure 2 As shown, the metallographic structure diagram of large diameter S22053 duplex stainless steel seamless pipe.

[0045] It can be seen from the above implementation process that the present invention does not require the S22053 duplex stainless steel bar to be subjected to conventional forging treatment or complex deformation processes such as perforation + cold rolling in the industry, and can produce S22053 duplex stainless steel seamless pipes that meet national standards, and the mechanical properties and metallographic structure of the material are ideal, and at the same time meet the uniform structure requirements of the industry trend, thereby greatly reducing the production process difficulty of large-diameter S22053 duplex stainless steel seamless pipes, increasing the long-term stability of product quality, and greatly reducing the production cost of large-diameter S22053 duplex stainless steel seamless pipes.

[0046] Finally, it should be noted that: The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments may still be modified, or some or all of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for producing large-diameter duplex stainless steel seamless pipes by a cycle rolling mill, comprising processing a rod billet through a hole to produce a hollow tube billet, performing oblique rolling and perforation processing on the hollow tube billet to produce a rough tube, forging rolling the rough tube by a cycle rolling mill to produce a rough tube, performing a solution heat treatment on the rough tube, and processing the rough tube by a machine to produce a seamless steel pipe, characterized in that: The specific steps include: S1. The hollow tube is made by machining the through hole of the rod blank. The size of the through hole of the rod blank is determined according to the deformation amount of the oblique rolling and piercing. S2. Hollow tube billets are processed by oblique rolling and piercing to form rough tubes. After precise calculation and three-dimensional simulation, combined with the rolling characteristics of large-diameter billets, the oblique rolling and piercing are slightly expanded and deformed, the longitudinal grooves at the entrance of the piercing roller are lengthened and deepened, the friction between the bar and the roller surface is increased, the roller drag force is increased, and the total piercing reduction rate, top front reduction rate and roller speed are set to be smaller, which effectively improves the stability of the piercing process, reduces the uneven deformation during the piercing process, and effectively inhibits the occurrence of internal and external surface defects of the product; S3, the rough pipe is rolled into a rough pipe by a forging rolling mill. In one cycle, the rough pipe is forged to a diameter reduction of 100mm to 200mm by the forging function, and then the rolled piece is rolled to the designed size by finishing rolling and final rolling; S4, solution heat treatment of rough pipe; S5. The rough pipe is machined into seamless steel pipe. Large lathes and deep hole boring machines are used to process the forged rough pipe into finished seamless steel pipe by internal boring and external boring technology.

2. The method for producing large-diameter duplex stainless steel seamless pipes by a periodic rolling mill according to claim 1, characterized in that: The diameter of the through hole in S1 is between Φ80 mm and Φ120 mm.

3. The method for producing large-diameter duplex stainless steel seamless pipes by a cycle rolling mill according to claim 1, characterized in that: The S3 medium-cycle tube mill adopts a continuous segmented longitudinal forging process, and performs forging rolling on the rolled piece in a concentrated deformation zone. The process parameters of the periodic forging rolling are as follows: hole type = outer diameter of rough tube - 0 to 15 mm; mandrel = outer diameter of rough tube - wall thickness of rough tube × 2; roller speed = 33 to 45 rpm; feed amount = 30 to 50 mm / time.

4. The method for producing large-diameter duplex stainless steel seamless pipes by a periodic rolling mill according to claim 1, characterized in that: In S4, the process steps of charging furnace temperature ≤ 400° C., heating time ≥ 3 h; heating to 1080° C. and keeping warm for 3 h; and rapidly hanging into quenching pool for strong cooling after being taken out of the furnace are carried out to achieve solution annealing of the steel pipe.

5. The method for producing large-diameter duplex stainless steel seamless pipes by a periodic rolling mill according to claim 1, characterized in that: The machining of the rough pipe in S5 includes the following processes: frame positioning, rough boring, semi-finishing boring, fine boring, rough turning, semi-finishing turning, fine turning, outer surface polishing, flat end face chamfering and inner surface polishing.

6. The method for producing large-diameter duplex stainless steel seamless pipes by a cycle rolling mill according to claim 1, characterized in that: After the rough pipe is processed into seamless steel pipe by machining, the finished product inspection is also included, and the finished product inspection includes the inspection of the chemical composition of the seamless steel pipe, dimensional deviation and error, mechanical property inspection, metallographic structure inspection, grain size inspection and non-destructive inspection.

7. The method for producing large-diameter duplex stainless steel seamless pipes by a cycle rolling mill according to claim 1, characterized in that: The duplex stainless steel is S22053 duplex stainless steel, which is a complex stainless steel composed of 22% Cr, 2.5% Mo and 4.5% Ni-N alloy.

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