A method for manufacturing a large diameter thin-walled seamless steel pipe for low temperature use

Abstract

The present invention relates to a method for manufacturing a large-diameter thin-wall seamless steel pipe for low temperature use, the manufacturing method comprising the following steps: (1) heating, punching and continuous rolling a tube blank in sequence to obtain a raw tube; (2) subjecting the raw tube obtained in step (1) to a first heat expansion treatment to obtain a first steel tube; (3) subjecting the first steel tube obtained in step (2) to a normalizing treatment to obtain a second steel tube; (4) subjecting the second steel tube obtained in step (3) to a second heat expansion treatment to obtain a finished product. The manufacturing method provided by the present invention can ensure the out-of-roundness of the steel tube, improve the problems of bending and deformation of the steel tube, make up for the deficiencies of the straightening equipment, and make the outer dimensions and mechanical properties of the steel tube meet the standard requirements.

Description

Technical Field

[0001] The invention relates to the technical field of manufacturing seamless steel pipes, and in particular to a method for manufacturing a large-diameter, thin-walled seamless steel pipe for low temperature use. Background Art

[0002] Large diameter thin-walled seamless steel pipes for low temperature are mainly used for pipelines or devices in low temperature environments below -45℃. In recent years, with the development of industries such as petroleum and petrochemicals, devices and pipelines have tended to be large-scale. In order to meet the flow rate of the medium, the diameter demand for seamless steel pipes for low temperature has also changed from small to large. In order to make the performance of the material, especially the low temperature performance, meet the requirements of the standard, the steel pipe needs to be normalized, and the temperature is generally 880-920℃. The material of large diameter thin-walled seamless steel pipes for low temperature contains only a small amount of alloy components, and the strength at high temperature is very low. In addition, the characteristics of large diameter and thin wall thickness of the steel pipe make it easy for the steel pipe to have pits, out of roundness, deformation and bending during the heat treatment process.

[0003] For problems such as pits, out-of-roundness, deformation and bending generated during the heat treatment process, the steel pipe needs to be reshaped later. The main method for shaping pits is to use supporting equipment such as oil cylinders to push out the depressed parts; the main method for shaping out-of-roundness, deformation and bending is to use straightening equipment. However, the maximum outer diameter range of most multi-roll straightening machines is ≤720mm, which cannot meet the straightening requirements of large-diameter thin-walled seamless steel pipes for low temperature use, that is, it cannot ensure that the size of the steel pipe meets the standard requirements.

[0004] CN104946976B discloses a method for producing a large-diameter, thin-walled, precision seamless steel pipe for a rotary drilling rig drill rod, which sequentially smelts steel billets, rolls billet tubes, heat expands, anneals, and straightens to obtain a large-diameter, thin-walled, precision seamless steel pipe. However, this method easily causes problems such as pits, out-of-roundness, deformation, and bending of the steel pipe, and the straightening process is greatly limited by the straightening equipment.

[0005] CN102179450A discloses a seamless steel pipe thermal expansion process, which first thermally expands one end of the seamless steel pipe, then places an inner ring at the end, clamps the steel pipe, and then continues to expand the rest of the steel pipe. However, the process is not suitable for large-diameter thin-walled seamless steel pipes for low temperature use, because the wall thickness of large-diameter thin-walled seamless steel pipes for low temperature use is relatively thin, and the process is likely to cause pits or deformation of the steel pipe.

[0006] Therefore, it is of great significance to provide a method for manufacturing large-diameter thin-walled seamless steel pipes for low temperature use. Summary of the invention

[0007] In view of the above problems, the purpose of the present invention is to provide a method for manufacturing large-diameter thin-walled seamless steel pipes for low temperature use. Compared with the prior art, the manufacturing method provided by the present invention can ensure the out-of-roundness of the steel pipe, improve the problems of bending and deformation of the steel pipe, make up for the shortcomings of the straightening equipment, and make the external dimensions and mechanical properties of the steel pipe meet the standard requirements.

[0008] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0009] The present invention provides a method for manufacturing a large-diameter thin-wall seamless steel pipe for low temperature use, the manufacturing method comprising the following steps:

[0010] (1) The tube blank is heated, pierced and continuously rolled in sequence to obtain a raw tube;

[0011] (2) subjecting the raw tube obtained in step (1) to a first heat expansion treatment to obtain a first steel tube;

[0012] (3) normalizing the first steel pipe obtained in step (2) to obtain a second steel pipe;

[0013] (4) subjecting the second steel pipe obtained in step (3) to a second heat expansion treatment to obtain a finished product.

[0014] The manufacturing method provided by the present invention can effectively prevent the occurrence of pits, out-of-roundness, deformation and bending problems in large-diameter thin-walled seamless steel pipes for low temperature by sequentially performing the combined operation of the first heat expansion treatment, the normalizing treatment and the second heat expansion treatment. The present invention firstly enlarges the diameter of the steel pipe by the first heat expansion treatment, and strictly controls the conditions of the first heat expansion treatment, so that the performance of the steel pipe will not change greatly while the diameter is initially enlarged, and then the normalizing treatment is performed to improve the metallographic structure of the steel pipe, so that the grain size can be uniform and refined, and the mechanical properties such as low-temperature impact are improved; especially importantly, the second heat expansion treatment is finally performed to further expand the pipe, so that the steel pipe will not be greatly deformed and bent, and the out-of-roundness can be guaranteed to make up for the shortcomings of straightening. At present, the manufacturing method of large-diameter thin-walled seamless steel pipes for low temperature usually adopts the operation of first performing heat expansion treatment and then performing normalizing treatment, which not only easily causes pits, out-of-roundness, deformation and bending problems in the steel pipe, but also increases the difficulty of the subsequent straightening process. The manufacturing method provided by the present invention is not limited by straightening, can greatly improve production efficiency, and make the performance and size of the steel pipe meet the required standards.

[0015] In the present invention, the large diameter thin wall seamless steel pipe for low temperature, "low temperature" generally refers to below -45°C, "large diameter" refers to a diameter ≥711mm, and "thin wall" refers to a wall thickness ≤14mm.

[0016] Preferably, the chemical components contained in the tube blank in step (1) by mass percentage include: C≤0.30%, S≤0.025%, P≤0.025%, 0.29%≤Mn≤1.35%, Si≥0.10%.

[0017] The C≤0.30%, for example, may be 0.30%, 0.29%, 0.28% or 0.27%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0018] The S≤0.025%, for example, may be 0.025%, 0.024%, 0.023% or 0.022%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0019] The P≤0.025%, for example, may be 0.025%, 0.024%, 0.023% or 0.022%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0020] The 0.29%≤Mn≤1.35% may be, for example, 0.29%, 0.5%, 0.6%, 0.8%, 1% or 1.35%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0021] The Si is ≥ 0.10%, such as 0.1%, 0.2%, 0.3%, 0.4% or 0.5%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0022] Preferably, the manufacturing process of the tube blank comprises sequentially subjecting the molten steel to converter smelting, LF ladle refining, RH vacuum degassing and continuous casting.

[0023] In the present invention, it is preferred to control the molten steel to be successively subjected to converter smelting, LF ladle refining, RH vacuum degassing and continuous casting to manufacture tube blanks. In particular, RH vacuum degassing can be used to remove O, H and N gases in the molten steel. After vacuum degassing, the purity of the molten steel can be improved, the longitudinal and transverse mechanical properties can be uniform, and the elongation, cross-sectional shrinkage and impact toughness can be improved. For some steel types that require heat treatment, the heat treatment time can also be shortened after degassing.

[0024] Preferably, the heating temperature in step (1) is ≥1200°C, for example, it can be 1200°C, 1210°C, 1220°C, 1230°C, 1240°C or 1250°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0025] Preferably, the residual temperature of the formed steel pipe after continuous rolling is ≥880°C, for example, it can be 880°C, 890°C, 900°C, 920°C, 940°C, 960°C, 980°C or 1000°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable. Then, the raw pipe is air-cooled on a cooling bed to obtain the original pipe.

[0026] In the present invention, the tube blank is heated to reach the processing temperature required for piercing and continuous rolling.

[0027] Preferably, the temperature of the first heat expansion treatment in step (2) is 720-800°C, for example, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C or 800°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0028] The temperature setting of the first heat expansion treatment in the present invention is considered to be above the material phase transition temperature Ac3, thereby ensuring that the performance of the steel pipe (especially the low-temperature impact value which is difficult to control) will not change significantly.

[0029] In the present invention, after the debris on the inner surface of the original tube is cleared before the first heat expansion treatment, a lubricant is sprayed, and then the original tube is heat expanded by a two-step push-type hydraulic medium frequency heating tube expander.

[0030] Preferably, the speed of the first thermal expansion treatment is ≤120 mm / min, for example, it can be 120 mm / min, 110 mm / min, 100 mm / min, 90 mm / min, 80 mm / min or 70 mm / min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0031] In the present invention, it is preferred to control the speed of the first heat expansion treatment within a specific range, so that the advancement speed of the heat expansion mandrel can be controlled to allow the steel pipe to have sufficient heating time.

[0032] In the present invention, the temperature and speed during the first heat expansion treatment and the second heat expansion treatment generally fluctuate within a small range in actual working conditions, and the fluctuation value is generally ±10° C. or ±10 mm / min.

[0033] Preferably, the diameter change rate of the first heat expansion treatment is ≤1.8, for example, it can be 1.8, 1.7, 1.6, 1.5, 1.4, 1.3 or 1.2, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0034] Preferably, the first thermal expansion treatment adopts a PLC automatic temperature control system.

[0035] In the present invention, the first heat expansion process adopts a PLC automatic temperature control system, including: a far-infrared thermometer, a temperature and electrical signal exchanger, a DCR intelligent control core, an intermediate frequency heating device, a data recording, storage, printing and transmission module and a speed collector. At the set temperature, when the heat expansion core rod is inserted into the steel pipe for heat expansion processing, the actual temperature is collected by the far-infrared thermometer. When the voltage, current or other external factors cause the actual temperature to deviate from the set temperature, the temperature and electrical signal exchanger converts the temperature change signal into an electrical signal and transmits it to the DCR intelligent control core for calculation and analysis and issues instructions to control the intermediate frequency heating device so that the output power rises and falls with the change of temperature, thereby maintaining the constant temperature of the pipe expansion. The speed collector can also collect and set the pushing speed so that the pipe expansion process proceeds at a uniform speed. Among them, the end face of the heat expansion core rod is circular, which can further prevent the steel pipe from being out of round.

[0036] Preferably, the diameter of the induction heating coil of the first heat expansion treatment is greater than the diameter of the first steel pipe, and the difference between the two is ≤100mm, for example, it can be 80mm, 82mm, 85mm, 88mm, 90mm, 92mm, 95mm, 98mm or 100mm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0037] The present invention preferably controls the induction heating coil within a specific range, which can further control the temperature stability during heat expansion and save energy consumption.

[0038] Preferably, the temperature of the normalizing treatment in step (3) is 880-920°C, for example, it can be 880°C, 885°C, 890°C, 895°C, 900°C, 905°C, 910°C, 915°C or 920°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0039] Preferably, the holding time of the normalizing treatment is ≥30 min, for example, it can be 31 min, 32 min, 33 min, 34 min or 35 min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0040] Preferably, after the normalizing treatment in step (3), straightening treatment, shot blasting treatment and flattening treatment are sequentially performed to obtain the second steel pipe.

[0041] In the present invention, the straightening treatment is manual straightening, and its purpose is to adjust the shape of the steel pipe so that it does not affect the second heat expansion treatment. In the present invention, the shot blasting treatment is to remove the oxide scale on the inner and outer surfaces of the steel pipe, on the one hand to check the surface quality of the steel pipe, and on the other hand to reduce the risk of surface straightening during the second heat expansion treatment. In the present invention, the flat end treatment refers to straightening the steel pipe with uneven ends.

[0042] Preferably, the full-length straightness of the second steel pipe after the straightening treatment is ≤30 mm, for example, it can be 30 mm, 29 mm, 28 mm, 27 mm, 26 mm or 25 mm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0043] Preferably, the out-of-roundness of the second steel pipe after the straightening treatment is ≤20 mm, for example, it can be 20 mm, 19 mm, 18 mm, 17 mm, 16 mm or 15 mm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0044] Preferably, in step (4), the temperature of the second heat expansion treatment is greater than the temperature of the first heat expansion treatment.

[0045] Preferably, the temperature of the second thermal expansion treatment is 880-920°C, for example, it can be 885°C, 890°C, 895°C, 900°C, 905°C, 910°C, 915°C or 920°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0046] In the present invention, the temperature of the second heat expansion treatment is preferably controlled within a specific range, which can further avoid problems such as pits, out-of-roundness, deformation and bending of the steel pipe, thereby improving the mechanical properties of the steel pipe.

[0047] Preferably, the speed of the second thermal expansion treatment is ≤120 mm / min, for example, it can be 120 mm / min, 115 mm / min, 110 mm / min, 105 mm / min, 100 mm / min, 95 mm / min or 90 mm / min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0048] In the present invention, it is preferred to control the speed of the second heat expansion treatment within a specific range, which can further allow the steel pipe to have sufficient heating time.

[0049] Preferably, the diameter change rate of the second heat expansion treatment is ≤1.2, for example, it can be 1.2, 1.1, 1, 0.9, 0.8, 0.7, 0.6 or 0.5, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0050] In the present invention, the diameter change rate of the second heat expansion treatment is preferably controlled within a specific range, so that the steel pipe will not be greatly deformed and bent, and the out-of-roundness can be ensured to compensate for the deficiency of straightening.

[0051] Preferably, the second thermal expansion treatment adopts a PLC automatic temperature control system.

[0052] Preferably, the diameter of the induction heating coil subjected to the second heat expansion treatment is greater than the diameter of the finished product, and the difference between the two is ≤100 mm, for example, it can be 80 mm, 82 mm, 85 mm, 88 mm, 90 mm, 92 mm, 95 mm, 98 mm or 100 mm, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0053] Preferably, the diameter of the finished product in step (4) is ≥711 mm, for example, it can be 711 mm, 712 mm, 715 mm, 718 mm or 720 mm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0054] Preferably, the wall thickness of the finished product is ≤14 mm, for example, it can be 14 mm, 13 mm, 12 mm, 11 mm, 10 mm, 9 mm, 8 mm, 7 mm, 6 mm or 5 mm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0055] Preferably, after the second heat expansion treatment in step (4), physical and chemical property inspection, secondary straightening, secondary shot blasting and appearance inspection are also carried out to obtain a finished product.

[0056] In the present invention, the physical and chemical property inspection refers to the inspection of the chemical composition, mechanical properties and dimensions of the steel pipe. The secondary straightening refers to manual straightening again to ensure that the straightness of the steel pipe meets the standard requirements. The secondary shot blasting refers to shot blasting the inner and outer surfaces of the steel pipe again.

[0057] As a preferred technical solution of the present invention, the manufacturing method comprises the following steps:

[0058] (1) The tube blank is heated at ≥1200°C, then pierced and continuously rolled, the residual temperature of the formed steel pipe after continuous rolling is ≥880°C, and then air-cooled on a cooling bed to obtain the original tube;

[0059] The manufacturing process of the tube blank comprises the following steps: smelting the molten steel in a converter, refining it in a LF ladle, degassing it in a RH vacuum, and continuously casting it;

[0060] The chemical components contained in the tube blank, measured by mass percentage, include: C≤0.30%, S≤0.025%, P≤0.025%, 0.29%≤Mn≤1.35%, Si≥0.10%;

[0061] (2) subjecting the raw tube obtained in step (1) to a first heat expansion treatment at 720-800° C. using a PLC automatic temperature control system to obtain a first steel tube;

[0062] The speed of the first heat expansion treatment is ≤120 mm / min, the diameter change rate of the first heat expansion treatment is ≤1.8, the diameter of the induction heating coil of the first heat expansion treatment is greater than the diameter of the first steel pipe, and the difference between the two is ≤100 mm;

[0063] (3) normalizing the first steel pipe obtained in step (2) at 880-920° C., wherein the holding time of the normalizing treatment is ≥ 30 min, and then straightening, shot blasting and flattening are sequentially performed to obtain a second steel pipe;

[0064] (4) subjecting the second steel pipe obtained in step (3) to a second heat expansion treatment at 880-920° C. using a PLC automatic temperature control system, and then sequentially subjecting the steel pipe to physical and chemical property inspection, secondary straightening, secondary shot blasting, and appearance inspection to obtain a finished product; the diameter of the finished product is ≥711 mm, and the wall thickness of the finished product is ≤14 mm;

[0065] The speed of the second heat expansion treatment is ≤120mm / min, the diameter change rate of the second heat expansion treatment is ≤1.2, the diameter of the induction heating coil of the second heat expansion treatment is greater than the diameter of the finished product, and the difference between the two is ≤100mm.

[0066] Compared with the prior art, the present invention has the following beneficial effects:

[0067] (1) The manufacturing method provided by the present invention can effectively prevent the occurrence of pits, out-of-roundness, deformation, and bending problems in large-diameter thin-walled seamless steel pipes for low temperature use by sequentially performing a combination of a first heat expansion treatment, a normalizing treatment, and a second heat expansion treatment.

[0068] (2) The manufacturing method provided by the present invention is not restricted by straightening, can greatly improve production efficiency, and make the mechanical properties and dimensions of the steel pipe meet the required standards. The tensile strength of the finished product is ≥492MPa, the yield strength is ≥292MPa, the elongation after fracture is ≥36.0%, and the low-temperature impact value of -48°C is above 22J; under better conditions, the tensile strength is ≥498MPa, the yield strength is ≥304MPa, the elongation after fracture is ≥36.0%, and the low-temperature impact value of -48°C is above 64J. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Figure 1 Schematic diagram of a PLC automatic temperature control system in the first heat expansion process in Example 1 of the present invention;

[0070] Among them, 1- far-infrared thermometer; 2- temperature and electrical signal exchanger; 3- DCR intelligent control core; 4- medium frequency heating device; 5- data recording, storage, printing and transmission module; 6- speed collector; 7- induction heating coil; 8- original tube; 9- hot expansion core rod. DETAILED DESCRIPTION

[0071] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0072] Example 1

[0073] This embodiment provides a method for manufacturing a large-diameter thin-walled seamless steel pipe for low temperature use, the manufacturing method comprising the following steps:

[0074] (1) The molten steel is subjected to converter smelting, LF ladle refining, RH vacuum degassing and continuous casting in sequence to obtain a tube billet; the material of the tube billet is ASME SA333 Gr.6; the chemical composition of the tube billet is: C is 0.14%, P is 0.011%, S is 0.002%, Mn is 1.25%, and Si is 0.32%; the gas content after RH vacuum degassing is: [H] is 1.5ppm, [O] is 20ppm, and [N] is 35ppm; the diameter of the tube billet is Φ500mm;

[0075] The heating is carried out in a ring furnace, and the heating temperature is controlled to be 1220℃, and then piercing and continuous rolling are carried out. The residual temperature of the formed steel pipe after continuous rolling is 885℃, and then it is air-cooled on a cooling bed to obtain the original pipe;

[0076] The specification of the original tube is Φ508×13.5mm; the inner and outer surfaces of the original tube are inspected by magnetic leakage flaw detection, and no defects such as cracks are found;

[0077] (2) After removing the impurities from the inner surface of the original tube obtained in step (1), spraying lubricant, performing a first heat expansion treatment at 740° C. (temperature fluctuation is ±10° C.) using a PLC automatic temperature control system, the speed of the first heat expansion treatment is 100 mm / min (fluctuation is ±10 mm / min), the diameter change rate of the first heat expansion treatment is 1.8 (the material of the mandrel is 2520, the specification is Φ890 mm), and the diameter of the induction heating coil of the first heat expansion treatment is 950 mm, thereby obtaining a first steel tube;

[0078] The specification of the first steel pipe is Φ914×11mm;

[0079] The schematic diagram of the PLC automatic temperature control system used in the first heat expansion process is as follows: Figure 1 As shown, from Figure 1It can be seen that the PLC automatic temperature control system includes: a far-infrared thermometer 1, a temperature and electrical signal exchanger 2, a DCR intelligent control core 3, an intermediate frequency heating device 4, a data recording, storage, printing and transmission module 5 and a speed collector 6. At the set temperature, when the heat expansion core rod 9 is inserted into the original tube 8 for the first heat expansion treatment, the actual temperature is collected by the far-infrared thermometer 1. When the voltage, current or other external factors cause the actual temperature to deviate from the set temperature, the temperature and electrical signal exchanger 2 converts the temperature change signal into an electrical signal and transmits it to the DCR intelligent control core 3 for calculation and analysis and sends instructions to the intermediate frequency heating device 4, so that the output power of the induction heating coil 7 rises and falls with the change of temperature, thereby keeping the expansion temperature constant. The speed collector 6 can also be used to collect and set the pushing speed so that the expansion process proceeds at a uniform speed.

[0080] (3) normalizing the first steel pipe obtained in step (2) at 900° C. for 30 min, and then straightening the first steel pipe until the straightness of the entire length is 30 mm and the out-of-roundness is 20 mm, and then the inner and outer surfaces are shot blasted and the two ends of the steel pipe are flattened to obtain a second steel pipe;

[0081] (4) After removing the impurities from the inner surface of the second steel pipe obtained in step (3), spraying lubricant, and performing a second heat expansion treatment at 890° C. (temperature fluctuation is ±10° C.) using a PLC automatic temperature control system, the speed of the second heat expansion treatment is 80 mm / min (fluctuation is ±10 mm / min), the diameter change rate of the second heat expansion treatment is 1.06 (the material of the heat expansion core rod is 2520, the specification is Φ941 mm), and the diameter of the induction heating coil of the second heat expansion treatment is 1020 mm;

[0082] Then the physical and chemical properties are tested, and then the secondary correction is carried out until the straightness of the entire length is 4mm, the straightness per meter is 0.5mm / m, and the out-of-roundness is 6mm. After that, the inner and outer surfaces are subjected to secondary shot blasting and appearance inspection to obtain the finished product;

[0083] The specifications of the finished product are Φ965×9.53mm and the length is 8.5m.

[0084] Example 2

[0085] This embodiment provides a method for manufacturing a large-diameter thin-walled seamless steel pipe for low temperature use, the manufacturing method comprising the following steps:

[0086] (1) The molten steel is subjected to converter smelting, LF ladle refining, RH vacuum degassing and continuous casting in sequence to obtain a tube billet; the material of the tube billet is ASME SA333 Gr.6; the chemical composition of the tube billet is: C is 0.14%, P is 0.011%, S is 0.002%, Mn is 1.25%, and Si is 0.32%; the gas content after RH vacuum degassing is: [H] is 1.5ppm, [O] is 20ppm, and [N] is 35ppm; the diameter of the tube billet is Φ500mm;

[0087] The heating is carried out in a ring furnace, and the heating temperature is controlled to be 1220℃, and then piercing and continuous rolling are carried out. The residual temperature of the formed steel pipe after continuous rolling is 880℃, and then it is air-cooled on a cooling bed to obtain the original pipe;

[0088] The specification of the original tube is Φ508×13.5mm; the inner and outer surfaces of the original tube are inspected by magnetic leakage flaw detection, and no defects such as cracks are found;

[0089] (2) After removing the impurities from the inner surface of the original tube obtained in step (1), spraying lubricant, performing a first heat expansion treatment at 760° C. (temperature fluctuation is ±10° C.) using a PLC automatic temperature control system, the speed of the first heat expansion treatment is 90 mm / min (fluctuation is ±10 mm / min), the diameter change rate of the first heat expansion treatment is 1.8 (the material of the mandrel is 2520, the specification is Φ890 mm), and the diameter of the induction heating coil of the first heat expansion treatment is 950 mm, thereby obtaining a first steel tube;

[0090] The specification of the first steel pipe is Φ914×11mm;

[0091] (3) normalizing the first steel pipe obtained in step (2) at 880° C. for 30 min, and then straightening the first steel pipe until the straightness of the entire length is 28 mm and the out-of-roundness is 16 mm, and then the inner and outer surfaces are shot blasted and the ends of the steel pipe are flattened to obtain a second steel pipe;

[0092] (4) After removing the impurities from the inner surface of the second steel pipe obtained in step (3), spraying lubricant, and performing a second heat expansion treatment at 900° C. (temperature fluctuation is ±10° C.) using a PLC automatic temperature control system, the speed of the second heat expansion treatment is 70 mm / min (fluctuation is ±10 mm / min), the diameter change rate of the second heat expansion treatment is 1.06 (the material of the heat expansion core rod is 2520, the specification is Φ941 mm), and the diameter of the induction heating coil of the second heat expansion treatment is 1020 mm;

[0093] Then the physical and chemical properties are tested, and then the secondary correction is carried out until the straightness of the entire length is 4.5mm, the straightness per meter is 0.5mm / m, and the out-of-roundness is 5.5mm. After that, the inner and outer surfaces are subjected to secondary shot blasting and appearance inspection to obtain the finished product;

[0094] The specifications of the finished product are Φ965×9.53mm and the length is 8.67m.

[0095] Example 3

[0096] This embodiment provides a method for manufacturing a large-diameter thin-walled seamless steel pipe for low temperature use, the manufacturing method comprising the following steps:

[0097] (1) The molten steel is subjected to converter smelting, LF ladle refining, RH vacuum degassing and continuous casting in sequence to obtain a tube billet; the material of the tube billet is ASME SA333 Gr.6; the chemical composition of the tube billet is: C is 0.14%, P is 0.011%, S is 0.002%, Mn is 1.25%, and Si is 0.32%; the gas content after RH vacuum degassing is: [H] is 1.5ppm, [O] is 20ppm, and [N] is 35ppm; the diameter of the tube billet is Φ500mm;

[0098] The heating is carried out in a ring furnace, and the heating temperature is controlled to be 1220℃, and then piercing and continuous rolling are carried out. The residual temperature of the formed steel pipe after continuous rolling is 880℃, and then it is air-cooled on a cooling bed to obtain the original pipe;

[0099] The specification of the original tube is Φ508×13.5mm; the inner and outer surfaces of the original tube are inspected by magnetic leakage flaw detection, and no defects such as cracks are found;

[0100] (2) After removing the impurities from the inner surface of the original tube obtained in step (1), spraying lubricant, performing a first heat expansion treatment at 790° C. (temperature fluctuation is ±10° C.) using a PLC automatic temperature control system, the speed of the first heat expansion treatment is 80 mm / min (fluctuation is ±10 mm / min), the diameter change ratio of the first heat expansion treatment is 1.8 (the material of the mandrel is 2520, the specification is Φ890 mm), and the diameter of the induction heating coil of the first heat expansion treatment is 950 mm, thereby obtaining a first steel tube;

[0101] The specification of the first steel pipe is Φ914×11mm;

[0102] (3) normalizing the first steel pipe obtained in step (2) at 920° C. for 30 min, and then straightening the first steel pipe until the straightness of the entire length is 28 mm and the out-of-roundness is 18 mm, and then the inner and outer surfaces are shot blasted and the ends of the steel pipe are flattened to obtain a second steel pipe;

[0103] (4) After removing the impurities from the inner surface of the second steel pipe obtained in step (3), spraying lubricant, and performing a second heat expansion treatment at 910° C. (temperature fluctuation is ±10° C.) using a PLC automatic temperature control system, the speed of the second heat expansion treatment is 60 mm / min (fluctuation is ±10 mm / min), the diameter change rate of the second heat expansion treatment is 1.06 (the material of the heat expansion core rod is 2520, the specification is Φ941 mm), and the diameter of the induction heating coil of the second heat expansion treatment is 1020 mm;

[0104] Then the physical and chemical properties are tested, and then the secondary correction is carried out until the straightness of the entire length is 6mm, the straightness per meter is 0.5mm / m, and the out-of-roundness is 5mm. After that, the inner and outer surfaces are subjected to secondary shot blasting and appearance inspection to obtain the finished product;

[0105] The specifications of the finished product are Φ965×9.53mm and the length is 8.47m.

[0106] Example 4

[0107] This embodiment provides a method for manufacturing a large-diameter thin-walled seamless steel pipe for low temperature use. The only difference compared with Example 1 is that the speed of the second heat expansion treatment is controlled to 160 mm / min (fluctuation is ±10 mm / min).

[0108] Example 5

[0109] This embodiment provides a method for manufacturing a large-diameter thin-walled seamless steel pipe for low temperature use. The difference compared with Example 1 is that the specifications of the finished product are guaranteed to remain unchanged, and the diameter change rate of the second heat expansion treatment is adjusted to be equal to the diameter change rate of the first heat expansion treatment and both are 1.38, that is, the first heat expansion treatment is to Φ700×12.6mm, and the second heat expansion treatment is to Φ965×9.53mm.

[0110] Example 6

[0111] This embodiment provides a method for manufacturing a large-diameter thin-walled seamless steel pipe for low temperature use. The only difference compared with Example 1 is that the temperature of the second heat expansion treatment is controlled to 840°C (temperature fluctuation is ±10°C).

[0112] Example 7

[0113] This embodiment provides a method for manufacturing a large-diameter thin-walled seamless steel pipe for low temperature use. The only difference compared with Example 1 is that the temperature of the second heat expansion treatment is controlled to 960°C (temperature fluctuation is ±10°C).

[0114] Comparative Example 1

[0115] This comparative example provides a method for manufacturing a large-diameter thin-walled seamless steel pipe for low temperature use. The only difference compared with Example 1 is that the diameter change rate of the first heat expansion treatment is controlled to 1.89 so as to obtain a finished product with the same specifications as Example 1, and the second heat expansion treatment is not performed. The second steel pipe obtained is subjected to physical and chemical property inspection, secondary straightening, secondary shot blasting and appearance inspection to obtain a finished product.

[0116] The steel pipe obtained in this comparative example has pits, and the out-of-roundness and straightness cannot meet the required standards, and the appearance dimensions are unqualified.

[0117] The performance of the second steel pipe after normalizing treatment in Example 1-7 was tested (the -48°C low temperature impact value is the result of three measurements), and the results are shown in Table 1.

[0118] The finished products in Examples 1-7 were subjected to performance testing (wherein the -48°C low temperature impact value is the result of three measurements), and the results are shown in Table 2.

[0119] The finished products in Examples 1-7 were inspected for appearance and dimensions, and the results are shown in Table 3.

[0120] Table 1

[0121]

[0122] Table 2

[0123]

[0124]

[0125] Table 3

[0126]

[0127]

[0128] Where D is the nominal outer diameter and S is the nominal wall thickness.

[0129] From Table 1-3, we can see the following points:

[0130] (1) It can be seen from the data of Examples 1-7 that the tensile strength of the finished product obtained by the manufacturing method provided by the present invention is ≥492MPa, the yield strength is ≥292MPa, the elongation after fracture is ≥36.0%, and the low-temperature impact value of -48°C is above 22J; under better conditions, the tensile strength is ≥498MPa, the yield strength is ≥304MPa, the elongation after fracture is ≥36.0%, and the low-temperature impact value of -48°C is above 64J. The mechanical properties and appearance dimensions meet the standard requirements. After the second heat expansion treatment, the impact performance of the steel pipe decreases very little compared to that after normalizing treatment, and the values ​​are uniform.

[0131] (2) A comprehensive comparison of the data of Example 1 and Example 4 shows that the speed of the second heat expansion treatment in Example 1 is 80 mm / min, compared with the speed of the second heat expansion treatment in Example 4 of 160 mm / min. The -48°C low-temperature impact value in Example 1 is significantly higher than that in Example 4, and the wall thickness of the steel pipe in Example 1 meets the standard requirements, while the lower limit of the local wall thickness in Example 4 exceeds the standard requirements, and the impact value of the steel pipe decreases significantly. This shows that the preferred control of the speed of the second heat expansion treatment in the present invention can further improve the mechanical properties of the steel pipe and make the appearance size of the steel pipe meet the requirements.

[0132] (3) A comprehensive comparison of the data of Example 1 and Example 5 shows that the difference between Example 5 and Example 1 is that the diameter change rate of the second heat expansion treatment is equal to the diameter change rate of the first heat expansion treatment and both are 1.38. The -48°C low-temperature impact value in Example 5 is significantly lower than that in Example 1. This shows that the present invention preferably controls the diameter change rate of the second heat expansion treatment, which can further improve the mechanical properties of the steel pipe.

[0133] (4) Comprehensive comparison of the data of Example 1 and Example 6-7 shows that the temperature of the second heat expansion treatment in Example 1 is 890°C (temperature fluctuation is ±10°C), compared with 840°C (temperature fluctuation is ±10°C) and 960°C (temperature fluctuation is ±10°C) in Example 6-7, respectively. The mechanical properties in Example 1 are significantly higher than those in Example 6, and the appearance dimensions in Example 1 can better meet the standard requirements than those in Example 7, which shows that when the temperature of the second heat expansion treatment is low, the mechanical properties of the steel pipe are greatly reduced; when the temperature of the second heat expansion treatment is high, although the high mechanical properties can be maintained, due to the high temperature, large diameter of the steel pipe and thin wall thickness, the steel pipe will be warped during the heat expansion process, and the appearance will show slight (about 8mm height) wavy ring patterns, resulting in unqualified appearance dimensions. It can be seen that the preferred control of the temperature of the second heat expansion treatment in the present invention can effectively improve the mechanical properties of the steel pipe, while further ensuring that the appearance dimensions meet the requirements.

[0134] (5) A comprehensive comparison of the data of Example 1 and Comparative Example 1 shows that the only difference between Comparative Example 1 and Example 1 is that the second heat expansion treatment is not performed, and only the first heat expansion treatment is used and the diameter change rate of the first heat expansion treatment is adjusted to 1.89, so as to obtain a finished product with the same specifications as Example 1. However, pits appear in the finished product of Comparative Example 1, and the out-of-roundness and straightness cannot meet the required standards, and the appearance dimensions are unqualified. This shows that the manufacturing method provided by the present invention can effectively prevent the occurrence of pits, out-of-roundness, deformation, and bending in the steel pipe.

[0135] In summary, the manufacturing method provided by the present invention can effectively prevent problems such as pits, out-of-roundness, deformation and bending in large-diameter thin-walled seamless steel pipes for low temperature use, and the mechanical properties and dimensions meet the required standards.

[0136] The applicant declares that the above is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention shall fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for manufacturing a large-diameter thin-walled seamless steel pipe for low temperature use, characterized in that: The manufacturing method comprises the following steps: (1) The tube blank is heated at ≥1200°C, then pierced and continuously rolled. The residual temperature of the formed steel pipe after continuous rolling is ≥880°C, and then air-cooled on a cooling bed to obtain the original tube; (2) subjecting the raw tube obtained in step (1) to a first heat expansion treatment at 720-800° C. using a PLC automatic temperature control system to obtain a first steel tube, wherein the speed of the first heat expansion treatment is ≤120 mm / min, and the diameter change ratio of the first heat expansion treatment is ≤1.8; (3) normalizing the first steel pipe obtained in step (2) at 880-920° C., wherein the holding time of the normalizing treatment is ≥ 30 min, and then straightening, shot blasting and flattening are sequentially performed to obtain a second steel pipe; (4) subjecting the second steel pipe obtained in step (3) to a second heat expansion treatment at 880-920° C. using a PLC automatic temperature control system to obtain a finished product; The diameter of the finished product is ≥711 mm, and the wall thickness of the finished product is ≤14 mm; The speed of the second heat expansion treatment is ≤120 mm / min, and the diameter change rate of the second heat expansion treatment is ≤1.2; Step (4) The temperature of the second heat expansion treatment is greater than the temperature of the first heat expansion treatment.

2. The manufacturing method according to claim 1, characterized in that: The chemical components contained in the tube blank in step (1) by weight percentage include: C≤0.30%, S≤0.025%, P≤0.025%, 0.29%≤Mn≤1.35%, Si≥0.10%.

3. The manufacturing method according to claim 1, characterized in that: The manufacturing process of the tube blank comprises the following steps: sequentially subjecting molten steel to converter smelting, LF ladle refining, RH vacuum degassing and continuous casting.

4. The manufacturing method according to claim 1, characterized in that: The diameter of the induction heating coil of the first heat expansion treatment is greater than the diameter of the first steel pipe, and the difference between the two is ≤100 mm.

5. The manufacturing method according to claim 1, characterized in that: The straightness of the entire length of the second steel pipe after the straightening treatment is ≤30 mm.

6. The manufacturing method according to claim 1, characterized in that: The out-of-roundness of the second steel pipe after the straightening treatment is ≤20 mm.

7. The manufacturing method according to claim 1, characterized in that: The diameter of the induction heating coil subjected to the second heat expansion treatment is greater than the diameter of the finished product, and the difference between the two is ≤100 mm.

8. The manufacturing method according to claim 1, characterized in that: Step (4) After the second heat expansion treatment, physical and chemical property inspection, secondary straightening, secondary shot blasting and appearance inspection are also carried out to obtain the finished product.

9. The manufacturing method according to any one of claims 1 to 8, characterized in that: The manufacturing method comprises the following steps: (1) The tube blank is heated at ≥1200°C, then pierced and continuously rolled. The residual temperature of the formed steel pipe after continuous rolling is ≥880°C, and then air-cooled on a cooling bed to obtain the original tube; The manufacturing process of the tube blank comprises the following steps: smelting the molten steel in a converter, refining it in a LF ladle, degassing it in a RH vacuum, and continuously casting it; The chemical components contained in the tube blank, calculated by mass percentage, include: C≤0.30%, S≤0.025%, P≤0.025%, 0.29%≤Mn≤1.35%, Si≥0.10%; (2) subjecting the raw tube obtained in step (1) to a first heat expansion treatment at 720-800° C. using a PLC automatic temperature control system to obtain a first steel tube; The speed of the first heat expansion treatment is ≤120 mm / min, the diameter change rate of the first heat expansion treatment is ≤1.8, the diameter of the induction heating coil of the first heat expansion treatment is greater than the diameter of the first steel pipe, and the difference between the two is ≤100 mm; (3) normalizing the first steel pipe obtained in step (2) at 880-920° C., wherein the holding time of the normalizing treatment is ≥ 30 min, and then straightening, shot blasting and flattening are sequentially performed to obtain a second steel pipe; (4) subjecting the second steel pipe obtained in step (3) to a second heat expansion treatment at 880-920° C. using a PLC automatic temperature control system, and then sequentially subjecting the second steel pipe to physical and chemical property inspection, secondary straightening, secondary shot blasting, and appearance inspection to obtain a finished product; the diameter of the finished product is ≥711 mm, and the wall thickness of the finished product is ≤14 mm; The speed of the second heat expansion treatment is ≤120mm / min, the diameter change rate of the second heat expansion treatment is ≤1.2, the diameter of the induction heating coil of the second heat expansion treatment is greater than the diameter of the finished product, and the difference between the two is ≤100mm.

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