A 15NiCuMoNb5-6-4 steel pipe and a method for manufacturing the same

Abstract

The application provides a 15NiCuMoNb5-6-4 steel pipe and a preparation method thereof, and relates to the technical field of heat-resistant steel pipe manufacturing.The chemical composition and mass percentage of the steel pipe are as follows: C: 0.11%-0.16%, Si: 0.25%-0.45%, Mn: 0.90%-1.20%, Cr: 0.15%-0.30%, Mo: 0.25%-0.40%, Ni: 1.00%-1.30%, Cu: 0.50%-0.70%, Nb: 0.015%-0.035%, Alt: 0.015%-0.035%, P≤0.015%, S≤0.005%, N≤0.020%, and the rest is Fe and inevitable impurities; wherein, Ni / Cu≥2, and preferably, the Ni / Cu value ranges from 2.0 to 2.2.The application obtains a steel pipe with excellent comprehensive performance.

Description

Technical Field

[0001] This invention relates to the field of heat-resistant steel pipe manufacturing technology, and in particular to a 15NiCuMoNb5-6-4 steel pipe and its preparation method. Background Technology

[0002] 15NiCuMoNb5-6-4 is a Ni-Mo-Cu type low-alloy high-strength steel. Copper precipitation strengthening improves the material's strength and corrosion resistance, while adding a certain amount of Ni eliminates the high-temperature red brittleness tendency caused by the presence of copper. Simultaneously, the addition of a certain amount of dispersed Nb further refines the grain size, improving the material's overall performance. This material operates at 450℃ and is mainly used in medium- and high-temperature components such as feedwater pipelines, start-up steam-water separators, and water-cooled pipe headers in high-parameter thermal power units and nuclear power units.

[0003] As a type of Cu-containing steel pipe used in the main feedwater pipeline of supercritical power plant boilers, in addition to ensuring the strength indicators specified in the standards, it is also necessary to maintain excellent impact toughness.

[0004] Within the standard-specified composition range (C: ≤0.17%, Si: 0.25%~0.50%, Mn: 0.80%~1.20%, Cr: ≤0.30%, Mo: 0.25%~0.50%, Ni: 1.00%~1.30%, Cu: 0.50%~0.80%, Nb: 0.015%~0.045%, Alt: ≤0.050%, P≤0.025%, S≤0.010%, the remainder being Fe and unavoidable impurities), with a Cu content of 0.50%~0.80%, and using the standard-specified heat treatment process, with normalizing temperature of 880℃~980℃ and air cooling; and tempering temperature of 580℃~680℃ and air cooling, the resulting steel pipe exhibits large fluctuations in impact energy and fails to meet impact energy standards. When the heat treatment process is not properly selected, the hardness fluctuates significantly, especially for steel pipes with a wall thickness ≥30mm.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a 15NiCuMoNb5-6-4 steel pipe and its preparation method. Through reasonable composition design and the use of ASSEL unit to manufacture the steel pipe, the invention adopts a normalizing + tempering heat treatment process, which can improve the comprehensive performance of the material. Practice has proved that the steel pipe provided by this invention has high strength, good and stable impact toughness, and uniform hardness, thus obtaining a steel pipe with excellent comprehensive performance.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a 15NiCuMoNb5-6-4 steel pipe, wherein the chemical composition and mass percentage of the steel pipe are as follows: C: 0.11%–0.16%, Si: 0.25%–0.45%, Mn: 0.90%–1.20%, Cr: 0.15%–0.30%, Mo: 0.25%–0.40%, Ni: 1.00%–1.30%, Cu: 0.50%–0.70%, Nb: 0.015%–0.035%, Alt: 0.015%–0.035%, P≤0.015%, S≤0.005%, N≤0.020%, with the remainder being Fe and unavoidable impurities; Wherein, Ni / Cu≥2, preferably Ni / Cu values ​​in the range of 2.0 to 2.2.

[0008] Furthermore, the thickness of the steel pipe is 30mm to 90mm.

[0009] Furthermore, the outer diameter × wall thickness of the steel pipe is (108~530)mm × (30~90)mm.

[0010] In a second aspect, the present invention provides a method for preparing the 15NiCuMoNb5-6-4 steel pipe described in the first aspect, wherein the following steps are performed using a continuously cast billet as the billet: rolling, normalizing and tempering heat treatment.

[0011] Furthermore, the rolling process uses an ASSEL mill to roll the steel pipe, including billet heating, piercing, ASSEL mill rolling, and sizing. The heating temperature of the billet is 1230℃~1270℃, and the temperature before piercing and rolling is not lower than 1150℃.

[0012] Furthermore, the total reduction of the tube blank during the piercing deformation is controlled within 15%, the wall thickness reduction of the ASSEL mill is ≤10mm, the total reduction rate of the reducing mill is ≤4%, and the final rolling temperature is not lower than 900℃.

[0013] Furthermore, in the heat treatment process, the holding temperature during the normalizing process is 910℃~950℃, and the holding time is 1.5min / mm~2.5min / mm depending on the wall thickness of the steel pipe. After exiting the furnace, the steel pipe is cooled with water mist to a surface temperature of 120℃~300℃, and then air-cooled to room temperature.

[0014] Furthermore, the cooling rate of the water mist cooling is 60℃ / min to 120℃ / min.

[0015] Furthermore, in the heat treatment process, the holding temperature during tempering is 630℃~650℃, and the holding time is 3min / mm~4min / mm depending on the steel pipe wall thickness. After exiting the furnace, the steel pipe is air-cooled to room temperature.

[0016] The 15NiCuMoNb5-6-4 steel pipe described in the first aspect of the present invention or the 15NiCuMoNb5-6-4 steel pipe prepared by the preparation method in the second aspect of the present invention has a yield strength ≥480MPa, a room temperature tensile strength of 620MPa~750MPa, an impact absorption energy of ≥100J at 20℃, a hardness of HBW200-250, a microstructure of tempered bainite + ferrite, and a grain size ≥5.

[0017] The present invention provides a 15NiCuMoNb5-6-4 steel pipe and its preparation method, the beneficial effects of which are: (1) By controlling the ratio of Ni to Cu, the presence of Cu solid solution is minimized, and the adverse effects of Cu on microstructure and properties are eliminated; at the same time, the grain-refining element Al is added in appropriate amounts to maintain the fine grain size of steel, improve toughness, and achieve stable impact energy; and Cr is added quantitatively to improve the corrosion resistance and high-temperature oxidation resistance of steel pipes. (2) The steel pipes were manufactured using the ASSEL unit and the heat treatment process of normalizing and tempering was adopted. Water mist cooling was used for normalizing, and the cooling water flow rate was controlled at 80m³ / h to 160m³ / h and the final temperature of the steel pipe mist cooling was 120℃ to 300℃. The adoption of these measures greatly improved the strength and impact toughness of the material, and the material hardness was uniform, resulting in steel pipes with excellent comprehensive performance. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 The metallographic structure diagram of the steel pipe provided in the embodiment of the present invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0021] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0022] According to a first aspect of the present invention, the present invention provides a 15NiCuMoNb5-6-4 steel pipe, wherein the chemical composition and mass percentage of the steel pipe are as follows: C: 0.11%~0.16%, Si: 0.25%~0.45%, Mn: 0.90%~1.20%, Cr: 0.15%~0.30%, Mo: 0.25%~0.40%, Ni: 1.00%~1.30%, Cu: 0.50%~0.70%, Nb: 0.015%~0.035%, Alt: 0.015%~0.035%, P≤0.015%, S≤0.005%, N≤0.020%, with the remainder being Fe and unavoidable impurities; Wherein, Ni / Cu ≥ 2, preferably a Ni / Cu value range of 2.0 to 2.2 (e.g., 2.01, 2.02, 2.03, 2.04, 2.05, 2.06, 2.07, 2.08, 2.09, 2.10, 2.11, 2.12, 2.13, 2.14, 2.15, 2.16, 2.17, 2.18, 2.19). Ni / Cu refers to the ratio of the mass content of Ni to the mass content of Cu in the steel pipe.

[0023] The steel pipe described in this invention has a Cu content of 0.50% to 0.70% by mass. Although adding a certain amount of Ni can eliminate the tendency for high-temperature red brittleness caused by the presence of copper, improper proportions and inappropriate heat treatment processes can still negatively impact performance, such as unacceptable impact energy and large fluctuations in hardness. By combining Ni and Cu and controlling the Ni / Cu ratio within a suitable range, the presence of Cu solid solution is minimized, eliminating the adverse effects of Cu on microstructure and properties. Simultaneously, an appropriate amount of Al, a grain-refining element, is added to refine the grain size of the steel pipe, improve its toughness, and stabilize the impact energy.

[0024] The addition of Cr forms a dense chromium oxide protective layer, effectively preventing oxygen and corrosive media from further contacting the interior of the steel pipe and delaying high-temperature oxidation reactions. Cr also forms a stable Cr2O3 protective film on the steel surface, preventing oxygen and acidic media from corroding the substrate. Especially in high-temperature and high-pressure scenarios such as main steam pipes in nuclear power plants, increasing the Cr content can significantly reduce the rate of flow-accelerated corrosion. Simultaneously, Cr and Mo work synergistically to maintain good mechanical properties above 400℃, meeting the requirements for long-term operation of the steel pipe. Therefore, the quantitative addition of Cr is necessary, but it must not exceed the upper limit of 0.30% specified in the EN10216-2:2013 standard.

[0025] As an optional embodiment of the 15NiCuMoNb5-6-4 steel pipe of the present invention, the thickness of the steel pipe is 30mm to 90mm (e.g., 40mm, 50mm, 60mm, 70mm, 80mm).

[0026] As an optional embodiment of the 15NiCuMoNb5-6-4 steel pipe of the present invention, the outer diameter × wall thickness of the steel pipe is (108~530)mm × (30~90)mm.

[0027] In this invention, the steel pipe specifications are within the range of (108~530)mm×(30~90)mm in outer diameter × wall thickness. In actual demand, there is no steel pipe with a wall thickness greater than 90mm. If steel pipe with a thickness greater than 90mm is required, this method is also applicable.

[0028] According to a second aspect of the present invention, the present invention provides a method for preparing the 15NiCuMoNb5-6-4 steel pipe described in the first aspect, wherein the following steps are performed using continuously cast billets as raw materials: rolling, normalizing and tempering heat treatment.

[0029] This invention controls the composition of 15NiCuMoNb5-6-4 steel pipe, uses steel pipe manufactured by ASSEL unit, and adopts a unique heat treatment process to improve the comprehensive performance of the material. Practice has proven that this method is effective, greatly improving the strength and impact toughness of the material, and also producing uniform hardness, resulting in steel pipe with excellent comprehensive performance.

[0030] As an optional embodiment of the preparation method of the 15NiCuMoNb5-6-4 steel pipe of the present invention, the rolling is carried out by rolling the steel pipe using an ASSEL mill, including billet heating, piercing, ASSEL mill rolling, and sizing. The homogenization temperature of the billet heating is 1230℃~1270℃ (e.g., 1232℃, 1234℃, 1236℃, 1238℃, 1240℃, 1242℃, 1244℃, 1246℃, 1248℃, 1250℃, 1252℃, 1254℃, 1256℃, 1258℃, 1260℃, 1262℃, 1264℃, 1266℃, 1268℃), and the temperature before piercing and rolling is not lower than 1150℃ (e.g., 1155℃, 1160℃, 1165℃, 1170℃, etc.).

[0031] In this invention, if the homogenization temperature of the billet before rolling is too high, it will easily produce overheated structure; if the temperature is too low, it will increase the resistance to processing deformation and increase the manufacturing difficulty.

[0032] As an optional embodiment of the preparation method of the 15NiCuMoNb5-6-4 steel pipe of the present invention, the total reduction of the billet (the billet during piercing deformation) during the rolling process is controlled within 15%, the wall thickness reduction of the ASSEL rolling mill is ≤10mm, the total diameter reduction rate of the reducing mill is ≤4%, and the final rolling temperature is not lower than 900℃ (e.g., 910℃, 920℃, etc.). Here, the total reduction is the ratio of the billet reduction value in the piercing process to that of the round billet; the final rolling temperature is the temperature after the hot rolling process is completed and the diameter is set.

[0033] As an optional embodiment of the preparation method of the 15NiCuMoNb5-6-4 steel pipe of the present invention, in the heat treatment process, the holding temperature during the normalizing process is 910℃~950℃ (e.g., 912℃, 914℃, 916℃, 918℃, 920℃, 922℃, 924℃, 926℃, 928℃, 930℃, 932℃, 934℃, 936℃, 938℃, 940℃, 942℃, 944℃, 946℃, 948℃), and the holding time is 1.5 min / mm~2.5 min / mm according to the steel pipe wall thickness (e.g., 1.6 min / mm, 1.7 min / mm, 1.8 min / mm, 1.9 min / mm, 2.0 min / mm, 2.1 min / mm, 2.2 min / mm, 2.3 min / mm, 2.4 min / mm). After exiting the furnace, the steel pipe is cooled with water mist to a surface temperature of 120℃~300℃ (e.g., 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃), and then air-cooled to room temperature.

[0034] When the holding temperature during the normalizing process is too low, austenitization will be insufficient for steel pipes with a wall thickness of 30mm or more, resulting in incomplete microstructure transformation after normalizing. Simultaneously, the temperature must be controlled to prevent excessively high temperatures and grain growth. Water mist cooling, with controlled cooling termination temperature, must ensure the final temperature is below the Ms point to guarantee sufficient microstructure transformation (austenite to bainite). Furthermore, a certain amount of pre-existing temperature must be reserved to reduce microstructure stress and prevent stress cracking.

[0035] As an optional embodiment of the preparation method of the 15NiCuMoNb5-6-4 steel pipe of the present invention, the cooling rate of the water mist cooling is 60℃ / min~120℃ / min (e.g. 70℃ / min, 80℃ / min, 90℃ / min, 100℃ / min, 110℃ / min); this is to control the microstructure, ensure the comprehensive performance of the material, and control the bainite content to be above 40%; for steel pipes with a wall thickness of 30mm, normalizing requires accelerated cooling, but it should not be too fast to avoid the formation of tempered sorbite.

[0036] As an optional embodiment of the preparation method of the 15NiCuMoNb5-6-4 steel pipe of the present invention, in the heat treatment process, the holding temperature during tempering is 630℃~650℃ (e.g., 632℃, 634℃, 636℃, 638℃, 640℃, 642℃, 644℃, 646℃, 648℃), the holding time is 3~4 min / mm (e.g., 3.1 min / mm, 3.2 min / mm, 3.3 min / mm, 3.4 min / mm, 3.5 min / mm, 3.6 min / mm, 3.7 min / mm, 3.8 min / mm, 3.9 min / mm), and after exiting the furnace, it is air-cooled to room temperature.

[0037] In this invention, the lower limit of the holding temperature during the tempering process can ensure that the hardness of the bainitic structure is reduced, so that the copper precipitates are in a fine-particle state; however, the holding temperature should not be too high, so as to ensure that the strength index of the material is within a reasonable range.

[0038] The 15NiCuMoNb5-6-4 steel pipe described in the first aspect of the present invention or the 15NiCuMoNb5-6-4 steel pipe prepared by the preparation method in the second aspect has a yield strength ≥480MPa, a room temperature tensile strength of 620MPa~750MPa, an impact absorption energy of ≥100J at 20℃ (preferably ≥150J, 180J, 200J, 220J), a hardness of HBW of 200-250, a microstructure of tempered bainite + ferrite, and a grain size ≥5.

[0039] The present invention will be further described in detail below with reference to specific embodiments and comparative examples. The steel pipes prepared in the embodiments and comparative examples have a specification (outer diameter × wall thickness) of 508 mm × 60 mm. The chemical composition and mass percentage of the steel pipes of Examples 1 and 2 or the continuously cast billets used to produce steel pipes are shown in Table 1, with the balance being Fe. The continuously cast billets used to produce steel pipes are obtained by conventional methods in the art, and will not be described in detail here.

[0040] Table 1

[0041] Example 1 (1) The steel pipe is rolled using the ASSEL mill, including billet heating, piercing, ASSEL mill rolling, and sizing.

[0042] During the rolling process of the ASSEL unit, the homogenization temperature of the billet is 1260℃, and the temperature before piercing and rolling is 1160℃. The rough tube obtained after piercing has an outer diameter of 550.2mm × 67mm, the rolled tube has an outer diameter of 522mm × 60mm, and the sizing tube has an outer diameter of 508mm × 60mm.

[0043] During the piercing deformation process of the ASSEL rolling mill, the total reduction of the billet is 9%, the wall thickness reduction of the ASSEL rolling mill is 7mm, the total diameter reduction is 2.7%, and the final rolling temperature is 950℃.

[0044] (2) The steel pipe after being rolled by ASSEL is sent into a walking beam normalizing furnace for normalizing heat treatment. The normalizing temperature is 910℃ and the holding time is 120min. After exiting the furnace, the steel pipe is cooled by water mist at a rate of 95℃ / min. The water mist cooling is controlled to cool the surface temperature of the steel pipe to 200℃, and then air-cooled to room temperature.

[0045] (3) The steel pipe cooled to room temperature is sent into a walking beam tempering furnace for heating, the tempering temperature is controlled at 630℃, the holding time is 190min, and after it is taken out of the furnace, it is air cooled to room temperature. (4) Obtain products with good overall performance.

[0046] Example 2 The difference between this embodiment and Embodiment 1 is that the normalizing temperature is 950°C and the tempering temperature is 650°C.

[0047] Comparative Example 1 The main difference between this comparative example and Example 1 is that the Ni / Cu ratio is controlled to be 1.8.

[0048] The chemical composition and mass percentage of the steel pipe or the continuously cast billet used to produce the steel pipe are shown in Table 2, with the balance being Fe. Table 2

[0049] Comparative Example 2 The main difference between this comparative example and Example 1 is that the Ni / Cu ratio is controlled to be 2.4.

[0050] The chemical composition and mass percentage of the steel pipe or the continuously cast billet used to produce the steel pipe are shown in Table 3, with the balance being Fe. Table 3

[0051] Comparative Example 3 The main difference between this comparative example and Example 1 is that the Alt content is 0.05 wt.%.

[0052] The chemical composition and mass percentage of the steel pipe or the continuously cast billet used to produce the steel pipe are shown in Table 4, with the balance being Fe. Table 4

[0053] Comparative Example 4 The only difference between this comparative example and Example 1 is that water mist cooling is used for normalizing, and the water mist cooling is controlled to the surface temperature of the steel pipe is 100°C.

[0054] Comparative Example 5 The only difference between this comparative example and Example 1 is that the tempering temperature is 670°C.

[0055] Comparative Example 6 The only difference between this comparative example and Example 1 is that the Cr content is 0.10 wt.%.

[0056] Comparative Example 7 The only difference between this comparative example and Example 1 is that the normalizing temperature is 970°C.

[0057] Comparative Example 8 The only difference between this comparative example and Example 1 is that the normalizing temperature is 900°C.

[0058] Comparative Example 9 The only difference between this comparative example and Example 1 is that the tempering temperature is 610°C.

[0059] Comparative Example 10 The only difference between this comparative example and Example 1 is that air cooling is used after normalizing to allow the surface temperature of the steel pipe to be cooled to room temperature.

[0060] The performance testing methods refer to the following standards: The standard for room temperature tensile testing is EN ISO 6892-1; The standard for room temperature impact energy testing is EN ISO 148-1; The hardness testing standard is GB / T 231.1; The testing standard for the organization is GB / T 13298; The standard for grain size testing is GB / T 6394.

[0061] The steel pipes obtained in Examples 1 and 2 and Comparative Examples 1-10 were subjected to relevant performance tests using the above-mentioned performance testing methods. The test results are shown in Table 2.

[0062] Table 2

[0063] Among them, the data on the impact absorption energy at 20℃ are the test data of three different samples; the hardness data, from front to back, are the test data of the inner side, center and outer side of the steel pipe wall thickness section; the hardness standard value, microstructure requirements and grain size requirements are quoted from GB / T 5310-2023, and the grain size range shall not exceed 3.

[0064] Based on the data in the table above, it can be seen that when the Ni / Cu ratio is unsuitable, the impact energy of the material fluctuates significantly. In Comparative Example 1, when the Ni / Cu ratio is 1.8, the impact absorption energy at 20℃ is significantly reduced and fluctuates greatly. In Comparative Example 2, when the Ni / Cu ratio is 2.4, the impact absorption energy performance at 20℃ is improved, but the fluctuation is still relatively large, and the overall performance is not in the optimal range. Moreover, increasing the Ni / Cu value will also increase the manufacturing cost. In Comparative Example 3, when the Alt content is too high, the difficulty in controlling inclusions increases, leading to a decrease in the overall performance of the material. The impact absorption energy at 20℃ and the hardness value decrease significantly, and the surface quality of the steel pipe is also affected. In Comparative Example 4, lowering the normalizing water mist cooling temperature not only increases the risk of stress cracking in the steel pipe, but also increases the yield strength and tensile strength, decreases the plasticity, and causes tempered sorbite to appear in the microstructure. In Comparative Example 5, when water mist cooling is used, the tempering temperature is too high, resulting in a significant decrease in the yield strength and tensile strength of the material, and the hardness is lower than the standard requirement. In Comparative Example 6... Reducing Cr to 0.10% has no significant impact on the material's performance, but it reduces its oxidation resistance and service life. In Comparative Examples 7 and 8, the normalizing temperatures are within the standard heat treatment temperature range (880℃~980℃). In Comparative Example 7, increasing the normalizing temperature to 970℃ still allows the material's performance to meet the standard, but grain growth occurs, the range increases, and the impact absorption energy at 20℃ decreases and becomes unstable. In Comparative Example 8, reducing the normalizing temperature to 900℃ results in varying degrees of decrease in the material's strength and hardness, a slight decrease in plasticity, and a decrease in the impact absorption energy at 20℃. In Comparative Example 9, reducing the tempering temperature to 610℃ increases the material's yield strength and tensile strength, with the tensile strength exceeding the standard, but the plasticity deteriorates significantly, and the hardness increases significantly, even exceeding the standard range. In Comparative Example 10, air cooling after normalizing results in insufficient cooling intensity; although the steel pipe meets the standard requirements for plasticity, its tensile strength fails to meet the standard requirements, and its hardness is below the controlled lower limit.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a 15NiCuMoNb5-6-4 steel pipe, characterized in that, The following processes are performed on the continuously cast billet as the tube blank: rolling, normalizing and tempering heat treatment process; The chemical composition and mass percentage of the steel pipe are as follows: C: 0.11%–0.16%, Si: 0.25%–0.45%, Mn: 0.90%–1.20%, Cr: 0.15%–0.30%, Mo: 0.25%–0.40%, Ni: 1.00%–1.30%, Cu: 0.50%–0.70%, Nb: 0.015%–0.035%, Alt: 0.015%–0.035%, P≤0.015%, S≤0.005%, N≤0.020%, with the remainder being Fe and unavoidable impurities; The Ni / Cu ratio ranges from 2.0 to 2.

2. After normalizing, the steel pipe is cooled with water mist to a surface temperature of 120℃~300℃, and then air-cooled to room temperature; the cooling rate of water mist cooling is 60℃ / min~120℃ / min.

2. The method for preparing 15NiCuMoNb5-6-4 steel pipe according to claim 1, characterized in that, The thickness of the steel pipe is 30mm to 90mm.

3. The method for preparing 15NiCuMoNb5-6-4 steel pipe according to claim 1, characterized in that, The outer diameter × wall thickness of the steel pipe is (108~530)mm × (30~90)mm.

4. The method for preparing 15NiCuMoNb5-6-4 steel pipe according to claim 1, characterized in that, The rolling process uses an ASSEL mill to roll steel pipes, including billet heating, piercing, ASSEL mill rolling, and sizing. The billet heating temperature is 1230℃~1270℃, and the temperature before piercing and rolling is not lower than 1150℃.

5. The method for preparing the 15NiCuMoNb5-6-4 steel pipe according to claim 4, characterized in that, The total reduction of the billet during piercing is controlled within 15%, the wall thickness reduction of the ASSEL mill is ≤10mm, the total reduction rate of the reducing mill is ≤4%, and the final rolling temperature is not lower than 900℃.

6. The method for preparing 15NiCuMoNb5-6-4 steel pipe according to claim 1, characterized in that, In the heat treatment process, the holding temperature during the normalizing process is 910℃~950℃, and the holding time is 1.5min / mm~2.5min / mm depending on the steel pipe wall thickness.

7. The method for preparing 15NiCuMoNb5-6-4 steel pipe according to claim 1, characterized in that, In the heat treatment process, the holding temperature during tempering is 630℃~650℃, and the holding time is 3min / mm~4min / mm depending on the steel pipe wall thickness. After exiting the furnace, the pipe is air-cooled to room temperature.

8. The steel pipe prepared by the method for preparing 15NiCuMoNb5-6-4 steel pipe according to any one of claims 1-7, characterized in that, The steel pipe has a yield strength ≥480MPa, a room temperature tensile strength of 620MPa~750MPa, an impact absorption energy of ≥100J at 20℃, a hardness of HBW200-250, a microstructure of tempered bainite + ferrite, and a grain size ≥5.

Images