A method for manufacturing ultra-low carbon martensitic stainless steel tube blank

Abstract

The present invention relates to the technical field of martensitic stainless steel, and specifically discloses a method for manufacturing ultra-low carbon martensitic stainless steel tube billets. The present invention adopts a stacking slow cooling method to replace the traditional red-feeding process, which effectively reduces the risk of cracking of martensitic stainless steel ingots due to structural transformation and casting residual stress, and adds a grinding process after slow cooling to clean the defects of the ingot riser line, nozzle end and ingot body, and changes the surface treatment method before heating the ingot, which greatly improves the yield rate of the ingot, and the yield rate is increased to more than 80%. In addition, the present invention adopts a one-stage low-temperature annealing process. By controlling the specific annealing temperature and time, a lamellar martensite with a stable metallographic structure and uniformity can be obtained. The hardness of the ingot after annealing is moderate, and energy consumption is reduced while ensuring the performance and processing performance. Compared with the traditional two-stage annealing, the annealing cost is reduced by 150 to 260 yuan / ton, achieving the purpose of reducing costs and increasing efficiency, and has a high promotion and application value.

Description

Technical Field

[0001] The present invention relates to the technical field of martensitic stainless steel, and in particular to a method for manufacturing an ultra-low carbon martensitic stainless steel tube blank. Background Art

[0002] Super 13Cr is a type of martensitic steel. Compared to ordinary martensitic stainless steel, Super 13Cr has a significantly lower carbon content. By reducing the carbon content (maximum carbon content is 0.03%) and increasing the nickel (4.5-5.5%) and molybdenum (1.5-2.5%) contents, Super 13Cr not only increases its strength and hardness, but also improves its corrosion resistance. Due to its excellent overall performance, Super 13Cr is widely used in pipeline steel for oil and natural gas extraction and transportation in harsh environments, effectively replacing the original more expensive duplex stainless steel and austenitic stainless steel products.

[0003] At present, the production process of ultra-low carbon martensitic stainless steel tube billets is: electric furnace smelting-AOD refining-LF+VD refining-die casting-red delivery-heating-forging-annealing-inspection and warehousing. The above process has the following two problems: First, the red delivery (referring to the transfer of the steel ingot to the heating furnace as soon as possible after demolding, and the temperature of the steel ingot should be above 150°C when loading the furnace) has a low yield, and the yield has been hovering around 60% to 70%. The second problem is that the current annealing process is complicated to operate, generally adopting a two-stage or three-stage annealing process, and generally requires annealing at medium and high temperatures (about 800°C), with high energy consumption and low production efficiency. Therefore, it is necessary to develop a new production process for ultra-low carbon martensitic stainless steel to reduce costs and increase efficiency while significantly improving the yield of steel billets. Summary of the Invention

[0004] Aiming at the problems of low yield, high energy consumption and low production efficiency of existing ultra-low carbon martensitic stainless steel, the present invention provides a method for manufacturing an ultra-low carbon martensitic stainless steel tube blank.

[0005] To solve the above technical problems, the technical solutions provided by the embodiments of the present invention are:

[0006] A method for manufacturing an ultra-low carbon martensitic stainless steel tube blank comprises the following steps:

[0007] S1, slowly cooling the ingot to below 50°C by stacking, grinding, and obtaining a cast billet;

[0008] S2, after heating and forging the ingot, air-cooling it to 100°C to 150°C, then heating it to 630°C to 660°C at a rate of 70°C / h to 120°C / h, holding it for T time, then cooling it at a rate of 10°C / h to 30°C / h, taking it out of the furnace and air-cooling it to obtain an ultra-low carbon martensitic stainless steel tube billet;

[0009] Wherein, T=15+(M-50) / 7.5, T is the holding time, unit is h; M is the mass of the ingot, unit is ton.

[0010] At present, the red feeding process is widely used for ultra-low carbon martensitic stainless steel. The main reason is that martensitic stainless steel is highly sensitive to cracks. During the cooling process, it is very easy for the steel ingot to crack due to organizational transformation and casting residual stress. However, the red feeding process results in defects such as cracks, heavy skin, flying fins, floating sand, and sharp corners near the riser line of the steel ingot, which can only be simply treated by air planing. After subsequent forging, the riser line defects are transformed into folding cracks. Therefore, the riser end can only be cut off, resulting in a yield rate of around 73%.

[0011] In order to further improve or eliminate the various structural defects and residual stresses generated during the forging process, two-stage or three-stage annealing is currently used after forging. This process is complex, consumes a lot of gas energy, and has low production efficiency. The annealing process must meet the following requirements: (1) improving or eliminating the various structural defects and residual stresses generated during the forging process to prevent cracking; and (2) reducing the hardness of the ingot to below 300 HBW to meet the subsequent cutting performance requirements. In addition to meeting the above two points, from the perspective of large-scale production, the cost, production efficiency, and operational complexity of the annealing process must also be considered.

[0012] In response to the above problems, the manufacturing method of ultra-low carbon martensitic stainless steel tube billets provided by the present invention adopts a stacking slow cooling method instead of the traditional red-feeding process, which effectively reduces the risk of cracking of martensitic stainless steel ingots due to structural transformation and casting residual stress, and adds a grinding process after slow cooling to clean the defects of the ingot riser line, nozzle end and ingot body, and changes the surface treatment method before heating of the ingot, which greatly improves the yield rate of the ingot. The yield rate is increased from about 60% to 70% in the original traditional process to more than 80%, effectively reducing production costs and avoiding waste of raw materials.

[0013] Another improvement of the present invention is that the medium-high temperature (about 800°C) multi-stage annealing is changed to a one-stage low-temperature (630°C to 660°C) annealing. By controlling the specific annealing temperature and time, a stable and uniform lath-shaped martensite metallographic structure can be obtained. After annealing, the hardness of the ingot is moderate, ranging from 260HBW to 290HBW, taking into account both mechanical properties and processing properties. Moreover, since the annealing temperature and time are significantly reduced, the annealing cost is reduced by 150 to 260 yuan per ton compared with the traditional two-stage annealing, and the annealing efficiency is also significantly improved.

[0014] It should be noted that the manufacturing method of the ultra-low carbon martensitic stainless steel tube provided by the present invention is suitable for the preparation of existing common super 13Cr stainless steel tubes, such as common SUPER 13Cr, Super13Cr, SUP13Cr, S41426, S41427, M13Cr, HP13Cr, E13Cr, etc. can all adopt the above-mentioned process provided by the present invention, and can achieve the purpose of effectively improving the yield rate.

[0015] Specifically, the method for manufacturing an ultra-low carbon martensitic stainless steel tube blank provided by the present invention is suitable for preparing tube blanks with specifications ranging from Ø150 mm to Ø500 mm.

[0016] It should be noted that when T=15+(M-50) / 7.5 is used to calculate the holding time of the ingot, if the obtained time is not an integer, it is rounded up.

[0017] Preferably, in S1, the specific method of stacking is: placing the billets into several stacks, each stack has several layers of billets, the billets of two adjacent layers are staggered, and the interval between adjacent billets in the same layer is 10mm to 100mm; the bottom billet is placed on an iron pad.

[0018] Preferably, in S1, the angle between two adjacent layers of ingots is 70° to 90°.

[0019] The preferred stacking method can effectively reduce the risk of cracking of martensitic stainless steel ingots due to structural transformation and casting residual stress, which is beneficial to improving the yield rate of steel ingots.

[0020] Preferably, in S1, a grinding machine is used to grind away defects within 100 mm above and below the ingot riser line, as well as defects on the nozzle end and the ingot body.

[0021] Since the stacking slow cooling method is used instead of the traditional hot-feeding process, it not only improves the operational simplicity and safety of the process, but also changes the surface treatment method of the steel ingot before heat treatment. By grinding, the defects of the ingot riser line, nozzle end and ingot body can be cleaned, which can improve the yield rate of the subsequent heating process.

[0022] It should be noted that the stacking should be placed away from the wind vents.

[0023] It should be noted that a grinding machine is used to grind away defects such as cracks, heavy skin, flying fins, floating sand, sharp corners, etc. within 100 mm above and below the ingot riser line, and other defects on the nozzle end and the ingot body are ground away.

[0024] Preferably, in S2, the heating adopts a three-stage heating method.

[0025] Preferably, in S2, the specific heating process is: keeping the ingot at 500℃~600℃ for 2h~3h, then heating it to 750℃~850℃ at a rate of 70℃ / h~120℃ / h, keeping it for 2h~3h, and finally heating it to 1130℃~1160℃ at a rate of 70℃ / h~120℃ / h, and keeping it for 4h~8h.

[0026] By controlling the specific conditions of the heating process, the content of delta-ferrite in the steel ingot can be effectively controlled (≤5%), reducing defects in subsequent forging, increasing the yield rate, and also helping to improve the mechanical properties of the martensitic stainless steel ingot.

[0027] Preferably, the forging ratio is 4-7.

[0028] It should be noted that the forging process in the present invention is an existing technology and can adopt conventional processes of existing ultra-low carbon martensitic stainless steel technology, and the present invention does not make any special limitation.

[0029] Preferably, the ingot is prepared through electric furnace smelting, AOD refining, LF refining, VD refining and die casting processes.

[0030] The electric furnace smelting, AOD refining, LF refining, VD refining and die casting processes described in the present invention are all existing technologies, and conventional processes of existing ultra-low carbon martensitic stainless steel processes can be adopted, and the present invention does not make special limitations.

[0031] Preferably, the chemical composition of the ultra-low carbon martensitic stainless steel tube blank is: C≤0.03%, Mn≤1.00%, Si≤0.50%, S≤0.015%, P≤0.020%, Ni 3.50%~6.50%, Cr11.50%~13.50%, V≤0.30%, Mo 0.8%~3.0%, Al≤0.050%, Ti≤0.50%, Cu≤0.20%, N≤0.050%; the balance is Fe and unavoidable impurities.

[0032] The present invention also provides an ultra-low carbon martensitic stainless steel tube blank, characterized in that it is prepared by any of the above-mentioned methods for manufacturing the ultra-low carbon martensitic stainless steel tube blank.

[0033] The manufacturing method of the ultra-low carbon martensitic stainless steel tube blank provided by the present invention adopts the stacking slow cooling + grinding method to replace the red feeding process of the traditional process, and adopts a one-stage low-temperature annealing process, which effectively improves the yield rate of the tube blank, and the yield rate can reach more than 80%, effectively solving the bottleneck problem that the yield rate of the existing ultra-low carbon martensitic stainless steel cannot exceed 73%. The improved process operation is safer and simpler, reduces the requirements for production worker skills, and significantly reduces production energy consumption, achieving the purpose of reducing costs and increasing efficiency, and has high practical value. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is the metallographic structure diagram of the 3MALV22734 stainless steel tube prepared in Example 1;

[0035] Figure 2 This is the metallographic structure diagram of the 2MALV21533 stainless steel tube prepared in Example 2;

[0036] Figure 3 This is the metallographic structure diagram of the 4MALV20278 stainless steel tube prepared in Example 3;

[0037] Figure 4 This is the metallographic structure diagram of the 3MALV20137 stainless steel tube prepared in Comparative Example 1. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0039] In order to better illustrate the present invention, further examples are given below.

[0040] The hardness test of the finished tube blanks in the following examples and comparative examples refers to GB / T 231.1-2018.

[0041] Example 1

[0042] The embodiment of the present invention provides a 3MALV22734 stainless steel tube blank, the chemical composition of which is:

[0043] C 0.012%, Mn 0.38%, Si 0.26%, S 0.001%, P 0.015%, Ni 5.47%, Cr11.85%, V 0.06%, Mo 1.59%, Al 0.010%, Ti 0.028%, Cu 0.08%, N 0.046%, and the balance is Fe and inevitable impurities.

[0044] The preparation steps of the above 3MALV22734 stainless steel tube are as follows:

[0045] S1, using electric furnace smelting, AOD refining, LF refining, VD refining and die casting processes to obtain a 26-inch steel ingot (ingot weight 4.7 tons) with the above chemical composition;

[0046] S2: After the ingot is demoulded, it is stacked and slowly cooled. The ingot is cooled to 30-45°C and then ground. The cracks, heavy skin, flying fins, floating sand, sharp corners and other defects within 100mm above and below the ingot riser line are ground clean by a grinding machine. Other defects on the nozzle end and the ingot body are also ground clean.

[0047] The specific method of stacking slow cooling is as follows: the billets are placed into several stacks, each stack has several layers of billets, the billets of two adjacent layers are placed vertically, and the interval between adjacent billets in the same layer is 10mm to 100mm; the bottom billet is placed on an iron pad so that the bottom billet is suspended in the air;

[0048] S3, keep the casting at 550℃ for 2.5h, then heat it to 820℃ at a rate of 80℃ / h, keep it for 2.5h, and finally heat it to 1150℃ at a rate of 80℃ / h, keep it for 6h, and then take it out of the furnace for forging;

[0049] S4, adopts one-fire forming process, forged by fast forging machine, total forging ratio is 4.8, and tube size is 299mm.

[0050] S4, after forging, air-cool to 100℃~150℃ and then anneal in a furnace with a loading capacity of 52t. Then, heat to 680℃ at a rate of 80℃ / h, keep warm for 15h, then cool at 20℃ / h until the material temperature drops to 90~120℃ and air-cool out of the furnace to obtain 3MALV22734 stainless steel tube billet.

[0051] The yield rate from steel ingot to finished tube is 83%, with hardness of 278 / 272HBW.

[0052] The metallographic structure of the annealed billet is shown in Figure 2. Figure 1 As shown in Figure 3, its metallographic structure is mainly martensite + retained austenite + δ-ferrite.

[0053] Example 2

[0054] The embodiment of the present invention provides a 2MALV21533 stainless steel tube blank, the chemical composition of which is:

[0055] C 0.014%, Mn 0.32%, Si 0.32%, S 0.001%, P 0.013%, Ni 4.70%, Cr 12.03%, V 0.07%, Mo 1.64%, Al 0.010%, Ti 0.021%, Cu 0.09%, N 0.016%, and the balance is Fe and inevitable impurities.

[0056] The preparation steps of the above 2MALV21533 stainless steel tube are as follows:

[0057] S1, using electric furnace smelting, AOD refining, LF refining, VD refining and die casting processes to obtain a 30-inch steel ingot (ingot weight 7.1 tons) with the above chemical composition;

[0058] S2: After the ingot is demoulded, it is stacked and slowly cooled. The ingot is cooled to 23-30°C and then ground. The cracks, heavy skin, flying fins, floating sand, sharp corners and other defects within 100mm above and below the ingot riser line are ground clean by a grinding machine. Other defects on the nozzle end and the ingot body are also ground clean.

[0059] The specific method of stacking slow cooling is as follows: the billets are placed into several stacks, each stack has several layers of billets, the billets of two adjacent layers are placed vertically, and the interval between adjacent billets in the same layer is 10mm to 100mm; the bottom billet is placed on an iron pad so that the bottom billet is suspended in the air;

[0060] S3, keep the ingot at 500℃ for 3h, then heat it to 850℃ at a rate of 70℃ / h, keep it for 2h, and finally heat it to 1130℃ at a rate of 70℃ / h, keep it for 8h, and then take it out of the furnace for forging;

[0061] S4, adopts one-fire forming process, forging with a fast forging machine, total forging ratio is 4.0, and the tube specification is Ф415mm;

[0062] S4, after forging, air-cool to 150℃~200℃ and then anneal in a furnace with a loading capacity of 100t. Then, heat to 630℃ at a rate of 70℃ / h, keep warm for 22h, then cool at 10℃ / h until the material temperature drops to 90~120℃ and then air-cool out of the furnace to obtain 2MALV21533 stainless steel tube billet.

[0063] The yield rate from steel ingot to finished tube is 81%, with a hardness of 272 / 272HBW.

[0064] The metallographic structure of the annealed billet is shown in Figure 2. Figure 2 As shown in Figure 3, its metallographic structure is mainly martensite + retained austenite + δ-ferrite.

[0065] Example 3

[0066] The embodiment of the present invention provides a 4MALV20278 stainless steel tube blank, the chemical composition of which is:

[0067] C 0.016%, Mn 0.42%, Si 0.30%, S 0.0006%, P 0.009%, Ni 3.87%, Cr12.75%, V 0.07%, Mo 0.89%, Al 0.010%, Ti 0.015%, Cu 0.08%, N 0.030%, and the balance is Fe and inevitable impurities.

[0068] The preparation steps of the above 4MALV20278 stainless steel tube are as follows:

[0069] S1, using electric furnace smelting, AOD refining, LF refining, VD refining and die casting processes to obtain a 28-inch steel ingot (ingot weight 5.3 tons) with the above chemical composition;

[0070] S2: After the ingot is demoulded, it is stacked and slowly cooled. The ingot is cooled to 15-20℃ and then ground. The cracks, heavy skin, flying fins, floating sand, sharp corners and other defects within 100mm above and below the ingot riser line are ground clean by a grinding machine. Other defects on the nozzle end and the ingot body are also ground clean.

[0071] The specific method of stacking slow cooling is as follows: the billets are placed into several stacks, each stack has several layers of billets, the billets of two adjacent layers are placed vertically, and the interval between adjacent billets in the same layer is 10mm to 100mm; the bottom billet is placed on an iron pad so that the bottom billet is suspended in the air;

[0072] S3, keep the ingot at 600℃ for 2h, then heat it to 750℃ at a rate of 120℃ / h, keep it for 3h, and finally heat it to 1160℃ at a rate of 120℃ / h, keep it for 4h, and then take it out of the furnace for forging;

[0073] S4, adopts one-fire forming process, forging with a fast forging machine, total forging ratio is 5.8, and the tube size is 280mm.

[0074] S4, after forging, air-cool to 150℃~200℃ and then anneal in a furnace with a loading capacity of 180t. Then, heat to 660℃ at a rate of 120℃ / h, keep warm for 32h, then cool at 30℃ / h until the material temperature drops to 90~120℃ and air-cool out of the furnace to obtain 4MALV20278 stainless steel tube billet.

[0075] The yield rate from steel ingot to finished tube billet is 82%, with a hardness of 285 / 269HBW.

[0076] The metallographic structure of the annealed billet is shown in Figure 2. Figure 3 As shown in Figure 3, its metallographic structure is mainly martensite + retained austenite + δ-ferrite.

[0077] Comparative Example 1

[0078] This comparative example provides a 3MALV20137 stainless steel tube blank, the chemical composition of which is:

[0079] C 0.012%, Mn 0.35%, Si 0.22%, S 0.001%, P 0.011%, Ni 4.47%, Cr12.59%, V 0.07%, Mo 0.98%, Al 0.009%, Ti 0.015%, Cu 0.07%, N 0.015%, and the balance is Fe and inevitable impurities.

[0080] The preparation steps of the above 3MALV20137 stainless steel tube are as follows:

[0081] S1, using electric furnace smelting, AOD refining, LF refining, VD refining and die casting processes to obtain a 22-inch steel ingot (ingot weight 3.0 t) with the above chemical composition;

[0082] S2, after the steel ingot is demoulded, it is cleaned of defects such as heavy skin, flying fins, floating sand, and sharp corners by carbon arc gouging, and then sent to hot delivery. The average ingot temperature before loading into the furnace is 530℃;

[0083] S3, keep the casting at 550℃ for 2.5h, then heat it to 820℃ at a rate of 80℃ / h, keep it for 2.5h, and finally heat it to 1150℃ at a rate of 80℃ / h, keep it for 6h, and then take it out of the furnace for forging;

[0084] S4, adopts one-fire forming process, forging with a fast forging machine, total forging ratio is 7.0, and the tube size is 210mm;

[0085] S4, after forging, air-cooled to 300℃~350℃, then charged into the furnace for annealing, heated to 680℃ at a rate of 80℃ / h, kept warm for 15h, then cooled at 50℃ / h, taken out of the furnace and cooled at 250℃ for 8h, then annealed for the second time, heated to 680℃ at a rate of 80℃ / h, kept warm for 25h, then cooled to 250℃ at a rate of 20℃ / h, taken out of the furnace, and obtained 3MALV20137 stainless steel tube billet.

[0086] The yield rate from steel ingot to finished tube is 68%, with hardness of 265 / 270HBW.

[0087] The metallographic structure of the annealed billet is shown in Figure 2. Figure 4 As shown in Figure 3, its metallographic structure is mainly martensite + retained austenite + δ-ferrite.

[0088] In summary, the process of the present invention adopts the stacking slow cooling + grinding method to replace the red delivery process of the traditional process, which effectively improves the yield rate of the tube billet, and the yield rate can reach more than 80%, effectively solving the bottleneck problem that the yield rate of the existing ultra-low carbon martensitic stainless steel cannot exceed 73%. In addition, the one-stage low-temperature annealing process is adopted to reduce energy consumption while ensuring the performance and processing performance, reduce costs, and improve production efficiency, which has a high promotion and application value.

[0089] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for manufacturing an ultra-low carbon martensitic stainless steel tube, characterized in that: The steps include: S1, slowly cooling the ingot to below 50°C by stacking, and using a grinding machine to clean defects within 100 mm above and below the riser line of the ingot, as well as defects at the nozzle end and the ingot body, to obtain a cast billet; S2, after heating and forging the ingot, air-cooling it to 100°C to 150°C, then heating it to 630°C to 660°C at a rate of 70°C / h to 120°C / h, holding it for T time, then cooling it at a rate of 10°C / h to 30°C / h, taking it out of the furnace and air-cooling it to obtain an ultra-low carbon martensitic stainless steel tube billet; Wherein, T=15+(M-50) / 7.5, T is the holding time, in hours; M is the mass of the ingot, in tons; In S2, the specific heating process is: keeping the ingot at 500℃~600℃ for 2h~3h, then heating it to 750℃~850℃ at a rate of 70℃ / h~120℃ / h, keeping it for 2h~3h, and finally heating it to 1130℃~1160℃ at a rate of 70℃ / h~120℃ / h, keeping it for 4h~8h.

2. The method for producing an ultra-low carbon martensitic stainless steel tube according to claim 1, wherein: In S1, the specific stacking method is: placing the billets into several stacks, each stack has several layers of billets, the billets of two adjacent layers are staggered, and the interval between adjacent billets in the same layer is 10mm to 100mm; the bottom billet is placed on an iron pad.

3. The method for producing an ultra-low carbon martensitic stainless steel tube according to claim 2, wherein: In S1, the angle between two adjacent layers of ingots is 70° to 90°.

4. The method for producing an ultra-low carbon martensitic stainless steel tube according to claim 1, wherein: The forging ratio of the forging is 4-7.

5. The method for producing an ultra-low carbon martensitic stainless steel tube according to claim 1, wherein: The ingot is prepared through the processes of electric furnace smelting, AOD refining, LF refining, VD refining and die casting.

6. The method for producing an ultra-low carbon martensitic stainless steel tube according to claim 1, wherein: The chemical composition of the ultra-low carbon martensitic stainless steel tube is: C≤0.03%, Mn≤1.00%, Si≤0.50%, S≤0.015%, P≤0.020%, Ni 3.50%~6.50%, Cr 11.50%~13.50%, V≤0.30%, Mo 0.8%~3.0%, Al≤0.050%, Ti≤0.50%, Cu≤0.20%, N≤0.050%; the balance is Fe and unavoidable impurities.

7. An ultra-low carbon martensitic stainless steel tube, characterized in that: The ultra-low carbon martensitic stainless steel tube is prepared by the method for manufacturing the ultra-low carbon martensitic stainless steel tube according to any one of claims 1 to 6.

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