High-strength seamless tube and manufacturing method thereof

Abstract

The invention provides a high-strength seamless tube, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: 0.19 to 0.23 percent of C, 0.1 to 0.6 percent of Si, 0.6 to 1.5 percent of Mn, 0.1 to 0.4 percent of Cr, 0.1 to 0.18 percent of Mo, 0.02 to 0.04 percent of Nb, 0.01 to 0.03 percent of Ti, 0.0015 to 0.005 percent of B, 0.01 to 0.05 percent of Al and 0.0005 to 0.005 percent of Ca. The invention also provides a manufacturing method of the seamless tube, which comprises the following steps of: smelting and continuously casting to obtain a tube blank; heating, perforating, rolling and sizing; heat treatment is conducted, specifically, the austenitizing temperature is 900-930 DEG C, quenching is conducted after heat preservation is conducted for 30-60 min, then tempering is conducted at the temperature of 550-650 DEG C, and the heat preservation time is 50-80 min; and hot sizing and straightening.

Description

Technical Field

[0001] The present invention relates to a steel material and a manufacturing method thereof, and in particular to a seamless pipe and a manufacturing method thereof. Background Art

[0002] High-strength structural pipe products are widely used in the field of engineering machinery. They are used as core components of truck cranes and crawler cranes. They are required to have good strength, low-temperature toughness, good surface quality and weldability.

[0003] At present, conventional high-strength seamless pipes contain relatively high levels of W and Ni alloy elements. By adding such low-carbon equivalent contributing elements, its low-temperature toughness can be improved, and a certain strength and low carbon equivalent level can be guaranteed.

[0004] For example, the Chinese patent document with publication number CN101586450A, publication date November 25, 2009, and titled “A high-strength and high-toughness oil casing and its manufacturing method” discloses that the composition system of oil casing is: C: 0.16-0.28%, Si: ≤0.50%, Mn: 0.30-1.10%, Cr: 0.50-1.10%, Mo: 0.60-0.95%, Al: 0.015-0.0 60%, of which acid-soluble Als / Al≥0.80%, Ni<0.60%, Cu: 0.05%~0.25%, V: 0.060~0.20%, Ca>0.0015%, Nb:≤0.05%, Ti:≤0.05%, P<0.010%, S<0.002%, O:<0.0024%, H<0.0002%, N<0.008%, B:0.0000%~0.005%, and the rest is iron. It can be seen that the patent document adds Ni element, and its Cr content is also relatively high.

[0005] However, the cost of such elements is relatively high. In the context of homogeneous competition in domestic engineering machinery, it is hoped to provide a high-strength seamless pipe with an innovative component combination design, which can reduce or remove high-cost alloy elements while maintaining high-strength performance, thereby significantly reducing the cost of engineering machinery components. Summary of the invention

[0006] One of the purposes of the present invention is to provide a high-strength seamless pipe, which, through reasonable chemical element composition design supplemented by process design, reduces or removes high-cost alloy elements to obtain higher strength, and at the same time has excellent low-temperature impact toughness and good weldability.

[0007] In order to achieve the above object, the present invention provides a high-strength seamless pipe, which contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages:

[0008] C: 0.19-0.23%, Si: 0.1-0.6%, Mn: 0.6-1.5%, Cr: 0.1-0.4%, Mo: 0.1-0.18%, Nb: 0.0 2-0.04%, Ti: 0.01-0.03%, B: 0.0015-0.005%, Al: 0.01-0.05%, Ca: 0.0005-0.005%;

[0009] The high-strength seamless pipe does not contain Ni and W elements.

[0010] Accordingly, the present invention also provides a high-strength seamless pipe, the mass percentage of each element of which is:

[0011] C: 0.19-0.23%, Si: 0.1-0.6%, Mn: 0.6-1.5%, Cr: 0.1-0.4%, Mo: 0.1-0.18%, Nb: 0.02-0.04%, Ti: 0.01-0.03%, B: 0.0015-0.005%, Al: 0.01-0.05%, Ca: 0.0005-0.005%; the balance is Fe and other inevitable impurities.

[0012] In the high-strength seamless pipe of the present invention, the design principles of the chemical elements are as follows:

[0013] C: In the high-strength seamless pipe of the present invention, the C element is a carbide-forming element, which can improve the strength of steel. When the C element content is lower than 0.19%, the hardenability will be reduced, thereby reducing the toughness. When the C element content is higher than 0.23%, it will cause embrittlement in the heat-affected zone during welding, reducing the welding performance. Therefore, the mass percentage of the C element is controlled between 0.19-0.23% in the present invention.

[0014] Si: In the high-strength seamless pipe of the present invention, the Si element can be dissolved in ferrite to improve the yield strength of the steel. However, the Si element content should not be too high, as it will deteriorate the processing and toughness. When the Si element content is less than 0.1%, the steel will be easily oxidized. Therefore, the mass percentage of the Si element is controlled between 0.1-0.6% in the present invention.

[0015] Mn: In the high-strength seamless pipe described in the present invention, Mn is an austenite-forming element, which can improve the hardenability of steel. In this steel system, when the Mn content is less than 0.6%, the hardenability of the steel is significantly reduced, the martensite ratio is reduced, and thus the toughness is reduced; when the Mn content is greater than 1.5%, the carbon equivalent will be high, which will deteriorate the welding performance. Therefore, the mass percentage of the Mn element is controlled between 0.6-1.5% in the present invention.

[0016] Cr: In the high-strength seamless pipe described in the present invention, Cr is an element that strongly improves hardenability and a strong carbide-forming element. It can precipitate carbides during tempering to improve the strength of steel. However, when the Cr content is higher than 0.4%, it will lead to an increase in carbon equivalent and deteriorate welding performance. When the Cr content is lower than 0.1%, it is difficult to improve hardenability, resulting in a significant decrease in the matching of product strength and low-temperature toughness. Therefore, the present invention controls the mass percentage of the Cr element to between 0.1-0.4%.

[0017] Mo: In the high-strength seamless pipe described in the present invention, the Mo element can improve the strength and tempering stability of the steel through carbides and solid solution strengthening, and improve the low-temperature impact toughness. In this steel system, when the Mo element content is higher than 0.18%, the carbon equivalent will increase and the welding performance will deteriorate. When the Mo element content is lower than 0.1%, the hardenability will be significantly reduced, resulting in a significant decrease in strength and low-temperature toughness. Therefore, the present invention controls the mass percentage of the Mo element to be between 0.1-0.18%.

[0018] Nb: In the high-strength seamless pipe described in the present invention, Nb is a fine-grained and precipitation-strengthening element, which can make up for the decrease in strength caused by the reduction of carbon. At the same time, during the welding process, the Nb element can inhibit the grain growth in the heat-affected zone, and has a certain inhibitory effect on the performance deterioration of this area. When the Nb element content is less than 0.02%, the effect is not obvious. When the Nb element content is higher than 0.04%, it is easy to form coarse Nb (CN), thereby reducing toughness. Therefore, the present invention controls the mass percentage of the Nb element to between 0.02-0.04%.

[0019] Ti: In the high-strength seamless pipe described in the present invention, Ti is a strong carbonitride-forming element, which significantly refines the austenite grains and can compensate for the decrease in strength caused by the reduction of carbon. At the same time, the carbonitride formed by Ti has high thermal stability, and has a good effect on suppressing the coarsening of the structure in the heat-affected zone of welding during welding, thereby improving the welding performance. When the Ti element content is higher than 0.03%, coarse TiN is easily formed, which reduces the low-temperature impact toughness; if the Ti element content is too low, Ti will not be able to fully form TiN with N, and B and N in the steel will form BN brittle phase, reducing the toughness of the material. Therefore, the present invention controls the mass percentage of the Ti element between 0.01-0.03%.

[0020] B: In the high-strength seamless pipe of the present invention, the B element can significantly improve the hardenability. When the B element content is less than 0.0015%, the effect of improving the hardenability is not significant, and when the B element content is higher than 0.005%, the cold crack sensitivity of the material will be increased. Therefore, the mass percentage of the B element is controlled between 0.0015-0.005% in the present invention.

[0021] Al: Al is mainly used for deoxidation treatment, which can effectively control the oxygen content in the material. Based on this, the present invention controls its content to 0.01-0.05%.

[0022] Ca: In the high-strength seamless pipe of the present invention, Ca can purify the molten steel, promote the spheroidization of MnS, and improve the impact toughness. However, when the Ca content is too high, it is easy to form coarse non-metallic inclusions. Therefore, the mass percentage of Ca is controlled between 0.0005-0.005% in the present invention.

[0023] Furthermore, among the inevitable impurities in the high-strength seamless pipe of the present invention: P≤0.015%, S≤0.003%, and N≤0.007%.

[0024] It should be noted that the unavoidable impurity elements in the high-strength seamless pipe of the present invention mainly include S, P and N. Under the condition that technical conditions permit, in order to obtain seamless steel pipes with better performance and better quality, the content of impurity elements in the seamless steel pipe should be reduced as much as possible, and the lower the content, the better. Among them:

[0025] When the N content is higher than 0.007%, more Ti nitrides will be precipitated in the structure. The precipitated phase is larger and has sharp angles, which will significantly reduce the low-temperature impact toughness. Therefore, in some embodiments, the present invention controls the mass percentage of Ti element to N≤0.007%.

[0026] Too high content of P and S will affect the toughness of the steel. Therefore, in some embodiments, the mass percentages of P and S elements are controlled to P≤0.015% and S≤0.003%, respectively.

[0027] Furthermore, in the high-strength seamless pipe described in the present invention, its carbon equivalent is ≤0.55.

[0028] Furthermore, in the high-strength seamless pipe of the present invention, its microstructure is tempered troostite.

[0029] Furthermore, in the high-strength seamless pipe of the present invention, the grain size is 10-18 μm.

[0030] Furthermore, in the high-strength seamless pipe described in the present invention, its performance meets the following requirements: yield strength ≥ 770 MPa, tensile strength is 820-1000 MPa, elongation ≥ 23%, and -40°C impact toughness KV8 ≥ 45J.

[0031] Correspondingly, another object of the present invention is to provide a method for manufacturing a high-strength seamless pipe. The manufacturing method adopts a unique heat treatment process, combined with the composition design of the present invention, which can reduce or remove high-cost alloy elements to obtain higher strength, while having excellent low-temperature impact toughness and good weldability.

[0032] In order to achieve the above object, the present invention provides a method for manufacturing a high-strength seamless pipe, which comprises the steps of:

[0033] Smelting and continuous casting to obtain tube billets;

[0034] Heating, perforating, rolling and sizing;

[0035] Heat treatment: austenitizing temperature is 900-930℃, keep warm for 30-60min, then quench, then temper at 550-650℃, keep warm for 50-80min;

[0036] Heat sizing and straightening.

[0037] In the manufacturing method described in the present invention, the heat treatment adopts an austenitizing temperature of 900-930°C, which is kept for 30-60 minutes and then quenched, and then tempered at 550-650°C for 50-80 minutes. This is because: the present invention re-austenizes and water-quenches the rolled tube, which on the one hand refines the original austenite grains, and on the other hand makes the element distribution more uniform, and the precipitation strengthening phase is more evenly dispersed through tempering, thereby improving its strength-toughness matching performance.

[0038] Furthermore, in the continuous casting step of the manufacturing method described in the present invention, the superheat of the molten steel is controlled to be lower than 30° C., and the continuous casting speed is 1.8-2.2 m / min.

[0039] In some embodiments of the present invention, the superheat of the molten steel can be controlled below 30° C. during the casting process, and the continuous casting speed is ensured to be 1.8-2.2 m / min, which can effectively reduce component segregation.

[0040] Furthermore, in the heating step of the manufacturing method described in the present invention, the heating is performed to a soaking temperature of 1200-1240°C.

[0041] Furthermore, in the perforation step of the manufacturing method described in the present invention, the perforation temperature is 1180-1240°C.

[0042] Furthermore, in the rolling step of the manufacturing method described in the present invention, the final rolling temperature is 900-950°C.

[0043] Furthermore, in the sizing step of the manufacturing method described in the present invention, the sizing temperature is 850-900°C. Furthermore, in the manufacturing method described in the present invention, the temperature of hot sizing and straightening is 450-550°C.

[0044] The high-strength seamless pipe and the manufacturing method thereof of the present invention have the following advantages and beneficial effects:

[0045] The high-strength seamless pipe of the present invention has a higher C content and uses B to increase hardenability in composition design, replacing W and Ni alloy elements used in conventional steels to increase hardenability, while also reducing the addition of Cr and Mo elements. Therefore, the material alloy cost used is low and can have good economic benefits.

[0046] The heat treatment process of the high-strength seamless pipe of the present invention is simple and easy to implement in large-scale production.

[0047] The high-strength seamless pipe described in the present invention can be used to manufacture 770MPa grade structural pipe products, and has good strength, low-temperature toughness and welding performance. In some embodiments, it can meet the following indicators: yield strength is ≥770MPa, tensile strength is in the range of 820-1000MPa, and -40°C impact toughness KV8 ≥45J. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 The metallographic structure of the high-strength seamless pipe of Example 1 of the present invention is shown.

[0049] Figure 2 The grain size of the high-strength seamless pipe of Example 1 of the present invention is shown. DETAILED DESCRIPTION

[0050] The high-strength seamless pipe and the method for manufacturing the same according to the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an improper limitation on the technical solution of the present invention.

[0051] Examples 1-6 and Comparative Examples 1-5

[0052] The high-strength seamless pipes of Examples 1-6 are prepared by the following steps:

[0053] (1) Smelting and continuous casting to obtain tube blanks:

[0054] In some embodiments, a batching scheme of scrap steel + blast furnace molten iron can be adopted: for example, the proportion of molten iron is 50-60%, the molten steel is smelted in an electric furnace, refined outside the furnace, vacuum degassed and stirred with argon, and then treated with Ca to denature the inclusions, which can reduce the O and H contents.

[0055] In some preferred embodiments, the superheat of the molten steel can be controlled to be lower than 30° C., and the continuous casting speed can be 1.8-2.2 m / min.

[0056] (2) heating, punching, rolling and sizing the tube blank to obtain a tube body;

[0057] In the heating step of some embodiments, the heating is performed to a soaking temperature of 1200-1240°C, the piercing temperature is controlled to 1180-1240°C, the final rolling temperature is controlled to 900-950°C, and the sizing temperature is controlled to 850-900°C.

[0058] (3) Heat treatment: austenitizing temperature is controlled at 900-930°C, kept at this temperature for 30-60 min, then quenched, and then tempered at 550-650°C, and kept at this temperature for 50-80 min;

[0059] (4) Heat sizing and straightening: In some embodiments, the temperature of heat sizing and straightening can be controlled to 450-550°C.

[0060] It should be noted that the chemical element composition and related process design of the high-strength seamless pipes of Examples 1-6 of the present invention meet the design specification requirements of the present invention. While in Comparative Examples 1-5, although the steps and processes are roughly the same as those of the present invention, the chemical element content or specific process parameters thereof are not in accordance with the present invention.

[0061] Table 1 lists the mass percentages of the chemical elements of the high-strength seamless pipes of Examples 1-6 and the control pipes of Comparative Examples 1-5.

[0062] Table 1. (wt%, the balance is Fe and other inevitable impurities except P, S, N)

[0063] serial number C Si Mn Cr Mo Nb Ti B Al Ca P S N Carbon Equivalent Example 1 0.19 0.2 1.5 0.1 0.12 0.03 0.02 0.0015 0.01 0.0005 0.009 0.002 0.004 0.48 Example 2 0.19 0.1 1 0.2 0.18 0.02 0.025 0.002 0.04 0.001 0.010 0.001 0.005 0.43 Example 3 0.23 0.4 1.3 0.3 0.15 0.03 0.03 0.003 0.05 0.005 0.010 0.003 0.006 0.54 Example 4 0.2 0.6 0.8 0.4 0.1 0.03 0.03 0.004 0.03 0.003 0.012 0.002 0.007 0.43 Example 5 0.22 0.25 0.6 0.2 0.16 0.04 0.03 0.005 0.02 0.002 0.013 0.002 0.005 0.39 Example 6 0.19 0.51 1.1 0.25 0.15 0.02 0.01 0.0025 0.03 0.003 0.010 0.0015 0.006 0.45 Comparative Example 1 0.25 0.26 0.8 0.2 0.17 0.04 0.02 0.005 0.023 0.002 0.007 0.003 0.007 0.46 Comparative Example 2 0.21 0.33 1.8 0.15 0.1 0.03 0.01 0.003 0.04 0.002 0.008 0.003 0.005 0.55 Comparative Example 3 0.23 0.2 1.5 0.4 0.18 0.02 0.02 0.004 0.04 0.001 0.010 0.001 0.006 0.60 Comparative Example 4 0.19 0.3 1.2 0.3 0.4 0.04 0.02 - 0.05 0.003 0.010 0.003 0.006 0.53 Comparative Example 5 0.2 0.35 1 0.28 0.15 0.03 0.025 0.0035 0.003 0.004 0.01 0.002 0.009 0.45

[0064] Note: Carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) 15, where each chemical element is replaced by the value before the percentage sign of its mass percentage.

[0065] Table 2-1 and Table 2-2 list the process parameters of the high-strength seamless pipes of Examples 1-6 and the control pipes of Comparative Examples 1-5.

[0066] Table 2-1.

[0067]

[0068] Table 2-2.

[0069]

[0070] The high-strength seamless pipes of Examples 1-6 and the comparative pipes of Comparative Examples 1-5 were sampled and tested, and the test results are listed in Table 3. The relevant test process is as follows:

[0071] (1) Tensile test: The room temperature tensile property test is carried out according to GB / T 228.1-2000.

[0072] (2) Microstructure observation: Take the cross section of the tube for structure observation, grind and polish the sample, and use 4% nitric acid + alcohol to etch the metallographic structure. The structure is observed using a metallographic microscope.

[0073] (3) Impact test: -40℃ impact energy test is carried out according to GB / T 229-2007 “Metallic Materials Charpy Pendulum Impact Test Method”.

[0074] (4) Welding performance testing: Welding evaluation shall be carried out in accordance with NB / T-47014-2011 Welding procedure assessment for pressure-bearing equipment.

[0075] Table 3 lists the test results of the high-strength seamless pipes of Examples 1-6 and Comparative Examples 1-5.

[0076] Table 3.

[0077]

[0078] It can be seen from Table 3 that the yield strength of the high-strength seamless pipes of Examples 1-6 of the present invention is ≥780MPa, the tensile strength is between 860-930MPa, the -40°C impact toughness KV8 is ≥80J, the elongation is ≥23%, the grain size is between 10-18μm, and it also has good weldability.

[0079] also, Figure 1 The metallographic structure of the high-strength seamless pipe of Example 1 of the present invention is shown. Figure 1 As shown, the microstructure of the high-strength seamless pipe is a uniform and fine tempered martensite structure.

[0080] Figure 2 The microstructure of the high-strength seamless pipe of Example 1 of the present invention is shown. Figure 2 It can be seen that the grains are small and uniform.

[0081] However, since the chemical element composition of Comparative Examples 1-5 does not meet the design requirements of the present invention, it is unable to achieve excellent low-temperature impact toughness and good weldability.

[0082] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0083] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with the contents disclosed by those skilled in the art from the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A high-strength seamless pipe containing Fe and inevitable impurities, characterized in that: It also contains the following chemical elements in the following mass percentages: C: 0.19-0.23%, Si: 0.1-0.6%, Mn: 0.6-1.5%, Cr: 0.1-0.4%, Mo: 0.1-0.18%, Nb: 0.02-0.04%, Ti: 0.01-0.03%, B: 0.0015-0.005%, Al: 0.01-0.05%, Ca: 0.0005-0.005%; The high-strength seamless pipe does not contain Ni and W elements.

2. The high-strength seamless pipe according to claim 1, characterized in that: The mass percentage of each element is: C: 0.19-0.23%, Si: 0.1-0.6%, Mn: 0.6-1.5%, Cr: 0.1-0.4%, Mo: 0.1-0.18%, Nb: 0.02-0.04%, Ti: 0.01-0.03%, B: 0.0015-0.005%, Al: 0.01-0.05%, Ca: 0.0005-0.005%; the balance is Fe and other inevitable impurities.

3. The high-strength seamless pipe according to claim 1 or 2, characterized in that: Among the unavoidable impurities: P ≤ 0.015%, S ≤ 0.003%, N ≤ 0.007%.

4. The high-strength seamless pipe according to claim 1 or 2, characterized in that: Its carbon equivalent is ≤0.

55.

5. The high-strength seamless pipe according to claim 1 or 2, characterized in that: Its microstructure is tempered troostite.

6. The high-strength seamless pipe according to claim 5, characterized in that: Its grain size is 10-18μm.

7. The high-strength seamless pipe according to claim 1 or 2, characterized in that: Its performance meets the following requirements: yield strength ≥770MPa, tensile strength 820-1000MPa, elongation ≥23%, -40℃ impact toughness KV8 ≥45J.

8. The method for manufacturing a high-strength seamless pipe according to any one of claims 1 to 7, characterized in that: It includes the steps of: Smelting and continuous casting to obtain tube billets; Heating, perforating, rolling and sizing; Heat treatment: austenitizing temperature is 900-930℃, keep warm for 30-60min, then quench, then temper at 550-650℃, keep warm for 50-80min; Heat sizing and straightening.

9. The manufacturing method according to claim 8, characterized in that: In the continuous casting step, the superheat of the molten steel is controlled below 30°C, and the continuous casting speed is 1.8-2.2 m / min.

10. The manufacturing method according to claim 8, characterized in that: In the heating step, the heating is performed to a soaking temperature of 1200-1240°C.

11. The manufacturing method according to claim 8, characterized in that: In the perforating step, the perforating temperature is 1180-1240°C.

12. The manufacturing method according to claim 8, characterized in that: In the rolling step, the final rolling temperature is 900-950°C.

13. The manufacturing method according to claim 8, characterized in that: In the sizing step, the sizing temperature is 850-900°C.

14. The manufacturing method according to claim 8, characterized in that: The temperature of hot sizing and straightening is 450-550℃.

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