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: 0.14 to 0.20 percent of C, 0.15 to 0.55 percent of Si, 0.5 to 1.3 percent of Mn, 0.5 to 1.0 percent of Cr, 0.35 to 0.85 percent of Mo, 0.06 to 0.1 percent of V, 0.02 to 0.06 percent of Nb, 0.01 to 0.05 percent of Ti, 0.001 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 further provides a manufacturing method of the seamless tube. The manufacturing method comprises the steps of smelting and continuous casting; 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, tempering is conducted at the temperature of 580-650 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: 0.14-0.20%, Si: 0.15-0.55%, Mn: 0.5-1.3%, Cr: 0.5-1.0%, Mo: 0.35-0.85%, V: 0.06-0.1 %, Nb: 0.02-0.06%, Ti: 0.01-0.05%, B: 0.001-0.005%, Al: 0.01-0.05%, Ca: 0.0005-0.005%.

[0009] Accordingly, the present invention also provides a high-strength seamless pipe, the mass percentage of each element of which is:

[0010] C: 0.14-0.20%, Si: 0.15-0.55%, Mn: 0.5-1.3%, Cr: 0.5-1.0%, Mo: 0.35-0.85%, V: 0.06-0.1%, Nb: 0.02-0.06%, Ti: 0.01-0.05%, B: 0.001-0.005%, Al: 0.01-0.05%, Ca: 0.0005-0.005%; the balance is Fe and other inevitable impurities.

[0011] Furthermore, in the high-strength seamless pipe of the present invention, the mass percentage of each element further satisfies at least one of the following items:

[0012] C: 0.15-0.19%;

[0013] Si: 0.15-0.35%;

[0014] Mn: 0.6-1.1%;

[0015] Cr: 0.8-1.0%;

[0016] Mo: 0.5-0.85%;

[0017] Ti: 0.015-0.035%.

[0018] The high-strength seamless pipe of the present invention is designed with low-carbon, Cr-Mo and micro-alloy steel grades, and the carbon equivalent of the steel grades is controlled to achieve high strength, excellent low-temperature impact toughness, and good weldability.

[0019] Specifically, the design principles of the chemical elements of the high-strength seamless pipe of the present invention are as follows:

[0020] C: C is a basic element to ensure strength and hardenability. At the same time, the C content will affect the carbon equivalent and welding performance. If the C content in the steel is less than 0.14%, the hardenability of the steel is poor and the strength cannot meet the requirements. If the C content in the steel exceeds 0.2%, the C content is too high, which will lead to poor welding performance of the steel and deteriorate its plasticity and toughness. Therefore, in the high-strength seamless pipe described in the present invention, the mass percentage of the C element is controlled between 0.14-0.20%.

[0021] In some embodiments, the mass percentage of C can be further controlled to be between 0.15% and 0.19%.

[0022] Si: In the high-strength seamless pipe described in the present invention, Si acts as an important deoxidizer in steel, and the Si element can be dissolved in ferrite to increase the yield strength of the steel. However, it should be noted that the Si content in the steel should not be too high. If the Si content in the steel is too high, higher than 0.55%, the processing performance and toughness of the steel will deteriorate; and if the Si content in the steel is too low, lower than 0.15%, the steel will become easily oxidized. Therefore, in the high-strength seamless pipe described in the present invention, the mass percentage of the Si element is controlled between 0.15-0.55%.

[0023] In some embodiments, the mass percentage of Si may be further controlled between 0.15% and 0.35%.

[0024] Mn: In the high-strength seamless pipe described in the present invention, the Mn element is a deoxidizing and desulfurizing agent, which has a great influence on the hardenability and toughness level of the steel. When the Mn content in the steel is above 0.5%, a better effect can be produced, but too much Mn will make the hardenability of the steel too high, reduce the toughness of the heat-affected zone of the steel welding, and cause center segregation during continuous casting, deteriorating the low-temperature impact toughness of the parent material. Therefore, in the high-strength seamless pipe described in the present invention, the mass percentage of the Mn element is controlled between 0.5-1.3%.

[0025] In some embodiments, the mass percentage of Mn may be further controlled between 0.6-1.1%.

[0026] Cr: In the high-strength seamless pipe described in the present invention, the Cr element can increase the strength and hardenability of the steel, and the Cr element and the Mo element have a better effect. However, it should be noted that if the Cr content in the steel is too high, the carbon equivalent will be significantly increased, resulting in increased sensitivity to welding cracking and reduced toughness of the welding heat-affected zone. Therefore, in the high-strength seamless pipe described in the present invention, the mass percentage of the Cr element is controlled between 0.5-1%.

[0027] In some embodiments, the mass percentage of Cr may be further controlled between 0.8% and 1%.

[0028] Mo: In the high-strength seamless pipe described in the present invention, Mo is one of the main added elements, which can improve the hardenability of steel. The composite effect of Mo and Cr can better improve the hardenability. In addition, the Mo element also has good precipitation strengthening and solid solution strengthening effects. Mo has a good interaction with micro-alloying elements, which can effectively refine the precipitated phase, increase the stability and volume fraction of the precipitated phase, and improve the strength and toughness of the welding heat affected zone. However, it should be noted that the Mo element is expensive. Adding too high a content of Mo to the steel will not only lead to an excessively high carbon equivalent, but also increase the alloy cost. Therefore, in the high-strength seamless pipe described in the present invention, the mass percentage of the Mo element is controlled between 0.35-85%.

[0029] In some embodiments, the mass percentage of Mo may be further controlled to be between 0.5% and 0.85%.

[0030] V: In the high-strength seamless pipe described in the present invention, the V element can refine the grains in the steel, and the carbides formed by it can greatly improve the strength of the steel. However, when the amount of V added to the steel reaches a certain level, its strengthening effect is not obvious, and it will cause the stability of low-temperature impact toughness to deteriorate. In addition, V is a relatively expensive alloy element and should not be added in excess. Therefore, in the high-strength seamless pipe described in the present invention, the mass percentage of V element is controlled between 0.06-0.1%.

[0031] Nb: In the high-strength seamless pipe of 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. When the content is less than 0.02%, the effect is not obvious, and when it is higher than 0.06%, it is easy to form coarse Nb (CN), thereby reducing toughness. Based on this, the high-strength seamless pipe of the present invention controls its content to 0.02-0.06%.

[0032] Al: In the high-strength seamless pipe of the present invention, Al is a good deoxidizing element, but adding too much Al into the steel will easily cause aluminum oxide inclusions. It is necessary to increase the proportion of acid-soluble aluminum to the total aluminum as much as possible, and then feed Al wire in appropriate amounts after vacuum degassing. In addition, in the present invention, Al can also be combined with N to form AlN, thereby ensuring the hardenability effect of B. Therefore, in the high-strength seamless pipe of the present invention, the mass percentage of Al element is controlled between 0.01-0.05%.

[0033] 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 of the material. However, it should be noted that the Ca content in the steel should not be too high. When the Ca content in the steel is too high, coarse non-metallic inclusions are easily formed, which will affect the performance of the steel. Therefore, in the high-strength seamless pipe of the present invention, the mass percentage of the Ca element is controlled between 0.0005-0.005%.

[0034] Ti: In the high-strength seamless pipe described in the present invention, Ti is a strong carbonitride-forming element, which can significantly refine the austenite grains in the high-temperature state. During the rapid cooling of the steel, the carbonitrides of Ti in the steel can act as nucleation points to promote the refinement of the rapid cooling structure, and then refine the final quenched and tempered structure, while preventing the coarsening of the structure of the heat-affected zone during welding. In addition, adding an appropriate amount of Ti element to the steel also contributes to the strength of the steel, which can make up for the decrease in the strength of the steel caused by the reduction of the C element content in the steel. It should be noted that the Ti content in the steel should not be too high. If the Ti content in the steel is higher than 0.05%, coarse TiN is easily formed, thereby reducing the toughness of the material; and if the Ti content in the steel is too low, the Ti element cannot effectively play a refining effect. In the present invention, the Ti element is combined with B, and the Ti element is combined with N, thereby ensuring that B plays an effect of improving hardenability and preventing B and N from forming a brittle phase. Therefore, the mass percentage of the Ti element in the high-strength seamless pipe of the present invention is controlled between 0.01-0.05%.

[0035] In some embodiments, the mass percentage of the Ti element may be further controlled to be between 0.015-0.035%.

[0036] B: In the high-strength seamless pipe described in the present invention, B is an element that strongly improves hardenability. Its hardenability effect can replace a certain amount of Mn, Mo, and Cr elements, thereby significantly reducing the alloy cost. It is used in combination with Al and Ti to ensure its hardenability effect.

[0037] Furthermore, among the inevitable impurities in the high-strength seamless pipe of the present invention: P≤0.01%, S≤0.002%, and N≤0.007%.

[0038] It should be noted that in the high-strength seamless pipe of the present invention, the inevitable impurity elements mainly include S, P and N. If 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.

[0039] Furthermore, the carbon equivalent of the high-strength seamless pipe of the present invention is ≤0.67.

[0040] Furthermore, the microstructure of the high-strength seamless pipe described in the present invention is tempered troostite.

[0041] Furthermore, the grain size of the high-strength seamless pipe of the present invention is 8-20 μm.

[0042] Furthermore, the properties of the high-strength seamless pipe of the present invention meet the following requirements: yield strength ≥ 890 MPa, tensile strength 960-1100 MPa, elongation ≥ 23%, -40°C impact toughness KV 8 ≥45J.

[0043] 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.

[0044] 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:

[0045] Smelting and continuous casting to obtain tube billets;

[0046] Heating, perforating, rolling and sizing;

[0047] Heat treatment: austenitizing temperature is 900-930℃, keep warm for 30-60min, then quench, then temper at 580-650℃, keep warm for 50-80min;

[0048] Heat sizing and straightening.

[0049] 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 580-650°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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] Furthermore, in the perforation step of the manufacturing method described in the present invention, the perforation temperature is 1180-1240°C.

[0054] Furthermore, in the rolling step of the manufacturing method described in the present invention, the final rolling temperature is 900-950°C.

[0055] 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 400-550°C.

[0056] The high-strength seamless pipe and the manufacturing method thereof of the present invention have the following advantages and beneficial effects:

[0057] The high-strength seamless pipe of the present invention adopts low-carbon, Cr-Mo and micro-alloy steel in composition design, adopts B to increase hardenability, and does not contain W and Ni alloy elements, thereby significantly reducing the material alloy cost and having good economic benefits.

[0058] The heat treatment process of the high-strength seamless pipe of the present invention is simple and easy to implement in large-scale production.

[0059] The high-strength seamless pipe of the present invention can be used to manufacture 890MPa 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 ≥890MPa, tensile strength is in the range of 960-1100MPa, and -40℃ impact toughness KV 8 ≥45J. DETAILED DESCRIPTION

[0060] 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.

[0061] Examples 1-6 and Comparative Examples 1-5

[0062] The high-strength seamless pipes of Examples 1-6 are prepared by the following steps:

[0063] (1) Smelting and continuous casting to obtain tube blanks:

[0064] 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.

[0065] The alloy is then cast into round billets. In some embodiments, the superheat of the molten steel can be controlled to be lower than 30° C. during the casting process, and the continuous casting speed can be 1.8-2.2 m / min to reduce component segregation.

[0066] (2) heating, punching, rolling and sizing the tube blank to obtain a tube body;

[0067] 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.

[0068] (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 580-650°C, with a holding time of 50-80 min;

[0069] (4) Heat sizing and straightening: In some embodiments, the temperature of heat sizing and straightening can be controlled to 400-550°C.

[0070] 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.

[0071] 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.

[0072] Table 1. (wt%, the balance is Fe and other inevitable impurities except P, S, N)

[0073]

[0074] 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.

[0075] 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.

[0076] Table 2-1.

[0077]

[0078] Table 2-2.

[0079]

[0080]

[0081] 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:

[0082] (1) Tensile test: The room temperature tensile property test is carried out according to GB / T 228.1-2000.

[0083] (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.

[0084] (3) Impact test: -40℃ impact energy test is carried out according to GB / T 229-2007 “Metallic Materials Charpy Pendulum Impact Test Method”.

[0085] (4) Welding performance testing: Welding evaluation shall be carried out in accordance with NB / T-47014-2011 Welding procedure assessment for pressure-bearing equipment.

[0086] Table 3 lists the test results of the high-strength seamless pipes of Examples 1-6 and Comparative Examples 1-5.

[0087] Table 3.

[0088]

[0089] 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 ≥960MPa, the tensile strength is above 1000MPa, and the -40℃ impact toughness KV 8 ≥70J, elongation ≥23%, grain size is between 8-18μm, and it also has good weldability.

[0090] However, since the chemical element composition of Comparative Examples 1-2 does not meet the design requirements of the present invention, Comparative Example 3 has too high a carbon equivalent, and Comparative Examples 4-5 do not meet the design requirements of the present invention due to the heat treatment process, they all fail to achieve the balance of high strength, excellent low-temperature impact toughness and good weldability.

[0091] 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.

[0092] 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 all should belong to 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: 0.14-0.20%, Si: 0.15-0.55%, Mn: 0.5-1.3%, Cr: 0.5-1.0%, Mo: 0.35-0.85%, V: 0.06-0.1 %, Nb: 0.02-0.06%, Ti: 0.01-0.05%, B: 0.001-0.005%, Al: 0.01-0.05%, Ca: 0.0005-0.005%.

2. The high-strength seamless pipe according to claim 1, characterized in that: The mass percentage of each element is: C: 0.14-0.20%, Si: 0.15-0.55%, Mn: 0.5-1.3%, Cr: 0.5-1.0%, Mo: 0.35-0.85%, V: 0.06-0.1%, Nb: 0.02-0.06%, Ti: 0.01-0.05%, B: 0.001-0.005%, 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: The mass percentage of each element further satisfies at least one of the following items: C：0.15-0.19％； Si: 0.15-0.35%; Mn: 0.6-1.1%; Cr：0.8-1.0％； Mo: 0.5-0.85%; Ti: 0.015-0.035%.

4. The high-strength seamless pipe according to claim 1 or 2, characterized in that: Among the unavoidable impurities: P ≤ 0.01%, S ≤ 0.002%, N ≤ 0.007%.

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

67.

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

7. The high-strength seamless pipe according to claim 6, characterized in that: Its grain size is 8-20μm.

8. The high-strength seamless pipe according to claim 1 or 2, characterized in that: Its performance meets the following requirements: yield strength ≥890MPa, tensile strength 960-1100MPa, elongation ≥23%, -40℃ impact toughness KV8 ≥45J.

9. The method for manufacturing a high-strength seamless pipe according to any one of claims 1 to 8, 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 580-650℃, keep warm for 50-80min; Heat sizing and straightening.

10. The manufacturing method according to claim 9, 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.

11. The manufacturing method according to claim 9, characterized in that: In the heating step, the heating is performed to a soaking temperature of 1200-1240°C.

12. The manufacturing method according to claim 9, characterized in that: In the perforating step, the perforating temperature is 1180-1240°C.

13. The manufacturing method according to claim 9, characterized in that: In the rolling step, the final rolling temperature is 900-950°C.

14. The manufacturing method according to claim 9, characterized in that: In the sizing step, the sizing temperature is 850-900°C.

15. The manufacturing method according to claim 9, characterized in that: The temperature of hot sizing and straightening is 400-550℃.

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