High-strength seamless tube and manufacturing method therefor

Abstract

The present disclosure provides a high-strength seamless tube, comprising Fe and inevitable impurities, and further comprising the following chemical elements in percentage by mass: 0.14-0.20% of C, 0.15-0.55% of Si, 0.5-1.3% of Mn, 0.5-1.0% of Cr, 0.35-0.85% of Mo, 0.06-0.1% of V, 0.02-0.06% of Nb, 0.01-0.05% of Ti, 0.001-0.005% of B, 0.01-0.05% of Al, and 0.0005-0.005% of Ca. The present disclosure further provides a manufacturing method for the seamless tube, comprising the following steps that are sequentially carried out: smelting and continuous casting; heating, piercing, rolling and sizing; heat treatment; austenitizing at the temperature of 900-930°C, performing heat preservation for 30-60 min, quenching, and then tempering at 580-650°C, the heat preservation time of tempering being 50-80 min; and hot sizing and straightening.

Description

A high-strength seamless pipe and its manufacturing method Technical Field

[0001] The present disclosure 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] Conventional high-strength seamless steel pipes currently contain high levels of W and Ni alloying elements. By adding low-carbon equivalent contributing elements such as tungsten and nickel, the low-temperature toughness of the steel can be improved while maintaining a certain strength and low carbon equivalent level.

[0004] For example, the Chinese patent document with publication number CN101586450A, published on November 25, 2009, and titled “A high-strength and high-toughness oil casing and its manufacturing method” discloses an oil casing with the following composition system: C: 0.16-0.28%, Si: ≤0.50%, Mn: 0.30-1.10%, Cr: 0.50-1.10%, Mo: 0.60-0.95%, Al: 0.015-0.015% and Cr: 0.015-0.015%. 0.060%, 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 remainder is iron. Ni is added to this oil casing, and the Cr content is also high.

[0005] However, the costs of elements such as Ni, W, and Cr are relatively high. Amidst the fierce competition in the homogeneous construction machinery market, there is a desire for high-strength seamless pipes with innovative component combinations that can maintain high strength while reducing or eliminating the content of high-cost alloying elements, thereby significantly reducing the cost of construction machinery components.

[0006] Summary of the Invention

[0007] One of the purposes of the present disclosure is to provide a seamless pipe that, through reasonable chemical element composition design, preferably combined with process design, can achieve higher strength on the basis of reducing the content of high-cost alloying elements or excluding high-cost alloying elements, while having excellent low-temperature impact toughness and good weldability.

[0008] In order to achieve the above-mentioned object, the present disclosure provides a seamless pipe, which, in addition to Fe and unavoidable impurities, further contains the following chemical elements in the following mass percentages:

[0009] 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%.

[0010] In addition, the present disclosure also provides a seamless pipe, which contains the following chemical elements in percentage by mass:

[0011] 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 unavoidable impurities.

[0012] In some embodiments, the mass percentage of each element of the seamless pipe of the present disclosure further satisfies at least one of the following: 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%.

[0013] In some embodiments, in the seamless pipe of the present disclosure, unavoidable impurities include P, S, and N, and P≤0.01%, S≤0.002%, and N≤0.007%.

[0014] In some embodiments, the carbon equivalent of the seamless pipe of the present disclosure is ≤0.68, preferably ≤0.67, and the carbon equivalent is calculated by the following formula: carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where the symbol of each element is substituted into the value before the percentage sign of the mass percentage of the corresponding chemical element.

[0015] In some embodiments, the microstructure of the seamless pipe of the present disclosure is tempered troostite.

[0016] In some embodiments, the seamless pipe of the present disclosure has an average grain size of 8-20 μm.

[0017] In some embodiments, the seamless pipe of the present disclosure has a yield strength ≥890 MPa, preferably a yield strength ≥960 MPa, a tensile strength of 960-1100 MPa, preferably a tensile strength of 1000-1100 MPa, an elongation ≥23%, and a -40°C longitudinal impact toughness KV8 ≥45 J, preferably KV8 ≥70 J.

[0018] Another object of the present disclosure is to provide a method for manufacturing seamless pipes. This method utilizes a unique heat treatment process, combined with the composition design of the seamless pipe disclosed herein, to produce seamless pipes with high strength, excellent low-temperature impact toughness, and good weldability while reducing or eliminating the content of high-cost alloying elements.

[0019] In order to achieve the above object, the present disclosure provides a method for manufacturing a seamless pipe, which comprises the following steps performed in sequence:

[0020] -Smelting and continuous casting to obtain tube billets;

[0021] -Heating, perforating, rolling and sizing the tube blank to obtain the tube body;

[0022] - Heat treatment of the tube body: austenitizing at 900-930°C for 30-60 min, quenching, and then tempering at 580-650°C for 50-80 min;

[0023] -Hot sizing and straightening.

[0024] In some embodiments, a mixture of scrap steel and blast furnace hot metal can be used during the smelting step. For example, the hot metal ratio is 50-60%. The molten steel is smelted in an electric furnace, subjected to external refining, vacuum degassing, and argon stirring, and then treated with Ca to denature inclusions, thereby reducing O and H content.

[0025] In the manufacturing method disclosed herein, the heat treatment step adopts the above-mentioned process parameters (i.e., the austenitizing temperature is 900-930°C, quenching after holding for 30-60 minutes, and then tempering at 580-650°C for 50-80 minutes) because: by re-austenitizing and water quenching the rolled tube, on the one hand, the original austenite grains can be refined, and on the other hand, the element distribution can be made more uniform. Then, by tempering, the precipitation strengthening phase is made more uniformly dispersed, thereby improving the strength-toughness matching performance.

[0026] In some preferred embodiments, in the continuous casting step of the manufacturing method disclosed herein, the superheat of the molten steel is controlled to be below 30° C., and the continuous casting speed is 1.8-2.2 m / min, thereby effectively reducing component segregation.

[0027] In some embodiments, in the heating step of the manufacturing method of the present disclosure, the tube blank is heated to a soaking temperature of 1200-1240°C.

[0028] In some embodiments, in the perforating step of the manufacturing method of the present disclosure, the perforating temperature is 1180-1240°C.

[0029] In some embodiments, in the rolling step of the manufacturing method of the present disclosure, the finishing rolling temperature is 900-950°C.

[0030] In some embodiments, in the sizing step of the manufacturing method of the present disclosure, the sizing temperature is 850-900°C.

[0031] In some embodiments, in the manufacturing method of the present disclosure, the temperature of the hot sizing and straightening is 400-550°C.

[0032] The seamless pipe and the manufacturing method thereof disclosed herein have the following advantages and beneficial effects:

[0033] The seamless pipe disclosed herein is designed with low carbon, Cr-Mo and micro-alloy steel grades, and uses B to increase hardenability. It does not contain W and Ni alloy elements, thereby significantly reducing material alloy costs and achieving good economic benefits.

[0034] The heat treatment process of the seamless pipe disclosed herein is simple and easy to implement in large-scale production.

[0035] The seamless pipe disclosed herein can be used to manufacture 890 MPa grade structural pipe products, and has good strength, low-temperature toughness and welding performance.

[0036] The seamless pipe disclosed herein is designed using low-carbon, Cr-Mo and micro-alloy steel grades, and the carbon equivalent of the steel grades can be controlled at a low level to achieve high strength while having excellent low-temperature impact toughness and good weldability. DETAILED DESCRIPTION

[0037] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

[0038] In this article, carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where the symbol of each element is substituted into the value before the percentage sign of the mass percentage of the corresponding chemical element.

[0039] In this paper, the yield strength, tensile strength and elongation were measured according to GB / T 228.1-2000.

[0040] In this paper, the -40℃ longitudinal impact toughness KV8 is measured according to the standard GB / T 229-2007 "Charpy pendulum impact test method for metallic materials".

[0041] In this paper, metallographic microscope was used to observe the microstructure.

[0042] Specifically, the design principles of the chemical elements of the seamless pipe disclosed herein are as follows:

[0043] Carbon (C): Carbon is an essential element for ensuring strength and hardenability. Its content also affects carbon equivalent and weldability. If the C content in steel is less than 0.14%, the steel's hardenability is poor and its strength fails to meet requirements. If the C content exceeds 0.20%, the excessive C content leads to poor weldability and reduced plasticity and toughness. Therefore, in the seamless pipe disclosed herein, the mass percentage of C is controlled between 0.14% and 0.20%.

[0044] In some embodiments, the mass percentage of C can be further controlled between 0.15-0.19%.

[0045] Si: In the seamless pipe disclosed herein, Si is an important deoxidizer in steel. The Si element can be dissolved in ferrite to increase the yield strength of the steel. However, it should be noted that the content of the Si element in the steel should not be too high. When the Si element content in the steel is too high, for example, higher than 0.55%, the processing performance and toughness of the steel will deteriorate. If the Si element content in the steel is too low, for example, lower than 0.15%, the steel will easily oxidize. Therefore, in the seamless pipe disclosed herein, the mass percentage of the Si element is controlled between 0.15-0.55%.

[0046] In some embodiments, the mass percentage of Si may be further controlled between 0.15-0.35%.

[0047] Mn: In the seamless pipe disclosed herein, Mn acts as a deoxidizing and desulfurizing agent, significantly impacting the hardenability and toughness of the steel. A Mn content of 0.5% or greater in the steel produces excellent results. However, excessive Mn can increase the hardenability of the steel excessively, reducing the toughness of the heat-affected zone (HAZ) of the weld. It can also cause center segregation during continuous casting, deteriorating the low-temperature impact toughness of the base material. Therefore, in the seamless pipe disclosed herein, the mass percentage of Mn is controlled between 0.5% and 1.3%.

[0048] In some embodiments, the mass percentage of Mn can be further controlled between 0.6-1.1%.

[0049] Cr: In the seamless pipe disclosed herein, Cr increases the strength and hardenability of the steel, and its combination with Mo is particularly effective. However, it should be noted that excessive Cr content in the steel significantly increases the carbon equivalent, increasing weld cracking susceptibility and reducing the toughness of the weld heat-affected zone. Therefore, in the seamless pipe disclosed herein, the mass percentage of Cr is controlled between 0.5% and 1.0%.

[0050] In some embodiments, the mass percentage of Cr may be further controlled between 0.8-1.0%.

[0051] Mo: In the seamless pipe disclosed herein, Mo is one of the main added elements, which can improve the hardenability of steel. The combined effect of Mo and Cr is more effective in improving hardenability. In addition, the Mo element also has good precipitation strengthening and solid solution strengthening effects. Mo has a good interaction with microalloying elements, which can effectively refine the precipitate phase, increase the stability and volume fraction of the precipitate phase, and improve the strength and toughness of the weld heat affected zone. However, it should be noted that the Mo element is expensive. Adding too high a content of Mo to steel will not only lead to an excessively high carbon equivalent, but also increase the cost of the alloy. Therefore, in the seamless pipe disclosed herein, the mass percentage of the Mo element is controlled between 0.35-0.85%.

[0052] In some embodiments, the mass percentage of Mo can be further controlled between 0.5-0.85%.

[0053] V: In the seamless pipe disclosed herein, V can refine the steel grains, and the carbides it forms can significantly increase the steel's strength. However, after the V content reaches a certain level, further increases in V addition yield less significant reinforcement and can also lead to decreased low-temperature impact toughness stability. Furthermore, V is a relatively expensive alloying element and should not be added in excess. Therefore, in the seamless pipe disclosed herein, the mass percentage of V is controlled between 0.06% and 0.1%.

[0054] Nb: In the seamless pipe disclosed herein, Nb acts as a grain-refining and precipitation-strengthening element, compensating for the strength loss caused by reduced carbon content. When the Nb content is less than 0.02%, its effect is insignificant. When the Nb content exceeds 0.06%, coarse Nb(CN) particles tend to form, reducing toughness. Therefore, in the seamless pipe disclosed herein, the mass percentage of Nb is controlled between 0.02% and 0.06%.

[0055] Al: In the seamless pipe disclosed herein, Al is a preferred deoxidizing element. However, excessive Al addition to steel can easily form alumina inclusions. To maximize the proportion of acid-soluble aluminum in the total aluminum content, Al wire is typically added in appropriate amounts after vacuum degassing. Furthermore, in the seamless pipe disclosed herein, Al can also combine with nitrogen to form AlN, thereby ensuring the hardenability effect of B. Therefore, in the seamless pipe disclosed herein, the mass percentage of Al is controlled between 0.01-0.05%.

[0056] Ca: In the seamless pipe disclosed herein, Ca purifies the molten steel, promotes the spheroidization of MnS, and improves the material's impact toughness. However, it's important to note that the Ca content in the steel should not be too high. Excessive Ca content in steel can easily form coarse non-metallic inclusions, adversely affecting the steel's performance. Therefore, in the seamless pipe disclosed herein, the mass percentage of Ca is controlled between 0.0005% and 0.005%.

[0057] Ti: In the seamless pipe disclosed herein, Ti is a strong carbonitride-forming element that can significantly refine austenite grains at high temperatures. During the rapid cooling process of the steel, the Ti carbonitrides in the steel can act as nucleation sites to promote the refinement of the rapid cooling structure, thereby refining the final quenched and tempered structure, while preventing the coarsening of the heat-affected zone during welding. Furthermore, the addition of an appropriate amount of Ti to the steel also contributes to the strength of the steel, compensating for the decrease in strength caused by the reduction in the C content in the steel. It should be noted that the Ti content in the steel should not be too high. When the Ti content in the steel exceeds 0.05%, coarse TiN is easily formed, thereby reducing the toughness of the material. However, when the Ti content in the steel is too low, Ti cannot effectively achieve the refining effect. In the present disclosure, Ti is combined with B and N, thereby ensuring that B exerts its effect of improving hardenability and preventing B and N from forming brittle phases. Therefore, in the seamless pipe of the present disclosure, the mass percentage of the Ti element is controlled to be between 0.01-0.05%.

[0058] In some embodiments, the mass percentage of the Ti element may be further controlled to be between 0.015-0.035%.

[0059] B: In the seamless pipe disclosed herein, B is an element that significantly enhances hardenability. Its hardenability can replace a certain amount of Mn, Mo, and Cr, significantly reducing alloy costs. B, when used in conjunction with Al and Ti, ensures effective hardenability. However, excessive B content can lead to the formation of brittle phases and deteriorate impact toughness. Therefore, in the seamless pipe disclosed herein, the mass percentage of B is controlled between 0.001% and 0.005%.

[0060] It should be noted that the unavoidable impurity elements in the seamless pipe disclosed herein mainly include S, P and N. Where technical conditions permit, in order to obtain seamless steel pipes with better performance and higher 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.

[0061] DETAILED DESCRIPTION

[0062] The seamless pipe and the manufacturing method thereof disclosed herein will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an undue limitation to the technical solution disclosed herein.

[0063] Examples 1-6 and Comparative Examples 1-5

[0064] The seamless pipes of Examples 1-6 were prepared by the following steps:

[0065] (1) Smelting and continuous casting to obtain tube blanks:

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

[0067] (3) Heat treatment of the tube body: austenitizing temperature is 900-930℃, holding time is 30-60min, then quenching, and then tempering at 580-650℃, holding time of tempering is 50-80min;

[0068] (4) Hot sizing and straightening.

[0069] The comparative tubes of Comparative Examples 1-5 were prepared using the same steps as above, with the only difference being the component contents and / or specific process parameters.

[0070] Table 1 lists the mass percentage content of each chemical element in the seamless pipes of Examples 1-6 and the control pipes of Comparative Examples 1-5.

[0071] Table 1. (wt%, the balance is Fe and other inevitable impurities except P, S, and N) Note: Carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where the symbols of each element are substituted into the value before the percentage sign of the mass percentage of the corresponding chemical element.

[0072] Table 2-1 and Table 2-2 list the process parameters of the seamless pipes of Examples 1-6 and the control pipes of Comparative Examples 1-5.

[0073] Table 2-1.

[0074] Table 2-2.

[0075] The seamless pipes of Examples 1-6 and the control pipes of Comparative Examples 1-5 were sampled and tested, and the test results are listed in Table 3. The test process of the relevant properties is as follows:

[0076] (1) Tensile properties: The room temperature tensile properties were measured according to GB / T 228.1-2000.

[0077] (2) Microstructure: Take a cross section of the tube for microstructure observation. Grind and polish the sample, and use 4% nitric acid + alcohol to etch the metallographic structure. After etching, observe the structure using a metallographic microscope.

[0078] (3) Impact toughness: The longitudinal impact energy KV8 at -40°C is measured in accordance with GB / T 229-2007 “Charpy pendulum impact test method for metallic materials”.

[0079] (4) Welding performance: Welding evaluation shall be carried out in accordance with NB / T-47014-2011 Welding procedure assessment for pressure-bearing equipment.

[0080] Table 3 lists the test results of the seamless pipes of Examples 1-6 and the control pipes of Comparative Examples 1-5.

[0081] Table 3.

[0082] As shown in Table 3, the seamless pipes of Examples 1-6 all exhibited yield strengths ≥960 MPa, tensile strengths above 1000 MPa, -40°C impact toughness KV8 ≥70 J, elongation ≥23%, and grain sizes between 8 and 18 μm. Furthermore, they exhibited good weldability. In contrast, Comparative Examples 1-5 failed to achieve a balance of high strength, excellent low-temperature impact toughness, and good weldability.

[0083] All publications, patent applications, patents, and other references mentioned in this disclosure are incorporated by reference in their entirety.

[0084] Although the present disclosure has been illustrated and described with reference to certain preferred embodiments thereof, it should be understood by those skilled in the art that the above description is provided to further illustrate the present disclosure in conjunction with specific embodiments thereof, and that the present disclosure is not limited to these descriptions. Those skilled in the art may make various changes in form and detail, including simple deductions or substitutions, without departing from the spirit and scope of the present disclosure.

Claims

1. A seamless pipe, wherein: In addition to Fe and unavoidable impurities, the seamless pipe 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 seamless pipe according to claim 1, wherein: The seamless pipe contains the following chemical elements in percentage by mass: 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 unavoidable impurities.

3. The seamless pipe according to claim 1 or 2, wherein: The mass percentage of each element of the seamless pipe further satisfies at least one of the following: 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 seamless pipe according to any one of claims 1 to 3, wherein The unavoidable impurities include P, S and N, and P≤0.01%, S≤0.002%, and N≤0.007%.

5. The seamless pipe according to any one of claims 1 to 4, wherein The carbon equivalent of the seamless pipe is ≤0.68, preferably ≤0.67, and the carbon equivalent is calculated by the following formula: Carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where the symbol of each element is substituted into the value before the percentage sign of the mass percentage of the corresponding chemical element.

6. The seamless pipe according to any one of claims 1 to 5, wherein The microstructure of the seamless pipe is tempered troostite.

7. The seamless pipe according to any one of claims 1 to 6, wherein The average grain size of the seamless pipe is 8-20 μm.

8. The seamless pipe according to any one of claims 1 to 7, wherein The yield strength of the seamless pipe is ≥890 MPa, preferably ≥960 MPa, the tensile strength is 960-1100 MPa, preferably 1000-1100 MPa, the elongation is ≥23%, and the longitudinal impact toughness KV8 at -40°C is ≥45J, preferably KV8 is ≥70J.

9. A method for producing a seamless pipe according to any one of claims 1 to 8, wherein: The method comprises the following steps performed in sequence: - Smelting and continuous casting to obtain tube blanks; -Heating, perforating, rolling and sizing the tube blank to obtain the tube body; - Heat treatment of the tube body: austenitizing temperature is 900-930℃, holding time is 30-60min, quenching, and then tempering at 580-650℃, holding time of tempering is 50-80min; -Hot sizing and straightening.

10. The method of claim 9, wherein: In the continuous casting step, the superheat of the molten steel is below 30°C, and the continuous casting speed is 1.8-2.2 m / min.

11. The method according to claim 9 or 10, wherein: In the heating step, the tube blank is heated to a soaking temperature of 1200-1240°C.

12. The method according to any one of claims 9 to 11, wherein: In the perforating step, the perforating temperature is 1180-1240°C.

13. The method according to any one of claims 9 to 12, wherein: In the rolling step, the final rolling temperature is 900-950°C.

14. The method according to any one of claims 9 to 13, wherein: In the sizing step, the sizing temperature is 850-900°C.

15. The method according to any one of claims 9 to 14, wherein: The temperature of hot sizing and straightening is 400-550℃.

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