High-strength seamless pipe and manufacturing method therefor

Abstract

The present disclosure provides a seamless pipe, which contains Fe and unavoidable impurities, and further contains the following chemical elements in percentage by mass: 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%, and Ca: 0.0005-0.005%. The seamless pipe does not contain the elements Ni and W. The present disclosure further provides a manufacturing method for the seamless pipe, comprising the following steps: smelting and continuous casting, to obtain a pipe blank; heating, piercing, rolling, and sizing the pipe blank, to obtain a pipe body; performing heat treatment on the pipe body, the austenitizing temperature being 900-930°C and the holding time being 30-60 min, quenching, then tempering at 550-650°C, the tempering holding time 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: 0.19-0.23%, 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%;

[0010] The seamless pipe does not contain Ni and W elements.

[0011] In addition, the present disclosure also provides a seamless pipe, the mass percentage of each element of which is:

[0012] 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 unavoidable impurities.

[0013] In some embodiments, in the seamless pipe of the present disclosure, unavoidable impurities include P, S, and N, and P≤0.015%, S≤0.003%, and N≤0.007%.

[0014] In some embodiments, the seamless pipe of the present disclosure has a carbon equivalent of ≤ 0.55, preferably a carbon equivalent of ≤ 0.48.

[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 10-18 μm.

[0017] In some embodiments, the seamless pipe of the present disclosure has a yield strength of ≥770 MPa, a tensile strength of 820-1000 MPa, preferably 860-1000 MPa, an elongation of ≥23%, and a longitudinal impact toughness KV8 of ≥45 J at -40°C, preferably KV8 of ≥80 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:

[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 550-650°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 550-650°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 450-550°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 has a higher C content and uses B to increase hardenability in its composition design, replacing W and Ni alloy elements used in conventional steels to increase hardenability. At the same time, the addition amount of Cr and Mo elements is reduced. Therefore, the material alloy used has a low cost and can achieve 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 770 MPa-grade structural pipe products and has good strength, low-temperature toughness and welding performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG1 shows the metallographic structure of the seamless pipe of Example 1.

[0037] FIG2 shows the microstructure morphology (grain size) of the seamless pipe of Example 1. DETAILED DESCRIPTION

[0038] 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.

[0039] 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.

[0040] In this paper, the yield strength, tensile strength and elongation were measured according to GB / T 228.1-2000.

[0041] 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".

[0042] In this paper, metallographic microscope was used to observe the microstructure.

[0043] In the seamless pipe disclosed herein, the design principles of the various chemical elements are as follows:

[0044] C: In the seamless pipe disclosed herein, C is a carbide-forming element that increases the strength of the steel. When the C content is less than 0.19%, hardenability is reduced, thereby decreasing toughness. When the C content is greater than 0.23%, embrittlement occurs in the heat-affected zone during welding, reducing weldability. Therefore, the mass percentage of C in this disclosure is controlled between 0.19% and 0.23%.

[0045] Si: In the seamless pipe disclosed herein, Si can dissolve in ferrite to increase the yield strength of the steel. However, the Si content should not be too high, as it can degrade workability and toughness. Furthermore, when the Si content is below 0.1%, the steel is susceptible to oxidation. Therefore, the mass percentage of Si in the present disclosure is controlled between 0.1% and 0.6%.

[0046] Mn: In the seamless pipe disclosed herein, Mn is an austenite-forming element that improves the steel's hardenability. In this steel system, a Mn content less than 0.6% significantly reduces the steel's hardenability, lowering the martensite fraction and, consequently, toughness. A Mn content greater than 1.5% results in a high carbon equivalent and deteriorates weldability. Therefore, the present disclosure limits the mass percentage of Mn to between 0.6% and 1.5%.

[0047] Cr: In the seamless pipe disclosed herein, Cr is a strong carbide-forming element that significantly enhances hardenability. It precipitates carbides during tempering, increasing the steel's strength. However, a Cr content above 0.4% increases the carbon equivalent and degrades weldability. A Cr content below 0.1% makes it difficult to improve hardenability, significantly reducing the compatibility between product strength and low-temperature toughness. Therefore, the present disclosure limits the Cr content to between 0.1% and 0.4%.

[0048] Mo: In the seamless pipe disclosed herein, Mo improves steel strength and tempering stability through carbide formation and solid solution strengthening, while also improving low-temperature impact toughness. In this steel system, a Mo content above 0.18% increases the carbon equivalent and deteriorates weldability. A Mo content below 0.1% significantly decreases hardenability, leading to significant reductions in strength and low-temperature toughness. Therefore, the present disclosure limits the Mo content to between 0.1% and 0.18%.

[0049] Nb: In the seamless pipe disclosed herein, Nb is a grain refining and precipitation strengthening element that can compensate for the decrease in strength caused by the reduction in carbon content. At the same time, during the welding process, the Nb element can inhibit the growth of grains 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%, coarse Nb (CN) is easily formed, thereby reducing toughness. Therefore, the present disclosure controls the mass percentage of the Nb element to be between 0.02-0.04%.

[0050] Ti: In the seamless pipe disclosed herein, 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 in carbon content. At the same time, the carbonitride formed by Ti has high thermal stability, and during the welding process, it has a good effect on suppressing the coarsening of the structure in the heat-affected zone of the weld, 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 disclosure controls the mass percentage of the Ti element to be between 0.01-0.03%.

[0051] B: In the seamless pipe disclosed herein, element B significantly improves hardenability. When the B content is below 0.0015%, the hardenability improvement is insignificant. When the B content is above 0.005%, it increases the material's cold cracking susceptibility. Therefore, the present disclosure limits the mass percentage of B to between 0.0015% and 0.005%.

[0052] Al: Al is mainly used for deoxidation treatment, which can effectively control the oxygen content in the material. Based on this, the present disclosure controls the mass percentage of Al element to 0.01-0.05%.

[0053] Ca: In the seamless pipe disclosed herein, Ca purifies the molten steel, promotes the spheroidization of MnS, and improves impact toughness. However, excessive Ca content can easily form coarse non-metallic inclusions. Therefore, the mass percentage of Ca in the disclosed seamless pipe is controlled between 0.0005% and 0.005%.

[0054] It should be noted that the unavoidable impurity elements in the seamless pipe disclosed herein mainly include S, P and N. Under the conditions of technical conditions, 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:

[0055] When the nitrogen content exceeds 0.007%, a large amount of Ti nitrides will precipitate in the structure. These nitride precipitates are large and have sharp angles, which significantly reduces low-temperature impact toughness. Therefore, in some embodiments, the present disclosure controls the mass percentage of nitrogen to below 0.007%.

[0056] Excessive P and S content will affect the toughness of the steel. Therefore, in some embodiments, the present disclosure controls the mass percentages of P and S elements to P≤0.015% and S≤0.003%, respectively.

[0057] The seamless pipe and the manufacturing method thereof disclosed in the present invention 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 of the present invention.

[0058] Examples 1-6 and Comparative Examples 1-5

[0059] The seamless pipes of Examples 1-6 were prepared by the following steps:

[0060] (1) Smelting and continuous casting to obtain tube blanks:

[0061] (2) heating, punching, rolling, and sizing the tube blank to obtain a tube body;

[0062] (3) Heat treatment of the tube body: austenitizing temperature is 900-930℃, holding time is 30-60min, then quenching, and then tempering at 550-650℃, holding time of tempering is 50-80min;

[0063] (4) Hot sizing and straightening.

[0064] 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.

[0065] 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.

[0066] Table 1. (wt%, the balance is Fe and other inevitable impurities except P, S, and N)

[0067] 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.

[0068] 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.

[0069] Table 2-1.

[0070] Table 2-2.

[0071] 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:

[0072] (1) Tensile properties: The room temperature tensile properties were measured according to GB / T 228.1-2000.

[0073] (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.

[0074] (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”.

[0075] (4) Welding performance: Welding evaluation shall be carried out in accordance with NB / T-47014-2011 Welding procedure assessment for pressure-bearing equipment.

[0076] Table 3 lists the test results of the seamless pipes of Examples 1-6 and the control pipes of Comparative Examples 1-5.

[0077] Table 3.

[0078] As can be seen from Table 3, the seamless pipes of Examples 1-6 all have a yield strength ≥780 MPa, a tensile strength between 860-935 MPa, an impact toughness KV8 at -40°C ≥80 J, an elongation ≥23%, an average grain size between 10-18 μm, and good weldability.

[0079] In addition, Figure 1 shows the metallographic structure of the seamless pipe of Example 1. As shown in Figure 1 , the microstructure of the seamless pipe is a uniform and fine tempered sorbite structure.

[0080] FIG2 shows the microstructure of the seamless pipe of Example 1, from which it can be seen that the grains are fine and uniform.

[0081] Comparative Examples 1-5 cannot have both excellent low-temperature impact toughness and good weldability.

[0082] All publications, patent applications, patents, and other references mentioned in this disclosure are incorporated by reference in their entirety.

[0083] 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: 0.19-0.23%, 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%; The seamless pipe does not contain Ni and W elements.

2. The seamless pipe according to claim 1, wherein: The mass percentage of each element of the seamless pipe 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 unavoidable impurities.

3. The seamless pipe according to claim 1 or 2, wherein: The unavoidable impurities include P, S and N, and P≤0.015%, S≤0.003%, and N≤0.007%.

4. The seamless pipe according to any one of claims 1 to 3, wherein The carbon equivalent of the seamless pipe is ≤0.55, preferably ≤0.48, 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 content of the corresponding chemical element.

5. The seamless pipe according to any one of claims 1 to 4, wherein The microstructure of the seamless pipe is tempered troostite.

6. The seamless pipe according to any one of claims 1 to 5, wherein The average grain size of the seamless pipe is 10-18 μm.

7. The seamless pipe according to any one of claims 1 to 6, wherein The yield strength of the seamless pipe is ≥770MPa, the tensile strength is 820-1000MPa, preferably 860-1000MPa, the elongation is ≥23%, and the longitudinal impact toughness KV8 at -40°C is ≥45J, preferably KV8 ≥80J.

8. A method for manufacturing the seamless pipe according to any one of claims 1 to 7, comprising the following steps: - 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 550-650℃, holding time of tempering is 50-80min; -Hot sizing and straightening.

9. The method of claim 8, 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.

10. The method according to claim 8 or 9, wherein: In the heating step, the tube blank is heated to a soaking temperature of 1200-1240°C.

11. The method according to any one of claims 8 to 10, wherein: In the perforating step, the perforating temperature is 1180-1240°C.

12. The method according to any one of claims 8 to 11, wherein: In the rolling step, the final rolling temperature is 900-950°C.

13. The method according to any one of claims 8 to 12, wherein: In the sizing step, the sizing temperature is 850-900°C.

14. The method according to any one of claims 8 to 13, wherein: The temperature of hot sizing and straightening is 450-550℃.

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