High-strength seamless pipe and manufacturing method therefor

Abstract

The present disclosure provides a seamless tube, which comprises Fe and unavoidable impurities, and further comprises 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%, Ca: 0.0005-0.005%; wherein the seamless tube does not contain Ni or W. The present disclosure also provides a method of manufacturing the seamless tube, comprising the following steps: performing smelting and continuous casting to obtain a tube billet; subjecting the tube billet to heating, perforating, rolling, and sizing to obtain a tube body; subjecting the tube body to a heat treatment: austenitizing at a temperature of 900-930°C with a holding time of 30-60 min, quenching, and then tempering at a temperature of 550-650°C with a holding time of 50-80 min; and hot sizing and straightening.

Description

TECHNICAL FIELD

[0001] This disclosure relates to a steel material and a manufacturing method therefor, and more particularly to a seamless tube and a manufacturing method therefor.BACKGROUND

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

[0003] Currently, conventional high-strength seamless steel tubes all 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, and a certain strength and a low carbon equivalent level can be guaranteed.

[0004] For example, Chinese patent publication No. CN101586450A, published on November 25, 2009, entitled "HIGH-STRENGTH AND HIGH-TOUGHNESS OIL CASING AND MANUFACTURING METHOD THEREOF" discloses an oil casing, comprising the following components: C: 0.16-0.28%, Si: ≤0.50%, Mn: 0.30-1.10%, Cr: 0.50-1.10%, Mo: 0.60-0.95%, Al: 0.015-0.060%, wherein 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 being Fe. In the oil casing, the Ni element is added, and the Cr content is high.

[0005] However, the cost of elements such as Ni, W, and Cr is relatively high. In the context of a highly homogeneous and fiercely competitive engineering machinery industry, there is a need for a high-strength seamless tube with an innovative composition design, which can reduce the content of high-cost alloy elements or eliminate high-cost alloy elements while maintaining high strength performance, thereby significantly reducing the cost of engineering machinery components.SUMMARY

[0006] One of the objects of the present disclosure is to provide a seamless tube, which, through a reasonable design of the chemical composition, preferably in combination with process design, can achieve higher strength on the basis of reducing the content of high-cost alloy elements or eliminating high-cost alloy elements, while having excellent low-temperature impact toughness and good weldability.

[0007] In order to achieve the above object, the present disclosure provides a seamless tube, comprising, in addition to Fe and unavoidable impurities, 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%, Ca: 0.0005-0.005%; wherein the seamless tube does not contain Ni or W.

[0008] Furthermore, the present disclosure also provides a seamless tube consisting of 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%, Ca: 0.0005-0.005%; with the balance being Fe and unavoidable impurities.

[0009] In some embodiments, in the seamless tube according to the present disclosure, the unavoidable impurities comprise P, S, and N, with P ≤ 0.015%, S ≤ 0.003%, and N ≤ 0.007%.

[0010] In some embodiments, the seamless tube according to the present disclosure has a carbon equivalent of ≤0.55, preferably ≤0.48.

[0011] In some embodiments, the seamless tube according to the present disclosure has a microstructure of tempered sorbite.

[0012] In some embodiments, the seamless tube according to the present disclosure has an average grain size of 10-18 µm.

[0013] In some embodiments, the seamless tube according to 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 KV 8 at -40°C of ≥45 J, preferably KV 8 ≥80 J.

[0014] In addition, another object of the present disclosure is to provide a method of manufacturing a seamless tube. The manufacturing method employs a unique heat treatment process, together with the composition design of the seamless tube according to the present disclosure. Using this method, it is possible to obtain a seamless tube with high strength, excellent low-temperature impact toughness, and good weldability, while reducing or eliminating high-cost alloy elements.

[0015] In order to achieve the above object, the present disclosure provides a method of manufacturing a seamless tube, comprising the following steps: performing smelting and continuous casting to obtain a tube billet; subjecting the tube billet to heating, perforating, rolling, and sizing to obtain a tube body; subjecting the tube body to a heat treatment: austenitizing at a temperature of 900-930°C with a holding time of 30-60 min, quenching, and then tempering at a temperature of 550-650°C with a holding time of 50-80 min; and hot sizing and straightening.

[0016] In some embodiments, in the smelting step, an ingredient formulation of scrap steel + blast furnace molten iron may be adopted. For example, the proportion of the molten iron is 50-60%. The molten steel is smelted in the electric furnace, refined outside the furnace, vacuum degassed and stirred under argon gas, and then subjected to inclusion modification by Ca treatment, thereby reducing the content of O and H.

[0017] In the manufacturing method according to the present disclosure, the reason for using the above process parameters (i.e., austenitizing at a temperature of 900-930°C with a holding time of 30-60 min, quenching, and then tempering at a temperature of 550-650°C with a holding time of 50-80 min) in the heat treatment step is that: by subjecting the rolled tube to re-austenitizing and water-quenching, on the one hand, the original austenite grains can be refined, and on the other hand, the element distribution can be made more uniform, and then by performing the tempering, the precipitated strengthening phase is more uniformly dispersed, thereby improving the strength-toughness matching performance.

[0018] In some preferred embodiments, in the continuous casting step of the manufacturing method according to the present disclosure, a superheat of the molten steel is controlled to be 30 °C or below. and a continuous casting speed is controlled to be 1.8-2.2 m / min. Therefore, component segregation can be effectively reduced.

[0019] In some embodiments, in the heating step of the manufacturing method according to the present disclosure, the tube billet is heated to a soaking temperature of 1200-1240 °C.

[0020] In some embodiments, in the perforating step of the manufacturing method according to the present disclosure, a perforating temperature is 1180-1240°C.

[0021] In some embodiments, in the rolling step of the manufacturing method according to the present disclosure, a final rolling temperature is 900-950°C.

[0022] In some embodiments, in the sizing step of the manufacturing method according to the present disclosure, a sizing temperature is 850-900°C.

[0023] In some embodiments, in the manufacturing method according to the present disclosure, a temperature for heat sizing and straightening is 450-550°C.

[0024] The seamless tube and the manufacturing method therefor according to the present disclosure offer the following advantages and beneficial effects: In terms of composition design, the seamless tube according to the present disclosure increases the C content and utilizes B to enhance hardenability, thereby replacing the W and Ni alloying elements conventionally used for improving hardenability in conventional steel grades. Additionally, it reduces the addition amounts of Cr and Mo elements. Consequently, the alloy material cost is low, resulting in favorable economic benefits.

[0025] The heat treatment process of the seamless tube is simple and easy to implement in large-scale production.

[0026] The seamless tube according to the present disclosure can be used to manufacture structural tube products with a 770 MPa grade, offering excellent strength, low-temperature toughness, and weldability.BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG.1 shows the metallographic structure of the seamless tube in Example 1. FIG.2 shows the microstructural morphology (grain size) of the seamless tube in Example 1. DETAILED DESCRIPTION

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art to which the present disclosure pertains.

[0029] Herein, carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where each chemical element symbol is substituted with the numerical value before the percent sign of the mass percentage content of corresponding chemical element.

[0030] Herein, the yield strength, tensile strength and elongation are determined in accordance with the GB / T 228.1-2000 standard.

[0031] Herein, the longitudinal impact toughness KV 8 at -40°C is determined in accordance with the GB / T 229-2007 standard, "Metallic materials - Charpy pendulum impact test method."

[0032] Herein, the microstructure is observed using a metallographic microscope.

[0033] In the seamless tube according to the present disclosure, the design principles for each chemical element are as follows: C: In the seamless tube according to the present disclosure, C element is a carbide-forming element that can increase the strength of the steel. When the content of C element is less than 0.19%, hardenability is reduced, thereby lowering toughness. When the content of C element exceeds 0.23%, it can cause embrittlement in the heat-affected zone during welding, impairing weldability. Therefore, the present disclosure controls the mass percentage of C element to 0.19-0.23%.

[0034] Si: In the seamless tube according to the present disclosure, Si element is solid-dissolved in ferrite to enhance the yield strength of the steel. However, the content of Si element should not be too high, as excessive amounts can deteriorate workability and toughness. Moreover, when the content of Si element is less than 0.1%, the steel is prone to oxidation. Therefore, the present disclosure controls the mass percentage of Si element to 0.1-0.6%.

[0035] Mn: In the seamless tube according to the present disclosure, Mn is an austenite-forming element that can enhance the hardenability of the steel. Within the system of the present steel grade, when the Mn content is less than 0.6%, the hardenability of the steel is significantly reduced, lowering the martensite fraction and thereby diminishing toughness. When the Mn content exceeds 1.5%, it will lead to an excessively high carbon equivalent, impairing weldability. Therefore, the present disclosure controls the mass percentage of Mn element to 0.6-1.5%.

[0036] Cr: In the seamless tube according to the present disclosure, Cr is an element that strongly enhances hardenability and is a strong carbide-forming element, which can precipitate carbides during tempering to increase the strength of the steel. However, when the Cr content exceeds 0.4%, it causes an increase in carbon equivalent, deteriorating weldability. When the Cr content is less than 0.1%, it becomes difficult to improve hardenability, leading to a significant deterioration in the balance between strength and low-temperature toughness of the product. Therefore, the present disclosure controls the mass percentage of Cr element to 0.1-0.4%.

[0037] Mo: In the seamless tube according to the present disclosure, the Mo element improves the strength and temper stability of the steel through carbide and solid solution strengthening, while also enhancing low-temperature impact toughness. In the system of the present steel grade, when the content of Mo element exceeds 0.18%, it leads to an increase in carbon equivalent, deteriorating weldability. When the content of Mo element is less than 0.1%, the hardenability decreases significantly, resulting in a notable reduction in both strength and low-temperature toughness. Therefore, the present disclosure controls the mass percentage of Mo element to 0.1-0.18%.

[0038] Nb: In the seamless tube according to the present disclosure, Nb acts as a grain-refining and precipitation-strengthening element, which can compensate for the reduction in strength caused by a decrease in carbon content. Additionally, during the welding process, Nb element can inhibit grain growth in the heat-affected zone, thereby mitigating the performance degradation in this area. When the content of Nb element is less than 0.02%, its effect is not significant. When the content of Nb element exceeds 0.04%, coarse Nb(CN) precipitates tend to form, thereby reducing toughness. Therefore, the present disclosure controls the mass percentage of Nb element to 0.02-0.04%.

[0039] Ti: In the seamless tube according to the present disclosure, Ti is a strong carbonitride-forming element that significantly refines austenite grains, thereby compensating for the strength loss caused by the reduced carbon content. In addition, the carbonitrides formed by Ti exhibit high thermal stability and, during welding, effectively inhibit microstructural coarsening in the weld heat-affected zone, thereby improving weldability. When the content of Ti element exceeds 0.03%, coarse TiN precipitates tend to form, resulting in a reduction in low-temperature impact toughness. If the content of Ti element is too low, Ti cannot sufficiently combine with N to form TiN , and B in the steel tends to combine with N to form brittle BN phases, thereby reducing material toughness. Therefore, the present disclosure controls the mass percentage of Ti element to 0.01-0.03%.

[0040] B: In the seamless tube according to the present disclosure, B significantly enhances hardenability. When the content of B element is less than 0.0015%, its effect on improving hardenability is not significant. When the content of B element exceeds 0.005%, it increases the material's susceptibility to cold cracking. Therefore, the present disclosure controls the mass percentage of B element to 0.0015-0.005%.

[0041] Al: Al is primarily used for deoxidation treatment, effectively controlling the oxygen content in the material. Based on this, the present disclosure controls the mass percentage of Al element to 0.01-0.05%.

[0042] Ca: In the seamless tube according to the present disclosure, the Ca element can purify the molten steel and promote the spheroidization of MnS, thereby improving impact toughness. However, if the content of Ca element is too high, coarse non-metallic inclusions tend to form. Therefore, the present disclosure controls the mass percentage of Ca element to 0.0005-0.005%.

[0043] It should be noted that the unavoidable impurity elements in the seamless tube according to the present disclosure mainly comprise S, P, and N. Within technically feasible limits, to obtain seamless steel tubes with better performance and higher quality, the content of the impurity elements in the seamless tube should be minimized as much as possible, i.e., the lower, the better. Specifically: When the content of N element exceeds 0.007%, it leads to the precipitation of excessive Ti nitrides in the microstructure. These nitride precipitates are relatively large and have sharp angles, significantly reducing low-temperature impact toughness. Therefore, in some embodiments, the present disclosure controls the mass percentage of N element to be 0.007% or less.

[0044] Excessive P and S content can impair the toughness of the steel. Therefore, in some embodiments, the present disclosure controls the mass percentage of P and S element to P ≤ 0.015% and S ≤ 0.003%, respectively.

[0045] The seamless tube and manufacturing method therefor of the present disclosure will be further explained and illustrated below with reference to specific examples. However, such explanation and illustration shall not be construed as an improper limitation of the technical solution of the present disclosure.Examples 1-6 and Comparative Examples 1-5

[0046] The seamless tubes of Examples 1-6 were manufactured by the following steps: (1) performing smelting and continuous casting to obtain a tube billet: (2) subjecting the tube billet to heating, perforating, rolling, and sizing to obtain a tube body; (3) subjecting the tube body to a heat treatment: austenitizing at a temperature of 900-930°C with a holding time of 30-60 min, quenching, and then tempering at a temperature of 550-650°C with a holding time of 50-80 min; and (4) hot sizing and straightening.

[0047] The comparative tubes of Comparative Examples 1-5 were manufactured using the same steps described above, except for the compositional contents and / or specific process parameters.

[0048] Table 1 lists the contents of the chemical elements in percentage by mass for the seamless tubes of Examples 1-6 and the comparative tubes of Comparative Examples 1-5. Table 1. (wt%, the balance being Fe and unavoidable impurities other than P, S, and N)No.CSiMnCrMoNbTiBAlCaPSNCarbon equivalentExample 10.190.21.50.10.120.030.020.00150.010.00050.0090.0020.0040.48Example 20.190.110.20.180.020.0250.0020.040.0010.0100.0010.0050.43Example 30.230.41.30.30.150.030.030.0030.050.0050.0100.0030.0060.54Example 40.20.60.80.40.10.030.030.0040.030.0030.0120.0020.0070.43Example 50.220.250.60.20.160.040.030.0050.020.0020.0130.0020.0050.39Example 60.190.511.10.250.150.020.010.00250.030.0030.0100.00150.0060.45Comparative Example 10.25 0.260.80.20.170.040.020.0050.0230.0020.0070.0030.0070.46Comparative Example 20.210.331.8 0.150.10.030.010.0030.040.0020.0080.0030.0050.55Comparative Example 30.230.21.50.40.180.020.020.0040.040.0010.0100.0010.0060.60 Comparative Example 40.190.31.20.30.4 0.040.02-0.050.0030.0100.0030.0060.53Comparative Example 50.20.3510.280.150.030.0250.00350.003 0.0040.010.0020.0090.45Note: Carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where each chemical element symbol is substituted with the numerical value before the percent sign of the mass percentage content of corresponding chemical element.

[0049] Table 2-1 and Table 2-2 list the process parameters for the seamless tubes of Examples 1-6 and the comparative tubes of Comparative Examples 1-5. Table 2-1No.Superheat of molten steel (°C)Continuous casting speed (m / min)Soaking temperature (°C)Perforating temperature (°C)Final rolling temperature (°C)Sizing temperature (°C)Example 126212201180910850Example 2102.212401210900860Example 3202.112401220940870Example 4301.812301190950880Example 5251.812001240920900Example 6251.912401220920870Comparative Example 1201.912301210920900Comparative Example 2152.212201220930880Comparative Example 3201.912101230950870Comparative Example 4201.912201240920890Comparative Example 516212401240930860 Table 2-2 No.Austenitizing temperature (°C)Holding time (min)Tempering temperature (°C)Tempering holding time (min)Hot straightening temperature (°C)Example 19004065050550Example 29303058060480Example 39106055060450Example 49206057080490Example 59004061070520Example 69203060050510Comparative Example 19005064070480Comparative Example 29306058060500Comparative Example 39104056060510Comparative Example 49104057060470Comparative Example 59203059060500

[0050] Samples were taken from the seamless tubes of Examples 1-6 and the comparative tubes of Comparative Examples 1-5 respectively, and subjected to testing. The obtained test results are listed in Table 3. The testing procedures for the relevant properties are described below: (1) Tensile performances: Room-temperature tensile performances were measured in accordance with the GB / T 228.1-2000 standard. (2) Microstructure: Cross-section of the tube was taken for microstructure observation. The samples were ground and polished, and then the metallographic structure was etched using 4% nitric acid + alcohol. After etching, the microstructure was observed using a metallographic microscope. (3) Impact toughness: Longitudinal impact energy at -40°C was measured in accordance with the GB / T 229-2007 standard, "Metallic materials-Charpy pendulum impact test method." (4) Weldability: Weldability was evaluated in accordance with NB / T 47014-2011, "Welding procedure qualification for pressure equipment. "

[0051] Table 3 lists the test results of the seamless tubes from Examples 1-6 and the comparative tubes from Comparative Examples 1-5. Table 3No.Yield strength (MPa)Tensile strength (MPa)Elongation (%)Average grain size (µm)Longitudinal impact toughness KV 8 at - 40°C (j)WeldabilityExample 1780860251090GoodExample 28209002313100GoodExample 3850930241485GoodExample 4840935231395GoodExample 5790870231880GoodExample 68108902412110GoodComparative Example 1780860231750Poor Comparative Example 28409502625 40 GoodComparative Example 3860940251487Poor Comparative Example 4850940241635 GoodComparative Example 5820920251123 Poor

[0052] As shown in Table 3, the seamless tubes of Examples 1-6 all exhibit a yield strength of ≥780 MPa, a tensile strength of 860-935 MPa, an impact toughness at -40°C KV 8 of ≥80 J, an elongation of ≥23%, an average grain size of 10-18 µm, and also possess good weldability.

[0053] Furthermore, FIG. 1 shows the metallographic structure of the seamless tube from Example 1. As shown in FIG. 1, the microstructure of the seamless tube consists of a uniform and fine tempered sorbite structure.

[0054] FIG. 2 shows the microstructural morphology of the seamless tube from Example 1, revealing its fine and uniform grain.

[0055] Comparative Examples 1-5 cannot achieve both excellent low-temperature impact toughness and good weldability.

[0056] All publications, patent applications, patents, and other references mentioned in the present disclosure are hereby incorporated by reference in their entirety.

[0057] While the present disclosure has been illustrated and described with reference to certain preferred embodiments herein, it should be understood by those skilled in the art that the above description is provided in further detail in conjunction with specific embodiments and should not be construed as limiting the specific implementations of the present disclosure to these descriptions. Those skilled in the art may make various changes in form and detail, including several simple deductions or substitutions, without departing from the spirit and scope of the present disclosure.

Claims

1. A seamless tube, wherein the seamless tube comprises, in addition to Fe and unavoidable impurities, 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%, Ca: 0.0005-0.005%; wherein the seamless tube does not contain Ni or W.

2. The seamless tube according to claim 1, wherein the seamless tube consists of 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%, Ca: 0.0005-0.005%; with the balance being Fe and unavoidable impurities.

3. The seamless tube according to claim 1 or 2, wherein the unavoidable impurities comprise P, S, and N, with P ≤ 0.015%, S ≤ 0.003%, and N ≤ 0.007%.

4. The seamless tube according to any one of claims 1 to 3, wherein the seamless tube has a carbon equivalent of ≤0.55, preferably ≤0.48, wherein the carbon equivalent is calculated by the following formula: carbon equivalent = C + Mn / 6 + (Cr+Mo+V) / 5 + (Ni+Cu) / 15, where each chemical element symbol is substituted with the numerical value before the percent sign of the mass percentage content of corresponding chemical element.

5. The seamless tube according to any one of claims 1 to 4, wherein the seamless tube has a microstructure of tempered sorbite.

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

7. The seamless tube according to any one of claims 1 to 6, wherein the seamless tube 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 at -40°C of ≥45 J, preferably KV8 ≥80 J.

8. A method of manufacturing the seamless tube according to any one of claims 1 to 7, comprising the following steps: - performing smelting and continuous casting to obtain a tube billet; - subjecting the tube billet to heating, perforating, rolling, and sizing to obtain a tube body; - subjecting the tube body to a heat treatment: austenitizing at a temperature of 900-930°C with a holding time of 30-60 min, quenching, and then tempering at a temperature of 550-650°C with a holding time of 50-80 min; and - hot sizing and straightening.

9. The method according to claim 8, wherein in the continuous casting step, a superheat of a molten steel is 30°C or less, and a 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 billet 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, a perforating temperature is 1180-1240°C.

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

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

14. The method according to any one of claims 8 to 13, wherein a temperature for hot sizing and straightening is 450-550°C.

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