A method for manufacturing X80 steel pipe with good low-temperature toughness in the girth weld heat-affected zone

Abstract

The invention relates to a method for manufacturing an X80 steel pipe with good low-temperature toughness in a girth weld heat-affected zone, and belongs to the technical field of spiral welded pipe manufacturing. The invention relates to a manufacturing method of an X80 steel pipe with good low-temperature toughness in a girth weld heat-affected zone. The raw material of the welded pipe is pure molten steel, and the chemical composition thereof is as follows: C 0.040-0.050%, Si 0.20-0.25%, Mn 1.78-1.80%, P≤0.0050%, S≤0.0050%, Ni 0.10-0.20%, Cr 0.20-0.30%, MO 0.20-0.25%, Nb 0.034-0.056%, V 0.035-0.056%, Ti 0.010-0.025%, Pcm≤0.20%, IIW 0.42-0.48%, and the remainder is Fe element. The processing steps include: converter smelting, continuous casting, slab heating, rough rolling, finish rolling, controlled cooling, tempering and coiling. Beneficial effects: The microstructure of the welding heat-affected zone can maintain good low-temperature toughness. Through continuous casting, rough rolling, finish rolling and adding a heat preservation process during the cooling process, the temperature is precisely controlled to achieve the dispersion and precipitation of (Nb, V) C, thereby achieving the purpose of grain refinement of the X80 base material and improving the low-temperature toughness of the base material.

Description

Technical Field

[0001] The invention belongs to the technical field of spiral welded pipe manufacturing, and particularly relates to a method for manufacturing an X80 steel pipe with good low-temperature toughness in a girth weld heat-affected zone. Background Art

[0002] With the continuous development of the economy, the demand for oil and natural gas is constantly increasing. Pipeline steel, as the most efficient means of transporting oil and gas, is gaining increasing attention. X80 pipeline steel is currently the most widely used high-strength and high-toughness pipeline steel in pipeline construction. However, the increasing mileage and service life of pipelines, as well as the complex service environment surrounding pipelines, pose challenges to pipeline safety. Safety accidents caused by brittle fractures in pipeline girth welds due to fluctuating toughness and low toughness values ​​are common, resulting in significant economic losses, casualties, and adverse social impacts. Therefore, the service safety of girth welds has become a critical engineering issue facing the safe operation of pipelines.

[0003] Currently, X80 pipeline steel used in my country's oil and gas pipelines has achieved toughness standards through microalloying and controlled rolling and cooling technologies, meeting basic application requirements. However, during subsequent on-site girth welding, the low-temperature toughness of the girth weld and heat-affected zone (HAZ) significantly impacts the safety of the entire pipeline transportation process.

[0004] The mechanical properties evaluation results of X80 long-distance pipeline girth weld joints show that pipeline girth weld joints at different times all have large fluctuations in low-temperature toughness index parameters. The fluctuation in weld toughness is related to the welding materials and welding process, while the fluctuation in toughness in the heat-affected zone of the weld joint is related to the composition and structure of the base material.

[0005] Therefore, providing an X80 spiral welded pipe steel with excellent girth weld heat affected zone toughness is of great significance to my country's long-distance pipeline construction and energy strategic security.

[0006] At present, there are some studies on X80 pipeline steel welding and pipeline steel manufacturing technology at home and abroad. After searching, some patents and literature were found, but the relevant content focused on welding process parameters. There was no explanation for the protection or even optimization of the mechanical properties of the base material at the girth weld joint after being affected by welding heat. At the same time, the metallurgical composition, manufacturing method, and mechanical properties of the welding heat-affected zone of the product described in the present invention are significantly different from those in other patents or literature.

[0007] For example, CN 110695500 A discloses a method for preparing welded joints of X80 pipeline steel with excellent low-temperature performance. This patent primarily addresses submerged arc welding, requiring the creation of a double Y-shaped symmetrical groove on the butting surfaces of the two X80 steel pieces to be welded. The internal weld is welded using a three-wire, single-pass weld, while the external weld is welded using a multi-wire, multi-pass weld. This patent primarily examines the welding process and the variations in welding parameters, but does not specifically address the development process for X80 pipeline steel itself, nor does it specifically address X80 spiral welded pipe.

[0008] CN 111702334 B discloses a process for welding X80 pipeline steel plates. The patent requires a Y-shaped groove angle of 30°-60°. Laser arc hybrid welding is used. Before welding, a reagent is required to remove oil stains from the surface of the X80 pipeline steel and the oxide film on both sides of the joint. The patent also specifies requirements for the subsequent laser welding power and gas metal arc welding parameters. The patent primarily addresses the welding circuit, wire feed speed, welding voltage, and other welding parameters for cap-face gas metal arc welding. It does not address the girth welding process or the corresponding rolling process for the X80 pipeline steel.

[0009] CN 105817844 B discloses a method for manufacturing X80 pipeline steel spiral welded pipe. This patent primarily addresses the design of the weld groove and the redistribution of weld line energy between the inner and outer weld beads, thereby reducing the grain size of the critical coarse-grained region of the weld heat-affected zone (HAZ), reducing the size of the MA component, and improving the toughness of the HAZ. However, this patent utilizes a 21.4mm-thick X80 pipeline steel coil that has already been developed and does not address the steelmaking and rolling process for the coil. Furthermore, the patent does not address V in its elemental design, only proposing a 0.04-0.06wt.% Nb content. Summary of the Invention

[0010] In order to solve the above technical problems, the present invention provides a method for manufacturing an X80 steel pipe with good low-temperature toughness in the girth weld heat-affected zone. The microstructure of the welding heat-affected zone can maintain good low-temperature toughness. Through continuous casting, rough rolling, finish rolling and adding a heat preservation process during the cooling process, the temperature is precisely controlled to achieve the dispersion and precipitation of (Nb, V) C, thereby achieving the purpose of grain refinement of the X80 base material structure and improving the low-temperature toughness of the base material.

[0011] The present invention solves the above-mentioned technical problem with the following technical solution: a method for manufacturing an X80 steel pipe with good low-temperature toughness in a girth weld heat-affected zone, wherein the raw material of the welded pipe is pure molten steel, and the chemical composition thereof is as follows: C 0.040-0.050%, Si 0.20-0.25%, Mn 1.78-1.80%, P≤0.0050%, S≤0.0050%, Ni 0.10-0.20%, Cr 0.20-0.30%, Mo 0.20-0.25%, Nb 0.034-0.056%, V 0.035-0.056%, Ti 0.010-0.025%, Pcm≤0.20%, IIW 0.42-0.48%, and the remainder is Fe. The processing steps include: converter smelting, continuous casting, slab heating, rough rolling, finish rolling, controlled cooling, tempering and coiling.

[0012] Beneficial effects:

[0013] (1) The grain refinement effect is achieved mainly through the composite effect of Nb-V, so that after the steel pipe base material undergoes the girth welding process, the microstructure of the weld heat affected zone can maintain good low-temperature toughness, meeting the basic requirements for pipeline transportation safety. Appropriately reducing the Nb content and replacing part of the Nb with V can achieve the results of reducing costs while also improving the low-temperature toughness of the weld heat affected zone.

[0014] (2) Through continuous casting, rough rolling, finish rolling and adding a heat preservation process during the cooling process, the temperature is precisely controlled to achieve the dispersion and precipitation of (Nb, V)C, thereby achieving the purpose of grain refinement of the X80 base material and improving the low-temperature toughness of the base material. Furthermore, after the subsequent girth welding process, the grain size of the heat-affected zone remains small, thereby improving the low-temperature toughness of the heat-affected zone of the girth welding process.

[0015] (3) Appropriately adjust the rolling process parameters, adjust the number of passes and reduction rate, reduce production costs, weaken the excessive requirements for equipment, and improve the production efficiency of batch industrial production of high-strength and thick-gauge X80 pipeline steel.

[0016] Preferably, the converter smelting process is: LF and / or RH refining to obtain pure molten steel with target metallurgical composition and low P and low S impurity elements.

[0017] Preferably, the process of the continuous casting process is: pure molten steel is cast in a protective manner, and during the continuous casting process, a low superheat of 10 to 20°C above the liquidus, a low pulling speed of 1.5 to 2 m / min, and a dynamic light reduction process with a total reduction of 4 to 6 mm are adopted to finally obtain a slab with a thickness of 220 mm.

[0018] Preferably, the process of the slab heating step is: heating the slab to a furnace temperature of 1050-1130° C. for 180-280 minutes.

[0019] Preferably, the rough rolling process is as follows: the heated slab is rolled for 6 passes to obtain an intermediate slab with a thickness of 70 mm, with a final rolling reduction of 20% and a final rolling temperature of ≤960°C.

[0020] Preferably, the finishing rolling process is as follows: the intermediate billet is rolled for 5 passes, the finishing rolling inlet temperature is ≤ 950° C., and the cumulative finishing rolling reduction is 60%, to obtain a hot-rolled steel strip of target thickness.

[0021] Preferably, the process of the controlled cooling step is: rapidly cooling the hot-rolled steel strip, with a cooling time of 15 to 20 seconds and a cooling rate of 20 to 30° C. / s.

[0022] Preferably, the process of the tempering step is: when the hot-rolled steel strip is cooled to 450°C in the controlled cooling step, it is kept warm for 120s and then continued to cool, with the cooling temperature being 350-400°C and the cooling rate being 20-30°C / s.

[0023] Preferably, the coiling process is as follows: coiling the hot-rolled steel strip after tempering at a coiling temperature of 360-390°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a metallographic photograph of the X80 pipeline steel base material of the present invention;

[0025] Figure 2 This is a TEM image of (Nb, V)C dispersion and precipitation during the cooling and holding stage after finish rolling of the present invention;

[0026] Figure 3 A metallographic photograph of the microstructure of the heat-affected zone of the product provided by the present invention after girth welding;

[0027] Figure 4 These are SEM images of different positions affected by welding heat after girth welding of the X80 pipeline steel provided by the present invention. DETAILED DESCRIPTION

[0028] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0029] The embodiment of the present invention performs converter smelting, external refining, continuous casting, slab heating, rough rolling, finishing rolling, controlled cooling, tempering, and coiling according to the metallurgical composition and processing steps of the technical solution. The specific composition and parameters are shown below.

[0030] Table 1 Metallurgical composition of the present invention (wt.%)

[0031]

[0032] Table 1, Ceq=C+Mn / 6+(Cr+Mo) / 5+(Ni+Cu) / 15, Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B.

[0033] Table 2 Metallurgical composition of comparative examples (wt.%)

[0034]

[0035]

[0036] The main manufacturing process parameters of the embodiments of the present invention and the comparative examples are shown in Table 3.

[0037] Table 3 Manufacturing process parameters of the embodiment and the comparative example

[0038]

[0039] Table 4 shows the mechanical properties of the steel coils of the embodiments of the present invention and the comparative examples.

[0040] Table 4 Mechanical properties of steel coils of examples and comparative examples

[0041]

[0042] The mechanical properties of the steel pipes of the embodiments of the present invention and the comparative examples are shown in Table 5.

[0043] Table 5 Mechanical properties of steel pipes in examples and comparative examples

[0044]

[0045] The mechanical properties of the girth welds of the embodiments of the present invention and the comparative examples are shown in Table 6.

[0046] Table 6 Mechanical properties of girth welds of steel pipes in examples and comparative examples

[0047]

[0048] Comparing the metallurgical compositions of the examples and comparative examples in Tables 1 and 2 reveals the effects of changes in Nb-V content on the toughness of subsequent steel plates, coils, and the heat-affected zone (HAZ) of girth welds. Table 3 examines the effects of different rolling process parameters on the toughness of the X80 pipeline steel base material. The mechanical properties of the steel coils and pipes from the examples and comparative examples in Tables 4 and 5 demonstrate that the toughness of the steel remains essentially unchanged after the coiling process, meeting the basic requirements of X80 pipeline steel. The increase in Nb content in the comparative example results in a significant decrease in the corresponding Charpy impact energy at -20°C. Overall, the toughness of the examples is significantly superior to that of the comparative example. Therefore, reducing the Nb content to a certain extent and appropriately adding V can effectively improve the low-temperature toughness of the coils / pipes. In terms of Charpy impact energy (CVN) and DWTT, which reflect the low-temperature toughness of the base material, the examples outperform the comparative example. Comparing Examples 1 and 2, it is found that reducing the Nb content and replacing Nb with an appropriate amount of V significantly improves impact toughness.

[0049] After girth welding, the impact toughness of the weld seam in the Example slightly decreased, but overall still exceeded 200 J, indicating good impact toughness. The low-temperature toughness values ​​(Charpy impact energy and CTOD values) of the girth weld in the Example were significantly higher than those in the Comparative Example. This comparison of performance parameters demonstrates that the innovative metallurgical composition and production process of the present invention result in excellent low-temperature toughness in the weld heat-affected zone.

[0050] In the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0051] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for manufacturing an X80 steel pipe with good low-temperature toughness in the girth weld heat-affected zone, characterized in that: The raw material of the welded pipe is pure molten steel with the chemical composition of C 0.040-0.050%, Si 0.20-0.25%, Mn 1.78-1.80%, P≤0.0050%, S≤0.0050%, Ni 0.10-0.20%, Cr 0.20-0.30%, Mo 0.22-0.25%, Nb 0.034-0.056%, V 0.035%, Ti 0.010-0.025%, Pcm≤0.20%, IIW 0.42-0.48%, and the rest is Fe. The processing steps include: converter smelting, continuous casting, slab heating, rough rolling, finish rolling, controlled cooling, tempering and coiling. The process of the tempering step is as follows: when the hot-rolled steel strip is cooled to 450° C. in the controlled cooling step, it is kept warm for 120 seconds and then continued to cool at a cooling temperature of 350-400° C. and a cooling rate of 20-30° C. / s.

2. The method for manufacturing an X80 steel pipe with good low-temperature toughness in the girth weld heat-affected zone according to claim 1, characterized in that: The converter smelting process comprises: LF and / or RH refining to obtain pure molten steel with target metallurgical composition and low P and low S impurity elements.

3. The method for manufacturing an X80 steel pipe with good low-temperature toughness in the girth weld heat-affected zone according to claim 2, characterized in that: The continuous casting process is as follows: pure molten steel is cast in a protective manner, and during the continuous casting process, a low superheat of 10 to 20°C above the liquidus, a low drawing speed of 1.5 to 2 m / min, and a dynamic light reduction process with a total reduction of 4 to 6 mm are adopted, and finally a slab with a thickness of 220 mm is obtained.

4. The method for manufacturing an X80 steel pipe with good low-temperature toughness in the girth weld heat-affected zone according to claim 3, characterized in that: The process of the slab heating step is as follows: heating the slab to a furnace temperature of 1050-1130° C. for 180-280 minutes.

5. The method for manufacturing an X80 steel pipe with good low-temperature toughness in the girth weld heat-affected zone according to claim 4, characterized in that: The rough rolling process is as follows: the heated slab is rolled for 6 passes to obtain an intermediate slab with a thickness of ≥70 mm, the reduction in the final pass is ≥20%, and the rolling temperature in the final pass is ≤960°C.

6. The method for manufacturing an X80 steel pipe with good low-temperature toughness in the girth weld heat-affected zone according to claim 5, characterized in that: The finishing rolling process is as follows: the intermediate billet is rolled for 5 passes, the finishing rolling inlet temperature is ≤950° C., the finishing rolling cumulative reduction is ≥60%, and a hot-rolled steel strip of target thickness is obtained.

7. The method for manufacturing an X80 steel pipe with good low-temperature toughness in the girth weld heat-affected zone according to claim 6, characterized in that: The process of the controlled cooling step is: rapidly cooling the hot-rolled steel strip, with a cooling time of 15 to 20 seconds and a cooling rate of 20 to 30° C. / s.

8. The method for manufacturing an X80 steel pipe with good low-temperature toughness in the girth weld heat-affected zone according to claim 1, characterized in that: The coiling process is as follows: coiling the tempered hot-rolled steel strip at a coiling temperature of 360-390°C.

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