X80 pipeline steel with excellent toughness in heat-affected zone of girth weld and preparation method thereof

Abstract

The present application relates to a kind of X80 pipeline steel with excellent toughness in the heat-affected zone of girth weld and its preparation method, belong to low-carbon microalloy material technical field, solve the problem of insufficient low temperature toughness of X80 pipeline steel girth weld and its heat-affected zone in prior art.A kind of X80 pipeline steel with excellent toughness in the heat-affected zone of girth weld, the metallurgical composition of the pipeline steel is as follows: by mass percent, C:0.050~0.070%, Si:0.15~0.25%, Mn:1.65~1.80%, P≤0.012%, S≤0.0030%, Ni:0.30~0.45%, Cr:0.12~0.18%, Mo:0.10~0.16%, Nb:0.090~0.120%, Ti:0.012~0.020%, the rest is Fe and inevitable impurity element, and Pcm:0.17~0.19%.Realize the technical effect that the heat-affected zone of X80 steel pipe has high toughness after girth welding.

Description

Technical Field

[0001] This invention relates to the field of low-carbon microalloyed materials technology, and in particular to an X80 pipeline steel with excellent toughness in the heat-affected zone of a ring weld joint and its preparation method. Background Technology

[0002] With the continuous increase in demand for oil and gas resources, the mileage of oil and gas pipelines is constantly increasing, and they are developing towards higher strength levels, higher transmission pressures, and larger diameters. However, along with the increase in pipeline mileage and service life, the number of pipeline accidents is also increasing, causing huge economic losses, casualties, and adverse social impacts. In pipeline failure cases, more than half of the failures are due to defects in circumferential welds, making the service safety of circumferential welds a critical engineering issue facing pipeline safety. In particular, as oil and gas resource exploitation extends to high-altitude, polar, and deep-sea regions, higher requirements are placed on the low-temperature toughness of gas pipelines, especially circumferential welds.

[0003] Since the commencement of the West-East Gas Pipeline II in 2009, X80 grade pipeline steel has been widely used, and its manufacturing technology has developed rapidly. The tensile properties and low-temperature toughness of hot-rolled steel coils used to manufacture X80 steel pipes have reached excellent levels. However, the optimization of the performance indicators of the circumferential weld and its heat-affected zone during the post-pipeline welding process has not received sufficient attention. As a weak link in the pipeline's service life, the stability of the mechanical properties of the circumferential weld will have a significant impact on the entire pipeline project. Recent evaluation results of the mechanical properties of X80 long-distance pipeline circumferential welds indicate that circumferential welds from different periods exhibit significant fluctuations in low-temperature toughness parameters.

[0004] Therefore, providing an X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint and its preparation method is of great significance for my country's long-distance pipeline construction and energy strategic security.

[0005] Currently, there is some research on X80 pipeline steel and its manufacturing technology both domestically and internationally. A search revealed some patents and literature, but the relevant content mainly focuses on improving the mechanical properties of the base material. It does not explain how to ensure or even optimize the mechanical properties of the base material after it is affected by welding heat at the circumferential weld joint.

[0006] In summary, in order to ensure the mechanical properties of the base material at the welded joint of X80 pipeline steel rings after being affected by welding heat, it is necessary to develop X80 pipeline steel products with high toughness, especially low-temperature toughness, in the heat-affected zone after ring welding. Summary of the Invention

[0007] Based on the above analysis, the present invention aims to provide an X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint, in order to solve the problem of insufficient low-temperature toughness of the existing X80 pipeline steel circumferential weld and its heat-affected zone.

[0008] This invention provides an X80 pipeline steel with excellent toughness in the heat-affected zone of a ring weld joint. The metallurgical composition of the pipeline steel is as follows (by mass percentage): C: 0.050–0.070%, Si: 0.15–0.25%, Mn: 1.65–1.80%, P ≤ 0.012%, S ≤ 0.0030%, Ni: 0.30–0.45%, Cr: 0.12–0.18%, Mo: 0.10–0.16%, Nb: 0.090–0.120%, Ti: 0.012–0.020%, with the remainder being Fe and unavoidable impurity elements, and Pcm: 0.17–0.19%.

[0009] Furthermore, the X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential welded joint is prepared by the following steps: converter smelting—continuous casting—slab heating—rough rolling—finish rolling—controlled cooling—coiling.

[0010] Furthermore, the converter smelting step includes LF and RH refining.

[0011] Specifically, the continuous casting step adopts protective casting throughout the process, and the continuous casting process employs low superheat and dynamic light reduction technology.

[0012] Furthermore, in the secondary heating step of the slab, the secondary heating temperature at the furnace exit is 1180-1240℃, and the heating time is 180-360 minutes.

[0013] Furthermore, the roughing step involves 8 passes of rolling, with an intermediate billet thickness of 54-62 mm, a final pass reduction of ≥25%, and a final pass rolling temperature of ≤960℃.

[0014] The finishing rolling step involves 7 passes, with a finishing rolling inlet temperature ≤940℃ and a cumulative finishing rolling reduction ≥55%.

[0015] It should be noted that the cooling rate in the controlled cooling step is 15-25℃ / s; and the winding temperature in the winding step is 420-480℃.

[0016] Specifically, the parameters of the product base material and the heat-affected zone of the ring weld joint include: the yield strength R of the base material. t0.5 555-690MPa, tensile strength R m625-765MPa, yield strength ratio ≤0.92, Charpy impact energy ≥180J at -20℃; Charpy impact energy of the heat-affected zone of the welded joint ≥140J at -10℃; crack tip opening displacement CTOD of the welded joint ≥0.30mm at -10℃.

[0017] On the other hand, the present invention provides a method for preparing X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint, which is used to prepare X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint.

[0018] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0019] 1. This invention utilizes the composite strengthening effect of alloying elements such as Ni-Cr-Mo-Nb-Ti to ensure that the produced steel pipe has good comprehensive mechanical properties such as strength and low-temperature toughness.

[0020] 2. This invention achieves the technical effect of high toughness in the heat-affected zone of steel pipes after circumferential welding by precisely controlling the carbon element and carbon equivalent Pcm, the fine grain strengthening effect of Nb element, and the optimization of low-temperature toughness by Ni element. This results in the steel pipe having a refined lath bainite (BF), granular bainite (GB), and finely dispersed martensite / austenite components (M / A) in its heat-affected zone.

[0021] 3. This invention achieves good low-temperature toughness without the need to specifically add Cu, V or other rare earth elements, thus reducing the metallurgical cost of the product.

[0022] 4. This invention improves the preparation method by focusing on the detailed control of process parameters in continuous casting, roughing, and finishing rolling. By adjusting the pass reduction rate and key temperature parameters, the austenite grains in the rolling stage are refined. By appropriately increasing the coiling temperature, the matrix microstructure of refined acicular ferrite can also be obtained. This appropriately reduces the excessively high requirements on equipment capacity for low-temperature coiling, which can have a beneficial impact on equipment maintenance during the mass production of high-strength, thick-gauge pipeline steel.

[0023] 5. The X80 pipeline steel produced by the metallurgical composition and preparation method of the present invention has high and stable low-temperature toughness in the heat-affected zone after circumferential welding. Specifically, the Charpy impact energy of the heat-affected zone of the circumferential weld joint is ≥140J at -10℃ and the crack tip opening displacement (CTOD) is ≥0.30mm at -10℃.

[0024] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0025] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0026] Figure 1 Metallographic image of the microstructure of X80 pipeline steel in Embodiment 1 of the present invention;

[0027] Figure 2 The images show scanning electron microscope (SEM) images of the microstructure at different locations in the heat-affected zone (HAZ) of the X80 pipeline steel ring weld in Embodiment 1 of the present invention. Specifically, a is an SEM image of the microstructure in the fine-grained region of the HAZ of the X80 pipeline steel ring weld in Embodiment 1 of the present invention; b is an SEM image of the microstructure in the critical region of the HAZ of the X80 pipeline steel ring weld in Embodiment 1 of the present invention; and c is an SEM image of the microstructure in the tempered region of the HAZ of the X80 pipeline steel ring weld in Embodiment 1 of the present invention.

[0028] Figure 3 These are comparative images of the substructure of typical microstructures in the fine-grained region of the heat-affected zone in Example 2 and Comparative Example 6 of the present invention. In this image, a is a photograph of the substructure of typical microstructures in the fine-grained region of the heat-affected zone in Example 2 of the present invention; and b is a photograph of the substructure of typical microstructures in the fine-grained region of the heat-affected zone in Comparative Example 6 of the present invention.

[0029] Figure 4 The images show a comparison of SEM images of typical microstructures in the critical region of the heat-affected zone in Example 2 and Comparative Example 6 of the present invention. In Example 2, image a is a typical SEM image of the critical region of the heat-affected zone; and in Comparative Example 6, image b is a typical SEM image of the critical region of the heat-affected zone. Detailed Implementation

[0030] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0031] A specific embodiment of the present invention discloses an X80 pipeline steel with excellent toughness in the heat-affected zone of a ring weld joint. The metallurgical composition of the pipeline steel is as follows (by mass percentage): C: 0.050–0.070%, Si: 0.15–0.25%, Mn: 1.65–1.80%, P ≤ 0.012%, S ≤ 0.0030%, Ni: 0.30–0.45%, Cr: 0.12–0.18%, Mo: 0.10–0.16%, Nb: 0.090–0.120%, Ti: 0.012–0.020%, with the remainder being Fe and unavoidable impurity elements, and Pcm: 0.17–0.19%.

[0032] Preferably, the metallurgical composition of the pipeline steel is as follows by mass percentage: C: 0.055-0.065%, Ni: 0.35-0.40%, Nb: 0.095-0.110%, and Pcm: 0.170-0.185%.

[0033] Specifically, the formula for calculating the welding cold crack sensitivity index Pcm is as follows:

[0034] Pcm=w(C)+w(Si) / 30+[w(Mn)+w(Cr)+w(Cu)] / 20+w(Ni) / 60+w(Mo) / 15+w(V) / 10+5w(B), the unit is %.

[0035] The rationale for defining the composition of the X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint in this invention will be explained. Hereinafter, only the percentage of mass in the composition will be expressed as %.

[0036] Carbon (C) is the most economical and fundamental strengthening element in steel. It can significantly improve the strength of steel through solid solution strengthening and precipitation strengthening. However, increasing the C content negatively impacts the plasticity and toughness of steel. When the C content is too low, its precipitation strengthening effect, combining with elements such as Nb and V, cannot be fully utilized. Furthermore, excessively low C content can cause a large amount of coarse granular bainite to form in the heat-affected zone of welded joints, reducing toughness. Therefore, this invention sets the C content range to 0.050–0.070%.

[0037] Si promotes ferrite formation. However, excessive Si addition significantly reduces the toughness of the base material and its weld heat-affected zone. Therefore, the Si content in this invention is set at 0.15–0.25%.

[0038] Mn: Strengthening steel through solid solution is the most important and economical strengthening element to compensate for the strength loss caused by the reduction of carbon content. However, excessively high Mn content will increase segregation at the center of the continuously cast billet, increasing the anisotropy of the steel plate and reducing toughness. In order to ensure a balance between strength and low-temperature toughness and high billet quality, the Mn content range of this invention is designed to be 1.65% to 1.80%.

[0039] P and S are unavoidable impurity elements in steel. The lower their content, the better for performance improvement. However, considering the smelting cost, the upper limit of P content in this invention is designed to be 0.012% and the upper limit of S content is 0.0030%.

[0040] Ni has solid solution strengthening effect and can improve low temperature toughness, but Ni alloys are expensive. The design range of Ni content in this invention is 0.30 to 0.45%.

[0041] Mo and Cr: They can improve the stability of austenite and promote bainitic phase transformation, playing an important role in controlling the phase transformation microstructure; however, when the Mo content is high, a large number of martensite / austenite islands will appear in the heat-affected zone during welding, reducing the toughness of the heat-affected zone, and this phenomenon is not sensitive to Cr. In this invention, the Cr content is designed to be 0.12-0.18%, and the Mo content is designed to be 0.10-0.16%.

[0042] Nitrogen b (Nb): A precipitation strengthening element that inhibits austenite grain growth to achieve grain refinement. Simultaneously, Nb carbonitride precipitates provide precipitation strengthening. When the Nb content is around 0.075%, a large number of large-sized martensite / austenite components appear in the weld heat-affected zone, reducing toughness. However, when the Nb content increases to above 0.090%, the martensite / austenite components transform from long rod-shaped to fine-grained, with a significant reduction in size, resulting in a significant improvement in low-temperature toughness. On one hand, Nb can refine the grains before welding; on the other hand, during thermal cycling, Nb can effectively inhibit grain growth. Considering alloy cost, the Nb content in this invention is designed to be 0.090–0.120%.

[0043] Ti: As a strong nitrogen element, fine-particle TiN precipitation can provide precipitation strengthening. However, excessive Ti content will produce a large number of large-sized square TiN precipitations, which is detrimental to the low-temperature toughness of the base material and the heat-affected zone. Therefore, the Ti content of this invention is designed to be 0.012 to 0.020%.

[0044] Pcm: is the welding hydrogen-induced cracking sensitivity index. It is generally believed that when Pcm ≤ 0.22%, the welded joint is guaranteed to be free from cold cracking tendency. The Pcm of the present invention is in the range of 0.17 to 0.19%, which can avoid the appearance of coarse granular bainite structure and high proportion of martensite / austenite components in the heat-affected zone of the weld, thereby ensuring that the heat-affected zone of the weld has good low-temperature toughness.

[0045] On the other hand, the method for preparing X80 pipeline steel with excellent toughness in the heat-affected zone of the ring welded joint includes the following steps: converter smelting—continuous casting—slab heating—rough rolling—finish rolling—controlled cooling—coiling.

[0046] Specifically, the converter smelting steps include LF and RH refining to obtain pure molten steel with the target metallurgical composition and low levels of impurities such as P and S.

[0047] The continuous casting step employs protective casting throughout, and the continuous casting process utilizes a low superheat of 12–18°C above the liquidus and a dynamic light reduction process with a total reduction of 6–9 mm.

[0048] Furthermore, in the slab heating step, the cold slab is placed into the heating furnace for heating, and the furnace exit temperature is 1180-1240℃, with a heating time of 180-360 minutes.

[0049] Furthermore, the roughing step involves 8 passes of rolling, with an intermediate billet thickness of 54-62 mm, a final pass reduction of ≥25%, and a final pass rolling temperature of ≤960℃.

[0050] The finishing rolling step involves 7 passes, with a finishing rolling inlet temperature ≤940℃ and a cumulative finishing rolling reduction ≥55%.

[0051] It should be noted that the cooling rate in the controlled cooling step is 15-25°C / s; preferably, the cooling rate is 18-20°C / s.

[0052] Further, in the winding step, the winding temperature is 420-480°C; preferably, the winding temperature is 420-460°C.

[0053] By controlling the rolling and cooling parameters, austenite grain refinement was achieved. By appropriately increasing the coiling temperature, the matrix microstructure of refined acicular ferrite could also be obtained. This appropriately reduced the excessively high requirements of low-temperature coiling on equipment capabilities, which can have a beneficial impact on equipment maintenance during the mass production of high-strength, thick-gauge pipeline steel.

[0054] This invention utilizes the composite strengthening effect of alloying elements such as Ni-Cr-Mo-Nb-Ti, and the microstructure of the product base material is mainly acicular ferrite, which can ensure that the steel pipes produced have good comprehensive mechanical properties such as strength and low-temperature toughness.

[0055] Meanwhile, through precise control of C element and carbon equivalent Pcm, the fine grain strengthening effect of Nb element and the optimization of low temperature toughness by Ni element, the heat-affected zone microstructure of the steel pipe after ring welding is a mixed microstructure of lath bainite, granular bainite and martensite / austenite components. This microstructure can still ensure good low temperature toughness.

[0056] Specifically, the parameters of the product base material and the heat-affected zone of the ring weld joint include: the yield strength R of the base material. t0.5 555-690MPa, tensile strength R m 625-765MPa, yield strength ratio ≤0.92, Charpy impact energy ≥180J at -20℃; Charpy impact energy of the heat-affected zone of the welded joint ≥140J at -10℃; crack tip opening displacement CTOD of the welded joint ≥0.30mm at -10℃.

[0057] The advantages of precise control of the elemental chemical composition, content, and preparation process parameters of the present invention will be demonstrated below with specific examples.

[0058] This invention provides four types of 1-4#X80 pipeline steel, numbered 1-4, with metallurgical compositions shown in Table 1, all prepared using the following process:

[0059] Step 1: Converter smelting and ladle refining to obtain pure molten steel with the target metallurgical composition and low levels of impurities such as P and S;

[0060] Step 2, Continuous casting: Protective casting is used throughout the process. The continuous casting process adopts low superheat and dynamic light reduction technology to obtain the cast billet;

[0061] Step 3, slab heating: The cold slab is placed into the heating furnace for reheating at a temperature of 1180-1240℃ for 180-360 minutes.

[0062] Step 4, rough rolling: Perform 8 passes of rolling, with an intermediate billet thickness of 54-62 mm, a reduction of ≥25% in the last pass, and a rolling temperature of ≤960℃ in the last pass;

[0063] Step 5, Finishing rolling: Perform 7 passes of rolling, with the finishing rolling inlet temperature ≤940℃ and the cumulative reduction in finishing rolling ≥55%;

[0064] Step 6: Control cooling: The cooling rate is 15-25℃ / s;

[0065] Step 7, Winding: Winding is performed using a winding machine at a temperature of 420–480℃;

[0066] The specific process parameters are shown in Table 3.

[0067] The present invention provides two types of 5-6#X80 pipeline steel, numbered 5-6, with metallurgical composition as shown in Table 2. They are prepared by a process of converter smelting—ladle refining—continuous casting—slab heating—rough rolling—finish rolling—controlled cooling—coiling, with specific parameters as shown in Table 3.

[0068] Table 1. Metallurgical composition (wt.%) of steel in embodiments of the present invention

[0069] serial number C Si Mn P S Cr Ni Mo Nb Ti Pcm 1 0.060 0.20 1.70 0.0086 0.0020 0.16 0.35 0.15 0.095 0.015 0.176 2 0.055 0.18 1.75 0.0095 0.0018 0.15 0.32 0.16 0.105 0.014 0.172 3 0.065 0.19 1.72 0.0110 0.0015 0.18 0.42 0.14 0.110 0.015 0.183 4 0.058 0.22 1.75 0.0095 0.0016 0.14 0.38 0.12 0.115 0.016 0.174

[0070] Table 2. Metallurgical composition (wt.%) of the comparative examples

[0071] serial number C Si Mn P S Cu Cr Ni Mo Nb Ti Pcm 5 0.060 0.26 1.75 ≤0.015 0.0020 0.15 0.26 0.15 0.24 0.065 0.015 0.195 6 0.058 0.22 1.72 ≤0.015 0.0025 0.20 0.28 0.22 0.28 0.068 0.016 0.198

[0072] Table 3 Manufacturing process parameters for the examples and comparative examples

[0073] serial number Slab heating temperature Slab heating time intermediate billet thickness Roughing and finishing rolling temperatures Finishing rolling temperature winding temperature 1 1200℃ 225min 58 930℃ 825℃ 445℃ 2 1195℃ 195min 56 925℃ 830℃ 420℃ 3 1230℃ 260min 54 950℃ 847℃ 460℃ 4 1195℃ 245min 60 940℃ 835℃ 455℃ 5 1220℃ 215min 60 905℃ 802℃ 340℃ 6 1215℃ 230min 58 895℃ 810℃ 360℃

[0074] The mechanical properties of the steel coils in the embodiments and comparative examples of the present invention are shown in Table 4; the mechanical properties of the steel pipes in the embodiments and comparative examples of the present invention are shown in Table 5; the mechanical properties of the heat-affected zone of the circumferential welded joints of the steel pipes in the embodiments and comparative examples of the present invention are shown in Table 6.

[0075] Table 4 Mechanical properties of steel coils in the examples and comparative examples

[0076] serial number <![CDATA[R t0.5 / MPa]]> <![CDATA[R m / MPa]]> <![CDATA[R t0.5 / R m ]]> A / ％ -20℃CVN / J -15℃ DWTT / % 1 615 714 0.86 28.5 397 100 2 598 696 0.86 29.5 389 100 3 645 742 0.87 25.0 311 95 4 632 728 0.87 27.0 305 98 5 584 681 0.86 26.5 312 96 6 614 702 0.87 28.0 298 100

[0077] Table 5 Mechanical properties of steel pipes in the examples and comparative examples

[0078] serial number <![CDATA[R t0.5 / MPa]]> <![CDATA[R m / MPa]]> <![CDATA[R t0.5 / Rm]]> A / ％ -20℃CVN / J -15℃ DWTT / % 1 582 702 0.83 26.5 372 100 2 574 692 0.83 28.0 365 100 3 608 730 0.83 24.5 287 95 4 596 722 0.83 26.5 296 92 5 562 677 0.83 23.0 290 90 6 576 685 0.84 26.0 265 100

[0079] Table 6 Mechanical properties of the heat-affected zone of the welded joints of steel pipe rings in the examples and comparative examples.

[0080]

[0081]

[0082] The mechanical properties of the steel coils / pipes in the examples and comparative examples show that the yield strength R of the examples and comparative examples is similar. t0.5 and tensile strength R m In terms of indicators, the differences are not significant, with the strength of the product in this patent embodiment being slightly higher than that of the comparative example. Regarding the Charpy impact energy (CVN) and drop hammer tear area (DWTT) indicators, which reflect the low-temperature toughness of the base material, the embodiment is slightly superior to the comparative example, especially when the composition range of the embodiment is within the preferred range, the low-temperature toughness value is significantly improved compared to the comparative example. Table 6 shows that, comparing the mechanical properties of the circumferential weld after circumferential welding of the steel pipe, the low-temperature toughness value (Charpy impact energy and CTOD value) of the circumferential weld joint heat-affected zone of the product in the embodiment is much higher than that of the comparative example. Furthermore, the comparative example shows some fluctuation in the Charpy impact energy value, while the Charpy impact energy value of the embodiment is relatively stable.

[0083] Figure 3 a, Figure 3b represents the substructure of the typical fine-grained microstructure of the heat-affected zone in Example 2 and Comparative Example 6 of the present invention. The thick black line in the figure represents large-angle grain boundaries, and the thin red line represents small-angle grain boundaries. It can be seen from the comparison that the effective grain size of the heat-affected zone of the X80 pipeline steel produced by the alloy composition and preparation method of the present invention is smaller than that of the comparative example.

[0084] Figure 4 a, Figure 4 b are SEM images of typical microstructures of the critical region of the heat-affected zone in Example 2 and Comparative Example 6, respectively. The comparison shows that the distribution of the M / A components in the comparative example is finer and more diffuse, and the M / A components in the comparative example form an island chain structure that is detrimental to low-temperature toughness.

[0085] The comparison of the above performance parameters and microstructure shows that, through the innovative design of the metallurgical composition and production process of this invention, the product has excellent low-temperature toughness in the weld heat-affected zone.

[0086] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. An X80 pipeline steel exhibiting excellent toughness in the heat-affected zone of a circumferential weld joint, characterized in that, The metallurgical composition of the pipeline steel is as follows (by mass percentage): C: 0.050~0.070%, Si: 0.15~0.25%, Mn: 1.65~1.80%, P≤0.012%, S≤0.0030%, Ni: 0.30~0.45%, Cr: 0.12~0.18%, Mo: 0.10~0.16%, Nb: 0.090~0.120%, Ti: 0.012~0.020%, with the remainder being Fe and unavoidable impurity elements; and Pcm: 0.17~0.19%.

2. The X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint according to claim 1, characterized in that, The preparation method includes the following steps: converter smelting—continuous casting—slab heating—rough rolling—finish rolling—controlled cooling—coiling.

3. The X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint according to claim 2, characterized in that, The continuous casting step adopts protective casting throughout the process, and the continuous casting process adopts low superheat and dynamic light reduction process; the slab heating step is heated to a furnace temperature of 1180~1240℃ and a heating time of 180~360min.

4. The X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint according to claim 2, characterized in that, The roughing process involves 8 passes, with an intermediate billet thickness of 54-62 mm, a final pass reduction of ≥25%, and a final pass rolling temperature of ≤960℃.

5. The X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint according to claim 2, characterized in that, The finishing rolling step involves 7 passes, with a finishing rolling inlet temperature ≤940℃ and a cumulative finishing rolling reduction ≥55%.

6. The X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint according to claim 2, characterized in that, The cooling control step has a cooling rate of 15~25℃ / S; the winding step has a winding temperature of 420~480℃.

7. The X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint according to claim 2, characterized in that, The yield strength R of the X80 pipeline steel t0.5 555-690MPa, tensile strength R m 625-765MPa, yield strength ratio ≤0.92, Charpy impact energy at -20℃ ≥180J.

8. The X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint according to claim 2, characterized in that, The Charpy impact energy of the heat-affected zone of the welded joint is ≥140J at -10℃, and the crack tip opening displacement (CTOD) of the welded joint is ≥0.30mm at -10℃.

9. A method for preparing X80 pipeline steel with excellent toughness in the heat-affected zone of a circumferential weld joint, characterized in that, The method for preparing the X80 pipeline steel according to any one of claims 1-8 includes the following steps: converter smelting—continuous casting—slab heating—rough rolling—finish rolling—controlled cooling—coiling.

Images