X80 pipeline steel and rolling method thereof

Abstract

The invention discloses X80 pipeline steel and a rolling method thereof, and relates to the field of microalloy high-strength steel manufacturing. The pipeline steel comprises the following components in percentage by mass: 0.035%-0.05% of C, 0.3%-0.5% of Si and the like, the key components comprise 0.03%-0.04% of Ti and 0.008%-0.012% of Zr, the mass ratio of Zr to Ti is 0.2-0.4, and a precipitated phase is a (Zr, Ti) (C, N) composite phase with the particle size of 0.05-0.2 mu m. The rolling method comprises the steps of heating, rough rolling, finish rolling, cooling and quenching and tempering heat treatment, the rough rolling accumulative reduction rate is larger than or equal to 30%, and the finish rolling is larger than or equal to 40%. The yield strength of the steel ranges from 570 MPa to 600 MPa, the tensile strength ranges from 650 MPa to 680 MPa, the impact energy of a welding heat affected zone at the temperature of-40 DEG C is larger than or equal to 120 J, the API SPEC5L standard is met, and through a ternary microalloying collaborative system, Nb dependence and cost are reduced, and toughness and high-temperature structure stability are improved.

Description

Technical Field

[0001] This invention relates to the field of microalloyed high-strength steel manufacturing technology, and in particular to an X80 pipeline steel and its rolling method. Background Technology

[0002] In recent years, with the continuous growth of global demand for oil and gas across various industries, high-strength pipeline steel has been widely used in buried pipeline transportation. Among them, X80 pipeline steel, as a key representative of high-strength pipeline steel, boasts numerous advantages, including high operating efficiency, strong safety and reliability, low cost, continuous and stable operation, and environmental friendliness. Using X80 pipeline steel for oil and gas transportation can meet the requirements of long-distance transport and has significant engineering application value and strategic importance in the energy sector.

[0003] Pipeline steel is typically used in harsh environments with high pressure and complex media, thus placing high demands on its performance. Currently, X80 pipeline steel is usually produced using a combination of optimized alloy composition design and the TMCP (Transformer-Modified Plateau Process) technology. This involves two-stage rolling in the austenitic recrystallization zone and the non-recrystallization zone, followed by controlled cooling. Pipeline steel produced using the TMCP process exhibits excellent strength-toughness balance and superior performance.

[0004] However, in the existing alloy composition design of X80 pipeline steel, the addition of Ti is relatively conservative, usually used as an auxiliary microalloying element, with its addition amount generally controlled at around 0.01%. Compared with existing alloy systems characterized by high Nb content, Ti resources are abundant and inexpensive, making Ti a significant cost advantage for microalloying. Furthermore, Ti has strong chemical reactivity and is a powerful nitrogen-fixing element, effectively hindering the combination of Nb and N, increasing the solid solubility of Nb in austenite, thereby further leveraging Nb's significant grain refinement effect and moderate precipitation strengthening effect, and to some extent reducing dependence on Nb.

[0005] However, high TiN content can lead to coarsening during high-temperature rolling, heat treatment, or welding thermal cycling, potentially causing a decrease in steel toughness and affecting the microstructure stability during long-term service. This potential risk has not yet been effectively addressed. Meanwhile, how to further reduce Nb usage and expand cost advantages while maintaining or even improving the overall performance of the steel remains a critical technical bottleneck that urgently needs to be overcome in this field. Summary of the Invention

[0006] Given the conservative use of Ti in existing technologies and the problems associated with high-Ti steel during high-temperature rolling, heat treatment, or welding thermal cycling, this invention provides a superior, lower-cost, and more stable X80 pipeline steel alloy system and its rolling method by synergistically introducing extremely low amounts of Zr when the Ti content reaches near the upper limit (0.03%~0.04%), based on a "low-Nb-high-Ti" alloy system. Pipeline steel rolled using this alloy system meets the performance requirements for X80 pipeline steel in the API SPEC5L standard for yield strength, tensile strength, elongation, and yield ratio. Simultaneously, it significantly improves the toughness of the weld heat-affected zone and the high-temperature microstructure stability, further reducing Nb dependence and production costs.

[0007] To achieve the above objectives, the present invention provides an X80 pipeline steel, characterized in that the composition of the X80 pipeline steel, by mass percentage, is: C: 0.035%~0.05%, Si: 0.3%~0.5%, Mn: 1.6%~1.8%, Mo: 0.2%~0.3%, Ni: 0.2%~0.35%, Nb: 0.03%~0.06%, V: 0.025%~0.035%, Ti: 0.03%~0.04%, Zr: 0.008%~0.012%, P: ≤0.01%, S: ≤0.004%, O: ≤0.006%, N: ≤0.005%, with the balance being Fe and unavoidable trace impurities; the precipitated phases in the pipeline steel are (Zr,Ti)(C,N). The composite precipitate has a particle size of 0.05~0.2μm.

[0008] Preferably, the mass ratio of Zr to Ti is 0.2 to 0.4.

[0009] Preferably, the yield strength is 570MPa~600MPa, the tensile strength is 650MPa~680MPa, the elongation is 23%~33%, the reduction of area is 75%~85%, the yield strength ratio Rt0.2 / Rm is 0.85~0.9, the microhardness is 230HV~250HV, the crack sensitivity coefficient Pcm is 0.15%~0.17%, and Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+B; the impact energy of the weld heat-affected zone at -40℃ is ≥120J.

[0010] This invention also provides a rolling method for X80 pipeline steel, comprising the following steps: providing X80 pipeline steel as described above; placing the pipeline steel of the described composition into a heating furnace, with a heating temperature range of 1200℃~1250℃ and a heating time range of 2h~4h; rough rolling: rough rolling with fewer than or equal to 3 passes, with a rolling temperature range of 1050℃~1100℃; finish rolling: finish rolling with fewer than or equal to 3 passes, with a rolling temperature range of 820℃~950℃; cooling: air cooling to room temperature after rolling; quenching: heating to 900℃~950℃, holding for 45min~90min, and then water quenching; tempering: heating to 550℃~650℃, holding for 1h~2h, and then air cooling to room temperature.

[0011] Preferably, in the roughing rolling, the final rolling temperature is ≥1050℃ and the cumulative reduction rate of the roughing rolling is ≥30%.

[0012] Preferably, in the finishing rolling, the finishing rolling inlet temperature is ≤950℃, the final rolling temperature is ≥820℃, and the cumulative reduction rate of finishing rolling is ≥40%.

[0013] Preferably, the cooling process involves air cooling to room temperature.

[0014] Preferably, in the quenching process, the sample is heated to 900℃~950℃ at a heating rate of 10℃ / min, held at that temperature for 45min~90min, and then water-cooled.

[0015] Preferably, in the tempering process, the sample is heated to 550°C~650°C at a heating rate of 10°C / min, held at that temperature for 1h~2h, and then air-cooled to room temperature.

[0016] The core innovation of this invention lies in constructing a ternary microalloying synergistic system of "Ti as the primary nitrogen fixer, Zr as the auxiliary stabilizer, and Nb as the fully dissolved and exfoliated element": Ti preferentially combines with N to form TiN, solving the problem of Nb nitrogen consumption during fixation; Zr forms a (Zr,Ti)(C,N) composite precipitate phase with Ti, C, and N, effectively suppressing TiN coarsening and abnormal Nb (C,N) growth, stabilizing the grain boundary structure; Nb can still be fully dissolved and exfoliated at lower concentrations, playing a role in grain refinement and precipitation strengthening. This synergistic system further amplifies the effect of the original invention of "replacing Nb with Ti," significantly reducing Nb dependence while ensuring performance, resulting in a more prominent cost advantage.

[0017] Meanwhile, by optimizing the rough rolling process, the deformation degree is enhanced in the high-temperature austenite recrystallization zone, significantly refining the austenite grains. By optimizing the finish rolling process, dislocations accumulate in the non-recrystallization zone of austenite, forming flattened austenite grains, increasing the grain boundary area, providing more nucleation sites for the phase transformation process, and further refining the grains. During rolling and subsequent heat treatment, the (Zr,Ti)(C,N) composite precipitates and the fine precipitates such as Nb(C,N) and V(C,N) work synergistically to effectively pin grain boundaries and inhibit grain growth, while also producing a significant precipitation strengthening effect. Ultimately, this enables the X80 pipeline steel to achieve an excellent balance between strength and plasticity, while also possessing excellent weld heat-affected zone toughness and high-temperature microstructure stability.

[0018] The following will further explain the concept, specific structure, and technical effects of the present invention in conjunction with the accompanying drawings, so as to fully understand the purpose, features, and effects of the present invention. Attached Figure Description

[0019] Figure 1 This is a metallographic image of the microstructure of X80 pipeline steel in a preferred embodiment of the present invention; Figure 2 This is a SEM image of precipitates in X80 pipeline steel in a preferred embodiment of the present invention; Figure 3 This is a flowchart of a preferred embodiment of the rolling method for X80 pipeline steel according to the present invention. Detailed Implementation

[0020] The following description, with reference to the accompanying drawings, illustrates several preferred embodiments of the present invention to make its technical content clearer and easier to understand. The present invention can be embodied in many different forms, and the scope of protection of the present invention is not limited to the embodiments mentioned herein.

[0021] The preferred embodiments of the present invention will be described in further detail below with reference to the accompanying drawings: The rolling process of X80 pipeline steel includes: sequentially passing the raw materials through electric furnace smelting, LF refining, and RH vacuum degassing refining to minimize the content of harmful elements such as P, S, O, and N, where P ≤ 0.01%, S ≤ 0.004%, O ≤ 0.006%, and N ≤ 0.005%. During the smelting process, nano-sized Zr powder (particle size 20~50nm) is precisely added in the later stage of LF refining to ensure uniform distribution of Zr. After smelting, the steel is cast to obtain four X80 pipeline steel ingots with different Nb / Ti / Zr ratios, referred to as Examples 1 to 4. The steel ingots are Fe-based, and the other main chemical components are shown in Table 1.

[0022] Table 1. Other major chemical components (wt%) in various embodiments of the present invention

[0023] Among them, C is the most basic and economical strengthening element. When the C content is too high, it will reduce the toughness and weldability of the steel. When the content is too low (<0.01%), the reduction of interstitial carbon atoms and the precipitation of Nb (C, N) after welding thermal cycling will cause grain boundary weakening, leading to localized embrittlement in the weld heat-affected zone. In this invention, the C content is controlled at 0.035%~0.05%.

[0024] Si is an essential element for deoxidation in steelmaking and also has a certain solid solution strengthening effect. In this invention, the Si content is controlled at 0.3%~0.5%.

[0025] Mn (nitrogen) lowers the phase transformation temperature of austenite to ferrite, increases the rolling range of the non-recrystallized region, promotes grain refinement, and improves strength and toughness. When the Mn content is below 1.3%, these effects are not significant. When the Mn content is above 2.0%, it reduces the toughness and weldability of the steel, and severe banding may occur. In this invention, the Mn content is controlled at 1.6%~1.8%.

[0026] Mo can inhibit the formation of pearlite and polygonal ferrite, and promote the formation of acicular ferrite, thereby improving strength and toughness. In this invention, the Mo content is controlled at 0.2%~0.3%.

[0027] Ni is an element that can significantly improve low-temperature toughness, but due to its high price, it should not be added in large quantities. In this invention, the Ni content is controlled at 0.2%~0.35%.

[0028] Nb plays a role in solute dragging and pinning of austenite grain boundaries during phase transformation, inhibiting the recrystallization of deformed austenite and thus refining the grains. However, excessively high Nb content can increase the production cost of pipeline steel. This invention further releases the strengthening potential of Nb through the synergistic effect of Zr and Ti, controlling the Nb content to a lower range of 0.03% to 0.06%, maximizing cost advantages while ensuring strengthening effect.

[0029] V: Primarily enhances the strength of pipeline steel through carbide precipitation strengthening within ferrite. Excessive V content can also increase the production cost of pipeline steel. In this invention, the V content is controlled between 0.025% and 0.035%.

[0030] Ti is a strong nitrogen-fixing element. TiN, formed at high temperatures, can inhibit grain growth during rolling, thereby improving the toughness of steel. When Ti content is below 0.006%, the nitrogen-fixing effect is poor; when it exceeds 0.04%, the nitrogen-fixing effect reaches saturation, and excess Ti will lead to a deterioration in the toughness of the steel. In this invention, the Ti content is controlled within the high Ti range of 0.03% to 0.04%, providing a sufficient Ti source for the formation of a composite precipitate phase with Zr, while ensuring the full utilization of the main nitrogen-fixing function.

[0031] Zr acts as a "grain boundary and precipitate stabilizer," exhibiting stronger nitride and carbide formation capabilities than Ti. Zr's core role is not direct strengthening, but rather substitutional solid solution interaction with Ti, C, and N to form more stable and finer (Zr,Ti)(C,N) composite precipitates. Based on the TiN lattice structure, Zr atoms replace some Ti atoms in the TiN lattice while incorporating a small amount of C atoms, forming a "Zr-Ti-CN" quaternary composite structure. During high-temperature rolling and heat treatment, this composite phase more effectively pins austenite grain boundaries and ferrite / bainite lath boundaries after phase transformation, inhibiting TiN coarsening and abnormal Nb(C,N) growth. When the Zr content is below 0.008%, it is difficult to form a sufficient composite phase, resulting in insufficient stabilization. When it is above 0.012%, coarse ZrC inclusions may form, which may reduce toughness. Therefore, in this invention, the Zr content is precisely controlled at 0.008% to 0.012%, and the mass ratio of Zr to Ti is limited to 0.2 to 0.4 to ensure the formation of a uniform (Zr,Ti)(C,N) composite phase and avoid Zr precipitation alone.

[0032] P, S, O, and N are impurity elements in steel that easily form defects such as segregation and inclusions, adversely affecting the toughness and hot workability of steel plates. Their content should be minimized. This invention controls P to ≤0.01%, S to ≤0.004%, O to ≤0.006%, and N to ≤0.005%.

[0033] Hot rolling is employed on a rolling mill: the smelted pipeline steel is placed in a heating furnace and heated to 1200℃~1250℃; the roughing stage rolling temperature is 1050℃~1100℃, with one pass and a total reduction of not less than 30%; the finishing stage rolling temperature is 820℃~950℃, with two passes and a total reduction of not less than 40%. After rolling, the steel plate is air-cooled to room temperature. Specific rolling process parameters are shown in Table 2.

[0034] Table 2 Hot rolling process parameters of various embodiments of the present invention

[0035] Tempering heat treatment was performed in a heat treatment furnace: The four types of pipeline steel with different compositions after rolling were placed in the furnace for heat treatment. First, the samples were heated to 900℃~950℃ at a heating rate of 10℃ / min, held at that temperature for 45min~90min, and then water-cooled. Then, the samples were heated to 550℃~650℃ at a heating rate of 10℃ / min, held at that temperature for 1h~2h, and then air-cooled to room temperature.

[0036] The mechanical properties of X80 pipeline steel obtained according to the composition design and rolling methods of the above embodiments are shown in Table 3.

[0037] Table 3 Mechanical properties of various embodiments of the present invention

[0038] It can be seen that the mechanical properties of the X80 pipeline steel obtained according to the composition design and rolling methods of the various embodiments of the present invention all meet and partially exceed the performance requirements of X80 pipeline steel in the API SPEC5L standard, including yield strength, tensile strength, yield ratio, elongation, reduction of area, and microhardness. The yield strength is between 570MPa and 600MPa, the tensile strength is between 650MPa and 680MPa, the yield ratio is between 0.88 and 0.9, the elongation can reach 26% to 30%, and the microhardness is around 240HV. At the same time, the impact energy of the weld heat-affected zone at -40℃ is ≥130J, and the grain size growth rate after holding at 600℃ for 2h is ≤4.5%, achieving good strength-plasticity matching, excellent weldability, and high-temperature microstructure stability.

[0039] The microstructure of the X80 pipeline steel obtained in the above embodiments mainly consists of bainitic ferrite (BF) and granular bainite (GB), such as... Figure 1 As shown, the BF (Bladder Foil) consists of several parallel lath-shaped ferrite bundles. Its main characteristic is the high dislocation density within the laths. These dislocations are intertwined, hindering the movement of new dislocations, thus significantly improving the strength of the steel. Small M / A islands are uniformly distributed between the lath bundles. M / A islands are brittle-hard phases that improve strength; simultaneously, the fine and dispersed M / A islands can hinder crack propagation, and the retained austenite within them provides favorable toughness, making BF the optimal microstructure type combining high strength and good ductility and toughness. GB (Gross Grain Foil) consists of a ferrite matrix with fine granular or equiaxed M / A islands, also exhibiting good strength and toughness.

[0040] The precipitates in the X80 pipeline steel obtained in the above embodiments are mainly (Zr,Ti)(C,N) composite precipitates and microalloyed carbonitrides such as Nb(C,N) and V(C,N). Figure 3As shown, the (Zr,Ti)(C,N) composite precipitates, with a particle size of 0.05~0.2μm, are uniformly dispersed and can effectively hinder dislocation movement and grain boundary migration, thereby improving the strength of pipeline steel and inhibiting excessive grain growth. At the same time, in synergy with other precipitates, they further optimize the ductility, toughness, weldability, and long-term service stability of the steel.

[0041] In summary, this invention introduces critical-range Zr elements into a "low Nb-high Ti" system to construct a ternary microalloying synergistic system. Combined with hot rolling and tempering heat treatment processes, it fully leverages the strengthening and stabilizing effects of microalloying elements, comprehensively utilizing multiple strengthening mechanisms such as solid solution strengthening, precipitation strengthening, and grain refinement strengthening. This ensures that the mechanical properties of pipeline steel meet the performance requirements of X80 pipeline steel in the API SPEC5L standard, while significantly improving the toughness of the weld heat-affected zone and the stability of the high-temperature microstructure. Furthermore, by further reducing the Nb content, production costs are effectively reduced while maintaining the overall material performance, demonstrating significant engineering application value. The preferred embodiments of this invention have been described in detail above.

[0042] It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of this invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of this invention through logical analysis, reasoning, or limited experimentation on the basis of the prior art should be within the scope of protection defined by the claims.

Claims

1. An X80 pipeline steel, characterized in that, The composition of the X80 pipeline steel, by mass percentage, is as follows: C: 0.035%~0.05%, Si: 0.3%~0.5%, Mn: 1.6%~1.8%, Mo: 0.2%~0.3%, Ni: 0.2%~0.35%, Nb: 0.03%~0.06%, V: 0.025%~0.035%, Ti: 0.03%~0.04%, Zr: 0.008%~0.012%, P: ≤0.01%, S: ≤0.004%, O: ≤0.006%, N: ≤0.005%, with the balance being Fe and unavoidable trace impurities; the precipitated phase in the pipeline steel is a (Zr,Ti)(C,N) composite precipitate with a particle size of 0.05~0.2μm.

2. The X80 pipeline steel according to claim 1, characterized in that, The mass ratio of Zr to Ti is 0.2 to 0.

4.

3. The X80 pipeline steel according to claim 1, characterized in that, The yield strength is 570MPa~600MPa, the tensile strength is 650MPa~680MPa, the elongation is 23%~33%, the reduction of area is 75%~85%, the yield strength ratio Rt0.2 / Rm is 0.85~0.9, the microhardness is 230HV~250HV, the crack sensitivity coefficient Pcm ranges from 0.15% to 0.17%, and Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+B; the impact energy of the weld heat-affected zone at -40℃ is ≥120J.

4. A rolling method for X80 pipeline steel, characterized in that, The steps include: providing X80 pipeline steel as described in claim 1; placing the pipeline steel of the described composition into a heating furnace, with a heating temperature range of 1200℃~1250℃ and a heating time range of 2h~4h; rough rolling: the number of rough rolling passes is less than or equal to 3, and the rolling temperature range is 1050℃~1100℃; finish rolling: the number of finish rolling passes is less than or equal to 3, and the rolling temperature range is 820℃~950℃. Cooling: After rolling, air cool to room temperature; Quenching: Heat to 900℃~950℃, hold for 45min~90min and then water quench; Tempering: Heat to 550℃~650℃, hold for 1h~2h, then air cool to room temperature.

5. The rolling method for X80 pipeline steel according to claim 4, characterized in that, In the roughing process, the final rolling temperature is ≥1050℃ and the cumulative reduction rate of the roughing process is ≥30%.

6. The rolling method for X80 pipeline steel according to claim 4, characterized in that, In the finishing mill, the entry temperature is ≤950℃, the final rolling temperature is ≥820℃, and the cumulative reduction rate is ≥40%.

7. The rolling method for X80 pipeline steel according to claim 4, characterized in that, In the cooling process, air cooling is used to cool to room temperature.

8. The rolling method for X80 pipeline steel according to claim 4, characterized in that, In the quenching process, the sample is heated to 900℃~950℃ at a heating rate of 10℃ / min, held at that temperature for 45min~90min, and then water-cooled.

9. The rolling method for X80 pipeline steel according to claim 4, characterized in that, In the tempering process, the sample is heated to 550℃~650℃ at a heating rate of 10℃ / min, held at that temperature for 1h~2h, and then air-cooled to room temperature.

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