An ultra-thick L360 pipeline steel with excellent low-temperature toughness

Abstract

The present invention discloses an ultra-thick L360 pipeline steel with excellent low-temperature toughness, belonging to the field of materials and metallurgy technology. The steel comprises: C: 0.050% to 0.058%, Si: 0.10% to 0.20%, Mn: 1.0% to 1.1%, Nb: 0.040% to 0.050%, Ti: 0.019% to 0.021%, Cr: 0.18% to 0.24%, Ni: 0.15% to 0.20%, P≤0.015%, S≤0.003%, N≤0.004%, and Pcm: 0.118% to 0.130%, with the remainder being Fe and unavoidable impurities. The present invention adopts a low-carbon, low-manganese, low-Pcm alloy design, adds Nb and Ti to improve strength, obtains an acicular ferrite structure through the synergistic effect of Cr and Ni and a suitable cooling system, and achieves a good balance of strength and toughness.

Description

Technical Field

[0001] The present invention relates to the field of materials and metallurgical technology, and more particularly to an ultra-thick L360 pipeline steel with excellent low-temperature toughness. Background Art

[0002] With the rapid development of the economy and society, the demand for oil and natural gas energy continues to grow, and pipeline transportation volumes are also gradually increasing. To meet the needs of long-distance, high-pressure transportation, oil and gas pipeline steel is developing in the direction of larger diameters, thicker walls, and higher strength and toughness. However, for hot-rolled pipeline steel products with thicknesses exceeding 20mm, insufficient compression ratios force major steel mills to pay particular attention to the product's strength and toughness during assembly, especially low-temperature toughness. This has become a common technical difficulty in the production of ultra-thick products.

[0003] Currently, public data shows that the lowest impact test temperature for L360-grade pipeline steel hot-rolled coils is -20°C. Chinese patent application number 201610803301.7 discloses a thick-gauge X52 pipeline steel and its production method, where the -20°C impact energy is only 120J, significantly lacking in toughness. Furthermore, the high Cr content in this invention leads to high alloy costs. The finished product has a thickness of 12-20mm and a coiling temperature of 350-520°C. Low-temperature coiling presents significant risks when producing thick-gauge pipeline steel. Furthermore, this invention does not describe a production method for X52 steel with a thickness exceeding 20mm. Chinese patent application number 201310585644.7 discloses a low-cost production method and pipeline steel for X52 pipelines. The steel's composition, by weight, includes C: 0.08%-0.12%, Si ≤ 0.35%, Mn: 1.10%-1.40%, S ≤ 0.025%, P ≤ 0.025%, and Ti: 0.008%-0.022%. The finished product of this invention has a maximum thickness of only 10mm, a yield strength of 387-417MPa, and an impact energy of only 153J at -20°C. There is no drop hammer requirement. However, after spiral forming, the product loses strength due to the Bauschinger effect, resulting in a significant risk of yield strength falling below 360MPa. Chinese patent application number 201010243258.6 discloses an X52 pipeline steel and its production method. The steel's composition, by weight, includes C: 0.07%-0.09%, Si: 0.15%-0.30%, Mn: 1.10%-1.30%, S≤0.006%, P≤0.020%, Nb: 0.02%-0.04%, Ti: 0.01%-0.02%, N≤0.008%, and Als: 0.020%-0.040%. This invention limits the S content to below 0.006%, resulting in high steelmaking costs. The soaking temperature is 1200-1240°C, which coarsens the austenite grains and affects subsequent tensile properties. After rough rolling, the steel undergoes a 20-50 second swing-warming phase before final rolling, resulting in low production efficiency. The steel's thickness specifications are not specified. Furthermore, its mechanical properties only meet the API 5L standard, and it lacks low-temperature toughness. Chinese patent application number 201910949817.6 discloses an ultra-thick hot-rolled coil of X52 pipeline steel and its production method. This invention skips the warming stage after rough rolling and proceeds directly to finish rolling, which can easily cause mixed crystals and affect toughness. Therefore, the invention only describes 0°C impact performance and does not address drop hammer performance, especially at low temperatures. The prior art, "Research and Development of Hot-Rolled Medium-Wide Strip Steel X52 for Oil and Gas Pipelines," proposes a method for producing 5-7mm thick hot-rolled coil of X52-grade pipeline steel. This technology does not address the production of thick (≥20mm) X52 steel strip from thin slabs. Summary of the Invention

[0004] The present invention aims to overcome the above-mentioned deficiencies of the prior art and provide an ultra-thick L360 pipeline steel with excellent low-temperature toughness. By combining a production process of "molten iron pretreatment + converter smelting + refining outside the furnace + LF refining + continuous casting + heating + rolling, cooling, and coiling" with alloy composition design, the pipeline steel produced has a thickness of 20 mm to 24.2 mm, a drop weight shear area (SA) of 96% or greater at -20°C, and an impact energy of 263 J or greater at -40°C, exhibiting excellent low-temperature toughness.

[0005] To achieve the above object, the technical solution of the present invention is as follows:

[0006] A super-thick L360 pipeline steel with excellent low-temperature toughness comprises the following components in weight percentage: C: 0.050%-0.058%, Si: 0.10%-0.20%, Mn: 1.0%-1.1%, Nb: 0.040%-0.050%, Ti: 0.019%-0.021%, Cr: 0.18%-0.24%, Ni: 0.15%-0.20%, P≤0.015%, S≤0.003%, N≤0.004%, and Pcm: 0.118%-0.130%, with the remainder being Fe and unavoidable impurities; a method for preparing the super-thick L360 pipeline steel with excellent low-temperature toughness comprises the following steps: molten iron pretreatment, converter smelting, refining outside the furnace, LF refining, continuous casting, heating, rolling, cooling and coiling; the said processing During hot rolling, rolling, cooling and coiling, the continuous casting slab is heated to 1140℃~1150℃ and then subjected to a two-stage controlled rolling and cooling technology. Subsequently, five rough rolling passes are completed, with the final rolling temperature ≥1015℃. The reduction rate in the rough rolling stage increases with each pass, with the first pass's reduction rate being 20%~20.8%, the second pass's reduction rate being 20.6%~21.1%, the third pass's reduction rate being 20.9%~21.8%, the fourth pass's reduction rate being 21.4%~22.8%, and the fifth pass's reduction rate being 21.8%~24.2%, with a cumulative reduction rate of 69.4%~70.6%. The intermediate slab formed after rough rolling is heated to 950℃~958℃ before entering finish rolling, with the final rolling temperature being 790℃~810℃. After rolling, the intermediate slab is cooled to 522℃~543℃ at a rate of 20.7℃~22.8℃ / s and then coiled.

[0007] Optionally, the thickness of the L360 pipeline steel is 20 mm to 24.2 mm.

[0008] Optionally, the yield strength of the L360 pipeline steel is ≥407 MPa, the tensile strength is ≥496 MPa, the yield strength ratio is 0.79-0.84, and the A 50 The temperature is 48%~53%, the drop hammer shear area SA at -20℃ is ≥96%, and the impact energy at -40℃ is ≥263J.

[0009] Optionally, the metallographic structure of the L360 pipeline steel is acicular ferrite and dispersed Mahão islands, wherein the volume fraction of the acicular ferrite is 97% to 98%.

[0010] Optionally, the thickness of the intermediate blank is 50 mm to 52 mm.

[0011] Optionally, the finishing rolling passes are 7 times, the reduction rate of the first finishing pass is 13.5%~15.8%, the reduction rate of the second pass is 11.8%~15.1%, the reduction rate of the third pass is 10.9%~14.0%, the reduction rate of the fourth pass is 9.7%~12.9%, the reduction rate of the fifth pass is 9.4%~11.7%, the reduction rate of the sixth pass is 8.1%~11.2%, the reduction rate of the last pass is 5.1%~8.0%, and the cumulative reduction rate of finishing rolling is 53.5%~60.0%.

[0012] Optionally, during the off-furnace refining and LF refining, the molten steel discharged from the converter is subjected to RH vacuum treatment, followed by LF furnace light desulfurization treatment and calcium treatment.

[0013] Optionally, during the continuous casting, electromagnetic stirring or dynamic soft reduction is applied to the refined molten steel to obtain a continuous casting billet with a thickness of 170 mm.

[0014] The implementation of the present invention will have the following beneficial effects:

[0015] This invention relates to an ultra-thick L360 pipeline steel with excellent low-temperature toughness, particularly suitable for hot-rolled coils used in oil and gas pipelines. Through a production process consisting of "hot metal pretreatment + converter smelting + refining outside the furnace + LF refining + continuous casting + heating + rolling, cooling, and coiling," combined with alloy composition design, the resulting pipeline steel has a thickness of 20 mm to 24.2 mm, a drop shear area (SA) of ≥96% at -20°C, and an impact energy of ≥263 J at -40°C, demonstrating excellent low-temperature toughness.

[0016] The present invention utilizes a low-carbon, low-manganese, low-Pcm alloy design, adding Nb and Ti to enhance strength. Through the synergistic effects of Cr and Ni, combined with a suitable cooling system, an acicular ferrite structure is achieved, thereby achieving an excellent balance between strength and toughness. This design not only addresses the technical challenge of poor strength-toughness matching when producing thick-gauge products from thin billets, but also ensures product performance stability and excellent low-temperature toughness performance. Furthermore, the present invention utilizes a low-Mn, low-Si composition design, providing a foundation for the excellent strength and toughness of the finished product. Furthermore, a precise cooling system further optimizes the microstructure and mechanical properties. DETAILED DESCRIPTION

[0017] The present invention will be further described below with reference to specific examples, but the present invention is not limited thereto in any way.

[0018] 1. Chemical composition and mechanical properties

[0019] The invention discloses an ultra-thick specification L360 pipeline steel with excellent low-temperature toughness, comprising the following components in weight percentage: C: 0.050%-0.058%, Si: 0.10%-0.20%, Mn: 1.0%-1.1%, Nb: 0.040%-0.050%, Ti: 0.019%-0.021%, Cr: 0.18%-0.24%, Ni: 0.15%-0.20%, P≤0.015%, S≤0.003%, N≤0.004%, and Pcm: 0.118%-0.130%, with the remainder being Fe and unavoidable impurities.

[0020] Compared to the existing L360 pipeline steel, the ultra-thick, high-toughness L360-grade pipeline steel of the present invention adopts a low-C, low-Mn design, utilizes the Nb-Ti microalloying mechanism, and adds Cr to improve the uniformity of the thick-gauge product structure. At the same time, it is combined with an appropriate proportion of Ni in a cooling process to obtain an acicular ferrite structure, ensuring that the product has appropriate strength and good low-temperature toughness. Specifically, the mechanism of action of each alloy component in the ultra-thick L360 pipeline steel with excellent low-temperature toughness of the present invention is as follows:

[0021] Carbon: Carbon is the most important element in steel after iron and is the most economical element for increasing product strength. However, increasing carbon content directly increases the Pcm value, significantly reducing the steel's plasticity, toughness, and weldability. Low carbon content is crucial to the alloy formulation of this invention and fundamentally ensures the product achieves a good balance of strength and toughness. Therefore, the carbon content is controlled to 0.050% to 0.058%.

[0022] Si: Silicon is generally introduced as a reducing agent and deoxidizer during the steelmaking process. In addition, Si can also be dissolved in ferrite and austenite to play a strengthening role and improve the strength of the steel, but at the same time, plasticity and toughness will be lost. Therefore, a low Si design is also adopted in the present invention. Therefore, the Si content of the present invention is controlled to 0.10%~0.20%.

[0023] Mn: Manganese is a strengthening element in steel second only to C. It has a solid solution strengthening effect, increases austenite stability, promotes the formation of a certain amount of M / A, and is also beneficial for improving hardenability, effectively ensuring the strength of the steel. However, excessive manganese content can lead to severe segregation, forming segregated banded structures and significantly reducing toughness. In addition, Mn and S easily form MnS inclusions that deform along the rolling direction, splitting the matrix and reducing toughness. Therefore, in steels with strict toughness requirements, the Mn content should not be too high. In the present invention, the manganese content is controlled to 1.0% to 1.1%.

[0024] P, S, N: are inevitable impurity elements in steel. The lower the better, but too low a requirement will increase production costs. In the present invention, P≤0.015%, S≤0.005%, N≤0.006%.

[0025] Ti: Titanium is an extremely strong nitride-forming element with a significant nitrogen-fixing effect. The stoichiometric Ti / N ratio is 3.42, and only about 0.012% Ti is needed to fix all 40 ppm of nitrogen in the present invention. Under the high temperature conditions of continuous casting, stable and fine TiN particles can be formed. On the one hand, they can effectively prevent the growth of austenite during the reheating of the ingot, and on the other hand, they can improve the impact toughness of the weld heat-affected zone. The addition of Ti is particularly important for welded structural steel. In addition, because Ti has a higher binding force with S than Mn, Ti can "snatch" S from MnS inclusions to form Ti4C2S2, which is not easy to deform. This does not destroy the continuity of the matrix and thus improves the toughness index. Since the present invention adopts a low Mn and low S design, only a small amount of MnS is formed. Therefore, in addition to the 0.012% Ti used for nitrogen fixation, an additional addition of about 0.006% Ti can modify MnS to Ti4C2S2. Therefore, the Ti content of the present invention is 0.019% to 0.021%.

[0026] Nb: Niobium is a key element used in controlled rolling in modern microalloyed pipeline steels. It significantly increases the austenite recrystallization temperature and expands the unrecrystallized zone, allowing the rough rolling process window to operate at higher temperatures and reducing mill loads. The strain-induced precipitation of NbC generated during this stage hinders the recovery and recrystallization of the deformed austenite, continuously refining the original austenite grain structure and thus contributing to grain refinement. During the subsequent finishing rolling and cooling stages, Nb can also enhance steel performance through various strengthening mechanisms, including precipitation strengthening, precipitation strengthening, and phase transformation strengthening. However, due to the relatively low carbon content in the present invention, even excessive Nb additions will not be effective and will instead form "insoluble Nb lumps," impacting the overall performance of the material. Therefore, the present invention controls the Nb content to 0.040% to 0.050%.

[0027] Cr: Chromium is the most economical element for improving hardenability. Cr can effectively improve the uniformity of the through-thickness structure of thick-gauge products. Mo can promote the formation of acicular ferrite, but it is expensive. Cr, however, is only 1 / 9 the price of Mo. When added together with 1.2 times the Ni content and combined with a suitable cooling process, it can effectively promote the formation of acicular ferrite and Mahão Island (M / A) structures, improving the toughness of the product while also reducing strength loss during the pipe manufacturing process. Therefore, the Cr content in this invention is 0.18% to 0.24%.

[0028] Ni: Nickel is an austenite stabilizing element second only to C, N, and Mn. It can effectively prevent grain growth at high temperatures, maintain a fine grain structure, and ensure that the product has small original austenite grains. Ni can also play a role in solid solution strengthening and improve strength. In addition, Ni can also promote cross-slip of dislocations, effectively improving the toughness index of the product. In addition, the present invention uses a suitable ratio of Cr and Ni, combined with a cooling process, to obtain an acicular ferrite structure, further improving strength and toughness. However, the cost of Ni alloys is relatively high, and excessive addition will affect the economic efficiency of the product. Therefore, the Ni content of the present invention is 0.15% to 0.20%.

[0029] In a specific embodiment, the thickness of L360 pipeline steel is 20 mm to 24.2 mm.

[0030] In a specific embodiment, the final mechanical properties of the L360 pipeline steel coil are yield strength ≥407MPa, tensile strength ≥496MPa, yield strength ratio 0.79~0.84, A 50 The temperature is 48%~53%, the drop hammer shear area SA at -20℃ is ≥96%, and the impact energy at -40℃ is ≥263J.

[0031] In a specific embodiment, the metallographic structure of L360 pipeline steel is acicular ferrite and a small amount of dispersed Mahō islands, wherein the volume fraction of acicular ferrite is 97%-98%, and the volume fraction of Mahō islands is 2%-3%.

[0032] 2. Production Technology

[0033] The present invention also discloses a method for preparing ultra-thick L360 pipeline steel with excellent low-temperature toughness, as in any embodiment of the present invention, comprising the following steps: molten iron pretreatment, converter smelting, refining outside the furnace, LF refining, continuous casting, heating, rolling, cooling, and coiling. Specifically, the method comprises the following steps:

[0034] S1. In the molten iron pretreatment, the molten iron is desulfurized to obtain desulfurized molten iron.

[0035] S2. During converter smelting, the desulfurized molten iron is smelted in a converter and steel is tapped.

[0036] Specifically, the molten iron pretreatment and converter smelting steps of the present invention can be performed according to conventional preparation methods.

[0037] In S3, refining outside the furnace and LF refining, the molten steel after being discharged from the converter is subjected to RH vacuum treatment, followed by light desulfurization treatment in the LF furnace and calcium treatment to control the morphology of inclusions and improve the ductility, toughness and cold bending properties of the steel.

[0038] S4. During continuous casting, the refined molten steel is subjected to electromagnetic stirring or dynamic soft reduction to obtain a continuous casting billet with a thickness of 170 mm.

[0039] S5, during heating, rolling, cooling and coiling, after the continuous casting billet is heated to 1140℃~1150℃, a two-stage controlled rolling and controlled cooling technology is adopted. Since the Nb content in the present invention is not high, at a lower heating temperature, it can ensure that the added Nb, Ti, Cr, Ni and other alloys are fully dissolved, and play a role in the subsequent controlled rolling and controlled cooling; then quickly complete 5 rough rolling passes, the final rolling temperature is ≥1015℃, and the reduction rate in the rough rolling stage increases pass by pass. The reduction rate of the first pass is 20%~20.8%, the reduction rate of the second pass is 20.6%~21.1%, and the reduction rate of the third pass is 20.6%~21.1%. The reduction rate is 20.9%~21.8%, the reduction rate of the fourth pass is 21.4%~22.8%, the reduction rate of the fifth pass is 21.8%~24.2%, and the cumulative reduction rate is 69.4%~70.6%. This stage ensures that recrystallization occurs in all five rough rolling passes and the structure is continuously refined; the intermediate billet with a thickness of 50mm~52mm is formed after rough rolling. After the temperature reaches 950℃~958℃, it enters the finishing rolling. The final rolling temperature is 790℃~810℃. After rolling, it is cooled to 522℃~543℃ at a rate of 20.7℃~22.8℃ / S for coiling.

[0040] In a specific embodiment, the finishing rolling passes are 7 times, the reduction rate of the first finishing pass is 13.5%~15.8%, the reduction rate of the second pass is 11.8%~15.1%, the reduction rate of the third pass is 10.9%~14.0%, the reduction rate of the fourth pass is 9.7%~12.9%, the reduction rate of the fifth pass is 9.4%~11.7%, the reduction rate of the sixth pass is 8.1%~11.2%, the reduction rate of the last pass is 5.1%~8.0%, and the cumulative reduction rate of finishing rolling is 53.5%~60.0%. The reduction rate in the finishing rolling stage is lower, resulting in a lower density of dislocation and other substructures, which can ensure excellent toughness indicators of the product.

[0041] The present invention adopts the synergistic effect of Cr-Ni. Under this cooling process, it can promote the formation of acicular ferrite structure and generate 2% to 3% of Mahalanobis (MA) dispersed in the acicular ferrite matrix, which can ensure that the hot-rolled coil has good strength and low-temperature toughness.

[0042] The following are specific embodiments

[0043] The method for preparing ultra-thick L360 pipeline steel with excellent low-temperature toughness in this embodiment includes the following steps: molten iron pretreatment and converter smelting can be carried out according to conventional preparation methods, and then the molten steel after the converter smelting is subjected to RH vacuum treatment, followed by LF furnace light desulfurization treatment and calcium treatment to control the morphology of inclusions and improve the ductility, toughness and cold bending performance of the steel. The refined molten steel is subjected to electromagnetic stirring or dynamic light reduction to obtain a continuous casting billet with a thickness of 170 mm, and finally heated, rolled, cooled and coiled to obtain the product.

[0044] Examples 1-10

[0045] The chemical compositions of Examples 1-10 are shown in Table 1, the heating, rolling, and cooling process parameters are shown in Tables 2-4, and the mechanical property test results are shown in Table 5.

[0046] Table 1 Chemical composition of Examples 1-10 (%)

[0047]

[0048] Table 2 Production process parameters of steels in Examples 1-10

[0049]

[0050] Table 3 Production process parameters of steels in Examples 1-10

[0051]

[0052] Table 4 Production process parameters of steels in Examples 1-10

[0053]

[0054] Table 5 Mechanical properties and tissue ratios of Examples 1-10

[0055]

[0056] As can be seen from Tables 1-5, the ultra-thick L360 pipeline steel hot coil produced by the production process of the present invention has excellent strength and toughness, and the low-temperature toughness index is particularly excellent.

[0057] The above-described embodiments merely represent several implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A thick gauge L360 pipeline steel with excellent low temperature toughness, characterized in that: The invention comprises the following components in weight percentage: C: 0.051% to 0.058%, Si: 0.11% to 0.19%, Mn: 1.0% to 1.1%, Nb: 0.040% to 0.050%, Ti: 0.019% to 0.021%, Cr: 0.22% to 0.24%, Ni: 0.18% to 0.20%, P≤0.015%, S≤0.003%, N≤0.004%, Cr / Ni=1.2, Pcm: 0.118% to 0.130%, and the rest is Fe and unavoidable impurities; The method for preparing the thick-gauge L360 pipeline steel with excellent low-temperature toughness comprises the following steps: molten iron pretreatment, converter smelting, off-furnace refining, LF refining, continuous casting, heating, rolling, cooling and coiling; During the heating, rolling, cooling and coiling, the continuous casting slab is heated to 1142° C. to 1150° C. and then subjected to a two-stage controlled rolling and controlled cooling technology. Subsequently, five rough rolling passes are performed, with a final rolling temperature of ≥1015° C. The reduction rate in the rough rolling stage increases with each pass, with the first pass having a reduction rate of 20% to 20.8%, the second pass having a reduction rate of 20.6% to 21.1%, the third pass having a reduction rate of 20.9% to 21.8%, the fourth pass having a reduction rate of 21.4% to 22.8%, and the fifth pass having a reduction rate of 21.8% to 24.2%, with a cumulative reduction rate of 69.4% to 70.6%. The intermediate slab formed after the rough rolling is heated to 950° C. to 958° C. and then subjected to finish rolling, with a final rolling temperature of 791° C. to 810° C. After rolling, the intermediate slab is cooled to 522° C. to 543° C. at a rate of 20.7° C. to 22.8° C. / s and then coiled. The finishing rolling is performed seven times, with a first-pass reduction of 13.5% to 15.8%, a second-pass reduction of 11.8% to 15.1%, a third-pass reduction of 10.9% to 14.0%, a fourth-pass reduction of 9.7% to 12.9%, a fifth-pass reduction of 9.4% to 11.7%, a sixth-pass reduction of 8.1% to 11.2%, a final-pass reduction of 5.1% to 8.0%, and a cumulative finishing reduction of 53.5% to 60.0%. The thickness of the L360 pipeline steel is 20 mm to 24.2 mm; The yield strength of the L360 pipeline steel is ≥407MPa, the tensile strength is ≥496MPa, the yield strength ratio is 0.79~0.84, and the A 50 48%~53%, -20℃ drop hammer shear area SA≥96%, -40℃ impact energy≥263J; The metallographic structure of the L360 pipeline steel is acicular ferrite and dispersed Mahão islands, wherein the volume fraction of the acicular ferrite is 97% to 98%.

2. The thick gauge L360 pipeline steel with excellent low temperature toughness according to claim 1, characterized in that: The thickness of the intermediate blank is 50 mm to 52 mm.

3. The thick gauge L360 pipeline steel with excellent low temperature toughness according to claim 1, characterized in that: In the above-mentioned refining outside the furnace and LF refining, the molten steel discharged from the converter is subjected to RH vacuum treatment, and then subjected to LF furnace light desulfurization treatment and calcium treatment.

4. The thick gauge L360 pipeline steel with excellent low temperature toughness according to claim 1, characterized in that: During the continuous casting, electromagnetic stirring or dynamic soft reduction is applied to the refined molten steel to obtain a continuous casting billet with a thickness of 170 mm.

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