Thin-gauge, low-yield-strength-ratio, low-temperature service L450M pipeline steel and its production method

Abstract

This invention relates to a thin-gauge, low-yield-strength-ratio, low-temperature service L450M pipeline steel and its production method. The chemical composition of the steel, calculated by weight percentage, is as follows: C 0.062%–0.078%, Si 0.15%–0.25%, Mn 1.50%–1.60%, Nb 0.032%–0.043%, Ti 0.026%–0.029%, Mo 0.06%–0.12%, Cr 0.06%–0.12%, Als 0.015%–0.045%, P≤0.015%, S≤0.004%, N≤0.004%, with the remainder being iron and unavoidable impurities. This invention, by employing an economical and reasonable alloy design and matching appropriate production processes, enables the continuous casting and rolling of thick slabs (200-230mm) to produce thin-gauge (7-9mm) L450M pipeline steel hot-rolled coils.

Description

Technical Field

[0001] This invention belongs to the field of high-strength low-alloy steel in metallic materials, and particularly relates to a thin-gauge, low-yield-strength ratio, low-temperature service L450M pipeline steel and its production method. Background Technology

[0002] Pipeline transportation is the most economical way to transport oil and natural gas over long distances. With the increasing global demand for oil and natural gas, the pressure on main pipeline steel is also gradually increasing. To ensure the safety and stability of oil and gas pipeline transportation, the requirements for material thickness and toughness are gradually increasing. However, branch lines such as urban pipe networks often use thin-walled steel. As the thickness decreases, the strength increases, leading to a higher yield strength ratio. An excessively high yield strength ratio reduces the material's strain hardening ability, resulting in lower yield strength and load-bearing capacity, making the steel pipe more prone to failure during service. Therefore, international standards limit the yield strength ratio of pipeline steel. Currently, domestic and international pipeline specifications require a yield strength ratio of 0.90 or even 0.93 for X65 (L450M), X70 (L485M) grades and higher. This makes controlling the yield strength ratio even more difficult for thin-gauge, high-grade pipeline steel.

[0003] Prior to this invention, there were few documents or patents concerning low yield strength ratio, thin-gauge L450M pipeline steel, and they were all significantly different from this invention. The following is a brief introduction to several that are relatively close to this invention:

[0004] 1) Application No. CN201510828336.1, entitled "Low Yield-to-Tear Ratio Hot-Rolled Pipeline Steel Coil or Opening Plate and Preparation Method Thereof", discloses a low yield-to-tear ratio hot-rolled pipeline steel coil or opening plate and preparation method thereof. The composition of this patent is C: 0.036-0.057%; Si: 0.22-0.30%; Mn: 1.02-1.25%; P≤0.016%; S≤0.0015%; Cr: 0.15-0.25%; Nb: 0.020-0.044%; Ti: 0.011-0.015%; the rest is Fe and unavoidable impurities. The patent has a low sulfur content, resulting in higher steelmaking costs. Furthermore, it does not specify thickness specifications, and the high cooling rate of 64℃ / S used in production easily produces more hardened structures that affect overall performance. The impact and drop hammer test temperatures are -20℃, and the toughness index in low-temperature environments below -20℃ is not described.

[0005] 2) Patent application CN201910312138.8, entitled "A Low Yield-to-Strength Ratio L360M Hot-Rolled Steel Coil for Oil and Gas Pipelines and Its Preparation Method," discloses an L360M hot-rolled steel coil for oil and gas pipelines with a low yield-to-strength ratio and its preparation method. The composition (by weight percentage) is: C: 0.05–0.09%, Si: 0.15–0.30%, Mn: 1.10–1.25%, P≤0.025%, S≤0.010%, Nb: 0.008–0.030%, with the balance being Fe and unavoidable impurities. This patent is for the L360M grade and does not describe the thickness specifications or low-temperature toughness.

[0006] 3) Application CN201910821750.8, entitled "A Thin-Gross Straight-Seam Low-Yield-Strength Ratio High-Toughness Pipeline Coil and Its Manufacturing Method," discloses a thin-gauge straight-seam low-yield-strength ratio high-toughness pipeline coil and its manufacturing method. Its chemical composition (wt%) is: C: 0.08%–0.15%, Si: 0.05%–0.14%, Mn: 1.30%–1.60%, P≤0.018%, S≤0.005%, Ti: 0.01%–0.03%, Al: 0.03%–0.06%, Nb≤0.05%, V≤0.05%, Cr: 0.31%–0.50%, N≤0.005%, with the remainder being Fe and unavoidable elements. This patent contains V, has a different alloy system, and a relatively high Cr content, making it less economical, and its strength level is 415 MPa.

[0007] 4) Application CN202310771173.2, entitled "A High-Toughness Thin-Gross Pipeline Coil Steel with Low Yield-to-Toughness Ratio and Its Production Method," discloses a high-toughness thin-gauge pipeline coil steel with low yield-to-toughness ratio and its production method. The composition by weight percentage is: C 0.06-0.08%, Si 0.15-0.25%, Mn 0.80-0.90%, P≤0.015%, S≤0.005%, Ti 0.010-0.025%, Cr 0.30-0.33%, Ceq≤0.39, Pcm≤0.21, with the balance being Fe and unavoidable impurities. This invention has a low strength level and relatively lenient low-temperature toughness specifications.

[0008] 5) Application CN201910706357.4, entitled "A Manufacturing Method for Thin-Gauge Pipeline Steel with Low Yield-to-Strength Ratio," discloses a manufacturing method for thin-gauge pipeline steel with a low yield-to-strength ratio. The chemical composition is C 0.075-0.095, Si 0.2-0.3, Mn

[0009] The alloy composition is as follows: 1.65-1.75%, Al 0.02-0.04%, Nb 0.045-0.055%, Ti 0.01-0.02%, Cr 0.20-0.30%, S≤0.0008%, Ca / S≥2, with the balance being Fe and unavoidable impurities. This invention uses high Nb and high Cr alloys, resulting in higher alloy costs and poor low-temperature toughness, achieving only 100 J at -40℃.

[0010] The low yield strength ratio pipeline steels disclosed in the above literature basically use high Cr in alloy design. However, excessive Cr content can easily produce hardened structure. Although the yield strength ratio is reduced, low temperature toughness is lost. Moreover, the key components and processes for controlling the yield strength ratio in the above inventions are hardly described. Summary of the Invention

[0011] The purpose of this invention is to provide a low-temperature toughness, low yield strength ratio, thin-gauge L450M pipeline steel and its production method by adopting an economical and reasonable alloy design and matching an appropriate production process. This method enables the continuous casting and rolling of thick slabs (200-230mm) to produce thin-gauge (7-9mm) L450M pipeline steel hot-rolled coils.

[0012] To achieve the above objectives, the present invention employs the following technical solution:

[0013] The L450M pipeline steel, characterized by its thin gauge, low yield strength ratio, and low-temperature service, has the following chemical composition by weight percentage: C 0.062%–0.078%, Si 0.15%–0.25%, Mn 1.50%–1.60%, Nb 0.032%–0.043%, Ti 0.026%–0.029%, Mo 0.06%–0.12%, Cr 0.06%–0.12%, Als 0.015%–0.045%, P≤0.015%, S≤0.004%, N≤0.004%, with the remainder being iron and unavoidable impurities.

[0014] The pipeline steel has a yield strength of 450–484 MPa, a tensile strength of 533–582 MPa, an elongation after fracture of ≥34.5%, and a yield strength ratio of ≤0.84.

[0015] The pipeline steel has a Charpy impact energy (average of 3 samples) of ≥313J at -60℃ and an average drop hammer tear test shear area (average of 2 samples) of ≥89% at -60℃.

[0016] The hot-rolled coils for pipeline steel are produced in specifications of 7-9mm.

[0017] The L450M pipeline steel of this invention adopts a C-Mn-Nb alloy design, with appropriate amounts of Cr, Ti, and Mo added. A fine and uniform acicular ferrite (AF) structure is obtained through a precisely controlled two-stage rolling and cooling process to ensure the pipeline steel has excellent low-temperature toughness and a low yield strength ratio. The roles and rationale for the selection of its main elements are as follows:

[0018] C is the second most important element in steel after iron. It directly affects the strength, plasticity, toughness, and weldability of steel. C significantly improves the strength of steel through solid solution strengthening and precipitation strengthening. C contributes more to tensile strength than yield strength. Therefore, as the amount of C increases, the yield strength ratio tends to decrease. However, as the C content increases, the toughness and weldability of steel gradually deteriorate. Therefore, this invention controls the C content to be 0.062% to 0.078%.

[0019] Si is an important reducing agent and deoxidizer in the steelmaking process. Many materials in carbon steel contain less than 0.5% Si. This Si is generally introduced during the steelmaking process as a reducing agent and deoxidizer. In addition, Si can also be dissolved in ferrite and austenite to improve the strength of steel, but at the same time, it will reduce plasticity and toughness. Therefore, the Si content should not be too high. Thus, the Si content in this invention is controlled at 0.15% to 0.25%.

[0020] Mn: Manganese has solid solution strengthening properties and can lower the γ-α phase transformation temperature, thereby refining ferrite grains. It is also the most important and economical strengthening element in pipeline steel to compensate for the strength loss caused by the reduction in carbon content. However, excessive manganese content can lead to severe segregation, and Mn readily combines with sulfur to form long, strip-shaped MnS inclusions, resulting in a loss of toughness. Therefore, the content should not be too high. This invention controls the manganese content to 1.50%–1.60%.

[0021] Nitrogen (Nb) is the most important element in controlled rolling of modern microalloyed pipeline steel. NbC strain-induced precipitation hinders the recovery and recrystallization of deformed austenite, lowers the phase transformation temperature, expands the recrystallization zone rolling process window, and promotes the formation of acicular ferrite and MA islands. The grain refinement effect of Nb is the most important strengthening mechanism in pipeline steel, but grain refinement leads to an increase in yield strength ratio. Therefore, this invention uses a lower Nb addition to weaken the grain refinement strengthening effect of Nb, thereby reducing the yield strength ratio. Thus, this invention controls the Nb content to 0.032% to 0.043%.

[0022] Ti is a very strong nitride-forming element. The stoichiometric ratio of Ti / N is 3.42. Only about 0.017% Ti is needed to fix all 50 ppm of N in this invention. The fine TiN particles that are stable at high temperatures can effectively prevent the growth of austenite during the reheating of the billet and improve the impact toughness of the heat-affected zone of the weld. In addition, Ti has a much stronger affinity for S than Mn. Therefore, Ti can "remove" S from MnS to form Ti4C2S2. The spherical Ti4C2S2, which is not easily deformed during rolling, completely replaces the MnS formed by the stretching during rolling, which can significantly improve the uniformity of the product structure and has a significant effect on improving the toughness index. The addition of Ti must take into account the content of S and N in this invention. In this invention, the Ti required to form Ti4C2S2 with the upper limit of 0.004% S is about 0.025%. It is also necessary to consider the formation of a certain amount of TiN. Therefore, the Ti content in this invention is 0.026% to 0.029%.

[0023] Mo: Mo promotes the formation of acicular ferrite and a certain number of Mao islands (M / A). The contribution of M / A to tensile strength is higher than that to yield strength. An appropriate amount of Mo is beneficial for controlling a low yield strength ratio. However, when the Mo content exceeds 0.12%, the combined addition of Mo and Nb will form (NbMo)4C3, which can increase the system fraction of precipitates more than NbC alone. This can increase the number of nucleation nuclei, dislocation density, and precipitation hardening ability, thus increasing the yield strength ratio. Therefore, the Mo content in this invention is 0.06% to 0.12%.

[0024] Cr: When used in combination with Mo, Cr promotes the formation of Mao islands (M / A), which reduces the yield strength ratio and also minimizes strength loss during tube manufacturing due to the Bauschinger effect. Furthermore, when combined with rapid cooling after rolling, Cr also promotes the formation of acicular ferrite (AF), improving the product's strength and toughness. However, Cr is significantly cheaper than Mo. Therefore, the Cr content in this invention is 0.06%–0.12%.

[0025] Als: A deoxidizing element. Adding an appropriate amount of aluminum can form fine and dispersed AlN particles, which is beneficial for refining grains and improving the strength and toughness of steel. When the solid solution Al content exceeds 0.015%, a fine austenitic grain structure can be obtained from the steel during welding and reheating, improving weldability. Therefore, the Al content of this invention is 0.015% to 0.045%.

[0026] P: It is highly prone to segregation during the solidification of molten steel, forming a banded FP structure. P also significantly reduces the benefits of lower carbon content in pipeline steel, resulting in a loss of steel toughness. As a harmful element in pipeline steel, it should be minimized, but excessively low levels will increase costs. Therefore, the P content in this invention is controlled below 0.015%.

[0027] S: An unavoidable impurity element in steel, and lower levels are desirable. High-grade, dual-resistance pipeline steels require lower S content, generally controlled below 0.002%. However, excessively low levels increase production costs. In this invention, S content control is relatively lenient, relying on the addition of Ti to form Ti4C2S2, thereby reducing the detrimental effects of elongated MnS on the steel's mechanical properties, especially toughness. Therefore, the S content in this invention is ≤0.004%.

[0028] Nitrogen (N): Increasing the nitrogen content can significantly improve the strength of steel, but it also significantly reduces plasticity, especially toughness, worsens weldability, and exacerbates cold brittleness. As a harmful element in steel, its content should be as low as possible. Therefore, the N content in this invention is ≤0.004%.

[0029] The production method of L450M pipeline steel with thin gauge, low yield strength ratio and low temperature service includes the following process: hot metal pretreatment, converter smelting, ladle refining (RH+LF+calcium treatment), continuous casting, slab heating, rolling, laminar flow cooling, and coiling.

[0030] 1) Smelting and continuous casting process: hot metal pretreatment, converter smelting adopts top blowing or top and bottom combined blowing; ladle refining adopts RH vacuum treatment, LF furnace light desulfurization treatment and calcium treatment to control the quantity and morphology of inclusions; continuous casting adopts dynamic light pressure method, and the thickness of the continuous casting billet is 200-230mm.

[0031] 2) Rolling process: The continuously cast slab is heated to 1142-1156℃ in a walking beam furnace. The lower slab heating temperature can refine the original austenite grain structure. Grain refinement is the only way to improve both strength and toughness at the same time. Low-temperature heating provides a microstructure guarantee for the slab to obtain an excellent strength-toughness ratio. Then, it is rolled in two stages by roughing and finishing mills. The roughing finishing temperature is ≥1020℃ and the roughing reduction is ≥80%. This ensures that crystallization occurs in each pass of the roughing stage, thereby continuously refining the microstructure and improving strength and toughness. After the roughing is rolled to 40-43mm, it is allowed to stand for 15-20 seconds before entering the finishing mill. The finishing mill starting temperature is 980-1000℃ and the finishing finishing temperature is 906-917℃. The finishing mill reduction is relatively small, resulting in fewer dislocations and other substructures. The dislocation strengthening effect is weakened, and the improvement in tensile strength is small. Therefore, a lower yield strength ratio can be obtained.

[0032] 3) Laminar flow cooling: Final cooling to 530-550℃ at a rate of 27.3-29.20℃ / s before winding. This invention uses a composite addition of Cr and Mo. This cooling rate promotes the formation of M / A, and because the original austenite structure is fine and uniform, the M / A components can be dispersed, further reducing the yield strength ratio.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1) In terms of alloy design, this invention uses a lower Nb to weaken the grain refinement and strengthen the alloy, adds Mo and Cr to promote the formation of Mao islands (M / A) to ensure a lower yield strength ratio of the product; and uses a low carbon design and precise Ti content to form near-spherical Ti4C2S2 to ensure low-temperature toughness.

[0035] 2) In terms of rolling process, this invention uses low-temperature calcination to obtain fine original austenite grain structure; the large deformation in the roughing stage can ensure that the austenite grains are completely broken and recrystallized multiple times, providing a fine microstructure for the finishing stage; on the other hand, it can reduce the deformation in the finishing stage, thereby ensuring that the finished product has a low dislocation density; by using a higher finishing rolling inlet temperature, high-temperature and rapid rolling is carried out in the finishing stage to reduce the dislocation density, followed by appropriate cooling rate to enter the coiling to promote M / A formation. The above process can further ensure that the product has a low yield strength ratio.

[0036] 3) The product has excellent strength and toughness matching, with a yield strength of 450~484MPa, tensile strength of 533~582MPa, elongation after fracture ≥34.5%, yield strength ratio ≤0.84; Charpy impact energy at -60℃ (average of 3 specimens) Akv≥313J, and average drop hammer tear test shear area at -60℃ (average of 2 specimens) SA≥89%. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0038] The chemical composition of the steel of the present invention is shown in Table 1, the rolling process of the steel of the present invention is shown in Table 2, and the mechanical properties of the steel of the present invention are shown in Table 3.

[0039] Table 1. Chemical composition (wt, %) of the steel in the examples

[0040] Example C Si Mn P S Nb Ti Mo Cr Als N 1 0.077 0.15 1.57 0.015 0.0039 0.041 0.026 0.09 0.12 0.38 0.0038 2 0.075 0.16 1.51 0.014 0.0038 0.032 0.029 0.10 0.12 0.4 0.0040 3 0.063 0.17 1.6 0.013 0.0040 0.034 0.026 0.12 0.11 0.32 0.0039 4 0.076 0.18 1.59 0.012 0.0039 0.043 0.029 0.11 0.07 0.35 0.0040 5 0.062 0.20 1.58 0.012 0.0040 0.042 0.027 0.06 0.10 0.44 0.0037 6 0.076 0.22 1.52 0.015 0.0040 0.043 0.028 0.12 0.06 0.45 0.0039 7 0.078 0.25 1.50 0.014 0.0038 0.038 0.026 0.09 0.11 0.40 0.0040 8 0.066 0.23 1.53 0.015 0.0040 0.042 0.027 0.11 0.06 0.43 0.0040 9 0.065 0.22 1.58 0.013 0.0039 0.033 0.029 0.09 0.07 0.29 0.0040 10 0.062 0.19 1.52 0.015 0.004 0.032 0.026 0.12 0.08 0.44 0.0040

[0041] Table 2. Steel Process Regulations (Examples)

[0042]

[0043] Table 3 Main mechanical properties of steel in the examples

[0044]

[0045]

[0046] As can be seen from Table 3, by creatively utilizing the role of Ti, the cost of desulfurization in steelmaking can be reduced. By adopting an economical and reasonable alloy design and precise controlled rolling and cooling process, the product has an extremely excellent strength and toughness ratio.

[0047] This invention employs a C-Mn-Nb alloy system, precisely adding Ti to modify MnS into Ti4C2S2, thereby improving toughness. Adding an appropriate amount of Cr reduces the yield strength ratio without generating a hardened structure. Adding an appropriate amount of Mo further promotes the formation of the M / A ratio, further reducing the yield strength ratio. Furthermore, through rough rolling with a high reduction rate, the grains are completely broken and recrystallized during the rough rolling stage to improve strength and toughness. During the finish rolling stage, the proportion of dislocation strengthening is reduced. After rapid rolling at a relatively high finish rolling temperature range, appropriate cooling to medium-temperature coiling promotes the formation of the M / A ratio and reduces grain size, thus lowering the yield strength ratio. This invention employs refined measures in both composition and process design to control the low-temperature toughness and yield strength ratio of the product, providing technical assurance for achieving excellent low-temperature toughness and a low yield strength ratio.

Claims

1. A thin-gauge, low-yield-strength-ratio, low-temperature service L450M pipeline steel, characterized in that, The chemical composition of the steel, calculated by weight percentage, is as follows: C 0.062%~0.078%, Si 0.15%~0.19%, Mn 1.50%~1.59%, Nb 0.032%~0.043%, Ti 0.026%~0.029%, Mo 0.06%~0.12%, Cr 0.06%~0.12%, Als 0.015%~0.045%, P≤0.015%, S≤0.004%, N≤0.004%, with the remainder being iron and unavoidable impurities; the pipeline steel has a yield strength ratio ≤0.84, Charpy impact energy at -60℃ ≥313J, and average drop hammer tear test shear area SA≥89% at -60℃. The production method for the aforementioned thin-gauge, low-yield-strength-ratio, low-temperature service L450M pipeline steel includes the following process: hot metal pretreatment—converter smelting—laundry refining—continuous casting—slab heating—rolling—laminar cooling—coiling; the specific method is as follows: 1) Continuous casting: Dynamic light reduction is adopted, and the thickness of the continuously cast billet is 200-230mm; 2) Rolling: The continuously cast slab is heated to 1142~1156℃, and then rolled in two stages by roughing and finishing mills; the roughing finishing temperature is ≥1020℃, the roughing reduction rate is ≥80%, the finishing finishing temperature is 983~1000℃, the finishing finishing temperature is 906~917℃, and then it is cooled to 530~549℃ at a rate of 27.3~29.20℃ / s and coiled. The hot-rolled coil production specifications are 7~9mm.

2. The thin-gauge, low-yield-strength-ratio, low-temperature service L450M pipeline steel according to claim 1, characterized in that, The pipeline steel has a yield strength of 450–484 MPa, a tensile strength of 533–582 MPa, and an elongation after fracture of ≥34.5%.

3. A method for producing thin-gauge, low-yield-strength-ratio, low-temperature service L450M pipeline steel as described in claim 1 or 2, the production process comprising: Hot metal pretreatment—converter smelting—laundry refining—continuous casting—slab heating—rolling—laminar cooling—coiling; characterized in that... 1) Continuous casting: Dynamic light pressure is used, and the thickness of the continuously cast billet is 200-230mm. 2) Rolling: The continuously cast slab is heated to 1142~1156℃, and then rolled in two stages by roughing and finishing mills; the roughing finishing temperature is ≥1020℃, the roughing reduction rate is ≥80%, the finishing finishing temperature is 983~1000℃, the finishing finishing temperature is 906~917℃, and then it is cooled to 530~549℃ at a rate of 27.3~29.20℃ / s and coiled. The hot-rolled coil production specifications are 7~9mm.

4. The production method of thin-gauge, low-yield-strength-ratio, low-temperature service L450M pipeline steel according to claim 3, characterized in that, The ladle refining process employs RH vacuum treatment, LF furnace desulfurization treatment, and calcium treatment.

5. The method for producing thin-gauge, low-yield-strength-ratio, low-temperature service L450M pipeline steel according to claim 3, characterized in that, The continuous casting slab is heated using a walking beam furnace.

6. The method for producing thin-gauge, low-yield-strength-ratio, low-temperature service L450M pipeline steel according to claim 3, characterized in that, In step 2), the thickness of the intermediate billet after rough rolling is 40-43 mm.

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