A hot-rolled L450 grade pipeline steel coil for ground gathering and transportation and a production method thereof

Abstract

The invention relates to an L450 grade pipeline steel hot-rolled plate coil for ground gathering and transportation and a production method thereof. The chemical composition of the steel is C: 0.061%-0.070%, Si: 0.15%-0.25%, Mn: 0.39%-0.48%, P≤0.012%, S: 0.0028%-0.003%, Als: 0.030%-0.045%, Nb: 0.032%-0.038%, Ti: 0.020%-0.023%, Cu: 0.34%-0.42%, Cr: 0.32%-0.39%, N≤0.004%, and the remainder is iron and impurities. The yield strength of the hot-rolled coil product is 465-478 MPa, the tensile strength is 572-588 MPa, the elongation after fracture is ≥36.5%; the Charpy impact energy Akv at -40°C is ≥308 J; the corrosion rate is 0.0234-0.0270 mm / a, the corrosion potential is ≥-0.452 V, and the corrosion current density is ≤1.06×10 ‑5 A.cm ‑2 .

Description

Technical Field

[0001] The present invention relates to the technical field of low alloy hot rolled coil production, and in particular to an L450 grade pipeline steel hot rolled coil for ground gathering and transportation and a production method thereof. Background Art

[0002] Surface gathering and transportation pipelines are primarily used to transport crude oil, sewage, and natural gas. Because the transport medium contains a certain amount of chloride ions, pipeline corrosion failures such as perforations, leaks, and bursts are frequent. Currently, surface gathering and transportation pipelines used by major oil and gas fields mostly use 20# steel, 16Mn, stainless steel, or corrosion-resistant alloys. Stainless steel and corrosion-resistant alloys are prohibitively expensive to use on a large scale. Surface gathering and transportation pipelines made of 20# and 16Mn steel require the addition of large amounts of Mo, Cr, Ni, and Cu to ensure corrosion resistance due to their high carbon design, resulting in relatively high alloy costs. Therefore, there is an urgent need to develop a steel for surface gathering and transportation pipelines that is low-cost, highly corrosion-resistant, and possesses excellent comprehensive mechanical properties.

[0003] Chinese patent application number CN201310368061.9 discloses "Chloride-ion corrosion-resistant steel for pipelines and its preparation method." The steel's composition (by weight percentage) is: C 0.008% to 0.02%, Si 0.05% to 0.015%, Mn 0.50% to 0.85%, S ≤ 0.010%, P ≤ 0.01%, Mo 0.8% to 2.0%, Al 0.001% to 0.020%, Ni 0.50 to 1.2%, W 0.02 to 0.08% by weight, with Cr satisfying the following inequality: 2.4 - 0.49 [Mo] + 11.8 [C] + 12.2 [W] ≤ Cr ≤ 3.2 - 0.57 [Mo] + 9.8 [C] + 7.2 [W]. However, the addition of Mo, Ni, and W to the pipeline steel results in a high alloy cost.

[0004] The Chinese patent application with application number CN201910420753.0 discloses "a steel resistant to corrosion by multiple media with a yield strength of 500 MPa and a preparation method thereof". The composition of the steel is (by weight percentage): C 0.06%~0.10%, Si 0.15%~0.40%, Mn 0.70%~1.20%, P≤0.025%, S≤0.01%, Cu 0.20%~0.40%, Ni 0.20~0.50%, Cr 0.50~1.0%, Sb 0.02~0.12%, Sn 0.005~0.030%, Nb 0.02~0.05%, Ti 0.015%~0.025%, Ca 0.001%0~0.003%, N≤0.006%. The addition of Sb, Sn and Ni to the steel results in a higher alloy cost. The yield strength of the steel is ≥500MPa, but the impact energy at -40°C is only 80J. The strength and toughness of the product are poorly matched, making it not a preferred material for ground gathering and transportation pipelines.

[0005] Chinese patent application number CN201510534708.X discloses a "thick-gauge X65 pipeline steel and its production method." The steel's composition (by weight) is as follows: C 0.07%-0.09%, Si 0.10%-0.30%, Mn 1.40%-1.60%, P ≤ 0.018%, S ≤ 0.005%, Nb 0.035%-0.039%, V 0.06%-0.008%, Ti 0.010%-0.017%, and Cr 0.20%-0.30%. This application describes a conventional pipeline steel alloy design, significantly different from the present invention. Furthermore, the application does not describe the corrosion resistance of the product.

[0006] Chinese patent application number CN201010243241.0 discloses "An X65 pipeline steel and its production method." The steel's composition (by weight) is: C 0.055% to 0.090%, Si 0.15% to 0.35%, Mn 1.50% to 1.65%, S ≤ 0.005%, P ≤ 0.020%, Nb 0.040% to 0.055%, V 0.040% to 0.070%, and Ti 0.010% to 0.025%. This patent also describes a conventional pipeline steel alloy design, significantly different from the present invention. Furthermore, the patent does not describe the corrosion resistance of the product.

[0007] Chinese patent application number CN200710028175.3 discloses a "Method for Low-Cost Production of X65 Pipeline Steel." The steel's composition (by weight) is: C 0.046%-0.060%, Si 0.15%-0.30%, Mn 1.40%-1.60%, S 0.001%-0.008%, P 0.006%-0.015%, Nb 0.045%-0.050%, and Ti 0.015%-0.025%. This also describes a conventional pipeline steel alloy design, significantly different from the present invention. Furthermore, the patent application does not describe the corrosion resistance of the product.

[0008] Chinese patent application number CN20131217916.8 discloses a "CO2 corrosion-resistant pipeline steel for surface gathering and transportation and its preparation method." The steel's composition (by weight percentage) is as follows: C 0.01% to 0.08%, Si 0.10% to 0.50%, Mn 0.50% to 1.50%, P ≤ 0.020%, S ≤ 0.006%, Nb + V + Ti ≤ 0.1%, Cr 1.0 to 3.0%, Mo 0.10% to 0.30%, Cu 0.10% to 0.50%, and Ni 0.10% to 0.50%. However, the patent application relates to the production of medium and thick plates, and incorporates precious metals such as Mo and Ni to achieve excellent corrosion resistance and low-temperature toughness, with a yield strength exceeding 530 MPa.

[0009] Chinese patent application number CN201410247308.6 discloses "5Cr corrosion-resistant steel and its production method." The steel's composition (by weight) is: C 0.05% to 0.07%, Si 0.10% to 0.30%, Mn 0.50% to 0.80%, P ≤ 0.020%, S ≤ 0.01%, Cr 4.5% to 5.5%, and N 0.02% to 0.05%. This application relates to hot-rolled rebar steel, and its mechanical properties differ significantly from those of hot-rolled coil products.

[0010] The paper "Corrosion Resistance Screening and Evaluation Test of Oil Casing and Gathering Pipeline Materials" (authored by Duan Fangwei et al., "Oil and Gas Field Environmental Protection" Issue 6, 2018, pp. 24-28) mentions steel for gathering pipelines. The 20 steel, 16Mn steel, and X65 steel described in the paper are all high-carbon, Nb-free, and compositely added with Cr, Ni, Mo, and Cu alloy designs. The uniform corrosion rate data provided show that the corrosion level of the steel reaches moderate or even severe corrosion. In addition, the paper does not record the production process and mechanical properties of the steel.

[0011] The steels for ground gathering and transportation pipelines involved in the above-mentioned public materials are all medium and thick plate products, and the precious metals Mo and Ni are basically added in the alloy design. Except for the patent applications "A steel with a yield strength of 500MPa and resistance to corrosion of multiple media and its preparation method" and "Preparation method of CO2 corrosion-resistant pipeline steel for ground gathering and transportation", no Nb is added. The L450 grade pipeline steel hot-rolled coil for ground gathering and transportation described in the present invention breaks the inherent idea of ​​high-carbon design of ground gathering and transportation steel, and opens up a new way to use low-carbon steel to obtain sufficient strength and good corrosion resistance. Summary of the Invention

[0012] The present invention provides a hot-rolled coil of L450-grade pipeline steel for ground gathering and transportation, and a production method thereof. By adding relatively low contents of C, Mn and Si, and simultaneously adding an appropriate amount of Ti to form nearly spherical and non-deformable Ti4C2S2, the product obtains excellent toughness indicators; the effect of fine grain strengthening is enhanced by adding Nb; the most economical composite addition of Cr and Cu is adopted to obtain excellent corrosion resistance, reduce the corrosion rate, and increase the corrosion potential; in addition, a two-stage controlled rolling and controlled cooling technology is adopted to ensure that the final product obtains a fine and uniform microstructure; the small-thickness intermediate billet rolling technology is adopted to break the restriction that the thickness of the intermediate billet of pipeline steel is more than 3.6 times the thickness of the finished product, and the reduction in the finishing rolling stage can be reduced, thereby reducing the dislocation density and further improving the corrosion resistance.

[0013] In order to achieve the above object, the present invention adopts the following technical solutions:

[0014] The invention discloses an L450 grade pipeline steel hot-rolled coil for ground gathering and transportation. The chemical composition of the steel is, by weight percentage, C: 0.061%-0.070%, Si: 0.15%-0.25%, Mn: 0.39%-0.48%, P≤0.012%, S 0.0028%-0.003%, Als: 0.030%-0.045%, Nb: 0.032%-0.038%, Ti: 0.020%-0.023%, Cu: 0.34%-0.42%, Cr: 0.32%-0.39%, N≤0.004%, and the remainder is iron and unavoidable impurities.

[0015] A method for producing L450 grade pipeline steel hot-rolled coils for ground gathering and transportation includes the following process steps: molten iron pretreatment, converter smelting, refining outside the furnace, continuous casting, slab heating, rolling, laminar cooling, and coiling; wherein the following processes are controlled:

[0016] 1) Converter smelting; using top blowing or top and bottom combined blowing;

[0017] 2) Refining outside the furnace: light desulfurization and calcium treatment in LF furnace;

[0018] 3) Continuous casting; using dynamic soft reduction method;

[0019] 4) Slab heating: Continuous casting slab is heated to 1173-1182°C;

[0020] 5) Rolling; adopting a two-stage controlled rolling and controlled cooling method; the rough rolling and finishing temperature is 1040-1055°C, the rough rolling is performed for at least 5 passes, the first pass reduction rate is 18.4%-19.7%, and the reduction rate of each subsequent pass gradually increases, and the reduction rate of the last pass is 21.3%-22.9%; after rolling, the temperature is allowed to drop below 960°C before entering the finishing rolling, the finishing rolling temperature is 770-781°C, and the finishing rolling is performed for at least 7 passes, the first pass reduction rate is 17.6%-19.1%, and the reduction rate of each subsequent pass gradually decreases, and the last pass reduction rate is 6.1%-8.7%;

[0021] 6) Laminar cooling and coiling: Cool to 500-520℃ at a rate of 26.3-29.4℃ / s for coiling, and finally air cool to room temperature.

[0022] Furthermore, the yield strength of the hot-rolled coil product is 465-478 MPa, the tensile strength is 572-588 MPa, the elongation after fracture is ≥36.5%; the Charpy impact energy Akv at -40°C is ≥308 J; the corrosion rate is 0.0234-0.0270 mm / a, the corrosion potential is ≥-0.452 V, and the corrosion current density is ≤1.06×10 -5 A.cm -2 .

[0023] Furthermore, in step 5), the ratio of the thickness of the rolling intermediate billet to the thickness of the finished steel plate is ≤3.0.

[0024] Furthermore, the thickness of the continuous casting slab is 172-175 mm, the thickness of the rolling intermediate slab is 54-56 mm, and the thickness of the finished steel plate is 18-20 mm.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1) Breaking the inherent idea of ​​high-carbon design for existing surface gathering and transportation steel, we adopt a low-carbon, low-manganese, low-sulfur and low-phosphorus design to ensure the product has excellent corrosion resistance and weldability;

[0027] 2) Accurately control the Ti content to completely modify the long strips of MnS into nearly spherical Ti4C2S2, thereby improving the toughness and corrosion resistance of the product;

[0028] 3) No precious metals Ni and Mo are added, and Cu and Cr are added in combination to ensure the corrosion resistance and strength of the product, giving the product a greater advantage in terms of economy;

[0029] 4) Two-stage controlled rolling and controlled cooling is adopted to ensure that the product has good strength and toughness matching;

[0030] 5) The precisely distributed pass reduction ratio, combined with the small-thickness intermediate billet rolling technology, breaks the limitation that the intermediate billet thickness of pipeline steel must be more than 3.6 times the finished product thickness, reduces the reduction in the finishing rolling stage, thereby reducing the dislocation density and further improving the corrosion resistance of the steel. DETAILED DESCRIPTION

[0031] The present invention discloses an L450 grade pipeline steel hot-rolled coil for ground gathering and transportation, wherein the chemical composition of the steel is, by weight percentage, C: 0.061% to 0.070%, Si: 0.15% to 0.25%, Mn: 0.39% to 0.48%, P≤0.012%, S0.0028% to 0.003%, Als: 0.030% to 0.045%, Nb: 0.032% to 0.038%, Ti: 0.020% to 0.023%, Cu: 0.34% to 0.42%, Cr: 0.32% to 0.39%, N≤0.004%, and the remainder is iron and unavoidable impurities.

[0032] The present invention provides a method for producing a hot-rolled coil of L450 grade pipeline steel for ground gathering and transportation, comprising the following process steps: molten iron pretreatment, converter smelting, refining outside the furnace, continuous casting, slab heating, rolling, laminar cooling, and coiling; wherein the following processes are controlled:

[0033] 1) Converter smelting; using top blowing or top and bottom combined blowing;

[0034] 2) Refining outside the furnace: light desulfurization and calcium treatment in LF furnace;

[0035] 3) Continuous casting; using dynamic soft reduction method;

[0036] 4) Slab heating: Continuous casting slab is heated to 1173-1182°C;

[0037] 5) Rolling; adopting a two-stage controlled rolling and controlled cooling method; the rough rolling and finishing temperature is 1040-1055°C, the rough rolling is performed for at least 5 passes, the first pass reduction rate is 18.4%-19.7%, and the reduction rate of each subsequent pass gradually increases, and the reduction rate of the last pass is 21.3%-22.9%; after rolling, the temperature is allowed to drop below 960°C before entering the finishing rolling, the finishing rolling temperature is 770-781°C, and the finishing rolling is performed for at least 7 passes, the first pass reduction rate is 17.6%-19.1%, and the reduction rate of each subsequent pass gradually decreases, and the last pass reduction rate is 6.1%-8.7%;

[0038] 6) Laminar cooling and coiling: Cool to 500-520℃ at a rate of 26.3-29.4℃ / s for coiling, and finally air cool to room temperature.

[0039] Furthermore, the yield strength of the hot-rolled coil product is 465-478 MPa, the tensile strength is 572-588 MPa, the elongation after fracture is ≥36.5%; the Charpy impact energy Akv at -40°C is ≥308 J; the corrosion rate is 0.0234-0.0270 mm / a, the corrosion potential is ≥-0.452 V, and the corrosion current density is ≤1.06×10 -5 A.cm -2 .

[0040] Furthermore, in step 5), the ratio of the thickness of the rolling intermediate billet to the thickness of the finished steel plate is ≤3.0.

[0041] Furthermore, the thickness of the continuous casting slab is 172-175 mm, the thickness of the rolling intermediate slab is 54-56 mm, and the thickness of the finished steel plate is 18-20 mm.

[0042] The L450 grade pipeline steel for surface gathering and transportation described in the present invention adopts a C-Mn-Nb-Ti-Cr-Cu alloy design. Through two-stage controlled rolling and controlled cooling, a uniform and fine ferrite-pearlite (FP) structure is obtained to ensure that the product has excellent strength and toughness. Cu and Cr are added in combination to ensure that the product has excellent corrosion resistance. The functions and reasons for the selection of the main elements in the steel are as follows:

[0043] Carbon is the most economical element in steel for increasing its strength. However, as the carbon content increases, the steel's toughness and weldability gradually deteriorate. Excessive carbon content also negatively impacts corrosion resistance. Therefore, a low carbon content is essential for ensuring excellent strength, toughness, weldability, and corrosion resistance in surface gathering and transportation pipeline steel. In this invention, the carbon content is controlled within a range of 0.061% to 0.070%.

[0044] Si: It is an important reducing agent and deoxidizer in the steelmaking process. Many carbon steels contain less than 0.5% Si. Si can significantly improve the strength of the ferrite-pearlite structure type, but a high Si content will reduce the plasticity and toughness of the material. Therefore, the present invention controls the Si content to a lower level, that is, the Si content is controlled to 0.15% to 0.25%.

[0045] Mn: Manganese has a solid solution strengthening effect and can also reduce the γ-α phase transformation temperature, thereby refining the ferrite grains. However, manganese is prone to segregation. Excessive manganese content will cause severe segregation, resulting in a loss of toughness and corrosion resistance. Therefore, the present invention controls the manganese content to 0.39% to 0.48%.

[0046] P: It is very easy to highly segregate during solidification of molten steel, forming a banded FP structure. P also greatly reduces the benefits of reducing C in the steel, reducing the steel's toughness and corrosion resistance. Therefore, P, as a harmful element, should be minimized. In the present invention, the P content is controlled to below 0.012%.

[0047] S: is an inevitable impurity element in steel, which will reduce the toughness and corrosion resistance of the steel material. Generally, the lower the better. The present invention utilizes the effect of Ti element to modify MnS, so that Ti and a certain amount of S in the molten steel form Ti4C2S2 with greater stability than MnS, which can improve the corrosion resistance and toughness indicators of the product; therefore, the present invention controls the S content to 0.0028% to 0.003%.

[0048] Als: is an inevitable deoxidizing element in steel. 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. Therefore, the Als content in the present invention is set at 0.030% to 0.045%.

[0049] Nb: A key element for controlled rolling in modern microalloyed pipeline steels, it can lower the γ-α transformation temperature, increase the austenite recrystallization temperature, and promote austenite and ferrite refinement, resulting in grain refinement and strengthening, while also improving strength and toughness. Furthermore, the NbC formed after hot rolling can also contribute to precipitation strengthening. However, Nb is a precious element, and its strengthening effect is diminished beyond a certain level. Therefore, the present invention limits the Nb content to 0.032% to 0.038%.

[0050] Ti: A strong nitride-forming element, Ti not only fixes nitrogen but also transforms long MnS into nearly spherical Ti4C2S2, depending on the S and N content of the product, improving the toughness and corrosion resistance of the product. However, excessive Ti content can reduce the toughness of the steel. Therefore, the Ti content is controlled to 0.020% to 0.023% in this invention.

[0051] Cu: When an appropriate amount of copper is added to steel, it will precipitate secondary precipitation on its surface to form a cathode, promoting anodic passivation of the steel in contact with it and forming a rust layer on the surface, thereby increasing corrosion resistance. Copper can also increase the overall potential of the steel, thereby improving its corrosion resistance. However, once the Cu content exceeds 0.5%, the plasticity of the steel will decrease significantly, and when Cu is greater than 0.2%, the tendency to copper embrittlement will increase. The present invention utilizes the addition of other components and the adjustment of process parameters to avoid copper embrittlement. To ensure that the steel has good corrosion resistance and mechanical properties, the present invention controls the Cu content to 0.34% to 0.42%.

[0052] Cr: It effectively improves hardenability and increases product strength, especially tensile strength. It also effectively enhances steel's oxidation resistance, forming a dense passivation film and thus increasing the steel's corrosion resistance. However, excessive addition can lead to a loss of plasticity and toughness. Therefore, the present invention limits the Cr content to 0.32% to 0.39%.

[0053] N: An increase in nitrogen content can significantly improve the strength of steel, but also significantly reduce plasticity, especially toughness, while also worsening weldability and increasing cold brittleness. Therefore, the lower the nitrogen content, the better. Since too low a content will increase steelmaking costs, the present invention controls N ≤ 0.004%.

[0054] The present invention describes a method for producing L450-grade hot-rolled coils of pipeline steel for ground gathering and transportation. The production process includes: molten iron pretreatment - converter smelting - refining (LF+Ca treatment) - continuous casting - slab heating - rolling - laminar cooling - coiling. Unlike existing technologies, the following processes are controlled:

[0055] 1) Smelting to continuous casting process: After molten iron pretreatment, converter smelting adopts top blowing or top and bottom combined blowing; LF furnace refining is adopted for refining outside the furnace, and light desulfurization treatment and calcium treatment are used to control the number and morphology of inclusions to improve the plasticity and toughness of the steel; dynamic light reduction method is adopted for continuous casting to reduce central porosity and improve the quality of continuous casting billets.

[0056] 2) Heating, rolling and laminar cooling process: The continuous casting slab is heated to 1173-1182℃ in a walking beam furnace. Due to the effect of Nb in the steel, the austenite grain coarsening is not significant at this heating temperature, and all Nb is ensured to be in solid solution. Then, two-stage controlled rolling and controlled cooling are adopted. The rough rolling and final rolling temperature are 1040-1055℃. The rough rolling is performed for at least 5 passes. The first pass reduction rate is 18.4%-19.7%. The reduction rate of each subsequent pass increases gradually, and the reduction rate of the last pass is 21.3%-22.9%. After rolling, the temperature is lowered to below 960℃ before finishing rolling. The finishing rolling temperature is 770-781°C, and the finishing rolling is performed for at least 7 passes. The reduction rate of the first pass is 17.6%-19.1%, and the reduction rate of each subsequent pass gradually decreases, and the reduction rate of the last pass is 6.1%-8.7%. No recrystallization occurs in the finishing rolling stage, so that the dislocations caused by deformation in the finishing rolling stage are reduced (defects such as dislocations are microscopic substructures that are weak to corrosion) and the dislocation density is reduced, which can improve the corrosion resistance of the finished product. Subsequently, laminar cooling is adopted at a speed of 26.3-29.4°C / s to finally cool to 500-520°C for coiling, and finally air-cooled to room temperature.

[0057] The L450 grade pipeline steel hot-rolled coil for ground gathering and transportation disclosed in the present invention has excellent strength, toughness and corrosion resistance: yield strength of 465-478 MPa, tensile strength of 572-588 MPa, elongation after fracture ≥36.5%; -40°C Charpy impact energy (average of 3 samples) Akv ≥308 J, corrosion rate of 0.0234-0.0270 mm / a, corrosion potential ≥-0.452 V, and corrosion current ≤1.06×10 -5 A.cm -2 .

[0058] The present invention combines conventional corrosion tests and electrochemical corrosion tests to evaluate the corrosion performance of the product; the corrosion test conditions are as follows:

[0059] 1) Place the sample in a saturated sodium chloride solution for a full immersion test for 6 months. After the test, calculate the corrosion rate in mm / a.

[0060] 2) Electrochemical tests were conducted in a 3.5% NaCl solution containing a corrosive medium using a three-electrode system. The working electrode was the test sample, the auxiliary electrode was a Pt wire mesh, and the saturated calomel electrode was the reference electrode. The corrosion potential and corrosion current were calculated using the Tafel curve.

[0061] The following examples are implemented on the premise of the technical solution of the present invention, and provide detailed implementation methods and specific operating processes, but the protection scope of the present invention is not limited to the following examples.

[0062] [Example]

[0063] The chemical composition of the steel in each example is shown in Table 1, the main production process parameters of the steel in each example are shown in Table 2, the main pass reduction ratio of each example is shown in Table 3, and the main performance parameters of the products in each example are shown in Table 4.

[0064] Table 1 Chemical composition of steel (wt, %)

[0065] Example C Si Mn P S Als Nb Ti Cu Cr N 1 0.069 0.16 0.44 0.012 0.0029 0.039 0.033 0.023 0.42 0.39 0.0038 2 0.069 0.15 0.40 0.011 0.0028 0.041 0.032 0.022 0.35 0.39 0.004 3 0.061 0.15 0.46 0.010 0.003 0.030 0.037 0.023 0.36 0.37 0.0039 4 0.068 0.19 0.48 0.009 0.0029 0.032 0.038 0.023 0.34 0.33 0.004 5 0.061 0.21 0.48 0.009 0.003 0.043 0.038 0.022 0.42 0.33 0.0037 6 0.068 0.25 0.45 0.012 0.003 0.044 0.037 0.023 0.35 0.32 0.0039 7 0.070 0.23 0.39 0.011 0.0028 0.039 0.038 0.020 0.41 0.38 0.004 8 0.064 0.24 0.42 0.012 0.003 0.045 0.032 0.020 0.42 0.39 0.004 9 0.065 0.25 0.47 0.010 0.0029 0.037 0.033 0.022 0.34 0.34 0.004 10 0.063 0.20 0.41 0.012 0.003 0.042 0.033 0.020 0.41 0.32 0.004

[0066] Table 2 Main production process parameters of steel

[0067]

[0068] Table 3 Main pass reduction rates of various examples

[0069]

[0070]

[0071] Table 4 Main performance parameters of the product

[0072]

[0073] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for producing L450 grade pipeline steel hot-rolled coil for ground gathering and transportation, characterized in that: The chemical composition of the steel is as follows by weight: C: 0.061% to 0.070%, Si: 0.15% to 0.25%, Mn: 0.39% to 0.48%, P≤0.012%, S: 0.0028% to 0.003%, Als: 0.030% to 0.045%, Nb: 0.032% to 0.038%, Ti: 0.020% to 0.023%, Cu: 0.34% to 0.42%, Cr: 0.32% to 0.39%, N≤0.004%, and the rest is iron and unavoidable impurities; The production method of the L450 grade pipeline steel hot-rolled coil includes the following process steps: molten iron pretreatment, converter smelting, refining outside the furnace, continuous casting, slab heating, rolling, laminar cooling and coiling; wherein the following processes are controlled: 1) Converter smelting; using top blowing or top and bottom combined blowing; 2) Refining outside the furnace: light desulfurization and calcium treatment in LF furnace; 3) Continuous casting; using dynamic soft reduction method; 4) Slab heating: Continuous casting slab is heated to 1173-1182°C; 5) Rolling; adopt a two-stage controlled rolling and controlled cooling method; the rough rolling temperature is 1040-1055°C, the rough rolling is performed for at least 5 passes, the first pass reduction rate is 18.4%-19.7%, and the reduction rate of each subsequent pass gradually increases, and the reduction rate of the last pass is 21.3%-22.9%; after rolling, the temperature is lowered to below 960°C before finishing rolling, and the ratio of the thickness of the rolling intermediate billet to the thickness of the finished steel plate is ≤3.0; the finishing rolling temperature is 770-781°C, and the finishing rolling is performed for at least 7 passes, the first pass reduction rate is 17.6%-19.1%, and the reduction rate of each subsequent pass gradually decreases, and the last pass reduction rate is 6.1%-8.7%; 6) Laminar cooling and coiling: Cool to 500-520℃ at a rate of 26.3-29.4℃ / s, then coil, and finally air cool to room temperature; The thickness of the continuous casting slab is 172-175 mm, the thickness of the rolling intermediate slab is 54-56 mm, and the thickness of the finished steel plate is 18-20 mm. The yield strength of the hot-rolled coil product is 465-478 MPa, the tensile strength is 572-588 MPa, the elongation after fracture is ≥36.5%; the Charpy impact energy Akv at -40℃ is ≥308 J; the corrosion rate is 0.0234-0.0270 mm / a, the corrosion potential is ≥-0.452 V, and the corrosion current density is ≤1.06×10 -5 A.cm -2 .

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