High-strength steel for transmission pipeline flange and production method thereof

Abstract

The present invention relates to high-strength steel for transmission pipeline flanges and its production. The flange steel has the following chemical composition and mass percentages: C 0.08%-0.13%, Si 0.22%-0.30%, Mn 1.20%-1.35%, P ≤ 0.010%, S ≤ 0.002%, Cr 0.20%-0.30%, Mo 0.15%-0.20%, V 0.03%-0.06%, Nb 0.02%-0.05%, Al 0.020%-0.040%, N 0.0090%-0.0130%, Al / N 2-3, Ca / S ≥ 1. The production method comprises a BOF converter, LF refining, VD vacuum treatment, and CC continuous casting process. The transmission pipeline flange steel provided by the present invention has excellent resistance to hydrogen-induced cracking (HIC) and sulfur stress corrosion cracking (SSC).

Description

Technical Field

[0001] The present invention belongs to the technical field of steel material production, and in particular relates to high-strength steel for transmission pipeline flanges and a production method thereof. Background Art

[0002] High-strength transmission pipeline flanges are mainly used in harsh environments such as oceans and deserts to connect high-pressure pipelines that transmit liquids / gases such as oil and natural gas. The transmitted liquids / gases such as oil and natural gas usually contain toxic and corrosive gases such as hydrogen sulfide (H2S). Transmission pipeline flanges are required to have high strength and good mechanical properties, as well as high resistance to hydrogen-induced cracking (HIC) and sulfur stress corrosion cracking (SSC). Summary of the Invention

[0003] The present invention aims to provide a high-strength steel for transmission pipeline flanges and a production method thereof. The provided steel for transmission pipeline flanges has good performance and good resistance to hydrogen-induced cracking (HIC) and sulfur stress corrosion cracking (SSC).

[0004] To achieve the above objectives, the present invention provides the following technical solutions:

[0005] A high-strength steel for transmission pipeline flanges, having the following chemical composition and mass percentage: C 0.08%-0.13%, Si 0.22%-0.30%, Mn 1.20%-1.35%, P≤0.010%, S≤0.002%, Cr 0.20%-0.30%, Mo 0.15%-0.20%, V 0.03%-0.06%, Nb 0.02%-0.05%, Al 0.020%-0.040%, N 0.0090%-0.0130%, Al / N 2-3, Ca / S ≥1.

[0006] The high-strength steel for transmission pipeline flanges of the present invention has a chemical composition of O≤10ppm and H≤1.2ppm.

[0007] The production method of high-strength steel for transmission pipeline flanges of the present invention comprises the steps of BOF converter-LF refining-VD vacuum treatment-CC continuous casting.

[0008] In the BOF converter process described herein, slag is retained in the converter after the previous smelting cycle is completed, and nitrogen is blown from the bottom to protect the furnace through slag splashing. The splashing period lasts for 3-5 minutes. The amount of scrap steel added to the converter is 85-95% of the molten iron and 5-15% of the scrap steel. High-quality scrap steel is used. Before smelting, the converter slag near the converter mouth and cap is cleaned. If splashing occurs during the blowing process, residual converter slag on the cap is cleaned again to ensure that the cap is clean and free of slag before tapping. After 3-5 minutes of blowing in the converter, the previous slag is poured out and slag material is added for secondary slag smelting. Slag removal is strictly prohibited during the tapping process. During the tapping process, 2.5-3.5 kg / t of steel core aluminum, ferromanganese, ferrochrome, ferromolybdenum, and other alloys are added, along with 0.9-1.0 kg / t of steel lime and 0.6-0.7 kg / t of steel refined slag. A small amount of steel is retained in the furnace, and slag removal is strictly prohibited.

[0009] The LF refining process of the present invention prioritizes adjusting the Mn, Cr, Mo, and V components to ensure uniform molten steel composition. The LF refining time is ≥45 minutes, and the white slag retention time is ≥20 minutes. Aluminum granules and ferrosilicon powder are used for diffusion deoxidation during the LF process, improving the reducibility of the slag, reducing the oxygen partial pressure in the furnace gas, and minimizing secondary oxidation during the LF process.

[0010] During the later stages of LF, when the molten steel temperature is high, nitrogen is blown into the ladle bottom to increase nitrogen. When the molten steel temperature reaches ≥1600°C, the Mn, Cr, Mo, and V composition is adjusted, and nitrogen is blown into the molten steel through the slit-type air bricks at the bottom of the ladle, with the nitrogen pressure controlled at 0.2-0.4 MPa. At this point, the LF refining slag has become white slag, with the molten steel's [O] content ≤10 ppm and [S] content ≤0.003%. Diffusion deoxidation continues, followed by further deoxidation and desulfurization, while nitrogen is added to the molten steel. A bottom nitrogen blow time of ≥25 minutes ensures a molten steel nitrogen content of 150-200 ppm after LF.

[0011] Refining slag control range: CaO ≥ 60%, SiO2 ≤ 5.3%, Al2O3 27-29%, basicity R ≥ 11.5, TFe + MnO ≤ 0.4%, MgO ≤ 5.0%, which can ensure good deoxidation and desulfurization effects, as well as good inclusion adsorption effects, and control high molten steel purity.

[0012] In the VD vacuum treatment process of the present invention, the high vacuum holding time should be controlled at 5-7 minutes; after the VD vacuum treatment, aluminum wire is fed according to an Al content of 0.020-0.040%; the soft blowing time is ≥15 minutes, and the nitrogen flow rate is ≤10L / min.

[0013] Research has shown that vacuum level significantly affects hydrogen removal, while degassing time significantly affects nitrogen removal. Therefore, during the VD furnace vacuum treatment process, while maintaining a high vacuum level, the vacuum treatment time should be shortened to achieve the metallurgical effect of dehydrogenation and nitrogen retention. For the production of high-strength transmission pipeline flanges, the VD high vacuum hold time should be controlled within 5-7 minutes. This ensures excellent dehydrogenation and nitrogen retention while also guaranteeing a nitrogen content of 90-130 ppm in the finished product. After VD vacuum treatment, aluminum wire is fed at an Al content of 0.020-0.040%, ensuring an Al / N ratio of 2-3 in the finished product. After feeding the aluminum wire, the nitrogen flow rate is adjusted to enter the soft-blow stage, with a soft-blow time of ≥15 minutes and a nitrogen flow rate of ≤10 L / min. This promotes the floating of inclusions while preventing secondary oxidation caused by excessive liquid level fluctuations during the soft-blow process.

[0014] The CC continuous casting process described in this invention utilizes argon-sealed casting throughout the casting process to minimize secondary oxidation. This ensures that oxygen is not burned during ladle opening, thereby controlling the purity of the molten steel. The casting speed is constant at 0.43-0.45 m / min, the mold water flow rate is 3100-3300 L / min / flow, the specific water volume is 0.13-0.15 L / kg, the mold electromagnetic stirring intensity is 150-200 A, and the end-stage electromagnetic stirring intensity is 100-150 A. Use special protective slag for low carbon steel: CaO 22.5-23.8%, SiO2 20.9-23.6%, MgO 5.6-6.9%, Al2O3 14.7-17.6%, Na2O 8.7-10%, F-3.3-5%, C 15-15.8%, melting point 1200-1225℃, viscosity 0.71-0.85Pa.s / 1300℃. The continuous casting billet is slowly cooled off the line. After leaving the slow cooling pit, the surface inspection and cleaning of the billet should be strengthened.

[0015] The principle of the technical solution of the present invention is as follows:

[0016] High-strength transmission pipeline flanges typically require low phosphorus (P) and sulfur (S) contents, a high Ca / S ratio, an appropriate nitrogen (N) content, and a certain Al / N ratio. P is a readily segregating element. High P content increases the hardness of the segregation band, reducing the material's resistance to hydrogen-induced cracking (HIC). P also significantly reduces the steel's low-temperature impact toughness and raises the steel's brittle transition temperature, causing the flange to become cold-brittle. High-strength transmission pipeline flanges are typically required to have a P content of ≤0.010%. Sulfur (S) is the primary element affecting resistance to hydrogen-induced cracking (HIC) and sulfur stress corrosion cracking (SSC). When the S content is ≤0.002%, HIC is significantly reduced, even negligible. Lowering the sulfur content can also significantly improve impact toughness. When the S content in steel is between 0.002% and 0.005%, HIC susceptibility decreases as the Ca / S ratio increases. However, when the Ca / S ratio reaches a certain value, CaS inclusions form, significantly increasing HIC. Therefore, when the S content is high, the Ca / S ratio should be controlled within an extremely narrow range; otherwise, the steel's HIC resistance will be significantly weakened. When the S content is ≤0.002%, even if CaS inclusions form, the relatively low S content allows the Ca / S ratio to be controlled within a wider range, or even eliminated. High-strength transmission pipeline flanges typically require a S content of ≤0.002% and a Ca / S ratio of ≥1.

[0017] In high-strength transmission pipeline flanges, nitrogen plays a beneficial role. It acts as a solid solution strengthening element, expanding and stabilizing the austenite structure, improving the steel's mechanical properties and corrosion resistance. It also promotes the precipitation of V (C, N) in the austenite region, providing heterogeneous nucleation centers for submicron precipitates. High-strength transmission pipeline flanges typically require a nitrogen content of 90-130 ppm. A suitable Al / N ratio can refine grains and prevent mixed crystals. For high-strength transmission pipeline flanges, maintaining an Al / N ratio of 2-3 achieves ideal material properties and grain size uniformity.

[0018] After derivation and calculation, the relationship affecting molten steel [N] is as follows:

[0019]

[0020] That is, the [N] content in molten steel is directly proportional to the nitrogen blowing pressure and temperature, and inversely proportional to the element's nitrogen interaction coefficient. Research has shown that when the molten steel temperature is higher and the bottom-blowing nitrogen pressure is greater, the content of elements such as Mn, Cr, Mo, and V is higher, while the oxygen and sulfur contents are lower. Using LF bottom-blowing nitrogen to increase nitrogen can effectively avoid the low yield of nitrided manganese iron in the LF process. The large amount of Al2O3 generated by the wire feed after VD vacuum treatment can improve the fluidity and purity of the molten steel during the continuous casting process.

[0021] The technical solution of the present invention has the following beneficial technical effects:

[0022] The present invention adopts the process flow of "BOF converter-LF refining-VD vacuum treatment-CC continuous casting" to produce high-quality and high-strength steel for transmission pipeline flanges. By optimizing the refining slag system, lower P and S contents in molten steel can be ensured. By optimizing the LF bottom blowing process and the VD vacuum treatment process, the purity of the molten steel can be improved, the fluidity of the molten steel in the continuous casting process can be guaranteed, and the resistance of the high-strength steel for transmission pipeline flanges to hydrogen-induced cracking (HIC) and sulfur stress corrosion cracking (SSC) is improved. DETAILED DESCRIPTION

[0023] The technical solution of the present invention will be described in detail below through specific embodiments.

[0024] Examples 1-8

[0025] The composition of the steel plates in each example is shown in Table 1.

[0026] Table 1 Chemical composition of steel for high-strength transmission pipeline flanges in various embodiments (%)

[0027] C Si Mn P S Cr Mo V Nb Ca Al N Al / N Ca / S Example 1 0.09 0.26 1.25 0.007 0.0015 0.25 0.16 0.05 0.02 0.0018 0.020 0.0090 2.0 1.2 Example 2 0.1 0.26 1.29 0.007 0.0013 0.22 0.19 0.03 0.02 0.0015 0.030 0.0105 2.9 1.2 Example 3 0.09 0.25 1.28 0.009 0.0011 0.25 0.18 0.03 0.02 0.0012 0.022 0.0097 2.3 1.1 Example 4 0.11 0.30 1.20 0.007 0.002 0.20 0.15 0.03 0.02 0.002 0.035 0.013 2.7 1.0 Example 5 0.09 0.25 1.29 0.006 0.0015 0.26 0.16 0.05 0.03 0.0017 0.025 0.0115 2.2 1.1 Example 6 0.09 0.22 1.22 0.008 0.0016 0.25 0.20 0.03 0.05 0.0016 0.028 0.0121 2.3 1.0 Example 7 0.08 0.26 1.35 0.007 0.0017 0.30 0.19 0.06 0.02 0.0019 0.040 0.0130 3.0 1.1 Example 8 0.13 0.25 1.29 0.008 0.0013 0.25 0.18 0.03 0.02 0.0017 0.029 0.0105 2.8 1.3

[0028] The oxygen, nitrogen and hydrogen contents of the steel for high-strength transmission pipeline flanges in various embodiments are shown in Table 2.

[0029] Table 2 Oxygen, nitrogen and hydrogen content of steel for high-strength transmission pipeline flanges in various embodiments (ppm)

[0030] O N H Example 1 8.1 98 1.0 Example 2 8.0 105 1.0 Example 3 7.9 97 0.9 Example 4 8.2 130 1.1 Example 5 7.5 115 1.0 Example 6 7.9 121 1.1 Example 7 7.2 113 0.8 Example 8 7.8 105 1.1

[0031] The production method of the steel for transmission pipeline flanges in each embodiment includes the steps of BOF converter - LF refining - VD vacuum treatment - CC continuous casting.

[0032] BOF converter process: After the previous furnace smelting is completed, steel slag is left in the converter, and nitrogen is blown from the bottom to splash slag and protect the furnace. The splashing time is 3 to 5 minutes. The amount of scrap steel added to the converter molten iron is: 85 to 95% of the molten iron, 5 to 15% of the scrap steel, and high-quality scrap steel is used. Before smelting, the converter slag near the converter mouth and the furnace cap is cleaned. If splashing occurs during the blowing process, the converter slag remaining on the furnace cap is cleaned a second time to ensure that the furnace cap is clean and there is no residue before tapping. After 3 to 5 minutes of blowing in the converter, the early slag is poured out, and slag is added for secondary slag smelting. Slag is strictly prohibited during the tapping process. During the tapping process, 2.5-3.5 kg / t steel of steel core aluminum, ferromanganese, ferrochrome, ferromolybdenum and other alloys are added in sequence, 0.9-1.0 kg / t steel of white ash, and 0.6-0.7 kg / t steel of refined slag are added. A small amount of steel is left in the furnace and slag is strictly prohibited. The BOF converter process parameters of each embodiment are shown in Table 3;

[0033] Table 3 BOF converter process parameters of each embodiment

[0034]

[0035] LF refining process: give priority to adjusting the components of Mn, Cr, Mo, and V to ensure uniform composition of the molten steel. LF refining time ≥45min, white slag holding time ≥20min. Aluminum particles and ferrosilicon powder are used for diffusion in the LF process. When the temperature of the molten steel is high in the later stage of LF, nitrogen is blown from the bottom of the ladle to increase nitrogen. When the temperature of the molten steel is ≥1600℃, the adjustment of the components of Mn, Cr, Mo, and V is completed, and the composition of the molten steel is basically uniform, nitrogen is blown from the bottom of the ladle into the molten steel. The nitrogen pressure is controlled at 0.2-0.4MPa, and the bottom blowing time is ≥25min. The LF refining process parameters of each embodiment are shown in Table 4; the composition of the refining slag system used is shown in Table 5.

[0036] Table 4 LF refining process parameters of each embodiment

[0037]

[0038]

[0039] Table 5 Composition of the refined slag system used in each embodiment (%)

[0040] TFe Cao <![CDATA[SiO2]]> MgO <![CDATA[Al2O3]]> MnO <![CDATA[TiO2]]> <![CDATA[P2O5]]> S R Example 1 0.33 62.7 5.2 4.1 27.1 0.07 0.13 0.02 0.35 12.1 Example 2 0.26 60.5 5.2 4.3 29.0 0.07 0.21 0.01 0.45 11.6 Example 3 0.23 62.1 5.3 4.3 27.6 0.05 0.12 0.01 0.29 11.7 Example 4 0.27 62.5 5.3 4.0 27.5 0.05 0.10 0.01 0.27 11.8 Example 5 0.32 60.8 5.0 4.3 29.0 0.06 0.13 0.01 0.38 12.2 Example 6 0.27 60.8 5.1 5.0 28.3 0.05 0.12 0.02 0.34 11.9 Example 7 0.25 61.5 5.0 4.3 28.5 0.05 0.12 0.01 0.27 12.3 Example 8 0.23 61.6 5.3 4.3 28.1 0.05 0.11 0.01 0.30 11.6

[0041] VD vacuum treatment process: High vacuum holding time is controlled at 5-7 minutes; after VD vacuum treatment, aluminum wire is fed at an Al content of 0.020-0.040%; soft blowing time is ≥15 minutes, and nitrogen flow rate is ≤10 L / min. VD vacuum treatment process parameters for various examples are shown in Table 6.

[0042] Table 6 VD vacuum treatment process parameters

[0043]

[0044]

[0045] CC continuous casting process: The continuous casting process is carried out under argon sealing protection to minimize secondary oxidation during the casting process. It is ensured that the ladle does not burn oxygen when it opens automatically, and the purity of the molten steel is controlled. The casting speed is constant at 0.43-0.45m / min, the crystallizer water flow rate is 3100-3300L / min·flow, the specific water volume is 0.13-0.15L / kg, the crystallizer electromagnetic stirring intensity is 150-200A, and the end electromagnetic stirring intensity is 100-150A. Special protective slag for low carbon steel is used: CaO

[0046] 22.5-23.8%, SiO2 20.9-23.6%, MgO 5.6-6.9%, Al2O3 14.7-17.6%, Na2O 8.7-10%, F-3.3-5%, C 15-15.8%, melting point 1200-1225°C, viscosity 0.71-0.85 Pa.s / 1300°C. The continuous casting slabs were slowly cooled off the production line. After exiting the slow cooling pit, the surface of the slabs was inspected and cleaned. The CC continuous casting process parameters for each example are shown in Table 7, and the continuous casting mold slag composition is shown in Table 8.

[0047] Table 7CC continuous casting process parameters

[0048]

[0049] Table 8 Composition of continuous casting mold slag in various examples

[0050]

[0051]

[0052] The low-magnification grade test results of the continuous casting steel slabs for high-strength transmission pipeline flanges provided in each embodiment are shown in Table 9, and the high-magnification grade test results are shown in Table 10.

[0053] Table 9 Low-level grade test results of continuous casting slabs in various embodiments

[0054] Example Central looseness (grade) Other defects Example 1 1.0 none Example 2 1.0 none Example 3 1.0 none Example 4 1.0 none Example 5 1.0 none Example 6 1.0 none Example 7 1.0 none Example 8 1.0 none

[0055] As can be seen from Table 9, the high-strength transmission pipeline flange steel continuous casting billet provided by the present invention has only 1.0 level central looseness at low magnification, and no other low-magnification defects.

[0056] Table 10 Test results of high-magnification grade of rolled materials in various embodiments

[0057]

[0058]

[0059] As can be seen from Table 10, the high-magnification inclusion detection level of the steel rolled material for the strength transmission pipeline flange prepared by the technical solution provided by the present invention is low, and good molten steel purity can be achieved.

Claims

1. A high-strength steel for transmission pipeline flanges, characterized in that: Its chemical composition and its mass percentage are: C 0.08%-0.13%, Si 0.22%-0.30%, Mn 1.20%-1.35%, P≤0.010%, S≤0.002%, Cr 0.20%-0.30%, Mo 0.15%-0.20%, V 0.03%-0.06%, Nb 0.02%-0.05%, Al 0.020%-0.040%, N 0.0090%-0.0130%, Al / N 2-3, Ca / S≥1, and the rest are Fe and unavoidable impurities.

2. The high-strength steel for transmission pipeline flange according to claim 1, characterized in that: Its chemical composition includes O≤10ppm and H≤1.2ppm.

3. A method for producing high-strength transmission pipeline flange steel according to claim 1, characterized in that: The process comprises the following steps: BOF converter - LF refining - VD vacuum treatment - CC continuous casting; in the LF refining process, the refining slag system is controlled within the following ranges: CaO ≥ 60%, SiO2 ≤ 5.3%, Al2O3 27-29%, basicity R ≥ 11.5, TFe+MnO ≤ 0.4%, and MgO ≤ 5.0%.

4. The method for producing high-strength steel for transmission pipeline flanges according to claim 3, characterized in that: In the BOF converter process, after the previous smelting is completed, steel slag is left in the converter, and nitrogen is blown from the bottom to splash the slag to protect the furnace, and the splashing time is 3 to 5 minutes. The amount of scrap steel added to the converter molten iron is: 85 to 95% of the molten iron, and the amount of scrap steel is 5 to 15%. Before smelting, the converter slag near the converter mouth and the furnace cap is cleaned. If splashing occurs during the blowing process, the converter slag remaining on the furnace cap is cleaned a second time to ensure that the furnace cap is clean and free of residue before tapping. After blowing the converter for 3 to 5 minutes, the previous slag is poured out, and slag material is added for secondary slag making and smelting. Slag addition is strictly prohibited during the tapping process.

5. The method for producing high-strength steel for transmission pipeline flanges according to claim 3, characterized in that: In the BOF converter process, 2.5-3.5 kg / t steel of steel core aluminum, 0.9-1.0 kg / t steel of white ash, and 0.6-0.7 kg / t steel of refined slag are added in sequence during the steel tapping process.

6. The method for producing high-strength steel for transmission pipeline flanges according to claim 3, characterized in that: In the LF refining process, the LF refining time is ≥45 min, and the white slag holding time is ≥20 min.

7. The method for producing high-strength steel for transmission pipeline flanges according to claim 3, characterized in that: In the LF refining process, when the molten steel temperature is ≥1600°C, the Mn, Cr, Mo, and V components are adjusted, and bottom nitrogen is blown, the nitrogen pressure is controlled at 0.2-0.4 MPa, and the bottom nitrogen blowing time is ≥25 minutes.

8. The method for producing high-strength steel for transmission pipeline flanges according to claim 3, characterized in that: In the VD vacuum treatment process, the high vacuum holding time is controlled at 5-7 minutes; after the VD vacuum treatment, aluminum wire is fed according to the Al content of 0.020-0.040%; the soft blowing time is ≥15 minutes, and the nitrogen flow rate is ≤10L / min.

9. The method for producing high-strength steel for transmission pipeline flanges according to claim 3, characterized in that: In the CC continuous casting process, the casting speed is constant at 0.43-0.45 m / min, the water flow rate of the crystallizer is 3100-3300 L / min, the specific water volume is 0.13-0.15 L / kg, the electromagnetic stirring intensity of the crystallizer is 150-200 A, and the end electromagnetic stirring intensity is 100-150 A.

10. The method for producing high-strength steel for transmission pipeline flanges according to claim 3, wherein: The CC continuous casting process uses a special protective slag for low carbon steel: CaO 22.5-23.8%, SiO2 20.9-23.6%, MgO 5.6-6.9%, Al2O3 14.7-17.6%, Na2O 8.7-10%, F - 3.3-5%, C 15-15.8%, melting point 1200-1225℃, viscosity 0.71-0.85 Pa.s / 1300℃.

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