A kind of steel for aging-resistant double-layer welded pipe and its manufacturing method

Abstract

This invention discloses a steel for anti-aging double-layer welded pipes and its manufacturing method. The chemical composition of the steel, by weight percentage, is as follows: C: 0.010%–0.030%, Si ≤ 0.030%, Mn: 0.10%–0.30%, P ≤ 0.010%, S ≤ 0.012%, N ≤ 0.003%, Ti: 0.035%–0.055%, B: 0.004%–0.008%, with the remainder being Fe and unavoidable impurities. By optimizing the chemical composition and manufacturing process, this invention produces a steel with a yield strength of 170 MPa–250 MPa, a tensile strength of 310 MPa–390 MPa, an elongation ≥ 40%, and a grain size ≤ 9.5. The mechanical properties after 180 days meet the following requirements: yield strength 190MPa~260MPa, tensile strength 320MPa~400MPa, elongation ≥37%, yield strength increase ≤20MPa, and elongation decrease ≤3%. After 360 days, the mechanical properties meet the following requirements: yield strength 210MPa~280MPa, tensile strength 340MPa~420MPa, elongation ≥34%, yield strength increase ≤40MPa, and elongation decrease ≤6%. This steel plate exhibits excellent resistance to aging and meets the requirements for pipe manufacturing.

Description

Technical Field

[0001] This invention relates to the field of manufacturing and using steel for double-layer welded pipes, and specifically to an anti-aging double-layer welded pipe steel and its manufacturing method. Background Technology

[0002] The manufacturing process of double-layer welded pipe involves copper plating ordinary cold-rolled steel sheets, followed by two layers of tubing and brazing at high temperatures. The product is mainly used for brake lines, condenser tubes in refrigeration, etc. The product is extremely delicate, and its mechanical properties, surface quality, and thickness variations are significantly affected by the cold-rolled sheet. Otherwise, incomplete welds may occur, affecting the product's lifespan. Furthermore, since the tubing is in service during production, it is prone to aging over time, leading to increased strength, decreased elongation, and reduced service life, posing a risk of failure and endangering life and property. Therefore, double-layer welded pipe must possess good formability and uniform sheet thickness, as well as a certain degree of resistance to aging.

[0003] The domestic patent CN 102925794 B, entitled "Cold-rolled strip steel for double-layer welded pipe and its manufacturing method," provides a cold-rolled strip steel for double-layer welded pipe with a yield strength controlled at 170-260 MPa, a tensile strength guaranteed at 270-370 MPa, an elongation after fracture >32%, and a hardness (HR30T) guaranteed in the range of 40-52, and its manufacturing method. Its composition is: C: 0.015-0.054%, Si≤0.034%, Mn: 0.15-0.25%, P≤0.020%, S≤0.020%, Alt: 0.015-0.050%, with the balance being Fe and unavoidable impurities. While this composition meets the requirements for pipe manufacturing, it does not meet the requirements for resistance to aging.

[0004] The domestic patent CN 110629125 B, entitled "A Cold-Rolled Steel Sheet with Excellent Aging Resistance for Continuously Brazed Double-Layer Coiled Pipes," provides a cold-rolled steel sheet with excellent aging resistance for continuously brazed double-layer coiled pipes and its production method. The chemical composition of this steel by weight percentage is as follows: C: 0.03–0.07%, Si ≤ 0.05%, Mn: 0.10–0.25%, P ≤ 0.025%, S ≤ 0.020%, Alt: 0.03–0.06%, N ≤ 0.003%, with the remainder being Fe and unavoidable impurities. This patent has a yield strength ≥ 230 MPa, offers no improvement in aging resistance, and cannot meet customer processing requirements. Summary of the Invention

[0005] This invention addresses the issue of aging resistance in steel for double-layer welded pipes by proposing an innovative aging-resistant double-layer welded pipe steel and its manufacturing method. The aim is to ensure that the material maintains excellent mechanical properties even after long-term storage or use through carefully designed chemical composition and strictly controlled production process, especially preventing the degradation of elongation properties caused by aging.

[0006] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution:

[0007] This invention proposes a steel for anti-aging double-layer welded pipes. The chemical composition of the steel for anti-aging double-layer welded pipes, by weight percentage, is as follows: C: 0.010%~0.030%, Si≤0.030%, Mn: 0.10%~0.30%, P≤0.010%, S≤0.012%, N≤0.003%, Ti: 0.035%~0.055%, B: 0.004%~0.008%, with the remainder being Fe and unavoidable impurities.

[0008] Preferably, the steel for the anti-aging double-layer welded pipe meets the following properties: yield strength of 170MPa~250MPa, tensile strength of 310MPa~390MPa, elongation ≥40%, and grain size grade of 9.0-9.5; the mechanical properties after 180 days meet the following requirements: yield strength of 190MPa~260MPa, tensile strength of 320MPa~400MPa, elongation ≥37%, yield strength increase ≤20MPa, and elongation decrease ≤3%; the mechanical properties after 360 days meet the following requirements: yield strength of 210MPa~280MPa, tensile strength of 340MPa~420MPa, elongation ≥34%, yield strength increase ≤40MPa, and elongation decrease ≤6%.

[0009] Preferably, the content relationship of chemical components C, N, Ti and B satisfies the following condition: 5B + Ti > C + 3.14N.

[0010] This invention also provides a method for manufacturing steel for anti-aging double-layer welded pipes, comprising the following steps: molten iron pretreatment → converter → alloy fine-tuning station → RH → continuous casting → hot rolling and coiling → slow cooling pit insulation → pickling and cold rolling → high-temperature annealing → leveling → finished product.

[0011] Preferably, in smelting and continuous casting: pre-slag removal and post-slag removal measures are taken during the hot metal pretreatment process to adjust the sulfur element; self-circulating scrap steel is added in the early and middle stages of decarburization to modify the ladle top slag.

[0012] Preferably, in hot rolling and coiling: the heating temperature is controlled at 1200℃~1250℃, the holding time is ≥180min, the final rolling temperature is 850℃~890℃, and the coiling temperature is 710℃~750℃.

[0013] Preferably, during the slow cooling pit insulation process: after the hot-rolled coil production is completed, the high-temperature coil is directly hoisted into the insulation pit for 72 hours of insulation.

[0014] Preferably, in pickling and cold rolling: pickling and cold rolling adopt five-stand continuous rolling, pickling reduction rate ≥85%, and the end stand adopts smooth roll rolling with roughness ≤0.5μm and RPc≥220.

[0015] Preferably, in the high-temperature annealing: the annealing temperature is 800℃~840℃, the strip speed in the furnace is ≤200mpm, the cooling time in the heating section + soaking section is ≥300s, the temperature cooling rate in the rapid cooling section is ≤20℃ / s, cooling from 650℃ to 350℃, and then heating to 380℃ in the over-aging section.

[0016] Preferably, during leveling: the leveling elongation is 0.6-1.2%, the surface roughness is 0.5μm≤Ra≤1.0μm, and RPc≥120.

[0017] The role of alloying elements in steel grades in this invention is mainly based on the following principles:

[0018] 1) Carbon (C): Because double-layer welded pipes require the use of aluminum-killed steel and cannot use ultra-low carbon compositions, and as the carbon content increases, the steel plate's resistance to aging weakens. At the same time, a small amount of carbon can improve the strength and deformation resistance of the steel. Therefore, the carbon content is controlled at 0.010% to 0.030%.

[0019] 2) Silicon (Si): If the Si content is too high, the iron oxide scale on the surface of the steel plate is not easy to remove, and microcracks are easily formed on the surface due to the indentation of oxides. In addition, high Si content is not good for copper electroplating and affects its electroplating quality. Therefore, Si ≤ 0.030% in this invention.

[0020] 3) Manganese (Mn): Mn can lower the austenite-ferrite phase transformation temperature (which can compensate for the increase in the austenite-ferrite phase transformation temperature caused by the decrease in C element content), expand the hot working temperature range, and reduce the risk of mixed crystals in hot rolling; however, if the Mn content is too high, it will be detrimental to plasticity, stamping performance, and fatigue performance. Taking all factors into consideration, the Mn percentage content in this invention is controlled within the range of 0.10% to 0.30%.

[0021] 4) Phosphorus (P): Excessive P in steel is prone to segregation, which is detrimental to the formability and aging resistance of steel plates. Therefore, the content of P element in this invention is controlled within the range of ≤0.010%.

[0022] 5) Sulfur (S): Sulfur is a harmful element in battery casing steel, causing hot brittleness, reducing ductility and toughness, and making it prone to cracking during rolling. Furthermore, S is detrimental to weldability and reduces corrosion resistance. Therefore, this invention controls the S content in the steel to within the range of S ≤ 0.012%.

[0023] 6) Titanium (Ti): Ti is a strong carbide and nitride forming element, playing a role in carbon and nitrogen fixation. It can reduce the precipitation of free C and N elements in ferrite, forming TiC and TiN, thus providing good resistance to aging. However, excessive second-phase precipitates can also affect ferrite grain growth. Therefore, this invention controls the Ti content of the steel within the range of 0.035% to 0.055%.

[0024] 7) Nitrogen (N): Nitrogen is an impurity element in steel. Since free nitrogen can cause aging and is not conducive to resisting aging, this invention controls the nitrogen content to N≤0.003%.

[0025] 8) Boron (B): Boron acts similarly to Ti in steel grades, fixing carbon and nitrogen and slowing down aging. However, excessive boron significantly affects plasticity. Therefore, this invention controls boron content to the range of 0.004–0.008%. The formula 5B + Ti > C + 3.14N ensures sufficient precipitation of second-phase particles, resulting in interstitial grains.

[0026] The manufacturing method for this anti-aging double-layer welded pipe requires the following steps: slab smelting and continuous casting, hot rolling and coiling, heat preservation, pickling and cold rolling, continuous annealing, and leveling. Its technological characteristics are as follows:

[0027] (1) Slab smelting and continuous casting: Taking pre-slag removal and post-slag removal measures during the hot metal pretreatment process to adjust the [S] element can reduce harmful elements in the molten steel. Adding self-circulating scrap steel in the early and middle stages of decarburization to modify the ladle top slag helps to reduce impurity elements in the molten steel.

[0028] (2) Hot rolling and coiling: The heating temperature is controlled at 1200℃~1250℃, and the holding time is ≥180min to ensure sufficient and uniform heating. The final rolling temperature is controlled at 850℃~890℃, which ensures that the final rolling temperature is above the austenitic temperature and avoids the mixed crystal phenomenon caused by rolling in the two-phase region. Laminar flow cooling is selected with less water at the rear end, and the coiling temperature is set at 710℃~750℃.

[0029] (3) Heat preservation: After the hot-rolled coil production is completed, the high-temperature coil is directly hoisted into the heat preservation pit for 72 hours to allow carbides and nitrides to fully precipitate at high temperature and grains to grow fully, reducing the presence of supersaturated ferrite.

[0030] (4) Pickling and cold rolling: Pickling and cold rolling are carried out using five-stand continuous rolling with a reduction rate of ≥85%. A large reduction rate can increase the grain distortion energy in the steel, reduce the recrystallization temperature, and facilitate sufficient recrystallization and growth in the annealing furnace, thereby reducing aging. The last stand uses smooth roll rolling with a roughness control of ≤0.5μm and RPc≥220. This measure can effectively improve the surface quality of the finished plate and enhance the adhesion of electroplating on the steel plate.

[0031] (5) High-temperature annealing: Annealing temperature 800℃~840℃, strip furnace speed ≤200mpm, cooling time in the heating and soaking sections ≥300s, rapid cooling rate ≤20℃ / s, cooling from 650℃ to 350℃, and then heating to 380℃ in the over-aging section. This process not only allows the C and N second phases to fully precipitate, but also promotes sufficient grain growth. The final product properties can achieve a yield strength of 170MPa~250MPa, a tensile strength of 310MPa~390MPa, and an elongation ≥36%.

[0032] (6) Smoothing: Laser texturing rollers are used for smoothing, and the smoothing elongation is controlled at 0.6-1.2%. A smoothing roller with Ra of 1.6μm is used to ensure that the surface roughness is 0.5μm≤Ra≤1.0μm and RPc≥120. This roughness is beneficial to the smooth surface and strong adhesion after electroplating.

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

[0034] This invention mainly produces anti-aging double-layer welded pipe steel by precisely controlling the composition of steel and through smelting, hot rolling, pickling, annealing and leveling processes.

[0035] Precise control of chemical composition: Carbon (C) is controlled between 0.010% and 0.030%, balancing the requirement for non-ultra-low carbon steel in double-layer welded pipes with the need for aging resistance. This ensures the necessary strength and deformation resistance of the steel while minimizing aging sensitivity. The contents of elements such as silicon (Si), phosphorus (P), and sulfur (S) are strictly limited (Si≤0.030%, P≤0.010%, S≤0.012%) to reduce surface defects, improve electroplating quality, alleviate hot brittleness, and enhance ductility. The manganese (Mn) content is optimized between 0.10% and 0.30% to balance strength, plasticity, and hot working properties, and reduce the risk of mixed crystal formation. Ti (0.035%–0.055%) and B (0.004%–0.008%) within a specific range work together to fix C and N, effectively suppressing the free precipitation of carbon and nitrogen by forming second phases such as TiC and TiN, enhancing resistance to aging, and simultaneously satisfying the condition 5B+Ti>C+3.14N to ensure sufficient precipitation of second phase particles.

[0036] Production process optimization: Every step in the entire production process, from molten iron pretreatment to finished product, is meticulously designed to ensure the performance of the final product. Molten iron pretreatment and continuous casting: Through precise pre- and post-slag removal and ladle top slag modification, the content of harmful elements is significantly reduced, laying a pure base material foundation for subsequent processes. Hot rolling and coiling: Specific heating temperatures and holding times ensure uniform heating and full austenitization. Controlled final rolling temperature avoids mixed crystals, while a reasonable coiling temperature setting facilitates subsequent processing. Slow cooling pit holding: Direct high-temperature holding for 72 hours promotes the full precipitation of carbides and nitrides and uniform grain growth, effectively alleviating the problem of supersaturated ferrite. Pickling and cold rolling: The application of five-stand continuous rolling and a large reduction rate, combined with smooth roll rolling, improves surface quality and facilitates grain recrystallization during subsequent annealing, effectively controlling the aging effect. High-temperature annealing: Precisely controlled annealing temperature and cooling rate ensure the full precipitation of the second phase and complete grain growth, significantly improving the mechanical properties of the finished product. Smoothness: Laser roughening and strictly controlled roughness further improve the surface quality and electroplating adhesion of the sheet material.

[0037] The double-layer welded pipe steel produced according to the method of this invention has a yield strength of 170MPa~250MPa, a tensile strength of 310MPa~390MPa, an elongation ≥40%, and a grain size ≤9.5. After 180 days, its mechanical properties meet the following requirements: yield strength 190MPa~260MPa, tensile strength 320MPa~400MPa, elongation ≥37%, yield strength increase ≤20MPa, and elongation decrease ≤3%. After 360 days, its mechanical properties meet the following requirements: yield strength 210MPa~280MPa, tensile strength 340MPa~420MPa, elongation ≥34%, yield strength increase ≤40MPa, and elongation decrease ≤6%. Therefore, this double-layer welded pipe steel exhibits excellent resistance to aging while meeting the requirements for mechanical properties.

[0038] In summary, this invention, through carefully designed chemical composition and meticulous manufacturing process, achieves innovation in anti-aging double-layer welded pipe steel, effectively improves anti-aging performance and other mechanical properties, ensures the stability of mechanical properties of the material during long-term aging, solves key problems in the industry, and demonstrates significant technological progress and creative value. Attached Figure Description

[0039] Figure 1 This is a metallographic diagram of the cold-rolled steel sheet of Embodiment 4 of the present invention. Detailed Implementation

[0040] To enable those skilled in the art to better understand the technical solutions of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples. However, these should not be construed as limiting the present invention and are merely examples.

[0041] Unless otherwise specified, the test methods or experimental methods described in the following examples are conventional methods; unless otherwise specified, the reagents and materials are obtained from conventional commercial sources or prepared by conventional methods.

[0042] The present invention will be further described in detail below with reference to specific embodiments.

[0043] The chemical composition and content of the molten steel in Examples 1-6 and Comparative Examples 1-7 of this invention are shown in Table 1. The remainder consists of Fe and unavoidable impurity elements.

[0044] The production processes of molten steel in Examples 1-6 and Comparative Examples 1-7 of the present invention are shown in Table 2. Among them, Comparative Examples 1-7 were not smelted entirely according to the production process of the present invention.

[0045] Table 1 Chemical composition, wt%

[0046]

[0047]

[0048] All production processes were trial-produced according to Table 2, and the thickness of the finished product was 0.35mm.

[0049] Table 2 Production Process

[0050]

[0051] All performance characteristics of Examples 1-6 meet the design requirements. Figure 1 It can be seen that the grain size level of the anti-aging double-layer welded pipe prepared in Example 4 is 9.0. In Comparative Examples 1 and 2, B element was added, but Ti element was not added. Furthermore, high-temperature coiling and holding processes were not used, and high-temperature annealing was also not employed; the annealing speed was also fast, resulting in fine grains. Although the mechanical properties meet the pipe manufacturing requirements, the anti-aging performance is weak. Comparative Examples 3 and 4 used aluminum-killed steel in their composition design, and without the addition of B and Ti elements, the final yield strength and tensile strength met the requirements, but the elongation did not, and the grain size was too small. Comparative Examples 5, 6, and 7 used low-temperature coiling processes and conventional annealing temperatures; the final product yield strength values ​​were all too high, and the elongation did not meet the design requirements.

[0052] Table 3 Production Testing Performance Values

[0053]

[0054]

[0055] After 180 days of natural aging, the yield strength increase of the embodiments was ≤20 MPa, the elongation decreased by ≤3%, and the elongation was still ≥40%. In contrast, the yield strength increase of the comparative embodiments was >20 MPa, and the elongation decreased by >3%. After 360 days of natural aging, the yield strength increase of the embodiments was ≤40 MPa, and the elongation decreased by ≤6%. In contrast, the yield strength increase of the comparative embodiments was >40 MPa, and the elongation decreased by >6%.

[0056] Table 4 Performance test values ​​after natural aging at 180 and 360 days

[0057]

[0058] The above description is only a specific example of the present invention. It should be noted that the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the technical concept and technical solution of the present invention, or the direct application of the technical concept and technical solution of the present invention to other occasions without modification, are all within the protection scope of the present invention.

Claims

1. A type of steel for anti-aging double-layer welded pipes, characterized in that, The chemical composition of the steel for anti-aging double-layer welded pipes, by weight percentage, is as follows: C: 0.010%~0.030%, Si≤0.030%, Mn: 0.10%~0.30%, P≤0.010%, S≤0.012%, N≤0.003%, Ti: 0.035%~0.055%, B: 0.004%~0.008%, with the remainder being Fe and unavoidable impurities. The steel for anti-aging double-layer welded pipes meets the following properties: yield strength of 170MPa~250MPa, tensile strength of 310MPa~390MPa, elongation ≥40%, and grain size grade 9. Grade 0-9.5; Mechanical properties after 180 days: yield strength 190MPa~260MPa, tensile strength 320MPa~400MPa, elongation ≥37%, yield strength increase ≤20MPa, elongation decrease ≤3%; Mechanical properties after 360 days: yield strength 210MPa~280MPa, tensile strength 340MPa~420MPa, elongation ≥34%, yield strength increase ≤40MPa, elongation decrease ≤6%; The content relationship of chemical composition C, N, Ti and B satisfies the following condition: 5B+Ti>C+3.14N.

2. A method for manufacturing steel for anti-aging double-layer welded pipes as described in claim 1, characterized in that, The process includes the following steps: molten iron pretreatment → converter → alloy fine-tuning station → RH → continuous casting → hot rolling and coiling → slow cooling pit insulation → pickling and cold rolling → high temperature annealing → leveling → finished product.

3. The method for manufacturing steel for anti-aging double-layer welded pipes according to claim 2, characterized in that, In smelting and continuous casting: Before and after slag removal are carried out during the hot metal pretreatment process to adjust the sulfur element; self-circulating scrap steel is added in the early and middle stages of decarburization to improve the ladle top slag.

4. The method for manufacturing steel for anti-aging double-layer welded pipes according to claim 2, characterized in that, In hot rolling and coiling: heating temperature is controlled at 1200℃~1250℃, holding time is ≥180min, final rolling temperature is 850℃~890℃, and coiling temperature is 710℃~750℃.

5. The method for manufacturing steel for anti-aging double-layer welded pipes according to claim 2, characterized in that, During the slow cooling pit insulation process: After the hot-rolled coil production is completed, the high-temperature coil is directly hoisted into the insulation pit for 72 hours of insulation.

6. The method for manufacturing steel for anti-aging double-layer welded pipes according to claim 2, characterized in that, In pickling and cold rolling: pickling and cold rolling adopt five-stand continuous rolling, pickling reduction rate ≥85%, and the end stand adopts smooth roll rolling with roughness ≤0.5μm and RPc≥220.

7. The method for manufacturing steel for anti-aging double-layer welded pipes according to claim 2, characterized in that, During high-temperature annealing: the annealing temperature is 800℃~840℃, the strip speed in the furnace is ≤200mpm, the cooling time in the heating section + soaking section is ≥300s, the temperature cooling rate in the rapid cooling section is ≤20℃ / s, cooling from 650℃ to 350℃, and then heating to 380℃ in the over-aging section.

8. The method for manufacturing steel for anti-aging double-layer welded pipes according to claim 2, characterized in that, During leveling: Leveling elongation is 0.6~1.2%, surface roughness is 0.5μm≤Ra≤1.0μm, RPc≥120.

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