Steel for single-layer precision welded pipe and manufacturing method thereof

Abstract

The invention discloses steel for a single-layer precision welded pipe and a manufacturing method of the steel. The steel for the single-layer precision welded pipe comprises the following chemical components in percentage by weight: 0.003-0.006% of C, 0.28-0.40% of Mn, 0.025-0.075% of Als, 0.035-0.055% of Ti, less than or equal to 0.03% of Si, less than or equal to 0.015% of P, less than or equal to 0.012% of S, less than or equal to 0.003% of N and the balance of Fe and inevitable impurities. The yield strength ranges from 200 MPa to 240 MPa, the tensile strength ranges from 340 MPa to 400 MPa, the ductility is larger than or equal to 35%, the same-plate difference is smaller than or equal to 15 micrometers, and the steel plate has high strength, good welding performance and good drawing performance and can be used for the field of products such as condenser pipes and connecting pipes in the household appliance industry.

Description

Technical Field

[0001] The present invention belongs to the technical field of cold-rolled single-layer precision welded steel pipes, and particularly relates to a single-layer precision welded steel pipe and a manufacturing method thereof. Background Art

[0002] A single-layer precision welded steel pipe refers to a precision welded steel pipe produced by coiling cold-rolled strip steel after slitting, roll-forming it at 360°, high-frequency welding it into a steel pipe, then continuously cold-reducing the diameter, on-line heat treatment, and finally galvanizing or copper-plating the surface of the welded pipe. It is generally used in product fields such as condenser pipes and connecting pipes in the household appliance industry. The common requirements for steel pipes in these fields are small diameter, thin wall thickness, high precision, high ductility, good drawing performance, good anti-aging performance, and high flatness of the strip steel. At the same time, it needs to meet the welding, drawing, and galvanizing / copper-plating processes during the production of welded pipes.

[0003] Previously, most of the condenser pipes used in the household appliance industry were expensive copper pipes and cold-drawn seamless steel pipes, with high production costs. In recent years, with the demand for energy conservation and cost reduction in the household appliance and automotive industries, domestic and foreign pipe manufacturers have adopted cold-reduced single-layer precision welded steel pipes after galvanizing or copper-plating to replace copper pipes and cold-drawn seamless steel pipes. Currently, the strength requirements for precision welded pipes in the domestic market are relatively low, generally requiring the tensile strength of the finished welded pipe to be ≤340 MPa. Since the tensile strength of the steel plate increases to a certain extent after being made into a welded pipe, domestic generally requires the yield strength of the steel for pipe making to be 130 - 160 MPa, the tensile strength to be 270 - 300 MPa, and the elongation rate to be ≥45%. While the foreign market requires precision welded pipes to have higher strength to avoid deformation during subsequent processing such as bending and twisting or during service. Therefore, high-strength steel plates are generally used for pipe making, requiring the yield strength of the steel for pipe making to be 200 - 240 MPa, the tensile strength to be 340 - 400 MPa, and the elongation rate to be ≥35%.

[0004] Chinese Patent CN 114921724 A discloses a steel plate for producing a single-layer welded pipe for high-speed stretching and a manufacturing method thereof. The chemical composition and weight percentage of the steel plate are: C≤0.004%, Si≤0.010%, Mn: 0.10 - 0.15%, P≤0.015%, S≤0.013%, Als≤0.060%, Ti: 0.04 - 0.08%, B: 0.0002 - 0.0010%, N≤0.005%, and the rest are Fe and unavoidable impurities. The yield strength of the steel coil in this patent is 140 - 160 MPa, the tensile strength is 270 - 320 MPa, and the elongation rate is ≥39%, which cannot meet the high-strength requirements.

[0005] Chinese Patent CN 102747283 A discloses a production method for steel strips for precision copper-plated welded pipes. Its chemical composition (mass fraction) is: C ≤ 0.002%, Si ≤ 0.010%, Mn: 0.11 - 0.15%, P ≤ 0.01%, S ≤ 0.01%, Alt: 0.02 - 0.04%, Ti: 0.05 - 0.08%, N ≤ 30 ppm, O ≤ 30 ppm, and the balance is Fe and trace elements. The thickness specification of the finished steel strip for precision copper-plated welded pipes in this patent is 0.45 - 0.80 mm; its yield strength is 170 - 230 MPa, tensile strength is 280 - 320 MPa, and elongation after fracture A50 is greater than 45.0%, which also cannot meet the requirements of high strength. Summary of the Invention

[0006] To solve the above technical problems, the present invention provides a steel for single-layer precision welded pipes and its manufacturing method. This steel grade has high strength, good welding performance, and good drawing performance, and can be used in product fields such as condenser pipes and connecting pipes in the household appliance industry.

[0007] To achieve the above objectives, the technical solutions adopted by the present invention are as follows:

[0008] The present invention provides a steel for single-layer precision welded pipes. The chemical composition and weight percentage of the steel for single-layer precision welded pipes are: C 0.003 - 0.006%, Mn 0.28 - 0.40%, Als 0.025 - 0.075%, Ti 0.035 - 0.055%, Si ≤ 0.03%, P ≤ 0.015%, S ≤ 0.012%, N ≤ 0.003%, and the rest is Fe and inevitable impurities.

[0009] The metallographic structure of the steel for single-layer precision welded pipes is ferrite.

[0010] The yield strength of the steel for single-layer precision welded pipes is 200 - 240 MPa, the tensile strength is 340 - 400 MPa, and the elongation is ≥ 35%. It has high strength and can avoid deformation of the steel pipe during subsequent processing such as bending and twisting or during service.

[0011] The difference in the same plate of the steel for single-layer precision welded pipes is ≤ 15 μm, and it has good welding performance, effectively solving problems such as poor welding, weld leakage, and springback after bending of the welded pipe during the pipe manufacturing process.

[0012] The present invention also provides a manufacturing method for the steel for single-layer precision welded pipes. The manufacturing method includes the following steps: hot metal smelting, slab continuous casting, hot rolling, pickling and cold rolling, continuous annealing, skin pass rolling, and coiling.

[0013] In the hot rolling step, the slab heating and discharging temperature is controlled at 1200°C to 1260°C, and the holding time is ≥180 min. The second-phase particles dissolved in solid solution at high temperature are beneficial to rolling, and fine particles will precipitate again during coiling, which is beneficial to improving the stamping performance of the material. The final rolling temperature is controlled at 870°C to 930°C. This temperature can ensure that the final rolling temperature is controlled above the austenite temperature, avoiding the mixed grain phenomenon caused by rolling in the two-phase region.

[0014] In the hot rolling step, the coiling temperature is controlled at 640°C to 680°C to achieve fine and uniform precipitates and grain size. If the coiling temperature is too high, it will cause coarse grain size, reduce the effect of fine grain strengthening, and make the material strength low. By hot rolling at high temperature and coiling at low temperature, combined with the cold rolling and annealing process, the strength and drawing performance of the steel are improved.

[0015] In the hot rolling step, the hot rolling crown is controlled at 10μm to 30μm, and the wedge is controlled at -20μm to 20μm. This can ensure that the finished product has good flatness difference within the same plate.

[0016] In the pickling and cold rolling step, five-stand continuous rolling is adopted, and the total cold rolling reduction rate is ≥80%. A large reduction rate can increase the grain distortion energy in the steel, lower the recrystallization temperature, be beneficial to grain refinement, and improve the strength and drawing performance of the steel plate. To ensure the flatness difference within the same plate, edge trimming is required once during acid rolling, and the bilateral edge trimming amount is ≥16 mm.

[0017] In the continuous annealing step, the annealing soaking temperature is controlled at 720°C to 740°C, and the soaking time is 50 s to 130 s. Too high annealing temperature will cause coarse grains, thus reducing the material strength. The present invention adopts low-temperature annealing to improve the strength of the final finished steel plate. To ensure the flatness difference within the same plate, edge trimming is required twice after continuous annealing, and the bilateral edge trimming amount is ≥14 mm.

[0018] In the temper rolling step, the temper rolling elongation is controlled at 0.6% to 1.2% to eliminate the yield plateau of the strip, improve the strip shape, and provide a certain roughness for the strip, ensuring the stability of the welding and drawing production processes. Appropriate roughness can improve the coating adhesion ability of the welded pipe during galvanizing or copper plating.

[0019] To ensure that the strength of the steel for single-layer precision welded pipes meets the design requirements and has excellent drawing performance and excellent welding performance, the composition design is mainly based on the following principles:

[0020] 1) Carbon C: C is an economical strengthening element, which directly affects the strength, plasticity, toughness, and welding performance of the steel plate, etc. Generally, with the increase of carbon content, the strength and hardness of the steel increase, while the plasticity and toughness decrease, and the welding performance also decreases significantly. To ensure the necessary strength, the present invention conducts composition design based on the ultra-low carbon composition system framework, and the C content is controlled at 0.003% to 0.006%.

[0021] 2) Silicon (Si): Si is added as a reducing agent and deoxidizer during the steelmaking process. It can also play a role in solid-solution strengthening. Since the binding ability of Si with O is stronger than that of Fe, it is easy to form low-melting-point silicates during welding, increasing the fluidity of the slag and molten metal, causing splashing phenomena and affecting the welding quality. Therefore, it is not recommended to have too high Si content, and it is controlled at ≤0.03%.

[0022] 3) Manganese (Mn): Mn is a common desulfurizing and deoxidizing element in steel. It combines with S in the steel to form MnS, which can reduce the generation of FeS, thereby reducing the risks of hot brittleness and welding hot cracks. At the same time, Mn can dissolve infinitely in ferrite and austenite, and improve the strength of the steel plate in the way of solid-solution strengthening. However, too high Mn content will reduce the plasticity of the material. Considering the steel plate strength and welding quality, the Mn content is controlled at: 0.28 - 0.40%.

[0023] 4) Phosphorus (P): P is prone to segregation, thus reducing the plasticity, low-temperature toughness and welding performance of the steel. Although the increase of P element can improve the strength and hardness of the steel, it will also cause cold brittleness of the steel. Especially at low temperatures, the steel will become significantly brittle. Due to its embrittlement effect, weld brittleness or weld cracks are likely to occur during the welding process. Therefore, P needs to be controlled in a lower range, and this patent controls it at ≤0.015%.

[0024] 5) Sulfur (S): S combines with Fe in the steel and exists in the steel in the form of FeS, making the steel have hot brittleness. FeS and Fe will form a compound with a low melting point (985°C). Since the steel is generally hot-worked at temperatures above 1150 - 1200°C, FeS will melt in advance during the heating process, thus reducing the ductility and toughness of the steel and being prone to cracks during rolling. This invention controls it at ≤0.012%.

[0025] 6) Titanium (Ti): Ti has a very strong affinity with both C and N. It can form TiC and TiN with C and N, causing them to precipitate from ferrite, thereby reducing the free C and N elements in the solid solution and playing a good anti-aging role. TiN and TiC in the steel will slowly dissolve into the solid solution only when heated above 1000°C. Before dissolving into the solid solution, TiN and TiC particles can prevent grain growth, thereby improving toughness and strength. However, too high Ti content will form large-size precipitates in the structure, which are prone to cause crystal distortion, and too high Ti content will increase the manufacturing cost. Therefore, the Ti content is required to be 0.035 - 0.055%.

[0026] (7) Aluminum (Al): It is a deoxidizer and degassing agent that can prevent the generation of other oxides and prevent the occurrence of porosity defects in molten steel. Al has a strong affinity for N, which can fix the nitrogen in the steel and thus reduce the aging tendency. However, excessive Al will form large-sized alumina inclusions with oxygen in the steel, and the presence of inclusions will damage the welding performance of the steel and make its weldability poor. Therefore, in the present invention, Als is controlled within 0.025 - 0.075%.

[0027] (8) Nitrogen (N): The influence of N on the properties of steel is similar to that of C and P. As the nitrogen content increases, the strength of the steel can be significantly improved, while the plasticity and toughness are significantly reduced, and the weldability becomes poor. At the same time, it will also increase the aging tendency, cold brittleness and hot brittleness, and damage the welding performance and cold bending performance of the steel. Therefore, it is necessary to control the nitrogen content in the steel within a relatively small range as much as possible. In the present invention, it is controlled at ≤0.003%.

[0028] Since the steel for single-layer precision welded pipes needs to go through processes such as welding, drawing, and subsequent bending and folding during the manufacturing process, and requires good welding performance and drawing performance, therefore, in the composition design, the present invention is based on an ultra-low carbon composition system, adding a small amount of Mn and Ti. The strength is improved through the solid solution strengthening of Mn and the precipitation strengthening and grain refinement strengthening of Ti. The addition of Ti can also capture free C and N elements in the solid solution, playing a good anti-aging role. Strictly controlling the contents of elements such as Si, P, S, and N that are harmful to welding can effectively improve the welding performance of the material.

[0029] Through the composition design of the present invention, combined with reasonable hot rolling processes, cold rolling processes and annealing processes, a kind of steel for single-layer precision welded pipes with good welding and drawing performance can be produced, and its performance meets the requirements of yield strength of 200 - 240 MPa, tensile strength of 340 - 400 MPa, and elongation rate ≥ 35%.

[0030] The present invention ensures that the finished product has good flatness and flatness difference through the consistent control technology of flatness, avoids virtual welding caused by thickness fluctuation during the welding process, and improves the welding quality and the stability of the pipe manufacturing process.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] Compared with the steel for single-layer precision welded pipes with relatively low strength currently circulating in the country, the yield strength and tensile strength of the steel for single-layer precision welded pipes provided by the present invention have been greatly improved. The mechanical properties of the finished product meet the requirements: yield strength of 200 - 240 MPa, tensile strength of 340 - 400 MPa, and elongation rate ≥ 35%. It has high strength, good welding performance and good drawing performance, and solves problems such as poor welding, weld leakage and springback after bending of the welded pipe during the pipe manufacturing process. Description of the Drawings

[0033] Figure 1 It is the metallographic structure diagram of the steel for single-layer precision welded pipes in Example 1. Detailed implementation mode

[0034] A kind of steel for single-layer precision welded pipes provided by the present invention, its chemical composition and weight percentage are: C 0.003-0.006%, Mn 0.28-0.40%, Als 0.025-0.075%, Ti 0.035-0.055%, Si≤0.03%, P≤0.015%, S≤0.012%, N≤0.003%, and the rest are Fe and inevitable impurities.

[0035] The manufacturing method of the steel for single-layer precision welded pipes includes the following steps: hot metal smelting, slab continuous casting, hot rolling, pickling cold rolling, continuous annealing, skin pass rolling, and coiling.

[0036] In the hot rolling step, the slab heating and discharging temperature is controlled at 1200°C - 1260°C, the holding time is ≥180 min, the final rolling temperature is controlled at 870°C - 930°C; the coiling temperature is controlled at 640°C - 680°C; the hot rolling crown is controlled at 10μm - 30μm, and the wedge is controlled at -20μm - 20μm.

[0037] In the pickling cold rolling step, five-stand continuous rolling is adopted, the total cold rolling reduction rate is ≥80%, and one-time edge trimming is carried out after cold rolling, and the bilateral edge trimming amount is ≥16 mm.

[0038] In the continuous annealing step, the annealing soaking temperature is controlled at 720°C - 740°C, and the soaking time is 30 s - 300 s; secondary edge trimming is carried out after continuous annealing, and the bilateral edge trimming amount is ≥14 mm.

[0039] In the skin pass rolling step, the skin pass elongation is controlled at 0.6 - 1.2%.

[0040] The present invention will be described in detail below with reference to the embodiments.

[0041] The molten steel chemical compositions of the steel for single-layer precision welded pipes in each embodiment and comparative example are shown in Table 1, and the rest are Fe and inevitable impurity elements.

[0042] Table 1 Chemical compositions of embodiments and comparative examples, wt%

[0043] C Si Mn P S Als N Ti Example 1 0.004 0.02 0.28 0.012 0.009 0.045 0.002 0.045 Example 2 0.004 0.01 0.40 0.012 0.007 0.048 0.002 0.044 Example 3 0.004 0.03 0.34 0.013 0.008 0.044 0.002 0.035 Example 4 0.004 0.03 0.30 0.011 0.009 0.057 0.003 0.055 Example 5 0.003 0.03 0.33 0.011 0.009 0.046 0.002 0.045 Example 6 0.006 0.02 0.35 0.015 0.009 0.049 0.002 0.047 Comparative Example 1 0.004 0.02 0.24 0.013 0.009 0.046 0.003 0.043 Comparative Example 2 0.003 0.02 0.45 0.012 0.008 0.049 0.002 0.045 Comparative Example 3 0.004 0.03 0.32 0.014 0.009 0.045 0.003 0.030 Comparative Example 4 0.004 0.02 0.34 0.013 0.007 0.048 0.002 0.065 Comparative Example 5 0.004 0.02 0.35 0.012 0.008 0.046 0.002 0.045 Comparative Example 6 0.004 0.02 0.34 0.012 0.009 0.047 0.002 0.047

[0044] After continuous casting of molten steel, through hot rolling, pickling cold rolling, continuous annealing, and skin pass rolling processes, the main process parameters are shown in Table 2:

[0045] Table 2 Production process parameters

[0046]

[0047] The properties, same-plate difference, and usage effects of the steel for single-layer precision welded pipes produced in each example and comparative example are shown in Table 3.

[0048] Table 3 Mechanical properties of the finished product, same-plate difference, and usage effects

[0049]

[0050]

[0051] In Table 3, the evaluations of the welding effect and drawing effect of the steel in all examples and comparative examples were carried out under the same operation. After welding, there are no virtual solder joints at the weld, and when a uniform pressure is applied perpendicular to the weld and the welded pipe is pressed until the inner walls are in contact, and there are no cracks at the weld, it is considered to meet the requirements; after drawing, the diameter of the pipe is uniform, without wavy bends or breakage during drawing, it is considered to meet the requirements.

[0052] As can be seen from the above examples, the steel for single-layer precision welded pipes produced according to the solution of the present invention has mechanical properties that meet a yield strength of 200 - 240 MPa, a tensile strength of 340 - 400 MPa, and an elongation rate of ≥ 35%. It has high strength and good drawing properties, with a same-plate difference of ≤ 15 μm and good welding properties, effectively solving problems such as poor welding, weld leakage, and springback after bending of the welded pipe during the pipe manufacturing process. In the comparative examples, due to the lack of control in accordance with the requirements of the present invention, it was impossible to fully meet the above requirements.

[0053] The detailed description of a steel for single-layer precision welded pipes and its manufacturing method with reference to the examples above is illustrative rather than restrictive. Several examples can be listed within the defined scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the protection scope of the present invention.

Claims

1. A steel for single-layer precision welded pipe, characterized in that: The chemical composition and weight percentage of the steel for the single-layer precision welded pipe are: C 0.003-0.006%, Mn 0.28-0.40%, Als 0.025-0.075%, Ti0.035-0.055%, Si≤0.03%, P≤0.015%, S≤0.012%, N≤0.003%, and the rest are Fe and unavoidable impurities.

2. The steel for single-layer precision welded pipe according to claim 1, characterized in that: The metallographic structure of the steel for the single-layer precision welded pipe is ferrite.

3. The steel for single-layer precision welded pipe according to claim 1, characterized in that: The steel for the single-layer precision welded pipe has a yield strength of 200-240 MPa, a tensile strength of 340-400 MPa, and an elongation of ≥35%.

4. The method for manufacturing steel for single-layer precision welded pipe according to any one of claims 1 to 3, characterized in that: The manufacturing method comprises the following steps: molten iron smelting, slab continuous casting, hot rolling, pickling and cold rolling, continuous annealing, flattening and coiling.

5. The manufacturing method according to claim 4, characterized in that: In the hot rolling step, the slab heating furnace temperature is controlled at 1200°C to 1260°C, the holding time is ≥180min, and the final rolling temperature is controlled at 870°C to 930°C.

6. The manufacturing method according to claim 4, characterized in that: In the hot rolling step, the coiling temperature is controlled at 640°C to 680°C.

7. The manufacturing method according to claim 4, characterized in that: In the hot rolling step, the hot rolling convexity is controlled within a range of 10 μm to 30 μm, and the wedge shape is controlled within a range of -20 μm to 20 μm.

8. The manufacturing method according to claim 4, characterized in that: In the pickling and cold rolling step, the total cold rolling reduction ratio is ≥80%, and a trimming is performed after the cold rolling, and the double-sided trimming amount is ≥16 mm.

9. The manufacturing method according to claim 4, characterized in that: In the continuous annealing step, the annealing soaking temperature is controlled at 720° C. to 740° C., and the soaking time is 50s to 130s. After the continuous annealing, secondary trimming is performed, and the double-sided trimming amount is ≥14mm.

10. The manufacturing method according to claim 4, characterized in that: In the flattening step, the flattening elongation is controlled at 0.6-1.2%.

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