A high-corrosion-resistance precision seamless steel tube for new energy vehicles and a production process thereof

Abstract

This application relates to the field of metal material processing technology, and discloses a high corrosion-resistant precision seamless steel pipe for new energy vehicles and its production process. The process includes preparing a billet using steel with the stated chemical composition; hot rolling the billet to obtain a seamless steel pipe base; online controlled normalizing of the seamless steel pipe base, including heating and heat preservation; and after heat preservation, subjecting the base pipe to two-stage controllable cooling, which includes forced air cooling at a first cooling rate to a first predetermined temperature, followed by slow cooling at a second cooling rate to room temperature. Through the synergistic effect of specific chemical composition design and the two-stage controllable cooling process, the obtained steel pipe has a fine ferrite and pearlite structure, giving its matrix excellent corrosion resistance. Simultaneously, the fine grain strengthening effect combines high strength and good plasticity, and eliminates the need for a secondary surface anti-corrosion treatment process, simplifying the production flow.

Description

Technical Field

[0001] This invention relates to the field of metal material processing technology, specifically to a high corrosion-resistant precision seamless steel pipe for new energy vehicles and its production process. Background Technology

[0002] With the rapid development of the new energy vehicle industry, the performance requirements of its key components are increasing. Precision seamless steel pipes are key materials in areas such as battery temperature control systems and chassis structural components of new energy vehicles, and their reliability directly affects the safety and service life of the entire vehicle.

[0003] However, new energy vehicles operate in complex environments, often facing challenges such as humidity, alternating high and low temperatures, and corrosive media like de-icing agents in winter. This places stringent requirements on the corrosion resistance of steel pipes. Currently, the commonly used solution is to perform secondary anti-corrosion treatments on the surface of ordinary carbon steel seamless pipes, such as galvanizing, electrophoretic painting, or phosphating. This surface-treatment-based protection method has technical drawbacks: on the one hand, it adds extra production steps, leading to longer production cycles, increased energy consumption and manufacturing costs, and processes such as electroplating and phosphating generate waste acid and waste liquid, burdening the environment; on the other hand, the outer protective layer is easily damaged by bumps and scratches during transportation, assembly, or use. Once the protective layer is damaged, the steel substrate, lacking inherent corrosion resistance, will quickly undergo localized corrosion, causing pipeline leaks or structural failures, posing safety hazards.

[0004] Furthermore, to improve driving range, the lightweight design trend in new energy vehicles places higher demands on the strength of materials. Steel pipes produced using traditional processes struggle to simultaneously meet the dual standards of high strength and high toughness in terms of mechanical properties, limiting their application potential in lightweight structural design.

[0005] Therefore, developing a precision seamless steel pipe with high corrosion resistance, excellent comprehensive mechanical properties, and simplified production process is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a high corrosion-resistant precision seamless steel pipe for new energy vehicles and its manufacturing process, which solves the problem that existing seamless steel pipes for new energy vehicles rely excessively on surface treatment due to insufficient corrosion resistance of the base material, and that their comprehensive mechanical properties are difficult to meet lightweight requirements.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] The first aspect of this invention provides a high corrosion-resistant precision seamless steel pipe for new energy vehicles, which, by weight, is composed of the following components:

[0009] C: 0.08~0.15 parts;

[0010] Si: 0.20~0.45 parts;

[0011] Mn: 1.10~1.50 parts;

[0012] P: ≤0.020 copies;

[0013] S: ≤0.015 parts;

[0014] Cr: 0.60~1.10 parts;

[0015] Cu: 0.25~0.45 parts;

[0016] Mo:0.15~0.25 parts;

[0017] Nb: 0.02~0.05 parts;

[0018] The balance is Fe and unavoidable impurities.

[0019] In this chemical composition design, the technical roles of each element are as follows:

[0020] Cr, Cu, and Mo act synergistically as core corrosion-resistant elements. Cr forms a dense and stable Cr2O3 passivation film on the steel surface, which can prevent the contact between the corrosive medium and the steel substrate. Mo can further enhance the stability of the passivation film in a chloride-containing environment and inhibit the occurrence and development of pitting corrosion. Cu accumulates on the substrate surface under the action of corrosive media, which can effectively inhibit the cathodic process of corrosion reaction.

[0021] As a microalloying element, Nb precipitates in the form of Nb(C,N) carbonitrides during subsequent heat treatment. These precipitates can effectively pin austenite grain boundaries during the austenitization stage, inhibit austenite grain growth, and lay the foundation for obtaining a fine-grained structure.

[0022] C, Si, and Mn are elements that ensure the basic mechanical properties of steel pipes. Their content ranges are carefully selected to achieve the required strength without compromising the material's toughness and machinability. P and S, as impurity elements, are strictly controlled at low levels to reduce harmful inclusions within the material and decrease their potential to act as corrosion initiation points.

[0023] In one specific embodiment, the metallographic structure of the steel pipe is ferrite and pearlite of grade not lower than 9.0. The fine grains increase the total area of ​​the grain boundaries, homogenize the corrosion current density, and reduce the tendency for localized corrosion.

[0024] In one specific embodiment, the outer surface roughness Ra of the steel pipe is ≤0.8μm. The smooth surface reduces the adhesion of corrosive media and the microscopic sites for corrosion nucleation.

[0025] A second aspect of the present invention provides a manufacturing process for producing the above-mentioned high corrosion-resistant precision seamless steel pipes for new energy vehicles, the process comprising the following steps:

[0026] S1: Prepare tube blanks using steel with the above chemical composition;

[0027] S2: The tube blank is hot-rolled to obtain a seamless steel pipe mother tube;

[0028] S3: Perform online controlled normalizing on the seamless steel pipe mother tube, wherein the online controlled normalizing includes heating and heat preservation of the seamless steel pipe mother tube;

[0029] S4: After the insulation is completed, the seamless steel pipe mother pipe is subjected to two-stage controllable cooling. The two-stage controllable cooling includes forced air cooling in the first cooling stage and slow cooling in the second cooling stage.

[0030] In one specific embodiment, the heating in S3 involves heating the seamless steel pipe to a temperature range of 930°C to 970°C. This temperature range is designed to completely transform the steel pipe microstructure into austenite and partially dissolve Nb in the austenite, providing conditions for subsequent precipitation and grain refinement.

[0031] In one specific implementation, the heat preservation in S3 is calculated based on the steel pipe wall thickness t, and ranges from 1.2 min / mm·t to 1.8 min / mm·t. This heat preservation time ensures uniform temperature across the steel pipe cross-section and sufficient microstructure transformation.

[0032] In one specific embodiment, the heating and heat preservation in S3 are carried out under a protective atmosphere of nitrogen or a nitrogen-hydrogen mixture to prevent oxidation and decarburization of the steel pipe surface at high temperatures.

[0033] In one specific implementation, the two-stage controllable cooling in S4 has the following specific process parameters: Forced air cooling in the first cooling stage is performed at a cooling rate of 12℃ / s to 25℃ / s until the steel pipe temperature reaches 760℃ to 800℃; then, slow cooling in the second cooling stage is performed at a cooling rate of 1.5℃ / s to 4.0℃ / s. The technical mechanism of this cooling process is as follows: the rapid cooling in the first cooling stage allows austenite to quickly pass through the high-temperature zone, effectively inhibiting the growth of austenite grains and obtaining an undercooled austenite structure; the slow cooling in the second cooling stage allows the undercooled austenite to undergo a phase transformation at a greater degree of undercooling, transforming into fine ferrite and pearlite structures. Simultaneously, this cooling rate avoids the formation of non-equilibrium structures such as martensite, ensuring the material's plasticity and toughness.

[0034] In one specific embodiment, the tube blank is heated to 1150°C to 1250°C before hot rolling of S2.

[0035] In one specific embodiment, the process further includes a finishing step after S4: centerless grinding of the outer surface of the steel pipe to make its surface roughness Ra≤0.8μm.

[0036] In summary, this application includes at least one of the following beneficial technical effects:

[0037] 1. The seamless steel pipe of the present invention has excellent matrix corrosion resistance. This is due to the synergistic effect of Cr, Cu and Mo elements in the chemical composition, which form a dense and stable passivation film on the surface of the steel pipe; at the same time, the fine and uniform ferrite and pearlite microstructure obtained by the two-stage controllable cooling process reduces the tendency of micro-galvanic corrosion. This corrosion resistance mechanism from the inside out allows it to meet the long service life requirements of new energy vehicles in harsh environments such as humidity and salt spray without relying on external coatings.

[0038] 2. This invention, through the addition of the microalloying element Nb and the combination of an online controlled normalizing process, particularly the rapid cooling in the first stage effectively refines the austenite grains, while the slow cooling in the second stage promotes the formation of fine-grained ferrite and pearlite. This fine-grained structure, through the grain boundary strengthening effect, enables the steel pipe to maintain good plasticity and toughness while possessing higher strength, which helps to achieve lightweight design of automotive components.

[0039] 3. Because the steel pipe substrate itself has high corrosion resistance, the present invention eliminates the secondary surface treatment processes such as galvanizing and coating required in traditional processes. This not only shortens the production cycle and saves equipment investment and energy consumption, but also avoids the emission of pollutants such as waste acid and waste liquid that may be generated in electroplating and other processes, which meets the requirements of green and sustainable development of the manufacturing industry. Detailed Implementation

[0040] Example:

[0041] This invention provides a high corrosion-resistant precision seamless steel pipe for new energy vehicles and its manufacturing process, including...

[0042] Example 1

[0043] This embodiment provides a method for preparing a high corrosion-resistant precision seamless steel pipe, and the specific steps are as follows:

[0044] (1) The raw material preparation adopts the smelting process of electric furnace + ladle refining + vacuum degassing. The chemical composition is as follows: C 0.12 parts, Si 0.33 parts, Mn 1.30 parts, P 0.015 parts, S 0.010 parts, Cr 0.85 parts, Cu 0.35 parts, Mo 0.20 parts, Nb 0.04 parts, with the balance being Fe and unavoidable impurities. The qualified molten steel is continuously cast into round tube billets.

[0045] (2) Billet preparation and hot rolling: The above-mentioned round billet is heated to 1200°C in a heating furnace. After being kept at a uniform temperature, it is hot rolled by a piercing mill and a continuous rolling mill, and then prepared into a seamless steel pipe with a wall thickness of 5mm by a tension reducing mill.

[0046] (3) The steel pipe mother tube after normalizing and hot rolling is directly put into the online heat treatment line. Under the protective atmosphere of nitrogen-hydrogen mixed gas, it is heated to 950℃ in a walking beam induction furnace and held at this temperature for 7.5 minutes. After the holding period, the steel pipe is subjected to two-stage controllable cooling: first, forced air cooling is carried out at a cooling rate of 18℃ / s until the steel pipe temperature reaches 780℃; then it is transferred to a closed slow cooling channel and slowly cooled to room temperature at a cooling rate of 2.8℃ / s.

[0047] (4) Finishing: The cooled steel pipe is straightened, and then the outer surface of the steel pipe is precision ground by a centerless grinder to obtain a surface with Ra of 0.6μm.

[0048] Example 2

[0049] This embodiment provides a method for preparing a high corrosion-resistant precision seamless steel pipe, and the specific steps are as follows:

[0050] (1) The raw material preparation adopts a smelting process of electric furnace + ladle refining + vacuum degassing, and the chemical composition is as follows:

[0051] C 0.08 parts, Si 0.20 parts, Mn 1.10 parts, P 0.018 parts, S 0.012 parts, Cr 0.60 parts, Cu 0.25 parts, Mo 0.15 parts, Nb 0.02 parts,

[0052] The balance consists of Fe and unavoidable impurities. The qualified molten steel is then continuously cast into round tube billets.

[0053] (2) Tube blank preparation and hot rolling: The above round tube blank is heated to 1150°C in a heating furnace. After being kept at a uniform temperature, it is hot rolled by a piercing mill and a continuous rolling mill, and then prepared into a seamless steel tube with a wall thickness of 5mm by a tension reducing mill.

[0054] (3) The steel pipe mother tube after normalizing and hot rolling is directly put into the online heat treatment line. Under the protection of nitrogen, it is heated to 930°C in a gas-fired radiant tube heater and held at this temperature for 6 minutes. After the holding period, the steel pipe is subjected to two-stage controllable cooling: first, forced air cooling is performed at a cooling rate of 12°C / s until the steel pipe temperature reaches 760°C; then, it is transferred to a closed slow cooling channel and slowly cooled to room temperature at a cooling rate of 1.5°C / s.

[0055] (4) Finishing: The cooled steel pipe is straightened, and then the outer surface of the steel pipe is precision ground by a centerless grinder to obtain a surface with Ra of 0.8μm.

[0056] Example 3

[0057] This embodiment provides a method for preparing a high corrosion-resistant precision seamless steel pipe, and the specific steps are as follows:

[0058] (1) The raw material preparation adopts the smelting process of electric furnace + ladle refining + vacuum degassing. The chemical composition is as follows: C 0.15 parts, Si 0.45 parts, Mn 1.50 parts, P 0.010 parts, S 0.008 parts, Cr 1.10 parts, Cu 0.45 parts, Mo 0.25 parts, Nb 0.05 parts, with the balance being Fe and unavoidable impurities. The qualified molten steel is continuously cast into round tube billets.

[0059] (2) Tube blank preparation and hot rolling: The above round tube blank is heated to 1250°C in a heating furnace. After being kept at a uniform temperature, it is hot rolled by a piercing mill and a continuous rolling mill, and then prepared into a seamless steel tube with a wall thickness of 5mm by a tension reducing mill.

[0060] (3) The steel pipe mother tube after normalizing and hot rolling is directly put into the online heat treatment line. Under the protective atmosphere of nitrogen-hydrogen mixed gas, it is heated to 970℃ in a walking beam induction furnace and held at this temperature for 9 minutes. After the holding period, the steel pipe is subjected to two-stage controllable cooling: first, forced air cooling is performed at a cooling rate of 25℃ / s until the steel pipe temperature reaches 800℃; then it is transferred to a closed slow cooling channel and slowly cooled to room temperature at a cooling rate of 4.0℃ / s.

[0061] (4) Finishing: The cooled steel pipe is straightened, and then the outer surface of the steel pipe is precision ground using a centerless grinder to obtain a surface with Ra of 0.5μm. At the same time, the inner surface of the steel pipe is honed.

[0062] Comparative Example 1: Compared with Example 1, the difference is that it was prepared using national standard 20# steel as raw material, and its chemical composition is: C 0.17-0.24 parts, Si 0.17-0.37 parts, Mn 0.35-0.65 parts, and the rest are the same.

[0063] Comparative Example 2: Compared with Example 1, the difference is that in the online controlled normalizing step, after the heat preservation is completed, the seamless steel pipe mother tube is naturally cooled to room temperature in still air, without two-stage controlled cooling, and the rest are the same.

[0064] Comparative Example 3: Compared with Example 1, the difference is that the chemical composition of the raw materials does not include the three corrosion-resistant elements Cr, Cu and Mo, while the rest are the same.

[0065] Comparative Example 4: Compared with Example 1, the difference is that Nb element is not added to the chemical composition of the raw materials, but all other aspects are the same.

[0066] Test Example 1: Metallographic Structure Comparison Analysis

[0067] 1. Experimental Procedure

[0068] (1) Sampling: Take 10 mm long pipe section samples from each group of steel pipes prepared in Examples 1-3 and Comparative Examples 1-4 in a direction perpendicular to the pipe axis.

[0069] (2) Mounting: The cut pipe section sample is placed in the mounting machine and mounted with thermosetting phenolic resin at 150℃ and 20MPa to make a cylindrical metallographic sample with a diameter of 30mm. The surface to be observed is the cross-section of the steel pipe.

[0070] (3) Grinding: Use 240#, 400#, 800#, 1200# and 2000# wet sandpaper to manually or automatically grind the surface of the sample to be observed until the scratches from the previous pass are completely removed. Rotate the sample 90 degrees each time you change the sandpaper.

[0071] (4) Polishing: On a polishing machine, the sample is initially polished using a polishing cloth containing diamond polishing paste with a particle size of 3μm. Then, the polishing cloth is replaced and the sample is finally polished using diamond polishing paste with a particle size of 1μm until the surface to be observed presents a scratch-free mirror finish.

[0072] (5) Etching: Clean the polished sample surface with alcohol and blow it dry. Then immerse it in a 4% (v / v) nitric acid alcohol solution for chemical etching. The etching time is controlled between 5 and 10 seconds until the tissue is clearly visible. After etching, immediately rinse with water and clean with alcohol and blow dry.

[0073] (6) Observation and rating: The prepared metallographic sample was placed under an optical microscope and its microstructure was observed at 500x magnification. Subsequently, according to the comparative method in GB / T6394-2017 "Method for Determination of Average Grain Size of Metals", the microstructure in the field of view was compared with the standard spectrum to evaluate its ferrite grain size level. Five different fields of view were randomly selected for rating each sample, and the average value was taken as the final result.

[0074] 2. Experimental data are shown in Table 1.

[0075] Table 1. Metallographic structure and grain size test results of each embodiment and comparative example.

[0076] Sample number Metallographic structure description Grain size level Example 1 Fine ferrite and pearlite, uniform microstructure 10 Example 2 Fine ferrite and pearlite, evenly distributed 9.5 Example 3 Extremely fine ferrite and pearlite, diffusely distributed 10.5 Comparative Example 1 Ferrite + pearlite, with banded structure and uneven grain size. 6.5 Comparative Example 2 Ferrite grains are relatively coarse, and some are blocky. 7.5 Comparative Example 3 Fine ferrite and pearlite, uniform microstructure 9.5 Comparative Example 4 Ferrite + pearlite, with coarser grains than in Example 1. 7

[0077] As shown in Table 1, the steel pipes prepared in Examples 1, 2, and 3 all exhibited a fine ferrite and pearlite microstructure of grade 9.5 or higher. This is due to the two-stage controllable cooling process employed in their production: the forced air cooling in the first cooling stage, with a cooling rate of 12°C / s to 25°C / s, allowed austenite to rapidly pass through the high-temperature zone, increasing the nucleation rate of supercooled austenite and inhibiting grain growth; subsequently, the slow cooling in the second cooling stage ensured that the phase transformation was completed under a larger degree of supercooling, thereby precipitating fine ferrite and pearlite.

[0078] Comparing the results of Example 1 (grain size grade 10.0) and Comparative Example 2 (grain size grade 7.5), both have the same chemical composition, but Comparative Example 2 used natural air cooling. Under natural cooling conditions, the steel pipe spends a longer time in the high-temperature zone, resulting in sufficient austenite grain growth and a smaller phase transformation driving force, ultimately transforming into a coarse ferrite and pearlite structure. This set of comparative data shows that two-stage controlled cooling is a key process step in obtaining a fine-grained structure.

[0079] Comparing the results of Example 1 (grain size grade 10.0) and Comparative Example 4 (grain size grade 7.0), both had the same production process parameters, but Comparative Example 4 did not contain Nb. During the austenitization process at 930°C to 970°C, Nb partially dissolved and precipitated as Nb(C,N) particles. These particles pinned the austenite grain boundaries, effectively hindering austenite grain growth. Due to the lack of Nb pinning effect, Comparative Example 4, even with the same two-stage cooling process, had coarser initial austenite grains, resulting in a significantly coarser final ferrite-pearlite microstructure than Example 1. This comparison confirms that the synergistic effect of specific chemical composition and specific production process is a necessary condition for achieving microstructure refinement.

[0080] Test Example 2: Corrosion Resistance Comparison Test

[0081] 1. Experimental Procedure

[0082] (1) Sample preparation: 150 mm long pipe sections were cut from the steel pipes prepared in Examples 1-3 and Comparative Examples 1-4 as samples. The sample surface was ultrasonically cleaned and degreased with acetone or ethanol to remove oil stains, and then dried. The two end faces of the sample were sealed with corrosion-resistant epoxy resin or paraffin.

[0083] (2) Test Equipment and Conditions: The test was conducted in a neutral salt spray chamber according to GB / T10125-2021 standard. The test parameters were set as follows: Sodium chloride solution concentration: 50 g / L ± 5 g / L; Solution pH: 6.5 to 7.2; Chamber temperature: 35℃ ± 2℃; Spray deposition rate: 1.0 mL / (h·80 cm²) to 2.0 mL / (h·80 cm²); Spray method: continuous spraying.

[0084] (3) Sample placement: Place the prepared sample on the sample rack of the test chamber, so that the tube axis of the sample forms an angle of 20°±5° with the vertical direction.

[0085] (4) Test process and observation: Start the test equipment and begin continuous spraying. Every 24 hours, briefly open the test chamber and observe the surface condition of the sample with the naked eye or with the aid of a magnifying glass, and record the time when the first red rust spot with a diameter greater than 0.5 mm first appears (recorded as the time of red rust appearance).

[0086] (5) Corrosion rating: After 720 hours of testing, all samples were removed, the surface was gently rinsed with running water and dried. The corrosion area was rated according to GB / T6461-2002 standard to determine the protection level.

[0087] 2. Experimental data are shown in Table 2.

[0088] Table 2. Neutral salt spray test results for each example and comparative example.

[0089] Sample number Time to red rust appearance (h) Protection rating (Rp) after 720 hours Example 1 740 10 Example 2 710 9 Example 3 785 10 Comparative Example 1 18 3 Comparative Example 2 450 6 Comparative Example 3 36 4 Comparative Example 4 580 7

[0090] The test data in Table 2 show that the samples from Examples 1, 2, and 3 all took more than 700 hours to develop red rust, significantly longer than all the comparative samples. This result indicates that the steel pipe of the present invention has higher corrosion resistance. Through comparative analysis, this effect is attributed to the combined effect of chemical composition and microstructure.

[0091] Comparative Example 1 and Comparative Example 3 share the same production process, but Comparative Example 3 lacks Cr, Cu, and Mo elements, and its red rust formation time is only 36 hours. This indicates that the combined addition of Cr, Cu, and Mo is the chemical basis for achieving corrosion resistance. Cr forms a dense passivation film on the steel surface, Mo enhances the stability of this passivation film, while Cu accumulates at defects in the early stages of corrosion, inhibiting further corrosion. Without these elements, the steel substrate cannot form an effective passivation protective layer, leading to rapid corrosion in salt spray environments.

[0092] Comparative Example 1 and Comparative Example 2 have identical chemical compositions. However, due to the use of natural air cooling in Comparative Example 2, its microstructure consists of coarse ferrite and pearlite. The time to red rust on the sample in Example 1 was 740 hours, while that in Comparative Example 2 was 450 hours. This indicates that the fine, uniform microstructure obtained by the two-stage controlled cooling process can further improve the corrosion resistance of the material. The mechanism is that the fine grains increase the total grain boundary area, making the electrochemical activity on the material surface more uniform, reducing the probability of forming micro-corrosion cells with large potential differences, thereby inhibiting the initiation of local pitting corrosion.

[0093] Test Example 3: Comparison Test of Mechanical Properties

[0094] 1. Experimental Procedure

[0095] (1) Sample preparation: pipe sections were cut along the pipe axis from the steel pipes prepared in Examples 1-3 and Comparative Examples 1-4, and processed into P4 type full-section pipe tensile specimens in accordance with the requirements of GB / T228.1-2021 standard.

[0096] (2) Dimensional measurement: Use vernier calipers and wall thickness gauges to accurately measure the inner and outer diameters of the original gauge length of each sample and calculate its original cross-sectional area S0.

[0097] (3) Tensile test: At room temperature, the specimen is clamped on an electronic universal testing machine. The stress rate is 10 MPa / s to the yield point, and then the strain rate is continued at 0.002 s⁻¹ until the specimen breaks.

[0098] (4) Data Acquisition and Calculation: The testing machine automatically records the force-displacement data throughout the process. Based on the recorded data, the tensile strength, yield strength, and elongation after fracture of each specimen are calculated and obtained. Three specimens are tested in each group, and the average value of the results is taken.

[0099] 2. Experimental data are shown in Table 3.

[0100] Table 3. Mechanical property test results of each embodiment and comparative example.

[0101] Sample number Yield strength ReL (MPa) Tensile strength Rm (MPa) Elongation after fracture A (%) Example 1 453 582 27.8 Example 2 425 549 29.5 Example 3 481 615 26.2 Comparative Example 1 265 433 32.1 Comparative Example 2 382 508 30.3 Comparative Example 3 435 565 28.1 Comparative Example 4 360 495 31.5

[0102] The test data in Table 3 show that the steel pipes prepared in Examples 1, 2, and 3 have higher yield strength and tensile strength than all comparative sample samples (except for Comparative Example 1, whose higher elongation after fracture is an inherent characteristic of low-carbon steel), while maintaining an elongation after fracture of not less than 26%. This result indicates that the steel pipe of the present invention achieves an effective balance between strength and plasticity in its mechanical properties, the mechanism of which is rooted in the fine-grain strengthening effect obtained through the synergistic control of composition and process.

[0103] Comparing the data of Example 1 (yield strength 453 MPa) and Comparative Example 2 (yield strength 382 MPa), under the condition of completely identical chemical composition, Example 1, which uses a two-stage controlled cooling process, has a significantly higher strength than Comparative Example 2, which uses natural air cooling. This is because the two-stage controlled cooling process increases the supercooling degree of the austenite-to-ferrite transformation through the first stage of forced air cooling, thereby increasing the nucleation rate, inhibiting grain growth, and ultimately obtaining fine ferrite grains. According to the Hall-Petch relation, the smaller the grain size, the higher the yield strength of the material.

[0104] Comparing the data of Example 1 (yield strength 453 MPa) and Comparative Example 4 (yield strength 360 MPa), both used the same heat treatment process, but Comparative Example 4 did not contain Nb. The strength of Example 1 was significantly higher than that of Comparative Example 4, confirming the effect of Nb microalloying. During the normalizing heating stage from 930°C to 970°C, the carbonitrides formed by Nb were dispersed at the austenite grain boundaries, effectively pinning the grain boundaries and inhibiting austenite grain growth. This resulted in the initial austenite grains of Example 1 being refined before entering the two-stage cooling process. Therefore, the final strength improvement was the result of the combined effect of two mechanisms: the initial grain refinement brought about by Nb and the phase transformation microstructure refinement brought about by the two-stage controlled cooling.

Claims

1. A high corrosion-resistant precision seamless steel pipe for new energy vehicles, characterized in that, It consists of the following components in parts by weight: C: 0.08~0.15 parts; Si: 0.20~0.45 parts; Mn: 1.10~1.50 parts; P: ≤0.020 copies; S: ≤0.015 parts; Cr: 0.60~1.10 parts; Cu: 0.25~0.45 parts; Mo:0.15~0.25 parts; Nb: 0.02~0.05 parts; The balance is Fe and unavoidable impurities.

2. The high corrosion-resistant precision seamless steel pipe for new energy vehicles according to claim 1, characterized in that, The metallographic structure of the steel pipe is ferrite and pearlite of grade not lower than 9.

0.

3. The high corrosion-resistant precision seamless steel pipe for new energy vehicles according to claim 2, characterized in that, The outer surface roughness Ra of the steel pipe is ≤0.8μm.

4. A production process for manufacturing high corrosion-resistant precision seamless steel pipes for new energy vehicles as described in any one of claims 1-2, characterized in that, Includes the following steps: S1: Steel blanks are prepared using steel with equal proportions of raw material components; S2: The tube blank is hot-rolled to obtain a seamless steel pipe mother tube; S3: Perform online controlled normalizing on the seamless steel pipe mother tube, wherein the online controlled normalizing includes heating and heat preservation of the seamless steel pipe mother tube; S4: After the insulation is completed, the seamless steel pipe mother pipe is subjected to two-stage controllable cooling. The two-stage controllable cooling includes forced air cooling in the first cooling stage and slow cooling in the second cooling stage.

5. The production process according to claim 4, characterized in that, The two-stage controllable cooling in S4 has the following specific process parameters: forced air cooling in the first cooling stage is performed at a cooling rate of 12℃ / s to 25℃ / s until the steel pipe temperature reaches 760℃ to 800℃. Then, the second cooling stage is carried out at a slow cooling rate of 1.5℃ / s to 4.0℃ / s.

6. The production process according to claim 4, characterized in that, The heating in S3 involves heating the seamless steel pipe base to a temperature range of 930°C to 970°C.

7. The production process according to claim 6, characterized in that, The insulation time is calculated based on the steel pipe wall thickness t, and is between 1.2 min / mm·t and 1.8 min / mm·t.

8. The production process according to claim 6, characterized in that, The heating and heat preservation are carried out under a protective atmosphere of nitrogen or a nitrogen-hydrogen mixture.

9. The production process according to claim 4, characterized in that, Before the hot rolling of S2, the tube blank is heated to 1150°C to 1250°C.

10. The production process according to claim 4, characterized in that, The process further includes a finishing step after S4: The outer surface of the steel pipe is centerlessly ground to achieve a surface roughness Ra≤0.8μm.