A steel material having an ultra-high strength of 2000 MPa or more, a hot expanded tube member, a welded tube, and a production process

Abstract

This application provides a steel material with ultra-high strength of 2000MPa and above, a hot gas expansion tube component, a welded pipe, and a manufacturing process. The component, welded pipe, and steel material, by mass percentage, contain: C: 0.35~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%; the sum of Si, Cr, and Al content is 1.0~2.2%, and (Si+Cr) / Al is 2.0~3.5; the thickness t, alloying elements, and critical cooling rate v of the component, welded pipe, or steel material are also specified. C Satisfies the formula: , v C With a temperature range of 10~21℃ / s, the hot gas expansion tube component of this invention possesses ultra-high strength and good toughness.

Description

Technical Field

[0001] This invention relates to the field of steel materials technology, and in particular to a steel material with ultra-high strength of 2000MPa and above, a hot gas expansion tube component, a welded pipe, and a manufacturing process. Background Technology

[0002] As the automotive industry moves towards lightweighting, high safety, and low energy consumption, the requirements for the integration, connection strength, and space utilization of tubular components with closed cross-sections in vehicle body structures are constantly increasing. Because hot gas expansion tube technology can overcome the forming limits of cold-state processes, simplify structural design, and improve material utilization, it has become an important technological approach to solving these problems.

[0003] Hot gas expansion tube technology is an advanced process that integrates thermoplastic forming and internal high-pressure bulging principles. Its core technology utilizes the synergistic effect of heating and internal pressure to heat a tubular blank to full austenitization, causing plastic deformation. This allows the blank to expand radially and tightly conform to a pre-set mold cavity, resulting in automotive components with specific cross-sectional shapes, dimensional accuracy, and high strength. The main production process for hot gas expansion components includes welded tube bending and preforming, welded tube heating, gas expansion forming, and cooling and setting. If the welded tube surface lacks a coating, shot peening and oiling are required after cooling and setting to remove surface oxide scale generated during the forming process. Currently, hot gas expansion tube technology is mainly used in the manufacturing of critical load-bearing and safety components such as vehicle body frames (e.g., sill beams, pillars, chassis longitudinal beams) and anti-collision systems (e.g., front and rear anti-collision beams, energy-absorbing boxes), and has become one of the key processes for achieving lightweight and high-strength automotive structures.

[0004] However, with the increasing demands on the performance of hot gas expansion tube components, the performance requirements for the welded pipe blanks used in their manufacture are also becoming increasingly stringent. Through extensive production practice, the inventors have discovered that during the cooling and shaping stage of hot gas expansion tube components, the outer contour is cooled via a mold cavity with cooling water channels, while the interior relies on high-pressure cooling gas to dissipate heat through convection. This internal cooling method often suffers from insufficient cooling rate, leading to incomplete martensitic transformation in the wall thickness direction and affecting the final mechanical properties of the component. Furthermore, during the welding and pipe-making stage, a decarburized softening zone easily forms near the weld fusion line. After hot gas expansion, this zone becomes a weak point in the entire component. When the hot gas expansion tube component is subjected to impact and compression, the decarburized softening zone is prone to deformation and stress concentration, leading to weld cracking and failure. Furthermore, the weld seam area of ​​the welded pipe is dominated by hard and brittle quenched martensite, which makes it prone to cracking during the cold deformation process of hot gas expansion tube components preformed from the welded pipe. Cracked welded pipes cannot continue gas expansion forming due to air leakage. Simultaneously, fluctuations in the properties of the welded pipe billet itself affect the springback behavior of the preformed component, thus impacting the forming accuracy and shape stability of the hot gas expansion tube component. These numerous problems severely restrict the production and performance improvement of hot gas expansion tube components and the welded pipes used to manufacture them. Therefore, it is urgent to propose a solution for hot gas expansion tube components, welded pipes, steel, and manufacturing processes. This solution involves optimizing the alloy composition and manufacturing process of the steel, welded pipes, and components to meet the increasingly stringent comprehensive performance requirements of automotive structural components. Summary of the Invention

[0005] In view of this, the present invention aims to propose a steel material with ultra-high strength of 2000MPa and above and its hot gas expansion tube component, to solve the key problem of reduced weld hardness while ensuring that the base material of the hot gas expansion tube component has a strength of 2000MPa and above; and to provide a process for preparing welded pipes with this material, which solves the problems of poor cold formability of welded pipes and poor springback stability of welded pipes.

[0006] To achieve the aforementioned objectives, the technical solution of this application is implemented as follows: In a first aspect, the present invention provides a hot gas expansion tube component, wherein the chemical composition of the hot gas expansion tube component, by mass percentage, comprises: C: 0.35~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of the contents of Si, Cr, and Al is 1.0~2.2%, and the (Si+Cr) / Al ratio is 2.0~3.5; The weld of the hot gas expansion tube component includes a decarburized softened zone and a non-softened zone, and the ratio of the average hardness of the decarburized softened zone to the non-softened zone is ≥0.80.

[0007] Furthermore, the wall thickness t of the hot gas expansion tube component and the alloy element composition satisfy the following formula: , Among them, the critical cooling rate v C The temperature range is 10~21℃ / s, and the wall thickness t is 1.2~3.0mm.

[0008] Furthermore, the critical cooling rate v C The temperature is 11~19℃ / s.

[0009] Furthermore, the ratio of the average hardness of the decarburized softened zone to that of the non-softened zone is ≥0.85.

[0010] Furthermore, the chemical composition of the hot gas expansion tube component, by mass percentage, comprises: C: 0.35~0.41%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.3~2.2%, and (Si+Cr) / Al is 2.0~3.5.

[0011] Furthermore, the chemical composition of the hot gas expansion tube component, by mass percentage, comprises: C: 0.41~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.0~2.2%, and (Si+Cr) / Al is 2.0~3.5.

[0012] Furthermore, the surface of the hot gas expansion tube component has an aluminum alloy coating with a thickness of 15~40μm.

[0013] Furthermore, the yield strength of the hot gas expansion tube component is 1550~1720MPa, the tensile strength is 1950~2300MPa, the elongation after fracture is ≥5%, the average hardness is ≥570HV, and the standard deviation of the average hardness in different regions is ≤30HV.

[0014] Furthermore, the volume content of martensite in the base material of the hot gas expansion tube component is ≥95%, preferably ≥97%; the volume content of ferrite is ≤3%, preferably ≤1%, and even more preferably ≤0.5%.

[0015] Furthermore, the chemical composition of the hot gas expansion tube component, by mass percentage, further comprises: W: 0.01~0.20%, Cu: 0.01~0.20%, Co: 0.01~0.50%, Ta: 0.001~0.100%, Sn: 0.001~0.050%, Sb: 0.001~0.050%, As: 0.001~0.050%, Mg: 0.001~0.010%, Ca: 0.001 The content of Ni, Mo, W, Cu, Co, Ta, Sn, Sb, Ca, As, Mg, Zr, and REM is 0.001~0.500%, with Zr content of 0.001~0.100%, O content of 0.001~0.020%, REM content of 0.001~0.050%, P content of 0.001~0.020%, S content of 0.001~0.010%, and N content of 0.001~0.010%, and the sum of the contents of Ni, Mo, W, Cu, Co, Ta, Sn, Sb, Ca, As, Mg, Zr, and REM is 0.001~0.500%.

[0016] Secondly, the present invention also provides a welded pipe for preparing hot gas expansion tube components, wherein the chemical composition of the welded pipe, by mass percentage, comprises: C: 0.35~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of the contents of Si, Cr, and Al is 1.0~2.2%, and the (Si+Cr) / Al ratio is 2.0~3.5; The combined volume content of ferrite and pearlite in the base material of the welded pipe is ≥90%, and the weld of the welded pipe contains ≥90% tempered martensite and ≤3% retained austenite by volume.

[0017] Furthermore, the wall thickness t of the welded pipe and the alloy element composition satisfy the following formula: , Among them, the critical cooling rate v C The temperature range is 10~21℃ / s, and the wall thickness t is 1.2~3.0mm; Furthermore, the critical cooling rate v C The temperature is 11~19℃ / s.

[0018] Furthermore, the chemical composition of the welded pipe, by mass percentage, comprises: C: 0.35~0.41%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.3~2.2%, and (Si+Cr) / Al is 2.0~3.5.

[0019] Furthermore, the chemical composition of the welded pipe, by mass percentage, comprises: C: 0.41~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.0~2.2%, and (Si+Cr) / Al is 2.0~3.5.

[0020] Furthermore, the surface of the welded pipe has an aluminum alloy coating with a thickness of 10~30μm.

[0021] Furthermore, the welded pipe has a yield strength of 400~550MPa, a tensile strength of 550~800MPa, and an elongation after fracture of ≥22%.

[0022] Furthermore, the average hardness of the base material of the welded pipe is ≤250HV, the weld of the welded pipe includes a decarburized softened zone and a non-softened zone, the ratio of the average hardness of the decarburized softened zone to the non-softened zone is ≥0.85, and the average hardness of the non-softened zone is ≤550HV.

[0023] Furthermore, the ratio of the average hardness of the decarburized softened zone to that of the non-softened zone is ≥0.90.

[0024] Furthermore, the chemical composition of the welded pipe, by mass percentage, further comprises: W: 0.01~0.20%, Cu: 0.01~0.20%, Co: 0.01~0.50%, Ta: 0.001~0.100%, Sn: 0.001~0.050%, Sb: 0.001~0.050%, As: 0.001~0.050%, Mg: 0.001~0.010%, Ca: 0.001~0.050%. The content of the following components is 0.010%, Zr: 0.001~0.100%, O: 0.001~0.020%, REM: 0.001~0.050%, P: 0.001~0.020%, S: 0.001~0.010%, N: 0.001~0.010%, and the sum of the contents of Ni, Mo, W, Cu, Co, Ta, Sn, Sb, Ca, As, Mg, Zr, and REM is 0.001~0.500%.

[0025] Thirdly, the present invention also provides a manufacturing process for a hot gas expansion tube component, comprising the following steps: S1. Preforming The welded pipe used for preparing the hot gas expansion tube component of the second aspect of the present invention is subjected to mechanical preforming treatment. S2. Austenitizing heating Heat the welded pipe to 850~1050℃; S3. Air Inflation Molding and Mold Cooling The heated welded pipe is placed in a mold, high-pressure nitrogen is introduced into the welded pipe to form it, and it is then demolded after cooling to below 100°C in the mold to obtain a hot gas expanded pipe component.

[0026] Furthermore, for welded pipes with aluminum alloy coatings, the dew point is controlled below -15℃ during the austenitizing heating process.

[0027] Furthermore, after the hot gas expansion tube components are installed and welded, they are coated and baked. The baking process involves heating at a temperature of 150~180℃ and holding for 15~50 minutes.

[0028] Fourthly, the present invention also provides a steel for preparing hot gas expansion tube components and welded pipes, wherein the chemical composition of the steel, by mass percentage, comprises: C: 0.35~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of the contents of Si, Cr, and Al is 1.0~2.2%, and the (Si+Cr) / Al ratio is 2.0~3.5.

[0029] Furthermore, the plate thickness t of the steel and the alloy element composition satisfy the following formula: lnv C =5.0-3.5C-0.8Mn-0.3Si-0.6Cr-0.4Ni-1.1Mo-1.8B+0.5lnt, Among them, the critical cooling rate v C The speed is 10~21℃ / s.

[0030] Furthermore, the critical cooling rate v C The temperature is 11~19℃ / s.

[0031] Furthermore, the chemical composition of the welded pipe, by mass percentage, comprises: C: 0.35~0.41%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.3~2.2%, and (Si+Cr) / Al is 2.0~3.5.

[0032] Furthermore, the chemical composition of the welded pipe, by mass percentage, comprises: C: 0.41~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.0~2.2%, and (Si+Cr) / Al is 2.0~3.5.

[0033] Furthermore, the steel surface has an aluminum alloy coating with a thickness of 10~30μm.

[0034] Furthermore, the chemical composition of the steel, by mass percentage, further comprises: W: 0.01~0.20%, Cu: 0.01~0.20%, Co: 0.01~0.50%, Ta: 0.001~0.100%, Sn: 0.001~0.050%, Sb: 0.001~0.050%, As: 0.001~0.050%, Mg: 0.001~0.010%, Ca: 0.001~0.050%. The content of the following components is 0.010%, Zr: 0.001~0.100%, O: 0.001~0.020%, REM: 0.001~0.050%, P: 0.001~0.020%, S: 0.001~0.010%, N: 0.001~0.010%, and the sum of the contents of Ni, Mo, W, Cu, Co, Ta, Sn, Sb, Ca, As, Mg, Zr, and REM is 0.001~0.500%.

[0035] Fifthly, the present invention also provides a process for preparing a welded pipe for manufacturing a hot gas expansion tube component, comprising the following steps: S1. Steel slitting and roll forming The steel is slit according to the required diameter of the welded pipe, and the slit steel is rolled into a pipe blank using a roll forming die; the steel used in the fourth aspect of this invention is the steel used to prepare welded pipes. S2. High-frequency welding The tube blank interface is heated and welded using an induction coil, and weld defects inside and outside the welded tube are removed after welding. S3. Post-weld online tempering The welded pipe is subjected to online tempering treatment at a temperature of 400~600℃; S4. Fixed diameter and fixed length Adjust the diameter of the welded pipe to the target diameter and cut it to a fixed length.

[0036] Compared with the prior art, the present application provides a steel material with ultra-high strength of 2000MPa and above, a hot gas expansion tube component, a welded pipe, and a manufacturing process, which have at least the following beneficial effects: (1) Through extensive experiments, this invention has determined the quantitative calculation formula relationship between the wall thickness, alloy element composition and critical cooling rate of hot gas expansion tube components. By selecting the optimal combination of component wall thickness and critical cooling rate, the carbon content is controlled at 0.35~0.50%, and the alloy element compositions such as C, Mn, Si, Cr, Ni, Mo, and B are optimized according to the calculation formula. Thus, a hot gas expansion tube component with ultra-high strength of over 2000MPa and good toughness is obtained.

[0037] (2) By controlling the content of Si, Cr and Al elements, the present invention achieves hardness control of the decarburized softened zone of the weld of the hot gas expansion tube component. By controlling the average hardness ratio of the decarburized softened zone to the non-softened zone to ≥0.80, when the hot gas expansion tube component is subjected to a three-point bend test, even if the pressure head displacement reaches 12mm, it can still ensure that the weld of the component does not fail and crack. Thus, the weld of the hot gas expansion forming component has excellent performance and is not prone to premature cracking and failure when subjected to collision and extrusion.

[0038] (3) Based on a carbon content of 0.35~0.50%, this invention implements online tempering treatment after welding and strictly controls the tempering temperature to actively regulate the microstructure of the weld in the welded pipe. The microstructure of the weld is controlled to be mainly tempered martensite with a certain amount of residual austenite, so that the tempered martensite content is ≥90% and the residual austenite content is controlled to ≤3%. This improves the cold formability of the weld by increasing the tempered martensite content, so that the hardness of the non-softened zone of the weld is ≤550HV, ensuring that no cracking occurs at the weld when the welded pipe is bent.

[0039] (4) Based on a carbon content of 0.35~0.50%, this invention controls the yield strength of the welded pipe to be 400~550MPa, the standard deviation of the yield strength to be ≤35MPa, the tensile strength to be 550~800MPa, and the elongation after fracture to be ≥22%, so that the welded pipe has stable overall tensile properties, thereby ensuring the rebound stability of the welded pipe during the bending process and avoiding rebound fluctuations after bending. Attached Figure Description

[0040] To more clearly illustrate the technical solutions of the embodiments of this invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 This is the EBSD map of the weld in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the test locations for the weld hardness distribution of the present invention; Figure 3 This is a schematic diagram of the hot gas expansion tube component of the present invention; Figure 4 The microstructure of the parent material of the hot gas expansion tube component in Embodiment 2 of the present invention; Figure 5 The microstructure of the parent material of the hot gas expansion tube component in Comparative Example 1 of this invention is shown. Detailed Implementation

[0042] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. It should be noted that the described embodiments are only some embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0043] Unless otherwise stated, all temperatures described herein are in degrees Celsius (°C), all component contents are expressed as weight percentages (wt%), and all tissue structure contents are expressed as volume percentages (vol%). When numerical ranges are mentioned in the specification and claims, the upper and lower limits of the range are included by default. Preferred technical solutions can be freely combined as needed, unless specifically specified. Those skilled in the art should understand that the specific data and parameters described in the embodiments are illustrative and do not constitute a limitation of the invention. In the following embodiments and comparative examples, the equipment involved are all standardized equipment known in the art, which can be obtained commercially or prepared using conventional techniques.

[0044] In a first aspect, the present invention provides a hot gas expansion tube component, wherein the chemical composition of the hot gas expansion tube component, by mass percentage, comprises: C: 0.35~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of the contents of Si, Cr, and Al is 1.0~2.2%, and the (Si+Cr) / Al ratio is 2.0~3.5; The weld of the hot gas expansion tube component includes a decarburized softened zone and a non-softened zone, and the ratio of the average hardness of the decarburized softened zone to the non-softened zone is ≥0.80.

[0045] As an optional implementation, the ratio of the average hardness of the decarburized softened zone to that of the non-softened zone is ≥0.85.

[0046] Regarding the chemical composition of the hot gas expansion tube component of this invention, carbon is an important interstitial solid solution strengthening element in steel, enabling the material to achieve ultra-high strength and hardness. Therefore, this invention adds more than 0.35% carbon to ensure that the hot gas expansion tube component has a strength of 2000 MPa or higher. However, adding too much carbon will promote the formation of brittle twinned martensite, severely deteriorating toughness. Therefore, this invention preferably controls the carbon content to 0.35~0.50%. As an optional embodiment, the carbon content in the hot gas expansion tube component of this invention is preferably 0.41~0.50%, which ensures that the yield strength of the hot gas expansion tube component reaches more than 1650 MPa, the tensile strength reaches more than 2100 MPa, and the average hardness of different regions is ≥600 HV, exhibiting even better performance.

[0047] Manganese (Mn) is an economical and effective element for improving the hardenability of steel. The addition of Mn significantly expands the austenite phase region, thus affecting the martensitic transformation process. Therefore, an appropriate amount of Mn is commonly added to hot-stamped steel. However, when the carbon (C) content reaches the range of 0.35-0.50%, the addition of Mn easily leads to banded segregation in the material, causing uneven distribution of C, Mn, and other elements along the thickness direction of the steel plate, thereby worsening the bending toughness of the final hot-expansion tube component. Simultaneously, severe banded segregation also affects the uniformity of weld hardness in high-frequency welded pipes, thus impacting the cold forming performance of the weld. Therefore, this invention preferably controls the Mn content to be between 0.20% and 1.20%.

[0048] Si (silicon) has the effect of solid solution strengthening of the matrix, and its appropriate addition can significantly improve the strength of the martensitic matrix. However, as an easily oxidized element, excessive addition of Si can lead to the formation of oxides on the steel surface during production that are difficult to eliminate, thus affecting the surface quality of the final product. Therefore, this invention preferably controls the Si content to be between 0.25% and 0.85%.

[0049] Cr (Cr) significantly delays the pearlite transformation by hindering cementite nucleation and growth, shifting the isothermal transformation C-curve of steel to the right. Furthermore, Cr reduces the austenite decomposition rate, thereby lowering the critical cooling rate of the steel and promoting martensite formation. Moreover, Cr's effect on lowering the martensite transformation temperature is significantly less than that of Mn; therefore, Cr is more conducive to promoting the formation of dislocation-type martensite with good toughness. Simultaneously, Cr is also an easily oxidized element. During high-frequency welding of welded pipes, a dense oxide film can form on the steel substrate surface in the weld area. This oxide film effectively hinders the combination of the steel matrix with oxygen in the air, thus suppressing weld decarburization, increasing weld hardness, and effectively controlling the weld decarburization softening zone. However, excessive Cr addition not only increases alloy costs but also forms an oxide scale on the surface of hot-rolled steel coils that is difficult to completely remove through pickling, thus affecting the surface quality of the final product. Therefore, this invention preferably controls the Cr content to 0.20~0.70%.

[0050] The addition of Al can combine with Nitrogen (N) to form AlN, effectively fixing Nitrogen and preventing Nitrogen from combining with Beta (B), thus ensuring the improved hardenability of B. Furthermore, Al can increase the martensitic transformation temperature, helping to reduce the formation of brittle twinned martensite. Simultaneously, Al is also an easily oxidized element. During high-frequency welding of welded pipes, it can form a dense oxide film on the steel substrate surface in the weld area. This oxide film effectively hinders the combination of the steel matrix with oxygen in the air, thereby inhibiting weld decarburization, increasing weld hardness, and effectively controlling the decarburization softening zone of the weld. However, excessive Al addition can cause excessive resistance at the crystallizer inlet during continuous casting, affecting billet production and increasing the difficulty of controlling alumina inclusions in the steel. Therefore, this invention preferably controls the Al content to be between 0.25% and 0.65%.

[0051] Ni is a hardenability-improving element and can also improve the low-temperature toughness of steel. However, the addition of large amounts of Ni will inevitably increase the cost of the alloy and will also lead to a decrease in carbon content. eq and Mn eq The increase in Ni content enhances the hydrogen embrittlement sensitivity of the laser-cut edge. Therefore, the present invention preferably controls the Ni content to be between 0.01% and 0.40%.

[0052] Mo can improve the hardenability of steel plates and prevent temper brittleness; however, excessive addition of Mo can also increase carbon content.eq and Mn eq This leads to an increase in the hydrogen embrittlement susceptibility of the final hot gas expansion tube component, and also increases costs. Therefore, the present invention preferably controls the Mo content to be between 0.01% and 0.20%.

[0053] Boron (B) readily segregates at austenite grain boundaries, suppressing the austenite-to-ferrite phase transformation. Even a low B content can significantly improve the hardenability of steel. However, excessive B can lead to boron embrittlement, which is detrimental to performance. Therefore, this invention preferably controls the B content to 0.001~0.010%.

[0054] As an optional embodiment, the chemical composition of the hot gas expansion tube component of the present invention may further include, by mass percentage, W: 0.01~0.20%, Cu: 0.01~0.20%, Co: 0.01~0.50%, Ta: 0.001~0.10%, Sn: 0.001~0.05%, Sb: 0.001~0.05%, As: 0.001~0.05%, Mg: 0.001~0.010%, Ca: 0.001~0.010%, Zr: 0.001~0.10%, O: 0.001~0.02%, REM: 0.001~0.05%, and the sum of the contents of Ni, Mo, W, Cu, Co, Ta, Sn, Sb, Ca, As, Mg, Zr, and REM is 0.001~0.50%.

[0055] The addition of W can significantly improve the strength of steel plates, and W-containing precipitates can serve as hydrogen capture sites; therefore, it is preferable to add 0.01% or more. However, W is a strong carbide-forming element, and when the W content exceeds 0.20%, coarse and unevenly distributed carbides will form, severely impairing the performance of hot gas expansion tube components. Therefore, the W content is preferably set below 0.20%.

[0056] Cu can improve the hardenability of steel sheets, thereby increasing the strength of hot gas expansion tube components. To achieve this effect, it is preferable to set the Cu content to 0.01% or more. However, Cu tends to segregate at grain boundaries and cause copper embrittlement, reducing the hot workability of the steel sheet. Therefore, it is preferable to set the Cu content to 0.20% or less.

[0057] The addition of Co can increase the martensitic transformation temperature, which is beneficial for the self-tempering of martensite and thus improves its toughness. Therefore, it is preferable to set the Co content to 0.01% or more. However, Co is a rare and valuable element, and excessive addition will increase the cost of steel plates. In addition, Co can promote the graphitization of carbon in steel, which will damage the toughness of hot gas expansion tube components. Therefore, the Co content is preferably set to 0.50% or less.

[0058] Ta, as a strong carbide-forming element, can form stable carbides with carbon, thus refining the grain size and enhancing the strength and toughness of the steel plate. To achieve this effect, the Ta content is preferably set to 0.001%. Furthermore, when the Ta content exceeds 0.100%, a large amount of carbides will precipitate and easily accumulate at grain boundaries, leading to a decrease in the toughness of the steel plate. Therefore, the Ta content is preferably set to 0.100% or less.

[0059] The addition of Sn can refine the grain size and improve the machinability of steel plates by combining with elements such as S. Therefore, the Sn content is preferably 0.001% or more. However, Sn tends to segregate at grain boundaries, increasing the brittleness of the steel. Therefore, the Sn content is preferably set to 0.050% or less.

[0060] Sb also has the effect of refining grains, and to achieve this effect, the Sb content is preferably 0.001% or higher. However, excessive Sb tends to agglomerate at grain boundaries, which can adversely affect the properties of steel, especially its plasticity and toughness. Therefore, the Sb content is preferably set to 0.050% or lower.

[0061] The addition of arsenic (As) can strengthen steel to a certain extent, thereby increasing its strength. However, As tends to accumulate at grain boundaries, leading to increased brittleness of the steel plate and deteriorating weldability. Therefore, this invention preferably controls the As content to 0.001~0.050%.

[0062] Mg and Ca are commonly used for deoxidation in steel smelting and can also form sulfides with S, thereby improving the quantity and morphology of inclusions in the steel sheet microstructure. Therefore, the content of both Mg and Ca is preferably set to 0.001% or more. However, when the content of both Mg and Ca exceeds 0.010%, large-sized inclusions will form in the steel sheet microstructure, damaging the toughness of hot gas expansion tube components. Therefore, the content of both Mg and Ca is preferably set to 0.010% or less.

[0063] As a strong carbide-forming element, Zr has a similar effect to elements such as V, Nb, and Ti. Adding a small amount can refine the grain size and improve the low-temperature toughness of the steel plate. Therefore, the Zr content is preferably set at 0.001% or higher. However, when the Zr content exceeds 0.100%, large-sized carbides will form within the steel plate structure, leading to a decrease in the toughness of the hot gas expansion tube component. Therefore, the Zr content is preferably set at 0.100% or lower.

[0064] O element forms coarse oxides in steel, which is detrimental to the toughness of hot gas expansion tube components. Therefore, the lower the O content, the better. The O content is preferably controlled at 0.001~0.020%, and more preferably below 0.006%.

[0065] REM elements can improve deformability and toughness of hot gas expansion tube components by inhibiting oxide formation. Therefore, the REM content is preferably set to 0.001% or more. At the same time, the REM content should be controlled to not exceed 0.050% to avoid the formation of coarse oxides. In this invention, REM refers to a total of 17 elements including Sc, Y, and La series elements, and REM content refers to the total content of these elements.

[0066] V, Nb, and Ti can form carbides, nitrides, or carbonitrides with carbon and nitrogen. The uniform and fine precipitates can refine the grains at the austenite grain boundaries, improving the strength and toughness of the steel. Simultaneously, the precipitates are dispersed throughout the steel matrix, providing precipitation strengthening and acting as hydrogen traps to inhibit diffusible hydrogen in the matrix, reducing its segregation in stress concentration areas at the cutting edges of components and lowering the hydrogen embrittlement sensitivity of the final thermal expansion tube components. In particular, V can precipitate in large quantities during heating, further consuming carbon in the matrix and promoting the formation of dislocation-type martensite. Furthermore, Ti has a strong binding force with N; adding an appropriate amount of Ti can form TiN with N in solid solution in the steel, preventing the combination of N and B, thus consolidating N and protecting B, ensuring the hardenability of the base material. However, excessive addition of the above three microalloying elements will lead to a significant increase in cost. Therefore, the present invention preferably controls the V content to 0.001~0.200% and the sum of Nb and Ti contents to 0.001~0.100%.

[0067] In addition, the hot gas expansion tube component of the present invention also contains unavoidable impurity elements.

[0068] Polymer (P) is an unavoidable impurity element. On the one hand, P, as a solid solution strengthening element, can relatively inexpensively improve the strength of steel plates. On the other hand, when the P content exceeds 0.020%, P will segregate at grain boundaries, leading to adverse effects such as a significant decrease in toughness. Therefore, the P content is preferably below 0.020%, more preferably below 0.010%. Considering that a P content of less than 0.001% will increase smelting costs, the P content is preferably above 0.001%, more preferably above 0.004%.

[0069] Similar to phosphorus (P), sulfur (S) is an unavoidable impurity element in steel. S reacts with manganese (Mn) and becomes an inclusion in the steel as MnS. When the S content exceeds 0.010%, the large amount of MnS will severely impair the ductility and toughness of the steel, leading to deterioration in workability. Therefore, the S content in this invention is preferably below 0.010%, more preferably below 0.005%. Similarly, considering that an S content below 0.001% will increase smelting costs, the S content is preferably above 0.001%.

[0070] Nitrogen (N) is an unavoidable impurity element in steel, especially for steel containing boron (B). It can combine with boron and thus significantly reduce the effect of boron on improving hardenability. Therefore, it is necessary to minimize the N content. In this invention, the N content is preferably controlled at 0.001~0.010%.

[0071] Typically, hot-air expansion tube components are manufactured from high-frequency welded tubes with welded seams using a hot-air expansion process. The high-frequency welded tube manufacturing process involves heating the edges of a metal sheet with a high-frequency current until they reach a high temperature, then applying pressure to weld them into a tube. This process boasts significant advantages such as high production efficiency, low cost, and stable product quality. This process leverages the skin effect and proximity effect of high-frequency current. The skin effect causes the high-frequency current to concentrate on the surface of the metal material, while the proximity effect causes the current to accumulate at adjacent metal edges, rapidly heating the sheet edge to the welding temperature, which reaches 1200-1400℃, close to the melting point of the steel sheet. At this point, pressure is applied to the steel sheet edge by the extrusion rollers of the welded tube forming equipment, causing the near-molten metal atoms to diffuse and combine, ultimately forming a strong welded joint and completing the tube manufacturing. During the welding process, severe decarburization occurs at the near-molten steel sheet edge, resulting in a softened weld zone with low carbon and low hardness near the fusion line at the center of the weld after welding. For hot gas expansion components fabricated using high-frequency welded pipes with a characteristic weld softening zone, this zone becomes the weakest point of the entire component. When subjected to impact and compression deformation, the weld softening zone preferentially deforms and generates stress concentration, leading to premature cracking and failure near the weld, thus affecting overall vehicle safety. With increasing carbon content in the pipe blank material, especially when the carbon content is in the range of 0.35% to 0.50%, decarburization during high-frequency welding becomes more pronounced, further highlighting the weld softening zone problem on the final component. Therefore, it is necessary to control the weld softening zone of hot gas expansion components.

[0072] Through extensive experimentation, the inventors discovered that if the average hardness ratio of the softened zone to the non-softened zone of the weld in a hot gas expansion tube component is controlled to ≥0.80, the weld does not fail even when the pressure head displacement reaches 12mm during a three-point bend test. To control the softened zone of the weld in hot gas expansion tube components, the alloy composition of the base material needs to be optimized to control the width and hardness of the softened zone during the welding process. This invention, by adding easily oxidizable elements such as Si, Cr, and Al, enables the formation of a dense composite oxide film of SiO2, Cr2O3, and Al2O3 on the edge surface of the plate at high temperatures. These oxide films are characterized by high and dense melting points, reaching 1700℃, higher than the heating temperature of the steel plate during high-frequency welding. In the initial heating stage, a mixed outer layer of Cr2O3 and SiO2 preferentially forms on the surface of the heated steel plate. Utilizing the rapid film-forming properties of Cr2O3 and the filling effect of SiO2, a dense barrier is quickly constructed. During the high-temperature stage, a continuous thin layer of Al2O3 forms between the steel plate substrate and the pre-formed oxide film. Because Al has a higher affinity for oxygen than Cr and Si, a more stable Al2O3 protective layer is formed between the substrate and the outer oxide film, further hindering oxygen diffusion into the substrate. Ultimately, a double-layer composite film consisting of an outer layer of Cr2O3 and SiO2, and an inner layer of Al2O3, is formed on the steel plate surface. This composite oxide film adheres tightly to the steel plate surface, effectively slowing the diffusion of oxygen from the environment into the steel plate substrate and hindering the combination of oxygen and carbon in the substrate. This reduces the degree of decarburization at the edges of the plate before welding and controls the width of the softened zone and the average hardness of the weld.

[0073] Based on this, the present invention preferably controls the content of Si, Cr and Al elements as follows: (1) The Si content is 0.25~0.85%, the Cr content is 0.20~0.70%, and the Al content is 0.25~0.65%; (2) The sum of Si, Cr and Al content is 1.0~2.2%, preferably 1.3~2.2%; (3) (Si+Cr) / Al is 2.0~3.5.

[0074] As an optional implementation, if the surface of the component base material has an aluminum alloy coating, during induction heating, the aluminum alloy coating can form a high-temperature resistant intermetallic compound through the interdiffusion of Fe and Al, which can also play a role in anti-oxidation and anti-decarburization. Therefore, the aluminum alloy coating can also play a role in controlling the softening zone of the weld seam in high-frequency welded pipes and improving the hardness of the softening zone of the weld seam in hot gas expansion tube components. Regarding the thickness of the aluminum alloy coating, if the coating is too thin, it will increase the production difficulty of the coated steel plate and easily lead to incomplete coating defects, affecting the surface quality; if the coating is too thick, the aluminum alloy coating will be severely liquefied during high-frequency welding, increasing the risk of aluminum liquid flowing into the weld seam and aluminum liquid splashing, affecting the welding quality. Therefore, the present invention preferably controls the thickness of the aluminum alloy coating on the surface of the hot gas expansion tube component to be 15~40μm.

[0075] The wall thickness t and alloy element composition of the hot gas expansion tube component of the present invention preferably satisfy the following formula: lnv C =5.0-3.5C-0.8Mn-0.3Si-0.6Cr-0.4Ni-1.1Mo-1.8B+0.5lnt, Among them, the critical cooling rate v C The temperature is 10~21℃ / s, preferably 11~19℃ / s, and the wall thickness t is 1.2~3.0mm.

[0076] To obtain hot-expanded tube components with specific cross-sectional shapes, dimensional accuracy, and high strength, the gas expansion forming and cooling / setting stages are exceptionally important in the hot-expanding process. Gas expansion forming is the core stage of "giving shape" to the component, using high temperature and pressure to induce plastic deformation in the welded tube and bring it into contact with a mold, thereby obtaining a high-precision component shape. Cooling / setting is the key stage of "locking in the shape," ensuring dimensional accuracy and performance through cooling and solidification while maintaining pressure. The synergistic effect of gas expansion forming and cooling / setting enables the hot-expanded tube technology to achieve efficient and precise forming.

[0077] Through extensive production practice, the inventors realized that the cooling characteristics of hot-puff expansion tube components during the cooling and shaping stage differ significantly from the in-mold cooling method of traditional hot stamping components. In the in-mold cooling process of hot stamping components, the formed component cools rapidly under the full contact of the punch and die with a water channel design, resulting in consistent cooling and heat transfer across the upper and lower surfaces and high dimensional accuracy. However, in the cooling and shaping stage of hot-puff expansion tube components, due to the hollow structure of the tube, the outer contour is cooled through the mold cavity with pre-designed cooling channels, while the interior of the tube blank is cooled by convection cooling of high-pressure cooling gas, which removes heat from the inner wall. The pressure of the high-pressure cooling gas determines the degree of contact between the tube blank and the mold, as well as the component's accuracy. Since the heat conduction of gas is weaker than that of traditional molds, the cooling conditions for hot-puff expansion tubes differ from those of traditional hot stamping components, and the cooling effects on the inner and outer walls also differ.

[0078] In addition, hot air expansion tube components are mainly used in key structural components such as vehicle body frames (such as door sill beams, pillars, chassis longitudinal beams) and anti-collision systems. While ensuring that the hot air expansion tube components have extremely high collision safety, in order to meet the requirements of lightweight design, automotive hot air expansion tube components are mostly thin-walled parts, with a preferred wall thickness of 1.2~3.0mm.

[0079] During the cooling and shaping process, the inconsistency in cooling rates inside and outside the tube will lead to differences in heat transfer along the wall thickness, resulting in uneven microstructure and hardness along the wall thickness of the hot gas expansion tube, which in turn affects the performance of the final product. When the wall thickness is thicker, the hardening effect of the material is weakened, requiring a smaller critical cooling rate to ensure that the microstructure of the hot gas expansion tube component has a sufficient proportion of martensite, thereby avoiding problems such as low component strength, low average hardness, and large average hardness standard deviation. When the wall thickness is thinner, the hardening effect of the material is enhanced, and the critical cooling rate should not be too small. Otherwise, martensite structure is easily formed during the production of steel plates, resulting in excessively high steel plate strength. This increases the difficulty of cold working such as cold rolling, trimming, annealing, or coating, leading to low yield and significantly increased production costs.

[0080] To ensure sufficient hardenability of the hot gas expansion tube component of this invention, the critical cooling rate is used as the core indicator. The critical cooling rate refers to the minimum cooling rate required to ensure the formation of a specific proportion of martensite after austenitization during the cooling and shaping process. Therefore, to improve the uniformity of microstructure and hardness along the wall thickness direction of the hot gas expansion tube, this invention uses the critical cooling rate v... CThe temperature rate should be controlled at 10~21℃ / s, preferably 11~19℃ / s. When designing the alloy composition, the influence of material thickness on the hardenability of the designed components must be fully considered. Through extensive experimentation, the inventors have determined that the quantitative relationship between alloy element content, material thickness (i.e., wall thickness), and critical cooling rate satisfies the following calculation formula: .

[0081] For hot gas expansion tube components with a wall thickness of 1.2~3.0mm, using an alloy composition design that satisfies the above formula ensures that the critical cooling rate during the cooling and shaping stage is controlled within the optimized range, thereby obtaining a hot gas expansion tube component with excellent performance. Furthermore, to obtain ultra-high strength and toughness exceeding 2000MPa, the carbon content should always be maintained within the range of 0.35~0.50% when optimizing the material alloy composition as described above. This carbon content is fundamental to ensuring that the base material of the hot gas expansion tube component of this invention possesses ultra-high strength exceeding 2000MPa.

[0082] Furthermore, the yield strength of the hot gas expansion tube component of the present invention is preferably 1550~1720MPa, the tensile strength is preferably 1950~2300MPa, the elongation after fracture is preferably ≥5%, the average hardness is preferably ≥570HV, and the standard deviation of the average hardness in different regions is preferably ≤30HV. The volume content of martensite in the base material of the hot gas expansion tube component of the present invention is preferably ≥95%, more preferably ≥97%, and the volume content of ferrite is preferably ≤3%, more preferably ≤1%, and even more preferably ≤0.5%.

[0083] Secondly, the present invention also provides a welded pipe for preparing hot gas expansion tube components, wherein the chemical composition of the welded pipe, by mass percentage, comprises: C: 0.35~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of the contents of Si, Cr, and Al is 1.0~2.2%, more preferably 1.3~2.2%, and the (Si+Cr) / Al ratio is 2.0~3.5; The combined volume content of ferrite and pearlite in the base material of the welded pipe is ≥90%, and the weld of the welded pipe contains ≥90% tempered martensite and ≤3% retained austenite by volume.

[0084] The wall thickness t and alloy element composition of the welded pipe of the present invention preferably satisfy the following formula: , Among them, the critical cooling rate vC The temperature is 10~21℃ / s, preferably 11~19℃ / s, and the wall thickness t is 1.2~3.0mm.

[0085] To meet the design requirements of hot-expansion tube components, pre-forming processes such as pre-bending, flattening, or flaring of the high-frequency welded pipe are necessary before the hot-expansion forming stage. Through cold forming, the welded pipe acquires certain shape characteristics, which are then used to form the final shape after subsequent heating and gas expansion. However, to achieve ultra-high strength of 2000 MPa and above, 0.35–0.50% carbon should be added to the design composition of the hot-expansion tube component. Maintaining this carbon content means that the weld seam is essentially re-heat-treated after high-frequency welding. The post-weld cooling process significantly affects the weld seam hardness, thus influencing its microstructure and formability, and may even lead to cracking during the pre-forming stage. The high-carbon welded pipe of this invention, without special treatment, results in a hard and brittle quenched martensitic weld seam, while the base material remains the original annealed structure of the steel plate. The original annealed structure of the steel plate has a hardness of approximately 200 HV, while the microhardness of the quenched martensitic structure is above 600 HV. This means the weld seam is the hardest region of the welded pipe, thus posing an extremely high risk of cracking during preforming. Once a preform cracks, during the subsequent gas expansion molding stage, the injection of high-pressure gas into the pipe cavity will cause leakage, preventing further molding and ultimately rendering the component unusable.

[0086] To prevent preforming cracking, this invention, through extensive experimental research, has found that, while ensuring a carbon content of 0.35-0.50% in the design composition, adding an induction heating device during the post-weld cooling stage of the high-frequency welded pipe to temper and soften the weld seam can reduce the hardness of the quenched martensite structure. This results in a hardness of ≤550HV in the non-softened zone of the weld seam. At this point, a 90° bending test on the welded pipe will no longer result in failure cracking. In this configuration, the weld seam microstructure is dominated by tempered martensite, with a volume content ≥90%, and a retained austenite volume content ≤3%. The combined volume content of ferrite and pearlite in the welded pipe is ≥90%. Welds with this microstructure exhibit low hardness, good cold formability, and effectively reduce preforming cracking. Therefore, the average hardness of the base material of the welded pipe of the present invention is preferably ≤250HV, the ratio of the average hardness of the decarburized softened zone to the non-softened zone of the weld is preferably ≥0.85, more preferably ≥0.90, and the average hardness of the non-softened zone is preferably ≤550HV.

[0087] In the pre-forming stage of high-frequency welded pipes, controlling the springback of the bend is equally important for the subsequent hot gas expansion forming stage. If the springback fluctuation of the bend is large, the pre-formed bend will not be able to be smoothly placed into the cavity of the gas expansion forming mold after heating, resulting in the scrapping of the component. Therefore, it is desirable to control the springback fluctuation of the bend by controlling the performance stability of the welded pipe billet. Based on this, the inventors found through extensive experiments that, when the C content in the design composition is 0.35~0.50%, and when the yield strength is 400~550MPa, the standard deviation is ≤35MPa, the tensile strength is 550~800MPa, and the elongation after fracture is ≥22%, the springback stability of the welded pipe during the bending process can be controlled by the overall tensile performance stability of the high-frequency welded pipe, thereby reducing the springback fluctuation and ensuring the yield of hot gas expansion pipe components.

[0088] As an optional implementation, the carbon content in the welded pipe of the present invention is preferably 0.41~0.50%, which can ensure that the yield strength of the final product of the hot gas expansion pipe component reaches more than 1650MPa, the tensile strength reaches more than 2100MPa, and the average hardness of different regions is ≥600HV, thus having better performance.

[0089] As an optional embodiment, the chemical composition of the welded pipe of the present invention may further include, by mass percentage, W: 0.01~0.20%, Cu: 0.01~0.20%, Co: 0.01~0.50%, Ta: 0.001~0.100%, Sn: 0.001~0.050%, Sb: 0.001~0.050%, As: 0.001~0.050%, Mg: 0.001~0.010%, Ca: 0. The content of each of the following components is 0.001~0.010%, Zr: 0.001~0.100%, O: 0.001~0.020%, REM: 0.001~0.050%, P: 0.001~0.020%, S: 0.001~0.010%, N: 0.001~0.010%, and the sum of the contents of Ni, Mo, W, Cu, Co, Ta, Sn, Sb, Ca, As, Mg, Zr, and REM is 0.001~0.500%.

[0090] As an optional embodiment, the surface of the welded pipe of the present invention may have an aluminum alloy coating, the coating thickness of which is preferably 10~30μm.

[0091] Thirdly, the present invention also provides a manufacturing process for a hot gas expansion tube component, comprising the following steps: S1. Preforming The welded pipe used for preparing hot gas expansion tube components of the second aspect of the present invention is subjected to mechanical preforming treatment, and preforming processes such as bending and flattening of the welded pipe are completed. S2. Austenitizing heating Heat the welded pipe to 850~1050℃; The heating method of this invention can employ rapid heating methods such as induction heating and electric heating, or heating in a heating furnace. If rapid heating is selected, the preferred heating temperature is 900~1050℃, and the preferred heating time is 30~150s; if heating in a heating furnace is selected, the preferred heating temperature is 850~1000℃, and the preferred heating time is 150~600s. S3. Air Inflation Molding and Mold Cooling The heated welded pipe is placed in a mold, high-pressure nitrogen is introduced into the welded pipe to form it, and it is then demolded after cooling to below 100°C in the mold to obtain a hot gas expanded pipe component.

[0092] For welded pipes with aluminum alloy coatings, the austenitizing heating process preferably uses a heating furnace, with the dew point controlled below -15°C, and more preferably below -20°C. For welded pipes without coatings, it is preferable to shot-peened and oiled the cooled and shaped components to remove iron oxide scale from the surface of the hot gas expansion components and to prevent rust.

[0093] As an optional implementation, the hot gas expansion tube component is painted and baked after being installed and welded. The baking process is a baking temperature of 150~180℃ and a holding time of 15~50min, with the holding time preferably being 20~50min.

[0094] Fourthly, the present invention also provides a steel for preparing hot gas expansion tube components and welded pipes, wherein the chemical composition of the steel, by mass percentage, comprises: C: 0.35~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of the contents of Si, Cr, and Al is 1.0~2.2%, preferably 1.3~2.2%, and the (Si+Cr) / Al ratio is 2.0~3.5.

[0095] The plate thickness t and alloy element composition of the steel of the present invention preferably satisfy the following formula: lnv C =5.0-3.5C-0.8Mn-0.3Si-0.6Cr-0.4Ni-1.1Mo-1.8B+0.5lnt, Among them, the critical cooling rate v C The value is 10~21℃ / s, preferably 11~19℃ / s.

[0096] As an optional implementation, the carbon content in the steel of the present invention is preferably 0.41~0.50%, which can ensure that the yield strength of the final product of the hot gas expansion tube component reaches more than 1650MPa, the tensile strength reaches more than 2100MPa, and the average hardness of different regions is ≥600HV, thus having better performance.

[0097] As an optional embodiment, the steel surface of the present invention has an aluminum alloy coating with a coating thickness of 10~30μm.

[0098] As an optional embodiment, the chemical composition of the steel of the present invention, by mass percentage, further comprises: W: 0.01~0.20%, Cu: 0.01~0.20%, Co: 0.01~0.50%, Ta: 0.001~0.100%, Sn: 0.001~0.050%, Sb: 0.001~0.050%, As: 0.001~0.050%, Mg: 0.001~0.010%, Ca: 0.00 1~0.010%, Zr: 0.001~0.100%, O: 0.001~0.020%, REM: 0.001~0.050%, P: 0.001~0.020%, S: 0.001~0.010%, N: 0.001~0.010%, and the sum of the contents of Ni, Mo, W, Cu, Co, Ta, Sn, Sb, Ca, As, Mg, Zr, and REM is 0.001~0.500%.

[0099] Fifthly, the present invention also provides a process for preparing a welded pipe for manufacturing a hot gas expansion tube component, comprising the following steps: S1. Steel slitting and roll forming The steel is slit according to the diameter requirements of the welded pipe, and the slit steel is rolled into a pipe blank using a roll forming die; the steel used is the steel for preparing welded pipes according to the fourth aspect of the present invention; in order to ensure the uniformity of roll forming, it is preferred to roll 7 to 14 times. It should be noted that during the process of rolling steel into a tube blank using a roll forming die, and during the process of hot-expanding the tube blank into a hot-expanded tube component using a die, the thickness of the base material will decrease slightly due to plastic deformation. According to industry practice, this thinning amount is controlled to be no more than 10% of the original steel thickness. Since the size of the hot-expanded tube component of this invention is limited, the thinning amount of the base material during hot-expanding is extremely small and can be ignored. The steel thickness is equivalent to the welded pipe wall thickness and the wall thickness of the hot-expanded tube component.

[0100] S2. High-frequency welding The tube blank interface is heated and welded using an induction coil, with a preferred heating temperature of ≥1000℃, and then weld defects inside and outside the welded tube are removed. In an optional embodiment, the present invention can use internal and external scrapers to remove weld defects on the inside and outside of the welded pipe after high-frequency welding. After scraping, the ratio of the average thickness of the five weld points to the thickness of the base plate is ≥0.9, preferably ≥0.95, and more preferably ≥1, to prevent excessive thinning of the weld position during the hot gas expansion forming of the welded pipe. At the same time, the weld thickness should not be too large; after scraping, the weld thickness should not exceed 1.1 times the thickness of the base plate to ensure that weld defects are completely removed. S3. Post-weld online tempering The welded pipe is subjected to online tempering treatment at a temperature of 400~600℃; This invention employs online tempering treatment on welded pipes to adjust the room temperature microstructure of the weld and improve its formability. When the welded pipe cools to below 200°C, the weld forms a microstructure dominated by quenched martensite. Induction heating is then used to rapidly heat the weld and its surrounding area to 400-600°C, followed by final cooling to room temperature. This promotes the tempering and softening of the quenched martensite, improving weld formability. During this process, a certain proportion of retained austenite inevitably forms. This retained austenite has poor mechanical stability. When the welded pipe undergoes cold bending deformation, the retained austenite in the weld will undergo a martensitic transformation, forming a high-carbon, hard, and brittle martensite structure, which is extremely detrimental to the subsequent cold forming of the welded pipe. Therefore, this invention designs the heating temperature for online tempering treatment to soften the martensite and control the content of retained austenite to ≤3%. Therefore, the heating temperature setting for the online tempering process of this invention must take into account both the tempering degree of martensite and the content of residual austenite. After a large number of experiments, the inventors have summarized that the tempering temperature is preferably set to 400~600℃. S4. Fixed diameter and fixed length Adjust the diameter of the welded pipe to the target diameter and cut it to a fixed length.

[0101] The present invention will now be described in more detail with reference to exemplary embodiments. The following embodiments or experimental data are intended to illustrate the present invention by way of example, and those skilled in the art should understand that the present invention is not limited to these embodiments or experimental data.

[0102] Preparation of steel for pipe making Prepare steels with the chemical compositions of each group in Table 1. Table 1 shows the alloy composition (mass %) and critical cooling rate v of each group of steels. C (℃ / s), plate thickness t (mm), the preparation process includes the following steps: S1. Steelmaking The ingredients are prepared according to the chemical composition of each group in Table 1, smelted in a vacuum induction furnace, and then obtained by continuous casting or forging to meet the requirements of each component in Table 1. S2. Hot-rolled Each group of steel billets was heated to 1150℃, held at that temperature for 2 hours, and then hot-rolled at 880℃. Finally, the billets were coiled at 550℃ to obtain each group of hot-rolled steel coils. S3. Pickling Each group of hot-rolled steel coils was pickled to remove the oxide scale generated during the hot rolling process; S4. Cold rolled Each group of pickled hot-rolled steel coils was cold-rolled with a cold rolling reduction of 30-60%, resulting in cold-rolled steel coils with different thicknesses as shown in Table 1. S5. Coating or annealing The cold-rolled steel coils in groups D and E in Table 1 were coated. The coating was an aluminum alloy coating. By mass percentage, the coating contained 9.5% Si, 1.8% Fe, and the balance was Al and unavoidable impurities. The coating thickness of group D was 10 μm and that of group E was 30 μm. In Table 1, the cold-rolled steel coils of each group except for groups D and E have no coating on their surface. The cold-rolled steel coils of each group except for groups D and E are annealed to prepare the steel for pipe making.

[0103] Table 1 Alloy composition of the steel substrate of the present invention (mass%, balance being Fe and unavoidable impurities)

[0104] As shown in Table 1, the alloy composition of the steel substrates in groups A to I all meet the requirements of this invention for chemical composition, critical cooling rate, and wall thickness (plate thickness). Groups J to M serve as control groups. The critical cooling rate v in groups J and K is... C The critical cooling rate is 10~21℃ / s, while Si+Cr+Al is not in the range of 1.0~2.2%, and (Si+Cr) / Al is not in the range of 2.0~3.5; the critical cooling rate v in groups L and M is 10~21℃ / s. C Outside the 10~21℃ / s range, the Si+Cr+Al content is 1.0~2.2%, and the (Si+Cr) / Al ratio is 2.0~3.5. It should be noted that due to the excessively good hardenability of group M steels (v... C The temperature drop of <10℃ / s caused multiple strip breaks in the M group steel during the cold rolling stage of step S4, reducing the yield of steel coils and hindering actual production.

[0105] Preparation of high-frequency welded pipe Example 1 The process of preparing 30mm diameter welded pipes using Group A steel coils includes the following steps: S1. Steel coil slitting and roll forming According to the diameter requirements of the welded pipe, the steel coils of Group A are slit, and the slit steel is rolled into pipe blanks using a roll forming die. The roll forming is done in 7 passes. At this time, the thinning of the base material is very small and can be ignored. The wall thickness of the welded pipe is approximately the thickness of the steel. S2. High-frequency welding The tube blank interface is heated and welded using an induction coil at a temperature of 1000℃. Internal and external scrapers are used to remove weld defects inside and outside the welded tube after high-frequency welding. After scraping, the ratio of the average thickness of the five welds to the thickness of the base plate is approximately 0.95. S3. Post-weld online tempering The welded pipe is subjected to online tempering treatment at a tempering temperature of 500℃; S4. Fixed diameter and fixed length The outer diameter of the welded pipe is controlled to 30±0.5mm by a sizing device, and the welded pipe is processed into the corresponding length according to the final component size.

[0106] Comparative Example 1 30mm diameter welded pipes were prepared using group A steel coils. The preparation process of this comparative example was the same as that of Example 1, except that step S3, post-weld online tempering, was not performed.

[0107] Comparative Example 2 30mm diameter welded pipes were prepared using group A steel coils. The preparation process of this comparative example was the same as that of Example 1, except that the tempering temperature of the online tempering after welding in step S3 was 300℃.

[0108] Example 2 30mm diameter welded pipes are prepared using group B steel coils. The preparation process in this embodiment is the same as that in embodiment 1, except that the tempering temperature in step S3, which involves online tempering after welding, is 400℃.

[0109] Comparative Example 3 30mm diameter welded pipes were prepared using group B steel coils. The preparation process of this comparative example was the same as that of Example 1, except that the tempering temperature of the online tempering after welding in step S3 was 300℃.

[0110] Example 3 30mm diameter welded pipes are prepared using group C steel coils. The preparation process in this embodiment is the same as that in embodiment 1, except that the tempering temperature in step S3, which involves online tempering after welding, is 600℃.

[0111] Comparative Example 4 30mm diameter welded pipes were prepared using group C steel coils. The preparation process of this comparative example was the same as that of Example 1, except that step S3, post-weld online tempering, was not performed.

[0112] Comparative Example 5 30mm diameter welded pipes were prepared using group C steel coils. The preparation process of this comparative example was the same as that of Example 1, except that the tempering temperature of the online tempering after welding in step S3 was 720℃.

[0113] Example 4 30mm diameter welded pipes are prepared using group D steel coils. The preparation process in this embodiment is the same as that in embodiment 1, except that the tempering temperature of the online tempering after welding in step S3 is 450℃.

[0114] Comparative Example 6 30mm diameter welded pipes were prepared using group D steel coils. The preparation process of this comparative example was the same as that of Example 1, except that step S3, post-weld online tempering, was not performed.

[0115] Example 5 30mm diameter welded pipes are prepared using group E steel coils. The preparation process in this embodiment is the same as that in embodiment 1, except that the tempering temperature for online tempering after welding in step S3 is 450℃.

[0116] Examples 6-9 30mm diameter welded pipes were prepared using steel coils from Group F to Group I. The preparation process of Examples 6 to 9 was the same as that of Example 1, except that the tempering temperature of the online tempering after welding in step S3 was 600℃.

[0117] Comparative Examples 7-10 30mm diameter welded pipes were prepared using steel coils from groups J to M. The preparation process of Comparative Examples 7 to 10 was the same as that of Example 1, except that the online tempering temperature after welding in step S3 was 600℃.

[0118] To verify the performance of the welded pipes prepared in Examples 1-9 and Comparative Examples 1-10, the welded pipes of Examples 1-9 and Comparative Examples 1-10 were subjected to the following tests: (1) Perform tensile performance testing on the welded pipe as a whole, and record the yield strength, tensile strength and elongation after fracture. The test results are the average of five points.

[0119] (2) The base material and weld of the welded pipe were observed under a microscope, and the weld microstructure was observed using EBSD. The content of retained austenite was quantitatively obtained through more than five fields of view. As a typical EBSD test result, the EBSD characterization of Example 1 is as follows: Figure 1 As shown.

[0120] (3) Test the microhardness distribution of the entire weld in the welded pipe. The specific method is as follows: take the center line of the weld as the starting point and conduct hardness tests on both sides. The test interval is 0.25 mm, the indenter load is 0.3 kg·F, and at least 5 different weld positions are selected. The corresponding test point distribution is as follows. Figure 2 As shown. Figure 2In the diagram, W0 corresponds to the decarburized and softened zone of the weld, W0.25 and W0.50 correspond to the non-softened zone of the weld, W0.75 corresponds to the heat-affected zone near the base metal, and W1 corresponds to the base metal. The ratio of the average values ​​of all W0 and W0.25 is defined as the average hardness ratio of the decarburized and softened zone to the non-softened zone of the weld. (4) Perform a bending test on the welded pipe. The specific method is as follows: place the weld seam at the neutral line position (between the inner and outer sides of the bending area), select the bending core diameter as 6 times the outer diameter of the pipe. The diameter of the welded pipe in this invention is 30mm, so the bending core diameter is 180mm; bend the welded pipe at 90° and test at least 3 pipes; if there are no visible cracks in the bending area, no defects with a length >3mm on the tensile surface, and no cracks or peeling in the weld seam, and no thinning or sharp dents in the adjacent area, then the welded pipe is deemed qualified.

[0121] The online tempering temperatures of Examples 1-9 and Comparative Examples 1-10 of this invention, as well as the tensile properties, microstructure, microhardness, and 90° bending test results of the prepared welded pipes, are shown in Table 2.

[0122] Table 2 Performance test results of welded pipes in each embodiment and comparative example

[0123] As shown in Table 2, the online tempering temperature of Comparative Examples 2 and 3 was 300℃, while Comparative Examples 1, 4 and 6 did not use the online tempering process. As a result, the weld hardness of the welded pipe was too high, the elongation of the welded pipe was too low, the yield strength of the welded pipe fluctuated significantly, and the welded pipe cracked during the bending test, which was not conducive to the forming of the welded pipe.

[0124] The online tempering temperature of Comparative Example 5 was 720℃. The high tempering temperature caused partial austenitization of the welded pipe. During subsequent cooling, the base material could not guarantee sufficient ferrite and pearlite structure, resulting in high base material hardness, high welded pipe strength, and insufficient elongation. The welded pipe failed during bending tests.

[0125] The average hardness ratio of the decarburized softened zone to the non-softened zone in Comparative Examples 7 and 8 was only 0.79~0.81, significantly lower than other embodiments and comparative examples. This situation mainly stems from the unreasonable design of Si+Cr+Al and (Si+Cr) / Al in the welded pipe steel substrate. During the high-frequency welding process of the welded pipe, the three easily oxidized elements, Si, Cr, and Al, did not fully play their role in forming a dense composite oxide film to hinder the combination of O in the air and C in the substrate. Consequently, they failed to prevent decarburization at the weld edge during induction heating, resulting in a significant low-hardness decarburized softened zone at the center of the weld. Furthermore, there was a significant hardness difference between this decarburized softened zone and the non-softened zone in the weld. This uneven distribution of C content in the weld leads to differences in weld hardness, which in turn causes significant performance problems in the components after hot-expansion molding.

[0126] The critical cooling rate v of the annealed steel sheet for tube making in Comparative Example 10 C If the size is too small, the hardenability is good, which leads to the high strength of the steel plate base material, poor formability, increased difficulty in pipe manufacturing, and finally the finished welded pipe also fails during bending test.

[0127] After online tempering treatment at 400~600℃, the welded pipes of the other embodiments meet all the requirements, including: yield strength (YS) of 400~550MPa, YS standard deviation ≤35MPa, tensile strength (TS) of 550~800MPa, elongation after fracture (A) ≥22%; ferrite (F) and pearlite (P) content in the base metal ≥90%, the weld is composed of tempered martensite and less than 3% of retained austenite (RA); base metal hardness ≤250HV, the weld center is a decarburized softened zone, the ratio of its average hardness to the average hardness of the non-softened zone of the weld is ≥0.85, and the average hardness of the non-softened zone of the weld is ≤550HV. The steel coils of the welded pipes in Examples 4 and 5 have an AlSi coating. The AlSi coating can also prevent the weld edge from contacting the air during high-temperature heating, further enhancing the effect of inhibiting decarburization. As a result, the optimal average hardness ratio between the decarburized softened zone and the non-softened zone of the weld is obtained, reaching more than 0.96.

[0128] Preparation of hot gas expansion tube components Hot gas expansion tube components were prepared using the welded pipes that passed the bending test as listed in Table 2. The hot gas expansion tube components are as follows: Figure 3 As shown. Due to the limited size of the hot gas expansion tube component in the various embodiments and comparative examples of the present invention, the amount of thinning of the base material during the hot gas expansion forming process is extremely small and can be ignored. The steel thickness of the steel coil is approximately the wall thickness of the welded pipe, and approximately the wall thickness of the hot gas expansion tube component.

[0129] Example 1 The process of preparing a hot gas expansion tube component using the welded tube of Example 1 includes the following steps: S1. Preforming Complete the pre-forming processes such as bending and flattening of welded pipes; S2. Austenitizing heating The welded pipe is heated to 930℃ using electric heating. S3. Air Inflation Molding and Mold Cooling The heated welded pipe is placed in a mold, high-pressure nitrogen is introduced to form it, and it is cooled to below 100°C in the mold to obtain a hot gas expansion pipe component. S4. Coating and Baking After the components are loaded and welded, they are painted and baked. The baking process is a baking temperature of 150℃ and a holding time of 50 minutes.

[0130] Example 2 The hot gas expansion tube component is prepared using the welded tube of Example 2, including the following steps: S1. Preforming Complete the pre-forming processes such as bending and flattening of welded pipes; S2. Austenitizing heating The welded pipe is heated to 950℃ using electric heating. S3. Air Inflation Molding and Mold Cooling The heated welded pipe is placed in a mold, high-pressure nitrogen is introduced to form it, and it is cooled to below 100°C in the mold to obtain a hot gas expansion pipe component. S4. Coating and Baking After the components are loaded and welded, they are painted and baked. The baking process is to bake at 170℃ and keep warm for 30 minutes.

[0131] Example 3 The hot gas expansion tube component is prepared using the welded tube of Example 3, including the following steps: S1. Preforming Complete the pre-forming processes such as bending and flattening of welded pipes; S2. Austenitizing heating The welded pipe is heated to 970℃ using electric heating. S3. Air Inflation Molding and Mold Cooling The heated welded pipe is placed in a mold, high-pressure nitrogen is introduced to form it, and it is cooled to below 100°C in the mold to obtain a hot gas expansion pipe component. S4. Coating and Baking After the components are loaded and welded, they are painted and baked. The baking process is to bake at 180℃ and keep warm for 20 minutes.

[0132] Example 4 The hot gas expansion tube component is prepared using the welded tube of Example 4, including the following steps: S1. Preforming Complete the pre-forming processes such as bending and flattening of welded pipes; S2. Austenitizing heating The welded pipe is heated to 970℃ using a roller heating furnace with controllable dew point, while the dew point is controlled at -20℃ during the heating process. S3. Air Inflation Molding and Mold Cooling The heated welded pipe is placed in a mold, high-pressure nitrogen is introduced to form it, and it is cooled to below 100°C in the mold to obtain a hot gas expansion pipe component. S4. Coating and Baking After the components are loaded and welded, they are painted and baked. The baking process is 160℃ and 40min.

[0133] Example 5 The hot gas expansion tube component is prepared using the welded tube of Example 5, including the following steps: S1. Preforming Complete the pre-forming processes such as bending and flattening of welded pipes; S2. Austenitizing heating The welded pipe is heated to 930°C using a roller heating furnace with controllable dew point, while the dew point is controlled at -20°C during the heating process. S3. Air Inflation Molding and Mold Cooling The heated welded pipe is placed in a mold, high-pressure nitrogen is introduced to form it, and it is cooled to below 100°C in the mold to obtain a hot gas expansion pipe component. S4. Coating and Baking After the components are loaded and welded, they are painted and baked. The baking process is a baking temperature of 170℃ and a holding time of 40 minutes.

[0134] Examples 6-9 and Comparative Examples 7-9 Hot gas expansion tube components were prepared using the welded tubes of Examples 6-9 and Comparative Examples 7-9, respectively, including the following steps: S1. Preforming Complete the pre-forming processes such as bending and flattening of welded pipes; S2. Austenitizing heating The welded pipe is heated to 950℃ using electric heating. S3. Air Inflation Molding and Mold Cooling The heated welded pipe is placed in a mold, high-pressure nitrogen is introduced to form it, and it is cooled to below 100°C in the mold to obtain a hot gas expansion pipe component. S4. Coating and Baking After the components are loaded and welded, they are painted and baked. The baking process is 160℃ and 40min.

[0135] To verify the various properties of the hot gas expansion tube components prepared in Examples 1-9 and Comparative Examples 7-9, the hot gas expansion tube components of Examples 1-9 and Comparative Examples 7-9 were subjected to the following tests: (1) For hot gas expansion tube components, tensile properties were tested by sampling from the plane, avoiding the weld seam, and hardness tests were conducted in the wall thickness direction by sampling from three random base material areas of the component. The test points included the position 0.15 mm from the outer diameter surface, the position 1 / 4 wall thickness from the outer diameter surface, the center, the position 1 / 4 wall thickness from the inner diameter surface, and the position 0.15 mm from the inner diameter surface. The average hardness and standard deviation of the three areas were calculated.

[0136] (2) Microscopic observation of the hardness test specimen of the base material.

[0137] (3) Samples were taken from the welds near the sampling locations of the hardness test specimens of the component base material to test the microhardness of the welds. The specific method was as follows: starting from the center line of the weld, hardness tests were conducted on both sides with a test interval of 0.25 mm and an indenter load of 0.3 kg·F. At least 5 different weld locations were selected, and the corresponding test point distribution was as follows. Figure 2 As shown. Figure 2 In the diagram, W0 corresponds to the decarburized and softened zone of the weld, W0.25 and W0.50 correspond to the non-softened zone of the weld, W0.75 corresponds to the heat-affected zone near the base metal, and W1 corresponds to the base metal. The ratio of the average values ​​of all W0 and W0.25 is defined as the average hardness ratio of the decarburized and softened zone to the non-softened zone of the weld. (4) Perform a three-point bend test on the components. The specific method is as follows: Take at least three relatively straight sections of hot gas expansion tube components and perform a three-point bend on the components. Fix both ends of the components using corresponding fixtures, with a span of approximately 200 mm. Adjust the component weld to be directly below the pressure head. The pressure head diameter is 30 mm. Starting from the pressure head pressing down and contacting the weld, when the pressure head displacement reaches 12 mm, determine whether the component weld has failed by visual inspection or by the change in pressure head load. If it fails, it is determined that the component weld does not meet the performance requirements.

[0138] The tensile properties, microstructure, microhardness, and three-point bend test results of the hot gas expansion tube components prepared in Examples 1-9 and Comparative Examples 7-9 of this invention are shown in Table 3. Typical microstructures of the hot gas expansion tube components in Example 2 and Comparative Example 7 are as follows: Figure 4 and Figure 5 As shown.

[0139] Table 3 Performance test results of the hot gas expansion tube components of each embodiment and comparative example

[0140] As shown in Table 3, the hot gas expansion tube components of Embodiments 1-9 of the present invention have the following properties: (1) After coating and baking, YS reaches 1550~1720MPa, TS reaches 1950~2300MPa, A is not less than 5%, and the combination design of alloy composition and plate thickness (wall thickness) is reasonable, the hardenability is better, the performance uniformity of the whole component is better, the average hardness of different areas is ≥570HV, and the standard deviation is ≤30HV. It should be noted that for the components of Examples 6~9, the higher carbon content design (C is 0.41~0.49%) makes the yield strength of the component reach more than 1650MPa, the tensile strength reach more than 2100MPa, and the average hardness of different areas is ≥600HV, showing better performance; (2) Due to the reasonable design of the alloy composition and plate thickness (wall thickness), corresponding to the location of the base material hardness test, its microstructure is 95% or more martensite (M), with no ferrite (F, proportion ≤1%), such as Figure 4 As shown; (3) After thermal expansion, the hardness ratio between the decarburized and softened zone and the non-softened zone of the weld is ≥0.81, and when the content of Si+Cr+Al reaches 1.4% or more, the hardness ratio is ≥0.85.

[0141] Furthermore, it should be noted that for Examples 4 and 5, since the welded pipe surface is coated with AlSi coatings of different thicknesses, after hot gas expansion, the coating thicknesses are 15μm and 40μm, respectively. At the same time, the beneficial effect of controlling weld decarburization is also manifested in the hot gas expanded pipe component, with the hardness ratio of the weld decarburized softened zone to the non-softened zone reaching 0.87 and 0.88, respectively.

[0142] As shown in Table 3, the hot gas expansion tube components of Comparative Examples 7-9 of the present invention have the following properties: (1) For Comparative Examples 7 and 8, due to the unreasonable design of Si+Cr+Al and (Si+Cr) / Al, the weld center of the welded pipe has a serious decarburization and softening problem. This problem extends to the hot gas expansion tube component and eventually causes cracking when the component is bent at three points. (2) For Comparative Example 9, due to the unreasonable design of the plate thickness (wall thickness) and alloy combination, the critical cooling rate was too high, resulting in a large amount of bainite (B) appearing in the microstructure of the hot gas expansion tube component, such as Figure 5 As shown, the proportion of bainite (B) reached over 10%, therefore, the yield strength of the hot gas expansion tube component was only 1483 MPa, and the tensile strength was only 1901 MPa, both lower than those in the embodiments of the present invention. In addition, the hardness of the base material of Comparative Example 9 component was less than 570 HV in many places, and there was also a large standard deviation of hardness, indicating that the phase transformation in the wall thickness direction was non-uniform during the component cooling process.

[0143] It should be understood that the process parameters (such as heating temperature, cooling temperature, and holding time) and structural dimensions (such as pipe diameter) described herein are merely examples to illustrate the principles of this disclosure and are not intended to constitute limitations. These parameters can be adaptively adjusted or changed according to actual production conditions, equipment models, and the shape of the target component without departing from the core inventive concept of this disclosure. One of the key improvements of this disclosure lies in the high-frequency welded pipe manufacturing process, where the temperature range for online tempering after welding is explicitly limited to 400~600℃. This temperature range plays a crucial role in improving the formability of high-carbon content welds and preventing pre-forming cracking.

[0144] Furthermore, it should be understood that the method for calculating the critical cooling rate v provided by this invention... CThe formula is an empirical formula derived from a large amount of experimental data, aiming to reveal the quantitative relationship and trend of changes between alloy element content, plate thickness, and material hardenability. In practical application, the calculation result provided by this formula is a theoretical guideline or a central estimate. Due to unavoidable fluctuations in composition, process variations, and measurement errors in actual steel production, and the potential subtle differences in the microstructure of materials between different production batches or furnaces, the value of v calculated based on this formula in practical applications should be considered more nuanced. C Reasonable deviations, acceptable to those skilled in the art, are permissible. Such deviations do not deviate from the core inventive concept of this invention, namely, controlling the combination of alloy composition and plate thickness to achieve the critical cooling rate v of the material. C The pressure falls within the target range of 10~21℃ / s (preferably 11~19℃ / s), thus adapting to the cooling characteristics of the hot gas expansion process and achieving the required high strength and uniform microstructure. Those skilled in the art will understand that the core value of the formula lies in providing clear design guidance and optimization paths, rather than requiring absolutely precise mathematical fit.

[0145] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, and process improvements that fall within the spirit and principles of the present invention are included within the scope of protection of the claims of the present invention.

Claims

1. A hot gas inflatable tube member, characterized by, The chemical composition of the hot gas expansion tube component, by mass percentage, comprises: C: 0.35~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.0~2.2%, and (Si+Cr) / Al is 2.0~3.5; The weld of the hot gas expansion tube component includes a decarburized softened zone and a non-softened zone, and the ratio of the average hardness of the decarburized softened zone to the non-softened zone is ≥0.

80.

2. The hot gas inflatable tube member of claim 1, wherein, The wall thickness t of the hot gas expansion tube component and the alloy element composition satisfy the following formula: ， Wherein, the critical cooling rate v C is 10~21℃ / s, and the wall thickness t is 1.2~3.0mm.

3. The hot gas inflatable tube member of claim 2, wherein, The critical cooling rate v C is 11-19 °C / s.

4. The hot gas inflatable tube member of claim 1, wherein, The ratio of the average hardness of the decarburized softened zone to that of the non-softened zone is ≥0.

85.

5. The hot gas inflatable tube member of claim 1, wherein, The chemical composition of the hot gas expansion tube component, by mass percentage, includes: C: 0.35~0.41%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.3~2.2%, and (Si+Cr) / Al is 2.0~3.

5.

6. The hot gas inflatable tube member of claim 1, wherein, The chemical composition of the hot gas expansion tube component, by mass percentage, comprises: C: 0.41~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.0~2.2%, and (Si+Cr) / Al is 2.0~3.

5.

7. A hot gas inflatable tube member according to any one of claims 1-6, characterized in that The surface of the hot gas expansion tube component has an aluminum alloy coating with a thickness of 15~40μm.

8. The hot gas expansion tube component according to any one of claims 1-6, characterized in that, The yield strength of the hot gas expansion tube component is 1550~1720MPa, the tensile strength is 1950~2300MPa, the elongation after fracture is ≥5%, the average hardness is ≥570HV, and the standard deviation of the average hardness in different regions is ≤30HV.

9. The hot gas expansion tube component according to any one of claims 1-6, characterized in that, The volume content of martensite in the base material of the hot gas expansion tube component is ≥95%.

10. The hot gas expansion tube component according to any one of claims 1-6, characterized in that, The chemical composition of the hot gas expansion tube component, by mass percentage, further comprises: W: 0.01~0.20%, Cu: 0.01~0.20%, Co: 0.01~0.50%, Ta: 0.001~0.100%, Sn: 0.001~0.050%, Sb: 0.001~0.050%, As: 0.001~0.050%, Mg: 0.001~0.010%, Ca: 0.001~0.050%. The content of the following components is 0.010%, Zr: 0.001~0.100%, O: 0.001~0.020%, REM: 0.001~0.050%, P: 0.001~0.020%, S: 0.001~0.010%, N: 0.001~0.010%, and the sum of the contents of Ni, Mo, W, Cu, Co, Ta, Sn, Sb, Ca, As, Mg, Zr, and REM is 0.001~0.500%.

11. A welded pipe for manufacturing hot gas expansion tube components, characterized in that, The chemical composition of the welded pipe, by mass percentage, comprises: C: 0.35~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.0~2.2%, and (Si+Cr) / Al is 2.0~3.5; The combined volume content of ferrite and pearlite in the base material of the welded pipe is ≥90%, and the weld of the welded pipe contains ≥90% tempered martensite and ≤3% retained austenite by volume.

12. The welded pipe according to claim 11, characterized in that, The wall thickness t of the welded pipe and the alloy element composition satisfy the following formula: ， wherein the critical cooling rate v C is 10-21 °C / s.

13. The welded pipe according to claim 12, characterized in that, The critical cooling rate v C is 11-19 °C / s.

14. The welded pipe according to claim 11, characterized in that, The chemical composition of the welded pipe, by mass percentage, includes: C: 0.35~0.41%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.3~2.2%, and (Si+Cr) / Al is 2.0~3.

5.

15. The welded pipe according to claim 11, characterized in that, The chemical composition of the welded pipe, by mass percentage, includes: C: 0.41~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.0~2.2%, and (Si+Cr) / Al is 2.0~3.

5.

16. The welded pipe according to any one of claims 11-15, characterized in that, The surface of the welded pipe has an aluminum alloy coating with a thickness of 10~30μm.

17. The welded pipe according to any one of claims 11-15, characterized in that, The welded pipe has a yield strength of 400~550MPa, a tensile strength of 550~800MPa, and an elongation after fracture of ≥22%.

18. The welded pipe according to any one of claims 11-15, characterized in that, The average hardness of the base material of the welded pipe is ≤250HV. The weld of the welded pipe includes a decarburized softened zone and a non-softened zone. The ratio of the average hardness of the decarburized softened zone to the non-softened zone is ≥0.

85. The average hardness of the non-softened zone is ≤550HV.

19. The welded pipe according to claim 18, characterized in that, The ratio of the average hardness of the decarburized softened zone to that of the non-softened zone is ≥0.

90.

20. The welded pipe according to any one of claims 11-15, characterized in that, The chemical composition of the welded pipe, by mass percentage, further includes: W: 0.01~0.20%, Cu: 0.01~0.20%, Co: 0.01~0.50%, Ta: 0.001~0.100%, Sn: 0.001~0.050%, Sb: 0.001~0.050%, As: 0.001~0.050%, Mg: 0.001~0.010%, Ca: 0.001~0.0 10%, Zr: 0.001~0.100%, O: 0.001~0.020%, REM: 0.001~0.050%, P: 0.001~0.020%, S: 0.001~0.010%, N: 0.001~0.010%, and the sum of the contents of Ni, Mo, W, Cu, Co, Ta, Sn, Sb, Ca, As, Mg, Zr, and REM is 0.001~0.500%.

21. A manufacturing process for a hot gas expansion tube component, characterized in that, Includes the following steps: S1. Preforming The welded pipe for preparing the hot gas expansion tube component according to any one of claims 11-20 is subjected to mechanical preforming treatment. S2. Austenitizing heating Heat the welded pipe to 850~1050℃; S3. Air Inflation Molding and Mold Cooling The heated welded pipe is placed in a mold, high-pressure nitrogen is introduced into the welded pipe to form it, and it is then demolded after cooling to below 100°C in the mold to obtain a hot gas expanded pipe component.

22. The manufacturing process of the hot gas expansion tube component according to claim 21, characterized in that, For welded pipes with aluminum alloy coating, the dew point is controlled below -15℃ during the austenitizing heating process.

23. The manufacturing process of the hot gas expansion tube component according to claim 21, characterized in that, After being installed and welded, the hot gas expansion tube components are coated and baked. The baking process involves heating at 150~180℃ and holding for 15~50 minutes.

24. A type of steel for manufacturing hot gas expansion tube components and welded pipes, characterized in that, The chemical composition of the steel, by mass percentage, comprises: C: 0.35~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.0~2.2%, and (Si+Cr) / Al is 2.0~3.

5.

25. The steel according to claim 24, characterized in that, The thickness t of steel plate and the alloy element composition satisfy the following formula: lnv C = 5.0 - 3.5 C - 0.8 Mn - 0.3 Si - 0.6 Cr - 0.4 Ni - 1.1 Mo - 1.8 B + 0.5 Int, wherein the critical cooling rate v C is 10-21 °C / s.

26. The steel according to claim 25, characterized in that, The critical cooling rate v C is 11-19 °C / s.

27. The steel according to claim 24, characterized in that, The chemical composition of the steel, by mass percentage, comprises: C: 0.35~0.41%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.3~2.2%, and (Si+Cr) / Al is 2.0~3.

5.

28. The steel according to claim 24, characterized in that, The chemical composition of the steel, by mass percentage, comprises: C: 0.41~0.50%, Mn: 0.20~1.20%, Si: 0.25~0.85%, Cr: 0.20~0.70%, Al: 0.25~0.65%, Ni: 0.01~0.40%, Mo: 0.01~0.20%, B: 0.001~0.010%, V: 0.001~0.200%, Nb+Ti: 0.001~0.100%, with the balance being Fe and unavoidable impurities; wherein the sum of Si, Cr, and Al content is 1.0~2.2%, and (Si+Cr) / Al is 2.0~3.

5.

29. The steel according to any one of claims 24-28, characterized in that, The steel surface has an aluminum alloy coating with a thickness of 10~30μm.

30. The steel according to any one of claims 24-28, characterized in that, The chemical composition of the steel, by mass percentage, further comprises: W: 0.01~0.20%, Cu: 0.01~0.20%, Co: 0.01~0.50%, Ta: 0.001~0.100%, Sn: 0.001~0.050%, Sb: 0.001~0.050%, As: 0.001~0.050%, Mg: 0.001~0.010%, Ca: 0.001~0.0 10%, Zr: 0.001~0.100%, O: 0.001~0.020%, REM: 0.001~0.050%, P: 0.001~0.020%, S: 0.001~0.010%, N: 0.001~0.010%, and the sum of the contents of Ni, Mo, W, Cu, Co, Ta, Sn, Sb, Ca, As, Mg, Zr, and REM is 0.001~0.500%.

31. A process for manufacturing a welded pipe for producing a hot gas expansion tube component, characterized in that, Includes the following steps: S1. Steel slitting and roll forming The steel is slit according to the required diameter of the welded pipe, and the slit steel is rolled into a pipe blank using a roll forming die; the steel used is the steel described in any one of claims 24-30. S2. High-frequency welding The tube blank interface is heated and welded using an induction coil, and weld defects inside and outside the welded tube are removed after welding. S3. Post-weld online tempering The welded pipe is subjected to online tempering treatment at a temperature of 400~600℃; S4. Fixed diameter and fixed length Adjust the diameter of the welded pipe to the target diameter and cut it to a fixed length.

Images