A method for ultra-low temperature forming of a stainless steel monobloc tank bottom

Abstract

The present application relates to the technical field of metal plate forming, and discloses a stainless steel integral box bottom ultralow-temperature forming method.The method comprises the following steps: providing a stainless steel slab; according to the ultralow-temperature phase transformation strengthening rule and the box bottom deformation distribution, inversely calculating the target deformation degree and the required cooling temperature of each region; placing the slab into a mold and cooling to the target temperature; first, compressing the flange with high pressure side force, so that the center region deforms to fit the mold under the condition of double tensile stress; then, gradually reducing the side force, so that the flange region deforms to fit the mold under the condition of tensile stress to the target depth; finally, opening the mold and taking out the part.The method solves the coordinated control problem of wrinkling and cracking during the integral forming of an ultrathin and ultrahigh-strength stainless steel complex curved surface through the synergistic control of ultralow-temperature phase transformation strengthening and a staged stress path, and realizes the manufacture of a high-reliability rocket box bottom with large size, consistent strengthening of welds and base material.

Description

Technical Field

[0001] This invention relates to the field of metal sheet forming technology, and in particular to a method for cryogenic forming of an integral stainless steel box bottom. Background Technology

[0002] With the development of reusable and low-cost launch vehicles, stainless steel has gradually become the preferred material for next-generation rocket body structures due to its excellent comprehensive properties. The bottom of the rocket fuel tank, as a key ellipsoidal load-bearing component, directly affects the reliability and cost of the rocket due to its manufacturing technology.

[0003] Stainless steel has a density approximately three times that of aluminum alloy. To achieve a structural coefficient similar to that of aluminum alloy, the main structural material of currently operational rockets, the thickness of the stainless steel structure needs to be reduced to less than one-third that of aluminum alloy, while its strength needs to be nearly three times that of aluminum alloy. Therefore, the bottom of a stainless steel tank is an ultra-thin, ultra-high-strength ellipsoidal curved surface structure. For example, the bottom wall thickness of the propellant tank of a 4-meter diameter rocket is less than 2 millimeters, and its room temperature tensile strength requirement exceeds 1200 MPa, which far exceeds the limits of traditional integral molding.

[0004] The existing process is forced to first form multiple segments and a top cover from hard stainless steel sheets, which typically make up the bottom of the propellant tank of a 4-meter diameter rocket. These segments are then welded together. This method has many problems, such as long assembly and welding cycles (nearly twenty days per bottom), loss of weld performance due to the hard stainless steel being formed before welding, irreparable welding defects, and low reliability and lightweighting. It is difficult to meet the development requirements of low-cost, high-efficiency, and reusable rockets.

[0005] If an integrated stainless steel slab is used for integral forming, the traditional cold forming or hot forming techniques cannot solve the wrinkling and cracking problems during the forming process due to the ultra-thin and ultra-high strength requirements of the slab.

[0006] Therefore, there is an urgent need to develop a new method that can achieve integral forming of stainless steel box bottoms, applicable to both welded slabs and integral slabs, and solve the problem of coordinated control of wrinkling and cracking in the forming of ultra-thin and ultra-high strength stainless steel curved parts through ultra-low temperature phase transformation strengthening and staged stress path control. Summary of the Invention

[0007] The purpose of this invention is to provide a method for forming an integral stainless steel box bottom at ultra-low temperature, so as to solve the problems existing in the prior art and realize the integral forming of large-size, ultra-thin and ultra-high strength stainless steel box bottom curved parts.

[0008] To achieve the above objectives, the present invention provides the following solution:

[0009] This invention provides a method for cryogenic forming of a stainless steel integral tank bottom. Utilizing the cryogenic phase transformation strengthening effect of stainless steel and its welds, and through cryogenic cooling and staged stress path control, a thin stainless steel sheet welded in a flat state is formed into an integral tank bottom with welds that are uniformly reinforced with the base material. The method includes the following steps:

[0010] S1: Provide stainless steel slabs;

[0011] S2: Based on the relationship between the ultra-low temperature phase transformation strengthening of stainless steel and the deformation distribution law of the bottom of the box, the required degree of deformation and the required ultra-low temperature cooling temperature of each target area of ​​the slab after forming are calculated in reverse; and the required forming depth is obtained based on the required degree of deformation.

[0012] S3: Place the stainless steel slab into the forming mold and cool the stainless steel slab to the ultra-low temperature determined in step S2 using a low-temperature medium;

[0013] S4: Press the flange area of ​​the blank with the first blank holder force, and at the same time control the punch to move downward, so that the central area of ​​the blank is deformed under bidirectional tensile stress and comes into contact with the punch, until the deformation of the central area reaches the required forming depth calculated in step S2.

[0014] S5: Gradually reduce the blank holder force from the first blank holder force to the second blank holder force, while controlling the punch to continue to descend, so that the flange area blank is deformed and attached to the mold under the combined tensile and compressive stress state, until the deformation of the flange area reaches the required forming depth calculated in step S2.

[0015] S6: Open the mold and remove the formed stainless steel box bottom.

[0016] Preferably, in step S1, the stainless steel slab is a one-piece integral slab, or an integral slab formed by welding multiple stainless steel plates together in a flat state.

[0017] Preferably, the welding is performed using one of laser welding, argon arc welding, resistance welding, or submerged arc welding.

[0018] Preferably, in step S1, the stainless steel slab is a solution-treated stainless steel slab or a hardened stainless steel slab.

[0019] Preferably, in step S3, the cryogenic medium is one or more of liquid argon, liquid nitrogen, or liquid helium, forming a gas-liquid mixture.

[0020] Preferably, in step S3, the ultra-low temperature cooling temperature is from room temperature to -196°C.

[0021] Preferably, step S3 includes: after mold closing, forming a sealed cavity between the blank and the mold, introducing the low-temperature medium into the sealed cavity for cooling; and after the blank is cooled to a set temperature, pressurizing the low-temperature medium and using the low-temperature medium to pre-form the blank to a set height, so that the blank in the central area is fully deformed.

[0022] Preferably, in step S3, the cooling temperature and forming pressure of the slab are controlled by controlling the gas-liquid mixing ratio of the cryogenic medium and / or by using the vaporization pressurization of the cryogenic medium.

[0023] The present invention also provides a cryogenic forming apparatus for the method described above, comprising: a mold unit including a die, a punch and a blank holder; and a cryogenic medium supply unit for providing cryogenic medium and pressurization.

[0024] Preferably, the cryogenic medium supply unit includes a cryogenic pump, a storage tank, and a cryogenic valve for regulating the flow rate, pressure, and temperature of the cryogenic medium.

[0025] The present invention achieves the following technical effects compared to the prior art:

[0026] This invention successfully solves the problem of manufacturing ultra-thin, ultra-high-strength box bottoms using a single slab by combining "phase transformation strengthening of stainless steel through ultra-low temperature cooling" with "staged stress path control." The effect is derived as follows:

[0027] First, deformation at ultra-low temperatures provides the conditions for martensitic transformation of austenitic stainless steel, which is the physical basis for the material to obtain ultra-high strength (>1000MPa). Furthermore, the ultra-low temperature hardening characteristics can be utilized to prevent concentrated deformation and make the deformation transfer more uniform.

[0028] Secondly, the integrated slab avoids the potential weaknesses of welds and the extended welding cycle caused by splicing, pursuing structural integrity and manufacturing efficiency from the outset. However, the large size and thinness of the integrated slab make it more prone to overall instability during forming. To address this, a stress path control method of double tension followed by tension-compression provides a targeted solution: the initial high-pressure edge force at the center double tension forming places the material in a stress state most conducive to uniform extension and inhibiting buckling, fundamentally solving the "early wrinkling" problem, while simultaneously achieving phase transformation strengthening in the central region through ultra-low temperature; the subsequent release of the edge force and tension-compression forming allow the flange material to flow into the mold cavity under controlled conditions, thus avoiding excessive thinning and cracking in the central region, solving the "late cracking" problem. Finally, quantitative control based on the target inverse ensures that the deformation in each region accurately reaches the design value, thereby achieving a uniform distribution of the phase transformation strengthening effect and obtaining an integral box bottom with excellent overall performance and precise shape. This method breaks the traditional path of requiring large-size thin-walled parts to be formed in segments.

[0029] Furthermore, considering the width limitations of stainless steel plates in industrial production, large slabs can be welded together in a flat state, which is far more difficult to operate and produces higher welding quality than welding on complex curved surfaces. Straight welds are easy to automate and extremely fast, reducing the manufacturing cycle from "tens of days / month" of traditional interlocking welding to at least "one day / month," significantly improving production efficiency and cost advantages. More importantly, the plate is in a solution-treated (soft) state during welding, resulting in high weld metallurgical quality. Subsequent overall cryogenic forming allows the weld and base material to experience the same low-temperature environment and coordinated deformation. During this process, the microstructure of the weld zone also undergoes martensitic transformation strengthening, achieving equal strength matching between the weld and the base material (joint coefficient can reach 1.0), completely eliminating the inherent weak points of the welded structure. This "integration before strengthening" approach overturns the traditional "strengthening (hardened plate) before integration (welding weakening)" process, logically ensuring the performance integrity of the final product. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a schematic diagram of welding multiple stainless steel plates.

[0032] Figure 2 This is a schematic diagram of the structure of a stainless steel slab.

[0033] Figure 3 This is a schematic diagram of the cryogenic forming of stainless steel slabs under bitensile stress.

[0034] Figure 4 This is a schematic diagram of the cryogenic forming of stainless steel slabs under tensile and compressive stress.

[0035] Figure 5 This is a schematic diagram of the overall box bottom structure after molding;

[0036] Figure 6 A schematic diagram of the traditional melon-shaped welded box bottom;

[0037] Figure 7 A schematic diagram of the forming process for a spherical box-shaped curved surface component;

[0038] Figure 8 This is a schematic diagram of the numerical simulation model;

[0039] Figure 9This is a schematic diagram of the equivalent strain distribution after the bottom of a stainless steel box with a diameter of 4.2m and a wall thickness of 2.0mm is formed.

[0040] Figure 10 The graph shows the relationship between the room temperature yield strength of 304L stainless steel at different temperatures and degrees of deformation.

[0041] In the figure: 1-Stainless steel slab; 2-Welding device; 3-Weld seam; 4-Die; 5-Pressure ring; 6-Punch; 7-Sealing element; 8-Cryogenic valve; 9-Cryogenic pump; 10-Storage tank; 11-Ultra-low temperature medium; 12-Integral tank bottom; 13-Traditional melon-shaped welded tank bottom; 101-Stainless steel sheet.

[0042] 1-1 Flange area; 1-2 Rounded corner area; 1-3 Suspended area; 1-4 Mold application area. Detailed Implementation

[0043] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] The purpose of this invention is to provide a method for forming an integral stainless steel box bottom at ultra-low temperature, so as to solve the problems existing in the prior art and realize the integral forming of large-size, ultra-thin and ultra-high strength stainless steel box bottom curved parts.

[0045] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0046] The following is combined with Figures 1 to 10 The following describes embodiments of the present invention.

[0047] Example 1

[0048] This invention provides a method for cryogenic forming of a stainless steel integral box bottom. Utilizing the cryogenic phase transformation strengthening effect of stainless steel and its welds, and through cryogenic cooling and staged stress path control, a thin stainless steel sheet welded in a flat state is formed into an integral box bottom 12 with welds 3 consistent with the base material. The method includes the following steps:

[0049] S1: Provide stainless steel slab 1;

[0050] S2: Based on the relationship between the ultra-low temperature phase transformation strengthening of stainless steel and the deformation distribution law of the bottom of the box, the required degree of deformation and the required ultra-low temperature cooling temperature of each target area of ​​the stainless steel slab 1 after forming are calculated in reverse; and the required forming depth is obtained based on the required degree of deformation. Specifically, the degree of deformation of each area of ​​the stainless steel slab 1 after forming into an integral bottom of the box 12 without cracking or wrinkling is calculated in reverse through mechanical analysis and numerical simulation. Then, based on the correspondence between the degree of deformation, temperature and strength, the ultra-low temperature cooling temperature required to achieve the target strength performance is determined.

[0051] S3: Place the stainless steel slab 1 into the forming mold and cool the stainless steel slab 1 to the ultra-low temperature determined in step S2 using a low-temperature medium.

[0052] S4: With the first blank holder force ( Figure 3 F in 合模1 The flange area of ​​the slab is pressed, and the punch 6 is controlled to move downward, so that the central area of ​​the stainless steel slab 1 is deformed under biaxial tensile stress and comes into contact with the punch 6 until the deformation of the central area reaches the required forming depth calculated in step S2.

[0053] S5: Gradually reduce the blank holder force from the first blank holder force to the second blank holder force. Figure 4 F in 合模2 At the same time, the punch 6 is controlled to continue to descend, so that the flange area blank is deformed under the combined stress of tension and compression and comes into contact with the mold until the deformation of the flange area reaches the required forming depth calculated in step S2.

[0054] S6: Open the mold and remove the formed stainless steel integral box bottom 12.

[0055] This embodiment successfully solves the problem of manufacturing ultra-thin, ultra-high-strength box bottoms using a single-piece slab by combining "phase transformation strengthening of stainless steel through ultra-low temperature cooling" with "staged stress path control." The effect is derived as follows: First, the ultra-low temperature environment provides conditions for the martensitic phase transformation of austenitic stainless steel, which is the physical basis for the material to achieve ultra-high strength (>1000MPa). Second, the single-piece slab avoids the potential weaknesses of weld seams and the increased welding cycle caused by splicing, pursuing structural integrity and manufacturing efficiency from the source. However, the single-piece slab is large in size and thin in thickness, making it more prone to global instability during forming. To address this, a targeted solution is found in the stress path control method of first performing double tension followed by tension-compression: the initial double tension forming at the center under high pressure edge force places the material in a stress state most conducive to uniform extension and suppressing buckling, fundamentally solving the "early wrinkling" problem. Simultaneously, ultra-low temperature is used to achieve phase transformation strengthening in the central region. Subsequent release of the edge force and tension-compression forming allow the flange material to flow into the mold cavity under controlled conditions, thus avoiding excessive thinning and cracking in the central region, solving the "late cracking" problem. Finally, quantitative control based on the target inverse calculation ensures that the deformation in each region accurately reaches the design value, resulting in a uniform distribution of the phase transformation strengthening effect and obtaining a monolithic box bottom 12 with excellent overall performance and precise shape. This method breaks away from the traditional path of requiring segmented forming for large-size thin-walled parts.

[0056] The above S2 can be further specified as follows:

[0057] Establish deformation geometry and mechanics model: Based on the target shape of the box bottom and the geometric parameters of the forming mold, and based on the principle of constant volume and the theory of plate and shell forming, analyze and calculate the area changes, stress distribution and strain distribution of the flange area, fillet area, suspended area and mold-attached area during the forming process, and obtain the theoretical deformation degree of each region.

[0058] Numerical simulation to correct strain distribution: A refined numerical model is established using finite element software (such as Abaqus), taking into account actual factors such as wall thickness reduction, friction, and changes in blank holder force, to simulate the forming process of the slab without cracking or wrinkling, and to correct and output the actual deformation distribution of each region.

[0059] Establish the relationship between "deformation degree-temperature-strength" of materials: Through tensile tests of wide plates at different temperatures (room temperature to -196℃) and different pre-deformation amounts, test the strength and elongation of the specimens at the service temperature, construct a performance database of materials, and clarify the minimum cryogenic cooling temperature required to achieve the target performance at each deformation degree.

[0060] Inverse process parameter determination: Based on the deformation degree and material property database of each region obtained from the comprehensive numerical simulation, and according to the target performance indicators of each region at the bottom of the chamber (such as room temperature yield strength ≥950MPa), the minimum cryogenic cooling temperature required for each region is deduced; at the same time, the corresponding forming depth is determined according to the required deformation degree. In engineering, cooling is usually controlled uniformly by the lowest temperature of the most stringent region.

[0061] For ease of understanding, please refer to the appendix. Figure 7 Taking the forming of a spherical box-bottom curved surface part as an example, the specific details are as follows:

[0062] 1-Calculation of geometric parameters of the deformation zone:

[0063] Based on the principle of constant volume and ignoring wall thickness changes during slab forming, the sum of the areas of the four typical deformation regions—flange area, fillet area, overhang area, and mold-attached area—when the box bottom is formed to different depths, is consistent with the initial area of ​​the slab, satisfying the following:

[0064] (1);

[0065] In the formula: R 0 The initial radius of the slab; S Ⅰ The area of ​​the annular region 1-1 in the flange area; S Ⅱ The area of ​​rounded corner region 1-2; S Ⅲ The area of ​​the suspended region 1-3 is the cone surface area; S Ⅳ This refers to the area of ​​the molding area 1-4.

[0066] The area of ​​each region can be determined based on the known forming depth. H And obtained from the already confirmed mold geometry. Among them, based on a certain forming depth. H The geometric relationship at that time can be used to derive the radius of gyration of the molded surface in the molding area 1-4. R α With cone angle α :

[0067] (2);

[0068] In the formula: R p The radius of the punch is 6. H For forming depth; r d The radius of the 4-corner radius of the die cavity.

[0069] Based on this, the height of cones 1-3 in the suspended region was further derived. H u :

[0070] (3);

[0071] Based on geometric relationships, the real-time areas of rounded corner area 1-2, suspended area 1-3, and molded area 1-4 can be calculated:

[0072] (4);

[0073] In the formula, substitute equation (4) into equation (1), and use the area of ​​the flange region 1-1 annular circle. S Ⅰ Inverse calculation of the instantaneous outer diameter of the slab R :

[0074] (5);

[0075] The circumferential strain in each region can be analyzed based on the instantaneous radius of the slab.

[0076] 2-Mechanical analysis of each deformation zone:

[0077] The strain distribution after the box bottom is formed is qualitatively determined through mechanical analysis to serve as a criterion for determining the cryogenic cooling temperature. According to sheet metal forming theory, during the forming of box-shaped curved parts, the radial stress in the suspended area 1-3 is always tensile. Due to the material flowing from the flange area 1-1 into the mold during the blank forming process, the circumferential dimension shrinks, resulting in compressive stress; the mold-fitting area 1-4 undergoes bulging deformation, resulting in circumferential dimension expansion, resulting in tensile stress. Therefore, the circumferential stress gradually changes from the compressive stress at the fillet tangent point of the die 4 to the tensile stress at the tangent point of the punch 6. The radial tensile stress is always greater than the circumferential tensile stress and is the first principal stress. According to the material hardening function, the magnitude of the radial tensile stress can be used to qualitatively calculate the degree of deformation in each region of the material.

[0078] During the forming of the box bottom, the slab is mainly divided into three deformation zones: the first is the flange area 1-1 where the die 4 and the pressure ring 5 are pressed together; the second is the suspended area 1-3 between the flange pressing and the mold; and the third is the mold-fitting area 1-4 where it is already attached to the punch 6. The radial tensile stress in the flange area 1-1... The radial force balance during the deformation of the slab in flange zone 1-1 can be used to determine the instantaneous outer diameter R of the slab.

[0079] (6);

[0080] In the formula, This indicates the hardening stress (equivalent stress) at the edge of flange zone 1-1. r Let be the radial radius at any position. μ b This indicates the friction factor between the mold and the slab in the flange area. F bThis indicates the blank holder force applied to flange area 1-1.

[0081] Radial stress in suspended regions 1-3 The radial tensile stress at the entrance of die 4 can be determined based on the radial force balance during the deformation of the slab in the suspended zone 1-3.

[0082] (7);

[0083] In the formula, This indicates the radial stress in the suspended region 1-3. This indicates the radial stress at the inlet of die 4. t Indicates the slab wall thickness.

[0084] After simplifying equation (7), the radial stress at any point in the suspended region 1-3 can be obtained:

[0085] (8);

[0086] To draw the material from flange area 1-1 to die 4, the radial stress at the entrance of die 4 is subject to the radial stress from flange area 1-1. In addition, there is the blank holder force. F b Friction generated on the material surface in flange area 1-1 σ m The bending resistance generated when the material in flange area 1-1 flows through the fillet of die 4 σ w And the material in flange area 1-1 bypasses the die cavity 4 fillet to overcome resistance. T e μα .

[0087] (9);

[0088] (10);

[0089] In the formula, μ The friction factor between the rounded corner slab and the mold. α This refers to the corner radius at the rounded corner of the die.

[0090] Radial tensile stress at any point in the rounded corner region 1-2:

[0091] (11);

[0092] Radial tensile stress at any point in the suspended region 1-3:

[0093] (12);

[0094] Radial tensile stress at any point in the molding area 1-4:

[0095] (13);

[0096] The strain-stress relationship of the material satisfies:

[0097] (14);

[0098] In the formula, In response, For stress.

[0099] Substituting equations 11-13 into equation 14 yields the strain distribution in each region.

[0100] 3-Numerical simulation of each deformation zone:

[0101] Considering that the mechanical analysis did not take into account wall thickness reduction, numerical simulation was used to further correct and optimize the strain distribution of the slab after it was formed into a box bottom. Commercial simulation software such as Abaqus, Ansys, and Dynaform can be used for numerical simulation. Taking the numerical simulation of the strain distribution of a 4.2m diameter stainless steel box bottom as an example: the numerical simulation was performed using the Abaqus 6.14 software platform, using a 1 / 4 model. The slab was a three-dimensional deformable body, divided into four layers of hexahedral free meshes along the thickness direction, with a mesh size of 5mm. The three forming dies—die 4, blank holder 5, and punch 6—were all discrete rigid bodies, divided into quadrilateral structure meshes with a mesh size of 5mm. The contact relationship between the die 4, blank holder 5, and punch 6 and the slab was a surface-to-surface adaptive contact, with tangential behavior being a penalty function, and the corresponding friction coefficient was 0.12. The material properties of the slab were 304L stainless steel stress-strain relationship. The blank holder force was applied through a pressure load, with 7MPa for the first 60% of the forming depth and 5MPa for the last 40% of the forming depth. It should be noted that the application of blank holder force should take into account the avoidance of defects and obtain a strain distribution for the box bottom forming that does not cause wrinkling or cracking.

[0102] 4. Correspondence between deformation, temperature, and strength:

[0103] Tensile tests were conducted on wide plates at different temperatures to obtain specimens with varying degrees of deformation. Single-stretch specimens were then cut from these pre-deformed wide plate specimens. Specimen preparation referenced the national standard (GB / T 228.1-2021). The yield strength, tensile strength, and elongation, among other mechanical properties, were tested at the corresponding service temperatures to establish the relationship between the degree of deformation, deformation temperature, and strength. For example, to obtain the relationship between the degree of deformation, deformation temperature, and strength of a 2mm thick 304L solution-treated stainless steel plate: the wide plate specimen was 20mm wide and 80mm long; the deformation temperatures were 0℃, -40℃, -80℃, -120℃, -160℃, and -190℃; the corresponding deformation degrees were 0.07, 0.10, 0.15, and 0.20; the width of the cut mechanical property test specimen was 6mm and the length was 30mm. Through testing or literature review, the room temperature yield strength relationship of 304L stainless steel at different temperatures and degrees of deformation was obtained as follows: Figure 10 .

[0104] 5. Setting the ultra-low temperature cooling temperature:

[0105] Combining the strain distribution of the box bottom formed by mechanical analysis and numerical simulation calculations with the deformation temperature-deformation degree-strength relationship of the slab, and then based on the required strength performance indicators for each region of the box bottom, the cryogenic cooling temperature of the slab is comprehensively determined. Taking a 304L stainless steel box bottom with a diameter of 4.2m and a wall thickness of 2mm as an example, the room temperature yield strength of the entire box bottom region needs to be greater than 950MPa. Based on the strain distribution calculated by numerical simulation (… Figure 9 It can be seen that the deformation degree in the bottom area is relatively small, at 0.12; according to the room temperature yield strength relationship of 304L stainless steel under different temperatures and deformation degrees ( Figure 10 As can be seen, to achieve a yield strength of over 950 MPa at room temperature, the bottom needs to be cooled to below -180°C; similarly, the ultra-low temperature cooling temperatures for other regions are determined. Considering engineering control and stability, the slab temperature is usually controlled by cooling to the lowest possible temperature.

[0106] In some embodiments, in step S1, the stainless steel slab 1 is an integral slab, or an integral slab formed by welding multiple stainless steel plates 101 together in a flat state.

[0107] This embodiment provides a practical solution for situations where ultra-wide integrated slabs are unavailable, offering unique advantages. The effects are deduced as follows: Welding in a flat plate state significantly reduces operational difficulty and improves welding quality compared to welding on complex curved surfaces. Straight weld 3 is easily automated and extremely fast, reducing the manufacturing cycle from "tens of days / month" in traditional interlocking welding to at least "one day / month," greatly improving production efficiency and cost advantages. More importantly, the plate is in a solution-treated (soft) state during welding, resulting in high metallurgical quality for weld 3. Subsequent cryogenic forming ensures that weld 3 and the base material experience identical low-temperature environments and coordinated deformation. During this process, the microstructure of weld 3 also undergoes martensitic transformation strengthening, achieving equal strength matching between weld 3 and the base material (joint coefficient up to 1.0), completely eliminating inherent weaknesses in the welded structure. This "integration before strengthening" approach overturns the traditional "strengthening (hard plate) before integration (welding weakening)" process, logically guaranteeing the performance integrity of the final product.

[0108] The methods for achieving butt welding are alternative. Welding methods are not limited to laser welding or TIG welding; for specific thicknesses and materials, resistance welding, submerged arc welding, or electron beam welding are also options. The sheet metal splicing pattern can be optimized according to the sheet metal specifications and the unfolded diagram of the box bottom, not limited to simple butt joints, but also using "Y"-shaped or "cross"-shaped splicing to optimize the weld seam layout. Furthermore, non-destructive testing and defect repair can be performed in the flat state before welding, an advantage that curved surface welding cannot match.

[0109] In some embodiments, in step S1, the stainless steel slab 1 is a solution-treated stainless steel slab or a hardened stainless steel slab. Specifically, the stainless steel slab 1 is 301, 304, 316, or other solution-treated or hardened stainless steel.

[0110] Choosing solution-treated slabs as raw materials is a key preferred option in this invention, with significant advantages. The reasoning is as follows: the microstructure of solution-treated stainless steel is a single, uniform austenite with a low dislocation density, giving it excellent plastic forming capabilities. In the subsequent "weld-then-form" process, this excellent plasticity ensures that the welded slab can withstand the large deformation required for overall forming without cracking, a prerequisite for the process's implementation. More importantly, this uniform austenitic structure provides optimal initial conditions for the ultra-low temperature phase transformation strengthening effect. During low-temperature deformation, it can transform into high-strength martensite with higher efficiency and greater uniformity, thereby achieving the maximum performance leap from "extremely soft" to "extremely strong," most easily reaching ultra-high strength indicators above 1200 MPa. Therefore, solution-treated slabs are the best carrier for realizing the core concept of this invention: "obtaining shape through a soft state and strength through phase transformation."

[0111] Using hardened slabs expands the applicability of this method, making it suitable for specific supply chains or scenarios requiring initial stiffness. The effect is deduced as follows: Hardened stainless steel sheets are typically already undercold-deformed, containing a certain amount of deformation-induced martensite and high dislocation density, resulting in high initial strength. The cryogenic forming process of this method has two effects: First, it further induces martensitic transformation in the residual austenite portion of the slab, producing additional strengthening; second, the new, controlled overall plastic deformation can adjust and homogenize the existing dislocation structure and introduce new dislocations. This process not only pushes the final strength of the material to the target value (e.g., 1200 MPa), but more importantly, it allows all regions of the slab (especially the heat-affected zone of the weld seam in welded plates) to undergo a synchronous "re-strengthening," effectively improving the performance inhomogeneity caused by the previous cold rolling or welding of the hardened sheet, thereby enhancing the overall reliability of the component. Furthermore, since the hardened sheet already has deformation strengthening, it can reduce the degree of deformation in the subsequently formed components, facilitating engineering implementation.

[0112] The definition of "hard state" can encompass different states such as 1 / 4 hard, 1 / 2 hard, and fully hard obtained by different cold rolling deformation amounts.

[0113] In some embodiments, in step S3, the cryogenic medium is one or more of liquid argon, liquid nitrogen, or liquid helium, which are gas-liquid mixtures.

[0114] In some embodiments, in step S3, the cryogenic cooling temperature is from room temperature to -196°C.

[0115] In some embodiments, step S3 includes: forming a sealed cavity between the blank and the mold after mold closing, introducing a low-temperature medium into the sealed cavity for cooling; and pressurizing the low-temperature medium after the blank has cooled to a set temperature, using the low-temperature medium to pre-form the blank to a set height, so that the blank in the central area is fully deformed.

[0116] This embodiment adds the optimized step of "low-temperature medium pressure forming," a sophisticated strategy for proactively managing strain distribution and enhancing forming limits. The effect is derived as follows: Before the punch 6 contacts the slab, the fluid pressure within the sealed cavity allows the slab to freely bulge, inducing initial deformation under ideal conditions with no contact friction. At this point, the material is in its purest and most uniform biaxial tensile stress state, maximizing its forming potential. This step brings two decisive benefits: First, it pre-consumes a portion of the required overall deformation, particularly ensuring sufficient and high-quality pre-deformation of the central region, which is the most difficult and least prone to deformation in subsequent processes, thus significantly reducing the workload and cracking risk during the deep drawing stage of punch 6. Second, the low-temperature medium pressure forming process itself is an effective low-temperature deformation, inducing martensitic phase transformation in the surface material earlier and more uniformly, giving the slab a certain degree of "pre-strengthening" before contacting punch 6, enhancing its resistance to subsequent complex stress states. This step is crucial for solving cracking, improving material utilization, and enhancing top performance.

[0117] The control endpoint for cryogenic medium pressurization molding can be a set molding height or reaching a predetermined pressure value. The pressurization power can come entirely from an external high-pressure air source or hydraulic pump, or it can cleverly utilize the natural pressure increase generated by the heat absorption and vaporization of the cryogenic medium in a closed space (i.e., "vaporization pressurization") to reduce the energy consumption of the external system.

[0118] The formation of a sealed cavity can be achieved through various sealing elements 7, such as O-rings, polytetrafluoroethylene sealing strips, etc.

[0119] In some embodiments, in step S3, the cooling temperature and forming pressure of the slab are controlled by controlling the gas-liquid mixing ratio of the cryogenic medium and / or by utilizing the vaporization pressurization of the cryogenic medium. This is mainly achieved in real time by controlling the volume of the injected cryogenic medium and by continuously monitoring the pressure and temperature of the sealed cavity.

[0120] This embodiment achieves direct temperature control and pressurization of the cryogenic medium 11 by controlling the gas-liquid mixing ratio and vaporization pressurization, eliminating the need for an environmental chamber for temperature regulation and reducing cryogenic medium consumption.

[0121] The specific execution steps are as follows:

[0122] Step 1: Slab design and plate welding:

[0123] Based on the final shape and size of the target box bottom, unfold calculations are performed to design and cut multiple (e.g., three) stainless steel plates 101.

[0124] In the flat state, these plates are welded into a whole slab using a welder 2 (preferably laser welding, argon arc welding, resistance welding, or submerged arc welding). Preferably, a base material of appropriate width is selected according to the diameter of the slab to reduce the number of welds 3.

[0125] Radiographic inspection is performed immediately after welding. The control standard is that the micropore size generated in weld 3 must be less than 0.05 times the slab thickness. If defects exceeding the standard are detected, local re-welding is performed using the same welding process, with a re-welding length greater than 10mm. This inspection and repair in the flat state fundamentally solves the industry problem of the difficulty in repairing weld 3 defects after the curved component is formed.

[0126] Step Two: Reverse Engineering Design of Process Parameters

[0127] Based on the ultra-low temperature phase transformation strengthening mechanical model of the stainless steel material used (i.e. the variation law of material strength and plasticity with temperature and deformation), and combined with the deformation distribution simulation analysis of the target shape of the bottom of the box, the degree of slab deformation required for different areas of the bottom of the box to achieve the target performance and the corresponding optimal ultra-low temperature cooling temperature are calculated in reverse, providing a data basis for subsequent precise shape and property control.

[0128] Step 3: Slab positioning and low-temperature cooling:

[0129] The welded slab is transferred to the forming equipment and precisely positioned on the die 4.

[0130] Drive the pressure ring 5 downward to press the slab flange area.

[0131] The driving punch 6 moves down to a position where it engages with the straight wall of the pressure ring 5, and a sealed cavity is established on the upper side of the blank through the sealing element 7 that cooperates with the mold.

[0132] A cryogenic medium (such as liquid nitrogen) is introduced into the cavity. By adjusting the mixing ratio of the gaseous and liquid states of the medium, the slab is precisely cooled to the cryogenic temperature (e.g., the range of -150°C to -196°C) required in step two.

[0133] Step 4: Bi-stretching and strengthening of the central area:

[0134] Increase the blank holder force to fully lock the material in the flange area.

[0135] Before the punch 6 descends, the cryogenic pump 9 is started to pressurize the cryogenic medium in the storage tank 10 and inject it into the sealed cavity, causing the slab to be pre-formed to the predetermined height. This avoids contact friction between the punch 6 and the top area, allowing it to fully deform under pure bitensile stress, improving the phase transformation strengthening effect in this area, and solving the problems of insufficient top deformation and low performance in traditional compression molding.

[0136] As the punch 6 begins to descend, it works in conjunction with the internal medium pressure to cause the central slab to continuously deform under a state of double tensile stress until it completely adheres to the surface of the punch 6, thus completing the shape forming and phase transformation strengthening of the central area (dome) of the box bottom.

[0137] During the forming process, the pressure inside the cavity can be maintained or adjusted by utilizing the vaporization and pressurization effect of the cryogenic medium itself. This method can reduce the pressurization load and medium consumption of the cryogenic pump 9, thereby reducing the system's rated pressure requirements and production costs.

[0138] Step 5: Flange area tension forming and final molding:

[0139] The blank holder force is gradually reduced according to the preset blank holder force-displacement curve.

[0140] As punch 6 continues to descend, the slab in the flange area begins to flow into the die cavity under reduced blank holder force, and the stress state changes from pure tension to a combined tension and compression state. This process causes the slab (especially the sidewalls and opening areas) to gradually come into contact with punch 6, completing the final shape of the box bottom. This step, by introducing a controllable compressive stress component, avoids excessive thinning and cracking that could occur if the slab is in a double-tension state throughout the process.

[0141] Step Six: Component Removal

[0142] After forming, the pressure is released, the mold is opened (punch 6 returns, pressure ring 5 is lifted), and the obtained stainless steel integral box bottom 12 can be taken out. After the weld 3 of this component is strengthened by low temperature phase transformation, its strength is consistent with that of the base material (joint coefficient can reach 1.0), and the overall shape is accurate, without wrinkles or cracks.

[0143] Example 2

[0144] The present invention also provides a cryogenic forming apparatus for the method in Embodiment 1, comprising:

[0145] The mold unit includes a die 4, a punch 6, and a blank holder 5;

[0146] The cryogenic medium supply unit is used to provide cryogenic medium and pressurize it.

[0147] In some embodiments, the cryogenic medium supply unit includes a cryogenic pump 9, a storage tank 10, and a cryogenic valve 8 for regulating the flow rate, pressure, and temperature of the cryogenic medium.

[0148] Example 3

[0149] The present invention also provides a stainless steel integral box bottom, which is made by the method described in Embodiment 1 above. The box bottom has an ellipsoidal curved surface structure, a wall thickness of less than 2 mm, a room temperature yield strength of more than 1200 MPa, and when the box bottom is made of welded slab blanks, the joint coefficient of weld seam 3 can reach 1.0.

[0150] Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this invention. Furthermore, those skilled in the art will recognize that, based on the ideas of this invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. A method for cryogenic forming of a single stainless steel tank bottom, characterized in that, By using ultra-low temperature phase transformation strengthening and staged stress path control, stainless steel sheets welded in a flat state are formed into an integral box bottom with weld seams that are uniformly reinforced with the base material. This process includes the following steps: S1: Provide stainless steel slab blanks; S2: Based on the relationship between the ultra-low temperature phase transformation strengthening of stainless steel and the deformation distribution law of the bottom of the box, the required degree of deformation and the required ultra-low temperature cooling temperature of each target area of ​​the slab after forming are calculated in reverse, and the required forming depth is obtained according to the required degree of deformation. S3: Place the stainless steel slab into the forming mold and cool the stainless steel slab to the ultra-low temperature determined in step S2 using a low-temperature medium; S4: Press the flange area of ​​the blank with the first blank holder force, and at the same time control the punch to move downward, so that the central area of ​​the blank is deformed under bidirectional tensile stress and comes into contact with the punch, until the deformation of the central area reaches the required forming depth calculated in step S2. S5: Gradually reduce the blank holder force from the first blank holder force to the second blank holder force, while controlling the punch to continue to descend, so that the flange area blank is deformed and attached to the mold under the combined tensile and compressive stress state, until the deformation of the flange area reaches the required forming depth calculated in step S2. S6: Open the mold and remove the formed stainless steel box bottom.

2. The method for cryogenic forming of a single stainless steel tank bottom according to claim 1, characterized in that: In step S1, the stainless steel slab is a one-piece integral slab, or an integral slab formed by welding multiple stainless steel plates together in a flat state.

3. The method for cryogenic forming of a single stainless steel tank bottom according to claim 2, characterized in that: The welding process employs one of the following methods: laser welding, argon arc welding, resistance welding, or submerged arc welding.

4. The method for cryogenic forming of a single stainless steel tank bottom according to claim 1, characterized in that: In step S1, the stainless steel slab is a solution-treated stainless steel slab or a hardened stainless steel slab.

5. The method for cryogenic forming of a single stainless steel tank bottom according to claim 1, characterized in that: In step S3, the cryogenic medium is one or more of liquid argon, liquid nitrogen, or liquid helium, which are gas-liquid mixtures.

6. The method for cryogenic forming of a single stainless steel tank bottom according to claim 1, characterized in that: In step S3, the low-temperature cooling temperature is from room temperature to -196°C.

7. The method for cryogenic forming of a single stainless steel tank bottom according to claim 1, characterized in that: Step S3 includes: after mold closing, a sealed cavity is formed between the blank and the mold, and the low-temperature medium is introduced into the sealed cavity for cooling; after the blank is cooled to the set temperature, the low-temperature medium is pressurized, and the low-temperature medium is used to pre-form the blank to the set height, so that the blank in the central area is fully deformed.

8. The method for cryogenic forming of a single stainless steel tank bottom according to claim 7, characterized in that: In step S3, the cooling temperature and forming pressure of the slab are controlled by controlling the gas-liquid mixing ratio of the cryogenic medium and / or by using the vaporization pressurization of the cryogenic medium.

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