Manufacturing method of pickling-free high-strength cold-formed thin-walled trapezoidal steel pipe

Abstract

This invention discloses a method for manufacturing high-strength cold-formed thin-walled trapezoidal steel pipes without pickling. It includes the following steps: Step 1: heating the steel strip and cleaning away any rust-preventive oil that has seeped into the surface of the steel strip; Step 2: cold-forming the steel strip into a rough shape; Step 3: sealing and extruding the cold-formed steel pipe, and then welding the two welded edges together using high-frequency welding to form a cold-formed closed-end steel pipe, removing the external burrs from the weld seam; Step 4: sizing and finishing the cold-formed closed-end steel pipe after removing the external burrs; Step 5: straightening the welded cold-formed closed-end steel pipe; Step 6: sawing the straightened steel pipe with a flying saw; Step 7: applying online oil to the surface of the steel pipe, air-drying, adding padding, packaging, draining offline, plastic wrapping, and warehousing. This invention improves the yield of acid-free cold-formed high-strength thin-walled trapezoidal steel pipes and reduces scrap losses caused by difficulties in forming, poor surface quality, and poor welding quality.

Description

Technical Field

[0001] This invention belongs to the field of cold-formed steel pipe manufacturing technology, specifically relating to a method for manufacturing high-strength cold-formed thin-walled trapezoidal steel pipes without pickling. Background Technology

[0002] Acid-free cold-bending high-strength thin-walled trapezoidal steel pipes are used in bus chassis supports (such as...). Figure 1 As shown, due to environmental protection requirements, the pickling process for this type of product has been eliminated, requiring the use of pickled steel coils to produce steel pipes. However, because the surface of the pickled steel coils lacks the protection of iron oxide scale, it is prone to scratches and corrosion. Therefore, the surface of the pickled steel coils is coated with anti-rust oil. Since the steel coil surface is oily and lacks iron oxide scale protection, surface oil can enter the rolling mill's circulating water tank during steel pipe manufacturing, causing the rolling mill to slip, resulting in poor rolling speed stability and affecting welding quality. Furthermore, the lack of iron oxide scale protection leads to surface scratches during steel pipe manufacturing. Simultaneously, due to the material's high tensile strength (≥700MPa), thin wall thickness (<2.0mm), aspect ratio >2, and small forming angle (<70°), forming is difficult. These factors result in a yield rate of less than 90% for this type of product.

[0003] Existing high-strength cold-formed steel pipes have moderate wall thickness (3-12mm), centered weld seams, and relatively low physical properties such as yield strength (235-600MPa) and high elongation (≥20%), making them easy to cold-form and weld. However, pickling-free cold-formed high-strength thin-walled trapezoidal steel pipes present challenges in rough forming, welding, and finishing due to their high material strength, thin wall thickness, and asymmetrical shape.

[0004] The production of acid-free cold-bent high-strength thin-walled trapezoidal steel pipes needs to address quality issues such as welding misalignment and incomplete welding caused by edge waviness during the cold bending process due to thin wall thickness and high strength; it also needs to solve problems such as asymmetrical shape, poor forming stability, and difficulty in actual bending due to a small fillet angle.

[0005] Pickling-free cold-formed high-strength thin-walled trapezoidal steel pipes use high-strength steel strips with no iron oxide scale on the surface after pickling. The steel strip surface is coated with anti-rust oil. However, when the anti-rust oil enters the rolling mill, it contaminates the rolling lubricant in the circulating water pool. As a result, during the production process, oil stains adhere to the surface of the rolls, reducing the friction between the rolls and the steel strip, and causing the rolling mill to slip. The slippage of the rolling mill will scratch the steel strip without iron oxide scale protection, and will also cause the rolling speed to be unstable, affecting high-frequency welding, and causing pinholes and cracks in the weld.

[0006] After forming, the surface of the acid-free cold-bent high-strength thin-walled trapezoidal steel pipe is easily scratched due to the lack of iron oxide scale protection, and the inner and outer surfaces will rust quickly, affecting normal use. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the aforementioned background technology and provide a method for manufacturing high-strength cold-formed thin-walled trapezoidal steel pipes without pickling.

[0008] The technical solution adopted in this invention is: a method for manufacturing high-strength cold-formed thin-walled trapezoidal steel pipes without pickling, comprising the following steps:

[0009] Step 1: Heat the steel strip to allow the anti-rust oil on the surface of the pickled and oiled steel strip to overflow, and clean the overflowed anti-rust oil from the surface of the steel strip.

[0010] Step 2: Perform cold bending rough forming of the steel strip according to the external dimensions of the acid-free cold-bent high-strength thin-walled trapezoidal steel pipe;

[0011] Step 3: The cold-formed steel pipe is closed by extrusion, and the two welding edges are extruded and welded by high-frequency welding to form a cold-formed closed steel pipe. The external burrs of the weld seam of the cold-formed closed steel pipe are removed.

[0012] Step 4: The cold-bent closed steel pipe after deburring is sized and finished.

[0013] Step 5: Straighten the welded cold-bent closed steel pipe;

[0014] Step 6: Cut the straightened steel pipe with a flying saw;

[0015] Step 7: Apply oil to the surface of the steel pipe online, air dry, add padding, pack it, drain it offline, wrap it in plastic, and put it into storage.

[0016] In step 1 above, a thermoelectric heater is used to heat the steel strip.

[0017] In step 1 above, wool felt pads are used to clean the rust-preventive oil from the heated steel strip surface.

[0018] In step 2 above, cold bending rough forming includes single-bend forming, double-bend forming, and closed forming. During the single-bend forming process from the first to the third batch, the forming angle on one side is 30°, 40°, and 45° respectively. During the double-bend forming process from the fourth to the seventh batch, the forming angle on one side is 20°, 35°, 50°, and 60° respectively. During the closed forming process from the eighth to the tenth batch, the forming angle on one side is 70°, 80°, and 86° respectively for the bottom angle. The welded edge angle depends on... The angles for the second bend are 70°, 75°, and 82°. During the first to third bend forming processes, the other side forming angles for each bend are 30°, 40°, and 50°. During the fourth to seventh bend forming processes, the other side forming angles for each bend are 20°, 35°, 50°, and 68°. During the eighth to tenth closed forming processes, the other side forming angles for each bend are: bottom angle 86°, 100°, and 110°; welded edge angle 62°, 62°, and 62°.

[0019] The sixth and seventh rolling stands use bearing rolls, with a bearing connection between the roll and the shaft. These rolls rotate passively during rolling, preventing scratches on the steel pipe surface.

[0020] In step 3 above, the cold-formed steel pipe is subjected to closed extrusion through an upper extrusion roller and two side extrusion rollers. The upper extrusion roller is a vertical roller with an arc, which is convenient for controlling the stability of the welding of the two welding sides. Due to the asymmetry of the two sides of the steel pipe, the two side extrusion rollers are box rollers, one of which is embedded, which is convenient for tightly wrapping the steel pipe during welding and avoiding the vibration of the steel pipe from affecting the welding stability.

[0021] In step 4 above, a four-roller arc-wrapping forming method is used for sizing and finishing. This facilitates the steel pipe's dimensional accuracy to meet standard requirements, and in particular, the arc-wrapping forming of the small beveled edge ensures the precision of this arc value.

[0022] In step 3 above, a curved blade is used to remove the burrs on both sides of the weld.

[0023] In step 7 above, a water-spraying plate, an oil scraper, a rust-preventing oil spraying device, and a compressed air blowing pipe are used to coat the surface of the steel pipe with oil and allow it to air dry.

[0024] In step 2 above, the cold bending rough forming is cooled and lubricated using rolling lubricant with rust-preventing function.

[0025] This invention solves the quality problems such as welding misalignment and incomplete welding caused by edge waviness during the forming process of acid-free cold-bent high-strength thin-walled trapezoidal steel pipe, which affect the stability of welding quality.

[0026] This invention solves problems such as shape asymmetry, poor molding stability, and difficulty in bending due to a small fillet angle.

[0027] This invention solves the problem that when the anti-rust oil on the surface of the steel strip enters the rolling mill, it contaminates the rolling lubricating fluid in the circulating water tank, causing the rolling mill to slip, scratching the surface of the steel pipe, and also causing large fluctuations in rolling speed, unstable high-frequency welding, and the formation of pinholes and cracks in the weld.

[0028] This invention solves the problem that after steel pipes are formed, the surface of the steel pipe is easily scratched and the inner and outer surfaces will rust quickly because there is no iron oxide scale to protect the surface.

[0029] The specific beneficial effects of this invention are as follows:

[0030] (1) Using dedicated COPRA software to simulate the forming roller pattern of each frame: the forming angle, rounded corners and forming center were precisely designed to ensure the accuracy of the steel pipe forming process and solve the problems of bending and twisting and poor dimensional accuracy caused by asymmetrical forming.

[0031] (2) The minimum strain downhill method at the edge in the dedicated COPRA software was used to calculate the downhill volume of each flight, which solved the problem of uneven edge extension and wavy shape affecting welding quality caused by the thin wall thickness and high strength of the steel strip.

[0032] (3) The use of baking and surface cleaning devices removes the anti-rust oil on the surface of the steel strip, which solves the adverse effects of the anti-rust oil on the production of cold-bent steel pipes caused by the anti-rust oil itself on the steel strip (the anti-rust oil enters the circulating water pool, causing the rolling mill to slip, resulting in rolling speed fluctuations, unstable welding quality, and the speed difference between the steel pipe and the rolling mill caused by slippage, which can easily cause the rolling mill to scratch the surface of the steel pipe).

[0033] (4) Key stands, such as stands with a large forming angle, are prone to scratching the surface of steel pipes without iron oxide scale protection after pickling because the linear speed of the forming roll is inconsistent with that of the rolling mill. The lower roll design adopts bearing rolls. Since the rolling roll and the shaft are matched with bearings, the bearing rolls are passively rotated during the rolling process and will not scratch the surface of the steel pipe.

[0034] (5) The closed-formed steel strip is extruded and formed by a three-roll extrusion roller, and the two welding sides are finally closed and formed. After high-frequency welding is completed by extrusion, a closed hole steel pipe is formed. Among them, the upper extrusion roller is a vertical roller with an arc, which is convenient to control the stability of the welding of the two welding sides. Due to the asymmetry of the two sides of the steel pipe, the two side extrusion rollers are box rollers, one of which is embedded, which is convenient to wrap the steel pipe tightly during welding and avoid the steel pipe shaking from affecting the welding stability.

[0035] (6) The use of sizing precision forming (outer arc of the beveled side forming) solves the problem of R-crack quality caused by the easy deformation of the outer arc of the beveled side during sizing precision forming.

[0036] (7) The use of online oiling and drying equipment not only solves the problem of easy corrosion on the surface of acid-free steel pipes due to the lack of iron oxide scale protection, but also solves the problems of uneven oiling, heavy oil, and oil accumulation.

[0037] After the implementation of this invention, the yield of acid-free cold-bent high-strength thin-walled trapezoidal steel pipes has been improved, and the scrap loss caused by difficult forming, poor surface quality and poor welding quality of this type of product has been reduced. The annual production and sales volume is about 2,000 tons, generating a benefit of 600,000 yuan per year. Attached Figure Description

[0038] Figure 1 This is a cross-sectional diagram of a steel pipe using existing technology;

[0039] Figure 2 This is a cross-sectional view of the acid-free cold-bent high-strength thin-walled trapezoidal steel pipe of the present invention;

[0040] Figure 3 This is a schematic diagram of the rough forming process of the acid-free cold-bending high-strength thin-walled trapezoidal steel pipe of the present invention;

[0041] Figure 4 This is a schematic diagram of the edge minimum strain downhill method used in the cold bending high-strength thin-walled trapezoidal steel pipe of the present invention, which is free from acid washing.

[0042] Figure 5 This is a schematic diagram of the extrusion and welding process for the acid-free cold-bent high-strength thin-walled trapezoidal steel pipe of the present invention;

[0043] Figure 6 This is a schematic diagram of the sizing and finishing process of the acid-free cold-bent high-strength thin-walled trapezoidal steel pipe of the present invention;

[0044] Figure 7 This is a schematic diagram illustrating the surface baking and cleaning process of the acid-free cold-bent high-strength thin-walled trapezoidal steel strip of the present invention;

[0045] Figure 8 This is a schematic diagram of the surface oiling and air drying process of the acid-free cold-bent high-strength thin-walled trapezoidal steel pipe of the present invention;

[0046] Figure 9 This is a schematic diagram of the one-bending forming process of the acid-free cold-bending high-strength thin-walled trapezoidal steel strip of the present invention.

[0047] Figure 10 This is a process diagram of the acid-free cold bending high-strength thin-walled trapezoidal steel strip double bending forming process of the present invention;

[0048] Figure 11 This is a schematic diagram of the closed forming process of the acid-free cold bending high-strength thin-walled trapezoidal steel strip of the present invention. Detailed Implementation

[0049] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments to facilitate a clear understanding of the present invention, but these descriptions do not constitute a limitation on the present invention.

[0050] The overall process flow of this invention includes: uncoiling → feeding → shearing → butt welding → baking → surface cleaning → rough forming (343 asymmetric forming method) → extrusion welding (high frequency welding) → planing of outer weld seam → cooling and lubrication → sizing and finishing → straightening and shaping → flying saw cutting → oiling and air drying → packaging and collection → draining → plastic packaging and warehousing.

[0051] Specifically, the following steps are included:

[0052] (a) Based on the calculated width of the steel strip B ( Figure 2 (As shown) The strip is uncoiled, fed, sheared, butt-welded, and then fed to the feed roller by the pinch rollers. After being baked by the thermoelectric heater 2-1, the anti-rust oil that has penetrated into the steel strip B is allowed to overflow. Then, the surface of the heated steel strip B is cleaned using felt pads 2-2 (see...). Figure 7 (As shown); This solves the adverse effects of the rust-preventive oil inherent in steel strip B on the production of cold-bent steel pipes (rust-preventive oil entering the circulating water pool causes the rolling mill to slip, resulting in rolling speed fluctuations, unstable welding quality, and the speed difference between the steel pipe and the rolling mill caused by slippage, which easily causes the rolls to scratch the surface of the steel pipe).

[0053] (b) Calculate the width, forming centerline, forming angle, arc value, and forming passes. Based on the external dimensions of the acid-free cold-bent high-strength thin-walled trapezoidal steel pipe A, use professional analysis software to design, simulate, and optimize key process parameters such as the cold bending forming passes, forming angle of each pass, bending amount, and downhill amount of the steel strip B, to obtain the cold bending forming passes and the optimal forming parameters for each pass. Perform cold bending deformation according to the forming passes and optimal forming parameters until the optimal parameter values ​​before welding are reached to complete the rough forming. During the production process, use rolling lubricant with anti-rust function for cooling and lubrication.

[0054] In this embodiment: the forming roll pattern for each stand is designed with the vertical line of the centroid of section A of the steel pipe as the center line (see...). Figure 3 As shown), the forming roller pattern for each pass was simulated using dedicated COPRA software (see...). Figure 3 (As shown): Precise design of forming angles, fillets, and forming centers ensures the accuracy of the steel pipe forming process and solves the problems of bending, twisting, and poor dimensional accuracy that easily occur in asymmetrical forming. The downhill method with minimum edge strain in the COPRA software is adopted (see...). Figure 4 As shown in the figure, the descent volume of each flight was calculated, and the forming of each flight was carried out. This solved the problem of poor forming stability caused by asymmetrical forming. It also solved the problem of uneven edge extension and wavy shape affecting welding quality caused by the thin wall thickness and high strength of steel strip B.

[0055] Rough forming is achieved through 343 asymmetric forming methods (single-bend forming, double-bend forming, and closed forming, see...) Figure 9 — Figure 11 As shown), Figure 3 — Figure 4 As shown, during the first to third sorties of bending and forming, the forming angles for each sortie were 30° (30°), 40° (40°), and 45° (50°) respectively; during the fourth to seventh sorties of bending and forming, the forming angles for each sortie were 20° (20°), 35° (35°), 50° (50°), and 60° (68°) respectively.

[0056] Rolling forming "closed forming": During the cold bending forming process from the eighth to the tenth pass (8-10), the forming angles for each stand are as follows: the bottom angle is 70° (86°), 80° (100°), and 86° (110°); the welding edge angle is 70° (62°), 75° (62°), and 82° (62°).

[0057] In this embodiment, critical stands (forming stands six and seven), such as stands with large forming angles, are prone to scratching the surface of the steel pipe without oxide scale protection after pickling because the linear speed of the lower forming rolls 1-3 is inconsistent with that of the rolling mill. Therefore, the lower rolls 1-3 are designed to use bearing rolls 1-5 (see...). Figure 10 As shown), bearing rollers 1-5 are passively rotated during the rolling process because the rollers and shafts are fitted with bearings, which will not scratch the surface of the steel pipe.

[0058] "Extrusion and welding forming": The two ends of the rough forming and closure are subjected to high-frequency welding through extrusion rollers, using a three-roller extrusion roller (see...). Figure 5 As shown, the steel strip B, which has been closed and formed, is extruded to finally close the two welding edges. After high-frequency welding is completed by extrusion, a closed-hole steel pipe A is formed. Among them, the upper extrusion roller 1-1-1 is a vertical roller with an arc, which is convenient to control the stability of the welding of the two welding edges. Due to the asymmetry of the two sides of the steel pipe A, the two side extrusion rollers 1-2-1 are box rollers, one of which is embedded, which is convenient to tightly wrap the steel pipe A during welding and avoid the vibration of the steel pipe A from affecting the welding stability.

[0059] (c) "External weld seam planing": For the external weld seams 1-4 formed by the two welding edges of the steel pipe A which are "extruded and welded", use a curved blade to plan off the burrs on both sides of the weld to ensure that the weld surface is flat and smooth.

[0060] (d) "Cooling and lubrication": During the forming process, water-based anti-rust rolling lubricant is sprayed onto the surface of the rolls in contact with steel pipe A through a spray nozzle during each stand. This ensures rolling stability on the one hand and rust prevention on the surface of steel pipe A on the other.

[0061] (e) Sizing and finishing: Sizing and finishing is performed using a four-roll (including upper roll 1-1, two side rolls 1-2, and lower roll 1-3) arc-wrap forming method (see...). Figure 6 As shown in the figure, this facilitates the steel pipe A to meet the standard requirements for dimensional accuracy. In particular, the forming of the small arc on the bevel ensures the accuracy of the arc value.

[0062] (f) "Straightening and straightening": Straighten and straighten the steel pipe A after "sizing and finishing" to ensure that the longitudinal straightness of the steel pipe A meets the customer's standard requirements.

[0063] (g) Flying saw cutting: The steel pipe A is cut with a flying saw using a washing saw cutting equipment to ensure that it meets the customer's length requirements.

[0064] (h) Oiling and Plastic Packaging Collection: An online oiling and drying device (including a water-hanging plate 3-1, an oil scraper 3-2, a rust-preventing oil spraying device 3-3, and a compressed air blowing pipe 3-4) is used. Figure 8 As shown in the figure, the X direction is the rolling direction. After the steel pipe A is coated with oil, air-dried, and padded, it is packaged, drained, plastic-wrapped, and put into storage.

[0065] The contents not described in detail in this specification are existing technologies known to those skilled in the art.

Claims

1. A method for manufacturing high-strength cold-formed thin-walled trapezoidal steel pipe without pickling, characterized in that: Includes the following steps: Step 1: Heat the steel strip to allow the anti-rust oil on the surface of the pickled and oiled steel strip to overflow, and clean the overflowed anti-rust oil from the surface of the steel strip. Step 2: Perform cold bending rough forming of the steel strip according to the external dimensions of the acid-free cold-bent high-strength thin-walled trapezoidal steel pipe; Step 3: The cold-formed steel pipe is closed by extrusion, and the two welding edges are extruded and welded by high-frequency welding to form a cold-formed closed steel pipe. The external burrs of the weld seam of the cold-formed closed steel pipe are removed. Step 4: The cold-bent closed steel pipe after deburring is sized and finished. Step 5: Straighten the welded cold-bent closed steel pipe; Step 6: Cut the straightened steel pipe with a flying saw; Step 7: Apply oil to the surface of the steel pipes online, air dry, add padding, pack them, drain them offline, wrap them in plastic, and put them into storage; In step 2 above, cold bending rough forming includes single-bend forming, double-bend forming, and closed forming. During the single-bend forming process from the first to the third batch, the forming angle on one side of each batch is 30º, 40º, and 45º respectively. During the double-bend forming process from the fourth to the seventh batch, the forming angle on one side of each batch is 20º, 35º, 50º, and 60º respectively. During the closed forming process from the eighth to the tenth batch, the forming angle on one side of each batch is: bottom angle 70º, 80º, and 86º respectively; the welding edge angle depends on... The angles for the second bend are 70º, 75º, and 82º. During the first to third bend forming processes, the other side forming angles for each bend are 30º, 40º, and 50º respectively. During the fourth to seventh bend forming processes, the other side forming angles for each bend are 20º, 35º, 50º, and 68º respectively. During the eighth to tenth closed forming processes, the other side forming angles for each bend are: bottom angle 86º, 100º, and 110º respectively; welding edge angle 62º, 62º, and 62º respectively.

2. The method for manufacturing a pickling-free, high-strength, cold-formed, thin-walled trapezoidal steel pipe according to claim 1, characterized in that: In step 1 above, a thermoelectric heater is used to heat the steel strip.

3. The method for manufacturing a pickling-free high-strength cold-formed thin-walled trapezoidal steel pipe according to claim 1, characterized in that: In step 1 above, wool felt pads are used to clean the rust-preventive oil from the heated steel strip surface.

4. The method for manufacturing a pickling-free high-strength cold-formed thin-walled trapezoidal steel pipe according to claim 1, characterized in that: The sixth and seventh rolling mills use bearing rollers, and the rollers and shafts of the bearing rollers are fitted with bearings.

5. The method for manufacturing a pickling-free high-strength cold-formed thin-walled trapezoidal steel pipe according to claim 1, characterized in that: In step 3 above, the cold-bent steel pipe is subjected to closed extrusion through an upper extrusion roller and two side extrusion rollers; wherein, the upper extrusion roller is a vertical roller with an arc, and the two side extrusion rollers are box rollers, one of which is embedded.

6. The method for manufacturing a pickling-free high-strength cold-formed thin-walled trapezoidal steel pipe according to claim 1, characterized in that: In step 4 above, a four-roller arc forming method is used for sizing and finishing.

7. The method for manufacturing a pickling-free high-strength cold-formed thin-walled trapezoidal steel pipe according to claim 1, characterized in that: In step 3 above, a curved blade is used to remove the burrs on both sides of the weld.

8. The method for manufacturing a pickling-free high-strength cold-formed thin-walled trapezoidal steel pipe according to claim 1, characterized in that: In step 7 above, a water-spraying plate, an oil scraper, a rust-preventing oil spraying device, and a compressed air blowing pipe are used to coat the surface of the steel pipe with oil and allow it to air dry.

9. The method for manufacturing a pickling-free high-strength cold-formed thin-walled trapezoidal steel pipe according to claim 1, characterized in that: In step 2 above, the cold bending rough forming is cooled and lubricated using rolling lubricant with rust-preventing function.

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