Production method of 700MPa-grade ERW welded square and rectangular pipe

Abstract

The invention discloses a production method of a 700MPa-grade ERW welded square and rectangular pipe, and belongs to the technical field of production of low-alloy thick-specification high-strength square and rectangular pipes. A base material of the square and rectangular pipe is a hot-rolled coiled plate. The production process comprises the steps of molten iron pretreatment, converter combined blowing, LF + RH refining, continuous casting and hot continuous rolling. The steel comprises the following chemical components in percentage by mass: 0.05 to 0.07 percent of C, 0.05 to 0.15 percent of Si, 1.55 to 1.75 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.005 percent of S, 0.015 to 0.035 percent of Alt, 0.045 to 0.055 percent of Nb, 0.070 to 0.090 percent of Ti, less than or equal to 0.0050 percent of N and the balance of Fe and inevitable impurities. According to the invention, low-carbon component design is adopted, and the ERW welded square and rectangular pipe with the thickness of 700MPa and low forming cracking rate is produced.

Description

Technical Field

[0001] The invention belongs to the technical field of production of low-alloy thick-gauge high-strength square and rectangular tubes, and in particular relates to a production method of 700MPa-grade ERW welded square and rectangular tubes. Background Art

[0002] Rectangular steel pipe is the most widely used and most common special-shaped steel pipe. Compared with other non-circular steel pipes with the same cross-sectional area, it has the advantages of light weight, high strength, large bending section modulus, material saving and easy installation.

[0003] Large cold-formed square and rectangular steel pipes are mostly produced using the "direct forming" method, which involves bending hot-rolled coils. During "direct forming," the bending process generally employs two basic forming methods. Large square and rectangular steel pipes with a thickness greater than 8.0 mm and an unfolded width greater than 300 mm are often produced using the "direct forming" method. While this process offers advantages such as high roll versatility, low rolling energy consumption, and excellent surface quality, its products often suffer from issues such as poor shape and suboptimal weld quality.

[0004] The production process of the currently published technology is to use hot-rolled coil as the base material, which is formed by cold bending and then high-frequency welding. The production process is divided into two links. The first link is to produce hot-rolled coils in the steel plant. The production process is: molten steel → continuous casting billet → rough rolling → finishing rolling → laminar cooling → coiling → slow cooling. The second process link is to produce cold-bent steel in the steel pipe plant. The production process is: steel coil uncoiling → edge trimming → disc storage → continuous roll forming → high-frequency welding → straightening → fixed-length cutting.

[0005] Although cold bending and welding processes have long been mature, standardized, and regulated in China, the quality of high-frequency welded pipes is affected by a combination of factors due to the complex production processes and conditions. Factors influencing weld quality include not only welding process parameters but also raw material quality. The main factors affecting high-frequency welding quality include weld edge condition, welding pressure, frequency, welding method, output power, welding speed, inductor and impedance, and water cooling conditions. These factors interact with each other, making the quality of high-frequency welded pipes difficult to control. Welds often exhibit defects such as external cracks, sand holes, incomplete penetration, misaligned welds, and excessive weld bumps. Among these factors, weld edge condition and welding pressure have the most significant impact on weld quality and have a significant impact on welding quality.

[0006] At the same time, the inherent quality of the steel strip, such as defects such as metallographic segregation and delamination, has a more subtle impact on quality and is more harmful. The biggest difference between the cracking morphology caused by metallographic segregation of raw materials and the cracking caused by unreasonable welding process parameters is that the cracks in the former develop transversely along the parent material, while the crack direction in the latter coincides with the fusion line. The crack morphology characteristics of the weld caused by severe inherent quality segregation of the raw materials or the presence of interlayers are different from the crack morphology of the weld caused by welding process parameters. The crack direction in the former is "turned" and develops transversely along the parent material, while the crack in the latter coincides with the weld fusion line or cracks perpendicular to the parent material surface near the fusion line in the heat-affected zone. The inherent quality stratification and composition segregation of raw materials of high-frequency welded pipes can easily cause crack defects in the weld, and when flattened, the crack is not parallel to the weld, but extends to the parent material in a transverse or oblique direction.

[0007] Patent application CN 112048663A, published as "A method for producing low-cost, high-strength rectangular steel tubes," uses direct forming to produce 700 MPa-grade rectangular tubes. However, the chemical composition design limits the carbon content to 0.15-0.20%, which is relatively high and affects welding performance. The nitrogen content is not specified, and if the control is unstable, TiN is easily generated, affecting the impact performance of the weld.

[0008] The published paper "Analysis of the Influence of Raw Material Segregation and Stratification on High-Frequency Welded Pipe Welds" provides a comprehensive discussion and uses Q355B low-alloy steel as the material, without involving welded steel pipes with higher strength levels.

[0009] In summary, the above patents and documents do not form a comprehensive method for evaluating the quality of 700MPa grade ERW welded square tubes, and cracking caused by weld quality problems often causes economic losses to enterprises. Summary of the Invention

[0010] The present invention aims to provide a production method for 700MPa grade ERW welded square and rectangular tubes. The method adopts a comprehensive evaluation method including low carbon composition design, reasonable control of base material mechanical properties, banded segregation level, inclusion level, and weld fracture morphology analysis to produce 700MPa grade thick gauge ERW welded square and rectangular tubes with a low forming cracking rate.

[0011] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0012] The present invention discloses a production method for a 700MPa grade ERW welded square tube. The chemical composition of the square tube is, by mass, C: 0.05-0.07%, Si: 0.05-0.15%, Mn: 1.55-1.75%, P≤0.015%, S≤0.005%, Alt: 0.015-0.035%, Nb: 0.045-0.055%, Ti: 0.070-0.090%, N≤0.0050%, and the balance is Fe and unavoidable impurities. In the present invention, the weight percentage of nitrogen is not greater than 0.0050%, the purpose of which is to reduce the liquid phase precipitation of (Ti, Nb) (C, N) and prevent its coarsening and growth, because precipitation strengthening of Ti is the main strengthening method for improving the strength of the 700MPa grade square tube, but coarse TiN inclusions precipitated by liquid precipitation easily reduce the strength of the weld.

[0013] The parent material of the square tube is hot-rolled coil, and the production process of the hot-rolled coil includes: molten iron pretreatment, converter composite blowing, LF+RH refining, continuous casting, and hot rolling; the hot rolling process includes slab heating, rough rolling, finishing rolling, laminar cooling, and slow cooling of the steel coil; the rough rolling adopts 3+3 passes; the finishing rolling adopts 7 passes of hot rolling: wherein:

[0014] To ensure the uniformity of the mechanical properties of the welded square tube base material, the Mn element or pearlite banding segregation is controlled, the overheating degree in the continuous casting area is controlled at 15-30°C, the crystallizer liquid level fluctuation range is controlled at ±5mm, the thickness of the continuous casting billet is 225-235mm, the soaking temperature during slab reheating is 1210-1250°C, the soaking time is 35-50min, the thickness of the intermediate billet is 40-55mm, the final rolling temperature is 820-860°C, the coiling temperature is 580-610°C, the thickness of the rolled product is 8-10mm, and the coil is stacked and slowly cooled.

[0015] Furthermore, according to the size of the final square tube, the hot-rolled coil for producing the square tube is uncoiled, trimmed and longitudinally sheared. When longitudinally shearing the strip, the clearance of the disc shear blade ensures that the burr length of the upper and lower edges is ≤0.5mm and the width tolerance of the same coil is ≤4mm.

[0016] Furthermore, after continuous roll forming, the end shape of the directly formed square tube at the high-frequency welding is as follows: the edge of the strip forms two angles: the range of the opening angle α is: 3°-8°, the V-shaped angle is θ, the smaller the V-shaped angle, the better, and the two welding surfaces remain parallel, such as Figure 6 shown.

[0017] Furthermore, during the extrusion roller welding, the extrusion roller applies an extrusion force according to the thickness, strength and welding speed of the base material to squeeze out the weld inclusions and ensure that the weld edges are welded together without oxidation of the molten metal.

[0018] Furthermore, the weld is water-cooled, the cooling water temperature range is: 10-40℃, and the weld temperature range is: 20-100℃.

[0019] Furthermore, the mechanical properties of the hot-rolled coil of the parent material of the square tube meet the following requirements: tensile strength: 635-685 MPa, tensile strength: 720-780 MPa, and elongation after fracture ≥ 18.0%.

[0020] Furthermore, the mechanical properties of the hot-rolled coil of the parent material of the square tube meet the following requirements: microhardness HV0.1: 230-280, banded structure level: ≤2.0.

[0021] Compared with the prior art, the present invention has the following beneficial technical effects:

[0022] The present invention comprehensively proposes a production method for ERW welded square and rectangular tubes with a strength level of 700 MPa and a thickness specification of 8 to 10 mm from the perspectives of chemical composition design, particularly the nitrogen content control range, base material production process, tensile properties, hardness, pearlite banding level, slitting edge quality, direct forming welding angle, weld inclusion level, and weld room temperature impact absorption energy.

[0023] This invention patent comprehensively considers various factors that affect the quality of welds, and can ensure that 700MPa-grade ERW welded square tubes have good welding performance and cold forming performance on the basis of ensuring strength, reduce the cracking rate of high-strength ERW welded square tubes after welding, and proposes a solution to improve the welding adaptability of 700MPa-grade ERW welded square tubes, reduce the cracking rate, and increase the yield rate.

[0024] The present invention takes into account both process and cost, integrates technical means to improve the quality of ERW welded square and rectangular tube welds, realizes the stable batch supply of high-strength and thick-gauge square and rectangular tube steel, and provides technical support for the welding of square and rectangular tube steels with other strength grades and thickness specifications. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be further described below with reference to the accompanying drawings.

[0026] Figure 1 This is the metallographic structure diagram of the weld in Example 1 of the present invention;

[0027] Figure 2 The morphology and energy spectrum of TiN inclusions in the weld of Comparative Example 1 of the present invention are shown;

[0028] Figure 3 The morphology of both sides of the weld in comparative example 2 of the present invention is shown below;

[0029] Figure 4 The oxide in the middle of the weld of Comparative Example 3 of the present invention;

[0030] Figure 5 The inclusions remaining in the weld of Comparative Example 4 of the present invention;

[0031] Figure 6 Schematic diagram of direct forming welding angle. DETAILED DESCRIPTION

[0032] A production method for 700MPa grade ERW welded square tubes, specifically comprising:

[0033] (1) The chemical composition and weight percentage of the square tube base material are as follows: C: 0.05-0.07%, Si: 0.05-0.15%, Mn: 1.55-1.75%, P≤0.015%, S≤0.005%, Alt: 0.015-0.035%, Nb: 0.045-0.055%, Ti: 0.070-0.090%, N≤0.0050%, and the balance is Fe and unavoidable impurities.

[0034] (2) The production process of 700MPa grade ERW welded square tube is as follows: molten iron pretreatment - 260t converter smelting - LF+RH refining - slab continuous casting - heating furnace heating - high-pressure water descaling - rough rolling - hot rolling - laminar cooling - stacking slow cooling - steel coil uncoiling - disc storage - continuous roll forming - high-frequency induction heating - extrusion roller welding - outer surface weld slag scraping - weld water cooling - straightening and sizing - cut to length - weld inclusion and room temperature impact performance testing.

[0035] (3) Production process of 700MPa grade ERW welded square tube base material: overheat control in continuous casting area: 15~30℃, crystallizer liquid level fluctuation range: -5~+5mm, continuous casting billet thickness range: 225~235mm, slab heating soaking temperature: 1210~1250℃, soaking time: 35~50min, intermediate billet thickness: 40~55mm, final rolling temperature: 820~860℃, coiling temperature: 580~610℃, rolled product thickness: 8~10mm, stacked and slowly cooled after coiling.

[0036] (4) Longitudinal mechanical properties of 700MPa grade ERW welded square tube base material: tensile strength: 635~685MPa, tensile strength: 720~780MPa, elongation after fracture ≥18.0%, microhardness HV0.1: 230~280, banded structure level: ≤2.0.

[0037] (5) The hot-rolled coils of the 700MPa grade square tube parent material are unrolled, trimmed and longitudinally sheared to ensure that the burr length of the upper and lower edges is ≤0.5mm and the width tolerance of the same coil is ≤4mm.

[0038] (6) In the production process of 700MPa grade ERW welded square tubes, two angles are formed at the edge of the strip at the high-frequency welding: the range of the opening angle α is 3°~8°, and the V-angle is θ. The smaller the V-angle, the better. The two weld surfaces remain parallel.

[0039] (7) In the production process of 700MPa grade ERW welded square tubes, the extrusion force applied by the extrusion roller squeezes out the weld inclusions during welding, ensuring that the weld edge is not oxidized and the molten metal is completely welded.

[0040] (8) In the production process of 700MPa grade ERW welded square tubes, the cooling water temperature range during weld water cooling is: 10~40℃, and the weld temperature range is: 20~100℃.

[0041] (9) For the inclusion and room temperature impact absorption energy detection of 700MPa grade ERW welded square tube welds, the level of Class B inclusions at the welds is ≤2.0, the level of Class C inclusions is ≤2.0, the level of Class D inclusions is ≤2.0, and the room temperature V-type impact absorption energy of 7.5mm thick welds is ≥10J.

[0042] The present invention is described in detail below through specific examples. The examples are intended to help understand the present invention but are not intended to limit the present invention.

[0043] Table 1 Chemical composition (wt%) of the parent materials of Examples 1-4 of the present invention and Comparative Examples 1-4

[0044]

[0045]

[0046] Table 2 Process parameters for the production of parent materials of Examples 1-4 of the present invention and Comparative Examples 1-4

[0047]

[0048] Table 3 Mechanical properties and longitudinal shear quality of the base materials of Examples 1-4 of the present invention and Comparative Examples 1-4

[0049]

[0050]

[0051] Table 4 ERW welding process parameters and conditions of Examples 1-4 of the present invention and Comparative Examples 1-4

[0052]

[0053] Table 5 Welding properties of Examples 1-4 of the present invention and Comparative Examples 1-4

[0054]

[0055] The quality of the weld determines the quality of the ERW welded square tube. Since there are many factors that affect the quality of the weld, including the chemical composition of the base material, mechanical properties, degree of band segregation, welding process parameters, etc., the comparison is mainly conducted from the factors affecting the weld quality.

[0056] Comparative Example 1 mainly studies the effect of N element content on the welding quality of 700MPa grade ERW. When the mass fraction of the N content exceeds 0.0050%, a large number of TiN particles are present in the base material and the weld. During ERW welding, due to the high melting point of TiN, TiN particles will reduce the plasticity and toughness of the weld. When the 700MPa grade ERW welded square tube is straightened, it is easy to serve as a crack source, causing cracking or reducing the impact toughness of the weld.

[0057] Comparative Example 2 mainly studies the influence of whether the welding surface is parallel on the weld morphology and performance during ERW welding. Figure 2 As shown in the figure, if the two sides of the base material are not parallel during welding, the weld will easily form misalignment, reducing the weld bonding strength.

[0058] Comparative Example 3 mainly studies the influence of weld inclusions extruded during ERW welding on weld quality. Figure 3 As shown in the figure, when the ERW welding process is improper, the oxidized molten metal is not completely squeezed out of the weld during high-frequency induction heating, and inclusions will remain at the edge of the weld, reducing the strength of the weld.

[0059] Comparative Example 4 mainly studies the effect of the opening angle α of the strip edge during ERW welding on the weld quality. When the opening angle is small, the silicate inclusions formed during welding are not easily squeezed out of the weld and will remain in the middle of the weld, reducing the weld strength and making it easy to crack during forming.

[0060] From the results of the above embodiments, it can be seen that the 700 MPa grade ERW welded square tube of the present invention has excellent performance and high yield rate.

[0061] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for producing 700MPa grade ERW welded square tubes, characterized in that: The chemical composition of the square tube is calculated by mass fraction as follows: C: 0.05-0.07%, Si: 0.05-0.15%, Mn: 1.55-1.75%, P≤0.015%, S≤0.005%, Alt: 0.015-0.035%, Nb: 0.045-0.055%, Ti: 0.070-0.090%, N≤0.0050%, and the balance is Fe and unavoidable impurities; The parent material of the square tube is hot-rolled coil, and the production process of the hot-rolled coil includes: molten iron pretreatment, converter composite blowing, LF+RH refining, continuous casting, and hot rolling; the hot rolling process includes slab heating, rough rolling, finishing rolling, laminar cooling, and slow cooling of the steel coil; the rough rolling adopts 3+3 passes; the finishing rolling adopts 7 passes of hot rolling: wherein: To ensure the uniformity of the mechanical properties of the welded square tube base material, the Mn element or pearlite banding segregation is controlled, the overheating degree in the continuous casting area is controlled at 15-30°C, the crystallizer liquid level fluctuation range is controlled at ±5mm, the thickness of the continuous casting billet is 225-235mm, the soaking temperature during slab reheating is 1210-1250°C, the soaking time is 35-50min, the thickness of the intermediate billet is 40-55mm, the final rolling temperature is 820-860°C, the coiling temperature is 580-610°C, the thickness of the rolled product is 8-10mm, and the coil is stacked and slowly cooled.

2. The method for producing a 700MPa grade ERW welded square tube according to claim 1, characterized in that: According to the final size of the square tube, the hot rolled coil for producing the square tube is uncoiled, trimmed and longitudinally sheared. When longitudinally shearing the strip, the clearance of the disc shear blade is to ensure that the burr length of the upper and lower edges is ≤0.5mm, and the width tolerance of the same coil is ≤4mm.

3. The method for producing a 700MPa grade ERW welded square tube according to claim 1, characterized in that: After continuous roll forming, the end shape of the directly formed square tube at the high-frequency welding is as follows: the edge of the strip forms two angles: the range of the opening angle α is: 3°-8°, the V-shaped angle is θ, the smaller the V-shaped angle, the better, and the two welding surfaces remain parallel.

4. The method for producing a 700MPa grade ERW welded square tube according to claim 1, wherein: During extrusion roller welding, the extrusion roller applies extrusion force according to the thickness, strength and welding speed of the base material to squeeze out the weld inclusions and ensure that the weld edges are not oxidized and the molten metal is welded together.

5. The method for producing a 700MPa grade ERW welded square tube according to claim 1 or 4, characterized in that: The weld is water-cooled, the cooling water temperature range is 10-40℃, and the weld temperature range is 20-100℃.

6. The method for producing a 700MPa grade ERW welded square tube according to claim 1, characterized in that: The mechanical properties of the hot-rolled coil of the parent material of the square tube meet the following requirements: tensile strength: 635-685 MPa, tensile strength: 720-780 MPa, and elongation after fracture ≥18.0%.

7. The method for producing a 700 MPa grade ERW welded square tube according to claim 6, characterized in that: The transverse mechanical properties of the hot-rolled coil of the parent material of the square tube meet the following requirements: microhardness HV0.1: 230-280, banded structure level: ≤2.0.

Images