Method for producing hot-rolled seamless square tube

Abstract

This invention provides a method for preparing hot-rolled seamless square tubes. The method includes: Step 1, smelting metallurgical raw materials according to a certain ratio to obtain a billet; Step 2, piercing the billet to obtain a rough tube; Step 3, rolling the rough tube to obtain a blank tube; Step 4, cooling the blank tube to obtain a cooled blank tube; Step 5, heating the cooled blank tube in a walking beam furnace to A… c3 Step 6: Descaling the primary steel pipe using a double water ring descaling device to obtain a descaled steel pipe; Step 7: Sequentially sizing the descaled steel pipe into a round shape and then into a square shape to obtain a sized steel pipe; Step 8: Air cooling the sized steel pipe to obtain a hot-rolled seamless square tube. This application not only enables the hot-rolled seamless square tube to obtain excellent strength and toughness properties, but also reduces defects such as pitting, roughness, and severe iron oxide scale on the surface of the steel pipe.

Description

Technical Field

[0001] This invention relates to the field of seamless steel pipe manufacturing technology, and more specifically, to a method for manufacturing hot-rolled seamless square tubes. Background Technology

[0002] The hot-rolling process for producing seamless square tubes primarily utilizes existing hot-rolled steel pipe production lines. After piercing and rolling, the seamless steel pipes with circular cross-sections are directly squared into hot-rolled seamless square tubes using a three- or four-roll squaring and sizing mill. Compared to the traditional cold-drawing process, hot rolling allows for one-step forming, saving numerous production steps such as pickling, scaling, saponification, and cold drawing. This not only significantly improves production efficiency and reduces production costs but also greatly reduces environmental pollution.

[0003] Currently, hot-rolled seamless square tubes are produced using existing hot-rolled steel pipe production lines. During the sizing process, seamless steel pipes with circular cross-sections are directly sizing into finished hot-rolled seamless square tubes using a three- or four-roll sizing mill. To improve the strength and toughness of the final hot-rolled seamless square tube, alloying elements such as Cr, Ni, V, Nb, and Ti are added. The tubes are then cooled to a certain temperature on a small cooling bed before being further heated in a furnace. However, this process still has shortcomings. First, alloying elements such as Cr, Ni, V, Nb, and Ti cause a dense iron oxide scale to form on the surface of the steel pipe. Furthermore, to ensure the complete dissolution of microalloying elements or alloying elements into austenite, the rough pipe needs to be held at a certain temperature for a period of time. This increases the thickness of the iron oxide scale and forms pitting and roughness, making it difficult to remove the iron oxide scale from the steel pipe surface in subsequent processes. Second, to ensure the strength and toughness of the final hot-rolled seamless square pipe, the steel pipe needs to be reheated in the furnace, increasing the probability of pitting and roughness forming on the surface. Third, the iron oxide scale remaining on the surface of the round pipe is rolled into the surface of the steel pipe during the subsequent rounding and squaring processes, ultimately forming defects such as pitting and roughness on the finished hot-rolled square pipe. These defects not only accelerate the wear of the sizing mill rolls, but also cause iron oxide scale to adhere to the roll surface, leading to surface damage to the steel pipe. Moreover, they result in quality problems such as pitting and roughness on the surface of the final hot-rolled seamless square pipe, affecting the final product's performance. Therefore, in order to ensure that the finished steel pipe has good surface quality, the rough pipe must undergo a descaling process on its outer surface during the sizing stage.

[0004] Conventional descaling methods involve using a high-pressure water jet driven by a high-pressure water pump, spraying it through a nozzle on a single water ring. The powerful impact of the high-pressure water jet removes the iron oxide scale from the surface of the steel pipe. However, due to the dense oxide scale on microalloyed or alloy steel pipes, the bending or squaring process of the hot-rolled pipe makes center alignment difficult, and the high-pressure water jet coverage is uneven across different parts of the pipe, resulting in poor descaling performance. Therefore, there is an urgent need to develop equipment and methods for producing hot-rolled seamless square tubes with excellent external surface quality. This would solve the aforementioned defects such as pitting and roughness that occur during the squaring process, ultimately achieving the production of hot-rolled seamless square tubes with superior external surface quality. Summary of the Invention

[0005] The main objective of this invention is to provide a method for preparing hot-rolled seamless square tubes, so as to solve the problem of severe defects such as pitting and roughness on the outer surface of hot-rolled seamless square tubes in the prior art.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for preparing a hot-rolled seamless square tube is provided, the method comprising: step 1, smelting metallurgical raw materials according to a certain proportion to obtain a billet; step 2, piercing the billet to obtain a rough tube; step 3, rolling the rough tube to obtain a blank tube; step 4, cooling the blank tube to obtain a cooled blank tube; and step 5, heating the cooled blank tube in a walking beam furnace to a temperature A. c3 Step 6: Descaling the primary steel pipe at 50-60℃ yields a primary steel pipe; Step 7: Descaling the primary steel pipe using a double water ring descaling device yields a descaled steel pipe; Step 8: Sequentially sizing the descaled steel pipe into a round pipe and then into a square pipe, yielding a sizing steel pipe; Step 9: Air-cooling the sizing steel pipe yields a hot-rolled seamless square pipe; wherein, by mass percentage, the metallurgical raw materials include: 0.15-0.25% C, 0.20-0.50% Si, 1.20-1.60% Mn, and 0.05-0.12% [unspecified element]. The composition consists of V, 0.02-0.06% Nb, 0.015-0.06% Al, ≤0.15% Cr, ≤0.15% Mo, ≤0.10% Ni, ≤0.015% P, ≤0.005% S, ≤0.0010% B, and 0.0060-0.012% N, with the total mass of Cr, Mo, and Ni ≤0.3%, and the balance being Fe and unavoidable impurities; the cooling temperature is ≤600℃.

[0007] Furthermore, in step 4, the rough tube is cooled on a small cooling bed at a temperature of 450℃~600℃.

[0008] Furthermore, in step 5, the cooled rough tube is heated to 860℃~920℃ in a walking beam furnace and held for 20~60 minutes.

[0009] Further, in step 6, the dual-water-ring descaling device includes a first water ring and a second water ring arranged coaxially in sequence. The primary steel pipe passes through the first water ring and the second water ring in sequence. The inner surfaces of the first water ring and the second water ring are each independently provided with multiple nozzles, and the multiple nozzles of the first water ring and the second water ring are staggered in the circumferential direction; and / or, the high-pressure water sprayed by the nozzles of the first water ring has a pressure of 15~30MPa and a flow rate of 20~60m³. 3 / h; and / or, the high-pressure water jet from the nozzle of the second water ring is 20~35MPa, and the water flow rate is 30~70m³ / h. 3 / h.

[0010] Furthermore, in step 7, the reduction in diameter for round sizing is 5-15%; and / or, the reduction in diameter for round-to-square sizing is 5-30%.

[0011] Furthermore, before piercing, the preparation method also includes: heating the billet in a ring furnace in multiple stages to obtain a heated billet; the temperature of the multiple stages of heating is 1210℃~1300℃, the total heating time of the multiple stages is ≥3.5 hours, and the furnace exit temperature of the heated billet is 1200℃~1290℃.

[0012] Furthermore, the annular furnace includes, in sequence, a heat recovery section, a preheating section I, a heating section I, a heating section II, a heating section III, a soaking section I, and a soaking section II; the temperature in the heat recovery section is the furnace temperature, and the holding time is 20-25 min; the temperature in the preheating section I is T1, and the holding time is 25-30 min; the temperature in the heating section I is T2, and the holding time is 30-35 min; the temperature in the heating section II is T3, and the holding time is 25-35 min; the temperature in the heating section III is T1, and the holding time is 25-35 min; the temperature in the heating section III is T2, and the holding time is 30-35 min; the temperature in the heating section II is T3, and the holding time is 25-35 min; the temperature in the heating section III is T1, and the holding time is 25-35 min; the temperature in the heating section II is T1 ... The temperature is T4, and the holding time is 25~35 min; the temperature of the heat equalization zone I is T5, and the holding time is 35~40 min; the temperature of the heat equalization zone II is T6, and the holding time is 30~35 min; among them, 400℃≤T1≤650℃; 650℃<T2≤850℃; 850℃<T3≤1120℃; 1120℃<T4≤1300℃; 1210℃<T5≤1300℃; 1210℃<T6≤1300℃.

[0013] Furthermore, the billet is pierced by a tapered expansion method, and the piercing temperature is 1170℃~1270℃; and / or, the total deformation of the piercing is 30~85%; and / or, the wall thickness of the tube is 7~30mm.

[0014] Furthermore, the air cooling rate is controlled to be ≤15℃ / min; and / or, rolling is carried out using a continuous rolling mill or an Assel mill, with a rolling temperature of 960℃~1160℃.

[0015] By applying the technical solution of this invention, on the one hand, this application optimizes the design of the steel pipe composition and adjusts the ratio between alloying elements, thereby reducing the amount of alloying elements while ensuring the final strength and toughness of the steel pipe, and reducing the tendency for the formation of dense iron oxide scale on the surface of the steel pipe. On the other hand, before entering the walking beam furnace, it is first cooled to below 600°C, and then reheated in the walking beam furnace to A. c3 The above-mentioned temperature of 50-60℃ not only endows hot-rolled seamless square tubes with excellent strength and toughness, but also ensures that only a loose oxide scale forms on the surface of the steel pipe after heating, thus avoiding defects such as dense oxide scale, pitting, or roughness. Simultaneously, the use of a double-water-ring descaling device achieves full coverage of the blind areas of the steel pipe, thereby improving the descaling effect on the outer surface. Therefore, through the synergistic effect of optimized composition design, secondary heating process control in a walking beam furnace, and descaling by a double-water-ring device, defects such as roughness and pitting on the steel pipe surface are effectively reduced, ultimately resulting in hot-rolled seamless square tubes with excellent outer surface quality and superior strength and toughness. Attached Figure Description

[0016] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0017] Figure 1 This shows a schematic diagram of the structure of the steel pipe being descaled on the double water ring descaling device in Embodiment 1 of this application;

[0018] The above figures include the following reference numerals:

[0019] 1. First water ring; 2. Second water ring; 3. Nozzle; 4. Primary steel pipe. Detailed Implementation

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0021] As analyzed in the background section of this application, the existing hot-rolled seamless square tubes have serious defects such as pitting and roughness on their outer surface. In order to solve the above problems, this application provides a method for preparing hot-rolled seamless square tubes.

[0022] In a typical embodiment of this application, a method for preparing a hot-rolled seamless square tube is provided. The method includes: Step 1, smelting metallurgical raw materials according to a certain ratio to obtain a billet; Step 2, piercing the billet to obtain a rough tube; Step 3, rolling the rough tube to obtain a blank tube; Step 4, cooling the blank tube to obtain a cooled blank tube; Step 5, heating the cooled blank tube in a walking beam furnace to A… c3 Step 6: Descaling the primary steel pipe at 50-60℃ yields a primary steel pipe; Step 7: Descaling the primary steel pipe using a double water ring descaling device yields a descaled steel pipe; Step 8: Sequentially sizing the descaled steel pipe into a round pipe and then into a square pipe, yielding a sizing steel pipe; Step 9: Air-cooling the sizing steel pipe yields a hot-rolled seamless square pipe; wherein, by mass percentage, the metallurgical raw materials include: 0.15-0.25% C, 0.20-0.50% Si, 1.20-1.60% Mn, and 0.05-0.12% [unspecified element]. The composition consists of V, 0.02-0.06% Nb, 0.015-0.06% Al, ≤0.15% Cr, ≤0.15% Mo, ≤0.10% Ni, ≤0.015% P, ≤0.005% S, ≤0.0010% B, and 0.0060-0.012% N, with the total mass of Cr, Mo, and Ni ≤0.3%, and the balance being Fe and unavoidable impurities; the cooling temperature is ≤600℃.

[0023] On the one hand, this application optimizes the steel pipe composition and adjusts the ratio of alloying elements to reduce the amount of alloying elements while ensuring the final strength and toughness of the steel pipe, and also reduces the tendency for dense iron oxide scale to form on the surface of the steel pipe. On the other hand, before entering the walking beam furnace, it is first cooled to below 600°C, and then reheated in the walking beam furnace to A. c3 The above-mentioned temperature of 50-60℃ not only endows hot-rolled seamless square tubes with excellent strength and toughness, but also ensures that only a loose oxide scale forms on the surface of the steel pipe after heating, thus avoiding defects such as dense oxide scale, pitting, or roughness. Simultaneously, the use of a double-water-ring descaling device achieves full coverage of the blind areas of the steel pipe, thereby improving the descaling effect on the outer surface. Therefore, through the synergistic effect of optimized composition design, secondary heating process control in a walking beam furnace, and descaling by a double-water-ring device, defects such as roughness and pitting on the steel pipe surface are effectively reduced, ultimately resulting in hot-rolled seamless square tubes with excellent outer surface quality.

[0024] In one embodiment of this application, in step 4, the rough tube is cooled on a small cooling bed at a temperature of 450°C to 600°C.

[0025] The preferred cooling temperature helps to enhance its coordination with the heating temperature in the walking beam furnace, so that only a loose oxide scale forms on the surface of the steel pipe after heating, thereby reducing the risk of defects such as a large amount of dense oxide scale, pitting, or crazing.

[0026] In one embodiment of this application, in step 5, the cooled rough tube is heated to 860°C~920°C in a walking beam furnace and held for 20~60 minutes.

[0027] The above-mentioned temperature and time control helps to suppress the continuous thickening trend of the formation of dense iron oxide scale by the reaction of alloying elements with oxygen at high temperature, thereby improving the surface quality stability of the rough tube and achieving dual optimization of the surface finish and process controllability of the hot-rolled seamless square tube.

[0028] In one embodiment of this application, in step 6, the dual-water-ring descaling device includes a first water ring 1 and a second water ring 2 arranged coaxially in sequence. The primary steel pipe 4 passes through the first water ring 1 and the second water ring 2 in sequence. The inner surfaces of the first water ring 1 and the second water ring 2 are each independently provided with a plurality of nozzles 3, and the plurality of nozzles 3 of the first water ring 1 and the second water ring 2 are staggered in the circumferential direction; and / or, the high-pressure water sprayed by the nozzles of the first water ring 1 has a pressure of 15~30MPa and a water flow rate of 20~60m³. 3 / h; and / or, the high-pressure water pressure injected by the nozzle of the second water ring 2 is 20~35MPa, and the water flow rate is 30~70m³ / h. 3 / h.

[0029] The above-described dual-water-ring descaling device helps to achieve full coverage of the blind area of ​​the steel pipe, thereby improving the descaling effect on the outer surface of the steel pipe. The combined effect of the high-pressure water pressure and flow rate sprayed by the nozzles of the first water ring and the high-pressure water pressure and flow rate sprayed by the nozzles of the second water ring helps to enhance the dynamic impact force and coverage density of the water flow on the iron oxide scale on the outer surface of the primary steel pipe. In particular, it achieves efficient and uniform mechanical peeling and rinsing removal effect for the dense and loose oxide scale structure formed by alloying elements (such as V and Nb) during the heating process.

[0030] In one embodiment of this application, in step 7, the reduction in diameter for round sizing is 5-15%; and / or, the reduction in diameter for round-to-square sizing is 5-30%.

[0031] The combined effect of the above-mentioned reduction in diameter helps to apply strong extrusion and shearing force to the dense iron oxide scale that remains on the surface of the steel pipe after descaling. This helps to break and peel off the oxide layer that was not completely removed due to insufficient reduction in diameter, thereby effectively reducing defects such as pitting and roughness caused by residual iron oxide scale.

[0032] In one embodiment of this application, before piercing, the preparation method further includes: heating the billet in a ring furnace in multiple stages to obtain a heated billet; the temperature of the multiple stages of heating is 1210℃~1300℃, the total heating time of the multiple stages is ≥3.5 hours, and the furnace exit temperature of the heated billet is 1200℃~1290℃.

[0033] The aforementioned multi-stage heating process helps to achieve sufficient compositional homogenization and microstructure stabilization of the billet before piercing, promotes the full solid solution of alloying elements and the formation of uniform and fine austenite grains. On the one hand, it reduces the risk of local oxidation reaction rate differences caused by microstructure inhomogeneity during subsequent rough tube cooling and walking beam reheating. On the other hand, it helps to suppress the abnormal formation of dense iron oxide scale.

[0034] In one embodiment of this application, the annular furnace includes, sequentially arranged, a heat recovery section, a preheating section I, a heating section I, a heating section II, a heating section III, a soaking section I, and a soaking section II; the temperature of the heat recovery section is the furnace temperature, and the holding time is 20-25 min; the temperature of the preheating section I is T1, and the holding time is 25-30 min; the temperature of the heating section I is T2, and the holding time is 30-35 min; the temperature of the heating section II is T3, and the holding time is 25-35 min; The temperature of Heat Zone III is T4, and the holding time is 25~35 min; the temperature of Heat Zone I is T5, and the holding time is 35~40 min; the temperature of Heat Zone II is T6, and the holding time is 30~35 min; among them, 400℃≤T1≤650℃; 650℃<T2≤850℃; 850℃<T3≤1120℃; 1120℃<T4≤1300℃; 1210℃<T5≤1300℃; 1210℃<T6≤1300℃.

[0035] In this embodiment, precise gradient control of temperature and time in different zones of the annular furnace helps to achieve a stable and uniform temperature rise of the billet from low to high during the heating process. This reduces the risk of severe oxidation of alloying elements and formation of dense iron oxide scale caused by sudden temperature changes or local overheating, while also reducing energy consumption and improving thermal efficiency. Simultaneously, this temperature gradient helps to match the phase transformation behavior and diffusion kinetics of elements such as C, Mn, V, and Nb in the billet, thereby contributing to a uniform microstructure and an ideal plastic temperature window in the billet before it enters the piercing process.

[0036] In one embodiment of this application, the blank is pierced by a tapered expansion method, the piercing temperature is 1170℃~1270℃; and / or, the total deformation of the piercing is 30~85%; and / or, the wall thickness of the tube is 7~30mm.

[0037] Using a tapered diameter expansion method for piercing helps to obtain a larger total deformation and helps to obtain a thinner tube, thus ultimately enabling the production of a thinner hot-rolled seamless square tube.

[0038] In one embodiment of this application, the cooling rate of air cooling is controlled to be ≤15℃ / min; and / or, rolling is performed using a continuous rolling mill or an Assel mill, with a rolling temperature of 960℃~1160℃.

[0039] The above rolling methods and conditions help to produce uniform microstructure, small wall thickness tolerance, and low initial density of oxide layer on the surface of the rough tube, thereby forming a uniform and loose iron oxide scale during the subsequent cooling process, reducing the risk of dense oxide layer accumulation caused by local stress concentration or uneven surface oxidation.

[0040] In one embodiment of this application, after non-destructive testing and manual inspection, no defects such as pitting, scratches, or residual oxide scale were found on the outer surface of the hot-rolled seamless square tube, and the surface roughness Ra of the hot-rolled seamless square tube was ≤6.3μm.

[0041] The hot-rolled seamless square tubes prepared by the above methods have improved their appearance quality and adaptability to subsequent processing, fundamentally solving the technical problem of frequent surface defects caused by incomplete descaling in traditional processes, thus realizing the controllability and standardization of the surface quality of hot-rolled seamless square tubes.

[0042] The beneficial effects of this application will be further illustrated below with reference to embodiments and comparative examples.

[0043] Example 1

[0044] The technical solution of this invention is as follows: billet blanking → ring furnace heating → piercing → rolling → small cooling bed cooling → walking beam furnace reheating → double water ring descaling device descaling → round tube sizing → round to square sizing → large cooling bed air cooling → head and tail trimming → straightening → physical and chemical testing → non-destructive testing → manual inspection → lettering → packaging and warehousing.

[0045] The specific steps are as follows:

[0046] Step 1: The metallurgical raw materials are smelted to obtain a billet. The metallurgical raw materials, by mass percentage, include: 0.2% C, 0.3% Si, 1.5% Mn, 0.08% V, 0.04% Nb, 0.03% Al, 0.10% Cr, 0.10% Mo, 0.10% Ni, 0.010% P, 0.005% S, 0.0010% B, and 0.008% N. The sum of the mass of Cr, Mo, and Ni is 0.3%, and the balance is Fe and unavoidable impurities.

[0047] Step 2: The blank is pierced to obtain a tube;

[0048] The billet is heated in a ring furnace, passing through seven heating zones in sequence: heat recovery section, preheating zone I, heating zone I, heating zone II, heating zone III, soaking zone I, and soaking zone II. The heating temperatures of the seven zones are: furnace temperature (20 min), 500℃ (25 min), 700℃ (30 min), 1000℃ (35 min), 1200℃ (35 min), 1250℃ (35 min), and 1300℃ (30 min). The billet exits the furnace at 1290℃, and the total heating time is 3.5 hours.

[0049] The billet is subjected to tapered expansion piercing at a temperature of 1200℃. Using tapered expansion piercing, a tube with a total deformation of 65% and a wall thickness of 25mm can be obtained.

[0050] Step 3: Roll the tube to obtain a rough tube;

[0051] The rough tube is obtained by rolling the tube blank at a temperature of 1000℃ using a continuous rolling mill.

[0052] Step 4: Cool the raw tube to 450℃ on a small cooling bed;

[0053] Step 5: Reheat the cooled rough tube to 920°C in a walking beam furnace and hold for 50 minutes;

[0054] Step 6: Descaling the steel pipes after furnace exit is performed on a double-water-ring descaling device. This descaling device has two water rings, and multiple nozzles are installed on the inner surface of the two water rings. The nozzles in the two water rings are arranged alternately. When the steel pipe enters the first water ring 1, it then enters the second water ring 2. The double-water-ring descaling device can fully cover the blind area of ​​the primary steel pipe 4. The high-pressure water sprayed by the nozzle 3 of the first water ring has a pressure of 25MPa and a flow rate of 45m³ / h. 3 / h; the high-pressure water jet from nozzle 3 of the second water ring is 30MPa, and the water flow rate is 55m³ / h. 3 / h;

[0055] Step 7: Perform a round sizing on the descaled steel pipe, reducing the diameter by 15%;

[0056] Step 8: Perform a square sizing on the steel pipe after round sizing, with a diameter reduction of 5%;

[0057] Step 9: The cooled steel pipe is transported to a large cooling bed for air cooling, and the cooling rate of the air cooling is controlled at 15℃ / min; finally, a hot-rolled seamless square tube with excellent outer surface quality is obtained.

[0058] Example 2

[0059] The difference from Example 1 is that in step 4, the rough tube is cooled to 600°C on a small cooling bed to finally obtain a hot-rolled seamless square tube.

[0060] Example 3

[0061] The difference from Example 1 is that in step 4, the rough tube is cooled to 200°C on a small cooling bed to finally obtain a hot-rolled seamless square tube.

[0062] Example 4

[0063] The difference from Example 1 is that in step 5, the cooled rough tube is reheated to 860°C in a walking beam furnace to finally obtain a hot-rolled seamless square tube.

[0064] Example 5

[0065] The difference from Example 1 is that in step 5, the walking beam furnace heating temperature is 750°C, and the hot-rolled seamless square tube is finally obtained.

[0066] Example 6

[0067] The difference from Example 1 is that in step 6, the nozzle arrangement of the double water ring descaling device is such that only the axes of the double water rings are on the same horizontal line, and the nozzles on the water rings are not circumferentially staggered, ultimately resulting in a hot-rolled seamless square tube.

[0068] Example 7

[0069] The difference from Example 1 is that the high-pressure water sprayed by the nozzle of the first water ring is 15 MPa, and the high-pressure water sprayed by the nozzle of the second water ring is 20 MPa, ultimately resulting in a hot-rolled seamless square tube.

[0070] Example 8

[0071] The difference from Example 1 is that the high-pressure water sprayed by the nozzle of the first water ring is 30 MPa, and the high-pressure water sprayed by the nozzle of the second water ring is 35 MPa, ultimately resulting in a hot-rolled seamless square tube.

[0072] Example 9

[0073] The difference from Example 1 is that the high-pressure water sprayed by the nozzle of the first water ring is 10 MPa, and the high-pressure water sprayed by the nozzle of the second water ring is 10 MPa, ultimately resulting in a hot-rolled seamless square tube.

[0074] Example 10

[0075] The difference from Example 1 is that in step 7, the descaled steel pipe is sized by rounding, with a diameter reduction of 5%, and finally hot-rolled seamless square pipe is obtained.

[0076] Example 11

[0077] The difference from Example 1 is that in step 7, the descaled steel pipe is sized by rounding, with a diameter reduction of 2%, and finally hot-rolled seamless square pipe is obtained.

[0078] Example 12

[0079] The difference from Example 1 is that in step 8, the steel pipe after round sizing is sizing from round to square, with a diameter reduction of 30%, and finally hot-rolled seamless square pipe is obtained.

[0080] Example 13

[0081] The difference from Example 1 is that in step 8, the steel pipe after round sizing is sizing from round to square, with a diameter reduction of 40%, and finally hot-rolled seamless square pipe is obtained.

[0082] Example 14

[0083] The difference from Example 1 is that in step 2, the heating temperatures of the seven zones are as follows: furnace temperature (20 min), 200℃ (25 min), 400℃ (30 min), 1300℃ (35 min), 900℃ (35 min), 1000℃ (35 min), and 1350℃ (30 min), respectively. The billet exit temperature is 1320℃, and the total heating time is 3.5 hours, finally obtaining hot-rolled seamless square tubes.

[0084] Comparative Example 1

[0085] The difference from Example 1 is that in step 1, the metallurgical raw materials contain 0.30% C, 0.2% V, 0.1% Nb, 0.20% Cr, 0.20% Mo, and 0.15% Ni; the remaining components are the same as in Example 1, and a hot-rolled seamless square tube is finally obtained.

[0086] Comparative Example 2

[0087] The difference from Example 1 is that in step 1, the metallurgical raw materials contain 0.20% Mo and 0.15% Ni; the remaining components are the same as in Example 1, and a hot-rolled seamless square tube is finally obtained.

[0088] Comparative Example 3

[0089] The difference from Example 1 is that in step 4, the rough tube is cooled to 800°C on a small cooling bed to finally obtain a hot-rolled seamless square tube.

[0090] The difference between Comparative Example 4 and Example 1 is that step 4 is omitted, that is, the rough tube is not cooled on the small cooling bed, and the hot-rolled seamless square tube is finally obtained.

[0091] Performance testing:

[0092] Physical and chemical properties of hot-rolled seamless square tubes: tensile and impact properties were tested according to GB / T 228.1-2021 and GB / T 229 standards, respectively.

[0093] Yield strength R p0.2 (MPa): Performed according to GB / T 228.1-2021. Specimens were cut longitudinally from hot-rolled seamless square tubes, and the original cross-sectional area S0 and gauge length L0 were measured. The yield strength R was determined by tensile testing at a strain rate of 0.00025 / s using a universal testing machine. p0.2 Tensile strength R m (MPa) and elongation after fracture A (%).

[0094] Impact energy KV2 (J) at -40℃: Performed according to GB / T 229. V-notch Charpy impact specimens (10mm × 10mm × 55mm) were longitudinally cut from hot-rolled seamless square tubes. The specimens were placed in a -40℃ cooling medium for 5 minutes, then removed and impacted on a pendulum impact testing machine within 5 seconds. The absorbed energy KV2 was recorded. Three specimens were taken from each test group, and the arithmetic mean was calculated.

[0095] To check for residual defects such as pitting, cratering, rolling marks, and oxide scale on the outer surface of hot-rolled seamless square tubes: Under 500 Lux illumination, the outer surface of each square tube is visually inspected to check for pitting, cratering, rolling marks, and residual oxide scale.

[0096] The surface roughness Ra of hot-rolled seamless square tubes is measured using a surface roughness tester according to GB / T 1031-2009. At least three different locations are selected on the outer surface of the square tube, and the surface is scanned along the axial direction. Five sampling lengths are taken for evaluation, and the arithmetic mean deviation Ra is calculated.

[0097] The hot-rolled seamless square tubes prepared in the above embodiments and comparative examples were subjected to the above tests, and the test results are listed in Table 1.

[0098] Table 1

[0099]

[0100] Among them, a small amount of residual oxide scale or shallow pits, a large amount of oxide scale and pits, a large amount of pits and iron oxide scale, and severe pits and oxide scale residue indicate an increasing degree of defect.

[0101] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0102] On the one hand, this application optimizes the steel pipe composition and adjusts the ratio of alloying elements to reduce the amount of alloying elements while ensuring the final strength and toughness of the steel pipe, and also reduces the tendency for dense iron oxide scale to form on the surface of the steel pipe. On the other hand, before entering the walking beam furnace, it is first cooled to below 600°C, and then reheated in the walking beam furnace to A. c3 The above-mentioned temperature of 50-60℃ not only endows hot-rolled seamless square tubes with excellent strength and toughness, but also ensures that only a loose oxide scale forms on the surface of the steel pipe after heating, thus avoiding defects such as dense oxide scale, pitting, or roughness. Simultaneously, the use of a double-water-ring descaling device achieves full coverage of the blind areas of the steel pipe, thereby improving the descaling effect on the outer surface. Therefore, through optimized composition design, controlled secondary heating process of the walking beam furnace, and the synergistic effect of the double-water-ring descaling device, defects such as roughness and pitting on the steel pipe surface are effectively reduced, ultimately resulting in hot-rolled seamless square tubes with excellent outer surface quality and superior strength and toughness.

[0103] The above are merely embodiments of the present invention and are not intended to limit the invention. Those skilled in the art will recognize that the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a hot-rolled seamless square tube, characterized in that, The preparation method includes: Step 1: After mixing the metallurgical raw materials according to a certain ratio, the metallurgical raw materials are smelted to obtain a billet; Step 2: Pierce the blank to obtain a tube; Step 3: Roll the tube to obtain a rough tube; Step 4: Cool the raw pipe to obtain a cooled raw pipe; Step 5: Heat the cooled rough tube to A in a walking beam furnace. c3 The primary steel pipe is obtained at a temperature of 50~60℃. Step 6: Use a double water ring descaling device to descale the primary steel pipe to obtain a descaled steel pipe; Step 7: Perform circular sizing and circular-to-square sizing on the descaled steel pipe in sequence to obtain a sized steel pipe; Step 8: Air-cool the sized steel pipe to obtain the hot-rolled seamless square tube; The metallurgical raw materials, by mass percentage, include: 0.15-0.25% C, 0.20-0.50% Si, 1.20-1.60% Mn, 0.05-0.12% V, 0.02-0.06% Nb, 0.015-0.06% Al, ≤0.15% Cr, ≤0.15% Mo, ≤0.10% Ni, ≤0.015% P, ≤0.005% S, ≤0.0010% B, and 0.0060-0.012% N, wherein the sum of the masses of Cr, Mo, and Ni is ≤0.3%, and the balance is Fe and unavoidable impurities. The cooling temperature is ≤600℃.

2. The preparation method according to claim 1, characterized in that, In step 4, the rough tube is cooled on a small cooling bed at a temperature of 450°C to 600°C.

3. The preparation method according to claim 1, characterized in that, In step 5, the cooled rough tube is heated to 860℃~920℃ in the walking beam furnace and held for 20~60 minutes.

4. The preparation method according to any one of claims 1 to 3, characterized in that, In step 6, the double water ring descaling device includes a first water ring (1) and a second water ring (2) arranged coaxially in sequence. The primary steel pipe (4) passes through the first water ring (1) and the second water ring (2) in sequence. The inner surfaces of the first water ring (1) and the second water ring (2) are each independently provided with a plurality of nozzles (3), and the plurality of nozzles (3) of the first water ring (1) and the second water ring (2) are staggered in the circumferential direction. And / or, the high-pressure water sprayed by the nozzle (3) of the first water ring (1) is 15~30MPa, and the water flow rate is 20~60m³ / h. 3 / h; And / or, the high-pressure water sprayed by the nozzle (3) of the second water ring (2) is 20~35MPa, and the water flow rate is 30~70m³ / h. 3 / h.

5. The preparation method according to any one of claims 1 to 3, characterized in that, In step 7, the reduction in diameter for the round sizing is 5-15%; and / or, the reduction in diameter for the round-to-square sizing is 5-30%.

6. The preparation method according to any one of claims 1 to 3, characterized in that, Prior to the perforation, the preparation method further includes: The billet is heated in multiple stages in a ring furnace to obtain a heated billet; The temperature of the multi-stage heating is 1210℃~1300℃, the total heating time is ≥3.5 hours, and the furnace exit temperature of the heated billet is 1200℃~1290℃.

7. The preparation method according to claim 6, characterized in that, The annular furnace includes a heat recovery section, a preheating section I, a heating section I, a heating section II, a heating section III, a soaking section I, and a soaking section II arranged sequentially. The temperature of the heat recovery section is the same as the furnace temperature, and the holding time is 20-25 minutes. The temperature of the preheating section I is T1, and the holding time is 25~30min; The temperature of heating zone I is T2, and the holding time is 30~35min; The temperature of heating zone II is T3, and the holding time is 25~35 minutes; The temperature of heating zone III is T4, and the holding time is 25~35 minutes; The temperature of the heat equalization zone I is T5, and the heat preservation time is 35~40min; The temperature of the heat dissipation zone II is T6, and the heat preservation time is 30~35 minutes; Among them, 400℃≤T1≤650℃; 650℃<T2≤850℃; 850℃<T3≤1120℃; 1120℃<T4≤1300℃; 1210℃<T5≤1300℃; 1210℃<T6≤1300℃.

8. The preparation method according to any one of claims 1 to 3, characterized in that, The blank is pierced by a tapered expansion method, and the piercing temperature is 1170℃~1270℃; and / or, the total deformation of the piercing is 30~85%; and / or, the wall thickness of the tube is 7~30mm.

9. The preparation method according to any one of claims 1 to 3, characterized in that, The cooling rate of the air cooler is controlled to be ≤15℃ / min; and / or, the rolling is performed using a continuous rolling mill or an Assel mill, and the rolling temperature is 960℃~1160℃.

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