Welded circular steel tube with high production efficiency, low cost and low yield ratio and production method thereof

Abstract

The invention belongs to the technical field of metallurgy, and particularly relates to a high-production-efficiency, low-cost and low-yield-ratio welded round steel pipe and a production method thereof. The welded round steel pipe comprises the following components in percentage by mass: 0.16 to 0.22 percent of C, 0.10 to 0.25 percent of Si, 0.75 to 1.0 percent of Mn, less than or equal to 0.025 percent of P, less than or equal to 0.010 percent of S, 0.015 to 0.065 percent of Al, 0.25 to 0.45 percent of Cr and the balance of Fe and inevitable impurities. The production method comprises the steps of molten iron pretreatment, converter / electric furnace primary smelting, LF refining, slab continuous casting, slab heating, high-pressure water descaling, rough rolling, finish rolling, vertical roll edge pressing, multi-roll forming to form a round steel pipe, first fixed-length cutting, water cooling, cooling bed cooling, straightening, high-frequency welding and second fixed-length cutting. According to the invention, consistent production is adopted, the process flow is short, the production period is short, the production efficiency is high, the product delivery period of enterprises is greatly prolonged, and the fund turnover efficiency is improved.

Description

Technical Field

[0001] The invention belongs to the field of metallurgical technology, and specifically relates to a high-production-efficiency, low-cost, low-yield-to-strength ratio welded round steel pipe and a production method thereof. Background Art

[0002] A round steel pipe is a circular tubular steel material with openings at both ends and a hollow concentric circle cross-section, and a large ratio of length to circumference. According to the production method, it can be divided into seamless round steel pipe and welded round steel pipe. At present, the materials of welded steel pipe are mainly Q195A, Q215A, Q235A, Q345, etc., which are mainly used for pipelines, thermal equipment, mechanical industry, and steel structure engineering. The current main production process is to use hot-rolled coil as the base material, cold-bend it into a circle, and then weld it by high-frequency welding. The manufacturing process is divided into two stages. The first stage is the raw material production stage, which is to smelt and roll hot-rolled coils in the steel plant. The production process is: blast furnace molten iron → converter / electric furnace steelmaking → molten steel refining → ingot casting → rough rolling → finishing rolling → cooling → coiling → slow cooling → packaging → delivery. The second stage is the pipe forming stage, which is to purchase steel strips (steel coils) from the steel plant in the pipe making plant and cold-bend them. The production process is: steel coil uncoiling → trimming → disc storage → continuous roll forming → high-frequency welding → straightening → fixed-length cutting. The two-stage production results in the steel being transported multiple times, which increases the risk of surface damage such as scratches and bumps during transportation and lifting. It also has a long delivery period, complicated processes, many hidden dangers, and a long turnover cycle for production funds.

[0003] In the current pipe making technology, most round steel pipes are produced by cold bending forming process. As the degree of roll forming increases, the hot rolled steel strip is subjected to greater bending stress, resulting in work hardening. The last few rolls are very likely to induce cracking. The thicker the pipe is and the smaller the diameter is, the greater the uneven deformation of the inner and outer surfaces of the pipe will be during the roll forming process, which is more likely to cause cracking. Figure 1 Therefore, the steel used to make round steel pipes is required to have good mechanical properties. At present, steel mills mostly use low-carbon, high-manganese, and add expensive niobium, vanadium, titanium, molybdenum and other component systems to achieve the toughness and plasticity requirements during cold bending. There are many types of smelting alloys and high production costs.

[0004] At present, when producing round steel pipes, pipe mills need to trim the steel strips. This is because the original edges of the hot-rolled steel strips are in an arc-shaped chamfered shape (when the hot-rolled steel strips are rolled, the pressure of the rollers will cause the edges of the steel strips to bulge, causing the original edges to be in an arc-shaped chamfered shape, such as Figure 2 As shown in the figure, when the steel tube is roll-formed and the butt-side welding is performed, the chamfered sides cannot be in close contact, which restricts the welding effect. Therefore, the edge of the steel strip needs to be cut off to make the edge of the steel strip flat to ensure smooth butt welding on both sides. However, trimming reduces the yield rate and causes a certain amount of waste of the parent steel strip. In addition, a shearing process is required, which increases the process flow and personnel and equipment costs.

[0005] The yield strength ratio of steel is the ratio of the yield strength of steel to its tensile strength. In the fields of building structures, engineering machinery, etc., it is expected that steel has a suitable yield strength ratio. If the yield strength ratio is too high, the material has a strong ability to resist deformation and is not prone to plastic deformation. The structure and parts are prone to brittle failure. The structure is destroyed without obvious deformation during the failure, which is difficult to prevent. This form of failure is strictly prohibited in the fields of construction, etc. If the yield strength ratio is small, the plasticity of the material is good, the reliability of the structure and parts is high, the material is stressed to the yield strength and the strength continues to increase, and the structure undergoes plastic deformation. This deformation is visible to the naked eye, and the precursor of structural failure appears, which can be discovered and prevented in advance to avoid sudden damage. At present, hot-rolled steel strips are welded into round tubes. During the rolling process, the steel undergoes plastic deformation and work hardening. Compared with the parent material, the strength of the round tube after forming and welding is improved, especially the yield strength is much higher than the tensile strength, the elongation after fracture (absolute value) is reduced by 5-10%, and the yield strength ratio is as high as 0.90-0.98. Once the round tube undergoes plastic deformation, it will break quickly, which is extremely unsafe when used. Therefore, it is very necessary to design a production method for welded round steel tubes that has the characteristics of low yield strength ratio on the basis of meeting the strength and toughness, and has low cost, short process, few types of alloy materials, and high compactness of manufacturing process in terms of production process. Summary of the invention

[0006] In view of the deficiencies of the prior art, the object of the present invention is to provide a high production efficiency, low cost and low yield strength ratio welded round steel pipe and a production method thereof.

[0007] On one hand, the present invention provides a high-production-efficiency, low-cost, low-yield-to-strength ratio welded round steel pipe, the components of which are calculated by mass fraction as follows: C: 0.16-0.22%, Si: 0.10-0.25%, Mn: 0.75-1.0%, P: ≦0.025%, S: ≦0.010%, Al: 0.015-0.065%, Cr: 0.25-0.45%, and the remainder is Fe and unavoidable impurities.

[0008] Preferably, a high production efficiency, low cost and low yield strength ratio welded round steel pipe has the following composition by mass fraction: C: 0.18%, Si: 0.17%, Mn: 0.8%, P: 0.016%, S: 0.010%, Al: 0.050%, Cr: 0.35%, and the remainder is Fe and unavoidable impurities.

[0009] Another aspect of the present invention provides a production process for a welded round steel pipe with high production efficiency, low cost and low yield strength ratio, comprising the following steps:

[0010] (1) After the molten steel is initially smelted in a converter / electric furnace, refined in LF, and continuously cast into a slab with a thickness of 150 to 250 mm, the slab is heated to 1120 to 1220°C in a heating furnace for a holding time of (5 to 9) × the slab thickness value (in cm) min. The heating temperature should not be too high, and the holding time should not exceed 9 × the slab thickness value min to avoid overheating and overburning of the slab, excessive growth of austenite grains, and affect the surface quality and mechanical properties of the final product. After being taken out of the furnace, rough rolling is carried out, the starting rolling temperature of rough rolling is 1080-1165℃, and the rolling is performed for 5-7 passes. The thickness of the intermediate billet is 5-10 times the thickness of the finished hot-rolled steel strip, and the thickness is about 25-45mm. The intermediate billet is heated in the hot coil box, and the temperature of the intermediate billet is 1000-1100℃. After the intermediate billet is uncoiled, it enters the finishing rolling unit for 6-7 rolling passes, the final rolling temperature is 850-900℃, the total reduction rate of finishing rolling is 80-90%, and the thickness of the hot-rolled steel strip is 5-20mm.

[0011] (2) After finishing rolling, the hot-rolled steel strip enters a rolling device consisting of an upper roll, a lower roll, a left vertical roll, and a right vertical roll (see Appendix). Figure 3 ), the opening thickness between the upper roller and the lower roller is the product thickness of the hot-rolled steel strip, and the opening thickness between the left vertical roller and the right vertical roller is 10-15mm less than the width of the hot-rolled steel strip. Taking advantage of the good plasticity, low deformation resistance and easy metal flow of the finish-rolled strip at high temperature, the vertical roller is used to extrude the side surface of the high-temperature strip after finish rolling into a plane to obtain a hot-rolled steel strip with a flat side surface. The temperature of the hot-rolled steel strip is 840-900℃.

[0012] (3) The hot-rolled steel strip with a flat side enters the continuous forming roller press unit, which gradually bends and rolls the hot-rolled steel strip into a cylindrical shape. The temperature of the round steel pipe is 730-830°C. The round steel pipe is cut to size for the first time on the plasma cutting line. The length of the round steel pipe after cutting should match the length of the cooling bed.

[0013] The continuous forming roller press unit is a device in the prior art for cold bending a cold hot-rolled steel strip into a circle.

[0014] (4) The round steel pipe is passed through a water tank for cooling treatment at a cooling rate of 5 to 20°C / S. The final cooling temperature of the round steel pipe is 550 to 650°C to obtain a structure mainly composed of pearlite or granular bainite + ferrite.

[0015] The water tank is a device used in the prior art for performing water cooling treatment on round steel and threaded steel.

[0016] (5) After water cooling, the round steel pipe is cooled to below 100°C on a cooling bed. After the round pipe is straightened, high-frequency resistance welding is performed, and then the steel pipe is cut to length for the second time by plasma cutting equipment to the order specification length.

[0017] The invention provides a production process of a high-production-efficiency, low-cost and low-yield-to-strength ratio welded round steel pipe, and the process flow is: molten iron pretreatment → converter / electric furnace primary refining → LF refining → slab continuous casting → slab heating → high-pressure water descaling → rough rolling → finishing rolling → vertical roller edge pressing → multi-roller forming into a round steel pipe → first fixed-length cutting → water cooling → cooling bed → straightening → high-frequency welding → second fixed-length cutting.

[0018] Preferably, in step (1), the molten steel is subjected to converter, LF refining, and slab continuous casting to obtain a slab with a thickness of 175 to 200 mm. The slab is heated to 1130 to 1200° C. in a heating furnace and then rough rolled to obtain an intermediate billet with a thickness of 30 mm and a temperature of 1000 to 1080° C. The intermediate billet is finish rolled to obtain a hot-rolled steel strip with a thickness of 10 mm and a temperature of 855 to 890° C.

[0019] Preferably, the temperature of the circular steel pipe in step (3) is 770°C.

[0020] Preferably, in step (4), the circular steel pipe is passed through a water tank for cooling treatment, the cooling rate is 10°C / S, and the final cooling temperature of the circular steel pipe is 555-65°C, so as to obtain a circular steel pipe with pearlite or granular bainite + ferrite as the main structure.

[0021] Steel composition design:

[0022] C: Elements that improve hardenability can greatly improve the strength of steel. However, as the carbon content in the steel increases, the plasticity, toughness and weldability of the steel will decrease. Therefore, the carbon content of the present invention is in the range of 0.16-0.22%.

[0023] Si: has the effects of deoxidation and solid solution strengthening. It has a high solid solubility in steel, can enhance the volume fraction of ferrite, refine the grains, and improve the strength of steel. However, when the Si content is ≥0.30%, it is easy to form fayalite phase, increase the adhesion of iron sheet, and affect the surface quality of the finished steel strip. Therefore, the silicon content of the present invention is in the range of 0.10-0.25%.

[0024] Mn: significantly improves the hardenability and hardenability of steel. Increasing the manganese content can promote the enhancement of austenite stability, increase the toughness and strength of steel, effectively reduce the phase transition temperature, and refine the microstructure. However, excessive Mn will increase the degree of internal segregation and low-temperature toughness and reduce weldability. Therefore, the manganese content of the present invention is in the range of 0.75-1.0%.

[0025] P: a harmful element that increases the cold brittleness of steel. Its content should be reduced as much as possible. However, excessive pursuit of low P will increase smelting costs or process costs and affect the economic efficiency of the product. In the present invention, the content is controlled at ≤0.025%.

[0026] S: a harmful element that seriously affects the high-temperature plasticity of steel and easily reacts with Mn to form MnS, resulting in a waste of alloy Mn, causing fluctuations in product performance, and inducing hot brittleness of steel. Therefore, the lower the S content, the better, but the cost of the deep desulfurization process is relatively high. Therefore, the S content of the present invention is controlled at ≤0.010%.

[0027] Al: a strong deoxidizing element, which can fix free N in steel and improve the low-temperature toughness of the steel strip matrix and the welding heat-affected zone; the dispersed AlN can inhibit the growth of austenite grains during heating, refine the grain size, and improve the impact toughness; but excessive aluminum will cause a surge in the number of type B inclusions, resulting in poor internal quality of the steel strip and reduced weldability and machinability of the steel strip. Therefore, the Al content of the present invention is controlled at 0.015-0.065%.

[0028] Cr: Chromium can strongly improve the toughness of steel and increase the stability of austenite. It is low in cost and relatively cheap. At the same time, chromium can also significantly improve the corrosion resistance and oxidation resistance of steel. However, a significant increase in chromium will increase the alloy cost, reduce elongation, and deteriorate weldability. Therefore, the chromium content of the present invention is in the range of 0.25-0.45%.

[0029] Beneficial effects of the present invention

[0030] 1. The present invention utilizes hot-rolled steel strips at high temperatures to roll them into round tubes online, making full use of the advantages of steel materials at high temperatures, such as good toughness and plasticity, low deformation resistance, and good metal fluidity. The edge of the steel strip is "planarized" by a "vertical roller" extrusion method, which reduces the trimming operation of the finished steel strip, shortens the process flow, and reduces the consumption of raw steel.

[0031] 2. The steel strip is rolled into a round tube at high temperature. At high temperature, the steel strip is in an austenite phase with excellent toughness and plasticity, which can better meet the toughness and plasticity requirements of the rolling operation. There is no need to add expensive alloys to ensure the toughness of the steel matrix. While reducing production costs, the high-temperature steel strip forming process is not easy to crack, which significantly reduces the cracking rate during roll forming and improves the yield rate.

[0032] 3. The present invention utilizes hot-rolled steel strips at high temperatures to perform online rolling and round pipe making operations. The steel strips do not need to be stored and transported offline, and are then hoisted online to be rolled and pressed into pipes. From the production of the parent steel strip to the forming of the terminal round pipe, consistent production is achieved, with a short process flow, a short production cycle, and high production efficiency, which greatly improves the product delivery period of the enterprise and improves the capital turnover efficiency.

[0033] 4. The present invention utilizes the characteristics of the formed steel pipe that it does not need to bear axial load during use and is not prone to deformation. It does not have high requirements for toughness and plasticity. The round steel pipe is cooled online to efficiently obtain pearlite or granular bainite structure with good comprehensive strength and toughness properties to meet the strength and toughness requirements of the round pipe.

[0034] 5. The yield strength of the round tube mother material steel strip obtained by the present invention is relatively low. When the round tube is overloaded, it can withstand more plastic deformation before yielding deformation to fracture, which increases the stress-bearing space of the steel tube, prolongs the time from deformation to fracture, and is less likely to cause dangerous brittle fracture. It gains more protective processing time to deal with safety risks such as sudden fracture, and improves safety and reliability during use. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The present invention will be further described below in conjunction with the accompanying drawings.

[0036] Figure 1 This is a picture of cracking of steel round tube in the prior art.

[0037] Figure 2 This is a diagram showing that the original edge of the steel strip after rolling is in an arc-shaped chamfered shape.

[0038] Figure 3 Schematic diagram of the rolling device; in the figure: 1, upper roller; 2, lower roller; 3, left vertical roller; 4, right vertical roller; 5, hot-rolled steel strip.

[0039] Figure 4 This is a metallographic structure diagram of the weld of the steel round tube in Example 1 of the present invention at a scale of 100 μm.

[0040] Figure 5 This is the metallographic structure diagram of the steel round tube base material in Example 1 of the present invention at a scale of 100 μm.

[0041] Figure 6 This is the metallographic structure diagram of the steel round tube base material in Example 2 of the present invention at a scale of 100 μm.

[0042] Figure 7 This is the metallographic structure diagram of the steel round tube base material at a scale of 50 μm in Example 3 of the present invention.

[0043] Figure 8 This is the metallographic structure diagram of the steel round tube base material in Comparative Example 1 of the present invention at a scale of 100 μm.

[0044] Fig. 9 This is the metallographic structure diagram of the steel round tube base material at a scale of 50 μm in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0045] The present invention will be further described in detail below in conjunction with the accompanying drawings, but the protection scope is not limited thereto.

[0046] The invention provides a high-production-efficiency, low-cost, low-yield-strength-ratio welded round steel pipe, the components of which are calculated by mass fraction as follows: C: 0.16-0.22%, Si: 0.10-0.25%, Mn: 0.75-1.0%, P: ≤0.025%, S: ≤0.010%, Al: 0.015-0.065%, Cr: 0.25-0.45%, and the balance is Fe and unavoidable impurities.

[0047] The process flow of a high-production-efficiency, low-cost, low-yield-to-strength ratio welded round steel pipe provided by the present invention is as follows: molten iron pretreatment → converter / electric furnace primary smelting → LF refining → slab continuous casting → slab heating → high-pressure water descaling → rough rolling → finishing rolling → vertical roller edge pressing → multi-roller forming into a round steel pipe → first fixed-length cutting → water cooling → cooling bed → straightening → high-frequency welding → second fixed-length cutting.

[0048] Specifically, the present invention provides a production process for a high-production-efficiency, low-cost, and low-yield-to-strength ratio welded round steel pipe:

[0049] The sulfur content of the molten iron entering the furnace is controlled at ≤0.020% by mass, and the slag on the surface of the molten iron is scraped off after desulfurization.

[0050] The basicity of the final slag of converter smelting is controlled at R=3.0-3.5, and the slag is blocked by a slide plate to prevent slag from falling, and the steel discharge time is ≥3min. When the molten steel is discharged to 1 / 4, ferrochrome, ferroaluminum manganese, silicon manganese and ferrosilicon are evenly added, and the addition is completed when the molten steel is discharged to 3 / 4, and the alloy is added to the impact zone of the steel flow.

[0051] After the ladle arrives at LF refining, the temperature is measured, oxygen is determined, and argon is blown immediately. The argon pressure and flow rate are suitable for slightly turning the slag surface without exposing the molten steel. Aluminum iron and calcium carbide are used for deoxidation. The top slag is white slag before leaving the station, and the final slag basicity is controlled above 2.5. After making white slag, titanium iron or titanium wire is used for titanium alloying. Before the molten steel leaves the station, 100-150m of calcium aluminum wire is fed into each furnace for calcification treatment according to the aluminum content in the steel. The LF refining cycle is not less than 35 minutes, of which the soft blowing time is not less than 12 minutes. Soft blowing requires that the molten steel in the ladle moves slightly and the molten steel must not be exposed.

[0052] The continuous casting adopts full-process protective pouring, the cross-sectional specifications of the ingot are (150~250)mm*(750~1500)mm, medium carbon steel protective slag is used, attention is paid to the adsorption of inclusions by the protective slag, and the slag is replaced in time. The secondary cooling of the continuous casting machine adopts weak cooling mode, and the continuous casting machine pulling speed is 1.10~1.25m / min.

[0053] The heating time of the ingot in the heating furnace is 120 to 150 minutes, and the temperature of the ingot out of the heating furnace is 1130 to 1200°C.

[0054] The rough rolling unit is used for 5 to 7 reversible rolling. The temperature of the last rough rolling pass is 1000 to 1100℃. The deformation of the first rough rolling pass is ≥20%, and the cumulative deformation is controlled at 70%. The slab is descaled with high-pressure water at the entrance of the rough rolling. The nozzle pressure is ≥18MPa, and the number of injections, pressure and flow are adjusted in time to fully remove the iron oxide scale. The thickness of the intermediate slab after rough rolling is 25 to 45mm.

[0055] The cumulative deformation of the intermediate billet during finishing rolling is controlled at 55-65%. Before finishing rolling, the intermediate billet is descaled using two descaling water collecting pipes, and the nozzle pressure is ≥18MPa to remove the oxidized iron scale on the surface of the intermediate billet. The finishing rolling temperature is 850-900℃, and the terminal thickness of the hot-rolled steel strip is 5-20mm.

[0056] like Figure 3 As shown, after the finish rolling, the hot rolled steel strip 5 enters into a rolling device composed of an upper roller 1, a lower roller 2, a left vertical roller 3, and a right vertical roller 4. The opening thickness between the upper roller 1 and the lower roller 2 is the thickness of the hot rolled steel strip 5, and the opening thickness between the left vertical roller 3 and the right vertical roller 4 is 10-15 mm less than the width of the hot rolled steel strip 5. By utilizing the advantages of good plasticity, low deformation resistance, and easy metal flow of the strip steel after the finish rolling at high temperature, the left vertical roller 3 and the right vertical roller 4 are used to extrude the side surface of the edge of the high temperature strip steel 5 after the finish rolling into a plane, so as to obtain a hot rolled steel strip 5 with a flat side surface. There is no need to trim the hot rolled steel strip 5, thereby reducing the loss of raw materials. The temperature of the hot rolled steel strip 5 is 840-890°C.

[0057] The hot-rolled steel strip with a flat side enters the continuous forming roller press unit, and is bent into a round steel pipe by the continuous forming roller press unit. The steel strip is in austenite phase at high temperature, and the toughness and plasticity of the austenite phase are good. Without adding a large amount of expensive alloy elements to improve toughness and plasticity, the high-temperature steel strip can also meet the toughness and plasticity requirements of the rolling into round pipe operation. It is not easy to crack, and can greatly reduce the cracking rate of rolling into round pipe. The temperature of the round steel pipe after bending is 730-830℃. During the bending process, the rollers are sprayed with water for cooling, and then the round steel pipe is cut to size for the first time by plasma cutting equipment. The length of the round steel pipe after cutting matches the length of the cooling bed.

[0058] The continuous forming roller press unit is a device for cold bending and forming a cold hot-rolled steel strip in the prior art.

[0059] The round steel pipe passes through a water tank for cooling treatment, the cooling rate is 5-20℃ / S, the water tank length is 30-50m, the temperature of the round steel pipe before entering the water tank is 730-830℃, which is an austenite structure, and the final cooling temperature of the round steel pipe out of the water tank is 550-650℃, and a round steel pipe with pearlite or granular bainite + ferrite as the main structure is obtained. The yield strength of the obtained steel pipe is 355-500Mpa, the tensile strength is 470-780Mpa, the elongation is 18-30%, the impact energy at 20℃ is 50-200J, and the impact energy at 0℃ is 30-100J.

[0060] The cooling water tank is a device used in the prior art for performing water-penetrating cooling treatment on round steel and threaded steel.

[0061] The cooled round steel pipe is cooled to below 100°C on a cooling bed, and then straightened and high-frequency welded. The steel pipe is then cut to length for the second time using plasma cutting equipment to the length required by the customer. Specific embodiment:

[0063] (1) The molten steel composition corresponding to the embodiment is shown in Table 1.

[0064] Table 1. Molten steel composition corresponding to the examples (%)

[0065]

[0066] In Table 1, Examples 1, 2 and 3 are the compositions of molten steel smelted by the present invention.

[0067] Comparative Examples 1 and 2 are compositions of molten steel produced from base materials for manufacturing existing round steel pipes.

[0068] As shown in Table 1, the alloy elements used in Examples 1, 2, and 3 of the present invention are simple and less than those in the current technology. In the current technology, Nb is an expensive alloy, and Ti has extremely active chemical properties, and the composition design window is relatively wide. According to the alloy price calculation, the cost of the molten steel alloy of the present invention is 30 to 50 yuan / ton steel lower than that of the comparative alloy.

[0069] (2) The production process parameters corresponding to the embodiments and comparative examples are shown in Table 2.

[0070] Table 2. Process parameters corresponding to the examples

[0071]

[0072] In Table 2, Examples 1, 2 and 3 are round steel pipes produced by the present invention.

[0073] Comparative Examples 1 and 2 are round steel pipes produced according to the prior art. The specific process is: the pipe making plant purchases hot-rolled steel strips, which are formed by cold bending and rolling, and are made into round pipe products by high-frequency welding. The manufacturing process is divided into two stages. The first stage is the raw material production stage, which is to smelt and roll hot-rolled coils in the steel plant. The production process is: blast furnace molten iron → converter / electric furnace steelmaking → molten steel refining → ingot casting → rough rolling → finishing rolling → cooling → coiling → slow cooling → packaging → delivery. The second stage is the pipe forming stage, which is to purchase steel strips (steel coils) from the steel plant in the pipe making plant and cold-bend them. The production process is: steel coil uncoiling → trimming → disc storage → continuous roll forming → high-frequency welding → straightening → fixed-length cutting, and the rolling and round pipe making speed is 10 to 25 m / min.

[0074] (3) Mechanical properties of circular steel pipes produced in examples and comparative examples.

[0075] The test specimens are tested for longitudinal tensile properties and longitudinal impact properties.

[0076] The standard adopted for the tensile test is: GB / T 228.1 Tensile test of metallic materials Part 1: Room temperature test method.

[0077] The impact test adopts the standard: GB / T 229 Charpy pendulum impact test method for metal materials.

[0078] The performance is shown in Table 3;

[0079] Table 3 Mechanical properties of steel produced in examples and comparative examples

[0080]

[0081]

[0082] Embodiments 1, 2, and 3 are produced using the process requirements of the present invention. All operations from molten steel smelting to pipe forming are performed online in an integrated manner. The entire production process takes only 4 to 6 hours to be put into storage. However, in comparative example 1, the traditional process has a long production cycle and requires transportation between two production bases. It takes several days or even longer from manufacturing to product warehousing and delivery. The circular steel pipe provided by the present invention has a short production cycle, less transportation and hoisting times, a small probability of surface damage, high manufacturing efficiency, and a high capital turnover rate.

[0083] In Examples 1, 2, and 3, the hot-rolled steel strip is rolled into a round tube online at high temperature. At this time, the steel strip is in an austenite phase with good plasticity and toughness. No cracking or crack defects occur during rolling into a round tube, and no offline trimming is required, so the overall yield rate of the steel tube is high. The comparative example adopts an offline cold pressing process, which is lengthy, and the cold strip is pearlite or bainite structure with high resistance to deformation, and there is a risk of cracking. According to on-site feedback, the cracking rate is about 3-5%.

[0084] The organization photos of the round steel pipes produced in Examples 1, 2 and 3 are shown below. Figure 4 , 5 , 6, and 7 show pearlite + ferrite structure.

[0085] The organization photos of the round steel pipes produced in Comparative Examples 1 and 2 are shown below. Figure 8 , 9 As shown, it is a ferrite + pearlite + granular bainite structure.

[0086] In summary, Examples 1, 2, and 3 can produce circular steel pipes online using the technology provided by the present invention. The base material has a good yield strength ratio. The obtained circular pipe can withstand more plastic deformation from yielding to breaking, and has higher safety and reliability. It does not crack during the roll forming process, and its strength is comparable to the current technology, but its plasticity is better, and there are fewer alloy types, so the cost control is lower. Although the impact energy is slightly lower than the current process, the steel pipes after forming are mostly used as structural parts, and basically do not withstand circumferential impact during the service process, and do not need to be deformed, and mainly play the role of supporting the skeleton. Therefore, the impact function meets the current use requirements.

[0087] The process parameters (such as temperature, time, etc.) of the present invention can realize the method by taking upper and lower limits and interval values, and the embodiments are not listed one by one here.

[0088] Any contents not described in detail in the present invention can be based on the conventional technical knowledge in the art.

[0089] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention and should be included in the scope of the claims of the present invention.

Claims

1. A high production efficiency, low cost and low yield strength ratio welded round steel pipe, characterized in that: The composition of the welded round steel pipe is calculated by mass fraction as follows: C: 0.16-0.22%, Si: 0.10-0.25%, Mn: 0.75-1.0%, P: ≤0.025%, S: ≤0.010%, Al: 0.015-0.065%, Cr: 0.25-0.45%, and the remainder is Fe and unavoidable impurities.

2. The high production efficiency, low cost and low yield strength ratio welded round steel pipe according to claim 1 is characterized in that: The composition of the welded round steel pipe is calculated by mass fraction as follows: C: 0.18%, Si: 0.17%, Mn: 0.8%, P: 0.016%, S: 0.010%, Al: 0.050%, Cr: 0.35%, and the remainder is Fe and unavoidable impurities.

3. A method for producing a welded round steel pipe with high production efficiency, low cost and low yield strength ratio, the production method comprising: Hot metal pretreatment → primary smelting in converter / electric furnace → LF refining → slab continuous casting → slab heating → high-pressure water descaling → rough rolling → finishing rolling → vertical roller edge pressing → multi-roller forming to round steel pipe → first cut to length → water cooling → cooling bed → straightening → high-frequency welding → second cut to length.

4. The production method according to claim 3, characterized in that: The molten steel is subjected to primary refining in a converter / electric furnace, LF refining, and slab continuous casting to obtain slabs with a thickness of 150 to 250 mm.

5. The production method according to claim 3, characterized in that: During the slab heating process, the heating furnace is heated to 1120-1220° C., and the insulation time is min: (5-9)×the slab thickness value.

6. The production method according to claim 3, characterized in that: The starting temperature of rough rolling is 1080-1165℃, and the rolling is performed for 5-7 passes. The thickness of the intermediate billet is 5-10 times the thickness of the finished hot-rolled steel strip, which is 25-45mm. The intermediate billet is heated in the hot coil box. The temperature of the intermediate billet is 1000-1100℃. After the intermediate billet is uncoiled, it enters the finishing rolling unit for 6-7 passes. The final rolling temperature is 850-900℃, the total reduction rate of finishing rolling is 80-90%, and the thickness of the hot-rolled steel strip is 5-20mm.

7. The production method according to claim 3, characterized in that: The vertical roller edge pressing adopts a rolling device composed of an upper roller, a lower roller, a left vertical roller, and a right vertical roller. The opening between the upper roller and the lower roller is the product thickness of the hot-rolled steel strip, and the opening between the left vertical roller and the right vertical roller is 10 to 15 mm less than the width of the hot-rolled steel strip. The vertical roller is used to extrude the side of the edge of the high-temperature strip steel after finish rolling into a plane to obtain a hot-rolled steel strip with a flat side. The temperature of the hot-rolled steel strip is 840 to 900°C.

8. The production method according to claim 3, characterized in that: The multi-roll forming into a round steel pipe is as follows: the hot-rolled steel strip with a flat side is fed into a continuous forming roller press unit, and the hot-rolled steel strip is gradually bent and rolled into a cylindrical shape by the continuous forming roller press unit. The temperature of the round steel pipe is 730-830°C.

9. The production method according to claim 3, characterized in that: Water cooling is as follows: the round steel pipe passes through a water tank for cooling treatment, the cooling rate is 5-20℃ / S, the final cooling temperature of the round steel pipe is 550-650℃, and a round steel pipe with pearlite or granular bainite + ferrite as the main structure is obtained.

10. The production method according to claim 3, characterized in that: The obtained round steel pipe has a yield strength of 355-500Mpa, a tensile strength of 470-780Mpa, an elongation of 18-30%, an impact energy of 50-200J at 20°C, and an impact energy of 30-100J at 0°C.

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