600 mpa-grade low-cost steel for welded pipe, welded pipe and manufacturing method therefor

Abstract

Provided are steel for a welded pipe, a welded pipe and a manufacturing method therefor. The steel for a welded pipe comprises the following chemical elements in percentages by weight: C: 0.05-0.09%, Si≤0.1%, Mn: 1.2-1.6%, P≤0.02%, S≤0.010%, Ti: 0.015-0.06%, Nb: 0.006-0.06%, and the balance of Fe and other inevitable impurities, wherein the element content of the steel satisfies the following relation expressions (1) and (2): 93×Ti+48×Nb≥4.25% (expression 1), and 372×C-93×Ti-48×Nb≥13.4% (expression 2). The steel for a welded pipe and a welded pipe made of the steel for a welded pipe have excellent mechanical properties, and can satisfy the comprehensive performance requirements for a welded pipe for structures in terms of formability, connection strength, etc., in the field of engineering machinery.

Description

600MPa grade low-cost steel for welded pipe, welded pipe and manufacturing method thereof TECHNICAL FIELD

[0001] The present disclosure relates to a steel material for welded pipe, welded pipe made of the steel material for welded pipe and a manufacturing method thereof applicable to the field of engineering machinery, in particular, a low-cost steel for welded pipe with a tensile strength of 600 MPa grade, welded pipe made of the steel for welded pipe and a manufacturing method thereof. BACKGROUND

[0002] Structural steel pipes are widely used in the field of engineering machinery, and the mechanical properties, flattening properties and butt joint strength of the steel pipes are required to be high due to their use in load-bearing safety structures. At present, seamless steel pipes are mainly used in this field, although they have excellent performance, but have problems such as long production process, high cost, poor dimensional accuracy and geometric tolerance. With the continuous development of high-frequency welding (HFW) technology, high-frequency welded pipes are increasingly attracting attention due to their high weld quality, good pipe shape, uniform wall thickness and low manufacturing cost. Under the background of China's "double carbon" strategy and the enterprises' desire to reduce costs and increase efficiency, engineering machinery manufacturing enterprises are gradually showing their willingness to replace seamless pipes with high-frequency welded pipes.

[0003] High-frequency welding (HFW) is a welding process that heats the edge of the pipe blank through the skin effect and proximity effect of high-frequency current, melts it and realizes forging under the action of extrusion. High-frequency welded pipes are usually made of hot-rolled steel plates or steel coils, which are formed by roll bending, high-frequency welding, weld heat treatment, sizing and sizing, etc. They have the characteristics of low manufacturing cost and high dimensional accuracy, and have been widely used in the fields of petroleum, submarine pipelines and medium transportation.

[0004] In the prior art, there are several patents that disclose steel materials for welded pipes or casings and manufacturing methods thereof. For example:

[0005] CN102912245B discloses a N80 grade resistance-welded oil casing steel, which has a composition system of C-Si-Mn with the addition of Nb, Ti and Cr, and contains, by weight percentage, C: 0.05-0.09%, Si: 0.15-0.35%, Mn: 1.70-1.95%, P≤0.020%, S≤0.008%, Ti: 0.010-0.050%, Als: 0.02-0.06%, Nb: 0.05-0.08%, Cr: 0.20-0.30%, N≤0.008%, and the balance of Fe and inevitable elements. The N80 grade oil casing steel is prepared by a hot rolling process.

[0006] CN105695882B discloses a steel for J55 grade electric resistance welded casing pipe, which has a composition system of C-Si-Mn with the addition of Nb, Ti and Cr, and contains, by weight percent: C: 0.090-0.128%, Si: 0.10-0.35%, Mn: 1.02-1.25%, P≤0.015%, S≤0.008%, Nb: 0.01-0.03%, Ti: 0.010-0.030%, Cr: 0.30-0.40%, Als: 0.02-0.06%, N≤0.008%, and the balance of Fe and inevitable elements. The J55 grade casing pipe steel is prepared by a process of smelting, continuous casting, rolling and cooling, and has a microstructure of ferrite-pearlite-a small amount of M / A structure.

[0007] CN115369312A discloses a high-strength high-frequency resistance welded pipe, which has a composition system of C-Si-Mn with the addition of Nb, Ti, Ni, Cr and Mo, and contains, by mass percent: C: 0.060-0.085%, Si: 0.10-0.30%, Mn: 1.60-1.85%, Ni: 0.01-0.12%, Cr: 0.10-0.30%, Mo: 0.01-0.15%, Nb: 0.040-0.065%, Ti: 0.005-0.020%, Al: 0.020-0.050%, Ca: 0.001-0.004%, and the balance of Fe and other inevitable impurities. The 600-750 MPa grade high-frequency welded pipe is prepared by a process of plate rolling, high-frequency welding and weld heat treatment.

[0008] CN109536847B discloses a hot-rolled steel plate for a 390 MPa grade welded pipe, which has a composition system of C-Si-Mn with the addition of Nb and Ti, and contains, by weight percent: C: 0.05-0.07%, Si: 0.08-0.15%, Mn: 0.80-0.90%, P≤0.015%, S≤0.004%, N≤0.0060%, Al: 0.010-0.040%, Ti: 0.01-0.02%, Nb: 0.035-0.045%, and the balance of Fe and inevitable inclusions. The 390 MPa grade welded pipe hot-rolled steel plate is prepared by a hot-rolling process, and has a microstructure of ferrite + a small amount of pearlite.

[0009] CN114457282A discloses a hot-rolled steel plate for longitudinal split welded pipe with a yield strength of 415 MPa, which has a composition system of C-Si-Mn and adds Nb and Ti, and contains, by weight percentage, C: 0.04-0.06%, Si: 0.09-0.16%, Mn: 0.90-1.00%, P≤0.015%, S≤0.004%, N≤0.0060%, Al: 0.010-0.045%, Ti: 0.01-0.02%, Nb: 0.045-0.055%, Mn / Si is 5-10, and the balance is iron and unavoidable inclusions. The hot-rolled steel plate for welded pipe with a yield strength of 415 MPa is prepared by a hot-rolling process, and the metallographic structure is ferrite + pearlite.

[0010] The welded pipe products of the above prior art are mainly applied in the fields of oil casing, submarine pipelines and medium transportation, and mainly bear internal medium pressure during service, so the technical focus is mainly concentrated on the strength and toughness of the pipe material. In the field of engineering machinery, steel pipes are usually used as structural load-bearing members, which not only require the steel pipe itself to have high strength and good toughness, but also require the welded pipe to have excellent flattening performance and butt joint strength. At present, the welded pipe prepared from the existing hot-rolled steel plate is still difficult to meet the use requirements of replacing seamless steel pipes in engineering machinery in terms of comprehensive performance. SUMMARY

[0011] In view of the above defects and deficiencies of the prior art, the purpose of the present disclosure is to provide a low-cost 600 MPa grade welded pipe steel, a welded pipe made of the welded pipe steel and a manufacturing method thereof.

[0012] Therefore, in a first aspect, the present disclosure provides a 600 MPa grade welded pipe steel, wherein the steel contains, in addition to Fe and unavoidable impurities, the following chemical elements in the following weight percentage: C: 0.05-0.09%, Si≤0.1%, Mn: 1.2-1.6%, P≤0.02%, S≤0.010%, Ti: 0.015-0.06%, Nb: 0.006-0.06%, and the element content of the steel satisfies the following relationship formula (1) and relationship formula (2):

[0013] In the formula, each element symbol is substituted into the weight percentage content of the corresponding element.

[0014] In a preferred embodiment, the element content of the steel for welded pipe of the present disclosure also satisfies the following relationship (3): 0.42% ≤ C + 0.25 × Si + 0.25 × Mn + 1.07 × P + 0.13 × Cu + 0.05 × Ni + 0.23 × Cr + 0.2 × Mo + 0.2 × V ≤ 0.52%

[0015] (Formula 3). In the case where the steel does not contain Cu, Ni, Cr, Mo and / or V, the content of the corresponding element is calculated as 0%.

[0016] In a preferred embodiment, the steel for welded pipe of the present disclosure contains the following chemical elements in the following weight percentage: C: 0.05-0.09%, Si ≤ 0.1%, Mn: 1.2-1.6%, P ≤ 0.02%, S ≤ 0.010%, Ti: 0.015-0.06%, Nb: 0.006-0.06%, and the balance being Fe and unavoidable impurities.

[0017] In a preferred embodiment, the steel for welded pipe of the present disclosure further contains at least one of the following chemical elements in the following weight percentage: Mo ≤ 0.3%, Cr ≤ 0.2%, V ≤ 0.15%, Ni ≤ 0.1%, Ca ≤ 0.015%.

[0018] In a preferred embodiment, the microstructure of the steel for welded pipe of the present disclosure is ferrite + pearlite. In a preferred embodiment, the ferrite is in the shape of polygon or quasi-polygon. In a preferred embodiment, the grain size of the ferrite is ≥ 12 grade. In a preferred embodiment, the pearlite is distributed in the form of elongated strips or small blocks at the grain boundary of the ferrite. In a preferred embodiment, the volume fraction of the pearlite is ≤ 15%. In a more preferred embodiment, the volume fraction of the pearlite is 3%-15%.

[0019] In a second aspect, the present disclosure also provides a welded pipe made of the above-mentioned steel.

[0020] In a preferred embodiment, the welded pipe is made of the above-mentioned steel for welded pipe by slitting, roll bending and high-frequency welding.

[0021] The welded pipe of the present disclosure has the following excellent mechanical properties: the yield strength of the pipe body is ≥ 550 MPa, the tensile strength is ≥ 600 MPa, and the elongation is ≥ 20%; in the 90° laying flattening test, the weld is free of cracks when flattened to 2 / 3 × D (D is the outer diameter of the welded pipe); and the strength of the welded joint (i.e. the joint strength when welded in the butt welding mode) is ≥ 590 MPa.

[0022] In a third aspect, the present disclosure also provides a method for manufacturing the above-mentioned welded pipe, comprising the following steps:

[0023] 1) smelting, casting according to a predetermined chemical composition to obtain a casting billet or casting ingot;

[0024] 2) heating the casting billet or casting ingot;

[0025] 3) rolling and coiling the heated casting billet or casting ingot to obtain a finished steel coil;

[0026] 4) high-frequency welding to obtain a welded pipe:

[0027] forming: the finished steel coil is slit longitudinally and then formed, and the distance between the two edges of the formed open pipe is L≤2×T;

[0028] high-frequency welding: the high-frequency welding power is 100-250 KHz, the extrusion amount is 0.1×T-0.9×T, and the opening angle is 3-6°,

[0029] wherein T represents the wall thickness of the welded pipe, in mm.

[0030] In a preferred embodiment, the wall thickness T of the welded pipe is 1.5-16 mm.

[0031] In a preferred embodiment, in step 2), the heating temperature is 1210-1300℃, and the heating time is 120-300 min.

[0032] In a preferred embodiment, in step 3), the heated casting billet or casting ingot is subjected to 3-5 rough rolling passes, and the rough rolling exit temperature is 950-1080℃; then, the finished steel coil is obtained by finish rolling and coiling, the finish rolling temperature is 800-900℃, the cooling rate after finish rolling is ≥20℃ / s, and the coiling temperature is 500-630℃. In a more preferred embodiment, in step 3), the cooling rate after finish rolling is 20-150℃ / s, preferably 20-120℃ / s

[0033] The welded pipe steel of the present application adopts a low-carbon, low-silicon basic system in the design of the composition of the steel material, and cooperates with an appropriate amount of manganese (Mn) and micro-alloy elements titanium (Ti) and niobium (Nb). The design system requires fewer types and lower amounts of micro-alloy elements, avoids the use of or reduces the content of valuable alloy elements such as molybdenum (Mo), vanadium (V), and nickel (Ni), and still realizes excellent strength and toughness matching of the steel material.

[0034] The high-frequency welded pipe prepared from the welded pipe steel of the present application not only has excellent flattening performance, but also has good butt joint strength of the welded pipe, and can meet the comprehensive performance requirements of structural welded pipes in the forming property, connection strength, and the like in the field of engineering machinery. The welded pipe can be used to replace seamless steel pipes of the same grade, has significant cost and manufacturing efficiency advantages, and has good application prospects. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 shows a microstructure photograph of the high-frequency welded pipe body of Example 3.

[0036] Figure 2 shows a topography photograph of the high-frequency welded pipe of Example 5 and Example 11 in a 90° laying flat flattening test, flattened to 2 / 3 x D.

[0037] Figure 3 shows a schematic diagram of the opening angle (V).

[0038] Figure 4 shows a schematic diagram of the distance L between the two sides of the open pipe. DETAILED DESCRIPTION

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

[0040] As used herein, the term "and / or" refers to any and all possible combinations of one or more of the associated listed items.

[0041] In this context, the extrusion amount refers to the difference between the circumference of the raw pipe before extrusion and the circumference after extrusion.

[0042] In this context, the opening angle (V) refers to the included angle of the V-shaped region formed when the steel strip is shaped by the extrusion roller. By measuring the edge of the V-shaped region per unit length and then measuring the length perpendicular to the edge, the opening angle is calculated using the Tan function.

[0043] In this context, the yield strength, tensile strength, and elongation are determined in accordance with GB / T 228.1-2010 "Metallic Materials Tensile Testing at Ambient Temperature".

[0044] In this context, the welded joint strength of the welded pipe (butt welding method) is determined in accordance with "ISO 15614-1 2017 Welding Procedure Specification and Evaluation for Metallic Materials".

[0045] In this context, the volume fraction of pearlite and the grain size of ferrite are determined in accordance with GB / T 15749-2008 "Quantitative Metallographic Determination Method".

[0046] In this context, "quasi-polygon" refers to the outline of the grain that roughly presents a polygonal structure, but part of the edge may be slightly curved or not completely regular, and compared with the strict geometric polygon, the boundary shape has a certain natural deformation or slight undulation. Although it does not completely meet the characteristics of each side of the standard polygon being a straight line, it can still be identified as a closed, polygonal structural unit as a whole.

[0047] In this context, the shape of ferrite or pearlite can be observed by a metallographic microscope.

[0048] In the steel for welded pipe of the present disclosure, the design principles of each chemical element are as follows:

[0049] Carbon (C): Carbon is the base element in steel and one of the most important strengthening elements in the present application. As an interstitial atom, carbon has a significant effect on improving the strength of steel. In addition, carbon can also form carbide precipitates with titanium (Ti) and niobium (Nb), further improving the strength of hot-rolled steel plates, high-frequency weld seams, and welded joints. However, too high a carbon content will significantly reduce the weldability and low-temperature toughness of the steel. Therefore, considering the balance between strength and weldability, the C content in the present application is controlled at 0.05-0.09%.

[0050] Silicon (Si): Silicon can improve the strength of steel through solid solution strengthening and has the ability to inhibit the formation of Fe3C. However, too high a silicon content will generate Fe2SiO4 during the heating process, making it difficult to remove the oxide scale and affecting the surface quality of the steel plate. At the same time, an increase in silicon content is also not conducive to the stability of the high-frequency welding process. Therefore, the Si content in the present application is controlled at 0.1% or less.

[0051] Manganese (Mn): Manganese is an important toughening element that can expand the austenite region, stabilize the austenite structure, delay the transformation of austenite to pearlite, and reduce the critical cooling speed of martensitic transformation, thereby improving the hardenability of the steel. In addition, manganese also has a certain solid solution strengthening effect. However, too high a manganese content will cause excessive hardenability, increase the difficulty of structure control, and be detrimental to weldability. Therefore, the Mn content in the present application is controlled at 1.2-1.6%.

[0052] Phosphorus (P): Phosphorus is an impurity element in steel that easily segregates at grain boundaries and forms a low-melting eutectic compound Fe2P with iron, thereby significantly reducing the plasticity and toughness of the steel. Therefore, the lower the content, the better. Therefore, the P content in the present application is controlled at 0.02% or less.

[0053] Sulfur (S): Sulfur is an impurity element in steel that usually combines with Mn to form MnS inclusions. Although MnS has a certain plasticity, it is easily elongated and deformed along the rolling direction during rolling, disrupting the continuity of the matrix and reducing the transverse performance of the steel plate. Therefore, the lower the S content in the steel, the better. The S content in the present application is controlled at 0.010% or less.

[0054] Titanium (Ti): Titanium is a strong carbonitride-forming element. The unsolved carbonitride of Ti can effectively pin the austenite grain boundaries during heating of the steel, controlling the growth of austenite grains. The TiN and TiC precipitated during high-temperature austenite rough rolling can effectively inhibit the growth of austenite grains, while the fine TiC second-phase particles precipitated in ferrite can prevent dislocation movement, thereby greatly improving the strength of the steel. Therefore, the Ti content in the present application is controlled at 0.015-0.06%.

[0055] Niobium (Nb): Nb is a strong carbonitride forming element, which improves the strength of the steel through fine grain strengthening and precipitation strengthening. Nb can increase the austenite recrystallization temperature, and realize the austenite non-recrystallization rolling at a higher temperature, so that the rolling deformation of the rolled piece can be completed at a higher temperature while obtaining a fine structure. However, Nb is relatively expensive, and therefore, the content of Nb in the present application is controlled to be 0.006-0.06%.

[0056] In addition, the steel for welded pipe of the present application can also contain one or more of Mo, Cr, V, Ni, Ca, for example, Mo≤0.3%, Cr≤0.2%, V≤0.15%, Ni≤0.1%, Ca≤0.015%. The addition of these elements, respectively or in combination, can interact with the above-mentioned added elements, improve the purity of the steel, refine the grain size, promote the precipitation of second phase particles, slow down the coarsening of the grain and second phase particles, and thus further improve the mechanical properties of the steel plate.

[0057] The steel for welded pipe of the present application adopts a low-carbon, low-silicon basic system in the composition design of the steel material, and cooperates with an appropriate amount of manganese (Mn) and micro-alloy elements titanium (Ti) and niobium (Nb). The setting of low C and appropriate Mn helps to reduce the hardenability of the steel, thereby facilitating the accurate regulation of the steel structure and improving the weldability. If the contents of C and Mn are too high, the hardenability of the steel will be significantly improved, the ferrite phase change in the layer cooling stage after finish rolling will be inhibited, and bainite or martensite will be easily formed in the final structure, thereby making the strength of the steel too high and the elongation decreases. In addition, high hardenability is also not conducive to the formation of polygonal ferrite or quasi-polygonal ferrite, thereby weakening the precipitation strengthening effect of second phase particles such as TiC and NbC. At the same time, when the contents of C and Mn are too high, the carbon equivalent of the steel increases, and the weldability becomes poor.

[0058] In view of the low contents of C and Si in the steel, and the moderate content of Mn, in order to achieve the required microstructure and performance indicators, appropriate amounts of Ti and Nb are also added to the steel. The effects of adding Ti and Nb mainly manifest in the following two aspects: 1. pinning the austenite grain boundary at high temperature, preventing the rapid growth of austenite grains, and avoiding the formation of coarse austenite grains, which is not conducive to the formation of fine ferrite structure in the final steel; 2. forming second phase particles containing Ti and Nb in the steel during the layer cooling and post-cooling processes, and improving the strength of the steel through second phase precipitation.

[0059] In view of the different mechanisms of Ti and Nb in grain refinement and precipitation strengthening, in order to fully exert the grain refinement and precipitation strengthening effects of Ti and Nb, achieve the required microstructure and performance indicators, and at the same time make the alloy cost of the steel plate as low as possible, the present application uses the following two empirical formulas to control the contents of Ti and Nb: (1) 93xTi+48xNb≥4.25% (Formula 1)

[0060] The Ti and Nb alloys added in the steel are ensured to reach a certain content by the above-mentioned formula 1, so as to ensure the grain refinement and precipitation strengthening effects of the two. Considering that the cost of the Nb alloy raw material is relatively high, the Ti content can be appropriately increased and the Nb content can be reduced under the premise of meeting the formula, but in order to ensure the necessary fine-grain strengthening effect, the Nb content should be ≥0.006%. (2) 372xC-93xTi-48xNb≥13.4% (Formula 2)

[0061] The Ti and Nb added in the steel are ensured to be combined with C to form second phase particles by the above-mentioned formula 2, so as to fully play the roles of Ti and Nb.

[0062] In addition, considering that the steel will undergo high-frequency welding (HFW) and metal gas shielded welding (MAG) in the subsequent use process, in order to further ensure the weldability and post-weld strength of the steel and welded pipe, the alloy elements in the steel preferably also meet: 0.42%≤C+0.25xSi+0.25xMn+1.07xP+0.13xCu+0.05xNi+0.23xCr+0.2xMo+0.2xV≤0.52% (Formula 3)

[0063] The above-mentioned formula 3 is based on the carbon equivalent formula CE=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 of the International Institute of Welding, and comprehensively considers the requirements of high-frequency welding and MAG welding on the carbon equivalent. In the high-frequency welding process, since no welding wire is added and there is no gas protection, elements such as Si and Mn which are easy to oxidize and elements such as P which are easy to form low-melting-point compounds are particularly sensitive. Therefore, the present application particularly considers the influence of the contents of these elements, sets an upper limit to make the steel and steel pipe have good weldability, and sets a lower limit to make the steel have better post-weld strength, higher weld seam strength of the welded pipe and welded joint strength.

[0064] When calculating the values of the above-mentioned formulas (1) to (3), the weight percentage contents of the corresponding elements are substituted into each formula, and then rounding off is performed to retain the corresponding decimal point number of digits.

[0065] In one embodiment, the method for manufacturing a welded pipe of the present application comprises the following steps:

[0066] 1) Smelting, casting

[0067] Smelting, refining, continuous casting into a billet or ingot are carried out according to the predetermined chemical composition;

[0068] 2) Heating

[0069] The billet or ingot is heated in a heating furnace, the heating temperature is 1210-1300℃, and the heating time is 120-300min;

[0070] 3) Rolling, coiling

[0071] The heated casting billet or ingot is subjected to 3-5 rough rolling passes, and the rough rolling exit temperature is 950-1080℃,

[0072] Then, finish rolling and coiling are performed to obtain a finished steel coil, the finish rolling temperature is 800-900℃, the cooling rate after finish rolling is ≥20℃ / s, and the coiling temperature is 500-630℃.

[0073] 4) High-frequency welding pipe making

[0074] Forming: After the finished steel coil is longitudinally cut into a predetermined width, the plate is formed by a roll bending forming in a high-frequency welding unit, and it is ensured that the plate edges are fully formed during the forming process, and the distance L between the two edges of the formed open pipe is ≤2×T;

[0075] High-frequency welding: the high-frequency welding power is 100-250KHz, and the welding speed and welding power are adjusted according to the outer diameter and thickness to ensure that the edge of the steel strip is fully melted, the extrusion amount is 0.1×T-0.9×T, the opening angle is 3-6°, and the welded pipe is obtained.

[0076] Wherein, T represents the wall thickness of the welded pipe, and the unit is mm.

[0077] In the manufacturing method of the present disclosure:

[0078] The casting billet or ingot is heated in a heating furnace, the heating temperature is 1210-1300℃, and the heating time is 120-300min. Using a higher heating temperature helps to fully austenitize the casting billet (or ingot) and promote the full dissolution of the second phase containing Ti and Nb in the steel billet. The dissolved Ti and Nb are precipitated in the form of second phases such as TiC and NbC during subsequent rolling and layer cooling to improve the strength of the steel.

[0079] The rough rolling temperature is controlled to be 950-1080℃ to minimize the precipitation of TiC, NbC and other particles in the austenite, so that they are precipitated in the subsequent ferrite to fully play the role of second phase precipitation strengthening. The finish rolling temperature is controlled to be 800-900℃ to form more ferrite phase deformation nucleation points in the steel to create conditions for the final formation of relatively small grains.

[0080] After finish rolling, the steel is cooled at a cooling rate of ≥20 ℃ / s by using a pre-cooling process, so that the high-temperature steel is rapidly cooled to a lower temperature to cause austenite to transform into ferrite, so as to form fine and uniform ferrite. If the cooling rate is too low, the transformation of ferrite cannot be inhibited, and the formed ferrite grains are coarse. The coiling temperature of the steel coil is set to be 500-630 ℃. If the temperature is too high, the ferrite grains are prone to grow and coarsen after coiling, and if the temperature is too low, bainite and martensite are formed in the steel, which is not beneficial to the plasticity of the steel. The finally formed microstructure of the steel is polygonal ferrite and / or quasi-polygonal ferrite + a small amount of pearlite, wherein the volume fraction of the pearlite is ≤15%, the ferrite grain size grade is ≥12, and the pearlite is distributed in the form of fine strips or small blocks at the grain boundaries of the ferrite.

[0081] In the pipe manufacturing process, the high-frequency induction welding technology is adopted. The welding frequency is set to make the edges of the steel strip fully melt, and the edges of the steel plate are fully shaped, so as to ensure that the internal stress of the steel plate is fully released and the edges of the plate can be well butted during high-frequency welding. The opening angle is controlled to be 3-6° and the extrusion amount is controlled to be 0.1×T-0.9×T (T is the wall thickness of the welded pipe, unit: mm), so as to fully remove the oxides formed during the welding process and ensure good welding quality.

[0082] In the performance evaluation of the butt joint of the welded pipe, the welding process parameters are reasonably controlled, so as to ensure that the steel pipe has the strength performance meeting the use requirements under the corresponding welding conditions.

[0083] Compared with the prior art, the present application has the following beneficial effects:

[0084] The steel for welded pipe in the present application adopts a low-carbon and low-silicon basic system, and is matched with a proper amount of manganese (Mn) and micro-alloy elements titanium (Ti) and niobium (Nb). The design system requires less types and lower amounts of micro-alloy elements, avoids the use of precious alloy elements such as molybdenum (Mo), vanadium (V) and nickel (Ni), forms a microstructure of polygonal ferrite and / or quasi-polygonal ferrite + a small amount of pearlite, avoids the appearance of bainite or martensite in the structure, realizes the excellent matching of strength and toughness of the steel, controls the alloy elements in the steel to meet the above relationship (1) and relationship (2), and preferably meets relationship (3), so that the precipitation strengthening effect of TiC, NbC and other second phases can be fully exerted, the carbon equivalent is low, the steel plate has good welding performance, the strength of the steel, the strength of the high-frequency welded seam and the strength of the welded joint of the welded pipe meet the requirements, and the production cost is greatly reduced.

[0085] The welded pipe prepared from the pipe steel of the present application through longitudinal slitting, roll bending and high frequency welding has the following excellent mechanical properties: the yield strength of the pipe body is ≥550 MPa, the tensile strength is ≥600 MPa, and the elongation is ≥20%; in the 90° laying flattening test, the weld is free of cracks when flattened to 2 / 3×D (D is the outer diameter of the welded pipe); and the strength of the welded joint is ≥590 MPa. The welded pipe has good strength and toughness, and the flattening performance of the welded pipe and the strength of the butt welded joint of the welded pipe are also high, which can meet the requirements of the engineering machinery field for the flattening of the welded pipe and the butt joint strength of the welded pipe, and can replace seamless pipes of the same grade in the engineering machinery field.

[0086] On the basis of the component design, the preparation method of the present application controls the key process parameters such as the heating temperature, heating time, rough rolling temperature, finish rolling temperature, layer cooling speed and coiling temperature of the billet to ensure that Ti and Nb in the steel plate are precipitated in the form of second phases such as TiC and NbC in the subsequent rolling and layer cooling process, fully play the precipitation strengthening effect of the second phases, and form a large number of ferrite phase transformation nucleation points in the steel plate, form fine and uniform ferrite, and obtain a microstructure of polygonal ferrite and / or quasi-polygonal ferrite + a small amount of pearlite, wherein the volume fraction of the pearlite is ≤15%, the ferrite grain size grade is ≥12, and the pearlite is distributed in the form of fine strips or small blocks at the grain boundary of the ferrite, so that the welded pipe realizes the above-mentioned excellent mechanical properties.

[0087] The steel coil described in the present disclosure can be directly produced by a TMCP process, and no additional heat treatment is required after rolling. After slitting, the steel coil can be directly used for high-frequency welding pipe production, and the slitted steel plate does not need to be milled, and the weld after welding also does not need to be heat treated. The steel grade and the high-frequency welded pipe produced therefrom have the advantages of short production process, simple process, low manufacturing cost, etc., and are suitable for large-scale industrial application.

[0088] The welded pipe steel, welded pipe and manufacturing method of the present disclosure will be further explained and described below in combination with specific examples and the accompanying drawings of the specification, but the explanation and description do not constitute undue limitations on the technical solutions of the present application.

[0089] Examples 1-12 and Comparative Examples 1-2

[0090] The following steps are used to produce the welded pipes of Examples 1-12 and Comparative Examples 1-2:

[0091] 1) smelting and casting to produce a cast billet or ingot;

[0092] 2) heating the cast billet or ingot;

[0093] 3) rolling and coiling the heated cast billet or ingot to produce a finished steel coil;

[0094] 4) high frequency welding pipe manufacturing, and the welded pipe is obtained:

[0095] forming: the finished steel coil is longitudinally cut and slitted, and then is formed, and the distance between the two edges of the formed open pipe is L≤2×T;

[0096] high frequency welding: the high frequency welding power is 100-250 KHz, the extrusion amount is 0.1×T-0.9×T, and the opening angle is 3-6°,

[0097] wherein T represents the wall thickness of the welded pipe, and the unit is mm.

[0098] Table 1 lists the mass percentage of each chemical element in the steel material of Examples 1-12 and Comparative Examples 1-2. Table 2 lists the specific process parameters of the steel material of Examples 1-12 and Comparative Examples 1-2. Table 3 lists the specific process parameters of the welded pipe of Examples 1-12 and Comparative Examples 1-2. Table 4 lists the performance measurement results of the welded pipe of Examples 1-12 and Comparative Examples 1-2.

[0099] The measurement methods of the performance and microstructure are as follows:

[0100] welded joint strength measurement: the welded joint strength of the welded pipe is measured according to ISO 15614-1 2017 Metal Materials Welding Procedure and Evaluation. A certain length of the welded pipe is cut, and is assembled by aligning and overlapping the longitudinal high frequency welds of the welded pipe or at an angle of 90° along the radial direction, and is butt-jointed by using the MAG welding method. During the welding process, the welding wire with a diameter of Φ1.2mm to 1.6mm is selected to match the strength grade of the welded pipe, and the welding heat input is controlled within the range of 0.5-1.5kJ / mm.

[0101] mechanical property measurement: the yield strength, tensile strength and elongation of the pipe body are measured according to GB / T 228.1-2010 Metal Materials Room Temperature Tensile Test Method.

[0102] microstructure: observed by a metallographic microscope. The volume fraction of pearlite and the grain size of ferrite are measured according to GB / T 15749-2008 Quantitative Metallographic Determination Method.

[0103] Figure 1 shows the microstructure photograph of the high frequency welded pipe body of Example 3. As can be seen from Figure 1, the steel plate of this example has a microstructure of polygonal ferrite and / or quasi-polygonal ferrite + a small amount of pearlite, wherein the volume fraction of the pearlite is ≤15%, and is distributed in the form of elongated strips or small blocks at the grain boundary of the ferrite, and the ferrite grain grade is 12.5 grade.

[0104] Figure 2 shows the appearance of the weld of the pipe of Example 5 and Example 11 when flattened to 2 / 3 x D in the 90° lay flat flattening test. As can be seen from Figure 2, the pipe of the Examples had no cracks in the weld when flattened to 2 / 3 x D (D is the outer diameter of the pipe) in the 90° lay flat flattening test.

[0105] In contrast, the pipe of Comparative Example 1 had a lower strength, the weld cracked, and the pipe had a lower strength for the welded joint. The pipe of Comparative Example 2 had a lower elongation, the weld cracked.

[0106] All publications, patent applications, patents, and other references mentioned in this disclosure are incorporated by reference in their entirety.

[0107] While the disclosure has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, since the disclosure includes all modifications and equivalents falling within the scope of the disclosure. Appropriately, changes in the amount of form and detail can be made by persons of ordinary skill in the art, including a number of simple deductions or replacements, without departing from the spirit and scope of the disclosure.

Claims

1. A steel for 600 MPa grade welded pipe, wherein, The steel contains the following chemical elements with the following weight percentage: C: 0.05-0.09%, Si≤0.1%, Mn: 1.2-1.6%, P≤0.02%, S≤0.010%, Ti: 0.015-0.06%, Nb: 0.006-0.06%, and the balance of Fe and inevitable impurities, and the element content of the steel satisfies the following relationship (1) and relationship (2): 93×Ti+48×Nb≥4.25% (Formula 1), 372×C-93×Ti-48×Nb≥13.4% (Formula 2). In the formula, each element symbol is substituted with the weight percentage of the corresponding element.

2. The steel according to claim 1, wherein The steel contains the following chemical elements with the following weight percentage: C: 0.05-0.09%, Si≤0.1%, Mn: 1.2-1.6%, P≤0.02%, S≤0.010%, Ti: 0.015-0.06%, Nb: 0.006-0.06%, and the balance of Fe and inevitable impurities.

3. The steel according to claim 1 or 2, wherein, The element content of the steel also satisfies the following relationship (3): 0.42%≤C+0.25×Si+0.25×Mn+1.07×P+0.13×Cu+0.05×Ni+0.23×Cr+0.2×Mo+0.2×V≤0.52% (Formula 3), in which each element symbol is substituted with the weight percentage of the corresponding element.

4. The steel according to any one of claims 1 to 3, wherein, The steel also contains at least one of the following chemical elements with the following weight percentage: Mo≤0.3%, Cr≤0.2%, V≤0.15%, Ni≤0.1%, Ca≤0.015%.

5. The steel according to any one of claims 1 to 3, wherein, The microstructure of the steel is ferrite + pearlite.

6. The steel according to claim 5, wherein The ferrite is in a polygonal or quasi-polygonal shape, and the grain size grade of the ferrite is ≥12 grade.

7. Steel according to claim 5 or 6, wherein The pearlite is in an elongated strip or small block shape and is distributed at the grain boundary of the ferrite, and the volume fraction of the pearlite is ≤15%, preferably the volume fraction of the pearlite is 3%-15%.

8. A welded pipe, wherein, The welded pipe is made of the steel according to any one of claims 1-7.

9. The welded pipe according to claim 8, wherein, the yield strength of the pipe body of the welded pipe is ≥550 MPa, the tensile strength is ≥600 MPa, and the elongation is ≥20%, in a 90° laying flattening test, the welded pipe is flattened to 2 / 3×D without cracks in the weld, where D is the outer diameter of the welded pipe, the strength of the welded joint of the welded pipe is ≥590 MPa.

10. A method for manufacturing the welded pipe according to claim 8 or 9, comprising the following steps: 1) smelting and casting according to a predetermined chemical composition to obtain a cast blank or ingot; 2) heating the cast blank or ingot; 3) rolling and coiling the heated cast blank or ingot to obtain a finished steel coil; 4) high-frequency welding to obtain a welded pipe: forming: slitting the finished steel coil longitudinally, and then forming, the distance between the two sides of the formed open pipe is L≤2×T; high-frequency welding: the high-frequency welding power is 100-250 KHz, the extrusion amount is 0.1×T-0.9×T, and the opening angle is 3-6°, wherein T represents the wall thickness of the welded pipe, in mm.

11. The method of claim 10, wherein, The wall thickness T of the welded pipe is 1.5-16 mm.

12. The method of claim 10 or 11, wherein, In step 2), the heating temperature is 1210-1300℃, and the heating time is 120-300 min.

13. The method of any one of claims 10 to 12, wherein, In step 3), The heated cast blank or ingot is subjected to 3-5 rough rolling passes, and the rough rolling exit temperature is 950-1080℃, Then, finish rolling and coiling are carried out to obtain finished steel coils, the finish rolling temperature is 800-900℃, the cooling speed after finish rolling is ≥20℃ / s, and the coiling temperature is 500-630℃.

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