800 mpa-grade high-strength steel for welded pipes, welded pipe, and manufacturing method therefor

Abstract

Provided in the present invention are steel for welded pipes, a welded pipe and a manufacturing method therefor. The steel for welded pipes comprises the following chemical elements in percentage by weight: 0.04-0.10% of C, ≤0.15% of Si, 1.5-2.5% of Mn, ≤0.02% of P, ≤0.010% of S, 0.06-0.18% of Ti, 0.1-0.3% of Mo, and the balance of Fe and other inevitable impurities, the element contents of the steel satisfying the following relational equation (1) and relational equation (2): 2×Ti+Mo≥0.35% (equation 1), 0.5≤2×Ti / Mo≤3.1 (equation 2). The steel for welded pipes and the welded pipe made of the steel for welded pipes have excellent mechanical properties, and can meet the comprehensive performance requirements for structural welded pipes in terms of formability and connection strength, etc., in the field of engineering machinery. Thus, the welded pipe can be used for replacing seamless pipes of the same grade in the field of engineering machinery.

Description

800MPa grade high-strength steel for welded pipe, welded pipe and manufacturing method thereof TECHNICAL FIELD

[0001] The present disclosure relates to a steel material for welded pipe, a welded pipe made of the steel material for welded pipe and a manufacturing method thereof applicable to the field of engineering machinery, in particular, an 800MPa grade high-strength steel for welded pipe, a welded pipe made of the steel material for welded pipe and a manufacturing method thereof. BACKGROUND

[0002] Structural steel pipes are widely used in the field of engineering machinery, and are required to have high mechanical properties, flattening properties and butt joint strength because they are often used in load-bearing safety structures. At present, seamless steel pipes are mainly used in this field, although they have excellent properties, 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 by 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 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 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 conveying, 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, flaring performance and butt joint strength. SUMMARY

[0011] In view of the above defects and deficiencies of the prior art, the purpose of the present disclosure is to provide a high-strength welded pipe steel with tensile strength of 800 MPa, 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 800 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: 0.04-0.10%, Si≤0.15%, Mn: 1.5-2.5%, P≤0.02%, S≤0.010%, Ti: 0.06-0.18%, Mo: 0.1-0.3%, the element content of the steel satisfies the following relationship formula (1) and relationship formula (2): 2×Ti+Mo≥0.35% (Formula 1), 0.5≤2×Ti / Mo≤3.1 (Formula 2),

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

[0014] In a preferred embodiment, the element content of the welded pipe steel of the present disclosure also satisfies the following relationship formula (3): 0.5%≤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≤0.76%

[0015] (Formula 3), wherein each element symbol represents the weight percentage content of the corresponding element. In the case where the steel does not contain Cu, Ni, Cr 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 comprises the following chemical elements in the following weight percentage contents: C: 0.04-0.10%, Si≤0.15%, Mn: 1.5-2.5%, P≤0.02%, S≤0.01010%, Ti: 0.06-0.18%, Mo: 0.1-0.3%, 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 contents: Nb≤0.065%, 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 + bainite + pearlite. In a preferred embodiment, the ferrite is in quasi-polygonal shape, acicular or elongated. In a preferred embodiment, the volume fraction of bainite is≤15%. In a more preferred embodiment, the volume fraction of bainite is 3-5%. In a preferred embodiment, the volume fraction of pearlite is≤1%.

[0019] In a second aspect, the present disclosure further 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≥700MPa, the tensile strength is≥800MPa, and the elongation is≥18%; in a 90° laying flattening test, the weld is free of cracks when flattened to 1 / 2×D (D is the outer diameter of the welded pipe); and the strength of the butt joint (i.e. the joint strength when welded in a butt welding manner) is≥760MPa.

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

[0023] 1) smelting and casting according to a predetermined chemical composition to obtain a cast blank or ingot;

[0024] 2) heating the cast blank or ingot;

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

[0026] 4) Pipe making, producing welded pipes:

[0027] Forming: The finished steel coil is longitudinally cut into strips, and then formed. The distance between the two sides of the formed open tube 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] Where 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 to 16 mm, preferably 1.5 to 4 mm.

[0031] In a preferred embodiment, in step 2), the heating temperature is 1234–1300°C, preferably 1240–1300°C, and the heating time is 120–300 min.

[0032] In a preferred embodiment, in step 3), the heated billet or ingot undergoes 3 to 5 passes of rough rolling, with an exit temperature of 950 to 1080°C; then it is finished rolled and coiled to obtain the finished steel coil, with a final rolling temperature of 780 to 880°C, a cooling rate ≥15°C / s after finish rolling, and a coiling temperature of 520 to 660°C. In a more preferred embodiment, in step 3), the cooling rate after finish rolling is 15°C / s to 170°C / s, more preferably 15°C / s to 150°C / s.

[0033] The welded pipe steel of the present invention adopts a low-carbon, low-silicon basic system in steel composition design, and is combined with appropriate amounts of manganese (Mn) and microalloying elements titanium (Ti) and molybdenum (Mo), thereby obtaining high strength and toughness.

[0034] The high-frequency welded pipes prepared using the steel for welded pipes according to this invention not only possess excellent flattening performance but also exhibit good butt weld strength, meeting the comprehensive performance requirements of structural welded pipes in the engineering machinery field in terms of formability and connection strength. These welded pipes can replace seamless steel pipes of the same grade, offering significant cost and manufacturing efficiency advantages and demonstrating promising application prospects. Attached Figure Description

[0035] Figure 1 shows a microstructure photograph of the high-frequency welded pipe body of Example 4.

[0036] Figure 2 shows the morphological photographs of the high-frequency welded pipes of Examples 4, 7, and 12 when they were flattened to 1 / 2×D in a 90° placement flattening test.

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

[0038] Figure 4 shows a schematic diagram of the open tube two-side spacing L. 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 the present disclosure, the extrusion amount refers to the difference between the circumference of the raw pipe before extrusion and the circumference after extrusion.

[0042] In the present disclosure, 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. The opening angle is calculated by measuring the edge of the V-shaped region per unit length, then measuring the length perpendicular to the edge, and using the Tan function.

[0043] In the present disclosure, 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 the present disclosure, the welded joint strength of the welded pipe (butt welding method) is determined in accordance with ISO 15614-1 2017 "Metallic Materials Welding Procedure Specification and Qualification".

[0045] In the present disclosure, the volume fraction of bainite or pearlite is determined in accordance with GB / T 15749-2008 "Quantitative Metallography Determination Method".

[0046] In the present disclosure, "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. Compared with the strict geometric polygon, the boundary shape has a certain natural deformation or slight undulation. Although it does not completely conform to the characteristics that each side of the standard polygon is a straight line, it can still be identified as a closed, polygonal structural unit as a whole.

[0047] In the present disclosure, the shape of ferrite 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 invention. 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 molybdenum (Mo), 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 invention is controlled at 0.04-0.10%.

[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 Fe2SiO2 during heating, 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 invention is controlled at 0.15% 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 invention is controlled at 1.5-2.5%.

[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 invention 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 invention 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 invention is controlled at 0.06-0.18%.

[0055] Molybdenum (Mo): Mo can effectively improve the hardenability, inhibit the generation of pearlite, and also can play a role in refining the phase change structure, effectively improve the strength and toughness of the steel plate, especially help to improve the strength of the weld. In addition, Mo has strong affinity with C, compared with the steel only adding Ti, the thermal stability of (Ti, Mo)C is higher than that of TiC, so the coarsening degree of (Ti, Mo)C is significantly lower than that of TiC during high temperature coiling, which is more conducive to play the precipitation strengthening effect of the second phase. However, Mo is a precious alloying element, and excessive content will significantly increase the manufacturing cost of the steel. Therefore, the Mo content in the present application is controlled at 0.1-0.3%.

[0056] In addition, the steel for welded pipe of the present disclosure can also contain one or more of Nb, Cr, V, Ni, Ca, such as Nb≤0.065%, 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 elements that have been added, improve the purity of the steel, refine the grain size, promote the precipitation of the second phase particles, slow down the coarsening of the grain and the second phase particles, and 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 appropriate amount of manganese (Mn) and micro-alloy strengthening elements titanium (Ti) and molybdenum (Mo). The setting of low-carbon, low-silicon and appropriate amount of Mn helps to reduce the hardenability of the steel, thereby facilitating the accurate regulation of the steel structure and improving its weldability. If the contents of C, Si and Mn are too high, the hardenability of the steel will be significantly improved, so that full bainite / martensite structure is prone to appear in the original steel or the heat-affected zone of the welded pipe during the layer cooling process after finish rolling and after high-frequency welding and MAG welding, which makes the steel strength too high and the brittleness larger, which is not conducive to the pipe deformation or subsequent service.

[0058] In view of the low C and Si contents and the moderate Mn content in the steel, in order to achieve the required microstructure and performance indicators, appropriate amounts of Ti and Mo are also added to the steel. The purpose of adding Ti is to form TiC second phase particles in the steel, and to improve the strength of the steel through TiC second phase. The purpose of adding Mo is to refine the phase change structure, improve the strength and toughness of the steel. In addition, after adding Mo, (Ti, Mo)C second phase can be formed. (Ti, Mo)C has high thermal stability and low coarsening rate during high temperature coiling and other thermal processes such as high-frequency welding and butt welding, which can better play the precipitation strengthening effect of the second phase and ensure the strength and toughness of the steel after rolling and welding. In order to fully play the precipitation strengthening and anti-coarsening effects of Ti and Mo and make the mechanical properties of the steel meet the index requirements, the present application uses the following two empirical formulas to control the contents of Ti and Mo: (1) 2xTi+Mo≥0.35% (Formula 1)

[0059] The Ti and Mo alloy added in the steel is ensured to reach a certain content by the above-mentioned formula 1, so as to ensure the precipitation strengthening of the second phase and the anti-coarsening effect of the second phase particles.

[0060] The atomic number ratio of Ti and Mo added in the steel is ensured to be between 0.5 and 3 by the above-mentioned formula 2, so that Mo and the second phase containing Ti are fully combined, the rapid coarsening of the second phase particles is prevented, and the strengthening effect of the second phase is fully exerted.

[0061] 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 the welded pipe, the alloy elements in the steel preferably also satisfy: 0.5%≤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≤0.76% (formula 3)

[0062] 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 content of these elements, sets an upper limit to make the steel and the steel pipe have good weldability, and sets a lower limit to make the steel have better post-weld strength.

[0063] When calculating the values of the above-mentioned formula (1) to formula (3), the weight percentage content of the corresponding element is substituted into each formula, and then rounding off is performed to retain the corresponding decimal point number of digits.

[0064] The steel produced by using the above-mentioned designed composition and combining the process of the present application has a microstructure of quasi-polygonal / needle-shaped / elongated ferrite+beadite+pearlite, wherein the volume fraction of the beadite is ≤15%, and the volume fraction of the pearlite is ≤1%. The quasi-polygonal / needle-shaped / elongated ferrite in the structure has good resistance to the softening effect of the welding heat affected zone. The beadite structure makes the steel have good strength and toughness matching. The as-rolled steel with this structure has high strength and good toughness, good cold formability, and is not prone to cracking during the rolling and pipe-making process. After the high-frequency welding and welding heat cycle of the pipe-making process, the welding heat affected zone can still maintain good strength and toughness, so the strength of the welded joint of the welded pipe made of the steel plate can still reach ≥760 MPa or more.

[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 casting blank or ingot according to a predetermined chemical composition;

[0068] 2) Heating

[0069] The casting blank or ingot is heated in a heating furnace, the heating temperature is 1240-1300℃, and the heating time is 120-300min;

[0070] 3) Rolling, coiling

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

[0072] Then, using six or seven stands, finish rolling and coiling are performed to obtain a finished steel coil, the finish rolling temperature is 780-880℃, the cooling speed after finish rolling is ≥15℃ / s, and the coiling temperature is 520-660℃;

[0073] 4) High-frequency welding pipe making

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

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

[0076] Wherein, T represents the wall thickness of the welded pipe, in mm.

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

[0078] The casting blank or ingot is heated in a heating furnace, the heating temperature is 1240-1300℃, and the heating time is 120-300min. Using a higher heating temperature helps the casting blank(or ingot) to be fully austenitized, and promotes the second phase containing Ti and Mo in the steel blank to be fully dissolved. The dissolved Ti and Mo are precipitated in the form of second phases such as(Ti, Mo)C during subsequent rolling and layer cooling to improve the strength of the steel plate.

[0079] The rough rolling temperature is controlled to be 950-1080℃ to minimize the precipitation of TiC, (Ti, Mo)C and other particles in the austenite, so that they are precipitated in the subsequent ferrite, and the precipitation strengthening effect of the second phase is fully played. The finish rolling temperature is controlled to be 780-880℃ to form more ferrite phase deformation nucleation points in the steel plate to create conditions for the final formation of relatively small grains.

[0080] After finish rolling, the steel plate is cooled at a cooling rate of ≥15℃ / s using a pre-cooling process, so that the high-temperature steel plate 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 ferrite transformation cannot be inhibited, and the formed ferrite grains are coarse. The coiling temperature of the steel coil is set to 520-660℃. 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, martensite and other structures are formed in the steel plate, which is not conducive to the plasticity of the steel plate.

[0081] In the pipe manufacturing process, high-frequency induction welding technology is used. 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 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 that the oxides formed during welding are fully removed, and good welding quality is ensured.

[0082] In the performance evaluation of the welded pipe butt joint, by reasonably controlling the welding process parameters, it is ensured 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 welded pipe steel of the present application adopts a low-carbon and low-silicon basic system in the composition design of the steel material, cooperates with appropriate amounts of manganese (Mn) and titanium (Ti) and molybdenum (Mo), forms a microstructure of quasi-polygonal / needle-shaped / elongated ferrite + a small amount of bainite + a very small amount of pearlite, realizes excellent strength and toughness matching of the steel material. At the same time, the alloy elements in the steel satisfy the above relationship (1) and relationship (2), and preferably also satisfy relationship (3), so that the precipitation strengthening effect of TiC, (Ti, Mo)C and other second phases can be fully played, and the steel plate has a lower carbon equivalent, a better welding performance, a required steel strength, a high-frequency welding seam strength and a welded pipe welded joint strength, and the production cost is greatly reduced.

[0085] The welded pipe steel of the present application has the following excellent mechanical properties: the yield strength of the pipe body is ≥700 MPa, the tensile strength is ≥800 MPa, and the elongation is ≥18%; in the 90° laying and flattening test, the weld is free of cracks when flattened to 1 / 2×D (D is the outer diameter of the welded pipe); and the butt joint strength is ≥760 MPa (using a welding wire matched with the strength grade of the welded pipe, and using MAG welding method for butt joint). The welded pipe has good strength and toughness, and the flattening performance of the welded pipe and the butt joint strength of the welded pipe are also high, meeting the requirements of the engineering machinery field for the flattening, flaring and 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, so as to ensure that Ti and Mo in the steel plate are precipitated in the form of second phases such as TiC and (Ti, Mo)C in the subsequent rolling and layer cooling process, fully play the precipitation strengthening effect of the second phase, and form a large number of ferrite phase transformation nucleation points in the steel plate, to obtain a microstructure of quasi-polygonal / needle-shaped / elongated ferrite+ bainite+pearlite, wherein the volume fraction of bainite is ≤15%, and the volume fraction of pearlite is ≤1%, 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 the steel plate does not need to be subjected to additional heat treatment after rolling. After the steel coil is slit longitudinally, it can be directly used for high-frequency welding pipe production, and the slit steel plate does not need to be subjected to edge milling treatment, and the weld after welding is also not required to be subjected to heat treatment. 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 thereof 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 limitation 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 making, and the welded pipe is made:

[0095] forming: after longitudinal cutting and slitting of the finished steel coil, forming is performed, and the distance between the two sides of the open pipe after forming 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 performance and microstructure are as follows:

[0100] butt joint strength measurement: the butt joint strength of the welded pipe is measured according to ISO 15614-1 2017 Metal materials - Welding - Procedure qualification and verification. A certain length of the welded pipe is cut, and it 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 MAG welding is used for butt joint. During welding, a welding wire matching the strength grade of the welded pipe is selected, the welding wire diameter is Φ1.2mm to 1.6mm, 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 - Tensile testing at room temperature.

[0102] microstructure: observed by 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 4. As can be seen from Figure 1, the steel plate of this example has a microstructure of quasi-polygonal / needle-shaped / elongated ferrite + bainite + pearlite, wherein the volume fraction of bainite is ≤15%, and the volume fraction of pearlite is ≤1%.

[0104] Figure 2 shows the appearance of the weld of the welded pipe of Example 4, Example 7, and Example 12 when flattened to 1 / 2 x D in the 90° lay-flat flattening test. As can be seen from Figure 2, the welded pipe of the Examples had no cracks in the weld when flattened to 1 / 2 x D (D is the outer diameter of the welded pipe) in the 90° lay-flat flattening test.

[0105] In contrast, the welded pipe of Comparative Example 1 was too weak, the weld cracked, and the welded pipe was too weak at the welded joint. The welded pipe of Comparative Example 2 was too low in elongation, and 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 type of 800MPa grade welded pipe steel, wherein, In addition to Fe and unavoidable impurities, the steel contains the following chemical elements in the following weight percentages: C: 0.04~0.10%, Si≤0.15%, Mn: 1.5~2.5%, P≤0.02%, S≤0.010%, Ti: 0.06~0.18%, Mo: 0.1~0.3%. The elemental content of the steel satisfies the following relationships (1) and (2): 2×Ti+Mo≥0.35% (Equation 1), 0.5≤2×Ti / Mo≤3.1 (Equation 2). The symbols of each element in the formula are respectively substituted with the weight percentage content of the corresponding element.

2. The steel as claimed in claim 1, wherein, The steel contains the following chemical elements in weight percentages as follows: C: 0.04-0.10%, Si ≤ 0.15%, Mn: 1.5-2.5%, P ≤ 0.02%, S ≤ 0.01010%, Ti: 0.06-0.18%, Mo: 0.1-0.3%, with the balance being Fe and unavoidable impurities.

3. The steel as described in claim 1 or 2, wherein, The elemental content of the steel also satisfies the following relationship (3): 0.5%≤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≤0.76% (Equation 3), where each element symbol is replaced by the corresponding weight percentage content of the 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 in weight percentages: Nb≤0.065%, 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 + bainite + pearlite.

6. The steel as claimed in claim 5, wherein, The ferrite is in the form of a quasi-polygonal shape, needle-like shape, or elongated shape.

7. The steel as claimed in claim 5 or 6, wherein, The volume fraction of bainite is ≤15%, and the volume fraction of pearlite is ≤1%.

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

9. The welded pipe as described in claim 8, wherein, The welded pipe has a yield strength ≥700MPa, a tensile strength ≥800MPa, and an elongation ≥18%. In the 90° flattening test, when the welded pipe is flattened to 1 / 2 × D, no cracks appear in the weld, where D is the outer diameter of the welded pipe. The weld joint strength of the welded pipe is ≥760MPa.

10. A method for manufacturing the welded pipe according to claim 8 or 9, comprising the following steps: 1) Smelting and casting according to the predetermined chemical composition to obtain billets or ingots; 2) Heating the billet or ingot; 3) The heated billet or ingot is rolled and coiled to obtain finished steel coils; 4) Pipe making, producing welded pipes: Forming: The finished steel coil is longitudinally cut into strips, and then formed. The distance between the two sides of the formed open tube 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°. in, T indicates 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 to 16 mm, preferably 1.5 to 4 mm.

12. The method of claim 10 or 11, wherein, In step 2), the heating temperature is 1234–1300℃, preferably 1240–1300℃, and the heating time is 120–300 min.

13. The method according to any one of claims 10 to 12, wherein, In step 3), The heated billet or ingot undergoes 3 to 5 passes of rough rolling, with an exit temperature of 950 to 1080℃. Then, the finished steel coils are obtained by finishing rolling and coiling. The final rolling temperature is 780-880℃, the cooling rate after finishing rolling is ≥15℃ / s, and the coiling temperature is 520-660℃.

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