Hot-rolled coil plate for low-cost 450MPa-grade welded pipe and manufacturing method of hot-rolled coil plate

Abstract

The invention discloses a low-cost 450MPa-grade hot-rolled coil plate for a welded pipe and a manufacturing method thereof, and the low-cost 450MPa-grade hot-rolled coil plate comprises the following chemical components: 0.08 to 0.10 percent of C, 0.2 to 0.4 percent of Si, 1.4 to 1.6 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.003 percent of S, 0.01 to 0.03 percent of Ti, 0.020 to 0.040 percent of Nb, 0.003 to 0.005 percent of RE, 0.15 to 0.25 percent of Cr, 0.002 to 0.004 percent of Mg, 0.01 to 0.04 percent of Als, less than or equal to 0.003 percent of Ca, less than or equal to 0.008 percent of N, less than or equal to 0.15 percent of (Nb + Ti + C + N) and the balance of Fe and inevitable elements. The manufacturing method comprises the steps of smelting, slab continuous casting, casting blank heating, rolling, cooling and coiling. The Rt < 0.5 > of the steel is 450-550 MPa, the Rm is 550-650 MPa, the A50mm is larger than or equal to 30%, the ratio of the Rt < 0.5 > to the Rm is smaller than or equal to 0.90, and the impact energy average value Akv at the temperature of-30 DEG C is larger than or equal to 200 J.

Description

Technical Field

[0001] The present invention relates to the field of low-alloy high-strength steel, and in particular to a low-cost hot-rolled coil for 450MPa grade welded pipe produced from medium-thickness ingot and having good strength and toughness, and a manufacturing method thereof. Background Art

[0002] With the rapid development of high-quality strip steel continuous rolling and advancements in welding and inspection technologies, welded steel pipe has gradually become one of the most widely used steel materials. Currently, welded steel pipe serves as a pipeline for transporting media such as water, natural gas, air, oil, and coal slurry, as well as structural components in bridges, docks, and buildings. It is also used in the manufacture of automobiles and construction machinery. Welded steel pipe comes in a wide variety of types and specifications, depending on its manufacturing method and application. Currently, oil and gas pipelines are used in the largest quantities and have the highest technical requirements.

[0003] To ensure the safety and cost-effectiveness of welded pipe production, the quality of the hot-rolled coil used in its manufacture is crucial. Currently, the quality of steel used in welded pipe is continuously improving and production costs are decreasing through the adoption of clean steel smelting technologies, continuous rolling of defect-free slabs, microalloying technologies, and efficient hot rolling technologies. Therefore, fully leveraging the potential of precious microalloys and manufacturing processes while ensuring quality is key to enhancing product competitiveness.

[0004] Currently, there are many patents for 450MPa grade welded pipe steel, especially pipeline steel, but they are significantly different from the present invention. The following briefly introduces the patents that are relatively close to the present invention: (1) Chinese patent CN201510141936.0 “A production method for X65M pipeline steel”. The composition includes C: 0.03-0.08%, Si: 0.15-0.35%, Mn: 1.10-1.60%, P: 0-0.012%, S: 0-0.003%, Nb: 0.02-0.06%, Ti: 0.015-0.025%, Mo: 0.12-0.25%, Cu: 0.05-0.35%, Ni: 0.08-0.30%, Cr: 0-0.20%, Al: 0.015-0.050%, and the rest is Fe and unavoidable impurities. This patent adds a lot of precious metal elements such as Mo and Ni, which results in a higher alloy cost. At the same time, the alloy composition design is different from that of this application. In addition, this patent uses thicker ingots of 250 mm to 300 mm, with more rolling passes and a more complicated process, which is significantly different from this application.

[0005] (2) Chinese patent CN200710028175.3 “A method for producing X65 pipeline steel at low cost”. The composition includes C: 0.046-0.060%, Si: 0.15-0.30%, Mn: 1.40-1.60%, P: 0.006-0.015%, S: 0.001-0.008%, Nb: 0.045-0.050%, Ti: 0.015-0.025%, N: 0.005-0.007%, and the balance is Fe and unavoidable impurities. This patented composition utilizes a high-Nb, micro-Ti treated alloy design without the addition of Cr, Mg, or rare earth elements, which differs from the alloy system of this design. Furthermore, the lower P and S contents increase steelmaking production costs. While this patented composition does not include precious alloys such as Mo and Ni, it can meet the strength and toughness requirements of X65-grade pipeline steel. However, it is only suitable for the production of extremely thin X65 pipeline steel (under 10 mm) in CSP production lines and is not suitable for various complex and extreme service environments.

[0006] (3) Chinese patent CN202410512741.1 “Thin gauge low yield ratio low temperature service L450M pipeline steel and its production method”. The composition contains C: 0.062~0.078%, Si: 0.15~0.25%, Mn: 1.50~1.60%, Nb: 0.032~0.043%, Ti: 0.026~0.029%, Mo: 0.06~0.12%, Cr: 0.06~0.12%, Als: 0.015~0.045%, P < 0.015%, S < 0.004%, N < 0.004%, and the rest is iron and unavoidable impurities. This patented composition adds Mo precious metal elements, but does not add Mg and rare earth elements. The alloy system is different and the alloy cost is higher. In addition, this patent uses thicker ingots of 200 mm to 230 mm, the final product specifications are thinner, the number of rolling passes is more, and the process is more complicated.

[0007] (4) Chinese patent CN201110179745.5 “A thick-walled, low-cost X65 hot-rolled coil for spiral welded pipe and its manufacturing method”. The composition contains C: 0.05-0.10%, Si: 0.10-0.30%, Mn: 1.50-1.65%, P: <0.020%, S: <0.008%, Ti: 0.008-0.020%, Als: 0.02-0.06%, Nb: 0.04-0.07%, Cr: 0.10-0.20%, V: 0.03-0.06%, N: <0.008%, Ceq: <0.40%, Pcm: <0.21%, and the rest are Fe and unavoidable elements. The patented composition contains V element, without adding Mg and rare earth elements, and the alloy design system is different; the patented cooling process uses laminar cooling, which cannot fully utilize water to replace the alloy, thereby reducing costs and improving strength and toughness; in addition, the patented low-temperature toughness index requirements are also relatively loose, the impact test temperature is 0°C, and no low-temperature DWTT test is performed.

[0008] (5) Chinese patent CN201110179945.0 “X65 pipeline steel with excellent low-temperature toughness and its manufacturing method”. The composition contains C: 0.020-0.055%, Si: 0.10-0.25%, Mn: 1.50-1.70%, Nb: 0.060-0.080%, Cr: 0.20-0.35%, V: 0.020-0.040%, Ti: 0.010-0.020%, Als: 0.015-0.040%, P≤0.018%, S≤0.005%, N≤0.0060%, and the balance is iron and unavoidable impurities. The patented composition contains V elements and a high content of Nb elements, resulting in a higher alloy cost. Mg alloy and rare earth elements are not added, and the alloy design system is different. The patented cooling process uses laminar cooling, which cannot fully utilize water to replace the alloy, thereby reducing costs and improving strength and toughness. In addition, the patent uses a thinner ingot thickness of 130mm~170mm, which cannot guarantee the compression ratio and reduces the strengthening effect.

[0009] (6) Chinese patent CN201410092743.6, “An economical X65 oil and gas pipeline steel and its production method”. The composition includes C: 0.116-0.14%, Si: 0.15-0.40%, Mn: 1.10-1.40%, P: <0.015%, S <0.0020%, Nb: 0.025-0.045%, V: 0.057-0.060%, Ti: 0.005-0.020%, Al: 0.020-0.050%, N: <0.008%, and the balance is Fe and unavoidable inclusions. The patent contains V and a high C content. Cr, Mg, and rare earth elements are not added. The alloy composition system is different, which reduces the strengthening effect. The thickness of the ingot and the cooling method are not specified. In addition, the actual low-temperature toughness of the patent is insufficient.

[0010] (7) Chinese Patent No. 201710269780.3, “A Low-Temperature L450M Pipeline Steel and Its Manufacturing Method.” The composition includes C: 0.050-0.070%, Si: 0.10-0.25%, Mn: 1.61-1.70%, Nb: 0.056-0.070%, Cr: 0.21-0.30%, Ti: 0.008-0.020%, Al: 0.015-0.045%, P < 0.020%, S < 0.006%, N < 0.008%, and the remainder is iron and unavoidable impurities. This patent has a high Nb content, resulting in high alloy cost. Mg and rare earth elements are not added, and the alloy composition system is different. The ingot thickness is 135 mm to 170 mm, which makes it impossible to guarantee the compression ratio and reduces the strengthening effect. Furthermore, this patent uses two-stage cooling, which is a different cooling method.

[0011] Although there are many patents for 450 MPa grade welded pipe steel, especially pipeline steel, most of them contain precious elements such as Ni and Mo in their composition designs, which results in high alloy costs. Secondly, they use laminar cooling, which cannot fully utilize water to replace the alloy, thereby reducing costs and improving strength and toughness. Summary of the Invention

[0012] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a low-cost 450MPa grade hot-rolled coil for welded pipe and a manufacturing method thereof.

[0013] One of the technical solutions of the present invention is to provide a low-cost 450MPa grade hot-rolled coil for welded pipe, the chemical composition (weight, %) of which is as follows: C: 0.08-0.10%, Si: 0.2-0.4%, Mn: 1.4-1.6%, P: ≤0.02%, S: ≤0.003%, Ti: 0.01-0.03%, Nb: 0.020-0.040%, RE: 0.003-0.005%, Cr: 0.15-0.25%, Mg: 0.002-0.004%, Als: 0.01-0.04%, Ca: ≤0.003%, N: ≤0.008%, wherein (Nb+Ti+C+N) ≤0.15%, and the remainder is Fe and unavoidable elements.

[0014] Preferably, the optimal composite mass ratio of La and Ce is La / Ce=2.9~3.2.

[0015] Preferably, after rare earth treatment, more than 80% of the intracrystalline acicular ferrite nuclei have a size of 1-4 μm.

[0016] The present invention selects the above alloying element types and contents because: C: It is the most basic and economical strengthening element. On the one hand, C can dissolve in the iron matrix to form an interstitial solid solution, causing distortion in the matrix lattice and significantly improving the strength and hardness of the steel through solid solution strengthening. On the other hand, it forms carbide precipitation with elements such as Nb and Ti. These precipitates hinder the slip and expansion of grains, further improving the strength of the steel through precipitation strengthening. However, excessive C content can easily form pearlite banding, reducing the steel's plasticity and toughness. It can also reduce the steel's weldability, increase welding difficulty, and increase the risk of welding defects. The optimal range is 0.08-0.10%.

[0017] Si: It can be dissolved in ferrite and austenite, and enhance the strength and hardness of steel through solid solution strengthening. It is also an important deoxidizing element in steel, which helps to improve the purity of steel. It can significantly improve the tensile strength of steel and reduce the yield ratio. It has a positive effect on the fatigue strength and oxidation resistance of steel. However, too high a content will deteriorate the welding performance of steel and is not conducive to the removal of hot-rolled iron oxide scale. The optimal range is 0.2~0.4%.

[0018] Mn: It has the functions of solid solution strengthening, grain refinement and phase transformation strengthening, and is the most important and economical strengthening element. cThe Mn content in the phase region prevents the formation of proeutectoid ferrite during the finishing rolling process and promotes bainite transformation, thereby helping to obtain fine phase transformation products, significantly improving the overall strength of the steel. It can also significantly improve the stability of austenite and the hardenability of the steel. However, excessive Mn content will increase the tendency of central segregation in the continuous casting billet. In severe cases, banded martensite structure may form, which will crack during the subsequent deformation process and cause non-compliance with the flaw detection. The optimal range is 1.4-1.6%.

[0019] P, S, and N are inevitable impurity elements in steel. The lower the content, the better. However, excessive reduction in steel purity will significantly increase production costs. Under the premise of ensuring the toughness index of this product, P of the present invention is ≤0.02%, S ≤0.003%, and N ≤0.008%.

[0020] Ti: Ti is a strong nitrogen-fixing element. Adding approximately 0.015% Ti forms high-temperature stable, fine TiN precipitates during slab continuous casting. This fine TiN precipitates at austenite grain boundaries and pins them, effectively hindering austenite grain growth during high-temperature heating of the continuous-cast slab. It also improves the steel's weldability, reduces crack sensitivity in the weld heat-affected zone, and enhances the toughness of the welded joint. The strong precipitation strengthening effect of a small amount of TiC prevents significant grain growth during subsequent normalizing heat treatment, thereby ensuring high pipe strength. However, excessive TiC content not only increases costs but also easily forms large inclusions. The optimal range is 0.01-0.03%.

[0021] Nb: It has significant grain refinement and precipitation strengthening effects. During hot rolling, Nb precipitates through strain-induced precipitation to form fine Nb(C, N) particles, which pin grain boundaries and inhibit the growth and recrystallization of deformed austenite. Through controlled rolling and controlled cooling, the deformed austenite is transformed into a fine product with a high density of dislocations. Because the precipitation of Nb and Ti is closely related to the elements C and N, and affects the heating temperature design of the steel, the optimal range is 0.02-0.04%, and the requirement is that (Nb+Ti+C+N) ≤ 0.2%.

[0022] RE: RE, added to steel, acts as a deoxidizer, desulfurizer, and modifier. Composed of La and Ce, RE improves steel purity while generating numerous high-melting-point, dispersed, and fine inclusions. These inclusions serve as heterogeneous nucleation centers, thereby refining the grain size. Furthermore, rare earth inclusions can induce the nucleation of intragranular acicular ferrite, refining the steel's microstructure and improving its strength and toughness. Rare earth composite treatment outperforms single rare earth additions, with the optimal La / Ce mass ratio being between 2.9 and 3.2. After rare earth treatment, over 80% of the intragranular acicular ferrite nuclei are within the 1-4 μm range, with an optimal range of 0.003-0.005%.

[0023] Cr: It improves the hardenability of steel, inhibits the formation of polygonal ferrite and pearlite, and promotes the formation of ferrite or bainite with a large intragranular dislocation distribution in the medium and low temperature ranges, ensuring uniform microstructure and performance throughout the product's thickness. This effect is even more pronounced when used in combination with Nb. Furthermore, a Cr content of 0.20% or above significantly improves the steel's corrosion resistance. However, excessive Cr content tends to form coarse carbides, significantly increasing the steel's tensile strength and hardness, but providing limited improvement in yield strength and deteriorating the steel's toughness and weldability. Therefore, the optimal range is 0.15-0.25%.

[0024] Mg: Mg has strong chemical activity and a strong affinity with O and S. It is an effective refining agent that can reduce the O and S content and the number of inclusions in steel, purify the molten steel, and greatly reduce the damage of inclusions to steel properties. It can also significantly modify the inclusions in the steel, improving the steel's performance. Trace amounts of Mg can reduce the number of inclusions in steel, reduce their size, evenly distribute them, and improve their morphology. It can also increase yield strength and tensile strength by more than 5%, while maintaining essentially unchanged plasticity, thereby reducing the yield-to-strength ratio and improving the low-temperature toughness of steel. The appropriate range is 0.002-0.004%.

[0025] Al: Al is a strong nitride-forming element that is used to fix nitrogen atoms, which is beneficial for improving the material's aging resistance. Al is also an indispensable deoxidizer in steelmaking. Adding a small amount of Al to steel can refine grains and improve strength and impact toughness. The Al content in the present invention is 0.01-0.04%.

[0026] Ca: It has a strong affinity with sulfur in steel, playing a role in desulfurization. Calcium treatment can also change the morphology of sulfides, improve the anisotropy of the steel, and denature inclusions, achieving the purpose of spheroidizing inclusions and ensuring the toughness of the steel. However, excessive Ca content can lead to the formation of large inclusions, and its optimal range should be controlled below 0.003%.

[0027] The second technical solution of the present invention is to provide a low-cost method for manufacturing hot-rolled coil for 450 MPa grade welded pipe, which includes smelting, slab continuous casting, slab heating, rolling, cooling, and coiling, and is characterized by: Slab Heating: The continuous casting slab is heated to 1120-1160°C in a walking beam furnace. Within this temperature range, the Nb element is fully dissolved back, and the austenite grains do not grow significantly due to the pinning of Ti (C, N) particles at the grain boundaries, which is conducive to obtaining a fine and uniform structure.

[0028] Rolling: Rough rolling is performed at a final rolling temperature of 1030-1100°C over five passes, with a reduction of ≥18% for the first pass and ≥20% for the remaining four passes, with the reduction gradually increasing. Finish rolling begins at a temperature of 960-1010°C and ends at a temperature of 800-840°C, with a cumulative reduction of 60%-65%. This final rolling temperature range is conducive to Nb precipitation, refines grain size, and improves strength and toughness. High reductions generate a large number of dislocations and twins, which contribute to dislocation strengthening and significantly improve the yield strength and tensile strength of the steel.

[0029] Preferably, the thickness of the finishing intermediate billet is 48-58 mm, and the thickness of the finished product is 10-14 mm.

[0030] Cooling: After rolling, ultra-fast cooling is adopted, with the final cooling temperature of 450~520℃ and the cooling rate of 25~45℃ / s, and finally transformed into acicular ferrite and martensite / MA mixed structure, which has high strength, low yield ratio and good low-temperature toughness.

[0031] Coiling: After ultra-fast cooling, a strong coiler is used for coiling. The coil tension coefficient is 1.4~2.2, the auxiliary roll pressure is 350~400 MPa, the guide pressure is 30kN~50kN, the coil speed is 0.5 times, the number of skipping circles is 4~6, the MD lead rate is 10~12, the WR lead rate is 14~15, and the PR lead rate is 10~11. The coil shape is good without hard bends.

[0032] Furthermore, after the molten iron is pretreated, top blowing or top and bottom combined blowing process is adopted for converter smelting, followed by off-furnace refining, RH vacuum treatment, LF furnace light desulfurization treatment and calcium treatment. To ensure the spheroidization effect of sulfide and oxide inclusions, the feed line speed is ≥3.5 m / s, the net argon blowing time is ≥5 min, and the calming time is ≥10 min.

[0033] Furthermore, during continuous casting of slabs, the pouring is protected throughout, and electromagnetic stirring or dynamic soft reduction is used to reduce the center segregation of the continuous casting slab. The slab adopts a medium thickness of 190~210 mm, which can not only ensure the compression ratio but also make the slab quickly and evenly heated through.

[0034] The hot-rolled coil for welded pipe produced using the above chemical composition and process has excellent comprehensive properties, including yield strength of 450~550 MPa, tensile strength of 550~650 MPa, elongation after fracture ≥30%, yield strength ratio ≤0.90, average impact energy value Akv ≥200 J at -30°C, average drop weight test (DWTT) at -20°C ≥90%, hardness value (HV10) ≤220, and qualified cold bending performance. It can be widely used in various medium conveying pipelines and engineering machinery manufacturing.

[0035] Compared with the existing technology, the alloy composition design of this scheme adopts a low C-Nb-Ti design, and at the same time adds rare earth elements Ce+La and cheap Mg alloy. While reducing production costs, by adding Mg, Ce+La elements, the steel is purified, the grain structure is refined and inclusions are improved, which can significantly improve the overall performance of pipeline steel, enable it to meet higher usage requirements, and show higher adaptability and safety in various complex and extreme service environments, providing more reliable protection for pipeline projects.

[0036] Compared with the prior art, the present invention has the following beneficial effects: (1) Breaking the existing design concept of 450MPa grade welded pipe steel alloy, composite addition of rare earth elements and cheap Mg alloy, through its microalloying, reducing the use of precious metals such as Nb and Ti, and reducing production costs; (2) The composite addition of Mg and rare earth elements Ce + La plays a role in purifying steel, refining inclusions, and improving the morphology and distribution of inclusions, thereby improving mechanical properties, ensuring that the product has good strength and toughness, and making it safer in pipeline engineering applications; (3) Using medium-thick continuous casting billets of 190~210 mm, while ensuring the compression ratio, the continuous casting billets can be burned through quickly and evenly, achieving the effect of energy saving and consumption reduction, and making the production process more flexible and efficient. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. The following description of the exemplary embodiments is actually only illustrative and is in no way intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present invention.

[0038] The following examples are intended only to provide some preferred embodiments of the present invention and are not intended to limit the scope of the invention or its technical approach. Table 1 lists the chemical compositions of Examples 1-8, Table 2 lists the heating, rolling, cooling, and coiling process parameters for each example, Table 3 lists the AF nucleation size for each example, and Table 4 lists the mechanical property test results for each example.

[0039] Table 1 Chemical composition of example steel (wt, %)

[0040] Table 2 Process parameters for heating, rolling, cooling and coiling of steel in various examples

[0041] Table 3 AF nucleation size of each example steel

[0042] Table 4 Main mechanical properties of steel in various examples

Claims

1. A low-cost 450MPa grade hot-rolled coil for welded pipe, characterized in that: The chemical composition of the steel plate is as follows: C: 0.08%~0.10%, Si: 0.2%~0.4%, Mn: 1.4%~1.6%, P: ≤0.02%, S: ≤0.003%, Ti: 0.01%~0.03%, Nb: 0.020%~0.040%, RE: 0.003%~0.005%, Cr: 0.15%~0.25%, Mg: 0.002%~0.004%, Als: 0.01%~0.04%, Ca: ≤0.003%, N: ≤0.008%, of which (Nb+Ti+C+N) ≤0.15%, and the rest are Fe and unavoidable elements.

2. The low-cost 450MPa grade hot-rolled coil for welded pipe according to claim 1, characterized in that: The optimal composite mass ratio of La and Ce is La / Ce=2.9~3.

2.

3. The low-cost 450MPa grade hot-rolled coil for welded pipe according to claim 1, characterized in that: After rare earth treatment, more than 80% of the intracrystalline acicular ferrite nucleation size is between 1 and 4 μm.

4. The low-cost 450MPa grade hot-rolled coil for welded pipe according to claim 1, characterized in that: The yield strength of the steel is 450~550MPa, the tensile strength is 550~650MPa, the elongation after fracture is ≥30%, the yield strength ratio is ≤0.90, the average impact energy Akv at -30℃ is ≥200J, the average DWTT of the drop hammer at -20℃ is ≥90%, the hardness value HV10 is ≤220, and the cold bending performance is qualified.

5. A method for manufacturing a low-cost 450 MPa grade hot-rolled coil for welded pipe according to any one of claims 1 to 4, comprising smelting, slab continuous casting, slab heating, rolling, cooling, and coiling, characterized in that: Slab heating: The slab is heated to 1120~1160℃ in a walking beam furnace; Rolling: rough rolling and finishing temperature 1030~1100℃, finishing rolling starting temperature 960~1010℃, finishing temperature 800~840℃, cumulative reduction rate 60%~65%; Cooling: After rolling, ultra-fast cooling is adopted, with the final cooling temperature at 450~520℃ and the cooling rate at 25~45℃ / s; Coiling: After cooling, it is coiled on a strong coiler with a roll tension coefficient of 1.4~2.2, an auxiliary roll pressure of 350~400MPa, and a guide ruler pressure of 30kN~50kN.

6. The method for manufacturing low-cost 450 MPa grade hot-rolled coil for welded pipe according to claim 5, characterized in that: The thickness of the finishing intermediate billet is 48~58mm, and the thickness of the finished product is 10~14mm.

7. The method for manufacturing low-cost 450 MPa grade hot-rolled coil for welded pipe according to claim 5, characterized in that: During continuous casting of slabs, the pouring is protected throughout the entire process, and electromagnetic stirring or dynamic soft pressure is used. The thickness of the slab is 190~210mm.

Images