Low-yield-ratio ultrahigh-strength X100 pipeline steel coil plate and manufacturing method thereof

Abstract

The invention discloses a low-yield-ratio ultrahigh-strength X100 pipeline steel coil plate which comprises the following chemical components in percentage by weight: 0.09%-0.15% of C, 0.3%-0.6% of Si, 1.8%-2.2% of Mn, less than or equal to 0.01% of P, less than or equal to 0.003% of S, 0.01%-0.03% of Ti, 0.020%-0.035% of Nb, 0.04%-0.07% of V, 0.4%-0.7% of Cr, 0.002%-0.004% of Mg, 0.1%-0.2% of W, 0.50%-0.80% of Cr + W, 3-8% of Cr / W, 0.02%-0.05% of Als, less than or equal to 0.006% of Ca, less than or equal to 0.008% of N and the balance of Fe and inevitable elements. Mg improves inclusions and effectively reduces the yield ratio.

Description

Technical Field

[0001] The present invention relates to the manufacture of hot-rolled coil sheets of pipeline steel for oil and gas transportation pipelines, and particularly to a low yield ratio ultra-high strength X100 grade pipeline steel and a manufacturing method thereof, with a yield ratio reaching below 0.80. Background Art

[0002] With the increasing demand of mankind for oil and gas energy, the amount of pipeline laying is also increasing rapidly. Currently, the total length of transportation pipelines in the world has exceeded 2 million kilometers, and China has also laid more than 20,000 kilometers of oil and gas pipelines. It is estimated that in the next ten years, China will also build oil and gas pipelines with a total length of 100,000 - 200,000 kilometers of long-distance pipelines.

[0003] The demand for energy in various countries of the world increases year by year. As the most economical, safe and uninterrupted long-distance transportation tool for transporting oil and natural gas, pipelines have also developed rapidly. High-pressure and large-diameter transportation is considered the most economical development trend. Therefore, in order to match transportation economy, service strength and safety, etc., high-grade pipeline steel has become an inevitable pursuit goal. Various countries compete to develop higher-strength pipeline steel. Currently, the production technology of X80 pipeline steel is relatively mature. On the basis of existing production equipment, ultra-high strength and low yield ratio pipeline steel X100 coil sheets are developed to ensure the reliability of the compression and tensile ultimate capacity.

[0004] Currently, there are few reports on ultra-high strength X100 pipeline steel coil sheets. The following briefly introduces the patents and literatures relatively close to the present invention:

[0005] 1) Chinese Patent CN102162072A, ultra-high strength X100 pipeline steel and its production method. The composition contains C: 0.02% - 0.08%, Si ≤ 0.6%, Mn: 1.50% - 2.50%, P ≤ 0.015%, S ≤ 0.0030%, Nb: 0.04% - 0.10%, V ≤ 0.10%, Ti: 0.005% - 0.03%, Alt ≤ 0.06%, N ≤ 0.01%, Mo: 0.2% - 0.6%, Cu ≤ 0.5%, Ni: 0.1% - 1.0%, Cr: 0.1% - 1.0%. This invention is a production method of ultra-high strength X100 disclosed by Shagang. The product is rolled in the austenite recrystallization zone and the non-recrystallization zone, and the finish rolling temperature and finish cooling temperature are controlled to produce X100 grade hot-rolled plates. This invention has a relatively high content of precious alloying elements such as Ni and Cu, resulting in a high alloy cost and a low strength of the final product.

[0006] 2) Chinese Patent CN102304667A, an X100 pipeline steel plate with excellent low-temperature toughness and its preparation method. The composition contains C: 0.03% - 0.07%, Si: 0.10% - 0.45%, Mn: 1.50% - 1.79%, P ≤ 0.012%, S ≤ 0.0030%, Alt: 0.02% - 0.06%, Nb: 0.05% - 0.10%, V: 0.02% - 0.08%, Ti: 0.005% - 0.040%, Ni: 0.2% - 0.5%, Mo: 0.2% - 0.5%, N ≤ 0.008%. This invention is a production method of an X100 steel plate with excellent low-temperature toughness disclosed by Shougang, including processes of hot metal desulfurization, converter smelting, secondary refining, continuous casting, heating, rolling, cooling, and straightening. This invention has a relatively high content of precious alloy Ni and precious elements, resulting in high alloy cost and low strength of the final product. Summary of the Invention

[0007] Aiming at the technical problems existing in the current production of X100 pipeline steel coils, such as high alloy cost, high yield ratio, and poor safety, the purpose of this invention is to provide a low yield ratio ultra-high strength X100 grade pipeline steel coil and its manufacturing method, with a yield strength above 700 MPa, a tensile strength above 900 MPa, and a yield ratio below 0.80, used as a pipeline steel coil for oil and gas transportation. This invention adopts a low-carbon Nb-V-Ti composite design, with low Nb content and low alloy cost; the combination of Cr and W can effectively improve the tensile strength and reduce the yield ratio; Mg can improve inclusions and effectively reduce the yield ratio, having higher safety in pipeline engineering.

[0008] One of the technical solutions of this invention is to propose a low-cost low yield ratio ultra-high strength X100 pipeline steel coil. Its chemical composition by weight percentage is: C: 0.09% - 0.15%, Si: 0.3% - 0.6%, Mn: 1.8% - 2.2%, P: ≤ 0.01%, S: ≤ 0.003%, Ti: 0.01% - 0.03%, Nb: 0.020% - 0.035%, V: 0.04% - 0.07%, Cr: 0.4% - 0.7%, Mg: 0.002% - 0.004%, W: 0.1% - 0.2%, Cr + W: 0.50% - 0.80%, Cr / W: 3 - 8, Als: 0.02% - 0.05%, Ca ≤ 0.006%, N: ≤ 0.008%, and the rest are Fe and inevitable elements.

[0009] Compared with the prior art, this invention adopts a low-carbon Nb-V-Ti composite design, with low Nb content and low alloy cost; the combination of Cr and W can effectively improve the tensile strength and reduce the yield ratio; Mg can improve inclusions and effectively reduce the yield ratio, having higher safety in pipeline engineering.

[0010] C: It is a carbide-forming element, the most effective element for ensuring strength. It can improve hardenability and guarantee the strength and hardness of the material. Carbon significantly increases strength through solid-solution strengthening and phase transformation strengthening. Only by ensuring sufficient carbon can sufficient acicular ferrite and martensite-retained austenite (M-A) structures be formed. If the carbon content is too low, the strength and hardness of the material cannot be guaranteed; if it is too high, the weldability, plasticity, and impact toughness of the product will be affected. The optimal range is 0.09% - 0.15%.

[0011] Si: It can dissolve into ferrite and austenite, playing a certain role in solid-solution strengthening. It can significantly increase the hardness and tensile strength of steel, reduce the yield ratio, and improve fatigue strength and fatigue ratio. However, if the content is too high, the plasticity and toughness of steel will be significantly reduced. The optimal range is 0.3% - 0.6%.

[0012] Mn: Manganese has a solid-solution strengthening effect and can also increase the stability of austenite, which is also beneficial to improving hardenability and effectively guarantees the strength of steel. Manganese can compensate for the strength decline caused by the decrease in carbon content and is the most important and economical strengthening element. Manganese can shift the C curve to the right, promote the transformation of bainite, and is conducive to the formation of acicular ferrite, significantly increasing strength with little decrease in toughness. However, if the manganese content is too large, it can increase the tendency of center segregation in continuous casting billets, increase the banded structure in steel plates, increase the brittleness of steel plates, and reduce plasticity. The optimal range is 1.8% - 2.2%.

[0013] P, S, N: They are inevitable impurity elements in steel, and the lower the better. However, requiring too low levels will increase production costs. In the present invention, P ≤ 0.01%, S ≤ 0.003%, and N ≤ 0.008%.

[0014] Ti: Titanium is a strong nitrogen-fixing element. When about 0.015% Ti is added, fine and high-temperature stable TiN precipitation phases can be formed during slab continuous casting. These fine TiN precipitation phases can effectively prevent the growth of austenite grains during the heating process of continuous casting billets, and at the same time, have an obvious effect on improving the toughness of the heat-affected zone during steel welding. A small amount of precipitated TiC produces a strong precipitation strengthening effect, which can ensure that the grains do not grow significantly during the subsequent normalizing heat treatment of pipe making, thus ensuring the high strength of the steel pipe. However, if the content is too high, the effect is not obvious and large particle inclusions are easily formed. The optimal range is 0.01% - 0.03%.

[0015] Nb: Niobium is one of the important elements in low-carbon microalloyed steel. Niobium can significantly increase the austenite recrystallization temperature of steel, expand the range of the non-recrystallized zone, and can also inhibit the growth of austenite grains, with significant grain refinement strengthening and precipitation strengthening effects. In high-strength bainite steel, niobium promotes the formation of martensite-retained austenite (M-A) islands, increases the proportion of undercooled structures, and increases the tensile strength, achieving the effect of reducing the yield ratio. Niobium belongs to precious metals, and if the content is too high, the alloy cost will increase. The optimal range is 0.020% - 0.035%.

[0016] V: Vanadium narrows the austenite phase region and is infinitely soluble in sigma iron. It is a strong carbide and nitride forming element. Dissolved in austenite, it can improve the hardenability of steel; vanadium has strong precipitation strengthening and general grain refinement strengthening effects, which can supplement the deficiency of niobium precipitation strengthening. It dissolves into the solid solution during the rolling stage, increases the hardenability, promotes the formation of low-temperature supercooled structures, and improves the strength of the steel, especially the tensile strength. Excessive content is likely to cause an increase in the ductile-brittle transition temperature of the steel, and its optimal range is 0.04% - 0.07%.

[0017] Cr: Chromium can improve the strength by solid solution strengthening and reduce the yield ratio. Cr can dissolve into the solid solution like Mn, improve the hardenability of steel, and play a role in increasing the strength. After the Cr element dissolves into austenite, it increases the stability of supercooled austenite, shifts the C curve to the right, promotes the formation of supercooled structures martensite-retained austenite (M-A), and improves the strength and hardness of the steel. However, too high chromium content will significantly increase the brittle transition temperature of the steel, reduce the elongation rate, and easily form coarse carbides, resulting in deterioration of toughness. The appropriate range is 0.4% - 0.7%.

[0018] Mg: Magnesium has strong chemical activity and a strong affinity for oxygen and sulfur. It is an effective refining agent, which can reduce the oxygen and sulfur content and the number of inclusions in steel, purify the molten steel, and can also significantly modify the inclusions in steel, improving the impact and drop weight properties of the steel. Trace amounts of magnesium can reduce the inclusions in steel, decrease their size, make their distribution uniform, and improve their morphology. It can also increase the yield strength and tensile strength by more than 5% while basically keeping the plasticity unchanged, achieving the effect of reducing the yield ratio. The appropriate range is 0.002% - 0.004%.

[0019] W: Tungsten can form carbides in steel and partially dissolve into iron to form a solid solution. The effect of tungsten on the hardenability of steel is not as strong as that of molybdenum and chromium, and the increase in yield strength is not as obvious as the increase in tensile strength. It has a very high hardness in steel, which can improve the hardness and tensile strength of the steel, reduce the yield ratio. At the same time, with an appropriate addition amount, tungsten can improve the toughness of the steel, increase its fracture toughness, and improve the safety of steel use. However, a relatively high tungsten content will cause a significant decrease in the toughness of the steel. The appropriate range is 0.1% - 0.2%.

[0020] Als: Aluminum is a commonly used deoxidizer. Adding a small amount of aluminum to steel can refine the grains, improve the strength and impact toughness. The Als content in this invention is 0.02% - 0.05%.

[0021] Ca: Added during the secondary refining process, mainly to modify the inclusions to achieve the purpose of spheroidizing the inclusions and ensuring the toughness of the steel. Excessive content will form large particle inclusions, and its optimal range is below 0.006%.

[0022] The thickness of the coiled sheet of the present invention is 14 - 20 mm.

[0023] The ultimate mechanical properties of the coiled sheet of the present invention are as follows: the yield strength is above 700 MPa, preferably 700 - 730 MPa; the tensile strength is above 900 MPa, preferably 900 - 940 MPa; the yield ratio is below 0.80, preferably 0.76 - 0.78; the elongation is ≥18%, preferably 18% - 20%; the impact energy at -20 °C is ≥180 J, preferably 180 - 230 J; the DWTT drop weight at -10 °C is ≥90%, preferably 90% - 95%. It belongs to the X100 steel grade specified in API Spec 5L. API Spec 5L stipulates that for the X100 grade, the yield strength is 690 - 840 MPa, the tensile strength is 760 - 990 MPa, the elongation is ≥14%, the yield ratio is ≤0.97, the average impact energy Akv at 0 °C is ≥54 J, and the average DWTT drop weight at 0 °C is ≥85%.

[0024] The final microstructure of the coiled sheet of the present invention is a mixed microstructure of polygonal ferrite (F), bainite (B), and martensite - retained austenite (M - A). By volume percentage, the proportion of polygonal ferrite (F) is 1% - 3%, the proportion of bainite (B) is 92% - 97%, and the proportion of martensite - retained austenite (M - A) is 2% - 5%.

[0025] The coiled sheet of the ultra - high - strength pipeline steel of the present invention is a hot - rolled coiled sheet of ultra - high - strength X100 pipeline steel with a low yield ratio.

[0026] The second technical solution of the present invention is to provide a manufacturing method for a coiled sheet of ultra - high - strength X100 pipeline steel with a low yield ratio, which includes the following steps:

[0027] 1) Smelting and continuous casting process: Molten iron pretreatment, converter smelting, secondary refining (including LF furnace refining and RH furnace refining). In LF furnace, light desulfurization treatment is carried out to control S: ≤0.003% and calcium treatment is carried out to control the inclusion morphology and improve the ductility, toughness and cold bending performance of the steel. The molten steel is continuously cast into a slab, and electromagnetic stirring or dynamic soft reduction of 3 - 6 mm is adopted. The thickness of the continuously cast slab is 170 - 230 mm.

[0028] 2) Rolling, cooling, and coiling process: The continuous casting slab is heated in a reheating furnace to 1100 - 1200 °C and then hot mechanical rolling is carried out. This temperature range is conducive to the massive precipitation of Ti, refining the austenite grain size, which is beneficial to improving the yield and tensile strength. The finishing rolling temperature of rough rolling is 960 - 1050 °C, and the reduction per pass is greater than 20%. This temperature is conducive to the precipitation of TiC, hindering the growth of austenite grains and refining the grains to increase strength; the starting rolling temperature of finish rolling is 900 - 950 °C, the finishing rolling temperature is 800 - 850 °C, and the cumulative reduction is 60% - 65%. The temperature range of finishing rolling is conducive to the precipitation of Nb, refining the grain size, and improving strength and toughness; adopting a large reduction rate can generate a large number of dislocations and twins, playing a role in dislocation strengthening and significantly increasing the yield strength and tensile strength. After rolling, two-stage cooling is adopted. The cooling rate in the first stage is 5 - 10 °C / s, and it is cooled to 600 - 700 °C. This temperature is conducive to the precipitation of V element and generates a small amount of polygonal ferrite soft phase structure, ensuring a moderate yield strength; then laminar cooling is carried out, and the final cooling temperature is 350 - 450 °C, and the cooling rate is 25 - 45 °C / s to obtain bainite hard phase structure and high tensile strength. After cooling, coiling is carried out; finally, it is transformed into a mixed structure of polygonal ferrite, bainite, and martensite - retained austenite (M - A), with high strength, low yield ratio, and good low-temperature toughness.

[0029] The beneficial effects of the present invention are as follows:

[0030] 1) Adopting a low-carbon Nb-V-Ti composite design, with a low Nb content and low alloy cost;

[0031] 2) Adding Cr and W jointly, controlling the range of Cr + W at 0.50% - 0.80% and the range of Cr / W ratio at 3 - 8. The increase in tensile strength is greater than that of the yield strength, and the effect of reducing the yield ratio is good;

[0032] 3) Mg improves inclusions, increases the yield strength and tensile strength, effectively reduces the yield ratio, and has higher safety in pipeline engineering. Brief Description of the Drawings

[0033] Figure 1 It is the microstructural diagram of the low yield ratio ultra-high strength X100 pipeline steel coil plate prepared in Example 2 of the present invention. The volume percentages of polygonal ferrite, bainite, and M - A are 95%, 1%, and 4% respectively. Detailed Embodiments

[0034] The following non-limiting embodiments can enable those of ordinary skill in the art to understand the present invention more comprehensively, but do not limit the present invention in any way.

[0035] Examples 1 - 8

[0036] In Examples 1 to 8, the chemical compositions of the steel are shown in Table 1, the heating, rolling, and cooling process parameters in the manufacturing method are shown in Table 2, and the mechanical properties and microstructure test results of the prepared steel are shown in Table 3.

[0037] Table 1 Chemical Compositions of Examples wt%

[0038]

[0039] Table 2 Heating, Rolling, and Cooling Processes

[0040]

[0041] Table 3 Mechanical Properties and Microstructure Ratios

[0042]

[0043]

[0044] As can be seen from Tables 1 - 3, by adopting the composition design and rolling and coiling processes of the present invention, the produced low yield ratio ultra-high strength X100 pipeline steel coil plates meet the mechanical property requirements of API SPEC 5L standard X100.

Claims

1. A low yield ratio ultra-high strength X100 pipeline steel coil, characterized in that: Its chemical composition by weight percentage is: C: 0.09%-0.15%, Si: 0.3%-0.6%, Mn: 1.8%-2.2%, P: ≤0.01%, S: ≤0.003%, Ti: 0.01%-0.03%, Nb: 0.020%-0.035%, V: 0.04%-0.07%, Cr: 0.4%-0.7%, Mg: 0.002%-0.004%, W: 0.1%-0.2%, Cr+W: 0.50%-0.80%, Cr / W: 3-8, Als: 0.02%-0.05%, Ca≤0.006%, N: ≤0.008%, and the rest are Fe and unavoidable elements.

2. The low yield ratio ultra-high strength X100 pipeline steel coil according to claim 1 is characterized in that: The thickness of the rolled plate is 14-20mm.

3. The low yield ratio ultra-high strength X100 pipeline steel coil according to claim 1 is characterized in that: The yield strength of the coil is above 700MPa, the tensile strength is above 900MPa, the yield strength ratio is below 0.80, the elongation is ≥18%, the impact energy at -20℃ is ≥180J, and the DWTT drop hammer at -10℃ is ≥90%.

4. The low yield ratio ultra-high strength X100 pipeline steel coil according to claim 1 is characterized in that: The final structure of the rolled plate is a mixed structure of polygonal ferrite, bainite, and martensite-parallel. Calculated by volume percentage, polygonal ferrite is 1%-3%, bainite is 92%-97%, and martensite-parallel is 2%-5%.

5. The method for manufacturing the low yield ratio ultra-high strength X100 pipeline steel coil according to any one of claims 1 to 4, characterized in that: The steps include: 1) Smelting and continuous casting process: the molten iron is pre-treated, smelted in a converter, refined outside the furnace, and continuously cast into slabs; 2) Rolling, cooling and coiling process: The continuous casting slab is heated to 1100-1200°C in a heating furnace, and then thermomechanical rolling is adopted, the rough rolling and final rolling temperature is 960-1050°C, the finishing rolling start temperature is 900-950°C, the final rolling temperature is 800-850°C, and the cumulative reduction rate is 60%-65%; after rolling, two-stage cooling is adopted, the cooling rate of the first stage is 5-10°C / s, cooling to 600-700°C, and then laminar cooling is carried out, the final cooling temperature is 350-450°C, the cooling rate is 25-45°C / s, and coiling is carried out after cooling.

6. The manufacturing method according to claim 5, characterized in that: In step 1), the refining outside the furnace includes LF refining and RH refining. The LF refining adopts light desulfurization treatment to control S: ≤ 0.003% and performs calcium treatment.

7. The manufacturing method according to claim 5, characterized in that: In step 1), the continuous casting adopts electromagnetic stirring or dynamic soft pressure to reduce 3-6 mm, and the thickness of the continuous casting slab is 170-230 mm.

8. The manufacturing method according to claim 5, characterized in that: In step 2), the reduction ratio of the rough rolling pass is greater than 20%.

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