Ultrahigh-strength X100 pipeline steel coil plate and manufacturing method thereof

Abstract

The invention discloses an ultrahigh-strength X100 pipeline steel coil plate and a manufacturing method thereof. The ultrahigh-strength X100 pipeline steel coil plate comprises the following chemical components in percentage by weight: 0.05%-0.12% of C, 0.1%-0.4% 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, 0.06%-0.10% of Ti, 0.06%-0.09% of Nb, 0.02%-0.04% of V, 0.4%-0.6% of Mo, 0.002%-0.004% of B, 0.02%-0.05% of Als, less than or equal to 0.006% of Ca, less than or equal to 0.008% of N, less than 0.25% of Pcm and the balance of Fe and inevitable elements. The low-carbon Nb-Ti-V-B composite design is adopted, the alloy cost is low, the yield strength and tensile strength of the material are high, and the low-temperature toughness meets the requirement.

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 an ultra-high-strength X100 pipeline steel and its manufacturing method, with a yield strength of over 750 MPa, a tensile strength of over 900 MPa, and an impact energy at -20°C greater than 160 J. Background Art

[0002] With the increasing demand of mankind for oil and gas energy, the amount of pipeline laying has also increased 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 decade, China will also build oil and gas pipelines with a total length of 100,000 - 200,000 kilometers of long-distance pipelines.

[0003] Pipeline transportation has become the most economical and safest way of transporting oil and gas recognized in the world today. To improve transportation efficiency and reduce project investment, the development of pipeline steel for long-distance oil and gas transportation pipelines towards higher steel grades has become a trend. Currently, X70, which has been the highest steel grade in international standards for a long time, has been replaced by X80. Although X90 / X100 and higher grades are currently in the preliminary development stage, due to their advantages in material conservation, increasing transportation pressure, reducing construction volume, lowering maintenance costs, and optimizing the overall plan, etc., the cost of long-distance oil and gas pipelines can be saved by 5% - 12%. The huge economic benefits make the development of X100 and higher grade pipeline steels extremely urgent.

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

[0005] 1) Chinese Patent CN102162072A, an 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, controlling the finish rolling temperature and the finish cooling temperature to produce X100 grade hot-rolled sheets. 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 hot metal desulfurization, converter smelting, secondary refining, continuous casting, heating, rolling, cooling, and straightening processes. This invention has a relatively high content of precious alloy Ni, resulting in a high alloy cost and a low strength of the final product. Summary of the Invention

[0007] Aiming at the technical problems of high alloy cost, low strength, and poor safety in the current production of X100 pipeline steel coils, the purpose of the present invention is to provide a low-cost ultra-high-strength X100 grade pipeline steel coil and its manufacturing method, with a yield strength of more than 750 MPa, a tensile strength of more than 900 MPa, an impact energy at -20°C greater than 160 J, and used as a pipeline steel coil for oil and gas transportation. The present invention adopts a low-carbon Nb-Ti-V-B composite design, with a low alloy cost, high yield and tensile strengths of the material, and low-temperature toughness meeting the standard requirements, having higher safety in pipeline engineering.

[0008] One of the technical solutions of the present invention is to propose a low-cost ultra-high-strength X100 pipeline steel coil, and its chemical composition by weight percentage is: C: 0.05% - 0.12%, Si: 0.1% - 0.4%, Mn: 1.8% - 2.2%, P: ≤ 0.01%, S: ≤ 0.003%, Ti: 0.06% - 0.10%, Nb: 0.06% - 0.09%, V: 0.02% - 0.04%, Mo: 0.4% - 0.6%, B: 0.002% - 0.004%, Als: 0.02% - 0.05%, Ca ≤ 0.006%, N: ≤ 0.008%, and the rest is Fe and inevitable elements.

[0009] Compared with the prior art, the present invention adopts a low-carbon Nb-Ti-V-B composite design, with a low alloy cost, high yield and tensile strengths of the material, and low-temperature toughness meeting the standard requirements, 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 precipitation strengthening. Only by ensuring sufficient carbon can sufficient granular bainite, martensite, and 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 0.05% - 0.12%.

[0011] Si: It can dissolve into ferrite and austenite, playing a certain role in solid-solution strengthening. It can significantly increase the hardness and strength of steel, 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 0.1% - 0.4%.

[0012] Mn: Manganese has a solid-solution strengthening effect and can also increase the stability of austenite, which is also beneficial for improving hardenability and effectively guaranteeing the strength of steel. Manganese can compensate for the strength reduction 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 lath bainite and granular bainite, significantly increasing strength with little decrease in toughness. However, if the manganese content is too large, it can increase the tendency of central 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 element for fixing nitrogen and sulfur, forming nitrides and sulfides with them, which improves the strength, plasticity, and toughness of steel. When the titanium content is about 0.015%, 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. When the Ti content is greater than 0.06%, more TiC particles can be obtained, which can significantly increase the yield strength and tensile strength of steel through strain-induced precipitation and phase transformation precipitation. At the same time, the precipitated TiC produces a strong precipitation strengthening effect, which can ensure further refinement of weld grains after subsequent normalizing heat treatment of pipe making. The carbides formed by precipitation can increase the strength of the weld and can also significantly improve the plasticity and impact toughness at the weld. This high-strength steel with such a titanium content has good mechanical properties and process properties. However, if the content is too high, the effect is not obvious and large particle inclusions are easily formed. The optimal range is 0.06% - 0.10%.

[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, facilitate hot rolling under high temperature, reduce the mill load. At the same time, niobium can also inhibit the growth of austenite grains, and has significant grain refinement strengthening and precipitation strengthening effects. In high-strength bainitic steel, niobium promotes the formation of martensite-retained austenite (M-A island), increasing the yield strength and tensile strength. During hot rolling, the solid-solved Nb undergoes strain-induced precipitation to form Nb(N, C) particles, pinning the grain boundaries and inhibiting the growth of deformed austenite. Through controlled rolling and controlled cooling, the deformed austenite transforms into fine products with a high dislocation density, achieving a grain size of over grade 13. However, if the Nb content is too low, the dispersion precipitation effect is not obvious, and it cannot play the role of refining grains and strengthening the matrix; niobium belongs to precious metals, and too high content increases the alloy cost. The optimal range is 0.06%-0.09%.

[0016] V: Vanadium narrows the austenite phase region, is infinitely soluble in σ iron, and is a strong carbide and nitride forming element. Solute in austenite 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, and can inhibit grain growth during finish rolling and coiling stages, improving the strength of steel. Too high content is likely to increase the ductile-brittle transition temperature of steel. The optimal range is 0.02%-0.04%.

[0017] Mo: Molybdenum improves the strength of the base metal by increasing the hardenability of steel. Mo is an element that expands the γ phase region, can reduce the γ→α phase transformation temperature of steel, and with the increase of Mo content, the phase transformation temperature gradually decreases, which can effectively promote bainite transformation and play a role in phase transformation strengthening, obtaining finer lath bainite, granular bainite and martensite-retained austenite (M-A) phase transformation structures. In high-strength low-alloy steel, the yield strength increases with the increase of Mo content. Too high content of Mo increases the alloy cost and is detrimental to plasticity and toughness. The optimal range is 0.4%-0.6%.

[0018] B: The main role of boron in steel is to increase the hardenability of steel, thus saving other rare and precious metals such as nickel, copper, chromium, molybdenum, etc. Boron can replace nickel, chromium, and part of molybdenum. Adding a small amount of B can significantly inhibit the nucleation of ferrite at the austenite grain boundaries, and at the same time make the bainite transformation curve become flat, so that even in the case of low carbon, bainite and martensite structures can be obtained within a large cooling rate range, replacing precious alloys such as Ni and Cu, achieving low-cost production of ultra-high strength pipeline steel. However, after the addition amount reaches a certain level, the effect of B reaches saturation and is prone to cause embrittlement of steel. The optimal range is 0.002%-0.004%.

[0019] Als: Aluminum is a commonly used deoxidizer. Adding a small amount of aluminum to steel can refine the grain size, improve the strength and impact toughness. The Als content in the present invention is 0.02% - 0.05%.

[0020] Ca: Added during secondary refining process, mainly to modify inclusions to achieve the purpose of spheroidizing inclusions and ensure the toughness of steel. Excessive content will form large particle inclusions, and its optimal range is below 0.006%.

[0021] In the coil plate of the present invention, Pcm is lower than 0.25%.

[0022] The thickness of the coil plate of the present invention is 14 - 20 mm.

[0023] The final mechanical properties of the coil plate of the present invention are as follows: yield strength is above 750 MPa, preferably 750 - 800 MPa; tensile strength is above 900 MPa, preferably 900 - 950 MPa; yield ratio ≤ 0.85, preferably 0.83 - 0.85; elongation ≥ 16%, preferably 16% - 20%; impact energy at -20 °C is greater than 160 J, preferably 170 - 230 J; -10 °C DWTT drop weight ≥ 90%, preferably 90% - 95%, belonging to the X100 steel grade specified in API Spec 5L. API Spec 5L stipulates that for the X100 grade, yield strength is 690 - 840 MPa, tensile strength is 760 - 990 MPa, elongation ≥ 14%, yield ratio ≤ 0.97, average impact energy Akv at 0 °C ≥ 54 J, and average 0 °C drop weight (DWTT) ≥ 85%.

[0024] The final microstructure of the coil plate of the present invention is a mixed microstructure of lath bainite (LB), granular bainite (GB), and martensite - retained austenite (M - A). By volume percentage, the proportion of lath bainite is 60% - 70%, the proportion of granular bainite is 23% - 40%, and the proportion of martensite - retained austenite is 2% - 5%.

[0025] The ultra - high strength X100 pipeline steel coil plate of the present invention is a hot - rolled coil plate of ultra - high strength X100 pipeline steel.

[0026] The second technical solution of the present invention is to propose a manufacturing method for a low - cost ultra - high strength X100 pipeline steel coil plate, including 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), LF furnace light desulfurization treatment to control S: ≤ 0.003% and calcium treatment to control the inclusion morphology and improve the ductility, toughness and cold bending performance of steel; the molten steel is continuously cast into slab, electromagnetic stirring or dynamic soft reduction of 3 - 6 mm is adopted during continuous casting, and the thickness of the continuous casting slab is 170 - 230 mm.

[0028] 2) Rolling, cooling and coiling process: The continuously cast slab is heated in a reheating furnace to 1100 - 1200 °C and then hot mechanical rolling is adopted. 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 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 the strength. The starting rolling temperature of finish rolling is 850 - 950 °C, the finishing temperature is 700 - 800 °C, and the cumulative reduction is 65% - 70%. This temperature range is conducive to the precipitation of Nb, refining the grain size, and improving the tensile strength and toughness. The combination of the finishing temperature range and large reduction 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, ultra-fast cooling is adopted in the front section, with a cooling rate of 30 - 50 °C / s. After cooling, coiling is carried out, and the coiling temperature is 200 - 400 °C. At high cooling rates and low coiling temperatures, a mixed structure of lath bainite, granular bainite, and martensite - retained austenite (M - A) can be obtained, significantly increasing the yield strength and tensile strength.

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

[0030] 1) Low-carbon Nb - Ti - V composite design, with a carbon equivalent Pcm lower than 0.25%, having good weldability. In addition, it is also beneficial to inhibit the growth of grains in the weld and heat-affected zone during the welding of subsequent steel pipe making, improving the weld strength and the plasticity and impact toughness at the weld.

[0031] 2) Trace element B significantly inhibits the nucleation of ferrite at the austenite grain boundary, and the bainite transformation curve becomes flat, enabling low-carbon design X100 to obtain bainite and martensite structures within a relatively large cooling rate range, ensuring a sufficiently high yield strength and tensile strength, thus replacing precious alloys such as Ni and Cu, and having a low alloy cost.

[0032] 3) The material has high yield and tensile strengths. The yield strength can reach above 750 MPa, the tensile strength is above 900 MPa, and the impact energy at - 20 °C is greater than 160 J, having higher safety in pipeline engineering. Brief Description of the Drawings

[0033] Figure 1 It is the microstructural diagram of the ultra-high strength X100 pipeline steel coil plate prepared in Example 5 of the present invention. The volume percentages of LB, GB, and M - A are 69%, 27%, and 4% respectively. Detailed Embodiments

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

[0035] Examples 1 - 8

[0036] In Examples 1 - 8, the chemical composition of the steel is 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 composition of examples wt%

[0038] Example C Si Mn P S Nb Ti V Mo B Als Ca N Pcm 1 0.08 0.15 1.95 0.010 0.002 0.08 0.08 0.03 0.45 0.003 0.03 0.002 0.003 0.23 2 0.11 0.10 1.82 0.010 0.002 0.06 0.06 0.03 0.40 0.002 0.05 0.001 0.005 0.24 3 0.09 0.30 1.97 0.009 0.003 <![CDATA 0 . 06 > 0.07 0.02 0.50 0.002 0.04 0.002 0.005 0.24 4 0.07 0.20 2.05 0.008 0.003 <![CDATA 0 . 08 > 0.08 0.03 0.49 0.003 0.02 0.003 0.004 0.23 5 0.08 0.25 2.03 0.010 0.002 <![CDATA 0 . 07 > 0.10 0.03 0.44 0.002 0.02 0.002 0.005 0.23 6 0.05 0.35 2.10 0.009 0.001 0.09 0.09 0.03 0.58 0.004 0.03 0.001 0.006 0.23 7 0.10 0.17 1.85 0.006 0.002 0.06 0.07 0.02 0.42 0.003 0.02 0.003 0.006 0.24 8 0.06 0.33 2.14 0.007 0.002 0.09 0.08 0.04 0.55 0.003 0.04 0.001 0.007 0.23

[0039] Table 2 Heating, rolling, and cooling processes

[0040]

[0041] Table 3 Mechanical properties and tissue ratio

[0042]

[0043] As can be seen from Tables 1 - 3, by using the composition design and rolling and coiling processes of the present invention, low - cost ultra - high - strength X100 pipeline steel coils are produced, meeting the mechanical property requirements of API SPEC 5L standard X100.

Claims

1. An ultra-high strength X100 pipeline steel coil plate, characterized in that, Its chemical composition by weight percentage is as follows: C: 0.05% - 0.12%, Si: 0.1% - 0.4%, Mn: 1.8% - 2.2%, P: ≤0.01%, S: ≤0.003%, Ti: 0.06% - 0.10%, Nb: 0.06% - 0.09%, V: 0.02% - 0.04%, Mo: 0.4% - 0.6%, B: 0.002% - 0.004%, Als: 0.02% - 0.05%, Ca ≤0.006%, N: ≤0.008%, and the rest is Fe and inevitable elements.

2. The ultra-high strength X100 pipeline steel coil plate according to claim 1, characterized in that, Pcm is lower than 0.25%.

3. The ultra-high strength X100 pipeline steel coil plate according to claim 1, characterized in that, The thickness of the coiled sheet is 14 - 20 mm.

4. The extra-high-strength X100 pipeline steel coil sheet according to claim 1, wherein The yield strength of the coiled sheet is above 750 MPa, the tensile strength is above 900 MPa, the impact energy at -20°C is greater than 160 J, the yield ratio is ≤0.85, the elongation is ≥16%, and the -10°C DWTT drop weight is ≥90%.

5. The extra-high-strength X100 pipeline steel coil plate according to claim 1, wherein The microstructure of the coiled sheet is a mixed microstructure of lath bainite, granular bainite, and martensite - retained austenite. By volume percentage, lath bainite is 60% - 70%, granular bainite is 23% - 40%, and martensite - retained austenite is 2% - 5%.

6. The manufacturing method of the ultra-high strength X100 pipeline steel coil plate according to any one of claims 1-5, characterized in that, It includes the following steps: 1) Smelting and continuous casting process: Molten iron undergoes pretreatment, converter smelting, secondary refining, and continuous casting into slab billets. 2) Rolling, cooling, and coiling process: The continuous casting slab billets are heated in a heating furnace to 1100 - 1200°C, followed by thermo - mechanical rolling. The finishing rolling temperature of rough rolling is 960 - 1050°C, the starting rolling temperature of finish rolling is 850 - 950°C, the finishing rolling temperature is 700 - 800°C, and the cumulative reduction ratio is 65% - 70%. After rolling, ultra - rapid cooling is adopted in the front section, with a cooling rate of 30 - 50°C / s. After cooling, coiling is carried out, and the coiling temperature is 200 - 400°C.

7. The manufacturing method according to claim 6, characterized in that, In step 1), secondary refining includes LF refining and RH refining. LF refining adopts mild desulfurization treatment to control S: ≤0.003% and calcium treatment.

8. The manufacturing method according to claim 6, characterized in that, In step 1), electromagnetic stirring or dynamic soft reduction of 3 - 6 mm is adopted during continuous casting, and the thickness of the continuous casting slab billet is 170 - 230 mm.

9. The manufacturing method according to claim 6, characterized in that, In step 2), the reduction ratio of each rough rolling pass is greater than 20%.

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