X70 pipeline steel and preparation method thereof

Abstract

The invention provides X70 pipeline steel and a preparation method thereof, and belongs to the technical field of pipeline steel. The X70 pipeline steel comprises the following chemical components in percentage by mass: 0.03 to 0.045 percent of C, 0.2 to 0.30 percent of Si, 1.0 to 1.2 percent of Mn, less than or equal to 0.01 percent of P, less than or equal to 0.001 percent of S, 0.015 to 0.035 percent of Al, 0.20 to 0.25 percent of Cr, 0.017 to 0.022 percent of Ti, 0.005 to 0.07 percent of Nb, 0.03 to 0.05 percent of V, 0.0005 to 0.0008 percent of Mg, 0.0009 to 0.0015 percent of Ca and the balance of Fe and other inevitable impurity elements. According to the invention, magnesium (Mg) and calcium (Ca) elements are added into the X70 pipeline steel, so that the aggregation of free hydrogen is reduced, and the risk of hydrogen induced cracking is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of pipeline steels, and particularly relates to an X70 pipeline steel and a preparation method thereof. Background Art

[0002] X70 pipeline steel is a high-strength pipeline steel. The "X" in its name indicates high-strength pipeline steel, while "70" indicates that its minimum yield strength is 70,000 psi (about 483 MPa). X70 pipeline steel is usually used to transport high-pressure gas or liquid, such as natural gas and petroleum products. Due to its high strength and corrosion resistance, X70 pipeline steel can reduce the thickness of the pipeline wall, reduce engineering costs, and ensure the safe operation of the pipeline. X70 pipeline steel is widely used in the oil and gas industry, especially in pipeline projects that require high strength and corrosion resistance. Since X70 pipeline steel usually needs to operate under harsh environmental conditions, such as high pressure, high temperature, corrosive media, etc. In these environments, X70 pipeline steel is susceptible to hydrogen corrosion. Hydrogen can come from a variety of sources, such as pickling and electroplating during production and processing, as well as hydrogen atoms generated by corrosion reactions during service. Once these hydrogen atoms enter the interior of the pipeline steel, they may cause hydrogen-induced cracking. Hydrogen-induced cracking will seriously reduce the mechanical properties and service life of pipeline steel, posing huge safety hazards to energy transportation. Summary of the invention

[0003] In view of this, the present invention provides an X70 pipeline steel and a preparation method thereof, wherein magnesium (Mg) and calcium (Ca) elements are added to the X70 pipeline steel to reduce the aggregation of free hydrogen and the risk of hydrogen-induced cracking.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions: On the one hand, the present invention provides an X70 pipeline steel, comprising the following chemical components in mass percentage: C 0.03-0.045%, Si 0.2-0.30%, Mn 1.0-1.2%, P≤0.01%, S≤0.001%, Al 0.015-0.035%, Cr0.20-0.25%, Ti 0.017-0.022%, Nb 0.005-0.07%, V 0.03-0.05%, Mg 0.0005-0.0008%, Ca 0.0009-0.0015%, and the remaining components are Fe and other inevitable impurity elements.

[0005] Preferably, the mass ratio of Mg to Ca elements is 1:(1.1-3).

[0006] On the other hand, a method for preparing the X70 pipeline steel is also provided, comprising the following steps: S1, pre-treating molten iron by desulfurization, dephosphorization and desiliconization; S2, smelting the pretreated molten iron in a converter; S3, after the smelting in step S2, the molten iron is transferred to the off-furnace refining furnace and the RH furnace for refining to obtain molten steel; S4, continuously casting, continuously rolling and cooling the molten steel to obtain X70 pipeline steel; During RH furnace refining, Mg and Ca are added in batches in sequence, and the time interval between adding Ca and Mg is 2-5 minutes.

[0007] Preferably, in step S2, oxygen is top-blown and bottom-blown on the pretreated molten iron while stirring; and / or, In step S2, when the molten steel is tapped after smelting, a slag blocking operation is used to prevent rephosphorization.

[0008] Preferably, in step S3, the step of transferring the molten iron to an off-furnace refining furnace for refining after smelting in step S2 is as follows: using quartz sand and argon as a carrier gas to spray the bottom of the molten steel.

[0009] Preferably, in step S3, the steps of transferring the molten iron to the RH furnace for refining after smelting in step S2 include: controlling the vacuum degree in the RH furnace to ≤1 mbar, adding Mg and Ca elements in batches; and / or, In step S3, after RH furnace refining is completed, the H content in the molten steel is ≤0.00002wt%, and the N content is ≤0.0020wt%.

[0010] Preferably, in step S4, the molten steel is continuously cast and rolled using a thin plate continuous casting machine.

[0011] Preferably, in step S4, the molten steel is subjected to continuous casting and continuous rolling, and the specific process is: rough rolling and finish rolling are performed in sequence, wherein the rough rolling temperature is 1080-1170°C, and the finish rolling temperature is 870-920°C.

[0012] Preferably, in step S4, during the continuous casting and rolling of the molten steel, argon gas at a pressure of 0.6-0.7 MPa is used to purge the interior of the tundish for 7-13 minutes; and / or, In step S4, a low-carbon tundish covering agent having a carbon content of no more than 0.004% is used to coat the surface of the molten steel.

[0013] Preferably, in step S4, the cooling conditions are as follows: a starting cooling temperature of 750-780°C, a cooling rate of 25-33°C, and a final cooling temperature of 390-400°C.

[0014] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention adds magnesium (Mg) and calcium (Ca) elements to X70 pipeline steel. By using magnesium and calcium elements to treat molten steel, the morphology of inclusions in the steel is improved, ensuring that the inclusions are small, dispersed, spherical or ellipsoidal. These fine inclusions effectively provide irreversible hydrogen traps, reducing the content of free hydrogen in the steel. Reducing the aggregation of free hydrogen reduces the risk of hydrogen-induced cracking, especially in pipeline steels working in high-pressure, high-temperature, and corrosive environments.

[0015] (2) The inclusions of the present invention are surrounded by a Mn-poor zone. Mn is one of the important elements that affect the growth of M / A islands (Martensite-Austenite Island). A reduction in the content of Mn will inhibit the growth of M / A islands. The elements and processes involved in the present invention can also make M / A smaller and more dispersed. The fine M / A islands help reduce local stress concentration. The high-density dislocations inside the M / A islands and inclusions are used to provide dispersed hydrogen capture traps, so that the hydrogen is evenly distributed in the steel, avoiding local accumulation of hydrogen, and making the pipeline steel have a higher hydrogen-induced cracking stress threshold.

[0016] (3) The microstructure of the pipeline steel prepared by the preparation method of the present invention is mainly composed of ferrite, pearlite and M / A island, of which 77-89% is ferrite, 8-15% is M / A island, and 3-8% is pearlite. The average ferrite grain size is 3μm, and the M / A island size is 5μm. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The scanning electron microscope images of typical inclusions in the X70 pipeline steel provided in Examples 1-3 and Comparative Examples 1-4 of the present invention; Figure 2 The 10 mm diameter X70 pipeline steel provided in Examples 1-3 and Comparative Examples 1-4 of the present invention is 2 Statistical results of typical inclusions in the interior; Figure 3 The 10 mm diameter X70 pipeline steel provided in Examples 1-3 and Comparative Examples 1-4 of the present invention is 2 Statistical results of typical inclusion size distribution; Figure 4 This is a graph showing the observation result of the effect of inclusions on the refinement of the M / A island structure of the X70 pipeline steel provided in Example 1 of the present invention under a scanning electron microscope at 5000 times magnification; Figure 5 This is a diagram showing the slow rate stretching results of the X70 pipeline steel provided in Example 1 of the present invention and Comparative Example 1 before and after hydrogen charging.

[0018] Note: Figure 1In the figure, A is a scanning electron microscope morphology of a typical inclusion in the X70 pipeline steel of Example 1 at 2000 times; B is a scanning electron microscope morphology of a typical inclusion in the X70 pipeline steel of Example 2 at 2000 times; C is a scanning electron microscope morphology of a typical inclusion in the X70 pipeline steel of Example 3 at 2000 times; D is a scanning electron microscope morphology of a typical inclusion in the X70 pipeline steel of Comparative Example 1 at 2000 times; E is a scanning electron microscope morphology of a typical inclusion in the X70 pipeline steel of Comparative Example 2 at 2000 times; F is a scanning electron microscope morphology of a typical inclusion in the X70 pipeline steel of Comparative Example 3 at 2000 times; G is a scanning electron microscope morphology of a typical inclusion in the X70 pipeline steel of Comparative Example 4 at 2000 times. Figure 5 (a) is the slow rate tensile stress-strain curve of Example 1 before and after hydrogen charging; Figure 5 (b) is the slow rate tensile stress-strain curve of comparative example 1 before and after hydrogen charging. DETAILED DESCRIPTION

[0019] The present invention is further described in detail below in conjunction with specific embodiments so that those skilled in the art can understand the present invention more clearly.

[0020] X70 pipeline steel is a high-strength pipeline steel. The "X" in its name indicates high-strength pipeline steel, while "70" indicates that its minimum yield strength is 70,000 psi (about 483 MPa). X70 pipeline steel is usually used to transport high-pressure gas or liquid, such as natural gas and petroleum products. Due to its high strength and corrosion resistance, X70 pipeline steel can reduce the thickness of the pipeline wall, reduce engineering costs, and ensure the safe operation of the pipeline. X70 pipeline steel is widely used in the oil and gas industry, especially in pipeline projects that require high strength and corrosion resistance. Since X70 pipeline steel usually needs to operate under harsh environmental conditions, such as high pressure, high temperature, corrosive media, etc. In these environments, X70 pipeline steel is susceptible to hydrogen corrosion. Hydrogen can come from a variety of sources, such as pickling and electroplating during production and processing, as well as hydrogen atoms generated by corrosion reactions during service. Once these hydrogen atoms enter the interior of the pipeline steel, they may cause hydrogen-induced cracking. Hydrogen-induced cracking will seriously reduce the mechanical properties and service life of pipeline steel, posing huge safety hazards to energy transportation.

[0021] In order to solve the above technical problems, the present invention provides an X70 pipeline steel, comprising the following chemical components in mass percentage: C 0.03-0.045%, Si 0.2-0.30%, Mn 1.0-1.2%, P≤0.01%, S≤0.001%, Al0.015-0.035%, Cr 0.20-0.25%, Ti 0.017-0.022%, Nb 0.005-0.07%, V 0.03-0.05%, Mg0.0005-0.0008%, Ca 0.0009-0.0015%, and the remaining components are Fe and other inevitable impurity elements.

[0022] It should be noted that carbon is a key element in steelmaking, and its content affects the performance of steel. An appropriate amount of carbon can improve strength and hardness to meet the needs of different uses; but too much carbon will reduce toughness and weldability. The carbon content must be precisely controlled during steelmaking, and its distribution must be adjusted through subsequent processes to obtain an ideal organizational structure and ensure that the overall performance of the steel is good. Silicon is indispensable in steelmaking. It is an effective deoxidizer that can remove oxygen from molten steel and improve the purity of steel. Silicon can also increase the strength and hardness of steel, and improve its elastic limit, yield strength and fatigue strength. At the same time, silicon affects the toughness and weldability of steel to a certain extent, and its content needs to be reasonably controlled according to the specific use of the steel. Manganese can reduce the hot brittleness of steel and improve the forgeability and rollability of steel. It can strengthen ferrite and refine pearlite, greatly improve the strength and hardness of steel, and also increase the hardenability of steel, so that the steel has good comprehensive mechanical properties. Chromium (Cr) can improve the hardenability of steel and significantly enhance the strength, hardness and wear resistance of steel. Chromium can form a dense and stable oxide film, which effectively improves the corrosion resistance of steel and makes it more stable in an oxidizing environment. Magnesium plays a major role in deoxidation and desulfurization, controlling the morphology of inclusions, and refining grains in steelmaking. It can generate fine dispersed inclusions or change the properties of inclusions, reduce the impact of harmful inclusions, improve the mechanical properties and hot working properties of steel, and may also help improve the resistance to hydrogen-induced cracking. Calcium plays a major role in deoxidation and desulfurization in steelmaking, which can reduce the oxygen and sulfur content and reduce harmful inclusions; it can also spheroidize inclusions, improve their morphology and distribution, reduce the splitting effect on the steel matrix, improve the quality, plasticity, toughness, fatigue resistance and corrosion resistance of steel, and is an important element for improving the quality of steel. Vanadium contributes to strength mainly through precipitation strengthening, while improving the organizational structure of steel. When combined with other alloying elements, vanadium can indirectly improve the corrosion resistance of steel and enhance the stability of the passivation film. Titanium (Ti) is a strong deoxidizer that can reduce the oxygen content in steel and reduce oxide inclusions. Ti can form fine precipitates such as carbides and nitrides, refine grains, and improve strength and toughness. It can also improve the corrosion resistance of steel and stabilize the austenite structure. It is indispensable in the production of a variety of high-quality steels and helps improve the overall performance of steel. During the steelmaking process, niobium (Nb) can combine with carbon, nitrogen and other elements in steel to form fine and stable carbides, nitrides or carbonitrides. These precipitates effectively inhibit the growth of austenite grains during the heating and cooling of steel, refine the grain structure, and make the steel have better comprehensive mechanical properties such as strength, toughness and plasticity.

[0023] First, the present invention adds magnesium (Mg) and calcium (Ca) elements to X70 pipeline steel. By using magnesium and calcium elements to treat molten steel, the morphology of inclusions in the steel is improved, ensuring that the inclusions are small, dispersed spherical or ellipsoidal. These fine inclusions effectively provide irreversible hydrogen traps and reduce the content of free hydrogen in the steel. Reducing the aggregation of free hydrogen reduces the risk of hydrogen-induced cracking, especially in pipeline steels working under high pressure, high temperature and corrosive environments. Second, the present invention has a Mn-poor zone around the inclusions. The Mn element is one of the important elements that affect the growth of M / A islands (Martensite-Austenite Island). A reduction in the content of the Mn element will inhibit the growth of M / A islands. The elements and processes involved in the present invention can also make M / A smaller and more dispersed. The small M / A islands help reduce local stress concentration. The high-density dislocations and inclusions inside the M / A islands are used to provide dispersed hydrogen capture traps, so that the hydrogen is evenly distributed in the steel, avoiding local accumulation of hydrogen, and making the pipeline steel have a higher hydrogen-induced cracking stress threshold.

[0024] In some embodiments, the mass ratio of Mg to Ca elements is 1:(1.1-3).

[0025] The present invention also provides a method for preparing the X70 pipeline steel, comprising the following steps: S1, pre-treating molten iron by desulfurization, dephosphorization and desiliconization; Exemplarily, in some embodiments, the desulfurization conditions are: adding a desulfurizer (such as limestone, aluminum alloy or other suitable desulfurizer) to molten iron. The temperature is usually controlled at about 1500°C for desulfurization. During the reaction, sulfur reacts with the desulfurizer to generate sulfide (such as CaS), which is removed. The S content is reduced to ≤0.002wt%.

[0026] In some embodiments, the conditions for dephosphorization are as follows: Dephosphorization is usually performed by adding a phosphorus remover (such as iron oxide or aluminum alloy, etc.) to absorb or react phosphorus. The phosphorus content can also be reduced by oxidation and reduction reactions in the furnace, so that the P content is reduced to ≤ 0.012 wt%.

[0027] In some embodiments, the desiliconization condition is: reducing the silicon content in the molten iron by adding a reducing agent (such as aluminum or carbon). Usually, the molten iron is heated to an appropriate temperature (1500-1600°C) and treated in a furnace, and the silicon reacts with oxygen to form silicates, thereby removing the silicon content, so that the Si content is reduced to ≤0.3wt%.

[0028] S2, smelting the pretreated molten iron in a converter; For example, in some embodiments, the molten iron after desulfurization, dephosphorization and desiliconization is transferred to a converter to start the oxygen blowing process. Oxygen is added to the converter, and the oxidation reaction oxidizes the impurities (such as carbon, silicon, manganese, etc.) in the molten iron into oxides, thereby removing them. According to the requirements of the steel grade, the blowing time and oxygen amount are adjusted to control the composition of the molten steel to meet the requirements of the qualified carbon content and other elements in the molten steel.

[0029] S3, after the smelting in step S2, the molten iron is transferred to the off-furnace refining furnace and the RH furnace for refining to obtain molten steel; Exemplarily, in some embodiments, refining outside the furnace: In the refining furnace outside the furnace, the molten steel is further treated to remove some residual impurities and adjust the composition of the molten steel. The refining furnace operation usually includes adding deoxidizers, alloying elements (Mg and Ca are added in the present invention) and other auxiliary agents to further improve the purity, uniformity, temperature, etc. of the steel. After refining, the S content in the molten iron is reduced to ≤0.06wt%, the Si content is reduced to ≤0.3wt%, and the P content is reduced to ≤0.008wt%.

[0030] In some embodiments, after refining outside the furnace, the molten steel is transferred to the RH furnace (i.e., vacuum degassing furnace) for refining. In the RH furnace, the gaseous dissolved substances (such as hydrogen, nitrogen, oxygen, etc.) in the molten steel are removed by vacuum extraction to further improve the purity of the steel. During the RH furnace refining process, the chemical composition of the molten steel is adjusted by high temperature and high vacuum state to ensure that the alloy elements and carbon content in the molten steel reach the specified level. After refining, the H content is ≤0.00002wt%, and the N content is ≤0.0020wt%.

[0031] S4, continuously casting, continuously rolling and cooling the molten steel to obtain X70 pipeline steel; During RH furnace refining, Mg and Ca are added in batches in sequence, and the time interval between adding Ca and Mg is 2-5 minutes.

[0032] Exemplarily, in some embodiments, the continuous casting process: after the molten steel flows out of the RH furnace, it is sent to the continuous casting machine for casting through the submerged nozzle. During the continuous casting process, the molten steel is gradually solidified into a billet through a series of cooling devices, and the cross-sectional shape of the billet can be adjusted according to demand (such as square billet, round billet, etc.). Continuous rolling process: the billet obtained after continuous casting enters the continuous rolling mill, and through a multi-pass rolling process, it is gradually rolled into the required shape and size of the pipeline steel. During the continuous rolling process, it is necessary to accurately control the rolling temperature, pressure, speed and other parameters to ensure that the mechanical properties and dimensions of the steel meet the standards. Cooling process: the rolled steel is quickly cooled or slowly cooled by the cooling system to meet the specified microstructural requirements. By controlling the cooling rate, the grain size of the steel can be adjusted to ensure the strength, toughness and other properties of the steel.

[0033] It should be noted that the time interval between the addition of Mg and Ca is 2-5 minutes to avoid premature reaction of the two, thereby losing their due deoxidation effect. The addition needs to be precisely controlled so that Mg and Ca can play their role at the appropriate time and amount.

[0034] Further, in step S2, oxygen is top-blown and bottom-blown on the pretreated molten iron while stirring; and / or, In step S2, when the molten steel is tapped after smelting, a slag blocking operation is used to prevent rephosphorization.

[0035] Furthermore, in step S3, the molten iron is transferred to an off-furnace refining furnace for refining after being smelted in step S2, and the step is: using quartz sand and argon as a carrier gas to spray the bottom of the molten steel.

[0036] Further, in step S3, the molten iron is transferred to the RH furnace for refining after being smelted in step S2, and the steps are: controlling the vacuum degree in the RH furnace to ≤1 mbar, adding Mg and Ca elements in batches; and / or, In step S3, after RH furnace refining is completed, the H content in the molten steel is ≤0.00002wt%, and the N content is ≤0.0020wt%.

[0037] Furthermore, the molten steel is continuously cast and rolled using a thin plate continuous casting machine.

[0038] Furthermore, in step S4, the molten steel is subjected to continuous casting and continuous rolling, and the specific process is: rough rolling and finish rolling are carried out in sequence, wherein the temperature of the rough rolling is 1080-1170°C, and the temperature of the finish rolling is 870-920°C.

[0039] Further, in step S4, during the continuous casting and rolling of the molten steel, argon gas with a pressure of 0.6-0.7 MPa is used to purge the interior of the tundish for 7-13 minutes; and / or, In step S4, a low-carbon tundish covering agent having a carbon content of no more than 0.004% is used to coat the surface of the molten steel.

[0040] Furthermore, in step S4, the cooling conditions are as follows: a starting cooling temperature of 750-780°C, a cooling rate of 25-33°C, and a final cooling temperature of 390-400°C.

[0041] Example 1 The invention provides an X70 pipeline steel with a total mass of 100 kg, which includes the following components by mass fraction: C 0.045%, Si 0.24%, Mn 1.12%, P 0.0034%, S 0.001%, Al 0.020%, Cr 0.24%, Ti 0.017%, Nb 0.057%, V 0.041%, Mg 0.0007%, Ca 0.0012%, and the remaining components are Fe and other inevitable impurity elements. The mass ratio of Mg to Ca elements is 1:1.7.

[0042] The preparation method is: (1) Hot metal pretreatment: The hot metal is desulfurized, dephosphorized and desiliconized to reduce the S content in the hot metal to ≤0.002wt%, the Si content to ≤0.4wt%, and the P content to ≤0.012wt%.

[0043] (2) Converter smelting: Converter smelting is carried out simultaneously with top blowing, bottom blowing and stirring. Slag blocking operation is used during steelmaking to prevent phosphorus reversion.

[0044] (3) LF furnace (out-of-furnace refining furnace) refining: Quartz sand is used to spray argon as a carrier gas to the bottom of the molten steel. After the molten iron is pretreated, the S content in the molten iron is reduced to ≤0.06wt%, the Si content is reduced to ≤0.3wt%, and the P content is reduced to ≤0.008wt%.

[0045] (4) RH furnace refining: After the molten steel RH enters the station, the vacuum degree in the furnace is controlled to be ≤1mbar. According to the composition design, the corresponding alloy deoxidizing elements are added in batches. When the molten steel leaves the RH station, the H content in the molten steel is ≤0.00002wt%, and the N content is ≤0.0020wt%.

[0046] During RH furnace refining, Mg and Ca were added in batches, and the time interval between the addition of Mg and Ca was 3 minutes.

[0047] (5) Continuous casting and rolling process: The medium and thin plate continuous casting and rolling process is adopted, the slab thickness is 140 mm, the rough rolling temperature is 1120°C, the finishing rolling temperature is 890°C, and the final hot rolled plate thickness is 16 mm.

[0048] (6) The accelerated cooling method after rolling was adopted, with the starting cooling temperature being 770°C, the cooling rate being 28°C, and the final cooling temperature being 420°C.

[0049] Through the above preparation process, the obtained structure is mainly composed of ferrite, pearlite and M / A island, of which 86.3% is ferrite, 9.7% is M / A island and 4% is pearlite. The average grain size of ferrite is 2.7μm and the average grain size of M / A is 1.6μm.

[0050] Example 2 The invention provides an X70 pipeline steel with a total mass of 100 kg, which includes the following components by mass fraction: C 0.045%, Si 0.25%, Mn 1.11%, P 0.0032%, S 0.001%, Al 0.020%, Cr 0.24%, Ti 0.017%, Nb 0.059%, V 0.041%, Mg 0.0005%, Ca 0.0009%, and the remaining components are Fe and other inevitable impurity elements. The mass ratio of Mg to Ca elements is 1:1.8.

[0051] The preparation method is: (1) Hot metal pretreatment: The hot metal is desulfurized, dephosphorized and desiliconized to reduce the S content in the hot metal to ≤0.002wt%, the Si content to ≤0.4wt%, and the P content to ≤0.012wt%.

[0052] (2) Converter smelting: Converter smelting is carried out simultaneously with top blowing, bottom blowing and stirring. Slag blocking operation is used during steelmaking to prevent phosphorus reversion.

[0053] (3) LF furnace (out-of-furnace refining furnace) refining: Quartz sand is used to spray argon as a carrier gas to the bottom of the molten steel. After the molten iron is pretreated, the S content in the molten iron is reduced to ≤0.06wt%, the Si content is reduced to ≤0.3wt%, and the P content is reduced to ≤0.008wt%.

[0054] (4) RH furnace refining: After the molten steel RH enters the station, the vacuum degree in the furnace is controlled to be ≤1mbar. According to the composition design, the corresponding alloy deoxidizing elements are added in batches. When the molten steel leaves the RH station, the H content in the molten steel is ≤0.00002wt%, and the N content is ≤0.0020wt%.

[0055] During RH furnace refining, Mg and Ca were added in batches, and the time interval between the addition of Mg and Ca was 3 minutes.

[0056] (5) Continuous casting and rolling process: The medium and thin plate continuous casting and rolling process is adopted, the slab thickness is 145 mm, the rough rolling temperature is 1130°C, the finishing rolling temperature is 900°C, and the final hot rolled plate thickness is 15 mm.

[0057] (6) The method of accelerated cooling after rolling was adopted, with the starting cooling temperature being 780°C, the cooling rate being 29°C, and the final cooling temperature being 410°C.

[0058] Through the above preparation process, the obtained structure is mainly composed of ferrite, pearlite and M / A island, of which 83% is ferrite, 12% is M / A island and 5% is pearlite. The average grain size of ferrite is 2.8μm and the average grain size of M / A is 1.8μm.

[0059] Example 3 The invention provides an X70 pipeline steel with a total mass of 100 kg, which includes the following components by mass fraction: C 0.044%, Si 0.25%, Mn 1.11%, P 0.0032%, S 0.001%, Al 0.020%, Cr 0.24%, Ti 0.017%, Nb 0.059%, V 0.041%, Mg 0.0005%, Ca 0.00125%, and the remaining components are Fe and other inevitable impurity elements. The mass ratio of Mg to Ca elements is 1:2.5.

[0060] The preparation method is: (1) Hot metal pretreatment: The hot metal is desulfurized, dephosphorized and desiliconized to reduce the S content in the hot metal to ≤0.002wt%, the Si content to ≤0.4wt%, and the P content to ≤0.012wt%.

[0061] (2) Converter smelting: Converter smelting is carried out simultaneously with top blowing, bottom blowing and stirring. Slag blocking operation is used during steelmaking to prevent phosphorus reversion.

[0062] (3) LF furnace (out-of-furnace refining furnace) refining: Quartz sand is used to spray argon as a carrier gas to the bottom of the molten steel. After the molten iron is pretreated, the S content in the molten iron is reduced to ≤0.06wt%, the Si content is reduced to ≤0.3wt%, and the P content is reduced to ≤0.008wt%.

[0063] (4) RH furnace refining: After the molten steel RH enters the station, the vacuum degree in the furnace is controlled to be ≤1mbar. According to the composition design, the corresponding alloy deoxidizing elements are added in batches. When the molten steel leaves the RH station, the H content in the molten steel is ≤0.00002wt%, and the N content is ≤0.0020wt%.

[0064] During RH furnace refining, Mg and Ca were added in batches, and the time interval between the addition of Mg and Ca was 3 minutes.

[0065] (5) Continuous casting and rolling process: The medium and thin plate continuous casting and rolling process is adopted, the slab thickness is 150 mm, the rough rolling temperature is 1150°C, the finishing rolling temperature is 920°C, and the final hot rolled plate thickness is 20 mm.

[0066] (6) The accelerated cooling method after rolling was adopted, with the starting cooling temperature being 760°C, the cooling rate being 33°C, and the final cooling temperature being 440°C.

[0067] Through the above preparation process, the obtained structure is mainly composed of ferrite, pearlite and M / A island, of which 84% is ferrite, 15% is M / A island and 7% is pearlite. The average grain size of ferrite is 2.7μm and the average grain size of M / A is 1.8μm.

[0068] Example 4 The present invention provides an X70 pipeline steel, the components and preparation method of which are the same as those of Example 1, except that the mass ratio of Mg to Ca elements is 1:1.1.

[0069] Example 5 The present invention provides an X70 pipeline steel, the components and preparation method of which are the same as those of Example 1, except that the mass ratio of Mg to Ca elements is 1:3.

[0070] Comparative Example 1 The present invention provides an X70 pipeline steel with a total mass of 100 kg, including the following components by mass fraction: C 0.04%, Si 0.20%, Mn 1.2%, P 0.001%, S 0.001%, Al 0.004%, Cr 0.28%, Ti 0.008%, Nb 0.006%, V 0.041%, Mg 0.002%, Ca 0.004%, and the remaining components are Fe and other inevitable impurity elements. The mass ratio of Mg to Ca is 1:2, and compared with Examples 1-3, the Ca and Mg elements are not within the provided range.

[0071] Preparation method: (1) Hot metal pretreatment: The hot metal is subjected to desulfurization, dephosphorization and desiliconization treatments respectively, so that the S content in the hot metal is reduced to ≤0.06wt%, the Si content is reduced to ≤0.12wt%, and the P content is reduced to ≤0.008wt%; (2) Converter smelting: Converter smelting is carried out simultaneously with top blowing and bottom stirring. Slag blocking is used during steel tapping to prevent rephosphorization. The carbon content is controlled at 0.03-0.045wt% and the P content is less than 0.005wt%; (3) LF furnace refining, using silicon calcium alloy powder and argon as carrier gas to spray to the bottom of molten steel, maintaining the bottom argon blowing flow rate of 55-65L / min, argon blowing powder spraying time of 25min, reducing the sulfur content to ≤0.005wt%; (4) RH furnace refining: after the molten steel RH enters the station, the vacuum degree in the furnace is controlled to be ≤1mbar. According to the composition design, the corresponding alloy deoxidizing elements are added in batches. When the molten steel leaves the station, the H content in the molten steel is ≤0.0001wt%, and the N content is ≤0.0025wt%; During RH furnace refining, Mg, Ti and Ca were added in batches, and the time interval between the addition of Mg and Ti was 1 min, and the time interval between the addition of Ca and Ti was 3 min; (5) After the slab is poured, continuous casting and rolling process is carried out, using full protection pouring technology, dynamic light pressure reduction technology at the end of solidification of the slab after casting, deep-time long water inlet and strict sealing technology between ladle, tundish and crystallizer are adopted, and the overheating temperature is controlled at 25-30℃; (6) The medium-thin slab continuous casting and rolling process is adopted, with a slab thickness of 150 mm, a rough rolling temperature of 1150 °C, a finishing rolling temperature of 940 °C, and a final hot-rolled plate thickness of 30 mm.

[0072] Through the above preparation process, the obtained structure is mainly ferrite and pearlite, of which 82% is ferrite and 17% is pearlite, and the average grain size of ferrite is 3.1μm.

[0073] Comparative Example 2 The invention provides an X70 pipeline steel with a total mass of 100 kg, which includes the following components by mass fraction: C 0.045%, Si 0.26%, Mn 1.2%, P 0.0035%, S 0.001%, Al 0.020%, Cr 0.24%, Ti 0.017%, Nb 0.040%, V 0.043%, Ca 0.0015%, and the remaining components are Fe and other inevitable impurity elements. Among them, Mg element is not added.

[0074] The preparation method is similar to that of Example 1.

[0075] Comparative Example 3 The invention provides an X70 pipeline steel with a total mass of 100 kg, which includes the following components by mass fraction: C 0.045%, Si 0.26%, Mn 1.13%, P 0.0034%, S 0.001%, Al 0.020%, Cr 0.24%, Ti 0.018%, Nb 0.042%, V 0.043%, and the remaining components are Fe and other inevitable impurity elements. Among them, Mg and Ca elements are not added.

[0076] The preparation method is similar to that of Example 1.

[0077] Comparative Example 4 The invention provides an X70 pipeline steel with a total mass of 100 kg, which includes the following components by mass fraction: C 0.045%, Si 0.26%, Mn 1.13%, P 0.0034%, S 0.001%, Al 0.020%, Cr 0.24%, Ti 0.017%, Nb 0.042%, V 0.043%, Mg 0.0015%, and the remaining components are Fe and other inevitable impurity elements. Among them, no Ca element is added.

[0078] The preparation method is similar to that of Example 1.

[0079] Performance testing and results The pipeline steels prepared in Examples 1-3 and Comparative Examples 1-4 were observed under a scanning electron microscope. The results are as follows: Figure 1 As shown. In each sample, 10 mm 2 The typical inclusions in the Figure 2 , 3 As shown, Figure 2 10mm in each steel 2 Statistical diagram of the number of internal inclusions distribution, Figure 3 The statistical diagram of the size distribution of inclusions in each steel is shown in Figure 2. Figure 4 is the distribution of M / A islands around the inclusions in Example 1. The performance of the prepared pipeline steel hot-rolled slab was further evaluated according to NACE standard TM 0284-2016, and the results are shown in Table 1 below: Table 1

[0080] Note: CLR stands for cleanliness, CTR stands for conversion rate, and CSR stands for carbon dissolution rate.

[0081] like Figure 1 A, B, Figure 2 and 3 It can be seen that in Example 1-2, when the Ca-Mg element ratio is 1:(1.1-3), most of the inclusions are 1-3μm, and there are no inclusions larger than 5μm. Inclusions are relatively large in number, and the inclusions are spherical or ellipsoidal in shape. The overall feature is that the inclusions are finely dispersed in the steel. Figure 4 In the steel, the M / A islands around the inclusions are smaller than those on the matrix. The fine and dispersed M / A islands not only improve the strength and toughness of the steel, but also are conducive to the uniform distribution of hydrogen and improve its hydrogen resistance.

[0082] Comparative Examples 2 and 4 (such as Figure 1 E and G, Figure 2 and 3 Mg or Ca is added alone in Example 1 and Example 2, but the size of the inclusions is larger than that in Example 1 and Example 2, and the spheroidization effect is not good. A small number of inclusions have sharp corners, which are easy to become stress concentration areas, which is not good for the material's resistance to hydrogen-induced cracking. Figure 1 As shown in F, Figure 2 and 3 The inclusions in the sample (as shown) are relatively large in size and have many chain-shaped inclusions, which are not finely dispersed and easily cause HIC cracks.

[0083] The results of the HIC sensitivity test are shown in Table 1. The CLR, CTR, and CSR of the pipeline steels prepared in Examples 1-3 are all 0, and there is no visible HIC crack at the specified magnification, which shows good HIC resistance.

[0084] The mechanical test results show that although the pipeline steels prepared in Examples 1-3 and Comparative Examples 1-4 all meet the mechanical property requirements specified in national standards, the various mechanical property indicators of Examples 1-3 are more excellent.

[0085] like Figure 5 The slow rate stretching before and after hydrogen filling is compared with Example 1 and Comparative Example 1. ( Figure 5 (a) The strength and impact toughness of Example 1 are higher than those of the comparative example. The tensile curve after hydrogen charging shows that the strength and impact toughness of Example 1 are higher than those of the comparative example 1 after hydrogen charging. Figure 5 (b) The slow rate tensile plasticity loss after hydrogen charging is smaller and the hydrogen embrittlement resistance is better. The pipeline steel prepared by the method of the present invention has better performance in mechanical properties and hydrogen embrittlement resistance.

[0086] The specific raw materials not described in the present invention are all existing materials and can be directly purchased from the market.

[0087] The above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An X70 pipeline steel, characterized in that: The invention comprises the following chemical compositions in mass percentage: C 0.03-0.045%, Si 0.2-0.30%, Mn 1.0-1.2%, P≤0.01%, S≤0.001%, Al 0.015-0.035%, Cr 0.20-0.25%, Ti 0.017-0.022%, Nb 0.005-0.07%, V 0.03-0.05%, Mg 0.0005-0.0008%, Ca 0.0009-0.0015%, and the rest are Fe and other inevitable impurity elements.

2. The X70 pipeline steel according to claim 1, characterized in that: The mass ratio of Mg and Ca elements is 1:(1.1-3).

3. The method for preparing X70 pipeline steel according to claim 1 or 2, characterized in that: The following steps are involved: S1, pre-treating molten iron by desulfurization, dephosphorization and desiliconization; S2, smelting the pretreated molten iron in a converter; S3, after the smelting in step S2, the molten iron is transferred to the off-furnace refining furnace and the RH furnace for refining to obtain molten steel; S4, continuously casting, continuously rolling and cooling the molten steel to obtain X70 pipeline steel; During RH furnace refining, Mg and Ca are added in batches in sequence, and the time interval between adding Ca and Mg is 2-5 minutes.

4. The method according to claim 3, characterized in that In step S2, oxygen is top-blown and bottom-blown on the pretreated molten iron while stirring; and / or, In step S2, when the molten steel is tapped after smelting, a slag blocking operation is used to prevent rephosphorization.

5. The method according to claim 3, characterized in that: In step S3, the molten iron is transferred to an off-furnace refining furnace for refining after being smelted in step S2, and the steps are as follows: quartz sand is sprayed onto the bottom of the molten steel using argon as a carrier gas.

6. The method according to claim 3, characterized in that In step S3, the molten iron is transferred to the RH furnace for refining after being smelted in step S2, and the steps are: controlling the vacuum degree in the RH furnace to ≤1 mbar, adding Mg and Ca elements in batches; and / or, In step S3, after RH furnace refining is completed, the H content in the molten steel is ≤0.00002wt%, and the N content is ≤0.0020wt%.

7. The method according to claim 3, characterized in that In step S4, the molten steel is continuously cast and rolled using a thin plate continuous casting machine.

8. The method according to claim 7, characterized in that In step S4, the molten steel is continuously cast and rolled, and the specific process is: rough rolling and finish rolling are performed in sequence, wherein the rough rolling temperature is 1080-1170°C, and the finish rolling temperature is 870-920°C.

9. The method according to claim 7, characterized in that: In step S4, during the continuous casting and rolling of the molten steel, argon gas at a pressure of 0.6-0.7 MPa is used to purge the interior of the tundish for 7-13 minutes; and / or, In step S4, a low-carbon tundish covering agent having a carbon content of no more than 0.004% is used to coat the surface of the molten steel.

10. The method according to claim 7, characterized in that In step S4, the cooling conditions are as follows: the starting cooling temperature is 750-780°C, the cooling rate is 25-33°C, and the final cooling temperature is 390-400°C.

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