Steel plate for pipeline with excellent hydrogen resistance and casting method thereof

Abstract

The invention discloses a pipeline steel plate with excellent hydrogen resistance and a casting method thereof. The steel plate comprises the following chemical components: 0.01 to 0.04 percent of C, 0.12 to 0.18 percent of Si, 0.45 to 0.6 percent of Mn, less than or equal to 0.008 percent of P, less than or equal to 12 ppm of S, 0.12 to 0.22 percent of Cr, 0.08 to 0.18 percent of Ni, 0.11 to 0.21 percent of Cu, 0.025 to 0.035 percent of Nb, 0.016 to 0.028 percent of Ti, 0.021 to 0.049 percent of Alt, 5 to 12 ppm of Mg, 12 to 42 ppm of Ca and the balance of iron. In the casting method, RH incoming molten steel is 1620-1660 DEG C, argon is blown from the bottom, vacuum standing is performed, and vacuum is broken to feed a calcium line, so that Ca / S is regulated to be 2-4, and Ca / Alt is greater than or equal to 0.06; and then statically stirring for 2-3 minutes, and adding the magnesium alloy.

Description

Technical Field

[0001] The invention belongs to the technical field of steel material preparation, and relates to a pipeline steel plate with excellent hydrogen resistance and a casting method thereof. Background Art

[0002] As one of the cleanest energy sources, hydrogen is becoming a leading alternative to non-renewable energy sources due to its high energy conversion efficiency and environmentally friendly properties. The storage and transportation of hydrogen through pipelines, primarily made of steel, is a key component of the hydrogen energy industry.

[0003] The use of hydrogen pipelines presents severe challenges to steel. Specifically, hydrogen can enter steel through adsorption, dissociation, and permeation, where it can accumulate locally, leading to performance degradation such as hydrogen-induced plasticity loss, hydrogen-induced cracking, and hydrogen-induced delayed fracture. Diffusible hydrogen in steel is a key factor influencing hydrogen embrittlement behavior; the higher the diffusible hydrogen content in steel, the more severe the hydrogen embrittlement.

[0004] Previously, a common practice to delay hydrogen penetration was to add a coating on the surface of steel to block hydrogen from entering the interior of the steel. This does have a certain effect in preventing hydrogen penetration, but: pipeline steel needs to undergo pipe making, welding and long-term service in a high-pressure environment, and the coating is easy to fall off; and the construction of coating on the surface of pipeline steel is difficult and costly, making it difficult to promote.

[0005] How to fundamentally improve the hydrogen resistance of steel itself is an important issue faced in the development and production of steel for hydrogen pipelines. Summary of the Invention

[0006] The object of the present invention is to provide a pipeline steel plate having excellent hydrogen resistance and a casting method thereof.

[0007] To achieve the above-mentioned object, one embodiment of the present invention provides a pipeline steel plate. The chemical composition of the steel plate, in percentage by mass, includes: C 0.010-0.040%, Si 0.12-0.18%, Mn 0.45-0.60%, P ≤ 0.008%, S ≤ 0.0012%, Cr 0.12-0.22%, Ni 0.08-0.18%, Cu 0.11-0.21%, Nb 0.025-0.035%, Ti 0.016-0.028%, Alt 0.021-0.049%, Mg 0.0005-0.0012%, Ca 0.0012-0.0042%, with the remainder being iron and unavoidable impurities.

[0008] As a further improvement of one embodiment, in the steel plate, some or all of the non-metallic inclusions are of a core-shell structure, wherein the core is MgO·Al2O3 and the shell is MnS, CaS, TiN coated on the outer surface of the inner shell.

[0009] As a further improvement of one embodiment, the density of non-metallic inclusions in the steel plate is 40 to 70 per mm 2 The average diameter is ≤2μm, the diameter ≥15μm accounts for 0%, 10-15μm accounts for ≤1%, 6-10μm accounts for ≤5%, and 0-6μm accounts for ≥94%.

[0010] As a further improvement of one embodiment, the chemical composition of the steel plate further satisfies, in terms of mass percentage: 0.02≤Mg / Alt≤0.04.

[0011] As a further improvement of one embodiment, the chemical composition of the steel plate further satisfies, in terms of mass percentage: 0.01%≤Mg×Alt / S≤0.06%.

[0012] As a further improvement of one embodiment, the carbon segregation ratio of the steel plate is ≤105%, the manganese segregation ratio is ≤105%, the phosphorus segregation ratio is ≤104%, and the sulfur segregation ratio is ≤104%;

[0013] The steel plate has A, B, and C inclusions of ≤0.5 level, and D and Ds inclusions of ≤1 level.

[0014] To achieve the above-mentioned object, one embodiment of the present invention provides a method for casting a steel plate for pipelines. The casting method comprises sequentially performing molten iron pre-desulfurization, converter smelting, LF refining, RH refining, and continuous casting to produce the steel plate.

[0015] The chemical composition of the steel plate includes, by mass percentage, C 0.010-0.040%, Si 0.12-0.18%, Mn 0.45-0.60%, P≤0.008%, S≤0.0012%, Cr 0.12-0.22%, Ni 0.08-0.18%, Cu 0.11-0.21%, Nb 0.025-0.035%, Ti 0.016-0.028%, Alt 0.021-0.049%, Mg 0.0005-0.0012%, Ca 0.0012-0.0042%, and the remainder is iron and unavoidable impurities;

[0016] During RH refining: the temperature of the incoming molten steel is 1620-1660℃, firstly 3 / h flow rate of argon blowing at the bottom for 1 to 2 minutes, then vacuum standing, and then breaking the vacuum to feed calcium wire to adjust the Ca / S mass ratio of the molten steel to 2 to 4 and the Ca / Alt mass ratio to above 0.06; then stirring for 2 to 3 minutes, adding magnesium alloy to adjust the Mg mass proportion in the molten steel to 0.0005 to 0.0012%, and finally blowing argon at the bottom.

[0017] As a further improvement of one embodiment, the “re-vacuum standing” includes: standing for 25 to 30 minutes at a vacuum degree of ≤30 Pa;

[0018] The "final bottom blowing of argon" includes: 3 Bottom blowing of argon at a flow rate of less than / h.

[0019] As a further improvement of one embodiment, during continuous casting: the superheat of the molten steel during casting is 8 to 20°C, and the billet drawing speed is 0.1×L / F-0.05 to 0.1×L / F+0.05m / s, where L and F are the circumference and area of ​​the cross section of the steel plate obtained by continuous casting, respectively.

[0020] As a further improvement of one embodiment, in the pre-desulfurization of molten iron: magnesium powder and lime powder are mixed and sprayed into the molten iron to control the sulfur content in the molten iron to within 0.0012% by mass; the injection amount of magnesium powder is 0.35-0.45 kg per ton of molten iron;

[0021] In converter smelting: make slag with a basicity of 3.0-3.6 and control the P content within 0.005%;

[0022] During LF refining: After alloying, aluminum wire is fed to control the O mass ratio within 0.0040%, and then the steel is tapped.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: in terms of chemical composition, through the design of low-carbon and low-alloy components, trace magnesium is added to modify the inclusions, so that a large number of tiny irreversible hydrogen traps are obtained in the steel to adsorb hydrogen atoms in the lattice gaps or dislocations, thereby reducing the diffusible hydrogen in the steel; more importantly, it also avoids the ultra-high aggregation of hydrogen at the interfaces of some non-metallic inclusions, promotes the uniform distribution of hydrogen in the material, reduces the hydrogen aggregation in the irreversible hydrogen traps in the steel (that is, it will not cause an excessive increase in hydrogen pressure), and thus improves the hydrogen resistance of the steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a metallographic diagram of inclusions in the steel plate of Example 1 of the present invention;

[0025] Figure 2 This is a metallographic structure diagram of the steel plate according to Example 1 of the present invention. DETAILED DESCRIPTION

[0026] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0027] One embodiment of the present invention provides a steel material, specifically a steel plate, such as a steel plate having a thickness of ≤230 mm.

[0028] The steel plate may be obtained by continuous casting, for example, a continuous casting billet with a thickness of 210 to 230 mm; or, it may be obtained by further rolling a continuous casting billet with a thickness of 210 to 230 mm, for example, the steel plate is a hot-rolled plate with a thickness of less than 25 mm.

[0029] The chemical composition of the steel plate includes, by mass percentage, C 0.010-0.040%, Si 0.12-0.18%, Mn 0.45-0.60%, P≤0.008%, S≤0.0012%, Cr 0.12-0.22%, Ni 0.08-0.18%, Cu 0.11-0.21%, Nb 0.025-0.035%, Ti 0.016-0.028%, Alt 0.021-0.049%, Mg 0.0005-0.0012%, Ca 0.0012-0.0042%, and the rest is iron and unavoidable impurities.

[0030] As discussed in the background art, diffusible hydrogen is an important factor affecting hydrogen embrittlement behavior. The existing technology for hydrogen pipeline steel does not consider the correlation between chemical composition and diffusible hydrogen.

[0031] In this application, through the design of chemical composition, an appropriate amount of hydrogen traps are introduced into the steel to capture hydrogen, thereby reducing diffusible hydrogen and further improving the hydrogen resistance of the steel.

[0032] However, according to research, hydrogen traps are divided into two categories: reversible hydrogen traps and irreversible hydrogen traps, among which:

[0033] Reversible hydrogen traps (such as vacancies, dislocations, and low-angle grain boundaries) have low binding energy with hydrogen and can both capture and release hydrogen. Therefore, the hydrogen in reversible hydrogen traps is a harmful hydrogen source that easily diffuses and participates in the hydrogen-induced cracking process.

[0034] Irreversible hydrogen traps (such as large-angle grain boundaries, precipitated phases, retained austenite and inclusions, etc.) have high binding energy with hydrogen. Once hydrogen atoms enter the trap, they will not easily leave. In this way, the hydrogen in the irreversible hydrogen trap is difficult to escape, and the diffusion movement of hydrogen is inhibited, thereby improving the hydrogen resistance of steel.

[0035] Based on the above research results, in this application, through the design of chemical composition, the hydrogen traps introduced into the steel are mainly irreversible hydrogen traps, such as precipitates and inclusions.

[0036] However, the inventors also found in their research that the interface of non-metallic inclusions (such as MnS and Al2O3) in inclusions is an irreversible hydrogen trap, and its binding ability with hydrogen is very strong. Hydrogen easily accumulates here and generates hydrogen pressure. When the hydrogen pressure exceeds the material's tolerance limit, it will cause material failure, such as hydrogen-induced cracking.

[0037] In this way, in terms of chemical composition, the present application designs low-carbon and low-alloy components and adds trace magnesium to modify the inclusions, so that a large number of tiny irreversible hydrogen traps are obtained in the steel to adsorb hydrogen atoms in the lattice gaps or dislocations, thereby reducing the diffusible hydrogen in the steel; more importantly, it also avoids the ultra-high aggregation of hydrogen at the interfaces of some non-metallic inclusions, promotes the uniform distribution of hydrogen in the material, reduces the hydrogen aggregation in the irreversible hydrogen traps in the steel (that is, it will not cause excessive increase in hydrogen pressure), and thus improves the hydrogen resistance of the steel.

[0038] The functions of the various chemical elements in this embodiment are described in detail below.

[0039] C: It is an important element for improving the strength of steel; however, as the carbon content increases, the low-temperature impact toughness and welding performance of the steel decrease, which is not conducive to the application of the steel in hydrogen pipelines; in addition, the carbon element can precipitate soluble carbon at the grain boundaries. This grain boundary carbide is an active channel for hydrogen atoms. An increase in carbon content will reduce the hydrogen resistance of the steel when used in hydrogen pipelines; taking all factors into consideration, in one embodiment, the carbon content is controlled at 0.010~0.040%, preferably 0.025~0.040%.

[0040] Si: It mainly plays a role in solid solution strengthening, but excessive addition of silicon will lead to a significant deterioration of plasticity and toughness. In one embodiment, the silicon content is controlled at 0.12-0.18%.

[0041] Mn: It can improve the hardenability of steel and play a role in solid solution strengthening, compensating for the strength loss caused by low carbon. However, as the manganese content increases, it is easy to cause structural segregation and form banded structure, resulting in a decrease in the hydrogen resistance of the steel when used in hydrogen pipelines. Taking all factors into consideration, in one embodiment, the manganese content is controlled at 0.45-0.60%.

[0042] P: A harmful element that not only deteriorates the mechanical properties of steel but also causes phosphorus segregation to form a ferrite-pearlite banded structure, forming hydrogen transport channels, resulting in a decrease in the hydrogen resistance of the steel when used in hydrogen pipelines. In one embodiment, the phosphorus content is limited to ≤0.008%.

[0043] S: A harmful element. Sulfur and manganese form soft MnS inclusions, which are rolled into long strips during the rolling process. Hydrogen easily accumulates around the tips of these inclusions, resulting in excessive hydrogen pressure and hydrogen-induced cracking. In one embodiment, the sulfur content is controlled to ≤0.0012%.

[0044] Cr: It can be enriched in the corrosion product film. The chromium-containing corrosion product film has anion selectivity and can be quickly repaired after damage, thereby reducing local hydrogen sensitivity. However, adding a large amount of chromium will reduce toughness and welding performance. Taking all factors into consideration, in one embodiment, the chromium content is controlled at 0.12-0.22%.

[0045] Ni: can improve strength while maintaining good plasticity and toughness; in addition, nickel can significantly increase the corrosion potential of steel and inhibit the dissociation of hydrogen molecules; however, nickel is a relatively scarce resource and is expensive; taking all factors into consideration, in one embodiment, the nickel content is limited to 0.08-0.18%.

[0046] Cu: It can promote the formation of a passivation film and reduce hydrogen intrusion; however, excessive addition of copper will bring the risk of structural segregation, reduce the hydrogen resistance of the steel when used in hydrogen pipelines, and also cause hot brittleness; taking all factors into consideration, in one embodiment, the copper content is controlled at 0.11-0.21%.

[0047] Nb and Ti: Nb can significantly refine the grains and play a role in solid solution strengthening; Ti is not only beneficial for deoxidation, but also can form a small, stable TiN precipitate phase, which is a beneficial irreversible hydrogen trap; if added in excess, the beneficial effect cannot be equivalently improved; taking all factors into consideration, in one embodiment, the niobium and titanium contents are controlled at 0.025-0.035% and 0.016-0.028% respectively.

[0048] Mg: A trace amount of magnesium can modify Al2O3 into fine magnesium-aluminum spinel (MgO·Al2O3); however, when excessive magnesium is added, excessive Al-Mg-O-Mn-S inclusions are produced. These inclusions are difficult to disperse in the molten steel and tend to aggregate and merge into larger inclusions, even forming chain inclusions, which is not conducive to improving hydrogen resistance. After research, in one embodiment, the magnesium content is controlled at 0.0005-0.0012%.

[0049] Alt: It is a strong deoxidizing element that preferentially combines with oxygen in the molten steel to form Al2O3 with a high melting point. The fine Al2O3 and AlN particles can act as hydrogen traps (irreversible traps), capturing hydrogen atoms and reducing their diffusion ability, reducing the enrichment of hydrogen at grain boundaries or defects, and thus reducing the sensitivity to hydrogen-induced cracking. Excessive addition will form large-sized Al2O3, which is extremely detrimental to the hydrogen resistance. Taking all factors into consideration, in one embodiment, the aluminum content is controlled at 0.021-0.049%.

[0050] Ca: It can modify the morphology of sulfides and oxides and improve the toughness of steel. The interfacial energy of spherical inclusions after Ca treatment is low, which may form weak hydrogen traps and reduce the local enrichment of hydrogen. In addition, calcium treatment can indirectly optimize the distribution of inclusions and reduce the migration rate of hydrogen under stress. If excessive amount is added, the beneficial effects cannot be equivalently improved. Taking all factors into consideration, in one embodiment, the calcium content is controlled at 0.0012~0.0042%.

[0051] In one embodiment, in the steel plate, some or all of the non-metallic inclusions are in a core-shell structure, where the core is MgO·Al2O3 and the shell is MnS, CaS, TiN and covers the outer surface of the inner shell.

[0052] As mentioned above, the present application modifies the inclusions by using trace magnesium, for example, Al2O3 is modified into spherical inclusions MgO·Al2O3, and combined with the content design of Mn, S, Ca, Ti, and N, sulfides and nitrides are induced to precipitate on the surface of the spherical inclusions, so that dispersed composite inclusions are formed in the steel, that is, some or all of the non-metallic inclusions are core-shell structures. Such composite inclusions constitute irreversible hydrogen traps, and not only the interface between the core and the shell, but also the interface between the core or shell and the steel matrix can effectively capture hydrogen atoms without causing an increase in hydrogen pressure.

[0053] Preferably, among all the non-metallic inclusions in the steel plate, the number of non-metallic inclusions with a core-shell structure accounts for more than 95%, and more preferably more than 98%.

[0054] In one embodiment, the density of non-metallic inclusions in the steel plate is 40 to 70 per mm. 2 The average diameter is ≤2μm, the diameter ≥15μm accounts for 0%, 10-15μm accounts for ≤1%, 6-10μm accounts for ≤5%, and 0-6μm accounts for ≥94%.

[0055] In this way, through the design of trace magnesium, the inclusions in the steel are refined, softened, and spheroidized, forming dispersed "core-shell" structure inclusions, and forming a large number of tiny irreversible hydrogen traps, which not only reduces diffusible hydrogen, but also avoids excessive hydrogen pressure, thereby truly improving the hydrogen resistance.

[0056] Preferably, the density of non-metallic inclusions in the core-shell structure in the steel plate is 42 to 68 per mm. 2 , average diameter ≤1.8μm.

[0057] Furthermore, in one embodiment, the chemical composition of the steel plate satisfies the following relationship in terms of mass percentage: 0.02≤Mg / Alt≤0.04. In this way, the inclusion Al2O3 can be completely modified.

[0058] Furthermore, in one embodiment, the chemical composition of the steel plate, measured in mass percentage, further satisfies the following: 0.01% ≤ Mg × Alt / S ≤ 0.06%. This, in addition to modifying the inclusions with trace amounts of magnesium, further ensures the formation of sufficient nucleation sites to promote the formation of soft sulfides and prevent the formation and agglomeration of elongated sulfides. This increases and optimizes the number of non-metallic inclusions with a core-shell structure, improving hydrogen resistance.

[0059] In one embodiment, the center segregation of the continuous casting billet is ≤C1.5 or B1.0, type A segregation is not allowed to occur, the center porosity is ≤0.5 level, and the Al2O3 inclusion diameter is ≤1.0 level.

[0060] Here, central segregation, central porosity, and Al2O3 inclusions can be observed after cold acid etching according to YB / T 4003-2016, "Grading Chart for Macrostructure Defects in Continuously Cast Steel Slabs." Type A segregation is continuous, Type B is discontinuous (strip-like segregation exceeding 3mm in length), and Type C is dotted.

[0061] Furthermore, the steel plate has a carbon segregation ratio ≤105%, a manganese segregation ratio ≤105%, a phosphorus segregation ratio ≤104%, and a sulfur segregation ratio ≤104%.

[0062] Here, the carbon segregation ratio, manganese segregation ratio, phosphorus segregation ratio, and sulfur segregation ratio can be specifically measured in accordance with GB / T33165-2016 "Quantitative Analysis Method for Center Segregation of High Carbon Steel Wire Rod".

[0063] For example, the segregation ratio is calculated as M max / M ave × 100%, where M max 、M ave are the maximum and average values ​​of the element contents in steel, respectively. For example, the carbon segregation ratio is M max-C / M ave-C ×100%,M max-C 、M ave-C are the maximum and average values ​​of C content in steel respectively.

[0064] Furthermore, the A, B, and C inclusions of the steel plate are all ≤ level 0.5, and the D and Ds inclusions are all ≤ level 1.

[0065] Here, the rating of A, B, C, D, and Ds inclusions can be specifically based on GB / T 10561-2023 "Microscopic inspection method for the determination of the content of non-metallic inclusions in steel - standard rating chart", and the rating can be compared with the standard chart under a microscope.

[0066] Furthermore, an embodiment of the present invention also provides a casting method of the above-mentioned steel plate.

[0067] In this casting method, a steel plate is prepared through sequential molten iron pre-desulfurization, converter smelting, LF refining, RH refining and continuous casting.

[0068] That is, in this casting method, the obtained steel plate can also be called a continuous casting billet.

[0069] Among them, RH refining:

[0070] The temperature of the molten steel entering the station is 1620-1660℃, firstly with 4-5Nm 3 / h flow rate of argon blowing for 1 to 2 minutes. In this way, the large flow rate of argon stirring at high temperature causes obvious agitation of the molten steel, which can promote the floating of large initial inclusions; and it can also avoid long refining time, thereby preventing excessive temperature drop of the molten steel;

[0071] After blowing argon from the bottom, the steel is left to stand in vacuum. In this way, the oxygen content in the molten steel can be reduced to below 0.003% in a vacuum environment, thus reducing the generation of secondary inclusions.

[0072] Then, the vacuum is broken and the calcium wire is fed to control the Ca / S mass ratio in the molten steel to be 2-4 and the Ca / Alt mass ratio to be above 0.06. This calcium treatment can completely transform the Al2O3 in the molten steel into liquid or semi-liquid calcium aluminate, thereby promoting the spheroidization of inclusions.

[0073] After that, the steel is stirred statically for 2 to 3 minutes. This can improve the conversion rate of Al2O3 into liquid or semi-liquid calcium aluminate, for example, the conversion rate can be increased to more than 90%, and the formed liquid or semi-liquid calcium aluminate can be more easily modified into fine and spherical magnesium aluminum spinel (MgO·Al2O3); then, magnesium alloy is added to adjust the mass proportion of Mg in the molten steel to 0.0005 to 0.0012%, and the liquid or semi-liquid calcium aluminate can be fully modified into fine and spherical magnesium aluminum spinel (MgO·Al2O3); finally, argon is blown from the bottom to promote uniform distribution of inclusions.

[0074] Among them, in the "breaking vacuum and feeding calcium wire", the amount of feeding calcium wire can be controlled to be 200-400m, but is not limited thereto.

[0075] Optionally, in RH refining, the "bottom blowing argon followed by vacuum standing" is performed by standing for 25 to 30 minutes at a vacuum degree of ≤30 Pa. Such a high vacuum degree can further reduce the oxygen content in the water.

[0076] In one embodiment, during RH refining, the magnesium alloy is preferably a nickel-magnesium alloy having a magnesium content of 20 to 40%. This not only allows the liquid or semi-liquid calcium aluminate to be fully modified into fine, spherical magnesium aluminate spinel (MgO·Al2O3), but also helps reduce the magnesium vapor pressure and improve the magnesium yield.

[0077] In one embodiment, during RH refining, the "finally, bottom blowing argon" is performed at a rate of 1 Nm 3 Blow argon at a flow rate of less than / h, and then stir weakly for 15 to 20 minutes. In this way, the flow rate of bottom blowing argon is small, which can stabilize the distribution of inclusions.

[0078] Furthermore, during continuous casting: the superheat of the molten steel during casting is 8 to 20°C, and the casting speed is 0.1×L / F-0.05 to 0.1×L / F+0.05 m / s, where L and F are the circumference and area of ​​the cross section of the steel plate obtained by continuous casting, respectively.

[0079] In this way, through low superheat casting and appropriate drawing speed, the low-magnification quality of the steel plate obtained by continuous casting (i.e., continuous casting billet) can be greatly improved, and the segregation and central porosity in the continuous casting billet can be reduced.

[0080] In addition, during continuous casting, a strong cooling mode can be used, and soft reduction can be used at the end of continuous casting. This can prevent inclusions from agglomerating during the solidification process of the molten steel, reduce the segregation of carbon, manganese, phosphorus, and sulfur, and reduce the rating of A, B, C, D, and Ds inclusions.

[0081] Optionally, the thickness of the steel plate obtained in the continuous casting may be less than 230 mm, for example, 210 to 230 mm.

[0082] An optional implementation of continuous casting is introduced above. However, it can be understood that the present invention is not limited to the specific operation of continuous casting, and any feasible continuous casting technology in the art can be used for implementation.

[0083] Furthermore, in one embodiment, the molten iron is pre-desulfurized by mixing magnesium powder and lime powder and spraying them into the molten iron to control the mass proportion of S in the molten iron to within 0.0012%; the spraying amount of magnesium powder is 0.35-0.45 kg per ton of molten iron.

[0084] In one embodiment, converter smelting: slag with a basicity of 3.0 to 3.6 is produced, and the P mass ratio is controlled within 0.005%.

[0085] Specifically, during converter smelting, oxygen can be blown from the top and argon from the bottom. By adjusting the blowing pressure, the stirring intensity of the molten pool can be controlled to 0.8 to 1.2 Nm 3 / min·t.

[0086] Furthermore, during converter smelting, lime and dolomite can be used to produce slag with a basicity of 3.0 to 3.6.

[0087] In one embodiment, LF refining: After the gold treatment is completed, aluminum wire is fed to control the O mass ratio to be within 0.0040%, and then the steel is tapped.

[0088] The specific amount of the aluminum wire fed is not limited in this application, and is based on ensuring the Alt content in the final molten steel. For example, it can be 100 to 300 m of aluminum wire.

[0089] The above introduces an optional implementation method of molten iron pre-desulfurization, converter smelting, and LF refining. However, it can be understood that the present invention does not limit the specific operations of molten iron pre-desulfurization, converter smelting, and LF refining, and can also be implemented using any feasible technology in the field.

[0090] In summary, the beneficial effects of one embodiment of the present invention are: in terms of chemical composition, through the design of low-carbon and low-alloy components, trace magnesium is added to modify the inclusions, so that a large number of small irreversible hydrogen traps are obtained in the steel to adsorb hydrogen atoms in the lattice gaps or dislocations, thereby reducing the diffusible hydrogen in the steel; more importantly, it also avoids the ultra-high aggregation of hydrogen at the interfaces of some non-metallic inclusions, promotes the uniform distribution of hydrogen in the material, reduces the hydrogen aggregation in the irreversible hydrogen traps in the steel (that is, it will not cause excessive increase in hydrogen pressure), and thus improves the hydrogen resistance of the steel.

[0091] Thus, when the steel plate of the present invention is used in a hydrogen pipeline, it can greatly improve the hydrogen resistance of the hydrogen pipeline and reduce the incidence of defects such as hydrogen-induced plasticity loss, hydrogen-induced cracking, and hydrogen-induced delayed fracture in the hydrogen pipeline.

[0092] Here, let me explain in principle:

[0093] First, trace amounts of magnesium are used to modify calcium aluminate into magnesium aluminate spinel (MgO·Al2O3). Magnesium aluminate spinel exists in a solid state in molten steel and does not undergo a polymerization growth process. Therefore, the size of inclusions can be reduced and the alumina is modified from an irregular shape to a nearly spherical shape. At the same time, small inclusions have low buoyancy in molten steel and are difficult to float, resulting in an increase in the number of inclusions retained in the steel, forming diffusely distributed irreversible hydrogen traps.

[0094] Secondly, MnS uses magnesia-alumina spinel as the nucleation core to form Al-Ti-Mg-O-Mn-S-Ca composite inclusions that are soft on the outside and hard on the inside, with a "core-shell" and "core-shell-like" structure. Furthermore, this "core-shell" structured inclusion, on the one hand, can prevent the hard alumina and magnesia-alumina spinel from cutting the steel matrix and reduce the generation of microcracks in the steel. On the other hand, the soft MnS has a hard core, making the inclusions in the steel approximately round, reducing hydrogen-induced cracking caused by excessive hydrogen pressure due to hydrogen enrichment.

[0095] The test results show that after rolling, the hydrogen diffusion coefficient of the steel plate of the present application is D≤1.5×10 -6 cm 2 / s, the diffusible hydrogen concentration on the cathode side C0≤3.0×10 -6 mol / cm 3 .

[0096] Here, the hydrogen diffusion coefficient D can be measured according to the method disclosed in ISO 17081:2014; the cathode side diffusible hydrogen concentration C0 can be measured according to the method disclosed in GB / T 34542.3-2018 "Test method for hydrogen embrittlement sensitivity Part 3: Electrochemical hydrogen charging method".

[0097] The detailed description listed above is only a specific description of the feasible implementation methods of the present invention. The specific implementation methods of the present invention are introduced below through several specific examples.

[0098] Example 1

[0099] This embodiment provides a steel plate with a thickness of 15 mm.

[0100] The chemical composition of the steel plate includes, by mass percentage, C 0.035%, Si 0.15%, Mn 0.60%, P 0.006%, S 0.0008%, Cr 0.21%, Ni 0.12%, Cu 0.15%, Nb 0.028%, Ti 0.020%, Alt 0.025%, Ca 0.0022%, Mg 0.0010%, and the rest is iron and unavoidable impurities.

[0101] In the steel plate, 98% of the non-metallic inclusions are in a core-shell structure, with the core being MgO·Al2O3 and the shell being MnS, CaS, and TiN covering the outer surface of the inner shell; for example Figure 1 , showing the non-metallic inclusions of the core-shell structure in the metallographic structure of the steel plate of this embodiment.

[0102] Non-metallic inclusion density 66 pieces / mm 2The average diameter is 1.3μm, diameter ≥15μm accounts for 0%, 10-15μm accounts for 0.2%, 6-10μm accounts for 3.8%, and 0-6μm accounts for 96.0%.

[0103] The central segregation of the ingot is C1.0, the central porosity is 0.5, and the Al2O3 inclusion diameter is 0.5 level; in the finished steel plate, the carbon-carbon segregation ratio is 101%, the manganese segregation ratio is 102%, the phosphorus segregation ratio is 102%, and the sulfur segregation ratio is 102%; A, B, and C inclusions are all level 0, and D and Ds inclusions are all level 0.5.

[0104] The casting method of the steel plate is as follows:

[0105] Hot metal pre-desulfurization: Magnesium powder and lime powder are mixed and sprayed into the hot metal for desulfurization. The injection rate of magnesium powder is 0.39 kg per ton of hot metal. The sulfur content of the hot metal at the time of leaving the station is 0.0010%.

[0106] Converter smelting: slag with a basicity of 3.2 is used, and the P content after tapping is 0.0011%;

[0107] LF refining: After alloying, 260m of aluminum wire is fed, with an O mass ratio of 0.0038%, and then the steel is tapped;

[0108] During RH refining: the temperature of the incoming molten steel is 1650℃, firstly 3 / h flow rate of argon blowing for 1.3min, and then stand at 25Pa vacuum for 26min, active oxygen 0.0023%; then break the vacuum to feed calcium wire, adjust the Ca / S mass ratio of molten steel to 2.7, Ca / Al mass ratio to 0.09; then stir for 2.2min, add magnesium alloy to adjust the Mg mass ratio in the molten steel, and finally stir at 1.2Nm 3 / h flow rate of argon blowing bottom, weak stirring for 16 minutes, tapping;

[0109] Continuous casting: During casting, the superheat of molten steel was 12°C, the casting speed was 0.95 m / s, and a billet with a thickness of 220 mm was obtained.

[0110] The steel sheets with a thickness of 15 mm are produced by heating the continuous casting billet, hot rolling and cooling.

[0111] Afterwards, the hydrogen resistance of the steel plate was tested and the hydrogen diffusion coefficient D was found to be 0.8×10 -6 cm 2 / s, and the diffusible hydrogen concentration C0 on the cathode side is 1.6×10 -6 mol / cm 3 , it can be seen that the hydrogen resistance is excellent.

[0112] In addition, the above steel plates with a thickness of 15 mm were tested for their microstructure and properties. Figure 2 ; Mechanical properties were measured, yield strength was 405MPa, tensile strength was 515MPa, elongation was 42%, -40℃ impact energy KV2 was 410J, -15℃ DWTT drop hammer shear area fraction was 100%.

[0113] Example 2

[0114] This embodiment provides a steel plate with a thickness of 18 mm.

[0115] The chemical composition of the steel plate includes, by mass percentage, C 0.039%, Si 0.14%, Mn 0.55%, P 0.007%, S 0.0010%, Cr 0.20%, Ni 0.10%, Cu 0.13%, Nb 0.032%, Ti 0.022%, Alt 0.038%, Ca 0.0028%, Mg 0.0009%, and the rest is iron and unavoidable impurities.

[0116] In the steel plate, 99% of the non-metallic inclusions are in a core-shell structure, wherein the core is MgO·Al2O3 and the shell is MnS, CaS, and TiN coated on the outer surface of the inner shell.

[0117] Non-metallic inclusion density 62 pieces / mm 2 The average diameter is 1.5μm, diameter ≥15μm accounts for 0%, 10-15μm accounts for 0.1%, 6-10μm accounts for 3.2%, and 0-6μm accounts for 96.7%.

[0118] The central segregation of the continuous casting billet is C1.0, the central porosity is 0.5, and the Al2O3 inclusion diameter is 0.5 level; in the finished steel plate, the carbon segregation ratio is 102%, the manganese segregation ratio is 102%, the phosphorus segregation ratio is 102%, and the sulfur segregation ratio is 102%; A, B, C, and D inclusions are all level 0, and Ds inclusions are all level 0.5.

[0119] The casting method of the steel plate is as follows:

[0120] Hot metal pre-desulfurization: Magnesium powder and lime powder are mixed and sprayed into the hot metal for desulfurization. The injection rate of magnesium powder is 0.36 kg per ton of hot metal. The sulfur content of the hot metal at the time of leaving the station is 0.0009%.

[0121] Converter smelting: slag with a basicity of 3.4 is used, and the P content after tapping is 0.0018%;

[0122] LF refining: After alloying, 280m of aluminum wire is fed, with an O mass ratio of 0.0036%, and then the steel is tapped;

[0123] During RH refining: the temperature of the incoming molten steel is 1655℃, firstly with 4.5Nm 3 / h flow rate of argon blowing for 1.5min, and then stand at 25Pa vacuum for 27min, active oxygen 0.0022%; then break the vacuum to feed calcium wire, adjust the Ca / S mass ratio of molten steel to 2.8, Ca / Al mass ratio to 0.07; then stir for 2.6min, add magnesium alloy to adjust the Mg mass ratio in molten steel, and finally stir at 1.3Nm 3 / h flow rate of argon blowing bottom, weak stirring for 18 minutes, tapping;

[0124] Continuous casting: During casting, the superheat of molten steel was 15°C, the casting speed was 0.95 m / s, and a steel billet with a thickness of 220 mm was obtained.

[0125] The steel sheets with a thickness of 18 mm are produced by heating the continuous casting billet, hot rolling and cooling.

[0126] Afterwards, the hydrogen resistance of the steel plate was tested and the hydrogen diffusion coefficient D was found to be 0.6×10 -6 cm 2 / s, and the diffusible hydrogen concentration C0 on the cathode side is 1.8×10 -6 mol / cm 3 , it can be seen that the hydrogen resistance is excellent.

[0127] In addition, the mechanical properties of the 15 mm thick steel plate were measured, with yield strength of 400 MPa, tensile strength of 505 MPa, elongation of 45%, impact energy KV2 of -40°C of 390 J, and DWTT drop weight shear area fraction of -15°C of 100%.

[0128] Comparative Example

[0129] This comparative example provides a steel plate having a thickness of 220 mm.

[0130] The chemical composition of the steel plate includes, by mass percentage, C 0.033%, Si 0.15%, Mn 0.56%, P 0.008%, S 0.0009%, Cr 0.19%, Ni 0.11%, Cu 0.12%, Nb 0.035%, Ti 0.021%, Alt 0.032%, Ca 0.0032%, and the rest is iron and unavoidable impurities.

[0131] In the steel plate, the non-metallic inclusions are single Al2O3, MnS, CaS, TiN. The density of non-metallic inclusions is 42 per mm 2 The average diameter is 4.5μm, the diameter ≥15μm accounts for 1.6%, 10-15μm accounts for 12.3%, 6-10μm accounts for 16.1%, and 0-6μm accounts for 70.0%.

[0132] The central segregation of the continuous casting billet is C1.0, the central porosity is 0.5, and the Al2O3 inclusion diameter is 0.5 level; in the finished steel plate, the carbon segregation ratio is 105%, the manganese segregation ratio is 104%, the phosphorus segregation ratio is 104%, and the sulfur segregation ratio is 105%; A, B, C, and D inclusions are all level 0, and Ds inclusions are all level 0.5.

[0133] The casting method of the steel plate is as follows:

[0134] Hot metal pre-desulfurization: Magnesium powder and lime powder are mixed and sprayed into the hot metal for desulfurization. The injection rate of magnesium powder is 0.42 kg per ton of hot metal. The sulfur content of the hot metal at the time of leaving the station is 0.0009%.

[0135] Converter smelting: slag with a basicity of 3.3 is used, and the P content after tapping is 0.002%;

[0136] LF refining: After alloying, 250m aluminum wire is fed, with an O mass ratio of 0.0038%, and then the steel is tapped;

[0137] During RH refining: the temperature of the incoming molten steel is 1658℃, firstly with 4.6Nm 3 / h flow rate of argon blowing for 1.4min, and then stand at 26Pa vacuum for 22min, active oxygen 0.0028%; then break the vacuum to feed calcium wire, adjust the Ca / S mass ratio of molten steel to 3.6, Ca / Al mass ratio to 0.1; then at 1.2Nm 3 / h flow rate of argon blowing bottom, weak stirring for 19 minutes, tapping;

[0138] Continuous casting: During casting, the superheat of the molten steel was 12° C. and the casting speed was 1.00 m / s, to obtain a steel billet with a thickness of 220 mm, i.e., the steel plate of this comparative example.

[0139] The steel plate of this comparative example was heated, hot rolled, and cooled by continuous casting, and the specific operations were the same as those in Examples 1 and 2 above to produce a steel plate with a thickness of 18 mm. Afterwards, the hydrogen resistance of the steel plate was tested, and the hydrogen diffusion coefficient D was found to be 15×10 -6 cm 2 / s, the diffusible hydrogen concentration C0 on the cathode side is 22×10 -6 mol / cm 3 , it can be seen that the hydrogen resistance is poor.

[0140] In addition, the mechanical properties of the 15 mm thick steel plate were measured, with yield strength of 395 MPa, tensile strength of 495 MPa, elongation of 36%, impact energy KV2 of -40°C of 360 J, and -15°C DWTT drop hammer shear area fraction of 98%.

Claims

1. A steel plate for pipeline, characterized in that: The chemical composition of the steel plate includes, by mass percentage, C 0.010-0.040%, Si 0.12-0.18%, Mn 0.45-0.60%, P≤0.008%, S≤0.0012%, Cr 0.12-0.22%, Ni 0.08-0.18%, Cu 0.11-0.21%, Nb 0.025-0.035%, Ti 0.016~0.028%, Alt 0.021~0.049%, Mg 0.0005~0.0012%, Ca 0.0012~0.0042%, and the rest are iron and inevitable impurities.

2. The pipeline steel plate according to claim 1, characterized in that: In the steel plate, more than 95% of the non-metallic inclusions are in a core-shell structure, wherein the core is MgO·Al2O3 and the shell is MnS, CaS, and TiN coated on the outer surface of the inner shell.

3. The pipeline steel plate according to claim 1, characterized in that: In the steel plate, the density of non-metallic inclusions is 40 to 70 per mm 2 The average diameter is ≤2μm, the diameter ≥15μm accounts for 0%, 10-15μm accounts for ≤1%, 6-10μm accounts for ≤5%, and 0-6μm accounts for ≥94%.

4. The pipeline steel plate according to claim 1, characterized in that: The chemical composition of the steel plate further satisfies the following in terms of mass percentage: 0.02≤Mg / Alt≤0.

04.

5. The pipeline steel plate according to claim 1, characterized in that: The chemical composition of the steel plate further satisfies, in terms of mass percentage: 0.01%≤Mg×Alt / S≤0.06%.

6. The steel plate for pipeline according to claim 1, characterized in that: The carbon segregation ratio of the steel plate is ≤105%, the manganese segregation ratio is ≤105%, the phosphorus segregation ratio is ≤104%, and the sulfur segregation ratio is ≤104%; The steel plate has A, B, and C inclusions of ≤0.5 level, and D and Ds inclusions of ≤1 level.

7. A method for casting a steel plate for a pipeline, characterized in that: The casting method comprises preparing a steel plate by sequentially performing molten iron pre-desulfurization, converter smelting, LF refining, RH refining and continuous casting; The chemical composition of the steel plate includes, by mass percentage, C 0.010-0.040%, Si 0.12-0.18%, Mn 0.45-0.60%, P≤0.008%, S≤0.0012%, Cr 0.12-0.22%, Ni 0.08-0.18%, Cu 0.11-0.21%, Nb 0.025-0.035%, Ti 0.016-0.028%, Alt 0.021-0.049%, Mg 0.0005~0.0012%, Ca 0.0012~0.0042%, the rest is iron and inevitable impurities; During RH refining: the temperature of the incoming molten steel is 1620-1660℃, firstly 3 / h flow rate of argon blowing at the bottom for 1 to 2 minutes, then vacuum standing, and then breaking the vacuum to feed calcium wire to adjust the Ca / S mass ratio of the molten steel to 2 to 4 and the Ca / Alt mass ratio to above 0.06; then stirring for 2 to 3 minutes, adding magnesium alloy to adjust the Mg mass proportion in the molten steel to 0.0005 to 0.0012%, and finally blowing argon at the bottom.

8. The casting method of pipeline steel plate according to claim 7, characterized in that: The "re-vacuum standing" includes: standing for 25 to 30 minutes at a vacuum degree of ≤30Pa; The "final bottom blowing of argon" includes: 3 Bottom blowing of argon at a flow rate of less than / h.

9. The casting method of pipeline steel plate according to claim 7, characterized in that: During continuous casting: the superheat of the molten steel during casting is 8 to 20°C, and the casting speed is 0.1×L / F-0.05 to 0.1×L / F+0.05 m / s, where L and F are the circumference and area of ​​the cross section of the steel plate obtained by continuous casting, respectively.

10. The casting method of pipeline steel plate according to claim 7, characterized in that: In the pre-desulfurization of molten iron: After mixing magnesium powder and lime powder, spray it into the molten iron to control the sulfur content in the molten iron to within 0.0012% by mass; the injection rate of magnesium powder is 0.35-0.45 kg per ton of molten iron; In converter smelting: make slag with a basicity of 3.0-3.6 and control the P content within 0.005%; During LF refining: After alloying, aluminum wire is fed to control the O mass ratio within 0.0040%, and then the steel is tapped.

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