Steel for hydrogen pipelines and its production method

Abstract

The present invention discloses a steel for hydrogen transmission pipeline and its production method. The chemical composition of the continuous casting billet is as follows: C 0.01 - 0.04%, Si 0.12 - 0.18%, Mn 0.45 - 0.6%, 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 5 - 12 ppm, Ca 12 - 42 ppm. In the production method, the heating temperature of the billet is T<subgt;MnS< / subgt> - 10°C to T<subgt;MnS< / subgt> + 20°C. During controlled rolling, the finishing rolling temperature is Ar3 - 30°C to Ar3; during controlled cooling, the water inlet temperature is ≥ Ar3 - 80°C, the cooling rate is 10 - 20°C / s, and the final cooling temperature is Bs - 140°C to Bs - 70°C.

Description

Technical Field

[0001] The invention belongs to the technical field of steel material preparation, and relates to steel for hydrogen transmission pipelines and a production 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 for steel. Specifically, steel used in hydrogen pipelines must possess excellent mechanical properties, low-temperature toughness, and excellent hydrogen resistance. This is because during the use of hydrogen pipelines, hydrogen can enter the steel through adsorption, dissociation, and permeation, and can accumulate locally within the steel, leading to performance degradation such as hydrogen-induced plasticity loss, hydrogen-induced cracking, and hydrogen-induced delayed fracture.

[0004] In the existing technology, either it is impossible to take into account mechanical properties, low-temperature toughness and hydrogen resistance at the same time, such as poor hydrogen resistance, or as in CN113862549A, although quenching + tempering can be used to ensure hydrogen resistance, the additional heat treatment process leads to a long production cycle and high cost.

[0005] Therefore, how to improve the mechanical properties, low-temperature toughness and hydrogen resistance of steel used for hydrogen pipelines and avoid problems such as high production costs and low efficiency will have huge safety and economic benefits in the development of steel used for hydrogen pipelines. Summary of the Invention

[0006] The object of the present invention is to provide steel for hydrogen transmission pipeline and a production method thereof.

[0007] To achieve the above-mentioned purpose, one embodiment of the present invention provides a method for producing steel for hydrogen transmission pipelines. The production method comprises:

[0008] A continuous casting billet is prepared by smelting and continuous casting; the chemical composition of the continuous casting billet comprises, 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;

[0009] Continuous casting billet heating: The heating temperature is T MnS -10°C to T MnS +20°C, where T MnS = 11625 / (5.02 - lg([Mn]×[S])) - 272.15;

[0010] Controlled rolling: After the continuous casting billet exits the heating furnace, two-stage rolling is carried out; the starting rolling temperature in the first stage is T MnS -130°C to T MnS -30°C, and the starting rolling temperature in the second stage is A r3 +20°C to A r3 +60°C, and the finishing rolling temperature is A r3 -30°C to A r3 , and the thickness of the rolled steel plate in the first stage is more than 4 times that of the rolled steel plate in the second stage;

[0011] Controlled cooling: The steel plate obtained by controlled rolling is cooled, the water inlet temperature ≥ A r3 -80°C, the cooling rate is 10 - 20°C / s, and the final cooling temperature is Bs - 140°C to Bs - 70°C.

[0012] Preferably, for the continuous casting billet heating: According to the thickness of the continuous casting billet, the heating duration is 1.1 - 1.5 min / mm.

[0013] Preferably, A r3 = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni];

[0014] Bs = 630 - 45[Mn] - 35[Si] - 30[Cr] - 20[Ni];

[0015] In the formula, [C], [Mn], [Cu], [Cr], [Ni], [Si] are respectively the mass percentages of C, Mn, Cu, Cr, Ni, Si in the continuous casting billet.

[0016] Preferably, the smelting includes: in sequence, hot metal pre-desulfurization, converter smelting, LF refining, RH refining; among them,

[0017] The RH refining includes: the temperature of the molten steel entering the station is 1620 - 1660°C, first bottom-blow argon at a flow rate of 4 - 5 Nm 3 / h for 1 - 2 min, then vacuum standstill, and then break the vacuum and feed in the calcium wire to adjust the Ca / S mass ratio in the molten steel to be 2 - 4 and the Ca / Alt mass ratio to be above 0.06; then stir statically for 2 - 3 min, add magnesium alloy to adjust the Mg mass ratio in the molten steel to be 0.0005 - 0.0012%, and finally bottom-blow argon.

[0018] Preferably, the "re-vacuum static setting" includes: statically setting for 25 - 30 min under a vacuum degree of ≤ 30 Pa.

[0019] Preferably, the "final bottom blowing of argon" includes: bottom blowing argon at a flow rate of 1 Nm 3 / h or less.

[0020] Preferably, the hot metal pre-desulfurization includes: after mixing magnesium powder and lime powder, injecting them into the hot metal to control the mass ratio of S in the hot metal within 0.0012%; the injection amount of magnesium powder is 0.35 - 0.45 Kg per ton of hot metal.

[0021] Preferably, the converter smelting includes: making a slag with an alkalinity of 3.0 - 3.6 and controlling the mass ratio of P within 0.005%.

[0022] Preferably, the LF refining includes: after alloying is completed, feeding in aluminum wire to control the mass ratio of O within 0.0040%, and then tapping the steel.

[0023] Preferably, the continuous casting includes: when casting, the superheat of the molten steel is 8 - 20 °C, and the casting speed is 0.1×L / F - 0.05 - 0.1×L / F + 0.05 m / s, where L and F are respectively the perimeter and area of the cross-section of the continuous casting billet.

[0024] To achieve the above-mentioned invention object, an embodiment of the present invention provides a pipeline steel. The chemical composition of the steel in mass percentage 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%, and the rest is iron and inevitable impurities.

[0025] Preferably, the steel is a steel plate with a thickness of ≤ 25 mm, the hydrogen diffusion coefficient D ≤ 1.5×10 -6 cm 2 / s, the yield strength is 380 - 450 MPa, the tensile strength is 480 - 550 MPa, the elongation is ≥ 40%, the yield ratio is ≤ 0.88, the impact energy at -40 °C is ≥ 300 J, and the drop weight tear test shear area ratio at -15 °C is ≥ 95%.

[0026] Preferably, the steel is a steel plate with a thickness of ≤ 25 mm, and the diffusible free hydrogen concentration C₀ on the cathode side ≤ 3.0×10 -6 mol / cm 3 with a yield strength of 380 - 450 MPa, a tensile strength of 480 - 550 MPa, an elongation of ≥40%, a yield ratio of ≤0.88, an impact energy at -40°C of ≥300 J, and a DWTT drop weight shear area ratio at -15°C of ≥95%.

[0027] Preferably, some or all of the non-metallic inclusions in the steel have a core-shell structure, where the core is MgO·Al2O3 and the shell is MnS, CaS, TiN, which coats the outer surface of the inner shell.

[0028] Preferably, the density of the non-metallic inclusions in the steel is 40 - 70 per mm 2 , with an average diameter of ≤2 μm, a proportion of diameters ≥15 μm of 0%, a proportion of diameters 10 - 15 μm of ≤1%, a proportion of diameters 6 - 10 μm of ≤5%, and a proportion of diameters 0 - 6 μm of ≥94%.

[0029] Preferably, the chemical composition of the steel also satisfies any one or both of the following two conditions by mass percentage:

[0030] 0.02 ≤ Mg / Alt ≤ 0.04;

[0031] 0.01% ≤ Mg×Alt / S ≤ 0.06%.

[0032] Preferably, the central segregation of the continuous casting billet is ≤C1.5 or B1.0, A-class segregation is not allowed, the central porosity is ≤0.5 level, and the diameter of Al2O3 inclusions is ≤1.0 level;

[0033] 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%;

[0034] The inclusions of A, B, and C classes in the steel plate are all ≤0.5 level, and the inclusions of D and Ds classes are all ≤1 level.

[0035] Preferably, the microstructure of the steel is quasi-polygonal ferrite + pearlite, the volume fraction of quasi-polygonal ferrite is ≥90%, the volume fraction of pearlite is ≤10%, the average grain size of quasi-polygonal ferrite is 6 - 12 μm, and the banded structure is ≤0.5 level.

[0036] Compared with the prior art, the beneficial effects of the present invention are:

[0037] (1) In terms of chemical composition, through the design of low-carbon and low-alloy compositions, trace amounts of magnesium are added to modify inclusions, enabling a large number of fine and irreversible hydrogen traps to be obtained in the steel. These traps adsorb hydrogen atoms in lattice interstices or dislocations, reducing the diffusible hydrogen in the steel. More importantly, it also avoids the excessive accumulation of hydrogen at the interfaces of some non-metallic inclusions, promotes the uniform distribution of hydrogen in the material, and reduces the hydrogen accumulation in the irreversible hydrogen traps in the steel (i.e., it will not cause an excessive increase in hydrogen pressure), thereby enhancing the hydrogen resistance of the steel. For example, it reduces the incidence of defects such as hydrogen-induced plastic loss, hydrogen-induced cracking, and hydrogen-induced delayed fracture in hydrogen transmission pipelines.

[0038] (2) Based on the chemical composition, combined with the specific settings of continuous casting billet heating, controlled rolling, and controlled cooling, not only can the precipitation strengthening effects of Nb, Ti, and Al be fully exerted to make up for the strength loss caused by low carbon, but also the grain size can be refined through a low final rolling temperature. Additionally, through a large cooling rate and a low final cooling temperature, the segregation of pearlite and the formation of banded structures can be avoided. In summary, steel with excellent mechanical properties, low-temperature toughness, and hydrogen resistance is obtained, and the production method of the present application does not require subsequent heat treatment processes such as quenching and tempering, having the advantages of a short process flow, low production cost, and fast delivery. Brief Description of the Drawings

[0039] Figure 1 is the metallographic structure diagram of the steel plate in Example 1 of the present invention;

[0040] Figure 2 is the metallographic diagram of the inclusions of the steel plate in Example 1 of the present invention. Detailed Embodiments

[0041] To make the objectives, technical solutions, and advantages of the present invention clearer, 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 described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0042] In response to the application requirements of hydrogen transmission pipelines, an embodiment of the present invention provides a steel and a production method for the steel.

[0043] In terms of chemical composition, the chemical composition of the steel 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 balance is iron and inevitable impurities.

[0044] The functions of each chemical element in this embodiment are introduced in detail below.

[0045] C: It is an important element to improve the strength of the steel; however, with the increase of carbon content, the low-temperature impact toughness and welding performance of the steel decrease, which is not conducive to the application of the steel in hydrogen transmission pipelines; in addition, carbon elements can precipitate soluble carbon at grain boundaries, and this grain boundary carbide is an active channel for hydrogen atoms. The increase of carbon content will reduce the hydrogen resistance of the steel when applied to hydrogen transmission pipelines; considering comprehensively, in one embodiment, the carbon content is controlled at 0.010 - 0.040%, preferably at 0.025 - 0.040%.

[0046] Si: It mainly plays a role in solid solution strengthening, but excessive addition of silicon will cause significant deterioration of plasticity and toughness; in one embodiment, the silicon content is controlled at 0.12 - 0.18%.

[0047] Mn: It can improve the hardenability of the steel and play a role in solid solution strengthening at the same time, making up for the strength decline caused by low carbon; however, with the increase of manganese content, it is easy to cause tissue segregation and form banded structure, resulting in a decrease in the hydrogen resistance of the steel when applied to hydrogen transmission pipelines; considering comprehensively, in one embodiment, the manganese content is controlled at 0.45 - 0.60%.

[0048] P: It is a harmful element, which not only deteriorates the mechanical properties of the steel, but also causes phosphorus segregation to generate ferrite - pearlite banded structure, forming a hydrogen transmission channel, resulting in a decrease in the hydrogen resistance of the steel when applied to hydrogen transmission pipelines; in one embodiment, the phosphorus content is limited to ≤ 0.008%.

[0049] S: It is a harmful element. Sulfur reacts with manganese to form soft MnS inclusions, which will be rolled into long strips during the rolling process. Hydrogen is easy to accumulate around its tip, resulting in too high hydrogen pressure and causing hydrogen-induced cracking; in one embodiment, the sulfur content is controlled at ≤ 0.0012%.

[0050] 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%.

[0051] 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%.

[0052] 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%.

[0053] 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.

[0054] 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%.

[0055] 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%.

[0056] Trace amounts of magnesium can modify Al2O3 into fine magnesium aluminate spinel (MgO·Al2O3); however, when excessive magnesium is added, excessive Al-Mg-O-Mn-S inclusions will be generated, and these inclusions are not easily dispersed in the molten steel and are prone to aggregation and merger into larger-sized inclusions, and even chain-like inclusions may appear, which is not conducive to the improvement of hydrogen resistance performance; through research, in one implementation, the magnesium content is controlled at 0.0005~0.0012%.

[0057] As mentioned in the background art, excellent mechanical properties, low-temperature toughness, and hydrogen resistance performance are all necessary properties of steel for hydrogen transmission pipelines. Among them, especially the hydrogen resistance performance is an important factor affecting the safety and service life of hydrogen transmission pipelines.

[0058] Diffusible hydrogen is an important factor affecting hydrogen embrittlement behavior. However, in the existing technology of steel for hydrogen transmission pipelines, the correlation between chemical composition and diffusible hydrogen is not considered.

[0059] In this application, through the design of chemical composition, an appropriate amount of hydrogen traps is introduced into the steel to capture hydrogen, thereby reducing diffusible hydrogen, and then, on the basis of ensuring mechanical properties and low-temperature toughness, greatly improving the hydrogen resistance performance of the steel.

[0060] However, according to the research of the inventor, hydrogen traps are divided into two categories: reversible hydrogen traps and irreversible hydrogen traps, among which:

[0061] Reversible hydrogen traps (such as vacancies, dislocations, small-angle grain boundaries, etc.) have a low binding energy with hydrogen. It can both capture hydrogen and release hydrogen. Thus, the hydrogen in reversible hydrogen traps is a harmful hydrogen source, which is easy to diffuse and participate in the hydrogen-induced cracking process;

[0062] Irreversible hydrogen traps (such as large-angle grain boundaries, precipitation phases, retained austenite, and inclusions, etc.) have a high binding energy with hydrogen. Once a hydrogen atom enters the trap, it is not easy to leave. Thus, the hydrogen in irreversible hydrogen traps is difficult to escape, and the diffusion movement of hydrogen is inhibited, thereby improving the hydrogen resistance performance of the steel.

[0063] 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 precipitation phases and inclusions.

[0064] However, the inventor also found in the research that among inclusions, the interfaces of non-metallic inclusions (such as MnS and Al2O3) are irreversible hydrogen traps and have a very strong binding ability with hydrogen. Hydrogen is prone to accumulate here and generate hydrogen pressure. When the hydrogen pressure exceeds the material's bearing limit, it will cause material failure, such as hydrogen-induced cracking.

[0065] Based on this, in terms of chemical composition, this application designs with low carbon and low alloy components, adds trace magnesium to modify inclusions, so as to obtain a large number of fine irreversible hydrogen traps in the steel, adsorb hydrogen atoms in lattice interstices or dislocations, reduce 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 (i.e., does not cause excessive increase in hydrogen pressure), and thus improves the hydrogen resistance of the steel.

[0066] Further, in one embodiment, the chemical composition of the steel in mass percentage also satisfies: 0.02 ≤ Mg / Alt ≤ 0.04. In this way, the complete modification of inclusion Al2O3 can be achieved.

[0067] Still further, in one embodiment, the chemical composition of the steel in mass percentage also satisfies: 0.01% ≤ Mg×Alt / S ≤ 0.06%. In this way, on the basis of modifying inclusions with trace magnesium, it further ensures the formation of sufficient nucleation points to promote the generation of soft sulfides, avoid the generation and agglomeration of long strip sulfides, thereby increasing and optimizing non-metallic inclusions with core-shell structures and improving the hydrogen resistance.

[0068] The steel in one embodiment of the present invention can be in the form of a steel plate, for example, a steel plate with a thickness ≤ 25 mm, and for example, the specific thickness can be 10 - 25 mm.

[0069] In one embodiment, the steel is excellent in mechanical properties, with a yield strength of 380 - 450 MPa, a tensile strength of 480 - 550 MPa, an elongation rate ≥ 40%, and a yield ratio ≤ 0.88.

[0070] Here, the measurements of the above-mentioned yield strength, tensile strength, and elongation rate can be based on GB / T 228.1 - 2021.

[0071] The steel is also very excellent in low-temperature toughness, with an impact energy at - 40 °C ≥ 300 J.

[0072] Here, the measurement of the above-mentioned impact energy at - 40 °C can be based on GB / T 229 - 2020.

[0073] From another perspective, in terms of low-temperature toughness, the steel has a DWTT drop-weight shear area ratio at - 15 °C ≥ 95%.

[0074] Here, the measurement of the above-mentioned DWTT drop-weight shear area ratio at - 15 °C can be based on GB / T 8363 - 2018.

[0075] The steel is excellent in hydrogen resistance, with a hydrogen diffusion coefficient D ≤ 1.5×10 -6 cm2 / s.

[0076] Herein, the above hydrogen diffusion coefficient D can be measured in the manner disclosed in ISO 17081:2014.

[0077] From another perspective, the diffusible free hydrogen concentration C0 on the cathode side of the steel in terms of hydrogen resistance performance satisfies C0 ≤ 3.0×10 - 6 mol / cm 3 .

[0078] Herein, the above diffusible hydrogen concentration C0 on the cathode side can be measured in the manner disclosed in GB / T 34542.3-2018 "Test Methods for Hydrogen Embrittlement Sensitivity - Part 3: Electrochemical Hydrogen Charging Method".

[0079] In one embodiment, some or all of the non-metallic inclusions of the steel have a core-shell structure, the core is MgO·Al2O3, and the shell is MnS, CaS, TiN and coats the outer surface of the inner shell.

[0080] As described above, in the present application, trace magnesium is used to modify the inclusions. For example, Al2O3 is modified into spherical inclusions MgO·Al2O3, and combined with the content design of Mn, S, Ca, Ti, N, sulfides and nitrides are induced to precipitate on the surface of the spherical inclusions, so that composite inclusions are formed in the steel and are diffusely distributed, that is, some or all of the non-metallic inclusions have a core-shell structure. Such composite inclusions form irreversible hydrogen traps. Not only the interface between the core and the shell, but also the interface between the core or the shell and the steel matrix can effectively capture hydrogen atoms and will not cause an increase in hydrogen pressure.

[0081] Preferably, among all the non-metallic inclusions of the steel, the proportion of the number of non-metallic inclusions with a core-shell structure is more than 95%, and more preferably more than 98%.

[0082] In one embodiment, the density of the non-metallic inclusions of the steel is 40 - 70 per mm 2 , the average diameter is ≤ 2 μm, the proportion of those with a diameter ≥ 15 μm is 0%, the proportion of those with a diameter of 10 - 15 μm is ≤ 1%, the proportion of those with a diameter of 6 - 10 μm is ≤ 5%, and the proportion of those with a diameter of 0 - 6 μm is ≥ 94%.

[0083] Thus, through the design of trace magnesium, the inclusions in the steel are refined, softened and spheroidized, forming diffusely distributed "core-shell" structure inclusions, and constituting a large number of small irreversible hydrogen traps, which not only reduce the diffusible hydrogen, but also avoid excessive hydrogen pressure, thereby truly improving the hydrogen resistance performance.

[0084] Preferably, the density of the non-metallic inclusions with a core-shell structure of the steel is 42 - 68 per mm 2, the average diameter ≤ 1.8 μm.

[0085] In one embodiment, the central segregation of the continuous casting billet ≤ C1.5 or B1.0, A-type segregation is not allowed to occur, the central porosity ≤ 0.5 grade, and the diameter of Al2O3 inclusions ≤ 1.0 grade.

[0086] Here, for central segregation, central porosity, and Al2O3 inclusions, they can be specifically observed after cold acid etching according to YB / T 4003-2016 "Rating Diagram of Macrostructure Defects for Continuous Cast Steel Slabs". Among them, A-type segregation shows a continuous distribution, B-type segregation shows an intermittent distribution (strip-shaped segregation with a length exceeding 3 mm), and C-type segregation shows a dot-like distribution.

[0087] Furthermore, the carbon segregation ratio of the steel plate ≤ 105%, the manganese segregation ratio ≤ 105%, the phosphorus segregation ratio ≤ 104%, and the sulfur segregation ratio ≤ 104%.

[0088] Here, for the carbon segregation ratio, manganese segregation ratio, phosphorus segregation ratio, and sulfur segregation ratio, they can be specifically determined according to GB / T 33165-2016 "Quantitative Analysis Method for Central Segregation of High Carbon Steel Wire Rods".

[0089] For example, the calculation method of the segregation ratio is M max / M ave ×100%, where M max and M ave are respectively the maximum value and the average value of the element content in the steel. For example, the carbon segregation ratio is M max-C / M ave-C ×100%, and M max-C and M ave-C are respectively the maximum value and the average value of the C element content in the steel.

[0090] Furthermore, for A, B, and C types of inclusions in the steel, they are all ≤ 0.5 grade, and for D and Ds types of inclusions, they are all ≤ 1 grade.

[0091] Here, for the rating of A, B, C, D, and Ds types of inclusions, it can be specifically rated under a microscope by comparing with the standard atlas according to GB / T 10561-2023 "Determination of the Content of Non-Metallic Inclusions in Steel - Microscopic Examination Method Using Standard Rating Diagrams".

[0092] In one embodiment, the structure of the steel is quasi-polygonal ferrite + pearlite.

[0093] Among them, the volume fraction of quasi-polygonal ferrite ≥ 90%, and the volume fraction of pearlite ≤ 10%.

[0094] Furthermore, the average grain size of quasi-polygonal ferrite is 6 - 12 μm.

[0095] In addition, the banded structure ≤ 0.5 grade.

[0096] Here, the banded structure of the steel can be rated in accordance with GB / T 34474.1-2017 "Evaluation of banded structure in steel - Part 1: Standard rating diagram method".

[0097] The chemical composition, structure and properties of the steel in an embodiment of the present application have been introduced above. An embodiment of the present application also provides a production method of the steel.

[0098] Specifically, the production method includes a technological process of smelting, continuous casting, heating of continuous casting billet, controlled rolling and controlled cooling in sequence to prepare a steel plate with a thickness ≤ 25 mm.

[0099] In an embodiment of the present application, a continuous casting billet is prepared through smelting and continuous casting.

[0100] It can be understood that the chemical composition of the continuous casting billet is as described above, that is, consistent with the chemical composition of the steel of the present application.

[0101] The continuous casting billet heating process includes: the heating temperature is T MnS -10 °C ~ T MnS +20 °C, where T MnS takes the value of T MnS = 11625 / (5.02 - lg([Mn] × [S])) - 272.15.

[0102] In this way, through low-temperature heating, excessive growth of austenite can be avoided. At the same time, precipitation of intragranular acicular ferrite induced by inclusions during controlled rolling can also be avoided.

[0103] The controlled rolling process includes: after the continuous casting billet exits the heating furnace, two-stage rolling is carried out.

[0104] Among them, the starting rolling temperature T MnS -130 °C ~ T MnS -30 °C, the starting rolling temperature A r3 +20 °C ~ A r3 +60 °C, and the finishing rolling temperature A r3 -30 °C ~ A r3 .

[0105] The thickness of the rolled steel plate in the first stage is more than 4 times the thickness of the rolled steel plate in the second stage. It can be understood that the thickness of the rolled steel plate in the second stage is the thickness of the finally obtained steel, and as described above, the thickness ≤ 25 mm.

[0106] The controlled cooling process includes: cooling the steel plate obtained by controlled rolling.

[0107] Among them, during cooling, the water inlet temperature ≥ A r3-80 °C, cooling rate 10 - 20 °C / s, final cooling temperature Bs - 140 °C to Bs - 70 °C.

[0108] Here, A r3 represents the critical temperature at which austenite (γ-Fe) begins to transform into ferrite (α-Fe) during cooling.

[0109] A r3 The specific value of A can be obtained in various ways in this field. In one embodiment, optionally but not limited to this, A r3 = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni]. Here, [C], [Mn], [Cu], [Cr], [Ni] are the mass percentages of C, Mn, Cu, Cr, and Ni in the continuous casting billet respectively. For example, if the mass percentage of C in the continuous casting billet is 0.010%, then [C] is the mass percentage 0.010.

[0110] Furthermore, Bs represents the bainite transformation start temperature.

[0111] The specific value of Bs can be obtained in various ways in this field. In one embodiment, optionally but not limited to this, Bs = 630 - 45[Mn] - 35[Si] - 30[Cr] - 20[Ni]. Here, [Mn], [Cr], [Ni], [Si] are the mass percentages of Mn, Cr, Ni, and Si in the continuous casting billet respectively. For example, if the mass percentage of Mn in the continuous casting billet is 0.45%, then [Mn] is the mass percentage 0.45.

[0112] In the above production method of one embodiment, on the basis of low-temperature heating, through the specific settings of controlled rolling and controlled cooling, not only can the precipitation strengthening effects of Nb, Ti, and Al be fully exerted to make up for the strength loss caused by low carbon. For example, the yield strength of the obtained steel plate is 380 - 450 MPa, and the tensile strength is 480 - 550 MPa; at the same time, the grain size can be refined by the low final rolling temperature. For example, the average ferrite grain size is 6 - 12 μm; in addition, through the large cooling rate and low final cooling temperature, the pearlite segregation and banded structure can be avoided. For example, the banded structure ≤ 0.5 grade.

[0113] In summary, steel with excellent mechanical properties, low-temperature toughness, and hydrogen resistance can be obtained, and the production method of the present application does not require subsequent heat treatment processes such as quenching and tempering, and has the advantages of short process flow, low production cost, and fast delivery.

[0114] Furthermore, the continuous casting billet heating process further includes: according to the thickness of the continuous casting billet, the heating duration is 1.1 - 1.5 min / mm.

[0115] That is, for each millimeter of the thickness of the continuous casting billet, the heating duration is 1.1 - 1.5 minutes. From another perspective, if the thickness of the continuous casting billet is h millimeters, the heating duration is (1.1 - 1.5)×h minutes.

[0116] Furthermore, the controlled cooling process can be carried out by water cooling on an ultra-rapid cooling system.

[0117] Specifically, in terms of the water volume control of the ultra-rapid cooling system, the headers in zones A, B, C, and D can be opened (i.e., all the headers of the ultra-rapid cooling system are opened), the water pressure is 0.23 - 0.27 MPa, the upper and lower water ratio of each header is 0.75 - 0.95, the flow rate of the headers in zone A is 520 - 560 L / s, the total flow rate of the four zones is 1600 - 3600 L / s, and the water temperature ≤ 28°C.

[0118] In addition, the speed of the cooling roller table of the ultra-rapid cooling system is 1.8 - 2.2 m / s, and the roller table acceleration is 0.005 m / s 2 , but this application is not limited thereto.

[0119] Furthermore, the production method further includes a natural cooling process after the controlled cooling process.

[0120] That is, the final cooling temperature is the water outlet temperature of the steel plate. After the steel plate exits the water (i.e., leaves the ultra-rapid cooling system), the steel plate is naturally cooled until it cools to room temperature.

[0121] Furthermore, the smelting process of the production method specifically includes: in sequence, hot metal pre-desulfurization, converter smelting, LF refining, and RH refining.

[0122] Among them, the RH refining process includes:

[0123] The temperature of the molten steel entering the station is 1620 - 1660°C. First, argon gas is blown from the bottom at a flow rate of 4 - 5 Nm 3 / h for 1 - 2 minutes. In this way, through the agitation of a large flow rate of argon gas at high temperature, obvious agitation of the molten steel can be achieved, which can promote the floating of large-sized initial inclusions; and it can also avoid long-term refining, thereby avoiding excessive temperature drop of the molten steel;

[0124] After blowing argon gas from the bottom, then vacuum standing is carried out. In this way, the oxygen content in the molten steel can be reduced to less than 0.003% in a vacuum environment, reducing the generation of secondary inclusions;

[0125] Then, calcium wire is fed after breaking the vacuum to adjust the Ca / S mass ratio in the molten steel to 2 - 4 and the Ca / Alt mass ratio to be above 0.06; such calcium treatment can make the Al2O3 in the molten steel completely form liquid or semi-liquid calcium aluminates, thereby promoting the spheroidization of inclusions;

[0126] After that, stir statically for 2 - 3 min. In this way, the conversion rate of Al2O3 into liquid and semi - liquid calcium aluminate can be increased. For example, the conversion rate can be increased to over 90%, and the formed liquid and semi - liquid calcium aluminate can be more easily modified into fine, spherical magnesium aluminate spinel (MgO·Al2O3). Thus, next, add magnesium alloy to adjust the mass ratio of Mg in the molten steel to 0.0005 - 0.0012%. The liquid and semi - liquid calcium aluminate can be fully modified into fine, spherical magnesium aluminate spinel (MgO·Al2O3). Finally, blow argon from the bottom to promote the uniform distribution of inclusions.

[0127] Among them, in the "feeding calcium wire under broken vacuum", the amount of calcium wire fed can be controlled within 200 - 400 m, but not limited to this.

[0128] Optionally, in RH refining, when "after blowing argon from the bottom, then statically vacuum - hold", hold for 25 - 30 min under a vacuum degree of ≤30 Pa. Such a high vacuum degree can further reduce the oxygen content in the water.

[0129] In one embodiment, in RH refining, the magnesium alloy is preferably a nickel - magnesium alloy with a magnesium content of 20 - 40%. This can not only fully modify the liquid and semi - liquid calcium aluminate into fine, spherical magnesium aluminate spinel (MgO·Al2O3), but also help reduce the magnesium vapor pressure and improve the magnesium recovery rate.

[0130] In one embodiment, in RH refining, when "finally, blow argon from the bottom", blow argon from the bottom at a flow rate of 1 Nm 3 / h or less, and then weakly stir for 15 - 20 min. In this way, the flow rate of blowing argon from the bottom is small, which can stabilize the distribution of inclusions.

[0131] Furthermore, in one embodiment, the hot metal pre - desulfurization includes: mixing magnesium powder and lime powder and then injecting them into the hot metal to control the mass ratio of S in the hot metal within 0.0012%; the injection amount of magnesium powder is 0.3 to 0.45 Kg per ton of hot metal.

[0132] In one embodiment, the converter smelting includes: making a slag with an alkalinity of 3.0 - 3.6 and controlling the mass ratio of P within 0.005%.

[0133] Specifically, during converter smelting, oxygen can also be blown from the top and argon from the bottom. By adjusting the blowing pressure, control the stirring intensity of the molten bath at 0.8 - 1.2 Nm 3 / min·t.

[0134] Moreover, during converter smelting, lime and dolomite can be used to make a slag with an alkalinity of 3.0 - 3.6.

[0135] In one embodiment, the LF refining includes: after the gold addition is completed, feeding aluminum wire, controlling the mass ratio of O within 0.0040%, and then tapping the steel.

[0136] The specific amount of aluminum wire fed is not limited in this application, and it is based on ensuring the Alt content in the final molten steel. For example, it can be 100 - 300 m of aluminum wire.

[0137] The above separately introduces an optional embodiment of hot metal pre - desulfurization, converter smelting, and LF refining. However, it can be understood that the present invention does not limit the specific operations of hot metal pre - desulfurization, converter smelting, and LF refining, and any feasible technology in the art can also be used for implementation.

[0138] Furthermore, the continuous casting includes: when casting, the superheat of the molten steel is 8 - 20 °C, and the casting speed is 0.1×L / F - 0.05 - 0.1×L / F + 0.05 m / s, where L and F are respectively the perimeter and area of the cross - section of the steel plate obtained by continuous casting.

[0139] In this way, by low - superheat casting and appropriate casting speed, the macrostructure quality of the steel plate (i.e., continuous casting billet) obtained by continuous casting can be greatly improved, and the segregation and central porosity in the continuous casting billet can be reduced.

[0140] In addition, the continuous casting also includes: a strong cooling mode can be adopted, and soft reduction is used at the end of continuous casting. This can avoid the segregation of inclusions during the solidification of molten steel and reduce the segregation ratings of carbon, manganese, phosphorus, sulfur, and inclusions of types A, B, C, D, and Ds.

[0141] Optionally, the thickness of the steel plate obtained in continuous casting can be below 230 mm, for example, the thickness is 210 - 230 mm.

[0142] The above introduces an optional embodiment of continuous casting. However, it can be understood that the present invention does not limit the specific operations of continuous casting, and any feasible continuous casting technology in the art can also be used for implementation.

[0143] In summary, the beneficial effects of one embodiment of the present invention are as follows:

[0144] (1) In terms of chemical composition, through low - carbon and low - alloy composition design, adding trace amounts of magnesium to modify inclusions, a large number of fine irreversible hydrogen traps are obtained in the steel to adsorb hydrogen atoms in lattice interstices or dislocations, reducing the diffusible hydrogen in the steel; more importantly, it also avoids the excessive 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 (i.e., does not cause excessive increase in hydrogen pressure), and thus improves the hydrogen resistance of the steel, such as reducing the incidence of defects such as hydrogen - induced plastic loss, hydrogen - induced cracks, and hydrogen - induced delayed fracture in hydrogen - transporting pipelines;

[0145] (2) Based on the chemical composition and combined with the specific settings of continuous casting billet heating, controlled rolling, and controlled cooling, not only can the precipitation strengthening effects of Nb, Ti, and Al be fully exerted to make up for the strength loss caused by low carbon, but also the grain size can be refined by a low finish rolling temperature. In addition, through a high cooling rate and a low finish cooling temperature, the segregation of pearlite and the formation of banded structure can be avoided. In summary, steel with excellent mechanical properties, low-temperature toughness, and hydrogen resistance can be obtained, and the production method of this application does not require subsequent heat treatment processes such as quenching and tempering, having the advantages of a short process flow, low production cost, and fast delivery.

[0146] Here, regarding the improvement of hydrogen resistance, an explanation will be given from the principle:

[0147] First, through trace magnesium, calcium aluminate is modified into magnesium aluminate spinel (MgO·Al2O3), and magnesium aluminate spinel exists in a solid state in molten steel and has no process of aggregation and growth. Therefore, the size of inclusions can be reduced, and alumina can be modified from an irregular shape to an approximately spherical shape. At the same time, the buoyancy of small particle inclusions in molten steel is small and it is difficult to float, resulting in an increase in the number of inclusions retained in the steel, forming a dispersion distribution of irreversible hydrogen traps.

[0148] Second, and MnS takes magnesium aluminate spinel as the nucleation core to form an Al-Ti-Mg-O-Mn-S-Ca composite inclusion with a soft outer layer and a hard inner layer, showing a "core-shell" and a similar "core-shell" structure. Furthermore, this "core-shell" structure inclusion can, on the one hand, avoid the splitting of the steel matrix by hard alumina and magnesium aluminate spinel, reducing 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 circular, reducing hydrogen-induced cracking caused by excessive hydrogen pressure due to hydrogen enrichment.

[0149] The detailed descriptions listed above are only specific descriptions of the feasible implementation modes of the present invention. The following will introduce the specific implementation modes of the present invention through several specific examples.

[0150] Example 1

[0151] This example provides a steel plate with a thickness of 15 mm.

[0152] The chemical composition of the steel plate in mass percentage includes: 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 inevitable impurities.

[0153] The production process of the steel plate is as follows:

[0154] Hot metal pre-desulfurization: After mixing magnesium powder and lime powder, they are blown into the hot metal for desulfurization. The blowing rate of magnesium powder is 0.39 Kg per ton of hot metal; the mass ratio of S in the hot metal when leaving the station is 0.0010%;

[0155] Converter smelting: Slag with an alkalinity of 3.2 is made, and the mass ratio of P after tapping is 0.0011%;

[0156] LF refining: After alloying is completed, 260 m of aluminum wire is fed in. The mass ratio of O is 0.0038%, and then tapping is carried out;

[0157] In RH refining: The temperature of the molten steel entering the station is 1650 °C. First, argon is blown from the bottom at a flow rate of 4.6 Nm 3 / h for 1.3 min, then it is left standing for 26 min under a vacuum degree of 25 Pa, and the active oxygen is 0.0023%; then the vacuum is broken and calcium wire is fed in to control the Ca / S mass ratio in the molten steel to be 2.7 and the Ca / Alt mass ratio to be 0.09; after that, it is stirred statically for 2.2 min, magnesium alloy is added to control the mass ratio of Mg in the molten steel, and finally argon is blown from the bottom at a flow rate of 1.2 Nm 3 / h, and weakly stirred for 16 min, then tapping is carried out;

[0158] Continuous casting: When casting, the superheat of the molten steel is 12 °C, and the casting speed is 0.95 m / s, obtaining a continuous casting billet with a thickness of 220 mm;

[0159] Heating of the continuous casting billet: The heating temperature is 1120 °C, and the heating time is 270 min;

[0160] Controlled rolling: After the continuous casting billet exits the heating furnace, two-stage rolling is carried out; the starting rolling temperature in the first stage is 1050 °C, and the thickness of the rolled steel plate after rolling is 65 mm; the starting rolling temperature in the second stage is 880 °C, the finishing rolling temperature is 820 °C, and the thickness of the rolled steel plate after rolling is 15 mm;

[0161] Controlled cooling and natural cooling: The steel plate obtained by controlled rolling is water-cooled on the ultra-rapid cooling system. The water inlet temperature is 780 °C, the cooling rate is 18 °C / s, and the final cooling temperature is 480 °C; after discharging water, natural cooling is carried out until room temperature.

[0162] The steel plate of this embodiment is subjected to microstructure detection, as Figure 1 , and the detection result is:

[0163] The microstructure is quasi-polygonal ferrite + pearlite. The volume fraction of quasi-polygonal ferrite is 92%, the volume fraction of pearlite is 8%, and the average grain size of quasi-polygonal ferrite is 10 μm;

[0164] The center segregation of the continuous casting billet is C1.0, the center porosity is 0.5, and the diameter of Al2O3 inclusions is grade 0.5; in the finished steel plate, the carbon segregation ratio is 101%, the manganese segregation ratio is 102%, the phosphorus segregation ratio is 102%, and the sulfur segregation ratio is 102%; the inclusions of types A, B, and C are all grade 0, and the inclusions of types D and Ds are all grade 0.5;

[0165] 98% of the non-metallic inclusions have a core-shell structure. The core is MgO·Al2O3, and the shell is MnS, CaS, and TiN, which coat the outer surface of the inner shell; for example Figure 2 , showing the non-metallic inclusions with a core-shell structure in the metallographic structure of the steel plate of this embodiment;

[0166] The density of non-metallic inclusions is 66 / mm 2 , the average diameter is 1.3 μm, the proportion of diameters ≥ 15 μm is 0%, the proportion of 10 - 15 μm is 0.2%, the proportion of 6 - 10 μm is 3.8%, and the proportion of 0 - 6 μm is 96.0%.

[0167] The mechanical properties of the steel plate of this embodiment are measured as follows: the yield strength is 405 MPa, the tensile strength is 515 MPa, the elongation is 42%, the impact energy KV2 at -40 °C is 410 J, and the shear area fraction of the DWTT drop hammer at -15 °C is 100%.

[0168] In addition, the hydrogen resistance of the steel plate of this embodiment is detected, and the hydrogen diffusion coefficient D is 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 , indicating excellent hydrogen resistance.

[0169] Example 2

[0170] This embodiment provides a steel plate with a thickness of 18 mm.

[0171] The chemical composition of the steel plate in mass percentage includes: 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 are iron and inevitable impurities.

[0172] The production process of the steel plate is as follows:

[0173] Hot metal pre-desulfurization: After mixing magnesium powder and lime powder, they are blown into the hot metal for desulfurization. The blowing rate of magnesium powder is 0.36 Kg per ton of hot metal; the mass ratio of S in the hot metal when leaving the station is 0.0009%;

[0174] Converter smelting: Slag with an alkalinity of 3.4 is made, and the mass ratio of P after tapping is 0.0018%;

[0175] LF refining: After alloying is completed, 280 m of aluminum wire is fed in. The mass ratio of O is 0.0036%, and then tapping is carried out;

[0176] During RH refining: The temperature of the molten steel entering the station is 1655 °C. First, argon is blown from the bottom at a flow rate of 4.5 Nm 3 / h for 1.5 min, then it is left standing for 27 min under a vacuum degree of 25 Pa, and the active oxygen is 0.0022%; then the vacuum is broken and calcium wire is fed in to adjust the Ca / S mass ratio in the molten steel to 2.8 and the Ca / Alt mass ratio to 0.07; then it is stirred statically for 2.6 min, magnesium alloy is added to adjust the mass ratio of Mg in the molten steel, and finally argon is blown from the bottom at a flow rate of 1.3 Nm 3 / h, and weakly stirred for 18 min, then tapping is carried out;

[0177] Continuous casting: When casting, the superheat of the molten steel is 15 °C, the casting speed is 0.95 m / s, and a continuous casting slab with a thickness of 220 mm is obtained;

[0178] Heating of continuous casting slab: The heating temperature is 1125 °C and the heating time is 290 min;

[0179] Controlled rolling: After the continuous casting slab exits the heating furnace, two-stage rolling is carried out; the starting rolling temperature in the first stage is 1040 °C and the thickness of the rolled steel plate after rolling is 80 mm; the starting rolling temperature in the second stage is 870 °C, the finishing rolling temperature is 815 °C, and the thickness of the rolled steel plate after rolling is 18 mm;

[0180] Controlled cooling and natural cooling: The steel plate obtained by controlled rolling is water-cooled on the ultra-fast cooling system. The water inlet temperature is 775 °C, the cooling rate is 16 °C / s, and the final cooling temperature is 460 °C; after discharging water, natural cooling is carried out until room temperature.

[0181] The microstructure of the steel plate in this example is detected, and the detection results are as follows:

[0182] The microstructure is quasi-polygonal ferrite + pearlite. The volume ratio of quasi-polygonal ferrite is 93%, the volume ratio of pearlite is 7%, and the average grain size of quasi-polygonal ferrite is 9 μm;

[0183] The center segregation of the continuous casting billet is C1.0, the center porosity is 0.5, and the diameter of Al2O3 inclusions is grade 0.5; in the finished steel plate, the carbon segregation ratio is 102%, the manganese segregation ratio is 102%; the phosphorus segregation ratio is 102%, the sulfur segregation ratio is 102%; the inclusions of types A, B, C, and D are all grade 0, and the inclusions of type Ds are all grade 0.5;

[0184] 99% of the non-metallic inclusions have a core-shell structure, the core is MgO·Al2O3, and the shell is MnS, CaS, TiN and coats the outer surface of the inner shell;

[0185] The density of non-metallic inclusions is 62 / mm 2 , the average diameter is 1.5μm, the proportion of diameters ≥15μm is 0%, the proportion of 10 - 15μm is 0.1%, the proportion of 6 - 10μm is 3.2%, and the proportion of 0 - 6μm is 96.7%.

[0186] The mechanical properties of the steel plate in this example were measured: the yield strength was 400MPa, the tensile strength was 505MPa, the elongation was 45%, the impact energy KV2 at -40°C was 390J, and the shear area fraction of the DWTT drop hammer at -15°C was 100%.

[0187] In addition, the hydrogen resistance of the steel plate in this example was detected, and the hydrogen diffusion coefficient D was 0.6×10 -6 cm 2 / s, the diffusible hydrogen concentration C0 on the cathode side was 1.8×10 -6 mol / cm 3 , showing excellent hydrogen resistance.

[0188] Comparative example

[0189] This comparative example provides a steel plate with a thickness of 18mm.

[0190] The chemical composition of the steel plate in mass percentage includes: 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.

[0191] The production process of the steel plate is as follows:

[0192] Hot metal pre-desulfurization: After mixing magnesium powder and lime powder, it is blown into the hot metal for desulfurization, and the blowing amount of magnesium powder is 0.42 Kg per ton of hot metal; the mass fraction of S in the hot metal when leaving the station is 0.0009%;

[0193] Converter smelting: Make a slag with an alkalinity of 3.3, and the mass fraction of P after tapping is 0.002%;

[0194] LF refining: After alloying is completed, 250 m of aluminum wire is fed in, the mass fraction of O is 0.0038%, and then the steel is tapped.

[0195] During RH refining: The temperature of the molten steel entering the station is 1658 °C. First, argon is blown from the bottom at a flow rate of 4.6 Nm 3 / h for 1.4 min, then it is left standing for 22 min under a vacuum degree of 26 Pa, and the activity oxygen is 0.0028%; Then the vacuum is broken and calcium wire is fed in to adjust the Ca / S mass ratio in the molten steel to 3.6 and the Ca / Alt mass ratio to 0.1; After that, argon is blown from the bottom at a flow rate of 1.2 Nm 3 / h for weak stirring for 19 min, and then the steel is tapped.

[0196] Continuous casting: During casting, the superheat of the molten steel is 12 °C, the casting speed is 1.00 m / s, and a continuous casting slab with a thickness of 220 mm is obtained.

[0197] Heating of the continuous casting slab: The heating temperature is 1180 °C and the heating time is 295 min.

[0198] Controlled rolling: After the continuous casting slab exits the heating furnace, two-stage rolling is carried out; In the first stage, the rolling start temperature is 1045 °C and the thickness of the rolled steel plate is 80 mm; In the second stage, the rolling start temperature is 870 °C, the finishing rolling temperature is 820 °C, and the thickness of the rolled steel plate is 18 mm.

[0199] Controlled cooling and natural cooling: The steel plate obtained by controlled rolling is water-cooled on the ultra-fast cooling system, the water inlet temperature is 776 °C, the cooling rate is 17 °C / s, and the final cooling temperature is 455 °C; After exiting the water, natural cooling is carried out until room temperature.

[0200] The steel plate of this example is subjected to microstructure detection, and the detection results are as follows:

[0201] The microstructure is quasi-polygonal ferrite + pearlite. The volume fraction of quasi-polygonal ferrite is 91%, the volume fraction of pearlite is 9%, and the average grain size of quasi-polygonal ferrite is 12 μm.

[0202] The center segregation of the continuous casting slab is C1.0, the center porosity is 0.5, and the diameter of Al2O3 inclusions is grade 0.5; 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%; The inclusions of types A, B, C, and D are all grade 0, and the inclusions of type Ds are all grade 0.5.

[0203] The non-metallic inclusions are single Al2O3, MnS, CaS, TiN.

[0204] The density of non-metallic inclusions is 42 pieces / mm 2, with an average diameter of 4.5 μm, the proportion of diameters ≥ 15 μm is 1.6%, the proportion of 10 - 15 μm is 12.3%, the proportion of 6 - 10 μm is 16.1%, and the proportion of 0 - 6 μm is 70.0%.

[0205] The mechanical properties of the steel plate in this embodiment were measured. The yield strength is 395 MPa, the tensile strength is 495 MPa, the elongation is 36%, the impact energy KV2 at -40 °C is 360 J, and the drop weight tear test (DWTT) shear area fraction at -15 °C is 98%.

[0206] In addition, the hydrogen resistance of the steel plate in this embodiment was detected, and the hydrogen diffusion coefficient D was obtained as 15×10 -6 cm 2 / s, and the diffusible hydrogen concentration C0 on the cathode side was 22×10 -6 mol / cm 3 , indicating poor hydrogen resistance.

Claims

1. A production method of steel for hydrogen pipelines, characterized in that, The production method includes: Preparing a continuous casting billet through smelting and continuous casting; the chemical composition of the continuous casting billet 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 balance is iron and inevitable impurities; Continuous casting billet heating: The heating temperature is T MnS - 10°C to T MnS + 20°C, where T MnS = 11625 / (5.02 - lg([Mn]×[S])) - 272.15; Controlled rolling: After the continuous casting billet exits the heating furnace, two-stage rolling is carried out; the starting rolling temperature in the first stage is T MnS -130 °C to T MnS -30 °C, and the starting rolling temperature in the second stage is A r3 +20 °C to A r3 +60 °C, and the finishing rolling temperature is A r3 -30 °C to A r3 , and the thickness of the steel plate after rolling in the first stage is more than 4 times that of the steel plate after rolling in the second stage; A r3 = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni]; Controlled cooling: Cool the steel plate obtained by controlled rolling, with the water inlet temperature ≥ A r3 -80 °C, the cooling rate is 10 - 20 °C / s, the final cooling temperature is Bs - 140 °C to Bs - 70 °C, Bs = 630 - 45[Mn] - 35[Si] - 30[Cr] - 20[Ni]; In the formula, [C], [Mn], [Cu], [Cr], [Ni], [Si] are the mass percentages of C, Mn, Cu, Cr, Ni, and Si in the continuous casting billet respectively.

2. The production method of the steel for hydrogen transmission pipeline according to claim 1, characterized in that Heating the continuous casting billet: according to the thickness of the continuous casting billet, the heating duration is 1.1 - 1.5 min / mm.

3. The production method of the steel for hydrogen transmission pipeline according to claim 1, characterized in that The smelting includes: in sequence, hot metal pre-desulfurization, converter smelting, LF refining, and RH refining; among them, The RH refining process includes: the temperature of the molten steel entering the station is 1620 - 1660 °C. First, argon is blown from the bottom at a flow rate of 4 - 5 Nm 3 / h for 1 - 2 minutes, then it is vacuum - static, and then the vacuum is broken and a 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; after that, it is statically stirred for 2 - 3 minutes, magnesium alloy is added to control the Mg mass fraction in the molten steel to be 0.0005 - 0.0012%, and finally argon is blown from the bottom.

4. The production method of the pipeline steel according to claim 3, characterized in that, The re-vacuum standing includes: standing for 25 - 30 min under a vacuum degree of ≤ 30 Pa.

5. The production method of the pipeline steel according to claim 3, characterized in that, The last argon bottom blowing described above includes: bottom blowing argon at a flow rate of 1 Nm 3 / h or less.

6. The production method of the pipeline steel according to claim 3, characterized in that The hot metal pre-desulfurization includes: after mixing magnesium powder and lime powder, spraying them into the hot metal to control the mass ratio of S in the hot metal within 0.0012%; the spraying amount of magnesium powder is 0.35 - 0.45 Kg per ton of hot metal.

7. The production method of the pipeline steel according to claim 3, characterized in that The converter smelting includes: making a slag with an alkalinity of 3.0 - 3.6 and controlling the mass ratio of P within 0.005%.

8. The production method of the pipeline steel according to claim 3, characterized in that The LF refining includes: after the alloying is completed, feeding aluminum wire to control the mass ratio of O within 0.0040%, and then tapping the steel.

9. The production method of the pipeline steel according to claim 1, characterized in that The continuous casting includes: when casting, the superheat of the molten steel is 8 - 20 °C, and the casting speed is 0.1×L / F - 0.05 - 0.1×L / F + 0.05 m / s, where L and F are the perimeter and area of the cross-section of the continuous casting billet respectively.

10. A steel pipe for pipelines, characterized in that, The steel is prepared by the production method of the pipeline steel according to claim 1, and the chemical composition of the steel 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. The steel is a steel plate with a thickness ≤ 25 mm, a hydrogen diffusion coefficient D ≤ 1.5×10 -6 cm 2 / s, a yield strength of 380 - 450 MPa, a tensile strength of 480 - 550 MPa, an elongation ≥ 40%, a yield ratio ≤ 0.88, an impact energy at -40°C ≥ 300 J, and a DWTT drop weight shear area ratio at -15°C ≥ 95%.

11. A pipeline steel, characterized in that, The steel is prepared by the production method of the pipeline steel described in claim 1. The chemical composition of the steel, by mass percentage, 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%, and the balance is iron and inevitable impurities; The steel is a steel plate with a thickness ≤ 25 mm, the diffusible free hydrogen concentration C0 on the cathode side ≤ 3.0×10 -6 mol / cm 3 , yield strength 380 - 450 MPa, tensile strength 480 - 550 MPa, elongation ≥ 40%, yield ratio ≤ 0.88, impact energy at -40°C ≥ 300 J, DWTT drop weight shear area ratio at -15°C ≥ 95%.

12. The pipeline steel according to claim 10 or 11, characterized in that, Some or all of the non-metallic inclusions in the steel have a core-shell structure. The core is MgO·Al2O3, and the shell is MnS, CaS, TiN and coats the outer surface of the inner shell.

13. The pipeline steel according to claim 10 or 11, characterized in that, The density of non-metallic inclusions in the steel is 40 - 70 pieces / mm 2 , the average diameter is ≤2μm, the proportion with a diameter ≥15μm is 0%, the proportion with a diameter of 10 - 15μm is ≤1%, the proportion with a diameter of 6 - 10μm is ≤5%, and the proportion with a diameter of 0 - 6μm is ≥94%.

14. The pipeline steel according to claim 10 or 11, characterized in that, The chemical composition of the steel, by mass percentage, also satisfies any one or both of the following two conditions: 0.02 ≤ Mg / Alt ≤ 0.04; 0.01% ≤ Mg×Alt / S ≤ 0.06%.

15. The pipeline steel according to claim 10 or 11, characterized in that, The center segregation of the steel is ≤ C1.5 or B1.0, A-type segregation is not allowed, the center porosity is ≤ 0.5 grade, and the diameter of Al2O3 inclusions is ≤ 1.0 grade; 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 inclusions of types A, B, and C in the steel plate are all ≤ 0.5 grade, and the inclusions of types D and Ds are all ≤ 1 grade.

16. The pipeline steel according to claim 10 or 11, characterized in that, The microstructure of the steel is quasi-polygonal ferrite + pearlite. The volume fraction of quasi-polygonal ferrite is ≥ 90%, the volume fraction of pearlite is ≤ 10%, the average grain size of quasi-polygonal ferrite is 6 - 12 μm, and the banded structure is ≤ 0.5 grade.

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