A steel pipe for offshore machinery made of niobium-containing low-carbon manganese steel and its manufacturing method

Abstract

The present invention provides a steel pipe for offshore machinery made of niobium-containing low-carbon manganese steel, which comprises chemical components in the following mass percentages: C: 0.08-0.16%, Si: 0.20-0.50%, Mn: 1.10-1.70%, P≤0.025%, S≤0.010%, V≤0.10%, Nb≤0.10%, Ti≤0.10%, Ni≤0.30%, Mo≤0.10%, Cu≤0.20%, and the balance is Fe and inevitable trace impurities. The steel pipe for offshore machinery made of niobium-containing low-carbon manganese steel of the present invention has the properties of low production cost, high strength, easy machining and cutting, excellent cold working ductility, corrosion resistance, easy welding, etc.

Description

Technical Field

[0001] The present invention relates to the technical field of corrosion-resistant offshore mechanical pipes, and particularly to a niobium-containing low-carbon manganese steel offshore mechanical pipe and a manufacturing method thereof. Background Art

[0002] At present, fossil energy such as petroleum and natural gas still occupies the main position in the world's energy system. In recent years, the recovery and slow growth of the world economy have greatly driven the rapid growth of the demand for fossil energy such as petroleum and natural gas. Land-based petroleum and natural gas resources have been exploited for hundreds of years and are becoming increasingly depleted. Therefore, humans have gradually turned their attention to the field of offshore oil and gas.

[0003] The characteristics of offshore oil and gas fields are that most petroleum or natural gas contains certain acidic gases such as H 2 S, which are likely to cause corrosion to pipeline transportation and various oilfield pipeline mechanical components. The main corrosion methods are two types: HIC (hydrogen-induced cracking) and SSC (stress corrosion). At present, anti-HIC and SSC pipeline pipes are widely used, but there are few reports on the application of offshore mechanical pipes that are easy to process and have excellent cold-working ductility.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a niobium-containing low-carbon manganese steel offshore mechanical pipe and a manufacturing method thereof. The niobium-containing low-carbon manganese steel offshore mechanical pipe has the properties of low production cost, high strength, easy machining and cutting, excellent cold-working ductility, corrosion resistance, and easy welding.

[0006] In order to achieve the above purpose, the present invention provides the following technical solutions:

[0007] The present invention provides a niobium-containing low-carbon manganese steel offshore mechanical pipe, which includes the following chemical components in mass percentage: C: 0.08 - 0.16%, Si: 0.20 - 0.50%, Mn: 1.10 - 1.70%, P ≤ 0.025%, S ≤ 0.010%, V ≤ 0.10%, Nb ≤ 0.10%, Ti ≤ 0.10%, Ni ≤ 0.30%, Mo ≤ 0.10%, Cu ≤ 0.20%, and the balance is Fe and inevitable trace impurities.

[0008] Furthermore, it includes chemical components with the following mass percentages: C: 0.10 - 0.16%, Si: 0.25 - 0.45%, Mn: 1.10 - 1.45%, P ≤ 0.02%, S ≤ 0.008%, V: 0.03 - 0.08%, Nb: 0.01 - 0.08%, Ti: 0.02 - 0.08%, Ni: 0.02 - 0.20%, Mo: 0.01 - 0.1%, Cu: 0.01 - 0.20%, and the balance is Fe and unavoidable impurities; preferably, it includes chemical components with the following mass percentages: C: 0.11 - 0.15%, Si: 0.25 - 0.40%, Mn: 1.20 - 1.45%, P: 0.015 - 0.02%, S: 0.006 - 0.008%, V: 0.05 - 0.08%, Nb: 0.04 - 0.07%, Ti: 0.04 - 0.08%, Ni: 0.03 - 0.05%, Mo: 0.01%, Cu: 0.04 - 0.06%, and the balance is Fe and unavoidable impurities.

[0009] Furthermore, in the chemical components of the steel pipe, the mass content ratio of C and Nb is (1.5 - 8):1;

[0010] And / or, in the chemical components of the steel pipe, V + Nb + Ti ≤ 0.25%; preferably, V + Nb + Ti ≤ 0.20%;

[0011] And / or, the wall thickness of the steel pipe is 5 - 76.2 mm;

[0012] And / or, the yield strength Rp0.2 of the steel pipe is 400 - 550 MPa, the tensile strength Rm ≥

[0013] 540 MPa, the elongation rate ≥ 30%, the surface hardness ≤ 258 HBW, and the longitudinal full-size impact ≥ 80 J.

[0014] In addition, the present invention also provides a manufacturing method for the above-mentioned niobium-containing low-carbon manganese steel offshore machinery steel pipe, including the following steps:

[0015] (1) Smelt to obtain molten steel, and the chemical components of the molten steel are by weight percentage: C: 0.08 - 0.16%, Si: 0.20 - 0.50%, Mn: 1.10 - 1.70%, P ≤ 0.025%, S ≤ 0.010%, V ≤ 0.10%, Nb ≤ 0.10%, Ti ≤ 0.10%, Ni ≤ 0.30%, V + Nb + Ti ≤ 0.25%, Mo ≤ 0.10%, Cu ≤ 0.20%, and the balance is Fe and unavoidable trace inclusions;

[0016] (2) Continuously cast the molten steel obtained in step (1) to obtain round tube blanks;

[0017] (3) The round tube blanks obtained in step (2) are successively subjected to ring furnace heating, tube rolling, cooling on the cooling bed, and preliminary straightening to obtain semi-finished tubes.

[0018] (4) The semi-finished tubes obtained in step (3) are heated to 840 - 940 °C and then quenched with water. After the steel tubes are cooled to below 50 °C, tempering is carried out. After tempering, warm straightening is carried out to obtain the niobium-containing low-carbon manganese steel offshore mechanical steel tubes.

[0019] Further, in step (1), the smelting includes electric furnace steelmaking and LF furnace refining.

[0020] Preferably, during the electric furnace steelmaking, the addition amount of the deoxidizer is 2.0 - 6.5 kg / t, and the addition amount of Al blocks is 2.5 - 4.5 kg / t; during the LF furnace refining, the usage amount of the synthetic slag is ≥500 kg / t.

[0021] Preferably, in step (1), ferrotitanium alloy is added within 4 - 15 minutes before tapping from the LF furnace, and the addition amount is 0.6 kg / t.

[0022] Preferably, at the end of the LF furnace refining, 0.3 - 0.9 kg / t of Ca-Si wire or 0.15 - 0.25 kg / t of pure Ca wire is fed before the quiet blowing; more preferably, the quiet blowing time is ≥18 min.

[0023] Preferably, in step (1), the tapping temperature of the LF furnace is 1560 - 1595 °C.

[0024] Further, during the continuous casting in step (2), the tundish temperature is controlled at 1540 - 1600 °C.

[0025] Further, in step (3), the heating temperature of the ring furnace is controlled at 1220 - 1280 °C.

[0026] And / or, the tube rolling in step (3) successively includes piercing, ASSSEL rolling or continuous rolling, and stretch reducing.

[0027] And / or, the outer diameter tolerance of the semi-finished tubes obtained in step (3) is -0.5% - +1% D, the wall thickness tolerance is ≥ -12.5% t, the ovality of the tube body is ≤80%, the wall thickness non-uniformity is ≤16%, the straightness is controlled such that the overall straightness of the whole length is ≤0.2% L, and the straightness at the tube end is ≤2.5 mm within the 1.5 m length range at the tube end.

[0028] Further, in step (4), during the process of heating the semi-finished tubes to 840 - 940 °C, there are three heating sections. Among them, the heating temperature of the first heating section is controlled at 840 - 900 °C, the heating temperature of the second heating section is controlled at 900 - 920 °C, and the temperature of the third heating section, i.e., the soaking section, is controlled at 920 - 940 °C.

[0029] Further, in the step (4), the quenching is carried out by means of water quenching with external spraying and internal axial flow. Among them, the external spraying time is 15 - 60 seconds, and the internal axial flow time is 15 - 60 seconds; the external spraying water volume is 600 - 2500 m 3 / h, and the internal axial flow water volume is 400 - 1200 m 3 / h.

[0030] Further, in the step (4), the temperature at the end of temperature straightening is controlled to be greater than 450 °C;

[0031] And / or, the tempering heating temperature range is 540 - 640 °C, and the tempering time is (3 - 4 min / mm) × D, where D is the wall thickness of the steel pipe in mm.

[0032] Compared with the prior art, the technical solution of the present invention has at least the following technical effects:

[0033] (1) For the niobium-containing low-carbon manganese steel corrosion-resistant offshore mechanical steel pipe according to the present invention, the alloy content is relatively low, and the production cost is relatively low.

[0034] (2) For the niobium-containing low-carbon manganese steel corrosion-resistant offshore mechanical steel pipe according to the present invention, the surface hardness is relatively low, and it is easy to carry out cutting processing.

[0035] (3) For the niobium-containing low-carbon manganese steel corrosion-resistant offshore mechanical steel pipe according to the present invention, the strength is relatively high, and the ductility of metal cold processing is excellent, which is conducive to metal cold processing such as cold spinning or cold expanding.

[0036] (4) For the niobium-containing low-carbon manganese steel corrosion-resistant offshore mechanical steel pipe according to the present invention, it has excellent HIC and SSC resistance.

[0037] (5) In the present invention, after the steel pipe is tempered, temperature straightening is beneficial to reducing the residual stress of the material and reducing or avoiding the deformation of the steel pipe after processing; at the same time, controlling the temperature at the end of temperature straightening can ensure that most of the oxide scale on the outer surface of the steel pipe is straightened off. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The specification drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. Among them:

[0039] Figure 1 It is the microstructure picture of the steel pipe after quenching and tempering heat treatment in Example 1 of the present invention;

[0040] Figure 2 It is the microstructure picture of the steel pipe after quenching and tempering heat treatment in Example 2 of the present invention;

[0041] Figure 3 It is the microstructure picture of the steel pipe after quenching and tempering heat treatment in Embodiment 3 of the present invention;

[0042] Figure 4 It is the external shape diagram of the steel pipe in Embodiment 1 of the present invention after the HIC test;

[0043] Figure 5 It is the external shape diagram of the steel pipe in Embodiment 2 of the present invention after the HIC test;

[0044] Figure 6 It is the external shape diagram of the steel pipe in Embodiment 3 of the present invention after the HIC test. Detailed implementation manners

[0045] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Those skilled in the art should understand that the described embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention. 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. The process parameters without specific conditions noted in the following embodiments are usually in accordance with conventional conditions.

[0046] In the present invention, the endpoints and any values within the disclosed ranges are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0047] According to the first aspect of the present invention, a steel pipe for offshore machinery made of niobium-containing low-carbon manganese steel is provided, including chemical components with the following mass percentages: C: 0.08 - 0.16%, Si: 0.20 - 0.50%, Mn: 1.10 - 1.70%, P ≤ 0.025%, S ≤ 0.010%, V ≤ 0.10%, Nb ≤ 0.10%, Ti ≤ 0.10%, Mo ≤ 0.10%, Cu ≤ 0.20%, Ni ≤ 0.30%, and the balance is Fe and unavoidable trace impurities.

[0048] In the present invention, the composition design principle of the steel pipe is to use appropriate amounts of C and Mn, and by adding trace microalloying elements such as Nb, V, and Ti and their synergistic effects, and at the same time adding a small amount of elements such as Mo, Cu, and Ni, combined with a specific quenching and tempering process, so as to finally ensure various mechanical properties, especially excellent HIC resistance and SSC resistance.

[0049] As a preferred embodiment, the steel pipe for offshore machinery made of niobium-containing low-carbon manganese steel of the present invention comprises chemical components with the following mass percentages: C: 0.10 - 0.16%, Si: 0.25 - 0.45%, Mn: 1.10 - 1.45%, P ≤ 0.02%, S ≤ 0.008%, V: 0.03 - 0.08%, Nb: 0.01 - 0.08%, Ti: 0.02 - 0.08%, Ni: 0.02 - 0.20%, Mo: 0.01 - 0.1%, Cu: 0.01 - 0.20%, and the balance is Fe and inevitable impurities; more preferably, it comprises chemical components with the following mass percentages: C: 0.11 - 0.15%, Si: 0.25 - 0.40%, Mn: 1.20 - 1.45%, P: 0.015 - 0.02%, S: 0.006 - 0.008%, V: 0.05 - 0.08%, Nb: 0.04 - 0.07%, Ti: 0.04 - 0.08%, Ni: 0.03 - 0.05%, Mo: 0.01%, Cu: 0.04 - 0.06%, and the balance is Fe and inevitable impurities.

[0050] As a preferred embodiment, in the chemical components of the steel pipe, the content ratio of C and Nb is (1.5 - 8):1 (for example: 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, 5.5:1, 6:1, 6.5:1, 7:1, 7.5:1, 8:1 and the numerical range between any two points).

[0051] Optionally, in the chemical components of the steel pipe, V + Nb + Ti ≤ 0.25%; preferably, V + Nb + Ti ≤ 0.20%.

[0052] The addition principle of the main basic elements of the steel pipe for offshore machinery made of niobium-containing low-carbon manganese steel of the present invention is as follows:

[0053] C: C is the lowest-cost and most basic strengthening element in steel. It can improve the strength of steel through solid solution strengthening and precipitation strengthening. However, too high carbon content has an adverse effect on the toughness and welding performance of steel. Therefore, the development trend of steel pipes for offshore machinery is to reduce the C content as much as possible. However, considering the matching relationship between strength and toughness, the C content is controlled not to exceed 0.16% (for example, 0.10%, 0.12%, 0.14%, 0.15%).

[0054] Mn: Mn can improve the strength of steel through solid solution strengthening. Mn is added to pipes for marine engineering machinery to compensate for the decrease in strength caused by the decrease in C content. Mn can expand the γ phase region and reduce the γ→α phase transition temperature of steel, which is conducive to obtaining fine phase transformation products, thereby improving the toughness of steel and reducing the toughness-brittleness transition temperature. Mn is also an element that improves the hardenability of steel. Considering that Mn segregation is unfavorable to the anti-HIC performance and taking into account the strength requirements, the Mn content in the present invention is designed to be 1.10-1.70% (for example, 1.15%, 1.20%, 1.30%, 1.40%, 1.50%, 1.60%) to reduce the central segregation caused by the high Mn content, and the light reduction technology is used during continuous casting.

[0055] Nb: Nb is one of the most important microalloying elements in pipes for offshore machinery, and its effect of grain refinement is very obvious. Through the solid solution of Nb and the precipitation of Nb during hot rolling, the recovery and recrystallization of deformed austenite can be reduced, so that the steel has high strength and high toughness. After many tests, it has been proved that the ratio of C and Nb content has a certain influence on the performance of steel pipes. If the Nb content is too high relative to the C content, the cost will increase, the effect of grain refinement will not be significantly improved, and Nb element enrichment will also be caused; on the contrary, if it is too low, the effect of grain refinement will not be significantly improved, affecting the grain size and uniformity of the organization. Therefore, it is necessary to control the mass content ratio of C and Nb within the range of (1.5-8):1. Furthermore, through the above relationship between C and Nb content, the Nb content range is determined to be Nb≤0.10% (for example, 0.04%, 0.05%, 0.07%, 0.08%).

[0056] V: has precipitation strengthening and grain refining effects. When the three microalloying elements Nb, V and Ti act in combination, V mainly plays a precipitation strengthening role. In the present invention, the V content is designed to be V≤0.10% (for example, 0.04%, 0.05%, 0.07%, 0.08%).

[0057] Ti: Ti is a nitrogen-fixing element. The content stoichiometric ratio of Ti / N is 3, that is, about 0.02% of Ti can fix N below 60 ppm in steel. During the continuous casting of tube billets, TiN precipitation phases can be formed, which can also effectively prevent the growth of austenite grains during the heating of tube billets, is beneficial to increasing the solid solubility of Nb in austenite, and can also improve the impact toughness of the heat-affected zone of welding. It is an indispensable element for tubes used in offshore machinery. However, excessive Ti will form large TiN inclusions, affecting the impact toughness performance. Therefore, in the present invention, the Ti content is controlled not to exceed 0.10% (for example, 0.04%, 0.05%, 0.07%, 0.08%). In addition, the content of the three microalloying elements V+Nb+Ti can be controlled below 0.25%; more preferably, the content of the three microalloying elements V+Nb+Ti can be further controlled below 0.2%, which not only saves raw material costs but also can obtain steel with more excellent properties.

[0058] Ni: Nickel can improve the corrosion resistance of alloys and can resist the erosion of various corrosive media such as acids, alkalis, and salts. Therefore, adding nickel to alloys can improve their corrosion resistance. Nickel strengthens ferrite and refines pearlite in steel. The overall effect is to increase strength, and the impact on plasticity is not significant. While increasing the strength of steel, nickel has less damage to the toughness, plasticity, and other process properties of steel compared to other alloying elements.

[0059] Mo: Molybdenum elements can form strengthening phases with Fe, improve the continuity of the grain boundary interface of steel, and increase the shear strength of the grain boundary equilibrium interface, thereby enhancing the hardness and strength of steel. Mo elements can form strengthening phases and precipitates, hindering the movement and expansion of lattice defects, thereby improving the fatigue resistance of steel. Mo elements can improve the corrosion resistance of steel, mainly because Mo can form a protective layer to prevent the erosion of oxygen, chlorine, sulfur, etc. to steel. Molybdenum increases the hardenability of steel and the tempering stability of steel. When present as a single alloying element, it increases the temper brittleness of steel; when coexisting with manganese, etc., molybdenum can reduce or inhibit the temper brittleness caused by other elements.

[0060] Cu: Copper can strengthen the strength and hardness of steel, especially low-carbon steel. This is mainly because copper can improve the cold work hardening ability and tempering stability of steel, promoting grain boundary retardation and the formation of dispersed precipitation phases. This can significantly increase the tensile strength, yield strength, and hardness of steel while maintaining the good plasticity and toughness of steel. Copper forms copper compounds (such as Cu2S, etc.) in steel, and these compounds have high resistance to oxidation corrosion and sulfide corrosion. Therefore, copper can improve the corrosion resistance of steel, especially in corrosive media containing sulfides, such as seawater, etc. In addition, copper can also reduce the tendency of intergranular corrosion and stress corrosion cracking of steel.

[0061] S and P: S and P are harmful impurity elements inevitable in the pipes for marine engineering machinery. The lower, the better. The morphology of sulfides can be changed through ultra-low sulfur and Ca treatment, which can endow the pipes for marine engineering machinery with higher impact toughness.

[0062] In the steel of the present invention, other inevitable impurities involved include: A-type, B-type, C-type and D-type coarse and fine inclusions. The impurity content is preferably for the coarse type: A-type ≤ 2.0 grade, B-type ≤ 2.0 grade, C-type ≤ 1.0 grade, D-type ≤ 1.0 grade, and A + B + C + D for the coarse type ≤ 6.0; the impurity content is preferably for the fine type: A-type ≤ 2.0 grade, B-type ≤ 2.0 grade, C-type ≤ 1.0 grade, D-type ≤ 1.0 grade, and A + B + C + D for the fine type ≤ 6.0.

[0063] As a preferred embodiment, the thickness of the steel pipe for marine engineering machinery made of the niobium-containing low-carbon manganese steel of the present invention is 5 - 76.2 mm;

[0064] Preferably, the yield strength Rp0.2 of the steel pipe is 400 - 550 MPa, the tensile strength Rm ≥ 540 MPa, the elongation ≥ 30%, the surface hardness ≤ 258 HBW, and the longitudinal full-size impact ≥ 80 J.

[0065] According to the second aspect of the present invention, there is provided a manufacturing method of the above-mentioned steel pipe for marine engineering machinery made of niobium-containing low-carbon manganese steel, comprising the following steps:

[0066] (1) Smelting to obtain molten steel, and the chemical composition of the molten steel is by weight percentage: C: 0.08 - 0.16%, Si: 0.20 - 0.50%, Mn: 1.10 - 1.70%, P ≤ 0.025%, S ≤ 0.010%, V ≤ 0.10%, Nb ≤ 0.10%, Ti ≤ 0.10%, Ni ≤ 0.30%, Mo ≤ 0.10%, Cu ≤ 0.20%, with the balance being Fe and containing inevitable trace inclusions;

[0067] (2) Making the molten steel obtained in step (1) into round tube blanks through continuous casting;

[0068] (3) Sequentially heating the round tube blanks obtained in step (2) in a ring furnace, rolling the tubes, cooling on a cooling bed, and pre-straightening to obtain semi-finished tubes;

[0069] (4) Heating the semi-finished tubes obtained in step (3) to 840 - 940 °C and then quenching with water, tempering after the steel pipe is cooled to below 50 °C, and performing warm straightening after tempering to obtain the steel pipe for marine engineering machinery made of the niobium-containing low-carbon manganese steel of the present invention.

[0070] As a preferred embodiment, in step (1), the smelting includes electric furnace steelmaking and LF furnace refining;

[0071] Preferably, during the electric furnace steelmaking, the addition amount of the deoxidizer is 2.0 - 6.5 kg / t, and the addition amount of Al blocks is 2.5 - 4.5 kg / t; during the refining in the LF furnace, the consumption amount of the synthetic slag is ≥500 kg / t; more preferably, the deoxidizer is a silicon carbide deoxidizer; the components and their mass ratios of the synthetic slag are: CaO is 50%, Al 2 O 3 is 20%, and SiO 2 is 30%.

[0072] Preferably, in the step (1), ferro-titanium alloy is added into the molten steel within 4 - 15 minutes before tapping from the LF furnace until the titanium content in the molten steel meets the requirements of the steel composition. For example, the addition amount is 0.6 kg / t; the purpose of adding ferro-titanium alloy is to ensure the Ti content. Further, adding it before tapping is more convenient for dissolution and uniform stirring and diffusion.

[0073] Preferably, at the end of the refining in the LF furnace, 0.3 - 0.9 kg / t of Ca-Si wire or 0.15 - 0.25 kg / t of pure Ca wire is fed before the quiet blowing; more preferably, the quiet blowing time is ≥18 min;

[0074] Preferably, in the step (1), the tapping temperature of the LF furnace is 1560 - 1595 °C.

[0075] As a preferred embodiment, during the continuous casting in the step (2), the tundish temperature is controlled at 1540 - 1600 °C;

[0076] As a preferred embodiment, in the step (3), the heating temperature of the ring furnace is controlled at 1220 - 1280 °C;

[0077] As a preferred embodiment, the tube rolling in the step (3) includes piercing, ASSSEL rolling or continuous rolling, and stretch reducing in sequence.

[0078] As a preferred embodiment, the outer diameter tolerance of the green tube obtained in the step (3) is -0.5% - +1% D, the wall thickness tolerance is ≥ -12.5% t, the ovality of the tube body is ≤80%, the wall thickness non-uniformity is ≤16%, the straightness is controlled such that the overall straightness is ≤0.2% L, and the straightness at the tube end is ≤2.5 mm within the 1.5 m length range at the tube end.

[0079] As a preferred embodiment, in the step (4), when heating the capillary tube to 840 - 940 °C, it includes three heating sections. Among them, the heating temperature of the first heating section is controlled at 840 - 900 °C, and the heating time is (30 - 50 s / mm) × D, where D is the wall thickness of the steel pipe in mm; the heating temperature of the second heating section is controlled at 900 - 920 °C, and the heating time is (40 - 60 s / mm) × D, where D is the wall thickness of the steel pipe in mm; the temperature of the third heating section, i.e., the soaking section, is controlled at 920 - 940 °C, and the heating time is (50 - 70 s / mm) × D, where D is the wall thickness of the steel pipe in mm. The present invention optimizes the furnace temperature system of the quenching furnace, and uses segmented temperature gradient heating for temperature control, reducing the oxidation of the inner and outer surfaces of the steel pipe in the heating section.

[0080] As a preferred embodiment, in the step (4), the quenching is carried out by means of water quenching with external spraying and internal axial flow. Among them, the external spraying time is 15 - 60 seconds, and the internal axial flow time is 15 - 60 seconds; the external spraying water volume is 600 - 2500 m 3 / h, and the internal axial flow water volume is 400 - 1200 m 3 / h.

[0081] As a preferred embodiment, in the step (4), the temperature at the end of warm straightening is controlled to be greater than 450 °C; ensuring that the temperature at the end of warm straightening is greater than 450 °C can ensure that most of the scale on the outer surface of the steel pipe is straightened off.

[0082] As a preferred embodiment, in the step (4), the tempering heating temperature range is 540 - 640 °C, and the tempering time is (3 - 4 min / mm) × D, where D is the wall thickness of the steel pipe in mm.

[0083] The present invention will be described in detail below in conjunction with the specification drawings and embodiments of the present invention. Each example is provided by way of explanation of the present invention rather than limitation of the present invention. In fact, those skilled in the art will clearly understand that modifications and variations can be made to the present invention without departing from the scope or spirit of the present invention. For example, features shown or described as part of one embodiment can be used in another embodiment to yield yet another embodiment. Therefore, it is desirable that the present invention includes such modifications and variations that fall within the scope of the appended claims and their equivalents.

[0084] Examples 1 - 3

[0085] Examples 1 - 3 provide a niobium-containing low-carbon manganese steel steel pipe for offshore machinery, with production specifications of a pipe for welded connection of offshore pipeline valve equipment with an outer diameter × wall thickness of 537 × 28 mm and a pipe for welded connection of an 80Ksi steel grade offshore pipeline booster equipment with an outer diameter × wall thickness of 406.4 mm × 24.1 mm.

[0086] The main chemical components and their contents (wt%) of the steel grades in Examples 1-3 are shown in Table 1:

[0087] Table 1

[0088]

[0089] The production process flow of the niobium-containing low-carbon manganese steel offshore mechanical steel pipes in Examples 1-3 is as follows: steelmaking → Ф500 continuous casting → piercing → hot rolling by ASSEL mill → stretch reducing → cooling → pre-straightening → magnetic flux leakage inspection → quenching and tempering heat treatment → warm straightening → inspection → finishing inspection → chamfering of pipe ends → general packaging. According to customer requirements, double-length sawing requirements can be added between the cooling and straightening processes in the above process flow.

[0090] Specifically, the manufacturing method of the steel pipe includes the following steps:

[0091] (1) Through electric furnace steelmaking and LF furnace refining, the target molten steel with the composition shown in Table 1 is obtained; among them, the raw materials are: hot metal, pig iron and high-quality scrap steel with low harmful elements such as As and Sn and low S and P, and the carbon addition amount is controlled at 1.0-2.0%; the tapping temperature of the electric furnace is 1560-1595°C; during the LF refining process, ferrotitanium 0.6 kg / t is added within 4-16 minutes before tapping;

[0092] (2) The molten steel obtained by smelting is continuously cast into a round billet (i.e., round pipe billet), and the temperature of the arc continuous casting tundish is controlled at 1550±10°C;

[0093] (3) The round pipe billet is heated in a rotary hearth furnace, pierced, rolled by ASSEL mill, stretch reduced, cooled on a cooling bed, and sawed in sequence. Among them, the heating temperature of the rotary hearth furnace is controlled at 1230±10°C; the geometric dimensions of the steel pipe are controlled by piercing, ASSEL rolling or continuous rolling, stretch reducing, cooling on a cooling bed, and straightening to an outer diameter tolerance of -0.5% to +0.9%D, a wall thickness tolerance of ≥-12%t, a pipe body ovality of ≤80%, a wall thickness non-uniformity of ≤16%, a full-length straightness of ≤0.15%L, and a pipe end straightness of ≤2.0 mm within a 1.5 m length range at the pipe end;

[0094] (4) The quenching and tempering heat treatment is carried out on a continuous walking beam furnace production line. The steel pipe is heated to 840 - 940 °C and then quenched with water. After the steel pipe is cooled to below 50 °C, tempering is carried out. The tempering heating temperature range is 580 ± 15 °C (i.e., 565 - 595 °C). After tempering, warm straightening is carried out. Among them, the heating of the steel pipe before quenching is achieved through 3 heating sections. The heating temperature of the first heating section is controlled at 870 ± 15 °C (i.e., 855 - 885 °C), the heating temperature of the second heating section is controlled at 910 ± 10 °C (i.e., 900 - 920 °C), and the temperature of the third heating section, namely the soaking section, is controlled at 930 ± 10 °C (i.e., 920 - 940 °C); the quenching is carried out by means of external spraying plus internal spraying; during cooling, the external spraying is started first, and then the internal spraying is started; when the water spraying ends, the internal spraying ends first, and then the external spraying ends; the external spraying time is 15 - 20 seconds, and the internal spraying time is 15 - 20 seconds; the external spraying water volume is 2000 - 2300 m 3 / h, and the internal axial water volume is 700 - 1000 m 3 / h;

[0095] (5) After the warm straightening process, the steel pipes with a temperature not lower than 500 °C are fed forward by the transmission chain and roll in the circumferential direction. Subsequently, the steel pipes are purged online with high-pressure gas to make the scale fall off. The warm straightening ensures good outer surface quality and straightness of the steel pipes.

[0096] Table 2 lists the main process parameters in Examples 1 - 3

[0097] Table 2

[0098]

[0099] For each example, two analysis specimens are extracted, numbered as Example 1A, Example 1B, Example 2A, Example 2B, Example 3A, and Example 3B, and their mechanical properties are tested. The test results are shown in Table 3:

[0100] Table 3

[0101]

[0102] As can be seen from Table 3, for the niobium-containing low-carbon manganese steel steel pipes for offshore machinery prepared in Examples 1 - 3, their mechanical properties reach the following indexes: Rp0.2 is 400 - 550 MPa, the tensile strength Rm is ≥540 MPa, the elongation is ≥30%, and the high elongation performance enables excellent cold working or hot working performance; the surface hardness is ≤258 HBW and it is easy to machine; it has excellent HIC and SSC resistance. Impact toughness: longitudinal full size ≥80 J. It can be seen that all mechanical properties meet the standard requirements. The metallographic structure is shown in Figures 1-3 , and the structure is tempered sorbite structure.

[0103] The steel pipes produced in Examples 1-3 were subjected to HIC test standards and SSC tests, and the test results are shown in Table 4 as follows:

[0104] Table 4

[0105]

[0106] As can be seen from Table 4, the niobium-containing low-carbon manganese steel offshore mechanical steel pipes prepared in Examples 1-3 all achieved excellent HIC and SSC resistance performance. The external shape diagrams after the HIC test are as Figures 4-6 shown.

[0107] Comparative Example 1

[0108] This comparative example provides a niobium-containing low-carbon manganese steel offshore mechanical steel pipe, the manufacturing method step parameters of which are all the same as those in Example 1, the difference being that: the content ratio of C and Nb in the main chemical components of the steel grade is 1:1;

[0109] The main components and their contents (wt%) of the steel grade in this comparative example are: C: 0.11%, Si: 0.25%, Mn: 1.20%, P: 0.02%, S: 0.006%, V: 0.05%, Nb: 0.1%, Ti: 0.04%, Ni: 0.03%, Mo: 0.01%, Cu: 0.06%.

[0110] Comparative Example 2

[0111] This comparative example provides a niobium-containing low-carbon manganese steel offshore mechanical steel pipe, the manufacturing method step parameters of which are all the same as those in Example 1, the difference being that: the content ratio of C and Nb in the main chemical components of the steel grade is 11:1;

[0112] The main components and their contents (wt%) of the steel grade in this comparative example are: C: 0.11%, Si: 0.25%, Mn: 1.20%, P: 0.02%, S: 0.006%, V: 0.05%, Nb: 0.01%, Ti: 0.04%, Ni: 0.03%, Mo: 0.01%, Cu: 0.06%.

[0113] Comparative Example 3

[0114] This comparative example provides a niobium-containing low-carbon manganese steel offshore mechanical steel pipe, the manufacturing method step parameters of which are all the same as those in Example 2, the difference being that: the content of V+Nb+Ti in the main chemical components of the steel grade is 0.3%;

[0115] The main components and their contents (wt%) of the steel grade in this comparative example are as follows: C: 0.15%, Si: 0.4%, Mn: 1.45%, P: 0.015%, S: 0.008%, V: 0.1%, Nb: 0.1%, Ti: 0.1%, Ni: 0.04%, Mo: 0.01%, Cu: 0.05%.

[0116] Comparative Example 4

[0117] This comparative example provides a steel pipe for offshore machinery made of niobium-containing low-carbon manganese steel. The process parameters of its manufacturing method are all the same as those in Example 1, except that: after the warm straightening process in step (5), the temperature of the steel pipe is 440 °C. The oxide scale on the surface of the steel pipe obtained in this comparative example is thick.

[0118] Comparative Example 5

[0119] This comparative example provides a steel pipe for offshore machinery made of niobium-containing low-carbon manganese steel. The process parameters of its manufacturing method are all the same as those in Example 1, except that: in step (4), before quenching, the steel pipe is directly heated to 930 ± 10 °C without being heated in three heating sections.

[0120] The mechanical properties of the steel pipes produced in Comparative Examples 1-5 were tested, and the test results are shown in Table 5 as follows:

[0121] Table 5

[0122]

[0123] The steel pipes produced in Comparative Examples 1-5 were subjected to the HIC test standard and the SSC test, and the test results are shown in Table 6 as follows:

[0124] Table 6

[0125]

[0126] As can be seen from Table 5, in Comparative Examples 1 and 2, the content ratio of C and Nb is not within the range of (1.5 - 8):1, and the yield strength of the obtained steel pipes will decrease, and the low-temperature impact energy decreases and is unstable; in Comparative Example 3, due to the excessive content of V+Nb+Ti, it is easy to form composite inclusions and cannot pass the SSC test. Therefore, in the SSC test, the steel pipe breaks within 120 hours (as shown in Table 6); in Comparative Example 4, the oxide scale on the surface of the obtained steel pipe is thick, which affects the results of the tensile properties. Generally, the surface of the impact specimen is processed, and it has little impact on the impact test results; in Comparative Example 5, because heating is not carried out in stages, it is easy to cause uneven performance of the steel pipe, and the impact performance is unevenly obvious.

[0127] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A steel pipe for offshore machinery made of niobium-containing low-carbon manganese steel, characterized in that, it comprises chemical components in the following mass percentages: C: 0.11 - 0.15%, Si: 0.25 - 0.40%, Mn: 1.20 - 1.45%, P: 0.015 - 0.02%, S: 0.006 - 0.008%, V: 0.05 - 0.08%, Nb: 0.04 - 0.07%, Ti: 0.04 - 0.08%, Ni: 0.03 - 0.05%, Mo: 0.01%, Cu: 0.04 - 0.06%, and the balance is Fe and inevitable impurities; in the chemical components of the steel pipe, the mass content ratio of C and Nb is (1.5 - 8):1; in the chemical components of the steel pipe, V + Nb + Ti ≤ 0.25%; the elongation of the steel pipe ≥ 39.5%; the structure of the steel pipe is tempered sorbite structure; a manufacturing method of the niobium-containing low-carbon manganese steel steel pipe for offshore machinery, comprising the following steps: (1) Smelting to obtain molten steel, and the chemical components of the molten steel are by weight percentage: C: 0.11 - 0.15%, Si: 0.25 - 0.40%, Mn: 1.20 - 1.45%, P: 0.015 - 0.02%, S: 0.006 - 0.008%, V: 0.05 - 0.08%, Nb: 0.04 - 0.07%, Ti: 0.04 - 0.08%, Ni: 0.03 - 0.05%, Mo: 0.01%, Cu: 0.04 - 0.06%, and the balance is Fe and inevitable impurities; in the step (1), ferro-titanium alloy is added within 4 - 15 minutes before tapping from the LF furnace, and the addition amount is 0.6 kg / t; (2) Continuously casting the molten steel obtained in the step (1) to obtain a round tube blank; (3) Heating the round tube blank obtained in the step (2) in a rotary hearth furnace, rolling the tube, cooling on a cooling bed and pre-straightening to obtain a rough tube; (4) Heating the rough tube obtained in the step (3) to 840 - 940 °C and then quenching with water, after the steel pipe is cooled to below 50 °C, tempering is carried out, and after tempering, warm straightening is carried out to obtain the niobium-containing low-carbon manganese steel steel pipe for offshore machinery; in the step (4), the process of heating the rough tube to 840 - 940 °C includes three heating sections, wherein the heating temperature of the first heating section is controlled at 840 - 900 °C, the heating temperature of the second heating section is controlled at 900 - 920 °C, and the temperature of the third heating section, i.e., the soaking section, is controlled at 920 - 940 °C; in the step (4), the tempering heating temperature range is 565 - 595 °C.

2. The niobium-containing low-carbon manganese steel steel pipe for offshore machinery according to claim 1, characterized in that, in the chemical components of the steel pipe, V + Nb + Ti ≤ 0.20%; and / or, the wall thickness of the steel pipe is 24.1 - 76.2 mm; and / or, the yield strength Rp0.2 of the steel pipe is 400 - 550 MPa, the tensile strength Rm ≥ 540 MPa, the surface hardness ≤ 258 HBW, and the longitudinal full-size impact ≥ 80 J.

3. A manufacturing method of a steel pipe for offshore machinery made of niobium-containing low-carbon manganese steel as described in any one of claims 1-2, characterized in that, it includes the following steps: (1) Smelt to obtain molten steel, and the chemical composition of the molten steel is by weight percentage: C: 0.11-0.15%, Si: 0.25-0.40%, Mn: 1.20-1.45%, P: 0.015-0.02%, S: 0.006-0.008%, V: 0.05-0.08%, Nb: 0.04-0.07%, Ti: 0.04-0.08%, Ni: 0.03-0.05%, Mo: 0.01%, Cu: 0.04-0.06%, and the balance is Fe and unavoidable impurities; In the step (1), ferrotitanium alloy is added within 4-15 minutes before tapping from the LF furnace, and the addition amount is 0.6 kg / t; (2) The molten steel obtained by smelting in step (1) is made into round tube blanks through continuous casting; (3) The round tube blanks obtained in step (2) are sequentially heated in a rotary hearth furnace, rolled, cooled on a cooling bed, and pre-straightened to obtain rough tubes; (4) The rough tubes obtained in step (3) are heated to 840-940 °C and then quenched with water. After the steel pipes are cooled to below 50 °C, tempering is carried out, and after tempering, warm straightening is carried out to obtain the steel pipe for offshore machinery made of niobium-containing low-carbon manganese steel; In the step (4), during the process of heating the rough tubes to 840-940 °C, it includes three heating sections. Among them, the heating temperature of the first heating section is controlled at 840-900 °C, the heating temperature of the second heating section is controlled at 900-920 °C, and the temperature of the third heating section, that is, the soaking section, is controlled at 920-940 °C; In the step (4), the tempering heating temperature range is 565-595 °C.

4. According to the manufacturing method described in claim 3, characterized in that, in the step (1), smelting includes electric furnace steelmaking and LF furnace refining.

5. According to the manufacturing method described in claim 4, characterized in that, during the electric furnace steelmaking, the addition amount of deoxidizer: 2.0-6.5 kg / t, the addition amount of Al blocks: 2.5-4.5 kg / t; during the LF furnace refining, the consumption amount of synthetic slag ≥ 500 kg / t.

6. According to the manufacturing method described in claim 4, characterized in that, at the end of the LF furnace refining, 0.3-0.9 kg / t of Ca-Si wire or 0.15-0.25 kg / t of pure Ca wire is fed before static blowing.

7. According to the manufacturing method described in claim 6, characterized in that, the static blowing time ≥ 18 min.

8. According to the manufacturing method described in claim 4, characterized in that, in the step (1), the tapping temperature of the LF furnace is 1560-1595 °C.

9. According to the manufacturing method described in claim 3, characterized in that, during the continuous casting process of the step (2), the tundish temperature is controlled at 1540-1600 °C.

10. According to the manufacturing method described in claim 3, characterized in that, in the step (3), the heating temperature of the rotary hearth furnace is controlled at 1220-1280 °C; And / or, the tube rolling in step (3) sequentially includes piercing, ASSSEL rolling or tandem rolling, and stretch reducing; And / or, the outer diameter tolerance of the shell tube obtained in step (3) is -0.5% to +1%D, the wall thickness tolerance is ≥ -12.5%t, the ovality of the tube body is ≤ 80%, the wall thickness unevenness is ≤ 16%, the straightness is controlled such that the overall straightness is ≤ 0.2%L, and the straightness at the tube end is ≤ 2.5 mm within the 1.5 m length range at the tube end.

11. The manufacturing method according to claim 3, characterized in that, In the step (4), the quenching is carried out by means of water quenching with external spraying and internal axial flow, wherein the external spraying time is 15 to 60 seconds, and the internal axial flow time is 15 to 60 seconds; the external spraying water volume is 600 to 2500 m 3 / h, and the internal axial flow water volume is 400 to 1200 m 3 / h.

12. The manufacturing method according to claim 3, characterized in that, in step (4), the temperature at the end of controlled thermal straightening is greater than 450°C; And / or, the tempering time is (3 - 4 min / mm) × D, where D is the wall thickness of the steel pipe in mm.

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