A smelting method of Ni36LNG

Abstract

The application discloses a Ni36LNG smelting method and relates to the technical field of precision alloy manufacturing, and comprises the following steps: S1, adding 35t of low-phosphorus raw material and 20t of pure nickel into an alloy melting furnace and heating and melting; S2, tapping the alloy melting furnace at a tapping temperature of greater than or equal to 1580 DEG C; S3, transferring the molten steel into an AOD furnace, blowing oxygen to decarburize, and adding lime in batches; S4, adding full-aluminum deoxidizing agent to reduce and drawing slag; S5, after tapping the AOD furnace, positioning an LF furnace, building slag with low-carbon lime, adopting silicon-iron powder slag surface deoxidization, S6, drawing slag and transferring the ladle into a VD; S7, strongly stirring the molten steel, then adding 100kg of nickel-magnesium alloy into the molten steel, weakly stirring again, and realizing the deoxidization of the molten steel by adding the full-aluminum deoxidizing agent in the step S4, adding the low-carbon lime in batches in the step S5 and the silicon-iron powder, adsorbing and reacting to remove the oxides in the molten steel, avoiding the secondary oxidation of the molten steel, and guaranteeing the smelting quality and reducing the production cost of the steel.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of precision alloy manufacturing, in particular to a smelting method of Ni36LNG. BACKGROUND

[0002] Ni36LNG steel for liquefied natural gas is an ultralow-temperature high-nickel steel specially used for liquefied natural gas (LNG) storage and transportation facilities. The main components of Ni36LNG are nickel (Ni) and iron (Fe), wherein the content of nickel is about 36%, and the balance is iron and other trace elements. It has an extremely low thermal expansion coefficient and can maintain dimensional stability in an environment with large temperature changes. It also has good strength, toughness and corrosion resistance.

[0003] In the prior art, due to the material properties and process complexity of Ni36LNG, the production technology requirements for the modified steel grade are extremely high. At the same time, it has not been found that the smelting of this steel grade can be realized by die casting. If the smelting method is by continuous casting, a large number of equipment are needed, and the loss is high during the smelting process, resulting in high cost. In addition, when the steel grade is applied in different environments, it is difficult to control the content and proportion of C (carbon, which regulates compressive strength) and Ni (nickel, which is used to regulate low-temperature toughness) through the production process, so that the steel grade is difficult to be specifically suitable for different environments. SUMMARY

[0004] The purpose of the present application is to provide a smelting method of Ni36LNG to solve the above problems in the prior art.

[0005] In order to achieve the above purpose, the present application provides the following technical scheme:

[0006] A smelting method of Ni36LNG, comprising the following steps:

[0007] S1: adding 35t of low-phosphorus raw material and 20t of pure nickel into an alloy melting furnace, and heating and melting into molten steel;

[0008] S2: tapping the alloy melting furnace, sampling and detecting before tapping, and the tapping temperature is greater than or equal to 1580℃;

[0009] S3: transferring the molten steel to an AOD furnace, blowing oxygen to decarburize, and adding lime in batches according to the carbon content in the molten steel;

[0010] S4: when the mass percentage of carbon is less than or equal to 0.01%, adding 829kg of full-aluminum deoxidizer for reduction, and drawing slag after reduction;

[0011] S5: after the AOD furnace is tapped, the ladle is positioned at the LF furnace, 300-500kg of low-carbon lime is added in batches to form slag, and 40kg of silicon-iron powder is used for slag surface deoxidization to adjust the basicity and fluidity of the slag;

[0012] S6: removing slag and transferring the ladle to VD;

[0013] S7: strongly stirring the molten steel, adding 100 kg of nickel-magnesium alloy into the molten steel after the strong stirring, and then weakly stirring, and hoisting the pouring trolley when the temperature reaches 1500-1505 DEG C.

[0014] Preferably, in step S2, the mass percentage of phosphorus is ≤0.008% when tapping, and the sampling temperature is controlled to ≥1500 DEG C.

[0015] Preferably, in step S3, 800-1200 kg of lime is added and oxygen blowing is performed, 1000-1200 kg of lime is added when the mass percentage of carbon is ≤1.00% and ≥0.50%, 1000-1200 kg of lime is added when the mass percentage of carbon is ≤0.50% and ≥0.20%, and 500-1000 kg of lime is added when the mass percentage of carbon is ≤0.20%.

[0016] Preferably, before the ladle is transferred in S6, the mass percentage of sulfur is detected and confirmed to be ≤0.0015%, and 125 m of pure calcium wire is fed.

[0017] Preferably, the temperature of the molten steel is 1610-1700 DEG C when the ladle is transferred in S6.

[0018] Preferably, in S7, the strong stirring is performed by charging argon gas through the bottom gas brick of the ladle, and the duration is ≥15 min.

[0019] Preferably, in S7, the weak stirring is performed by charging argon gas through the bottom gas brick of the ladle, and the duration is ≥35 min.

[0020] In the above technical solution, the Ni36LNG smelting method provided by the application has the following beneficial effects:

[0021] 1. By adding full-aluminum deoxidizer in step S4, and adding low-carbon lime and silicon-iron powder in batches in S5, the deoxidation of the molten steel is realized, the oxides in the molten steel are adsorbed and reacted to be removed, the phenomenon that the molten steel is easily re-oxidized in the production process is avoided, the smelting quality is ensured, the production process of the entire production line does not need too many devices, and the operation is simple, and the production cost of the steel grade is greatly reduced.

[0022] 2. By controlling the different contents of low-carbon lime, the contents of C and Ni can be progressively increased or decreased in a proportional manner, so that the steel material is more suitable for the performance of each component of the LNG equipment, i.e., different structural parts and thin / thick walls of the storage tank, different pressure sections of the pipeline, so as to more accurately match the required performance requirements, greatly improving the practicality of each component of the LNG equipment during use.

[0023] It should be understood that the foregoing general description and the following detailed description are merely exemplary and explanatory, and are not intended to limit the present disclosure.

[0024] The present application file provides an overview of various implementations or examples of the technology described in this disclosure and is not intended to be all inclusive or to provide an exhaustive list of features of the disclosed technology. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the embodiments of the present disclosure clearer, the technical scheme of the embodiments of the present disclosure will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present disclosure, rather than all the embodiments. Based on the described embodiments of the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present disclosure.

[0026] A smelting method of Ni36LNG, comprising the following steps:

[0027] S1: adding 35t of low-phosphorus raw material and 20t of pure nickel into an alloy melting furnace, and heating and melting into molten steel;

[0028] The phosphorus in the molten steel is converted into phosphorus oxide (diphosphorus pentoxide) by oxidation reaction with oxygen, and then combined with the basic components (calcium oxide, magnesium oxide, etc.) in the slag to form stable phosphate into the slag, thereby separating from the molten steel, so as to reduce the phosphorus content in the molten steel.

[0029] S2: tapping the alloy melting furnace, sampling and detecting before tapping, and the tapping temperature is ≥1580℃;

[0030] S3: transferring the molten steel to an AOD furnace, blowing oxygen to decarburize, and adding lime in batches according to the carbon content in the molten steel;

[0031] Decarburization: (C+O2=CO2↑ or 2C+O2=2CO↑);

[0032] Desulfurization: CaO+[S]+[C]=CaS+CO↑ (CaS enters the slag to reduce the sulfur content in the steel);

[0033] 5CaO+2[P]+3(FeO)=5CaO·P2O5+3Fe (generate stable calcium phosphate complex into the slag to reduce the phosphorus content in the steel);

[0034] Carbon in molten steel and oxygen blowing into the furnace constantly occur oxidation reaction, at the same time, other elements in molten steel will also occur oxidation reaction with oxygen, constantly produce oxides, in addition, while the oxidation reaction, the temperature in the furnace will continue to rise, lime melting is an endothermic process, can reduce the temperature of molten steel, so need to add lime multiple times, to avoid the phenomenon of local overheating or supercooling in the furnace, while decarburization, maximize the effect of dephosphorization and desulfurization, maintain the continuous progress of oxidation reaction.

[0035] S4: when the carbon mass percentage is less than or equal to 0.01%, 829 kg of full aluminum deoxidizer is added for reduction, and slag is drawn after reduction;

[0036] When carbon is less than or equal to 0.01%, full aluminum deoxidizer is added for reduction reaction, that is, by adding full aluminum deoxidizer into the furnace, the strong reducing property of aluminum can reduce the dissolved oxygen in molten steel to a very low level, generate Al2O3 (solid inclusions, dispersed in molten steel in the form of fine particles), avoid secondary oxidation due to high oxygen content of molten steel in the subsequent LF stage, at the same time, full aluminum deoxidizer contacts with the slag (oxides such as FeO, MnO) produced in S3, displacement reaction occurs, metal elements in the slag are reduced back to molten steel, reduce metal loss in the smelting process, reduce production cost, generated Al2O3 enters the slag (absorbed by alkaline components such as lime), reduce the oxidizing property of the slag, avoid the diffusion of oxygen in the slag to the molten steel, Al2O3 exists in the slag in the form of solid particles (easily float up due to the density less than molten steel), convenient for slagging, full aluminum deoxidizer is pure aluminum particles, aluminum content is greater than or equal to 99.5%.

[0037] S5: after the AOD furnace is tapped, the ladle is positioned at the LF furnace, 300-500 kg of low-carbon lime is added in batches for slagging, 40 kg of ferrosilicon powder is added for slag surface deoxidization, adjust the basicity and fluidity of the slag;

[0038] The silicon in ferrosilicon powder deoxidizes on the slag surface to generate silicon dioxide, reduces oxidizing components (FeO, MnO, etc.) in the slag, and reduces the oxidizing property of the slag. If the oxidizing property of the slag is too high, the slag will supply oxygen to the molten steel in the opposite direction, which will destroy the deep deoxidization result in the AOD stage. After the deoxidization of ferrosilicon powder, the oxidizing substances in the slag are reduced, and the re-oxygenation of the molten steel can be avoided. The main function of low-carbon lime is desulfurization. At the same time, after the addition of low-carbon lime, the basicity of the slag is increased, the high-basicity slag contacts with Al2O3 (solid inclusions) in S4, so that the Al2O3 is wrapped and adsorbed by the slag. Moreover, the lime reacts with Al2O3 to generate low-melting-point calcium aluminate, which floats to the slag, further purifying the molten steel. Through the reaction of low-carbon lime and silicon dioxide, the slag is stabilized in the high-basicity range, which can inhibit the corrosion of the molten steel to the carbon-containing refractory material, indirectly reducing the introduction of carbon (the carbon content will still increase, and the carbon comes from the corrosion of the furnace lining, the ladle, and the carbon in the low-carbon lime. Since the low-carbon lime can reduce the introduction of carbon, the increase of low-carbon lime reduces the introduction of carbon by the molten steel), and further strengthens the desulfurization effect of the molten steel. Moreover, the reaction of lime and silicon dioxide generates calcium silicate and adjusts the slag composition, such as generating CaO-SiO2-Al2O3 ternary slag system, to avoid high viscosity of the slag, so that the slag layer has good fluidity, covers the surface of the molten steel, isolates air, and avoids pollution of the molten steel.

[0039] S6: slagging, and transferring the ladle to the VD;

[0040] S7: strong stirring of the molten steel, after the end of the strong stirring, adding 100 kg of nickel-magnesium alloy to the molten steel, and then weak stirring, when the temperature reaches 1500-1505℃, lifting the pouring trolley.

[0041] The function of the strong stirring is: 66.7 Pa high vacuum degree, under this pressure, the dissolved gas (H2, N2) in the molten steel will diffuse to the liquid surface and escape due to the difference in partial pressure, avoiding defects such as gas holes and hydrogen embrittlement in the later stage of the steel; argon stirring in a vacuum environment produces bubbles that can adsorb inclusions, and the vacuum reduces the surface tension of the molten steel, accelerating the aggregation and growth of inclusions and floating them to the slag phase, improving the cleanliness of the molten steel;

[0042] Adding 100 kg of nickel-magnesium alloy: nickel is used to supplement the component loss in the early smelting, and magnesium reacts with the residual oxygen in the molten steel to generate MgO for deep deoxidization;

[0043] Adding 100 kg of nickel-magnesium alloy after strong stirring makes the molten steel in a reducing atmosphere after the end of the vacuum state of strong stirring, so that the subsequent weak stirring can promote the uniform diffusion of Mg, avoiding the addition of Mg too early, which leads to a large amount of evaporation in the vacuum stage;

[0044] The weak stirring is to realize uniformity of components and temperature and floating of micro-inclusions under the condition of non-vacuum argon stirring, and to make the Ni-Mg alloy fully dissolved, the reaction of Mg and inclusions complete and the inclusions stable.

[0045] Specifically, in step S2, the mass percentage of phosphorus is ≤0.008% when tapping, and the sampling temperature is controlled to ≥1500℃.

[0046] The sampling is performed when the temperature of the molten steel is ≥1500℃, at which the molten steel has good fluidity, the sampling is more representative, and the phenomenon of "rephosphorization" is avoided, and meanwhile, the temperature is close to the tapping temperature, so that the composition at the final tapping can be better predicted.

[0047] In the further provided embodiment of the application, in step S3, 800-1200 kg of lime is added and oxygen blowing is performed for decarburization, when the mass percentage of carbon is ≤1.00% and ≥0.50%, 1000-1200 kg of lime is added, when the mass percentage of carbon is ≤0.50% and ≥0.20%, 1000-1200 kg of lime is added, and when the mass percentage of carbon is ≤0.20%, 500-1000 kg of lime is added; before the transfer of the molten steel ladle in S6, the mass percentage of sulfur is detected and confirmed to be ≤0.0015%, and 125 m of pure calcium wire is fed;

[0048] In step S3, the carbon content in the molten steel is monitored in real time during the oxygen blowing decarburization process, when the mass percentage of carbon reaches the interval of the added lime, i.e., when the lime needs to be added, the lime amount is determined by detecting the basicity of the slag, the higher the basicity, the less the lime amount, and the lower the basicity, the more the lime amount, and the lime amount in each stage will not deviate from the corresponding interval of the carbon content in S3.

[0049] The function of the calcium wire feeding is to improve the deoxidization effect and deeply deoxidize so as to further reduce the oxygen content in the molten steel.

[0050] Further, the temperature of the molten steel is 1610-1700℃ when transferred in S6.

[0051] Still further, in S7, the strong stirring is VD controlled vacuum degree 66.7 Pa, and argon stirring is performed through the bottom gas brick of the molten steel ladle, and the duration is ≥15 min.

[0052] In the further provided embodiment of the application, in S7, the weak stirring is argon stirring performed through the bottom gas brick of the molten steel ladle, and the duration is ≥35 min.

[0053] The following are embodiments of adding different amounts of low-carbon lime in S5:

[0054] Example Low carbon lime addition C Si Mn Cr Ni P S Ti Al N 1 300 kg 0.04 0.15 0.30 0.08 35.0 0.007 0.0010 0.05 0.02 0.0025 2 400 kg 0.035 0.18 0.35 0.05 35.5 0.007 0.0008 0.04 0.03 0.0020 3 500 kg 0.03 0.20 0.40 0.08 36.5 0.008 0.0010 0.05 0.02 0.0020

[0055] In summary of the above examples 1-3, the C content is between 0.03-0.04%, which ensures the strength and makes the low temperature impact toughness (-163°C) more stable, at the same time, the Ni content covers 35.0-36.5, which meets the standard of Ni36LNG steel, and the performance of anti-brittle fracture is ensured by controlling P and S, and by controlling the different content of low-carbon lime, the content of C and Ni can be increased or decreased in a proportional form, so as to make the performance of LNG equipment more suitable, that is, different structural parts and different pressure sections of the thin wall / thick wall of the storage tank and the pipeline can be more accurately matched with the required performance requirements, avoiding the performance redundancy / deficiency caused by "one-size-fits-all" components, and the engineering practicability is stronger.

[0056] As for examples 1-3, by controlling the content of C and Ni, example 3 is the optimal scheme.

[0057] Comparative example 1:

[0058] The difference between this comparative example and example 3 is that 1220kg ferrosilicon is used instead of full aluminum deoxidizer in S4.

[0059]

[0060] Comparative example 2:

[0061] The difference between this comparative example and example 3 is that 500kg standard lime is used instead of low-carbon lime in S5.

[0062] C Si Mn Cr Ni P S Ti Al N 0.05 0.25 0.40 0.008 35.8 0.008 0.0010 0.05 0.05 0.0025

[0063] By comparing the C and Ni in any one of the comparative examples 1 and 2 with the examples, none of them can meet the requirements of the steel produced in the examples of the present application. Although the C and Ni in comparative example 1 meet the standard of the steel, the Ni content is only 35.2%, and the Si and N in the examples of the present application are lower than those in comparative example 1. Therefore, in combination with the Ni, Si and N in comparative example 1, the low temperature toughness of example 3 of the present application is better than that of comparative example 1.

[0064] In comparative example 2, by using standard lime instead of low-carbon lime in example 3, the C content is too high, which affects the low temperature toughness, and the Ni content is also lower than that in example 3, so the low temperature toughness is far inferior to that of example 3 of the present application.

[0065] The foregoing merely illustrates some exemplary embodiments of the application, and it will be appreciated that those skilled in the art will be able to devise various modifications without departing from the spirit and scope of the application. The described embodiments are to be considered in all respects as illustrative only and not restrictive in character, and the scope of the application is indicated not by the foregoing description but by the claims that follow.

Claims

1. A method of smelting Ni36LNG, characterized by, It comprises the following steps: S1: adding 35t of low-phosphorus raw material and 20t of pure nickel into an alloy melting furnace, and heating and melting into molten steel; S2: tapping from the alloy melting furnace, and sampling and detecting before tapping, with a tapping temperature of ≥1580℃; S3: transferring the molten steel into an AOD furnace, blowing oxygen to decarburize, and adding lime in batches according to the carbon content in the molten steel; S4: when the carbon mass percentage is ≤0.01%, adding 829kg of full-aluminum deoxidizer to reduce, and drawing slag after reduction; S5: after tapping from the AOD furnace, positioning the ladle in the LF furnace, adding 300-500kg of low-carbon lime in batches to build slag, and adding 40kg of ferrosilicon powder to deoxidize the slag surface; S6: drawing slag, and transferring the ladle into a VD; S7: strongly stirring the molten steel, adding 100kg of nickel-magnesium alloy into the molten steel after strong stirring, and weakly stirring, and hoisting the pouring trolley when the temperature reaches 1500-1505℃.

2. A method of smelting Ni36LNG as claimed in claim 1, wherein, In the step S2, the phosphorus mass percentage is ≤0.008% when tapping, and the sampling temperature is controlled to ≥1500℃.

3. A method of smelting Ni36LNG as claimed in claim 1, wherein, In the step S3, 800-1200kg of lime is added, and oxygen is blown to decarburize, 1000-1200kg of lime is added when the carbon mass percentage is ≤1.00% and ≥0.50%, 1000-1200kg of lime is added when the carbon mass percentage is ≤0.50% and ≥0.20%, and 500-1000kg of lime is added when the carbon mass percentage is ≤0.20%.

4. A method of smelting Ni36LNG as claimed in claim 1, wherein, Before the ladle is transferred in the step S6, the sulfur mass percentage is detected and confirmed to be ≤0.0015%, and 125m of pure calcium wire is fed.

5. The method of smelting Ni36LNG as claimed in claim 1, wherein, When the ladle is transferred in the step S6, the molten steel temperature is 1610-1700℃.

6. A method of smelting Ni36LNG as claimed in claim 1, wherein, In the step S7, the strong stirring is controlled by the VD to have a vacuum degree of 66.7Pa, argon is filled through the air brick at the bottom of the ladle to stir, and the duration is ≥15min.

7. The method of smelting Ni36LNG as claimed in claim 1, wherein, In the step S7, the weak stirring is to fill argon through the air brick at the bottom of the ladle to stir, and the duration is ≥35min.

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