A raw steel and a method for manufacturing the same

Abstract

The application provides a raw material steel and a preparation method thereof, and belongs to the field of metallurgical industry.The chemical components of the raw material steel are as follows: C 0.25-0.32wt%, Si 0.8-1.2wt%, Mn≤0.1wt%, P≤0.005wt%, S≤0.001wt%, Cr 0.8-1.2wt%, Ni≤0.3wt%, Mo 0.35-0.45wt%, Cu≤0.10wt%, Al≤0.025wt%, As≤0.008wt%, Pb≤0.001wt%, Sb≤0.003wt%, Sn≤0.004wt%, N≤0.006wt%, O≤0.0015wt% and the balance of Fe.The preparation method comprises the following steps: molten iron pretreatment, converter smelting, LF refining, RH vacuum degassing, soft blowing, continuous casting, slow cooling and heating rolling into finished products.The molten iron is pretreated by desulfurization, low-temperature P removal is carried out in converter smelting, deep P removal and S removal are carried out in ladle refining LF, and the molten steel with ultra-low P and ultra-low S is obtained, then the molten steel is continuously cast and one-fire rolled into finished products, so that the process cost of the steel is greatly reduced and the production efficiency is improved.The raw material steel produced by the process method can be put into a vacuum induction furnace after rust removal, and high-purity low-cost ultra-high-strength steel for aviation and aerospace can be smelted.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical industry, and specifically relates to a raw material steel and its preparation method. Background Technology

[0002] The demand for low-alloy ultra-high strength steel used in the aerospace field is increasing with the development of aerospace technology, leading to a corresponding increase in the demand for its raw materials. Previously, due to the low demand for raw materials, electric arc furnace casting was typically used for production. The process flow was: electric arc furnace + LF + VD + soft blowing + casting 3t ingots --- hot-rolled round steel --- shearing. However, with the increasing consumption of raw materials, which has exceeded 10,000 tons, the original electric arc furnace casting process can no longer meet the needs. There is an urgent need for a new production process, continuous casting, to replace casting, in order to meet the demand with large-scale, high-efficiency production. However, since low-alloy ultra-high strength steel is smelted using a dual process of vacuum induction and vacuum consumable steel, it has high requirements for the purity of the raw steel. In electric arc furnace production, the purity can be ensured by multiple slag formation and removal processes due to the slow production pace of ingot casting. However, continuous casting production is fast-paced. If multiple slag formation and removal processes are still used in electric arc furnaces to ensure purity, it will be impossible to keep up with the production pace of continuous casting. Therefore, a converter plus continuous casting mode can be used for production. However, the converter faces the problem of difficult dephosphorization, making it difficult to guarantee the ultra-purity requirements of the raw steel.

[0003] Therefore, how to provide a raw steel with high production efficiency, low cost, and high purity to meet the production needs of low alloy ultra-high strength steel for aerospace is an urgent problem to be solved. Summary of the Invention

[0004] This invention aims to provide a raw material steel and its preparation method, which uses a continuous casting first-stage casting process instead of a die casting process, thereby solving the problems of low production efficiency, high cost, and poor supply capacity of current raw material steel.

[0005] In a first aspect, the present invention provides a method for preparing raw material steel, comprising the following steps:

[0006] (1) Pre-desulfurization of molten iron:

[0007] Control the P in molten iron to be ≤0.11wt%, S to be ≤0.03wt%, and the molten iron temperature to be ≥1350℃. Use KR mechanical stirring to perform pre-desulfurization so that S is ≤0.008wt%. Then, perform slag removal operation before and after desulfurization with desulfurizing agent so that the S in molten iron leaving the station is ≤0.005wt%.

[0008] (2) Converter smelting:

[0009] Using molten iron, scrap steel, Mo alloy, and Ni alloy as raw materials, the materials are added to the furnace. The dephosphorization temperature is controlled at 1550-1580℃, and lime is added for preliminary dephosphorization to ensure that P ≤ 0.008wt%. When tapping steel from the converter, the tapping temperature is controlled at 1560-1640℃, and the carbon content of the tapped steel is 0.04-0.10wt%, with P ≤ 0.008wt%. During the tapping process, dephosphorizing agent, fluorite, and lime are added to ensure that P ≤ 0.005wt%, followed by slag removal.

[0010] (3) LF Refining:

[0011] The dephosphorization temperature is controlled at 1550-1580℃, and the deep dephosphorization is reduced to ≤0.002wt%. After removing the oxidized slag, refining slag is added, and then deoxidizer is added to deoxidize and produce white slag. The temperature is raised to 1580-1600℃, and deep desulfurization is carried out to S≤0.001wt%. Various alloys are added according to the target composition to adjust the composition.

[0012] (4) RH vacuum degassing and soft argon blowing:

[0013] After RH vacuum degassing, silicon-calcium wire is added to purify the molten steel, followed by soft blowing argon treatment.

[0014] (5) Continuous casting: The continuously cast billet is cooled in place or sent to a hot place instead of annealing;

[0015] (6) Rolling: The continuously cast billet is heated at 1220-1280℃ for 5-20 hours and then hot rolled. The hot-rolled round steel is then cooled slowly instead of annealed.

[0016] Furthermore, in step (1) pre-desulfurization of molten iron, the composition of the molten iron before desulfurization is required to be as follows: Si 0.2-0.4, As≤0.008, Pb≤0.001, Sb≤0.003, Sn≤0.004;

[0017] And / or, the mass ratio of the desulfurizing agent to the molten iron is 1 to 1.5:115;

[0018] And / or, the desulfurizing agent comprises components in the following mass ratio: CaO:CaF2 = 7:3;

[0019] And / or, the stirring time and stirring speed of the KR mechanical stirrer are controlled at 40-50 min and 100-120 r / min, respectively.

[0020] Furthermore, in the converter smelting in step (2), the mass ratio of the scrap steel to the molten iron is 23-26:115, and the Mo and Ni alloys are added into the furnace along with the scrap steel hopper, according to the lower limit of the raw material steel composition.

[0021] And / or, during the initial dephosphorization, the ratio of the mass of the lime added to the mass of the molten iron is 9-10:115;

[0022] And / or, the ratio of the mass of the lime added during the tapping process to the mass of the molten iron is 1:143 to 145;

[0023] And / or, in the converter smelting in step (2), the mass ratio of the fluorite to the molten iron is 1:3833-3840;

[0024] And / or, the mass ratio of the dephosphorizing agent to the molten iron is 1:383-385;

[0025] And / or, the dephosphorizing agent comprises: CaO 15-25wt%, MgO ≤10wt%, SiO2 ≤6wt%, Al2O3 ≤10wt%, Fe2O3 ≥50wt%, P ≤0.1wt%, S ≤0.1wt%.

[0026] Furthermore, in step (3) LF refining, the mass ratio of the refining slag to the molten iron is 1:383-385;

[0027] And / or, the binary basicity of the refining slag is 6-9;

[0028] And / or, the refining slag composition is: 50-60 wt% CaO, 8-15 wt% SiO2, 3-8 wt% MgO and 25-30 wt% Al2O3;

[0029] And / or, the deoxidizer includes aluminum and silicon carbide, the aluminum includes aluminum wire and aluminum granules, the ratio of aluminum wire to molten iron is 3-3.5m:1t, and the mass ratio of aluminum granules to molten iron is 1:1900-2900;

[0030] And / or, when adding deoxidizer, the argon gas is controlled at 300-400 NL / min. First, aluminum wire is fed in for precipitation deoxidation, and then aluminum particles and silicon carbide slag are added for surface diffusion deoxidation.

[0031] And / or, while adding alloys to adjust the composition, lime and fluorite are added to form slag, so as to avoid the slag being too thick and affecting the desulfurization effect;

[0032] And / or, the alloy includes low-titanium high-chromium, ferrosilicon, nickel plate, and ferromolybdenum.

[0033] Furthermore, in step (4) RH vacuum degassing and soft argon blowing,

[0034] Perform RH vacuum degassing treatment for no less than 20 minutes under vacuum conditions ≤76pa to remove N and O from the molten steel;

[0035] And / or, the soft-blown argon treatment time is not less than 20 minutes;

[0036] And / or, the main components of the silicon-calcium wire are: 30-40 wt% Ca, 15-25 wt% Si, and 10-20 wt% Al.

[0037] Furthermore, the RH vacuum degassing treatment includes a deep circulation treatment of molten steel for no less than 15 minutes at a vacuum degree ≤67Pa;

[0038] And / or, the argon flow rate is controlled at 50-150 NL / min during the soft-blown argon treatment.

[0039] Furthermore, in step (5) continuous casting, an arc-shaped continuous casting machine is used for casting, with an arc radius of 16.5m, a superheat of 20-40℃, and a casting speed of 410×530mm: 0.4~0.5m / min, and the casting is carried out under full protection.

[0040] Furthermore, in the rolling process of step (6), during heating, the furnace is first cooled down to: ≤800℃ in the preheating section, 800-1180℃ in the first heating section, 1220-1280℃ in the second heating section, and 1220-1280℃ in the soaking section; after the furnace is filled with material, the heating rate is controlled at ≤100℃ / h, and the final temperature in the furnace is controlled at: ≤900℃ in the first heating section, ≤1240℃ in the second heating section, and ≤1280℃ in the soaking section.

[0041] And / or, during hot rolling, a 1350 mill is used for billet preparation, and a 750 mill is used for finishing, with a finished product specification of Φ120mm;

[0042] And / or, in order to reduce the surface hardness of the steel, the rolled steel is placed in an insulation hood for slow cooling for ≥26 hours, and after exiting the insulation hood, the steel is quickly placed in a pit for cooling for ≥10 hours.

[0043] Furthermore, the lime contains ≥95wt% CaO and the fluorite contains ≥97wt% CaF2.

[0044] Secondly, the present invention provides a raw material steel prepared by the above-mentioned preparation method, wherein the chemical composition of the raw material steel is as follows: C 0.25-0.32wt%, Si 0.8-1.2wt%, Mn≤0.1wt%, P≤0.005wt%, S≤0.001wt%, Cr 0.8-1.2wt%, Ni≤0.3wt%, Mo 0.35-0.45wt%, Cu≤0.10wt%, Al≤0.025wt%, As≤0.008wt%, Pb≤0.001wt%, Sb≤0.003wt%, Sn≤0.004wt%, N≤0.006wt%, O≤0.0015wt%, and the balance Fe.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] 1. The raw material steel preparation method of this invention employs pre-desulfurization of molten iron, low-temperature desulfurization in a converter, and deep desulfurization and desaturation in a ladle (LF) to obtain ultra-low P and ultra-low S steel. Continuous casting and single-fire rolling are then used to produce finished steel, significantly reducing the process cost and improving production efficiency. The main improvement lies in the optimized process flow, replacing ingot casting with converter continuous casting, which greatly increases production efficiency. The original ingot casting smelting production efficiency was 2.5 t / h, while the converter production efficiency is now 40 t / h. Simultaneously, cost reduction and efficiency improvement are achieved; compared to ingot casting with two firings, converter continuous casting with one firing reduces the cost per ton of steel by ≥2000 yuan / t.

[0047] 2. The raw steel obtained by the preparation method of the present invention is low in cost and high in purity, with P≤0.005wt% and S≤0.001wt%. In particular, the phosphorus content is more than an order of magnitude lower than that of ordinary steel, which can meet the production needs of vacuum induction smelting of low alloy ultra-high strength steel for aerospace. The raw steel produced by this method can be put into a vacuum induction furnace for smelting high-cleanliness and low-cost low alloy ultra-high strength steel for aerospace after rust removal. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. The embodiments of this invention are implemented based on the technical solutions of this invention, and detailed implementation methods and processes are given. However, the scope of protection of this invention is not limited to the following embodiments. Those skilled in the art should understand that the embodiments are merely helpful in understanding this invention and should not be considered as specific limitations on this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0049] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0050] In this invention, unless otherwise specified and / or stated, all numerical values ​​relating to component amounts are by weight throughout. Process parameters in the following examples, unless otherwise specified, are generally performed under conventional conditions. The raw materials described in the following examples are all available from publicly available commercial sources.

[0051] The raw material steel of this invention, abbreviated as YLG-D, is a process material used for smelting ultra-high strength steel in a vacuum induction furnace. It can provide pure Fe element and can be understood as an Fe alloy raw material for vacuum induction furnace smelting.

[0052] This invention aims to ensure the production of D406A, a low-alloy ultra-high-strength steel for aerospace applications. It researches and improves the production process of YLG-D, the ultra-pure raw material steel required for the large-scale vacuum induction smelting of D406A, to achieve high production efficiency, low cost, and guaranteed quality of YLG-D. This invention employs a converter continuous casting and single-fire rolling process as a new production method, replacing the old electric furnace ingot casting and two-fire rolling process. The specific flow is as follows: molten iron pretreatment – ​​converter smelting – LF refining – RH vacuum degassing – soft blowing – continuous casting – slow cooling – heated rolling. Specific implementation methods are provided below:

[0053] Firstly, a method for preparing raw material steel, comprising the following steps:

[0054] (1) Pre-desulfurization of molten iron:

[0055] The composition of molten iron should be controlled as follows: P ≤ 0.11 wt% (specifically, 0.10 wt%, 0.09 wt%, 0.08 wt%, 0.07 wt%, 0.06 wt%, 0.05 wt%, 0.04 wt%, 0.03 wt%, 0.02 wt%, 0.01 wt%, etc.), S ≤ 0.03 wt% (specifically, 0.029 wt%, 0.027 wt%, 0.025 wt%, 0.023 wt%, 0.021 wt%, 0.019 wt%, 0.017 wt%, 0.015 wt%, 0.013 wt%, 0.01 wt%, etc.), and the molten iron temperature ≥ 1350℃ (specifically, 1355℃, 1360℃, 1370℃, 1380℃, etc.). Pre-desulfurization is carried out at temperatures of 390℃, 1400℃, 1410℃, 1420℃, etc., using KR mechanical stirring to achieve S≤0.008wt% (specifically such as 0.0075wt%, 0.007wt%, 0.006wt%, 0.005wt%, 0.004wt%, 0.003wt%, 0.002wt%, 0.001wt%, etc.). Slag removal is then performed before and after desulfurization with the desulfurizing agent to ensure that the S at the molten iron outlet is ≤0.005wt% (specifically such as 0.0045wt%, 0.004wt%, 0.0035wt%, 0.003wt%, 0.0025wt%, 0.002wt%, 0.0015wt%, 0.001wt%, etc.).

[0056] (2) Converter smelting:

[0057] Using molten iron, scrap steel, Mo alloy, and Ni alloy as raw materials, these are added to the furnace. The dephosphorization temperature is controlled at 1550-1580℃ (specifically, 1555℃, 1560℃, 1565℃, 1570℃, 1575℃, etc.). Lime is added for preliminary dephosphorization, resulting in P ≤ 0.008wt% (specifically, 0.0075wt%, 0.007wt%, 0.006wt%, 0.005wt%). (wt%, 0.004wt%, 0.003wt%, 0.002wt%, 0.001wt%, etc.); the tapping temperature during converter tapping is controlled at 1560-1640℃ (specifically 1565℃, 1570℃, 1580℃, 1590℃, 1600℃, 1610℃, 1620℃, 1630℃, 1635℃, etc.), and the carbon content of the tapped steel is 0.04-0.1%. 0 wt% (specifically 0.045 wt%, 0.05 wt%, 0.06 wt%, 0.07 wt%, 0.08 wt%, 0.09 wt%, 0.095 wt%), P ≤ 0.008 wt% (specifically 0.0075 wt%, 0.007 wt%, 0.006 wt%, 0.005 wt%, 0.004 wt%, 0.003 wt%, 0.002 wt%, 0.001 wt%), dephosphorizing agent, fluorite and lime are added during the tapping process to make P ≤ 0.005 wt% (specifically 0.0045 wt%, 0.004 wt%, 0.0035 wt%, 0.003 wt%, 0.0025 wt%, 0.002 wt%, 0.0015 wt%, 0.001 wt%), and then slag removal is performed;

[0058] (3) LF Refining:

[0059] The dephosphorization temperature is controlled at 1550-1580℃ (specifically 1555℃, 1560℃, 1565℃, 1570℃, 1575℃, etc.), with deep dephosphorization to ≤0.002wt% (specifically 0.0018wt%, 0.0016wt%, 0.0014wt%, 0.0012wt%, 0.001wt%, 0.0008wt%, 0.0006wt%, 0.0004wt%, etc.). After removing the oxidation slag, refining slag is added, followed by the addition of a deoxidizer to produce white slag. The temperature is then raised to 1580-1600℃ (specifically 1582℃, 1584℃, 1586℃, 1588℃, etc.). Temperatures of 1590℃, 1592℃, 1594℃, 1596℃, 1598℃, etc. are used to deeply desulfurize to S≤0.001wt% (specifically such as 0.00095wt%, 0.0009wt%, 0.00085wt%, 0.0008wt%, 0.00075wt%, 0.0007wt%, 0.00065wt%, 0.0006wt%, 0.00055wt%, 0.0005wt%, 0.00045wt%, 0.0004wt%, 0.0002wt%, 0.0001wt%, etc.), and various alloys are added to adjust the composition according to the target composition.

[0060] (5) RH vacuum degassing and soft-blown argon:

[0061] Under vacuum conditions ≤76 Pa (specifically 75 Pa, 74 Pa, 72 Pa, 70 Pa, 68 Pa, 66 Pa, 64 Pa, 62 Pa, 60 Pa, etc.), perform RH vacuum degassing treatment for no less than 20 min (specifically 21 min, 23 min, 25 min, 27 min, 29 min, 31 min, 33 min, 35 min, etc.) to remove N and O from the molten steel. Then, add a silicon-calcium wire to purify the molten steel. After purification, perform soft blowing argon treatment for no less than 20 min (specifically 21 min, 23 min, 25 min, 27 min, 29 min, 31 min, 33 min, 35 min, etc.).

[0062] (5) Continuous casting: The continuously cast billet is cooled in place or sent to a hot place instead of annealing;

[0063] (6) Rolling: The continuously cast billet is heated at 1220-1280℃ (specifically 1225℃, 1230℃, 1235℃, 1240℃, 1245℃, 1250℃, 1255℃, 1260℃, 1265℃, 1270℃, 1275℃, etc.) for 5-20 hours (specifically 6 hours, 8 hours, 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, 19 hours, etc.) and then hot-rolled. The hot-rolled round steel is then cooled slowly instead of annealed.

[0064] The present invention relates to a continuous casting production process for YLG-D raw material steel. Without compromising quality, it aims to solve the problem of difficult desulfurization in the converter, meet the ultra-pure requirements of YLG-D, and ensure the production rhythm of the continuous casting process. Therefore, sufficient molten iron preparation for the converter is required. Secondly, the ladle refining (LF+RH) requires rapid desulfurization and composition adjustment to ensure the continuous casting rhythm, thereby ensuring high-quality, fast-paced, and low-cost production of YLG-D.

[0065] Currently, converter smelting of similar alloy structural steels typically meets the requirements of P ≤ 0.015 wt% and S ≤ 0.005 wt%. On the one hand, converters raise the temperature through the chemical heat of the oxygen-carbon-oxygen reaction, resulting in a narrow controllable temperature range for P removal compared to electric arc furnaces. On the other hand, the rapid production pace of converter continuous casting leaves limited time for P removal. Therefore, the technical challenge of converter smelting of YLG-D lies in achieving P ≤ 0.005 wt% and S ≤ 0.001 wt%. In the preparation method of this invention, during P removal, a low-temperature, oxygen-rich environment is controlled for dephosphorization. First, the low-temperature range of the converter smelting process is controlled at 1550-1580℃, and initial P removal (P ≤ 0.008 wt%) is achieved by adding active lime. Then, no alloys or deoxidizers are added during tapping to maintain the oxidizing environment of the molten steel and ensure the dephosphorization effect. After adding the dephosphorizing agent, the dephosphorization temperature in the LF furnace is controlled at 1550-1580℃, allowing the slag to continue reacting with the phosphorus in the molten steel. This further dephosphorizes phosphorus in an oxidizing environment until it is ≤0.002wt%. Subsequent alloying to adjust the chemical composition may introduce some phosphorus, but the finished product will still meet the requirement of ≤0.005wt%. Compared to the normal process, no alloys or deoxidizers are added during tapping. Lime and dephosphorizing agent are added after the converter tapping begins, which is a crucial step in reducing phosphorus to ≤0.005wt%. After tapping, slag removal is performed to prevent the reduction of phosphorus in the P2O5 in the slag. During desulfurization, an oxygen-deficient high-temperature environment is controlled to achieve desulfurization. First, the sulfur content in the molten iron should be ≤0.03wt%. Then, pre-desulfurization is carried out using KR mechanical stirring to reduce sulfur content to ≤0.008wt%. Slag removal is performed both before and after desulfurization with the desulfurizing agent. Before desulfurization, the molten iron slag is removed, and after desulfurization, the desulfurization slag is removed to ensure that the sulfur content of the molten iron is ≤0.005wt% when it leaves the converter. After desulfurization in the above-mentioned oxidizing environment, the oxidizing slag is removed, refining slag is added, deoxidizing agent is added, white slag is produced, and the temperature is raised to 1580-1600℃ for deep desulfurization to reduce sulfur content to ≤0.001wt%.

[0066] As an optional implementation, in step (1) pre-desulfurization of molten iron, the composition of the molten iron before desulfurization is also required to be as follows: Si 0.2-0.4 wt% (specifically 0.22 wt%, 0.24 wt%, 0.26 wt%, 0.28 wt%, 0.30 wt%, 0.32 wt%, 0.34 wt%, 0.36 wt%, 0.38 wt%, etc.), As ≤ 0.008 wt% (specifically 0.0075 wt%, 0.007 wt%, 0.006 wt%, 0.005 wt%, 0.004 wt%, 0.003 wt%, 0.002 wt%, 0.001 wt%, etc.), Pb ≤ 0.001 wt% (specifically 0.0009 wt%, 0.0008 wt%, 0.0007 wt%, 0.0006 wt%, 0.0005 wt%, 0.0004 wt%, 0.0003 wt%). 0.0002wt%, 0.0001wt%, etc.), Sb≤0.003wt% (specifically such as 0.0029wt%, 0.0027wt%, 0.0025wt%, 0.0023wt%, 0.0021wt%, 0.0019wt%, 0.0017wt%, 0.0015wt%, 0.0013wt%, 0.001wt%, etc.), Sn≤0.004wt% (specifically such as 0.0039wt%, 0.0037wt%, 0.0035wt%, 0.0033wt%, 0.0031wt%, 0.0029wt%, 0.0027wt%, 0.0025wt%, 0.0023wt%, 0.0021wt%, etc.).

[0067] As an optional implementation, in step (1) pre-desulfurization of molten iron, the mass ratio of the desulfurizing agent to the molten iron is 1 to 1.5:115 (specifically, 1.1:115, 1.2:115, 1.3:115, 1.4:115, etc.).

[0068] As an optional implementation, in step (1) pre-desulfurization of molten iron, the desulfurizing agent comprises components in the following mass ratio: CaO:CaF2 = 7:3.

[0069] As an optional implementation, in step (1) of hot metal pre-desulfurization, the desulfurizing agent is added in two stages. The specific addition is adjusted according to the initial sulfur content of the hot metal and the actual site conditions; it can be added in smaller amounts first and then more, more first and then less, or the same amount can be added.

[0070] As an optional implementation, in step (1) pre-desulfurization of molten iron, the stirring time and stirring speed of the KR mechanical stirring are controlled at 40-50 min (specifically 41 min, 42 min, 43 min, 44 min, 45 min, 46 min, 47 min, 48 min, 49 min, etc.) and 100-120 r / min (specifically 112 r / min, 113 r / min, 114 r / min, 115 r / min, 116 r / min, 117 r / min, 118 r / min, 119 r / min, etc.).

[0071] As an optional implementation, in the converter smelting step (2), the mass ratio of the scrap steel to the molten iron is 23 to 26:115 (specifically, 23.5:115, 24:115, 24.5:115, 25:115, 25.5:115, etc.), and the Mo and Ni alloys are added into the furnace along with the scrap steel hopper, according to the lower limit of the raw material steel composition.

[0072] As an optional implementation, in the converter smelting step (2), the ratio of the mass of the lime added during the initial dephosphorization to the mass of the molten iron is 9 to 10:115 (specifically, 9.1:115, 9.3:115, 9.5:115, 9.7:115, 9.9:115, etc.).

[0073] As an optional implementation, in the converter smelting step (2), the ratio of the mass of the lime added during the tapping process to the mass of the molten iron is 1:143 to 145 (specifically, 1:143.3, 1:143.5, 1:143.7, 1:144, 1:144.3, 1:144.5, 1:144.7, 1:144.9, etc.).

[0074] As an optional implementation, in the converter smelting step (2), the mass ratio of fluorite to molten iron is 1:3833 to 3840 (specifically, 1:3834, 1:3835, 1:3836, 1:3837, 1:3838, 1:3839, etc.).

[0075] In the preparation process of this invention, the lime used contains ≥95wt% CaO (specifically, 95.5wt%, 96wt%, 97wt%, 98wt%, 99wt%, 100wt%, etc.), and the fluorite used contains ≥97wt% CaF2 (specifically, 97.5wt%, 98wt%, 98.5wt%, 99wt%, 99.5wt%, 100wt%, etc.). The main function of CaO is for dephosphorization and desulfurization, specifically dephosphorization at low temperatures in an oxygen-rich environment and desulfurization at high temperatures in an oxygen-deficient environment. The main function of fluorite is to improve the fluidity (viscosity) of molten steel and promote physicochemical reactions in the molten steel, such as facilitating the flotation of inclusions. The combined use of these two materials ultimately achieves the dephosphorization and desulfurization requirements of the molten steel, completing the purification process.

[0076] As an optional implementation, in the converter smelting step (2), the mass ratio of the dephosphorizing agent to the molten iron is 1:383 to 385 (specifically, 1:383.3, 1:383.5, 1:383.7, 1:384, 1:384.3, 1:384.5, 1:384.7, 1:384.9, etc.).

[0077] As an optional implementation, in the converter smelting step (2), the dephosphorizing agent includes: CaO 15-25wt% (specifically, 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, 21wt%, 22wt%, 23wt%, 24wt%, etc.), MgO ≤10wt% (specifically, 9wt%, 8wt%, 7wt%, 6wt%, 5wt%, 4wt%, 3wt%, 2wt%, 1wt%, etc.), SiO2 ≤6wt% (specifically, 5wt%, 4wt%, 3wt%, 2wt%, 1wt%, etc.), Al2O3 ≤10wt% (specifically, 9wt%, 8wt%, 7wt%, 6wt%, 5wt%, 4wt%, 3wt%). % (e.g., 2wt%, 1wt%), Fe2O3≥50wt% (e.g., 55wt%, 60wt%, 65wt%, 70wt%, 75wt%, 80wt%, 85wt%, 90wt%), P≤0.1wt% (e.g., 0.09wt%, 0.08wt%, 0.07wt%, 0.06wt%, 0.05wt%, 0.04wt%, 0.03wt%, 0.02wt%, 0.01wt%), S≤0.1wt% (e.g., 0.09wt%, 0.08wt%, 0.07wt%, 0.06wt%, 0.05wt%, 0.04wt%, 0.03wt%, 0.02wt%, 0.01wt%).

[0078] As an optional implementation, in the converter smelting step (2), the dephosphorizing agent and the fluorite are added together and in three batches, with one-third added in each batch.

[0079] As an optional implementation, in step (3) LF refining, the mass ratio of the refining slag to the molten iron is 1:383-385 (specifically, 1:383.3, 1:383.5, 1:383.7, 1:384, 1:384.3, 1:384.5, 1:384.7, 1:384.9, etc.), the binary basicity of the refining slag is 6-9 (specifically, 6.5, 7, 7.5, 8, 8.5), and the composition of the refining slag is: 50-60 wt% CaO (specifically, 51 wt% CaO, ...). 52wt%, 53wt%, 54wt%, 55wt%, 56wt%, 57wt%, 58wt%, 59wt%, etc.), 8-15wt% SiO2 (specifically 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, etc.), 3-8wt% MgO (specifically 4wt%, 5wt%, 6wt%, 7wt%, etc.), and 25-30wt% Al2O3 (specifically 26wt%, 27wt%, 28wt%, 29wt%, etc.).

[0080] As an optional implementation, in step (3) LF refining, the deoxidizer includes aluminum and silicon carbide, the aluminum includes aluminum wire and aluminum granules, the ratio of aluminum wire to molten iron is 3-3.5m:1t (specifically such as 3.1m:1t, 3.2m:1t, 3.3m:1t, 3.4m:1t, etc.), and the mass ratio of aluminum granules to molten iron is 1:1900-2900 (specifically such as 1:2000, 1:2100, 1:2200, 1:2300, 1:2400, 1:2500, 1:2600, 1:2700, 1:2800, etc.).

[0081] As an optional implementation, in step (3) LF refining, when adding deoxidizer, argon gas is controlled at 300-400NL / min (specifically 310NL / min, 320NL / min, 330NL / min, 340NL / min, 350NL / min, 360NL / min, 370NL / min, 380NL / min, 390NL / min, etc.). First, aluminum wire is fed in for precipitation deoxidation, and then aluminum particles and silicon carbide slag are added for surface diffusion deoxidation. Preferably, the silicon carbide used for deoxidation is added to the slag surface throughout the deoxidation process, in three batches: the first batch is 30-40 kg (specifically, 31 kg, 33 kg, 35 kg, 37 kg, 39 kg, etc.) of silicon carbide; the second batch is 40-50 kg (specifically, 41 kg, 43 kg, 45 kg, 47 kg, 49 kg, etc.) of silicon carbide; and the third batch is 40-50 kg (specifically, 41 kg, 43 kg, 45 kg, 47 kg, 49 kg, etc.) of silicon carbide.

[0082] As an optional implementation, in step (3) LF refining, lime and fluorite are added simultaneously with the addition of alloy to adjust the composition, in order to avoid the slag being too thick and affecting the desulfurization effect. Preferably, lime and fluorite are added in three batches: the first batch is 200 kg of lime and 60 kg of fluorite; the second batch is 300 kg of lime and 60 kg of fluorite; and the third batch is 300 kg of lime and 30 kg of fluorite, for a total of 600 kg of lime and 150 kg of fluorite. The amount of lime can be increased according to the desulfurization situation, and the total amount of lime is controlled at 800-1200 kg (specifically, 850 kg, 900 kg, 950 kg, 1000 kg, 1050 kg, 1100 kg, 1150 kg, etc.).

[0083] As an optional implementation, in step (3) LF refining, the alloy includes low titanium high chromium, ferrosilicon, nickel plate, ferromolybdenum, etc.

[0084] As an optional implementation, in step (4) RH vacuum degassing and soft argon blowing, the RH vacuum degassing treatment includes a deep circulation treatment of molten steel for no less than 15 min (specifically, 16 min, 18 min, 20 min, 22 min, 24 min, 26 min, 28 min, 30 min, etc.) and a vacuum degree ≤ 67 Pa (specifically, 66 Pa, 64 Pa, 62 Pa, 60 Pa, 58 Pa, 56 Pa, 54 Pa, 52 Pa, 50 Pa, etc.).

[0085] As an optional implementation, in step (4) RH vacuum degassing and soft blowing argon, the argon flow rate is controlled at 50-150 NL / min (specifically, 55 NL / min, 60 NL / min, 65 NL / min, 70 NL / min, 80 NL / min, 90 NL / min, 100 NL / min, 110 NL / min, 120 NL / min, 130 NL / min, 140 NL / min, 145 NL / min, etc.).

[0086] As an optional implementation, in step (4) RH vacuum degassing and soft argon blowing, the main components of the silicon-calcium wire are: 30-40wt% Ca (specifically 31wt%, 32wt%, 33wt%, 34wt%, 35wt%, 36wt%, 37wt%, 38wt%, 39wt%, etc.), 15-25wt% Si (specifically 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, 21wt%, 22wt%, 23wt%, 24wt%, etc.), and 10-20wt% Al (specifically 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, etc.).

[0087] As an optional implementation, in step (5) continuous casting, an arc-shaped continuous casting machine is used for casting, with an arc radius of 16.5m, a superheat of 20-40℃ (specifically 22℃, 24℃, 26℃, 28℃, 30℃, 32℃, 34℃, 36℃, 38℃, etc.), and a casting speed of 0.4~0.5m / min for 410×530mm (specifically 0.41m / min, 0.43m / min, 0.45m / min, 0.47m / min, 0.49m / min, etc.), with full-process protective casting.

[0088] As an optional implementation, in step (6) rolling, during heating, the furnace is first cooled down to: a preheating section ≤ 800℃ (specifically, 790℃, 770℃, 750℃, 730℃, 710℃, 690℃, etc.), a first heating section 800-1180℃ (specifically, 850℃, 900℃, 950℃, 1000℃, 1050℃, 1100℃, 1150℃, etc.), a second heating section 1220-1280℃ (specifically, 1225℃, 1230℃, 1240℃, 1250℃, 1260℃, 1270℃, 1275℃, etc.), and a soaking section 1220-1280℃ (specifically, 1225℃, 1230℃, 1240℃, 1250℃, 1260℃, 1270℃, 1275℃, etc.). When the furnace is filled with material, the heating rate is controlled at ≤100℃ / h (specifically 95℃ / h, 90℃ / h, 80℃ / h, 70℃, 60℃, 50℃, etc.). The final temperature inside the furnace is controlled as follows: heating stage ≤900℃ (specifically 890℃, 870℃, 850℃, 830℃, 810℃, 790℃, etc.), heating stage ≤1240℃ (specifically 1230℃, 1220℃, 1210℃, 1200℃, 1190℃, etc.), and soaking stage ≤1280℃ (specifically 1270℃, 1260℃, 1250℃, 1240℃, 1230℃, 1220℃, 1210℃, 1200℃, etc.).

[0089] As an optional implementation, in step (6) rolling, a 1350 mill is used for billet opening and a 750 mill is used for forming, with the finished product specification being Φ120mm.

[0090] The rolling process does not require microstructure control; it only ensures that the surface is free of cracks and the internal structure of the steel is free of oxidation. Because YLG-D steel undergoes surface rust removal before being used in a vacuum induction furnace, if the surface cracks and the interior is oxidized, the rust removal process cannot completely remove the oxidation, affecting the purity control of the vacuum induction melting process.

[0091] As an optional implementation, in (6) rolling, in order to reduce the surface hardness of the steel, the rolled steel is placed in an insulation hood for slow cooling for ≥26h (specifically, 27h, 28h, 29h, 30h, 31h, 32h, 33h, 34h, 35h, etc.), and after exiting the insulation hood, the steel is quickly placed in a pit for cooling for ≥10h (specifically, 11h, 12h, 13h, 14h, 15h, 16h, 17h, etc.).

[0092] Secondly, a raw material steel obtained by the above preparation method, wherein the chemical composition of the raw material steel is as follows: C 0.25-0.32wt% (specifically, 0.26wt%, 0.27wt%, 0.28wt%, 0.29wt%, 0.30wt%, 0.31wt%, etc.), Si 0.8-1.2wt% (specifically, 0.85wt%, 0.9wt%, 0.95wt%, 1.0wt%, 1.05wt%, 1.1wt%, 1.15wt%, etc.), Mn ≤0.1wt% (specifically, 0.09wt%, 0.08wt%, 0.07wt%, 0.06wt%, 0.05wt%, 0.04wt%, 0.03wt%, 0.02wt%, 0.01wt%, etc.). P ≤ 0.005 wt% (specifically, 0.001 wt%, 0.0015 wt%, 0.002 wt%, 0.0025 wt%, 0.003 wt%, 0.0035 wt%, 0.004 wt%, 0.0045 wt%, etc.), S ≤ 0.001 wt% (specifically, 0.00095 wt%, 0.0009 wt%, 0.00085 wt%, 0.0008 wt%, 0.00075 wt%, 0.0007 wt%, 0.00065 wt%, 0.0006 wt%, 0.00055 wt%, 0.0005 wt%, 0.00045 wt%, 0.0004 wt%, 0.0002 wt%, 0.0001 wt%, etc.), Cr 0.8-1.2 wt% (specifically 0.85 wt%, 0.9 wt%, 0.95 wt%, 1.0 wt%, 1.05 wt%, 1.1 wt%, 1.15 wt%, etc.), Ni ≤ 0.3 wt% (specifically 0.29 wt%, 0.28 wt%, 0.27 wt%, 0.26 wt%, 0.25 wt%, 0.24 wt%, 0.23 wt%, 0.22 wt%, 0.21 wt%, etc.), Mo 0.35-0.45 wt% (specifically 0.36 wt%, 0.37 wt%, 0.38 wt%, 0.39 wt%, 0.40 wt%, 0.41 wt%, 0.42 wt%, 0.43 wt%, 0. 0.44wt%, Cu≤0.10wt% (specifically 0.09wt%, 0.08wt%, 0.07wt%, 0.06wt%, 0.05wt%, 0.04wt%, 0.03wt%, 0.02wt%, 0.01wt%, etc.), Al≤0.025wt% (specifically 0.024wt%, 0.022wt%, 0.020wt%, 0.018wt%, 0.016wt%, 0.014wt%, 0.012wt%, 0.01wt%, etc.), As≤0.008wt% (specifically 0.0075wt%, 0.007wt%, 0.006wt%, 0.005wt%, 0.008wt%).0.004wt%, 0.003wt%, 0.002wt%, 0.001wt%, etc.), Pb≤0.001wt% (specifically such as 0.00095wt%, 0.0009wt%, 0.00085wt%, 0.0008wt%, 0.00075wt%, 0.0007wt%, 0.00065wt%, 0.0006wt%, 0.00055wt%, 0.0005wt%, ...1wt%), etc. 0.45wt%, 0.0004wt%, 0.0002wt%, 0.0001wt%, etc.), Sb≤0.003wt% (specifically such as 0.0029wt%, 0.0027wt%, 0.0025wt%, 0.0023wt%, 0.0021wt%, 0.0019wt%, 0.0017wt%, 0.0015wt%, 0.0013wt%, 0.001wt%, etc.), Sn≤0. 0.004 wt% (specifically such as 0.0039 wt%, 0.0037 wt%, 0.0035 wt%, 0.0033 wt%, 0.0031 wt%, 0.0029 wt%, 0.0027 wt%, 0.0025 wt%, 0.0023 wt%, 0.002 wt%, etc.), N ≤ 0.006 wt% (specifically such as 0.0059 wt%, 0.0057 wt%, 0.0055 wt%, 0.005... 3 wt%, 0.0051 wt%, 0.0049 wt%, 0.0047 wt%, 0.0045 wt%, 0.0043 wt%, 0.004 wt%, etc.), 0 ≤ 0.0015 wt% (specifically 0.0014 wt%, 0.0012 wt%, 0.0010 wt%, 0.0008 wt%, 0.0006 wt%, 0.0004 wt%, 0.0002 wt%, etc.) and the balance Fe.

[0093] The raw material steel of this invention has high purity, with P≤0.005wt% and S≤0.001wt%. In particular, the phosphorus content is more than an order of magnitude lower than that of ordinary steel, which can meet the production needs of vacuum induction smelting of low alloy ultra-high strength steel for aerospace applications.

[0094] The present invention will now be described in further detail with reference to specific embodiments.

[0095] Example 1

[0096] A method for preparing YLG-D raw material steel, the specific steps of which are as follows:

[0097] (1) Pre-desulfurization of molten iron

[0098] The molten iron contained 0.03 wt% sulfur, ≤0.11 wt% phosphorus, and other elements including 0.2 wt% Si, 0.0058 wt% As, 0.0006 wt% Pb, 0.0014 wt% Sb, and 0.0025 wt% Sn. The molten iron temperature was 1353 ± 2℃. Pre-desulfurization was performed using KR mechanical stirring. Before desulfurization, the molten iron slag was removed, and then desulfurization was carried out. Based on the initial sulfur content of the molten iron, a total of 1200 kg of desulfurizing agent was added twice. The stirring time and stirring speed were controlled at 44 min and 110 r / min, respectively. After desulfurization, the desulfurization slag was removed, and the sulfur content at the outlet was 0.004 wt%.

[0099] The desulfurizing agent comprises components in the following mass ratio: CaO:CaF2 = 7:3.

[0100] (2) 120t converter smelting

[0101] Using molten iron and scrap steel as raw materials, 23 tons of scrap steel and 115 tons of molten iron were added to the furnace along with the scrap steel, according to the lower limit of the raw steel composition. The initial dephosphorization temperature was controlled at 1560±2℃, and 9t of lime was added for initial dephosphorization to make P≤0.008wt%. The final steel composition and tapping temperature were controlled at the converter tapping point: carbon 0.04wt%, phosphorus 0.006wt%, and 1580℃.

[0102] After the steel is tapped from the converter, add 300 kg of dephosphorizing agent, 30 kg of fluorite, and 800 kg of quicklime in sequence.

[0103] The dephosphorizing agent includes: CaO 15-25wt%, MgO≤10wt%, SiO2≤6wt%, Al2O3≤10wt%, Fe2O3≥50wt%, P≤0.1wt%, and S≤0.1wt%.

[0104] When the converter reaches 20 tons of steel output, 300 kg of dephosphorizing agent and 30 kg of fluorite are manually added in three batches (100 kg of dephosphorizing agent and 10 kg of fluorite per batch). When the steel output reaches 50 tons, 800 kg of active lime is added as top slag. Compared with the normal process, no alloying or deoxidizing agents are added during the steel output process to ensure the dephosphorizing effect.

[0105] (3) 120t LF refining

[0106] After adding the dephosphorizing agent, the furnace is heated in the LF furnace and the temperature is raised to the dephosphorization temperature of 1578±2℃, so that the slag and P in the molten steel continue to react and further dephosphorize to 0.002wt% in an oxidizing environment. Then the oxidizing slag is removed and 300Kg of refining slag is added. The basicity of the refining slag is 6-9. The composition of the refining slag is: 50-60wt% CaO, 8-15wt% SiO2, 3-8wt% MgO and 25-30wt% Al2O3.

[0107] After slag is removed and the furnace is energized, argon gas is controlled at 350 NL / min, 375m of aluminum wire is fed in for precipitation and deoxidation, 50kg of aluminum granules are added for surface deoxidation, and silicon carbide diffusion deoxidation is performed to create white slag. The temperature is raised to 1598±2℃ for deep desulfurization to S≤0.001wt%. Subsequently, special alloys such as low-titanium high-chromium, ferrosilicon, nickel plates, and ferromolybdenum are added to adjust the composition according to the target requirements. Lime and fluorite are added simultaneously with the alloys to form slag and prevent the slag from becoming too thick, which would affect the desulfurization effect. The lime and fluorite are added in three batches: the first batch contains 200kg of lime and 60kg of fluorite; the second batch contains 300kg of lime and 60kg of fluorite; and the third batch contains 300kg of lime and 30kg of fluorite, for a total of 600kg of lime and 150kg of fluorite. Silicon carbide for deoxidation is added to the slag surface throughout the process, in three batches: the first batch is 35 kg of silicon carbide; the second batch is 45 kg of silicon carbide; and the third batch is 45 kg of silicon carbide.

[0108] (4) 120t RH vacuum treatment

[0109] Under a vacuum degree ≤76 Pa, the total treatment time for molten steel was 20 min, including a 15 min deep circulation treatment of the molten steel under a vacuum degree ≤67 Pa. After the vacuum treatment, 70 m of silicon-calcium wire was fed. The main components of the silicon-calcium wire were: 30-40 wt% Ca, 15-25 wt% Si, and 10-20 wt% Al. Afterwards, the argon flow rate for soft blowing of the molten steel was controlled at 125 NL / min, and the soft blowing time was 20 min.

[0110] (5) Continuous casting

[0111] Casting was carried out using an arc-shaped continuous casting machine with an arc radius of 16.5m, a superheat of 30℃, a casting speed of 410×530mm: 0.4m / min, and full-process protective casting with billet stack cooling.

[0112] (6) Rolling

[0113] Heating: First, cool down the furnace to the following temperatures: preheating section ≤ 800℃, heating section 800-1180℃, heating section 1220-1280℃, and soaking section 1220-1280℃. After filling the furnace with material, control the heating rate at ≤ 100℃ / h. The final furnace temperature is controlled at: heating section ≤ 900℃, heating section ≤ 1240℃, and soaking section 1280℃. The total heating and holding time is 900min.

[0114] Rolling: The billet is opened using a 1350 mill and the finished product is formed using a 750 mill, with a finished product specification of Φ120mm. The rolling process does not require control of the microstructure, only ensuring that the surface is free of cracks.

[0115] Post-rolling: In order to reduce the surface hardness of the steel, the rolled steel is placed in an insulation hood for slow cooling for 26 hours. After exiting the insulation hood, the steel is quickly placed in a pit for cooling for 10 hours.

[0116] The chemical composition of YLG-D obtained in this embodiment is shown in Table 1 below. Furthermore, its inclusion coefficient, as determined by Aspex scanning analysis, is 3.5.

[0117] Example 2: A method for preparing YLG-D raw material steel, the specific steps of which are as follows:

[0118] (1) Pre-desulfurization of molten iron

[0119] The molten iron contained 0.028 wt% sulfur, ≤0.11 wt% phosphorus, and other elements including 0.4 wt% Si, 0.0052 wt% As, 0.0005 wt% Pb, 0.0012 wt% Sb, and 0.0028 wt% Sn. The molten iron temperature was 1370 ± 2℃. Pre-desulfurization was performed using KR mechanical stirring. Before desulfurization, the molten iron slag was removed, and then desulfurization was carried out. Based on the initial sulfur content of the molten iron, a total of 1300 kg of desulfurizing agent was added twice. The stirring time and stirring speed were controlled at 45 min and 120 r / min, respectively. After desulfurization, the desulfurization slag was removed, and the sulfur content at the outlet was 0.003 wt%.

[0120] The desulfurizing agent comprises components in the following mass ratio: CaO:CaF2 = 7:3.

[0121] (2) 120t converter smelting

[0122] Using molten iron and scrap steel as raw materials, with 26 tons of scrap steel and 115 tons of molten iron, Mo and Ni alloys are added to the furnace along with the scrap steel, according to the lower limit of the raw steel composition. The initial dephosphorization temperature is controlled at 1553±2℃, and 10t of lime is added for initial dephosphorization to make P≤0.008wt%. The final steelmaking composition and tapping temperature are controlled as follows: carbon 0.06wt%, phosphorus 0.007wt%, and 1570℃.

[0123] The remaining settings in step (2) are the same as in Example 1.

[0124] (3) 120t LF refining

[0125] After adding the dephosphorizing agent, the furnace is heated in the LF furnace and the temperature is raised to the dephosphorization temperature of 1570±2℃, so that the slag and P in the molten steel continue to react and further dephosphorize to 0.0018wt% in an oxidizing environment. Then the oxidizing slag is removed and 300Kg of refining slag is added. The basicity of the refining slag is 6-9. The composition of the refining slag is: 50-60wt% CaO, 8-15wt% SiO2, 3-8wt% MgO and 25-30wt% Al2O3.

[0126] After the slag is removed and put into the pit, power is supplied, argon gas is controlled at 350NL / min, 400m of aluminum wire is fed in for precipitation and deoxidation, 55Kg of aluminum granules are added for slag surface deoxidation, and silicon carbide diffusion deoxidation is used to create white slag. The temperature is raised to 1590±2℃ for deep desulfurization to S≤0.001wt%.

[0127] The remaining settings in step (3) are the same as in Example 1.

[0128] (4) 120t RH vacuum treatment

[0129] Under vacuum conditions of ≤76 Pa, the total treatment time for molten steel is 21 min, including 15 min of deep circulation treatment of molten steel under vacuum conditions of ≤67 Pa. After vacuum treatment, 60 m of silicon-calcium wire is fed.

[0130] The remaining settings in step (4) are the same as in Example 1.

[0131] (5) Continuous casting

[0132] Superheat: 35°C, and the remaining settings of step (5) are the same as in Example 1.

[0133] (6) Rolling

[0134] Heating: First, cool down the furnace to the following temperatures: preheating section ≤ 800℃, heating section 800-1180℃, heating section 1220-1280℃, and soaking section 1220-1280℃. After filling the furnace with material, control the heating rate at ≤ 100℃ / h. The final furnace temperature is controlled at: heating section ≤ 900℃, heating section ≤ 1240℃, and soaking section 1270℃. The total heating and holding time is 840 minutes.

[0135] The rolling process and post-rolling treatment are the same as in Example 1.

[0136] The chemical composition of YLG-D obtained in this embodiment is shown in Table 1 below. Furthermore, its inclusion coefficient, as determined by Aspex scanning analysis, is 4.3.

[0137] Table 1. Chemical composition of YLG-D obtained in Examples 1, 2 and Comparative Examples 1-10

[0138]

[0139]

[0140] Comparative Example 1

[0141] The difference from Example 1 is that, between steps (2) and (3), alloys and aluminum ingot deoxidizers are added during the converter tapping process, with 1.2-2.4 kg of aluminum ingot deoxidizer added per ton of molten steel, and the feeding of aluminum wire is cancelled in step (3). All other settings are the same as in Example 1, and the P content of the raw steel obtained is 0.012 wt%.

[0142] Comparative Example 2

[0143] The difference from Example 1 is that in step (1), the molten iron is not subjected to KR mechanical stirring pre-desulfurization treatment. All other settings are the same as in Example 1. The content of the raw steel obtained is 0.004wt% and the content of S is 0.010wt%.

[0144] Comparative Example 3

[0145] The difference from Example 1 is that in step (1), the molten iron is not subjected to KR mechanical stirring pre-desulfurization treatment; between steps (2) and (3), alloy and aluminum ingot deoxidizer are added during the converter tapping process, with 1.2-2.4 kg of aluminum ingot deoxidizer added per ton of molten steel, and the feeding of aluminum wire is cancelled in step (3). The rest of the settings are the same as in Example 1. The P content of the obtained raw steel is 0.015 wt%, and the S content is 0.008 wt%.

[0146] Comparative Example 4

[0147] The difference from Example 1 is that in step (4), no silicon-calcium wire treatment was added, and the resulting raw steel had an inclusion coefficient of 7.2 after Aspex scanning analysis.

[0148] Comparative Example 5

[0149] The difference from Example 1 is that in step (2), the temperature of the initial dephosphorization is controlled at 1600±2℃, and the rest of the settings are the same as in Example 1.

[0150] Comparative Example 6

[0151] The difference from Example 1 is that in step (2), the temperature of the initial dephosphorization is controlled at 1500±2℃, and the rest of the settings are the same as in Example 1.

[0152] Comparative Example 7

[0153] The difference from Example 1 is that in step (3), the temperature for deep dephosphorization is controlled at 1600±2℃, while the rest of the settings are the same as in Example 1.

[0154] Comparative Example 8

[0155] The difference from Example 1 is that in step (3), the temperature for deep dephosphorization is controlled at 1500±2℃, while the rest of the settings are the same as in Example 1.

[0156] Comparative Example 9

[0157] The difference from Example 1 is that in step (3), the temperature of deep desulfurization is controlled at 1560±2℃, and the rest of the settings are the same as in Example 1.

[0158] Comparative Example 10

[0159] The difference from Example 1 is that in step (3), the temperature of deep desulfurization is controlled at 1650±2℃, and the rest of the settings are the same as in Example 1.

[0160] A comparison of the data in Table 1 for Examples 1, 1, 2, and 3 shows that whether or not alloys and deoxidizers are added during the tapping process affects both dephosphorization and desulfurization. Furthermore, not adding alloys and deoxidizers during tapping is crucial for reducing phosphorus (P) to ≤0.005 wt%, and is also an important step in further ensuring desulfurization to 0.001 wt% or lower. KR mechanical stirring pre-desulfurization treatment of molten iron is a necessary step to ensure desulfurization to 0.001 wt% or lower, but it does not affect the dephosphorization effect.

[0161] A comparison of the data in Table 1 between Example 1 and Comparative Example 4 shows that the addition of silicon-calcium wire can reduce the inclusion coefficient of the raw steel, but has no effect on dephosphorization and desulfurization.

[0162] A comparison of the data in Table 1 between Example 1 and Comparative Examples 5 and 6 shows that when the initial dephosphorization temperature exceeds the range of 1550-1580℃, whether too high or too low, the dephosphorization effect is poor, and the final dephosphorization effect cannot be lower than 0.005wt%, affecting the purity of the raw steel and causing it to fail to meet the standard.

[0163] A comparison of the data in Table 1 between Example 1 and Comparative Examples 7 and 8 shows that when the deep dephosphorization temperature exceeds the range of 1550-1580℃, whether too high or too low, the dephosphorization effect is poor, and the final dephosphorization effect cannot be lower than 0.005wt%, affecting the purity of the raw steel and causing it to fail to meet the standard.

[0164] A comparison of the data in Table 1 between Example 1 and Comparative Examples 9 and 10 shows that when the deep desulfurization temperature exceeds the range of 1580-1600℃, whether too high or too low, the desulfurization effect is poor, failing to achieve a final desulfurization efficiency below 0.001wt%, thus affecting the purity of the raw steel and causing it to fail to meet standards. The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are within the scope of protection of the pending claims.

Claims

1. A method of producing a raw steel, characterized by, The method comprises the following steps: (1) hot metal pre-desulphurization: controlling P≤0.11wt%, S≤0.03wt% in hot metal, hot metal temperature≥1350℃, pre-desulphurization by KR mechanical stirring to make S≤0.008wt%, slagging operation before and after desulphurization by desulphurizer to make S≤0.005wt% in hot metal; the desulphurizer comprises components with the following mass ratio: CaO:CaF2=7:3; the mass ratio of the desulphurizer to the hot metal is 1-1.5:115; (2) converter smelting: hot metal, scrap steel, Mo alloy and Ni alloy are added into the converter, the dephosphorization temperature is controlled at 1550-1580℃, lime is added to make preliminary dephosphorization to make P≤0.008wt%, the tapping temperature is controlled at 1560-1640℃, the tapping C content is 0.04-0.10wt%, P≤0.008wt%, dephosphorization agent, fluorite and lime are added during tapping to make P≤0.005wt%, and then slagging operation is performed; the dephosphorization agent comprises: CaO 15-25wt%, MgO≤10wt%, SiO2≤6wt%, Al2O3≤10wt%, Fe2O3≥50wt%, P≤0.1wt%, S≤0.1wt%; the mass ratio of the dephosphorization agent to the hot metal is 1:383-385; (3) LF refining: the dephosphorization temperature is controlled at 1550-1580℃, deep dephosphorization is performed to≤0.002wt%, after removing the oxidized slag, refining slag is added, deoxidizing agent is added to deoxidize and form white slag, the temperature is raised to 1580-1600℃, deep desulphurization is performed to S≤0.001wt%, and various alloys are added according to the target composition to adjust the composition; (4) RH vacuum degassing and soft argon blowing: after RH vacuum degassing treatment, silicon-calcium wire is added to purify the molten steel, and then soft argon blowing treatment is performed; (5) continuous casting: the continuous casting billet is replaced by annealing by using cold stacking or red delivery; (6) rolling: the continuous casting billet is heated at 1220-1280℃ for 5-20h, and then hot rolling is performed to obtain hot rolled round steel which is replaced by annealing by using slow cooling.

2. The production method according to claim 1, wherein In the step (1) hot metal pre-desulphurization, the hot metal composition before desulphurization also requires the following: Si 0.2-0.4wt%, As≤0.008wt%, Pb≤0.001wt%, Sb≤0.003wt%, Sn≤0.004wt%; and / or, the stirring time and stirring speed of the KR mechanical stirring are controlled at 40-50min and 100-120r / min respectively.

3. The production method according to claim 1, wherein In the step (2) converter smelting, the mass ratio of the scrap steel to the hot metal is 23-26:115, Mo and Ni alloy are added into the converter with the scrap steel, and are added according to the lower limit of the raw material steel composition; and / or, the ratio of the mass of the lime added to the mass of the hot metal during preliminary dephosphorization is 9-10:115; and / or, the ratio of the mass of the lime added to the mass of the hot metal during tapping is 1:143-145; and / or, in the step (2) converter smelting, the mass ratio of the fluorite to the hot metal is 1:3833-3840.

4. The production method according to claim 3, wherein In the LF refining step (3), the mass ratio of the refining slag to the molten iron is 1:383-385; and / or, the binary basicity of the refining slag is 6-9; and / or, the refining slag comprises 50-60wt% CaO, 8-15wt% SiO2, 3-8wt% MgO and 25-30wt% Al2O3; and / or, the deoxidizer comprises aluminum and silicon carbide, the aluminum comprises aluminum wire and aluminum particles, the ratio of the aluminum wire to the molten iron is 3-3.5m:1t, and the mass ratio of the aluminum particles to the molten iron is 1:1900-2900; and / or, when the deoxidizer is added, the argon is controlled at 300-400NL / min, the aluminum wire is fed first for precipitation deoxidation, and then the aluminum particles and the silicon carbide slag are added for surface diffusion deoxidation; and / or, the lime and the fluorite are added for slagging at the same time when the alloy is added to adjust the composition, so as to avoid the influence of the thickening slag on the desulfurization effect; and / or, the alloy comprises low-titanium high-chromium, ferrosilicon, nickel plate and molybdenum iron.

5. The production method according to claim 1, wherein In the RH vacuum degassing and soft argon blowing step (4), the RH vacuum degassing treatment is performed under a vacuum degree of ≤76Pa for not less than 20min to remove N and O in the molten steel; and / or, the soft argon blowing treatment time is not less than 20min; and / or, the main components of the calcium-silicon wire are 30-40wt% Ca, 15-25wt% Si and 10-20wt% Al.

6. The production method according to claim 5, wherein The RH vacuum degassing treatment comprises a cyclic deep treatment of the molten steel under a vacuum degree of ≤67Pa for not less than 15min; and / or, the argon flow is controlled at 50-150NL / min during the soft argon blowing treatment.

7. The production method according to claim 1, wherein In the continuous casting step (5), an arc-shaped continuous casting machine is used for casting, the arc radius is 16.5m, the superheat is 20-40℃, the casting speed is 410×530mm:0.4-0.5m / min, and the whole process is protected casting.

8. The production method according to claim 1, wherein In the rolling step (6), during the heating, the temperature is first lowered in the furnace, and the temperature is lowered to ≤800℃ in the preheating section, 800-1180℃ in the first heating section, 1220-1280℃ in the second heating section and the soaking section; the temperature rising speed is controlled at ≤100℃ / h after the furnace is filled with materials, and the final temperature in the furnace is controlled at ≤900℃ in the first heating section, ≤1240℃ in the second heating section and ≤1280℃ in the soaking section; and / or, the 1350 unit is used for blooming and the 750 unit is used for forming the material in the hot rolling; and / or, in order to reduce the surface hardness of the steel, the steel is slowly cooled in the holding cover for ≥26h after rolling, and the steel is rapidly cooled in the pit for ≥10h after leaving the holding cover.

9. The production method according to any one of claims 1 to 8, characterized by, The CaO content in the lime is ≥95wt%, and the CaF2 content in the fluorite is ≥97wt%.

10. A raw steel produced by the production method according to any one of claims 1 to 9, characterized in that, The chemical composition of the raw steel is as follows: C 0.25-0.32wt%, Si 0.8-1.2wt%, Mn≤0.1wt%, P≤0.005wt%, S≤0.001wt%, Cr 0.8-1.2wt%, Ni≤0.3wt%, Mo 0.35-0.45wt%, Cu≤0.10wt%, Al≤0.025wt%, As≤0.008wt%, Pb≤0.001wt%, Sb≤0.003wt%, Sn≤0.004wt%, N≤0.006wt%, O≤0.0015wt% and the balance of Fe.

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