Low-nitrogen cold heading steel and method for producing the same

Abstract

The application belongs to the technical field of steelmaking, and provides a low-nitrogen cold upsetting steel and a production method thereof. Desulfurized molten iron and scrap steel are used as raw materials, and the molten iron is desulfurized in a blast furnace, and then subjected to an electric furnace smelting process, an LF refining process and a continuous casting process. The desulfurization of the molten iron in the blast furnace reduces the sulfur content in the molten iron, and the sulfur content after treatment is required to be less than or equal to 0.003%. At the same time of the desulfurization of the molten iron, the nitrogen content is also significantly reduced. By moving the desulfurization process to the pretreatment of the molten iron, the serious increase of nitrogen caused by the desulfurization in the refining LF process is avoided. The desulfurized molten iron is added in a proportion of more than 45%, and the purpose of controlling the nitrogen content in the cold upsetting steel without vacuum treatment in the electric furnace process is achieved.

Description

Technical Field

[0001] This application belongs to the field of steelmaking technology, specifically relating to a low-nitrogen cold heading steel and its production method. Background Technology

[0002] Cold heading steel is a type of steel manufactured using the cold heading process. It is widely used in the production of various standard mechanical parts and fasteners such as screws, pins, and nuts. Therefore, it must have good cold forging performance and requires low content of impurities such as S, P, and N in the steel.

[0003] Traditional electric arc furnace (EAF) steelmaking processes typically involve EAF smelting, LF refining, and continuous casting. EAF steel produced from ordinary molten iron and scrap steel has high sulfur (S) and nitrogen (N) content. To reduce sulfur content, deep deoxidation with Al alloys is performed during EAF tapping and LF refining to lower oxygen content before desulfurization. However, deep deoxidation and desulfurization processes result in significant nitrogen absorption. Therefore, current technologies employ an additional vacuum treatment process: primary furnace smelting, VD furnace vacuum refining, and continuous casting, to reduce the N content in the molten steel.

[0004] However, vacuuming and maintaining a vacuum will cause the temperature of the molten steel to drop, so the temperature of the LF when it leaves the station is required to be high, which will increase the power consumption and electrode consumption of the LF. Moreover, high temperature molten steel reduces the life of the ladle, and vacuum treatment increases the smelting cost, etc. Summary of the Invention

[0005] In view of this, in order to solve the problems of high power consumption and electrode consumption, poor ladle durability, and high smelting cost caused by vacuum treatment in the existing cold heading steel preparation process, this application proposes a production method for low-nitrogen cold heading steel. Instead of using vacuum treatment, it reduces energy consumption and production cost in the cold heading steel preparation process by controlling several other key factors.

[0006] To address the aforementioned deficiencies, this application also provides a low-nitrogen cold heading steel, in which the contents of nitrogen, aluminum, and sulfur are all relatively low.

[0007] This application provides the following technical solution:

[0008] In a first aspect, this application provides a method for producing low-nitrogen cold heading steel, the method comprising: using desulfurized molten iron and scrap steel as raw materials, and performing electric furnace smelting, LF refining and continuous casting processes;

[0009] The sulfur content of the desulfurized molten iron is ≤0.003wt%; the nitrogen content of the desulfurized molten iron is ≤0.0025wt%.

[0010] The amount of desulfurized molten iron added is 45 wt% or more;

[0011] The nitrogen content of the scrap steel is ≤40ppm.

[0012] In one optional implementation, 11-13 kg of desulfurizing agent is added per ton of molten iron during the preparation of desulfurized molten iron.

[0013] Preferably, the desulfurization time is 8-12 minutes;

[0014] More preferably, the desulfurizing agent is a mixture of lime and fluorite;

[0015] More preferably, the ratio of lime to fluorite is (8-10):1.

[0016] In one optional embodiment, after the molten iron is desulfurized, a slag removal process is performed, wherein the exposed area of ​​the molten iron surface during the slag removal process is ≥75%.

[0017] Preferably, the desulfurized molten iron is added at a rate of 9-11 t / min.

[0018] In one alternative embodiment, the nitrogen content of the steel smelted in the electric furnace is ≤20ppm, preferably below 18ppm;

[0019] Preferably, the temperature at which steel is smelted in an electric furnace is above 1640°C;

[0020] More preferably, the temperature of the molten pool is raised to above 1500°C within 25 minutes during the electric furnace smelting process.

[0021] In one alternative implementation, when the electric furnace melts 3 / 5 to 4 / 5 of the steel, 0.8-1.2 kg of aluminum blocks are added per ton of steel;

[0022] Preferably, the aluminum block contains ≥99.5% Al;

[0023] More preferably, the particle size of the aluminum blocks is 10-50 mm.

[0024] In one alternative implementation, at the end of LF refining, the aluminum content in the molten steel is ≤0.03%.

[0025] In one optional embodiment, the length and width of the scrap steel are both ≤50cm;

[0026] Preferably, the thickness of the scrap steel is ≤1cm.

[0027] In one alternative embodiment, the nitrogen content of the resulting low-nitrogen cold heading steel is ≤40ppm.

[0028] In one optional embodiment, the low-nitrogen cold heading steel is grade ML08Al.

[0029] Preferably, the composition of the steel grade ML08Al by weight percentage includes: C 0.05-0.08%; Si < 0.1%; Mn 0.30-0.38%; P < 0.018%; S < 0.01%; Al 0.020-0.055%; Cu < 0.12%; Cr < 0.08% and N ≤ 0.004%.

[0030] Secondly, this application also provides a low-nitrogen cold heading steel obtained by the aforementioned production method.

[0031] The technical solution of this application has the following advantages:

[0032] 1. The production method of low-nitrogen cold heading steel in this application uses desulfurized molten iron and scrap steel as raw materials, and involves electric furnace smelting, LF refining, and continuous casting processes. The sulfur content of the desulfurized molten iron is ≤0.003wt%; the nitrogen content is ≤0.0025wt%; the amount of desulfurized molten iron added is ≥45wt%; and the nitrogen content of the scrap steel is ≤40ppm. By moving the desulfurization process forward to the molten iron pretreatment stage, the severe nitrogen increase caused by desulfurization during the LF refining process is avoided. By controlling the composition and amount of raw materials, the goal of controlling the nitrogen content in cold heading steel without vacuum treatment in the electric furnace process is achieved.

[0033] 2. This application further improves the cold heading steel by controlling the tapping temperature to a higher level (≥1640℃); adding aluminum blocks for weak deoxidation during electric furnace smelting; and controlling the Al content to within 0.03% before continuous casting, thereby resulting in a lower nitrogen content in the prepared cold heading steel. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0035] Figure 1 The N content of each step in the preparation of ML08Al in Example 1 of this application;

[0036] Figure 2 The N content of each step in the preparation of ML08Al using conventional methods. Detailed Implementation

[0037] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0038] Unless otherwise stated or in case of contradiction, the terms or phrases used herein shall have the following meanings:

[0039] In this application, the selection range of "and / or", "or / and", and "and / or" includes any one of two or more related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. It should be noted that when at least three items are connected by at least two conjunctions selected from "and / or", "or / and", and "and / or", it should be understood that the technical solution undoubtedly includes technical solutions connected by "logical AND", and also undoubtedly includes technical solutions connected by "logical OR".

[0040] For example, "A and / or B" includes three parallel solutions: A, B, and A+B. Another example is the technical solution "A, and / or, B, and / or, C, and / or, D," which includes any one of A, B, C, and D (i.e., all connected by "logical OR"), any and all combinations of A, B, C, and D, including combinations of any two or three of A, B, C, and D, and combinations of all four of A, B, C, and D (i.e., all connected by "logical AND").

[0041] In this application, the terms "multiple", "various", "multiple times", "multi-dimensional", etc., unless otherwise specified, refer to a quantity greater than or equal to 2. For example, "one or more" means one or more or more.

[0042] In this application, the terms "combinations thereof", "any combination thereof", and "any combination thereof" include all suitable combinations of any two or more of the listed items.

[0043] In this application, the term "suitable" as used in phrases such as "suitable combination," "suitable method," and "any suitable method" refers to the ability to implement the technical solution of this application, solve the technical problem of this application, and achieve the expected technical effect of this application.

[0044] In this application, terms such as "preferred," "better," "more suitable," and "ideal" are used only to describe implementation methods or embodiments with better effects, and should be understood not to constitute a limitation on the scope of protection of this application.

[0045] In this application, terms such as "further," "even further," and "particularly" are used for descriptive purposes to indicate differences in content, but should not be construed as limiting the scope of protection of this application.

[0046] In this application, the terms "optionally," "optionally," and "optional" refer to options that are optional, meaning they can be selected from either "with" or "without." If multiple "optional" options appear in a technical solution, unless otherwise specified and there are no contradictions or mutual constraints, each "optional" option is independent.

[0047] In this application, the terms "first aspect," "second aspect," "third aspect," and "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," and "fourth," etc., serve only a non-exhaustive enumeration purpose and should be understood not to constitute a closed limitation on quantity.

[0048] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0049] In this application, numerical intervals (i.e. numerical ranges) are involved. Unless otherwise specified, the selected numerical distributions within the above numerical intervals are considered continuous and include the two numerical endpoints (i.e., the minimum and maximum values) of the numerical range, as well as every value between these two numerical endpoints.

[0050] Unless otherwise specified, the temperature parameters in this application are permitted to be either constant-temperature treatment or variations within a certain temperature range. It should be understood that the constant-temperature treatment allows temperature fluctuations within the precision range of the instrument control, such as ±5℃, ±4℃, ±3℃, ±2℃, or ±1℃.

[0051] In this application, percentage content refers to mass percentage for solid-liquid mixtures and solid-phase-solid mixtures, and volume percentage for liquid-phase-liquid mixtures, unless otherwise specified.

[0052] In this application, unless otherwise specified, percentage concentrations refer to final concentrations. The final concentration refers to the percentage of the added component in the system after its addition.

[0053] In this application, % (w / w) and wt% both represent weight percentage, % (v / v) refers to volume percentage, and % (w / v) refers to mass-volume percentage.

[0054] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0055] In existing technologies, without vacuum treatment, the electric arc furnace (EAF) smelting + LF refining + CC continuous casting process produces molten steel with high sulfur (S) and nitrogen (N) content from ordinary molten iron and scrap steel. To achieve an S content of ≤0.01% in the molten steel composition, deep deoxidation with Al alloy is required during EAF tapping and LF refining to reduce oxygen content before desulfurization. However, this deep deoxidation and desulfurization processes result in significant nitrogen absorption. Therefore, adding a vacuum treatment step, i.e., EAF + LF + VD(RH) + CC, can reduce N. However, adding a vacuum treatment step increases LF power consumption and electrode consumption, reduces the service life of the ladle due to the high temperature of the molten steel, increases smelting costs, and adds production steps, resulting in increased costs.

[0056] In some more specific embodiments, a method for producing low-nitrogen cold heading steel includes: using desulfurized molten iron and scrap steel as raw materials, and performing electric furnace smelting, LF refining and continuous casting processes;

[0057] The sulfur content of the desulfurized molten iron is ≤0.003wt%; the nitrogen content of the desulfurized molten iron is ≤0.0025wt%.

[0058] The amount of desulfurized molten iron added is 45 wt% or more;

[0059] The nitrogen content of the scrap steel is ≤40ppm.

[0060] This application presents a method for producing low-nitrogen cold heading steel using desulfurized molten iron and scrap steel as raw materials, through electric furnace smelting, LF refining, and continuous casting processes. The molten iron is first desulfurized to reduce its sulfur content, requiring an S content ≤0.003% after treatment. Simultaneously, the nitrogen content is significantly reduced during molten iron desulfurization. By moving the desulfurization process forward to the molten iron pretreatment stage, the severe nitrogen increase caused by desulfurization during the LF refining process is avoided. By controlling the composition and amount of raw materials, the goal of controlling the nitrogen content in cold heading steel through an electric furnace process without vacuum treatment is achieved.

[0061] Tests showed that the nitrogen content in molten iron fluctuated little, therefore, there is no need to limit the nitrogen content in molten iron.

[0062] This application uses molten iron with a proportion of over 45%, and the remainder is high-quality scrap steel, which can provide sufficient carbon content and physical heat, and can control the nitrogen content of the steel to ≤20ppm.

[0063] In some specific embodiments, 11-13 kg of desulfurizing agent is added per ton of molten iron during the preparation of desulfurized molten iron;

[0064] Preferably, the desulfurization time is 8-12 minutes;

[0065] More preferably, the desulfurizing agent is a mixture of lime and fluorite;

[0066] More preferably, the ratio of lime to fluorite is (8-10):1.

[0067] In one optional embodiment, after the molten iron is desulfurized, a slag removal process is performed, wherein the exposed area of ​​the molten iron surface during the slag removal process is ≥75%, in order to prevent excessive desulfurization slag from entering the electric furnace and causing sulfur increase in the molten steel.

[0068] In order to reduce the sulfur content in molten iron, this application selects to add a mixture of lime and fluorite during the desulfurization process. Under the action of the vortex field formed by the rotating stirring head (80 revolutions / minute), the lime and sulfur in the molten iron react fully to form iron slag. The desulfurization slag is then removed by a slag remover, thereby reducing the sulfur content in the molten iron.

[0069] Preferably, the desulfurized molten iron is added at a rate of 9-11 t / min.

[0070] In one alternative embodiment, the nitrogen content of the steel smelted in the electric furnace is ≤20ppm;

[0071] Preferably, the temperature at which steel is smelted in an electric furnace is above 1640°C;

[0072] More preferably, the temperature of the molten pool is raised to above 1500°C within 25 minutes during the electric furnace smelting process.

[0073] In the electric arc furnace (EAF) smelting process, molten iron is added via a chute at the rear of the furnace, with a controlled addition rate to prevent prolonged exposure and ensure proper heat preservation, thereby reducing energy consumption. Simultaneously, oxygen lances at the furnace door and walls, along with carbon lances, are used for fluxing and electrode heating. Energy is supplied to the furnace within 25 minutes, raising the molten pool temperature to over 1500℃ before the decarburization stage begins. The CO generated during EAF decarburization carries some nitrogen (N) elements. The strong oxidizing atmosphere within the furnace and the presence of 20% to 30% (FeO) content foam slag, which completely covers the molten steel, control the N content in the EAF to below 20 ppm. The tapping temperature is controlled above 1640℃, thus reducing the LF energizing time.

[0074] In one alternative implementation, when the electric furnace smelting reaches 3 / 5-4 / 5 of the steel output... ， Add 0.8-1.2 kg of aluminum blocks per ton of steel; adding them too early will cause strong deoxidation of the molten steel, resulting in severe nitrogen absorption.

[0075] Preferably, the aluminum block contains ≥99.5% Al;

[0076] More preferably, the particle size of the aluminum blocks is 10-50 mm.

[0077] In some embodiments, at the end of LF refining, the aluminum content in the molten steel is ≤0.03%.

[0078] During the electric arc furnace tapping process, the longer the contact time and the larger the contact surface area between the molten steel and air, the more nitrogen is absorbed into the molten steel. This application employs late and weak deoxidation during the tapping process. When approximately 2 / 3 of the steel has been tapped, aluminum blocks are added for deoxidation to prevent the free oxygen content of the molten steel from being too low before LF treatment, thus reducing the amount of nitrogen absorbed during tapping. Reducing the amount of deoxidized aluminum during tapping can also lower the amount of nitrogen absorbed.

[0079] Meanwhile, to prevent spillage at the tapping spout, argon blowing at the bottom of the ladle is not activated during tapping to avoid molten steel churning and contact with air. During electric arc furnace smelting and tapping, aluminum blocks are added for weak deoxidation.

[0080] During the refining process, the generated electric arc ionizes nitrogen in the air, some of which dissolves into the molten steel. To reduce temperature drop, the refining energizing time is reduced by increasing the electric furnace tapping temperature, promptly transporting ladles containing molten steel from the electric furnace to the refining process, and using hot-turnaround ladles. Al in the steel combines with N to form AlN, which has nitrogen-fixing properties; therefore, the aluminum content is controlled to below 0.03% to minimize nitrogen fixation.

[0081] The Al content is controlled to be less than 0.03% before continuous casting, so that the nitrogen content of the cold heading steel is low.

[0082] In some embodiments, the length and width of the scrap steel are both ≤50cm;

[0083] Preferably, the thickness of the scrap steel is ≤1cm. Limiting the length, width, and thickness of the scrap steel is from the perspective of its entry into the furnace, to avoid bridging of the scrap steel entering the furnace and causing the electric furnace lid to fail to close; at the same time, it avoids the scrap steel not being completely melted before the end of smelting.

[0084] To reduce the nitrogen content of cold heading steel, routine protective measures must be implemented during continuous casting, including: argon sealing of the long nozzle in the continuous casting ladle for protection during casting; checking the argon blowing pipe for leaks after each heat; timely replacement of the long nozzle sealing ring; thorough cleaning of the nozzle's upper rim; and timely replacement of the long nozzle with a new one. Other aspects include: ensuring the ladle opens automatically; maintaining the required black slag thickness with the tundish covering agent; and purging the tundish with argon gas before casting begins.

[0085] In some embodiments, the nitrogen content of the low-nitrogen cold heading steel produced is ≤40ppm.

[0086] In some embodiments, the low-nitrogen cold heading steel is grade ML08Al.

[0087] Preferably, the composition of the steel grade ML08Al by weight percentage includes: C 0.05-0.08%; Si < 0.1%; Mn 0.30-0.38%; P < 0.018%; S < 0.01%; Al 0.020-0.055%; Cu < 0.12%; Cr < 0.08% and N ≤ 0.004%.

[0088] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0089] The required composition range for preparing grade ML08Al steel is shown in Table 1. The nitrogen content for each step in the conventional production process of ML08Al is shown in Table 1. Figure 2 .

[0090] Table 1. Composition Control Range for Steel Grade ML08Al

[0091]

[0092] Example 1

[0093] This embodiment provides a low-nitrogen cold heading steel and its production method, including electric furnace smelting, LF refining, and continuous casting processes; details are as follows:

[0094] (1) Analysis of the original blast furnace molten iron showed that the sulfur content was 0.026% and the nitrogen content was 38 ppm. Then, the KR method was used for pretreatment desulfurization. The desulfurizing agent used in the KR treatment was a mixture of 90 wt% lime and 10 wt% fluorite, with an addition rate of 12 kg / ton of molten iron. Under the influence of the vortex field created by the rotating stirring head (80 revolutions / minute), the lime and sulfur in the molten iron reacted fully to form slag. The desulfurization slag was then removed by a slag remover for 10 minutes. The molten iron ladle was then tilted for slag removal, with 78% of the molten iron surface exposed during the process, for 15 minutes. Further analysis showed that the sulfur content in the molten iron decreased to 0.0026% and the nitrogen content to 25 ppm.

[0095] (2) Open the furnace lid of the electric furnace and load 60 tons of scrap steel (high-quality clean scrap steel of steel grade ML08Al, nitrogen content ≤40ppm, length and width of scrap steel ≤50cm, thickness ≤1cm) from the top. Close the furnace lid. Add 55 tons of desulfurized molten iron from step (1) to the chute behind the furnace. Start blowing oxygen with the oxygen lance on the furnace wall and start smelting at the same time. After smelting for 42 minutes, the temperature is measured at 1640℃. Electric furnace final S content: 0.005%, N content: 17ppm (e.g. Figure 1 ).

[0096] (3) When the electric arc furnace output reaches 70 tons, 100 kg of aluminum blocks are added to the ladle. The nitrogen content is measured to be 24 ppm before refining. During the refining process (LF), alloys are added and the temperature is increased. The nitrogen content is measured to be 36 ppm when the steel leaves the station. Then it is hoisted to the continuous casting machine for casting. The nitrogen content of the finished product is measured to be 39 ppm (e.g., ...). Figure 1 The aluminum content is 0.025%, the sulfur content is 0.005%, and the Si content is 0.04%.

[0097] Example 2

[0098] This embodiment provides a low-nitrogen cold heading steel and its production method, including electric furnace smelting, LF refining, and continuous casting processes; details are as follows:

[0099] (1) Analysis of the original blast furnace molten iron showed that the sulfur content was 0.026% and the nitrogen content was 38 ppm. Then, the KR method was used for pretreatment desulfurization. The desulfurizing agent used in the KR treatment was a mixture of 85 wt% lime and 15 wt% fluorite, with an addition rate of 11 kg / ton of molten iron. Under the influence of the vortex field created by the rotating stirring head (80 revolutions / minute), the lime and sulfur in the molten iron reacted fully to form slag. The desulfurization slag was then removed by a slag remover for 10 minutes. The molten iron ladle was then tilted for slag removal, with 77% of the molten iron surface exposed during the slag removal process for 15 minutes. A second sample analysis showed that the sulfur content in the molten iron decreased to 0.003% and the nitrogen content to 26 ppm.

[0100] (2) Open the furnace lid of the electric furnace and load 60 tons of scrap steel (high-quality clean scrap steel of grade ML08Al, nitrogen content ≤40ppm, length and width of scrap steel ≤50cm, thickness ≤1cm) from the top. Close the furnace lid. Add 55 tons of desulfurized molten iron from step (1) to the chute behind the furnace. Start blowing oxygen with the oxygen lance on the furnace wall and start smelting at the same time. After smelting for 42 minutes, the temperature is measured at 1642℃. The final S content of the electric furnace is 0.005%, and the N content is 18ppm.

[0101] (3) When the electric arc furnace output reaches 70 tons, 100 kg of aluminum blocks (10-50 mm in diameter) are added to the ladle. The nitrogen content is measured to be 24 ppm before refining. During the refining process (LF), alloys are added and the temperature is increased. The nitrogen content is measured to be 36 ppm when the steel leaves the station. The steel is then hoisted to the continuous casting plant for casting. The finished product is sampled and the nitrogen content is measured to be 38 ppm, the aluminum content is 0.028%, the sulfur content is 0.0047%, and the silicon content is 0.041%.

[0102] Comparative Example 1

[0103] The comparative model is produced using standard procedures, as follows:

[0104] (1) The original blast furnace molten iron was sampled and analyzed. The S content was 0.027% and the N content was 38ppm.

[0105] (2) Open the furnace lid of the electric furnace and load 60 tons of ordinary scrap steel (steel grade ML08Al, which is conventional ordinary scrap steel with a length and width of ≤100cm and a thickness of ≤7cm) from the top. Close the furnace lid. Add 55 tons of conventional molten iron from step (1) to the chute at the back of the furnace. Start blowing oxygen into the oxygen lance on the furnace wall and start smelting at the same time. After smelting for 40 minutes, the temperature is measured at 1633℃. The final S content of the electric furnace is 0.033%, and the N content is 35ppm.

[0106] (3) When the electric arc furnace output reaches 50 tons, 300 kg of aluminum blocks (10-50 mm in diameter) are added to the ladle. The nitrogen content is measured to be 50 ppm before refining. During the refining process (LF), alloys are added and the temperature is increased. The nitrogen content is measured to be 75 ppm when the steel leaves the station. The steel is then hoisted to the continuous casting plant for casting. The finished product is sampled and the nitrogen content is measured to be 79 ppm, the aluminum content is 0.039%, the sulfur content is 0.005%, the Si content is 0.07%, and the contents of other components meet the requirements of Table 1.

[0107] The difference between the comparative example and the embodiment is that the molten iron was not pretreated for desulfurization; instead, desulfurization was performed in the refining LF furnace after smelting in the electric arc furnace. Before desulfurization, 3 kg of aluminum blocks were added per ton of steel for strong deoxidation. The S content of the steel tapped from the electric arc furnace was 0.033%, and the N content was 35 ppm. During the tapping and refining processes, the cumulative nitrogen addition for desulfurization was 40 ppm, reaching 75 ppm before continuous casting. Figure 2 The nitrogen content differs significantly from that of the examples. Other preparation processes are the same as in the examples.

[0108] The results showed that, compared with the control example (using undesulfurized molten iron), Example 1, which used desulfurized molten iron, exhibited a significant difference in nitrogen content, with the resulting cold heading steel containing 39 and 79 ppm of nitrogen, respectively. Other component indicators of the steel produced by both methods were normal, with S content at 0.005% and C, Mn, and P contents all within the control range. Differences existed in Al and Si content; the Al content in control example 1 was 0.039% and Si content was 0.07%, while in example 1, the Al content was 0.025% and Si content was 0.04%.

[0109] The above indicates that the low-nitrogen cold heading steel obtained by the production method of this application has a nitrogen content of less than 40 ppm; the Al content and Si content are both low.

[0110] As can be seen from the above experiments, this application can achieve the goal of significantly reducing the nitrogen content in cold-forged steel without vacuum treatment under electric arc furnace steelmaking process conditions.

[0111] The above embodiments use ML08Al steel, which is the most difficult to control nitrogen in cold heading steel, as an example. The inventors have also tested other cold heading steels, and the results show that the production method of this application is applicable to all cold heading steels with nitrogen control requirements.

[0112] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A method for producing low-nitrogen cold heading steel, characterized in that, The production method includes: using desulfurized molten iron and scrap steel as raw materials, and carrying out electric furnace smelting, LF refining and continuous casting processes; The sulfur content of the desulfurized molten iron is ≤0.003wt%; the nitrogen content of the desulfurized molten iron is ≤0.0025wt%; during the preparation of desulfurized molten iron, 11-13kg of desulfurizing agent is added per ton of molten iron; the desulfurizing agent is a mixture of lime and fluorite, and the ratio of lime to fluorite added is (8-10):1; after desulfurization, the molten iron undergoes slag removal treatment, and the exposed area of ​​the molten iron surface during the slag removal treatment is ≥75%; The amount of desulfurized molten iron added is 45 wt% or more; The nitrogen content of the scrap steel is ≤40ppm; The nitrogen content of steel produced by electric furnace melting is ≤20ppm, and the temperature of steel produced by electric furnace melting is above 1640℃; during the electric furnace melting process, the temperature of the molten pool is raised to above 1500℃ within 25 minutes; When the electric furnace smelting reaches 3 / 5-4 / 5 of the steel output, add 0.8-1.2 kg of aluminum blocks per ton of steel; the Al content in the aluminum blocks should be ≥99.5%; the particle size of the aluminum blocks should be 10-50 mm. At the end of LF refining, the aluminum content in the molten steel is ≤0.03%; The nitrogen content of the low-nitrogen cold heading steel produced is ≤40ppm; The low-nitrogen cold heading steel is grade ML08Al, and the composition of grade ML08Al by weight percentage includes: C 0.05~0.08; Si < 0.1; Mn 0.30~0.38; P < 0.018; S < 0.01; Al 0.020~0.055; Cu < 0.12; Cr < 0.08 and N ≤ 0.

004.

2. The method for producing low-nitrogen cold heading steel according to claim 1, characterized in that, The desulfurization time is 8-12 minutes.

3. The method for producing low-nitrogen cold heading steel according to claim 2, characterized in that, The desulfurized molten iron is added at a rate of 9-11 t / min.

4. The method for producing low-nitrogen cold heading steel according to claim 1, characterized in that, The nitrogen content of steel smelted in an electric furnace is below 18 ppm.

5. The method for producing low-nitrogen cold heading steel according to claim 1, characterized in that, The length and width of the scrap steel are both ≤50cm; The thickness of the scrap steel is ≤1cm.

6. Low-nitrogen cold heading steel obtained by any one of the production methods described in claims 1-5.

Images