A method for producing free-cutting steel and the free-cutting steel itself.

Abstract

This invention provides a method for producing free-cutting steel and the free-cutting steel itself, belonging to the field of continuous casting production technology for free-cutting steel. The production method includes: obtaining molten steel with a Pb mass fraction ≤0.40%; continuously casting the molten steel in a crystallizer with a protective slag to obtain free-cutting steel; the viscosity of the protective slag at 1000-1350℃ is 0.3-0.8 Pa·s, and the negative slip time of the crystallizer is 0.15-0.2s; the consumption of the protective slag is 0.25-0.35 L / kg, and / or the C mass fraction in the protective slag is 4-7%. Using the production method provided by this invention, the production process of free-cutting steel avoids crystallizer leakage and slag entrapment problems, and the billet surface quality is good.

Description

Technical Field

[0001] This invention belongs to the field of continuous casting production technology of free-cutting steel, and particularly relates to a production method of free-cutting steel and free-cutting steel. Background Technology

[0002] Free-cutting steel is an alloy steel containing one or more elements from sulfur, phosphorus, lead, calcium, selenium, and tellurium to improve its machinability. With the increasing automation, speed, and precision of machining processes, good machinability is crucial for steel. This type of steel is mainly used in automatic machine tools and is therefore considered a special-purpose steel. Based on the free-cutting elements it contains, it can be classified into sulfur-based free-cutting steel, lead-based free-cutting steel, and calcium-based free-cutting steel, among others.

[0003] In the existing technology, high-viscosity protective slag is often used for protective casting in the continuous casting production of free-cutting steel. For example, CN202011548636.1 discloses a special protective slag for crystallizer of low-carbon tellurium high-sulfur free-cutting steel and its preparation method. The viscosity of the protective slag is 1.6 to 2.4 Pa·s, which can easily cause steel leakage when the initial billet shell exits the crystallizer. Summary of the Invention

[0004] To solve the above-mentioned technical problems, the present invention provides a method for producing free-cutting steel and free-cutting steel. Using the production method provided by the present invention, there is no problem of steel leakage in the crystallizer or slag entrapment in the production process of free-cutting steel, and the surface quality of the slab is good.

[0005] In a first aspect, the present invention provides a method for producing free-cutting steel, comprising:

[0006] To obtain molten steel with a Pb mass fraction ≤ 0.4%;

[0007] The molten steel enters a crystallizer with a protective slag for continuous casting to obtain free-cutting steel; the viscosity ν of the protective slag at 1000-1350℃ is 0.3-0.8 Pa·s, and the negative slip time of the crystallizer is 0.13-0.2s;

[0008] The consumption of the protective slag is 0.25-0.35 L / kg, and / or the mass fraction of C in the protective slag is 4-7%.

[0009] In some embodiments, the viscosity ν of the protective slag and the negative slip time t of the crystallizer meet the following condition: ν = kt, where the value of k ranges from 2 to 5.

[0010] In some embodiments, the amplitude of the crystallizer is ±4.2-4.5 mm, the negative slip time of the crystallizer is 0.15-0.16 s, and the vibration frequency of the crystallizer is 170-180 times / min.

[0011] In some embodiments, the specific water flow rate of the crystallizer is 1700-1900 L / min.

[0012] In some embodiments, during the continuous steel casting process, the billet is cooled in a secondary cooling zone, which includes multiple cooling sections arranged sequentially along the process, and the cooling intensity of each cooling section is 0.3-1.2 L / kg.

[0013] In some embodiments, the cooling intensity of the plurality of cooling sections decreases sequentially.

[0014] In some embodiments, the melting point of the protective slag is 1050-1080°C.

[0015] In some embodiments, the mass fraction of sulfur (S) in the molten steel is 0.01-0.40%, and the mass fraction of carbon (C) in the molten steel is 0.4-1.12%.

[0016] In some embodiments, the casting speed during the continuous casting process is 1.5-2.5 m / min; the cross-sectional dimensions of the free-cutting steel are 100-200 mm × 100-200 mm.

[0017] On the other hand, the present invention also provides a free-cutting steel obtained by the aforementioned production method.

[0018] One or more technical solutions in this application have at least the following technical effects or advantages:

[0019] This invention provides a method for producing free-cutting steel, comprising: obtaining molten steel with a Pb mass fraction ≤0.4%; the molten steel entering a crystallizer with a protective slag for continuous casting to obtain free-cutting steel; the viscosity ν of the protective slag at 1000-1350℃ is 0.3-0.8 Pa·s, the negative slip time of the crystallizer is 0.13-0.2s; the consumption of the protective slag is 0.25-0.35 L / kg, and / or the mass fraction of C in the protective slag is 4-7%. By controlling the viscosity of the protective slag, the negative slip time of the crystallizer, and the slag consumption, the protective slag is promoted to fully fill the air gap between the primary billet shell and the inner wall of the crystallizer, improving the uniformity of heat transfer in the primary billet shell, ensuring the uniformity of the thickness of the primary billet shell, and avoiding the problem of steel leakage in the crystallizer caused by uneven primary billet shell. Using the production method provided by this invention, there is no problem of steel leakage in the crystallizer during the production of free-cutting steel, and the surface quality of the slab is good. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a process flowchart of a method for producing free-cutting steel according to an embodiment of the present invention. Detailed Implementation

[0022] The present invention will be described in detail below with reference to specific embodiments and examples, thereby making the advantages and various effects of the present invention more clearly apparent. Those skilled in the art should understand that these specific embodiments and examples are for illustrative purposes only and are not intended to limit the present invention.

[0023] Throughout this specification, unless otherwise specified, the terminology used herein should be understood as having the meaning commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In the event of any conflict, this specification shall prevail.

[0024] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0025] It should be noted that in this article, relational terms such as “first” and “second” are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations.

[0026] The technical solution provided in this application aims to solve the above-mentioned technical problems, and the overall approach is as follows:

[0027] Please see Figure 1 In a first aspect, embodiments of the present invention provide a method for producing free-cutting steel, characterized in that it includes:

[0028] S1. Obtain molten steel with a Pb mass fraction ≤ 0.4%;

[0029] Pb in free-cutting steel can improve its machinability. However, Pb has a relatively low melting point of 327℃ and is highly volatile in molten steel, causing significant fluctuations in the liquid level within the mold. Therefore, high-viscosity mold flux is often used in continuous casting to prevent it from being drawn into the molten steel during these fluctuations. Slag entrainment can easily degrade the quality of the free-cutting steel. Furthermore, after the molten steel enters the mold, the solidification and shrinkage create air gaps between the primary billet shell and the inner wall of the mold. High viscosity Pb... The protective slag is difficult to fill the air gaps, resulting in different rates of heat conduction from the initial billet shell to the outside. In the air gaps without slag filling, the initial billet shell relies on air for heat conduction, which has a low thermal conductivity, resulting in a thinner initial billet shell. In the air gaps with slag filling, the initial billet shell relies on slag for heat conduction, which has a high thermal conductivity, resulting in a thicker initial billet shell. This leads to uneven thickness of the initial billet shell on the four sides (the four sides of the billet excluding the two end faces). In the areas with thinner initial billet shells, under the influence of reheating and latent heat of solidification, the problem of steel leakage in the crystallizer will occur.

[0030] S2. The molten steel enters a crystallizer with a protective slag for continuous casting to obtain free-cutting steel; the viscosity ν of the protective slag at 1000-1350℃ is 0.3-0.8 Pa·s, and the negative slip time of the crystallizer is 0.11-0.2s.

[0031] The consumption of the protective slag is 0.25-0.35 L / kg, and / or the mass fraction of C in the protective slag is 4-7%.

[0032] Because the viscosity of the protective slag is not particularly high and its fluidity is good, it can fill the air gaps evenly and fully. The thermal conductivity of the air gaps is lower than that of the protective slag, thus affecting the solidification rate of the primary billet shell. The protective slag, distributed in the air gaps between the primary billet shell and the mold sidewalls, can, on the one hand, increase the solidification rate of the primary billet shell and increase its thickness; on the other hand, it can ensure the uniformity of the thickness of the primary billet shell on all four sides, avoiding the problem of steel leakage from the mold. If the viscosity of the protective slag is too high, its distribution within the air gaps will be uneven. This means that the air gaps contain both the highly thermally conductive protective slag and the less thermally conductive air, resulting in an uneven distribution of the primary billet shell thickness on all four sides. The protective slag with a viscosity of 0.3-0.8 Pa·s provided in this application can be evenly distributed within the air gaps, ensuring that there is no air in the air gaps. This makes the heat transfer rate of the primary billet shell the same on all four sides, thus ensuring the uniformity of the billet thickness on all four sides and avoiding the problem of steel leakage at the exit of the mold. If the viscosity of the protective slag is too low, the presence of low-melting-point and easily volatile Pb element in the molten steel will cause fluctuations in the liquid level within the crystallizer. During continuous casting, the protective slag is easily drawn into the molten steel, affecting the internal quality of the cast billet. Negative slip refers to the downward movement of the crystallizer relative to the cast billet, and the negative slip time refers to the time it takes for the crystallizer to move downward relative to the cast billet. During this downward movement, the protective slag can come into contact with the exposed primary billet shell. When the crystallizer moves upward relative to the cast billet, the protective slag in contact with the primary billet shell, due to its good fluidity, will enter the air gap between the inner wall of the crystallizer and the primary billet shell. This ensures that the four sides of the primary billet shell are evenly coated with liquid protective slag, guaranteeing uniform heat dissipation from the four sides of the primary billet shell. However, for free-cutting steels containing Pb, the volatilization of Pb within the crystallizer leads to extremely uneven temperature distribution and large fluctuations in the liquid level. The slag in this application has a low viscosity, making it prone to slag entrapment. However, by controlling the appropriate negative slip time under the slag viscosity conditions specified in this application, slag entrapment can be avoided. Excessive negative slip time in the crystallizer easily leads to slag entrapment. The carbon content in the protective slag can reflect its consumption to some extent; controlling a certain level of slag consumption further ensures that the air gaps are filled with protective slag. By matching the viscosity, negative slip time, and consumption of the protective slag, slag entrapment is prevented, and the protective slag completely fills the air gaps between the primary billet shell and the inner wall of the crystallizer, ensuring the uniformity of the primary billet shell thickness and avoiding crystallizer leakage.Excessive viscosity of the protective slag increases the risk of steel leakage from the crystallizer; insufficient viscosity increases the risk of slag entrapment in the molten steel. Excessive negative slip time in the crystallizer results in a more bent initial solidified billet shell, leading to deeper oscillation marks and poorer billet surface quality. Conversely, insufficient negative slip time limits the efficiency of liquid protective slag entering the space between the billet shell and the crystallizer's inner wall during negative slip. This results in both air gaps and liquid protective slag between the billet shell and the crystallizer's inner wall, affecting heat transfer uniformity, reducing slag consumption, and impacting billet lubrication, thus increasing the likelihood of steel leakage from the crystallizer.

[0033] 1000-1350℃ is the temperature of the surface of the molten steel in the crystallizer, which means that the viscosity of the liquid slag formed after the protective slag melts in the molten steel in the crystallizer is 0.3-0.8 Pa·s.

[0034] In some embodiments, the viscosity ν of the protective slag and the negative slip time t of the crystallizer meet the following condition: ν = kt, where k ranges from 2 to 5. The lower the viscosity, the shorter the negative slip time; the higher the viscosity, the longer the negative slip time, to further ensure that the molten steel does not have slag entrainment problems and that the thickness of the initial billet shell is uniform; that is, viscosity and negative slip have a certain positive correlation.

[0035] In some embodiments, the amplitude of the crystallizer is ±4.2-4.5 mm, the negative slip time of the crystallizer is 0.13-0.16 s, and the vibration frequency of the crystallizer is 170-180 times / min. When the amplitude of the crystallizer is ±4.2-4.5 mm, the maximum stroke of the crystallizer is twice the amplitude. An amplitude of ±4.2-4.5 mm indicates a maximum stroke of 9 mm and a minimum stroke of 8.4 mm. For example, an amplitude of ±4.3 mm indicates a maximum stroke of 8.6 mm. A smaller amplitude results in less flux intake, affecting the lubrication of the copper tubes in the crystallizer and the shell removal of the continuously cast billet, easily leading to sticking and leakage accidents. A larger amplitude results in a longer negative slip time, greater and more difficult-to-control sway, deeper vibration marks, and a higher risk of transverse cracks and breakage accidents on the surface of the cast billet. The vibration curve of the crystallizer can be a sine curve or a non-sine curve; there is no specific limitation. By controlling the amplitude, negative slip time, and vibration frequency to match each other, we can avoid the problem of slag entrapment caused by the volatilization of Pb element in the crystallizer, which leads to extremely uneven temperature field distribution and large liquid surface fluctuations. We can also ensure that the liquid protective slag is fully distributed in the air gap between the primary billet shell and the inner wall of the crystallizer, thus ensuring the heat transfer uniformity of the primary billet shell and achieving uniform thickness of the primary billet shell on all four sides, avoiding the problem of steel leakage in the crystallizer due to local thin spots.

[0036] In some embodiments, the specific water flow rate of the crystallizer is 1700-1900 L / min. A higher specific water flow rate in the crystallizer results in excessively rapid solidification shrinkage, making cracks more likely; conversely, a lower specific water flow rate results in a thinner initial billet shell, increasing the risk of runoff from the crystallizer.

[0037] In addition, the protective slag can consist of the following components by mass fraction: CaO: 25-30%, SiO2: 25-40%, Al2O3: 5-8%, C: 4-7%, F: 4-6%, MgO: 5-8%, Na2O: 3-5%, with the remainder being unavoidable impurities; the viscosity of the protective slag at 1000-1350℃ is 0.3-0.8 Pa·s. The functions of each component in the protective slag are:

[0038] CaO and SiO2: These two phases are the basic components of the protective slag, capable of adsorbing inclusions in the molten steel. The ratio of CaO / SiO2 determines the basicity of the protective slag, which is an important indicator of the adsorption of inclusions and the quality of its lubrication performance. Generally, higher basicity enhances the slag's ability to adsorb inclusions but reduces lubrication performance and thermal conductivity compared to heat transfer through contact between the billet shell and the inner wall of the crystallizer. Al2O3 increases the viscosity of the protective slag. Carbon (C) is the skeletal material of the protective slag, controlling its melting rate. A higher C mass fraction results in a slower melting rate and slower slag consumption; a lower C mass fraction results in a faster melting rate and faster slag consumption. Fluorine (F) is the solvent material and the active component of the protective slag. F reduces the viscosity of the protective slag, thereby reducing slag surface tension and improving its fluidity. A higher F mass fraction results in a lower viscosity, while a lower F mass fraction results in a higher viscosity. MgO is used as a solvent to reduce the viscosity of the protective slag; Na2O is used as a solvent and is the active component of the slag to reduce the viscosity and melting point of the protective slag and reduce the surface tension of the slag.

[0039] In some embodiments, during the continuous steel casting process, the billet is cooled in a secondary cooling zone, which includes multiple cooling sections arranged sequentially along the process flow. The cooling intensity of each cooling section is 0.3-1.2 L / kg. The solidification of the billet occurs gradually from the outside in, and a liquid core remains in the secondary cooling zone. Controlling the cooling intensity of each cooling section ensures that the billet gradually cools and solidifies from the outside in, avoiding steel leakage in the secondary cooling zone.

[0040] In some embodiments, the cooling intensity of the plurality of cooling sections decreases sequentially along the process. The size of the liquid core of the billet at the second cooling zone decreases sequentially along the process; therefore, the cooling intensity of the cooling sections also decreases sequentially. Specifically, the cooling sections may have three, four, five, or other numbers, without any specific limitation.

[0041] In some embodiments, the melting point of the protective slag is 1050-1080°C.

[0042] In some embodiments, the mass fraction of sulfur (S) in the molten steel is 0.01%-0.40%, and the mass fraction of carbon (C) in the molten steel is 0.4%-1.12%. High C content in the molten steel increases the range of the two-phase region (liquid-solid region). When this free-cutting steel solidifies in the crystallizer, it causes uneven billet shell thickness. When the free-cutting steel exits the crystallizer, the initial billet shell will reheat. Furthermore, the release of latent heat of solidification will cause steel leakage in areas with thinner billet shells. Additionally, high-carbon steel has poor high-temperature plasticity, resulting in low initial billet shell strength in the crystallizer. Under the static pressure of the molten steel, the billet shell and crystallizer wall are in close contact, further hindering the flow of protective slag into the space between the billet shell and the crystallizer wall. The initial billet shell of high-carbon steel is also prone to adhesion to the inner wall of the crystallizer during solidification. Therefore, controlling the melting point of the protective slag, the negative slip time of the crystallizer, and the amplitude of the crystallizer is crucial to ensure that the slab exits the crystallizer without slag entrapment or steel leakage, resulting in good billet quality.

[0043] In some embodiments, the end face size of the free-cutting steel is 100-200mm × 100-200mm, and the velocity during the continuous casting process is 1.5-2.5m / s.

[0044] On the other hand, embodiments of the present invention also provide a free-cutting steel, obtained by the aforementioned method for producing free-cutting steel.

[0045] The protective slag for free-cutting steel and the production method of free-cutting steel of this application will be further described below with reference to specific embodiments.

[0046] Examples 1 to 5 and Comparative Examples 1 to 3

[0047] Examples 1 to 5 and Comparative Examples 1 to 3 each provide a method for producing free-cutting steel, wherein the chemical composition of the free-cutting steel is shown in Table 1 and the chemical composition of the protective slag is shown in Table 2.

[0048] The free-cutting steel molten steel is continuously cast. The consumption of protective slag is shown in Table 2. The cross-section of the billet, the casting speed, the cooling intensity of the crystallizer and the cooling intensity of the secondary cooling zone are also shown in Table 2. The vibration curves of the crystallizer are sinusoidal curves.

[0049] Table 1

[0050] serial number Brand C / ％ Si / % Mn / % S / ％ Pb / % Example 1 1144 0.44 0.02 1.45 0.32 0.002 Example 2 C45Pb 0.43 0.03 0.68 0.029 0.15 Example 3 Y85Pb 0.82 0.02 0.85 0.14 0.22 Example 4 Y95Pb 0.91 0.03 0.96 0.16 0.31 Example 5 Y100Pb 1.01 0.03 1.13 0.16 0.37 Comparative Example 1 1144 0.44 0.02 1.45 0.32 0.002 Comparative Example 2 Y95Pb 0.91 0.03 0.96 0.16 0.31 Comparative Example 3 Y100Pb 1.01 0.03 1.13 0.16 0.30

[0051] Table 2

[0052]

[0053] Table 3

[0054]

[0055] The production methods provided in Examples 1 to 5 produce slabs without slag entanglement or steel leakage. Comparative Example 1 uses a traditional free-cutting steel protective slag with high viscosity and low slag consumption. Although the vibration parameters of this invention are adopted, the matching effect is poor, resulting in a slab leakage rate of approximately 10 times / thousand tons. Comparative Example 2 has a lower slag viscosity; with the vibration of the crystallizer, slag is entangled into the slab, with a slag entanglement rate reaching 5%. Although continuous casting proceeds smoothly, the slab quality declines. Comparative Example 3 uses the protective slag composition designed in this invention, but the cooling intensity, especially the secondary cooling intensity, is low. In the first stage of secondary cooling, a steel leakage accident caused by slab reheating occurs.

[0056] The production method for free-cutting steel provided by this invention promotes the full filling of the air gap between the primary billet shell and the inner wall of the crystallizer by controlling the viscosity of the protective slag, the negative slip time of the crystallizer, and the slag consumption. This improves the uniformity of heat transfer in the primary billet shell, ensures the uniformity of the primary billet shell thickness, and avoids the problem of steel leakage in the crystallizer caused by uneven primary billet shell thickness. Using the production method provided by this invention, there is no problem of steel leakage in the crystallizer or slag entrapment in the production process of free-cutting steel, and the surface quality of the slab is good.

[0057] Finally, it should be noted that the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0058] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0059] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for producing a free-cutting steel, characterized in that, The method comprises the following steps: obtaining a molten steel with a mass fraction of Pb less than or equal to 0.4%; the molten steel is continuously cast in a crystallizer with a protective slag to obtain free-cutting steel; the viscosity of the protective slag is 0.3-0.8 Pa·s at 1000-1350℃, the negative strip time of the crystallizer is 0.124-0.2 s, the amplitude of the crystallizer is ±4.2-4.5 mm, and the vibration frequency of the crystallizer is 170-180 times / min; the viscosity of the protective slag and the negative strip time of the crystallizer satisfy the following condition: ν=k t, the value range of k is 2-5, so that no slag is wrapped and no breakout occurs; the consumption of the protective slag is 0.25-0.35 L / kg, and / or the mass fraction of C in the protective slag is 4-7%.

2. The production method of free-cutting steel according to claim 1, characterized in that, The negative strip time of the crystallizer is 0.15-0.16 s.

3. The production method of free-cutting steel according to any one of claims 1-2, characterized in that, The specific water consumption of the crystallizer is 1700-1900 L / min.

4. The method of producing a free-cutting steel according to claim 3, characterized in that, In the continuous casting process, the secondary cooling zone of the cast blank is cooled, the secondary cooling zone comprises a plurality of cooling sections arranged in sequence along the process, and the cooling intensity of each cooling section is 0.3-1.2 L / kg.

5. The method of producing a free-cutting steel according to claim 4, characterized in that, The cooling intensity of the plurality of cooling sections decreases in sequence along the process.

6. The production method of free-cutting steel according to any one of claims 1 to 2, characterized in that, The melting point of the protective slag is 1050-1080℃.

7. The production method of free-cutting steel according to any one of claims 1 to 2, characterized in that, The mass fraction of S in the molten steel is 0.01-0.40%, and the mass fraction of C in the molten steel is 0.4-1.12%.

8. The production method of free-cutting steel according to any one of claims 1 to 2, characterized in that, In the continuous casting process, the pulling speed is 1.5-2.5 m / min; and the section size of the free-cutting steel is 100-200 mm×100-200 mm.

9. A free machining steel characterized in that, The production method is obtained by using any one of claims 1-8.

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