Method for controlling tower-shaped hairline of medium-carbon boron-containing steel

Abstract

The invention specifically discloses a control method for tower-shaped hairline of medium-carbon boron-containing steel, which sequentially comprises the following steps: KR desulfurization, LD converter, LF external refining, RH vacuum degassing and continuous casting, and the content of S is not more than 0.002% by KR desulfurization; the RH limit vacuum degree is smaller than or equal to 67 Pa, the vacuum circulation time is longer than or equal to 25 min, and the continuous soft blowing time is longer than or equal to 20 min; an R30 crystallizer with a larger R angle is used, a small nozzle is used for aerial fog cooling in a secondary cooling area, the specific water flow of secondary cooling water is 0.15 L / Kg, the water flow of the crystallizer is 140 m < 3 > / h, and the water flow proportion of a foot roller is 0.80; before continuous casting production, casting stopping is organized for secondary cooling chamber maintenance; low-superheat-degree casting is adopted, and the superheat degree is controlled to be 15-25 DEG C; during production, a crystallizer is adopted for production in the early stage, the steel passing amount of the crystallizer does not exceed 150 furnaces, and finally the 35MnB tower type hairline inspection results all meet the technical requirements of customers.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and particularly relates to a method for controlling tower-shaped graining of medium-carbon boron-containing steel 35MnB. Background Art

[0002] Medium-carbon boron-containing steel 35MnB is often used to make track links for construction machinery. Track links made of 35MnB steel connect track plates and transmit power. Due to the special working environment conditions of construction machinery such as excavators, loaders, cranes, and large dump trucks, the track steel must have high purity, uniform composition, good organizational structure, hardenability and surface quality to provide guarantees for subsequent processing of the track.

[0003] A key inspection item for medium-carbon boron-containing 35MnB steel is the pickling hairline test. Hairline is a macroscopic defect in steel. It is a linear crack formed by pinholes, pores, and non-metallic inclusions in the steel, which are extended along the forging direction during deformation. Hairline seriously compromises the mechanical properties of the steel, especially fatigue strength, and significantly affects the performance of the product. Typically, 35MnB hairline testing requires compliance with the standard "GB / T15711-2018 Test of Non-Metallic Inclusions in Steel - Tower Hairline Pickling Method": The starting length of the hairline is 1mm, the total number of hairlines is ≤7, the maximum length is ≤4mm, and the total length is ≤15mm.

[0004] However, when using 240mm×300mm continuous casting billets to produce Φ50-80mm 35MnB steel for customers, the pass rate of 35MnB steel crackle inspection is only about 85%, which greatly affects product quality, resulting in material downgrading and re-judgment, or even scrapping, causing great losses to the company. Therefore, it is urgent to develop a new tower-shaped crackle control process for medium-carbon boron-containing steel 35MnB. Summary of the Invention

[0005] The present invention aims to provide a method for controlling tower-shaped graining in medium-carbon and boron-containing steel. The method comprises the following steps: controlling the S content at a relatively low level during smelting of a continuous casting billet with a specification of 240 mm×300 mm rolled into round steel, improving the vacuum degree, the molten steel circulation time, and the soft blowing time; stopping casting before production for secondary cooling chamber maintenance; using an R30 crystallizer, mist cooling of the billet with a small nozzle; adopting an early crystallizer for production, with the crystallizer passing no more than 150 furnaces of steel; reducing the secondary cooling water ratio, the crystallizer water volume, and the water volume ratio of the foot roll; and performing low superheat casting. Ultimately, the method ensures that the inspection results of the tower-shaped graining in 35MnB steel meet the technical requirements of customers.

[0006] To achieve the above object, the present invention provides the following technical solutions: A method for controlling tower-shaped graining in medium-carbon boron-containing steel comprises the following steps: KR desulfurization → LD converter → LF refining → RH vacuum degassing → continuous casting, wherein the RH ultimate vacuum degree is ≤67Pa, the vacuum cycle time is ≥25min, and the continuous soft blowing time is ≥20min; an R30 crystallizer with a larger R angle is used, a small nozzle mist cooling is used in the secondary cooling zone, the secondary cooling water specific water volume is 0.15L / Kg, and the crystallizer water volume is 140m 3 / h, ensuring the "weak cooling + uniform" mode of the crystallizer, and at the same time, the water volume ratio of the foot roller is 0.80 to prevent poor cooling effect of the foot roller and ensure weak cooling and uniform cooling; low superheat casting is adopted, and the superheat is controlled at 15-25℃, with a target of 20℃, to ensure uniform cooling of the ingot and reduce the level of banded structure; in the early stage of production, the crystallizer is used for production, and the amount of steel overload in the crystallizer does not exceed 150 furnaces to prevent excessive steel overload in the crystallizer, which will cause serious wear and deformation of the copper tube, and ensure uniform cooling of the continuous casting ingot in the crystallizer.

[0007] Preferably, the S content of the KR desulfurization is no more than 0.002%, ensuring that the S content of the finished product is no more than 0.005%.

[0008] Preferably, argon is blown throughout the bottom blowing process of the LD converter. In order to reduce the O content in the steel, the end point C is required to be ≥0.12%. During the steel tapping process, aluminum iron is added first, and then ferrosilicon, silicon manganese, and ferroboron alloy are added in sequence.

[0009] Preferably, the LF refining furnace produces white slag and maintains it all the time, while ensuring the fluidity of the top slag. In order to prevent FeO and MnO in the slag from adding oxygen to the molten steel, the FeO+MnO in the slag is required to be ≤0.50%. Depending on the Al content, aluminum wire can be added once in the early stage of refining, and aluminum addition operations are strictly prohibited in the later stage.

[0010] Preferably, the RH is strictly prohibited from adding alloys and returning to the LF furnace to increase the temperature.

[0011] Preferably, before the continuous casting production, the pouring is stopped to carry out the maintenance of the second cooling chamber to ensure that the arc of the crystallizer is centered, the spray rack and the nozzle are centered, the nozzle is cleaned in time to prevent the nozzle from being blocked, and the arc is ensured to be centered and the cooling water meets the process requirements. At the same time, the copper tube and the protective slag are inspected more strictly to ensure a good cooling effect of the crystallizer.

[0012] Preferably, the continuous casting adopts an integral tundish, the tundish is cleaned, and emptied by filling with argon before pouring, the argon blowing time is ≥2 minutes, the casting is protected throughout, and pouring is started outside the tundish.

[0013] Compared with the prior art, the present invention has the following beneficial effects: During smelting, the S content was controlled at a low level, the vacuum degree, molten steel circulation time and soft blowing time were improved, and before production, the pouring was stopped for secondary cooling chamber maintenance. An R30 crystallizer was used, and the ingot was cooled with aerosol from a small nozzle. Production was carried out using the early crystallizer, and the crystallizer steel flow rate did not exceed 150 furnaces. The secondary cooling water ratio, the crystallizer water volume and the water volume ratio of the full roller were reduced, and casting was carried out at a low superheat. Ultimately, the 35MnB tower-type hairline inspection results met the customer's technical requirements, solving the technical difficulties described in the background. DETAILED DESCRIPTION

[0014] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below through examples.

[0015] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0016] Secondly, the term "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive of other embodiments. Example

[0017] The present invention is further described below with reference to an embodiment. This embodiment provides a method for controlling tower-shaped graining in medium-carbon boron-containing steel. The required range of chemical composition of 35MnB steel with a specification of Φ50-80 mm in the embodiment of the present invention is shown in Table 1.

[0018] Table 1 Chemical composition (wt%)

[0019] Example 1: KR desulfurization, S content is 0.001%; LD converter bottom blowing is argon-blown throughout the process, the final C content is 0.23%, aluminum iron is added first during the steelmaking process, and then silicon iron, silicon manganese, boron iron and other alloys are added in sequence, the LF refining furnace produces white slag and keeps it all the time, while ensuring the fluidity of the top slag, the FeO+MnO content in the slag is 0.32%, according to the Al content, aluminum wire is added once in the early stage of refining, and no aluminum addition operation is performed in the later stage; RH does not add alloys and returns to the LF furnace for temperature increase, the ultimate vacuum degree is 52Pa, The vacuum treatment time was 26 minutes, the continuous soft blowing time was 21 minutes, and the pouring was stopped before production. The secondary cooling chamber maintenance was carried out to ensure that the arc of the crystallizer was aligned, the spray rack and the nozzle were aligned, and the nozzle was cleaned in time to prevent the nozzle from being blocked. It was ensured that the arc alignment and cooling water met the process requirements. The R30 crystallizer was used, and the crystallizer passed 3 furnaces of steel. At the same time, the copper tube and protective slag were inspected more strictly. The secondary cooling zone used small nozzles for mist cooling. The secondary cooling water volume was 0.15L / Kg, and the crystallizer water volume was 140m 3 / h, a full-roll water ratio of 0.80, and a superheat of 18°C. The continuous casting process employed an integral tundish, which was cleaned and purged with argon before pouring began. The argon purge lasted for 3 minutes, ensuring full protection during the casting process. Pouring began outside the tundish. Electromagnetic stirring was performed in the mold and at the end of solidification. 240 mm × 300 mm continuous casting slabs were drawn and then rolled into Φ50 mm 35MnB round steel. Low-magnification samples of two randomly selected slabs were taken for corner crack rating, both receiving a grade of 0.5. A randomly selected Φ50 mm 35MnB round steel was tested for pickling tower cracking, and the results met the customer's technical requirements. The finished product contained 0.003% sulfur, with very low O, N, and H contents. The specific test results are shown in Table 2.

[0020] Example 2: KR desulfurization, S content is 0.002%; LD converter bottom blowing is argon-blown throughout the process, the final C content is 0.21%, aluminum iron is added first during the steelmaking process, and then silicon iron, silicon manganese, boron iron and other alloys are added in sequence. The LF refining furnace produces white slag and keeps it all the time, while ensuring the fluidity of the top slag. The FeO+MnO content in the slag is 0.37%. According to the Al content, aluminum wire is added once in the early stage of refining, and no aluminum addition operation is performed in the later stage. No alloy is added to the RH and the temperature is returned to the LF furnace for heating. The ultimate vacuum degree is 57Pa, and the vacuum is 100%. The air treatment time was 27 minutes, and the continuous soft blowing time was 23 minutes. Before production, the pouring was stopped and the secondary cooling chamber maintenance was carried out to ensure that the arc of the crystallizer was aligned, the spray rack and the nozzle were aligned, and the nozzle was cleaned in time to prevent the nozzle from being blocked. It was ensured that the arc alignment and cooling water met the process requirements. The R30 crystallizer was used, and the crystallizer passed 76 furnaces of steel. At the same time, the copper tube and protective slag were inspected more strictly. The secondary cooling zone used small nozzles for mist cooling. The secondary cooling water ratio was 0.15L / Kg, and the crystallizer water volume was 140m 3 / h, a full-roll water ratio of 0.80, and a superheat of 21°C. The continuous casting process employed an integral tundish, which was cleaned and purged with argon before pouring began. The argon purge lasted for two minutes, ensuring full protection during the casting process. Pouring began outside the tundish. Electromagnetic stirring was performed in the mold and at the end of solidification. 240 mm × 300 mm continuous casting billets were drawn and then rolled into 60 mm 35MnB round steel. Low-magnification samples of two randomly selected billets were taken for corner crack rating, both receiving a grade of 0.5. A randomly selected 60 mm 35MnB round steel was tested for pickling tower cracking, and the results met the customer's technical requirements. The finished product contained 0.004% sulfur, with very low O, N, and H contents. The specific test results are shown in Table 2.

[0021] Example 3: KR desulfurization, S content is 0.001%; LD converter bottom blowing is argon-blown throughout the process, the final C content is 0.27%, aluminum iron is added first during the steelmaking process, and then silicon iron, silicon manganese, boron iron and other alloys are added in sequence, the LF refining furnace produces white slag and keeps it all the time, while ensuring the fluidity of the top slag, the FeO+MnO content in the slag is 0.34%, according to the Al content, aluminum wire is added once in the early stage of refining, and no aluminum addition operation is performed in the later stage; RH does not add alloys and returns to the LF furnace for temperature increase, the ultimate vacuum degree is 61Pa, vacuum The processing time was 26 minutes, and the continuous soft blowing time was 22 minutes. Before production, the pouring was stopped and the secondary cooling chamber maintenance was carried out to ensure that the arc of the crystallizer was aligned, the spray rack and the nozzle were aligned, and the nozzle was cleaned in time to prevent the nozzle from being blocked. It was ensured that the arc alignment and cooling water met the process requirements. The R30 crystallizer was used, and the crystallizer steel flow rate was 142 furnaces. At the same time, the copper tube and protective slag were inspected more strictly. The secondary cooling zone used small nozzles for mist cooling. The secondary cooling water volume was 0.15L / Kg, and the crystallizer water volume was 140m 3 / h, a full-roll water ratio of 0.80, and a superheat of 23°C. The continuous casting process employed an integral tundish, which was cleaned and purged with argon before pouring began. The argon purge lasted for 3 minutes, ensuring full protection during the casting process. Pouring began outside the tundish. Electromagnetic stirring was performed in the mold and at the end of solidification. 240 mm × 300 mm continuous casting billets were drawn and then rolled into 80 mm 35MnB round steel. Two randomly selected low-magnification specimens of the ingots were subjected to corner crack ratings of 0.5 and 1.0. One 80 mm 35MnB round steel was randomly selected for pickling tower crack testing, and the results met the customer's technical requirements. The finished product contained 0.004% sulfur, with very low O, N, and H contents. Specific test results are shown in Table 2.

[0022] Table 2 Test results of the embodiment

[0023] As can be seen from the above embodiments, the present invention controls the S content at a low level when smelting medium-carbon boron-containing steel for 35MnB track links with a specification of Φ50-80mm, thereby improving the vacuum degree, molten steel circulation time, and soft blowing time. Before production, the pouring is stopped for secondary cooling chamber maintenance, an R30 crystallizer is used, and small nozzles for air mist cooling of the ingot are used. Production is carried out using the early crystallizer, the crystallizer steel flow rate does not exceed 150 furnaces, the secondary cooling water ratio, the crystallizer water volume, and the water volume ratio of the foot roll are reduced, and low superheat casting is performed. The tower-shaped hairline test results of 35MnB successfully meet the customer's technical requirements.

[0024] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for controlling tower-shaped graining in medium-carbon boron steel, comprising the steps of KR desulfurization → LD converter → LF refining → RH vacuum degassing → continuous casting, characterized in that: The RH limit vacuum degree is ≤67Pa, the vacuum cycle time is ≥25min, and the continuous soft blowing time is ≥20 minutes; an R30 crystallizer with a larger R angle is used, a small nozzle mist cooling is used in the secondary cooling zone, the secondary cooling water ratio is 0.15L / Kg, and the crystallizer water volume is 140m 3 / h, ensuring the "weak cooling + uniform" mode of the crystallizer, and at the same time, the water volume ratio of the foot roll is 0.80 to prevent poor cooling effect of the foot roll and ensure weak cooling and uniform cooling; low superheat casting is adopted, and the superheat is controlled at 15-25℃, with a target of 20℃, to ensure uniform cooling of the ingot and reduce the level of banded structure; in the early stage of production, crystallizer production is used, and the crystallizer steel overload does not exceed 150 furnaces to prevent excessive steel overload in the crystallizer, which will cause serious wear and deformation of the copper tube, and ensure uniform cooling of the continuous casting in the crystallizer.

2. The method for controlling tower-shaped graining of medium-carbon-boron steel according to claim 1, characterized in that: The S content of the KR desulfurization is no more than 0.002%, ensuring that the S content of the finished product is no more than 0.005%.

3. The method for controlling tower-shaped graining of medium-carbon-boron steel according to claim 1, wherein: The LD converter bottom blowing is carried out with argon blowing throughout the whole process. In order to reduce the O content in the steel, the end point C is required to be ≥0.12%. During the steel tapping process, aluminum iron is added first, and then silicon iron, silicon manganese, and boron iron alloy are added in sequence.

4. The method for controlling tower-shaped graining of medium-carbon-boron steel according to claim 1, wherein: The LF refining furnace produces white slag and maintains it all the time while ensuring the fluidity of the top slag. In order to prevent FeO and MnO in the slag from adding oxygen to the molten steel, the FeO+MnO in the slag is required to be ≤0.50%. Depending on the Al content, aluminum wire can be added once in the early stage of refining, and aluminum addition operations are strictly prohibited in the later stage.

5. The method for controlling tower-shaped graining of medium-carbon-boron steel according to claim 1, characterized in that: It is strictly forbidden to add alloy to the RH and return it to the LF furnace to increase the temperature.

6. The method for controlling tower-shaped graining of medium-carbon-boron steel according to claim 1, characterized in that: Before the continuous casting production, the pouring is stopped to carry out the maintenance of the second cooling chamber to ensure that the arc of the crystallizer is centered, the spray rack and the nozzle are centered, the nozzle is cleaned in time to prevent the nozzle from being blocked, and the arc is ensured to be centered and the cooling water meets the process requirements. At the same time, the copper tube and the protective slag are inspected more strictly to ensure a good cooling effect of the crystallizer.

7. The method for controlling tower-shaped graining of medium-carbon-boron steel according to claim 1, characterized in that: The continuous casting adopts an integral tundish, which is cleaned and emptied by filling with argon before pouring. The argon blowing time is ≥2 minutes, and the casting is protected throughout the whole process. The pouring is started outside the tundish.

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