A metallurgical process for reducing the point segregation of GCr15SiMn bearing steel ingot

Abstract

The application discloses a metallurgical process for reducing point segregation in GCr15SiMn bearing steel ingot, and the obtained ingot has high cleanliness, and meanwhile, the ingot has a composite inclusion with MgO-Al2O3 oxide as a core and MnS as a peripheral wrapping. The ingot cleanliness requires that the oxygen content in the steel is controlled to be 6-10 ppm, and the titanium content is less than or equal to 25 ppm. The composite inclusion has a core oxide with a size of 0.1-2 microns, and a peripheral wrapping MnS inclusion, and the total size is controlled to be less than 5 microns, and the number density is greater than or equal to 10 per mm 2 . The composite inclusion is spontaneously formed in the solidification process of the molten steel and serves as a nucleation core of early austenite, thereby inhibiting the enrichment of solute elements such as C and Si between the solidification front dendrites, and achieving the purpose of reducing the point segregation of the ingot. The EAF-LF-VD-VC process route is adopted, and the methods such as strictly controlling the end composition of the electric furnace tapping, the Al content in the whole process, the composition of the refining slag and the double vacuum process are adopted to realize the purpose.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of steel ingot smelting, and particularly relates to a metallurgical process for reducing point segregation of GCr15SiMn bearing steel ingot. BACKGROUND

[0002] GCr15SiMn belongs to high-carbon chromium bearing steel, and its main chemical components are: ω[C]: 0.95% to 1.05%, ω[Si]: 0.45% to 0.75%, and ω[Mn]: 0.95% to 1.25%. The steel has excellent hardenability, wear resistance and elastic limit, and is widely used in the fields of wind power, heavy machinery and mine machinery. Due to the harsh environment, the reliability of bearing materials is strictly required. The C and Si contents of the steel are relatively high, and the element segregation is serious during solidification. Meanwhile, the high Si content can further aggravate the segregation of C. On the other hand, when producing large-size (more than 250 mm) GCr15SiMn bearing steel bars, steel ingot casting is usually adopted, but the slow cooling speed during the solidification of the steel ingot can further aggravate the segregation of C and Si elements. In actual production, due to the difficulty in controlling the segregation of solute elements, the GCr15SiMn steel ingot often produces “point segregation” defects (also known as “A” type segregation) caused by the enrichment at the front of the solidification interface, which seriously affects the continuity and uniformity of the bearing materials.

[0003] In order to improve the alloy composition and organizational uniformity of the steel and reduce the macro “point segregation” defects of the steel ingot, Chinese patent CN200810010341.1 “A manufacturing method of low-segregation large steel ingot” proposes that a steel ball is added during the steel ingot casting process to increase the nucleation points inside the steel ingot and accelerate the cooling of the steel ingot. However, in actual production, the adding method, adding time and adding amount of the steel ball are difficult to control. Chinese patent CN201510956350.X “A method for reducing macro-segregation of large steel ingot” adds metal powder with the same composition as the steel in the casting process to increase the nucleation points inside the steel ingot, reduce the superheat degree of the molten steel, and improve the cooling rate of the molten steel. However, the addition of the metal powder in this process not only increases the smelting process, but also makes it difficult to homogenize the powder added to the molten steel.

[0004] Chinese patent CN201410078431.X “Production method of low-macro-segregation large roll blank steel ingot” proposes that a preheated cold bar with the same composition is fixed and hung at the center of the inner cavity of the steel ingot mold to accelerate the cooling speed of the center molten steel during the solidification process. However, in actual process, the preheating and fixing of the cold bar are relatively difficult, and the surface of the cold bar is required to be smooth and crack-free, which is difficult to operate and has high cost. In addition, a coagulation accelerating member for accelerating the solidification of the molten steel is arranged in the center of the steel ingot mold, which is also difficult to operate.

[0005] Chinese patent CN200910012415.X, "A Method for Suppressing Macroscopic Segregation of Large Steel Ingots by Mechanical Stirring," utilizes a carbon rod for mechanical stirring during the solidification process of the steel ingot to break columnar crystals, increase nucleation points inside the ingot, accelerate the cooling rate, and suppress the convection of solute elements. However, this method is prone to carbon rod burnout, leading to increased carbon content in the steel, and the stirring rate of the carbon rod is difficult to control. Chinese patent CN200810011408.3, "A Manufacturing Method for Obtaining Low-Segregation Large Steel Ingots by Accelerating Bottom and Sidewall Cooling," proposes using gray cast iron for the ingot mold and introducing compressed gas for cooling and protection at the bottom and sidewalls of the mold to increase the cooling rate of the ingot. However, in practice, introducing compressed gas for cooling and protection on the surface of the ingot is difficult to solve the problem of internal segregation in large steel ingots.

[0006] Chinese patent CN201210251784.6, "A Method for Controlling A-Type Segregation in Steel Ingots through Steel Purification," utilizes purification smelting and casting control technologies to reduce inclusions in molten metal and eliminate A-type segregation in steel ingots. However, this invention requires a vacuum carbon deoxidation process, which can only control the oxygen content to no more than 15 ppm. Furthermore, the lack of composite inclusion nuclei during steel ingot solidification prevents the refinement of the solidification structure and the complete elimination of A-type segregation in the steel ingot.

[0007] For GCr15SiMn steel ingots, using the above methods to reduce the point segregation defects in the ingots is not only difficult to operate, but also relatively costly, thus making it difficult to achieve the desired results. Summary of the Invention

[0008] This invention discloses a metallurgical process for reducing point segregation in GCr15SiMn bearing steel ingots, in order to solve any of the above-mentioned technical problems in the prior art and other potential problems.

[0009] To achieve the above objectives, the present invention employs the following technical solution: a metallurgical process for reducing point segregation in GCr15SiMn bearing steel ingots, comprising the following steps: EAF+LF+VD+VC, specifically:

[0010] In the EAF process:

[0011] After smelting, the composition of the molten steel is controlled to meet the following requirements: ω[C] 0.15%–0.30%, ω[Ti] ≤ 0.0007%, ω[S] ≤ 0.020%, and the steel is tapped at a certain temperature.

[0012] During the steelmaking process, a certain amount of Al, low-titanium alloy and carbon raiser are added;

[0013] In the LF process:

[0014] In the initial stage of refining, a certain amount of slagging agent is added to control the initial Al content of LF at 0.02% to 0.03%, and to ensure that the Al content at the end of refining is 0.015% to 0.020%.

[0015] In the VD process:

[0016] The first degassing treatment was performed, and at the end of VD degassing, the Al content in the molten steel was 0.012% to 0.015%, ω[H] ≤ 0.00015%, and ω[N] ≤ 0.0060%.

[0017] In the VC process:

[0018] A second degassing treatment is performed, followed by casting using the VC vacuum casting method. During solidification, specific fine composite inclusions are spontaneously generated. After casting, a steel ingot with a uniform structure and no segregation is obtained.

[0019] Furthermore, the composite inclusions in the GCr15SiMn bearing steel ingot have MgO-Al2O3 oxides as the internal core and MnS wrapped around them; and the number density of the composite inclusions is ≥10 inclusions / mm². 2 .

[0020] Furthermore, the core oxide of the composite inclusion has a size of 0.1–2 μm, and the total inclusion size is less than 5 μm.

[0021] Furthermore, the tapping temperature in the EAF process is 1620–1660°C;

[0022] The amount of Al added during the steelmaking process is 0.6–0.8 kg / ton of steel;

[0023] The amount of low-titanium alloy and carburizer added is the lower limit required by the steel alloy grade;

[0024] The low-titanium alloys include ferrosilicon, ferromanganese, and ferrochrome.

[0025] Furthermore, in the LF process, the amount of slag-forming agent added is 8-10 kg / ton of steel;

[0026] After LF refining for 40 minutes, the slag composition meets the following requirements: ω(CaO): 46%–56%, ω(SiO2): 5%–15%, ω(Al2O3): 20%–30%, ω(MgO): 3%–10%, ω(T.Ti): ≤0.07%; the remainder are unavoidable impurities.

[0027] Furthermore, the process parameters for the first degassing step in the VD process are as follows:

[0028] The argon flow rate for bottom blowing in the ladle is 60-80 L / min, the ultimate vacuum degree is ≤67 Pa, and the degassing time is controlled at 16-20 min.

[0029] Furthermore, the process parameters for the second degassing step in the VC process are as follows:

[0030] The vacuum level is controlled to be ≤67Pa, the pouring temperature is 1540~1550℃, the pouring speed is 2~6 tons of steel / min, and the pouring time is 10~30min.

[0031] Furthermore, no Ca or Mg alloys are added during the smelting process, wherein the Mg content comes from magnesium oxide in the slag and / or MgO in the ladle refractory.

[0032] Furthermore, the final composition of the GCr15SiMn steel ingot obtained at the end of the VC casting process satisfies the following: ω[C]: 0.95%~1.05%, ω[Si]: 0.45%~0.75%, ω[Mn]: 0.95%~1.25%, ω[Cr]: 1.40%~1.65%, ω[Al]: 0.012%~0.015%, ω[O]: 0.0006%~0.0010%, ω[S]: 0.0020%~0.0050%, ω[Ti]: <0.0025%, ω[N]: <0.0050%, ω[H]: <0.0001%, ω[Ca]≤0.0002%, ω[Mg]: 0.0003%~0.0006%, with the remainder being Fe and unavoidable impurity elements;

[0033] A GCr15SiMn bearing steel ingot, wherein the GCr15SiMn bearing steel ingot is prepared by the above-mentioned metallurgical process.

[0034] The technical principle of this invention is as follows:

[0035] Furthermore, Al is a key component for controlling the O content in molten steel. When ω[Al] > 0.015% in molten steel, the dissolved O in the molten steel before solidification is very low, and the molten steel easily generates a large amount of Al2O3-like oxides, which is not conducive to obtaining the ideal composite core effect. When ω[Al] < 0.01% in molten steel, the steel will be dominated by large-sized SiO2-MnO-Al2O3-like oxides, which is not conducive to controlling the cleanliness of the steel, nor is it conducive to the uniform distribution of MnS. In addition, Al is also a key component in the formation of MgO-Al2O3 oxides during solidification. Therefore, the Al content needs to be controlled within the range of 0.012% to 0.015%.

[0036] Furthermore, oxygen (O) is an important indicator for measuring the cleanliness of bearing steel, and its range is controlled between 0.0006% and 0.0010%. When ω[O] > 0.0010% in molten steel, Al2O3 oxide is easily formed prematurely and aggregates and grows to form inclusions; when ω[O] < 0.0006%, the O content is too low to form a sufficient number of heterogeneous nucleation sites.

[0037] Furthermore, the Ti content is controlled below 0.0025%, and the N content is controlled below 0.0050%. However, when ω[Ti] > 0.0025% and ω[N] > 0.0050% in the molten steel, large TiN inclusions larger than 5 μm are easily formed during solidification. TiN is a hard and brittle inclusion with sharp edges, which can easily scratch the matrix of the steel during use, seriously affecting the fatigue life of bearing steel.

[0038] Furthermore, sulfur (S) is a key component in the formation of MnS nuclei, and both S and Mn are prone to segregation. Ultimately, if ω[S] in the molten steel is ≥0.0050%, large MnS inclusions will form, reducing the steel's performance; conversely, if ω[S] is <0.0020%, sufficient MnS inclusions cannot be formed. Controlling the S content within the range of 0.0020% to 0.0050% ensures the formation of fine, complex inclusions in the steel, with a quantity ≥10 inclusions / mm. 2 .

[0039] Furthermore, the Ca content is controlled to be less than 0.0002%. The Ca content in molten steel is mainly controlled through the slag composition. When ω[Ca] in molten steel exceeds 0.0002%, it not only easily forms CaO-Al2O3 inclusions in the molten steel, but also the composite oxides generated during solidification are surrounded by CaS or (Ca,Mn)S. These inclusions have poor mismatch with austenite and cannot serve as heterogeneous nuclei for austenite. By controlling the composition of the refining slag system, especially the CaO content in the slag, the Ca content can be kept below 0.0002%.

[0040] Furthermore, the Mg content is controlled between 0.0003% and 0.0006%. The Mg content in molten steel is mainly affected by the MgO content in the refractory materials or slag. When ω[Mg] in molten steel > 0.0006%, large-sized MgO-Al2O3 inclusions will be formed in the steel, and uniformly distributed fine composite inclusion nuclei cannot be obtained. If ω[Mg] in molten steel < 0.0003%, the number of MgO-Al2O3 oxides in the steel decreases, and a sufficient number of composite inclusion nuclei cannot be formed.

[0041] Furthermore, the hydrogen (H) content is below 0.0001%. The H content in molten steel is primarily removed using vacuum equipment. When ω[H] in molten steel exceeds 0.0001%, H2 bubbles will form, which will disturb the distribution of solute elements at the solidification interface during solidification, exacerbating point segregation defects in the ingot.

[0042] Other elements, including C, Si, Mn, Cr, etc., shall comply with the national standard GB / T 18254-2016. The remaining elements are Fe and other unavoidable impurity elements.

[0043] Furthermore, the composition of the molten steel at the end of the electric arc furnace tapping process is strictly controlled. The composition of the molten steel before tapping is: ω[C] 0.15%~0.03%, ω[S]≤0.020%, ω[Ti]≤0.0007%, in order to reduce the degree of over-oxidation in the electric arc furnace and alleviate the tasks of deoxidation, desulfurization and titanium control in the refining process.

[0044] Furthermore, the Al content in the steel is controlled throughout the entire process. First, during the EAF tapping process, 0.6–0.8 kg / ton of steel is added, with its content controlled at approximately 0.03%, to reduce the oxygen content in the steel and prevent the formation of large-sized SiO2-MnO-Al2O3 inclusions. Second, during the LF refining process, the Al content is controlled within the range of 0.015%–0.025%, both to further reduce the oxygen content in the steel and to prevent Al from reducing Ca, Mg, and Ti in the slag and entering the molten steel. Finally, at the end of VD degassing, aluminum is added as little as possible to maintain the Al-Al2O3 balance between the molten steel and the slag during refining, thus controlling the Al content at 0.012%–0.015%.

[0045] Furthermore, during the initial stage of EAF tapping or LF refining, slag-forming agents such as lime and synthetic slag are added at a rate of 8–10 kg / ton of steel. After 40 minutes of refining, the slag composition is ensured to be: ω(CaO): 46%–56%, ω(SiO2): 5%–15%, ω(Al2O3): 20%–30%, ω(MgO): 3%–10%, ω(T.Ti): ≤0.07%; the remainder being unavoidable impurities such as MnO and FeO. After refining, the molten steel contains ω[Ti] ≤0.0025%, ω[S] 0.0020%–0.0050%, and ω[Mg] 0.0003%–0.0006%.

[0046] Furthermore, during the VD vacuum degassing process, the argon flow rate was controlled at 60–80 L / min, the ultimate vacuum degree was ≤67 Pa, and the degassing time was controlled at 16–20 min. After degassing, the trace elements in the molten steel met the following requirements: ω[O] 0.0006%–0.0010%, ω[Ca] ≤0.0002%, ω[H] ≤0.00015%, ω[N] ≤0.0060%.

[0047] Furthermore, in VC vacuum casting, the casting temperature is controlled at 1540–1550℃, and the casting speed is 2–6 tons of steel / min. After secondary vacuum degassing, the ω[H] in the molten steel is <0.0001%, and the ω[N] is <0.0050%.

[0048] The beneficial effects of this invention in controlling point segregation in GCr15SiMn steel ingots are as follows:

[0049] (1) By strictly controlling the final composition of EAF steel tapping, the Al content during the refining process, the component composition of the refining slag, and the double vacuum process, the oxygen content in the steel can be controlled at 6-10 ppm, the titanium content at less than 25 ppm, and there are basically no inclusions larger than 5 μm in the steel. The reduction of O and Ti content in the steel will significantly reduce the number and size of oxide inclusions and TiN, thereby mitigating the point segregation of steel ingots to a certain extent.

[0050] (2) By strictly controlling the content of elements such as Al, O, Ca, Mg, and S in the molten steel, a large number of fine composite inclusions were obtained during the solidification process of the molten steel. These composite inclusions consist of MgO-Al2O3 magnesium aluminum spinel oxide as the core, with a size of 0.1–2 μm, surrounded by MnS inclusions, with a total size of less than 5 μm and a quantity of ≥10 inclusions / mm. 2 These fine composite inclusions can serve as nucleation sites for primary austenite at the solidification front, inhibiting the enrichment of solute elements such as C and Si between dendrites at the solidification front and suppressing point segregation in steel ingots. Attached Figure Description

[0051] Figure 1 This is a schematic diagram illustrating the metallurgical process mechanism of a GCr15SiMn bearing steel ingot with low point segregation according to the present invention.

[0052] Figure 2a This is a morphology diagram of a typical composite inclusion (MgO-Al2O3)-MnS in steel according to an embodiment of the present invention.

[0053] Figure 2b The image shows the surface scan results of the composite (MgO-Al2O3)-MnS inclusions in the steel in the embodiments of the present invention.

[0054] Figure 3A low-magnification microstructure image of a steel cross-section was prepared for an embodiment of the present invention.

[0055] Figure 4 The results show the detection results of multiple groups of oxygen content in the steel of Comparative Example 1.

[0056] Figure 5 The image shows the morphological characteristics of TiN inclusions in the steel of Comparative Example 1.

[0057] Figure 6 Low-magnification microstructure images of the cross-section of the steel were prepared for Comparative Example 1.

[0058] Figure 7 The morphological characteristics of CaO-Al2O3-MgO inclusions in steel prepared for Comparative Example 2 were studied.

[0059] Figure 8a The morphological characteristics of the composite (MgO-Al2O3)-CaS inclusions in the steel prepared for Comparative Example 3 are shown.

[0060] Figure 8b The surface scan results are for the composite (MgO-Al2O3)-CaS inclusions in the steel prepared for Comparative Example 3.

[0061] Figure 9 Low-magnification microstructure of the cross-section of the forged steel was prepared for Comparative Example 3. Detailed Implementation

[0062] To enable those skilled in the art to better understand the technical problems, technical solutions, and advantages of the present invention, the process of the present invention will be described in detail with reference to specific embodiments.

[0063] This invention discloses a metallurgical process for reducing spot segregation in GCr15SiMn bearing steel ingots. The metallurgical process specifically includes the following steps:

[0064] (1) The electric arc furnace steel tapping process is as follows:

[0065] The molten steel produced from the electric arc furnace should meet the following composition requirements: ω[C] 0.15%–0.3%, ω[S] ≤ 0.02%, ω[Ti] ≤ 0.0007%, and the temperature should be 1620–1660℃ before tapping.

[0066] During the EAF tapping process, add 0.6–0.8 kg / ton of steel Al;

[0067] Meanwhile, ferrosilicon, ferromanganese, ferrochrome, and carbon raisers are added according to the lower limit of the alloy. Among them, the residual titanium content requirements for low titanium alloys and slag-forming materials are as follows: ferrochrome (Ti≤0.025%), ferrosilicon (Ti≤0.01%), ferromanganese (Ti≤0.03%), carbon raisers (Ti≤0.03%), and slag-forming materials (Ti≤0.035%).

[0068] (2) The LF furnace refining process is as follows:

[0069] After LF arrives at the station,

[0070] First, add lime, synthetic refining slag and other slag-forming agents, at a rate of 8-10 kg / ton of steel;

[0071] Secondly, selectively add Al to ensure that the Al content is in the range of 0.02% to 0.03%; in the LF refining process, choose to add as little or no aluminum as possible so that the Al content in the LF refining is controlled in the range of 0.015% to 0.025%.

[0072] The slag is energized and heated, and ferrosilicon, ferromanganese, and ferrochrome are added for fine-tuning of the composition. At the same time, Si-C and carbon powder mixed deoxidizers are added in small batches and multiple times on the slag surface for diffusion deoxidation. After refining for 40 minutes, the slag composition is guaranteed to be ω(CaO): 46%–56%, ω(SiO2): 5%–15%, ω(Al2O3): 20%–30%, ω(MgO): 3%–10%, ω(T.Ti): ≤0.07%; the remainder consists of unavoidable impurities such as FeO and MnO.

[0073] After LF refining, ω[Ti]≤0.0025%, ω[Ca]≤0.0003%, and ω[Mg]0.0003~0.0006%.

[0074] (3) The VD vacuum degassing process is as follows:

[0075] When the vacuum pressure is reduced to ≤67Pa, the blowing flow rate is preferably 60~80L / min, and the extreme vacuum time is maintained for 16~20min; at the end of VD, the final steel composition meets the following requirements: ω[Al] 0.012~0.015%, ω[O] 0.0006~0.0010%, ω[Ca] ≤0.0002%, ω[S] 0.0020%~0.0050%, ω[H] ≤0.00015%, ω[N] ≤0.0060%, and the ladle is used.

[0076] (4) The VC vacuum casting process is as follows:

[0077] The casting was performed using the VC vacuum casting method, with the casting temperature controlled at 1540–1550℃; the casting speed for the ingot body was 4–6 tons of steel / min, and the casting speed for the riser was 2–3 tons of steel / min; the total casting time was controlled at 10–30 minutes. After casting, the ω[H] in the ingot was ≤0.0001%, and the ω[N] was ≤0.0050%.

[0078] The process route of this invention adopts "EAF+LF+VD+VC". By strictly controlling the final composition of EAF steel tapping, the Al content during refining, the component composition of refining slag, and the double vacuum process, the oxygen content in the steel can be controlled at 6-10 ppm, the titanium content at less than 25 ppm, and there are virtually no inclusions larger than 5 μm in the steel. Simultaneously, the composite inclusions generated during solidification are MgO-Al2O3 magnesium aluminum spinel oxides with a core size of 0.1-2 μm, surrounded by MnS inclusions with a total size of less than 5 μm and a number ≥10 / mm. 2 These fine composite inclusions can act as nucleation sites for primary austenite at the solidification front, inhibiting the enrichment of solute elements such as C and Si between dendrites at the solidification front and suppressing point segregation in steel ingots. Figure 1 As shown;

[0079] This process is suitable for suppressing macroscopic point segregation defects in GCr15SiMn steel ingots or similar special steels.

[0080] Example

[0081] This embodiment adopts the "EAF+LF+VD+VC" process flow. By strictly controlling the content of elements such as Al, O, Ti, N, Ca, Mg, S, and H in the steel, the oxides and TiN inclusions larger than 5μm in the steel are greatly reduced, the cleanliness of the molten steel is improved, and a large number of fine (MgO-Al2O3)-MnS composite inclusions are obtained, which effectively reduces the probability of point segregation defects in GCr15SiMn steel ingots.

[0082] (1) EAF Beginner Training

[0083] At the end of the electric furnace smelting, the temperature was 1650℃, the composition of the molten steel was: ω[C] 0.20%, ω[Si] 0.01%, ω[Mn] 0.15%, ω[Cr] 0.07%, and the tapping volume was 37.4t;

[0084] During the 1 / 3 tapping process, 0.8 kg / ton of steel Al is added; at the same time, the steel composition is in accordance with the lower limit requirements: Si-Fe, Mn-Fe, Cr-Fe, and carbon raiser.

[0085] (2) LF Refining

[0086] Upon arrival at the LF refining station, temperature was measured and samples were taken. The steel sample composition, measured after 10 minutes, was as follows: ω[C] 0.94%, ω[Si] 0.51%, ω[Mn] 1.0%, ω[Cr] 1.33%, ω[S] 0.014%, ω[Al] 0.027%. Based on the composition test results, low-titanium ferrosilicon, ferromanganese, and ferrochrome were added during the LF process for fine-tuning of the composition, but no additional Al was required.

[0087] Then, lime and synthetic refining slag, etc., are added in an amount of 360.5 kg.

[0088] During the intermediate stage of LF refining, after 40 minutes of refining, the slag composition was ω(CaO) 55%, ω(SiO2) 12.9%, ω(Al2O3) 21.8%, ω(MgO) 4.5%, and ω(T.Ti) 0.06%; the remainder consisted of unavoidable impurities such as FeO.

[0089] Before the LF refining process was completed, temperature was measured and samples were taken. The steel sample composition was determined 10 minutes later as follows: ω[C] 0.99%, ω[Si] 0.58%, ω[Mn] 1.07%, ω[Cr] 1.50%, ω[S] 0.0040%, ω[Al] 0.018%, ω[Ti] 0.0022%, ω[Ca] 0.0003%, ω[Mg] 0.0004%.

[0090] (3) VD vacuum degassing

[0091] After VD enters the station, the flow rate of argon gas blown into the ladle is 70 L / min; then, after starting the vacuum pump, when the pressure drops to 67 Pa, the ultimate vacuum is maintained for 18 min; after VD ends, temperature is measured and samples are taken. The composition of the steel sample detected after 10 min is: ω[Al] 0.012%, ω[O] 0.0008%, ω[Ca] 0.0001%, ω[S] 0.0025%, ω[H] 0.00012%, ω[N] 0.0053%, ladle.

[0092] (4) The VC vacuum casting process is as follows:

[0093] The VC vacuum casting method was used for pouring, with a pouring temperature of 1550℃. The average pouring rate for the ingot body was 5 tons of steel / min, and the average pouring rate for the riser was 2 tons of steel / min. The total pouring time was 15 minutes. After pouring, the weight of the obtained steel ingot was 40 tons. The element content of each element in the molten steel before VC pouring in this embodiment is shown in Table 1.

[0094] Table 1. Chemical composition of molten steel before VC casting in the embodiments of the present invention, %

[0095]

[0096] During the LF refining process of the invention, the Al content and slag composition in the molten steel were within the specified range; at the end of VD refining, the O, Ti, Ca, Mg, and S contents in the molten steel also reached the target values. After VC vacuum casting, the N and H elements in the molten steel also met the specified range. The O and Ti contents in the steel ingot prepared using this invention were controlled at 8 ppm and 23 ppm, respectively, and the molten steel cleanliness was relatively high. At this time, the typical inclusion in the steel ingot was (MgO-Al2O3)-MnS, such as...Figure 2a As shown, its surface scan results are as follows: Figure 2b As shown. These composite inclusions can serve as nucleation sites for primary austenite at the solidification front, refining the solidification structure and suppressing point segregation in the steel ingot. Figure 3 The figure shows the macroscopic low-magnification microstructure of the steel ingot prepared according to the present invention. As can be seen from the figure, the GCr15SiMn steel ingot produced by the present invention does not exhibit macroscopic point-like segregation defects.

[0097] Comparative Example 1

[0098] This comparative example is basically the same as the invention example, except that the Al content control throughout the entire process is unstable. Initially, the Al content in the LF process was 0.008%. A large amount of aluminum was added during the LF refining process, and the Al content was controlled at 0.032% at the end of refining. After the VD vacuum casting, the Al content was 0.019%. The above Al content control exceeds the requirements of this invention. The elemental contents in the molten steel in Comparative Example 1 before VC vacuum casting are shown in Table 2.

[0099] Table 2 Chemical composition of molten steel before vacuum casting in Comparative Example 1, %

[0100]

[0101] In Comparative Example 1, the oxygen content in the steel ranged from 9 to 14 ppm, exhibiting a large fluctuation range. The test results are as follows: Figure 4 As shown; at the same time, the Ti content also exceeds the scope of the claims, approximately 28 ppm, at which point large-sized TiN inclusions exist in the steel, with morphology as shown. Figure 5 As shown, during solidification, these two types of inclusions promote the formation of point segregation defects during ingot solidification. The low-magnification microstructure of the steel in Comparative Example 1 is shown below. Figure 6 As shown in the figure, it is easy to see from the figure that the surface of the steel ingot produced in the comparative production has a large number of black spots, indicating that the steel has severe point segregation.

[0102] Comparative Example 2

[0103] Comparative Example 2 is essentially the same as the Invention Example, except that in the LF refining stage, the refining slag composition is: 62% CaO, 12.5% ​​SiO2, 18% Al2O3, 4.5% MgO; the remainder is unavoidable impurities such as FeO. The CaO and Al2O3 content in the above slag exceeds the requirements of this invention. Before VC casting, the elemental content in the molten steel of Comparative Example 2 is shown in Table 3.

[0104] Table 3 Chemical composition of molten steel before vacuum casting in Comparative Example 2, %

[0105]

[0106] In Comparative Example 2, due to the high CaO content in the slag, Al facilitates the incorporation of Ca and Mg elements into the molten steel during refining. This results in the formation of large CaO-Al₂O₃-MgO inclusions ≥10 μm in size, with morphological characteristics as follows: Figure 7 As shown, an increase in the number of large inclusions in steel can also affect the point segregation of steel ingots.

[0107] Comparative Example 3

[0108] Comparative Example 3 is basically the same as the Invention Example, except that VC casting was not used in the refining stage after VD. The content of each element in the molten steel of Comparative Example 3 before ingot casting is shown in Table 3.

[0109] Table 4. Chemical composition of molten steel before vacuum casting in Comparative Example 3, %

[0110]

[0111] In Comparative Example 3, since a vacuum casting process was not used, the Ca, H, and N content in the molten steel did not meet the requirements of this invention before casting. Therefore, the morphology of the composite inclusions formed in the steel during solidification was as follows: Figure 8a As shown in the figure. The core oxide is MgO-Al2O3, and the core is surrounded by CaS inclusions. The surface scan results are shown in the figure. Figure 8b As shown. These inclusions cannot act as nucleation sites for primary austenite, thus failing to suppress point segregation in the ingot. Therefore, the macroscopic segregation defects in the ingot are not improved either. Figure 9 As shown.

[0112] The above provides a detailed description of a metallurgical process for reducing point segregation in GCr15SiMn bearing steel ingots, as provided in the embodiments of this application. The descriptions of the embodiments above are merely for the purpose of helping to understand the method and core ideas of this application; furthermore, those skilled in the art will recognize that, based on the ideas of this application, there will be changes in specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this application.

[0113] Certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that hardware manufacturers may use different names to refer to the same component. This specification and claims do not distinguish components based on differences in name, but rather on differences in function. The terms "comprising" and "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising / including but not limited to". "Approximately" means that within an acceptable margin of error, those skilled in the art can solve the technical problem and substantially achieve the technical effect within a certain margin of error. The following descriptions in the specification are preferred embodiments for carrying out this application; however, these descriptions are for the purpose of illustrating the general principles of this application and are not intended to limit the scope of this application. The scope of protection of this application shall be determined by the appended claims.

[0114] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a product or system comprising a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a product or system. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the product or system that includes said element.

[0115] It should be understood that the term "and / or" used in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this article generally indicates that the preceding and following related objects have an "or" relationship.

[0116] The foregoing description illustrates and describes several preferred embodiments of this application. However, as previously stated, it should be understood that this application is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the application concept described herein through the foregoing teachings or techniques or knowledge in related fields. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of this application should be within the protection scope of the appended claims.

Claims

1. A metallurgical process for reducing point segregation in GCr15SiMn bearing steel ingots, characterized in that, The metallurgical process includes the following steps: EAF+LF+VD+VC, specifically: In the EAF process: After smelting, the composition of the molten steel is controlled to meet the following requirements: ω[C] 0.15%–0.30%, ω[Ti] ≤ 0.0007%, ω[S] ≤ 0.020%, and the steel is tapped at a certain temperature. During the steelmaking process, a certain amount of Al, low-titanium alloy and carbon raiser are added; The tapping temperature in the EAF process is 1620–1660°C; The amount of Al added during the steelmaking process is 0.6–0.8 kg / ton of steel; The amount of low-titanium alloy and carburizer added is the lower limit required by the steel alloy grade; The low-titanium alloys include ferrosilicon, ferromanganese, and ferrochrome; In the LF process: In the initial stage of refining, a certain amount of slagging agent is added to control the initial Al content of LF at 0.02% to 0.03%, and to ensure that the Al content at the end of refining is 0.015% to 0.020%. The amount of slag-forming agent added is 8-10 kg / ton of steel; After LF refining for 40 minutes, the slag composition meets the following requirements: ω(CaO): 46%–56%, ω(SiO2): 5%–15%, ω(Al2O3): 20%–30%, ω(MgO): 3%–10%, ω(T.Ti): ≤0.07%; The remainder are unavoidable impurities; In the VD process: The first degassing treatment was performed, and at the end of VD degassing, the Al content in the molten steel was 0.012% to 0.015%, ω[H] ≤ 0.00015%, and ω[N] ≤ 0.0060%. In the VC process: A second degassing treatment was performed, and the VC vacuum casting method was used for casting. During the solidification process, specific fine composite inclusions were spontaneously generated. After casting, a GCr15SiMn bearing steel ingot with uniform structure and no segregation was obtained. The process parameters for the second degassing step in the VC process are as follows: The vacuum level is controlled to be ≤67Pa, the pouring temperature is 1540~1550℃, the pouring speed is 2~6 tons of steel / min, and the pouring time is 10~30min; The composite inclusions in the GCr15SiMn bearing steel ingot have MgO-Al2O3 oxides as the internal core and MnS wrapped around them; and the number density of the composite inclusions is ≥10 inclusions / mm². 2 ; The core oxide of the composite inclusion has a size of 0.1 to 2 μm, and the total inclusion size is less than 5 μm.

2. The metallurgical process according to claim 1, characterized in that, The process parameters for the first degassing step in the VD process are as follows: The argon flow rate for bottom blowing in the ladle is 60-80 L / min, the ultimate vacuum degree is ≤67 Pa, and the degassing time is controlled at 16-20 min.

3. The metallurgical process according to claim 1, characterized in that, No Ca or Mg alloys are added during the smelting process; the Mg content comes from magnesium oxide in the slag and / or MgO in the ladle refractory.

4. The metallurgical process according to claim 1, characterized in that, The final composition of the GCr15SiMn steel ingot obtained at the end of the VC casting process satisfies the following: ω[C]: 0.95%~1.05%, ω[Si]: 0.45%~0.75%, ω[Mn]: 0.95%~1.25%, ω[Cr]: 1.40%~1.65%, ω[Al]: 0.012%~0.015%, ω[O]: 0.0006%~0.0010%, ω[S]: 0.0020%~0.0050%, ω[Ti]: <0.0025%, ω[N]: <0.0050%, ω[H]: <0.0001%, ω[Ca]≤0.0002%, ω[Mg]: 0.0003%~0.0006%, with the remainder being Fe and unavoidable impurity elements.

5. A GCr15SiMn bearing steel ingot, characterized in that, The GCr15SiMn bearing steel ingot is prepared using the metallurgical process described in any one of claims 1-4.

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