A method for preparing a hot-rolled medium bar for C-Mn high-carbon wear-resistant steel ball based on continuous casting small square billet

Abstract

The application provides a method for preparing a hot-rolled medium bar for C-Mn high-carbon wear-resistant steel ball based on continuous casting small billet, and belongs to the field of special steel metallurgy. The application optimizes the continuous casting-rolling production process of high-carbon wear-resistant ball steel: 160mm*160mm section small billet continuous casting, billet heating, rough rolling and finish rolling, water tank+cold bed cooling, shearing, finishing, slow cooling, and 40mm-50mm hot-rolled medium bar. In the continuous casting process, a strong crystallizer and a solidification end electromagnetic stirring technology are used. In the heating process, a high-temperature diffusion technology for preventing oxidation and decarburization is used. In the rolling process, two passes of large deformation reduction and a synergistic controlled cooling technology are used. The center shrinkage hole of the hot-rolled medium bar is eliminated, the center porosity and the network carbide are obviously improved, the complicated process of continuous casting large billet, high-temperature long-time heating and cogging process are omitted, the processing flow is shortened, and the production efficiency is improved. The application realizes large-scale rolling by using a small compression ratio, so that the purpose of ensuring product quality while reducing production cost is achieved.

Description

Technical Field

[0001] This invention belongs to the field of special steel metallurgy, specifically relating to a hot-rolled bar for C-Mn series hot-rolled high-carbon wear-resistant steel balls based on continuously cast small square billets and its preparation method. Background Technology

[0002] C-Mn type hot-rolled high-carbon wear-resistant steel balls, due to their high hardness and good hardenability, are one of the important grinding materials used in ball mills and are widely used in industries such as mining, metallurgy, power plants, and cement. The diameter of these balls is less than or equal to 80mm, and they are generally produced from hot-rolled bars using a "skew rolling → quenching" process. The skew rolling process is similar to "ball rolling," with a compression ratio of almost zero. The center of the bar is subjected to tensile stress. If residual shrinkage cavities and severe network carbides exist in the center of the steel ball, cracks are easily induced during the ball forming process, drop tests, and service life. Therefore, high internal quality requirements are placed on the bars used to produce high-carbon wear-resistant steel balls.

[0003] The C-Mn type high-carbon wear-resistant ball steel has a C content of 0.72~1.05% and a Mn content of 0.7~1.1%, making it a typical hypereutectoid steel. During continuous casting solidification, it is prone to severe central shrinkage cavities and segregation, which are inherited by the rolled product, causing residual shrinkage cavities and carbon network in the core. In existing technologies, the production of hot-rolled bars for high-carbon wear-resistant ball steel with a diameter of Ø40mm~Ø50mm generally uses larger cross-section rectangular billets or large square billets (over 200mm × 200mm) as raw materials to ensure the compression ratio during rolling. This process is complex, has a low yield, and high operating costs. Currently, the market for wear-resistant ball steel is highly competitive. How to reduce the production cost of hot-rolled bars while ensuring their quality has become one of the urgent problems to be solved in this competitive market. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a method for preparing hot-rolled intermediate bars for C-Mn series high-carbon wear-resistant steel balls based on continuously cast small square billets. The continuous casting process employs "low superheat + suitable casting speed + weak secondary cooling + strong crystallizer and electromagnetic stirring process at the solidification end" to control center segregation and shrinkage cavities in the billet. The heating process reduces billet stress and achieves homogenization through "slow heating in the low-temperature section + rapid heating in the high-temperature section," and applies a high-temperature oxidation-resistant coating to the billet surface to reduce the decarburized layer. The rolling process employs "single-pass large reduction rolling force core penetration" to control center shrinkage cavities and porosity. The rolling process employs "light water piercing combined with cooling at a suitable cooling bed rate after rolling" to control the precipitation of network carbides in the center.

[0005] To achieve the above objectives, the technical solutions adopted in the embodiments of the present invention are as follows: This invention provides a method for preparing hot-rolled intermediate bars for C-Mn series high-carbon wear-resistant steel balls based on continuously cast small square billets. The method includes the following steps: Step S1: Control the superheat of the molten C-Mn series high carbon wear-resistant steel ball steel before entering the crystallizer to be below the preset temperature; before casting, set the crystallizer stirring current and frequency to make the corresponding central magnetic induction intensity 488~585Gs, set the solidification end electromagnetic stirring current and frequency to make the corresponding central magnetic induction intensity 891~1070Gs, and adopt the continuous stirring mode; after casting, the casting speed is used to make the solid fraction of the billet center at the installation position of the solidification end electromagnetic stirrer 0.1~0.2; the secondary cooling zone adopts a weak cooling mode with a specific water volume of 0.6~0.7L / Kg to cool and solidify the billet with liquid core, and the end electromagnetic stirrer stirs the pasty area containing the billet that is not completely solidified, to obtain a 160mm×160mm cross-section continuous casting small square billet; Step S2: After applying a high-temperature resistant and anti-oxidation coating to the surface of the continuously cast small square billet, it passes through the preheating section, heating section one, heating section two, and soaking section of the heating furnace in sequence, with the furnace set in a reducing atmosphere. Step S3: The uniformly heated continuous casting small square billet is subjected to rough rolling and finish rolling in sequence, and hot-rolled medium bar is obtained after rolling. Step S4: The hot-rolled bar is cooled by a water tank and a cooling bed. The water volume in the cooling tank after finishing rolling, the turning speed of the cooling bed, and the ventilation rate are set to ensure the temperature of the hot-rolled bar is greater than 850℃ before it enters the cooling bed, and that it passes quickly after entering the cooling bed, avoiding point A. c1 ~A cm Linear temperature; Step S5 involves shearing, finishing, and slow cooling of the cooled hot-rolled bar to obtain hot-rolled bars with a diameter of Ø40mm~Ø50mm for C-Mn series high-carbon wear-resistant steel balls.

[0006] In a preferred embodiment of the present invention, in step S1, the stirring frequency is calculated based on the set central magnetic induction intensity and the skin effect of the crystallizer copper tube and the solidified shell of the cast billet on the electromagnetic field.

[0007] As a preferred embodiment of the present invention, the formula for calculating the stirring frequency is as follows: (1) In equation (1), T is the electromagnetic torque, and σ is the conductivity of the conductor. f B is the stirring frequency, and B is the measured magnetic flux density at the center of the stirrer. r 1 is the equivalent radius of the liquid core, and L is the effective working length of the stirrer; The stirring frequency corresponding to the maximum value of electromagnetic torque T f The optimal stirring frequency is [the specified frequency].

[0008] In a preferred embodiment of the present invention, the superheat of the molten steel temperature before entering the crystallizer is controlled to be below 25°C in step S1.

[0009] In a preferred embodiment of the present invention, in step S2, the temperature of the preheating section in the heating furnace is set to 40~720℃, the temperature of the first heating section is set to 700~900℃, the temperature of the second heating section is set to 1060~1160℃, and the temperature of the soaking section is set to 1110~1160℃.

[0010] In a preferred embodiment of the present invention, the initial rolling temperature in step S3 is set to 1110~1130℃, the single-pass reduction is 62~68mm, and the two-pass deformation is ≥40%.

[0011] In a preferred embodiment of the present invention, the shearing temperature in step S5 is greater than 380°C.

[0012] In a preferred embodiment of the present invention, the slow cooling is performed by immersion cooling, with an immersion cooling temperature ≥280℃, and the lid is opened after slow cooling for more than 56 hours, with the exit temperature ≤60℃.

[0013] In a preferred embodiment of the present invention, the component content of the hot-rolled intermediate bar for C-Mn series high-carbon wear-resistant steel balls is as follows: C: 0.72~1.05%; Si: 0.22~0.32%; Mn: 0.7~1.10%; Cr: 0.2~0.8%; S: ≤0.01%; P: ≤0.02%; Ni and Cu: ≤0.03%, and are residual elements; Al t 0.01~0.03%, the remainder being Fe and other unavoidable impurities.

[0014] The method for preparing hot-rolled intermediate bars for C-Mn series high-carbon wear-resistant steel balls based on continuously cast small square billets provided in this invention has the following beneficial effects: (1) The C-Mn series hot-rolled high-carbon wear-resistant balls with specifications of Ø40mm~Ø50mm provided have a center segregation, shrinkage cavity and porosity of less than 0.5 grade in the hot-rolled bar, and a core grain size greater than 8 grade; the center network carbide is less than 1.0 grade, and the metallographic structure is mainly sorbite and pearlite, without martensite and bainite structures. The average hardness in the hot-rolled state is 28HRC. Wear-resistant balls made from this raw material by quenching-skew rolling process have a surface and core hardness greater than 60HRC with a small difference.

[0015] (2) By systematically optimizing the various processes of continuous casting of small billets, heating furnace, rolling and controlled cooling after rolling, it is possible to produce high homogeneous, high-density hot-rolled bars with a large cross-section using small billets as raw materials with a smaller compression ratio. This reduces costs while ensuring quality and improving the market competitiveness of the products. At the same time, it effectively improves the wear resistance, toughness and fatigue life of the steel balls hot-rolled from these billets.

[0016] (3) Considering the differences in performance and process design of various small billet continuous casting machines, heating furnaces, and rolling mills, a method for replicable process parameters was developed, eliminating the need for extensive testing, thus improving efficiency and saving time and costs. The process flow of this invention is simple and easy to implement, and can be applied to large-scale industrial production. At the same time, it ensures that in actual production, the improvement in central network carbides, central shrinkage porosity, and grain size of hot-rolled bars with the same cross-section under similar equipment configurations is relatively small.

[0017] Of course, implementing any product or method of the present invention does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a flowchart of the method for preparing hot-rolled intermediate bars for C-Mn series high carbon wear-resistant steel balls based on continuously cast small square billets, as described in an embodiment of the present invention. Figure 2 The solid fraction distribution at the center of the billet calculated by the solidification heat transfer model in Embodiment 1 of the present invention at a casting speed of 1.9 m / min is shown. Figure 3 This is a 100x metallographic image of the surface of the Ø50mm hot-rolled bar in Embodiment 1 of the present invention; Figure 4 This is a 100x metallographic image of the center of the Ø50mm hot-rolled bar in Embodiment 1 of the present invention; Figure 5 This is a diagram of the austenite grains at the center of the cross section of a hot-rolled bar with a diameter of Ø50mm in Example 1 of the present invention. Figure 6 This is a 100x metallographic image of the surface of the Ø40mm hot-rolled bar in Embodiment 2 of the present invention; Figure 7 This is a 100x metallographic image of the center of the Ø40mm hot-rolled bar in Embodiment 2 of the present invention; Figure 8This is a diagram of the austenite grains at the center of the cross section of a hot-rolled bar with a diameter of Ø40mm in Example 2 of the present invention. Detailed Implementation

[0020] After discovering the aforementioned problems, the inventors of this application conducted a detailed study on the existing process for preparing hot-rolled intermediate bars for C-Mn series hot-rolled high-carbon wear-resistant steel balls. The study found that using small square billets (160mm × 160mm cross-section) as raw materials to produce hot-rolled intermediate bars of high-carbon wear-resistant ball steel with a specification of Ø40mm~Ø50mm can simplify the production process. Furthermore, if the quality of the hot-rolled intermediate bars can be effectively controlled, production costs can be significantly reduced, and market competitiveness can be improved.

[0021] Currently, special steel enterprises mainly use high-carbon steel (50#~82B, etc.) in small square billets (160mm~165mm×160~165mm cross-section) as raw materials to produce high-carbon hard wire rods and bearing steel GCr15 hot-rolled wire rods through the process of "hot rolling of high-speed wire rod + Stellmore air cooling". Since the compression ratio is large in the wire rolling process, the continuous casting process mainly controls the center segregation of the billet and does not need to consider the center shrinkage cavity and porosity. Therefore, small square billets can be used for production. However, due to the high variety and content of carbon and alloying elements, high-carbon wear-resistant spherical steel is prone to defects such as central segregation, shrinkage cavities, and porosity in the cast billet during solidification. Compared with large billets, the continuous casting process of small billets is more prone to central segregation, shrinkage cavities, and porosity because the two-phase region is narrow and the feeding difference is poor. In addition, it is difficult to control the superheat in the production process stably and at a low level. Using large reduction is beneficial to the densification of core shrinkage cavities and porosity. The rolling speed of hot-rolled bar is fast, and the large reduction and inaccurate rolling control will cause steel accumulation and uneven deformation of the rolled material. In order to control the core carbon network of high-carbon steel rolled material, controlled rolling and controlled cooling are used for wire rod and bars with smaller cross sections (specifications below Ø40mm). However, for larger hot-rolled bars, the large cross section makes it impossible to cool evenly and accurately control the cooling rate, which can cause the core to not cool through or the cooling rate to be too high, resulting in abnormal martensitic structure. Due to the above-mentioned technical difficulties in actual production, the hot-rolled bars of high-carbon wear-resistant ball steel with small billets as raw materials have serious carbon network and non-dense residual shrinkage cavities in the center, and the quality of the steel is difficult to meet the requirements for producing large-sized hot-rolled wear-resistant balls.

[0022] It should be noted that the defects in the above-mentioned prior art solutions are all the result of the inventors' practice and careful research. Therefore, the discovery process of the above problems and the solutions proposed by the embodiments of the present invention in the following text should be the inventors' contributions to the present invention.

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. It should be noted that, without conflict, the embodiments and features in the embodiments of the present invention can also be combined with each other.

[0024] It should be noted that similar reference numerals and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. In the description of the embodiments of the present invention, the terms "first," "second," "third," "fourth," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance. In addition, sometimes a subscript such as W1 may be written in a non-subscript form such as W1, and their meanings are consistent unless the distinction is emphasized.

[0025] It should be understood that the term "and / or" 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, or B existing alone. A and B can be singular or plural. Additionally, the character " / " in this article generally indicates an "or" relationship between the preceding and following related objects, but it can also represent an "and / or" relationship. Please refer to the context for a more accurate understanding.

[0026] After the above in-depth analysis, the inventors of this application discovered that to produce hot-rolled intermediate bars of high-carbon wear-resistant ductile steel with a cross-section of 160mm×160mm as raw material, the continuous casting process needs to obtain a highly homogeneous and dense billet (with low ratings for center segregation, shrinkage cavities, and porosity). The heating process further homogenizes the billet and creates a smaller decarburized layer. The rolling process achieves a denser core and controls the precipitation of carbon in the core. Post-rolling cooling further reduces the carbon content in the core of the hot-rolled intermediate bar. By coordinating and controlling the processes before and after rolling, high homogeneity and high density of the hot-rolled intermediate bar can ultimately be achieved. Based on this, the present invention provides a hot-rolled bar for C-Mn series hot-rolled high-carbon wear-resistant steel balls based on continuously cast small square billets and its preparation method. First, qualified molten steel is sequentially stirred, cooled and solidified, straightened and flame-cut in a multi-flow small square billet continuous casting machine equipped with a crystallizer and an electromagnetic stirrer at the solidification end to become a fixed-length billet with good internal quality. It is then conveyed to a walking beam furnace via a conveyor roller. Before entering the furnace, a high-temperature oxidation-resistant coating is applied to the surface of the billet. After entering the furnace, it undergoes preheating, heating, and holding before exiting the furnace. After exiting the furnace, it is descaled by a high-pressure descaling machine using high-pressure water. The descaled billet is then rolled into a bar by a rolling mill with alternating horizontal and vertical short stress lines, followed by roughing, intermediate rolling, pre-drilling and water cooling, finishing, and sizing. After rolling, it is lightly drilled onto a water cooling bed. The hot-rolled bar is sheared after rapid walking on the cooling bed. The fixed-length finished product enters a slow cooling pit and is then closed. After the time condition is met, it is removed from the pit.

[0027] The main components of the C-Mn series high carbon wear-resistant ball steel, C, Mn and Cr, have mass percentage contents of 0.72~1.05%, 0.7~1.10% and 0.55~0.65%, respectively, while the contents of S and P are all less than 0.01%. This invention uses 160mm×160mm cross-section small square billets as raw materials, which are then hot-rolled and controlled-cooled to produce high-homogeneity and high-density hot-rolled bars of C-Mn series high-carbon wear-resistant spherical steel with a cross-section of Ø50mm and below. The continuous casting process, characterized by "low superheat + suitable casting speed + weak secondary cooling + strong crystallizer and electromagnetic stirring at the end of solidification," controls the center segregation and shrinkage of the billet. The heating process, characterized by "slow heating in the low-temperature section + rapid heating in the high-temperature section," reduces stress and homogenizes the billet. The application of a high-temperature oxidation-resistant coating controls the decarburization layer of the billet. The rolling process, characterized by "single-pass large reduction rolling force and core penetration," controls the center shrinkage and porosity. The post-rolling controlled-cooling process, characterized by "light water piercing + suitable cooling bed speed," controls the precipitation of network carbides in the center. This invention achieves synergistic optimization of the three processes—continuous casting, rolling, and post-rolling cooling—in the production of high-carbon wear-resistant ball hot-rolled bars with a cross-section of Ø50mm. The cross-section exhibits center segregation, shrinkage cavities, and porosity below grade 0.5, a center carbon segregation index below 1.07, and a core grain size greater than grade 8. The surface decarburized layer is less than 0.2D%, the central microstructure is primarily sorbite and pearlite, the central network carbides are below grade 1.0, and there are no abnormal martensite or bainite structures. The average hardness in the hot-rolled state is 28 HRC. This invention enables the production of high-homogeneity, high-density, and large-sized hot-rolled bars from small billets using a relatively low compression ratio, while effectively improving the wear resistance, toughness, and fatigue life of the hot-rolled balls.

[0028] The hot-rolled bar for C-Mn series high-carbon wear-resistant steel balls, by weight percentage, has the following composition of C: C: 0.72~1.05%; Si: 0.22~0.32%; Mn: 0.7~1.10%; Cr: 0.2~0.8%; S: ≤0.01%; P: ≤0.02%; Ni and Cu: ≤0.03%, and are residual elements; Al t The content of the impurities is 0.01~0.03%, with the remainder being Fe and other unavoidable impurities. The hot-rolled bar has a diameter of Ø40mm~Ø50mm.

[0029] like Figure 1 As shown, the method for preparing hot-rolled intermediate bars for C-Mn series high-carbon wear-resistant steel balls based on continuously cast small square billets includes the following production process: continuous casting of small square billets, heating in a heating furnace, rolling of intermediate bars, cooling in a water tank after rolling, cooling on a cooling bed, finishing, and slow cooling in a pit. Specifically, it includes the following steps: Step S1: Control the superheat of the molten steel before entering the crystallizer to be below 25℃; before casting, set the crystallizer stirring current and frequency to make the corresponding central magnetic induction intensity 488~585Gs, and set the solidification end electromagnetic stirring current and frequency to make the corresponding central magnetic induction intensity 891~1070Gs, and adopt a continuous stirring mode; after casting, the casting speed is used to make the solid fraction of the billet center at the installation position of the solidification end electromagnetic stirrer 0.1~0.2; the secondary cooling zone adopts a weak cooling mode with a specific water volume of 0.6~0.7L / Kg to cool and solidify the billet with liquid core, and the end electromagnetic stirrer stirs the pasty area containing the incompletely solidified billet to obtain a 160mm×160mm cross-section continuous casting small square billet.

[0030] In this step, the magnetic induction intensity at the center of the crystallizer and the electromagnetic stirrer at the solidification end is measured using a gaussmeter with an alternating low-frequency mode, and the corresponding stirring process parameters (stirring current and stirring frequency) are then determined accordingly. The casting speed is chosen to ensure that the solid fraction at the center of the billet at the installation position of the electromagnetic stirrer at the solidification end is 0.1~0.2. The secondary cooling process needs to be determined based on a solidification heat transfer model simulation calculation verified by nail guns and temperature measurement. The continuous casting process adopts a control strategy of "low superheat + suitable casting speed + weak secondary cooling + strong crystallizer and solidification end electromagnetic stirring process" to produce billets with low central carbon segregation, shrinkage cavities and porosity, and chemical composition that meet the requirements through continuous casting of 160mm×160mm cross-section small square billets.

[0031] This step calculates the stirring frequency based on the set central magnetic induction intensity and the skin effect of the crystallizer copper tube and the solidified billet shell on the electromagnetic field; the formula for calculating the stirring frequency is as follows: (1) In equation (1), T is the electromagnetic torque, and σ is the conductivity of the conductor. f B is the stirring frequency, and B is the measured magnetic flux density at the center of the stirrer. r 1 represents the equivalent radius of the liquid core, and L represents the effective working length of the stirrer. The stirring frequency corresponding to the maximum value of the electromagnetic torque T is... f The optimal stirring frequency is [the specified frequency].

[0032] Step S2: After applying a high-temperature resistant and anti-oxidation coating to the surface of the continuously cast small square billet, it passes through the preheating section, heating section 1, heating section 2, and soaking section of the heating furnace in sequence, with the furnace set to a reducing atmosphere; the preheating section temperature is set to 40~720℃, the heating section 1 temperature to 700~900℃, the heating section 2 temperature to 1060~1160℃, and the soaking section temperature to 1110~1160℃, respectively.

[0033] In this step, the heating furnace process adopts a control strategy that combines slow heating in the low-temperature section and rapid heating in the high-temperature section with the application of a high-temperature resistant and anti-oxidation coating on the surface of the billet. This reduces the internal stress of the 160mm×160mm cross-section small square billet, prevents austenite grain growth, achieves homogenization of alloying elements, and reduces the decarburized layer on the surface of the billet.

[0034] Step S3: The uniformly heated continuous casting small square billet is subjected to rough rolling and finish rolling in sequence. The initial rolling temperature is set to 1110~1130℃, the single-pass reduction is 62~68mm, and the two-pass deformation is ≥40%. After rolling, a hot-rolled bar with a specification of Ø40mm~Ø50mm is obtained.

[0035] In this step, the rolling process adopts the three-roll continuous rolling and four-roll reducing sizing rolling technology of short stress line rolling mill. The rolling process adopts the process control strategy of "single-pass large reduction and double-pass large deformation rolling force core penetration" to achieve high density of the center (low rating of low magnification center shrinkage cavity and porosity in the cross section of hot rolled bar).

[0036] Step S4: Cool the hot-rolled bar. Set the water volume of the cooling tank after finishing rolling, the turning speed of the cooling bed, and the ventilation rate to ensure that the temperature of the hot-rolled bar is greater than 850℃ before it enters the cooling bed, and that it passes through the cooling bed quickly after entering, avoiding point A. c1 ~A cm Linear temperature.

[0037] In this step, after the hot-rolled bars with a diameter of Ø40mm~Ø50mm are rolled, the water volume in the water tank is set before cooling after finishing rolling. After cooling begins, the hot-rolled bars are sequentially cooled by atomization through the water tank nozzles. The temperature of the hot-rolled bars before being placed on the cooling bed is greater than 850℃. The A content of the high-carbon wear-resistant ball steel is obtained through thermal expansion tests. c1 and A cm Linear temperature, control the final cooling temperature of the upper cooling bed to be greater than 850℃ and set the cooling bed ventilation rate to ensure that the hot-rolled bars pass through the cooling bed quickly and avoid A c1 ~A cm Linear temperature. The post-rolling cooling adopts a process control strategy of "light water piercing + suitable cooling bed cooling rate" to reduce and avoid the precipitation of central secondary network cementite, thereby achieving a low rating of central network carbides in hot-rolled bars.

[0038] Step S5: After rolling and cooling, the hot-rolled bar is rapidly moved along a cooling bed to shear and finish the cooled bar, followed by slow cooling to obtain hot-rolled bars of Ø40mm~Ø50mm for C-Mn series high-carbon wear-resistant steel balls; and the shearing temperature is greater than 380℃. During slow cooling, the finished hot-rolled bar is hoisted into the slow cooling pit by an overhead crane.

[0039] In this step, slow cooling is achieved by immersion cooling in a pit, with an immersion cooling temperature ≥280℃. After slow cooling for more than 56 hours, the lid is opened, and the temperature upon exiting the pit is ≤60℃. The slow cooling process uses a higher immersion temperature and a longer slow cooling time to ensure stress release and prevent cracking.

[0040] The technical solution of the present invention will be further described below with reference to the accompanying drawings, specific embodiments, and comparative examples, and will be described in further detail.

[0041] Specific embodiments 1-2 and comparative examples 1-3 all provide a hot-rolled bar for C-Mn series high-carbon wear-resistant steel balls based on continuously cast small square billets and its preparation method. The actual steel composition of embodiments 1-2 and comparative examples 1-3 is shown in Table 1. The production process and method are the same, all prepared according to steps S1-S6 in the aforementioned technical solution. The difference lies in the specific steel composition and production process parameters of the different embodiments and comparative examples. The casting machine is a full-arc 8-strand small square billet casting machine with an arc radius of 12m and a metallurgical length of 34m. It is equipped with secondary cooling air mist cooling, built-in crystallizer electromagnetic stirring and external solidification end electromagnetic stirring. The heating furnace is a 28m×13m walking beam furnace that can control the reducing atmosphere and achieve controllable heating speed; the high-pressure water descaling machine has a maximum pressure of 30Mpa; the rolling process includes a 21-stand alternating horizontal and vertical short stress line rolling mill, a cooling water tank, a KOCKS four-stand three-roll reducing and sizing mill, a diameter measuring instrument, a cooling bed, etc.

[0042] Table 1. Main chemical composition (wt%) of C-Mn series high carbon wear-resistant ball steel

[0043] To further explain and illustrate the technical solution of this invention, the magnetic induction intensity at the center of the electromagnetic stirrer under different stirring currents and frequencies at the crystallizer and solidification end was measured using the alternating low-frequency mode of a Lakeshore 475DSP gaussmeter, as shown in Tables 2 and 3. Only a portion of the data is listed in the table below. Based on the skin effect of the crystallizer copper tube and the solidified billet shell on the electromagnetic field, the optimal stirring frequency can be obtained by combining formula (1).

[0044] (1)

[0045] In equation (1), T is the electromagnetic torque and σ is the conductivity of the conductor; f B is the stirring frequency; B is the measured magnetic flux density at the center of the stirrer. r 1 is the equivalent radius of the liquid core; L is the effective working length of the stirrer; the stirring frequency corresponding to the maximum value of the electromagnetic torque T is the optimal stirring frequency.

[0046] Table 2. Magnetic induction intensity at the center of the electromagnetic stirrer in the crystallizer under different stirring parameters / Gs

[0047] Table 3. Magnetic induction intensity at the center of the electromagnetic stirrer at the solidification end under different stirring parameters / Gs

[0048] To further explain and illustrate the technical solution of the present invention, the solidification heat transfer model for calculating the solid fraction at the center of the billet in the embodiments and comparative examples was verified by a nail-shooting test. The model calculated the solid fraction at the center of the billet when the drawing speed of C-Mn series high-carbon wear-resistant ball steel was 1.9 m / min, as shown below. Figure 2 As shown.

[0049] Example 1

[0050] This embodiment provides a method for preparing hot-rolled intermediate bars for C-Mn series high-carbon wear-resistant steel balls based on continuously cast small square billets. The main components of the C-Mn series high-carbon wear-resistant steel balls are shown in Table 1. The cross-section of the continuously cast small square billet is 160mm×160mm, the superheat of the tundish is 22℃, the casting speed is 1.9m / min, the secondary cooling water ratio is 0.68L / Kg, the electromagnetic stirring current of the crystallizer is 450A, the stirring frequency is 4Hz, the magnetic induction intensity at the center of the stirrer is 535Gs, the electromagnetic stirring position at the solidification end is 9.78m, the electromagnetic stirring current at the solidification end is 550A, the stirring frequency is 6Hz, the stirring mode is alternating 10s-7s-10s, the magnetic induction intensity at the center of the stirrer is 980Gs, and the corresponding solid fraction at the center of the billet is 0.15.

[0051] Before entering the heating furnace, a high-temperature resistant and anti-oxidation coating is applied to the surface of the billet. During the heating process, the preheating temperature is less than 720℃, the average temperature of the first heating stage is 800℃, the average temperature of the second heating stage is 1100℃, and the average temperature of the soaking stage is 1135℃, with a total heating time of 240 minutes. The furnace atmosphere is reducing. The rolling process adopts a three-roll continuous rolling + four-roll reduction sizing rolling technology using a short stress line mill. The initial rolling temperature is 1120℃, the single-pass reduction is 66mm, and the two-pass deformation is 45%.

[0052] After finishing rolling, a light water-passing process controlled by a water tank is adopted, with a flow rate of 220m³ / h for water tank #1. 3 / h, the A of JSB3 was obtained through thermal expansion testing. c1 and A cm The linear temperatures were 733.6℃ and 501.9℃ respectively, and the final cooling temperature on the upper cooling bed was controlled at 872℃. The cooling bed was well-ventilated and the steel was passed through quickly, avoiding A... c1 ~A cm Linear temperature. During the finishing process, the cooling bed moves rapidly to ensure that the shearing temperature of the hot-rolled bar at the cold shear is 395℃; slow cooling is achieved by pit cooling, with a pit cooling temperature of 320℃. After 60 hours of slow cooling, the cover is opened, and the temperature at the exit of the pit is 40℃.

[0053] The metallographic structure and central austenite grain diagram of the Ø50mm hot-rolled bar obtained in this embodiment are shown below. Figures 3-5 As shown, the central shrinkage cavity, porosity, and segregation are all grade 0.5; the grain size is grade 8, and the central carbon segregation index is 1.07. The surface decarburized layer is 0.18D%, and the central metallographic structure consists of sorbite and pearlite, with a central carbon grade less than 1, and no martensite or bainite. The average hardness in the hot-rolled state is 27.6 HRC. Wear-resistant balls made from this raw material through a quenching-skew rolling process have surface and central hardness greater than 62 HRC with a small difference.

[0054] Example 2

[0055] This embodiment provides a method for preparing hot-rolled intermediate bars for C-Mn series high-carbon wear-resistant steel balls based on continuously cast small square billets. The steel composition for C-Mn series high-carbon wear-resistant balls is shown in Table 1. The cross-section of the continuously cast small square billet is 160mm×160mm, the superheat of the tundish is 24℃, the casting speed is 1.85m / min, the secondary cooling water ratio is 0.66L / Kg, the electromagnetic stirring current of the crystallizer is 400A, the stirring frequency is 4Hz, the magnetic induction intensity at the center of the stirrer is 488Gs, the electromagnetic stirring position at the solidification end is 9.78m, the electromagnetic stirring current at the solidification end is 500A, the stirring frequency is 6Hz, the stirring mode is alternating 10s-7s-10s, the magnetic induction intensity at the center of the stirrer is 891Gs, and the corresponding solid fraction at the center of the billet is 0.17.

[0056] Before entering the heating furnace, a high-temperature resistant and anti-oxidation coating is applied to the surface of the billet. During the heating process, the preheating temperature is less than 700℃, the average temperature of the first heating stage is 780℃, the average temperature of the second heating stage is 1080℃, and the average temperature of the soaking stage is 1110℃. The total heating time is 2020 min. The furnace atmosphere is reducing. The rolling process adopts a three-roll continuous rolling + four-roll reduction sizing rolling technology using a short stress line mill. The initial rolling temperature is 1100℃, the single-pass reduction is 62mm, and the two-pass deformation is 40%.

[0057] After finishing rolling, a light water-passing process controlled by a water tank is adopted, with a flow rate of 190m³ / h in water tank #1. 3 / h, the A of JSB3 was obtained through thermal expansion testing. c1 and A cm The linear temperatures were 733.6℃ and 501.9℃ respectively, and the final cooling temperature on the upper cooling bed was controlled at 860℃. The cooling bed was well-ventilated and the steel was passed through quickly, avoiding A... c1 ~A cm Linear temperature. During the finishing process, the cooling bed moves rapidly to ensure that the shearing temperature of the hot-rolled bar at the cold shear is 365℃; slow cooling is achieved by pit cooling, with a pit cooling temperature of 340℃. After 50 hours of slow cooling, the cover is opened, and the temperature at the exit of the pit is 60℃.

[0058] The metallographic structure and central austenite grain diagram of the Ø40mm hot-rolled bar obtained in this embodiment are shown below. Figures 6-8 As shown, the central shrinkage cavity, porosity, and segregation are all grade 0.5, and the central carbon segregation index is 1.08; the surface decarburized layer is 0.21D%, the central microstructure is sorbite and pearlite, the central network carbon grade is 1, and there is no martensite or bainite. The grain size is grade 8; the average hardness in the hot-rolled state is 27.4 HRC. Wear-resistant balls made from this raw material through a quenching-skew rolling process have surface and central hardness greater than 60 HRC with a small difference.

[0059] The process parameters for the production of Comparative Examples 1 to 3 are shown in Table 4.

[0060] Table 4. Production process parameters of hot-rolled intermediate bars of C-Mn series high-carbon wear-resistant spherical steel for Comparative Examples 1-3

[0061] Samples were taken and analyzed after applying the production process parameters of the C-Mn series high-carbon wear-resistant spherical steel provided in Comparative Examples 1-3 above. Hot-rolled bar samples with a cross-section of Ø50mm were used for evaluation. The evaluation was conducted according to GB / T13298-2015 "Metallic Microstructure Examination Method", GB / T224-2019 "Determination of Decarburized Layer Depth in Steel", and ASTM E112-2013 "Standard Test Method for Determination of Average Grain Size". Center segregation was assessed using the standard YB / T4413-2014 "Metallographic Evaluation Method for Center Segregation in High-Carbon Steel Wire Rod". The analytical results are summarized in Table 5.

[0062] Table 5. Low-magnification, microstructure, and property analysis results of the hot-rolled bars obtained in Comparative Examples 1-3

[0063] Based on the content of Tables 4-5 above, combined with the appendix Figure 3 ~Appendix Figure 8It can be seen that the Ø40~50mm C-Mn series high carbon wear-resistant spherical steel hot-rolled bars provided in Examples 1~2 of this application have central porosity and central segregation of less than 0.5 grade, and central carbon content is less than 1.0 grade, indicating good performance of the hot-rolled bars. They meet the requirements of C-Mn series high carbon wear-resistant spherical steel. In Comparative Example 1, the continuous casting process has a high casting speed of 2.0m / min and a small current in the crystallizer and end electromagnetic stirring, which causes severe central segregation of the billet, which is inherited by the hot-rolled bars, resulting in a central carbon segregation index of 1.15 and a central segregation grade of 1.5. Although the post-rolling water quenching, cooling rate, and slow cooling process are normal, the core carbon content still reaches grade 2. In Comparative Example 2, the heating furnace heating system is unreasonable, with low heating section temperature and short heating time, resulting in slow diffusion of carbon and alloying elements, leading to poor homogenization and a smaller decarburized layer thickness. Although the post-rolling water quenching, cooling rate, and slow cooling process are normal, the core carbon content still reaches grade 1.5. Comparative Example 3 has an unreasonable rolling process, with a low initial rolling temperature and significantly small single-pass reduction and two-pass deformation, which means that the rolling force cannot be fully applied to the core, resulting in poor improvement of central porosity and grain size.

[0064] Therefore, this invention addresses the problems of homogenization and central densification in the production of Ø40mm~Ø50mm high-carbon wear-resistant spherical steel hot-rolled bars from 160mm×160mm cross-section small square billets. This includes optimizing and improving key continuous casting process parameters such as the rules for determining the electromagnetic stirring process parameters at the crystallizer and solidification end; determining the temperature regime for furnace preheating, heating, and homogenization; determining the high-temperature resistant oxide coating; implementing large-reduction rolling during the rolling process; and determining the post-rolling light water piercing and cooling bed cooling rates. Through the synergistic optimization of the three upstream and downstream processes of continuous casting, rolling, and post-rolling cooling, this invention enables the production of highly homogeneous, highly dense, and larger-section hot-rolled bars from small square billets using a smaller compression ratio. Simultaneously, it effectively improves the wear resistance, toughness, and fatigue life of the steel balls hot-rolled from these billets.

[0065] It should be understood that, in various embodiments of the present invention, the order of the above-mentioned process numbers does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0066] The above description is merely a preferred embodiment of the present invention and an explanation of the technical principles employed, and is not intended to limit the scope of the claimed invention, but merely to illustrate preferred embodiments of the invention. Those skilled in the art should understand that the scope of the invention is not limited to the specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the inventive concept. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing hot-rolled intermediate bars for C-Mn series high-carbon wear-resistant steel balls based on continuously cast small square billets, characterized in that, The continuously cast small square billet is a 160mm×160mm cross-section small square billet, and the hot-rolled medium bar cross-section has a center carbon segregation index of less than 1.07, and the center segregation, shrinkage cavity and porosity low magnification rating are all less than 0.

5. The method includes the following steps: Step S1: Control the superheat of the molten steel for C-Mn series high carbon wear-resistant steel balls before entering the crystallizer to be below the preset temperature; before casting, set the crystallizer stirring current and frequency to make the corresponding central magnetic induction intensity 488~585Gs, set the solidification end electromagnetic stirring current and frequency to make the corresponding central magnetic induction intensity 891~1070Gs, and adopt a continuous stirring mode; after casting, the casting speed is used to make the solid fraction of the billet center at the installation position of the solidification end electromagnetic stirrer 0.1~0.2; the secondary cooling zone adopts a weak cooling mode with a specific water volume of 0.6~0.7L / Kg to cool and solidify the billet with liquid core, and the solidification end electromagnetic stirrer stirs the pasty area containing the billet that is not completely solidified, to obtain a 160mm×160mm cross-section continuous casting small square billet; Step S2: After applying a high-temperature resistant and anti-oxidation coating to the surface of the continuously cast small square billet, it passes through the preheating section, heating section one, heating section two, and soaking section of the heating furnace in sequence, with the furnace set in a reducing atmosphere. Step S3: The uniformly heated continuous casting small square billet is subjected to rough rolling and finish rolling in sequence, and hot-rolled medium bar is obtained after rolling. Step S4: The hot-rolled bar is cooled by a water tank and a cooling bed. The water volume in the cooling tank after finishing rolling, the turning speed of the cooling bed, and the ventilation rate are set to ensure the temperature of the hot-rolled bar is greater than 850℃ before it enters the cooling bed, and that it passes quickly after entering the cooling bed, avoiding point A. c1 ~A cm Linear temperature; Step S5 involves shearing, finishing, and slow cooling of the cooled hot-rolled bar to obtain hot-rolled bars with a diameter of Ø40mm~Ø50mm for C-Mn series high-carbon wear-resistant steel balls.

2. The method according to claim 1, characterized in that, In step S1, based on the set central magnetic induction intensity, the stirring frequency is calculated according to the skin effect of the crystallizer copper tube and the solidified shell of the billet on the electromagnetic field.

3. The method according to claim 2, characterized in that, The formula for calculating the stirring frequency is as follows: (1) In equation (1), T is the electromagnetic torque, σ is the conductivity of the conductor, f is the stirring frequency, B is the measured magnetic induction intensity at the center of the stirrer, r1 is the equivalent radius of the liquid core, and L is the effective working length of the stirrer. The stirring frequency f corresponding to the maximum value of the electromagnetic torque T is the optimal stirring frequency.

4. The method according to claim 1, characterized in that, In step S1, the superheat of the molten steel before entering the crystallizer is controlled to be below 25°C.

5. The method according to claim 1, characterized in that, In step S2, the temperature of the preheating section in the heating furnace is set to 40~720℃, the temperature of the first heating section is set to 700~900℃, the temperature of the second heating section is set to 1060~1160℃, and the temperature of the soaking section is set to 1110~1160℃.

6. The method according to claim 1, characterized in that, In step S3, the initial rolling temperature is set to 1110~1130℃, the single-pass reduction is 62~68mm, and the two-pass deformation is ≥40%.

7. The method according to claim 1, characterized in that, The shearing temperature in step S5 is greater than 380℃.

8. The method according to claim 1, characterized in that, The slow cooling process involves immersion cooling in a pit, with an immersion cooling temperature ≥280℃. After slow cooling for more than 56 hours, the lid is opened, and the temperature upon exiting the pit is ≤60℃.

9. The method according to claim 1, characterized in that, The composition of the hot-rolled intermediate bar for C-Mn series high-carbon wear-resistant steel balls is as follows: C: 0.72~1.05%; Si: 0.22~0.32%; Mn: 0.7~1.10%; Cr: 0.2~0.8%; S: ≤0.01%; P: ≤0.02%; Ni and Cu: ≤0.03%, and are residual elements; Al t 0.01~0.03%, the remainder being Fe and other unavoidable impurities.

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