A medium wide band rolling mill backup roll and centrifugal composite casting method

Abstract

The application discloses a medium wide-band rolling mill supporting roller and a centrifugal composite casting method thereof, and belongs to the technical field of metallurgical rolling; and solves the quality problems of the existing technology, such as peeling caused by poor combination of the working layer of the roller body, insufficient strength of the roll neck and fracture, etc. The wide-band rolling mill supporting roller comprises an outer layer, an intermediate layer and a core portion from outside to inside; the outer layer is made of 45Cr4NiMoV steel, and the intermediate layer and the core portion are made of nodular cast iron. The centrifugal composite casting method of the medium wide-band rolling mill supporting roller comprises the following steps: step 1, smelting and slag washing of the outer layer molten steel; step 2, refining and pouring of the outer layer molten steel; step 3, smelting, inoculation and pouring of the intermediate layer molten iron; step 4, stopping and assembling; step 5, smelting, spheroidizing inoculation and pouring of the core portion molten iron; and step 6, quenching and tempering heat treatment. The medium wide-band rolling mill supporting roller has the advantages of low production cost, short production cycle, high accident resistance, etc.

Description

Technical Field

[0001] This invention relates to the field of support roll casting technology, and in particular to a support roll for a medium-width rolling mill and a centrifugal composite casting method. Background Technology

[0002] Currently, most medium-width and wide hot strip mills in China use conventionally cast semi-alloy steel rolls as support rolls, while some companies use small forged steel support rolls. Narrow strip mills also utilize centrifugally composite semi-alloy steel rolls. During online rolling of cast rolls, issues such as outer metallurgical quality and bonding layer problems often lead to roll body spalling, shoulder breakage, and cracking, resulting in direct roll scrapping, impacting rolling efficiency, and increasing roll consumption costs for the user. Forged steel rolls, due to their complex manufacturing processes, low material yield, and long production cycle, are expensive and difficult for most private enterprises in the medium-width and narrow-width strip mills to afford.

[0003] In existing technologies, some have adopted advanced computer simulation technology to optimize the design of the gating system by using a tangential ingate rotary filling method, iron mold sand covering process, sliding roller neck, and heat-insulating riser technology, and have successfully produced cast steel support rollers, making it possible to produce large support rollers domestically. However, due to the long assembly time of the fixtures and low production efficiency, they have not been able to form an industrialized product.

[0004] Although some have successfully produced support rolls for large hot continuous rolling mills using three-layer centrifugal composites, the working layer is made of high-chromium steel and centrifugally cast, making it denser and more wear-resistant. The middle layer and core are made of low-alloy steel, which has excellent strength and toughness. The overall service life of the rolls is 30% longer than that of statically manufactured cast steel support rolls, meeting the requirements of large hot continuous rolling mills. However, the initial equipment investment is large, and due to the characteristics of vertical centrifugal casting, the working layer thickness at both ends of the roll body is uneven.

[0005] Some manufacturers use a horizontal centrifugal double-layer composite casting method, where the working layer is centrifugally cast and the inner layer is statically cast to produce high-alloy centrifugal composite cast steel support rollers. This effectively improves the wear resistance of the outer layer. However, horizontal centrifuges have speed limitations, resulting in smaller roller sizes. Furthermore, the outer layer is made of a high-alloy steel core and ductile iron, which are two layers with significantly different physical properties. This can easily lead to problems such as scrap, poor bonding between the core and working layer due to excessive carbides, and large differences in the thickness of the working layer, making production difficult.

[0006] Some designs employ a three-layer centrifugal composite structure, with an outer layer made of low-chromium alloy and a middle and core layer made of ductile iron. This effectively improves the uniformity of the working layer thickness. However, because the outer layer uses high-quality low-phosphorus and low-sulfur scrap steel, the raw material cost is high. Furthermore, the composite structure between the outer and middle layers lacks precise control, making it prone to forming a mixed layer at the bonding layer, resulting in a large amount of carbides. This leads to poor bonding or low tensile strength at the bonding area, making it easy to peel off from the bonding area during rolling and use. The use of low-chromium alloy and the limitations imposed by centrifuge speed inevitably reduce the wear resistance of the working layer. Summary of the Invention

[0007] Based on the above analysis, the present invention aims to provide a medium-width strip mill support roll and a centrifugal composite casting method to solve the technical problems of poor wear resistance of the working layer of the roll body, long production cycle and high production cost of existing medium-width strip mill support rolls.

[0008] The objective of this invention is mainly achieved through the following technical solutions:

[0009] On one hand, the present invention provides a medium-width strip mill support roll, which has a three-layer composite structure, comprising an outer layer, an intermediate layer and a core from the outside to the inside; the outer layer is made of 45Cr4NiMoV steel, and the intermediate layer and the core are made of ductile iron.

[0010] In one possible design, the core's chemical composition is: C: 2.8-2.9%, Si: 2.7-2.8%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Re: 0.01-0.03%, Mg: 0.03-0.06%.

[0011] In one possible design, the chemical composition of the intermediate layer is: C: 2.8-2.9%, Si: 1.9-2.0%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Mg: 0.01-0.03%.

[0012] On the other hand, the present invention also provides a centrifugal composite casting method for medium-width strip mill support rolls, used to prepare the aforementioned medium-width strip mill support rolls. The centrifugal composite casting method includes the following steps:

[0013] Step 1: Smelting and slag washing of the outer layer of molten steel;

[0014] Step 2: Refining and casting of the outer layer of molten steel;

[0015] The outer layer of molten steel is heated in an induction furnace and taken out of the furnace when the temperature of the outer layer of molten steel reaches the range of 1660-1670℃; after taking out of the furnace, it is refined and argon-blown for 10-15 minutes; when the temperature of the outer layer of molten steel is 1560-1580℃, it is poured.

[0016] Step 3: Smelting, inoculation, and casting of the intermediate layer of molten iron;

[0017] Using an intermediate frequency furnace, pig iron and scrap steel are smelted to form an intermediate layer of molten iron, and the chemical composition of the intermediate layer of molten iron is adjusted. The intermediate layer of molten iron is held at 1400-1450℃, and after holding at this temperature, it is taken out of the furnace. Then, surface inoculation, slag removal, cooling and temperature measurement are carried out before casting.

[0018] Step 4: Shutdown and assembly;

[0019] When the temperature of the intermediate layer of molten steel in the mold cavity drops to 1040-1090℃, the horizontal centrifuge is decelerated, stopped, and the roll body mold is assembled in sequence; the assembly time of the roll body mold is controlled within 10 minutes.

[0020] Step 5: Core molten iron smelting, spheroidization inoculation and casting;

[0021] Pig iron and scrap steel are smelted in an intermediate frequency furnace to form molten iron in the core. The tapping temperature of the molten iron in the core is 1530-1545℃. Spheroidizing agent, inoculant and iron filings are put into the bottom of the core ladle in sequence for spheroidizing and inoculation treatment. After slag removal, it is poured. The core filling temperature during pouring is 1420-1430℃. After pouring, it is kept at the temperature for 5-7 days to obtain the support roll of the medium-width rolling mill.

[0022] Furthermore, in step 1, the outer layer of molten steel is 45Cr4NiMoV molten steel. The raw material for smelting the outer layer of molten steel is the waste material from the sprue of the support roll steel ingot. After the outer layer of molten steel is melted and cleared, the outer layer of molten steel is refined by slag washing. The slag washing tapping temperature is greater than or equal to 1650℃.

[0023] Furthermore, in step 1, during the slag washing and refining process, 0.2%-1% of slag washing agent is placed at the bottom of the slag washing bag.

[0024] Furthermore, in step 2, the centrifuge used during casting rotates at 1000-1200 rpm and has a centrifugal gravity coefficient of 100-120g.

[0025] Furthermore, in step 3, the timing of casting the intermediate layer is determined: first, the solidification time of the outer layer of molten steel is determined, and the intermediate layer is cast 3 minutes after the outer layer of molten steel has solidified or 17-25 minutes after the outer layer of molten steel has been cast. The furnace exit temperature of the intermediate layer of molten iron is 1600-1620℃, and the casting temperature is 1520±5℃.

[0026] Furthermore, in step 5, the interval between the core molten iron being tapped from the furnace and the pouring is controlled to be within 10 minutes.

[0027] Furthermore, it also includes step 6, which involves quenching and tempering the support rolls of the medium-width rolling mill;

[0028] The quenching and tempering heat treatment includes the following sub-steps:

[0029] Step 61: Perform overall heating and heat preservation on the support rolls of the medium-width strip mill;

[0030] Step 62: After the heat preservation is completed, the medium-width strip mill support rolls are taken out of the furnace and hoisted to the spray quenching bed;

[0031] Step 63: Perform zone quenching on a spray quenching bed, during which the wide strip mill support rolls reciprocate.

[0032] Step 64: The quenched medium-width strip mill support rolls are hoisted into the trolley-type electric furnace for loading, and then tempered.

[0033] Step 65: After tempering and exiting the furnace, perform hardness testing on the support rolls of the medium-width strip mill;

[0034] Step 66: If the hardness test result meets the technical requirements of the support roller, proceed to the next process for processing; if it does not meet the hardness test requirements, return to step 64 for supplementary tempering.

[0035] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0036] (1) This invention solves the quality problem of using small rolling mill rolls, improves the wear resistance of the working layer of the roll body, reduces costs, shortens the manufacturing cycle, improves the cost performance of the rolls, and effectively solves the problem of using medium-width and narrow-width rolling mill support rolls.

[0037] (2) This invention controls the timing and temperature of the intermediate layer pouring to ensure that the outer layer and intermediate layer are not mixed, forming a diffusion fusion without the formation of carbides, thus improving the bonding quality. After the core molten iron is poured, the intermediate layer acts as a buffer. After the core molten iron completely melts the intermediate layer, it forms a diffusion fusion with the outer layer, ensuring that the fusion is complete and the outer layer thickness is extremely uniform (the thickness of the outer layer at both ends is more uniform when using a horizontal casting centrifugal casting machine compared to a vertical casting centrifugal casting machine). The casting stress of the medium-width support roller after casting is low, saving one stress-relief annealing process, reducing costs and shortening the process cycle. The roller body has low hardness after casting, making it easy to process and improving processing efficiency. Only one tempering heat treatment is required, reducing costs while improving production efficiency.

[0038] (3) The outer matrix structure of the medium-width support roller of the present invention is tempered martensite and carbides, the roller body hardness is 63-69HSD, the roller body uniformity is within ±1.5HSD, and there is no obvious hardness reduction in the working layer of the roller body. The three-layer composite part of the roller body is subjected to ring cutting test. The transition zone structure is: tempered martensite + tempered bainite + granular carbides + pearlite. The thickness of the middle layer is 1.5-2.5mm, no strip carbides are formed, and the bonding is good.

[0039] (4) Take off the roll neck body Tensile testing revealed that the matrix structure of the roll neck body consisted of pearlite, ferrite, graphite, and carbides, with a pearlite content greater than 90% and a carbide content of 1-2%. The tensile strength was greater than 500 MPa, the elongation was 1-3%, and the roll neck hardness was 35-45 HS. The roll body exhibited good metallurgical quality, and flaw detection met the GB / T1503-2008 technical standard.

[0040] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained through the embodiments described and the accompanying drawings, which are particularly pointed out. Attached Figure Description

[0041] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0042] Figure 1 The transition zone structure of the medium-width support roller of the present invention Figure 1 ;

[0043] Figure 2 The transition zone structure of the medium-width support roller of the present invention Figure 2 ;

[0044] Figure 3 The transition zone structure of the medium-width support roller of the present invention Figure 3 ;

[0045] Figure 4 The transition zone structure of the medium-width support roller of the present invention Figure 4 ;

[0046] Figure 5 The transition zone structure of the medium-width support roller of the present invention Figure 5 ;

[0047] Figure 6 The outer layer structure of the transition zone of the medium-width support roller of the present invention Figure 1 ;

[0048] Figure 7 The outer layer structure of the transition zone of the medium-width support roller of the present invention Figure 2 ;

[0049] Figure 8 The outer layer structure of the transition zone of the medium-width support roller of the present invention Figure 3 ;

[0050] Figure 9The outer layer structure of the transition zone of the medium-width support roller of the present invention Figure 4 ;

[0051] Figure 10 The core structure of the medium-width support roller of the present invention Figure 1 ;

[0052] Figure 11 The core structure of the medium-width support roller of the present invention Figure 2 ;

[0053] Figure 12 The core structure of the medium-width support roller of the present invention Figure 3 ;

[0054] Figure 13 The core structure of the medium-width support roller of the present invention Figure 4 . Detailed Implementation

[0055] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0056] The present invention provides a medium-width strip mill support roll, which has a three-layer composite structure, comprising an outer layer, an intermediate layer and a core from the outside to the inside; the outer layer is made of 45Cr4NiMoV steel, and the intermediate layer and the core are made of ductile iron.

[0057] The core has the following chemical composition: C: 2.8-2.9%, Si: 2.7-2.8%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Re: 0.01-0.03%, Mg: 0.03-0.06%. The intermediate layer has the following chemical composition: C: 2.8-2.9%, Si: 1.9-2.0%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Mg: 0.01-0.03%.

[0058] This invention provides a centrifugal composite casting method for support rolls of medium-width rolling mills, the casting method comprising the following steps:

[0059] Step 1: Smelting and slag washing of the outer layer of 45Cr4NiMoV steel;

[0060] The outer layer steel smelting process includes: the outer layer uses the sprue waste of forged steel support roll ingots made of YB-65 or YB-70 material from China First Heavy Industries. The smelting process of this ingot is ladle refining and vacuum casting, which ensures the metallurgical quality of the sprue waste.

[0061] When using YB-65 forged steel support roll ingots as sprue waste, remelting is performed directly in an induction furnace. When using YB-70 forged steel support roll ingots as sprue waste, 20-30% scrap steel needs to be added to adjust the C and Cr content. After the original molten steel is completely melted, it undergoes slag washing refining, with the slag washing tapping temperature not lower than 1650℃.

[0062] It should be noted that during slag washing, the slag washing ladle needs to be baked at a temperature not lower than 800℃. 0.2%-1% of slag washing agent (a mixture of fluorite and lime) should be placed at the bottom of the ladle. After slag washing, the slag should be removed, and the molten steel should be reintroduced into the furnace. Si and Mn alloys should be added to adjust the alloy composition of the molten steel to meet the requirements of the outer layer composition in Table 1. The overheating temperature should be 1530℃-1550℃, and should not exceed 1580℃. The temperature should be maintained.

[0063] In step 1 above, the slag washing furnace exit temperature is required to be greater than 1650℃. The present invention controls the overheating temperature of the alloy composition to 1530℃-1550℃ in order to prevent excessive oxidation during the smelting process. The overheating temperature should not exceed 1580℃ in order to avoid silicon-manganese burn-off and reduce costs.

[0064] In step 1 above, waste steel and scrap steel of YB-65 and YB-70 forged steel rolls are used as raw materials. The outer layer of 45Cr4NiMoV steel is smelted through medium frequency, slag washing refining and plug rod bottom leak slag trapping process. This ensures metallurgical quality while greatly reducing the loss of alloying elements and reducing raw material costs.

[0065] Step 2: Refining and casting of the outer layer of molten steel;

[0066] After determining the tapping timing (based on the centrifuge status), the medium-frequency furnace is rapidly heated (to full load). Tapping occurs when the outer layer of molten steel reaches 1660-1670℃. A plug-rod bottom-sink ladle is used, requiring preheating to a temperature not lower than 800℃ to minimize temperature drop and allow sufficient time for argon refining. Argon blowing time should be at least 10 minutes, followed by final deoxidation of the ladle bottom. After tapping, argon refining is performed for 10-15 minutes. Pouring is then carried out when the molten steel temperature reaches 1560-1580℃. The horizontal centrifuge used during pouring operates at 1000-1200 rpm with a centrifugal gravity coefficient of 100-120g. Different roll specifications require different speeds to ensure outer layer density. Glass slag is added promptly after pouring the outer layer of molten steel, at a rate of 2-4 kg / m³. 2 The purpose of adding glass slag is to prevent the outer layer of molten steel from oxidizing during solidification.

[0067] Step 3: Smelting, inoculation, and casting of the intermediate layer of molten iron;

[0068] Pig iron and scrap steel (scrap steel accounting for 20-30% of the weight) are combined and pre-charged in the furnace. The furnace is preheated using gas in a medium-frequency induction furnace. At the designated smelting time, electricity is supplied to raise the temperature, initially using low power (20-30% of full power). Once molten iron forms, the temperature is increased to full load. After the molten iron is completely melted, the chemical composition is tested. Based on the weight of the intermediate layer of molten iron, the composition of the melted iron, and the target composition, the amount of alloy to be added is determined. Si, Mn, Ni, and other alloys are added, and the adjusted chemical composition meets the following requirements: The required composition is as follows: C: 2.8-2.9%, Si: 1.9-2.0%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Mg: 0.01-0.03%. After the molten iron is held at 1400-1450℃ for a certain period of time, the intermediate layer of molten iron is heated to 1600-1620℃, then removed from the furnace, surface inoculated, slag removed and cooled, temperature measured and poured, and the pouring timing and temperature are coordinated according to the production rhythm.

[0069] When determining the timing of pouring the intermediate layer, first determine the solidification time of the outer layer. Pour the intermediate layer 3 minutes after the outer layer has solidified, or 17-25 minutes after the outer layer has been poured. The composition of the intermediate layer should conform to Table 1. The furnace exit temperature of the intermediate layer should be 1600-1620℃, and the pouring temperature should be 1520±5℃. This ensures that the intermediate layer and the outer layer of molten steel diffuse and fuse together, without any mixing or the formation of a large amount of carbides. Glass slag should be added promptly after pouring the intermediate layer of molten iron, at a rate of 2-4 kg / m³. 2 This prevents the outer layer of molten iron from oxidizing during solidification.

[0070] In step 3 above, when determining the timing of pouring the intermediate layer, because the liquidus of the outer 45Cr4NiMoV molten steel is 1479℃ and the solidus is 1416℃, the solidification time of the outer layer exceeds the range of the water-cooled thermocouple. Pouring the intermediate layer too early will cause mixing of the bonding layer and the formation of a large amount of carbides, while pouring it too late will result in poor bonding. Therefore, this invention uses infrared thermography to calculate the solidification time of the outer layer for different materials and roll types, thereby determining the timing of pouring the intermediate layer.

[0071] Step 4: Shutdown and assembly;

[0072] When the temperature of the intermediate layer cavity drops to 1040-1090℃, the centrifuge is slowed down, stopped, and assembled. The assembly time of the roller body mold is controlled within 10 minutes.

[0073] In step 4 above, after the intermediate layer is poured, a water-cooled thermocouple is inserted into the mold cavity to measure the temperature. When the cavity temperature drops to 1040-1090℃, the centrifuge slows down and stops. During this process, the core molten iron tapping time needs to be determined in advance so that the core molten iron can be heated, tapped, and spheroidized.

[0074] Step 5: Core molten iron smelting, spheroidization inoculation and casting;

[0075] Pig iron and scrap steel (20-30% by weight) are pre-charged into the medium-frequency furnace and preheated with gas. At the smelting time, electricity is supplied to raise the temperature, initially using low power (20%-30% of full power). Once molten iron forms, the temperature is raised to full load. After the molten iron melts completely, the chemical composition is tested. Based on the weight of the core molten iron, the melt composition, and the target composition, the amount of alloy added is determined. Si, Mn, Ni, and other alloys are added, and the adjusted chemical composition meets the following specifications: C: 2.8-2.9%, Si: 2.7-2.8%, Mn: <0.50%, Ni: 0.5-0.6%, P <0.1%, S <0.02%, Re: 0.01-0.03%, Mg: 0.03-0.06%. After the core molten iron composition meets the requirements, it is held at 1400-1450℃.

[0076] When the centrifuge slows down, the molten iron in the core is required to be rapidly heated to 1530-1545℃ in the induction furnace before being taken out of the furnace. The spheroidizing agent, inoculant, and iron filings are placed in the bottom of the ductile iron ladle in the core for spheroidizing and inoculation treatment. After slag removal, the core is poured. The core filling temperature is 1420-1430℃. After pouring, the core is kept at the temperature for 5-7 days.

[0077] Specifically, the centrifuge assembly should be completed within 10 minutes. During assembly, the molten core should be tapped from the furnace according to process requirements at a temperature of 1530℃. ReMg spheroidizing agent and FeSi75 inoculant should be placed at the bottom of the ladle in sequence, and then covered with iron filings. The core filling temperature should be 1420-1430℃. The interval between the tapping of the molten core and the pouring should be controlled within 10 minutes to avoid spheroidization and inoculation fading.

[0078] It is important to emphasize that the ReMg spheroidizing agent, FeSi75 inoculant, and iron filings are placed sequentially at the bottom of the ductile iron ladle. The purpose of this is as follows: ReMg spheroidizing agent will burn violently upon contact with molten steel, causing the molten steel to boil, which is a relatively intense process. In contrast, the reaction between FeSi75 inoculant and molten steel is not very intense. To ensure that ReMg spheroidizing agent gradually contacts and reacts with molten steel, ReMg spheroidizing agent is placed at the very bottom of the ladle, while FeSi75 inoculant and iron filings are placed above ReMg spheroidizing agent.

[0079] The technical solution adopted in this invention is as follows: a three-layer composite casting is used, with the outer layer made of 45Cr4NiMoV (JBT4120-2017 Technical Conditions for Large Forged Alloy Steel Support Rollers), and the middle layer and core made of ductile iron. The specific composition of the material is shown in Table 1.

[0080] Table 1 Chemical Composition Control Table for Centrifugal Composite Cast Steel Support Rollers

[0081]

[0082] Compared with the prior art, the medium-width support roller provided by the present invention has good metallurgical quality. The flaw detection complies with the GB / T1503-2008 technical standard. The test results show that the thickness of the intermediate layer is 1.5-2.5mm, no strip-shaped carbides are formed, and the bonding is good.

[0083] Take off the roll neck body Tensile testing revealed that the matrix structure of the roll neck body consisted of pearlite, ferrite, graphite, and carbides, with a pearlite content greater than 90% and a carbide content of 1-2%. The tensile strength was greater than 500 MPa, the elongation was 1-3%, and the roll neck hardness was 35-45 HS. Medium-width support rolls manufactured using the above technical solution exhibited good metallurgical quality, and flaw detection met the GB / T1503-2008 technical standard.

[0084] The outer layer is made of 45Cr4NiMoV, with corresponding heat treatment process (see step 6 for tempering heat treatment). The matrix structure is tempered martensite and carbides. The roll body hardness is 63-69HSD, the roll body uniformity is within ±1.5HSD, and there is no obvious hardness reduction in the working layer of the roll body. The roll neck is micro-alloyed. The matrix structure is pearlite + ferrite + graphite + carbides. Its pearlite content is greater than 90%, carbide content is 1-2%, hardness is 35-45HS, tensile strength is greater than 500MPa, and elongation is 1-3%.

[0085] Compared with existing technologies, the outer layer uses forged steel roll ingot waste, which, through medium-frequency refining and bottom-leaking slag-trapping processes, ensures metallurgical quality while significantly reducing raw material costs. The three-layer composite centrifugal casting (outer layer, intermediate layer, and core) controls the pouring timing and temperature of the intermediate layer, enabling diffusion fusion between the outer and intermediate layers without any mixing layers, thus improving bonding quality. After the core molten iron is poured, the intermediate layer acts as a buffer, allowing the core molten iron to completely melt the intermediate layer before forming diffusion fusion with the outer layer. (From a microscopic or metallographic perspective, the transition zone refers to the bonding layer between the core and the outer layer (working layer); see the microstructure diagram of the transition zone for details.) Figures 1 to 5 See the tissue diagram of the outer layer of the transition zone. Figures 6 to 9 See the microstructure diagram of the core. Figures 10 to 13 This ensures excellent uniformity of the outer layer thickness during melting, preventing the formation of excessive carbides. The transition zone microstructure consists of tempered martensite + tempered bainite + granular carbides + pearlite, resulting in a high-quality bonding layer. Low casting stress after casting eliminates the need for a stress-relieving annealing process, reducing costs and shortening the production cycle. The low hardness of the cast roll body facilitates machining, improving processing efficiency. Only one tempering heat treatment is required, further reducing costs and shortening the production cycle.

[0086] With appropriate quenching and tempering heat treatment, the outer matrix structure consists of tempered martensite and carbides, with a roll body hardness of 63-69 HSD and a roll body uniformity within ±1.5 HSD. The working layer of the roll body shows no significant hardness reduction during testing. Ring-cutting tests are performed on the three-layer composite section of the roll body. The transition zone structure is: tempered martensite + tempered bainite + granular carbides + pearlite. The roll neck strength is 512-547 MPa at the sprue end and 540-541 MPa at the riser end. The intermediate layer has a bonding thickness of 1.5-2.5 mm, without the formation of strip-shaped carbides, indicating good bonding.

[0087] It should be emphasized that the centrifugal composite casting method for medium-width strip mill support rolls of the present invention further includes step 6, which involves quenching and tempering heat treatment of the medium-width strip mill support rolls; specifically, it includes the following sub-steps:

[0088] Step 61: Use a trolley-type gas furnace or electric furnace to heat and maintain the overall temperature of the support rolls of the medium-width rolling mill;

[0089] In step 61 above, the support roller is loaded into the trolley-type gas furnace. Before loading, the surface quality of the support roller is checked to avoid sharp corners, cracks and other defects that affect the heat treatment quality. During loading, in addition to leveling and compacting the roller body, the roller neck also needs to be leveled and compacted. After the height of the shims is ≥500mm, the entire roller is heated and kept warm.

[0090] In step 61 above, taking a bogie-type gas furnace as an example, the overall heating and heat preservation process is divided into the following five sub-steps:

[0091] Step 611: After loading the medium-width strip mill support rolls into the trolley-type gas furnace, keep them at 250-300℃ for 4-6 hours;

[0092] Step 612: After the heat preservation is completed, increase the temperature to 630-670℃ at a rate of ≤30℃ / h;

[0093] Step 613: After heating to 630-670℃, keep the temperature at 630-670℃ for 20-25 hours to make the temperature of the inner and outer layers of the roller uniform.

[0094] Step 614: After the heat preservation is completed, increase the temperature to 910-960℃ at a rate of ≤30℃ / h;

[0095] Step 615: Heat to 910-960℃ and keep warm for 10-15 hours.

[0096] It is important to emphasize that by controlling the specific conditions and methods of overall heating and heat preservation, the temperature of each part of the support roller is made more uniform, especially the core and the outside of the support roller, thereby reducing thermal stress, allowing the core to reach the set temperature more quickly, and reducing gas consumption and time.

[0097] It should be noted that there is a row of burners on each side of the trolley-type gas furnace, which is heated by burning natural gas. During heating, a flame of a certain length and height will be formed. In order to prevent the flame from burning the support roller, the support roller needs to be raised to a height of ≥500mm. In addition, raising the support roller is also conducive to the circulation of gas in the furnace and the furnace temperature is more uniform.

[0098] Step 62: After the heat preservation is completed, the support rolls of the medium-width strip mill are taken out of the furnace and hoisted to the spray quenching bed within a certain time.

[0099] In step 62 above, the total time t for unloading, air cooling, and hoisting must be ≤10 min. Specifically, the air cooling time during the hoisting of the support roller from the trolley-type gas furnace to the spray quenching bed must be ≤10 min. Before hoisting, the spray quenching bed pre-adjusts the valves of the nozzles at the corresponding roller body and neck positions according to the overall dimensions of the support roller. The compressed air and water valves for the nozzles at the roller body position are open, while the compressed air valve for the nozzles at the roller neck position is open, and the water valve is closed. After hoisting the support roller to the marked position on the spray quenching bed, the roller body and neck are quenched using different cooling media. The nozzles at the roller body and neck positions are arranged as follows: spray pipes are arranged on both sides of the spray quenching bed, with multiple nozzles on each pipe, spaced at intervals. Each nozzle controls the air-water ratio by controlling its corresponding valve switch. The total time for unloading, air cooling, and hoisting is controlled within 10 min to reduce the temperature drop and heat loss of the support roller.

[0100] Step 63: Perform zone quenching on the spray quenching bed, with the support rollers reciprocating during the zone quenching process;

[0101] Step 64: Hoist the quenched support rollers into the bogie-type electric furnace for loading, and then perform tempering.

[0102] Step 65: After tempering and exiting the furnace, test the hardness of the support rollers;

[0103] Step 66: If the hardness test result meets the technical requirements of the support roller, proceed to the next process for further processing; if it does not meet the hardness test requirements, return to step 4 for supplementary tempering. The supplementary tempering temperature is determined based on the actual hardness test result and technical requirements.

[0104] Compared with the prior art, the present invention addresses the different materials of the roll body and roll neck of the support roll of the medium-width rolling mill by using different cooling media to perform tempering heat treatment in different areas. This reduces the risk of scrapping the support roll after tempering heat treatment and ensures that the hardness of the cast steel layer of the support roll body reaches 60-70HSD, the hardness uniformity is ≤4HSD, and the hardness of the ductile iron of the roll neck reaches 35-45HSD.

[0105] In step 63 above, spray cooling is used for the area corresponding to the roll body, and forced air cooling is used for the area corresponding to the roll neck. The entire spray quenching process is divided into three stages. After a certain period of time, the air pressure and water pressure are adjusted according to the set parameters, specifically including:

[0106] Step 631: The spray quenching time for the first stage is 23-27 minutes. When spraying the roller body, the water pressure is 0.18-0.22 MPa and the air pressure is 0.18-0.22 MPa; when spraying the roller neck, the water pressure is 0 MPa and the air pressure is 0.18-0.22 MPa.

[0107] Step 632, the second stage of spray quenching time is 43-47 min. When spraying the roller body, the water pressure is 0.08-0.12 MPa and the air pressure is 0.08-0.12 MPa. When spraying the roller neck, the water pressure is 0 MPa and the air pressure is 0.08-0.12 MPa.

[0108] Step 633, Stage 3: Air-blast quenching. When air-blasting is performed on the roll body, the water pressure is 0 MPa and the air pressure is 0.08-0.12 MPa; when air-blasting is performed on the roll neck, the water pressure is 0 MPa and the air pressure is 0.08-0.12 MPa. At this time, infrared temperature measurement needs to be performed on the roll body and both roll necks respectively. When the infrared temperature measurement value of the roll neck is ≤500℃, stop the blowing, and the quenching is completed. If temperature measurement conditions are not available (meaning that there is no temperature measurement equipment on site, or temperature measurement cannot be performed due to other reasons), based on production experience, setting the time for Stage 3 to 30-40 minutes can meet the requirements to ensure that the core of the ductile iron in the roll neck completes the microstructure transformation.

[0109] When performing quenching and tempering heat treatment on medium-width strip mill support rolls, different cooling methods are used for different areas of the roll body and roll neck materials. The cooling medium for roll body quenching is initially a water mist made of a mixture of tap water and compressed air, which is later changed to compressed air. The cooling medium for roll neck quenching is always compressed air. This regional cooling method can ensure the production of high-quality centrifugal composite cast steel support roll products, thereby solving the problems of roll neck cracking or roll body hardness not meeting the requirements after existing centrifugal composite cast steel support rolls undergoing quenching and tempering heat treatment.

[0110] It should be noted that the entire quenching process of the roller body in this invention is divided into three stages: the first and second stages involve spraying the roller body with air, and the third stage involves spraying air into the roller body. The purpose of spraying air into the third stage is different from that of spraying the roller body with air in the first and second stages: the purpose of spraying the roller body with air in the first and second stages is to cool the roller body quickly, complete the microstructure transformation, and make its hardness meet the requirements; the roller body temperature in the third stage has already dropped to a lower temperature, at which point the water pressure is adjusted (the water is turned off) to become air spray, which can reduce quenching stress and avoid cracking.

[0111] This invention employs different cooling media for the roll body and roll neck. The outer layer of the roll body is made of ZG45Cr4NiMoV material, while the inner layer and roll neck are made of ductile iron, with an intermediate layer between the inner and outer layers. Compared to conventional centrifugal composite cast steel support rolls that use integral oil quenching, spray quenching, or forced-air quenching, this invention uses forced-air quenching at the roll neck, which effectively reduces the quenching stress generated during the quenching process and lowers the risk of cracking and scrapping due to the poor plasticity of the ductile iron in the roll neck. The spray quenching method used for the cast steel layer of the roll body effectively improves the hardness and hardened layer depth of the roll body material after quenching, avoiding product defects caused by insufficient roll body hardness or excessive hardness reduction, thus improving the production quality of centrifugal composite cast steel support rolls.

[0112] In step 63 above, the rotation of the support roller is a reciprocating rotation, that is, it first rotates forward for 120s-180s, then rotates in the opposite direction for 120s-180s, and then rotates forward again, and so on. The entire reciprocating rotation process starts from the beginning of the spray blowing and stops when the spray blowing ends, that is, when the quenching ends.

[0113] Compared with the prior art, the purpose of the present invention to make the support roller reciprocate is to reduce the axial movement of the support roller on the spray quenching bed, avoid water from getting into the transition area of ​​the roller neck and roller body, and at the same time make the quenching process of the roller body more uniform.

[0114] In step 64 above, when loading the support roller into the furnace for tempering, it is required that it be leveled and firmly supported during loading. A thermocouple should be placed in the middle of the roller body. During tempering, the tempering holding time is based on the thermocouple temperature (the starting point for calculating the tempering holding time is when the thermocouple temperature reaches the specified tempering temperature). The entire tempering process is performed in four stages: holding, heating, holding, and cooling. The specific process is as follows:

[0115] Step 641: After tempering and loading into the furnace, maintain the temperature at 250-300℃ for 4-6 hours;

[0116] Step 642: After the heat preservation is completed, increase the temperature at a rate of ≤10℃ / h;

[0117] Step 643: After the furnace temperature is raised to 500-550℃, start temperature equalization. When both the furnace temperature and the thermocouple temperature are within 500-550℃, keep the temperature for 40-45 hours.

[0118] It should be noted that in step 643, if the tempering furnace is a non-circulating furnace, it needs to be held at a thermocouple temperature of 500-550℃ for 23-27 hours, and then the support roller is rotated 180° after exiting the furnace. The holding time is then calculated cumulatively. Specifically, after rotating the support roller 180° after exiting the furnace, it is put back into the furnace. Once the furnace temperature reaches 500-550℃, the holding time is calculated cumulatively. When rotating the support roller after exiting the furnace, the furnace door needs to be opened, the support roller lifted out and rotated 180° on the ground, and then lifted back into the heat treatment furnace. This process takes time, and the furnace temperature will drop. Therefore, the holding time should be calculated only after the furnace temperature has risen back to its original level.

[0119] Step 644: After the heat preservation is completed, cool down at a rate of ≤10℃ / h. Remove from the furnace when the furnace temperature is ≤150℃. The tempering process is then complete.

[0120] Step 65: After tempering, perform hardness testing on the support rollers.

[0121] In step 65 above, the hardness testing process is as follows: the hardness of the roll body is measured on four generatrices that are 90° apart, with five measurements taken on each generatrices; the hardness of the roll neck is measured on two generatrices that are 180° apart, with four measurements taken on each generatrices, and five measurements taken at each location. The average value is taken as the hardness at that location.

[0122] Step 66: If the hardness test result meets the technical requirements of the support roller, proceed to the next process for further processing; if not, perform supplementary tempering. The supplementary tempering temperature is determined based on the hardness test result and technical requirements.

[0123] Example 1

[0124] This embodiment uses waste YB-65 forged steel support roll ingots as raw material, cut into square pieces and fed into the furnace. Medium-frequency power is used for heating, initially at low power for 30 minutes, then at full power until the molten steel is completely melted. The steel is then removed from the furnace at 1665℃ for slag washing. The slag washing ladle is baked at 810℃, and 0.2% slag washing agent is added to the bottom of the ladle. The slag is skimmed off, and the molten steel is returned to the furnace. The alloy composition (by weight percentage) is adjusted by adding 3 kg of electrode powder, 23 kg of ferrosilicon, and manganese. 31 kg of iron and 9 kg of nickel plate were used. The target composition was: C: 0.4-0.5%, Si: 0.4-0.7%, Mn: 0.6-0.8%, Cr: 3.5-4.5%, Ni: 0.4-0.8%, Mo: 0.4-0.7%, V: 0.05-0.15%, P<0.015%, S<0.015%. The mixture was superheated to 1530-1550℃ and held at that temperature before being heated to 1670℃ and then unloaded. The outer bottom ladle was hot-rolled, and the molten steel was refined by blowing argon for 12 minutes to cool it to 1563℃ before pouring the outer layer of molten steel. The centrifuge speed was 1200 rpm, and the gravity coefficient was 110 g.

[0125] After the outer layer of molten steel is poured, immediately add 23 kg of AS72 type protective slag. After the outer layer solidifies for 22 minutes, pour the intermediate layer (the chemical composition of the intermediate layer is: C: 2.8-2.9%, Si: 1.9-2.0%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Mg: 0.01-0.03%). The tapping temperature of the intermediate layer of molten steel is 1600℃, and the pouring temperature is 1520℃. A hot ladle is used. After the intermediate layer of molten iron is poured, add 3 kg of YY1 type protective slag. A water-cooled thermocouple is inserted into the mold cavity to measure the temperature.

[0126] When the cavity temperature drops to 1053℃, the centrifuge is slowed down and stopped. Molten iron is tapped from the core at 1532℃ (the chemical composition of the core molten iron is: C: 2.8-2.9%, Si: 2.7-2.8%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Re: 0.01-0.03%, Mg: 0.03-0.06%). 1.65% rare earth spheroidizing agent and 0.7% ferrosilicon inoculant are placed at the bottom of the ladle, covered with iron filings. Slag is removed from the slag pit and the temperature is measured. The ladle is then hoisted to the pouring position, where the temperature is measured at 1455℃. Pouring is then performed, with the pouring speed controlled so that the molten iron level is always 2 / 3 of the way up the pouring cup. The ladle is held at this temperature for 7 days before being unpacked.

[0127] The dimensions of the roller blank are: After tempering heat treatment, the outer matrix structure of the medium-width support roller in this embodiment consists of tempered martensite and carbides. The roller neck hardness is 37-44 HS, the roller body hardness is 64-68 HS, and the roller body uniformity is within ±1.5 HSD. A ring-cutting test was performed on the roller body bonding layer; the bonding layer thickness is 1.5-1.8 mm, no strip-shaped carbides are formed, and the bonding is good. The transition zone structure consists of tempered martensite + tempered bainite + granular carbides + pearlite. The roller neck strength is 512-547 MPa at the sprue end and 540-541 MPa at the riser end.

[0128] Take off the roll neck body The tensile test bar test showed that the matrix structure of the roll neck body is pearlite + ferrite + graphite + carbide, with pearlite content greater than 90% and carbide content of 1%. The tensile strength is 520MPa and the elongation is 3%. The metallurgical quality of the roll body is good, and the flaw detection meets the technical standard GB / T1503-2008.

[0129] Example 2

[0130] Using YB-70 forged steel support roll ingot waste as raw material, it is cut into square pieces and fed into the furnace. 25% channel steel scrap is added. Medium-frequency power is supplied for heating. Initially, heating is carried out at low power for 30 minutes, then at full power until the molten steel is clear. The molten steel is removed from the furnace at 1680℃ and slag is washed. The slag washing ladle is baked at 700℃. 0.2% slag washing agent is placed at the bottom of the ladle. The slag is removed, and the molten steel is returned to the furnace. 32 kg of ferrosilicon, 34 kg of ferromanganese, and 10 kg of nickel plate are added. Adjustments are made. Alloy composition (weight percentage): Target composition: C: 0.4-0.5%, Si: 0.4-0.7%, Mn: 0.6-0.8%, Cr: 3.5-4.5%, Ni: 0.4-0.8%, Mo: 0.4-0.7%, V: 0.05-0.15%, P<0.015%, S<0.015%. The alloy is superheated to 1530-1550℃ and held, then heated to 1661℃ before being unloaded. The outer bottom ladle is a hot ladle; the molten steel is refined by argon blowing for 10 minutes and cooled to 1556℃ before pouring the outer layer of molten steel. The centrifuge speed is 1000 rpm, and the gravity coefficient is 105g.

[0131] After the outer layer of molten steel is poured, immediately add 17 kg of AS72 type protective slag. After the outer layer solidifies for 24 minutes, pour the intermediate layer. The intermediate layer of molten steel (chemical composition of the intermediate layer of molten steel: C: 2.8-2.9%, Si: 1.9-2.0%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Mg: 0.01-0.03%) tapping temperature is 1620℃, pouring temperature is 1525℃, and a hot ladle is used. After the intermediate layer of molten iron is poured, add 3 kg of YY1 type protective slag. A water-cooled thermocouple is inserted into the mold cavity to measure the temperature. When the cavity temperature drops to 1044℃, the centrifuge is slowed down and stopped.

[0132] The roll body mold is assembled in the container. Molten iron is tapped from the core at 1545℃ (the chemical composition of the molten iron in the core is: C: 2.8-2.9%, Si: 2.7-2.8%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Re: 0.01-0.03%, Mg: 0.03-0.06%). 1.65% rare earth spheroidizing agent and 0.7% ferrosilicon inoculant are placed at the bottom of the ladle, covered with iron filings. Slag is removed from the slag pit and the temperature is measured. The ladle is then hoisted to the pouring position, where the temperature is measured at 1449℃. Pouring is then performed, with the pouring speed controlled so that the molten iron level is always 2 / 3 of the way up the pouring cup. After holding at this temperature for 7 days, the container is unpacked.

[0133] The dimensions of the roller blank are: After heat treatment, the outer matrix of the medium-width support roller in this embodiment consists of tempered martensite and carbides. The roller neck hardness is 37-40 HS, and the roller body hardness is 66-70 HS. A ring-cutting test was performed on the roller body bonding layer; the bonding layer thickness was 1.5-2.2 mm, and no strip-shaped carbides were formed, indicating good bonding. The transition zone microstructure consists of tempered martensite + tempered bainite + granular carbides + pearlite. The roller neck strength at the sprue end is 533-563 MPa with an elongation of 2.0-3.0%, and the riser end strength is 578-588 MPa with an elongation of 1.5-3.0%. The pearlite content at the roller neck is >90%, and the carbide content is approximately 1%.

[0134] Take off the roll neck body The tensile test bar test showed that the matrix structure of the roll neck body is pearlite + ferrite + graphite + carbide, with a pearlite content of 94% and a carbide content of 2%. The tensile strength is 520MPa and the elongation is 2%. The metallurgical quality of the roll body is good, and the flaw detection meets the technical standard GB / T1503-2008.

[0135] Example 3

[0136] Using YB-70 forged steel support roll ingot waste as raw material, 25% channel steel scrap was added, cut into square pieces and fed into the furnace. Medium-frequency power was used for heating, initially at low power for 30 minutes, then at full power until the molten steel was completely melted. The steel was then removed from the furnace at 1676℃ for slag washing. The slag washing ladle was baked at 700℃, and 0.2% slag washing agent was added to the bottom of the ladle. The slag was skimmed off, and the molten steel was returned to the furnace. 16 kg of ferrosilicon, 21 kg of ferromanganese, and 4 kg of nickel plate were added, and the mixture was adjusted. Gold composition (weight percentage), target composition: C: 0.4-0.5%, Si: 0.4-0.7%, Mn: 0.6-0.8%, Cr: 3.5-4.5%, Ni: 0.4-0.8%, Mo: 0.4-0.7%, V: 0.05-0.15%, P<0.015%, S<0.015%. Overheat to 1530-1550℃ and hold, then heat to 1661℃ and remove from the furnace.

[0137] The outer bottom leak-proof liner is a hot-rolled liner covered with an insulating agent. The molten steel is refined by blowing argon for 10 minutes to cool it to 1556℃, and then the outer layer of molten steel is poured in. The centrifuge speed is 1150 rpm and the gravity coefficient is 120g. After the outer layer of molten steel is poured, immediately add 15 kg of AS72 protective slag to prevent surface oxidation. After the outer layer solidifies for 17 minutes, pour the intermediate layer (the chemical composition of the intermediate layer is: C: 2.8-2.9%, Si: 1.9-2.0%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Mg: 0.01-0.03%). The tapping temperature of the intermediate layer of molten steel is 1610℃, and the pouring temperature is 1525℃. A hot ladle is used. After the intermediate layer of molten iron is poured, add 3 kg of O slag and 1 kg of borax as protective slag to prevent oxidation. A water-cooled thermocouple is inserted into the mold cavity to measure the temperature. When the cavity temperature drops to 1044℃, the centrifuge is slowed down and stopped.

[0138] The roll body mold is assembled in the box. Molten iron is tapped from the core at 1540℃ (the chemical composition of the molten iron in the core is: C: 2.8-2.9%, Si: 2.7-2.8%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Re: 0.01-0.03%, Mg: 0.03-0.06%). 1.65% rare earth spheroidizing agent and 0.7% ferrosilicon inoculant are placed at the bottom of the ladle, covered with iron filings. Slag is removed and the temperature is measured in the slag removal pit. The ladle is then hoisted to the pouring position, where the temperature is measured at 1530℃. Pouring is then performed, with the pouring speed controlled so that the molten iron level is always 2 / 3 of the way up the pouring cup. After holding at this temperature for 6 days, the ladle is unpacked.

[0139] The dimensions of the roller blank are: After heat treatment, the roll neck hardness is 36-39HS, the roll body hardness is 63-66HS, the outer layer thickness difference is <8mm, and the bonding is good. The roll neck strength at the sprue end is 538-612MPa with an elongation of 1.0-1.5%, and the riser end strength is 554-585MPa with an elongation of 2.0-1.5%. The roll neck pearlite content is >90%, and the carbide content is 1-2%.

[0140] The outer matrix structure of the medium-width support roll cast in this embodiment is tempered martensite and carbides. The roll body uniformity is within ±1.5HSD, and there is no significant decrease in hardness in the working layer of the roll body. The three-layer composite part of the roll body is subjected to ring cutting test. The transition zone structure is: tempered martensite + tempered bainite + granular carbides + pearlite. The thickness of the middle layer is 1.5-2.5mm. No strip carbides are formed, and the bonding is good.

[0141] According to the above embodiments, the present invention can produce centrifugal composite cast steel support rolls with a roll body diameter of 550-1050mm and a total roll length of 2000-3900mm.

[0142] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A centrifugal composite casting method for support rolls of medium-width rolling mills, characterized in that, The centrifugal composite casting method includes the following steps: Step 1: Smelting and washing of the outer layer of molten steel; The outer layer of molten steel is 45Cr4NiMoV molten steel. The raw material for smelting the outer layer of molten steel is the waste material from the sprue of the support roll steel ingot. After the outer layer of molten steel is melted and cleared, the outer layer of molten steel is refined by slag washing. The slag washing tapping temperature is greater than or equal to 1650℃. When using YB-65 forged steel support roll ingots as sprue waste, remelting is performed directly in an intermediate frequency furnace; when using YB-70 forged steel support roll ingots as sprue waste, 20-30% scrap steel is added to adjust the C and Cr content. Step 2: Refining and casting of the outer layer of molten steel; The outer layer of molten steel is heated in an induction furnace and taken out of the furnace when the temperature of the outer layer of molten steel reaches the range of 1660-1670℃; after taking out of the furnace, it is refined and argon-blown for 10-15 minutes; when the temperature of the outer layer of molten steel is 1560-1580℃, it is poured. Step 3: Smelting, inoculation, and casting of the intermediate layer of molten iron; Using an intermediate frequency furnace, pig iron and scrap steel are smelted to form an intermediate layer of molten iron, and the chemical composition of the intermediate layer of molten iron is adjusted. The intermediate layer of molten iron is held at 1400-1450℃, and after holding at this temperature, it is taken out of the furnace. Then, surface inoculation, slag removal, cooling and temperature measurement are carried out before casting. Determine the timing of pouring the intermediate layer: First determine the solidification time of the outer layer of molten steel. Pour the intermediate layer 3 minutes after the outer layer of molten steel has solidified or 17-25 minutes after the outer layer of molten steel has been poured. The tapping temperature of the intermediate layer of molten iron is 1600-1620℃, and the pouring temperature is 1520±5℃. Step 4: Shutdown and assembly; When the temperature of the intermediate layer of molten steel in the mold cavity drops to 1040-1090℃, the horizontal centrifuge is decelerated, stopped, and the roll body mold is assembled in sequence; the assembly time of the roll body mold is controlled within 10 minutes. Step 5: Core molten iron smelting, spheroidization inoculation and casting; Pig iron and scrap steel are smelted in an intermediate frequency furnace to form molten iron in the core. The tapping temperature of the molten iron in the core is 1530-1545℃. Spheroidizing agent, inoculant and iron filings are put into the bottom of the core ladle in sequence for spheroidizing and inoculation treatment. After slag removal, it is poured. The core filling temperature during pouring is 1420-1430℃. After pouring, it is kept at the temperature for 5-7 days to obtain the support roll of the medium-width rolling mill. It also includes step 6, which involves quenching and tempering the support rolls of the medium-width rolling mill; this includes the following sub-steps: Step 61: Use a trolley-type gas furnace or electric furnace to heat and maintain the overall temperature of the support rolls of the medium-width rolling mill; When using a bogie-type gas furnace, the overall heating and heat preservation processes are divided into the following five sub-steps: Step 611: After loading the medium-width strip mill support rolls into the trolley-type gas furnace, keep them at 250-300℃ for 4-6 hours; Step 612: After the heat preservation is completed, increase the temperature to 630-670℃ at a rate of ≤30℃ / h; Step 613: After heating to 630-670℃, keep the temperature at 630-670℃ for 20-25 hours to make the temperature of the inner and outer layers of the roller uniform. Step 614: After the heat preservation is completed, increase the temperature to 910-960℃ at a rate of ≤30℃ / h; Step 615: After heating to 910-960℃, maintain the temperature for 10-15 hours; Step 62: After the heat preservation is completed, the medium-width strip mill support roll is taken out of the furnace and hoisted to the spray quenching bed within a certain time. The total time for taking out of the furnace, air cooling and hoisting is ≤10min. That is, the air cooling time during the process of hoisting the support roll from the trolley-type gas furnace to the spray quenching bed should be ≤10min. Before hoisting, the valve switches of the nozzles at the corresponding positions of the roll body and roll neck of the spray quenching bed have been pre-adjusted according to the overall size of the support roll. The valves of the compressed air pipeline and the tap water pipeline of the nozzles corresponding to the roll body position are in the open state, while the valve of the compressed air pipeline of the nozzles corresponding to the roll neck position is in the open state and the valve of the tap water pipeline is in the closed state. After the support roll is hoisted to the marked position on the spray quenching bed, the roll body and the roll neck are quenched using different cooling media. The nozzles at the roll body and roll neck positions are set up as follows: spray pipes are arranged on both sides of the spray quenching bed, and multiple nozzles are provided on the spray pipes. There is a gap between the nozzles. Each nozzle controls the air and water ratio by controlling the corresponding valve switch. Step 63: Perform zone quenching on the spray quenching bed, with the support rollers reciprocating during the zone quenching process; In step 63, the rotation of the support roller is a reciprocating rotation, that is, first rotating forward for 120s-180s, then rotating in the opposite direction for 120s-180s, then rotating forward again, and so on. The entire reciprocating rotation process starts from the beginning of the spray blowing and stops when the spray blowing ends, that is, when the quenching ends. In step 63, spray cooling is applied to the area corresponding to the roll body, and forced air cooling is applied to the area corresponding to the roll neck. The entire spray quenching process is divided into three stages, including: Step 631: The spray quenching time for the first stage is 23-27 minutes. When spraying the roller body, the water pressure is 0.18-0.22 MPa and the air pressure is 0.18-0.22 MPa; when spraying the roller neck, the water pressure is 0 MPa and the air pressure is 0.18-0.22 MPa. Step 632, the second stage of spray quenching time is 43-47 min. When spraying the roller body, the water pressure is 0.08-0.12 MPa and the air pressure is 0.08-0.12 MPa. When spraying the roller neck, the water pressure is 0 MPa and the air pressure is 0.08-0.12 MPa. Step 633, Stage 3: Air-blast quenching. When air-blasting is performed on the roll body, the water pressure is 0 MPa and the air pressure is 0.08-0.12 MPa; when air-blasting is performed on the roll neck, the water pressure is 0 MPa and the air pressure is 0.08-0.12 MPa. At this time, infrared temperature measurement is performed on the roll body and the two roll necks respectively. When the infrared temperature measurement value of the roll neck is ≤500℃, the air blowing is stopped and the quenching ends. If the temperature measurement conditions are not available, the time of Stage 3 is set to 30-40 minutes to ensure that the core of the ductile iron in the roll neck completes the microstructure transformation. Step 64: Hoist the quenched support rollers into the bogie-type electric furnace for loading, and then perform tempering. In step 64, the entire tempering process is performed in four stages: holding, heating, holding again, and cooling. The process is as follows: Step 641: After tempering and loading into the furnace, maintain the temperature at 250-300℃ for 4-6 hours; Step 642: After the heat preservation is completed, increase the temperature at a rate of ≤10℃ / h; Step 643: After the furnace temperature is raised to 500-550℃, start temperature equalization. When both the furnace temperature and the thermocouple temperature are within 500-550℃, keep the temperature for 40-45 hours. Step 644: After the heat preservation is completed, cool down at a rate of ≤10℃ / h, and remove from the furnace when the furnace temperature is ≤150℃. The tempering process is then complete. Step 65: After tempering and exiting the furnace, test the hardness of the support rollers; Step 66: If the hardness test result meets the technical requirements of the support roller, proceed to the next process for processing; if it does not meet the hardness test requirements, return to step 64 for supplementary tempering.

2. The centrifugal composite casting method for medium-width strip mill support rolls according to claim 1, characterized in that, In step 1, during the slag washing and refining process, 0.2%-1% of slag washing agent is placed at the bottom of the slag washing bag.

3. The centrifugal composite casting method for medium-width strip mill support rolls according to claim 1, characterized in that, In step 2, the centrifuge used during casting rotates at 1000-1200 rpm / min and has a centrifugal gravity coefficient of 100-120g.

4. The centrifugal composite casting method for medium-width strip mill support rolls according to claim 3, characterized in that, In step 5, the interval between the core molten iron being tapped from the furnace and the pouring is controlled to be within 10 minutes.

5. A support roll for a medium-width rolling mill, characterized in that, The medium-width rolling mill support roll is prepared by the centrifugal composite casting method according to any one of claims 1 to 4; the medium-width rolling mill support roll comprises, from the outside to the inside, an outer layer, an intermediate layer and a core; The outer layer is made of 45Cr4NiMoV steel, and the middle layer and core are made of ductile iron.

6. The medium-width strip mill support roll according to claim 5, characterized in that, The chemical composition of the core is as follows: C: 2.8-2.9%, Si: 2.7-2.8%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Re: 0.01-0.03%, Mg: 0.03-0.06%.

7. The medium-width strip mill support roll according to claim 5, characterized in that, The chemical composition of the intermediate layer is as follows: C: 2.8-2.9%, Si: 1.9-2.0%, Mn: <0.50%, Ni: 0.5-0.6%, P<0.1%, S<0.02%, Mg: 0.01-0.03%.

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