Two intermediate rolls for a twenty-roll rolling mill, and a method for optimizing the composition and heat treatment process thereof

Abstract

The present application belongs to the technical field of rolling mill equipment, and particularly relates to a two-intermediate-roller for a 20-roller rolling mill, a composition optimization method thereof and a heat treatment process. The composition optimization method draws a temperature phase diagram by using a material performance simulation software, determines an optimal quenching temperature interval, sets an optimal quenching temperature to draw a CCT curve by using the software, determines a cooling rate of material quenching, determines an optimal alloy element addition amount by the changes of material hardness and yield strength after quenching under different alloy addition amounts in the software, configures the determined optimal quenching temperature and optimal alloy element addition amount into the software, analyzes carbide precipitation amount and carbide size under different tempering temperatures and holding times, and determines an optimal tempering process. The present application optimizes the alloy composition and heat treatment process of the two-intermediate-roller by using a material performance simulation software, and improves the quality of the two-intermediate-roller.

Description

Technical Field

[0001] The invention belongs to the technical field of rolling mill equipment, and in particular relates to two intermediate rolls for a twenty-high rolling mill, a composition optimization method and a heat treatment process thereof. Background Art

[0002] The Sendzimir 20-high mill is a high-precision rolling mill widely used in the production of thin strip materials such as stainless steel, silicon steel, and copper alloys. Its core advantage lies in its ability to achieve high-precision rolling of extremely thin strips, meeting the strict requirements of modern industry for material surface quality, dimensional accuracy, and mechanical properties. The second intermediate roll, as an important component of the Sendzimir mill, is responsible for driving the entire roll system. While bearing pressure, it is also subject to extremely large torque. In particular, when an emergency stop occurs during the rolling process, the instantaneous torque it bears is extremely large, which can easily cause accidents such as roll breakage. Therefore, higher requirements are placed on the hardness, wear resistance, and fatigue resistance of the second intermediate roll. To improve the performance of the second intermediate roll, it is necessary to optimize the alloy composition of the roll and supplement it with a suitable heat treatment process. Currently, in actual production processes, multiple trial and error methods are generally used to optimize the alloy composition and heat treatment process. However, this method has problems such as high trial and error costs and long R&D cycles. In addition, the mechanism of the influence of alloy composition on performance is not clear, which to some extent limits the improvement of the quality of the second intermediate roll. Summary of the Invention

[0003] The purpose of the present invention is to provide a second intermediate roll for a twenty-high rolling mill, a composition optimization method and a heat treatment process thereof, and to optimize the alloy composition and heat treatment process of the second intermediate roll through material property simulation software to improve the hardness, wear resistance, fatigue resistance and quality of the second intermediate roll.

[0004] The technical solution of the present invention is: a method for optimizing the composition of two intermediate rolls for a twenty-high rolling mill, comprising the following steps:

[0005] S1. Based on the known material composition of the second intermediate roll for the twenty-high mill and the different contents of a single alloying element to be added, a temperature phase diagram of the single alloying element at different addition amounts is drawn using material property simulation software;

[0006] S2. Based on the drawn temperature phase diagram, determine the optimal quenching temperature range by the percentage of carbon element in the quenched martensite, wherein the optimal quenching temperature range is a set of optimal quenching temperatures at different addition amounts of a single alloying element;

[0007] S3. Based on the material composition of the configured two intermediate rollers and the different contents of the single alloying elements to be added, the optimal quenching temperature is set using material property simulation software, and the CCT curve is drawn to further determine the cooling rate of the material quenching;

[0008] S4. Configuring the determined cooling rate and optimal quenching temperature into the material properties simulation software, and determining the optimal alloying element addition amount by analyzing the changes in hardness and yield strength of the material after quenching at different alloying element addition amounts;

[0009] S5. Configure the determined optimal quenching temperature and optimal alloying element addition amount into the material property simulation software. Analyze the carbide precipitation amount and carbide size at different tempering temperatures and holding times to determine the optimal tempering process.

[0010] Preferably, the material performance simulation software is JMatPro software.

[0011] Furthermore, the specific steps of step S1 are:

[0012] S1.1. Input the known material composition of the two intermediate rolls for a twenty-high mill and the different contents of a single alloying element to be added into the material properties simulation software. Set the material phase transition temperature range and plot the temperature phase diagram for different addition amounts of the single alloying element.

[0013] S1.2. Determine the Ac1 and Ac3 points of the material in the temperature phase diagram that has been drawn, and combine the transformation law of the carbide phase in the diagram to preliminarily predict the range of quenching temperature.

[0014] Furthermore, the specific steps of step S2 are:

[0015] S2.1. Based on the temperature phase diagram, use material properties simulation software to draw a graph showing the variation of the content of different elements in austenite with temperature.

[0016] S2.2. According to the martensite strengthening theory, when the mass fraction of carbon content in martensite is 0.6%, the hardness reaches the maximum value, and then the optimal quenching temperature range under different addition amounts of a single alloying element is found.

[0017] Furthermore, the specific steps of step S3 are:

[0018] S3.1. Based on the material composition of the configured two intermediate rolls and the different contents of the single alloying element to be added, use material property simulation software to set the optimal quenching temperature and draw the CCT curve;

[0019] S3.2. Based on the drawn CCT curve, the starting transformation temperature and time of martensite at different cooling rates can be obtained, and then the cooling rate of the material can be determined to obtain more martensite structure.

[0020] Preferably, the second intermediate rollers are Cr8 type rollers, and the material composition of the second Cr8 type intermediate rollers is: C 1.0~1.8wt%, Si 0.2~0.8wt%, Mn 0.2~0.6wt%, P ≤0.03wt%, S ≤0.02wt%, Cr 7.5~8.5wt%, Mo 1.2~2.5wt%, V 1.2~3.0wt%, W 0.8~1.5wt%, and the rest is Fe; the single alloy element to be added is Nb, and the addition amount of Nb is 0~1.0wt%.

[0021] Preferably, when the addition amount of Nb is 0-1.0wt%, the material phase transition temperature range of the Cr8 type second intermediate roller is 0-1200°C, the optimal quenching temperature range is 1098-1112°C, and the cooling rate is 10°C / s.

[0022] Preferably, when the addition amount of Nb is 0.6wt%, the material phase change temperature range of the Cr8 type second intermediate roller is 0-1200℃, the quenching temperature is 1108℃, the cooling rate is 10℃ / s, and the tempering process is to heat to 550-560℃ at a heating rate of 10℃ / s, keep warm for 180min, and then air cool to room temperature.

[0023] The heat treatment process of the Cr8 type second intermediate roller is determined based on the above-mentioned composition optimization method for the second intermediate roller of the twenty-high rolling mill. The Cr8 type second intermediate roller uses Nb with an addition amount of 0.6wt%. Its heat treatment process specifically includes: heating the Cr8 type roller to 1108℃ in a heating furnace at a heating rate of 10℃ / s and keeping it warm for 120 minutes, then cooling it by water quenching at a cooling rate of 10℃ / s, and then heating it to 550-560℃ at a heating rate of 10℃ / s and keeping it warm for 180 minutes, and finally air cooling it to room temperature.

[0024] The Cr8 type second intermediate roller is prepared based on the heat treatment process of the Cr8 type second intermediate roller; 0.6wt% Nb is added to the Cr8 type second intermediate roller, and the average hardness of the Cr8 type second intermediate roller is 66.95HRC and the average fracture toughness is 35.79MPam 1 / 2 , the specific wear rate is 0.631×10 -5 mm 3 N -1 m -1 .

[0025] The beneficial effects of the present invention are:

[0026] 1. High efficiency and cost savings: Material performance simulation software can quickly predict the microstructure evolution and performance changes of roll materials under different heat treatment conditions, significantly shortening the R&D cycle and reducing experimental costs.

[0027] 2. Multi-dimensional performance prediction: Material performance simulation software can simultaneously predict multiple performance indicators of the roll, including hardness, strength, toughness, phase change behavior, thermal expansion coefficient, etc., providing comprehensive data support for process optimization;

[0028] 3. Dual analysis of complex processes: Material performance simulation software can simulate complex heat treatment process paths, including quenching and tempering processes, predict their effects, optimize and evaluate processes, and provide reliable guidance for actual production;

[0029] 4. Data visualization: Material performance simulation software can provide a variety of charts and visualization tools, which can intuitively display the changing trends of material properties during heat treatment, facilitating result analysis and process optimization. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0031] Figure 1 The temperature phase diagram of the Cr8 type second intermediate roller with different Nb addition amounts is shown in Figure 2. Figure 1 (a) is the temperature phase diagram when the Nb addition amount is 0wt%, Figure 1 (b) is the temperature phase diagram when the Nb addition amount is 0.2wt%, Figure 1 (c) is the temperature phase diagram when the Nb addition amount is 0.4wt%, Figure 1 (d) is the temperature phase diagram when the Nb addition amount is 0.6wt%, Figure 1 (e) is the temperature phase diagram when the Nb addition amount is 0.8wt%, Figure 1 (f) Temperature phase diagram when Nb addition is 1.0wt%;

[0032] Figure 2 is the original composition content of high temperature austenite under different Nb addition amounts of Cr8 type two intermediate rollers, where Figure 2 (a) is the original composition content of high temperature austenite when Nb addition is 0wt%, Figure 2 (b) is the original composition content of high temperature austenite when the Nb addition amount is 0.2wt%, Figure 2 (c) is the original component content of high temperature austenite when Nb addition is 0.4wt%, Figure 2 (d) is the original composition content of high temperature austenite when the Nb addition amount is 0.6wt%, Figure 2 (e) is the original component content of high temperature austenite when Nb addition is 0.8wt%, Figure 2 (f) is the original composition content of high-temperature austenite when the Nb addition amount is 1.0wt%;

[0033] Figure 3 The continuous cooling transformation curve of the material under different Nb addition amounts of the Cr8 type second intermediate roller is shown in FIG. Figure 3 (a) is the continuous cooling transformation curve of the material with Nb addition of 0wt%, Figure 3 (b) is the continuous cooling transformation curve of the material with Nb addition of 0.2wt%. Figure 3 (c) is the continuous cooling transformation curve of the material with Nb addition of 0.4wt%. Figure 3 (d) is the continuous cooling transformation curve of the material with Nb addition of 0.6wt%. Figure 3 (e) is the continuous cooling transformation curve of the material with Nb addition of 0.8wt%, Figure 3 (f) is the continuous cooling transformation curve of the material with a Nb addition of 1.0wt%;

[0034] Figure 4 The bar graph of the quenching hardness and yield strength of the Cr8 type second intermediate roller with different Nb addition amounts is shown in the figure. Figure 4 (a) is the bar graph of the quenching hardness of materials with different Nb addition amounts, Figure 4 (b) is the bar graph of material yield strength at different Nb addition amounts;

[0035] Figure 5 The carbide precipitation amount and carbide size of the Cr8 type second intermediate roller at different tempering temperatures and holding times when the Nb addition amount is 0.6wt%. Figure 5 (a) is the amount of carbide precipitation of Cr8 type two intermediate rollers at different tempering temperatures and holding times. Figure 5 (b) Carbide size of Cr8 type two intermediate rollers at different tempering temperatures and holding times;

[0036] Figure 6 Schematic diagram of the heat treatment schedule for the Cr8 type second intermediate roll when the Nb addition is 0.6wt%;

[0037] Figure 7 This is the actual microstructure diagram obtained for the Cr8 type second intermediate roller, where Figure 7 (a) is the microstructure of the Cr8 type second intermediate roller without Nb addition, Figure 7 (b) Microstructure of the second intermediate roller of Cr8 (0.6% Nb). DETAILED DESCRIPTION

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0039] In the actual production process, the method of multiple trials and errors is generally used to optimize the alloy composition and heat treatment process. However, this method has problems such as high trial and error costs and long R&D cycles, and the mechanism of the influence of alloy composition on performance is not clear, which to a certain extent limits the improvement of the quality of the two intermediate rolls. In view of this, the inventors of this application provide a composition optimization method and heat treatment process for the two intermediate rolls for a twenty-high rolling mill, which can optimize the alloy composition and heat treatment process of the two intermediate rolls through material performance simulation software, thereby improving the quality of the two intermediate rolls for the twenty-high rolling mill.

[0040] like Figure 1-7 As shown, the method for optimizing the composition of two intermediate rolls for a twenty-high rolling mill includes the following steps:

[0041] S1. Based on the known material composition of the two intermediate rolls for the twenty-high mill and the different contents of a single alloying element to be added, a temperature phase diagram of the single alloying element at different addition amounts is drawn using material property simulation software.

[0042] The specific steps of step S1 are:

[0043] S1.1. Input the known material composition of the two intermediate rolls for a twenty-high mill and the different contents of a single alloying element to be added into the material properties simulation software. Set the material phase transition temperature range and plot the temperature phase diagram for different addition amounts of the single alloying element.

[0044] S1.2. Determine the Ac1 and Ac3 points of the material in the drawn temperature phase diagram, and combine the transformation law of the carbide phase in the diagram to preliminarily estimate the range of quenching temperature.

[0045] It should be noted that the material performance simulation software is JMatPro software.

[0046] S2. Based on the temperature phase diagram that has been drawn, the optimal quenching temperature range is determined by the percentage of carbon element in the quenched martensite.

[0047] The specific steps of step S2 are:

[0048] S2.1. Based on the temperature phase diagram, use material properties simulation software to draw a graph showing the variation of the content of different elements in austenite with temperature.

[0049] S2.2. According to the martensite strengthening theory, when the carbon content in martensite is 0.6% by mass, the hardness reaches its maximum value, and then the optimal quenching temperature range under different addition amounts of a single alloying element is found.

[0050] Among them, the optimal quenching temperature range is the set of optimal quenching temperatures under different addition amounts of a single alloying element.

[0051] S3. Based on the material composition of the configured two intermediate rollers and the different contents of the single alloying elements to be added, the optimal quenching temperature is set using material property simulation software, and the CCT curve is drawn to further determine the cooling rate of the material quenching.

[0052] The specific steps of step S3 are:

[0053] S3.1. Based on the material composition of the configured two intermediate rolls and the different contents of the single alloying element to be added, use material property simulation software to set the optimal quenching temperature and draw the CCT curve;

[0054] S3.2. Based on the drawn CCT curve, the starting transformation temperature and time of martensite at different cooling rates can be obtained, and then the cooling rate of the material can be determined to obtain more martensite structure.

[0055] S4. Configure the determined cooling rate and optimal quenching temperature into the material property simulation software, and determine the optimal alloying element addition amount by observing the changes in the hardness and yield strength of the material after quenching at different alloy addition amounts.

[0056] S5. Configure the determined optimal quenching temperature and optimal alloying element addition amount into the material property simulation software. Analyze the carbide precipitation amount and carbide size at different tempering temperatures and holding times to determine the optimal tempering process.

[0057] In this embodiment, the two intermediate rollers are Cr8 type rollers. The material composition of the two Cr8 type intermediate rollers is shown in Table 1:

[0058] ;

[0059] The single alloying element to be added is Nb, and the different amounts of Nb to be added are 0% to 1.0% by mass. Specifically, in this example, the amounts of Nb to be added may be 0wt%, 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, and 1.0wt%.

[0060] According to the material composition of the Cr8 type second intermediate roller and the different contents of Nb to be added, the material performance simulation software is input and the material phase change temperature range is set to 0-1200℃. The following is drawn through the material performance simulation software: Figure 1 The temperature phase diagram shown; respectively Figure 1 (a) Figure 1 (b) Figure 1 (c) Figure 1 (d) Figure 1 (e) Figure 1 In (f), the Ac1 point and Ac3 point of the material are determined, and combined with the transformation law of the carbide phase in the figure, the range of the quenching temperature is preliminarily estimated.

[0061] Based on the above embodiment, when the material reaches Ac1 temperature, M 23 C6 carbide begins to dissolve, increasing the C content of austenite; then in the temperature range of 950-1180℃, M7C3 carbide gradually dissolves, further increasing the C content of austenite. Therefore, it is preliminarily estimated that the optimal quenching temperature should occur during the dissolution of M7C3 carbide, that is, 950-1180℃.

[0062] In this embodiment, according to Figure 1 The temperature phase diagram shown in the figure is drawn using material performance simulation software. Figure 2 The graph showing the change of different element contents in austenite with temperature is shown in the figure. Figure 2 The different elements displayed include: Mn, Mo, Si, V, W, C, P, S and Nb; according to the martensite strengthening theory, when the carbon content in martensite is 0.6wt%, the hardness reaches the maximum value, and then the optimal quenching temperature range of Nb at different addition amounts is found to be 1098-1112℃; among them, the optimal quenching temperature range 1098-1112℃ is the set of optimal quenching temperatures of Nb at different addition amounts, for example, when the Nb addition amount is 0wt%, the optimal quenching temperature of the material is 1098℃, when the Nb addition amount is 0.6wt%, the optimal quenching temperature of the material is 1108℃, and when the Nb addition amount is 1.0wt%, the optimal quenching temperature of the material is 1112℃.

[0063] In this embodiment, based on the material composition of the configured Cr8 type second intermediate roller and different amounts of Nb added, the optimal quenching temperature is set using material property simulation software, and the following is drawn: Figure 3 The CCT curve shown in Figure 3 Four different water quenching cooling rates are shown, namely: 100°C / s, 10°C / s, 1°C / s, and 0.1°C / s.

[0064] The CCT curve plotted reveals the martensite transformation start temperature and transformation time at different cooling rates. It can be seen that as the cooling rate increases from 0.1°C / s to 100°C / s, the martensite transformation start time decreases. Therefore, to obtain a larger amount of martensite, a conventional water quenching method with a cooling rate of 10°C / s is sufficient. While higher cooling rates can produce more martensite, they are very costly and are therefore not considered. Therefore, the present invention uses 10°C / s instead of 100°C / s.

[0065] In this embodiment, the cooling rate of 10°C / s and the optimal quenching temperature are configured into the material property simulation software, and the following is obtained: Figure 4 The transformation diagram of the hardness and yield strength of the material after quenching at different Nb addition amounts is shown to determine that when the Nb addition amount is 0.6wt%, the material reaches the highest hardness of 53.24HRC and yield strength of 1539.91MPa after quenching.

[0066] In this embodiment, the determined quenching temperature of 1108°C and the optimal Nb addition of 0.6wt% are configured into the material property simulation software, and the following is obtained: Figure 5 The graphs showing the changes in MC carbide precipitation and size at tempering temperatures of 500-600°C and holding times of 180-240 minutes show that more fine carbides provide better structural strengthening, but increased carbide size generally reduces the material's mechanical properties. At 540°C, while the carbide content reaches its highest, the carbides also reach their largest size. Therefore, to achieve a high content and small size, this temperature should be avoided. Therefore, the tempering process was determined to be heating at a rate of 10°C / s to 550-560°C, holding for 180 minutes, and then air cooling to room temperature. The 10°C / s heating rate used here reflects the heating rate used in conventional tempering processes.

[0067] The heat treatment process of the Cr8 type second intermediate roll is set according to the above-mentioned method for optimizing the composition of the second intermediate roll for the twenty-high rolling mill, such as Figure 6 As shown in FIG, the heat treatment process of the Cr8 type second intermediate roller with a Nb addition of 0.6 wt% specifically includes: heating the Cr8 type roller to 1108°C in a heating furnace at a heating rate of 10°C / s and keeping it warm for 120 min, then cooling it by water quenching at a cooling rate of 10°C / s, and then heating it to 550-560°C at a heating rate of 10°C / s and keeping it warm for 180 min, and finally air cooling it to room temperature.

[0068] After the Cr8 (0.6% Nb) second intermediate roller was successfully prepared by the above-mentioned heat treatment process of the Cr8 type second intermediate roller, a plurality of 10mm×10mm×10mm small sample pieces were cut from the middle of the roller body surface by wire cutting technology. The cross section of the square after grinding and polishing was corroded with 4.0% nitric acid alcohol solution by volume fraction for 10-15s, cleaned and dried, and the microstructure was observed by Sigma300 field emission scanning electron microscope. The Cr8 type roller without Nb was heated to 1108℃ in a heating furnace at a heating rate of 10℃ / s and kept warm for 120min. n, and then cooled by water quenching at a cooling rate of 10℃ / s, and then heated to 550-560℃ at a heating rate of 10℃ / s and kept warm for 180min. Finally, it was air-cooled to room temperature to obtain a Cr8 type second intermediate roller without Nb addition. A plurality of 10mm×10mm×10mm small samples were cut from the middle position of the roller body surface by wire cutting technology. The cross section of the square blocks after grinding and polishing was corroded with 4.0% nitric acid alcohol solution by volume for 10-15s, cleaned and blown dry, and the microstructure was observed by Sigma300 field emission scanning electron microscope.

[0069] The microstructure of the Cr8 type second intermediate roller without Nb addition and the Cr8 (0.6% Nb) second intermediate roller is as follows: Figure 7 As shown in the figure, in the Cr8 type second intermediate roller without Nb addition, the tempered lath martensite decomposes to form a tempered bainite structure, and the C atoms inside the original martensite are redistributed and combined with alloying elements such as Cr, Mo and V to form high chromium carbides, as shown in point 1; in the structure of the Cr8 (0.6% Nb) second intermediate roller, more NbC particles appear at the grain boundaries, as shown in point 2, with a grain size of about 8μm. This is a carbide precipitation phase with high hardness, which can effectively resist wear and improve the wear resistance of the material; and after precipitation at the grain boundary, through the pinning effect, the coarsening of the tempered bainite is effectively suppressed during tempering, thereby improving the material's resistance to tempering softening and obtaining better mechanical properties.

[0070] Several 10mm×10mm×10mm small specimens were cut from the middle of the roller surface of two Cr8 type intermediate rollers. After grinding and polishing, the squares were tested on a HR-150A Rockwell hardness tester. The hardness of the two Cr8 type rollers is shown in Table 2:

[0071] ;

[0072] The average hardness of the Cr8 type roller without Nb addition is 65.81HRC, and the average hardness of the Cr8 (0.6%Nb) roller is 66.95HRC, which is significantly improved.

[0073] At the middle of the roller surface of two Cr8 type intermediate rollers, multiple specimens were prepared according to the national standard GB4161-2007 Metallic Material Plane Strain Fracture Toughness KIC Test Method. The fracture toughness test was conducted on a three-point bending tester. The fracture toughness of the two Cr8 type rollers is shown in Table 3:

[0074] ;

[0075] The average fracture toughness of Cr8 type roll without Nb addition is 43.01MPam 1 / 2 The average fracture toughness of Cr8 (0.6% Nb) roll is 35.79 MPa 1 / 2 , there is a slight decrease, but it can be seen that the fracture toughness value of the Cr8 type roll without Nb addition fluctuates greatly, while the fracture toughness value of the Cr8 (0.6%Nb) roll is relatively stable.

[0076] Several 10mm×10mm×10mm small specimens were cut from the middle of the roller surface of the two Cr8 type intermediate rollers. After grinding and polishing, the specimens were tested for wear resistance on the MS-M9000 multifunctional friction and wear testing machine. The wear resistance of the two Cr8 type rollers is shown in Table 4:

[0077] ;

[0078] The specific wear rate of the Cr8 type roll without Nb addition is 1.129×10 -5 mm 3 N -1 m -1 The specific wear rate of Cr8 (0.6% Nb) roller is 0.631×10 -5 mm 3 N -1 m -1 , wear resistance increased by 44.11%.

[0079] After multiple sets of mechanical properties experimental tests, the results show that the optimized cold rolling roller has improved hardness, toughness uniformity and wear resistance.

[0080] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for optimizing the composition of two intermediate rolls for a twenty-high rolling mill, characterized in that: The steps include: S1. Based on the known material composition of the second intermediate roll for the twenty-high mill and the different contents of a single alloying element to be added, a temperature phase diagram of the single alloying element at different addition amounts is drawn using material property simulation software; The specific steps of step S1 are: S1.

1. Input the known material composition of the two intermediate rolls for a twenty-high mill and the different contents of a single alloying element to be added into the material properties simulation software. Set the material phase transition temperature range and plot the temperature phase diagram for different addition amounts of the single alloying element. S1.

2. Determine the Ac1 and Ac3 points of the material in the temperature phase diagram, and preliminarily predict the quenching temperature range based on the carbide phase transformation law in the diagram; S2. Based on the drawn temperature phase diagram, determine the optimal quenching temperature range by the percentage of carbon element in the quenched martensite, wherein the optimal quenching temperature range is a set of optimal quenching temperatures at different addition amounts of a single alloying element; The specific steps of step S2 are: S2.

1. Based on the temperature phase diagram, use material properties simulation software to draw a graph showing the variation of the content of different elements in austenite with temperature. S2.

2. According to the martensite strengthening theory, the hardness reaches its maximum when the mass fraction of carbon in martensite is 0.6%. The optimal quenching temperature range for different addition amounts of a single alloying element can be found. S3. Based on the material composition of the configured two intermediate rollers and the different contents of the single alloying elements to be added, the optimal quenching temperature is set using material property simulation software, and the CCT curve is drawn to further determine the cooling rate of the material quenching; The specific steps of step S3 are: S3.

1. Based on the material composition of the configured two intermediate rolls and the different contents of the single alloying element to be added, use material property simulation software to set the optimal quenching temperature and draw the CCT curve; S3.

2. Based on the plotted CCT curve, the martensite transformation temperature and time at different cooling rates can be obtained, and the cooling rate of the material can be determined to obtain a larger amount of martensite structure. S4. Configuring the determined cooling rate and optimal quenching temperature into the material properties simulation software, and determining the optimal alloying element addition amount by analyzing the changes in hardness and yield strength of the material after quenching at different alloying element addition amounts; S5. Configuring the determined optimal quenching temperature and optimal alloying element addition amount into the material properties simulation software, and analyzing the amount of carbide precipitation and carbide size at different tempering temperatures and holding times to determine the optimal tempering process; The material performance simulation software is JMatPro software.

2. The method for optimizing the composition of the second intermediate rolls for a twenty-high rolling mill according to claim 1, characterized in that: The second intermediate roll is a Cr8 type roll, and the material composition of the Cr8 type second intermediate roll is: C 1.0~1.8wt%, Si 0.2~0.8wt%, Mn 0.2~0.6wt%, P ≤0.03wt%, S ≤0.02wt%, Cr 7.5~8.5wt%, Mo 1.2~2.5wt%, V 1.2~3.0wt%, W 0.8~1.5wt%, and the rest is Fe; the single alloy element to be added is Nb, and the addition amount of Nb is 0~1.0wt%.

3. The method for optimizing the composition of the second intermediate rolls for a twenty-high rolling mill according to claim 2, wherein: When the addition amount of Nb is 0~1.0wt%, the material phase transition temperature range of the Cr8 type second intermediate roller is 0-1200℃, the optimal quenching temperature range is 1098-1112℃, and the cooling rate is 10℃ / s.

4. The method for optimizing the composition of the second intermediate rolls for a twenty-high rolling mill according to claim 3, characterized in that: When the addition amount of Nb is 0.6wt%, the material phase change temperature range of the Cr8 type second intermediate roller is 0-1200℃, the quenching temperature is 1108℃, the cooling rate is 10℃ / s, and the tempering process is to heat to 550-560℃ at a heating rate of 10℃ / s, keep warm for 180min, and then air cool to room temperature.

5. The heat treatment process of the Cr8 type second intermediate roll is determined based on the composition optimization method for the second intermediate roll of the twenty-high rolling mill according to claim 4, and is characterized in that: The heat treatment process of the Cr8 type second intermediate roller specifically includes: heating the Cr8 type rolling roller to 1108℃ in a heating furnace at a heating rate of 10℃ / s and keeping it warm for 120 minutes, then cooling it by water quenching at a cooling rate of 10℃ / s, and then heating it to 550-560℃ at a heating rate of 10℃ / s and keeping it warm for 180 minutes, and finally air cooling it to room temperature.

6. Cr8 type second intermediate roller, characterized by: The Cr8 type second intermediate roller is prepared based on the heat treatment process of the Cr8 type second intermediate roller according to claim 5; the average hardness of the Cr8 type second intermediate roller is 66.95HRC, and the average fracture toughness is 35.79MPam 1 / 2 , the specific wear rate is 0.631×10 -5 mm 3 N -1 m -1 .

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