Method for preventing loop fluctuation between saw back steel finishing mill frames

Abstract

The invention belongs to the technical field of metallurgical steel rolling, and particularly relates to a method for preventing loop fluctuation between saw back steel finishing mill racks, which comprises the following steps of: (1) determining different furnace staying time of a second heating section and a soaking section according to different Mo contents of various steel types of saw back steel, and increasing the furnace temperature of a high-temperature section as much as possible in a performance allowable range; the higher the Mo content is, the longer the furnace staying time is; (2) in order to effectively avoid precipitation of M7C3 short rod type alloy carbides in a finish rolling area, the target temperature of a rough rolling outlet and the target temperature of a finish rolling outlet of each steel grade are determined; and (3) according to the thickness of each steel type, determining the AGC compensation coefficient of each rack when each steel type has no loop fluctuation and the thickness precision completely meets the requirement. According to the method, the Mo content is distinguished, the furnace staying time and the target furnace temperature of the high-temperature section of the heating furnace are determined, the rough rolling outlet temperature and the finish rolling outlet temperature are determined again, the AGC compensation coefficient of each rack is adjusted and optimized, and the loop fluctuation problem occurring when the saw back steel is rolled on the finish rolling rack is essentially solved.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical steel rolling, and in particular relates to a method for preventing the fluctuation of inter-stand loopers of a saw back steel finishing mill. Background Art

[0002] Saw back steel is a high-end variety in the high alloy series, mainly used as the back material of bimetallic composite saw blades. It has high alloy content and excellent performance. It is used to match precious saw blade teeth and is used for sawing precious hard materials with high fatigue life and high processing accuracy. The main production process of the hot rolling production line is that the slab is heated in the heating furnace according to the temperature specified by the process, and then enters the rough rolling mill for rolling after heating to the target temperature. The rough rolling vertical roll controls the width and the flat roll controls the thickness. Reversible rolling is carried out in the rough rolling unit, generally 5 to 7 times; after rolling in the rough rolling unit, the strip reaches the preset target thickness, width and temperature; then enters the finishing mill for seven-stand flat roll continuous rolling, so that the strip reaches the preset target thickness, mesh width, target temperature, target convexity and target straightness, etc., and finally the strip is formed into a steel coil by the coiler. The seven stands of the finishing mill are F0, F1, F2, F3, F4, F5, and F6. During the continuous rolling process of the seven stands, the flow rate per second between the stands must be equal to ensure the stability of the loopers between the stands. During the entire rolling process of the strip, the stability of the looper is the premise for the stable rolling of the strip and the guarantee of the rolling accuracy. The biggest difficulty affecting the stability of the saw back steel rolling is the poor stability of the loopers between the finishing mill stands and the large fluctuation of the loopers. Specifically, after the strip is threaded, the rolling pressure fluctuations of the F4, F5, and F6 stands will be irregular, resulting in the 4# looper between F4~F5 and the looper between F5~F6. The 5# looper angle fluctuates between the stands, and the rolling pressure fluctuates (the difference between the peak and the trough is 1000~3000KN), causing the looper angle to fluctuate greatly (15°~35°), and the highest angle can reach more than 40°; the looper fluctuation causes the strip rolling to be extremely unstable. On the one hand, it will cause the thickness, width, temperature and other product precision indicators to fluctuate greatly, resulting in the finished product size and performance not meeting user requirements; on the other hand, when the looper angle between the stands fluctuates greatly, strip rolling, steel piling and other faults will frequently occur. The specific reasons for the fluctuation of the looper between the finishing mill stands are as follows: (1) Saw back steel undergoes a carbide solid solution process in the high temperature section of the heating furnace, which allows the carbides to fully dissolve into the austenite matrix, making the strip grains uniform, and the hardness and strength uniform. However, due to the high Mo content (≥1.0%) of saw back steel, the required solid solution time is long. If the strip is not kept in the furnace for an insufficient time in the high temperature section, the temperature uniformity is poor, resulting in poor solid solution effect and uneven hardness of the strip. During the continuous thinning and rolling process, it is easy to cause pressure fluctuations. The higher the Mo content, the more obvious this phenomenon is. (2) At about 900℃ (890~920℃), M7C3 short rod-shaped alloy carbides will precipitate on the saw back steel. The precipitation of carbides will cause uneven hardness of the strip and cause large fluctuations in the rolling pressure of the rolling mill. At the current rolling process temperature, the precipitation area is located in the F5 and F6 frames. When the control temperature is too low, it will extend to the F4 frame. (3) Influence of AGC action: AGC refers to automatic thickness control. The AGC function mainly achieves the purpose of dynamically adjusting the thickness by dynamically adjusting the rolling pressure. The main control parameter of the AGC function is the AGC compensation coefficient. Under normal control conditions, the larger the AGC compensation coefficient, the faster the rolling pressure correction and the higher the rolling accuracy; conversely, the smaller the AGC compensation coefficient, the slower the rolling pressure correction and the lower the rolling accuracy. When rolling sawback steel, since the user's requirements for the longitudinal thickness difference are higher than those for normal rolled strip steel (the sawback steel requires the thickness fluctuation to be within the range of ±0.03mm), the AGC coefficient takes the maximum value. However, in the actual rolling process, due to the large fluctuations in rolling pressure, when the AGC compensation coefficient is too large, it will cause a larger fluctuation in rolling pressure and a larger fluctuation in looper; at the same time, it will also cause a large fluctuation in thickness, and the thickness accuracy will be reduced, which cannot meet the user's requirements. Summary of the invention

[0003] The purpose of the present invention is to provide a method for preventing the fluctuation of the looper between the frames of the saw back steel finishing mill, so as to solve the problem that the rolling of the strip steel is extremely unstable due to the irregular fluctuation of the rolling pressure and the fluctuation of the looper angle between the frames in the finishing mill.

[0004] In order to achieve the above object, the present invention adopts the following technical scheme: A method for preventing the fluctuation of the inter-stand looper of the saw back steel finishing mill comprises the following contents: (1) According to the different Mo contents of various saw back steel grades, the residence time of the second heating section and the soaking section is determined. At the same time, in order to further improve the solid solution effect, the high temperature section furnace temperature is increased as much as possible within the performance allowable range; the higher the Mo content, the longer the residence time; (2) In order to effectively avoid the precipitation of M7C3 short rod-shaped alloy carbides in the finishing rolling area, the rough rolling outlet target temperature and the finishing rolling outlet target temperature of each steel grade are determined; (3) According to the influence of the AGC compensation coefficient of each stand on the rolling accuracy and looper fluctuation, the AGC compensation coefficient of each stand is determined according to the thickness of each steel grade when there is no looper fluctuation and the thickness accuracy fully meets the requirements. When the thickness of saw back steel TSJ31 is ≥4.0mm, the AGC compensation coefficients of stands F0~F6 are 1.2, 1.0, 1.0, 0.85, 0.8, 0.75 and 0.7 respectively; when the thickness of saw back steel TSJ31 is 3.0mm~4.0mm, the AGC compensation coefficients of stands F0~F6 are 1.17, 0.95, 0.9, 0.75, 0.7, 0.65 and 0.6 respectively; when the thickness of saw back steel TSJ31 is <3.0mm, the AGC compensation coefficients of stands F0~F6 are 0.97, 0.85, 0.8, 0.7, 0.6, 0.55 and 0.5 respectively; when the thickness of saw back steel TJB When the thickness of saw back steel TSJ33 is the same as that of saw back steel TSJ31, the AGC compensation coefficients of each frame are the same; when the thickness of saw back steel TSJ33 is ≥4.0mm, the AGC compensation coefficients of frames F0~F6 are 1.2, 1.1, 1.0, 0.88, 0.83, 0.77 and 0.72 respectively; when the thickness of saw back steel TSJ33 is 3.0mm~4.0mm, the AGC compensation coefficients of frames F0~F6 are 1. 19, 1.0, 0.9, 0.77, 0.72, 0.65 and 0.6; when the thickness of saw back steel TSJ33 is less than 3.0mm, the AGC compensation coefficients of frames F0~F6 are 0.99, 0.87, 0.83, 0.7, 0.6, 0.55 and 0.5 respectively; when the thickness of saw back steel TSJ35, saw back steel TJB34 and saw back steel TSJ33 are the same, the AGC compensation coefficients of each frame are the same.

[0005] Preferably, the Mo content of the saw back steel TJB32 in (1) is 1.29%-1.34%, the second furnace temperature is 1270±20℃, the second furnace dwelling time is not less than 70 minutes, and the temperature of the soaking furnace is 1260±20℃. The soaking furnace dwelling time is not less than 65 minutes; the Mo content of the saw back steel TJB34 is 1.05%-1.20%, the second furnace temperature is 1290±20℃, the second furnace dwelling time is not less than 60 minutes, and the soaking furnace temperature is 1280±20℃. The soaking furnace dwelling time is not less than 50 minutes; the Mo content of the saw back steel TSJ31 is 1.28%-1.33%, the second furnace temperature is 1290±20℃, the second furnace dwelling time is not less than 68 minutes, and the soaking furnace temperature is 1280±20℃. The soaking time in the furnace shall not be less than 65 minutes; the Mo content of saw back steel TSJ33 is 1.25%-1.30%, the second furnace temperature is 1290±20℃, the second furnace time is not less than 65 minutes, and the soaking furnace temperature is 1280±20℃. The soaking time in the furnace shall not be less than 60 minutes; the Mo content of saw back steel TSJ35 is 1.05%-1.25%, the second furnace temperature is 1280±20℃, the second furnace time is not less than 60 minutes, and the soaking furnace temperature is 1270±20℃. The soaking time in the furnace shall not be less than 55 minutes.

[0006] Preferably, the rough rolling outlet temperature of the saw back steel TJB32 in (2) is 1110±20℃, and the finishing rolling outlet temperature is 920±20℃, the rough rolling outlet temperature of the saw back steel TJB34 is 1130±20℃, and the finishing rolling outlet temperature is 930±20℃, the rough rolling outlet temperature of the saw back steel TSJ31 is 1130±20℃, and the finishing rolling outlet temperature is 930±20℃, the rough rolling outlet temperature of the saw back steel TSJ33 is 1130±20℃, and the finishing rolling outlet temperature is 930±20℃, and the rough rolling outlet temperature of the saw back steel TSJ35 is 1120±20℃, and the finishing rolling outlet temperature is 930±20℃.

[0007] Compared with the prior art, the present invention has the following beneficial effects: (1) The method of the present invention determines the residence time and target furnace temperature of the high temperature section of the heating furnace according to the difference in Mo content. The residence time of the strip in the high temperature section is sufficient, the temperature uniformity is good, the solid solution effect is improved, the hardness of the strip is uniform, and large fluctuations in pressure and looper angle during rolling are avoided; (2) The method of the present invention re-determines the rough rolling outlet temperature and the finishing rolling outlet temperature to prevent the precipitation of M7C3 short rod-shaped alloy carbides, make the strip hardness uniform, and avoid causing large fluctuations in the rolling pressure of the rolling mill and fluctuations in the looper angle; (3) The AGC compensation coefficient of each stand is determined according to the thickness of each steel grade, and the rolling pressure adjustment rate is corrected to prevent a larger fluctuation in the rolling pressure. At the same time, a larger fluctuation in the looper angle is avoided, and the rolling accuracy is improved. This essentially solves the problem of large fluctuations in the looper angle when the saw back steel is rolled on the finishing mill. DETAILED DESCRIPTION

[0008] The technical solution of the present invention is described clearly and completely below in conjunction with the embodiments, but the present invention is not limited to the scope of the embodiments.

[0009] The experimental methods in the following examples without specifying specific conditions were carried out according to conventional methods and conditions. Example 1

[0010] The saw back steel grade TJB32 rolled in this embodiment has a steel coil number of 941053201, a blank thickness of 220 mm, and a blank width of 1030 mm. The main chemical components (%) are: C: 0.307; Si: 0.238; Mn: 0.998; Cr: 3.982; Ni: 0.751; Mo: 1.315, and the target thickness of the finished product is 3.4 mm, and the target width of the finished product is 1005 mm.

[0011] A method for preventing the fluctuation of the inter-stand looper of the saw back steel finishing mill comprises the following contents: (1) The furnace temperature and residence time in the high temperature section are controlled as shown in the following table:

[0012] (2) Rough rolling target temperature: 1110°C; actual temperature after rough rolling is: 1113°C; finishing rolling target temperature: 920°C; (3) The AGC compensation coefficients of the finishing mills F0~F6 are: 1.17, 0.95, 0.9, 0.75, 0.7, 0.65, 0.6. After the finishing mill calculation is completed, the relevant calculation parameters of each mill are as follows:

[0013] Note: All rack outlet thicknesses are thermal values.

[0014] During the rolling process of this steel block, there is no pressure fluctuation in the rolling pressure of each stand, and the looper angle between each stand is stable and has no fluctuation. The actual temperature at the finishing outlet is 924℃, the actual thickness of the finished product is 3.40mm, and the actual width is 1012mm. The rolling accuracy meets the user's requirements. Example 2

[0015] The saw back steel grade TJB32 rolled in this embodiment has a steel coil number of 943127309, a blank thickness of 230 mm, and a blank width of 1030 mm. The main chemical composition contents (%) are: C: 0.314; Si: 0.257; Mn: 0.994; Cr: 3.95; Ni: 0.759; Mo: 1.332. The target thickness of the finished product is 4.0 mm, and the target width of the finished product is 1005 mm.

[0016] A method for preventing the fluctuation of the inter-stand looper of the saw back steel finishing mill comprises the following contents: (1) The furnace temperature and residence time in the high temperature section are controlled as shown in the following table:

[0017] (2) Rough rolling target temperature: 1110°C; the actual temperature after rough rolling is: 1114°C, and the finishing rolling target temperature is: 920°C; (3) The AGC compensation coefficients of the finishing mills F0~F6 are: 1.2, 1.0, 1.0, 0.85, 0.8, 0.75, 0.7. After the finishing mill calculation is completed, the relevant calculation parameters of each mill are as follows:

[0018] Note: All rack outlet thicknesses are thermal values.

[0019] During the rolling process of this steel block, there is no pressure fluctuation in the rolling pressure of each stand, and the angle of the looper between the stands is stable and has no fluctuation. The actual temperature at the finishing outlet is 922℃, the actual thickness of the finished product is 4.03mm, and the actual width is 1013mm. The rolling accuracy meets the user's requirements. Example 3

[0020] The saw back steel grade TJB34 rolled in this embodiment has a steel coil number of 942161702, a blank thickness of 222 mm, and a blank width of 1030 mm. The main chemical composition contents (%) are: C: 0.353; Si: 0.27; Mn: 0.955; Cr: 3.975; Ni: 0.755; Mo: 1.13. The target thickness of the finished product is 3.5 mm, and the target width of the finished product is 1005 mm.

[0021] A method for preventing the fluctuation of the inter-stand looper of the saw back steel finishing mill comprises the following contents: (1) The furnace temperature and residence time in the high temperature section are controlled as shown in the following table:

[0022] (2) Rough rolling target temperature: 1130°C; the actual temperature after rough rolling is: 1128°C, and the finishing rolling target temperature is: 930°C; (3) The AGC compensation coefficients of the finishing mills F0~F6 are: 1.19, 1.0, 0.9, 0.77, 0.72, 0.65, 0.6. After the finishing mill calculation is completed, the relevant calculation parameters of each mill are as follows:

[0023] Note: All rack outlet thicknesses are thermal values.

[0024] During the rolling process of this steel block, there is no pressure fluctuation in the rolling pressure of each stand, and the angle of the looper between the stands is stable and has no fluctuation. The actual temperature at the finishing outlet is 932℃, the actual thickness of the finished product is 3.51mm, and the actual width is 1011mm. The rolling accuracy meets the user's requirements. Example 4

[0025] The saw back steel grade TSJ31 is rolled in this embodiment, with the steel coil number: 943110901, the blank thickness: 230 mm, and the blank width: 1030 mm. The main chemical composition content (%) is: C: 0.333; Si: 0.253; Mn: 1.005; Cr: 3.928; Ni: 0.752; Mo: 1.322. The target thickness of the finished product is 2.9 mm, and the target width of the finished product is 1000 mm.

[0026] A method for preventing the fluctuation of the inter-stand looper of the saw back steel finishing mill comprises the following contents: The furnace temperature and residence time in the high temperature section are controlled as follows:

[0027] (2) Rough rolling target temperature: 1130°C; the actual temperature after rough rolling is: 1132°C, and the finishing rolling target temperature is: 930°C; (3) The AGC compensation coefficients of the finishing mills F0~F6 are: 0.97, 0.85, 0.8, 0.7, 0.6, 0.55, 0.5. After the finishing calculation is completed, the relevant calculation parameters of each mill are as follows:

[0028] Note: All rack outlet thicknesses are thermal values.

[0029] During the rolling process of this steel block, there is no pressure fluctuation in the rolling pressure of each stand, and the angle of the looper between the stands is stable and has no fluctuation. The actual temperature at the finishing outlet is 931℃, the actual thickness of the finished product is 2.89mm, and the actual width is 1008mm. The rolling accuracy meets the user's requirements. Example 5

[0030] The saw back steel grade TSJ33 rolled in this embodiment has a steel coil number of 941063902, a blank thickness of 230 mm, and a blank width of 1090 mm. The main chemical composition contents (%) are: C: 0.327; Si: 0.27; Mn: 1.019; Cr: 3.981; Ni: 0.754; Mo: 1.276. The target thickness of the finished product is 3.0 mm, and the target width of the finished product is 1070 mm.

[0031] A method for preventing the fluctuation of the inter-stand looper of the saw back steel finishing mill comprises the following contents: (1) The furnace temperature and residence time in the high temperature section are controlled as shown in the following table:

[0032] (2) Rough rolling target temperature: 1130°C; the actual temperature after rough rolling is: 1129°C, and the finishing rolling target temperature is: 930°C; (3) The AGC compensation coefficients of the finishing mills F0~F6 are: 1.19, 1.0, 0.9, 0.77, 0.72, 0.65, 0.6. After the finishing mill calculation is completed, the relevant calculation parameters of each mill are as follows:

[0033] Note: All rack outlet thicknesses are thermal values.

[0034] During the rolling process of this steel block, there is no pressure fluctuation in the rolling pressure of each stand, and the angle of the looper between the stands is stable and has no fluctuation. The actual temperature at the finishing outlet is 930℃, the actual thickness of the finished product is 2.98mm, and the actual width is 1077mm. The rolling accuracy meets the user's requirements. Example 6

[0035] The saw back steel grade TSJ35 rolled in this embodiment has a steel coil number of 949065101, a blank thickness of 230 mm, and a blank width of 1050 mm. The main chemical composition contents (%) are: C: 0.34; Si: 0.30; Mn: 1.001; Cr: 4.102; Ni: 0.825; Mo: 1.151. The target thickness of the finished product is 2.8 mm, and the target width of the finished product is 1030 mm.

[0036] A method for preventing the fluctuation of the inter-stand looper of the saw back steel finishing mill comprises the following contents: (1) The furnace temperature and residence time in the high temperature section are controlled as shown in the following table:

[0037] (2) Rough rolling target temperature: 1120°C; the actual temperature after rough rolling is: 1127°C, and the finishing rolling target temperature is: 930°C; (3) The AGC compensation coefficients of each finishing stand F0~F6 are: 0.99, 0.87, 0.83, 0.7, 0.6, 0.55, 0.5. After the finishing calculation is completed, the relevant calculation parameters of each stand are as follows:

[0038] Note: All rack outlet thicknesses are thermal values.

[0039] During the rolling process of this steel block, there is no pressure fluctuation in the rolling pressure of each stand, and the angle of the looper between the stands is stable and has no fluctuation. The actual temperature at the finishing outlet is 934℃, the actual thickness of the finished product is 2.78mm, and the actual width is 1039mm. The rolling accuracy meets the user's requirements.

[0040] After the implementation of this technology, the amount of scrap caused by the fluctuation of the loops between the frames has been reduced by more than 1,800 tons per month. At the same time, the varieties and specifications of saw back steel have been further expanded, and the thickness has been expanded from more than 3.0mm rolling to 2.8mm rolling, with obvious economic benefits.

Claims

1. A method for preventing the fluctuation of the inter-stand looper of the saw back steel finishing mill, characterized in that: It includes the following: (1) According to the different Mo contents of various saw back steel grades, the residence time of the second heating section and the soaking section is determined. At the same time, in order to further improve the solid solution effect, the high temperature section furnace temperature is increased as much as possible within the performance allowable range; the higher the Mo content, the longer the residence time; (2) In order to effectively avoid the precipitation of M7C3 short rod-shaped alloy carbides in the finishing rolling area, the rough rolling outlet target temperature and the finishing rolling outlet target temperature of each steel grade are determined; (3) According to the influence of the AGC compensation coefficient of each stand on the rolling accuracy and looper fluctuation, the AGC compensation coefficient of each stand is determined according to the thickness of each steel grade when there is no looper fluctuation and the thickness accuracy fully meets the requirements. When the thickness of saw back steel TSJ31 is ≥4.0mm, the AGC compensation coefficients of stands F0~F6 are 1.2, 1.0, 1.0, 0.85, 0.8, 0.75 and 0.7 respectively; when the thickness of saw back steel TSJ31 is 3.0mm~4.0mm, the AGC compensation coefficients of stands F0~F6 are 1.17, 0.95, 0.9, 0.75, 0.7, 0.65 and 0.6 respectively; when the thickness of saw back steel TSJ31 is <3.0mm, the AGC compensation coefficients of stands F0~F6 are 0.97, 0.85, 0.8, 0.7, 0.6, 0.55 and 0.5 respectively; when the thickness of saw back steel TJB When the thickness of saw back steel TSJ33 is the same as that of saw back steel TSJ31, the AGC compensation coefficients of each frame are the same; when the thickness of saw back steel TSJ33 is ≥4.0mm, the AGC compensation coefficients of frames F0~F6 are 1.2, 1.1, 1.0, 0.88, 0.83, 0.77 and 0.72 respectively; when the thickness of saw back steel TSJ33 is 3.0mm~4.0mm, the AGC compensation coefficients of frames F0~F6 are 1. 19, 1.0, 0.9, 0.77, 0.72, 0.65 and 0.6; when the thickness of saw back steel TSJ33 is less than 3.0mm, the AGC compensation coefficients of frames F0~F6 are 0.99, 0.87, 0.83, 0.7, 0.6, 0.55 and 0.5 respectively; when the thickness of saw back steel TSJ35, saw back steel TJB34 and saw back steel TSJ33 are the same, the AGC compensation coefficients of each frame are the same.

2. A method for preventing the fluctuation of the inter-stand looper of the saw back steel finishing mill according to claim 1, characterized in that: The Mo content of the saw back steel TJB32 in (1) is 1.29%-1.34%, the second furnace temperature is 1270±20℃, the second furnace dwelling time is not less than 70 minutes, and the uniform heating furnace temperature is 1260±20℃. The uniform heating furnace dwelling time is not less than 65 minutes; the Mo content of the saw back steel TJB34 is 1.05%-1.20%, the second furnace temperature is 1290±20℃, the second furnace dwelling time is not less than 60 minutes, and the uniform heating furnace temperature is 1280±20℃. The uniform heating furnace dwelling time is not less than 50 minutes; the Mo content of the saw back steel TSJ31 is 1.28%-1.33%, the second furnace temperature is 1290±20℃, the second furnace dwelling time is not less than 68 minutes, and the uniform heating furnace temperature is 1280±20℃. The soaking time in the furnace shall not be less than 65 minutes; the Mo content of saw back steel TSJ33 is 1.25%-1.30%, the second furnace temperature is 1290±20℃, the second furnace time is not less than 65 minutes, and the soaking furnace temperature is 1280±20℃. The soaking time in the furnace shall not be less than 60 minutes; the Mo content of saw back steel TSJ35 is 1.05%-1.25%, the second furnace temperature is 1280±20℃, the second furnace time is not less than 60 minutes, and the soaking furnace temperature is 1270±20℃. The soaking time in the furnace shall not be less than 55 minutes.

3. A method for preventing the fluctuation of the inter-stand looper of the saw back steel finishing mill according to any one of claims 1 or 2, characterized in that: In (2), the rough rolling outlet temperature of the saw back steel TJB32 is 1110±20℃, and the finishing rolling outlet temperature is 920±20℃, the rough rolling outlet temperature of the saw back steel TJB34 is 1130±20℃, and the finishing rolling outlet temperature is 930±20℃, the rough rolling outlet temperature of the saw back steel TSJ31 is 1130±20℃, and the finishing rolling outlet temperature is 930±20℃, the rough rolling outlet temperature of the saw back steel TSJ33 is 1130±20℃, and the finishing rolling outlet temperature is 930±20℃, and the rough rolling outlet temperature of the saw back steel TSJ35 is 1120±20℃, and the finishing rolling outlet temperature is 930±20℃.

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