Cold rolling method and method for manufacturing cold-rolled steel sheet

Abstract

This invention provides a cold rolling method that suppresses the adhesion of scum to the work rolls, thereby suppressing the occurrence of roll defects while enabling cold rolling. [Solution] A cold rolling method using a coolant containing rolling oil, wherein the rolling oil concentration of the coolant is 1.0% by mass or more and 1.4% by mass or less.

Description

Technical Field

[0001] The present invention relates to a cold rolling method capable of suppressing the occurrence of roll defects in cold rolled steel sheets and a method for manufacturing cold rolled steel sheets.

Background Art

[0002] When a defect occurs in a work roll of a cold rolling mill that rolls cold rolled steel sheets or the like due to foreign matter being bitten in, the defect is transferred to the cold rolled steel sheet rolled by the work roll, resulting in roll defects. To prevent the occurrence of such roll defects, it is effective to prevent foreign matter from adhering to the work roll.

[0003] As a technique for preventing the occurrence of roll defects in cold rolled steel sheets due to foreign matter adhering to the work roll, Patent Document 1 discloses a cold rolling method for steel sheets in which a work roll is washed with water or the like for a tandem rolling mill composed of a plurality of rolling stands. According to Patent Document 1, it is said that scum, which is foreign matter adhering to the work roll, can be removed by washing the work roll with water or the like.

[0004] In particular, even if a defect occurs in the work roll of the upper stand and the defect is transferred to the steel sheet, there is a possibility that the defect will disappear when it is rolled in the subsequent stand. Therefore, mainly the work roll of the final stand of the tandem rolling mill is washed to peel off the adherents from the work roll surface to prevent the occurrence of roll defects. Also, there is a method of previously cutting the front end and the rear end of the coil before cold rolling with a shear to remove heges or the like that cause roll defects.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the work roll cleaning method disclosed in Patent Document 1, scum that causes roll defects cannot be removed from the work rolls without stopping the line and increasing the number of cleaning cycles. Therefore, this method of cleaning work rolls has the problem of reducing production efficiency.

[0007] Furthermore, even if the scum adhering to the work rolls can be removed by washing, the iron generated during cold rolling, which causes scum formation, cannot be removed from the coolant. Therefore, even if the scum adhering to the work rolls is removed by washing, if cold rolling continues, the scum will adhere to the work rolls again, so this does not provide a fundamental solution.

[0008] The present invention was made in view of the problems of the prior art, and its objective is to provide a cold rolling method that suppresses the adhesion of scum to the work rolls, rather than cleaning the scum adhering to the work rolls, thereby suppressing the occurrence of roll defects. Another objective of the present invention is to provide a method for manufacturing cold-rolled steel sheets using the cold rolling method. [Means for solving the problem]

[0009] The means to solve the above problems are as follows: (1) A cold rolling method using a coolant containing rolling oil, wherein the rolling oil concentration of the coolant is 1.0% by mass or more and 1.4% by mass or less. (2) The cold rolling method according to (1), wherein the iron content of the coolant is 50 ppm or more and 200 ppm or less. (3) A method for producing a cold-rolled steel sheet, comprising cold-rolling a rolled material using the cold-rolling method described in (1) or (2). [Effects of the Invention]

[0010] In the cold rolling method according to the present invention, a coolant with a rolling oil concentration of 1.0% by mass or more and 1.4% by mass or less is used, which allows iron to be dispersed in the coolant, increasing the amount of iron that can be incorporated into the coolant. As a result, the amount of aggregated iron that accumulates on the work rolls, which causes scum to form, is reduced, and it becomes possible to suppress the adhesion of scum to the work rolls and the occurrence of roll defects in the cold-rolled steel sheet caused by the scum getting caught between the work rolls and the material being rolled. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a schematic diagram showing an example of a cold rolling mill that can implement the cold rolling method according to this embodiment. [Figure 2] Figure 2 is a graph showing the relationship between the rolling oil concentration and iron content of the coolant. [Figure 3] Figure 3 is a schematic diagram of the iron removal device. [Figure 4] Figure 4 is a graph showing the relationship between the daily operating time of the iron removal device and the iron content of the coolant. [Figure 5] Figure 5 is a graph showing the number of roll defects that occurred in the example. [Modes for carrying out the invention]

[0012] The present invention will be described in detail below through embodiments of the present invention. The following embodiments are preferred examples of the present invention, and the present invention is not limited in any way by these embodiments. Figure 1 is a schematic diagram showing an example of a cold rolling mill 100 on which the cold rolling method according to this embodiment can be carried out.

[0013] The cold rolling equipment 100 includes a tandem rolling mill 10 with five rolling stands 12 (#1 to #5std), a dirty tank 20, a filter device 22, a clean tank 24, an iron removal device 26, and an oil concentration meter 28. Each rolling stand 12 has a pair of upper and lower work rolls 14 and a pair of upper and lower backup rolls 16. Cold-rolled steel sheets are produced by cold-rolling the material to be rolled (hot-rolled steel sheet) using this pair of upper and lower work rolls 14.

[0014] Coolant is supplied by being sprayed from the inlet and outlet sides of a pair of upper and lower work rolls 14 toward the surface of the work rolls 14 during rolling. Alternatively, the coolant may be sprayed toward the surface of the material being cold-rolled. An oil pan 18 is provided below the backup rolls 16. The coolant supplied to the work rolls 14 and the material being rolled is recovered by the oil pan 18. The recovered coolant is transferred through piping to a dirty tank 20.

[0015] The dirty tank 20 is a tank that stores the coolant recovered and transferred from the oil pan 18 of each rolling stand 12. The filter device 22 is a device that removes coarse foreign matter contained in the coolant. The clean tank 24 is a tank that stores the coolant from which coarse foreign matter has been removed by the filter device 22. The coolant stored in the dirty tank 20 has coarse foreign matter removed by the filter device and is then stored in the clean tank 24.

[0016] In the clean tank 24, the rolling oil concentration and iron content of the coolant are measured. The coolant stored in the clean tank 24 is then transferred again to each rolling stand 12 through piping, but a portion of it is transferred to the oil concentration meter 28, where the rolling oil concentration of the coolant is measured. The oil concentration meter 28 is, for example, an ultrasonic oil concentration meter, which measures the rolling oil concentration of the coolant by measuring the propagation speed of ultrasonic waves, which differ depending on the oil concentration of the coolant. The coolant that has been transferred to the oil concentration meter 28 and whose rolling oil concentration has been measured is then transferred to the dirty tank 20.

[0017] Further, the rolling oil concentration of the coolant may be measured by analyzing the coolant withdrawn from the clean tank 24. Specifically, by heating the withdrawn coolant to evaporate the moisture, the mass of the rolling oil and the mass of the moisture are determined, and the rolling oil concentration of the coolant is calculated from these two masses. Further, by adding an acid solution and brine to the withdrawn coolant and heating to separate the rolling oil, the volume of the rolling oil is determined, and the rolling oil concentration of the coolant is calculated from the volume. The rolling oil concentration of the coolant may be measured only by the oil concentration meter 28, or may be measured in combination with the measurement of the rolling oil concentration by extraction in addition to the measurement by the oil concentration meter 28.

[0018] There are mainly two purposes for using the coolant in cold rolling. The first is to cool the temperature of the steel sheet increased by the large amount of heat generated by plastic processing and friction by the coolant. The second is to impart lubricity between the work roll 14 and the steel sheet to reduce the rolling load during cold rolling and mitigate the heat generation due to friction.

[0019] In order to suppress the generation of roll marks by the work roll 14 while maintaining the above functions required for the coolant, the rolling oil concentration of the coolant in the clean tank 24 is adjusted using the measured rolling oil concentration of the coolant. In the cold rolling method according to the present embodiment, the rolling oil concentration of the coolant in the clean tank 24 is adjusted to be within the range of 1.0 mass% or more and 1.4 mass% or less.

[0020] For example, when the rolling oil concentration of the coolant in the clean tank 24 is less than 1.0 mass%, rolling oil is added to the coolant in the clean tank 2 to increase the rolling oil concentration. On the other hand, when the rolling oil concentration of the coolant in the clean tank 24 is higher than 1.4 mass%, moisture is added to the coolant in the clean tank 24 to lower the rolling oil concentration. In this way, the rolling oil concentration of the coolant in the clean tank 24 is adjusted to be within the range of 1.0 mass% or more and 1.4 mass% or less.

[0021] FIG. 2 is a graph showing the relationship between the rolling oil concentration and the iron content of the coolant. In FIG. 2, the horizontal axis represents the rolling oil concentration (mass %) of the coolant, and the vertical axis represents the iron content (ppm) of the coolant. By setting the rolling oil concentration of the coolant within the range of 1.0 mass % or more and 1.4 mass % or less, it is possible to suppress the occurrence of slip and steel sheet contamination during cold rolling, and to suppress the adhesion of scum, which causes roll defects, to the work roll 14. As a result, it becomes possible to cold roll the material to be rolled while suppressing the occurrence of roll defects on the cold-rolled steel sheet due to the scum being bitten between the work roll 14 and the steel sheet.

[0022] In particular, by setting the rolling oil concentration of the coolant to 1.0 mass % or more, the iron content contained in the coolant can be more dispersed in the coolant. If the iron content can be dispersed in the coolant, the amount of iron that can be taken into the coolant without aggregation increases. As a result, the amount of aggregated iron accumulated on the work roll 14 decreases, so that the adhesion of scum to the work roll 14 can be suppressed. That is, if the iron content of the coolant is the same, the higher the rolling oil concentration of the coolant, the more the iron content is dispersed in the coolant, so that the aggregated iron accumulates on the work roll 14, and scum adheres thereto, causing roll defects on the cold-rolled steel sheet can be suppressed. As a result, it becomes possible to stably manufacture a cold-rolled steel sheet by cold rolling the material to be rolled.

[0023] On the other hand, when the rolling oil concentration of the coolant is less than 1.0 mass %, the iron content contained in the coolant is not dispersed in the coolant, and the iron aggregated on the work roll 14 accumulates, scum adheres, and roll defects are likely to occur on the cold-rolled steel sheet. Further, when the rolling oil concentration of the coolant is higher than 1.4 mass %, the lubricity between the work roll 14 and the material to be rolled becomes too high, slip occurs during cold rolling, or contamination occurs on the cold-rolled steel sheet.

[0024] Next, we will explain how to measure the iron content of the coolant. The iron content of the coolant is measured by analyzing the coolant extracted from the clean tank 24. The iron content of the coolant can be measured using the following procedure 1 to 6.

[0025] 1. Filter the coolant extracted from the clean tank 24 using filter paper. 2. Add water to the filtered coolant to prepare a fixed amount of a mixed solution of water and rolling oil. 3. Prepare two test tubes and add the reagents to both test tubes. 4. Drop the filtered coolant into one of the test tubes to allow it to develop color. 5. Add the Fe standard solution dropwise to the other test tube until it matches the color of the first test tube. 6. Calculate the iron content of the coolant from the amount of Fe standard solution added.

[0026] The iron content of the coolant in the clean tank 24 is adjusted using the iron content of the coolant measured in the procedure described above. In the cold rolling method according to this embodiment, it is preferable to adjust the iron content of the coolant to be between 50 ppm and 200 ppm. This suppresses the occurrence of linear defects in the cold-rolled steel sheet and prevents the accumulation of aggregated iron on the work roll 14, which in turn prevents scum from adhering to it and causing roll defects in the cold-rolled steel sheet.

[0027] On the other hand, if the iron content of the coolant exceeds 200 ppm, the amount of aggregated iron increases and accumulates on the work roll 14, and scum adheres to it, causing roll defects in the cold-rolled steel sheet, which is undesirable. Also, the iron in the coolant is adsorbed by the oil droplets in the coolant. As a result, the particle size of the oil droplets in the coolant increases. Larger oil droplets reduce the coefficient of friction between the work roll 14 and the rolled material, thus increasing the lubricity of the coolant. For this reason, if the iron content of the coolant is less than 50 ppm, the particle size of the oil droplets in the coolant decreases, reducing the lubricity of the coolant and making it easier for linear defects to occur at the edges of the cold-rolled steel sheet, which is undesirable.

[0028] Cold rolling increases the iron content of the coolant. Therefore, the iron content of the coolant is adjusted by removing the iron from the coolant that has exceeded 200 ppm using the iron removal device 26.

[0029] Figure 3 is a schematic diagram of the iron removal device 26. The iron removal device 26 consists of a cylindrical upper ball piece 40 and lower ball piece 41, which are magnetic pole irons and have multiple holes 42, a magnetic filter 46, and a pair of electromagnetic coils 44 provided on the left and right sides of the magnetic filter 46. The magnetic filter 46 is fixed by being sandwiched from above and below by the upper ball piece 40 and the lower ball piece 41.

[0030] The coolant before cleaning is supplied from below the iron removal device 26. The coolant before cleaning is supplied through multiple holes 42 in the lower ball piece 41 and the iron is removed as it passes through the magnetic filter 46. The cleaned coolant, from which the iron has been removed, is discharged through multiple holes 42 in the upper ball piece 40. In this way, the iron in the coolant is removed by the iron removal device 26.

[0031] Figure 4 is a graph showing the relationship between the daily operating time of the iron removal device 26 and the iron content of the coolant. In Figure 4, the horizontal axis represents the daily operating time of the iron removal device 26 (minutes / day), and the vertical axis represents the iron content of the coolant (ppm). The graph shown in Figure 4 is the result of removing iron from the coolant using a high-gradient electromagnetic filter (DEM) manufactured by Daido Steel Co., Ltd., which has the same configuration as the iron removal device 26 shown in Figure 3.

[0032] As shown in Figure 4, increasing the operating time of the iron removal device 26 reduces the iron content in the coolant. Therefore, the iron content of the coolant is adjusted by adjusting the daily operating time of the iron removal device 26 so that the iron content of the coolant becomes 125 ppm, which is the median value within the target range. In other words, the operating time of the iron removal device 26 can be calculated by subtracting the daily operating time corresponding to the measured iron content of the coolant from the daily operating time at which the iron content of the coolant becomes 125 ppm.

[0033] In this way, the rolling oil concentration and iron content of the coolant stored in the clean tank 24 are adjusted. The coolant with adjusted rolling oil concentration and iron content is then transferred again through piping to each rolling stand 12 and supplied by being sprayed onto the work rolls 14 and the material to be rolled at each rolling stand. The supplied coolant is then collected again by the oil pan 18 and transferred to the dirty tank 20. In this way, the coolant circulates within the cold rolling mill 100 while its rolling oil concentration and iron content are adjusted in the clean tank 24.

[0034] As described above, the cold rolling method according to this embodiment uses a coolant whose rolling oil concentration is adjusted to within the range of 1.0% by mass or more and 1.4% by mass or less. This suppresses the occurrence of slip and steel sheet contamination during cold rolling, and also suppresses the adhesion of scum, which causes roll defects, to the work roll 14. As a result, it becomes possible to manufacture cold-rolled steel sheets by cold-rolling the material to be rolled while suppressing the occurrence of roll defects in the cold-rolled steel sheets. [Examples]

[0035] Next, we will describe an example in which the occurrence of roll defects in cold-rolled steel sheets was confirmed using the cold rolling equipment 100 shown in Figure 1. In this example, the occurrence of roll defects was confirmed for cold-rolled steel sheets manufactured in Comparative Example, Invention Example 1, and Invention Example 2.

[0036] The comparative example is a manufacturing example in which cold rolling was performed using a coolant with a rolling oil concentration of less than 1.0% by mass and an iron content that was not controlled. Invention Example 1 is a manufacturing example in which cold rolling was performed using a coolant with a rolling oil concentration in the range of 1.0% by mass or more and 1.4% by mass or less and an iron content that was not controlled. Invention Example 2 is a manufacturing example in which cold rolling was performed using a coolant with a rolling oil concentration in the range of 1.0% by mass or more and 1.4% by mass or less and an iron content in the range of 50 ppm or more and 200 ppm or less.

[0037] Figure 5 is a graph showing the number of roll defects that occurred in the examples. In Figure 5, the horizontal axis represents the implementation period for the Comparative Example, Invention Example 1, and Invention Example 2, and the vertical axis represents the number of roll defects that occurred per month (times / month).

[0038] As shown in Figure 5, the number of roll defects per month during the comparative example's implementation period was higher than the number of defects during the implementation periods of Invention Examples 1 and 2. The average number of roll defects per month during the comparative example's implementation period was 41.8 times / month. In contrast, the average number of roll defects per month during Invention Example 1's implementation period was 25.0 times / month. From these results, it was confirmed that Invention Example 1 can manufacture cold-rolled steel sheets while suppressing the occurrence of roll defects more effectively than the comparative example.

[0039] The average number of roll defects per month during the implementation period of Invention Example 2 was 18.0 times / month, which is fewer than the number of roll defects per month during the implementation period of Invention Example 1. From these results, it was confirmed that Invention Example 2 can manufacture cold-rolled steel sheets while suppressing the occurrence of roll defects more effectively than Invention Example 1.

[0040] In Invention Example 1, the rolling oil concentration in the coolant was increased to a concentration that prevented the rolled material from slipping, thereby dispersing the iron contained in the coolant. This suppressed the aggregation of iron in the coolant, and prevented the accumulation of aggregated iron on the work rolls, which would lead to the formation of scum. As a result, it was confirmed that in Invention Example 1, cold-rolled steel sheets could be produced by cold-rolling the rolled material while suppressing the occurrence of roll defects compared to the comparative example.

[0041] In Invention Example 2, in addition to improving the rolling oil concentration of the coolant, the iron content of the coolant was controlled to a range in which linear defects do not occur in the cold-rolled steel sheet and the occurrence of roll defects is suppressed. As a result, the aggregation of iron in the coolant was further suppressed, and the accumulation of aggregated iron on the work rolls and the generation of scum were further suppressed. As a result, it was confirmed that in Invention Example 2, cold-rolled steel sheets can be manufactured by cold-rolling the material to be rolled while suppressing the occurrence of roll defects more effectively than in Invention Example 1. [Explanation of Symbols]

[0042] 10 Tandem Rolling Mill 12 Rolling Stand 14 Work Roles 16 Backup Roles 18 Oil pan 20 Dirty Tank 22 Filter device 24 Clean Tanks 26 Iron removal device 28 Oil concentration meter 40 Upper ball piece 41 Lower ball piece 42 holes 44 Electromagnetic coil 46 Magnetic filter 100 Cold rolling equipment

Claims

1. A cold rolling method using a coolant containing rolling oil, A cold rolling method wherein the rolling oil concentration of the coolant is 1.0% by mass or more and 1.4% by mass or less.

2. The cold rolling method according to claim 1, wherein the iron content of the coolant is 50 ppm or more and 200 ppm or less.

3. A method for producing a cold-rolled steel sheet, comprising cold-rolling a rolled material using the cold-rolling method described in claim 1 or claim 2.

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