JP2000061502A - Method for cold-rolling austenitic stainless steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the generation of edge cracks, chatter marks and breakage of a sheet and to obtain a steel sheet excellent in high glossiness and surface properties by executing rolling by controlling the temp. of the steel sheet so that the temp. of the steel sheet on the inlet side of the 2nd pass of the rolling becomes not less than a specified temp. SOLUTION: The generation of the edge cracks and chatter marks is closely related to the amount M (volume %) of martensite and temp. of the steel sheet on the inlet side of the 2nd pass and prevented by making the temp. of the steel sheet on the inlet side of the 2nd pass as not less than T deg.C shown by the undermentioned formula: T( deg.C) =1.68×M+63. When the temp. of the steel sheet is raised to not less than T deg.C on the inlet side of the 2nd pass, because the temp. does not become less than T deg.C by working heat on and after the 3rd pass, it is not necessary to specially heat the steel sheet. More-over, though the temp. of the steel sheet on the inlet side of the 2nd pass is restricted to not less than T deg.C, the upper limit is specially not restricted. But, when the temp. is too high, burning is generated and there is the possibility of resulting in surface defects, so it is desirable to take the temp. of the steel sheet as <=80 deg.C.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for cold rolling an austenitic stainless steel sheet, and more particularly, to prevent occurrence of edge cracks and chatter marks during rolling. The present invention relates to a cold-rolling method for austenitic stainless steel sheets that can be performed efficiently and obtain good steel sheet surface quality. [0002] Generally, in cold rolling of a stainless steel sheet, a rolled product is required to have a high surface gloss. Since stainless steel has a high deformation resistance and is easily work-hardened, a small diameter work roll, a Sendzimir mill, has been used, which has a small amount of rolling oil introduced into a roll bite and can obtain a high rolling pressure. In this rolling, a water-insoluble rolling oil (hereinafter, referred to as neat oil) using a low-viscosity mineral oil as a base oil or a rolling oil obtained by emulsifying the same is used. However, the Sendzimir mill has two rolling rolls.
The rolling mill has a complicated structure and the roll diameter is 5
Since the diameter is as small as 0 to 80 mm, there is a problem that the rolling speed is restricted and productivity is low. [0004] In recent years, in order to improve productivity, high gloss rolling using a tandem mill having a large roll diameter has been attempted. [0005] Recently, a cluster mill (roll diameter: 80 to 120 mm) and a six-stage UC mill provided with 12-stage rolling rolls having a simple structure and a good shape control function have been developed. High speed rolling has been attempted. When a mineral oil-based neat oil is used in these rollings, there are problems such as lack of cooling ability and lubricity, generation of seizure flaws, and ignition of the rolling oil at the time of a breakage accident. [0006] When a highly lubricating rolling oil is used, the amount of oil introduced between the rolling roll and the material to be rolled (roll bite) increases, which causes a problem of a decrease in glossiness. On the other hand, when the emulsion oil is used, there is no danger of ignition, but the steel sheet being rolled is easily cooled by the emulsion oil. [0007] In the rolling of an austenitic stainless steel plate, work-induced martensite is generated by rolling and work-hardened, which makes the rolling more difficult. It is known that this work-induced martensite (hereinafter simply referred to as martensite) is more likely to occur at lower temperatures. Japanese Patent Application Laid-Open No. 64-21013 discloses a method of rolling by increasing the temperature of a steel sheet in an initial pass to 50 to 250 ° C., and Japanese Patent Publication No. 6-6206 discloses a method of rolling a steel sheet before rolling. It shows a method of uniformly increasing the temperature of a steel sheet to about 100 ° C. with time. However, in these methods, raising the temperature of the steel sheet is effective in suppressing the formation of work-induced martensite, but has a problem such as occurrence of seizure in an intermediate pass. . [0010] Further, in the rolling of austenitic stainless steel, there is a problem that ear cracks are easily generated and chatter marks are easily generated as compared with ferritic stainless steel. Chattering is a phenomenon in which a striped pattern is generated in the width direction on the surface of a rolled material. There are various theories such as a cause caused by a lubricating state and a mechanical cause, but it is not clear yet. Either of these cases occurred more remarkably in emulsion rolling than in neat oil rolling, and tended to occur more easily when the plate thickness was reduced. [0011] In particular, since the rolling reduction is large and the rolling rate is increased, the problem occurs remarkably. Therefore, the only countermeasure is light rolling or low-speed rolling. It was necessary to reduce the rolling efficiency and further reduce the rolling efficiency at a low rolling reduction, which significantly reduced the rolling efficiency. In particular, when an emulsion oil is used in a reverse type rolling mill, excessive cooling occurs in the acceleration rolling section and the deceleration rolling section in each rolling pass, and ear cracks and chattering are remarkably generated. In winter, when the outside air temperature was low, the material temperature of the initial pass was low, which was further promoted. The above-mentioned accelerated rolling section is a portion from the start of rolling to a steady rolling speed, and the slow rolling is a portion from the steady rolling speed to the stop of rolling, and the rolling speed is lower than the steady portion. When chattering occurs, the rolling becomes unstable, and a striped pattern of about twice the sheet thickness in the sheet width direction, that is, a so-called chatter mark, is generated on the rolled material surface to deteriorate the surface quality. There was also. As described above, in the cold rolling of austenitic stainless steel, cracks and chatter marks are generated in addition to maintaining the glossiness. Therefore, in a reverse type rolling mill, the speed is higher than 300 m / min. The current situation is that high efficiency rolling cannot be performed. SUMMARY OF THE INVENTION An object of the present invention is to provide an austenitic stainless steel using an oil-in-water emulsion oil in reverse rolling, even if high-speed rolling at a speed of 300 m / min or more is performed. An object of the present invention is to provide a cold rolling method for a stainless steel sheet which can prevent chatter marks and sheet breakage and can obtain a steel sheet having high gloss and excellent surface properties. The gist of the present invention relating to a method of cold rolling a stainless steel plate is as follows. "When an austenitic stainless steel sheet is cold-rolled by a reverse rolling mill using an oil-in-water emulsion oil, the temperature of the steel sheet on the entry side of the second pass of the rolling is expressed by the following equation (1). Cold rolling method for austenitic stainless steel sheet, characterized in that the steel sheet temperature is controlled so as to be not lower than ℃ C. T (° C.) = 1.68 × M + 63 (1) Here, M is the amount of martensite (volume%) at the second pass entry side. ”The present inventors have determined that in order to prevent ear cracks and chatter marks generated during cold rolling of an austenitic stainless steel sheet, As a result of repeated examinations of reverse rolling using emulsion oil, the following findings were obtained, and the present invention was completed. (1) Chatter marks are not caused by the vibration phenomenon of a general rolling mill but are related to the internal quality of a steel sheet. Chatter marks and ear cracks are basically the same phenomenon. . (2) Ear cracks are inclined 45 to 50 ° from the steel sheet surface at the steel sheet edge, and are continuously generated in a saw-tooth shape, and are a phenomenon derived from periodic shear deformation. (3) Chatter marks are traces in which the shearing deformation that has caused ear cracks has spread toward the center of the width of the steel sheet and has reached the steel sheet surface. (4) The amount of martensite generated in a steel plate in which ear cracks and chatter marks have occurred is greater than that in the case of a steel plate in which ear cracks and chatter marks do not occur. (5) Periodic shear deformation that causes chatter marks is such that the amount of martensite generation exceeds 10% and 1
It easily occurs when the rolling reduction per pass is 15% or more, the total rolling reduction is 50% or more, and the rolling speed is about 300 m / min or more.
In general, it is likely to occur in winter when the outside temperature is low. (6) Martensite occurs remarkably in the first to third passes and is affected by the temperature of the steel sheet on the entry side of those passes. However, the generation of martensite in the first pass is relatively small, and may not occur even when the temperature of the steel sheet on the first pass side is low. (7) The occurrence of ear cracks and chatter marks is closely related to the amount of martensite M (volume%) on the entry side of the second pass and the steel sheet temperature. It can be prevented by raising the temperature to T ° C. or higher as shown in the following formula. T (° C.) = 1.68 × M + 63 (8) If the temperature of the steel sheet is raised to T ° C. or higher at the second pass entry side, the temperature does not become lower than T ° C. due to the processing heat after the third pass. There is no particular need to heat the steel sheet. Figure 1 shows the SU
It is a plane | planar external view of the chatter mark produced | generated by the cold rolling of S304. FIG. 2 shows a state in which a cross section in the rolling direction of the chatter mark generating portion of the SUS304 cold-rolled steel sheet is observed with a microscope. As shown in FIG. 2, the shear deformation occurs continuously at an angle of 45 to 50 ° in the thickness direction from the surface of the steel sheet, and a chatter mark is formed when the shear deformation reaches the surface of the steel sheet. It has occurred. FIG. 3 shows that the steel sheet of SUS304 has a total rolling reduction of 5%.
It is an observation figure of the edge crack part which occurred at the edge of the steel plate at the time of rolling at 9%. FIG. 3A is a side view of an edge portion of the steel sheet,
FIG. 3B is a plan view of the lower surface of FIG. From FIG. 3A, it can be seen that chatter marks are generated where the shear deformation reaches the steel sheet surface. Embodiments of the cold rolling method of the present invention will be described below. The steel sheet temperature on the entry side of the second pass is defined as T (° C.)
The reason for setting = 1.68 × M + 63 or more is as follows. First, the temperature of the steel sheet on the entry side of the second pass is controlled when the amount of martensite generated in the first pass is 10%.
%, Which is relatively small, and does not pose a problem if the generation amount is controlled in the second and subsequent passes. It is also conceivable to heat the steel sheet on the entry side of the first pass, but if emulsion oil is spray-supplied to the heated steel sheet, uneven adhesion of oil to the steel sheet is likely to occur, and one type of seizure during rolling There is a risk that certain galling and uneven gloss may occur. Further, naturally, before the first pass, no work-induced martensite has been generated, so that it cannot be measured and its generation amount cannot be predicted. If the temperature of the steel sheet is controlled to T.degree. C. or more before the second pass, it becomes T.degree. C. or more by the processing heat after the third pass. Next, the temperature of the steel sheet on the entry side of the second pass is defined as T
The reason why the temperature is set to not lower than C is that the formation of martensite cannot be suppressed at a temperature lower than T ° C. T (° C.) = 1.68 × M + 63 is an equation obtained by various experiments. FIG. 4 shows an example of the test result obtained from the above equation, and shows the relationship between the temperature on the second pass side, the amount of martensite, and the chatter mark. In this test, a SUS304 hot-rolled steel sheet (annealed and pickled material) was reverse-rolled using emulsion oil, and the rolling speed was set to 50, 1 in the first pass.
Rolling was performed at a rate of 00, 150, 200, 250, and 300 m / min, and the amount of martensite (measured value by a ferrite scope: M%) and the steel sheet temperature (T) were measured on the entry side of the second pass. The rolling conditions were as follows. (Pass No.) Rolling speed (m / min) Reduction rate (total reduction rate)% 1 50,100,150,200,250,300 18 (18) 2 50-300 15 (30.3) 3 300 15 (40.8) 4 300 15 (49.6) 5 300-600 15 (57.2) In addition, by changing the rolling speed in the first pass, the temperature of the steel sheet during rolling changes due to heat generated by processing, frictional heat, cooling by emulsion oil, etc., and the amount of martensite Can be adjusted. Also, 2
The temperature of the steel sheet at the pass was varied according to the change of the rolling speed (entry steel sheet speed), the supply amount of the emulsion oil, and the external heating. After cold rolling under the above conditions, the occurrence of cracks and chatter marks on the steel sheet was examined. As a result, the occurrence of chatter marks of about 3 mm from the edge has a distribution as shown by the triangle in FIG. 4 and is linear. This straight line is T = 1.68 × M +
63. Therefore, in order to prevent chatter marks, when the temperature of the steel sheet at the entry side of the second pass is lower than T, the steel sheet is raised to at least T ° C. by an external heating means to thereby prevent chatter marks exceeding 3 mm from the edge. Can be prevented from occurring. The temperature of the steel sheet on the entry side of the second pass is set to T ° C.
Although the above is controlled, the upper limit is not particularly limited,
If the temperature is too high, seizure may occur and surface defects may occur. If the temperature of the entering steel sheet in the second pass is equal to or higher than T ° C., even if the steel sheet temperature in the first pass is lower than 15 ° C. in winter when the outside air temperature is low, the total rolling reduction exceeds 50%. The amount of martensite does not exceed 10%, and ear cracks and chatter marks are reduced to a level that does not pose a problem in practical use. In order to increase the temperature of the material to be rolled in the second pass, it is possible to utilize the heat generated during processing by strongly reducing the first pass. In winter, when the temperature of the steel sheet on the first pass is low, a rolling reduction of 30% or more is required, which exceeds the rolling capacity of the rolling mill. Is more than T ° C. As means for heating the steel sheet, high-temperature steam is preferably used. The first reason is that it is relatively easy to use industrially and is economical, and secondly, in other methods using radiant heat or induction heating, the oil content is remarkably deteriorated by heating, and the lubricity is reduced and the glossiness is reduced. Causes decline. Furthermore, in the case of high-temperature steam, oil and abrasion powder on the steel plate can be blown off by a mechanical surface cleaning effect. This is because the hot-rolled stainless steel plate subjected to cold rolling has a surface roughness of R due to an attack by shot or acid at the time of descaling.
a is as coarse as about 3 μm, and abrasion powder is easily generated in an initial pass during cold rolling. Abrasion powder tends to remain on the steel plate when rolled with emulsion oil, plate stains and uneven gloss,
Furthermore, it causes seizure flaws and indentation flaws. On the other hand, in the case of high-temperature steam, oiliness and abrasion powder on the steel plate can be blown off, so that the glossiness is rather improved. It is also conceivable to preheat the temperature of the steel sheet from the first pass entry side. However, when the oil content of the emulsion oil adheres to the steel sheet, if the temperature is high, the adhesion tends to be uneven, and one type of seizure may occur. Certain galling and uneven gloss are likely to occur. The heating of the steel sheet before rolling in the second pass is performed at a rolling speed of 50 to 300 m / sec in accelerated rolling or decelerated rolling.
It is preferred to do this in the minute part. In the rolling in the acceleration / deceleration section, the rolling speed is lower than the speed of the steady rolling. Therefore, since the steel sheet is easily cooled by the emulsion oil and the temperature is lowered,
It is preferable to heat the steel sheet to T ° C or more in this portion. The temperature control of the steel sheet may be performed before rolling in the second pass, and the same temperature control may be performed on the entry side of the third pass, or may be performed in the latter half of the rolling. The temperature control in the latter half of the rolling is preferably performed to prevent the temperature of the steel sheet from excessively rising due to the processing heat. The high-temperature steam used in the present invention will be described. High-temperature steam here is not special,
The pressure is about 2-7 kg / cm ² and the temperature is 100-15
0 ° C. Also, in terms of flow rate, per plate width of 100 mm,
It may be about 30 to 200 liter / min. The nozzle is desirably installed so as to be opposed to the surface of the steel sheet in the direction of travel rather than perpendicularly, and the distance from the steel sheet is preferably as short as possible in order to raise the temperature and remove oil and wear powder. Furthermore, it is more efficient to arrange a plurality of rows of nozzles. As a method for measuring the amount of martensite, a ferrite scope (product name: MP3C, FE8e3) manufactured by FISCHER, Germany may be used simply. The measurement principle is DIN59981, ASTMB499, IS
This is a magnetic induction method defined by O2178 or the like, in which a low-frequency AC excitation current is generated from a measurement probe, and a low-frequency magnetic field is generated on the surface of the measurement sample (1.5 to 2.0 mm in the thickness direction). The magnetic flux density is measured by a measurement probe (pickup coil), and the measurement signal is converted into a ferrite amount or a martensite amount based on the probe characteristics and an appropriate mathematical conversion model. The signal of the amount of martensite and the measurement signal of the steel sheet temperature are sent to an arithmetic circuit, and a target set steel sheet temperature is calculated from the relationship between the two by the above-mentioned equation (1). The method of the present invention can be realized by automatically or manually controlling the opening and closing of the flow valve of the high-temperature steam by a signal. The amount of martensite: M% is basically measured at the same timing as the measurement of the steel sheet temperature T on the entry side of the second pass. However, if the position of the steel sheet in the rolling direction can be confirmed, 1%. This can be done before winding the pass. According to this method, since T is determined in advance, there is an advantage that the operation instruction for opening and closing the flow valve of the high-temperature steam during the second pass rolling can be executed without delay of the calculation time. Further, the composition and properties of the rolling oil of the emulsion oil used in the present invention will be described. The composition and properties of the rolling oil used in the present invention are not particularly limited, but the following can be used. The composition of the stock solution of the emulsion oil is obtained by adding a low-viscosity mineral oil, a synthetic hydrocarbon or the like to a base oil and adding a synthetic ester thereto. For example, a monoester of a fatty acid having 10 to 18 carbon atoms such as lauric acid and palmitic acid and an alcohol having 1 to 18 carbon atoms. Further, monoesters, diesters and triesters of the above-mentioned fatty acids with polyhydric alcohols such as trimethylolpropane. Further examples include diesters of dibasic acids such as adipic acid and the above-mentioned alcohols. The amount of the synthetic ester is preferably adjusted so that the saponification value of the whole emulsion oil crude oil is in the range of 50 to 120 mgKOH / g, and the viscosity of the stock solution is 7 cSt to 1 at 40 ° C.
5 cSt. The concentration when dispersing in water is 5
-20% is preferable, and the average diameter of the oil particles is 1.5-5 μm.
Is preferable. The rolling oil temperature during use is 4
0-50 ° C is preferred. EXAMPLES The effects of the present invention will be described in more detail with reference to examples. The material is SKD11, the diameter is 100 mm,
Work roll with surface roughness Ra 0.13 μm and diameter 3
The following austenitic stainless steel and SUS304 hot-rolled steel sheet (annealed and pickled) were rolled by a 4Hi reverse rolling mill equipped with a 50 mm backup roll. Thickness: 3.2 mm, width: 100 mm, length 300 m FIG. 5 is a diagram showing the arrangement of rolling devices and measuring devices used in the examples. FIG. 3 is a diagram showing a state where the steel plate 9 unwound from the coil 2 by the 4Hi reverse rolling mill 1 is rolled in the second pass. The temperature of the steel sheet before rolling is measured by the thermometer 3 a, and the measured value is sent to the set steel sheet temperature calculator 7 via the temperature signal converter 5. At the same time, the martensite amount of the steel sheet is measured by the martensite amount measuring device 4, and the measured value is also sent to the set steel plate temperature calculator 7 via the martensite amount signal converter 6.
In order to heat the steel sheet temperature to the set temperature obtained by this computer, a signal from the computer is sent to the steam valve indicator 12 to control the steam amount changing valve 8, and the steam injected from the nozzle 11 Heating is performed so that the temperature is equal to or higher than T ° C. Immediately before rolling, the temperature of the steel sheet is measured again by the thermometer 3b. If the measured value is sent to the set steel sheet temperature calculator 7 via the temperature signal converter 5 and the temperature does not reach T ° C. or higher, Will be corrected. The emulsion oil is supplied to the surface of the work roll and the surface of the steel sheet from rolling oil spray nozzles 10 provided above and below the steel sheet on the side of the rolling mill. Table 1 shows the rolling conditions. For the rolling oil, neat oil was used for comparison with the two emulsion oils shown in Table 2. [Table 1] [Table 2] For heating the steel sheet, the temperature shown in the footnote of Table 3
The test was performed using three types of high-temperature steam and high-frequency induction coils having different pressures. The temperature of the steel sheet was measured by a contact-type thermometer (manufactured by Anritsu Keiki Co., Ltd .: long life type 211K) at the entry side of the first and second passes, and the amount of martensite was measured by a Fischer (Germany). A ferrite scope (product name: MP3C) manufactured by FISCHER) was used. Tables 3 to 5 show the steel sheet temperature before the first pass rolling and the second pass rolling, the amount of martensite (M) and the target steel sheet heating temperature before the second pass rolling, and the steel sheet temperature after the heating. [Table 3] [Table 4] [Table 5] In rolling Nos. 3, 6, and 9, the temperature was controlled even in the second pass. The steam flow rate in eight passes of rolling No. 6 was set to 100 liter / min irrespective of the rolling speed. The rolled steel sheet was visually inspected for uneven gloss and seizure. Further, the width of the chatter mark generated from the edge in the rolling speed range where the degree of chatter mark generation was the strongest, and the amount of martensite by a ferrite meter were measured. Further, the presence or absence of a sheet break and the occurrence path in the rolling up to the final 7 passes are determined based on the following criteria based on Tables 3 to
5 is shown. In addition, when the plate was broken by the last pass, the measurement was performed in the break pass. (Degree of edge crack depth) ：: No occurrence, □: Minor occurrence (within allowable range),
▲: remarkable occurrence (defective) (chatter mark width) ○: unrecognizable, □: slight (1 to 3 mm), no quality problem, ▲: remarkable (5 mm or more), inappropriate quality (degree of uneven gloss) ○: confirmation Not possible, □: slight and no problem in quality,
▲: remarkable and unsuitable quality (degree of seizure) ○: no occurrence, □: slight occurrence (within allowable range),
：: remarkable occurrence (defective) (presence of sheet break) ○: no occurrence, ▲: occurrence As is clear from the examples of the present invention, the temperature of the steel sheet on the entry side of the second pass is determined by the equation (1). If it is lower than T,
When the steel sheet is heated to T ° C. or higher and rolled in the second pass, the amount of martensite is reduced, and chatter marks are not generated or are extremely light. According to the examples of the present invention, even in the winter season when the outside air temperature is particularly low, even when rolling is performed at a high rolling speed of 300 m / min or more by a reverse rolling mill, ear cracks are generated and chatter marks are formed. It is possible to perform high-efficiency rolling without sheet breakage, and to obtain a steel sheet having high gloss and high surface properties.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan external view of a surface defect called chatter mark generated on the surface of a SUS304 cold-rolled steel sheet. FIG. 2 is a view showing a longitudinal section of a SUS304 cold-rolled steel sheet in which chatter marks are generated. FIG. 3 is a diagram showing edge cracks and chatter marks generated at the edge of a SUS304 cold-rolled steel sheet. FIG. 4 is a graph showing the relationship between the entering steel sheet temperature in the second pass, the martensite, and the degree of chatter marks generated in a rolling pass in which the total draft exceeds 50%. FIG. 5 is a diagram showing a measurement of a steel sheet temperature, a martensite amount, and a heating state of the steel sheet by steam. [Description of Signs] 1 Rolling machine 8 Steam valve opening / closing indicator 2 Coil 10 Rolling oil spray nozzle 3a, 3b Thermometer 11 Steam nozzle 4 Martensite amount measuring machine 5 Temperature signal converter 6 Martensite amount signal converter 7 Set steel plate temperature Arithmetic unit

Claims (1)

Claims 1. When cold rolling an austenitic stainless steel sheet by a reverse rolling mill using an oil-in-water emulsion oil, the temperature of the steel sheet on the entry side of the second pass of the rolling is calculated by the following equation. A cold rolling method for an austenitic stainless steel sheet, wherein the steel sheet temperature is controlled so as to be equal to or higher than the temperature T ° C shown in (1). T (° C.) = 1.68 × M + 63 (1) where M is the amount of martensite (volume%) at the second pass entry side.