Method for the controlled rolling of cold rolled steel sheet

Abstract

Provided is a method for performing temper rolling of a cold-rolled steel sheet without causing defects such as a jump in a wet-type temper rolling process, which can correspond to changes in concentration or load of a temper rolling liquid and can be applied to both single-stand and multi-stand rolling mills. In a method for performing wet-type temper rolling with respect to a cold-rolled steel sheet after annealing, a tension per unit cross-sectional area of the steel sheet in the temper rolling process is set based on a carbon content C (mass %) of the cold-rolled steel sheet. 2 ).

Description

Technical Field

[0001] The present invention relates to a method for temper rolling a cold-rolled steel sheet, and in particular to a method for setting tension during wet temper rolling. Background Art

[0002] Cold-rolled steel sheets are produced by rolling hot-rolled steel sheets to the desired thickness at room temperature. Because the steel sheet undergoes work hardening during this process, annealing is sometimes necessary to soften it. Tempering rolling is then performed to eliminate yield elongation, correct shape, adjust surface roughness, and adjust the material quality.

[0003] Temper rolling involves wet temper rolling using a tempering fluid and dry temper rolling without a tempering fluid. In the past, dry temper rolling was the mainstream method, particularly in the field of can steel sheets, due to its aesthetic appeal. However, with the increasing demand for different steel sheet specifications, wet temper rolling has become a popular method, enabling the production of various materials by controlling elongation through adjustment of the tempering fluid's properties.

[0004] One of the purposes of temper rolling is to adjust the surface roughness of the steel plate by transferring the roughness of the work rolls to the steel plate. In order to stably adjust the surface roughness of the steel plate, it is necessary to determine the work roll roughness and rolling load independently for the desired surface roughness of the steel plate. On the other hand, material adjustment is implemented by controlling parameters such as the elongation of temper rolling to specified values. As mentioned above, the surface roughness of the work rolls and the rolling load are determined independently according to the surface roughness of the steel plate. Here, the elongation is defined as the ratio of the difference between the input side plate thickness and the output side plate thickness relative to the plate thickness on the output side. Therefore, the elongation is generally adjusted by controlling the plate thickness on the input and output sides by controlling the tension of the steel plate before and after the rolling mill.

[0005] However, wet temper rolling suffers from a phenomenon known as jumping. Jumping refers to abnormal elongation, where the elongation fluctuates unsteadily, particularly when the elongation drops below 5%. If this jumping occurs, the thickness and material properties of the steel plate can fluctuate significantly.

[0006] As a method for preventing this jump, Patent Document 1 discloses a method of adjusting the concentration of the temper rolling solution according to the material and elongation. Furthermore, Patent Document 2 discloses a method of combining wet temper rolling and dry temper rolling in a rolling mill composed of multiple stands.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Literature 1: Japanese Patent Application Laid-Open No. 2016-150353

[0010] Patent Literature 2: Japanese Patent Application Laid-Open No. 2018-015801 SUMMARY

[0011] PROBLEMS TO BE SOLVED BY THE INVENTION

[0012] However, the method described in Patent Literature 1, which prevents the occurrence of the jump by adjusting the concentration of the temper rolling liquid, has a problem that the proportion of the roughness pattern transferred from the work roll to the steel sheet changes due to the difference in the concentration, and the roughness and appearance of the steel sheet change, so that it is difficult to reliably prevent only the occurrence of the jump. Also, the method described in Patent Literature 2, which combines the wet-type temper rolling and the dry-type temper rolling, has a problem that it cannot be used in a single stand rolling mill.

[0013] An object of the present application is to provide a temper rolling method of a cold-rolled steel sheet, which can correspond to the change in the concentration of a temper rolling liquid or the change in a rolling load in wet-type temper rolling, and which can be applied to both a single stand rolling mill and a multiple stand rolling mill without causing a jump.

[0014] TECHNICAL SOLUTION FOR SOLVING THE PROBLEMS

[0015] As described above, the surface roughness of the work roll and the rolling load are individually determined depending on the desired surface roughness of the steel sheet. In addition to performing the temper rolling at a prescribed elongation, it is necessary to appropriately set the tension. In the case where the tension is set too large, abnormal elongation, i.e., the jump, occurs. Also, in the case where the tension is set too small, insufficient elongation or a shape defect called a bellows shape occurs.

[0016] The inventors of the present application earnestly investigated the relationship between the carbon content and the jump, focusing on the fact that the mechanical properties of the steel sheet are strongly affected by the carbon content of the steel sheet, in order to solve the above-described problems. As a result, it was found that the tension in the temper rolling is related to the carbon content of the steel sheet, and finally a temper rolling method of a cold-rolled steel sheet that solves the above-described problems was obtained.

[0017] The gist of the present application is as follows.

[0018] [1] A temper rolling method of a cold-rolled steel sheet, in a temper rolling method of wet-type temper rolling performed with respect to a cold-rolled steel sheet after annealing, a tension T (kgf / mm2) at the time of the temper rolling is set based on a carbon content C (mass%) of the cold-rolled steel sheet. 2 ).

[0019] [2] According to the tempering rolling method of cold-rolled steel sheet described in [1], the tension T is set based on the carbon content C of the cold-rolled steel sheet and further based on the thickness t (mm) of the cold-rolled steel sheet, the load w (tonf / mm) per unit width and the surface roughness a (μmRa) of the working roll.

[0020] [3] The temper rolling method for cold-rolled steel sheet according to [2], wherein the tension T is set based on the following formula (1):

[0021] t×w×(-200×a-90) / (a×(1-logC))+17.1≤T≤t×w×(-200×a+10) / (a×(1-logC))+17.1·····(1),

[0022] Where, t is the thickness of the steel plate (mm), w is the load per unit width (tonf / mm), a is the surface roughness of the work roll (μmRa), C is the carbon content of the steel plate (mass%), T is the tension per unit cross-sectional area (kgf / mm 2 ).

[0023] Effects of the Invention

[0024] The present invention prevents the occurrence of sudden changes in elongation, poor elongation, or bellows-like shape defects known as cross-buckling during wet temper rolling, even when the temper rolling fluid concentration or load changes. Furthermore, the present invention is applicable to both single-stand and multi-stand rolling mills. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of a temper rolling facility showing one embodiment of the temper rolling method of the present invention. DETAILED DESCRIPTION

[0026] Hereinafter, the present invention will be described in further detail.

[0027] Embodiments of the present invention will be described using the drawings. Figure 1 This schematic diagram illustrates one embodiment of the present invention in a method for temper rolling a cold-rolled steel sheet. It outlines a temper rolling facility for temper rolling an annealed cold-rolled steel sheet. Temper rolling is performed using work rolls 1 that apply pressure to a steel sheet 3 from above and below, and backup rolls 2 that support the work rolls 1. Temper rolling fluid is supplied from temper rolling fluid supply nozzles 5 to perform rolling.

[0028] In order to measure the tension T (kgf / mm 2: Tension per unit cross-sectional area), tension gauges (not shown) are installed before and after the work rolls 1. The steel plate 3 is pressed by the upper and lower work rolls 1 at a predetermined load, advancing in the travel direction 4 while being subjected to the set tension. If the tension is set too high, the elongation fluctuates unstably, resulting in sudden changes. If the tension is set too low, insufficient elongation or cross-buckling may occur.

[0029] In this temper rolling, the tension T set based on the various factors described below is applied by adjusting the rotational speed of the work rolls 1 .

[0030] The inventors of the present invention have repeatedly conducted experiments and discussions on various factors affecting the occurrence of defects in the rolling conditions of temper rolling, and have found that the following factors have an influence.

[0031] Specific factors include the thickness of the steel plate t (mm), the load per unit width w (tonf / mm), the surface roughness a of the work roll (μmRa), the carbon content C of the steel plate (mass%), the tension per unit cross-sectional area T (kgf / mm 2 ).

[0032] Investigations into the relationship between these factors and the occurrence of jumps revealed that jumps are more likely to occur with increasing thickness t and tension T. On the other hand, it was found that jumps are more likely to occur with decreasing work roll surface roughness a and carbon content C.

[0033] However, all factors other than tension T depend on the specifications of the product steel sheet and are therefore set within a specific range. Therefore, their settings cannot be freely changed. In other words, if factors other than tension T can be kept constant and tension T is set to the optimal value, the occurrence of sudden changes can be effectively suppressed.

[0034] Here, how to set the tension T to the most appropriate value is discussed.

[0035] As mentioned above, high tension T is prone to sudden changes, while low tension T is prone to shape defects known as cross buckling. In temper rolling, there's a demand for high tension T to maintain shape stability. Given its relationship with the occurrence of sudden changes, it's important to determine the upper limit of tension setting. Furthermore, from the perspective of preventing shape defects, it was concluded that setting a lower limit for tension setting is also necessary, and various experiments were conducted. As a result, the following method for setting tension T was developed.

[0036] The present invention is characterized in that, in a temper rolling method for performing wet temper rolling on an annealed cold-rolled steel sheet,

[0037] The tension T (kgf / mm) during the temper rolling is set based on the carbon content C (mass %) of the cold-rolled steel sheet. 2 ), further on the basis of the carbon content C (mass %), it is set based on the thickness t (mm) of the cold-rolled steel plate, the load w (tonf / mm) per unit width and the surface roughness a (μmRa) of the working roll, and more preferably, the tension T is set based on the following formula (1).

[0038] t×w×(-200×a-90) / (a×(1-logC))+17.1≤T≤t×w×(-200×a+10) / (a×(1-logC))+17.1·····(1),

[0039] Where, t is the thickness of the steel plate (mm), w is the load per unit width (tonf / mm), a is the surface roughness of the work roll (μmRa), C is the carbon content of the steel plate (mass%), T is the tension per unit cross-sectional area (kgf / mm 2 ). Also, log in the formula is the natural logarithm.

[0040] The above equation (1) is the result of multiple regression analysis and other methods based on data compiled from numerous experimental examples and simulation results. The lower and upper limits of the tension T are determined to satisfy equation (1). By setting the tension T within this range and performing temper rolling, it is possible to obtain a high-quality steel plate without causing jumps or other defects.

[0041] Incidentally, in the aforementioned equation (1), observing the relationship between each factor and tension T, it is found that the plate thickness t and load w have a negative correlation with tension T. As t and w increase, the value of tension T at which a sudden change occurs decreases. On the other hand, the surface roughness a and carbon content C of the work roll have a positive correlation with tension T. Therefore, as a and C increase, the value of tension T at which a sudden change occurs increases.

[0042] The following describes the tension T of the present invention, the various factors used to set it (C, t, w, a), and parameters related to the operating conditions of temper rolling. The cold-rolled steel sheets targeted by the present invention include automotive steel sheets, can steel sheets, and other general cold-rolled steel sheets.

[0043] About Zhang Li T

[0044] The range of the tension T of the present invention is 5.0 kgf / mm 2 ~30.0kgf / mm 2 If it is outside this range, sufficient temper rolling cannot be performed, and defects such as jump or poor shape may occur. Preferably 12kgf / mm 2 ~16.0kgf / mm2 .

[0045] The tension T set based on various factors during temper rolling is applied by adjusting the rotational speed of the work rolls 1 as described above.

[0046] About carbon content C

[0047] The carbon content C (mass %) of the cold-rolled steel sheet is a factor that significantly affects the tension T. The carbon content C of the cold-rolled steel sheet of the present invention is preferably 0.0005 mass % or more and preferably 0.1 mass % or less. It is more preferably 0.001 mass % or more and more preferably 0.08 mass % or less.

[0048] The carbon content C can be analyzed in accordance with JIS G 1211-3.

[0049] About plate thickness t

[0050] The thickness t (mm) of the cold-rolled steel sheet of the present invention is preferably 0.1 mm or more and preferably 1.0 mm or less, more preferably 0.1 mm or more and more preferably 0.6 mm or less.

[0051] The plate thickness t can be measured using a gamma-ray plate thickness meter, an X-ray plate thickness meter, or the like.

[0052] About load w

[0053] The load w (tonf / mm) per unit width is preferably 0.1 tonf / mm or greater and preferably 1.5 tonf / mm or less. Outside this range, sufficient temper rolling cannot be performed, and defects such as jumping and poor shape may occur. It is more preferably 0.2 tonf / mm or greater and more preferably 1.0 tonf / mm or less.

[0054] The load w can be measured using a load cell or the like.

[0055] About surface roughness a

[0056] The surface roughness a (μmRa) of the work roll is preferably 0.20 μmRa or more and preferably 2.00 μmRa or less, more preferably 0.25 μmRa or more and further preferably 1.80 μmRa or less.

[0057] Note that Ra is one of the parameters that express surface roughness and is also a parameter that expresses arithmetic mean roughness. The surface roughness of the work roll can be measured in accordance with JIS B 0601.

[0058] Furthermore, the surface roughness of the work roll can be adjusted by electrospark machining, grinding, or the like.

[0059] About annealing conditions

[0060] First, the annealing step, which is the first stage of the temper rolling step of the present invention, will be described.

[0061] In the typical cold-rolled steel sheet annealing process, a continuous annealing line equipped with an annealing furnace for annealing is used before the temper mill for quenching and tempering. This continuous annealing line comprises multiple uncoilers for uncoiling the cold-rolled steel sheet coils (steel strips), a welding machine, a cleaning device, an annealing furnace, a tempering mill, and multiple reels for winding the steel sheet.

[0062] The uncoiler unwinds the steel sheet from the coiled steel strip. The steel sheet is unwound from the uncoiler and transported in the longitudinal direction.

[0063] Here, for example, when there are two uncoilers, after one uncoiler finishes uncoiling, the other uncoiler starts uncoiling, and the welding machine welds the tail end of the previous steel plate and the front end of the next steel plate, thereby continuously processing the steel plates.

[0064] It should be noted that the present invention is not limited to having a plurality of uncoilers, and the steel plate may be uncoiled from one uncoiler.

[0065] The welding machine welds the tail end of the first unwinding steel plate and the front end of the second unwinding steel plate to form a whole. This allows continuous processing of steel plates that are longer than the length of the coil loaded on one unwinder.

[0066] The cleaning device is a device that cleans and removes oil and dirt attached to the surface of the steel plate. The cleaning method of the steel plate in the cleaning device is not limited, and various cleaning methods used in steel plate processing equipment such as electrolytic degreasing and alkaline degreasing can be used.

[0067] An annealing furnace is a device (furnace) for annealing cleaned steel sheets. An annealing furnace is a common annealing furnace that performs heating, soaking, and cooling.

[0068] The heat treatment conditions are preferably 600°C or higher and 850°C or lower, for 20 seconds or higher and 100 seconds or lower, more preferably 650°C or higher and 800°C or lower, for 25 seconds or higher and 90 seconds or lower.

[0069] The soaking conditions are preferably 600°C or higher and 800°C or lower, 5 seconds or higher and 60 seconds or lower, more preferably 650°C or higher and 750°C or lower, 180 seconds or higher and 55 seconds or lower.

[0070] As the cooling treatment conditions, it is preferable that the cooling rate be 5°C / sec or more, preferably 30°C / sec or less, and that the cooling be performed at 100°C or more and to 200°C or less. Further, it is preferable that the cooling rate be 10°C / sec or more, preferably 25°C / sec or less, and that the cooling be performed at 120°C or more and to 180°C or less.

[0071] Operation conditions for the temper rolling

[0072] The temper rolling mill is not only applicable to the four-stage mill as shown in Figure 1 Fig. 1, but also to a six-stage mill or the like. Further, it is applicable to a single stand or a multi-stand of wet type and dry type.

[0073] Work roll diameter It is preferable that the work roll diameter be 450 mm or more and 600 mm or less. In particular, it is more preferable that the work roll diameter be 500 mm or more and 550 mm or less.

[0074] The temper rolling liquid supply nozzles 5 are provided on the surface side and the back side of the steel sheet 3, and supply the temper rolling liquid between the steel sheet 3 and the work roll 1 from the upstream side (input side) of the traveling direction of the steel sheet 3. That is, the temper rolling liquid supply nozzles 5 supply the temper rolling liquid to the surface side and the back side of the steel sheet 3. By supplying the temper rolling liquid between the steel sheet 3 and the work roll 1, it is possible to prevent foreign matter from being mixed between the work roll 1 and the steel sheet 3, and to prevent defects from being generated on the steel sheet 3.

[0075] In the Figure 1 temper rolling liquid supply nozzles 5 are provided on the input side, and supply the temper rolling liquid between the steel sheet 3 and the work roll 1, but the present application is not limited thereto. The temper rolling liquid supply nozzles 5 can be provided on the surface of the work roll 1 or between the work roll 1 and the backup roll 2 to supply the temper rolling liquid. Further, in the case of a six-stage mill, the temper rolling liquid supply nozzles 5 can be provided between the intermediate roll provided between the work roll 1 and the backup roll 2 and the work roll 1 to supply the temper rolling liquid. Further, the temper rolling liquid supply nozzles can be provided on the output side as well as on the input side.

[0076] The kind of the temper rolling liquid of the present application is not particularly limited, and specifically, it is possible to cite a surfactant or a fatty acid or the like.

[0077] The supply temperature of the temper rolling liquid is preferably adjusted to 10°C or more and 60°C or less. It is more preferable that the supply temperature be 20°C or more and 50°C or less.

[0078] Examples

[0079] Hereinafter, the present application will be more specifically described based on examples, but the present application is not limited to these examples.

[0080] First, use Figure 1 Working roll diameter shown The four-stage temper mill adjusts the surface roughness of the work rolls by grinding with a grinding stone. Furthermore, a tempering fluid composed of a surfactant or fatty acid is adjusted to 20°C to 40°C and supplied from the mill input side.

[0081] As target steel sheets, low carbon steel having a carbon content C of 0.04 mass % and ultra-low carbon steel having a carbon content C of 0.0014 mass % or 0.0024 mass % were used.

[0082] Steel plates were prepared with thicknesses t of 0.2 mm, 0.25 mm, and 0.3 mm. The load w per unit width applied to the steel plates was 0.3 tonf / mm, 0.5 tonf / mm, and 0.6 tonf / mm. The surface roughness a of the work rolls was adjusted to 0.28 μm Ra, 0.47 μm Ra, and 0.88 μm Ra.

[0083] The above-mentioned combination of factors was performed, and the calculated value obtained from the tension setting formula (1) based on the above factors was compared with the actual set tension value. The results of the investigation on the occurrence of actual jump and shape defects are shown in Table 1.

[0084] The presence of jumps is determined by the elongation calculated from the difference in circumferential speed between the front and rear rolls of the temper mill. Specifically, jumps are determined to have occurred if the elongation is 5% or greater. Furthermore, the presence of shape defects is determined by the height of the undulations on the steel sheet surface. The undulations on the steel sheet surface are measured using a stylus-type profilometer; a shape defect is determined if the height difference is 0.1 mm or greater.

[0085] As Example 1, steel plates with carbon contents C of 0.04 mass%, 0.0024 mass%, and 0.0014 mass% were used, and the other factors, namely, the plate thickness t, the load w, and the work roll surface roughness a, were fixed at 0.2 mm, 0.3 tonf / mm, and 0.28 μmRa. These values ​​were substituted into equation (1) to determine the lower and upper limits of the tension T. When the actual operating tension was set within the upper and lower limits (Examples 1-1, 1-5, and 1-8), no sudden change or shape defects occurred. When the tension was set to a value exceeding the upper limit (Examples 1-2, 1-6, and 1-9), a sudden change occurred, and when the tension was set to a value exceeding the lower limit (Examples 1-3, 1-4, and 1-7), shape defects occurred.

[0086] Next, as Example 2, using steel sheets having a plate thickness t of 0.2 mm, 0.25 mm, 0.3 mm, the other elements, namely the carbon content C, was fixed at 0.04 mass%, the load w was fixed at 0.3 tonf / mm, and the work roll surface roughness a was fixed at 0.28 μm Ra, and this was implemented. Substituting these values into Equation (1), the lower limit value and the upper limit value of the tension T were calculated. In the case where the tension actually operated was set to a range of this upper and lower limit values (Example 2-1, 2-3, 2-6), neither the jump nor the shape defect occurred, in the case where it was set to a value exceeding the upper limit value (Example 2-4, 2-7), the jump occurred, and in the case where it was set to a value exceeding the lower limit value (Example 2-2, 2-5), the shape defect occurred.

[0087] Next, as Example 3, the load w was set to 0.3 tonf / mm, 0.5 tonf / mm, 0.6 tonf / mm, the other elements, namely the carbon content C, was fixed at 0.04 mass%, the plate thickness t was fixed at 0.2 mm, and the work roll surface roughness a was fixed at 0.28 μm Ra, and this was implemented. Substituting these values into Equation (1), the lower limit value and the upper limit value of the tension T were calculated. In the case where the tension actually operated was set to a range of this upper and lower limit values (Example 3-1, 3-3, 3-6), neither the jump nor the shape defect occurred, in the case where it was set to a value exceeding the upper limit value (Example 3-4, 3-7), the jump occurred, and in the case where it was set to a value exceeding the lower limit value (Example 3-2, 3-5), the shape defect occurred.

[0088] Finally, as Example 4, using work rolls having a surface roughness a of 0.28 μm Ra, 0.47 μm Ra, 0.88 μm Ra, the other elements, namely the carbon content C, was fixed at 0.04 mass%, the plate thickness t was fixed at 0.2 mm, and the load w was fixed at 0.3 tonf / mm, and this was implemented. Substituting these values into Equation (1), the lower limit value and the upper limit value of the tension T were calculated. In the case where the tension actually operated was set to a range of this upper and lower limit values (Example 4-1, 4-3, 4-6), neither the jump nor the shape defect occurred, in the case where it was set to a value exceeding the upper limit value (Example 4-4, 4-7), the jump occurred, and in the case where it was set to a value exceeding the lower limit value (Example 4-2, 4-5), the shape defect occurred.

[0089] From the above results, it was found that if the tension in the actual operation was set to a range of the upper and lower limit values of the tension calculated according to Equation (1) of the present application, the aforementioned jump and the like defects did not occur, and good temper rolling could be performed.

[0090] Table 1

[0091]

[0092] Description of Reference Numerals

[0093] 1 Working roll, 2 Backup roll, 3 Steel plate, 4 Arrow indicating direction of travel, 5 Quenching and tempering rolling fluid supply nozzle.

Claims

1. A tempering rolling method for cold-rolled steel sheets, characterized in that: In a temper rolling method in which wet temper rolling is performed on an annealed cold-rolled steel sheet, The tension T during the temper rolling is set based on the following formula (1): t×w×(-200×a-90) / (a×(1-lnC))+17.1≤T≤t×w×(-200×a+10) / (a×(1-lnC))+17.1·····(1), Where, t is the thickness of the cold-rolled steel sheet in mm, w is the load per unit width in tonf / mm, a is the surface roughness of the work roll in μmRa, C is the carbon content of the cold-rolled steel sheet in mass%, and T is the tension per unit cross-sectional area in kgf / mm. 2 .

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