JPH0118132B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for cooling metal strips using a cooling roll contact method. [Prior Art] In continuous annealing equipment for metal strips, a method is known in which the strip is cooled by bringing it into contact with a cooling roll. FIG. 1 shows a continuous annealing facility using this type of cooling system, in which cooling roll zones 6 and 8 each having a plurality of cooling rolls 12 are provided at the rear of a soaking zone 5 and an over-temperature treatment zone 7. There is. This cooling roll 1
2, for example, as shown in FIG. 2, a structure having a spiral refrigerant liquid passage 14 inside the roll shell 13 is used, and a passage 15 provided in the roll shaft of the cooling roll and a roll shaft are used. A rotary joint 16 connected to the end allows a refrigerant liquid such as cooling water to flow into the refrigerant liquid passage 14. FIG. 3 shows the thermal cycle in the continuous annealing equipment shown in FIG. 1, in which the metal strip After being sent to the tropical zone 5 and heated to about 700°C, it is rapidly cooled to about 400°C in a cooling roll zone 6 at a cooling rate of 100 to 200°C/sec. Then metal strip
After being treated in the overaging treatment zone 7, it is transferred to the cooling roll zone 8 again.
It is cooled, passed through an exit looper 9 and a temper mill 10, and then wound onto a tension reel 11. When the metal strip X is cooled by the cooling roll as described above, the shape of the metal strip X is easily distorted due to thermal stress generated during the cooling process.
There is a drawback that buckling is likely to occur in the non-contact portion between the cooling rolls 12. In extreme cases, this buckling causes vertical wrinkles in the line direction called "squeezing", which becomes a serious problem that may lead to breakage of the metal strip X within the line. [Problems to be Solved by the Invention] Conventionally, regarding the thermal stress generated in such metal strips and the resulting buckling/restriction, buckling-prone areas are locations where compressive stress occurs in the width direction of the strip, If this value exceeds the buckling resistance value, there is an extremely high risk of buckling and constriction, as disclosed in, for example, JP-A-56-5930 and JP-A-59-28532. It cannot be said that sufficient elucidation has been made regarding specific measures for prevention. [Means for Solving the Problem] The present invention was made in view of the above circumstances, and as a result of careful study of the buckling and drawing characteristics during roll contact cooling of the strip, the drawing was also developed based on a specific index. discovered that by regulating the cooling conditions and line conditions in each cooling roll, the metal strip could be cooled without throttling. That is, the present invention controls the line conditions and cooling conditions so that the following equation is satisfied when a metal strip is cooled by roll contact. V/α・{a・σ _e /E(R・t/W) ² +b・φ _s 1/
1-ν ² (t/W) ³ }>1 However, v: Line speed [mpm] α: Overall heat transmission coefficient of roll cooling (Kcal/m ² h℃) E: Young's modulus [Kg/mm ² ] σ _e : Yield stress [Kg/ ^mm2 ] R: Radius of water-cooled roll (m) t: Plate thickness (m) W: Plate width (m) ν: Poisson's ratio φ _s : Buckling resistance coefficient of biaxial stress field a , b: constant value specific to the roll cooling device The details of the present invention will be described below. When a metal strip is cooled by roll contact, compressive thermal stress is generated in both the longitudinal and width directions at the center of the strip width where the strip starts contacting the cooling roll, which can lead to buckling and even squeezing. It causes FIG. 4 shows the change in shape of the strip during roll contact cooling, and it can be considered that the change in shape of the cooled strip is determined by the conditions shown in the figure. In other words, when the compressive stress (σ ₁ ) ^ave (uniform compressive stress in the sheet width direction) in the buckling-prone area reaches the critical buckling stress σ _B , buckling occurs, resulting in a slight bulge. Furthermore, when the compressive stress (σ ₁ ) ^ave of this buckling-prone area exceeds σ _B and reaches the critical stress σ _P , a constriction occurs. FIG. 5 shows various schematic diagrams of uniform biaxial stress states, and FIG. 5 shows critical buckling stress values for those states. Assuming that the strip roll is cooling,
If the y direction in Figure 5 is the line direction and the x direction is the sheet width direction, buckling will not occur unless the sheet width direction compression _σ I understand that. The critical buckling stress σ _cr when a rectangular plate whose periphery is simply supported with one side B and a plate thickness t is uniformly compressed only in one direction can be expressed by the following equation. σ _cr = 4x ² E/12 (1-ν ² ) (t/B) ² However, E: Young's modulus ν: Poisson's ratio According to this, when tensile stress in the line direction is applied in addition to the compressive stress in the width direction , indicating that buckling does not occur unless a larger compressive stress in the width direction is applied. For example, when line direction stress σ _L =σ _cr is applied, the buckling limit stress σ _cr due only to compression in the width direction
Buckling does not occur unless the stress in the width direction increases from σ to σ. This ratio σ/σ _cr is defined as the coefficient φ _s (buckling resistance coefficient of biaxial stress field) of biaxial stress field buckling with respect to uniaxial stress field buckling. φ _s is expressed by the formula shown in Table 1.

【table】

〔Example〕

A steel strip heated to approximately 700°C was roll-cooled to a temperature of 600 to 400°C using a cooling roll apparatus as shown in FIG. 8, and the presence or absence of squeezing was examined. Table 2 shows the results, line/cooling conditions, and aperture PI, and it can be seen that the occurrence of aperture is appropriately suppressed by performing cooling with the aperture PI>1 according to the method of the present invention.

〔Effect of the invention〕

According to the present invention described above, there is an effect that a metal strip can be cooled without causing throttling regardless of its size, and it can be said that it is particularly suitable for continuous annealing of steel strips.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing a continuous annealing line of a strip. FIG. 2 is a longitudinal sectional view of the cooling roll. FIG. 3 shows a thermal cycle in the continuous annealing line of FIG. 1. FIG. 4 is an explanatory diagram showing the change in shape of the strip during roll contact cooling. Figure 5- is a schematic diagram of a uniform biaxial stress state;
Figure 5- shows the critical buckling stress values for those conditions. FIG. 6 is an explanatory view showing the buckled shape of the strip during roll contact cooling, and FIG. 7 is an explanatory diagram showing the constricted shape. FIG. 8 is an explanatory diagram showing a cooling roll device used in an embodiment of the present invention. FIGS. 9A and 9B are explanatory views showing how the cooling roll device shown in FIG. 8 is used, respectively. In the figure, 12, 12a, 12b are cooling rolls, and X is a metal strip.

Claims (1)

[Scope of Claims] 1. A roll contact cooling method for a metal strip, which comprises controlling line conditions and cooling conditions so as to satisfy the following formula when performing roll contact cooling of a metal strip using a cooling roll. V/α・{a・σ _e /E(R・t/W) ² +b・φ _s 1/
1-ν ² (t/W) ³ }>1 However, v: Line speed [mpm] α: Overall heat transmission coefficient of roll cooling (Kcal/m ² h℃) E: Young's modulus [Kg/mm ² ] σ _e : Yield stress [Kg/ ^mm2 ] R: Radius of water-cooled roll (m) t: Plate thickness (m) W: Plate width (m) ν: Poisson's ratio φ _s : Buckling resistance coefficient of biaxial stress field a , b: constant value specific to the roll cooling device