JP3380354B2 - Method for suppressing in-furnace flaws on stainless steel strip in vertical bright annealing furnace - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention prevents deformation of a stainless steel strip in a vertical bright annealing furnace and prevents generation of scratches caused by contact with a furnace wall. Regarding the method of suppressing. 2. Description of the Related Art In a bright annealing furnace, particularly in the case of a thin and wide material, the strip is liable to undergo out-of-plane deformation due to thermal stress generated in the strip passing through the furnace. Abrasion may occur due to contact. In particular, when a stainless steel strip is heat-treated in a vertical bright annealing furnace in which no support roll is disposed in the furnace, there is a high risk of scratches. [0003] As a conventional technique for preventing such deformation of the stainless steel strip inside the furnace, for example, Japanese Patent Laid-Open Publication No. Sho 49-17314.
In the publication, a method of cooling a high-temperature stainless steel strip as slowly as possible is shown, but it is not clear how slow cooling is necessary depending on conditions such as a sheet width and a sheet thickness. . Further, as disclosed in Japanese Patent Publication No. 53-35848, there is a method of correcting a sheet warpage in a furnace by blowing a gas having a temperature difference between the front and back of a stainless steel strip passing through the furnace. However, this method has the problem that when buckling occurs due to buckling due to compressive stress acting in the in-plane direction of the plate, the buckling mode is easily reversed, especially in the case of a thin plate, and control is difficult. There is. [0004] As described above, in the vertical bright annealing furnace, in order to suppress the in-furnace deformation, in the method of correcting the once deformed one, the deformation itself is an unstable phenomenon called buckling. It is difficult to correct. SUMMARY OF THE INVENTION [0005] The present invention has been made to solve the above-mentioned problems of the prior art, and uses a stainless steel strip without using secondary shape correcting means. An object of the present invention is to provide a method for preventing deformation of a sheet in a vertical bright annealing furnace in a furnace and suppressing generation of scratches in the furnace. According to the present invention, when heat-treating a stainless steel strip using a vertical bright annealing furnace having a heating zone and a cooling zone, a gap in the height direction of the cooling zone is provided . The temperature distribution of the stainless steel strip is set by a polygonal line, and the temperature gradient change amount of the stainless steel strip and the average tension applied to the stainless steel strip are defined by the following equation: σ _t (y) −4.9 × 10 ^{− 2} w · q · E · α ≧ 0, where y: furnace height position [m] σ _t (y); average tension at furnace height position y [Pa] w; plate width [m] q ; temperature gradient variation [℃ / m] = (ΔT / Δy) 2 - (ΔT / Δy) 1 (ΔT / Δy) 1; in the cooling zone, the height direction, stainless
Set the temperature distribution of the strip with a polygonal line, and
Temperature gradient at position 1 in the area where the temperature changes
Distribution [° C / m] (ΔT / Δy) ₂ ; stainless steel in cooling zone, height direction
Set the temperature distribution of the strip with a polygonal line, and
Temperature at a certain position 2 in the region where the degree does not change
Gradient [° C./m] E; Young's modulus of stainless steel strip at plate temperature at furnace height y [Pa] α; Linear expansion coefficient of stainless steel plate at plate temperature at furnace height y [/ ° C.] This is a method for suppressing in-furnace scratches on a stainless steel strip in a vertical plate bright annealing furnace, characterized in that thermal deformation of the stainless steel strip is suppressed by being contained in the stainless steel strip. [0007] The present inventors first conducted a thermal stress analysis by the finite element method in order to quantitatively grasp the influence of the change in the temperature gradient of the plate temperature distribution on the thermal stress generated in the stainless steel strip. Was carried out. In order to grasp the basic causal relationship, the following thermal stress is analyzed by analyzing the thermal stress acting in the plane when the temperature distribution is set by the broken line shown by the solid line in FIG. 3, that is, the plate temperature curve 1 composed of two straight lines. Obtained knowledge. (1) A local thermal stress is generated around a position where a temperature gradient changes. (2) The area where the stress is generated should be about the width of the plate from the position where the temperature gradient changes. (3) If the change in temperature gradient is convex upward, compressive stress acts near the center of the sheet width, and tensile stress acts on the end of the sheet width. (4) When the change in the temperature gradient is convex downward, a tensile stress acts near the center of the sheet width and a compressive stress acts on the end of the sheet width. (5) The change in temperature gradient and the peak stress value must be proportional. (6) The plate width and peak stress value are proportional. (7) Young's modulus and peak stress value should be proportional. (8) The coefficient of linear expansion is proportional to the peak stress value. (9) When the Young's modulus and linear expansion coefficient do not have temperature dependency,
The peak value of the compressive stress generated by the change of the temperature gradient shall be calculated by the following formulas (1) and (2). _{σ x, peak = k x w} · q · E · α .................. (1) σ y, peak = k y w · q · E · α .................. (2) Here, sigma _{x, peak} : Peak value of vertical stress in the width direction [Pa] σ _{y, peak} ; Peak value of vertical stress in the height direction [Pa] x; Index y indicating the width direction; Index w indicating the height direction; m] q; temperature gradient change [° C./m]=(ΔT/
Δy) ₂ − (ΔT / Δy) ₁ (ΔT / Δy) ₁ ; temperature gradient at position 1 (y <9 m in FIG. 3) in the height direction of the cooling zone [° C./m] (ΔT / Δy) ₂ ; cooling Temperature gradient at position 2 in the height direction of the belt (in FIG. 3, y> 9 m) [° C./m] E; Young's modulus [Pa] α; Linear expansion coefficient [/ ° C.] k _x ; constant (7.5 × 10 ^-2) ) K _y ; constant (4.9 × 10 ⁻² ) In the above, the case of the sheet temperature curve 1 in which the temperature gradient changes digitally at one point has been described. It is considered that the temperature gradient continuously and gradually changes as shown by the sheet temperature curves 2 and 3 indicated by the dashed line. Even in the case of such a sheet temperature curve, a basically similar stress state is obtained such that, when the sheet is convex upward, a compressive stress acts on the center of the sheet width and a tensile stress acts on an end portion of the sheet width. However, for example, as shown in FIG. 4, when the vertical direction stress at the center of the sheet width is plotted on the vertical axis and the position in the height direction is plotted on the horizontal axis, the stress distribution curve 4 shown by the solid line is obtained. As described above, the stress distribution having a sharp peak in the case of the sheet temperature curve 1 composed of two straight lines is different from the stress distribution curve indicated by the broken line and the one-dot chain line in the case of the sheet temperature curves 2 and 3 in which the temperature gradient is gradually changed. There is a difference that the peaks become dull and smooth, and the peak level is greatly reduced, as shown in FIGS. On the other hand, when the vertical stress at the center of the plate width is plotted on the vertical axis, the stress distribution curves 7, 8, and 9 shown by the solid line, the broken line, and the one-dot chain line in FIG. In both cases of the plate temperature curve 1 composed of two straight lines and the plate temperature curves 2 and 3 in which the temperature gradient is gradually changed, the peak is smooth, and the decrease in the peak value by gradually changing the temperature gradient is small. Therefore, the absolute value of the compressive stress generated in the stainless steel strip due to the change in the temperature gradient is greater in the height direction, and the absolute value of the compressive stress in the height direction is greater than that in the height direction due to the change in the temperature gradient. If a large tension is applied,
The possibility of out-of-plane deformation of the stainless steel strip due to buckling is small. If the temperature gradient changes smoothly, change the temperature gradient
Since the compressive stress in the height direction due to
Out-of-plane deformation is possible compared to the case where the temperature distribution is set with a polygonal line.
The performance becomes even smaller. Therefore, the sheet temperature distribution is broken
Assuming that the average tension at the height position y of the furnace in the case of setting is σ _t (y), the following equation (3) is used: σ _t (y) −4.9 × 10 ⁻² w · q · E · α ≧ 0 …………… (3) Within the range that satisfies (3), the temperature gradient change q and the average tension σ
By determining _t (y), deformation of the stainless steel strip in the furnace can be suppressed, and occurrence of in-furnace scratches can be reduced. The average tension σ _t (y) can be expressed by the following equation (4). Σ _t (y) = σ _t (y ₀ ) + ρ · g · (y−y ₀ ) (4) where y ₀ is the furnace outlet position [m], σ _t ( y ₀ ) is the average tension at the furnace outlet [Pa], ρ is the density of the stainless steel strip [kg / m ³ ],
g is the gravitational acceleration [m / sec ² ]. An embodiment of the present invention will be described below with reference to FIG. As shown in this figure, the vertical bright annealing furnace 10
Is constituted by a chute 11, two in-furnace deflector rolls 12, a heating zone 13 to which an electric heater or the like is attached, and a cooling zone 14 to which a cooling gas nozzle or the like is attached.
The stainless steel strip S sent from the previous process is fed into the chute 11 via the entrance bridle roll 15 and the entrance deflector roll 16, passes through the in-furnace deflector roll 12, passes through the heating zone 13, and the cooling zone 14. And the exit side deflector roll 1
7. It is sent to the next process via the outlet bridle roll 18. In this vertical bright annealing furnace 10, due to the configuration of the furnace,
Since the space between the furnace walls in the furnace in the cooling zone 14 is narrow, there is a high risk of occurrence of scratches especially near the inlet 14a of the cooling zone 14. The method of the present invention was applied to heat treatment of a stainless steel strip S made of a stainless steel ferrite special steel. The annealing temperature at this time is 950 ° C, the sheet width is 1.3m, and the sheet thickness is 0.2
mm. The Young's modulus E of the stainless steel strip S in the vicinity of the entrance of the cooling zone 14 which is a problem is 120 GPa, and the linear expansion coefficient α is
Assuming that 1.5 × 10 ⁻⁵ , the relationship between the average tension σ _t (y) near the inlet of the cooling zone 14 and the temperature gradient variation q is q ≦ 8.7 × 10 ⁻⁶ σ _t (y)... …… (5) This relationship is shown in FIG. Therefore, since an excessive tension may lead to breakage of the plate, the outlet tension is set so that the average tension σ _t (y) near the inlet of the cooling zone 14 is 5 MPa. Further, the maximum value of the temperature gradient change q near the inlet of the cooling zone 14 is indicated by a circle within the range of the present invention in FIG.
The stainless steel strip S was manufactured by setting the plate temperature distribution in the height direction to be ° C / m. As a result, the rate of occurrence of scratches was reduced to 1/3 of that before application of the present invention. In order to achieve a temperature gradient change of 40 ° C./m near the inlet of the cooling zone 14, the temperature gradient change near the inlet of the cooling zone 14 is set by setting the plate temperature distribution to start cooling from the latter half of the heating zone 13. Could be kept low. The maximum value of the temperature gradient change q in the vicinity of the inlet 14a of the cooling zone 14 is 80 ° C./m (marked by X in FIG. 2).
The operation was performed under the same conditions as above, and as a result, a large amount of scratches were generated. Further, the average tension σ _t (y) is set to 2.5 MPa, and the temperature gradient change q
When the maximum value was set to 40 ° C./m and 80 ° C./m, a large amount of scratches were similarly generated. As described above, according to the present invention,
Since the amount of change in the temperature gradient of the sheet temperature curve of the stainless steel strip and the average tension applied to the stainless steel strip at an arbitrary position in the height direction of the cooling zone are controlled so as to satisfy the above-mentioned formula (3). The deformation of the stainless steel strip passing through the vertical bright annealing furnace is suppressed without using a conventional secondary shape correcting means that blows a cooling gas so as to have a temperature difference between the front and the back of the stainless steel strip. The generation of flaws can be reduced, which can greatly contribute to the improvement of product quality and yield.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing a cross section of a vertical bright annealing furnace according to an embodiment of the present invention. FIG. 2 is a characteristic diagram illustrating a relationship between an average tension and a temperature gradient change amount. FIG. 3 is a characteristic diagram showing an example of a sheet temperature curve set for examining a basic effect of a sheet temperature curve in a longitudinal direction of a stainless steel strip. FIG. 4 is a characteristic diagram showing a distribution of a vertical stress in a width direction of the stainless steel strip corresponding to the sheet temperature curve of FIG. 3; FIG. 5 is a characteristic diagram showing a distribution of a vertical stress in a height direction of a stainless steel strip corresponding to the sheet temperature curve of FIG. 3; [Explanation of Signs] 1 Sheet temperature curve in height direction (case in which temperature gradient is changed at one point at a stroke) 2 Sheet temperature curve in height direction (case in which temperature gradient is continuously and gradually changed) 3 Height Temperature curve in the direction (case in which the temperature gradient is changed more slowly than the sheet temperature curve 2) 4 Stress distribution curve of the vertical stress generated in the width direction center of the stainless steel strip in the sheet temperature curve 1 5 Sheet temperature curve The stress distribution curve of the widthwise vertical stress generated at the center of the stainless steel strip in the width direction in 2, the stress distribution curve of the widthwise vertical stress generated in the widthwise center of the stainless steel strip in the sheet temperature curve. Stress distribution curve of vertical stress generated in the center of width direction of stainless steel strip in 1; stress distribution curve of vertical stress generated in width direction center of stainless steel strip in temperature curve 2; Of stainless steel strip in temperature curve 3 Stress distribution curve of vertical stress generated in the center in the width direction 10 Vertical bright annealing furnace 11 Chute 12 Furnace deflector roll 13 Heating zone 14 Cooling zone 14a Cooling zone entrance 15 Inlet bridle roll 16 Inlet deflector roll 17 Exit Side deflector roll 18 Exit side bridle roll S stainless steel strip

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-78755 (JP, A) JP-A-64-75630 (JP, A) JP-B-62-15613 (JP, B1) (58) Field (Int.Cl. ⁷ , DB name) C21D 9/52-9/66

Claims (1)

(57) [Claims 1 At the time of heat treatment of stainless steel strip by using a vertical bright annealing furnace with a heating zone and a cooling zone, the height direction of the stainless band of the cooling zone The temperature distribution of the plate is represented by a line
By setting the amount of change in the temperature gradient of the stainless steel strip and the average tension applied to the stainless steel strip within the range of the following formula, thermal deformation of the stainless steel strip is suppressed, and the vertical bright annealing is characterized in that: A method for suppressing in-furnace scratches on stainless steel strip in a furnace. σ _t (y) −4.9 × 10 ⁻² w · q · E · α ≧ 0, where y: furnace height position [m] σ _t (y); average tension at furnace height position y [ Pa] w; strip width [m] q; temperature gradient variation [℃ / m] = (ΔT / Δy) 2 - (ΔT / Δy) 1 (ΔT / Δy) 1; in the cooling zone, the height direction ,stainless
Set the temperature distribution of the strip with a polygonal line, and
Temperature gradient at position 1 in the area where the temperature changes
Distribution [° C / m] (ΔT / Δy) ₂ ; stainless steel in cooling zone, height direction
Set the temperature distribution of the strip with a polygonal line, and
Temperature at a certain position 2 in the region where the degree does not change
Gradient [° C./m] E; Young's modulus of stainless steel strip at plate temperature at furnace height y [Pa] α; Linear expansion coefficient of stainless steel plate at plate temperature at furnace height y [/ ° C.]