JP3129967B2 - How to remove scale from hot rolled steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing scale from a hot-rolled steel sheet, and more particularly to a technique for producing a hot-rolled steel sheet having low red scale and good surface properties in high Si steel.

[0002]

2. Description of the Related Art To manufacture a hot-rolled steel sheet, a raw material slab is inserted into a heating furnace in an oxidizing atmosphere, and usually 1100-1.
After heating at a temperature of 300 ° C. for several hours, hot rolling is performed.
If the primary scale generated during this heating and the secondary scale generated after extraction from the heating furnace are rolled without being sufficiently removed in the so-called descaling, the scale will bite into the surface of the product steel sheet, causing scale flaws. It remains. The scale flaws significantly impair the surface properties of the steel sheet and also serve as starting points for cracks during bending, and thus have a significant effect on product quality. In particular, in a steel type having a high Si (silicon) content, Si is selectively oxidized during high-temperature oxidation in a heating furnace, thereby producing firelite (2FeO.SiO ₂ ) having high thermoplasticity, which is complicated at a ground iron interface. Since a complicated subscale layer is formed, the peelability of the primary scale is significantly reduced. And the scale of the high Si steel left without being removed is
It is destroyed by the subsequent rolling and becomes powder, and remains as a scale defect called a red scale or a linear scale on the steel sheet surface.

The following method is conceivable as a means for solving this problem. (1) Apply an antioxidant to the slab surface
249214). (2) Set the heating temperature of the slab to the melting point of firelite (about 1
170 ° C.) or lower (Japanese Patent Laid-Open No. 55-110727, Japanese Patent Publication No. 58-1167). (3) Rolling in a completely non-oxidized state (Japanese Patent Publication No. 60-15 / 1985)
684). (4) The temperature before rolling and the temperature during rolling are set to a high temperature (about 1000
℃ or more). (5) Complete scale removal of the generated scale is performed.

However, the above methods (1) to (4) have the following disadvantages. In the case of (1), not only a complicated coating operation is increased, but also a processing agent is expensive and a manufacturing cost is high. In (2), since rolling is performed at a low temperature, the load on the rolling mill increases, and there are standards that cannot be applied depending on the type of steel from the viewpoint of securing material properties. (3) is not realistic because the equipment cost is enormous. (4) Since extraction is performed at a high temperature from the heating furnace, the calorific value increases, and the scale loss increases.

On the other hand, with respect to the method (5), a method for determining a collision pressure at which no red scale occurs based on an equation for estimating the collision pressure from the water injection pressure (Japanese Patent Laid-Open No. 6-31571).
No. 3), a method of determining the collision pressure of water that does not generate red scale from the wt% of P and Si (JP-A-6-19043)
No. 3) and a method of calculating the collision energy of high-pressure water that has reached the surface of the steel sheet and removing the scale under conditions that provide collision energy that does not generate red scale (Japanese Patent Laid-Open No. 6-114432). Is disclosed.

However, in the method of determining the collision pressure of water only from the steel composition and the injection pressure, an equation whose theoretical basis is unclear is used. There is a problem that the expression cannot be applied as it is. In addition, the method of determining the scale removal conditions of high Si steel based on the collision energy of water has a problem in that when the Si component and the heating conditions are different, the change in the difficulty of scale peeling from the steel material is not considered.

[0007]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention uses a collision pressure determination formula having a theoretical basis that can be applied even when the water injection shape or the injection distance changes, It is an object of the present invention to provide a method for removing the scale of a hot-rolled steel sheet, which can efficiently remove scale without wasting water consumption, considering the difficulty of scale peeling from steel material even if the components and heating conditions are different. .

[0008]

Means for Solving the Problems The inventor of the present invention has made intensive studies to achieve the above object, and has found that the dewatering capacity of the jet water is not only the total impact force (F) on the steel sheet surface but also the unit impact force (S). Also convinced that we needed to consider. On the other hand, on the steel side, there is a scale separation difficulty that depends on the steel material composition and heating conditions.Therefore, the scale removal conditions are set by comparing the scale removal capacity of the jet water with the scale separation difficulty on the steel side. The present invention was invented.

That is, according to the present invention, when water is sprayed to remove the scale of a hot-rolled steel sheet, the scale removal capability of the sprayed water (B1) and the scale peeling difficulty of the hot-rolled steel sheet (B2)
B1 = f ₁ (F, S, T) B2 = f ₂ ( K, R) at R = 100 A scale that satisfies B2 <B1 under the condition of S where B1> 0. This is a method for removing scale from a hot-rolled steel sheet, which comprises injecting water under removal conditions.

Here, F: total impact force of spray water on the steel sheet surface [N] F = f ₃ (D , P ₀ ) (1) S: unit impact force of spray water on the steel sheet surface [Pa] S = f ₄ (P ₀ , D, L, β) (2) T: Scale removal time [sec] R: Scale exfoliation rate [%] In the present invention, the Si content of the hot-rolled steel sheet is set to 0.1%. 1wt%
As described above, the impact condition of the jet water on the steel sheet surface is F> 240
[N] and S> 0.2 × 10 ⁶ [Pa] or more.

Further, the present invention relates to a method for removing the scale (B1) of the jet water and the scale peeling difficulty (B) of the hot-rolled steel sheet.
The method for removing the scale of a hot-rolled steel sheet is characterized in that 2) is performed by the following equations (3) to ( 5 ).

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, when water is sprayed to remove the scale of a hot-rolled steel sheet, the scale removal capability of the sprayed water (B1) and the scale peeling difficulty of the hot-rolled steel sheet (B2)
B1 = f ₁ (F, S, T) and B2 = f ₂ ( K, R)
at R = 100 The condition of S for satisfying B1> 0 was determined, and under this condition of S, water was jetted under the scale removal condition satisfying B2 <B1. The descaling ability to be performed and the scale peeling difficulty determined by the steel type and heating conditions can be evaluated using the same index. Therefore, even when the steel type and the heating conditions change, it becomes possible to accurately determine F and S necessary for complete scale removal. As a result, in the production of a hot-rolled steel sheet, the occurrence of scale flaws can be stably prevented under the minimum equipment conditions, and further, the absolute values of F and S, various parameters related to descale, and the flow rate of water Q
[M ³ / s], particle diameter D [m] of water droplet, injection pressure P ₀
The relationship among [Pa], the injection distance L [m], the coefficient β [-] in consideration of the injection shape, and the like is quantified, and the design of the descaler becomes easy. In addition, the present invention enables efficient scale removal without wasting water usage.

Here, F: total impact force of spray water on the steel sheet surface [N] F = f ₃ (D , P ₀ ) (1) S: unit impact force of spray water on the steel sheet surface [Pa] S = f ₄ (P ₀ , D, L, β) (2) T: Scale removal time [sec] R: Scale exfoliation rate [%] In the present invention, the Si content of the hot-rolled steel sheet is set to 0.1%. 1wt
% And the impact condition of the sprayed water on the steel sheet surface is F> 240.
[N] and S> 0.2 × 10 ⁶ [Pa] or more to enable scale removal from the steel sheet surface,
The selection of necessary F and S completed the scale removal from the high Si steel.

Hereinafter, the contents of the present invention will be supplemented with reference to FIG. First, regarding the water injection to the steel plate surface, assuming the water drop model shown in FIG. 1, the total impact force (F) and the unit impact force (S) of the jet water of the above equations (1) and (2) are calculated as follows. And embodied in equations (1) ′ and (2) ′.

[0015]

(Equation 4)

[0016]

(Equation 5)

Using the formulas (1) ′ and (2) ′, the descaler's scale removal capability based on various parameters (for example, flow rate, droplet diameter, spray pressure, spray distance, spray shape, etc.), Quantification of F and S becomes possible. The parameters used in the expressions (1) ′ and (2) ′ are those shown in FIG. 1 and ρ is the water droplet density [kg / m
³ ], α is a coefficient [−] in consideration of the collision area when the droplet of the water hits the plate, t is the time for the shock wave to propagate through the droplet (droplet diameter / sonic speed), and C is the sound speed [m / s].

Next, the scale removal capability B1 of the jet water is
Using aluminum specimens that show plastic deformation similar to high-temperature steel sheets,
In advance, the erosion amount per unit area of F, S, T and the test material (weight loss when water is sprayed (weight of plate before deske-weight of plate after deske) B [10 ^-3 g / mm ² ]) Was quantified to obtain equation (3).

[0019]

(Equation 6)

In the present invention, the erosion amount B is determined as the evaluation index B1 of the descaler injection water scale removal ability. Subsequently, regarding the scale peeling difficulty B2 on the steel material side, the erosion amount B and the red scale peeling rate R are in a linear relationship, and the slope K of the straight line indicates the heating conditions (heating time, heating temperature, heating furnace temperature) of the steel material. Atmosphere O ₂ concentration) and Si concentration,
It was found by experiments that the value changed with the scale removal time T, and the expressions (4) and (5) were obtained.

[0021]

(Equation 7)

[0022]

(Equation 8)

Here, Ti: heating time [min], T
e: heating temperature [° C], Si: steel material Si concentration [wt%],
O ₂ : atmosphere O ₂ concentration [%] in the heating furnace. In the present invention, the erosion amount B satisfying R = 100 [%] in the equation (5) is determined to be used as the scale peeling difficulty B2 of the steel material. Therefore, the present invention compares the above B1 and B2,
With B1> B2 as the basic conditions for scale removal, F and S required for complete red scale removal are determined according to heating conditions (heating time, heating temperature, atmosphere O ₂ concentration in the heating furnace) and steel Si concentration, and scale removal. It was done.

In the present invention, the Si content of the hot-rolled steel sheet is set to 0.1 wt% or more, the condition of the impact of the injection water on the steel sheet surface is F> 240 [N], and S> 0.2 × 10 ⁶ [Pa]
It is most preferable to use under the above conditions. The reason is that in the region of F ≦ 240 or S ≦ 0.2 × 10 ⁶ [Pa], erosion of the aluminum test material does not occur, and therefore, it is impossible to remove red scale from Si steel. .

[0025]

EXAMPLES The hot-rolled steel sheet descaling method according to the present invention was applied to hot-rolling of Si steels having Si contents of 0.1, 0.5 and 1.6 wt%, respectively. First, as the descaler, the relationship between F and S was determined using the above equations (1) ′ and (2) ′, and a four-bank structure having different relationships was used. That is, four rows of nozzle headers for injecting water were arranged from upstream to downstream of the line . Specifically, from the upstream side, bank # 1 has F = 240 [N] and S = 0.10
× 10 ⁶ [Pa], # 2 bank F = 400 [N], S
= 0.81 × 10 ⁶ [Pa], bank # 3 has F = 480
[N], S = 1.5 × 10 ⁶ [Pa], # 4 bank is F
= 960 [N] and S = 1.93 × 10 ⁶ [Pa].

Next, the unit erosion amount B was determined by erosion for 30 seconds with the above-mentioned aluminum test material, and B1 was determined by applying equation (3) to each of the above banks. The B1 is shown in FIG.
On the lower side , a vertical line (dotted line) raised from the F and S values of each bank is represented by an intersection that intersects the horizontal axis (amount of aluminum erosion). On the other hand, in the actual operation, the operation is performed by changing the use pattern of the descaler according to the heating condition and the Si content of the steel material. Therefore, Table 1 shows an example in which the heating conditions of the steel material are fixed and (F) and (S) necessary for complete removal of the red scale are determined for each Si content. Then, the Si concentrations, heating conditions, and arbitrary scale peeling rates R of the various Si steels in Table 1 are substituted into equations (4) and (5), and R
= Amount of erosion that satisfies 100% (hardness of scale peeling on the steel material side) is obtained, and is defined as B2. FIG. 2 shows an example of the B2.
Is shown above. That is, a vertical line is drawn down from the uppermost end (corresponding to R = 100%) of three straight lines having different Si contents in the figure to the horizontal axis, and the intersection with the horizontal axis is B2.

[0027]

[Table 1]

Therefore, based on FIG. 2, the descaling condition satisfying B1> B2 becomes clear. In particular,
With 0.1% Si steel, only # 2 bank, with 0.5% Si steel, only # 3 bank, and with 1.6% Si steel, using only # 4 bank, complete red scale It can be seen that it can be removed. Further, the relationship of FIG.
Considering the equation (2) ′, it is more advantageous to increase the water amount Q and the injection pressure P ₀ or to reduce the water droplet diameter D in order to improve the scale removal ability. It has also been found that an increase of ₀ is the most advantageous in improving the scale removing ability because the F and S can be increased simultaneously. However, in practice,
Since the increase in P ₀ leads to an increase in running cost, in order to secure F, a policy of increasing the water amount Q should be given priority in consideration of the temperature drop of the steel first. First, the policy of shortening the water injection distance L should be given priority.

FIG. 3 shows a comparison between the red scale peeling rate of a hot-rolled steel sheet after adopting the scale removing method according to the present invention and the peeling rate obtained by a conventional operation method. The target steel sheet had a Si content of 1.6 wt% and used # 4 banks in the application of the present invention. However, in the conventional method, a descaler corresponding to about # 1 bank having a low scale removing ability was used. As shown in FIG. 3, 90% of the steel sheets had a red scale peeling rate of 80% or less, which was significantly reduced to 2%, and the surface quality was significantly improved.

[0030]

As described above, according to the present invention, the scale removal capacity determined by the impact pressure F and the unit impact pressure S of the jet water on the steel sheet and the scale on the steel sheet side determined by the steel type and heating conditions The difficulty of peeling can be evaluated with the same index, and scale can be sufficiently removed even when the steel type or the heating conditions change. As a result, stable scale flaws can be prevented in the production of a hot-rolled steel sheet.

Since the relationship between the absolute values of F and S and the scale removal conditions Q, D, P, L, and β is quantified, the design of the scale removal device is facilitated.

[Brief description of the drawings]

FIG. 1 is a diagram showing a model of collision of water droplets on a steel plate in scale removal by jet water.

FIG. 2 Red scale peeling rate R, total impact force of spray water (F)
FIG. 3 is a diagram showing a relationship between the unit impact force and the unit impact force (S).

FIG. 3 is a diagram illustrating a red scale prevention effect according to an embodiment of the present invention.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-71330 (JP, A) JP-A-6-114432 (JP, A) JP-A-7-144213 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) B21B 45/08

Claims (3)

(57) [Claims] When removing the scale of a hot-rolled steel sheet by injecting water, the scale removal capability (B1) of the sprayed water and the scale peeling difficulty (B2) of the hot-rolled steel sheet are determined by the following equation: B1 = f ₁ (F, S, T) B2 = f ₂ ( K, R) at R = 100 Under the condition of S where B1> 0, water is jetted under scale removal conditions that satisfy B2 <B1. Method for removing scale from hot-rolled steel sheet. Here, F: total impact force of spray water on the steel sheet surface [N] F = f ₃ (D , P ₀ ) (1) S: unit impact force of spray water on the steel sheet surface [Pa] S = f ₄ (P ₀ , D, L, β) (2) T: Scale removal time [sec] R: Scale peeling rate [%] 2. The hot rolled steel sheet has a Si content of 0.1% by weight or more, and the condition of impact of the injection water on the steel sheet surface is F> 240.
2. The method for removing scale from a hot-rolled steel sheet according to claim 1, wherein [N] and S> 0.2 × 10 ⁶ [Pa] or more. 3. The descaling ability of the jet water (B1).
The scale peeling difficulty (B2) of the hot-rolled steel sheet is calculated by the equations (3) to ( 5 ).
For removing hot-rolled steel sheet scale. (Equation 1) (Equation 2) (Equation 3) Here, Ti: heating time [min], Te: heating temperature [° C.], Si: steel material Si concentration [wt%], O ₂ : atmosphere in heating furnace O ₂ concentration [%].