JPH11267741A - Manufacture of hot rolled steel plate having excellent surface property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a descaling condition excellent in scale removability in hot rolling even in a high Si steel plate whose content of Si is <=0.15 mass %. SOLUTION: A transfer speed of a steel piece and a rolling stock is specified to <=150 m/min when a high pressure water jet is injected on the surface of the steel piece to execute primary descaling just before starting rough rolling and further the high pressure water jet is injected on the surface of a rolling stock to execute secondary descaling just before starting finish rolling. Also, a colliding force U (MPa) of the high pressure water jet per unit area on the surface of stationary steel is set to >=U1 which is calculated by the formula: U1=0.98+3.26([Si]-0.2) in the primary descaling and set to >=U2 which is calculated by the formula: U2=0.65+0.017×[Si]/([P]+[S]) in the secondary descaling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for descaling a hot-rolled steel sheet (including a hot-rolled steel sheet used as a base material of a cold-rolled steel sheet and a surface-treated steel sheet), particularly, excellent in scale removal property. .

[0002]

2. Description of the Related Art In hot rolling in steel sheet production,
The slab is usually heated to a high temperature of 1000 ° C. or higher, and then subjected to rough rolling and finish rolling to roll to a predetermined thickness. At the time of the high-temperature heating, a primary scale is generated on the surface of the billet, and a secondary scale is generated from the start of rough rolling to the start of finish rolling, but when these scales are rolled without being removed, In addition, the scale penetrates into the product surface and becomes a scale flaw.

In order to prevent the occurrence of scale flaws, a scale remover (HDS) is provided immediately before the rough rolling mill, and the scale remover (FSS) is also provided immediately before the finish rolling mill.
B) is provided, and a high-pressure water jet is jetted from the high-pressure water jet nozzle provided on the HDS and FSB to the surface of a steel slab or a rolled material to remove a primary scale and a secondary scale.

[0004] It is known that the removability of scale is affected by the steel content, particularly the Si content.
When Si becomes high Si steel of 0.15 mass% or more, the removability of scale is rapidly deteriorated. This is because in high Si steel, firelite, which is a low melting point oxide, is formed between the iron oxide and the steel during heating of the steel slab, and this remains without being removed. A defect called scale is generated, which causes scale flaws.

Various descaling conditions have been devised in order to effectively remove the scale (descaling) of the high Si steel. For example, Japanese Patent Application Laid-Open No. Hei 6-71330 specifies the impact force per unit area of a high-pressure water jet immediately before finish rolling, the flow rate, and the descaling time. In order to improve the surface properties of products, a method has been proposed in which residual primary scale generated during slab heating and secondary scale generated during rough rolling are collectively removed immediately before finish rolling. Japanese Patent Application Laid-Open No. 6-114432 discloses that the primary scale is sufficiently removed from the viewpoint that, when the primary scale remains, the removability of the secondary scale is deteriorated and the scale removal immediately before finish rolling becomes difficult. Thus, a technique for improving the surface properties has been proposed.

[0006]

However, in order to implement the former technique, it is necessary to finely define the production conditions before the start of finish rolling, which is inferior in operability and has a sufficient scale removing effect. It is hard to say. Further, it cannot be said that a sufficient scale removing effect is obtained by the latter technique for the following reasons, and the scale flaw is not completely eliminated.

[0007] The removability of scale immediately before finish rolling mainly depends on the amount of Si added to the steel, the degree of residual primary scale, and the contents of P and S in the steel. That is, in the case of the Si-added steel, the primary scale is likely to remain, and the remaining primary scale is crushed by rolling and at the same time is taken into the secondary scale generated by oxidation during the rough rolling, and the Si oxide becomes secondary. Takes a structure dispersed in the scale. The secondary scale in which Si oxide is dispersed has a higher strength than when it is not included,
Since it is very difficult to peel off, an extremely high impingement force of a high-pressure water jet is required to completely remove it.

On the other hand, the removability of the secondary scale in which the Si oxide is not dispersed, unlike the primary scale, is less affected by the amount of Si added, but is affected by the P and S contents. In a steel sheet requiring high workability, the P and S levels need to be kept low. In such a case, the removability of the secondary scale is deteriorated.

Therefore, even if the primary scale can be surely removed, depending on the content of P and S, the secondary scale generated thereafter tends to remain and the surface properties of the product deteriorate. In other words, it is necessary to add Si to secure the strength and to reduce the P and S contents to secure the workability. In such a case, the scale immediately before the finish rolling has the Si oxide dispersed therein. Further, since the P and S contents are low, the removal becomes very difficult. Therefore, a sufficient quality of the surface properties of the steel sheet cannot be achieved only by the scale removing operation just before the finish rolling or by the scale removing operation only before the start of the rolling.

The present invention has been made in view of such a problem, and provides a descaling condition excellent in scale removability in hot rolling even for a high Si steel sheet having a Si content of 0.15 mass% or more. It is another object of the present invention to provide a method for easily producing a hot-rolled steel sheet free from scale defects.

[0011]

According to the method of the present invention for producing a hot-rolled steel sheet having excellent surface properties, a steel slab containing at least 0.15 mass% of Si is heated and then subjected to rough rolling and finish rolling. Immediately before the start of rough rolling, a high-pressure water jet is sprayed on the surface of the billet to perform primary descaling, and immediately before the start of finish rolling, a high-pressure water jet is sprayed on the surface of the rolled material.
In a method for producing a hot-rolled steel sheet for performing the next descaling,
The moving speed of the billet and the rolled material at the time of performing the primary descaling and the secondary descaling is set to 150 m / min or less, and the collision force U (MPa) per unit area on the stationary steel surface of the high-pressure water jet is applied. ) Is set equal to or larger than U1 calculated by the following equation 1 in the primary descaling, and the following equation 2 is set in the secondary descaling.
A method for producing a hot-rolled steel sheet having excellent surface properties set to U2 or more calculated by the following equation. U1 = 0.98 + 3.26 ([Si] -0.2) Equation 1 U2 = 0.65 + 0.017 × [Si] / ([P] + [S]) Equation 2 where [X] Represents the mass% of the element X.

First, the scale removal conditions before the start of rough rolling will be described. Removal of scale immediately before the start of rough rolling
In the case of the Si-added steel, when the addition amount is 0.15 mass% or more, the amount rapidly decreases. This is because, as described above, firelite caused by Si is generated in the primary scale generated by heating in an oxidizing atmosphere performed before the rough rolling. The degree of residual scale after descaling immediately before the start of rough rolling has a significant effect on the scale removability immediately before finish rolling.

Therefore, the present inventor has determined that the Si content (mass
%) Using high Si steels of 0.2% and 1.0%, heating the steel slab to 1200 ° C. in an oxidizing atmosphere, and then moving the steel slab at a speed of 150 m / min. The force was set variously, descaling was performed immediately before the start of rough rolling, and the residual scale thickness distribution after rapid cooling and scale removal was investigated. The result is shown in FIG.

The impact force of the high-pressure water jet in the present invention means the impact force U (MPa) per unit area of the high-pressure water jet on the stationary steel surface defined by the following equation. This equation is an empirical equation determined off-line by determining the relationship between the measured value of the impact and each parameter.
The cumulative rate (%) on the vertical axis in FIG. 1 is, for example, 4% of the scale when the cumulative rate for a scale thickness of 60 μm is 40%.
0% or more means that the thickness is 60 μm or more.

U = A × P × (Q ^0.64 / L ^1.38 ) × (α /
27) ^K x 0.098 (MPa) K = 0.339 ln (H)-2.7903 where A: constant (determined off-line by a pressure sensor or the like so that the calculated value matches the actually measured value); P: injection pressure (kgf / cm ² ) of high-pressure water jet nozzle, Q: flow rate per nozzle (l / min), L: distance (mm) from steel surface to nozzle tip (see FIG. 3), α: nozzle Injection angle (degree), H: vertical distance (mm) between steel surface and nozzle tip (see Fig. 3)

FIG. 1 shows that the residual scale becomes thicker as the amount of Si added increases at a constant impact force. In addition, the residual scale thickness distribution was investigated by changing the impact force, and the residual scale thickness did not fall below a certain value even when the impact force was increased for both the Si content of 0.2 mass% and 1.0 mass%. It has been found that the limit is an average thickness of approximately 50 μm.

From the above results, the impact force required to reduce the average scale thickness to the limit value of about 50 μm, and the condition under which the frequency of existence of the thick scale is reduced to 0.
2% Si-0.98MPa, 1.0% Si-3.59MPa
Accordingly, the collision force U1 (MPa) of the high-pressure water jet required for removing the primary scale was set as in the following equation 1. What this formula means is that in order to limit the average thickness of the scale to the limit, it is necessary to increase the necessary collision force at a constant rate as the Si addition amount increases. Is that even if the collision force is increased beyond the value of Equation 1, the residual scale does not become thin. U1 = 0.98 + 3.26 ([Si] -0.2) Formula 1 where [X] indicates the mass% of the element X. The same applies to Expression 2 described later.

Next, the scale removal conditions immediately before the start of the finish rolling will be described. The impact force required for scale removal immediately before the start of finish rolling was determined in view of the primary scale removal limit. That is, even if the average thickness of the residual scale after scale removal immediately before rough rolling is around the lower limit of 50 μm, the impact force of the high-pressure water jet required for scale removal immediately before finish rolling comes from the residual primary scale. It is affected by the dispersion of the Si oxide in the secondary scale and the P and S contents in the steel. If the primary scale is completely removed and the dispersion of the Si oxide does not occur in the secondary scale, the scale removability immediately before finish rolling does not depend on the amount of Si in the steel, but rather, P, The removability changes depending on the amount of S. Therefore, when the primary scale remains, it is necessary to consider the amounts of P and S in addition to the influence of Si oxide dispersion. Even when the residual scale thickness distribution is the same, the amount of firelite generated during slag heating is
Because it depends on the amount, it depends on the amount of Si.

Therefore, when the scale removal immediately before the start of rough rolling is performed with a collision force indicated by U1 or more, an investigation was conducted to determine the collision force required for scale removal immediately before the start of finish rolling. This is because, using steels having various components shown in Table 1, the slab moving speed when passing through the HDS is 100 m / min.
After removing the primary scale by setting the impact force of the high-pressure water jet on the HDS to 2 MPa, the impact force of the FSB high-pressure water jet and the [Si]
/ ([P] + [S]). In this study, the moving speed of the rolled material when passing through the FSB was 5
0 to 100 m / min.

FIG. 2 shows the results of the investigation. According to FIG.
It has been found that the generation of red scale can be prevented by setting the collision force U of the high-pressure water jet at or above U2 (MPa) calculated by the following equation (2). U2 = 0.65 + 0.017 × [Si] / ([P] + [S]) Equation 2

[0021]

[Table 1]

Next, the moving (passing) speed at the time of descaling of the slab and the rolled material (both are referred to as steel materials) at the time of scale removal will be described.

The moving speed of the steel material is a very important factor. A high-pressure water jet is composed of high-velocity water particles, and the impact force depends on the frequency of the water droplet particles arriving per unit time and the kinetic energy of the water droplets. Usually, since the descaling is performed while moving the steel material, the number of water particles reaching the unit area is actually smaller than the impact in the stationary state. Therefore, if the impact force of a high-pressure water jet on a stationary steel surface is applied as the impact force during scale removal during steel movement, it cannot be strictly said that there is no effect on scale removal. According to the experiment of the above, if the moving speed of the steel material is 150 m / min or less, even if the impact force of the high-pressure water jet on the stationary steel surface is applied as the impact force at the time of actual scale removal, the scale removal performance It was confirmed that there was no problem.

Since the moving speed of the rolled material at the entrance of the finishing mill is usually 150 m / min or less, in actual operation, the moving speed of the steel material when passing through the HDS is set to 150 m / min or less, and the high-pressure water jet is used. By setting the collision force U to be equal to or greater than the values U1 and U2 calculated by the equations 1 and 2, it is possible to reliably prevent the occurrence of scale flaws.

[0025]

EXAMPLE A slab having the components shown in Table 1 was heated in a heating furnace for 120 minutes.
After heating to 0 to 1250 ° C, the moving speed when passing through HDS, H
Rough rolling and finish rolling were performed by setting the collision force U per unit area of the high-pressure water jet on the stationary steel surface in DS and FSB under the conditions shown in Table 2. In addition, the moving speed of the rolled material when passing through the FSB was set to 50 to 150 m / min.
Table 2 also shows the collision forces U1 and U2 calculated by Expressions 1 and 2.

The obtained hot-rolled steel sheet was examined for scale flaws, and the surface properties were evaluated by (flaw occurrence frequency × size). The flaw occurrence frequency was determined by dividing the entire length of the coil into six areas and determining the number of flaw occurrence areas / 6 according to the presence or absence of scale flaws in each area.
For example, if flaws are found in two areas, the frequency is 2
/6=3.3. In addition, the size of the scale flaw was indicated by a rank in which the size of the flaw was divided into three stages. Size 0 = no flaw, size 1 = scattered, size 2 = narrow band, and size 3 = wide band. Therefore, the evaluation of the surface properties includes a case of 0 (no scale flaw), and is a total of 19 levels.

[0027]

[Table 2]

According to Table 2, scale flaws are reliably prevented by setting the collision force U of the high-pressure water jet in the HDS and FSB to be equal to or higher than the limit collision force U1, U2 determined off-line by Equation (1) or (2). It was confirmed that it was possible.

On the other hand, HDS and / or FS
The impact force of the high-pressure water jet at B is U1 or U2
If it is less than 1, generation of scale flaws will be recognized. Further, even when the collision force of the high-pressure water jet in the HDS and FSB was equal to or higher than the limit collision force, when the steel material moving speed at the time of passing through the HDS was 200 m / min, scattered scale flaws were observed in a relatively wide range.

[0030]

According to the manufacturing method of the present invention, by setting the collision force U of the high-pressure water jet in the HDS to be equal to or greater than U1 determined by the Si content, the average thickness of the primary scale can be easily adjusted to near the limit value. Can be removed,
Further, the collision force U of the high-pressure water jet in the FSB is S
By setting the collision force to be equal to or higher than the predetermined collision force U2 in consideration of the i, P, and S contents, it is possible to reliably prevent the occurrence of scale flaws. Further, the impact force of the high-pressure water jet on the HDS and FSB can be easily set off-line, and the implementation is easy because there is no restriction on the rolling operation.

[Brief description of the drawings]

FIG. 1 is a graph showing the accumulation rate of residual scale after primary descaling due to the difference in the Si content and the impact force of a high-pressure water jet.

FIG. 2 is a graph showing the relationship between the FSB high-pressure water jet impact force affecting the generation of red scale after primary scale removal and [Si] / ([P] + [S]) in steel.

FIG. 3 is an explanatory diagram showing a state of spraying a high-pressure water jet on a steel material surface.

Claims (1)

[Claims] 1. A steel slab containing at least 0.15 mass% of Si is heated and then subjected to rough rolling and finish rolling. -Performing the ring, and jetting a high-pressure water jet to the surface of the rolled material just before the start of the finish rolling, and
In the method for manufacturing a hot-rolled steel sheet to be subjected to a ring, the moving speed of the steel slab and the rolled material at the time of performing the primary descaling and the secondary descaling is set to 150 m / min or less, and the stationary steel material surface of the high-pressure water jet is provided. In the first order descaling, the collision force U (MPa) per unit area is set to be equal to or more than U1 calculated by the following equation 1, and in the second order descaling, the following equation 2
A method for producing a hot-rolled steel sheet having excellent surface properties set to U2 or more calculated by the following equation. U1 = 0.98 + 3.26 ([Si] -0.2) Equation 1 U2 = 0.65 + 0.017 × [Si] / ([P] + [S]) Equation 2 where [X] Represents the mass% of the element X.