JPH06114432A - Manufacture of high silicon hot rolled steel sheet - Google Patents

Abstract

PURPOSE:To provide a practical method capable of manufacturing a high silicon hot rolled steel sheet reduced in scale flaws. CONSTITUTION:A manufacture of high silicon hot rolled steel sheet is obtained by a method wherein after the sheet is extracted from a heating furnace, high pressure water having a discharge pressure 300-700kgf/cm<2> is sprayed to a billet in which an upper layer part made of primary scale, mainly, a wustite layer is removed before a rough rolling or after >=1 pass of rough rolling under such a condition that collision energy (E) indicated by an expression 1) as follows exceeds 50kJ/m<2>, the remaining primary scale is removed, then, hot rolling is exerted. E=98 (PXQ)/(WXV)...1), where E: collision energy (kJ/m<2>), P: discharge pressure (kgf/cm<2>) per nozzle, Q: flow rate (liter/min) per nozzle, W: injecting width (mm) per nozzle, V: running speed (m/min) of billet. In this way, the remaining rate of scale is lowered and a product free from red scale and scale flaw can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled steel sheet of high Si steel having a small amount of scale flaws and a good surface quality, and particularly to a scale having a difficulty in peeling (so-called red scale) which causes scale flaws. -) Is efficiently removed.

[0002]

2. Description of the Related Art In the production of hot-rolled steel sheet, it is necessary to load a steel slab into a heating furnace and heat it to 1100-1400 ° C. before rolling. Since this heating atmosphere is oxidizing, an oxide film, that is, a scale, is formed on the surface of the steel slab. This scale is mainly composed of iron oxides such as Fe ₂ O ₃ , Fe ₃ O ₄ , FeO, etc., and depending on the alloy composition, it is composed of oxidation products containing Cr ₂ O ₃ , SiO ₂ , Al ₂ O _3, etc. The thickness of the scale (primary scale) when extracted from the heating furnace is 0.5 to 3.0 mm.
It also becomes.

If the primary scale produced in the heating furnace or the secondary scale newly produced after the removal of the primary scale is not sufficiently removed, the scale is eroded on the surface of the product. It remains as a scale flaw. The scale flaws of the steel plate in which such a scale bites not only deteriorates the appearance of the as-rolled product (black leather product), but also the product after the scale is completely removed by pickling. In addition, it becomes a cause of leaving uneven defects.

For the purpose of preventing the above scale flaws, the primary scale and the secondary scale are removed prior to rough rolling and finish rolling. Fig. 1 shows an example of the arrangement of the descaling device in the hot-rolled steel sheet production line.
Of the high pressure water from the scale breaker VSB equipped with a vertical roll arranged in front of the rough rolling mill and the nozzle A immediately behind the scale breaker VSB. That is, VSB
The vertical roll is used to apply a reduction of several% to several tens% in the width direction to raise the scale layer on the surface of the slab and make it easy to peel off, then inject 100-150 kgf / cm ² of high-pressure water. To remove the primary scale. The flow rate per nozzle is usually about 50 to 200 liters / min.

The removal of the secondary scale is carried out by a jet jet of high pressure water of 100 to 150 kgf / cm ² from nozzles (BG) arranged immediately before each stand (R1 to R6) of the rough rolling mill. , 2 to 4 rows of nozzles (H, I, J) installed immediately before the finish rolling mill
Etc.) from the jet jet. The water injection pressure here is also usually 100 to 150 kgf / cm ² .

The quality of the peelability of the scale is greatly influenced by the composition and structure of the scale, and in the case of the scale of steel having a large Si content (hereinafter referred to as high Si steel), the scale is greatly peeled. It is known that sex becomes worse. In high-Si steel, low-melting-point oxide firelite (2FeO ・ SiO ₂ ) is formed between wustite (FeO) and steel during heating of the billet, and the eutectic temperature of firelite and wustite (1170 ℃) )
With the above, a layer in which firelite and wustite are intricately entangled with each other is formed, so that the peelability of the scale is significantly deteriorated. Therefore, a scale residue that is not removed by the above-mentioned descaling exists on the surface of the billet of high Si steel, and it becomes a defect called so-called red scale.

As a method of preventing the red scale of the hot rolled steel sheet of high Si steel, (a) reducing the Si content in the steel, (b)
Measures such as keeping the heating temperature of the billet below 1170 ° C can be considered. However, in order to ensure the desired steel plate properties (e.g. workability), it is sometimes necessary to contain Si in a certain amount or more, and because there is no cheap component that has the same function and effect in place of Si, Method a) is not practical.

In the method (b) of heating at 1170 ° C. or lower to prevent the formation of a eutectic of firelite and wustite, the rolling temperature is lowered, and the rolling load and rolling torque are increased. Therefore, for example, in the case of a wide material or a thin material, there is a problem that it cannot be produced because the rolling condition exceeds the equipment specifications.

Japanese Unexamined Patent Publication (Kokai) No. 63-68214 proposes a method of removing the scale by high-pressure water jet with an impact pressure of 25 g / mm ² or more before finish rolling. But this method
Since the slab extracted from the heating furnace is descaled after being stretched 10 times by rough rolling, the descaling area is large and it is not economical, and the equipment before the finishing rolling mill, that is, the existing equipment It is difficult to make equipment such as a descaling device, a thickness gauge, a temperature sensor, a length gauge, and a crop cutting device, and there is also a problem that the sprayed water becomes a disturbance factor of various sensors. It cannot be said that it is practical.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a hot rolled steel sheet of high Si steel which is difficult to descale.
In particular, it is an object of the present invention to provide a manufacturing method in which descaling at the time of rolling is effectively performed by a practical method and the occurrence of flaws in the product steel sheet due to scale is reduced.

[0011]

The gist of the present invention is the following method for producing a hot rolled steel sheet.

(1) After extraction from a heating furnace, a steel piece from which the upper layer mainly consisting of the wustite layer of the primary scale has been removed is subjected to high pressure water with a discharge pressure of 300 to 700 kgf / cm ² before rough rolling. The collision energy (E) expressed by the formula is 50 kJ / m
^A method for producing a hot-rolled steel sheet of high Si steel, comprising spraying under a condition of ² or more, removing the remaining primary scale, and then hot rolling.

E = (P × Q) / (W × V), where E: collision energy (kJ / m ² ) P: nozzle discharge pressure (kgf / cm ² ) Q: per nozzle Flow rate (liter / min) W: Injection width per nozzle (mm) V: Travel speed of steel slab (m / min) (2) After extraction from the heating furnace, the upper layer consisting mainly of the wustite layer of the primary scale The steel strip from which the parts have been removed is subjected to at least one pass of rough rolling, and then the discharge pressure is 300 to 700 kgf / c.
Collision energy (E) expressed by the above formula for high-pressure water of m ²
Of 50 kJ / m ² or more, the remaining primary scale is removed, and then the remaining pass of rough rolling and finish rolling are performed. Manufacturing method of rolled steel sheet.

Here, high Si steel means that Si is approximately 0.10.
It means steel containing at least wt%.

In such a steel, the scale having poor peelability is generated as described above, and the scale flaw becomes a problem.
The Si content varies depending on the steel type, but steel containing about 10% by weight of Si is also a subject of the present invention. The present invention was made based on the following findings obtained by detailed investigation on the descaling property of high Si steel.

(1) The primary scale of high Si steel has a discharge pressure of 300
It cannot be completely removed by spraying with a high-pressure water jet of about 1000 kgf / cm ² . However, most of the wustite layer, which is the upper layer of the primary scale, can be removed even with the discharge pressure of 100 to 150 kgf / cm ² , which is the usual descaling condition. Usually high Si
The primary scale of steel consists of a thin layer of firelite at the interface with the steel and a thick layer of Wustite above it.
The upper layer, wustite, has good peelability, and the discharge pressure is 100-150k.
It can be removed by jetting high-pressure water of about gf / cm ² .

However, since the firelite layer is intricately entangled with steel and wustite, it is 100 to 150 kgf / cm ²
It cannot be removed with moderate discharge pressure. Discharge pressure from 300 to 10
Even if the spray is increased to about 00 kgf / cm ² , if it is sprayed from above the thick wustite layer, the collision energy does not reach the firelight layer, and the firelight layer remains and scales.
Defects occur.

(2) However, the upper wustite layer, which constitutes the majority of the primary scale, is removed by, for example, reduction with a vertical roll and high-pressure water jet with a discharge pressure of about 100 to 150 kgf / cm ² , and then rough rolling. Before, or after performing one or more passes of rough rolling, further inject high-pressure water with a discharge pressure of 300 kgf / cm ² or more under the condition that the collision energy of the above formula is 50 kJ / m ² or more. The impinging pressure of water effectively acts on the firelite layer, causing it to mechanically delaminate and be completely removed.

[0019]

The structure and operation of the present invention will be described in detail.

In the method of the present invention, the billet is heated at a normal heating temperature that does not hinder rolling, for example, 1200 to 1300 ° C. This heating creates a primary scale of 2-3 mm thickness on the surface of the high Si steel billet. At this time, between the steel and the wustite layer that constitutes the atmosphere side portion (upper layer) of the scale,
As mentioned above, a low-melting oxide called firelite (2FeO ・ SiO ₂ ) is generated and penetrates into the grain boundary of the steel to strongly bond the steel-wustite interface and remove wustite by complicatedly entangled with wustite. It also worsens sex.

In the method of the present invention, first, immediately after the steel slab is extracted from the heating furnace, the upper layer of the primary scale, which is mainly composed of wustite, is removed. Hereinafter, this step is referred to as "preliminary primary scale removal". The conditions for this preliminary removal of primary scale may be the conditions for normal descaling. For example, with a vertical roll, width reduction of several% to several tens% and discharge pressure of 100%
Conditions such as high-pressure water injection of 150 kgf / cm ² can be adopted.
However, this pre-primary scale removal alone did not completely remove the firelite and its mixture layer with wustite, and when the rolling was continued in that state, the remaining scale was observed as red scale. ,Also,
These cut into the surface of the product and become scale flaws.

Therefore, after removing most of the primary scale mainly composed of wustite by the above-mentioned preliminary primary scale removal, high-pressure water having a discharge pressure of 300 kgf / cm ² or more is sprayed. At this time, the conditions are set so that the collision energy of the high-pressure water, that is, E defined by the above equation is 50 kJ / m ² or more. Most of the fire lights can be mechanically removed by the collision energy of this high-pressure water, so that no red scale occurs and no scale flaw occurs.

If the rolling is performed for one pass or more before the high-pressure water having a discharge pressure of 300 kgf / cm ² or more is sprayed, the remaining firelite layer is thinly spread and fine cracks are introduced into the layer. As a result, the effect of scale removal by the next high-pressure water injection is increased. Descaling by high-pressure water injection after completion of all passes of rough rolling improves the descaling property further.However, as described above, since the steel sheet extended by rolling is processed, descaling is performed. The area increases, the energy consumption increases, and the cost increases.

Here, the discharge pressure of the jet water is 300 kgf / c.
The reason why the pressure of m ² or more is necessary is that the peeling of the firelight is not sufficiently performed at a pressure lower than this, as shown in Examples 2 and 4 described later. The upper limit of the water discharge pressure is set to 700 kgf / cm ² from the viewpoint of energy saving and equipment cost reduction. A practical value of 700 kgf / cm ² or less is sufficient to mechanically separate the firelight.

Next, the reason why E defined by the formula must be 50 kJ / m ² or more will be described. The formula is introduced based on the new finding of the present inventor that it is optimal to use the collision energy of high-pressure water per unit area of the steel strip as a parameter representing the strength of descaling by high-pressure water. Is. As shown in FIGS. 3 and 5 in which the test results of Examples described later are summarized, when the collision energy E defined by the equation is 50 kJ / m ² or more,
The scale residual rate is 10% or less, which is not a problem in actual production. Even with this collision energy, it is not possible to remove even the part where the firelite penetrates into the grain boundaries of steel, but since its proportion to the entire surface of the grain boundaries is low, it does not cause harmful scale flaws, and also in terms of appearance. Since it does not become a red scale, it does not impair the commercial value of hot rolled steel sheet.

The billet from which the primary scale has been removed by the method described above is then sent to the steps of rough rolling (or a subsequent pass of rough rolling) and finish rolling. There, rolling is carried out while removing the secondary scale by the usual method described with reference to FIG.

The method of the present invention removes primary scale prior to rolling. Therefore, the descaling area is much smaller than the method of performing descaling before finish rolling, and the occurrence of irregularities on the surface is small. In addition, it is easy to interact with existing equipment and is highly practical.

[0028]

Example 1 A billet of high Si steel of steel type A shown in Table 1 (size
200 mm L × 70 mm W × 30 mm T) was heated at 1220 ° C. and rolled in a hot-rolled steel sheet rolling test line consisting of three stands shown in FIG.

First, high-pressure water having a discharge pressure of 120 kgf / cm ² was sprayed from the nozzle 1 onto the steel slab 7 extracted from the heating furnace to remove the primary scale in advance, and subsequently the high-pressure water was sprayed from the nozzle 2. As the conditions for jetting from this nozzle 2, discharge pressure is 150 to 1000 kgf / cm ² , flow rate is 20 to 60 liters / mi.
n, spray width is variously changed in the range of 40 to 300 mm, and the striping speed is changed by changing the strip running speed of the steel strip 6 in the range of 30 to 100 m / min.
I went to Le. Then set the rolling start temperature to 1000 ° C and set No. 1
Stand (3), No. 2 stand (4) and No. 3 stand
Three-stand tandem rolling in (5) was performed to obtain a hot-rolled steel sheet having a thickness of 6 mm. FIG. 3 also shows changes in plate thickness at each stand.

[0030]

[Table 1]

The scale condition of the steel sheet surface obtained as described above was investigated. The descaling property was evaluated by using a 100 μm mesh grid point (100
The presence or absence of the red scale below the points) was counted and expressed as an area ratio. The result is shown in FIG.

The horizontal axis of FIG. 3 is the collision energy E defined as described above. As shown in the figure, the red scale residual rate sharply decreases with the increase of E. And E is 50 k
If it is J / m ² or more, the red scale residual rate is 10% or less, which is the quality level required for ordinary products.

[0033]

Example 2 Steel pieces A to C in Table 1 (size 220 mmt × 11)
(50 to 1300 mm W) is hot-rolled in an actual production line to a thickness of 3.2
mm steel plate. The heating temperature of the steel slab was set to 1250 ° C, where red scale is most likely to occur, the primary scale of the steel slab extracted from the heating furnace was removed, and this was rolled to a plate thickness of 30 to 35 mm with a 6-stand rough rolling machine, and then It was rolled to a final plate thickness as described above with a 7-stand finishing mill at an outlet temperature of 870 to 900 ° C. and wound up.

The removal of the primary scale is carried out by preliminarily removing the primary scale by injecting high-pressure water having a discharge pressure of 140 kgf / cm ² after width reduction with a vertical roll, and then injecting high-pressure water under the conditions shown in Table 2. I went the way. The secondary scale was removed by injecting 140 kgf / cm ² of high-pressure water on the entrance side of each stand.

A 100 mm square sample was cut out from the center of the obtained hot-rolled steel sheet, and a grid-like grid with a grid width of 10 mm was drawn to determine the area ratio by the presence or absence of a red scale at the grid points, and the red scale. It was defined as the residual rate. The results are also shown in Table 2. As can be seen from this table, in the conventional method (No. 1), the red scale residual rate was 96%, which was observed as red scale unevenness in the visual appearance, but No. 4 to No. With No. 13, almost no red scale remained, and a steel sheet having a good appearance was obtained.

No. 2 and No. 3 have a discharge pressure of 300 kgf / cm ²
It is a lower comparative example. In this case, the red scale residual rate is
It is 95% and 97%, and the effect of sufficient descaling is not obtained.

[0037]

[Table 2]

[0038]

Example 3 A piece of high Si steel of steel type A shown in Table 1 (size 200 mm L × 70 mm W × 30 mm T) was heated at 1220 ° C., and FIG.
Rolling was carried out on a hot-rolled steel sheet test rolling line consisting of four stands shown in.

First, high-pressure water with a pressure of 120 kgf / cm ² was sprayed from the nozzle (1) to the steel slab 7 extracted from the heating furnace to remove the primary scale beforehand, and then the No. 1 stand (3) was used for 1 pass. After that, high-pressure water was subsequently jetted from the nozzle (2). The conditions of jetting from this nozzle (2) and the plate passing speed are the same as in the first embodiment. Then set the rolling start temperature to 1000 ° C, and set No. 2 stand (4), No. 3 stand (5).
And 4-stand tandem rolling was performed on No. 4 stand (6) to obtain a hot rolled steel sheet having a thickness of 6 mm. FIG. 4 also shows changes in plate thickness at each stand.

The scale condition of the steel sheet surface obtained as described above was evaluated by the same method as in Example 1, and the results are shown in FIG. As in the case of Fig. 3, when E is 50 kJ / m ² or more, the red scale residual rate is 10% or less, which is the quality level required for ordinary products. The decrease is more remarkable than in the case of FIG.

[0041]

Example 4 Similarly to Example 2, steel pieces (size 220 mmt × 1150 to 1300 mm W) having chemical compositions A to C in Table 1 were used.
3.2mm thick by heating at 50 ℃ and hot rolling in the actual production line
Steel plate. However, in this example, the pre-primary scale removal of the steel slab extracted from the heating furnace (the conditions are those of Example 2).
Same as the above), and then rolled in 1 to 3 passes of the rough rolling mill,
Then, high-pressure water was sprayed under the conditions shown in Table 3 to remove the remaining primary scale, followed by rough rolling for 5 to 3 passes and rolling to a plate thickness of 30 to 35 mm. The rough rolling has a total of 6 passes. Then the exit temperature was 8 on a 7-stand finishing mill.
It was rolled to a final plate thickness of 70 to 900 ° C. and wound up. The secondary scale was removed by injecting 140 kgf / cm ² of high-pressure water at the entrance side of each stand, as in Example 2.

With respect to the obtained hot-rolled steel sheet, the red scale residual rate was measured in the same manner as in Example 2, and the results are shown in Table 3. In addition, in Table 3, the results of the conventional example (No. 1) in Table 1 are also shown. As can be seen from this table, in Nos. 4 to 13 corresponding to the examples of the present invention, steel sheets having a good appearance with almost no red scale remaining were obtained.

[0043]

[Table 3]

[0044]

EFFECTS OF THE INVENTION According to the method of the present invention, even if it is a high Si steel, the scale produced in the heating furnace, in particular, the scale containing the low melting point oxide, which is difficult to remove, is almost completely removed, and then rough rolling and finish rolling are performed. Done. Therefore, it is possible to manufacture a hot-rolled steel sheet having good surface properties without scale defects due to the residual primary scale.

[Brief description of drawings]

FIG. 1 is a schematic view of a general hot-rolled steel sheet production line showing an arrangement of descaling equipment.

FIG. 2 is a schematic view of a hot-rolled steel sheet test rolling line used in Examples.

[FIG. 3] Collision energy of high-pressure water measured in an example
It is a figure which shows the relationship between (E) and a red scale residual rate.

FIG. 4 is a schematic view of a hot-rolled steel sheet test rolling line used in another example.

FIG. 5: Impact energy of high-pressure water measured in another example
It is a figure which shows the relationship between (E) and a red scale residual rate.

[Explanation of symbols]

A to J: High pressure water jet nozzle, VSB: Vertical roll scale breaker R1 to R6: Rough rolling stand, F1 to F7: Finishing rolling stand 1, 2: High pressure water jet nozzle, 3 to 6: Rolling stand,
7: Billet

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuhiro Nojima 4-53-3 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Inventor Yukio Matsuda 4-chome, Kitahama, Chuo-ku, Osaka City, Osaka Prefecture 5-33 Sumitomo Metal Industries, Ltd. (72) Inventor Tomoki Fukagawa 4-53-3 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd.

Claims (2)

[Claims] 1. A steel slab obtained by removing an upper layer portion mainly composed of a wustite layer of a primary scale after extraction from a heating furnace,
Before rough rolling, high-pressure water with a discharge pressure of 300 to 700 kgf / cm ² has a collision energy (E) of 50 kJ / m ² expressed by the formula below.
A method for producing a hot-rolled steel sheet of high Si steel, comprising spraying under the above conditions, removing the remaining primary scale, and then hot rolling. E = 98 (P × Q) / (W × V) ・ ・ ・ ・ ・ However, E: Collision energy (kJ / m ² ) P: Nozzle discharge pressure (kgf / cm ² ) Q: Flow rate per nozzle ( Liter / min) W: injection width per nozzle (mm) V: traveling speed of billet (m / min) 2. After extraction from a heating furnace, a steel slab from which the upper layer mainly consisting of the wustite layer of the primary scale has been removed is subjected to rough rolling for at least one pass, and then the discharge pressure is set to 300-.
700 kgf / cm ² of high-pressure water was sprayed under the condition that the collision energy (E) represented by the above formula was 50 kJ / m ² or more, the remaining primary scale was removed, and the remaining pass of rough rolling was used. A method for producing a hot-rolled steel sheet of high Si steel, characterized by performing finish rolling.