JP2001105021A - Method of manufacturing hot-rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a hot-rolled steel sheet excellent in surface property without scale defects by executing descaling by jetting high-pressure water jet from two rows of nozzles to high silicon and high nickel steel containing Si and Ni of 0.2-2.0 wt.% respectively in a hot-rolling stage. SOLUTION: By using a descaling device on which two rows of headers 4a, 4b are arranged at a proper interval in the advancing direction of the line and setting the collision pressure for descaling at >=3 kgf/cm2 and the attack angles θ of the descaling nozzles of each row respectively at 0-5 deg., descaling is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet having excellent surface properties by performing descaling by injecting a high-pressure water jet onto a steel material surface during hot rolling to prevent scale flaws and scale patterns from occurring. It relates to a manufacturing method.

[0002]

2. Description of the Related Art In the production of a steel sheet, it is general that a steel material is heated in a heating furnace at a temperature of usually 1100 to 1300 ° C. for several hours and then hot-rolled in a hot strip mill. At this time, if the steel material is rolled until the oxide scale generated on the surface of the steel material is not sufficiently removed, the oxide scale is pushed into the surface of the product and remains as scale flaws or scale patterns.

[0003] Such scale flaws and scale patterns are
Not only deteriorates the appearance of the rolled product (black scaled product), but also causes uneven defects to remain on the surface of the product (white scaled product) after removing the oxide scale by pickling.
In addition, it becomes a starting point of cracks during bending, and the difference in cooling capacity between the scale remaining part and the peeled part during forced cooling of the steel sheet in the hot rolling process causes unevenness in the mechanical property values of the steel sheet. It has a serious negative effect on quality.

Conventionally, as a method for preventing the occurrence of such scale flaws, a descaling device comprising a header having nozzles attached at regular intervals has been installed on the exit side of a heating furnace, on the side of a rough rolling mill, on the entrance side of a finishing mill, or the like. A method is adopted in which a high-pressure water jet is sprayed from each nozzle onto the steel surface to separate and remove oxide scale on the steel surface. In particular, for steel materials that emphasize surface quality, as shown in FIG. It is common practice to arrange two or more rows of descaling headers 2 on the entry side of the finishing mill 1 and to simultaneously inject high-pressure water from a plurality of rows.

The angle of attack θ of the descaling nozzle with respect to the line traveling direction is usually about 15 °. This is because the high-pressure water jet is jetted in the direction opposite to the steel material advancing direction to increase the speed component of the collision force of the high-pressure water jet in the line direction, thereby improving the scale peelability. The high-pressure water jet sprayed from the nozzle is also sprayed at a torsion angle γ with respect to a direction orthogonal to the line advancing direction as shown in FIG. 2 in order to make the impact force on the steel material surface uniform in the width direction of the steel material. It is wrapped with an adjacent high-pressure water jet with a constant wrap width a. In a row of nozzles, the temperature becomes high in portions other than the wrap portion, so that a high-temperature scale easily occurs before the finishing stand is engaged and between the finishing stands. Therefore, normally, the nozzles are arranged in two rows in front and back as shown in FIG. 2, and furthermore, the nozzles in the front row and the rear row are shifted in a direction orthogonal to the line advancing direction, and the injection part of the high-pressure water jet is shifted so that the width in the steel material width direction is The temperature is made uniform.

[0006]

FIG. 3 shows an example in which the headers for descaling are arranged in two rows.
As shown in the line, the high-pressure water jet from the nozzle of the header 4a installed at the front interferes with the high-pressure water jet from the nozzle of the header 4b located at the rear along the steel material surface, The impact force of the jet against the steel surface is reduced. According to the observation in the off-line test, in particular, as shown in Fig. 4, the high-pressure water jet injected from the nozzle collides with the steel surface and a part of the high-pressure water flowing in the direction of the back arrow is blocked by the next high-pressure water jet. When the water flows backward to generate a strong water flow, a strong water flow area 3 as shown by the diagonal lines in FIG. 5 is generated, and a phenomenon is seen in which this water penetrates the high-pressure water jet injected from the rear nozzle. It was found that the impact force of the jet in this area was significantly reduced.

An object of the present invention is to provide a method for manufacturing a hot-rolled steel sheet which can solve the above-mentioned problem that occurs when a plurality of rows of descaling nozzles are arranged.

[0008]

According to the first aspect of the present invention, a plurality of rows of descaling nozzles are arranged at appropriate intervals in the line traveling direction when hot rolling a steel material to produce a hot rolled steel sheet. In a method for producing a hot-rolled steel sheet in which a high-pressure water jet is sprayed onto a steel material surface to perform descaling, the descaling is performed by setting the impact pressure on the steel surface to 3 kgf / cm ² or more and forward in the line advancing direction. The angle of attack of the descaling nozzle with respect to the steel material is set to 0 to 5 °.

The invention according to claim 2 is characterized in that, in the invention according to claim 1, all of the descaling nozzles in each of the plurality of rows have an angle of attack of 0 to 5 °. The present inventors have developed a line progression for each nozzle when the descaling nozzles are arranged in two rows at appropriate intervals in the line progression direction with respect to the Si-containing steel and the Ni-containing steel, which are difficult-scale release materials. Various investigations were carried out by changing the angle of attack θ with respect to the direction and the collision pressure of the high-pressure water jet during descaling.

The investigation was carried out on steel materials A and B having the compositions shown in Table 1, and as a result, the injection of high-pressure water from the nozzle located at the rear in the line advancing direction was stopped, and the high-pressure water jets were aligned. In this case, even if the collision pressure is 3 kgf / cm ² or more and the angle of attack θ is 5 ° or less, the scale residual ratio increases,
When two rows of high pressure water jets are used, the collision pressure is 3kgf /
cm is less than ² angle of attack θ is 5 ° even the scale residual rate increases will increase the impact pressure to 3 kgf / cm ² or more, when the angle of attack θ on 5 ° or less, the scale residual rate is less than 1% I found that.

The reason why the upper limit of the angle of attack θ of the nozzle located in front with respect to the line traveling direction is set to 5 ° is that if it exceeds 5 °, it interferes with the high-pressure water jet from the nozzle located in the rear, and In order to reduce the drop in collision force due to water flow interference, the angle of attack θ should be set to 0.
° is desirable. Incidentally, when the nozzles are arranged in two rows, the angle of attack θ of the nozzles located at the front and the rear is 0 °.
As shown in FIG. 6, the high-pressure water jet injected from the nozzle of the rear header 4b hits the surface of the steel material 5 and flows toward the rear of the line, and the high-pressure water jet injected from the nozzle of the front header 4a is The amount of water flowing in front of the line on the surface is equalized, so that the front and rear water flows are interfered at an intermediate point between the two headers, thereby preventing the collision force of the high-pressure water jet ejected from the rear nozzle from being reduced.

The lower limit of the angle of attack θ is set to 0 °
°, that is, when the nozzle is oriented in the line travel direction,
This is because the oxide scale that has peeled off from the steel material surface due to the descaling and that has been removed is caught in the rolls of the rolling mill, thereby significantly deteriorating the surface quality of the product. To change the angle of attack of the nozzle, first disconnect the connection at the flange that connects the header and the pipe, rotate the header around the axis by the required amount, adjust the direction, and then remove the flange. It is good to fasten. This eliminates the need to replace a header with a different nozzle direction only by adjusting the angle of the header, which is economical.

According to a third aspect of the present invention, the descaling according to the first or second aspect of the present invention is performed such that the Si content is 0.2 to 2.0.
According to a fourth aspect of the present invention, the descaling according to the first or second aspect is performed on a steel material having a Ni content of 0.2 to 2.0% by weight. It is characterized by being carried out. When the Si content of the Si-containing steel is 0.2% by weight or more, firelite 2FeO.SiO ₂ is generated in the scale and the secondary scale during heating and penetrates deeply into the ground iron interface, making scale peeling difficult. Becomes Also, when the Ni content of the Ni-containing steel is 0.2% by weight or more, the unevenness of the ground iron interface becomes severe, and scale peeling becomes difficult.

According to the first and second aspects of the present invention, Si and N
It was found that the scale peeling effect was sufficient even when the test was carried out on steel materials in which the content of i was 0.2% by weight or more. In the invention according to claim 3, the upper limit of the Si content does not need to be limited, but when the Si content increases, the weldability and cold workability deteriorate.
The content was 2.0% by weight or less.

In the invention according to claim 4, the upper limit of the Ni content is not necessarily limited, but the Ni content is set to 2.0% by weight in consideration of toughness, ductility, economy and the like. It was as follows.

[0016]

EXAMPLES Steel materials A and B shown in Table 1 below were descaled under the conditions shown in Table 2 using a descaling apparatus having headers arranged in two rows, and then subjected to finish rolling. After the coil is recoiled and pickled, the scale remaining rate per unit area on the front and back of the coil is%.
Was visually inspected. The results are shown in Table 2.

[0017]

[Table 1]

[0018]

[Table 2]

Here, the collision pressure is obtained by measuring the load due to water jetted from one nozzle by a pressure receiving sensor and obtaining the collision pressure per unit area from the load / collision area of the water jet by actual measurement. In the table, nozzle A indicates a nozzle located in the front in the line traveling direction, and nozzle B indicates a nozzle located in the rear in the line traveling direction.

The angle of attack θ is 0 ° when the high-pressure water jet is injected perpendicularly to the steel, + when the high-pressure water jet is injected in the direction opposite to the line traveling direction, and − when the high-pressure water jet is injected in the same direction. And As shown in Table 2, No. 1 within the scope of the invention described in claim 1 was obtained. 2, No. 6 and no. 8
The steel materials of No. 1 each have a scale residual ratio of less than 1%, and are within the scope of the invention according to claim 2. 6 and no. 8
In the case of the steel material of No. 1, the scale remaining ratio further decreased, and when the attack angles θ of the front and rear nozzles were each set to 0 °, the scale remaining ratio was 0%. On the other hand, No. 1 in which descaling was performed with one row of nozzles. 5 and No. 5 Steel No. 10 has scale residual ratios of 5.9% and 12.2%, respectively, and has a collision pressure out of the range of the invention according to claim 1.
5 and No. 5 Steel No. 9 has a scale residual ratio of 6.
No. 8% and 9.5%, and the angle of attack exceeded the range of the invention according to claim 1. 1, No. 3 and No. 3 7 have a scale residual ratio of 11.2% and 29.
Reached 9% and 14.9%. In particular, in the case of No. In the case of the steel material No. 3, the descaled scale pieces were bitten into the finish rolling mill, and the roll flaws generated at that time were transferred to the finished product, and the scale bite flaws became large.

[0021]

According to the present invention, a plurality of rows of descaling headers are arranged for a high Si content steel and a Ni content steel, which have conventionally been considered to be difficult to remove scale, and the descaling impact pressure is 3 kgf / cm. By performing descaling by setting the angle of attack of the descaling nozzle located at ² or more and in front to 0 to 5 °, it is possible to obtain a hot-rolled steel sheet having good surface properties with few scale flaws. As in the invention described in Item 2, when the angle of attack of each row of nozzles is set to 0 to 5 °, a hot-rolled steel sheet having better surface properties with less scale flaws can be obtained. In particular, when the angle of attack of the nozzles in each row is set to 0 °, it is possible to most effectively prevent a decrease in the collision pressure due to interference of the high-pressure water jet water flow.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a rolling line provided with a descaling device.

FIG. 2 is a diagram showing a collision area of a high-pressure water jet.

FIG. 3 is a diagram showing a flow of a high-pressure water jet ejected from two rows of descaling nozzles.

FIG. 4 is a view showing the operation of a high-pressure water jet.

FIG. 5 is a diagram showing a high-pressure water jet water flow.

FIG. 6 is a diagram showing a flow of a high-pressure water jet injected from a descaling nozzle of the present invention.

[Explanation of symbols]

 1. Finishing mill 2. Descaling header 3. Strong water basin 4a, 4b Header 5. Steel

Claims (4)

[Claims] When a steel material is hot-rolled to produce a hot-rolled steel sheet, a high-pressure water jet is jetted onto the surface of the steel material from a plurality of rows of descaling nozzles arranged at appropriate intervals in the line traveling direction. In the method for producing a hot-rolled steel sheet for descaling, the descaling is performed with respect to a steel material of a descaling nozzle having a collision pressure against a steel material surface of 3 kgf / cm ² or more and located forward with respect to a line traveling direction. A method for producing a hot-rolled steel sheet, wherein the method is performed at an angle of 0 to 5 °. 2. A method for manufacturing a hot-rolled steel sheet according to claim 1, wherein the angles of attack of all the descaling nozzles in each of the plurality of rows are set to 0 to 5 °. 3. The method for producing a hot-rolled steel sheet according to claim 1, wherein the method is applied to a steel material having a Si content of 0.2 to 2.0% by weight. 4. The method for producing a hot-rolled steel sheet according to claim 1, wherein the method is carried out on a steel material having a Ni content of 0.2 to 2.0% by weight.