JPH09103818A - Manufacture of hot rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a hot rolled steel sheet excellent in scale defect free surface property by executing descaling in appropriate condition even to Si-containing steel and Ni-containing steel being hardly descaling material. SOLUTION: Slab total length is sent out from a roughing mill 1 after finishing the last rough rolling and thereafter descaling is carried out by injecting high pressure water jet onto the slab surface with collision pressure 4-30kfg/cm<2> from FSB3 in the state passed a steel product 4 so that elapsed time till passing through FSB3 on entry side of finish mill 2 may be 30-300 seconds and the draft of each pass at rough rolling may be 25% or more and cumulative draft from a starting point of rolling to before finish rolling may be >=85%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet having excellent surface properties, which is produced by jetting a high-pressure water jet to the surface of a steel material during hot rolling for descaling to prevent the occurrence of scale flaws and scale patterns. It relates to a method of manufacturing.

[0002]

2. Description of the Related Art In the production of a steel sheet, a slab of a raw material is usually heated in a heating furnace in an oxidizing atmosphere in a range of 1100 to 1300.
Generally, it is heated at a temperature of ℃ for several hours and then hot-rolled in a hot strip mill.At this time, the steel material is rolled until the scale generated on the slab surface is not sufficiently removed. Then, the scale is pushed into the surface of the product and remains as a scale flaw or a scale pattern.

Such scale flaws and scale patterns are
Not only does the appearance of the rolled product (black leather product) deteriorate, but it also causes uneven defects to remain on the surface of the product (white leather product) after removing the scale by pickling, and during bending. It becomes a starting point of cracks, and during forced cooling of the steel plate in the hot rolling process, the difference in the cooling capacity between the scale remaining part and the peeling part causes unevenness in the mechanical property values of the material. It has an adverse effect.

Generally, a method for preventing the occurrence of such scale flaws is 100 to 150 kg on a rolling line.
A method is used in which a descaling device using a water jet with a discharge pressure of f / cm ² is installed, and the scale on the steel material surface is peeled off and removed, and then rolling is performed. However, the quality of the peelability of the scale is greatly influenced by the composition and structure of the scale, and particularly in the case of a steel scale having a high Si content (hereinafter referred to as high Si steel) or a steel scale containing a large amount of Ni, the peelability is significantly poor. Is known to be.

[0005] This is because the Si in the steel during high-temperature oxidation is
Is selectively oxidized to form 2FeO.SiO ₂ (firelite) at the interface between FeO (wustite) and the base iron, which is in a molten state due to its low melting point (1170 ° C.) and penetrates into the scale and the base iron in a wedge shape. Therefore, a scale layer having a unique structure in which the scale and the ground iron interface are complicatedly formed is formed. When the steel contains Ni, as the oxidation progresses, the concentrated portion of Ni remains as a convex shape and the interface shape becomes uneven. Therefore, the peelability of the scale becomes poor.

[0006] Such an adverse effect depends on the chemical composition of the steel. Particularly, the influence of Si is large, and is significantly increased when the Si content is 0.2% or more. In the case of cold rolling, it was extremely difficult to completely prevent the occurrence of scale flaws. As a means for improving this, for example, the heating temperature is set to the melting point of the firelite (1).
170 ° C.) or less, a method of modifying the scale on the slab surface before heating and applying a chemical agent for improving peelability (JP-A-57-6493), a machine using a brush roll. Method for peeling off the scale manually
No. -13926) has been proposed, but there are problems that they are complicated and inferior in workability, there is a problem in terms of manufacturing cost, and finish rolling is restricted due to temperature decrease. It is not a drastic measure to prevent it.

[0007]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and by performing descaling under optimal conditions, there are no scale flaws and good surface properties. An object of the present invention is to provide a method for manufacturing a hot-rolled steel sheet.

[0008]

Means for Solving the Problems It is important for the present inventors to concentrate pores at the base metal interface of a steel material in order to improve the peeling property due to descaling of the scale generated on the surface of the steel material during hot rolling. As a factor for forming pores, a scale breaker after final rough rolling and before finish rolling (hereinafter referred to as “F
Focusing on the elapsed time of the slab before reaching "SB"), the relationship between this elapsed time and the pore was investigated. As a result, after the final rough rolling of the slab has been completed and after being sent from the rough rolling mill, the amount of pores generated is small when the elapsed time before passing through the FSB is 30 seconds or less, and the scale peeling property after descaling is small. If the scale is decreased and the scale remains, and if the elapsed time is 300 seconds or more, the scale layer becomes thick and the descaling property deteriorates, and productivity is significantly deteriorated, and the temperature decrease becomes large, so that the rolling property is improved. However, it has been found that problems such as deterioration of the sheet passing property are caused. From this, the above elapsed time is 30 to 300 seconds,
It has been found that the time is preferably 50 to 120 seconds.

The present inventors also investigated the impact pressure on the slab surface during descaling and the scale peeling property. When the impact pressure was set to 4 kgf / cm ² or more, Si, N
It has been found that even with difficult scale peeling materials such as Si-containing steel and i-containing steel containing 0.2% or more of i, the scale can be almost peeled off. That is, for the steel materials A, B and C having the compositions shown in Table 1, as a result of investigating the relationship between the impact pressure on the slab surface during descaling and the scale residual rate in the product after rolling, the steel materials A, B and C were obtained. Both, collision pressure 4kg
It has been found that when the f / cm ² or less, the scale residual rate per unit area increases, but when the collision pressure exceeds 4 kgf / cm ² , the scale residual rate becomes approximately less than 1%.

The first invention was devised based on the above findings, and the elapsed time from the completion of the final rough rolling of the slab after being sent out from the rough rolling mill to the passage of the FSB is 30 times. ˜300 seconds, preferably 50 to 120 seconds, and the descaling is performed at a collision pressure of 4 to 30 kgf / cm ² on the slab surface by the high-pressure water jet injected from the FSB.

In the present invention, the discharge pressure of the nozzle for ensuring the collision pressure during descaling of 4 to 30 kgf / cm ² is to reduce the injection area of high-pressure water from the nozzle or lower the height of the nozzle. If the discharge pressure of the nozzle can be set low,
Equipment costs and installation space can be reduced, such as downsizing of motors due to lower pressure of pumps and reduction of wall thickness of headers and pipes.

In the present invention, the upper limit of the collision pressure is set to 30.
kgf / cm ² means that even if the scale residual rate can be reduced, the motor becomes larger due to the higher pressure of the pump, the header,
This is because the equipment becomes large and the cost increases due to an increase in the wall thickness of the piping. Si-containing steel has a Si content of 0.2%
If the above is the case, firelite penetrates deeply into the ground iron interface in the scale and secondary scale during heating, making scale exfoliation extremely difficult, and Ni-containing steel also has a Ni content of more than 0.2%. The unevenness of the iron interface becomes severe and the scale releasability becomes difficult, but the present invention is not limited to the above steel materials A, B and C.
As described above, there is a sufficient scale peeling effect even for steel materials having Si and Ni contents of 0.2% or more, respectively.

Therefore, the second invention is characterized in that the descaling of the above invention is carried out on a steel material having a Si content of 0.2 to 2.0%, and the third invention is the descaling of the above invention. It is characterized in that it was carried out for a steel material having a Ni content of 0.2 to 2.0%. Si content is 0.2
When the content is more than%, fire light is generated in the scale during heating and the secondary scale, so that the fire light penetrates deeply into the interface of the base metal, and scale peeling becomes extremely difficult.
According to the method of the present invention, it is possible to perform more effective descaling on a steel sheet containing 0.2% or more of Si, which is difficult to scale off. Although there is no need to limit the upper limit, since the weldability and cold workability deteriorate, the Si content is limited to 2.0% or less. On the other hand, when Ni exceeds 0.2%, the unevenness of the interface of the base metal becomes remarkable, and scale peeling becomes difficult. According to the method of the present invention, for such a steel plate,
Descaling becomes possible more effectively. The upper limit is not necessarily limited, but the Ni content is set to 2.0% or less by comprehensively judging toughness, ductility, economy and the like.

The descaling according to the first aspect of the present invention is also preferably performed on a steel material having a steel material temperature of 850 to 1000 ° C. When the descaling is performed at 850 ° C. or less, the scale strength becomes higher than the base steel strength, and scale peeling becomes difficult. Further, if the steel sheet temperature is excessively lowered, the rolling property and the threading property may be deteriorated and the rolling may become impossible. When the temperature is 850 ° C. or higher, the cooling strain effect easily occurs, the scale peeling effect increases, and efficient descaling is possible. On the other hand, it was found that the situation of production scale was different at 1000 ° C or higher. As a result of observing a lot of generated scales, the present inventors have found that the pores in the scale have a great influence on the quality of scale releasability. That is, when the temperature is 1000 ° C. or higher, the progress of oxidation is significantly accelerated, the scale thickness increases, and large pores are generated in the scale, which makes it difficult to exfoliate the scale and the scale easily remains. Become.

Therefore, a fourth preferred invention is characterized in that the descaling of the above invention is performed on a steel material having a steel material temperature of 850 to 1000 ° C. In order to improve the scale peeling property by descaling, the present inventors also make the base metal and the scale interface flat, and
We considered that it is necessary to generate cracks on the steel surface, and investigated the relationship between the reduction ratio and scale releasability, which is considered to be the cause. That is, with respect to the rolling reduction of each pass in rough rolling before finish rolling and the cumulative rolling reduction up to finish rolling, the scale remaining rate when the rolling reduction of each pass and the cumulative rolling reduction was changed was investigated. As a result, the reduction ratio of each pass before finish rolling and the cumulative reduction ratio before finish rolling are steel products in which the reduction ratio of each pass of the former is 25% or more and the cumulative reduction ratio of the latter is 85% or more. On the other hand, when the above-mentioned descaling was carried out, the scale residual rate was remarkably reduced. It is considered that this is because the rolling with the above-mentioned reduction rate facilitated the formation of cracks on the surface of the steel material and the easy peeling of the scale.

Therefore, the fifth preferred invention is the descaling of the above invention, wherein the rolling reduction of each pass in rough rolling before finish rolling is 25% or more, and the cumulative rolling reduction before finish rolling is 85% or more. It is characterized by performing to.

BEST MODE FOR CARRYING OUT THE INVENTION On the inlet side of a finish rolling mill 2 shown in FIG.
mm) is arranged, and the elapsed time from when the slab full length has finished the final rough rolling and after being sent from the rough rolling mill 1 to passing through the FSB 3 on the entry side of the finish rolling mill 2 is 30
~ 300 seconds, and the rolling reduction of each pass in the rough rolling is 25% or more, and in the state where the steel material 4 is threaded so that the cumulative reduction from the rolling start point to before the finish rolling is 85% or more, the high-pressure water jet to the slab surface than FSB3, injected at impact pressure of 4~30kgf / cm ^2, for descaling.

[0017]

EXAMPLE FIG. 1 shows the steel materials A, B and C shown in Table 1.
After descaling the steel material 4 sent from the rough rolling mill 1 under the operating conditions shown in Table 2 from the FSB 3 on the entry side of the finishing rolling machine 2, finish rolling is performed, and the scale residual rate of the obtained steel material surface is measured. It was evaluated quantitatively. The results are shown in Table 2.

[0018]

[Table 1]

[0019]

[Table 2]

As shown in Table 2, No. 1 within the scope of the present invention. The steel materials of 2 to 4, 6, 7, and 9 to 12 each had a scale residual rate of less than 1%, and the collision pressure, the reduction rate, the elapsed time until the descaling after the rough rolling, and the descaling temperature were adjusted. ． No scale remains in the steel materials 2, 4, 7, and 9. However, due to the collision pressure, No. 3 which is outside the range of the present invention. 1 and F from the end of rough rolling
N that is outside the range of the present invention in the elapsed time until reaching SB
o. The scale residual rates of the steel materials of Nos. 5 and 8 were No. 5, respectively.
67% with No. 1 steel, No. Steel No. 5 of 39%, No. 5 8
Steel material reached 23%. In addition, No. In the steel materials of No. 8, the elapsed time until descaling was long, narrowing occurred during finish rolling, and it could not be a finished product.

[0021]

EFFECTS OF THE INVENTION According to the present invention, even for high Si content steels and Ni content steels, which were conventionally considered to be difficult to remove scale, the entire length of the slab has passed the final rough rolling and passed through the FSB. Scale removal is performed by setting the time to 30 to 300 seconds and descaling by setting the collision pressure on the slab surface to 4 to 30 kgf / cm ² , and the hot rolled steel sheet with good surface quality without scale flaws. In addition, the discharge pressure of the nozzle for securing the collision pressure of 4 to 30 kgf / cm ² can be reduced by reducing the injection area of high-pressure water from the nozzle or lowering the height of the nozzle. If the discharge pressure of the nozzle can be set low, the equipment cost and the installation space can be reduced, such as downsizing of the motor due to the low pressure of the pump and reduction of the wall thickness of the header and piping.

The rolling reduction of the steel material to be descaled is 25% for each pass from rough rolling to finish rolling.
As described above, by setting the cumulative reduction rate from the rolling start point to 85% or more, the peelability due to descaling can be further improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a rolling line.

[Explanation of symbols]

 1 ・ ・ Rough rolling mill 2 ・ ・ Finishing rolling mill 3 ・ ・ FSB 4 ・ ・ Steel material

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiya Kurita 11-1 Showa-cho, Kure-shi, Hiroshima Nisshin Steel Co., Ltd. Kure Steel Works (72) Inventor Akifumi Hiramatsu 11-11 Showa-cho, Kure-shi, Hiroshima (72) Inventor Koji Ozako 11-11 Showa-cho, Kure City, Hiroshima Prefecture Nisshin Steel Co., Ltd. 11-12 Showa-cho, Kure City, Nisshin Steel Co., Ltd. Tororou Yamada 11-11 Showa-cho, Kure City, Hiroshima Prefecture Nisshin Steel Co., Ltd.

Claims (5)

[Claims] 1. When manufacturing a hot-rolled steel sheet by hot rolling a slab, the entire length of the slab is sent from a rough rolling machine after final rough rolling, and then passes through a scale breaker before finish rolling. The slab is passed through so that the elapsed time is 30 to 300 seconds, and the descaling by the high-pressure water jet is performed immediately before finish rolling so that the collision pressure on the slab surface is 4
A method for manufacturing a hot-rolled steel sheet, characterized in that the hot-rolled steel sheet is carried out at -30 kgf / cm ² . 2. The method for producing a hot-rolled steel sheet according to claim 1, wherein the steel material has a Si content of 0.2 to 2.0%. 3. The method for producing a hot-rolled steel sheet according to claim 1, wherein the steel material has a Ni content of 0.2 to 2.0%. 4. A steel material temperature of 850 to 100 for descaling.
The method for producing a hot-rolled steel sheet according to any one of claims 1 to 3, which is performed on a steel material at 0 ° C. 5. Descaling is performed on a steel material having a rolling reduction of 25% or more in each pass from rough rolling to finish rolling and a cumulative rolling reduction of 85% or more from a rolling start point. The method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 4.