JP2001288491A - Hot rolling process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolling process whereby low-friction hot rolling can be realized at a coefficient of friction of 0.15 or below without encountering poor bite at the stand of each finishing rolling mill provided with abrasion- resistant cast iron work rolls, and the surface roughening of work rolls can be prevented. SOLUTION: Provided is process for hot-rolling steel in such a manner that the tail of each preceding rolled stock is joined to the head of the following workpiece between a roughing rolling mill and a plurality of finishing rolling mills, wherein all of the rolling stands of finishing rolling mills are composed of abrasion-resistant work rolls, and a lubricant containing 20-70 mass % alkaline earth metal carbonate and 30-80 mass % grease of a consistency (an index representing the viscosity of a grease) of 0 is applied to each rolling roll in a coating amount of 2 g/m2 or above to prevent roll surface roughening and to lower the coefficient of friction to 0.15 or below during rolling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolling of steel, in which a tail end portion of a preceding rolling material and a leading end portion of a succeeding rolling material are joined and connected between a rough rolling device and a finishing rolling device to finish the steel. The present invention relates to a hot rolling method for preventing the surface roughness of a roll surface when using a wear-resistant work roll in all stands of a rolling mill row and for reducing a friction coefficient.

[0002]

2. Description of the Related Art In a conventional thin sheet hot rolling process, a rolled material is extracted one by one from a heating furnace, rolled to a predetermined thickness by rough rolling, and successively arranged by a plurality of continuously arranged finishing mills. After being rolled, it is wound on a winder to form a hot coil. In the finish rolling process, the leading end of the rolled material is sequentially bitten by the work roll of each finish rolling mill. At this time, if, for example, a slip occurs between the leading end of the rolled material and the roll and the roll does not bite into the leading end of the rolled material, subsequent rolling cannot be performed, and the rolled material becomes mis-rolled and becomes waste. Is done.

[0003] When the coefficient of friction between the rolling roll and the rolled material is large, poor biting is unlikely to occur, so that large rolling and high-speed rolling are facilitated. Also, if the coefficient of friction between the rolled material and the roll is large, there is no slippage between the roll and the rolled material, that is, a so-called sticking friction state, and the rolled material is processed by shear deformation and is uneven in the thickness direction. Undergoes severe deformation. A rolled material manufactured by undergoing such non-uniform deformation in the thickness direction has a drawback of poor deep drawability. However, when the friction coefficient between the rolling roll and the rolled material is reduced by performing sufficient lubrication, the rolled material is hardly subjected to shear deformation, so-called uniform deformation, and as a result, a rolled material excellent in deep drawability is obtained. . However, high lubrication has a disadvantage that poor biting is likely to occur.

In order to solve such a defect, for example, Japanese Patent Laid-Open Publication No. 1-258802 and Japanese Patent Laid-Open Publication No. 5-7902, etc., sequentially bond the rear end of a coarse bar and the front end of a subsequent coarse bar to finish. A method is described in which rolling does not cause biting failure even when high lubrication is performed. However, in these methods, since the rough bars are joined and continuously rolled, the biting failure is reduced, but a specific lubrication method for reducing the friction coefficient to a specified value or less, that is, the type of the lubricant used, The application amount and application method are not mentioned.

[0005] Further, in the hot rolling of conventional steel, adamite cast steel, high chromium cast iron,
And three types of rolls made of nickel-grain cast iron have been mainly used.
6, JP-A-5-169217, JP-A-5-1692
No. 19, JP-A-5-169220, etc., rolls of high-speed steel (hereinafter referred to as high-speed steel) based rolls having excellent wear resistance have been developed and have become the mainstream of rolls. . The high-speed rolling rolls have extremely high hot wear resistance because carbides such as Cr, W, V, Mo, and Nb having a high high-temperature hardness are precipitated in the metal structure. Further, similarly to the conventional roll, as the rolling progresses, Fe ₂ O ₃ and Fe ₃ O called black scale are formed on the roll surface.
An oxide film composed mainly of ₄ is formed. The oxide film may peel off as the rolling proceeds. In that case,
The portion where the oxide film has peeled off has a step difference with the portion where the oxide film has remained, resulting in roll surface roughness. The roll surface roughness is transferred to the rolled material to reduce the surface quality, or the peeled oxide film is pushed into the rolled material to generate a surface defect involving the oxide film. Therefore, it is necessary to replace or grind the rolling rolls, and it is not possible to enjoy the high wear resistance of the high-speed hot rolling rolls.

As a means for solving this problem, as described in JP-A-6-200282, a highly basic alkaline earth metal sulfonate, for example, a highly basic calcium salt, is provided between a rolling roll and a rolled material. There is a method of supplying a sulfonate. The highly basic calcium sulfonate contains CaCO _3, and after the CaCO ₃ adheres to the rolling roll, the CaCO ₃ comes into contact with a high-temperature rolled material and is decomposed into CaO. Such an alkaline earth metal oxide reacts with the scale present on the surface of the rolled material and changes the surface scale to CaO.
And has the effect of modifying the scale to a scale mainly containing a solid solution of FeO and FeO. Since the scale mainly composed of the solid solution of CaO and FeO does not adhere to the surface of the rolling roll, it is possible to suppress the formation of the black scale, and as a result, the peeling of the black scale is suppressed. However, in order to reduce the friction coefficient to 0.15 with this lubricant, a very large amount of lubricant supply was required.

[0007]

SUMMARY OF THE INVENTION A method for continuously hot-rolling by joining and joining a tail end of a preceding rolled material and a leading end of a succeeding rolled material between a rough rolling facility and a finishing rolling facility. Although the problem of poor biting does not occur, the specific method for reducing the friction coefficient to the target value, that is, the type, application amount, application method, etc. of the lubricant is not disclosed. Also, unlike conventional hot rolling of one slab, hot rolling is performed continuously. Therefore, the use of a high-speed roll having wear resistance is expected for the work roll.
There is a problem of rough skin caused by black scale generated on the roll surface. As a countermeasure for the occurrence of the roughened surface, the rolling roll having the roughened surface is immediately replaced with a polished rolling roll. However, the exchange of the rolling rolls is a very serious problem because the rolling work for a long time has to be stopped and productivity is greatly reduced.

[0008] An object of the present invention is to prevent the occurrence of biting failure even when high lubrication rolling is performed with a reduced friction coefficient to a predetermined value.
Another object of the present invention is to provide a hot rolling method capable of preventing the roughening of the rolling roll.

[0009]

SUMMARY OF THE INVENTION The gist of the present invention is to join a tail end portion of a preceding rolled material and a leading end portion of a succeeding rolled material between a rough rolling equipment and a finish rolling equipment, thereby hot-rolling steel. In the rolling method, wear-resistant work rolls are used for all rolling stands of the finishing mill, and the alkaline earth metal carbonate 20 is used.
By supplying a lubricant containing 30 to 80% by mass of grease having a grease viscosity of 0 to 70% by mass and a degree of 0 (index of grease viscosity) to the wear-resistant work roll, 2 g / m ² or more is supplied from a lubrication nozzle to the roll surface A hot-rolling method for steel, characterized in that the formation of an oxide film is suppressed and the rolling coefficient is reduced to 0.15 or less by hot rolling. This is a method for hot rolling steel. Further, in the hot rolling method, the thickness of the oxide film formed on the surface of the wear-resistant work roll is 0.1 μm to 20 μm.

That is, in order to solve the above problems, the present inventors joined the tail end of the preceding rolled material and the front end of the succeeding rolled material between the rough rolling equipment and the finish rolling equipment. Concatenate,
It is desirable to use a high-speed hot-rolling roll with excellent wear resistance, and at the same time perform a continuous hot-rolling process using a lubricant that achieves a low coefficient of friction and suppresses the formation of black scale. Was. The reason is that by joining and connecting the tail end of the preceding rolled material and the leading end of the succeeding rolled material between the rough rolling equipment and the finish rolling equipment, "biting of the slab tip" is eliminated, so This is because even if lubricating rolling is performed, poor biting does not become a problem. In order to continue hot rolling continuously, it is considered that a high-speed roll excellent in wear resistance is desirable as a roll. At this time, in order to avoid roughening due to peeling of the black scale generated when a high-speed roll excellent in wear resistance is used, and to realize low-friction-coefficient rolling, the preparation containing calcium carbonate and having a 0 Rolling lubricants are preferred. That is, C
When aCO ₃ comes into contact with the hot rolled material, CaCO ₃ becomes C
The CaO is decomposed into aO and CO ₂ , and the decomposed CaO changes the scale of the rolled material into a solid solution of CaO and FeO to suppress the formation of black scale on the roll surface.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. FIG. 1 is a diagram showing an embodiment of the present invention.
In the present invention, the rear end 3 of the preceding rolled material and the front end 4 of the subsequent rolled material are joined between the rough rolling mill 1 and the finishing rolling mill 2. This joining is performed sequentially on the rolled material that is further continued. By this joining, the succeeding material is integrated with the preceding material, so that the problem of biting at the tip of the slab is eliminated unlike the conventional rolling in which the slab bites one by one. Therefore, even in rolling with high lubrication, rolling slip due to poor biting does not occur. The rear end 3 of the preceding material and the front end 4 of the following material are joined by, for example, a laser welding method or an upset welding method. Moreover, you may install the coil box 6 as needed.

In this hot rolling method, in order to continuously roll the rolled material and pursue the productivity as much as possible, it is necessary to reduce the frequency of replacing the rolling rolls as much as possible. Therefore, the rolls used are required to have excellent wear resistance. A high-speed roll 5 is desirable as a roll that meets such requirements. The high-speed roll is a roll in which the material of at least the surface layer of the roll is made of high-speed steel, and is manufactured by various manufacturing methods such as a continuous casting method, a centrifugal casting method, and a powder sintering method. . In many cases, only the surface layer of the roll is made of high-speed steel, but an integral roll made of high-speed steel for the entire roll may be used. Also in the high-speed roll 5, an oxide film containing Fe ₂ O ₃ and Fe ₃ O ₄ as main components, a so-called black scale, is formed on the roll surface as the rolling progresses, similarly to the conventional roll. When the black scale peels off, a step is generated between the peeled part and the non-peeled part, so that the roll surface becomes rough. In order to avoid this, grease having a preparation of 0 containing calcium carbonate may be sprayed from the lubrication nozzle 7 to the rolling roll on the entry side of each stand, and the calcium carbonate in the grease contacts the hot rolled material, It is decomposed into CaO and CO ₂ , and the decomposed CaO changes the scale of the rolled material into a solid solution of CaO and FeO, and has the effect of suppressing the formation of black scale on the roll surface.

FIG. 2 shows the relationship between the friction coefficient and the injection amount of the supplied lubricant (calcium carbonate 30% by mass, grease 70% by mass) when a hot rolling experiment was performed. Here, the injection amount is “a supply amount of grease and carbonate of alkaline earth metal supplied per minute (g / min) / (rolled material width (m) × rolling speed (m / min)) In the following, the supply amount of the lubricant refers to this. The physical meaning of the unit of g / m ² means how much lubricant is supplied to the steel sheet area passing per unit time.
Further, the front and back surfaces of the rolled material are lubricated from the upper and lower nozzles, and the supply amount described here indicates the supply amount per one surface. As can be seen from FIG. 2, the friction coefficient is reduced by supplying the lubricant, and the friction coefficient of 0.15 or less can be obtained by supplying the lubricant at 2 g / m ² or more.

Therefore, when the present invention is applied, in the hot rolling step, at least one stand, preferably all the stands, have a friction coefficient of 0.15 or less, have no biting failure, and do not cause rough skin. As described in JP-A-1-38855, it becomes possible to manufacture a hot-rolled steel sheet excellent in formability with high efficiency.

In the present invention, an alkaline earth metal carbonate is used in an amount of 20.
The reason why the content is limited to about 70% by mass is that if the content is less than 20% by mass, the effect of suppressing the oxide film of the alkaline earth metal carbonate is insufficient.
If it exceeds, the effect is saturated and it is uneconomical.
Therefore, in the present invention, the carbonate of the alkaline earth metal is 20 to
It was limited to 70% by mass. In addition, the grease of the furnishing 0 is 30
Since the lubricating effect is exhibited from mass% or more, if it exceeds 80%, the lubricating state becomes excessive and rolling slip occurs. Therefore, in the present invention, the grease of the preparation 0 is limited to 30 to 80%.

The reason why the supply amount of the lubricant is limited to 2 g / m ² or more is that the friction coefficient can be reduced to 0.15 or less. By setting the friction coefficient to 0.15 or less, the rolled material is in a uniformly compressed state, and a steel sheet excellent in workability can be obtained. Therefore, in the present invention, the lubricant supply amount is 2 g / m ²
As described above, the friction coefficient is limited to 0.15 or less.

Further, the thickness of the oxide film on the rolling roll is set to 0.1.
The reason for limiting the thickness to 20 μm is that if no oxide film is present on the surface of the roll, seizure occurs between the roll and the rolled material. It must be at least 1 μm. However, when the thickness of the oxide film exceeds 20 μm, the oxide film easily peels off rapidly and causes the roughening of the roll surface. Therefore, in the present invention, the thickness of the oxide film on the surface of the rolling roll is limited to 0.1 to 20 μm.

[0018]

(Example 1) As Example 1 of the present invention, Table 1 was used.
Hot rolling was performed under the conditions shown in Table 2 using a lubricant containing 30% by mass of calcium carbonate, 70% by mass of grease having a degree of preparation of 0, and a lubricant supply amount shown in Table 2. The state of skin formation and the state of rough skin due to peeling of the black scale were investigated. When the lubricant supply amount is 0 g / m ² , no lubricant is supplied, and the roll cooling water (5 L / m 2 per roll) is used.
in) only. As a rolling mill, a single-stand experimental rolling mill was used, and a coiled rolled material was used. Further, assuming continuous hot rolling, the rolled material was heated in a state where the leading end of the rolled material had been wound on a winder in advance, and was subjected to an experiment. The coefficient of friction is described in the 1978 29th Plastic Working Alliance Lecture Meeting, P.S. 139, using the rolling analysis model
The advance rate and rolling load determined in the experiment were determined so as to match the analysis results. The degree of skin roughness was visually determined, and a good case without skin roughness was evaluated as ○, a slight skin roughness was evaluated as Δ, and the occurrence of remarkable skin roughness was evaluated as ×. The stable rolling property was evaluated based on the occurrence of slip during rolling, and the case where no slip occurred was evaluated as ×, and the case where slip occurred was evaluated as x.
Table 3 shows the results of the survey.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

From Table 3, it can be seen that Invention Examples 1 to 4 have a sufficient supply of the lubricant, exhibit an effect of suppressing black scales, and as a result, no rough skin is observed. Furthermore, the friction coefficient is 0.15 or less in each case, no slip occurs, and stable low-friction hot rolling can be realized. Also, the thickness of the black scale at this time cut out a sample from the rolling roll,
It was determined from observation of the cross section. At this time, the thickness of the black scale is 3 μm.
Met. In Comparative Example 1, since no lubricant was supplied, no slippage occurred during rolling, but the friction coefficient was 0.30.
It shows a high value. Further, since no effect of inhibiting the formation of black scale was observed, remarkable skin roughness occurred. When the black scale thickness was measured by the same method as described above, the black scale thickness at this time was 40 μm. Further, in Comparative Examples 2 and 3, it can be seen that the friction coefficient was reduced by the supply of the lubricant.
However, due to insufficient supply, no slippage during rolling was observed, but remarkable roughening or slight roughening was observed. At this time, the thickness of the black scale was 30 μm in Comparative Example 2 and 25 μm in Comparative Example 3.

(Example 2) Next, as Example 2, hot rolling was performed under the rolling conditions and lubrication conditions shown in Table 4 using a hot-roll finishing tandem rolling mill consisting of seven stands. Also in this example, assuming continuous hot rolling, the lubricant was supplied after the leading end of the rolled material was wound as in Example 1. The supply amount of the lubricant per minute was set so that a predetermined amount (unit: g / m ² ) was supplied to each of the upper and lower rolls of each stand in accordance with the change in the rolling speed and the sheet width. . The lubricant was supplied by an air atomizing method.

[0024]

[Table 4]

Table 5 shows the results of the investigation at this time. The surface condition of the rolling roll was determined by pulling out the rolling roll from the stand after the completion of the rolling, and visually examining the condition. The friction coefficient at this time was calculated in the same manner as in Example 1. Further, the occurrence of rolling slip was determined in Example 1.
Judgment was made in the same manner as above.

[0026]

[Table 5]

As can be seen from Table 5, in Invention Examples 1 and 2, low friction hot rolling with a friction coefficient of 0.15 or less was realized, and no occurrence of slip was observed. In addition, no rough skin caused by black scale was observed. On the other hand, in Comparative Example 1, no slip was generated during rolling because no lubricant was supplied, but the coefficient of friction showed a high value of about 0.3 at all stands. Further, in all stands, rough skin caused by peeling of black scale and pear-skin-like rough skin occurred. In Comparative Example 2, although no slip occurred during rolling, the coefficient of friction was lower than in Comparative Example 1. However, since the effect of the lubricant on inhibiting black scale was not sufficiently exhibited, rough skin caused by significant peeling of black scale was observed in F1 to F4. At the F5 to F7 stands, satin-like rough skin occurred.

[0028]

By using the hot rolling method according to the present invention, low-friction hot rolling can be efficiently performed in all stands without causing biting defects, and roll surface roughening does not occur. It is possible to improve productivity, reduce the unit consumption of rolls, the unit consumption of electric power, and the like, thereby enabling significant cost reduction. Furthermore, since the coefficient of friction during hot rolling can be greatly reduced, it is also possible to create new products such as hot-rolled steel sheets having excellent deep drawability.

[Brief description of the drawings]

FIG. 1 is a diagram showing an outline of rolling equipment of a hot rolling method of the present invention.

FIG. 2 is a diagram showing a relationship between a friction coefficient and a lubricant supply amount in hot rolling.

[Description of Signs] 1 Rough rolling mill 2 Finish rolling mill 3 Trailing end of preceding rolled material 4 Leading end of trailing rolled material 5 High-speed rolling roll of finishing stand 6 Coil box 7 Lubrication nozzle

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C10N 20:02 C10N 20:02 40:24 40:24 Z 50:10 50:10 (72) Inventor Inoue Go 20-1 Shintomi, Futtsu-shi, Chiba F-term in the Technology Development Division of Nippon Steel Corporation (reference) 4E002 AD02 AD04 CB03 4H104 AA01C AA13A AA13C EA02A FA02 PA26 QA01 QA18

Claims (3)

[Claims] 1. A method according to claim 1, wherein the rough rolling equipment and the finishing rolling equipment are
In the method of hot-rolling steel by joining the tail end of the preceding rolled material and the tip end of the subsequent rolled material, a wear-resistant work roll is used for all rolling stands of the finish rolling mill, and the alkaline earth metal By supplying a lubricant containing 20 to 70% by mass of carbonate and 30 to 80% by mass of grease having a degree of preparation of 0 (an index indicating the viscosity of grease) to the wear-resistant work roll by 2 g / m ² or more from a lubrication nozzle. A hot rolling method for steel, comprising suppressing formation of an oxide film on a roll surface and performing hot rolling with a friction coefficient at the time of rolling being 0.15 or less. 2. The hot rolling method for steel according to claim 1, wherein calcium carbonate is used as a carbonate of the alkaline earth metal. 3. The hot-rolling roll used in the hot-rolling method according to claim 1, wherein the thickness of the oxide film formed on the surface of the wear-resistant work roll is 0.1 μm to 20 μm.
m, a hot-rolling roll.