JP3445015B2 - Hot rolling method for stainless steel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is suitable for obtaining a steel sheet product having good surface properties by preventing seizure of a material to be rolled against a rolling roll during hot rolling of stainless steel. The present invention relates to a hot rolling method for stainless steel.

[0002]

2. Description of the Related Art Since stainless steel products are required to have a beautiful surface, it is necessary to prevent the occurrence of surface defects in the manufacturing process thereof. However, in the conventional hot rolling equipment such as a tandem mill, when hot rolling stainless steel, the stainless steel seizes on the surface of the rolling roll, and defects due to the seizure often occur on the surface of the steel sheet. .

It is said that this seizure is caused by the metal surface of the hot rolling roll and the metal surface of the hot rolling material being in close contact (metal-metal touch). Particularly in stainless steel, an oxide film (scale) is hard to form on the surface, and even if it forms, the rate of its formation is slow, so the chances of the above metal-metal touch occur particularly during hot rolling of high Cr stainless steel. The seizure on the roll tends to occur frequently. When the seizure occurs, the surface quality of the roll deteriorates, and the deteriorated shape is transferred to the surface of the material to be rolled, so that the surface quality of the material to be rolled deteriorates.

When a flaw is generated on the surface of the hot rolled stainless steel sheet due to the seizure, even if the flaw is a slight flaw,
This product is not suitable for applications such as mirror finishing in the final process. In addition, a separate process for removing surface flaws by polishing the surface is required. In some cases, if the degree of flaw is remarkable, it cannot be used as a product and is scrapped. Therefore, the defects cause a decrease in the manufacturing yield of steel sheet products and increase the manufacturing cost.

In order to avoid such problems, various attempts have conventionally been made to reduce the rolling load of the rolling rolls, select the rolling conditions, select the roll material, and select the lubricant.

[0006] For example, for the lubricant, animal fats and oils,
Various methods have been proposed to prevent seizure by supplying rolling oil such as vegetable oil, mineral lubricating oil or synthetic lubricating oil to the surface of the rolling roll. A method is also being investigated in which powder having a lubricating function is dispersed and mixed in rolling oil and then supplied to the surface of the rolling roll by an injection method. However, these methods cannot completely prevent seizure under the severe conditions for seizure due to high Cr content and thin gauge of stainless steel, and sometimes surface flaws occur.

Japanese Unexamined Patent Publication No. 64-83309 discloses a lubricant for hot rolling of stainless steel in which iron oxide powder is dispersed in a viscous aqueous solution in an amount of 1 to 30% by weight. According to the invention of this publication, as described in the publication, if the occurrence of oxide scale on the surface is a cause of roll seizure during hot rolling of stainless steel, the iron oxide powder is positively replenished from the outside. This is based on the idea that this cause will be eliminated. The iron oxide powder used includes Fe ₂ O ₃ and Fe ₃ O ₄ powder having a particle size of 10 μm or less. As the viscous aqueous solution, a thickener such as an acrylic acid polymer or a carboxyvinyl polymer is dissolved in water. What was done is illustrated.

Japanese Unexamined Patent Publication (Kokai) No. 63-254195 discloses a lubricating oil or a composition in which iron oxide powder is suspended in a liquid of a water-insoluble polymeric substance, and a lubricating oil for hot rolling of stainless steel. It is described that the lubricating oil can also suppress the seizure on the roll.

On the other hand, different from the idea of supplying iron oxide from the outside between the roll and the material to be rolled, there is also the idea of actively oxidizing the roll surface in the case of hot rolling of ordinary steel. there were. For example, Japanese Patent Publication No. 54-35985 describes a method in which iron hydroxide is supplied to the roll surface during rolling through water as a medium to form a coating film containing triiron tetraoxide as a main component on the roll surface. . In this case, as the roll material, adamite (the alloy name of cast iron for rolls), which is easily oxidized, is used.
When 5% suspension water of ferric hydroxide was sprayed from the roll water cooling nozzle, and when 10-20% suspension water of ferric hydroxide was in contact with the roll surface, it was passed through a felt installed in a wiper shape. It is described that an artificial black skin was formed on the roll surface in each case when applied by coating. However, this method oxidizes the roll surface for hot rolling of ordinary steel, and this concept cannot be applied to rolls (high speed steel or high chromium) for rolling stainless steel.

Further, JP-A-60-184405 discloses a method of positively oxidizing the metal surface on the material to be rolled with a hydroxide of iron, as opposed to oxidizing the roll surface described above. In the hot rolling process of stainless steel for rough rolling and finish rolling, iron hydroxide is supplied to the material to be rolled between the rough rolling passes or between the final rough rolling pass and the first finish rolling pass. By doing so, we propose a method in which the oxide scale on the surface of the rolled material peels off when passing through the previous pass, and oxide scale is generated in the part where the metal surface is exposed, and the metal surface is not exposed before passing through the next pass. is doing.
It is described that in this way, when the rolled material is regenerated with an oxide scale and then passed through a hot rolling roll, seizure on the rolling roll is suppressed and the surface of the rolled material is improved.

[0011]

In recent years, since stainless steel used for exhaust gas members for automobiles is required to have excellent high temperature oxidation resistance, it is necessary to have a Cr content (specifically, a Cr equivalent). ) Tends to be increased. As a result, the scale thickness generated on the surface during hot rolling becomes extremely thin and seizure flaws become noticeable. These C
A technique capable of completely preventing seizure even for a material having a high r equivalent has not been completed.

For example, a lubricant prepared by dispersing iron oxide powder in a viscous aqueous solution as disclosed in JP-A-64-83309 is stably supplied to the roll surface unless the dispersibility and dispersion retention are solved. I can't. For this reason, the anti-seizure effect cannot be sufficiently achieved especially for stainless steel having a high Cr content. Therefore, if the concentration of the powdered iron powder is increased or the grain size is limited within a narrower range. ，
There is a problem with manufacturability and supplyability, and it becomes impossible to operate stably.

The method of oxidizing the surface of the adamite roll by ferric hydroxide during cooling as disclosed in Japanese Patent Publication No. 54-35985 is applicable to hot rolling of ordinary steel, but not hot rolling of stainless steel. However, since the roll material and rolling conditions are different, the effect of preventing seizure cannot be expected. In addition, JP-A-6
In the method of supplying iron hydroxide to the surface of the material to be rolled before being bitten by the roll, as in Japanese Patent Publication No. 0-184405,
In high-speed rolling, there is not enough time to form an oxide film, and most of the iron hydroxide supplied to the surface tends to flow out of the system without being used for forming an oxide film. There is a problem that the yield becomes low and it is not possible to cope with fluctuations in hot rolling conditions.

Many of the conventionally proposed techniques for preventing seizure of stainless steel have been studied from the viewpoint of lubricant composition and properties. Regarding the relationship between the hot rolling conditions of stainless steel and the lubricant, There were many unknowns. Therefore, an object of the present invention is to clarify the relationship between the optimum lubricant for preventing seizure during hot rolling of stainless steel and the hot rolling condition, and to reduce the cost of using the lubricant while maintaining high Cr equivalent stainless steel. An object of the present invention is to provide a hot rolling method capable of enhancing the seizure flaw prevention effect even in hot rolling of steel.

[0015]

According to the present invention, during hot rolling of stainless steel, water is applied to the surface of the work roll of the hot rolling stand in which the temperature of the rolled material passing therethrough is in the range of 900 to 1100 ° C. A method for hot rolling stainless steel, which comprises applying or spraying a fluid containing iron oxide.

At that time, hot rolling is performed by ordinary rough rolling and finish rolling, but the cumulative reduction ratio of finish rolling is 90%.
It is preferable that the rolling reduction be made to be not more than%, and the rolling reduction of each pass in the finish rolling be made to be not more than 30%.

In the method of the present invention, the fluid to be applied or sprayed on the surface of the work roll is a fluid prepared by mixing 5 to 30% by weight of iron hydroxide powder having an average particle size of 0.1 μm or more and less than 5 μm in an aqueous medium. Is particularly preferable. The aqueous medium can be obtained by blending water with a thickener composed of a water-soluble polymer.

[0018]

According to the method of the present invention, it is possible to completely prevent seizure on the roll when hot-rolling stainless steel having a Cr equivalent of up to 25%, and even with a higher Cr equivalent, there is an effect of preventing seizure. . As a result, a hot-rolled steel sheet with good surface properties can be obtained.

The seizure prevention effect of the method of the present invention is brought about by appropriately selecting the lubricant and hot rolling conditions as described below.

The present inventors have been studying roll seizure behavior in hot rolling of stainless steel for many years.
The cause of roll seizure in hot rolling of stainless steel is that, because stainless steel has excellent oxidation resistance, the rate of scale formation is slow, and when a new surface occurs in the preceding stage of multi-step rolling, this new surface is protected. I learned that there is not enough scale layer to be sent to the next stage without being generated. In addition, this phenomenon remarkably appeared in the material having high Cr equivalent and excellent in high temperature oxidation resistance. Based on this knowledge, we came up with the idea of continuously supplying iron hydroxide to the roll bite (work roll surface) as an alternative to the lack of scale. The iron hydroxide to be replenished is convenient for post-treatment of sludge discharged from rolling equipment.

It is possible to use oils and fats and lubricating oils as the medium for supplying the iron hydroxide, but since the iron hydroxide powder does not necessarily have lipophilicity, it is difficult to disperse it uniformly and hot Since the friction coefficient during rolling decreases, slippage and other operational problems are likely to occur.
Therefore, in the present invention, an aqueous medium is used as the iron hydroxide medium. A thickener may be added if necessary from the viewpoint of dispersibility and retention of the iron hydroxide powder. There are various forms of iron hydroxide, and among them, iron oxyhydroxide powder is preferable.

In the case of high Cr stainless steel, seizure is particularly likely to occur because the thin scale formed on the surface is a scale having a high Cr ₂ O ₃ or Al ₂ O ₃ concentration, which is why during hot rolling. This is because the scale is likely to crack. When a crack occurs in the scale, the exposed metal part of the crack contacts the roll, and the metal-metal touch between the roll and the material to be rolled causes adhesion, resulting in significant seizure. In this metal-metal touch, a fluid mixed with fine iron hydroxide having an average particle size of 0.1 to 5 µm is applied or sprayed onto the roll surface or roll bite, and this fluid is supplied to the roll and the material to be rolled. It was found that it can be effectively prevented if it is caught in between.

Probably, the iron hydroxide wound between the roll and the material to be rolled is decomposed by the heat of the material to be rolled according to the following equation, and since this decomposition reaction is an endothermic reaction, 2FeO (OH) → Fe ₂ O ₃ + H ₂ O Due to its endothermic effect, heat is removed from the outer surface of the roll surface,
It is considered that this heat removal causes a decrease in diffusion temperature at which metal-to-metal touch adhesion occurs, and acts to suppress the adhesion.

In order to suppress this adhesion and effectively suppress seizure, it is extremely important to supply the fluid under appropriate hot rolling conditions. Hereinafter, if the iron hydroxide-containing fluid is simply referred to as a “lubricant”, this lubricant is supplied to the work roll surface of the stand where the material temperature of the material to be rolled is 900 to 1100 ° C. It has become clear from a number of experimental facts that this is necessary. In addition, the seizure flaw is closely related to the rolling reduction and the rolling reduction amount of the finish rolling. By setting the cumulative rolling reduction of the finishing rolling to 90% or less and the rolling reduction of each pass in the finishing rolling to 30% or less, It was found that the effect of preventing seizure flaws can be maximized.

Lubricants and hot rolling conditions required to obtain such effects will be individually described below.

Types of Iron Hydroxide in Lubricants Iron hydroxide includes FeO (OH), Fe (OH) ₂ and Fe.
Some are represented by chemical formulas such as (OH) ₃ . Although any iron hydroxide may be used in the lubricant of the present invention, industrially, FeO (OH) powder is the most stable and practically available. Further, if an appropriate amount of iron hydroxide is contained, an appropriate amount of iron oxide powder may be mixed. There are iron oxides represented by the chemical formulas FeO, Fe ₂ O ₃ and Fe ₃ O ₄ , of which Fe ₂ O ₃ has a high hardness. Therefore, in terms of hardness involved in roll wear, etc., FeO and F
Although e ₃ O ₄ seems to be slightly preferable, Fe ₂ O ₃ may be used in terms of preventing seizure of the roll. It is difficult to obtain FeO industrially stably. For this reason, it is practical to use Fe ₃ O ₄ or Fe ₂ O ₃ or a mixture of these as iron oxide, including the cost aspect. It is possible to reduce the cost as it is.

However, for steel types having a high Cr equivalent, iron hydroxide alone is superior as an anti-seizure effect in mixing iron oxide. It is considered that this is because iron hydroxide has an action of the above-described decomposition endothermic reaction, which is not present in ordinary iron oxide, and the iron oxide film formed by this reaction easily adheres to the roll. The purity of iron hydroxide and iron oxide does not necessarily have to be high, and oxides such as SiO ₂ and MnO and those containing impurities such as metallic iron can also be used.

The particle size of the particle size iron hydroxide iron hydroxide in the lubricant, in terms of stability over time of uniform dispersibility and dispersion in an aqueous dispersing agent (viscous solution), more supplied to the rolling rolls It is extremely important for the uniform dispersibility on the supply surface and the wrapping property in the case of the above.

When the average particle size of iron hydroxide is 5 μm or more,
Since the ratio of the area where the particles cover the supply surface to the weight of the iron hydroxide particles becomes small, the coating efficiency of the supply surface decreases when the same weight of iron hydroxide is supplied. Although it is desirable that the average particle diameter of the iron hydroxide powder is 1 μm or less, it has been found that by optimizing the hot rolling conditions as in the present invention, a sufficient effect up to less than 5 μm can be obtained. Therefore, the average particle size of the iron hydroxide powder needs to be less than 5 μm.

When the average particle size of iron hydroxide is small, it is advantageous in terms of uniform dispersibility and coating efficiency on the supply surface, but when the average particle size is less than 0.1 μm, fine powder is generally used. Since the iron hydroxide powder has a property of easily agglomerating, even if each iron hydroxide powder is fine, the agglomeration force of the fine powder is strong and it is extremely difficult to uniformly disperse the powder. Therefore, the average particle diameter of the iron hydroxide powder needs to be 0.1 μm or more. Also, 0.
An ultrafine powder having a particle diameter of less than 1 μm is not practical because it is industrially difficult to manufacture and causes an increase in cost. Therefore, the average particle size of iron hydroxide in the lubricant must be 0.1 μm or more and less than 5 μm.

The content of iron hydroxide in the amount lubricants iron hydroxide in the lubricant, suitability for obtaining the prevention effect with a desired burn, also in order to ensure the supply of stable lubricant Must be adjusted to. When the hot rolling conditions are selected as in the present invention, even with stainless steel having a high Cr equivalent, the seizure prevention effect can be stably obtained if the content of the iron hydroxide powder is 5% by weight or more. When the Cr equivalent is low, the powder content may be further reduced to 1% by weight. As described above, the content of the hydroxide powder is actually selected according to the Cr equivalent, but if the content exceeds 30% by weight, the stability of the lubricant with time tends to decrease. Furthermore, increasing the content causes an increase in the manufacturing cost of the lubricant, which is very disadvantageous in terms of cost. That is,
By combining the hot rolling conditions in which the lubricant effectively acts as in the present invention, seizure flaws can be obtained even on a stainless steel having a high Cr equivalent without containing a large amount of solid lubricant exceeding 30% by weight. It can be prevented. Therefore, the content of iron hydroxide in the lubricant is preferably 5 to 30% by weight.

Aqueous Medium Dispersing Iron Hydroxide The lubricant of the present invention is applied or sprayed on the surface of a work roll in a state in which iron hydroxide is dispersed in an aqueous medium. The aqueous medium is suitable for water. It is preferable to add a suitable thickener to maintain the viscosity of the lubricant at an appropriate level. Although various water-soluble polymers are known as thickeners, according to the experiments conducted by the present inventor, the dispersibility of iron hydroxide powder, its dispersion retention, and the adhesion of lubricant to rolls are It has been found that hydrophilic acrylic polymers such as those of cross-linked acrylic acid polymers and sodium polyacrylate are better than those of ordinary cellulose ethers. It was also found that xanthan gum, which is a biogum, also has good retention of iron hydroxide powder. The amount of such a thickener added is such that the viscosity of the obtained lubricant is 1000 to 5000.
It is preferable that it is 0 centipoise (measured value by a B-type viscometer at 25 ° C.).

Hot rolling conditions (lubricant operating temperature range ) The lubricant of the present invention has a rolling material temperature of 900 to
When applied or sprayed on the surface of the work roll of the hot rolling stand in the range of 1100 ° C., the anti-seizure effect is advantageously achieved even in high Cr stainless steel. That is,
The use temperature range of the lubricant of the present invention is preferably 900 to 1100 ° C. Although the detailed reason for this is not clear, seizure is effectively prevented in the range of 900 ° C. to 1100 ° C., as shown in Examples described later.

Hot Rolling Conditions (Cumulative Rolling Reduction in Finish Rolling ) Hot rolling of stainless steel usually consists of rough rolling and finish rolling, but the cumulative rolling reduction in finish rolling is closely related to seizure flaws. . The lubricant of the present invention produces good results when the cumulative rolling reduction in finish rolling is 90% or less, as shown in Examples described later.

[0035]Hot rolling conditions (reduction of each pass during finish rolling)
rate) The rolling reduction of each pass is similar to the cumulative rolling reduction during finish rolling.
It is closely related to burn-in defects. During finish rolling
If there is a stand with a reduction rate of more than 30% per pass,
The reason is that the number of new surfaces generated during rolling increases
Although it is considered that the use of the lubricant of the present invention prevents seizure
May not be satisfactorily performed, especially when the Cr equivalent is high.
This tendency is high in stainless steel. Therefore,
The bright lubricant has a reduction ratio of 30% or less in each pass during finish rolling.
It is used under the rolling schedule of rolling reduction.
Is desirable.

When the lubricant of the present invention is applied under the hot rolling conditions as described above, surface defects due to seizure defects can be eliminated even in stainless steel containing high Cr. Of course, those having a low Cr content can completely prevent seizure. For example, if the Cr equivalent is 25% or less, seizure can be almost completely prevented. In the present specification, the Cr equivalent is a value calculated from each component value in steel by the following formula. Cr equivalent (%) = [Cr] +2 [Si] +2.5 [A
l] +1.2 [Mo] -30 [C] -15 [N] -2
[Ni]-[Mn]

Method of Supplying Lubricant In the present invention, in order to supply the lubricant to the surface of the rolling roll, first, the manufactured lubricant is stored in a storage tank, and from this tank, a nozzle is provided through a conduit in the vicinity of the rolling roll. The nozzle is continuously pumped from the nozzle toward the surface of the upper and lower work rolls in the rolling stand during rolling. The rolling stand using this lubricant is appropriately selected from a finishing rolling stand and a rough rolling stand. However, in order to make the lubricant effectively work, the material temperature is in the range of 900 to 1100 ° C as described above. Choose a stand for. The lubricant may be sprayed onto the roll surface over the entire width in contact with the rolled material, or may be selectively sprayed near the edge of the rolled material where seizure is likely to occur.

Amount of Lubricant Supply The amount of lubricant supplied to the rolling rolls is adjusted in consideration of the liquid feeding pressure, the nozzle function, and the rolling speed of the rolling rolls. About 0.1 liter / m ² to several liters / m ² against the contact area
^{About 2} is preferable. In addition, instead of spraying from the nozzle,
A method of applying the lubricant to the surface of the rotating roll by using an application means such as a brush may be used.

The effects of the present invention will be specifically described below with reference to representative examples.

[0040]

[Example] Thickness 200 mm, width 1030 to 1240 m
After heating a slab of ferritic stainless steel having a unit weight of 10 to 14 tons to 1200 to 1250 ° C., a rough bar was used as a rough bar, and a finishing rolling machine group consisting of 7 stands was used to obtain a thickness of 2.8 to 4 Rolling was carried out by changing the type of lubricant and hot rolling conditions into a hot coil of 0.9 mm.

The stainless steel used for rolling was 0.0
1-0.06% C, 0.22-0.68% Si, 0.18-
0.74% Mn, 17.9 to 22.1% Cr, 0.111
0.34% Ni, 0.03 to 0.95% Mo, 0.02 to
It has a chemical composition value of 3.7% Al, 0.006 to 0.013% N, and these are Cr according to the formula described in the text.
Equivalent weight of about 19-24% and 25-29% of 2
The lubricants were classified into types, and 20 lubricants having any Cr equivalent were rolled using each lubricant shown in Table 1.

At that time, after changing the work rolls of the finishing rolling mill group (replacement with grinding-finished rolls), 20 slabs were continuously heated for each Cr equivalent with any lubricant. Rolling was carried out for one cycle, and the lubrication condition and hot rolling condition were kept constant during one cycle of these 20 rolls.

A lubricant prepared by dispersing iron oxyhydroxide in a viscous aqueous solution was used, and the particle size and content of the iron hydroxide powder were changed as shown in Table 1. As the viscous aqueous solution, a crosslinked acrylic acid polymer (Junron, trade name, manufactured by Nippon Pure Chemical Co., Ltd.) was dissolved in water, and the viscosity of the final lubricant was adjusted to about 4000 to 6000 centipoise.

The lubricant is supplied to the work roll from the lubricant storage tank by a plunger type pump at 40 kgf /
Liquid was sent to the nozzle at the tip of the conduit with a pressure of mm ² , and a plurality of nozzles (3 in one stand and 4 in one stand)
The lubricant was sprayed on the upper and lower work rolls so that the total amount was about 0.3 liter / m ² .

After hot rolling, the steel was passed through a continuous annealing pickling line to be descaled, and the surface texture of each obtained coil was observed, and the presence or absence of surface defects due to roll seizure was inspected. Table 1 shows the number of the 20 wound coils of each lubricant in which the surface flaws were observed. In addition, the evaluation of each example was comprehensively evaluated. This comprehensive evaluation standard evaluates the anti-seizure effect on three levels: extremely good (◎ mark), good (○ mark) and insufficient effect (× mark).
The results are also shown in Table 1.

[0046]

[Table 1]

As shown in Table 1, in the hot rolling of Nos. 1 to 5, in each case where the Cr equivalent was in the range of 19 to 24%, 2 was obtained in each example.
None of the 0 coils had seizure flaws.
Good results were also obtained with Cr equivalents of 25 to 29%, and seizure flaws did not occur particularly with Nos. 3 and 4 having a cumulative rolling reduction of 90% or less.

On the other hand, even if the lubricant is the same as that of the present invention, the seizure flaw was generated in most of the coils in Comparative Examples Nos. 6 and 7 whose operating temperature range was out of 900 to 1100 ° C.
Further, even if the operating temperature range of the lubricant was appropriate, seizure flaws occurred in most of the coils in Comparative Example No. 8 in which the content of iron hydroxide powder in the lubricant was small.

[0049]

As described above, according to the method of the present invention, even in the hot rolling of the high Cr equivalent stainless steel excellent in the high temperature oxidation resistance, the roll seizure can be stably prevented. In addition, as a result of preventing the seizure of the rolls, the damage of the rolling rolls is reduced and the basic unit for roll restoration is reduced.
Since the obtained hot-rolled sheet has good surface properties, it is possible to manufacture a stainless steel sheet having high added value such as a mirror-finishing application with good productivity without performing surface grinding on the hot-rolled sheet.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI C10M 145: 14 C10M 125: 10 125: 10) C10N 10:16 C10N 10:16 20:06 Z 20:06 40:24 Z 40 : 24 (56) References JP-A-6-136380 (JP, A) JP-A 64-83309 (JP, A) JP-A 4-130194 (JP, A) JP-A 57-22802 (JP, A) ) JP-A-60-184405 (JP, A) JP-A-52-37535 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B21B 3/02 B21B 27/10 B21B 45 / 00 C10M 173/00

Claims (5)

(57) [Claims] 1. A hot-rolling stainless steel roll is coated or sprayed with a fluid containing iron hydroxide on the surface of a work roll of a hot-rolling stand in which the temperature of the material to be rolled is in the range of 900 to 1100 ° C. A method for hot rolling stainless steel, which is characterized in that: 2. The hot rolling method according to claim 1, wherein the hot rolling comprises rough rolling and finish rolling, and the cumulative reduction ratio of the finish rolling is reduced to 90% or less. 3. The hot rolling method according to claim 1, wherein the hot rolling includes rough rolling and finish rolling, and the reduction ratio of each pass in the finish rolling is distributed to 30% or less. 4. The fluid has an average particle size of 0.1 in an aqueous medium.
The hot-rolling method according to claim 1, wherein the hot-rolling method is a fluid containing 5 to 30% by weight of iron hydroxide powder having a size of not less than 5 μm and less than 5 μm. 5. The hot rolling method according to claim 4, wherein the aqueous medium is a mixture of water and a thickener made of a water-soluble polymer.