JPH0457402B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for hot rolling ferritic stainless steel that prevents the occurrence of surface defects on a rolled material during the hot rolling process. Stainless steel products are required to have a beautiful surface appearance, and even minute scratches are easily noticeable. However, since stainless steel has high deformation resistance, if flaws occur during the hot rolling process, the flaws do not disappear easily even after subsequent cold rolling, and minute flaws persist. For this reason, conventional hot rolling of stainless steel has required strict control to prevent the occurrence of flaws. However, conventional stainless steel has a Cr content of 16
% or more, surface defects such as cracks and cracks due to rolling roll seizure often occur. Generally, when such surface flaws occur, the only way to salvage them is to scrap them if they are significant or to treat them by polishing if they are minor.
When surface defects occur, yield and production efficiency are significantly reduced. Furthermore, when seizing occurs on a rolling roll, the roll cannot be used and rolling must be stopped and replaced, resulting in an increase in the roll unit consumption and rolling cost. Conventional measures to prevent such surface defects include raising the rolling temperature of the rolled material to reduce the deformation resistance during rolling, thereby lowering the rolling load, and increasing the number of rolling passes to reduce the rolling rate per pass. Methods have been used to reduce the surface flaws, but these have the drawback of not being able to completely prevent surface flaws, and leading to a decrease in production efficiency and an increase in production costs. The present invention was developed in view of the fact that in recent years, in order to increase production efficiency, reduce production costs, and improve yield, the unit weight of slabs has been increased and rolling has to be carried out under harsh conditions. To provide a rolling method in which surface flaws do not occur even when rolling is performed under rolling conditions that would otherwise cause surface flaws. The present inventors investigated the causes of surface flaws such as cracks and cracks due to roll seizure that occur during the hot rolling process of stainless steel by changing the rolling conditions as shown below. It was found that the cause was that the oxide scale on the material surface had completely peeled off, exposing the metal surface of the material to be rolled. Table 1 shows a thickness of 30 mm and a width for investigating the cause of surface flaws.
This figure shows the conditions when a material to be rolled with a length of 100 mm and a length of 200 mm is heated at 1150° C. for 1 hour in a heating furnace in an atmospheric atmosphere, and then hot rolled.

[Table] In this Table 1, rolling conditions A, B, and C are all set so that rolling in the width direction is performed first and rolling in the thickness direction is performed later. The conditions between the rolling passes in the width direction and the thickness direction are changed to vary the state of oxide scale adhesion. In other words, rolling condition A is such that the rolling amount in the width direction is reduced to 2 mm, so that of the oxidized scale on the surface generated during heating, only the surface layer side is peeled off, and the thin layer on the base metal side remains. be. This remaining thin oxide scale does not change after a short time of air cooling, so during rolling in the thickness direction, the sheet is rolled while being surrounded by the thin oxide scale. On the other hand, in both rolling conditions B and C, the rolling amount in the width direction was increased to 10 mm, so that most of the oxide scale generated during heating was peeled off from the base material, leaving an exposed part of the metal surface. It is something.
Then, in the case of rolling condition B, the exposed part is maintained even during rolling in the thickness direction, and in the case of rolling condition C, it is oxidized by heating to 1100°C, and a thin oxide scale is regenerated. be. Therefore, in the case of rolling conditions A and C, the material to be rolled is rolled in a state covered with a thin oxide scale during rolling in the thickness direction, but in the case of rolling condition B, there are exposed parts of the metal surface. It will be rolled. Table 2 shows the results obtained by hot rolling stainless steels with different chromium contents under the rolling conditions set above.
This study investigated the surface conditions of the rolled material and rolling rolls after rolling.

[Table] (Note) ○ indicates no abnormality As shown in Table 2, in the case of rolling condition B in which rolling is performed in the thickness direction with the metal surface of the rolled material exposed, the Cr content of the rolled material is If it is 16% or more, seizing and cracking of the rolling rolls will occur. However, when Table 2 is examined, (1) rolling conditions A and C in which rolling is performed in the thickness direction while the material to be rolled is covered with a thin oxide scale, and (2) rolling condition B. Even when rolled with low Cr steel, seizing and cracking of the rolling rolls did not occur in all cases where the material to be rolled was low Cr steel. Therefore, when considering the reason why rolling rolls do not seize or crack in these cases, it is thought that in (1) the thin oxide scale plays the role of lubrication, and in (2) it is thought that the
Cr steel has poor high-temperature oxidation resistance and has the property of regenerating oxide scale in a short period of time, and it is thought that this oxide scale plays a similar role. Therefore, it can be seen that in order to prevent seizure and cracking of the rolling rolls, it is sufficient to regenerate a thin oxide scale on the exposed metal surface of the rolled material. Also, the rolled material
Cr steel with a Cr content of 16% or more causes seizing and cracking of rolling rolls because it has excellent high-temperature oxidation resistance, so oxide scale does not regenerate on exposed metal surfaces and has high frictional resistance. I understand that this is because of this. Conventionally, regarding oxidized scale, if it is rolled while still attached to the rolled material, it will be pushed into the surface of the rolled material during rolling and cause indentation flaws, so after extraction from the heating furnace and before rolling, high-pressure water injection etc. are used to completely remove it. It was considered preferable to descale and completely descale what was generated during rolling. However, as is clear from the above investigation, if the base metal surface is completely descaled and exposed, surface flaws will occur during hot rolling, so it is not enough to simply remove the oxide scale; It is necessary to control the amount of the base metal, and if the base metal surface is exposed, it must be regenerated. Therefore, the present invention provides a rolling method in which a thin oxide scale is regenerated on the exposed metal surface of a material to be rolled during hot rolling to prevent surface defects. As mentioned above, in stainless steel with a Cr content of 16% or more, oxide scale is difficult to regenerate in a short period of time in the atmospheric environment. Therefore, in the present invention, an organic acid is supplied to promote oxidation so that the oxidized scale is regenerated in a short time. However, in the present invention, since it is not preferable to use sulfonic acids that leave inorganic acid groups in view of the corrosion resistance of stainless steel and equipment, carboxylic acids are used and supplied in water or rolling lubricating oil. In other words, the present invention provides hot rolling of ferritic stainless steel with a Cr content of 16% or more,
By supplying an aqueous solution containing a carboxylic acid, a rolling lubricating oil containing a carboxylic acid, or both to the rolled material between passes of rough rolling or between passes of rough rolling and finish rolling, it adheres to the surface of the rolled material during rolling. The present invention provides a method for hot rolling stainless steel, characterized in that a thin oxide scale is regenerated in a portion where the oxidized scale is peeled off and the metal surface is exposed, and then the next pass of rolling is performed. In the present invention, the water-soluble carboxylic acid or the carboxylic acid contained in the rolling lubricating oil can be of any kind, as long as it promotes oxidation of the exposed metal surface of the rolled material, and may be aliphatic or aromatic. . However, since polybasic acids of tricarboxylic acid or higher are expensive, mono- and dicarboxylic acids are preferred. Of course, in the case of the present invention, the aqueous solution or rolling lubricating oil may contain not only one type of carboxylic acid, but also two or more types of carboxylic acids having different molecular weights and structures. In the present invention, the oxidized scale that is regenerated by promoting oxidation with carboxylic acid is used to obtain a lubricating effect, and if it is made thick like the oxidized scale that is generated during the heating process before hot rolling, it will cause indentation defects. Make sure it is thin, as it may cause damage. In addition, even in the hot rolling of the present invention, the oxidized scale that forms on the rolled material during the heating process before hot rolling is extremely thick, and if it is rolled with the scale still attached, indentation scratches will occur. Then, descale using high-pressure water injection. However, when descaling, it is necessary to control the metal surface so that it is not exposed and surface defects such as seizure and cracks occur. The present invention will be explained below with reference to Examples. Table 3 shows a slab of SUS430 steel with a Cr content of 17.1% (thickness 200 mm, width 1050 mm, weight 15 tons) in a heating furnace.
After soaking at 1100℃, rough rolling to 25mm and width 1060mm, and then finish rolling to make a hot-rolled coil with thickness of 3.6mm and width of 1060mm. This study investigated whether or not a carboxylic acid-containing aqueous solution and rolling lubricating oil are supplied to the material, and how this changes depending on the supply conditions. The carboxylic acid-containing aqueous solution and rolling lubricating oil (hereinafter referred to as organic acid solution) were supplied to the rolled material through an injection nozzle, and the organic acid solution was injected over the entire width of the rolled material and near the edges. The reason why we targeted spraying only near the edge is because the oxidized scale that adheres to the rolled material during rolling generally peels off due to strain during vertical rolling (width direction rolling). This is because when the amount is small, oxide scale peeling occurs concentrated near the edges. The rough rolling was performed according to the following pass schedule, and the organic acid solution was supplied according to Examples 1 and 2.
In the case of Examples 3 to 5, the rolling was performed between the rough rolling passes, and in the cases of Examples 3 to 5, the rolling was performed between the rough rolling and finish rolling passes. (Heating) → Vertical rolling 1 → Horizontal rolling 1 → Horizontal rolling 2 → Vertical rolling 2 → Horizontal rolling 3* ---→ Horizontal rolling 4 → Vertical rolling 3 → Horizontal rolling 5 → Vertical rolling 4 → Horizontal rolling 6** ---→ (Finish rolling) *Supply of organic acid solutions in Examples 1 and 2 **Supply of organic acid solutions in Examples 3 to 5

[Table] Table 3 shows that in the case of the conventional method in which an organic acid solution is not supplied to the rolled material, the oxide scale peels off during rolling and no oxide scale is formed on the exposed metal surface, resulting in seizure defects. Cracks and cracks may occur. However, the flaws disappeared when an organic acid solution was supplied. In Examples 2, 4, and 5, the organic acid solution was supplied only near the edges, but surface flaws were prevented from occurring. If the material to be rolled is wide and the amount of rolling in the width direction is small, surface flaws can be prevented by simply supplying an organic acid solution near the edges, so it is less expensive if the amount of rolling in the vertical direction is small. It can be prevented. In the above embodiments, the organic acid solution contains one type of carboxylic acid, but it is clear that the same effect can be obtained even if a solution containing two or more types of carboxylic acids is supplied. Further, in the above embodiment, a case of hot rolling of a steel plate is shown, but it is clear that the present invention can be applied to hot rolling of other steel materials such as stainless steel sections and wire rods. As described above, according to the present invention, surface flaws do not occur even when hot rolling is performed under rolling conditions that conventionally cause surface flaws. It can be rolled, yield and productivity are improved and production costs are reduced. In the present invention, since surface flaws are prevented by supplying an organic acid solution to the rolled material during rolling, the production cost associated with supply increases slightly. However, this increase in production cost is small compared to the reduction in production cost due to improvements in yield and productivity, so the overall production cost is reduced.

Claims (1)

[Claims] 1. During hot rolling of ferritic stainless steel with a Cr content of 16% or more, the rolled material is treated with an aqueous solution containing carboxylic acid or rolling lubrication containing carboxylic acid between passes of rough rolling or between passes of rough rolling and finish rolling. By supplying oil or both, the oxide scale adhering to the surface of the material to be rolled during rolling is peeled off and a thin oxide scale is regenerated on the exposed metal surface, after which the next pass of rolling is performed. A method for hot rolling stainless steel, characterized by: