JPS60170503A - Hot rolling method of stainless steel - Google Patents

Abstract

PURPOSE:To prevent a rolling material from producing flaw in its surface by removing oxidized scales stuck on the surface during rolling, before performing the next pass rolling by forming a thin oxidized scale on it, in rolling a medium or high Cr stainless steel. CONSTITUTION:In a hot rolling of stainless steel containing >=11% Cr, oxidized scales stuck on the surface of a material to be rolled are peeled by a high pressure water spray. The exposed metallic surface of material is successively oxidized by heating the material, etc. and then the material is rolled at the next pass after reforming a thin oxidized scale on it. In this way, the material is prevented from producing surface flaw, and the yield and productivity of a product are improved, even when the material is hot rolled under a severe condition.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for hot rolling fully polished stainless steel to prevent surface flaws occurring during the hot rolling process.

Stainless steel products are required not only to have a beautiful surface appearance, but also to have even the slightest flaws.

Since the stainless steel housing has a large resistance to deformation, if there are flaws in the hot rolled state, the flaws do not disappear easily even after cold rolling, and minute flaws persist. Is this why conventional hot rolling is used? However, when hot rolling stainless steel with a Cr content of 11% or more, surface defects such as scratches and cracks due to rolling roll seizure occur. There was a tendency for this to occur frequently. Generally, when such surface flaws occur, the only way to repair them is to scrap them if they are serious, or to take care of mild ones in addition to the oxidation process. 9 problems, which can significantly reduce yield and productivity when they occur, are rare. If seizing occurs on a rolling roll, the roll will wear out during use and must be replaced, resulting in problems such as an increase in the unit consumption of the roll and an increase in rolling costs due to breakage of the rolling body.

The occurrence of such surface flaws can be prevented by increasing the rolling temperature of the temporarily rolled material to reduce the deformation resistance during rolling, by decreasing the rolling load, or by increasing the number of rolling steps. It can be prevented by lowering the rolling ratio of the pass effort or by increasing Tc, but it is rarely possible to prevent it completely, and moreover, it often has the disadvantage of lower production efficiency and an increase in heavy costs.

In recent years, the present invention has been developed to increase the single Mk of slabs for all purposes such as increasing production efficiency, reducing production costs 2 and improving yield. In view of the fact that rolling is also carried out under harsh conditions and actual m-tr is impossible, the present inventors have devised a method of providing a hot rolling method that does not generate surface defects even when rolled under harsh conditions. They explained that surface defects such as cracks and split defects caused by seizing of rolling rolls that occur during hot rolling of stainless steel are caused by the complete peeling off of oxidized schools on the surface of the rolled material, as well as box grains caused by various causes. It was found that the cause was dew tF5 on the gold surface of the pre-rolled material.

For example, in the case of hot rolling a steel plate, the material to be rolled is extracted from a heating furnace and then descaled by jetting high-pressure water or the like before rolling. The second rolling is performed as seven rough rolling and finishing rolling, and the oxidized scale generated during these rolling is decooled during the rolling. Conventionally, this method is effective in preventing indentation defects compared to hot rolling because when pre-rolled material No. 11 with thick oxide scale stuck to it is rolled, the oxidation school is pushed into the surface by rolling and many indentation defects occur. I couldn't help but think. However, the present inventors conducted various investigations regarding this descaling and found that if descaling is complete and hot rolling is performed with the metal surface of the rolled material exposed, surface flaws such as seizure flaws and cracking flaws will occur ( It was also found that high-instep Cr stainless steel has excellent high-temperature oxidation resistance, so oxidation schools do not easily regenerate on the exposed metal surface, resulting in surface defects.

Therefore, when a thin acid slag was regenerated on the exposed metal surface, no surface flaws were observed and f'L disappeared.

The present invention has been made based on the above research, and the gist of the invention is that when hot rolling stainless steel with a CrMM content of 11% or more is carried out, the oxide scale attached to the surface of the rolled sheet material is peeled off during rolling. Then, the oxidized school is regenerated in the area where the metal surface of the pressed earth is exposed, and then the next rolling process is carried out.

The oxide scale generated on the rolled material during the heating process before the hot rolling of the present invention is extremely thick, and if the thick oxidized scale remains attached during rolling, it will cause school indentation defects. During rolling, thick oxide scales generated during the heating process before rolling are completely descaled by jetting high-pressure water, etc., as in the conventional method. However, it is necessary to control this decool so that the metal surface is not exposed and surface flaws such as seizures and cracks occur. In addition, the oxidation schools that are regenerated on the exposed parts of the metal surface due to peeling off during rolling are used to obtain lubricated young fruit, and the thick oxide scales that are generated during the heating process before rolling are rather a problem. Because it's harmful, it's made into something.

Next, the first moat, which will be explained using an example, has a thickness of 30W+ and a width of 100+11111. length 2
00 threshold rolled material at 1150℃ in a heating furnace in an atmospheric atmosphere.
This figure shows the condition of the hot-rolled yarn, which is set so that there is a difference in the adhesion state of trap oxide scale when hot-rolling is performed after heating for 1 hour.

In Table 1, rolling conditions A, Bh, and Cri are all set to carry out rolling in the width direction completely and to perform rolling in the thickness direction later. The rolling and grooving conditions in the direction and thickness direction were changed to vary the state of oxide scale adhesion. In other words, under rolling condition A, the total amount of rolling in the width direction is small (two scales in total) (therefore, of the oxidized scale generated on the tile surface during heating, only the surface layer peels off, and the thin layer on the base metal side remains). This remaining thin oxidation school does not change with subsequent air cooling for a short time, so when rolling in the thickness direction, it is rolled with a thin oxidation school wrapped in 1f'. For both rolling conditions B and C, the rolling amount in the width direction is k l Owm
The oxidized scale generated during heating is largely peeled off from the base metal, leaving an exposed portion of the metal surface. Then, when the exposed part is rolled under rolling condition H, it is maintained at 8°C during thickness direction rolling, and when rolling condition C is used, it is heated to 1100°C to oxidize it.
This will allow the thin oxide school to regenerate. Therefore, in the case of rolling conditions A and C, the material to be rolled is wrapped in a thin oxide school during rolling in the thickness direction, but in the case of rolling condition B, the exposed metal surface is rolled. The material will be rolled in the same condition.

Table 2 VX, chromium@ under the rolling conditions set as above
Stainless steels with different amounts of M were fully hot-rolled, and the surface conditions of the rolled material and rolling rolls after rolling were investigated.

Table 2 (Note) ○ indicates no abnormality.

As is clear from Table 2 (if the material to be rolled is covered with a thin oxide scale during thickness direction rolling, seizing of the rolling rolls will not occur and no cracks will occur. If the metal surface is exposed, seizing and cracking of the rolling roll will occur.In this way, seizing and cracking of the rolling roll may or may not occur depending on the presence or absence of thin oxide scale. The oxidized school plays the role of a general lubricant, and if it is not present, it is thought that the rubbing resistance will increase and seize and cracks in the rolling roll will occur.

By the way, even if the rolling problems such as seizing and cracking of the rolling rolls are rare in B, the lower the Cr content of the material to be rolled, the more likely they are to occur ((,
It stops occurring when the Cr content reaches 8%;
The lower the r content, the poorer the high temperature oxidation resistance (because % 1
This is thought to be because the oxidized school was regenerated during the 5 seconds of air cooling.

As described above (according to the present invention), surface flaws do not occur even if hot rolled under conventional rolling conditions that would cause surface flaws, so even under harsh rolling conditions that increase the unit weight of the slab. Hot rolling improves yield and productivity, reducing production costs.
Ru.

patent applicant Nisshin Steel Co., Ltd. agent Mitsuru Shindo

Claims (1)

[Claims] During hot rolling of stainless steel with a Cr content of 11% or more, the oxidized school that adhered to the surface of the rolled material during rolling peels off and the metal surface of the rolled sheet becomes mist. -5, then a famous pass rolling? A stainless steel hot rolling method with all the features to be carried out.