JPS6311083B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for completely preventing grain boundary oxidation and preventing edge cracking during hot rolling of nickel-containing steel. Conventionally, high alloy steels, especially high nickel alloys such as amber and inconel, were easily oxidized when heated.
It was said that internal oxidation is particularly pronounced in a heavy oil-based combustion gas atmosphere with a high sulfur content. The reason for this is that the sulfur in the combustion gas reacts with the nickel in the steel, forming a low melting point phase such as Ni-
It was thought that Ni ₃ S ₂ eutectic etc. were formed in the scale and promoted oxygen penetration. These phenomena occur as a problem of scale loss in heating furnaces, and when products such as boilers are used at temperatures of 700 to 1000℃.
It was treated as a range of high temperature corrosion problems. There are few examples of hot workability problems such as edge cracking during hot rolling being investigated. In other words, it has rarely been recognized as a grain boundary cracking phenomenon based on grain boundary oxidation that occurs deep inside the metal and occurs in high-temperature environments such as 1000 to 1300°C, where the reaction rate is extremely fast. Also, known methods for preventing oxidation of steel, which is thought to be easily oxidized during heating, include applying an antioxidant to the surface of the steel slab, or covering the surface of the steel slab with a steel plate to reduce contact with the atmosphere. There is. However, there are many mechanistic aspects of the oxidation prevention in these methods that are unclear, and there have been cases where the oxidation prevention effect has not been achieved due to errors in use. for example,
When a slag-based antioxidant containing silicon oxide as a main component was applied to a slab, no effect was observed on the vertical surfaces, and cracks often occurred during hot rolling. The inventor experimentally investigated the mechanism of the above antioxidant and found that it forms a multi-component low melting point compound or eutectic in a high temperature range exceeding 1000℃, and exhibits an antioxidant effect in the molten state. I understand. Therefore, most of the antioxidant applied to vertical surfaces melts away at high temperatures that are effective for preventing oxidation, so it is thought that it has no antioxidant effect. In addition, we conducted thermal oxidation experiments on steel slabs with systematically varying nickel and chromium contents, and found that the degree of grain boundary oxidation was more pronounced as the difference in oxidation ease, or oxidation resistance, between the grain interior and the grain boundary was larger. It turns out that there is something. In other words, in iron-nickel steel that does not contain chromium, the oxidation resistance within the grains and at the grain boundaries is about the same.
The oxidation proceeds as a whole surface oxidation, and although there is a lot of scale loss, only the grain boundaries are not particularly deeply oxidized. On the other hand, when chromium is contained, as the content increases, the oxidation resistance within the grain increases, and since the increase in the oxidation resistance at the grain boundary is extremely small, the grain boundary is selectively oxidized, resulting in hot processing. This causes deep grain boundary oxidation that becomes the starting point for cracks. It is most noticeable when the content is 2 to 10% by weight of chromium. Furthermore, as the chromium content increases, the grain boundary oxidation resistance increases significantly, the tendency for general oxidation becomes stronger, and scale loss decreases. Further, for 2 to 10% by weight chromium-nickel steel (5% by weight or more Ni), which has significant grain boundary oxidation, conventional steel plate coatings have no effect in preventing oxidation. Furthermore, since the crystal grains in the solidified structure of continuously cast slabs and the like are coarse, grain boundary oxidation progresses faster than in cast materials. As mentioned above, oxidation prevention during heating of nickel steel (Ni 5% by weight or more) and high nickel-chromium steel decomposed slabs and continuously cast slabs, which are prone to grain boundary oxidation, can be done using conventional steel plate coating methods or oxidation prevention methods. Since sufficient effects cannot be obtained with the chemical coating method, strong anti-oxidation measures are desired. In view of the current situation, the present invention proposes a method for preventing oxidation during heating of steel that is susceptible to intergranular oxidation. This invention involves applying an antioxidant to the surface of a high nickel steel or high nickel-chromium steel slab containing 5% by weight or more of nickel, and covering a part or all of the surface with a steel plate to prevent heating during heating. The main purpose is to prevent grain boundary oxidation and prevent edge cracking during hot rolling, and furthermore, the steel billet contains one or more of calcium, magnesium, yttrium, and rare earth elements.
This also includes a method in which the grain boundary oxidation resistance of the material itself is improved by containing 0.1% by weight or less, and then the above-mentioned oxidation prevention method is carried out. According to the implementation of this invention, even if the antioxidant applied to the vertical surface of the steel slab is molten in a high temperature range, it will not melt off because it is covered with the steel plate, and the intergranular oxidation prevention will be completely prevented. It is done. In addition, adding one or more of calcium, magnesium, yttrium, and rare earth elements (lanthanum, cerium, etc.) to steel improves the grain boundary oxidation resistance of the material itself, making oxidation prevention more effective. This is because it is effective for Conventionally, it has been said that high-temperature corrosion resistance deteriorates when the sulfur content in steel is high, but the above-mentioned additive elements have a sulfur removal effect, which is thought to improve intergranular oxidation resistance. It is also believed that the high temperature deformability is improved and that this has the effect of preventing cracks from expanding during rolling. Note that if these additive elements are contained in a total amount exceeding 0.1% by weight, hot workability will deteriorate.
It is desirable that the content be 0.1% by weight or less. Although this invention is primarily intended for steels that tend to undergo grain boundary oxidation during heating, it goes without saying that it can be applied to all other types of steel. Next, embodiments of the invention will be described. A rolled test piece (thickness 20 mm x width
30cm x length 100mm), applied anti-oxidation measures under the conditions shown in Table 2, charged it into a heating furnace and heated it to 1150℃, and then the thickness was 20mm → 12mm → 8mm → 5mm → 3.2
It was hot rolled at 800℃ and finished at 800℃. The occurrence of edge cracks after rolling was then investigated. The results are shown in Table 2.

【table】

【table】

[Table] From the above results, it can be seen that the steel coated with only the steel sheet coating and the steel coated with only the oxidation agent had the same deep cracks as the steel without any countermeasures, indicating that the conventional method of preventing oxidation is ineffective. On the other hand, with the steel according to the present invention, microcracks of 0.5 mm or less were observed, but the conventional deep cracks were completely prevented. Furthermore, all of the steels to which calcium, magnesium, and rare earth elements were added had no cracks, indicating that these added elements further improved the antioxidation effect.

Claims (1)

[Claims] 1. Antioxidant is applied to the surface of a high nickel steel slab containing 5% by weight or more of nickel, and part or all of the surface is covered with a steel plate to prevent grain boundary oxidation during heating. A method for preventing edge cracking during hot rolling of steel. 2. The high nickel steel contains one or more of calcium, magnesium, yttrium, and rare earth elements.
A method for preventing edge cracking during hot rolling of steel according to claim 1, characterized in that a steel piece containing 0.1% by weight or less is used. 3 Applying an antioxidant to the surface of a high nickel-chromium steel piece containing 5% by weight or more of nickel,
A method for preventing edge cracking during hot rolling of steel, characterized by covering part or all of its surface with a steel plate to prevent grain boundary oxidation during heating. 4. The high nickel-chromium steel contains one of calcium, magnesium, yttrium, and rare earth elements.
4. A method for preventing edge cracking during hot rolling of steel according to claim 3, characterized in that a steel billet containing 0.1% by weight or less of 0.1% by weight or more is used.