JPS6043411B2 - Method for producing austenitic steel sheet with excellent oxidation resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high Si-containing austenitic steel sheet with excellent oxidation resistance by continuous casting.

In recent years, high Si-containing austenitic steel sheets have been used for various heat-resistant applications such as automobile exhaust gas purification devices, and SLJSXM15JI has also been standardized, and the demand for them is increasing.

By the way, this type of so-called high-grade steel has conventionally been manufactured by the ingot-blowing method.

However, today's remarkable development of continuous casting technology has largely replaced the ingot making process, and it is now being used to produce this high-grade steel. Manufacturing austenitic steel sheets with excellent oxidation resistance using the continuous casting method has clear advantages over the ingot-forming method in terms of yield, production efficiency, energy consumption, etc. It can be used as an advantage.

However, the steel for the above-mentioned purpose had the disadvantage that when it was made into a slab through continuous casting and then hot-rolled to form a hot-rolled coil, cracks in the coil edges occurred, which significantly reduced the value of the finished product. . In order to manufacture a high Si-containing austenitic steel sheet with excellent oxidation resistance by continuous casting at a stable quality and at a low cost, the above-mentioned drawbacks must be overcome.

This invention is a high Si-containing austenitic steel that does not contain high Cr and high Ni like SUS310S, but has relatively low Cr and Ni contents, has corrosion resistance equivalent to conventional SUS3lμs, and has low Cr and Nl contents. The purpose of this method is to significantly reduce manufacturing costs by reducing the occurrence of coil edge cracks that occur when hot-rolling a cast piece produced by the above-mentioned continuous casting method to make a hot-rolled coil. High S due to continuous casting that can prevent
This paper proposes a method for manufacturing an i-containing austenitic steel sheet. This invention is based on CO. 1O% or less, Si5% or less,
Mn 2% or less, Nl 9-15%, Crl 7.5-25%
, S.O. Either we melt the steel with less than 3% OO, less than 3% remaining and unavoidable impurities; 1O% or less, Si5% or less, Mn2% or less, Nl
9-15%, Crl 7.5-25%, SO. OO3% or less, and CaO. A steel containing OOlO ~ 0.0060% and the balance consisting of Fe and unavoidable impurities is produced,
After continuous casting into a slab, heating temperature 1200-12
The gist of this invention is a method for producing an austenitic steel sheet with excellent oxidation resistance, which is characterized by hot rolling at a 50° C1 rolling reduction ratio of 30 or less.

The rolling ratio here refers to slab thickness/product thickness.

Before explaining the manufacturing method according to the present invention, the target austenitic steel will be explained. This steel was specially applied for in 1972.
Although it was already proposed in No. 122836, austenite has the following characteristics and has excellent oxidation resistance equivalent to SUS31O5 despite relatively low Cr and Ni contents. It is steel. That is, this invention W4. Although they are high Si-containing austenitic steels and have the same composition, S
This is based on the knowledge that oxidation resistance varies significantly depending on the amount adjusted. In other words, according to the inventors' research, oxidation resistance can be improved by reducing the amount of S in steel. is significantly improved, and the oxidation resistance is further improved by the Ca treatment. S in steel exists as MnS,
MnS on the steel surface changes from 800℃ to Mn-Cr-0-
After passing through S, it finally changes to Cr-Mn-0.

Therefore, S is liberated and, conversely, Cr and O are concentrated.
Moreover, a local Cr-depleted layer is generated around the inclusion, and an S-rich atmosphere is initially formed. In other words, high S content steel becomes an oxide scale mainly composed of spinel-type oxides containing localized Fe, Cr, and Si in a uniform oxide scale mainly composed of Cr2O3,
After a long period of oxidation, the oxidized scale generating portion containing Fe becomes the starting point of abnormal oxidation. On the other hand, with ultra-low S steel, a uniform oxide scale consisting of Cr2O3 is formed on the surface, there are very few inclusions, and a very small amount of carbon is present.
A-based oxysulfides are stable even at 1000'C, and 12
At 00'C, S is liberated and becomes an oxide, but no concentration of Cr is observed.

Therefore, there is no abnormal oxidation after long-term oxidation. Also,
It has been found that even in high-Si content austenitic steels containing the same amount of S, the amounts of Cr and Si affect the oxidation resistance.

That is, when Si is 1.5% or more, there is an effect of reducing MnS, and in steel containing 22.5% or more (Si+Cr), the corrosion resistance is improved. Therefore, by reducing the S content to an extremely small amount and performing Ca treatment as necessary, (Ca+Si)
By setting the content to 22.5% or more, Cr. N
Despite being a steel with relatively low i content, Cr
, it is possible to obtain oxidation resistance equivalent to that of SUS31OS, which has a high Ni content.

Next, the reasons for limiting each component will be explained.

C improves strength, but improves workability, stress corrosion cracking resistance,
In particular, it deteriorates oxidation resistance, so the upper limit of its content should be 0.1%.
shall be.

Si is necessary as a deoxidizing agent and, as mentioned above, is extremely effective in improving oxidation resistance, so it is actively added, but if it exceeds 5%, weldability deteriorates, so the content should not exceed 5%. do.

Mn is a necessary element for acting as a deoxidizing agent and improving workability, but if added in excess, high temperature oxidation resistance deteriorates, so its content is set to 2.0% or less.

Ni and Cr are necessary to maintain the characteristics of austenitic steel with excellent oxidation resistance, but are limited as follows in order to manufacture at the lowest possible cost. Ni is set at 9% or more to ensure a stable austenite structure, and at 15% or less to avoid a price increase. Cr needs to be contained in an amount of 17.5% or more to ensure improved oxidation resistance.

However, if it exceeds 25%, 15% or more of Ni must be added to stabilize the austenite phase, so in order to provide it at a low price, it needs to be 25% or less.
It was set at 17.5 to 25%. S is 0.003% to reduce MnS in the steel as much as possible and improve oxidation resistance.
The following shall apply.

The amount of MnS precipitated decreases significantly below about 0.001%; however, when it is close to the upper limit, MnS
In order to completely prevent the precipitation of S, it is desirable to change S to a compound that is stable even at high temperatures. Ca is an effective element for improving oxidation resistance, and the effect can be obtained at 0.0010% or more, but if it exceeds 0.0060%, it impairs the weldability of steel, so Ca is an element that is effective in improving oxidation resistance. %.

Next, the manufacturing method will be explained.

Steel having the above-mentioned limited components is melted in various steelmaking furnaces such as converters or electric furnaces, but in order to contain the extremely small amount of S that is characteristic of this steel, for example, CO is added by gas dilution. Sufficient desulfurization is possible by using an out-of-furnace refining method such as oxidation refining and reduction refining by lowering the partial pressure.

After the steel produced in this way is made into slabs by continuous casting,
High S with excellent oxidation resistance is achieved by hot rolling at a heating temperature of 1200 to 12500C and a reduction ratio of 30 or less.
1-containing austenitic steel sheet.

The reason why the heating temperature is limited to 1200-1250'C is that if the temperature is less than 1200°C, the deformation resistance increases, resulting in a marked decrease in hot workability and frequent occurrence of edge cracks during coil rolling. It has also been found that when the temperature exceeds 1250°C, grain boundary oxidation occurs on the slab surface, causing edge cracking of the coil during hot rolling. Therefore, the heating temperature was limited to 1200 to 12500C. The reduction ratio was limited to 30 or less.
If it exceeds the above heating temperature (1200-1250°C)
In this case, edge cracking occurs due to a decrease in the surface temperature of the coil during processing, so the rolling reduction ratio is set to 30 or less.

After forming a slab through continuous casting, it is hot-rolled within a limited range as described above to form a hot coil, resulting in excellent oxidation resistance due to continuous casting that does not cause cracks on the coil edges. It becomes possible to manufacture austenitic steel sheets.

The effects of the manufacturing method according to the present invention will be explained based on the following examples.

Steel with various composition changes within the above-mentioned limited range was melted and made into a slab using a continuous casting machine.

Steel types 1 to 8 are steels without Ca addition during melting,
Its component composition is as shown in Table 1. Steel type 9-1
Steel No. 2 has the composition shown in Table 2 and is Ca-added. For steel types 1 to 4, the continuous casting conditions are cooling with a specific water amount of 1.9 to 1.4e/K9St, and hot rolling is outside the limited range of this invention method (comparative example) .

For steel types 5 to 12, the manufacturing method according to the present invention is carried out, and the cooling is slow cooling with a specific water amount of 1.4'/K9st. Each steel was hot-rolled under the conditions shown in Table 3 to form hot coils, and the coil edges were evaluated for cracking.

The results are shown in Table 3, but steels 1 to 4 according to comparative examples were all 10 m! ! Edge cracks of t or more were observed, but steels 5 to 12 according to the present invention did not show any edge cracks, and VA9 to 12 to which Ca was added during melting did not show any cracks at all. Ta.

Steels 5, 6, 9, 10, 1 manufactured by carrying out this invention
1 was repeatedly heated 1200'C x 5 Ollr x (1 to 6) times, and changes in weight due to oxidation were investigated.

The results are shown in FIG. 1 in comparison with SUS3lO5. From this result, although the Cr and Nl contents of the present invention are relatively low, SU
It can be seen that the weight decrease is approximately the same as that of S3lμs, and that it has the same oxidation resistance. As described above, since the method of the present invention can completely prevent edge cracking of hot coils, it is a manufacturing method that can manufacture an austenitic steel sheet with excellent oxidation resistance by continuous casting, which is highly productive in mass production.

[Brief explanation of drawings]

Figure 1 shows a comparison button U with steel produced by carrying out this invention.
1 is a chart showing the results of an oxidation resistance test on S3lOS.

Claims (1)

[Claims] 1 C0.10% or less, Si 5% or less, Mn 2% or less,
Ni9-15%, Cr17.5-25%, S0.003
% or less, the balance is Fe and unavoidable impurities, and after making a slab by continuous casting, the heating temperature is 1200℃.
A method for producing an austenitic steel sheet with excellent oxidation resistance, characterized by hot rolling at ~1250°C and a rolling reduction ratio of 30 or less. 2 C0.10% or less, Si5% or less, Mn2% or less,
Ni9-15%, Cr17.5-25%, S0.003
% or less, and further contains 0.0010 to 0.0060% of Ca, with the balance consisting of Fe and unavoidable impurities. After forming a slab by continuous casting, the steel is heated to a temperature of 1200 to 1200.
A method for producing an austenitic steel sheet with excellent oxidation resistance, characterized by hot rolling at 1250°C and a rolling reduction ratio of 30 or less.