JPH0215611B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing a ferritic stainless steel sheet having excellent corrosion resistance and containing impurity elements such as C, N, and O as much as possible. (Prior Art) Stainless steels that are generally referred to as highly corrosion-resistant stainless steels often have a high content of Mo, and are often evaluated by the Cr equivalent shown by the following empirical formula as an index of corrosion resistance. Cr eq=%Cr+3×%Mo Here, the higher the Cr eq, the better the corrosion resistance. The ferritic stainless steel in the present invention refers to stainless steel in which the Cr equivalent content in the above formula is 25% or more. Conventional stainless steel has been susceptible to pitting corrosion, crevice corrosion, stress corrosion cracking, etc. in environments where halogen ions such as Cl ions are present, such as in seawater, and its corrosion resistance has not been satisfactory. In recent years, welded stainless steel pipes have begun to be used for condenser pipes in nuclear power plants, etc., but this stainless steel is characterized by high Cr and high Mo, with the above-mentioned Cr equivalent being about 35. . Also, on the other hand, reduce the amount of C+N, or reduce the amount of C+N.
When the amount of N is around 300 ppm, Nb and Ti stabilize C and N, increasing the corrosion resistance of stainless steel. There have been many studies on the corrosion resistance of these stainless steels, such as Streicher.
Corrosion83,． No.70, April (1983) describes this in detail. However, one of the drawbacks of high-purity, high-Cr ferrite stainless steel, which has these excellent properties, is manufacturing problems. Already published in 1978
Publication No. 128464, Japanese Patent Application Laid-Open No. 1983-39732, Japanese Patent Application Publication No. 1987-39732
As disclosed in Publication No. 60-2628, Japanese Patent Application Laid-open No. 60-2622, etc., these steels are
Alternatively, during surface care after cooling, or even during transport to the heating furnace for the next hot rolling process, a phenomenon in which the slab is bent or damaged (referred to as cracking) may occur. It may become impossible to manufacture. Furthermore, it is known that this high Cr, high Mo ferrite stainless steel becomes brittle extremely quickly, and that when it is wound into a hot coil, it hardens and cracks, making it impossible to unwind it. Regarding this point,
As stated in Publication No. 24055, the coiled hot coil is rapidly cooled and held at a temperature below 450°C to reduce embrittlement, making it possible to coil it, and making it possible to manufacture strips by annealing and rapid cooling. There is. (Problems to be solved by the invention) These high-purity, high-Cr, high-Mo ferritic stainless steels are extremely brittle during the manufacturing process and difficult to handle. We experienced a cracking phenomenon in which cracks occur during the handling of slabs. The inventors took measures to prevent this cracking as proposed in Japanese Patent Application No. 196806/1983. We also experienced cases where hot-rolled coils became brittle during winding and broke during unwinding. Regarding this embrittlement of hot coils, we have established a technology that can prevent it, as proposed in Japanese Patent Application No. 113834/1983. However, these high-purity, high-Cr, high-Mo stainless steels require hot coil annealing in order to be subjected to the cold rolling process, and for this reason, the surface layer Cr is removed during annealing.
is oxidized and a Cr-free layer is formed. This escape
The higher the Cr, the larger the Cr layer, and in the case of seawater-resistant stainless steel, it is not easy to remove the Cr layer, and in the pickling process, the steel is forced to pickle at a higher temperature and for a longer period of time than conventional stainless steel. In this way, annealing is performed to ensure toughness, and pickling is performed for a long time to remove the Cr-free layer produced by this annealing, which leads to a decrease in productivity and yield, resulting in high prices and high prices. This is one of the reasons why high-purity Cr and high-Mo stainless steels become even more expensive. (Means for Solving the Problems) Therefore, the present inventors conducted research to establish a manufacturing method for this high purity ferritic stainless steel, and established a manufacturing method as summarized below. That is, C and N in the steel are kept as low as possible, and the necessary minimum amount of Ti and Nb is added to ensure intergranular corrosion and corrosion resistance of welded parts. When cooling CC slabs and slabs that have been subjected to blooming rolling, embrittlement is minimized by air cooling or at a cooling rate faster than air cooling until the temperature reaches 600°C. These slabs are heated to over 1100℃ during hot rolling, and after hot rolling is finished at over 800℃, water is poured on a runout table to force cooling and rapidly cool the brittle areas, thereby preventing the embrittlement of the hot coil. minimize
Ferritic stainless steel that avoids embrittlement during cooling of the hot coil by performing low-temperature winding at 550℃ or less, and can be pickled and cold-rolled in the normal cold rolling process without annealing the hot coil. Established a method for manufacturing steel. The manufacturing method of the present invention will be described in detail below. Select 25Cr−4Ni−4Mo as the main component, Nb,
Small steel ingots with varying amounts of Ti and C+N are melted in a laboratory, heated to 1170°C, hot rolled to a thickness of 5 mm, the rolling end temperature is 800°C or higher, and then water cooled to various winding temperatures. As a simulation of winding, hot-rolled sheets were annealed after being kept at the winding temperature for 1 hour (20℃/hr), and hot-rolled sheets were omitted. The process was implemented and a comparative study was conducted. Table 1 shows the components of the test steel, and Table 2 shows the conditions and circumstances of each process. As a result, C and N are lower than conventional steel.
Although the annealing of the hot-rolled material is omitted, the toughness of the material is maintained, and problems such as cracking do not occur during the cold-rolling process, making it possible to obtain an extremely good cold-rolled sheet. was completed. In addition, with regard to the removal of Cr from hot-rolled sheets, when hot-rolled sheets are omitted, the result is less than half that of when hot-rolled sheets are annealed, making it possible to significantly shorten the pickling time for hot-rolled coils. It has been found. As mentioned above, for steel types that are highly embrittled, forced cooling by water injection to prevent embrittlement after hot rolling not only makes handling of the hot coil easier, but also improves the composition, especially Products that reduce C and N as much as possible and add necessary amounts of stabilizing elements such as Nb and Ti do not cause any problems in subsequent processes even if annealing of the hot-rolled sheet is omitted, making it equivalent to annealed materials. We were able to obtain the above materials. Furthermore, as shown in Table 2, the desorption performed in the pickling process
The Cr layer was found to be easier to remove compared to annealed hot-rolled sheets, and the pickling time could be shortened, which solved the problem of removing the Cr-free layer, which previously caused a decrease in productivity. It was found that this solution greatly contributed to improving productivity and lowering prices.

【table】

【table】

[Table] The reasons for the limitations of the present invention will be described below. When cooling high Cr and high Mo CC slabs, slabs made by blooming CC slabs, or slabs after blooming steel ingots, Laves that exist at 800℃ are used.
We investigated ways to suppress precipitation as much as possible by speeding up the cooling of the phase precipitation region. As a result, it was found that cooling of these slabs can be done by cooling the slabs to below 600°C by air cooling or using air cooling or higher. Next, during hot rolling, the above slab is heated to 1100°C or higher in a heating furnace to dissolve the precipitates,
After the second phase particles present in the slab are made into a solid solution, normal hot rolling is performed. This rolling is completed at a temperature of 800℃ or higher, water-cooled immediately after rolling to prevent the precipitation of the Laves phase with a precipitation nose at 800℃, and the hot-rolled steel strip is rolled at a temperature of 550℃ or lower. Even during reheating and slow cooling, it prevents the formation of intermetallic compounds such as the Laves phase and carbides, prevents embrittlement, and provides toughness. Also, if possible after winding the hot-rolled coil,
By immersing the material in water for less than a minute, the cooling rate can be accelerated and toughness can be further improved. The above phenomenon was further investigated in a broader range of component systems through laboratory experiments, and it was found that it holds true for the following component system. Cr: 18-55% (wt%) Mo: 0.5-6% Ni: 0-8% C: 0.01% or less N: 0.02% or less Nb or Ti type 1 or 2 and 0.6% or less O: 0.01% or less Si: 2% or less Mn: 2% or less S: 0.005% or less P: 0.04% or less Deoxidizing components such as Al and Ca 0.06% or less Next, the reason for limiting the steel composition range of the present invention will be explained. Ferritic stainless steels like this one, which can be used even in seawater environments, have high corrosion resistance.
It needs to be a Cr and high Mo steel, and Cr eq (=%Cr+3×%Mo) needs to be 25% or more. Further, from the viewpoint of corrosion resistance and toughness, it is necessary to reduce impurities such as C, N, O, S, etc. in the steel components as much as possible. In particular, if the amounts of C and N are large, carbonitrides are likely to form, reducing toughness and at the same time causing intergranular corrosion, which are harmful elements. Furthermore, as shown in Figure 1, they are harmful from the viewpoint of manufacturability. The lower the content, the more desirable it is, and the content is limited to C: 0.01% or less and N: 0.02% or less. Furthermore, it is desirable to keep O as low as possible.
It was set to 0.01% or less. Nb and Ti are effective elements for fixing C and N, improving toughness and preventing intergranular corrosion.As shown in Figure 2, the value of (Nb+Ti)/(C+N) The larger the value, the faster the embrittlement tendency becomes. Excessive Nb and Ti promote embrittlement in high temperature ranges, so the amount added should be kept to a minimum.
(Nb+Ti)/C+N was limited to 20 or less, and one or two types were limited to 0.6% or less. Ni is effective in improving toughness, and if necessary 8%
It can be added below. If the Cr-free layer remains in the product, it will significantly reduce corrosion resistance. Therefore, it is necessary to completely remove it during pickling of hot rolled sheets. If annealing of the hot-rolled sheet is omitted, this Cr-free layer will be significantly reduced.
The pickling process can also be simplified, but this pickling process
It is necessary to completely remove the Cr layer. (Example) Steel with the components shown in Table 3 was produced by sufficiently decarburizing and denitrifying by the VOD method. These are CC slabs,
It was charged into a heating furnace to form a blooming rolled slab and air cooled to 500°C. After that, it is charged into a hot rolling heating furnace, heated to 1170℃ for more than 10 minutes, hot rolled as usual, and after leaving the final stand at 850℃, it is water cooled and rolled to a temperature of 320 to 380℃.
It was carried out in Afterwards, the coil undergoes cold refining.
After mechanical descaling, the annealed product and the non-annealed product were pickled, cold rolled, and finally annealed.
A cold-rolled steel sheet was obtained by pickling.

[Table] In order to completely remove the Cr-free layer when hot-rolled coils are pickled, annealed coils are pickled in high-temperature H ₂ SO ₄ for 3 minutes.
The annealing time for the omitted material was 1.5 minutes. Similar to annealed materials, there are no problems with hot-rolled sheets subjected to the cold rolling process without annealing.
A cold-rolled steel sheet with excellent corrosion resistance was manufactured. (Effect of the invention) As described above, according to the present invention, high Cr, high
There is no decrease in toughness during the manufacturing process of Mo-based ferritic stainless steel, and by omitting annealing of hot-rolled sheets, it is possible to save energy and improve productivity by shortening pickling time. Since the yield is also improved, it becomes possible to manufacture highly corrosion-resistant stainless steel plates and welded steel pipes at even lower cost.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the influence of winding temperature and C+N on the impact properties of a hot rolled sheet, and FIG. 2 is a diagram showing the influence of (Nb+Ti)/(C+N) on embrittlement at high temperatures.

Claims (1)

[Claims] 1. C: 0.01% or less, N: 0.02% or less,
O: 0.01% or less, Cr eq=(%Cr+3×%Mo):
25% or more, one or two types of Nb and Ti: 0.6% or less as the main components, and a ferritic stainless steel slab that satisfies (Nb+Ti)/(C+N)≦20 from casting temperature or blooming temperature. The slab is cooled to 600°C or lower with air cooling or at a cooling rate higher than air cooling, then heated to 1100°C or higher, hot rolled, and after finishing hot rolling at 800°C or higher, it is rapidly cooled by water injection to 550°C. In order to suppress the formation of a Cr-free layer, the hot-rolled steel plate is pickled without annealing, and after the generated Cr-free layer is completely removed, the steel plate is produced in a cold rolling process. A method for producing a high-purity ferritic stainless steel sheet. 2 C: 0.01% or less, N: 0.02% or less, in weight%
O: 0.01% or less, Cr eq=(%Cr+3×%Mo):
25% or more, one or both of Nb and Ti: 0.6% or less,
Ni: Main component is 8% or less, (Nb+Ti)/(C
＋N）≦20 from casting temperature or blooming temperature.
After cooling the slab to 600℃ or less with air cooling or a cooling rate higher than air cooling, then heating the slab to 1100℃ or higher, hot rolling and finishing hot rolling at 800℃ or higher,
Rapid cooling by water injection, winding and unrolling at below 550℃
In order to suppress the formation of a Cr layer, the hot rolled steel sheet is pickled without annealing, and after the generated Cr-free layer is completely removed, the steel sheet is produced in a cold rolling process. A method for producing high-purity ferritic stainless steel sheets.