JPH0422677B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD This invention relates to a method for manufacturing a duplex stainless steel clad steel plate using austenitic-ferritic stainless steel (hereinafter referred to as duplex stainless steel) as a laminated material and carbon steel or low alloy steel as a base material. . Conventional technology The cladding method is known as a method of laminating two or more metal plates to take advantage of their respective properties.The steel base material is carbon steel or low alloy steel, stainless steel, nickel, or Ni. -There is a clad steel sheet clad with Cr-Fe alloy, etc. There are rolling methods, explosion rolling methods, casting methods, forge welding methods, and other methods to manufacture clad steel sheets, but the rolling method, which is most commonly used, involves assembling slabs of cladding material and base material. This is heated, hot-rolled, finished to a predetermined thickness, and left to cool to produce a clad steel plate. Stainless steel has structures such as austenite, martensite, and austenite + ferrite, depending on its composition, processing, heat treatment, etc. Among these, duplex stainless steel with austenite + ferrite is heated to 950 to 1100 °C and then rapidly cooled. By subjecting it to solution heat treatment, it has excellent corrosion resistance and good resistance to localized corrosion such as stress corrosion cracking and intergranular corrosion, so it is used in various pollution prevention equipment, chemical industry equipment, etc. In order to manufacture clad steel plates using duplex stainless steel with excellent properties as a laminating material, hot-rolled clad steel plates are heated to 950-1100°C and then rapidly cooled. It is necessary to impart corrosion resistance. However, when this solution heat treatment is applied, the base metal, carbon steel or low alloy steel, is simultaneously reheated to the austenite region and then rapidly cooled, making it difficult to ensure the desired performance of the base metal. Become. For this reason, it has been considered difficult to manufacture clad steel plates made of duplex stainless steel by the rolling method. Problems to be Solved by the Invention In view of the current situation, this invention uses duplex stainless steel as a laminated material, which can impart excellent properties without reheating and solution heat treatment to duplex stainless steel. This paper proposes a manufacturing method for clad steel sheets. Means for Solving the Problems The inventor believes that one purpose of solution heat treatment of duplex stainless steel is to prevent the precipitation of the highly brittle σ phase, which is a Fe-Cr intermetallic compound, but Solution heat treatment requires heating to a high temperature in order to dissolve the precipitated σ phase into a solid solution in a relatively short period of time, but in the continuous cooling process that follows hot rolling in the cladding process, the complete The precipitation of the σ phase from the solid solution state is slightly delayed, and the rapid cooling treatment to prevent the precipitation of the σ phase is possible in a low temperature range, and cooling is performed at a relatively slow cooling rate without damaging the performance of the base material. It has been found that the precipitation of the σ phase can be prevented without any problem. Based on this knowledge, we have come to make this invention. In other words, this invention involves heating a material made of austenitic/ferritic stainless steel as a laminated material and nickel interposed between a base material of carbon steel or low alloy steel to 1,100 to 1,250°C within 15 minutes. After hot rolling at a temperature of 850℃ or higher, cooling to 700℃ at a cooling rate of 30℃/min or higher,
The main point is to continue to let it cool. The components of the duplex stainless steel used as the laminating material in this invention do not need to be particularly restricted, and any generally known stainless steel can be used. An example of its chemical composition is C0.08% or less,
Si1.00% or less, Mn2.00% or less, Cr21.0~28.0%,
Ni3.0~7.5%, Mo1.0~4.0%, Cu2.0% or less,
Contains W1.0% or less, N0.3% or less, and if necessary, other trace elements may be added, and the balance is Fe.
and unavoidable impurities. Further, the nickel interposed between the laminate material and the base material is effective in forming an alloy phase on the bonding surfaces of the two to complete the bonding, and a sheet as thin as foil is used. The hot rolling in this invention is carried out by a commonly used method, and the rolling temperature is 1100°C.
If the temperature is less than 1,100 to 1,250°C, the deformability decreases, the deformation resistance becomes large, and the rollability decreases, and even if it exceeds 1,250°C, the deformability decreases and the rollability is poor, so a range of 1,100 to 1,250°C is desirable. Further, the reason why the hot rolling time and the cooling rate in the cooling process after hot rolling were controlled is as follows. Duplex stainless steel with the above chemical composition was heated at 1250℃ x 30
FIG. 1 shows the isothermal transformation curve of the σ phase when the sample was heated for 1 minute and then cooled with water. From this figure, about 35 to 830℃
It can be seen that 2.5% σ phase precipitates when held for a minute. Therefore, in order to prevent the precipitation of the σ phase during the cooling process after hot rolling, it is necessary to control the cooling rate so that it does not mix with the curve in which the σ phase does not precipitate, which is determined based on the isothermal transformation curve of the σ phase. There is. Now, the cooling curves for the case where rolling is finished at 1000°C (Line A) and the case where rolling is completed from the heating temperature to 850°C and rolling is finished at 850°C (Line B) are based on the isothermal transformation curve of the σ phase. Figure 2 shows the relationship when the cooling rate is controlled so as not to intersect the 0% curve (indicated by a chain line) determined by the equation. In other words, when the rolling end temperature is set to 850°C or higher, hot rolling (indicated by the tooth profile curve) is completed within 15 minutes, and the temperature is increased to a safe 700°C without precipitation of the σ phase.
It can be seen that it is necessary to cool within 1 minute (cooling rate of 30°C/min or more), and that normal cooling is sufficient for temperatures below 700°C, which is outside the region of σ phase precipitation. Based on the above results, hot rolling is completed at a temperature of 850°C or higher, and the cooling rate is continued to a temperature of 700°C or higher.
It is rapidly cooled at a rate of 30°C/min or more and then left to cool. Incidentally, if it is necessary to correct the flatness of the strip, it may be cooled while leveling is performed in the above-mentioned forced cooling process. Effects of the Invention This invention prevents the precipitation of the σ phase by rapidly cooling the σ phase precipitation area after hot rolling in the production of clad steel sheets made of duplex stainless steel as laminated materials, thereby ensuring the properties of the base material. Corrosion resistance equivalent to that achieved by solution heat treatment can be achieved without the need for reheating or solution heat treatment, which is harmful to corrosion. Therefore, there is no need for reheating solution heat treatment, which is normally required to impart excellent corrosion resistance to clad steel made of dual-phase stainless steel, which has not been practical in the past. This is not necessary and can also contribute to energy savings. Examples Next, examples of the present invention will be described. Example 1

[Table] A slab of duplex stainless steel whose chemical composition is shown in Table 1 with a thickness of 15 mm, a width of 1000 mm, and a length of 1500 mm was used as the laminating material, and a 110 mm thick slab of 50 Kg/mm grade ² carbon steel,
A slab with a width of 1000 mm and a length of 1500 mm is used as the base material, and a Ni foil with a thickness of 100 μm is interposed between them.
Heated to 1200℃, rolled for 3 minutes, finished at 950℃,
Thickness is base material 22mm, laminated material 3mm, total 25mm, width 1500
After forming a clad steel plate with a length of 5000 mm, it was immediately rapidly cooled to 350°C at a cooling rate of 50°C/min, and then allowed to cool. For comparison, the same assembled slab as above was hot rolled by the conventional method and then left to cool (Comparative Example 1).
In addition, after performing the same rolling as in Comparative Example 1 and letting it cool, the temperature was reduced to 1050°C.
The sample was reheated and subjected to solution heat treatment (Comparative Example 2).
Then, pitting corrosion tests were conducted on these laminated materials using artificial seawater (80°C Ar deaerated). The result is the second
Shown in the table.

[Table] From the above results, it can be seen that the corrosion resistance of the clad steel sheet according to the present invention is superior to Comparative Example 2, which was subjected to reheating solution heat treatment, although duplex stainless steel with the same composition was used. Example 2

[Table] A slab of duplex stainless steel whose chemical composition is shown in Table 3 with a thickness of 24 mm, a width of 1740 mm, and a length of 3000 mm was used as the laminated material, and a slab of 40 Kg/mm ^2nd class carbon steel with a thickness of 176 mm,
A slab with a width of 1740 mm and a length of 3000 mm is used as the base material, and a Ni foil with a thickness of 100 μm is interposed between them.
Heated to 1240℃, rolled for 8 minutes, finished at 870℃,
The base material is 14.5mm thick, the laminated material is 2mm, the shape is 16.5mm, and the width is 16.5mm.
After forming a clad steel plate with a length of 3500 mm and a length of 18000 mm, it was immediately cooled to 550 °C at a cooling rate of 70 °C/min while leveling, and then allowed to cool. For comparison, the same assembled slab as above was hot-rolled by the conventional method and then allowed to cool while leveling (Example 3), and the same rolling as in Comparative Example 3 was performed and the slab was allowed to cool while leveling. 1020℃
The sample was reheated to undergo solid solution heat treatment (Comparative Example 4).
Then, a pitting corrosion test for these laminated materials was conducted in the same manner as in Example 1. The results are shown in Table 4.

[Table] From the above results, it can be seen that similarly to Example 1, the clad steel sheet according to the present invention has excellent corrosion resistance equivalent to that of Comparative Example 4, which was subjected to reheating solution heat treatment. Based on the above examples, the relationship between the pitting corrosion potential of the laminated material and the Cr+3Mo (%) contained in the laminated material is shown in FIG. 3. From this figure,
It can be seen that the corrosion resistance when duplex stainless steel is used as a cladding material becomes better as the Cr content increases. Therefore, the Cr content of the duplex stainless steel may be adjusted so as to obtain the required corrosion resistance depending on the purpose of use.

[Brief explanation of drawings]

Fig. 1 is a graph showing the isothermal transformation curve of the σ phase, Fig. 2 is a graph showing the relationship between the cooling curve after hot rolling and σ phase precipitation in this invention, and Fig. 3 is a graph showing the relationship between the Cr+3Mo content of the laminate and the σ phase precipitation. It is a graph showing the relationship with corrosion resistance.

Claims (1)

[Claims] 1 A combination material made of austenitic/ferritic stainless steel with nickel interposed between the base material of carbon steel or low alloy steel,
℃ and hot rolled within 15 minutes to 850℃
After finishing rolling at a temperature above, the cooling rate is 30℃/
A method for manufacturing a duplex stainless steel clad steel sheet, characterized by cooling to 700°C or less for at least 1 minute, and then allowing it to cool.