JPS62110880A - Production of two-phase stainless steel clad steel - Google Patents

Abstract

PURPOSE:To impart excellent characteristics to a two-phase stainless steel without subjecting said steel to reheating and soln. heat treatments by controlling the temp. and time for hot rolling of a combined stock formed by using the stainless steel as a cladding metal and interposing nickel between the cladding metal and base metal, the cooling rate, etc., after the end of rolling. CONSTITUTION:The combined stock formed by using the austenite ferritic stainless steel (2-phase stainless steel) as the cladding metal and interposing the nickel between the cladding metal and carbon alloy or low-alloy steel is heated to 1,100-1,250 deg.C and is subjected to hot rolling within 15min. After the rolling is ended at >=850 deg.C, the stock is cooled down to <=700 deg.C at >=30 deg.C/min cooling rate and is then allowed to cool. Then the need for the reheating and soln. heat treatments which are normally required to impart excellent corrosion resistance to a clad steel using a two-phase stainless steel as a cladding metal is eliminated and the need for a reheating installation is eliminated; in addition, this method contributes to the economization on energy as well.

Description

Detailed Description of the Invention: Industrial Field of Application The present invention is directed to a two-phase stainless steel in which austenitic-ferritic stainless steel 'a (hereinafter referred to as duplex stainless steel) is used as a laminated material and carbon steel or low alloy steel is used as a base material. This invention relates to a method for manufacturing stainless steel clad steel plates.

Conventional technology The cladding method is known as a method of laminating two or more types of metal plates to take advantage of their respective properties.The steel base material is carbon steel or low alloy steel, stainless steel, nickel, N, etc. - There is a clad steel plate clad with a cr-re gold alloy.

There are rolling methods, explosion rolling methods, casting methods, forge welding methods, and other methods to manufacture clad steel plates, but the most commonly used rolling method is to assemble 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 950-1100°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 sidewall corrosion and intergranular corrosion, so it is used in various pollution prevention equipment, chemical industry equipment, etc.

In order to manufacture a clad steel plate using duplex stainless steel with excellent properties as a laminating material, the clad steel plate made by hot rolling is subjected to solution heat treatment in which the clad steel plate is heated to 950 to 1100°C and 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, the present invention provides a method of using duplex stainless steel as a laminated material, which can impart excellent properties to the duplex stainless steel without reheating and solution heat treatment. This paper proposes a method for manufacturing clad steel plates.

Means for Solving the Problem 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 reheating and solidifying 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.However, in the continuous cooling process that follows hot rolling in the cladding process, complete solidification during heating The precipitation of the σ phase from the molten 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. We have discovered that the precipitation of the σ phase can be prevented.Based on this knowledge, we have come up with this invention.

That is, this invention uses a material made of austenitic/ferritic stainless steel as a laminated material and nickel interposed between it and a base material of carbon steel or low alloy steel.
Heated to 250°C and hot rolled for less than 15 minutes85
After finishing rolling at a temperature of 0°C or higher, the cooling rate is 30°C/
The gist is to cool down to 700°C for at least 1 minute and then leave to 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 components is: CO 0.08% or less, Si 1.00% or less, Mn 2.00% or less, Cr 21.0-28.0%, N
i3°O~7°5%, Ho1. Contains O~4.0%, Cu2°0% or less, Wl, 0% or less, No, 3% or less,
If necessary, other trace elements may be added and the remainder may be 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 practiced method, but if the rolling temperature is lower than 1100°C, the deformability will decrease, the deformation resistance will be large, the rollability will decrease, and the rolling temperature will decrease.
If the temperature exceeds 250°C, the deformability will decrease and the rolling properties will be poor, so a temperature in the range of 1100 to 1250°C is desirable.

The reason why the hot rolling time and the cooling rate in the cooling process after hot rolling were controlled is as follows.

FIG. 1 shows the isothermal transformation curve of the σ phase when the two-phase stainless steel having the above chemical composition was heated to 1250° C. for 3Q minutes and then cooled with water. From this figure, it can be seen that 2.5% of the σ phase is precipitated when the temperature is maintained at 830° C. for about 35 minutes.

Therefore, in order to prevent the precipitation of the σ phase in the cooling process after hot rolling, it is necessary to control the cooling rate so that it is consistent 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 are based on the isothermal transformation curve of the σ phase when rolling is finished at 1000°C (line A) and when rolling is completed from the heating temperature to 850°C and rolling is finished at 850°C (line B). 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 cooling is performed within 5 minutes to a safe 700″C where the σ phase does not precipitate (cooling rate 30° C./min or more), and it can be seen that normal cooling is sufficient at temperatures below 700° C., which is out of the region of σ phase precipitation.

Based on the above results, the hot rolling is completed at a temperature of 850° C. or higher, and then the product is rapidly cooled to a temperature of 700° C. or lower at a cooling rate of 30° C./min or higher, and then allowed to cool.

In addition, 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 region after hot rolling in the production of clad steel sheets made of duplex stainless steel as a laminated material, 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 from duplex stainless steel, which could not be put to practical use in the past, so no reheating equipment is required. , it can also contribute to energy savings.

Example Next, embodiments of the invention will be described.

Example 1 Table 1 Chemical composition (wt%) Thickness 15 of duplex stainless steel whose chemical composition is shown in Table 1
Order, width 1000mm, length 1500mm. The slab is used as the laminating material, and the same <50Kg 4m class carbon steel with a thickness of 110mm
.

A slab with a width of 1000 mm and a length of 1500 mw is used as the base material, and a slab with a thickness of 100 mm is used as the base material. overlaid with Ni foil interposed,
Heated to 1200°C, rolled for 3 minutes and finished at 950°C, total thickness 25mm with base material 22mm and laminated material 3mm.
After forming a clad steel plate with a width of 1500 mm and 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 allowed to cool, and Comparative Example 1) was also rolled in the same manner as in Comparative Example 1, allowed to cool, and then reheated to 105°C. Comparative Example 2).

Then, the pitting corrosion test of these laminated materials was carried out in artificial seawater (80℃
This was done using a vacuum cleaner. The results are shown in Table 2.

Table 2 Test Results From the above results, it can be seen that the corrosion resistance of the clad steel plate 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 components was used.

(Left below) 9 Example 2 Table 3 Chemical composition (wt%) Thickness 24 of duplex stainless steel whose chemical composition is shown in Table 3
A slab with a width of 1,740 mm and a length of 3,000 mm is used as the laminating material, and the same (40 kg grade 4 carbon steel with a thickness of 176
A slab with a width of 1740 mm and a length of 3000 mm was used as the base material, and a Ni foil with a thickness of 100 ρ was interposed between the slabs, which were then heated to 1240°C, rolled for 8 minutes, and finished at 870°C. The base material is 14.5 mm and the laminated material is 2 tnm, making a total of 16.5 wuu and a width of 3500 mm.

After forming a clad steel plate with a length of 18,000 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. It was reheated to 0.degree. C. and subjected to solution heat treatment (Comparative Example 4).

Then, a pitting corrosion test was conducted on these laminated materials in the same manner as in Example 1. The results are shown in Table 4.

Table 4 Test Results From the above results, it can be seen that similar to Example 1, the clad steel plate 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 pitting corrosion potential of the laminated material and the C contained in the laminated material
The relationship with r+3MO (%) is shown in Figure 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, in order to obtain the required corrosion resistance depending on the purpose of use, we
What is necessary is to adjust the r content.

[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 the present invention, and Fig. 3 is a graph showing the Cr+3)1o content of the laminated material. It is a graph showing the relationship between the amount and corrosion resistance. Applicant: Sumitomo Metal Industries, Ltd. Figure 1 Time (Hr) Figure 2 Cr+3M□←0

Claims (1)

[Claims] A composite material made of austenitic ferritic stainless steel and a base material of carbon steel or low-alloy steel with nickel interposed between them is heated to 1100 to 1250°C.
A duplex stainless steel cladding characterized in that after being hot rolled for 5 minutes or less and finishing the rolling at a temperature of 850°C or higher, it is cooled to 700°C or lower at a cooling rate of 30°C/min or higher, and then allowed to cool. Method of manufacturing steel plates.