JP2748814B2 - Manufacturing method of high purity ferritic stainless steel clad steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-purity ferritic stainless steel clad steel sheet having excellent surface properties after processing such as a head processing.

[0002]

2. Description of the Related Art High-purity ferritic stainless steels represented by SUS444 are characterized by having excellent resistance to chloride stress corrosion cracking, which is a disadvantage of general-purpose austenitic stainless steels such as SUS304 and SUS316. Use is expanding in containers and the like. However, since it becomes difficult to secure toughness as the sheet thickness increases, the demand for clad steel sheets for use in thick materials is increasing. However, the high-purity ferritic stainless steel clad steel plate manufactured by the conventional method has problems such as rough surface on the surface of the laminated material after the manufacture, and the occurrence of miscellaneous echoes in the ultrasonic flaw detection test, making the flaw detection test impossible. was there. In addition, high-purity ferritic stainless steel clad steel sheet is often used after being processed into a head plate, such as the head of a hot water tank or a pressure vessel. There is a problem that it takes time and effort.

It is well known that the deterioration of the surface properties after bending or tensioning such as mirror processing is caused by coarse crystal grains, and a similar phenomenon is thin ridging. Ridging is considered to be caused by a coarse elongation structure and a texture that are generated when a ferritic stainless steel is subjected to high pressure, and JP-A-55-134128 and the like have been proposed as a countermeasure. In this method, in order to destroy a coarse elongation structure, a large pressure is applied in a relatively low temperature range of a hot rolling step, and a fine recrystallized structure is obtained by annealing thereafter. In general, as a technique for reducing crystal grains, a method of passing a transformation point between a rolling start temperature and a finishing temperature can be considered. On the other hand, in the production of high-purity ferritic stainless steel clad steel sheet by the conventional rolling method, generally, the slab is heated to as high a temperature as possible because the higher the slab heating temperature before clad rolling is, the higher the bonding strength is obtained. The method of rolling and manufacturing is mainly adopted, and normalizing heat treatment may be applied after hot rolling from the viewpoint of the mechanical properties of the base material, if necessary. However, inconveniences such as wrinkles on the surface of the laminated material after processing were disadvantages of the high-purity ferritic stainless steel clad steel sheet.

[0004]

In the production of a conventional high-purity ferritic stainless steel clad steel sheet, the above-described technique for countermeasures for thin sheet ridging or transformation point utilization cannot be applied as it is. In other words, the method of applying a large reduction in the low temperature range of the hot rolling process, which is a countermeasure for thin plate ridging, is contrary to the method of heating the assembled slab as high as possible in the conventional clad steel plate manufacturing process and continuously rolling from there. Since the amount of reduction in the high-temperature region is limited within the limited amount of reduction due to the slab design and the rolling capacity, a problem such as a high joining strength cannot be obtained. On the other hand, heating the slab to a high temperature in order to increase the bonding strength promotes the coarsening of the crystal grains and makes it difficult to refine the structure in the subsequent steps. Further, the method of passing the transformation point during rolling cannot be applied because high-purity ferritic stainless steel is almost a ferrite single phase from room temperature to its melting point and does not undergo phase transformation. The present invention has been made to solve the above problems, and has as its object to provide a method for producing a high-purity ferritic stainless steel-clad steel sheet having excellent surface properties after processing.

[0005]

In order to achieve the above-mentioned object, the method of the present invention comprises, by weight%, Cr: 16.0 to 3%.
A high-purity ferritic stainless steel clad steel sheet is manufactured by hot rolling a slab in which a high-purity ferritic stainless steel containing 2.0% and Mo: 2.5% or less and carbon steel or low-alloy steel are superposed. In the method, 1100
After heating the slab to 1200 ° C, the first stage hot rolling was performed to 9
After performing at a high temperature of 50 ° C. or more, the rolling was interrupted once and 900
This is a method for producing a high-purity ferritic stainless steel-clad steel sheet having excellent surface properties after processing, wherein hot rolling is performed again at a temperature of not more than 50 ° C. and a cumulative rolling reduction of 50% or more.
Further, there is provided a method for producing a high-purity ferritic stainless steel-clad steel sheet having excellent surface properties after processing, wherein a normalizing heat treatment is performed at 850 to 1000 ° C. after the hot rolling.

The features of this manufacturing method are as follows. In order to simultaneously obtain high bonding strength and good surface properties after processing in high-purity ferritic stainless steel clad steel sheet, the hot rolling process is divided into two different purposes, and these are performed under optimal conditions. We found that combining was effective. That is, the hot rolling process is divided into two processes, a first hot rolling process in a high temperature range and a second hot rolling process in a low temperature range, and a high bonding strength is obtained in the previous process. By combining these steps under optimum conditions, it is possible to obtain both high bonding strength and a fine structure. Here, if the conditions of the first-stage hot rolling step are inappropriate, high bonding strength cannot be obtained, or fine structure in the subsequent steps is hindered and good surface texture after processing is obtained. Can not get. Also, if the conditions of the second stage hot rolling step are inappropriate,
Either a fine structure with good surface properties after processing cannot be obtained, or a sufficient reduction in the first-stage hot rolling that contributes to joining cannot be ensured. That is, if any one of the conditions of the rolling process divided into two is inappropriate, it becomes impossible to obtain high bonding strength and / or good surface texture after processing. It is necessary to optimize within the combination of all processes.

In the case of a high-purity ferritic stainless steel clad steel sheet, normalizing heat treatment may be performed after hot rolling from the viewpoint of the mechanical properties of the base material. In this case, if the normalizing heat treatment conditions or the hot rolling conditions are inappropriate, the fine structure obtained by the hot rolling becomes coarse, and good surface properties cannot be obtained after processing. Therefore, it is necessary to prevent coarsening of the structure even when normalizing heat treatment is performed after hot rolling. For that purpose, the strain energy accumulated in the structure in the second stage hot rolling process in the low temperature region in the hot rolling process causes a fine recrystallized structure to be formed in the normalizing heat treatment, and the crystal grains are formed. Coarsening can be prevented. At that time, in the second hot rolling step, if the rolling reduction is small and sufficient strain energy is not accumulated in the structure, recrystallization does not occur or the number of nucleation sites for recrystallization is reduced. It is too small to obtain a fine recrystallized grain structure. Further, in the normalizing heat treatment, if the heat treatment temperature is low, recrystallization does not sufficiently occur. Conversely, if the heat treatment temperature is high, crystal grains become coarse due to grain growth after recrystallization. Therefore, it is necessary to optimize the conditions of the hot rolling step and the normalizing heat treatment.

[0008]

The present invention will be described below in detail. First, the reason why the components of the high-purity ferritic stainless steel of the joining material are limited will be described. Conventionally, austenitic stainless steel excellent in corrosion resistance has been widely used in chemical plants and the like, but austenitic stainless steel has a risk of generating stress corrosion cracking in a chloride environment. On the other hand, ferritic stainless steel is inferior in corrosion resistance to austenitic stainless steel, but is characterized by being excellent in stress corrosion cracking resistance. recent years,
High-purity ferritic stainless steels in which the amount of impurity elements such as C and N in ferrite stainless steels are reduced and corrosion resistance is improved have been developed and used in chemical plants, hot water tanks and the like. For high-purity ferritic stainless steel to have the same corrosion resistance as austenitic stainless steel, Cr needs to be at least 16% by weight. Similarly, addition of Mo is desirable from the viewpoint of corrosion resistance. However, if Cr and Mo are added in large amounts, the productivity, weldability, economy, etc. are impaired.
Mo has an upper limit of 2.5%.

Next, the reason why the slab heating temperature before clad rolling is set to 1100 to 1200 ° C. will be described. In general, in clad rolling, as the slab heating temperature is increased, a higher bonding strength can be stably obtained even at a lower reduction ratio. Therefore, as the lower limit temperature of the slab heating, a stable high bonding strength can be obtained in normal rolling.
0 ° C. On the other hand, as the upper limit temperature, when heated to a temperature exceeding 1200 ° C., the crystal grains of the composite material are coarsened, and a fine structure of the composite material can be obtained no matter how much reduction is applied in the subsequent rolling process. The temperature was set to 1200 ° C. because the surface disappeared and the surface became rough after production and processing.

Next, the reason why the rolling conditions and the normalizing heat treatment conditions are limited will be described. Table 1 shows the chemical components of the composite material and the base material of the test high-purity ferritic stainless steel clad steel sheet. The amount of C and the amount of N are 0.007wt each in the composite material.
%, 0.0082 wt% and high purity 18 wt% Cr-2
SUS444 of wt% Mo system was used. SS4 for base material
00 was used. Table 2 shows the surface properties of the composite material obtained by performing a table bending test using a bending radius of a plate thickness t for a material obtained by combining the above composite material and the base material and rolling under three conditions. When hot rolling was continuously performed from a high temperature and rolling was performed at a finish of 900 ° C., the surface of the laminated material after the bending test was roughened. On the other hand, the first stage hot rolling was temporarily interrupted at a temperature of 950 ° C. or more, and then rolled under the conditions of hot rolling at a cumulative rolling reduction of 50% or more at 900 ° C. or less again. After the bending test, the surface of the composite material did not become rough and had good surface properties. Table 2
After rolling under the three conditions shown in FIG.
FIG. 1 shows the changes in the recrystallization ratio and the crystal grain size when heat-treated by normalizing at a temperature of 30 minutes. The recrystallization behavior during normalizing heat treatment after rolling is greatly affected by rolling conditions.
The recrystallization is almost completed by normalizing heat treatment at 850 ° C for those subjected to reduction at 0 ° C or less, while normalizing heat treatment at 1050 ° C is required for 900 ° C finished material to obtain a complete recrystallized structure. Met. In the case where the rolling was performed at 900 ° C or lower, a fine recrystallized structure could be obtained by normalizing heat treatment at 850 ° C to 1000 ° C after rolling.

[0011]

[Table 1]

[0012]

[Table 2]

FIG. 1 also shows whether ultrasonic flaw detection was performed at 2 MHz and 5 MHz. Those having a structure with a recrystallization ratio of 90% or more and a crystal grain size of 90 μm or less were ultrasonically feasible, but those with a structure with a recrystallization ratio of less than 90% or a crystal grain size of more than 90 μm. In this case, echo was generated and ultrasonic testing was not possible. Those having a structure capable of ultrasonic testing were generally good in the surface properties of the laminated material after the test in the surface bending test in which the bending radius was set to the plate thickness t. On the other hand, in the case of a structure having a structure that cannot be subjected to ultrasonic flaw detection, surface roughness was observed on the surface of the composite material after the surface bending test. That is, in the case where the normalizing heat treatment is performed, it is necessary to obtain a fine recrystallized grain structure that can be subjected to ultrasonic flaw detection in order to prevent the surface of the laminated material after processing from being roughened.

In FIG. 2, after the slab is heated to 1200 ° C., the hot rolling is temporarily stopped at 1000 ° C.
Rolling at a rolling reduction of 10% to 50% at 50 ° C or less,
The change in the recrystallization ratio and the crystal grain size when the heat treatment was performed at 910 ° C. for 30 minutes was shown. In order to obtain a structure having a recrystallized ratio of 90% or more and a crystal grain size of 90 μm or less for improving the surface properties of the laminated material after processing, a cumulative rolling reduction at 900 ° C. or less is required to be 50% or more. You can see that. Therefore, in order to obtain a fine structure that improves the surface properties of the laminated material after processing, hot rolling at a cumulative draft of 50% or more at 900 ° C. or less and subsequent normalizing heat treatment are performed. If the heat treatment temperature is 85
It is necessary that the temperature be 0 ° C to 1000 ° C. Where 9
If the rolling reduction at 00 ° C. or lower becomes large, the first-stage rolling reduction ratio that contributes to joining cannot be sufficiently obtained. Conversely, if excessive reduction is applied in the first rolling, 90
The rolling reduction at 0 ° C. or less cannot be secured, and the structure cannot be miniaturized. Therefore, since the rolling at 950 ° C. or less does not contribute to the joining, the first stage is used in order to efficiently distribute the rolling at 950 ° C. or more that contributes to the joining under the total pressure and the rolling at 900 ° C. or less for the refinement of the structure. It is desirable to perform the eye rolling intensively at 950 ° C. or higher and temporarily suspend the rolling.

[0015]

EXAMPLES In order to clarify the features of the present invention, a SUS444 clad steel plate manufactured according to the present invention and a SUS444 clad steel plate manufactured under conditions not applicable to the conventional method and the present invention are described.
The characteristics of the clad steel plate were compared. Table 3 shows the components of SUS444 used for the bonding material. All base materials are SS4
00 was used. Table 4 shows the manufacturing conditions of the test clad steel sheet when the normalizing heat treatment is not performed. Table 5 shows the properties of the steel sheet manufactured according to Table 4. Table 6 shows the manufacturing conditions of the test clad steel sheet when the normalizing heat treatment was performed.
Table 7 shows the properties of the steel sheet manufactured according to Table 6. The shear strength in Tables 5 and 7 was measured by the shear strength test method of JIS G0601. The surface property of the composite material after the production was such that the maximum height Rmax of the composite material surface measured by a roughness meter was 0.1.
The case of less than mm was regarded as good. The surface properties of the composite material after bending were also evaluated according to the surface properties of the composite material after rolling.
Further, the ultrasonic flaw detection in Table 7 was performed at 2 MHz and 5 MHz.

[0016]

[Table 3]

[0017]

[Table 4]

[0018]

[Table 5]

[0019]

[Table 6]

[0020]

[Table 7]

A1 to A5 and N1 to N6 are clad steel sheets produced according to the present invention and have high shear strength, good surface properties of the laminated material after production and after bending, and N1 to N6.
In No. 6, ultrasonic testing was also possible. On the other hand, A6 to A8 and N
7 to N9 had high bonding strength, but the surface of the composite material was roughened after bending, and N7 to N9 was further impossible to perform ultrasonic flaw detection due to generation of miscellaneous echoes. In addition, A6
A7 and N7 had rough surfaces on the surface of the composite material even after production. A9 to A13 of comparative steel sheets not according to the method of the present invention
In addition, N10 to N16 were inferior in bonding strength, surface properties of the laminated material after production or bending, or ultrasonic inspection was impossible.

A9 and N10 were inferior in shear strength due to low slab heating temperature. Since the slab heating temperature was high for A10 and N11, the surface of the composite material after production and after bending was roughened, and N11 could not be further subjected to ultrasonic flaw detection. A11 and N12 were 95% of the second stage hot rolling.
Starting at 0 ° C., the cumulative rolling reduction was 50%.
The cumulative rolling reduction at a temperature of not more than ℃ was insufficient, the surface of the laminated material after bending was roughened, and further ultrasonic testing was not possible with N12. For A12 and N13, the first-stage hot rolling was performed to 907 ° C. and 919 ° C., respectively, and the second-stage rolling reduction could not be ensured. With N13, further ultrasonic testing was not possible. A13 and N14 had insufficient rolling reduction in the second-stage hot rolling, N15 had a low normalizing heat treatment temperature, and N16 had a high normalizing heat treatment temperature. Occurs and N14
In addition, ultrasonic inspection was impossible for N16. In order to confirm the surface properties of the clad material of the clad steel sheet manufactured according to the present invention after the head plate processing, the head plate processing by cold spinning was actually performed on N1 in Table 6 and the surface properties of the bonded material after the processing were investigated. The shape of the head plate was a semi-elliptical shape having an inner diameter of 1250 mm. The surface properties of the composite material after processing were good without roughening, and it was confirmed that the steel of the present invention had excellent surface properties of the composite material even after processing the end plate.

[0023]

The high-purity ferritic stainless steel clad steel sheet manufactured according to the present invention has superior surface properties after processing as compared with the steel sheet manufactured by the conventional method. Therefore, according to the present invention, it is possible to shorten or omit the polishing step of the bonding material after the processing such as the end plate processing, and as a result, it is possible to greatly reduce the cost.

[Brief description of the drawings]

FIG. 1 is a diagram showing a change in structure due to rolling conditions and normalizing heat treatment conditions.

FIG. 2 is a diagram showing a structural change according to rolling conditions.

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI C21D 9/46 C21D 9/46 Z C22C 38/00 302 C22C 38/00 302H 38/22 38/22 (72) Inventor: Hideki Kimura Process 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) Reference JP-A-60-216984 (JP, A) JP-A-3-277542 (JP, A) JP-A-4-1822081 (JP, A) JP-A-5-154672 (JP, A)

Claims (2)

(57) [Claims] 1. Cr: 16.0 to 32.0% by weight
%, Mo: a method of manufacturing a high-purity ferritic stainless steel clad steel sheet by hot rolling a slab obtained by laminating a high-purity ferritic stainless steel containing not more than 2.5% and carbon steel or low-alloy steel, 1100-120
After heating the slab to 0 ° C, the first stage hot rolling was performed at 950 ° C.
A high-purity ferritic stainless steel clad steel sheet having excellent surface properties after processing, characterized in that the rolling is interrupted once after being performed at the above high temperature, and the hot rolling is performed again at a temperature of 900 ° C. or less and a cumulative rolling reduction of 50% or more. Manufacturing method. 2. A high-purity ferritic stainless steel-clad steel sheet having excellent surface properties after processing, which is subjected to normalizing heat treatment at 850 to 1000 ° C. after hot rolling according to claim 1. Manufacturing method.