JPS63220904A - Pack rolling method for boron added austenitic stainless steel - Google Patents

Abstract

PURPOSE:To prevent rolling crack and to obtain a good product by using a material whose deformation resistant value is under the value of a material to be rolled as a cover material, heating it in a specific temp. range and performing finishing rolling at >= a fixed temp. CONSTITUTION:The material having the deformation resistant value or less of a B contg. austenitic stainless steel is used as a cover material 2, a pack rolling stock is heated at 1,100 deg.C min. and 1,175 deg.C max. and a finishing rolling is performed at higher temp. than the temp. shown in the equation. With this method, the crack generation at rolling time is prevented and the product in specified plate thickness and of excellent quality may be obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a pack rolling method for austenitic stainless steel with poor hot workability, and is a method for rolling austenitic stainless steel to a predetermined thickness without causing cracks due to rolling. This relates to a rolling method for the purpose of

[Prior art] Boron is added to a concentration of 0.3 to 2. Ovt%-added austenitic stainless steel has excellent thermal neutron absorption properties and is therefore used as a material for fuel baskets in casks for transporting and storing spent nuclear fuel.

However, the addition of boron forms hard and brittle borides in stainless steel, and as the amount of boron added increases, hot workability significantly decreases.

Usually, boron-added austenitic stainless steel plates and thin steel plates with a thickness of 10 mm or less are used as thermal neutron absorbers.

Therefore, when a normal rolling method is applied, it becomes difficult to secure a rolling temperature that maintains hot workability, and cracks occur due to deterioration of hot workability. In this case, it is possible to raise the rolling temperature by increasing the heating temperature, but since the aforementioned boride melts at 1200°C or higher, the heating temperature is 1
It is not possible to raise the temperature higher than 175℃. The contents and problems of conventional techniques that have been implemented with these points in mind are described below.

[Problems to be Solved by the Invention] Boron-added austenitic stainless steel has a narrow temperature range that can be hot worked, so it is manufactured by repeating heating and processing many times to maintain this temperature range. The increase in number significantly increases manufacturing costs.

Also, in this method, boron-added austenitic stainless steel sheets are often used for medium-thick sheets of 1101I or less or thin sheets of 3 mm or less, so the temperature drop due to thinner sheets is large and hot processing is difficult. It is not possible to process the plate to the specified thickness within the possible temperature range, resulting in cracks.

This cracking occurs from the edges of the steel plate and progresses toward the center of the steel plate as the degree of processing increases. The cracked portion cannot be used, resulting in a decrease in yield and an increase in manufacturing costs.

On the other hand, Ti is added to boron-added austenitic stainless steel.
Attempts have been made to change the composition and morphology of borides by adding (titanium) or vanadium (V) to improve hot workability, but since these elements are ferrite-forming elements, boride In addition, delta ferrite is also formed, which does not lead to a dramatic improvement in hot workability and prevents the occurrence of cracks.

The present invention is intended to solve the above-mentioned problems of the prior art, and aims to provide a method for rolling boron-added austenitic stainless steel that can be rolled to a predetermined thickness without causing cracks due to rolling. purpose.

[Means for Solving the Problems] The present invention consists of laminating one or more boron-containing austenitic stainless steel materials coated with a release agent on the surface, covering the top and bottom with cover materials, surrounding them with spacers, and welding them. In the method for producing a boron-containing austenitic stainless steel material, which comprises hot rolling a pack-rolled material assembled with a steel sheet, a material having a deformation resistance value equal to or less than the deformation resistance value of the boron-containing austenitic stainless steel material is selected as the cover material; The method for rolling a boron-added austenitic stainless steel is characterized in that the pack-rolled material is heated at 1100° C. or higher and 1175° C. or lower, and finish rolling is performed at a temperature higher than or equal to the temperature expressed by the following formula.

T Cod 53B + 870 However, T: Finishing rolling temperature limit (°C) B: Boron content (wt%) [Function] The present invention was made as a result of detailed study of the manufacturing technology of boron-added austenitic stainless steel sheets by pack rolling. It is.

First, regarding hot rolling workability, as shown in the examples below, the higher the amount of boron added, the more borides there are, and the higher the critical temperature at which cracks occur, but the amount of boron added (wt%) Accordingly, there is a temperature range in which hot rolling is possible.

Therefore, the rolling finish temperature of this material is changed by boron addition Q (wt
%), the material can be produced by hot rolling by setting the rolling temperature T (° C.) or higher given by the following formula (1).

T-53B+870 (1) Therefore, in order to satisfy the above formula (1), that is, to prevent the temperature drop of the material, the present inventors came up with the following pack rolling method. It is.

The pack rolling method, as shown in Figure 1, involves stacking one or more layers of boron-added austenitic stainless steel and wrapping the outer periphery with other materials to maintain the thickness of the rolled material above a certain level. This method prevents the temperature of the material from dropping and hot-rolls the thickness of each layer to the finished rolling dimension.

The present rolling method will be described below.

1) Basic configuration of assembly FIG. 1 is an explanatory diagram of the basic configuration of assembly in pack rolling. In the figure, 1 is a core material, which is manufactured by hot rolling or hot forging a boron-added austenitic stainless steel ingot or continuously cast slab within a range that satisfies the above-mentioned formula (1). Therefore, the finished thickness of the core material is necessarily thick.

Furthermore, it is also possible to use an ingot or continuously cast slab of the present material as the core material immediately without going through the manufacturing process of the core material.

2 is called a cover material, and 3 is called a spacer, and they are made of a weldable material such as low carbon steel. 4 is a release agent, which prevents pressure bonding between layers due to rolling, and also acts as a heat transfer resistance to prevent a drop in temperature of the core material 1.

5 is a welded portion between the cover material 2 and the spacer 3.

2) Basic characteristics of pack rolling method Figures 3 and 4 are diagrams showing the relationship between (elongation of cover material) - (elongation of core material), shape characteristics and plate thickness characteristics, respectively.
Based on these figures, the results of a theoretical study of the basic characteristics of the laminated rolling method will be qualitatively explained.

■Shape characteristics As shown in Figure 3, if the deformation resistance of the core material is greater than that of the cover material, that is, if the core material is harder than the cover material, the elongation of the soft cover material will be greater. This difference in elongation increases as the ratio of the deformation resistance of the core material to the deformation resistance of the cover material (hereinafter referred to as deformation resistance ratio) increases.

Under such conditions, the core material is stably rolled inside the cover material and has a good shape.

When the deformation resistance ratio is 1, that is, when the hardness of the core material and the cover material are the same, the elongation of each material matches, and stable rolling occurs in exactly the same condition as rolling a single material, resulting in a good shape. Become.

On the other hand, when the deformation resistance ratio is less than 1, the elongation of the core material is greater than the elongation of the cover material.

Therefore, the deformation of the core material is restrained by the cover material, and generally plastic buckling occurs, resulting in a defective shape.

That is, Deformation resistance ratio (k’) – Deformation resistance of core material and cover r Defining it as the ratio of the deformation resistance of the material, k≧1: Good shape ■r k　く１；Poor shape r Therefore, a stable pack rolling method is established when k≧1. It became clear that

■ Plate thickness characteristics As shown in Figure 4, when k < 1, shape defects do not occur, but the ratio of the thickness of the core material to the thickness of the pack rolled material (referred to as the total thickness) The difference between the elongation of the material and the elongation of the core material is different.

As shown in the figure, when the ratio of the core material thickness to the total thickness (hereinafter referred to as the laminated sheet thickness ratio) is small, the elongation of the cover material is promoted and the difference between the elongation of the core material and the core material is large. This elongation difference decreases as the thickness ratio increases.

That is, when this is expressed as a change in plate thickness, the rolling reduction ratios of the core material and the cover material are as shown in FIG. 5 with respect to the rolling reduction ratio (referred to as the total rolling ratio) of the pack rolled material.

Therefore, in order to obtain a predetermined core material thickness, it is necessary to set the final target thickness of the pack rolled material according to the deformation resistance ratio and the laminated sheet thickness ratio.

In the case of k-1, r The total rolling reduction ratio - the rolling reduction ratio of the core material - the rolling reduction ratio of the cover material.

In practical terms, the productivity of the product thickness of the core material and the number of laminated sheets,
In view of the welding strength of the cover material, the total plate thickness, etc., the combined plate thickness ratio is preferably about 0.3 to 0.8.

3) Selection of cover material In order to embody this invention, we investigated the deformation resistance of a 1.00% boron-added austenitic stainless steel plate (Table 1, Nα2) and considered an appropriate cover material. .

An example is shown in Figure 56.

As is clear from the figure, the average deformation resistance of the 1.00% boron-added austenitic stainless steel plate (SUS304B) is approximately 30 to 40% greater than that of the 5US304 material.

In addition, for carbon steel (C-0, 21%), 1.7 to q It is about twice as large.

Therefore, it is predicted that no matter which material is selected as the cover material, it is possible to perform pack rolling without causing shape defects.
Considering that the 5US304 material itself is an expensive material and that the average deformation resistance is higher than that of carbon steel, which increases the rolling load, it is industrially preferable to select low carbon steel as the cover material.

Furthermore, in actual rolling, the temperature of the cover material falls below the temperature of the core material, so care must be taken to prevent the average deformation resistance of the cover material from becoming larger than that of the core material due to the temperature drop.

4) Pack assembly conditions (optimization of cover material thickness) 1.00
% boron-added austenitic stainless steel sheet (SUS
304B) by the pack rolling method, the rolling temperature range T≧923°C according to the above-mentioned formula (1) and the average deformation resistance of 5US304B during rolling≧carbon steel (Ceq=0.
21%) (core material) (cover material) It is necessary to consider pack assembly conditions (optimization of cover material thickness) that simultaneously satisfy the following requirements.

An example of the results of calculation simulation is shown in FIG. The calculation conditions are: Core material (SUS304B) dimensions: 108 x 1310 x 1810' - Rolling 1
0.4t x 1840' Number of core materials 1 piece pack assembly dimensions t x 1470' x 1970
'Heating temperature: 1150°C Bus schedule: Cross rolling (including tentering) using one plate mill.

From FIG. 7, it can be seen that the appropriate thickness of the cover material in this case is 56 mm/side or more. However, as mentioned above in 2) (2), it goes without saying that the thinner the cover material is, the easier it is to achieve the target rolling dimension.

By taking into account the examination process as exemplified in 3) and 4) above, the results are as follows: 0.3 to 2. The conditions for manufacturing Owt% boron-added austenitic stainless steel sheets using the pack rolling method have been established.

Next, the reason for limiting the heating temperature will be described.

The heating temperature was set to 1100°C or higher and 1175°C or lower because
Heating below 1100°C is not preferable because the finishing temperature will drop significantly and the temperature range that can be rolled will be narrowed.
Furthermore, if heated above 1175°C, the boride will melt and crack during rolling, so the heating temperature should be between 1100°C and 1100°C.
The temperature was 175°C.

In addition, boron-added austenitic stainless steel has B: 0.3 to 2. Owt%, C: 0.08vt% or less, Si: 2. Ovt% or less, Mn: 2. Ovt.
% or less, P: 0.05vt% or less, s: o, oav
t% or less, Cr: 1B, 0-20. Owt%, N
i:8. O~15. Owt%, Mo: 3.0wt%
Below, N: 0.15wt% or less is contained, and the balance is Fe.
and steel consisting of unavoidable impurities.

Next, examples of the present invention will be described.

〔Example〕

First, boron-added austenitic stainless steel having the composition shown in Table 1 was used as a raw material and heated at a heating temperature of 1150° C., and its hot rolling workability was evaluated.

The results are shown in FIG.

Table 1 (vt%) As shown in Fig. 1, the following can be seen from the relationship between the B addition rate (vt%) and the finishing rolling temperature (°C).

The higher the amount of boron added, the more borides there are, and the higher the critical temperature at which cracks occur, but the boron addition rate (wt%)
) There is a temperature range in which hot rolling is possible, and as shown in FIG. 1, the following equation (1) is established from the relationship between the finishing rolling temperature and the boron addition rate (vt1%).

T-53B+870 (1) From equation (1), it can be seen that this material can be manufactured by hot rolling by keeping the rolling temperature above the limit rolling temperature (T° C.).

Example 1 A boron-added austenitic stainless steel having the composition shown in Table 2 below was tested as a core material.

Pack rolling was performed as shown in FIG. The conditions in that case are shown below.

Core material: Boron-added austenitic stainless steel plate
108 mm (thickness) x 1310 mm (width) x
181hm (length) x 1 cover material; carbon steel (C-0, 21%) q pack rolled material; 240 +a+s (thickness) x 1
47h+s (width) X 1970 length (length) Laminated plate thickness ratio, 0.45 Rolling dimension of core material;
1g40! I11 (Width) At a heating temperature of 1120°C or higher, 12 baths were processed (cross rolling) using a plate rolling mill. After rolling, the core material was dismantled and investigated. As a result, cracks were found in the core material. No occurrence was observed.

FIG. 8 shows the simulation results of the average temperature of the core material and the cover material, the actually measured surface temperature of the cover material, and the average resistance value of the core material and the cover material.

According to this, the deformation resistance ratio k>1.

■Example 2 12 baths (including cross rolling) were performed in a plate rolling mill under the conditions shown below.

Core material: Boron-added austenitic stainless steel plate
108 m+s (thickness) X1310+am (width)
1890u+ (long) x 1 cover material; carbon steel (C-
0.21%) q Pack rolled material; 300 mm (thickness) x 1470 dragon (width) x 1850+*m (length) Laminated plate thickness ratio, 0.36 Rolled dimensions of core material; 5.4 m+i (thickness) x 1
840mm (width) Heating temperature: 1150°C After rolling, it was dismantled and the core material was examined, and as a result, no cracks were observed in the core material.

As in Example 1, the same specifications are shown in FIG.

Comparative Example 12 baths (cross rolling) were carried out in a plate rolling mill under the conditions shown below.

Core material: boron-containing austenitic stainless steel plate
10g +am (thickness) x131hm (width) x 1
810mm (length) x 1 cover material; carbon steel (C-0, 21%) q hack rolled cable material; 160 srs (thickness) x 14
70mm (width) x 1970! I11 (Long) Laminated plate thickness ratio: 0.875 Heating temperature: 1150"c After rolling, the core material was dismantled and examined, and as a result, cracks were observed at the edges of the core material.

Compared to Examples 1 and 2 of the present invention, the combined plate thickness ratio is larger, that is, the thickness of the cover material is thinner in the comparative example, so the insulation effect on the core material is weaker, as shown in FIG. 10. This is because the finish rolling temperature was 915°C, which was lower than the limit rolling temperature.

In this case, and as shown in FIG. 10, the deformation resistance ratio of the core material and the cover material is k<1r for the above-mentioned reason. (4 passes to 10 passes) [Effects of the Invention] According to the pack rolling method for boron-added austenitic stainless steel of the present invention, it is possible to roll the boron-added austenitic stainless steel to a predetermined thickness without causing cracks due to rolling. be.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of the pack rolling method, Figure 2 is a graph of the relationship between the amount of B added and the finishing temperature of rolling, Figure 3 is an explanatory diagram of the shape characteristics which are the basic characteristics of the pack rolling method, Figures 4 and Figure 5 is an explanatory diagram of the shape characteristics, which are also the basic characteristics of the pack rolling method, and Figure 6 is a graph of the relationship between rolling temperature and average deformation resistance ratio.
Figure 7 shows rolling finishing temperature and cover material thickness/one side (+s+s
), FIG. 8 is a graph of operational results in Example 1, FIG. 9 is a graph of operational results in Example 2, and FIG. 10 is a graph of operational results in Comparative Example.

Claims (1)

[Claims] One or more sheets of boron-containing austenitic stainless steel materials coated with a release agent on the surface are laminated, the upper and lower sides are covered with cover materials, the surroundings are surrounded by spacers, and the assembled packed rolled materials are heated. In the method for manufacturing the boron-containing austenitic stainless steel material, the material is selected as the cover material and the deformation resistance value is equal to or less than the deformation resistance value of the boron-containing austenitic stainless steel material, and the pack-rolled material is rolled at 1100°C. A pack rolling method for boron-added austenitic stainless steel, which comprises heating at a temperature of 1175° C. or lower and finishing rolling at a temperature not lower than the following formula. T=53B+870 However, T: Finishing limit rolling temperature (℃) B: Boron content (wt%)