JPH0929391A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method for SUS304 stainless steel containing B which is excellent in the thermal and mechanical properties in high temperature range. SOLUTION: In a continuous casting method for SUS304 stainless steel slab containing 0.4-2.00 mass % B which is used for a cell of a spent fuel rack of a nuclear power station, the casting speed is 0.4-0.8m/min, the interval in the casting direction of a support roll is <=300mm, and the solid phase ratio of the center part in the thickness of the slab in the final position of the secondary cooling zone is <=0.2. The slab free from any surface and internal defects can be manufactured without generating the breakout.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for preventing the generation of surface and internal defects of SUS304 stainless steel slab containing B (boron) in a specific range for a specific purpose.

[0002]

2. Description of the Related Art Continuous casting of B-containing SUS304 stainless steel slabs used in cells of spent fuel racks of nuclear power plants has been realized, energy saving by omitting steps, productivity improvement by high yield, and product defects. Controlling outbreaks has not yet been achieved.

[0003] Usually, the production of B-containing SUS304 stainless steel slab is carried out through the production of an ingot and the subsequent slabbing process. Ingot manufacturing method is SUS3
This is a process of casting molten stainless steel 04 into a cast iron mold and completely completing solidification in the mold to produce a steel ingot.
After cooling this ingot to a workable temperature for several days, the upper end and the lower end of the ingot having a large amount of inclusions are cut.

At this time, the yield of the ingot is 90%.
To a degree. Before the hot rolling step, the surface of the ingot is cared for, then the ingot is charged into a heating furnace, the temperature is raised to about 1000 ° C., and slab rolling is performed.

Since the size of the cast iron mold for producing the ingot is limited, the melted SUS304 stainless steel melt must be divided into several molds and agglomerated. As described above, in the ingot manufacturing method, it takes man-hours for cooling in the mold, the yield of the ingot is not high, and a large amount of energy is required to reheat the steel ingot after cooling, which improves productivity and saves energy. Contrary to.

In order to manufacture the B-containing SUS304 stainless steel slab with high productivity while conserving energy, it is necessary to realize continuous casting, but until now, continuous casting has been difficult. This is because the surface cracks of the slab in the mold that occur during continuous casting, breakout, bulging of the slab between the support rolls, and the occurrence of internal cracks at the center of the slab cause difficult problems. .

In order to determine the continuous casting conditions, the thermal and mechanical properties in the high temperature range including the solidification temperature range of the steel for which the continuous cast slab is to be obtained are grasped, the casting speed and the casting of the support roll are performed. It is necessary to determine the interval in the direction (hereinafter referred to as the support roll interval) and the solid fraction of the central portion of the slab that passes through the final position of the secondary cooling zone. The casting speed controls the strength of the initial solidified shell in the mold, that is, the thickness of the initial solidified shell. It governs and determines the surface and internal quality of the slab.

However, since the thermal and mechanical properties of B-containing SUS304 stainless steel in the high temperature region including the solid-liquid coexisting region have not been clarified in detail up to now, it is impossible to determine the above continuous casting conditions. Met.

[0009]

The object of the present invention is to
Clarifying the criteria of continuous casting conditions with good thermal and mechanical properties of SUS304 stainless steel containing 4 to 2.0 mass% B in high temperature region including solid-liquid coexistence region, as well as surface and internal quality of slab. The present invention provides a continuous casting method of the above-mentioned stainless steel slab for use in a cell of a spent fuel rack of a nuclear power plant.

[0010]

The gist of the present invention resides in the following continuous casting method.

A continuous casting method of 0.4 to 2.0 mass% B-containing SUS304 stainless steel slab for use in a cell of a spent fuel rack of a nuclear power plant, with a casting speed of 0.
4 to 0.8 m / min, a support roll interval of 300 mm or less, and a solid casting rate of 0.2 or less in the central portion of the thickness of the slab at the final position of the secondary cooling zone. .

The term "central part of the thickness of the slab" as used herein means a central part in a cross section perpendicular to the casting direction of the slab, and is a part where solidification is delayed.

"Solid phase fraction" is defined by the following definition.

Solid phase ratio: The ratio of the length of dendrite grown from the surface of the slab toward the center of the thickness to the thickness of the slab.

The solid state is set to 0 in the completely liquid state. The slab is cooled in four directions, and when the tip of the dendrite grown from the surface of the slab reaches the central part of the thickness of the slab, it is in a completely solidified state, and the solid fraction is 1.0. Usually 0.9
A solidified state is obtained when the degree is 9 or more.

A desirable value for the support roll spacing is 250
mm, a desirable lower limit is about 150 mm, and a desirable lower limit of the solid fraction at the final position of the secondary cooling zone is about 0.05.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The steel to be used in the continuous casting method of the present invention is 0.4 to 2.0 mass% B-containing SUS304 stainless steel used in the cells of a spent fuel rack of a nuclear power plant. SU containing B in such a range
In the S304 stainless steel, the characteristic of the crystallization morphology of the solidification structure in the solidification process is that a δ ferrite phase having a dendrite shape as a primary crystal is crystallized and a (Fe, Cr) B type eutectic structure is formed in the dendrite gap. It is to crystallize.

The solidus temperature of SUS304 stainless steel when the B content is 0.4 to 2.0 mass% is about 12.
It is 70K. Since the liquidus temperature decreases as the B content increases, the solid-liquid coexistence temperature range, which is the difference between the liquidus temperature and the solidus temperature, decreases as the B content increases. The solid-liquid coexistence temperature range is 1 when the B content is 0.4 mass%.
It is 50K and 70K in the case of 1.2 mass%.
However, when the B content is 2.0 mass%, there is no solid-liquid coexistence temperature range because the (Fe, Cr) B type eutectic structure is crystallized directly from the liquid phase.

Taking SUS304L stainless steel for comparison, its liquidus temperature is 1463K, solidus temperature is 1415K, and solid-liquid coexistence temperature range is 48K. As described above, the solidus temperature of the B-containing SUS304 stainless steel is about 14 higher than that of SUS304L stainless steel.
It is 5K lower.

The solid-liquid coexistence temperature range and solidus temperature peculiar to such a steel are determined by the strength of the shell formed in the initial stage of solidification,
That is, it affects the tensile strength at high temperature.

According to the continuous casting method of the present invention, the casting conditions are 0.4 to 0.8 m / min at the casting speed, 300 mm or less between the support rolls of the secondary cooling zone, and the final position of the secondary cooling zone is passed. The solid fraction at the center of the thickness of the cast slab is 0.2 or less.

A preferred type of continuous casting apparatus for carrying out the method of the present invention is a vertical bending die (bending radius 5
m). Arrangement of the support roll is 2 directly under the mold.
250 mm in the casting direction within the range of 00 mm to 10 m
It is desirable to have multiple steps at mm intervals. The desirable lower limit of this support roll interval is about 150 mm, and the desirable diameter of the support roll is about 150 mm. The position of the secondary cooling zone is preferably within the range of 100 mm to 5 m immediately below the mold and is the gap between the support rolls.

In order to eliminate breakout in the mold and achieve continuous casting, the thickness of the initial solidified shell formed in the mold is set to 15 mm or more and 30 mm or less, and the strength to withstand the pulling force downward to the mold. It is necessary to have a thickness having For this purpose, the casting speed must be 0.4 to 0.8 m / min in consideration of the solid-liquid coexistence temperature range and the solidus temperature of the B-containing SUS304 stainless steel.

At this time, the desirable superheat degree of molten steel in the tundish is about 20 ° C., and the desirable mold cooling capacity is 150 to
400 kcal / m ² s, desirable secondary cooling water is 100-25
0 liter / min, desirable mold vibration width is 3 to 6 m, and desirable mold vibration period is 40 to 100 cpm.

Under each of the above conditions, the casting speed was 0.4 m / mi.
If it is less than n, the thickness of the initial solidified shell in the mold becomes larger than 30 mm, and the solid fraction in the central portion of the thickness of the slab at the final position of the secondary cooling zone does not become 0.2. On the other hand, 0.8m / min
If it exceeds, the thickness of the initially solidified shell in the mold becomes smaller than 15 mm, so that the strength of the shell is weak and breakout occurs.

Since the B-containing SUS304 stainless steel has a structure morphology accompanied by eutectic, the morphology of the solid-liquid interface is almost smooth. Therefore, the slab does not have strength in the thickness direction until the solid phase ratio becomes 1 at the central portion of the thickness of the continuously cast slab. At this time, the arrangement of the support rolls in the secondary cooling zone is not optimal, that is, when the support roll interval exceeds 300 mm, bulging occurs and internal cracking occurs in the thickness central portion of the cast slab. Therefore, the support roll interval is 300 mm to suppress the occurrence of bulging.
It was as follows.

Further, in order to prevent the porosity from being generated in the central portion of the thickness of the slab, the solid fraction of the central portion of the thickness of the slab at the final position of the secondary cooling zone is set to 0.2 or less and the central portion of the slab is controlled. It is necessary to suppress the reheat of the slab due to the liquid phase remaining near the part.

If the solid phase ratio exceeds 0.2, it becomes difficult for the molten steel to flow due to solidification shrinkage, and porosity occurs. A desirable lower limit of the solid phase ratio is about 0.05. This is because when the solid fraction is smaller than about 0.05, the final solidification position is separated from the final position of the secondary cooling zone by 2 m or more.

[0029]

Example First, prior to the continuous casting experiment, the B-containing S
US304 stainless steel and SUS30 as a comparative example
The liquidus temperature and solidus temperature of 4 L stainless steel were measured by differential thermal analysis. The results are shown in Table 1.

[0030]

[Table 1]

Further, a test piece having a diameter of 10 mm and a length of 100 mm was used to measure the tensile strength which governs the strength of the solidified shell of the cast piece by using a high temperature tensile tester capable of measuring in the solidification process. The result is shown in FIG.

FIG. 1 is a graph showing the relationship between the tensile strength of the test piece and the temperature. As shown, the B-containing SUS3
The tensile strength of 04 stainless steel is extremely small just above the solidus temperature, and rapidly increases at the solidus temperature. On the contrary,
The tensile strength of SUS304L stainless steel appears at a solid phase ratio of 0.7, and the tensile strength increases with a decrease in temperature, and the tensile strength becomes about 5 MPa at the solidus temperature.

As described above, it is clear that the tensile strength development behavior of the B-containing SUS304 stainless steel having a eutectic solidification structure and the SUS304L stainless steel having a normal dendrite structure are significantly different.

Next, continuous casting experiments were conducted under various conditions based on the measurement results of the thermal and mechanical properties of the B-containing SUS304 stainless steel. Table 2 shows the experimental conditions.

[0035]

[Table 2]

The B content is 0.4 to 2.0 mass%, the casting speed is 0.2 to 1.2 m / min, and the support roll interval is 50.
The solid phase ratio in the central portion of the thickness of the slab at the final position of the secondary cooling zone was 400 mm to 400 mm, and the solid fraction of the slab was varied from 0.1 to 0.8, and the defect occurrence state of the slab was investigated.

The results are summarized in Table 3.

[0038]

[Table 3]

As shown in Table 3, the casting speed was 0.2 m / mi.
At n and 1.2 m / min, in both cases of B content, breakout or surface cracks occurred on the surface of the slab in the continuous casting mold, making continuous casting impossible. Alternatively, a slab with good surface properties could not be obtained.

When the support roll interval is 400 mm,
Internal cracks were generated in the vicinity of the center of the thickness of the slab regardless of the B content. Alternatively, bulging occurred on the slab between the support rolls.

When the solid fraction in the central portion of the thickness of the slab at the final position of the secondary cooling zone was 0.3, internal cracking due to porosity occurred in the central portion of the slab.

From the above results, as the casting conditions, the casting speed is 0.4 to 0.8 m / min and the support roll interval is 300.
mm or less, and if the solid fraction of the central portion of the slab passing through the final position of the secondary cooling zone is 0.2 or less, 0.4 to 2.0
It is apparent that it becomes possible to manufacture continuously cast slabs of SUS304 stainless steel containing mass% B.

[0043]

According to the method of the present invention, 0.4 without surface and internal defects without causing breakout.
It is possible to manufacture continuously cast slabs of SUS304 stainless steel containing 2.0 mass% B.

[Brief description of drawings]

FIG. 1 B-containing SUS304 stainless steel and SUS
It is a figure which shows the relationship between the tensile strength of 304L stainless steel, and temperature.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G21F 1/12 G21F 1/12 (72) Inventor Fumio Kawato 4-5 Kitahama, Chuo-ku, Osaka-shi, Osaka No. 33 Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] 1. A continuous casting method of 0.4 to 2.0 mass% B-containing SUS304 stainless steel slab for use in a cell of a spent fuel rack of a nuclear power plant, wherein a casting speed is 0.
4 to 0.8 m / min, the distance between the support rolls in the casting direction is 300 mm or less, and the solid fraction in the thickness center of the slab at the final position of the secondary cooling zone is 0.2 or less. Continuous casting method.