JP3188625B2 - B-containing austenitic stainless steel excellent in hot workability and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel material used for storing, transporting or neutron shielding plates of neutron-emitting nuclear waste, and in particular, a high B (boron) -containing austenitic stainless steel excellent in hot workability. And its manufacturing method.

[0002]

Austenitic stainless steel containing B (boron) is effective and widely used as a material having thermal neutron shielding properties and corrosion resistance.
However, austenitic stainless steel containing B in an amount of 0.5% or more is poor in hot workability, and is particularly susceptible to splitting during hot rolling. In addition, since the tensile strength and elongation at room temperature are small and the variation is large, this is a major problem in the characteristics of the product.

Conventionally, B-containing austenitic stainless steels, in particular, as a technique for improving the hot workability thereof, are disclosed in, for example, Japanese Patent Application Laid-Open No. 57-45464 (Austenitic stainless steel for boron-containing nuclear reactor having excellent hot workability). A method of improving the hot workability by adding Al to a B-containing austenitic stainless steel;
No. 89459 (boron-containing stainless steel excellent in corrosion resistance and workability) proposes a method of improving hot workability by adding V to B-containing stainless steel. However, the addition of these alloy components has not sufficiently improved the hot workability, and it has been difficult to produce a B-containing austenitic stainless steel sheet by a normal hot rolling method.

The tensile properties of a B-containing austenitic stainless steel sheet at room temperature are described, for example, in Japanese Patent Application Laid-Open No. 6-207207 (Production of boron-containing stainless steel for baskets in spent nuclear fuel casks excellent in ductility, toughness and corrosion resistance). The steel sheet manufactured by the powder metallurgy disclosed in (Method) exhibits excellent room-temperature tensile properties (high ductility and high strength). However, the powder metallurgy method is not suitable for mass production and is also unsuitable for manufacturing large objects such as structures.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to improve the hot workability of a B-containing austenitic stainless steel. Another object of the present invention is to provide a neutron which, in addition to the above-described improvement in hot workability, further improves mechanical properties such as tensile properties and elongation at room temperature, and exhibits stable properties at room temperature. It is to propose a B-containing austenitic stainless steel material effective as a shielding steel material and a method for producing the same.

[0006]

Means for Solving the Problems As means for solving the above problems, the present invention provides: (1) C: 0.1 wt% or less, Si: 1.0 wt% or less, Mn: 2.0 wt% or less, B: 0.05 -5.0 wt%, Cr: 11.0-27.0 wt%, Ni: 7.0-12.0 wt%, and N: 0.3 wt% or less, and the fine boride consisting of Fe and unavoidable impurities and having a size of 65 µm or less ,
In the slab stage, from the surface of the slab in the thickness direction
B analysis value of the part up to a depth of 10 mm and in the slab thickness direction
The difference from the B analysis value at the center is indicated by the value obtained by dividing the difference by the ingot sizing B analysis value.
B-containing austenitic stainless steel material excellent in hot workability formed by uniformly dispersing and precipitating in the sheet thickness direction so as to be 0.10 or less when subjected to heat treatment. (2) C: 0.1 wt% or less, Si: 1.0 wt% or less , Mn: 2.0 wt% or less, B: 0.05 to 5.0 wt%, Cr: 11.0 to 27.0 wt%, Ni: 7.0 to 12.0 wt%, Mo: 3.0 wt% or less, and N: 0.3 wt% or less, with the balance being the balance In the slab stage, a fine boride composed of Fe and unavoidable impurities and having a thickness of 65 μm or less
Analysis value of the part up to a depth of 10 mm in the direction and the thickness of the slab
The value obtained by dividing the difference from the B analysis value at the central part by the lumped B analysis value
The present invention proposes a B-containing austenitic stainless steel material which is excellent in hot workability and is uniformly dispersed and precipitated in the thickness direction so as to be 0.10 or less when indicated by.

Further, the present invention provides a method for producing a semiconductor device comprising: C: 0.1 wt% or less;
1.0 wt% or less, Mn: 2.00 wt% or less, B: 0.05 to 5.0 wt
%, Cr: 11.0 to 27.0 wt% Ni: 7.0 to 25.0 wt% and N:
In a method of producing a B-containing austenitic stainless steel material using a forged slab of high B-containing austenitic stainless steel for neutron shielding, containing 0.3 wt% or less, the balance being Fe and unavoidable impurities, an ingot prior to hot rolling. In the slab stage, the forging is performed under the conditions that the forging temperature is in the range of 800 to 1200 ° C. and the rolling reduction per rolling is 5% or more, and the average grain size is 65 μm or less . The surface of the slab
B analysis value of the part from the
The difference from the B analysis value at the center in the thickness direction
Control so that it is less than 0.10 when divided
Thus, a method for producing a B-containing austenitic stainless steel material having excellent hot workability, characterized by being uniformly dispersed in the thickness direction, is proposed.

[0008] In the above manufacturing method of the present invention,
As a forged slab of the B-containing austenitic stainless steel, C: 0.1 wt% or less, Si: 1.0 wt% or less, Mn: 2.
0 wt% or less, B: 0.05 to 5.0 wt%, Cr: 11.0 to 27.0 wt
%, Ni: 7.0 to 25.0 wt%, Mo: 3.0 wt% or less and N:
It is characterized by using a B-containing austenitic stainless steel containing 0.3 wt% or less, the balance being Fe and unavoidable impurities.

Further, in the method of the present invention, as the forged slab of the B-containing austenitic stainless steel, C: 0.1 wt% or less, Si: 1.0 wt% or less, M:
n: 2.0 wt% or less, B: 0.05 to 5.0 wt%, Cr: 11.0 to 27.0
It is characterized by using a B-containing austenitic stainless steel containing wt%, Ni: 7.0 to 12.0 wt%, Mo: 3.0 wt% or less, and N: 0.3 wt% or less, with the balance being Fe and unavoidable impurities.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention provides an ingot forging process prior to hot rolling of B-containing stainless steel, in which the rolling reduction in one rolling is controlled to 5% or more, so that the average grain size is 65 μm or less. This is an effective technique for uniformly dispersing the boron having a size of at least in the thickness direction of the plate, and particularly for uniformly and finely dispersing the slab in the thickness direction of the slab. B-containing austenitic stainless steel having stable mechanical properties can be produced.

[0011] The inventors of the present invention have found that when hot rolling the austenitic stainless steel containing a large amount of B, the front and rear ends of the stainless steel crack (two cracks occur at the center of the sheet thickness). The cause was studied in detail. as a result,
This double crack is caused by coarse boron (B) which is a compound of B and (Fe, Cr) existing in the center of the forged slab. During hot rolling, a crack is generated from the boron as a starting point. I found one that would break. That is, during hot rolling, strain concentrates around the harder boron than the austenitic phase, which is the matrix, but the strain is more concentrated as the grain size of the boron increases, and as a result, the interface between the boron and the matrix separates. Alternatively, the boride itself is destroyed and cracks occur.

When the size of these cracks is large or the number of them is large, that is, when coarse borides are present or when a large amount of borides are present, the cracks grow and connect to each other. It turned out to be cracked. Regarding the size of the boride that causes the double split, in the case of stainless steel containing 1.3% B, the boride having a grain size of 70 μm or more was the starting point of the crack. Therefore, in order to prevent splitting during hot rolling, it is effective to forge under appropriate conditions before hot rolling to make the boron finer and to uniformly disperse it at least in the thickness direction.

Therefore, the inventors conducted various tests and studies on the room temperature tensile strength of the B-containing austenitic stainless steel. As a result, coarse (300 μm)
m) The presence of boride reduces the room temperature tensile strength.
However, it has been found that when forging is performed under appropriate conditions before hot rolling, the room-temperature tensile strength does not decrease and the variation in the strength level also decreases. In other words, the fracture at room temperature of the austenitic stainless steel containing B caused by
Occurrence originates from the boride itself or the interface between the boride and the matrix.Especially, strain is easily concentrated around the coarse boride, and internal cracks are likely to occur. It breaks prematurely.

However, even for high-B content austenitic stainless steel, if this steel is subjected to forging under appropriate conditions prior to hot rolling, the boron becomes finer, and consequently the concentration of strain on the boron. As a result, it was found that the tensile elongation was increased and the tensile strength was increased, and at the same time the variation in the tensile strength was reduced.
That is, by performing such a treatment, a B-containing austenitic stainless steel material having stable mechanical properties can be manufactured.

As described above, in order to stabilize the mechanical properties at room temperature, it can be said that forging is performed under appropriate conditions before hot rolling to make the boron finer. Then, the inventors also studied conditions for ingot forging in which the above-described effects can be expected. That is, the structure, hot workability, and room temperature tensile strength of a B-containing austenitic stainless steel manufactured under various forging conditions were investigated. As a result, the forging temperature becomes 800-
When the rolling reduction in one rolling at 1200 ° C. is controlled to 5% or more, the boron at the center of the forged slab is finely and uniformly dispersed, and good hot workability is obtained. It was found that splitting could be prevented. In addition, it was also found that the tensile strength of the final product at room temperature was improved and the dispersion was reduced, and that the mechanical properties at room temperature were stabilized.

Next, the reasons for limiting the material components used in the present invention will be described. C: 0.1 wt% or less C forms Cr carbide in stainless steel and reduces Cr contributing to corrosion resistance and oxidation resistance. Therefore,
It is practical to make it 0.1 wt% or less.

Si: 1.0 wt% or less Si is a deoxidizing element and is necessary for refining. In addition, it is an element that improves the corrosion resistance and oxidation resistance of stainless steel. However, if it exceeds 1.0 wt%, hot workability deteriorates.
Therefore, a preferable addition range is 0.10 to 1.0 wt%.

Mn: 2.0 wt% or less Mn is effective as a deoxidizing element like Si, and needs to be added to some extent. Although it is an element used as a substitute for Ni, the addition of a large amount increases the residual induced radioactivity as a neutron shielding material. Therefore, the upper limit is 2.0 wt%. Preferably it is 0.10-2.0 wt%.

B: 0.05 to 5.0 wt% B is an essential element in the present invention in order to secure neutron absorption ability. In order to exhibit this neutron absorption ability, it is necessary to add at least 0.05 wt%.
However, if B is added in an amount of 5.0 wt% or more, many practical problems such as an increase in material strength and deterioration in workability occur. Therefore, the upper limit is 5.0 wt%. More preferably, 0.1 to 5.0 wt% is desired from the viewpoint of workability. More preferably 0.3
~ 5.0 wt%.

N: 0.3 wt% or less N is an element that improves the strength and corrosion resistance of stainless steel. However, if added in excess of 0.3 wt%, workability degradation due to an increase in strength poses a problem. Therefore, it is limited to 0.3 wt% or less.

Cr: 11.0 to 27.0 wt% Cr is a basic component in stainless steel and improves corrosion resistance. In order to expect a remarkable improvement in corrosion resistance, a Cr content exceeding at least 11 wt% is necessary, and the higher the Cr content, the better the corrosion resistance. However, when Cr is added in an amount exceeding 27 wt%, the material becomes extremely brittle, which is not preferable for practical use.
Therefore, the range of Cr is limited to 11.0 to 27.0 wt%. More preferably, it is desirably 16 wt% or more that exhibits excellent corrosion resistance and 25 wt% or less that does not cause embrittlement.

Ni: 7.0 to 25.0 wt% Ni is a basic component of stainless steel together with Cr, and is an essential element for stabilizing the austenite phase. In B-added stainless steel, Ni hardly dissolves in the boron and is hardly consumed, so that the addition of 7 wt% can sufficiently stabilize austenite. But 25wt%
If the addition exceeds the limit, the cost becomes extremely high, so the upper limit is set to 25 wt%. More preferably, the content is 7.0 to 12.0 wt% at which austenite becomes stable.

Mo: 3.0 wt% or less Mo exhibits about three times the effect of improving corrosion resistance as compared with Cr, and is an extremely effective element for improving corrosion resistance. In order to obtain a preferable effect of improving pitting corrosion resistance, addition of 0.5 wt% or more is desirable. However, if it is added in excess of 3.0 wt%, the cost increases and the embrittlement becomes significant, which is not preferable in practical use. Therefore, the addition range was limited to 3.0 wt% or less. The preferred range is 0.5-3.0 wt%, more preferably
1.0 to 3.0 wt%.

Next, the manufacturing conditions according to the present invention will be described. As described above, the present invention first performs ingot forging prior to hot rolling. In this forging, as the rolling reduction (forging ratio) in one rolling is increased, the finer the boron is, Is effective in obtaining a B-containing stainless steel excellent in hot workability. However, when the rolling reduction is greater than 50%, the load on the forging machine increases. On the other hand, when the rolling reduction is 5% or less, the fineness and uniform dispersion of the boron are not sufficient, and the effect of improving the hot workability cannot be obtained. Therefore, in the present invention, this forging has a reduction rate of 5% in one reduction.
Above. Preferably 5% to 50%, more preferably 5%
It is 30%, more preferably 10 to 30%. In the present invention, it is necessary to reduce B by 5% or more in order to refine B, but on the other hand, the shape may be adjusted at a reduction ratio of 5% or less for the purpose of adjusting the shape of the slab. .

Next, the forging temperature is set in the range of 800 to 1200 ° C. The reason for this is that in the forging of B-containing stainless steel, in order to obtain a crack-free slab, the forging temperature must be at least 800 ° C., and if the forging temperature is lower than this temperature, cracking will occur. This is because a sound slab cannot be obtained and the strength is increased, so that the load on the forging machine is increased, which is a practical problem. On the other hand, if the forging temperature is higher than 1200 ° C., the boron partially melts and the ingot is broken. Therefore, the forging temperature is 800-1200
° C. Preferably it is 950 to 1180 ° C.

In the present invention, the size of the boride to be controlled is large.
The size, that is, the particle size is in the range of 65 μm or less. Borai
If the size of the alloy is controlled by changing the forging conditions,
At the same time, it is evenly dispersed in the thickness direction of the slab,
The extent of the part from the slab surface to the depth of 10 mm in the thickness direction
Of the B analysis value of the slab and the B analysis value at the center in the thickness direction of the slabdifference
Divided by the B analysis value of the ingot is 0.10 or less.
You.

[0027]

[Example] SUS 304 (Austenitic stainless steel)
And SUS 316 (austenitic stainless steel containing 1 to 3.0 wt% Mo) as a base,
Austenitic stainless steel was melted by adding wt%, 1.0 wt%, 3.0 wt%, and 5.0 wt% as shown in Table 1.
A 6-ton ingot of this molten steel was forged under a forging condition shown in Table 1 using a 2500-ton press to obtain a forged slab. The forged slab was heated to 1150 ° C and subjected to hot rolling. The obtained hot-rolled sheet was pickled and annealed, and then examined for a grain size of a boride, hot rollability and strength. The results are shown in Table 1.

As shown in Table 1, among the comparative examples, those having low or high casting temperatures out of the conditions of the present invention (Nos. 1 and 2) showed poor forging,
In the cases where the rolling reduction during forging was small (Nos. 3 and 4), the grain size of the boride was large, and double cracks occurred during hot rolling and the tensile strength was low. In addition, the one with a relatively large rolling reduction (No. 14) had a large forging load, had a high equipment risk, and caused slight cracking.However, it was possible to perform hot rolling after grinding with a grinder . Of the present invention (No. 6
In each of (13) to (13), a product having no split at the time of hot rolling and having high tensile strength at room temperature and little variation was obtained.

[0029]

[Table 1]

[0030]

As described above, according to the present invention, there is provided a stable machine which is free from splitting during hot rolling and has a high tensile strength at room temperature and a small variation, which are observed in high B content stainless steel. B-containing austenitic stainless steel having characteristic properties can be produced.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Makoto Oikawa 4-2 Kojimacho, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Nippon Yakin Kogyo Co., Ltd. Research and Development Headquarters Technical Research Institute (56) References JP-A-7-188744 (JP) JP-A-5-320750 (JP, A) JP-A-5-263133 (JP, A) JP-A-2-170947 (JP, A) JP-A-63-286555 (JP, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/06 B21J 5/00 C21D 8/00-8/02

Claims (5)

(57) [Claims] C: 0.1 wt% or less, Si: 1.0 wt% or less, Mn: 2.0 wt% or less, B: 0.05 to 5.0 wt%, Cr: 11.0 to 27.0 wt%, Ni: 7.0 to 12.0 wt%, And N: a fine boride containing not more than 0.3 wt%, the balance being Fe and unavoidable impurities and having a size of not more than 65 μm in the slab stage from the surface of the slab to the thickness direction.
Analysis value of the part up to a depth of 10 mm in the direction and the thickness of the slab
The value obtained by dividing the difference from the B analysis value at the central part by the lumped B analysis value
A B-containing austenitic stainless steel material excellent in hot workability formed by uniformly dispersing and precipitating in the thickness direction so as to be 0.10 or less when indicated by . 2. C: 0.1 wt% or less, Si: 1.0 wt% or less, Mn: 2.0 wt% or less, B: 0.05 to 5.0 wt%, Cr: 11.0 to 27.0 wt%, Ni: 7.0 to 12.0 wt%, Mo: 3.0% by weight or less and N: 0.3% by weight or less, the balance consisting of Fe and unavoidable impurities ,
In the slab stage, from the surface of the slab in the thickness direction
B analysis value of the part up to a depth of 10 mm and in the slab thickness direction
The difference from the B analysis value at the center is indicated by the value obtained by dividing the difference by the ingot sizing B analysis value.
A B-containing austenitic stainless steel material which is excellent in hot workability and is uniformly dispersed and precipitated in the plate thickness direction so as to be 0.10 or less when subjected to heat treatment. 3. A method for producing a B-containing austenitic stainless steel material using a forged slab as a material, wherein forging of an ingot prior to hot rolling is performed at a forging temperature of 800-12.
Within the range of 00 ° C., and the rolling reduction per rolling is 5 to 3
By performing the process under the condition of 0 % or more, fine borides of 65 μm or less can be formed in the slab stage.
B analysis value of the part from the surface to the depth of 10 mm in the thickness direction
And the difference between the B analysis value at the center of the slab thickness direction
So that when divided by the analysis value, it is less than 0.10
A method for producing a B-containing austenitic stainless steel material having excellent hot workability, characterized by controlling and uniformly dispersing in the thickness direction. 4. The forged slab according to claim 3, wherein C: 0.
1 wt% or less, Si: 1.0 wt% or less, Mn: 2.0 wt% or less, B:
0.05 to 5.0 wt%, Cr: 11.0 to 27.0 wt% Ni: 7.0 to 25.0 wt%
% And N: 0.3% by weight or less, and a B-containing austenitic stainless steel material excellent in hot workability, characterized in that it is a B-containing austenitic stainless steel containing the balance of Fe and unavoidable impurities. 5. The forged slab according to claim 3, wherein C: 0.
1 wt% or less, Si: 1.0 wt% or less, Mn: 2.0 wt% or less,
B: 0.05 to 5.0 wt%, Cr: 11.0 to 27.0 wt%, Ni: 7.0 to
B-containing austenitic stainless steel containing 25.0 wt%, Mo: 3.0 wt% or less and N: 0.3 wt% or less, with the balance being Fe and unavoidable impurities, characterized by excellent hot workability. Manufacturing method of austenitic stainless steel.