JPH09263890A - B-containing austenitic stainless steel material excellent in hot workability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a B-containing austenitic stainless steel material excellent in hot workability and having stable mechanical properties at room temp. by specifying temp. and draft at the time of forging of ingot prior to hot rolling of B-containing austenitic stainless steel material. SOLUTION: The steel material is produced by using, as a stock, a forged slab of high B austenitic stainless steel for neutron shield, which has a composition consisting of, by weight, <=0.1% C, <=1.0% Si, <=2.0% Mn, 0.05-5.0% B, 11.0-27.0% Cr, 7.0-12.0% Ni, <=0.3% N, and the balance Fe and containing, if necessary, <=3.0% Mo. In this method, the forging of ingot prior to hot rolling is carried out under the conditions of 800-1200 deg.C forging temp. and >=5% draft per rolling reduction. By this method, the desired B-containing austenitic stainless steel material, in which fine borides of <=65μm average grain size are uniformly dispersed at least in a plate thickness direction, can be obtained.

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 storage and transportation of neutron-emitting nuclear reactor waste or for neutron shielding plates, in particular, a high B (boron) -containing austenitic stainless steel material excellent in hot workability. And its manufacturing method.

[0002]

2. Description of the Related Art Austenitic stainless steel containing B (boron) is effective and widely used as a material having thermal neutron shielding properties and corrosion resistance.
However, an austenitic stainless steel containing B in an amount of 0.5% or more has a poor workability during hot working, and is particularly prone to cracking in the hot rolling process, which is a serious problem in manufacturing. In addition, the tensile strength and elongation at room temperature are small and the variations are large, which is a serious problem in terms of product characteristics.

Conventionally, as a technique for improving the hot workability of B-containing austenitic stainless steel, especially Japanese Patent Laid-Open No. 57-45464 (Austenitic stainless steel for boron-containing nuclear reactor excellent in hot workability) Steel), a method of improving hot workability by adding Al to B-containing austenitic stainless steel, and JP-A-55-
Japanese Patent No. 89459 (boron-containing stainless steel excellent in corrosion resistance and workability) proposes a method of adding V to B-containing stainless steel to improve hot workability. However, the addition of these alloy components does not sufficiently improve the hot workability, and it has been difficult to produce a B-containing austenitic stainless steel sheet by a normal hot rolling method.

Regarding the tensile properties of the B-containing austenitic stainless steel sheet at room temperature, for example, JP-A-6-207207 (manufacture of boron-containing stainless steel for a basket in a spent nuclear fuel cask, which is excellent in ductility, toughness, and corrosion resistance). The steel sheet produced by powder metallurgy disclosed in the method) exhibits excellent room temperature tensile properties (high ductility and high strength). However, the powder metallurgy method is unsuitable for mass production, and also unsuitable for manufacturing large-scale objects such as structures.

[0005]

An object of the present invention is to improve the hot workability of a B-containing austenitic stainless steel material. Another object of the present invention is to improve the hot workability, further improve mechanical properties such as tensile properties and elongation at room temperature, and other neutrons exhibiting 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-mentioned 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 to 5.0 wt
%, Cr: 11.0 to 27.0 wt%, Ni: 7.0 to 12.0 wt%, and N: 0.3 wt% or less, and balance Fe and unavoidable impurities, and a fine boride of 65 μm or less in at least the plate thickness direction. A B-containing austenitic stainless steel material excellent in hot workability obtained by uniformly dispersing and precipitating, (2) 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%, Mo: 3.0 wt% or less and N: 0.3 wt% or less, balance Fe and unavoidable impurities, and 6
We propose a B-containing austenitic stainless steel material having excellent hot workability, which is obtained by uniformly dispersing and precipitating fine boride of 5 μm or less in at least the plate thickness direction.

In the present invention, C: 0.1 wt% or less, Si:
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:
A method for producing a B-containing austenitic stainless steel material using a forged slab of a high B-containing austenitic stainless steel for neutron shielding, which contains 0.3 wt% or less and the balance Fe and unavoidable impurities, and is an ingot prior to hot rolling. By performing the forging under the conditions that the forging temperature is in the range of 800 to 1200 ° C and the reduction rate per reduction is 5% or more, fine boride with an average grain size of 65 μm or less is at least in the plate thickness direction. A method for producing a B-containing austenitic stainless steel material having excellent hot workability, which is characterized by being uniformly dispersed, is proposed.

In the above manufacturing 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, Mn: 2.
0 wt% or less, B: 0.05 to 5.0 wt%, Cr: 11.0 to 27.0 wt
%, Ni: 7.0-25.0 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 and the balance Fe and unavoidable impurities.

Further, in the above-mentioned manufacturing 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
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, and the balance Fe and unavoidable impurities is used.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, in the ingot forging process prior to hot rolling of B-containing stainless steel, the average grain size is 65 μm or less by controlling the reduction rate in one reduction to be 5% or more. It is an effective technology to evenly disperse boride of the same size at least in the plate thickness direction, and in particular, to finely disperse it uniformly in the slab thickness direction. It is possible to manufacture a B-containing austenitic stainless steel material having stable physical properties.

Regarding the austenitic stainless steel containing a large amount of B, the inventors of the present invention have produced two cracks at the front and rear ends of the austenitic stainless steel when it is hot-rolled (a crack occurs in the central portion of the plate thickness to form a two-plate shape). ) The cause was investigated in detail. as a result,
This double cracking is caused by a boride which is a coarse compound of B and (Fe, Cr) existing in the central part of the forged slab, and a crack originates from this boride during hot rolling. I found one to break. That is, during hot rolling, strain is concentrated around the harder boride as compared to the austenite phase that is the matrix, but the larger the grain size of this boride, the greater the concentration of strain, resulting in the exfoliation of the interface between the boride and matrix. Alternatively, the boride itself is destroyed and cracks occur.

When the size of these cracks is large or the number of cracks is large, that is, when there are coarse boride or a large amount of boride, the cracks grow and are connected to each other. It turned out to be a crack. Regarding the size of the boride that causes the double crack, 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 double cracks during hot rolling, it is effective to perform forging under appropriate conditions before the hot rolling to make the boride fine and at least uniformly disperse in the plate thickness direction.

Therefore, the inventors have conducted various test studies on the room temperature tensile strength of the austenitic stainless steel containing B. As a result, coarse (300 μ
m) The presence of boride reduces the room temperature tensile strength.
However, it was found that when forging under appropriate conditions is performed before hot rolling, the room temperature tensile strength does not decrease and the variation in strength level also decreases. That is, the fracture due to the tensile of the austenitic stainless steel containing B at room temperature is
It originates from the boride itself or the interface between the boride and the matrix.In particular, strain is easily concentrated around the coarse boride and internal cracks are likely to occur, which reduces the tensile elongation and reduces the work hardening stage. It will break early.

However, even for a high B content austenitic stainless steel, if this steel is subjected to forging under appropriate conditions prior to hot rolling, the boride becomes fine, and consequently the strain is concentrated on the boride. It was found that as a result, the tensile elongation increased and the tensile strength increased, and at the same time, the variation in tensile strength also decreased.
That is, when such a treatment is performed, 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 it is effective to perform forging under appropriate conditions before the hot rolling to make the boride fine. Therefore, the inventors have also studied the conditions of ingot forging in which the above-mentioned effects can be expected. That is, the B-containing austenitic stainless steels produced under various forging conditions were investigated for their structure, hot workability, and room temperature tensile strength. As a result, the forging temperature is 800-
When the rolling reduction at one rolling at 1200 ° C. is controlled to 5% or more, the boride at the center of the forged slab is finely and uniformly dispersed, and good hot workability is obtained. It turned out that it is possible to prevent sheet breakage. Moreover, it was also found that the tensile strength of the final product at room temperature was improved and the variation was reduced, and the mechanical properties at room temperature were stable.

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

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

Mn: 2.0 wt% or less Mn is effective as a deoxidizing element similarly to Si, and needs to be added to some extent. In addition, it is an element used as a substitute for Ni, but if it is added in a large amount, the amount of residual induced radioactivity will increase as a neutron shielding material, so 2.0 wt% was made the upper limit. It is preferably 0.10 to 2.0 wt%.

B: 0.05 to 5.0 wt% B is an essential element in the present invention in order to secure the neutron absorbing ability. In order to develop this neutron absorption capacity, it is necessary to add at least 0.05 wt%.
However, if B is added in an amount of 5.0 wt% or more, practical problems such as increase in material strength and deterioration of workability occur, so 5.0 wt% is set as the upper limit. More preferably, 0.1 to 5.0 wt% is desired in terms 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, but if it is added in excess of 0.3 wt%, workability deterioration due to increased strength becomes 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 of at least 11 wt% is required, and the higher the Cr content, the better the corrosion resistance. However, when Cr is added in an amount of more than 27 wt%, embrittlement of the material is remarkable, which is not preferable for practical use.
Therefore, the Cr range is limited to 11.0 to 27.0 wt%. More preferably, it should be 16 wt% or more, which exhibits excellent corrosion resistance, and 25 wt% or less, which 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 is hardly dissolved in boride and is hardly consumed, so that addition of 7 wt% can sufficiently stabilize austenite. But 25wt%
If added in excess of 5, the cost becomes extremely high, so the upper limit was made 25 wt%. More preferably, 7.0 to 12.0 wt% is preferable, which makes austenite stable.

Mo: 3.0 wt% or less Mo exhibits an effect of improving corrosion resistance about three times that of 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 becomes high and embrittlement becomes remarkable, which is not preferable in practice. Therefore, the addition range is limited to 3.0 wt% or less. The preferred range is 0.5-3.0 wt%, more preferred
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, the larger the rolling reduction (forging ratio) in one rolling, the finer the boride and its dispersion. Is uniform and is effective in obtaining B-containing stainless steel having excellent hot workability. However, when the rolling reduction is set to be larger than 50%, the load on the forging machine becomes large. On the other hand, when the rolling reduction is 5% or less, the fineness and uniform dispersion of the boride are not sufficient, and the effect of improving hot workability cannot be seen. Therefore, in the present invention, this forging has a reduction rate of 5% in one reduction.
Above. Preferably 5-50%, more preferably 5-5
It is 30%, more preferably 10 to 30%. In the present invention, in order to make B finer, it is necessary to reduce it by 5% or more. On the other hand, for the purpose of adjusting the shape of the slab, the shape may be adjusted at a reduction rate of 5% or less. .

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, the forging temperature must be at least 800 ° C or higher in order to obtain a crack-free slab. If the forging temperature is lower than this temperature, cracking will occur. This is because a healthy slab cannot be obtained and the strength increases, which increases the load on the forging machine and poses a practical problem. On the other hand, when the forging temperature is higher than 1200 ° C, the boride is partially melted and the ingot is destroyed. Therefore, the forging temperature is 800-1200
Limited to ℃. It is preferably 950 to 1180 ° C.

In the present invention, the size of the boride to be controlled, that is, the particle size is within the range of 65 μm or less. When the size of the boride is controlled by changing the forging conditions, the grain size becomes fine and at the same time, it is uniformly dispersed in the thickness direction of the slab. The extent of this is the B analysis value and the slab from the slab surface to the depth of 10 mm in the thickness direction. When the work of the B analysis value in the central portion in the thickness direction of is divided by the B analysis value of the ingot, it is 0.10 or less.

[0027]

[Example] SUS 304 (austenitic stainless steel)
And SUS 316 (1 to 3.0 wt% Mo-containing austenitic stainless steel) as a base, and B to 0.5
As shown in Table 1, wt%, 1.0 wt%, 3.0 wt%, and 5.0 wt% were added to produce austenitic stainless steel.
A 6-ton ingot of this molten steel was forged under a forging condition shown in Table 1 using a 2500-t press to obtain a forged slab. The forged slab was heated to 1150 ° C. and hot-rolled. The hot-rolled sheet obtained was pickled and annealed, and then the boride grain size, hot-rollability and strength were investigated. The results are shown in Table 1.

As shown in Table 1, of the comparative examples, the ones whose casting temperature deviates from the conditions of the present invention and is low or high (No. 1 and 2) are all poor in forging, and
Those with a small reduction rate during forging (Nos. 3 and 4) had a large grain size of boride, which caused double cracking during hot rolling, and at the same time had a low tensile strength. In addition, those with a relatively large reduction (No. 14) had a large forging load, high equipment risk, and slight cracks occurred, but after grinding with a grinder, hot rolling could be performed. . Of the present invention (No. 6
In all of 13 to 13), a product without double cracking during hot rolling, high tensile strength at room temperature and little variation was obtained.

[0029]

[Table 1]

[0030]

As described above, according to the present invention, a stable machine which does not have double cracks during hot rolling which is found in high B content stainless steel, has high tensile strength at room temperature, and has little variation can be obtained. B-containing austenitic stainless steel having specific properties can be manufactured.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Makoto Oikawa 4-2 Kojima-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa Nihon Metallurgical Industry Co., Ltd. Technical Research Center

Claims (5)

[Claims] 1. 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: 0.3 wt% or less, the balance Fe and unavoidable impurities, and the fine boride of 65 μm or less uniformly dispersed and precipitated at least in the plate thickness direction, which is excellent in hot workability, and contains B-containing austenitic stainless steel. Steel material. 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%, Hot workability that contains Mo: 3.0 wt% or less and N: 0.3 wt% or less, consists of balance Fe and unavoidable impurities, and uniformly deposits fine boride of 65 μm or less at least in the plate thickness direction. Excellent B-containing austenitic stainless steel. 3. A method of producing a B-containing austenitic stainless steel material using a forged slab as a raw material, the forging of an ingot prior to hot rolling, the forging temperature being 800 to 12
Hot working characterized in that fine boride of 65 μm or less is uniformly dispersed at least in the plate thickness direction by performing the rolling in a range of 00 ° C and the rolling reduction rate per rolling is 5% or more. A method for producing a B-containing austenitic stainless steel material having excellent properties. 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 2
A B-containing austenitic stainless steel containing 5.0 wt% and N: 0.3 wt% or less, and the balance Fe and unavoidable impurities, and having excellent hot workability.
Method for producing contained austenitic stainless steel. 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: 0.05-5.0 wt%, Cr: 11.0-27.0 wt%, Ni: 7.0-
A B-containing austenitic stainless steel containing 25.0 wt%, Mo: 3.0 wt% or less and N: 0.3 wt% or less, and the balance Fe and unavoidable impurities. Manufacturing method of austenitic stainless steel.