JP2953331B2 - Method for producing B-containing austenitic stainless 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 B-containing austenitic stainless steel sheet having a small surface roughness, which is used for a spent fuel storage rack for nuclear power generation, a transport casting, and the like.

[0002]

2. Description of the Related Art Austenitic stainless steel containing a large amount of B in an amount of 0.3 to 2% by weight has a large neutron absorption cross section and excellent corrosion resistance. And used for transport casting.

The surface roughness of a B-containing austenitic stainless steel plate used for nuclear power generation has a target value of Ra <1.0 μm in order to ensure corrosion resistance.

[0004] Austenitic stainless steel sheets are usually
After ingot smelting, it is manufactured through various processes such as ingot, hot rolling, annealing, shot blasting, and pickling. Austenitic stainless steel containing a large amount of B is manufactured in the same process as general austenitic stainless steel.

[0005] B-containing austenitic stainless steel is
After hot rolling as compared with general austenitic stainless steel, a thick oxide scale layer is formed on the surface of the steel sheet, and the metal base is partially subjected to internal oxidation to make the metal base uneven. For this reason,
After hot rolling and annealing, it is difficult to completely remove the scale by descaling only by shot blasting or pickling.

When the pickling process is repeated, the scale is completely descaled, but the surface becomes uneven, and the surface roughness Ra increases.
The target value cannot be obtained.

Therefore, the surface roughness of the product is set to Ra <1.0.
In actuality, the surface of the steel plate after pickling is polished and then pickled again to make the product into μm.

Japanese Patent Application Laid-Open No. 06-010172 discloses a method for effectively removing the surface oxide scale and scale flaws of an austenitic stainless steel strip by applying a hot-rolled austenitic stainless steel strip to a belt or roll. After performing the grinding process by mechanical grinding means with a shape grinder to remove at least the grain boundary oxide layer,
A method of pickling with a nitric hydrofluoric acid pickling solution is disclosed.

[0009]

When a mechanical grinding step and a pickling step after grinding are included in the descaling process of a hot-rolled steel sheet, the production efficiency deteriorates and the cost increases.
An object of the present invention is to descaling completely without mechanical grinding or shot blasting of the annealed steel sheet surface or the like, and Ra <1.
An object of the present invention is to provide a method for producing a B-containing austenitic stainless steel sheet having a surface roughness of 0 μm.

[0010]

Means for Solving the Problems In the present invention and the like, attention has been paid to the fact that oxidation proceeds during annealing after hot rolling and the oxide scale becomes thicker, and intensive studies have been made on a method for preventing oxidation during annealing.

The annealing of a general austenitic stainless hot rolled steel sheet is usually performed at 1100 ° C. to 1150 ° C. A general austenitic stainless steel sheet has a surface roughness Ra <1.0 μm after pickling even at such an annealing temperature.

The B-containing austenitic stainless steel sheet is also usually annealed at the same temperature as a general austenitic stainless steel sheet, but as a result of various investigations from the viewpoint of suppressing the oxide scale thickness and the unevenness of the internal oxidation during annealing, It has been found that there is an annealing temperature range where the progress of the oxide scale is extremely slow without impairing the mechanical properties. That is, it was clarified that the oxide scale thickness and the unevenness of internal oxidation were significantly reduced by considerably lowering the annealing temperature to 900 to 1050 ° C., and a uniformly descaled surface was obtained. The gist is as follows: "After hot rolling austenitic stainless steel containing 0.3 to 2% B by weight, annealed in a temperature range of 900 ° C to 1050 ° C, and then subjected to descaling treatment. A method for producing a B-containing austenitic stainless steel sheet having a small surface roughness after pickling.

[0013]

Next, the reason for limiting the method for producing a B-containing austenitic stainless steel sheet having a small surface roughness after pickling according to the present invention as described above and the effect thereof will be described.

[0014] Annealing is performed to make a non-uniform structure generated by hot rolling into a uniform recrystallized structure. Conventionally, annealing was performed at 1100 to 1150 ° C.
Even when lowered, a uniform recrystallized structure is obtained, and the desired mechanical properties can be obtained. Therefore, the lower limit of the annealing temperature is 900 ° C.
And

On the other hand, if the annealing temperature exceeds 1050 ° C. (
(Fe, Cr) ₂ B precipitates, resulting in Fe, Cr
Is reduced, the oxidation resistance is inferior, the oxide scale thickness and internal oxidation unevenness are significantly increased, the descaling property is inferior, the descaling cannot be sufficiently performed only by the pickling process, and the surface roughness can be reduced without mechanical grinding. The upper limit was set to 1050 ° C. because it could not be <1.0 μm.

The reason why the surface roughness after pickling is reduced by limiting the annealing temperature to 900 to 1050 ° C. is that the metal base is oxidized and scales such as Fe ₂ O ₃ and Cr ₂ O ₃ are formed. You.

It is presumed that the oxidation progress rate of these scales is low because the annealing temperature is low, so that the scale thickness and unevenness of internal oxidation are suppressed.

Further, in the formation of these scales, since the pickling solution permeates into the inside in a short time, it is assumed that even if shot blasting is omitted, the descaling is uniformly performed in a short time.

The term "annealing" as used in the present invention includes annealing other than final annealing. For example, intermediate annealing in hot rolling, annealing after hot rolling, descaling, skin pass rolling, and annealing after hot rolling when a final annealing step is performed. It is even better to apply the method of the present invention to such annealing.

Next, the B content is limited because if it is less than 0.3, the thermal neutron absorption capacity is small and it becomes unsuitable as a nuclear material. On the other hand, if it exceeds 2%, the hot workability becomes remarkable. Inferior, hot rolling impossible, 0.3% to 2%
And

The components of the austenitic stainless steel to which the present invention is applied are not limited because the effect is not greatly affected by the difference in the component composition. However, the preferred components for nuclear power generation are as follows. .

C: 0.08% or less, Si: 1% or less, M
n: 2% or less, P: 0.045% or less, S: 0.03%
Hereinafter, Cr: 18 to 20%, Ni: 8 to 10.5%, and Mo: 0.3 to 0.75% as necessary.

[0023]

EXAMPLE 3500 kg of a B-containing austenitic stainless steel ingot having the chemical composition shown in Table 1 was melted and pressed with a press.
A slab having a thickness of 6 mm was produced, and then a steel sheet having a thickness of 6 mm was produced by hot rolling.

[0024]

[Table 1]

From this steel sheet, the following materials for oxidation resistance test pieces and mechanical property test pieces were cut out and annealed under the following conditions. The mechanical property test pieces were collected such that the length of the test piece was in the rolling direction in the hand direction.

(1) Specimen Oxidation resistance specimen Specimen dimensions: 6.0mm thick, 40mm wide, 60mm long Mechanical properties specimen Tensile specimen dimensions: 6.0mm thickness, JIS-13B bending specimen Dimensions: board thickness 6.0mm, width 20mm, length 100mm (2) Annealing Heating temperature by electric furnace: 900-1150 ℃ Heating atmosphere: In air Heating time: 16.5 minutes After annealing, pickling under the following conditions As the mechanical property test pieces shown, a normal temperature tensile test piece and a bending test piece were prepared and subjected to the test.

Pickling liquid: 3% HF + 15% HNO ₃ Pickling liquid temperature: 50 ° C. Pickling time: 30 minutes The oxidation resistance test was performed by measuring the thickness of the oxidized scale after annealing by cross-sectional microscopic observation, and then under the above conditions. Pickling was performed, and the surface roughness after the pickling was measured. The test results are shown in Tables 2 and 3.

[0028]

[Table 2]

[0029]

[Table 3]

FIG. 1 shows the relationship between the annealing temperature and the oxide scale thickness in Table 2.

FIG. 2 shows the relationship between the annealing temperature and the surface roughness Ra in Table 2. The surface roughness was measured in two directions perpendicular to the rolling direction and the rolling direction. Table 3 shows the results of the tensile test and the bending test.

As is clear from FIG. 1, the lower the annealing temperature, the smaller the thickness of the oxide scale. Further, as is clear from Table 2 and FIG. 2, descaling after pickling results in residual scale when the annealing temperature exceeds 1050 ° C., and the surface roughness Ra sharply increases.

As apparent from Table 3, even if the annealing temperature is lower than 900 to 1050 ° C. than the conventional annealing temperature, if the tensile test and the bendability are 900 ° C. or more, 1100 ° C.
Satisfies the target value in the same manner as in the conventional example annealed.

From these results, the annealing was performed at 900 to 105.
The B-containing austenitic stainless steel sheet applied at 0 ° C.
It can be seen that the oxide scale is thin, is descaled uniformly by pickling, has a small surface roughness Ra, and has no problem in mechanical properties.

[0035]

According to the production method of the present invention, a polishing step in the descaling step is not required, the production efficiency is increased, and a B-containing austenitic stainless steel sheet having a small surface roughness after pickling can be produced. Can be used.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of annealing temperature on oxide scale thickness.

FIG. 2 is a diagram showing a relationship between an annealing temperature and a surface roughness Ra after pickling.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-50429 (JP, A) JP-A-56-158819 (JP, A) JP-A-63-96221 (JP, A) 23001 (JP, A) JP-A-7-51724 (JP, A) JP-A-7-51727 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21D 7/00-9 / 48

Claims (1)

(57) [Claims] An austenitic stainless steel containing 0.3 to 2% B by weight is hot-rolled and then heated at 900 ° C.
A method for producing a B-containing austenitic stainless steel sheet having a small surface roughness after pickling, comprising annealing in a temperature range of -1050 ° C, then pickling and subjecting to descaling treatment.