JPH08104921A - Method of manufacturing austenitic stainless steel plate with excellent end face corrosion resistance - Google Patents

Abstract

(57) [Summary] [Purpose] To produce austenitic stainless steel plate with no internal defects and a perfect recrystallized structure and excellent in corrosion resistance at any part in the plate thickness direction by low pressure reduction. [Structure] Continuous cast slab (if necessary, 950-80
After hot rolling with a cumulative reduction of 10 to 50% (in a temperature range of 0 ° C.), it is performed in a temperature range of 1200 to 1300 ° C. (1150 to 1300 ° C. when rolling at 950 ° C. or less) for 1 hour or more. Heat and high shape ratio rolling: SQR (roll diameter x 1-pass reduction amount) / average plate thickness ≥ 0.6 is applied for 1 pass or more, and if necessary, cumulative reduction ratio 10 to 30% in a temperature range of 950 to 800 ° C. Hot rolled steel sheet with a total reduction ratio of 1.5 to 5.0 from the slab to the final thick steel sheet
Solution heat treatment is performed in a temperature range of 00 to 1100 ° C. [Effect] It has become possible to manufacture a thick steel plate having a plate thickness of 70 mm or more, which has been impossible in the past using a continuous cast slab.

Description

Detailed Description of the Invention

[0001]

INDUSTRIAL APPLICABILITY The present invention has a perfect recrystallized structure without internal defects and is excellent in corrosion resistance at any portion in the plate thickness direction, that is, an end face corrosion resistant austenitic stainless steel plate is manufactured at a low pressure reduction ratio. It is about how to do it.

[0002]

BACKGROUND OF THE INVENTION Austenitic stainless steel is widely used as a structural material for industrial and consumer use because it has both excellent corrosion resistance and weldability. With the increase in size of structural members, the plate thickness of the austenitic stainless steel sheet used tends to further increase. For example, a dam water discharge pipe uses austenitic stainless steel having a plate thickness of more than 70 mm.

Since the austenitic stainless steel has no phase transformation, grain refinement due to the phase transformation due to heating and cooling does not occur. Therefore, in general, when a thick steel plate is produced using a continuous cast slab, the total reduction ratio from the continuous cast slab to the final plate thickness cannot be sufficiently taken, and a coarse solidification structure of unrecrystallized remains. . Here, the total pressure reduction ratio is
It is "continuous cast slab thickness / final plate thickness".

The properties of materials are closely related to grain size. For example, as the crystal grains become coarser, the strength decreases and the strength or ductility becomes anisotropic. Therefore, the strength and ductility in a specific direction are insufficient, and when the product is bent, a wrinkle pattern with severe irregularities appears on the surface. Also, with the coarsening of crystal grains,
Since the permeability of ultrasonic waves is remarkably reduced, there is a problem that ultrasonic flaw detection cannot be performed to detect internal defects in thick steel plates. The coarsening of the crystal grains with the increase of the plate thickness as described above greatly reduces the use performance and workability of the material, and therefore, it is necessary to devise a fine grain.

Generally, an austenitic stainless steel plate is heated to 1150 ° C to 1285 ° C and then heated to 1250 ° C.
Hot rolling is performed in a temperature range of 950 ° C, and 1000 ° C to 1
Solution heat treatment is performed in a temperature range of 120 ° C. However, when manufactured by such a general method, in order to obtain a completely recrystallized structure, the total pressure reduction ratio from the cast to the final product is required to be 5.0 or more.

Therefore, conventionally, there has been disclosed a manufacturing method in which a coarse solidified structure formed in a casting solidification process is subjected to a large plastic deformation in hot rolling to perform recrystallization by rolling (Japanese Patent Laid-Open No. 63-309307). Issue). However, in the disclosed method, the paths must be held for a predetermined time or more,
Not only the productivity is impaired, but when the total reduction ratio from the cast piece to the final product is as low as 5 or less, a sufficiently uniform and fine perfect recrystallized structure cannot be obtained.

In the case of continuously cast slabs, Cr, Ni,
Negative segregation of Mo and the like appears near the 1/4 · t portion of the plate thickness (t), and this negative segregation cannot be eliminated by hot rolling at a low pressure reduction ratio. Therefore, after subjecting the slab to breakdown rolling,
Soaking diffusion heat treatment was performed at a high temperature, and hot rolling with a large reduction ratio was performed (Japanese Patent Laid-Open No. 5-320755). However, also in this case, the total reduction ratio from the cast piece to the final product had to be 5.0 or more.

From the above, 1/4 · t of the plate thickness (t)
In order to produce an austenitic stainless steel sheet having a plate thickness of more than 70 mm, which is excellent in corrosion resistance in the vicinity of the portion, that is, excellent in end face corrosion resistance, has no internal defects, and has been uniformly and completely recrystallized, a cast piece having an initial thickness of more than 350 mm is required. Such an extremely thick plate could not be manufactured with a continuously cast slab having a maximum slab thickness of 350 mm, and it was necessarily manufactured using an IC ingot.

[0009]

However, in order to produce a high-strength austenitic stainless steel having a large product thickness to some extent, it is necessary to use a large IC ingot capable of achieving a large total reduction ratio by the final thickness. is there. Therefore, the product yield is remarkably reduced as compared with the case where the continuous casting slab is used, and the manufacturing cost is increased. Further, even if a large ingot is used, there is a problem that the upper limit of the product size is significantly restricted. Therefore,
It has been desired to develop a method for producing an austenitic stainless steel plate having a perfect recrystallized structure, which has no internal defects, is excellent in corrosion resistance of the steel plate end face, and has a small total reduction ratio.

[0010]

DISCLOSURE OF THE INVENTION The inventors of the present invention are directed to a thermal processing process capable of eliminating negative segregation with a small total pressure reduction ratio and completely destroying a solidified structure to form a uniform fine recrystallized structure. As a result of detailed laboratory study on the segregation diffusion behavior and recrystallization behavior of austenitic stainless steel, the diffusion behavior and the recrystallization behavior largely depend on the working degree in the non-recrystallization temperature range and the subsequent working conditions. With this in mind, the present invention has been accomplished.

As this heat processing process, a continuous cast slab is hot-rolled in a temperature range of 950 ° C. to 800 ° C. and then subjected to soaking diffusion heat treatment for 1 hour in a temperature range of 1100 ° C. to 1300 ° C. It was set and the product macrostructure was investigated in a laboratory. FIG. 1 shows the changes in the product macrostructure due to the cumulative rolling reduction of 950 ° C. or less and the reheating temperature. Here, ◯ indicates a uniform fine recrystallized structure in which no coarse solidified structure remains at all, and X indicates an unrecrystallized structure in which a coarse solidified structure remains on the entire surface.

When the cumulative rolling reduction in the temperature range of 950 ° C. or lower is less than 10%, the product macrostructure is a non-recrystallized structure regardless of the subsequent reheating temperature. On the other hand, 9
When the cumulative rolling reduction in the temperature range of 50 ° C. or less is 10% or more, reheating to 1100 ° C. or more results in a completely recrystallized structure. The reason for this is as follows. When the rolling temperature is 950 ° C. or lower, the strain applied by rolling remains without being released, and becomes the recrystallization driving force at the time of reheating to 1100 ° C. or higher, and it is presumed that recrystallization is promoted. However, the cumulative rolling reduction is 10%
When the amount is less than the above, it is considered that the processing strain required for recrystallization is not accumulated, and the unrecrystallized solidified structure remains even after the subsequent reheating.

FIG. 2 shows changes in the presence or absence of internal defects in the thick product steel sheet depending on the rolling pass conditions. The ∘ mark indicates that the thick steel plate after rolling has no internal defect, and the X mark indicates that the internal defect remains. Further, the variable D included in the parameters on the horizontal axis represents the diameter (mm) of the rolling roll, and the variable ΔH represents the reduction amount (the thickness difference mm between the inlet side and the outlet side of the rolling pass).
(D · ΔH) ^1/2 / Rolling with an average plate thickness ≧ 0.6 is carried out for one or more passes, so that internal defects are completely eliminated.

FIG. 3 shows changes in the segregation degree of Mo and Ni of the SUS316 product plate according to the soaking diffusion heat treatment (SP process) temperature. Before soaking and diffusion heat treatment, 950
Hot rolling with a cumulative reduction of 20% is performed at a temperature of ℃ or less, and after soaking and diffusion heat treatment, hot rolling with a total reduction ratio of 3.0 and solution heat treatment at 1050 ° C are performed to obtain a product slab. did. The holding time of the soaking diffusion heat treatment was set to 1 hour. When the soaking diffusion heat treatment temperature is less than 1150 ° C, Ni
Alternatively, the segregation degree of Mo is large and is not so different from that of the steel sheet not subjected to the soaking diffusion heat treatment. On the other hand, it was found that the segregation degree is sharply reduced when the soaking diffusion heat treatment temperature is 1150 ° C. or higher. As a result, the corrosion resistance of the end face is greatly improved.

From the above knowledge, in order to manufacture an austenitic stainless steel extra thick plate having a plate thickness of 70 mm or more using a continuous cast slab with a total reduction ratio of 5.0 or less, 10 at a temperature range of 950 ° C. or less. % After processing more than 1%
While heating above ℃ for 1 hour or more (D ・ ΔH) ^1/2
It has been found that it is essential to carry out rolling with an average plate thickness ≧ 0.6 in one pass or more. As a result, it has become possible to manufacture an austenitic stainless steel plate with a total reduction ratio that is dramatically smaller than in the past, no internal defects, excellent end face corrosion resistance, and a uniform fine recrystallized structure.

Next, the reason why the content range of each component defined by the present invention is limited will be described. C: If the content of C exceeds 0.08%, the intergranular corrosion resistance of the weld heat affected zone is significantly processed, so the upper limit is 0.0.
8%. Si: Deoxidizing agent remained during steel making. Although it is an element that increases the strength by solid solution strengthening by containing it, on the other hand, since it is also an element that lowers the hot cracking property during welding and the N solid solution amount during solidification, its upper limit is 2.0. %. Mn: Deoxidizing agent remained during steel making. Although it is an element that increases the solubility of N by including it,
If contained in a large amount, corrosion resistance and hot workability are impaired, so the upper limit was made 5.0%.

Cr: It is a basic element of stainless steel, and it is necessary to contain at least 15% or more in order to obtain excellent corrosion resistance. However, if the content exceeds 27%,
Since the δ / γ phase balance at high temperature is impaired and the σ phase precipitates and the toughness deteriorates significantly, the upper limit is set to 2
It was set to 7%. Ni: a basic element of austenitic stainless steel, and in order to obtain excellent corrosion resistance and austenitic structure, the content of Ni is 6.0% or more. However, 16.0%
If it is contained in excess of 10%, the weld cracking at the time of welding, hot workability, etc. are deteriorated, so the upper limit was made 16.0%.

Mo: An element that is extremely effective in improving corrosion resistance and has an effect of suppressing the occurrence of pitting corrosion. However, if added in a large amount, not only will alloy alloy cost rise, but also the σ phase will be likely to form and the impact toughness will increase. Therefore, the upper limit was set to 4.0% because it causes deterioration of corrosion resistance. N: It is a major strengthening element in interstitial solid solution strengthening and refinement of crystal grains by precipitation of carbonitrides of Nb and Ti.
However, if the content exceeds 0.35%, the hot workability deteriorates, and blowholes are easily generated in the welded portion during solidification, so the upper limit was made 0.35%.

The rolling temperature range of the cumulative rolling reduction of 10 to 50% is set to 950 ° C to 800 ° C because the processing strain remains due to the rolling at 950 ° C or less, and the coarse solidified structure is destroyed by the subsequent reheating treatment. This is because a uniform and fine perfect recrystallized structure can be obtained. Rolling at a temperature higher than 950 ° C. releases the processing strain, so that recrystallization is not sufficiently caused by subsequent reheating, and a coarse solidified structure remains unrecrystallized. Therefore, the upper limit is set to 950 ° C. However, if the temperature is less than 800 ° C, the rolling reaction force remarkably increases, although the effect on the accumulation of work strain does not change.
The lower limit is set to 800 ° C. because it causes a defective rolling shape and an increase in load on the rolling mill.

The cumulative rolling reduction in the temperature range of 950 ° C. to 800 ° C. is set to 10% or more, because the work strain required as a driving force for breaking the cast structure to form a uniform fine recrystallized structure is caused in the steel. This is to accumulate. When the cumulative rolling reduction is less than 10%, the accumulated work strain is insufficient and a uniform and fine recrystallized structure cannot be obtained. However, the cumulative rolling reduction is 50
Even if it exceeds%, the strain necessary for recrystallization is sufficiently added, so an extra load is applied to the rolling mill.
It was set to 0%.

The heating temperature after hot rolling from 950 ° C. to 800 ° C. is set to 1150 ° C. to 1300 ° C. to promote the diffusion of the negative segregation portion formed in the continuously cast slab and improve the corrosion resistance of the end face of the product thick plate. This is because heating at 1150 ° C. or higher is required to achieve this. However, when heated above 1300 ° C,
The upper limit was set to 1300 ° C. because the crystal grains are remarkably coarsened and the product strength is lowered. The heating time is set to 1 hour or more because heating for 1 hour or more is required to eliminate the negative segregation of the continuously cast slab.

The temperature of the reheating treatment or the solution heat treatment subsequent to the rolling with the cumulative reduction of 10 to 50% in the temperature range of 950 ° C. or lower is set to 1000 ° C. or higher because the Cr carbide is sufficiently dissolved in the matrix. , 950 ° C
This is because the coarse columnar solidified structure is uniformly and completely recrystallized by using the working strain accumulated in the following rolling as a driving force. If the reheating temperature is less than 1000 ° C, Cr
Since the carbide does not completely form a solid solution, not only the intergranular corrosion resistance deteriorates, but also a coarse columnar solidified structure remains, so that the anisotropy of mechanical properties becomes large. However, in the case of solution heat treatment, heating above 1100 ° C. causes growth of crystal grains and lowers strength, so the upper limit was made 1100 ° C.

The reason why the total pressure reduction ratio from the slab to the product is set to 1.5 or more is to break the coarse casting structure and obtain a uniform fine recrystallization structure. If the total pressure reduction ratio is less than 1.5, coarse unrecrystallized columnar solidified structure remains, and the anisotropy of mechanical properties becomes large.

The rolling of (DΔH) ^1/2 / average plate thickness ≧ 0.6 is performed for one or more passes by crimping internal defects (zaku) generated inside the continuous casting in the solidification process of casting, This is to eliminate internal defects in the final product. In the case of (D · ΔH) ^1/2 / average plate thickness <0.6, the working amount is small and the inside of the slab is not sufficiently deformed and internal defects are not crimped, so the lower limit is 0. It was set to 6. In the formula, D is the diameter of the rolled work / roll, ΔH is the difference between the inlet side plate thickness and the outlet side plate thickness of the rolling mill, and the average plate thickness is the average of the inlet side plate thickness and the outlet side plate thickness.

[0025]

[Example] After austenitic stainless steel having the chemical composition shown in Table 1 was melted in an electric furnace, the mold thickness was 3
CC slabs were cast by a vertical continuous casting machine of 00 mm and 190 mm. This slab was hot-rolled under the conditions shown in Table 2 (Table 2-1 and Table 2-2) and subjected to solution heat treatment, and then the presence or absence of internal defects in the thick product steel sheet and the macrostructure were investigated. In the examination result of the macrostructure, ◯ indicates a uniform fine recrystallized structure in which no solidified structure remains at all, and X indicates an unrecrystallized structure in which a solidified structure remains on the entire surface. The pitting corrosion starting temperature in the ferric chloride solution is an index showing pitting corrosion resistance (corrosion resistance), and the lower the starting temperature, the poorer the pitting corrosion resistance.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

As a result, as shown in Table 2, No. The thick steel sheets 1 to 8 have no internal defects at all, exhibit a uniform and fine complete recrystallization structure, and have a pitting corrosion resistance equivalent to that of the surface layer and 1 / 2t portion even in the negative segregation portion. That is, every part of the cross section shows uniform and excellent corrosion resistance. On the other hand, in soaking diffusion heat treatment, 11
No. that did not heat above 50 ° C for 1 hour or more 9, 10,
No. 13 and No. 14 steel plates which were not rolled at 950 ° C. or less in BD rolling. It can be seen that the thick steel sheets of Nos. 11, 13, and 15 have portions where the negative segregation of the end face is not sufficiently eliminated and the pitting corrosion resistance is poor, and the corrosion resistance in the steel sheet cross-section thickness direction is non-uniform. Further, (D · ΔH) ^1/2 / average plate thickness <0.
No. 6 No. 12 thick steel plate did not undergo hot rolling at 950 ° C. or less at all, because internal defects remained in the central portion of the product thickness.
No. The thick steel plate of No. 13 had a coarse solidification structure while the macrostructure of the product was not recrystallized.

As described above, the temperature conditions of hot rolling and the amount of reduction are closely related and act extremely effectively. With a slight total reduction ratio, there are no internal defects, and the end face has excellent corrosion resistance and is uniform. It is clear that it is possible to manufacture an austenitic stainless steel plate having a fine completely recrystallized structure.

[0031]

As is apparent from the above examples, according to the present invention, it becomes possible to manufacture an austenitic stainless steel plate having excellent end face corrosion resistance with a very low total reduction ratio as compared with the conventional method. That is, it is possible to manufacture an austenitic stainless steel plate having a plate thickness of 70 mm or more by using a continuously cast slab. Therefore, compared with the case where the IC ingot is used for manufacturing, the yield is remarkably improved, the manufacturing process can be simplified, and the austenitic stainless steel plate can be supplied at an extremely low cost. Furthermore, it has become possible to significantly reduce energy consumption and labor. The industrial significance of these improvements is extremely great.

[Brief description of drawings]

FIG. 1 is a diagram showing the effects of a cumulative reduction of 950 ° C. or less and a reheating temperature on a product macrostructure.

FIG. 2 is a diagram showing the effect of rolling conditions on the presence or absence of internal defects in a product.

FIG. 3 is a diagram showing the effect of soaking diffusion temperature on the segregation degree of the final product.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Fuminori Eme 1-1 No. 1 Tobita-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Prefecture Nippon Steel Corporation Yawata Works

Claims (2)

[Claims] 1. By weight%, C: 0.08% or less, Si: 2.0% or less, Mn: 5.0% or less, Ni: 6.0-1.
Continuous casting slab of austenitic stainless steel containing 6.0%, Cr: 15 to 27%, Mo: 4.0% or less, N: 0.35% or less is accumulated in a temperature range of 950 ° C to 800 ° C. Reduction rate 10
After hot rolling at 50%, it is heated in the temperature range of 1150 to 1300 ° C. for 1 hour or more, and (D · ΔH) ^1/2 / average plate thickness ≧ 0.6 (where D: rolling work roll Hot rolling that includes one or more passes of hot rolling satisfying the conditions of diameter (mm), ΔH: reduction amount (mm)), and the total reduction ratio from the slab to the final thick steel plate is 1.5 to 5.0. A method for manufacturing an austenitic stainless steel plate having excellent end face corrosion resistance, which comprises performing solution heat treatment in a temperature range of 1000 ° C to 1100 ° C after rolling. 2. By weight%, C: 0.08% or less, Si: 2.0% or less, Mn: 5.0% or less, Ni: 6.0-1.
Continuous casting slab of austenitic stainless steel containing 6.0%, Cr: 15 to 27%, Mo: 4.0% or less, N: 0.35% or less is accumulated in a temperature range of 950 ° C to 800 ° C. Reduction rate 10
After hot rolling at 50%, it is heated in the temperature range of 1150 to 1300 ° C. for 1 hour or more, and (D · ΔH) ^1/2 / average plate thickness ≧ 0.6 (where D: rolling work roll 950 ℃ including one or more passes of hot rolling satisfying the conditions of diameter (mm), ΔH: rolling reduction (mm))
After performing hot rolling with a cumulative reduction of 10 to 50% in the temperature range of ˜800 ° C. and hot rolling with a total reduction ratio of 1.5 to 5.0 from the cast slab to the final thick steel plate. 10,
A method for producing an austenitic stainless steel plate having excellent end face corrosion resistance, which comprises performing solution heat treatment in a temperature range of 00 ° C to 1100 ° C.