JP2716937B2 - High corrosion resistant austenitic stainless steel with excellent hot workability - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-corrosion-resistant austenitic stainless steel having excellent hot workability, particularly when used as a material for a bleaching process of a seawater heat exchanger or a papermaking plant. The present invention proposes a stainless steel which is suitable for so-called acid resistance, pitting corrosion resistance and crevice corrosion resistance, and has excellent resistance to corrosion by chlorides, and also excellent hot workability. 2. Description of the Related Art In recent years, the level of quality required for corrosion-resistant materials has become extremely high from the viewpoint of safety and cost performance due to maintenance-free operation. Is growing. [0003] Regarding the corrosion resistance of such stainless steel,
There are indexes such as pitting corrosion, crevice corrosion, stress corrosion cracking, general corrosion, and intergranular corrosion. Among these quality indices, pitting corrosion and crevice corrosion are the most frequently encountered indices related to the use of stainless steel, especially when the chloride ion concentration is high and the temperature is high, such as in seawater heat exchangers. What has good corrosion resistance under various environmental conditions is particularly important. Therefore, conventionally, as a method of improving pitting corrosion resistance and crevice corrosion resistance, it has been known to increase the contents of Cr and Mo. However, Cr as an alloying element,
If the Mo content is high, intermetallic compounds such as the σ phase are likely to precipitate, and it is difficult to obtain stable quality in terms of corrosion resistance, etc. In addition, hot workability is deteriorated and manufacturing problems are hindered. The problem remains. Therefore, for a high alloy containing high Cr and high Mo, it is necessary to design a comprehensive alloy in consideration of corrosion resistance, microstructure stability against σ phase precipitation, and hot workability. Known techniques are inadequate. [0006] As a technique for overcoming this point, a conventional technique has been disclosed in
Japanese Patent Application No. 60-23185 proposes an improved hot workability. However, this prior art also requires extremely high workability when considering mass production, and the effect of improvement is still insufficient. In general, when an intermetallic compound such as the σ phase is precipitated, the mechanical properties are deteriorated and the corrosion resistance is also deteriorated. Therefore, it is necessary to stabilize the austenite structure, and it is necessary to contain an austenite-forming element such as Ni or N in a predetermined amount or more. In addition, as an industrial material, easy production is an indispensable factor in addition to quality such as corrosion resistance and mechanical properties.
If a large amount of r is contained, the hot workability is reduced, so that a solution in this respect is necessary for mass production. In short, the present invention proposes a highly corrosion-resistant alloy containing much higher Cr and Mo than SUS304 and SUS316, that is, excellent in all aspects of corrosion resistance, structural stability against σ phase precipitation, and hot workability. It is an object of the present invention to provide an austenitic stainless steel having high hot workability necessary for mass production, in particular. SUMMARY OF THE INVENTION The present invention relates to an austenitic structure which is excellent in corrosion resistance such as pitting corrosion resistance and crevice corrosion resistance and in which a heterogeneous phase is hardly precipitated even in terms of structure. In order to obtain an excellent product, the present inventors have completed the present invention based on the finding that when the oxygen level in the steel is low, B acts very effectively and is effective. That is, if the steel contains high Cr and high Mo, the high-temperature strength increases, the workability deteriorates, and grain boundary cracks easily occur during hot working. However, when B is added, the B precipitates at the grain boundaries and improves hot workability. But this B is
Since it is easily associated with oxygen in steel, when the oxygen level is high, the amount of free B that strengthens the grain boundaries decreases, and the effect of B cannot be sufficiently exhibited. The inventors have found that the effect on the hot workability of the addition of B changes depending on the O level.
It has been found that when the concentration is reduced to 60 ppm or less, the invention is remarkably improved. That is, the object of the present invention is that C ≦
0.030% by weight, Si ≦ 2.0% by weight, Mn ≦ 1.0
% By weight, Cr: 19 to 30% by weight, Ni: Over 20% by weight, 30% by weight or less, Mo: Over 3.5% by weight,
Less than 6.0% by weight, B: 0.001 to 0.010% by weight
And at least one of W, V and Cu as an accessory component in a total amount of 2.0% by weight or less, And N: 0.10 to 0.40% by weight, O ≦
0.0060% by weight, P ≦ 0.040% by weight, S ≦ 0.
It is an object of the present invention to provide a highly corrosion-resistant austenitic stainless steel which is 005% by weight, the balance being Fe and inevitable impurities and having excellent hot workability. Another object of the present invention is that C ≦ 0.030.
Wt%, Si ≦ 2.0 wt%, Mn ≦ 1.0 wt%, Cr:
19-30% by weight, Ni: more than 20% by weight and 30% by weight or less, M
o: More than 3.5% by weight and less than 6.0% by weight, B: 0.001 to
0.010% by weight and Mg ≦ 0.05% by weight, and
2.0% by weight in total of at least one of W, V and Cu
Cr + 3Mo + 20N ≧ 40 Cr + 2Mo + 8Si + 2Mn + W + 3V ≦ Ni + Cu + 50N + 6.4, and N: 0.10 to 0.40% by weight, O ≦ 0.0060% by weight, P ≦ 0.040% by weight, S ≦ 0.005% by weight, and the balance Is to provide a high corrosion resistant austenitic stainless steel having excellent hot workability comprising Fe and unavoidable impurities. Further, according to the present invention, in the high corrosion resistant austenitic stainless steel described above, Mg
Can be contained at 0.05 wt% or less. Here, at least one of W, V and Cu is contained as a component for improving corrosion resistance. In this case, the condition for the stability of the structure is as follows: It is necessary to The reasons for limiting the composition of the austenitic stainless steel of the present invention will be described below. C: If it is higher than 0.03 wt% (hereinafter abbreviated as “%”), chromium carbide precipitates in a heat-affected zone such as welding, and the grain boundary becomes sharp, resulting in deterioration of corrosion resistance. Si: Effective for corrosion resistance such as pitting corrosion resistance, but if it exceeds 2.0%, precipitation of intermetallic compounds such as σ phase is remarkably promoted, and corrosion resistance and toughness are rather deteriorated. Mn: If it exceeds 1.0%, corrosion resistance deteriorates, and
Promotes precipitation of σ phase and the like. Cr: an element indispensable for corrosion resistance. If it is less than 19%, the characteristics of a high corrosion resistance alloy are lost. On the other hand, Cr promotes the formation of the σ phase and the like.
It is necessary to add a large amount such as i. Ni: Ni is extremely effective as a structure stabilizing element for suppressing the precipitation of a different phase such as the σ phase. When the content is 20% or less, the structure becomes unstable. Additions above 30% are expensive. Mo: Like Cr, it is an element indispensable for improving corrosion resistance.
If the content is less than 3.5%, the original corrosion resistance cannot be obtained. However, like Cr, when the content is 6% or more, precipitation of a different phase such as a σ phase is promoted.
Is required in large quantities. B: An element indispensable for improving hot workability, and 0.001% or more is required to exhibit its effect. Also, 0.010
%, On the contrary, the workability deteriorates. Fixes Mg: S and improves hot workability. However, 0.
If it exceeds 05%, the compound precipitates at the grain boundary at a high temperature, and on the contrary, the workability deteriorates. N: Extremely effective for stabilizing tissue and pitting resistance. However, if the content is less than 0.10% by weight, sufficient effects cannot be obtained.
Addition exceeding 40% generates blowholes during casting,
The workability is degraded due to the extremely high temperature strength. O: The O content has a significant effect on the hot workability improving effect of B, and the limitation of the O content under B addition has an important meaning. That is, in a perfect austenite structure such as the alloy of the present invention, impurity elements such as S are likely to precipitate at grain boundaries and deteriorate hot workability. However, by adding B, harmful effects such as S can be suppressed. . This is high temperature (hot working temperature)
B is easy to move in the alloy, and B is deposited before S precipitates at the grain boundary.
Precipitates to suppress deterioration of hot workability due to precipitation of S at the grain boundary. However, B also has a high affinity for O, and if the O content in the alloy is high, B and O are combined to reduce free B precipitated at grain boundaries, and the effect of improving hot workability of B is reduced. You. That is, depending on the O content, there is a remarkable change in the effect of improving the hot workability of B. In order to secure free B in the alloy that contributes to the improvement of the hot workability, it is necessary to keep the O content low. On the other hand, it is conceivable to increase the free B in the alloy by adding B in excess of the B amount fixed by O. However, when the O content is high, the B addition amount increases, and B becomes a component and a compound in the alloy. And, conversely, deteriorates hot workability. Therefore, in order to ensure hot workability, it is necessary to combine B and O in appropriate amounts as shown in FIG. Therefore, in the steel of the present invention, in order to significantly increase the effect of B and secure extremely excellent hot workability, O
It is necessary to limit the content to 0.0060% or less. P: Hot workability when P exceeds 0.040%,
Deterioration of weldability. S: If S exceeds 0.005%, hot workability, weldability, and corrosion resistance are significantly deteriorated. S is preferably 0.001% or less. Examples Next, examples in which the characteristics of the steel of the present invention were examined will be described. Table 1 shows the component compositions of the test materials used in this example. This test material (steel of the present invention, comparative steel) was obtained as a 10 kg steel ingot in an induction furnace, hot forging, annealing, hot rolling and annealing. The characteristic test was performed according to the following method. (1) Hot workability evaluation: (70t × 100w ×
l) A 10 kg ingot was heated to 1250 ° C., and made into 10 t × 100 w × l with a hammer, and evaluated by the maximum crack length generated at the end. (2) Tissue stability: (2t) mm plate solution treated with 10N KOH
, And the amount of the precipitate was measured by a lattice point method by microscopic observation. (3) Corrosion resistance [Equation 6] All were evaluated by the degree of corrosion. The crevice corrosion test piece is shown in FIG. 1 is a rubber band and 2 is a gasket (Teflon pillar). The test results are described below. (1) Regarding hot workability As shown in Table 1, each steel of the present invention
No cracks occurred during hot forging: hot workability was very good. On the other hand, in the comparative steels, steels A and C containing no boron and steels containing boron but having an oxygen content of more than 60 ppm (B, E) or steels having a boron content of more than 0.01% All of (D, F, G) have cracks. In particular, it has been found that when the oxygen content in steel increases, boron, which is effective in improving workability, does not work effectively. Even if the O concentration is lower than 60 ppm, B
The steel (I) to which Cr is not added has cracks. (2) Structure Stability and Corrosion Resistance The structure stability was evaluated by the amount of the precipitated phase such as the σ phase precipitated on the steel, and the results are shown in Table 1. Table 2 shows the results of the corrosion resistance. In the case of the steels of the present invention, the amount of precipitated phase was as low as 1% or less, and the corrosion resistance was as low as 0.1 g / m ² · h or less in pitting corrosion test and crevice corrosion test, and 2.0 g / m m ² · h or less. On the other hand, in the comparative steel, the amount of the precipitated phase exceeded 1%, the degree of corrosion in the pitting and crevice corrosion tests exceeded 0.1 g / m ² · h, and the comparative steels A, B exceeded ² g / m ² · h. FIG. 2 is a graph showing the effect of the B and O contents on the hot workability of the steel of this example. The hot workability depends on the amounts of B and oxygen. Is effective for improving the workability, but when it exceeds 0.01%, the workability deteriorates conversely, and when the O content exceeds 60 ppm, the workability is remarkably deteriorated, and B <0.001% In the case of the component, it can be seen that cracking occurs even if oxygen (O) is low. FIG. 3 is a diagram showing the effect of each element on the σ precipitation amount. It was found that the amount of precipitation rapidly increased when the temperature reached. FIG. 4
Indicates the effect of each element on the corrosion resistance.
It was confirmed that the corrosion resistance was improved when + 3Mo + 20N was 40% or more. [Table 1] [Table 2] As described above, according to the present invention, B
The austenitic stainless steel, which has excellent corrosion resistance such as pitting corrosion and crevice corrosion resistance, as well as excellent hot workability, in addition to acid resistance, is manufactured by the unique alloy design of the present invention in which the contents of chromium and O are strictly adjusted. It can be obtained at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 (a) and (b) are a front view and a side view of a crevice corrosion test piece. FIG. 2 is a graph showing the influence of B and O on hot workability. FIG. 3 is a graph showing the relationship between the σ phase precipitation amount and the composition of each component. FIG. 4 is a graph showing the relationship between pitting corrosion resistance and composition of each component.

Claims (1)

(57) [Claims] C ≦ 0.030% by weight, Si ≦ 2.0% by weight, Mn
≦ 1.0% by weight, Cr: 19 to 30% by weight, Ni: 20
More than 30% by weight, Mo: more than 3.5% by weight, less than 6.0% by weight, B: 0.001 to 0.010
% By weight, and at least one of W, V and Cu as an accessory component in a total amount of 2.0% by weight or less. And N: 0.10 to 0.40% by weight, O ≦
0.0060% by weight, P ≦ 0.040% by weight, S ≦ 0.
005% by weight, the balance being Fe and unavoidable impurities. 2. C ≦ 0.030% by weight, Si ≦ 2.0% by weight, Mn
≦ 1.0% by weight, Cr: 19 to 30% by weight, Ni: 20
More than 30% by weight, Mo: more than 3.5% by weight and less than 6.0% by weight, B: 0.001 to 0.010% by weight and Mg ≦ 0.05% by weight, and , W,
Containing at least one kind of V or Cu in a total amount of 2.0% by weight or less; And N: 0.10 to 0.40% by weight, O ≦
0.0060% by weight, P ≦ 0.040% by weight, S ≦ 0.
005% by weight, the balance being Fe and unavoidable impurities.