JPH08269550A - Production of austenitic stainless steel excellent in intergranular stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE: To provide a chemical composition of austenitic stainless steel excellent in stress corrosion cracking resistance and its production. CONSTITUTION: The chemical composition of an Ni-Cr austenitic stainless steel, in which the content of C is limited to <=0.03% in order to inhibit the precipitation of carbides, causing intergranular stress corrosion cracking, in the grain boundaries and also N having a high degree of ability of entering solid solution is incorporated by <=0.15%, is set. Subsequently, at the time of producing this steel, a slab of this steel is heated at a temp. in the range between 1100 and 1300 deg.C, thereby, the amount of precipitation of carbides per unit grain boundary is reduced and also the amount of depletion of Cr in the vicinity of grain boundaries is reduced and further a Cr-depleted region is dispersed. By the chemical composition and manufacturing method mentioned above, the austenitic stainless steel excellent in intergranular stress corrosion cracking resistance can be provided.

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 austenitic stainless steel having excellent intergranular stress corrosion cracking resistance.

[0002]

2. Description of the Related Art In an austenitic stainless steel used in a high temperature and high pressure water environment, the intergranular stress corrosion cracking resistance is important. It is known that this intergranular corrosion is caused by a Cr-deficient layer accompanying the precipitation of Cr-based carbides at the grain boundaries. Therefore, in order to prevent the precipitation of this carbide, for example, "Handbook of Stainless Steel" (January 24, 1995)
It is said that reducing the amount of carbon (C) is effective as shown in “4) High-temperature water” on page 269 of the Japanese issue).

However, the solid solubility of carbon in austenitic stainless steel is extremely small, for example, 60.
It is only a few ppm at 0 ° C. Since it is difficult to reduce the amount of C to 0.01% or less by the current refining technology on an industrial scale, supersaturated C is present even at 600 ° C. although it is slight. Therefore, there is a possibility that Cr-based carbide may be precipitated at grain boundaries during cooling after heat treatment or during welding. This leads to the formation of a Cr-deficient layer in the vicinity of the grain boundary, which becomes a factor that limits the life of the material.

[0004]

As described above, the conventional steel is
During the cooling after the heat treatment or during welding, precipitation of Cr-based carbides occurs at the grain boundaries, leaving the possibility of intergranular stress corrosion cracking due to Cr deficiency. An object of the present invention is to provide a method for producing an austenitic stainless steel exhibiting excellent intergranular stress corrosion cracking resistance even in the presence of a slight amount of supersaturated C.

[0005]

The cause of the above-mentioned Cr deficiency is related to the fact that C existing in the steel precipitates as Cr-based carbides in the grain boundaries during cooling after heat treatment or during welding. ing. That is, C that precipitates at grain boundaries
It is known that the r-based carbide forms a Cr-deficient layer in the vicinity of the grain boundary and causes the grain boundary stress corrosion. Therefore, the amount of C is reduced to suppress the precipitation.

However, there is a limit to the reduction of C in stainless steel, and although the amount of C has been greatly reduced by the development of vacuum refining technology such as VOD, it is said to be several ppm.
It is difficult to make the solubility of C at 0 ° C. or lower.
Although it is possible to make the carbon content extremely low by means such as EB melting, it is not practically applicable because it requires a significantly high cost.

The present inventors have found that the C content is 0.03% or less and low C
By observing the fine structure of the Ni-Cr austenitic stainless steel of the system, it was clarified that the Cr-based carbides do not precipitate at all the grain boundaries but at the grain boundaries with a certain period. As a result of a detailed examination of this periodicity, it was found that Cr-based carbides were precipitated at the grain boundaries that overlapped with the Cr concentrated segregation zone. That is, it is considered that since the amount of C is small, it preferentially precipitates at the grain boundaries where the amount of Cr, which is a carbide forming element, is large. Since a Cr-deficient layer is continuously present in the vicinity of the grain boundary where such carbide is intensively deposited, grain boundary stress corrosion cracking occurs.

On the other hand, in a sufficiently homogenized sample, the precipitation of Cr-based carbides at the grain boundaries is dispersed in all the grain boundaries, so that the amount of carbides per unit grain boundary becomes small. Therefore, in the formed Cr-deficient layer, the amount of decrease in the Cr concentration is smaller and the length along the grain boundary is shorter than that in the above-described portion where the Cr is segregated heavily. As a result, the intergranular stress corrosion cracking resistance of the sufficiently homogenized material is further enhanced. That is, FIG. 1 shows 0.025% C-0.
5% Si-0.9% Mn-0.021% P-12.0%
The result of having evaluated the influence of the billet heating temperature and heating time on the grain boundary stress corrosion cracking resistance of Ni-17.5% Cr-0.041% N steel by the low speed tensile test in high temperature water is shown. As is clear from this figure, the intergranular stress corrosion cracking resistance is greatly improved by heating for a time in the temperature range of 1100 to 1300 ° C. for a period of time or longer.

The present invention has been made based on the above findings, and the gist thereof is% by weight.
And C: 0.030% or less, Si: 2.0%
Hereinafter, Mn: 3.0% or less, P: 0.0
4% or less, N: 0.15% or less, Ni: 6
.About.16%, Cr: 15 to 22%, or a further 1% by weight of a steel slab containing 100% or less of Mo: 3.0% or less and the balance of Fe and unavoidable impurities.
The method for producing an austenitic stainless steel having excellent intergranular stress corrosion cracking resistance is characterized in that after heating at a temperature of 1300 ° C. to 1300 ° C. for t hours or more, rolling or forging is performed.

[0010]

First, in the component system of the present invention, since C precipitates as carbides at the grain boundaries during high temperature use, C is present near the grain boundaries.
It is an element that forms an r-deficient layer and impairs intergranular stress corrosion cracking resistance. From such a viewpoint, the C content is limited to 0.030% or less. Next, both Si and Mn are necessary as deoxidizing agents, but if Si is present in excess of 2.0% and Mn is present in excess of 3.0%, hot working is performed. The former is set to 2.0% or less and the latter is set to 3.0% or less because the property is impaired.

P is an element that tends to segregate at the grain boundaries and may impair the corrosion resistance of the grain boundaries, so the upper limit was made 0.04%. Ni is an essential element as an austenite forming element, and is a ferrite forming element Cr.
The necessary amount for forming an austenite structure in terms of component equilibrium with respect to the amount is in the range of 6 to 16%. Cr is an element that improves the corrosion resistance, and for that purpose 15% or more is required, but if it exceeds 22%, embrittlement due to high temperature heating such as repeated welding occurs, so the upper limit was made 22%.

N, together with C, is a strengthening element for austenitic stainless steel. Since N has a higher solubility than C, it can stably exist in a solid solution state during cooling after heat treatment or during welding. Therefore, if N is used within the range of solubility, a strengthening effect can be expected and grain boundary embrittlement due to nitride does not occur. From such a viewpoint, the upper limit of the amount of N is set to 0.15%. The reason for not setting the lower limit is to control the strength by changing the amount of N according to the application, but the level of 0.01 in normal industrial scale melting is used.
The lower the percentage, the lower limit. Mo is an element having an action of enhancing corrosion resistance, but if it is added in an amount exceeding 3.0%, rolling or forging becomes difficult because it increases the resistance to hot deformation. Therefore, the content is set to 3.0% or less.

Before rolling or forging a steel slab having a chemical composition within the above range, it is necessary to reduce the segregation of the components such as Cr by heating it at a high temperature. The heat treatment required for this purpose, as shown in FIG. 1, is 1100 ° C. or lower, so that a sufficient diffusion effect cannot be obtained. on the other hand,
Heating at a temperature over 1300 ° C causes an increase in the delta ferrite phase, and rather expands the component variation, so 1300 ° C was made the upper limit. Also, the required heating time varies depending on the temperature, and the higher the temperature, the shorter the homogenization is achieved. The required minimum time, t, is defined by equation 1 as a function of temperature.

The steel of the present invention based on the chemical composition and the manufacturing method as described above is manufactured by various electric furnaces,
Ordinary ingot or continuous casting is used to make steel ingots or ingots, steel ingots are made into steel ingots by rolling or forging, and ingots are made directly or by rolling into steel ingots. Is used as a steel material of various shapes. The effects of the present invention will be described more specifically below based on examples.

[0015]

EXAMPLES Table 1 shows the chemical composition of the steel of the present invention and the comparative steel and the heating conditions of the billet. Parallel part diameter from these samples: 3 m
m, gage length: 20mm low speed tensile test piece
Tested in hot water at 300 ° C. Table 2 shows the results of these tests. As is clear from the results of these characteristic investigations, the steel of the present invention is superior in intergranular stress corrosion cracking resistance to the comparative steel. On the other hand, in the comparative steel, for example, alloy No. 8 has a low heating temperature and thus grain boundary cracking occurs, and alloy No. 9 has 1200 ° C. heating, but the heating time is short, so that the grain boundary cracking still occurs. There is.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

As described above, the steel of the present invention is a material having excellent intergranular stress corrosion cracking resistance due to the low carbon content and the heat treatment of the billet, and the corrosion resistant material used in high temperature water or the like. Is industrially extremely effective.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of heating temperature and heating time of a steel piece on intergranular stress corrosion cracking resistance.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/58 C22C 38/58

Claims (2)

[Claims] 1. By weight%, C: 0.030% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.04% or less, N: 0.15% or less, Ni : 6 to 16%, Cr: 15 to 22%, the balance consisting of Fe and inevitable impurities is heated in the range of 1100 to 1300 ° C for at least t hours and then rolled. Alternatively, a method for producing an austenitic stainless steel excellent in intergranular stress corrosion cracking resistance, characterized by forging. t = 1/25 (1300-T) +2 (1) where T: temperature (° C) 2. The component according to claim 1, further comprising Mo:
The method for producing an austenitic stainless steel excellent in intergranular stress corrosion cracking resistance according to claim 1, characterized in that steel containing 3.0% or less is used.