JPH10317104A - Austenitic stainless steel excellent in intergranular stress corrosion crack resistance ant its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the austenitic stainless steel excellent in intergranular stress corrosion resistance by heating a stainless steel to a specified temp., subjecting to rolling or forging, solution treatment, then heat treatment at a specified temp. for a prescribed time and cooling at a specified cooling rate. SOLUTION: A stainless steel ingot, which contains, by weight, 0.030-0.080% C, <=3.0% Si, <=3.0% Mn, <=0.03% P, <=0.15% N, 6-16% Ni, 15-22% Cr or/further <=3.0% Mo, is subjected to reduction at >=10% draft at a finish temp. of >=950 deg.C and is made into a steel slab. The steel slab is heated for >=2 hr at 1100-1300 deg.C and is rolled or forged, successively, after solution treatment and then heated at 550-800 deg.C and 1000-1100 deg.C for a prescribed time, is cooled in hot water at a cooling rate of >=3 deg.C/sec. A Cr concentration in a crystal grain boundary and within 0.01 μm both side is regulated to 1.3 times of a Cr average concentration of the whole steel. A Cr enriched layer is formed on a crystal grain boundary, the generation of intergranular stress corrosion crack is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an austenitic stainless steel excellent in intergranular stress corrosion cracking resistance and a method for producing the same.

[0002]

2. Description of the Related Art In austenitic stainless steels used in a high-temperature, high-pressure water environment, etc., resistance to intergranular stress corrosion cracking is regarded as 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 carbides, for example, as shown in “4) High-temperature water” on page 269 of the Stainless Steel Handbook (issued on January 24, 1995), carbon (C) It is said that reducing the amount is effective.

However, the solid solubility of C in austenitic stainless steel is extremely small, for example, 600 ° C.
Is only a few ppm. It is difficult to reduce the C content to 0.01% or less with the current industrial-scale refining technology.
Although slightly, even at 600 ° C., supersaturated C is present. Therefore, there is a possibility that Cr-based carbide precipitates at grain boundaries during cooling after heat treatment or during welding. This leads to the formation of a Cr deficient layer near the grain boundaries, which is a factor that limits the life of the material.

[0004]

As described above, the conventional steel is
During cooling after heat treatment or during welding, precipitation of Cr-based carbides occurs at grain boundaries, leaving the possibility of intergranular stress corrosion cracking due to Cr deficiency. The present invention is intended to solve such a conventional problem.
An object of the present invention is to provide an austenitic stainless steel excellent in intergranular stress corrosion cracking resistance capable of suppressing the formation of r deficiency and a method for producing the same.

[0005]

The cause described above is related to the fact that C present in steel precipitates as a Cr-based carbide at grain boundaries during cooling after heat treatment or during welding. That is, the Cr-based carbide precipitated at the grain boundary is
It is known that a deficient layer is formed and causes intergranular stress corrosion. For this reason, the amount of carbon is reduced to suppress precipitation. However, there is a limit to the reduction of C in stainless steel, and with the development of vacuum refining technology such as VOD, C
Although the amount has been greatly reduced, it is said to be a few ppm. Although it is possible to reduce the carbon content to an extremely low level by means such as EB dissolution or the like, it is extremely expensive, so that it is impossible to lower the solubility of C at 600 ° C. or less in a normal production process.

The present inventors have investigated the solid solution process of Cr carbide precipitated at the grain boundaries. As a result, it has been found that the Cr carbide disappears by heating at the solution temperature, but when the holding time is short, the Cr concentration at the grain boundaries and near the grain boundaries is high. It is considered that the reason for this is that the diffusion of Cr is slower than that of C, so that Cr remains in the vicinity even after the solid solution of Cr carbide. The intergranular stress corrosion cracking property of such a material in which Cr was concentrated in the grain boundary and in the vicinity of the grain boundary by heating to a carbide precipitation temperature range was investigated. That is, 0.045% C-0.6%
Si-0.9% Mn-0.015% P-0.001% S
-9.2% Ni-18.5% Cr-0.041% N Using a sample in which the Cr concentration in the vicinity of the grain boundary was changed, 600
After precipitating Cr carbide at the grain boundaries by heating at 1 ° C. for 1 hour, the grain boundary stress corrosion cracking properties were investigated. FIG. 1 shows the relationship between the Cr concentration in the vicinity of the grain boundary and the intergranular stress corrosion cracking property in 300 ° C. high-temperature water. Recognize. This is presumably because, when Cr is concentrated near the grain boundaries, formation of a Cr-deficient layer due to precipitation of Cr carbide due to subsequent heating is suppressed.

It has been found that the formation of such a Cr-enriched layer in the vicinity of the grain boundary can be controlled by adjusting the grain boundary precipitation treatment of Cr carbide and the subsequent solution heat treatment. That is, a sufficient amount of Cr carbide is firstly precipitated at the grain boundary, and then the solution heat treatment conditions are controlled so that the Cr carbide is dissolved but the Cr-enriched region remains. Furthermore, it is necessary to keep the variation in Cr concentration small in order to form a Cr-agricultural part uniformly over all grain boundaries, but this can be achieved by a combination of fracture of the solidification structure and heat treatment for homogenizing the steel slab. .

[0008] The present invention has been made based on the above findings, and the gist thereof is as follows. (1) By weight%, C: 0.030 to 0.080%, Si: 3.0% or less, Mn: 3.0% or less, P: 0.03% or less, N: 0.15% or less, Ni: 6 to 16%, Cr: 15 to 22%, if necessary, Mo: 3.0% or less, the balance being Fe and unavoidable impurities, within 0.01 μm of the grain boundary and both sides of the grain boundary An austenitic stainless steel excellent in intergranular stress corrosion cracking corrosion resistance, characterized in that the concentration of Cr is 1.3 times or more the average value. (2) By weight%, C: 0.030 to 0.080%, Si: 3.0% or less, Mn: 3.0% or less, P: 0.03% or less, N: 0.15% or less, Ni: 6 to 16%, Cr: 15 to 22%, if necessary, Mo: 3.0% or less, the balance being Fe and inevitable impurities. Finishing temperature 9
A steel slab manufactured by applying a reduction of 10% or more at 50 ° C or more is heated in a range of 1100 to 1300 ° C for 2 hours or more, then rolled or forged, and subjected to a solution heat treatment. after heating defined as the time · t ₁ above equation 1 in range, after heating 80% to 100% of the time period · t ₂ defined in formula 2 in the range of 1000 to 1100 ° C., 3 ° C. / sec or higher A method for producing an austenitic stainless steel excellent in intergranular stress corrosion cracking resistance, characterized by cooling at a cooling rate of. Formula 1: t ₁ = −0.04T + 32.2 Formula 2: t ₂ = −0.009T + 10 where t ₁ , t ₂ : time (hr), T: temperature (° C.)

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION First, in the component system of the present invention, C
Precipitates as Cr carbides at the crystal grain boundaries by precipitation heat treatment, and C
element necessary for forming a region having a high r concentration,
For that purpose, 0.030% or more is required. On the other hand, the addition of a large amount makes the solid solution of the carbide difficult, so that it is not desirable, so the upper limit was made 0.080%.

Next, both Si and Mn are necessary as deoxidizing materials. However, when Si is present in excess of 2.0%, and when Mn is present in excess of 3.0%, Since the hot workability is impaired, the former is set to 2.0% or less, and the latter is set to 3.0% or less.

P is an element that tends to segregate at the grain boundaries and may impair the corrosion resistance of the grain boundaries. Therefore, the upper limit is set to 0.03%.

Ni is an essential element as an austenite-forming element, and the amount required for forming an austenitic structure in terms of component balance is 6 with respect to the amount of Cr, which is a ferrite-forming element.
% To 16%.

[0013] Further, Cr is an element for improving the corrosion resistance. 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 is made 22%.

N is a strengthening element of austenitic stainless steel together with C. Since N has a higher solubility than C, it can stably exist in a solid solution state even 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, N
The upper limit of the amount was 0.15%. The reason for not setting the lower limit is to control the strength by changing the N amount according to the application.
Is the lower limit.

Mo is an element having an effect of improving corrosion resistance, and is added as necessary. At that time, if added in excess of 3.0%, hot deformation resistance is increased, so that rolling or forging becomes difficult. Therefore, when the content can be varied, the content is set to 3.0% or less.

It is necessary to prevent the formation of a Cr-deficient layer in order to ensure intergranular stress corrosion cracking resistance even when Cr carbide is precipitated by reheating such as welding. For this purpose, it is necessary to increase the concentration of Cr in and near the grain boundaries in advance. If the amount is at least 1.3 times the average value of the Cr concentration, the formation of a Cr deficient layer due to reprecipitation of carbide can be prevented. The Cr concentration is a value measured using an electron microscope and EDS (Energy Dispersive Spectroscopy) using a thin film sample.

In order to reduce the solidification segregation of steel having the above chemical composition range, first, a steel slab is manufactured by rolling at a finishing temperature of 950 ° C. or more and a rolling reduction of 10% or more for the purpose of breaking the solidification structure of a steel ingot or a continuously cast slab. There is a need to. This hot working is indispensable for making the subsequent homogenizing heat treatment effective. Before rolling or forging the slab thus manufactured, the slab is heated at a high temperature so that
Needs to be reduced. For this purpose, the necessary heat treatment is performed at a temperature below 1100 ° C, so that a sufficient diffusion effect cannot be obtained. Therefore, the lower limit is set at 1100 ° C. 1300 ℃
Was set as the upper limit. If the heating time is less than 2 hours, sufficient homogenization cannot be performed, so that the heating time is set to 2 hours or more.

As a treatment for increasing the Cr concentration at the grain boundary and the vicinity of the grain boundary, first, 550 is deposited for precipitation of Cr carbide.
It is necessary to heat at a temperature in the range of ８００800 ° C. for the time shown in the formula 1. At 550 ° C or lower, diffusion is slow.
Even at a temperature of 800 ° C. or more, the degree of supersaturation of C becomes small, so that a sufficient amount of precipitation cannot be obtained. Next, as a solid solution treatment of Cr carbide, a temperature of 100 to 1100 ° C. and a time of 80 to
Heating for 100% of the time is required. That is, 10
If the temperature is lower than 00 ° C., undissolved carbides remain, and if the temperature is higher than 1100 ° C., the diffusion of Cr becomes active and the Cr-enriched layer disappears.
Since the r-enriched layer disappears, it is necessary to limit the temperature to this range.

The steel of the present invention based on the chemical composition range and the production method as described above is made into a steel ingot or slab by ordinary ingot or continuous casting after steel making by various electric furnaces or the like, and then rolled or forged. After that, the steel slab is subjected to a homogenizing heat treatment, and then rolled or forged to be used as steel materials of various shapes. The effects of the present invention will be more specifically described below based on examples.

[0020]

EXAMPLES Table 1 shows the chemical composition of the test material steel. Table 2 shows the draft in the production of steel slabs, the heat treatment conditions for homogenizing the steel slabs, the heat treatment conditions for carbide precipitation, the solution heat treatment, the Cr concentration near the grain boundaries, and the heat treatment at 600 ° C. for 1 hour for these samples. , A low-speed tensile test piece having a parallel part diameter of 3 mm and a gauge length of 20 mm was sampled, and the result of a test in high-temperature water at 300 ° C. is shown.

As is clear from the results of these characteristic investigations,
The example of the present invention is superior in the intergranular stress corrosion cracking resistance to the comparative steel. In contrast, in the comparative examples, for example, alloy No. 8 has a low carbide precipitation temperature, and alloy No. 11 has a high precipitation temperature. Grain boundary cracking has occurred. Alloy No. 9 has a low solution heat treatment temperature and Alloy No. 10 has a high temperature, so that the Cr concentration near the grain boundaries is insufficient and grain boundary cracking is also caused.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

As described above, the steel of the present invention is a material having excellent intergranular stress corrosion cracking resistance due to heat treatment of steel slabs and precipitation and solid solution treatment of carbides. It is industrially extremely effective as a corrosion-resistant material to be used.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of Cr concentration near grain boundaries on intergranular stress corrosion cracking.

Claims (4)

[Claims] C: 0.030 to 0.080%, Si: 3.0% or less, Mn: 3.0% or less, P: 0.03% or less, N: 0.15% by weight% In the following, Ni: 6 to 16%, Cr: 15 to 22%, the balance being Fe and unavoidable impurities, the grain boundary and the Cr concentration within 0.01 μm on both sides of the grain boundary being 1.3 of the average value. Austenitic stainless steel excellent in intergranular stress corrosion cracking corrosion resistance, characterized in that it is twice or more. 2. The stainless steel according to claim 1, further comprising:
The austenitic stainless steel having excellent intergranular stress corrosion cracking resistance according to claim 1, characterized by containing Mo: 3.0% or less by weight. 3. In% by weight, C: 0.030 to 0.080%, Si: 3.0% or less, Mn: 3.0% or less, P: 0.03% or less, N: 0.15% Hereinafter, a steel ingot or a continuously cast slab containing 6 to 16% of Ni and 15 to 22% of Cr was subjected to a finishing temperature of 9%.
A steel slab manufactured by applying a reduction of 10% or more at 50 ° C or more is heated in a range of 1100 to 1300 ° C for 2 hours or more, then rolled or forged, and subjected to a solution heat treatment. after heating defined as the time · t ₁ above equation 1 in range, after heating 80% to 100% of the time period · t ₂ defined in formula 2 in the range of 1000 to 1100 ° C., 3 ° C. / sec or higher A method for producing an austenitic stainless steel excellent in intergranular stress corrosion cracking resistance, characterized by cooling at a cooling rate of. Formula 1: t ₁ = −0.04T + 32.2 Formula 2: t ₂ = −0.009T + 10 where t ₁ , t ₂ : time (hr), T: temperature (° C.) 4. The austenitic steel having excellent stress corrosion cracking resistance according to claim 3, wherein the steel ingot or the slab according to claim 3 further contains Mo: 3.0% or less by weight. Method for producing stainless steel.