JP2712702B2 - Steel for pressure vessels - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a steel for a pressure vessel which exhibits excellent strength, toughness, corrosion resistance and the like in various environments.

<Conventional Technology> In recent years, a great number of types of pressure vessels have been used in various environments against the background of remarkable technological innovations in various fields of the industrial field. With the increasing demand for pressure vessels, stricter attention has been paid to the design and manufacture of the pressure vessels. At present, many standards related to pressure vessels are used in Japanese Industrial Standards (JIS) and American Society of Mechanical Engineers (ASME). Standards have been established.

However, all of the above standards are intended for use in the atmosphere at room temperature or medium / high temperature, and when the contents of the container are corrosive substances, the corrosion resistance of stainless steel etc. should be taken into account. Needless to say, the material is applied, but basically, the specification of the material applied to the pressure vessel is also in accordance with the above-mentioned assumed condition.

However, with the recent demands for increasing the operating pressure and reducing the weight of pressure vessels, waves of increasing strength have been rushing to vessel materials, and at present, for example, JIS G3204, "Temperature alloy steel for pressure vessels" SA for “Boiler & Pressure Vessel Code Section II” of high-strength steel and ASME specified as “forged products”
High-strength steel specified as -723 is a typical pressure vessel material. The characteristics of the high-strength steel for pressure vessels are basically only in the composition of the chemical components, and utilize the strength of the medium carbon martensite structure to increase the thickness of the body of the pressure vessel to ensure hardenability. Alloy elements are added accordingly. However, at this time, in order to obtain sufficient toughness, JIS specifies that the tempering temperature be 610 ° C or higher, and ASME specifies that the tempering temperature be 540 ° C or higher.
And ASME are specified to be 1.2 minutes or more for a thickness of 1 mm.

On the other hand, recently, the demand for pressure vessels used in seawater, such as oxygen tanks for divers and gas storage vessels for diving survey vessels, has been increasing remarkably, and pressure vessels applied to environments different from those in the atmosphere have been increasing. Studies on materials for use are also being actively conducted. Needless to say, there has been a great demand for pressure vessels used under relatively low pressure in seawater, but C: 0.25 to 0.30% (hereinafter,% indicating component ratio is weight %), Si: 0.10 to 0.35%, Mn: 0.65% or less, P: 0.05% or less, S: 0.05% or less, Cr: 2.5 to 3.5%, Mo: 0.30 to 0.70%, Fe and inevitable impurities: balance Yield strength: 70kgf / mm ² (686MPa) or more, Tensile strength: 85kgf / mm ² (833MPa) or more, Elongation: 15% or more, Drawing: 25% or more, 0 ° C Charpy impact value: Steel materials having a mechanical property of 60 J / cm ² or more have been generally applied.

However, when attempting to increase the strength of such steel for pressure vessels with the aim of increasing the applied pressure and further reducing the weight, the toughness is insufficient, and trying to satisfy the toughness avoids the inconvenience of insufficient strength. It was concluded that the conventional high-strength steel for pressure vessels could not satisfy the recent required performance.

Therefore, the use of higher strength steel for pressure vessels (for example, SA-723 material of ASME) for use in seawater was also studied, but as described above, these materials were considered for use in seawater.強度 σ _B > 125kgf / mm ² (1225MP
a) In ②, there was a risk of delayed fracture, and it was only confirmed that it was not suitable for use in seawater.

Thus, when considering a practical material for pressure vessels used in seawater, increasing the strength of conventional pressure vessel steel to increase the working pressure will cause disadvantages such as deterioration of toughness and delayed fracture. It was inadequate and, after all, there was no practical pressure vessel material suitable as a material for "high-pressure vessels used in seawater" which could sufficiently meet recent demands.

Therefore, an object of the present invention is not only to have sufficient strength and toughness to cope with recent demands for high pressure and light weight, but also to show excellent corrosion resistance and delayed fracture resistance to seawater, and to be used in seawater. It was put into providing a satisfactory and practical pressure vessel steel.

<Means for Solving the Problems> Based on the results of a number of experiments performed to achieve the above object, the present inventors first responded to recent demands for “pressure vessels used in seawater” by: a. ) Strength: strength required to reduce the weight of the existing pressure vessel by 15% or more, b) Toughness: toughness that does not cause brittle fracture even at 1.4 times the current pressure, c) Delayed fracture resistance: delayed fracture in seawater D) Corrosion resistance: It was concluded that the development of pressure vessel steel with the characteristics that corrosion resistance in seawater would not be lower than existing materials was indispensable. The inventors of the present invention who have continued research based on such recognition have found that, when focusing on use in seawater, to reliably prevent brittle fracture that has not been quantitatively grasped so far. It was clarified that it was particularly important to introduce the concept of "fracture toughness" into steel materials for pressure vessels, and as a result of further research, the following new findings could be obtained.

That is, if the chemical composition of steel is devised, the yield strength: 95 kgf / mm ² (930 MPa) or more, and the tensile strength: 125 kgf / mm ² (1225 MPa), depending on the forging ratio and heat treatment conditions (quenching and tempering conditions). Below, elongation: 15% or more, drawing: 40% or more, 0 ° C Charpy impact value: 60 J / cm ² or more, 0 ° C plane strain fracture toughness value: It has been found that a property of crystal grain size: 7 or more can be ensured, and that the corrosion resistance in seawater exceeds that of the above-mentioned conventional steel.

The present invention has been made on the basis of the above findings. "The steel for pressure vessels is C: 0.25 to 0.40%, Si: 0.10 to 0.40%, Mn: 0.30 to 1.20%, P: 0.010% or less, S: Including 0.010% or less, Ni: 2.0 to 3.0%, Cr: 0.80 to 1.50%, Mo: 0.30 to 0.80%, V: 0.015 to 0.20%, sol.Al: 0.015 to 0.060%, N: 0.006 to 0.015% (However, Ni + Cr ≤ 4.0%), with the balance being substantially composed of Fe, yield strength: 95 kgf / mm ² or more, tensile strength: 125 kgf / mm ² or less, elongation: 15% or more , Drawing: 40% or more, 0 ° C Charpy impact value: 60 J / cm ² or more, 0 ° C plane strain fracture toughness value: 355 kgf / mm ^3/2 or more, Grain size: 7 or more And excellent corrosion resistance and delayed fracture resistance. "

Next, the reason why the content ratio of each component of the steel in the present invention is limited as described above will be described together with its operation.

<Action> CC is the main strength controlling element in the martensitic structure, and to ensure the required strength as steel for pressure vessels, it is necessary to use 0.1%.
It is necessary to add 25% or more. On the other hand, C content is 0.40%
If the content exceeds 1, the toughness will be impaired, so the C content is determined to be 0.25 to 0.40%.

Si Si must be added in an amount of 0.10% or more from the viewpoint of deoxidation and hardenability of steel, but at the same time, Si lowers the toughness of the grain boundaries and the matrix, so the upper limit of the content was set to 0.40%. .

Mn Mn has the effect of improving the deoxidation, desulfurization and hardenability of steel, but if its content is less than 0.30%, the desired effects of the above-mentioned effects cannot be obtained, while the content exceeds 1.20%. Therefore, the Mn content is determined to be 0.30 to 1.20% because there is a possibility that nonmetallic inclusions remain.

P, SP and S are all harmful impurity elements that lower the cleanliness of the steel, and it is desirable to minimize their contents as much as possible to improve the resistance to delayed fracture. But P
Since it is not economical to keep the S and S contents too low, the upper limit of each content is set to 0.010% from this viewpoint.

Ni Ni has the effect of improving hardenability without impairing the toughness of steel, but if its content is less than 2.0%, the desired hardenability cannot be ensured. The upper limit was determined from the viewpoint of the effect, and the Ni content was limited to 2.0 to 3.0%.

Cr Cr has an effect of improving corrosion resistance in addition to the same effect as Ni, but if its content is less than 0.80%, the effect of the above-mentioned effect is not sufficient. The upper limit was further set, and the Cr content was limited to 0.80 to 1.50%.

In this case, the sum of the Ni content and the Cr content is 4.0%
If it exceeds 500, the fracture toughness value starts to decrease.
Cr ≦ 4.0% ”.

Mo Mo has the effect of improving the hardenability and toughness of steel, and is an element that is particularly effective in suppressing the harmfulness of P and improving delayed fracture resistance. However, if the content is less than 0.30%, the desired effect due to the above-mentioned effects cannot be expected. On the other hand, the upper limit is determined from the viewpoint of economy and addition effect, and the Mo content is 0.30 to 0.80%.
%.

V V has the effect of raising the yield point of steel, but if its content is less than 0.015%, the desired effect of the above-mentioned effect cannot be obtained. On the other hand, if its content exceeds 0.20%, the toughness is reduced. , V content is determined to be 0.015 to 0.20%.

sol.Al Al has an effect on deoxidation and grain refinement of steel and has an effect of improving delayed fracture resistance, but the sol.Al content is 0.015
%, The effect of the above effect is not sufficient, while the effect of 0.
If the content exceeds 060%, nonmetallic inclusions may remain. Therefore, the sol.Al content is set to 0.015 to 0.060%.

NN has an effect of forming a compound with Al to refine the crystal grains, but if the content is less than 0.006%, the desired effect due to the above-mentioned effect cannot be obtained, while the content exceeds 0.015%. If this is done, coarse AlN will remain and the above effect will be reduced. Therefore, the N content is set to 0.005 to 0.015%.

The steel for pressure vessels according to the present invention has a yield strength of 95 kgf / mm ² or more, a tensile strength of 125 kgf / mm ² or less, an elongation of 15% or more, a drawing of 40% or more, and a 0 ° C Charpy impact value of 60 J. / cm ² or more, 0 ° C plane strain fracture toughness: 355 kgf / mm ^3/2 or more, grain size: 7 or more The purpose is to secure mechanical and metallurgical properties. The reasons are as follows. It is.

Yield Strength [σ _y ] The pressure vessel is basically designed such that the stress (σ) generated in the body is sufficiently lower than the yield strength (σ _y ) of the material. Further, the body plate thickness (t) is t = f (P · D · σ _y ), where P: pressure and D: outer diameter of the container. If the container has the same working pressure and size, the yield strength is high. By using a material having high thickness, the plate thickness can be reduced and the weight can be reduced. If the yield strength of the material is 95 kgf / mm ² (930 MPa) or more, it is possible to sufficiently cope with the current demand for weight reduction.

Tensile strength [σ _B ] If a steel material having a tensile strength exceeding 125 kgf / mm ² (1225 MPa) is used in seawater, delayed fracture may occur during use. Therefore, the tensile strength is preferably adjusted to 125 kgf / mm ² or less.

Elongation, drawing, and Charpy impact values If the elongation, drawing, and Shapri impact values are higher than the actual values of the current material, satisfactory performance as a pressure vessel can be ensured, and the values are "elongation: 15% or more." , “Aperture: 40% or more”, “0 ° C Charpy impact value: 60 J /
cm ² or more ”was used as the reference value.

Fracture toughness The conditions under which brittle fracture does not occur in pressure vessels Is true. Note that k is represented by k = f (σ · a), where a: the size of a defect, and increases as the stress or the defect increases. The maximum of K is when a (the size of the defect) penetrates the plate thickness. Even in this case, if the brittle fracture is not performed, the contents leak and the internal pressure decreases, so that catastrophic fracture occurs. Does not reach. In the present invention, the required condition of the pressure vessel used in seawater is σ = 43 kgf / mm ² (420 MPa), t = 20 m
from what I thought was m Therefore, this was set as the lower limit of the required fracture toughness value of the material.

Grain size The susceptibility to delayed fracture is different for steels having the same tensile strength, and one of the factors is the grain size. In general, steel having the same chemical composition has a greater resistance to delayed fracture as the crystal grains become finer. However, steel having a tensile strength of 125 kgf / mm ² or less exhibits sufficient delayed cracking resistance if the crystal grain size is 7 or more, and thus was used as a reference value.

By the way, the following are standard forging ratios and heat treatment conditions for achieving the above-mentioned target values of the mechanical properties of the steel of the present invention.

Forging ratio: 3 or more, Quenching: Hold in a temperature range of 820 to 920 ° C for at least 30 minutes per 1 cm of thickness before quenching. Tempering: Temper at 560 to 630 ° C.

Next, the effects of the present invention will be described more specifically with reference to examples.

<Examples> Example 1 First, four types of steel ingots having the chemical composition shown in Table 1 were melted, and then hot forging was performed at a forging ratio of 9.3 to produce test materials.

In Table 1, Comparative Steel I is a low-carbon steel with a large amount of Ni added, Comparative Steel II is a high-carbon steel with a small amount of Ni added, and Comparative Steel III is a conventional steel.

Next, the test material was heated to 910 ° C. and held for 2 hours, and then oil-quenched. Then, the test material was cut into five test pieces, each of which was subjected to a tempering treatment at a different temperature from 500 to 660 ° C. for 5 hours.

Then, the yield strength (σ _y ), tensile strength (σ _B ) and fracture toughness (K _IC ) of each test piece after the above treatment were examined, and the results are shown in FIG. Here, the tensile test was performed according to JIS Z2241, the impact test was performed according to JIS Z2242, and the fracture toughness test was performed according to ASTM E399.

As is clear from the results shown in FIG. 1, the steel for pressure vessels according to the present invention has sufficiently satisfactory mechanical properties, while the chemical composition is out of the range specified in the present invention. For steel, pressure It can be seen that the recent mechanical property requirements for force vessels are not fully adequate.

Next, the steel of the present invention and the comparative steel III were subjected to a seawater corrosion test (ASTM G31), and the results are shown in Table 2.

As is evident from the results shown in Table 2, repeated dry and wet corrosion, which is actually a problem and has a large amount of corrosion,
It can be confirmed that the corrosion amount of the steel of the present invention was 0.855 g, whereas that of the comparative steel III was 2.163 g.

Further, the steel of the present invention was subjected to a delayed fracture test of 4000 hours at k ₁ = 300 kgf / mm ^3/2 . Under this condition, delayed fracture did not occur, and it was found that the steel had sufficiently good delayed fracture resistance. Was.

Example 2 In this example, "several steel ingots having different chemical component compositions within the range specified by the present invention" and "steel ingots of comparative steel" were used to investigate the effects of variations in chemical composition. Two types were melted to produce test materials. The chemical composition of these ingots is the third As shown in the table.

Next, these ingots were hot forged at a forging ratio of 4.8, and the steel of the present invention was heated to 910 ° C. and held for 2 hours, followed by “oil quenching” and “tempering at 610 ° C. for 5 hours”
The comparative steel was processed at 900 ° C. for 2 hours and then subjected to “oil quenching” and “tempering at 620 ° C. for 5 hours”, respectively.

Then, after each of the above treatments, the mechanical properties were investigated and the corrosion resistance test was performed for each steel material. The results are shown in Table 4.

As is clear from the results shown in Table 4, the steel of the present invention has sufficiently satisfactory mechanical properties and corrosion resistance as in the case of Example 1, while the chemical composition is not It can be seen that all of the comparative steels that do not meet the requirements of the invention have inferior properties.

From the test results described above, it can be confirmed that according to the present invention, a steel for a pressure vessel exhibiting sufficiently satisfactory performance even in seawater can be realized.

In addition, the steel of the present invention can be applied not only to the pressure vessel itself used in seawater but also to ancillary equipment (valves, piping, etc.) of the vessel, and can be used for similar equipment in fresh water and the atmosphere. Of course.

<Summary of effects> As described above, according to the present invention, it is possible to further increase or reduce the working pressure of the pressure vessel product,
In addition, it can provide steel for pressure vessels that can eliminate concerns such as delayed fracture, and greatly contributes to improving the performance of various pressure vessels used in seawater, such as oxygen cylinders for divers and air vessels for diving survey vessels. And industrially very useful effects.

[Brief description of the drawings]

FIG. 1 is a graph in which the relationship between the tempering temperature and the mechanical properties of the steel material obtained in the examples is compared between the steel subject to the present invention and the comparative steel.

Claims (1)

(57) [Claims] [Claim 1] C: 0.25 to 0.40%, Si: 0.10 to 0.40%, Mn: 0.30 to 1.20%, P: 0.010% or less, S: 0.010% or less, Ni: 2.0 to 3.0%, Cr: : 0.80 ~ 1.50%, Mo: 0.30 ~ 0.80%, V: 0.015 ~ 0.20%, sol.Al: 0.015 ~ 0.060%, N: 0.006 ~ 0.015% (Ni + Cr ≦ 4.0%), with the balance being substantial Pressure vessel steel consisting of Fe and having an austenite grain size of 7 or more.