JPH03229826A - Production of pressure vessel used in seawater - Google Patents

Abstract

PURPOSE:To obtain the pressure vessel having sufficient strength and toughness and also superior corrosion resistance to seawater by forging a low alloy steel with the prescribed composition into a vessel shape so that the forging ratio of a barrel part becomes the prescribed value, heating this vessel-shaped product, holding it at the heating temp., and carrying out hardening and further tempering. CONSTITUTION:A steel having a composition which contains, by weight, 0.25-0.35% C, 0.1 0-0.40% Si, 0.50-1.20% Mn, <=0.020% P, <=0.020% S, 2.2-3.0% Ni, 0.80-1.40% Cr, 0.30-0.80% Mo, 0.01 5-0.20% V, 0.01 5-0.060% sol.Al, and 0.006-0.015% N and in which Ni+Cr is regulated to 3.50-4.0 is formed into a shape of a vessel forged so that the forging ratio of a barrel part becomes 4-15. This vessel-shaped product is heated up to 850-920 deg.C, by which carbides are allowed to enter sufficiently into solid solution in an austenitic structure. Successively, this vessel-shaped product is held at the above temp. for 30min per centimeter of plate thickness of the barrel part, hardened, and further tempered at 580-650 deg.C, by which the pressure vessel for use in seawater having excellent mechanical properties of the barrel part can be obtained.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a "pressure vessel used in seawater" suitable as a storage container for air or other gas, such as an oxygen cylinder for a diver. This relates to a manufacturing method.

<Prior art> At present, very strict attention is paid to the design and manufacture of various pressure vessels, and pressure vessels are Many standards have been established. However, each of the above standards assumes use in the atmosphere at room temperature or medium/high temperature, and the applicable materials have only been specified from the viewpoint of the assumed conditions. However, if the content of the container is a corrosive substance, it goes without saying that a corrosion-resistant material such as stainless copper is used in consideration of this.

In recent years, in response to the gradual increase in the pressure vessels used and the demand for lighter weight, the strength of vessel materials has been increased. High-strength steel specified as "forged steel products" and ASME's rBoiler & Pressure
Vessel Code 5ection ■ J
High-strength steel specified as SA-723 is a typical material for pressure vessels.

The characteristics of the above-mentioned high-strength steels for pressure vessels basically lie only in their chemical composition; they utilize the strength of the medium-carbon martensitic structure, and increase the thickness of the body of the pressure vessel to ensure hardenability. Accordingly, alloying elements are added. However, in order to obtain sufficient toughness at this time, JIS requires a tempering temperature of 610.
℃ or higher, and ASME specifies the tempering temperature to be 540℃ or higher, and the holding time during heat treatment is specified by JIS and AS.
Both ME and ME are defined as 1.2 minutes or more for a thickness of 1 mm.

On the other hand, recently there has been a noticeable increase in demand for pressure vessels used in seawater, such as oxygen tanks for divers and gas storage containers for research submersibles. In the past, the following steel materials were the mainstream for pressure vessels used in seawater under low pressure, which were in relatively high demand.

Chemical composition C of steel material: 0.25 to 0.30% (hereinafter, % indicating the component ratio
(% by weight).

Si: 0.10-0.35%, gate n: 0.6
5% or less P: 0.05% or less, S: 0.05%
Below Cr: 2.5-3.5%, Mo: 0.
30-0.70% Fe and unavoidable impurities: remainder.

Mechanical properties of steel yield strength: 70kgf/urn! (686MPa) or more.

Tensile strength: 85 kgf/m% (833 MPa) or more.

Elongation: 15% or more, reduction of area 225% or more, 0°C Charpy impact value: 60J/cfl1 or more.

However, the desire to increase the applied pressure and reduce weight has reached the pressure vessels used in seawater without exception.
The development of even stronger materials continued. Therefore, when attempting to apply the above-mentioned high-strength steel for pressure vessels, this material has the inconvenience of insufficient toughness when trying to obtain the desired strength, and insufficient strength when attempting to satisfy the toughness. It was concluded that it was not suitable for the above uses.

Therefore, even higher strength pressure vessel steel, such as the above-mentioned ASME
The application of 5A-723 was also considered, but as mentioned above,
Since these materials are not designed for use in seawater, they are in the high strength range (σm > 125kgf/mm” (1225M
Pa)), there was a risk of delayed failure, and it was not suitable for seawater applications.

In this way, when considering practical pressure vessel materials for use in seawater, conventional pressure vessel steels suffer from disadvantages such as toughness deterioration and delayed fracture when the strength is increased in order to increase the working pressure. It was unsuitable, and in the end, it had to be said that there was no known economical method for manufacturing a "high pressure vessel for use in seawater" that could fully meet recent demands.

Therefore, the object of the present invention is to not only have sufficient strength and toughness to meet the recent demands for higher pressure and lighter weight, but also to exhibit excellent corrosion resistance and delayed fracture resistance against seawater, and to be suitable for use in seawater. The aim was to provide a stable means of manufacturing a fully satisfactory and practical pressure vessel.

<Means for Solving the Problems> Based on the results of numerous experiments conducted to achieve the above objectives, the present inventors have determined that in order to meet the recent demands for "pressure vessels used in seawater": Materials for pressure vessels: a) Strength: Strength necessary to reduce the weight of current pressure vessels by 15% or more; b) Toughness: Toughness that does not cause brittle fracture even under pressure 1.4 times the current pressure.

C) Delayed fracture resistance: Delayed fracture does not occur in seawater.

d) Corrosion resistance: Corrosion resistance in seawater is no lower than current materials.

They concluded that it is essential to develop steel materials with these characteristics, and furthermore, from a survey of the current state of pressure vessel manufacturing, they concluded that ``Pressure vessel manufacturing requires that current production equipment can be used almost as is.'' In order to further satisfy the economical conditions, it is necessary to use low-alloy steel as the container material, hot forge it to obtain a rough shape of the seamless container, and then machine it to the specified dimensions. However, we have come to strongly recognize that there is a need for a means that can follow a process that further ensures the required performance through heat treatment.

In order to achieve this, it is essential to find the optimal conditions from the above viewpoint in each process and investigate whether the product performance when combining these satisfies the required characteristics. We clarified the importance of introducing the concept of "fracture toughness value" to reliably prevent brittle fracture, which has not been quantitatively understood until now, as a necessary mechanical property that should be used as a standard, and further research. As a result of continuing,
We were able to obtain the following knowledge.

In other words, by devising and appropriately combining the chemical composition of low-alloy steel, the forging ratio when forming it into a container, and the heat treatment conditions (quenching and tempering conditions), it is possible to create a steel structure that is suitable for pressure vessels used in seawater. Stably achieved, yield strength: 95 kgf/mm2 (930 MPa) or more.

Tensile strength: 125 kgf/ad (1225 MPa) or less.

Elongation = 25% or more.

Aperture = 40% or more.

0°C Charpy impact value: 60J/co? that's all.

0℃ plane strain fracture toughness value: 355 kgf/m"" (110 MPa5) or more.

It has been discovered that not only is it possible to mass-produce pressure vessels constructed of steel materials having a

The present invention has been made based on the above findings, r C: 0.25 to 0.35%, Si: 0.10
~0.40%Mn: 0.50~1.20%,
P: 0.020% or less S: 0.020% or less,
Ni: 2.2-3.0% Leaf: 0.80-1.4
0%, Mo: 0.30-0.80%V:
0.015-0.20%, sol, Afl
: 0.015~0.060%N: 0.006
~0.015% (however, Ni + cr-3,50~4.0%}), and the balance is substantially Fe, when the forging ratio of the body is 4.
After forging it to a container shape of ~15, it is 850
Heat to ~920℃, hold at that temperature for 30 minutes or more per 1cm of body plate thickness, and then quench, and further heat to 580~650℃.
By tempering at ℃, the mechanical properties of at least the body have a yield strength of 95 kgf/no+I or more.

Tensile strength: 125 kgf/mm2 or less Elongation: 15% or more.

Aperture: 25% or more.

0°C Charpy impact value: 60 J/cra or more.

It has become possible to stably manufacture a pressure vessel suitable for use in seawater that exhibits a 0°C plane strain fracture toughness value of 400 kgf/Tsuru 3'' or more, and also has excellent corrosion resistance and delayed fracture resistance. It has the characteristics of "point".

Next, in the present invention, the reason why the chemical composition 1 mechanical properties and manufacturing conditions of the pressure vessel are limited as described above will be explained together with their effects.

8) Chemical component composition (a) C C is the main strength controlling element in the martensitic structure, and in order to ensure the required strength as a pressure vessel, 0.25
It is necessary to add more than %. On the other hand, C content is 0.35
%, the toughness is impaired, so the C content was set at 0.25 to 0.35%.

(b) 5i It is necessary to add 0.10% or more of Si from the viewpoint of deoxidizing the steel and ensuring hardenability, but at the same time, Si reduces the toughness of grain boundaries and matrix, so the upper limit of the content should be set. It was set at 0.40%.

(C) Mn Mn has the effect of improving the deoxidation, desulfurization, and hardenability of steel, but if its content is less than 0.50%, the desired effects due to the above effects cannot be obtained; If the Mn content exceeds 20%, nonmetallic inclusions may remain, so the Mn content is set at 0.50 to 1.20%.

(d) P and S Both P and S are harmful impurity elements that reduce the cleanliness of steel, and it is desirable to keep their content as low as possible, especially in order to improve resistance to delayed fracture. However, it is not economical to keep the P and S contents too low, so from this point of view, the upper limit of both contents was set at 0.020%.

(e) Ni Ni has the effect of improving the hardenability of steel without impairing its toughness, but if its content is less than 2.2%, the desired hardenability cannot be secured, and there is no solution. , the upper limit was set from the point of view of economy and addition effect, and the Ni content was 2.2 to 3.0%.
limited to.

(f) Cr In addition to the same effect as Ni, Cr also has the effect of improving corrosion resistance, but if its content is less than 0.80%, the effect of the above effect is not sufficient, and on the other hand, the economic efficiency The upper limit is set based on the effect of addition, and the Cr content is 0.80~
It was limited to 1.40%.

In this case, the sum of Ni content and Cr content is 3.
If it is less than 50%, the desired yield strength may not be stably secured; on the other hand, if the sum of Ni content and Cr content is 4%
．． Since the fracture toughness value starts to decrease when it exceeds 0%,
Ni + Cr was limited to 3.50 to 4.0%.

(g) M.

Mo has the effect of improving the hardenability and toughness of steel, and is an element that is particularly effective in suppressing the harmful effects of P and improving delayed fracture resistance. However, if the Mo content is less than 0.30%, the desired effect due to the above action cannot be expected.On the other hand, an upper limit was set from the viewpoint of economical efficiency and the effect of addition, and the Mo content was set to 0.30%.
It was limited to 30-0.80%.

(h) V (1) has the effect of increasing the yield point of steel, but if its content is less than 0.015%, the desired effect of said effect cannot be obtained; on the other hand, if it is contained in excess of 0.20%, Since this causes a decrease in toughness, the content should be from 0.015 to
It was set at 0.20%.

(Il sat, AN A1 has the effect of deoxidizing and grain refining of steel, and has the effect of improving delayed fracture resistance, but if the sof, AI! content is less than 0.015%, the effect of The sof and Af contents were determined to be 0.015 to 0.060% because the effect is not sufficient and, on the other hand, if the content exceeds 0.060%, nonmetallic inclusions may remain.

(J) N N has the effect of forming a compound with M and refining crystal grains, but if its content is less than 0.006%, the desired effect of the above effect cannot be obtained; If the content exceeds 0.015%, coarse AfN will remain and reduce the above effect, so the N content should be 0.005 to 0.
．． It was set as 0.015%.

B) Mechanical properties (al Yield strength [σ,] Pressure vessels are basically designed so that the stress (σ) generated in the body is sufficiently lower than the yield strength (σy) of the material.

Further, the body plate thickness (1) is, and if the container has the same working pressure and size, the plate thickness can be reduced and the weight can be reduced by using a material having a high yield strength. In the present invention, from the viewpoint of ensuring the required sufficient weight reduction effect, the yield strength of at least the container body is set to 95 kgf/mnI.
(930MPa) or more.

(b) Tensile strength [σ8] If the tensile strength of the steel material constituting the pressure vessel of the present invention exceeds 125 kgf/mm2 (1225 MPa), delayed fracture may occur when used in seawater. The tensile strength was determined to be 125 kgf/mm2 or less.

(C) Elongation, reduction of area, and Charpy impact value If the elongation, reduction of area, and Charpy impact value are equal to or higher than the actual values of current materials, sufficiently satisfactory performance as a pressure vessel can be ensured. % or more", "Aperture: 25% or more", "0℃ Charpy impact value: 60 J/c
m2 or more" was set as the standard value.

(dl Fracture toughness value) The condition under which brittle fracture does not occur in a pressure vessel used in seawater is that k < k holds true in any case. K becomes maximum when a (the size of the defect) penetrates the plate thickness, and even in this case, if there is no brittle fracture, the contents will leak and the internal pressure will drop, resulting in a catastrophic failure. In the present invention, the required conditions for pressure vessels used in seawater are 6-43 kgf/mm2 (42 kgf/mm2).
0MPa), t = 201 k”
X = 3""kgf/m"" (110MPa5), so r k +c = 400 kgf/m""
(124 MPa 5) J was taken as the lower limit of the required fracture toughness value of the material.

C) Forging ratio When forming a container shape from a steel ingot by forging, increasing the forging ratio increases the strength (improves toughness). In other words, Figure 1 shows the forging ratio of low-alloy steel and 0°C This is a graph showing the relationship with the Charpy impact value.
The figure also shows that the Charpy impact value increases as the training ratio increases.

In order to stably secure the above-mentioned Charpy impact value of the current material, it is necessary to set the above-mentioned forging ratio to 4 or more. It was determined that However, as the training ratio increases, the anisotropy of mechanical properties including the impact value becomes noticeable, and in extreme cases, the impact value may decrease as the training ratio increases. In particular, since the pressure vessel according to the present invention is seamlessly forged in one piece and the forging ratio of each part of the vessel is different, it is important to ensure good impact values in all parts and in all directions. The upper limit of the training ratio was set, and as a result, the range of the training ratio was limited to 4-15.

D) Heating temperature and holding time during quenching If the heating temperature during quenching is less than 850°C, solid solution of carbide into the osnanite structure may be insufficient.
On the other hand, if the heating temperature exceeds 920°C, the austenite crystal grains will become larger and the structure after quenching will become coarser, reducing toughness.
The temperature was set at 920°C.

In addition, the heating holding time is 30 times per 1 cm of body plate thickness.
If the heating time is less than 30 minutes, sufficient quenching may not be achieved and the desired strength and toughness may not be stably ensured.

E) Tempering Temperature If the tempering temperature is less than 580°C, the strength will be higher than necessary and the toughness will also be poor; on the other hand, if the tempering temperature is higher than 650°C, the required strength will not be obtained. Therefore, the tempering temperature was determined to be 580 to 650°C.

Next, the present invention will be explained in more detail with reference to Examples.

<Example> First, two types of steel ingots having the chemical composition shown in Table 1 were melted, and then hot horizontal press was performed so that the body forging ratio was 4.5 to 9 as shown in Fig. 2. We created a seamless container like this one.

Next, this seamless container was heated to 910°C, held for 2 hours, and then oil quenched. Subsequently, this seamless container was subjected to tempering treatment, but the tempering temperature was 610°C.
℃.

The yield strength, tensile strength, impact value, fracture toughness value, and seawater corrosion resistance of each pressure vessel thus obtained were investigated, and the results are shown in Table 2. Here, the tensile test is based on JIS Z2241, and the fracture toughness test is based on ASTM.
E399, and the seawater corrosion test is ASTM G 31.
Each was carried out according to the following.

As is clear from the results shown in Table 2, the pressure vessels manufactured according to the conditions stipulated by the present invention have sufficiently satisfactory mechanical properties, whereas the manufacturing conditions It can be seen that the conventional products that are outside the range do not exhibit sufficient mechanical properties.

In addition, corrosion resistance is also a practical problem, and dry and wet repeated corrosion, which causes a large amount of corrosion, is 0.703 g in the pressure vessel according to the present invention, while it is 0.919 g in the conventional container. It can be confirmed that it was as large as g.

Furthermore, regarding the pressure vessel according to the present invention, there is a +
A delayed fracture test was carried out for 6,000 hours under = 170 MPa, and it was confirmed that no delayed fracture occurred under this condition, and that it had sufficiently good delay resistance performance.

From the above test results, it is clear that according to the present invention, a pressure vessel for use in seawater with sufficiently satisfactory performance can be stably obtained.

Note that the means for manufacturing a pressure vessel according to the present invention is not limited to manufacturing the pressure vessel itself used in seawater, but can also be applied to manufacturing incidental equipment for the vessel (valve, piping, etc.). Of course, it can also be applied to similar equipment inside.

Summary of Effects> As explained above, the present invention provides a highly durable, high-performance product that can sufficiently increase operating pressure and reduce weight compared to conventional products, and does not cause delayed fracture. This brings about extremely useful effects industrially, such as making it possible to stably provide pressure vessels for use in seawater at low cost.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the forging ratio and Charpy impact value of low alloy steel. FIG. 2 is a schematic explanatory diagram showing the shape of a pressure vessel prototyped in an example.

Claims (1)

[Claims] Weight percentage: C: 0.25-0.35%, Si: 0.10-0.40
%, Mn: 0.50-1.20%, P: 0.020% or less, S: 0.020% or less, Ni: 2.2-3.0%,
Cr: 0.80-1.40%, Mo: 0.30-0.8
0%, V: 0.015-0.20%, sol. Al: 0
．． 015-0.060%, N: 0.006-0.015
% {however, Ni + Cr = 3.50 to 4.0%}, and the balance is substantially Fe, when the forging ratio of the body is 4.
After forging it to a container shape of ~15, it is 850
It is heated to ~920℃, held at that temperature for 30 minutes or more per 1cm of body plate thickness, and then quenched, and further heated to 580~650℃.
The mechanical properties of the body are: yield strength: 95 kgf/mm^2 or more, tensile strength: 125 kgf/mm^2 or less, elongation: 15% or more, reduction of area: 25% or more, A method for manufacturing a pressure vessel used in seawater that exhibits a 0°C Charpy impact value of 60 J/cm^2 or more and a 0°C plane strain fracture toughness value of 400 kgf/mm^3^/^2 or more.