KR20240086394A - Method for manufacturing low-temperature high-pressure tank using steel having high yield ratio - Google Patents

Abstract

The present invention relates to a method of manufacturing a low-temperature, high-pressure tank using high yield ratio steel.

Description

Method of manufacturing a low-temperature, high-pressure tank using high yield ratio steel {METHOD FOR MANUFACTURING LOW-TEMPERATURE HIGH-PRESSURE TANK USING STEEL HAVING HIGH YIELD RATIO}

The present invention relates to a method of manufacturing a low-temperature, high-pressure tank using high yield ratio steel.

Tanks used as storage containers for liquefied gas such as Liquefied Petroleum Gas (LPG) must withstand high pressure at low temperature (-52°C for LPG). Therefore, when such a low-temperature, high-pressure tank is manufactured by welding steel materials, it is required to secure excellent low-temperature toughness not only in the steel material but also in the welded area.

When manufacturing a tank by welding the tank steel materials, it is important to remove residual stress in the weld zone. As such, methods for removing stress from welded areas include the PWHT (Post Welding Heat Treatment) method using heat treatment, and the Mechanical Stress Relief (MSR) method which applies physical force by applying a certain pressure to the welded area. there is.

When manufacturing liquefied gas storage tanks, PWHT was mainly used to remove residual stress in the weld zone. However, in recent years, gas tanks have been basically enlarged, making it difficult to apply PWHT, and thus the mechanical stress relief (MSR) method is preferred.

In Non-Patent Document 1, in order to apply the MSR method in the process of manufacturing an actual low-temperature and high-pressure tank, the yield ratio (YS/TS), which is the ratio of yield strength (YS) and tensile strength (TS), is 0.8 or less. The scope of application is limited to steel (IGC code).

A high yield ratio of a material means that the breaking load can be reached if only a little more load is applied after the material has yielded. In general, the MSR method is a method of relieving residual stress through plastic deformation of the material, so when a load for plastic deformation is applied to a material with a high yield ratio, yield occurs in a local area (weld zone where tensile residual stress exists), that is, Because there is a high possibility that fracture will occur when the tensile strength is reached, the non-patent document 1 (IGC code) restricts the use of high yield ratio steel materials.

However, in order to apply MSR, if the yield ratio of steel for tank manufacturing is limited to 0.8 or less, the steel materials that can be used for tank manufacturing will inevitably be extremely limited. Therefore, the range of steel materials that can be provided to manufacturers is reduced, so the unit price of applied steel materials may increase, which reduces the supply and demand rate of steel materials, and as a result, the unit delivery price of the manufactured tank also increases.

Therefore, there is a demand for the development of technology that can manufacture low-temperature, high-pressure tanks by applying MSR to steel materials with a yield ratio exceeding 0.8 (high-yield ratio steel materials).

RESOLUTION MSC.370(93) (adopted on 22 May 2014) AMENDMENTS TO THE INTERNATIONAL CODE FOR THE CONSTRUCTION AND EQUIPMENT OF SHIPS CARRYING LIQUEFIED GASES IN BULK (IGC CODE)

One aspect of the present invention is to provide a method of manufacturing a tank by applying a mechanical stress relief (MSR) method even when using a high yield ratio steel material with a yield ratio (YS/TS) exceeding 0.8. It is done.

The object of the present invention is not limited to the above-mentioned matters. The additional problems of the present invention are described throughout the specification, and those skilled in the art will have no difficulty in understanding the additional problems of the present invention from the content described in the specification of the present invention.

One aspect of the present invention is a method of manufacturing a tank by welding steel materials,

Preparing steel for tank manufacturing;

Performing a mechanical stress relief (MSR) preliminary evaluation on steel whose yield ratio (YS/TS) exceeds 0.8;

determining structural safety by performing the MSR preliminary evaluation; and

It relates to a method of manufacturing a tank, including the steps of manufacturing a tank by welding a steel material with suitable structural safety and performing mechanical stress relief (MSR) treatment.

According to the method of the present invention, MSR is performed on steel materials exceeding a yield ratio of 0.8, which could not be applied previously, to manufacture low-temperature pressurized tanks used for liquefied gas storage, etc., thereby enabling application of various steel materials, thereby reducing tank manufacturing cost and efficiency. can increase.

The various and beneficial advantages and effects of the present invention are not limited to the above-described content, and may be more easily understood through description of specific embodiments of the present invention.

Figure 1 is a flowchart of a conventional method of manufacturing a low-temperature, high-pressure tank.
Figure 2 is a flowchart of a method for manufacturing a low-temperature, high-pressure tank according to an embodiment of the present invention.
Figure 3 is a flowchart of mechanical stress relief (MSR) preliminary evaluation according to an embodiment of the present invention.
Figure 4 shows the physical characteristics of the test material in an embodiment of the present invention.
Figure 5 is a photograph and graph showing the effect of residual stress relaxation for establishing MSR conditions in an embodiment of the present invention.
Figure 6 is a graph of the ECA procedure and instability failure evaluation results for establishing MSR conditions in an embodiment of the present invention.
Figure 7 is a true stress-true strain graph according to the ASME code in an embodiment of the present invention.
Figure 8 shows the results of regional plastic collapse evaluation in an embodiment of the present invention.
Figure 9 shows local plastic collapse evaluation results in an embodiment of the present invention.

The terms used in this specification are for describing the present invention and are not intended to limit the present invention. Additionally, as used herein, singular forms include plural forms unless the relevant definition clearly indicates the contrary.

The meaning of “including” used in the specification specifies a configuration and does not exclude the presence or addition of another configuration.

Unless otherwise defined, all terms, including technical and scientific terms, used in this specification have the same meaning as commonly understood by a person of ordinary skill in the technical field to which the present invention pertains. Terms defined in the dictionary are interpreted to have meanings consistent with related technical literature and current disclosure.

Hereinafter, the present invention will be described with reference to the drawings. The drawings and description below are provided for detailed explanation to those skilled in the art and are not intended to limit embodiments of the present invention.

Figure 1 is a flow chart showing a conventional method for manufacturing a low-temperature, high-pressure tank to which PWHT cannot be applied.

In order to manufacture a low-temperature, high-pressure tank, the tank design values, such as the type and capacity of the required cargo, are considered, appropriate steel materials are selected, and the steel materials are welded to manufacture the tank. When performing the above welding, a process for relieving the stress of the weld zone is required. Post Welding Heat Treatment (PWHT) is usually performed to relieve stress, but recently, the size of tanks has become larger, making it difficult to apply PWHT. Therefore, Mechanical Stress Relief (MSR) is being used instead of PWHT.

However, according to classification rules (IGC code), welding and MSR are applied only to steel materials with a yield ratio (YS/TS) of 0.8 or less to produce tanks. Therefore, in the process of selecting the steel material, it is determined whether the yield ratio of the steel material is 0.8 or less. If it is more than 0.8, it is excluded from tank manufacturing and the steel material must be selected again, and the steel material that can be applied is limited, so it is difficult to manufacture the tank. There are difficulties.

Accordingly, the present inventors have studied in depth a method of manufacturing a low-temperature, high-pressure tank by applying welding and MSR processes to steel materials with a yield ratio exceeding 0.8, and have arrived at the present invention.

Figure 2 is a flowchart showing an example of a method for manufacturing a low-temperature, high-pressure tank of the present invention. Referring to FIG. 2, an embodiment of a method for manufacturing a low-temperature, high-pressure tank of the present invention will be described in detail.

As a method of manufacturing a tank by welding steel materials, the required design value of the tank is secured (S1) and steel materials for this are selected (S2). The yield ratio (YS/TS) of the selected steel material is checked (S3), and if the yield ratio is 0.8 or less, a tank is manufactured (S5) by performing welding and mechanical stress relief (MSR) as usual procedures.

As an example of a steel material for manufacturing the low-temperature, high-pressure tank of the present invention, LT-FH-36 steel material can be used. According to the design purpose, the LT-FH-36 steel is recognized as a material capable of MSR with a yield strength (YS) of 355 MPa, a tensile strength (TS) of 490 MPa, and a yield ratio of 0.72. However, when confirmed through actual physical property tests, the yield strength is at the level of 420 to 470 MPa, the tensile strength is at the level of 510 to 550 MPa, and the yield ratio is sometimes around 0.83 to 0.88. In the conventional method, in this case, MSR was evaluated as an impossible material.

Tank manufacturing consists of single pipe and double pipe work. In Korea, welding techniques such as SAW (Submerged Arc Welding) and FCAW (Flux Cored Arc Welding) are used for single pipe and double pipe work, and tanks containing welded parts welded in this way are manufactured. Relieve residual stress using the MSR method. The MSR method is a method of relieving residual stress through plastic deformation by applying a load exceeding the design pressure inside the tank through a hydraulic test method after completion of tank manufacturing.

In the present invention, the yield ratio (YS/TS) of the selected steel is checked (S3), and if the yield ratio (YS/TS) of the steel exceeds 0.8 as described above, a mechanical stress relief (MSR) preliminary evaluation is performed. , Tanks can be manufactured through welding and mechanical stress relief (MSR) processing for steel materials that meet the structural safety evaluation standards for steel materials. However, for steel materials that do not meet the structural safety evaluation, reexamination is desirable.

As shown in Figure 3, it is desirable to derive safety evaluation criteria through the mechanical stress relief (MSR) preliminary evaluation as follows.

An appropriate MSR condition is established by evaluating the residual stress relaxation effect and ECA (Engineering Critical Assessment) based on fracture mechanics considering the residual stress for the steel material (steel material with a yield ratio exceeding 0.8).

The purpose of PWHT is to relieve residual stress in the weld zone, and it is known that most residual stresses are alleviated when PWHT is actually performed. Likewise, the purpose of MSR is to relieve residual stress. However, MSR is a method of relieving residual stress by applying internal pressure to induce plastic deformation, so the degree of relieving residual stress varies depending on the degree of internal pressure. The ECA is a fracture mechanics-based crack propagation and unstable fracture evaluation. Therefore, the level of residual stress that must be alleviated is defined through ECA evaluation under the operating environment considering the level of residual stress, and from this, pressurization conditions are established when performing MSR.

When performing MSR according to the MSR conditions established above, structural stability is evaluated.

When performing MSR under the above conditions, it is evaluated whether it is structurally stable through elastoplastic-based finite element analysis. This includes safety assessments against global collapse and local failure. The structural stability evaluation evaluates structural soundness when applying the internal pressure set during actual MSR performance, and the degree of soundness is evaluated according to material properties.

As a result of the structural safety evaluation, steel materials that meet the safety evaluation standards are welded and MSR is performed to manufacture a tank. The above safety evaluation criteria considers that stability during MSR performance is secured and it is evaluated that welding can be performed by applying the relevant steel material.

Hereinafter, embodiments of the present invention will be described. Of course, various modifications to the following examples can be made by those skilled in the art without departing from the scope of the present invention. The following examples are for understanding of the present invention, and the scope of the present invention should not be limited to the following examples, but should be determined by the claims described below as well as their equivalents.

(Example)

As shown in Figure 4, LT-FH-36 steel is a material capable of MSR at the design value, but when looking at the actual mill data, the yield ratio is 0.85, which makes MSR application impossible in some cases. In this example, MSR preliminary evaluation was performed on the steel material with the yield ratio of 0.85 (hereinafter referred to as experimental material) to confirm the applicability of MSR.

1. Establishing MSR conditions

To establish the MSR conditions of the test material, evaluation of the residual stress relaxation effect and ECA evaluation considering residual stress were performed. Figure 5 shows the effect of residual stress relaxation according to the MSR pressure level, and shows the effect of relaxation of residual stress in the weld zone when pressure is applied from 1.5 Po (Po = design pressure), which is the minimum pressure level suggested by the IGC code, to 2.0 Po. . At this time, the residual stress relaxation effect is obtained through finite element analysis by simulating the welding process and the MSR process. Specifically, the temperature distribution of the steel during welding is obtained through transient heat transfer analysis, and then the thermal carbon dioxide is calculated by considering the physical properties of the steel by temperature. Welding residual stress and post-MSR residual stress are derived through thermo-elasto-plastic analysis. Although the residual stress effect increases as the pressure increases, it was determined that there was a sufficient residual stress relaxation effect even when performing MSR at a pressure of 1.5 Po, so the 1.5 Po condition was tentatively selected as the MSR condition.

When the residual stress was relaxed under the tentatively selected MSR conditions, fracture mechanics-based ECA evaluation was performed considering the residual stress to verify structural safety.

This includes analysis of applied stress through finite element analysis considering the operating environment, definition of initial defect size according to non-destructive (NDT) method, evaluation of crack propagation, and evaluation of unstable fracture. Figure 6 shows the procedure and evaluation location of the ECA evaluation. As a result of the unstable failure evaluation of one Y-joint, it was evaluated that unstable failure did not occur during the design life, and the final MSR condition of 1.5 Po was established.

2. Structural stability evaluation

Structural stability evaluation was performed using MSR pre-evaluation items using the established MSR conditions, and specifically, elastoplastic-based structural stability evaluation was performed. This includes assessment of global collapse and local failure. Figure 7 is a true stress-true strain diagram determined according to the actual yield strength and tensile strength values of the test material according to the ASME code, and structural stability was evaluated by performing MSR based on the corresponding physical properties.

The regional plastic collapse criterion is an item that evaluates the convergence of the analysis under the relevant loading conditions. If the analysis converges, it is evaluated that regional plastic collapse does not occur. In this example, an excessive load was intentionally applied to derive the point at which the analysis did not converge (diverge), and safety was evaluated by dividing the load value at the point of divergence by the load value under MSR conditions. The results are shown in Figure 8, and it was proven that safety against wide-area plastic collapse was sufficient with a safety rating of 2 or higher.

Meanwhile, the local plastic collapse criterion is that if the sum of the equivalent plastic strain and forming strain in the analysis exceeds the limit strain calculated by the triaxial stress state and material properties in the analysis, local plastic collapse occurs. was considered to have occurred.

)에 의하여 계산되는 한계 변형률(

)보다 작은지 확인한 것이다. Figure 9 shows the results of evaluating local plastic collapse for the Y-joint, which is one of the evaluation positions, and the sum of the equivalent plastic strain and forming residual strain at the point (node) where the maximum equivalent plastic strain occurs is the triaxiality at that point. Stress state (σ _1~3 ) and uniaxial tensile limit strain (

), the limiting strain calculated by (

) was checked to see if it was smaller than .

As can be seen in Figure 9, as a result of evaluating the test material with a yield ratio of 0.85, it was confirmed to have sufficient stability, and through this, the tank manufactured using steel with a strength ratio of 0.85 was evaluated to be structurally safe as a result of the MSR preliminary evaluation. can see.

From the above results, it can be confirmed that the test material in this example has a yield ratio of 0.85, which exceeds the yield ratio of 0.8, but welding and MSR for manufacturing a low-temperature and high-pressure tank can be applied.

Claims (4)

A method of manufacturing a tank by welding steel materials,
Preparing steel for tank manufacturing;
Performing a mechanical stress relief (MSR) preliminary evaluation on the steel material whose yield ratio (TS/YS) exceeds 0.8;
determining structural safety by performing the MSR preliminary evaluation; and
Welding the steel material with suitable structural safety and performing mechanical stress relief (MSR) treatment to manufacture the tank.
Tank manufacturing method comprising.
In claim 1,
The MSR preliminary evaluation is,
establishing MSR conditions; and
Performing structural safety assessment under established MSR conditions
Tank manufacturing method comprising.
In claim 2,
The step of establishing the MSR conditions is a tank manufacturing method obtained by evaluating the effect of residual stress relaxation and evaluating ECA (Engineering Critical Assessment) based on fracture mechanics considering residual stress.
In claim 2,
The structural stability evaluation is a tank manufacturing method including evaluation of global collapse and local failure.

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