JP2001079667A - Welded steel structure excellent in resistance to weld cold cracking - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welded steel structure which can be welded without preheating and has better resistance to weld cold cracking and strength than the conventional welded steel structure. SOLUTION: In the welded structure formed by welding a martensite stainless steel or a low alloy steel, the vol. ratio of a retained austenite in respective welded metal part and weld heat affected part satisfies the formula, YS <950-500log(C-30γ). Wherein, YS is a yield stress (MPa) of the base material, γis the vol. ratio of struce of welded metal part to that of weld heat affected part expressed in terms of the vol. ratio of remaining austenite to residual structure and C is initial hydrogen concn. (ppm) in the welded metal part or the weld heat affected part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welded steel pipe produced by welding a steel plate or the like, a line pipe or a tank used for transportation and storage of petroleum or natural gas, etc., which has excellent resistance to low-temperature cracking at welding. It relates to a welded steel structure.

[0002]

2. Description of the Related Art Recently, in order to reduce oil and gas production and transportation costs, from the viewpoint of reducing material costs of oil well pipes and line pipes, such as 13Cr steel and improved 13Cr steel containing low C-Ni,
Inexpensive martensitic stainless steel is being used. In particular, in an area where platform construction is difficult, it is considered that the well is transported from the well to the existing platform through the above-described corrosion-resistant steel pipe for processing.

[0003] A large-diameter steel pipe such as a UO pipe or a spiral pipe is used for a pipeline, and such a pipe is formed by forming a steel sheet into a tube and then welding a seam.
It is laid by connecting the ends of steel pipes of a certain length by girth welding.

Further, tanks in chemical industry plants,
Alternatively, a penstock used for hydroelectric power generation is often manufactured by bonding thick plates by welding. In addition, steel structures such as bridges are also welded.

[0005] In these steel welds, welding cold cracking involving hydrogen is often a problem. Here, hydrogen refers to diffusible hydrogen that diffuses in steel at room temperature (hereinafter referred to as room temperature diffusible hydrogen).

It is well known that the higher the amount (concentration) of this room-temperature diffusible hydrogen contained in steel, the more easily low-temperature welding cracks occur.

[0007] When a coated welding rod is used, this hydrogen source is chemically contained in a fixing material component such as an organic substance as a coating material component, water of crystallization contained in a mineral component, and sodium silicate. Moisture, moisture on the joint surface, etc. Therefore, in order to reduce the diffusible hydrogen concentration at room temperature and prevent low-temperature cracking in welding, preheating for the purpose of drying the welding rod and drying the joint surface (heat to about 100 ° C before welding)
Is being performed.

However, this preheating requires an increase in the number of welding steps, equipment for preheating, management of the preheating temperature, and the like, which causes an increase in manufacturing cost.

[0009] Therefore, studies have been made from a long time on the chemical composition of steel so that the prevention of low-temperature cracking of welding can be achieved at a preheating temperature as low as possible. A typical study result is the introduction of Pcm (weld cold crack susceptibility index).

[0010] This Pcm is, like C equivalent, C, S
It is a polynomial expressed by the following equation, with the amount of alloy elements such as i and Mn as terms.

Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 2
0 + Mo / 15 + V / 10 + 5B As can be seen from this equation, the Pcm value increases as the amount of alloying elements in the steel increases. Experimentally, Pcm, preheating temperature,
The correlation with the amount of diffusible hydrogen at room temperature has been clarified, and steel having a Pcm suitable for each welding method is used. Low Pcm steels have been developed because a lower Pcm is advantageous for preventing welding cold cracking.

However, when high strength is to be obtained with a low Pcm steel, for example, in the case of a thick plate, quenching, tempering,
Transformation strengthening by accelerated cooling after rolling is used,
Due to the small number of alloying elements, there is naturally a limit to strengthening.
Further, since Pcm itself is an index indicating a limit, it is shown from the beginning that welding exceeding the limit is not possible. Further, the low Pcm steel has a problem of softening in the heat affected zone (hereinafter, referred to as HAZ).

[0013] In recent years, from the viewpoint of weight reduction and cost reduction, higher strength has been desired, and welding can be performed without preheating (called preheating free) without preheating, without using Pcm. There is a demand for the development of steel having excellent low-temperature cracking resistance.

[0014]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a welded steel structure which can be welded without preheating and which is superior to conventional welded steel structures in terms of low-temperature crack resistance and high strength. .

[0015]

SUMMARY OF THE INVENTION The present inventors have proposed a method of drying a welding rod for reducing hydrogen and preheating steel, if the molten metal at the time of welding or the hydrogen dissolved in HAZ (hereinafter referred to as "these metals"). Are collectively referred to as insoluble hydrogen), and various experiments and examinations were conducted with attention paid to the relationship between the solute hydrogen and welding stress, and the following findings were obtained.

A) In order to lower the room temperature diffusible hydrogen concentration in the weld metal portion or HAZ, it is preferable to trap solid solution hydrogen as much as possible in the cooling process up to 200 ° C. after welding to form room temperature non-diffusible hydrogen. .

B) Oxides have conventionally been considered to be effective as hydrogen trap sites, but they have no effect in the cooling process from the high temperature region of the molten metal immediately after welding to 200 ° C.

C) The structure of steel includes ferrite composed of a BCC crystal lattice and austenite composed of an FCC crystal lattice. It is known that the latter has a higher solid solubility of hydrogen than the former. However, general low-alloy steels are transformed into an austenitic phase at high temperatures and into a ferrite phase at low temperatures, so it was thought that hydrogen dissolved in the austenite phase diffuses into the ferrite phase together with the ferrite transformation. It is effective to generate retained austenite to form trap sites.

D) When residual austenite is used as a trap site, room temperature diffusible hydrogen can be represented by the following formula. The larger the amount of retained austenite, the smaller the room temperature diffusible hydrogen. Room temperature diffusible hydrogen = [Solute hydrogen content-30 x Retained austenite volume fraction (%)] e) However, since austenite itself has low strength,
As the retained austenite increases, the strength decreases,
It is necessary to keep the amount of retained austenite that can prevent low-temperature cracking of the weld while ensuring the strength of the steel.

F) Welding cold cracking is related to the yield stress and the amount of diffusible hydrogen at room temperature, and can be summarized by the following equation. If the austenite amount satisfies this equation, the strength can be secured.
Welding low-temperature cracking can be prevented.

YS <950-500log (C-30γ) where YS: Yield stress (MPa) of the base metal γ: Volume ratio of the following structures of the weld metal or weld heat affected zone: Retained austenite volume / remainder structure The volume occupied by C: the solid solution hydrogen concentration (ppm) of the molten metal portion or the weld heat affected zone at the time of welding The present invention has been made based on such knowledge,
The summary is as follows.

(1) By weight%, C: 0.05% or less, C
A welded steel structure formed by welding a martensitic stainless steel containing r: 9 to 15% and Ni: 2 to 10%, wherein a retained austenite in a weld metal portion and a weld heat affected zone is formed. A welded steel structure excellent in low-temperature cracking resistance with a volume ratio satisfying the following formula.

(2) By weight%, C: 0.4% or less, N
i: a welded steel structure formed by welding low alloy steel containing 10% or less and 50% or more of the metal structure is bainite, martensite, or a mixed structure of bainite and martensite. A welded steel structure having excellent low-temperature cracking resistance in which the volume fraction of retained austenite in the weld metal portion and the weld heat affected zone satisfies the following expression.

YS950-500log (C-30γ) where YS: yield stress (MPa) of the base metal γ: volume ratio of the following structures in the weld metal or weld heat affected zone: occupied by the volume of residual austenite volume / remainder structure Volume C: solid solution hydrogen concentration (ppm) in the molten metal part or the heat affected zone during welding (ppm) The solid solution hydrogen concentration C in the equation (3) is set to the following value according to the welding method, or (2) A welded steel structure having excellent low-temperature cracking resistance.

C: maximum solid solution hydrogen concentration (ppm) in the molten metal portion or the weld heat affected zone during welding Covered arc welding (SMAW): C value during moisture absorption = 18 Covered arc welding (SMAW): Normal C Value = 9 Smooth arc welding (SAW): Normal C value = 5 Metal active welding (MAG): Normal C value = 2 metal inert gas welding (MIG): Normal C value = 2 tank stainless inert gas welding (MIG) TIG): Normal C value = 2 The term "welded steel structure" as used in the present invention refers to all articles formed by welding steel. And connected line pipes, tanks,
Examples include penstocks and bridges used in hydroelectric power generation.

Hereinafter, embodiments of the present invention will be described.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION The welded structure of the present invention is made of martensitic stainless steel or low alloy steel, and is formed by welding these steels as described above.

Martensitic stainless steel having the following chemical composition is suitable for welded structures such as line pipes and tanks.

The following low alloy steels are suitable for welded structures such as penstocks and bridges for hydraulic power generation.

The reasons for defining the chemical composition of each steel are as follows. In addition, all percentages of the chemical composition indicate% by weight.

1) Martensitic stainless steel C: C is an element which remarkably hardens the weld metal part and the weld heat-affected zone to increase the low-temperature cracking susceptibility.
Must be 5% or less. Preferably, it is as low as 0.03% or less.

Cr: Cr is an element that enhances the resistance to corrosion of carbon dioxide gas, and at least 9% is required to obtain its effect. On the other hand, if it exceeds 15%, the effect will be saturated,
The upper limit was set to 15%. Preferably it is 10.5 to 13%.

Ni: Ni is a low C-Cr containing martensitic stainless steel and is an essential element for obtaining a martensitic structure, and 2% or more is required to obtain its effect. On the other hand, if it exceeds 11%, the effect is saturated and the strength is reduced, so the upper limit is set to 11%. Preferably it is 4 to 9%.

Further, if necessary, the following elements can be contained.

Si: Si has a deoxidizing effect and is an element effective for strengthening. When the content is 0.01%, the effect is small, and when it exceeds 1%, the effect is saturated and the toughness is reduced.
It is preferably set to １1%.

Mn: Mn has a deoxidizing effect and is effective as an austenite-forming element for controlling the structure of martensite and retained austenite. When Mn is contained, the content is preferably in the range of 0.02 to 2%.

Mo: Mo is effective for improving corrosion resistance, particularly hydrogen sulfide cracking resistance, and is preferably 0.1 to 3%. Cu: Cu has an effect of improving corrosion resistance and is effective for adjusting the structure, and is preferably about 0.1 to 1%. Al: Al has a deoxidizing effect, and is preferably about 0.01 to 1%.

Ti: Ti has an effect of stabilizing the strength by fixing C, and when it is contained, it is contained in an amount of 0.01 to 0.3%.
2% is preferred.

2) Low-alloy steel C: C is effective in promoting the generation of retained austenite, but it increases the hardness of the welded portion, so it must be 0.4% or less. The lower limit is desirably 0.05%. Ni: Ni is an element effective for generating retained austenite, and is contained in an amount of 10% or less. If it exceeds 10%, the amount of retained austenite becomes excessive, so the upper limit was made 10%. The lower limit is not particularly limited, but is preferably 0.2%.

Further, if necessary, the following elements can be contained.

Si, Al and Co are effective for producing retained austenite, and when they are contained, one or more of them is preferably in the range of 0.1 to 4.

Mn, Cu, Cr and Mo are effective elements for increasing the strength, and preferably 0.05% or more. However, if contained in a large amount, the toughness of the welded portion is deteriorated. It is better to make it 05% or less.

V, Nb, Ti and B have the effect of increasing the hardness of the welded portion, and it is preferable that each of them is 0.05% or less.

Although N is effective in forming retained austenite, it is preferable that the content of N is 0.007% or less, and more preferably 0.005% or less, because it makes the ferrite phase brittle.

In the above low alloy steel, C and Ni are important for generating retained austenite, respectively, and therefore C and Ni are particularly specified. Metal structure: The metal structure of low alloy steel is such that 50% or more of the metal structure is bainite, martensite, or a mixed structure of bainite and martensite for the following reasons.

In order to form residual austenite, it is necessary to suppress the precipitation of cementite in the bainite structure or to set the martensite end temperature to room temperature or lower. In such a steel, when the content of bainite or martensite or both is less than 50%, retained austenite is not generated, and it is impossible to prevent low-temperature cracking in welding.
Other than bainite and / or martensite are ferrite, pearlite, retained austenite, and the like.

3) YS <950-500log (C-30γ) Weld low-temperature cracking occurs in the range of about 200 ° C. or less to room temperature, and room-temperature diffusible hydrogen in the weld metal or HAZ causes cracking. The above equation is an equation for preventing the occurrence of low-temperature cracking in welding. YS is the yield stress (MPa) of the base metal, γ is the volume ratio of the following structure of the weld metal or HAZ, and C is the melting when welding. Shows the solid solution hydrogen concentration (ppm) of the metal part or HAZ.

Γ = volume of retained austenite / volume occupied by the remaining structure (C-30γ) on the right side of this equation is a term indicating the diffusible hydrogen concentration at room temperature, and the calculated value is the critical stress. The critical stress is a maximum stress at which a low-temperature welding crack does not occur in a weld metal portion or a HAZ. If the residual stress generated after welding is not less than the critical stress, low-temperature welding cracks will occur.

The left side of the equation is the yield stress of the base steel itself, Y
S. Since the maximum value of the residual stress after welding can be assumed to be a stress equal to the yield stress, unless the critical stress shown on the right side exceeds YS, low-temperature welding cracks occur. Therefore, YS is the largest YS of the standard of the steel type used.
It is desirable to keep it.

As understood from the equation, the larger the term of the diffusible hydrogen concentration at room temperature, the lower the critical stress. That is, cracking easily occurs with low stress.

By setting the amount of retained austenite so as to satisfy this equation, it is possible to prevent the occurrence of low-temperature cracking in welding.

In order to form retained austenite in the weld metal portion or HAZ, alloy elements such as Mn and Ni for stabilizing austenite may be increased, and by adjusting the content of the austenite stabilizing element,
The amount of retained austenite can be adjusted. To determine the retained austenite volume fraction, the weld metal and H
A thin film test piece can be obtained from AZ and determined by X-ray diffraction.

As described above, in the present invention, the alloying element can be increased as opposed to the low Pcm steel which lowers the alloying element,
As a result, it is possible to easily obtain high strength while ensuring low-temperature crack resistance against welding.

An accurate value can be obtained by measuring the solid solution hydrogen concentration in the molten metal or HAZ at the time of this welding in a state in which the diffusion of diffusible hydrogen is prevented by rapid cooling immediately after welding. The dissolved hydrogen concentration in Examples described later is an actually measured value. From the viewpoint of control or material design, it is desirable that the maximum concentration of the solute hydrogen be determined in advance in consideration of the atmosphere and welding conditions in each welding method. By doing so, there is no need to analyze solid solution hydrogen during actual welding. The dissolved hydrogen concentration may be set to the following value (unit: ppm). This is not the case when the hydrogen concentration can be controlled through the welding conditions.

Covered arc welding (SMAW): C value at the time of moisture absorption = 18 Covered arc welding (SMAW): C value at the time of normal = 9 Supmer sheet arc welding (SAW): C value at the time of normal = 5 metal active welding ( MAG): Normal C value = 2 Metal inert gas welding (MIG): Normal C value = 2 Tank stainless steel gas welding (TIG): Normal C value = 2

[0056]

(Example 1) Martensitic stainless steel having the six chemical compositions shown in Table 1 was melted in a small vacuum melting furnace of 150 kg to form an ingot, hot forged and then hot-rolled. A steel plate having a thickness of 25 mm was manufactured. 9 each steel plate
Heating was carried out at 50 ° C. for 1 hour, followed by water quenching, followed by heating at 640 ° C. for 1 hour and tempering to adjust the strength to X80 class.

[0057]

[Table 1]

On the other hand, a welding wire was produced by drawing a part of the forged material.

Using these steel plates and welding wires,
Weld cracking susceptibility was evaluated by a y-cracking test specified in JIS Z3158. Welding is performed using SAW, which is a general method for welding pipes, and T, which is generally used as a welding method for pipes
IG and SMAW.

The presence or absence of cracks was determined by an optical microscope at a magnification of 100, and the starting points were distinguished between the weld metal and the HAZ.

The solid solution hydrogen concentration analysis was carried out by
A test steel plate having a size of 2 mm, a width of 25 mm, and a length of 100 mm was sampled and subjected to bead-on welding under the same welding conditions, the same atmosphere, and the same opportunity as in the y crack test to produce a weld metal.
The concentration of dissolved hydrogen in the weld metal of low alloy steel is determined according to JIS.
The amount of diffusible hydrogen was measured according to the gas chromatography method according to the provisions of Z3118. In addition, HA of low alloy steel
Z was measured by a temperature rising analysis method. The reason why the method prescribed in JIS Z3118 is not used for martensitic stainless steel is that hydrogen is not released sufficiently at a temperature prescribed in JIS due to slow release of hydrogen.

The temperature rising analysis method means that a test piece is heated in a vacuum from room temperature to about 1000 ° C. at a temperature rising rate of 1 to 20 ° C./min, and the amount of released hydrogen gas relative to the temperature is increased. This is a method in which a hydrogen release curve is obtained by plotting the peaks and a hydrogen release curve is obtained, and the amount of released hydrogen in one peak portion of the curve including a peak formed at around 100 to 200 ° C. is defined and analyzed as a diffusible hydrogen amount.

The concentration of dissolved hydrogen in the weld metal of martensitic stainless steel was measured by a temperature rising analysis method. Also,
The solid solution hydrogen concentration of HAZ in martensitic stainless steel was measured by a temperature rising analysis method using the same test piece as the thin film test piece for measuring retained austenite shown below.

For analysis of retained austenite, a thin film specimen was sampled from the weld metal and HAZ, and the volume fraction of retained austenite was determined by X-ray diffraction. Note that H
A test piece from AZ was taken along the melting line within 1 mm from the melting line after macroetching.

Table 2 shows the obtained dissolved hydrogen concentration, residual austenite volume ratio, and y-crack test results. Also,
A tensile test is performed on each steel sheet to determine the yield stress (YS).
Their maximum YS was 660 Mpa.

[0066]

[Table 2]

Example 2 HT80-grade carbon steel high-strength low-alloy steels having the six chemical compositions shown in Table 3 were melted in a small vacuum melting furnace of 150 kg to form ingots and hot forged. A steel plate having a thickness of 50 mm was manufactured by post-hot rolling.
Each steel sheet was heated at 900 ° C. for 1 hour and water-quenched, and further heated at 600 ° C. for 1 hour and tempered. The maximum YS was 920 MPa. For the welding wire, test numbers 8 to 1
2 was prepared from Example 1B, test number 1
For No. 3, a commercially available wire for HT80 was used.

[0068]

[Table 3]

Using the steel sheet and the welding wire, welding was performed by the MAG method in a low hydrogen atmosphere. Although SMAW is generally used as a welding method, the MAG method was used because the hydrogen concentration in this welding method was too high and there was no method that could prevent cracking without preheating.

After welding, a Y crack test was performed in the same manner as in Example 1, and the concentration of dissolved hydrogen and the volume fraction of retained austenite were determined. Further, a tensile test was performed on each steel sheet to determine a yield stress (YS), and the maximum YS was 900 MPa. Table 4 shows the results.

[0071]

[Table 4]

As is apparent from Tables 2 and 4, YS <
In the region satisfying 950-500 log (C-30γ), no crack occurs in both the weld metal portion and the HAZ. Further, as a conventional example, a result of welding of a general HT80 is shown. In the preheating-free condition, cracking occurred even in the MAG method.

[0073]

According to the present invention, it is possible to perform preheating-free welding that is easy to carry out welding work, and it is not necessary to use a low Pcm steel. A steel structure can be obtained.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masahiko Hamada 4-33, Kitahama, Chuo-ku, Osaka-shi, Osaka Prefecture Inside Sumitomo Metal Industries, Ltd. (72) Kazuya Fujiwara 4-chome, Kitahama, Chuo-ku, Osaka-shi, Osaka No.5-33 F-term in Sumitomo Metal Industries, Ltd. (reference) 4E001 AA03 BB01 BB05 BB07 BB08 BB12 CA03 CC03

Claims (3)

[Claims] (1) C: 0.05% or less, Cr: 9% by weight
A welded steel structure formed by welding martensitic stainless steel containing 〜15% and Ni: 2 ： 11%, wherein the volume fraction of each retained austenite in the weld metal part and the weld heat affected zone is A welded steel structure having excellent low-temperature cracking resistance, characterized by satisfying the following formula. YS <950-500log (C-30γ) where, YS: Yield stress of base metal (MPa) γ: Volume ratio of the following structures in the weld metal or weld heat affected zone: Retained austenite volume / volume occupied by remaining structure C: concentration of dissolved hydrogen (ppm) in the molten metal part or the heat affected zone during welding 2. C: 0.4% or less, Ni: 10% by weight
% Or less, and 50% or more of the metal structure is a welded steel structure formed by welding a low alloy steel that is bainite, martensite, or a mixed structure of bainite and martensite, A welded steel structure having excellent low-temperature cracking resistance, characterized in that the volume fraction of retained austenite in a weld metal part and a weld heat affected zone satisfies the following equation. YS <950-500log (C-30γ) where, YS: Yield stress of base metal (MPa) γ: Volume ratio of the following structures in the weld metal or weld heat affected zone: Retained austenite volume / volume occupied by remaining structure C: concentration of dissolved hydrogen (ppm) in the molten metal part or the heat affected zone during welding 3. A welded steel structure having excellent low-temperature cracking resistance according to claim 1, wherein the solid solution hydrogen concentration C in the formula is set to the following value according to the welding method. . C: Maximum dissolved hydrogen concentration (ppm) in the molten metal part or welding heat affected zone during welding Covered arc welding (SMAW): C value during moisture absorption = 18 Covered arc welding (SMAW): Normal C value = 9 Sulfur Seat Arc Welding (SAW): Normal C value = 5 Metal Active Welding (MAG): Normal C value = 2 Metal inert gas welding (MIG): Normal C value = 2 Tank stainless steel gas welding (TIG): Normal C value = 2