JP3624758B2 - A welded steel structure with excellent cold cracking resistance - Google Patents

Description

【００５８】
一方、鍛造材の一部を用いて伸線により溶接ワイヤーを作製した。
【００５９】
これらの各鋼板と溶接ワイヤーを用いて、ＪＩＳ Ｚ３１５８に規定されるｙ割れ試験によって溶接低温割れ感受性を評価した。溶接は、製管溶接法として一般的なＳＡＷ、その管の周溶接法として一般的なＴＩＧ及びＳＭＡＷとした。
【００６０】
割れの発生有無は、１００倍の光学顕微鏡により判定すると共に、発生起点を溶接金属とＨＡＺで区別した。
【００６１】
固溶水素濃度分析は、上記鋼板から厚さ１２ｍｍ、幅２５ｍｍ、長さ１００ｍｍの試験鋼板を採取し、ｙ割れ試験と同じ溶接条件、同じ雰囲気、同じ機会に、ビードオン溶接をおこない、溶接金属を作製した。そして、低合金鋼の溶接金属の固溶水素濃度は、ＪＩＳ Ｚ３１１８の規定に従って、拡散性水素量をガスクロマトグラフ法に従って測定した。また、低合金鋼のＨＡＺは、昇温分析法により測定した。なお、マルテンサイトステンレス鋼に対しては、ＪＩＳ Ｚ３１１８に規定の方法を用いないのは、ＪＩＳに規定の温度では、水素の放出が遅くて十分放出されないためである。
【００６２】
なお、昇温分析法とは、真空中で試験片を室温から１０００℃近傍まで１〜２０℃／分の昇温速度で昇温して、放出される水素ガス量を温度に対してプロットして水素放出曲線を求め、１００〜２００℃近傍にできるピークを含む曲線一山部分の放出水素量を拡散性水素量と定義して分析する方法である。
【００６３】
マルテンサイトステンレス鋼の溶接金属部の固溶水素濃度は、昇温分析法により測定した。また、マルテンサイトステンレス鋼のＨＡＺの固溶水素濃度は、以下に示す残留オーステナイト測定用の薄膜試験片と同じ試験片を用いて昇温分析法により測定した。
【００６４】
また、残留オーステナイトの分析は、溶接金属およびＨＡＺから薄膜試験片を採取して、Ｘ線回析法により残留オーステナイト体積率を求めた。なお、ＨＡＺからの試験片は、マクロエッチした後、溶融線から１ｍｍの範囲で溶融線に沿って採取した。
【００６５】
求めた固溶水素濃度および残留オーステナイト体積率およびｙ割れ試験結果を表２に示す。また、各鋼板の引張試験をおこない降伏応力（ＹＳ）を求め、それらの最大のＹＳを６６０Ｍｐａとした。
【００６６】
【表２】

【００６７】
（実施例２）
表３に示す６種の化学組成のＨＴ８０級の炭素鋼系高強度低合金鋼を、１５０キロの小型真空溶解炉で溶製してインゴットとし、熱間鍛造した後熱間圧延によって板厚５０ｍｍの鋼板を製造した。各鋼板を９００℃で１時間加熱して水焼入れし、さらに、６００℃で１時間加熱して焼き戻した。最大ＹＳは９２０ＭＰａとした。溶接ワイヤーは、試験番号８〜１２については実施例１Ｂから作製したもの、試験番号１３については市販のＨＴ８０用のワイヤーを用いた。
【００６８】
【表３】

【００６９】
この鋼板と溶接ワイヤとを用いて低水素雰囲気のＭＡＧ法で溶接をおこなった。溶接方法としてはＳＭＡＷが一般的であるが、この溶接方法では水素濃度が高くなりすぎて、予熱しないで割れを防止できる方法が見あたらないので、ＭＡＧ法とした。
【００７０】
溶接後、実施例１と同様の方法でＹ割れ試験をおこなうと共に、固溶水素濃度、および残留オーステナイト体積率を求めた。また、各鋼板の引張試験をおこない降伏応力（ＹＳ）を求め、それらの最大のＹＳを９００ＭＰａとした。結果を表４に示す。
【００７１】
【表４】

【００７２】
表２および４から明らかなように、ＹＳ＜９５０−５００ｌｏｇ（Ｃ−３０γ）を満足する領域では、溶接金属部およびＨＡＺ共に割れが発生していない。また、従来例として、一般的なＨＴ８０の溶接結果を示した。予熱フリーでは、ＭＡＧ法でも割れが生じた。
【００７３】
【発明の効果】
本発明によれば、溶接施工が容易な予熱フリーの溶接ができ、低Ｐｃｍ鋼とする必要はなく、低Ｐｃｍ鋼より耐溶接低温割れ性に優れ、かつ、高強度の溶接鋼構造物を得ることができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a welded steel structure excellent in welding cold cracking resistance, such as a welded steel pipe manufactured by welding steel plates and the like, a line pipe and a tank used for transportation and storage of oil and natural gas, and the like.
[0002]
[Prior art]
Recently, in order to reduce oil and gas production and transportation costs, low-cost martensitic stainless steels such as 13Cr steel and modified 13Cr steel with low C-Ni content are used from the viewpoint of reducing material costs for oil well pipes and line pipes. Steel is being used. In particular, in an area where it is difficult to build a platform, it is also considered that the existing platform is transported from the well through the corrosion-resistant steel pipe as described above.
[0003]
The pipe line is a large-diameter steel pipe such as a UO pipe or a spiral pipe. Such a pipe is formed by welding a seam after forming a steel plate into a tubular shape. Are connected by circumferential welding.
[0004]
Moreover, a tank in a chemical industry plant or a hydraulic iron pipe used for hydroelectric power generation is often manufactured by laminating thick plates by welding. In addition, steel structures such as bridges are also welded.
[0005]
In these steel welds, cold cracking involving hydrogen is often a problem. Here, hydrogen is diffusible hydrogen that diffuses in steel at room temperature (hereinafter referred to as room temperature diffusible hydrogen).
[0006]
It is well known that the higher the amount (concentration) of this room temperature diffusible hydrogen contained in the steel, the easier the welding cold cracking occurs.
[0007]
This hydrogen source, when a coated welding rod is used, is an organic substance that is a coating material component, crystallization water contained in a mineral component, moisture contained chemically in a fixing material component such as sodium silicate, The moisture on the surface of the joint portion. Therefore, in order to reduce the room temperature diffusible hydrogen concentration and prevent cold welding cracking, preheating (heating to around 100 ° C. before welding) for the purpose of drying the welding rod and the surface of the joint is performed. ing.
[0008]
However, this preheating requires an increase in the welding process, equipment for preheating, management of the preheating temperature, and the like, which increase the manufacturing cost.
[0009]
Therefore, studies have been made from the viewpoint of the chemical composition of steel for a long time so that welding cold cracking prevention can be achieved at as low a preheating temperature as possible. A typical study result is the introduction of Pcm (welding cold cracking susceptibility index).
[0010]
This Pcm is a polynomial represented by the following equation with the amount of alloying elements such as C, Si, Mn, etc. as the term, such as C equivalent.
[0011]
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
As can be seen from this equation, the Pcm value increases as the amount of alloy elements in the steel increases. Experimentally, correlation with Pcm, preheating temperature, and room temperature diffusible hydrogen content has been clarified, and steel having Pcm suitable for each welding method is used. Low Pcm steel has been developed because a lower Pcm is advantageous for preventing welding cold cracking.
[0012]
However, when high strength is to be obtained with low Pcm steel, for example, in thick plates, transformation strengthening by quenching, tempering and accelerated cooling after rolling is utilized, but naturally there is little alloying element to strengthen. There is a limit. Further, since Pcm itself is an index indicating a limit, it is indicated from the beginning that welding exceeding the limit is impossible. Furthermore, in the low Pcm steel, there is a problem of softening at the heat affected zone (hereinafter referred to as HAZ).
[0013]
Recently, from the viewpoint of weight reduction and cost reduction, further enhancement of strength is desired, and the preheating temperature can be lowered without relying on Pcm, and if possible, welding can be performed without preheating (referred to as preheating free). Development of steel with excellent crackability is desired.
[0014]
[Problems to be solved by the invention]
An object of the present invention is to provide a welded steel structure that can be welded without preheating and is superior to conventional welded steel structures in welding cold cracking resistance and high strength.
[0015]
[Means for Solving the Problems]
In the case where the welding rod is not dried to reduce hydrogen or the steel is not preheated, the inventors of the present invention will refer to molten metal in welding or hydrogen dissolved in HAZ (hereinafter collectively referred to as solute hydrogen). ) Is inevitable, and as a result of various experiments and examinations focusing on the relationship between solid solution hydrogen and welding stress, the following knowledge was obtained.
[0016]
a) In order to reduce the room temperature diffusible hydrogen concentration in the weld metal part or HAZ, it is preferable to trap the solute hydrogen as much as possible in the cooling process up to 200 ° C. after welding to make room temperature non-diffusible hydrogen.
[0017]
b) Conventionally, it was considered that an oxide was effective as a hydrogen trap site, but there is no effect at all in the cooling process from a high temperature region of molten metal immediately after welding to 200 ° C.
[0018]
c) The structure of steel includes ferrite composed of BCC crystal lattice and austenite composed of FCC crystal lattice. The latter is known to have higher hydrogen solubility than the former. However, since general low alloy steel transforms into an austenite phase at high temperatures and a ferrite phase at low temperatures, hydrogen that was dissolved in the austenite phase was thought to diffuse in the ferrite phase along with the ferrite transformation. It is effective to generate retained austenite to form a trap site.
[0019]
d) When residual austenite is used as a trap site, room temperature diffusible hydrogen can be expressed by the following formula, and the amount of room temperature diffusible hydrogen can be reduced as the amount of residual austenite increases.
Room temperature diffusible hydrogen = [amount of dissolved hydrogen-30 x volume fraction of retained austenite (%)] e) However, since austenite itself has low strength, the strength decreases as the amount of retained austenite increases, ensuring the strength of the steel. However, it is necessary to make the amount of retained austenite capable of preventing welding cold cracking.
[0020]
f) Weld cold cracking is related to the yield stress and the amount of room temperature diffusible hydrogen, and can be organized by the following formula. If the amount of austenite satisfies this formula, the strength can be secured and weld cold cracking should be prevented. Can do.
[0021]
YS <950-500 log (C-30γ)
Where YS: Yield stress (MPa) of the base material
γ: volume ratio of retained austenite of the structure shown below in the weld metal or weld heat affected zone / volume occupied by the remaining structure C: solid solution hydrogen concentration (ppm) in the molten metal or weld heat affected zone during welding Was made based on such findings, and the gist is as follows.
[0022]
(1) A welded steel structure formed by welding martensitic stainless steel containing C: 0.05% or less, Cr: 9-15% and Ni: 2-10% by weight% A welded steel structure excellent in welding cold cracking resistance in which the volume fraction of retained austenite in the weld metal part and weld heat affected zone satisfies the following formula.
[0023]
(2) By weight%, C: 0.4% or less, Ni: 10% or less, 50% or more of the metal structure is bainite or martensite, or a mixed structure of bainite and martensite It is a welded steel structure formed by welding a low alloy steel, and the volume fraction of retained austenite in the weld metal part and weld heat-affected zone is excellent in welding cold cracking resistance satisfying the following formula Welded steel structure.
[0024]
YS950-500log (C-30γ)
Where YS: Yield stress (MPa) of the base material
γ: volume ratio of retained austenite of the structure shown below in the weld metal or weld heat affected zone / volume occupied by the remaining structure C: solid solution hydrogen concentration (ppm) in the molten metal or weld heat affected zone during welding (3 The welded steel structure having excellent weld cold cracking resistance according to (1) or (2), wherein the solid solution hydrogen concentration C in the formula is set to the following value according to the welding method.
[0025]
C: Maximum solute hydrogen concentration (ppm) in the molten metal part or welding heat affected zone during welding
Covered arc welding (SMAW): C value at the time of moisture absorption = 18
Covered arc welding (SMAW): C value at normal time = 9
Submerged arc welding (SAW): C value at normal time = 5
Metal active welding (MAG): Normal C value = 2
Metal inert gas welding (MIG): Normal C value = 2
Tungsten inert gas welding (TIG): Normal C value = 2
The welded steel structure as used in the present invention refers to all articles formed by welding steel. Representative examples include welded pipes, line pipes connected by circumferential welding of steel pipes, tanks, hydraulic power. Examples include hydraulic iron pipes and bridges used in power generation.
[0026]
Embodiments of the present invention are described below.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
The welded structure of the present invention is made of martensitic stainless steel or low alloy steel, and is a steel structure as described above formed by welding these steels.
[0028]
Martensitic stainless steel having the following chemical composition is suitable for welded structures such as line pipes and tanks.
[0029]
The following low alloy steels are suitable for welded structures such as hydraulic iron pipes and bridges for hydroelectric power generation.
[0030]
The reason for defining the chemical composition of each steel is as follows. In addition,% display of chemical composition shows weight% altogether.
[0031]
1) Martensitic stainless steel C:
C is an element that significantly hardens the weld metal part and the weld heat-affected zone to increase the sensitivity to weld cold cracking, and needs to be 0.05% or less. Preferably, 0.03% or less is better.
[0032]
Cr:
Cr is an element that enhances the resistance to carbon dioxide gas corrosion, and 9% or more is necessary to obtain the effect. On the other hand, if it exceeds 15%, the effect is saturated, so the upper limit was made 15%. Preferably it is 10.5 to 13%.
[0033]
Ni:
Ni is a low C—Cr-containing martensitic stainless steel and is an essential element for obtaining a martensite structure, and 2% or more is necessary to obtain the effect. On the other hand, if it exceeds 11%, the effect is saturated and the strength is lowered, so the upper limit was made 11%. Preferably it is 4 to 9%.
[0034]
Furthermore, the following elements can be contained if necessary.
[0035]
Si:
Si is an element that has a deoxidizing action and is effective for strengthening. The effect is small at 0.01%, and if it exceeds 1%, the effect is saturated and the toughness is also reduced, so 0.01 to 1%. Is preferred.
[0036]
Mn:
Mn has a deoxidizing action and is effective as an austenite-forming element because it adjusts the structure of martensite and retained austenite. When it is contained, the range of 0.02 to 2% is preferable.
[0037]
Mo:
Mo is effective in 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 action and is preferably about 0.01 to 1%.
[0038]
Ti:
Ti has the effect of fixing C and stabilizing the strength. When Ti is contained, 0.01 to 0.2% is preferable.
[0039]
2) Low alloy steel C:
C is effective in promoting the formation of retained austenite, but it increases the hardness of the welded portion, so it needs to be 0.4% or less. The lower limit is preferably 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%.
[0040]
Furthermore, the following elements can be contained if necessary.
[0041]
Si, Al, and Co are effective for the production of retained austenite, and when included, any one or more of them should be in the range of 0.1 to 4.
[0042]
Mn, Cu, Cr and Mo are effective elements for increasing the strength and are preferably 0.05% or more. However, if contained in a large amount, the toughness of the welded portion is deteriorated. It is good to make it.
[0043]
V, Nb, Ti and B have the effect of increasing the hardness of the welded portion, and it is preferable that all be 0.05% or less.
[0044]
N is effective for the formation of retained austenite, but it makes the ferrite phase brittle, so it should be 0.007% or less, preferably 0.005% or less.
[0045]
In the low alloy steel, C and Ni are particularly important for producing retained austenite, so C and Ni are particularly specified.
Metal structure:
The metal structure of the low alloy steel is bainite or martensite for 50% or more of the metal structure for the following reasons, or a mixed structure of bainite and martensite.
[0046]
In order to produce residual austenite, it is necessary to suppress the precipitation of cementite in the bainite structure or to lower the martensite finish temperature to room temperature or lower. In such steel, when bainite, martensite, or both are less than 50%, retained austenite is not generated and welding cold cracking cannot be prevented. Other than bainite and / or martensite, ferrite, pearlite, retained austenite, and the like.
[0047]
3) YS <950-500 log (C-30γ)
Weld cold 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 formula is for preventing the occurrence of welding cold cracking, YS is the yield stress (MPa) of the base metal, γ is the volume ratio of the following structure of the weld metal part or HAZ, and C is the melting when welding The solid solution hydrogen concentration (ppm) of the metal part or HAZ is shown.
[0048]
γ = residual austenite volume / volume occupied by the remaining structure (C-30γ) on the right side of this equation is a term indicating the room temperature diffusible hydrogen concentration, but the calculated value is the critical stress. The critical stress is the maximum stress at which a low temperature weld crack is not generated in the weld metal part or HAZ. If the residual stress generated after welding is equal to or greater than the critical stress, low temperature weld cracks will occur.
[0049]
The left side of the equation is the yield stress, YS, of the base steel itself. Since the maximum value of the residual stress after welding can be assumed to be a stress equal to the yield stress, low-temperature weld cracking occurs unless the critical stress indicated on the right side exceeds YS. Therefore, it is desirable to set YS as the maximum YS in the standard of the steel type used.
[0050]
As can be understood from the equation, the critical stress decreases as the term of the room temperature diffusible hydrogen concentration increases. That is, it becomes easy to crack at a low stress.
[0051]
By setting the amount of retained austenite so as to satisfy this equation, it is possible to prevent the occurrence of welding cold cracking.
[0052]
In order to generate retained austenite in the weld metal part or HAZ, it is only necessary to increase alloy elements such as Mn and Ni that stabilize austenite. By adjusting the content of the austenite stabilizing element, the amount of retained austenite Can be adjusted. In order to obtain the retained austenite volume fraction, a thin film test piece is collected from the weld metal part and HAZ, and can be obtained by an X-ray diffraction method.
[0053]
As described above, in the present invention, it is possible to increase the alloy element as opposed to the low Pcm steel that lowers the alloy element, and as a result, it is possible to easily obtain high strength while ensuring the resistance to welding cold cracking.
[0054]
An accurate value can be obtained for the solid solution hydrogen concentration in the molten metal or HAZ at the time of welding if it is measured immediately after welding to prevent diffusion of diffusible hydrogen. The solid solution hydrogen concentration in the examples described later is an actual measurement value. From the viewpoint of management or material design, the solid solution hydrogen concentration is desirably determined in advance as a maximum value in consideration of the atmosphere and welding conditions in each welding method. By doing so, it is not necessary to analyze solute hydrogen during actual welding. It is better to set the solid solution hydrogen concentration to the following value (unit: ppm). However, this is not the case when the hydrogen concentration can be controlled through the welding conditions.
[0055]
Covered arc welding (SMAW): C value at the time of moisture absorption = 18
Covered arc welding (SMAW): C value at normal time = 9
Submerged arc welding (SAW): C value at normal time = 5
Metal active welding (MAG): Normal C value = 2
Metal inert gas welding (MIG): Normal C value = 2
Tungsten inert gas welding (TIG): Normal C value = 2
[0056]
【Example】
(Example 1)
A martensitic stainless steel having six chemical compositions shown in Table 1 was melted in an ingot by a 150 kg small vacuum melting furnace, hot forged, and then hot rolled to produce a steel plate having a thickness of 25 mm. Each steel plate was heated at 950 ° C. for 1 hour and water quenched, and further heated at 640 ° C. for 1 hour and tempered to adjust the strength to X80 grade.
[0057]
[Table 1]

[0058]
On the other hand, a welding wire was produced by drawing using a part of the forged material.
[0059]
Using each of these steel plates and welding wires, the sensitivity to welding cold cracking was evaluated by a y-cracking test specified in JIS Z3158. Welding was SAW, which is a common pipe-welding method, and TIG and SMAW, which are general pipe-welding methods.
[0060]
The presence / absence of cracks was determined by a 100 × optical microscope, and the generation start point was distinguished from weld metal and HAZ.
[0061]
Solid solution hydrogen concentration analysis is to collect a test steel plate with a thickness of 12 mm, a width of 25 mm, and a length of 100 mm from the above steel plate, and perform bead-on welding in the same welding conditions, the same atmosphere and the same opportunity as the y-cracking test. Produced. And the solid solution hydrogen concentration of the weld metal of low alloy steel measured the amount of diffusible hydrogen according to the gas chromatograph method according to prescription | regulation of JISZ3118. The HAZ of the low alloy steel 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, at the temperature prescribed in JIS, the release of hydrogen is slow and not sufficiently released.
[0062]
The temperature rise analysis method is a method in which the test piece is heated in a vacuum from room temperature to around 1000 ° C. at a rate of 1 to 20 ° C./min, and the amount of released hydrogen gas is plotted against the temperature. Thus, a hydrogen release curve is obtained, and the amount of released hydrogen at the peak portion of the curve including the peak formed at around 100 to 200 ° C. is defined as the amount of diffusible hydrogen and analyzed.
[0063]
The solid solution hydrogen concentration in the weld metal part of martensitic stainless steel was measured by a temperature rising analysis method. Moreover, the solid solution hydrogen concentration of HAZ of martensitic stainless steel was measured by the temperature rising analysis method using the same test piece as the thin film test piece for residual austenite shown below.
[0064]
For analysis of retained austenite, a thin film specimen was collected from the weld metal and HAZ, and the volume fraction of retained austenite was determined by X-ray diffraction. In addition, the test piece from HAZ was extract | collected along the melting line in the range of 1 mm from a melting line, after carrying out macroetching.
[0065]
The obtained solid solution hydrogen concentration, residual austenite volume fraction, and y crack test results are shown in Table 2. Moreover, the tensile test of each steel plate was performed, the yield stress (YS) was calculated | required, and those maximum YS was set to 660 Mpa.
[0066]
[Table 2]

[0067]
(Example 2)
HT80 grade carbon steel high-strength low-alloy steels with six chemical compositions shown in Table 3 were melted in an ingot by a 150 kg small vacuum melting furnace, hot forged, and then hot-rolled to a thickness of 50 mm The steel plate was manufactured. Each steel plate was heated and quenched at 900 ° C. for 1 hour, and further tempered by heating at 600 ° C. for 1 hour. The maximum YS was 920 MPa. For the test numbers 8 to 12, the welding wire prepared from Example 1B was used, and for the test number 13, a commercially available wire for HT80 was used.
[0068]
[Table 3]

[0069]
Using this steel plate and welding wire, welding was performed by the MAG method in a low hydrogen atmosphere. SMAW is generally used as the welding method. However, since the hydrogen concentration is too high in this welding method and there is no method that can prevent cracking without preheating, the MAG method was used.
[0070]
After welding, a Y crack test was performed in the same manner as in Example 1, and the solid solution hydrogen concentration and the retained austenite volume fraction were determined. Moreover, the tensile test of each steel plate was performed, yield stress (YS) was calculated | required, and those maximum YS was 900 MPa. The results are shown in Table 4.
[0071]
[Table 4]

[0072]
As apparent from Tables 2 and 4, in the region where YS <950-500 log (C-30γ) is satisfied, neither the weld metal part nor HAZ cracks. Moreover, the welding result of general HT80 was shown as a prior art example. In preheating free, cracking occurred even in the MAG method.
[0073]
【The invention's effect】
According to the present invention, preheating-free welding that is easy to weld can be performed, and it is not necessary to use low Pcm steel, and a welded steel structure that is superior to low Pcm steel in welding cold crack resistance and that has high strength is obtained. be able to.

Claims (3)

A welded steel structure formed by welding martensitic stainless steel containing, by weight%, C: 0.05% or less, Cr: 9-15%, and Ni: 2-11%, and weld metal A welded steel structure excellent in welding cold crack resistance, characterized in that the volume fraction of retained austenite in the weld zone and the weld heat affected zone satisfies the following formula:
YS <950-500 log (C-30γ)
Where YS: Yield stress (MPa) of the base material
γ: Volume ratio of retained austenite of the structure shown below in the weld metal or weld heat affected zone / volume occupied by the remaining structure C: Solid solution hydrogen concentration (ppm) in the molten metal or weld heat affected zone during welding Low alloy containing, by weight%, C: 0.4% or less, Ni: 10% or less, and 50% or more of the metal structure is bainite or martensite, or a mixed structure of bainite and martensite It is a welded steel structure formed by welding steel, and the welded low temperature cracking resistance is characterized in that the volume fraction of retained austenite in the weld metal part and the weld heat affected zone satisfies the following formula: Excellent welded steel structure.
YS <950-500 log (C-30γ)
Where YS: Yield stress (MPa) of the base material
γ: Volume ratio of retained austenite of the structure shown below in the weld metal or weld heat affected zone / volume occupied by the remaining structure C: Solid solution hydrogen concentration (ppm) in the molten metal or weld heat affected zone during welding 3. The welded steel structure having excellent weld cold crack resistance according to claim 1, wherein the solid solution hydrogen concentration C in the formula is set to the following value according to a welding method.
C: Maximum solute hydrogen concentration (ppm) in the molten metal part or welding heat affected zone during welding
Covered arc welding (SMAW): C value at the time of moisture absorption = 18
Covered arc welding (SMAW): C value at normal time = 9
Submerged arc welding (SAW): C value at normal time = 5
Metal active welding (MAG): Normal C value = 2
Metal inert gas welding (MIG): Normal C value = 2
Tungsten inert gas welding (TIG): Normal C value = 2