JP2596868C - - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welded structure such as a spherical tank exposed to a hydrogen sulfide atmosphere.
More specifically, the present invention relates to a welded structure having excellent HIC resistance and SSC resistance. [0002] Welded structures such as LPG and gas storage spherical tanks exposed to a hydrogen sulfide atmosphere may have hydrogen-induced cracking (HIC) or sulfide stress corrosion cracking (SSC) during use. When these occur, they have a great influence on the reliability of the structure, so that various strict requirements are made. As concrete examples of steel and steel HAZ, NACE is proposed to reduce non-metallic inclusions such as sulfides (MnS) and to control the form thereof to prevent HIC, and to prevent SSC, The maximum hardness is 22 or less in Rockwell C hardness (equivalent to Vickers hardness of 248 or less, hereinafter Hv248
Below), and Ni to 1% or less. In order to satisfy these characteristics, a method of securing the characteristics such as the strength of the steel material by using a method such as a controlled rolling control cooling process while suppressing the addition elements to a low level is currently adopted. On the other hand, in the case of a weld metal, the same characteristics as those of a base metal and a heat affected zone are required in order to ensure the reliability of a structure. Regarding weld metal, unlike steel,
It is said that non-metallic inclusions are spherical because they do not go through a rolling process, and that HIC does not occur because they are very fine compared to steel. As with the base metal, SSC can occur under certain conditions, and the danger increases as hardenability increases. However, since the properties of the weld metal are almost determined by the cooling process and the components of the weld metal determined by the welding conditions, the same properties as the base metal cannot often be obtained with the same composition. Therefore, conventionally, in order to obtain the same characteristics as the base metal, the alloy element of the weld metal has been set higher than that of the base metal to solve this problem. Although this method is indispensable for securing the strength and toughness of the weld metal, the addition of an alloy element has the effect of increasing the hardenability of the weld metal, and is not preferred from the viewpoint of SSC resistance. This tendency becomes more remarkable especially when the weld metal is hardened by the influence of heat by the subsequent welding. According to the prior art, as disclosed in Japanese Patent Application Laid-Open No. 63-2588, the requirement that the maximum hardness proposed by NACE be set to Hv248 or less is applied to a weld metal to secure SSC resistance. However, it has been usual to secure properties such as toughness. But,
The method of securing strength and toughness while maintaining a low hardness of Hv248 or less is naturally limited in view of the fact that the originally contradictory characteristics are compatible. In particular, when the strength of the base metal is improved and the same strength is required for the weld metal, it is extremely difficult to achieve both low hardness and high strength. This problem becomes more difficult when performing repair welding or the like during the use of a structure, because it is likely that the weld metal, which has more added elements than the base metal, will harden under the influence of the heat. . [0005] In order to overcome such a problem, not only does the weld metal satisfy Hv 248 or less, but even if Hv 248 or less is not satisfied, the weld metal has a low hardness. (Hv 248 or less) must be achieved, and the base material HAZ must have properties equivalent to the SSC resistance. In other words, there is a need for an invention of a weld metal in which the highest hardness that does not cause SSC (hereinafter, simply referred to as the limit hardness) exceeds Hv248. This is significant when repair welding becomes necessary during the use of a welded structure, considering that the weld metal may not satisfy the condition of Hv248 or less depending on the welding conditions. That is, an object of the present invention is to provide a welded structure characterized by having a weld metal exhibiting good SSC resistance even when the condition of Hv248 or less is not satisfied. Means for Solving the Problems The present inventors have paid attention to the above circumstances, and since the weld metal has a difference that the amount of oxygen is much larger than that of the base metal, I am convinced that the SSC sensitivity must be different from that of the base metal, and I have mainly studied the effect of components in the weld metal on the SSC sensitivity. The present invention has been completed as a result of such research, and the constitution is that the base material portion is, by weight%, C: 0.02 to 0.06%, Si: 0.6% or less, Mn; 1.0 to 1.6%, P: 0.020% or less, S: 0.001% or less,
Al: 0.001 to 0.060%, Nb: 0.005 to 0.04%, Ti;
0.005 to 0.030%, Ca; 0.001 to 0.006%, N: 0.006% or less, if necessary, Mo; 0.05 to 0.5%, Ni; 0.05 to 0. 5%, C
u: 0.05 to 0.5%, V: 0.01 to 0.10%, one or more kinds are contained, the balance being iron and unavoidable impurities, and the molten metal is represented by the formula (1) Pwm: 0.25 to 0.43%, and C: 0.03 to 0.14%, Si: 0.1 to 0.6%, Mn: 0.70 to 1.8 %, P; 0.02% or less, S;
0.02% or less, Ti: 0.005 to 0.05%, B: 0.0006 to 0.00
5%, O: 0.015 to 0.05%, Al: 0.01 to 0.055%, and if necessary, Cr: 0.5% or less, Mo: 0.5% or less, Ni: 1 0.0% or less, C
u; a welded structure containing 1.0% or less of one or more kinds, and the balance being iron and unavoidable impurities. The hardness of the weld metal exceeds 248 in Vickers hardness, and in a NACE environment. Not causing SSC, or Pwm; 0.25
-0.55%, C: 0.03-0.14%, Si: 0.1-0.6%, Mn: 0
. 70 to 1.8%, P: 0.02% or less, S: 0.02% or less, B: 0.000
Less than 6%, O: 0.015 to 0.08%, Al: 0.01 to 0.055%,
If necessary, one or more of Cr: 0.5% or less, Mo: 0.5% or less, Ni: 1.0% or less, and Cu: 1.0% or less are contained, with the balance being iron and A gist exists for a welded structure comprising unavoidable impurities, wherein the hardness of the weld metal is greater than 248 in Vickers hardness and does not cause SSC in a NACE environment. Pwm = C + Si / 24 + Mn / 5 + Cr / 7 + Mo / 4 + Ni / 10 + Cu /
14 (1) Hereinafter, the present invention will be described in detail. First, the base material will be described. The HIC resistance and SSC resistance of the base material have been conventionally adopted. Reduction of sulfide in the base material and control of its morphology, and achievement of the maximum hardness of the base material and HAZ by securing Hv248 or less. The component range for that will be described below. C and Mn are indispensable components for securing the strength toughness of the base material. But,
Excessive addition increases the hardenability too much, so the range is 0.02 to 0, respectively.
. 06%, and 1.0 to 1.6%. If the added amount of Si is too large, the HAZ toughness deteriorates, so the upper limit was made 0.6%. Although P and S are impurities in the present invention, they degrade the toughness of the base material and HAZ, and S forms sulfides.
1%. [0008] Al is used in an amount of 0.0 from the viewpoint of the amount required for deoxidation and the amount that does not cause deterioration in toughness.
01-0.060%. Nb is required to be 0.005% in order to improve the strength due to the precipitation effect.
The upper limit is set to 0.04% to suppress the hardness. Ti is indispensable as TiN, which is effective for miniaturization of the base material and HAZ. However, excessive addition of both Ti and N deteriorates the toughness of the base material and the HAZ, so the range is made 0.005 to 0.030% and 0.006% or less. Ca is necessary for controlling the form of sulfides and for improving toughness. However, if CaO and CaS are generated in large amounts, on the contrary, toughness is deteriorated. 006%. Although the above description is about the basic components, the base material portion of the structure of the present invention may further contain one or more of the following additional components as required. [0009] Mo improves the strength and toughness of the base material. However, too much Mo causes deterioration of toughness and weldability. The lower limit is set to 0.05% in the sense that a substantial effect can be obtained. Regarding the lower limit, the same applies to Ni and Cu. The upper limit of Ni is set to 0.5% from the viewpoint that HAZ toughness is not adversely affected. Cu has an upper limit of 0.5% from the viewpoint of preventing Cu cracks during the production of the base material. V contributes to the precipitation effect similarly to Nb, but does not perform as much as Nb.
The range was set to 0.01 to 0.10%. The steel having the above component range is subjected to controlled rolling control cooling or quenching and tempering as required, so that the strength is increased to 50 kgf / mm ² or more, and good HIC resistance and low hardness (Hv248) are obtained. Below), that is, it is possible to ensure good SSC resistance. Next, the weld metal will be described. It is said that HIC does not occur in the weld metal as described above. SS
As for C, good SSC resistance has been ensured by satisfying Hv248 or less for the base material and HAZ, but good SSC resistance is exhibited for the weld metal even if the condition of Hv248 or less is not satisfied. Must be set. First, with respect to C, Si, Mn, Cr, Mo, Ni, and Cu, all of these components have the highest hardness (critical hardness) that does not cause SSC, considering the SSC resistance of the weld metal. Work). However, due to the different degrees of influence, (1
) The index Pwm calculated by the formula was introduced. By setting Pwm ≦ 0.55%, good SSC resistance can be obtained. Conversely, if Pwm> 0.55%, SSC may not be prevented even if Hv248 or less proposed by NACE is satisfied. It is for this reason that the upper limit of Pwm is set to 0.55%. However, the upper limit of Pwm also varies depending on other elements. That is, the upper limit of Pwm is reduced by B described later. However, since B is added to the weld metal together with Ti and is very effective in improving toughness, the present inventors considered that it was unreasonable not to add B to the weld metal. Thus, 0.43% has been found as the upper limit at which good SSC resistance can be obtained even when B is added. On the other hand, if Pwm ≦ 0.43% is satisfied, Hv2 is satisfied regardless of whether B is added or not.
Although good SSC resistance can be obtained without satisfying 48 or less, Pwm <
In the range of 0.25%, it is difficult to secure the same strength as the base metal, and the maximum hardness is Hv248 or less even at a low heat input that can be considered during normal welding. This is the same as securing SSC resistance by the conventional technology. Therefore, P
If the range of wm <0.25% is out of the range of the present invention, the lower limit of Pwm is set to 0.
. 25%. Next, each component of the weld metal will be described for reasons for limiting its range. If C is excessively added, it hardens excessively, causes solidification cracking during welding, and sometimes lowers SSC resistance.
%. The lower limit of 0.03% is the minimum value for securing strength and hardenability. Mn, like C, is indispensable for securing strength and toughness. The lower limit of Mn, 0.70%, is the minimum value necessary to secure these. Conversely, Mn is 1
. If it exceeds 8%, excessive hardenability occurs and the limit hardness becomes Hv248 or less. Therefore, this value was made the upper limit. [0015] Si is a deoxidizing element, and greatly affects the amount of O described later. In addition, Si
Has a great influence on workability during welding work. From the viewpoint of ensuring SSC resistance, the lower the Si content, the better (the greater the O content, the better). However, the lower limit is set to 0 from the viewpoint of ensuring sufficient workability for practical use and preventing excessive O increase. 0.1%. The upper limit is set to 0.6% from the viewpoint of excessive increase in hardenability, deterioration in toughness, and decrease in SSC resistance due to decrease in O content. [0016] P and S are impurities in the present invention. However, P and S are likely to segregate at the grain boundaries in the weld metal, which may cause problems of SR embrittlement and deterioration of toughness. Therefore, the upper limits are each set to 0.020%. B lowers the critical hardness by adding it. Therefore, the lower limit of Pwm shown in the equation (1) may be set to 0.55% when the function of B is not large and less than 0.0006%, but the upper limit of Pwm is set to 0 when adding more. .43
%. This means that by adding B, C, Si, Mn, Cr,
This means that the selection range of Mo, Ni, and Cu is narrowed, which is not preferable. However, when B is added together with Ti, the toughness of the weld metal can be remarkably improved. In addition, the decrease in the critical hardness due to the addition of B has a very small effect on the critical hardness of B when B is 0.001% or more. From the above, the present inventors considered that B addition was practically effective even when the range of Pwm was narrowed. Lower limit of B addition
0006% is the minimum value for achieving a substantial effect. However, if B is added in excess, it segregates at the austenite grain boundaries and suppresses the growth of proeutectoid ferrite.
Since excessive hardenability is caused, the upper limit of the addition of B is set to 0.005%. O is the only component among the weld metal components handled in the present invention that can increase the limit hardness by increasing the addition amount, that is, can improve the SSC resistance. However, most of O is present as nonmetallic inclusions in the weld metal and acts as nuclei for producing ferrite. Therefore, if added excessively, the hardenability becomes too low, and the possibility that predetermined toughness cannot be ensured arises. come. Furthermore, when B is present in the weld metal, O combines with B to form B ₂ O ₃ , which may reduce effective B. Therefore, when adding B, the upper limit of O is set to 0.05 as a range that does not impair the effect.
%. In the case where B is not added, 0.1 is set as the upper limit for securing the predetermined toughness.
08%. The lower limit of 0.015% is the minimum value for obtaining good SSC resistance. Al forms nonmetallic inclusions with O. If the amount of Al is insufficient, sufficient deoxidation is not performed, and excess O of the weld metal reacts with C, Si, Mn, or the like, so that a predetermined hardenability may not be secured. The lower limit of 0.01% of Al is the minimum value for sufficient deoxidation. On the other hand, excessive addition of Al may cause excessive deoxidation, and the amount of O in the weld metal may not be in an appropriate range. The upper limit of Al was set to 0.055% as a maximum value for securing an appropriate amount of O. The above is the description of the basic components in the weld metal. In addition, one or more of Cr, Mo, Ni, Cu, and Ti can be added as necessary. Cr and Mo are elements to be added in order to make use of the excellent characteristics of the present invention. However, Cr and Mo, like C and Mn, have the function of increasing the hardenability and are more expensive than C and Mn. Therefore, the upper limit of each is set to 0.5%. Cu has a small effect on the SSC resistance as can be understood by comparing the coefficient of Cu in Pwm with that of Mn, Cr, Mo and the like. Therefore, the selection range can be made wider than Cr and Mo. However, this does not mean that there is no effect at all, and excessive addition causes the toughness of the molten metal and further the weld metal after SR to be deteriorated, so the upper limit was made 1.0%. Ni has a relatively small coefficient in Pwm like Cu. However, excessive addition increases the hardenability of the weld metal excessively and causes deterioration of toughness. Therefore, the upper limit is set to 1.0%. Ti should be added together with B and used for improving the toughness of the weld metal. Ti
The lower limit of 0.005% is the minimum amount necessary to obtain a substantial effect. Ti becomes TiN, TiO, etc. and acts as a nucleus for producing ferrite, and has an effect of forming a fine structure. On the other hand, it forms precipitates such as TiC, and excessive presence thereof causes deterioration of toughness of weld metal. Therefore, the upper limit is set to 0.05%. Although the component ranges of the base metal and the weld metal have been described above, the component ranges of the weld metal are not limited by the welding method. Means for limiting the components of the weld metal to a predetermined range include limiting the components of the welding wire or controlling the components of the welding flux, but the weld metal component after welding falls within the predetermined range. If there is, it does not depend on these means. Example A base material having the components shown in Table 1 was subjected to a maximum hardness test of a heat affected zone in accordance with JIS Z3101, and the maximum hardness of the base material and HAZ was measured. These were evaluated for SSC resistance. According to the present invention, the maximum hardness of the weld heat affected zone is Hv
It can be seen that it does not exceed 248 and shows good SSC resistance. Furthermore, the HIC cracking rate (CAR) in a NACE environment measured by ultrasonic testing was also shown. Compared with the comparative example, the present invention does not generate HIC and shows good HIC resistance. Also, in the case of the present invention, no crack was observed in the case of the present invention, even when the four-point bending SSC test was performed while applying a bending stress corresponding to the actual yield stress. To make various welded joints, two beads are placed as shown in FIG.
An SSC test piece 3 was collected from the portion shown in FIG. In FIG. 1, the second bead 2
Is for giving a thermal effect to the first bead 1, and the maximum hardness of the first bead can be controlled by changing the heat input amount of the second bead. The three welding methods used for the first bead are submerged arc welding (SAW), manual welding (SMAW), and gas shield welding (GMAW). These three welding methods are general and typical welding methods. Except for TIG welding on stainless steel and self-shielded welding where it is difficult to secure the oxygen content in the weld metal, most of the welding structures used in sour environments are constructed using these three welding methods. Is done. The second bead is used to control the maximum hardness of the first bead and the base material heat-affected zone, but when SSC is generated from the second bead, the first bead or the base material heat-affected zone is formed. Since it is no longer an SSC test, for the second bead, the heat input 5k
The combination of the Y-3 wire and the shielding gas in Table 2D that did not cause SSC even at j / cm was used. Tables 2A, 2B and 2C show the components of the welding material used in each welding method. Table 2 shows welding conditions and welding materials used for the first bead. Tables 3A and B are S
It is a summary of SC test results. The chemical composition of the weld metal refers to the composition of the first bead. The limit hardness is equivalent to the actual yield stress, by preparing a welded joint in which the maximum hardness of the first bead is changed by the second bead, collecting an SSC specimen from each joint as shown in FIG. Bending stress was applied in a NACE environment and determined as the highest hardness without SSC. According to the present invention, SSC does not occur in the base metal and the base metal heat-affected zone, and the critical hardness of the weld metal is all higher than Hv248, which is satisfactory without satisfying the condition of Hv248 or less proposed by NACE. It can be seen that excellent SSC resistance can be obtained. On the other hand, in the comparative example, YC3, YC4, YD2, YE2, YE3, YF2, YG
2, MA3, SA, SF, etc., SSC or HIC occurs in the base metal or base metal HAZ before reaching the limit hardness of the molten metal, and the limit hardness of the weld metal cannot be determined; The critical hardness of the weld metal is Hv24, like SL, SU, SV, SY
Some of them are below 8. [Table 1] [Table 2A] [Table 2B] [Table 2C] [Table 2D] [Table 2E] [Table 3A] [Table 3B] According to the present invention, the standard for preventing SSC, Hv248, which has been widely believed so far, has been used.
A welded structure having a weld metal exhibiting good SSC resistance can be provided without necessarily satisfying the following conditions. This means that the range of choice of the weld metal component is widened, and it is expected to improve the reliability of the welded structure, such as securing SSC resistance and other characteristics, and improve the efficiency of repair welding work. The usefulness is enormous.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing a welded joint subjected to a welding test; [Description of Signs] 1 First bead 2 Second bead 3 SSC test piece 4 Base material

Claims (1)

Claims: 1. The base material portion contains, by weight%, C: 0.02 to 0.06%, Si;
6% or less, Mn: 1.0 to 1.6%, P: 0.020% or less, S: 0.001%
Hereinafter, Al: 0.001 to 0.060%, Nb: 0.005 to 0.04%, Ti
0.005-0.030%, Ca; 0.001-0.006%, N; 0.00
6% or less, the balance being steel consisting of iron and unavoidable impurities, wherein the weld metal satisfies the index Pwm represented by the following formula (1);
C: 0.03 to 0.14%, Si: 0.1 to 0.6%, Mn: 0.70 to 1.8
%, P: 0.02% or less, S: 0.02% or less, Ti: 0.005 to 0.05%
, B: 0.0006 to 0.005%, O: 0.015 to 0.05%, Al;
0.01 to 0.055%, with the balance being a welded structure composed of iron and unavoidable impurities, wherein the hardness of the weld metal exceeds 248 in Vickers hardness and does not cause SSC in a NACE environment. Welded structures. Pwm = C + Si / 24 + Mn / 5 + Cr / 7 + Mo / 4 + Ni / 10 + Cu
/ 14 (1) 2. The welded structure according to claim 1, wherein the weld metal further comprises Cr: 0.5% or less, Mo: 0.5% or less, Ni; 0% or less, Cu;
A welded structure further comprising 1.0% or less of one or more types. 3. A steel material equivalent to the base material according to claim 1, wherein the weld metal satisfies an index Pwm of 0.25 to 0.55% in weight% and C; 0.
03-0.14%, Si; 0.1-0.6%, Mn; 0.70-1.8%, P;
0.02% or less, S: 0.02% or less, B: less than 0.0006%, O: 0.01
5 to 0.08%, Al; 0.01 to 0.055%, the balance being iron and inevitable impurities. 4. The welded structure according to claim 3, wherein the weld metal further contains, by weight%, Cr: 0.5% or less, Mo: 0.5% or less, Ni: 1.0% or less, Cu: A welded structure further containing 1.0% or less of one or more of them. 5. The welded structure according to claim 1, wherein the base metal is M
o: 0.05 to 0.5%, Ni: 0.05 to 0.5%, Cu: 0.05 to 0.5
%, V; a welded structure further comprising one or more kinds in the range of 0.01 to 0.10%. 6. The welded structure according to claim 2, wherein the base material comprises M
o: 0.05 to 0.5%, Ni: 0.05 to 0.5%, Cu: 0.05 to 0.5
%, V; a welded structure further comprising one or more kinds in the range of 0.01 to 0.10%. 7. The welded structure according to claim 3, wherein the base material is M
o: 0.05 to 0.5%, Ni: 0.05 to 0.5%, Cu: 0.05 to 0.5
%, V; a welded structure further comprising one or more kinds in the range of 0.01 to 0.10%. 8. The welded structure according to claim 4, wherein the base metal is M
o: 0.05 to 0.5%, Ni: 0.05 to 0.5%, Cu: 0.05 to 0.5
%, V; a welded structure further comprising one or more kinds in the range of 0.01 to 0.10%.