JP4134974B2 - High toughness UOE steel pipe for low temperature - Google Patents

Description

  The present invention relates to a low temperature high toughness UOE steel pipe, and in particular, in a low temperature high toughness UOE steel pipe manufactured by double-sided single-layer submerged arc welding, the low temperature high toughness UOE having excellent low temperature toughness on both the inner surface side and the outer surface side. It relates to steel pipes.

  In recent years, energy development has extended to extremely cold regions, and line pipes used there require extremely high low temperature toughness. As the most effective method for ensuring the low temperature toughness of the weld metal in this line pipe, for example, as disclosed in Patent Document 1, welding is performed using a welding material containing Mo, Ti, and B. It is known that it is effective to obtain a fine weld metal structure, and Mo—Ti—B based weld metal is widely used for welded portions of steel pipes that require low temperature toughness.

  Patent Documents 2 and 3 disclose techniques for ensuring low temperature toughness by optimizing not only the composition of weld metal components but also the subsequent heat treatment applied to the line pipe.

However, when the Mo-Ti-B weld metal is used, in the case of double-sided single-layer welding, the inner surface side weld metal is reheated by the outer surface side welding, and as a result, there is a problem that it becomes brittle. As a countermeasure against such reheat embrittlement, the oxygen content of the weld metal is measured (Patent Document 4), the addition of Mn and Mo is suppressed, and Ni is added instead (Patent Document 5). A technique for optimizing rolling conditions after limiting (Patent Document 6) is disclosed.
JP-A-53-63238 Japanese Patent Publication No. 1-38851 Japanese Patent Publication No.2-11654 JP 59-156599 JP-A-62-34694 JP-A-61-266126

  However, none of the above conventional techniques has been an essential solution because the components are adjusted by regarding the weld metal as the same on the inner and outer surfaces.

  The inventors diligently pursued the cause of embrittlement due to reheating of the Mo-Ti-B weld metal. A part of the result is shown in FIG. The figure shows the relationship between the toughness of the weld metal reheat zone and the maximum heating temperature when a reproducible thermal cycle corresponding to welding of the outer surface is applied to a single-sided, single-layer Mo-Ti-B weld metal. From this, the present inventors can obtain good toughness when the Mo-Ti-B weld metal is welded (hereinafter referred to as AW), but in a region reheated to a temperature around 800 to 1200 ° C. It was clarified that the toughness deteriorates significantly.

  That is, as a result of detailed examination of this point, the above-mentioned conventional weld metal having the Mo-Ti-B composition has a fine acicular ferrite (hereinafter referred to as AF) structure in the as-welded portion. It showed toughness, but it changed to upper bainite (hereinafter referred to as UB) having poor toughness in the portion reheated to a temperature in the vicinity of 800 to 1200 ° C., revealing that the toughness was greatly deteriorated. That is, the deterioration of toughness due to reheating is due to the formation of the UB structure, which is considered to be caused by the high hardenability of the weld metal.

  Therefore, the present invention overcomes the above-mentioned deficiencies in the double-sided single-layer submerged arc welding method of a high-toughness UOE steel pipe for low temperature, and is an effective submerged arc welding technique that can prevent embrittlement even by heating by external surface welding And to provide a low temperature high toughness UOE steel pipe having excellent low temperature toughness on both the inner surface side and the outer surface side.

In the present invention, the base material composition is C: 0.10 wt% or less, Si: 0.5 wt% or less, Mn: 2.0 wt% or less, Cu: 0.5 wt% or less, Cr: 0.5 wt% or less, Ni: 0.5 wt% or less , Nb: 0.07 wt% or less, V: 0.07 wt% or less, Mo: 0.3 wt% or less, Ti: 0.1 wt% or less, B: 0.003 wt% or less The steel material comprising the remaining Fe and unavoidable impurities is subjected to double-sided single-layer submerged arc welding using a Ti-B welding steel wire and a SiO ₂ —CaO—CaF ₂ flux.
The composition of the weld metal formed on the inner surface is C: 0.08 wt% or less, Si: 0.5 wt% or less, Mn: 0.8 to 1.8 wt%, Cu: 0.20 wt% or less, Cr: 0.20 wt% or less, Ni : 0.20 wt% or less, V: 0.05 wt% or less, Nb: 0.025 wt% or less, Mo: 0.2 wt% or less, Ti: 0.005 to 0.03 wt%, B: 0.0005 to 0.0030 wt%, N: 0.0080 wt% or less, O: 0.035 wt% or less, balance Fe and inevitable impurities, and Pcm represented by the following formula is 0.110 to 0.170 wt%,
The composition of the outer surface side weld metal formed later is C: 0.10 wt% or less, Si: 0.5 wt% or less, Mn: 0.8 to 1.8 wt%, Cu: 0.20 wt% or less, Cr: 0.20 wt% or less, Ni: 0.20 wt% or less, V: 0.05 wt% or less, Nb: 0.020 wt% or less, Mo: 0.1 to 0.4 wt%, Ti: 0.005 to 0.05 wt%, B: 0.0005 to 0.0060 wt%, N: 0.0080 wt% or less, O: A tough UOE steel pipe for low temperature, characterized by being 0.035 wt% or less, the balance Fe and unavoidable impurities, and Pcm represented by the following formula being 0.140 to 0.200 wt%.
Record

  According to the present invention, the chemical composition of the base material of the low temperature high toughness UOE steel pipe and the chemical composition of the inner surface side and outer surface side weld metal formed by double-sided single layer submerged arc welding are distinguished and limited, respectively. A high-temperature toughness UOE steel pipe having excellent low-temperature toughness can be obtained.

  The reasons for limiting the chemical components of the base material as described above are as follows.

  C: An element that has a great influence on the strength and toughness of the base metal. If it exceeds 0.10 wt%, the toughness and ductility are adversely affected.

  Si: Si is an element that is inevitably contained in the deoxidation process of steel, but should be 0.5 wt% or less in order to improve the toughness of the HAZ part.

  Mn: An extremely important element that improves the strength and toughness of the base material at the same time. However, if it exceeds 2.0 wt%, the upper limit is set to 2.0 wt% because it adversely affects the steel material due to segregation and the like.

  Cu: An element necessary for ensuring the strength of the base material. If the content exceeds 0.5 wt%, the base material and the HAZ portion are hardened, so 0.5 wt% was set as the upper limit.

  Cr: An element necessary for ensuring the strength of the base metal, but if contained in excess of 0.5 wt%, the upper limit is set to 0.5 wt% because the toughness of the HAZ part is deteriorated.

  Ni: An element that improves the strength and toughness of the base metal. However, if the content exceeds 0.5 wt%, the HAZ portion is cured, so 0.5 wt% was made the upper limit.

  Nb, V: It is added to ensure the strength and toughness of the base metal and the HAZ part, but if both Nb and V exceed 0.07wt%, the toughness will be adversely affected. It was set to 0.07 wt% or less.

  Mo: Mo is an element necessary for ensuring the strength of the base material. However, if the content exceeds 0.3 wt%, the HAZ portion is hardened, so 0.3 wt% was set as the upper limit.

  Ti: Ti is an element necessary for securing the toughness of the base material. However, if the Ti content exceeds 0.1 wt%, the upper limit is set to 0.1 wt% in order to deteriorate the toughness of the base material.

  B: B segregates at the austenite grain boundaries during rolling to increase the hardenability, but if it exceeds 0.003 wt%, the upper limit is set to 0.003 wt% in order to deteriorate the toughness of the HAZ part.

  The reason for limiting the component composition of the weld metal is as follows.

  C: C is a highly hardenable component, and its increase generates carbides and martensite and decreases toughness. Therefore, it is necessary to make 0.08 wt% or less on the inner surface side and 0.10 wt% or less on the outer surface side.

  Si: Si is added as a deoxidizer and is also contained in the weld metal, but if it exceeds 0.5 wt% on both the inner surface side and the outer surface side, the toughness decreases.

  Mn: Mn is necessary as a deoxidizer and hardenability component, but if it is less than 0.8 wt% on both the inner and outer surfaces, the effect is poor, while if it exceeds 1.8 wt%, UB is generated and toughness decreases. .

  Cu: Cu is a hardenability component, and it is a component mixed from base metal dilution and wire plating. However, if it exceeds 0.20 wt% on both the inner and outer surfaces, the hardenability becomes excessive and the toughness is impaired. The upper limit was 0.20 wt%.

  Cr: Cr is a hardenability component and is contained by dilution of the base material. However, if it exceeds 0.20 wt% on both the inner and outer surfaces, the hardenability becomes excessive and the toughness is impaired, so the upper limit is 0.20 wt%. It was.

  Ni: Ni is a hardenability component and is contained by dilution of the base material. However, if it exceeds 0.20wt% on both the inner and outer surfaces, the hardenability becomes excessive and the toughness is impaired, so the upper limit is 0.20wt%. It was.

  V: V is a hardenability component and is contained by dilution of the base material. However, if it exceeds 0.05 wt% on both the inner and outer surfaces, the hardenability becomes excessive and the toughness is impaired, so 0.05 wt% is the upper limit. It was.

  Nb: Nb is contained by dilution of the base material, but if it exceeds 0.025wt% on the inner surface side and 0.020wt% on the outer surface side, it will damage toughness, so from the viewpoint of securing toughness, 0.025wt% on the inner surface side. On the outer surface side, the upper limit was 0.020 wt%.

  Mo: Mo is a hardenability component and refines the structure to improve toughness. However, it promotes the formation of UB by reheating, which reduces the toughness of the weld metal on the inner surface, so 0.2 wt% on the inner surface side. It is necessary to keep it below. On the other hand, at least 0.1 wt% is necessary on the outer surface side to improve toughness by refining the structure, but the hardenability added exceeding 0.4 wt% becomes a martensite structure and harms the toughness, so the upper limit is 0.4. wt%.

  Ti: Ti improves the toughness by forming fine ferrite. However, this effect is insufficient if the inner surface and the outer surface are less than 0.005 wt%, so at least 0.005 wt% is required. On the other hand, if it exceeds 0.03 wt% on the inner surface side and 0.05 wt% on the outer surface side, the solid solution Ti increases and the toughness decreases due to precipitation of carbides and nitrides.

  B: B is a high hardenability component, and by synergistic effects with Ti, fine AF is formed and toughness is improved. However, if both the inner surface and outer surface are less than 0.0005 wt%, this effect is poor and the inner surface If it exceeds 0.0030wt% and the outer surface exceeds 0.0060wt%, martensite is generated and the toughness decreases.

  N: N is a component inevitably contained in the weld metal, but nitriding B promotes the formation of intergranular ferrite and reduces toughness, so the inner side and the outer side are both 0.0080 wt% or less. There is a need.

  O: O is a component inevitably contained in the weld metal, but if it exceeds 0.035 wt% on both the inner surface side and the outer surface side, the toughness decreases due to an increase in inclusions in the weld metal.

  Pcm: Pcm indicates the hardenability of the entire weld metal composition. When the hardenability is insufficient, proeutectoid ferrite is precipitated and the toughness is deteriorated. For this reason, 0.140 wt% or more is required on the outer surface side. On the other hand, the inner surface side is reheated by the outer surface side welding, so it must be 0.110 wt% or more. On the other hand, if it exceeds 0.200 wt%, the hardenability becomes excessive and the toughness deteriorates even with welding. Furthermore, in the range of 0.170 to 0.200 wt%, UB precipitates during reheating and the toughness deteriorates. Therefore, it was limited to 0.110 to 0.170 wt% on the inner surface side subjected to reheating, and limited to 0.140 to 0.200 wt% on the outer surface side not subjected to reheating.

  In addition, as a welding wire to be used, C: 0.08 wt% or less, Si: 0.5 wt% or less, Mn: 1.00 to 2.00 wt%, Mo: 0.8 wt% or less, Ti: 0.10 to 0.30 wt%, B: 0.01 to A Ti-B welding wire (welding steel wire) composed of 0.03 wt%, the balance Fe and inevitable impurities is preferable.

  The reason is as follows.

  C: When it exceeds 0.08 wt%, the hardenability of the weld metal becomes excessive and the toughness deteriorates.

  Si: If it exceeds 0.5 wt%, the hardenability of the weld metal becomes excessive and the toughness deteriorates.

  If Mn is less than 1.00 wt%, the strength and toughness of the weld metal are not sufficient, and if it exceeds 2.00 wt%, the hardenability of the weld metal becomes excessive and the toughness deteriorates.

  Mo: If it exceeds 0.8 wt%, the hardenability of the weld metal becomes excessive and the toughness deteriorates.

  If Ti is less than 0.10 wt%, the minimum Ti content of the weld metal cannot be ensured, and if it exceeds 0.30 wt%, toughness deteriorates due to solid solution hardening and precipitation hardening.

  B: If the content is less than 0.01 wt%, the minimum B content of the weld metal cannot be ensured. If the content exceeds 0.03 wt%, the hardenability becomes excessive and the toughness deteriorates.

The SiO ₂ -CaO-CaF ₂ based welding flux _{used, SiO 2: 20~35wt%, CaO} : 15~40wt%, CaF 2: preferably those containing 10 to 30 wt%. This is for increasing the basicity and reducing the amount of oxygen in the weld metal.

  When welding API standard X60 class UO pipe steel plates PA and PB with a thickness of 23.8mm having the chemical composition shown in Table 1, an example according to the present invention method and a comparative example that does not satisfy the limiting requirements of the present method method and We conducted welding under the same welding conditions and compared their toughness. In this comparative test, four-electrode double-sided one-layer submerged arc welding was performed by combining molten fluxes having different chemical compositions as shown in Table 2 and wires having different chemical compositions as shown in Table 3. The welding conditions in this case are as shown in Table 4, and the groove shape is as shown in FIG.

  After the completion of welding, as shown in FIG. 3, 5 mm-sized Charpy impact test pieces were sampled from the inner and outer surface weld metals, respectively, and subjected to Charpy impact tests. Table 5 shows the chemical composition of the weld metal. The impact test results based on the samples taken from the inner surface and outer surface side weld metals are as shown in Table 6.

  As is apparent from Table 6, the weld metal according to the present invention exhibits high toughness on both the inner surface side and the outer surface side that are reheated by the outer surface.

  Incidentally, in changing the composition and Pcm of the weld metal on the inner surface side and the outer surface side, in this example, the welding conditions on the inner and outer surfaces were made the same and different welding compositions were used. In arc welding, since the penetration of the base metal is large, the welding wire composition used on the inner and outer surfaces may be the same and the welding conditions may be changed.

  INDUSTRIAL APPLICABILITY The present invention can be used for pipeline steel pipes, particularly pipeline steel pipes used in extremely cold regions.

It is a characteristic view which shows the relationship between the toughness of a weld metal, and the maximum heating temperature. It is sectional drawing which shows the groove shape in the Example of this invention. It is sectional drawing which shows the Charpy impact test piece collection position in the Example of this invention.

Explanation of symbols

1 Outer side weld metal impact test piece 2 Inner side weld metal impact test piece

Claims (1)

Base material composition is C: 0.10 wt% or less, Si: 0.5 wt% or less, Mn: 2.0 wt% or less, Cu: 0.5 wt% or less, Cr: 0.5 wt% or less, Ni: 0.5 wt% or less, Nb: 0.07 wt% or less, V: 0.07wt% or less, and
Ti: B-based welding of steel material containing one or more selected from Mo: 0.3 wt% or less, Ti: 0.1 wt% or less, B: 0.003 wt% or less, and the balance Fe and inevitable impurities and use steel wire, the submerged arc welding duplex one layer with a flux of SiO ₂ -CaO-CaF ₂ based,
The composition of the weld metal formed on the inner surface is C: 0.08 wt% or less, Si: 0.5 wt% or less, Mn: 0.8 to 1.8 wt%, Cu: 0.20 wt% or less, Cr: 0.20 wt% or less, Ni : 0.20 wt% or less, V: 0.05 wt% or less, Nb: 0.025 wt% or less, Mo: 0.2 wt% or less, Ti: 0.005 to 0.03 wt%, B: 0.0005 to 0.0030 wt%, N: 0.0080 wt% or less, O: 0.035 wt% or less, balance Fe and inevitable impurities, and Pcm represented by the following formula is 0.110 to 0.170 wt%,
The composition of the outer surface side weld metal formed later is C: 0.10 wt% or less, Si: 0.5 wt% or less, Mn: 0.8 to 1.8 wt%, Cu: 0.20 wt% or less, Cr: 0.20 wt% or less, Ni: 0.20 wt% or less, V: 0.05 wt% or less, Nb: 0.020 wt% or less, Mo: 0.1 to 0.4 wt%, Ti: 0.005 to 0.05 wt%, B: 0.0005 to 0.0060 wt%, N: 0.0080 wt% or less, O: A high toughness UOE steel pipe for low temperature, comprising 0.035 wt% or less, balance Fe and unavoidable impurities, and Pcm represented by the following formula being 0.140 to 0.200 wt%.
Record