Patent Application
20240051070
This application relates to a submerged arc welded joint, particularly, to a welded joint in a steel structure for cryogenic environment use that is formed by welding of a high-Mn content steel material and that is resistant to the occurrence of hot cracking during the welding process and has high strength and excellent cryogenic impact toughness.
Environmental regulations are more rigorous in recent years. The demand is increasing for liquefied natural gas (hereinafter, also written as LNG) that does not contain sulfur and is thus regarded as a clean fuel without emission of air pollutants, such as sulfoxides. To ensure that LNG can be transported or stored, LNG transportation or storage containers (tanks) are required to maintain excellent cryogenic impact toughness at or below a temperature of −162° C. that is the liquefaction temperature of LNG.
To satisfy excellent cryogenic impact toughness that is required, for example, aluminum alloys, 9% Ni steel, and austenite stainless steel are conventionally used as materials for containers (tanks) for the above and other purposes.
However, aluminum alloys have low tensile strength and entail increasing of the wall thickness of a structure that is designed. Aluminum alloys are also low in weldability. Furthermore, 9% Ni steel is economically disadvantageous because an expensive Ni-based material should be used as the welding material. Furthermore, austenite stainless steel has drawbacks in that it is expensive, and the strength of the base material is low.
Due to these problems, recent studies of materials for LNG transportation or storage containers (tanks) are directed to high-Mn content steel containing about 10 to 35% Mn in terms of mass % (hereinafter, also written as “high-Mn steel”). High-Mn steel is characterized in that the steel has an austenite phase even at a cryogenic temperature and does not undergo brittle fracture, and also in that the steel has high strength compared with austenite stainless steel. Thus, there are demands for the development of welding materials capable of stably welding such high-Mn content steel materials.
To meet such demands, for example, Patent Literature 1 discloses a “cryogenic high-Mn steel material”. The “cryogenic high-Mn steel material” disclosed in Patent Literature 1 is such that the steel material contains, by mass %, C: 0.001 to 0.80%, Mn: 15.0 to 35.0%, S: 0.001 to 0.01%, Cr: 0.01 to 10.0%, Ti: 0.001 to 0.05%, N: 0.0001 to 0.10%, and O:0.001 to 0.010%; the P content is limited to P: 0.02% or less; the steel material further contains one or both of Si: 0.001 to 5.00% and Al: 0.001 to 2.0%, and further contains one, or two or more of Mg: 0.01% or less, Ca: 0.01% or less, and REM: 0.01% or less in a total content of 0.0002% or more; the steel material satisfies 30C+0.5Mn+Ni+0.8Cr+1.2Si+0.8Mo≥25 . . . (Formula 1) and O/S≥1 (Formula 2); the balance is Fe and incidental impurities; the austenite volume fraction is 95% or more; the austenite grain size is 20 to 200 μm; and the carbide coverage ratio at austenite grain boundaries is 50% or less. In the high-Mn steel material disclosed in Patent Literature 1, the austenite grain size is controlled to an appropriate size to ensure that carbides that have occurred at grain boundaries will not serve as fracture origins or as cracking propagation pathways. The austenite grain size is appropriately controlled by appropriately controlling the amounts of the alloying elements as well as the balance thereof, and further by appropriately controlling the amounts of S and O and by adding Mg, Ca, and REM. The literature describes that this control can also reduce the coarsening of the grain size in welded heat affected zones.
Furthermore, Patent Literature 2 discloses a “cryogenic steel plate”. The “cryogenic steel plate” disclosed in Patent Literature 2 is such that the steel plate contains, by mass %, C: 0.30 to 0.65%, Si: 0.05 to 0.30%, Mn: more than 20.00% and less than 30.00%, Ni: 0.10% or more and less than 3.00%, Cr: 3.00% or more and less than 8.00%, Al: 0.005 to 0.100%, and N: 0.0050% or more and less than 0.0500%; the P, S, and O contents are limited to P: 0.0040% or less, S: 0.020% or less, and O:0.0050% or less; the balance is Fe and impurities; the Mn segregation ratio XMn (XMn=Mn1/Mn0) calculated from the Mn concentration Mn1 at a Mn-rich portion and the Mn concentration Mn0 at a Mn-poor portion is 1.6 or less; the yield stress and the tensile stress at room temperature (25° C.) are 400 MPa or more and 800 MPa or more, respectively; and the Charpy impact absorbed energy (vE−196) of a welded heat affected zone is 70 J or more. According to the technique described in Patent Literature 2, the steel plate as hot-rolled can be provided as a material for LNG transportation or storage containers (tanks).
Furthermore, Patent Literature 3 discloses a “high-strength welded joint having excellent cryogenic impact toughness” and a “flux-cored arc welding wire for the welded joint”. The “flux-cored arc welding wire” disclosed in Patent Literature 3 is a wire that has a composition including, by wt %, C: 0.15 to 0.8%, Si: 0.2 to 1.2%, Mn: 15 to 34%, Cr: 6% or less, Mo: 1.5 to 4%, S: 0.02% or less, P: 0.02% or less, B: 0.01% or less, Ti: 0.09 to 0.5%, N: 0.001 to 0.3%, TiO2: 4 to 15%, a total of one or more selected from SiO2, ZrO2, and Al2O3: 0.01 to 9%, a total of one or more selected from K, Na, and Li: 0.5 to 1.7%, and one or more of F and Ca: 0.2 to 1.5%, the balance being Fe and incidental impurities. Patent Literature 3 describes that welding with the flux-cored arc welding wire disclosed therein can effectively produce a welded joint that has excellent low-temperature toughness with an absorbed energy of 27 J or more in a Charpy impact test at a test temperature of −196° C. and has high strength with a room-temperature tensile strength of more than 400 MPa. Furthermore, it is described that the welded joint can attain excellent hot cracking resistance by virtue of the Mo content in the wire composition being controlled to Mo: 1.5% or more.
Furthermore, Patent Literature 4 discloses a “gas metal arc welding solid wire”. The “gas metal arc welding solid wire” disclosed in Patent Literature 4 is a wire that has a composition including, by mass %, C: 0.2 to 0.8%, Si: 0.15 to 0.90%, Mn: 17.0 to 28.0%, P: 0.03% or less, S: 0.03% or less, Ni: 0.01 to 10.00%, Cr: 0.4 to 4.0%, Mo: 0.01 to 3.50%, B: less than 0.0010%, and N: 0.12% or less, the balance being Fe and incidental impurities. Where necessary, the wire may contain one, or two or more selected from V, Ti, and Nb, and one, or two or more selected from Cu, Al, Ca, and REM. Patent Literature 4 describes that welding with the gas metal arc welding solid wire disclosed therein generates less fume and can produce a high-strength welded joint that has high strength with a room-temperature yield strength (0.2% proof stress) of 400 MPa or more and has excellent cryogenic impact toughness with an absorbed energy vE−196 of 28 J or more in a Charpy impact test at a test temperature of −196° C.
Furthermore, Patent Literature 5 discloses a “method for manufacturing a cryogenic high-strength welded joint”. In the “method for manufacturing a cryogenic high-strength welded joint” disclosed in Patent Literature 5, workpieces of a steel material that has a steel composition including, by mass %, C: 0.10 to 0.70%, Si: 0.05 to 1.00%, Mn: 18 to 30%, P: 0.030% or less, S: 0.0070% or less, Al: 0.01 to 0.07%, Cr: 2.5 to 7.0%, N: 0.0050 to 0.0500%, and O (oxygen): 0.0050% or less, the balance being Fe and incidental impurities, are welded by gas metal arc welding while using a welding solid wire that has a wire composition including, by mass %, C: 0.2 to 0.8%, Si: 0.15 to 0.90%, Mn: 17.0 to 28.0%, P: 0.03% or less, S: 0.03% or less, Ni: 0.01 to 10.0%, Cr: 0.4 to 4.0%, Mo: 0.02 to 2.5%, Al: 0.1% or less, N: 0.12% or less, and O (oxygen): 0.04% or less, the balance being Fe and incidental impurities. In the method for manufacturing a cryogenic high-strength welded joint, the Mn and Cr contents in the solid wire are lower than the Mn and Cr contents in the steel material, respectively, and the gas metal arc welding is performed while controlling welding conditions so that the dilution ratio, defined by the formula (1) below, of the steel material into the weld metal in the first layer of the multilayer weld metal portion is 35 to 60%.
Dilution ratio (%)=100×{(content of component elements contained in the weld metal in the first layer: mass %)−(content of component elements contained in the solid wire: mass %)}/{(content of component elements contained in the steel material: mass %)−(content of component elements contained in the solid wire: mass %)} . . . (1). Where necessary, the steel material may contain, by mass %, one, or two or more selected from Mo: 2.0% or less, V: 2.0% or less, and W: 2.0% or less, and/or one or two selected from REM: 0.0010 to 0.0200% and B: 0.0005 to 0.0020%, and the solid wire may contain, by mass %, one, or two or more selected from V: 1.0% or less, Ti: 1.0% or less, and Nb: 1.0% or less. According to the manufacturing method described in Patent Literature 5, a welded joint can be easily produced that has a multilayer weld metal portion having high strength with a room-temperature yield strength (0.2% proof stress) of 400 MPa or more and excellent cryogenic impact toughness with an absorbed energy vE % 196 of 28 J or more in a Charpy impact test at a test temperature of −196° C. Due to these high strength and excellent cryogenic impact toughness, the welded joint is described to be suited for welded steel structures used in cryogenic environments.
However, studies by the inventors have shown that hot cracking occurs during submerged arc welding of workpieces of the high-Mn steel material described in Patent Literature 1 or 2 with the welding wire described in Patent Literature 3 or 4 under the dilution ratio conditions described in Patent Literature 5.
An object of the disclosed embodiments is to solve the problems in the art discussed above and provide a submerged arc welded joint that can be formed with reduced occurrence of hot cracking during the welding process and has not only high strength but also excellent cryogenic impact toughness to favorably serve as a welded joint for high-Mn content steel materials used in cryogenic environments.
The term “high strength” as used herein means that a weld metal prepared in accordance with the requirements specified in JIS Z 3111 has a yield strength (a 0.2% proof stress) of 400 MPa or more and a tensile strength of 660 MPa or more at room temperature (25° C.), and also means that a welded joint prepared in accordance with the requirements specified in JIS Z 3121 has a tensile strength of 660 MPa or more at room temperature (25° C.). The term “excellent cryogenic impact toughness” means that a welded joint prepared in accordance with the requirements specified in JIS Z 3128 has an absorbed energy vE−196 of 28 J or more in a Charpy impact test at a test temperature of −196° C. with respect to a weld metal and a welded heat affected zone of the welded joint.
In order to achieve the above object, the inventors carried out extensive studies first on factors that would affect hot cracking during submerged arc welding. As a result, the inventors have found that one of the factors giving rise to hot cracking is the segregation of P into the last-solidified region of the weld metal. Submerged arc welding is a highly efficient welding process that inputs a large amount of heat (J) compared to gas metal arc welding. In particular, the inventors have found that the diffusion of elements is promoted due to the low cooling rate and the consequent long residence time at high temperatures, and thus phosphorus tends to be segregated into the last-solidified region. The inventors have found that the P segregation into the last-solidified region of the weld metal can be suppressed and further the occurrence of hot cracking can be reduced by introducing 6.0 mass % or more Cr into the weld metal to ensure that chromium forms Cr phosphide in the weld metal in the liquid phase state.
The inventors next studied a steel composition and a weld metal composition that would be necessary for a welded joint prepared by submerged arc welding in accordance with the requirements specified in JIS Z 3121 to attain desired high strength and desired excellent cryogenic impact toughness at the same time. As a result, the inventors have found that a submerged arc welded joint needs to be designed so that it involves a high-Mn content steel material having a chemical composition that is controlled to the ranges of, by mass %, C: 0.10 to 0.80%, Si: 0.05 to 1.00%, and Mn: 18.0 to 30.0%, has a P content and a S content lowered to P: 0.030% or less and S: 0.0070% or less, is further controlled to the specific ranges of Al: 0.010 to 0.070%, Cr: 2.5 to 7.0%, and N: 0.0050 to 0.0500%, and has an O content lowered to O (oxygen): 0.0050% or less, and further so that the weld metal formed by submerged arc welding has a chemical composition that is controlled to the ranges of, by mass %, C: 0.10 to 0.80%, Si: 0.05 to 1.00%, and Mn: 15.0 to 30.0%, has a P content, a S content, and an Al content lowered to P: 0.030% or less, S: 0.030% or less, and Al: 0.100% or less, is further controlled to the specific range of Cr: 6.0 to 14.0%, and has a N content lowered to N: 0.100% or less.
The disclosed embodiments have been completed based on the above findings and further studies.
A summary of the disclosed embodiments is as follows.
[1] A submerged arc welded joint in a high-Mn content steel material, wherein
[2] The submerged arc welded joint described in [1], wherein the chemical composition of the high-Mn content steel material further includes, by mass %, one, or two or more selected from:
[3] The submerged arc welded joint described in [1] or [2], wherein the chemical composition of the high-Mn content steel material further includes, by mass %, one or two selected from:
[4] The submerged arc welded joint described in any one of [1] to [3], wherein the high-Mn content steel material has a yield strength of 400 MPa or more in a tensile test at room temperature and a Charpy impact absorbed energy vE−196 of 28 J or more at a test temperature of −196° C.
[5] The submerged arc welded joint described in any one of [1] to [4], wherein the chemical composition of the weld metal further includes, by mass %, one or two selected from:
[6] The submerged arc welded joint described in any one of [1] to [5], wherein the chemical composition of the weld metal further includes, by mass %, one, or two or more selected from:
[7] The submerged arc welded joint described in any one of [1] to [6], wherein the chemical composition of the weld metal further includes, by mass %, one, or two or more selected from:
[8] The submerged arc welded joint described in any one of [1] to [7], wherein the weld metal has a yield strength of 400 MPa or more and a tensile strength of 660 MPa or more in a tensile test at room temperature, the welded joint has a tensile strength of 660 MPa or more at room temperature, and the weld metal and a welded heat affected zone in the welded joint have a Charpy impact absorbed energy vE−196 of 28 J or more at a test temperature of −196° C.
The disclosed embodiments can reduce the occurrence of hot cracking in a welded joint during submerged arc welding of a high-Mn content steel material and allows for easy production of a submerged arc welded joint having high strength and excellent cryogenic impact toughness, thus attaining significant effects in industry.
The disclosed embodiments pertain to a submerged arc welded joint that is suited as a joint in a cryogenic steel material formed by submerged arc welding of workpieces of a high-Mn content steel material. The welded joint of the disclosed embodiments is a welded joint prepared by submerged arc welding evaluated in accordance with, for example, the requirements specified in JIS Z 3121. The welded joint is a submerged arc welded joint that can be formed with reduced occurrence of hot cracking during the welding process and has not only high strength but also excellent cryogenic impact toughness. Specifically, the yield strength (the 0.2% proof stress) and the tensile strength of the weld metal at room temperature (25° C.) are 400 MPa or more and 660 MPa or more, respectively, and the tensile strength of the welded joint at room temperature (25° C.) is 660 MPa or more. Furthermore, a weld metal and a welded heat affected zone of a welded joint prepared in accordance with the requirements specified in JIS Z 3128 have an absorbed energy vE−196 of 28 J or more in a Charpy impact test at a test temperature of −196° C.
Submerged arc welding (hereinafter, also written as “SAW”) is a welding process in which an electrode wire is continuously fed into a granular flux supplied beforehand on a base material (in the disclosed embodiments, a high-Mn content steel material), and an arc is struck between the tip of the electrode wire and the base material to weld them continuously. This submerged arc welding is advantageous in that welding can be performed efficiently by applying a large current to increase the deposition rate of the wire.
First, the steel material that is used will be described. Incidentally, “%” in the “chemical composition” below means “mass %”.
The steel material used in the disclosed embodiments is a high-Mn content steel material that has a chemical composition including C: 0.10 to 0.80%, Si: 0.05 to 1.00%, Mn: 18.0 to 30.0%, P: 0.030% or less, S: 0.0070% or less, Al: 0.010 to 0.070%, Cr: 2.5 to 7.0%, N: 0.0050 to 0.0500%, and O (oxygen): 0.0050% or less, the balance being Fe and incidental impurities. The reasons as to why the chemical composition is thus limited will be described below.
Carbon is an inexpensive and important element that acts to stabilize austenite phases. In order to obtain such an effect, the C content needs to be 0.10% or more. Thus, the C content is limited to 0.10% or more. The C content is preferably 0.20% or more, more preferably 0.25% or more, still more preferably 0.30% or more, and most preferably 0.35% or more. If, on the other hand, the C content is more than 0.80%, Cr carbide is excessively formed to cause a decrease in cryogenic impact toughness. Thus, the C content is limited to 0.80% or less. The C content is preferably 0.75% or less, more preferably 0.70% or less, still more preferably 0.65% or less, and most preferably 0.63% or less.
Silicon is an element that acts as a deoxidizing agent and contributes to increasing the strength of the steel material by being dissolved in the steel to effect solid solution hardening. In order to obtain such effects, the Si content needs to be 0.05% or more. Thus, the Si content is limited to 0.05% or more. The Si content is preferably 0.07% or more, more preferably 0.10% or more, still more preferably 0.15% or more, and most preferably 0.20% or more. If, on the other hand, the Si content is more than 1.00%, weldability is lowered. Thus, the Si content is limited to 1.00% or less. The Si content is preferably 0.80% or less, more preferably 0.70% or less, still more preferably 0.65% or less, and most preferably 0.50% or less.
Manganese is a relatively inexpensive element that acts to stabilize austenite phases. In the disclosed embodiments, this element is important for achieving high strength and excellent cryogenic impact toughness at the same time. In order to obtain such effects, the Mn content needs to be 18.0% or more. Thus, the Mn content is limited to 18.0% or more. The Mn content is preferably 20.0% or more, more preferably 22.0% or more, and still more preferably 24.0% or more. On the other hand, adding more than 30.0% manganese results in the saturation of the effect of enhancing the cryogenic impact toughness and is economically disadvantageous because the effect appropriate to the amount added cannot be expected. If, furthermore, the Mn content is as large as more than 30.0%, cuttability is lowered, and Mn segregation is promoted to increase the probability of the occurrence of stress corrosion cracking. Thus, the Mn content is limited to 30.0% or less. The Mn content is preferably 29.0% or less, more preferably 28.0% or less, and still more preferably 27.0% or less.
Phosphorus is an element that is segregated as an impurity at grain boundaries to serve as origins of stress corrosion cracking. It is therefore desirable in the disclosed embodiments to remove as much phosphorus as possible. Up to 0.030% phosphorus is acceptable. Thus, the P content is limited to 0.030% or less. The P content is preferably 0.028% or less, more preferably 0.024% or less, still more preferably 0.020% or less, and most preferably 0.015% or less. Extreme dephosphorization to less than 0.002% entails prolonged refining and raises the refining cost. Thus, the P content is preferably 0.002% or more from an economical viewpoint.
In the steel, sulfur is present as sulfide inclusions and lowers the ductility and the cryogenic impact toughness of the steel material and the weld metal. It is therefore desirable to remove as much sulfur as possible. Up to 0.0070% sulfur is acceptable. Thus, the S content is limited to 0.0070% or less. The S content is preferably 0.0050% or less, and more preferably 0.0040% or less. Extreme desulfurization to less than 0.0005% entails prolonged refining and raises the refining cost. Thus, the S content is preferably 0.0005% or more from an economical viewpoint.
Aluminum is an element that acts as a deoxidizing agent and is used most commonly in the molten steel deoxidizing process for steel materials. In order to obtain such an effect, the Al content needs to be 0.010% or more. Thus, the Al content is limited to 0.010% or more. The Al content is preferably 0.020% or more, and more preferably 0.030% or more. If, on the other hand, the Al content is more than 0.070%, aluminum is mixed into the weld metal portion during the welding process to cause a decrease in the toughness of the weld metal. Thus, the Al content is limited to 0.070% or less. The Al content is preferably 0.060% or less, and more preferably 0.050% or less.
Chromium is an element that stabilizes austenite phases and effectively contributes to enhancements in cryogenic impact toughness and steel material strength. Furthermore, this element is effective for forming fine crystal regions. In order to obtain such effects, the Cr content needs to be 2.5% or more. Thus, the Cr content is limited to 2.5% or more. The Cr content is preferably 3.0% or more, more preferably 3.3% or more, still more preferably 3.5% or more, and most preferably 4.0% or more. If, on the other hand, the Cr content is more than 7.0%, Cr carbide is formed to cause a decrease in cryogenic impact toughness and a decrease in stress corrosion cracking resistance. Thus, the Cr content is limited to 7.0% or less. The Cr content is preferably 6.8% or less, more preferably 6.5% or less, and still more preferably 6.0% or less.
Nitrogen is an element that acts to stabilize austenite phases, and effectively contributes to enhancing the cryogenic impact toughness. In order to obtain such effects, the N content needs to be 0.0050% or more. Thus, the N content is limited to 0.0050% or more. The N content is preferably 0.0060% or more, more preferably 0.0070% or more, and still more preferably 0.0080% or more. If, on the other hand, the N content is more than 0.0500%, nitrides or carbonitrides are coarsened to cause a decrease in cryogenic impact toughness. Thus, the N content is limited to 0.0500% or less. The N content is preferably 0.0400% or less, more preferably 0.0300% or less, and still more preferably 0.0200% or less.
In the steel, oxygen (O) is present as oxide inclusions and lowers the cryogenic impact toughness of the steel material. It is therefore desirable to remove as much oxygen (O) as possible. Up to 0.0050% oxygen is acceptable. Thus, the O (oxygen) content is limited to 0.0050% or less. The O content is preferably 0.0045% or less, and more preferably 0.0040% or less. Extreme deoxidation to an O (oxygen) content of less than 0.0005% entails prolonged refining and raises the refining cost. Thus, the O (oxygen) content is preferably 0.0005% or more from an economical viewpoint. The O content is more preferably 0.0006% or more.
The composition described above is the basic chemical composition. Where necessary, in addition to the basic chemical composition, the chemical composition may include one, or two or more selected from Mo: 2.00% or less, V: 2.0% or less, and W: 2.00% or less, and may further include one or two selected from REM: 0.0010 to 0.0200% and B: 0.0005 to 0.0020%.
Molybdenum, vanadium, and tungsten are each an element that contributes to austenite phase stabilization and also contributes to enhancements in the strength and the cryogenic impact toughness of the steel material. One, or two or more may be selected and added as required. When molybdenum, vanadium, and tungsten are added to obtain such effects, the Mo, V, and W contents are preferably each 0.001% or more. The Mo, V, and W contents are more preferably each 0.003% or more. If, on the other hand, the Mo and W contents are each more than 2.00% and the V content is more than 2.0%, an increased amount of coarse carbonitrides are formed and serve as fracture origins to cause a decrease in cryogenic impact toughness. Thus, when molybdenum, vanadium, and tungsten are present, the contents are limited to Mo: 2.00% or less, V: 2.0% or less, and W: 2.00% or less. The contents are preferably Mo: 1.70% or less, V: 1.7% or less, and W: 1.70% or less, and more preferably Mo: 1.50% or less, V: 1.5% or less, and W: 1.50% or less.
REM indicates rare earth metals, such as Sc, Y, La, and Ce. Through shape control for inclusions, these elements act to enhance the toughness of the steel material and to enhance the ductility and the sulfide stress corrosion cracking resistance. Boron is an element that is segregated at grain boundaries to contribute to enhancing the toughness of the steel material. One or two kinds of these elements may be selected and added as required.
In order to obtain the above effects, the REM content needs to be 0.0010% or more. Thus, when rare earth metals are present, the REM content is limited to 0.0010% or more. The REM content is preferably 0.0015% or more. If, on the other hand, the REM content is more than 0.0200%, the amount of nonmetallic inclusions is increased to cause decreases in toughness, ductility, and sulfide stress cracking resistance. Thus, when rare earth metals are present, the REM content is limited to 0.0200% or less. The REM content is preferably 0.0180% or less.
In order to obtain the effect described above, the B content needs to be 0.0005% or more. Thus, when B is present, the B content is limited to 0.0005% or more. The B content is preferably 0.0008% or more. If, on the other hand, the B content is more than 0.0020%, the amount of coarse nitride and carbide is increased to cause a decrease in toughness. Thus, when boron is present, the B content is limited to 0.0020% or less. The B content is preferably 0.0018% or less.
The balance of the chemical composition described above is Fe and incidental impurities. Examples of the incidental impurities include Ca, Mg, Ti, Nb, and Cu. Up to 0.05% incidental impurities in total are acceptable. The steel material may contain elements other than those described above as long as the basic composition and the composition of the optional components described above are satisfied. Such embodiments are also within the technical scope of the disclosed embodiments.
A preferred method for producing the high-Mn content steel materials used in the disclosed embodiments will be described.
Molten steel having the above-described steel composition is obtained by a common steelmaking process, such as a converter or an electric arc furnace, and is cast into a steel material, such as slab having a predetermined size, by a common casting process, such as a continuous casting process or an ingot making-slabbing process. It is needless to mention that the steel obtained by the steelmaking process may be subjected to secondary refining in, for example, a vacuum degassing furnace. The steel material obtained is further heated, hot-rolled, and then cooled to give a steel material of predetermined size. A steel material having excellent cryogenic impact toughness may be obtained by performing heating at a heating temperature in the range of 1100 to 1300° C., finishing hot rolling at a finishing delivery temperature of 790 to 980° C., and immediately subjecting the steel to post treatment, such as cooling. It is needless to mention that heat treatment, such as annealing treatment, may be further performed to control characteristics of the steel material.
Preferred characteristics of the high-Mn content steel materials used in the disclosed embodiments will be described.
The steel material is preferably a cryogenic high-strength steel material that has the steel composition described above and that has a plate thickness of, for example, 6 to 100 mm, a yield strength (a 0.2% proof stress) of 400 MPa or more in a tensile test at room temperature (25° C.), and a Charpy impact absorbed energy vE−196 of 28 J or more at a test temperature of −196° C. Furthermore, it is more preferable that the tensile strength be 800 MPa or more.
In the disclosed embodiments, workpieces of the high-Mn content steel material described above are welded together by submerged arc welding to form a welded joint that includes a weld metal portion composed of a single layer of a weld metal or a plurality of layers of a weld metal.
The weld metal of the disclosed embodiments has a basic chemical composition including C: 0.10 to 0.80%, Si: 0.05 to 1.00%, Mn: 15.0 to 30.0%, P: 0.030% or less, S: 0.030% or less, Al: 0.100% or less, Cr: 6.0 to 14.0%, and N: 0.100% or less, the balance being Fe and incidental impurities. The reasons as to why the chemical composition is thus limited will be described below.
Carbon is an element that acts to increase the strength of the weld metal by solid solution hardening. Furthermore, carbon stabilizes austenite phases to enhance the cryogenic impact toughness of the weld metal. In order to obtain such effects, the C content needs to be 0.10% or more. Thus, the C content is limited to 0.10% or more. The C content is preferably 0.20% or more, and more preferably 0.25% or more. If, however, the C content is more than 0.80%, carbides are precipitated to cause a decrease in cryogenic impact toughness and to increase the probability of hot cracking during the welding process. Thus, the C content is limited to 0.80% or less. The C content is preferably 0.75% or less, more preferably 0.70% or less, still more preferably 0.65% or less, and most preferably 0.63% or less.
Silicon acts as a deoxidizing agent to increase the yield of Mn, and also increases the viscosity of the molten metal to effectively allow a bead to maintain the shape stably. In order to obtain such effects, the Si content needs to be 0.05% or more. Thus, the Si content is limited to 0.05% or more. The Si content is preferably 0.10% or more, more preferably 0.15% or more, still more preferably 0.20% or more, and most preferably 0.25% or more. If, however, the Si content is more than 1.00%, the cryogenic impact toughness of the weld metal is lowered. Furthermore, silicon is segregated during solidification to form liquid phases at interfaces of solidified cells, causing a decrease in hot cracking resistance. Thus, the Si content is limited to 1.00% or less. The Si content is preferably 0.80% or less, more preferably 0.75% or less, and still more preferably 0.70% or less.
Manganese is an element that stabilizes austenite phases at low cost, and needs to be contained at 15.0% or more in the disclosed embodiments. If the Mn content is less than 15.0%, ferrite phases are formed in the weld metal to cause a significant decrease in cryogenic impact toughness. Thus, the Mn content is limited to 15.0% or more. The Mn content is preferably 17.0% or more, and more preferably 18.0% or more. If, on the other hand, the Mn content is more than 30.0%, manganese is segregated excessively during solidification to induce hot cracking. Thus, the Mn content is limited to 30.0% or less. The Mn content is preferably 28.0% or less, and more preferably 27.0% or less.
Phosphorus is an element that is segregated at grain boundaries to induce hot cracking. It is therefore preferable in the disclosed embodiments to remove as much phosphorus as possible. Up to 0.030% phosphorus is acceptable. Thus, the P content is limited to 0.030% or less. The P content is preferably 0.020% or less, more preferably 0.018% or less, still more preferably 0.016% or less, and most preferably 0.014% or less. Excessive dephosphorization raises the refining cost. Thus, the P content is preferably controlled to 0.002% or more.
In the weld metal, sulfur is present as sulfide inclusion MnS. MnS serves as fracture origins and lowers the cryogenic impact toughness. Thus, the S content is limited to 0.030% or less. The S content is preferably 0.025% or less, more preferably 0.020% or less, and still more preferably 0.017% or less. Excessive desulfurization raises the refining cost. Thus, the S content is preferably controlled to 0.001% or more.
Aluminum acts as a deoxidizing agent and has an important action to increase the viscosity of the molten metal and allow the bead shape to be maintained stably. Furthermore, aluminum narrows the temperature range of the solid-liquid coexistence region of the molten metal to contribute to the suppression of the occurrence of hot cracking in the weld metal. These effects are marked when the Al content is 0.001% or more. Thus, the Al content is preferably 0.001% or more. If, however, the Al content is more than 0.100%, the viscosity of the molten metal is so increased that a bead does not spread to increase the probability of defects, such as fusion failure. Thus, the Al content is limited to 0.100% or less. The Al content is preferably 0.060% or less, more preferably 0.050% or less, still more preferably 0.040% or less, and most preferably 0.030% or less.
Chromium acts as an element that stabilizes austenite phases at cryogenic temperatures to enhance the cryogenic impact toughness of the weld metal. Chromium also acts to enhance the strength of the weld metal. Furthermore, chromium narrows the temperature range of the solid-liquid coexistence region of the molten metal to effectively suppress the occurrence of hot cracking. Furthermore, chromium forms Cr phosphide in the liquid phase to suppress hot cracking caused by phosphorus. In order to obtain such effects, the Cr content needs to be 6.0% or more. If the Cr content is less than 6.0%, the above effects cannot be ensured. Thus, the Cr content is limited to 6.0% or more. The Cr content is preferably 6.5% or more, more preferably 7.0% or more, still more preferably 7.5% or more, and most preferably 8.0% or more. If, on the other hand, the Cr content is more than 14.0%, Cr carbide is formed to cause a decrease in cryogenic impact toughness. Thus, the Cr content is limited to 14.0% or less. The Cr content is preferably 13.0% or less, more preferably 12.0% or less, still more preferably 11.5% or less, and most preferably 11.0% or less.
Nitrogen is an element that is incidentally mixed. Similarly to carbon, nitrogen effectively contributes to an enhancement in the strength of the weld metal and stabilizes austenite phases to contribute to a stable enhancement in cryogenic impact toughness. These effects are marked when the N content is 0.003% or more. Thus, the N content is preferably 0.003% or more, more preferably 0.004% or more, and still more preferably 0.006% or more. If, on the other hand, the N content is more than 0.100%, nitrides are formed to cause a decrease in low-temperature toughness. Thus, the N content is limited to 0.100% or less. The N content is preferably 0.090% or less, more preferably 0.080% or less, still more preferably 0.070% or less, and most preferably 0.050% or less.
The composition described above is the basic chemical composition of the weld metal in the disclosed embodiments. Where necessary, in addition to the above chemical composition, the weld metal may further contain optional components, specifically, may selectively contain Mo: 3.50% or less and/or Ni: 10.00% or less, one, or two or more selected from V: 1.60% or less, Ti: 1.00% or less, Nb: 1.00% or less, and W: 1.00% or less, and one, or two or more selected from Cu: 1.00% or less, Ca: 0.010% or less, B: 0.0100% or less, and REM: 0.020% or less.
Both molybdenum and nickel are elements that strengthen austenite grain boundaries. Either or both may be selected and added as required.
Molybdenum is an element that strengthens austenite grain boundaries, and is segregated at grain boundaries to enhance the strength of the weld metal. Furthermore, molybdenum also acts to enhance the strength of the weld metal by solid solution hardening. If, on the other hand, the Mo content is more than 3.50%, molybdenum is precipitated as carbide, which serves as fracture origins to cause a decrease in cryogenic impact toughness. Thus, when molybdenum is present, the Mo content is limited to 3.50% or less. The Mo content is preferably 3.20% or less, more preferably 3.00% or less, and still more preferably 2.50% or less. When molybdenum is present, the Mo content is preferably 0.005% or more to ensure that the above effects are obtained. The Mo content is more preferably 0.30% or more, still more preferably 0.50% or more, and most preferably 1.00% or more.
Nickel is an element that strengthens austenite grain boundaries, and is segregated at grain boundaries to enhance the cryogenic impact toughness. Furthermore, nickel also has an effect of stabilizing austenite phases. Thus, an increase in the Ni content leads to stabilization of austenite phases and enhances the cryogenic impact toughness of the weld metal. Thus, when nickel is present, the Ni content is preferably 0.02% or more, more preferably 0.05% or more, and still more preferably 1.00% or more. However, increasing the Ni content in excess of 10.00% is economically disadvantageous because of the expensiveness of the element. Thus, when nickel is present, the Ni content is limited to 10.00% or less. The Ni content is preferably 8.00% or less, more preferably 7.00% or less, still more preferably 6.50% or less, and most preferably 6.00% or less.
Vanadium, titanium, niobium, and tungsten are each an element that easily forms carbide and contributes to an enhancement in the strength of the weld metal. One, or two or more may be selected and added as required.
Vanadium is a carbide-forming element and is precipitated as fine carbide to contribute to an enhancement in the strength of the weld metal. In order to obtain such effects, the content of vanadium, when present, is preferably 0.001% or more. If, however, the V content is more than 1.60%, the carbide is coarsened and serves as fracture origins to cause a decrease in cryogenic impact toughness. Thus, when vanadium is present, the V content is limited to 1.60% or less. The V content is preferably 1.00% or less, more preferably 0.80% or less, and still more preferably 0.60% or less.
Titanium is a carbide-forming element and is precipitated as fine carbide to contribute to an enhancement in the strength of the weld metal. Furthermore, titanium is precipitated as carbide at interfaces of solidified cells of the weld metal, and thereby contributes to the suppression of the occurrence of hot cracking. In order to obtain such effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. If the Ti content is more than 1.00%, the carbide is coarsened and serves as fracture origins to cause a decrease in cryogenic impact toughness. Thus, when titanium is present, the Ti content is limited to 1.00% or less. The Ti content is preferably 0.80% or less, more preferably 0.60% or less, and still more preferably 0.50% or less.
Niobium is a carbide-forming element and is precipitated as carbide to contribute to an enhancement in the strength of the weld metal. Furthermore, niobium is precipitated as carbide at interfaces of solidified cells of the weld metal, and thereby contributes to the suppression of the occurrence of hot cracking. In order to obtain such effects, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more. If, however, the Nb content is more than 1.00%, the carbide is coarsened and serves as fracture origins to cause a decrease in cryogenic impact toughness. Thus, when niobium is present, the Nb content is limited to 1.00% or less. The Nb content is preferably 0.80% or less, more preferably 0.70% or less, still more preferably 0.60% or less, and most preferably 0.50% or less.
Tungsten is a carbide-forming element and is precipitated as carbide to contribute to an enhancement in the strength of the weld metal. Furthermore, tungsten contributes to the stabilization of austenite phases and enhances the cryogenic impact toughness. Furthermore, tungsten is precipitated as carbide at interfaces of solidified cells of the weld metal, and thereby contributes to the suppression of the occurrence of hot cracking. In order to obtain such effects, the content of tungsten, when present, is preferably 0.001% or more. The W content is more preferably 0.002% or more, and still more preferably 0.005% or more. If, on the other hand, the W content is more than 1.00%, the carbide is coarsened and serves as fracture origins to cause a decrease in cryogenic impact toughness. Thus, when tungsten is present, the W content is limited to 1.00% or less. The W content is preferably 0.80% or less, more preferably 0.60% or less, and still more preferably 0.40% or less.
Copper is an element that contributes to austenite stabilization. Calcium, boron, and rare earth metals are elements that contribute to workability enhancement. One, or two or more kinds of these elements may be selected and added as required. Copper is an element that stabilizes austenite phases, and stabilizes austenite phases even at cryogenic temperatures to enhance the cryogenic impact toughness of the weld metal. In order to obtain such effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.04% or more. If, however, the Cu content is more than 1.00%, copper is segregated during solidification to induce hot cracking during the welding process. Thus, when copper is present, the Cu content is limited to 1.00% or less. The Cu content is preferably 0.80% or less.
Calcium binds to sulfur in the molten metal to form the high-melting point sulfide CaS. CaS has a higher melting point than MnS and thus contributes to the suppression of the occurrence of hot cracking of the weld metal. These effects are marked when the Ca content is 0.001% or more. Thus, when calcium is present, the Ca content is preferably 0.001% or more. If, on the other hand, the Ca content is more than 0.010%, the arc is disturbed during the welding process to make it difficult to perform welding stably. Thus, when calcium is present, the Ca content is limited to 0.010% or less. The Ca content is preferably 0.008% or less, and more preferably 0.006% or less.
Boron is an element that is incidentally mixed, and is segregated at austenite grain boundaries. If more than 0.0100% boron is mixed, boron nitride is formed at austenite grain boundaries to lower the strength, and further the boron nitride serves as fracture origins to cause a decrease in cryogenic impact toughness. Thus, when boron is present, the B content is limited to 0.0100% or less. The B content is preferably 0.0080% or less, and more preferably 0.0050% or less. On the other hand, excessive boron removal raises the refining cost. Thus, when boron is present, the B content is preferably 0.0001% or more.
Rare earth metals are powerful deoxidizing agents and are present in the form of REM oxides in the weld metal. The REM oxides serve as nucleus formation sites at the time of solidification, and thereby reduce the size of crystal grains and contribute to an enhancement in the strength of the weld metal. These effects are marked when the REM content is 0.001% or more. Thus, when rare earth metals are present, the REM content is preferably 0.001% or more. The REM content is more preferably 0.002% or more, and still more preferably 0.003% or more. If, however, the REM content is more than 0.020%, the arc stability is lowered. Thus, when rare earth metals are present, the REM content is limited to 0.020% or less. The REM content is preferably 0.018% or less, and more preferably 0.015% or less.
The balance of the chemical composition described above is Fe and incidental impurities. Examples of the incidental impurities include H, O, Mg, Zn, and Re. Up to 0.0100% incidental impurities in total are acceptable. The weld metal may contain elements other than those described above as long as the basic composition and the composition of the optional components described hereinabove are satisfied. Such embodiments are also within the technical scope of the disclosed embodiments.
The chemical composition of the weld metal is mainly determined by the inflow proportions of the base metal and the welding material, such as a SAW wire.
Next, a method for producing the welded joint according to the disclosed embodiments will be described.
First, workpieces of a high-Mn content steel material having the chemical composition described above are provided. Edge preparation is then performed so that the workpieces of the steel material provided will have a predetermined groove shape. The groove that is formed may have any shape without limitation. Examples include normal single V grooves, double V grooves, and double bevel grooves for welded steel structures that conform to JIS Z 3001-1.
Subsequently, the edge-prepared workpieces of the steel material are subjected to submerged arc welding to form a single layer of a weld metal or a plurality of layers of a weld metal. A welded joint is thus formed. Any welding material may be used as long as the weld metal portion that is formed has the desired characteristics. The first choice is a welding wire that has the chemical composition described hereinabove. The type of the wire and the type of a welding flux are not particularly limited.
The types of the wires include solid wires, and flux-cored wires that include a wire flux within the wires. Any of these wires may be used in the disclosed embodiments. A flux-cored wire for use herein is produced so that the total of the chemical compositions of the steel skin, the metal powder, and the wire flux powder that are used will be the target chemical composition of the welding material.
Next, methods for manufacturing the SAW wires (solid wires and flux-cored wires) will be described.
The solid wire is preferably manufactured by a casting step in which a molten steel having the target composition is obtained in a common melting furnace, such as an electric furnace or a vacuum melting furnace, and is cast into, for example, a mold having a predetermined shape; a heating step in which the steel ingot obtained is heated to a predetermined temperature; a hot rolling step in which the steel ingot heated is hot rolled to give a steel material (a steel rod) having a predetermined shape; and a cold rolling step in which the steel material (the steel rod) obtained is cold rolled (cold drawn) several times and, where necessary, is subjected to an annealing step at an annealing temperature of 900 to 1200° C. to give a wire having a desired size. The chemical composition of the solid wire is not particularly limited but is preferably such that, by mass %, C: 0.20 to 0.80%, Si: 0.20 to 1.00%, Mn: 16.0 to 30.0%, P: 0.030% or less, S: 0.030% or less, Cr: 6.0 to 15.0%, Mo: 0.01 to 4.0%, Ni: 0.01 to 10.0%, O:0.050% or less, and N: 0.150% or less.
For example, the flux-cored wire is preferably manufactured as follows. A steel sheet (sheet thickness: 0.5 mm) as a steel skin material that has a composition including 0.05 to 0.20% C, 0.15 to 0.30% Si, 0.2 to 1.2% Mn, and the balance of Fe is subjected to cold bending in the width direction to form a U shape. Subsequently, a metal powder and a wire flux powder that have been conditioned so that the target wire composition will be obtained are sealed in the steel skin. The steel skin is then cold drawn to give a SAW flux-cored wire.
The chemical composition of the above metal powder is not particularly limited. The powder is a metal powder or an alloy powder that includes metal components which are added supplementarily to the chemical composition of the steel skin material so as to obtain the desired composition of the welding wire as a whole. The chemical composition of the flux-cored wire is not particularly limited but is preferably such that, by mass %, C: 0.20 to 0.80%, Si: 0.20 to 1.00%, Mn: 16.0 to 30.0%, P: 0.030% or less, S: 0.030% or less, Cr: 6.0 to 15.0%, Mo: 0.01 to 4.0%, Ni: 0.01 to 9.0%, 0: 0.200% or less, and N: 0.100% or less. The components in the wire flux powder are not particularly limited. The components in the flux powder may be the same as or similar to the components in a welding flux described below. [Welding fluxes]
The welding flux that is used together with the above SAW wire (the solid wire or the flux-cored wire) is not particularly limited and may be a generally known sintered flux or pre-fused flux. Specifically, a powder material may be used that has a chemical composition including, for example, SiO2: 20 to 40%, MnO: 8 to 15%, TiO2: 5 to 10%, Al2O3: 10 to 20%, and MgO: 20 to 30%. As an example, the composition may include 38% SiO2, 11% MnO, 8% TiO2, 16% Al2O3, and 27% MgO. However, the welding fluxes in the disclosed embodiments are not limited to those described above.
The heat-affected zone in the welded joint described hereinabove is a region that has been affected by the heat introduced during the welding process and has been denatured from the characteristics of the unaffected zone of the base material or the unaffected zone of the weld metal due to, for example, a change in crystal structure, the occurrence of new phases, a change in grain size, the diffusion of elements, or the recovery of dislocation. The reasons that give rise to a change in characteristics are not limited to those described above.
The disclosed embodiments will be further described below based on EXAMPLES. EXAMPLES below are only illustrative of detailed examples of the disclosed embodiments and are not intended to limit the scope of this disclosure.
Molten steels were obtained in a vacuum melting furnace, cast into a mold, and then bloomed into slabs (wall thickness: 150 mm) having a chemical composition described in Table 1. Steel materials were thus obtained. Next, the steel materials obtained were charged into a heating furnace, heated to 1250° C., hot rolled at a finishing delivery temperature of 850° C., and immediately water-cooled. Steel plates (high-Mn content steel materials) having a plate thickness of 20 mm were thus obtained.
Next, molten steels having a chemical composition (a wire composition) described in Table 2 were obtained in a vacuum melting furnace and cast to give steel ingots. The steel ingots obtained were heated to 1200° C., hot rolled, and subsequently cold rolled to give 4.0 mm submerged arc welding solid wires.
Separately, flux-cored wires were produced that had a steel skin, and a metal powder and a flux powder included within the steel skin. The steel skin material was a steel sheet (sheet thickness: 0.5 mm) that had a composition including 0.1% C, 0.2% Si, 0.5% Mn, and the balance of Fe. The steel skin material was subjected to cold bending in the width direction to form a U shape. Subsequently, a metal powder and a flux powder that had been conditioned so that the chemical composition described in Table 3 would be obtained were sealed in the steel skin. The U shape sealed steel skin was then cold drawn to give a welding flux-cored wire (diameter: 3.2 mm). The chemical compositions described in Table 3 are each the total of the steel skin, the metal powder, and the flux powder.
Next, workpieces of the steel material obtained (plate thickness: 20 mm) were butted against each other and restrained in accordance with JIS Z 3121, and a 45° single V groove was formed. Submerged arc welding was performed using the solid wire or the flux-cored wire as the welding material to form a weld metal in the groove. Welded joints were thus obtained. At the time of welding, a flux that had a composition including 38% SiO2, 11% MnO, 8% TiO2, 16% Al2O3, and 27% MgO was used.
In the submerged arc welding, the solid wire (diameter: 4.0 mm ϕ) or the flux-cored wire (diameter: 3.2 mm ϕ) having a composition described in Table 2 or Table 3 was fed in a down-pointing attitude, and the workpieces of the steel material described in Table 1 were welded without being preheated under conditions with current: 450 to 650 A (DCEP), voltage: 28 to 36 V, welding speed: 20 cm/min, and interpass temperature: 100 to 150° C. The combinations of the materials are described in Table 4.
Test pieces for tensile test in accordance with JIS Z 2241 and Charpy impact test specimens (V-notch) in accordance with JIS Z 2242 were taken from the high-Mn content steel materials obtained and were subjected to a tensile test and an impact test. In the tensile test, three test pieces were tested at room temperature, and the values obtained (0.2% proof stress) were averaged to give a tensile characteristic of the high-Mn content steel material. In the Charpy impact test, three test specimens were tested to measure the absorbed energy vE−196 at a test temperature of −196° C., and the average of the results was taken as the cryogenic impact toughness of the high-Mn content steel material.
To analyze the chemical composition of the weld metal, chippings as a sample were taken from a central portion of the weld metal and were analyzed to determine the components by a combustion-infrared absorption method, an inert gas fusion-thermal conductivity method, an inert gas fusion-infrared absorption method, an absorptiometric method, an ICP-AES method, a precipitation analysis method, a volumetric method, and a wet chemical analysis method.
After the welding process, a 10 mm thick macro test specimen was sampled from a central location in the weld line direction with a micro cutter in such a manner that a cross section perpendicular to the weld line would be observable. The cross section of the weld metal was observed with an optical microscope (×30) to determine the presence or absence of hot cracks. In the optical micrograph of the microstructure, elongated black regions 25 μm or more in width×80 μm or more in length were judged to be hot cracks. When hot cracks were present, the hot cracking resistance was low and was rated as “×”. When there were no hot cracks, the hot cracking resistance was excellent and was rated as “∘”.
The welded joints obtained were cut to give test pieces (diameter of parallel part: 6 mm ϕ) for tensile test of the weld metal, and Charpy impact test specimens (V-notch) of the weld metal in accordance with the requirements specified in JIS Z 3111. The test pieces and the test specimens were subjected to a tensile test and an impact test. In the tensile test, three test pieces were tested at room temperature, and the values obtained (0.2% proof stress, tensile strength) were averaged to give a tensile characteristic of the weld metal of the welded joint. In the Charpy impact test, similarly, three test specimens were tested to measure the absorbed energy vE−196 at a test temperature of −196° C., and the average of the results was taken as the cryogenic impact toughness of the weld metal of the welded joint.
Furthermore, the welded joints were tensile tested at room temperature in accordance with the requirements specified in JIS Z 3121. Test pieces were taken along a direction perpendicular to the weld axis so that the weld axis would be located at the center of the length in the parallel part of the test piece. The thickness of the test piece was equal to the total thickness of the welded joint. That is, the test pieces were test pieces No. 1A. Three test pieces were tested, and the results obtained were averaged to give a tensile characteristic of the welded joint.
Furthermore, the welded heat affected zones of the welded joints were tested by a Charpy impact test in accordance with the requirements specified in JIS Z 3128. The test specimens were V-notched in a direction perpendicular to the surface of the base material. The test specimens were taken from a location at the middle of the plate thickness, corresponding to the center of the weld metal, and 1 mm from the fusion line. Three test specimens were tested, and the average of the results obtained was taken as the cryogenic impact toughness of the welded heat affected zone of the welded joint.
As described hereinabove, the target values in the disclosed embodiments are 400 MPa or more yield strength (0.2% proof stress) at room temperature (25° C.) and 28 J or more absorbed energy vE−196 in a Charpy impact test at a test temperature of −196° C. for the high-Mn content steel materials; 400 MPa or more yield strength (0.2% proof stress) and 660 MPa or more tensile strength at room temperature (25° C.) for the weld metals; 660 MPa or more tensile strength at room temperature (25° C.) for the welded joints; and 28 J or more absorbed energy vE−196 in a Charpy impact test at a test temperature of −196° C. for the weld metals and the welded heat affected zones in the welded joints. The results obtained are described in Tables 1 to 5.
F
17.4
13
14.2
14
1.30
31.3
5.1
15
0.066
3.80
16
4.5
17
0.89
18
16.1
22
0.037
0.031
23
1.9
26
1.27
32.2
4.8
33
F
34
F
35
F
13
22.5
14
x
23.3
15
20.4
16
x
388.7
648.2
18.6
17
x
26.1
18
4.2
22
x
26.3
23
x
364.9
632.9
655.2
16.6
26
x
395.6
21.2
33
18.2
34
10.0
35
14.4
In all EXAMPLES, the welded joints were excellent in hot cracking resistance and suffered no hot cracks during the welding process, and the weld metals had a good weld bead appearance.
In all EXAMPLES, furthermore, the welded joints and the weld metals attained high strength and excellent cryogenic impact toughness. Specifically, the high-Mn content steel materials had a yield strength (a 0.2% proof stress) of 400 MPa or more at room temperature (25° C.) and an absorbed energy vE−196 of 28 J or more in a Charpy impact test at a test temperature of −196° C.; the weld metals had a yield strength (a 0.2% proof stress) of 400 MPa or more and a tensile strength of 660 MPa or more at room temperature (25° C.); the welded joints had a tensile strength of 660 MPa or more at room temperature (25° C.); and the weld metals and the welded heat affected zones in the welded joints had an absorbed energy vE−196 of 28 J or more in a Charpy impact test at a test temperature of −196° C.
In COMPARATIVE EXAMPLES falling outside the scope of the disclosed embodiments, the weld metals failed to achieve high strength and excellent cryogenic impact toughness at the same time because the hot cracking resistance was low, and the welded joint suffered hot cracks; because the weld metal had a yield strength (a 0.2% proof stress) of less than 400 MPa or a tensile strength of less than 660 MPa at room temperature (25° C.); because the welded joint had a tensile strength of less than 660 MPa at room temperature (25° C.); or because the weld metal or the welded heat affected zone had an absorbed energy vE−196 of less than 28 J in the Charpy impact test at a test temperature of −196° C.
In the welded joint No. 13 representing a COMPARATIVE EXAMPLE, the Mn content in the weld metal was below the range of the disclosed embodiments, and the austenite phases in the weld metal were unstable. As a result, excellent cryogenic impact toughness that was desired was not obtained and the absorbed energy vE−196 of the weld metal at a test temperature of −196° C. was less than 28 J.
In the welded joint No. 14 representing a COMPARATIVE EXAMPLE, the Si and Mn contents in the weld metal were above the ranges of the disclosed embodiments, and further the Cr content in the weld metal was below the range of the disclosed embodiments. As a result, silicon, manganese, and phosphorus were segregated into the last-solidified region during the welding process, thus causing hot cracks. Furthermore, excellent cryogenic impact toughness that was desired was not obtained and the absorbed energy vE−196 of the weld metal at a test temperature of −196° C. was less than 28 J.
In the welded joint No. 15 representing a COMPARATIVE EXAMPLE, the S and Mo contents in the weld metal were above the ranges of the disclosed embodiments, and MnS and Mo carbide possibly serving as fracture origins were formed. As a result, excellent cryogenic impact toughness that was desired was not obtained and the absorbed energy vE−196 of the weld metal at a test temperature of −196° C. was less than 28 J.
In the welded joint No. 16 representing a COMPARATIVE EXAMPLE, the Cr content in the weld metal was below the range of the disclosed embodiments, and consequently the weld metal had a 0.2% proof stress of less than 400 MPa and a tensile strength of less than 660 MPa, thus failing to achieve desired high strength. Furthermore, phosphorus was segregated into the last-solidified region during the welding process, thus causing hot cracks. Furthermore, excellent cryogenic impact toughness that was desired was not obtained and the absorbed energy vE−196 of the weld metal at a test temperature of −196° C. was less than 28 J.
In the welded joint No. 17 representing a COMPARATIVE EXAMPLE, the C content in the weld metal was above the range of the disclosed embodiments, and consequently carbides were formed in the weld metal to cause hot cracks. Furthermore, excellent cryogenic impact toughness that was desired was not obtained and the absorbed energy vE−196 at a test temperature of −196° C. was less than 28 J.
In the joint No. 18 representing a COMPARATIVE EXAMPLE, the Cr content in the weld metal was above the range of the disclosed embodiments, and consequently Cr carbide was formed. As a result, excellent cryogenic impact toughness that was desired was not obtained and the absorbed energy vE−196 of the weld metal at a test temperature of −196° C. was less than 28 J.
In the welded joint No. 22 representing a COMPARATIVE EXAMPLE, the P content in the weld metal was above the range of the disclosed embodiments, and phosphorus was segregated into the last-solidified region of the weld metal, thus causing hot cracks. Furthermore, the S content in the weld metal was above the range of the disclosed embodiments, and MnS possibly serving as fracture origins was precipitated. As a result, excellent cryogenic impact toughness that was desired was not obtained and the absorbed energy vE−196 of the weld metal at a test temperature of −196° C. was less than 28 J.
In the welded joint No. 23 representing a COMPARATIVE EXAMPLE, the Cr content in the weld metal was below the range of the disclosed embodiments, and consequently the weld metal had a 0.2% proof stress of less than 400 MPa and a tensile strength of less than 660 MPa, thus failing to achieve desired high strength. Furthermore, phosphorus was segregated into the last-solidified region during the welding process, thus causing hot cracks. Furthermore, excellent cryogenic impact toughness that was desired was not obtained and the absorbed energy vE−196 of the weld metal at a test temperature of −196° C. was less than 28 J. Furthermore, the welded joint had a tensile strength of less than 660 MPa, thus failing to achieve desired high strength.
In the welded joint No. 26 representing a COMPARATIVE EXAMPLE, the Si and Mn contents in the weld metal were above the ranges of the disclosed embodiments, and further the Cr content in the weld metal was below the range of the disclosed embodiments. As a result, silicon, manganese, and phosphorus were segregated into the last-solidified region during the welding process, thus causing hot cracks, and the weld metal had a 0.2% proof stress of less than 400 MPa, thus failing to achieve desired high strength. Furthermore, excellent cryogenic impact toughness that was desired was not obtained and the absorbed energy vE−196 of the weld metal at a test temperature of −196° C. was less than 28 J.
In the welded joints Nos. 33, 34, and 35 representing COMPARATIVE EXAMPLES, the Mn content in the steel material was below the range of the disclosed embodiments, and the austenite phases in the steel material were unstable. As a result, excellent cryogenic impact toughness that was desired was not obtained and the absorbed energy vE−196 of the welded heat affected zone at a test temperature of −196° C. was less than 28 J.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-031513 | Mar 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/008117 | 2/28/2022 | WO |
