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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite

Definitions

• the present invention relates to a high Mn steel suitable for use in a structure used in a cryogenic environment such as a tank for a liquefied gas storage tank, and a manufacturing method thereof.
• the steel plates used for this type of structure are required to have high strength and toughness at extremely low temperatures.
• the steel plates used for this type of structure are required to have high strength and toughness at extremely low temperatures.
• a hot-rolled steel sheet is used for a liquefied natural gas storage tank, it is necessary to ensure excellent toughness at an extremely low temperature of liquefied natural gas boiling point: -164°C or less. If the low temperature toughness of the steel material is inferior, the safety as a structure for a cryogenic storage tank may not be maintained, so there is a strong demand for improving the low temperature toughness of the applied steel material.
• austenitic stainless steel 9%Ni steel, or 5000 series aluminum alloy, which has a main structure of austenite that does not show brittleness at extremely low temperatures, has been conventionally used.
• these steels and alloys are high in alloy cost and manufacturing cost, there is a demand for a steel material that is inexpensive and has excellent low temperature toughness.
• Patent Document 1 As a new steel material replacing the conventional cryogenic steel, use of a relatively inexpensive, high Mn steel containing a large amount of Mn, which is an austenite stabilizing element, as a structural steel in a cryogenic environment.
• Patent Document 1 As a new steel material replacing the conventional cryogenic steel, use of a relatively inexpensive, high Mn steel containing a large amount of Mn, which is an austenite stabilizing element, as a structural steel in a cryogenic environment.
• Patent Document 1 proposes to control the carbide coverage of austenite grain boundaries. Further, Patent Document 2 proposes to control the austenite crystal grain size by adding a carbide coating and Mg, Ca, and REM.
• the austenitic steels used as the cryogenic steels described in Patent Document 1 and Patent Document 2 described above have large work hardening from the initial stage of deformation at the time of tensile deformation until reaching the maximum stress (tensile strength), and plasticity. Due to its excellent deformability, it has excellent ductility up to the middle stage of deformation. On the other hand, the deformability in the latter stage of deformation after the stress measured in the tensile test reaches the maximum (tensile strength) is also an important characteristic as a structural member. This is because the deformation performance in the latter stage of deformation is the performance at the final stage leading to the final destruction. From this point of view, it is necessary to secure sufficient ductility in the latter stage of deformation, especially the reduction value, and from the perspective of ensuring the ductility of high strength steel, a reduction value of 50% or more is desirable.
• the object of the present invention is to provide a high Mn steel which is excellent not only in high strength and low temperature toughness but also in ductility, and a manufacturing method thereof.
• the "high strength” means having a yield strength of 400 MPa or more and a tensile strength of 800 MPa or more at room temperature.
• the "excellent low temperature toughness” means a steel plate with a thickness of 10 mm or more, which was subjected to a Charpy impact test according to JIS Z2242 (1998) at -196°C, and a full size test piece (10 mm x 10 mm In the case of using a half size test piece (10 mm ⁇ 5 mm ⁇ 55 mm), the Charpy impact absorption energy (average value) is 100 J or more (a steel sheet having a plate thickness of less than 10 mm) in the base material. , 20 J or more by the Charpy V-notch half size test).
• excellent in ductility means having a diaphragm value of 50% or more.
• the inventors of the present invention have conducted the earnest research on a method for solving the above-mentioned problems, targeting high-Mn steel, and have obtained the following findings. That is, in the high Mn steel, by controlling the morphology of Ca-based inclusions, it is possible to improve the toughness and ensure the ductility (drawing value) during tensile deformation. It has been found that it is effective to keep the balance with the S amount within an appropriate range.
• the heating temperature of the steel material, the finish rolling finish temperature, and the average cooling rate from the temperature of (finish rolling finish temperature-100°C) or higher to the temperature range of 300°C or higher and 650°C or lower are set. It has been found that by limiting the amount, it is possible to control the crystal grain size, suppress the precipitate, and improve the low temperature toughness.
• Cu has an effect of improving the chloride stress corrosion cracking resistance under a low chloride concentration environment.
• Cu adversely affects chloride stress corrosion cracking resistance in a high chloride concentration environment.
• the inventors have optimized the balance between the Cu content and the Ni content in the high Mn steel containing Cu to add Ni, so that even in a high chloride concentration environment. It was found that excellent chloride stress corrosion cracking resistance can be exhibited. Thereby, excellent chloride stress corrosion cracking resistance can be imparted to the high Mn steel containing Cu regardless of the chloride concentration.
• the chloride stress corrosion cracking means that the tensile stress applied to the high Mn steel is the tensile strength of the high Mn steel in the corrosive environment peculiar to the high Mn steel, especially in the environment where chloride ions are present. Even if it is not more than the above, it means a phenomenon that the high Mn steel is cracked or fractured.
• the chloride stress corrosion cracking resistance means resistance to the chloride stress corrosion cracking.
• the present invention has been made by further studying the above findings, and the summary thereof is as follows. 1. In mass %, C: 0.10% or more and 0.70% or less, Si: 0.10% or more and 0.90% or less, Mn: 20% or more and 30% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.01% or more and 0.07% or less, Cr: 1.8% or more and 7.0% or less, Ni: 0.01% or more and less than 1.0%, Ca: 0.0005% or more and 0.010% or less, N: 0.0050% or more and 0.0500% or less, O: 0.0050% or less, Ti: 0.0050% or less and Nb: 0.0050% or less are contained, the following formula (1) is satisfied, and the balance has a component composition of Fe and unavoidable impurities and a structure having austenite as a matrix phase.
• the yield strength is 400 MPa or more
• composition of the components is% by mass, Cu: less than 2.0%, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, 1.
• a method for producing a high Mn steel which comprises performing a cooling treatment with an average cooling rate of 0.5° C./s or more from a temperature of finish rolling ending temperature ⁇ 100° C. or higher to a temperature range of 300° C. or higher and 650° C. or lower.
• C 0.10% or more and 0.70% or less
• Si 0.10% or more and 0.90% or less
• Mn 20% or more and 30% or less
• P 0.030% or less
• S 0.0070% or less
• Al 0.01% or more and 0.07% or less
• Cr 1.8% or more and 7.0% or less
• Cu 0.2% or more and less than 2.0%
• Ni 0.1% or more and less than 1.0%
• Ca 0.0005% or more and 0.010% or less
• N 0.0050% or more and 0.0500% or less
• O 0.0050% or less
• hot rolling is performed at a finish rolling end temperature of 750° C. or higher and lower than 950° C., and then (finishing A method for producing a high Mn steel, which comprises a cooling treatment at an average cooling rate of 0.5° C./s or more from a temperature equal to or higher than the rolling end temperature ⁇ 100° C.) to a temperature range of 300° C. or higher and 650° C. or lower.
• the present invention it is possible to provide a high Mn steel having high strength, excellent low temperature toughness particularly in an extremely low temperature range, and excellent ductility. Therefore, by using the high Mn steel of the present invention, it is possible to improve the safety and life of a steel structure used in a cryogenic environment such as a tank for a liquefied gas storage tank, which is an industrially significant effect. Play. Further, according to another aspect of the present invention, it is possible to provide a high Mn steel that exhibits excellent chloride stress corrosion cracking resistance regardless of the chloride concentration.
• the high Mn steel of the present invention will be described in detail.
• the "%" display in the component composition means “mass%”.
• C 0.10% or more and 0.70% or less
• C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite. In order to obtain the effect, it is necessary to contain C by 0.10% or more.
• the amount of C is set to 0.10 to 0.70%.
• the C content is preferably 0.20% or more, preferably 0.60% or less, and more preferably 0.20% or more and 0.60% or less.
• Si acts as a deoxidizer and is not only necessary for steelmaking, but also has the effect of forming a solid solution with steel to strengthen the steel sheet by solid solution strengthening. .. To obtain these effects, Si must be contained at 0.10% or more. On the other hand, if Si is contained in excess of 0.90%, the weldability deteriorates and the low temperature toughness, especially the toughness at extremely low temperatures, becomes low. Therefore, the Si amount is set to 0.10% or more and 0.90% or less. The amount of Si is preferably 0.12% or more, preferably 0.70% or less, and more preferably 0.12% or more and 0.70% or less.
• Mn 20% to 30% Mn is a relatively inexpensive austenite stabilizing element.
• Mn is an important element for achieving both strength and cryogenic toughness. In order to obtain the effect, Mn needs to be contained at 20% or more. On the other hand, even if Mn exceeds 30%, the effect of improving the low temperature toughness is saturated, and the alloy cost is increased. In addition, weldability and cuttability are deteriorated. Therefore, the Mn content is set to 20% or more and 30% or less.
• the Mn content is preferably 23% or more, preferably 28% or less, and more preferably 23% or more and 28% or less.
• the amount of P is preferably 0.002% or more.
• the P amount is preferably 0.005% or more, preferably 0.028% or less, and more preferably 0.024% or less. Further, the amount of P is more preferably 0.005% or more and 0.028% or less.
• the S amount is 0.0070% or less. It should be noted that excessive reduction of S increases the refining cost and is economically disadvantageous. Therefore, the S amount is preferably 0.001% or more.
• the amount of S is preferably 0.0020% or more, preferably 0.0060% or less, and more preferably 0.0020% or more and 0.0060% or less.
• Al acts as a deoxidizing agent, and is most commonly used in the molten steel deoxidizing process of steel sheets. In order to obtain such an effect, it is necessary to contain Al in an amount of 0.01% or more. On the other hand, if Al is contained in excess of 0.07%, it mixes in the weld metal during welding and deteriorates the toughness of the weld metal, so the Al content is made 0.07% or less. Therefore, the Al amount is 0.01% or more and 0.07% or less.
• the Al content is preferably 0.02% or more, preferably 0.06% or less, and more preferably 0.02% or more and 0.06% or less.
• Cr 1.8% or more and 7.0% or less
• Cr is an element that stabilizes austenite with an appropriate amount of addition and is effective in improving low temperature toughness and base material strength. In order to obtain such effects, it is necessary to contain Cr at 1.8% or more. On the other hand, when Cr is contained in an amount of more than 7.0%, low temperature toughness and stress corrosion cracking resistance are deteriorated due to the formation of Cr carbide. Therefore, the amount of Cr is set to 1.8% or more and 7.0% or less.
• the Cr content is preferably 2.0% or more, preferably 6.7% or less, and more preferably 2.0% or more and 6.7% or less. Further, in order to improve the stress corrosion cracking resistance, the Cr content is more preferably 2.0% or more and 6.0% or less.
• Ni 0.01% or more and less than 1.0%
• Ni has the effect of forming a solid solution in steel and strengthening the steel sheet by solid solution strengthening, and at the same time, has the effect of improving low temperature toughness, especially toughness at extremely low temperatures. Therefore, it is contained at 0.01% or more.
• the amount of Ni added is less than 1.0%.
• the Ni content is preferably 0.03% or more, preferably 0.8% or less, and more preferably 0.03% or more and 0.8% or less.
• stainless steels such as SUS304 and SUS316 as austenitic steels excellent in low temperature toughness, but these steels are optimized for Ni equivalent and Cr equivalent as an alloy design for obtaining an austenitic structure. , A large amount of Ni is added.
• the present invention is an austenitic material that has been made inexpensive by minimizing the Ni content. The necessary minimum amount of Ni was realized by optimizing the amount of Mn added.
• Ni 0.1% or more and less than 1.0%
• the high Mn steel contains a predetermined amount of Cu
• Ni by adding Ni by optimizing the balance between the Cu amount and the Ni amount, the chloride concentration can be increased. Therefore, it is possible to exert excellent chloride stress corrosion cracking resistance.
• the Ni content is 0.1% or more and less than 1.0%, as described later. If the Ni content is less than 0.1%, the effect on stress corrosion cracking cannot be obtained, and if the Ni content is 1.0% or more, the cost increases.
• Ca 0.0005% or more and 0.010% or less
• Ca improves the toughness by controlling the morphology of inclusions described below, and effectively acts to secure ductility (drawing value) during tensile deformation. To obtain such an effect, Ca needs to be 0.0005% or more.
• the amount of Ca is set to 0.0005% or more and 0.010% or less.
• the amount of Ca is preferably 0.0010% or more, preferably 0.0090% or less, and more preferably 0.0010% or more and 0.0090% or less.
• Ca/S ⁇ 1.0 It is important to control the morphology of Ca-based inclusions by further setting Ca/S within the appropriate range in the above Ca amount and S amount. That is, by setting Ca/S ⁇ 1.0, by promoting complex precipitation of MnS in the crystal grains with Ca-based inclusions as nuclei, precipitation/coarsening of MnS on the grain boundaries is suppressed, It is effective for improving the toughness and ensuring the ductility during tensile deformation, specifically for reducing the drawing value to 50% or more. In order to obtain such an effect, Ca/S needs to be 1.0 or more. Preferably, Ca/S is 1.7 or more.
• N is an austenite stabilizing element and is an element effective in improving low temperature toughness. In order to obtain such effects, it is necessary to contain N in an amount of 0.0050% or more. On the other hand, when N is contained in an amount of more than 0.0500%, the nitride or carbonitride is coarsened and the toughness is reduced. Therefore, the N content is set to 0.0050% or more and 0.0500% or less.
• the N content is preferably 0.0060% or more, preferably 0.0400% or less, and more preferably 0.0060% or more and 0.0400% or less.
• O 0.0050% or less O deteriorates the low temperature toughness due to the formation of an oxide. Therefore, O is set to 0.0050% or less. Preferably, the O content is 0.0045% or less. It should be noted that excessive reduction of the amount of O increases the refining cost and is economically disadvantageous, so the amount of O is preferably 0.0003% or more.
• Ti and Nb contents Suppressing Ti and Nb contents to 0.0050% or less, respectively.
• Ti and Nb form carbonitrides with high melting points in steel and suppress coarsening of crystal grains. As a result, the starting point of fracture and crack propagation become a route.
• it is necessary to intentionally suppress Ti and Nb because it hinders the microstructure control for enhancing the low temperature toughness and improving the ductility. That is, Ti and Nb are components inevitably mixed from raw materials and the like, and are generally mixed in the range of Ti: more than 0.005 to 0.010% and Nb: more than 0.005 to 0.010%. Is.
• the content of Ti and Nb is preferably less than 0.0050%, more preferably 0.003% or less.
• Cu 0.2% or more and less than 2.0%
• Cu has an effect of improving chloride stress corrosion cracking resistance under a low chloride concentration environment. From this viewpoint, it is effective to contain Cu in an amount of 0.2% or more. On the other hand, Cu worsens the chloride stress corrosion cracking resistance under the high chloride concentration environment. Therefore, when Cu is contained, the Cu content is less than 2.0%. If the Cu content is less than 0.2%, the effect on the stress corrosion cracking property cannot be obtained, and if the Cu content is 2.0% or more, the cost is increased in addition to the above problems.
• the Cu content is preferably 0.3% or more, 0.8% or less, and more preferably 0.3% or more and 0.8% or less.
• the balance other than the above essential components is iron and inevitable impurities.
• the unavoidable impurities here include H and the like, and it is acceptable if the total is 0.01% or less.
• the following elements may be contained, if necessary, in addition to the above essential components.
• Mo 2.0% or less
• V 2.0% or less
• W 2.0% or less
• Mg 0.0005 to 0.0050%
• REM 0.0010 to 0.0200%, one or two. More than seed
• Mo, V, W 2.0% or Less
• Mo, V, W contribute to the stabilization of austenite and to the improvement of the base metal strength. In order to obtain such effects, it is preferable that Mo, V and W be contained in an amount of 0.001% or more.
• Mo, V, and W when Mo, V, and W are each contained in excess of 2.0%, coarse carbonitrides may be generated, which may be the starting point of fracture, and pressure on the manufacturing cost. Therefore, when these alloy elements are contained, the content is 2.0% or less.
• the amount of each of Mo, V and W is more preferably 0.003% or more, preferably 1.7% or less, and more preferably 1.5% or less. Further, the respective amounts of Mo, V and W are preferably 0.003% or more and 1.7% or less, more preferably 0.003% or more and 1.5% or less.
• Mg 0.0005 to 0.0050%
• REM 0.0010 to 0.0200%
• Mg and REM are elements useful for controlling the morphology of inclusions, and can be contained if necessary.
• the morphology control of inclusions means that expanded sulfide-based inclusions are made into granular inclusions.
• the ductility, toughness and sulfide stress corrosion cracking resistance are improved by controlling the morphology of the inclusions.
• it is preferable that the Mg content is 0.0005% or more and the REM content is 0.0010% or more.
• the content of Mg is 0.0005 to 0.0050%
• the content of REM is 0.0010% to 0.0200%.
• the amount of Mg is more preferably 0.0010% or more, more preferably 0.0040% or less, and further preferably 0.0010% or more and 0.0040% or less.
• the REM amount is more preferably 0.0020% or more, more preferably 0.0150% or less, and further preferably 0.0020% or more and 0.0150% or less.
• the steel may cause brittle fracture in low temperature environment and is suitable for use in low temperature environment.
• the matrix phase of the steel material is an austenite structure having a face-centered cubic structure (fcc) as the crystal structure.
• fcc face-centered cubic structure
• “using austenite as the base phase” means that the austenite phase has an area ratio of 90% or more.
• the balance other than the austenite phase is a ferrite phase or a martensite phase, but needless to say, the austenite phase may be 100%.
• the method for producing a high Mn steel of the present invention includes a step of heating a steel material having the above-described composition, a step of hot rolling the heated steel material, and a hot rolled sheet that has been hot rolled. It includes a step of performing a cooling process. And in the method for producing a high Mn steel of the present invention, the temperature range in the step of heating the steel material is set to 1100° C. or higher and 1300° C. or lower, and the finish rolling end temperature in the step of performing the hot rolling is 750° C. or higher. The temperature is less than 950° C., and the average cooling rate from the temperature of (finishing rolling end temperature-100° C.) or more to the temperature range of 300° C. or more and 650° C. or less in the step of performing the cooling treatment is 0.5° C./s. The above is characterized.
• molten steel having the above-described composition can be melted by a known melting method such as a converter or an electric furnace. Further, secondary refining may be performed in a vacuum degassing furnace. At that time, in order to limit the Ti and Nb which hinder the preferable microstructure control to the above-mentioned range, it is necessary to avoid the inevitable mixture of Ti and Nb from the raw materials and the like, and to take measures to reduce the content thereof. Need to take. For example, by reducing the basicity of the slag during the refining stage, these alloys are concentrated into slag and discharged to reduce the Ti and Nb concentrations in the final slab product.
• a method may be used in which oxygen is blown in to oxidize and the Ti and Nb alloys are floated and separated during reflux.
• a steel material such as a slab having a predetermined size is preferably formed by a known casting method such as a continuous casting method or an ingot making method. Note that the slab after continuous casting may be subjected to slab rolling to obtain a steel material.
• the manufacturing conditions for forming the above steel material into a steel material excellent in high strength, low temperature toughness, and ductility will be specifically specified.
• Steel material heating temperature 1100° C. or more and 1300° C. or less
• the heating temperature before hot rolling is set to 1100° C. or more.
• the upper limit of the heating temperature is set to 1300° C.
• the temperature control here is based on the surface temperature of the steel material.
• Finish rolling end temperature 750° C. or higher and lower than 950° C.
• the cumulative rolling reduction at high temperature. That is, when hot rolling is performed at a low temperature, the microstructure becomes fine, and excessive working strain is introduced, resulting in a decrease in low temperature toughness. Therefore, the lower limit of the finish rolling end temperature in hot rolling is set to 750° C. as the surface temperature of the steel sheet.
• finishing is performed in a temperature range of 950° C. or higher, the crystal grain size becomes excessively large and desired yield strength cannot be obtained. Therefore, it is necessary to carry out final finishing rolling of less than 950°C for one pass or more.
• the cooling end temperature is set to a temperature range of 300° C. or higher and 650° C. or lower. This is because cooling to the above temperature range can suppress the precipitation of carbides, which causes a decrease in toughness.
• the average cooling rate of the steel sheet surface from the surface temperature of the steel sheet (finishing rolling end temperature-100°C) or higher to the temperature range of 300°C or higher and 650°C or lower. Is 0.5° C./s or more.
• the average cooling rate is preferably 200° C./s or less.
• the cooling rate is calculated as an average cooling rate of the steel sheet by simulation calculation based on the temperature change of the surface.
• the cooling time in the temperature range of 1400°C to 1300°C as the surface temperature of the steel during cooling it is preferable to control the cooling time in the temperature range of 1400°C to 1300°C as the surface temperature of the steel during cooling to 100 s or less.
• the cooling time in the casting step as described above, the complex precipitation of MnS with Ca-based inclusions such as Ca(O,S) as nuclei is promoted, and the number of (Ca,Mn)S increases.
• the proportion of elongated MnS is reduced without MnS growing at or within the grain boundaries.
• By controlling the morphology of such Ca-based inclusions it is possible to obtain a high Mn steel having a good reduction value of 51% or more.
• a steel slab having the composition shown in Table 1 was produced as a steel material by a converter-ladle refining-continuous casting method. Then, the obtained steel slab was subjected to slab rolling and hot rolling under the conditions shown in Table 2 to obtain a steel plate having a maximum thickness of 32 mm.
• the tensile properties, toughness, and microstructure evaluations of the steel sheets were carried out as follows.
• the average value of the three absorbed energies is 100 J or more, and the low temperature toughness of the base material is excellent.
• a Charpy V-notch half size test piece was sampled and a similar Charpy impact test was performed.
• an average value of 20 J or more was defined as excellent in low temperature toughness of the base material.
• Samples 32 and 33 were subjected to a boiling magnesium chloride stress corrosion cracking test in accordance with ASTM G36.
• the test piece was a U-bending test piece according to ASTM G30 Example a.
• a test piece of 2.5 mm thickness ⁇ 20 mm width ⁇ 80 mm length was sampled from the position of 1 mm below the surface of the steel sheet in the C direction, and the central portion of the test piece in the longitudinal direction was bent at 5R for the test.
• the test time was 400 hours.
• the test piece in which no crack was confirmed on the surface was judged to have excellent resistance to chloride stress corrosion cracking.
• Table 3 the case where no cracks were visually confirmed was indicated as ⁇ , and the case where cracks were visually confirmed was indicated as x.
• the high Mn steel according to the present invention has the above-mentioned target performances (yield strength of base material is 400 MPa or more, drawing value is 50% or more, low temperature toughness is 100 J or more as an average value of absorbed energy (vE ⁇ 196 ) (half size test). In the case of one piece, it was confirmed that 20 J or more)) was satisfied. On the other hand, in Comparative Examples outside the scope of the present invention, at least one of the yield strength, the drawing value and the low temperature toughness does not satisfy the above target performance.
• Sample 32 containing Cu and Ni so that Cu/Ni was within the predetermined range exhibited excellent chloride stress corrosion cracking resistance.
• sample 33 in which Cu/Ni was out of the predetermined range sufficient chloride stress corrosion cracking resistance could not be confirmed.

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