JPH0245695B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention has excellent rust resistance in a humid environment, and
This relates to a cryogenic tough steel having a yield strength of 1100 MPa or more and a Charpy absorbed energy of 50 J or more at the following cryogenic temperatures. (Prior Technology) In recent years, experimental nuclear fusion reactors, particle accelerators, magnetic levitation trains, etc. using superconducting electromagnets have been constructed in Japan, Europe, and the United States, and are expected to be put into practical use in the near future. Among these facilities, especially superconducting electromagnets, superconductor holding materials, and surrounding structural materials are
Since it is cooled to an extremely low temperature of around 4.2K, it is inevitably cooled to the same temperature, and at the same time, due to the increase in electromagnetic force due to the huge magnet capacity, it is used under high stress that could not be predicted in the past. I am trying to do.
Therefore, such structural materials are required to have high strength and good toughness at extremely low temperatures around 4.2K in order to withstand these usage conditions. Furthermore, such structural materials may be used in contact with vacuum after being given a metallic luster to the surface as part of a heat insulating structural member, and the surface is required to maintain its metallic luster in order to maintain a high vacuum. Ru. Therefore, in such cases, rust resistance is also an important characteristic, that is, that the shiny metal surface does not rust due to condensation or ice formation due to exposure to the atmosphere or when the atmosphere enters due to deterioration of the vacuum level of the equipment. . In other cases as well, rust resistance is considered one of the characteristics that cannot be ignored. To improve low-temperature strength in response to current demands.
Ni-Cr based austenitic stainless steels with higher N content than usual, such as SUS304LN and 316LN, are used. Although these steels have extremely excellent rust resistance, they have the disadvantage that the austenite structure becomes unstable at extremely low temperatures around 4.2K, resulting in a decrease in toughness, and their strength does not meet the recent demands for higher strength. It is not always sufficient, and it is expensive because it contains a large amount of Ni. On the other hand, recently, Japanese Patent Application Publication No. 57-2868
High Mn austenitic steels have been developed as low-temperature structural steels, as shown in Japanese Patent Application Laid-Open No. 58-144418, but they do not have the above-mentioned rust resistance and have insufficient strength. Furthermore, it has the disadvantage that machining such as cutting and drilling is extremely difficult. At present, both of these steels
It is generally considered difficult to obtain the high strength and high toughness that are the objectives of the present invention at temperatures around 4.2K. (Problems to be Solved by the Invention) The present invention eliminates the drawbacks of existing steels, does not rust when exposed to the atmosphere or when condensed, and has a yield strength of more than 1100 MPa and good toughness at extremely low temperatures around 4.2 K. The purpose of this project is to make low-cost, high-strength steel for cryogenic use that has excellent workability and excellent workability. (Means for Solving the Problems) The purpose of the present invention is to achieve C0.20% or less, Si0.05 to 2.5%,
Mn16~35%, Cr10~20%, Ni0.1~8.0%,
Contains N0.10~0.50%, Al0.001~0.20%, S0.003% or less, or as necessary, Mo0.05~4.0
%, one or more of Cu, W, Co in total
Contains 0.01 to 4.0%, one or more of Nb, Ti, and V in a total of 0.005 to 2.0%, with the remainder consisting of iron and unavoidable impurities, and has excellent rust resistance.
This is achieved by providing a high manganese tough steel for cryogenic use that has a yield strength of 1100 MPa or more and a Charpy absorbed energy of 50 J or more. First, the reasons for limiting the steel components in the present invention will be described.The mechanical properties described here are properties at 25K or lower. C is an extremely effective austenite stabilizing element in steels containing large amounts of Mn and Cr, and by stabilizing austenite it improves toughness at cryogenic temperatures below 25K and further increases tensile strength. However, if its content exceeds 0.2%, workability will be significantly deteriorated, and Cr carbides will precipitate in areas heated to 600 to 800°C during welding or stress relief annealing, reducing rust resistance. It also reduces toughness. Therefore, the amount of C was set to 0.20% or less. Si is an effective component for increasing strength, and if it is less than 0.05%, the effect is insufficient, and if it exceeds 2.5%, it destabilizes austenite and reduces toughness, so its content was set at 0.05 to 2.5%. Mn stabilizes the austenite phase of steel containing 10 to 200% Cr and improves its strength and toughness at cryogenic temperatures, but if it is less than 16%, a sufficient amount of austenite cannot be obtained, and as the amount of Mn increases, the strength increases. , toughness improves, but if it exceeds 35%, it becomes difficult to melt the steel, and the amount of Mn alloy input increases, making it expensive, so it was set to 35% or less. Cr is an active ingredient that provides rust resistance and stabilizes austenite, and if it is less than 10%, sufficient rust resistance cannot be obtained and rust will inevitably occur when exposed to the atmosphere. Furthermore, since it is not possible to obtain a sufficient amount of austenite to stabilize toughness by generating a ferrite phase exceeding 20%, the amount is set to 10 to 20%. Ni has the effect of preventing the transformation from austenite to α' martensite, improving strength and toughness, and reducing the amount of Mn and N, which are problems in melting. However, if it is less than 0.1%, no significant effect is obtained. I can't. On the other hand, Ni has less reinforcing effect than Mn, and also reduces the solubility of N, so the upper limit is
It was set at 8.0%. Note that since Ni deteriorates rust resistance, it is advantageous to keep it preferably at 6.0% or less. N causes austenite to be included and has a remarkable effect on increasing strength, especially proof stress, at extremely low temperatures. In particular, the steel of the present invention contains a large amount of Mn and Cr, which increase the solubility of N, so Cr-Ni contains a large amount of Ni, which has the effect of reducing the solubility of N.
Compared to austenitic stainless steels, it is possible to obtain high strength with a small amount of N, which is extremely advantageous for increasing strength. Furthermore, low C high like the steel of the present invention
N also has a remarkable effect on improving the rust resistance of Mn-high Cr steel. The target lower limit yield strength of the steel of the present invention at 4.2K is 1100 MPa, but if N is less than 0.10%, a yield strength of 1100 MPa cannot be obtained, and sufficient rust resistance cannot be obtained. Increasing the amount of N increases strength and improves rust resistance, but if N exceeds 0.5%, toughness decreases and it becomes difficult to melt or weld the steel, so the range of N is 0.10 to 0.50%. limited to. Al is used to improve the hot workability of Mn-Cr austenitic steel and to refine the austenite grains to increase the strength.
If the content is less than 0.20%, the effect will be insufficient, while if it exceeds 0.20%, the toughness will decrease, so the content should be reduced to 0.001 to 0.20%.
%. It is a major feature of the present invention that the S content is 0.003 or less. That is, the present inventors have developed steel that satisfies the composition range of the present invention except for S.
As a result of investigating the influence of S on the Charpy absorbed energy at 4.2K, we found that its content was
It has been found that the energy can be significantly improved by reducing the content to 0.003% or less. Figure 1 shows 0.02%C-0.3%Si-25%Mn-15%
A 13mm thick plate of Cr-1%Ni-0.20%N steel
After solid solution heat treatment at 1100℃, JIS No. 4 Charpy impact test was taken, and the results of the impact test at 4.2K are shown. As is clear from these results, it can be seen that when the amount of S is 0.003% or less, the Charpy absorbed energy is significantly improved. From this, the amount of S was set to 0.003% or less. Furthermore, in the present invention, Mo, Cu,
The strength and toughness of steel can be improved by containing W, Co, Nb, Ti, and V. As a result of numerous experiments, it has been found that adding an appropriate amount of Mo to high Mo-high Cr steel with extremely low carbon content, such as the one used in the present invention, is effective in annealing and welding after stress relief.
It is extremely effective in preventing the deterioration of low-temperature toughness caused by the precipitation of Cr carbides, which generally occurs when heated to a temperature range of 600 to 800°C. Figure 2 shows 0.03%C-0.3%Si-25%Mn-15%
By adding Mo to Cr-3%Ni-0.20%N steel, it is further added to the steel that has been subjected to solution heat treatment.
This figure shows the change in toughness when sensitized at 700°C. The values of 0.2% proof stress and Charpy impact absorption energy are obtained using a round bar tensile test piece with a parallel part diameter of 7 mm and a gauge length of 45 mm, respectively, and JIS4
These are the results of testing the No. 1 impact test piece in liquid helium (4.2K). These results show that when Mo is added in an amount of 1% and 2%, the reduction in Charpy absorbed energy due to the sensitization treatment is significantly reduced compared to when Mo is not added at all. Such an effect is observed with Mo of 0.05% or more, so the lower limit is set at 0.05%. However, if it exceeds 4%, a ferrite phase is formed and the toughness decreases, so the upper limit is set at 4.0%.
%. Cu, W, and Co have the effect of improving toughness and strengthening the base at 0.01% or more, but when it exceeds 4.0%, toughness begins to deteriorate, so the total content of these components is
Must be between 0.01 and 4.0%. Nb, Ti, and V increase strength through precipitation hardening, and also have the effect of refining the grains of steel and improving toughness. However, if the total content of these elements exceeds 2.0%, the toughness decreases, so the total content of these elements was set to 0.005 to 2.0%. A temperature below 25K is used as the cryogenic temperature, because most current superconducting electromagnets are cooled with liquefied helium (4.2K), and liquefied helium was used as the coolant for the convenience of the test. The actual operating temperatures range from the temperature of superfluid liquefied helium (2.1K) to the temperature of liquefied hydrogen (20.4K). It has a yield strength of 1100MPa or more (at 0.2 permanent elongation) at a temperature of 25K or less. This is because the operating stress of the equipment mentioned above is approximately 1000 MPa at maximum, and in practical terms
It is desirable to provide a yield strength of 1200MPa or more.
For this purpose, it is necessary to have a yield strength of 330 MPa or more at room temperature. Similarly, these facilities require toughness of 50 J or more of Sharpy absorbed energy (based on JIS No. 4 test piece) at 4.2 K.
The steel of the present invention also has a good absorbed energy of 50 J or more, but preferably 80 J or more. Traditionally, structural steels used in such applications have been required to be non-magnetic because they are used in strong magnetic fields, and therefore required to be fully austenitic at service temperatures around 4.2K. However, the steel of the present invention does not necessarily need to be completely non-magnetic at around 4.2K, and if it has rust resistance, proof stress of 1100MPa or more at 4.2K, and sharpy absorbed energy of 50J or more, it becomes austenite. This does not preclude the presence of a ferromagnetic ferrite phase in addition to the phase. For this purpose, it is sufficient that the austenite phase exists in an amount of 80% or more, and the alloying elements can be increased, decreased, or selected within this limit. Of course, it is possible to make completely austenitic steel at 4.2K, and in this case Cr
is 10-18%, Mn is 20-27%, N is 0.20-0.40%
It is desirable to do so. The steel of the present invention is manufactured according to the steps described below. That is, molten steel of a specified composition is obtained using a melting furnace such as a converter or an electric furnace, or if necessary, a vacuum degassing method, a ladle refining method, a remelting method, etc.
After forming this into a steel ingot, it is heated to 1000 to 1250°C and subjected to blooming rolling, forging, or continuous forging to obtain a steel billet. This piece of steel is processed using the usual method.
After being reheated to 1,000 to 1,250℃, or immediately after producing the billet, it is processed by rolling or forging without being cooled down to low temperatures to produce thick plates, shaped steel, steel bars, wire rods, hot-rolled sheets, etc. Ru. Cooling after rolling or forging may be allowed to cool naturally as is usually done, but rapid cooling such as water cooling may be performed to prevent deterioration of toughness and rust resistance due to precipitation of carbonitrides during cooling. The steel of the present invention may be rolled or forged, but in order to make the rust resistance and toughness more stable,
It is desirable to perform solution treatment by heating to 1150°C. Furthermore, the hot-rolled thin sheets produced in this way are cold-rolled using a normal method to
It can be heat treated at 1150°C for recrystallization and solid solution formation to form a cooling thin plate. Further, the steel of the present invention can be processed such as cutting and bending, and welded without any problem. It can also be used as a bolt by hot or cold forming. (Example) Next, an example of the present invention will be described. Table 1 shows the chemical composition, mechanical properties, and corrosion resistance of the steel of the present invention and comparative steel. Among these, the steel of the present invention is produced by melting it in an electric furnace, heating it to a steel ingot at 1100-1200°C, rolling it into a steel billet, heating it again to 1100-1200°C, and reducing the thickness. 10~50mm
These are the results of corrosion resistance tests by tensile, impact, and outdoor exposure on plates that were rolled into thick plates and then subjected to solution heat treatment at 1000 to 1150°C.
However, only Steel 2 was water quenched directly at 1000°C after thick plate rolling, and was not subjected to solution heat treatment thereafter. Among the comparison steels, steel No. 38 is SUS304LN and No. 39
The steel No. 10 is a low C-25% Mn steel developed as a low temperature steel with a small coefficient of thermal expansion. As can be seen from this result, Nos. 1 to 37 of the present invention
The steel has a 0.2% yield strength of over 1100MPa and a absorbed energy of over 138J at 4.2K, showing extremely excellent strength and toughness. Also, for example 60
No rust was observed in the outdoor exposure test for several days. Furthermore, these steels were immersed in air-saturated water for 30 days, but no rust occurred. On the other hand, steels No. 38 and 39 listed as comparative steels have a high yield strength.
Lower than 1100MPs, No. 40 steel also has poor rust resistance. Further, the steel of the present invention was completely non-magnetic and had a transmittance of 1.005 or less. (Effects of the invention) As is clear from the above experimental results, the steel of the present invention has superior strength, rust resistance, and toughness compared to conventional high-Mn steel, and is also superior to austenitic stainless steel with high Ni and Cr contents. In addition to equivalent rust resistance, it has superior strength and properties at extremely low temperatures than stainless steel, and can be supplied at a lower cost than stainless steel, which is of great industrial benefit.

【table】

【table】 [Brief explanation of the drawing]

Figure 1 shows 0.02%C-0.3%Si-25%Mn-15%Cr
-1%Ni-0.20%N A thick plate (13mm) of N steel was solution heat treated at 1100℃ and then subjected to an impact test at 4.2K. Figure 2 is 0.03%C-0.3%Si-25% Mn
- Adding Mo to 15% Cr - 3% Ni - 0.2% N steel,
FIG. 2 is a diagram showing changes in toughness when a steel plate that has been subjected to solution heat treatment is further subjected to sensitization treatment at 700°C.

Claims (1)

[Claims] 1 C 0.20% or less, Si 0.05 to 2.5%, Mn 16 to 35%,
Cr10~20%, Ni0.1~8.0%, N0.10~0.50%,
A rust-resistant cryogenic high manganese tough steel containing 0.001 to 0.20% Al and 0.003% or less S, with the balance consisting of iron and inevitable impurities. 2 C0.20% or less, Si0.05~2.5%, Mn16~35%,
Cr10~20%, Ni0.1~8.0%, N0.10~0.50%,
Al0.001~0.20%, S0.003% or less and Mo0.05~
4.0%, with the balance consisting of iron and unavoidable impurities. 3 C0.20% or less, Si0.05~2.5%, Mn16~35%,
Cr10~20%, Ni0.1~8.0%, N0.10~0.50%,
Contains Al0.001~0.20%, S0.003% or less, and also contains one or more of Cu, W, and Co in total.
Rust-resistant high manganese tough steel for cryogenic use, characterized by containing 0.01 to 4.0%, with the remainder consisting of iron and inevitable impurities. 4 C0.20% or less, Si0.05~2.5%, Mn16~35%,
Cr10~20%, Ni0.1~8.0%, N0.10~0.50%,
Contains Al0.001~0.20%, S0.003% or less, and also contains one or more of Nb, Ti, and V in total.
Rust-resistant high manganese tough steel for cryogenic use, characterized by containing 0.005 to 2.0%, with the remainder consisting of iron and inevitable impurities. 5 C0.20% or less, Si0.05~2.5%, Mn16~35%,
Cr10~20%, Ni0.1~8.0%, N0.10~0.50%,
Contains Al0.001~0.20%, S0.003% or less, and also contains one or more of Cu, W, and Co in total.
0.01 to 4.0%, and one or two of Nb, Ti, and V
Rust-resistant high manganese tough steel for cryogenic use, characterized in that it contains 0.005 to 2.0% in total of 0.005% to 2.0%, with the balance consisting of iron and unavoidable impurities. 6 C0.20% or less, Si0.05~2.5%, Mn16~35%,
Cr10~20%, Ni0.1~8.0%, N0.10~0.50%,
Al0.001~0.20%, S0.003% or less and Mo0.05~
4.0%, and further contains one or more of Cu, W, and Co in a total of 0.01 to 4.0%, with the balance consisting of iron and inevitable impurities. Manganese tough steel. 7 C0.20% or less, Si0.05~2.5%, Mn16~35%,
Cr10~20%, Ni0.1~8.0%, N0.10~0.50%,
Al0.001~0.20%, S0.003% or less and Mo0.05~
4.0%, and further contains one or more of Nb, Ti, and V in a total of 0.005 to 2.0%, with the balance consisting of iron and inevitable impurities. Manganese tough steel. 8 C0.20% or less, Si0.05-2.5%, Mn16-35%,
Cr10~20%, Ni0.1~8.0%, N0.10~0.50%,
Al0.001~0.20%, S0.003% or less and Mo0.05~
4.0%, and further contains one or more of Cu, W, and Co in a total of 0.01 to 0.4%, and Nb, Ti,
A rust-resistant, high manganese tough steel for cryogenic use, which contains one or more types of V in a total amount of 0.005 to 2.0%, with the remainder consisting of iron and inevitable impurities.