JPH0765143B2 - Cryogenic non-magnetic austenitic stainless steel with excellent reheat resistance - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a nonmagnetic austenitic stainless steel for cryogenic use which is used in a solution state, and more specifically, has a strength even when subjected to reheating after solution treatment. The present invention relates to a cryogenic non-magnetic austenitic stainless steel that does not deteriorate in toughness and the like.

[Prior Art] Solution heat treatment is a basic heat treatment of austenitic stainless steel for cryogenic use, and it is important to enhance mechanical properties, especially toughness, by solid solution of Cr carbide or embrittlement phase such as σ phase. Plays a role. However, welding means etc. will be adopted to remanufacture cryogenic containers etc. using solution heat treated austenitic stainless steel, and as a result, reheating such as heat affected zone near the welded part will be performed. Grain boundary precipitation of carbides from the solution state occurs at the receiving site, and corrosion resistance and low temperature toughness deteriorate.
Therefore, there is a demand for an ultra-low temperature austenitic stainless steel that does not cause deterioration in properties, especially low temperature toughness and corrosion resistance, even when subjected to reheating such as welding.

On the other hand, recently, due to the demand for a structural material for housing a superconducting magnet, expectations for an austenitic stainless steel that is non-magnetic and that exhibits high strength and high toughness in a cryogenic environment are increasing. However, for example, in manufacturing a high magnetic field superconducting magnet using Nb ₃ Sn as a superconductor,
Cu-Sn in the matrix by heat treatment after processing the complex is embedded, the Nb core, by reacting Sn and Nb in the matrix Nb ₃ S
Such a heat treatment is often performed after the composite is incorporated into the constituent materials, although it forms n superconductors. However, since the heat treatment temperature at this time is usually 650 to 750 ° C., when the cryogenic austenitic stainless steel is used as the structural material, grain boundary precipitation of carbide is very likely to occur, and under cryogenic conditions It cannot be used at all. Therefore, as a structural material for superconducting magnets, SUS6
A so-called superalloy such as age-hardening heat-resistant stainless steel or Ni-based heat-resistant alloy such as 60 is used, or SUS321 or SUS347 containing Nb or Ti is preliminarily subjected to stabilization treatment for carbon fixation. After that, measures such as applying the heat treatment are taken.

However, in the case of the above superalloy, there is a problem that the cost of the superalloy becomes very high because it contains a very expensive element and requires special heat treatment. In addition, there is a problem that the weight of an ingot that can be manufactured is limited due to the difficulty of the casting technique, and there are various problems that it is a difficult-to-cut material and lacks workability and that welding is not easy. On the other hand, SUS321
When using SUS347 or SUS347 with stabilizing treatment, the heat treatment cost is added, so that the material is also expensive, and the material with different working history and welding history is always subjected to optimal stabilizing treatment. Is extremely difficult,
It is difficult to obtain the expected characteristics (low temperature toughness, etc.) by heat treatment, and there is a problem that strength is generally low.

[Problems to be Solved by the Invention] The present invention has been made in view of such circumstances, and does not require addition of expensive special components or special heat treatment in advance, and can be reheated at about 600 to 800 ° C. To provide a non-magnetic cryogenic austenitic stainless steel that has almost no deterioration in cryogenic properties even when heated, that is, has a balance of yield strength and fracture toughness value equal to or higher than that of a normal austenitic stainless steel. It is intended.

[Means for Solving the Problem] However, the nonmagnetic austenitic stainless steel for cryogenic use of the present invention which has achieved the above-mentioned object is C: 0.05% (meaning weight%, the same applies hereinafter) or less Mn: 1 to 10% Si: 0.5% or less P: 0.03% or less S: 0.01% or less Cr: 13 to 20% Ni: 10 to 15% Nb: 0.02 to 0.10% N: 0.1 to 0.25% Mo: 1.5 to 4.5% B: 0.002 to 0.006 % Remainder: The first point is that it consists of Fe and inevitable impurities, and one or more selected from Ca, Mg, Zr, and Ce in addition to the above structure: 0.001 to 0 in total.
It has a second gist in that it contains 01%.

[Operation] The chemical components of the cryogenic austenitic stainless steel according to the present invention and the reasons for limitation thereof will be described.

C is an element effective for stabilizing austenite and improving yield strength at extremely low temperatures, but since it causes precipitation of grain boundary carbides by reheating and causes deterioration of toughness at extremely low temperatures, it is 0.05% or less, preferably 0.03% or less. I have to hold back. Precipitation of grain boundary carbides due to reheating tends to occur more easily as the reheating temperature increases. If the reheating temperature is up to about 650 ° C, it is sufficient to suppress the C content to 0.05% or less. When the heating temperature is considered to be about 700 ° C. or higher, it is desirable to limit the C content to 0.03% or less.

Mn has the effect of stabilizing austenite and increasing the solid solubility limit of nitrogen, and also has the effect of improving hot workability, but even if added more than necessary, the effect does not rise any further, so the amount added It is specified to 1 to 10%.

Cr is an essential component in forming austenitic stainless steel together with Ni, but has the effect of suppressing high temperature intergranular corrosion at the time of reheating, and withstands the temperature range below room temperature in relation to the object of the present invention. It also has the function of increasing rust. However, if added more than necessary, it causes precipitation of σ phase during reheating and causes deterioration of toughness. Therefore, in the present invention, the Cr amount is defined as 13 to 20%.

Ni has the effect of improving ductility and toughness at extremely low temperatures, but even if added more than necessary, the effect does not increase, so in the present invention, the Ni content was made 10 to 20%. Ni is an essential component for stabilizing austenite, and its effect is
The equivalent can be expressed by the following formula. In the steel of the present invention, the reduction of the solid solution element due to reheating can be almost ignored, but the solid solution carbon is reduced by the reheating, so that the term of carbon in the formula (1) below is substantially omitted. The Ni equivalent formula is valid.

Ni equivalent = Ni + 0.5 Mn + 30C + 30N (1) On the other hand, when the α'phase is generated by processing such as cold cutting or bending, it changes to a ferromagnetic material, so it is used as a structural material for superconducting magnets. Requires a condition for obtaining stable austenite that does not generate an α'phase, and it is desirable to adjust the component composition so that the Ni equivalent satisfies the following formula (2).

Ni equivalent ≧ 18 + | Cr-18 | (2) Nb fixes carbon as NbC fine precipitates during reheating,
Further, it has an effect of fixing Cr as fine precipitates of NbCrN, and as a result, it is possible to suppress grain boundary precipitation of Cr ₂₃ C ₆ , which is a cause of grain boundary embrittlement. However, if added excessively, the size of the precipitate becomes coarse and the toughness deteriorates rather. For the above reason, the Nb content needs to be 0.02 to 0.1%. The reason for limiting the amount of Nb in the present invention is not to completely fix carbon with Nb / C of 10 or more like SUS347, but to finely and uniformly precipitate NbC and NbCrN to produce Cr and Nb. fixed suppresses grain boundary precipitation of Cr ₂₃ C _6, even if there is some Cr ₂₃ C ₆ (Nb / C is at 10 or less) which covers the NbC and NbCrN of fine precipitation, mechanical It is based on the finding that the characteristics will not be degraded.

N has the effect of stabilizing austenite and significantly improving the proof stress at extremely low temperatures, and is an important component in the present invention for ensuring high strength. Also, during reheating, carbon easily diffuses into the grain boundaries and precipitates Cr ₂₃ C ₆ , but since nitrogen is the same interstitial element as carbon, it competes with carbon to suppress its diffusion, and Cr ₂₃ C ₆ It has the effect of preventing precipitation. However, since it is difficult to add 0.25% or more due to the solid solubility limit, the N content was set to 0.1 to 0.25%.

Mo has the effect of suppressing the diffusion of impurities to the grain boundaries during reheating and suppressing the grain boundary embrittlement, but if added excessively, Mo compounds will precipitate and cause grain boundary embrittlement. To 1.
Specified as 5 to 4.5%. The effect of suppressing the grain boundary embrittlement due to the addition of Mo tends to increase as the reheating temperature increases. If the reheating temperature is about 700 ° C or higher, the Mo addition amount of 1.5% or more is sufficient. When the heating temperature is up to about 650 ° C, it is desirable to increase the amount of added Mo to 2.5% or more. On the other hand, since the precipitation of Mo compounds is more likely to occur as the reheating temperature becomes higher, it is desirable to keep the Mo addition amount at 2.5% or less when the reheating temperature is about 700 ° C or higher, while the reheating temperature is 650 Even if the temperature is up to about ℃, it must be kept below 4.5%.

B is an element that tends to segregate preferentially at grain boundaries, and in the solution state, segregation of B deteriorates the cryogenic toughness.
It has the effect of preventing the precipitation of carbides at the grain boundaries when it is reheated for a long time. In order to obtain the above effect, 0.002% or more must be added, but excessive addition has the adverse effect of lowering the cryogenic toughness in the solution state and worsens hot workability, so it is limited to 0.006% or less. There is a need.

Si is required as a deoxidizing component during steel making, but it is a component that promotes deterioration of toughness during reheating, so the addition amount must be kept to the minimum necessary. For this reason, the Si content is 0.
Must be kept below 5%. Since P promotes deterioration of toughness during reheating similarly to Si, P is a better component if it is small, so in the present invention, it is limited to 0.03% or less.

S is a component that forms a sulfide to lower the cryogenic toughness and accelerates the deterioration of the toughness during reheating, and also deteriorates the hot workability, so it is desirable to limit the content as much as possible. The amount of S must be kept below 0.01%.

In addition to the above component composition, if necessary, one or more elements selected from Ca, Mg, Zr, and Ce are added in a total amount of 0.001% or more, so that the toughness can be further improved. That is, since these elements have a deoxidizing action and a desulfurizing action and have an effect of removing inclusions during steelmaking, they improve toughness as a result of cleaning. If the addition amount is less than 0.001%, there is no effect, but if added in excess, the hot workability deteriorates.
It is necessary to keep it below%.

[Example] Example 1 A steel having the composition shown in Table 1 was hot-rolled to a thickness of 28
After processing into a steel plate of mm, the solution treatment was performed at 1050-1100 ° C. Then, after reheating at 700 ° C for 75 hours, -269
When the tensile test and the fracture toughness test were performed at a temperature of ° C to evaluate the properties, the results shown in Table 2 were obtained. Table 2 also shows the results of observing the morphology of the fracture surface of the fracture toughness test piece with a scanning electron microscope and the results of magnetic permeability measurement.

As shown in Tables 1 and 2, the example steels A to F had a temperature of −2 even after reheating.
The yield strength at 69 ℃ is 1100MPa or more, and the fracture toughness value is It was confirmed that the above values were high. It was also confirmed that the fracture surface had a good magnetic permeability and that it showed stable non-magnetism even after processing.

On the other hand, Comparative Steel G is an example containing no Mo and B and has good tensile properties but a fracture toughness value. The intergranular fracture was shown below. Comparative Example Steel H is a steel close to SUS304, and Comparative Example Steel I is a steel close to SUS316L, but it has low yield strength and low fracture toughness. Comparative Example Steel J
Steel equivalent to SUS347 has low yield strength, but partially exhibits intragranular fracture surface due to the effect of Nb addition, and the fracture toughness value is also higher than those of comparative steels H and I. However, since the composition of the components is not proper such as not containing Mo and B and C and Nb being excessive, the yield strength and fracture toughness value are inferior to those of the examples, and a stable non-magnetic state is not shown.

Example 2 A steel having the chemical composition shown in Table 3 was treated in the same manner as in Example 1 and its properties were measured. The results shown in Table 4 were obtained. The solution treatment is 1100 ° C for 1 hour, and the reheat treatment is
It was carried out at 650 ° C for 75 hours.

As shown in Tables 3 and 4, the example steels K to P were -269 even after reheating.
Yield strength at 1100MPa or more, fracture toughness value The above values were high. The steel reheated at 650 ° C showed higher fracture toughness values than the solution-annealed steel (Sample k * 3).

EFFECTS OF THE INVENTION The present invention is configured as described above, and is capable of obtaining an austenitic stainless steel exhibiting excellent ductility and fracture toughness in a cryogenic environment even if it is reheated after solution treatment. did it. Moreover, since it is not necessary to add an expensive component or to perform a special heat treatment before reheating, the desired stainless steel can be economically obtained. Thus, it was possible to economically provide a cryogenic non-magnetic austenitic stainless steel that can withstand the heat treatment of superconductors such as Nb ₃ Sn.

Claims (2)

[Claims] 1. C: 0.05% or less (meaning weight%; the same applies hereinafter) Mn: 1-10% Si: 0.5% or less P: 0.03% or less S: 0.01% or less Cr: 13-20% Ni: 10- 15% Nb: 0.02 to 0.10% N: 0.1 to 0.25% Mo: 1.5 to 4.5% B: 0.002 to 0.006% Balance: Fe and unavoidable impurities Magnetic austenitic stainless steel. 2. C: 0.05% Mn: 1-10% Si: 0.5% or less P: 0.03 or less S: 0.01% or less Cr: 13-20% Ni: 10-15% Nb: 0.02-0.10% N: 0.1 〜0.25% Mo: 1.5〜4.5% B: 0.002〜0.006% One or more selected from Ca, Mg, Zr, Ce: 0.001〜0 in total.
01% balance: Fe and unavoidable impurities. Non-magnetic austenitic stainless steel for cryogenic use with excellent reheat resistance.