JPS6063317A - Production of high-mn non-magnetic steel for extra-low temperature service - Google Patents

Abstract

PURPOSE:To develop a non-magnetic structural material having excellent toughness and yield strength at an extra-low temp. by subjecting an Mn steel contg. a specific amt. of V to hot working then to an aging treatment under specific conditions. CONSTITUTION:A raw material for an Mn steel contg. >0.10% C and >0.05% N in which <1.0% (C+2N), contg. 26-30% Mn and <10% Cr, contg. >=0.1% V and contg. the above-mentioned elements within the range of >=0.2(C%+0.5N%+ 0.05Cr%) and <=5(C%+0.5N%+0.05Cr%) is used as an extra-low temp. non- magnetic structural material for technology to utilize superconductivity including thermonuclear power generation and particularly a superconductive magnet. Such material is subjected to hot working at 600-900 deg.C finish temp. and >10% finish reduction ratio, then to an aging treatment. Otherwise the material is subjected to a soln. heat treatment and cold working at >=10% after hot working then to an aging treatment. The relation satisfying the equation (1) is maintained between the temp. T and the time (t) in the stage of such aging treatment.

Description

[Detailed Description of the Invention] This invention relates to superconductivity utilization technology including nuclear fusion power generation,
In particular, it relates to non-magnetic structural materials used at extremely low temperatures, such as cryogenic structural materials for superconducting magnets.

As is well known, in various superconductor utilization technologies such as nuclear fusion power generation, elementary particle accelerators, and superconducting power storage, superconducting magnets are used to flow large persistent currents that generate strong magnetic fields. Inside such a superconducting magnet, there is a powerful
Electromagnetic forces are induced, and normally liquid helium
．． Since it is held at an extremely low temperature of 2 K, materials for superconducting magnet support structures are required to withstand strong electromagnetic force at an extremely low temperature of approximately 4.2 K. In other words, among the properties required for superconducting magnet support structure materials, the most important properties are a high yield stress (yield strength) at 42K;
Specifically, it is required to have a toughness of 1001 mm or more, and also to ensure cryogenic toughness, specifically, to have a Charpy impact absorption energy value of several Yum or more. - This type of structural material also needs to have some degree of good machinability and weldability at room temperature. Furthermore, it is absolutely necessary that the superconducting magnet support structure material be non-magnetic so as not to interact with the magnetic field, and from an economic standpoint, it is desirable that it be as inexpensive as possible.

By the way, materials conventionally considered as superconducting magnet support structure materials include austenitic stainless steel, high nickel steel, aluminum alloy, titanium alloy, and FRP. However, among these, austenitic stainless steels have a yield stress, which is the most important point, of only about 7°C.

In addition, some high Ni steels have a yield stress of 100 ki! There are some materials that exceed this, but they have the drawbacks of being prone to weld cracking and being extremely expensive, and titanium alloys are also similar in this respect. Furthermore, aluminum alloy has a yield stress of several tens) ψ
Although FRP has the advantage of having a low specific gravity and being easy to handle, it has a low yield stress and is expensive. As described above, all of the various materials that have traditionally been considered candidates for superconducting magnet support structures have insufficient properties, and there is therefore a strong need for the development of new materials that can satisfy the above-mentioned required properties. It was rare.

This invention was made against the background of the above-mentioned circumstances, and it is necessary to develop a material that has characteristics suitable for use as a support structure material for superconducting magnets, that is, it has a significantly high yield stress at cryogenic temperatures, poor cryogenic toughness, workability at room temperature, and It is an object of the present invention to provide a method for manufacturing a non-magnetic material that has good weldability and is inexpensive.

As mentioned above, various materials can be considered as cryogenic structural materials for superconducting magnets, but the present inventors have selected high-Mn materials from the viewpoints of magnetic stability, yield strength dependence, economic efficiency, etc.
We speculated that steel was the most promising material, and conducted various experiments and studies to find a way to dramatically improve the properties of high-Mn steel, especially the cryogenic strength (yield strength), compared to conventional high-Mn steel.
It is effective to add ■, and in that case, simply V
In addition to adding V, it is also necessary to specify the V content according to the C, N, and Cr contents in the steel, and to allow hot or cold working strain to remain to precipitate fine V carbides. We have newly found that this is effective in significantly improving yield strength. To explain this point in more detail, methods to increase the cryogenic yield strength of high Mn steel include: ■ Utilizing solid solution hardening with components in the steel; ■ Utilizing precipitation hardening with carbides, nitrides, intermetallic compounds, etc. do, ■
Methods such as using cold working can be considered, but according to detailed experiments by the present inventors, it is possible to significantly improve the cryogenic yield strength of conventional materials by using methods It is difficult to simply apply C, N, Cr
In addition to solid solution hardening, V is added in an amount corresponding to the content of these three elements, and prior to aging treatment (precipitation hardening treatment) to precipitate carbides in By introducing an appropriate processing strain and performing precipitation hardening treatment at a temperature and time depending on the amount added, it is possible to reduce the It was discovered that the cryogenic strength could be significantly improved at a lower cost than that of conventional high-rise steel, and this invention was achieved.

The metallurgical reason why cryogenic strength can be achieved economically by the above-mentioned means without impairing nonmagnetism, toughness, workability, weldability, etc. is as follows. That is, C, N, Cr
If V is added in an appropriate amount and subjected to hot or cold strain, then heat treatment (aging treatment) is performed to form nitrides and C.
V carbides precipitate uniformly and finely within the crystal grains while the formation of R carbonitrides is minimized, and as a result, V carbides significantly reduce the yield of the material during cryogenic deformation. Although this structural change may be delayed and cause an increase in yield strength, it does not impair nonmagnetism, toughness, workability, or weldability.

Specifically, the method for producing a high Mn nonmagnetic steel for cryogenic use of the present invention includes C and N such that C is at least 0.010 cm and N is 0.0
5 chi or more, and C (rice cake) + 2
Contains r 10% or less, O 1% or more and -H(C(%)+o, 5xN(s)+o, osxcr(
%)) or more, 5 X (C(%) +〇, 5 XN (
%)+0.05XCr(%)) or less, the steel material is hot worked at a finishing temperature of 600 to 900°C and a finishing degree of 10 inches or more, and then subjected to aging treatment, or When performing aging treatment after solution treatment after hot working and cold working of 10% or more, the temperature (T) and time (1) of the aging treatment are determined as follows: ≦(650+100
XV Excellent)) The temperature is within the range defined by 10hr≦t≦100j+r.

The manufacturing method of the present invention will be explained in more detail below.

First, the reason for limiting the material components will be explained.

C and N: Both of these are interstitial solid solution elements and are necessary to cause solid solution hardening, and in order to obtain this effect, C0.101 or more and N0905% or more are required. , C (%) + 2 N (%) If it exceeds 1.0%, the toughness will deteriorate, so C (%) + 2 N (%)
) shall be regulated to 1.0% or less.

Mn: An element that characterizes the wood tone, 26% or more is required to stabilize the austenite phase, that is, to ensure magnetic stability and workability, while if it exceeds 30%, it improves toughness and liquid resistance. Since the properties deteriorate, it was limited to a range of 26 to 30%.

Cr: It is desirable to add Cr to cause solid solution hardening and at the same time provide a certain degree of induction resistance, but if it exceeds 10 Cr, it inhibits the stability of the austenite phase and has a negative impact on toughness and stress corrosion cracking susceptibility. Therefore, the upper limit of the amount added is set to 10.

(2) = An element that characterizes this invention, and plays a role in bringing about extremely high cryogenic strength when combined with the heat treatment conditions described below. For that purpose, at least 0.1
In addition, the content of V must be
v (%) is IC (%) + 0.5N (%) + 〇, 05Cr (%) depending on the content of C, N, Cr, i.e. C (%), N (%), Cr (%) ))
≦V(%)≦5(c(%)+0.5N(%)+0.05
Cr (It is necessary to satisfy Q. The reason is -1(C(%)+o, 5N(%)+o, oscr(%)
), the intragranular fine and uniform precipitation of V carbides will not occur sufficiently, resulting in insufficient strength increase;
If it exceeds 0.5N (chi) + 0.05 Cr (%)), more V will be contained than is required for solid solution hardening and V carbide precipitation, resulting in deterioration of ductility and weldability. .

Note that each of the above components, namely C, N, Cr, Mn
.

The remainder to (1) is Fe and unavoidable impurities.

In the method of the present invention, a slab of high Mn steel containing the components specified above is either hot rolled and subjected to aging treatment, or if it is not yet hot rolled, it is solution treated and then cold rolled. Processing and subsequent aging treatment. In the former case, that is, when solution treatment is not performed after hot rolling, the hot finishing temperature in the hot rolling process is set at 600 to 900.
℃, and the degree of finishing must be 10 inches or more.

If the hot finishing temperature exceeds 900°C, the processing strain will recover, and the aging treatment in the next process will not be able to significantly increase the strength, while if the hot finishing temperature is less than 600°C, shape control may become difficult. be. Further, the working degree in this hot finish rolling must be 10q or more, and if this value is not reached, it is not possible to introduce the necessary strain to effectively increase the strength during the aging treatment in the next step. On the other hand, in the latter case, after hot rolling, solution treatment is performed and then cold working is performed.
When aging treatment is performed thereafter, there is no need to set any particular restrictions on the hot rolling conditions, but when performing cold working after solution treatment, the degree of working must be 10% or more. In this case, if the degree of cold working is less than 10 inches, it may not be possible to introduce the necessary strain to effectively increase the strength in the aging treatment in the next process, or the strain that is required to increase the strength effectively in the aging treatment in the next process may not be able to be applied. There is an optimum range of temperature conditions for aging treatment after introducing processing strain depending on the content. That is, the present inventors, CO, 5%,
N O, 18%, Cr 7゜2%, Mn2
High Mn steel slabs containing 8.0% and 05% or 1.0% V were hot rolled at a finishing temperature of 850°C and a finishing degree of 15mm, and aged for 50 hours at various temperatures. 4, and it was found that the aging temperature and proof stress have a relationship as shown in FIG. 1 depending on the V content. From these results, the aging treatment temperature should be set at [35 o-tooxv (%)) °C or higher, (
650 + 100XV (chi)) or less, 100k
g/A71! It is clear that a high cryo-temperature yield strength is obtained above a certain level, and that sufficient cryo-temperature yield strength cannot be obtained either below or exceeding that range. Further, the aging treatment time needs to be within the range of 10 to 100 hours. That is, the present inventors hot-rolled a material slab with the same components as above under the same finishing conditions as above, and obtained 650
When the aging treatment was carried out at different temperatures at 4K and the relationship between the 0.2% proof stress and the aging time was investigated, the results shown in FIG. 2 were obtained. As is clear from FIG. 2, if the aging time is less than 10 hours, age hardening is not sufficient, while if it exceeds 100 hours, the V carbide becomes coarse, resulting in over-aging, resulting in a decrease in strength. Therefore, in order to obtain sufficient cryogenic strength, especially a yield strength of 1000 k or more at 4K, the aging time must be set to 10 to 100 hours.

Examples of the present invention will be described below along with conventional examples and reference examples.

The present invention example, the conventional example, and the reference example with conditions slightly deviating from the limiting conditions of the present invention, whose chemical components are shown in Table 1, were melted in a normal converter and then in an extra-furnace vacuum refining furnace. #'# The product was wrought, bloomed, and hot rolled, and in some cases, after hot rolling, it was subjected to bath treatment and cold rolling, and then subjected to aging treatment to obtain a product with a thickness of 4.0 m. The properties of the obtained product, such as cryogenic properties at liquid helium temperature (4K), were investigated. The process conditions after finish rolling in the hot rolling process and the results of the characteristic tests are also shown in Table 1.

As is clear from Table 1, the cryogenic single-needle blades of each conventional example, except for the Ti alloy, have a temperature of only 70 to 80 kVfnfL @ degrees. Although the cryogenic yield strength of the Ti alloy is close to the lowest value of the examples of the present invention, the elongation and toughness (Charpy absorbed energy) are extremely low, and the workability is still poor. On the other hand, all of the examples of the present invention have extremely high cryogenic strength, which is at least 150 kgn or more, and can reach 200 kg/mrl in some cases. In the case of the example of the present invention, although the cryogenic yield strength is high as mentioned above, the ductility (elongation) and toughness (Charpy absorbed energy) are also relatively good at cryogenic temperatures, and the machinability and machinability at room temperature are also good. Weldability is also good.

On the other hand, among the reference examples that do not slightly satisfy the condition range of the present invention, example 169 does not undergo precipitation hardening because no V is added, and the cryogenic yield strength remains at the same value as the conventional example. In addition, the example of 1610 contains a large amount of V added which exceeds the limiting conditions of this invention, and in this case, the cryogenic strength is sufficient, but the ductility, workability, and weldability are poor.

All of All to No. 13 are examples in which the hot rolling finishing process conditions are outside the scope of the present invention, and all of these have cryogenic yield strength other than T1 alloy (although higher than the conventional example, the present invention example has However, a significantly large value of 1 in 150 countries has not been obtained. Also, in case 14, the aging treatment conditions are outside the scope of the present invention, but the same applies in this case.

As is clear from the above examples, according to the method of the present invention, the ingredients of the high Mn steel material are limited, an appropriate amount of V is added, and the rolling treatment process conditions are set to introduce and maintain working strain. By specifying the precipitation aging treatment conditions of the final V carbide according to the V content, the yield strength at extremely low temperatures such as liquid helium temperature is significantly higher than that of conventional ones, and it is possible to It has become possible to inexpensively produce high-Mn steel that does not impair ductility, workability at room temperature, and weldability, and also has nonmagnetic properties.

The non-magnetic steel produced by the method of this invention can be used for structural materials that require low-temperature strength, such as structural materials using superconductivity technology such as superconducting magnet support structures, as well as structural materials for liquefied hydrogen technology and liquefied natural gas technology. All are applicable.

[Brief explanation of the drawing]

Figure 1 is a correlation diagram showing the influence of precipitation aging treatment temperature on 0 and 2% proof stress at 4K, and Figure 2 is the same (correlation diagram showing the influence of precipitation aging treatment 114 hours on 02% yield strength at 4K). .Applicant Kawasaki Steel Co., Ltd. Agent Patent attorney Toyoda (and 1 other person)

Claims (1)

[Claims] Contains C and N within the range of C010 or more, N O,05 or more, and C (chi) + 2N (correspondence) is 10 or less, and Mn 26 to 30%, Cr Contains 10% or less, and 0.1% or more and above (C (%) 100
．． 5 N (%) + o, o5cr (%)) or more, 5 (
C (%) 100.5N (%) + 0.05Cr (%)) or more
A high Mn steel containing V in an amount of F is used as a raw material, and the steel cable material is hot worked at a finishing temperature of 600 to 900°C and a finishing degree of 10 inches or more, and then subjected to aging treatment, or the steel cable material is hot worked. When performing aging treatment after solution treatment after processing and cold working of 10% or more, the aging treatment temperature (T
) and time (1), (650-toov(%))0
C≦T≦(650+100V(%Month℃10hr≦t≦1
A method for manufacturing high Mn nonmagnetic steel for cryogenic use within the range specified by 00 hr.