JP3439062B2 - High Mn stainless steel with excellent hot workability and cryogenic toughness - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high Mn stainless steel material which is excellent in hot workability and has high strength and toughness at an extremely low temperature of 4K (-269 ° C) absolute temperature.

[0002]

2. Description of the Related Art In recent years, M using a large superconducting magnet
Technologies related to HD power generation and fusion reactors are making rapid progress. Since the superconducting magnet coils used in these devices are operated in a state where they are cooled by liquid helium to an absolute temperature of 4K, the structures such as these supports and structures are also cooled to the same temperature. Therefore, steel materials such as steel plates used for these structures are required to have characteristics different from those of ordinary steel materials, and these requirements are becoming more and more severe.

First, when the superconducting magnet coil forms a high magnetic field in a superconducting state, a strong electromagnetic force acts on the support and the structure, and high stress is repeatedly applied in the strong magnetic field. Therefore, as a steel material used for such an application, a non-magnetic steel having a high yield strength and excellent fracture toughness at an extremely low temperature is required, including the welded portion.

Next, the temperature of the superconducting magnet coil is set to 4
In order to reach K, the atmosphere of the support or the structure needs to be vacuumed for heat insulation, and therefore the surface of the support or the structure must not be rusted. Therefore, the steel used for such an application further needs a non-magnetic steel having excellent rust resistance including the welded portion.

Further, this superconducting magnet coil has become larger in size with the increase in output in recent years, and for manufacturing its support and structure, for example, 110 mm thickness × 2500 mm width × 10000 mm length or more is required. Thick and large area steel is required.

[0006] And, more importantly, in addition to these characteristics, for steel materials for large superconducting magnet supports and structures, at 4K, a value of 1200 N / mm ² or more of 0.2
The point is that the% yield strength (YS) and the fracture toughness value K _IC of 200 MPam ^1/2 or more are required. In addition, the high strength and high toughness have been changed to the guarantee at the central portion of the plate thickness, not the guarantee of the material at the t / 4 (t: plate thickness) position of the steel material as in the past. This property requirement is a very strict requirement for a thick and large-area steel material in which the material in the central part of the plate thickness is more difficult to control, and it was not easy to stably satisfy such requirements. .

Up to now, as the prior art of the thick-walled steel material for this application, there is 18% Cr-8% Ni SUS304LN which is a high nitrogen type austenitic stainless steel. % Yield strength is 80
0N / mm ² only about without the required 1200 N / mm ² or more 0.2
% Yield strength cannot be satisfied, and the recent severe requirement for characteristics cannot be satisfied.

On the other hand, it has been proposed to apply a high nitrogen type and high Mn type non-magnetic steel material.
As described in the column of the prior art of Japanese Patent Publication No. 55-51423, there is a big problem that the hot workability such as rolling and forging is extremely poor. Specifically, when hot working this high Mn-based non-magnetic steel material, for example, hot rolling of a slab after continuous casting, slab rolling of a steel ingot after ingot casting, or hot rolling to a thick plate is performed. In the above, a large crack is generated on the entire surface of the slab or the steel ingot, or a crack is generated on the entire surface of the steel plate or the end portion in the width direction.

In particular, in the case of hot working using a large mass steel ingot, slab or steel plate, the above-mentioned superconducting magnet coil can be made larger due to the occurrence of the cracks.
There is a problem that thick, wide, and long steel materials cannot be collected from the rolled material after processing. This problem is an obstacle to the increase in size of the superconducting magnet coil and its structure, and the practical application of the fusion power generation facility.

Therefore, conventionally, various techniques have been proposed in order to improve the hot workability of this kind of high Mn non-magnetic steel material. First, a typical one of the technologies is reduction of impurities that render hot workability of steel products harmless.

For example, Japanese Patent Publication No. 55-51423 discloses a technique of using austenitic steel having excellent low temperature toughness as low temperature steel for LNG tanks and tankers. Then, in this conventional technique, in order to improve the hot workability of austenitic steel, Al and Ca are added in combination to inhibit the hot workability, the fixation of S and O in the steel and the morphological change of inclusions. This prevents hot work cracking of steel materials.

In Japanese Patent Laid-Open No. 59-229471 and Japanese Patent Publication No. 3-59136, in order to prevent cracking of a high nitrogen type austenitic stainless steel during slab rolling, in addition to reducing S and O in the steel, , A technique for reducing P in steel from the usual 0.020 to 0.035% content to 0.020% or less is disclosed.

Japanese Examined Patent Publication No. 2-45695 and Japanese Patent Laid-Open No. 61-170545 disclose that the former is S in order to obtain Charpy impact absorption energy of 50 J or more at 4K for rust-resistant high-Mn steel for cryogenic use. The amount is 0.003% or less, and the latter has a Ca content of 0.003%.
A technique of adding 1 to 0.020% is disclosed, and both of them add Al in order to improve hot workability.

Japanese Unexamined Patent Publication No. 2-15151 discloses a high Mn austenitic steel for a superconducting magnet structure,
In order to obtain cryogenic characteristics (high fracture toughness at 4K) after Nb ₃ Sn formation heat treatment, Nb and Ti are co-containing with Mo for the purpose of suppressing grain boundary precipitation of Cr carbide, and Sn, Sb, As The technology that regulates the total amount to 0.020% or less is disclosed. Also in this technique, the amounts of P and S in steel are limited in order to improve hot workability.

Various techniques have also been proposed for preventing surface cracks by controlling the hot working itself. For example, in Japanese Patent Laid-Open No. 7-90366, in order to prevent the deterioration of hot workability and toughness due to the formation of δ ferrite in high Mn stainless steel non-magnetic steel, N
In order to prevent surface cracking due to hot working while increasing the amount of i, the steel ingot is heated in a narrow area of 1150 to 1250 ° C, and the slabbing rolling is finished at a surface temperature of 1000 ° C. The technology that also specifies the condition is disclosed. Also in this technique, P and S in the steel are limited in order to improve hot workability.

Japanese Patent Publication No. 36482/1993 has a 0.2% proof stress (YS) of 1200MPam (N / mm ² ) or more and a fracture toughness value K _IC of 150MPam ^1/2 or more at a liquid He temperature (4K). In order to improve the hot workability of a high Mn austenitic steel for a superconducting magnet structure of a fusion reactor, the above-mentioned Japanese Patent Laid-Open No.
Similar to the -90366 publication, after heating the steel ingot in a narrow region of 1150 to 1250 ° C, slab rolling is completed at 950 ° C or higher, and then heating in a narrow region of 1150 to 1250 ° C and then 950 to 1010 ° C.
Discloses a technique for finishing the plate rolling. Also,
This technique also limits P and S in steel in order to improve hot workability of steel.

[0017]

However, among the hot workability improving techniques described in these prior arts, first, a technique for reducing impurities in steel that impedes hot workability, that is, Al, Ca, etc. Is added to fix or reduce P, S, and O in the steel or to control the morphology of inclusions, as will be described later, it is thick and wide in correspondence with the enlargement of the superconducting magnet coil. However, hot work cracking of long steel cannot be completely prevented.

Further, in the hot workability improving technique for controlling the heating temperature and the rolling end temperature in a specific range, the setting range of the heating temperature and the rolling end temperature is necessarily 100 ° C. as described above.
It becomes the following narrow area. Therefore, in response to the large size of the superconducting magnet coil, particularly when using a large mass steel ingot or slab, when manufacturing a wide, long steel material, the temperature drop during processing is large, within the set conditions, It is extremely difficult to insert the entire steel ingot or slab being processed. Therefore, a steel ingot portion that is below the appropriate working temperature and falls outside the set conditions is inevitably generated, so that cracks occur at this portion and hot work cracking cannot be completely prevented.

More importantly, in these prior arts, hot work cracking is completely prevented, and 1200 N / mm at 4K of austenite single phase high Mn type stainless steel material.
It is not considered to secure or secure 0.2% proof stress of ² or more and fracture toughness value of 200MPam ^1/2 or more both stably, and in the central part of the steel plate thickness. Is the point that has not achieved. That is, the prevention of hot work cracking and the high-strength and high-toughness materials at 4K are technical issues that are contradictory to each other, and are extremely strict required characteristics particularly for steel materials having a large mass or a large area. Conventionally, there has been no steel material that stably satisfies such requirements.

Actually, for example, the above-mentioned Japanese Patent Laid-Open No. 2-15151 discloses both 0.2% proof stress at 4K and the fracture toughness value K _IC as characteristics of this seed steel. Although 0.2% proof stress is barely 1200 N / mm ² , the fracture toughness value is 172 M at maximum.
It is only about Pam ^1/2, and the fracture toughness value of 200MPam ^1/2 or more is not obtained. Moreover, the thickness of this steel material itself is 30 mm.
Although the test piece sampling position of the steel material characteristics is not specified, it is the position of t / 4 (t: plate thickness) of a normal steel material, and not the central portion of the plate thickness targeted by the present invention. It is presumed that.

Further, in the above Japanese Patent Publication No. 36482/1993,
In the examples, as a characteristic of a thick plate having a steel material thickness of 70 mm, 0.2% proof stress (YS) of 1200 N / mm ² or more at 4K.
And a fracture toughness value (K _IC ) of about 210 MPam ^{1/2 at} maximum. However, although this steel material also has a large plate thickness, there is no description about the width and the length, and the present invention targets 50 mm thickness x 2000 mm width x 10000 mm length or more,
It is presumed that it is more preferable to target a steel material having a thickness of 110 mm × 2000 mm width × 10000 mm or more, which is thicker and smaller than a steel material having a large area. It is stipulated that the position is the t / 4 (t: plate thickness) position of the steel material, and it is not the center part of the plate thickness targeted by the present invention.

That is, the central portion of the plate thickness of the steel material is a portion that is less likely to be affected by heat treatment and processing than the portion closer to the surface layer, the material is the most difficult to control, and the material is easily deteriorated. In addition, as the steel material has a larger area, the characteristics are inferior as compared with the t / 4 position, which is the normal test piece sampling position of the steel material. Therefore, the thicker the steel material and the larger the area, the t
Even if the characteristics at the / 4 position are excellent, there is no guarantee that the center part of the plate thickness is equally excellent. If the properties at the center part of the plate thickness are to be guaranteed, the plate thickness is not at the t / 4 position. It is necessary to guarantee directly in the central part.

Therefore, the present invention is based on these conventional high Mn.
Considering the problems of stainless steel and high Mn austenitic steel,
0.2% proof stress of 1200N / m at 4K cryogenic temperature
High strength of m ² or more and fracture toughness value evaluated by K _IC value is 2
It is an object of the present invention to provide a high Mn stainless steel material having a high toughness of 00 MPam ^1/2 or more and, at the same time, preventing hot work cracking and having excellent hot workability.

[0024]

[Means for Solving the Problem] Preventing hot work cracking and stably securing high strength and high toughness at 4K in the central portion of the plate thickness means that a thick and wide material can be obtained. As described above, the cryogenic steel materials have conflicting problems. However, the present inventors diligently studied this problem, and as a result, first, as the steel type of the steel material, a high Mn stainless steel that can overcome the low yield strength, which is the drawback of austenitic steel, was selected.

Next, as a countermeasure against hot work cracking, it was selected to improve the strength and ductility of dendrite grain boundaries. Hot rolling of as-cast steel ingots and continuous cast slabs made of high Mn stainless steel with austenite single phase,
The cracks generated during the hot rolling of the breakdown slab are commonly generated at the dendrite grain boundaries processed in the austenite unrecrystallized region. From this, it is effective to improve the strength and ductility of the dendrite grain boundaries in order to prevent hot work cracking.

As a means for improving the strength and ductility of this dendrite grain boundary, in the present invention, the solid-liquid coagulation temperature range is narrowed in order to suppress impurity elements and segregation at the dendrite grain boundary during the solidification process of casting. Focusing on
We paid attention to controlling the alloying amount of austenite stabilizing element appropriately, and finding out the harmful impurity element segregating at the dendrite grain boundary during solidification and controlling the amount to a low level. In addition to these means, in order to exert high yield strength and high toughness in the solidified metal at 4K, the Ni equivalent (Ni
_eq ) and Cr equivalent (Cr _eq ) are properly controlled,
It was decided to control the average austenite grain size and the elongation.

As will be described later, the chemical components of the high Mn stainless steel are controlled to an appropriate amount so as to satisfy all of the above items (1) to (5) at the same time, thereby preventing the occurrence of hot work cracks and providing a sound surface and internal quality. It was found that the steel material of No. 1 can be obtained at a high yield and can satisfy the required values of the yield strength and the fracture toughness value at 4K, and the present invention has been completed.

In the present invention, based on the above technical idea, a high M
n composition of stainless steel in mass%, C: 0.03 to 0.
10%, Si: 0.10 to 0.50%, Mn: 18 to 30%, Ni: 5
~ 8%, Cr: 12-18%, Mo: 0.50-3.00%, A
1: 0.01 to 0.07%, N: 0.10 to 0.28%, and
35% ≤ Ni _eq + 0.8 x Cr _eq ≤ 40% (however, Ni _eq = N
i% + 30 × C% + 30 × N% + 0.5 × Mn%, Cr _eq = C
r% + Mo% + 1.5 × Si% + 0.5 × Nb%), and P: 0.008% or less, S: 0.003% or less, P
% + S% ≦ 0.011%, O: 0.0050% or less, Pb: 5 × 10
^-4 % or less, cleanliness d (However, according to JIS G 0555 measurement method): 0.
Each content is regulated to 1% or less, and the balance is made up of Fe and inevitable impurities. Furthermore, the average austenite grain size (
(However, according to the measurement method on ASTM E112-1995, 13.5, page 234)
Of 0.05 to 0.17 mm and the elongation of austenite grains AI _l
(However, according to the measuring method on ASTM E112-1995, 16.3.5, page 237) 1.2 or less.

Further, as a selective addition element, Nb,
You may contain 1 or more types from V and Ti, and 1 or more types from B and Ca.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical components in the present invention will be described below. C is an element effective for securing non-magnetism and increasing proof stress at 4K through stabilizing austenite. If the content is less than 0.03%, such effects are poor, while if it exceeds 0.10%, precipitation of Cr carbides at the dendrite grain boundaries becomes noticeable during casting cooling, and the toughness and corrosion resistance at 4K deteriorate. To do. Therefore, the C content is set to the range of 0.03 to 0.10%.

Si is an essential element for deoxidizing molten steel, and is effective for increasing the yield strength, but if it is less than 0.10%, the effect is insufficient, and if it exceeds 0.50%, the high temperature ductility decreases and welding At the same time, high temperature cracking is likely to occur and toughness is deteriorated. Therefore, the Si content is 0.10-0.
The range is 50%.

Mn is an element which is indispensable for improving the toughness at 4K as an austenite forming element in place of (substituting for) Ni which adversely affects the weld hot cracking resistance. It also increases the solid solubility limit of N and contributes to stabilization of the austenite structure. If the content is less than 18%, sufficient toughness at 4K (fracture toughness value of 200 MPam ^1/2 or more) cannot be obtained, and α'martensite and the like tend to precipitate, resulting in loss of non-magnetism. On the other hand, if the content exceeds 30% and is excessive, the hot workability is deteriorated, the yield is deteriorated, and the cost is increased. Therefore, the Mn content is in the range of 18 to 30%.

Ni suppresses the crystallization of δ-ferrite during the solidification process of the weld metal, stabilizes the austenite structure, and improves the toughness at 4K (the fracture toughness value is 200MPam.
It is an essential element for obtaining ( ^1/2 or more). Ni is 5
If the content is less than%, such an excellent effect cannot be obtained. However, on the other hand, excessive Ni content expands the solid-liquid solidification temperature range in the complete austenite structure,
It promotes the segregation of the low melting point impurity element to the dendrite grain boundaries, and reacts with S to precipitate a low melting point NiS compound at the grain boundaries of the weld metal, deteriorating the ductility of the grain boundaries of the solidified metal. Therefore, since an excessive Ni content adversely affects the weld hot cracking resistance, the upper limit content is 8%.
And the Ni content is in the range of 5-8%.

Cr provides rust resistance, stabilizes austenite, and is necessary for increasing the yield strength at 4K, and also increases the solid solution limit of N. Contains 12
If it is less than 18%, this effect does not exist, and if it exceeds 18%, δ ferrite crystallizes in the solidification process of the weld metal and the complete austenite structure cannot be maintained, resulting in a decrease in toughness and a sufficient toughness at 4K (fracture toughness value). But 200MPam ^1/2 or more) cannot be obtained. Therefore, the Cr content is in the range of 12 to 18%.

Further, in order to secure proof stress, toughness and non-magnetism at 4K, Ni and Cr are limited to the content of each element, and Ni equivalent (N
i _eq ) and Cr equivalent (Cr _eq ) are Ni _eq + 0.8 × Cr
_It is necessary to make _eq 35% or more. If it is less than 35%, the required toughness at 4K cannot be obtained even with a completely austenitic structure.

On the other hand, in order to improve the hot cracking susceptibility at the time of welding, it is necessary to set an upper limit constraint on Ni _eq in accordance with Cr _eq , and Ni _eq + 0.8 × Cr _eq
40% or less. If it exceeds 40%, the susceptibility to hot cracking will increase. However, in this Ni _eq and Cr _eq , Ni _eq = Ni% + 30 x C% + 30 x N% + 0.5 x Mn
%, Cr _eq = Cr% + Mo% + 1.5 x Si% + 0.5 x N
b%.

Like C, N is an element effective in stabilizing austenite and improving the proof stress at 4K. C is Cr
Although toughness is deteriorated due to precipitation of grain boundaries of carbide, N does not have such an adverse effect. In order to exert the above effects, the content of 0.15% or more is required, but if it exceeds 0.28%, the toughness is significantly deteriorated and the weld hot cracking resistance becomes poor. Therefore, the N content is set in the range of 0.15 to 0.28%.

Al is a deoxidizing element for molten steel, captures solid solution oxygen, and prevents the formation of blowholes.
It is an element necessary for improving the toughness at 4K. 0.01%
If it is less than 0.05%, a sufficient effect cannot be obtained, while if it exceeds 0.07%, the toughness at 4K is deteriorated due to an increase in alumina inclusions. Therefore, the Al content is 0.01-0.
The range is 07%.

Mo is a solid solution strengthening element, and is 0 at 4K.
It is an element that improves the yield strength by 2%. Further, it is also effective in preventing deterioration of toughness due to precipitation of grain boundaries of Cr carbide, but such an effect cannot be obtained at less than 0.50%,
When it exceeds 3.00%, the proof stress improving effect is saturated, but
The toughness at 4K deteriorates. Therefore, the Mo content is 0.50
The range is to 3.00%.

Next, the significance of the inclusion of the selectively added element of the present invention and the reason for limiting the content will be described. Nb, V, Ti
Causes carbonitrides to precipitate and 0.2% at 4K due to precipitation strengthening.
% It is an element effective for improving the yield strength, but the content of each element is 0.
If it is less than 01%, its effect is poor. On the other hand, if any of the contents exceeds 0.50%, it deteriorates the toughness at 4K. Therefore, the total content of one or more of Nb, V, and Ti is 0.01 to 0.50%.

Ca exerts a strong deoxidizing effect during the melting of steel, traps and reduces solid solution oxygen harmful to the toughness at 4K, and is effective for refining nonmetallic inclusions. If the content is less than 0.0010%, such effects are poor, and conversely
If the content exceeds 0.0035%, the cleanliness of steel deteriorates. Therefore, the content of Ca is set to the range of 0.0010 to 0.0035%.

B segregates at the austenite grain boundaries to strengthen the grain boundaries and improve the proof stress at 4K, but if the content is less than 0.0001%, such an effect is poor, and conversely. If the content exceeds 0.0030%, the toughness of steel deteriorates. Therefore, the content of B is 0.0001 to 0.0030.
The range is%.

Next, regarding the regulation of impurities, the present invention is characterized in that P, S, O, and Pb are regulated together in order to prevent hot work cracking. When the content of these impurities is high, as described above, the low melting point impurity element is segregated in the dendrite grain boundaries during the solidification process, and the hot work cracking resistance is deteriorated.

First, P and S are elements that deteriorate the hot work cracking resistance. To secure the hot work cracking resistance of a high Mn stainless steel ingot, P: 0.008% or less,
It is necessary to regulate S: 0.003% or less.

O remains as solid solution oxygen, blowholes and non-metallic inclusions and deteriorates the toughness at 4K. Further, since the hot work cracking resistance of the high Mn stainless steel ingot is deteriorated, the yield is deteriorated and the cost is increased. Therefore, the O content needs to be 0.0050% or less.

The regulation of Pb is an important regulation in the present invention. In the present invention, in addition to the impurities such as P, S, and O, Pb is regulated in order to secure the hot work cracking resistance of the high Mn stainless steel ingot. The present inventors have found that the high Mn stainless steel ingot is
It was found that there is a positive correlation with the Pb content. Pb is
Since it is contained in the metallic Cr of the ingot raw material, it is easily mixed in the steel together with the addition of the metallic Cr. Therefore, in order to reduce the hot work cracking susceptibility of a high Mn stainless steel ingot, the amount of Pb having a low melting point, which is inevitably included from the raw material and the manufacturing process, segregates at the dendrite grain boundaries in the solid-liquid coagulation solidification region is reduced. This is very important. For this reason,
It is important to regulate Pb to 5 × 10 ^-4 % or less.

Further, in the present invention, in order to secure the hot work cracking resistance and to stably secure the high strength and high toughness at 4K, the cleanliness d (according to JIS G 0555 measurement method) is used. , Average austenite grain size (However, ASTM E112 −199
5, Lineal Intercept Procedure 13.5, according to the measurement method on page 234), and the elongation AI _l (However, ASTM E112 −1995, Speci
mens with Nonequiaxed Grain Shapes 16.3.5, page 237). By the way, these are the characteristic values in the steel sheet after hot working,
By specifying these characteristic values of the steel sheet after hot working, it is possible to secure the hot workability of the steel sheet itself, and also stably secure the high strength and high toughness at 4K of the steel sheet after hot working. it can.

First, the average austenite grain size of steel is
It affects the 0.2% proof stress at 4K and the fracture toughness value,
The smaller the diameter, the higher the 0.2% proof stress, but on the contrary, destruction
The toughness value decreases. 200MPam^1/2that's all
Fracture toughness value K_{I c}In order to secure
A uniform austenite grain size is required and also 1200 N / mm ²
To secure the above 0.2% proof stress (YS), 0.17 mm or less
The lower average austenite grain size is required. According to
The 0.2% proof stress at 4K and the fracture toughness value together.
In order to achieve an average austenite grain size of 0.05 to 0.17 mm,
There is a need to. This average austenite grain size is
 E112 −1995, Lineal Intercept Procedure 13.5,234
As shown in the measuring method on the page, austenite in a rectangle with a side of 100 mm
The number of particles is measured, and the value divided by the magnification of the microscope is
It is measured as the average austenite grain size.

Next, the fracture toughness value at 4K has a negative correlation with the elongation of austenite grains. Fracture toughness value at 4K
In order to satisfy 200MPam ^1/2 or more, ASTM E112 −
1995,16.3.5, the elongation AI _l exhibition by 237 pages of the measurement method 1.2
Must be: This extensibility AI _l is ASTM E112
−1995, Specimens with Nonequiaxed Grain Shapes 16.
It is specified as follows in the measuring method on page 237, 3.5.

[0050]

[Equation 1]

The presence of non-metallic inclusions in the steel is 4
It adversely affects the fracture toughness value at K. In the present invention, the non-metallic inclusions in the steel material are defined by the cleanliness d according to the JIS G 0555 measuring method, and the cleanliness d is 0.1% in order to satisfy the fracture toughness value of 200 MPam ^1/2 or more at 4K. Below. JIS
The cleanliness d by the G 0555 measurement method is d = n / (p × f) ×
Expressed as 100% (where p is the total number of grid points on the inserted glass plate in the microscope field, f is the number of fields, and n is the number of grid point centers occupied by all the inclusions in f fields).
That is, the cleanliness d indicates the area ratio of the non-metallic inclusions occupied by the base material cloth.

Next, a method for producing the high Mn stainless steel of the present invention will be described. The high-Mn stainless steel of the present invention can be manufactured by a normal manufacturing method of high-Mn stainless steel, and is manufactured by performing slab rolling, hot forging, hot working such as plate rolling after melting of steel. . However, the high Mn stainless steel of the present invention has markedly improved hot workability as compared with the conventional high Mn stainless steel, but is inferior in hot workability to carbon steel and low alloy steel. Therefore, if the heating temperature or finishing temperature of slab rolling or thick plate rolling is too low, cracks may occur on the surface of the steel ingot or the steel plate, resulting in a decrease in yield, regardless of the present invention. In order to prevent this, it is desirable to raise the heating temperature and finishing temperature.
It is desirable to set the temperature to 50 ℃ and the finishing temperature to 900 ℃ or higher. Cooling after rolling may be air cooling or forced cooling.

In the present invention, as will be described later, the optimum working temperature range is widened (230 ° C. or higher). Therefore, even under the above-mentioned desirable conditions, the temperature range is set to the conventional heating temperature or rolling end temperature (100 ° C.). It is much wider than Therefore, even if a particularly large mass (wide and long) steel ingot or slab is processed under this more preferable temperature condition, the steel ingot or slab falls in temperature during processing and falls below the appropriate processing temperature,
Does not cause hot work cracking.

[0054]

【Example】

Example 1 Next, the significance of each requirement of the high Mn stainless steel material of the present invention described above will be described with reference to examples.

Flat steel ingots having the chemical components, cleanliness, average austenite grain size, and elongation shown in Tables 1, 2, and 3 were melted, and the hot work crack susceptibility of each of these steel ingots was investigated. . The hot work cracking susceptibility of a steel ingot was evaluated by taking a tensile test piece from the surface layer of the steel ingot, performing a high-temperature high-speed tensile test (Gleeble test), and measuring the cross-sectional shrinkage ratio (RA) of the fractured part.
In this case, it has been empirically known that when RA is 40% or more, cracking does not occur in slabbing, and in this test, RA is 40% or more is a critical condition under which cracking does not occur. To the standard.

First, the influence of the amount of Pb on the hot work crack susceptibility of the steel ingot will be described with reference to FIGS. Figure 1
No. in Table 1 above. The sample materials having different Pb amounts of 1 to 4 are heated to each processing temperature (for example, 1160 to 1225 ° C in the sample material of No. 1 marked with ◯) at a rate of 10 ° C / s and held for 10 minutes, remains in this heated state, when processed by strain rate and the second (On heating), shows a relationship between the section of shrinkage and (RA) and processing temperature. As is clear from the influence of the Pb amount on the cross-sectional shrinkage ratio (RA) in FIG. 1, the lower the Pb amount of the test material, the higher the upper limit of the heating temperature at which the hot working crack does not occur (RA is 40% or more). You can see that it spreads to the side.

FIG. 2 shows the data of No. 1 in Table 1 above. The highest processing temperature (RA in FIG.
No. (Processing temperature at which RA of specimens 1-4 is 40%)
10 ° C. / s and held at a rate of heating after 10 minutes, if the the heating temperature after cooled to a specific temperature at a cooling rate of 5 ° C. / s and held between 60s, and processed subsequently by strain rate of 2 to (On The relationship between the cross-sectional shrinkage rate (RA) and the processing temperature is shown. As is clear from the influence of the Pb amount on the cross-sectional shrinkage ratio (RA) in FIG. 2, the lower the Pb amount of the test material is, the lower the lower limit of the working temperature at which the hot working crack does not occur becomes lower.

From the results shown in FIGS. 1 and 2, RA is 40%.
FIG. 3 shows a summary of the relationship between the processing temperature range ΔT and the Pb amount as described above. From FIG. 3, the test material N of the present invention
o. When the amount of Pb is reduced to 5 × 10 ⁻⁴ % or less as in Nos. 3 and 4, the test material Nos. Of the conventional example or the comparative example in which the amount of Pb is larger than this amount. No hot work cracking, compared to 1 and 2,
Good processing temperature range is widened.

In the usual slab rolling, when a wide and long steel material is manufactured using a large-mass steel ingot or slab corresponding to the above-mentioned increase in size of the superconducting magnet coil, processing is performed within the set conditions. The processing temperature range (ΔT) must be at least 230 ° C. or higher in order to put the entire steel ingot or slab in the inside and prevent a portion out of the set condition from occurring.

In this respect, the test material No. Sample materials Nos. 3 and 4 of the conventional example or the comparative example are shown. ΔT compared to 1 and 2
However, it can be seen that 230 ° C. or higher can be secured and the working temperature range is widened, so that heating at a higher temperature and accompanying rolling finish at a higher temperature are possible. As a result, in the case of producing a wide and long steel material, even if the temperature of the steel ingot or the slab decreases during processing, a steel material portion that is out of the set condition does not occur, and hot work cracking can be prevented.

Next, the influence of the amount of Ni on the hot work crack susceptibility of the steel ingot will be described with reference to FIGS. Figure 4
No. in Table 1 above. The test materials having different Ni contents of 5 to 9 are heated to each heating temperature (for example, 1175 to 1220 ° C. for the test material of No. 5 marked with ◯) at a rate of 10 ° C./s and held for 10 minutes, remains in this heated state, when processed by strain rate and the second (On heating), shows a relationship between the section of shrinkage and (RA) and processing temperature. As is clear from the effect of the Ni content on the cross-sectional shrinkage ratio (RA) in FIG. 4, the lower the Ni content, the higher the upper limit of the heating temperature at which the hot working crack does not occur (RA is 40% or more). You can see that it spreads to the side.

FIG. 5 shows that No. 1 in Table 1 above. 5 to 9 sample materials, the maximum processing temperature (RA in FIG.
No. Processing temperature at which RA of specimens 5 to 9 is 40%)
10 ° C. / s and held at a rate of heating after 10 minutes, if this from the heating temperature at a cooling rate of 5 ° C. / s and held for 60 seconds after cooled to a particular temperature, and processed subsequently by strain rate of 2 to (On The relationship between the cross-sectional shrinkage rate (RA) and the processing temperature is shown. As is clear from the influence of the Ni content and the parameter X value consisting of Ni equivalent + 0.8 × Cr equivalent on the cross-sectional shrinkage (RA) in FIG. 5, the lower the Ni content, the lower the test material (when the Ni content was reduced). It can be seen that the lower limit of the working temperature at which hot working cracks do not occur spreads to the lower temperature side.

Further, from the results of FIGS. 4 and 5, FIG. 6 shows a summary of the relationship between the processing temperature range ΔT in which RA is 40% or more and the Ni content. 6, the parameter X value consisting of Ni amount of 5 to 8 and Ni equivalent + 0.8 × Cr equivalent was set to 35 to 40%. The test materials of 3 and 7 to 9 are N
i amount and parameter X value N out of this range
o. Compared with the test materials of Nos. 5 and 6, a good working temperature range that does not cause hot working cracking can be secured at ΔT of 230 ° C. or higher, and heating at higher temperature and accompanying rolling at higher temperature Finishing is possible.

As described above with reference to FIGS. 1 to 6, the Pb amount is set to 5
× 10 ^-4 % or less, Ni content 8% or less, and Ni equivalent +
It can be seen that the processing temperature range ΔT of RA 40% or more can be set to 230 ° C. or more if the condition that the parameter X value of 0.8 × Cr equivalent is 40% or less is provided. Therefore,
As a result, hot working cracks can be prevented in the range of normal hot working conditions.

Next, using flat steel ingots having the chemical composition, cleanliness, average austenite grain size, and elongation shown in Tables 1, 2, and 3, the slabs were actually slab-rolled into slabs of 220 mm. The situation of occurrence of cracks was investigated. The results are shown in Tables 4, 5 and 6 together with the slab rolling conditions. N in Tables 1-3
o. Are Nos. In Tables 4 to 6, respectively. It corresponds to. Table 4,
As is clear from 5 and 6, the Pb content, the Ni content and the Ni content
No. of the conventional example or the comparative example in which the parameter X value consisting of the equivalent + 0.8 × Cr equivalent deviates from the standard. 1, 2,
In Nos. 5, 6, and 27, cracking occurred during slabbing.

Except for the case where cracks occurred during the slabbing, only those without cracks during the slabbing were successively subjected to thick plate rolling, and test pieces were taken from the center of the thickness of the 110 mm thick plate. Sampled, 4K (-269 ° C), tensile test (shape: JIS 14A) and ASTM E813-1
A J _1C test according to 989 was conducted to obtain 0.2% proof stress (YS), tensile strength (TS), and fracture toughness value K _IC . The results are shown in Tables 4, 5, and 6 together with the plate rolling conditions.

0.2% yield strength (YS) and toughness (K
_IC value), Ni content and Ni _eq +0.8 x Cr _eq (=
Regarding the relationship with the parameter of (X), in Tables 4 to 6, particularly No. The results arranged using the data of the test materials of 3 and 7 to 9 are shown in FIGS. As is clear from FIGS. 7 and 8, the 0.2% proof stress (YS) and toughness (K _IC value) at 4K increase as the Ni content and the parameter X increase.
From this, as described above, 1200 N / mm ² at 4K
In order to obtain the 0.2% proof stress (YS) and the fracture toughness value K _IC of 200 MPa m ^1/2 or more, it is necessary to set the Ni content to 5% or more and the parameter X to 35% or more.

However, as described above, hot work cracking is prevented.
From the viewpoint of stopping, the Ni content is 8% or less and the parameter
X needs to be 40% or less. Therefore, the present invention
At the same time, 0.2% at 4K
% Yield strength (YS) and toughness (K _{I c}Value)
Is a Ni content of 5 to 8% and a parameter X of 35 to 40%.
It turns out that it needs to be a range.

Next, the relationship among 0.2% proof stress (YS) and toughness (K _IC value) at 4K, cleanliness d, average austenite grain size, and elongation austenite grain e will be described.

9 and 10 show 0.2% proof stress (YS) at 4K.
And the toughness (K _IC value) and the cleanliness d of steel are shown below. 9 and 10 show Tables 4 and 5, especially No.
The results are summarized by using the data of 0.2% proof stress and toughness at 4K of 8 and 10 to 12 test materials and the cleanliness of steel. As is clear from FIG. 9, the 0.2% proof stress at 4K is not significantly affected by the cleanliness d of the steel. But,
As is clear from FIG. 9, the toughness (K _IC value) at 4K is greatly affected by the cleanliness d of steel, and the cleanliness d of steel is as low as 0.1 or less (high cleanliness of steel). However, the toughness (K _IC value) at 4K is high. Therefore, 200MP at 4K
In order to obtain a fracture toughness value K _IC of am ^1/2 or more, it is understood that the cleanliness d of steel needs to be 0.1 or less.

Next, FIGS. 11 and 12 show the relationship between 0.2% proof stress (YS) and toughness (K _IC value) at 4K and the average austenite grain size. FIGS. 11 and 12 show the specimens of Tables 4 and 5, especially No. 8, 13 to 19 at 4K.
The results are summarized using the data of 0.2% proof stress, toughness and average austenite grain size of steel. As is clear from FIG. 11, the 0.2% proof stress at 4K has a positive correlation with the reciprocal of the square root of the average austenite grain size (d) of steel (increases as the average austenite grain size decreases). Therefore, in order to obtain 0.2% proof stress (YS) of 1200 N / mm ² or more at 4K, the average austenite grain size is 0.17 mm.
Must be:

On the contrary, the toughness at 4K has a negative correlation with the reciprocal of the square root of the average austenite grain size (d) of the steel, as is clear from FIG. To increase). Therefore, 2 at 4K
To obtain a fracture toughness value K _IC of 00 MPam ^1/2 or more,
The average austenite grain size must be 0.05 mm or less. As described above, from the results of FIGS. 11 and 12, 12 at 4K
In order to satisfy both the 0.2% proof stress of 00 N / mm ² or more and the fracture toughness value K _IC of 200 MPa m ^1/2 or more, it is necessary to set the average austenite grain size of steel in the range of 0.05 to 0.17 mm. I know there is.

Further, in FIGS. 13 and 14, 0.2% resistance at 4K
Force (YS) and toughness (K_{I c}Value) and the elongation
And show each. 13 and 14 show the results of Tables 4 and 5, especially N.
o. 0.2% proof stress at 8K of 8 and 20-22 test materials and
And toughness and the elongation of steel
Is. As is clear from Fig. 13, 0.2% proof stress at 4K
Has a positive correlation with the elongation of steel (the greater the elongation,
To increase). However, on the contrary, the toughness at 4K is
As is clear from the above, there is a large negative correlation with the elongation of steel.
(The greater the elongation, the more it decreases rapidly). Therefore,
200MPam at 4K ^1/2Fracture toughness value above K_{I c}Get
If so, it is necessary to set the extensibility to 1.2 or less.
I understand.

The above description of FIGS. 9 to 14 can be summarized as 4
0.2% proof stress of 1200N / mm ² or more at K and 200MPam
If both of the fracture toughness values K _IC of ^1/2 or more are to be satisfied,
The Ni content is 5% or more, the parameter X is 35% or more, the cleanliness d of the steel is 0.1 or less, and the average austenite grain size is in the range of 0.05 to 0.17 mm. It can be seen that the degree must be 1.2 or less.

In addition, No. 6 in Table 6 The test materials 23 and 24 are for examining the influence of the Ca content. No.
The Ca content of No. 23 is 0.0035%, which includes the upper limit of the Ca content of the present invention, and the fracture toughness value K _{IC at} 4K is as high as 225 MPa m ^1/2 in the central portion of the plate thickness, whereas No. The Ca content of 24 is 0.0100%, which exceeds the upper limit of the Ca content of the present invention, and the fracture toughness value K _{IC at} 4K in the central portion of the plate thickness.
Is also low at 190MPam ^1/2 . This is No. This is because the Ca content of 24 is too large and the amount of non-metallic inclusions in the steel becomes large, which deteriorates the fracture toughness at 4K.

Further, in Table 6, No. 25, 26, 28, 2
The test materials of Nos. 9 and 30 are examples of the present invention, and since they satisfy the requirements of the present invention, hot workability and 0.2% proof stress at 4K (Y
S) and toughness (K _IC value) are also good. On the other hand, No. In the comparative example of No. 27, since the Si amount, the parameter X, and the like deviate from the upper limits of the present invention, cracks occur in the slabbing and the hot workability is poor. Also,
No. of Table 6 In the comparative example No. 31, the total amount of Nb and V deviates from the upper limit of the present invention, so that the fracture toughness value K _{IC at} 4K is as low as 160 MPam ^1/2 .

[0077]

[Table 1]

[0078]

[Table 2]

[0079]

[Table 3]

[0080]

[Table 4]

[0081]

[Table 5]

[0082]

[Table 6]

[0083]

EFFECTS OF THE INVENTION According to the present invention, in the central part of the plate thickness, 0.2% proof stress at an extremely low temperature of 4K is 1200 N / mm ² or more, and high fracture toughness value is 200 MPa m ^1/2 or more. Have and at the same time
It is possible to provide high-Mn stainless steel that prevents hot work cracking and has excellent hot workability, and as a result, it is possible to increase the size of structural materials for superconducting magnets used in MHD power generation and fusion reactors. The industrial value is great in that it does.

[Brief description of drawings]

FIG. 1 is an amount of Pb and a cross-sectional shrinkage rate (RA) in the present invention.
It is explanatory drawing which shows the relationship with and processing temperature.

FIG. 2 is an amount of Pb and cross-sectional shrinkage ratio (RA) in the present invention.
It is explanatory drawing which shows the relationship with and processing temperature.

FIG. 3 is an amount of Pb and cross-sectional shrinkage ratio (RA) in the present invention.
It is explanatory drawing which shows the relationship with the processing temperature which becomes 40% or more.

FIG. 4 Ni content and Ni _eq +0.8 in the present invention
× Cr _eq (= X) parameter and cross-sectional shrinkage (RA)
It is explanatory drawing which shows the relationship between processing temperature.

FIG. 5: Ni content and Ni _eq +0.8 in the present invention
× Cr _eq (= X) parameter and cross-sectional shrinkage (RA)
It is explanatory drawing which shows the relationship between processing temperature.

FIG. 6 Ni content and Ni _eq +0.8 in the present invention
× Cr _eq (= X) parameter and cross-sectional shrinkage (RA)
It is explanatory drawing which shows the relationship with the processing temperature which becomes 40% or more.

FIG. 7: Ni content in the present invention and 0.2% at 4K
It is explanatory drawing which shows the relationship with yield strength.

FIG. 8 is an explanatory diagram showing the relationship between the Ni content and the fracture toughness value K _{IC at} 4K in the present invention.

FIG. 9: Cleanliness of steel according to the invention and 0.2% at 4K
It is explanatory drawing which shows the relationship with yield strength.

FIG. 10 is an explanatory diagram showing the relationship between the cleanliness of steel and the fracture toughness value K _{IC at} 4K in the present invention.

FIG. 11 is an explanatory diagram showing the relationship between the average austenite grain size of steel and the 0.2% proof stress at 4K in the present invention.

FIG. 12 is an explanatory diagram showing the relationship between the average austenite grain size of steel and the fracture toughness value K _{IC at} 4K in the present invention.

FIG. 13: Elongation of steel according to the invention and 0.2 at 4K
It is explanatory drawing which shows the relationship with% yield strength.

FIG. 14 is an explanatory view showing the relationship between the elongation of steel in the present invention and the fracture toughness value K _{IC at} 4K.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-9-41087 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (6)

(57) [Claims] 1. In mass%, C: 0.03 to 0.10%, Si:
0.10 to 0.50%, Mn: 18 to 30%, Ni: 5 to 8%, C
r: 12-18%, Mo: 0.50-3.00%, Al: 0.01-0.
07%, N: 0.15 to 0.28%, and 35% ≤ Ni _eq +
0.8 x Cr _eq ≤ 40% (however, Ni _eq = Ni% + 30 x C%
+30 x N% + 0.5 x Mn%, Cr _eq = Cr% + Mo% +
1.5 x Si% + 0.5 x Nb%), and
P: 0.008% or less, S: 0.003% or less, O: 0.0050% or less, Pb: 5 × 10 ⁻⁴ % or less, cleanliness d (However, JIS G 0555
(By measuring method): Each is regulated to 0.1% or less, consisting of balance Fe and inevitable impurities, and average austenite grain size
(However, according to the measuring method of ASTM E112-1995, 13.5, 234 pages) is 0.05 to 0.17 mm, and the elongation of austenite grains is
AI _l (according However ASTM E112 -1995,16.3.5,237 page measurement method) is 1.2 or less, the sheet thickness central part, in the absolute temperature 4K, 1200 N / mm ² or more 0.2% proof stress and 200MPa
A high Mn stainless steel material excellent in hot workability and cryogenic toughness, characterized by having a fracture toughness value of m ^1/2 or more. 2. Further, as a selective addition element, Nb: 0.01 to
0.20%, V: 0.01 to 0.50%, Ti: 0.01 to 0.50
%, The total amount of one or more of 0.01% to 0.50%
A high Mn stainless steel material containing the excellent hot workability and cryogenic toughness according to claim 1. 3. As a selective additive element, B: 0.0001 to
A high Mn stainless steel material having excellent hot workability and cryogenic toughness according to claim 1 or 2, containing 0.0030%. 4. Ca: 0.0010 as a selective additive element.
The high Mn stainless steel material having excellent hot workability and cryogenic toughness according to any one of claims 1 to 3, containing 0.0035% to 0.0035%. 5. The cross-sectional shrinkage ratio (RA) of the fractured portion is 40% or less.
Heat from the high-temperature high-speed tensile test (greeble test) above
The high Mn stainless steel material excellent in hot workability and cryogenic toughness according to any one of claims 1 to 4, having a hot working temperature range ΔT of 230 ° C or higher. 6. A high-Mn stainless steel material excellent in hot workability and cryogenic toughness according to claim 1, wherein the steel material is used for a structure of a superconducting magnet coil.