JPS6013022A - Production of nonmagnetic steel plate - Google Patents

Abstract

PURPOSE:To maintain excellent strength and toughness at an ultra-low temp. as well in producing a nonmagnetic steel plate of high Mn austenitic stainless steel by subjecting the steel to a heat treatment under specific conditions. CONSTITUTION:A nonmagnetic steel material of a high Mn austenitic stainless steel contg. 0.01-0.20% C, 0.01-2.0% Si, 10-30% Mn, 0.01-10.0% Ni, 12-20% Cr and 0.01-0.30% N, satisfying the formula 0.20%<=C+N<=0.40% or contg. further 1 or >=2 kinds among Cn, Mo, W, Nb, V, Ti, Al, Ca, Cl and Zr at 0.01- 3% in total in the case of Cu, Mo and W, 0.01-1.0% in total in the case of Nb, V, Ti and Al or 0.001-0.10 in total in the case of Ca, Cl and Zr as a structural material for supporting a superconductive magnet used in a cryogenic temp. such as liquid He or the like is held for 1-60min at 800 deg.C or is quickly cooled down to a temp. region of <=500 deg.C after air cooling upon ending of hot rolling at 950 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a non-magnetic steel sheet, and more particularly, to a method for manufacturing a non-magnetic steel sheet having high strength and high toughness at extremely low temperatures.

Superconducting magnets required for nuclear fusion reactors, superconducting magnet power storage installations, superconducting power generators, etc. are cooled to liquid helium temperature (-269°C) and subjected to severe stress repeatedly in strong magnetic fields. operate under conditions.

Therefore, the supporting structure material of the superconducting magnet is -269°
C has high yield strength and high toughness, and is also required to be non-magnetic. Of course, when rust develops, the insulation efficiency of the magnet decreases, so excellent induction resistance is also required.

SUS is a typical non-magnetic steel.
Although austenitic stainless steels such as 304.316 have excellent induction resistance and high toughness, they have low yield strength, and have the disadvantage that austenite has poor stability and magnetic permeability easily increases due to plastic deformation. . On the other hand, high Mn-based austenitic steels developed for cryogenic applications, such as 25Mn-lNi-5Cr steel, 32Mn--
7Cr steel and the like cannot be said to be sufficient in terms of yield strength or resistance to induction. Therefore, these conventional steels are still insufficient as superconducting magnet support structure materials.

The present invention was made in view of the drawbacks and problems of non-magnetic steel as a superconducting magnet support structure material explained above. An object of the present invention is to provide a method for manufacturing a high Mn austenitic stainless steel nonmagnetic steel sheet that is useful as a superconducting magnet support structure material that has high induction resistance and small variations in mechanical properties in the thickness direction. The method for manufacturing such a non-magnetic steel sheet includes (1) CO,
01~0.20 t%, Si0.01~2.00w1%
, Mn1O-30+ut%, N i , 0. Ol-10
, oou+L%, Cr 12-20u+t%, NO,
Contains 001~0.300u+t%, 0.2
0u+t%≦C+N≦0.40u+t% A steel ingot or billet consisting of residual iron and unavoidable impurities that satisfies 95%
Finish hot rolling at a temperature of 0°C or higher, then roll to 800°C
The first invention provides a method for manufacturing a non-magnetic steel sheet, which is characterized in that it is subjected to an isothermal holding or air cooling process at the above temperature for 1 to 60 minutes, and then rapidly cooled to a temperature of 500°C or less,
) CO, 01-0.20IIIL%, Si0.01-2
,0(but%, Mn 1O-30u+t%, NiO
,01-10,00u+L%, Cr 12-20wt%
, contains 0,001-0,300% of NO,
0.20u+L%≦c1N≦0.40u+t%, and furthermore, Cu, Mo, W, Nb, V, Ti, AI,
One or more selected from Ca, Ce, and Zr.”
2, the total amount of Cu, Mo, and w is 0.01 to 3
．． 00u+t%, Nl), ■, Ti is '0.01 in total amount
-1.00 wt%, and the total amount of AI, Ca%Ce, and Zr is 0.001 to 0.1000wt%, with the balance being iron and unavoidable impurities. Finish hot rolling at temperature, then
Two inventions in which the second invention is a method for manufacturing a non-magnetic steel sheet characterized by carrying out isothermal holding or air cooling at a temperature of 800°C or higher for 1 to 60 minutes, and then rapidly cooling to a temperature of 500°C or lower. It is more than that.

The method for manufacturing a non-magnetic steel sheet according to the present invention will be described in detail below.

First, the components and component ratios of the steel used in the method for manufacturing a non-magnetic steel sheet according to the present invention will be explained.

C is an element that is effective in stabilizing austenite and improving its yield strength. If the content is less than 0.0III%, this effect will be small, and if the content exceeds 0.20u+t%, the toughness will decrease and This will impair the resistance to induction. Therefore, the C content is set to 0.01 to 0.20 wt%.

Si is necessary for deoxidation during steel making, and is an element that increases the fluidity of molten steel during ingot making, reduces internal defects in steel ingots, and is also effective in improving direction resistance. , less than OhL%, there is no such effect, and 2.0On+L%
If the content exceeds 100%, the high temperature ductility will be inhibited and the toughness will be reduced. Therefore, the Si content is 0.01 to 2,00
Let it be wt%.

Mn is effective in stabilizing austenite and improving toughness, and if the content is less than 10 wt%, this effect will be small, and if the content is in excess of 30 wt%, hot workability and toughness will decrease. Therefore, the Mn content is 10~
It is set to 30Illt%.

Ni is an element effective in stabilizing austenite, improving toughness, and improving resistance to induction, and its content is 0.01u+L%.
If it is less than 8, there will be no such excellent effect. OO
If the content exceeds +ut%, the toughness will be saturated, and any further content will be wasteful. Therefore, the Ni content is 0.0
1 to 8.00w1%.

Cr is necessary for imparting resistance to induction, and is an element that improves yield strength.If the content is less than 12u+t%, this effect will not occur, and if the content exceeds 20wL%, the hot Decreases workability and toughness.

Therefore, the Cr content is set to 12 to 20 wt%.

N is an element that is effective in stabilizing austenite and increasing its yield strength, but if the content is less than 0.001%, it will not have this effect, and if it is contained in more than 0.300u+j%, the toughness will decrease. is large. Therefore, the N content is 0.001~
0.30 axis L%.

Both C and N are strong austenite stabilizing elements,
In addition, it has a large effect on the crack resistance toughness.
When the C+N content is less than 0.20 wt%, the effect of stabilizing austenite and increasing yield strength is small;
Excessive content exceeding u+t% significantly reduces toughness. Therefore, 0.20≦C+N≦0.40u+t%.

In addition to the above components, Cu, Mo, W, Nb, V, Ti,
Contains one or more of AI, Ca, Ce, and Zr. Cu, Mo, and W are effective in strengthening the austenite base and increasing the yield strength, but when the content is 0.0bu
If the content is less than t%, there will be no effect, and if the content exceeds 3.00wt%, hot workability and toughness will deteriorate. Therefore, C
The total amount of u%Mo and W is 0.01 to 3.0%. In addition, Nb, V, Ti, and AI form carbonitrides,
It is effective for directional resistance due to precipitation strengthening, and the content is 0.
O], less than ulL% has no effect; 1. OOwL
If the content exceeds %, the toughness will deteriorate. Therefore, N
b,■,T1,ノ\I content is 0.01-1.0 in total
It is set to 0wt%. Furthermore, Ca, Ce, and Zr have the effect of cleaning steel, refining inclusions, and spheroidizing the steel when the content is O.
If the content exceeds 0.001%, the cleanliness will be adversely affected.

Therefore, the total content of Ca, Ce, and Zr is 0.0
01 to 1.001%.

Next, manufacturing conditions in the method for manufacturing a non-magnetic steel sheet according to the present invention will be explained.

The inventor has obtained 0.05C22Mn 5N i-+3cr
Using -0,22N steel, the above liquid helium 1.
The influence of manufacturing conditions on various mechanical properties at temperature (-269°C) was investigated and will be explained.

Figures 11 and 2 show the influence of manufacturing conditions on the yield strength and Charpy absorbed energy value at -269°C. In FIGS. 4 and 2, 1 indicates a water-cooled material, 2 indicates an air-cooled material, and 3 indicates a solution-treated material after air cooling. In FIG. 1 and FIG. 2, the toughness of the air-cooled material after rolling increased as the rolling finishing temperature increased, but the yield strength significantly decreased. Furthermore, it was found that when this material was solution treated, the finishing temperature dependence was eliminated and the toughness was improved, but the yield strength was significantly reduced. On the other hand, we discovered a new fact that materials that are water-cooled immediately after rolling have better yield strength and toughness than air-cooled materials. That is, it was found that high yield strength and high toughness can be obtained at -269°C by high-temperature finishing → water cooling.

However, the material subjected to this treatment is shown in Figure 3 (4 in the figure).
1000°C finish → water cooling; 5 indicates 950°C finish → water cooling. ), there is a large change in toughness depending on where the specimen is taken in the thickness direction, and a significant decrease in toughness is observed near the surface layer, and there is a strong possibility that such a heterogeneous material will remain a problem in practice.

Next, the present inventor investigated the cause of the decrease in toughness of the surface layer of a water-cooled material after high-temperature finishing, and this will be explained.

Figure 4 shows the hardness distribution in the plate thickness direction (in the figure, 1 indicates water-cooled material after rolling, 2 indicates air-cooled material, and 3 indicates solution-treated material after air-cooling). It was revealed that the hardness change was small in the case of the water-cooled material, whereas the hardness and L of the water-cooled material increased significantly from the 1/4 part to the surface part.

In addition, Fig. 6 is a microscopic micrograph of each part in the thickness direction of the steel plate (Fig. 6 (, ) is near the surface layer of the steel plate, Fig. 6 (I
)) is 1/4 part, Figure 6 (c) is 1/2 part)
As is clear from this photograph, recrystallization is not sufficient in the vicinity of the surface layer, and it can be seen that unrecrystallized grains remain and at the same time, the recrystallized grains become finer. This means that the surface layer has a high cooling rate during water cooling and is not sufficiently recrystallized, and as shown in Figure 5 (A is 22Mn-5N
1-13Cr-0,22N steel, T3 is 0.6 (21
4Mn 2Ni-2Cr steel, C indicates 5S41. )
Niostenitic steel has much higher hot deformation resistance than ferritic steel such as 5S41, and during hot open rolling, the reduction amount per pass is not sufficient and the processing concentrates on the surface layer. It is considered that the organization has become a major organization.

Based on the various new findings of the present inventors as explained above,
A steel ingot or billet having the above-mentioned components and ratios is hot-rolled at a temperature of 950°C or higher, and instead of being rapidly cooled immediately thereafter, it is rolled until recrystallization of the surface layer is completed, that is, at 800°C. After holding isothermally for 1 to 60 minutes at a temperature of 1 to 60 minutes or slowly cooling by air cooling, it is possible to prevent the precipitation of carbides that impair toughness and resistance, and at the same time to prevent excessive coarsening of γ grains (reduction in yield strength). Excellent mechanical properties were obtained by carrying out a rapid cooling treatment to a temperature of 500° C. or less to prevent the material from sagging.

In this case, the hot rolling is terminated at a temperature of 950° C. or higher because, below this temperature, toughness deteriorates significantly due to grain refinement.

Further, the reason why the quenching start temperature is set to 800° C. or higher is that below this temperature, precipitation of Cr carbides at grain boundaries becomes significant, resulting in deterioration of toughness and reduction of induction resistance.

Furthermore, rapid cooling to a temperature of 500℃ or less requires 500℃.
This is because there is a concern that a large amount of Cr carbide will precipitate if cooling is stopped at a temperature of 0.degree. C. or higher.

As a result of the above, as shown in Fig. 7 (6 indicates 1000℃ finish → air cooling → 900℃ → water cooling, 7 indicates 1000℃ finish → water cooling), by adding a slow cooling process before water cooling, the area near the surface layer The researchers discovered a new fact that had not been seen before: the toughness of the steel plate was significantly improved, and the variation in toughness in the thickness direction was almost eliminated, and at the same time, an improvement in toughness was observed even in the 1/4E and 1/2 portions. . As shown in the micrograph of the microstructure at various parts in the thickness direction in Figure 9, the γ grains in the surface layer have become slightly finer, but recrystallization has been completed, and as shown in Figure 8, (8 is 1000°C finish → air cooling → 900°C
"C→Water cooling, 9 indicates 1000°C finish - water cooling.)
The variation in plate lv force direction hardness has also been significantly improved.

Examples of the method for manufacturing a non-magnetic steel sheet according to the present invention will be described together with comparative examples.

Molten steel with the ingredients and ingredient ratios shown in Table 1.
The agglomerates were made into steel slabs, and the treatments shown in Table tjS2 were performed. The results are shown in Table 3.

As explained above, since the method for manufacturing a non-magnetic steel sheet according to the present invention has the above configuration, it has high yield strength, high toughness, and low magnetic permeability at an extremely low temperature of -269°C, and also has high durability. A non-magnetic steel plate suitable as a superconducting magnet support structure material with excellent rust resistance can be manufactured.

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams showing the relationship between rolling finishing temperature, proof stress, and Charpy absorbed energy value, Figure 3 is a diagram showing the relationship between specimen sampling position in the plate thickness direction and toughness, and Figure 4 is a diagram showing the relationship between the plate toughness and the test piece sampling position in the plate thickness direction. Figure 5 is a diagram showing the hardness distribution in the thickness direction, Figure 5 is a diagram showing the relationship between rolling temperature and deformation resistance, Figure 6 is a micrograph showing the metal structure of each part in the thickness direction, and Figure 7 is a diagram showing the specimen collection (fi position). Figure 8 is a diagram showing the relationship between toughness and toughness, and Figure 8 is a diagram showing the hardness distribution in the plate thickness direction.
FIG. 9 is a micrograph showing the metal structure of each part in the thickness direction of the plate. (J de・/l゛舅) 躬9doichi=doζ0ヱ(bu'Not) AH 毛 [F] (と"l"A)Ig dandoku 1 "Me <゛・ -9

Claims (1)

[Claims] (],)CO,O]~0.20u+t%, Si0.01
~2.001%, Mloo-30111t%, Ni
O, 01-10, OOu+t%, Cr 12-20+l
lt%, NO,001~0.300+ut%, and 0.20+++L%≦C1N≦0.40+
Hot rolling of a steel ingot or billet consisting of residual iron and unavoidable impurities that satisfies lIt% at a temperature of 950°C or higher, followed by isothermal holding at a temperature of 00°C or higher for 1 to 60 minutes. Alternatively, a method for producing a non-magnetic steel sheet, which is characterized by rapidly cooling to a temperature of 500°C or less after undergoing an air cooling process. (2) CO, 01-0, 20u+L%, Si 0.
01-2. OOu+L%, Mn 1O-30u+L%,
NiO, 01-10,00u+L%, Cr 12~・
Contains 20%, NO,001~0.300%,
and satisfies 0.20wt%≦C1N 60.40wt%, and furthermore, Cu, Mo, W, Nb, \'STi, A
1.Ca. One or more selected from Ce and Zr, C
The total amount of u, Mo, and W is 0.01 to 3.00 Il.
The total amount of lt%, Nb, V, Ti%A1 is 0.01 to 1.
00IllL%, and the total amount of Ca, Ce, and Zr is 0.
001-0. ] 000wt, with the balance consisting of iron and unavoidable impurities, hot rolling is completed at a temperature of 950°C or higher, followed by isothermal holding at a temperature of 800°C or higher for 1 to 60 minutes, or A method for producing a non-magnetic steel sheet, which comprises air cooling and then rapid cooling to a temperature of 500°C or less.