JPH0619110B2 - Method for producing high Mn austenitic stainless steel for cryogenic use - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a high Mn austenitic stainless steel for cryogenic use, which has high yield strength, high toughness and excellent magnetic properties at cryogenic temperatures.

(Prior Art) Superconducting magnets used in fusion reactors, linear motor cars, electromagnetic propulsion vessels, large accelerators, etc. are cooled to liquid helium temperature (-269 ° C) during operation, and are repeatedly used in a strong magnetic field. It is exposed to a harsh environment where high stress acts. Therefore, a non-magnetic steel having high yield strength and high toughness at -269 ° C is required for the supporting structure material of the superconducting magnet.

Conventionally, as cryogenic non-magnetic steel, SUS 304L, 304L
Austenitic stainless steels such as N and 316L have been used, but these stainless steels have excellent toughness at -269 ° C, but have a major drawback of low yield strength. Therefore, in order to use the conventional austenitic stainless steel as a structural material of a superconducting magnet, it is unavoidable to increase the wall thickness, and as a result, the cooling efficiency by liquid helium becomes extremely poor, or the superconducting magnet. However, there is a problem in that the size and weight must be increased.

(Object of the Invention) The present invention has been made to solve the problems in the conventional austenitic stainless steel as the above-mentioned non-magnetic steel for cryogenic use, and has high yield strength, high toughness and excellent High Mn for cryogenic temperature that combines magnetic properties
An object is to provide a method for producing austenitic stainless steel.

(Structure of the Invention) The method for producing a high-Mn austenitic stainless steel for cryogenic use according to the present invention is, by weight%, C 0.01 to 0.15%, Si 0.10 to 2.00%, Mn 16 to 30%, Heat a steel ingot or billet containing Ni 0.1-8.0%, Cr 12-20%, P 0.03% or less, S 0.02% or less, and N 0.10-0.35%. Total rolling reduction of 1 after rolling
Cold rolling at 0% or more, then 900-115
After the solution treatment at a temperature of 0 ° C., the total rolling reduction is 0.5
It is characterized in that cold rolling is performed at 10%.

First, the reason for limiting the chemical composition in the steel of the present invention will be described.

C is an element effective in stabilizing austenite and improving yield strength. If the addition amount is less than 0.01%, the above effect is poor. On the other hand, if the addition amount exceeds 0.15%, the toughness of the steel is deteriorated and the rust resistance is impaired. Become. Therefore, in the present invention, the amount of C is set to the range of 0.01 to 0.15%.

Si is necessary for deoxidation during steel melting, and is also effective for improving yield strength. In order to effectively develop such effects, it is necessary to add 0.10% or more. However, when it is added in excess of 2.00%, it impairs the high temperature ductility of the steel and reduces the toughness. Therefore,
In the present invention, the addition amount of Si is 0.10 to 2.00.
The range is%.

Like C, Mn also has the effect of stabilizing austenite, improves toughness, and increases the solid solubility limit of N. However, in the amount of Ni within the range specified in the present invention, when the amount of Mn added is less than 16%, austenite cannot be sufficiently stabilized at an extremely low temperature. On the other hand, if it is added in excess of 30%, δ-ferrite is likely to be formed, and the hot workability, toughness and magnetic properties of the steel are deteriorated. Therefore, in the present invention, the amount of Mn added is in the range of 16 to 30%.

Since P and S are impurity elements that impair the hot workability, weldability and toughness of steel, it is preferable to suppress the contents thereof as much as possible in the steel of the present invention. However, in consideration of economical efficiency in steel making, the content of P is 0.03% or less, and the content of S is 0.02% or less.

Ni is effective in stabilizing austenite and improving toughness, but at least 0.1% must be added in order to effectively exhibit such effects. However, in the range of Mn amount defined in the present invention, even if Ni is added excessively in excess of 8.0%, the above effect is saturated and the economical efficiency is impaired. The range is from 0.1 to 8.0%.

Cr is an element that imparts rust resistance to steel and is also effective in improving yield strength and increasing N solid solution limit. In order to effectively obtain these effects, in the steel of the present invention, at least 1
It is necessary to add 2%. On the other hand, a large amount of addition exceeding 20% promotes the formation of δ-ferrite and deteriorates hot workability, toughness, magnetic properties, etc., so the addition amount is 12
-20% range.

As with C, N is extremely effective as an interstitial solid solution element for stabilizing austenite and improving yield strength of steel, but in order to effectively exhibit the above effect, the addition amount is 0.10.
% Or more is required. However, if over 0.35% is added, the toughness and weldability are significantly deteriorated. Therefore, the amount of N added is in the range of 0.10 to 0.35%.

Further, the steel of the present invention contains, in addition to the above-mentioned elements, one or more elements selected from the group consisting of Cu, Mo and W in a total amount of 0.01 to 2.00%. You can All of these elements strengthen the austenite matrix and are effective in increasing the yield strength. However, when the total amount added is less than 0.01%, the above effect is poor,
On the other hand, when the total amount exceeds 2.00% and is excessively added, the toughness of the steel is deteriorated, so the total amount added is 0.
The range is from 01 to 2.00%.

Further, the steel of the present invention is, in addition to the above-mentioned elements or independently of the above-mentioned elements, Nb 0.01 to 0.50%, V 0.01 to 0.50%, and Ti 0.01 to 0. At least one element selected from the group consisting of 0.50% can be contained.

Nb, V and Ti are all effective in improving the yield strength of steel by solid solution strengthening or precipitation strengthening. In order to effectively develop such effects, 0.0
It is necessary to add 1% or more. However, if the added amount exceeds 0.50%, the toughness is deteriorated, so the upper limit of the added amount is 0.50% for each element.

Further, the steel of the present invention, together with or independently of the above-mentioned elements, has a total amount of at least one element selected from the group consisting of Al, Ca, Ce and Zr of 0.001 to 0.10.
You may contain in the range of 0%. Each of these elements
It has the effects of cleaning steel, refining inclusions, and spheroidizing,
Improves the hot workability and toughness of steel, but the total amount is 0.0
If the addition amount is less than 01%, the above effect cannot be effectively obtained. On the other hand, if the addition amount exceeds 0.100% in total, the cleanliness of the steel is deteriorated, and
It deteriorates the toughness of steel.

According to the method of the present invention, a steel ingot or a steel slab having the above chemical composition is hot-rolled, then cold-rolled at a total reduction of 10% or more, and then at a temperature of 900 to 1150 ° C. After solution treatment, the total rolling reduction is 0.5 to 10%.
Then, cold rolling is performed to obtain a high Mn austenitic stainless steel for cryogenic temperatures.

First, cold rolling with a total reduction of 10% or more after hot rolling is
It is necessary to refine the austenite grains of the steel after the solution treatment described later and to improve the dimensional accuracy of the product. The solution treatment after the cold rolling is 900 to 1150.
It is carried out at a temperature of ° C. When it is lower than 900 ℃,
It is insufficient for the solid solution of the precipitate and the sizing of the austenite crystal grains, and the rust resistance and toughness deteriorate. But 115
When the solution treatment is carried out in a high temperature range exceeding 0 ° C., the austenite crystal grains are remarkably coarsened and the yield strength is greatly reduced.

In the method of the present invention, after the solution treatment, cold rolling is further performed at a total reduction of 0.5 to 10%.
The present inventors have found that this cold rolling increases the yield strength of the austenitic stainless steel at -269 ° C and, at the same time, improves the ductility and toughness.

Steel with a chemical composition of steel type A in the table is 4.0 mm thick
Hot-rolled, then cold-rolled to a thickness of 2.0 mm, and
Solution treatment was performed at 1050 ° C. These steel sheets were further cold-rolled in the range of 0 to 20% in total rolling reduction, and the thus obtained steel sheets were subjected to a tensile test and a notched tensile test at -269 ° C.

The results are shown in FIG. 1. The present inventors have found that the yield strength and tensile strength at -269 ° C. improve as the cold rolling rate increases, while the elongation is about 2 to the cold rolling rate. It has been found that it rises up to the range of 5% and then decreases as the cold rolling rate increases. That is, the steel sheet cold-rolled in the range of the cold-working rate of 0.5 to 10% has higher yield strength and tensile strength than the steel sheet not subjected to such cold-rolling, and the ductility is the same or higher. is there.

On the other hand, the toughness defined as the breaking strength by the notched tensile test / 0.2% proof stress by the smooth tensile test also shows the same tendency as the elongation, and the cold working ratio is in the range of 0.5 to 10%. The steel sheet has a toughness equal to or higher than that of a steel sheet that is not subjected to cold working.

(Effect of the invention) N strengthened high Mn austenitic stainless steel is originally
Even as-solution-treated, SUS 304L, compared to steel such as 304 LN, has a high yield strength, but according to the present invention, by treating a steel having a predetermined chemical composition under predetermined conditions,
Higher yield strength can be obtained, and at the same time, ductility and toughness are also improved. Therefore, according to the austenitic stainless steel according to the present invention, for example, it is possible to achieve thinning and weight saving of the superconducting magnet.

Incidentally, the manufacturing method according to the present invention is not limited to a steel plate, but a wire rod,
It can also be applied to steel bars, forged products, die steels, etc.

(Examples) Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited to these examples.

Example 1 Inventive Steels 1 to 8 and Comparative Steel 9 having the chemical compositions shown in the table
Nos. 16 to 16 were melted in a vacuum melting furnace, hot forged and then hot rolled to manufacture a hot rolled plate having a thickness of 4.0 mm. Then
This is pickled and then cold rolled to a thickness of 2.0-3.
8mm cold rolled steel sheet is manufactured, and
Solution treatment was performed at 0 to 1200 ° C. After pickling these cold-rolled sheets, they are further cold-rolled at a total reduction ratio of 0 to 20%. Was carried out to obtain a test material. Tensile tests and notched tensile tests were performed on these test materials at liquid helium temperature (-269 ° C).

The steels 1 to 8 of the present invention according to the method of the present invention have high strengths of yield strength 120 kgf / mm ² or more and tensile strength 160 kgf / mm ² or more at -269 ° C, and at the same time, have an elongation of 35% or more,
It has excellent ductility. Further, the toughness (definition is the same as above) is also excellent, and shows a high value of 1.5 or more.

Further, the magnetic permeability at −269 ° C. after cold rolling is as low as 1.0 or less, and it also has excellent magnetic properties.

On the other hand, the comparative steels 9 to 15 have the chemical composition within the range specified by the present invention, but the manufacturing method does not satisfy the conditions specified by the present invention, and therefore does not have predetermined characteristics.

That is, Comparative Steel 9 is inferior to the steel of the present invention in strength, ductility, and toughness because cold rolling after solution treatment is not performed. Comparative steels 10 and 11 have high cold rolling ratios after solution treatment of 15% and 20%, respectively, and thus have high strength but low ductility and toughness. Comparative Steel 12 has a particularly low yield strength because the solution heat treatment temperature is too high at 1200 ° C. On the contrary, Comparative Steel 13 has an extremely low ductility and toughness because the solution treatment temperature is too low, the solid solution of carbonitride is not sufficient, and the austenite is mixed grains. Comparative steel 14 has a cold rolling rate of 5% after hot rolling.
Therefore, the austenite grains after the solution treatment are coarsened, and the yield strength and tensile strength are not sufficient.

Comparative steel 15 has low ductility and toughness because the cold rolling rate after solution treatment is as high as 15%. Also, comparative steel 16
Since the N content is as low as 0.05%, the yield strength and tensile strength are not sufficient, and the magnetic permeability at -269 ° C is also poor.

[Brief description of drawings]

The drawing shows that the steel having the chemical composition shown as steel type A in the table is hot-rolled to a plate thickness of 4.0 mm, cold-rolled to a plate thickness of 2.0 mm, and further subjected to solution treatment at 1050 ° C. Using the steel sheet thus obtained, further cold rolling was performed in the range of 0 to 20% of the total rolling reduction, and the steel sheet thus obtained had 0.2% proof stress at -269 ° C and tensile strength. It is a graph which shows the influence of the cold-rolling rate which acts on the elongation, elongation, and toughness.

Claims (4)

[Claims] 1. C 0.01-0.15% by weight%, Si 0.10-2.00%, Mn 16-30%, Ni 0.1-8.0%, Cr 12-20%, After hot rolling a steel ingot or a steel slab containing P 0.03% or less, S 0.02% or less, and N 0.10 to 0.35%, the total rolling reduction is 1
Cold rolling at 0% or more, then 900-115
A method for producing a high-Mn austenitic stainless steel for cryogenic use, which comprises performing solution treatment at a temperature of 0 ° C. and further performing cold rolling at a total reduction of 0.5 to 10%. 2. A weight percentage of (a) C 0.01-0.15%, Si 0.10-2.00%, Mn 16-30%, Ni 0.1-8.0%, Cr 12- 20%, P 0.03% or less, S 0.02% or less, and N 0.10 to 0.35%, and (b) Nb 0.01 to 0.50%, V 0.01. To 0.50%, and at least one element selected from the group consisting of Ti 0.01 to 0.50%, and (c) one or two elements selected from the group consisting of Al, Ca, Ce and Zr. The total amount of the above elements is 0.001 to 0.1
After hot rolling a steel ingot or a steel slab containing 100%, cold rolling is performed at a total reduction of 10% or more, and then 900
After the solution treatment at a temperature of ~ 1150 ° C,
A method for producing a high-Mn austenitic stainless steel for cryogenic use, which comprises performing cold rolling at a total reduction of 0.5 to 10%. 3. A weight percentage of (a) C 0.01-0.15%, Si 0.10-2.00%, Mn 16-30%, Ni 0.1-8.0%, Cr 12- 20%, P 0.03% or less, S 0.02% or less, and N 0.10 to 0.35%, and (b) one or more selected from the group consisting of Cu, Mo, and W, or The total amount of two or more elements is 0.01 to 2.00%, and the total amount of one or more elements selected from the group consisting of (c) Al, Ca, Ce and Zr is 0.001. ~ 0.1
After hot rolling a steel ingot or a steel slab containing 100%, cold rolling is performed at a total reduction of 10% or more, and then 900
After the solution treatment at a temperature of ~ 1150 ° C,
A method for producing a high-Mn austenitic stainless steel for cryogenic use, which comprises performing cold rolling at a total reduction of 0.5 to 10%. 4. A weight percentage of (a) C 0.01-0.15%, Si 0.10-2.00%, Mn 16-30%, Ni 0.1-8.0%, Cr 12- 20%, P 0.03% or less, S 0.02% or less, and N 0.10 to 0.35%, and (b) one or more selected from the group consisting of Cu, Mo, and W, or The total amount of two or more elements is 0.01 to 2.00, and (c) Nb 0.01 to 0.50%, V 0.01 to 0.50%, and Ti 0.01 to 0.50. And at least one element selected from the group consisting of (d) Al, Ca, Ce, and Zr in a total amount of 0.001 to 0.1
After hot rolling a steel ingot or a steel slab containing 100%, cold rolling is performed at a total reduction of 10% or more, and then 900
After the solution treatment at a temperature of ~ 1150 ° C,
A method for producing a high-Mn austenitic stainless steel for cryogenic use, which comprises performing cold rolling at a total reduction of 0.5 to 10%.