JPH0657867B2 - High strength non-magnetic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a non-magnetic stainless steel having excellent corrosion resistance and high strength, which is used in various devices and apparatuses that function by utilizing magnetic properties.

<Prior art and its problems> Cr-Ni-based austenitic stainless steel represented by SUS304 has good corrosion resistance and a non-magnetic austenitic structure in the annealed state. Therefore, it is used as a non-magnetic steel for electric and precision instrument parts. ing. Since some parts require strength, they are used after cold working. However, since the austenite phase of SUS304 steel is metastable, martensitic transformation occurs during cold working and becomes magnetic. Therefore, for such a purpose, SUS31 having a more stable austenite phase is used.
6 or SUS3 with high N content as high strength steel
04, SUS316N is used.

However, these Cr-Ni austenitic steels were not originally developed as non-magnetic steels, but general steels were merely diverted to non-magnetic applications. For example,
The SUS316 series, which has a more stable austenite phase, contains a large amount of expensive Ni and Mo. Mo has an excellent effect on corrosion resistance, but it does not contribute to high strength or non-magnetism. In addition, SUS316N having a high N content also has a small increase in strength due to cold working, and cannot be said to be a high strength material.

High Si content steels include heat resistant steels and stress corrosion cracking steels, and these steel types are based on the fact that Si exerts an excellent effect on oxidation resistance or stress corrosion cracking prevention. Further, as a steel having excellent scratch resistance, Japanese Patent Publication No. 56-3238
There is steel disclosed in Japanese Patent No. 7 and its chemical composition is Cr: 12 to 19
%, Ni: 4-12%, Mn7-12%, Si3-5
%, C: 0.01 to 0.12%, N: 0.03 to 0.3%
In order to secure the austenite structure during annealing,
Austenite forming element N in an amount directly proportional to the content of
i is included.

Furthermore, as a steel having seizure resistance and scratch resistance, Japanese Patent Publication No. 56
-11379, there is a steel of which the chemical composition is Cr:
13-25%, Ni: 5-15%, Mn: 0.5-5.
5%, Si: 2.5 to 5.0%, C: 0.15% or less,
N: 0.05 to 0.20%, which can be used as a sliding member under the condition that a lubricant cannot be used. It strengthens the geology of Si and N, and produces an oxide film by Si and its self-recovery property. It uses the enhancement.

However, these high Si content austenitic steels are not considered at all in terms of weldability and ensuring non-magnetism in a state hardened by cold working. That is, these steels are prone to high temperature cracking during welding depending on the component system, and become magnetic due to the formation of martensite by cold working, and cannot be used as non-magnetic steels.

As outlined above, the present situation is that Cr-Ni austenitic stainless steel as work-hardening type high strength non-magnetic stainless steel having good weldability has not yet been provided.

<Means for Solving Problems> The present inventors have investigated the effects of alloying elements and cold working and heat treatment on the hardness and magnetic permeability of Cr-Ni austenitic stainless steels for many years, and as a result, Si and N were cold worked. It has been found that it has a significant effect on the increase in strength after heat treatment and after appropriate heat treatment after cold working. Further, it was found that V not only increases the strength after annealing by forming precipitates with C and / or N but also brings about a remarkable increase in strength by cold working or cold working followed by moderate heat treatment. . Therefore, the strength of steel is increased by adding cold-working or cold-working and subsequent heat treatment to the steel in which the solid solution strengthening elements of Si and N are added to the Cr-Ni base, and further V which is the precipitation strengthening element is added. By designing and adding an austenite phase stabilizing element, the composition of the austenitic steel that maintains the non-magnetic property after cold working was designed. In the present invention, the term “non-magnetic” means that the magnetic permeability is 1.01 or less.

<Structure and Action of the Invention> According to the present invention, C: 0.12% or less, Si: 3 to 6%, and Mn: by weight.
3 to 9%, Ni: 12 to 16%, Cr: 12 to 22%,
N: 0.1 to 0.25%, V: 0.5% or less,
The balance is Fe and impurities, and the A (γ) value defined by the following formula A (γ) = Ni + 0.5Mn + 30 (C + N) -1.3Cr-2.6Si + 13V + 11.8 is -10.0 <A ( γ) <0 is satisfied, and the Ni equivalent defined by Ni equivalent = Ni + 0.6Mn + 9.69 (C + N) + 0.18Cr-0.11Si ² + 2.3V, Y = exp (0 .45X-4.60) (where Y is the cold rolling ratio (%) and X is the Ni equivalent) in the formula, cold working was carried out below the curve AA line in FIG. A high strength non-magnetic stainless steel is provided.

In the steel of the present invention, C is a strong austenite phase stabilizing element similar to N and is an element effective for improving strength. On the other hand, C significantly reduces corrosion resistance and weldability. In consideration of the above, in the case of the steel of the present invention, the upper limit is 0.12%.

Si is a useful element that achieves the high strength, which is the main feature of the steel of the present invention, and at least 3% is necessary to achieve its purpose, but when the Si content increases, it also increases after cold working. The upper limit is set to 6% because the steel becomes magnetized and the hot workability deteriorates.

Mn, like Ni, is an essential element for ensuring non-magnetism after cold working. Further, Mn is also an element that enhances the solid solubility of N. 3% or more is necessary to exert these performances, and the content of Mn together with Ni needs to be adjusted according to the Si content in order to maintain the non-magnetic property after cold working. In order to reduce the hardness after cold working and to increase the hot crack sensitivity during welding, the upper limit is 9%.

Ni is a basic component of austenitic steel and is an element that contributes to stabilization of the austenitic phase. 12% or more is necessary to maintain non-magnetic property after cold working.
It is necessary to adjust the Ni content as described above according to the i and Mn contents. However, a large amount of Ni brings about a decrease in hardness after cold working similarly to Mn, and increases the hot crack sensitivity during welding, so the upper limit is made 16%.

Cr is a basic component of stainless steel, and it is necessary to contain 16% or more in order to obtain good corrosion resistance. However, if it is contained in a large amount, a large amount of delta ferrite is generated and the hot workability is deteriorated. However, the upper limit is set to 22% because non-magnetism cannot be secured.

N is an element effective for maintaining the non-magnetism, which is the main feature of the steel of the present invention, and for obtaining high strength. In order to exhibit these performances, it is necessary to contain 0.1% or more. However, if it exceeds 0.25%, a sound steel ingot cannot be obtained, so this is made the upper limit.

V is an element effective for obtaining high strength, which is a main feature of the steel of the present invention, but as shown in FIG. 7, even if a large amount of V is added, the strength is not increased so much, and conversely. The upper limit is set to 0.5% because the amount of delta ferrite generated increases and non-magnetism cannot be secured.

The formulas for A (γ) and Ni equivalents and the numerical range of A (γ) are derived from insights from the experimental results.

That is, in the present invention, Cr-Ni-based austenitic stainless steel is made to contain a large amount of Si, N and Mn, and an appropriate amount of V is added to adjust the composition as described above.
By subjecting this to cold working, or by subjecting it to cold working and heat treatment, the strength is increased and the magnetic permeability after cold working is 1.
It can be suppressed to 01 or less. The heat treatment after cold working is preferably performed at 300 to 600 ° C.

<Detailed Description of the Invention> Next, the present invention will be described in detail with reference to the drawings. According to the knowledge of the present inventors, there is the following relationship between the hardness and magnetic permeability of Cr-Ni-based austenitic stainless steel and the strength after cold rolling or cold rolling followed by heat treatment and alloying elements. .

FIG. 1 shows hardness and Si, N after 60% cold working of 17Cr-14Ni-5Mn-ySi-0.05C-xN-0.15V steel (x and y are variables) of the present invention.
The relationship of the content of is shown. FIG. 2 shows the relationship between the hardness and the Si and N contents of the same steel after 60% cold working and heat treatment at 500 ° C. for 1 hour. That is, FIG.
It shows the mutual relationship between Si and N that affects the hardness of Cr-5Mn-ySi-0.05C-xN-0.15V steel (x and y are variables). That is, the figure shows the interrelationship of Si and N and the content ratio of HV420 in the cold working state of 60%. Therefore, if the ratio of Si and N is lower than that of the line of HV420, the hardness decreases, and for example, the Si obtained by HV400,
The ratio of N is shown in the diagram. On the contrary, the higher the ratio of Si and N, the higher the hardness.

FIG. 2 shows the Si and H content ratios at which HV500 or HV480 is obtained when the same steel is cold worked by 60% and then treated at 500 ° C. for 1 hour. As can be seen from these figures, both Si and N contribute greatly to the cold work and increase in hardness after aging. As you can see from these figures, S
i and N complementarily increase the hardness after cold working.

Figure 3 shows the change in hardness of 17Cr-14Ni-5Mn-4.7Si-0.05C-0.15N composition V-free steel and V-added steel after heat treatment at 500 ℃ for 1 hour in 60% cold working. It is a diagram showing. As can be seen from this figure, the V-added steel has a greater hardness after cold rolling and cold rolling heat treatment than the V-free steel.

FIG. 4 shows 16.5Cr-14.0Ni-5.0Mn-5.0Si-0.10C-0.
It is a diagram showing the effect of heat treatment temperature (soaking time, 1 hour) on the hardness of 16N-0.22V steel after 60% cold rolling.
As can be seen from this figure, the heat treatment after cold working is 300 ~
It should be done at 600 ° C. There is no curing effect in the lower temperature range, and softening occurs in the higher temperature range.

Fig. 5 shows a comparative steel, 16Cr-13Ni-1.65Mn-xSi-0.05C-0.0.
It is a diagram which shows the influence of Si on the magnetic permeability after cold rolling of 4N-0.10V steel (x is a variable). Conventionally, it was said that Si contributes to stabilization of the austenite phase against cold working, but as shown in this figure, Si increases the magnetic permeability after cold working, and the effect increases as the content increases. Is remarkable. Therefore, it is necessary to adjust the content of the austenite phase stabilizing element according to the content of Si, which is an element essential for strengthening. That is, it was found that the value of Ni equivalent which affects the magnetic permeability after cold working is defined by the above-mentioned formula.

FIG. 6 shows the relationship between the reduction ratio of the cold rolling applied and the minimum Ni equivalent value required to maintain non-magnetism in the steel of the present invention. The minimum value of Ni equivalent weight as defined above required to maintain non-magnetic properties after cold working is given by curve A-A in FIG. 6 and is a highly cold rolled steel. It has been found that it must have a reasonably high Ni equivalent value.

Figure 7 shows the effect of V on the hardness of 19Cr-14Ni-5Mn-5Si-0.06C-0.15N-xV steel (x is a variable) after cold rolling and after heat treatment at 500 ° C for 1 hour. FIG. From this figure, it can be seen that the effect of V on the hardness of the steel of the present application is saturated at about 0.15%.

Further, the stainless steel containing high Si, high Mn, and high Ni is one kind of high alloy steel, and in order to obtain good weldability, it is necessary to design the composition so as to generate an appropriate amount of delta ferrite during welding. . As a result of repeated experiments, the stability index of the austenite phase, A (γ), was defined as described above, and the value was adjusted so that it was between -10.0 and 0 as described above. It turned out to be good. The A (γ)
If the value is less than -10.0, a large amount of delta ferrite is generated and non-magnetism is not maintained, and if it is 0 or more, good weldability is not secured.

<Example> The characteristics of the steel of the present invention will be clarified by an example as compared with the conventional steel and the comparative steel.

The steel shown in Table 1 was melted. Samples A1 and A2 are conventional steels, A1 is SUS304 steel, A2 is SUS316 steel, and the amount of Ni is close to the upper limit of the standard. C1 to C3 are comparative steels, C1 is S
The i content is high, and C2 has a high Si, N and Mn content, but the Ni content is as low as 9%. C3 has a component content within the range of the present invention, but has an A (γ) value outside the range of the present invention. B1 to B6 are the steels of the present invention.

Each steel was melted in a 30 kg high frequency induction melting furnace.
Each steel was forged to a thickness of 10 mm and a width of 120 mm, heat treated, cold rolled to 3 mm, intermediate annealed, further cold rolled to 1.5 mm, and finally annealed.
The test piece was 20 × 300 mm.

These samples were subjected to the target cold rolling, and then the Vickers hardness was measured with a load of 20 kg, and the magnetic permeability was determined by a Shimadzu magnetic balance M.
The measurement was performed using a B-3 type under a magnetic field of 1000 Oe.

Table 2 shows 20%, 40%, 6% after annealing of each steel in Table 1.
Evaluations of Vickers hardness after 0% cold rolling and after heat treatment at 500 ° C. for 1 hour after cold rolling, magnetic permeability after 60% cold rolling, and weldability are shown.

Here, the weldability was evaluated by performing a color check after TIG welding, and ◯ indicates that no cracks were observed, and x indicates that many cracks were observed.

As is known from Table 2, A1 steel (SUS304) is excellent in hardness by Hv455 and Hv486 by 60% cold rolling, but has Ni equivalent of 13.77. Low, very high magnetic permeability after cold working. That is, SUS304 cannot be used as a non-magnetic steel because the martensite phase is generated by cold working and the magnetic permeability increases. A2 steel (SUS316) is 60
% Non-magnetic after 1.0% cold rolling, the hardness after 60% cold rolling is Hv376 and the hardness after heat treatment is as low as Hv407, which is high strength material. Not enough. C1 steel and And C2 steel have high strength levels like SUS304, but have high magnetic permeability after cold rolling. C3 steel has a high hardness after being subjected to 60% cold rolling and has a magnetic permeability of 1.01 or less, which is non-magnetic, but the A (γ) value is a positive value and the weldability is high. Is extremely bad.

On the other hand, the B1 to B6 steels of the present invention are Si,
High N content, including V, 60% hardness after cold rolling is Hv 420 or more, and 60% after cold rolling 500 ° C.
The hardness becomes Hv500 or more by performing the heat treatment at. Regarding the magnetic permeability, it contains appropriate amounts of Ni, Mn and N so that the Ni equivalent given by the above formula becomes 19.0 or more, and the magnetic permeability is 1. even after cold rolling of 60%. It is 01 or less and is excellent in non-magnetism. Furthermore, the A (γ) value given by the above formula is -10.0 to
The composition is adjusted so as to be 0, and the weldability is also good.

<Effects of the Invention> The steel of the present invention has good corrosion resistance, is excellent in hardness after being subjected to heat treatment after cold working and after cold working, and has a magnetic permeability of 1.01 or less after cold working. (EN) Provided is a non-magnetic stainless steel having stable non-magnetism and having extremely high practicality as a material for electric and electronic equipment parts and devices requiring high strength.

[Brief description of drawings]

FIG. 1 shows the relationship between hardness and Si and N contents of 60Cr-14Ni-5Mn-ySi-0.05C-xN-0.15V steel (y and x are variables) of the present invention after 60% cold working. It is a diagram. FIG. 2 is a diagram showing the relationship between hardness and Si and N contents after heat-treating the same steel at 60 ° C. for 1 hour after cold working. Figure 3 shows the change in hardness of 17Cr-14Ni-5Mn-4.7Si-0.05C-0.15N composition V-free steel and V-added steel after heat treatment at 500 ℃ for 1 hour in 60% cold working. It is a diagram showing. FIG. 4 shows 16.5Cr-14.0Ni-5.0Mn-5.0Si-0.11C-0.
It is a diagram which shows the influence of the heat treatment temperature (soaking time 1 hour) on the hardness after 16% cold working of 16N-0.22V steel. Fig. 5 shows a comparative steel, 16Cr-13Ni-1.5Mn-xSi-0.05C-0.04.
It is a diagram which shows the influence of Si which acts on the magnetic permeability after cold rolling of N-0.15V steel (x is a variable). FIG. 6 shows the relationship between the Ni equivalent required to maintain non-magnetism and the upper limit of the cold rolling rate in the steel of the present invention. Figure 7 shows the effect of V on the hardness of 19Cr-14Ni-5Mn-5Si-0.06C-0.15N-xV steel (x is a variable) after cold rolling and after heat treatment at 500 ° C for 1 hour. It is a diagram showing.

Claims (1)

[Claims] 1. C: 0.12% or less by weight, Si: 3 to
6%, Mn: 3-9%, Ni: 12-16%, Cr: 1
2 to 22%, N: 0.1 to 0.25%, V: 0.5% or less, and the balance Fe and impurities. The following formula A (γ) = Ni + 0.5Mn + 30 (C + N) -1.3Cr-2.6Si + 13V + 11.8, A (γ) value satisfies -10.0 <A (γ) <0, and Ni equivalent = Ni + 0.6Mn + 9.69 (C + N) For the Ni equivalent defined by + 0.18Cr-0.11Si ² + 2.3V, the empirical formula Y ≦ exp (0.45X−4.60) (where Y is the cold rolling rate (%), X Is a Ni equivalent), and is a high-strength non-magnetic stainless steel characterized by being cold-worked below the curve AA line in FIG.