JP3182995B2 - High Mn non-magnetic steel with excellent stress corrosion cracking resistance and mechanical properties - 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 non-magnetic steel having excellent resistance to stress corrosion cracking and mechanical properties.

[0002]

2. Description of the Related Art Conventional high-Mn non-magnetic steel, 1%
C-13% Mn-based steel (Hadfield steel) and 0.4% C-18
% Mn-5% Cr-based steel (ASTM A289 class B steel) is cheaper than austenitic stainless steel, has higher proof stress, and has stable magnetic permeability to cold working. Instead, they are used for structural members such as lifting magnets, generators and electric motors. However, it is known that such steel materials are susceptible to stress corrosion cracking due to the presence of low tensile stress even in extremely weak corrosive environments wetted by industrial water or dew condensation. It has been reported.

[0003]

The occurrence of such stress corrosion cracking can be prevented by surface coating or cathodic protection. However, there are problems with the surface coating, such as the fact that some structures cannot be completely coated, and that the coating peels off during use. On the other hand, cathodic protection has problems in that it cannot be applied depending on the use or place, and that a large facility cost is required.

[0004] The present invention has been made to solve the above-mentioned problems, and the stress corrosion cracking resistance is improved by adjusting the chemical composition without depending on the surface coating or cathodic protection, thereby improving the material properties. Another object of the present invention is to provide a high-Mn nonmagnetic steel having excellent mechanical properties.

[0005]

[Means for solving the problems] C: 0.40 to 0.65%, Mn: 1
3.0-20.0%, Si: 0.1-1.5%, Mo: 0.01-3.0%, Al:
0.005 to 1.0%, N: 0.001 to 0.10%, and C
Is a high Mn non-magnetic steel having a relationship of 25% ≦ 20 × C% + Mn% ≦ 29% with the balance being Fe and unavoidable impurities and having excellent stress corrosion cracking resistance and excellent mechanical properties.

Nb: 0.01-1.0%, V: 0.01-1.0%, Ti: 0.
2. The composition of claim 1, which contains at least one selected from 005 to 0.2%.
Mn nonmagnetic steel.

[0007] Ca: 0.0005-0.020%, S: 0.03-0.15%, Z
The high Mn non-magnetic steel according to claim 1 or 2, which contains at least one element selected from the group consisting of 0.005 to 0.5% of r: stress corrosion cracking resistance and mechanical properties.

[0008]

[Function] 1% C-13%, which is a typical high Mn non-magnetic steel
Mn-based steel and 0.4% C-18% Mn-5% Cr-based steel, etc., generate stress corrosion cracking due to low tensile stress even in extremely weak corrosive environments wetted by industrial water or condensation. It is known that there are two types of stress corrosion cracking of steel, namely, intergranular cracking and intragranular cracking. Therefore. In order to prevent the occurrence of stress corrosion cracking, it is necessary to take measures against both cracks.

It is considered that grain boundary cracks occur when a Cr deficient layer is formed around Cr carbides precipitated at crystal grain boundaries, and this portion is selectively corroded. On the other hand, intragranular cracking is caused by the austenite forming elements C, Mn and
If a large amount of Ni is contained, it is likely to occur. Therefore, as a result of repeated studies, as shown in FIG. 1, the present inventor did not add Cr and Ni, C: 0.40 to 0.65%, Mn: 1
In the range of 3.0 to 20.0%, the chemical components are placed in the vicinity of the boundary line in the austenite (γ) single phase region in the Fe-C-Mn alloy phase diagram, that is, in the region of 25% ≦ 20 × C% + Mn% ≦ 29%. It has been found that the adjustment can prevent generation of intergranular cracks and intragranular cracks while maintaining a stable austenite structure.

That is, FIG. 1 shows the relationship between the C and Mn contents and stress corrosion cracking in a C-Mn series steel. In the figure, white marks indicate those in which stress corrosion cracking did not occur, and black marks indicate that. This indicates that stress corrosion cracking has occurred. As shown in FIG. 1, the steel in the hatched region in the figure maintains a stable austenite structure and has not generated stress corrosion cracking. In addition, when the content of C or Mn is out of this range, although it has a stable austenite structure, it can be seen that intragranular cracking easily occurs. The stress corrosion cracking test was 1.5 mm thick x 15 mm wide.
× A test piece having a length of 65 mm was loaded with a load of 245 N / mm ² by a four-point support bending method, and the test was carried out under three kinds of corrosion conditions of immersion in seawater and tap water and exposure to the atmosphere outdoors. The corrosion period is 90 days.

[0011] However, although the steel type defined by the above chemical composition range has excellent stress corrosion cracking resistance, it does not contain Ni, which is an austenite forming element, and stabilizes austenite by adding an appropriate amount to improve the proof stress. Cr
, The stability of the magnetic permeability to 0.2% proof stress and cold working is insufficient. Further, there is a problem that when the temperature is maintained at a constant temperature of about 600 ° C. for a long time as in the stress relief annealing treatment, the toughness is remarkably deteriorated and the magnetic permeability is increased.

As a solution to such a problem, the present inventor has found that as shown in FIG. 2, the addition of appropriate amounts of Mo and Al exerts a remarkable improvement effect. Figure 2 shows 0.6%
C-15% Mn-0.5% Mo-0.03% Al steel, 0.6% C-15% Mn-0.5%
This figure shows the change in magnetic permeability due to cold working of the as-hot-rolled Mo steel and 0.6% C-15% Mn steel. As shown in the figure, the addition of an appropriate amount of Mo improves the stability of the magnetic permeability for cold working,
It can be seen that the addition of does not adversely affect the stability of the magnetic permeability.

That is, the addition of an appropriate amount of Mo increases the 0.2% proof stress and stabilizes austenite, thereby improving the stability of magnetic permeability to cold working. In addition, the generation of pearlite-like precipitates during long-time constant temperature maintenance is significantly suppressed, so that deterioration in toughness and increase in magnetic permeability are significantly reduced. However, the toughness of the as-rolled material tends to deteriorate with an increase in strength, and becomes insufficient.

Therefore, when an appropriate amount of Al is added in addition to the addition of Mo, the toughness is improved by the action of cleaning the austenite ground, and the deterioration of the toughness caused by the increase in strength due to the addition of Mo is compensated. Similarly, in order to suppress the generation of pearlite-like precipitates during long-time constant temperature holding, it was revealed that deterioration in toughness and increase in magnetic permeability were further reduced. It was also found that the addition of an appropriate amount of these elements does not adversely affect stress corrosion cracking susceptibility.

[0015] Therefore, as shown in Examples Nos. 1 to 4 below, a steel obtained by adding an appropriate amount of Mo and Al to the component system determined by the above-described chemical composition range for preventing stress corrosion cracking has excellent resistance. while maintaining the stress corrosion cracking resistance, and a 0.2% yield strength 300N / mm ² or more, Charpy impact value at ₀ ℃ (vE 0) is
It has excellent mechanical properties of 180J or more. Furthermore, even if cold work of about 50% is performed, the magnetic permeability (μ)
1.02 or less, and the degree of increase in magnetic permeability is small. Also, 60
Even if stress relief annealing treatment is performed at 0 ° C for about 4 hours, vE
It was found that ₀ maintained a high toughness of 140 J or more and a low magnetic permeability of μ = 1.002.

Next, the reasons for limiting the component elements and the amounts added in the present invention will be described.

C is a strong austenite-forming element and has the effect of forming a solid solution in austenite to stabilize non-magnetism and increasing strength and toughness.
Requires addition of 0.40% or more. When the addition amount is less than 0.40%, the strength becomes insufficient and the austenite becomes unstable, so that the cold permeability increases the magnetic permeability. On the other hand, if added in excess of 0.65%, intragranular cracking is likely to occur, and when held at a constant temperature of around 600 ° C. for a long time, pearlite-like precipitates are likely to occur,
The toughness tends to deteriorate and the magnetic permeability tends to increase. Therefore, the C content is set to 0.40 to 0.65%.

Mn is a main austenite forming element together with C in the steel of the present invention, and has the effect of forming a solid solution in austenite ground to stabilize non-magnetism. Therefore, Mn needs to be added at 13.0% or more. Even if it is less than 13.0%, it has an austenitic structure, but when cold worked, austenite becomes unstable and the magnetic permeability increases. On the other hand, 20.0
%, It tends to generate intragranular stress corrosion cracking, and the hot workability tends to deteriorate. Therefore, Mn
The content is 13.0 to 20.0%.

A C-Mn steel not containing Ni and Cr and comprising 0.40 to 0.65% of C and 13.0 to 20.0% of Mn is composed of C and Mn.
The amount is near the boundary line in the austenite single phase region in the Fe-C-Mn alloy phase diagram, that is, 25% ≦ 20 × C% + Mn% ≦
By adjusting the chemical composition to a region of 29%, it is possible to maintain a stable austenite structure and prevent stress corrosion cracking. That is, 20 x C% + Mn% is 25%
If it is less than 2%, a two-phase structure of austenite (γ) and epsilon (ε) is obtained, so that a stable austenite structure cannot be ensured. On the other hand, if 20 × C% + Mn% exceeds 29%, a stable austenite structure , Stress corrosion cracking is likely to occur. Therefore, the relationship between C and Mn is 2
Regulate at 5% ≦ 20 × C% + Mn% ≦ 29%.

Si is an element that has a deoxidizing action during steel melting and is effective in improving strength. For that purpose, addition of 0.1% or more is required. On the other hand, if it exceeds 1.5%, the susceptibility to stress corrosion cracking increases and the hot workability tends to deteriorate. Therefore, the Si content is set to 0.1 to 1.5%.

Mo forms a solid solution in the austenite ground to increase the strength, particularly the 0.2% proof stress, and stabilizes the austenite by adding an appropriate amount, so that the stability of the magnetic permeability to cold working is improved. Furthermore, when kept at a constant temperature of about 600 ° C. for a long period of time, the generation of pearlite-like precipitates is suppressed, which has the effect of reducing the deterioration of toughness and the increase in magnetic permeability. Further, it is added more positively because it does not adversely affect the susceptibility to stress corrosion cracking. Addition amount is 0.01%
If it is less than the above, the above effects cannot be sufficiently exhibited. Meanwhile, 3.0
%, The effect at the time of maintaining the temperature is not remarkable, and the cost is disadvantageous. Therefore, Mo
The content is 0.01 to 3.0%.

Al has a deoxidizing effect at the time of steel melting and improves toughness by cleaning austenite ground.
Compensates for the deterioration of toughness caused by the increase in strength due to the addition of Mo. Further, when it is kept at a constant temperature of about 600 ° C. for a long time, it has the effect of suppressing the generation of pearlite-like precipitates and reducing the deterioration of toughness and the increase in magnetic permeability. Also,
It is added more positively because it does not adversely affect the susceptibility to stress corrosion cracking. If the added amount is less than 0.005%, the above effects cannot be sufficiently exhibited. On the other hand, if it exceeds 1.0%, hot workability tends to deteriorate. Therefore, Al
The content is 0.005 to 1.0%.

N does not adversely affect stress corrosion cracking susceptibility. This element is an austenite-forming element, and forms a solid solution in austenite ground to increase the strength. For that purpose, it is necessary to add 0.001% or more. On the other hand, if added in excess of 0.1%, the solid solubility limit of nitrogen in molten steel is exceeded, and the soundness of the steel ingot is impaired. Therefore, the N content should be 0.001 to 0.1%.

In the present invention, by adding the following chemical components in addition to the above basic chemical components, the strength or machinability can be significantly improved without impairing the above-mentioned properties.

Nb and V do not adversely affect stress corrosion cracking in the content range of the present invention. These elements strengthen the austenitic ground and add Nb-based and
Precipitation of V-based carbides significantly increases strength, especially 0.2% proof stress. Therefore, 0.01% or more is added. On the other hand, when added excessively, when the temperature is kept constant at a temperature around 600 ° C. for a long time, precipitation of carbides becomes remarkable, and the toughness tends to deteriorate. Therefore, the upper limit is set to 1.0%. Therefore, the contents of Nb and V are set to 0.01 to 1.0%.

Since Ti has a coarsening suppression effect up to a high temperature range, it is effective in improving toughness. For that, 0.005%
The above addition is required. On the other hand, if added excessively, Ti
Since the amount of nonmetallic inclusions in the oxide increases and the toughness is impaired, the upper limit is made 0.2%. Therefore, the Ti content is 0.005
To 0.2%.

[0027] Ca does not adversely affect stress corrosion cracking susceptibility. This element acts as a deoxidizer and desulfurizer. In addition, the formation of Ca-containing nonmetallic inclusions reduces the anisotropy of the mechanical properties and significantly improves the machinability, especially the surface machinability. Therefore, 0.0005% or more is added. On the other hand, if added excessively, the cleanliness of the steel is impaired and the mechanical properties are deteriorated, so the upper limit is made 0.020%. Therefore,
The Ca content is 0.0005 to 0.020%.

S does not adversely affect stress corrosion cracking susceptibility. This element is effective in improving machinability, especially drilling, by forming a compound with Mn. For that purpose, 0.03% or more is added. On the other hand, if added excessively, hot workability and toughness are degraded, so the upper limit is made 0.15%. Therefore, the S content is set to 0.03 to 0.15%.

Zr does not adversely affect stress corrosion cracking susceptibility. This element is effective for improving the machinability such as drilling ability and surface machinability by forming a compound with S. Therefore, 0.005% or more is added. On the other hand, if added excessively, the toughness is degraded, so the upper limit is made 0.5%. Therefore, the Zr content is set to 0.005 to 0.5%.

[0030]

Embodiments of the present invention will be described below.
High-Mn steel having the chemical components shown in Table 1 was melted in a high-frequency furnace and ingot into 90 kg ingots. The ingot was forged and then hot-rolled to a steel plate having a thickness of 20 mm. The test material was of two types: as-rolled material (history A) and a material subjected to stress-relieving annealing at 600 ° C. for 4 hours in furnace cooling (history B). For each test material, a stress corrosion cracking test and a mechanical test were performed, and the magnetic permeability was measured.

The stress corrosion cracking test was carried out by applying a load of 245 N / mm ² to a test piece having a thickness of 1.5 mm, a width of 15 mm, and a length of 65 mm collected from each test material by a four-point support bending method, using seawater and tap water. The test was performed under three types of corrosion conditions: water immersion and outdoor exposure to the atmosphere. The corrosion period is 90 days. Table 2 shows the results of the stress corrosion cracking test.
Table 3 shows the mechanical test results and the magnetic permeability.

As is apparent from Table 2, stress corrosion cracking did not occur in any of the examples of the present invention under any of the corrosion conditions. On the other hand, No. 13 of the comparative example was Cr, and No. 14 of the comparative example was Ni.
, The grain boundary cracks and intragranular cracks easily occur, respectively, and stress corrosion cracking occurs in seawater immersion.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

Further, in Comparative Example No. 16, Cr was replaced by N in Comparative Example.
Since it contains more than o.13, stress corrosion cracking (grain boundary cracking) occurs under any of the corrosion conditions.
Also, No. 17 of Comparative Example contains C beyond the limited range,
Since 25% ≦ 20 × C% + Mn% ≦ 29% is not satisfied, stress corrosion cracking (intragranular cracking) occurs under any of the corrosion conditions.

As is clear from Table 3, the examples of the present invention have excellent mechanical properties and magnetic permeability both in the as-rolled material (history A) and in the stress-relieved annealing material (history B). In particular, No. 5 of the present invention has high toughness (vE ₀ ) because it contains Ti,
o. 6, 7, 9, and 10 are high because they contain Nb or V.
Has 2% proof stress. Also, although not shown in the table,
Nos. 8, 9, and 10 of the present invention contained any of Ca, S, and Zr, and thus had excellent machinability.

On the other hand, No. 11 of the comparative example does not contain Mo and Al, so that the history A has a low 0.2% proof stress and the history B has a low yield strength.
The 0.2% proof stress and vE ₀ are low, and the magnetic permeability is increased. Since No. 12 of the comparative example does not contain Al, the history B shows that vE ₀
Is low. In Comparative Example No. 13, which does not contain Mo and Al, the history B has a low vE ₀ and an increased magnetic permeability. No. 14 of the comparative example does not contain Mo and Al, so that the history B has a low vE ₀ . No. 15 of the comparative example has a low C content,
Since it does not contain Al, the histories A and B have a low 0.2% proof stress, and the histories B have a low vE ₀ and an increased magnetic permeability.
No. 17 of the comparative example has a high C content and does not contain Mo and Al. Therefore, in the history B, vE ₀ is low and the magnetic permeability is increased.

As described above, according to the present invention, both the as-rolled material and the stress-relieved annealing material have excellent stress corrosion cracking resistance,
Although it has mechanical properties and magnetic permeability, none of the comparative examples simultaneously satisfies all the properties of stress corrosion cracking resistance, mechanical properties and magnetic permeability.

[0040]

As is apparent from the above description, since the steel of the present invention has excellent stress corrosion cracking resistance even without surface coating or cathodic protection, even if used in a corrosive environment, the steel of the present invention has a high stress corrosion cracking resistance. No corrosion cracking occurs. Even when cold working and stress relieving annealing are performed, the mechanical properties are excellent, so the 0.2% proof stress is high, the decrease in toughness is small, and the increase in magnetic permeability is slight.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between C and Mn contents and stress corrosion cracking in a C-Mn steel.

[Figure 2] 0.6% C-15% Mn-0.5% Mo-0.03% Al steel, 0.6% C
It is a figure which shows the change of the magnetic permeability by cold working of the hot rolled material of -15% Mn-0.5% Mo steel, 0.6% C-15% Mn steel.

Claims (3)

(57) [Claims] [Claim 1] C: 0.40 to 0.65%, Mn: 13.0 to 20.0%, S
i: 0.1-1.5%, Mo: 0.01-3.0%, Al: 0.005-1.0
%, N: 0.001 to 0.10%, and the relationship between C and Mn
A high Mn non-magnetic steel excellent in stress corrosion cracking resistance and mechanical properties, wherein 25% ≦ 20 × C% + Mn% ≦ 29%, the balance being Fe and unavoidable impurities. 2. Nb: 0.01 to 1.0%, V: 0.01 to 1.0%, T
2. The high-Mn nonmagnetic steel according to claim 1, which comprises one or more selected from i: 0.005 to 0.2%. 3. Ca: 0.0005-0.020%, S: 0.03-0.15
3. The high-Mn non-magnetic steel according to claim 1 or 2, further comprising at least one selected from the group consisting of 0.005 to 0.5% and Zr: 0.005 to 0.5%.