JPS6057500B2 - Mo-saving austenitic stainless steel with excellent seawater resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a Mo-saving material with excellent seawater resistance, mechanical properties, and hot workability for use in various marine-related equipment such as marine propeller shafts, seawater pump shafts, and seawater valve stems. It relates to austenitic stainless steel.

Conventionally, SUS316 containing about 2% Mo has been generally used as the material for the various marine-related equipment mentioned above.

However, in recent years, as equipment has become significantly larger and faster, performance has required not only seawater resistance but also high strength, and SUS316 has had the problem of insufficient strength.

Furthermore, since the price of Mo has skyrocketed in recent years, SUS316 containing 2% or more of Mo has become an extremely expensive seawater-resistant stainless steel. There was a strong demand for the development of seawater-resistant stainless steel.The seawater-resistant austenitic stainless steel developed to meet these demands was 1.
9%Cr-10%Ni-1%Mo-1.5%Si-O,
There is 1%N steel.

However, although this steel also has a reduced Mo content of 1%, it is still expensive, and although its tensile strength has improved considerably from 66 to 91-, it is still lacking. are doing. The present invention overcomes the drawbacks of conventional steels, and the inventors have discovered that Cr)Mo)N,
As a result of investigating the influence of each element of Ni, Si, and Liao, Cr is 1
Seawater resistance is significantly improved in the range of 6 to 25%, and when the amount of Cr is in the above range, Mo is 2.5 times that of Cr, and N
It was found that i had the effect of increasing the seawater resistance by 0.4 times, N had the effect of increasing the seawater resistance by 2 times, and Si, Mn had almost no effect on the seawater resistance.

Then, by integrating the effects of each alloying element mentioned above on seawater resistance, the seawater resistance of austenitic stainless steel is:
It has been found that evaluation can be made using Cr equivalent (■Cr+O, 4Ni+20N). FIG. 1 shows the calculation of the Cr equivalent of conventional steel and evaluation of its seawater resistance. As is known from Figure 1, in order to obtain sufficient seawater resistance, it is sufficient to adjust the alloy content so that the Cr equivalent is 28 or more. It can be seen that in order to obtain a MO seawater resistant stainless steel, it is sufficient to increase the Cr equivalent to 28 or more, which is comparable to SUS3l6, by setting high Cr and high N.

However, if we simply use high Cr and high N steel, high Cr impairs the austenite-ferrite balance at high temperatures, and high N increases deformation resistance at high temperatures, resulting in hot workability. This may cause problems such as a decrease in Moreover, high N tends to cause bubbles to form in the steel ingot during solidification, so it is necessary to increase the amount of solid solution by increasing the Cr content.
Increasing the amount of delta ferrite precipitation will conversely increase the risk. As described above, the balance between high Cr and high N content is a very delicate and difficult problem. The inventors have discovered that high C
As a result of considering these problems associated with increasing r-N,
The amount of Cr is 20.5-22.5%, and the amount of N is 0.18-0.
As a combination of 30% and Cr equivalent of 2? By making it the above, it has seawater resistance equivalent to S33l6, has excellent castability and hot workability, and has a tensile strength of 7.
M-saving with excellent mechanical properties of about 0k91w11t
We have successfully developed an austenitic stainless steel that is resistant to seawater.

The present invention will be explained in detail below.

The first invention steel has a weight ratio of CO. O8% or less, SlO
．． 6O% or less, Mn 3.5% or less, Ni & 2 to 12.5
%, Cr2O. 5-22.5%, NO. l8%~0.3
0% and has a Cr equivalent of 28 or more, according to the second invention. The steel is the first invention steel plus CUO. 2-1
．． 0% and CaO. The hot workability of the first invention steel is further improved by containing 02% or less.

The reasons for limiting the composition of the steel of the present invention will be explained below.

Since C impairs the general corrosion resistance of stainless steel, including seawater resistance, its upper limit was set at 0.08%.

In addition, in order to obtain sufficient seawater resistance, it is desirable that the content be 0.04% or less. Cr is a basic element that imparts seawater resistance to the steel of the present invention, and is also an element that increases the amount of solid solution of N to the required range.5 As shown in Figure 1, SUS3
In order to obtain seawater resistance comparable to 16, it is necessary to have a Cr equivalent of 28 or more, and the lower limit of the Cr content was set at 20.5%.

However, excessive Crjl content impairs the austenite-ferrite balance and reduces hot workability, so the upper limit was set at 22.5%. Ni is a basic element of austenitic stainless steel, and is an element that improves seawater resistance, cold workability, and hot workability. In order to obtain an austenitic structure with the composition balance of the steel of the present invention, it is necessary to contain at least 8.2% N1, and the lower limit is 8.2% or more.
．． It was set at 2%. However, excessive Ni content not only increases the price of steel but also reduces hot workability, so the upper limit was set at 12.5%. In addition, in the steel of the present invention,
It is desirable to keep the Ni amount as low as possible, and ≦9.8
% or less. N is a main element in the steel of the present invention, and is an essential element in order to obtain excellent seawater resistance, strength, and austenite phase.

In order to fully exhibit these properties, the content must be 0.18% or more. However, the inclusion of a large amount of N significantly increases the deformation resistance during hot working, making hot rolling difficult, and also increases the risk of generating bubbles in the steel ingot during ingot making, so the upper limit has been set to 0.30. %.

Although Si is an element necessary for deoxidation during steel manufacturing, the inclusion of more than necessary is harmful to the steel of the present invention.

That is, Si is a ferrite-forming element twice as strong as Cr and significantly impairs the austenite-ferrite balance, and Si lowers the amount of solid solution of N, so the upper limit was set at 0.60%. In addition, in manufacturing, it is desirable to limit the amount of Si to 0.30 to 0.50% and increase the amount of CrlN as much as possible. Mn is deoxidized during steelmaking,
It is an element necessary for desulfurization and also contributes to increasing the amount of N solid solution without impairing seawater resistance.

However, since adding more than necessary impairs hot workability, the upper limit was set at 3.5%. Although Cu has no effect on seawater resistance, it is an element that improves general corrosion resistance such as sulfuric acid resistance, and can be contained in an amount of 0.20% or more depending on the application.

However, if the content exceeds 1.0%, hot workability will be impaired, so the upper limit was set at 1.0%. Ca is an element that improves the hot workability of the steel of the present invention.

However, if it is contained in large quantities, it will impair the cleanliness of the steel.
On the contrary, the hot workability is deteriorated, so the upper limit of CaO. O2%. Next, the characteristics of the steel of the present invention will be clarified through examples in comparison with conventional steel. Table 1 is
The chemical composition of these test steels is shown.

In Table 1, the Al-A gear is conventional steel, and A1 is SUS3.
Otun A2 is SUS31 A3 is 19%Cr-10%Ni
, -0.10%N-1%MO-1.5%Si steel, A4 is SUS3O4N, . A5 is SUS31U tone.

B1 to B4W4 are the first invention steels, and Belil4 is the second invention steel. Table 2 shows the strength, hot workability, and seawater resistance of A1 to A, B1 to B, and B, which were subjected to the solid solution heat treatment in Table 1. Regarding strength, JI
Proof tensile strength and elongation were measured using a No. S4 test piece.

Regarding hot workability, a high-temperature torsion test was conducted by heating to 1200°C, and the torsion value was also evaluated.

For seawater resistance, 50y11IFeC13 at 50℃
This figure shows the corrosion loss when immersed in a 11/20 N-HCl aqueous solution for 48 hours. As is known from Table 2, A1 steel, which is a conventional steel, has excellent hot workability, but is inferior in strength, hot workability, and seawater resistance.

With increasing Ni content and 2.2% M for A1 steel
A′2H containing O! Regarding No. 4, there is a considerable improvement in seawater resistance and good hot workability, but the strength is inferior, and A3lA is S
By containing 1.5% of i, further increasing the amount of N to 0% and containing 1% of MO, it has excellent seawater resistance and hot workability, but it becomes an expensive steel. Although there has been a considerable improvement in strength, it is still lacking. The A4 steel contains 0.15% N compared to the Al steel, so there is some improvement in seawater resistance and strength, but it is still lacking in these areas. Aya greatly increased the amount of Ni and also increased the MO by 2.
By containing 8%, it has excellent seawater resistance, but it becomes an expensive seawater-resistant steel, and its strength is inferior to that of A1 steel.

In contrast to these, Bl-B4 and B6l, which are the steels of the present invention,
liJ is Cr, . Ni. N is appropriately contained and the Cr equivalent is 28J). As for seawater resistance, the corrosion loss is 1.7 to 11.0 f1
r, A2, A3 containing large amounts of Ni and MO,
It has excellent seawater resistance equivalent to A5 steel, and has a yield strength of 34k91Td or more and a tensile strength of 67k9.
Excellent with 1Tn1t or more and elongation of 52% or more, and
The hot workability is also excellent, with a twist value of 1 (2) or more. This shows that the steel of the present invention is excellent not only in seawater resistance but also in strength and hot workability.

As mentioned above, the steel of the present invention minimizes the use of expensive Ni in order to obtain excellent seawater resistance at low cost.
By increasing the Cr equivalent and increasing its value to 2? Top and
In addition, the strength is improved by N, and the CrsN content is increased by low Si, and hot workability is improved, making it an austenitic stainless steel with excellent seawater resistance, strength, and hot workability. We succeeded in obtaining steel, and it has high practicality as a member for various marine-related equipment such as propeller shafts for ships, pump shafts for seawater, and valve stems for seawater.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the influence of Cr equivalent on seawater resistance.

Claims (1)

[Claims] 1 Weight ratio: C 0.08% or less, Si 0.60% or less Mn 3.5% or less, Ni 8.2 to 12.5%, Cr2
Contains 0.5-22.5%, N0.18-0.30%, the balance consists of Fe and impurity elements, and has a Cr equivalent of 28
A Mo-saving austenitic stainless steel with excellent seawater resistance characterized by the above properties. 2 Weight ratio: C 0.08% or less, Si 0.60% or less, Mn 13.5% or less, Ni 8.2-12.5%, C
r20.5-22.5%, N0.18-0.30%,
Mo-saving austenitic stainless steel with excellent seawater resistance, containing 0.2 to 1.0% Cu and 0.02% or less Ca, with the balance consisting of Fe and impurity elements, and having a Cr equivalent of 28 or more. steel.