JPS6057501B2 - 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, equipment has become larger and faster, and SUS31
6 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 stainless steel. The seawater resistant austenitic stainless steel developed to meet this requirement is 19% Cr.
-10%Ni-1%Mo-1.5%Si-O, 1%N steel and 19%Cr-9%Ni-1.2%Si-O, 15%
Stainless steel cast steel, etc., in which appropriate amounts of Ca, Nb, and Ti are added to N, have been developed.

Although the former type of steel has excellent seawater resistance,
It is expensive because it contains as much as 1% Mo, and although its tensile strength has been considerably improved to 66 to 9/1, it is still lacking. In addition, the latter steel regulates the upper limit of Cr and adds N to suppress the sigma phase and at the same time improves weldability by making an appropriate amount of delta ferrite exist.
Furthermore, this stainless steel cast steel is characterized by adding Ti and Nb to suppress the formation of Cr carbides, and adding Ca to improve machinability, and it has excellent seawater resistance comparable to SUS3l6. In addition, hot rolling is difficult due to the high Si content to improve hot metal flowability and the presence of delta ferrite. As described above, an inexpensive seawater-resistant steel that has seawater resistance equivalent to SUS3l6 and excellent strength has not yet been developed. The present inventors previously investigated the influence of various alloying elements on seawater resistance, and found that the seawater resistance of austenitic stainless steel is Cr equivalent = Cr + 2.5 M0 +
It was discovered that it could be evaluated with 0.4Ni+20N, and MO
21%Cr-9%Ni-0. was replaced with high Cr and high N.
Succeeded in developing 2%N steel (Japanese Patent Application No. 145983/1983). The above steel is an inexpensive seawater resistant steel with seawater resistance comparable to SUS3l6 and meets basic requirements, but Ni
It has a relatively high content of 9%, making it a slightly more expensive steel than SUS3O4. Therefore, the present inventors considered substituting Mn for Ni with the aim of obtaining a seawater-resistant stainless steel that maintains excellent seawater resistance and strength and is as inexpensive as SUS3O. It has been found that Mn has an austenite phase forming ability 0.4 times that of Ni, and that an amount of Mn of 5% or less has almost no effect on seawater resistance. (1st
Furthermore, as a result of investigating the influence of impurity elements on seawater resistance, we found that 0, S, and C are elements that particularly impair seawater resistance;
is 100 times that of Cr, and S and C have an influence of 2 times as much, and the seawater resistance of austenitic stainless steel is determined by C biequivalent = C
It has been found that evaluation can be made using r equivalent -20(C-S)-1000.

Figure 2 shows the evaluation of seawater resistance by calculating the C biequivalent of conventional steel. From this result, in order to obtain stainless steel that is inexpensive and has excellent seawater resistance, it is necessary to reduce the contents of MO and Ni, which are expensive elements, as much as possible, and to increase the content of high Cr and high N.
It has been found that it is sufficient to increase the C biequivalent to 25 or more by reducing O, S, and C. In addition, the steel of the present invention has a low Ni-encounter
By creating a high-N steel, a sufficient N content is required to obtain an austenitic structure, but since N increases deformation resistance and makes hot rolling difficult, addition of 0.30% or more is not recommended. This is not preferable, and in order to obtain a C biequivalent of 25 or more, the Cr content must be relatively large, and the ferrite at high temperatures must be
Austenite balance tends to be impaired.

Therefore, 25 times as much Mn is contained instead of Ni, but the overall amount of alloy increases and hot workability decreases. It has improved characteristics. Based on the above knowledge, the present inventors have determined that N of 21%Cr-9%Ni-0.20%N steel
By replacing a part of the amount of i with an appropriate amount of Mn within a range of 5% or less, an austenitic structure is obtained, and at the same time, the accompanying decrease in seawater resistance and hot workability is reduced to low C, low O, and low S. By supplementing with
We have succeeded in developing a seawater-resistant austenitic stainless steel that is as inexpensive as SUS3O4 and has excellent mechanical properties of approximately K9. The present invention will be explained in detail below.

The first invention steel has a weight ratio of CO. O5% or less, SlO
．． 60% or less, Mrl 2-5%, Ni 6-8%, Crl
9-22.5%, NO. 18 to 0.30% and has a C biequivalent of 2S1). The second invention steel contains O of 50 PPM or less and SO. of the first invention steel. OIO%
The third invention steel is the first invention steel with CUO.
2-1.0%, CaO. The hot workability and corrosion resistance of the first invention steel are 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 0.05%.

Note that in order to obtain sufficient seawater resistance, the content is desirably 0.03% 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 within a necessary range. As shown in FIG. 2, in order to obtain seawater resistance comparable to that of SUS3l6, the C biequivalent must be 25 or more, and the lower limit of the Cr content was set at 19%. However, excessive Cr content impairs the austenite-ferrite balance and reduces hot workability, so the upper limit is set at 22.5%.
And so. 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 in the composition balance of the steel of the present invention, the amount of Ni must be at least It is necessary to contain 6% or more, and the lower limit is set at 6%.

However, excessive Ni content increases the price of steel, so the upper limit was set at 8%. N is the main element of the steel of the present invention,
It is an indispensable element for obtaining 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 further increases the risk of generating bubbles in the steel ingot during ingot making, so the upper limit is set to 0.
It was set at 30%.

Although Si is an element necessary for deoxidation during steel manufacturing, the inclusion of more than necessary S1 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, the amount of Si is limited to 0.30 to 0.50%, and the amount of Cr. ．．
It is desirable to increase the amount of N. Mn is an element necessary for deoxidation and desulfurization during steel manufacturing, and is an element that contributes to increasing the amount of N solid solution without impairing water resistance, and must be contained in an amount of at least 2% or more.

However, since adding more than necessary impairs hot workability, the upper limit was set at 5%. Although Cu has no effect on seawater resistance, it is an element that improves general corrosion resistance such as sulfuric acid resistance.
It 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.
Since it actually deteriorates hot workability, the upper limit was set at 0.02%. Next, the characteristics of the steel of the present invention will be clarified through examples in comparison with conventional steel. Table 1 shows the chemical composition of these test steels.

In Table 1, A1 to A are conventional steel, and A1 is SUS3.
O4, A2 is SUS3l, A3 is 19%Cr-10%N
i-0.10%N-1%MO-1.5%Si steel, A4 is SUS3O4NlA5 is SUS3l6S.

B1 to B5 and B7 steels are the invention steels, B1 to B4A are the first invention steels, B5 steel is the second invention steel, and B7 steel is the third invention steel. Table 2 shows A1 subjected to the solid solution heat treatment shown in Table 1.
The strength, hot workability, seawater resistance, and corrosion fatigue resistance of ~A5$AlBl~Bya and B7 steels are shown.

Regarding strength, proof tensile strength and elongation were measured using a JIS No. 4 test piece.

Regarding hot workability, a high temperature torsion test was conducted by heating to 12000C, and the torsion value was also evaluated.

For seawater resistance, 50g11-FeCl at 500C
3+ shows the corrosion weight loss when immersed in Kawano 20N-HCl aqueous solution for 48 hours. Corrosion fatigue resistance was evaluated by performing a rotating bending fatigue test using an artificial seawater drip method, and using a fatigue strength of 107.

As is known from Table 2, A1 steel, which is a conventional steel, has excellent hot workability, but is inferior in strength, seawater resistance, and corrosion fatigue resistance. A steel with increased Ni content and 2.2% MO content compared to A1 steel showed considerable improvement in seawater resistance and corrosion fatigue resistance, and also had good hot workability. However, it is inferior in strength, and A3J4 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, corrosion fatigue resistance, and hot workability. Although it is an expensive steel and shows considerable improvement in strength, it still lacks something. Although A4 steel contains 0.15% N compared to Al steel, it shows some improvement in seawater resistance and strength, but it is still lacking in these areas.

M steel has a large amount of Ni and a 2.8% increase in Ni content.
By including it, it is excellent in seawater resistance, corrosion resistance, fatigue resistance, and hot workability, but it becomes an expensive seawater-resistant steel, and its strength is inferior to that of A1 steel.

On the other hand, B1 to B5 and B7 steels, which are the steels of the present invention, contain Cr, Ni, N, and Mn appropriately, and have a Cr equivalent of 6 or more, thereby improving their corrosion resistance in seawater. Weight loss is 2.0-10.8g/Wt・7,
In addition, its corrosion fatigue resistance is 29k9/i, which is equivalent to that of A2, A3, and A5 steels containing large amounts of Ni and iron. Also, yield strength 34k9/i or more, tensile strength 67k
g/Tlrlt or higher, elongation is 55% or higher, and the hot workability is also excellent with a twist value of 1(2) or higher.

This shows that the steel of the present invention is excellent not only in seawater resistance but also in corrosion fatigue resistance 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.
and by increasing the C biequivalent to 25 or more by making it low 0, low C, and low S, and improving the strength with N,
Furthermore, by lowering the Si content, we can increase the Cr and N contents and improve hot workability, creating an austenitic stainless steel with excellent seawater resistance, corrosion fatigue resistance, strength, and hot workability. This is what I was able to successfully obtain.

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]

Figure 1 is a diagram showing the influence of Mn on seawater resistance.
FIG. 2 is a diagram showing the influence of C biequivalent on seawater resistance.

Claims (1)

[Claims] 1. C0.05% or less, Si 0.60% or less, Mn 2-5%, Ni 6-8%, Cr 19-22.5 in terms of weight ratio
%, N0.18 to 0.30%, the remainder being Fe and impurity elements, and having a Cr' equivalent of 25 or more. 2 Weight ratio: C 0.05% or less, Si 0.60% or less, Mn 2-5%, Ni 6-8%, Cr 19-22.5
%, N0.18 to 0.30%, O50PPM or less, S0010% or less, and the balance consists of Fe and impurity elements, and has a Cr' equivalent of 25 or more. Mo saving with excellent seawater resistance
Austenitic stainless steel. 3 Weight ratio: C0.05% or less, Si0.60% or less, Mn2-5%, Ni6-8%, Cr19-22.5
%, N0.18-0.30%, O50PPM or less, S
0.010% or less, and further Cu0.2 to 1.0%,
A Mo-saving austenitic stainless steel having excellent seawater resistance, containing 0.02% or less of Ca, the remainder consisting of Fe and impurity elements, and having a Cr' equivalent of 25 or more.