JPH09302446A - Duplex stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an economical duplex stainless steel suitable for use in a marine structure, excellent in seawater corrosion resistance, and having high strength. SOLUTION: This steel has a composition containing, as alloying elements, <=0.06%, by mass, C, 1.50-3.50% Si, 0.1-3.0% Mn, 2.0-8.0% Ni, 18.0-28.0% Cr, 0.03-0.20% N, and 0.1-0.8% Mo and having the balance Fe with inevitable impurities. Moreover, the area ratio of austenitic phase is regulated to 20-80% in this steel. This steel can further contain, by mass, 0.1-1.5% Cu, 0.1-0.8% W, 0.01-2.0% Nb, 0.01-2.0% Ti, and 0.01-2.0% V.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an economical duplex stainless steel having excellent seawater corrosion resistance and high strength, which is suitable for marine structures.

[0002]

2. Description of the Related Art Conventionally, SUS 42 has been used as a material for applications requiring corrosion resistance such as shaped steel used for sluice gates and stern shafts for ship propulsion, particularly corrosion resistance and strength against seawater.
Martensitic stainless steel represented by 0J2 and S
Although precipitation hardening stainless steels such as US630 have been used, they have sufficiently high strength but are not sufficient in terms of corrosion resistance to seawater. Nitrogen-containing austenitic stainless steels have been developed as stainless steels with excellent corrosion resistance and high strength, but these contain a large amount of expensive Ni and complicated control is required to achieve high strength. Since rolling is required, there is a problem of high cost such as poor productivity.

Duplex stainless steel represented by SUS329J2L, which is composed of two phases of an austenite phase and a ferrite phase, is a steel excellent in corrosion resistance. However, the conventional duplex stainless steel is expensive because it contains a large amount of Mo, and in terms of strength, a shaped steel used for offshore structures such as floodgates, a stern shaft for ship propulsion, which is the object of the present invention. It is insufficient for yield strength: 600 MPa or more and tensile strength: 750 MPa or more, which are required by applications such as.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and an object thereof is to provide corrosion resistance to seawater, yield strength: 600 MPa or more, tensile strength: 750 MPa or more. It is to provide a duplex stainless steel having strength.

[0005]

In order to achieve the above object, the duplex stainless steel of the present invention comprises: (1) the content of alloying elements in mass% is C: 0.06
% Or less, Si: 1.50 to 3.50%, Mn: 0.1
3.0%, Ni: 2.0 to 8.0%, Cr: 18.0
28.0%, N: 0.03 to 0.20%, Mo: 0.
1 to 0.8%, consisting of residual Fe and unavoidable impurities, and having an austenite phase area ratio of 20 to 80.
%. (2) In addition to the above alloy elements, in mass%, Cu: 0.1
.About.1.5% and W: 0.1 to 0.8%, any one kind or two kinds is contained. (3) In addition to the alloy element of (1) or (2) above, in mass%, Nb: 0.01 to 2.0%, Ti: 0.01 to
2.0%, V: 0.01 to 2.0%, any one kind or two or more kinds is contained.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the alloy composition in the duplex stainless steel of the present invention will be described below. C: 0.06% or less C is an element that enhances the strength of steel and stabilizes the austenite phase, and is effective in saving expensive Ni. However, the content of Cr carbide is set to 0.06% or less because it forms Cr carbide and precipitates at the grain boundary to lower the corrosion resistance. Si: 1.50 to 3.50% Si is also effective as a deoxidizing agent for steel, is a ferrite forming element, and controls the amount of the ferrite phase in the duplex stainless steel. It is an essential element for strengthening. Therefore, the lower limit of the content rate is 1.5
0%. However, if the content is excessive, the precipitation of intermetallic compounds is accelerated, and the steel is embrittled and the manufacturability thereof is significantly impaired. Therefore, the upper limit of the Si content is set to 3.50%. Mn: 0.1 to 3.0% Mn acts as a deoxidizer for steel, increases the amount of N dissolved in steel, and is effective as an alternative element to expensive Ni.
Include 1% or more. However, if the Mn content is too high, the manufacturability of the steel is deteriorated, so the upper limit of the content is set to 3.
0%. Ni: 2.0 to 8.0% Ni is a strong austenite forming element and is an important element for controlling the amount of austenite phase in duplex stainless steel. In addition, it is an essential element for ensuring corrosion resistance, and at least 2.0% for that purpose.
It is necessary to contain the above. However, since it is an expensive element, the upper limit of the content rate is set to 8.0% in consideration of economy. Cr: 18.0 to 28.0% Cr is an essential element for ensuring corrosion resistance,
Further, it is a ferrite forming element and is an element necessary for controlling the amount of ferrite phase in the duplex stainless steel. For that purpose, it is necessary to contain at least 18.0% or more. However, if the content is too high, the effect is saturated and the productivity is rather lowered. Therefore, the upper limit of the content is set to 28.0%. N: 0.03 to 0.20% N is an element that enhances the strength of steel and stabilizes the austenite phase similarly to C, and is effective in saving expensive Ni. Therefore, 0.03% or more is contained.
However, if excessively added, blowholes are generated during steel melting and the quality of the ingot is deteriorated, so the upper limit of the N content is set to 0.20%. Mo: 0.1 to 0.8% Mo is added to improve corrosion resistance, particularly in a reducing environment. For that, at least 0.1
% Must be contained. However, the cost of steel only increases even if it is contained in a large amount. Therefore, the upper limit of the Mo content is 0.8%. Cu: 0.1 to 1.5%, W: 0.1 to 0.8% Cu and W are both elements that improve the corrosion resistance, and in order to exert their effects, the content of each of them is 0.
1% or more is required. However, if it is contained excessively, the manufacturability of the steel is deteriorated, so Cu is 1.5%, and W is 0.8% as the upper limits of the content rates, and either one or two types are selected. Shall be included. Nb: 0.01 to 2.0%, Ti: 0.01 to 2.0
%, V: 0.01 to 2.0% Nb, Ti, and V all form carbonitrides to increase the strength of steel. For that purpose, the content rate of each is 0.01%
The above is necessary. However, if it is contained excessively, the cleanliness of the steel will be deteriorated and the corrosion resistance will be impaired. Therefore, any one kind or two or more kinds shall be contained with the upper limit of the content ratio being 2.0%.

The duplex stainless steel of the present invention satisfies the above alloy composition range and, at the same time, its metallic structure is
It shall consist of two phases, an austenite phase and a ferrite phase. When the austenite phase area ratio is less than 20%, the corrosion resistance is insufficient, the mechanical strength is low, and the high strength required for the application of the present invention cannot be achieved. When the austenite phase area ratio exceeds 80%, the corrosion resistance is excellent, but the mechanical strength is insufficient. Therefore, the duplex stainless steel of the present invention contains an austenite phase area ratio of 20 to 80% and is substantially composed of two phases of an austenite phase and a ferrite phase.

[0008]

Embodiments of the present invention will be described below. Using a high frequency induction furnace, the steel having the chemical composition shown in Table 1 was melted into a 50 kg steel ingot. This was made into a round bar having a diameter of 20 mm by hot forging, and subjected to solution heat treatment at 1050 ° C. water cooling to obtain a test material. In addition, Comparative Example 4 having a high N content
Because a blowhole was generated in the steel ingot and a sound steel ingot was not obtained, and Comparative Example 3 having a high Si content caused cracks during hot forging, and therefore, subsequent tests were performed on these. There wasn't.

The metal structure was revealed on the cross section of the test material, and the area ratio of the austenite phase was measured using an image processing device. Table 1 shows the results.

[0010]

[Table 1]

From the test material, a JI having a parallel part diameter of 10 mm
A No. S4 tensile test piece was cut out and a tensile test was performed at room temperature to examine the tensile properties. The diameter of the parallel part is 8 mm and the length is 40 m.
A torsion test piece of m was cut out, and a static torsion test was performed at room temperature to examine the torsion strength. Furthermore, regarding the test piece whose surface was polished with # 120 emery paper, JIS Z2371
The salt spray test was carried out for 196 hours according to the above method to examine the corrosion resistance. As for the corrosion resistance, as a result of the salt water spray test, the one that did not rust at all was evaluated as A, the one that caused spotted rust having an area ratio of 10% or less was evaluated as B, and the one that rusted when the area ratio exceeded 10% was evaluated as C. .

The results of these tests are shown in Table 2.

[0013]

[Table 2]

As can be seen from the results in Table 2, Comparative Example 1 in which the area ratio of the austenite phase is large beyond the range of the present invention.
(Equivalent to SUS304), Comparative Example 5 (equivalent to SUS316)
Has excellent corrosion resistance but low mechanical strength such as proof stress, tensile strength and torsional strength. Comparative Example 6 having a low Ni and N content and an area ratio of the austenite phase smaller than the range of the present invention has low mechanical strength and poor corrosion resistance. Even if the area ratio of the austenite phase is within the range of the present invention, Comparative Example 2 and Comparative Example 7 (SUS329JL) having a low Si content rate.
2) is not sufficiently high in mechanical strength. In Examples 1 to 12 satisfying the chemical composition of the present invention and the area ratio of the austenite phase, the yield strength targeted by the present invention is 600 MPa.
As described above, the tensile strength is 750 MPa or more, the mechanical strength and the excellent corrosion resistance to seawater are provided, and the torsional strength is also excellent.

[0015]

As described in detail above, according to the present invention,
It is possible to provide an economical duplex stainless steel having excellent seawater corrosion resistance and high strength, which is suitable for offshore structures.

Claims (3)

[Claims] 1. The content of alloying elements in mass% is C: 0.06% or less, Si: 1.50 to 3.50%, Mn: 0.1 to 3.0%, Ni: 2. 0 to 8.0%, Cr: 18.0 to 28.0%, N: 0.03 to 0.20%, Mo: 0.1 to 0.8%, consisting of residual Fe and inevitable impurities. And, the austenite phase area ratio is 20 to 80%, a duplex stainless steel. 2. In addition to the alloying elements, at least one of Cu: 0.1 to 1.5% and W: 0.1 to 0.8% by mass% is contained. The duplex stainless steel according to claim 1, characterized in that. 3. In addition to the alloy elements, in mass%, Nb: 0.01 to 2.0%, Ti: 0.01 to 2.0%, and V: 0.01 to 2.0%. , Any one type, or two or more types are contained, The duplex stainless steel of any one of Claim 1 and Claim 2 characterized by the above-mentioned.