JPH07109548A - Alustenitic stainless steel excellent in hot workability - Google Patents

Abstract

PURPOSE:To obtain an austenitic stainless steel for seawatet resistance excellent in hot workability and free from deterioration in corrosion resistance and mechanical properties even in a thick or round product by specifying the chemical components in a steel and controlling its structure. CONSTITUTION:This steel has a compsn. contg., by weight, <=0.040% C, 40.80% Si, 1.00 to 5.00% Mn, <=0.040% P, <=0.030% S, <=1.00% Cu, 18.0 to 22.0% Ni, 20.0 to 28.0% Cr, 4.0 to 8.0% Mo, 0.15 to 0.40% N, 0.001 to 0.10% Mg, and the balance substantial Fe and has a structure in which the content of delta ferrite is regulated to <=10%. Thus, the austenitic stainless steel for seawater resistance excellent in hot workability can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic stainless steel for seawater having excellent hot workability.

[0002]

2. Description of the Related Art Stainless steel is used in many fields because of its excellent corrosion resistance. However, when used in the presence of chlorine ions such as seawater, for example, SUS316 and SUS316L have problems such as pitting corrosion and crevice corrosion.
Therefore, a material having more pitting corrosion resistance is required.

Therefore, Fe-20Cr-2 in which the contents of Mo and Cr are increased and N is added to improve the corrosion resistance.
Stainless steels such as 5Ni-7Mo-0.1N have been developed. However, by adding a large amount of alloying elements such as Mo, Cr, N, etc. and making them highly alloyed, there is a drawback that hot workability is impaired, and processing at high temperatures such as decomposition rolling and decomposition forging becomes difficult. .

[0004]

SUMMARY OF THE INVENTION The object of the present invention has been made in view of the above circumstances, and it has sufficient corrosion resistance even in a corrosive environment containing chloride ions, and has excellent hot workability. It is an object of the present invention to provide an austenitic stainless steel for seawater resistance that does not impair corrosion resistance and mechanical properties even when manufacturing thick or thick round products.

[0005]

The austenitic stainless steel for seawater having excellent hot workability according to the present invention has an alloying element content of wt%, C: 0.040% or less, S
i: 0.80% or less, Mn: 1.00 to 5.00%, P
: 0.040% or less, S: 0.030% or less, C
u: 1.00% or less, Ni: 18.0 to 22.00
%, Cr: 20.00 to 28.00%, Mo: 4.00
~ 8.00%, N: 0.15-0.40%, and M
g: 0.001 to 0.10%, the balance Fe and unavoidable impurities, and 10% or less of the δ ferrite phase.

The stainless steel of the present invention may contain one or both of the following alloy elements in addition to the above alloy elements. 1) wt%, Sn: 0.02 to 0.10%, Ti:
0.10 to 2.00%, Nb: 0.10 to 2.00%,
Ta: 0.10 to 2.00%, Co: 0.10 to 3.0
0%, and W: 0.10 to 3.00%, and any one or more of them, and 2)% by weight, Ca: 0.001 to 0.020%, A
1: 0.01-0.07%, and B: 0.001-
Any one or more of 0.10%.

The reasons for limiting the alloy components will be described below. C: 0.040% or less C is a strong austenite forming element and expensive Ni
Although it is effective in omitting the above, since the carbides are formed to deteriorate the corrosion resistance and toughness of the steel, the upper limit is 0.04 in the present invention.
0%

Si: 0.80% or less Si is effective as a deoxidizing agent for steel, but since it promotes the formation of intermetallic compounds, the upper limit is 0.80%. Mn: 1.00 to 5.00% Mn acts as an austenite forming element and is also effective as a deoxidizing agent for steel. Further, since it is effective as a component for stabilizing N, 1% or more is added. However, if added excessively, the corrosion resistance of steel deteriorates, so the upper limit is set to 5.
00%.

P: 0.040% or less P promotes stress corrosion damage and deteriorates corrosion resistance, so P must be 0.040% or less. Desirably 0.
020% or less. S: 0.030% or less Since S remarkably deteriorates corrosion resistance and hot workability, 0.0
It should be 30% or less. Desirably 0.005%
And

Cu: 1.00% or less Cu is particularly effective in improving the corrosion resistance in a low pH region, but it is not necessary to add Cu in such a large amount in a seawater environment having a pH of about 8 and rather deteriorates the hot workability. In the present invention, the upper limit is 1.00%. Ni: 18.0 to 22.00% Ni is effective as an austenite forming element and is indispensable. Used together with Cr and Mo when high corrosion resistance is required for seawater resistance. Corrosion resistance and oxidation resistance are improved as the Ni content is higher, but a large amount of addition makes the metal structure of the steel an austenite single phase and reduces hot workability. The Ni content is set to 18.0 to 22.00% in order to secure a δ ferrite phase preferable for improving workability.

Cr: 20.00 to 28.00% Cr is an element which improves the corrosion resistance and is particularly effective against pitting corrosion and crevice corrosion in a seawater environment. Further, in the steel of the present invention, it forms a δ ferrite phase necessary for improving hot workability. Therefore, a content rate of 20.00% or more is added. However, if added excessively, an intermetallic compound is formed to cause embrittlement of the steel, and the δ ferrite phase is crystallized in an unnecessarily large amount to impair the hot workability of the steel. Is 28.00%.

Mo: 4.00 to 8.00% Mo has an effect of more than Cr on the corrosion resistance and is a ferrite forming element. In the present invention, the content ratio is 4. because the corrosion resistance is improved and the hot workability due to the δ ferrite phase is improved.
Add more than 00%. However, Mo increases the rate of formation of intermetallic compounds, so excessive addition causes embrittlement of steel,
Furthermore, since the δ ferrite phase is crystallized in an unnecessarily large amount to impair the hot workability of the steel and cause an increase in cost,
The upper limit of the content rate is 8.00%.

N: 0.15 to 0.40% N is effective for improving the corrosion resistance and also for improving the strength, and has a strong function as an austenite forming element, so N is contained in an amount of 0.15% or more. However, if added excessively, blowholes are formed during the ingoting of steel and the manufacturability is impaired, so the upper limit of the content is made 0.40%.

Mg: 0.001 to 0.10% Mg is an essential element for improving hot workability in the steel of the present invention. This effect is caused by containing 0.001% or more of Mg, but even if added in a large amount, the effect is saturated, so the upper limit of the content is made 0.10%. Sn: 0.02 to 0.10% Sn improves the acid resistance of stainless steel, so the content may be added in the range of 0.10% or less.

Ti, Nb and Ta: 0.10 to 2.0
0%, Ti, Nb and Ta all fix C in the steel as a carbide and improve the corrosion resistance of the steel. In particular, since the effect of improving the crevice corrosion resistance of the welded portion is great, the content rate is 0.10
You may add in the range of 2.00%. Co and W: 0.10 to 3.00% Since Co and W improve the corrosion resistance of stainless steel, the content may be added in the range of 0.10 to 3.00%.

Ca: 0.001 to 0.020% Since Ca acts as a deoxidizing and desulfurizing agent for steel and improves hot workability, it is added in a content range of 0.001 to 0.020%. May be. Al: 0.01 to 0.07% Al acts as a strong deoxidizing agent for steel, but if added in excess, it deteriorates the corrosion resistance and hot workability of steel, so the content rate is 0.01 to 0.07%. You may add in the range of.

B: 0.001 to 0.10% Since B improves the hot workability of steel, its content is 0.001 to 0.001%.
You may add in 0.10% of range. In the steel of the present invention, the δ ferrite phase plays an important role in improving the hot workability of the steel. The amount is mainly determined by the distribution of austenite-forming elements such as Ni, C, N, Mn, and Cu and ferrite-forming elements such as Cr, Si, Mo, and Al, but the chemical composition is within the above-mentioned component content range. Then, the δ ferrite phase is adjusted to be within 10%.

The δ-ferrite phase is present at the austenite grain boundaries and forms a solid solution with P and S which impede the hot workability of steel, and eliminates the adverse effect. The effect is exhibited even with a small amount of δ-ferrite, but it is preferable to be 2% or more to bring out a sufficient effect. However, the presence of an excessive amount of δ ferrite phase rather impairs the hot workability of steel, so the upper limit is made 10%.

[0019]

EXAMPLE Steels having the chemical composition shown in Table 1 were melted in a vacuum induction furnace to obtain a 50 kg steel ingot.

[0020]

[Table 1]

Approximately 1 from the vicinity of the surface of the height center of the steel ingot
A 0 mm square sample material was cut out and used for measurement of the amount of δ ferrite. The amount of δ ferrite was measured by a method in which a cross section of a steel ingot was used as a surface to be processed, which was buffed, corroded with a 5% picric acid + hydrochloric acid alcohol solution, and mirrored at 200 times to obtain an area ratio. . The results are shown in Table 1. Further, a test piece having a diameter of 6 mm and a length of 110 mm was cut out from the vicinity of the surface of the steel ingot and subjected to a greeble test. An example of the greeble test results is shown in FIG. It can be seen that the aperture value is improved by adding an appropriate amount of Mg and the δ ferrite phase.

Separately, the above steel ingot (average diameter 100 mm)
Was heated to 1200 ° C., and forged into 50 mm square with 1 heat up to 1000 ° C., and the presence or absence of cracking was examined to evaluate the forgeability. Further, the steel ingot is hot-forged to have a diameter of 2
It is processed into 0 mm steel bar and subjected to solution treatment, and a test piece with a diameter of 18 mm and a length of 25 mm is cut out from this, and the end surface is 1
It was ground with a No. 000 emery and subjected to a corrosion resistance test.

The corrosion resistance test is 6% FeC adjusted to pH 1.
The test piece was immersed in the l ₃ solution for 24 hours, and the surface condition was visually observed. If corrosion did not occur, raise the liquid temperature in increments of 2.5 ° C and perform the same operation. Temperature (ie, the temperature at which critical pitting occurs) (CP
T). Table 2 shows the results of the forgeability and corrosion resistance test.

[0024]

[Table 2]

In Table 2, forgeability ○: no forge cracking, forgeability ×: forge cracking, and corrosion resistance ○: critical pitting corrosion temperature 50 ° C or higher, corrosion resistance ×: critical pitting corrosion temperature 47.5 ° C or less Represents.

[0026]

EFFECTS OF THE INVENTION According to the present invention, by selecting a unique alloy composition, hot workability is good, and corrosion resistance and mechanical properties are not impaired even when manufacturing thick products or thick round products. It is possible to provide austenitic stainless steel which has sufficient corrosion resistance even in an environment containing chlorine ions and is suitable for components such as pumps, shafts and bolts for seawater, and wires for seawater.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a test temperature and an aperture value in a greeble test.

Claims (4)

[Claims] 1. The content of alloying elements is% by weight, C: 0.040% or less, Si: 0.80% or less, Mn: 1.00 to 5.00%, P: 0.040% or less, S: 0.030% or less, Cu: 1.00% or less, Ni: 18.0 to 22.00%, Cr: 20.00 to 28.00%, Mo: 4.00 to 8.00%, N : 0.15 to 0.40%, and Mg: 0.001 to 0.10%, consisting of the balance Fe and unavoidable impurities, and containing 10% or less of a δ ferrite phase. Austenitic stainless steel for seawater with excellent workability. 2. In addition to the alloy element according to claim 1, in weight%, Sn: 0.02 to 0.10%, Ti: 0.10 to 2.00%, Nb: 0.10 to 2. 00%, Ta: 0.10 to 2.00%, Co: 0.10 to 3.00%, and W: 0.10 to 3.00%, and any one kind or two or more kinds is contained. , A balance of Fe and inevitable impurities, and containing 10% or less of a δ ferrite phase, an austenitic stainless steel for seawater having excellent hot workability. 3. In addition to the alloy elements according to claim 1, in weight%, Ca: 0.001 to 0.020%, Al: 0.01 to 0.07%, and B: 0.001 to 0. Out of 10%, the seawater-resistant excellent in hot workability is characterized by containing any one or more of 10% and the balance Fe and unavoidable impurities and containing 10% or less of a δ ferrite phase. For austenitic stainless steel. 4. In addition to the alloy element according to claim 1, in weight%, Sn: 0.02 to 0.10%, Ti: 0.10 to 2.00%, Nb: 0.10 to 2. 00%, Ta: 0.10 to 2.00%, Co: 0.10 to 3.00%, and W: 0.10 to 3.00%, and any one or more of them, and Ca. : 0.001 to 0.020%, Al: 0.01 to 0.07%, and B: 0.001 to 0.10%, and any one or more of them is contained, and the balance Fe and An austenitic stainless steel for seawater, which is excellent in hot workability and is characterized by containing unavoidable impurities and 10% or less of a δ ferrite phase.