JPH06293942A - High nitrogen stainless steel having high strength and high corrosion resistance and excellent in hot workability - Google Patents

Abstract

PURPOSE:To produce a high nitrogen stainless steel having high strength and high corrosion resistance and excellent in hot workability by preparing a high nitrogen stainless steel having a specific composition in which respective contents of N, B, O, and Nb are specified. CONSTITUTION:A high nitrogen stainless steel, having a composition consisting of, by weight ratio, 0.03-0.10% C, 0.10-1.00% Si, 1.00-3.00% Mn, <=0.04% P, <=0.02% S, 7-10% Ni, 17-20% Cr, 0.18-0.25% N, <=0.05% Ti, 20-40ppm B, <=50ppm O, 0.25X(C+N) to 0.25% Nb, and the balance Fe with inevitable impurities, is prepared. By this preparation, the austenitic stainless steel excellent in hot workability and having high strength and high corrosion resistance, in spite of being a high nitrogen stainless steel containing >=0.18% N, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to large-scale industrial equipment and large-scale structures, which require high strength and high corrosion resistance, that is, chemical,
High strength used in various plants such as ocean and nuclear power
The present invention relates to high nitrogen stainless steel having high corrosion resistance and excellent hot workability.

[0002]

2. Description of the Related Art Recently, the size of various industrial equipments and structures has been increased, and it has been strongly demanded to improve the strength of structural stainless steels. By adding nitrogen, the strength of solid solution is strengthened by its solid solution strengthening action. SUS304N2, SUS304LN, SU
Austenitic stainless steels such as S316N and SUS316LN are known. The invention of Japanese Examined Patent Publication No. 4-45576 is disclosed as further improving the strength after the solid solution strengthening heat treatment.

However, the austenitic stainless steel reinforced by adding nitrogen in an amount of 0.18% or more as described above has a poor hot workability, so that it is easy to form a large-sized work suitable for a large structure. It wasn't.

[0004]

DISCLOSURE OF THE INVENTION The present invention provides a nitrogen gas of 0.
Despite being a high nitrogen content stainless steel containing 18% or more, it provides an austenitic stainless steel with excellent hot workability and high strength and corrosion resistance for use in large industrial equipment and large structures. That is.

[0005]

The present invention is a solid solution strengthening component in structural stainless steel for use in large industrial equipment and large structures exposed to harsh environmental conditions. In order to improve strength and corrosion resistance by adding N of 0.18 to 0.25% and improve hot workability in spite of containing N of 0.18% or more, B is added in an amount of 20 to 40 pp.
m is contained, O is 50 ppm or less, and Nb added to improve the strength has an upper limit of 0.25% in order to secure hot workability, and a lower limit has a tensile strength of 750 to 8
0.25 × (C + N)% to obtain 00 (N / mm ² )
And

Therefore, the composition of the high-nitrogen stainless steel of the present invention, which is a means for solving the problems, and which is excellent in hot workability, has a weight ratio of C: 0.03 to 0.10.
%, Si: 0.10 to 1.00%, Mn: 1.00 to 3.0
0%, P: 0.04% or less, S: 0.02% or less, Ni:
7-10%, Cr: 17-20%, N: 0.18-0.2
5%, Ti: 0.05% or less, B: 20-40 ppm,
O: 50 ppm or less, Nb: 0.25 × (C + N) to 0.2
5%, and the balance is Fe and inevitable impurities.

[0007]

In the present invention, Nb and B are contained and the upper limit of the Nb content is suppressed although the nitrogen content is increased in the austenitic stainless steel to strengthen the solid solution. , Stainless steel with excellent hot workability, high strength and high corrosion resistance, suitable for large industrial equipment and large structures exposed to the severe conditions required.

Next, the reasons for limiting the components of the present invention will be explained.

At least 0.03% is necessary because C is a strong austenite-forming element and at the same time very effective in strengthening the austenite matrix.
Therefore, the lower limit of C is set to 0.03%. But 0.10
%, The intergranular corrosion is significantly promoted, so the upper limit is made 0.10%.

Si is an effective deoxidizing agent, essential for steelmaking operations, and very effective for strengthening the austenite matrix, so at least 0.10% is necessary, so the lower limit is 0.10. %. But 1.00
If it is contained in excess of%, defects and cracks are likely to occur during production, so the upper limit is made 1.00%.

Mn is an element which acts as a deoxidizing and desulfurizing agent and stabilizes the austenite phase. Also, the solubility of nitrogen is remarkably increased. Therefore, in the present invention, at least 1.00% or more is necessary, and the lower limit is 1.00.
%. However, if added in excess of 3.00%, corrosion resistance, oxidation resistance, and hot workability deteriorate, so the upper limit is made 3.00%.

If P is contained in excess of 0.04%, hot workability and corrosion resistance are significantly deteriorated, so P must be made 0.04% or less.

If S is contained in excess of 0.02%, the hot workability and corrosion resistance are significantly deteriorated, so S must be made 0.02% or less.

Ni is a basic element of austenitic stainless steel, and it is necessary to contain Ni in an amount of 7% or more in order to obtain excellent corrosion resistance. However, in excess of 10%, a large amount of Ni
If it contains Al, the weld cracking resistance, hot workability, corrosion resistance and the like during welding are deteriorated, so the upper limit is made 10%.

Cr is a basic element of stainless steel and provides excellent corrosion resistance. Further, the solid solubility of nitrogen is remarkably increased and it is useful for strengthening the matrix by adding nitrogen. Since these effects become remarkable at 17% or more, the lower limit is set to 17%. However, if the Cr content exceeds 20% and is too large, the balance of the δ / γ structure at high temperature is impaired, so the upper limit is made 20%.

N has an interstitial solid solution strengthening effect and a grain refining and precipitation strengthening effect due to Nb (C, N) precipitation. Together with C, it is a major strengthening element of the matrix and also contributes to the improvement of corrosion resistance. It is an element. 0.1 to obtain these effects
It is necessary to contain 8% or more. However, if it is contained too much in excess of 0.25%, the hot workability is impaired, so
0.25% or less.

B is an important element for improving the hot workability when N is contained in an amount of 0.18% or more, and exerts an effect regardless of the hot working temperature conditions. Since the effect is remarkable at 20 to 40 ppm as shown in FIG. 1, the content is limited to 20 to 40 ppm in the present invention.

O remains in the steel as non-metallic inclusions,
It deteriorates mechanical properties and hot workability. As shown in FIG. 2, in the present invention, when the content exceeds 50 ppm, the reduction value (hot workability) is significantly deteriorated. Therefore, the upper limit of O is 5
Set to 0 ppm.

Ti is TiN and Ti (C, N) in the crystal grains.
As a result, a large amount of N precipitates, so that the solid solution amount of N is reduced and the corrosion resistance is reduced. The effect becomes remarkable when it exceeds 0.05%, so the upper limit is made 0.05%.

Nb is an element that fixes the residual C as NbC, improves the corrosion resistance, and improves the grain refinement and strength by precipitation of Nb (C, N). That is, tensile strength 750
To satisfy (N / mm ² ) or more, 0.25 × (C +
N)% or more is necessary, but if it exceeds 0.25%, dynamic recrystallization is delayed and hot workability is deteriorated, so the upper limit is made 0.25%.

[0021]

[Table 1]

[0022]

EXAMPLES Next, the characteristics of the steel of the present invention will be clarified by examples compared with the conventional steel and the comparative steel.

Table 1 shows the test materials of the present invention steel and the comparative steel.
It shows chemical components other than Fe.

First, the hot workability will be described.

FIG. 1 shows the effect of B on the hot workability, and FIG.
Shows the effect of O. All the test materials used were steels that had been water-cooled after being held at 1250 ° C for 3 hours. Test at 1000 ℃, 5cm / sec
That is, the high-temperature tensile test was performed and the aperture value was measured.

From FIG. 1, by adding 20 to 40 ppm of B, all the reduction values are 60% or more and the hot workability can be greatly improved. Since the comparative steels 18 and 19 contain a large amount of Nb, the hot workability is deteriorated. B segregates at the grain boundaries to suppress the grain boundary segregation of other impurities such as S and P, and prevents grain boundary embrittlement, but if it is less than 20 ppm, it is not effective. Further, if added in excess of 40 ppm, boride is formed and the hot workability at high temperature is deteriorated.

When O is also set to 50 ppm or less, all the drawing values become 75% or more, and the hot workability can be greatly improved. Generally, if a large amount of O is contained, the cleanliness of the γ grain boundary is impaired at high temperatures and the elongation performance of the γ grain boundary is deteriorated.
It has been found that the hot workability required in the present invention can be secured.

Next, the tensile strength will be described. Table 2
Shows the tensile strength of the steel that has been subjected to solution heat treatment (1100 ° C. × 10 min water cooling). The test was performed using a JIS No. 4 test piece. All of those satisfying 0.25 × (C + N) ≦ Nb ≦ 0.25 have tensile strength of 750 to 800 N
/ Mm ² is shown. Sufficient tensile strength cannot be obtained for steels containing no Nb and steels containing comparatively small amounts of Nb such as Comparative Steels 15 and 16. The comparative steels 18 and 19 that contain a large amount of Nb show high tensile strength. However, as described above, the hot workability is deteriorated.

[0029]

[Table 2]

Further, the corrosion resistance will be described. In Table 2,
Solution heat treatment (1100 ℃ × 10min water cooling) and sensitization heat treatment (1100 ℃ × 10min water cooling → 650 ℃ × 2hr. Air cooling)
The steel which has been subjected to the above-mentioned test shows pitting corrosion resistance. The test is 35 ℃
The corrosion degree in the 6% ferric chloride solution was measured. Solid solution

[0031]

[Table 3] In the activated state, when the amount of N is large, the corrosion resistance is improved. Addition of Nb has almost no effect. In the sensitized state, it is greatly improved by adding Nb. This is due to the fixation of C by Nb. Comparative steels 20 and 21 have very poor pitting corrosion resistance in the sensitized state, but since both contain Ti in excess of 0.05%, nitrogen is fixed by Ti and solute nitrogen decreases. It is thought that it was done.

Table 3 shows a comprehensive evaluation of the above results. As can be seen from Table 3, the steels of the present invention are excellent in hot workability, tensile strength and corrosion resistance in all the test materials, and all of them are excellent in overall evaluation. On the other hand, the comparative steel is not excellent in all of hot workability, tensile strength and corrosion resistance, and is inferior to the steel of the present invention in the comprehensive evaluation.

[0033]

As described above, the high nitrogen stainless steel of the present invention has high strength and high corrosion resistance without deteriorating hot workability despite containing N of 0.18% or more. It is an extremely excellent structural stainless steel that can be used for machines and structures that have grown in size.

[Brief description of drawings]

FIG. 1 is a graph showing experimental results on the effect of B on hot workability.

FIG. 2 is a graph showing experimental results on the effect of O on hot workability.

Claims (1)

[Claims] 1. A weight ratio of C: 0.03 to 0.10.
%, Si: 0.10 to 1.00%, Mn: 1.00 to 3.0
0%, P: 0.04% or less, S: 0.02% or less, Ni:
7-10%, Cr: 17-20%, N: 0.18-0.2
5%, Ti: 0.05% or less, B: 20-40 ppm,
O: 50 ppm or less, Nb: 0.25 × (C + N) to 0.2
High nitrogen stainless steel with high strength, high corrosion resistance, and excellent hot workability, characterized by containing 5% and the balance Fe and unavoidable impurities.