JP2000328198A - Austenitic stainless steel with excellent hot workability - Google Patents

Abstract

(57) [Problem] To provide a Cu-containing austenitic stainless steel having good high-temperature strength and excellent hot workability. SOLUTION: Cu: 2 to 6%, Y, La, Ce and N
d or a combination of two or more thereof in a total of 0.01 to 0.1.
2%, and the W value represented by the following formula is -2000 to +2
Austenitic stainless steel in the range of 000. W = ｛(Mn + 283Mg + 192Ca + 25Y + 18La + 19Ce + 23Nd) × 10Al /
S｝-(85900Cu × S)

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a Cu-containing austenitic stainless steel excellent in hot workability.

[0002]

2. Description of the Related Art Conventionally, SUS304 has been used as an apparatus material in a boiler, a chemical plant, or the like used in a high temperature environment.
H, SUS316H, SUS321H and SUS34
18-8 austenitic stainless steels such as 7H have been used. However, in recent years, the operating conditions of the apparatus under such a high-temperature environment have become severely severe, and the required performance for the materials used has become severe, and the high-temperature strength of the conventionally used 18-8 austenitic stainless steel is insufficient. It is becoming.

[0003] Therefore, as steel having improved high-temperature strength without adding a large amount of expensive elements, the present inventors have developed austenitic steel having good high-temperature strength by adding Cu, Nb and N, which are elements that improve creep rupture strength, in a complex manner. (Japanese Patent Publication No. 8-30247, Japanese Unexamined Patent Publication No. 8-1)
No. 3102).

[0004] However, such Cu-containing steel has poor hot workability as compared with conventional 18-8 austenitic stainless steel, and its improvement is required in actual production.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a Cu-containing austenitic stainless steel having good high-temperature strength and excellent hot workability.

[0006]

The gist of the present invention resides in the following austenitic stainless steels (1) and (2).

(1) C: 0.03 to 0.15% by weight
%, Si: 1.5% or less, Mn: 0.1 to 2%, P:
0.05% or less, S: 0.01% or less, Cr: 15 to 2
5%, Ni: 6 to 25%, Cu: 2 to 6%, Nb: 0.
1-0.8%, Al: 0.001-0.1%, N: 0.
05 to 0.3%, one of Y, La, Ce and Nd
Species or two or more species in total of 0.01 to 0.2%, B: 0
０．０１0.01%, Mg and Ca: 0 to 0.015% each
However, an austenitic stainless steel containing Mg + Ca ≦ 0.015%, and having a W value represented by the following formula in the range of −2000 to +2000, with the balance being Fe and inevitable impurities and having excellent hot workability. .

W = ｛(Mn + 283Mg + 192Ca + 25Y + 18La + 19Ce + 23N
d) × 10Al / S｝-(85900Cu × S) Here, the element symbol indicates the content (% by weight) of each element.

(2) The austenitic stainless steel according to (1), further containing one or two of Mo: 0.3 to 2% and W: 0.5 to 4%.

The present inventors have conducted intensive experiments and studies on the hot workability of Cu-containing austenitic stainless steel in order to solve the above-mentioned problems. As a result, the present inventors have obtained the following findings and completed the present invention. Reached.

A) In Cu-added austenitic stainless steel, Cu promotes the segregation of S at the grain boundaries and significantly reduces hot workability.

B) The W value defined by the following equation is -2000 to
+2000, Y and L in addition to Mn.
By containing one or more of a, Ce and Nd and, if necessary, one or both of Mg and Ca, by fixing S segregated at grain boundaries due to Cu as sulfide. The hot workability of a Cu-containing austenitic stainless steel is remarkably improved.

W = ｛(Mn + 283Mg + 192Ca + 25Y + 18La + 19Ce + 23N
d) × 10Al / S｝-(85900Cu × S)

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the chemical composition of the austenitic stainless steel of the present invention will be described below. In addition, the following percentages of the chemical components indicate% by weight.

C: 0.03 to 0.15% C is an element effective for securing the tensile strength and creep rupture strength required when used in a high temperature environment. However, when C is contained in an amount exceeding 0.15%, only the amount of undissolved carbide in a solution state increases, which not only does not contribute to improvement in high-temperature strength but also deteriorates mechanical properties such as toughness. I do. Therefore, the upper limit of the C content is set to 0.15%. In the steel of the present invention, since N is also contained, C
Although the content may be lower, the lower limit needs to be 0.03% in order to obtain the above effect. Preferably it is 0.05% or more.

Si: 1.5% or less Si is an element used as a deoxidizing agent and is an element effective for improving oxidation resistance. However, when the content is large, weldability and hot workability deteriorate. I do. Further, since the steel of the present invention also contains N, if it is added in a large amount, the amount of nitride precipitated during use at a high temperature increases, leading to a decrease in toughness and ductility. Therefore, the Si content is set to 1.5% or less. When toughness and ductility are emphasized, the content is preferably 0.5% or less, and more preferably 0.3% or less. When the deoxidizing action is sufficiently ensured by another element, it is not necessary to substantially contain the element.

Mn: 0.1 to 2% Mn is an element used as a deoxidizer like Si,
Further, Cu is an important element that fixes S whose grain boundary segregation is promoted and improves hot workability. In order to obtain the effect sufficiently, it is necessary to contain 0.1% or more. However, if it exceeds 2%, precipitation of an intermetallic compound such as a σ phase is caused, and high-temperature strength and mechanical properties are reduced. Therefore,
The Mn content was 0.1 to 2%. More preferably, 0.
3 to 2%, 0.5 to 1.
5% is preferred.

P: 0.05% or less P is contained as an unavoidable impurity and significantly deteriorates hot workability. For this reason, it is preferable to set the temperature as low as possible. Desirably, it is 0.04% or less.

S: 0.01% or less S is also contained as an unavoidable impurity, and significantly degrades hot workability. 0.01% to prevent this deterioration
It was as follows. The lower the better, the more preferable it is 0.005% or less.

Cr: 15 to 25% Cr is an element necessary for improving oxidation resistance and corrosion resistance at high temperatures, and its performance is improved as the content increases. To obtain these effects sufficiently, 15% or more is required. On the other hand, if it exceeds 25%, the austenite structure becomes unstable. Therefore, the Cr content is 15 to
25%.

Ni: 6 to 25% Ni is an essential component for securing a stable austenite structure, and its optimum content is Cr, Mo,
It is determined by the contents of ferrite-forming elements such as W and Nb and austenite-forming elements such as C and N. In the steel of the present invention, if the content is less than 6%, it is difficult to stabilize the austenite structure. On the other hand, if the content exceeds 25%, it is economically disadvantageous, so the content was set to 6 to 25%.

Cu: 2 to 6% Cu precipitates consistently with the austenite matrix as a fine Cu phase in steel used at a high temperature, and greatly contributes to the improvement of creep rupture strength. 2%
It is necessary to contain the above. However, when the content exceeds 6%, creep rupture ductility and workability deteriorate.
Therefore, the Cu content was set to 2 to 6%.

Nb: 0.1-0.8% Nb is an element that improves the creep rupture strength by strengthening the dispersed precipitation of fine carbonitrides. However, if the content is less than 0.1%, a sufficient effect cannot be obtained.
If it is excessively contained in excess of 8%, weldability and workability are deteriorated, and the amount of undissolved carbonitride increases in N-containing steel, and mechanical properties are also deteriorated. Therefore, the content of Nb is set to 0.1 to 0.8%.

Al: 0.001 to 0.1% An element used as a deoxidizing material, and it is necessary to contain 0.001% or more in order to obtain the effect. But,
When the content exceeds 0.1%, the precipitation of an intermetallic compound such as a σ phase is promoted when used in a high temperature range for a long time, and the toughness is deteriorated. Therefore, the content of Al is 0.001 to
0.1%. Preferably it is 0.001 to 0.06, more preferably 0.001 to 0.03%.

N: 0.05 to 0.3% N, like C, is an element effective for improving the tensile strength and creep rupture strength. However, if its content is less than 0.05%, a sufficient effect is obtained. It cannot be demonstrated. On the other hand, N is C
Since the solid solubility limit is larger than that of, even if it is contained in a relatively large amount, it forms a solid solution in the solution state, and the toughness reduction accompanying nitride precipitation generated during aging is relatively small. If it is contained in excess, the toughness after aging decreases. Therefore, the content of N is set to 0.05 to 0.3%.

B: 0 to 0.01 B can be contained as necessary, because it contributes to the improvement of creep rupture strength by strengthening the finely dispersed precipitation of carbonitrides and strengthening the grain boundaries. If included, 0.00
If it is less than 1%, the effect is not exhibited, while 0.010
%, The weldability deteriorates. Therefore, when it is contained, the content is made 0.001 to 0.01%.
Desirably, it is 0.001 to 0.008%.

Mg, Ca: 0 to 0.015% These elements mainly fix S as sulfides and improve hot workability. However, if it is contained excessively, the hot workability decreases, so the upper limit is 0.015% each.
And The lower limit is not particularly limited, but is preferably 0.001% or more.

The desirable content of these elements when they are contained is 0.002 to 0.01%, respectively. These elements may be contained singly or in combination of two. However, these elements basically have the same action and effect. The upper limit needs to be 0.015% in total.

Y, La, Ce, Nd: 0.01 to 0.2
% These elements combine with S to form a sulfide, and the solubility product of the sulfide is smaller than that of the sulfide of Mn, so that the fixation of S becomes stronger and the hot workability is improved. . In addition, the sulfides of these elements are finely dispersed as compared with the sulfides of Mn, and thus effectively act to suppress the growth of γ grains. To obtain these effects, a total of 0.01% or more is required. However, if it exceeds 0.2%, the sulfides may be agglomerated and coarsened and cracks may be generated from the starting point during hot working, and conversely, the hot workability decreases, so the upper limit is 0.2%.
And Therefore, the content of these elements is 0.01 to
0.2%. In order to further improve the hot workability, the content is preferably set to 0.03 to 0.15%.

One of these elements may be contained alone, or two or more thereof may be contained in combination. Since all of these elements have the same effect, two or more of these elements are contained simultaneously. In this case, the total of these elements is 0.01 to 0.
The range is 2%.

Mo: 0.3 to 2%, W: 0.5 to 4% Since these elements have an effect of improving high-temperature strength, one or two of these elements are contained as necessary. . When Mo is contained, if the Mo content is less than 0.3% or the W content is less than 0.5%, the effect is not sufficiently exhibited. On the other hand, if Mo is contained in excess of 2% and W is contained in excess of 4%, the effect tends to be saturated and the structure stability and workability deteriorate. Therefore, if you want to include
0.3 to 2% for Mo, 0.5 to 4 for W
%.

W = ｛(Mn + 283Mg + 192Ca + 25Y + 18La + 19Ce + 23N
d) × 10Al / S｝-(85900Cu × S) In steels containing Cu, Cu promotes grain boundary segregation of S, so lowering S alone is not enough to improve hot workability. S must be completely fixed. For this purpose, the W value obtained by various experiments and represented by the following equation is −20.
It is necessary to contain one or more of Y, La, Ce and Nd in addition to Mn and, if necessary, one or both of Mg and Ca so as to fall within the range of 00 to +2000. is there.

If W is less than -2000, the fixation of S is incomplete, and a small amount of grain boundary segregation of S remains to deteriorate the hot workability. On the other hand, if it exceeds +2000, the number of sulfides decreases and the γ grains become coarse, and at the same time, the hot workability deteriorates due to the increase of oxide-based inclusions and the superposition of the precipitation of the low-melting intermetallic compound. Therefore, the W value is -2000 to +2.
000.

[0034]

EXAMPLES Austenitic stainless steels having 30 kinds of chemical compositions shown in Table 1 were melted into 50 kg ingots.

[0035]

[Table 1]

In the table, symbols 1 to 23 are steels of the present invention, and A to G are comparative steels having W values out of the range specified in the present invention.

The following tensile test pieces and rolled test pieces for evaluating hot workability were cut out from each ingot as cast.

Tensile test piece: diameter 10 mm, length 130 m
m round bar test piece Rolled test piece: thickness 20 mm, width 100 mm, length 250
In the mm tensile test, a high-speed tensile test at a strain rate of 1 / s was performed by heating to 1000 ° C., and the drawing ratio was determined from the fracture surface after the test to evaluate hot workability. In addition, the rolling test piece is 120
After heating to 0 ° C., hot rolling was performed to finish the sheet to a thickness of 10 mm, the state of occurrence of cracks after rolling was examined, and workability in hot rolling was evaluated.

The results are shown in Table 1.

FIG. 1 is a diagram showing the relationship between the aperture ratio and the W value shown in Table 1. As is clear from FIG. 1, when the W value is in the range of -2000 to +2000 specified in the present invention, a drawing ratio of 60% or more can be obtained in the high-speed tensile test. On the other hand, when the W value was less than -2000 or more than +2000, a drawing ratio of 60% or more could not be obtained.

Further, as shown in the workability column showing the results of cracking after hot rolling in Table 1, the steels of the present invention (symbols 1 to 5) were used.
In No. 23), no crack occurred.

On the other hand, when the W value is -2000 to +200
The steels of Comparative Examples (symbols A to G), which are out of the range of 0 and exhibit a drawing ratio of less than 60% in the high-speed tensile test, all cracked in hot rolling. This is because, as shown in Table 1, the steel of the comparative example usually contains a sufficient amount of Mn,
Y, La, Ce, Nd, etc.
This shows that although S and Ca were contained, the W value was out of the range specified in the present invention, so that S segregation at the grain boundaries by Cu was not completely prevented.

[0043]

Industrial Applicability The steel of the present invention has excellent hot workability while having good high-temperature strength, and does not crack even when hot-worked into members of various shapes, so that products can be manufactured with high yield. Is possible.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a W value and an aperture ratio.

Claims (2)

[Claims] (1) C: 0.03 to 0.15% by weight, S
i: 1.5% or less, Mn: 0.1 to 2%, P: 0.05
%, S: 0.01% or less, Cr: 15 to 25%, N
i: 6-25%, Cu: 2-6%, Nb: 0.1-0.
8%, Al: 0.001 to 0.1%, N: 0.05 to
0.3%, one or more of Y, La, Ce and Nd in a total amount of 0.01 to 0.2%, and B: 0 to 0.
01%, Mg and Ca: each containing 0 to 0.015%, but Mg + Ca ≦ 0.015%, and the W value represented by the following formula is in the range of -2000 to +2000, with the balance being Fe and inevitable Austenitic stainless steel with excellent hot workability composed of chemical impurities. W = ｛(Mn + 283Mg + 192Ca + 25Y + 18La + 19Ce + 23Nd) × 10Al /
S｝-(85900Cu × S) Here, the element symbol indicates the content (% by weight) of each element. 2. C: 0.03 to 0.15% by weight, S
i: 1.5% or less, Mn: 0.1 to 2%, P: 0.05
%, S: 0.01% or less, Cr: 15 to 25%, N
i: 6-25%, Cu: 2-6%, Nb: 0.1-0.
8%, Al: 0.001 to 0.1%, N: 0.05 to
0.3%, a total of 0.01 to 0.2% of one or more of Y, La, Ce and Nd, Mo: 0.3
２2% and W: one or two of 0.5 to 4%
Species, B: 0-0.01%, Mg and Ca: 0-0.
Austenite containing 015%, but Mg + Ca ≦ 0.015%, and having a W value represented by the following formula in the range of -2000 to +2000, with the balance being Fe and unavoidable impurities and having excellent hot workability. Series stainless steel. W = ｛(Mn + 283Mg + 192Ca + 25Y + 18La + 19Ce + 23Nd) × 10Al /
S｝-(85900Cu × S) Here, the element symbol indicates the content (% by weight) of each element.