JP2000073145A - Austenitic stainless steel excellent in hot workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Cu-contg. austenitic stainless steel high in high temp. strength and having hot workability equal to that of 18-8 series austenitic stainless steel. SOLUTION: This steel has a chemical compsn. contg., by weight, 0.03 to 0.15% C, <=1.5% Si, 0.1 to 2% Mn <=0.05% P, <=0.01% S, 15 to 25% Cr, 6 to 25% Ni, 2 to 6% Cu, 0.1 to 0.8% Nb, 0.003 to 0.1% Al, 0.05 to 0.3% N and O to 0.015% Mg or/and Ca, in which X value obtd. by X=[(Mn+283Mg+192 Ca)×10Al/S]-(85,900Cu×S)]} is -2,000 to 2,000.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an austenitic stainless steel excellent in hot workability.

[0002]

2. Description of the Related Art Conventionally, in a boiler or a chemical plant used in a high temperature environment, SUS is used as a material for an apparatus.
304H, SUS316H, SUS321H and SU
18-8 austenitic stainless steels such as S347H 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. Accordingly, the conventionally used 18-8 austenitic stainless steel has been used. , High temperature strength is becoming insufficient.

As an austenitic stainless steel capable of meeting the above-mentioned requirements, for example, Japanese Patent Application Laid-Open No. 62-133048
As disclosed in Japanese Unexamined Patent Application Publication No. Hei 8-13102, rather than adding a large amount (about 5% or more) of expensive alloy elements such as Mo and W, Cu, a relatively inexpensive alloy element, There is one in which high-temperature strength is improved by adding Nb and N in combination.

However, the Cu-added steel as described above has a poor hot workability as compared with the conventional 18-8 austenitic stainless steel, so that an immediate improvement is required.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a hot working material comparable to a conventional 18-8 austenitic stainless steel despite the fact that Cu is a Cu-added steel from the viewpoint of high-temperature strength. An object of the present invention is to provide an austenitic stainless steel having workability.

[0007]

The gist of the present invention resides in the following (1) austenitic stainless steel having excellent hot workability.

(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 to 0.8%, Al: 0.003 to 0.1%, N: 0.
05 to 0.3%, B: 0 to 0.01%, Mg: 0 to 0.
015%, Ca: 0 to 0.015%, the balance being Fe
An austenitic stainless steel excellent in hot workability, characterized by having a chemical composition comprising an unavoidable impurity and having an X value determined by the following formula of -2000 to 2,000.

X = [(Mn + 283 × Mg + 192 × C)
a) × 10 × Al / S] − (85900 × Cu × S) Here, the element symbols represent the contents (% by weight) of the respective elements in the steel.

[0010] The steel of the present invention described above, instead of a part of Fe,
Mo: 0.3% to 2% or W: 0.5% to 4% may be contained.

The present invention has been completed based on the following findings. The present inventors have conducted intensive studies to improve the hot workability of Cu-added austenitic stainless steel, and have obtained the following findings.

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

The grain boundary segregation of S by Cu is represented by the formula [[(Mn)
+ 283 × Mg + 192 × Ca) × 10 × Al / S] −
(85900 × Cu × S) ”is −20.
Mn that satisfies 00 to 2000, or Mn in addition to Mn.
g and / or Ca are added, and S
When these are fixed as sulfides or the like, S segregation at grain boundaries is eliminated and hot workability is dramatically improved. In addition,
The symbol of the element in the above formula is the content (% by weight) of each element in the steel.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the reasons for defining the chemical composition of steel in the present invention as described above will be described in detail.
In the following, unless otherwise specified, "%"
Means "% by weight".

C: C is an element effective for securing the required tensile strength and creep rupture strength when used in a high-temperature environment. However, if the content exceeds 0.15%, only the amount of undissolved carbide in the solution state increases, which not only does not contribute to the improvement in high-temperature strength but also deteriorates mechanical properties such as toughness. On the other hand, in the present invention, as will be described later, the C content may be lower in order to contain a relatively large amount of N.
%, The above effect is not exhibited. Therefore, the C content is set to 0.03 to 0.15%. A preferred range is 0.05 to 0.15%, and a more preferred range is 0.05 to 0.13%.

Si: Si is added as a deoxidizing agent for molten steel and is an effective element for improving oxidation resistance. However, if its content is large, weldability and hot workability are deteriorated. Further, as described above, since the steel of the present invention contains a relatively large amount of N, the presence of a large amount of Si promotes the precipitation of a large amount of nitride during use at a high temperature, and the toughness and This leads to a decrease in ductility. Therefore, the Si content is set to 1.5% or less. When importance is placed on toughness and ductility, 0.5%
The content is preferably set to not more than 0.3%, more preferably not more than 0.3%. When the deoxidation is sufficiently ensured by another element, the Si content may be substantially 0 (zero).

Mn: Mn has a deoxidizing effect on molten steel, similar to Si described above, but in the present invention, S inevitably contained in steel is fixed as sulfide (MnS) or the like,
Cu is a very important element required to prevent segregation of S at grain boundaries and to improve hot workability. In order to sufficiently obtain the effect, a content of 0.1% or more is required. However, when the content 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 range of the Mn content is 0.1 to
2%. In addition, a preferable range is 0.3 to 2%. A preferred range when importance is placed on tissue stability is 0.5 to 1.5%.

However, the content thereof must be such that the X value obtained by the above equation satisfies -2000 to 2000. This is because if the X value is less than -2000, S
Is incompletely fixed, and a small amount of S segregates at the grain boundaries, thereby deteriorating hot workability. Conversely, when the X value exceeds 2000,
The sulfides and the like are coagulated and coarsened, and the effect of suppressing the coarsening of the γ particle size is lost. In addition, the hot workability is deteriorated by the sulphides and the like which are coagulated and coarsened. Therefore, the X value was -2000 to 2000.

P: P is an element contained in steel as an unavoidable impurity and significantly deteriorates hot workability. For this reason, the lower the content, the better, but excessive reduction is costly, and if it is 0.05% or less, there is no particular problem.
It was as follows. A preferred upper limit is 0.04%, and a more preferred upper limit is 0.03%.

S: S is sometimes positively added from the viewpoint of machinability and weldability, but is usually contained in steel as an unavoidable impurity as in the case of P, and It is an element that significantly deteriorates the properties. For this reason, from the viewpoint of improving the hot workability, which is a problem to be solved by the present invention, the lower the content, the better, but the excessive reduction requires a cost, which is in view of the cost of steel production. 0.01%
It was as follows. A preferred upper limit is 0.005%.

In the Cu-added steel of the present invention, as described above, Cu promotes S segregation at grain boundaries,
Simply reducing the S content is not enough to improve the hot workability, and Mn, and in addition to Mn, Mg and Ca so that the X value satisfies -2000 to 2000.
It is necessary to fix S by adding one or both of the following.

Cr: Cr is an element necessary for improving the oxidation resistance and corrosion resistance at high temperatures, and its performance is improved as its content increases. However, its content is 1
If it is less than 5%, a sufficient effect cannot be obtained, and if it exceeds 25%, the austenite structure becomes unstable.
Therefore, the Cr content was set to 15 to 25%.

Ni: Ni is an element necessary to secure a stable austenite structure, and its optimum content is ferrite-forming elements such as Cr, Mo, W, and Nb contained in steel, and C and N Determined by the content of the austenite-forming element. In the Cu-added 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%, the effect is saturated and the economy is impaired. Therefore, the Ni content is set to 6 to 25%.

Cu: Cu forms fine C during use at high temperatures.
It is an element that precipitates consistently with the austenite matrix as the u phase and greatly contributes to the improvement of the creep rupture strength, but it is necessary to contain 2% or more in order to exert its effect. However, when the content exceeds 6%, creep rupture ductility and workability deteriorate. Therefore, the Cu content is 2
To 6%.

Nb: 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. Conversely, if the content exceeds 0.8%, not only does the weldability and workability deteriorate, but also the N-added steel of the present invention In this case, the amount of undissolved carbonitride increases, and the mechanical properties deteriorate. Therefore, the content of Nb is set to 0.1 to 0.8%.

Al: Al is an element added as a deoxidizing agent for molten steel. To obtain the effect, 0.003%
It is necessary to contain the above. However, when the Al content is 0.1.
If it exceeds 1%, when used for a long time under high temperature conditions, precipitation of intermetallic compounds such as σ phase is promoted, and toughness is deteriorated. Therefore, the Al content was set to 0.003 to 0.1%. A preferred range is 0.003 to 0.06%, and a more preferred range is 0.003 to 0.03%.

N: Like N, N is an element effective for improving the tensile strength and creep rupture strength.
When the content is less than 5%, a sufficient effect is not exhibited. On the other hand, since N has a larger solid solubility limit than C, even if it is contained in a relatively large amount, it forms a solid solution in a solution state, and the toughness decrease accompanying precipitation of nitride generated during aging is relatively small. 0.3
%, The toughness after aging decreases. Therefore, the content of N is set to 0.05 to 0.3%.

B: It is not necessary to add B, but when added, it forms a fine carbonitride and is an element that contributes to the improvement of creep rupture strength by its dispersive precipitation strengthening action and grain boundary strengthening action. is there. Therefore, if it is desired to obtain the effect, it can be added. However, if the content is less than 0.001%, the above effect is not exhibited. Deteriorates. Therefore, the content of B when added is 0.001 to
It is preferably set to 0.01%. A more preferred range is
0.001 to 0.008%.

Mg, Ca: These elements do not necessarily need to be added similarly to the above B, but when added, they mainly form sulfides and the like to fix S in the steel,
It prevents grain boundary segregation of S and contributes to improvement of hot workability.
Therefore, when it is desired to obtain the effect, either one or both of them can be added. However, in order to obtain the effect, it is necessary that each element contains 0.001% or more. However, if its content (when both are added, the total content) exceeds 0.015%, the hot workability will conversely decrease. Therefore, when M is added,
Content of g and Ca (Total content if both are added)
Is preferably set to 0.001 to 0.015%. A more preferred range is 0.002 to 0.01%.

However, the content thereof is, as in the case of the above-mentioned Mn, such that the X value obtained by the above equation is -2000 to 2000.
Must be satisfied. This is because the X value is -200
If it is less than 0, the fixation of S is incomplete and a small amount of S
Are segregated at the grain boundaries, and the hot workability is reduced. On the other hand, when the X value exceeds 2,000, the sulfides and the like are agglomerated and coarse, and the effect of suppressing the coarsening of the γ particle size is lost. It is.

Mo, W: These elements correspond to the above B, M
Like g and Ca, they do not necessarily need to be added, but have the effect of improving high-temperature strength. For this reason,
In order to obtain the effect, one or both of them can be added. However, when the content is less than 0.3% in the case of Mo and less than 0.5% in the case of W, the above-mentioned effects are not sufficiently exhibited. On the other hand, if the content exceeds 2% in the case of Mo and exceeds 4% in the case of W, not only the effect is saturated, but also the stability of the structure and the workability deteriorate. Therefore, when added, the content is preferably 0.3 to 2% for Mo and 0.5 to 4% for W.

The steel of the present invention is analyzed for its chemical composition at the final stage of refining molten steel obtained by smelting according to a conventional method, and based on the analysis results, Mn, Cu and S, and further Mg or / It can be easily manufactured by finely adjusting the content of Ca and Ca.

[0033]

EXAMPLE 30 having the chemical composition shown in Tables 1 and 2
Various types of test steels were smelted using a vacuum high-frequency induction furnace, and the obtained molten steel was cast into a mold to have a diameter of 160 mm and a length of 30 mm.
A 50 mm ingot of 0 mm was obtained. In Tables 1 and 2, Nos. 1 to 23 are steels of the present invention, and Nos. A to G are steels of comparative examples.

[0034]

[Table 1]

[0035]

[Table 2]

Next, from each ingot as cast,
A round bar-shaped tensile test piece having a diameter of 10 mm and a length of 130 mm was collected and subjected to a high-speed tensile test at 1000 ° C. (strain rate 1 / sec.
The sample was subjected to c), and the drawing ratio (%) of the fractured surface was examined.

Further, a material having a thickness of 20 mm was cut out from the ingot after the above-mentioned tensile test piece was sampled, and the material was heated to 1200 ° C., and then subjected to hot rolling to reduce the thickness to 10 mm, thereby generating cracks. Was examined.

The results of the above investigation are shown in Tables 1 and 2. FIG. 1 shows the relationship between the drawing ratio (%) of the fractured surface, which is the result of the high-speed tensile test, and the X value obtained by the above equation.

First, as is apparent from the results of FIG.
Only when the value is -2000 to 2000 defined in the present invention, a drawing ratio of 60% or more can be obtained in the high-speed tensile test.
It can be seen that the above-described aperture ratio cannot be obtained.

Further, as is clear from the results shown in Tables 1 and 2, the X value is in the range of -2000 to 20 as defined in the present invention.
In the case of the steel of the present invention (No. 1 to 23) having a reduction ratio of 60% or more in a high-speed tensile test within a range of 00, no cracks occurred in actual hot rolling.

On the other hand, the X value is defined by the present invention -2.
Out of the range of 000-2000, 60% in high-speed tensile test
Steels of comparative examples showing no less than the draw ratio (No. AG)
In any case, cracks occurred in actual hot rolling. This is because, as shown in Table 1, the steels of Comparative Examples (Nos. A to G) usually contain a sufficient amount of Mn.
Furthermore, although containing Mg or Ca, the X value “= [(Mn +
283 x Mg + 192 x Ca) x 10 x Al / S]-
(85900 × Cu × S) ”deviates from the range specified in the present invention, so that S segregation at the grain boundary by Cu is not completely prevented.

[0042]

According to the present invention, the hot workability is extremely good, and it does not crack even when hot worked into various shapes of members, and has a high temperature strength capable of producing a product with a high yield. A high Cu-containing austenitic stainless steel can be provided at low cost.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between an X 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: 0.003 to 0.1%, N: 0.05 to
0.3%, B: 0 to 0.01%, Mg: 0 to 0.015
%, Ca: 0 to 0.015%, and the balance consists of Fe and unavoidable impurities, and the X value obtained by the following equation is −
An austenitic stainless steel excellent in hot workability, having a chemical composition of 2000 to 2000. X = [(Mn + 283 × Mg + 192 × Ca) × 10 ×
Al / S] − (85900 × Cu × S) Here, the element symbols represent the contents (% by weight) of the respective elements in the steel. 2. The method according to claim 1, further comprising one or more of Mo: 0.3 to 2% and W: 0.5 to 4% by weight. Austenitic stainless steel with excellent hot workability.