JPS5964752A - Austenitic steel excellent in weldability and high- temperature strength - Google Patents

Abstract

PURPOSE:To improve the weldability, high-temperature strength and corrosion resistance of austenitic steel, by letting the austenitic steel have a specified composition comprising in addition to C, Si, Mn, Cr and Ni, Al or Mg, and as impurities P and S, and the balance of substantially Fe. CONSTITUTION:The austenitic steel contains 0.01-0.20% C, >=3% Si, >=10% Mn, 20-30% Cr, 10-30% Ni, 0.04-0.30% N and 0.01-0.5% Al and/or 0.001-0.05% Mg. If required, the austenitic steel further contains 0.001-0.020% B and/or 0.01-1.0% Nb. It contains as impurities P and S such as that P <=0.020% and S <=0.010%, which must satisfy the formula I or II, with the balance being Fe and unavoidable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an austenitic stainless steel that has excellent weldability, high-temperature strength, and corrosion resistance.

In materials for equipment such as boilers and chemical plant equipment used in high-temperature environments, not only high-temperature strength but also corrosion resistance and weldability are important considerations.

Conventionally, 18-8 series austenitic stainless steel has been mainly used for this type of application, but in recent years, the usage conditions for such applications have become significantly harsher, and the required material performance has become more sophisticated. The above-mentioned 18-8 yarn used for knotting has become insufficient in both high temperature strength and corrosion resistance.

Generally, improved corrosion resistance is achieved by JIR resistance of Cr. By the way, if the amount of Ni is increased, the amount of Ni will have to be increased to maintain the austenite phase.
Such high alloying can improve corrosion resistance by t, but the high temperature strength is higher than 18-8 thread stainless steel.
It is sufficient to maintain the 1il level, and in many cases it is 5il level.
As seen in the example of US310 steel, a decrease occurs. It is undeniable that such high alloying tends to lead to deterioration of weldability.

The inventors of the present invention have conducted extensive experiments and research to find a method for improving high-temperature strength and weldability under the condition of increasing the amount of Cr necessary for improving corrosion resistance, and as a result, the following results have been obtained. I gained knowledge.

■ Under Cr enrichment, N to maintain austenite λ・l-1
By using f:, the increase in 1'Ji amount is suppressed, and high temperature strength is improved by solid solution strengthening of N, and B-Nl
) can be added singly or in combination to strengthen the glaze precipitation of carbonitrides and improve high-temperature strength.

■ By adding A# and w, high temperature strength, ductility and toughness can be improved.

(2) Weldability is improved by controlling the levels of P and S as impurities to a low level under specific conditions, taking into consideration the amounts of each other (O) and (2) the amounts of B and Nb.

That is, the present invention is based on the above knowledge, and its gist is that C0,01~Q, 20%, Si
3% or less, 1VIn 10% or less, Cr20-30%,
1\1lO-30%, NO,04-0,30%,A
60.01~0.5%, +vl! 0.001-0.0
One side of Section 5 includes both sides, f30.00 as necessary.
Contains 1 to 0020 Hiro NbO, 01 to 1.0 or both, P as an impurity, 5 or Po, 020%
Below, 50.010% or less, and the following formula, 20(3P(@+40O5(%)+4N b(%)
It is an austenitic stainless steel that satisfies +150B (%)≦7.0, and the balance is Fe and unavoidable impurities. This invention steel has superior high-temperature strength to the current 18-8 series austenitic steel and high Cr-Ni series S [JS310 steel, and has weldability equivalent to or better than the 18-8 series mentioned above. Moreover, its corrosion resistance is equivalent to or better than that of 5US310 steel.

The reasons for limiting the ingredients in the present invention will be described below.

C: An effective component to ensure the tensile strength and creep rupture strength necessary for heat-resistant steel, and must contain 0.01% or more.
Even if the coefficient exceeds 0.20, unsolidified carbide remains in the solution treatment state, which has no effect on high-temperature strength, but rather on toughness after aging. NUN is an austenite-forming element similar to C, and is also an effective element for improving high-temperature strength, so 0.04% is required to achieve its effect. The above is necessary. But 0.30
If the percentage is listed above, a large amount of nitride or raw material will result in a toughness of 1 degree less than that after aging, so it was set at 0.04 to 0.30%.

Sl: It is an effective element as a deoxidizing agent and also for increasing oxidation resistance.If it exceeds 3%, weldability deteriorates and the structure becomes unstable, so it was set to 3% or less.

1νh1: It is effective in deoxidizing and improving processability, and is also useful in austenite formation, and it is possible to remove part of Ni by IV.
It can be replaced with In. Furthermore, although it is effective in improving high temperature strength, excessive addition causes deterioration of heat resistance properties, so it is necessary to limit it to 10% or less.

Cr: Shows great effects in improving high-temperature strength, oxidation resistance, and corrosion resistance, but if it is less than 20%, sufficient corrosion resistance cannot be obtained, and if it exceeds 30%, workability is insufficient and a stable complete austenite phase is formed. Therefore, in the present invention, it becomes difficult to obtain 2
Limited to 0-30 staff.

N1: This is an essential element for obtaining a stable austenite structure, and is determined from the relationship with the N content and Cri. In the present invention, it is suitably between 10 and 30.

A4: Is it a deoxidizing component or has an effect on high temperature strength, ductility, and toughness?
- The effect is saturated when it is necessary or 0.5% twice. It used to be 0.01-0.5%.

IVI! ? : O1 is a necessary element for deoxidation and workability [two good qualities, and also contributes to improving elongation and toughness; excessive addition may actually impair workability;
It was set as O○1 to 0.05. The effects of 〇 and +vIY above are most prominently exhibited by the addition of the additives.

I3: Is it an effective element for improving high-temperature strength properties through precipitation strengthening ν and grain boundary strengthening of carbides?
Is it effective at less than 0.001%? However, adding too much will cause deterioration of weldability.

Nl): The fine fractional precipitation of carbonitrides greatly contributes to the improvement of high-temperature strength, or if the N content is large, the amount of undissolved Nl) carbonitrides in the solution treatment state. Therefore, it is necessary to adjust the amount added according to the amount of N. Adding an excessive amount is not preferable from the viewpoint of weldability and high-temperature strength.

Previously, N1] was limited to 0.01 to 1.0%.

1), S general p and S levels are respectively 0.
020 to 0.030%, 0.005 to 0.015%, or in the case of a high Cr high alloy composition system such as the steel of the present invention, in the case of a normal level of P/S content in 2. Shows great susceptibility to hot cracking during welding. To ensure the same level of weldability as 18-8 series austenitic steel, at least Po
, 020chi or less, and 50.010% or less. Furthermore, it is desirable to suppress P to 0.010% or less and S to 0.005% or less. this is,
This is because a reduction in P and S contributes to not only weldability but also workability. Furthermore, the limit P/S level at which the same level of weldability as that of the 18-8 series can be achieved is 13, which deteriorates weldability, and changes in relation to the amount of Nb, so the following formula should be calculated within the range of adding Mf7a1) And is it necessary to suppress the S level?

206 P (%) + 40 OS (person in charge) + 4 N l
)(%)+15013(Part)≦7.0 Next, the present invention will be specifically explained with reference to Examples.

Table 1 shows the chemical composition of the sample materials. (A) to (Y) are comparative steels, and (1) to (A) are the present invention 711'i1.

These test materials can be obtained using the following procedure. That is,
After melting in the atmosphere at 25°C, forging and then cold rolling, both the comparison steel and the invention steel had a Nl additive-free steel of 1150'
The C1Nb-added steel (was subjected to solution treatment at 1200°C. The adjustment of the 1HAS level in the steel (was carried out by changing the blending ratio of the melting raw materials).

A creep rupture test at 700°C was conducted for each of the sample materials, and the rupture strength at 700°C for 13,000 hours was determined.

In addition, in order to evaluate the liquid properties, paleotrain test and T
A weld cracking test was conducted using IG welding. Palestrain test is a test in which a flat test piece is welded and separated without the use of a filler rod, and during welding, strain is applied along a jig with a fixed radius to artificially cause cracks in the welded part. The hot cracking susceptibility is evaluated based on the length of the crack. The test piece plate thickness was 8 mm, the welding conditions were Q200AX15V, welding speed 15cT + 〆min, and two rounds of A and J machining.

The weld cracking test by TIG welding uses a flat plate test piece with 60 grooves as shown in Figure 1.
Welding was performed without using a filler rod at V, 10 crn/min (welding speed) K, and the presence or absence of bead cracking was examined and evaluated.

The results are summarized in Table 2. Figure 2 also shows the results of multiple regression analysis of the weld metal crack length by the above-mentioned Palestrain test and the weld metal crack test # (hereinafter referred to as T
The results of the IG welding test are shown below.

In the figure, ○ and e marks indicate Cr type (20-30%), Δ mark indicates 1
8-8 series Tear, ○: No bead cracking in the TIG welding test, O2: There was bead cracking as well, respectively, and the suffix to the same description corresponds to the steel description in Table 1.

As is clear from Table 2 and Figure 2, in high Cr austenite l-tel such as the composition system of the present invention, the hot cracking susceptibility during welding is influenced by the amount of P, S, Nb and B, The yeast welded metal crack length in the test is
It can be sufficiently organized by the value parameter, and as the X value increases, the cracking susceptibility tends to increase almost linearly. In this case, if the X value exceeds 7.0, bead cracking will be observed in the TIG welding test. Even if the X value is 7.0 or less (within the range of the present invention), results with no bead cracking can be obtained in the TIG welding test, and the weld metal crack length in the Palestrain test is also lower than that of high Cr systems (with an X value of over 7.0). 20-30%Cr
) IN -0vD and CQM
Comparative steel C (corresponding to 31661111) can maintain cracking susceptibility comparable to that of λ0 (n).

Figure 3 is a plot diagram based on the test results in Table 2.
B amount and 700°cx3000h clip rupture strength (0,
05 intensity (ky'mA) is the value at the coordinates of the ○ mark plotted in the figure in the form of a subscript of the ○ mark. ○The markings inside the egg correspond to the steel symbols in Table 1. As is clear from this, no particular change in creep rupture strength was observed depending on the amount of P and B, and as shown in Table 2, all of the steels of the present invention (1) to (c) were equivalent to 5US316 steel (N ( 0) Steel, S
It shows significantly superior creep rupture strength compared to (Q) steel equivalent to US 304 steel and (Q) steel equivalent to 5 US 310 steel.

Furthermore, /! Test results are shown in FIGS. 4 and 5, which clarify the effects of A6 and Mg addition, which are one of the characteristics of the invention, on high strength, ductility, and inA property improvement. Figure 4 shows the results of the creep rupture test under a stress of 114 kg/m at 700'C.
The Charpy impact test results for the 00h aging stage are shown, and the numbers in the figures indicate the test steel or the steel with the same numbers as in Figure 1.

From both countries, it is clear that the addition of ACIV[P alone or in combination improves the fracture life, fracture ductility, and toughness after aging in any component system, and in particular, the improvement effect on ductility and toughness is remarkable. I understand. Also, A7
1. This effect of Mf becomes particularly remarkable in the case of combined addition of A6 and IVIS'.

The steel of the present invention has a high temperature strength far superior to that of the 18-8 austenitic steel and high Cr-Ni S[JS310 steel, and is equivalent to or higher than the 18-8 series mentioned above. It has excellent weldability, anti-corrosion properties equivalent to or better than 5US310 steel, and also has good ductility and toughness, making it particularly suitable for use in high-temperature equipment such as boilers and chemical plant equipment. It is extremely effective.

[Brief explanation of the drawing]

Figure 1 is a perspective view showing a TIG welding test piece, Figure 2 is a diagram showing the relationship between the X value of the sample steel and the Palestrain test and TIG welding test results, and Figure 3 is the p, A diagram showing s amount and creep rupture strength (700'C13000h),
Figure 4 shows the creep rupture test (700'C, σ-
Figure 6 is a diagram showing the results of the Charpy impact test of the sample steel after aging at 700'C for 11000 hours. Figure 1 Figure 2 Figure 5 (Llo) Figure 3

Claims (2)

[Claims] (1) C0.01-0.20%, Si3% or less, M
nlO% or less, Cr20-30%, Ni1O-30%,
NO, 04-0°30%, A710.01-0.5%
%1MP 0.001 to 0.05% or both, P and S as impurities are Po, 020% or more, and oio% or less, and the following formula: 2061)(%)+40O5( %)<7.0,
An austenitic steel with good weldability and high-temperature strength, characterized in that the remainder consists of Fe and unavoidable impurities. (2) C0.01-0.20%, Si3% or less, 1
vInlO% or less, Cr2O~30%, Ni1O-30
%, NO,04-0,30%, NO,01-0,5%
, rVIy 0.001-0.05%, and further contains 80.001-0.020%, Nb0.
01-1.0, and the I content as an impurity is l) 0.020% or less, 80.010%
Below, and the following formula, 206 P (%) + 40 OS (
%)+ 4 N L+(%)+ 150 B(%)<
An austenitic steel with good weldability and high-temperature strength, which satisfies 7.0 and is characterized by being composed of Fe and unavoidable impurities.