JP2696584B2 - Ferrite heat-resistant stainless steel with excellent low-temperature toughness, weldability and heat resistance - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a ferritic heat-resistant stainless steel used for exhaust gas purification materials of various internal combustion engines or various combustion devices.

[Background of the Invention and Prior Art]

In recent years, air pollution caused by gas emitted from automobiles or factories has become a major problem. For example NO _x exhaust gas from the viewpoint of prevention of pollution automobiles, HC, the amount of such CO have been regulated, recently there more stringent tends regulations in view of acid rain, improved exhaust gas purification efficiency Is needed.

On the other hand, in automobiles, in addition to improvement of purification efficiency, demands for higher output and higher performance of engines are increasing, and the temperature of exhaust gas tends to increase. Due to this background, the exhaust gas system components become extremely hot during operation, and mechanical stress fluctuations due to machine vibrations or external vibrations, or cooling and heating cycles depending on the operation pattern, and even in cold regions They will be exposed to extremely harsh conditions, such as undergoing temperature fluctuations due to the temperature drop in winter.

When heat-resistant steel such as stainless steel is used in these applications, it is obvious that it is excellent in heat resistance, but it requires welding work using either sheet material or pipe, so it is necessary to have excellent weldability. It is necessary to have excellent workability when processing them. Therefore, in these applications, it is important to simultaneously have heat resistance, low temperature toughness, weldability and workability.

Austenitic stainless steel represented by SUS304 is considered to be a promising material for the above applications because of its excellent workability and good weldability. However, since austenitic stainless steel has a large coefficient of thermal expansion, there is a concern about thermal fatigue fracture due to thermal stress generated during use in applications that undergo heating and cooling.
In addition, since austenitic stainless steel has a large thermal expansion difference from the surface oxide, the surface oxide is easily separated by heating and cooling. For this reason, in some applications
A Ni-based alloy typified by Inocnel 600 is used. This alloy material is a promising material because of its low coefficient of thermal expansion, excellent high-temperature oxidation resistance such as adhesion of surface oxides, and excellent high-temperature strength. It has not been used in general.

On the other hand, ferritic stainless steel is less expensive than austenitic stainless steel, and has excellent thermal fatigue characteristics due to its small coefficient of thermal expansion. Therefore, heating
It is considered to have excellent characteristics in an application that undergoes a temperature cycle of cooling. For this reason, ferritic stainless steels represented by Type 409 and SUS430 have begun to be used for some applications. However, since the strength of these materials is significantly reduced at 900 ° C or higher, there are problems such as high temperature fatigue failure due to insufficient strength and abnormal oxidation when exceeding the oxidation resistance limit. To solve these problems, it is possible to add various alloying elements that improve high-temperature strength and to improve oxidation resistance by increasing the amount of Cr. Generally, the impact toughness of steel is significantly deteriorated, and the weldability and workability are also significantly deteriorated.

As described above, at present, no material has been developed that can simultaneously satisfy various properties such as high-temperature strength, oxidation resistance, heat resistance, toughness, weldability, and workability. In order to cope with increasingly severe use conditions and environments with the progress of higher output and higher performance of internal combustion engines, it has heat resistance such as high temperature strength and thermal fatigue characteristics and high temperature oxidation, as well as manufacturability and workability. There is a demand for materials having excellent weldability and low-temperature toughness. If a ferritic stainless steel with excellent heat resistance and excellent manufacturability, workability, weldability and low-temperature toughness can be obtained, a material that is extremely promising for special applications as described above can be obtained. It is thought that it is possible.

[Object of the invention]

The present invention has excellent high-temperature strength and high-temperature oxidation resistance, improves low-temperature toughness which is a drawback of ferritic stainless steel, and also prevents weld hot cracking of welds, which is a problem in production and construction. The purpose is to develop a ferritic heat-resistant stainless steel.

[Configuration of the invention]

In the present invention, C: 0.03% or less, Si: 0.1 to 0.8%, Mn: 0.6 to 2.0%, S: 0.006% or less, Ni: 4% or less, Cr: 17.0 to 25.0%, Nb: 0.2% by weight. 0.8%, Mo: more than 1.0 to 4.5% Cu: 0.1 to 2.5%, N: 0.03% or less However, in the above range, the ratio of Mn% / S% is 200 or more, [Nb] = Nb% -8 % (C% + N%) These elements are contained so that [Nb] satisfies the relationship of 0.2 or more and Ni% + Cu% ga4 or less, with the balance being
Provide ferritic heat-resistant stainless steel with excellent low-temperature toughness, weldability and heat resistance consisting of Fe and inevitable impurities in production.

In addition, the present invention further provides the above steel with Al: 0.5% or less, Ti: 0.6% or less, V: 0.5% or less, Zr: 1.0% or less, W: 1.5% or less, B: 0.01% or less, REM: To provide ferritic heat-resistant stainless steel containing 0.1% or less of one or more types and having excellent low-temperature toughness, weldability and heat resistance.

[Detailed Description of the Invention]

The inventors of the present invention have conducted test and research in order to achieve the above object, and have obtained the following findings.

Fig. 1 shows the results of examining the effect of Mo and Cu on the high-temperature tensile strength, based on the basic composition of Fe-18% Cr-0.45% Nb from the viewpoint of high-temperature strength, which is an important property required for materials. It is a thing. As can be seen in the figure, the high-temperature strength is improved by adding more than 1% of Mo. In addition, the high-temperature strength is higher than that of Mo alone due to the composite addition of Mc-Cu. Therefore, it was found that composite addition of Mo and Cu is effective for materials that require high-temperature strength.

FIG. 2 shows the effect of Mn on the scale peeling resistance among the high-temperature oxidation characteristics, which is another important characteristic. The test is based on the basic composition of Fe-18% Cr-0.45% Nb.
Change the amount of Mn to 1 at 900 ° C and 1000 ° C in air.
The continuous oxidation was performed for 00 hours, and the scale peeling amount was examined. As a result, scale peeling was suppressed by adding Mn at 0.6% or more at any test temperature. Therefore, it was found that Mn raises the oxidation resistance limit of ferritic stainless steel.

Fig. 3 shows the basic composition of Fe-18% Cr-0.45% Nb, with the addition of an appropriate amount of Mo and Cu, which were effective in Fig. 1, and varied the Mn and S to reduce welding hot cracking. The effect of the Mn / S ratio was examined. In the welding hot cracking test, a 1.2 mm thick cold-annealed plate was prepared and processed into a 40 mm x 20 mm test piece.
TIG welding was performed in a state where tensile stress was applied in the longitudinal direction while holding both ends of the test piece, and the minimum strain at which cracking began to occur was defined as the critical strain, which was used as an index of the hot cracking susceptibility. As can be seen from FIG. 3, in the case of adding Mo—Cu, when Mn / S is 200 or more, the amount of critical strain increases, and the effect of improving weldability was observed. As a result, in order to improve the hot cracking of the weld, an appropriate amount of M
We have found that it is effective to add n.

Fig. 4 shows the characteristics of Fe-18 to determine the toughness of the product.
5 shows the results of a Charpy impact test conducted to examine the effects of Mo and Cu based on the basic composition of% Cr-0.45% Nb. Mo
It is the same as the conventionally known results that the impact value decreases with the addition of Cu, but a new finding was found that the toughness was improved by further adding Cu in combination. In particular, it was found that the impact value was sufficiently improved by adding Cu in combination even for steels with extremely low impact toughness, such as steel with 4% Mo. In addition, it was found that the composite toughness with Ni and Mo can improve the impact toughness at a low temperature, as shown in Examples below. This is a significant finding and is considered to be particularly effective for components exposed to low-temperature environments in winter, and it can be used under increasingly severe conditions expected in the future, leading to new applications of ferritic stainless steel. It is considered something.

Based on these findings, the present invention is to provide a ferrite stainless steel excellent in high-temperature strength, thermal fatigue properties and oxidation resistance, and excellent in weldability and low-temperature toughness and excellent in total balance.

The reasons for limiting the content of each chemical component value in the steel of the present invention will be described below.

C and N: C and N are generally important elements for increasing the high-temperature strength, but when the content is increased, the oxidation resistance, workability and toughness decrease. C and N are Nb
To reduce the effective Nb content in the ferrite phase. Therefore, C and N are desirably low, and each is set to 0.03% or less.

Si: Si is an element effective for improving oxidation resistance. However, if it is added excessively, the hardness increases and the workability and toughness are expected to decrease.

As shown in the above test results, Mn: Mn fixes S harmful to hot cracking in the form of MnS, removes S in the weld metal,
Let it decrease. Reduction of S itself is effective, but Mn / S ≧ 200
It was found that if the above relationship was satisfied, it was good. On the other hand, M
As described above, n is added in an amount of 0.6% or more in terms of scale peeling resistance, thereby improving the scale peeling resistance. Therefore, it is necessary that Mn be in the range of 0.6 to 2.0% and satisfy the relationship of Mn / S ≧ 200.

Since S: S is harmful to welding hot cracking as described above, it is desirable that S be as low as possible. However, the lower the S, the higher the manufacturing cost. In the steel of the present invention, S is 0.00
Even if it is allowed to be up to 6%, the effect of Mn has sufficient welding hot crack resistance as described above, so the upper limit of S is made 0.006%.

Ni: As can be seen from the examples, Ni has the same toughness improving effect as Cu. However, when added excessively, precipitation of austenite phase or the like occurs at high temperatures, and there is a concern that thermal fatigue characteristics may be reduced due to an increase in thermal expansion coefficient and the like. Therefore, it was found that Ni + Cu must satisfy the relationship of 4% or less in the composite addition with Cu, which is an austenite forming element. From this result, the upper limit was set to 4%.

Cr: Cr is an element indispensable for improving corrosion resistance and oxidation resistance. The lower limit is set to 17% because it is necessary to add 17% or more to maintain oxidation resistance of 900 ° C or more.
From the viewpoint of oxidation resistance, the higher the content of Cr, the better. However, if added in excess, the steel will become brittle and the workability will deteriorate due to the increase in hardness, so the upper limit is set to 25%.

Nb: Nb is an element necessary for maintaining high-temperature strength. In addition, it has a favorable effect on the improvement of workability and oxidation resistance, and on the pipe formability by high frequency welding. As can be seen from the results of the high-temperature tensile test shown in Table 2 below, it is necessary to add at least 0.2% in order to improve the high-temperature strength. However, since Nb forms a compound of C and N, even if the lower limit is simply set to 0.2%, the amount of C and N decreases the dissolved Nb, and the effect of Nb on the high temperature strength decreases. Therefore, it is necessary to satisfy the relationship that [Nb] according to the equation [Nb] = Nb% -8 (C% + N%) is 0.2% or more. On the other hand, when Nb is added excessively, the susceptibility to welding hot cracking increases. The upper limit of Nb is set to 0.8% so as to maintain sufficient high-temperature strength and not significantly affect the hot cracking susceptibility.

Mo: Mo increases the high-temperature strength as added as described in the above test results. It is also effective in improving high-temperature oxidation resistance and corrosion resistance. On the other hand, if it is added excessively, the toughness at low temperature is remarkably reduced, and the manufacturability and workability are lowered.

Cu: Cu is also a very effective element in terms of toughness as described in the above test results, and is an important element of the steel of the present invention. Since the effect of improving toughness requires 0.1% or more as shown in FIG. 4, the lower limit is set to 0.1%. On the other hand, if added excessively, it becomes hard and impairs workability. In addition, the upper limit is set to 2.5% because it has a significant adverse effect on hot workability.

Al: Al improves high temperature oxidation resistance. However, if added excessively, it will cause problems in manufacturability and weldability, so the upper limit is 0.5%
And

Ti: Ti increases high-temperature strength and improves workability. However, as in the case of Al, an excessive addition causes problems in manufacturability and weldability, so the upper limit is made 0.5%.

V: V, like Ti, also increases the high-temperature strength and improves workability. However, when added in excess, the strength is adversely reduced.
Therefore, the upper limit is set to 0.5%.

Zr: Zr increases high-temperature strength and improves high-temperature oxidation characteristics. However, if added in excess, the strength is reduced, so the upper limit is made 1.0%.

W: W, like Ti and V, increases high-temperature strength and improves workability. However, if added excessively, the strength is reduced, so the upper limit is made 1.5%.

B: B improves hot workability, increases high-temperature strength, and also improves workability. However, an excessive addition causes a reduction in hot workability, so the upper limit is made 0.01%.

REM: Rare earth element improves the hot workability by adding a trace amount, and improves oxidation resistance, especially scale adhesion. However, an excessive addition will adversely affect hot workability. Therefore, the upper limit is set to 0.1%.

〔Example〕

Table 1 shows the chemical component values of the test materials. M1 to M21 are steels of the present invention, and M22 to M30 are comparative steels. In the laboratory, 30kg ingots of these steels were prepared and forged into 25mmφ round bars and 25mm thick plates. After annealing at 950 ℃ ~ 1100 ℃ in round bar, it was processed into JIS standard high temperature tensile test piece. After cutting, the forged plate is subjected to hot rolling by extraction at 1200 ° C, hot rolling of 5 mm ^t , 950 ° C to 1100 ° C
After annealing in, a part was processed into a Charpy impact test specimen as it was. The remaining part was repeatedly cold rolled and annealed, and a high temperature oxidation test was performed at a thickness of 2 mm _{t, and} a hot crack test was performed at a thickness of 1.2 mm ^t .

Table 2 shows the high-temperature tensile strength obtained by the high-temperature tensile test performed in accordance with the JIS standard, the amount of scale peeling obtained by continuous oxidation tests at 900 ° C and 100 ° C for 100 hours, and the criticality at the time of welding obtained by the hot crack test described in the text. The results of the Charpy impact test performed on a V-notch Charpy impact test specimen with a strain amount and a plate thickness of 4.5 mm _t are shown.

The results in Table 2 show that the addition of Nb, Mo, and Ni increases the high-temperature strength. In addition, the high-temperature strength of the composite steel containing Mo and Cu further increases.
The results of the continuous high-temperature oxidation test show that when the Mn content exceeds 0.6% at both 900 ° C. and 1000 ° C., the scale peeling resistance is significantly improved. It can also be seen that the critical strain in the welding hot cracking test is significantly improved when Mn / S exceeds 200. On the other hand, the Charpy impact test results show that although the impact toughness decreases with the addition of Mo, the addition of Cu improves the toughness, and the addition of Ni has the same effect.

[Effects] As described above, according to the present invention, a ferritic heat-resistant material having excellent high-temperature strength and high-temperature oxidation resistance, excellent resistance to high-temperature cracking at welding, and improved low-temperature toughness, which is a disadvantage of ferritic stainless steel. This is a stainless steel for industrial use, and it can make a great contribution as an exhaust gas system material that can respond to the high output and high performance of internal combustion engines in the future.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amount of Mo and high-temperature tensile strength showing an example of the results of a high-temperature tensile test which led to the present invention, and FIG. Fig. 3 shows the relationship between Mn / S and critical strain, showing an example of the results of welding hot cracking test. Fig. 4 shows an example of the results of Charpy impact test.
FIG. 4 is a relationship diagram between a Cu amount and a Charpy impact test value.

Claims (2)

(57) [Claims] C .: 0.03% or less, Si: 0.1 to 0.8%, Mn: 0.6 to 2.0%, S: 0.006% or less, Ni: 4% or less, Cr: 17.0 to 25.0%, Nb: 0.2 to 0.8%, Mo: more than 1.0 to 4.5%, Cu: 0.1 to 2.5%, N; 0.03% or less, provided that the ratio of Mn% / S% is 200 or more, [Nb] = Nb% According to the formula of -8% (C% + N%), these elements are contained so that [Nb] satisfies the relation of 0.2 or more and Ni% + Cu% satisfies the relation of 4 or less, and the balance is
Ferritic heat-resistant stainless steel with excellent low-temperature toughness, weldability and heat resistance consisting of Fe and inevitable impurities in production. 2. In% by weight, C: 0.03% or less, Si: 0.1 to 0.8%, Mn: 0.6 to 2.0%, S: 0.006% or less, Ni: 4% or less, Cr: 17.0 to 25.0%, Nb: 0.2 to 0.8%, Mo: over 1.0 to 4.5%, Cu: 0.1 to 2.5%, N: 0.03% or less, and Al: 0.5% or less, Ti: 0.6% or less, V: 0.5% or less, Zr : 1.0% or less, W: 1.5% or less, B: 0.01% or less, REM: 0.1% or less, and in the above range, the ratio of Mn% / S% is 200 or more, [ According to the formula of [Nb] / Nb% -8 (C% + N%), these elements are contained so that [Nb] satisfies the relationship of 0.2 or more, and Ni% + Cu% satisfies the relationship of 4 or less.
Ferritic heat-resistant stainless steel with excellent low-temperature toughness, weldability and heat resistance consisting of Fe and inevitable impurities in production.