JPH03274245A - Ferritic heat-resistant stainless steel excellent in low temperature toughness, weldability and heat resistance - Google Patents

Abstract

PURPOSE:To improve the low temp. toughness of a stainless steel and to prevent the weld high temp. cracks in a weld zone by compositely adding Mo and Cu to a steel and specifying the Mn/S ratio, Nb content or the like therein. CONSTITUTION:The compsn. of a ferritic heat-resistance stainless steel is formed of, by weight, <=0.03% C, 0.1 to 0.8% Si, 0.6 to 2.0% Mn, <=0.006% S, <=4% Ni, 17.0 to 25.0% Cr, 0.2 to 0.8% Nb, 1.0 to 4.5% Mo, 0.1 to 2.5% Cu, <=0.003% N and the balance Fe with inevitable impurites. In this compositional range, the Mn%/S% ratio is regulated to >=200, the content of Nb in accordance with a formula [Nb]=Nb%-8(C%+N%) is regulated to >=0.2 and the relationship of <=4 Ni%+Cu% is satisfied. If required, optimum amounts of one or more kinds among Al, Ti, V, Zr, W, B and rare earth elements are incorporated therein. This steel can be used for an exhaust gas purifying material in an internal combustion engine, a burning appliance or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a ferritic heat-resistant stainless steel used for exhaust gas cleaning materials of various internal combustion engines or various combustion equipment.

[Background of the invention and prior art]

In recent years, air pollution from gases emitted from automobiles and factories has become a major problem.For example, the amount of N08HC, Go, etc. in automobile exhaust gas has been regulated from the perspective of pollution prevention, but recently, acid rain, etc. As regulations tend to become more stringent in this regard, it has become necessary to improve exhaust gas purification efficiency.

On the other hand, in automobiles, in addition to improving purification efficiency, demands for higher engine output or improved performance are increasing, and exhaust gas temperatures tend to rise. Due to this background, exhaust gas system members become extremely hot during operation, and are also subject to mechanical stress fluctuations due to machine vibrations and external vibrations, cooling/heating cycles depending on the operating pattern, and even in cold regions. They will be exposed to extremely harsh conditions, including temperature fluctuations caused by the drop in winter temperatures.

When heat-resistant steel such as stainless steel is used for these purposes, it is a thermal theory that it has excellent heat resistance, but since welding is required regardless of whether plate material or vibrator is used, it is necessary to have excellent weldability. It is necessary to have excellent workability during processing. Therefore, in these applications, it is important to simultaneously improve heat resistance, low-temperature toughness, weldability, and workability.

Austenitic stainless steel, represented by Sυ5304, has excellent workability and good weldability, and is therefore considered to be a promising material for the above uses. However, since austenitic stainless steel has a large coefficient of thermal expansion, there is a concern about thermal fatigue failure due to thermal stress generated during use in applications where it is subjected to heating and cooling. In addition, since austenitic stainless steel has a large difference in thermal expansion from the surface oxide, the surface oxide tends to peel off when heated and cooled. alloy is used. This alloy material is heat WM17! It is a promising material because it has a low coefficient of oxidation, excellent high-temperature oxidation resistance such as surface oxide adhesion, and excellent high-temperature strength, but it is an extremely expensive material so it is not widely used. This has not yet been achieved.

On the other hand, 1 ferritic stainless steel is cheaper than austenitic stainless steel, and has a small coefficient of thermal expansion, so it has excellent thermal fatigue properties.

Therefore, it is considered to have excellent characteristics in applications that undergo heating-cooling temperature cycles. Therefore, for some applications, Type 409 and 5I
Ferritic stainless steel, typified by IS430, is beginning to be used. However, these materials are
If the temperature exceeds the oxidation resistance limit, the strength will drop significantly, causing problems such as high-temperature fatigue failure due to the lower level of 3-strength strength, and abnormal oxidation occurring if the oxidation resistance limit is exceeded. To address 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, but the addition of such alloying elements and increasing the amount of Cr are generally The impact toughness of the steel is significantly deteriorated, and the weldability and workability are also significantly deteriorated.

As mentioned above, currently it has high temperature strength and oxidation resistance.

No material has yet appeared that can simultaneously satisfy multiple properties such as heat resistance, toughness, weldability, and workability.

In order to respond to usage conditions and environments that will become increasingly severe as the efficiency of exhaust gas purification increases and the output and performance of internal combustion engines increases.

There is a demand for materials that have high-temperature strength, thermal fatigue properties, and high-temperature oxidation resistance, as well as excellent manufacturability, workability, weldability, and low-temperature toughness. If a ferritic stainless steel with excellent heat resistance and excellent manufacturability, workability, weldability, and low-temperature toughness could be obtained, it would be possible to obtain a material that is extremely promising for the special uses mentioned above. It is thought that this can be done.

[Purpose 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 in welded parts, which is a problem in manufacturing and construction. The aim is to develop a heat-resistant ferritic stainless steel.

[Structure of the invention] The invention is at 2% by weight.

C: 0.03% or less.

Sl: 0.1-0.8%.

Mn: 0.6-2.0% S: 0.006% or less.

Ni+4% or less.

Cr: 17.0-25.0%.

Nb: 0.2-0.8%.

Mo: 1.0-4.5%.

Cu: 0.1-2.5%.

N: 0.03% or less.

However, within the above range.

The ratio of Mn%/S% is 200 or more.

[Nb] is 0.2 or more according to the formula [Nb] - Nb% - 8 (C% - N%), and Ni% + Cu
These elements are contained so that % satisfies the relationship of 4 or less.

Provided is a ferritic heat-resistant stainless steel having excellent low-temperature toughness, weldability, and heat resistance, the balance of which is Fe and impurities unavoidable during manufacturing.

In addition, one aspect of the present invention further provides the above-mentioned steel. AIl, 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: Provides a ferritic heat-resistant stainless steel containing one or more of 0.1% or less and having excellent low-temperature toughness, weldability, and heat resistance.

(The presenters who explained the invention in detail conducted repeated tests and studies to achieve the above-mentioned objective, and were able to obtain the following knowledge.

Figure 1 shows the results of investigating the effects of MO and Cu on high-temperature tensile strength using Fe-18%Cr-0.45%Nb as the basic composition from the viewpoint of high-temperature strength, which is an important property required for materials. This is what is shown. As seen in the same figure,
High temperature strength is improved by adding 1% or more of Mo. Furthermore, the combined addition of 2M◇-Cu increases the high-temperature strength compared to the single addition of Mo. Therefore, it has been found that combined addition of MO and Cu is effective for materials requiring high-temperature strength.

FIG. 2 shows an investigation of the influence of Mfl on scale peeling resistance, which is another important property, high temperature oxidation property. In the test, the basic composition was Fe48%Cr-0.45%Nb, the amount of Mn was varied, and continuous oxidation was carried out for 100 hours at 900"C and 1000°C in the air, and the amount of scale peeled off was investigated.Results At all test temperatures, scale peeling was suppressed by adding 0.6% or more of Mn.

Therefore, it was found that Mn increases the oxidation resistance limit of ferritic stainless steel.

Figure 3 shows Fe-18%Cr-0,45%Nb as the basic rule and an appropriate amount of Mo, which was found to be effective in Figure 1.
The effect of the Mn/S ratio on weld hot cracking was investigated by adding a combination of Mn and Cu and varying Mn and S. In the welding hot cracking test, a 1.2s-thick cold-rolled annealed plate was processed into a 40m x 200m test piece.
TIG welding was performed with both ends of the specimen held and tensile stress applied in the longitudinal direction, and the minimum strain at which two cracks began to occur was defined as the critical strain, and this was used as an index of weld hot cracking susceptibility. As seen in Figure 3, Mo-Cu
In the case of composite addition, when Mn/S is 200 or more, the critical strain increases and weldability is improved. As a result, in order to improve weld hot cracking, Mn/S must be 2
It has been found that it is effective to add an appropriate amount of Mn that is 0.00 or more.

Figure 4 shows Fe-1 in order to understand the toughness of the product.
The basic composition is 8%Cr-0,45%Nb, Mo and C
These are the results of a Charpy impact test conducted to investigate the influence of u. It is the same as the previously known result that the impact value decreases when Mo is added, but in addition, the impact value decreases when Mo is added.
We were able to obtain new knowledge that toughness can be improved by adding . Among them, even steels with extremely low impact toughness, such as steel with 4% Mo added, can be sufficiently improved by adding CII in combination! It was found that the value was improved. Furthermore, as shown in the Examples below, it was found that the impact toughness at low temperatures can be improved by adding Ni and Mo in combination. This is an important finding, and is thought to be particularly effective for components exposed to low-temperature environments during the winter.
It will be possible to use it under increasingly harsh conditions expected in the future, and is expected to lead to new and expanded applications for ferritic stainless steel.

Based on these findings, the present invention provides an austenitic stainless steel with a good total balance, which has excellent high-temperature strength, thermal fatigue properties, and oxidation resistance, as well as excellent weldability and low-temperature toughness.

The reason 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 high-temperature strength, but on the other hand, when their content increases, oxidation resistance, workability, and toughness decrease. N
It forms a compound with b and reduces the effective amount of Nb in the ferrite phase. Therefore, it is desirable that C(!:N) be low, and each should be 0.03% or less.

Si: Si is an effective element for improving oxidation resistance. but,! ! If added in excess, hardness increases and workability and @ properties are expected to decrease, so the content is set in the range of 0.1 to 0.8%.

Mn: As shown in the above test results, Mn fixes S, which is harmful to weld hot cracking, in the form of MnS, thereby removing and reducing S in the weld metal. Reducing S itself is also effective, but Mn
It was found that the condition is good if the relationship of /S≧200 is satisfied. On the other hand, as mentioned above, the scale peeling resistance is improved by adding 0.6% or more of Me.

Therefore, it is necessary that Mn be in the range of 0.6 to 2.0% and satisfy the relationship of Mn/S≧200.

As mentioned above, SO3 is harmful to weld hot cracking, so it is desirable to keep it as low as possible, but the lower it is, the higher the manufacturing cost will be. In the steel of the present invention, S is 0
．． Even if it is allowed up to 0.006%, as mentioned above, the action of Mn will provide sufficient resistance to welding hot cracking, so the upper limit of S should be set at 0.006%.
006%.

Ni: As can be seen from the examples, Ni has the same toughness improvement effect as Cu, but when added in excess, austenite phase precipitation occurs at high temperatures, leading to a decrease in thermal fatigue properties due to an increase in the coefficient of thermal expansion, etc. etc! Q: I am reminded of this. For this reason, it has been found that in the composite addition with Cu, which is an austenite forming element, it is necessary to satisfy the relationship of Ni+Cu within 4%. Based on this result, the upper limit is 4%
And so.

Cr: Cr is an essential element for improving corrosion resistance and oxidation resistance. The lower limit was set at 17% because it is necessary to add 17% or more in order to maintain oxidation resistance at temperatures of 900° C. or higher. From the viewpoint of oxidation resistance, the higher the Cr content, the more preferable it is, but adding too much Cr causes embrittlement of the steel and also deteriorates workability due to increased hardness, so the upper limit is set at 25%.

Nb: Nb is an element necessary to maintain high temperature strength. It also has a positive effect on the improvement of workability and oxidation resistance, as well as on the pipe formability by high-frequency welding.As can be seen from the high temperature tensile test results in Table 2 below, in order to improve high temperature strength % needs to be added. However, Nb
creates a compound with C and N, so simply set the lower limit to 0.
Even at 2%, solid Nb decreases depending on the amount of C and N, and the effect of Nb on high temperature strength decreases. Therefore [N
It is necessary to satisfy the relationship that [Nb] is 0.2% or more according to the formula: b]-Nb%-8 (C%-N%). On the other hand, excessive addition of Nb increases the weld hot cracking susceptibility. The upper limit of Nb is set to 0.8% in order to maintain sufficient high-temperature strength and not to significantly affect weld hot cracking susceptibility.

Mo: As mentioned in the above test results, the more Mo is added, the higher the high temperature strength is. It is also effective in improving high temperature oxidation resistance and corrosion resistance. On the other hand, if added in excess, the toughness at low temperatures will be significantly lowered, and the manufacturability and workability will also be lowered, so the content was set at 1.0 to 4.5%.

Cu: As mentioned in the above test results, Cu is also a very effective element in terms of toughness and is an important element in the steel of the present invention. Since 0.1% or more is required to obtain the effect of improving toughness as shown in FIG. 4, the lower limit was set at 0.1%. On the other hand, if added in excess, it becomes hard and impairs workability. Furthermore, since it has a significant negative effect on hot workability, the upper limit is set at 2.5%.

AI! AI improves high temperature oxidation resistance. However, adding too much will cause problems in manufacturability and weldability, so the upper limit should be set at 0.
5%.

Ti: Ti increases high temperature strength and also improves workability.

However, like AI, adding too much causes problems in manufacturability and weldability, so the upper limit is set at 0.5%.

■: V also increases high-temperature strength on the same level as Ti and improves workability. However, adding too much leads to a decrease in strength, so the upper limit is set at 0.5%.

Zr: Zr increases high temperature strength and improves high temperature oxidation properties. However, adding too much leads to a decrease in strength, so the upper limit is set at 1.0%.

Like Ti and V, WOW also increases high temperature strength and improves workability. However, adding too much leads to a decrease in strength, so the upper limit is set at 1.5%.

BIB improves hot workability, increases high temperature strength, and also improves workability. However, since adding too much leads to a decrease in hot workability, the upper limit is set at 0.01%.

REM: Rare earth elements improve hot workability and improve oxidation resistance, especially scale adhesion, by adding a small amount of rare earth elements. However, since adding too much leads to a decrease in hot workability, the upper limit is set at 0.1%.

[Example] Table 1 shows the chemical composition values of the test materials. M1 to M21 are steels of the present invention, and 1M22 to M30 are comparative steels.

These steels were made into 30 kg 1i blocks in the laboratory, and 2
Forged into a 5 φ round bar and a 251 thick plate. Round bar is 950
After annealing at 1700°C to 1700°C, it was processed into JIS standard high temperature tensile test pieces. 12 After cutting, the plate was hot-rolled by extraction at 1200°C to make a 5mm' hot-rolled plate.95
After annealing at 0°C to 1700°C, some of the specimens were processed into Charpy impact test pieces as they were. The remaining part was repeatedly cold rolled and annealed.

A high-temperature oxidation test was conducted with the plate thickness of 2-1. :2m
A welding hot cracking test was conducted with a plate thickness of s+'.

Table 2 shows the high temperature tensile strength, 10
Amount of scale peeling due to continuous oxidation test for 0 hours 9 Amount of cracking field strain during welding according to the weld hot cracking test described in the main text, and Charpy tli impact conducted on a V-notch Charpy 1ji impact test piece with a plate thickness of 4.5 m The test results were shown.

From the results in Table 2, it can be seen that the high temperature strength is increased by adding Nb, Mo and Ni. In addition, steels with composite additions of Mo and Cu show a further increase in high-temperature strength. The continuous high temperature oxidation test results show that the scale flaking resistance is significantly improved at both 900'C and 1000C when the Mn content exceeds 0.6%. Furthermore, it can be seen that the amount of field strain in the fa weld hot cracking test is significantly improved when Mn/S exceeds 200. On the other hand, the results of the Charpy impact test show that although the impact toughness decreases as Mo is added, the toughness is improved by adding Cu, and the same effect is obtained by adding Ni.

〔effect〕

As described above, according to the present invention, a ferritic heat-resistant stainless steel has excellent high-temperature strength and high-temperature oxidation resistance, and has excellent welding hot cracking resistance, as well as improved low-temperature toughness, which is a drawback of ferritic stainless steel. The obtained material can make a great contribution as an exhaust gas system material that can respond to future increases in output and performance of internal combustion engines.

[Brief explanation of drawings]

Figure 1 is a relationship diagram between the amount of Mo and high temperature tensile strength, showing an example of the high temperature tensile test results that led to the present invention. Figure 2 is a relationship diagram between the amount of Mn and the amount of scale exfoliation, showing an example of the results of a high temperature oxidation test. ,
Figure 3 is a diagram showing the relationship between Mn'/S and the amount of direct rise strain, showing an example of the results of a welding hot cracking test. Figure 4 is a diagram showing the relationship between Cu content and Charpy impact test value, showing an example of the results of the Charpy impact test. . Person in charge Nissin Steel Co., Ltd. Figure 1 Figure 2 Figure 4 Figure 4 (%) Figure procedure amendment (voluntary) April 8, 1991

Claims (2)

[Claims] (1) In 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: 17.0-25.0%, Nb: 0.2-0.8%, Mo: 1.0-4.5%, Cu: 0.1-2.5%, N: 0.03 % or less, but within the above range, the ratio of Mn%/S% is 200 or more, [Nb] is 0.2 or more according to the formula [Nb] = Nb% - 8 (C% + N%), and Ni% Low-temperature toughness, containing these elements so that +Cu% satisfies the relationship of 4 or less, with the remainder being Fe and unavoidable impurities during manufacturing;
Ferritic heat-resistant stainless steel with excellent weldability and heat resistance. (2) In 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: 17.0-25.0%, Nb: 0.2-0.8%, Mo: 1.0-4.5%, Cu: 0.1-2.5%, 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 addition, in the above range, the ratio of Mn%/S% is 200 or more, [Nb] = Nb%-8 (C % + N%) Contains these elements so that [Nb] satisfies the relationship of 0.2 or more and Ni% + Cu% of 4 or less according to the formula, and the remainder consists of Fe and unavoidable impurities during manufacturing. low temperature toughness,
Ferritic heat-resistant stainless steel with excellent weldability and heat resistance.