KR0180206B1 - Heat resistant ferritic stainless steel excellent in low-temperature, weldability & heat resistance - Google Patents

Abstract

C: 0.03% or less, Si: 0.1 ~ 0.8%, Mn for improving the low temperature toughness of ferritic stainless steel and preventing welding high temperature cracks to be used as a material for the exhaust path of automobiles, especially the path between the engine and the converter. : 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: 1.0 to 4.5%, Cu: 0.1 to 2.5%, N: 0.03 % Or less, and in some cases, one or two or more of Al, Ti, V, Zr, W, B, and REM, and then the ratio of Mn% / S% is 200 or more, [Nb] = Nb% [Nb] according to the formula of -8 (C% + N%) containing these elements so as to satisfy the relationship of 0.2 or more and Ni% + Cu% of 4 or less, and the remaining part is composed of Fe and unavoidable impurities It provides ferritic stainless steel with excellent low temperature toughness, weldability and heat resistance.

Description

[Name of invention]

Ferritic stainless steel with excellent low temperature toughness, weldability and heat resistance

[Technical Field]

[Industrial use]

The present invention relates to a ferritic heat resistant stainless steel having excellent low temperature toughness, weldability and heat resistance. Specifically, the present invention relates to a part of an exhaust gas path of an automobile engine, particularly a heat resistant stainless steel suitable for constituting a path of a high temperature part from an engine to a converter. It's about the river.

Background of the Invention and Prior Art

In recent years, air pollution caused by automobile exhaust gas has become a big problem, and the amount of NOx, HC, CO, etc. in exhaust gas is regulated from the viewpoint of pollution prevention. Tend to be. Therefore, it has become necessary to further improve the purification efficiency of exhaust gas.

On the other hand, in addition to the improvement of the purification efficiency, the exhaust gas temperature of automobiles tends to increase even when the demand for higher engine power or performance is increased. From this background, the members of the exhaust gas system are exposed to higher temperatures during operation. In particular, a member between the engine and the converter of the exhaust gas purifying apparatus, such as an exhaust manifold or a double tube, cannot be avoided to become higher.

Such a member is subjected to mechanical stress fluctuations in response to vibrations caused by driving and running of the engine, and also to a large temperature, such as a heating / cooling cycle depending on an operation pattern or, in some cases, cooling to a low temperature in a cold district. They will be exposed under very harsh conditions, both mechanically and thermally, such as fluctuations.

When heat-resistant steel such as stainless steel is applied to these applications, it is not only excellent in heat resistance but also such a member has a welded joint (in a pipe, it is common to make a pipe by welding, and the connection with another member is also constructed by welding. In many cases, it is necessary to be excellent in weldability, and also to be excellent in workability when processing them.

Therefore, besides the original corrosion resistance function called stainless steel, it is an important subject to combine heat resistance, low temperature toughness, weldability, and workability simultaneously in such applications.

Austenitic stainless steels, represented by SUS 304, are considered to be promising materials for such applications because of their excellent workability and good weldability.

However, because austenitic stainless steels have a large coefficient of thermal expansion, thermal fatigue due to thermal stress generated during use is feared in applications such as heating and cooling. In addition, the austenitic stainless steel has a large thermal expansion difference with the surface oxide, so that the surface oxide is easily peeled off by heating and cooling. From this fact, Ni-based alloys, represented by Inconel 600, have been applied to part of the vehicle exhaust path.

This alloy material is a promising material because it has a low coefficient of thermal expansion, is excellent in high temperature oxidation characteristics such as adhesion of surface oxides, and has excellent high temperature strength. However, because it is an extremely expensive material, it is not widely used.

On the other hand, ferritic strain steel is cheaper than austenitic stainless steel. In addition, since the thermal expansion coefficient is small, the thermal fatigue property is excellent, so it is considered to be suitable for applications such as receiving a heat-cooling temperature cycle.

For this reason, ferritic stainless steels, such as Type 409 or SUS 430, are being used for a part of the vehicle exhaust path.

However, these materials have a property that the strength is significantly lowered at 900 ° C. or higher, which causes problems such as high temperature fatigue failure due to lack of strength, and problems of abnormal oxidation when exceeding the oxidation resistance limit.

To this problem, it is possible to add various alloying elements to improve the high temperature strength, and to improve the oxidation resistance by increasing the amount of Cr, but the addition of such alloying elements and the increase of the amount of Cr generally affect the impact of steel. The toughness is significantly degraded, and the weldability and workability are remarkably inferior.

In this way, stainless steel materials suitable for the stringent conditions, such as improvement of exhaust gas purification efficiency, high engine output and high performance, that is, high temperature strength, oxidation resistance, heat resistance, toughness, weldability and workability, A variety of properties can be satisfied at the same time, and economical materials are not present in both the austenitic and ferritic systems.

Extremely promising materials for such special applications, if ferritic stainless steels with excellent heat resistance and high temperature strength, and excellent manufacturability, weldability and low temperature toughness can be obtained while utilizing the above-described characteristics of ferritic stainless steels. Can be obtained.

Japanese Patent Laid-Open No. 64-8254 discloses a ferritic stainless steel for such a purpose, but is unknown about low temperature toughness.

On the other hand, Japanese Unexamined Patent Publications No. 59-52226 and 61-44121 disclose ferrite-based stainless steels which have improved rust generation resistance and acid resistance to chlorine ions by drastically reducing S on Cu and Ni. However, high temperature strength, heat resistance, weldability, low temperature toughness, etc. are not taught at all.

[Purpose of invention]

It is therefore an object of the present invention to obtain ferritic stainless steels simultaneously satisfying the above-mentioned severe properties required by the members of the vehicle exhaust path, in particular the members exposed to high temperatures between the engine and the converter, in particular The purpose of the present invention is to obtain a ferritic heat resistant stainless steel that can improve low temperature toughness, which is an inherent drawback of ferritic stainless steel, and can also prevent welding high temperature cracks in welding parts, which is a manufacturing and construction problem.

[Description of the Invention]

The present invention is in weight percent

C: 0.03% or less,

Si: 0.1 to 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, in the above-mentioned range, the ratio of Mn% / S% is 200 or more, [Nb] according to the formula of [Nb] = Nb% -8 (C% + N%) is 0.2 or more, and Ni% + Cu% A ferritic heat-resistant stainless steel containing these elements so as to satisfy the relationship of 4 or less, and the remaining portions are made of Fe and unavoidable impurities in manufacturing, and excellent in low temperature toughness, weldability and heat resistance.

In addition, the present invention is in addition to the aforementioned steel

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: Provides ferritic heat-resistant stainless steel excellent in low temperature toughness, weldability and heat resistance containing one or two or more kinds of 0.1% or less.

[Brief Description of Drawings]

1 is a relationship between the Mo content and the high temperature tensile strength showing an example of the results of the high temperature tensile test to complete the present invention.

2 is a relationship diagram between the amount of Mn and the amount of scale peeling showing an example of the results of the high temperature oxidation test.

3 is a relationship between Mn / s and the critical strain amount showing an example of the weld hot crack test results.

4 is a relationship between the amount of Cu and the Charpy impact test value showing an example of the Charpy impact test results.

Detailed description of the invention

The inventors of the present invention were able to obtain the following insights by repeating the test research to achieve the above object.

FIG. 1 shows the results of investigating the effects of Mo and Cu on the high temperature tensile strength at the indicated temperature using Fe-18% Cr-0.45% Nb as a basic composition from the viewpoint of high temperature strength, which is an important characteristic required for materials. As can be seen from this figure, the high temperature strength is improved by adding 1% or more of Mo. The high temperature strength is higher than that of Mo alone alone due to the complex addition of Mo-Cu. Therefore, the finding that composite addition of Mo and Cu is effective as a material requiring high temperature strength is obtained.

FIG. 2 examines the effect of Mn on the scale peeling resistance among high temperature oxidation characteristics, which is another important characteristic. In the test, the amount of Mn was changed based on Fe-18% Cr-0.45% Nb, and the amount of Mn was changed, and the scale peeled amount was investigated by performing 100 hours of continuous oxidation at 900 degreeC and 1000 degreeC in air | atmosphere. As a result, scale peeling was suppressed by adding 0.6% or more of Mn at any test temperature. Therefore, Mn has gained the knowledge that it raises the oxidation resistance limit of ferritic stainless steel.

FIG. 3 shows Fe-18% Cr-0.45% Nb as the basic composition, and an appropriate amount of Mo and Cu compounded in FIG. 1 is added, and then Mn / S is changed to Mn / The effect of the S ratio is investigated. The welding hot crack test was performed by making a 1.2 mm thick cold rolled annealing plate, processing it into a 40 mm x 200 mm test piece, and then applying TIG welding while applying tensile stress in the longitudinal direction while maintaining both ends of the test piece. The minimum amount of deformation at which cracking starts to occur was set as the critical amount of deformation, and this was used as an index of weld hot cracking susceptibility. As shown in FIG. 3, in the case of Mo-Cu composite addition, when the Mn / S is 200 or more, the critical deformation amount is increased to improve the weldability.

As a result, in order to improve the welding high temperature crack, the knowledge that it is effective to add the appropriate amount of Mn which becomes Mn / S 200 or more was obtained.

FIG. 4 shows the results of Charpy impact test for Fe-18% Cr-0.45% Nb as a basic composition to investigate the toughness as a product, and to investigate the effect of Mo and Cu. The effect of lowering the impact value when Mo is added is the same as the conventionally known result. However, new insight into the toughness is improved by the addition of complex Cu again. Among them, it was found that even if the impact toughness, such as 4% Mo-added steel, was significantly low, the impact value was sufficiently improved by adding Cu compositely. In addition, it was found that the toughness at low temperature can be improved by adding Ni and Mo as shown in Examples described later.

This fact is of great insight and is particularly useful for members exposed under synchronous low-temperature environments (engine-directed members that are subject to mechanical vibrations at low temperatures, such as engine startup in low-temperature environments, specifically manifolds or double-tubing). This makes it possible to use even in the more stringent conditions expected in the future.

Based on the findings of the present invention, the present invention provides a ferritic stainless steel having a good overall balance with excellent high temperature strength, thermal fatigue characteristics and oxidation resistance, and excellent weldability and low temperature toughness.

Next, the reason which limited content of each chemical component value in the steel of this invention is demonstrated.

C and N: C and N are generally important elements for increasing the high temperature strength, while higher contents lead to deterioration of oxidation resistance, workability and toughness. In addition, C and N form a compound with Nb and reduce the amount of effective Nb in the ferrite phase. Therefore, C and N are preferably low, and are preferably 0.03% or less.

Si: Si is an effective element for improving oxidation resistance. However, if excessively added, the hardness rises and the workability and toughness decrease, so the range is 0.1 to 0.8%.

Mn: Mn fixes S, which is harmful to welding hot cracking, in the form of MnS, as shown in the above test results, to remove and reduce S in the weld metal. Although the reduction of S itself is also effective, it has been found that satisfactory when the relationship of Mn / S? 200 is satisfied. On the other hand, Mn improves scale peel resistance by adding 0.6% or more in terms of scale peel resistance as described above.

Therefore, it is necessary to set Mn to 0.6 to 2.0% and satisfy | fill the relationship of Mn / S≥200.

S: Since S is harmful to welding high temperature crack as described above, the lower one is preferable, but the lower one, the higher the production cost.

In the present invention steel, even though S is allowed up to 0.006%, the upper limit of S is set to 0.006% because S has sufficient weld high temperature cracks by the action of Mn as described above.

Ni: As can be seen from the examples, Ni has the same toughness improving effect as Cu. However, when excessively added, an austenite phase precipitates at high temperatures, which may cause a decrease in thermal fatigue characteristics due to an increase in the coefficient of thermal expansion. For this reason, it turned out that Ni + Cu must satisfy | fill the relationship within 4% in the complex addition with Cu which is an austenite formation element. From this result, the upper limit was 4%.

Cr: Cr is an indispensable element for improving corrosion resistance and oxidation resistance.

The lower limit is 17% because the addition of 17% or more is necessary to maintain the oxidation resistance at 900 ° C or higher. Higher Cr is preferable in terms of oxidation resistance, but excessive addition of steel leads to weakening of the steel, and deterioration of workability as the hardness increases, so the upper limit is made 25%.

Nb: Nb is an element necessary for maintaining high temperature strength. In addition, it has a good effect on the improvement of workability and oxidation resistance and the manufacture of pipe by high frequency welding. As can be seen from the results of the high temperature tensile test of the second table described later, it is necessary to add at least 0.2% to improve the high temperature strength. However, since Nb forms compounds by C and N, even if only the lower limit is 0.2%, the amount of solid solution Nb decreases by the amount of C and N, and the effect of Nb on high temperature strength decreases.

Therefore, it is necessary to satisfy the relationship that [Nb] according to the formula of [Nb] = Nb%-8 (C% + N%) becomes 0.2% or more. On the other hand, when Nb is added excessively, the welding hot cracking sensitivity becomes high.

The upper limit of Nb is made 0.8% so that sufficient high temperature strength is maintained and furthermore, it does not affect welding hot cracking susceptibility very much.

Mo: Mo increases the high temperature strength as added as described in the above test results. It is also effective for improving high temperature oxidation and corrosion resistance. On the other hand, when excessively added, the toughness at low temperature is remarkably lowered, and the productivity and workability are lowered. Therefore, 1.0%-4.5%, preferably 2.0-4.5%, more preferably more than 2.5%-4.5%.

Cu: Cu is also an extremely effective element in terms of toughness and an important element of the steel of the present invention, as described in the above test results.

In order to obtain the toughness improving effect, as shown in FIG. 4, 0.1% or more is required since the lower limit is required. On the other hand, when it adds excessively, it will become hard and will damage workability. In addition, the upper limit is 2.5% because it significantly affects hot workability.

Al: Al improves high temperature oxidation resistance. However, when it adds excessively, it becomes a problem in manufacturability and weldability, so an upper limit is made into 0.5%.

Ti: Ti increases the high temperature strength and also improves the workability. However, when added excessively like Al, it becomes a problem in manufacturability and weldability, so an upper limit is made into 0.5%.

V: V, like Ti, increases the high temperature strength and improves the workability. However, excessive addition causes a decrease in strength. Therefore, an upper limit is made into 0.5%.

Zr: Zr increases the high temperature strength and improves the high temperature oxidation characteristics. However, excessive addition causes a decrease in strength, so the upper limit is made 1.0%.

W: W, like Ti and V, increases the high temperature strength and improves the workability. However, excessive addition will cause a decrease in strength, so the upper limit is made 1.5%.

B: B improves hot workability, raises high temperature strength and improves workability. However, since excessive addition will cause a decrease in hot workability, the upper limit is made 0.01%.

REM: Rare earth element improves hot workability by trace addition, and improves oxidation resistance, especially adhesion of scale. However, since excessive addition will cause a decrease in hot workability, the upper limit is made 0.1%.

EXAMPLE

Table 1 shows the chemical composition values of the samples.

M1-M21 is a steel of this invention, M22-M30 is a comparative steel.

These steels were made in a laboratory by making a 30 kg steel mass and forging them with 25 mm round rods and 25 mm thick plates. The round rods were annealed at 950 ° C.-1100 ° C. and then processed into high temperature tensile test pieces of JIS standard.

The forged plate was cut and then hot-rolled at 1200 ° C. to obtain a hot rolled plate having a thickness of 5 mm, annealed at 950 ° C. to 1100 ° C., and then partly processed into a Charpy impact test piece. The remaining part was cold rolled and annealed to make a sheet thickness of 2 mm, a high temperature oxidation test was performed, and a weld hot crack test was performed at a plate thickness of 1.2 mm.

Table 2 shows the high temperature tensile strength according to the high temperature tensile test conducted according to JIS standard, scale peeling amount by 100 hours continuous oxidation test at 900 ° C and 1000 ° C, and critical deformation amount during welding by welding high temperature crack test described in the text. And the result of the Charpy test conducted with the V notch Charpy impact test piece at a plate thickness of 4.5 mm.

From the results in Table 2, it can be seen that the high temperature strength is increased as Nb, Mo, and Ni are added.

In addition, it can be seen that the high temperature strength of the composite additive steel of Mo and Cu is further increased. In the results of continuous high temperature oxidation test, it can be seen that scale peel resistance is remarkably improved when Mn content exceeds 900% in both 900 ° C and 1000 ° C. In addition, it can be seen that the critical deformation amount in the welding hot crack test is remarkably improved when Mn / S exceeds 200. On the other hand, the Charpy impact test results show that the impact toughness decreases as Mo is added, but the toughness is improved by adding Cu, and the same effect can be obtained by adding Ni.

As described above, according to the present invention, in addition to excellent high temperature strength and high temperature oxidation characteristics, it is excellent in solder joint high temperature cracking, and furthermore, low temperature toughness, which is a defect of ferritic stainless steel, is also improved. Obtained.

Therefore, it is possible to provide a novel product useful as an exhaust pipe system material, in which a pipe is manufactured by welding between an engine and a converter, especially an exhaust gas system material, in which future engines can cope with high output and high performance. .

Claims (3)

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% to 25.0%, Nb: 0.2 to 0.8 %, Mo: 1.0-4.5%, Cu: 0.1-2.5%, N: 0.03% or less, provided that the ratio of Mn% / S% is 200 or more, [Nb] = Nb% -8 (C % + N%), these elements are contained so as to satisfy the relationship of [Nb] of 0.2 or more and Ni% + Cu% of 4 or less, and the remaining part is composed of Fe and unavoidable impurities in manufacturing. Ferritic stainless steel with excellent low temperature toughness, weldability and heat resistance. 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% to 25.0%, Nb: 0.2 to 0.8 %, Mo: 1.0 to 4.5%, Cu: 0.1 to 2.5%, N: 0.03% or less, containing 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, in addition to containing one or two or more, the ratio of Mn% / S% in the above range is 200 or more, [Nb] = Nb% Contain these elements such that [Nb] according to the formula of 8 (C% + N%) is 0.2 or more and Ni% + Cu% is 4 or less, and the remaining portions are Fe and inevitable impurities Ferritic stainless steel for low temperature toughness, weldability and heat resistance. An exhaust gas pipe of an automobile manufactured from ferritic heat resistant stainless steel according to claim 1.