JP5141295B2 - Ferritic stainless steel with excellent thermal fatigue properties - Google Patents

Description

  The present invention relates to an environment subject to repeated thermal cycles, such as exhaust system members such as exhaust manifolds, exhaust pipes and converter cases of automobiles and motorcycles, exhaust ducts of thermal power plants, heat exchangers, and fuel cell-related members. The present invention relates to a ferritic stainless steel having excellent thermal fatigue characteristics used for members used below.

  Automotive exhaust system members represented by exhaust manifolds, exhaust pipes, converter cases, and the like are repeatedly subjected to a thermal cycle of temperature rise to a high temperature range and temperature fall to room temperature as the engine is started and stopped. For this reason, these members are restrained due to the surroundings of the members and the shape of the members, and free expansion and contraction are hindered. As a result, when a material inferior in thermal fatigue characteristics is used for the above member, the material is repeatedly used and breaks or exhaust gas leaks.

  Generally, the thermal fatigue experienced by the material used for the exhaust system member becomes more severe as the maximum temperature of the thermal cycle is higher. However, since the engine combustion temperature of automobiles tends to increase year by year, with the trend toward lower emissions, the materials used for exhaust system members are highly desired to have better thermal fatigue characteristics than ever before. ing.

  As a material for exhaust system members that can cope with higher exhaust temperature, SUS444 (19 mass% Cr-1.8 mass% Mo-0.2 mass% Nb), which is a ferritic stainless steel added with Nb or Mo, has been conventionally used. It has been known. Several other materials for exhaust system members have been developed. For example, Patent Document 1 discloses that oxidation resistance at a high temperature exceeding 900 ° C. is obtained by adding Si and Mo in combination after reducing Si to a certain amount or less and maintaining workability at room temperature. Enhanced ferritic stainless steel has been proposed.

  Further, in Patent Document 2, Cu is added to ferritic stainless steel, and Cu precipitates are finely precipitated in a medium temperature range of 600 to 750 ° C. Further, in a high temperature range of 800 ° C. or higher, fine precipitation is performed. The strength, heat fatigue resistance and oxidation resistance required for exhaust system parts used in exhaust gas environments in the high temperature range of 800 ° C or higher are provided by re-dissolving the Cu precipitates that have been re-dissolved to enhance solid solution. Ferritic stainless steel has been proposed.

  Further, Patent Document 3 discloses a ferrite having improved thermal fatigue characteristics by keeping the existence form of Cu precipitates (ε-Cu phase) in a specific state before being used for an exhaust gas member. Stainless steel has been proposed.

Furthermore, in Patent Document 4, in the Cr-containing steel containing Nb, the content of Si and Cr is reduced, and by adding an appropriate amount of Mo in relation to the Si content, precipitation of the Laves phase is suppressed, By making Mo into a solid solution-Mo main form, steel has been proposed in which the high temperature strength is improved without increasing the normal temperature strength and the occurrence of abnormal oxidation is suppressed.
JP 2004-018921 A JP 2000-297355 A JP 2006-117985 A Japanese Patent Laid-Open No. 2002-212585

  However, the steel of the above prior art cannot sufficiently cope with the recent increase in exhaust gas temperature, and development of a material having more excellent thermal fatigue characteristics is desired.

  Accordingly, an object of the present invention is to advantageously solve the above-mentioned problems of conventional exhaust system member materials, and to provide a ferritic stainless steel having much superior thermal fatigue characteristics than conventional materials.

  The inventors focused on the effect of additive elements on the thermal fatigue properties of ferritic stainless steel, and contained Cr: 15 mass%, Mo: 1.8 mass%, and Nb: 0.5 mass% described in Patent Document 4. It was studied to improve the thermal fatigue characteristics by adding W to this steel and further adding Cu thereto. As a result, it was found that a ferritic stainless steel excellent in thermal fatigue characteristics can be obtained by adding an appropriate amount of W and Cu to the above base component steel, and further studying this knowledge, The present invention was developed.

That is, the present invention includes C: 0.020 mass% or less, Si: 0.25 mass% or less, Mn: 2.0 mass% or less, P: 0.060 mass% or less, S: 0.008 mass% or less, Cr: 13.2. 0 to 16.0 mass%, Ni: 1.0 mass% or less, N: 0.020 mass% or less, Nb: 0.4 to 0.6 mass%, Cu: 1.3 to 1.9 mass%, Mo: 1.0 ~ 2.5 mass%, W: 2.0 to 2.93 mass%, B: 0.0005 to 0.0100 mass%, the balance being ferritic stainless steel having a component composition consisting of Fe and inevitable impurities is there.

  In addition to the above component composition, the ferritic stainless steel of the present invention further includes Ti: 0.5 mass% or less, Zr: 0.5 mass% or less, Co: 0.5 mass% or less, and V: 0.5 mass% or less. It contains 1 type or 2 types or more chosen from these.

  According to the present invention, it is possible to provide a ferritic stainless steel that has excellent thermal fatigue characteristics compared to conventional materials. Therefore, the ferritic stainless steel of the present invention is suitable as a material for exhaust system members such as automobiles.

The basic experiment that triggered the development of the present invention will be described.
C: 0.007 mass%, N: 0.008 mass%, Cr: 15 mass%, Nb: 0.5 mass%, Mo: 1.8 mass% and Cu: 1.4 mass%, W is 1 mass% and 3 mass% Further, the steel added by changing Cu in the range of 0 to 1.8 mass% to the two types of steel contained is melted in a vacuum melting furnace to form a 100 kg steel ingot, and then this steel ingot is heated to 1100 ° C. It was heated and hot-rolled to obtain a 30 mm-thick sheet bar, and further forged into a square bar having a thickness of 30 mm and a width of 30 mm. This square bar was held at a temperature of 1000 to 1150 ° C. for 1 to 5 minutes, and then subjected to a finish heat treatment for cooling at a cooling rate of 20 ° C./sec or more, and then the shape shown in FIG. 1 (the finest diameter 8 mmφ) was tested. The piece was processed into a thermal fatigue test.

  As shown in FIG. 2, in the thermal fatigue test, after holding for 90 seconds at a minimum temperature of 200 ° C., the temperature is raised to a maximum temperature of 880 ° C. at 6 ° C./sec. A heat treatment was repeatedly applied with a heat cycle that lowered the temperature at −6 ° C./sec until 1 cycle. At this time, the apparent strain was controlled by an electromechanical system to be 60% of the free thermal expansion / contraction strain (constraint rate was 40%). The thermal fatigue life was defined as the number of cycles when the maximum load was reduced to 90% with respect to the maximum load generated in the fifth cycle when the load-strain hysteresis loop was stabilized.

The results of the thermal fatigue test are shown in FIG. From this result, when Cu is added to steel containing 1.0 mass% and 3.0 mass%, the tendency of thermal fatigue life to be improved by adding Cu: 1.3 mass% or more is recognized in any steel. However, it has been found that the effect of improving the thermal fatigue life by adding Cu is larger in the steel containing W: 3 mass%, and the thermal fatigue life exceeding 1000 cycles can be obtained by adding Cu: 1.3 mass% or more.
The present invention is based on the above findings.

Next, the component composition that the ferritic stainless steel of the present invention should have will be described.
C: 0.020 mass% or less C is an element that increases the strength of steel, but if it exceeds 0.020 mass%, the deterioration of toughness and formability becomes significant. In particular, when emphasizing moldability, C is preferably as low as possible, so C: 0.020 mass% or less. Preferably it is 0.008 mass% or less.

Si: 0.25 mass% or less Si is added as a deoxidizer and as a steel strengthening element. However, excessive addition of Si reduces toughness, so it is made 0.25 mass% or less. However, if the amount is reduced too much, the oxidation resistance deteriorates, so it is preferably 0.01% by mass or more. More preferably, it is the range of 0.01-0.15 mass%.

Mn: 2.0 mass% or less Mn is an element that acts as a deoxidizer and improves the adhesion of the oxide film. However, when added excessively, coarse MnS is formed, and moldability and corrosion resistance are lowered. Therefore, in this invention, it is set as Mn: 2.0 mass% or less. Preferably it is 1.5 mass% or less.

P: 0.060 mass% or less P is an impurity inevitably mixed in steel, and is also a harmful element that lowers formability and toughness. Therefore, it is desirable to reduce it as much as possible. However, excessive reduction increases the de-P cost, so P: 0.060 mass% or less. Preferably, it is 0.030 mass% or less.

S: 0.008 mass% or less S is an impurity inevitably mixed in the steel, and is also a harmful element that lowers the corrosion resistance. Therefore, it is desirable to reduce it as much as possible. However, excessive reduction increases the S-removal cost, so S: 0.008 mass% or less. Preferably, it is 0.005 mass% or less.

Cr: 13.0 to 16.0 mass%
Cr is an important element that improves the corrosion resistance and oxidation resistance, which are the basic characteristics of ferritic stainless steel, and such an effect is recognized by addition of 13.0 mass% or more. However, excessive addition causes a decrease in toughness, so the upper limit is made 16.0 mass%. Preferably, it is in the range of 14.0 to 16.0 mass%.

Ni: 1.0 mass%
Ni is an element effective for improving toughness. However, excessive addition causes an increase in raw material cost, so 1.0 mass% or less. Preferably, it is the range of 0.01-0.8 mass%.

N: 0.020 mass% or less N, like C, is an element that increases the strength of steel. However, if the content is 0.020 mass% or more, the toughness and formability are significantly lowered. Therefore, the upper limit of N is 0.020 mass%. Preferably it is 0.010 mass% or less.

Nb: 0.4 to 0.6 mass%
Nb improves the formability and corrosion resistance of steel by fixing C and N. Moreover, it has the effect of increasing high-temperature strength by dissolving in steel. Such an effect is recognized when the content is 0.4 mass% or more. On the other hand, excessive addition causes a decrease in toughness, so the content is made 0.6 mass% or less. Preferably it is the range of 0.45-0.55 mass%.

Cu: 1.3-1.9 mass%
Since Cu precipitates finely in the thermal cycle that the exhaust system member receives and improves the yield strength of the steel, it has a function of solid-dissolving in the steel at a high temperature and strengthening the steel, so it is extremely important in the present invention. It is an element. However, Cr: 15 mass%, Mo: 1.8 mass%, Nb: 0.5 mass% as a base, in the component steel in which W is added thereto, the range in which the effect of the Cu addition is expressed is limited, Remarkably observed with addition of 1.3 mass% or more. However, excessive addition causes embrittlement of the steel, so the upper limit is made 1.9 mass%. Preferably it is the range of 1.4-1.8 mass%.

Mo: 1.0-2.5 mass%
Mo is an element that enhances high-temperature strength and oxidation resistance by dissolving in steel, and is an important component in the present invention. These effects are recognized when Mo: 1.0 mass% or more is added. However, excessive addition causes an increase in raw material cost, so it is limited to 2.5 mass% or less. Preferably, it is the range of Mo: 1.3-2.0 mass%.

W: 2.0-4.0 mass%
W, like Mo, is an element that improves the high-temperature strength and oxidation resistance by dissolving in steel, and is an important component in the present invention. These effects are recognized when W: 2.0 mass% or more is added. However, excessive addition causes an increase in raw material cost, so it is limited to 4.0 mass% or less. Preferably, it is the range of W: 2.0-3.5mass%.

B: 0.0005 to 0.0100 mass%
B is an element that improves workability, particularly secondary workability. Such an effect is recognized at 0.0005 mass% or more. However, when 0.0100 mass% or more is added, BN precipitates and the workability is lowered, so the upper limit is set to 0.0100 mass%. Preferably, it is in the range of 0.0005 to 0.0050 mass%.

In addition to the above components, the ferritic stainless steel of the present invention may further contain one or more selected from Ti, Zr, Co and V in the following range.
Ti: 0.5 mass% or less Ti is an element that improves formability, and has an effect of increasing the solid solution amount of effective Nb because the affinity with C and N is preferentially bonded with priority over Nb. Such an effect is recognized at Ti: 0.02 mass% or more, but when added exceeding 0.5 mass%, coarse Ti (C, N) precipitates and deteriorates the surface properties. Therefore, the upper limit of Ti is preferably 0.5 mass%. More preferably, it is the range of 0.02-0.4 mass%.

Zr: 0.5 mass% or less Zr is an element that improves formability, like Ti, and has an affinity for C and N that is strongly prioritized and bonded to Nb, and therefore increases the amount of solid solution of effective Nb. Such an effect is recognized at 0.02 mass% or more, but if it exceeds Zr: 0.5 mass%, a Zr intermetallic compound precipitates and the steel is embrittled. Therefore, the upper limit of Zr is preferably 0.5 mass%. More preferably, it is the range of 0.02-0.4 mass%.

Co: 0.5 mass% or less Co is an element effective for increasing the high-temperature strength, and can be added as necessary. Although this effect is recognized by addition of 0.1 mass% or more, excessive addition causes an increase in cost, so the upper limit is preferably set to 0.5 mass%. More preferably, it is the range of 0.3-0.5 mass%.

V: 0.5 mass% or less V is an element effective for improving moldability. However, excessive addition exceeding 0.5 mass% causes coarse V (C, N) to precipitate and deteriorates the surface properties. For this reason, when adding V, it is preferable to set it as 0.5 mass% or less.

  In the steel sheet of the present invention, the balance other than the above components is Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

  The method for producing the ferritic stainless steel according to the present invention is not particularly limited, and a known method can be applied. For example, steel having a component composition suitable for the present invention is melted by a known method such as a converter or an electric furnace, and further subjected to secondary refining such as ladle refining or vacuum refining, and then continuous casting. Steel strip (slab) is formed by the method or ingot-bundling rolling method. Then, it is preferable to make it a cold-rolled annealing board through each process, such as hot rolling, hot-rolled sheet annealing, pickling, cold rolling, finish annealing, pickling. The cold rolling may be performed once or twice or more with intermediate annealing. Further, in addition to cold rolling, finish annealing and pickling steps may be repeated. Moreover, you may abbreviate | omit hot-rolled sheet annealing. Furthermore, when glossiness is required on the steel sheet surface, skin pass rolling may be performed.

  Steel having the composition shown in Table 1 is melted in a vacuum melting furnace to form a 100 kg steel ingot, and then the steel ingot is heated to 1200 ° C. and hot-rolled into a 30 mm thick sheet bar. Forged into square bars with a thickness of 30 mm and a width of 30 mm. Next, after holding the plate at 1000 to 1150 ° C. for 1 to 5 minutes and then subjecting it to a finish heat treatment at a cooling rate of 20 ° C./sec or more, the test of the shape shown in FIG. The piece was processed into a thermal fatigue test.

  The thermal fatigue test shown in FIG. 2 was held at a minimum temperature of 200 ° C. for 90 seconds, then heated to 6 ° C./sec to a maximum temperature of 880 ° C., held at this temperature for 120 seconds, and again to a minimum temperature of 200 ° C. The heat treatment which makes the heat cycle which falls at 6 degrees C / sec 1 cycle was repeatedly provided. At this time, the apparent strain was controlled to be 60% (constraint rate: 40%) of the free thermal expansion / contraction strain by an electromechanical system. The thermal fatigue life is the number of cycles when the maximum load is reduced to 90% of the maximum load generated in the fifth cycle when the load-strain hysteresis loop is stabilized, and the number of cycles exceeds 1200. Was evaluated as being excellent in thermal fatigue properties (◯).

  The results of the thermal fatigue test are also shown in Table 1. From Table 1, it can be seen that all the ferritic stainless steels satisfying the component composition of the present invention have a thermal fatigue life exceeding 1200 cycles and have good thermal fatigue characteristics. On the other hand, the steel of the comparative example which deviates from the component composition of the present invention has a thermal fatigue life of not reaching 1200 cycles.

  Since the ferritic stainless steel of the present invention has excellent thermal fatigue characteristics, it can be used as an exhaust path member of a thermal power generation system or a solid oxide fuel cell member in addition to an exhaust system member of an automobile or the like. . Moreover, since the steel plate of this invention contains Cr and Mo, it can be used also as a corrosion-resistant steel plate.

It is a figure explaining a thermal fatigue test piece. It is a figure explaining the heating cycle of a thermal fatigue test, and a distortion cycle. It is a graph which shows the influence of W and Cu addition on the thermal fatigue life of Nb and Cr containing steel.

Claims (2)

C: 0.020 mass% or less,
Si: 0.25 mass% or less,
Mn: 2.0 mass% or less,
P: 0.060 mass% or less,
S: 0.008 mass% or less,
Cr: 13.0 to 16.0 mass%,
Ni: 1.0 mass% or less,
N: 0.020 mass% or less,
Nb: 0.4 to 0.6 mass%,
Cu: 1.3-1.9 mass%,
Mo: 1.0 to 2.5 mass%,
W: 2.0~ 2.93 mass%,
B: 0.0005 to 0.0100 mass% is contained,
The balance is ferritic stainless steel having a composition composed of Fe and inevitable impurities. In addition to the above component composition, Ti: 0.5 mass% or less, Zr: 0.5 mass% or less, Co: 0.5 mass% or less, and V: 0.5 mass% or less The ferritic stainless steel according to claim 1, comprising:

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